C 0 NFE REN C E RE PORTS

Fourth International Catalysis Congress‘*’

The Moscow Catalysis Congress from June 23 to 29, 1968 was the fourth in a series of four-yearly international con- gresses on heterogeneous catalysis. The kinetics and mecha- nisms of special reactions, new research methods, and the properties of little-known catalysts were dealt with in six plenary lectures and 86 discussion lectures, but the principal topic was the relation between the chemical composition, crystalline and electronic structure, and the activity and se- lectivity of catalysts.

1. The Active Center

Though many systems are now known in which the entire catalyst surface is uniformly active, the particularly high activity of special centers on the surface of others is partly due to geometric or topochemical factors. Uniformly active sur- faces have been found on metal oxides i n simple combustions and on metals in simple hydrogenations. This is so e.g. ac- cording to P. C. Aben, J . C. Platteeuw, and B. Stouthamer (Netherlands) (31) in the hydrogenation of benzene on Pt, Pd, and Ni catalysts supported on SiOz, AI2O3, A1203-Si02, and MgO-SiO2, and according to M . Kraft and H. Spindler (Leuna) (69) in the dehydrogenation of cyclohexane with Pt on A1203. However, these authors also found that the surface of the same catalyst is not uniformly active in the dehydro- cyclization reaction of n-heptane. R. van Hurdeveld and F. Hartog (Netherlands) (70) also point out that a catalyst can have several types of centers, which can act differently in dif- ferent reactions; nickel on aerosil is reported to have a uni- formly active surface in the deuteration of benzene to give cyclohexane, but not in the H/D exchange between benzene and Dz. Apart from crystallite corners, edges, sur- faces, and lattice defects, active centers in metal catalysts in- clude crystallite steps of atomic height (Bs sites), as was described by G. C . Bond (England) (67).

The importance of the topochemical factor was discussed by G. Purruvuno (USA) (11) for the 12C/14C exchange in CO&O mixtures and for the decomposition of Hz02 and NzO on cobalt ferrites having the composition c03-~ Fex04 (x =

1.9 . . . 2.1). Whereas the stoichiometric ferrite ( x = 2.0) gives a minimum rate for the first reaction, it is found to be the optimum catalyst for the other two reactions; this is attributed to the change in the distribution of ion vacancies with the Co/Fe ratio. B. V. Erofeev, N. V. Nikiforova, I. I . Urbunovich, and L. D. Dmitrieva (USSR) (47) reported that Cu and Mo on A1203, MgO, SiOz, BeO, ZnO, and some spinels are particularly effective in the dehydrogenation of cyclohexane or the dehydrocyclization reaction of n-hexane if the metals are atomically dispersed and situated in cation sites of supports with octahedrally arranged anions. Accord- ing to K. Turama, s. Yoshida, and Y. Doi (Japan) (13), Cr5+ ions, which are characterized by an unusual crystal field, are active in the copolymerization of ethylene on chromic oxide.

A geometric factor is particularly important for systems in which multi-point adsorption of the reacting materials occurs. For example, according to 0. V. Krylov and E. A. Fokina (USSR) (64)’ the activity of metal oxides in the reaction of acrolein with methanol increases with the lattice constant.

[*I In this report, the lecture number is given (in parentheses) in each review, cf. Proc. IV. Internat. Congress Catalysis, Moscow 1968, in press.

0. N . Bragin and A . L. Libermann (USSR) (27) showed that the hydrogenolysis of alkylcyclopentanes and cyclohexane on Ru, Rh, Os, and Ir, which proceeds by two-point adsorption, yields a large number of products, whereas on Pt, owing to six-point adsorption, only the alkylcyclopentanes, and not the cyclohexane, react. According to H. Noller, P. AndrPu, E. Schmitz, S. Serain, 0. Neufang, and J. Girdn (Hannover and Venezuela) (81), two-point adsorption is also responsible for the stereospecific action of many catalysts in the dehydro- chlorination and dehydrobromination of 2,3-dichloro- or 2.3-dibromobutane to form 2-halogenobutene. For example, while KB02 and K2CO3 give a high cis-trans ratio in the end products, CaC12 and Ca3(PO& give a low ratio; this is explained by differences in the adsorption of substrate halogen on the catalyst cation and of substrate hydrogen on the catalyst anion.

