Surface Ru–H Bipyridine Complexes-Grafted TiO2Nanohybrids for Efficient Photocatalytic CO2Methanation

A series of novel surface Ru–H bipyridine complexes-grafted TiO2nanohybrids were for the first time prepared by a combined procedure of surface organometallic chemistry with post-synthetic ligand exchange for photocatalytic conversion of CO2to CH4with H2as electron and proton donors under visible light irradiation. The selectivity toward CH4increased to 93.4% by the ligand exchange of 4,4′-dimethyl-2,2′-bipyridine (4,4′-bpy) with the surface cyclopentadienyl (Cp)–RuH complex and the CO2methanation activity was enhanced by 4.4-fold. An impressive rate of 241.2 μL·g–1·h–1for CH4production was achieved over the optimal photocatalyst. The femtosecond transient IR absorption results demonstrated that the hot electrons were fast injected in 0.9 ps from the photoexcited surface 4,4′-bpy–RuH complex into the conduction band of TiO2nanoparticles to form a charge-separated state with an average lifetime of ca. 50.0 ns responsible for the CO2methanation. The spectral characterizations indicated clearly that the formation of CO2•–radicals by single electron reduction of CO2molecules adsorbed on surface oxygen vacancies of TiO2nanoparticles was the most critical step for the methanation. Such radical intermediates were inserted into the explored Ru–H bond to generate Ru–OOCH species and finally CH4and H2O in the presence of H2.