EPFR formation from phenol adsorption on Al2O3 and TiO2: EPR and EELS studies

We have examined the formation of environmentally persistent free radicals (EPFRs) from phenol over alumina and titania using both powder and single-crystal samples. Electron paramagnetic resonance (EPR) studies of phenol adsorbed on metal oxide powders indicates radical formation on both titania and alumina, with both oxides forming one faster-decaying species (lifetime on the order of 50-100 h) and one slower-decaying species (lifetimes on the order of 1000 h or more). Electron energy loss spectroscopy (EELS) measurements comparing physisorbed phenol on single-crystal TiO2(1 1 0) to phenoxyl radicals on the same substrate indicate distinct changes in the pi-pi* transitions from phenol after radical formation. The identical shifts are observed from EELS studies of phenoxyl radicals on ultrathin alumina grown on NiAl(1 1 0), indicating that this shift in the pi-pi* transition may be taken as a general hallmark of phenoxyl radical formation. (c) 2012 Elsevier B.V. All rights reserved.