16 Structure and Texture of Catalysts

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Structure and Texture of Catalysts J. H. DE BOER Technische Hogeschool, Delft, The Netherlands, and Staatsmijnen i n Limburg, Central Laboratory, Geleen, The Netherlands Catalyst structure may be studied by numerous and widely varying methods. Apart from the crystallographic pattern, the structure of the outer surface or of the surface layers is especially important. Unfortunately, we do not know much about the real structure of the surface. It is an important question t o know to what degree the surface is a twodimensional replica of the three-dimensional regularities and irregularities of the lattice. Evaporated films probably show a lamellar structure; the surface planes are not identical with the main orientation of the film as are observed in electron diffraction patterns and through the electron microscope. Catalytic material on a carrier shows generally a microcrystalline structure, no indications of an “amorphous” phase with exceptional properties can be found. Unavoidable, as well as deliberately added, contaminations seem t o have an important influence on the structure of the catalytic surface. Closely related is the question of the role played by promoters and their distribution on the surface. Increase of the phase boundary between the solid catalyst and the reaction phase leads t o the development of porous structures in most technical catalysts. Many methods are in use t o characterize these pores. Acceptable values for the dimensions of the pores and the available surface area are obtained, although certain corrections will be shown t o be necessary. I n some instances, the genesis of the pores as well as some indications about their shape are available.

I. INTRODUCTION

In theoretical studies the surface of a solid is sometimes pictured as if an ideal lattice had been cut by an ideally sharp razor blade and as if the atoms of such a freshly cut surface would retain their places. Such a “theoretical” surface does not exist ; a mutual displacement of the surface atoms occurs, leading to the “ideal” surface. Section I1 deals with the structure of this “ideal” surface. Actual surfaces are, however, surfaces of the actual nonideal crystals, or conglomerations of crystals. The possible structure of the 131

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“actual” surface is treated in Section 111. Section IV deals with “contaminated” surfaces, as they occur or after being contaminated deliberately. A large surface area per unit of volume (or weight), a desired property for many catalysts, brings problems which are dealt with in the last section about the texture of catalysts.

11. THE“IDEAL” SURFACE 1 . The 8urjace of Polar Salts or Oxides a. Contraction of the Surface. Our knowledge of the “ideal” surfaces of

these compounds was recently reviewed (1) and may be summarized as follows. Specular diffraction spectra obtained with helium beams (2) reveal (1) that the mutual distances of the ions in the outer layers of LiF are the same as in the crystal. Ever since Born calculated the theoretical figures for the surface energies of the various crystal faces of polar salts (3),all theoretical approaches agree that the mutual distance between the outer layers should be smaller than the distance between the layers in the interior of the lattice. The predicted degree of this contraction on the surface depends on which repulsion law is used and whether or not the polarization and van der Waals’ forces are incorporated in the calculations. Recently a publication of the work of the late Dr. Nicolson (4) revealed that small cubes of MgO, prepared in vacuo and having a particle size of roughly 500 A., show a contraction of about 0.05 %, with respect to the normal crystal parameter. b . The Negative Double Layer. Even more important than the contraction of the surface is the nature of the outer layer. There are indications that the outside layer of all polar salts and oxides consists of negative ions. Older experimental studies of the absorption spectra of adsorbed molecules by the author and his collaborators (6) showed this to be the case for many salts and oxides, obtained by sublimation in vacuo. The configuration of salts like CaF2, or oxides such as TiOz makes it quite understandable that their surfaces-according to their cleaving p l a n e s 4 0 consist of the negative ions. Other salts, like NaC1, assume such an arrangement by the polarization of the outside layers (6); the negative ions of the surface are displaced outwards, the positive ions towards the inside of the lattice. The distance between the mid-point of the double layer thus formed and the next layer is smaller than the normal lattice distance, thus leading to the contraction of the surface mentioned above. Molecules adsorbed by physical adsorption forces on these surfaces will, consequently, be polarized to form dipoles pointing with their negative sides away from the surface. Polar molecules, possessing peripheric dipoles, such as OH-, NH2-, or COOH-groups, are selectively adsorbed with their positive ends in direct contact with the negative surface ions. It was recently shown (7) that the heat of immersion of the clean solid surface of

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rutile in many polar liquids is entirely due to the adsorption of the dipoles of the molecules of the first adsorbed layer. The average electric field of Ti02 , a t the point of the center of the dipole could be estimated to be

