Abstract:

Catalytic hydrogenation of CO₂ to methanol has attracted considerable attention due to its potential in alleviating global warming and mitigating the dependence on fossil fuels. Cu-based catalysts are widely used in industry because of their high activity for methanol production. However, the reaction still suffers from low methanol selectivity because of the generation of CO as a by-product via the reverse water gas shift reaction (RWGS). The formation of another by-product H₂O leads to inevitable Cu sintering, which decreases the methanol production rate. It is well known that CO can alter competitive molecular adsorption on the surface and the redox behavior of the active sites; hence, CO doping in feed gas might not only inhibit the RWGS but also minimize surface poisoning by the adsorbed oxygen. On the other hand, CO₂ hydrogenation to methanol over Cu-based catalysts is a structure-sensitive reaction, and a change in the precursor can have a remarkable influence on the structure and morphology of the catalyst, and ultimately, the catalytic performance. In this work, Cu/ZnO/Al₂O₃ catalysts have been prepared via a hydrotalcite-like precursor (CHT-CZA) and a complex phase precursor (CNP-CZA) using co-precipitation and ammonia evaporation methods. Subsequently, the performance of the two types of catalysts with different CO contents (CO₂: CO:H₂:N₂ = x:(24.5 - x):72.5:3) is compared at 250 ℃ and 5 MPa in order to explore the role of CO. The evaluation results show that both catalysts follow a similar trend in the conversion of CO and CO₂ as well as the space-time-yield (STY) of MeOH and H₂O. The conversions of CO₂ and STY_H2O decrease gradually with an increase in the CO volume, but STY_MeOH is positively correlated with the CO volume. Furthermore, X-ray photoelectron spectroscopy (XPS) analysis reveals that the amount of reduced Cu species on the surface increases with increasing CO content. Judging from these results, the introduction of CO inhibits the RWGS and enhances the methanol yield for both catalysts by removing the surface oxygen as the reducing agent and thereby facilitating the exposure of the active reduced Cu species. On the other hand, transmission electron microscopy (TEM) observations indicate the doped CO may cause agglomeration of particles due to over-reduction, leading to gradual catalyst deactivation. Compared with the traditional CNP-CZA, the catalyst derived from hydrotalcite-like compounds exhibits better activity and long-term stability under all atmospheres, at different CO doping levels. This is because the hydrotalcite-like layer structure helps maintain the active metal state and confine the structure by limiting the agglomeration of Cu species.

Key words: CO₂ hydrogenation, Methanol synthesis, Cu/ZnO/Al₂O₃ catalyst, CO doping, Hydrotalcite-like precursors