Revealing the intrinsic relationship between nano/electronic structure of CuCo/NC (NC drived from ZIF-67) and their catalytic performance for furfural selective hydrogenation

In this work, zeolitic imidazolate framework-67 (ZIF-67) is prepared by the hydrothermal method, and then carbon doping with nitrogen (NC)-supported cobalt is gained by the thermal treatment in N₂ at 900 °C. The CuCo/NC bimetallic catalysts with different Cu and Co loadings (various Cu/Co mass ratios) are obtained by the galvanic replacement method. A series of characterization results (such as aberration-corrected scanning transmission electron microscopy (AC-STEM) and in-situ XPS) could prove that the nanostructure of the as-synthesized CuCo/NC bimetallic catalysts is that Cu-related species in the form of co-existence of single atoms and clusters is supported on the Co nanoparticles then on NC, and there is a synergy effect among Cu-, Co–, and N-related species. The CuCo/NC-2 (Cu-9.6 wt%, Co-29.7 wt%) catalyst prepared without noble metals demonstrated remarkable catalytic activity under mild reaction conditions (3.0 MPa H₂, 100 °C, 6 h), achieving 51.3 % furfural (FF) conversion and 97.6 % furfuryl alcohol (FA) selectivity. In addition, density functional theory calculations further prove that Cu and Co form an electronic synergistic effect (electrons transferring from Co to Cu). On one hand, the introduction of Cu could significantly reduce the energy required for hydrogen dissociation. The energy required for hydrogen dissociation on CuCo/NC-2 is only 0.003 eV, much lower than the 0.382 eV required for Co/NC, so the active hydrogen species much more easily participate in the hydrogenation reaction. On the other hand, CuCo/NC has lower activation energy for the FF hydrogenation reaction, making the reaction easier to proceed. The activation energies required for the two-step reaction of adding H to the carbonyl group over CuCo/NC are only 1.657 eV and 1.245 eV, while Co/NC requires 2.410 eV and 2.405 eV, respectively. These two aspects play a crucial role in enhancing the catalytic performance (activity and selectivity to FA) of CuCo/NC for FF hydrogenation.