Kinetics of low-temperature methane activation on IrO2(1 1 0): Role of local surface hydroxide species

Abstract

The ability of the IrO2(1 1 0) surface to promote CH4 activation at low temperatures (~150 K) suggests possibilities for developing IrO2-based catalysts to selectively convert light alkanes to high-value chemicals. In this study, we present experimental results and microkinetic simulations, based on density functional theory (DFT) calculations, of temperature programmed reaction spectra (TPRS) obtained for CH4 σ-complexes adsorbed on IrO2(1 1 0), and focus on clarifying how surface OH groups modify the branching between CH4 desorption, activation and subsequent reactions during TPRS. Our DFT results predict that surface OH groups strongly destabilize CH4 σ-complexes on IrO2(1 1 0) in addition to deactivating surface O-atoms that are needed to achieve CH4 activation at low temperature. We demonstrate that a microkinetic model that incorporates the influence of surface OH groups on the CH4 binding and reactivity reproduces experimental TPRS results which show that adsorbed CH4 σ-complexes preferentially dissociate at low CH4 coverage, but that an increasing fraction of the adsorbed CH4 desorbs at low temperature (~120 K) with increasing initial CH4 or H coverage. The simulations reveal that low-temperature CH4 desorption during TPRS arises primarily from CH4 σ-complexes that are kinetically-trapped between adjacent surface OH groups, and are thus destabilized and unable to access reactive O-atoms. In addition, when the effect of adjacent OH groups is incorporated we find that the PBE-D3 functional incorporating dispersion provides CH4 binding energies that agree more closely with that obtained from the TPRS experiments. Our results demonstrate that the local effect of adjacent OH groups must be incorporated into any microkinetic models to properly capture the selectivity between extensive and partial oxidation in alkane conversion on IrO2(1 1 0) under reaction conditions.