Some General Aspects of Chemisorption and Catalysis

Some General Aspects of Chemisorption and Catalysis TAKA0 KWAN The Research Institute for Catalysis, Hokkaido University, Sapporo, J a p a n

Page I. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 11. The Rate of Chemisorption.. . . . . . . . . . . . . . . . . . . . . . . . . 1. Temperature Coefficient. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 2. Absolute Rates of Chemisorption . . . . . . . . . . . . . . . . . . 111. Chcmisorption Equilibria and Related Problems. . . . . . . . . . . . . . . . . . . . . . . 76 1. Determination of True Equilibria.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 2. The Isobar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 3. The Isothnrni.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4. Theoretical Aspects of the Adsorption Isotherm. . . . . . . . . . . . . . 5 . Chemisorbed States of Di- and Triatomic Molecules. . . . . . . . . . 6. Adsorption Heats of Hydrogen on Metallic Surfaces... . . . . . . . IV. The Nature of the Catalyst Surface.. . . . . . . . . . . . . . . . . . . . . . . . 1. Homogeneity of Metallic Surfaces for Chemisorption 2. Heterogeneity of Metallic Surfaces for Chcmisorptio 3. The Iron Catalyst for Ammonia Synthesis. . . . . . . . . 4. Oxide Catalysts.. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5. Poisoning of Catalysts . . . . . . . . . . . . . . . . . . . . . . . . V. Studies of Single Crystals of Metals in Adsorption and VI. Topochemistry in Heterogeneous Catalysis. . . . . . . . . . . . VII. The Mechanism of Heterogeneous Catalysis, . . . . . . . . . . . . . . . . . . . . . . . . . . 108 1. The Stoichiometric Number.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 2. Adsorption and Elcmentary Reaction Rates.. . . . . . . . . . . . . . . . . . . . . . . 111 3. Activation Energy and Frequency Factor. . . . . . . . . . . . . . . . . . . . . . . . . . 113 4. The Catalytic Hydrogenation of Ethylene.. . . . . . . . . . . . . . . . . . . . . . . . . 114 VIII. General Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 References.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

I. S C O P E It is generally accepted th at the specific rate of chemical reaction, k, in a homogeneous system can be expressed by the Arrhenius equation in the form, =

Ae-E/RT

(1)

where E is the activation energy and A the frequency factor, both of which are considered to be approximately constant. The physical meaning of these quantities is a t present accounted for by statistical mechanics, a t least for simple gaseous reactions. 67

68

TAK A0 KWAN

A catalyst accelerates the rate of reaction leaving the above relation still valid in most cases. However, the meaning of the two parameters E and A in the Arrhenius equation is not clear enough in many individual cases of heterogeneous catalysis. On the other hand, the LangmuirHinshelwood method, which treats a heterogeneous reaction as analogous t o a homogeneous one, allows us to interpret the pressure dependencay of the rate or the “order” of the heterogeneous reaction even though its applicability t o some heterogeneous reartions is still questionable. I t would be of 8ome value t o review here several investigations dealing with the pressure dependency of catalytic reaction rates as well as the relation between E and A and also to show how divergent findings and different views have been hitherto put forward. The Langmuir-Hinshelwood theory, for instance, for a bimolecular surface reaction between reactants a and b, where a is supposed t o be more strongly adsorbed, predicts that there is a maximum in the rate as the partial pressure of a increases. Such a maximurn was actually observed by Hinshelwood and Priehard (1) for the carbon dioxide pressure in the reaction between carbon dioxide and hydrogen on platinum. No corresponding maximum was found for the same reaction on tungsten. In the latter case the rate approaches a constant value with increasing carbon dioxide pressure. Schwab (2) failed to find a maximum, however, even in the case of a platinum catalyst. He attributed the falling off of the rate of hydrogenation of ethylene on nickel a t high ethylene pressure to irreversible poisoning, concluding that the observed mechanism is due either to the Rideal mechanism or t o the adsorption of reactants on separate parts of the catalyst surface. An objection to this view was made by Laidler (3)) who found that the rate of the exchange reaction between deuterium and ammonia on a promoted iron catalyst passed through a maximum with incirease of the ammonia pressure, contrary to the zero-order kinetics with respect to ammonia reported by Farkas (4). Schwab ( 5 ) points out a relationship between the parameters E and il of the Arrhenius equation which varies from one catalyst to the other for a definite reaction, for instance, for the dehydrogenation of alcohol, as expressed by A

= BeE/h

(2)

where B and h are constants. This means that the acceleration of the reaction, resulting from a decrease of E, is t o some extent compensated by LL simultaneous decrease of ,4. Schwab attributed this relation to the existence of different “active centers” in such a way that active centers with lower acvtivation energies are less abundant, or those with higher

GENERAL ASPECTS OF CHEMISORPTION AND CATALYSIS

69

activation energies are more frequent, on the catalyst surface. I n contrast to this hypothesis, the “plane” theory, which identifies the sites of the catalytic reaction with certain adjacent sets of atoms on the crystal surface of the catalyst, postulates that A remains constant when the catalyst surface is entirely covered or entirely vacant. No experimental data are at present available to settle this alternative. It is known, on the other hand, in the case of the decomposition of formic acid (6) that only A varies while E remains constant, i.e., that the reaction proceeds lo4times faster on rhodium than on glass, the value of activation energy being almost unchanged for both catalysts. Similar results were obtained by Beeck (7) with the hydrogenation of ethylene on evaporated films of nickel, iron, palladium, platinum, and rhodium. The variation of the catalytic activity by a factor of lo3from the least efficient catalyst iron to the best catalyst rhodium was entirely attributed to the change in A , the activation energy being 10.7 kcal./mole for all these catalysts. Eyring et al. (8) calculated A for the ethylene hydrogenation on the nickel film on the basis of the plane theory, in satisfactory agreement with the experimental values. In other cases, however, the theoretical deduction of A is still open to question. As further references to the same reaction the well-known work by zur Strassen (9) and Rideal (10) will be given. According to these authors the rate of the hydrogenation reaction on nickel goes through a maximum with increasing temperature, the activation energy or the inclination of log k against 1/T being assumed to reach a negative value at higher temperatures. As regards this reversal of the temperature coefficient Horiuti ( l l ) ,as will be described later, put forward an alternative hypothesis, differing from the “desorption” theory of Schwab (12). In the field of heterogeneous catalysis, the experimental observations regarding the pressure dependencies and the relations between activation energies and frequency factors appear so complicated that the kinetic study of the reaction seems insufficient for clarifying the underlying mechanism. Much more work ought to be done a t the present stage on the adsorption of the reactants and on the structure of catalyst surfaces, in order to elucidate the nature of the catalyst itself. We shall briefly review some significant adsorption studies and investigations of the catalyst surfaces. The knowledge gained in this way can be very helpful in judging the contradictory views described above. Adsorption studies, chiefly from the author’s laboratory, will be described at the outset (Sections I1 and 111). The physical properties of catalyst surfaces will then be discussed (Sections IV-VI). Finally, the “structure l 1 theory of heterogeneous catalysis proposed by Horiuti will be reviewed (Section VII).

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11. THE RATE

O F CI-IEMISORPTION

Coejicient The term activated adsorption” is, as originally designated by H. S. Taylor, a type of adsorption that takes place a t a measurably slow rate associated with a certain temperature coefficient. For the same type of adsorption, the term “ chemisorption” has been used more frequently, in later years. We shall use here the term “chemisorpt,ion” when the heat of adsorption is comparable with the heat evolved in ordinary chemical reactions. The rate of the chemisorption of hydrogen is immeasurably fast on most metallic catalysts unless the surfaces of the latter are contaminated with impurities. Extremely pure metal surfaces can be obtained by means of suitable techniques, for instance, by flashing of tungsten wire (Roberts, 13) a t very high temperatures or by evaporating a pure metal in vacuo (Beeck et al., 14). It has recently been found by Frankenburg (15) and also hy the present author (IG) that ordinary metal preparations can be purified t o a n extent approaching that of the extremely pure metallic wires and films. As a result, the chemisorption of hydrogen becomes almost instantaneous on a most carefully reduced tungsten powder, and on specimens of nickel or cobalt provided that these metals were reduced with highly purified hydrogen and kept free from any contamination. I n studying the chemisorption of hydrogen on carefully reduced nickel the author has actually observed that a minute quantity of the vapor of stop-cock grease or of mercury vapor from a pressure gage appreciably affect the rate of chemisorption in so far as these contaminants reduce considerably the rate of adsorption and produce the effects typical for the so-called activated adsorption. Incomplete reduction of nickel oxide t o the metal leads to a similar result. This can be avoided by repeated reduction and subsequent evacuations of the metal sample a t 400°C. for a week. A typical result obtained with a n exhaustively reduced nickel specimen is shown in Fig. 1. I n view of these findings, the activated adsorption of hydrogen on other reduced metal catalysts frequently reported in the earlier literature might have been caused by contamination effects. Results pointing in the same direction were obtained recently by Schuit and De Boer (17), who found th at activated adsorption of hydrogen occurs only on a partially oxidized surface of nickel supported on silica (3: 1) but not on a thoroughly reduced surface. According to Schuit and De Boer, very prolonged evacuation or heating of a “reduced” nickel catalyst in a n inert atmosphere leads to a slow “activated” hydrogen adsorption. This effect, however, disappears on renewed careful reduction 1. l‘emperat

lire

G E N E R A L ASPECTS O F CHEMISORPTION A N D CATALYSIS

71

of the catalyst. This is considered as evidence for a slow diffusion of oxygen from the interior of the catalyst to its surface during prolonged heating in a n inert atmosphere. Eucken (18) obtained, on the other hand, 6.6 kcal./mole as the temperature coefficient of the adsorption of hydrogen on 8 reduced nickel sample. It appears, however, th a t this result should be rechecked with due regard to the results of Schuit and De Boer, particularly since the nickel sample used by Eucken was reduced for a shorter time and a t a somewhat lower temperature than the samples investigated by the Dutch authors.

3t

t min.

FIG.1. Change of Hz pressure against time, subsequent to the initial rapid uptake on reduced nickel. Reproduced from Kwan (16).

The rate of the adsorption of hydrogen on a flashed tungsten wire, as observed by Roberts (13), was almost the same a t - 195" and 22"C., indicating that the temperature coefficient of adsorption is zero or, a t least, negligibly small. A similar result was obtained by Beeck et al. (14) with evaporated nickel films. Matsuda (19) of our laboratory followed the rate of adsorption of hydrogen on nickel wire of 5 0 0 0 - ~ m area . ~ by means of a Pirani gage and an electromagnetic oscillograph a t - 183" and 20"C., finding again the temperature coefficient t o be almost zero. As will be shown later, the rate of chemisorption of hydrogen on a completely clean nickel surface, as calculated by the absolute rate theory on the basis of a temperature coefficient equalling zero, was lo6times greater than the observed rate. Such a marked discrepancy has been pointed out also by Eyring et al. (20) using Roberts' data. This might be interpreted by assuming that the observed hydrogen "sorption" is controlled in its

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TAKA0 KWAK

rate by some other process slower than the chemisorption proper. This would mean that the observed temperature coefficient equalling zero might not be that of the chemisorption but rather that of the ratecontrolling slower process. We now turn to the numerous observations made on the activated adsorption of hydrogen on reduced copper. I n this case a definite, large temperature coefficient or activation energy A€* has been found. It was calculated by the conventional formula

where t , and t z are the respective times for the adsorption of a given amount ot the gas a t the absolute temperatures T I and Tf. The progress of adsorption observed by Ward (2l), Reebe et al. (22), and others is very complex. Beebe et al. found that the adsorption proceeds “autocatalytically” a t first, its rate increasing with increasing quantities of adsorbed hydrogen. A subsequent slower process, observed by several workers, has been interpreted in different ways: for instance, in terms of a diffusion of hydrogen along grain boundaries, of a solution of hydrogen in the bulk of the copper, or of the displacement of foreign adsorbed gases by adsorbed hydrogen. Activation energies derived according to equation (3), homever, vary widely with the extent and the temperature of adsorption and have obviously no definite physical meaning. Before attempting any explanation of the observed phenomena, one should investigate whether. in all these investigations, the surface of the copper was kept clean enough, rememhering that nickel had t o be reduced repeatedly a t 400°C. i n order t o obtain a clean metal surface, whereas the copper specimen used by the authors mentioned was prepared merely by repeated reduction and oxidation below 200°C. The possibility of a contamination of the copper specimens used in these invcxstigations is also indicated by the experimental fact that the adsorbed quantity of hydrogen per unit weight of copper was extremely small compared with that for other metals, i n spite of the large heat of adsorption whose value exceeded 30 kcal./mole according to Ward (21). These findings have to be correlated t o those of Beeck et al. (14), who observed that no perceptible amount of hydrogen is chemisorbed over an evaporated copper film which consisted probably of the pure metal. Kwan and Izu (23) investigated the chemisorption of hydrogen on carefully reduced copper, with results strikingly different from those in the earlier work discussed above. Kahlbaum’s “extra fine” copper and other samples of an equally high grade of purity were reduced a t 450OC. with hydrogen a t a pressure of several centimeters of mercury. Hydrogen

G E N E R A L ASPECTS O F CHEMISORPTION AND CATALYSIS

73

was consumed to some extent for the first few hours, after which no further consumption was observed. During the following period the copper was kept a t the same condition of reduction but the system was repeatedly evacuated and supplied with fresh hydrogen. The rate of ehemisorption of hydrogen on this copper specimens, after the reaction vessel had been evacuated a t 400°C. was found t o be very small even for

t, hr.

