Sr-doping effects on La2O3 catalyst for oxidative coupling of methane

Abstract

Density-functional theory calculations were carried out to study the strontium (Sr)-doping effect on methane activation over a lanthanum-oxide (La2O3) catalyst for the oxidative coupling of methane (OCM) using the cluster model. Eight Sr-doped La2O3 cluster models were built from pure La2O3 clusters that were used previously to model the La2O3 catalyst. These form two distinct categories, namely, those without a radical character (LaSrO2(OH), La2SrO4, La3SrO5(OH), and La5SrO8(OH)) and those with a radical character (LaSrO3, La2SrO4(OH), La3SrO6, and La5SrO9). The potential-energy surface for CH4 activation to form a CH3 radical at different Sr–O and La–O pair sites on these Sr-doped La2O3 clusters was calculated to study the Sr-doping effect on the OCM catalytic activity. CH4 physisorption and chemisorption energies, and activation barriers, and CH3 desorption energies were predicted. Compared with the pure La2O3 clusters, in general, the Sr-doped La2O3 clusters are thermodynamically and kinetically more reactive with CH4. For the Sr-doped La2O3 clusters without the radical character, the Sr–O pair site is more reactive with CH4 than the La–O pair site, although a direct release of the CH3 radical is also highly endothermic as in the case of the pure La2O3 clusters. In contrast, for the Sr-doped La2O3 clusters with a radical character, the activation of CH4 at the oxygen radical site and the release of the CH3 radical are much easier. Thus, our calculations suggest that the Sr dopant prompts the OCM catalytic activity of the La2O3 catalyst by providing a highly active oxygen-radical site and by strengthening the basicity of the M–O pair site, which leads to lower CH4 activation energies and lower CH3 desorption energies.