Inma Angurell

influence of the support on surface rearrangements of bimetallic nanoparticles in real catalysts

Supported nanoparticles make the reaction faster Several techniques now allow surface structures used as catalysts to be probed during exposure to reactive gases, as opposed to under vacuum conditions. Divins et al. used near-ambient-pressure x-ray photoelectron spectroscopy to compare the effect of reaction gases on unsupported palladium-rhodium nanoparticles versus ones on a reducible cerium oxide support. For the reaction of ethanol with steam to produce hydrogen, the supported nanoparticles were more reactive and less prone to reduction and surface rearrangement. Science, this issue p. 620 A cerium oxide support rendered palladium-rhodium nanoparticles more reactive and harder to reduce under reaction conditions. Catalysts used for heterogeneous processes are usually composed of metal nanoparticles dispersed over a high–surface-area support. In recent years, near-ambient pressure techniques have allowed catalyst characterization under operating conditions, overcoming the pressure gap effect. However, the use of model systems may not truly represent the changes that occur in real catalysts (the so-called material gap effect). Supports can play an important role in the catalytic process by providing new active sites and may strongly affect both the physical and chemical properties of metal nanoparticles. We used near-ambient pressure x-ray photoelectron spectroscopy to show that the surface rearrangement of bimetallic (rhodium-palladium) nanoparticles under working conditions for ethanol steam reforming with real catalysts is strongly influenced by the presence of a reducible ceria support.

Metallodendrimers containing both ruthenium (internal layer) and rhenium (external layer)

The reaction of [Re(bipy)(CO)3(CCpy)] with two carbosilane dendrimers containing terminal (p-cymene)Ru(triflate)2 units gives bimetallic dendrimers functionalised with (p-cymene)Ru(triflate)(pyCC)Re(bipy)(CO)3 units. Dendrimers of this kind can be envisaged as formed by two metal layers: the internal, constituted by ruthenium atoms, and the external, by rhenium metals. The luminescent properties of these species are only attributed to the rhenium moieties.

Unprecedented eight-palladium(I) crown-cycle with metal–metal unsupported bonds

Tri(N-pyrrolyl)phosphine reacted with the sigma/pi complex [Pd(mu-Cl)(COD-MeO)]2 to give the octa-cycle [Pd(mu-Cl)[P(pyrl)3]]8 containing four Pd(I)-Pd(I) unbridged bonds.

Synthesis and catalytic properties of neutral and cationic rhodium- and iridium-containing carbosilane dendrimers

The reaction of phosphanyl-terminated carbosilane dendrimers containing only one phosphorus ligand per arm with [MCl(cod)]2 (M = Rh, Ir; cod = cycloocta-1,5-diene) resulted in the grafting of MCl(cod) moieties on the surface of the dendrimer. However, dendrimers displaying two phosphorus ligands per arm gave very unstable species which were not isolated. On the other hand, the last group of dendrimers reacted with [MCl(cod)]2, in the presence of silver salts, to afford cationic dendrimers in which a metal fragment is attached simultaneously to both phosphorus atoms of the same arm. The substitution of cod with 1,1′-bis(diphenylphosphino)ferrocene (dppf) in one example of the latter group yielded a bimetallic Rh/Fe layer-containing dendrimer. The new metallodendrimers were tested as catalysts in the hydrogenation of 1-hexene.

Single and double metallic layer-containing ruthenium dendrimers. Synthesis and catalytic properties

The reaction of a series of phosphanyl-terminated carbosilane dendrimers displaying only one phosphorus ligand per arm with [RuCl(2)(p-cymene)](2) resulted in the grafting of RuCl(p-cymene) moieties on the periphery of the dendrimer. In these species, the chloride ligand is easily displaced by the organic bases pyridine, 4-cyanopyridine and 4,4[prime or minute]-bipyridine to afford new cationic metallodendrimers. NMR studies have confirmed the chirality of the ruthenium centre. The species containing 4,4[prime or minute]-bipyridine reacts through the uncoordinated pyridyl nitrogen with a new equivalent of [RuCl(2)(p-cymene)](2) or [RhCl(CO)(2)](2) to lead to homo- or hetero-bimetallic layer-containing dendrimeric systems. The ruthenodendrimers were tested as catalysts in the transfer hydrogenation of cyclohexanone by propan-2-ol and their activity compared with that of some analogous mononuclear ruthenium(ii) complexes.

Improved Stability of Pd/Al2O3 Prepared from Palladium Nanoparticles Protected with Carbosilane Dendrons in the Dimethyl Ether Steam Reforming Reaction

The dimethyl ether steam reforming reaction to generate hydrogen was tested over 1 wt % Pd/Al2O3 catalysts prepared from different precursors. The conventional catalyst prepared by incipient wetness impregnation from palladium nitrate underwent strong deactivation under reforming conditions at 823 K because of metal sintering and carbon deposition. The same occurred over the catalyst prepared from preformed Pd nanoparticles protected with dodecanethiol. In contrast, catalysts prepared from Pd nanoparticles protected with carbosilane dendrons showed enhanced stability and good performance for the production of hydrogen. This was because of the formation of SiO2 at the Pd–Al2O3 interface that acted as a pinning center and prevented Pd migration under the reaction conditions as well as the accumulation of carbon.