Metal precursors

Using metallic precursors, HF solutions with higher concentrations of tantalum or niobium can be achieved. It is possible to prepare solutions that have maximum concentrations of about 1000 g/1 tantalum oxide and about 600 g/1 niobium oxide (Me205). 

Ceramic boards are currently widely used in high-performance electronic modules as interconnection substrates. They are processed from conventional ceramic precursors and refractory metal precursors and are subsequently fired to the final shape. This is largely an art a much better fundamental understanding of the materials and chemical processes will be required if low-cost, high-yield production is to be realized (see Chapter 5). A good example of ceramic interconnection boards are the multilayer ceramic (MLC) stractures used in large IBM computers (Figure 4.11). These boards measure up to 100 cm in area and contain up to 33 layers. They can interconnect as many as 133 chips. Their fabrication involves hundreds of complex chemical processes that must be precisely controlled. 

Metals Precursor Cluster ProducKs) Transformation Modeled Ref. 

Mono or bis-carbene complexes are possible depending on the carbene/ metal precursor ratio and the steric bulk of the carbene. Most of the metal precursors and bases used for the synthesis of chiral complexes are presented below Metal precursors ... 

Aerosol flame synthesis is a mature technology. A solid phase is generated by dispersing the metal precursors in a flame. The first reports are dated from the 1970s to the 1980s [17-19]. Reviews can be found in [20, 21]. Three different approaches are identified, depending on the state of precursor ... 

Vapor fed aerosol flame synthesis (VAFS) the precursor is in gas phase by using volatile metal precursors such as chlorides. 

Wenkin, M., Touillaux, R., Ruiz, P., Delmon, B., and Devillers, M. (1996) Influence of metallic precursors on the properties of carbon-supported bismuth-promoted palladium catalysts for the selective oxidation of glucose to gluconic acid. Appl. Catal., A, 148, 181-199. 

The Effect of Support-Metal Precursor Interactions on the Surface Composition of Supported Bimetallic Clusters... 

Determination of Metal Precursor Mobilities During Pretreatment. Relative precursor mobilities were obtained by premixing the sllica-or alumina-supported metal catalysts with pure silica (Cab-O-Sll, grade M-5, Cabot Corp.) or pure alumina (Alon C, Cabot Corp.) In a 1 2 ratio prior to pretreatment. The catalyst and silica were ground together using a mortar and pestle for at least 0.5 hr. before they were placed in the Pyrex microreactor for pretreatment. 

A large increase In dispersion following pretreatment was explained by considering the migration of the metal precursor from the catalyst to the additional silica support during pretreatment. 

Metal precursor-support interactions in the case of alumina are quite different. The nature of the H2PtClg-Al203 interaction is still open to question. However, recent "in-situ ultraviolet studies (13-14) suggest the following ... 

The surface dynamics of metal precursor-support interactions dur-... 

Kan et al. reported preparation of Au-core/Pd-shell bimetallic nanoparticles by successive or simultaneous sonochemical irradiation of their metal precursors in ethylene glycol, respectively. In the successive method, Pd clusters or nanoparticles are first formed by reduction of Pd(N03)2, followed by adding HAUCI4 solution. As a result, Au-core/Pd-shell structured particles are formed, although Pd-core/Au-shell had been expected. In their investigations, the successive method was more effective than the simultaneous one in terms of the formation of the Au-core/Pd-shell nanoparticles [143]. 

PtRu nanoparticles can be prepared by w/o reverse micro-emulsions of water/Triton X-lOO/propanol-2/cyclo-hexane [105]. The bimetallic nanoparticles were characterized by XPS and other techniques. The XPS analysis revealed the presence of Pt and Ru metal as well as some oxide of ruthenium. Hills et al. [169] studied preparation of Pt/Ru bimetallic nanoparticles via a seeded reductive condensation of one metal precursor onto pre-supported nanoparticles of a second metal. XPS and other analytical data indicated that the preparation method provided fully alloyed bimetallic nanoparticles instead of core/shell structure. AgAu and AuCu bimetallic nanoparticles of various compositions with diameters ca. 3 nm, prepared in chloroform, exhibited characteristic XPS spectra of alloy structures [84]. 

CFPs are ideal supports to be metalated on chemical bases. One of the simplest ways is the treatment of the CFP with solutions of metal compounds. Under these conditions, metalation requires the previous swelling of the resin, the introduction of a metal precursor and the... 

The introduction of the metal precursor into the CFP can occur upon metal co-ordination or ion exchange (Scheme 2). 

In the first place the ease of these reactions depends on the swelling behavior of the polymeric support. If the liquid employed for dissolving the metal precursor swells the support to a relatively high extent, the interior of the swollen polymer will be readily accessible (Figure 7) [30]. 

If the support is devoid of functional groups apt to interact with the metal precursor, there are not chemical forces facilitating the metal uptake. Under these conditions, metal uptake is driven by absorption forces and can still occur, but it is controlled by simple diffusion. This situation can favor an eggshell radial distribution of the metal precursor over a homogeneous one [31]. 

Figure 7. SEM and XRMA microphotographs of palladium catalysts supported on the amphiphilic resin made by DMAA, MTEA, MBAA (cross-linker) [30]. Microphotographs (a) and (b) show an image and the radial palladium distribution after uptake of [Pd(OAc)2] from water/acetone the precursor diffuses only into the outer layer of the relatively little swollen CFP after reduction the nanoclusters remain close to the edge of the catalyst beads. Microphotographs (c) and (d) show the radial distribution of sulfur and palladium, respectively, after uptake of [PdCU] from water after reduction palladium is homogenously distributed throughout the catalyst particles. This indicates that under these conditions the CFP was swollen enough to allow the metal precursor to readily penetrate the whole of polymeric mass. (Reprinted from Ref. [30], 2005, with permission from Elsevier.)...

The observed distribution can be readily explained upon assuming that the only part of polymer framework accessible to the metal precursor was the layer of swollen polymer beneath the pore surface. UCP 118 was meta-lated with a solution of [Pd(AcO)2] in THF/water (2/1) and palladium(II) was subsequently reduced with a solution of NaBH4 in ethanol. In the chemisorption experiment, saturation of the metal surface was achieved at a CO/Pd molar ratio as low as 0.02. For sake of comparison, a Pd/Si02 material (1.2% w/w) was exposed to CO under the same conditions and saturation was achieved at a CO/Pd molar ratio around 0.5. These observations clearly demonstrate that whereas palladium(II) is accessible to the reactant under solid-liquid conditions, when a swollen polymer layer forms beneath the pore surface, this is not true for palladium metal under gas-solid conditions, when swelling of the pore walls does not occur. In spite of this, it was reported that the treatment of dry resins containing immobilized metal precursors [92,85] with dihydrogen gas is an effective way to produce pol-5mer-supported metal nanoclusters. This could be the consequence of the small size of H2 molecules, which... 

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