Tuning CO Hydrogenation toward Olefins rather than C2+ Alcohols by Highly Dispersed Mn-Modified Iron Nitride Catalysts

Abstract Given the similar electronic structure of N and C atoms, iron nitrides and iron carbides are both active for Fischer–Tropsch synthesis. In this work, a series of FeMn-xC3N4 catalysts were prepared via NH3 thermal decomposition of graphitic carbon nitride (g‑C3N4). Tuning the addition content of g-C3N4 can promote the dispersion of Mn species and increase the ratio of Fe2N to Fe4N, regulating product distribution from C2+ alcohols to olefins for syngas conversion. The iron nitride catalyst without g‑C3N4 shows the surface enrichment of Mn species, which promoted the formation of CHO* and transformation toward C2+ alcohol, resulting in the proportion of C2+ alcohols in C2+ products reaching up to 36.2%. The introduction of g‑C3N4 promoted the dispersion of Mn species and facilitated electron transfer from Mn to Fe, generating electron-rich Fe species that enhanced the dissociative adsorption of CO and boosted the conversion of CHO* to CH*. As a result, the olefin proportion in C2+ products remarkably increased to 74.0% while decreasing the proportion of C2+ alcohols to 10.2%. Density functional theory (DFT) indicated that the energy barrier from CHO* to CH* decreased on the surface of highly dispersed MnOx-modified Fe2N. Simultaneously, the adsorption energy of olefins decreased, leading to the increasing selectivity of olefins. This work acquired a better understanding of tuning reaction pathways from C2+ alcohol synthesis to olefin synthesis on iron nitride catalysts, aiming to provide guidance on the optimization of catalyst design for efficient syngas conversion.