Effects of Surface Impurities in Chemisorption and Catalysis

Abstract

It has long been recognized that the addition of impurities to metal catalysts can produce large effects on the activity, selectivity, and resistance to poisoning of the pure metal [8.1]. For example, the catalytic properties of metals can be altered greatly by the addition of a second transition or Group 1B metal or by the addition of impurities such as potassium or sulfur. On the other hand, catalytic processing is often plagued by loss of activity and/or selectivity due to the inadvertent contamination of catalysts by undesirable impurities. In either case, the catalytic properties are dramatically altered by the modification of the chemistry by the impurity. Although these effects are well recognized in the catalytic industry, the mechanisms responsible for surface chemical changes induced by surface additives are poorly understood. However, the current interest and activity in this area of research promises a better understanding of the fundamentals by which impurities alter surface chemistry. A pivotal question concerns the underlying relative importance of ensemble (steric or local) versus electronic (nonlocal or extended) effects. A general answer to this question will critically influence the degree to which we will ultimately be able to tailor-make exceptionally efficient catalysts by fine-tuning the electronic structure.