Tuning the Active Phase and Surface Chemistry of a CO2 Hydrogenation Co/TiO2 Catalyst with UV Light

Abstract

To improve catalytic performance, it is important to control the state of the active phase of catalysts (e.g., metallic, oxidic or carbide). However, conventional pretreatments (e.g., heating) often cause unwanted side-effects, like sintering and active phse encapsulation. Here, we propose UV light illumination as an alternative approach to tune the active phase under reaction conditions. We showcase this possibility using a Co/TiO2 catalyst for CO2 hydrogenation. UV light illumination resulted in an increase in the percentage of reduced Co (i.e., 93 vs. 97 % metallic cobalt for dark vs. light), as revealed by X-ray absorption spectroscopy. Furthermore, using operando Fourier transform infrared spectroscopy under CO2 hydrogenation conditions, we observed a light-induced change in surface coverage of different species, such as CO*, (bi)carbonates and formates. When switching between dark and light conditions, a reversible increase in activity was observed, accompanied by a decrease in the IR absorption bands of formates and CO*. We propose that the reduction of cobalt is due to a charge transfer mechanism from the conduction band of the titania support to the conduction band of the cobalt metal (oxide) nanoparticles. On the other hand, we attribute CO desorption to a metal-to-ligand charge transfer, from the d-band of the cobalt metal (oxide) nanoparticles to the adsorbed CO molecules. While UV light illumination is more commonly used to enhance catalytic performance, we demonstrate its application to tune the active phase of the catalyst, offering advantages over conventional methods that suffer from undesired effects, like metal (oxide) nanoparticle sintering.