Effect of Mn addition to Cu/ZnO/ZrO2 catalysts for CO2-to-methanol hydrogenation

Cu/ZnO/ZrO₂ catalysts are well known for promoting methanol synthesis via CO₂ hydrogenation. In this study, the effect of Mn addition to Cu/ZnO/ZrO₂ was investigated. XPS analysis revealed that the as-prepared catalysts contained both Mn³⁺ and Mn⁴⁺ species, whereas only Mn³⁺ was present after hydrogen reduction at 300 °C. These results suggest that Mn species existed as Mn₂O₃ under reaction conditions. Furthermore, the addition of Mn led to a decrease in the crystallite size of metallic Cu, indicating that Mn species promoted the dispersion of Cu species. This increased dispersion is considered to enhance the number of active sites on the Cu/ZnO/ZrO₂ catalyst. In addition, the presence of Mn species enhanced the methanol selectivity by suppressing the undesirable methanol decomposition to CO. As a result, the Mn-modified Cu/ZnO/ZrO₂ catalyst achieved a methanol yield of 9.3 % at 4 MPa and 250 °C.