FIG.2. Change of H2 pressure against time on reduced copper. a, 5 g. Kahlbaum’s copper; b, Kojima’s copper. Reproduced from Kwan (23).

the first portions of admitted gas. At temperatures <300°C., it took two or three days for the hydrogen pressure, as measured on a McLeod gage, to decrease measurably. The rates of adsorption were, in this range, strictly proportional t o the hydrogen pressures throughout,, and no later phase of a greatly diminished adsorption rate, as reported by other authors, was observed. These results are shown in Fig. 2. I n these measurements of the hydrogen uptake by thoroughly reduced copper, the quantities of hydrogen adsorbed per unit weight of the ad-

74

TAKA0 KWAN

sorbent were exceptionally large compared with any other result hitherto reported, in spite of the higher temperatures and the lower pressures used in these experiments. It can be shown th at the pressure decrease measured in this system is entirely due to chemisorption and not to a solution of hydrogen in the bulk of the copper on the basis of the solubility data reported by Sieverts (24). From the rate constant k determined a t various temperatures the activation energy was found to be 20.5 & 0.5 kcal./ mole according to the equation d ln k Ae* = R T ? (4) dT There exists comparatively little information on the activation energy of the chemisorption of hydrogen on other metallic surfaces. The only additional data reported are those for doubly promoted iron catalysts by Emmett and Harkness (25). They obtained 10.4 & 1.0 kcal./mole for Ae* by means of equation (3) for the initial 2 ~ muptake . ~ of hydrogen in the temperature range from -78.5' to -96.5"C. As regards the chemisorption of nitrogen, slow adsorption has been observed with doubly promoted iron (Emmett and Hrunauer, 26), tungsten powder (Davis, 27) and with evaporated iron film (Beeck, 28). I n the first two systems, the activation energies were found to increase with increasing adsorption, the extrapolated values for sparsely covered surfaces being 10 kcal./mole, in both cases. Our recent investigations, dealing with the system nitrogen-iron catalyst, indicated th a t the activation energies remain nearly constant a t 8 kcal./mole within a certain range of sparsely covered surface, and then increase IinearIy with the logarithm of adsorbed amounts. 2. Absolute Rates of Chemisorption

Attempts have been made by Eyring arid Sherman (29) and by Okamoto, Horiuti, and Hirota (30) to evaluate the activation energy for the chemisorption of hydrogen on carbon or nickel on the assumption that the surface atoms behave as isolated atoms. The calculated values, although too high, vary markedly with the spacing of surface atoms. Quantitatively, however, we must a t present rely upon experimental data. The absolute rates of chemisorption will be calculated hcre using the observed temperature coefficient. The absolute rate of chemisorption of gaseous molecules 6, J', on the bare and plane surface of a catalyst can be given, assuming th a t the transmission coefficient equals unity (3l),by

GENERAL ASPECTS O F CITEMISORPTION .4ND CATALYSIS

75

where G is the number of surface sites for the activated complex 6* per cm.2, q6*is the partition function of 6’, N 6 is the concentration of 6 in the gas phase, and QE is the partition function of 6 per unit volume. Assuming now that the activated complex &*is a t rest on the surface, and extracting the zero-point energy contribution from the partition functions, we obtain,

where Q: is expressed, for instance, for a diatomic molecule or a linear molecule as

where the symbols m, I et,c. have their usual significance. A c t in equation (6) is the potential energy difference per mole between 6 and fi* a t the absolute zero. The Arrhenius activation energy A€* = RT?

d In V / N 6 d 7’

is expressed according t o equations (6) and (7) as follows: A€*

=

A€,* - X R T

(8)

which enables us to determine A c t from the observed value of V . Determining A€,* either theoretically or experimentally and estimating G at per cm.2, we may calculate the absolute rate V and compare i t with the experimental value of this quantity. Work in this direction 11”s first done by Okamoto, Horiuti, and Hirota (30), who calculated the rate of hydrogen chemisorption on nickel, assuming this t o be the ratedetermining step of the hydrogen electrode process. Their results, together with corresponding results of other authors, are summarized in Table I. The calculated and observed values agree with each other within a factor of lo2except in the case of hydrogen adsorption on tungsten and nickel, where the rates observed, as mentioned above, are presumably not those of the adsorption process proper. Davis obtained a value of A€: of 12.68 kcal./mole from the observed chemisorption rate of nitrogen on tungsten and compared it with a value of A€* of 10 kcal./mole derived according to equation (3). A calculation from the relation that A€,* is 10 kcal. YdRT of the rate by means of equation (6) leads to the result that V is 7.1 X lo9 as shown in Table I. The initial rate of chemisorption calculated on the basis of a n activation energy deduced from the observed rate is not reliable because the rates

+

76

T A K A 0 KWAN

are usually too large for exact measurements. The exceptionally small rate of the chemisorption of hydrogen on reduced copper permits a n adequate measurement, even a t the initial stage. This observed rate agrees closely with the calculated value. TABLE I

Calculated and Observed Rates of Chemisorption on Bare Siirfuces of illetallic Cutulysls

Rates of chcmisorption, molecules set.-' Catalyst

Chemisorp- Temp., Pressure, A€*, tion of "C. mm. Hg kcal./mole Obs.

Kin Promotedb Fe Promotedh Fr

\v

Cll Ni

\v

Ni Q

b

Calc.

Ref.

Hz Hz

50 -78.5

690 760

15.4 10.4

6 . 0 X 1012 2 . 0 X lo1* 30 9 . 0 x 10" 2 . 8 X 1 O l o 25,20

N2

271

760

14.4

8 . 7 X 1 O l o 9 . 2 X 10" 26,20

0.6 0.59 192.5 10-4 10-3

10 8 . 5 X lo8 20.5 9.0 X 1Olo ~ ( c ~ I c . )3 . 4 x 1 0 1 2 0 5 . 9 x 1012 0 1 . 0 x 1013

100 400 20 - 195 -183

7 . 1 X lo8 27 4 . 8 X 1 O ' O 32 4 . 4 x 10'4 33,34 9 . 8 x 1017 13,20 1 . 0 x 1018 19

Hydrogen electrode proress. The rates are given for an approxiiiiatdy half-covered surface. Rcprodnced from Glasstone el al.

(2 0).

Keii (33) calculated the activation energy of the chemisorptioii of ethylene on nickel assuming two carbon atoms respectively bonded with two adjacent nickel atoms and found almost constant activation energies of 4 kcal./mo!e-assuming spacings between adjacent nickel atoms of 2.49 and 3.52 A, and Ni--C bond strengths t o vary from 38.2 t o 60 kcal./ mole. It is surprising that the rates calculated in this way are compatible with the experimental observations of Steace and Stove1 (34).

111.

CHEMISORPTION EQUILIBRIA .4ND R E L A T E D I'ROBLEMS

1. Determination of True Equilibria

According t o most early observations on insufficiently cleaned metal surfaces, chemisorption proceeds slowly and continues a t a decreasing rate, for several days, so that final attainment of equilibrium is never fully assured. Usually, the adsorbed amounts were determined a t a stage when the gas uptake had become very slow. As described in the foregoing sections, on clean metallic surfaccs chemisorption comes to a definite endpoint without a slow drift. However, even in such cases it is important to

QENEIWL ASPECTS O F CHEMISORPTION AND CATALYSIS

77

ensure t ha t true equilibria are attained without adsorption hysteresis, or decomposition of the adsorbate. It is known th a t ammonia, carbon monoxide, or ethylene suffer catalytic decomposition on metallic surfaces in a certain range of temperature and pressure subsequent t o their initial rapid adsorption. Benton and White (35) established the adsorption equilibria of hydrogen on reduced nickel by approaching them from both higher and lower pressure a t the upper part of the adsorption isotherm keeping the

t

min.

FIG.3. Kcversibility of IIz adsorption on reduced nickel, temperature. Reproduced from Kwan (16).

011 raising

or lowering the

temperature constant. Frankenburg (15) and Kwan (16) verified the reversibility of the adsorption isotherm of hydrogen on reduced metallic catalysts over the whole range of temperature and pressures covered, by approaching the adsorption equilibria from both the adsorption and desorption sides by lowering and raising the temperature. A typical example is shown in Fig. 3 for the adsorption of hydrogen on reduced nickel. The observed pressure is plotted against time and the figures on the plot indicate temperatures in degrees centigrade. It follows from Fig. 3 t ha t the same adsorption equilibria of hydrogen on reduced nickel were found, both by rapid desorption and by readsorption.

78

T A K A 0 KWAN

I n this connection the concept of an “immobile” adsorbed film of hydrogen on tungsten or nickel suggested by Roberts (36,37) and others might be considered. According to these workers adsorbed hydrogen atoms a t the surface of tungsten or nickel are immobile in the sense that they remain individually tixed on the sites on which they were first adsorbed without attaining a distribution equilibrium, owing t o their high heat of adsorption. This was deduced from the fact that, for instance, hydrogen hardly can be desorbed from a tungsten surface up t o 500°C. in a high vacuum. Roberts and Miller (37) showed th a t the theoretical relation between adsorption heat and surface coverage derived on the assumption of a mobile film fits equally well with the experimental findings as the relation derived for a n immobile film, but Miller (38) recently expressed the view that the latter concept correlates better with the experimental data. It will be shown later on the basis of statistical mechanics th a t the chemisorbed atoms are immobile in the sense that they are vibrating around the equilibrium position rather than undergoing two-dimensional translation. 2. The Isobar

Two types of adsorption are usually postulated, the one being the ‘(physical” adsorption which occurs a t a lower temperature and is associated with smaller adsorption heats of the magnitude of the latent heat of evaporation of the adsorbate, and the other the “chemical” adsorption or chemisorption occurring usually a t higher temperatures with larger heats of the magnitude of heats of chemical reactions. The adsorption heats being positive in both cases, it is thermodynamically t o be expect>edthat the adsorption isobars would steadily decrease with increasing temperatures without showing any maximum provided that true adsorption equilibria are established. The frequently reported maxima observed in isobars must be attributed to the failure to attain a true equilibrium within the period of observation, under conditions at which hoth types of adsorption are occurring simultaneously. A maximum may appear when either type of adsorption is in equilibrium whereas the other is not, and the increase of the latter, not equilibrated adsorption with increasing temperature is then due to the increase of its rate. Only above a certain temperature a t which the latter type of adsorption becomes fast enough to attain its true equilibrium, the isobars assume again their normal shape, i.e., a fall with increasing temperature. An interesting example of an isobar having two maxima at 80” and 218°C. has been reported in the case of zinc oxide-hydrogen by Taylor and Strother (39), and a similar one a t -80” and 100°C. in the case of

GENERAL ASPECTS OF CHEMISORPTION AND CATALYSIS

79

iron-hydrogen by Emmett and Harkness ( 2 5 ) . Eucken (18) obtained a similar result for the system nickel-hydrogen as shown in Fig. 4. Eucken assumed a homogeneous surface, contrary t o the above workers who assumed that heterogeneity of the surface caused the observed maxima. For an explanation of the maxima, Eucken postulated an adsorbed state of the hydrogen different from that of the purely physical adsorption and also from that of its chemisorption in atomic form. Such a state may be, in the author’s opinion, that of ionized hydrogen molecules. Hydrogen ions are assumed by Roriuti to be present on oxidized metal

FIG.4. Abnormal shapes of the adsorption isobars of Hz on zinc oxide, P = 760 mm. Hg (Taylor and Strother); promoted iron, P = 760 mm. Hg (Emmett and Harkness); reduced nickel, P = 0.01 mm. Hg (Eucken); and evaporated nickel film presintered a t 400”C., P = 0.1 mm. Hg. (The ordinates are arbitrarily), (Beeck).

surfaces, e.g., on a nickel surface contaminated with negative elements such as oxygen, sulfur, or selenium. It was experimentally shown by Kwan (40) that hydrogen ions are formed as intermediates in the hydrogenation of acetone with a sulfur-coated nickel catalyst. Eucken found also in his experiments with the hydrogen-nickel system that the isobars observed on decreasing the temperature lie below the isobars obtained on increasing the temperature, as shown in Fig. 4. This is contrary t o the observations of others who found the ((cooling curve to be almost identical and sometimes lying above, the heating curve.” Eucken contemplated the possibility that hydrogen atoms a t high temperatures may be uniformly distributed over the nickel surface and prevent, in this distribution the chemisorption of additional hydrogen (cooling isobar) whereas atoms adsorbed a t lower temperature would not

