Adsorption Phenomena

Adsorption Phenomena J. H. DE BOER Staatsmijnen in Liinburg, Central Laboratory, Geleen, The Netherlands Page I. Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Adsorption and Catalysis ..................................... 19 111. Physical Adsorption and isorption . . . . . . . . . . . . . IV. Difficulties Experienced in Calculating Adso 1. General Remarks.. . . . . . . . . . . . . . . . . . . . . . 2. The Structure of a Smooth Surface of an 3. The Distance between an Adsorbed Molecule and the Surface.. . . . . . . 24 4. The Influence of the Repulsion Forces.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 V. The Forces that Cause Adsorption .................... 29 1. The Nonpolar van der Waals’ Attraction Forces.. . . . . . . . . . . . . . . . . . . 29 2. Nonpolar van der Waals’ Forces on Conducting Surfaces. . . . . . . . . . . . . 3 1 3. The Adsorption of Ions on Metal Surfaces.. . . . . . . . . . . . . . . . . 32 4. The Adsorption of Ions on Dielectric Sur 5. The Adsorption of Polar Molecules.. . . . . . . . . . . . . . . . . . . . . . 35 ielectric Adsorbent. . 37 6. The Polarization of an Adsorbed Molecule 7. The Polarization of an Adsorbed Molecule by a Conducting Adsorbent. 38 8. Chemical Bonds in Adsorption Phenomena on Metals. . . . . . a. The Formation of Ions.. . . . .................... 39 b. Sharing of Electrons.. . . . . . 9. Heats of Adsorption and Desorption nected with Chemisorption on Metals. . . . . . . . . . . . . . . . . . . . . . . 48 10. The Influence of the Electronic Structu 11. Chemical Bonds in Adsorption Phenomena on Nonmetallic Surfaces.. . 57 57 a. Carbon Monoxide Adsorption.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .......................... 59 b. Hydrogen Adsorption. . . . . . c. Oxygen Adsorption.. . . . . . . ............... d. Hydrocarbons. . . . . . . . . . . . .................. e. Hydrogen Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 12. Active Spots. . . . . . . . . . . . . . .......................... 61 VI. Cooperation among Various Forces. . . . . . . . . . . . . . . . . . . . . . . . . 64 1. Physical Adsorption on Charcoal (and Metals). . . . . 2. The Adsorption on Ionic Surfaces .......................... 65 3. The Adsorption of Hydrogen.. . . . . . . . . . . . . . . . . . . . . . . . . . . 68 4. The Chemisorption of Oxygen.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 5. Optical and Other Physicochemical Changes by Adsorption.. . . . . . . . . . 79 VII. Mobility and Orientation. . . . . . . . . . . . . . . . . . . . . . 1. Mobility on Charcoal. . . . . . . . . ............................ 81 2. Orientation or Rotation., . . . . . . . . . . . . . . . . . . . . . . . 3. Hopping Molecules. . . . . . . . . ............................ 83 4. The Time of Adsorption.. . . . . . . . . . . . . . . . . . . 17

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Page 91 92 7. Solution in the Adsorbent (Catalyst). , . . . . . . . . . . . . . . . . . 96 VIII. Physical Adsorption Phenomena a t High ccupation.. . . . . . 98 1. General Remarks.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 2. The Heat of Adsorption as a Function of the Degree of Occupation in Physical Adsorption Phenomena on Conducting Adsorbents.. . . . . . . . . 98 3. The Heat of Adsorption as a Function of the Degree of Occupation in Physical Adsorption Phenomena on Ionic Surfaces. . . . . . . . . . . . . . . . . . 100 4. Two-Dimensional Condensation and Multimolecular Adsorption. . . . . . 104 IX. Chemisorption Phenomena at High Degrees of Occupation.. . . . . . . . . . . . . 107 1. The Decrease of the Heat of Chemisorption with Increasing Degrees of Occupation.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 2. Factors that Cause a Surface Heterogeneity for Chemisorption. . . . . . . 109 3. Experimental Methods to Study the Hete Character of a Surface for Chemisorption. . 4. The Heats of Adsorption of Cesium Atoms o .......................................... 116 e of the Bonds at High Degrees of Occupation. . 123 6. The Decrease of the Heat of Chemisorption with Increasing Degree of Coverage for Other Adsorptives. . . . . . . . . . . . . . . . . . . . . . . . 125 7. Advantages and Disadvantages of Metal Surfaces Prepared by Different Methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 8. Other ExpIanations for the Decrease f Chemisorption with Increasing Coverage. . . . . . . . . . ............................ 128 9. Changes in Activation Energy creasing Degree of Occupation. . 131 134 10. Equations for Chemisorption Isotherms. . . . . . . . . . . . . . . . . . . . . . . . . . . . 11. Some Effects in Chemisorption Phenomena that are Connected with ...................................... 136 Activation Energies, . 12. Restricted Chemisorption Caused by the Increase of the Activation Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 13. Some Final Remarks with Respect to the Decrease of the Heat of Chemisorption with Increasing Amount of Adsorbed Material. . X. Simultaneous Adsorption of Different Molecules or Atoms.. . . . . . . 1. Simultaneous Adsorption of Different Molecules in Physical Adsorption ..................... 140 2. Simultaneous Adsorption of Different Species in Chemisorption; the 141 Relative Amounts that are Chemisorbed. . . . . . . . . . . . . . . . . . . . . . . . . . 3. The Chemisorption of Different Atoms Giving Dipoles of the Same Sign 143 4. Contaminated Surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 5. Mutual Assistance of Chemisorbed Atoms.. . . . . . . . . . . . . . . . . . . . . . . . 147 XI. Some Remarks on Catalysis and Chemisorption.. . . . . . . 1. Heat of Chemisorption and Catalysis.. . . . . . . . . . . . . . 2. Endothermic Chemisorption ................................. 149 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150 5. Mobility and Reactivity.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Induced Mobility of Atoms of the Surface.. . . . . . . . . . . . . . . . . . . . . . . . .

I. INTRODUCTION The field of adsorption is usually divided into two main domains, viz., the domain of physical adsorption and the domain of chemical adsorption,

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or chemisorption. Both domains have already been treated, separately, in this series, physical adsorption by Hill ( I ) and, more recently, chemisorption by Kwan ( 2 ) . Both authors make definite statements with respect to the scope of their articles. Hill’s article gives recent aspects of the statistical thermodynamical theory of physical adsorption, which has been the field of much of his own work, and Kwan devotes a great part of his article to recent contributions by Japanese authors. We shall, therefore, try to focus our attention somewhat more closely on some other aspects in both domains of the field of adsorption. The author shall not give a detailed survey of his own work on adsorption, which started in 1925; nevertheless examples from his own work may be used t o illustrate points in the argument. The author will also take the opportunity to put forward, in this article, his own views on some aspects. These views have not been published before; some have proved their fruitfulness in the work of several of the author’s present collaborators, others may be considered to be of a tentative character. It will be clear that some “ established” views of 25 to 30 years ago may be less established now, and the author intends to concentrate attention on one or two cases where a completely new interpretation of experimental facts has revolutionized our ideas or is about to do so. 11. ADSORPTION AND CATALYSIS The present article is meant to throw some light on adsorption problems related to catalysis. It is obvious that only the so-called “heterogeneous” catalysis is concerned. Here the reaction takes place a t the interface between the catalyst (solid or liquid) and the phase which contains the reacting molecules (liquid or gas). Such a heterogeneous catalyst acts as an adsorbent for at least one of the reacting elements. Adsorption may be caused by “physical” forces, comparable with those which are responsible for liquefaction of gases, or by “chemical” forces, similar to those acting in the formation of normal chemical compounds. It is customary, consequently, to distinguish between physical adsorption (also called van der Waals’ adsorption, because of the nature of these physical cohesion forces) and chemical adsorption, or chemisorption. In some cases of heterogeneous catalysis it is the mere fact of adsorption which, by increasing the L‘concentration’)of the reacting molecules, accelerates the reaction. The expression for the rate v of a chemical reaction can be given as v =

f

a

e-E/RT.

where f is the so-called frequency factor, E the activation energy, R the gas constant, and T the absolute temperature. TC: is the product of the

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J. H. DE BOER

concentrations Ci,each t o its appropriate power n. It is the last concentration factor which is influenced in this case. The adsorption may be either of a physical or of a chemical nature. There are examples of both kinds of adsorption leading to catalysis by this mechanism. A homogeneous catalyst cannot act in this way. I n other cases catalysis is caused by the catalyst-homogeneous or heterogeneous-taking away the heat of reaction, thus influencing the frequency factor f. A heterogeneous catalyst is advantageous in this case, because it has far more possibilities of dissipating energy than a homogeneous catalyst. The adsorption, again, may be caused by physical as well as by chemical forces. I n most cases of catalysis, however, the increase in the rate of the reaction is caused by the catalyst lowering the activation energy, El of the reaction. I n order to do this, the catalyst, by entering into a chemical combination with one of the reactants, must alter the properties of its molecules. I n cases of homogeneous catalysis addition-compounds are formed between the catalyst and one of the reacting elements, or there is an exchange of an electron between the catalyst and the molecule which is influenced. I n heterogeneous catalysis the same happens. When a molecule of one of the reactants enters into a chemical combination with the surface of the catalyst (which means with one or more of the constituent atoms of the surface or with the surface as a whole), it is chemisorbed. There is, in chemisorption phenomena, very often an exchange of an electron between the adsorbed molecule-which may or may not break up into smaller units (atoms, ions, or radicals)-and the adsorbent. I n other cases of chemisorption the adsorbed atom or radical shares electrons with one of the constituent atoms of the adsorbing surface. One might be inclined to think that only chemisorption would lead to a n enhanced reactivity in this type of catalysis. Adsorption by physical forces only tends t o lower the reactivity of the adsorbed molecules. It is, however, difficult to give such definitions of physical adsorption and chemisorption that the fields are clearly separated. We shall, therefore, discuss some of the differences and similarities between these two kinds of adsorption.

111. PHYSICAL ADSORPTION AND CHEMISORPTION

It is in many cases difficult to decide whether a certain adsorption phenomenon belongs to the physical adsorption type or whether it is a case of chemisorption. If we define physical adsorption as the phenomenon which occurs when the molecules are bound t o the surface of the adsorbent by van der Waals’ cohesion forces in the widest sense of the word, hence

ADSORPTION PHENOMENA

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including quadrupole, permanent dipole, and induced dipole attraction, and if we define chemisorption as the phenomenon which occurs when the binding of the molecules to the surface is caused by an exchange or a sharing of electrons, we may have a distinguishing principle, but we certainly have not an easy means of analysis. There is sometimes a tendency t o believe that the heat of adsorption gives a n indication of the kind of adsorption. The forces acting in physical adsorption phenomena are the same as those which cause liquefaction. It may therefore be expected that the heats of adsorption will be of the same order of magnitude as the heats of liquefaction of gases, The forces which are responsible for chemisorption are the same as those leading t o chemical combinations, and one expects the heats of chemisorption t o be of the same magnitude as the heats of formation of these compounds. There is consequently a tendency to believe that the heats of adsorption in physical adsorption phenomena do not include high values and th a t chemisorption is characterized by high heats of adsorption. True, the heat of adsorption in physical adsorption phenomena does, generally speaking, not include very high values. Figures of 20 kcal./mole or higher do, however, occur. I n many cases the heat of adsorption in chemisorption phenomena is high; the heat of adsorption of oxygen on some metals may be of the order of magnitude of a few hundred kilocalories/mole. On the other hand the heat of adsorption in other cases of chemisorption may even be negative, as is the case with the negative heat of formation of endothermic compounds. Chemisorption is often identified with the so-called “activated ” adsorption, where the rate of adsorption is governed by an activation energy. We shall have t o conclude in our following discussions th a t this criterion is not valid. One might suppose that in chemisorption phenomena the adsorbed atoms or molecules occupy fixed places on the surface and that physically adsorbed molecules show some freedom of movement over the surface. We shall see, however, that “mobile” adsorption and adsorption on definite adsorption sites may occur in both cases. The absorption of light by certain molecules is sometimes drastically changed when they are adsorbed. This phenomenon undoubtedly has some relation t o the nature of the binding forces and one might be inclined t o consider it as chemisorption. We shall be compelled in our following discussions t o shatter this conception. It is, of course, in many cases quite easy to decide whether we are dealing with chemisorption forces or with forces of a physical nature only. There are, however, many other cases where the decision will be less obvio w . There is no sharp boundary between the two domains of adsorption.

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IV. DIFFICULTIES EXPERIENCED IN CALCULATING ADSORPTION ENERGIES

I. General Remarks If the forces causing adsorption were known with a suficiently high degree of accuracy and if their dependence on the distance of the adsorbed atom or molecule from the adsorbing surface were also exactly known, it would, theoretically, be possible to calculate the change in potential energy accompanying the act of adsorption. Though we know the exact laws of the mutual attraction of two ions and though we have also a fair knowledge of some other forces which may participate in adsorption phenomena, there are other forces, especially the repulsion forces, of which the magnitude and the dependence on distance are hardly known. It is true that all molecular and atomic forces ultimately find their root in the mutual behavior of the constituent parts of the atoms, viz., the nuclei and the electrons. They may theoretically all be derived from the fundamental wave equations. It is, however, convenient, as in other branches of physics and chemistry, t o treat the various forms of mutual interaction of atoms as different forces, acting independently. We shall therefore follow the usual procedure and treat such forces as the nonpolar van der Waals (dispersion) forces, the forces of the electrostatic polarization of atoms or molecules by ions or by dipoles, the mutual attraction or repulsion Coulomb forces of ions and of dipoles, the exchange forces leading t o covalent bonds, the repulsion forces due to interpenetration of electronic clouds, together with the Pauli principle, etc., all as different, independently acting forces. I n all cases a t least two of these forces act simultaneously; a summation of their contributions t o the adsorption energy over all participant atoms has t o be made for each of the participating forces separately. Such a summation can in principle be applied with any desired degree of accuracy t o some of the forces mentioned, provided that the distances between the participating atoms are exactly known. The application of this procedure to calculating or estimating the magnitude of the contribution toward the energy of adsorption due t o these forces cannot lead, however, t o a high degree of accuracy, because we have, first, hardly any knowledge of the real structure of the adsorbing surface (even of a smooth surface) and, second, we know little about the real distance of the adsorbed atom from the surface. We shall deal first with these two serious setbacks (3).

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2. The Structure of a Smooth Surface of an Adsorbent

Usually in theoretical calculations of adsorption energies the surface of the adsorbent is idealized as if it were obtained by cutting a crystal into two halves with an ideally sharp razor blade. It is then generally assumed t ha t the atoms (or ions) of these freshly created surfaces do not alter their positions. From specular reflection experiments with molecular beams of hydrogen or helium ( 4 ) it may be concluded that cleavage surfaces of LiF or NaCl are very smooth surfaces indeed, the inequalities being caused only by the temperature movement and amounting to the order of magnitude of cm. Diffraction spectra obtained with helium beams ( 5 ) , indicate moreover (6) that the distance between two fluorine ions in the outer layer of LiF is exactly the same as in the crystal. The distances, however, from the ions of the outer layer to those of the second layer may have undergone material alterations. I n a theoretical study Verwey ( 7 ) came to the conclusion th a t the negative surface ions of the free surfaces of alkali halide crystals are generally displaced, so that their distances from the next layers of the lattice are increased, and the positive surface ions are displaced toward the inside of the lattice. The numerical results of his calculations for NaCl are t ha t the distance between the sodium ions of the outer layer and the ions of the second layer is 2.66 8.;whereas the chloride ions of the outer layer are located a t a distance of 2;86 8. from the second layer, the normal distance in the lattice being 2.81 A. The electrical double layer formed by the negative chloride ions, being located in a plane 0.20 A. distant from the plane of the positive sodium ions, is almost compensated for by the effect of the dipoles set up in the negative ions. Displacement of the outer ions and polarization of the negative ions have been used by various authors in attempts to calculate the surface energy of NaC1. The various figures obtained in these calculations vary from 77 erg/cm.2 to 155 erg/cm.2 The latter figure, calculated by Shuttleworth (8), is obtained from a model which included surface distortion, while the influence of the van der Waals forces was also accounted for. Experimental data for the surface energy of NaC1, are, however, mostly higher than the theoretical figures. The most recent experimental figure of 305 erg/cm.2 a t 25°C. b y Benson and Benson (9) is based on the cubic face, as are the theoretical figures. We may, from the rather scarce theoretical and experimental evidence, conclude t ha t the distance between like atoms (ions) in the surface layers will probably be the same as in the lattice itself. The distances between

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J. H. DE 130ER

different ions will probably be materially different from those in the lattice, and also the distances of all atoms (ions) of the surface layer from those of the second layer are different from the distance in the lattice. We may expect the atoms of the outer layer of a metal surface to be somewhat less close to the second layer than corresponding to the distances in the metal lattice. The edge atoms of the two-dimensional layers constituting the lattice of graphite (charcoal) may be expected to lie a t a somewhat smaller distance from their neighboring atoms than the normal C-C-distance in graphite (I 0). Considering the fact that both single (gaseous) molecules of alkali halides and crystals of alkali halides consist of ions, we may conclude that the outer layers of the crystals also consist virtually of ions. The same may be true for crystals of the alkaline earth halides and of some fluorides of other metals, such as lead fluoride, or of some complex fluorides such as potassium zirconium fluoride (K2ZrF6)(11). Several oxides, such as the alkaline earth oxides, form in their crystalline state lattices consisting of ions. The single molecules of these oxides, however, must be considered as having largely a homopolar character (12). It is, therefore, not certain that the constituent atoms of the outside layers of the crystals of these oxides are in the same ionic state as the ions inside the crystal; a more homopolar character is certainly possible. The same holds for many halides of heavy metals, such as AgCl and AgI. When these halides are suspended in water (colloidal state), the binding may well be of a more ionic character, owing to the surface hydration. 3. The Distance between an Adsorbed Molecule and the Surface

A more serious setback in calculating adsorption energies is our complete lack of knowledge of the real distance of the adsorbed atom or molecule from the adsorbing surface. Unfortunately most of the forces which cause adsorption greatly depend on this distance. As a matter of fact, the equilibrium distance between the adsorbed atom and the surface is determined by the equilibrium of all attraction and repulsion forces acting on the atom. Calculations of lattice energies and of sublimation energies from the various simultaneously acting forces have been very successful, because the distances between the ions or molecules in the crystals are well known from other sources. I n adsorption phenomena the distance between the adsorbed atom or molecule and the surface is completely unknown. Very often this distance is assumed to be the sum of the “radii” of the adsorbed atom and a surface atom. These radii are then derived from experimentally determined figures of distances of the same atoms in other combinations. I n a calculation of the energy of physical adsorption the radius of the adsorbed atom or molecule is often taken from an-

25

ADSORPTION PHENOMENA

other combination of a similar physical nature (a so-called “van der Waals’ radius”). The same should be done for the radius of the surface atoms, but generally far smaller figures are used in t8hiscase. Many calculated figures of adsorption energies given in the literature which, because of their close approximation to experimental values, are said t o prove that a supposed force is responsible for the adsorption phenomenon, are in fact fallacious, because of the improbably low figures for the “distances” used in obtaining these calculated results ( I S ) .

_-_--_--

to B

FIG.1. When the adsorption force depends on the polarizability of the adsorbent, the acting distance between the adsorbed molecule and the surface is far smaller if the region of idcal polarizability starts a t the outer peripheries of thc atoms of the adsorbent (van dcr Waals’ radii) (I A ) than if this region starts at a plane through the centers of the outer atoms (I B ) .

Another difficulty is met with in adsorption on metallic surfaces. Metals, or rather conducting bodies, are considered as adsorbents with an ideal polarizability. Accepting this view as true does not make it clear whether the metallic properties leading to this ideal polarizability should be assumed t o start at the outer peripheries of the surface atoms of the metal or whether we must assume these properties to be found from & plane through the centers of the surface atoms. The choice of the outer boundary of the region of conducting electrons is very important, however, for the assessment of the “distance” of the adsorbed atom to the metal. As we shall see in Secs. V,7,8 and VI,l, the forces th a t cause adsorption on metal surfaces are exercised by the adsorbent body as a whole, rather than by the constituent atoms. It is, therefore, important t o know the

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J. H. DE BOER

distance between the adsorbed atom and the metal surface. A glance at Fig. 1 will reveal that, depending on the assumption of where the polarizability starts, this distance may vary by a factor of about 2.

4. The InJluence of the Repulsion Forces Even if the exact structure of the adsorbing surface and also the exact distance between the adsorbed molecule and the surface were known t o us, we still should not be able to calculate the energy of adsorption, mainly because of our lack of knowledge of the repulsion forces. These repulsion forces arise from the interpenetration of the electronic clouds of the atoms. This, together with the Pauli exclusion principle, prevents the atoms from approaching each other too closely. It is extremely difficult to formulate the resulting repulsion forces in a mathematical expression. They increase strongly with decreasing distance between the atoms, but certainly not according t o a simple law. For certain cases simplified equations have proved useful. I n calculations of the lattice energies of ionic lattices of the alkali halides and the oxides of the alkaline earths, good results are obtained with a simple expression for the contribution t o the potential energy, owing to the repulsion forces, suggested by Born and Mayer (14):

Ere, = +be-r/p

(1)

where b and p are constants and r is the distance between the atoms (15,16). (The expression for this contribution to the potential energy E is given a positive sign, because it causes an increase in potential energy; the contributions to the potential energy given by attraction forces will, consequently, be given negative signs.) When, however, the .formation of single molecules of the alkali halides and their condensation to crystals is studied, expression (1) fails completely, as may be seen from Fig. 2, where the ratio of the interionic distances in the alkali halide molecules rm and in the solid alkali halide crystals r, is given as a function of r,. An earlier empirical expression

where b and n are constants, gives excellent results in this case (I?'). It transpires t ha t for the larger distances between the ions, as used in the calculations of lattice energies-integration from infinitely large distances t o the equilibrium distance in the lattice-expression (1) can be used. For smaller distances, however, it is better t o use the older expression (2) with a high value of n.

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ADSORPTION PHENOMENA

As we shall see in the next section, the potential energy of two atoms attracting each other with nonpolar van der Waals’ forces will decrease according t o a Eaut = - (3) r6 where a is a constant and r is the distance between the atoms.

3.5

3.0

-

4.0

rc

FIG.2. Ratio r,,,/rc as a function of r,. The theoretical lines A and B are drawn with the repulsion law (2) for TZ = 12 and n = 10 respectively. Line C is drawn with repulsion law (1) and p = 0.345 A., as found from the lattice energies of alkali halides. Experimental points for several alkali halides of the NaCl type are shown a8

+.

When van der Waals’ forces and repulsion forces act together, the potential energy is given by a b E = Eattr Erep = - p 7 (4)

+

+

If n = 12, we see that the repulsion forces balance the attraction forces a t P = rm, when 6a - 12b ,‘I

-13

or

At the equilibrium distance rm the total decrease in potential energy is, therefore,

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J. H . DE BOER

FIQ.3. E as afunction of r [Eq. (4)].Curve A gives the energy of attraction, curve R the energy of repulsion; curve W is the resultant curve.

which is only half of the value which would have been found but for the contribution of the repulsion forces (Fig. 3). Using a more general expression yields a

E=-r"+T"

b

(5)

we obtain for the decrease of potential energy at the equilibrium distance rm,

Without having a better knowledge of the laws of the repulsion forces we cannot expect to make exact calculations of the energies of adsorption. The influence of the repulsion forces is often neglected or accounted for by subtracting a fixed percentage, say 40%, from the adsorption energy as calculated with the attraction forces only (18).When adsorption is caused by van der Waals' forces only, the contribution of the repulsion

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forces is nearly completely counterbalanced by the contribution of correction terms for the attraction forces (19-21)- (See Sec. V,l.)

V. THE FORCES THAT CAUSEADSORPTION 1. T h e Nonpolar van der Waals’ Attraction Forces (22)

The attraction forces which act most frequently in physical adsorption are the nonpolar van der Waals’ forces. Since London (23) described the close connection between their nature and the cause of optical dispersion, they may also be called dispersion forces. The main contribution to the nonpolar van der Waals’ forces arises from the interaction of continually changing inducing dipoles and induced dipoles. The interaction energy of a pair of atoms due to this contribution is inversely proportional to the sixth power of the distance:

E,-

C -T6

(7)

The constant C depends on the properties of the atoms. London (23) derived the following approximation for C:

where a1and a2 are the polarizabilities of the participating atoms, h is Planck’s constant, and v 1 and v 2 are characteristic frequencies of the optical dispersion curve of the atoms. The energies hvl and hv.2 are, therefore, characteristic energies in the dispersion equation; they are often approximately equal to the ionization energies of the atoms. If, therefore, no data on the dispersion are available, the following expression may be used :

where I1 and I , are the ionization energies of the atoms. Other approximations for C have been derived; of these we shall mention only

given by Slater and Kirkwood (24), where E and m are the charge and the mass, respectively, of an electron. ar and a z , as above, are the polarizabilities of the atoms, and nl and n2are the numbers of electrons in the outer shells of the atoms. The latter equation always results in somewhat higher figures than expression (8) or (9).

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J. H. DE BOER

Apart from the term in Eq. (7) there are other contributions toward the nonpolar van der Waals’ interaction energy arising from the interaction of continually changing quadrupoles with dipoles and quadrupoles with quadrupoles. The total expression should, therefore, be written as

where D and E are, like C, constants depending on the nature of the atoms. The contribution by the second term of this equation may amount t o from 15 t o 30% of the first term, and in some cases this contribution may be even higher than that of the first term. In numerical calculations of adsorption energies, however, expression (7) is mostly used. It is assumed that the two last terms of Eq. (11) are counterbalanced by the contribution of the repulsion forces (see Sec. IV,4). Expression (7) gives the interaction energy between two atoms. I n order t o evaluate the adsorption energy, the interaction energies of the adsorbed atom with all individual atoms of the adsorbent should be calculated and added together. This addition is allowed, as the dispersion forces are, a t a first approximation, additive. If a molecule instead of a n atom is adsorbed, the summation should be made for all atoms of the molecule. I n the latter case we may sometimes expect deviations from the additive law. Many molecules show different polarizabilities in different directions. If the position of an adsorbed molecule is fixed, the angles of its various axes of polariaability with respect t o the surface enter into the calculations (25). If, however, the molecule rotates freely, which is often the case in physical adsorption, this correction is not necessary. Instead of carrying out an elaborate calculation of all individual contributions, Polanyi and London (26) replaced the summation by a n integration :

where Na is the number of atoms of the adsorbent per cubic centimeter and ro is the shortest distance between the adsorbed atom and the surface. According to this expression the decrease in potential energy due t o the nonpolar van der Waals’ forces in the case of adsorption on a surface is inversely proportional to the third power of the distance. According t o London (27), integration is permissible only if T O >> l/(Na)VJ.I n practice the actual equilibrium distance in adsorption is always too small, and consequently expression (12) always gives too low values. A better method is to evaluate the interaction energies of the adsorbed

ADSORPTION PHENOMENA

31

atom with all the individual atoms of the adsorbent within a certain distance T D , to add all these contributions, and then to add the contributions of the rest of the atoms of the adsorbent beyond the chosen distance by integration (28). The values thus obtained turn out t o be about 2.5 times the values calculated by use of Eq. (12). The surface of an adsorbent is not smooth but shows a roughness of molecular or higher dimensions. Many catalysts used in practice are deliberately prepared to contain a great number of capillaries of submicroscopic dimensions. There are many places on the highly developed inner surface areas of such microporous adsorbents where the adsorbed molecules come into direct contact with many more atoms of the adsorbent than would be possible if the surface were an ideally smooth plane. Such places where an increased number of atoms of the adsorbent are in direct contact with the adsorbed molecules form “active places” or “active spots” for van der Waals’ adsorption (28-30). Crevices, cavities, the inside of cracks, recessed parts of the surface, and especially the inside of capillaries are all more “active” for van der Waals’ adsorption than is a plane surface. The heats of adsorption will be higher on these active spots and, therefore, the first molecules t o be adsorbed generally show a higher differential heat of adsorption. The heat of adsorption mostly tends to fall with increasing amount of adsorbed matter (see Sec. V,12). 2. Nonpolar van der Waals’ Forces on Conducting Surfaces

The adsorption by nonpolar van der Waals’ forces on metal surfaces demands a separate treatment. Many attempts have been made t o consider the metal as an ideally polarizable structure. As Margenau and Pollard (31) pointed out, there is a serious objection against such a use of the so-called “image” picture. The inducing fields of the continually changing dipoles in a nonpolar molecule change so rapidly that the conduction electrons in the metal are incapable of following their movements. With respect to van der Waals’ forces a metal behaves as a dielectric body. Margenau and Pollard describe the van der Waals’ interaction energy between an adsorbed atom and the adsorbing metal as a sum of two contributions : E, = Ei Ez (13)

+

where E l results from the polarization of the metal by the continually changing dipoles in the adsorbed atom and Ez arises from the polarization of the adsorbed atom by the electronic movements in the metal. El turns out to be positive instead of negative. This part, therefore, re-

32

J. H. DE BOER

sults in a repulsion instead of an attraction. The final expression is

where e is the charge of a n electron, a0 is the polarizability of the adsorbed atom, ro is the shortest distance of the atom t o the metal, h is Planck’s constant, ne is the number of conduction electrons per cubic centimeter in the metal, m is the mass of an electron, v o is the characteristic frequency of the adsorbed atom, C is a numerical constant approximately equal t o 2.5 and re is the radius of a sphere in the metal containing one conduction electron. We shall use this expression in estimating the nonpolar van der Waals’ interaction energy of physically adsorbed atoms or molecules on metals and on charcoal. We see that also in this expression the potential energy is inversely proportional to the third power of the distance. 3. The Adsorption of Ions on Metal Surfaces

When an ion is adsorbed on a metallic surface, the electric charge of the ion will polarize the metal in such a way that the action may be described as if an electric charge of opposite sign were formed (electric image) a t a distance below the surface equal to the distance between the actual inducing charge and the metal surface. The attraction which the adsorbed ion experiences by this phenomenon may, therefore, be described as the attraction between the ion and its image a t a distance 2 r, the distance between the ion and the surface being r . It is here that we meet the difficulty (Sec. IV,3) of not knowing where we should locate the surface of the metal, or rather the boundary of the region of conducting electrons. The image force being given by

where e is the charge of an electron and ni denotes the number of elementary charges of the ion, the contribution to the adsorption energy is

where r0 is the equilibrium distance between the ion and the metal surface. An ion adsorbed on 8 metal surface is also attracted by the normal van der Waals’ forces [Eq. (14)] and by some other minor forces, which we shall discuss presently in this section. It is, moreover, repelled by the

ADSORPTION PHENOMENA

33

repulsion forces [Eq. (2)], balancing the attraction forces at the equilibrium distance ro. When all these contributions are added together and numerically evaluated for the adsorption of a cesium ion on tungsten, the unknown value of ro can be estimated from the known value of the heat of adsorption of the Cs+-ion, viz., 54.5 kcal./mole. Then ro is found to be 1.74 A. (SZ), which seems to fit the assumption that the boundary of the region of ideal polarisability starts at a plane through the centers of the surface atoms of tungsten. The possibility of this boundary starting at the outer peripheries of the tungsten atoms is, however, not completely excluded (32).

