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

NH\u2083/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, NO\u2082 and H\u2082O have been carried out in lab scale, starting from room temperature. Further, the interaction of C\u2083H\u2086 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 H\u2082O. Thus, significant amounts of NO\u2082 can be produced over the SCR while H\u2082O of the exhaust condenses on the cold devices upstream. Further, the SCR catalyst can store NO\u2082 efficiently, so that a major part of NO x disappears. Finally, upon addition of H\u2082O 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 NO\u2082 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 NO\u2082 to NO over the SCR.