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Effect of Zn addition on the water–gas shift reaction over supported palladium catalysts

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Abstract.
The effect of Zn addition on the Pd turnover rate (TOR) for the water–gas shift (WGS) reaction was
studied on 2 wt% Pd on alumina with Zn content varying from 0 to 19% Zn by weight. Over 85% of
the Zn reacted with alumina to form zinc aluminate with a fraction reacting with Pd. The average Pd–
Zn coordination number as measured by extended X-ray absorption fine structure (EXAFS) increased
monotonically from 0 to 4.5 as the Zn loading on alumina increased. At a loading of 2% Pd and 19% Zn
on alumina the molar fraction of Zn in the Pd particles was 0.34, and the turnover rate increased by
a factor of 13, as compared with the pure metallic Pd supported on alumina, to 3.6 × 10−2 molH2 s−1
(mol exposed Pd)−1 at 280 ◦C, 6.8% CO, 8.5% CO2, 21.9% H2 O, and 37.4% H2 at 1 atm. The formation of
the ordered 1:1 PdZn intermetallic compound by deposition of Pd on zinc oxide increased the average
Pd–Zn coordination number to 5.9 and the WGS turnover rate by a factor of 20 to 5.3×10−2 molH2 s−1
(mol exposed Pd)−1. The bonding of adsorbed CO changed from 80% bridging on pure Pd to 90% linear
on the 1:1 PdZn intermetallic compound as determined by diffuse reflectance infrared spectroscopy
performed under WGS conditions. The fraction of CO binding linearly to Pd correlated linearly with the
WGS rate per Pd on the surface. Because the dispersion of the samples was about the same, this result
implies that the number of active sites can be counted by the fraction of CO binding linearly. A simple
random substitution model for PdZn distribution was used to estimate the concentration of Pd and Zn
on the surface, based on the coordination numbers measured by EXAFS. The correlation of the reaction
rate with the types of sites on catalysts with six different Pd–Zn compositions shows that the data is
consistent with an active site composed of surface Pd atoms with surface coordination to 2 Pd and 4 Zn
atoms, which is the coordination expected for the (111) plane of the 1:1 PdZn intermetallic compound.
An alternate explanation for the promotional effect of Zn on Pd/alumina is that the formation of zinc
aluminate breaks a deleterious Pd–alumina interaction.

Keywords:
Water–gas shift
Methanation
Palladium catalysts
Fuel cells
Zinc oxide
Catalyst promoters
Kinetics
Pd–Zn alloys
In situ IR spectroscopy
EXAFS

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Informasi Detail

Judul Seri

Journal of Catalysis

No. Panggil

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Penerbit

London : Elsevier Inc.., 2008

Deskripsi Fisik

Journal of Catalysis 257 (2008) 43–54

Bahasa

English

ISBN/ISSN

00219517

Klasifikasi

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Tipe Isi

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Tipe Media

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Tipe Pembawa

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Edisi

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Subjek

KIMIA

Info Detail Spesifik

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Pernyataan Tanggungjawab

agus

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