Understanding lanthanum oxide surface structure by DFT simulation of oxygen 1s calibrated binding energy in XPS after in situ treatment

Lanthanum oxide (La2O3) is a promising catalyst for the oxidative coupling of methane (OCM). Our recent work provided a detailed XPS characterization results of La2O3 after in situ interaction with OCM related species, including CO2 and H2O. In this study, the experimental results are further corroborated with DFT calculated binding energies. It helps revealing the catalyst surface structure under realistic process conditions. Five different low index surfaces and both pristine La2O3 and carbonate-covered surfaces have been simulated. The (001) pristine surface yields the best fitting results to the experimental data, which is ascribed as the calibrated XPS lattice oxygen O 1s peak with binding energy of 529.8 eV. Calculated O 1s values of (001) and (011) surfaces always demonstrate the best match with experimental XPS data. These two surfaces are also the most likely exposed La2O3 surfaces as proposed by previous stability calculation result. Additional simulations are performed for bulk structures of La2O2CO3 and La(OH)(3). Combining theoretical and experimental results provides more proper assignment of XPS features and suggests a carbonate formation pathway from surface to subsurface on La2O3(0 0 1). Correlating the surface structures with in situ XPS experimental data brings further insights into the La2O3 catalyzed OCM reaction mechanism.