In situ ESR and TPSR measurements of SrTi_0.975Li_0.025O_3-δ |nanocatalysts for oxidative coupling of methane at low temperature

Perovskitetype SrTi_1-xLi_xO_3-δ(x = 0, 0.025, 0.050) nano catalyst was prepared by using sol-gel method. Its structure was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), in-situ electron spin resonance (ESR), and temperature-programmed surface reaction (TPRS); its catalytic performance for oxidative coupling of methane (OCM) was measured in a microreactor. The results indicated that the OCM catalytic performance at low temperature (about 650℃) is enhanced by doping Li⁺ cation to the B site of perovskitetype SrTiO₃ nano catalysts; the SrTi_1-xLi_xO_3-δ nano catalyst with x = 0.025 performs best. Compare with the conventional catalysts of the same composition prepared by citric acid method, SrTi_0.975Li_0.025O_3-δ nano catalyst performs much better for OCM reaction at low temperature; higher C₂ selectivity is achieved at the same temperature. In-situ ESR results showed that there exist O^- with ESR signal at g = 2.0430 and F center with ESR signal at g = 2.0080 in the nano SrTi_0.975Li_0.025O_3-δ catalyst, while the conventional SrTi_0.975Li_0.025O_3-δ catalyst shows O^2- with the ESR signals at g^∥ = 2.014 and g^⊥ = 1.9906. The high catalytic performance of SrTi_0.975Li_0.025O_3-δ nano catalyst is related to F centers due to the low coordination surface atoms on the nano oxide particles. F center may activate CH₄ at lower temperature and lead to the formation of C₂ hydrocarbons with higher selectivity.