(762d) Core-Shell Pt/Al2O3@Cu/ZSM-5 Catalyst for Selective NH3 Oxidation: Synthesis, Evaluation, and Optimization

Authors: 
Rajat, G., University of Houston
Harold, M., University of Houston
Wang, D., Cummins Inc.
Le, T. T., University of Houston
Rimer, J. D., University of Houston
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Ghosh, Rajat-Subhra Normal Ghosh, Rajat-Subhra 2 1 2019-04-09T16:11:00Z 2019-04-09T16:11:00Z 1 998 5692 47 13 6677 16.00

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normal">Core-Shell
Pt/Al2O3@Cu/ZSM-5 Catalyst for Selective NH3
Oxidation:

normal">Synthesis,
Evaluation, and Optimization

normal">

Rajat
Subhra Ghosh*,
M. P. Harold*, Thuy Le*, J.D. Rimer*, and D. Wang**

left">

normal">*Dept.
of Chemical and Biomolecular Engineering, University of Houston, Houston, TX
77204-4004, USA

normal">**Cummins
Inc., 1900 McKinely Av., MC50197, Columbus, IN 47201,
USA

normal"> " times new roman>

text-align:left"> 11.0pt;mso-bidi-font-size:12.0pt;font-family:" times new roman>Introduction

justify;text-indent:.5in;line-height:normal">The ammonia slip
catalyst (ASC) is an essential aftertreatment unit in modern diesel vehicles.
The purpose of ASC is to prevent the release of unreacted NH3
leaving a selective catalytic reduction (SCR) unit. The state-of-the-art ASC
for selective oxidation of NH3 to N2 has a dual-layer
architecture comprising a Pt/Al2O3 bottom layer (PGM) and
a Cu/Fe-exchanged zeolitic (M-Z) top layer [1]. The dual-layer structure enables
a high N2 selectivity at high NH3 conversion. It does so
by converting NO, one of the main oxidation products of the PGM, through
reaction with counter-diffusing NH3. While recent studies
demonstrate the effectiveness of the dual layer design [2], further advances
are needed to reduce the PGM loading and ASC volume while enhancing low
temperature activity.

justify;text-indent:.5in;line-height:normal">In this work, the dual-layer concept
is scaled down to the level of a single catalyst particle with the intent to
meet the above-mentioned challenges. A core-shell (CS) particle comprising a
PGM core and a M-Z shell, specifically Pt/Al2O3@Cu/ZSM-5
(Figure 1a), was synthesized. Reaction testing of the CS catalyst showed
enhanced low temperature activity without compromising N2
selectivity on comparing with a physical mixture of catalyst having same Pt and
Cu loading. The CS catalyst had comparable light-off to a Pt/Al2O3
catalyst having 3x Pt loading. The enhanced activity is attributed to the
de-stabilization of platinum oxide (PtOx)
exposing the more active metallic platinum in the CS catalyst, supported by
independent temperature programmed reduction (H2-TPR) measurements.

justify;text-indent:.5in;line-height:normal">

text-align:left"> 11.0pt;mso-bidi-font-size:12.0pt;font-family:" times new roman>Materials
and methods

Core-shell
particle synthesis: 12.0pt;font-family:" times new roman>A Pt/Al2O3[Pt(0.05 w/w)/Al2O3] catalyst serving
as the core particle was synthesized by incipient wetness impregnation using tetraamine
platinum nitrate font-family:" times new roman>. Synthesis of the shell involves seeding
of core particles, followed by secondary growth [3]. The CS particle was
sequentially ion exchanged with NH4NO3 and Cu(NO3)2 to obtain the Cu-ZSM-5 shell
layer.

Reaction
testing: font-family:" times new roman> A bench flow fixed-bed reactor, described
elsewhere [2], was used to evaluate the CS powder catalyst. The feed containing
500 ppm NH3 + 5% O2 (balance Ar)
had a space velocity of 280k h-1. A FT-IR was used to measure effluent NO, NO2,
N2O, and NH3.

text-align:left"> font-family:" times new roman>

12.0pt;font-family:" times new roman>Results and Discussion

Catalyst
characterization: 10.0pt;font-family:" times new roman> The Si/Al ratio (SAR) of the
zeolite obtained from XPS and EDX data was found to be ~30. Figure 1b shows a SEM
image of prepared CS particles which are 40-50 microns in diameter with a shell
thickness of 1-1.5 microns. From EDX, the Cu weight percent in the shell is ~
2.9. Powder XRD confirmed the MFI lattice structure of the shell. " times new roman>

