Ammonia Oxidation On a Bi-Functional Pt/Al2O3 and Fe-Exchanged ZSM-5 Washcoated Monolith Catalyst

Ammonia Oxidation on a Bi-Functional Pt/Al₂O₃ and Fe-Exchanged ZSM-5 Washcoated Monolith Catalyst

Sachi Shrestha*, M. P. Harold*¹, K. Kamasamudram**² and A. Yezerets**

*Dept. of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204-4004, USA

**Cummins Inc., 1900 McKinely Av., MC50197, Columbus, IN 47201, USA

^*1mharold@uh.edu; *²krishna.kamasamudram@cummins.com

Introduction

The latest technology of choice to reduce NOx from the exhaust of the lean burn heavy duty vehicles is NH₃ - selective catalytic reduction, where NH₃ produced on board reduces NOx to benign N₂ over a metal (Cu or Fe) exchanged zeolite catalyst [1].  NH₃ emissions from this aftertreatment system may result from the over-supply of NH₃ during operation, dynamical effects, and a deactivated catalyst. In order to minimize the breakthrough of NH₃ an Ammonia Slip Catalyst (ASC) wherein NH₃ slipping out of the SCR catalyst is selectively oxidized to N₂.  Pt is a well-known highly active catalyst for NH₃ oxidation.  Unfortunately, the oxidation of NH₃ on Pt also leads to undesired (for this application) byproducts such as N₂O, NO, and NO₂.  

In order to improve the selectivity of NH₃ oxidation towards N₂, previous research has shown a benefit of coupling the oxidation of NH₃ with a SCR catalyst which is known to have high reduction activity of NOx to N₂ in presence of NH₃. The use of such a bi-functional catalyst with oxidative function to oxidize NH₃ (to NOx) and reductive function to improve selectivity (reduce NOx to N₂) towards N₂ is the catalyst of choice to mitigate slippage of NH₃ from the tailpipe.  Different configurations have been proposed for the ASC.  These include a SCR monolith modified with a short downstream zone containing an oxidation catalyst such as Pt.  Another configuration is a dual layer configuration, where Pt is applied as Pt/Al₂O₃ on the bottom layer and a metal-exchanged-zeolite is applied on top of the Pt/Al₂O₃ layer [2]. 

In this study we synthesize and evaluate various catalyst architectures that combine Pt/Al₂O₃ with Fe- zeolite, including a dual layer comprising a Pt/Al₂O₃ bottom layer and a Fe-zeolite top layer, and mixed and co-impregnated multi-component catalysts. Our objective is to elucidate the effect of the multi-component (Pt/Fe) catalyst architecture on NH₃ oxidation performance with goal of maximizing NH₃ conversion and N₂ yield over a wide range of conditions while minimizing the loading of Pt.

Materials and Methods

A series of catalyst were synthesized in the laboratory in order to study its NH₃ oxidation activity. Pt/Al₂O₃ catalyst was synthesized by incipient wetness impregnation method, and a commercial Fe-zeolite catalyst was provided by Sud Chemie (Clariant Int.).  Slurries containing the catalyst powders were washcoated onto 400 cpsi cordierite monoliths. The washcoat had either a single layer or dual layer architecture with the former comprising mixed layers of Pt/Al₂O₃ and Fe-zeolite and the latter comprising a bottom Pt/Al₂O₃ layer and a top Fe-zeolite layer. The weight loadings of Pt/Al₂O₃ and Fe-zeolite catalyst were varied systematically to study their effect on NH₃ oxidation activity and product selectivity. The Pt loading in Pt/Al₂O₃ catalyst was varied from 0.7-10.5 g/ft³, while the Fe-zeolite loading was varied from 0.5-1.5 g/in³. The single layer Pt/Al₂O₃ with the same loading served as the reference.

The bench scale reactor setup consisted of gas feed system, reactor system, and data analyzing system. The analyzing system consisted of FT-IR and QMS to measure the concentration of the effluent gas. The total flow rate was fixed as 1000 sccm, corresponding to the GHSV of 66K hr^-1 for 2 cm long monolith and 265K hr^-1 for 0.5 cm long monolith. For all the experiments, the feed concentration of 500 ppm NH₃ and 5% O₂ was used with varying levels of NO (0 – 500 ppm).

