Selective Ammonia Oxidation on Multi-Layer Cu-SSZ-13/Pt/Al2O3 Monoliths: Impact of Top Layer

摘要

Selective catalytic reduction (SCR) of NOx (NO + NO2) with NH3 on metal (Cu- and Fe-) exchanged zeolites is used to mitigate NOx emission from heavy duty diesel vehicles. [1]. NH3 is produced on-board by hydrolyzing urea and is injected upstream of the SCR catalyst. One challenge of this technology is that unreacted NH3 can escape from the catalyst due to desorption of the stored NH3 during an exhaust temperature increase, overdosing of NH3, or SCR catalyst deactivation [2]. Ammonia is corrosive and is easily detectable by human nose. ?Ammonia Slip Catalyst? (ASC) is positioned downstream of the SCR catalyst to selectively oxidize NH3 to N2, however, some undesirable product such as N2O and NOx can also be formed. Therefore, there is a need for further development of ASC technology to maximize NH3 conversion and minimize the by-product formation. The aftertreatment technology application requires the ASC to work under various challenging conditions such as low temperature, high space velocity, and widely varying exhaust gas composition. The state-ofthe- art ASC uses a dual-layer architecture with Pt/Al2O3 bottom layer and a Cu- or Fe- exchanged zeolitic top layer [3,4]. This architecture is used to increase the selectivity to desired products (N2) by reducing the undesired intermediate NOx formed on Pt/Al2O3 over SCR catalyst. Therefore the SCR component plays an important role in controlling the selectivity to desired/undesired products.