Study Of Cu/TiO₂ Catalysts For Methanol Synthesis From CO₂ Hydrogenation

Synthesis of methanol from the catalytic hydrogenation of the carbon dioxide is of great importance for the development of economy and the protection of environment. The process can synthesize useful chemical chemicals, which will utilize carbon dioxide more efficiently, and reduce the greenhouse simultaneously. In this paper, Cu/TiO2 catalysts were prepared and used for methanol synthesis from CO2 hydrogenation. The influences of the crystal phase of titanium dioxide, the amount of Cu loading and the effects of Ce promoters on the physicochemical and catalytic properties of Cu/TiO2 catalysts were investigated. The results obtained are mainly as follows.1. The influences of the TiO2 crystal phase on Cu/TiO2 catalysts were investigated. High purity anatase TiO2 (a-TiO2) and rutile TiO2 (r-TiO2) were prepared by hydrothermal synthesis and used as the support in Cu/TiO2 catalysts for methanol synthesis from CO2 hydrogenation. In comparison with the Cu/a-TiO2, the Cu/r-TiO2catalyst exhibits a higher CO2 conversion and CH3OH selectivity, though it possesses a smaller metallic copper surface area. Moreover, the higher amount of valid basic sites and stronger surface basicity were observed over the Cu/r-TiO2catalyst. The catalytic properties of Cu/TiO2 are determined by the surface basicity of the catalyst rather than the metallic copper surface area. These results further confirm that the "support" acts as the role of adsorbing and activating CO2, and provide an evidence and support for the dual-site mechanism.2. The influences of the Cu content on Cu/TiO2catalysts were studied. With the increase in copper content, the CuO crystallite size increases, and the reduction peak of CuO shifts toward the higher temperature. The Cu surface area in the reduced catalyst takes on a volcano varation trend. Furthermore, the number and the density of basic sites reduce with the addition of Cu contents. As the loads of copper is lower than 10%, CO2 conversion is relate to Cu surface area, while the conversion of CO2 is determined by the amount of valid basic sites when copper loading is higher than 10%. CH3OH selectivity is determined by the strength of basic sites, and a suitable strength of basic sites is beneficial to the improvement of the CH3OH selectivity.3. The effects of Ce on the catalytic performance of Cu/TiO2 catalysts were studied. With the introduction of Ce, the crystallization of CuO reduce, while the amount and strength of medium and strong basic sites increase. The activity and methanol selectivity of catalysts first increase and then decrease with the increase in the amount of Ce. As the amount of CeO2doping is 5 wt%, the highest CO2 conversion and methanol selectivity was obtained. The improvement of catalytic performance is related to the change of surface basicity.