Water promoted photocatalytic transfer hydrogenation of furfural to furfural alcohol over ultralow loading metal supported on TiO2

Photocatalytic hydrogenation of furfural offers an ideal method for selective biomass upgrading into value-added chemicals or fuel additives under mild conditions. However, it is still challenging to control the product selectivity due to side reactions of functional groups and reactive radical intermediates. Herein, photocatalytic transfer hydrogenation of furfural was studied using the TiO2-based photocatalysts with alcohols as both the solvent and hydrogen donor. Ultralow loading metal supported on TiO2, together with adding a small amount of water in the system, were demonstrated to greatly increase the selectivity of furfuryl alcohol product. Electron paramagnetic resonance (EPR), ultraviolet–visible spectroscopy (UV–Vis) and photoluminescence (PL) measurements gave evidence that ultralow loading Pt or Pd on TiO2 increase the oxygen vacancy concentration and the photogenerated charge separation efficiency, which accelerates the photocatalytic reduction of furfural. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and mechanistic studies confirmed that photogenerated holes and electrons are active species, with dissociatively adsorbed methanol being directly oxidized by holes, furfural hydrogenated by protons and electrons and H2O modifying the intermediate diffusion which contributes to high selectivity of furfuryl alcohol. This work demonstrates a simple approach to design photocatalysts and tune product selectivity in biomass valorization.