Formation and reduction of nitrogen oxides

One early controversy with regard to NO chemistry revolved around what was termed prompt NO. Prompt NO was postulated to form in the flame zone by mechanisms other than those thought to hold exclusively for NO formation from atmospheric nitrogen in the high-temperature zone of the flame or post-flame zone. Although the amount of prompt NO formed is quite 

The term prompt NO derives from the fact that the nitrogen in air can form small quantities of CN compounds in the flame zone. In contrast, thermal NO forms in the high-temperature post-flame zone. These CN compounds subsequently react to form NO. The stable compound HCN has been found in the flame zone and is a product in very fuel-rich flames. Chemical models of hydrocarbon reaction processes reveal that, early in the reaction, O atom concentrations can reach superequilibrium proportions and, indeed, if temperatures are high enough, these high concentrations could lead to early formation of NO by the same mechanisms that describe thermal NO formation. 

Although most early analytical and experimental studies focused on NO formation, more information now exists on N02 and the conditions under which it is likely to form in combustion systems. Some measurements in practical combustion systems have shown large amounts of N02, which would be expected under the operating conditions. Controversy has surrounded the question of the extent of N02 formation in that the N02 measured in some experiments may actually have formed in the probes used to capture the gas sample. Indeed, some recent high-pressure experiments have revealed the presence of N20. 

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Figure 2. Formation and reduction of nitrogen oxides in combustion mechanistic pathways.

C. Formation and Reduction of Nitrogen Oxides TABLE 1 Structure of Gaseous Nitrogen Compounds -... 

CH2 participates in the formation and reduction of nitrogen oxides, and CH2 may react with unsaturated hydrocarbons in fuel-rich flames to form higher hydrocarbons, which ultimately result in the soot formation referred to above. 

Adsorption of nitric and sulfuric acids on ice particles provides the sol of the nitrating mixture. An important catalyst of aromatic nitration, nitrous acid, is typical for polluted atmospheres. Combustion sources contribute to air pollution via soot and NO emissions. The observed formation of HNO2 results from the reduction of nitrogen oxides in the presence of water by C—O and C—H groups in soot (Ammann et al. 1998). As seen, gas-phase nitration is important ecologically. 

Krocher O, Elsener M, Jacob E (2009) A model gas study of ammonium formate, methanamide and guanidinium lormate as alternative ammonia precursor compounds for the selective catalytic reduction of nitrogen oxides in diesel exhaust gas. Appl Catal B 88(l-2) 66-82... 

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