Lifetime of combustion-generated environmentally persistent free radicals on Zn(ii)O and other transition metal oxides

Abstract

Previous studies indicated that Environmentally Persistent Free Radicals (EPFRs) are formed in the post-flame, cool zone of combustion. They result from the chemisorption of gas-phase products of incomplete combustion (particularly hydroxyl- and chlorine-substituted aromatics) on Cu(II)O, Fe(III)2O3, and Ni(II)O domains of particulate matter (fly ash or soot particles). This study reports our detailed laboratory investigation on the lifetime of EPFRs on Zn(II)O/silica surface. Similarly, as in the case of other transition metals, chemisorption of the adsorbate on the Zn(II)O surface and subsequent transfer of electron from the adsorbate to the metal forms a surface-bound EPFR and a reduced metal ion center. The EPFRs are stabilized by their interaction with the metal oxide domain surface. The half-lives of EPFRs formed on Zn(II)O domains were the longest observed among the transition metal oxides studied and ranged from 3 to 73 days. These half-lives were an order of magnitude longer than those formed on nickel and iron oxides, and were 2 orders of magnitude longer compared to the EPFRs on copper oxide which have half-lives only on the order of hours. The longest-lived radicals on Zn(II)O correspond to the persistency in ambient air particles of almost a year. The half-life of EPFRs was found to correlate with the standard reduction potential of the associated metal.

Effect of Particulate Matter Mineral Composition on Environmentally Persistent Free Radical (EPFR) Formation

“…5–7,35 Previous studies have also shown that different metal speciations result in varying radical yields and changes to their persistency in ambient air. 14 Among the studied metals, copper and iron have shown the highest activity of EPFR formation, while zinc and nickel indicated extremely long EPFR lifetimes in ambient air (in months).…”

Formation of environmentally persistent free radicals (EPFRs) on ZnO at room temperature: Implications for the fundamental model of EPFR generation

“…EPFRs were first proposed to form through the reduction of metal oxides by high-temperature exposure (above 150°C) to simple substituted aromatics, which electron and X-ray spectroscopy has shown to occur on CuO 12,13 and TiO 2 14 model systems. However, the longest-lived EPFR species observed to date are phenoxy radicals generated on ZnO 15 , which presents a significant challenge to the generic model of EPFR formation because it is usually not possible to create a Zn(I) oxidation state except under rare conditions 16–20 . The extraordinary longevity of ZnO-generated EPFRs – lifetimes of 73 days in ambient conditions– suggests that Zn-containing PM may present an ongoing environmental hazard throughout remediation processes.…”

Environmentally Persistent Free Radicals and Their Lifetimes in PM2.5

“…12,21–23 For example, organic precursors chemisorbed on CuO and Fe 2 O 3 displayed one decay with the longest lifetime on CuO (74 min) resulting from phenol. 12,21 When phenol was chemisorbed on Ni and Zn, two subsequent decays were observed, and these decays were also the longest for each metal.…”

“…23 All decays from the model EPFR system implied phenoxyl radicals are the short-lived species, while semiquinone radicals are the long-lived species. 12,23 Semiquinone radicals, however, were suggested to decompose into phenoxyl radicals. 12,23 The differences in the aforementioned decay behavior resulted from metal speciation and, although not specifically mentioned here, the adsorbed organic precursor.…”