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

Shrestha, S. - Presenter, University of Houston
Harold, M. P. - Presenter, University of Houston
Kumar, A. - Presenter, Cummins Inc.
Olsson, L. - Presenter, Chalmers University of Technology
Leistner, K. - Presenter, Chalmers University of Technology

Selective Ammonia
Oxidation on Multi-Layer

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;



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. [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 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.

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

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 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.

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


[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)

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

" src="https://www.aiche.org/sites/default/files/aiche-proceedings/conferences/..." height="47" class="documentimage">