 

Results and Discussion

 

We have conducted a large number of experiments on several different catalyst having different compositions and architectures.  Table 1 lists the catalysts used in the current study.  A typical set of results is shown in Figure 1.  Figure 1(a) and (b) shows the NH₃ conversion and product distribution for NH₃ oxidation reaction on a dual layer and mixed catalyst respectively at the GSHV 66K hr^-1.  It was seen that at this space velocity the configuration of the catalyst had minimal effect on NH₃ conversion capability of the catalyst.  Both dual layer and mixed catalyst showed the light-off (temperature of 50% conversion) at ~215 ^oC, with the complete conversion of reactant at ~235 ^oC.  However, there were several noticeable differences on the product distribution for same reaction over the two catalysts bed at temperature above 230 ^oC.  N₂O yield showed an interesting trend with mixed catalyst giving lower yield of N₂O than dual layer catalyst at the temperature range of 230-350 ^oC and dual layer catalyst giving lower yield at temperature above 350 ^oC.  N₂O is formed in a Pt/Al₂O₃ layer at temperature above 200 ^oC from the reaction between NO ad-species and N ad-species.  On mixed catalyst, NO ad-species formed on Pt catalyst which is an important intermediate for N₂O formation, can migrate to the Fe-zeolite catalyst where it can be reduced by store NH₃ to N₂, thus also bringing a slight improvement in N₂ yield at this temperature range.  However, above 350 ^oC, the decrease in N₂O yield for dual layer catalyst compared to the mixed catalyst is due decomposition of N₂O and N₂O-SCR reaction of the back diffusing N₂O formed on Pt/Al₂O₃ layer on Fe-zeolite layer.  Unlike, dual layer catalyst the presence of Pt catalyst on the gas-solid interface facilitates the desorption of N₂O directly to the flow channel without having to interact with the Fe-zeolite layer for the mixed catalyst, hence giving slightly higher N₂O yield at temperature above 350 ^oC. 

Additional data showed that the NOx (NO+NO₂) yield from dual layer catalyst was lower than that from mixed layer catalyst.  NOx is a dominant product of NH₃ oxidation on Pt/Al₂O₃, however the dual layer structure ensure all NOx formed on the bottom Pt catalyst to back diffuse through the top Fe-zeolite catalyst which are very active in reducing NOx to N₂ in presence of NH₃, where as the presence of Pt catalyst in the gas-solid interface for mixed catalyst facilitates the NOx formed on Pt catalyst to escape through the flow channel, thus giving slightly higher NOx yield and in turn lower N₂ yield.  Also, not shown here, the mixed catalyst were beneficial in terms of higher NH₃ conversion when residence time of the reactant became the dominant factor, because of the absence of diffusion barrier for NH₃ oxidation on a Pt/Al₂O₃ layer.

These and other data will be reported and a phenomenological model proposed that explains the main trends. 

 Significance

This work should benefit in understanding the coupling of oxidation and reduction/storage component of ASCs, thus, helping in further advancement of present commercial slip catalysts.

 References

 Kamasamudram, K., Currier, N., Castagnola, M., & Chen, H.-ying. (2011). New Insights into Reaction Mechanism of Selective Catalytic Ammonia Oxidation Technology for Diesel Aftertreatment Applications. SAE International Journal of Engines. 4(1), 1810-1821.

 Scheuer, a., Hauptmann, W., Drochner, a., Gieshoff, J., Vogel, H., & Votsmeier, M. (2011). Dual layer automotive ammonia oxidation catalysts: Experiments and computer simulation. Applied Catalysis B: Environmental, 111–112, 445-455.

 

Figure 1: Comparison of activity and product yield of catalyst comprising of Fe-zeolite loading of 1.5g/in³ and Pt loading of 10 g/ft³ for NH₃ oxidation reaction. (a) Dual Layer (b) Mixed. Reaction Conditions: 500 ppm NH₃, 5% O₂ and 66K hr^-1 GHSV.

 

Table 1. Catalyst Used for evaluation

Catalyst Name	Catalyst Description	Fe-zeolite Loading (g/in3)	Pt/Al2O3 Loading (g/in3)	Pt Loading (g Pt per 100 g washcoat)
LFeZ(1.5)Pt10)	Layered_FeZ(1.5)/Pt-Al2O3(1.3)	1.5	1.3	0.46
MFeZ(1.5)Pt10)	Mixed_FeZ(1.5)/Pt-Al2O3(1.3)	1.5	1.3	0.46