2.72 X lo6 e.8.u. 2. The Surface of Charcoal and of Metals a. Dilatation of the Surface. A recent investigation of Shishakov (8) indicates that metal films show a somewhat expanded lattice of 1 to 2 %. Mignolet (9) observed that the work function of metal films is lower than that of the same metal in its normal state. A lower work function points to a larger distance between the atoms. It is, therefore, possible that we have to assume that the mutual distance between the outside layers of a metal crystal is somewhat larger than in the interior of the lattice. b. The Positive Double Layer. Metal surfaces also show an electric double layer, caused by electrons protruding from the outside layer of atoms (ions). Molecules which are adsorbed by physical forces penetrate into this (diffuse) electron layer and are polarized in such a way that their dipoles have their positive poles pointing away from the surface. This polarization, which is, therefore, opposite in sign to that on salt or oxide surfaces, is experimentally shown by contact potential measurements (10) and by the mutual repulsion of the adsorbed molecules (11).The same electric double layer also results in a weaker adsorption of molecules with peripheric dipoles ( I d ) , quite contrary to their behavior on salt or oxide crystals. The electric field can be estimated, from these measurements, to be F = 6.2 X lo6 e.s.u. a t the center of a Nz molecule, adsorbed on charcoal.

F

=

111. THE“ACTUAL”SURFACE The properties of the actual surfaces are not opposed to those of an ideal surface, but are added to them. 1. Orientation

Various crystallographic planes of the same crystal may show appreciably large differencesin catalytic activity. As catalysis is not always restricted to the outer surface layer, but may penetrate to some depth, the question arises whether the orientation of the crystal or the arrangement of the outer layer will be determining. As these questions were recently reviewed ( l S ) , a short summary may be sufficient. The beautiful experiments of Gwathmey and collaborators (14) on spherical single crystals (since 1948) show that in some catalytic reactions the places of a copper sphere that are parallel to the most densely packed { 1 11 ] planes show the highest activity. The surface remains quite smooth a t these spots, but is seriously roughened on the parts which are parallel to the { 100) planes, where the catalytic reaction proceeds

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at a far slower rate. The roughening produces a multitude of small { 111] and { 110) planes, but does not lead to an increase of the rate of reaction. Other investigators also observed-for their reactions-the highest catalytic activity in the [ l l l ] directions of silver single crystal plates (15).It is, however, not always the most densely packed planes that favor the reaction. Gwathmey et al., using the same catalytic reaction, found that the (0001) region of a hexagonal structure-though having the same two-dimensional structure as the { 1111 planes of the face-centered cubic structure of copper-shows the lowest activity as ccmpared with other planes. Beeck (16) observed for a different catalytic reaction that the [110] directions of oriented nickel films show the highest catalytic activity, while the same directions in platinum films were less active than random films. Sachtler et al. (17), making electron diffraction and electron microscope investigations, concluded that the main orientation of the nickel films, as used by Beeck, is indeed as stated by this author, but that the denser { 1111 and (100) planes seem to be exposed to the gas phase. The complex experimental evidence points to the fact that difference in orientation, more than the actual arrangement of the outer planes, may govern the speed of a catalytic reaction. Whether a certain orientation promotes the speed or slows it down may well depend on the mechanism of the slowest of the various consecutive reactions of the whole sequence of the catalytic example which is studied. Orientation is, therefore, quite important for catalytic praxis. It may well be assumed that empirically found preferred methods for preparing active catalysts are often those methods leading to the most active orientation. Westrik and Zwietering (18) proved that the iron catalyst for ammonia synthesis, prepared by a careful slow reduction of magnetite is well oriented in the [lll]direction. 2. Lattice Defects

The “ideal” surface, discussed in Section 11, is the surface of an “ideal” lattice. A real lattice contains a considerable number of defects. Lattice vacancies or interstitial atoms (ions) occur and are inherent to the lattice a t a given temperature or may result from a temperature treatment and a “freezing in” before equilibrium is reached. We may, undoubtedly, expect to find similar defects at the surface. But here also we may not extrapolate the bulk situation to the surface. According to the Gibbs law, the equilibrium between the bulk phase and the surface will be determined by whether the distortions contribute to an increase or a decrease of the surface energy. Experimental data in the field of chemisorption, especially those indicating the heterogeneous character of the surface, have been explained on the hypothesis that surface defects are “frozen in” (19)and correspond with

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the temperature of preparation of the catalyst. Others (20) assume an equilibrium, given by the temperature and by the degree of saturation of the surface forces by adsorbed molecules. 3. Surface Heterogeneity (21)

