Electrochemical reduction of NOx - DTU Orbit

1.1 NOx pollution
1 Introduction
NO is formed in high-compression internal combustion engines in three ways 5 : 1) by oxidation of
nitrogen-containing hydrocarbons in the fuel; 2) by reaction between fuel hydrocarbons and N2
and subsequent oxidation, or 3) by reaction between N2 and O2 in the air through the Zeldovich
mechanism. The majority of the NO in engine exhaust is formed by the Zeldovich mechanism, in
which activated N* and O*creates a chain reaction as described by equation 1.1 and 1.2 6 :
N2+O* → NO + N* (1.1)
N*+O2→ NO + O*
After the NO has been formed part of the NO will react with O2 and form NO2 according to the
equilibrium:
2 NO + O2 2 NO2.
The NOx concentration in the exhaust and the ratio between NO and NO2 depend on the engine
and the driving mode of the engine, but typically a total NOx concentration in the range 300-1200
ppm 7, 8 is expected of which 5-10% is NO2 5, 9, 10 . With respect to the impact of NOx on health and
environment, NO2 is the most harmful substance, as NO2 may contribute to the formation of acid
rain and cause respiratory problems when the concentration is >0.3 ppm 11 . Furthermore both NO
and NO2 participate in the formation of photochemical smog, which is damaging to plants and
detrimental to human health 11 . Due to these negative impacts of NOx on health and environment,
increasingly stricter emission limits have been imposed on the NOx emission from automotive
exhaust. In the U.S this has been done by the introduction of the TIER 1-2 emission standards and
in Europe by the introduction of the Euro 1-6 emission standards 12 . For illustration, the NOx
emission standards according to Euro 1-6 are stated in table Table 1. The dramatic decrease in the
allowable NOx emission means NOx-removal technologies become a necessity as engine control of
diesel engines previously used to reduce the NOx emission no longer is enough to comply with the
NOx emission standards 13 .
2
(1.2)
(1.3)