Photo-enhanced thermal catalytic CO2 methanation activity and stability over oxygen-deficient Ru/TiO2 with exposed TiO2 {001} facets: Adjusting photogenerated electron behaviors by metal-support interactions

In this study, two Ru/TiO2 samples with different TiO2 facets were prepared to investigate their photo-thermal catalytic CO2 + H2 reaction behavior. Without UV irradiation, the Ru/TiO2 with 67% {001} facet (3RT) displayed improved thermal catalytic activity for CO2 methanation than that of Ru/TiO2 with 30% {001} facet (0RT). After H2 pretreatment, both samples exhibited enhanced thermal catalytic activities, but the H2-treated 3RT (3RT-H) exhibited superior activity to that of the H2-treated 0RT (0RT-H). Under UV irradiation, 3RT-H exhibited apparent photo-promoted thermal catalytic activity and stability, but the enhanced catalytic activity was lower than that of 0RT-H. Based on the characterization results, it is proposed that both the surface oxygen vacancies (Vos) (activating CO2) and the metallic Ru nanoparticles (activating H2) were mainly responsible for CO2 methanation. For 0RT, H2 pretreatment and subsequent UV irradiation did not promote the formation of Vos, resulting in low catalytic activity. For 3RT, on the one hand, H2 pretreatment promoted the formation of Vos, which were regenerated under UV irradiation; on the other hand, the photogenerated electrons from TiO2 transferred to Ru to maintain the metallic Ru nanoparticles. Both behaviors promoted the activation of CO2 and H2 and enhanced CO2 methanation. Moreover, the photogenerated holes favored the dissociated H at Ru migrating to TiO2, also promoting CO2 methanation. These behaviors occurring on 3RT-H may be attributed to the suitable metal-support interaction between the Ru nanoparticles and TiO2 {001}, resulting in the easy activation of lattice oxygen in TiO2 to Vos. With reference to the analysis of intermediates, a photo-thermal reaction mechanism is proposed for the Ru/TiO2 {001} facet sample.