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

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

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Evaluation, and Optimization

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

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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

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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 (PtO_x)
exposing the more active metallic platinum in the CS catalyst, supported by
independent temperature programmed reduction (H₂-TPR) measurements.

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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.

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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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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)