Photoreduction of CO2 using sol–gel derived titania and titania-supported copper catalysts

Carbon dioxide was photocatalytically reduced to produce methanol in an aqueous solution using 254 nm UV irradiation. Titania and Cu-loaded titania were synthesized by an improved sol–gel method using a homogeneous hydrolysis technique. The grain size of TiO2 and Cu/TiO2 were uniform and average diameters were approximately 20 nm. Photocatalytic reduction was conducted in a quartz reactor with a UV lamp irradiating at the center. XPS analysis reveals that Cu 2p3/2 is 933.4 eV indicating primary Cu2O species on the TiO2 supports. EDX and XPS revealed that most copper clusters were on the TiO2 surface. The optimum amount of copper loading was 2.0 wt.% for the highest dispersion among catalysts. The methanol yield of 2.0 wt.% Cu/TiO2 was 118 μmol/g following 6 h of UV illumination. The yield was much higher than those of sol–gel TiO2 and Degussa P25, whose yields were 4.7 and 38.2 μmol/g, respectively. The methanol yield reached a steady-state 250 μmol/g after 20 h of irradiation. Experimental results indicated that the methanol yield was significantly increased by adding NaOH. The caustic solution dissolved more CO2 than did pure water. In addition, the OH− in aqueous solution also served as a strong hole scavenger. The redistribution of the electric charge and the Schottky barrier of Cu and TiO2 facilitates electron trapping via supported Cu. The photocatalytic efficiency of Cu/TiO2 was markedly increased because of the lowering the re-combination probability for hole–electron pairs. The highest quantum and energy efficiencies achieved were 10 and 2.5%, respectively.