Hydrogen oxidation on oxygen-rich IrO<sub>2</sub>(110)

<p>We investigated the adsorption and oxidation of H<sub>2</sub> on O-rich IrO<sub>2</sub>(110) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. Our results show that H<sub>2</sub> dissociation occurs efficiently on O-rich IrO<sub>2</sub>(110) at low temperature and initiates from an adsorbed H<sub>2</sub> σ-complex on the coordinatively-unsaturated Ir atoms (Ir<sub>cus</sub>). We find that on-top oxygen atoms (O<sub>ot</sub>), adsorbed on the Ir<sub>cus</sub> sites, promote the desorption-limited evolution of H<sub>2</sub>O during subsequent oxidation of the adsorbed hydrogen on IrO<sub>2</sub>(110) while suppressing reaction-limited production of H<sub>2</sub>O via the recombination of bridging HO groups (HO<sub>br</sub>) (~500 to 750 K) during TPRS. The desorption-limited TPRS peak of H<sub>2</sub>O shifts from ~490 to 550 K with increasing O<sub>ot</sub> coverage, demonstrating that O<sub>ot</sub> atoms stabilize adsorbed OH and H<sub>2</sub>O species. DFT predicts that molecularly-adsorbed H<sub>2</sub> dissociates on O-rich IrO<sub>2</sub>(110) at low temperature and that the resulting H-atoms redistribute to produce a mixture of HO<sub>br</sub> and HO<sub>ot</sub> groups, with equilibrium favouring HO<sub>ot</sub> groups. Our calculations further predict that subsequent H<sub>2</sub>O evolution occurs through the recombination of HO<sub>br</sub>/HO<sub>ot</sub> and HO<sub>ot</sub>/HO<sub>ot</sub> pairs, and that these reactions represent desorption-limited pathways because the dissociative chemisorption of H<sub>2</sub>O is favoured over molecular adsorption on IrO<sub>2</sub>(110). The higher stability of HO<sub>ot</sub> groups and their preferred formation causes the higher-barrier HO<sub>ot</sub>/HO<sub>ot</sub> recombination reaction to become the dominant pathway for H<sub>2</sub>O formation with increasing O<sub>ot</sub> coverage, consistent with the experimentally-observed upshift in the H<sub>2</sub>O TPRS peak temperature.</p>