The photoreaction of TiO2 and Au/TiO2 single crystal and powder surfaces with organic adsorbates. Emphasis on hydrogen production from renewables

Abstract

TiO₂ and TiO₂-based materials are prototypes for photo-catalytic reactions as they have been shown for many decades to be active for total oxidation of hydrocarbons to clean the environment. In the last decade or so there has been a shift in the objectives for photoreactions mainly towards hydrogen production from renewables. Here we review the fundamentals behind the reactivity of model TiO₂ surfaces with simple organic compounds in the dark and under photo-irradiation, then we consider the case of ethanol photo-reaction to hydrogen at its fundamental and applied levels. The review starts with an overview of the bulk and surface structures of rutile TiO₂, as it is the most studied phase in surface science despite its lower activity for hydrogen production compared to the anatase phase. We then focus on the gold/TiO₂ system where both phases (anatase and rutile) of TiO₂ have received more attention. In the gold/TiO₂ system emphasis is mainly devoted to understanding the effect of particle dimensions on the electronic conduction as well as the oxidation state of Au particles. The most recent observation using environmental X-ray Photoelectron Spectroscopy indicated the absence of charge transfer from the support to the metal. The photoreaction of ethanol is presented in more detail. The rate for hydrogen production on Au/TiO₂ catalysts does not change if TiO₂ particles are of macro-size or nano-size once normalised by surface area. Also, it was clearly seen that Au/TiO₂ anatase is about two orders of magnitudes higher compared to a similar system where TiO₂ is in the rutile phase. The mechanism for hydrogen production is presented and discussed on the metal/semiconductor systems. Of particular interest is the observation of synergistic effect between the anatase and rutile nanoparticles and one explanation involving electron transfer from one phase to the other is invoked.