PH Dependence of MgO, TiO2, and γ-Al2O3Surface Chemistry from First Principles

We investigate the pH-dependent surface chemistry of three relevant and widely used oxides, MgO, TiO2, and γ-Al2O3, from ab initio molecular dynamics simulations at the level of density functional theory (DFT). We studied the MgO (001), anatase TiO2(101), and γ-Al2O3(001) low-index surfaces and considered solvation effects by explicitly simulating the solid/water interfaces. The analysis of the MgO/H2O, TiO2/H2O, γ-Al2O3/H2O interfaces allowed us to access atomistic details of the structure and the effects induced by water on these surfaces. We also investigated the pH dependence by means of the Grand Canonical formulation of species in solution. This allowed us to evaluate acid-base equilibrium constants and the point-of-zero charge (pHPZC) values. The calculated pHPZCvalues compare well with available experimental measurements. Based on the above result, we also predicted the fraction of charged species (H+and OH-) and neutral molecules (H2O) present on the surface as a function of pH, a fundamental aspect if one wants to identify suitable catalysts for water splitting reactions.