In most insulating catalysts and in reactions with uncharged adsorbed intermediates, acid groups on the catalyst surface act as active centers. This was shown by T. Yamaguchi and K. Tanaba (Japan) (80) in the hydrolysis of dichloromethane on NiS04, ZnS. SiOz, and A1203-Si02, by Y. Noto, K . Fu- kuda, T. Onishi, and K. Tamaru (Japan) (37) and J. M . Criado, J. Dominguez, F. Gonzhlez, G. Munuera, and J. M . Trill0 (Spain) (38) in the dehydration of formic acid on A1203, Si02, CrZ03, and Ti02 and by B. D. Flockhart. S. S. Uppal, I . R . Leith, and R. C . Pink (Ireland) (79) in the H/D exchange between n-propane and Dz on Al2O3. According to H . Bremer and K . H. Steinberg (Merseburg) (76), T. V. Antipina, 0. V. Bulgakov, and A . V. Uvarov (USSR) (77). and T. Nishizawa, H. Hattori, T. Uematsu, and T. Shiba (Japan) (55 ) . acid groups also bring about the dehydrogenation of isopropanol on MgO-SiO2 and cracking processes, as well as the polymerization of ethylene and propylene and double bond migrations in 1-butene on partly fluorinated or hydro- fluorinated A1203 catalysts. In all these cases the activity increases with the concentration of Lewis- or Brmsted centers.

2. The Catalyst-Substrate Bond

A knowledge of the nature and strength of the chemisorption bond is particularly useful for the establishment of reaction mechanisms and for the estimation of reaction rates. As was emphasized by J. L. Garnett, R . J. Hodges, and W. A. Sollich- Buumgartner (Australia) (l), there is an analogy between homogeneous and heterogeneous catalysis, as a result of which the interaction between individual substrate molecules and isolated catalyst structural units also approximately describes the chemisorption on the surface of solid catalysts. In the H/D exchange of monohalogenobenzenes, alkyl- benzenes, and polycyclic aromatic compounds with DzO on Pt, for example, the analogy is attributed to the n-complex mechanism occurring in homogeneous and heterogeneous catalysis. V. S. Feidblium (USSR) (I 5 ) referred to the analogy in the dimerization and isomerization of a-olefins on Ziegler catalysts. P. Cossee, P. Ros. and J. H . Schachtschneider (Netherlands) (14) presented results of quantum-chemical calculations that give a detailed picture of the energy rela- tionships of the catalyst-substrate complex in the first- mentioned case. A. Cimino, V. Indovinu, F. Pepe, and M. Schiuvello (Italy) (12) studied the action of isolated catalyst structural units on the decomposition of NzO on solids containing atomically dispersed CrzO3 as catalyst doped with

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various amounts of Liz0 in a catalytically inactive MgO matrix. H. H. Dunken and C . Opitz (Jena) (2) calculated the chemi- sorption bond of CO, HCN, OH, and hydrocarbons on metals by the MO-LCAO method. The authors did not examine only the interaction between substrate molecules and isolated metal atoms, but also considered small metal domains. The results correspond qualitatively to a number of important experimental results. Some authors also examined the interactions between sub- strates and the solid’s electrons. However, an electronic factor can be expected only with semiconducting catalysts and possibly with metallic catalysts, but not with insulators, and also only in reactions with complete electron exchange between substrate and catalyst (ionosorption). It is therefore not surprising e.g. that F. Bozon- Verduraz and S. J. Teichner (France) (6) found no change in the rate of hydrogenation of ethylene on ZnO whose electron concentration was varied by doping, since the rate-determining step in this case is not associated with an electron exchange. On the other hand, T. Freund, S . R . Morrison, and W. P . Gomes (USA) (4) found that in the oxidation of formic acid to Hz02 and COz on ZnO exposed to light, the concentration of electrons and holes on the catalyst surface determines the reaction rate. According to J. M. Criado. J. Dominguez, F. Gonzaler, G. Munuera, and J. M. Trill0 (Spain) (38). the rate of de- hydrogenation of formic acid on Crz03 and on Ti02 also increases with the electron concentration, which is varied by doping. C. Borgianni. F. Cramarossa, F. Paniccia, and E. Molinari (Italy) (7) found that p-semiconductors (CrzO3, NiO) are more active than n-semiconductors (ZnO, FezO3) in the combustion of Hz. E. G. Schlosser and W. Herzog (Frank- furt/Main) (9) presented a semiquantitative interpretation of the electronic factor on the basis of the combustion of CO and HZ on doped NiO catalysts. Taking into account the space-charge layers on the catalyst surface, they were able to show that both the increase in reaction rate with increasing hole concentration and the decrease in reaction rate with decreasing hole concentration are much smaller than ex- pected. J. Scheve and I . W . Schulz (Berlin) (84) believe that the electronic factor is due to energetic excitation of adsorbed molecules as a result of collision with electrons of the solid. The best catalyst is accordingly characterized by maximum energy transfer with agreement between the thermal energy of the electrons and special energy term differences of the molecule, as was shown for the decomposition of NzO on CuO/CrzO3 mixed catalysts. N . P. Keyer (USSR) (8) mentioned that the “field effect”, in which changes in the electron concentration on the surfaces of solids are brought about by an electric field, should be particularly useful for the investigation of electronic factors, since the catalyst mixture does not change in this case. Re- sults have already been obtained for the decomposition of isopropanol on TiOz. On the basis of investigations on the hydrogenation of benzene on Cu/Ni alloys, D. A . Cadenhead, N . J. Wagner, and R. L. Thorp (USA) (26) warn against overestimation of the electronic factor. Whereas the decrease in the reaction rate observed in this case with increasing Cu content had previously been attributed entirely to the filling of the electron vacancies in the 3d band, the authors now believe that the only important factor is the Ni concentration on the catalyst surface, which can differ from that in the interior as a result of a separation effect.

