Deep Eutectic Solvent-Assisted Synthesis of NiCuO x Catalyst with Enhanced Low-Temperature CO-SCR Performance: A Structure–Activity Relationship Study

Nitrogen oxides (NOx) are harmful air pollutants, and their selective catalytic reduction by CO (CO-SCR) offers the dual benefit of removing both NO and CO, thus realizing the concept of “waste-to-value”. However, most CO-SCR catalysts are effective only at ≥300 °C, limiting low-temperature applications. Herein, we explore Ni–Cu mixed oxides (NiCuOx) that leverage the synergistic roles of Cu+ species and the Ni3+/Ni2+ redox cycle and the surface synergistic oxygen vacancies (SSOVs) that facilitate NO dissociation. NiCuOx catalysts were synthesized using three conventional methods─sol–gel, coprecipitation, and hydrothermal─as well as a less conventional microwave-assisted deep eutectic solvent (DES) method, and their CO-SCR performance was systematically evaluated. Among them, the DES-derived catalyst (DES-NiCuOx) exhibits superior catalytic performance, achieving 100% NO conversion above 120 °C and 100% N2 selectivity above 150 °C, ranking among the highest reported to date for nonprecious metal catalysts. This outstanding activity is attributed to its larger specific surface area; enriched Cu+ species and OVs, particularly SSOVs; and improved redox properties. In situ DRIFTS analysis reveals that interfacial Ni3+–□–Cu+ (where □ represents an OV) structures in DES-NiCuOx promote NO dissociation and CO adsorption. Moreover, the concurrent involvement of Langmuir–Hinshelwood and Mars–van Krevelen mechanisms, facilitated by excellent oxygen mobility and redox properties, plays a vital role in enhancing low-temperature CO-SCR efficiency. This work highlights the advantages of the DES synthesis route for preparing highly dispersed mixed metal oxides and provides new insights into the rational design of efficient catalysts for low-temperature NOx abatement.