X-ray analysis of Pt?Sn catalysts supported on zinc-aluminium spinel

An x-ray diffraction analysis of Pt–Sn catalysts supported on zinc-aluminium spinel has shown that, after testing in paraffin dehydrogenation, they contain a tin-platinum solid solution with a face-centered cubic cell.

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, - , , .

     

Investigation of supported nickel catalysts by X-ray absorption spectrometry and X-ray diffraction using synchrotron radiation

In a previous paper a study of the supported nickel catalysts based on extended X-ray absorption fine structure spectroscopy was presented for analysis of the first coordination shell. The present study evidenced a strong deformation of the local structure of the metal due to its interaction with oxide support. The average particle size, microstrains and probability of faults, the particle size distribution function of supported Ni catalysts were determined by X-ray diffraction method. The method is based on Fourier analysis of experimental X-ray line profile (1 1 1), (2 0 0) and (2 2 0). The global structure is obtained with a fitting method based on the generalized Fermi function facilities for approximation. A chemisorption model was elaborated by correlation of the local and global structure connected with the specific surface areas. The results obtained on supported Ni catalysts which are used in H/D isotopic exchange reactions are reported. Both types of measurements were performed on the Beijing synchrotron radiation facilities.     

Determination of metal particle sizes in supported catalysts by small-angle X-ray scattering technique

Pt/SiO₂ catalysts have been studied by a small-angle X-ray scattering technique without commonly practiced packing of the carrier pores. The obtained Pt particle sizes are in good agreement with chemisorption and electron microscopy data.

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Pt/SiO₂ . Pt .

     

FT-IR emission studies of chemisorbed species on supported metal catalysts.

Infrared emission spectra of CO adsorbed on Rh/Al₂O₃ and Pt/Al₂O₃ were studied at 420K. The bands at 2092 and 2031 cm⁻¹ were assigned to symmetric and asymmetric stretching modes of surface Rh(CO)₂ groups, respectively. The RhC stretching modes are expected just below the range of observations (420 cm⁻¹. On Pt/Al₂O₃ the bands at 2043 and 457 cm⁻¹ were assigned to CO and PtC stretching vibrations of Pt-CO surface groups, respectively. Force constant calculations give 17.3 and 16.9 Ncm⁻¹ for CO stretching and 2.55 and 3.53 Ncm⁻¹ for MC stretching on the Rh and Pt catalysts, respectively.     

Kinetic modeling studies of ethylene polymerization reactions using supported chromium catalysts

Kinetic models for ethylene polymerization based on a general coordination–insertion mechanism, in which either a monocoordinated species or a bicoordinated species could lead to migratory insertion, were constructed. These models were implemented through the solution of a set of differential equations resulting from the material balances for all the species involved. The application of these kinetic models to monomer consumption for different supported catalysts produced very good fittings and allowed the estimation of the kinetic rate constants of each elementary step. Although the same kinetic scheme was used to describe all the observations, the results of the fitting showed that the supported chromium species behaved very differently according to the support. Only in the case of the silica‐supported catalysts was mechanical fragmentation of the particles observed during the course of the reaction, and this implied the inclusion of a new term in the model. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3464–3472, 2004     

X-ray diffraction studies of platinum-alumina catalysts in a high-temperature chamber

X-ray diffraction studies of platinum-alumina catalysts in a high-temperature chamber have revealed that for the catalysts obtained by the reduction of preactivated samples, no complete reduction of platinum takes place.

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, , , .

     

IR and X-ray photoelectron spectroscopy studies of polymer-supported palladium catalysts

IR and X-ray photoelectron spectroscopy studies of hydrogenation catalysts prepared by treating a phosphinated styrene-divinylbenzene copolymer with PdCl₂, have revealed two forms of supported palladium: bound Pd(II) and metallic Pd(0). The latter is shown to be the active component.

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, PdCl₂ . : - Pd(II) . , .

     

Crystallite size determination for supported metal catalysts by single X-ray profile fourier analysis

An X-ray diffraction method is presented capable of determining the average particle size, microstrains, the probability of faults as well as a particle size distribution function in crystalline materials. The method is based on the Fourier analysis of a single X-ray diffraction profile. Results obtained on supported platinum catalysts used in H/D isotopic exchange reactions are reported. On the basis of experimental spectra and our XRLINE1 code all structural parameters could be obtained.     

