Dissociative Chemisorption and Oxidation of H 2 on the Stoichiometric IrO 2 (110) Surface
In this work, we investigated the dissociative chemisorption and oxidation of H 2 and D 2 on the stoichiometric IrO 2 (110) surface (“s-IrO 2 (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-IrO 2 (110) during adsorption at 90 K, with ~90% of the dissociated H 2 oxidizing to H 2 O during TPRS and evolving in a broad feature between 400 and 800 K. We also observe small quantities of H 2 desorbing in TPRS peaks at 200 K and 530 K, and show that these peaks arise from the desorption of molecularly-adsorbed H 2 and the recombination of atomic hydrogen, respectively. Our results demonstrate that H 2 dissociation on s-IrO 2 (110) occurs by a precursor-mediated mechanism wherein H 2 molecules adsorb strongly on coordinatively-unsaturated Ir atoms (Ircus) and the resulting σ-complexes then serve as precursors for H 2 bond cleavage. Our DFT calculations predict that H 2 adsorbs strongly on an atop-Ircus site, and that the H 2 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 H 2 complex. We also find that the total hydrogen uptake on s-IrO 2 (110) saturates at an H 2 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 H 2 sigma-complex formation. Finally, we used DFT to examine pathways for H 2 O formation on s-IrO 2 (110) and find that steps leading directly to H 2 O formation are energetically demanding and likely determine the overall rate of H 2 oxidation on s-IrO 2 (110).