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Cold Start Effect Phenomena over Zeolite SCR Catalysts for Exhaust Gas Aftertreatment
Abstract:
NH₃/urea SCR is a very effective and widely used technology for
the abatement of NOx from diesel exhaust. The SCR mechanism is well
understood and the catalyst behavior can be predicted by
mathematical models - as long as operation above the temperature
limit for AdBlue® injection is considered. The behavior below this
level is less understood.
During the first seconds up to minutes after cold start,
complete NOx abatement can be observed over an SCR
catalyst in test bench experiments, together with a significant
increase in temperature after the converter (ca. 100 K).
In this work these effects have been investigated over a
monolith Cu-zeolite SCR catalyst. Concentration step experiments
varying NO, NO₂ and H₂O have been carried out in lab scale,
starting from room temperature. Further, the interaction of C₃H₆
and CO with NOx over the SCR has been investigated. The
effects observed during cold start have been reproduced
successfully at lab scale, providing insight into the prevailing
mechanisms.
It was found that low-temperature NO oxidation proceeds already
at room temperature, showing a negative temperature dependence.
However, it is strongly inhibited by H₂O. Thus, significant amounts
of NO₂ can be produced over the SCR while H₂O of the exhaust
condenses on the cold devices upstream. Further, the SCR catalyst
can store NO₂ efficiently, so that a major part of NOx
disappears. Finally, upon addition of H₂O vapor to the gas, it
adsorbs on the zeolite, causing a sudden temperature increase due
to the heat of adsorption. The results show that this increase
forces the stored NO₂ to desorb immediately. However, real engine
cold start measurements show NO desorption instead. The reason can
be reducing agents like CO and hydrocarbons that were indeed proven
able to reduce NO₂ to NO over the SCR.