Slip catalyst

Ammonia oxidation catalysts (sometimes called slip catalyst) are conventional oxidation catalysts based on precious metals. The most active types are based on Pt. Then-activity is strongly dependent on the temperature and, thus, relatively large catalyst volumes are required for the ammonia oxidation below 250°C. At rising temperatures, their oxidation power increases and this leads to the formation of N20 and NO. Especially undesired is their strong tendency to form N20 at intermediate temperatures (250-300°C) [2] if the gas coming from the SCR catalyst also contains unreacted NO, which allows for the reaction ... 

On the other hand, considering the practical application, slip catalysts are generally required at the outlet of the SCR reactor in order to remove secondary emissions which typically include a certain concentration of ammonia, isocyanic acid (originating from incomplete urea decomposition), nitrous oxide, and nitrohy-drocarbons. ... 

Dual-Layer Ammonia Slip Catalysts for Automotive SCR Exhaust Gas Aftertreatment An Experimental... 

To achieve this high level of efficiency, all aspects of the system need to be optimized. An SCR system will generally comprise an ammonia delivery system, and the catalyst system itself comprises the DOC (and typically DPF), SCR catalyst, and the ammonia slip catalyst. 

Fig. 1.10 Nitrous oxide formation in ammonia slip catalysts is promoted by high N0 NH3 ratios coming out of the SCR catalyst [45]...

Fig. 1.11 Generational improvements in ammonia slip catalysts. Early improvements dropped cost with minor compromise in selectivity. The latest generation recovers the performance with greatly reduced precious metal loading [34]...

Ammonia slip catalysts are improving in terms of cost and selectivity to nitrogen. Figure 1.11 from Walker [34] shows modem catalysts have similar performance to the first generation catalysts, but with only 20 % of the platinum loading. The latest catalyst has much better selectivity to nitrogen with less undesirable by-products, yet with half the precious metal of its predecessor. Formation of NO and NO2 is still an issue but improving. 

Figure 2.21 shows the cross-section of the installed DPF -I- SCR reactor. After entering the inlet chamber, the exhaust flow is divided into a right and a left path. First, the exhaust flows through a diesel oxidation catalyst (DOC) to convert some NO to NO2, which is a prerequisite for a continuous soot oxidation within the diesel particulate filter (DPF) and which additionally improves the SCR conversion rate, (see Fig. 2.22) [22]. The DPF placed behind the DOC reduces particulate matter (PM) with high eflftciency. Downstream of the DPF, a special designed mixing chamber gives the injected urea solution sufficient time for evaporation, mixing and thermolysis before entering the SCR catalyst. Finally, an ammonia slip catalyst is used to prevent any NH3 slip to the environment.

Fig. 2.21 Combined aftertreatment system for a shunting locomotive a DPF -I- SCR installed in one reactor housing, b gas flow (Picture MTU) with A exhaust gas inlet, 1 diesel oxidation catalyst (DOC), 2 diesel particulate filter (DPF), 3 urea solution Injection, 4 SCR catalyst, 5 ammonia slip catalyst (ASC), 6 urea mixing area, B exhaust gas outlet...

As can be seen in Fig. 3.7, at low ANR, the NH3 slip is close to zero and NOx conversion is close to proportional to ANR. At higher ANR (closer to 1), NH3 slip is increasing rapidly as the NOx conversion approaches 100 %. Since NH3 slip in the exhaust outlet needs to be minimized, this puts a limit to what NOx conversion can be achieved. By adding an ammonia slip catalyst (ASC) after the SCR catalyst, the NH3 slip can be lowered allowing for extra margin in the control [18]. 

The challenge for the NH3-SCR systems is to maximize the conversion efficiency by keeping at the same time the NH3 release to a minimum. However, under certain conditions an increase of the de-NO performance can only be achieved by increasing the reductant quantity and consequently allowing higher NH3 slip. In order to minimize the released NH3, the addition of an ammonia slip catalyst ASC (also referenced as ammonia oxidation catalyst AOC) downstream the SCR is considered as an efficient and reliable solution. 

Colombo M, Nova I, Tronconi E, Koltsakis G (2012) A Modeling Study of NH3 Slip Catalysts Analysis of the SCR/PGM Interactions. Topics in Catalysis. doi 10.1007/ si1244-013-9948-x... 

The reducing agent, NH3, is adsorbed onto the SCR and then consumed by NOx reduction reactions. The amount of ammonia fed to the reactor must be controlled depending on the operating conditions in order to maintain a high level of NH3 adsorption and NOx conversion of the SCR on the one hand. On the other hand, NH3 emissions from the aftertreatment system into the environment are not desired and have to be avoided. In order to reduce as much as possible the amount of released ammonia, the addition of another catalytic device downstream the SCR one can be an efficient and reliable solution. Furthermore, to maintain the system as compact as possible, one solution is to add an ammonia oxidation functionality directly in the rear part of an SCR monolith (Fig. 18.1a). In this way, an NH3 slip catalyst (ASC) is added after the SCR to oxidize NH3 leaving the SCR brick [2]. 

Finally, the SCR and PGM kinetics were implemented in a mathematical model of dual-layer monolithic catalysts, specifically developed for SCR -I- PGM NH3 slip catalysts [20] and validated against experimental data collected over the full dual-layer ASC monolith catalyst [19]. 

Like for the SCR component, the reacting systems prevailing over the PGM component of the Ammonia Slip catalyst under typical NH3-SCR operating conditions have been addressed according to an order of increasing complexity,... 

Scheuer A, Votsmeier M, Sehuler A, Gieshoff J, Droehner A, Vogel H (2009) NH3-Slip Catalysts Experiments Versus Meehanistie Modelling. Top Catal 52 (13) 1847-1851... 

A commercial application has been reported by Volkswagen for some EU6 vehicle applications of their new 1.6 and 2.0 1 TDI engines [72]. The system comprises a close coupled DOC followed in short distance by an SCR catalyst coated DPF. In addition, a slip catalyst with combined SCR and oxidation functionality is installed downstream of the DPF. The urea injection is located in-between the DOC and DPF components and a static mixer is used to improve distribution and evaporation within the limited space. Due to the high temperatures that can occur in this very close coupled position, a water-cooled urea injector was selected and the supply lines for the aqueous urea solution are made of highly temperature resistant materials. 

Tang et al. [50] report on experiments of a heavy-duty aftertreatment system comprising a DOC followed by a DPF with integrated SCR functionality and an ammonia slip catalyst. The size of the DPP was 12" diameter and 12" in length... 

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