An EPR investigation of acetonitrile reactivity with superoxide radicals on polycrystalline TiO2

Abstract

The interaction of acetonitrile with superoxide radicals over a polycrystalline TiO 2 (Degussa P25) surface was investigated using continuous-wave electron paramagnetic resonance (cw-EPR) spectroscopy. For the first time, a thermally unstable radical intermediate has been observed following the low-temperature exposure of acetonitrile to surface-adsorbed O 2 -radicals. The radical intermediate has been identified as an [O 2 -ÁÁÁCH 3 CN] type surface complex characterised by the g values of g 1 = 2.031, g 2 = 2.010 and g 3 = 2.003. This surface complex is thermally unstable and decomposes at temperatures of T [ 240 K. A second oxygen-centred species was also observed following acetonitrile adsorption, characterised by the spin Hamiltonian parameters of g 1 = 2.028, g 2 = 2.010, g 3 = 2.004, A 1 = 1.2 mT, A 2 = 1.0 mT and A 3 = 1.0 mT, and was assigned to a hydroperoxy radical (HO 2 • ).

An EPR characterisation of stable and transient reactive oxygen species formed under radiative and non-radiative conditions

“…Similar to the case reported for acetone [41], a thermally unstable ROS was observed when acetonitrile (CH 3 CN) was exposed to a TiO 2 sample that contained preformed O 2 − radicals (i.e. O 2 − /TiO 2 ); the O 2 − centres were readily formed by An EPR characterisation of stable and transient reactive oxygen… exposure of molecular oxygen to thermally reduced TiO 2 [42]. /TiO 2 at 77 K, followed by thermal annealing between T = 200 K and 240 K, a gradual decrease in the O 2 − signal intensity was observed.…”

“…In the above cases for methanol and toluene (and previously reported for acetonitrile [42] and acetone [41]), the organic substrates were exposed to the thermally − with acetonitrile are shown in Fig. 9.…”

“…Indeed, EPR has often been used to monitor the reactivity of the oxygen-centred radicals in various oxidation-based surface reactions [20]. In previous work, we have demonstrated how a series of transient organoperoxy radicals (ROO • ) can play a contributory role in the decomposition of acetone and acetonitrile over TiO 2 [41,42], but these species are only stable and therefore visible at temperatures lower than 200 K (unlike O 2 − which is readily detected at room temperature). Therefore, in this section, we will evidence how organic substrates react with superoxide radicals over polycrystalline TiO 2 surfaces under dark conditions leading to transient ROS.…”

“…This new signal was first observed at T ~ 210 K and begins to decay irreversibly at T > 250 K; this signal can once again be assigned to the transient [O 2 − …CH 3 CN] adduct. The main difference between the thermally treated [42] and photoirradiated samples ( Fig. 9) is the lower overall intensity of the [O 2 − …CH 3 CN] adduct on the photoirradiated sample.…”

Heterogeneous Photocatalytic Reactions of Sulfur Aromatic Compounds

“…Instead of the hydroxyl radical, the superoxide radical anion is formed from O 2 upon illumination of TiO 2 in CH 3 CN. [103] The products of photocatalytic reactions of sulfur aromatic compounds are also soluble in CH 3 CN, but can be selectively (vs. reactants) extracted by water. In principle, acetonitrile itself can be photocatalytically oxidized by O 2 in the presence of TiO 2 [104] if water is also present.…”