80

TAKA0 KWAN

have this effect (heating isobar). Following the contamination hypothesis mentioned at a n earlier point, it may also be possible that diffusion of oxygen from the interior of the metal to the surface during the heating diminishes the surface available for the “cooling isobar,” in accordance with the concepts of Schuit and lle Boer (17). With sintered nickel films a t 400°C. Beeck‘(28) obtained a result that resembles i n some respects those of Eucken. He attributed these effects to ail additional sintering of his films a t this temperature with a simultaneous formation of a new phase of hexagonally closepacked nickel. In regard to the amount of hydrogen adsorbed, Beeck’s observatioiis deviate considerably from those of Eucken; a n initial fast adsorption of hydrogen at 0.1 mm. Hg pressure and a t -195”C., according to Ueeck, covers almost the entire surface of the nickel film. Nevertheless, the amount of hydrogen adsorbed increases with temperature, exceeding by far the amount corresponding to a monolayer. Beeck considered the latter effect as a clear sign for an absorption of hydrogen into the bulk of the metal, i.e., as the exothermic formation of nickel hydride. With reduced massive nickel no similar sorption effects can be observed. 3. The Isotherm

Adsorption isotherms for gases on metallic catalysts aid on oxide catalysts have been measured by Frankenburg (15), Davis (27), and Kwan (1 6) with special precautions for securing the attainment of true thermodynamical equilibria. There are two striking features of these isotherms. First, the adsorbed quantity, if extrapolated to very high gas pressures, apparently reaches the same saturation value a t high pressures within a considerable range of temperatures, for instance, in the case of tungstcn-hydrogen a t temperatures from -195” t o 750°C. This is in contradktion to the concept (41) th at the saturation value of a given adsorbent decreases markedly with increasing temperature. It was further shown in the case of tungsten-hydrogen that the number of hydrogen atoms adsorbed a t saturation is approximately l O I 5 per cm.2 of surface area, iridivatiiig that all the tungsten atoms on the surface* are accessihle for adsorption. Secondly, the adsorption equilibrium on a sparsely covered surface is approximately proportional to the square root or t o the cube root of the equilibrium pressure, tending, however, to follow a lesser power of the equilibrium pressure at higher coverage. This nearly corresponds to the Freundlich type of adsorption isotherm. From this Kwan suggests * The lattci was iiirasured by low-trmperaturc, adsorption of nitrogen on the t tiiipbtcvi tiiii face, iising thr Rrrinaurr-T”memtt-T~llrr(R.E.T.) rncthod.

G E N E R A L ASPECTS OF CHEMISORPTION A N D CATALYSIS

81

t,hat the entire isotherm may be divided into two parts, i.e., one of the Langmuir type for adsorption on the sparsely covered surface and one of the Freundlich type on the highly covered surface. I n Figs. 5a and 5h are showii typical examples of adsorption isotherms for hydrogen 011 reduced metallic catalysts. It should be possible t o evaluate accurately measured adsorption isotherms with statistical mechanics in order to reach an understanding of adsorption and of the nature of adsorbent surfaces. Only a few attempts, however, have been made along this line. Wilkins (42) studied the van der Waals adsorption of helium, nitrogen, and other gases on platinum foil and attributed 1.0

C

3

% a

*d

o

-&

I U

2

SI

U

-I ?i

-1.0

(L1

5M

I

-

-2.0

-6 log P, mm. Hg

FIG. Sa. Adsorption isotherms of 112 on reduced tungsten. The dotted portions rc,prcsent extrapolated isotherms. Itrproduccd from Frankenburg (15).

the departure from the Langmuir type of adsorption to the mutual attraction of the adsorbed particles. Halsey and Taylor (43) analyzed the adsorption isotherm of hydrogen on tungsten, measured by Frankenburg, by means of a statistical-mechanical formula developed by Fowler and Guggenheim. They concluded that the “main” part of the isotherm which obeys the Freundlich formula, may be interpreted by assuming sites of exponentially-distributed adsorption energies, and th a t the alternative assumption of a mutual repulsion of adsorbed particles on a homogeneous surface appears less satisfactory. A similar treatment was carried out by Sips (44). I t seems very probable from the shape of the isotherms on a sparsely

82

T A K A 0 KWAN

covered surface (Langmui r type of adsorption) th a t homogeneous ndsorption sites do exist to a certain extent on the surface. Frankenburg’s measurements on the system tungsten-hydrogen show that such conditions hold up, in this particular system, to a coverage of roughly 0.8% of R

NI

-10-

250°C -20-

-30

-20

-10

0

-30

-20

-10

0

300’C

-30

-20

-10

0

log P . mm Hg

FIG.5b. Adsorption isotheims of H2 on reduced nickel (0.5 g., 12.5 m.?) platinum black (0.93 g., 260 and retliic~drolmlt (0.5 g., 7 m.z). Reproduced from Hwan (16). b

a

log P, mm. Hg

FIG.6. Adsorption isotherms of C2H4on reduced nickel (0.5 g., 13 m.?) by Matsusita (46).

saturation. A transition from the Langmuir type to the Freundlich type of isotherm has been observed at about 10 t o 20% of saturation in the system nickel-hydrogen (16), a t 7 % in the system carbon dioxide-nickel (45), and a t about 10% in the system riitrogen-tuiiRsteii (27).

GENERAL ASPECTS O F CHEMISORPTION AND CATALYSIS

a3

A very unusual shape of adsorption isotherm has recently been obtained by Matsusita of this laboratory (46) for the adsorption of ethylene on reduced nickel. The adsorption isotherms determined a t 130" and 140°C. in the range of to lop6mm. Hg equilibrium pressure consist of three linear parts in the log-log plot as shown in Fig. 6. The first break occurs a t the coverage 0.1 assuming that saturation corresponds, in this case, t o one ethylene molecuIe being adsorbed for every four nickel atoms in the surface. Since it seems difficult to decide from the shapes of the adsorption isotherms whether the catalyst surfaces are heterogeneous or homogenous with repulsive interaction operative, statistical mechanics have been applied t o the isotherm for such low surface coverings that a n y interaction of adsorbed particles can be ignored (see Section IV).

4. Theoretical Aspects of the Adsorption Isotherm I n this section we shall derive for later discussion a formula for the adsorption isotherm with special reference to the degree of dissociation of the adsorbate a t the catalyst surface. The general expression for a n adsorption isotherm of gaseous molecules 6 is given (31) by i=n

where n is the degree of dissociation of the molecule 6 a t the catalyst suris face and N6the concentration of 6 in the gas phase. The symbol the partition function of the adsorbed particle &(a*) and this may be expressed, provided th at the adsorbed particle possesses a vibrational mode of motion of frequencies v,,kJ as:

where W) is the lowest potential energy of &(a). Q6 is the partition function of the gaseous molecules 6 per unit volume and is expressed as

-_r6

Qa = Q,"e kT

(1 1)

where 2 is the lowest energy level of 6 and Q,",for a diatomic or linear molecule, is given already by equation (7). The differential heat of adsorption of 6 per mole, A€, is given by the difference between the partial molar enthalpy of the gaseous molecule A*

84

TAKA0 KWAX

1€76z(iA) 1

arid

of the adsorhetl set b,(o) of particles as

From equations (12), (13), and (14) we have the following relation, neglecting the cwntri\mt,ion from higher vibrational levels of the gaseous molecule :

where N a is tlhe Avogadro number. Substituting NAtsin equations (11) and (10) from (15) we have

or assuming t,hnt hv

>> 1rT i n q6,(")and hence that

me have the following pxpression when no

<< 1

The above equations give a relation between 8, AE,and N for a molecule which splits into n statistirally independent parts in the adsorbed state, and afford a method of determining n by putting into equation (18) the observed differential heat of adsorption by a sparsely covered surface. e is being determined from the adsorbed quantity and the total measured surface area of' the adsorbent. 5 . Chemisorbed States of

Di-and Triatomic Molecules

The existence of dissociated atoms has been demonstrated for hydrogen or nitrogen on the surface of metallic catalysts by means of isotopic

85

G E N E R A L ASPECTS OF CHEMISORPTION AND CATALYSIS

exchange reactions. We have, however, only a little knowledge about the adsorbed state of other molecules such as carbon monoxide and carbon dioxide. As regards the former, Craxford (47) concluded from his kinetic' investigation of the Fischer-Tropsch synthesis th a t CO remains undissociated when adsorbed on the surface of cobalt whereas Kodama and his co-workers (48) suggested dissociative adsorption on the basis of data obtained by kinetic analysis. Beeck et al. (14) measured the ratios of the adsorbed amounts of various gases t o that of adsorbed carbon monoxide on a n evaporated nickel film a t 0.1 mm. Hg pressure with the following result: CO/CO 1

Irl/co 35

s>/CO

C J I4/CO

'i

1;

O/CO 2

Since according to these authors a saturated monolayer is formed of all the adsorbed gases, under their experimental conditions, the ratios found seem t o suggest that each carbon monoxide molecule and also each dissociated hydrogen atom occupies one adsorption site, that carbon monoxide is adsorbed without dissociation, and further that nitrogen occupies two adsorption sites and ethylene four sites. A similar experiment conducted by Rideal and Trapnell(49) with carbon monoxide, hydrogen, and oxygen on a n evaporated tungsten film confirms Beeck's results except that for Oz/CO was found to be f $ instead of 2 . It is not yet clear, however, from these investigations whether any one of these gases is dissociated into statistically independent parts or not. On investigating the adsorbed state of carbon dioxide with particular reference t o the 0 - N relation of equation (18), Kwan and Fujita (50) suggest t ha t carbon dioxide when adsorbed on nickel dissociates into three statistically independent parts, or completely into its three atoms, whereas i t splits into two parts on copper ferrite and remains undissociated on ferric oxide. The adsorption isotherm plotted as the logarithm of adsorbed amounts of CO, against the logarithms of equilibrium pressures is shown, for these three adsorbents, i n Fig. 7. The partial dissociation on copper ferrite would presumably be a split into carbon monoxide and one oxygen atom, i.e., COz F1 CO 0. It was similarly demonstrated t ha t hydrogen (15,16) as well as nitrogen (27) dissociate into their atoms on the surface of metallic catalysts. According t o Matsusita (46) the adsorption of ethylene on reduced nickel proceeds in proportion to the 0.44th power of the equilibrium pressure on a sparsely covered surface as shown in Fig. 6. This may indicate dissociative adsorption of the ethylene molecule into two parts although the deviation from the square root type of adsorption (0.5th power) remains unexplained. The absence of the exchange reaction of deuterium

+

86

T A K A 0 KWAN

between ethylene and deutero-ethylene on a heated nickel filament as shown by Conn and Twigg (51), however, excludes the possibility of such type of dissociation. The adsorbed state of carbon monoxide on a platinum surface was determined by Kwan (52) as follows: using the differential heat of adsorption, the covered surface fraction was calculated by means of equation (I 8 ) assuming n either 1 or 2 . These values were compared with those derived from the adsorbed quantity and the surface area of the adsorbent assuming that 10l6 atoms per square centimeter of platinum surface are available NI

Fez03

-05-

-15-

-20-

400°C

-40

-20

0

-40

-20

-25-

0

-40

-20

0

log P. mrn. Hg

FIG.7 . Adsorption isotherms of COZ on reduced nickel (1 g., 20 m2),copper ferrite (4.2 g . , 210 m2), and ferric oxide (5 g., 110 m*) by Kwan and Fujita (50).

for adsorbing either one carbon monoxide molecule (n = 1) or one of the dissociated atoms ( n = 2 ) of this molecule. It was found that Bcalo. derived from equation (18) and Bobs. calculated from the surface area of the adsorbent and the adsorbed quantity agreed better for the case n = 2 . This result is compatible with the recent work of Eischens and Webb (53), who showed that oxygen exchange between COI3 and C 0 l 6 occurs on the surface of a reduced iron sample. A similar investigation of carbon dioxide adsorption on reduced nickel leads to the conclusion th at the best agreement is obtained under the assumption t ha t n is 3, i.e., that the COz molecule completely dissociates on adsorption (45). On the other hand, agreement is obtained for carbon dioxide adsorption on copper ferrite for n = 2 . On ferric oxide the assumption of n = I, tentatively suggested by the nearly direct proportionality

GENERAL ASPECTS OF CHEMISORPTION AND CATALYSIS

87

of the adsorbed quantities to the equilibrium pressures, leads to the marked discrepancy that 6 0 b s . / 6 c ~ i c , is about lo2, as shown in Fig. 8. The entire theoretical treatment on page 83 Section I11 was based on the assumption that the adsorbed particles are a t their lowest vibrational level and are devoid of any translational or rotational degrees of freedom. If this assumption is abandoned, #tala. a In q

6 N

can assume much larger values. The factor q6i(a)eT-s-Tin equation (17) is generally greater than unity and approaches unity as hv becomes much greater than k T . For instance, the calculation of this factor for an adsorbed hydrogen atom gives 7 a t 500", 5 a t 200", and 2 a t 20°C. on the basis of the calculated frequencies of the three vibrational modes of motion, 417, 479, and 1900 cm.-l (30). Ni

CuO'Fe,On

I

p 1 X

Q

A 2 4 6 8 1 0

d P,rnm. Hgx lo3

FIG.8. Adsorption isotherms of CO, on reduced nickel (7' = 200"C., A t = 22 kcal./mole), copper ferrite (1'= 300"C., Ae = 31 kcal./mole), and ferric oxide ( T = 300°C., Ae = 29.5 kcal./mole). Investigation of adsorption isotherms over a wide range of temperatures would reveal the extent of the validity of this treatment. In this connection the close agreement between Boslo. and Bobs., as shown in Fig. 13 for hydrogen adsorption on reduced cobalt at 300", 200", and lOO"C., might suggest its validity, a t least for this case of hydrogen adsorption. On the other hand a somewhat smaller value of Beaic. compared with in the case of carbon dioxide adsorption on nickelmay be probably attributed to the over-simplification of the mathematical treatment. The marked discrepancy in the case of carbon dioxide adsorption on ferric oxide seems likely to be due not only to the existence of excited vibrational levels but perhaps also t o rotational degrees of freedom in the adsorbed state.

