Dispersion supports

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

Transmission electron microscopy is one of the techniques most often used for the characterization of catalysts. In general, detection of supported particles is possible, provided that there is sufficient contrast between particles and support - a limitation that may impede applications of TEM on well-dispersed supported oxides. The determination of particle sizes or of distributions therein is now a routine matter, although it rests on the assumption that the size of the imaged particle is truly proportional to the size of the actual particle and that the detection probability is the same for all particles, independent of their dimensions. 

It is known that Au nanoparticles efficiently catalyze various reactions, but its activity greatly depends on the degree of dispersion, support, and preparation method. We tried to synthesize Au wires and particles by our photo- and H2-reduction. For the S5mthesis of Au nanowires, several groups use HAUCI4 as a precursor. 

Instead of the absorption of chiral modifiers on metal surfaces, a new method using a slightly different approach attaches chiral moieties directly to metal surfaces through chemical bonds. Chiral silyl ethers have been attached to Pd surface atoms these new catalysts have the form (Pd)s=Si-0-R(,< orS) 42 Their synthesis arose from studies of the effects of siliconation on the catalytic activities and selectivities of dispersed, supported Pd and Pt.43-47 The results from... 

Recently, ultrathin evaporated films have been used as models for dispersed supported metal catalysts, the main object being the preparation of a catalyst where surface cleanliness and crystallite size and structure could be better controlled than in conventional supported catalysts. In ultrathin films of this type, an average metal density on the substrate equivalent to >0.02 monolayers has been used. The apparatus for this technique is shown schematically in Fig. 8 (27). It was designed to permit use under UHV conditions, and to avoid depositing the working film on top of an outgassing film. ... 

The infrared spectrum of hydralazine hydrochloride (Figure 1) was obtained with a Beckman IR-12 spectrophotometer. A mineral oil dispersion between potassium bromide plates was scanned from 420 to 4000 cm-1, and a thicker layer of the dispersion, supported on polyethylene film,... 

The infrared spectrum of hydralazine hydrochloride base in a potassium bromide dispersion (Figure 2) was recorded from 400 to 4000 cm-1, and the 200 to 550 cm-1 region was obtained from a mineral oil dispersion supported on polyethylene film. The spectra of potassium bromide dispersions of the base are qualitatively identical to those of mineral oil dispersions. The assignment of absorption bands in the spectrum of the base is similar+to that of the hydrochloride except for the presence of N-H stretch absorption in the latter. A spectrum of the base has been published (6). 

A related approach to the preparation of highly dispersed supported bimetalhc catalysts involves the reaction of metal complexes with supported metal clusters or particles. The method is based on the idea that by careful choice of the metal complex and control of the reaction parameters it may be possible to cause the metal complex to react selectively with the supported metal but not with the support surface [13]. Because this approximation to the subject is the main focus of this chapter, it is thoroughly developed in the following sections. 

Ni(CO)4 is the sole binary carbonyl complex of the elements of group 10 that is stable (Table 8.1). However, very few studies in which Ni(CO)4 is used in the preparation of catalysts have been reported [43]. This is probably due to the difficulty of manipulation of Ni(CO)4 and its very high toxicity. However, surface Ni(CO)4 species have been identified after the interaction of CO with highly dispersed supported nickel catalysts prepared by other routes [44]. Recent interest in the use of Ni(CO)4 has focused on the controlled production of nickel nanoparticles for specific purposes, such as in automotive converters [45]. The use of nickel tetracarbonyl as an agent for the nucleation process in the growth of single-wall carbon nanotubes has also been reported [46]. 

The complex [N(C4H9)4]2[Pt6(CO)12] has also been used to prepare highly dispersed supported platinum aggregates.9,10... 

Structural promotion A highly dispersed support can provide and (or) stabilize a high surface area of the catalyst supported by it. A typical example is ammonia synthesis where the thermal sintering of the iron catalyst is inhibited by alumina (although the phase configuration is different). 

For studying supported catalysts, TEM is the commonly applied form of electron microscopy, and today images as shown in Figure 7.1 are obtained routinely. In general, the detection of supported particles is possible provided that there is sufficient contrast between the particles and the support. This may impede applications of TEM on well-dispersed supported oxides [11]. Contrast in the transmission mode is caused not only by the attenuation of electrons due to density and thickness variations over the sample, but also by diffraction and interference. For example, a particle in a TEM image may show less contrast than other identical... 

Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge fine structure (XANES) have been used with great success to characterize highly dispersed supported metals. EXAFS can provide information on the local structure of highly dispersed metal on a support, such as alumina, and XANES, measured at the same time, provides information on the valence state of the metal. 

Passivation often involves the controlled exposure of the catalyst to air at ambient temperature. Rapid exothermic reactions are prevented while forming stable layers which inhibit further rapid reaction upon air exposure. Similar exposure to other passivating reagents would also lead to air stable surface layers on the metallic surfaces. A typical example is the passivation of Ni catalysts which would oxidize catastrophically upon exposure to air, and of highly-dispersed supported Pt catalysts. Many methods or techniques are available ... 

A. Highly Dispersed Supported Vanadium Oxide The Influence of Resonance... 

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