Metal supported rhodium catalysts from

The argument of each sine contribution in (6-8) depends on k, which is known, r, which is to be determined, and the phase shift (f(k). The latter needs to be known before r can be determined. The phase shift is a characteristic property of the scattering atom in a certain environment, and is best derived from the EXAFS spectrum of a reference compound, for which all distances are known. For example, the phase shift for zero-valent rhodium atoms in the EXAFS spectrum of a supported rhodium catalyst is best determined from a spectrum of pure rhodium metal as in Fig. 6.13, while RI12O3 may provide a reference for the scattering contribution from oxygen neighbors in the metal support interface. 

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).

Table III shows the rate constants and activation energies for the hydrogenations over platinum and rhodium catalysts. For each compound the activation energy was greater for the hydrogenation with the supported catalyst. Activation energies were calculated from the least-squares , slopes. It is of interest to note that the values of fci.o for the supported rhodium catalyst are, at worst, only a factor of 2 to 3 less than for the corresponding values for pure platinum oxide. This indicates that the supported catalyst is far more active per unit weight of catalytic metal. Hydrogenation was attempted with pure rhodium oxide, but the reaction did not go at all under the conditions used for the other hydrogenations. It is likely that the conditions were not sufficient to reduce the oxide to the metal form.

The polyamide obtained by polycondensation of 2,6-diaminopyridine and 2,6-pyridine dicarboxylic acid was the first polymer to assemble itself into a double helix (DNA-type) in solution. The synthesis and physicochemical characterization of some polymer-supported rhodium catalysts based on polyamides containing 2,6- and 2,5-pyridine units were reported by Michalska and Strzelec (2000) these catalysts were used for the hydrosilylation of vinyl compounds such as phenylacetylene. Chevallier et al. (2002) prepared polyamide-esters from 2,6-pyridine dicarboxylic acid and thanolamine derivatives and investigated their polymer sorption behavior towards heavy metal ions. Finally, Scorlanu et al. (2006) also prepared a polymer with improved performance based on polyureas containing 2,6-pyridine moiety and polyparabanic acids, and polymethane-ureas containing 2,6-pyridine rings. 

Electron spin resonance (ESR) signals, detected from phosphinated polystyrene-supported cationic rhodium catalysts both before and after use (for olefinic and ketonic substrates), have been attributed to the presence of rhodium(II) species (348). The extent of catalysis by such species generally is uncertain, although the activity of one system involving RhCls /phosphinated polystyrene has been attributed to rho-dium(II) (349). Rhodium(II) phosphine complexes have been stabilized by steric effects (350), which could pertain to the polymer alternatively (351), disproportionation of rhodium(I) could lead to rhodium(II) [Eq. (61)]. The accompanying isolated metal atoms in this case offer a potential source of ESR signals as well as the catalysis. 

The studies discussed above deal with highly dispersed and therefore well-defined rhodium particles with which fundamental questions on particle shape, chemisorption and metal-support interactions can be addressed. Practical rhodium catalysts, for example those used in the three-way catalyst for reduction of NO by CO, have significantly larger particle sizes, however. In fact, large rhodium particles with diameters above 10 nm are much more active for the NO+CO reaction than the particles we discussed here, because of the large ensembles of Rh surface atoms needed for this reaction [28]. Such particles have also been extensively characterized with spectroscopic techniques and electron microscopy we mention in particular the work of Wong and McCabe [29] and Burkhardt and Schmidt [30], These studies deal with the materials science of rhodium catalysts that are closer to the ones used in practice, which is of great interest from an industrial point of view. 

Ohmic heating of catalyst is often used as a simple method of igniting the chemical reaction during reactor startup, for instance, in the oxidation of ammonia on platinum-rhodium gauze catalysts. Another application is the prevention of cold-start emissions from automotive catalysts responsible for much of the residual pollution still produced from this source (21). The startup times needed for the catalyst to attain its operating temperature can be cut by a factor of 5 or more by installing an electrically heated catalyst element with a metallic support upstream of the main catalyst unit. Direct electrical catalyst heating permits facile temperature control but requires a well-defined catalyst structure to function effectively. 

The per cent of dicyclohexylamine formed in hydrogenation of aniline increases with catalyst in the order ruthenium < rhodium platinum, an order anticipated from the relative tendency of these metals to promote double bond migration and hydrogenolysis (30). Small amounts of alkali in unsupported rhodium and ruthenium catalysts completely eliminate coupling reactions, presumably through inhibition of hydrogenolysis and/or isomerization. Alkali was without effect on ruthenium or rhodium catalysts supported on carbon, possibly because the alkali is adsorbed on carbon rather than metal (22). 

It is also possible to prepare supported metal catalysts from casy-to-rcduce elements. For example, [ Supp -0]Rh( /3-C3H5)2 (with supp - Al, Si, or Ti) reacts with H2 at room temperature to give rhodium metal particles (1.5 nm) [54, 58]. Diallyl compounds of Group 10 elements (Ni, Pd, Pt), forming species singly anchored onto silica or alumina, arc transformed into small metal particles ([Pg.175]

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