Cu@ZIF-8 derived inverse ZnO/Cu catalyst with sub-5 nm ZnO for efficient CO2 hydrogenation to methanol

Maximizing the Cu-ZnO interface is regarded as the best strategy to promote methanol synthesis activity over CuZn catalysts. Reducing the ZnO particle size can enhance the Cu-ZnO interaction, but its synthesis with a sub-5 nm size still remains a challenge. Moreover, the fabrication of an active powder catalyst with an inverse ZnO/Cu configuration is still very rare. In this work, we report a facile synthesis of inverse ZnO/Cu catalysts by direct calcination of Cu@ZIF-8 (zeolitic imidazolate framework-8). The particle size of ZnO and Cu can be tuned by changing the Cu loading as well as the calcination temperature. Importantly, inverse ZnO/Cu catalysts with sub-5 nm ZnO were obtained under gentle calcination at 623 K. In the CO2 hydrogenation reaction, the highest methanol synthesis activity reached 933 g kg(cat)(-1) h(-1) at 533 K and 4.5 MPa, with a high turnover frequency (TOF) of 0.013 s(-1), over an optimized C2Z-623 catalyst, which exceeds the commercial CuZnAl catalyst (HiFuel R120, Johnson Matthey) and has ranked the top reported value over advanced CuZn catalysts under comparable conditions. Such excellent activity is attributed to the inverse structure of Cu particles covered by sub-5 nm ZnO, which promotes the intimate Cu-ZnO interface with enhanced SMSI. Moreover, we find that the methanol turnover frequency (TOF) monotonically increases with decreasing the particle size of ZnO as well as Cu. It is proposed that reducing the ZnO size favors the spillover of reducible ZnOx species onto the Cu surface while lowering the Cu size is responsible for the enhanced amount of exposed Cu steps for accommodating Zn species, both of which could facilitate the formation of Zn covered Cu steps as active sites for methanol synthesis.