Effect of Doped Metals Rh, Pd, and Cu over the IrO2(110) Surface: Improving C2 Selectivity during Oxidative Coupling of Methane

Oxidative coupling of methane (OCM) is a core chemical process in which methane is directly produced to high-value-added products (ethane and ethylene). The density functional theory (DFT) method was used to study the OCM reaction on IrO2(110) and M/IrO2(110) (M = Rh, Pd, and Cu) surfaces to explore catalysts with high activity and C2 hydrocarbon selectivity. A pure IrO2(110) surface exhibits high activity, but the C2 selectivity is low because O2 is easily adsorbed and dissociated to form Oad, which will lead to the formation of the byproduct CO. Therefore, the catalytic performance of IrO2(110) surfaces doped with second metals (Rh, Pd, and Cu) was investigated. The results show that the doping of Cu and Pd is detrimental to the adsorption and dissociation of O2 and inhibits the formation of Oad. However, Rh doping has no obvious effect. Additionally, charge analysis shows that the doping of metals reduces the transfer of charge from the catalyst surface to the adsorbed O2 compared to the pure IrO2(110), which results in the relatively weak adsorption and high dissociation energy of O2. Moreover, the analysis of reaction rate constants also shows that the dissociation rate of O2 on Cu/IrO2(110) at the same temperature is much lower than that on IrO2(110), Rh/IrO2(110), and Pd/IrO2(110) surfaces. It can be seen that the doping of Cu can improve the C2 hydrocarbon selectivity of the IrO2 catalyst.