Formation of Environmentally Persistent Free Radicals on α‑Al 2 O 3 Niveen W. Assaf, † Mohammednoor Altarawneh,* ,† Ibukun Oluwoye, † Marian Radny,* ,‡ Slawomir M. Lomnicki, § and Bogdan Z. Dlugogorski † † School of Engineering and Information Technology, Murdoch University, 90 South Street, Murdoch, Western Australia 6150, Australia ‡ School of Mathematical and Physical Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia § Department of Environmental Sciences, Louisiana State University, Baton Rouge, Louisiana 70803, United States ⊥ Poznan University of Technology, Institute of Physics, Poznan, Poland * S Supporting Information ABSTRACT: Metal oxides exhibit catalytic activity for the formation of environmentally persistent free radicals (EPFRs). Here, we investigate, via first-principles calculations, the activity of alumina α-Al 2 O 3 (0001) surface toward formation of phenolic EPFRs, under conditions relevant to cooling down zones of combustion systems. We show that, molecular adsorption of phenol on α-Al 2 O 3 (0001) entails binding energies in the range of -202 kJ/mol to -127 kJ/mol. The dehydroxylated alumina catalyzes the conversion of phenol into its phenolate moiety with a modest activation energy of 48 kJ/mol. Kinetic rate parameters, established over the temper- ature range of 300 to 1000 K, confirm the formation of the phenolate as the preferred pathways for the adsorption of phenol on alumina surfaces, corroborating the role of particulate matter in the cooling down zone of combustion systems in the generation of EFPRs. 1. INTRODUCTION Environmentally persistent free radicals (EPFRs) represent a group of species that have prolonged life span in ambient environment. Over the last two decades, EPFRs have been the subject of tremendous interest in a wide range of scientific explorations. The major areas of interest have focused on the formation mechanisms and their associated impact on human health 1, 2 from chronic respiratory and cardiopulmonary dysfunction as a result of oxidative stress imposed by reactive oxygen species (ROS). These chemical species include hydroxyl radicals, hydrogen peroxide, and superoxide anion radicals that form in the redox cycling of EPFRs, in particulate matters of an aerodynamic diameter <2.5 μm (PM 2.5 ) generated in combustions, and thermal processes. The chemical makeup of PM 2.5 constitutes a key factor in clarifying the formation mechanisms of EPFRs. Metal oxides, present on surfaces of particulates from combustion processes, 3-6 promote the formation of ROS. 7-9 For this reason, considerable attention has also focused on investigating the catalytic role of metal oxides in forming EPFRs. Lomnicki et al. 10 and Vejerano et al. 11 performed a series of experimental studies to examine the surface-mediated formation of EPFRs over two transition metal oxides, Fe 2 O 3 and CuO, deposited on the silicon oxide surface. These researchers provided a detailed account of the physiochemical interaction of five different aromatic hydrocarbons (phenol, hydroquinone, 2-monochlorophenol, 1,2-dichlorobenzene, and catechol) with the selected metal oxide surfaces. They indicated that, in the progressive physisorption and chemisorption processes, the surface metal atoms transfer electrons to the adsorbed organic precursors, successively leading to the generation of persistent surface bound radicals. Because of the higher oxidation potential of Fe 2 O 3 , the authors proposed that the surface potentially produces more stable EPFRs with longer lifetime compared to those produced over the CuO surface. The same authors have also compared EPFRs formation over NiO 12 and ZnO 13 surfaces. In particular, ZnO exhibit potential for producing EPFRs with long lifetime, ranging from 3 to 73 days, such as semiquinone-type species. 13 Along the same line of enquiry, Patterson and co-workers 14 demonstrated that, the TiO 2 surfaces produce EPFRs, such as phenolate, from their respective stable molecules. Even seemingly inactive surfaces, such as those of silica can generate EPFR, as reported by Mosallanejad et al. 15 in a recent study on activation of 2-chlorophenols to yield polychlorinated dibenzo- p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). Alumina (Al 2 O 3 ) exists as one of the most abundant metal oxides in PM 2.5 encountered in combustion systems. 16-18 Its concentration in PM 2.5 varies between 13 and 16% by mass. 19 Received: May 25, 2016 Revised: August 30, 2016 Accepted: September 9, 2016 Published: September 9, 2016 Article pubs.acs.org/est © 2016 American Chemical Society 11094 DOI: 10.1021/acs.est.6b02601 Environ. Sci. Technol. 2016, 50, 11094-11102