Rh nanoparticles

The Rh complexes (cod)Rh[ jt-OSi(0 Bu)3] 2 and (nbd)Rh[ jt-OSi (0 Bu)3] 2 thermally decompose primarily via loss of HOSi(O Bu)3 and formation of Rh metal particles, rendering them ineffective for use as precursors to Rh/Si/0 materials however, these complexes are potentially useful as sources of Rh nanoparticles or site-isolated Rh species via grafting methods [99]. 

Finally, the term steric stabihzation coifid be used to describe protective transition-metal colloids with traditional ligands or solvents [38]. This stabilization occurs by (i) the strong coordination of various metal nanoparticles with ligands such as phosphines [48-51], thiols [52-55], amines [54,56-58], oxazolines [59] or carbon monoxide [51] (ii) weak interactions with solvents such as tetrahydrofuran or various alcohols. Several examples are known with Ru, Ft and Rh nanoparticles [51,60-63]. In a few cases, it has been estab-hshed that a coordinated solvent such as heptanol is present at the surface and acts as a weakly coordinating ligand [61]. 

The presence of soluble Rh nanoparticles after catalysis is demonstrated by TEM. The kinetic of the catalytic reaction was found to be zero-order in respect to the substrate and first order with respect to hydrogen and catalyst. Curiously, under the same conditions (60 °C, 7 bar H2), ethylcyclohexane is not detected at the end of phenylacetylene hydrogenation and the formation of methylcyclohexane from toluene was only obtained under drastic conditions 40 bar H2 and 80 °C. 

Finally, the groups of Chaudret and Choukroun have demonstrated that PVP-protected native Rh nanoparticles synthesized by an organometallic approach are active in the hydrogenation of benzene in a biphasic mixture. 

The XRD and TEM showed that the bimetallic nanoparticles with Ag-core/Rh-shell structure spontaneously form by the physical mixture of Ag and Rh nanoparticles. Luo et al. [168] carried out structure characterization of carbon-supported Au/Pt catalysts with different bimetallic compositions by XRD and direct current plasma-atomic emission spectroscopy. The bimetallic nanoparticles were alloy. Au-core/Pd-shell structure of bimetallic nanoparticles, prepared by co-reduction of Au(III) and Pd(II) precursors in toluene, were well supported by XRD data [119]. Pt/Cu bimetallic nanoparticles can be prepared by the co-reduction of H2PtClg and CuCl2 with hydrazine in w/o microemulsions of water/CTAB/ isooctane/n-butanol [112]. XRD results showed that there is only one peak in the pattern of bimetallic nanoparticles, corresponding to the (111) plane of the PtCu3 bulk alloy. 

Recently five monometallic (Au, Pd, Pt, Ru, Rh) nanoparticles were investigated as electron mediators together with four core/shell bimetallic (Au/Pd, Au/Pt, Au/Rh, Pt/ Ru) nanoparticles [53,194-196]. The linear relationship was observed between the electron transfer rate coefficients and the hydrogen generation rate coefficient as shown in Figure 15. 

Rhodium-catalyzed enantioselective hydrogenation of acctamido -cinnamic in water was also achieved using pyrphos bound to poly-acrylic acid as ligand.337 Roucoux described some Rh° nanoparticles which function as reusable hydrogenation catalyst for arene derivatives in a biphasic water-liquid system.338... 

Rhee and coworkers published the synthesis of bimetallic Pt-Pd nanoparticles [57] or Pd-Rh nanoparticles [58] within dendrimers as nanoreactors. These nanocatalysts showed a promising catalytic activity in the partial hydrogenation of 1,3-cyclooctadiene. The reaction was carried out in an ethanol/water mixture at 20 °C under dihydrogen at atmospheric pressure. The dendrimer-encapsulated nanoclusters could be reused, without significant loss of activity. 

Figure 3.32 Formation of an external hydrophilic layer at the periphery of a Rh nanoparticle, which accounts for the selective hydrogenation of hex-5-en-1-ol into hexane-1-ol with [Rhj3(T SnBu)/Si02.

On Rh Tj bonding of isocyanides is present on Rh/Al203, Rh(lll) and Rh nanoparticles. Only on the latter two surfaces are bridging isocyanides (to two or three metal atoms) also observed. 

Aggregation of the atoms or microclusters may give metal nuclei. The micro-cluster itself may work as the nucleus. Although the size of microcluster or nucleus is not clear, the nucleus may consist of 13 atoms, which is the smallest magic number, This idea may be supported by the structural analysis of PVP-stabilized Pt nanoparticles (64) and other systems. In fact, a particle of 13 atoms is considered an elemental duster. In the case of preparation of PVP-stabilized Rh nanoparticle dispersions by alcohol reduction, formation of very tiny particles, the average diameter of which is estimated to be 0.8 nm, was observed (66). These tiny particles in the metastable state may consist of 13 atoms each and easily increase in size to the rather nanoparticles with average diameter of 1.4 nm, i.e., the particles composed of 55 atoms. This observation again supports the idea that the elemental cluster of 13 atoms is the nucleus. 

