Electronic silica-supported

The performance of a supported metal or metal sulfide catalyst depends on the details of its preparation and pretreatraent. For petroleum refining applications, these catalysts are activated by reduction and/or sulfidation of an oxide precursor. The amount of the catalytic component converted to the active ase cind the dispersion of the active component are important factors in determining the catalytic performance of these materials. This investigation examines the process of reduction and sulfidation on unsupported 00 04 and silica-supported CO3O4 catalysts with different C03O4 dispersions. The C03O4 particle sizes were determined with electron microscopy. X-ray diffraction (XRD), emd... 

The author wishes to thank Dr. J.B. Cohen for supplying samples of Pt and Pd on alumina and silica and Drs. J. Schwank and A.K. Dayte for samples of Ru and Au on magnesia and silica. This work was supported by the US Department of Energy under Contract DMR-76ER02995 and has make use of the resources of the ASU Faoiltity for High Resolution Electron Microscopy, supported by NSF grant DMR 8306501. 

An apparent particle size effect for the hydrodechlorination of 2-chlorophenol and 2,4-dichlorophenol was observed by Keane et al. [147], Investigating silica supported Ni catalysts (derived from either nickel nitrate or nickel ethane-diamine) with particles in the size range between 1.4 and 16.8 nm, enhanced rates for both reactions were observed with increased size over the full range (Figure 13). As electronic factors can be ruled out in this dimension, the observed behavior is traced back to some sort of ensemble effect, known from CFC transformations over Pd/Al203... 

Figure 2. Electron diffraction patterns of silica supported Pt particles (a) as-grown and (b) after the reduction in El2 at 873 K.

The three-dimensional electron tomographical construction of silica-supported metallocene catalysts using conventional TEM (Steinmetz et al 2000), and a novel method for the automated acquisition of tilt series for electron tomography of nanoparticles using STEM have been reported (Zeisse et al 2000). The HAADE-STEM is shown to be capable of determining the compositions of individual nanoparticle catalysts of a few atoms supported on porous substrates (Vaughan et al 1999). 

Thermal activation of either pure (191, 192) or silica-supported Ce02 (45, 191) at 500°C in vacuo gives a signal at gav = 1.963 which has been attributed to Ce3+ ions or to electrons in the solid. The adsorption of oxygen on pure Ce02 gives a poorly resolved signal with g = 2.0312 and = 2.0137... 

Ai85,86 is discussed on p. 114. Agarwal et al.102 as well as Sharma et al.103 studied this reaction using silica-supported V2Os-alkali metal sulphate catalysts. A two-step oxidation-reduction mechanism gave the best description of the process. The activity increased with increasing atomic number of the added alkali metal for which no interpretation was offered. In an electron microscopic study of these catalysts Sharma et al.103 showed that K2 S04 and V2 05 are present as separate phases but that the sulphate causes the presence of a larger amount of V2 05 in the form of needle-like crystals which appear to be more active for the methanol oxidation. A similar result was obtained by these authors for catalytic oxidation of toluene over these catalysts.104... 

Fig. 7.1 Transmission electron microscopy images of a spherical model for a silica supported model catalysts for Fischer—Tropsch synthesis.

Catalytic superactivity of electron-deficient Pd for neopentane conversion was recently verified for Pd/NaHY (157, 170). The reaction rate was positively correlated with the proton content of the catalyst. Samples that contained all the protons generated during H2 reduction of the catalysts were two orders of magnitude more active than silica-supported Pd. Samples prepared by reduction of Pd(NH3)2+NaY displayed on intermediate activity. It was suggested that Pd-proton adducts are highly active sites in neopentane conversion. With methylcyclopentane as a catalytic probe, all Pd/NaY samples deactivated rapidly and coke was deposited. Two types of coke were found (by temperature-programmed oxidation), one of... 

EUROPT-1 is a silica-supported platinum catalyst containing 6.3 %Pt [1—8a]. This unusually high metal loading was chosen in order to facilitate the use of as many techniques of characterization as possible (see discussion following Ref. 8a) in particular, it is difficult to apply electron microscopy if the metal content is less than 1%. Samples were distributed to some 20 members of EUROCAT, and detailed results on its physical characterization were reported in five papers [1-5] and summaries presented in two conference proceedings [8a, 9a]. Results concerning its catalytic behavior were published later [6], and most recently a comprehensive review of work performed by other users of the catalyst has appeared [7], Publications describing its use continue to appear. 

Electron microscopy shows that polymerization starts at active centres on the surface of the particle. During this initial stage, a thin polymer cover is formed on and just below the outer surface of the silica support. This thin cover consists of highly crystalline polypropylene, which acts as a diffusion barrier for the monomer. Diffusion of propylene through this layer thus becomes rate-limiting for polymer formation consequently the high initial polymerization activity decreases sharply after a few minutes and a period of relatively low activity is reached. 

In summary, our study shows the power and the necessity of computational methods in unravelling the structural chemistry of silica-supported metal clusters. It has been shown that the stmcture of such clusters is highly sensitive to the processes by which the cluster-support interaction occurs and not just dependent on the strength of the binding interaction. Further studies are aimed at also investigating the electronic structure of supported clusters and the effects of ligands on cluster structure. 

Mono- or zerovalent nickel can be produced by single or successive one-electron transfer steps during the photoreduction of silica-supported nickel catalysts in a hydrogen atmosphere. 

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