NxOy Radicals

Studies of nitrogen oxide radicals in various condensed media by means of the EPR technique started about 45 years ago. Initial results were collected in [88, 28]. NxOy radicals are of interest first of all because of their toxicity and a key role in atmospheric chemistry. From this point of view, formation, stability and reactivity of these species adsorbed on the surface of nanosized metal-oxide semiconductor particles, which are photoactive and widely presented in atmosphere, are of essential importance. Principal values of g- and A-tensors for some cases are picked up in the following Table 8.4. 

Practically all experiments showed a case of three-axis anisotropy in EPR spectra, and the EPR parameters could be easily measured. Free radicals in the atmosphere could be detected by a method of matrix isolation and EPR suggested in [93]. Formation of the N022 ion-radical has been proved by using 15NO at the same g-values Az for 15N022 was equal to 54 G instead of 38 G for 14N022 [91]. One can also see from Fig. 2 in [91] that 

EPR spectra are not really axial, therefore in precise measurements gx / gy. The EPR parameters published in [100] and concerning the N02 radical in Argon matrix at helium temperatures (Table 8.4) are not correct because of the wrong interpretation of the spectrum presented in Fig. 12 [100]. Correct determination by the same spectrum gives gx =2.004, gy =1.992, gz =2.001 Ax = 58 G, Ay = 46 G, Az = 62 G, which correlate well with the rest of the parameters listed in Table 8.4. 

Rather often, scientists did not measure the EPR parameters of N02 radicals in their systems, but observed its very characteristic spectrum, measured concentrations and used these data for discussing structural properties and mechanisms of the chemical reactions occurred (for example, [96-99]). 

Our studies of Ti02 and Zr02 nano-sized particles prepared by a sol-gel precipitation method [101] (titration with NH4OH and further stabilization of the precipitate with HNO3) showed interesting difference between titanium and zirconium dioxides [89, 90]. Fig. 8.1 performs a typical EPR spectrum of the N02 radical adsorbed on the surface of NC Ti02 thermally treated for 1 h at 200° (parameters are listed in Table 8.4). A similar one with lower intensity has been observed in the case of Zr02 [90], 

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Thus, recording and analysis of EPR spectra of lattice metal ions in their paramagnetic state, changes of the spin-Hamiltonian parameters, absolute and relative concentration of the species as a result of influence of external conditions such as heat treatment, light irradiation, chemical reactions, gas evaporation, etc., provide a valuable information about the structure and properties of oxide semiconductor materials. The results of the EPR studies of On and NxOy radicals will be discussed below. 

After the adsorption of inorganic (02, 03, NO, N02, S02, CO, C02, etc.) or organic molecules onto the semiconductor surface and especially after further illumination of a sample prepared, different stable or relatively stable radicals are easily recorded by the EPR method. Several important systems in which charge separation created organic radicals were described in detail in Chapter 1 of this book. Some additional information concerning adsorbed pentane, methane, ethylene, benzene, methylbenzenes and m-dinitrobenzene can be found in publications [41, 60, 69-74]. Further, we will shortly discuss some structural features of paramagnetic centers formed under chemical activation or irradiation of the adsorbed oxygen or NxOy molecules. 

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