Enhanced Visible Light Photocatalytic Hydrogenation of CO2 into Methane over a Pd/Ce-TiO2 Nanocomposition

A series of Ce-doped TiO2 nanoparticles were prepared by a sol-gel process and characterized by XRD, SEM, TEM, EDX mapping, UV-vis DRS, Raman spectroscopy, N-2 adsorption-desorption, PL spectra, CO2-TPD, and XPS. It is found that Ce ions can enter the lattice matrix of TiO2 and occupy of Ti sites. This atom replacement leads to the formation of impurity energy levels in the band gap of TiO2, extending light absorption into the visible light region. Because Ce has a more flexible valence state, both Ce3+ and Ce4+ could be formed in the composites. The preference facilitates the photoinduced charge separation inside of the crystals. Moreover, Pd nanoparticles were then loaded as a co-catalyst on the surface of doped composites. As the trapping center of electrons, it can efficiently adsorb and activate CO2 molecules, promoting their transformation into CH4. These composites were then evaluated as photocatalysts for CO2 hydrogenation. While all of them could efficiently catalyze the reaction, 1.0% Pd/0.5% Ce-TiO2 catalysts show the best photocatalytic performance, with CH4 and CO yields up to 220.61 and 27.36 mu mol/g, respectively, under visible light irradiation of 3 h. The improved photocatalytic behavior could be possibly induced by the synergistic effect between Ce and Pd. A probable mechanism was thus proposed based on above characterizations and experimental results.