We examine a large number of DFT calculations regarding the chemistry of oxide surfaces and show that their qualitative conclusions can be predicted by using a few rules derived from the Lewis acid−base properties of the species involved. (1) The presence of a Lewis acid on an oxide surface increases substantially the binding energy of a Lewis base. ( 2) If an oxide has certain properties because it is a Lewis base, these properties can be suppressed by adsorbing a Lewis acid on the surface. (3) The presence of a Lewis base on an oxide surface diminishes the binding energy of another base, as compared to the binding energy on the same surface with no base on it. These rules also hold if the words "acid" and "base" are exchanged. We show that these rules apply to a large number of systems which seem to have no relationship to each other and which are important for catalysis by oxides.

The activity of electrocatalysts exhibits a strongly dependence on their electronic structures. Specifically, for perovskite oxides, Shao-Horn and co-workers have reported a correlation between the oxygen evolution reaction activity and the eg orbital occupation of transition-metal ions, which provides guidelines for the design of highly active catalysts. Here we demonstrate a facile method to engineer the eg filling of perovskite cobaltite LaCoO3 for improving the oxygen evolution reaction activity. By reducing the particle size to ∼80 nm, the eg filling of cobalt ions is successfully increased from unity to near the optimal configuration of 1.2 expected by Shao-Horn's principle. Consequently, the activity is significantly enhanced, comparable to those of recently reported cobalt oxides with eg∼1.2 configurations. This enhancement is ascribed to the emergence of spin-state transition from low-spin to high-spin states for cobalt ions at the surface of the nanoparticles, leading to more active sites with increased reactivity.

Many recent articles have suggested that density functional theory (DFT) with the generalized-gradient approximation does not provide the correct electronic structure for the oxides of titanium. The current opinion is that a Hubbard U correction improves the DFT results. There is no generally accepted method for deciding what the value of the U parameter should be, and we propose that, if one intends to study catalysis, U should be chosen to fit the reaction energy for the oxidation of Ti2O3 to TiO2. We show that the value of U derived in this manner provides additional improvements in the description of the electronic structure.

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