A novel process for the preparation of Cu/ZnO and Cu/ZnO/Al₂O₃ ultrafine catalyst: Structure, surface properties, and activity for methanol synthesis from CO₂ + H₂

Cu/ZnO and Cu/ZnO/Al₂O₃ ultrafine particle catalysts have been prepared by a novel oxalate gel coprecipitation method. XRD, TEM, TG/DTG, and EXAFS were used to characterize the phases, morphology, and particle sizes of the precursors of the catalysts and reduced catalysts. The results showed that isomorphous substitution took place between copper and zinc in the precipitates. The addition of Al to the binary system made the copper and zinc in the catalyst exist in much smaller crystallites and exhibit an amorphous-like structure. The effects of composition, structure, and surface property of the catalysts as well as the reaction condition on the activity and selectivity to methanol for methanol synthesis from CO₂ + H₂ were investigated. The activity was found to increase with the increase of surface area of metallic copper in the range of 10-60% of copper content, but it decreased as the copper content exceeded 60%, although the metallic copper surface area was higher. This was explained by the strong synergy between copper and zinc oxide. The effect of contact time on the relative selectivity (γ = S_CH3OH/S_CO) and selectivity of methanol was also investigated. The results indicated that methanol was formed directly from hydrogenation of CO₂.