I n their studies of carbon dioxide adsorption on copper oxide Garner and his associates (54) assume the formation of a "carbonate ion" held by forces of electrovalency t o the oxide surface. A decision for or against this concept may be obtainable by a study of the exchange reaction, for

88

TAKA0 KWAN

iiihtauce, l)etwceii carbon dioxide and a cwpper oxide contaiiiiiig 0'" Vainshtein (55) has objected to Garner's view by pointing out th a t no exchange reaction occurs betwren carbon dioxide and manganese dioxide, arid that varbon dioxide is formed from carbon monoxide and oxygen in the presence of clopper oxide, without any exchange reaction between the oxygen and the copper oxide. Garner's view conflicts, moreover, with the conclusion arrived a t by the present author that carbon dioxide dissociates into two parts on copper ferrite. 0'. Adsorption Heats o j Hydroyen on Metallic Surfaces

Kumerous determinations of differential heats of adsorption have been made, both by direct calorimetric aiid by indirect thermodynamical evaluations. Generally speaking, these determinations cannot be regarded as 'highly accurate. For the calorimetric measurement, difficulties are encountered due to the frequently low heat conductivity of the adsorbent a t a low pressure, leading to ahnormal heat curves as shown frequently in the earlier literature. This difficulty has been successlully overcome by Itoberts' technique and more rerently by that of I3eeck ct al. (56). lioberts used for his determinations of the adsorption heats of hydrogen on a tungsten wire the increase in electric resistance of the wire, aiid 13eec.k measured the adsorption heats evolved on a metallic* film deposited inside a thin glass tube, by means of a platinum resistance thermometer wound around the tube. Another reliable way for determining differential heats of adsorption is the indirect method based on evaluating, by Clausius-Clapayron, these heats from a family of reliable adsorption isotherms. Frankenburg, Davis, Kwan, and others have determined the differential heats of adsorption for hydrogen or nitrogen on various metallic catalysts. using this indirect procedure. A remarkable fact observed by the workers who used the indirect method is t ha t the differential adsorption heats remain nearly constant within a certain range of low coverage of the surfaces. For example, the adsorption heat of hydrogen on tungsten (15) was found to be independent of the adsorbed quantities below the coverage of 0.8% of saturated adsorption. Adsorption heats of hydrogen, carbon dioxide, and ethylene (16,45,46) on reduced nickel show the bame phenomenon. It is easily understandable that such initial values of constant differential adsorption heats might be very difficult to detect by direct calorimetry, particularly in the case of the system tungsten-hydrogen. One should keep in mind that adsorption heats as evaluated by direct calorimetric measurements are not truly differential heats but rather integrals of the differential heats over certain fractions of surface coverages.

GENERAL ASPECTS O F CHEMISORPTION AND CATALYSIS

89

The purity of metallic surfaces may undoubtedly be enhanced by “flashing” or “evaporating” the adsorbing metal, but this method is not applicable to the oxide type of catalysts. Differential adsorption heats on various oxide catalysts have been determined with the indirect method by the author and his co-workers. As a rule, the differential heats of adsorption found in this way proved larger than any of the heats of adsorption formerly reported on the basis of calorimetric measurements. h detailed discussion will now be given of the differential heats of adsorption of hydrogen on several metallic catalysts. Nickel. Adsorption heats of hydrogen on an evaporated nickel film as measured by Beeck (28) and on reduced nickel found by Eucken (18) and

Fraction of surface covered 0

9. Adsorption heats of H2 on nickel found by Beeck (evaporated film), Eucken (reduced nickel), Kwan el al. (reduced nickel), and Srhuit (reduced nickel supported on silica). 1’IC.

by Kwan (16) are shown in Fig. 9. The heat values of the first two workers were determined by the direct method whereas Kwan’s values were determined indirectly. No drastic decrease of the differential heats of adsorption with increasing coverage was observed by any of these authors. I n a n earlier investigation, Fryling (57) had observed a rapid fall of the differential heats, starting in the range of very low surface coverage. The three heat curves of Beeck, Eucken, and Kwan disagree in the values obtained for a sparsely covered surface. This might perhaps he attributed t o different states of purity of the nickel surfaces used by these workers. Since Eucken obtained his specimen by a short reduction a t 28OoC., i t may have contained unreduced oxide. According t o a theoretital investigation of the heat curve by Waitg (58) based on the 13ethe-Peierls approximation assuming mutual repulsion of the adsorbed particles, the heat curve should start flat at, small coverage and then

90

TAKA0 KWAN

decrease with increasing coverage, as actually found by Kwan. For exhaustively reduced nickel, supported on silica, an approximately linear heat curve has been obtained more recently by Schuit (59). Extrapolated to low surface coverings, the adsorption heats found by Schuit are close to the initial differential heats evaluated by Kwan. According to Beeck the sintering of an evaporated nickel film causes a lowering of the heat curve, as shown by the dotted curve of Fig. 9, which then approaches tha t found for reduced massive nickel. The discrepancy between this curve and t ha t of Kwan a t higher coverages might be explained by

Fraction of surface covered 0

FIG.10. Adsorption heats of H, on reduced cobalt.

assuming that the coverages plotted by Beeck are too high, a s pointed out by Tiley (60) and others. Cobalt. So far as the author is aware, the differential heats of adsorption for hydrogen on cobalt have not yet been reported. Data derived from adsorption isotherms measured by Kwan (16) are shown in Fig. 10. The heat coverage curve shows constant initial 19 kcal./mole 132 values of about within the first approximately 1% of coverage. Iron. Fig. 11 shows the adsorption heats of hydrogen on reduced iron and on a promoted iron catalyst as used by I. G. Farben for ammonia synthesis (61). The adsorption heats on both catalysts coincide, decreasing regularly with increasing coverage from 17.5 kcal. a t e = 0.01 to about 5 kcal. a t 8 = 0.1. It is interesting to note th a t the promoting oxides contained in the industrial iron catalyst do not affect the adsorption heat of hydrogen on the metal. Morozov’s values (62) found for reduced iron a t high hydrogen pressures are compatible with the heats of

GENERAL ASPECTS O F CHEMISORPTION AND CATALYSIS

91

adsorption observed in a lower pressure range. Contrary to all these data, the heats of hydrogen adsorption on evaporated iron films reported by Beeck (28) are appreciably higher, and are claimed to remain constant a t about 30 kcal./mole hydrogen for allcoverages ranging from very small values t o complete coverage. This discrepancy between the hydrogen adsorption on a n evaporated iron film and on a bulk sample of reduced iron is surprising, and calls for further investigation. Copper. Ward (21) found the adsorption heat of hydrogen on reduced copper t o be 32.9 kcal./mole Hz, and independent of the concentration of the adsorbed hydrogen. Baking-out ” of the adsorbent, however, ((

Fraction of surface covered, 0

FIG.11. Adsorption heats of H, on reduced iron (open circles), and on I.G. ammonia synthesis iron (double circles). Full circles indicate Morozov’s data.

resulted in lower values of the heats of adsorption, until, on repeated pretreatment of the copper, a final value of 9 kcal./mole Hz was reached. This decrease of the heats of adsorption, caused by the ‘(baking-out ” of the adsorbent, can hardly be reconciled with the measurements of the author, who found adsorption heats of hydrogen on a thoroughly outgassed copper specimen to exceed 35 kcal./mole (evaluated by equation (18)). This is the largest so far reported for hydrogen on copper, and is compatible with the value of 47 kcal./mole Hzderived from spectroscopic data for the dissociation of CuH. Incidentally, Nagasako and Izu (63) determined the adsorption isotherm of hydrogen on Raney-Cu (8 :92) and found t ha t the adsorption heat decreases steadily from a value of 26 kcal./mole. The adsorption heat extrapolated to a bare surface comes close to the value of 35 kcal./mole, in agreement with that estimated for a pure copper specimen as outlined above. Platinum. The adsorption heat of hydrogen on platinum derived from adsorption isotherms a t 280” and a t 300°C. by Kwan was found t o remain

92

T A K A 0 KWAN

constant a t 18 kcal./mole €I2 over the entire range of measurement, ill agreemelit with Maxted and Hassid (64), who found it constant at about 16 kcal./mole H2 by the direct calorimetric method. Kwan’s findings, however, do not agree with those of Kistiakowsky et al. (65), who reported a rapid fall of the adsorption heats as shown in Fig. 12. The platinum was prepared by all these authors from vhloroplatinic acid by reducing it with an alkaline formalin solution in the usual way. After this, the metal was heated by Kwan in an atmosphere of hydrogen at 350°C. for an extended period, whereas Maxted kept the temperatures below

cm ’/g. adsorbed gas, S.T.P

FIG. 12. Adsorption heath of 111 on platinuiii.

100°C. in order t o avoid any “stru(%umlchanges.” The somewhat smaller value obtained by the latter author might be intcrpreted by taking into consideration the term T $ I Z l ’ for the calculation of the adsorption heat (equation (15)). Thus, it appears th at the difference in heat treatment, employed by both workers had no influence upon the nature of the platinum surface.

IV. THEXATUREO F

THE

CATALYST SURFACE

1 . Homogeneity of Metallic Surfaces for Chemisorption

During the past several years Beeck and his associates (28) have determined adsorption heats of hydrogen on a variety of evaporated

GENEHAL ASPECTS OF CHEMISORPTION AND CATALYSIS

93

metal films and found them as a whole to decrease gradually with increasing coverage and to be nearly flat a t a low temperature. This undoubtedly affords a serious criticism of the original view of “active centers” which, it was assumed, account for the observed rapid decrease in adsorption heats with increasing coverage. To dissolve this discrepancy, H. S. Taylor proposed in his memorial lecture t o the Faraday Society in 1950 the interpretation that technical catalysts, which are mostly oxidized at their surfaces and frequently contain added ingredients, possess active centers regardless of whether highly purified metals such as Heeck’s metal films have such centers, or not. The shapes of the curves of the adsorption heats of hydrogen on reduced nickel as determined by Eucken, by Kwan, and also b y Schuit, as has been pointed out above, are roughly in accordance with the heat curve found for an evaporated nickel film, although the massive adsorbents used by the three investigators differed in such properties as surface areas. The decrease in the adsorption heats from 8 = 0 to 8 = 1 was found t o be 7-10 kcal. per g.-atom in all three cases. The calculation of the repulsive potential of hydrogen atoms ou nickel, taking 35% of the Morse potential, gives a value of 5.65 kcal. per g.-atom for a single atom n-ith two neighbors a t a separation of 2.49 A. (30). Remembering th a t more than two neighbors are present 0x1 a fully covered surface, for instance, 011 the (111) or the (110) plane of nickel, one might conclude that the observed decline of the heats of adsorption is due to the mutual repulsion between adsorbed hydrogen attoms on a homogeneous surface, rather than to surface heterogeneity. This view is supported by the statistical-mechanical interpretation of the adsorption isotherm a t such low coverages that the differential heat of adsorption of hydrogen is nearly constant a t 26 kcal./mole. From the agreement between the surface fraction calculated by means of cquation (18) with n = 2, and the experimentally found value cubs, for the hydrogen adsorption on nickel, Kwan et al. concluded that the surface of reduced nickel is homogeneous, or that every surface element is equally available for the chemisorption of hydrogen. Roginskii and Keier (GG) recently suggested means of differentiating between adsorption effects caused b y interaction of adsorbed particles with one another, and the effects caused by a heterogeneity of the adsorbent surface. They used a gas in two isotopic forms. Two samples of this gas, one of them being radioactive, were added successively to the catalyst and then, after the system was evacuated, the radioactive content of the desorbing gas was determined. If the surface were heterogeneous, the fraction added first should be removed last from the surface, while if the surface were homogeneous, the two isotopic forms should be re-

94

TAKA0 KWAN

moved in the same proportion as present on the surface. The experiment conducted by these workers on the adsorption of a hydrogen-deuterium mixture on the surface of reduced nickel led them to the conclusion th a t the surface of this nickel specimen was heterogeneous. The covered fraction of the surface was, in this experiment, 5 to 10% of the total surface and contains, according t o Roginskii and Keier, active sites differing in their adsorption heats for hydrogen and activation energies for the desorption of hydrogen. This coriclusion is obviously incompatible with Kwan's observations on the system Ni-€12 concerning the relation between adsorption heats and coverage. It can be correlated, however, to earlier experimental data, Ni

1

co

Pt

0.10

/

,

0.2

0.4

,

,

06

0.8

@
FIG.13. Adsorption isotherm of H2 on reduced nickel, cobalt, and platinum. The plot of Ocalo. and &bs. against the square root of cquilibrium pressure. Reproduced from Kwan (16).

such as the initial convex curvature of the adsorption heats as reported by Fryling. T o explain this discrepancy, it has t o be emphasized th a t the nickel specimens used by Roginskii and Keier, and b y Fryling, were not as pure as that of Kwan. This explanation is supported by recent results obtained by Schuit (67) with an exhaustively reduced nickel-silica catalyst using the Roginskii-Keier method described above. According t o Schuit, only an equilibrium mixture of H2, HD, and D2 could be desorbed from the catalyst, although the method by which H2 and D2 were adsorbed, the order of their admittance, and the technique of desorption were varied in various respects. The surface nature of reduced cobalt and platinum was also investigated by Kwan el al. by means of the statistical-mechanical interpretation of the adsorption isotherm of hydrogen, and the known surface areas

GENERAL ASPECTS OF CHEMISORPTION AND CATALYSIS

95

of the specimens. Again, the conclusion was drawn th a t the surface of these metals is homogeneous, 8cslc.being found t o be in satisfactory agreement with OoLs This is shown in Fig. 13. As for the above examples, the adsorption of hydrogen on reduced copper can also be regarded as occurring almost equally over the whole available surface inasmuch as Kwan and Kujirai ( 3 2 ) showed the active sites of the copper specimen to be identical with lattice points in studying the adsorption rate on the basis of the absolute rate theory. Consequently, the author has reached the conclusion that the surface of a number of reduced metallic catalysts is of a homogeneous nature for chemisorption as long as they are prepared by a very careful reduction and are kept free from any poisoning materials.

