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|Name||Nitric oxide||Nitrogen dioxide|
|Appearance||Colorless gas||Red-brown gas|
|Density||1.0367 (rel. to air)||...|
Nitrogen oxides—as defined by emission regulations and regulatory measurement protocols—include two gases: nitric oxide NO and nitrogen dioxide NO2. Physical properties of both gases are listed in the table on the right . From the chemical point of view, there are several other nitrogen oxides. One of them, nitrous oxide N2O—an unregulated emission from internal combustion engines—is discussed below.
Concentrations of NOx in diesel exhaust are typically between 50 and 1000 ppm. If concentrations are given in mass units, NOx is usually expressed as NO2 equivalent.
Nitric oxide (NO) is a colorless and odorless gas. In the laboratory, it may be synthesized directly from nitrogen and oxygen under high temperature and pressure:
(1) N2 + O2 ↔ 2NO - 182.4 kJ/mole
The negative heat effect represents an endothermic reaction. Equation (1) can also represent the overall reaction of NO formation from the elements in the engine cylinder, where temperature and pressure are high. The exact NOx formation mechanisms in the diesel engine, however, are much more complex; they were discussed in the paper on diesel NOx formation.
At low temperature and pressure, the chemical equilibrium moves to the left side of Equation (1). Thermodynamically, nitric oxide has a tendency to decompose to nitrogen and oxygen under conditions in diesel exhaust. The rate of decomposition, however, equals practically zero and NOx control from diesel engines requires sophisticated in-cylinder and/or aftertreatment solutions.
In older, naturally aspirated engines, approximately 95% of nitrogen oxides were composed of NO and only 5% of NO2. The proportion of NO2 in total NOx in turbocharged diesel engines (without aftertreatment) is typically higher, reaching up to about 15%. According to British data, the fraction of NO2 in vehicle NOx emissions (all fuels) increased from around 5-7% in 1996 to 15-16% in 2009 . NO can be easily oxidized by oxygen into nitrogen dioxide at ambient conditions:
(2) 2NO + O2 ↔ 2NO2 + 113.8 kJ/mole
The above reaction can be accelerated by catalysts, such as diesel oxidation catalysts and catalytic particulate filters. Therefore, the proportion of NO2 in total NOx can be much higher in engines equipped with catalysts. The average NO2/NOx ratio in US 2007 heavy-duty truck engines (with catalytic aftertreatment) tested by the ACES study was 68% . Production of NO2 according to Equation (2) also occurs spontaneously—but not instantaneously—in the NO-air mixture after exhaust gases are discharged into the atmosphere.
NO2 is a very toxic red-brown gas of an unpleasant irritating odor. NO2 is extremely reactive and exhibits strong oxidation properties. NO2 chemistry also plays a role in several types of emission control catalysts, where NO2 reactions may include oxidation of hydrocarbons, carbon monoxide as well as diesel particulates.
Nitrogen oxides are highly active ozone precursors playing an important role in the smog chemistry. Besides diesel particulates, NOx is considered one of the most critical pollutants found in diesel exhaust.