摘要:

CONCLUDING REMARKSby G.-M. SCHWABIt has already become a rule, that the General Discussions of the Faraday Societygive a strong impulse forward to the subject discussed. This has especially been thecase with the Liverpool Discussion on Heterogeneous Catalysis in 1951. There, theprevailing feature was the notion of electronic processes during catalysis which istoday generally named " the electronic factor ". At that time we were all veryexcited over these new notions which gave more or less for the first time a deeperinsight into the mechanism of the catalytic activation. The question arises whetherour discussion now, 1966, has given a similar impact to the research in the field ofchemisorption and catalysis. Very generally it may be said that this discussion wasnot so much characterized by the appearance of new theoretical points of view butrather and predominantly by the appearance and evaluation of new experimentaltechniques.Catalysis is by definition a kinetic phenomenon and problem.And for thisreason I have repeatedly emphasized that it should be studied by kinetic methods.This appears still to be the case, for, amongst the twenty-three papers discussed therewere still eight based mainly on kinetic measurements or chemisorption observations.However, the rest is mainly devoted to the introduction of a number of moderntechniques.A general remark which should be made in this connection is that a species whichcan be observed experimentally in the surface of an adsorbent or a catalyst may beproved to be present there, but it is not at all proved to be an intermediate step of thereaction considered.Another point is that every new method needs a number ofyears before it is adapted to special purposes. This is the case with most of the newmethods presented in this meeting.A very useful tool which now begins to play an increasing role in catalysis researchis theJieZd emission microscope. It can be used as an electron emission microscopefor the study of the general adsorption behaviour on different faces and as an ionemission microscope for the investigation of more intimate details in the surface.Here, different groups have agreed that an adsorbed gaseous species is not onlyinfluenced by the solid but it exerts in turn a certain influence on the solid penetratingbeneath the outermost layer and re-arrangeing atoms in its surroundings.Althoughthis re-arrangement is not beyond doubt it must be taken into account in future.Difficulties in the interpretation of electron micrographs occurred frequently becauseit is not always clear what is seen on the screen. Here, additional parameters, likework functions, (emerging from the microscopic work itself), resistance measurementsand chemical methods must be used.The next technique which is already an old one, however, and is still open todiscussion, is the use of evaporated thinJiZms. The old idea that this method offeredthe possibility of exposing a large and yet smooth surface to a gas proved not to bevalid. The inner surfaces and the porous structure of the film complicate the pictureconsiderably because adsorption and desorption are modified by delayed exchangebetween external and internal surfaces. At the same time, it is still questionablewhether the heat of chemisorption is a constant and even what is its exact value.Itmay also be doubted if the purity of a film surface fulfils the rigorous conditions ofultra-high vacuum techniques. The shape of a suitable calorimeter for film adsorption41 414 CONCLUDING REMARKSis also under discussion. The former simple interpretations connecting resistanceand electron concentrations must be modified by using simultaneous values of workfunction, resistance and heat of adsorption. Extraction of electrons need not neces-sarily result in an increase of resistance because the density of states near the Fermisurface is of importance.One interesting point is that on an evaporated nickel film for the first time aparamagnetic surface conversion of parahydrogen on a metal has been discovered.Instead of supposing that the sum of the inhomogeneous fields of surface atomsresults in a homogeneous field, one has to imagine a hopping of para-hydrogenmolecules from one surface atom to the next.A still older technique is the use of the Brunauer-Emmett-Teller equation for thedetermination of the inner surface of a porous solid.Although this method givesconsistent values of practical use its theoretical basis is as questionable as its relia-bility with non-porous solids. As an alternative, the potential theory as modified byDubinin and his co-workers has been discussed.Sometimes, surface areas deter-mined by these two methods coincide, but this appears to be rather fortuitous.Infra-red spectroscopy of