Low-Temperature CO Oxidation Catalyzed by Free Palladium Clusters: Similarities and Differences to Pd Surfaces and Supported Particles

The catalytic low-temperature oxidation of CO to CO2 with molecular oxygen is of particular industrial and ecological interest. Gas-phase reaction kinetics measurements in conjunction with first-principles calculations provide comprehensive insight into the mechanisms and energetics of the low-temperature CO combustion reaction catalyzed by small free palladium clusters Pd x + (x = 2–7). Similar to the cases of extended palladium single crystals and supported nanoparticles, the catalytic activity of the free palladium clusters was found to be largely determined by the fast adsorption and dissociation of molecular oxygen and the binding strength of carbon monoxide. In particular, Pd4 +, Pd5 +, and Pd6 + were found to catalyze the oxidation of CO at room temperature, with Pd6 + being most active. Detailed mechanistic investigations of the CO oxidation reaction catalyzed by Pd6 + reveal a Langmuir–Hinshelwood reaction mechanism, similar to that found earlier for CO oxidation on palladium single crystals, with comparable energetics. The main difference, however, between the cases of small clusters and extended surfaces arises from a considerably reduced bonding of CO to the Pd6 + cluster compared to the adsorption strength on the Pd(111) surface, as well as in comparison with the other investigated clusters. This lower CO binding energy prevents CO poisoning at, and below, room temperature, and enables effective low-temperature CO oxidation. Consequently, this study shows that free clusters can serve as model systems for mechanistic studies of catalytic reactions at the molecular level, in addition to opening new ways for the rational design of effective low-temperature CO oxidation catalysts through tunability of the reaction parameters by changing the number of constituent atoms.