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 CO 2 hydrogenation. Herein, two isomorphous zeolitic imidazolate frameworks (ZIF-67 and ZIF-8) were used as sacrificial templates to synthesize multi-layered Co 3 O 4 /ZnO catalysts with tunable core-shell structures and well-controlled spatial positions. The multi-layered catalysts were then evaluated for the catalytic performance for CO 2 hydrogenation. A series of characterization techniques were applied to investigate the morphology and structure of the Co 3 O 4 /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 Co 3 O 4 /ZnO nanocatalysts exhibited significantly enhanced activity as compared to their monometallic counterparts (pure Co 3 O 4 and ZnO). In addition, the ZnO@Co 3 O 4 with Co 3 O 4 as shell showed significantly higher CO 2 conversion (25%) than that of Co 3 O 4 @ZnO with ZnO as shell (0.82%). Furthermore, in-situ Diffuse Reflection Fourier Transform Infrared Spectroscopy (DRIFTS) analysis suggested that formate (HCOO*) and methoxy (CH 3 O*) species were the crucial intermediates in the CO 2 hydrogenation over the Co 3 O 4 /ZnO composite nanocatalysts. However, pure ZnO could not activate the CO 2 ; consequently, the use of the Co 3 O 4 /ZnO nanocatalysts derived from multiple-layered ZIFs structures provides new insights into improving the catalytic efficiency of CO 2 hydrogenation.