adsorbed layers is a very useful tool for identifyingadsorbed molecules. We know, however, from many examples that species visiblein the infra-red spectrum do not have any connection to the chemical. process studiedon the same surface. In addition, it is necessary to know exactly the nature of theadsorbing surface. Very often, chemisorbed states on metal surfaces have beenstudied by taking infra-red spectra of layers adsorbed on metals finely divided onsupports. It has been shown during this discussion that even after reduction of suchpreparations, sometimes no free metal is present and that the modifications of theinfra-red spectrum are brought about by the influence of non-reduced ions in thesurface.Partly reduced species of radical-like nature may also occur. Only thesimultaneous study of the changes the spectrum undergoes by chemical tieatment canlead to conclusions which agree also with electron-spin-resonance observation.An extremely useful tool for the clarification of reaction mechanism is the use ofradio-active or stdble isotopes, the latter being used in connection with mass-spectro-scopy. It is surprising that during this discussion this method has been applied onlyonce in the study of oxygen exchange in the gas phase and between the gas phase andsolid.It proved very fruitful in comparing rates of exchange with rate of hetero-geneous oxidation reaction.Electron spin resunance is a modern method for the study of adsorbing surfaces.It was applied to problems of this kind only a few years ago, and much discussion isstill active on the interpretation of the curves. Here, we have especially dealt withthe appearance of Cr5+ in chromia-alumina-surfaces. The interpretation of thespectra is now unanimous; however, it has not yet been possible to correlate thesignals obtained with the catalytic activity of the preparations. It appears thatparticles and molecules or radicals active in catalysis are not sufficiently stable to beobserved by e.s.r. Those who apply the method are well aware of the fact that alarge amount of preliminary and comparative work is needed for every new systemstudied.On the other hand, much new information on the types of adsorption canbe drawn from such experiments, e.g., the occurrence of 0, and 0; on titania surfaces.Finally, I want to mention the study of zeolites as catalysts and adsorbents. They areable to give useful information because here the number and the coordination of theactive centres is known a priori. It appears that chemisorbed metal ions may actthrough their electric fieldsCONCLUDING REMARKS 41 51 do not intend in this connection to sumarize the extensive discussions carriedout on the problem of the electronic factor in semiconductor catalysts. The measure-ment of the Hall-eflect can be regarded as making useful progress which generally hasconfirmed the conclusions of former investigations.In addition to experimental progress, it appears to me that there has also been sometheoretical progress during the last fifteen years, although perhaps not equally spec-tacular as the introduction of the electronic factor appeared to be.This factor ispresently subject to considerable refinement. It is no longer only the delocalizedelectronic states in Brillouin-bands, in d-holes, in semiconductor-bands, for whichattempts are made to connect them with catalytic activity, but rather localized statesin the surface. Detailed pictures have been drawn of orbitals emerging from thesurfLrce in a defined number and defined direction. These orbitals may be used toform chemisorption bonds with acceptor molecules or donor molecules, and especiallyalso with n-electrons.This will permit in future the application of the notions ofligand field theory to catalysis. We know already of examples where catalytic actionin solution and in surfaces is similar.The second important theoretical progress, in my opinion, is the emphasis laidon virtual pressures. In more cases than hitherto envisaged, the adsorption coverageof an intermediate species in a chemical reaction should be considered as dependenton the pressure which the respective entity would have in the gas phase if equilibriumin that phase were established.It is to be hoped that our discussion has shed considerable light on the applicabilityand on the limitations of a number of powerful new tools in catalysis. Therefore infuture we will know better how to use these tools, and new theoretical concepts willhelp towards a better interpretation of the results achieved by them. Provided wedraw all the necessary consequences from the results of our discussion it may be hopedthat when we meet again after another 15 years we shall know many more detailson the intimate mechanisms of heterogeneous catalysis