4. The Adsorption of Ions on Dielectric Surfaces When an ion is adsorbed on the surface of a dielectric, itself consisting of ions, we may expect Coulomb forces to act between the ions of the adsorbent and the adsorbed ion. A positive ion, adsorbed on top of a negative ion of an adsorbent, is attracted by this ion, but it is repelled by the ions surrounding the one with which it is in direct contact, attracted again by the then following ions, etc. The result is a rather weak attraction. Huckel (33) derived the following equation for the electrostatic field emanating from a cubic face of the surface of an alkali halide crystal :

where e is the charge of the ions, r, is the shortest interionic distance in the crystal, and r is the distance from the surface. If an ion is adsorbed on top of a surface ion of the crystal having an opposite charge t o its own, and its distance from this ion is exactly the same as the interionic distance, re, in the crystal, the energy contribution of the Coulomb forces, resulting in the force of Eq. (17), is 2 E , = -0.0662 (18) rc

This energy is only 6.6?4, of the energy contribution by Coulomb forces which we would have found if the adsorbing ion of the crystal surface had not been surrounded by all other ions of the crystal. The collaboration of all the ions of the crystal results in relatively small energy contributions and, moreover, in attraction forces of a very short range. At a distance r = 2rc the force is negligibly small. When an ion approaches the adsorbing crystal, foIlowing a line perpendicular to the surface and ending in a surface ion of the same charge as its own, it will be repelled. The electrostatic part of the repulsion

34

J. H. DE BOER

forces is also given by Eq. (17), the force acting in opposite direction. Surface spots exactly in between surface ions, do not exercise electrostatic forces on an adsorbed ion. Therefore, a t the short distances over which the electrostatic surface forces are active they cause a periodical unhomogeneous field. The movement of a single adsorbed ion over the surface may be hampered by these variations. I n all these electrostatic considerations the surface of the ionic crystal was idealized, as described in Sec. IV,2, as if it were cut by means of an ideally sharp razor blade. Our lack of knowledge of the structural deviations of the surface arrangements with respect t o the structure inside the crystal renders it impossible for us t o make any quantitative or semiquantitative statements regarding the actual adsorption energies caused by electrostatic forces. We can only say that in most ionic crystals negative ions i.e., halide ions or oxide ions, tend t o form the outside (adsorbing) surface. We shall have an opportunity (see, for example, Sees. V,5 and VI,5) t o revert t o this phenomenon. The foregoing electrostatic calculations hold, moreover, only for positions in the middle of a cubic face of a crystal of the NaCl type. Any deviation from this situation may result in a stronger electrostatic bond. Corners and edges of crystals, other crystallographic faces, lattice disturbances, etc., may all form “active spots” where the electrostatic adsorption of ions is relatively strong. We shall return t o the problem of active spots in Sec. V,12. I n Sec. V,3 we dealt with the adsorption of ions on metallic surfaces as the problem of the polarization of an ideally polarizable structure by the ion. Dielectrics have a more restricted polarizability, the polarization resulting in the shifting of the electrons in the atoms or in groups of atoms of the dielectric to which they belong or in the mutual shifting of ions as well (34). Instead of Eq. (16), which holds for an adsorbent of ideal polarizability, we obtain for the adsorption energy contribution due t o the electrostatic induction of a dielectric:

where K is the static polarizability of the dielectric. If the polarizability of the dielectric is due only to electronic shifts and not to a displacement of ions, we have

K

=

n:

where nr is the refractive index. Because of the relation between the mean polarizability E and refraction, a = -3 . Ln 2 - 1 4rN3 nf 2

+

ADSORPTION PHENOMENA

35

we obtain

where Na is, as in Eq. (12), the number of atoms (or centers of polarization) per cubic centimeter. I n all cases, however, where K > n:, Eq. (19) must be used instead of (21). This is the case for all metallic halides and oxides. The contribution of this polarization toward the adsorption energy of ions on plane surfaces of ionic dielectrics is far more important than the contribution by the Coulomb electrostatic attraction (34). 5 . The Adsorption of Polar Molecules Molecules that have dipoles, such as organic halides, ethers, ketones, nitrocompounds, etc., will be attracted by the electrostatic field emanating from the surface of an ionic crystal. The contribution toward the adsorption energy is given by E p = -Fp (22) where F is the electrostatic field, as given by Eq. (17), and p is the dipole moment of the molecule. As the field F is not very strong and decreases very strongly with increasing distance, a significantly high contribution to the adsorption energy can be expected only, if a dipole of sufficiently high dipole moment is situated a t such a place in the polar molecule that it can approach to within a very short distance of the surface. Hydrogen bearing compounds, like water, ammonia, organic hydroxy compounds (alcohols, phenols), organic amines and acids, possess dipoles between the hydrogen atoms of the functional groups and the atoms t o which these hydrogen atoms are bound. All these dipoles are situated near the periphery of the molecules. These peripheral dipoles, moreover, all point with their positive end toward the outside of the functional group. As the ion arrangement of most ionic surfaces is such th a t the negative ions form the outer layer (Secs. IV,2, and V,4), these peripheral dipoles show a strong tendency t o take up an oriented position perpendicular to the surface, the H atom of each dipole tending tQmake a close contact with one of the negative surface ions (a halide ion or a n oxide ion) (36).Surfaces of inorganic salts and oxides have, consequentiy, a great tendency to adsorb water molecules tenaciously (36),and organic alcohols, phenols, amines, and acids are also adsorbed remarkably well. Protein surfaces have negative parts pointing outward, such as the oxygen of the CO groups, and peripheral dipoles may be adsorbed in a way similar t o the case of inorganic surfaces. Carbohydrate surfaces and protein surfaces,

36

J. H. DE BOER

moreover, possess peripheral dipoles of their own (OH groups and NH groups) and they can easily attract dipole molecules, such as water. In this case the oxygen of an adsorbed water molecule comes into direct contact with the hydrogen atom of a surface dipole (37). In later years all these types of bonds formed by peripheral dipoles have often been classified as cases of hydrogen bonding (38). Calculations of the contributions toward the adsorption energy by these forces reveal them to be quite appreciable and, indeed, a strong and oriented adsorption will always be found. These calculations result in somewhat too low values when the dipole moment I.( is used, because of the short distances between the dipoles and the negatively charged attracting centers. I n many cases the actual contribution may be 10% higher than the calculated one (39). A dipole that is not situated near the periphery of the adsorbed molecule makes a far smaller contribution toward the adsorption energy than do peripheral dipoles. Such nonperipheral dipoles may, however, cause enough difference in adsorption energy t o effect a preferential adsorption between molecules that otherwise would show equal adsorption energies. They also may lead to a fixed and an oriented position of the adsorbed molecule instead of an adsorption of more or less freely moving and rotating molecules. A similar behavior may be found when the charge distribution in the molecules is more complex. In carbon dioxide the charge distribution is of the character of a quadrupole. Lenel(40) calculated the influence of the interaction of this quadrupole with the surface of an alkali halide crystal and reached the conclusion that a substantial contribution of roughly 3 kcal./mole is to be expected from this polar interaction. Recently Drain (41) succeeded in approaching the remarkable fact that the heat of adsorption of Nz on ionic crystals is often appreciably higher than that of O2and A, which is not the case when these gases are adsorbed on nonionic surfaces. He shows that the quadrupole of N2 may be responsible. We shall return t o this problem in Sec. VI,2. When a dipole molecule is adsorbed on the surface of a metal or on other conducting surfaces (charcoal), the attraction may be described by the image force. The energy of interaction is given by

E, = when the dipole is oriented perpendicularly to the surface and

(23)

ADSORPTION PHENOMENA

37

when the dipole is parallel to the surface. If the direction of the dipole forms an angle /3 with the normal to the surface, we obtain

All these contributions are very low and they may, in most cases, be neglected. The polarization of a dielectric adsorbent by an adsorbed dipole may, similarly, be neglected. 6 . The Polarization of a n Adsorbed Molecule by a Dielectric Adsorbent

The electrostatic field emanating from the surface of an ionic crystal will also polarize a molecule adsorbed on it. The energy of combination due to this effect is F2CX E,= - -

2

where P is the strength of the field (such as, given by Eq. (17) for a cubic face of NaC1) a t the center of polarisability of the adsorbed molecule, and a is the polarizability of that molecule. As we have seen in Sec. V,4, the field over a smooth face of an ionic crystal is small and falls very rapidly with the distance r . At the center of polarizability of the adsorbed molecule, F is usually so small that the contribution according to Eq. (26) is of minor importance. Calculating this contribution for an argon atom on top of a potassium ion of the cubic face of KC1, and taking the distance between the center of the argon atom and the center of the potassium ion as r = 3.14 A. (the same as the smallest interionic distance, re, in the KCl crystal), we obtain E , = 0.25 kcal./mole, if the ~ m . ~ polarizability of the argon atom is a = 1.68 X Lenel (40)and also Teller (42) have pointed out that the electrostatic field of the surface of the ionic lattice is too unhomogeneous for even an argon atom to be treated as a whole with one average value for the polarizability, centered in the center of the atom. More elaborate calculations, taking into account a more likely distribution of the field strength and of the polarizability, lead to a value of E, = 0.45 kcal./mole for the argon atom in this position. It is to be noted that an argon atom, adsorbed just over a center of a square formed by four surface ions of the cubic face of KCl, would, according to Eq. (26), not contribute to the adsorption energy by static polarization as the electric field is zero in this position. According to Lenel’s method, however, a contribution of 0.37 kcal./rnole is obtained. All these contributions, however, are certainly low. It is only at “active spots,” as already mentioned in Sec. V,4, that the electrostatic polar-

38

J. H. DE BOER

ization leads t o important contributions toward the adsorption energy. We shall return to this problem in Sec. V,12. 7. T h e Polarization of an Adsorbed Molecule by a Conducting Adsorbent Only a few years ago it was discovered by Mignolet (43) that a metal also polarizes a molecule which is adsorbed on its surface. Mignolet, by measuring contact potentials, found that nonpolar molecules, adsorbed on metallic surfaces by purely physical adsorption forces, show, nevertheless, rather appreciable dipole moments. T o give an example, Mignolet found a potential change of 0.85 volt by the adsorption of xenon on a nickel surface. Assuming that he had a fully occupied xenon-adsorption layer, this means that each xenon atom shows a n induced dipole moment e.s.u. (0.42 Debye). The direction of the dipoles is of ,u = 0.42 X such that the positive part is pointing away from the adsorbing surface. The same conclusion may be obtained from the study of the behavior of many gases adsorbed on charcoal. We shall discuss the mobility of adsorbed molecules in Sec. VII, but we may mention here one of the results of such studies. Many gases, such as A, Nz, 02,CO, CH4, etc., when adsorbed on charcoal, behave as two-dimensional nonideal gases (44). This behavior can be described by a two-dimensional van der Waals’ equation, from which a two-dimensional van der Waals’ constant a2 (comparable with the normal three-dimensional van der Waals’ a) may be derived. The two-dimensional van der Waals’ constants can also be calculated from the three-dimensional values of a (46). The experimental results show that the actual a2 constants for gases adsorbed on charcoal or on mercury are always far lower than the theoretical ones and are very often even negative (46). The adsorbed molecules tend t o repel each other instead of showing a mutual attraction. This behavior also points t o a polarization of the adsorbed molecules by the field of the charcoal or of the mercury (47). We may assume this polarization t o be caused by an electric double layer formed by an electron distribution over the surface of the conducting adsorbent and corresponding positive charges in the metal. The dipole moment induced in the adsorbed molecules by the field of this double layer may be calculated from the difference between the theoretical value of a2 and the actual value which is found. This difference forms the a2 contribution caused by the dipole and is given by the expression

where p is the dipole moment and d is the diameter of the molecules (48).The actual value of the field may then be derived in each case from p

39

ADSORPTION PHENOMENA

by using the expression

where a is the polarizability of the molecule. E q u a h n (26) could then be used t o calculate the contribution t o the adsorption energy due t o this polarization. It is not necessary to calculate F , however, as E, is immediately given by

Some figures obtained in this way for gases adsorbed on charcoal are given in Table I. TABLE I

Gas

Nz

GO

CH,

P,

Debye 1.1 1.1 1.16

C2He

1.7

GO 2

2.0 1.5

C3Hs

’””, ff,

10-24

cm.8

1.76 1.95 2.60 4.47 6.29 2.65

2a

kcal./mole 5.0 4.4 3.6 4.6 4.6 6.1

It is clear that this effect leads t o a rather important contribution t o the adsorption energy, if not t o the most important, in those cases of adsorption of gases on conducting surfaces where no chemical interaction results. 8. Chemical Bonds in Adsorption Phenomena on Metals a. The Formation of Ions. The adsorption of atoms of alkali metals, alkaline earth metals, and some other metals on surfaces of other metals may easily lead to the formation of ions from the adsorbed atoms. An electron is transferred from the adsorbed atom t o the adsorbing metal surface. The formation of ions from alkali atoms by a heated tungsten filament was discovered, nearly simultaneously, by Ives (@) and by Langmuir and Kingdon (50). The ions evaporate from the filament, and the condition for this conversion and the subsequent evaporation of the ions is t ha t the ionizing potential Vi, of the alkali atoms be less than the work function, ‘p, of the metal filament. A metal may absorb electrons as well as emit them, and for cases of chemisorption we may compare a metal as a whole to an atom, possessing an “ionization energy,” eVi = w, as well as a n electron affinity - Ed = --ep (61).If etp of the metal is

40

J. H. DE BOER

larger than eV, of the adsorbed alkali atom, a n electron will be transferred from the alkali atom to the metal and on desorption a t sufficiently high temperatures an alkali ion will evaporate. If the alkali atoms strike a metal surface with a lower temperature, they will still be ionized, and so the electrons will still be transferred to the adsorbing metal. At lower temperatures they will not evaporate but

4

8

12

16

ZO------aD

-c t i n A

FIQ.4. Potential curves relating t o the combination of sodium and chlorine to a molecule.

stay on the surface as adsorbed ions. Initially it was thought that Eq. (30) describes also the condition for this ionic adsorption. The author (66), however, showed that the condition is where Q. ia the adsorption energy of the alkali atom in atomic form and Qi its adsorption energy in ionic form. The adsorption of a sodium atom on a tungsten surface leading to an ionic adsorption may be compared directly to the formation of an ionic molecule of NaCl from the atoms of N a and C1. Figure 4 gives the change in potential energy as a function

ADSORPTION PHENOMENA

41

of the distance between an N a atom and a C1 atom. When the atoms (energy level A ) approach each other and the transfer of a n electron could be avoided, only a weak van der Waals' attraction (minimum B ) would result. The transfer of an electron from a n Na atom t o a remote C1 atom would mean a shift from energy level A to level D,the energy distance being given by (€Tii - Eel), viz. the difference between the ionization energy of the sodium atom and the electron affinity of the chlorine atom. When the ions (level 0 ) approach each other, a strong Coulomb attraction brings us t o the minimum €3, where the repulsion forces

FIQ.5. Potential curves relating to the adsorption of sodium on a tungsten surfme.

balance the attraction forces. The energy difference between level A (atoms) and the minimum E (ionic molecule) gives the heat of formation of the ionic molecule from the atoms. A similar curve may be drawn for the system of a sodium atom and a tungsten surface (Fig. 5). Minimum B (adsorption of N a in atomic form) would be appreciably lower than in the corresponding Fig. 4, and minimum E is higher than in Fig. 4. The difference between the energy levels A and D is here given by (cVi - ep), i.e., by the difference between the ionization energy of the sodium atom and the electron affinity of the tungsten metal. As level E is still appreciably lower than level B [condition (31)], the atom will adsorb in an ionic form; the real change in potential energy connected with the mutual approach of the sodium atom and the tungsten surface being given by the line ASEF. If, by heating, the adsorbed ion is desorbed, line E S A will be followed and the ion desorbs in atomic form while drawing a n electron from the metal

42

J. H. D E BOER

with it (see also See. IX,5). The condition for this atomic evaporation is given by the unequality (30), viz. the difference between the levels A and D; hence level A is lower than level D. When a cesium atom is adsorbed on a tungsten surface, level A is higher than level D (Fig. 6 ) and the desorption of the cesium is in ionic form, provided that no external electric fields are used that will force atoms to evaporate. The potential curves of Fig. 6 are completely comparable to the formation of the ionic molecule of CsF from the atoms of cesium and fluorine (Fig. 7).

ur i n A

FIG.6. Potential curves relating to the adsorption of cesium on a tungsten surface.

The adsorbing metal as such behaves in these cases as an atom of an electronegative element. Some years ago, in preparing an article about adsorption forces, the author (53) wrote: “One of the first statements that has to be made before discussing atomic forces and adsorption is that there are no special adsorption forces.” Generally speaking this statement may be true. The author, however, overlooked the special nature of the forces on conducting surfaces, as discussed in this section and in Sec. V,7. As the conduction electrons, which play an important role in the forces discussed in these two sections, do not belong t o definite atoms of the conducting body, but t o the body as a whole we may speak of special adsorption forces in their own rights. The question may arise whether, in forming adsorbed ions on surfaces, the metal may sometimes also act as the electron donor, resulting

ADSORPTION PHENOMENA

43

in the formation of adsorbed negative ions. From experiments of Rijanoff and Lukirsky (54) on selective photoelectric emission of potassium exposed t o atomic hydrogen, it may be concluded th a t hydrogen atoms when colliding with a surface of potassium will each receive a n electron from the metal and form an adsorbed layer of negatively charged hydrogen ions on the surface (55).This surface compound may be compared t o the ionic compound of lithium hydride or other hydrides. E

in kcal/rnolc

FIQ.7. Potential curves relating to the combination of cesium and fluorine atoms to a molecule.

b. Sharing of Electrons. I n the last example of the previous section we compared the adsorption of atomic hydrogen to the formation of a n alkali hydride. We may now ask whether surface combinations exist that may be compared t o chemical compounds of hydrogen in which this element is the electropositive partner, as is the case in hydrogen chloride. It is known that HC1 cannot be described as a purely ionic compound. Nevertheless some decades ago many attempts were made t o describe i t as a n ionic compound, in which the negative chloride ion was

44

3. H. DE BOER

polarized by the positive hydrogen ion to such a n extent that part of the electrons did belong to both partners. Later it became clear that only one pair of electrons, shared by both partners, was responsible for the bond, the electrons forming a pair when their spins are opposed. The resulting HC1 molecule, however, is a polar molecule, the electron distribution being such that the center of the negative end of the dipole is more on the side of the chlorine atom and the center of the positive end on the side of the hydrogen atom. It is now customary to describe the actual situation as a hybrid between the ionic molecule H+Cl- and the homopolar atomic molecule HCI, a situation which may be expressed by one of the systems of formulation used in these cases of “resonance” phenomena, such as H+C1- H HCl or H+C1 The second formula means merely that the HC1 molecule is a resonance hybrid between the ionic molecule H+Cl- and the molecule with the purely covalent bond, the direction of the arrow giving the direction in which the electrons have, on the average, been displaced (66). As, however, such an arrow is used by others (57), for indicating a coordinate link (semipolar double bond) caused by a lone electron pair of the donor atom, which likewise produces a dipole with its positive end on the donor side and its negative one on the acceptor side, the author suggests t h a t be used for the normal covalent bond, which, by resothe symbol nance with an ionic structure, possesses a dipole. The point of this half arrow also indicates the direction of the negative end of the dipole. The full arrow -+ will then be reserved for the coordinate link. Both links play their roles in chemisorption, and it may be useful for the purposes of this article t o introduce relatively simple symbols. According to this principle HC1 should be formulated as H-C1. I n many cases of chemisorption normal covalent bonds are formed, where an electron of the adsorbed atom and one of the metal form a pair. The adsorbed atoms share their electrons either with the atoms of the metal on which they are adsorbed or with the metal as a whole. They form dipoles on the surface of the metal, and the direction of these dipoles are of great importance for chemisorption and catalysis. Just as a metal may play the role of a halogen atom in the adsorption of alkali ions discussed in the previous section, it may in other cases act similarly to the chlorine atom in HCl and form a covalent bond. Hydrogen atoms adsorbed on the surface of platinum may serve as an example. The dipoles point with their positive ends away from the metal and may

-

ADSORPTION PHENOMENA

45

be regarded as resonance hybrids :

which, according to the above-mentioned suggestions, we can formulate as

a symbol t ha t will be simplified t o

I n other cases the adsorption of hydrogen atoms results in surface hydride dipoles pointing with their negative ends away from the metal, as is observed in the adsorption on nickel,

R

which is t o some extent comparable to the existence of alkali hydrides, or rather t o that of free molecules of metallic hydrides. The free molecules of metallic oxides can best be described as having covalent links, possessing dipoles (68). Similarly oxygen atoms adsorbed on metallic surfaces form covalent bonds, sharing two pairs of electrons with the metal (69)or with one specific atom or two atoms of the metal. Their dipoles point with the negative ends away from the metal. We may, t o give an example, express the situation of the adsorption of oxygen on silver by

,+, +, or

Similar covalent bonds may be f rm d between metal surfaces and many other atoms, including atoms forming part of molecules or radicals. I n many cases the dipoles point with their negative ends away from the metal surface. I n other cases, however, as with CzHzand CzH4on nickel, they form dipoles pointing with their positive poles away from the surface (60).

46

J. H. DE BOER

The surface hydrides, surface oxides, and other surface compounds, mentioned above, need not be formed by the action of free atoms with free valencies on metal surfaces, but, just as in normal chemical reactions, these compounds may result from the reaction of the metal surfaces with molecules. The chemisorption of an H z molecule on a metal surface may lead t o the chemisorption of two separate hydrogen atoms and so may the action of a n 0 2 molecule on a metal surface lead to the chemisorption of two oxygen atoms, the action of an NPmolecule t o the chemisorption of two nitrogen atoms, etc. Surface hydrides, oxides, and nitrides are, then, the result of normal chemical reactions of these gases with the surfaces of the metals. This does not mean that the molecules H z , 02, or Nz could not be chemisorbed in their molecular state without being split into atoms. We shall see in Sec. V,11 that molecular chemisorption also comes into the picture. Other molecules may also lead to such “dissociative” chemisorptions. I n some cases of chemisorption of NH3 this molecule may split into a n H atom and an NH2 radical which are separately adsorbed on the metal surface; sometimes the dissociation may go even further. I n this type of surface reaction CHI and other hydrocarbons may also split into H atoms a lid hydrocarbon radicals which are bound by chemisorption. Ethylene, when adsorbed on nickel or similar metals, may be supposed t o open its double bond and be adsorbed as HZC-CH, /

V”/’

/

but it is also possible that a dissociative chemisorption will take place as is given by

‘a’

H HC=CH

75

H

VL

The chemisorbed CzHzradical may, in such a case, be supposed to be stabilized by a resonance between structures such as + + HC -CH

HC =CH

HC-CH

*+-*++As already stated above, the dipole connected with chemisorbed ethylene points with its positive end away from the metal surface.

47

ADSORPTION PHENOMENA

Coordinate links between molecules having at least one lone pair of electrons and acting as the electron donor, and metal surfaces acting as electron acceptors may also lead to chemisorption of these molecules. Maxted (61) and co-workers found that many molecules with such lone pairs of electroils are extremely effective poisons for catalytic processes because of their strong adsorption on the metallic surfaces acting as catalysts. Maxted ascribes this strong adsorption to the coordinate links forming in these cases. We may give the following examples t o illustrate the type of chemisorption:

using the full arrow t o indicate the coordinate link. These donor elements,

S,As, N and also Se, P, etc., do not form catalyst-poison molecules when

they are in their highest state of valency; there are no lone pairs of electrons then. The same holds for ions of these elements. The sulfite ion acts as a poison; the sulfate ion does not:

The former is bound by a coordinate link, aided by an image force of the ionic charge, which, however, is rather weak here because of the relatively large distances of the negative excess charges from the metal. The sulfate ion exhibits this image force only. In both cases there are the van der Waals’ forces as well, which also will be weak here. Suhrmann (62) explains the strong increase of the normal photoelectric effect of metals caused by the adsorption of water molecules and also by the molecules of ammonia, by accepting similar coordinate links t o function in the chemisorption of these molecules. Dipoles are formed which point with their positive poles away from the surface, thereby decreasing the work function and, consequently, increasing the normal photoelectric effect: H

48

J. H . DE BOER

9. Heats of Adsorption and Desorption and Activation Energies

Connected with Chemisorption on Metals When sodium is adsorbed on a tungsten surface, it is transferred into an ion. The heat of adsorption can be read from Fig. 5 (Sec. V,8,a) as the difference between level A (the atom) and the minimum E of curve DEF. The difference in energy between levels D and E is given mainly by Eq. (16), Sec. V,3, and is modified by contributions from van der Wads’ forces, polarization forces, and repulsive forces. The total amount of this energy difference is about Q; = 77 kcal./mole. I n order to obtain the heat of adsorption, the energy difference between levels A and D , viz., tV;

- q o = 14 kcal./mole

must be subtracted from this value, the resulting value being (QJi = about 63 kcal./mole

symbolizes the heat of adsorption of the atom in the form of where (QJi an ion. The heat of desorption is also given by the same difference between levels E and A and is, consequently, equal to the heat of adsorption; Qsd..

=

This equality is nornial for all cases of physical adsorption and, as we have seen, also holds in the present case of chemisorption. When we now consider the adsorption of cesium on a tungsten surface, we see from Fig. 6 that the heat of adsorption of the atom is given by (Qa); =

- (tV; -

Qi

tq) =

54

+ 14.8 = 68.8 kcal./mole

In this case cesium is desorbed in the ionic form, the heat of desorption being no more than Qde.. = Q = i 54 kcal./mole which shows that the heat of desorption is smaller than the heat of adsorption. As a matter of fact we cannot speak of a purely reversible phenomenon here. We shall not discuss the details of the calculations of the energies involved in Figs. 5 and 6 of Sec. V,8,a; a reference to some earlier calculations by the author may suffice (63). When atoms of hydrogen are adsorbed on a metal surface, the changes in potential energy may be schematically represented by a single curve as shown in Fig. 8. Our present knowledge of the forces responsible for the formation of the covalent bond described in Sec. V,8,b does not enable

ADSORPTION PHENOMENA

49

us to calculate this curve. Once the heat of adsorption is known and some other figures are given, the form of the curve may be constructed as a socalled “Morse” curve (64). The calculation of the heat of adsorption may be approached in a semi-empirical way as was shown by Eley (66),who uses an equation of Pauling (66) for calculating the bond energy of a covalent bond between the atoms A and B:

- B ) = s { D ( A - A ) + D ( B - B ) ) + 2 3 . U 6 ( ~-~ X B ) ~ (32) where D ( A - B ) represents the bond energy between A and B in kcal./ D(A

mole and

ZA

and

ZB

are the “electronegativities” of A and B.

ro

I

I

-r

FIQ.8. Schematical representation of the potential curve of the adsorption of a hydrogen atom on a metal surface.

I n the case of the surface bond between an H atom and a tungsten surface, we obtain

D(W

-

H) = l/i(D(W

- W)

+ D(H - H)) + 23.06(zw - za)2

(32a)

D(H - H) is known to be 103.2 kcal./mole. Eley calculates D(W - W) from the sublimation energy of the metal and puts B(W - W) = %-6x (33) where S is the sublimation energy. (xW- zH) may be estimated with Pauling’s approximation, which assumes that this difference is equal t o the dipole moment of the bond, expressed in debyes (1 debye = 10-8 e.s.u.). This dipole moment may be obtained from the change in contact potential caused by the adsorbed layer. This value is mostly known experimentally for a fully occupied ad-

50

J. H. DE BOER

sorbed layer, and so for a complete layer of dipoles on the metal (at 0 0 being the degree of occupation). In that case we obtain

=

1,

where pe-1 is the dipole moment per bond a t 0 = 1; AV is the change in contact potential, and ne,l is the number of dipoles (adsorbed atoms) per square centimeter a t e = 1. As a matter of fact, for calculating the term (zw - zH)with (zw - z), = PO-0 (35) we need pe-0, this being the dipole moment for a surface bond a t 6 = 0. Eley assumes th at the difference between pe-1 and pe-o will not be so large as t o disturb the calculation seriously. The calculation of the terms of Eq. (32a) leads to x { D ( W - W) % { 33.8

+ D ( H - H) 1 + 23.06(~w+ 103.2 1 + 4.9 D(W - H)

hence t o

=

SH)'

D(W - H) 73.4

= =

73.4 kcal./mole

The experimental value is 74.1 kcal./mole. The result is fantastically good in this example and it cannot be expected t o fit so closely in other cases. I n Table I1 we have given some calculated and observed values for D ( M - H ) for different metals M , as calculated by Eley; the observed value for Cu, however, has been obtained from more recent data (67). TABLE I1

__

~

~

Bond Energies for Surface Hydrides in Kcal./Mole

D(M D(M

Metal

- H) calc. - H) obs.

Ta 67.6 71.1

W 73.4 74.1

Cr 59.5 74.1

Fe 60.1 67.6

Ni 60.2 67.1

Rh

63.1 65.6

cu

58.4 69.1(67)

It is remarkable that the observed figures differ so little, far less than the calculated ones. The formation of surface hydrides, oxides, and nitrides is usually a result of the dissociative chemisorption of the molecules of these gases. As can be seen from Fig. 9, which gives the potential curves for such a n adsorption, the heat of adsorption is given by the difference in energy level between A and E , this difference being given by (Qm)a

=

2Qa - Dm

(36)

ADSORPTION PHENOMENA

51

where (Q,), stands for the heat of adsorption of the molecule in atomic form, Q. for the heat of adsorption of the atom, and D, denotes the dissociation energy of the molecule. [Compare D(H - H) in Eq. (32a).] Calculated values of (Q,), can be easily derived from calculated values of Q. and the known values of D,. Trapnell (68) gives a survey of calculated and observed values of (&), for various gases on various metals. I n all these cases he gives the so-called “initial” heats of adsorption, i.e., the heat of adsorption on a bare surface. It is these figures which E in kcal/mole

-

rinA

FIG.9. Potential curves relating to the dissociative chemisorption of a molecule M (H2) on a metal Me without an activation energy,

have to be compared with the calculated ones. At higher degrees of occupation there is generally a strong decrease in heat of adsorption (see Sec. IX). According to Trapnell’s tables the experimental values of the initial heats of adsorption of various gases are highest when the gases are adsorbed on tantalum and lowest when they are adsorbed on copper or gold; they follow the order:

Ta

> W, Cr > Fe > Ni > Rh > Cu, Au

which is not the order which may be expected from calculations with the aid of the equations given in this section. We shall return to this problem in the next section. I n Fig. 9 the intersection point S of the curves ARC and DEF lies

52

J. H. DE BOER

lower than level A . This means that there is a constant decrease of potential energy during the mutual approach of the molecule and the metal ; there is, in other words, no activation energy involved in the chemical reaction :

M

f

F]

At At

A t the intersection point S , or rather a t a somewhat lower energy level (stabilization by resonance), we find the so-called “activated complex” of this chemical reaction and so the transition state between the original reaction partners and the reaction product. Metal surfaces seem to act as if they really have free valencies. There are no activation energies in chemisorption reactions between atoms and metal surfaces. I n reactions of molecules with metal surfaces, on the other hand, activation energies may be expected. If it is true, however, that metal surfaces behave as free atoms or as surfaces with free valencies, the activation energies should be small for we know from other chemical reactions that the reaction between molecules and free atoms or free radicals proceed without activation energies or with small values of these energies. From the behavior of Hz with chlorine atoms, for example (no activation energy), we may expect the activation energy for the chemisorption of H2 on metal surfaces to be very small or even negligible. Other molecules, such as O2 and N2, having far larger dissociation energies, may, however, need some activation in order to react with a metal surface. We may approach this problem somewhat more closely with the aid of potential curves. There is no activation energy in Figs. 5 and 9; the slope of the right part of curve DEF is such that the intersection point S does not come above level A . I n Fig. 10 the location of the intersection point S is such th a t a n activation energy E , will govern the speed of the reaction. It may be clear that point S will be higher the larger the difference between levels A and D, or the smaller the difference between levels D and E. The activation energy will also be higher the smaller the equilibrium distance 9-0 a t point E and the steeper the slope of the part DE of curve DEF. I n Fig. 11 the influence of these variations may be seen. The smaller the ionic contribution to the adsorption energy of the atom, the steeper will be part DE and the higher point S and, hence, the larger the activation energy. I n recent years more and more experimental evidence has come to point t o the conclusion that the chemisorption of Hz on really pure metal surfaces does not involve an activation energy (69-72) or at most in-

ADSORPTION PHENOMENA

53

volves such a small one that even a t very low temperatures the adsorption proceeds practically instantaneously. The adsorption of Nz on an iron surface, (73,74) even on a very pure iron surface, involves an activation energy, and the situation may be described by Fig. 10. The chemisorption of Nz on a tungsten surface, on the other hand, seems to proceed without an activation energy or a t most a very small one (76-77). E in kral/mole

FIQ.10. Potential curves relating to the dissociative chemisorption of a molecule M on a metal Me with an activation energy.

