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Structure sensitivity and insensitivity in catalysis
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He stayed then for two years as a research fellow at Argonne National Laboratory before becoming a senior scientist at the University of Hannover. In 1969 after receiving his habilitation degree he moved to Krupp Chemical Engineering Division (now: Krupp-Koppers) where he was head of chemical research and development. In 1974 he became full professor for chemical technology and reaction engineering at the Ruhr-University Bochum. Manfred Baerns is a member of the DECHEMA Board, chairman of DECHEMA's working party on industrial reactions and a member of its working party on catalysis. Besides being scientific coordinator of several research programmes, consultant for industrial companies and member of advisory boards in different research programs in Germany and abroad, he is an active researcher himself. Since taking over his professorship in Bochum, Manfred Baerns has addressed in his research different aspects of heterogeneous catalysis and catalytic reaction engineering. In the fundamental studies aimed at a better understanding of reaction mechanisms he applies such methods as XPS, PD/DSC, in-situ DRIFT spectroscopy and transient methods e.g., the Temporal Analysis of Products reactor. The results of these studies have been used in the development of catalysis for oxidative coupling of methane to C2+ hydrocarbons, partial oxidation of methane to synthesis gas, Fisher-Tropsch synthesis, selective oxidation of propane, butene and aromatic hydrocarbons to their oxygenates and more recently NO× reduction. In the field of catalytic reaction engineering his studies are aimed at developing kinetic models for various reactions as well as reactor modelling and simulations as means for the
applied catalysis A: General
optimization of reaction conditions and reactor design. This large research experience will certainly be very useful in his new, very challenging position. We wish Manfred Baerns every success in Berlin. L. MLECZKO Structure Sensitivity and Insensitivity in Catalysis
Exchange reactions with D2, and hydrogenation and dehydrogenation reactions of hydrocarbons on metal catalysts are regarded as structure insensitive, but hydrocracking as being structure sensitive. The first class involves only C-H bond breaking and making while the second consists of C-C bond fission processes. Classical examples are ethane/D2 exchange and ethane hydrocracking to methane, respectively. The term insensitive is usually used to describe a reaction where all the exposed surface metal atoms are believed to be catalytically active, even for polycrystalline materials. Recent work on paraffin/D2 exchange on Pt/TiO2 catalyst in the non-SMSI and SMSl states [A.C. Faro and C. Kemball, J. Chem. Soc., Faraday Trans., 91 (1995) 741; W. Patterson and J.J. Rooney, Catal. Today, 12 (1992) 113, and references therein] gave the surprising result that neither the rates nor the multiplicities vary greatly from one type of catalyst to the other. This was especially true of methane, the paraffin which is most difficult to activate. However, H2 chemisorption capacity falls dramatically by a factor of 104-105 from the non-SMSl to the SMSI catalyst. The only viable conclusion is that the sites for paraffin activation are very few and that Volume 131 No. 1 - - 14 October 1995
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the exchange reaction involves direct reaction of D2 (Eley-Rideal mechanism) with the adsorbed hydrocarbon species. The reaction is in reality structure sensitive, but the presence or absence of a vastly superior number of metallic sites capable of hydrogen chemisorption, but not of C-H activation, does not matter. The Ti02 support inthe SMSI catalyst is obviously highly selective in eliminating these weak sites leaving the very active ones, e.g. those required for methane exchange, largely intact. When we come to ethane hydrocracking the rates drop by a factor of 104-10 ~ from the non-S MSI to the SMSI catalyst, i.e. the same as for H2 chemisorption, but the activation energy remains the same. Are we now to conclude that, since C-H activation is really a structure sensitive reaction, C-C fission is supersensitive? The most sensible explanation of the distinction between these two types of reaction now appears to be that they are not due to different types of sites, as in the