2. Heterogeneity of Metallic Surfaces for Chemisorption The differential heat of adsorption of hydrogen on reduced iron found by Kwan et al. decreases regularly with increasing coverage even a t low coverage in marked contrast to the findings of the same authors for nickel, cobalt, and platinum. Kwan et al. evaluated Oealo. for iron in the same way as for nickel by introducing the observed adsorption heat 17.5 kcal./mole a t a sparsely covered surface into equation (18) with n = 2. The result of the calculation was th at Boalo. is found to be 70 times as large as Oobs., the latter value being derived from the B.E.T. area of the adsorbent and from the adsorbed quantity. This discrepancy is in contrast to the results obtained for nickel and cobalt for which Ocnlo. and cobs. were found toagree reasonably well. For the latter two metals, this agreement appears to prove the homogeneity of their surfaces. Analyzing also Frankenburg’s data for the tungsten-hydrogen system, Kwan found a n appreciable discrepancy between ecnlo.and gobs.as for iron, indicating that the tungsten powders used by Frankenburg, as well as the iron specimens studied by Kwan, had heterogeneous surfaces, or that two or more parts characterized by different adsorption heats are present on the surfaces of these iron and tungsten specimens. The flat portion of the heat curve of hydrogen for a sparsely covered surface of tungsten determined carefully by Frankenburg might be indicative of the presence of one of such parts. Assuming, for simplicity’s sake, that two parts of different surface types are exposed on the surface which is measured as the total B.E.T. to may be derived for every one of these parts. area, the ratio of eoLB. The directly observable adsorption heat, Ae, may generally be expressed by the weighted mean of the adsorption heat on the part 1 or Ael and t ha t on the part 2 or Aez (61). If the adsorption of hydrogen on a sparsely covered surface does not take place substantially on the part 2 , or 0 2 = 0, the observed heat of adsorption, Ae, should equal AeI. The

90

TAKAO RWAN

covered fraction of surface, Ooalo , detlermiried by statistical mechanics on the basis of the observed adsorption hrat,, A€, is i n that case identical with 01, i.e., Oc,,,o, = O1. On the other hand, Oobs is givtu OIL the basis oi the assumption that the B.E.T. area is the sum of the arcas of parts 1 and 2 and that e2 = 0, as :

(19)

where N1 and Nz are the numbers of sites for hydrogen, respectively, on the parts 1 and 2. The ratio 8obs./8oalr. or x is now given by

It follows that the ratio Oobs./Ocalc may be taken as giving directly the fraction of part 1 characterized by higher adsorption heat of hydrogen. Tahle I1 shows the ratio x for reduced iron and for tungsten, respectively. TAB1,E I1 7'hr ('alculatrd nnd Observed Practions of Su,:face Covered for Hydroqcn on Rediicrtl Iron at 50°C. and on Yungsten at 600°C. (Total pressure was 0.001 mm. Hg.)

Fe, Ae Bobs.

0.0028

=

W, Ae

17.5 kcal./mole

eoalc.

0.26

X

Ooba.

0.011

0.0028

=

48 kcal./molc

eoalo.

0.22

X

0.014

It might be interesting to analyze the adsorption data of Davis for nitrogen on tungsten, applying the method described above. The tungsten powder used by Davis was prepared in the same manner as Frankenburg's. The adsorption heat of nitrogen derived from the isotherms is almost constant (78 kcal./mole) until it attains 10% of the saturation value. Using the adsorption heat, Ocalo was evaluated in the same way as for hydrogen, and compared with &,be. The result is shown in Table 111. TABLE I11 The Pnlcdated and Observed Fraction of S u r f w e Covered for Nitrogen on 7'iinCqslarr. 760°C. (At = 78 kcal./mole; P = 0.0001 mm. Hg.) X

Bobs.'"

0.015

trl

0.14

0.11 ~~~~

%b,.

was obtained by assuming one adsorbed nitrogen atom for one tiingsten atom in the surface

aa suggested by Davis.

GENERAL ASPECTS O F CHEMISORPTION AND CATALYSIS

97

As shown in Table 111, the fraction of the active part, x, for the adsorption of nitrogen is appreciably greater than th a t for hydrogen. If both gases would behave identically on the surface of tungsten or, in other words, nitrogen were adsorbed preferentially on the same surface fraction that adsorbs hydrogen more efficiently, x ought to be the same for both gases, particularly since Frankenburg and Davis used the same powder specimen (W No. 9799) for their adsorption studies. Nichols and Herring (68) have recently studied the surface of tungsten by means of thermionic emission and found that various kinds of crystal planes are exposed, for instance, 2.2% of ( l l l ) , 12% of (110), etc. If this remarkable observation should be confirmed it would be of great value for clarifying the character of the heterogeneous surface shown by tungsten for the chemisorption of hydrogen and nitrogen. I t seems likely that the heterogeneity of the tungsten surface is partly responsible for the discrepancy between the calculated and observed rates of heterogeneous reactions obscrved on this material. Eley (69) calculated the rate of parahydrogcn conversion on the surface of platinum, palladium, nickcl, and tungsten assuming that there are 10l6 reaction sites available per squarc centimeter of the catalyst surface, all equally active. The discrepancy between the reaction rate calculated on this basis and the observed rate is comparatively greater for tungsten than for other metals. Similar deviations between the calculated and observed rates of adsorption and desorption were found for the system tungsten-nitrogen (27).

3. The Iron Catalyst for Ammonia Synthesis

Brunauer and Emmett (70) concluded from a n adsorption study of carbon monoxide, carbon dioxide, and nitrogen on doubly promoted iron catalysts that 1 wt. % of potassium oxide contained in the catalyst covers more than 50% of its total surface. Additional observations in agreement with these findings were made by Matsui (71). I n spite of this complex nature of the surface of the promoted iron catalyst, the adsorption heat of hydrogen on this catalyst is, as previously shown by Kwan et al., identical with th at on pure iron, suggesting th a t the promoters do not influence the adsorption of hydrogen. The adsorbed quantity of hydrogen per unit area of the I.G. catalyst is, however, five times as large as that on pure iron. The statistical-mechanical evaluation of the adsorption isotherms of hydrogen for both the I.G. catalyst and pure iron leads to the conclusion that the “active part” with respect t o hydrogen adsorption is five times greater on the I.G. catalyst than on unpromoted iron. The increase of the active part of the promoted iron catalyst can be correlated to the fact that the surface area of the promoted oxidic catalyst increases with progressing reduction, in marked contrast with that of unpromoted magnetite (61,70). The enhanced activity of the promoted

98

TAKA0 KWAN

iron catalyst compared with that of pure iron might be interpreted as an increase of the “active part” of the surface. Matsui has found for a n I.G. catalyst that the aluminum oxide at its surface is present in the form of ferrous aluminate which covers parts of the metal surface, consisting of a-iron. The same author prepared a small single crystal of iron coated by an oxide film arid studied its structure by the transmission method of electron diffraction. The pattern shows, Fig. 14, that the (111) plane of magnetite is produced parallel t o the (111) plane and also to the (110) plane of iron, suggesting th a t with the

Fro. 14. Electron-diRraction patterns of singlc-crystal iron coated with magnetite. Incident beam is pcrpendicular to the (110) plane (a) and the (111) plane (0) of iron. Reproduced from Matsui (71).

I.G. catalyst a similar orientation may exist between the crystals of

a-iron and the spinel ferrous aluminate. Yamaguti and Nakayama (172) conclude from electron-diff raction studies of a chromium steel covered by decomposition products of ammonia that nitrogen is chemisorbed on the (110) plane of iron, Fig. 15. If this is the case and, moreover, if the (111) plane is active for the hydrogen adsorption (73), no competition for adsorption would occur between hydrogen and nitrogen a t low degrees of adsorption. However, unpublished experiments of the present author and Takeuti show that the rate of hydrogen adsorption on the surface of an iron-ammonia catalyst is appreciably affected even when the coverage by ammonia decomposition products is only 1 or 2%. According to Brunauer and Emmett (73), the surface of the doubly promoted iron catalyst contains two types of sites corresponding, respectively, t o “Type A ” and “Type B ” of hydrogen adsorption (first arid

G E N E R A L ASPECTS OF CHEMISORPTION AND CATALYSIS

99

second maxima of the adsorption isobar in Fig. 4). The first type is, according t o these authors, the outermost plane of the iron crystal, appearing as any of the (110), (loo), and (111) planes. The second one comprises lower-lying planes, exposed as (100) and (111) planes. On this basis, Brunauer and Emmett proposed an explanation for the observed increase in the ratio of “Type A ” to “ Type €3 ” of hydrogen adsorption that occurs on sintering of the catalyst. The presence of various active parts on the surface of the ironammonia catalyst was demonstrated by Emmett and Kummer (74) by means of adsorbing radioactive and nonradioactive samples of carbon

FIG.15. xitrogen atoms adsorbed on the (110) surface of iron (schematic representatwn). The dlstanres given are in A-units. Reproduced from Yamaguti and Nakayama (72)

monoxide. The exchange reac*tioii between the two fractions of added carbon monoxide occurs very fast even at - 190°C.

4. Oxzde (’atalysts Heterogeneity of the surfaces of oxide catalysts such as chromium oxide, zinc oxide, and zinc-chromium oxide has been postulated by H. S. Taylor (75) on the basis of adsorption studies. I n the author’s view, Taylor’s experimental observations may be also explained without assuming a heterogeneous character of the oxide surfaces. Kwan and l‘ujita (76) investigated the adsorption of carbon dioxide on a copper ferrite catalyst of the composition CuO.FezOj,prepared by the usual precipitstioii method and ignited a t 500°C. in VUCZLO for about one month. Special precautions were made in the adsorption studies for attaining true adsorption equilibria. The adsorption isotherm for the OW

100

T AKA0 KWAN

range of adsorption is shown in Fig. 7. Taking the heat of adsorption of 31 kcal./mole, as derived from the isotherm, the covered fraction of the surface was calculated by means of equation (18) with n = 2, and compared with t ha t derived from the adsorbed quantity and the B.E.T. area of the adsorbent. Close agreement between the values of 0 can be attained if one assumes the presence of 5 X 10'4 metal ions on the unit surface area of the copper ferrite, in agreement with the spinel-type structure found by x-ray investigation. and Ooba. are plotted against the equilibrium pressures in Fig. 8. This numerical agreement suggests th a t the surface of copper ferrite is homogeneous.

P,mm.

of

Hg * l o 3

FIG.1G. Adsorption isothrrins of CO, on copper chromitc (1 g., 43 m.*). The plot against equilibrium pressure. T = 200"C., A€ = 21.4 kcal./mole. and

Bcaio.

