= ) of 40 % and 27 μmol CO2 g Fe À 1 s À 1 , respectively, were obtained at 3 wt% of K under the same reaction conditions. At 3 wt% of K and above, the overall selectivity to the olefins (inclusive of light olefins and heavy olefins) remained at as high as ca. 60 % at high CO 2 conversion of ca. 50 %. Comprehensive characterizations revealed that the K promoter enhanced the basicity of the catalyst, which facilitates the formation of χ-Fe 5 C 2 , the key active phase for CO 2 hydrogenation. A good linear relationship is established between the surface basicity and the content of χ-Fe 5 C 2 . Moreover, the K promoter weakened the adsorption of the olefins, which inhibited the secondary hydrogenation of the olefins, thus improving the selectivity to the olefins. This work demonstrates that K is a highly promising promoter for the tailoring of the types of the olefinic products in CO 2 hydrogenation.
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