Combined experimental and computational study to unravel the factors of the Cu/TiO2 catalyst for CO2 hydrogenation to methanol

The hydrogenation of CO2 to methanol over Cu-nanoparticles supported on TiO2 nanocrystals was studied at 30 bar pressure and 200−300 °C. 5 wt% Cu-TiO2 catalyst was synthesized by a modified hydrothermal method (Cu-TiO2HT) and by incipient wetness impregnation method (Cu-TiO2IMP). TEM analysis of the Cu-TiO2HT catalyst revealed the formation of Cu-nanoparticles (3-5 nm), while larger Cu particle sizes were observed on the Cu-TiO2IMP catalyst. The Cu-TiO2HT catalyst showed superior catalytic activity (CO2 conversion ∼ 9.4 %) and methanol selectivity (∼ 96 %) at 200 °C and 30 bar pressure. Low CO2 conversions (∼6%) and high CO selectivity (∼40 %) was obtained on the Cu-TiO2IMP catalyst. Density functional theory (DFT) calculations indicated the CO2 activation to methanol to proceed via a reverse water gas shift pathway with a significantly lower (93 kJ/mol) CO2 activation barrier on the Cu-nanoparticles, relative to the larger Cu particles (127 kJ/mol). In addition, the higher selectivity towards methanol over the Cu-TiO2HT catalyst was attributed to the higher CO and HCO stability on the Cu nanoparticles. Time of stream (TOS) study of the Cu-TiO2 catalysts showed no significant deactivation even after 150 h with molar feed ratio 1:3:1 (CO2:H2: N2) at 200 °C.