The presence of various crystallographic planes and various crystallographic directions, as well as the occurrence of lattice defects, undoubtedly causes a heterogeneous distribution of adsorption forces. This heterogeneity is far moreimportant for physical adsorption than for chemisorption. In the domain of chemisorption, moreover, surface heterogeneity is more important on surfaces of ionic compounds than on conducting surfaces. The wellknown strong decrease of the heat of chemisorption on metallic surfaces may not be ascribed-or to a limited extent only-to the heterogeneous character of the surfaces ( I S ) . IV. CONTAMINATED SURFACES 1. Inherent Contaminations

a . Metal Surfaces. It is very difficult to prepare and to maintain clean surfaces, free from contaminations. Tungsten filaments, heated for a prolonged time at a high temperature, are clean. Recent work (91) has shown how quickly the surface is contaminated again by the residual gases in the “vacuum.” Films of metals, produced by sublimation in vacuum, may be obtained in a clean state because most of the possible impurities are bound by the first layers that are evaporated. Owing to their very large surface areas, the films can be maintained in a clean state for a longer time than filaments can. Metal powders, obtained by thorough reduction of their oxides, may, sometimes, have a clean surface, but there is always a high probability that gases (hydrogen) are dissolved or occluded in the metal. A pure metallic surface is always completely wetted by mercury; if mercury does not spread over it, the surface is surely contaminated. b. Salts and Oxides. Salt films, obtained by sublimation in vacuum, have clean surfaces. For some salts, such as CaF2,sublimation is the only way to obtain a pure surface. Water molecules are so tightly adsorbed to the surface of CaFz that no means seem to exist to remove them without introducing another impurity. On heating in vacuum, HF evaporates, leaving OH groups behind (22). When, in the investigations of Nicolson (4), mentioned under 11-1,MgO is sublimated in air, the cubes do not show the contraction, previously mentioned; adsorbed molecules, presumably water molecules, saturate the surface forces. Oxides such as A1203 or SiOz can hardly be obtained without some OH groups still being chemisorbed on their surface. c. Semiconducting Oxides. Many oxides show semiconducting properties

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because of the coexistence of ions of the same atom (homonymous ions), but in different valencies, on crystallographically identical places (23). CuzOmay, when in contact with air, take up oxygen, in the form of 02-ions, converting Cu+ ions into Cu2+ions at the same time. The semiconductivity is raised by this process. Diffusion of ions causes a close relation between the bulk properties and the surface properties, so that the incorporating of extra oxygen at the surface is translated into an increased semiconductivity. Similarly, changes in the electric conductivity or also in the magnetic susceptibility may be caused by catalytic actions on the surface of such a semiconductor (24). Zinc oxide is a semiconductor because of its oxygen deficiency. When it is prepared in air, more oxygen is present in the surface region than in the interior; the surface is more stoichiometric than the bulk (25). 2. Modi$ers

In many cases the activity of a catalyst is due to small amounts of foreign material, modifying (26) qualitatively and quantitatively the chemisorption properties of the surface. When this modification leads to an increased catalytic activity, we speak of iipromotors”; if the activity is decreased, we talk about “poisons.” It depends often on the surface concentration whether a contamination acts as a promotor or as a poison. Sulfur atoms on the surface of a nickel hydrogenation catalyst may poison the normal hydrogenation, but they may also lead to the promotion of selective isomerization processes, involving chemisorbed hydrogen atoms. A selective hydrogenation of triple bonds can be obtained by carefully poisoned metallic catalysts, the I ‘ promoting” being performed with strongly adsorbed organic bases or by metallic contaminations (27). In many cases, the effect of small contaminations is strongly dependent on their distribution in microporous catalysts; an adsorption at the mouths of the pores may lead to a very strong poisoning effect, a homogeneous distribution to an increased selectivity (28). The qualitative nature of their effect often depends on the sign of the dipoles which they form on the surface (13). Some promotors serve to protect the well-developed surface area of a catalyst against a sintering process leading to a decreased surface area. This is, for example, the case with the Alz03addition to the iron catalyst for NHI synthesis (29), obtained by reduction of Fe304 containing some dissolved A1203. The reduction is remarkably more difficult in such a case, and it is doubtful whether or when a complete reduction is obtained. An iron catalyst, containing only 0.4% AIZO3and no other “promotors,” still showed a loss of weight of mg/g.h. after a prolonged reduction of 100 hrs. with pure hydrogen at 550” (SO).

