(425g) Elucidating the Role of Carbon in the Hydrodeoxygenation of Phenol in Iron Carbide-Based Catalysts

Iron-based catalysts exhibit an exceptional selectivity in cleaving C-O bonds, however it can easily deactivate via oxidation under HDO conditions. Recent studies suggests that the presence of a small amount of carbon results in the formation of metal carbides that can affect its chemical stability under an oxidizing environment. Metal carbides can be a cheaper alternative due to their resemblance with noble metals in HDO/hydrogenation reactions. Here, we propose to elucidate the vapor phase HDO mechanism for phenol on iron carbide surfaces. It is also important to consider the lateral interactions between carbon adatoms on Fe to enable the development of realistic multi-scale models of such complex systems. Thus, we quantified them using Hamiltonian lattice gas cluster expansion (LGCE) models. Our DFT-based results for the vapor phase HDO reaction mechanism of phenol Fe₃C(001) suggest that phenol deoxygenation occurs through the dehydroxylation mechanism. Under this pathway, the first C-O bond in phenol is cleaved in the presence of co-adsorbed hydrogen, to form surface phenyl and hydroxyl species. In the next step, hydrogenation of phenyl species occurs through C-H bonding between co-adsorbed surface hydrogen and carbon of the phenyl ring, leaving benzene as the end product. The dominant mechanism resembles the one occurring on bare iron surfaces, but with slightly higher barrier (~0.3 eV) on carbide surfaces. We also observed that the carburization of iron results in an increase in the activation energies for C-O bond cleavage indicating a reduction in surface oxophilicity. This indicates that presence of carbon protects iron carbides from oxidation but does not hinder the nature of catalytic activity as compared to pure iron surfaces. Our results from LGCE model suggest some important ground state structures including the c(2 × 2) ordered structure that occurs at ½ ML (Figure 1). This correlates well with the experimentally-observed LEED structure.