- Email: [email protected]
Heterogeneous catalysis — still magic or already science
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ing that none of these reactions had been observed experimentally. Recent experiments (D.P. Belyung, A. Fontijn and P. Marshall, J. Phys. Chem., 97 (1993) 3456) have now shown that a reaction between AIO and methane does indeed occur at high temperatures but it is not solely a simple hydrogen atom extraction. Belyung et al. used a high-temperature, fast-flow reactor in which AIO was generated by oxidation of AI vapour and then reacted downstream with a excess of methane over the range 590 to 1380 K The reaction was monitored by following AIO concentration. (As an incidental comment, these authors' treatment of their experimental and fitting errors should serve as a model to the many slapdash workers in heterogeneous catalysis.) A curved Arrhenius plot (with apparent activation energies forthe overall reaction rising from 46 kJ mor" at 100 Kt071 kJ mor" at 1380 K), and fitted rate equations which were not consistent with a single reaction step indicated the complexity of the overall reaction. From further ab initio and transition state calculations the authors suggest that direct abstraction of a hydrogen atom can occur at the higher temperatures but that one or more routes, possibly involving AIO insertion into a C-H bond to give, e.g., a CHaAlOH intermediate, dominate the initial attack step. Clearly this reaction is not directly comparable with that of methane dissociation on a solid alumina catalyst but it may have a suggestive relevance in view of the coordinative unsaturation of surface AI atoms in alumina at high temperatures.
applied catalysis A: general
Heterogeneous Catalyals - Stili Magic or Already SCience?
The editors of News Brief always warm to controversial articles on catalysis, even those from the most ethereal of surface scientists claiming that they can produce new industrial catalysts on the basis solely of their experiments with single crystals under UHV conditions. So an article with the title above (R. Schlogl, Angew. Chem. Int. Ed. Engl., 32 (1993) 381) was certain to attract our interest. Schlogl takes a surprisingly pessimistic view: in his final paragraph he states "... if we try to answer the question asked in the title the realistic reply must be - still magic. The obstacles to a scientific understanding of heterogeneous catalysis are considerable...." He starts by defining the "underlying problem" as "understanding the operation of a catalytic system at an atomic level; it is not adequate to have only a model". He cites ammonia synthesis as the sole example where the "culminating success of decades of research was a kinetic theory that allowed the precise prediction of the yield of ammonia under technical conditions using as parameters experimentally determined properties and a set of experimentally verified elementary reaction steps." There are those who still have considerable doubts about these predictions and the assumptions behind them, and it also depends on exactly what you mean by "precise". However this is only marginally relevant, for the main thrust of Schlogl's argument is "...this strategy was not successful in any other practical application as yet." Schlogl goes through the inevitable problems of catalysis research (and few would quarrel with him here): we must not make "... a priori distinctions between imVolume 104 NO.1 - 7 October 1993
N4
portant and unimportant components of the system but should consider all the constituents (active components, support, poisons, promoters) of a practical cataIyst"; " all analysis should be done in situ ... [but] only a fraction of the analyses are conducted under in situ conditions, since only few techniques can be used for such investigations ..."; "Activity and selectivity data do not allow conclusions concerning the reaction mechanism"; "The limitations of mass- and energy-exchange processes and non-catalytic consecutive reactions tend to obscure the relationship between kinetics and mechanism"; "Extrapolations from experimental to real operation conditions must be limited ...". Most of his examples are drawn from various studies on methanol synthesis and on ammonia synthesis, and workers in these areas will find much to stir them into argument. While the discussions of aspects of this article will no doubt keep going on, maybe one weakness of Schlogl's position is his apparent "all or nothing" position. He seems to believe that unless you can calculate everything for a catalytic reaction then catalyst design is "magic". One of the features of successful applied science is that "you do what you can do" and make the best use of it. So although we cannot complete a full a priori design exercise (even for an ammonia synthesis catalyst or process) much of it can nevertheless be done in a rational way on the basis of scientific knowledge (and this includes a judicious selection of the results from surface chemistry) and technical experience, leaving only part to be done empirically. I would not call this procedure "black magic". The human body Is many orders of magnitude more complex than any catalyst and our understanding of its function-
applied catalysis A: general
ing is correspondingly more fragmentary, but I think few medical men would like their profession to be described as "black magic". We would be happy to publish any contributions which carry these arguments forward.
