NO₂ is much more toxic than NO. The average proportion of NO₂ in
exhaust gases of vehicles increases significantly due to the use of
oxidation catalysts and catalytic coatings in the exhaust gas
systems during the last decades combined with generalization of
using low sulfur fuels. Diesel oxidation catalysts (DOC) and
Pt-containing DPF coatings are widely used to support the
regeneration of particle filters, being a source of strongly
increased production of NO₂.
The present work shows some examples and summarizes the
experiences in this matter performed at the Laboratories for
IC-Engines & Exhaust Emissions Control (AFHB) of the University
of Applied Sciences Biel-Bienne, Switzerland, during some research
activities on engine dynamometers in the years 2010-2012.
In general it can be stated: - there is a maximum of
NO₂/NOx - ratio with Pt-coated catalyst (DOC), or with
catalyzed soot filter (CSF), typically when the exhaust gas
temperature range is around 350°C; there is even a higher potential
for NO₂ formation with higher Pt-content in the coating, - lower
NO₂ production appears with higher spatial velocity, higher S
content in fuel and with used DOC/DPF and/or when these devices are
loaded with soot, - in some cases of semi active regeneration
systems (fuel injection upstream of DOC), or with the use of RME
conditions with higher NO₂ rates can appear.