Oxygen reaction with nitrogen spectroscopy

Automobile exhaust catalysts typically contain noble metals such as Pt, Pd and Rh with a ceria promoter supported on alumina. Traditionally, the principal function of the Rh is to control emissions of nitrogen oxides (NO ) by reaction with carbon monoxide, although the increasing use of Pd has been proposed. For example, recent X-ray absorption spectroscopy studies of Holies and Davis show that the average oxidation state of Pd was affected by gaseous environment with an average oxidation slate between 0 and +2 for a stoichiometric mixture of NO and CO. Exposure of Pd particles to NO resulted in the formation of chemisorbed oxygen and/or a surface oxide layer. 

The elucidation of the reaction mechanism of the SCR reaction has been carried out using a variety of techniques. Transient studies with isotopically (oxygen-18 and nitrogen-15) labeled molecules have been performed [86,90]. Spectroscopic studies of the working catalysts were performed by Went et al. [90] and Topsoe [91] using laser Raman spectroscopy and FTIR, respectively. 

There have been several papers dealing with the oxidation reactions of nitrogen and sulfur-based compounds. Hindered amines, such as substituted 2,2,6,6-tetramethylpiperidines, are easily oxidized by electron-transfer reactions to the corresponding cation, by the sulfate radical anion, and by sensitized electron transfer to carbonyl triplets. Radicals derived from tertiary piperidines were observed directly by optical spectroscopy and deprotonated to a-alkylamine radicals. The amine radical cation derived from secondary piperidines deprotonated to give aminyl radicals. In the presence of oxygen, both classes were oxidized to give nitroxyl radicals, but by different proposed mechanisms. Both oxidation and fragmentation pathways have been observed in the photochemical reaction of alkyl phenyl sulfides with tetranitromethane. The oxidation of various A-(arylthio)-4-substituted-2,6-diarylanilines (18) with PbOa yielded, in most cases, persistent radicals that could... 

Whereas the distribution profile of the radicals in the bulk cannot be determined, the distribution of the products of their reaction with oxygen can. In other words, the degree and distribution of oxidative degradation in the bulk can be determined by measuring the distribution of the hydroperoxides and of their decomposition products using infrared spectroscopy (FTIR) [43, 44]. In Figure 21.1 the oxidation products after NO (nitrogen monoxide) treatment for a new prosthesis are reported [41-42]. 

High precision isotope ratio mass spectrometry permits reliable measurement of the small changes in isotopic distributions which occur in the reactions of molecules with natural abundance incorporations, especially for experiments with isotopes of carbon, nitrogen or oxygen [35]. An alternative of growing importance is the use of NMR spectroscopy to monitor similar changes for 13C and deuterium at natural abundance [36]. 

The reaction chemistry is quite different with cysteine. Under neutral conditions, cysteine reduces vanadate within an hour or so, but vanadate also rapidly forms relatively highly favored complexes with the cysteine that can be studied by NMR spectroscopy within the reduction time. All the complexes studied have sulfur in the coordination sphere. Four such complexes have been identified (Figure 2.2), two bisligand (-243 ppm, -309 ppm) and two monoligand (-393 ppm, 405 ppm) complexes [43], These complexes have not been structurally characterized, but it is quite likely that they are octahedral complexes with a coordination similar to that depicted in Scheme 4.14. Because vanadium(V) displays a general propensity toward five-membered rather than six-membered chelates, coordination of the type depicted in Scheme 4.14b with a nitrogen rather a carboxylate oxygen in the coordination shell seems most likely. 

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