conventional jargon concerning structure sensitivity and insensitivity, but rather that the usually ignored "hydrogen factor" is responsible. The same sites cause both reactions but the C-H reactions involve mainly molecular D2 whereas the C-C fission reactions are dependent on the total surface pool of H adatoms. As this pool largely disappears for the SMSI catalyst, so also does hydrocracking activity. The same idea rather than that of ensembles also applies to similar changes in the catalytic activities of noble metal catalysts upon alloying with the coinage metals. The distinction is ultimately a mechanistic one since the intermediates involved in the C-C bond fission processes, e.g., surface carbenes, carbynes, and individual C atoms,
applied catalysis A: General
require the pool of mobile H adatoms for their removal, but the intermediates responsible for the C-H reactions do not, being capable of a direct Eley-Rideal type reaction with H2 (D2) from the gas phase. The general question arising out of all of this is the following. Are any metal catalyzed reactions structure insensitive? J.J. ROONEY R o u m a n i a n Literature
In an article entitled "Possibilities of ethene elimination from polluting gas mixtures by adsorption in selective solvents" [Rev. Chim. (Bucharest), 46 (1995) 335], F. Barca, N. Brindas, N. Moldovan, G. Paraschivescu, A. Marinescu and A. Popa discuss the possibilities of ethylene extraction from residual gases resulting from chlorination processing using an adequate and easily accessible solvent, 1,2-dichloroethane (1,2-DCE). The data obtained showed that the extraction of ethylene is possible to a degree of 99%, according with the working conditions established in the pilot plant. In a paper entitled "Hydrogen-oxygen stoichiometric mixtures catalytic burning at low temperatures" [Rev. Chim. (Bucharest), 4 (1995) 448], G. Ionita has presented the results of the tests of the catalytic combustion process of hydrogen, performed with six mixed type packings, each different in nature and composition. Demineralized water which flows directly over the catalytic packing, absorbing the heat of reaction from the catalytic burner, was used as cooling agent. The combustion was complete and was performed at a temperature below 50°C under conditions of full safety, elimiVolume 131 No. 1 - - 14 October 1995
He stayed then for two years as a research fellow at Argonne National Laboratory before becoming a senior scientist at the University of Hannover. In 1969 after receiving his habilitation degree he moved to Krupp Chemical Engineering Division (now: Krupp-Koppers) where he was head of chemical research and development. In 1974 he became full professor for chemical technology and reaction engineering at the Ruhr-University Bochum. Manfred Baerns is a member of the DECHEMA Board, chairman of DECHEMA's working party on industrial reactions and a member of its working party on catalysis. Besides being scientific coordinator of several research programmes, consultant for industrial companies and member of advisory boards in different research programs in Germany and abroad, he is an active researcher himself. Since taking over his professorship in Bochum, Manfred Baerns has addressed in his research different aspects of heterogeneous catalysis and catalytic reaction engineering. In the fundamental studies aimed at a better understanding of reaction mechanisms he applies such methods as XPS, PD/DSC, in-situ DRIFT spectroscopy and transient methods e.g., the Temporal Analysis of Products reactor. The results of these studies have been used in the development of catalysis for oxidative coupling of methane to C2+ hydrocarbons, partial oxidation of methane to synthesis gas, Fisher-Tropsch synthesis, selective oxidation of propane, butene and aromatic hydrocarbons to their oxygenates and more recently NO× reduction. In the field of catalytic reaction engineering his studies are aimed at developing kinetic models for various reactions as well as reactor modelling and simulations as means for the
applied catalysis A: General
optimization of reaction conditions and reactor design. This large research experience will certainly be very useful in his new, very challenging position. We wish Manfred Baerns every success in Berlin. L. MLECZKO Structure Sensitivity and Insensitivity in Catalysis
