Exploring the effect of the reaction conditions on the mechanism of the photocatalytic reduction of CO₂ in the vapor phase over Pt/TiO₂: an operando FTIR study

The utilization of photocatalysis for CO2 conversion into solar fuels holds significant promise for advancing clean energy solutions; however, there are still many uncertainties regarding the surface mechanisms of the reaction, even for the most commonly studied TiO2-based photocatalytic systems. Of special relevance is the origin of photoconverted products and the role played by adventitious carbon species on the photocatalyst surface, whose nature and origin lack unambiguous identification to date. In this study, we investigated the dynamic nature of vapor-phase photocatalytic CO2 reduction using a benchmark Pt/TiO2 photocatalyst. To identify carbon species on the photocatalyst surface, we reported a comprehensive analytical approach involving X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and operando Fourier-transform infrared (FTIR) spectroscopy during the activation and photocatalytic reduction of CO2. Through this multi-technique approach, we were able to differentiate initial carbonaceous surface species and identify active intermediates during reactions. Upon irradiation, carbon species in the form of carboxylates get involved in reactions with photocatalytically activated surface adsorbed water and can contribute to 40% of methane yields in the first few minutes of irradiation, therefore hindering a reliable quantification of CO2 conversion levels. This was confirmed by exposure of the catalyst to light and water vapor during ten irradiation cycles, which significantly reduced the amount of methane and C-species on the catalyst surface. Transient activity was identified as the dominant factor driving methane production. Moreover, reactivation of the catalyst can be achieved through periodic irradiation conditions, leading to a remarkable 60% increase in methane production yields during 180 minutes of irradiation. These findings shed light on the mechanisms occurring on the photocatalyst surface upon light/dark transition steps and demonstrate the potential for enhancing CO2 photoreduction performance through periodic irradiation strategies.