Effect of NaOH Concentration on Rapidly Quenched Cu–Al Alloy-Derived Cu Catalyst for CO₂ Hydrogenation to CH₃OH

By utilizing greenhouse gas CO₂ and renewable energy-sourced H₂ to produce methanol, the “methanol economy” can replace fossil fuels and H₂ as the energy storage medium, which not only reduces CO₂ emissions, but also mitigates the energy shortage issue. However, the traditional Cu-based catalysts for CO₂-to-methanol conversion suffer from low activity at low temperature and high vulnerability to sintering and deactivation. In this contribution, rapidly quenched skeletal Cu catalysts (RQ Cu) are prepared by leaching the RQ Cu–Al alloy with NaOH aqueous solutions of different concentrations. It is found that high NaOH concentration of 10 wt% favors the preparation of the RQ Cu-10 catalyst with higher porosity, lower residual Al content, and larger active Cu surface area (S_Cu) than the RQ Cu-3 catalyst leached with 3 wt% of NaOH solution. However, in aqueous-phase CO₂ hydrogenation at 473 K and 4.0 MPa, the CO₂ conversion over the RQ Cu-3 catalyst is more than two times greater than that over the RQ Cu-10 catalyst, and the selectivity and productivity of methanol are 1.20 and 2.69 times of the corresponding values over the RQ Cu-10 catalyst. At 5.0 MPa, the selectivity and productivity of methanol are further boosted to 97.9% and 1.329 mmol g_Cu^–1 h^–1 on the RQ Cu-3 catalyst. It is identified that the S_Cu of the RQ Cu-3 catalyst is well preserved after reaction, while dramatic growth of the Cu crystallites occurs for the RQ Cu-10 catalyst. The better catalytic performance and stability of the RQ Cu-3 catalyst are tentatively attributed to the presence of more residual Al species by using NaOH solution with lower concentration for Al leaching, which acts as the dispersant for the Cu crystallites during the reaction.