Catalytic CO oxidation is a seemingly simple reaction between CO and O₂ molecules, one of the reactions in automotive catalytic converters, and the fruit-fly reaction in model catalysis. Surprisingly, the phase responsible for the catalytic activity is still under debate, despite decades of investigations. We have performed a simple but yet conclusive study of single crystal Rh and Pd model catalysts, resolving this controversy. For Rh, the oxygen covered metallic surface is more active than the oxide, while for Pd, thin oxide films are at least as active as the metallic surface, but a thicker oxide is inactive. Apart from resolving a long-standing debate, our results are of great importance for oxidation catalysis in general and especially for avoiding extinction due to too high oxygen exposures.

CO₂ reduction reactions, which provide one route to limit the emission of this greenhouse gas, are commonly performed over Cu-based catalysts. Here, we use ambient pressure X-ray photoelectron spectroscopy together with density functional theory to obtain an atomistic understanding of the dissociative adsorption of CO₂ on Cu(100). We find that the process is dominated by the presence of steps, which promote both a lowering of the dissociation barrier and an efficient separation between adsorbed O and CO, reducing the probability for recombination. The identification of steps as sites for efficient CO₂ dissociation provides an understanding that can be used in the design of future CO₂ reduction catalysts.

CO₂ methanation over Rh/CeO₂ and Ni/CeO₂ highlighting the different surface speciation during reaction as deduced from our study.