Exchange reactions with D2, and hydrogenation and dehydrogenation reactions of hydrocarbons on metal catalysts are regarded as structure insensitive, but hydrocracking as being structure sensitive. The first class involves only C-H bond breaking and making while the second consists of C-C bond fission processes. Classical examples are ethane/D2 exchange and ethane hydrocracking to methane, respectively. The term insensitive is usually used to describe a reaction where all the exposed surface metal atoms are believed to be catalytically active, even for polycrystalline materials. Recent work on paraffin/D2 exchange on Pt/TiO2 catalyst in the non-SMSI and SMSl states [A.C. Faro and C. Kemball, J. Chem. Soc., Faraday Trans., 91 (1995) 741; W. Patterson and J.J. Rooney, Catal. Today, 12 (1992) 113, and references therein] gave the surprising result that neither the rates nor the multiplicities vary greatly from one type of catalyst to the other. This was especially true of methane, the paraffin which is most difficult to activate. However, H2 chemisorption capacity falls dramatically by a factor of 104-105 from the non-SMSl to the SMSI catalyst. The only viable conclusion is that the sites for paraffin activation are very few and that Volume 131 No. 1 - - 14 October 1995
N5
the exchange reaction involves direct reaction of D2 (Eley-Rideal mechanism) with the adsorbed hydrocarbon species. The reaction is in reality structure sensitive, but the presence or absence of a vastly superior number of metallic sites capable of hydrogen chemisorption, but not of C-H activation, does not matter. The Ti02 support inthe SMSI catalyst is obviously highly selective in eliminating these weak sites leaving the very active ones, e.g. those required for methane exchange, largely intact. When we come to ethane hydrocracking the rates drop by a factor of 104-10 ~ from the non-S MSI to the SMSI catalyst, i.e. the same as for H2 chemisorption, but the activation energy remains the same. Are we now to conclude that, since C-H activation is really a structure sensitive reaction, C-C fission is supersensitive? The most sensible explanation of the distinction between these two types of reaction now appears to be that they are not due to different types of sites, as in the conventional jargon concerning structure sensitivity and insensitivity, but rather that the usually ignored "hydrogen factor" is responsible. The same sites cause both reactions but the C-H reactions involve mainly molecular D2 whereas the C-C fission reactions are dependent on the total surface pool of H adatoms. As this pool largely disappears for the SMSI catalyst, so also does hydrocracking activity. The same idea rather than that of ensembles also applies to similar changes in the catalytic activities of noble metal catalysts upon alloying with the coinage metals. The distinction is ultimately a mechanistic one since the intermediates involved in the C-C bond fission processes, e.g., surface carbenes, carbynes, and individual C atoms,
applied catalysis A: General
require the pool of mobile H adatoms for their removal, but the intermediates responsible for the C-H reactions do not, being capable of a direct Eley-Rideal type reaction with H2 (D2) from the gas phase. The general question arising out of all of this is the following. Are any metal catalyzed reactions structure insensitive? J.J. ROONEY R o u m a n i a n Literature
In an article entitled "Possibilities of ethene elimination from polluting gas mixtures by adsorption in selective solvents" [Rev. Chim. (Bucharest), 46 (1995) 335], F. Barca, N. Brindas, N. Moldovan, G. Paraschivescu, A. Marinescu and A. Popa discuss the possibilities of ethylene extraction from residual gases resulting from chlorination processing using an adequate and easily accessible solvent, 1,2-dichloroethane (1,2-DCE). The data obtained showed that the extraction of ethylene is possible to a degree of 99%, according with the working conditions established in the pilot plant. In a paper entitled "Hydrogen-oxygen stoichiometric mixtures catalytic burning at low temperatures" [Rev. Chim. (Bucharest), 4 (1995) 448], G. Ionita has presented the results of the tests of the catalytic combustion process of hydrogen, performed with six mixed type packings, each different in nature and composition. Demineralized water which flows directly over the catalytic packing, absorbing the heat of reaction from the catalytic burner, was used as cooling agent. The combustion was complete and was performed at a temperature below 50°C under conditions of full safety, elimiVolume 131 No. 1 - - 14 October 1995