ACCOUNTS OF CHEXICAL RESEARCH" Registered i n US.Patent and Trademark Office; Copyright 1982 by the American Chemical Society
VOLUME 15
NUMBER 6
J U N E , 1982
EDITOR
JOSEPH F. BUNNETT ASSOCIATE EDITORS
Joel E. Keizer John E. McMurry EDITORIAL ADVISORY BOARD
Fred Basolo R. Stephen Berry Michel Boudart Maurice M. Bursey Edward A. Collins John T. Gerig Jenny P. Glusker Kendall N. Houk Jay K. 'Kochi Maurice M. Kreevoy Theodore Kuwana Ronald N. McElhaney Eva L. Menger Kurt Mislow John C. Polanyi Alexander Rich Anthony M. Trozzolo Gene G. Wubbels Published by the AMERICAN CHEMICAL SOCIETY 1155 16th Street, N.W. Washington, D.C. 20036
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Prediction, Explanation, and Design in Heterogeneous Catalysis Prediction of chemical reactivity remains an elusive goal, especially in catalysis, which implies a swift and repeated turnover around a delicately balanced catalytic cycle of elementary steps. Yet explanation of many catalytic reactions is now at hand, with details that seemed unattainable only a few years ago. This is particularly striking in heterogeneous catalysis as a result of the applications of surface science, e.g., to the understanding of ammonia synthesis, oxidation of carbon monoxide, and reduction of carbon monoxide to methane, on individual crystallographic planes of iron, palladium, and nickel, respectively. Results obtained with these model catalysts at pressures up to 20 atm provide the standards by which the corresponding industrial catalysts can be assessed. Indeed, heterogeneous catalysis has recently become quantitative and reproducible. Hence it is now a respectable branch of the science of chemical reactivity. But what about invention of new catalytic cycles and design of new solid catalysts? Many answers to that question can be provided, by the conservative stating that luck still favors the prepared mind, or the visionary claiming that tailor-made or molecularly engineered catalysts are within reach. Clearly the direction of a substantial amount of chemical research in industry and academe will depend on whether design of heterogeneous catalysis is deemed feasible or not. I believe it has become feasible for the following three reasons. First, new concepts are available to guide discovery and invention in heterogeneous catalysis. They have emerged from the subject itself, e.g., shape selectivity, bi- or multifunctionality, bi- or multimetallic clusters, metal support interactions, structure sensitivity. Or they can be adapted from organometallic, bioinorganic, or metalloenzyme chemistry, e.g., coordinative unsaturation, chiral ligands,,polynuclear metallic clusters, oxidative addition, fluxional intermediates, hydrophobic cavities hosting inclusion complexes. Second, new tools permit a characterization of structure and composition of bulk and surface species, sometimes near or under catalytic conditions. Thus catalyst activity and stability can be investigated at the molecular level, whereas in the past catalyst screening relied on the black box approach of conventional process research. Third, new materials from solid state chemistry can be used for the synthesis of catalytic solids that embody some of the new concepts listed above. Among them are hydrophobic or functionalized molecular sieves and layered structures, with controlled porosity and surface chemistry. The permutations accessible to a creative designer are left to his imagination. New concepts, tools, and materials reinforced by the newly gained understanding of reaction mechanisms, as a result of the impact of surface science, will accelerate discoveries in heterogeneous catalysis, as energy costs and environmental needs demand more active, selective, and resistant catalysts. Fortunately for the progress of heterogeneous catalysis, scientific design doea not require the ability to predict. I t rests on the ability to explain what is, in order to create new structures, processes, and systems. On that h i s , the future of heterogeneous catalysis appears very bright indeed. Michel Boudart Stanford University
