Insights into the role of water and surface OH species in methane activation on copper oxide: a combined theoretical and experimental study

Abstract

Despite its huge potential, the utilization of methane as a main feedstock for the synthesis of fuels and value-added chemicals is limited. Earth abundant transition metal oxides (TMOs) are promising catalysts for efficiently transforming methane to value-added products, but their activity is not satisfactory. The introduction of a small amount of water was reported to significantly change TMO's methane activation ability; however, the role of water remains unclear. In this study, a combined theoretical and experimental approach is used to elucidate how the presence of water influences thermodynamics and kinetics of methane activation on the CuO catalyst. Density functional theory (DFT) calculations show that water can be activated to form surface hydroxide species (OH*) on the CuO surface with a very low barrier of 5.6 kJ mol⁻¹. The presence of surface OH* opens a new reaction pathway for the C–H bond activation. DFT computed activation barriers for the first and second activations of methane in the presence of surface hydroxide species are 62 and 76 kJ mol⁻¹, significantly lower than the corresponding barriers of 82 and 154 kJ mol⁻¹ on CuO under dry conditions. FTIR experiments of the methane reaction on CuO at a temperature of 300 °C validate the theoretical prediction, showing that the moist reaction is faster and has lower induction times, indicating that adsorbed water is an initiator for methane activation. Besides, the stability of the CuO catalyst is also enhanced in the presence of water which helps to prevent the consumption of lattice oxygen of CuO and avoid the reduction of CuO to the inactive Cu metallic state.