Some Techniques for Studying the Surface of Solid Oxidation Catalysts

44 Some Techniques for Studying the Surface Oxidation of Organic Compounds Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 06/20/16. For personal use only.

of Solid Oxidation Catalysts S. W. WELLER Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, Ν. Y.

Selected techniques are discussed for characterizing transi tion metal oxide and metal catalysts for organic oxidations. Chromia is considered a representative transition metal oxide, platinum and silver as representative metals. The importance of catalyst characterization under reaction con ditions is emphasized since the chemical state of the catalyst surface depends on the competitive rates of adsorption, sur face reaction, and desorption of several gaseous constituents in the redox atmosphere existing during oxidation. More over, cyclic changes in surface chemistry during cyclic processes may significantly affect reaction stoichiometry as well as catalytic activity.

major objective of heterogeneous catalysis is interpreting catalytic behavior i n terms of the structure, texture, and chemistry of the catalyst surface. Achieving this objective requires detailed knowledge of the surface at an atomic level. The solid catalysts useful in organic reactions may be categorized loosely into three groups: acid catalysts, catalysts for hydrogénation reactions, and catalysts for oxidation reactions. The structure and action of solid acid catalysts are relatively well understood, thanks i n large part to the extensive pre-existing knowledge of carbonium ion chemistry and of the nature of an acid. Even here it is chastening to note the current controversy on the nature of acid catalysts based on synthetic zeolites. Hydrogénation catalysts, particularly metals, are also relatively well characterized because of extensive study. This has occurred partly for commercial reasons; catalyzed organic reactions of great industrial i m portance—e.g., hydrogénation and reforming—involve reducing condi tions. In part, however, this has occurred because of the esthetic attracA

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Oxidation of Organic Compounds Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SANTA BARBARA on 06/20/16. For personal use only.

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Surface of Catalysts

tiveness of studying clean metal surfaces with elegant experimental tools such as field ion microscopy and low energy electron diffraction. A considerable spectrum of techniques also exists for studying the surfaces of solid oxidation catalysts. Several of these are discussed below. No attempt has been made here to be comprehensive. For example, the powerful tool of optical absorption spectroscopy (particularly infrared) is now so well-known and widely used that there is no need here to emphasize its importance. Before proceeding a word of caution is in order. There is much ambiguity in the loose characterization of catalysts given above. First, in a general sense the same nominal catalyst may be effective for both hydrogenative and oxidative reactions—e.g., platinum and chromia. O n a finer scale, however, profound differences in the surface chemistry of a given catalyst may exist in oxidizing and reducing atmospheres, even though the same crystallographic bulk phase is present. This is true of both chromia and platinum at elevated temperatures. The active catalytic surface w i l l then be quite different in the two environments. This situa tion is further confounded in organic oxidation reactions because both reducing and oxidizing gaseous constituents are present. The actual state of the catalyst surface depends on the competitive rates of adsorption, surface reaction, and desorption of the several gaseous constituents. A t the present state of knowledge, our best recourse is to the experimental characterization of the catalyst surface under reaction conditions. Selected Techniques for Studying Catalyst Surfaces The following discussion of techniques is eclectic. T w o points are clear: (1) The various techniques are complementary, and as many of them as possible should be applied to the same catalysts; (2) Most of the reported work has been on catalysts studied after arbitrary pretreatment rather than under reaction conditions. For focus, the discussion is centered on two nominal classes of oxida tion catalysts—transition metal oxides, exemplified by chromia, and metals, exemplified by platinum and silver. Transition Metal Oxides (Chromia) The complexity of the situation may be illustrated by tracing the development of our knowledge of chromia catalysts. It is now clear that C r ions on the surface may occur in valence states from C r to C r ; all intermediate valence states have been shown to occur, and all are possible in a regime of temperature and gas composition where the thermodynamically stable bulk phase is C r 0 . 1 1

