Formation of Environmentally Persistent Free Radicals (EPFRs) on the Phenol-Dosed α-Fe<sub>2</sub>O<sub>3</sub>(0001) Surface
Nadra Sakr, Orhan Kizilkaya, Sierra F. Carlson, Simon Chan, Reuben A. Oumnov, Jaqueline Catano, Richard L. Kurtz, Randall W. Hall, E. D. Poliakoff, Phillip Sprunger
Abstract
Environmentally persistent free radicals (EPFRs) are a class of toxic air pollutants that are found to form by the chemisorption of substituted aromatic molecules on the surface of metal oxides. In this study, we employ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) to perform a temperature-dependent study of phenol adsorption on α-Fe2O3(0001) to probe the radical formation mechanism by monitoring changes in the electronic structure of both the adsorbed phenol and metal oxide substrate. Upon dosing at room temperature, new phenol-derived electronic states have been clearly observed in the UPS spectrum at saturation coverage. However, upon dosing at high temperature (>200 °C), both photoemission techniques have shown distinctive features that strongly suggest electron transfer from adsorbed phenol to Fe2O3 surface atoms and consequent formation of a surface radical. Consistent with the experiment, DFT calculations show that phenoxyl adsorption on the iron oxide surface at RT leads to a minor charge transfer to the adsorbed molecule. The experimental findings at high temperatures agree well with the EPFRs’ proposed formation mechanism and can guide future experimental and computational studies.