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

Abstract

Electron paramagnetic resonance (EPR) spectroscopy is the ideal method of choice when detecting and studying the wide variety of paramagnetic oxygen-centred radicals. For simple diatomic radicals, such as the superoxide (O 2 −) or peroxy (ROO •) species, the CW EPR profile (in particular the g-values) of these species can appear similar and indeed indistinguishable in some cases. Experiments using 17 O-enriched oxygen, revealing a rich 17 O hyperfine pattern, are therefore essential to distinguish between the two species. However, in many cases, particularly involving TiO 2 photocatalysis, the peroxy-type (ROO •) radicals or other intermediate species such as the [O 2 − …organic]-type adducts can be transient in nature and once again can produce similar g-values. In general terms, these reactive oxygen species (ROS) are formed and detected at low-temperature conditions. Hence, the application of EPR spectroscopy to studies of surface-stabilised oxygen-centred radicals must be performed under carefully selected conditions in order to confidently distinguish between the differing types of diatomic radicals, such as O 2 − , ROO • or [O 2 − …organic].

The Composites of PCL and Tetranuclear Titanium(IV)-oxo Complexes as Materials Exhibiting the Photocatalytic and the Antimicrobial Activity

“…[50]. On the other hand, titanium(III), due to a positive spin-orbit coupling constant, shows EPR signals at even lower g-values, below 1.99 [48][49][50][51][52]. This approach makes it possible to identify the presence of ROS of the type O 2 − and O − in PCL + (2) samples (Table 5).…”

Characterisation and reactivity of oxygen species at the surface of metal oxides

“…In principle the observed pattern could also be due to the simultaneous presence of two distinct adsorption sites with different degree of metalto-oxygen electron transfer. Recent analysis [65] have however demonstrated that there is a unique site where the superoxide adsorbs with a low-symmetry structure and an inequivalent electron spin density distribution over the adduct. The reason of such non-equivalency is not yet clear.…”

“…Several examples are provided, which document well selective oxidation reactions involving the O2or Ointermediates that are operating at low temperatures in catalytic gas-solid [84] and catalytic/photocatalytic solid-liquid systems in particular [85] EPR technique has often been applied to monitor surface reactions of oxygen species, but its applicability is limited to studies of radicals in the low temperature processes. Since most of ROS are unstable in nature, it is usually difficult to monitor directly their behavior at higher temperatures where they appear as transient intermediates [65].…”

“…Involvement of O2in isotopic exchange has been revealed clearly by EPR technique upon contacting the 16 O2radical trapped on the surface of a supported MoOx/SiO2 catalyst with the gas phase 17 O2[65]. Development of the characteristic hyperfine pattern due to the singly and doubly labelled dioxygen isotopomers, implies the following reactions 16 O2 -(ads) + 17 O2(g) → The corresponding second order kinetics of the decay of O2 species has been observed over the catalysts containing isolated and clustered molybdenum species supported on silica[20].…”

“…Similar CO2radicals (g1 = 2.0044, g2 = 2.0022, g3 = 1.998) are produced over the UV-irradiated silica gel upon interaction of CO the with hole O centers at 90-250 K, and the reaction follows the Langmuir-Hinshelwood-type mechanism with an activation energy of 18 kJ/mol[91].At low temperatures the reactivity of superoxide radicals is usually limited to formation of adducts with most inorganic and larger organic molecules. Several adducts of such type have been identified on the TiO2 surface by Murphy et al[65], including [O2 ZnO catalyst, reaction between O2 and CO at ambient conditions leads to the formation of CO3 radicals with gxx = 2.002, gyy = 2.007 and gzz = 2.028, which upon increasing of the temperature is transferred into CO2[93]. /O2 and The resultant methyl radicals can escape into the gas phase (at high temperatures), where they couple into ethane molecules, constituting an example of mixed hetero-homogeneous catalysis[97], or can be trapped on the surface in the form of methoxy intermediates (•CH3 + O 2-+ O2 → CH3O -+ O2 -), opening a rather complex network of various reactions, owing to cooperative action of several ROS species[95].…”