Dispersion changes and reactivity of platinum on silica during thermal treatment under vacuum and at low oxygen pressure

Wl,; adysŁ,; Romanowski, aw; Lamber, Ryszard

doi:10.1016/0040-6090(85)90220-2

Cited by 18 publications

(7 citation statements)

References 31 publications

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“…As the observations on Pt/ceria are very similar to those on Pt/silica and Pt/alumina, the formation of a Pt 3 Ce alloy is considered most likely to occur under the given experimental conditions. The formation of sharp-edged particles upon high-temperature reduction of (silica) supported Pt particles has been reported previously [7,8] and has been explained by a surface reconstruction under the influence of hydrogen, leading the system to a minimum surface energy [8]. However, alloy formation has also been reported, e.g.…”

Section: Resultsmentioning

confidence: 61%

Pt/ceria thin film model catalysts after high-temperature reduction: a (HR)TEM study

Penner

Rupprechter

Sauer

et al. 2003

Vacuum

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noble metals are widely used as valuable catalysts in deNOx and combustion technology. However, like most noble metal-reducible oxide combinations they are subjected to strong metal-support interaction upon high temperature reduction, resulting in changing catalytic properties [1] and eventually deactivation. An investigation of these phenomena and of structureactivity correlations is greatly facilitated by model systems with planar thin supports. In the present work, well-facetted Pt particles, obtained by epitaxial growth on (001) NaCl surfaces, were supported by thin films of crystalline ceria. These model catalysts are well suited for characterisation by electron microscopy and provide a defined reference state for studies of structural changes during catalyst treatment and reaction. Catalyst changes due to calcination and reduction have been studied after treating the samples in oxygen (673K, 1h), and after hydrogen exposure at increasing temperatures from 673K to 1073K for 1h, either in a flow system or in a circulating batch reactor. The morphology and composition were followed by (HR)TEM, electron diffraction and EELS. In their initial state, most Pt particles are truncated half octahedra with (001) faces perpendicular to the electron beam, exhibiting square or rectangular shapes at lower magnification [2]. They are surrounded by grains of CeO , partly in epitaxial correlation to the metal. In contrast, after a 2 reductive treatment at 723 K, Pt particles embedded in ceria recrystallise into a cubic or plateletlike structure, thereby losing in part their original azimuthal orientation. Some particles exhibit round edges but have increased in size, obviously due to agglomeration. At this reduction stage the bulk stoichiometry of the ceria support is still unchanged. With rising reduction temperature the average particle size increases but the platelet or cube like structure persists. On Pt/silica and Pt/alumina a similar transformation of the Pt particles was observed to occur at higher temperature and possibly connected with alloy formation [3]. These phase transformation are reversible by low-temperature oxidation and can be repeated by subsequent reduction. At present, HREM, image simulations and EELS measurements are carried out in order determine the origin of this effect. Further heating of the ceria-based Pt catalyst in hydrogen up to 1073K results in clustering of the Pt particles on large flat areas of a sintered support of partly reduced ceria, exhibiting large lattice periodicities between 0.8 and 1.2 nm. EELS measurements confirm the formation of a chemically stable Ce (III) oxide. These structural changes are connected to changes of the catalytic properties [4], as will be exemplified and discussed for the CO hydrogenation. This work was supported by the Austrian Science Fund (Project S 8105)[1] A. Trovarelli, Catal. Rev

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Section: Resultsmentioning

confidence: 61%

Pt/ceria thin film model catalysts after high-temperature reduction: a (HR)TEM study

Penner

Rupprechter

Sauer

et al. 2003

Vacuum

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“…Cube-like structures with sharp edges have been already reported before high-resolution EM became available [11,12], and were attributed to surface reconstruction under the influence of hydrogen, leading the system to minimum surface energy [13]. That alloy formation has to be taken into account was shown by Lamber et al [5] who observed diffraction features compatible with Pt 3 Si after heating a Pt/silica model catalyst in hydrogen at 840 K. Under the given experimental conditions alloy formation is thermodynamically feasible for all three described systems [14].…”

Section: Discussionmentioning

confidence: 99%

Platinum nanocrystals supported by silica, alumina and ceria: metal–support interaction due to high-temperature reduction in hydrogen

Penner

Wang

et al. 2003

Surface Science

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Regular Pt nanoparticles, obtained by epitaxial deposition on NaCl surfaces, were supported by thin films of silica, alumina and ceria and subjected to hydrogen reduction at temperatures up to 1073 K. The changes in morphology and composition were followed by (HR)TEM, electron diffraction and EELS, and the results were supported by theoretical calculations. The structural changes of the Pt particles upon reduction at 773 K and above are surprisingly similar despite the differing chemical properties of the three supports. Some platelet-and cube-like geometries exhibit double lattice periodicities in high resolution images and electron diffraction patterns. With increasing reduction larger aggregates of more complex appearance and structure are formed. Surface reconstruction under hydrogen and alloy formation are considered as responsible for this effect. Most likely, the first step is identical on all three systems and consists in the topotactic formation of Pt rich Pt3Me (Me = Si, Al, Ce) under the influence of hydrogen, followed by transformation into diverging structures of lower Pt content and different crystallography. Density functional calculations were performed for deriving energies of formation of PtMe and Pt3Me compounds.

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“…The formation of silicide under such conditions is rather surprising, though the presence of metal silicides in Pt/SiO 2 [1], Rh/SiO 2 [10] and Cu/SiO 2 [11] systems after annealing in vacuum was reported. It is known that high vacuum is a reducing environment because hydrogen constitutes the main part of the residual gas.…”

Section: Methodsmentioning

confidence: 98%

“…2) contains number of rings that could not be assigned to any form of metallic Co, cobalt oxide or cobalt silicate but which fit well the Co 2 Si with orthorhombic structure (ICDD file 01-073-6828) (c.f. Table 1 Co 2 Si in [1][2][3][4][5][6][7][8][9][10] orientation. Another example of Co 2 Si particle in [-101] orientation is shown in Fig.…”

Section: Methodsmentioning

confidence: 99%

“…An example is the formation of Pt, Pd, Ni and Rh silicides during high temperature treatment of metal-SiO 2 binary systems [1][2][3][4][5][6][7][8][9][10]. Usually, the process occurs in reducing gas atmosphere, though the formation of Pt [1], Rh [10] and Cu [11] silicides under vacuum was also reported. The strength of the metal-silica interaction is very sensitive to small changes of ambient experimental conditions.…”

Section: Introductionmentioning

confidence: 98%

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Interaction of Co nanoparticles with SiO2: silicide formation

Potoczna-Petru

Kępiński

Krajczyk

2009

React Kinet Catal Lett

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Dispersion changes and reactivity of platinum on silica during thermal treatment under vacuum and at low oxygen pressure

Cited by 18 publications

References 31 publications

Pt/ceria thin film model catalysts after high-temperature reduction: a (HR)TEM study

Pt/ceria thin film model catalysts after high-temperature reduction: a (HR)TEM study

Platinum nanocrystals supported by silica, alumina and ceria: metal–support interaction due to high-temperature reduction in hydrogen

Interaction of Co nanoparticles with SiO2: silicide formation

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