The Mechanism of NOS

Other metal oxide catalysts studied for the SCR-NH3 reaction include iron, copper, chromium and manganese oxides supported on various oxides, introduced into zeolite cavities or added to pillared-type clays. Copper catalysts and copper-nickel catalysts, in particular, show some advantages when NO—N02 mixtures are present in the feed and S02 is absent [31b], such as in the case of nitric acid plant tail emissions. The mechanism of NO reduction over copper- and manganese-based catalysts is different from that over vanadia—titania based catalysts. Scheme 1.1 reports the proposed mechanism of SCR-NH3 over Cu-alumina catalysts [31b],... 

SURFACE SCIENCE STUDIES OF THE MECHANISM OF NO CONVERSION CORRELATIONS BETWEEN KINETICS IN VACUUM VERSUS UNDER CATALYTIC CONDITIONS... 

The mechanism of NO release from N-diazeniumdiolates is depicted in Fig. 3.7. If R>, of the generic structure shown at the top is a cation, NO is generated spontaneously on protonation of the anionic portion along with the formation of dialkylamine. If R3 is covalently bound, it must be removed first to free the anion before spontaneous... 

With the discovery that N O plays a role in vasodilation it has become important that nitrovasodilators (i.e. substances that act as vasodilators by the release of NO) are well-characterised. This includes a clear understanding of the mechanism of NO release. The use of an inappropriate agent may give non-reproducible or false data. At least three crucial factors must be considered when an NO-donor drug is selected ... 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

Although NO does not itself use a G-protein for signalling, the mechanism of NO production in vascular endothelium is initiated by IP3 via a G-protein-linked acetylcholine receptor on the cell surface. The IP3 causes activation of nitric oxide synthase via calcium- calmodulin and the NO generated diffuses from the endothelial cell into the adjacent smooth muscle cell where cGMP is produced. 

Sodium nitroprusside was first prepared and investigated in the middle of the nineteenth century, and a comprehensive summary of the earlier chemical investigations has been published (17). Up to 1910-1930, the addition reactions of bases to NP were explored, involving the characterization of colored intermediates (e.g., with SH-, SR-, and SO3 ), useful for analytical purposes. The hypotensive action of NP was first demonstrated in 1929, and a considerable research effort has attempted to establish the mode of action of NP and its metabolic fate. Questions still arise on the mechanism of NO release from NP in the biological fluids, and we refer to them below. New accounts dealing with modern structural and reactivity issues associated with the coordination of nitrosyl in NP and other complexes have appeared (18-20). From the bioinorganic and environmental viewpoint, nitrosyl iron complexes have been studied with... 

Most gaseous fuels as well as some liquid fuels contain no or only small amounts of chemically bound nitrogen. In combustion of these fuels, the important source of NO is fixation of N2 in the combustion air. Molecular nitrogen, with its triple bond, is very stable, and only very reactive radicals may succesfully attack N2. The mechanisms of NO formation from N2 is quite well understood [274], and for many applications semiquantitative predictions of NO are within reach. 

The mechanism of NO autoxidation is dependent on reaction conditions. In the gas phase and in hydrophobic solvent, the initial intermediate is N02, which then rapidly establishes an equilibrium with N2O3 (1). 

The dissociation of NO is a crucial elementary step in the mechanism of NO reduction by CO in automotive exhaust catalysis. The overall reaction here is ... 

Laser-Fluorescence techniques for NO are of interest for studying the mechanisms of NO formation and its influence on chemical processes and pollutant formation in flames. In general, the optical fluorescence techniques provide very high detection sensitivities and good spatial resolution. 

One of the important hydrocarbon combustion reaction intermediates is the CH radical. Although CH chemiluminescence (.42 A — X2ir) has been observed in many hydrocarbon flames, the mechanism of CH formation and its reaction kinetics have been difficult to unravel in situ due to the low steady-state concentrations and the complex nature of combustion reactions. This project was undertaken to investigate a means of CH radical production and to study its reactions with various important species so that an overall picture of the oxidation processes, particularly with regard to the mechanism of NO formation, may be better understood. 

The mechanisms of NO release from organic nitrates are complex, and still not fully explored. Figure 10.6, based on the scheme of Van de Voorde [7], dates back to the early 1990s but can be considered still valid after slight modification based on additional data [42]. It summarizes the proposed mechanisms by which NO-... 

The mechanism of NO formation has been studied in detail and can be summarized as follows (Figure 5.25) ... 

To elucidate the mechanisms of NO-induced cellular stress, the effects of SIN-1 (32) on neuroblastoma cells were examined. SIN-1 induced a transient decline in ATP levels and the delayed loss of cell viability, with no significant increase in caspase-3 activity or DNA laddering. NO was suggested to be a potent toxin independent of peroxynitrite formation [41]. 

The establishment of the mechanism (however defined) of a heterogeneous reaction is an incomparably more difficult task than for a homogeneous reaction. The relative magnitude of the two tasks is shown by the fact that the mechanisms of many homogeneous reactions are agreed and their transition states quite well defined, whereas the mechanism of no heterogeneous reaction is as yet beyond dispute. 

Nitric oxide is formed in combustion engines by the interaction of oxygen and nitrogen in air at the high temperatures reached during the combustion cycle. The percentages of NO found in the exhaust gas is close to that calculated from the chemical equilibrium N2+02 = 2N0 at peak temperatures near 2500 K. Once NO is formed, its abundance seems to be effectively frozen in. The mechanism of NO formation is not precisely known. Most authors have adopted the reaction chain first proposed by Zeldovich et al. (1947) ... 

A large amount of N2O was formed from the initial stage over LaM03 (M = Co, Mn, Fe, Cr, Ni) at 573 K. The time course of the NO+CO reaction (performed in a batch recirculation system) reflects this situation. These results support a two-step reaction pathway in which N2O is an intermediate for nitrogen formation, deal et al. (1994) confirm the role of N2O as intermediate in this reaction over perovskite oxides. They used steady-state isotopic transient kinetic analysis to study the mechanism of NO + CO reaction over LaCo03. They concluded that N2O was an intermediate in the formation of N2 at T < 873 K. They also concluded that at high temperature CO2 desorption became the rate-limiting step of the overall reaction. This is likely due to the rapid formation and slow decomposition of very stable carbonates on the perovskite surface as reported by Milt et al. (1996). 

We have shown [2] that an investigation of the transient interaction between NO and Cu-ZSM5 at temperatures so low that the catalyst has no steady-state activity should contribute to elucidate the redox chemistry of copper in these systems, that is strictly related to the mechanism of NO decomposition. In this work the study was extended to Cu-HZSM5 catalysts at different copper content and Si/Al ratio. 

Fig. 10 The mechanism of NO reduction by hydrocarbons over Pt-based catalysts.

The good correlation between the Cu(I) content and NO decomposition rate is noteworthy. Since the measurement was carried out under reaction conditions, it confirms the conjecture by Hall et aL that a small fraction of cuprous ion is maintained in the working catalyst(7,P). Both the cuprous ion concentration and the catalyst efficiency vary in the same way with temperature. According to the redox mechanism proposed by Li and Hall(7), the rate of NO decomposition should be proportional to the partial pressure of NO and the number of the available active sites in Cu-ZSM-5, which they attribute to Cu(I) ions. The correlation that we observe seems to support their notion that Cu(I) participates in the mechanism of NO decomposition at elevated temperature. 

---