Reaction Kinetics and Polarization-Modulation Infrared Reflection Absorption Spectroscopy (PM-IRAS) Investigation of CO Oxidation over Supported Pd−Au Alloy Catalysts

Pd−Au bimetallic model catalysts, synthesized either as thin films on Mo(110) or as nanoparticles on a TiO2 thin film, were used in CO oxidation at both low (1 × 10−7 Torr) and elevated (8−16 Torr) CO pressures. Alloying with Au forms isolated Pd sites that are incapable of dissociating O2. This inability causes reactivity loss at low reactant pressures. Moreover, alloying also electronically modifies surface sites and affects the reaction activation energy. At elevated pressures, Pd preferentially segregates to the surface to form contiguous Pd sites and CO oxidation reactivity is regained. Under stoichiometric conditions and relatively low temperatures, Pd−Au alloys show superior reactivity compared with pure Pd due to more facile CO desorption (less CO inhibition). Under net oxidizing conditions, metallic Pd displays superior reactivity due to its higher capability of dissociating O2. However, pure Pd oxidizes and loses reactivity more readily than do Pd−Au alloys.