The cyclopenta[c]pyran system 6 is formed when allenyl propynyl ethers are heated in CO in the presence of Co/Rh nanoparticles a Pauson-Khand reaction is involved <07SL453>. An enantioselective Bronsted acid-catalysed Nazarov cyclisation of a dihydropyran-based divinylketone affords cyclopenta[/>]pyrans 7 <07AG(E)2097> and Sc and In triflates catalyse a similar reaction of 6-alkenoyl derivatives of both dihydropyran and thiopyran <07S1733>. 

Catalyst activation by oxidation/reduction can have a strong effect on activity/ selectivity, and HRTEM has been extensively applied to monitor the restructuring of, e.g., Rh nanoparticles [19, 20, 32, 33]. 

Fig. 15.5 Oxidation and reduction of epitaxially grown polyhedral Rh nanoparticles (mean size 5 nm) on alumina, monitored ex situ by HRTEM. In the as-prepared state, most of the Rh particles were half-octahedra with 111 and 100 surface facets, as revealed by combining results from HRTEM and WBDF (a, d), and SAED (b). Upon oxidation in 1 bar at 723 K, an epitaxial Rh-oxide shell developed on top of a Rh core (c, e). Reduction in 1 bar H at 523 and 723 K led to polycrystalline (f) and rounded crystalline (g) nanoparticles, respectively. The microstructural changes were correlated with changes in catalytic hydrogenolysis activity (see text for details) adapted in part from [20] with permission. Copyright (1998) Elsevier...

The preparation, atomic and electronic structure, adsorption and reaction properties of metal nanoparticles supported on AljOj/NiAlCllO) were thoroughly explored by Freund and coworkers [11, 43, 52], Most studies were carried out under UHV, including CO dissociation on Rh nanoparticles [39], alkene hydrogenation [53], and CO and methanol oxidation [52],... 

Fuchs M, Jenewein B, Penner S, Hayek K, Rupprechter G, Wang D, et al. (2005). Interaction of Pt and Rh nanoparticles with ceiia and silica supports ring opening of methylcyclobutane and CO hydrogenation after reduction at 373-723 K. Appl Catal A Gen, 294, 279... 

Hydrogenation reactions have been the most extensively studies for measuring the catalytic activities of nanoparticles. Noble monometallic (Pd, Pt, Rh) nanoparticles protected by linear polymers like PVP or polyvinylalcohol have high catalytic activities for hydrogenation of olefins [142-144]. We applied PVP-protected Pd/Pt [79,80] and Au/Pd [86] bimetallic nanoparticles prepared by the simultaneous... 

The same reaction was also investigated over Rh nanoparticles dispersed over pure and doped Ti02 and other porous supports of known work functions [136]. It was established that the influence of electrochemical promotion on kinetic parameters of the model reaction is identical to the influence of metal-support interactions, under conditions at which the change of the work function of the catalyst is the same, regardless of the means by which the alteration in the work function is achieved. 

The Pd-catalysed reaction of phenols with propynoic acids offers an atom-economic and green route to coumarins. Although the reaction with propynoic acid itself is not regioselective, 3-phenylpropynoic acid gives a single product from w-substituted phenols <04S1466>. The synthesis of coumarins both from alkynes, CO and iodophenols and by the reaction of phenols with propiolic esters is facilitated by Co/Rh nanoparticles <04SL2541>. 

The hydrido complex [(l,5-C8Hi2)RhH]4 can be used as a source of small rhodium crystallites. Aromatic hydrocarbon solutions of this complex are unstable when exposed to dihydrogen, allowing the formation of Z nm size, crystalline but agglomerated Rh nanoparticles, that show catalytic properties in aromatic hydrocarbures hydrogenation. ... 

Our initial studies have demonstrated that the Pd and Rh nanoparticles formed in the CO2 microemulsions are very effective catalysts for hydrogenation of olefins and arenes in supercritical CO2. Dispersing metal nanoparticles in supercritical CO2 utilizing the microemulsion is a new approach for homogenization of heterogeneous catalysis. This approach may have important applications for chemical synthesis in supercritical fluids. 

The citral conversion and the selectivity to unsaturated alcohols increased with the Ge content, this effect being most obvious on alumina supported catalysts (Fig. 9.8). The selectivity enhancement on alumina in comparison with silica was explained by Ge deposited on the alumina support in the close vicinity of Rh nanoparticles during the catalytic redox reaction. These Ge species would be partially reduced during the reduction step of the catalyst preparation and would promote the activation of the carbonyl group for hydrogenation. Then, the catalytic properties of the bimetallic Rh-Ge/Al203 catalysts prepared by the redox reaction strongly depended on their reduction temperature, which must be... 

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