(407e) Selective Ammonia Oxidation on Multi-Layer Cu-SSZ-13/Pt/Al2O3 Monoliths: Impact of Top Layer

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

^*1mharold@uh.edu; ^**krishna.kamasamudram@cummins.com;
^***louise.olsson@chalmers.se

Introduction

Selective catalytic reduction (SCR)
of NO_x (NO + NO₂) with NH₃ on metal (Cu- and
Fe-) exchanged zeolites is used to mitigate NO_x emission from heavy
duty diesel vehicles. [1]. 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 [2]. 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 NO_x 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 NO_x 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 [3], 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 NO_x,
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.  

Significance

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.

References

[1]        I. Masakazu, H. Hideaki,
Catalysis Today, 10 (1991) 67-71, 10 (1991) 67-71.

[2]        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.

[3]        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.

[4]        S. Shrestha, M.P.
Harold, K. Kamasamudram, A. Yezerets, Ammonia Oxidation on Structured Composite
Catalysts, Topics in Catalysis. 56 (2013) 182-186.