Controlling the Oxidation State of Fe-Based Catalysts through Nitrogen Doping toward the Hydrodeoxygenation of m-Cresol

Stabilizing the oxidation state of Fe is of great importance for the rational design of Fe-based catalysts. To this end, N-doped carbon composites (NC) are prepared with different N-doping contents and used as supports for Fe particles. We find N-doped carbonaceous materials enable an effective control of the Fe oxidation state via an electronic interaction between Fe and N. This interaction leads to a decrease in the Fe particle size when the N-doping content is increased, as shown by structural characterization of transmission electron microscopy and powder X-ray diffraction, and to a weakened oxygen affinity of the Fe particles in the presence of N-doped sites, as unveiled by H2-temperature-programmed reduction measurements with separate N2O and water vapor pretreatments. The weaker oxygen affinity correlates with the excellent long-term stability of Fe/NC catalysts during the hydrodeoxygenation of lignin-derived m-cresol to form valuable aromatic products, whereas obvious oxidative deactivation is observed for Fe/C. An investigation of the surface nitrogen composition of all N-doped samples shows that their deactivation rate constants are closely related to the nitrogen content anchoring Fe into the carbon architecture, which indicates unambiguously that embedding nitrogen into the carbon skeleton (mainly including pyridinic and pyrrolic nitrogen functional groups) plays a critical role in stabilizing Fe, while amino-N is inclined to suffering from wastage during the hydrodeoxygenation reaction. These results are further confirmed computationally through an electronic analysis of Fe–N complexes embedded into graphene, showing that skeletal nitrogen sites shield Fe from oxidation relative to bare carbon or aminic nitrogen.