Similar studies with copper chromite, CuO.CrzOa, zinc chromite, ZnOCrz03, and magnetite, Fe0.Fe203, were carried out by the same workers (76), leading again to the conclusion that the surfaces of these mixed oxides are homogeneous. For these systems, the plots of 8eale. against the equilibrium pressure were found to coincide closely with those of cob, as shown in Figs. 16 and 17. The investigation of zinc oxide-zinc chromite of the composition ZnO-ZnOCr20a(77), which reveals an x-ray pattern of zinc oxide plus a spinel structure, led to the result that eohsdoes not agree with any of the values of 8Lalo.evaluated for n = 1, 2, and 3 (Fig. 18). The differential heat of adsorption of carbon dioxide on this catalyst was found to be 43 kcal./ mole, i.e., equal to that on zinc oxide alone but by far greater than th a t

GENERAL ASPECTS O F CHEMISORPTION AND CATALYSIS

I

101

n=3

10 -

B 0

5-

0

5 P,mm. Hg I lo2

10

FIG.17. Adsorption isotherms of COZ on zinc chromite (3.5 g., 103 m.". The plot of Ocalo. and Oobs. against equilibrium pressure. T = lOO"C., As = 16 kcal./mole.

P,mm. Hg x lo3

PIG. 18. Adsorption isotherms of COz on zinc oxide-zinc chromite (4g . , 129 m.". The plot of eoalc.and &ba. against equilibrium pressure. !!' = 4OO0C., AE = 43 kcal./ mole.

102

T A K A 0 KWAN

on zinc chromite. This discrepancy might indicate a heterogeneous surface of this particular catalyst. For several oxide catalysts a chemical heterogeneity, caused by deficiencies or excesses of the oxygen content, may be important. Thus, i t was shown by Garner and his associates (54) and by ICinuyama and the present author that copper oxide shows a very low adsorption of carbon dioxide after outgassing in a high vacuum, and that the original adsorbing power for C O r is restored on oxidation of the outgassed specimen. With catalysts consisting of an oxide mixture, a formation of new compounds can occasionally occur on heating. Such effects are indicated by observations of Huttig (78) and by Ward and Erchak (79). Such specifically prepared catalysts may have heterogeneous surfaces, of which a limited part only may be decisive for their catalytic activity. 5 . Poisoning of Catalgsts

Additional information about the nature of catalyzing surfaces can be derived from suitably devised poisoning experiments in which the weakening of the catalyst activity is measured as a function of the amounts of poisons adsorbed at the catalyst surface. If poisoning molerules occupy a progressively increasing fraction of a homogeneous surface, the rate of catalytic reaction should decrease in proportion to the quantities of the adsorbed poison. On a heterogeneous surface, however, the adsorbed poison will affect the catalyst activity according to the extent to which it occupies selectively the active fractions of the catalytic surface and will have little influence on the catalytic reaction when it is adsorbed on less active fractions. Thus, a given catalyst can be strongly poisoned by one kind of poison while retaining its activity in the presence of another poisoning material. Evidence for both homogerieous and heterogeneous catalyzing surfaces has been attained by this criterion. Here some typical investigations of this kind will be referred to, in an attempt t o correlate them with the adsorption studies described above. Indications for the homogeneous nature of the surfaces of some catalysts have been obtained by Maxted and associates (80) in a series of poisoning experiments in which the rate of the hydrogenation of various organic substances over platinum, palladium, and nickel decwased linearly with increasing amounts of adsorbed poisons, according to 17,

=

V(1

- a(")

(21 1

lier re V , is the rate of hydrogenation for the (+oncentration( " of poison at the catalyst surface, V the rate in absenc*enf the poison, ant1 LY :I conHtant, the so-called sensitivity coefficient. It can 1 ) srcti ~ from rquatioir (21) that llttutcd's sensitivity ooefficient

103

GENERAL ASPECTS O F CHEMISORPTION AND CATALYSIS

(Y is the reciprocal of the quantity of poison which would completely suppress the activity of the catalyst. The values of 1/a calculated from the data of Maxted et al., on the hydrogenation of cyclohexene in the presence of unsintered and sintered platinum black poisoned b y means of methyl sulfide (81), are listed in the last column of Table IV.

TABLE IV The “Limitingl 1 Quantityof Poison or 1/ a Catalyst Unsintered Pt Sintered Pt

wt., g. 0.05 0.05

Surface Area, m2

Sensitivity Coeff., a,mole-‘

0.59 0.40

0.459 X lo6 0.680 X lo6

1/ff,

molecules em. 2.24 2.22

x 1014 x 1014

As shown in Table IV about 2 X 1014molecules of methyl sulfide per square centimeter of platinum surface are required to stop the hydrogenation reaction. According to Maxted and Evans (82) the relative toxicity or the sensitivity coefficient of methyl sulfide referred t o that of hydrogen sulfide is 7.1. The value ] / a of hydrogen sulfide may be hence obtained as 2.24 X loL4X 7.1 = 1.6 X lo1‘. If a single hydrogen sulfide molecule occupies one active site of the hydrogenation catalyst, ] / a may be regarded as the number of active sites on the platinum surface. The value of 1.6 X l o t 5is of the same magnitude as the number of lattice points per square centimeter of the metal surface. This leads t o the conclusion that the surface of platinum is catalytically homogeneous not only in part but entirely. The striking fact is that sintering causes no variation of the catalyt,ic activity of the platinum catalyst; i.e., the number of active sites is found to be the same for the sintered and the unsintered catalysts, in accordance with the conclusion derived from the adsorption measurements. A similar linear decrease in the rate of hydrogenation observed by the same workers on progressively poisoned nickel is also indicative of homogeneity of the surface. However, since the total surface area of the nickel samples employed in this study was not measured, it remains undecided whether the homogeneity applies to the total surface or to merely a part of the surface. Of the numerous poisoning experiments that indicate heterogeneity of catalyst surfaces, reference may be made first to a study of “selective” poisoning made by Russell et al. (83,84). Russell and Ghering found th a t the rate of the hydrogenation of ethylene over reduced copper was not markedly affected by nitrous oxide but that the rate of ethane formation decreased rapidly as soon as the adsorbed nitrous oxide was decomposed,

104

T A K A 0 KWAN

with the formation of an oxide film on the copper surface. They concluded that the parts of the surface active for ethane formation are the most active ones for the decomposition of nitrous oxide. Poisoning of copper that had been sintered a t 400°C. weakened its hydrogenating activity to a lesser extent than poisoning of unsintered copper, contradictory t o the case of platinum, mentioned above. A plot of the activity against the amounts of oxygen adsorbed per gram of the catalyst, reveals extensive horizontal portions especially on the sintered catalyst in spite of its probably diminished surface area. This led Russell and Ghering t o the conclusion t ha t sintering favors the development of surface areas th a t catalyze the decomposition of nitrous oxide, with the simultaneous formation of a surface oxide of copper. A similar phenomenon was observed by Russell and Loebenstein with reduced nickel supported on quartz. Here, the activity of the catalyst for the hydrogenation of carbon dioxide was markedly decreased by its treatment with methane, while it was only slightly weakened for the hydrogenation of nitrous oxide. On the basis of these observations Russell et al. suggest that two types of hydrogenations occur on mutually exclusive areas of the nickel surface. At first sight this concept is not compatible with the observations of Maxted et al. on the poisoning of nickel catalysts by sulfide, or with the conclusions of Kwan and of Schuit based on chemisorption studies for the system hydrogennickel. The results may, however, be reconciled by assuming th a t the reduction of nitrous oxide is catalyzed on a nickel surface covered with oxygen, while the reduction of carbon dioxide proceeds on the metallic surface. Unless the oxidized surface is playing a specific role in the nitrous oxide reduction, the finding of Russell et al. would indicate the existence of several types of nickel crystallites with different catalytic activities. The well-known poisoning of the iron catalyst, used in ammonia synthesis, by minute amounts of water vapor or oxygen seems t o be compatible with chemisorption measurements. According to Almquist and Black (85) only 10 to 15% poisoning of the total B.E.T. surface of this catalyst causes a decrease of its catalytic activity b y about 70%. Since only a part of the catalyst surface actively chemisorbs hydrogen, and, probably, nitrogen, the area active for the formation of ammonia can also be expected to be a mere fraction of the total surface.

V. STUDIESOF SINGLE CRYSTALSOF METALSIN ADSORPTIONAND CATALYSIS

It has been shown in Section IV that the surface of a reduced metallic catalyst as well as that of an oxide catalyst may be regarded as homogeneous, or a t least partly homogeneous. Edges and cracks of crystallites, which sometimes have been believed to be the active centers of catalysis,

GENERAL ASPECTS O F CHEMISORPTION AND CATALYSIS

105

appear to be t o this author of little importance. Studies of adsorption and catalysis on single crystals of a given metal appear therefore of fundamental importance for clarifying the surface nature of the usual metallic catalysts. Three slices of single-crystal nickel, 10 mm. in diameter and 2 mm. in thickness, respectively, parallel t o the (110), (loo), and (111) planes were prepared by S. Kaya and denoted, respectively, a, b, and c. They were studied by Matsuda (unpublished) about ten years ago with particular reference t o the rates of recombination of hydrogen atoms and t o the activation energies for the chemisorption of hydrogen. The latter had been theoretically estimated b y Horiuti et al., as mentioned elsewhere, a s functions of the different arrangement of the nickel atoms on the three main crystal planes mentioned above. The relative rates of recombination on the electropolished surfaces of these specimens observed by Matsuda a t room temperature followed the qualitative rule :

a-c>b Yamaguti (86) later investigated the boundary planes of these specimens, after having them etched with an ethyl alcohol-bromine solution (10: 1 by vol.) for 10-20 seconds, by means of electron diffraction, and by the oxide-replica method of electron microscopy. According to him, specimen a had as boundary zones the planes (110) and (lTO), specimen b the (001) and (110) planes, and specimen c the (111) plane. There is, however, no proof as to whether the atoms in the “polished” surfaces of these single-crystals were arranged in an ideally ordered pattern. Yamaguti noted that the etched surface of specimen a was oxidized in the air more quickly than that of b and of c, suggesting t h a t the (110) plane is more susceptible to oxidation. Important contributions to this topic have been made by Benton and Gwathmey (87), Leidheiser and Gwathmey (88,89), and more recently by Rhodin (90). These authors prepared approximately planar singlecrystals of metals and determined the adsorption characteristics and catalytic activities of these specimens. It was shown th a t the (100) plane of single-crystal copper i s more readily oxidized by oxygen than other planes. I n the case of single-crystal nickel, carbon was found t o be selectively deposited on the (11 1) plane during the catalytic decomposition of carbon monoxide. This is strong evidence for the “selective” poisoning of metallic catalysts on certain specific crystal planes exposed in the catalyst surf ace. Rhodin found slightly different adsorption heats of nitrogen on the three main crystal planes of copper. These heats remain nearly constant a t about 2 kcal./mole for any of the planes but go through distinct

106

TAK.\O

li\V:\N

maxima as the adsorbed quantity approaches a complete monolayer. The most striking observation of this author is that the differential heat of adsorption, derived from low-temperature adsorption isotherms for polycrystalline copper exceeds by far the heats of adsorption found for the uniform planes. Since i t is unlikely that this effect is due t o an oxide contamination of the polycrystalline samples, the higher heat of adsorption is possibly caused by the presence of steps or cracks in the polycrystalline surfaces. Ile Boer and ('usters (91), as well as Barrer (92), discussed a t an earlier time that such irregularities in surfaces may profoundly affect the magnitude of their heats of physical adsorption. The rate of reaction between hydrogen and oxygen on the (111) plane of single-crystal specimens of copper was measured by Gwathmey et al. and was found t o be about two times greater than th a t on the (100) plane. This suggests that the crystallographic orientation affects the catalytic activity, in accordance with the experimental results of Beeck and Ritchie (93), who showed that non-oriented metal films exhibit catalytic activities different from those of oriented metal films. Thus, in many cases, the heterogeneity of the surfaws of otherwise pure metallic catalysts may be caused by the abundance of different crystal planes. On the other hand, the catalytic homogeneity of a metallic surface does not necessarily mean that this surface is homogeneous in regard to its crystallographic arrangement. Preferred orientation of metal films can be clearly achieved by depositing the metal on the smooth surface of a well-crystallized solid in vacuo. For example, Uyeda (94) and Kainuma (95) obtained (111)-oriented films of nickel, copper, and platinum when these metals were deposited on a cleavage surface of molybdenite (0001) a t temperatures ranging from 20" t o 500°C. Miyake and Kubo (96) observed a temperature dependency of the orientation of deposited films of face-centered cubic metals when they were deposited on a cleavage surface of zinc blende (110). As revealed by electron-diffraction patterns, Beeck and Ritchir obtained (I 10)-oriented nickel films paralIel to their support in an atmosphere of inert gas a t a pressure of 1 mm. I-Ig, and non-oriented films in a high vacuum. From the B.E.T. area of these films available to hydrogen adsorption and from the number of chemisorbed atoms a t - 195°C:. and 0.1 mm. Hg pressure, which they consider t o represent the saturation value of adsorption, these authors conclude that only (1 10) planes are present on the accessible surfaces of the oriented films (93). A similar investigation with a non-oriented film of nickel led t o the conclusion that the area occupied by cheniisorbrd hydrogen atoms is compatible with the assumption that the non-oriented film exposes on its surface approximately ecyal fractions of the (loo), (1 lo), and (111) planes. There

GENERAL ASPECTS O F CHEMISORPTION AND CATALYSIS

107

are, however, certain objections against the validity of this cvxiclusion of Beeck. In conjunction with their studies of evaporated barium gettt.1. film, Oda arid Tanaka (97) investigated the relationships between the structure of a nickel film evaporated on a glass plate and the conditions of its preparation. These nickel films had a remarkable tendency to expose the (110) plane with increasing thickness even if made in a high vacuum. When the support on which the nickel vapor was condensed was heated, various kinds of crystal planes were observed to develop parallel to the support as a function of the temperature, e.g., the (I 10) plane a t 100°C., the (110) plane and (200) plane at ZOO’C., and the (200) plane at 300°C. A non-oriented surface was formed a t 350°C. From this, it seems reasonable to conclude that even ordinary metallic catalysts, including carriersupported catalysts, may preferentially expose crystal planes of various kinds, depending on their mode of preparation. In this connection reference may be made t o the work of Yamaguti and his co-workers (98), who noted the intensity changes of electron diffraction occurring for crystalline powders of nickel, magnetite, and magnesia with rising temperature as a possible means of determining the crystallographic indices of their boundary surfaces. These authors concluded t ha t (113) is the predominant plane on a surface of thoria, (110) and (100) for magnesia, (111) for magnetite, and (110) and (100) for reduced nickel. If these observations and their interpretation should he confirmed, they would be of great value for the determination of the surfape structure of powdered metallic and oxidic. catalysts.

