Understanding the role of metal supported on TiO2 in photoreforming of oxygenates

Abstract

To achieve net-zero targets regarding GHG emissions by 2050, the identification of sustainable energy vectors is critical. In this context, photoreforming presents a potential candidate for recycling and transforming widely available biomass-derived wastes into clean hydrogen fuel, such as crude glycerol from biodiesel and a potential future H₂ production opportunity from bioethanol. Many years of work has proved that TiO₂ is an excellent material for photoreforming of organics due to its stability, availability, and environmentally friendly characteristics as compared to other semiconductors. However, photoreforming faces several obstacles, including the comparatively low hydrogen generation under Sun-equivalent light sources and the need of expensive noble metals. Efforts have been made in several directions, such as extending light absorption by TiO₂ to the visible range, reducing the recombination rate of charge carriers, and preventing back reactions. To overcome these challenges, many methods have been proposed, such as controlling the phase and morphology of TiO₂ nanoparticles, decoration with various metal co-catalysts, doping with metal and non-metal ions, plasmonic enhancement, and preparation of composite systems. Although each approach has its own merits, metal loading has proven to be the most effective among them all. This review provides a deep insight into the underlying role of metal towards the enhancement of TiO₂ catalytic activity, focusing on the findings of recent published work. We discuss in detail the effect of various metals on TiO₂ electronic structure, preparation methods, role in light absorption (surface Plasmon resonance) and chemical changes during various photoreforming steps. Following this we extend our discussion to dye sensitized systems and catalyst testing benchmarking. At the end of the review, we provide possible future research directions to enhance the photocatalytic activity of TiO₂ based photocatalysts for photoreforming.