Contaminated or incompletely reduced metal surfaces, however, may also in their chemisorption reactions with Hz lead to appreciable values of activation energies governing the speed of the reaction. We shall discuss these phenomena later (Sec. X,4). In case an activation energy is noticed the heat of desorption is higher than the heat of adsorption. A glance a t Fig. 10 shows that the speed of the desorption reaction will be governed by the difference in height between level E and intersection point S , which is the sum of the heat of adsorption ( A to E ) and the activation energy ( A to 8). If the energy level of the minimum E lies above level A (Fig. 12), the heat of adsorption will be negative (increase of potential energy). Such an adsorption would be an endothermic one. I n physical adsorption p h e

54

J. H. DE BOER

nomena endothermic adsorption does not occur; as the entropy always decreases in physical adsorption phenomena, the heat of adsorption has t o be positive in order to enable adsorption to take place. I n chemisorption processes, however, endothermic adsorption cannot be excluded. b

I C

E

F'

F

E -r

-r

d

C

0

i E' E

E - 1

-r

FIG.11. Various possibilities for the relative situations of potential curves relating to dissociative chemisorption. (a) A higher value for D, gives a higher act. energy, (b) a higher value for &. gives a lower act. energy, (c) a shorter distance ro gives a higher act. energy, (d) a higher ionic contribution gives a lower act. energy.

Figure 12 indicates a possibility of such a case, which is comparable t o the existence of endothermic chemical compounds. Once formed, such a n endothermic surface molecule may have a certain time of existence before dissociating. This time may be long enough t o enable it t o react with other molecules. I n many catalytic processes the intermediate role

55

ADSORPTION PHENOMENA

of such endothermic surface compounds is not to be excluded. It may be expected that such a n endothermic chemisorption plays a rather important role in catalysis. The time of existence (for time of adsorption, see Sec. VII,4) depends on the heat of desorption (E',S),which is smaller than I

2

4

-r

6

8

10

12

FIG. 12. Schematic representation of potential curves relating t o anlendothermic chemisorption.

the heat of activation (A,X) in this case. We shall discuss possible cases of endothermic chemisorption in Secs. V,9, VI,3,4,5 and X,4. 10. The InJEuenceof the Electronic Structure of the Metals

I n the previous section we saw that the experimental heats of chemisorption of many gases are highest on tantalum and lower on other metals, the decrease following a certain order. The order, mentioned there, is not exactly the same as the order for the heats of sublimation (78).If,however, Eq. (32) is valid and the term of the electronegativities may be neglected or is the same for all metals, the term containing the bond strength between metal atoms would be the only one which differs from metal to metal, and the term would follow the heat of sublimation. It has struck many workers in the field that the transition metals and near-transition metals are the best catalysts for many gas reactions (78), and i t was suggested (79) th at probably the covalent bonds between the chemisorbed atoms and the metal were formed by sharing of an electron of the atom with a d electron of the metal. It is, indeed, not improbable that d electrons of the metals play a n important role in these chemisorption phenomena. It is known from their role in the bond strengths of complex chemical compounds that pairing of other electrons with d electrons leads to strong bonds. An incorporation of d elec-

56

J. H. DE BOER

trons from the metallic adsorbents would, therefore, lead to higher heats of adsorption (80). Beeck (82)drew attention to the fact that the order of decreasing heats of chemisorption on various metals is the same as the order of increasing d character of these metals. Increasing d character, according to Pauling (82),means that more d electrons are used for the mutual cohesion of the metal atoms in the metallic crystal lattices. According to Pauling’s metal theory these d electrons are not available for other chemical bond formations. An increasing d character, therefore, means fewer d electrons for chemisorption. The best indication that d electrons may play an important role in catalytic reactions and in chemisorption phenomena is obtained from those experiments where a reaction is studied, that is catalyzed by the surfaces of alloys of two metals composed in such a way that in the range of alloys the d band is completely filled from a certain composition onward. I n several cases a sudden change in catalytic activity is found a t such a critical composition (83,84). Another, more direct, indication is obtained from the work of Dilke, Eley, and Maxted (85),who found from the change in magnetic susceptibility of palladium on adsorption of dimethylsulfide that an electron of the sulfide had entered the d band of the metal. As we saw in Sec. Vl8,b, a coordinate link is formed in this case. Although the availability of d electrons will certainly influence the ease of formation and the strength of the covalent bonds in chemisorption phenomena on metals, we may not expect a simple relationship between the heat of chemisorption and some more or less simple property which is related to the d electrons. There will certainly be other properties of the metals which play a role. I n expression (32) the last term containing the electronegativities has some relation to the availability of electrons from the metal, and it must be said that the order in which the heats of chemisorption fall (See. V,8,b) is nearly the same as that in which the work function of the metals rises. I n the formation of the dipoles between the adsorbed atoms and the metal, work has to be done against the work function; we may expect that less work will be necessary to form these dipoles and that the dipole moment will be larger, the smaller the work function. I n the forming of these polar bonds, electrons of the metals are withdrawn from the metal. The binding of electrons can be shown by the increase of the secondary electron emission (86,8?), and conductivity measurements (88,89) and measurements of contact resistances (90,91) show that conduction electrons have been occupied by these bonds. The physical adsorption of a gas on a metallic surface, on the other hand, causes a slight increase of the conductivity of the metal (92,93).

ADSORPTION PHENOMENA

57

If. Chemical Bonds in Adsorption Phenomena on Nonmetallic Surfaces Cheniisorption on surfaces other than those of metals have been studied mostly on oxides or salts. Oxides may be either semiconductors or insulating dielectrics. Semiconductors may in some respects act similarly t o metals; we may speak about a work function, more or less free electrons in partially occupied energy bands for electrons, etc., as is customary in the more physical theories about metal structure. It is more convenient for our purposes to consider them from the chemical viewpoint and t o describe them as ionic crystals having ions of the same atom (homonymous ions), but in different valencies, on crystallographically identical places (94).Such ions may be statistically distributed among their sites in the crystal lattice. The surfaces of such crystals contain, therefore, also these ions of different valencies. I n some special cases the lattices of the oxides or salts may contain these ions already in their stoichiometric compositions (Fe3O4)(95), but in most cases homonymous ions of different valencies are linked with deviations of the stoichiometric composition or they are controlled by the addition of some ions of other metals having other valencies which form solid solutions in the crystals, a method devised by Verwey and collaborators (96,97). Homonymous ions of deviating valency (charge) may also be formed in some processes of chemisorption; they will then be formed on or near the surface but may subsequently diffuse into the interior of the cryst,al lattice. There is no boundary between semiconductors and insulators; the distinction has been made only for practical reasons and a arbitrary boundary has been chosen at a certain value of the electrical resistance. We may expect that also metal ions in so-called “insulators” can be transferred into ions of other valency or into atoms. Such reactions may proceed more easily a t the surface than in the interior of the crystal and we have t o be aware of this possibility in cheinisorption processes. a. Carbon Monoxide Adsorpiion. Normal CuzO, containing a slight excess of oxygen with respect to its stoichiometric composition, contains Cuzf ions and consequently is a semiconductor. The electron transfer in the lattice can be pictured as

cu+

+ cuz+ i3 cu2+ + c u +

At room temperature carbon monoxide is adsorbed readiIy while simultaneously the semiconductivity is decreased (98).Probably a covalent bond is formed between CO and the copper ions of the surface, thus immobilizing electrons for the electron transfer in the lattice. Whether the bonding of the CO molecule is caused by donating two electrons from the

58

J. H. DE BOER

CO t o a Cu2+ion or by sharing electrons with a Cu+ ion is not known. I n both cases d electrons (or d levels) would be involved. Carbon monoxide is also readily adsorbed in ZnO (99) and a donation of electrons to the Zn2f ions or a sharing of electrons with Zn+ ions or Zn atoms, which may both be present in ZnO (loo),might be considered. A similar bond may be formed in the chemisorption of CO on Cr 203a t low temperatures (liquid-air temperature). When the adsorbed gas is desorbed in this latter case (101),and also from ZnO and C u 2 0 , i t is desorbed as such. If i t is chemisorbed on Cr203 a t room temperature, however, it is desorbed as COz (10.2). Apparently a more complicated surface reaction has taken place in this case. The CO probably combines with two oxygen ions of the surface to form a CO:- ion while simultaneously some metal ions are reduced t o a lower valency. On the surface of Crz03this reaction may perhaps be pictured as CO

+ 202- + 2cr3+--t C0:- + 2Cr2+

When the adsorbed gas is desorbed by heating, the surface carbonate ions decompose: C O i - 4 COZ 0 2 -

+

and COz is desorbed instead of CO. Such a n irreversible chemisorption of CO has also been found with other oxides. I n all such cases the production of metal ions of lower valency results in a tendency to adsorb oxygen, or rather t o enhance the capacity to adsorb oxygen. Sometimes the extra amount of oxygen that may be adsorbed after an irreversible CO adsorption is-stoichiometrically-equal to half the amount of CO t ha t was adsorbed. Carbon monoxide on CuO, for example,

CO

+ 202- + 2CUZ+--t c0;- + 2 c u +

induces a n enhanced O2 adsorption

XOZ

+ 2Cu++

2Cu2+

+

02-

oxidizing practically all the Cu+ t o Cu2+ again (105).In other eases the enhancement of the oxygen chemisorption is less. The reversible adsorption of CO on ZnO mentioned above does not lead t o a n enhancement of oxygen adsorption. The net result of the irreversible adsorption of CO, followed by the adsorption of the right amount of O2is

CO

+

SO2

+

O2-+

c0:-

and i t has been proved (104) that a direct adsorption of C 0 2on the same oxides leads t o identical surface carbonate layers.

59

ADSORPTION PHENOMENA

b. Hydrogen Adsorption. Hydrogen, just as carbon monoxide, may either react with an oxide surface t o cause a reversible chemisorption or may be bound in a stronger way which, on desorption, leads to the evaporation of water molecules. Both ehenisorptions must be considered t o be of the dissociative type. The reversible adsorption of H z and of Dz on Crz03a t liquid-air temperatures, with initial heats of adsorption of 5.1 and 5.4 kcal./mole respectively (105), are active in the exchange of H z and DZand we must, consequently, assume that both gases are adsorbed in the atomic form. We may think of a sharing of electrons between Cr3+ ions and H atoms, while also a n electron transfer from the H atom t o a Cr3+ ion, forming a CrZf ion, may be considered. Both possibilities may contribute t o the real situation, the bond being a resonance between them:

ci

or

Cr*O3

At room temperature H z is chemisorbed by Crz03 in a different way

(106); the initial heat of adsorption is far higher, viz. 72 kcal./mole, and

on desorption HzO is given off. We must assume th a t on adsorption OHions are formed from Oz- ions and metal ions are reduced t o a lower valency Cr3t

02-

Cr3+

r l Cr2+ 0 Cr2+ 0

02-

(c.,o,I+n,-

rl-

On desorption HzO evaporates and an ion vacancy is left: Cr2+

o

Cr2+ O

Cr2f

T

l

... Crz+ +

02-

H

Z

O

After adsorption of Hz in this way an enhancement of the tendency to chemisorb O2 should be expected which, indeed, is found t o be the case. After desorption of water, one may expect chemisorbed oxygen t o fill u p

60

J. H. DE BOER

the vacant site in the surface, I n our schematical way of describing surface phenomena we tend t o place the adsorbed species in a layer above the layer of the surface atoms. In reality there are often open sites in the surface layers and adsorbed atoms or ions may tend to fill such gaps. We shall see in a later section (VI1,G) that adsorbed atoms or ions and original surface atoms or ions may often change places; adsorption is, therefore, not restricted strictly to the outer surface. c. Oxygen Adsorption. It will be obvious from what has been said molecules may also be expected t o chemisorb readily when metal t ha t 0% ions of lower valency are present. This is indeed found t o be the case, and when 0 2 is adsorbed on CuzO the semiconductivity is increased ( l o r ) ,because Cu2+ions are created simultaneously next to the Cuf ions. Zinc oxide is normally nonstoichiometric in composition, there being a n oxygen deficiency. When it is prepared in air, a large amount of chemisorljed oxygen is present on the surface, which is thus more stoichiometric than the bulk (108). Oxygen chemisorption penetrates gradually into deeper layers, as will be discussed in Sec. VI1,B. It may be stated here that the oxidation of metals by oxygen to form metallic oxides proceeds essentially along these lines. The surface reaction is a chemisorption of oxygen, which is dissociated and ionized into two 02-ions, while, simultaneously metal atoms or metal ions of lower valencies are oxidized t o higher valencies. Metal atoms or metal ions of lower valencies are continuously produced by the part of the metal that has not yet been oxidized; they diffuse through the already formed oxide layer and provide the electrons for transferring the 0 2 molecules into 02-ions. As oxygen may, a t a sufficiently high temperature, be desorbed from the oxide, it may be expected th at there will be a continual exchange between O2 of the gas and 02ions of the oxide surface, provided th a t the temperature is high enough to enable the diffusion in the lattice to take place. The diffusion in the lattice seems to be the rate-governing process in exchange reactions between O2 (containing the stable isotope 1 6 0 ) and oxygen ions of Crz03,A1203, MgO, T h o z , etc. The apparent activation energy is 27 kcal./mole for Tho2 between 440' and 540°C. and 29.5 kcal./mole for Cr203 at temperatures below 410°C.;a t higher temperatures the exchange with Crz03requires a n activation energy of about 1 kcal./mole only (109). d . Hydrocarbons. Dissociative chemisorption on the metal ions of transition metal oxides, similar to the low-temperature reversible chemisorption of HI?on these oxides, has been suggested for the chemisorption of saturated hydrocarbons on these oxides a t high temperature. This adsorption is suggested as the first step in the dehydrogenation and aromatization of paraffins (110).

ADSORPTION P H E N O M E N A

61

e. Hydrogen Ions. Many oxides adsorb hydrogen ions from acids. We may in many cases assume them to be bound to the OH groups of the surfaces, where they are bound as in H30+ions in aqueous solutions. An acidified alumina, therefore, may be pictured as

H H

I

\ / Of

H

H

H

0

0

I

I

the anion of the acid being bound in the neighborhood of the positive charge. I n the lattice of A1203we must assume the constituent atoms to be in ionic form, hence Al3+ and 0 2 - . The surface OH groups may be bound mainly by covalent forces (in resonance with the ionic bond). When H+ is bound to such an OH group an oxonium compound is formed. The synthetic cracking catalyst consisting of SiOz and A1203 has no additional anion, the negative charge being part of the surface itself. The bonds in the silica lattice have mainly a covalent character; A1 may replace Si on the surface, provided that it can assume a negative formal charge t o enable it to show four covalencies like Si; the electrons are provided by the oxygen atom of the water molecule assuming a positive formal charge itself.

1.2. Active Spots (111)

I n Sec. V , l we discussed the influence of crevices, cavities, the inside of cracks, recessed parts of the surface, and especially the inside of capillaries. I n all these “active spots” for nonpolar van der Waals’ forces the adsorbed molecules can find far more direct neighbors than on a plane surface, and consequently the heat of adsorption is far higher in these spots than on plane surfaces. Owing to their structure many dielectric adsorbents, adsorbing molecules with nonpolar van der Waals’ forces show a rather heterogeneous distribution of adsorption sites of various strengths. If this were not the case, no smooth adsorption isotherms would be found, but isotherms showing sudden jumps, separated by hori-

62

J. H. DE BOER

zontal parts. A so-called “stepwise adsorption” would occur. The very fact that smooth-running adsorption isotherms are found proves the heterogeneous character of the surfaces for physical adsorption (112). It is worth while to see whether this conclusion may also be drawn for the other types of adsorption discussed in Sec. V. It is, in principle, the same for the nonpolar van der Waals’ forces discussed in Sec. V,2. Charcoal within molecular dimensions, acting principally as a conducting adsorbent, shows by its very nature a rather flat adsorbing surface (113). There is, however, a marked difference between the basal faces and the hexagonal prism faces of the graphitic structure, and normal charcoal is therefore not homogeneous enough in its surface to show stepwise physical adsorption. When charcoal is graphitized at very high temperatures, however, an adsorbent of a homogeneous nature is obtained and recently stepwise adsorption isotherms have been found for krypton or graphitized carbon black (114). Step-wise adsorption results from two-dimensional condensation (Xec. VIII,4) on homogeneous surfaces. If, at the same time, multimolecular adsorption takes place, steps may be found in the building up of every successive layer. These steps, however, do not coincide with the filling up of every successive layer (215). The forces between ions and metal surfaces, discussed in Sec. V,3 are far less influenced by active spots. Those spots that are active for nonpolar van der Waals’ forces are not active here. According to the simplified picture described in Sec. V,3, all crystallographic faces should give the same attraction if the equilibrium distance ro were the same. This distance, however, will not be the same and for this reason as well as because of other minor differences, we may expect the actual surfaces also to be heterogeneous with respect t o this contribution of adsorption forces though quantitatively far less outspoken than for the nonpolar van der Waals’ forces. The Coulomb forces between an ion and a cubic face of an ionic crystal of the NaCl type are small, as we saw in Sec. V,4. The corresponding forces on other crystallographic faces may be appreciably stronger (three to four times). Crystal edges and especially crystal corners also exercise far stronger forces than the plane surface. Those spots on a crystal face where during the forming of the crystal the growth stopped show very high attractive fields for ions of opposite charge. This type of adsorption, therefore, which may be negligibly small on some smooth crystal faces, may be of major importance on active spots of the character just mentioned. Actual surfaces, therefore, will again show a heterogeneous distribution of adsorption strengths for this type of adsorption. It may, however, be noted that the active spots for van der Waals’ forces and

ADSORPTION PHENOMENA

63

those for Coulomb forces do not coincide. It is rather so that active spots for van der Waals’ forces are not active for Coulomb forces and vice versa (116). As discussed in Sec. V,4, ions also polarize dielectrics [Eqs. (19) and (21)], and we saw that the contribution toward adsorption energy arising from this effect may be even more important than that from the Coulomb forces. Just as in the case of the interactions of ions and metal surfaces, these polarization effects are far less influenced by active spots than Coulomb forces are. The adsorption of polar molecules on surfaces of ionic crystals (Sec. V,5) is influenced by active spots of the same kind as influence the action of Coulomb forces. The effect of these active spots is, quantitatively, less €or dipole-containing molecules than for ions. The eff ect of dipoles on metal surfaces is small (Sec. V,5), and active spots are not expected to give appreciably higher contributions. The effect of active spots on the polarization of adsorbed molecules by a dielectric absorbent (Sec. V,6) is very great. The nature of the active spots is the same as of those which affect the attraction of ions or dipoles. Edges or corners of crystals, other crystallographic faces, and especially those places where the growth of individual crystal faces stopped, as well as lattice disturbances in the surface, will be active. The polarization of adsorbed molecules by conducting surfaces (Sec. V,7) cannot be expected to be highly influenced by most active spots, which are effective for van der Waals’ forces, nor can such an influence be expected in the case of the formation of ions by transfer of an electron between the adsorbed atom or molecule and a metal surface (Sec. V,8,a and partially also b). As the forces that govern the transfer of electrons are related to the work function, however, we may expect these forces to differ for different crystallographic faces, as we know t h a t the different crystallographic faces show different work functions. It is for this reason that actual metal surfaces, consisting of various crystallographic faces, ivill show some heterogeneous character with respect t o these forces. The same holds for the covalent forces between adsorbed molecules and metal surfaces discussed in Sec. V,8,b. Other active spots will be of less importance again in this case. Covalent bonds are more individual than bonds caused by van der WaaIs’ forces or by Coulomb or dipole attraction, and cooperation of other surrounding atoms of the adsorbent has, therefore, less influence. This just means that active spots are less important for covalent bonds. Covalent bonds between adsorbed molecules and oxide or salt surfaces (Sec. V,ll) may perhaps be highly dependent on active spots, because the formation of these bonds alters seriously the distribution of

64

J. H. D E BOER

electric charges in their neighborhood. It is, therefore, not only the strength of the bond which is formed that concerns us in this case, but also the change in bond strength around it. These rather complicated effects may probably result in a rather heterogeneous distribution of heats of those chemisorptions on the surfaces of oxides and of salts where the surrounding ions suffer changes of character and charge simultaneously. Other crystallographic faces will also give variations in the strength of these chemisorptions (see Sec. VIII) . VI. COOPERATION AMONG VARIOUSFORCES The various forms of interaction between a molecule and a surface, which are conveniently (see Sec. IV,l) treated as different forces and were discussed in Sec. V, cooperate with the repulsion forces, dealt with in Sec. IV,4, t o create the phenomenon of adsorption. They determine the magnitude of the adsorption energy and the distance between the adsorbed molecule and the surface. Nonpolar van der Waals’ forces and repulsion forces are always present and are, therefore, always among the cooperating forces in every case of adsorption. There are, however, few cases where these two general forces are the only ones that operate. The adsorption of noble gases on nonpolar dielectric surfaces will be governed by these two forces only. I n practically all other cases one or more of the other forces of Sec. V will cooperate. We shall, in this section, discuss a few selected cases of such cooperation. I . Physical Adsorption on Charcoal (and Metals)

The adsorption of many gases on charcoal is mostly taken as an example of cooperation of van der Waals’ forces and repulsion forces only. When London used his equations for the dispersion forces to calculate adsorption energies [Eqs. (8) and (12)l he found (117) a good agreement between his figures and the experimental values of the heats of adsorption of gases such as He, A, Nz, CO, CHI, and CO, on charcoal. H e had unfortunately made a calculation error, and so his figures were ten times too high. The discrepancy could partly, but only partly, be bridged by the application of a summation instead of an integration, as described in Sec. V,1. I n 1934 we gave a solution (118) of the problem, namely that the adsorption of these gases occurs in pockets, tubes, and cavities of the charcoal, hence that the adsorption would mainly take place on active places. This view was generally accepted and refined by Brunauer (119) when he suggested that all adsorbed molecules, in the very narrow capillaries of charcoal, would be in contact with two layers of carbon atoms instead of one. This view is entireIy true, but the calculated figures for

ADSORPTION PHENOMENA

65

the adsorption energy still tended t o be too low. One has to bear in mind that the repulsion forces were neglected in these last calculations. In Sec. IV,4 we saw how big the influence of the repulsion forces may be. Recent investigations showed (120) that all the gases mentioned above, when adsorbed on charcoal, are very mobile and behave as two-dimensional gases. It was discovered (121) in the course of the same investigations that the gases were polarized by the electric field of the charcoal (Sec. V,7) and that this polarization resulted in a n important contribution t o the heat of adsorption, probably the most important one. Physical adsorption of these gases on charcoal, therefore, must be regarded as being caused by the cooperative action of a polarization by the field of the charcoal nonpolar van der Waals forces and repulsion forces. The same picture holds for physical adsorption on metal surfaces. The polarization of the adsorbed molecules causes dipoles pointing with their positive ends away from the metal surface. The work function of the metal will be lowered by this effect, and it seems as if the increase of the normal nonselective photoelectric emission of metals by the adsorption of water molecules (122) or molecules of organic substances such as pyridine, propionic acid, and benzene (123) or alcohol, diethyl ether, and acetone (124) is caused by this effect. The explanation, which, many years ago, was given by the author (125), viz., polarization by positive hydrogen ions which should still be present, may seem to be unnecessary and obsolete. As already mentioned in Sec. V,8,b, water molecules may even cause dipoles by forming a coordinate bond with the metal surface, these dipoles working in the same sense as those formed by polarization in the molecules themselves. When chemical action enters into the picture, in other words when chemisorption can also take place, more forces come into action, as in the above-mentioned adsorption of water molecules. We shall discuss the chemisorption of hydrogen and of oxygen on charcoal and on metal surfaces in subsequent sections. 2. The Adsorption on Ionic Surfaces

There have been numerous theoretical and experimental investigations on the adsorption of argon, oxygen, and nitrogen on potassium chloride (126-128) and in this connection we may refer to a survey in Brunauer’s book on physical adsorption (129). There seems to be a general agreement that the most favorable positions for the adsorbed atoms or molecules will be found just above the center of a lattice cell. The electrostatic polarization is minimum a t such spots, but the nonpolar van der Waals’ forces are at their maximum and dominate (130).Drain

66

J. H. DE BOER

(131) drew attention to the fact that the adsorption energy of nitrogen is generally higher than that of argon or oxygen when adsorbed on ionic surfaces, and t h at the energies are practically the same on nonionic adsorbents. He ascribes this effect to the quadrupole moment of nitrogen and calculates the contribution of the mutual attraction by the quadrupole of nitrogen and the field over a cubic face of KC1. According t o these calculations the sites just above the center of a lattice cell still represent the most favorable positions; the calculated contribution by the quadrupole attraction is of the right magnitude to explain the extra heat of adsorption of nitrogen on this surface as compared with that of oxygen and argon [about 500 cal./mole (128)l.Drain assumed the nitrogen molecules to lie flat on the surface. Drain and Morrison (132) studied the thermodynamic properties of O2 and Na on rutile experimentally and derived the conclusion that the adsorbed molecules do not freely move over the surface (see also Sec. VII,2) and that both rotational degrees of freedom of nitrogen are considerably hindered whilst oxygen has more freedom of rotation. We saw in Sec. V,5 that molecules with peripheral dipoles, such as OH, N H 2 and, COOH groups are attracted strongly by the electrostatic field of the surface. These dipole forces form the most important contributions toward the adsorption energy when such molecules are adsorbed on ionic surfaces, nonpolar van der Waals’ forces and electrostatic polarization giving smaller contributions (133). Healy et al. (134) studied experimentally the heats of adsorption of many polar and nonpolar gases on polar and nonpolar surfaces by means of their heats of immersion. It was found that the heat of immersion of rutile on a series of straight-chain compounds was a linear function of the dipole moment of the wetting liquid. In a later article (135)-this work was extended and it is shown that nearly the entire heat effect on immersion of the clean solid surface is due to adsorption of molecules in the first layer. From the slope of the line, giving the values found for the net heat of adsorption as a function of the dipole moments, the average field strength, F , of rutile can be found by means of Eq. (22). The experimental value found by these investigators is

F

=

2.72 X lo6 e.s.u.

which is the value of the field strength at the distance to the center of the dipole. By means of Hiickel’s equation for the dependence of the field strength on distance [Eq. (17)J the average distance between the center of the dipole and the rutile surface was calculated t o be 2.08 A. We may remark that the polarizing field of charcoal (Secs. V,7 and VI,1) has approximately the same strength a t this distance (136). I n the

ADSORPTION PHENOMENA

67

latter case the sign of the field is just opposed to th a t of the field over a rutile surface, and consequently peripheral dipoles having their positive poles (H atoms) pointing outward are strongly absorbed by the surface of rutile, but not attracted a t all, even repelled, by the surface of graphite (charcoal). This result may probably be generalized to the statement tha t the electrostatic field over an ionic crystal always attracts peripheric dipoles strongly (the negative ions practically always form the outside layer of these surfaces) and the polarizing field of metals repels them. A further analysis of the various contributions toward the adsorption energies (135) has revealed th at the adsorption energy of alcohol on rutile consists mainly of the contribution of the attraction of the dipole; the nonpolar van der Waals’ forces contribute less than 40% of this part and electrostatic polarization less than 10 %. The adsorption energies of hydrocarbons on rutile are mostly due to the van der Waals’ forces, and half the amount of the van der Waals’ contribution (one third of the total) originates from the electrostatic polarization. The adsorption of water, by its peripheral dipoles, on the surface of some inorganic salts, such as CaFz, is so strong that it cannot be desorbed as such. On heating, a reaction takes place and HF desorbs (1.37) instead of HzO. The resulting salt surface is left occupied with OH groups instead of F ions a t the outer layer. The physical adsorption of HzO molecules has been transformed into a chemisorption of OH groups. On further heating, HzO desorbs by the reaction of two OH groups t o form one HzO molecule and the surface hydroxide is converted into a surface oxide. The conversion of the surface layers of many oxides into surface hydroxide layers is the result of the chemisorption of water on these oxides. On heating, HzO desorbs and the OH groups are converted into 0 ions again. Some organic molecules with OH dipoles behave similarly to water molecules when adsorbed on salt surfaces such as CaFz, BaC12, or NaCl surfaces. On heating they do not desorb as such, but HF or HC1 evaporates and their ions are left behind as an adsorbed layer on the depleted salt surface. Many di- and polyhydroxy anthraquinones, like alizarin (I%), behave in this way. The chemisorption reaction is accompanied by characteristic color changes, which could be studied by their absorption spectra, using completely transparent adsorbent layers as obtained by vacuum sublimation of these salts. The reaction of these hydroxy anthraquinones is strictly confined to the surface layer of ions. With other substances, like picric acid, the reaction resulting in the formation of HF or HC1 and picric ions proceeds further into the lattice and ultimately the whole salt layer is converted

68

J. H. DE BOER

into a picrate layer (139). Other organic molecules, like monohydroxy anthraquinones or ortho- and paranitrophenol, are strongly adsorbed with their OH dipoles, but on heating they desorb as such. The surface reactions with HzO or with alizarin may be used for estimating the surface areas of such salt layers (140). Estimations of surface areas may in other cases also successfully be performed with the aid of the physical adsorption of other molecules with peripheric dipoles, such a s lauric acid molecules on alumina (141). 3. T h e Adsorption of Hydrogen

Various examples of chemisorption of hydrogen on metals or on oxides have already been mentioned in Sees. V,8,b through 11. I n this section we shall discuss some problems related t o the transformation of physically adsorbed hydrogen to chemisorbed hydrogen and the relations between various forms of chemisorbed hydrogen. We shall restrict ourselves to some selected problems and refer to the articles of Eley (142) and of Beeck (143) in this series for many other questions concerning chemisorption. Unlike other gases hydrogen seems t o be chemisorbed on charcoal a t very low temperatures. It is only a t extremely low temperatures, viz., in the neighborhood of 20"K., that we can speak of a physical adsorption of hydrogen on charcoal (144). The heat of adsorption is very low in this case, about 0.37 kcal./mole, which is comparable with the value for helium (0.36 kcal./mole). I n the temperature range from 60' to 90°K. hydrogen and deuterium are chemisorbed on charcoal and on graphite, the heat of adsorption being about 1.5 kcal./mole. The entropy of adsorption points to localized adsorption but it cannot be established whether one or two adsorption sites are occupied. A dissociative adsorption, however, with a free and random distribution of the hydrogen atoms has t o be excluded in this temperature range. It may be th a t dissociation has taken place but th at the atoms cannot move apart (see Sec. VII,3). At still higher temperatures (50" to 100°C.) the chemisorption of hydrogen has a dissociative character and the atoms have moved apart t o form an at-random distribution over the surface; the heat of adsorption is roughly 2.5 kcal./mole. This chemisorption probably does not lead t o the formation of covalent bonds between C atoms and H atoms; such bonds are formed a t higher temperature, hence by a chemisorption process with a n appreciably high activation energy and a far higher heat of adsorption. Burstein (146) in an extensive study of the exchange between Hz and D2 on charcoal found th at this process is governed by the dissociative adsorption of hydrogen in the range of temperatures between 500°K. and

