Nitrogen cycle dimerization

Anaerobic samples were prepared by degassing the solvent by three freeze-thaw cycles while solids and NMR tubes were degassed by placing under vacuum, then storing in a nitrogen atmosphere. If oxygen is not carefully excluded, the ferrous porphyrin obtained by the autoreduction may be rapidly oxidized to the oxo-bridged dimer. 

All these reactions are exothermic and the energy is released at each stage. The stages involve nitric oxide to nitrogen peroxide, nitrogen peroxide to its dimer, dimer solution to nitric acid, and the release of nitric oxide, so that the cycle begins again. 

The ultimate loss of the HAS activity occurs by destruction of the heterocycle initiated thermally, photochemically, chemically or by high-energy radiation. An intramolecular H-abstraction from the p-carbon atom in thermolysis of 2,2,6,6-tetramethyl-4-oxo-piperidinyl-l-oxyl 133 via a general reaction (Eq. 8) was proposed as a pathway of thermal selfdestruction of the piperidine cycle [25] (Scheme 25). The respective hydroxylamine was isolated in the yield of 66.5%. The biradical intermediate 137 either dimerizes to nitroxide 138 or thermolyses via 139 to a nitrogen-free fragment 140 (phorone) and nitric oxide. 

The concept of using a trimeric single source precursor for AIN can be derived from the observation of AI3N3 cycles as the smallest structural fragment in wurtzite AIN. However, detailed mechanistic studies [194] indicate that under gas phase thermolysis the trimeric precursor [Me2Al(NH2)]3 is in equilibrium with (or decomposes to) dimeric (20) and monomeric (21) compounds. Furthermore, nitrogen-poor species (22) were also observed by TOF-mass spectrometry. 

Briefly, it was found that the Fast SCR reaction proceeds via a sequential scheme, which implies NO2 dimerization, its disproportionation to surface nitrites and nitrates and their successive reactions with NH3, with formation of ammonium nitrites and nitrates. Ammonium nitrite then rapidly decomposes to nitrogen, while ammonium nitrate is reduced by NO to nitrites, forming NO2 [2, 3, 7]. It was further experimentally shown that such a crucial step proceeds via a redox cycle involving the very effective reoxidation of the reduced V-sites by surface nitrates [6, 9]. This explains the higher rate of the Fast SCR reaction as compared to the Standard SCR chemistry, in which the rate limiting catalyst reoxidation is carried out less rapidly by gaseous oxygen [6]. 

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