278717 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
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.
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.
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.
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