(454b) Ammonia Oxidation On a Bi-Functional Pt/Al2O3 and Fe-Exchanged ZSM-5 Washcoated Monolith Catalyst
- Conference: AIChE Annual Meeting
- Year: 2013
- Proceeding: 2013 AIChE Annual Meeting
- Group: Catalysis and Reaction Engineering Division
- Time: Wednesday, November 6, 2013 - 8:50am-9:10am
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**
Chemical and Biomolecular Engineering, University of Houston, Houston, TX
1900 McKinely Av., MC50197, Columbus, IN 47201, USA
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 .
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
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.
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).
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
These and other data will be reported and a phenomenological
model proposed that explains the main trends.
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.
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
Fe-zeolite Loading (g/in3)
Pt/Al2O3 Loading (g/in3)
Pt Loading (g Pt per 100 g washcoat)