3. Catalytic Activity and Thermodynamic Quantities

AS was shown by W. M . H. Sachtler, G. J. Dorgelo. J. Fahren- fort, and R . J. H . Voorhoeve (Netherlands) (34) for the oxida- tion of benzaldehyde to benzoic acid on MnOz, V205, and Vz05/SnO~ mixed catalysts, by V. E. Ostrovsky and N. N.

Dobrovolsky (USSR) (46) for the combustion of Hz on Cu, Ag, and Au, and by A . Cimino, V. Indovina, F. Pepe, and M . Schiavello (Italy) (12) for the decomposition of N20 on Cr203iMgO mixed catalysts doped with LizO, the activity of the catalysts increases as the chemisorbed oxygen becomes more loosely bound. This is not surprising, since the reaction consists essentially of two steps, i.e. the combination of the organic substrate or of hydrogen with chemisorbed oxygen and the re-chemisorption of oxygen on the catalyst, the first step being rate-determining. Similar relationships were found by G. K . Boreskov, V . V. Popovsky, and V. A. Suzonov (USSR) (33) in the 160:180 exchange in oxygen isotope mix- tures on oxides of the first series of transition metals and in the combustion of Hz and methane on cobaltites and chromites. D. G. Klissurski (Bulgaria) (36) confirmed these findings in the oxidation of methanol to formaldehyde o n transition metal oxides and on SbzO3, W03, and Sn02, and K. Kochloej, M . Kraus, and V. Baiant (CSSR) (85) mentioned in this connection the dehydration of secondary alcohols on SiO2, Ti02, ZrOz, and Alz03. In all these cases there is a linear relationship between the logarithm of the reaction rate or the activation energy and the bonding energy of the surface oxygen i.e. the free energy of surface oxygen release. This linear relationship, which is also known as the linear free energy relationship (LFER), may according to V. A. Roiter, G. I . Golodetz, and Y. I. Pyatnirzky (USSR) be regarded as a solid analog of the Hammett or Taft relationship found for homogeneous organic reactions. The authors also showed in the combustion of Hz and propane on transition metal oxides that the activity cannot be increased indefinitely by the use of catalysts having very low oxygen-binding energies; on the contrary, there is an optimum catalyst for which the oxygen-binding energy is roughly equal to half of the enthalpy of the overall reaction. If the oxygen-binding energy is further decreased the catalyst activity decreases again, since the oxidation of the catalyst now becomes rate-determining instead of its reduction. A plot of the activity against the oxygen-binding energy gives a diagram having the volcano shape described by Balandin. R. A. Gnrdner (USA) (83) also assessed the catalytic activity from the strength of the catalyst-substrate bond by evaluation of special bond strengths of the intermediates, which affect the IR vibration frequency, as shown for the combustion of Hz and the combination of H2 and CO.

4. Statistical Methods for the Discovery of Catalysts

The work of I . I. Joffe. V. S. Fyodorov, B. Y. Gurevitch, M . A . Ustrayh, I . S . Fux, and K . M . Muhenberg (USSR) (63) may be of special practical interest. The authors use the parameter calculation method known from statistics t o find active and selective catalysts, e.g. for the oxidation of hydrocarbons, and illustrated the process in the case of the combustion of CO on metal oxides. It was found that out of 20 tabulated physico- chemical quantities of the catalysts, only eight are significant for the catalytic activity. These quantities in order of increas- ing influence, are density, color index, melting point, vacancy volume, electronic structure factor, susceptibility, concentra- tion of d-electron vacancies, and ionization potential of the metal. On the basis of these quantities, a number of metal oxides that have not yet been tested were chosen as promising catalysts for the combustion of CO. D. A . Dowden, C. R. SchneN, and G. T. Walker (England) (62) showed how active and selective catalysts for complicated reactions can be found by the use of an arrangement scheme by means of which a large volume of experimental data can be taken into account, e.g. catalysts for the steam reforming

[VB 169 IE] of paraffins.

German version: Angew. Chem. 80,917 (1968)

Angew. Chem. internat. Edii. 1 Vol. 7 (1968) / No. 11 897