The Reactions and Role of Ammonia Slip Catalysts in Modern Urea-SCR Systems

The modern mobile aftertreatment systems (ATSs) include ammonia slip catalyst (ASC) located after the selective catalytic reduction (SCR) catalyst. Mainly copper, iron and vanadium SCR catalysts are applied commercially. SCR systems are mostly located after diesel particulate filter (DPF), which is thermally a demanding location for SCR and ASC. ASC properties are integrated to control strategy together with the SCR catalyst to reach NO_x conversions above 95%. Reactions by Pt loading, Pt layer radial location (bottom, top, full layer) and feed conditions were simulated experimentally in synthetic gases on Cu, Fe and V based SCR catalysts in this study. NH₃ slip after SCR unit was simulated in controlled (NH₃/NO ≤ 1) or uncontrolled (NH₃ only or NH₃/NO_x >> 1) conditions. The axial location of Pt relatively to the flow direction dominated to the radial location of Pt in reaction rates. The results showed significant differences in ammonia conversions and reaction selectivity (to N₂, NO, N₂O) by the Pt layer location (bottom or top) together with a thick SCR layer. The Pt loading was optimized to the range of 2–5 g/cft to keep a good selectivity to N₂ as well as to avoid N₂O/NO_x formation and the Pt contamination effect on SCR. Pt-ASC was necessary to remove remaining NH₃ and CO after SCR but the prevention of N₂O formation is an essential part of design. Experimental simulations with 2-layer ASC with a Cu-chabazite (CHA) catalyst resulted in a complete mapping by the Pt loading as a function of space velocity and temperature as fresh and hydrothermally aged.