ADSORPTION PHENOMENA

69

90°K. The charcoal had to be degassed thoroughly (at high temperature and for a long time). Adsorption of hydrogen at higher temperatures, viz. 5OO0C., poisons the surface for the exchange reaction, probably owing t o formation of covalent C-H bonds at the surface, as mentioned above. There is one indication that hydrogen may be adsorbed physically on pure-metal surfaces a t very low temperatures with a heat of adsorption comparable t o the value of 0.37 kcal./mole which was mentioned above for the physical adsorption on charcoal. Eucken and Hunsmann (146) give a heat of adsorption of Hz on Ni of 1.2 kcal./mole in the initial stages of adsorption at 20"K., a value that decreases to 0.4 kcal./mole on further adsorption. If the first value indicates active spots, the latter might give the real value. At liquid-air temperatures hydrogen adsorbs chemically on many metal surfaces, provided th a t they are really pure and not contaminated with oxygen or other gases (147). (See also Secs. V,9 and X,4.) The activation energy on pure-metal surfaces seems t o be very small. On oxides or on metal surfaces which are not completely pure, appreciably high activation energies may be found. We saw that hydrogen when adsorbed on charcoal can give rise t o different types of chemisorption. We may ask whether similar effects are found with metal surfaces. Emmett and Harkness (148) found two different sorts of chemisorption of hydrogen on an iron catalyst for NH3 synthesis. These chemisorptions were found in the neighborhood of - 100" and 100°C. respectively and were called type A and type B adsorption. The experiments were repeated by Gauchman and Royter (149), who obtained identical results. According t o Emmett (150) the chemisorption a t -100°C. can be obtained only when the iron is prepared by reduction of the oxide a t about 500°C. in a rapid stream of hydrogen which has been freed of oxygen and well dried before being passed into the reduction zone. It seems as if surface contaminations have something t o do with the type-A adsorption with the lower energy of activation. It is possible that type-A adsorption occurs on parts of the surface that are well reduced and free of contaminations or contain at, least fewer contaminations than the parts on which type-B adsorption takes place. We shall discuss the influence of these contaminations more fully in Sec. X,4. Another possible explanation was recently offered by Zwietering (151), who discusses the possibility th at type-A adsorption is due t o hydrogen atoms having a dipole with the negative pole pointing away from the surface, and that type-B represents the type where the dipole has a reversed direction. Type-A adsorption is comparable to chemisorption on pure-metal surfaces in the form of wires or of films obtained by sublimation; the sign of the dipoles of the hydrogen chemisorption at very low

+

70

J. H. DE BOER

temperatures is indeed such that the negative end of the dipole points away from the surface. Photoelectric measurements a t higher temperatures have often shown tha t chemisorbed hydrogen forms dipoles pointing with their positive ends away from the surface, also in those cases where in later investigations a t low temperatures (liquid-air temperatures) contact potential measurements revealed dipoles of reversed direction. It is, therefore, not to be excluded th at both types of chemisorption may occur at the same metal surfaces and it is t o be expected then t h a t the latter type, which in the case of iron is called A-type chemisorption:

occurs a t a lower temperature (lower activation energy) than the B type: B

Figure 13 may elucidate the energy relations. With type A the hydrogen is the electron acceptor and with type B it is the electron donor. As the ionization energy of a hydrogen atom has a high value (312 kcal./ mole) and energy is gained when an electron is joined to a hydrogen atom (16.4 kcal./mole) and as the work function of the metal is exactly the same as its electron affinity, the transfer of a n electron from the hydrogen atom t o the metal (type B ) costs more energy than the transfer in the opposite direction (type A). It is for this reason th a t the difference between the energy levels DA for type A and level A (the energy level for the Hz molecule; compare Figs. 9, 10, and 11) is far less than the difference between levels D g and A. As the H atom can approach nearer t o the metal in case B than in case A (a positively charged hydrogen atom has negligibly small dimensions), we may expect a higher activation energy and a larger value for the heat of adsorption in case B than in case A , just as has been observed by Emmett and Harkness and later investigators. We shall return to these possible explanations for type-A and type-B adsorption in Sec. X,5 but may remark here that both explanations offered above need not be considered as alternatives. The influence of the contaminations is probably such th at they facilitate the formation of the type-B dipole. Both type-A and type-B adsorption on iron have a poisoning effect on the Hz and Dz exchange on iron catalysts a t very low temperatures

71

ADSORPTION PHENOMENA

(- 196OC.). We have here the same relationship as we saw with the adsorption of hydrogen on charcoal. There is, apparently, a t those low temperatures a type of chemisorption leading to far smaller heats of adsorption and heats of desorption than are caused by the processes which dominate at higher temperatures (162). Whether this low-temperature chemisorption is of a dissociative type or not cannot be decided yet; because of the H2 and D2 exchange we might be inclined to believe it t o be of the dissociative type. The bond strength between the two H atoms must have been appreciably loosened anyhow. One might suppose that

L

I)

1

2

-r

3

4

5

6

7

FIG.13. Potential curves giving a possible explanation for the existence of two chemisorptions of hydrogen on metals.

a t liquid-nitrogen and liquid-air temperatures other electrons of the metals are involved in the chemisorption processes than at higher temperatures, where probably d electrons participate in the bonding. The Hz and Dz exchange also proceeds easily (153), that is, a t 90°K. on the surfaces of some oxides, such as Cr203.In a recent study Molinari and Parravano (164) showed ZnO t o be also a good catalyst for this reaction. Pure, nonsintered ZnO gives the exchange reaction only very slowly. As we saw in Sec. V , l l , the ZnO surface may be more stoichiometric than the bulk of the oxide. Molinari and Parravano succeeded in increasing the speed of the exchange reaction (and in lowering its activation energy) by various methods, as by sintering in vacuum, by a reducing activation,

72

J. H. DE B O E R

or by incorporating three-valent ions such as A13+ or Ga3+. All these measures result in changing the ratio between cations and anions in the ZnO lattice in favor of the cations. This means that more Zn ions of lower valency are produced. It is also stated th a t no hydrogen chemisorption could be detected on the nonsintered ZnO, in spite of its larger surface area. The more Zn ions of lower valency, the more sites will be available for hydrogen chemisorption and the exchange reaction. As the chemisorption leading to this exchange will be of the dissociative type, we may conclude that hydrogen atoms will form bonds with the metal ions (probably with those of lower valency). As the heat of adsorption is 5.1 kcal./mole in the case of Crz03, we must conclude th a t the bond strength between the adsorbed hydrogen atom and the chromium ion will be about 54 kcal./mole. Atomic hydrogen adsorbs very strongly on many surfaces including glass (155-157). From the desorption curve (167) (in the form of Hz molecules) i t may be concluded that the heat of adsorption of the hydrogen atoms will be about 43 t o 48 kcal./mole (168). Twice this amountfor the two hydrogen atoms that are formed from one molecule-is less than the dissociation energy of a hydrogen molecule. A dissociative chemisorption of Hi on glass will therefore be of an endothermic character. The activation energy, moreover, will be very high, as the activation energy for the desorption is already nearly 25 kcal./mole. Figure 14 gives potential curves which probably represent the relations in this case. The bond strength between the hydrogen atom and the glass surface is, neverbheless, remarkably high, namely of the same order of magnitude as the strengths of hydrogen in potassium hydride (44.5 kcal./mole) and in arsenic hydride (47.3 kcal./mole). The nature of this bond is unknown, but we may suggest that it is formed by a n electron transfer from the hydrogen atom to one of the metal ions of the glass surface (Ca2+, Na+, Pb2+) which the hydrogen atom can approach very closely. Hydrogen atoms are adsorbed far more strongly on CaFz films obtained by sublimation in a high vacuum (157). The CaFz surface when exposed to atomic hydrogen is covered with adsorbed atoms t o saturation, when there is one adsorbed hydrogen atom for every fluorine ion of the surface (159). The hydrogen is not desorbed at room temperature, and from the rate of desorption a t elevated temperatures a n activation energy of desorption of more than 40 kcal./mole may be estimated (160). The heat of adsorption of atomic hydrogen on CaFz may be estimated from the latter figure to be roughly 60 kcal./mole. As shown in Fig. 15, the dissociative chemisorption of molecular hydrogen on CaFz should be an exothermic process. The activation energy for adsorption, however, is extremely high, and this is probably the reason th a t this chemisorption phenomenon has never been found.

73

ADSORPTION PHENOMENA

125

E in kcrl/mole

I

10c

73

so

25

0

B 1

2

3

4

5

6

i

8

inn

-r

FIQ.14.Potential curves representing the adsorption of atomic hydrogen on glass. E in Kcal/,,,,lc 10c

SO

0

-25 1

2

-

3

4

rinA

5

6

7

FIQ. 15. Potential curves representing the adsorption of atomic hydrogen on CaFs films.

74

J. H. DE BOER

The high values of the activation energies connected with the interaction of hydrogen with glass surfaces or with a CaF2 surface may be caused partly by the large difference in distances between the hydrogen molecule and “the surface” on the one hand and between the hydrogen

AH* mo‘ecu‘e

f Hatorn

ions a-ions

etc

FIG. 16. Cross section through a CaF2lattice perpendicular t o an octahedral face. F ions form the outside surface. An Hzmolecule cannot come so near to a Ca ion as an H atom can.

atom and the metal ion to which it is bound on the other. The metal ions in CaFz-and in the majority of inorganic salts-are not situated on the outside of the surface; it is the negative ions that form the outside. The situation is visualized in Fig. 16 and the nature of the bond may be pictured as

r, C$+

t ,

or

CaF,

Other salt surfaces, as the surface of LiF, will also be covered to saturation when exposed to atomic hydrogen. Hydrogen atoms colliding on such a saturated unimolecular layer of adsorbed hydrogen atoms will not be adsorbed to any appreciable extent; they will even partly reflect specularly. It is in this way that an apparent discrepancy must be solved according t o which hydrogen atoms are extremely strongly adsorbed t o the surface of such inorganic saIts whiie on the other hand cleavage surfaces of LiF give specular reflection and even remarkably sharp diffraction patterns with beams of atomic hydrogen (161). Atomic hydrogen reflects on the first adsorbed layer of hydrogen atoms, which is so firmly bound that even the chemical reaction of a n impinging atom and a n

ADSORPTION PHENOMENA

75

adsorbed one t o form a hydrogen molecule is not a n important reaction (168). Atomic hydrogen is only weakly adsorbed on the surfaces of ice and paraffins. There is, nevertheless some adsorption on these surfaces at very low temperatures, and a combination reaction

H+H-+HZ takes place under these circumstances. This catalytic reaction is probably of great importance for the formation of molecular hydrogen from E in kcal/rnolc

-

rinA

FIG.17. An endotherniic chemisorption with low activation energies for adsorption and desorption.

the atomic hydrogen in the interstellar gas clouds on the surfaces of interstellar dust. On metallic oxides and salt surfaces, on the other hand, atomic hydrogen is adsorbed strongly and, depending on the relative position of the potential curves, this may lead t o an exothermic or endothermic dissociative adsorption of molecular hydrogen on these surfaces. For the exchange between Hz and DZ, it is necessary that a dissociative adsorption take place. But it is also a necessity that the activation energies for the adsorption and for the desorption be low enough t o allow the reaction t o proceed a t measurable speed. An endothermic adsorption as pictured in Fig. 17 may, therefore, also lead to this catalytic exchange rextion. It is possible t ha t such a situation governs the rather quick Hz and Dz ex-

76

J. H. D E BOER

change reaction on alumina (162), even at -8O"C., where no measurable adsorption is found. A measurable (exothermic) chemisorption is, therefore, not a necessity for catalysis.

4. The C h e ~ ~ s o r of ~ ~Oxygen ~on Whilst hydrogen enters into a chemisorptive bond with charcoal at very low temperatures, oxygen remains physically adsorbed unless relatively high temperatures are reached. At liquid-air temperatures the adsorption entropy of oxygens shows that the adsorbed molecules are completely free t o move and rotate over the surface (163). At room temperature and higher temperatures the van der Waals' adsorption changes slowly into a chemisorption (164). This behavior of oxygen is clearly shown by its properties t o catalyze-by its paramagnetic properties-the ortho-paraconversion of hydrogen. When adsorbed on charcoal a t low temperatures oxygen promotes this conversion, but when adsorbed a t higher temperatures it poisons the effect (165,166). The reaction of oxygen with the surface of charcoal, therefore, requires a n activation energy. I n the case of adsorption on metals, the activation energy may be zero or negligibly small. Oxygen spontaneously forms a chemisorbed layer of surface oxide molecules on a cesium surface a t liquid-air temperatures. It is, however, quite possible that this chemisorption is nondissociative (see later in this section). A molybdenum film, obtained by evaporation in a high vacuum, however, requires a higher temperature t o convert the physical adsorption into chemisorption. This can be shown by the decrease of conductivity of the film by chemisorption of oxygen (167). A similar behavior is found with the adsorption of oxygen on nickel or platinum (168). There are some cases where oxygen is chemisorbed without being dissociated into atoms. As a matter of fact one can easily understand that a molecular chemisorption may be the first step of chemisorption. This first step may well be the formation of an 0; ion, a n oxygen molecule that has taken up one electron. Oxygen molecules have a positive electron affinity of 2 to 3 kcal./mole (1691, which means that this energy is gained when electrons are taken u p by oxygen. 0; ions are found in the normal oxidation of the higher alkali metals, like potassium, when lattices of superoxides result from the reaction of the metal with oxygen (KOz). The paramagnetic and other physicochemical properties of these superoxides suggest them t o be ionic compounds, the negative ions being 0; ions. 0; ions are also formed, as a first step, in electrical reduction processes where O2is reduced at a cathode (170). It is quite reasonable to assume, therefore, th a t oxygen can be chemi-

ADSORPTION PHENOMENA

77

sorbed on some metals as 0; ions or as O2 molecules accepting an electron from the metal t o form a covalent bond with the metal surface. As already stated above, a cesium-metal surface, when exposed to oxygen a t liquid-air temperatures, is spontaneously covered with a chemisorbed layer of oxygen. As the fully oxidized product which is formed by oxidation a t higher temperatures proves t o be cesium superoxide (CsOz), we may assume that the chemisorbed layer formed a t - 180°C. also consists of chemisorbed 0; ions. As the work function of cesium has a low value, the difference in energy between levels A and D in Fig. 18 is rather small and, consequently, minimum E of the chemisorption curve will be appreciably lower than level A . E in kcal/mole

c

E I

1

2

3

4

5

----- m

rhA

FIG.18. Potential curves for the forming of a surface Cs+Oa- layer on Cs-metal.

Other metals, like silver, copper, or platinum, have far higher work functions and it is possible, even probable, th a t the corresponding minima E will be higher than level A (Fig. 19). We have again a n example of endothermic chemisorption. It will hardly be possible to obtain a measurable amount of chemisorbed oxygen in this way, but in catalytic processes other molecules may well be oxidized b y the very active 0; ions, which will form in great numbers when the activation energy E , can be easily obtained from thermal energy. Molecular oxygen ions depicted in Fig. 19 will be catalytically active, though their times of adsorption are very small; molecular oxygen ions depicted in Fig. 18 will not be catalytically active, because their heat of desorption is too high. I n constructing Figs. 18 and 19 we assumed th a t the distance be-

78

J. H. DE BOER

t

100-

,--

C

75.

/

/

/

/

c

/

/

I

/

/

0

/

.--0

/

,/

/

/

A

------1 1

D

2

3

4

5

----

U r i n A

FIG.20. An alternative set of curves for Fig. 19.

03

ADSORPTION PHENOMENA

79

tween the oxygen and the surface in minimum E is not only smaller than the minimum of the van der Waals’ curve ABC, but also th a t i t falls on the left-hand side of curve BC. It is not impossible that the size of the 0; ion is such that the situation would be better represented by the relative positions of the curves of Fig. 20. There is no principal difference between Figs. 20 and 19. The formation of some organic hydroperoxides b y oxidation with molecular oxygen is catalytically promoted by metals like silver or copper (171). A dissociative chemisorption of oxygen cannot be active in these processes; they probably proceed via the chemisorption of 0; ions (or O2 molecules forming a covalent bond resonating with a n ionic bond). 5 . Optical and Other Physicochemical Changes by Adsorption A potential curve of an endothermically chemisorbed atom or molecule represents an excited state with respect to the normal state of the physically adsorbed atom or molecule. When cesium atoms are adsorbed on salt layers or on cesium oxide, they are adsorbed as atoms and not, as they would be on metal surfaces, as ions. Ionization can be brought about by absorption of light (172) or by thermal excitation (173). The potential curves of the adsorption of cesium on a CaFz surface are given in Fig. 21, which shows that the curve for the ion represents a n endothermic chemisorption. By the absorption of light of suitable wave length the system is transferred from minimum B to a point P of the upper curve and an electron is freed and may be drawn off as a photoelectron. The phenomenon of the selective photoelectric effect could be fully explained by this photoionization process (174).By thermal excitation the transfer can be effected a t point S and this mechanism may serve t o explain the electron emission of oxide cathodes. Point S is reached by taking up a n amount of energy, which may be called the work function of the oxide cathode in this case but which is completely comparable with the energy of activation in chemisorption discussed in Sec. V,9 and subsequently. We shall not discuss these phenomena in .this article but refer t o a book of the author where these subjects are dealt with in detail (174). A similar relationship may be found in the adsorption of other atoms or molecules. Many organic substances with peripheric dipoles when adsorbed on salt layers or on the surfaces of metallic oxides show absorption spectra which are shifted appreciably to the red side of the spectrum. Thus p-nitrophenol, having a maximum of light absorption at 316 mp. when adsorbed on CaF2, has its absorption spectrum shifted to the red side and is yellow instead of colorless (175), its absorption maximum being at 365 mp. (176). Adsorbed on BaFz it shows a n absorption maxi-

80

J. H. DE BOER

mum at 413 my. (177). Similarly phenolphthalein when adsorbed on CaFz shows a bright red color and has a maximum of light absorption at 475 mp. but is red violet in color when adsorbed on BaFz with a maximum of absorption a t 536 mp. (17'8). I n all these cases the shift of the absorption spectrum has nothing to do with salt formation or ionization but results from the fact that, apparently, the excited state of the molecule is adsorbed more strongly than the ground state. The act of adsorption, therefore, decreases the energy difference between the ground level and the excited level (178).

i

3 -rinA

4

6

CO

FIG.21. Potential curves for the thermal and photo ionization of an adsorbed cesium atom.

Similar color changes were reported later by Weitz and his collaborators (179), apparently without knowing the older work of the author (180). They describe the bright-red coloration resulting from the physical adsorption of phenolphthalein. I n all these cases we are concerned with physically adsorbed molecules, pointing with their peripheric dipoles to the negative ions of the surface. The excited states of these molecules are far more polar in character; light absorption causes an electron shift in the molecule in a direction away from the surface, resulting in a far stronger bond with the negative ion on which the molecule is adsorbed (181). We may illustrate

ADSDRPTION PHENOMENA

81

this point with just one example. The structure of the ground state of p-hydroxyazobenzene is given mainly by the formula

with the normal resonance structure for the benzene rings and the OH group. The molecule is oriented with the OH group t o the negatively charged oxygen ions when adsorbed on alumina. The excited state will m:i.inly have the structure

light absorption causing a iransition of an electron from the 0 of the OH group t o an N of the azo group. The positively charged OH group of this excited structure is more strongly adsorbed on the alumina than the ground state, hence a shift of the absorption spectrum to longer wave lengths; p-hydroxyazobenzene is deep red in color when adsorbed on dry alumina. Its original yellow color can be shown by admission of water, water molecules replacing the organic molecules (182).

VII. MOEIILITY AND ORIENTATION 1 . Mobility o n Charcoal

The study of the adsorption entropies gives a great deal of information about the mobility of a.dsorbed atoms or molecules along the surface, as Kemball (185) has shown. A systematic study of the entropies of gases adsorbed on charcoal (184) along similar lines showed th a t many gases, including CO, 0 2 , N2, and many hydrocarbons behave as twodimensional gases, moving freely over the surface while rotating freely. At lower temperatures and at, higher degrees of occupation there is some restriction of the free movements, which increases in strength as the temperature is lower. The free translatory movement is the first t o be restricted at lower temperatures. The free rotation is hardly affected a t all. A distinction between the restrictions of the translation and the rotation could be made by comparing various gases, including the noble gases which have no rotations. I n drawing the conclusions we found th a t the area occupied per molecule also proved to be a n important entity. Rotating molecules show a molecular surface area which is approximately equal t o the value of the two-dimensional van der Waals’ b(b2) (185,186). Carbon disulfide molecules, methyl-, ethyl-, and n-propylchloride molecules, and also diethylether molecules, are more or less strongly hindered in their movements a t not too high temperatures. Their rota-

82

J.

n.

DE BOER

tions, however, are not hindered. The same holds for n-pentane and n-heptane molecules; these have lost their translatory movements, but not their rotations, when adsorbed on charcoal a t room temperature. The values for the molecular areas of these molecules as found experimentally on charcoal surfaces are compared with their bz-values in Table 111, which also shows some values of molecular areas occupied by the molecules on other, polar, surfaces. These figures have either been taken from the article by Livingston (185) or have been newly determined (18.4). TABLE 111 Molecular areas in A2 Gas

cs2

CzH,C1 C3H7C1 (C2Hs)zO C4Hio GHi2

b2

On charcoal

23.5 23.7 31.7 36.0 33.2 37.3 47.0

25.2 24.8 32.1 34.1 37.1 46.2

On polar adsorbents 37.9

44.6

> 44

59.6

The approximate equality of the bz value of the two-dimensional van der Waals’ equation and the molecular area is not so obvious as might be thought (186a). The bz value is by definition twice the surface area of a molecule, the diameter of which is d. This diameter is derived from the distance of approach of two colliding molecules. The molecular surface area, however, is derived from the density of liquids, each molecule being assigned its own sphere with a diameter dmin,on the assumption th a t the molecules are closely packed. As it happens t o be th a t for a great number of molecules d,, = about 1.37d a two-dimensional closely packed hexagonal arrangement will allow the molecules a surface area of

As

Nd&, fi = 0.865&

=

1.62d2

b z = g r d 2 = 1.57d2

we see that the surface area which the molecules really occupy-provided that they are freely rotating-is reasonably given by the bz value. 2. Orientation or Rotation There is, apparently, hardly any orientation of the molecules when they are adsorbed on charcoal. This is due t o the rather unspecific nature

ADSORPTION PHENOMENA

83

of the polarizing forces emanating from the charcoal surface (Sec. V,7). The same may hold for the physical adsorption on metals. In physical adsorption phenomena on dielectric ionic substances conditions are different. As we have seen already, there are more specific fields over such surfaces, reriulting in alternating fields when ions of opposite signs are being passed over via the centers of surface cells (Secs. V,4 and 5 ) . Many molecules having dipoles (peripheral or not) or quadrupoles may, therefore, tend to be oriented and to lose their rotations. As discussed in Sec. VI,2, Drain and Morrison (187) assume nitrogen molecules to lie flat on the surface of rutile, owing to their quadrupole moments. The molecular areas of other molecules, as experimentally found on polar substances, also suggest flat positions in many cases. Molecules with peripheral dipoles are directed with these dipoles to negative spots of the surfact:, as discussed in Secs. V,5 and VI,2 and 5. Such molecules may erect estch other so as to leave the dipoles directed to the surface and the rest of the molecules, being parallel to each other, pointing away from it (188). We shall not discuss these points in this article but only remark that they may be of great importance for the understanding of some selective catalytic processes. 3. .Flopping Molecules

If the entropy data indioate a localized or site adsorption, the adsorbed molecules need not be considered as immobile in the course of time. The mere existence of a,n adsorption equilibrium between a gas and an adsorbed layer implies a mobility of the adsorbed molecules along the surface. Owing to the regular pattern of crystalline matter, the surface of crystalline adsorbents will show periodical fluctuations. There will, consequently, be a regular alternation of spots where the strength of the adsorption forces is somewhat greater than the average and others where it is lower. The adsorption energy may, therefore, be different if the molecule is situated on the top of a surface ion or if it is just over the center of a surface cell. If, in the case of dynamical equilibrium, a molecule can pick up such an amount of energy from the thermal energy fluctuations that it can desorb, we may expect that by assembling a smaller amount of energy it will be able t o move from one spot to another without losing its contact with the surface altogether. In the case of normal dyi~amicalequilibrium of adsorption, a molecule striking the surface will stay there for an average time, T , which we may call the time of adsorption. During its stay of T sec. a t the surface, it remains for a much shorter time, T' sec., at a given site. After this average halting time, 7 ' ) it jumps to a neighboring site, where it stays again

84

J. H . DE BOER

for a n average time, T I , after which it jumps again, etc. During its adsorption time of 7 sec., it will hop T / T ' times to one of the neighboring sites, thus moving along the surface. The time of adsorption, 7 , may be related to the heat of adsorption by the expression 7 = T0eQ./RT (37) where T O is a constant, which we shall discuss in the next section and is the heat of adsorption. Similarly we may relate the halting time, with the activation energy for the hopping movement, Em, by

Qa

r',

The constant 7; is of the same order of magnitude as 7 0 . Em may be considered t o be the difference between the heat of adsorption when the molecule is adsorbed on a preferential site of the normal regular surface pattern and the heat of adsorption of the same molecule adsorbed on a spot just in between two such preferential sites. Em is, therefore, essentially smaller than Qd. If Em is very small with respect t o R T , the molecules will move freely over the surface; we are then dealing with the two-dimensional gases of Sec. VI1,l. I n many cases of physical adsorption on polar surfaces the value of Em may be of the order of one third t o one half the value of Qa. Let us assume Qa to he 10 kcal./mole and Em t o be 5 kcal./mole. The time of adsorption 7 a t room temperature is then sec. and the halting time T I is roughly 5 X 10-lo sec. roughly 3 X The molecule, therefore, will during its stay a t the surface make a n average of 6,000 hops from one spot to another. Let every hop be equal t o the shortest distance between the surface atoms of the adsorbent, hence about 3 8. The molecule would then cover a distance of roughly 2 X cm. This, of course, is not its rectilinear displacement, because the directions of the successive hops are arbitrarily distributed so that the displacement of the molecule is far smaller than the total length of all the hops together. Migration of adsorbed niolecules over the surface of the adsorbent in the case of physical adsorption or of adsorbed atoms or radicals in the case of chemisorption is a normal phenomenon provided the temperature is high enough to enable them to overcome the energy of activationif there is one. I n the case of adsorption of cesium ions on tungsten the energy of activation for migration is about 14 kcal./mole. A t room temperature the adsorbed ions may be considered t o be localized a t definite adsorption sites, but a t elevated temperatures there is a vivid migration. Oxygen atoms chemisorbed on a metal surface a t room temperature may be considered t o be bound to definite spots; a t higher temperatures, how-

85

ADSORPTION PHENOMENA

ever, they migrate and a t sufficiently high temperatures they may even behave as a free mobile two-dimensional gas. (See end of Sec. VII,4.) Migration of atoms over the surface of their own crystals plays a n important role in crystal growth. It is very likely that pairs of ions of opposite sign move far more freely over the surface of a n ionic crystal than single ions do (189). Such a migration of ionic pairs, probably by hopping movements, will play an important role in sintering phenomena of catalysts, leading t o a considerable reduction of the large surface area of the capillary systems of rnicroporous substances.

4. T h e T i m e of Adsorption I n the previous section we related the time of adsorption, heat of adsorption, Q a : 7 = eQz/RT

r,

t o the

(37)

This equation originates from Frenkel (190), who identified the constant the time of an oscillation of the adsorbed molecule, namely with the reciprocal frequency of tthe vibration perpendicular t o the surface. The identification of r o with the time of an oscillation of adsorbed molecules leads to the assumption th at r o will be about sec., the latter being the time of oscillation of bound atoms (191). I n many cases see., although this has of adsorption 70, indeed, proves t o be about nothing t o do with a time of oscillation of molecules. Frenkcl’s original derivation holds for the special case where the perpendicular vibration of the adsorbed molecules contributes t o the entropy, t ha t is, for the special case which Kemball (192) has termed “supermobile adsorption.” The adsorbed molecules, which in this case move freely over the surface, have not lost the third degree of translatory freedom altogether; this freedom is transformed into a freedom of vibration, perpendicular t o the surface. The reciprocal value of the frequency of the vibratory flight, which these molecules make over the surface, equals 1 0 . I n all other cases r o has the dimensions but not the meaning of a reciprocal frequency (193). The time of adsorption can be calculated by means of statistical mechanics from the partition functions of the gaseous and the adsorbed molecule (193). The equilibrium condition for the adsorption may be written as r o with

where J t r , and ofvihr stand for the partition functions of the translations, rotations and vibrations of the gaseous molecules respectively;

86

J. H. DE BOER

.frat, and are the corresponding functions for the adsorbed molecules. N, and Na stand for the number of molecules in the gaseous and adsorbed phases respectively and Qo is the heat of adsorption at absolute zero. The translational partition function of a n ideal gas is equal to

Jtr,

where m is the mass of a molecule, k and h are the constants of Boltzmann and of Planck respectively, and P is the pressure. Similarly we can write for the translational partition function of an ideal two-dimensional gas :

where 0 is the total area of the adsorbent. I n the evaluation of Eq. (38) the gas may be considered as an ideal gas. The two-dimensional gas-we consider mobile adsorption-need not be a n ideal one and we may write aftr

=

at e e t i

X

af t r I _

offme tr

Inserting this in Eq. (38) we obtain with (38), (39), and (40)

X

afrot

X

afvihr

X

eQoIRT

(41)

The internal vibrations of the gaseous and the adsorbed molecules may be taken to be equal. The adsorbed molecules, however, may have a vibration perpendicular to the surface which has taken the place of the lost translation (see above). We may, therefore, write afvzhr

=

ofvibr

X fs

(42)

where fz is the partition function of the vibration of the adsorbed molecule perpendicular to the surface. The amount of molecules adsorbed per unit area, CT = N,/O, may then be written as

According t o the kinetic theory of gases the number of molecules falling on 1 cm.2 in 1 sec. is equal to n=

n r

d 2 x T

(44)

ADSORPTION PHENOMENA 7

87

and n are related to u by (194) (45)

u = n7

Solving 7 from (45), (44), and (43) results in

Comparing (46) with (37) we see that the two equations differ in th a t the exponent of (46) contain,s the heat of adsorption a t absolute zero and not a t the temperature where the adsorption takes place and th a t the expression in brackets in (46) stands for r 0 : TO

=

&fh aftr

afmt

a ireetr gfrot

(47)

We may evaluate T O in a few selected cases. 1. For a freely translating and freely rotating adsorbed molecule, which moreover has retained part of the entropy of the third direction of translation, by converting it into an entropy contribution from the vibration perpendicular to the surface (fz > 1) (supermobile adsorption), we may put aftr

=

a5.e tr

afm

=

sfrot

TO

=

-fs

and

so that h kT

If the frequency of the supermobile adsorption v, is so low th a t hv, is much smaller than the kinlstic energy kT, the partition function

hence To

=

1

VZ

(49)

This is the case which Frenkel had in mind when he derived Eq. (37). We shall illustrate this case with a few examples. Cassel and Neugebauer (195) estimated the adsorption of xenon on mercury. An evaluation of their data (196) leads t o the conclusion that the adsorbed xenon molecules are not in their lowest state of vibrations perpendicular to the surface,

88

J. H. DE B O E R

but that the entropy contribution of this vibration is still 7.2 entropy units (at 283°K.). This means that the frequency of the vibration is v, = 4.3 )( 10" set.-'

and consequently TO =

2.3 X 10-'2 sec.

Argon adsorbed on charcoal a t 215'K. has still a slight supermobile character (197), the entropy contribution of the perpendicular vibration being 2 entropy units. Its frequency is and consequently

v, = 4.5 X 10l2 set.-' T~

=

2.2 X

sec.