ISSN:0366-9033    

DOI:10.1039/DF9664100413

出版商:RSC    

年代:1966

数据来源: RSC

摘要:

AUTHOR INDEX *Amberg, C. H., 175. 410.Amigues, P., 362.Angell, C. L., 328.Barry, T. I., 394.Bassett, D. W., 65, 73.Blyholder, G., 251.Bond, G. C., 200, 251, 253, 254, 255.Boreskov, G. K., 263, 307, 309.Brennan, D., 56, 67, 95, 106, 116, 187.Brocker, F. J., 87.Burwell, R. L., Jr., 215, 249, 258, 259, 309, 406.Byrne, J. J., 261.Cadenhead, D. A., 260.cernf, S., 102, 111, 117, 149.Chon, H., 380, 401, 407, 41 1.Cimino, A., 249, 350, 399.Clarke, J. K., 261.Codell, M., 323.Cornaz, P. F., 290.Cossee, P., 253, 277, 314.Davis, B. J., 43, 74.Degols, L., 410.Delchar, T. A., 72.Dewing, J., 399.Dowden, D. A., 256.Duell, M. J., 43, 74.Ehrlich, G., 7, 54, 57, 58, 61, 68, 102, 112.Eischens, R. P., 179.Eley, D. D., 135, 184, 185, 256, 262.Ellison, A., 315.Erkelens, J., 175.Ertl., G., 72, 252.Frennet, A., 114.Fujita, Y., 181, 407.Gay, I.D., 398.Geus, J. W., 62, 113.Gomer, R., 14, 61, 63.Graham, M. J., 95.Haber, J., 313, 406.Hansford, R. C., 394.Haul, R. A. W., 119, 308.Hayward, D. O., 75, 102.Heyne, H., 181.Hill, T., 314.Holscher, A. A., 29, 54, 62, 70, 73, 1 1 1 .Hooff, J. H. C. van, 290.Iyengar, D., 323.Joke, B. J., 223.Jirfi, P., 306, 307.Karra, J., 310, 321, 323.Kasai, P. H., 328.Kemball, C., 186, 190, 249, 258, 395.King, D. A., 63, 106, 109.Klemperer, D. F., 187.Klier, K., 306. 397.Knor, Z., 57, 63, 149.Kuchynka, K., 397.Liefkens, Th. J., 175.Lienard, G., 183.Mathieu, M. V., 177.Moss, R. L., 43, 74.Muller, J., 119, 186.Mykura, H., 59.Nelson, R.L., 322, 396.Norton, P. R., 135, 183.Novhkova, J., 307.Oudar, J., 58.Parkyns, N. D., 175.Peri, J. B., 121, 179.Ponec, V., 117, 149, 181, 184.Pluijm, F. J., 290.Prater, C. D., 380, 401, 407, 411.Rabo, J. A., 328, 396.Reijen, L. L. van, 252, 277, 308, 318.Rigby, L. J., 65.Roberts, M. W., 104, 162, 188, 189.Roginskii, S. Z., 65, 111, 319 400.Rooney, J. J., 223, 257, 258, 260.Rozendaal, A., 253, 254.Sachtler, W. M. H., 29, 67, 70, 109, 261, 262.Schiavello, M., 350.Schomaker, V., 328.Schrage, K., 215.Schnit, G. C. A., 290, 326.Schwab, G.-M., 252, 253, 256, 413.Sheppard, N., 177, 254.Sing, K. S. W., 120, 315.Steiner, H., 312.Stone, F. S., 306, 321, 350, 395, 398, 399.Szab6, Z. G., 305, 394.Taylor, N., 75.Teichner, S. J., 188, 362, 404, 407.Thomas, J. M., 71, 262, 398.Thomas, W. J., 176.Tompkins, F. C., 72, 75, 181, 186, 398.Turkevich, J., 181, 309, 310, 321, 323, 407.Volter, J., 56, 401.Webb, A. N., 178.Wedler, G., 87, 104, 108, 109.Wells, P. B., 223, 237, 257.Wells, B. R., 112, 162.Wilkinson, R. W., 396.Wilson, G. R., 223, 237.* The references in heavy type indicate papers submitted for discussion.41

ISSN:0366-9033    

DOI:10.1039/DF9664100416

出版商:RSC    

年代:1966

数据来源: RSC