Steam reforming of methane by titanium oxide photocatalysts with hollow spheres

Abstract

Steam reforming of methane (SRM) is one of the most useful techniques for methane (CH₄) conversion because of the large hydrogen yield per CH₄ molecule. However, this process is not commercially viable due to the high reaction temperature and associated energy costs. To decrease the SRM reaction temperature, the introduction of photochemical energy has been proposed; however, the charge recombination of photo-generated carriers must be suppressed to achieve higher activity. Here, TiO₂ photocatalysts with a hollow sphere structure are synthesized and loaded with spatially separated co-catalysts to achieve high charge separation in an attempt to improve SRM efficiency. The highest SRM activity is observed for hollow-sphere structured TiO₂ with Pt and Rh₂O₃ co-catalysts selectively deposited on the inner and outer TiO₂ surfaces, respectively. In situ electron spin resonance and photo-luminescence measurements clearly demonstrate that photo-excited electrons and holes are trapped at Pt and Rh₂O₃ sites, respectively, of Rh/hollow TiO₂/Pt, resulting in efficient charge separation and increased SRM activity. Taken together, these findings support our hypothesis that the spatial separation and heterogeneous loading of co-catalysts is a promising design strategy for photocatalytic methane conversion reactions.