The visible-light-assisted thermocatalytic methanation of CO2 over Ru/TiO(2-x)Nx

A Ru/TiO(2-x)Nx catalyst was prepared by impregnation-reduction method with loading Ru on the nitrogen doped TiO2 support. This catalyst’s performance for CO2 methanation was evaluated under visible light irradiation (435nm < λ < 465 nm) or not, which was compared with that of Ru/TiO2 catalyst. It was found that Ru/TiO(2-x)Nx exhibits a better catalytic activity than Ru/TiO2 under visible light irradiation or not, and the promoted effect of visible light on the former is much more significant than that on the latter’s. After comparing the results of chemisorption, temperature-programmed surface reaction, X-ray Photoelectron spectroscopy and photocurrent tests over Ru/TiO(2-x)Nx and Ru/TiO2, it was found that the photo-assisted role on Ru/TiO(2-x)Nx mainly comes from two cases: (i) The TiO(2-x)Nx itself is prone to adsorb CO2 and its activation into the CO species (as the intermediates of CO2 methanation reaction), which visible light can further promote the process by forming much more significant oxygen vacancies over TiO(2-x)Nx. (ii) The photo-generated electrons of TiO(2-x)Nx induced by visible light can transfer to the Ru nanoparticles, resulting in the increase in the surface electron density of Ru and then the promoted adsorption and activation of CO2. For Ru/TiO2, TiO2 itself cannot transform the adsorbed CO2 into the CO intermediates, which resulted in a lower thermo-catalytic activity for CO2 methanation. Under visible light irradiation, the extrinsic excitation of TiO2 cannot formation of new surface oxygen vacancies at TiO2, but the photo-generated electrons can transfer from TiO2 to Ru, which resulted in the increase of surface electron density and then the promoted adsorption and activation of CO2 at Ru sites. Thus, a weaker photo-assisted effect for catalyzing CO2 methanation can occur on Ru/TiO2.