Insight into the synthesis of alkali metal modified Co-doped ZnFe2O4 with faveolate spinel structure and their enhanced selectivity of olefins during CO2 hydrogenation

Emission reduction of CO2 allows of no delay, and CO2 hydrogenation strategy reveals the great potential to achieve target of "Carbon Neutral". Trapped with the low selectivity of olefins in our previous work of Co-doped ZnFe2O4, the alkali metal (Na, K) modified Co-doped ZnFe2O4 were synthesized, and their enhanced selectivity of olefins were present. The promotion caused by alkali metal modification revealed that more generation of metallic Fe and the additional generation of Fe5C2 species could be ascribed for the enhanced selectivity of C2- C 4 olefins and C 5 + products. The introduced alkali metal via the strong electronic interaction increased the outer electrons density around Fe atoms, which could promote the adsorption of CO2 and prohibited the secondary hydrogenation reaction of the formed olefins. Therefore, the enhanced selectivity of C2-C4 olefins over the alkali metal (Na, K) modified Co-doped ZnFe2O4 was emerged, while the CO2 conversion was suppressed (caused by the less generation of Fe3O4). Compared with Na-modified samples, the introduced K could make more generation of Fe5C2 to promote the carbon chain growth, due to its stronger alkalinity, thus leading to the highest selectivity of C 5 + products over K-modified samples while the highest selectivity of C2-C4 olefins over Namodified samples.