Free‐Radical Pathways in the Decomposition of Ketones over Polycrystalline TiO₂: The Role of Organoperoxy Radicals

Abstract

The oxidative decomposition of various ketones (including acetone, 2-butanone, 4-heptanone, cyclopentanone and cyclohexanone) over dehydrated TiO(2) (P25) powder is investigated by electron paramagnetic resonance (EPR) spectroscopy. For the first time, a series of thermally unstable radical intermediates are observed both on the activated and reduced TiO(2) surface, depending on the adopted experimental conditions. These radical intermediates are identified as organoperoxy-based species of general formula ROO(.-) and RCO(3) (.-). They are formed by reaction of photogenerated charge carriers (either trapped electrons or trapped holes) with the adsorbed ketones in the presence of molecular oxygen. The organoperoxy intermediates are thermally unstable and decompose at temperatures in the region of 180-250 K. This work demonstrates that free-radical pathways involving both organoperoxy and superoxide radicals can be responsible for the thermal- and photodecomposition of ketones over polycrystalline TiO(2) (P25).

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

“…It is important to note that the growth of species B is accompanied by the simultaneous loss of species A (due to O 2 -). Moreover, the reactivity between CH 3 CN and O 2 -appears to be site-specific, since the relative order of O 2 -loss follows the trend of site 2.019 (I) [ 2.023 (II) [ 2.026 (III); in other words, the O 2 -radicals responsible for the signal at g zz = 2.019 are first to react with the organic substrate, followed by sites II and III (in agreement with recent results reported for acetone reactivity) [22].…”

“…Owing to the improved resolution of this spectrum, computer simulations revealed that the g values for species B are g 1 = 2.031, g 2 = 2.010 and g 3 = 2.003 (with no observable hyperfine interaction). These g values may be compared to those of other known surface oxygen radicals originating from the interaction and reactivity of organic substrates with molecular oxygen over polycrystalline titania [22,27,28]. In the absence of any hyperfine coupling, the HO 2…”

“…10 mg) was slowly heated (over a 5-h period) under vacuum (10 -4 Torr) in an EPR cell to a final maximum temperature of 773 K. This vacuum-reduced sample, which is blue in colour (and hereafter referred to as the reduced sample), was then held at this temperature for a further 1 h. The reduced sample was then cooled to room temperature and exposed to oxygen (15 Torr) for 10 min before evacuation of the excess gas at 298 K. This treatment resulted in the formation of O 2 -radicals, as described in detail elsewhere [18,[21][22][23]. The sample was subsequently precooled to 77 K before the addition of CH 3 CN (7.3 Torr) at this temperature.…”

“…Several electron paramagnetic resonance (EPR) studies have already described the nature of the oxygen-centred radicals formed in these surface reactions [18][19][20][21] and their subsequent reactivity. For example, Carter et al [22] have recently shown how a series of transient organoperoxy radicals, indirectly involving O 2 -, play a contributory role in the decomposition of acetone over polycrystalline TiO 2 . Therefore, in the present study, the interaction of acetonitrile with superoxide radicals over polycrystalline TiO 2 will be explored, under dark conditions, in order to ascertain the nature of the radical intermediates formed in the decomposition process.…”

Titanium Dioxide in Photocatalysis

“…• radical generated by a first hole-mediated reaction [179]. The intermediates observed in another study are mainly acetate, formate, acetaldehyde, and formic acid corresponding to a C-C bond cleavage [180].…”