Crystallite size determination for supported metal catalysts by single X-ray profile fourier analysis

An X-ray diffraction method is presented capable of determining the average particle size, microstrains, the probability of faults as well as a particle size distribution function in crystalline materials. The method is based on the Fourier analysis of a single X-ray diffraction profile. Results obtained on supported platinum catalysts used in H/D isotopic exchange reactions are reported. On the basis of experimental spectra and our XRLINE1 code all structural parameters could be obtained.     

Studies of bulk and supported Cu?Cr catalysts in a high-temperature X-ray chamber

Bulk and -Al₂O₃-supported Cu–Cr catalysts have been studied in a high-temperature X-ray chamber in air, inert (He) and reducing (He+5% H₂) media. The different course of solid-phase reactions is due to the gas phase composition and the presence of a support.

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-Al₂O₃ , () (He+5%H₂) . , .

     

Diffuse reflectance spectroscopic studies of supported molybdenum oxide catalysts for propene metathesis

Diffuse reflectance spectra of 10 wt.% MoO₃ supported on -Al₂O₃ and SiO₂ have been measured after various steps of treatment. -Al₂O₃ was pretreated by 0.05 and 1.0 wt.% KOH. The comparison of spectral data and catalytic properties allows to predict the optimal oxidation state of molybdenum ions in surface clusters (approximately Mo⁵⁺), necessary to obtain supported MoO₃ catalysts for propene metathesis with high activity and selectivity.

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, -Al₂O₃ SiO₂ 10% . . -Al₂O₃ 0,05% 1,0% KOH. ( Mo⁵⁺), .

     

X-ray diffraction studies of reforming platinum-alumina catalysts in a high-temperature chamber

X-ray diffraction studies of platinum-alumina catalysts in a high-temperature chamber indicate that in a stream of hydrogen starting from 550–600 K, platinum is practically completely reduced. In several cases at higher temperatures a platinum hydride is also formed.

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, , 550–600 ; .

     

In situ X-ray diffraction studies of structural transformations in V-P-O catalysts

Structural transformations of V-P-O catalysts have been studied in situ in oxidative, inert and reducing atmosphere by the high-temperature X-ray diffraction method. Formation of vanadyl phosphates is shown to depend on the P/V ratio in the initial sample. It has been established that transformations in the phase composition of catalysts is independent of the reaction media at P/V=2. The effect of catalyst composition on catalytic properties is discussed.

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V-P-O , . , P/V . P/V=2. V-P-O .

     

Structural thermal and textural studies on cobalt oxide/alumina supported catalysts

CoO/Al₂O₃ catalysts containing amounts of cobalt ranging form 2 to 20% were prepared atpH 11 from neutral mesoporous alumina composed of γ-Al₂O₃ and poorly crystalline boehmite, and were then dried at 80?C. X-ray diffraction, DTA and TG techniques were used to study the structural changes produced upon thermal treatment up to 700?C. Soaking of the alumina in cobalt ammine complex solutions for a period of 10 days (the time required for equilibrium) resulted in a series of catalyst samples (I–V). Another sample (III-a) was soaked for a period of 5 days only in order to study the effect of the soaking time upon the equilibrium conditions. Cobalt aluminate (CoAl₂O₄) bands were characterized in all catalyst samples except III-a. They increased in intensity with increasing cobalt content. Surface species appeared in samples heated to 80?C, and others persisted at 150?C. Heating to temperatures above 200?C resulted in the formation of cobalt oxides, due to decomposition of the surface compounds. DTA and TG studies showed that this was more pronounced at higher concentrations of cobalt. Samples heated at 500?C and above did not undergo any further structural changes, except that the boehmite in the support was converted to γ-Al₂O₃. The variations in the surface parameters followed the same pattern as found previously [1], demonstrating that the catalyst samples are mesoporous, with retention of two ranges of pore size in most cases.     

Identification of CO adsorption sites in supported Pt catalysts using high-energy-resolution fluorescence detection X-ray spectroscopy

High-energy-resolution fluorescence detection (HERFD) X-ray spectroscopy is presented as a new tool for the identification of the bonding sites of reactants in supported metal catalysts. The type of adsorption site of CO on an alumina-supported platinum catalyst and the orbitals involved in the bonding are identified. Because X-ray absorption spectroscopy (XAS) is element-specific and can be used under high pressures and temperatures, in situ HERFD XAS can be applied to a swathe of catalytic systems, including alloys.     

