Ion Beams and Catalysis
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ION BEAMS AND CATALYSIS
J.A. CAIRNS Chemistry Division, AERE Harwell,
Didcot, Oxfordshire,
OX1l
ORA,
England
ABSTRACT Ion beam techniques have found increasing application in recent years to a wide variety of disciplines. It is the purpose of this paper to explore their potential in the field of catalysis, concentrating exclusively on solid heterogeneous catalysts. An initial description of the internal structure and external physical form of some typical catalysts is followed by an assessment of the use of ion beams for the preparation and interrogation of both "real" and model catalysts. These techniques are then compared to some of the modern tools used in current catalysis research. It emerges that ion beams can indeed be used to advantage in certain applications, such as detecting light elements selectively; measuring interaction effects between metals and supports in model systems; highlighting surface relaxation effects in metal single crystals; and even, in special circumstances, in synthesising catalysts.
INTRODUCTION The purpose of this paper is to explore the potential for interaction between ion beam techniques and the large and apparently distinct discipline of solid heterogeneous catalysis. In order to achieve this objective it is considered necessary to describe, admittedly in a simplified manner, the basic structural features of such catalysts, and to outline some of the techniques used in their preparation and characterisation. We will then be in a position to assess more objectively whether ion beam technology can be applied usefully, either to prepare new types of catalysts, or to interrogate existing ones to yield new information.
SOME BASIC FEATURES OF HETEROGENEOUS CATALYSTS [I] A catalyst may be designed to perform several functions, but the most basic one is usually to lower the activation energy for a particular reaction, so that it can proceed at a faster rate or at a lower temperature. The catalyst achieves this by adsorbing one or more of the reactants on to its surface, thereby converting them into a more active state for subsequent reaction. Hence typically the first objective in fabricating a catalyst is to prepare it in highly dispersed form - that is to increase its surface area. This is achieved by dispersing the active phase of the catalyst (which usually is a metal) over a high surface area carrier material, known as the support, thereby producing a so-called supported metal catalyst. The support is usually an oxide, such as silica or alumina, although for some reactions a high surface area carbon can be used. The method of preparation may involve impregnating the support with a solution of the appropriate metal salt, followed by drying, calcination, and perhaps reduction at an appropriate temperature, the final result being very finely divided metallic particles distributed over the support, and indeed perhaps within its pores. Fig. 1 shows a transmission electron micrograph of a Pt/Al 2 03 catalyst [ 2], where the highly dispersed metal is seen to be distributed through
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