Cold Start Effect Phenomena over Zeolite SCR Catalysts for Exhaust Gas Aftertreatment 2013-01-1064

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 NO_x 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 NO_x 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 NO_x 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.