Effect of nitrogen doping on the reducibility, activity and selectivity of carbon nanotube-supported iron catalysts applied in CO2 hydrogenation

• Carbon nanotubes are a highly suitable catalyst support for CO2 hydrogenation.
• Nitrogen doping of carbon nanotubes promotes the reduction of supported iron oxide nanoparticles.
• The low catalytic activity of Fe/SiO2 is due to strong metal–support interactions.
• An olefin selectivity of 11% in the C2–C5 range was achieved with unpromoted Fe/NCNT.

CO2 hydrogenation to short-chain hydrocarbons was investigated over iron catalysts supported on oxygen- and nitrogen-functionalized multi-walled carbon nanotubes (CNTs) and on silica, which were synthesized by the dry impregnation method using ammonium ferric citrate as precursor. The reduction of the calcined catalysts was examined in detail using temperature-programmed reduction in H2 and in situ X-ray absorption near-edge structure (XANES) analysis. The XANES results revealed that the mixture of hematite and magnetite was gradually transformed into wustite and metallic iron during heating in H2. Iron oxide nanoparticles supported on nitrogen-functionalized CNTs were easier to reduce compared to those on oxygen-functionalized CNTs indicating a promoting effect of the nitrogen functional groups. The interaction between iron oxide and silica was found to be much stronger inhibiting the reduction to metallic iron. As a result, the catalytic activity of iron nanoparticles supported on CNTs in CO2 hydrogenation at 360 °C, 25 bar and a H2:CO2 ratio of 3 was almost twofold higher compared with iron supported on silica. CO2 was converted into C1–C5 hydrocarbons with CO and methane as major products over all catalysts. The Fe/NCNT catalyst achieved the highest olefin selectivity of 11% in the hydrocarbons range of C2–C5. In contrast, mostly paraffins were formed over the Fe/SiO2 catalyst.

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