Recent Reviews
Recent reviews on catalysis and related subjects include: JA Rabo, Catalysis: Past, present and future, in L. Guczi, F. Solymosi and P. Tetenyi (Editors), New Frontiers in Catalysis, Proceedings 10th International Congress on Catalysis, Budapest, 1992 (Studies in Surface Science and Catalysis, Vol.75), Elsevier, Amsterdam, 1993, p. 1. S.L. Suib, Selectivity in catalysis. An overview, Am. Chem. Soc. Symp. Ser. ,517 (1993) 1. M. Che, Interfacial coordination chemistry: Concepts and relevance to catalysis phenomena, in L. Guczi, F. Solymosi and P. Tetenyi (Editors), New Frontiers in Catalysis, Proceedings 10th International Congress on Catalysis, Budapest, 1992 (Studies in Surface Science and Catalysis, Vol.75), Elsevier, Amsterdam, 1993, p. 31. K.J. Klabunde and V.X. Li, Clusters, alloys, and poisoning. An overview, Am. Chem. Soc. Symp. Ser., 517 (1993) 88. M. Misono, New catalytic aspects of heteropolyacids and related compounds - To the molecular design of practical catalysts, in L. Guczi, F. Solymosi and P. Tetenyi (Editors), New Frontiers in Catalysis, Proceedings 10th International Congress on Catalysis, Budapest, 1992 (Studies in Surface Science and Catalysis, Vol.75), Elsevier, Amsterdam, 1993, p. 69. Volume 104 NO.1 -7 October 1993
ing that none of these reactions had been observed experimentally. Recent experiments (D.P. Belyung, A. Fontijn and P. Marshall, J. Phys. Chem., 97 (1993) 3456) have now shown that a reaction between AIO and methane does indeed occur at high temperatures but it is not solely a simple hydrogen atom extraction. Belyung et al. used a high-temperature, fast-flow reactor in which AIO was generated by oxidation of AI vapour and then reacted downstream with a excess of methane over the range 590 to 1380 K The reaction was monitored by following AIO concentration. (As an incidental comment, these authors' treatment of their experimental and fitting errors should serve as a model to the many slapdash workers in heterogeneous catalysis.) A curved Arrhenius plot (with apparent activation energies forthe overall reaction rising from 46 kJ mor" at 100 Kt071 kJ mor" at 1380 K), and fitted rate equations which were not consistent with a single reaction step indicated the complexity of the overall reaction. From further ab initio and transition state calculations the authors suggest that direct abstraction of a hydrogen atom can occur at the higher temperatures but that one or more routes, possibly involving AIO insertion into a C-H bond to give, e.g., a CHaAlOH intermediate, dominate the initial attack step. Clearly this reaction is not directly comparable with that of methane dissociation on a solid alumina catalyst but it may have a suggestive relevance in view of the coordinative unsaturation of surface AI atoms in alumina at high temperatures.
applied catalysis A: general
Heterogeneous Catalyals - Stili Magic or Already SCience?
