(11b) Use of Sacrificial Agent to Enhance Gas Transport through the Washcoat

Use of Sacrificial Agent to Enhance Gas
Transport through the Washcoat

Pritpal Singh Dhillon, Michael
P. Harold*

Di Wang², Ashok
Kumar²

¹Dept. of Chemical & Biomolecular Engineering,
University of Houston, Houston, Texas

²Cummins Inc., Columbus, Indiana

*
corresponding
author e-mail: mpharold@central.uh.edu

Introduction

         The
Ammonia Slip Catalyst (ASC) is utilized to convert unreacted
ammonia selectively into N₂ while minimizing undesired products NO,
NO₂ and N₂O. The dual-layer ASC comprises a base layer of
an oxidation catalyst (Pt/Al₂O₃) and a top layer of a SCR
catalyst such as Cu-exchanged zeolite. The dual-layer ASC is known for higher nitrogen selectivity,
but it adversely affects ammonia conversion due to mass transfer limitations.
Increasing the top (SCR) layer loading improves nitrogen selectivity,
simultaneously it reduces ammonia conversion. To
increase nitrogen selectivity without affecting conversion, we prepared
modified dual-layer ASC materials   with
enhanced top washcoat macro-porosity. Sacrificial agents including yeast and
poly-(tert-butyl acrylate) were used to create voidage inside the top SCR layer
with the intent of reduced the mass transfer limitations for ammonia [1]. The
impact of the modified washcoat on the ammonia conversion and N₂
selectivity is the focus of the study. Reaction experiments were conducted
using the modified dual-layer ASC showed significant increase in nitrogen
selectivity and improvement in ammonia conversion simultaneously.

Materials and Methods

        Bottom Layer: The base layer of a
dual-layer ASC comprising Pt/Al₂O₃
was prepared through a sequence of steps. Catalyst powder was synthesized by
incipient wetness impregnation using chloroplatinic acid hexahydrate as the Pt
precursor. The Pt/Al₂O₃ slurry was prepared by adding
deionized water and boehmite solution (AlOOH: 20wt %)
to Pt/Al₂O₃ powder (mass ratio Pt/Al₂O₃:
water: boehmite = 8:15:2) and ball milled for 20 h. The monolith was dipped
into the Pt/Al₂O₃ slurry
for 30 s and excess slurry was blown for 10 s. Subsequently, the monolith was
dried in an oven at 120 ^oC for 2 h and the same dipping procedure
was repeated to get the required loading. Finally, the monolith was dried
overnight in an oven at 120 ^oC, followed by calcination at 550 ^oC
for 5 h with a temperature ramp rate of 0.5^oC/min. The freshly
prepared monolith was then treated with 2% H₂ and balance Ar for 1 h
at 500 ^oC, followed by degreening with 5% O₂ and balance
Ar at same temperature for 2 h.

       Conventional Dual-Layer: The dual-layer
ASC comprising a Cu/SSZ-13 top layer and Pt/Al₂O₃ bottom
layer was prepared as follows. Cu/SSZ-13 containing 3% Cu slurry was prepared
by adding deionized water and boehmite solution (ALOOH: 20wt %) to Cu/SSZ-13
powder (mass ratio Cu/SSZ-13: water: boehmite = 4:8:5) and ball milled for 20
h. The monolith which was already coated with Pt base layer was dipped into the Cu/SSZ-13slurry for 30 s
and excess slurry was blown for 10 s. Subsequently, the monolith was dried in
an oven at 120 ^oC for 2 h and the same dipping procedure was
repeated to get the required loading. Finally, the monolith was dried overnight
in an oven at 120 ^oC, followed by calcination at 550 ^oC
for 5 h with a temperature ramp rate of 0.5^oC/min. The type-I dual
layer ASC was prepared (Fig 1).

       Modified Dual-Layer: For the modified
dual-layer ASC, the Cu/SSZ-13 (Cu/SSZ-13 containing 5% Cu) slurry was prepared
by similar manner as above and ball milled for 20 h. Then, a calculated amount
of sacrificial agent (yeast/polymer) was added to the slurry, intended to
increase the porosity of top layer roughly by 20%. The monolith which was
already coated with Pt base layer was dipped into the Cu/SSZ-13slurry for 30 s and excess slurry was blown
for 10 s. Subsequently, the monolith was dried at room temperature for 4 h and
the same dipping procedure was repeated to get the required loading. Finally,
the monolith was dried overnight at room temperature, followed by calcination
in flow reactor (1000 sccm, 5% O₂ & balance Argon) at 550 ^oC
for 5 h with a temperature ramp rate of 1^oC/min. The type-II dual
layer ASC was prepared (Fig 2).

                                Figure 1: (Type-I: Dual Layer ASC)               Figure
2: (Type-II: Modified dual layer ASC)

Results and Discussion

       The dual layer samples (type I) with
varied top layer loadings were tested for NH₃ oxidation. The sample
with the single Pt/Al₂O₃ layer achieved the highest NH₃
conversion approaching 95% but had a poor N₂ selectivity (Fig 3).
The dual-layer sample with Cu/SSZ-13 loading of 3 g/in³ gave the
highest N₂ selectivity but had a lower NH₃ conversion
than the single layer sample (Fig 4). The results expectedly showed that the
increase in top zeolite layer loading increases the mass transfer
limitations  by restricting the amount of
NH₃ reaching the base Pt layer [2]. To resolve the problem, modified
dual-layer ASC is introduced.

            Figure 3: NH₃
conversion vs temperature                          Figure 4: N₂
selectivity vs temperature

       Two samples of almost same loadings were
prepared, a dual-layer ASC {Pt(10)CuZ(2.6)} and
modified dual-layer ASC {Pt(10)CuZ(2.7)} using the above-mentioned steps. The
modified dual-layer showed a notably higher N₂ selectivity and
somewhat improved  NH₃
conversion at high temperature (Fig 5 & 6).

  
         Figure 5: NH₃ conversion vs
temperature                          Figure 6: N₂
selectivity vs temperature       

        This is an ongoing study. The results
we get so far are quite encouraging and more experiments are being done to
check the consistency of reaction results. Most of the yeast or
poly-(tert-butyl acrylate) burn away at 550 ^oC (TGA data not shown
here for brevity). The catalyst characterization will be done to find if there
is any change in catalyst morphology due to yeast/polymer combustion.

References:

1.      Václavík, M., Dudák, M., Novák, V.,
Medlín, R., Štěpánek, F., Marek, M., & Kočí, P. (2014). Yeast
cells as macropore bio-templates enhancing transport properties and conversions
in coated catalyst layers for exhaust gas oxidation. Chemical Engineering Science.

2.      S. Shrestha, M. P. Harold, K.
Kamasamudram, A. Kumar, L. Olsson and K. Leistner, "Selective oxidation of
ammonia to nitrogen on bi-functional Cu-SSZ-13 and Pt/Al2O3 monolith
catalyst," Catalysis Today, 2016.