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A landmark in characterizing chromia-alumina catalysts at the molecular level was the pioneering work of Eischens and Selwood (9,10) on magnetic susceptibility-composition isotherms. The change in suscep tibility of the chromium, as the chromia content was varied, was found to be associated with an almost constant magnetic moment but a varying Weiss constant. The data were consistent with a model in which the chromia occurred as small crystallites on the surface of the alumina, the crystallite size decreasing with decreasing chromia concentration to the limit of a two-dimensional dispersion of chromium ions on the alumina surface. This model has proved useful in all subsequent work. The electrical conductivity of chromia decreases with decreasing oxygen pressure and becomes still lower in hydrogen (4). This result led to classifying chromia as an oxygen excess, p-type semiconductor. However, by observing that the sign of the thermoelectric potential changes i n going from an oxidizing to a reducing atmosphere, Chapman et al. (6) deduced that chromia becomes an η-type semiconductor in pure hydrogen. Voltz and Weller largely confirmed the results both of Bevan et al., and of Chapman et al. (21, 24). New facts were added, however, by studying the chemisorption of both oxygen and hydrogen i n cycling experiments and by attempts to measure, by direct iodometric titration, the extent to which surface C r ions were oxidized to higher valent forms by oxygen (22, 23). It appeared that the bulk of the surface ions are oxidized to higher valent forms i n 1 atm. oxygen at 500°C. and re duced in 1 atm. hydrogen. m

Matsunaga (15) applied the magnetic techniques of Eischens and Selwood and the chemisorption and chemical techniques of Voltz and Weller to a series of chromia-alumina catalysts. H e found that in the limit of low chromia contents, where Eischens and Selwood deduced a two-dimensional distribution of chromium ions, treatment with oxygen at 450°C. resulted in an average valence number of six for all of the chromium ions i n the sample. E S R techniques, introduced about 10 years ago, added an extra ordinarily powerful tool for characterizing in detail the individual pramagnetic species which might be present in the catalyst under varying conditions. C r , C r , C r , and C r are paramagnetic, although C r is not. W i t h their co-workers, Maclver, Cossee, and Voevodsky (7, 17, 18) found that catalysts prepared by progressive reduction of C r 0 3 - A l 0 contained substantial quantities of C r and C r ions. Cossee and V a n Reijen (7) state that hydrogen reduction above 300°C. gives the trivalent state as the stable final product. Pecherskaya et al. (81), however, ob serve that "rigorous reduction" with hydrogen results in a disappearance 2+

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r,+

6 +

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of the narrow line associated with C r , possibly by reduction to C r give the spinel C r A l 0 . 3+


2

2 +

fo

4

H o w much information concerning the surface can be obtained from thermodynamic considerations? There are some relevant data available for the thermodynamics of the bulk phases but few for the surface ( twodimensional) phases. Bulk C r 0 loses oxygen at fairly low temperatures to form a host of reported intermediate oxides. C r 0 , the best defined of these, decom poses to C r 0 with a decomposition pressure of 1 atm. at about 430°C. (5). A t temperatures of 450°-500°C., therefore, C r 0 is the stable bulk phase in oxygen pressures up to 1 atm. Bulk C r 0 is also resistant to reduction by hydrogen in this temperature range. A t 500°C. the equilib rium constant for the reaction C r 0 ( s ) + H ( g ) = 2 C r O ( s ) + H 0 ( g ) is 2.2 Χ 10" (12). For any reduction of the bulk phase to occur at 500°C. and a hydrogen partial pressure of 1 atm., therefore, the water partial pressure must be maintained below 1.7 Χ 10" mm. H g . 3

2

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8

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These results are useful but far from definitive in establishing the nature of the catalyst surface in contrast to the bulk. In oxygen, Matsunaga's data show, surprisingly, that all of the surface chromium ions are C r under conditions where C r 0 is the stable bulk phase (15). In 1 atm. dry hydrogen Weller and Voltz observed that the hydrogen chemisorption at 500°C. corresponded to almost one atom of hydrogen for each surface oxide ion (23). Clearly one must be concerned with a large swing i n valence number of the surface chromium ions from C r to C r , including all possible intermediates, in different atmospheres. Further more, in the complicated atmosphere occurring during organic oxidations, one is not able a priori to predict even whether the net environment at the surface is oxidizing or reducing i n character. 6 +

