Surface Ru-H Bipyridine Complexes-Grafted TiO 2 Nanohybrids for Efficient Photocatalytic CO 2 Methanation.

A series of novel surface Ru-H bipyridine complexes-grafted TiO 2 nanohybrids were for the first time prepared by a combined procedure of surface organometallic chemistry with post-synthetic ligand exchange for photocatalytic conversion of CO 2 to CH 4 with H 2 as electron and proton donors under visible light irradiation. The selectivity toward CH 4 increased 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 CO 2 methanation activity was enhanced by 4.4-fold. An impressive rate of 241.2 μL·g -1 ·h -1 for CH 4 production 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 TiO 2 nanoparticles to form a charge-separated state with an average lifetime of ca. 50.0 ns responsible for the CO 2 methanation. The spectral characterizations indicated clearly that the formation of CO 2 •- radicals by single electron reduction of CO 2 molecules adsorbed on surface oxygen vacancies of TiO 2 nanoparticles 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 CH 4 and H 2 O in the presence of H 2 .