In-situ FT-IR study on CO2 hydrogenation over Cu catalysts supported on SiO2, Al2O3, and TiO2 Original Research Article

For methanol synthesis from CO2 hydrogenation, TiO2-supported Cu catalyst (Cu/TiO2) showed the highest turnover frequency (specific rate per one active site) among three catalysts (Cu/Al2O3, Cu/SiO2, and Cu/TiO2). According to in-situ FT-IR observations, the peaks of adsorbed formate species observed during the reaction were suppressed for Cu/TiO2, although they were dominant for both Cu/Al2O3 and Cu/SiO2. In order to understand the relationship between IR spectra and the activity, as well as to find out the essential factors that control the activity, we investigated the reactivity of intermediate species mainly by in-situ FT-IR spectroscopy. Each Cu site itself had little ability to adsorb CO2; however, Al2O3 and TiO2 readily adsorbed CO2 by weak bondings on them. CO2 species adsorbed on Cu sites were rapidly converted to formate species under reaction conditions. Formate species generated on Cu sites of Cu/SiO2 remained under H2 at 473 K. On Cu/Al2O3 the formate species adsorbed on the support firmly, so it was difficult to distinguish the formate on Cu from that on the support (Al2O3), and they were stable under H2 at 473 K. On Cu/TiO2, hydrogenation of formate species occurred at 423 K. The behavior of IR spectra during reaction over Cu/TiO2 could be explained by the promotion of formate hydrogenation by the synergetic effect between Cu and TiO2.