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Iwasa, Nobuhiro; Mayanagi, Tomoyuki; Ogawa, Noriaki; Sakata, Kentaro; Takezawa, Nobutsune

doi:10.1023/a:1019056728333

Cited by 218 publications

(159 citation statements)

References 12 publications

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“…A correlation to systems in which hydride formation is excluded under otherwise similar reaction conditions may be particularly helpful. It is known that adding ZnO to a silica-supported Pd catalyst suppresses hydride formation over a wide range of temperatures [33]. This fact has been recently elucidated by the present authors [34], who showed that a topotactic, well-ordered PdZn alloy phase forms on regular Pd nanoparticles under reductive conditions.…”

Section: Introductionsupporting

confidence: 57%

Hydride formation and stability on a Pd–SiO2 thin-film model catalyst studied by TEM and SAED

Jenewein¹,

Penner²,

Gabasch³

et al. 2006

Journal of Catalysis

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Hydride formation was studied on Pd particles supported on SiO 2 , and the results were evaluated with reference to a corresponding ZnO-promoted Pd/SiO 2 catalyst reported recently. Pd particles (mean size, ~5 nm) were epitaxially grown on NaCl(001) cleavage faces and subsequently covered by a layer of amorphous SiO 2 , thereby maintaining their epitaxial orientation. The films were subjected to various hydrogen and annealing treatments in the temperature regime of 373-873 K, and their stability in an O 2 environment (373-573 K) was also tested. The structural and morphological changes were followed by transmission electron microscopy and selected area electron diffraction. Reduction at 523 K increased the mean particle size considerably and converted the Pd particles into an amorphous hydride phase that decomposed at and above 673 K either by reduction in hydrogen or by annealing in He environment, forming a crystalline Pd 2 Si phase. Oxidative treatments of the Pd-H phase at temperatures above 523 K induce transformation into partially oriented Pd particles. The Pd 2 Si phase was stable under reductive conditions up to 873 K and decomposed into disoriented Pd particles on oxidation at temperatures at and above 573 K. Although the Pd/SiO 2 catalyst can be readily converted into an amorphous hydride phase, it loses its hydrogen storage capability on entering the silicide state at 673 K, and hence no hydride formation was observed. These results are in contrast to previous observations on a corresponding Pd/ZnO/SiO 2 catalyst, where the formation of a well-ordered PdZn alloy prevented the formation of the hydride phase and significantly enhanced the structural stability of the catalyst and its resistance against sintering. In contrast, the Pd/SiO 2 system was converted into the amorphous hydride state under "real" catalytic conditions, for example, during CO 2 hydrogenation at around 523 K.

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Section: Introductionsupporting

confidence: 57%

Hydride formation and stability on a Pd–SiO2 thin-film model catalyst studied by TEM and SAED

Jenewein¹,

Penner²,

Gabasch³

et al. 2006

Journal of Catalysis

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“…The strong particle stabilization effect (SPSE) is of importance for catalytic processes such as methanol steam reforming and methanol synthesis on Pd/ZnO catalysts. In particular, the striking differences between the Pd/SiO 2 and Pd/ZnO/SiO 2 systems demonstrated in this work may lead to a better understanding of the drastic activity and selectivity differences between Pd/SiO 2 and Pd/ZnO reported in the literature 13,23 . According to recent single crystal experiments 33,38 and theory studies 38,39 it seems that this very stable PdZn alloy phase of stoichiometric surface composition is stable over a wide range of temperatures.…”

mentioning

confidence: 60%

“…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).…”

mentioning

confidence: 99%

“…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 .…”

mentioning

confidence: 99%

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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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“…However, instability of this catalyst under fuel processing conditions at elevated temperature has been a recurring problem. Among the precious metal and other group VIII metal catalysts, Pd/ZnO [16][17][18] and Pd-In/Al 2 O 3 [19] catalyst reduced at >300°C gives exceptionally high activity and selectivity in the SRM reaction, but a high Pd loading of~10 wt.% is required to achieve the activity and selectivity with a H 2 yield comparable to the CuZnAl catalyst. Therefore, there is a growing incentive to develop non-precious metal and non-copper catalysts with a particular emphasis on low cost and increasing catalytic activity and selectivity that can operate at high space velocity with high methanol conversion, high selectivity to H 2 and low CO selectivity.…”

Section: Introductionmentioning

confidence: 99%