Effect of CaO on the performance of Cu-ZnO-ZrO₂ catalyst for methanol synthesis from CO₂ and H₂

CuO:ZnO:ZrO₂=5:4:1 (molar ratio) catalysts were prepared with CaO doping of 0,1%, 2%, 4%, 8%, 16% (molar fraction) by cocurrent-flow co-precipitation. X-ray diffraction (XRD), thermal analysis(TG-DTG), Fourier infrared (FT-IR), N₂ adsorption desorption (BET), X-ray photoelectron spectroscopy (XPS), hydrogen temperature programmed reduction (H₂-TPR), CO₂ temperature programmed desorption (CO₂-TPD) and NH₃ temperature programmed desorption (NH₃-TPD) were used to characterize the catalysts. The catalyst activity was evaluated with a lab-made fixed bed reactor. Results show that CaO doping enhances Lewis acid and surface alkaline of the catalyst, increases the amount of high temperature carbonate in the catalysts, improves the thermal stability, reduces the CuO particle size, enhances the synergistic effect of Cu-Zn, increases the Cu specific surface area and the Cu dispersion. The catalyst activity is influenced by the surface acidity, the specific surface area of copper, the synergistic effect of Cu-Zn and the dispersion of copper. When the doping amount of CaO is 2%, the copper specific surface area is 79.3 m²/g, the dispersion degree of copper is 34.8%, the CO₂ conversion is 19.01%, the selectivity of methanol is 24.55% and the yield of methanol is 0.044 g/(g_cat·h), catalyst activity is the highest. With the amount of CaO increasing, the excessive CaO occupies the catalyst pore and covers the surface active sites, the Lewis acid and the surface alkaline of the catalysts become so strong that the effective contact of CuO and H₂ is reduced, CO₂ is difficult to desorb, resulting in decrease of catalytic activity. Therefore, the proper doping amount of CaO (2%) can promote the synthesis of methanol through CO₂ hydrogenation.