Enhancement of oxygen vacancy sites of La_2-xM_xCe₂O_7-δ (M = Ca, Ba, Sr) catalyst for the low temperature oxidative coupling of Methane: A combined DFT and experimental study

This study investigated the effect of different alkaline earth metals doped La_2-xM_xCe₂O_7-δ (M = Ca, Sr, Ba and x = 0.30) catalysts prepared by one-step hydrothermal method for the low-temperature methane oxidative coupling (OCM) to olefins. The presence of a disordered cubic fluorite phase of synthesized catalysts was detected by powder XRD and Raman analysis. The surface analysis by XPS and Raman shows the presence of oxygen vacancy sites, which are responsible for the formation of electrophilic superoxide O₂^– species in presence of molecular oxygen. We observed that the ratio of active oxygen species (O₂⁻ and O₂²⁻) to lattice oxygen (O^2–) was linearly correlated with the number of basic sites. The catalytic activity of different catalysts for the OCM reactions follows the order: La_1.7Ca_0.3Ce₂O_7-δ > La_1.7Sr_0.3Ce₂O_7-δ > La_1.7Ba_0.3Ce₂O_7-δ > La₂Ce₂O₇. The reactivity order follows the same trend as the amount of basic sites and reactive oxygen species. According to this study, the best catalyst is La_1.7Ca_0.3Ce₂O_7-δ, which gives the highest C₂ yield and C₂ selectivity of 23.6 % and 78.8 %, respectively, at 650 °C. DFT studies revealed that substitution of La by Ca in the La₂Ce₂O₇ catalyst significantly reduces the oxygen vacancy formation energy by about 100 kJ/mol compared to the undoped LaCe-oxide. We believe that the superior activity of La_1.7Ca_0.3Ce₂O_7-δ catalyst is attributed to the higher amount of basicity and electrophilic oxygen content, which are the governing factors in OCM for CH₄ conversion and C₂ product selectivity as well.