Effect of surface structure and Pd doping of Fe catalysts on the selective hydrodeoxygenation of phenol

Density functional theory (DFT) calculations were performed to investigate the effects of surface structure and Pd doping of Fe catalysts on the activity and selectivity of phenol hydrodeoxygenation (HDO). Phenol adsorption via binding with the aromatic ring was found to be energetically more stable than adsorbing through solely the hydroxyl group on Fe(110), Fe(111) and Fe(211) surfaces. The adsorption strength on monometallic Fe surfaces increased in the sequence of Fe(211) < Fe(110) < Fe(111). Pd doping did not have a significant influence on the adsorption stability of phenol. The (211) facet of Fe featured particular steps showed good activity toward aromatics production, as evidenced by the lower barriers associated with the dehydroxylation followed by H addition to form benzene than the direct hydrogenation on the aromatic ring. In contrast, the ring saturation products would be dominant on Fe(110) and Fe(111) surfaces due to lower hydrogenation barriers on the aromatic ring of phenol as compared to the C_Ar-O bond cleavage. The kinetically preferred pathway for phenol conversion on Pd doped Fe(110) and Fe(111) surfaces was the direct hydrogenation on the aromatic ring, same to that identified on monometallic (110) and (111) surfaces. In comparison, on Fe(211), the difference in kinetic barrier between dehydroxylation and ring hydrogenation pathways became larger in the presence of Pd, promoting the selectivity to benzene formation. In the presence of Pd dopant, the H₂ molecule was readily dissociated and the surface H* coverage was increased, leading to a lowering of the adsorption enthalpy of key surface species and ultimately enhancing the rates of HDO.