Vol.:(0123456789) 1 3 Topics in Catalysis (2018) 61:397–411 https://doi.org/10.1007/s11244-017-0877-y ORIGINAL PAPER Dissociative Chemisorption and Oxidation of H 2 on the Stoichiometric IrO 2 (110) Surface Tao Li 1  · Minkyu Kim 2  · Zhu Liang 1  · Aravind Asthagiri 2  · Jason F. Weaver 1 Published online: 12 December 2017 © Springer Science+Business Media, LLC, part of Springer Nature 2017 Abstract 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 fnd that the dissociative chemisorption of hydrogen occurs efciently 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 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 (Ir cus ) 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-Ir cus site, and that the H 2 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 H 2 complex. We also fnd 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 infuence of HO br groups on H 2 σ-complex formation. Finally, we used DFT to examine pathways for H 2 O formation on s-IrO 2 (110) and fnd 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). 1 Introduction Understanding the interactions of hydrogen with IrO 2 sur- faces is central to improving applications of electrocatalysis as well as exploiting the high-reactivity of IrO 2 for promot- ing methane activation. Several studies have demonstrated that iridium oxide is an efective electrocatalyst for the hydrogen evolution reaction (HER) [1–3]. Due to its high stability and electronic conductivity in the potential region of the oxygen evolution reaction (OER) [4], IrO 2 has also been shown to be one of the most active electrocatalysts for OER, especially in applications with polymer electro- lyte membrane (PEM) fuel cells [5–7]. Recently, Liang et al. have reported that methane undergoes highly facile C–H bond activation on the stoichiometric IrO 2 (110) surface at temperatures as low as 150 K [8], which may provide opportunities for developing catalytic processes for efcient methane functionalization to value-added products. In that study, stoichiometric constraints as well as a high stability of hydrogenated bridging oxygen (HO br ) groups on IrO 2 (110) are considered as key factors in determining the branching between CH 4 oxidation and recombination during TPRS [8]. The importance of IrO 2 in applications of electrocata- lytic water splitting as well as the exceptional reactivity of IrO 2 (110) in promoting CH 4 activation provide substantial motivation for investigating the adsorption and oxidation of H 2 on the s-IrO 2 (110) surface. Over the last two decades, detailed studies about the dis- sociative adsorption of hydrogen on late transition metal (TM) oxides have been reported for only two surfaces, i.e., Tao Li and Minkyu Kim have contributed equally to this work. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11244-017-0877-y) contains supplementary material, which is available to authorized users. * Jason F. Weaver weaver@che.uf.edu 1 Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA 2 William G. Lowrie Chemical & Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA