Complex redox chemistry on the RuO₂(1 1 0) surface: Experiment and theory

Employing temperature-programmed reaction and desorption, we studied the reduction of RuO₂(1 1 0) by CO exposure under various conditions. RuO₂(1 1 0) is mildly and heavily reduced by CO exposure when the reaction temperature is below 400 K and above 500 K, respectively. The restoration of the reduced RuO₂(1 1 0) surface was investigated by low energy electron diffraction and Auger electron spectroscopy. CO molecules adsorb over the under-coordinated Ru atoms and recombine with the under-coordinated lattice O atoms on the RuO₂(1 1 0) surface. The initial conversion probability for this process is as high as 80%. The mildly reduced RuO₂(1 1 0) surface is characterized by the removal of bridging O atoms. A mildly reduced surface, where all bridging O atoms are consumed, is inactive in oxidizing CO below 450 K. However, temperature treatment (≥ 550 K) or oxygen exposure at room temperature is able to reactivate the mildly reduced surface. Heavy reduction of RuO₂(1 1 0) leads to a roughening of the surface, which is partly restored by annealing at 700-800 K. The activation barriers of various reaction pathways, motivated by the present experiments, are determined by state-of-the-art density function theory calculations and compared with experiments.