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Iwasa, Nobuhiro; Mayanagi, Tomoyuki; Masuda, Satoshi; Takezawa, Nobutsune

doi:10.1023/a:1005668404401

Cited by 76 publications

(52 citation statements)

References 2 publications

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“…In line with previous studies on PdZn formation 8,[12][13][14] , the present electron microscopy study unambiguously reveals the onset of the formation of a tetragonal PdZn phase upon reduction at T ≥ 473 K. The formation of the alloy phase proceeds most likely via topotactic formation of PdZn on top of the Pd particles. This growth mechanism is probably favoured by the relatively small mismatch of the lattice constants of fcc Pd (0.388 nm) and tetragonal PdZn (0.410 nm).…”

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confidence: 91%

Growth and structural stability of well-ordered PdZn alloy nanoparticles

Penner

Jenewein

Gabasch

et al. 2006

Journal of Catalysis

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Despite many recent attempts to unravel the structure of novel PdZn alloys, promising catalysts in methanol steam reforming, a detailed study on the formation of Pd-Zn alloy particles and of their structural and thermal stability is still missing. We take advantage of the unique properties of epitaxially grown Pd particles embedded in layers of amorphous ZnO and mechanically stabilized by SiO 2 , and present an electron microscopy study of the preparation of well-ordered PdZn alloy nanoparticles at surprisingly low reduction temperatures. They are formed by topotactic growth on the surface of the Pd nanoparticles and are structurally and thermally stable in a broad temperature regime (473 -873 K). At and above 873K, partial decomposition of PdZn and beginning interaction with the SiO 2 support has been observed. Keywords:Thin film model catalyst, hydrogen reduction, alloy formation, electron microscopy, selected area electron diffraction ManuscriptMuch recent effort has been invested in the development of suitable catalysts for methanol steam reforming 1-15 as this is one of the most promising processes for hydrogen production with a high hydrogen-to-carbon ratio 16 . Cu/ZnO catalysts have been commercially used to produce hydrogen with high selectivity and activity 2-6 , but they suffer from deactivation at reaction temperatures above 573 K 17 . Recently, novel Pd/ZnO systems 3-15 (as well as Pd/Ga 2 O 3 and Pd/In 2 O 3 18,19 ) have attracted more attention because of their enhanced long-term and thermal stability 20,21 . As unsupported pure Pd exhibits only a poor selectivity 22 , the observed high activity and selectivity for CO 2 formation was ascribed to the formation of distinct PdZn, PdIn and PdGa alloys upon reductive activation at elevated temperatures 18 . Best characterized is the Pd/ZnO system, where alloy formation has been studied and confirmed by X-ray diffraction (XRD) 18,23 , temperature-programmed reduction (TPR) 18,23 and X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) [24][25][26] . Iwasa et al. 23 observed PdZn alloy formation upon reduction at very low T (≥ 473 K). Density functional studies revealed the close relationship between the electronic structure of PdZn and Cu-based catalysts giving rise to a similar catalytic performance in methanol steam reforming 27,28 . Comparatively few studies have been carried out on the structural characterization of the PdZn alloys by electron microscopy, and these were limited to overview imaging of powder catalysts in the as-prepared state and after hydrogen reduction, thereby mainly supporting XRD measurements 29,30 . The Pd-Zn system is known to form several stable bulk alloy phases 31 . The most important and thermally most stable is the PdZn (β1) phase, which crystallizes in a tetragonal (AuCu-type) L1 0 structure 32 . A key point for understanding the catalytic pecularities of the above-mentioned Pd-Zn alloy particles is also to determine their surface composition. Recent experiments in our laboratory 33 show that a well-orde...

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confidence: 91%

Growth and structural stability of well-ordered PdZn alloy nanoparticles

Penner

Jenewein

Gabasch

et al. 2006

Journal of Catalysis

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“…This result can be due to the fact that the tetrabridged structure of complex 1 is rather stable even at elevated temperatures, up to 100-150°C, keeping the Pd and Zn atoms together under conditions of hydrogen treatment (150-250°C). The reduction easily proceeds at low H 2 partial pressures (0.05-0.10 atm), under much milder conditions than have been used in traditional preparation procedures [9,11]. These facts demonstrate the advantage of using heterometallic carboxylate complexes as parent compounds for the synthesis of nanoalloys and mixed-metal catalysts.…”

Section: Discussionmentioning

confidence: 92%

“…The Pd-Zn catalytic system deserves a special interest because it is highly active and selective in methanol synthesis, acetylene hydrogenation, steam reforming and dehydrogenation of methanol, and related industrial processes [8][9][10][11][12]. Pd-Zn alloys, which are thought to be crucial for the catalytic behavior, are known to form upon reduction of the Pd/ZnO catalyst at rather high temperatures ( ‡ 500°C) when the catalyst was prepared according to the traditional procedure from the component salts.…”

Section: Introductionmentioning

confidence: 99%

An easy way to Pd–Zn nanoalloy with defined composition from a heterobimetallic Pd(μ–OOCMe)4Zn(OH2) complex as evidenced by XAFS and XRD

et al. 2006

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“…Iwasa et al were the first to report that Pd became highly active for reforming and selective towards CO 2 when supported on ZnO and reduced at temperatures above 300 ºC [4][5][6][7][8][9][10]. This was attributed to the bulk PdZn alloy which was formed by the spill-over of atomic hydrogen from the Pd metal to the ZnO, leading to facile reduction of the ZnO and migration of Zn to the metallic surface [5,9].…”

Section: Introductionmentioning

confidence: 99%

Stability of bimetallic Pd–Zn catalysts for the steam reforming of methanol

Conant

Karim

Lebarbier

et al. 2008

Journal of Catalysis

184

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ZnO-supported palladium-based catalysts have been shown in recent years to be both active and selective towards the steam reforming of methanol, although they are still considered to be less active than traditional copper-based catalysts. The activity of PdZn catalysts can be significantly improved by supporting them on alumina. Here we show that the Pd/ZnO/Al 2 O 3 catalysts have better long-term stability when compared with commercial Cu/ZnO/Al 2 O 3 catalysts, and that they are also stable under redox cycling. The Pd/ZnO/Al 2 O 3 catalysts can be easily regenerated by oxidation in air at 420 ºC followed by re-exposure to reaction conditions at 250 ºC, while the Cu/ZnO based catalysts do not recover their activity after oxidation. Reduction at high temperatures (>420 ºC) leads to Zn loss from the alloy nanoparticle surface resulting in a reduced catalyst activity. However, even after such extreme treatment, the catalyst activity is regained with time on stream under reaction conditions alone, leading to highly stable catalysts. These findings illustrate that the nanoparticle surface is dynamic and changes drastically depending on the environment, and that elevated reduction temperatures are not necessary to achieve high CO 2 selectivity.

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Cited by 76 publications

References 2 publications

Growth and structural stability of well-ordered PdZn alloy nanoparticles

Growth and structural stability of well-ordered PdZn alloy nanoparticles

An easy way to Pd–Zn nanoalloy with defined composition from a heterobimetallic Pd(μ–OOCMe)4Zn(OH2) complex as evidenced by XAFS and XRD

Stability of bimetallic Pd–Zn catalysts for the steam reforming of methanol

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