Correction: Understanding of binding energy calibration in XPS of lanthanum oxide by in situ treatment.

Correction for 'Understanding of binding energy calibration in XPS of lanthanum oxide by in situ treatment' by Jerry Pui Ho Li et al., Phys. Chem. Chem. Phys., 2019, 21, 22351-22358.

Understanding of binding energy calibration in XPS of lanthanum oxide by in situ treatment.

Rare earth oxides have seen increased usage over the years in batteries and catalysts. Due to their unique electronic properties, they are the subject of fundamental and practical interest. However, the complexity in their electronic structures makes unambiguous characterization, such as X-ray photoelectron spectroscopy (XPS), very challenging. Lanthanum oxide (La2O3) has attracted special attention as a promising catalyst for the oxidative coupling of methane (OCM) reaction. In this work, a new and reliable way of XPS calibration is developed by applying various in situ preparations for a nanorod La2O3 catalyst to intentionally form different lanthanum compounds, followed by XPS characterization and corroboration with first principles calculations. To form different compounds, five sample treatments were performed including heating in vacuum and treatment with O2, CH4, CO2, and H2O, which are all relevant to OCM reaction conditions. Adventitious carbon or lattice oxygen, as conventional calibration standard species for energy scale, is only suitable for one or few in situ prepared surfaces. Our results also clearly demonstrate the vital difference between performing the ex situ analysis after exposure of the sample to the atmosphere and the in situ analysis. By carefully comparing the spectra of various photoemission peaks of different compounds, we conclude that the binding energy of 102.2 eV for the La 4d7/2 peak can be used as the internal calibration standard for all considered samples. Furthermore, different oxygen species were unambiguously identified by matching the oxygen 1s binding energies from the in situ measurements and first principles predictions.

First principles studies of CO2 and O2 chemisorption on La2O3 surfaces.

Periodic density functional theory calculations were performed to study the surface structures and stabilities of the La2O3 catalyst in CO2 and O2 environments, relevant to the conditions of the oxidative coupling of methane (OCM) reaction. Thermodynamic stabilities of the clean surfaces were predicted to follow the order of (001) ≥ (011) ≫ (110) > (111) > (101) > (100), with their direct band gaps at the Γ point following the similar order of (001) > (011) > (110) > (111) > (100) > (101). Hubbard U corrections to the La 4f and 5d orbitals do not qualitatively change the predictions of surface energies and band gaps. For the most stable (001) surface, CO2 chemisorption to form carbonate species is exothermic by -0.60 eV with a negligible energy barrier of 0.07 eV, whereas O2 chemisorption to form peroxide species is endothermic by 0.64 eV with a considerable energy barrier of 1.29 eV. For the slightly less stable (011) surface, both CO2 and O2 chemisorption can occur at different surface sites, and the same applies to the other studied surfaces. Dissociation temperatures of surface carbonate species range from 300 to 1000 K at pCO2 of 1 bar, which follow the order of (101) ≈ (110) > (111) ≈ (100) ≈ (011) ≫ (001), showing their strong sensitivity to the surface structure. Dissociation temperatures of surface peroxide species are mostly lower than the room temperature except for those of the (011) and (111) surfaces, although the significant kinetic barriers predicted should prevent their facile dissociation. Insights into the temperature-programmed desorption experiments and the methane reactivity of La2O3 in the OCM reaction were also given based on the results of our calculations.