Catalysis: new perspectives from surface science

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Authors: D.W. Goodman and J.E. Houston
Date: Apr. 24, 1987
From: Science(Vol. 236)
Publisher: American Association for the Advancement of Science
Document Type: Article
Length: 6,201 words

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Catalysis: New Perspectives from Surface Science

THE APPLICATION OF MODERN SURFACE SCIENCE TECHniquesto fundamental catalytic studies has advanced our understanding of elementary surface reactions at the atomic level and in particular the relation between surface characteristics (that is, structure and composition) and catalytic properties. Most surface-sensitive techniques require an ultrahigh vacuum (UHV) environment and are best suited for studying well-characterized, low-surface-area materials such as metal single crystals. In contrast, many catalytic processes of practical interest are catalyzed by active metals dispersed on an oxide support with a high surface area (typically 100 to 200 m2/g) at pressures of 1 atm or greater. Because of these differences in the catalyst materials and in the pressure regimes, questions have been raised with regard to the applicability of the work conducted at low pressures on idealized surfaces of single crystals to catalysis at much greater pressures on supported catalysts with high surface area. Good correlations between the data for single crystals and for supported catalysts would allow the chemical information obtained from model catalyst studies to be used to gain a detailed understanding of "real' catalyst systems. This information could be useful in improving existing catalysts or in developing new ones.

As a step toward bridging the pressure gap alluded to above,experimental systems have been developed (1-3) in the last decade that consist of a reaction chamber linked to a UHV surface analysis system. With such an apparatus it is possible to conduct both kinetic measurements at atmospheric pressures and surface characterization under UHV conditions without removing the sample from the controlled environment. Such experimental systems have been used to measure the kinetics of several important catalytic reactions (4-9). Comparisons of kinetic data from these single crystal model catalysts with the reaction rates measured over supported catalysts in the same reactant environment have shown excellent correlations. Valuable information on reaction mechanisms has become available from surface analytical data obtained after reaction (4, 5, 7, 8, 10, 11). Furthermore, the role of surface impurities in either promoting or inhibiting certain surface reactions (9, 12, 13) and the origins of the enhanced catalytic properties of mixed-metal systems (14) have been explored.

In this article we highlight the application of surface science toseveral pivotal questions related to heterogeneous catalysis. This article is not intended to be a broad review of surface science or of its application to the area of catalysis. Rather, we concentrate on specific applications and use work by the authors to illustrate the impact that surface science can have on understanding surface-catalyzed reactions.

We begin with a brief discussion of the type of instrumentationused in this work. We then outline examples of reactions that illustrate the level of comparison that has been achieved over a broad class of "structure-insensitive' reactions, that is, those reactions for which catalyst morphology produces little change in catalytic activity (15). We then discuss the relation of single crystal work to reactions that are "structure-sensitive,' that is, those reactions that depend on metal particle size and on the nature...

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Gale Document Number: GALE|A4811104