Rare earth zirconates Ln-Zr-O (Ln=La, Pr, Gd, Tb) with a fluorite phase for the oxidative coupling of methane: The role of reactive sites and surface properties

Abstract

In this study, a series of rare earth zirconates Ln-Zr-O (Ln = La, Pr, Gd, Tb; named LZ, PZ, GZ, TZ) with a fluorite phase structure were successfully synthesized using the glycine nitrate combustion method for oxidative coupling of methane (OCM). Among these, PZ and TZ possessed redox sites, their surface A-site ions possessed multiple valencies, and their lattice oxygen was easily reduced and desorbed at low temperatures. The OCM reaction performance results revealed that the redox sites were not conducive to C₂ selectivity, although they did increase the concentration of oxygen vacancies. The unit cell free volume (V_f) was found to be an essential factor that influenced methane conversion in all catalysts. The influence of oxygen vacancies on C₂ selectivity was less important than that of surface basic sites. O₂^- and O₂^2- were identified as the reactive oxygen species in OCM, and their concentration were affected by the amount of moderate and strong basic sites on the catalyst surface: the more abundant these two types of basic sites, the more sites available to stabilize electron-deficient oxygen O₂^- and O₂^2-, which promotes C₂ selectivity. Lattice oxygen species O^2- were found to promote the deep oxidation of methane: the more abundant the oxygen vacancies, the more active the lattice oxygen, and the higher the selectivity of the deep oxidation product CO₂.