From these figures for 7 0 and the heats of adsorption of xenon on mercury (3,255 cal./mole) and of argon on charcoal (3,470 cal./mole) and the temperatures of the experiments (283°K. for xenon and 215°K. for argon) the times of adsorption may be calculated with Eq. (37) [or (46)]. We obtain T = 7.8 X 10-lo sec. for xenon on mercury and T = 8.0 X sec. for argon on charcoal Despite the somewhat larger heat of adsorption and the lower temperature, the time of adsorption of argon on charcoal therefore is practically the same as the corresponding figure for xenon on mercury. The higher the entropy, hence the more mobile the adsorbed molecule is, the longer is its time of adsorption, other quantities, suck as heat of adsorption and temperature, being equal. 2. If the perpendicular vibration is practically in its lowest state, because the vibrational quantum hv, is much greater than kT and, therefore, does not contribute to the entropy, the partition function f, = 1 and we obtain

This holds true for all adsorbed molecules which, on adsorption, have just lost one degree of translatory freedom and which, in the adsorbed state, move with complete freedom over the surface as molecules of a two-dimensional gas. They also rotate freely. It was mentioned in Sec. VII,1 that many gases which are physically adsorbed on charcoal belong t o this category and we may expect this to be also the case when they are physically adsorbed on metal surfaces.

ADSORPTION P H E N O M E N A

89

sec. and we see At 300°K. their value of T~ amounts to 1.60 X that, although the meaning is different, the numerical value of h / k T in the temperature range where the adsorption is usually measured is of the order of sec., which also happens t o be the order of a reciprocal molecular frequency. We shall just give one example. The adsorption of ethyl chloride on charcoal a t 331°K., measured by Pearce and Taylor (198), can be described as a case where the adsorbed molecule moves and rotates freely over the surface. When the surface is covered with adsorbed molecules t o about 27%, the heat of adsorption is 9.1 kcal./mole. The value of h/k?' is 1.45 X 10-13 sec.; hence which leads t o

TO = T =

1.45 X 10-13 sec. 1.45 X

sec.

3. If the two translations along the surface are seriously hindered, uftr

uffrce t r

and we obtain smaller values for T~ [Eg. (47)]. The same holds when rotations are hindered or lost during adsorption, because then UfFUt

< Jrat

Kemball (199) has shown that the adsorption of benzene on mercury may be described by assuniing the molecules to be oriented with their rings parallel to the mercury surface while moving freely over this surface and rotating in the plane of the ring. Numerically, we have (200) fi

and

,fro,

= 1 =

6.8 X lop4 X

Jrot

sec. 4. When, in the act of adsorption, all three translations are lost we are concerned with a case of localized adsorption. Then, T~ has to be calculated in a somewhat different way, because a localization partition function, taking account of the number of possible ways of distributing Nu molecules over N , adsorption sites, comes into the picture. Instead of Eq. (46) we obtain 70

=

1.1 X

where 8 indicates the fraction of the adsorption sites that is occupied by adsorbed molecules, 8 = N,/N, (52)

90

J. H . DE BOER

and f z and f, are the partition functions for the vibrations of the adsorbed molecule along the surface. The effect of the internal vibrations of the molecules does not (or hardly) contribute and has been left out of consideration. A formal application of Eq. (51) leads to the conclusion th a t the time of adsorption appears to be dependent on the amount adsorbed and drops to zero for a fully occupied layer (200). This is caused by the assumption of the “monolayer ”-conception, according to which molecules striking the occupied parts of that layer are supposed to reflect, without being adsorbed even for a moment. Equation (51), therefore, gives 7 as the average value for such an “ideal monolayer.” The real time of adsorption is given by Eq. (48) by omitting the factor (1 - 0) and we see that it can be written again i n the form of Eq. (46) by putting

If 7 0 is evaluated in such a way th at Jz = f, = fi = 1, and SO all vibrations of the adsorbed molecule (atom or radical) are in their ground state, we see that the value is lower than that of Eq. (50)-the case of the two-dimensional gas with free translations and free rotations-by an amount Taking the adsorption of water on charcoal (201) as an example and accepting for a moment that the adsorbed water molecules have retained their rotations and taking NB/O = 10i5/~n1.2, we obtain (300°K.) T~

=

sec.

As the water molecules have also lost some of their rotational degrees of freedom, the actual value is lower, viz., T~

=

10-lB sec. (200)

Comparing the adsorption of ethyl chloride on charcoal (see 2 above) with the adsorption of water on charcoal, mentioned here, we obtain TABLE IV C,€I&l

T Qa ro r

331°K.

9 . 1 kcnl./mole 1.45 X sec. 1.45 X lo-’ sec.

€120

300°K. 10.8 kcal./mole see. 1.8 X sec.

ADE,ORPTION PHENOMENA

91

Despite the somewhat lower temperature and the higher heat of adsorption, the time of adsorption of the water molecules is roughly ten times shorter than th at of ethyl chloride molecules. It is again the influence of the entropy on the free energy t h a t causes this effect. The time of adsorption is higher the higher the mobility of the adsorbed molecule. I n those cases of chemisorption where the adsorbed atoms or radicals are fixed by covalent bonds, each t o its own atom on the adsorbing surface, T~ will be appreciably lower than sec. At higher temperatures there is always an appreciable mobility of the adsorbed atoms. As long as their movements can be described by the concept of hopping molecules, which mesns th at the activation energy for their movements is appreciably higher than RT, this mobility has no effect on the time of adsorption. At still higher temperatures the activation energy for the mobility may be comparable with R T and migration of the adsorbed molecules may be better described as a more or less free mobility. Johnson and Vick (202) in 1935 measured the time of adsorption 7 for oxygen atoms a t a tungsten surface in the neighborhood of 2,200"C. They found the following figures: a t 2,548'K.: 7 = 0.36 sec. a t 2,362"K.: 7 = 3.49 sec. which can be represented by = 8 X 10-14 X e147.000/RT As a t this temperature h /kT = 2.2 X we see that the chemisorbed atoms seem to behave as a supermobile twodimensional gas under these circumstances. 5 . Mobility and Reactivity Two species of molecules reacting on the surface of a catalyst may both be bound by chemisorption forces, or it may be that only one of the reacting species is bound. I n the latter case-which is known as the Rideal-mechanism-both sorts of molecules hit the surface of the catalyst, but only one of the splxies is chemisorbed. The molecules of the other sort hit the chemisorbed molecules and form a n "activated complex" which leads t o reaction. They may, however, also be adsorbed by van der Waals' forces and react with the chemisorbed reaction partner from a van der Waals' layer. It may be stated th a t entropy considerations show that such reactions will proceed more easily the smaller the mobility of the adsorbed molecules is, other quantities, such as the activation energy of reaction, being the same.

92

J. H. DE BOER

This also holds for the other mechanism-known as the LangmuirHinshelwood mechanism-where the two reactant molecules have to be chemisorbed side by side. Such pairs can be formed statistically by the molecules hitting the surface and being bound by chemisorption forces a t the very spots where they hit (localized adsorption) or they may result from collisions of the molecules moving along the surface of the catalyst (mobile adsorption). It may again be stated th a t the formation of pairs in the case of immobile localized adsorption is more favorable for reaction than in the case of mobile adsorption. As a n example we shall take the unison of two chemisorbed hydrogen atoms forming a hydrogen molecule which desorbs. When hydrogen atoms are adsorbed on a glass wall they are very strongly bound; theii rate of desorption a t room temperature is negligible (Sec. V1,3). There is, however, a very slow desorption of molecular hydrogen, resulting from the combination of hydrogen atoms on the surface. The rate of this reaction may be calculated with the aid of Eyring’s (203) theory of absolute reaction rates. The experimental rate of the evolution of molecular hydrogen from a layer of hydrogen atoms on glass, which is only partially occupied, may be understood by accepting the concept of localized adsorption of the hydrogen atoms and an energy of activation for the reaction of 25.1 kcal. /mole. If, however, the hydrogen atoms are assumed t o move freely over the surface, this energy of activation would give a rate of production of hydrogen ten times lower than the observed rate. With mobile atoms the energy of activation is required to be lower; 23.7 kcal./mole would then give the observed rate ($04). 6 . Induced

Mobility of Atoms of the Surface

Potassium metal in bulk is not attacked by dry molecular hydrogen at room temperature. Atomic hydrogen, however, reacts with the surface of metallic potassium. About 20 years ago this reaction was studied thoroughly by Lukirsky and Rijanow (206). By considering the photoelectric behavior when known amounts of atomic hydrogen were taken up by potassium a t various temperatures they found that a t -180°C. only a unimolecular layer of chemisorbed hydrogen atoms is taken up. Thus, the photoelectric activity in visible light is reduced to a low value, which points t o the fact th at the hydrogen atoms form surface hydride molecules (206). The resulting electrical double layer, with its negative side pointing away from the metal, increases the work function of the metal to such a value t ha t visible light cannot release electrons from the metal. When,

ADSORPTION PHENOMENA

93

after the forming of this urlimolecular layer, the rest of the hydrogen is pumped away and the metal is heated up to room temperature, a change will occur in the surface structure which causes a high photoelectric sensitivity. The same high photoelectric activity is found when atomic hydrogen is taken u p by the surface of the potassium a t room temperature. The photoelectric activity-at room temperature-increases linearly with the amount of hydrogen atoms th at are chemisorbed, until a maximum is reached, after which the activity falls again t o a value which is lower than t hat of the pure metal (Fig. 22). The maximum is reached when the amount of hydrogen taken u p is, again, just sufficient t o form

-

time

FIG.22. Amount of hydrogen taken up by potassium (curve a) and photoelectric current of the layer (curve b ) ail a function of the time of exposure to hydrogen atoms (20,5).

one unimolecular layer of cliemisorbed hydride molecules. Apparently, a t room temperature another layer is adsorbed in addition to the first one. The explanation of these phenomena is that a t room temperature the mobility of the potassium atoms of the surface of the metal is high enough to cause a migration of these atoms onto and over every area of surface hydride which is formed by the take up of hydrogen atoms. Consequently the photoelectric current rises in direct proportion t o the number of atoms adsorbed and thus with time (Fig. 22). The photoelectric cathode which is formed by these phenomena may be represented by t,he symbol (206):

[K]-KH-K After the whole surface has been covered with such a composite layer built up of a unimolecular hydride layer and a superposed unimolecular layer of potassium, hydrogen is taken up by the latter film, converting it into

[K]-KH-KH

94

J. H. DE BOER

which shows no photoelectric sensitivity. This is the final state a t room temperature. The final state a t - 18OoC., however, is represented by

[K]-KH which, again, shows no photoelectric sensitivity. If, a s reported above, this system is heated t o room temperature, it converts t o [ K]-KH-K

which shows a maximum of photoelectric sensitivity and can take u p another amount of atomic hydrogen while losing its photoelectric sensitivity a t the same time. Other investigators have prepared similar and more complicated layers with hydrogen on top of metals along the same principles, hence the combined action of chemisorption and surface migration. A similar behavior may be shown by oxygen. When oxygen is adsorbed on cesium a t - 18OoC., the photoelectric current rises sharply, passes a maximum, and falls to zero when oxygen is supplied continuously (207). Apparently the same happens as in the system potassiumatomic hydrogen a t room temperature. At - 180°C. cesium atoms are mobile enough to migrate on top of the first layer of surface oxide, whereupon these atoms are oxidized in their turn. The final state a t - 180°C.on continuous supply of oxygen-seems t o be a bimolecular layer of cesium oxide on top of the cesium metal. The metal is now protected against further attack. When oxygen is acting on cesium a t room temperature the mobility of the cesium atoms is so high th at a polyatomic layer of cesium atoms forms on top of any oxide layer that has been formed. We may also say that, apparently, the cesium oxide is absorbed by the metal, i.e., it is dissolved in the cesium. The appearance of the surface is unchanged, and so is its photoelectric behavior. Only after the cesium has been almost completely oxidized on continuous supply of oxygen, does a very thin adsorbed cesium layer appear on the cesium oxide and does the photoelectric current temporarily rise sharply until, on further supply of oxygen, also these last cesium atoms are converted into oxide. The mobility of potassium is smaller than th a t of cesium. The exposure of potassium metal to oxygen a t room temperature leads to a complete oxidation of the metal, but, unlike cesium, the potassium atoms, during the oxidation, do not form a polyatomic layer on top of the oxide formed, but a layer of only one-atom thickness. Consequently the photoelectric current rises and continues to rise with the amount of oxide formed until a maximum is reached-when 4 X lop4 g. of oxygen has been taken up/cm.2 of potassium surface (208)-after which it decreases

ADElORPTION P H E N O M E N A

95

again. Apparently the thickness of the oxide layer forms such a hindrance for the passage of the electrons t h a t the photoelectric sensitivity has t o fall. ’ The mobilities of the atoms of alkali metals are high. Atonis of other metals, however, may also migrate onto and on surface oxide layers. The oxidation of many metals starts in a way similar to the mechanisms discussed above. An oxygen molecule chemisorbed on the metal surface is converted into an oxygen ion by electrons from the metal. A metal ion may move up to take a position next to the chemisorbed oxygen ion. Thus a unimolecular layer of metal oxide molecules may be formed. More metal ions can move on top of this layer and the oxidation may continue. Also other mechanisms of transport of ions (metal ions from the metal through the oxide layer via vacant places in the lattice of the oxide layer or oxygen ions moving in the opposite direction) may play a role. We touch here the general phenomena of the so-called “tarnishing” reactions, which have been studied from many sides during the last 20 to 25 years; we may refer to a few outstanding articles by Wagner (209) and M ot t (210) or refer to the excellent little monograph of Rees (211). I n special cases the action of oxygen does not lead to a complete oxidation of the metal but to a reaction which is restricted to the surface or t o the surface region. These are the cases which are of importance for catalysis. The chemisorbed, catalytically active species need not necessarily be bound to the very outside of the catalyst; we may also find them in the second or third layer under the surface; for instance, in the surface region, let us say in the region of the first four or five layers. We have already discussed some consequences of this conception in Sec. V,11 and it may suffic~: here t o point to the fact that the rate of diffusion from this surface region to the surface and in the reversed direction is often sufficiently high to consider the surface region of, say, four layers as a potential source for catalytically active species. I n zinc oxide and in a-FesOs such surface regions can play a role in the equilibrium with oxygen a t low pressures (212), giving rise t o an induced heterogeneity of the surface. Frumkin and collaborators (213) investigated the action of oxygen on iron which leads t o passivity. Here again, iron atoms can migrate on top of the first unimolecular layer of oxide, giving rise to an increased capacity for electron emission. Depending on temperature, the oxidation process ends after formation of two to four layers of oxide on the iron metal, which protect the metal against further oxidation in the same way as two layers of cesium oxide protect cesium at - 180°C. A t 100°C. a maximum in potential difference with tungsten (minimum in work function) appears when 22 X l0l4 moles oxygen are adsorbed/cm.2 of real surface

96

J. H . DE BOER

of the iron (Fig. 23); at 270°C. a maximum is found when 72 X l O I 4 moles of oxygen have been taken up. Afte,r the maximum has been passed, the original potential difference with tungsten is reached a t 60 X 1014 and 100 X 1014 moles 02/cm.2 respectively. The higher the temperature, the thicker the layer which is formed by the simultaneous action of chemisorption and migration. Summarizing, we may state th at by the migration of metal atoms (ions) onto and over oxide (or other) layers formed by chemisorption, oxygen (or other atoms or ions) may be incorporated into the metal lat-

FIG.23. Change of potential difference, a t two teniperatures, between W and iron when oxygen is taken up by the latter (213).

tide, or oxides (or other compounds) may be formed. There are many cases where the rate of diffusion in the surface region, hence in a region of three to four layers under the surface, is great enough to enable the adsorbed or absorbed atoms to take part in catalytic reactions. 7. Solution in the Adsorbent (Catalyst)

Many two-atomic gases can dissolve into metals. They are split into atoms and diffuse into the metal in atomic form. I n the dissolved state they behave as having a positive or a negative charge (Zf4). Hydrogen atoms, dissolved in palladium, nickeI or iron are, partly, present in the form of protons (215); oxygen atoms in solution in zirconium are, partly, negatively charged (216). I n many cases the dissolution of the gas into the metal may be an exothermic process; in other cases, however, including the dissolution of hydrogen into nickel, iron, and platinum, the process is of an endothermic nature. I n the latter case the solubility of the hydrogen increases with increasing temperature. It is a n important fact th at in many cases the rate of diffusion of the dissolved atoms in the metals is high as compared with the rate of dissolution of the gases into the metal or the rate of desorption from the

ADSORPTION P H E N O M E N A

97

metal. The temperature coefficient of the diffusion of hydrogen through nickel or platinum is entirely given by the heat of desorption from the surface of those metals (217),where molecular hydrogen has t o be formed on desorption. Molecular hydrogen does not dissolve easily into iron with a smooth surface a t temperatures below 200°C. Atomic hydrogen, however, enters the iron easily even a t room temperature (218). I n the case of molecular hydrogen it is the activation energy at the surface which governs the process. On a smooth or con1,aminated iron surface, it is the rate of chemisorption which governs the iota1 rate. We shall return t o this special case in Sec. X,4. Oxygen may be taken up by platinum. A study of this reaction has revealed that at some stageis at not too high temperatures, e.g., 200"C., there may be a temporary increase of the work function of the platinum, showing that some of the oxygen is still at the surface; after some time the work function returns to the proper value for platinum, the oxygen atoms having diffused into the metaI (219). Nitrogen may in atomic form dissolve in and diffuse through a! iron. Up to 0.4 atomic % may be taken up. Iron and nitrogen may form compounds of variable composition. e iron nitride, for example, has a close-packed hexagonal lattice of iron atoms with nitrogen atoms occupying some of the octahedral interstices in an ordered manner. The composition may be between 35.5 and 99.3 N atoms/100 iron atoms at 400°C. The N atoms diffuse readily through this lattice. Goodeve and Jack (620) studied the evolution of Nz gas from this solution and found that the nitrogen atoms on reaching the surface behave as a two-dimensional gas and combine to form Nz molecules, which evaporate. The rate-determining process is the unison of two ehemisorbed N atoms to form a N?:molecule. Oxygen and nitrogen may also dissolve in the lattice of zirconium or titanium, where again they are situated a t interstices of the metallic lattice. I n these metals the atoms are too strongly bound to be of any value for catalytic purposes. The diffusion of chemisorbed atoms into the metals and the migration phenomena of the previcms section may show that in chemisorption phenomena it is distinctly possible to find the adsorbed atoms below the surface of the adsorbent. Chemisorbed atoms of the surface region, hence including those below the actual surface, may take part in catalytic processes. When the nitrogen atoms of the t iron nitride phase meet hydrogen at the surface of their lattice, ammonia is formed. Hydrogen atoms dissolved in nickel may well react with chemisorbed olefines reacting from underneath. There is also the possibility of hydrogen compounds, for example, hydrocarbons, splitting off a hydrogen atom which immediately

98

J. H. DE BOER

disappears into the lattice of the catalyst, the rest of the molecules remaining chemisorbed at the surface. Such chemisorption phenomena play a role in catalytic isomerization processes of vegetable oils on certain nickel catalysts.

VIII. PHYSICAL ADSORPTIONPHENOMENA AT HIGH DEGREES OF OCCUPATION 1. General Remarks

Most of the preceding sections were concerned with singly adsorbed atoms or molecules. At higher degrees of occupation, when the distance between the adsorbed atoms or molecules decreases, the interaction forces between them may become strong enough t o influence the strength of the adsorption. These forces may be repelling forces or mutual attraction forces. I n theoretical derivations of adsorption isotherms, giving the amount of adsorbed molecules as a function of the equilibrium pressure of the coexistent gas or the equilibrium concentration in the coexistent solution, it is mostly assumed that the heat of adsorption is the same over the entire surface of the adsorbent. As we discussed in Sec. V,12 such a conception can hardly be maintained for practical adsorbents. Chemisorbed atoms or ions alter some properties of the adsorbents rather seriously. It is especially the work function for electron emission (and, therefore, also the electron affinity) which is either decreased or increased. Conversely this change affects again the adsorption energy. As a result of these various effects the strength of the adsorption may depend on the degree of occupation. The present section deals with physical adsorption phenomena a t high degrees of occupation. Chemisorption phenomena a t high degrees of coverage will be dealt with in See. IX. 2. T h e Heat of Adsorption as a Function of the Degree of Occupation in Physical Adsorption Phenomena on Conducting Adsorbents

As we discussed in Sec. VI,1 physical adsorption on charcoal and on metal surfaces is caused by the polarization of the adsorbed molecules in the electronic field over the surface of the conducting adsorbent (Sec. V,7) , together with the nonpolar van der Waals’ forces between the adsorbent and the adsorbed molecules (Sec. V,2). As mentioned in Sec. V,12, the magnitude of the polarization of the adsorbed molecules by the electronic field is not seriously influenced by so-called “active spots” or by surface heterogeneity. The contribution by the nonpolar van der Waals’ forces, however, is more influenced by a heterogeneous character of the surface of the adsorbent. As those forces cooperate and as the surface of a metallic

99

ADS0 RPTION PHENOMENA

adsorbent generally shows heterogeneity, we may expect the heat of adsorption t o decrease with inmcreasing amount of adsorption. This is true for the physical adsorption on normal, polycrystalline metals and metal powders. Usually the heat of adsorption starts a t low coverage with a figure appreciably higher than the heat of liquefaction of the adsorbed gas. With increasing degree of coverage, 0, the heat of adsorption decreases skadily and gradually approaches the heat of liquefaction. Rhodin (921) has succeeded in preparing three different well-defined crystallographic faces of copper and in measuring the adsorption of nitrogen on these faces a t various (low) temperatures. As may be seen from

I

Mo'

1

FIG.24. Heats of adsorption of nitrogen on copper as a function of t h e degree of coverage, 8. The solid lines are for the crystallographic faces, which:are indicated; the dashed line is for polycrystalline copper (221).

Fig. 24, i t is remarkable th at the heat of adsorption on all three crystallographic faces is practically constant a t low degrees of coverage and th a t a t 6 values higher than 0.5 the heats of adsorption increase considerably. At low 0 values we may coiiclude th at the nitrogen molecules, which repel each other by their dipoles induced by the electronic cloud of the surface and attract each other by their mutual van der Waals' forces, do not influence each other; this may be largely due to the counteraction between their forces. At higher 0 values the attraction by their mutual van der Waals' forces, leading to a two-dimensional condensation (Sec. V111,4), raises the heats of adsorption considerably. It may be noted that the three faces give rise to different heats of adsorption. The {IlO] face, which is the least densely packed with copper atoms, gives the smallest value; the most densely packed { 1 1 1 ) face gives the highe6t heat of adsorption. Polycrystalline copper, a t low

100

J. H. DE BOER

8 values, gives heats of adsorption which are higher than those on any of the :three faces. Apparently, active places, e.g. crystal boundaries, are responsible for this behavior (222). Charcoal, activated or graphitized, and graphite itself may be expected t o offer mainly their basic planes for adsorption. We may therefore expect a rather homogeneous character of the surface. The heat of adsorption of physically adsorbed molecules reflects this character. The heat of adsorption of many gases, including argon, nitrogen, oxygen, and many hydrocarbons, is practically constant (223a-e) . Sometimes heats of adsorption are reported which decrease slightly with increasing degree of occupation; the heats of adsorption for ethylchloride on charcoal, reported by Goldmann and Polanyi (ZdSe,%4), decrease from 12.5 kca1.l mole for 0 = 0.09 t o 9.5 kcal./mole for 0 = 0.60. The heats of desorption of n-pentane (bdSc), carbon disulfide (22%) , and diethylether (223e) on the same charcoal show exactly the same dependence on the degree of occupation. We may, therefore, conclude that in such a case the surface heterogeneity causes the decrease. Argon on graphite, a t higher e values, shows the same behavior as nitrogen on sinyle-crystal copper, as has been described above (223). The mutual van der Waals’ attraction forces may, in this case, 1ea.d t o two-dimensional condensation, as is also shown by the entropy data. We shall discuss two-dimensional condensation in Sec. VIII,4. I n all other cases the mutual repulsion of the induced dipoles and the mutual attraction by van der Waals’ forces, both rather weak in themselves, balance each other in the adsorption of gases on charcoal and graphite, causing the heat of adsorption t o be practically independent of the degree of occupation. It is only a t very low e values that the influence of active spots is noticeable; a t low degrees of coverage higher heats of adsorption are reported, which fall rapidly with increasing 0 values to practically constant values (625). 3. The Heat of Adsorption as a Function of the Degree of Occupation in Physical Adsorption Phenomena o n Ionic Surfaces

As we saw in Sec. VI,2 physical adsorption of normal gases on ionic surfaces results from a combined action of van der Waals’ forces and polarization of the molecules by the electric fields of the surface. Active spots (Sec. V, 12) influence both effects. Actual heterogeneous surfaces of ionic adsorbents, therefore, showing various Crystallographic faces, crystal boundaries, edges, vacant ionic sites and many other types of active places, will in all practical cases adsorb the first molecules with a relatively high heat of adsorption. The heat of adsorption will decrease appreciably with increasing degree of coverage (626). Crawford and Tom-

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101

kina (227), investigating the adsorption of SO2, COZ, and other gases on CaFz and BaFz, noticed a decrease in the heats of adsorption with increasing amounts of adsorbed molecules. They ascribe the effect t o the nonuniformity of the surfaces, in addition t o the presence of different crystallographic planes. Both SOz and C 0 2 possess quadrupole moments and it might be asked whether the main contiribution toward the heat of adsorption comes from the nonpolar van der Waals' forces or from the electrostatic forces.

FIQ.25. Isosteric heats of adsorption a t 0°K. for nitrogen on rutile (curve A ) and for argon on rutile (curve B ) ;contribution due to the quadrupole attraction of nitrogen (curve C); contributions due to the electrostatic polarization of argon (curve 0 ) and to the dispersion forces (curve E ) [data from Morrison et al. (23O)J.

Active spots for these effects do not coincide (Sec. V,l2). Active spots for van der WaaIs' forces are not active for electrostatic effects and vice versa. A distinction between the two effects has recently been made by Drain and Morrison (628). The heats of adsorption of argon, oxygen, and nitrogen on rutile decrease markedly with an increasing amount of adsorbed molecules (229). In Fig. 25 the isosteric heats of adsorption for nitrogen (curve A ) and argon. (curve B ) , reduced t o O'K., are given according t o the data of Morrison and collaborators (230). The curves are similar in character, and Drain and Morrison (268) succeeded in reducing them by linear transformations t o one single curve. The curve for oxygen could also be reduced t o the same curve. A single form of distri-

102

J. H. D E BOER

bution curve may be used for the three gases. The excess of heat of adsorption of nitrogen over that of argon varies greatly with the degree of occupation, 8, and Drain and Morrison assume that this excess of heat is caused by the attraction of the quadrupole of nitrogen by the electrostatic fields (see also Sec. V1,2) of the various parts of the surface. The difference between curves A and 3 (curve C ) may then be taken as an indication of the heterogeneity of the distribution of these electrostatic active spots. The known quadrupole moment of nitrogen enabled them t o calculate the electric field, acting a t the various surface coverages, whereupon, assuming argon to be adsorbed on the same electrostatic active spots, they calculated the contribution of the electrostatic polarization of argon by means of the known polarizability of this gas. Thus curve D results; curve El being the difference between B and D, gives the contribution of the nonpolar van der Waals’ forces. We see t ha t a t low # values the electrostatic polarization is more important than the attraction by van der Waals’ forces. This would, according t o our interpretation, mean that the electrostatic fields force the molecules t o be adsorbed on their active spots and not on the active spots of the van der Waals’ forces. The antagonism of the activities of both kinds of active spots is in this picture reflected in the increase of the van der Waals’ contributions (curve E ) with increasing degree of occupation. Relations of this kind can, of course, be found only when the molecules are not freely mobile over the surface but are actually localized on their active spots. Morrison and collaborators (228,230) found from their entropy figures that the adsorptions of argon, nitrogen, and oxygen on rutile are of the localized character; this does not mean (see Sec. VII,3) tha t the molecules cannot move over the surface. During the calorimetric experiments they reach their equilibrium positions by hopping movements. These experimeiits were done between 75” and 180°K. At still lower temperatures even the hopping movement may not be sufficiently strong for the molecules to reach their right positions; in such cases too low values are found for the heats of adsorption a t low 0 values. Pace, Dennis, Greene, and Heric ( M I ) found that the temperature should be higher than 73°K. in order t o obtain reversible adsorption, When we calculate the st.rengths of the electrostatic fields corresponding to the points a and b indicated in Fig. 25, we find the values 4.1 X lo6 e.s.u. and 0.9 X 106 e.s.u. respectively. These figures compare very well with the average value of the field over rutile, viz.,

F

=

2.72 X lose.s.u.

derived by Chessick, Zettlemoyer, Healey, and Young (232) from their

ADSORPTION PHENOMENA

103

completely different experiments, which were discussed in Sec. VI,2. As already stated in See. VI,2, this average field is of the same order of magnitude as the field caused by the electronic cloud distribution over charcoal (Secs. V,7 and V1,l). The field over charcoal-and metals-however, is far more homogeneously distributed over the surface than the electrostatic field emanating from the surface of rutile. Moreover, as already stated in Sec. VI,2, the two fields have opposite directions. Here we differ with Drain and Morrison (228), who tentatively ascribe the field t o the positive titanium ions. In th at case the peripheric dipoles of alcohols would not be so strongly adsorbed as they apparently are.

f

- e

FIG.26. Heats of adsorption of argon on cesium iodide (curve A ) and on potassium chloride (curve B ) as a function of 0 (233).