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3. The Distribution of Contaminants on the Surface

The t,otal surface area of a catalyst is measured by means of nonselective physical adsorption (V-1) ; selective physical adsorption-or chemisorption-measurements may give information about the part of the surface which is covered with contaminants. The free-iron surface of an iron catalyst containing A1,03 as a stabilizer is mostly measured by the nonactivated chemisorption of CO at low temperatures (31); when the rest of the total surface area is ascribed to the alumina covering, the conclusion may-in many cases-be drawn that this covering has a unimolecular character. A free-nickel surface may be measured by the nonactivated chemisorption of hydrogen at low temperatures (32); at higher temperatures, an activated chemisorption of hydrogen on the oxygen-covered parts of nickel renders this adsorption nonselective (33). An alumina addition to silica produces proton-active catalysts for “cracking” purposes. The selective adsorption of gases with proton affinity can be used to measure the surface area covered with protons (34).The aluminum ions seem to form a unimolecular layer on the surface of the silica (35). The amount of OH groups on an alumina surface may be measured by the selective adsorption of iodine from pentane solutions (36), while the OH-groups on silica give a selective adsorption of butyric acid from pentane solutions (37).

4. Catalysts on Carriers Another method for producing and maintaining a large surface area of the active catalyst is the application of the finely divided catalyst on carriers, such as nickel on silica or platinum on alumina, etc. Here again the carrier is catalytically not indifferent but may “modify” the catalyst.. Recent investigations, as for example, those of Selwood and collaborators (38) using their method of the “magnetic isotherm,” or quantitative x-ray investigations (39)as executed by Coenen, indicate that the active material is present in a microcrystalline state on the surface of the carrier. Selwood’s investigations also indicate a strong influence of the carrier on the valency of catalytically active metal oxides. Magnetic investigations inform us about the particle size and the degree of reduction of metallic catalysts on carriers. The reduction of a nickel silicate gel (40)leads to small nickel particles of, say, 50-A. particle size (41) on a silica carrier. They have grown from still smaller particles by a process of surface migration of nickel atoms (42). The reduction proceeds well only at relatively high temperatures, but the surface of the nickel particles can be obtained free from contaminations such as oxygen (33). The nickel particles are distributed at random and are independent of the structure of the carrier (4%’).Quantitative x-ray examinations and electron-microscopeobservations (39)confirm these results.

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V. THETEXTURE OF CATALYSTS

I. The Surface Area The universal introduction of the measurement of the total surface area by means of an adsorption isotherm of a physically adsorbed indifferent gas, preferably nitrogen, has had a great stimulating influence. The theoretical foundations of the Brunauer, Emmett, and Teller (B.E.T.) equation are such (43) that it may be better to consider it as a successful empirical equation. The practical figures obtained by this method mostly compare well with results obtained by other methods. However, we must be aware of exceptions. The B.E.T. method gives figures which are too high for adsorbents with very narrow pores and high surface areas (44). Better results are then obtained with the newly developed method of Halsey and collaborators (&), using rare gases at normal temperatures. This method, however, does involve elaborate calculations. We must also keep in mind that a sigmoid-shaped isotherm does not always indicate multimolecular adsorption (46). Owing to the presence of narrow capillaries, the total surface area is not always available for catalysis or for other applications. The dipole adsorption of lauric acid from pentane solutions on the surface of polar substances, such as A 1 2 0 3 ,indicates the surface area available in the wider pores (47'); a silica surface, however, is not polar enough for the general adsorption of lauric acid (3'7). 2. The Pore Volume

The pore volume-for pores with a radius smaller than 7.5 p-is mostly estimated by subtracting the specific volumes measured with mercury and with helium. When applied to microporous systems with a large surface area, however, the latter volume needs to be corrected because of the fact that the helium atoms have a volume of their own (46, 48). As the acting radius of the helium atom is not known and as a possible adsorption of helium slightly compensates the effect, it is difficult to estimate the actualvalue of the correction. It may, however, amount to a few per cent of the density. The pore volume amounts often to the same value as the volume of the real solid material; in many microporous systems it is even higher. S. The Wid& of the Pores

An average value for the width is often obtained from the figures for the total surface area (8)and the pore volume ( V ) ,viz., r (ord)

=

2V/S

where r is the radius in the case of cylindrical pores or d is the width of the clefts when we deal with fissure-shaped pores.

