The photocatalytic and thermal catalytic reduction of CO2 with H2 over Pt/TiO2 catalysts

Pt/TiO2 with Pt loading ranging from 0.5 to 2 wt.% were prepared by wet impregnation method. CO2 hydrogenation over Pt/TiO2 was investigated under light irradiation and thermal conditions respectively. Results indicated hydrogenation of CO2 could be catalyzed by Pt/TiO2 under both conditions, but the reaction occurred in the different ways. Under thermal condition, CO2 was converted to CO and CH4, the main product was CO at low temperature (100% of CO selectivity at 250 ？ over 0.5% Pt/TiO2). Increase of reaction temperature led to increase of CH4 selectivity from 0 to 60.94% when reaction temperature rose from 250 to 450 ？ over 1.5% Pt/TiO2. The increase of Pt loading also resulted in the increase of CH4 selectivity. On the contrary, CH4 was the only product under light irradiation. The CO2-TPD results indicated Pt served as CO2 adsorption centers. CO2 adsorbed on Pt site via carbonyl group. Under light irradiation, excited electrons were trapped by Pt and reduced surface adsorbed carbonyl species to form CH4 via formic acid intermediate. However, under thermal condition, H2 was first decomposed into H atoms over Pt, then reduced the carbonyl species adsorbed on Pt to form CO, and hydrogenated CO to produce CH4.