Investigating the Impact of Na2WO4 Doping in La2O3-Catalyzed OCM Reaction: A Structure–Activity Study via In Situ XRD-MS

Abstract

The La2O3 catalyst exhibits good performance in OCM reactions for its promising C2 selectivity and yield. Previous studies have affirmed that the formation of carbonates in La2O3 impedes the catalyst’s activity as a result of poisoning from CO2 exposure. In this study, a series of Na2WO4-impregnated La2O3 catalysts were synthesized to investigate the poisoning-resistant effect. The bulk phase and kinetics of the catalysts were analyzed in reactors employed with in situ XRD-MS and online MS, focusing on the CO2 adsorption on La2O3 and the phase transition process to La2O2CO3 in temperature zone correlated to OCM light-off. In situ XRD analysis revealed that, with Na2WO4 doped, CO2 exposure at elevated temperatures formed La2O2CO3 in tetragonal crystal phases, exhibiting distinctive differences from the hexagonal phase carbonates in undoped commercial La2O3. The ability to develop tetragonal or monoclinic La2O2CO3 was suggested as a descriptor to assess the sensitivity of La2O3 catalysts to CO2 adsorption, a tunable characteristic found in this study through varying Na2WO4 doping levels. Coupled XRD-MS analysis of CO2 adsorption uptake and phase change further confirmed a positive dependence between the resistivity of La2O3 catalyst to CO2 adsorption and its low-temperature C2 selectivity. The results extended the previous CO2 poisoning effect from multiple perspectives, offering a novel modification approach for enhancing the low-temperature performance of La2O3 catalysts in OCM.