Periodic DFT calculation for the formation of EPFRs from phenol on γ-Al2O3 (110): Site-dependent mechanism and the role of ambient water

The heterogeneous interfacial reaction on metal oxide surfaces in combustion and thermal processes is the crucial source of environmentally persistent free radicals (EPFRs). Al 2 O 3 is one of the most abundant metal oxides in fine particulate matter (PM) encountered in combustion systems. In the present work, the detailed formation mechanism of EPFRs from phenol on the γ-Al 2 O 3 (110) surface with different hydration levels was investigated by periodic theoretical calculations. The results show that γ-Al 2 O 3 has stronger catalytic activity for the formation of EPFRs compared with other metal oxides including α-Al 2 O 3 . Furthermore, the heterogeneity of the γ-Al 2 O 3 (110) surface has an impact on the formation energy barrier of EPFRs, even on the reaction route itself. When introducing ambient water, the formation pattern of EPFRs at various surface sites transforms from a single type of pathway to a mixed reaction mode and then changes to an almost new mixed mode with elevated water coverage. The multiple roles of ambient water on catalytic dissociation reaction, which had been rarely investigated in previous studies, was clarified. Although this work focuses on γ-Al 2 O 3 , it is expected to foster a crucial re-examination and integration of conventional EPFRs formation mechanism on the other metal oxide surfaces already reported in the literature, providing a novel insights into the heterogeneous generation of atmospheric PM-associated EPFRs. • Transition alumina (γ-Al 2 O 3 ) has strong catalytic activity for the formation of EPFRs. • The energy barrier of EPFRs formation varies on the distinct surface sites of γ-Al 2 O 3 (110). • The formation pattern of EPFRs changes with the evaluated water coverage. • The ambient water plays multiple roles in producing EPFRs on metal oxides.