(454b) 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