Cold Start Effect Phenomena over Zeolite SCR Catalysts for Exhaust Gas Aftertreatment

Abstract

<div class="section abstract"><div class="htmlview paragraph">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.</div><div class="htmlview paragraph">During the first seconds up to minutes after cold start, complete NO<sub>x</sub> 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).</div><div class="htmlview paragraph">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 NO<sub>x</sub> 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.</div><div class="htmlview paragraph">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 NO<sub>x</sub> 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.</div></div>