Formation of environmentally-persistent free radicals (EPFR) on α-Al₂O₃ clusters

Alumina oxides assume prominent catalytic applications in a wide range of industrial processes. However, alumina surfaces also serve as potent promoters in the heterogeneous formation of the notorious environmentally-persistent free radicals (EPFR). Herein, we theoretically examine dissociative adsorption mechanisms of phenol molecules over Al₂O₃ and hydrated Al₂O₃·nH₂O clusters that mimic dehydrated and hydrated alumina structures, respectively. We show that fission of the phenol's hydroxyl bond over dehydrated alumina systematically incurs lower energy barriers in reference to the hydrated structures. A 1,2-water elimination step marks the most feasible channel in the interaction of phenol with hydrated clusters. The relevance of the acidity sites to the catalytic activity of alumina is clearly supported by the finding that the catalytic activity of the alumina surface in producing the phenoxy/phenolate species reversibly correlates with the degree of hydroxyl coverage. Desorption of adsorbed phenolates requires sizable desorption energies, and thus is expected to facilitate surface-mediated condensation into dioxin-like moieties.