Fabrication of multi-layered Co3O4/ZnO nanocatalysts for spectroscopic visualization: Effect of spatial positions on CO2 hydrogenation performance

Controlled fabrication of spatial positions of different active sites on catalyst structures is critical to exploring the reaction mechanisms for CO2 hydrogenation. Herein, two isomorphous zeolitic imidazolate frameworks (ZIF-67 and ZIF-8) were used as sacrificial templates to synthesize multi-layered Co3O4/ZnO catalysts with tunable coreshell structures and well-controlled spatial positions. The multi-layered catalysts were then evaluated for the catalytic performance for CO2 hydrogenation. A series of characterization techniques were applied to investigate the morphology and structure of the Co3O4/ZnO composites with different spatial locations. Due to the unique multi-layered structure, the distinct interfacial structures, and the synergistic effects, the resultant core-shell structured Co3O4/ZnO nanocatalysts exhibited significantly enhanced activity as compared to their monometallic counterparts (pure Co3O4 and ZnO). In addition, the ZnO@Co3O4 with Co3O4 as shell showed significantly higher CO2 conversion (25%) than that of Co3O4@ZnO with ZnO as shell (0.82%). Furthermore, in-situ Diffuse Reflection Fourier Transform Infrared Spectroscopy (DRIFTS) analysis suggested that formate (HCOO*) and methoxy (CH3O*) species were the crucial intermediates in the CO2 hydrogenation over the Co3O4/ZnO composite nanocatalysts. However, pure ZnO could not activate the CO2; consequently, the use of the Co3O4/ZnO nanocatalysts derived from multiple-layered ZIFs structures provides new insights into improving the catalytic efficiency of CO2 hydrogenation.