It is remarkable th at the heats of adsorption of argon, oxygen, and nitrogen on rutile do not increase when the e value approaches unity, in other words, when the mutual distances of the molecules approach those of a completely filled unimolecular layer. Apparently the heterogeneity of the surface prevents this effect. I n Fig. 24 it is shown th a t such a n increase, caused by the mutual attraction of the adsorbed molecules, is found with the adsorption of nitrogen on copper. Such a n effect is also found by Orr ( W S ) , who estimated the differential heats of adsorption from adsorption isotherms of argon, oxygen, and nitrogen on potassium chloride and on cesium iodide a t about 80°K. Two of his curves for argon are given in Fig. 26. We see, again, th a t at low coverages the heats of adsorption fall with increasing e values, but th a t if @ approaches unity the heats of adsorption rise t o a maximum. This increase is caused by the mutual van der Waals’ attraction of the adsorbed molecules. Kemball (284) deduced from entropy data th a t the adsorbed molecules are freely mobile in this case, up to a 0 value of about 0.8. Iodine molecules, adsorbed on barium chloride layers obtained by sublimation, also show an increased heat of adsorption with increas-

104

J. H. DE BOER

ing degree of coverage (636),the heat of adsorption being 11.8 kcal./mole a t 6 = 0.57 and 13.7 kcal./mole at 0 = 0.74. The heat of sublimation of solid iodine is 15.5 kcal./mole; the adsorbed molecules moving freely over the surface have a far higher entropy than those of the solid. Again the mutual van der Waals' attraction of the adsorbed molecules causes the increase of heat of adsorption. The heterogeneous character of the surface of CaF2, SrF2, and BaFz layers, obtained by sublimation, is shown by the absorption spectra of iodine molecules which are adsorbed on these surfaces. The very first molecules show a very high absorption coefficient (236); molecules adsorbed at 0 values higher than e = 0.005 show far lower values for the absorption coefficient. The absorption maxima shift continuously t o longer wave lengths when the molecules are adsorbed at higher 0 values (23237)'

4. Two-Dimensional Condensation and Multirnolecular Adsorption The mutual van der Waals' attraction of adsorbed molecules may lead t o two-dimensional condensation phenomena. We shall not discuss these phenomena in detail but shall refer t o some recent reviews and general treatments (238,239). A few remarks will, therefore, be sufficient here, Two-dimensional condensation of a nonideal two-dimensional gas (mobile adsorption) requires a temperature lower than the two-dimensional critical temperature, which, depending slightly on the theory used, may be calculated to be either exactly half the normal three-dimensional temperature (two-dimensional van der Waals' equation) or a figure somewhere in this neighborhood. Experimental evidence corroborates this. Mutual orientation may play a n important role in two-dimensional condensation phenomena. When anisotropic molecules erect each other during their condensation, the two-dimensional critical temperature will be higher. When they are condensed while lying flat on the surface, the temperature will be lower than half the value of the three-dimensional critical temperature. The normal critical temperature of nitrogen being T, = 12G°K., the normal (isotropic) two-dimensional critical temperature would be T,, = 63°K. ; if, however, the nitrogen molecules are condensed while lying flat on the surface, T,,would be 5G.8"K. If the condensation led to an arrangement with all the long axes of the molecules parallel to each other and perpendicular to the surface, T,, would be 92°K. There are indications that the two-dimensional critical temperatures of N z and CO on steel are above 78°K. and a t about 93°K. respec-

ADSORPTION PHENOMENA

105

tively; this may mean that those molecules erect each other when condensing on the surface of steel. Two-dimensional condensation may even be an endothermic process when the difference between the heats of adsorption of the molecules in their flat positions and in their positions perpendicular t o the surface is larger than the contributicln of the mutual attraction of the van der Waals’ forces during two-di rnensional condensation (240). Parallel oriented dipoles lower the value of T,,, as does a decrease in the heat of adsorption caused by a heterogeneous surface. Hill (241) has discussed the two-dimensional condensation phenomena in localized adsorbed layers. Two-dimensional condensation from a dilute localized adsorbed layer to a relatively condensed localized layer may also occur in this case at temperatures lower than a two-dimensional critical temperature. Two-dimensional condensation-on homogeneous surfaces-leads to sudden jumps in the adsorption isotherm. These jumps may already be found at very low pressures of the gas which is in equilibrium with the adsorbed layer (242). Heterogeneous surfaces do not give rise t o sudden jumps but to gradual slopes (Sec. V,12). There is sometimes a tendency to consider such jumps as indications of multimolecular adsorption; this is not correct. It is of course true that stepwise adsorption can also occur together with multimoleculair adsorption. (See also Sec. V,12.) A condensed unimolecular layer may form a suitable new surface for another adsorbed layer t o be formed (243). A second layer, and more, may also be adsorbed on top of a supercritical densely packed unimolecular layer. I n many cases of di- or multimolecular adsorption, the isotherm shows a region where the slope changes from a rather steep course to a more gradual or sometimes nearly horizontal one. The adsorbed amount in this region often coincides approximately with the amount which fills a unimolecular layer. Any method, therefore, which analyzes mathematically or graphically the isotherm to produce a point of that region can be used for the estimation of surface areas. This explains the success of the well-known B.E.T. method for this analysis. After the excellent Idiscussion by Hill (244) of the B.E.T. and the Huttig theories, in which he points out the weaknesses of the first and the fallacy of the latter, and after the analysis by Halsey (.245),who indicates when a B.E.T. isotherm of “satisfactory ” character is obtained on a heterogeneous surface, little need he said here. It may, however, be of solme importance t o indicate once more (246) that all kinds of isotherms, urjually taken as proof of multimolecuIar adsorption, may be obtained with unirnolecular layers and gradual varia-

106

J. H. DE BOER

tion of adsorption forces on heterogeneous surfaces. The B.E.T. method, nevertheless, is very useful as a practical tool. Molecules can adsorb in a second layer only when their heat of adsorption in that layer is higher than the heat of liquefaction (or solidification) or when in the second layer the entropy is higher than in the liquid (or solid). This entropy effect can be of assistance for only the top layer, for if a third layer has to be adsorbed on top of the second one, the entropy in the second one may not be very high. Consequently multimolecular adsorption requires a heat of adsorption in the second layer and higher (up till the last but one) layers which is higher than the heat of liquefaction. Hill (247) and also Halsey (248) assume the van der Waals’ field of the surface t o transmit energy to the second and higher layers. In addition to this possibility it may be pointed out th a t in physical adsorption on charcoal and on metals as well as on ionic surfaces, the adsorbed molecules are polarized, as we have seen in the preceding sections. The field of their dipoles may also influence the molecules in a second layer, etc. This is essentially the basic idea of the oldest conception of multimolecular adsorption (2.49). Unfortunately, many authors in more recent literature have erroneously stated that, according t o our old conception, polarization had to do the entire job. It has, in fact, only to provide the small amount of extra energy above the heat of liquefaction t o enable a next layer to form. The forces responsible for a second or third layer to be adsorbed cannot be very strong. We may, therefore, not expect multimolecular adsorption t o occur at moderately low relative pressures on flat and homogeneous surfaces. By means of absorption spectra a bimolecular adsorption of p-nitrophenol could be established on CaFz or BaFz layers (250). The absorption spectrum of the first layer was quite different from th a t of p-nitrophenol itself, the absorption spectrum of the second layer being practically the same as th at of pure p-nitrophenol. The first layer is apparently sufficiently polarized by the salt surface, to enable a second layer t o adsorb; the second layer, however, cannot accumulate a third layer. It may not be entirely excluded, however, that there is no more place in the capillary space between the salt layers t o accommodate a third layer of p-nitrophenol. Recently Deryagin and Zorin (251), by optical means, investigated the adsorption of alcohols, water, benzene, nitrobenzene, and other gases on a n optically polished glass surface. Multimolecular adsorption did not occur until the relative vapor pressure was higher than 0.95. I n capillary systems of microporous substances more layers can easily be formed by the combined action of two or more walls, giving a gradual transition to capillary condensation (262).

107

ADSORPTION PHENOMENA

IX. CHEMISORPTION PHENOMENA AT HIGHDEGREES OF OCCUPATION 1. The Decrease of the H m t of Chemisorption with Increasing Degrees of Occupation

I n most cases of chemisorption it is experimentally found th a t the differential heat of chemkorption decreases seriously with increasing 0 values. This phenomenon has been the subject of many discussions during the last few years (253-258). In order t o draw the attention to the magnitude of the effect we shall give a few examples in Figs. 27 50 kcal/mole

t

40 30

20

I0

- (12_0.4

0.6

0.8

- 9

FIG.27. Heats of chemisorption of hydrogen on tungsten; A, on tungsten films (253,260), B, on tungsten filament [curve through the points of Roberts (261)],C, on tungsten powder (269).

and 28. Curves A and B of Fig. 27, giving the heat of chemisorption of hydrogen on tungsten films (259,260) and on tungsten filaments (261) respectively, show that the initial values of the heat of chemisorption (at e = 0) are practically the same on both forms of this adsorbent and that the decrease as a function of e also follows practically the same line. Curve C of the same figure shows the heats of chemisorption of hydrogen on tungsten powder according to Frankenburg (26.2).The initial heat is practically the same as on the other forins of the tungsten adsorbent, but the decrease, as a function of el is much steeper. As discussed fully by Beeck (254),it is not improbable that the surface of the tungsten powder used by Frankenburg was seriously contaminated with some impurities (BBS), such as, for example, silica (see Sec. IX,2). Although in the case of Fig. 27 the initial heats of chemisorption are

108

J. H. DE BOER

practically the same on the three forms of adsorbent used, this is not so in the examples of Fig. 28. I n this figure curve A gives the heat of chemisorption of hydrogen on a nickel film (264) and curve B the same on nickel powder obtained by reduction of nickel oxide (666).As the reduction of the oxide in the latter case consisted of a short reduction at the relatively low temperature of 280"C., a contamination of Euckens' nickel sample by oxide, or chemisorbed oxygen, is not impossible (see Sec. X,4). Curves C and D were also obtained with reduced nickel powder, at 100' and 300°C. respectively (266);the initial heats of those curves %re comparable with the heat of curve E, which was found for thoroughly reduced nickel supported on silica (267).

I

FIQ.28. Heats of chemisorption of hydrogen on nickel: A, on a nickel film a t room temperature (864);B, on nickel powder at 0°C. (86'6); C, on nickel powder a t 100°C. (266);D, on nickel powder at 300°C. (866);E, on nickel supported on silica a t 0°C. (67).

Whereas the heats of chemisorption of hydrogen, found on various nickel adsorbents, are higher on films than on reduced powder, copper films do not bind hydrogen by chemisorption (f?68,W69) , although copper powder does (270,271) (see Sec. X,4). We shall not mention more examples here. The same picture as shown by hydrogen is given by other gases when chemisorbed on metals. We may refer t o the literature mentioned in the beginning of this section. I n Secs. VIII,2 and 3 we saw that two phenomena may cause a decrease of the heat of physical adsorption of gases on metals or on ionic adsorbents: first the mutual repulsion of parallel oriented dipoles and, second, a heterogeneous character of the surface, Since we also learned that the mutual repulsion of dipoles does not materially contribute t o a decrease of the heat of adsorption and since i t can easily be proved that, also in the case of chemisorption on metals,

ADfIORPTION PHENOMENA

109

the dipoles of the chemisorbed atoms are not large enough by far to explain the effect (272), we shall not further consider the possibility of this cause. The mutual repulsion of the chemisorbed atoms by these dipoles can, in the most favorable case, account for only a minor part of the decrease of the heat of chemisorption with increasing degree of covering. A heterogeneous character of the adsorbing surface could, of course, also lead t o a decrease in the heat of chemisorption with increasing e if the heterogeneity is conceived as the existence of a wide distribution of strengths of chemical bonda between the chemisorbed atoms and the surface. Ever since Taylor (273) suggested the surface to be heterogeneous, many authors have accepted the view th at the surface would show varying properties from place t o place. Other authors, however, are inclined to consider metal surfaces which are carefully (physically as well as chemically) prepared as essentially homogeneous. Well-prepared filaments t ha t can be outgasseld a t high temperatures (such as the filaments of tungsten, used by Roberts (,%‘GI)), and films of metals, obtained by evaporation and condensation of metals in a high vacuum, would presumably show more homogeneous surfaces than powders, obtained by reduction of oxides, would do. Anyhow, one would not expect the same degree of heterogeneity t o result independently of the method of preparation of the metal. If the surface has t o be considered as a homogeneous one, another explanation must be given for the strong decrease of the heat of chemisorption. We shall discuss the influence of the heterogeneous character of the surface in See. IX,2, which will be followed (Sec. IX,3) by a discussion of experimental methods to distinguish between a heterogeneous and a homogeneous surface. I n subsequent sections we shall examine the decrease of the heats of chemisorption with increasing degrees of occupation on a surface of a homogeneous character. 2. Factors that Cause a Surface Heterogeneity for Chemisorption

The undeniable fact that; the surface may show a dominating heterogeneity for physical adsorption, hence for van der Waals’ attraction forces or for electrostatic polarization by local fields of the surface (Secs. VII1,B and 3), does not mean th at they should be heterogeneous for chemisorption as well. As was stated in Sec. V,12 the forces between ions and metal surfaces and the covalent forces between chemisorbed atoms or molecules and metal surfa,ces are far less influenced by “active places” of the surface than are some of the forces leading to physical adsorption. It is especially the cracks and fissures of the surface, which may give it a pronounced heterogeneoua character for physical adsorption, th a t do not influence the chemisorption bonds very much (274).

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J. H. D E BOER

It is, therefore, quite possible that a surface, heterogeneous in character for physical adsorption shows a homogeneous nature for chemisorption. As discussed in Sec. V, the work function of a metal measuring the energy which has to be provided to extract an electron from it and a t the same time indicating the electron affinity of the metal plays a n import a nt role in many heats of chemisorption. The actual value of the work function is different for different crystallographic faces of the metal. In a qualitative way this is shown spectacularly by the emission patterns obtained with Muller's field-emission microscope. I n 1937 Muller (275), using a tungsten single-crystal point, observed that the { 110) face shows the weakest emission of electrons; the emission from a 1211) face was stronger; a {loo) face emitted more strongly again, and the strongest emission was obtained from a { 111) face. It is undecided yet whether these crystal faces are really present or not (276a,b,277).It may be th a t surface migration takes place and results in the forming of really small crystal faces, the crystal tending to shape itself into a n equilibrium form (278). Such surface migrations of the tungsten atoms, even taking place in preferred directions, have been observed (276b,278) when tungsten atoms are condensed on the emitting point a t 1000 t o 1500°C. Similar phenomena are observed when nickel atoms are condensed on a n emitting nickel point (279), and we may accept, with Gomer, th a t the singlecrystal point has a spherical form with small crystal faces of low indexes, separated from each other b y bond regions without sharp edges. The influence of the natural roughness of the surface on the emission seems t o be rather small (280). All these observations seem t o indicate th a t the variation of work function with crystallographic direction is not necessarily connected with the actual presence of these faces a t the outside of the crystal. One may, perhaps, conclude th a t the work function depends on the direction in which the electron leaves the metal, the direction of leaving perpendicular to a { 1111 plane, hence in a [lll]direction in tungsten, resulting in a lower work function than in a [110] direction. Nichols (281) measured the work function of tungsten for various directions. His figures are reported in Table V, where also for the various directions the numbers are given of the nearest (Nl) neighbors of a tungsten atom of the surface as well as the numbers of the next following atoms (N,) and the numbers of the atoms that follow as third neighbors (Na);these numbers have been taken from Stranski and Suhrmann (278). There are indications that the work function for the [110] direction is far higher than indicated (280). It seems as if the number of neighbor atoms located a t short distances determines the magnitude of the work function; the larger the number of direct neighbors at the shortest and

111

ADSCiRPTION PHENOMENA

next shortest distances, the higher the work function. It must be stated, however, that Benjamin and Jenkins (282) found the emission of { 110) to be greater than that of { 1111. From a comparison between various modifications of metals it was already known that the work function of a metal (average work function over all directions) is higher the denser the metal (283). Recently Sachtler dkcovered a rough parallelism between the work function and the product of the density of the metal and the ionization potential of the individual metal atoms (684). The heat of chemisorption also depends on the orientation of the surface. I n a qualitative way this is clearly demonstrated by electron emission microscope pictures of metals on which various atoms are adsorbed. TABLE V Crystallographic direction [hkll

N1

N2

N3

6 4 5 4 4

4 5 3 3 3

7

8

7 9 7

'p

(e volt)

4.65 4.53 4.66 4.36 4.36

The various orientations of the crystallites of a metal specimen show up clearly, and i t can be shown that the electron emission depends on the degree of covering with adsorbed atoms, which itself depends on orientation and on temperature (2135-287). We may expect the adsorbability to depend on the work function and also on geometrical faciiors, such as for example the fitting in of the dimensions of the adsorbed rztoin or molecule in the two-dimensional pattern of the surface. Geometrical factors have played an important role in the catalytic literature a t various times (288,289). Both factors depend on the orientation of the crystal faces of the surface. They may either work in opposite directions or may collaborate in the same direction. The chemisorption of cetsium or other alkali metal atoms on tungsten, leading t o the formation of ions (Sec. V , l l ) , will be largely governed by the work function, As positive ions are formed, we may expect the heat of chemisorption to be larger, the higher the work function. Johnson and Shockley (g90) found that a tungsten filament, consisting of a single crystal, having a (110) plane perpendicular t o the axis and emitting electrons a t 2,OOO"K. shows 11111 and { 100) faces, which is in accordance with the work functions mentioned in Table V. When the tungsten is

112

J. H. DE BOER

heated a t lower temperatures in cesium vapor, other faces appear on the fluorescent screen, depending on temperature, viz., { 211) a t 900°C. and 1110) a t 850°C. The (211) faces adsorb cesium most strongly, next follow the (110) faces. At 700°K. the amount of cesium adsorbed on these two faces is already too large for maximal emission; the faces around (211) and (110) show the highest emission. { 100) and { 111))however, do not show up yet; apparently they do not adsorb cesium at this Gemperature and cesium pressure, These results are completely in harmony with the work-function figures of Table V. The higher the work function the better cesium is adsorbed. Similar results were obtained with tungsten single-crystal spheres (291). The adsorptions of sodium, barium and thorium on tungsten and molybdenum have been studied along similar lines. Sodium on tungsten and molybdenum and barium on molybdenum are preferably adsorbed on 12111 faces (292); barium on tungsten on 1111) faces; thorium, however, is preferably adsorbed on { 111) faces but not on { 110) faces (29.2). It seems as if other factors are operative besides the work function. Oxygen atonis, chemisorbed on tungsten or molybdenum, seem to prefer (100) faces (293). In the chemisorption of oxygen the metal has t o provide the binding electrons, and it might be expected that a low value of the work function would favor the process. Steric factors, however, may also play their role here. We may conclude that on polycrystalline material, where various crystallographic orientations will be present at the surface, a certain degree of heterogeneity will result from the different heats of adsorption on the different crystal faces. As we see, however, from the difference in the work functions of tungsten, these differences are not extremely large. We may perhaps expect th at a certain part of the observed decrease of heats of chemisorption with increasing 0 values may be ascribed to this heterogeneity, but it seems doubtful th at the whole effect should be caused by it. I n Secs. V,5 and VI1,6 we discussed the influence of irregularities of atomic dimensions in the surface. We also saw how chemisorption may cause new deviations of normal valencies or may create vacancies in the regular pattern of the surface layer or of the layers in the surface region beneath the surface. Vol’kenshtein (294) treated chemisorption phenomena on solids with surfaces that are provided with such sorts of microdefects (295). These microdefects possess a mobility which is characterized by a n energy of activation. They have the ability of interacting with one another and they give rise to new microdefects when chemisorption takes place on them. Instead of the concept of active centra, therefore, which are bound

ADSORPTION PHENOMENA

113

to definite sites, a picture is developed of nonlocalized active spots. Taylor (296) suggests that such active spots may be operative on the surfaces of actual catalysts and on metal surfaces which are contaminated. Vol'kenshtein considers the microdefects to be partly of a biographical origin, partly of a thermal oyigin. The creation of the latter sort may consume a certain amount of energy. Chemisorption means a reaction between the atom to be chemisorbed and a microdefect. The heat of chemisorption results from the cantributions by the heats of these reactions and the opposite contribution by the heats of creation of new (thermal) microdefects that form new adsorption sites. I n this conception the surface is not essentially of a heterogeneous character; the heterogeneities are created during and by the chemisorption process. As this creation consumes energy, the heat of chemisorption decreases. As already stated above, Taylor assumes the aforementioned microdefects t o be present on contaminated surfaces. Contaminated surfaces, however, may also show heterogeneity for chemisorption in a more classical way. As we have seen in Sec. V, oxides of metals may chemisorb gases, such as hydrogen, as well as metals do. Surface contaminations extending over small, but dej'inite, areas may, therefore, offer possibilities for chemisorption th at are different from those on clean or less contaminated parts of the surface. PL heterogeneity in this sense may, therefore, also be expected on contaminated surfaces. Roginskil (297) showed that in many cases catalytic activity may be given t o the catalyst b y very small amounts of gases acting as promotors. Heterogeneity of the kind #just indicated may, therefore, be expected with actual catalysts. 3. Experimental Methods to Study the Heterogeneous or Homogeneous Charucter of a Surface for Chemisorption

The occurrence of different crystal faces a t the surface of polycrystalline adsorbents may cause a cert,sin degree of heterogeneity. The differences, however, in heats of chemisorption on the various crystal faces will probably not be large enough to explain the general decrease of heats of adsorption with increasing 8 values. It is true that orientation may result in an appreciable diff erenee in catalytic activity or that catalytic activity is particularly shown b y selected crystal faces, but this does not necessarily point t o a difference in heat of adsorption. Beeck, Smith, and Wheeler (298) prepared nonoriented and oriented nickel films, the latter showing { 110] planes parallel to the substrate on which they were condensed. They showed that the catalytic hydrogenation of ethylene a t 0°C. proceeds five times more quickly on these oriented films than on randomly oriented films. The heats of chemisorption of

114

J. H. D E BOER

hydrogen on both types of films, however, are the same, which induced Beeck t o conclude that the difference in catalytic activity is caused by a difference in entropy of activation (299). Sachtler, Dorgelo, and van der Knaap (300) showed that oriented nickel layers, made according to the methods of Beeck, Smith, and Wheeler, are, indeed, oriented with a (110) plane parallel to the surface on which they are condensed but that the crystal faces forming the outside of the layers are not (110) planes. This could mean that the border faces of the oriented layers are randomly distributed, forming a surface just as heterogeneous as th at of nonoriented layers. If catalysis, like electron emission but unlike chemisorption, is governed more by orientation than by the actual presence of oriented bordering planes, differences in catalytic activity cannot be used to study the homogeneous or heterogeneous chemisorption character of a surface. Various studies of catalytic activities on single crystal spheres of copper (301) suggest that orientation may indeed be responsible for differences in the rates of the catalytic reactions. The reaction of hydrogen and oxygen shows the highest rate on those places of the surface of the copper sphere that are parallel to ( I l l ) directions. Those parts of the surface that are parallel to { 100) directions are seriously roughened by the reaction, though the rate of the reaction is lower there than on the ( 111) parts, which are not roughened (305). It is a s if on the parts which are parallel t o the { 100) planes both H and 0 atoms penetrate into the copper to some depth and react underneath the actual surface (see Secs. VII,6 and 7), and the quicker rate on the (111) parts (parallel t o the { 111) planes that are not actually present) prevents the atoms from penetrating. There is no direct parallelism between heats of chemisorption and catalytic activities. Another argument, used to prove the existence of an important heterogeneity of the adsorbing surface, is taken from the inhibition that catalytic reactions may suffer from the addition of quite small quantities of strongly adsorbed poisons (303). A chemisorption of carbon monoxide, covering only 1% of the surface of a copper-powder catalyst, may reduce the activity in ethylene hydrogenation at 0°C. by a factor of 9 (304).Since Wheeler (305) has shown that such drastic reductions of the reaction rates may quite reasonably be expected with porous catalysts when the poison is selectively adsorbed on the pore mouths, these arguments are not to be taken as proof of the existence of heterogeneity in chemisorption properties. The occurrence of slow chemisorptions following fast chemisorptions is in many cases explained by a heterogeneous character of the surface, The rate of these slow chemisorptioiw is governed b y a n activation en-

ADSORPTION PHENOMENA

115

ergy, and it has been observled th at the rate decreases exponentially with the amount that is adsorbed. The activation energy increases linearly with the degree of occupation 8. We shall return t o this relationship in Secs. IX,9 and 11, where we shall see that this phenomenon cannot necessarily be taken as proof of the existence of heterogeneity. Taylor and collaborators (306) have found that in many cases a n increase in temperature during such a slow adsorption causes a rapid desorption, followed by a slow readsorption. This effect has been observed for a number of adsorption3 on oxide and metal-powder surfaces, and also in chemisorption of hydrogen on tungsten films (307).This phenomenon, t o be sure, can be explained from a heterogeneous character of the surface if it is assumed that there are areas with relatively low heats of adsorption and activation energies as well as areas where higher heats of adsorption and activation energies are found. There are, however, also other possibilities to explain such phenomena, and the occurrence of this effect should, therefore, not be taken as an absolute proof of the existence of heterogeneity (Sec. IX,ll). An experimental method of investigation which may indicate in a direct way the existence or nonexistence of heterogeneity for chemisorption has been devised by ltoginskii and collaborators (308). By this method the adsorption is performed in two steps b y use of different isotopic forms. Should the surface be of a homogeneous nature, the adsorbed atoms would all be in the same condition. When, therefore, the gas is subsequently desorbed in two steps, the isotopic composition of the two portions should be the same. If, however, the surface has a heterogeneous character, the fra'ction of the gas added first during the adsorption process should be removed last from the surface during the desorption process and the desorbed fractions should have the same isotopic differences as the adsorbed portions. The method was used for the study of the chemisorption of hydrogen on nickel and on zinc oxide, and Keier and Roginskii could demonstrate the heterogeneous character of the surfaces of these adsorbents ($09). They could also demonstrate the heterogeneous character of the surface of charcoal for the chemisorption of hydrogen (310). Kummer and Emmett (31 1 ) studied the chemisorption of carbon monoxide on a n iron ammonia catalyst by the same method and found a partial mixing of the desorbed gas, as if the surface consisted of a heterogeneous complex of homogeneous parts. Similar results were obtained by Eischens (312). The chemisorption of nitrogen on an iron ammonia catalyst was also studied by Emmett and Kurrtmer (313),who found that the surface behaved as if it were of a homogeneous character.

116

J. H. D E BOER

Recently Schuit used the method for studying the chemisorption of hydrogen deuterium mixtures on thoroughly reduced nickel, supported by silica (314). He concluded that such a nickel surface behaves as a completely homogeneous surface. The various investigations do not, therefore, lead t o a definite conclusion. It is quite possible that some of the surfaces which were investigated were homogeneous for the type of chemisorption that was studied. A slight degree of heterogeneity, as may be expected for a polycrystalline material, may still lead to a homogenization of the isotopic mixture which is adsorbed, provided the activation energy for surface migration is not too high. It is, therefore, also quite understandable that experiments with one sorbate (nitrogen) may lead to the conclusion that the surface shows a homogeneous character, while the use of another sorbate (carbon monoxide) seems to prove the existence of a certain degree of heterogeneity, as we have seen from the results of Emmett and Kummer. The nature of the surface depends, undoubtedly, on the way the adsorbent has been prepared. There are various reasons to agree with the conclusion drawn by Kwan (315) that the surface of a number of metallic catalysts is of a homogeneous nature for chemisorption as long as these catalysts are prepared by a very careful reduction and are kept free from any poisoning materials. We shall see that it is very difficult to fulfill these conditions.

4. The Heats of Adsorption of Cesium Atoms on Tungsten at High Degrees of Occupation

I n Sec. V,9 we spoke about the heat of adsorption of a cesium atom chemisorbed in the form of an ion on a tungsten surface. The value (Q& = 68.8 kcal./mole holds for the adsorption on a bare tungsten surface. It has been known for many years that the heat of adsorption decreases with an increase in the amount of cesium atoms adsorbed (316). I n Fig. 29 the full-line curve shows the decrease according to the figures of Taylor and Langmuir (317). These authors could represent Q O (the heat of adsorption as a function of e) by an empirical equation: Qs =

1

64 kcnl./mole + 0.7148

This equation holds between e = 0.06 and e = 0.60; a t 6 values lower than 0.06 the heat of chemisorption increases more strongly with decreasing 0 (full-line curve) than was calculated with the equation (dotted curve). This deviation from the empirical equation was explained by Taylor and Langmuir by the presence of active areas on the tungsten

117

ADSORPTION PHENOMENA4

surface. If 0.5% of the surface consists of such active areas, the adsorption by the rest of the surface can be represented by the equation. Apart from this heterogeneity of 0.5%, they consider the surface to be homogeneous and Langmuir (328) explains the “normal” fall of the heat of adsorption from the mutual repulsion of the dipoles which are formed by the cesium ions and the negative charges in the metal opposite these ions. Applying the two-dimensional van der Waals’ equation, one can express the spreading force of the adsorbed film in terms of the dipole moment p and the degree of occupation 8.

40. 6

FIG.29. Heat of adsorption of cesium on tungsten (317).

By use of Gibbs’ adsorption equation, the spreading force can also he expressed as a function of {;herate of evaporation of the atoms. From the figures for the spreading force, the dipole moments pi can be calculated. In this way the following values were found by Langmuir: a t 0 = 0, pi at 8 = 0.50, pi a t 8 = 0.90, pi

= = =

16.16 X lo-‘* e.s.u. 8.28 X 10-l8 e.s.u. 6.06 X 10-l8 e.s.u.

The marked fall in dipole moment with increasing 8 values indicates a strong mutual depolarization of the dipoles. As the figures for the dipole moments, indicated above, are calculated on the assumption that the decrease of the heat of adsorption is caused by the mutual repulsion of the dipoles, they may not be used,

118

J. H. DE BOER

reversely, for calculating the magnitude of this decrease, as is sometimes done in literature. The presence of the dipoles pointing with their positive poles away from the metal facilitates the emission of electrons from the metal. The work function of the metal is lowered by an amount AV

=

4aeoopi

where uo is the maximum number of cesium atoms that can be adsorbed per square centimeter, 8 is the degree of occupation, and pi the dipole moment. The electrons are influenced by the forces of the dipole layer only after they have left the surface; hence Langmuir suggested that when they have covered half the distance between the positive and negative poles of the dipoles, only half the foregoing amount is responsible for the actual lowering of the work function. However, if we denote the effective part of the dipole protruding from the metal by p instead of p i , we can use A p = 4Tfbop (54)

As was pointed out by the author ( S 1 9 ) ,the lowering of the work function has a pronounced effect on the heat of adsorption. I n See. V,9 we learned that the heat of adsorption of a cesium atom, transformed into a cesium ion by the chemisorption process, is given by (&a);

= Qi

- (evi - W )

(55)

where ( Q a ) i symbolizes the heat of adsorption of the atom in the form of an ion, Qi is the heat of adsorption of a cesium ion, eVi is the ionization energy of the cesium atom, and p is the work function of the metal (see also Sec. V,8a). As soon as some cesium is adsorbed on the surface, the work function of the metal is lowered, which means that less energy is gained during the act of adsorption of the next atoms. Introducing Eq. (54) into (55) and denoting the heat of adsorption by the symbol Qo, t o indicate that it depends on 8, we obtain or, introducing (54),

Qe = Qi - (evi =

Q; -

-

Q(O

+ CAP)

+

47re~,p)

(56)

(57)

If Qi remained constant and if p did not change during the progress of further adsorption, there would be a linear decrease of the heat of adsorption with 8. The heat of adsorption of the cesium ion, Q;, however, does not remain constant, but increases during further adsorption. This effect is

119

ADSORPTION P H E N O M E N A

caused by the fact th at the dipole layer does not give a homogeneously distributed electric double layer, but an electric double layer where the dipoles are situated a t discrete spots (3dO19d1).An electric double layer with homogeneously distributed charges does not exert any electric force

+

-

2"

FIG.30. Change of potential when an electron passes through a double layer with homogeneously distributed electric charge.

outside the planes of its charges; there is only a potential gradient between those planes (Fig. 30). Jf, however, the dipoles are situated a t discrete spots, such as indicated in Fig. 31, the double layer also exerts forces outside the planes of the double layer. Positive charges are attracted from the side of the positive plane, negative E charges from the side of the negative plane. I n Fig. 32 t he change of potential is given for a n electron passing through such B discrete dipole layer; the distance between the dipoles is 26 in this case, if 6 is the length of the dipole. The solid line in the figure holds for the case when the electron passes through the planes between the charges, the charges being arranged in a regular square pattern. If the electron passes FIG.31. Two planes of along the line DE in Fig. 31, it meets one of the discrete charges; distance charges and is repelled, as is indicated by the 6; distance of the charges dotted lines in Fig. 32. in the planes d = 26. Line ABC passes through The positive charge of a cesium ion will the planes between the therefore be attracted owing to the discrete charges. Line DE passes distribution of dipoles brought about by the through the planes a t cesium ions already present. Consequently the points occupied by heat of adsorption of the cesium ion is increwsed. charges. This was already knownfrom the work of Becker (322) and could be explained by the author (323) by means of these considerations. The increase of the heat of adsorption of the cesium ion, as found by Becker, giving direct proof for the discreteness of the dipole distribution, means a less strong decrease of the heat of adsorption of the cesium atom. When we raise level D of Fig. 6 (Sec. V,8,a) by an amount

I;

120

J. H. DE BOER

4 . r r & ~ ~ level p, E should be consequently raised by a smaller amount. The difference between levels E and A gives the heat of adsorption of the atom. The theory developed in 1935 leads to the view that the effective decrease of the heat of adsorption of the atom depends more or less on the ratio between the length of the dipoles (6) and their distance from

E'\

FIG.32. Charge of potential when an electron passes through a dipole layer along lines ABC or DE of Fig. 31.

each other ( d ) . We may suggest that the effective decrease is approximated by 6 6 AQ,tr = - eAp -- 2e - dUop

d

2d

This leads t o

QB=

Qo

- AQeir

= Qo

-

where Qo is the heat of adsorption a t 0 = 0 [here As we may, again as an approximation, write and

(59) see Eq. ( 5 5 ) ] .

$$€6 = p

According t o this equation the decrease of the heat of adsorption should be proportional t o 0 3 4 ; hence &o = Qo -

a@."$

(611 The constant a can be calculated by means of the known values for uo = 3.56 X atoms/cm and p = 6.8 X 10-l8 e.s.u. (324) and amounts t o a = 56.4 kcal./mole

ADElORPTION P H E N O M E N A

121

Curve B in Fig. 33 is calculated by Eq. (61), by use of this value for a and Q0 = 68.8 kcal./mole, which is the experimental value for e = 0. Curve B should be compared with curve A , which gives the experimental data (this curve is the same as the solid curve of Fig. 29). We see th a t the two curves have complthely diff eredt characteristics.

