Potassium as a Versatile Promoter to Tailor the Distribution of the Olefins in CO2 Hydrogenation over Iron-Based Catalyst

The effect of K on the distribution and productivity of the olefins in CO2 hydrogenation over the single-walled carbon nanotubes (SWNTs) supported Fe catalysts was systematically investigated. The promotion of K not only effectively suppressed the formation of the undesired light paraffins (CH4 and C-2(0)-C-4(0)), but also significantly affected the distribution and the productivity of the olefins. The highest selectivity and productivity of light olefins (C-2(=) C-4(=)) of 27% and 16 mu mol(CO2)g(Fe)(-1) s(-1), respectively, were obtained at 7 wt% of K, while the highest selectivity and productivity of heavy olefins (C-5+(=)) of 40% and 27 mu mol(CO2)g(Fe)(-1) s(-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 CO2 conversion of ca. 50%. Comprehensive characterizations revealed that the K promoter enhanced the basicity of the catalyst, which facilitates the formation of chi-Fe5C2, the key active phase for CO2 hydrogenation. A good linear relationship is established between the surface basicity and the content of chi-Fe5C2. 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 CO2 hydrogenation.