Hydrogen oxidation on oxygen-rich IrO2(110)

Abstract

We investigated the adsorption and oxidation of H2 on O-rich IrO2(110) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. Our results show that H2 dissociation occurs efficiently on O-rich IrO2(110) at low temperature and initiates from an adsorbed H2 σ-complex on the coordinatively-unsaturated Ir atoms (Ircus). We find that on-top oxygen atoms (Oot), adsorbed on the Ircus sites, promote the desorption-limited evolution of H2O during subsequent oxidation of the adsorbed hydrogen on IrO2(110) while suppressing reaction-limited production of H2O via the recombination of bridging HO groups (HObr) (~500 to 750 K) during TPRS. The desorption-limited TPRS peak of H2O shifts from ~490 to 550 K with increasing Oot coverage, demonstrating that Oot atoms stabilize adsorbed OH and H2O species. DFT predicts that molecularly-adsorbed H2 dissociates on O-rich IrO2(110) at low temperature and that the resulting H-atoms redistribute to produce a mixture of HObr and HOot groups, with equilibrium favouring HOot groups. Our calculations further predict that subsequent H2O evolution occurs through the recombination of HObr/HOot and HOot/HOot pairs, and that these reactions represent desorption-limited pathways because the dissociative chemisorption of H2O is favoured over molecular adsorption on IrO2(110). The higher stability of HOot groups and their preferred formation causes the higher-barrier HOot/HOot recombination reaction to become the dominant pathway for H2O formation with increasing Oot coverage, consistent with the experimentally-observed upshift in the H2O TPRS peak temperature.