Local Environment of Superoxide Radical Formed on the TiO 2 Surface Produced From Ti(OiPr) 4 Exposed to H 2 O 2

A selective heterogeneous catalyst with the superoxide anion O 2 •− stable at room temperature, generated by treating Ti(OR) 4 ( R = iPr, nBu) with H 2 O 2 , was previously reported and used for the oxidation of organic compounds. It was proposed that exceptional stability of the O 2 •− is resulted from its stabilization near Ti +4 cation on the TiO 2 surface with the participation of water molecules and/or OH groups. In this article, we performed high-resolution pulsed EPR study on the O 2 •− in this catalyst with the aim to obtain quantitative information about its interactions with the surrounding molecules. Our data has shown that the O 2 •− is involved in weak hyperfine interactions with protons in the nearest environment, which do not exceed a value of 2 MHz. It corresponds to an O s …H distance of ≥ 3 Å that excludes the formation of hydrogen bonds. The analysis of possible structures of O 2 •− adsorption on TiO 2 surface taking into account our results and lack of radical mobility at room temperature previously observed in EPR spectra led us to suggest that the O 2 •− is stabilized in the form of superoxo bridge coordinated with two titanium atoms.