It would be a milestone in the field of energy conversion if the greenhouse gas CO2 could be recycled with just the light of the sun as energy source. However, current studies in photocatalysis are predominantly based on a trial-and-error methodology, providing little insight into the fundamental physical and chemical processes.
It is the main aim of our research to operate photoreactors under reaction conditions of highest purity with highly sensitive trace gas analysis and to establish reaction protocols enabling us to conduct kinetic and mechanistic studies of photocatalytic CO2 reduction and related reactions. As a first step, we managed to carry out photocatalytic CO2 reduction under continuous-flow conditions, reaching steady-state operation. This provided the basis to study the influence of CO2 concentration, light intensity, and the presence of oxygen on the formation rate of the main product methane. It was revealed that either the amount of charge carriers reaching the surface, or the number of catalytic active sites limit product formation. In a related study, evidence was obtained that the formation of CH4 from CO2 involved an intermediate C-C coupling step, leading to the formation of species such as acetic acid and acetaldehyde. Our studies can thus provide guidelines for future photocatalyst development.