How CO2 poisons La2O3 in an OCM catalytic reaction: A study by in situ XRD-MS and DFT

Abstract

Oxidative coupling of methane (OCM) is a catalytic partial oxidation process that converts methane directly into more valuable C2 products (ethane and ethylene). The mechanism related to selectivity and conversion is still not clear. Previous results suggested that the bulk structure change of the La2O3 catalyst is related to OCM activation performance. This work is focused on investigation of carbonates and the reaction products simultaneous formation over La2O3 catalyst utilizing coupled in situ XRD-MS. Semi quantitative analysis clearly reveals a positive dependence between CO2 and C2 products light-off temperature and the initial bulk La2O2CO3 concentration, indicating a strong correlation between carbonates and La2O3 catalyst poisoning. The activity of the La based catalyst is associated to its oxide form rather than to its oxycarbonate form. Based on operando results a two-step bulk La2O2CO3 decomposition model resulting in surface carbonates presenting on the catalyst surface between 500 °C and 800 °C is proposed. DFT calculations further confirm the proposed model and indicate surface carbonates do inhibit catalytic methane activation. It is concluded that CO32− species located in surface region of the catalyst operate as a direct La2O3 catalyst poison in the highly motivating OCM reaction.