FIG.33. Curve A : heat of adhorption of cesium on tungsten, curve B: theoretical curve according to Eq. (60) with H constant dipole moment, curve C: theoretical curve according to Eq. (60) with a variable dipole moment, b u t a constant polarizability, CY = 10 X curve D is curve C fitted on A at 8 = 0.3.

In our calculations the dipole moment p has been considered constant. This is not correct: i t decreases as a result of mutual depolarization of the dipoles and this may seriously affect the curves. The depolarization can be accounted for by an equation given by Roberts (325): p e == p0/[i

+ gLYe(~~i

(62)

where p e is the dipole moment a t the degree of occupation 8, po the dipole moment given by the first cesium ion on a bare tungsten surface, and a the “polarizability.” e and 130 have the same meaning as in the other equations. It is not known which value of LY should be used for the polarizability in a chemisorption case like this, neither whether it would be constant and independent of 8. Using a constant value a = 10 X and using the known value for p = 6.8 X loF2*at e = 0.07 (SW/i), we

122

J. H. DE BOER

have calculated pe as a function of 0. The figures, obtained in this way, have been substituted in Eq. (SO), which is now written in the following form: Qs = Qo - bpifP’ (63) where b =4~(~0)9$ b amounts t o b = 1.22 when p e is given in Debye units. The resulting curve for Qe is given as curve C in Fig. 33. It may be observed that the slope of this curve for higher 0 values is approximately

0.1

0.2

0.3

0.4

0.5

C

- 8

FIG.34. Curve il: heat of adsorption of cesium on tungsten, curve B : theoretical curve according to Eq. (611, curves C, 1),and E : calculated with Eq. (63) and (L = 10 X lopz4,o = 30 X U P 4 , and LY = 40 X 10-24 respectively. All curves are fitted on curve A at 0 = 0.2 and 0.6.

the same as the slope of curve A . If curve C is not started a t Qo = 68.8 kcal./mole but drawn in such a way that it fits on curve A at 0 = 0.3, we obtain curve D. This curve covers curve A well for e values between 0 = 0.2 and e = 0.6. Apart from a slight bend a t lower 6 values, it is practically a straight line. The first part of curve A cannot be represented by these curves. We may try whether Eq. (61) or (63) will give better results when we esti-

ADSORPTION PHENOMENA

123

mate the constants a orb, and also Qo, by fitting the curves at two points of the experimental one. We have chosen the points a t 6 = 0.2 and 0 = 0.6, and some of the results may be seen in Fig. 34, where curve A is the experimental curve again. Curve B is calculated by means of Eq. (61) and curves C, D,and E are calculated by means of Eq. (63). The values have been calculated assuming a = 10 X 30 X respectively, by means of a depolarization equation th a t and 40 X differs slightly from Eq. (62). We have used pe =

+

~ / [ l g ~ ( e d ~ i

(64)

which holds for mobile dipoles; whereas Eq. (62) holds for located dipoles. Curve C approaches curve -4 very well for B values higher than B = 0.2; it turns out t o be practically identical with curve D of Fig. 33. It seems not possible to devise an equation for Q Owhich covers curve A completely, a t least not ;along the lines of thought developed above. We may, probably, conclude th at the tungsten filaments used in these experiments, being of a polycrystalline nature, show different crystallographic faces or directions, which gives rise to slight differences of Q over the surface. The surface seeins to be slightly heterogeneous. 6. A Change in the Nature of the Bonds at High Degrees of Occupation

When, at increasing degree of occupation, level D of Fig. 6 (Sec. V,8,a) is raised by the growing dipole layer, it will, at a certain e value, reach level A . When this happens, condition (30) of Sec. V,8 will not be fulfilled any more. From this degree of occupation onward the adsorbed cesium, though still adsorbed in the ionic form, will not desorb in the ionic, but in the atomic form. It is at 0 = 0.07 th a t this happens (326). At a still higher degree of occupation level D of Fig. 6 (Sec. V,8,a) will be raised t o such an extent that it reaches the same height as the minimum B of the curve giving the adsorption of cesium in atomic form. This happens a t e = 0.134 (327), and a t higher 0 values no ions can be formed on the surface. 131 our earlier considerations (528) we suggested t ha t atoms would be adsorbed next t o the ions and be polarized by them. Though we stated there th at a sharp distinction between ions and atoms could not be made, we had better consider this old picture obsolete and replace it by thte picture of the adsorption of atoms, which are still strongly polarized by the metal surface. Even physically adsorbed atoms are polarized in the same direction when adsorbed on metal surfaces (Sees. V,7 and V1,l). The nature of the adsorption, therefore, changes at higher degrees of occupation. There is experimental evidence of this change. Mayer (329) bombarded with potassium ions filaments of platinum, copper, and aluminum,

124

J. H. DE BOER

on which sodium was adsorbed. He found a n emission of sodium light (sodium D line) during the induced evaporation of sodium. Apparently the adsorbed sodium ions are desorbed by the bombarding potassium ions and while evaporating they are neutralized to atoms via several excited states t hat cause light emission. Light emission was not observed when smaII amounts of sodium were adsorbed ; it became noticeable with increasing amounts of adsorbed sodium passed through a maximum and disappeared again when a larger amount of sodium was present. A t low 8 values the sodium ions are liberated as such, a t increasing 0 values the liberated ions are neutralized, and a t still higher 0 values sodium atoms are adsorbed which do not need to be neutralized when they are liberated. Other experimental evidence may be obtained from photoelectric measurements. The normal (nonselective) photoelectric emission of a tungsten filament on which sodium is adsorbed shows a marked increase with raising temperature at relatively low 0 values. The photo effect, however, decreases with rising temperature when the degree of covering is higher. Both effects are reversible (350). Apparently a t relatively low e values, when the adsorbed sodium is in the ionic form, a n increase in temperature means a slight increase of the mean distance of the ion from the surface, hence a slightly increased dipole moment and a slightly decreased work function. When the adsorbed sodium is in the atomic state the dipoles, which are now formed by polarization of the atoms by the field of the metal, decrease when their mean distance from the metal increases a t higher temperatures. The main differences between our present views and those held in 1934-1937 may be briefly stated. Contrary t o our older views we have t o accept a certain degree of heterogeneity of the tungsten surface. We cannot understand the form of curve A of Fig. 33 without accepting the presence of areas where the adsorbed ions are more strongly bound than on the rest of the surface. This heterogeneity is certainly not caused by impurities or foreign atoms; the presence of different crystallographic faces may be responsible. Second, we do not maintain the picture of atoms adsorbed next t o ions a t higher degrees of coverage. At low e values all atoms are adsorbed in the ionic state; at higher degrees of coverage the type of binding changes and from a certain @ value onward atoms are adsorbed as atoms. They are polarized by the field of the metal. At increasing coverage their dipole moments decrease by mutual polarization and a minimum is found for the work function when the decreasing dipole moment per atom is not compensated any more by the increasing amount of dipoles per square centimeter. It is likely th a t the polarizability which is effective in this depolarization is different for the various states of binding; hence it will not be a constant, independent of

ADSNORPTION PHENOMENA

125

coverage. This means that Eqs. (60) and consequently (61) cannot be used in a more quantitative way. 6. T h e Decrease of the Haat of Chemisorption with Increasing Degree of Coverage for Other Adsorptives

Most chemisorbed atoms form dipoles on the surface of their adsorbents. Either the positive or the negative poles of these dipoles may point away from the metal (Sec. V,8b). I n both cases the dipoles influence the work function of the metal, increasing it when the dipoles point with their negative poles away from the metal and decreasing it when the dipoles point in the other direction. As a negative dipole (negative pole pointing away from the surface) is formed by shifting a n electron from the metal t o the adsorbed atom, work is done against the work function. By the increase of the work: function more work will be required for the formations of new dipoles when the degree of coverage increases. T h e heat of chemisorption will therefore decrease. I n case positive dipoles are formed, the electron affinity of the metal facilitates the effect. As the electron affinity decreases with an increase in the amount of adsorbed atoms, the result is again t,hat the heat of chemisorption will decrease with increasing degree of coverage (331). The cause of the decrease of the heat of chemisorption is, therefore, the same a s we discussed in the preceding sections. Boudart (332) elaborated the idea recently, and he proved that it can explain the order of magnitude of the experimental figures for the decrease. I n his equations Boudart uses the conception of an electric double layer with homogeneously distributed charges. As, however, the decrease of the heat of chemisorption AQ is numerically smaller than the change in work function Acp, Boudart introduces the relation AQ =

(65)

and tries t o justify this by assuming that the electron is bound halfway between the planes of the dipole layer. This assumption is a strange one, because it is just these electrons th at have to contribute to the dipole layer. Mignolet (333), therefore, criticizes this assumption and shows that i t is superfluous if more correct data than Boudart's are used. I n his reply t o this criticism EIoudart (334) once more points out that his conception may be taken a3 only an approximate one, just t o give the order of magnitude. As we have seen in Sec. lX,4 the dipole layer may not be treated as a double layer with a continuous distribution of charge. Discrete charge distributions have t o be assumed. Gomer (335)criticizes Boudart on this point. H e evaluates potential curves for a discrete distribution of dipole

126

J. H. DE BOER

charges and, rightly, remarks that a t low 6 values the effect will be smaller than that calculated by Boudart’s method. It is, unfortunately, very difficult to compare the experimental data on the decrease of the heats of adsorption with the observed values for the changes in q o o . It is possible only in the case of the adsorption of alkali metals on tungsten filaments, where sufficient reliable data are available. Contact potentials, measured when the gases are adsorbed on filaments, are less reliable. Most investigations concerning contact potentials on films have yielded values of surface potentials that are known for nearly complete films, but not for adsorbed layers with low e values. There are, moreover, rather serious deviations among the experimental values published by different authors (336).

7. Advantages and Disadvantages of Metal Surfaces Prepared by Diflerent Methods Comparing the various surfaces that have been used for measurements of heats of adsorption and contact potentials, one may say t h a t none of them seems to be ideal for the purpose. It is true that filaments, especially those of tungsten, may be obtained in a pure state, free from contaminations. However, when no special precautions are taken, they do not stay in this condition. It is remarkable how quickly impurities are taken up from the residual gas in the highest vacuum that can be applied. Unless chemical binding agents are used (“getters”), the filaments will be contaminated during the cooling down from the flashing temperature which is used to clean them. A recent publication of Thomas and Schofield (337) on the accommodation coefficients of helium has revealed that even Roberts’s filaments were not clean, though i t was thought that they had unquestionably clean metal surfaces (SSS). It is very fortunate that in the experiments on the adsorption of atoms of alkali and alkaline earth metals on filaments these metals themselves produce and maintain a high vacuum. The only drawback of these filaments is that they are polycrystalline and, therefore, show a slight heterogeneity. Films of metals, produced by evaporation and condensation, will also take up impurities from the “vacuum.” During the preparation of the films the vacuum produced by the evaporation is very high. Afterward traces of gas, liberated from various parts of the apparatus, form a source of contamination. Owing to their very large surface areas, however, the films can be maintained in a clean state for a far longer time than filaments. Many films seem to have the additional advantage that they consist mainly of one crystallographic plane. The method of preparation provides a unique process of crystallization (3.39); it may be that the

ADSORPTION PHENOMENA

127

crystallographic face that develops is not the same as the faces found when the metal is prepared in other ways. There is, however, a disadvantage in the use of films. Because of their extremely large surface area per unit of weight of the metal (340),their surface energy is high. The average thickness of the primary laminae t h a t build up the total film is very small, and it is known th at the films do not behave electrically as normal metals (341). Many of these films show a somewhat expanded lattice of 1-2% (342). It ii3 only after thorough sintering th a t they approach a more normal metallic state. Mignolet (343) observed th a t the work function of films increases during sintering and approaches the value of the normal metal. During such a sintering process impurities may well be taken up. The development of a very large surface area, hence a n extremely open structure, deplends also on the rate of evaporation and condensation, Tungsten films will show a slightly more normal behavior than nonsintered nickel films. Metals produced by a thorough reduction of oxides may be obtained in such a state that their surface is clean. The possibility is not excluded, however, that these metals contain an appreciable amount of dissolved impurities (e.g., atoms of tlhe reducing gas, hydrogen) which may interfere with the chemisorption processes carried out afterward. Boudart (344) explains the peculiar behavior of the chemisorption of hydrogen on tungsten powder, as investigated by Frankenburg, from such an interference by dissolved atoms. I n Sec. IX,l we saw that the initial heat of chemisorption of hydrogen on this powder has the same value as obtained by the chemisorpticm on tungsten films and tungsten filaments (Fig. 27), but th at the decrease of the heat of chemisorption with increasing 8 values proceeds very quickly. It does not seem impro'bable th at thoroughly reduced metals, supported on a carrjer, give the best approach to pure metallic surfaces. The small, crystalline, metal particles cannot sinter together; they therefore cannot include impurities, and, moreover, they are formed under circumstances so nearly ideal that, being so small, they may consist of single crystals, probably showing one main crystallographic face. It is remarkable that with nickel on silica prepared in this way, Schuit and de Boer (345) obtained curve E of our Fig. 28 (Sec. IX ,l), which may be compared with curves C or D of Fig. 33. It is not improbable that the decrease of the heat of chemisorption in this case may be fully understood from the decrease in the work function together with the mutual depolarization of the dipoles. Schuit (346) found (Sec. IX,3) that this surface behaved as a homogeneous one. It may be remarked that curve A in Fig. 28, giving the heat of chemisorption of hydrogen on a nickel film, also resembles curves C and D of Fig. 33, but th at the absolute values are higher than

128

J. H. DE BOER

those of curve E . Curve A was obtained by Beeck with a nickel film of large surface area (see above). After sintering of this film the absolute values of the heats of chemisorption were decreased (347) to practically the same level as those of Schuit and de Boer. The higher heats of chemisorption of the negatively charged hydrogen atoms found with the unsintered film are in accordance with the lower work function of such a film. 8. Other Explanations for the Decrease of the Heat of Chemisorption with Increasing Coverage

Suggestions have been made to explain the decrease of the heat of adsorption with increasing amount of chemisorbed atoms in terms of kinetic energy of the electrons used or set free by the bonding of the atoms. Eley (348) remarked that when a chemical bond is formed on the surface, which involves an electron t o enter the metal, this electron will occupg the lowest available energy level. The following electron that enters the metal when the following atom is chemisorbed has to occupy the next higher energy level, thus having a somewhat higher kinetic energy in the electron gas in the metal. Conversely, when on the surface a chemical bond is formed that requires an electron to take part, this electron will come from the highest energy level in the metal; the next>comes from the next lower energy level, etc. In either case the differential heat of chemisorption will decrease with increasing number of atoms adsorbed. Similar remarks were made by Schwab (349) and by Mignolet (560). As the conductibility band belongs t o the collective conduction electrons of the whole metal, it seems rather unlikely that electrons entering its permitted levels or coming from its occupied levels would bring about such drastic effects when they are produced by or used in the production of chemical bonds on the surface of the metal. It is for this reason, th at Temkin (351) introduced the idea of a surface electron gas. He suggested the presence of a two-dimensional electron gas a t the surface of the metal, which apparently behaves in complete independence of the normal three-dimensional electron gas. Accepting the same exclusion principles and statistic distribution for this separate twodimensional electron gas as for the normal three-dimensional one, Temkin derives the following expression for AQ:

where h is Planck's constant and m is the mass of an electron, u is, as usual, the number of adsorbed atoms per square centimeter, ua is, again,

ADSORPTION PHENOMENA

129

the number of chemisorbed atoms in a completely covered unimolecular layer, and e = u/uo. This expression would give a linear decrease with 0. If uo = 3.56 X l O I 4 atoms/cm.2 were used in the case of the adsorption of cesium on tungsten (as was done before; see Sec. IX,4), AQ would amount t o 19.6 kcal./mole between B = 0 and 0 = 1. This value, though of the right order of magnitude, is too low; it would predict a decrease of 7.85 kcal./mole between 0 == 0.2 and 0 = 0.6, although the actual decrease (Fig. 29) is 11.7 kcal./mole. When applied to the chemisorption of hydrogen on nickel supported on silica, where (352) (Secs. IX,1 and 7) a t 0°C. AQ = 14.48 Eq. (66) demands th at uo should be 2.6 X l O I 4 atoms/cm.2 t o account for this figure. As 6 0 for hydrogen atoms on nickel will certainly be far higher than this, we reach the conclusion that in this case Eq. (66) gives too high a figure. Both examples show one of the imperfections of Eq. (66). Temkin, realizing that his AQ, generally speaking, would be too large, suggests that the mass of the electron, m, has to be replaced by a n I( effective mass.” Temkin’s theory, on account of its simplifications, does not contain anything of the specific character of the metals but the various values for uo. This is certainly too strong a simplification. A more serious remark, however, was made by Boudart (353), when he discussed the constancy of the heat of solution of gases in metals. From the fact that the heat of solution of hydrogen in the interior of metals does not fall with increasing concentration, Boudart drew the conclusion t ha t such an effect may not be expected at the surface either. The heat of solution of hydrogen in /3 titanium (354) has the constant value of 27.83 kcal./mole for concentrations lower than 10 atomic % and increases slowly t o only 28.3 kcal./mole for 30 atomic %. This increase is caused by the dilatation of the lattice. I n this instance of solution there will also be an exchange of electrons with the collective conduction electrons of the metal. If then, in the case of solution, this does not involve a change in heat, it may also bme expected that the exchange of electrons between chemisorbed atoms and the surface layer of electrons does not involve such a change. Temkin, on the other hand, refers to a study by Federova and Frumkin ( 3 5 1 ~who ) ~ found that the heat of solution of hydrogen in the P-phase of palladium depends largely on the concentration. There are some indications in literature, however, that may endorse the non-dependence. I n Sec. VII,6 we learned that the chemisorption of oxygen on iron was not restricted to the surface only; iron atoms (ions) moving on top of the chemisorbed oxygen atoms (ions) bring about a n

130

3. H. DE BOER

incorporation of the chemisorbed oxygen ions into the surface layers of the metal. I n a n extensive study of heat of chemisorption on iron films, Bagg and Tompkins (355) observed that the heat of sorption of oxygen by iron films a t room temperature was 71 kcal./mole, independent of coverage, They also observed that oxygen penetrated into the lattice during their measurements. A constancy of the heat of adsorption at increasing coverage may, t o be sure, also be found when the molecules or atoms d o not move freely over the surface before entering into chemisorption. When they stay at the first point of the surface that they reach, the adsorbed layer will be built up gradually from those parts of the microporous system (metal films, like films of inorganic salts, are microporous systems (356)) th a t

',I

I

L

0.2

0.4

0.6

0.11

1.0

- 8

FIG. 35. A : heat of chemisorption of hydrogen on an iron film a t 23°C.; B : the same at -180°C. (857).

are in direct contact with the gas and proceed gradually t o the interior. In this case the average value for the heat of chemisorption is found, independent of the amount adsorbed. It is only when the molecules move freely over the whole surface before being chemisorbed th a t the chemisorbed atoms will be distributed a t random or, by their mutual repulsion, take up positions as far apart from each other as possible, and it is only in these cases that the usual curves, showing a high initial heat of adsorption falling off with illcreased 0 values, are found. This effect is clearly demonstrated by the curves found by Beeck (357) for the heats of chemisorption of hydrogen on iron films a t - 180°C., where hydrogen does not move freely prior to chemisorption or after chemisorption, and a t 23"C., where it does move over the surface (Fig. 35). It is, therefore, understandable th at Bagg and Tompkins explain the constancy of'the heat of sorption th at they found for oxygen on iron films a t room temperature in terms of immobility. I n view of what we learned in Sec. VII,G, however, a solution of oxygen in the surface layers is more probable.

ADSORPTION PHENOMENA

131

Maxted and Hassid (35CI), and later Kwan (359),found the heat of sorption of hydrogen on platinum t o be independent of the amount th a t was taken up. It is quite probable that, a t the temperatures they used in their investigations, hydrogen penetrates the surface layers of the metal. As the atoms that penetrate into the surface layers of the metal do not produce a regular dipole layer on the surface, they do not change the work function in a regular way either. The constancy of the heats of solution and of the heats of sorption in those cases where, apparently, the atoms penetrate the surface layers of the metal does not indicate that the kinetic energy of the electrons in the metal plays the role th at T'emkin needs for his theory. We cannot, of course, entirely exclude the possibility that in all these cases, including the solution of hydrogen in 8-titanium, the constancy of the heats of sorption is caused by immobility of the sorbed atoms. We are, meanwhile, inclined t o think th at Ternkin's picture does not give the solution here. If i t did, the obvious conclusion would be that neither the changes in work function nor in contact potential would be caused by alleged dipole layers, but by the changes in the occupation of the energy levels of the surface electron gas. The consequences of the adoption of this idea would be far reaching. Alternatively, we might assume th a t part of the change in work function and in contact potential-say half the effectwould be due t o the change in the kinetic energy of the electrons in the sense described by Temkin, and the other part would be caused by surface dipoles.

9. Changes in Activation Energy with Increasing Degree of Occupation

It is not only the heat of chemisorption th a t changes during progressing adsorption; the attivation energy, if present, also gradually alters its value. I n general, the activation energy increases with increasing degree of occupation. Figure 36 shows the increase in activation energy of the chemisorption of nitrogen on iron in the temperature range between 200" and 250"C.,as measured by Zwietering and Roukens (360). As we saw in Sec. V,9, the chemisorption of nitrogen on iron is one of the rare cases where the chemisorption of a gas on a pure-metal surface involves a n activation energy. I n most other cases of chemisorption on pure metal surfaces the rate of chemisorption, even a t low temperatures, is so high that, apparently, there is no, or only a very small, activation energy. Even if there is no activation energy when the first atoms are being chemisorbed, there may be cine when the degree of occupation becomes higher. A study of potential curves (Fig. 37) reveals th a t the decrease in the heat of adsorptian with mereasing 0 i s likely t o be accompanied by

132

J. H. DE BOER

the deveIopment of an activation energy from a certain 6 value onward. E will then increase with decreasing Q. I n Fig. 37 curve 4 shows the occurrence of a small activation energy, smaller than the heat of chemisorption (Qk), and the activation energy of curve 5 (EL)is larger than the heat of adsorption ( Q s ) .

.-/

k-l/rdole

FIG,36. Activation energy of the chemisorption of nitrogen on a n iron catalyst (360).

It has recently been realized that in many cases a quickly proceeding chemisorption may be followed by a slower uptake of the same gas. When the pressure is raised, more gas is taken up and the additional uptake de-

.o

I

-

2

4

6

e x

Oistance from s u r f a c e

FIG.37. A gradual decrease in the heat of chemisorption Q may give rise t o the development of an activation energy E, the latter increasing with decreasing Q.

pends on the pressure. I n many cases this phenomenon has been studied with the aid of metal films obtained by evaporation and condensation of metal vapors. As these films have a microporous character and as even the rates of physical adsorption of gases on microporous systems, like charcoal, may show an activation energy (361) (caused by the activation

ADSORPTION PHENOMENA

133

energy of surface migration) , the results obtained with films may perhaps not be taken as conclusive evidence of the occurrence of an activation energy in the later stages of' adsorption. We shall return to this question in Sec. IX,11. A careful study of the ad,sorption of various gases on carefully cleaned mercury surfaces has led Kemball (362) to the conclusion that such gases as carbontetrachloride, hexachlorethane, and chloroform are chemisorbed

'O1

t

a,

0.1

0.3

- 8

FIG.38. Heats of chemisorption of nitrogen on a n iron film as a function of 0 (363).

on the surface of mercury. I n the initial stages there is no activation energy, but after roughly half the surface is covered (e slightly more than 0.5) an activation energy comes to govern the rate. This activation energy for CC14 on mercury is practically zero for e values up to 0 = 0.50 and assumes the values of 4.1 kcal./mole for 0 = 0.62, 9.6 kcal./mole for 0 = 0.69, and 19.2 kcal./mole for e = 0.76. I n all cases where an activation energy is already present at 0 = 0 (Nz on iron, H, on contaminated metal surfaces; see Sec. V,9) it increases with increasing 3 values. The increase of the activation energy is slower than the decrease of the heat of chemisorption. An examination of Fig. 37 shows that this should be so. The maxima in the potential curves are shifted to the left and the minima of the curves are either a t the same distance from the eurface-as we have assumed in our figure-

134

J. H . DE BOER

or are shifted t o the right when the bond with the surface becomes less strong. It is, therefore, t o be expected th at

< AQ

AE

(67)

Figure 38 gives the heats of chemisorption of nitrogen on iron films, recently published by Bagg and Tompkins (363),and we may compare the decrease of this curve with the increase shown by the curve of Fig. 36. 10. Equations for Chemisorption Isotherms

The activation energy for the chemisorption of nitrogen on a n iron catalyst, as measured by Zwietering and Roukens (360) (hence the curve of Fig. 36 just mentioned), can be represented by the expression

E

=

9.2

+ 72.78 kcal./mole

The rate of chemisorption a t a pressure of 20 cm. mercury could be represented by

'' -

dt-

446.5 X

e107.28/R

x e-(9200+7Z,700S)/RT

The entropy term partly counteracts the term containing the activation energy. Restricting ourselves to one temperature, we can write the equation as dB _ -- 446.5e at

0200 72,700 1072 -_ RT x e ( - X F + * ) e

= kle-keS

(68)

The constant kz in this equation contains that part of the equation for the activation energy that depends on 8, as well as the &dependent term of the entropy factor. . Equation (68) has the same form as a n empirical expression found by Zeldowitsch (364) in 1934; viz., vads

=

k,,pe-g8

(69)

where Veda is the rate of adsorption (chemisorption accompanied b y a n activation energy), p is the pressure, and k , and g are constants. This equation is, of course, valid only at constant temperature. Prior to the formulation of Eq. (69) it had been found by Langmuir (385) that a n empirical equation vdes =

kdehe

(70)

where k d and h are constants, offers a good expression for the rate of desorption (vdeJ of alkali atoms chemisorbed on metals. Both expressions (69) and (70) give a good representation of the

ADSORPTION PHENOMENA

135

rates of adsorption and desorption in many cases of chemisorption. At equilibrium, Usds

hence

kape-g8

=

vdss

(71)

= kdehe

Taking logarithms and rearranging, we obtain S(g

Introducing we obtain

g

k, + h) = In --p kd

+h =f

and

k,/kd

= Oo

1

6 = -In uop

f

This empirical equation of the adsorption isotherm, giving the relationship between e and the premure, excellently represents many characteristics of chemisorption. Equation (72) is introduced by Frumkin and Slygin (366),who derived it from their electrochemical investigations on hydrogen electrodes. The equation has played a n extensive role in the successful theory of ammonia catalysis of Temkin (367) and it has in literature been termed the Temkin equation (368), although Temkin himself and other Russian investigators call it the logarithmic adsorption isotherm. Equation (72) can, of course, represent e values as a function of the pressure p (or the concentration c) provided th a t these values are not too close t o zero or too close to unity. I n the range of medium values of 0, it adequately represents many cases of chemisorption (369). This isotherm equation will always fit the experimental data when the heat of chemisorption shows an approximately linear decrease with increasing 0 values; i t is not necessary that an activation energy be present. This may be seen from Eqs. (71) and (72); even when h = 0, Eq. (72) results. The validity of Eq. (72:) does, of course, reveal nothing about the cause of the decrease of the heat of chemisorption. Any time th a t the heat of chemisorption may be represented by

Q~ =

- ce

(73)

the logarithmic adsorption equation

e

RT

= - In C

(uop)

(74)

136

J. H. DE BOER

which is identical with Eq. (72), can be derived, whether it be with the aid of the conception of a heterogeneous or of a homogeneous surface. Any time that the heat of chemisorption does not decrease linearly with increasing 8 value, but may be represented by

Qe

=

const. - LY log

e

(75)

an isotherm equation of the form 0 = const. pRTIa

(76)

can be derived. This latter form is the well-known empirical exponential equation, generally called the Freundlich isotherm. We shall not here give the various derivations of these equations but refer to the publications of RoginskiI (370), Temkin ( 3 7 l ) , Brunauer, Love, and Keenan (37W), Zeldowitsch (373), Halsey and Taylor (374), Trapnell (376), and Bokhoven et al. (376). Practically all experimentally obtained data on chemisorption isotherms may, in fact, be expressed in either Eq. (74) or Eq. (76). 11. Some E$ects in Chemisorption Phenomena that are Connected

with Activation Energies I n Sec. IX,9 we discussed the phenomenon that the quick chemisorption of many gases on films of metals is often followed by a slow uptake of the same gas. With nickel films Beeck, Ritchie, and Wheeler (377) found that the amount of gas taken up in this slow sorption process is independent of the degree of sintering of the film. The surface areas of the films that are freely available to the gas-also t o a physically adsorbed gas such as krypton-markedly decrease on sintering. The amount of hydrogen taken up in the fast chemisorption process shows the same decrease. The amount of hydrogen taken up in slow chemisorption, however, is not connected with the surface area but with the weight or the volume of the films. Similar results with tungsten films were obtained by Trapnell (378). I n a more recent investigation Porter and Tompkins (379) found that, on sintering, the amount of hydrogen sorbed as a result of the slow uptake by iron films decreases roughly in proportion to the decrease observed on the fast chemisorption process and hence t o the freely available surface area. They could also prove that the slow process follows the fast chemisorption process continuously. It is, therefore, not a solution in the lattice of the metal films, as was originally suggested by Beeck. Such a dissolution process, moreover, is an endothermic process. Porter and Tompkins (379) suggest that the phenomenon is caused by surface heterogeneity and that fast chemisorption takes place, with-

ADSORPTION PHENOMENA

137

out the occurrence of an activation energy, on the most active parts of the surface and the slow uptake, which also is a chemisorption phenomenon, is accompanied by an activation energy. As we have seen in Sec. IX,9, it is not necessary t o introduce active and nonactive parts. I n the case of iron films, as investigated by Porter and Tompkins, both phenoinena apparently occur on the surface which is freely available. Fast chemisorption may quite normally be followed by SIOWchemisorption associated with a n activation energy when th e potential curves follow the picture laid down in Fig. 37. The activation energy is a normal consequence of the decrease in the heat of chemisorption. With the nickel films of Beeck, Ritchie, and Wheeler and the tungsten films of Trapnell, however, the situation is more complicated. Films of metals, like films of inorganic salts, offer surface areas th a t are proportional t o the weight of the films (380). On sintering, the surface areas decrease strongly (%I), but on adsorption of a strongly bound adsorptive the result of the sintering may be annihilated. CaFz films, previously sintered, showing a strong decrease in the amount of iodine th a t could be taken up by adsorption, did not show a decrease in the cesium-absorption capacity. Apparently cesium desintered the films, for, when after the adsorption of cesium i t was desorbed carefully-without raising the temperature-the original surface area was restored, as could be measured by the amount of iodine that could be adsorbed (382). Such a desintering might also be responsible for the slow uptake of hydrogen in the case of nickel and of tungsten films. The activation energy would then be due at least partly t o the work that is required to reopen the capillary space closed by the previous sintering. A desintering effect may also have played a role in the experiments of Taylor and collaborators, already mentioned in Sec. 1X,3, who found that in many cases a n increase in temperature during slow (activation energy) adsorption causes rapid desorption, followed by slow readsorption (383). As remarked in this section, this behavior has often been taken as a proof of heterogeneity for chemisorption. Areas were assumed with a relatively low heat of adsorption and a relatively low activation energy, together with areas where both the heat of chemisorption and the activation energy have higher values. The occurrence of these types of sites next t o each other on the same surface does not seem very probable. If during the experiments quick chemisorption were accompmied by a slow desintering process, the same phenomena would be observed. On the temperature being raised, desorption would occur, resulting in a slightly smaller degree of occupation corresponding with the temperature and pressure employed. This quick desorption would be followed by a slow uptake.of gas molecules th a t go

