Promotional Effect of ZnO and ZrO2 in K-Doped Fe Catalysts for CO2 Hydrogenation to Light Olefins

Promoters play a critical role in tuning the activity and selectivity of Fe catalysts in CO2 hydrogenation to produce light olefins, which are key building blocks in the petrochemical industry. Herein, by a combined experimental and theoretical approach, we show that high and stable performance of Fe catalysts could be achieved by taking advantage of the promotional effect of both Zn and Zr. Structural characterization indicates that ZnO could improve the dispersion and reducibility of Fe oxides and facilitate the formation of active Fe carbide species, whereas ZrO2 could stabilize the structure and catalytic performance, especially the selectivity of hydrocarbon products. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments suggest that carbonate, bicarbonate, formate, and methoxy are essential intermediates in the CO2 hydrogenation to hydrocarbon products, including paraffins and olefins. The conversion kinetics of each intermediate species are dependent on the type of promoters as well as the phase structure of the active Fe species. DFT calculations revealed a strong correlation between the formation energy of surface oxygen vacancies and that of Fe carbide species in promoted Fe oxides, in accordance with experimental results. Moreover, the calculated energy profiles of CO2 hydrogenation over different catalysts indicate that Zn could promote the activation of CO2 and its transformation to the oxygenate intermediates, while Zr could facilitate the conversion of oxygenates to hydrocarbon precursors. The discrepancies in the evolution trend of various intermediate species on promoted Fe catalysts in the transient DRIFTS experiments can be rationalized by the differences in energy barriers of elementary or rate limiting steps.