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

Abstract

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 (Ircus) and the resulting σ-complexes then serve as precursors for H2 bond cleavage. Our DFT calculations predict that H2 adsorbs strongly on an atop-Ircus site, and that the H2 complex can dissociate by a facile pathway involving H-transfer to a neighboring bridging O atom (Obr) to produce an H-Ircus/HObr 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 HObr 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).