The ligand polyhedral model approach to the mechanism of complete carbonyl exchange in [Rh4(CO)12] and [Rh6(CO)16].

The Ligand Polyhedral Model (LPM) is applied to the carbonyl scrambling observed on the NMR timescale for the two cluster carbonyls [Rh4(CO)12] and [Rh6(CO)16]. For [Rh4(CO)12] the NMR data is completely consistent with the libration of the Rh4 central core about a C3 - rotational axis within the icosahedral shell of twelve CO ligands leading to a single-step concerted exchange process. Rotation of the Rh4 unit is also found to lead to the formation of a new all terminally bonded isomer which retains the icosahedral shell of CO ligands. Importantly, this new isomer does not have the [Ir4(CO)12] structure which is known to possess an cube-octahedral ligand shell. With [Rh6(CO)16] and its substituted derivative [Rh6(CO)15L] exchange is considered to take place via a polyhedral inter-conversion in which the external tetra-capped truncated tetrahedral shell of carbonyl ligands observed in the ground-state undergoes rearrangement first to the 2 : 6 : 6' : 2 complementary geometry and then returns to the ground-state structure. This leads to a single-step fully concerted CO exchange. The LPM approach also provides an understanding of the variation of activation energies observed in going from [Rh4(CO)12] to [Rh6(CO)16] and of the driving force to the fluxional process.

Nanoparticulate PdZn--pathways towards the synthetic control of nanosurface properties.

This paper reports an in-depth structural investigation of PdZn nanoparticulates prepared over an entire compositional range. By using a combination of HRTEM, ICP-OES, EDX and XPS alongside PXRD, we are able to show how a liquid-type reduction process can be exploited to target different PdZn bimetallic structures while maintaining reproducibly narrow particle size distributions and average particle diameters of approximately 3 nm. Samples have been further analyzed by quantitative phase analysis of the Rietveld refined diffraction data, providing indications as to how variations in specific surface compositions are obtained when Zn is used as the alloying metal. The influence of nanolattice strain is investigated by geometric analysis of TEM data. Results suggest, in conjunction with previously published catalytic data, how different compositions of this specific bimetallic system may be exploited in catalytic processes to control substrate/product affinity. We thus demonstrate a new and simplified approach to PdZn bimetallics, which may offer novel perspectives for applications in industrial catalysis.

Nanoparticulate copper--routes towards oxidative stability.

A modified polyol-based reduction method in ethylene glycol that incorporates poly(N-vinylpyrrolidone) (PVP, M(av) = 10,000; 40,000; 55,000) as polymeric anti-agglomerant alongside a reducing additive (N(2)H(4) x H(2)O, NaBH(4), NaH(2)PO(2) x H(2)O) has been employed to investigate the influence of synthetic parameters on the purity, morphology and stability of an array of polymer-coated copper nanoparticles. While data point to ethylene glycol being capable of acting as a reductant in this system, the use of NaH(2)PO(2) x H(2)O as co-reductant in tandem with the presence of PVP (M(av) 40,000) has rendered nanoparticles with a mean size distribution of 9.6 +/- 1.0 nm that exhibit stability towards oxidation for several months. These data allow us to probe fundamentally how oxidatively stable nano-copper might be achieved.

Capillary microreactors wall-coated with mesoporous titania thin film catalyst supports.

A new method for catalyst deposition on the inner walls of capillary microreactors is proposed which allows exact control of the coating thickness, pore size of the support, metal particle size, and metal loading. The wall-coated microreactors have been tested in a selective hydrogenation reaction. Activity and selectivity reach values close to those obtained with a homogeneous Pd catalyst. The catalyst activity was stable for a period of 1000 h time-on-stream.

Confined palladium colloids in mesoporous frameworks for carbon nanotube growth.

Palladium colloidal nanoparticles with an average size of approximately 2.4 nm have been incorporated into mesoporous inorganic thin films following a multistep approach. This involves the deposition of mesoporous titania thin films with a thickness of 200 nm by spin-coating on titanium plates with a superhydrophilic titania outer layer and activation by calcination in a vacuum furnace at 573 K. Nanoparticles have been confined within the porous titania network by dip-coating noble metal suspensions onto these mesoporous thin films. Finally, the resulting nanoconfined systems were used as substrates for the growth of oriented carbon nanotubes (CNTs) using plasma-enhanced chemical vapour deposition at 923 K in order to enhance their surface area. These CNTs were tested in the hydrogenation of phenylacetylene by hydrogen in a batch reactor. The initial reaction rate observed on a CNT/TiO2 structured catalyst was considerably higher than that on 1 wt% Pd/TiO2 thin films.

Exploring the structural complexities of metal-metalloid nanoparticles: the case of Ni.B as catalyst.

Understanding of the structural complexities of metal-metalloid nanoparticles is at the heart of several proposals for investigating the physical properties and practical applications of these bi-elemental nanomaterials. To date, the most widely studied metal-metalloid is the nickel-boron (Ni.B) system; however, the exact nature of the structure of the material itself has remained unclear. Herein we show our systematic investigations of the material in an attempt to reveal its fascinating nanostructure. The relation between its high catalytic activity and the ultrafine structure is explored, and the work has been further extended to the formation of colloidal Ni.B nanoparticles. The results presented in this work may represent a substantial progress toward a full understanding of the nickel-boron chemistry.

Selective oxidation with dioxygen by gold nanoparticle catalysts derived from 55-atom clusters.

