Reduction of nitrogen dioxide

Ayer, R. J., Yonebayashi, T. Catalytic reduction of nitrogen dioxide by carbon monoxide. Atmos. Environ. 1, 307 (1967). — Gas Chromatog. Abstr. 1968, 797. 

Reaction between phenol and hydroxyl yields the dihydroxybenzenes, which can then undergo further oxidation (hydroquinone to benzoquinone, further hydroxylated to hydroxybenzoquinone, catechol and resorcinol to trihydroxybenzenes [79,100]). The condensation products, phenoxyphenols and dihydroxybiphenyls, most likely originate from the reaction between phenol and the phenoxyl radical [101]. Their presence indicates that some phenoxyl forms in the system, due to the reaction of phenol with OH or NO2. The possibility for NO2 to oxidise phenol to phenoxyl has been the object of a literature debate [102,103] in the context of nitration processes. The problem can be tackled upon consideration of the reduction potentials of the various species. The reduction potential of phenoxyl to undissociated phenol is E = 1.34 V - 0.059 pH [104], while for the reduction of nitrogen dioxide to nitrite it is E = 0.90 V [105]. Accordingly oxidation of phenol to phenoxyl would be possible above pH 7.5, and of course in the presence of phenolate (pH > 10 [106]). 

Briiggemann TC, Przybylski M-D, Balaji SP, Kell FJ (2010) Theoretical investigation of the mechanism of the selective catalytic reduction of nitrogen dioxide with ammonia on H-form zeolites and the role of nitric and nitrous acids as intermediates. J Phys Chem C 114 6567-6587... 

In other cases it is necessary to introduce two temperature coefficients depending on the field in order to accoimt for the influence of temperature. This is the case, for example, in the reduction of nitrogen dioxide by carbon monoxide. Figure 5.1 shows the shape of the curve in Arrhenius coordinates. 

Low temperatures strongly favor the formation of nitrogen dioxide. Below 150°C equiUbrium is almost totally in favor of NO2 formation. This is a slow reaction, but the rate constant for NO2 formation rapidly increases with reductions in temperature. Process temperatures are typically low enough to neglect the reverse reaction and determine changes in NO partial pressure by the rate expression (40—42) (eq. 13). The rate of reaction, and therefore the... 

These fragments either combine intramolecularly to form the ortho and para nitro compounds or dissociate completely and then undergo an intermolecular reaction to form the same products. The theory was not developed to include a detailed transition state and no mention was made of how the para isomer was formed. Reduction of the cation-radical could give the amine (which was observed experimentally76), but one would expect the concurrent formation of nitrogen dioxide and hence nitrite and nitrate ions however, the latter has never been... 

For a fuel, an electron source is needed. Water is the ultimate electron source from an economical point of view. Water photolysis is the simplest among the chemical conversion systems of solar energy. Photochemical reduction of nitrogen or carbon dioxide to produce ammonia or hydrocarbons with the electrons from water is also an attractive system of conversion. 

It is not possible to obtain titanium metal by the usual method of reduction of the dioxide with carbon, because a very stable carbide is formed. Moreover the metal is reactive towards nitrogen and oxygen at elevated temperatures, and hydrogen at 900 °C only reduces Ti02 as far as Ti3Os. Reduction of the dioxide with most reducing metals, e.g. Na, Al, Ca or Mg, seldom seems to yield a pure product the most common contaminants, and in some cases the principal products, are lower oxides of titanium. Reduction of the tetrachloride is, therefore, the basis of the preferred methods. 

In Fig. 30 it is seen that the effect with carbon dioxide is nearly proportional to the pressure. The reduction in quantum yield is due to the removal by collision of the excess energy of the excited nitrogen dioxide molecules before they can collide with other molecules of nitrogen dioxide and effect a chemical reaction. The energy is removed in the form of extra energy given to the colliding molecules, and thus converted into heat. 

Metal- and proton-exchanged zeolites have been recently attracted much attention because of their selective catalytic activity to efficiently reduce nitrogen monoxide (NO) by hydrocarbon in an 02-rich atmosphere [1]. The formation of nitrogen dioxide (NO2) from NO and O2 has been suggested as an important step in the selective reduction [2, 3] NO2 is one of rare stable paramagnetic gaseous molecules and has been subjected to electron spin resonance (ESR) studies [4-7]. The ESR parameters and their relation/to the electronic structure have been well established [4] and NO2 can be used as a "spin probe" for the study of molecular dynamics at the gas-solid interface by ESR. 

Early emission control methods were based on the use of a thermal reactor for hydrocarbon and carbon monoxide oxidation, combined with exhaust gas recirculation (EGR) for reduction of nitrogen oxide emissions (Fig. 3.2a). Hydrocarbons and carbon monoxide in the hot exhaust fed to the reactor, once heated, were rapidly oxidized to carbon dioxide and water by the additional pumped air which was fed to the reactor (e.g., Eqs. 3.3 and 3.4). 

Zirconium dioxide and zeolites of pentasil structure are widely used as catalysts and efficient carriers in many heterogeneous reactions, and particularly in the process of selective catalytic reduction of nitrogen oxides by hydrocarbons (SCR-process) [1,2]. Synthesis of new catalytic systems for NOx SCR-process by CnHm is therefore related with searching for their optimum composition and preparation methods to attain maximum activity in this reaction. 

Bollinger, M.J., R.E. Sievers, D.W. Fahey, and F.C. Fehsenfeld. 1983. Conversion of nitrogen dioxide, nitric acid, and w-propyl nitrate to nitric oxide by gold-catalyzed reduction with carbon monoxide. Anal. Chem. 55 1980-1986. 

Milford, J. B., Russell, A. G., and McRae, G. J. (1989) A new approach to photochemical pollution control implications of spatial patterns in pollutant responses to reductions in nitrogen dioxides and reactive organic gas emissions. Environ. Sci. Technoi., 23, 1290-1301. 

Other energy-producing reactions of organisms involve the reduction of oxygen to water, the reduction of nitrate to ammonia and nitrogen gas, the reduction of sulfate to sulfide, and the reduction of carbon dioxide to methane. All of these reactions can exert a profound effect on water quality especially when it is realized that the affected chemical species also engage in many other chemical reactions. For example, the sulfide ion forms precipitates with many heavy metals. The microbial reduction of sulfate to sulfide could be accompanied by a reduction in the dissolved heavy metal content in a natural water. 

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