Selective Ammonia Oxidation on Multi-Layer
Cu-SSZ-13/Pt/Al2O3 Monoliths: Impact of Top Layer
Sachi Shrestha*, M. P. Harold*1, K. Kamasamudram**, A. Kumar**,
K. Leistner***, and L. Olsson***
*Dept. of Chemical and Biomolecular Engineering, University of Houston, Houston, TX 77204-4004, USA
**Cummins Inc., 1900 McKinely Av., MC50197, Columbus, IN 47201, USA
***Chalmers University of Technology, 412 96 Gothenburg, Sweden
*firstname.lastname@example.org; **email@example.com; ***firstname.lastname@example.org
Selective catalytic reduction (SCR) of NOx (NO + NO2) with NH3 on metal (Cu- and Fe-) exchanged zeolites is used to mitigate NOx emission from heavy duty diesel vehicles. . NH3 is produced on-board by hydrolyzing urea and is injected upstream of the SCR catalyst. One challenge of this technology is that unreacted NH3 can escape from the catalyst due to desorption of the stored NH3 during an exhaust temperature increase, overdosing of NH3, or SCR catalyst deactivation . Ammonia is corrosive and is easily detectable by human nose. “Ammonia Slip Catalyst” (ASC) is positioned downstream of the SCR catalyst to selectively oxidize NH3 to N2, however, some undesirable product such as N2O and NOx can also be formed. Therefore, there is a need for further development of ASC technology to maximize NH3 conversion and minimize the by-product formation. The aftertreatment technology application requires the ASC to work under various challenging conditions such as low temperature, high space velocity, and widely varying exhaust gas composition. The state-of-the-art ASC uses a dual-layer architecture with Pt/Al2O3 bottom layer and a Cu- or Fe-exchanged zeolitic top layer [3,4]. This architecture is used to increase the selectivity to desired products (N2) by reducing the undesired intermediate NOx formed on Pt/Al2O3 over SCR catalyst. Therefore the SCR component plays an important role in controlling the selectivity to desired/undesired products.
In this contribution the investigations of the impact of catalyst architecture and diffusion of reactant through different catalyst support, on the NH3 conversion and product selectivity, over Cu-SSZ-13/Pt/Al2O3 coated monoliths will be presented.
Materials and methods
A 0.15wt% Pt/Al2O3 catalyst was synthesized by an incipient wetness impregnation method using H2PtCl6. 6H2O (Sigma Aldrich, USA). The Na-SSZ-13 synthesized by Chalmers was ion exchanged with NH4NO3 followed by Cu(NO3)2 to obtain Cu-SSZ-13. XRD was used to examine that correct structure was received and elemental analysis of the powder was conducted with ICP-SFMS, resulting in a Si/Al ratio of 3.63 and a copper loading of 3.1wt-%. The 0.8 cm diameter cordierite substrates of 400 cells per square inch were washcoated with the slurries containing the above catalysts in order to obtain monolith with various catalyst loading. When used the SCR catalyst loading on the monolith was between 0.85 to 3 g/in3 while the oxidation catalyst loading was fixed at 1.4 g/in3.Further, in order to characterize the effect of diffusion through these support materials, dual layer catalysts with bottom Pt/Al2O3 layer and top inert layer, either Al2O3 or Na-ZSM-5, was synthesized.
A bench top reactor, described elsewhere , was used for evaluating the activity of the catalysts. The total flow rate was maintained at 1000 sccm, corresponding to a GHSV of 66k h-1 for 2 cm long monolith and 265k h-1 for 0.5 cm long monolith. The feed concentration of 500 ppm NH3 was used with varying levels of NO (0 – 500 ppm) along with 5% O2, 2.5% H2O, 2% CO2 and balance Ar. An FT-IR was used to measure NO, NO2, N2O, NH3, CO2 and H2O species concentrations. The gas lines were heated to above 150 oC to avoid adsorption and condensation of H2O and NH3.
Results and discussions
The impact of the SCR catalyst loading, varied between 0 and 3g/in3, on the NH3 oxidation activity of dual layer Cu-SSZ-13/Pt/Al2O3 is shown in Figure 1(a). The NH3 conversion decreased with the increase in SCR catalyst layer thickness, which was more distinct between 250 and 350 oC. Below 350 oC the SCR catalyst contribution to direct NH3 oxidation by oxygen is minimal. Since most NH3 oxidation activity takes place in Pt/Al2O3 layer, the observed decrease in NH3 conversion is attributed to a lower mass transport of NH3 through the SCR layer which acts as a barrier. On the other hand, the presence of the CuSSZ-13 top layer dramatically increased the N2 yield compared to a discrete Pt/Al2O3 catalyst and above 350 oC with the selectivity to N2 increasing sharply with increase in SCR catalyst loading, Figure 1(b).
In the presentation the observed differences in NH3 conversion and N2 yield will be explained based on i) increased resistance to NH3 mass transfer to bottom layer Pt/Al2O3, ii) conversion of NOx, produced in Pt/Al2O3 bottom layer, through reaction with NH3 in the top layer SCR catalyst, iii) direct and selective oxidation of NH3 above 350 oC in the SCR catalyst which increases with an increase in SCR catalyst loading, and iv) species distribution leading to various redox reactions with different rates along the axial direction.
Additional experiments using Pt-catalyzed CO oxidation with varying levels of an Al2O3 top layer are being conducted to elucidate the mass transport resistance and morphological effects.
This work demonstrates the application of engineering principles to design catalyst with tunable activity and selectivity properties and to further the advancement of NH3 slip catalyst technology to mitigate the diesel engine emissions.
 I. Masakazu, H. Hideaki, Catalysis Today, 10 (1991) 67-71, 10 (1991) 67-71.
 J.W. Girard, G. Cavataio, C.K. Lambert, The Influence of Ammonia Slip catalyst on NH3 N2O and NO Emissions for Diesel Engines, SAE Technical Paper. (2007) 2007-01-1572.
 S. Shrestha, M.P. Harold, K. Kamasamudram, A. Yezerets, Selective oxidation of ammonia on mixed and dual-layer Fe-ZSM-5+Pt/Al2O3 monolithic catalysts, Catalysis Today. (2014) 1-11.
 S. Shrestha, M.P. Harold, K. Kamasamudram, A. Yezerets, Ammonia Oxidation on Structured Composite Catalysts, Topics in Catalysis. 56 (2013) 182-186.