138

J. H. DE BOER

on reopening capillaries th at had previously been closed in the sintering (“stabilizing”) process. The slow rate of desintering should then be ascribed t o the activation energy of the desintering and not to a n activation energy of the chemisorption proper. The fact th a t the desorption process is quicker than the slow chemisorption means that the heat of chemisorption must be lower than the activation energy acting in the slow process of uptake. However, there is still another possibility t o explain this behavior. We saw in Sec. VI,3 that hydrogen may be chemisorbed in more than one way. If the general scheme of Fig. 13 could be applied to the phenomena that we are discussing here, the fast chemisorption process would be a chemisorption of hydrogen in the minimum E A of curve ABEAFA; i t may be t hat the activation energy ( E J A is practically negligible. If the temperature is not too low, this chemisorption will be accompanied by a slow process along curve ABEBFB with the activation energy (EJB. Raising the temperature would produce a desorption of atoms adsorbed a t EA, but not of those a t E,. This desorption would be followed by the continuation of the slower process, leading to a n adsorption of atoms a t EB. If this picture should be accepted, we do have sites with a high heat of chemisorption together with a high activation energy as well as sites where both quantities are low. In this picture these sites do not exist next t o each other, however; there is only one type of site, on which the atoms may be bound in two ways. There is, of course, still the possibility of heterogeneity. The experiments, however, do not prove a heterogeneous character of the surface. 12. Restricted Chemisorption Caused by the Increase of the Activation Energy

If, with increasing degree of occupation, the activation energy becomes higher and higher (see Fig. 37) it may rise so high that no further chemisorption will take place, a t least not a t the temperature of the experiment. Higher temperature and higher pressure would result in a n increase of the chemisorption. According t o Fig. 36 the activation energy for the chemisorption of nitrogen on iron a t 0 = 0.2 is about 24 kcal./ mole. This means that even at a nitrogen pressure of 1 atm. a n amount of roughly lo6 molecules of nitrogen only would be chemisorbed/sec. and/cm.Z a t room temperature, leading t o a n increase of e with about 10-6/hr. This means that the chemisorption has practically come t o a standstill. Experimentally it was found b y Beeck (384) th a t a t room temperature the chemisorption of nitrogen on iron films does not proceed further than t o about 8 i=0.2. (See also Fig. 38.) At liquid-air tempera-

ADSORPTION PHENOMENA

139

tures quite a different type of chemisorption of nitrogen takes place on iron films. Chemisorption is then fast and there is no activation energy, and the heat of chemisorption is only 10 kcal./mole, decreasing to 5 kcal./mole with increasing: coverage. Theref ore any time an activation energy plays a role, increasing with increasing degree of occupation, we are not sure whether the whole surface will be occupied fully by a unimolecular chemisorbed layer. 13. Some Final Remarks with Respect to the Decrease of the Heat of Chemisorption with Increasing Amount of Adsorbed Material

I n the preceding pages of this section we have repeatedly been obliged t o draw attention to the disagreements of opinion with respect to the question of homogeneity or heterogeneity of the surface. Summarizing and reviewing this section we may draw the following conclusions. 1. The decrease of QS u ith increasing e values on metal surfaces is due mainly t o the change in the work function of the metals b y the discrete dipole layers formed by the chemisorbed atoms. The author wishes to state here that he does not recommend the phrase induced heterogeneity, introduced b y Boudart (585). He would rather recommend the term work-function effect or surface potential effect. 2. An a priori heterogeneity may be, and is mostly, superimposed on the surface potential effect This heterogeneity may be caused by the presence of different crystallographic faces or of impurities, by contaminations of the surface, or by dissolved impurities in the lattice or between the grains. 3. The mutual depolarization of the surface dipoles may be responsible for a third contribution. This effect is of importance in the chemisorption of atoms of alkali metals on metal surfaces. It causes, also in the chemisorption of other gases a minimum or a maximum in th e surface potential as a function of 8. 4. T he surface potential effect together with the depolarization may bring about a more or less linear decrease of QSwith 8. This leads to the logarithmic adsorption isotherm (74). A slight heterogeneity together with the surface potential effect may give the same result. 5. A pronounced contribution of surface heterogeneity may result in a more or less exponential decrease of QO with 8, leading t o th e exponential adsorption isotherm (76). 6. On the surface of semiconducting or insulating oxides th e decrease of Q Owith 0 may be caused by microdefects (see Sec. IX,2), a s discussed by Vol’kenshtein (386). If practically all of these microdefects were of biographical origin, th e heat of chemisorption would be constant; if, on

140

J. H. DE B O E R

the other hand, they were all of thermal origin, a n exponential adsorption isotherm would be obtained.

X. SIMULTANEOUS ADSORPTION OF DIFFERENT MOLECULES OR ATOMS I . Simultaneous Adsorption of Diferent Molecules in Physical Adsorption Phenomena

We shall make only a few remarks with respect t o the physical adsorption of different molecules on the same adsorbent. The physical adsorption of solvent molecules in catalytic reactions carried out in solutions is often overlooked. The author cannot resist the impression that in many of Maxted’s (387) experiments on the poisoning of catalysts the adsorption of solvent molecules plays a more important role than Maxted thinks. It may be expected that a critical study of the adsorption of the solvent molecules and their influence on the adsorption of the poison molecules will lead t o even more important conclusions on poisoning than have already been reached by Maxted and collaborators in their systematic and elaborate work. Another remark may be made with respect to those estimations of surface areas where use is made of the adsorption of strongly polar molecules, such as those of fatty acids. Generally a unimolecular layer of mutually erected long molecules with their polar ends on the surface is assumed t o be formed on adsorption from solutions. This holds only for surfaces of ionic or sufficiently strong polar character and even in those cases only if the solvent molecules do not interfere. Since the work of Harkins and Gans (388),benzene is frequently used as a solvent for the fatty acids. Houben (389),in his thesis, showed that even this solvent is not completely indifferent and that a mixed layer of benzene and lauric acid is formed on the surface of alumina. It is only with still less strongly adsorbed, nonpolar, solvents, such as n-pentane, that reliable results are obtained. Fortuin (390) found that lauric acid, from n-pentane solutions, is adsorbed t o form a complete unimolecular layer on alumina surfaces that are depleted of chemisorbed water (OH groups) as well as on those that are completely covered with OH groups. Lauric acid forms a complete unimolecular layer even on alumina surfaces possessing a complete chemisorbed water layer (OH groups), on top of which a second layer of physically bound water is adsorbed with strong polar forces. The lauric acid molecules do not replace any of these water molecules; they are only adsorbed on top of them. If, however, a multimolecular water layer is present (in capillaries wide enough to admit lauric acid molecules), lauric

ADSORPTION PHENOMENA

141

acid replaces all but the two water layers just mentioned and is adsorbed on top of these two. 2. Simultaneous Adsorption of Di$erent Species

in Chemisorption; the Relative Amounts that are Chemisorbed Catalysis offers numerous examples of the simultaneous chemisorption of different molecules or atoms. The extremely important problems of poisoning, of promoting and of selective catalysis depend on this phenomenon. I n the past numerical estimations of the relative amounts that will be bound by chemisorption in equilibrium were mostly based on the assumption that Langmuir’s adsorption isotherm would be valid. I n such a case the following equation is easily derived:

where el and O2 are the degrees of occupation for species 1 and species 2 respectively, p l and p2 the partial pressures of the two gases in equilibrium with the adsorbed layer, and Q 1 and Qz the heats of adsorption of the two species. I n the conceptions underlying Langmuir’s adsorption isotherm the heats of adsorption are constant and independent of the amount that is adsorbed. We learned, in Sec. IX, th at this does not hold for chemisorption and we shall, therefore, derive a n expression based on the logarithmic adsorption isotherm [Eq. (7 1) or (74)l. We shall assume th at the fall of the heats of chemisorption and the rise of the activation energies are caused by the surface potential effect (Secs. IX,4,6,9, and 13) and that these changes vary linearly with 8. If we now also assume that the dipoles of both sorts of chemisorbed atoms are directed in the same sense (hence either both are positive or negative), it is a logical consequence th at the decrease in the heat of chemisorption of species 2 caused by the dipoles of species 1 is the same as the decrease t ha t the dipoles of species 1 cause in the heat of chemisorption of their own species. And we shall, therefore, also assume th a t the activation energies of both species are influenced in the same way by the dipole layers of both species. Both activation energies and heats of chemisorption depend, therefore, on the sum of the degrees of occupation of both species: (el 8.J. Consequently we can write

+

+ +

(El)(e,+e,)= OEI Y l e l (E2)(8,+8,)

=

(&J(e,+e,)

OE2

= =

0‘21

(&2)(e1+e,)

oQ2

?’I&

+ +

yzez y202

- ciei - C Z ~ Z - C1e1 - ~ 2 0 2

(78) (79) (80) (81)

142

J. H . DE

BOER

where the symbols stand for the initial values of the activaand tion energies and heats of chemisorption respectively and y and c are constants. The rate of adsorption of species 1 may now be written as (vadJl = k a l p l ( l -

el - e 2 ) e - ( o E i + y i ~ i + r 2 ~ 2 ) / R T

(82)

and its rate of desorption as (vdea)

=

kdlOle- ( ~ Q I - c01-cz I

O~+aEi+yi Oi+yiOz)/RT

(83)

for, the activation energy for desorption is the sum of the heat of chemisorption and the activation energy for adsorption (See. V,9). Adsorption equilibrium means (uads)l = ( v d d l

On rearranging terms we obtain

A similar equation holds for

02:

Dividing (84) b y (85) gives

The product ( k a l / k d l ) ( k d 2 / k a 2 ) may be p u t equal to unity, just as is done in deriving Eq. (77). Its deviation from unity is negligible with respect t o the e power. We, therefore, obtain

The only difference between Eqs. (87) and (77) is th a t in Eq. (87) we have t o take the initial heats of chemisorption as they are when no surface potential effect is noticeable as yet. It may be remarked that the same result would have been obtained if we had started from the conception of Temkin (Sec. IX,8). Equation (87) can be used only when the dipoles of both species have the same electrical direction. If one of the species forms a positive dipole layer and the other a negative one, the one dipole layer will decrease the activation energy and increase the heat of chemisorption of the other. The two sorts of

ADSORPTION PHENOMENA

143

molecules (atoms) will, therefore, facilitate each other’s chemisorption, in regard t o both rate and drength. We may then write

and

+

= o ~ l Ylel

- Yzez

+ ( Q ~ ) ( ~ , + ~ ~ ~) clel + czez

(&)(61+&)

(E~)(~= , +o ~ ~~~z )ylel =

( Q ~ ) ( ~ , +=~ ~o , )~

yZez

+ clel - czez z

Instead of Eq. (87) we obtain

If 81 and 0 2 do not differ too much and the pressures are of the same order of magnitude we may probably write 2(ci81 - czez) =

OQI

-

oQz

(89)

Two sorts of chemisorbed, atoms, giving dipoles of opposite signs, will attract each other and no homogeneous distribution may be expected. The formation of pairs or of agglomerates on the surface th a t will result from this attraction will influence the validity of Eq. (88) or (89). These equations have never been put t o the test. The atoms will in any case, as noted above, facilitate each other’s chemisorption, in regard t o both rate and strength. 3. The Chemisorption of Digereat Atoms Giving Dipoles of the

Same Sign The simultaneous chemisorption of CzHz and C2H4 on nickel may serve as a n example. The initial heats of chemisorption are 67 kcal./mole for CzHz and 58 kcal./mole for CzH4. At equal pressures the proportion of chemisorption a t 50°C. will be [Eq. (87)] -~ eCoHz = 106 eC@~

The surface of the catalyst is, therefore, practically covered with acetylene only, and when a mixture of the two gases is hydrogenated it is only acetylene that is converted to ethylene until practically all acetylene has disappeared. The selective hydrogenation of nonsubstituted and substituted acetylene and ethylene mixtures may be ascribed to the selective chernisorption of the gases (991). A special type of selective catalytic reaction is the poisoning of the catalytic formation of ammonia by oxygen or oxygen-containing gases,

144

J. H. D E BOER

such as CO, COZ, or HzO. All these gases react readily with the surface of the iron catalyst, producing chemisorbed oxygen atoms on th a t surface (392). I n the reaction with hydrogen these oxygen atoms compete with the nitrogen atoms. As oxygen has a n appreciably higher heat of chemisorption than nitrogen, the presence of even a very small amount of oxygen in the gas mixture results in a serious poisoning effect. I n both cases mentioned above, the two gases, chemisorbed simultaneously, do not react with each other. When such reactions can occur, more complicated relationships may be expected. Relatively few measurements have been made by direct chemisorption studies. Beeck (393) studied the simultaneous chemisorption of nitrogen and hydrogen on iron films. As we have seen in Sec. IX,12, the activation energy for nitrogen adsorption, after it has covered 20% of the surface has practically become too high for further chemisorption a t room temperature. Beeck found that when hydrogen is first adsorbed to a degree of coverage O, the surface will take up less nitrogen, namely to a n amount of ON

=

0.2(1 - 6,)

Boudart (394) explains this by assuming th a t the dipole layer formed by hydrogen affects the activation energy of nitrogen in the same sense as does nitrogen itself, but quantitatively t o a smaller degree. If a completely covered hydrogen film affects the activation energy of nitrogen just as much as a film of nitrogen covering only 20% of the surface, and if both effects bear a linear relation to 0, the above-mentioned relationship may be understood. The heat of chemisorption of hydrogen, adsorbed on iron that has previously been covered with nitrogen u p to 6 = 0.18, is, indeed, lower than the heat of chemisorption of hydrogen adsorbed on a clean surface (395). The heat of chemisorption of CO on a n iron film partly covered with nitrogen is also lower than on a clean film, but Bagg and Tompkins (395) found t hat hydrogen when adsorbed on a film partly covered with CO shows a higher heat of adsorption than when adsorbed on a clean film. Beeck (393) as well as Bagg and Tompkins (395) has included oxygen as one of the gases in these investigations. As, however, oxygen penetrates easily into the surface layers of the adsorbents (Sec. VII,6), complications arise.

4.

Contaminated Surfaces

Contaminations or poisons on surfaces may have a far more import a nt effect than the blocking of a part of the surface. Apart from the quantitatively large influence that they may have when specifically adsorbed in a microporous system (396), they may also influence the giving

145

ADSOIZPTION PHENOMENA

off or taking up of electrons by the surface. Roginskii (3977, who has extensively studied the influence of surface contaminations, classifies them into four groups, the most important of which is constituted by the socalled “modifiers.” It is these modifiers that influence the electric properties of the catalysts (398). These modifying contaminations exercise a great influence also on chemisorption phenomena. We may give just one example. It is well

c-

d i s t a n c e in fhe m e t a l

-

d i s t a n c e f r o m the m e f a l

FIG.39. Potential curves indicating the chemisorption on and the dissolution in iron having a noncontaminated surface.

known t ha t the dissolution of hydrogen into iron is a n endothermic process (Sec. VII,7). Hydrogen atoms, either obtained in a gaseous atmosphere or produced by the action of an acid medium on iron, however, penetrate easily into iron. The presence of sulfide ions on the surface of the iron facilitates this proctm (S99). Sulfide ions may be assumed t o form a dipole layer with the negative poles pointing away from the metal. The sign of this dipole layer is, therefore, the same as that of the dipole layer formed by the chemisorption of hydrogen itself when this is adsorbed a t room temperature. Consequently, the heat of chemisorption of hydrogen on iron will be severely reduced by the presence of the sulfur contamination. At the same time a n energy of activation for the dissociative chemisorption of molecular hydrogen will result from the action of the sulfur layer. The relations are schematically shown in Figs. 39 and 40. I n Fig.

146

3. H. DE BOER

39 curve A B C D gives the dissociative chemisorption of molecular hydrogen; there is no activation energy on a pure iron surface (level C lower than level A ) . The heat of chemisorption is rather high, which involves a low level for the minimum D. Molecular hydrogen can penetrate (in atomic form) into iron, provided that the kinetic energy is high enough t o reachlevel E , from which it can move into the metal (level F ) . The exact place of the surface cannot be pictured in such a potential-curve scheme; it is somewhere in

-

distance in the m e t a l

-

distance from the rnclal

FIG.40. Potential curves indicating the chemisorption on and the dissolution in iron having a contaminated surface.

the region of the vertical lines indicated by dashes. Atomic hydrogen, coming from level G, has sufficient energy t o penetrate into the metal (curve GDEF) but, owing to the fact that level D is far lower than E , most of it stays a t the surface in chemisorbed form. When a contamination is present which produces a dipole layer, as sulfur does, the dissociative chemisorption of molecular hydrogen is given by (Fig. 40) ABC’D’; there is an activation energy (difference between levels C’ and A ) ;the heat of adsorption is severely reduced; it is even negative (endothermic chemisorption) . The dissolution of molecular hydrogen proceeds less easily than in the case of a pure-iron surface. The kinetic energy has t o be sufficient to overcome the difference between C’

ADSORPTION PHENOMENA

147

and A in this case. Atomic hydrogen coming from G will penetrate easily into the metal. Far fewer hydrogen atoms will stay a t the surface, because D’is not much lower than E’. ( I t may even be higher.) Surface contaminations that produce a dipole layer of the same electric sign as that produced by 1,he adsorbed atoms themselves decrease the heat of chemisorption and produce and increase a n activation energy. This is why, activation energies are always found for the chemisorption of hydrogen on metal surfaces which are not sufficiently reduced or which are contaminated with impurities forming negative dipoles or which are partially oxidized (Secs. V,9 and IX,9). The heats of chemisorption will be lower a t the same time. If the nickel powder of Eucken described in Sec. IX,l (Fig. 28, curve B) were contaminated with so many oxygen ions (insufficiently reduced) that these would produce a surface-potential effect of the same magnitude as would be created by a chemisorption of hydrogen itself of $ = 0.3 curve B of Fig. 28 would have t o be shifted t o the right over a distance of 0 = 0.3 and it would practically coincide with curve E of Schuit and de Boer. 6. Mutual Assistance of Chemisorbed Atoms

I n Sec. X,2 we learned that atoms producing dipoles of opposite character will facilitate each other’s chemisorption, in regard t o both rate and strength. There are many examples to be found in the literature on electron emission where this effect is obvious (400). When the work function of tungsten is strongly increased by the chemisorption of oxygen, not only the atoms of alkali metals and alkaline earth metals will be bound as ions, but even metal atoms with far higher ionization energies will be chemisorbed in the ionic state. Phenomena of this kind play a role in the action of oxygen on the surfaces of iron, copper, or nickel when iron, copper, or nickel ions move next, to or on top of chemisorbed oxygen ions (Sec. VII,6), causing the penetration of the chemisorbed oxygen into the surface layers of the metal and causing a reverse of the surface potential a t the same time. Similarly cesium, adsorbed “on top of” chemisorbed oxygen on tungsten, is far more strongly bound than cesium chemisorbed on a bare tungsten surface; as the result of the simultaneous chemisorption of both species the work function is decreased to such a low value as cannot be obtained with cesium alone. I n Sec. IX,1 we remarked th at copper films do not chemisorb hydrogen, whereas copper powder does. We must add here that also pure and thoroughly reduced copper powder does not chemisorb hydrogen (401). However, when copper powder is not completely reduced it shows a lower work function than does pure copper (40Z),because copper ions on

148

J. H . DE BOER

top of oxygen ions produce, together with these latter ions, such a surface potential that, just as in the case of iron (Sec. VII,6), the work function is decreased. This complex layer has, therefore, produced a dipole layer with its positive side pointing away from the surface. Such a dipole layer facilitates the chemisorption of hydrogen, forming dipoles of opposite direction (403). We may also say th a t the reduced work function, caused by the copper ions and the oxygen ions together, facilitates the formation and chemisorption of negatively charged hydrogen. Hydrogen chemisorption a t lower temperatures or even at room temperature leads to dipoles pointing with their negative poles away from the metal. At higher temperatures another type of hydrogen chemisorption ( B adsorption, Sec. VI,3) occurs, which is probably connected with the formation of positively charged hydrogen. It is not improbable th a t an oxygen contamination of the surface promotes the B adsorption or, as already stated in Sec. VI,3, prevents the A adsorption. A dipole layer, formed by oxygen, with the negative pole pointing away from the surface could indeed reduce the heat of chemisorption of negatively charged hydrogen atoms ( A adsorption) to such a low value th a t this type of adsorption would not take place any more, although on the other hand this layer would promote B adsorption. Technical ammonia catalysts, consisting of iron with “promotors” such as aluminum oxide, potassium oxide, etc., working a t high temperatures (400” t o 500°C.) most probably adsorb hydrogen in the positively charged B form. This form of chemisorption will facilitate the chemisorption of nitrogen, which is negatively charged in its chemisorbed state. It is well known that nitrogen can easily be taken up by iron when it is heated in ammonia, which dissociates into hydrogen and nitrogen. A presorption of hydrogen markedly facilitates also the chemisorption of molecular nitrogen and its dissolution into iron (404). Conversely a previous adsorption of nitrogen can give rise to a n increase in the chemisorption of hydrogen (405). X I . SOMEREMARKS ON CATALYSIS AND CHEMISORPTION 1. Heat of Chemisorption and Catalysis

The history of the views on the mechanism of the parahydrogen conversion is a very illustrative one. For a long time it was thought th a t the heat of chemisorption of the chemisorbed hydrogen atoms was too high t o enable the atoms to react together on the surface so as t o produce molecular hydrogen. The hydrogen chemisorption was even thought t o be irreversible at or below room temperature. Subsequently it was found (406) that a t sufficiently high hydrogen pressures a normal reversible

ADSOltPTION PHENOMENA

149

chemisorption could be measured. The decrease of the heat of chemisorption with increasing amount a,dsorbed proved to proceed so far th a t sufficiently low values were obtained a t such e values as are found during the catalytic reaction. The low values of the heat of chemisorption enable the catalytic reaction to take place. Similar effects may be obtained with “promotors ” producing such surfaee-potential effects that the heat of chemisorption is low enough for chemical action. Low heats of chemisorption are especially needed in the case of the Langmuir-Hinshelwood mechanism (Sec. VII,5). A promotor may also cause a sufficient amount of chemisorbed atoms to be present. Boudart (40’7) remarks that the presence of aluminum or potassium on an iron ammonia catalyst may produce a lower work function, resulting in the chemisorption of more nitrogen than would otherwise be possible (0 = 0.2; see Sec. IX,12). The first nitrogen atoms bound by such a “promotion” caused by aluminum or potassium and oxygen together (just as with copper promoted by oxygen; see Sec. IX,5) will be bound more firmly. At the same time, however, a larger amount of nitrogen can be chemisorbed and the heat of chemisorption is sufficiently decreased for all these nitrogen atoms to react. It may be remarked t ha t the increased hydrogen-B-chemisorption, promoted by the “promotors,” as discussed in Sec. X,5, may also facilitate the nitrogen chemisorption in the same sense. Here again it is of major importance t ha t the heat of chemisorption be reduced to a sufficiently low value. As the rate of the ammonia catalysis is governed by the rate of the chemisorption of nitrogen, hence by the activation energy of nitrogen, i t is important that this activation energy be sufficiently lowered. During this catalysis no adsorption equilibria will be reached. The nitrogen is taken away by the hydrogen a,t a quicker rate than by its own desorption. 2. Endothermic Chemisorption

The heat of chemisorption, which must be low in order t o enable catalysis t o take place, may even be negative. I n various sections we have seen that endothermic ohemisorption may play a n important role (Secs. V,9, VI,3,4,5, and X,4:).Figure 40 shows th a t surface contaminations can “promote” endothermic chemisorption. I n nickel, a s in iron, hydrogen atoms can be dissolved endothermically. It is highly probable that dissolved hydrogen atoms react from the metal phase with chemisorbed hydrocarbons. Hydrogen atoms can also be taken up from hydrocarbons by such catalysts, the hydrogen atomn immediately disappearing in the metal.

150

J. H. DE BOER

A measurable amount of chemisorbed atoms need not be present during catalysis. It is essential, however, that the heat of chemisorption be sufficiently low, or even negative.

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4. Fraser, R. G. J., “Molecular Rays.” Cambridge, New York, 1931. 6. Estermann, J., Frisch, R., and Stern, O., 2. Physik 78, 348 (1931). 6. de Boer, J. H., Advances in Colloid Sci. 8, 5 (1950).

?. Verwey, E. J. W., Rec. trav. chim. 66, 521 (1946). 8. Shuttleworth, R., Proc. Phys. SOC.(London) 62A, 167 (1949). 9. Benson, G. C., and Benson, G. W., Can. J . Chem. 33, 232 (1955). 10. de Boer, J. H., Rec. trav. chim. 69, 826 (1940). 11. de Boer, J. H., Koninkt. Ned. Akad. Wetenschap. Proc. 49, 1103 (1946). 12. de Boer, J. H., and Verwey, E. J. W., Rec. trav. chim. 66, 443 (1936). 13. de Boer, J. H., Advances in Colloid Sci. 3, 35, 38 (1950). 14. Born, M., and Mayer, J. E., 2. Physik 76, 1 (1932). 16. Mayer, J. E., and Maltbie, M., 2. Physik 76, 748 (1932). 16. de Boer, J. H., and Verwey, E. J. W., Rec. trav. chim. 66, 443 (1936). 17. Verwey, E. J. W., and de Boer, J. H., Rec. trav. chim. 69, 633 (1940). 18. Brunauer, S., “The Adsorption of Gases and Vapors,” Chapter IV. Oxford, New York, 1943. 19. de Boer, J. H., Trans. Faraday SOC.82, 10 (1936). 20. Hellmann, H., “Einfuhrung in die Quantenchemie,” Chapter V. Deuticke, Leipzig, 1937. 21. de Boer, J. H., Advances in Colloid Sci. 3, 21 (1950). 2%.For a further discussion, see Margenau, H., Revs. Mod. Phys. 11, 1 (1939); de Boer, J. H., Advances in Colloid Sci. 3, 20 (1950). 23. London, F., 2. Physik 63, 245 (1930). 24. Slater, J. C., and Kirkwood, J. G., Phys. Rev. 37, 682 (1931). 26. de Boer, J. H., and Heller, G., Physica 4, 1045 (1937). 26. Polanyi, M., and London, F., Naturwissenschaften 18, 1099 (1930). 27. London, F., 2. physik. Chem. B11, 246 (1931). 88. de Boer, J. H., and Custers, J. F. H., 2. physik. Chem. B26,225 (1934); de Boer, J. H., Trans. Faraday SOC.32, 10 (1936). 89. Brunauer, S., “Physical Adsorption of Gases and Vapors,” Chapter VII. Oxford,

New York, 1943.

5’0. de Boer, J. H., Advances in Colloid Sci. 3, 27, 46 (1950). 31. Margenau, H., and Pollard, W. G., Phys. Rev. 60, 128 (1941).

32. de Boer, J. H., Advances in Colloid Sci. 3, 44 (1950). 33. Huckel, E., “Adsorption und Kapillarkondensation,” p. 126. Akademieche

Verlagsges., Leipzig, 1928.

$4. For a more detailed treatment see de Boer, J. H., Advances in Coolloid Sci. 3, 13 (1950). 36. de Boer, J. H., and Custers, J. F. H., 2. physik. Chem. B26, 225 (1934). 36. de Boer, J. H., and Dippel, C. J., 2. physik. Chem. B26, 399 (1934). 37. de Boer, J. H., and Dippel, C. J., Rec. trav. chim. 62, 214 (1933).

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151

38. Wheland, G. W., “The Theory of Resonance.” Wiley, New York, 1945. 39. de Boer, J. H., Advances in Colloid Sci. 3, 33 (1950). 40. Lenel, F. V., Z. physik. Chem. B23, 379 (1933).

41. Drain, L. E., Trans. Faraday Xoc. 49, 650 (1953).

4.2. Brunauer, S., “The Adsorption of Gases and Vapors,” p. 28. Oxford, New York,

1943. 43. Mignolet, J. C. P., Discussions Faraday SOC.No. 8, 105 (1950); J . Chem. Phys. 21, 1298 (1953). 44. de Boer, J. H., and Kruyer, S., Koninkl Ned. Akad. Wetenschap. Proc. B66, 451 (1952); B66, 67 (1953); B66, 236 (1953); B66, 415 (1953); B67, 92 (1954); B68, 61 (1955). 46. de Boer, J. H., “The Dynamical Character of Adsorption,” p. 146. Oxford, New York, 1953. 46. de Boer, J. H., “The Dynamical Character of Adsorption,” p. 168. Oxford, New York, 1953; Ned. Tijdschr. Natuurk. 19, 283 (1953); Kruyer, S., Thesis, Delft, 1955. 47. cf. also Magnus, A., Z. physik. Chem. A142, 401 (1929); Trans. Faraday SOC.28, 386 (1932). 48. de Boer, J. H., “The Dynarnical Character of Adsorption,” p. 155. Oxford, New York, 1953. 49. Ives, H. E., Boston Meeting American Physical Society, Dec. 1922; see J . Franklin Inst. 201, 47 (1926). 60. Langmuir, I., and Kingdon, K. H., Science 67, 58 (1923). 61. de Boer, J. H., “Electron Emission and Adsorption Phenomena,” 58. Cambridge, New York, 1935. 52. de Boer, J. H., “Electron Emission and Adsorption Phenomena,” 520. Cambridge, New York, 1935; de Eloer, J. I<., and Veenemans, C. F., Physica 1, 753 (1934). 63. de Boer, J. H., Advances in Colloid Sci. 3, 2 (1950). 54. Riyanoff, S., Z. Physik 71,325 (1931); Lukirsky, P. I., and Riyanoff, S., ibid. 76, 249 (1932); Lukirsky, P. I., Physik. 2.Sowjetunion 4, 225 (1933). 65. de Boer, J. II., “Electron Emission and Adsorption Phenomena,” p. 215. Cambridge, New York, 1935. 66. Wheland, G. W., “The Theory of Resonance,” p. 229. Wiley, New York, 1945. 67. Sidgwick, N. V., “The Electron Theory of Valence.” Oxford, New York, 1927; see also Wells, A. F., “Structural Inorganic Chemistry,” p. 49. Oxford, New York; Pauling, L., “The Nature of the Chemical Bond,” p. 7. Cornell U. P., Ithaca, 1948. 58. de Boer, J. H., and Verwey, IS. J. W., Rec. trav. chim. 66, 443 (1936). 69. Verwey, E. J. W., and de Boer, J. H., Rec. trav. chim. 66, 675 (1936). 60. Mignolet, J. C. P., Discussions Faraday Soc. NO.8, 105 (1950). 61. Maxted, E. B., Advances in Catalysis 3, 129 (1951). 62. Suhrmann, R., 2. Elektrochem. 66, 351 (1952). 63. de Boer, J. H., “Elektronenemission und Adsorptionserscheinungen,” p. 59. Leipsig, 1937; see reference 32. 64. Morse, P. H., Phys. Rev. 34, 57 (1929). 66. Eley, D. D., Discussions Faraday SOC.NO. 8, 34 (1950). 66. Pauling, L., “The Nature of ,,he Chemical Bond,” p. 60. Cornell U. P., Ithaca, 1948. 67. Kwan, T., Advances in Catalysis 6, 91 (1954).

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