The editors of News Brief always warm to controversial articles on catalysis, even those from the most ethereal of surface scientists claiming that they can produce new industrial catalysts on the basis solely of their experiments with single crystals under UHV conditions. So an article with the title above (R. Schlogl, Angew. Chem. Int. Ed. Engl., 32 (1993) 381) was certain to attract our interest. Schlogl takes a surprisingly pessimistic view: in his final paragraph he states "... if we try to answer the question asked in the title the realistic reply must be - still magic. The obstacles to a scientific understanding of heterogeneous catalysis are considerable...." He starts by defining the "underlying problem" as "understanding the operation of a catalytic system at an atomic level; it is not adequate to have only a model". He cites ammonia synthesis as the sole example where the "culminating success of decades of research was a kinetic theory that allowed the precise prediction of the yield of ammonia under technical conditions using as parameters experimentally determined properties and a set of experimentally verified elementary reaction steps." There are those who still have considerable doubts about these predictions and the assumptions behind them, and it also depends on exactly what you mean by "precise". However this is only marginally relevant, for the main thrust of Schlogl's argument is "...this strategy was not successful in any other practical application as yet." Schlogl goes through the inevitable problems of catalysis research (and few would quarrel with him here): we must not make "... a priori distinctions between imVolume 104 NO.1 - 7 October 1993
N4
portant and unimportant components of the system but should consider all the constituents (active components, support, poisons, promoters) of a practical cataIyst"; " all analysis should be done in situ ... [but] only a fraction of the analyses are conducted under in situ conditions, since only few techniques can be used for such investigations ..."; "Activity and selectivity data do not allow conclusions concerning the reaction mechanism"; "The limitations of mass- and energy-exchange processes and non-catalytic consecutive reactions tend to obscure the relationship between kinetics and mechanism"; "Extrapolations from experimental to real operation conditions must be limited ...". Most of his examples are drawn from various studies on methanol synthesis and on ammonia synthesis, and workers in these areas will find much to stir them into argument. While the discussions of aspects of this article will no doubt keep going on, maybe one weakness of Schlogl's position is his apparent "all or nothing" position. He seems to believe that unless you can calculate everything for a catalytic reaction then catalyst design is "magic". One of the features of successful applied science is that "you do what you can do" and make the best use of it. So although we cannot complete a full a priori design exercise (even for an ammonia synthesis catalyst or process) much of it can nevertheless be done in a rational way on the basis of scientific knowledge (and this includes a judicious selection of the results from surface chemistry) and technical experience, leaving only part to be done empirically. I would not call this procedure "black magic". The human body Is many orders of magnitude more complex than any catalyst and our understanding of its function-
applied catalysis A: general
ing is correspondingly more fragmentary, but I think few medical men would like their profession to be described as "black magic". We would be happy to publish any contributions which carry these arguments forward.
Recent Reviews
Recent reviews on catalysis and related subjects include: JA Rabo, Catalysis: Past, present and future, in L. Guczi, F. Solymosi and P. Tetenyi (Editors), New Frontiers in Catalysis, Proceedings 10th International Congress on Catalysis, Budapest, 1992 (Studies in Surface Science and Catalysis, Vol.75), Elsevier, Amsterdam, 1993, p. 1. S.L. Suib, Selectivity in catalysis. An overview, Am. Chem. Soc. Symp. Ser. ,517 (1993) 1. M. Che, Interfacial coordination chemistry: Concepts and relevance to catalysis phenomena, in L. Guczi, F. Solymosi and P. Tetenyi (Editors), New Frontiers in Catalysis, Proceedings 10th International Congress on Catalysis, Budapest, 1992 (Studies in Surface Science and Catalysis, Vol.75), Elsevier, Amsterdam, 1993, p. 31. K.J. Klabunde and V.X. Li, Clusters, alloys, and poisoning. An overview, Am. Chem. Soc. Symp. Ser., 517 (1993) 88. M. Misono, New catalytic aspects of heteropolyacids and related compounds - To the molecular design of practical catalysts, in L. Guczi, F. Solymosi and P. Tetenyi (Editors), New Frontiers in Catalysis, Proceedings 10th International Congress on Catalysis, Budapest, 1992 (Studies in Surface Science and Catalysis, Vol.75), Elsevier, Amsterdam, 1993, p. 69. Volume 104 NO.1 -7 October 1993