N-Doped Fe@CNT for Combined RWGS/FT CO ₂ Hydrogenation

The conversion of CO ₂ into chemical fuels represents an attractive route for greenhouse gas emission reductions and renewable energy storage. Iron nanoparticles supported on graphitic carbon materials (e.g., carbon nanotubes (CNTs)) have proven themselves to be effective catalysts for this process. This is due to their stability and ability to support simultaneous reverse water-gas shift (RWGS) and Fischer-Tropsch (FT) catalysis. Typically, these catalytic iron particles are postdoped onto an existing carbon support via wet impregnation. Nitrogen doping of the catalyst support enhances particle-support interactions by providing electron-rich anchoring sites for nanoparticles during wet impregnation. This is typically credited for improving CO ₂ conversion and product selectivity in subsequent catalysis. However, the mechanism for RWGS/FT catalysis remains underexplored. Current research places significant emphasis on the importance of enhanced particle-support interactions due to N doping, which may mask further mechanistic effects arising from the presence or absence of nitrogen during CO ₂ hydrogenation. Here we report a clear relationship between the presence of nitrogen in the CNT support of an RWGS/FT iron catalyst and significant shifts in the activity and product distribution of the reaction. Particle-support interactions are maximized (and discrepancies between N-doped and pristine support materials are minimized) by incorporating iron and nitrogen directly into the support during synthesis. Reactivity is thus rationalized in terms of the influence of C-N dipoles in the support upon the adsorption properties of CO ₂ and CO on the surface rather than improved particle-support interactions. These results show that the direct hydrogenation of CO ₂ to hydrocarbons is a potentially viable route to reduce carbon emissions from human activities.