Effect of Chromium Doping on the Catalytic Behavior of Cu/ZrO2/CNTs-NH2 for the Synthesis of Methanol from Carbon Dioxide Hydrogenation

A series of Cu/ZrO2/CNTs-NH2 catalysts with various chromium dopings were prepared using a co-precipitation method for the synthesis of methanol by the hydrogenation of CO2. The impact of the addition of chromium on the catalytic performance of the Cu/ZrO2/CNTs-NH2 catalyst was investigated in a fixed-bed plug flow reactor. When the chromium loading was set to 1% of the total amount of Cu2+ and Zr4+, the methanol yield increased to a maximum of 7.78% (reaction conditions: 3.0 MPa, 260 degrees C, V(H-2): V(CO2): V(N-2)= 69: 23: 8 and gaseous hourly space velocity (GHSV)= 3600 mL center dot h(-1) center dot g(-1)). The catalysts were characterized by N-2 physisorption, X-ray diffraction (XRD), temperature-programmed desorption of H-2 (H-2-TPD), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption of CO2 (CO2-TPD), differential thermal analysis (DTA), and scanning electron microscopy (SEM). The results of these analyses indicated that the introduction of chromium reduced the size of the Cu nanoparticles, enhanced the dispersion of the Cu species, inhibited the phase transformation and sintering of ZrO2, increased the specific surface area, enhanced the amount of CO2 adsorbed, and promoted the conversion of weakly adsorbed CO2 species to strongly adsorbed CO2 species. Taken together, these factors lead to a high methanol yield. However, when the chromium loading was greater than 1%, the amount Cu and Zr on the surface, as well as the size of the Cu nanoparticle reduced considerably, which led to a significant reduction in the adsorption of CO2 species. This effect also facilitated the formation of strongly adsorbed CO2 species, leading to lower methanol yields.