Enhancing direct hydroxylation of benzene to phenol on Fe₁/PMA single-atom catalyst: a comparative study of H₂O₂vs. O₂-assisted reactions

In this study, systematic first-principles calculations were carried out to ascertain the optimal single-atom catalyst (SAC) among the 3d-transition metals (TM₁ = Fe₁, Co₁, Ni₁, Cu₁, and Zn₁) anchored on a phosphomolybdic acid (PMA) cluster for efficient benzene oxidation to phenol, which is otherwise challenging at ambient temperature. Strong binding due to substantial charge transfer between Fe₁ and PMA, and the adsorption energies of H₂O₂ and O₂ oxidants enabled significant bonding within the Fe₁/PMA cluster, facilitating enhanced catalytic performance compared to that of 3d-TM₁ (Co₁, Ni₁, Cu₁, and Zn₁). The Fe₁/PMA cluster demonstrated enhanced reactivity towards H₂O₂ supported by lower activation barriers and rate-determining steps for H₂O₂ (0.84 and 0.67 eV) compared to O₂ (1.02 and 0.66 eV). The spontaneous dissociation of H₂O₂ on Fe₁/PMA, in contrast to O₂, is a crucial step to initiate iron-oxo (Fe₁-O) active site formation, easing benzene to phenol oxidation at ambient temperature. Thus, the proficient coordination environment of Fe₁ atoms as SACs adsorbed on the PMA cluster is found to influence catalytic performance, especially in the case of H₂O₂. The proposed mechanism is reminiscent of hydrocarbon hydroxylation in enzymatic processes, establishing Fe₁/PMA as an environmentally friendly, heterogeneous and non-noble metal green catalyst for electrocatalytic phenol production.