2

3

6 +

2+

A particularly interesting situation occurs in reactions, such as butane dehydrogenation over chromia, which are normally operated cyclically without reaching steady state. The deposition of coke and drop i n catalyst temperature during the dehydrogenation period necessitate frequent catalyst regeneration by air. The surface chromium ions therefore vary regularly in valence number between 3 + and 6 + , on about a 15-minute cycle. Under these conditions, about 20% of the butane dehydrogenation to butadiene may be attributable to oxidation of the butane by the C r ( or equivalently, by the chemisorbed oxygen ) in the freshly regenerated catalyst [10 minute on-stream period, feed rate = 0.6 gram C H i / g r a m catalyst-hr. ( L H S V = 1), 1.4 Χ 10" moles chemisorbed 0 / gram of regenerated catalyst]. The question, "what is the catalyst?" here be comes difficult to answer. 6 +

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Silver)

Physical methods for characterizing clean surfaces of metals are generally well known and do not require elaboration here. One property of the solid deserves mention, however, because it has been investigated mostly by metallurgists and has not yet been studied adequately in catalysis. This property is the microstructure, in the sense of grain boundaries in a polycrystalline material, and probably more importantly, in the sense of the density of emergent dislocations (edge and screw) which are present at the surface even i n a single crystal. These represent high energy sites, where the catalytic activity may be expected to be atypical. A particularly useful technique for characterizing the microstructure of metal foils is transmission electron microscopy, which can be extended to studying all surfaces by using carbon extraction replicas (19). As yet no catalytic studies seem to have been published on samples in which dislocation density was determined directly. A provocative study has been published recently, however, on silver ribbon which was an nealed to various degrees after initial cold-working (20). The dislocation density was indirectly characterized by measuring hardness and thermo electric potential, both of which decreased as the annealing temperature was increased. Uhara et al. found that the catalytic activity for three reactions—decomposition of H 0 , oxidation of C H O H , and decompo sition of H C O O H — d e c r e a s e d with increasing annealing temperature i n a manner paralleling the decrease in hardness and thermoelectric poten tial. These results are not definitive, but they suggest that further direct study of the role of dislocations in catalysis w i l l prove rewarding. 2

2

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5

A fundamental problem in characterizing metal surfaces in oxidation catalysis is that, as with transition metal oxides, the chemistry of the surface is shaped by the reaction conditions. Margolis has taken the plausible position that most metal surfaces in oxygen are covered with oxygen and behave like metal oxides (13, 14). This is true even of platinum, a classical example of a "metal" catalyst, and here again pre dictions from bulk thermodynamics are unreliable with respect to the surface. The thermodynamic data for the platinum oxides are not well estab lished. However, a reasonable value for the free energy of formation of the lower oxide, PtO, at 527°C. is - 1 kcal./mole (5, 25). This corre sponds to an oxygen dissociation pressure at 527°C. of 0.28 atm.—i.e., bulk P t O is unstable toward decomposition to bulk Pt for oxygen pres sures below 0.28 atm. Bulk P t 0 is, of course, even less stable. Neverthe less, it has been reported that at this temperature and an oxygen pressure 2

