DFT Studies of Oxygen Vacancies on Undoped and Doped La2O3 Surfaces

La2O3 is one of the more efficient oxide catalysts for oxidative methane coupling. In this article, we examine the extent to which methane activation can be improved by replacing a La cation in the surface layer with other cations. The purpose of these substitutional dopants is to make the oxygen atoms in their neighborhood more reactive, which makes the doped oxide a better oxidant. We examined doping the surface layer of La2O3(001) and (011) with Cu, Zn, Mg, Fe, and Al. We have chosen dopants whose oxide formation enthalpy is less than that of La2O3. Some (Cu, Fe) are capable of having two different valence states, whereas some (Zn, Mg, Al) have only one. All of them lower substantially the energy of vacancy formation on the two faces. We use a moderation principle to suggest that Cu-doped La2O3 is not a good catalyst for methane activation despite lowering the energy of oxygen-vacancy formation the most. We propose that it is likely that the experimental value for the oxygen-vacancy formation energy might be affected substantially by the presence of adventitious dopants, which will then affect catalytic activity as well. We suggest that dopants affect the energy of vacancy formation in two ways: a local modification of the bond strength of the oxygen atoms to the oxide and a global effect due to a change in the Fermi level, which, in turn, can affect the charge of the oxygen vacancy and its energy of formation.