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

[Display omitted] • Multi-layered Co 3 O 4 /ZnO catalysts were designed and used for CO 2 hydrogenation. • CO 2 conversion of ZnO@Co 3 O 4 was 30 times higher than that of the Co 3 O 4 @ZnO. • CO 2 conversion of ZnO@Co 3 O 4 @ZnO was higher than that of ZnO@Co 3 O 4 (35% vs 25%). • Spatial position and interfacial area of Co 3 O 4 /ZnO are key to catalytic activity. • DRIFTS showed that formate and methoxy species were the crucial intermediates. 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. [ABSTRACT FROM AUTHOR]