Formation and Stabilization of Combustion-Generated Environmentally Persistent Free Radicals on Transition Metal Oxides Supported on Silica

The formation of environmentally persistent free radicals in combustion system was investigated from chemisorptions of chlorine- and hydroxy-substituted benzenes on transition metal oxide surface under post-combustion conditions. This manuscript reports the formation of EPFRs on silica particles containing 5% Fe(III)2O3, Ni(II)O, and Zn(II)O. The EPFRs are formed by the chemisorption of substituted aromatic molecular adsorbates on the metal cation center followed by electron transfer from the adsorbate to the metal ion at temperatures from 150 to 400 oC. Depending on the nature of the adsorbate and the temperature, two organic EPFRs were formed: a phenoxyl-type radical, which has a lower g-value of 2.0023-2.0043, and a second semiquinone-type radical, with a g-value of 2.0044-2.0085. In general, the yield of EPFRs on Ni(II)O are similar to Cu(II)O and differs by ~10× compared to Fe(III)2O3. The persistency of the EPFRs is highly dependent on the metals they are associated with. The half-lives of EPFRs on iron and nickel ranged from 24 to 111 and 36 to 128 h, respectively, compared to the half-lives on copper of 27 to 74 min. However, the half-lives measured for the EPFRs on zinc are remarkably higher which range from 72 to1752 h. The higher oxidation potential of the metals is believed to result in increased stabilization of the EPFR once formed, resulting in the longer half-lives. The half-life of EPFRs on BALF was ~2× higher than in serum. On average, twice as much DMPO-OH spin adducts was formed in the presence of EPFR-containing particle compared to non-EPFR-containing particles at higher incubation time. A 10:1 ratio of OH radical concentration formed to the number of EPFR radicals indicates a cyclic process. The inability to identify superoxide radicals in aqueous media, while a DMPO-O2- spin adduct is detected in the aprotic solvents, suggests a very quick transformation of the O2.- in aqueous environment, thus, the superoxide anions self-dismutates in protic environment and forms OH radicals