Observation of Molecular Hydrogen Produced from Bridging Hydroxyls on Anatase TiO2`(101)

Anatase TiO2 is used extensively in a wide range of catalytic and photocatalytic processes and is a promising catalyst for hydrogen production. Here, we show that molecular hydrogen was produced from bridging hydroxyls (HOb) on the (101) surface of single-crystal anatase (TiO2(101)). This stands in contrast to rutile TiO2(110), where HOb pairs react to form H2O. Electron bombardment at 30 K produced bridging oxygen vacancies in the surface. Deuterated bridging hydroxyls (DOb) were subsequently formed via dissociation of adsorbed D2O and confirmed by infrared reflection-absorption spectroscopy. During temperature-programmed desorption (TPD) spectroscopy, D-2 desorption was observed at 520 K. Density functional theory calculations show that both H-2 and H2O production from HOb are endothermic at 0 K on TiO2(101), but H-2 (H2O) desorption is entropically driven above 230 K (800 K). The calculated activation barrier for H-2 desorption is 1.40 eV, which is similar to the desorption energy obtained from analysis of the D-2 TPD spectra. The H-2 desorption likely proceeds in two steps: H atom diffusion on the surface and then recombination.