Decomposition of NO

NO decomposition has been studied over a variety of catalysts such as oxides of various metals, zeolites, perovskites, and noble metals. Pt on AI2O3, and unsupported oxides such as C03O4, CuO, NiO, Fe203, and Zr02 are suitable catalysts 

Zeolites are also able to decompose N2O. Li and Armor (11] prepared 25 zeohte samples by exchanging the cation Na+. These catalysts were tested for the catalytic decomposition of N2O. The Co- and Cu-exchanged zeolites are active at temperatures ranging from 623 K to 673 K. Rhodium and ruthenium supported on ZSM-5 are active catalysts between 523 and 573 K. 

In contrast to Co-ZSM-5, Cu-ZSM-5 and perovskites show very high catalytic activities for the decomposition of NO. 

According to Li and Hall [12] the selectivity to N2 on Cu-ZSM-5 is not 1. An amount of undecomposed NO and O2 react homogeneously at low temperatures downstream from the reactor. Based on their results the rate equation for the NO decomposition reaction can be written in Langmuir-Hinshelwood form as in eq 1. 

Valyon and Hall [13] employed isotopically labeled molecules such as 02 and N 0 to study the very interesting issue of the release of molecular oxygen during 

Membrane Configuration Electrode/ catalyst Temperature (°C) Main results Ref. 

SZYb Tube with one closed end Pt/Sr-AbOs 350 50% removal, 20% efficiency [45] 

SZYb = SrZro, 9Ybo.i03 sSCYb = SrCeo.95Ybo.O5O3- -5-  

Strontium cerate or strontium zirconate proton conductors have been tried as electrolyte membrane, with the results of nitrogen oxide reduction as summarized in Table 6.5. When Pt/Ba/Al203 or Pt/Sr/Al203 is used as working electrode, it is possible to reduce the NO, even in the presence of excess O2. The reduction of NO proceeds through the electrochemical reduction of NO absorbed in Sr/Al203 but not through the chemical reduction of NO by H2 gas. 

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The Pt-catalyzed decomposition of NO (into N2 and O2) is found to obey the experimental rate law... 

At the high temperatures found in MHD combustors, nitrogen oxides, NO, are formed primarily by gas-phase reactions, rather than from fuel-bound nitrogen. The principal constituent is nitric oxide [10102-43-9] NO, and the amount formed is generally limited by kinetics. Equilibrium values are reached only at very high temperatures. NO decomposes as the gas cools, at a rate which decreases with temperature. If the combustion gas cools too rapidly after the MHD channel the NO has insufficient time to decompose and excessive amounts can be released to the atmosphere. Below about 1800 K there is essentially no thermal decomposition of NO. 

The decomposition of NO is a very slow catalytic reaction. Amirazmi, Benson, and Boudart recently studied the kinetics over platinum and over oxides of copper, cobalt, nickel, iron and zirconium from 450 to 900°C. They found that the kinetics is first order in NO with concentrations from 1.5 to 15%, and that oxygen has a strong inhibiting effect. Even at these temperatures, the kinetics is about a factor of 1000 too low for automotive usage (97). 

Rabalais and coworkers (30) have reported on the SIMS of NO on Ni(lOO) as a function of temperature. They were not able to go to low enough temperature to observe (NO)2 condensation, but they did observe that the decomposition of NO to N and O fragments with temperature increase was accompanied by a decrease of NO containing clusters and an increase in N and O containing clusters. This result is, therefore, rather similar to that mentioned in this paper earlier for CO dissociation on Ni(lOO) (8). 

These surface kinetics studies initially focused on the dissociation of NO. For instance, Comelli and co-workers reported on the kinetics of the isothermal decomposition of NO on Rh(110) at temperatures ranging from 198 to 240 K and NO coverages below 0NO 0.3 ML [45], Auger electron spectroscopy (AES) lineshape analysis was used to measure the amount of undissociated NO as a function of time, and the resulting 0NO(t)... 

Groothaert et al., using operando UV-vis spectroscopy combined with online GC analysis [176] and operando X-ray absorption fine structure (XAFS) [177], presented the first experimental evidence for the formation of the bis( x-oxo)dicopper core in Cu-ZSM-5 and for its key role of intermediate in the sustained high activity of Cu-ZSM-5 in the direct decomposition of NO into N2 and 02. In particular, monitoring the catalytic conversion of NO and N20 above 673 K, they found that the bis( x-oxo)dicopper core is formed by the O abstraction of the intermediate N20 (Figure 4.14). Subsequently,... 

In each case, we are considering only the direction of reaction indicated. The reverse reaction may well be of a different order for example, the decomposition of NO, is second-order.)... 

Vasodilation is attributable to nitric oxide (NO), which is produced either directly from the nitroester or liberated by decomposition of NO intermediates (Feelisch and Noack 1987). Either glutathione in cells of vascular tissue or sulfhydryl groups of proteins in these tissues may be responsible for converting nitrates to NO. Nitric oxide activates guanylyl cyclase, which increases intracellular levels of cyclic guanosine 3 5 -monophosphate and thereby produces vasodilation (Kelly and Smith 1996 Robertson and Robertson 1996). 

RATE COEFFICIENTS FOR ELEMENTARY STEPS IN THE THERMAL DECOMPOSITION OF NO... 

At room temperature reaction (30) is slow and (29) is sufficiently fast250 to be effective even at very small conversions of N02 to NO. Reaction (30 ) is rapid and explains the increase in constant total pressure. It may be regarded249 as a decomposition of NO promoted by a rapid conversion of symmetrical N03 to the less stable peroxy form... 

Some of the important reactions in contemporary technology involve NO, which is a designation of N2O, NO, and NO2, and was one of the first examples in this book. The formation of these molecules in combustion processes is a major source of air pollution, and the catalytic oxidation of NH3 to NO on R surfaces is used to produce nitric acid, a major industrial chemical. The decomposition of NO, to N2 is a major process in the automotive catalytic converter. 

Recently a few reports have claimed that direct decomposition of NO is successfully catalyzed... 

Johnston et al. (1986), for example, examined kinetic data from laboratory studies reported in the literature and estimated a first-order rate constant at 1 atm pressure at room temperature of (3 + 2) X 10 3 s , corresponding to a lifetime for NO, with respect to decomposition of about 6 min. Subsequent attempts by Davidson et al. (1990) to measure this decomposition directly were complicated by a contribution from a wall-catalyzed decomposition of NO,. However, they suggest that their data support this conclusion weakly, with their data being not inconsistent with the rate constant suggested earlier by Johnston et al. (1986). 

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