Promotion in Heterogeneous Catalysis: A Topic Requiring a New Approach?

Abstract

Promoters, and their counterparts poisons, are two topics that have been studied extensively since heterogeneous catalysts have been used industrially. Commercial catalysts tend to involve multiple promoters to enhance the activity, selectivity, lifetime and structural integrity, in addition to guard beds or feedstock purification measures that are used to avoid the adsorption of poisons. Yet, at a fundamental level, there is still considerable debate as to how specific promoters function. Promoter effects tend to be specific to a particular catalyst formulation and generic effects are not common, yet this would be desirable. Generally, in acid catalysis and hydrogenation reactions, promoter effects can be dramatic and unexpected. However, in oxidation reactions of alkanes and alkenes, the promotion effects observed to date are somewhat limited. In this paper, the topic of promotion in heterogeneous catalysis is discussed. The complex interplay between structural and electronic effects of promoters is described using examples of both well defined metal surfaces (e.g., the promoted iron catalyst for ammonia synthesis) and multicrystalline metal, metal oxide and metal phosphate catalysts (e.g., Li-doped MgO for methane oxidation). Subsequently, a molecular approach for the promotion of heterogeneous catalysts is prepared and discussed. This is based on observations from well defined molecular catalysts for homogeneously catalysed processes of ligand accelerated reactions. Examples are described where the effects of ligand acceleration, increasing reaction rate by over two orders of magnitude, can be observed for heterogeneous catalysts for hydrogenation and acid catalysed reactions. The remaining challenge is to identify similar effects for partial oxidation reactions, and it is questioned whether a molecular approach can be developed to meet this challenge.