Active sites on oxides: From single crystals to catalysts

Metal oxides are ubiquitous in heterogeneous catalysis, serving as catalysts, as catalyst supports, and as modifiers and promoters, among other roles. The surface science approach to understanding the structure and reactivity of metal oxide surfaces has blossomed during the preceding decade. We consider here important concepts drawn from catalysis by metal oxides and their connections to reaction pathways and principles of oxide surface reactivity revealed by experimental and theoretical studies of well-defined oxide surfaces. The applications of reaction mechanisms and site requirements from surface science studies of carboxylic acid chemistry, in particular, to emerging catalysts for dehydration, coupling, and reduction reactions provide important examples of contributions to heterogeneous catalysis from oxide surface science. We also consider relationships between oxide surface reactivity and the physical and electronic properties of oxides as represented by characteristics such as electronegativity, bond energies, ionicity, Madelung potential, and polarizability. This analysis highlights the need for reaction data on well-defined single-crystal surfaces in order to distinguish structural and electronic effects when comparing patterns of catalytic activity and selectivity among different metal oxides. This review of the connetions between metal oxide single-crystal surfaces and high surface area catalysts (or their components) demonstrates the potential of surface science approaches to elucidate the chemical and physical bases for catalysis by metal oxides, in pursuit of the goal of catalyst design.