(126c) Ammonia Oxidation On Bifunctional Structured Catalysts

Ammonia Oxidation on Bifunctional Structured Catalysts

Sachi Shrestha*, Michael
P. Harold*¹, Krishna Kamasamudram**² and Aleksey
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

Ammonia Slip Catalyst (ASC) is applied
downstream of NH₃ based selective catalytic reduction catalyst to
minimize the breakthrough of ammonia from heavy duty diesel vehicles. A class
of Cu-exchanged zeolite catalyst with high ammonia sorption capacity and high
activity for NOx conversion to N₂, and low loading Pt-based catalyst
with high activity for ammonia oxidation is developed. In this study our goal
is to better understand the catalytic mechanism of post-SCR ammonia conversion
and product selectivity on low-loading Pt-based catalysts and in so doing
provide guidance in the development of a new class of ammonia slip catalysts
(ASCs).

Materials and Methods

ASCs
synthesized in our laboratory were used for evaluation of NH₃
oxidation. Pt was deposited onto the Al₂O₃ support by
incipient wetness impregnation while Cu was ion-exchanged into zeolite. These
powders were used for coating cordierite monoliths. Different configuration of
washcoat, mixed, layer or sequential, was applied onto 2 cm long, 0.8 cm
diameter monoliths. ASCs, containing Pt/Al₂O₃ and
Cu-ZSM-5/Al₂O₃ on the same monolith, were prepared by co-
or successive washcoating methods to obtain layered and mixed catalyst
structures. Approximately 1-2 g of Pt/Al₂O₃ and 1 to 3 g
of Cu-ZSM-5/Al₂O₃ were applied onto the monolith. Performance
of lab synthesized ASCs were compared with a commercial Fe-zeolite based ASC.

The performance
of the synthesized monolith ASCs were evaluated in a bench-scale reactor system
comprising a FTIR and mass spectrometer for gas composition measurements.

 Steady state performance with feed
gas containing 500 ppm NH₃, 5% O₂, and in presence and
absence of 500 ppm NO were evaluated over a wide temperature range. 

Results

Steady-state
NH₃ oxidation over the low-loading Pt/Al₂O₃
catalyst showed light-off between 175-200 ^oC (Figure 1). The
N-containing product distribution shifted from a mixture of N₂ and N₂O
at low temperature to a mixture of NO and NO₂ at high
temperature.  With increasing temperature
the N₂ decreased monotonically where as the NOx increased
monotonically. These data show that while complete NH₃ conversion is
achieved above 220 ^oC on the low loading Pt catalyst, N₂O
and NOx are major and unwanted products over a wide range of temperature. 

The NH₃
oxidation activity of in-house synthesized PGM catalyst, that contains only PGM
component, and ASC catalyst that contains two components, PGM and zeolite catalysts,
is shown in Figure 1 and Figure 2 respectively. Complete NH₃
conversion is achieved above 200 ^oC for both the catalysts. From
Figures 1 and 2, it is observed that the light off temperature of NH₃
conversion is slightly affected by presence of zeolite catalyst. However, the selectivity
to N₂ is high across the temperature range when the ASC contains
zeolite as one of the catalyst components. The ASC containing both PGM and
zeolite can serve as a dual functional, NO reduction and NH₃
oxidation catalyst. The washcoat configurations of the two catalytic components
are varied to further optimize activity and selectivity to N₂ and
the results will be discussed in detail.

References

Scheuer, a., W. Hauptmann,
et al. 2011. ?Dual
layer automotive ammonia oxidation catalysts: Experiments and computer
simulation.? Applied Catalysis B: Environmental. (111-112) 445-455