The iron–potassium catalysts supported on single- and multiwalled carbon nanotubes, FeK/SWNTs and FeK/MWNTs, were fabricated, characterized, and evaluated in CO2 hydrogenation. The FeK/SWNTs catalyst outperforms the FeK/MWNTs catalyst in selectivity during CO2 hydrogenation to olefins with the selectivities being 62.3% and 52.4%, respectively. Of particular interest is that the FeK/SWNTs catalyst is more selective toward heavy olefins (C5+ = selectivity, 39.8%), whereas the FeK/MWNTs catalyst is more selective to light olefins (C2–C4 = selectivity, 30.7%). The FeK/SWNTs catalyst also affords much lower selectivities to undesired CO and CH4 in addition to higher productivity of hydrocarbons, thus resulting in an unprecedentedly high productivity of heavy olefins of 27.6 μmolCO2 gFe –1 s–1. The more electron-enriched exterior of SWNTs with large curvature, higher abundance of Hägg carbide, and stronger interaction with light olefins may rationalize the superior catalytic behavior of the FeK/SWNTs catalyst in CO2 hydrogenation to heavy olefins. The FeK/SWNTs catalyst also exhibited a minor change in catalytic performance in a 120 h stability test, highlighting SWNTs as a promising catalyst support for producing value-added chemicals from greenhouse gas CO2.

The direct conversion of CO2 with renewable H2 to aromatics can transform greenhouse gas and intermittent reproducible energies into valuable organic building blocks. However, the catalytic efficiency for this purpose remains low on existing catalysts containing either metal oxides (the methanol route) or iron (the olefin route) as the CO2 hydrogenation component(s). In this contribution, benefitting from the exceptional activity of the honeycomb-structured graphene (HSG)-supported, potassium-promoted iron (FeK1.5/HSG) in hydrogenating CO2 to light olefins, and with the help of the tandem HZSM-5, CO2 was converted to aromatics with a high selectivity of 41% among all the carbon-containing products (inclusive of CO) or 68% among all the hydrocarbons at a CO2 single-pass conversion of 35% and high space velocity of 26000 mL h–1 gcat –1, which results in an unprecedentedly high space time yield of aromatics of 11.8 μmolCO2 gcat –1 s–1. Furthermore, the dual-layer packing configuration of the FeK1.5/HSG-zeolite catalyst enables flexible adjustment of the aromatics spectrum simply by changing the type of the tandem zeolite. This work shows promise for the realization of a high-efficiency and versatile CO2-to-aromatics technology.

= ) 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.