TiO2 -based photoreforming of alcohols as an alternative pathway for low-carbon H2 evolution

Abstract

Photocatalytic H2 production is a key reaction aiming at efficient solar energy conversion towards green fuels. While water splitting would be the ideal reaction for sustainable H2 production, thermodynamic and kinetic constraints have so far limited its applicability. Alternatively, photoreforming of organic derivatives, particularly those derived from renewable sources, can be an economic and technical solution to promote H2 evolution in parallel with the production of added-value oxidation products. Among different organic substrates, alcohol photoreforming stands out due to its large availability, costeffectiveness, and the possibility to produce key chemical feedstocks. Such reactions have been studied since the early 70s, starting with primary alcohols such as methanol and, more recently, involving polyols such as glycerol. Despite different semiconductors having been employed as photocatalysts, fundamental understanding of the reaction mechanism is a necessary step towards the development of more efficient systems. In this manuscript, the fundamental aspects of alcohol photoreforming are reviewed, focusing on TiO2 -based systems in which a vast literature is available. A systematic comparison of the light-driven reforming pathways of C1-C6 alcohols on TiO2 -based photocatalysts is presented, extending from methanol to more complex substrates such as glycerol and biomass-derived compounds. The aim is to highlight how molecular complexity influences oxidative pathways, intermediate formation, hydrogen evolution, and catalyst requirements. The main findings employing different characterization techniques are summarized and corelated with surface modifications of the oxide aiming at improved H2 evolution rates and selectivity for different oxidation products. The discussion can work as a tool to promote the rational development of more efficient sunlight-driven photocatalysts for photoreforming.