Transition metal catalyst, growth forms

The formation of filamentous carbon deposits on transition metal catalysts (Fe, Co, Ni) and their alloys have been investigated in some detail over the past two decades.21,38-40 Among them, nickel is the most promising candidate since it forms carbon deposits at temperatures as low as 723-823 K using CH4, C2H6 or CO + H2 feeds. Carbon fibres are usually produced during these reactions. Typical forms of the carbon produced from CH4 decomposition on silica-supported Ni catalysts are shown in Fig. 7.1. The pyrolysis of methane at temperatures somewhat lower than 873 K produces fish-bone type nanofibres.41 The Ni metal particles are present at the tip of each carbon fibre, and catalyse methane decomposition as well as growth... 

Carbon formation on steam reforming catalysts takes place in three different forms whisker-like carbon, encapsulated carbon, and pyrolytic carbon as described in Table 2.2 [1]. Whisker-like carbon grows as a fiber from the catalyst surface with a pear-shaped nickel crystal on the end. Strong fibers can even break down catalyst particles increasing the pressure drop across the reformer tubes [4], The carbon for whisker formation is formed by the reaction of hydrocarbons as well as CO over transition metal catalysts [1], The whisker growth is a result of diffusion through the catalyst and nucleation to form a long carbonaceous fiber. 

The quantitative aspect of the EXAFS technique is also well known and the literature gives several studies where chemisorption and EXAFS measurements are compared (see for example We can illustrate this particular contribution of the spectroscopy by a study of rare earth transition metal catalysts prepared from intermetallic LaNij-type compounds. The three classical preparation steps are here skipped with a carbon monoxide hydrogenation reaction. The intermetallic phase is transformed into a rare earth oxide upon which the transition metal is left as metallic clusters which form the active species. This transformation has been followed as a function of the time reaction In Fig. 5 we plot the Fourier transforms of CeNij at the nickel edge before the reaction (a), after 10 hours (b) and after 27 hours (c) under the CO + H2 mixture. These are all compared to elemental nickel (d). The increase of the amplitude of the first peak and the growth of three new ones at greater distances are the consequence of the formation of nickel particles. A careful analysis of these four shells has allowed us quantitatively to estimate the fraction of extracted nickel during the reaction as 30% after 10 hours and 80% after 27 hours on a CO + flux at 350 °C. 

With the introduction of a variety of surface-sensitive instrumental techniques and the use of transient techniques utilizing isotopic tracers, the carbide theory of chain growth was revived to represent a dominant viewpoint since 1980. Brady and Pettit showed that the decomposition of diazomethane on various transition metal catalysts and in the absence of CO and H2 primarily led to ethylene. In the presence of H2, hydrocarbons up to Cjg were formed and conformed to the Schulz-Flory distribution expected for FTS (for an example of a description of Schulz-Flory distribution, see Reference 23). It was... 

RAFT is a highly versatile method of polymerization that utilizes a chain-transfer agent (CTA) to produce the dormant form of the growing polymer chains. The CTA is commonly a di- or tri-thiocarbonylthio compound that forms a stabilized radical intermediate. Chain equilibration between actively growing and dormant chains leads to uniform rate of chain growth for all polymers, and is linearly related to conversion. Since transition metal catalysts are not required, RAFT has increased compatibility with a wider anay of monomer functionalities than other polymerization methods, hut the... 

For the CNTs growth from powder form catalysts, it is used as a support an inert and refractory oxide, and transition metals in the form of oxides, which will generate, during the reaction, the catalyst NP. To produce a homogeneous mixture between the oxide and metal oxide catalyst support, two methods are most commonly used solution combustion synthesis (SCS) [79] or the impregnation method [80]. Both use inorganic salts (usually nitrates) as the catalysts. 

It is now clear that, when propagation centers are formed, olefin polymerization by all solid catalysts (including the Phillips Petroleum catalyst from chromium deposited on oxides, and the Standard Oil catalyst of molybdenum oxide on aluminum oxide) essentially follows the same mechanism chain growth through monomer insertion into the transition-metal-carbon bond, with precoordination of the monomer. Interestingly,... 

It is also believed that the polymerisation catalysts herein are effective because the unpolymerised olefinic monomer can only slowly displace from the complex a coordinated olefin formed by /1-hydride elimination from the growing polymer chain, which is attached to the transition metal. The displacement can occur by associative exchange. An increase in the steric bulk of the ligand slows the rate of associative exchange and allows polymer chain growth. 

Step growth reactions involving the homo- and heteropolycondensation of various bifunctional unsaturated monomers in the presence of transition metal-based coordination catalysts have appeared to be a very useful synthetic tool for the preparation of low and high molecular weight polymers with an unsaturation in the polymer backbone. These reactions lead to unsaturated hydrocarbon and non-hydrocarbon polymers where polymeric chains are formed by carbon-carbon and carbon-heteroatom coupling respectively. 

It was long believed that the isobutylalane does not give the growth reaction at all 299). This, however, was due to the fact that the reaction vessels employed always contained traces of a transition metal that catalyzed the displacement. With isoprene, isobutene is split off and polymeric alkyl aluminum compounds are formed which still contain some alkenyl groups. These are said to be less sensitive to air and moisture than the trialkyl alanes. The replacement is hastened by Ni as catalyst 61). 

It is generally accepted that, in the polymerisation of dienes on lanthanide catalysts, the growing chain is attached to the transition metal by an 7t-allyl bond and that the chain growth occurs by incorporation of the monomer via the metal-carbon o-bond. In the case of neodymium catalysts, the delocalised 7t-allyl type structure of the terminal unit has been observed by spectroscopic methods [8, 26, 28, 58-60]. The results reported in these papers show that the relative contents of cis-l,A- and tri2ns-1,4-units in polydienes depend on the type of solvent used, the polymerisation temperature, structure of diene monomer, and the composition of lanthanide-based catalysts. These data can be interpreted in terms of the concept of isomerisation equilibrium between anti- and syn-forms of n-allyl terminal unit. One of the arguments in favour of the existence of this isomerisation... 

Catalyst systems that do not require the presence of MAO can be prepared by forming a cationic complex of the transition metal, which is associated with a stabilizing anion that can be easily displaced by an incoming monomer during chain growth. Two examples show how these can be prepared ... 

---