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of 10 cm. (0.13 atm.), platinum on alumina chemisorbs oxygen i n amount sufficient to convert 80% of the platinum completely to P t 0 (16). If one is to achieve better understanding of the catalytic behavior of metals i n oxidation reactions, more attention should be given to measuring surface thermodynamics and to establishing the surface chem istry under reaction conditions. There are striking elements of difference between metal behavior i n reducing and oxidizing conditions. As an interesting example, platinum is notoriously sensitive toward sulfur poisoning i n hydrogénation, presumably because of the stability (and relative inactivity) of PtS. B y contrast, platinum catalysts appear to be quite suitable for oxidizing sulfur-containing organic compounds (25). [Among the few literature references on this matter is an observation by Davy (8) that sulfur does not act on platinum in the presence of air.] This insensitivity to sulfur poisoning probably arises from the fact that the reactions PtS + 3/ 0 = P t O + S 0 and PtS + 0 — Pt + S 0 are favorable by some 55-60 kcal./mole. Almquist and Black and Emmett and Shultz 40 years ago pointed out how measurements of surface equilibria, as i n the surface reaction 3/4 F e + H 0 = 1/4 F e 0 + H , can yield values for the excess surface free energy of surface atoms ( J , 2, 11). F o r a single gaseous constituent (as for oxygen on platinum), the method reduces to determin ing chemisorption isotherms. In any case the free energy of formation of the surface phases w i l l be a function of the surface coverage (or gas composition), i n contrast to the thermodynamics for bulk phases. A particularly elegant application of this approach has been reported by Bénard ( 3 ) . In this work the surface reaction A g + H S = AgS - f - H at 400°C. was studied over silver single crystals, and the extent of sur face sulfiding as a function of the F H S / Î H 2 ratio i n gas phase was estab lished separately for the (100), (110), and (111) faces by using radio active sulfur i n the H S . The extent of surface coverage by sulfur at a given P H S / P H ratio varied greatly with surface face, decreasing i n the order (110) > (100) > (111). It is clear that equilibrium measurements of surface thermodynamics cannot predict surface composition under the dynamic conditions of catalytic oxidation. Nevertheless, such measurements w i l l provide a sounder base than bulk thermodynamics for understanding the surface chemistry and permit working backward, from direct measurements of surface chemistry during reaction, to predictions concerning the microenvironment at the surface under reaction conditions.


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Literature Cited (1) Almquist, J. A., Black, C. Α., J. Am. Chem. Soc. 48, 2814 (1926). (2) Almquist, J. Α., J. Am. Chem. Soc. 48, 2820 (1926).

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(3) Bénard, J., "Coloquio sobre Quimica Fisica de Proceso in Superficies Solidas," p. 375, Consejo Superior de Investigaciones Cientificas, Madrid, 1965. (4) Bevan, D. J. M., Shelton, J. P., Anderson, J. S., J. Chem. Soc. 1948 (1729). (5) Brewer, L., Chem. Rev. 52, 1 (1953). (6) Chapman, P. R., Griffith, R. H., Marsh, J. D. F., Proc. Roy. Soc. (London) A 224, 419 (1954). (7) Cossee, P., Van Reijen, L. L., Proc. Intern. Congr. Catalysis, Paris, 2, 1679 (1960). (8) Davy, E., Phil. Mag. 40, 31 (1812). (9) Eischens, R. P., Selwood, J. Am. Chem. Soc. 69, 1590, 2698 (1947). (10) Ibid., 70, 2271 (1948). (11) Emmett, P. H., Shultz, J. F., J. Am. Chem. Soc. 51, 3249 (1929). (12) Maier, C. G., U. S. Bur. Mines Bull. 436 (1942). (13) Margolis, L. Ya., Izv. Akad. Nauk, S.S.S.R., Otd. Khim. Nauk 1958, 1175. (14) Ibid., 1959, 225. (15) Matsunaga, Y., Bull. Chem. Soc. Japan 30, 868 (1957). (16) Mills, F. Α., Weller, S. W., Cornelius, Ε. B., Proc. Intern. Congr. Cataly sis, Paris, 2, 2221 (1960). (17) O'Reilly, D. E., Maclver, D.S.,J.Phys. Chem. 66, 276 (1962). (18) Pecherskaya, Y. I., Kazansky, V. B., Veovodsky, V. V., Proc. Intern. Congr. Catalysis, Paris, 2, 1694 (1960). (19) Thomas, G., "Transmission Electron Microscopy of Metals," Wiley, New York, 1962. (20) Uhara, I., Kishimoto, S., Yoshida, Y., Hikino, T., J. Phys. Chem. 69, 880 (1965). (21) Voltz, S. E., Weller, S. W., J. Am. Chem. Soc. 75, 5227 (1953). (22) Ibid., 76, 4701 (1954). (23) Ibid., p. 4695. (24) Weller, S. W., Voltz, S. Ε., Z. Physik. Chem. (N.F.) 5, 100 (1955). (25) Weller, S. W., unpublished results. RECEIVED April 8, 1968.

Some Techniques for Studying the Surface of Solid Oxidation Catalysts

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