Unlocking synergy between multi-valence rhodium species for promoted methanol photoreforming

Solar-powered photocatalytic methanol dehydrogenation to produce hydrogen (H₂) and formaldehyde provides a promising approach for storable H₂ fuel without carbon emissions. However, the different properties of C-H and O-H bonds in methanol molecules make it challenging to cleave both bonds effectively on a single catalytic active site during the methanol dehydrogenation process. This work proposes a strategy that constructs multi-valence metal species in the co-catalyst to address this challenge. In the case study of multi-valence rhodium species (Rh⁰ and Rh³⁺) on titanium dioxide (RhO _x /TiO₂) photocatalysts, an apparent turnover frequency (TOF, the H₂ evolution rate as a function of the co-catalyst amount) of 1236 h^-1 is achieved, outperforming that of most reported co-catalysts. Detailed investigations unveil that the synergy between Rh⁰ and Rh³⁺ not only facilitates the cleavage of both C-H and O-H bonds in methanol molecules but also facilitates the desorption of H₂ molecules, leading to improved efficiency. This work showcases an effective strategy for engineering co-catalysts to promote photocatalytic methanol dehydrogenation and provides insights into the mechanism of this reaction catalyzed by heterogeneous photocatalysts.