The mechanism of H2 and H2O desorption from bridging hydroxyls of a TiO2(110) surface

Abstract

The photocatalytic H₂ production from H₂O over TiO₂ has attracted tremendous attention in recent years, and great progress has been achieved in light adsorption and water dissociation. As a comparison, the H₂ and O₂ production over a pure TiO₂ surface has not been successful yet. Recently, the desorption of bridging hydroxyls (BBOHs) to form H₂ has been found to be possible on a TiO₂(110) surface. However, the yield of H₂ is low, and the majority of BBOHs desorb as H₂O. Here, for the first time, we have systematically studied the mechanism of H₂ desorption and the competition of H₂ and H₂O desorption on a TiO₂(110) surface with DFT methods. We found that the generally believed pathway, the direct coupling of BBOHs, is not the most facile pathway. We propose that the reaction undergoes a Ti–H key intermediate, which then forms an O–H^δ+⋯H^δ−–Ti type dehydrogenation transition state. On a stoichiometric surface, the barriers for H₂ desorption and H₂O desorption are 2.20 eV and 1.09 eV, respectively. However, with an increase in BBO vacancies (O_vs, created by H₂O desorption), the barrier for H₂ desorption decreases, while the barrier for H₂O desorption increases. Specifically, H₂ desorption was found to be easier than H₂O desorption with our two O_vs TiO₂ model. Therefore, our results predict that H₂O desorption is dominant at the beginning, then H₂ desorption becomes more and more competitive. This prediction perfectly fits with the experimental observations. Furthermore, the stability of the key Ti–H intermediates and the possible ways for changing the conditions of H₂ desorption have also been discussed.