VI. TOPOCHEMISTRY I N I ~ E T E R O G E N E O U SC A T 4 L Y S I S The catalytic activity of a powder catalyst should be proportional to its surface area in case its entire surface is equally effective for the catalytic reaction. On the other hand, this correlation does not apply to catalysts whosc active sites are located a t crystallographically exceptional positions such as edges and corners of its microcrystallies. Schwab and Rudoloph (99) studied as early as 1934 the “Topochemistry in Heterogeneous Catalysis,” and concluded that the active sites of several catalysts were located a t crystal boundaries in their surfaces. The chief evidence represented by these workers is that in various catalytic reactions, e.g., in the hydrogenation of ethyl cinnamate over different specimens of powdered nickel catalysts, the rates are proportional to a power of the catalyst surface areas lying between I .8 and 4.0. I n the preceding section, we have attempted t o point out, on the basis of statistical-mechanical interpretations of the adsorption isotherm and of the R.E.T. method for the measurement of the surface areas of cata-

108

T A K A 0 KWAN

lysts, that the surfaces of reduced nickel and cobalt are wholly available for the adsorption of hydrogen. Furthermore, poisoning experiments of Maxted et al. led to the conclusion that active sites, whatever their nature may be, are not localized a t a few limited regions of the catalyst surface. No convincing experimental evidence of metallic catalysts has been brought forward in this author's opinion that would prove th a t the boundary lines between surface crystallites are the seats of catalytic action. I n mixed oxide catalysts of the type MeO.MezOa,new phases can be formed, as shown by Ward and Erchak (79), arid it is possible that some of these phases have higher activities than any of the components used in preparing the catalyst. Morikawa (100) and also Koizumi (101) found that the polymerization of ethylene proceeds rapidly in the presence of a nickel-kieselguhr catalyst containing 15 "/o Ni, while both nickel or kieselguhr alone are extremely poor catalysts of this reaction. This might be interpreted by the presence of active sites localized a t the interface of nickel and kieselguhr since the formation of a homogeneous new phase between these two components is improbable. VII. THEMECHANISM OF HETEROGENEOUS CATALYSIS 1. The Stoichiometric Number

It is generally accepted that heterogeneous catalysis represents a sequence of elementary reactions such as the adsorption of the reactant on the catalyst surface, atomic rearrangements of the adsorbed particles, and desorption of the products, the overall reaction rate being governed by the slowest step of these elementary reactions. The rate of the slowest +

step in the forward direction, denoted by V , however, cannot always be --t

identified with the overall reaction rate v, the relation being expressed in general by -

nu

-

t

=

+

V

(22)

where n is the number of forward acts of the rate-determining step required t o transfer one reactant, denoted by the left-hand side of the relevant chemical equation, completely t o the product appearing on the t

t

right-hand side. The baclcward rate v bears a similar relation to V C

t

nu = V

GENERAL ASPECTS OF CHEMISORPTION AND CATALYSIS

109

The idea of n was first put forward by Horiuti (30) and designated as the stoichiometric number (" Kagaku-Ryosu ") . For instance, if we write down the ammonia synthesis reaction from the elements as: NZ 3 Hz = 2 NHD,we can particularize the elementary reactions involved in this process to the stoichiometric numbers n = 1, 2, and 3, respectively, as:

+

n

Nz = 2 N(a) N(a) H(a) = NH(a) NH(a) H(a) = NH,(a) NH3(a) = NH3, etc. HI = 2 H(a)

= 1,

+ +

n = 2, n = 3,

where (a) denotes the adsorbed states of the intermediates. We shall now express the stoichiometric number in the form accessible t o experimental determination. The net rate of reaction v, i.e., the excess of

4

over its reversal, is given by +

v=v-v

t

(24)

From equations (22), (23), and (24) it follows immediately th a t

or

v

+

=

t

(V - V ) / n c

Since all the elementary reactions except the slowest are usually assumed t o be in partial equilibrium, we have the relation

v where Ap is the free energy increase of the overall reaction. From equations (25), and (26) we have

Differentiating v in equation (27) with respect to Ap, we have

110

T AKA0 KWAN

At equilibrium or Ap = 0 it follows that

Equation (29) enables us t o determine n experimentally, for example, by -+

using a radioactive isotope, provided that the forward reaction rate v is observed in the neighborhood of equilibrium. The determination of the stoichiometric number was made by Horiuti and Ikushima (102) for the hydrogen electrode process on platinum and more recently by Horiuti and Enomoto (103) for the ammonia synthesis --t

on a promoted iron catalyst, and 21 was evaluated by means of deuterium in the former case and by ammonia containing heavy nitrogen in the latter. For the hydrogen electrode process, El2 = 2 H+ 2e, particular emphasis was laid by the former workers on the point that either of the following two mechanisms is fitting:

+

i)

H,

H,+

ii)

H1

2H,

*

&ere

+ e,

HI+ -+ 2H' H

+- e,

or

ft. H + + e

indicates the slowest step

The ionization process of Hz to form H$ is considered to be ratedetermining in one case, the formation of H+ in the other. The experiment has shown that n = 1, hence excluding the mechanism (ii) which holds when n = 2. The determination of the stoichiometric number in the case of ammonia synthesis, N z 3 H z = 2 NH3, was carried out a t 430°C. and a t a pressure of 40 cm. Hg of the (Nz 3 Hz) mixture, both in static and flowing systems. The results are shown in Table V.

+

The n-Vdue f o r

" Obtained

111

TABLE V

~4111~~10nLU Synthesis

Exp. No. 1 I1

+

2.2

over l'roniolerl Iron Calalysl

2

3

4

5"

2.4

2.1

1.7

2 .3

a flow system.

Table V ahows values of n close t o 2 . According to this, the adsorptioii step of nitrogen on the cntalyst, which has heen hr~licved by many

GENERAL ASPECTS OF CHEMISORPTION A N D CATALYSIS

111

workers t o be the slowest step i n ammonia synthesis, cannot be rate-determining, a t least, not under tlhr conditions employed in these experiments. 2. .I dsorption and Elementary Reaction Rates

Special attention has been given for a long time to the specific activities of catalysts as correlated with their adsorption characteristics. Severtheless, no complete theory has been developed regarding these relationships. We shall here briefly discuss the relationship between adsorption and the elementary reaction rates, with reference t o the work of Horiuti (11). The rate of the heterogeneous elementary reaction, V per unit area of catalyst surface, is given for the case when the surface is nearly vacant, and for the case when the surface is practically occupied by the dominant adsorbed molecule 6.l as

(30.H) (30.L) where V(H) and V(L) are the rates on a vacant and on an occupied surface, respectively. On a given catalyst, V(H) may in general be realizable a t “higher” temperatures and V(L) a t “lower” temperatures, because of the exothermic nature of the adsorption heat of the molecule 6”. From equations (30.H) and (30.L) we can deduce, regardless of the mutual interaction of adsorbed particles, the expressions which are identical with the Arrhenius equation in the form In V = In A’ - E / R T as N6’ Ae* ln V(H) = In lcTN __ eG - - (31.H) h Q$ RT lcTN N6’ Q6“ Ae* Ae6” (31.L) 111 V(L) = ln __ eG - ; I2 T h Q; N 6

+

where A€* is the difference of potential energy between the initial reactant 6’ and the activated complex 6”, and Ae6‘“is the adsorption heat of 6”. The deduction of equations (31) from (30) was carried out by replacing T as well as that in Q of equation (30) by TNel-TN/Twhich is correct in its value and its first derivative at the average T of the temperature range considered. The frequency factor of equation (31) is therefore “temperature independent.” It follows from equation (31) that the rate is proportional t o the concentration of reactant when the surface is vacant while inversely proportional to the concentration of the dominant adsorbed molecule when occupied. I n the latter case the rate is independent of the concentration of reactant when 6’ = 6m (zero-order). Ae* and Ac* of equation (31) are, respectively, the “apparent” and the ‘‘tnie” activation energies in the terminology of Hinshelwood.

+

112

TAKA0 KWAN

The value of N/QN of an ordinary gaseous molecule is about under the usual experimental conditions, In V(L) hence being much greater than In V(H) a t 1/RT = 0. Since the heat of adsorption, Ad"', is positive, In V(L) has a steeper slope with l / R T than In V(H) does, as shown in Fig. 19, and the two lines should cross. The temperature of the intersection of the lines, or T k , is given by equating In V(H) to In V(L) :

Actual elementary reaction rates tend t o follow the V(II) or V(L) straight lines at higher or lower temperatures but deviate from these lines I

I

3

5 M -

I

I

-----__

-

s

-

M

I

1IRT'

0

I

1

IFF

FIG. 19. Temperature dependence of an heterogeneous elemrntary reaction rate. Reproduced from Horiuti (1 1).

in the neighborhood of the intersection. The rate V a t the intersection can be given by = V(H)(I - e*-) = v(L)e6" (33) where 06" is the fraction of surface covered by 6'". It follows from equation (33) that V/V(H) or lT/V(L) is 35 as shown by the dotted curve in Fig. 19. This figure reveals characteristic features of heterogeneous catalysis in terms of the two parameters of the Arrhenius equation. First, the simple Arrheiiius relationship or the linearity of In V against 1/T does not hold urtiess the surface coverage is extremely small or very close to completion. In other words, the Arrhenius activation energy for heterogeneous catalysis tends t o decrease with rising temperature as long as the slowest step does not vary over the temperature range. Secondly, the adsorption heat of the dominant adsorbed molecule or A P does affect

v

113

GENERAL ASPECTS O F CHEMISORPTION AND CATALYSIS

the rate of heterogeneous catalysis reversely, depending on whether the surface is vacant or occupied, provided that the following relation between activation energy and the increment of adsorption heat holds for various types of catalysts (104). or

where A€,* is the activation energy for the homogeneous reaction, being a constant fraction. According to equations (31.H) and (34) or (31.L) and (35) the more Ae6" increases the more the In V(L) line is inclined against 1/T, causing the reaction rate to decrease whereas the relation is reversed for the In V(H) line, i.e., A€* becomes less and less with increasing AE" and finally might attain even a negative value. I n studying the dehydrogenation as well as the dehydration reaction of isopropyl alcohol over a series of nickel catalysts that had been treated with chlorine, bromine, and iodine, Kwan and Takasaki (105) found th a t the rate of the dehydrogenation increases with increasing electronegativity of the coating element, whereas the rate of dehydration shows the opposite behavior. The kinetics can be interpreted by means of equations (31.H) and (34), in the case of the dehydrogenation and by equations (31.L) and (35) in the dehydration reaction, H%Obeing assumed to be the dominant adsorbed molecule. (Y

3. Activation Energy and Frequency Factor

Schwab has pointed out that the following relationship between the two parameters of the Arrhenius equation is frequently encountered. A decrease in the activation energy of a given reaction, for a series of catalysts, often does not increase the reaction rate to the extent calculated, because of a simultaneous decrease of the frequency factor. Cremer (106) confirmed this for the decomposition of ethyl chloride on various chloride catalysts. These findings will be discussed here with due regard to the relation between adsorption and elementary reaction rates dealt with in the preceding section. The rate of a given catalytic reaction should change from a n expression by V(H) t o one by V(L) as we progress from a catalyst with a low adsorption heat for 6" to a catalyst with a high one. Further, the Arrhenius activation energy as well as the frequency factor will increase with increasing heats of adsorption. From this it can be expected that the change in the frequency factor, for instance, in a monomolecular decomposition reaction of a reactant such as formic acid or ethyl chloride, cannot exceed 1/QN or roughly loTz6.The highest value of the factor may be

114

TAKA0 KWAK

li T N e G or approximately loz8molecules set.-' crn? under the usual h experimental conditions. These figures have been confirmed by Schwab (107) for the dehydrogenation of formic acid over alloy catalysts of different composition. He obtained a frequency factor increasing from loz2t o lozsmolecules set.-' cm.? while the Arrhenius activation energy increased from 12 t o 30 kcal./mole. If, as Schwab believes, the reaction is of zero order over the entire temperature range investigated for each type of catalyst employed in his study, the frequency factor should be independent of the particular type of catalyst. T h a t this is not the case leads the present author t o the tentative view that the observed “zeroorder ” reaction needs further clarification. The log-log plot of the adsorption isotherm, which can possibly be correlated t o the pressure-dependency of the catalytic reaction rate, is very flat. The adsorption of ethylene on nickel increases only by 10%for an increase of the equilibrium pressure by a factor of 10, although the surface is still far from being covered by a monolayer. The work of Laidler et al. (3), who studied the ammonia-deuterium exchange reaction on a promoted iron catalyst by means of the “microwave method,” also throws doubt on the zero-order kinetics with respect to observations made by Farkas (4). !A sudden increase of activation energy parallel to th a t of the frequency factor with alloy catalysts of various composition was noted for the decomposition of hydrogen peroxide over a Ni-Cu alloy catalyst containing 30% Ni (log), for the decomposition of formic acid a t the Curie point over a Pd-Co alloy catalyst (log), and for the parahydrogen conversion over a Pd-Au alloy catalyst containing 40% Pd (110). These results might be interpreted along similar lines, i.e., the numerical values of the frequency factor obtained in these investigations usually lie within the two extreme cases given by equations (31.H) and (31.1,). The changes in the electronic structure of these alloy catalysts would presumably cause the adsorption heat of a reactant or of an intermediate to vary, leading the kinetic expression toward either equation (31.H) or (3 1,L). Alternative interpretation may also be possible by taking into consideration the entropy changes of the activated complex associated with its energy changes, hut no successful treatment to account for the observed relationships is seen yet in the literature.

