Impact of Hydrocarbons on the Dual (Oxidation and SCR) Functions of Ammonia Oxidation Catalysts

Ammonia oxidation (AMOX) catalysts are critical parts of most diesel aftertreatment systems around the world. These catalysts are positioned downstream of selective catalytic reduction (SCR) catalysts and remove unreacted NH₃ that passes through the SCR catalyst. In many configurations, the AMOX catalyst is situated after a diesel oxidation catalyst and catalyzed diesel particulate filter that oxidize CO and hydrocarbons. However, in Euro V and proposed Tier 4 final aftertreatment architectures there is no upstream oxidation catalyst. In this study, the impact of hydrocarbons is evaluated on two different types of AMOX catalysts. One has dual washcoat layers-SCR washcoat on top of PGM washcoat-and the other has only a PGM washcoat layer. Results are presented for NH₃ and hydrocarbon oxidation, NO_x and N₂O selectivity, and hydrocarbon storage. The AMOX findings are rationalized in terms of their impact on the individual oxidation and SCR functions. Finally, these new results are compared to recent hydrocarbon conversion measurements made on vanadium-based SCR catalysts in order to estimate the potential system-level reduction of hydrocarbons possible with a combined V-SCR+AMOX aftertreatment.