Rhodium nanoparticles

Cao D, Wieckowski A, Inukai J, Alonso-Vante N (2006) Oxygen reduction reaction on rathe-nium and rhodium nanoparticles modified with selenium and sulfur. J Electrochem Soc 153 A869-A874... 

Fig.3 PVP-protected rhodium nanoparticles a TEM micrograph b HREM micrograph...

Iridium and rhodium nanoparticles have also been studied in the hydrogenation of various aromatic compoimds. In all cases, total conversions were not observed in BMI PF6. TOFs based on mol of cyclohexane formed were 44 h for toluene hydrogenation with Ir (0) and 24 h and 5 h for p-xylene reduction with lr(0) or Rh(0) nanoparticles, respectively. The cis-1,4-dimethylcyclohexane is the major product and the cisitrans ratio depends on the nature of the metal 5 1 for lr(0) and 2 1 for Rh(0). TEM experiments show a mean diameter of 2.3 nm and 2.1 nm for rhodium and iridium particles, respectively. The same nanoparticle size distribution is observed after catalysis (Fig. 4). 

Similarly to Iridium and rhodium nanoparticle studies, Dupont describes benzene hydrogenation in various media by platinum(O) nanoparticles prepared by simple decomposition of Pt2(dba)3 in BMI PFe at 75 °C and under 4 bar H2 [68]. The Pt nanoparticles were isolated by centrifugation and char-... 

The catalytic lifetime was studied by reusing the aqueous phase for three successive hydrogenation runs of toluene, anisole and cresol. Similar turnover activities were observed during the successive runs. These results show the good stability of the catalytically active iridium suspension as previously described with rhodium nanoparticles. 

Finally, Jessop and coworkers describe an organometalhc approach to prepare in situ rhodium nanoparticles [78]. The stabilizing agent is the surfactant tetrabutylammonium hydrogen sulfate. The hydrogenation of anisole, phenol, p-xylene and ethylbenzoate is performed under biphasic aqueous/supercritical ethane medium at 36 °C and 10 bar H2. The catalytic system is poorly characterized. The authors report the influence of the solubility of the substrates on the catalytic activity, p-xylene was selectively converted to czs-l,4-dimethylcyclohexane (53% versus 26% trans) and 100 TTO are obtained in 62 h for the complete hydrogenation of phenol, which is very soluble in water. 

The size of the metal particles deposited from SMA is strongly dependent on the stabilization of the metal mi-croclusters present in the starting SMA solutions. Their stability can be conveniently modulated by addition of suitable ligands. Examples related to y-Al203-supported Platinum and Rhodium nanoparticles are reported here. 

NMR measurements are very useful to understand the properties of the stabilizing reagents of metal nanoparticles. Author s group reported the structure of stabilization of non-ionic and cationic surfactants on platinum nanoparticles [22] and that of ternary amines on rhodium nanoparticles [23]. Such information is considerably important for applications of nanoparticles such as... 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

Delmas et al. produced PVP-stabilized rhodium nanoparticles using the method reported by Hirai [32] to perform catalytic hydrogenation of oct-l-ene in a two-liquid-phase system [40]. These authors investigated the effect of various parameters on nanoparticle stability and activity under more or less severe conditions. It was also shown that PVP/Rh colloids could be reused twice or more, without any loss of activity. 

Catalytic studies and kinetic investigations of rhodium nanoparticles embedded in PVP in the hydrogenation of phenylacetylene were performed by Choukroun and Chaudret [90]. Nanoparticles of rhodium were used as heterogeneous catalysts (solventless conditions) at 60 °C under a hydrogen pressure of 7 bar with a [catalyst]/[substrate] ratio of 3800. Total hydrogenation to ethylbenzene was observed after 6 h of reaction, giving rise to a TOF of 630 h 1. The kinetics of the hydrogenation was found to be zero-order with respect to the al-kyne compound, while the reduction of styrene to ethylbenzene depended on the concentration of phenylacetylene still present in solution. Additional experi-... 

A new class of heterogeneous catalyst has emerged from the incorporation of mono- and bimetallic nanocolloids in the mesopores of MCM-41 or via the entrapment of pro-prepared colloidal metal in sol-gel materials [170-172], Noble metal nanoparticles containing Mex-MCM-41 were synthesized using surfactant stabilized palladium, iridium, and rhodium nanoparticles in the synthesis gel. The materials were characterized by a number of physical methods, showed that the nanoparticles were present inside the pores of MCM-41. They were found to be active catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene, and... 

Ghung has developed an effective protoeol for the cyclization/silylformylation of 1,6-enynes catalyzed by cobalt/ rhodium nanoparticles (Go2/Rh2). For example, reaction of dimethyl allylpropargylmalonate and triethylsilane catalyzed by Go2/Rh2 in dioxane at 105 °G under GO formed the corresponding silylated cyclopentane... 

Apart from the type of metal itself, structure sensitivity is another key effect determining the CO dissociation probability (e.g., Rh(l 11) and (100) single-crystal surfaces do not dissociate CO (373,374), whereas Rh(210) (375) and rhodium nanoparticles with diameters of about 2-3 nm do (1 0 1)). 

Rupprechter G, Hayek K, Hofmeister H (1998) Electron microscopy of thin film model catalysts Activation of alumina supported rhodium nanoparticles. J Catal 173 409... 

The use of elemental rhodium (e.g., rhodium surfaces, rhodium on carbon, rhodium nanoparticles) will not be discussed in this review. 

Water-in-C02 microemulsions with diameters in the order of several nanometers are prepared by a mixture of AOT and a Pn E-P04 co-surfactant. The CO2 microemulsions allow metal species to be dispersed in the nonpolar supercritical CO2 phase. By chemical reduction, metal ions dissolved in the water core of the microemulsion can be reduced to the elemental state forming nanoparticles with narrow size distribution. The palladium and rhodium nanoparticles produced by hydrogen reduction of Pd and Rh ions dissolved in the water core are very effective catalysts for hydrogenation of olefins and arenes in supercritical CO2. 

The formation and stabilization of noble metal colloids in the aqueous phase are widely known. Platinum and palladium are most widely used in hydrogenation of C=C bonds but some results have been described with rhodium. Generally, surfactants are investigated as stabilizers for the preparation of rhodium nanoparticles for biphasic catalysis in water. In many cases, ionic surfactants, such as ammonium salts, which provide sufficiently hydrophilic character to maintain the catalytic species within the aqueous phase, are used. The obtained micelles constitute interesting nanoreactors for the synthesis of controlled size nanoparticles due to the confinement of the particles inside the micelle cores. Aqueous colloidal solutions are then obtained and can be easily used as catalysts. 

Figure 2 TEM micrograph of rhodium nanoparticles, isolated before (left) and after (right) catalysis. (Reproduced from [26] with permission from John Wiley, Sons Inc ).

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