5. T h e Catalytic HydroyewatiorL of Ethylenr The catalytic hydrogenation of ethylene has been extensively studied, particularly in connection with investigations of t,he exchange reaction

GENERAL ASPECTS OF CHEMISORPTION AND CATALYSIS

115

with deuterium. The observed kinetics, however, reveal a complex and varied behavior depending on the working conditions and the kind of catalyst used. Thus, none of the various mechanisms which have been proposed is fully satisfactory. We shall deal here primarily with a few important characteristics of the kinetics of the ethylene hydrogenation, such as the relation of the two parameters of the Arrhenius equation, and with the pressure dependency of the reaction rate. According t o Horiuti and Polanyi ( I 11) the hydrogenation reaction of ethylene in the presence of a nickel catalyst consists of the following consecutive elementary steps: CH2

II

CHz

Is

CH2(4 cHz(a)]L2cHda~

Hz-

It,

H(a)

+

H (4

-I

111

CHa CHa

where (a) denotes the adsorbed states of the intermediates. I n this mechanism the exchange reaction is intimately connected with the hydrogenation reaction inasmuch as the former is assumed to occur through step I1 and its reverse or through the “half-hydrogenated” state I1 (associative mechanism). As far as the exchange reaction of unsaturated hydrocarbons with deuterium is concerned, the associative mechanism has been accepted as the principal mechanism rather than “ dissociative mechanism” put forward by Farkas and Farkas (112). On the other hand, Farkas (113) and Twigg and Rideal (114) believe that the hydrogenation follows another path. According to these authors, ethylene is attacked by molecular hydrogen or by two hydrogen atoms being simultaneously prescrit on the surface. Recent investigations along this line by Twigg (1 15), however, have indicated that the hydrogenation of ethylene by an equimolar mixture of Hz and D2 does produce CH2DCH1 which would be expected t o be formed only via the consecutive attack of ethylene by hydrogen atoms. A characteristic feature of the ethylene hydrogenation on nickel is that its rate has a maximum a t about 6OoC., above which the Arrhenius activation energy is negative (9). Zur Strassen and Schwab (12) ascribed this t o a desorption of ethylene from the surface of the nickel catalyst, while Twigg and Rideal attributed it to a desorption of hydrogen held over “gaps” in the layer of adsorbed ethylene molecules. Both these interpretations of the reversal in the sign of the temperature coefficient are based on the assumption that the rate is controlled by the desorption of the dominant adsorbed molecule.

116

TAKA0 KWAN

An alternative view t o these interpretations of the “temperature inversion” has been put forward by Horiuti (11). He concluded from the observed or, if not available, from the calculated adsorption heats that the concentrations of CeH4(a),H(a), and C2H6(a) are negligible over the range of zur Strassen’s experimental conditions, i.e., that Tk of any adsorbed molecular species calculated from equation (32) are much lower than the observed Tk values. Thus, Horiuti claims th a t the steady reaction rate v is compatible with zur Strassen’s experimental results in terms of the rates of the elementary reactions, in the following manner: 1- 1 v - V(1b)

-

1 +-V(II1)

where

According t o this concept, F’(1b) is much smaller a t low temperatures than V(III), and I b is the rate-determining step a t higher temperatures; V(I.11) is smaller, and 111 the rate-determining step. This mechanism satisfies the observed kinetic expression, i.e., v cy NH2 a t lower temperatures and 21 a NHt x NCzH4a t higher temperatures. From zur Strassen’s results, I-Ioriuti obtained the activation energies as Ae*(Ib) = 12 kcal./mole, Ae*(III) = -12 kcal./mole

A picture of the catalytic hydrogenation of ethylene as proposed by

Horiuti is shown in Fig. 20. I n this figure the velocities of the intermediate step reactions on nickel catalyst a t any set of values of N H zand NCzH4are obtained by shifting the straight lines parallel to themselves by appropriate distances. Rideal (10) found that when NHz NCzH4was 760 mm. Hg and NQH4 was about 30 mm. Hg the rate was proportional to NCzH4.He attained a maximum a t 137°C. whereas when N H 2 NCzH4was 760 mm. H g and NHzwas about 30 mm. Hg the rate was proportional t o NH1and attained no maximum up to 190°C. The diagram constructed for his experimental condition makes the 1-111 intersection shift to 120°C. for the former case and t o 180°C. for the latter case. The result obtained by Toyama (116) when NHa was 45 mm. Hg and NCZH4 was 50 mm. Hg, that a maximum rate is obtained in the neighborhood of 140°C., is in agreement with Horiuti’s diagram. According t o Twigg (114) the activation energy for the exchange

+

+

GENERAL ASPECTS O F CHEMISORPTION AND CATALYSIS

117

reaction with deuterium is by about 9 kcal./mole greater than that for hydrogenation over the temperature range 55-120OC. This was interpreted by Horiuti by deriving, for step Ib as the rate-determining process, t

the rate expression of the exchange reaction, V(Ib), i.e., the reverse step of Ib, as V(Ib)2 V(1b) = (39) 17 (II I) t

~

t

the activation energy for the exchange reaction, At*(Ib), being given by t

At*(Ib)

=

2At*(Ib) - Ae*(III)

(40)

4-

It follows from equation (40) that Ae*(Ib) is larger than At*(Ib) provided that At*(Ib) is greater than Aa*(III).

1’O

FIG. 20. The “structure” of the catalytic hydrogenation of ethylene on nickel. N C A = 0.03 mm. Hg. Full line and dotted line indicate calculated and observed rate (zur Strassen), respectively. Reproduced from Horiuti (11). NH2 =

A somewhat similar explanation of the difference between the activation energies for the exchange reaction and the hydrogenation was given by Twigg (115), who assumed that the adsorption equilibrium of hydrogen was not reached at lower temperatures. The only difference from Horiuti’s treatment is that, according to Twigg, the reaction proceeds by way of ail interaction between an adsorbed hydrogen molecule and a chemisorbed ethylene molecule which form an adsorbed ethyl radical and an adsorbed hydrogen atom.

118

T A K A 0 KWAN

According t o Beeck (7) the hydrogenation of ethylene over evaporated metal films is zero-order with respect to ethylene and first-order with respect t o hydrogen as in zur Strassen’s experiments carried out in a lower temperature range. To account for these kinetics, three different mechanisms have been hitherto proposed: (I) adsorption of reactants on adjacent sites of the catalyst surface according to Schwab, (11) a Rideal type of mechanism, i.e., a reaction between gaseous molecules or a van der Waals’ adsorbed molecule and a chemisorbed one, and (111) a Langmuir-Hinshelwood type of mechanism with insufficient covering of the catalyst by chemisorbed ethylene. Mechanism (I) seems unlikely in view of the foregoing arguments concerning the nature of the nickel catalyst. Mechanism (11) implies that the two reactants do not compete for the same surface: in other words, when the site for the activated complex is not occupied simultaneously by the two reactants, this mechanism would be valid. Allowing the step (Ib), or a modified (Ib) step, t o be the slowest step a t lower temperatures, Horiuti and Twigg caii explain satisfactorily the temperature coefficient of the exchange and hydrogenation reactions. General rate expressions were derived on the basis of the absolute rate theory by Laidler (117) for mechanisms (11) and (111). The frequency factor was found to be approximately the same for both mechanisms. For mechanism (111) Laidler investigated the steric factor (the ratio of the rate of reaction to the number of ethylene molecules striking the surface with the required energy of activation), which Beeck gives as for a nickel film. Eyring et al. (8) calculated the rate on the same basis and obtained a satisfactory agreement with the experiment. However, the remarkably divergent values of the frequency factor associated with a n approximately constant activation energy with a variety of metal films, found by Beeck, still constitute an unsolved problem. T-111. GENERALCONCLUSIONS

1. The present article deals primarily with the elucidation of the surface nature of common metallic and oxidic catalysts, and with statisticalmechanical investigations of the chemisorption equilibrium on these catalysts. The surface areas of these catalysts as determined by the Brunauer-Emmett-Teller method have been taken into consideration. It was shown that a number of certain metallic catalysts such as nickel, cobalt, and platinum and also oxide catalysts of the spinel type act as a n array of homogeneous active sites. There is no reason t o believe that a few limited regions of the surfaces of these catalysts, such as corners, edges, lattice defects, etc. are particularly important for their catalytic activity. This conclusion is in accordance with the poisoning experiments of Maxted et al. There is some evidence th at the surfaces of these catalysts

G E N E R A L ASPECTS O F CHEMISORPTION A N D CATALYSIS

119

can be modified by the presence of carriers or by their mode of preparation in such a way as to make them heterogeneous. Such “heterogeneous catalysts l 1 are, for instance, prepared by the insufficient reduction of the oxides of nickel or copper. 2. A heterogeneity was demonstrated for the surfaces of iron, tungsten, and also for oxide catalysts consisting of various phases, in accordance with poisoning experiments on these catalysts. One of the important functions of promoters is to develop active sites on the surface of certain catalysts, as, e.g., on the promoted iron catalysts used in the ammonia synthesis. 3. There is ample evidence to show that substrates, including poisons, are chemisorbed selectively on certain crystal planes of metallic catalysts and that the final catalytic activity depends on the kinds of crystal planes that are exposed on the surface. 4. It was found that chemisorption equilibrium is rapidly attained in most reacting systems through rapid desorption and readsorption. With a few exceptions, chemisorbed molecules can be regarded as immobile since statistical-mechanical calculations of the chemisorption equilibrium agree well with the experiment if two-dimensional translations and rotations of the chemisorbed molecules are assumed to be nonexistent. The chemisorbed state of di- or triatomic molecules can be “molecular” or (1 atomic,” depending on the nature of the adsorbent. For example, the carbon dioxide molecule is chemisorbed with complete dissociation into its three atoms on metallic surfaces, while on oxidic catalysts it is chemisorbed with only partial dissociation. 5. The well-known characteristic relationship between the Arrhenius activation energy and the frequency factor for a given reaction on different catalysts should not be ascribed to the heterogeneity of the catalyst surface, as Schwab and others believe, but to the extent at which the surface is covered by the dominant adsorbed molecular species. In this connection it was demonstrated that, quite apart from the mutual interaction of adsorbed particles, higher coverage at lower temperatures and lower coverage a t higher temperatures can profoundly affect the Arrhenius activation energy and the frequency factor. Due to this, the Arrhenius activation energy can even decrease with increasing temperature to a negative value. 6. The catalytic hydrogenation of ethylene on nickel, as explained by Horiuti, is based on four consecutive elementary reactions, vie., the chemisorption of the reactants to form adsorbed ethylene (Ia) and adsorbed hydrogen atoms (Ib), the reaction (11) between these adsorbents to give half-hydrogenated molecules, and the addition of another adsorbed hydrogen atom (111) to form ethane, In this mechanism, step (Ib) is

120

T A K A 0 KWAN

considered t o be the rate-determining step a t lower temperatures, step (111) a t higher temperatures. This mechanism explains the observed reversal of the temperature coefficient, the pressure dependency of the reaction rate, and the shift of the temperature a t which a maximum rate of hydrogenation occurs, a t various pressures. ACKNOWLEDGMENTS The author expresses his sincere gratitude to his colleagues in The Research Institute for Catalysis who were kind enough to offer him their unpublished data. Thanks are also due to Prof. J. Horiuti for his theoretical advice, and to Dr. W. G. Frankenburg for his kind help in preparing this paper.

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