The Critical Roles of Promoters in Heterogeneous Catalytic Hydrogenation

Promoters are widely used in industrial heterogeneous catalysts. Although catalytically inert themselves, even trace amounts of a promoter can disproportionately enhance critical performance metrics, such as activity, selectivity, and durability. Diverse classes of promoters, including alkali, alkaline earth, halogen, and molecular species, have been identified for various reactions. Their promotional effects are broadly categorized as structural or electronic. However, their low concentration, complex behavior, and dynamic nature within catalysts make it extremely challenging to reveal their atomic-scale functions, leading to frequent misinterpretations in the literature. Some of the effects have been incorrectly justified in common practice. This review revisits studies employing well-defined model catalysts and theoretical calculations to clarify the fundamental roles of promoters in hydrogenation reactions. By delineating their multiple atomic-scale functions, we aim to provide a framework for the rational design of advanced promoted heterogeneous hydrogenation catalysts.