Supported gold nanoparticles have excited much interest owing to their unusual and somewhat unexpected catalytic properties, but the origin of the catalytic activity is still not fully understood. Experimental work on gold particles supported on a titanium dioxide (110) single-crystal surface has established a striking size threshold effect associated with a metal-to-insulator transition, with gold particles catalytically active only if their diameters fall below approximately 3.5 nm. However, the remarkable catalytic behaviour might also in part arise from strong electronic interaction between the gold and the titanium dioxide support. In the case of industrially important selective oxidation reactions, explanation of the effectiveness of gold nanoparticle catalysts is complicated by the need for additives to drive the reaction, and/or the presence of strong support interactions and incomplete understanding of their possible catalytic role. Here we show that very small gold entities ( approximately 1.4 nm) derived from 55-atom gold clusters and supported on inert materials are efficient and robust catalysts for the selective oxidation of styrene by dioxygen. We find a sharp size threshold in catalytic activity, in that particles with diameters of approximately 2 nm and above are completely inactive. Our observations suggest that catalytic activity arises from the altered electronic structure intrinsic to small gold nanoparticles, and that the use of 55-atom gold clusters may prove a viable route to the synthesis of robust gold catalysts suited to practical application.

Crystal structure and growth mechanism of unusually long fullerene (C60) nanowires.

Exceptionally long C60 nanowires, with a length to width aspect ratio as large as 3000, are grown from a 1,2,4-trimethylbenzene solution of C60. They have been formed to possess a highly unusual morphology, with each nanowire being composed of two nanobelts joined along the growth direction to give a V-shaped cross section. The crystal structure of these nanowires is found to be orthorhombic, with the unit cell dimensions of a = 10.2 A, b = 20.5 A, and c = 25.6 A. Structural and compositional analyses enable us to explain the observed geometry with an anisotropic molecular packing mechanism that has not been observed previously in C60 crystal studies. The nanowires have been observed to be able to transform into carbon nanofibers following high-temperature treatment, but the original V-shaped morphology can be kept unchanged in the transition. A model for the nanowire morphology based upon the solvent-C60 interactions and preferential growth directions is proposed, and potentially it could be extended for use to grow different types of fullerene nanowires.

The unusual nanostructure of nickel-boron catalyst.

A highly unusual nanostructure of the nickel-boron particulate material, initially synthesised in the 1950s and well known to be an exceedingly active hydrogenation catalyst, has been identified for the first time.

Highly efficient catalysts for the hydrogenation of nitro-substituted aromatics.

Nanoparticles of Co and NiPd, derived from colloidal precursors and supported on commercially available non-ordered mesoporous silica, are highly effective, cheap, recyclable and industrially viable catalysts for the hydrogenation of a range of nitro-substituted aromatics under mild conditions.

Suppressed electron hopping in a Au nanoparticle/H2S system: development towards a H2S nanosensor.

We herein report and discuss electron transport within a Au/H2S nanoscale device and thereby highlight a phenomenon that may be used in the development of a novel on-chip H2S sensor.

Production of carbon nanofibers in high yields using a sodium chloride support.

A new route for the highly convenient scalable production of carbon nanofibers on a sodium chloride support has been developed. Since the support is nontoxic and soluble in water, it can be easily removed without damage to the nanofibers and the environment. Nanofiber yields of up to 6500 wt % relative to the nickel catalyst have been achieved in a growth time of 15 min. Electron microscopy (SEM, TEM) and thermal gravimetric analysis (TGA) indicated that the catalytically grown carbon had relatively little thermal over-growth and possessed either a herringbone or a semi-ordered nanostructure, depending on the growth conditions.

Direct conversion of iron stearate into magnetic Fe and Fe3C nanocrystals encapsulated in polyhedral graphite cages.

We report a direct salt-conversion approach for large-scale synthesis of carbon-encapsulated magnetic Fe and Fe3C nanoparticles.

A nanoscale dendrimer-based Fe24 cluster: synthesis and molecular self-assembly.

We report the synthesis, characterisation and self-assembly of a nanoscopic Fe24 cluster using an organic dendrimer as a molecular substrate for building up well-defined molecular nanostructures.

Analysis of low oxidation state transition metal clusters by laser desorption/ionization time-of-flight mass spectrometry.

A variety of homonuclear and heteronuclear transition metal carbonyl clusters have been analyzed by ultraviolet laser desorption/ionization time-of-flight mass spectrometry. The spectra were recorded in negative and positive ion modes, using both linear and reflective techniques. A range of different clusters based on different nuclearities, geometries, and ligand types, which include hydrides, phosphines, nitriles, and cyclopentadienyl ligands and naked main group atoms, were studied. These experiments have allowed us to construct a detailed picture of the technique for the analysis of transition metal carbonyl clusters and their derivatives. In general, extensive reactions are observed, cluster aggregation reactions in particular, and from a comparison of the spectra obtained, some mechanistic inferences concerning the aggregation processes have been drawn.

Constraining asymmetric organometallic catalysts within mesoporous supports boosts their enantioselectivity.

By constraining tethered asymmetric organometallic catalysts within the nanopores of silica supports so as to increase the interaction between the pore wall and the active center (and hence to restrict access of the reactant to the catalyst), a significant improvement in enantioselectivity is achieved. A schematic illustration of a cationic chiral, organometallic catalyst, [Rh(I)(COD)PMP] {(S)-(+)-1-(2-pyrrolidinylmethyl)-pyrrolidine and cyclooctadiene}, which is noncovalently anchored via a N-H...F hydrogen bond with the triflate ion, CF3SO3-, to the curved inner surface of a 38 A diameter pore of a silica support for the asymmetric hydrogenation of the C=O bond in methyl benzoylformate to the corresponding methyl mandelate is shown. Rh (purple); N (blue); H (white); F (green); C (gray); S (yellow); O (red).