Structural properties of alumina surfaces and their roles in the synthesis of environmentally persistent free radicals (EPFRs)

Alumina oxides have been widely utilised as independent catalysts or as support materials for other catalysts. From an environmental perspective, alumina nanoclusters dispersed on surfaces of particulate matter PM12, generated from various combustion processes, play a critical role in the synthesis of environmentally persistent free radicals (EPFRs). Of particular importance are phenoxy-type EPFRs that often act as building blocks for the formation of notorious pollutants. Herein, we present a systematic review of the literature pertinent to structural features of alumina surfaces at the nano-scale and their well-established role in the synthesis of EPFRs. Central to the capacity of alumina surfaces in mediating the formation of EPFRs are their active Lewis acid-base sites. The nature of these sites is very sensitive to hydration scenarios. As evident in electroparamagnetic resonance measurements, more than one category of EPFR forms on alumina surfaces. This generally entails the co-existence of various surface terminations, varying degrees of hydrations, and distinct underlying reaction pathways. The mechanisms for the formation of EPFRs over alumina surfaces involve interactions with terminal OH groups followed by creating genuine chemical bonds with Al3+ sites. Higher concentrations of EPFRs were often detected on alumina surfaces, in reference to other transition metal oxides. We envisage that future studies may focus on the generation of EPFRs from potential precursors other than phenols and catechol, such as brominated species and substituted thiophenols.