Dissociative Chemisorption and Oxidation of H2 on the Stoichiometric IrO2(110) Surface.

We investigated the dissociative chemisorption and oxidation of H₂ and D₂ on the stoichiometric IrO₂(110) surface (“s-IrO₂(110)”) using temperature programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. We find that the dissociative chemisorption of hydrogen occurs efficiently on s-IrO₂(110) during adsorption at 90 K, with ~ 90% of the dissociated H₂ oxidizing to H₂O during TPRS and evolving in a broad feature between 400 and 800 K. We also observe small quantities of H₂ desorbing in TPRS peaks at 200 and 530 K, and show that these peaks arise from the desorption of molecularly-adsorbed H₂ and the recombination of atomic hydrogen, respectively. Our results demonstrate that H₂ dissociation on s-IrO₂(110) occurs by a precursor-mediated mechanism wherein H₂ molecules adsorb strongly on coordinatively-unsaturated Ir atoms (Ir_cus) and the resulting σ-complexes then serve as precursors for H₂ bond cleavage. Our DFT calculations predict that H₂ adsorbs strongly on an atop-Ir_cus site, and that the H₂ complex can dissociate by a facile pathway involving H-transfer to a neighboring bridging O atom (O_br) to produce an H-Ir_cus/HO_br pair. For this pathway, we predict that the energy barrier for dissociation is ~ 65 kJ/mol lower than the binding energy of the adsorbed H₂ complex. We also find that the total hydrogen uptake on s-IrO₂(110) saturates at an H₂ coverage of ~ 0.65 ML during adsorption at 90 K, and present evidence that this limited uptake results from a strong influence of HO_br groups on H₂ σ-complex formation. Finally, we used DFT to examine pathways for H₂O formation on s-IrO₂(110) and find that steps leading directly to H₂O formation are energetically demanding and likely determine the overall rate of H₂ oxidation on s-IrO₂(110). [ABSTRACT FROM AUTHOR]