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The occurrence of hysteresis in adsorption phenomena, caused by capillary condensation, has led to the application of the Kelvin equation for the desorption branch of a complete adsorption isotherm and thus to a complete distribution curve of the widths of the various pores as a function of their volumes (49). The results are mostly expressed in the form of radii of cylindrical pores. The method may be applied for radii between 20 and 300 A. The figures obtained have to be corrected for the thickness of the multilayer adsorption on the surface of the nonfilled capillaries (50), and various calculation methods have recently been published (51) which need not be discussed here, since Wheeler (56) gave an excellent review recently. A completely different method for the measurement of the pore distribution was devised by Ritter and Drake (53)by using the penetration of mercury at higher pressures. This method can be applied to pores of a width of 7.5 p (1 atm.) down to, e.g., 75 A (1000 atm.) or lower. Many results agree very well with the above-mentioned method, based on capillary condensation (54),although the application of a constant contact angle seems to be somewhat arbitrary.

4. The Shape of the Pores The idea that the pores should be cylindrically shaped-a picture mostly used in literature-is in reality the most unlikely one. For any cross section, however, be it square, rectangular, or regularly or irregularly polygonal in shape, a circular one can be substituted, chosen in such a way that the volume of a pore of a length L is given by d L , r being the radius of the substituted circle. The surface area of such a pore is then given by

Sh = 2wFL where F may be called the shape factor of the pore (55).The average “radius” of the pores is, therefore, r / F = 2V/S which automatically gives d for fissure-shaped pores. It must be kept in mind that also the application of the Kelvin equation leads to an r / F value; a comparison of the average value and the Kelvin value, therefore, does not give any information about the shape of the pores. The other methods mentioned above also seem to be insensitive for the shape of the pores. A recent investigation (56) applying the desorption and the mercury penetration methods to various arrangements of spherically shaped particles of a uniform diameter of about 150 A showed that the total surface area calculated from the pore distribution, evaluated with the aid of the model of cylindrical pores (168 m.”g.) agreed not only with the surface area, measured with the B.E.T. method (165 m.”g.) but also with the geometrically estimated surface area, with the aid of electron-microscope photos of the

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spheres (175 m.”g.). Calculations (57) showed that the observed phenomena may just as well be described by capillary condensation in cylindrical pores as in the open spaces of arrangements of spherical particles. Sometimes, however, a comparison between the adsorption branch and the desorption branch may lead to a conclusion about the shape of the capillaries. An adsorption branch which has no inflexion point and gives a sharp rise only for relative pressures close to unity, combined with a desorption branch showing a definite inflexion point at medium values of relative pressures, indicates fissure-shaped capillaries (58). Hysteresis curves of this form are, for example, found with agglomerations which consist of disk- or plate-shaped particles, such as montmorillonites, and indeed hysteresis curves published by Barrer and MacLeod (59) show this behavior. Similar curves are found with the dehydration products of many well-crystallized metal oxide hydrates, such as those of the aluminum hydrates (gibbsite, bayerite, boehmite, and diaspore) (60). Optical (form birefringence) and x-ray examinations of these latter products indicate the existence of systems of mutually parallel oriented fissureshaped capillaries. Even after severe sintering such a parallel orientation is still at least partially present (61). The surface areas of cylindrical or pseudocylindrical pores are bound t o decrease when increasing amounts of strongly adsorbed matter are applied ; fissure-shaped capillaries should not show such an effect. Recently Fortuin has obtained some promising results with the lauric acid method of measuring surface areas on well-sintered samples of alumina, before and after the introduction of strongly bound OH-groups and water molecules (6%’). ACKNOWLEDGMENT Although this abbreviated survey could stress only some of the most important points, the author has taken the opportunity to incorporate some brief remarks on the recent results obtained by his research group a t Delft University and on the catalyst research group of the Central Laboratory of the Staatsmijnen a t Geleen (the Netherlands). He wishes to express his sincere thanks to Mr. P. Zwietering of the latter group for his assistance in preparing this review.

Received: February 27, 1966

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39. Coenen, J. W. E., Delft, to be published. 40. van Eijk vanvoorthuijsen, J. J. B., andFraneen, P., Rec. Irav. chim. 70,793 (1951). 41. Selwood, P.W., Adler, S., and Philips, T. R . , J . Am. Chem. SOC.7 7 , 1462 (1955): Sabatka, J. A., and Selwood, P. W., ibid. 77, 5799 (1955). 4.9. Heukelom, W., Broeder, J. J., and van Reyen, L. L., J . chim. phys. 61,474 (1954). 43. de Boer, J . H., “The Dynamic Character of Adsorption.” Oxford U. P., New York, 1953;Hill, T. L.,Advances i n Catalysis 4, 212 (1952);Halsey, G.D., ibid.

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