NH3
oxidation: NH3
oxidation was carried out on 4 samples: Pt(0.15)/Al2O3 [P(0.15)-A],
Pt(0.05)/Al2O3 [P(0.05)-A], physical mixture of P(0.05)-A
+ ‘x’ wt% Cu(2.9%)-ZSM-5 [CuZ,
x = 10, 20], and the core-shell Pt/Al2O3@Cu(2.9%)-ZSM-5 [CS].
The NH3 oxidation light-off temperatures (T50) were as follows: P(0.15)-A(260°C);
P(0.05)-A(310°C); P(0.05)-A+CuZ(~300°C) and CS (250°C),
as shown in Figure 1d. The T50 of 250°C for the core-shell sample was 50°C
lower than the samples having the same amount of Pt. Remarkably, the CS
light-off temperature was ~5°C less than the sample containing 3x higher Pt(P(0.15)-A).
Moreover, the CS achieved 100% conversion at much lower temperature (325°C)
than P(0.05)-A (450°C). The lower light off for CS
catalyst was due to the increase in NH3 oxidation activity in the
core of the catalyst. Reaction testing for steady state ammonia oxidation over
different stages of synthesis (core mso-ascii-font-family:" times new roman mso-bidi-font-family: wingdings>àseeded core symbol;mso-symbol-font-family:Wingdings">à " times new roman>CS) as seen in Figure 1f. The seeded core (P(0.05)-AS)
had a light off at 215°C, which is 95°C lower than P(0.05)-A.
The CS catalyst with fully developed shell of ZSM-5 lowered the light off to
250°C due to the diffusion resistance which is still lower than P(0.05)-A. The P(0.05)-A and
P(0.05)-AS have platinum dispersions of 39 and 25 %, respectively. The
dispersion measurement does not explain the lowering of light off. H2-TPR
showed the formation of H2O peaks at lower temperature for P(0.05)-AS compared to P(0.15)-A (Figure 1g). The oxides of
Pt are destabilized in seeded core, making it more susceptible to reduction
compared to P(0.05)-A. This exposes more metallic Pt
in the case of seeded core making it more reactive [4]. The destabilization
effect of platinum oxides is due to the modified metal-support interactions
[5,6]. The combined factors of destabilizing effect of platinum oxides,
suppression of platinum oxide formation due to the modification of γ -Al2O3
surface leads to the higher activity of the seeded core.

N2
selectivity: The CS
catalyst shows a very high N2 selectivity, achieving a maximum N2
yield of 95% at 300°C, with negligible N2O formation (yield < 3%)
(Figure 1c). At higher temperature (500°C), N2 yield drops to ~ 90 %
which is much higher than P(0.05)-A + CuZ physical mixture (Figure 1e). The high N2
selectivity of CS catalyst is due to the SCR chemistry taking place in the
Cu-ZSM5 shell [2].



These findings not only confirm
that the dual-layer ASC concept can be achieved at the single particle level,
but there is an apparent added advantage of lower light-off at reduced PGM
loading not obtained previously.

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normal">Figure
1:
(a)
Core-shell (CS) particle schematic (b) SEM micrograph of synthesized CS
catalyst (c) Steady state NH3 conversion and product yield for CS
catalyst (d & e )Steady state NH3 conversion/N2 yield
for various samples, CS, P(0.05)A, P(0.05)A + 10 wt %
Cu(2.9)-Z, P(0.05)A + 20 wt % Cu(2.9)-Z, P(0.15)A (f)
Steady state NH3 conversion over different stages of synthesis, core
[P(0.05)-A], seeded-core [P(0.05)-AS] and CS catalysts.[Feed: 500 ppm NH3,
5 % O2, balance Ar, 280 K h-1
and 0.18 g of catalyst] (g) Transient H2-TPR behavior as a function
of time over P(0.05)-AS (seeded core), P(0.05)-A and Al2O3
with 1 % H2, balance Ar, 30sccm and 100 mg
of catalyst.

normal">References

[1]        S. Shrestha et al., Topics in Catalysis,
56, 182-186 (2013)

[2]        S. Shrestha et al., Catalysis Today,
267, 130-144 (2016)

[3]        Ding W. et al., Chemie-Ingenieur-Technik, 87, 702-712 (2015)

[4]        Völter J. et
al., Journal of Catalysis, 104, 375-380 (1987)

[5]        Huizinga T., et al., Applied Catalysis,
10, 199-213 (1984)

[6]        Yazawa, Y., et al., Applied Catalysis A: General, 237,
149-148 (2002)