Metallic precursors, oxidation

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). 

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. 

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]. 

Dyrks, T., Dyrks, E., Hartmann, T., Masters, C.L. and Beyreuther, K. (1992). Amyloidogenicity of d A4 and /3 A4-bearing amyloid protein precursor fragments by metal-catalyzed oxidation. J. Biol. Chem. 267, 18210-18217. 

Generally, stable and well-dispersed metal NPs have been prepared in ILs by the simple reduction of the M(I-IV) complexes or thermal decomposition of the organometallic precursors in the formal zero oxidation state. Recently, other methods such as the phase transfer of preformed NPs in water or organic solvents to the IL and the bombardment of bulk metal precursors with deposition on the ILs have been reported. However, one of the greatest challenges in the NPs field is to synthesize reproducibly metal NPs with control of the size and shape. Selected studies of the preparation of metal NPs in ILs that, in some cases, provide NPs with different sizes and shapes are considered in this section. 

Using the results of an earlier study concerning enantioselective copper-catalyzed intramolecular C—H insertion of metal carbenoids,109 an interesting system for optimizing the proper combination of ligand, transition metal, and solvent for the reaction of the diazo compound (75) was devised (see Scheme 19).110 The reaction parameters were varied systematically on a standard 96-well microtiter/filtration plate. A total of five different ligands, seven metal precursors, and four solvents were tested in an iterative optimization mode. Standard HPLC was used to monitor stereoselectivity following DDQ-induced oxidation. This type of catalyst search led to the... 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

There are many documented synthetic routes to metal dichalcogeno-phosph(in)ates although the most common involve reaction of the alkali metal or ammonium salt of the ligand (see Section 5.2.5 for preparation of these species) with a suitable metal precursor (including metal chlorides, oxides,... 

Using this method, homogeneous alloys, segregated alloys, layered bi-metallics, and decorated particles are all readily accessible. An obvious advantage of the precursor concept over the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors may be tailored independent of the support. Further, the metal particle surface may be modified by lipophilic or hydrophilic protective shells and coated by intermediate layers, e.g., of oxide. The modification of the precursor by dopants is also possible. 

Instead of electrostatic (or physical) adsorption, metal uptake onto oxides might be considered chemical in nature. In chemical mechanisms, the metal precursor is envisioned to react with the oxide surface, involving as surface-ligand exchange [13,14] in which OH groups from the surface replace ligands in the adsorbing metal complex. In this section it will be shown that a relatively simple electrostatic interpretation of the adsorption of a number of catalyst precursors is the most reasonable one for a number of noble metal/oxide systems. 

On the other hand, a simpler and purely physical (electrostatic) mechanism might go toward describing the uptake of common noble metal precursors onto common oxide supports. The first model of Agashe and Regalbuto [26] was patterned after that of James and Healy [17] from colloid science literature. In the original James and Healy s model, a Langmuir isotherm was employed and... 

FIGURE 13.1. Different modes of adsorption between metallic precursor and the oxide surface. 

Loosely bound fj -cyclopentadienyl anions can also serve as the base to deproto-nate imidazolium salts. When chromocene is reacted with an imidazolium chloride in THF the metal precursor loses one molecule of cyclopentadiene to form the 14-electron complex [( 7 -C5H5)Cr(NHC)Cl] [Eq. (13)]. This complex can be further oxidized by CHCI3 to give [( 7 -C5H5)Cr(NHC)Cl2]. This route also works with nickelocene to generate the corresponding [( -C5H5)Ni(NHC)Cl] complex. ... 

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