Hydrogen oxidation on oxygen-rich IrO₂(110)

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 (Ir_cus). We find that on-top oxygen atoms (O_ot), adsorbed on the Ir_cus 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 (HO_br) (~500 to 750 K) during TPRS. The desorption-limited TPRS peak of H₂O shifts from ~490 to 550 K with increasing O_ot coverage, demonstrating that O_ot 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 HO_br and HO_ot groups, with equilibrium favouring HO_ot groups. Our calculations further predict that subsequent H₂O evolution occurs through the recombination of HO_br/HO_ot and HO_ot/HO_ot 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 HO_ot groups and their preferred formation causes the higher-barrier HO_ot/HO_ot recombination reaction to become the dominant pathway for H₂O formation with increasing O_ot coverage, consistent with the experimentally-observed upshift in the H₂O TPRS peak temperature.