Investigation of the influence of hydrocarbons on Cu-zeolite SCR behavior

Volker  Schmeisser, Daimler AG

NH3/urea SCR is a very effective and widely used technology for the abatement of NOx emissions from Diesel engines. Research on the SCR mechanisms over the last years focused not only on the basic paths of Standard, Fast and NO2 SCR, but also on the formation and decomposition of byproducts like N2O or ammonium nitrate. The SCR behavior is well understood and can be predicted by mathematical models. Currently a trend exists in placing the SCR washcoat on the particle filter for packaging and temperature reasons. However, this increases the risk of unburnt hydrocarbons getting onto the SCR washcoat. The effect of hydrocarbons on the SCR behavior can be significant, even for chabazite zeolites. For a detailed understanding and modeling of these effects a systematic investigation is necessary.

In this work a state-of-the-art Cu-SCR sample has been investigated with synthetic exhaust gas. The influence of different saturated and unsaturated gaseous hydrocarbons, namely C2H2, C3H6 and C3H8, has been tested. The experiments focused on the adsorption and desorption behavior of the hydrocarbons. Especially their interaction with NH3, i.e. the competitive aspect of the adsorption processes, has been adressed. Further, the effect of hydrocarbons on the NOx- and NH3-conversion behavior has been determined.

It was found that C3H8 does not adsorb on the SCR catalyst and has no influence on the NOx conversion. C2H2 and C3H6 adsorb on the catalyst, but only if water is absent from the exhaust gas. The adsorbed hydrocarbons can be substituted by NH3, if added to the feed gas, indicating that NH3 is a stronger adsorbent. If NH3 has been preasorbed, no hydrocarbon adsorption followes. The NH3 storage capacity is not affected by the hydrocarbons. During the adsorption of hydrocarbons at increased temperatures they obviously decompose, e.g. to CO as one product, resulting in an increased storage capacity for hydrocarbons. It seems that the decomposition product CO can adsorb on different sites. The NOx-conversion can be positively and negatively affected by C2H2 and C3H6, depending on the temperature and NO2/NOx-ratio. However, this is only the case when the hydrocarbons are present in the feedgas during the conversion experiment, even in the presence of water. As no hydrocarbon adsorption occurs under these conditions, their influence must be due to a different mode of affection.

The oral presentation will cover the methodical approach and representative results of the work.

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