Zero carbon footprint hydrogen generation by photoreforming of methanol over $Cu/TiO_{2}$ nanocatalyst

Hydrogen generation by photoreforming of methanol is being one of the most intensely investigated photochemical transformations in pursuing a sustainable, zero waste circular economy. This transformation usually produces a significant amount of gaseous carbon in the form of CO2 and CO that disqualifies many active photocatalysts as being fully sustainable. Here, we demonstrate the atomically dispersed Cu on TiO2 composite nanoparticles synthesized by the wet impregnation method that are highly active and selective in hydrogen production by photoreforming of methanol solution. Our photocatalyst exhibits an apparent quantum efficiency of 10% at 365 nm wavelength and the low power 0.45 W light emitting diode (LED) source. The only gaseous product found is molecular hydrogen, whereas all carbon is trapped in the downstream liquid mixture of formaldehyde and formic acid, making the process fully sustainable and clean. We used various experimental techniques and density functional theory calculations (DFT) to characterize our photocatalysts and provide an insight into the exceptional behavior of the Cu/TiO2 system. A combination of ab initio DFT and X–ray photoelectron spectroscopy (XPS) measurements allowed us to identify the Cu+–Cu0–Cu+ redox cycle under the reaction conditions. Bypassing the Cu2+ oxidation state is crucial to keep the oxidation potential of photogenerated holes low enough to prevent CO2 generation and keeping all carbon in the liquid phase. This work paves the way toward an efficient and clean generation of hydrogen by photoreforming of methanol over well–established and cheap Cu/TiO2 photocatalyst.