Precursor metallic

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

Metal clusters on supports are typically synthesized from organometallic precursors and often from metal carbonyls, as follows (1) The precursor metal cluster may be deposited onto a support surface from solution or (2) a mononuclear metal complex may react with the support to form an adsorbed metal complex that is treated to convert it into an adsorbed metal carbonyl cluster or (3) a mononuclear metal complex precursor may react with the support in a single reaction to form a metal carbonyl cluster bonded to the support. In a subsequent synthesis step, metal carbonyl clusters on a support may be treated to remove the carbonyl ligands, because these occupy bonding positions that limit the catalytic activity. 

When the metal nanoparticles are inserted into zeolite supercages, the size of the metal particles is confined according to the size of the supercage. However, after reduction of the precursor metal ions in a stream of hydrogen, the protons replacing the metal ions in the cation exchange position also interfere with the metal particles, influencing thereby their chemisorption and catalytic properties. 

Based on the molecular design of precursor metal complexes, the solvent-free controlled thermolysis of metal complexes may cause the thermal reduction and simultaneous attachment of organic moiety on the growing metal nuclei and give us a solution of the defects of ordinary... 

Metal carbonyl compounds are other suitable precursors for the synthesis of NPs by thermal decomposition. The main advantage is the formation of CO that is expelled from the IL phase due to its poor solubility. However, high temperatures are commonly used to decompose such precursors. Metal NPs of Cr(0), Mo(0), and W(0) were prepared by thermal or photolytic decomposition of their respective monometallic carbonyl compounds [M(CO)6] dispersed in ILs [52]. Similarly, the precursors [Fe2(CO)9], [Ru3(CO)i2], and [Os3(CO)12] were employed in order to obtain stable metal NPs (1.5-2.5 nm) in BMI.BF4 [53]. The same procedure was extended to the preparation of lr(0), Rh(0), and Co(0) NPs in ILs [54]. 

According to the general migratory-insertion mechanism proposed by Cossee,58 chain propagation (4.105) is a two-step process in which the precursor metal reagent (I) forms an intermediate alkene complex (II) that subsequently rearranges to the insertion product (III),... 

Catalysts were prepared by incipient wetness impregnation of commercial supports using cobalt nitrate as a precursor. Metallic cobalt species were active centers in the ethanol steam reforming. Over 90% EtOH conversion achieved. Nature of support influences the type of byproduct formation. Ethylene, methane and CO are formed over Co supported on A1203, Si02 and MgO, respectively... 

Many of these systems employ charged polymers or polyelectrolytes that confer on them particular properties due to the existence of electrical charges in the polymer structure. Oyama and Anson [14,15] introduced polyelectrolytes at electrode surfaces by using poly(vinylpiridine), PVP, and poly-(acrylonitrile) to coordinate metal complexes via the pyridines or nitrile groups pending from the polymer backbone. Thomas Meyer s group at North Carolina [16, 17[ also employed poly(vinylpyridine) to coordinate Ru, Os, Re and other transition-metal complexes by generating an open coordination site on the precursor-metal complex. 

All synthetic methods described up to now (ligand displacement, acac or halo-methyl precursors, metal-bonded carbene + nucleophile, metal-bonded nucleophile + carbene) result in a metal-bonded ylide through the Ca atom. The reactivity of ylides toward metallic systems is, however, greater than anticipated and other reaction pathways could compete with simple C-bonding. 

Precursor Metal % Peroxide conversion (%) Yield (%) Selectivity to epoxide (%) Reference... 

The purpose of the present work is to study the precursor (metallic chlorides or carbonyl compounds), particle size and support (silica, magnesia and titania) effects in the selective hydrogenation of carvone employing rhodium as active metal. 

No epitaxy could be obtained by reaction of the metal films with reactive gas for short reactions times. This is understandable as the carburization and nitridation reactions progress from the surface of the metal films to the substrate and occur with a change in crystal structure of the film (for instance bcc to hex). So even if the starting metal film is epitaxial, the final carbide or nitride compound could be polycrystalline. For high temperatures and for long time treatments (>15 h), however, perfect epitaxial Y Mo2N films could be obtained on MgO (100).17 In this last case, the crystalline state of the precursor metal film had no effect on the final parallel orientation of the nitride. 

Mx and M2 refer to the metals in the precursor metal carbonyl and in the oxide support, respectively.) Temperature-programmed-oxidation and -reduction experiments provide complementary information. 

Both its solubility and any potential side reaction with reductant dictate the choice of precursor metal salt or complex. In many cases the reductant is an aluminium alkyl or electropositive metal (Goups 1,2, 12 or 13, possibly amalgamated with mercury), requiring the use of anhydrous metal salts or complexes. Carbonyls of higher nuclearity are... 

Two main preparation methods have been used to synthesize Mo/V/Te/(Nb)/0 catalysts active in propane ammoxidation (a) the dry-up and (b) the hydrothermal synthesis. The dry-up method involves mixing aqueous slurries of metal oxide precursors followed by a gradual evaporation of the combined aqueous slurry. Solvent evaporation leads to nucleation and growth of precursor metal oxide phases, which require further heat treatment to obtain active catalysts. 

Table 4J. Faraday magnetic balance study of the redox behaviour of two ccria-supported rhodium catalysts prepared from Rh(NO))j (N) and RhCi> (Cl) metal precursors. Metal loading and BET surface area of the catalysts were 3 wt% and 49 m g respectively. Data taken from(i95).

Catalyst precursor Metal loading (wt%) Reaction temperature rc) Time on stream (h) CO Conversion (%) Product composition (mol%) ... 

Supported metal clusters are often prepared by (a) bonding of precursor metal carbonyl clusters followed by removal of carbonyl ligands attempting to maintain the metal clusters frame intact, (b) surface-mediated synthesis in which the supported... 

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