129Xe nuclear magnetic resonance studies of supported platinum-alumina catalysts

¹²⁹Xe nuclear magnetic resonance spectra of xenon adsorbed on supported platinum-alumina catalysts have been examined. The state of metallic platinum particles (Pt^o) was shown to be different for Pt/Al₂O₃ and Pt/Al₂O₃(Cl) samples. Some assumptions about the mutual arrangement of supported metallic particles have been made.

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¹²⁹Xe , , Pt⁰ Pt(II). , (Pt⁰) , . Pt⁰ Pt(II).

     

X-ray microtomography studies of nascent polyolefin particles polymerized over magnesium chloride-supported catalysts

Drastic changes occur during the initial stages of the α-olefin polymerization over heterogeneous catalysts. Fragmentation of the support takes place as polymer is formed at the active sites within the voids of the support/catalyst. Magnesium chloride-supported titanium catalyst/polymer particles have been analyzed employing high-resolution computed microtomography (CMT) using synchrotron radiation at Brookhaven National Laboratory. The changes in morphology, the spatial distribution of the support/catalyst fragments, porosity, and polymer distribution in single growing polypropylene and polyethylene particles have been studied. These studies documented considerable macroporosity ( > 2 μm in size) within the growing catalyst/support/polymer particles. The largest pores may be due to agglomeration of smaller subparticles. Our results confirm that the initial fragmentation of the support proceeds readily and uniformly to yield a multi-grain growth of subparticle agglomerates. The support/catalyst fragments appear to be distributed relatively uniformly within the growing polymer particle. The surface of the subparticle agglomerates is accessible through the void-space between growing catalyst/particle grains. This may facilitate monomer transport to the activate sites through the polymer/catalyst particles. © 1993 John Wiley & Sons, Inc.     

Ethylene polymerization studies with supported cyclopentadienyl,arene, and allyl chromium catalysts

Highly active catalysts for low pressure ethylene polymerization are formed when chromocene, bis (benzene)- or bis (cumene)-chromium or tris- or bis (allyl)-chromium compounds are deposited on high surface area silica-alumina or silica supports. Each catalyst type shows its own unique behavior in preparation, polymerization, activity, isomerization, and response to hydrogen as a chain transfer agent. The arene chromium compounds require an acidic support (silicaalumina) or thermal aging with silica to form a highly active catalyst. At 90°C polymerization temperature arene chromium catalysts produced high molecular weight polyethylene and showed, in contrast to supported chromocene catalysts, a much lower response to hydrogen as a chain transfer agent. An increase in polymerization temperature caused a significant decrease in polymer molecular weight. Addition of cyclopentadiene to supported bis (cumene)-chromium catalyst led to a new catalyst which showed a chain transfer response to hydrogen typical of a supported chromocene catalyst. Polymerization activity with tris- or bis (allyl)-chromium appears to depend on the divalent chromium content in the catalyst. Changes in the silica dehydration temperature of supported allyl chromium catalyst have a significant effect on the resulting polymer molecular weight. High molecular weight polymers were formed with catalysts that were prepared using silica dehydration temperatures below about 400°C. Dimers, trimers, and oligomers of ethylene were usually formed with catalysts that were prepared on silica dehydrated much above 400°C. The order of activity of the different types of catalysts was chromocene/silica > chromocene/silica-alumina > bis (arene)-chromium/silica-alumina ? allyl chromium/silica.     

New masking liquids for the analysis of supported catalysts and functional materials by small-angle X-ray scattering

A new contrasting procedure is proposed for studying supported catalysts and functional materials by small-angle scattering. The main advantage over earlier analogues is the use of new masking liquids, which possess very low X-ray absorption at a relatively high density. A number of supported catalysts were studied using the new procedure, and almost complete agreement of the particle-size distributions of supported metals with transmission electron-microscopic data was demonstrated. The use of the new contrasting procedure on a typical small-angle diffractometer makes it possible to obtain the particle-size distributions of metals in supported catalysts containing noble metals at a level to 0.1 wt %.