Nanometric NaYF4 as an Unconventional Support for Gold Catalysts for Oxidation Reactions.

The metal-support interaction plays an important role in gold catalysis. We employ here crystalline cubic (α-) and hexagonal (ß-) phases of heterometallic fluoride NaYF4 nanoparticles (NPs), obtained by the decomposition of a single source precursor [NaY(TFA)4(diglyme)] (TFA = trifluoroacetate), as nonoxide supports for gold catalysts. Using an isostructural gadolinium analogue, we also obtained doped α-NaYF4:Gd3+ and ß-NaYF4:Gd3+ NPs. A successful deposition of ∼1% by weight gold NPs of average size 5-6.5 nm on these doped and undoped metal fluorides using HAuCl4·3H2O afforded Au/NaYF4 catalysts which were thoroughly characterized by using several physicochemical techniques such as X-ray diffraction, Brunauer-Emmett-Teller analysis, high-resolution transmission electron microscopy, energy-dispersive X-ray spectrometry, and X-ray photoelectron spectroscopy. A comparative study of the above catalysts for different oxidation reactions show that while for the aerobic oxidation of trans-stilbene in solution phase, they are either better (in terms of stilbene conversion) or at par (in terms of trans-stilbene oxide yield) in comparison to the reference catalyst Au/TiO2 of the World Gold Council, their activity toward CO oxidation reactions in gas phase remains much less than that of gold catalysts supported on metal oxides.

One-pot deposition of palladium on hybrid TiO2 nanoparticles and catalytic applications in hydrogenation.

One-pot deposition of Pd onto TiO(2) has been achieved through directly contacting palladium(II) salt with nanosized functionalized TiO(2) support initially obtained by sol-gel process using titanium isopropoxide and citric acid. Citrate groups act as functional moieties able to directly reduce the Pd salt avoiding any further reducing treatment. Various palladium salts (Na(2)PdCl(4) and Pd(NH(3))(4)Cl(2)·H(2)O) and titanium to citrate (Ti/CA) ratios (20, 50, and 100) were used in order to study the effect of the nature of the precursor and of the citrate content on the final Pd particle size and catalytic properties of the as-obtained Pd/TiO(2) systems. Characterization was performed using N(2) adsorption-desorption isotherms, ICP-AES, FTIR, XRD, XPS, and TEM. The as-obtained hybrid Pd/TiO(2) catalysts were tested in the selective hydrogenation (HYD) of an α,ß-unsaturated aldehyde, i.e. cinnamaldehyde. Citrate-free Pd/TiO(2)-based catalysts present lower selectivity into saturated alcohol. However, citrate-functionalized Pd/TiO(2) catalyst seems to control the selectivity, the particle size and dispersion of Pd NPs leading to high intrinsic activity.

New synthesis approach for hybrid Gd(III)-loaded Nanocrystalline TiO2 as potential magnetic resonance imaging contrast agents.

A new aqueous "in situ and one pot" method to elaborate surface-functionalized Gd(III)-doped anatase TiO2 nanoparticles, starting from heterometallic/heteroleptic Gd-Ti alkoxides, is reported. Full characterization and thermal behaviour of the nanomaterials demonstrate that this molecular approach offers the convenience of working under soft temperature and pH conditions in order to allow fine tuning of the organic surface functionalization and gadolinium doping. In vitro phantom images exhibit potential MRI T1-contrast agent properties.

Aminoalkoxo-supported heteroleptic hexanuclear gallium(III) wheel as a synthon for group 13 heterometallics: a rare sol-gel precursor for mixed Al-Ga oxide as support for gold catalysts.

A new heteroleptic gallium(III) complex, Ga(6)Cl(6)(mdea)(6) (1 x 2 CHCl(3)) (mdeaH(2) = N-methyl diethanolamine) was prepared in good yield by a chloro-aminoalkoxo exchange reaction and used as a synthon for the synthesis of a novel group 13 heterometallic derivative, Ga(2)Al(4)(O)(2)(mdea)(2)(OPr(i))(10) (2 x 2 CHCl(3)), the latter acting as a facile single source precursor for the sol-gel preparation of the mixed Al-Ga oxide as a high surface area support for gold catalysts.

Aerobic methylcyclohexane-promoted epoxidation of stilbene over gold nanoparticles supported on Gd-doped titania.

Aerobic partial oxidations of alkanes and alkenes are important processes of the petrochemical industry. The radical mechanisms involved can be catalyzed by soluble salts of transition metals (Co, Cu, Mn...). We show here that the model methylcyclohexane/stilbene co-oxidation reaction can be efficiently catalyzed at lower temperature by supported gold nanoparticles. The support has little influence on gold intrinsic activity but more on the apparent reaction rates which are a combination of catalytic activity and diffusion limitations. These are here minimized by using gadolinium-doped titania nanocrystallites as support for gold nanoparticles. This material is obtained by mild hydrolysis of a new Gd(4)TiO(O(i)Pr)(14) bimetallic oxoalkoxide. It leads to enhanced wettability of the < 3 nm gold particles in the tert-butyl hydroperoxide (TBHP)-initiated epoxidation of stilbene in methylcyclohexane; Au/TiO(2):Gd(3+) is in turn as active as the state-of-the-art hydrophobic Au/SiO(2) catalyst. The rate-determining step of this reaction is identified as the gold-catalyzed homolytic decomposition of TBHP generating radicals and initiating the methylcyclohexane-mediated epoxidation of stilbene, yielding a methylcyclohexan-1-ol/trans-stilbene oxide mixture. Methylcyclohexan-1-ol can also be obtained in the absence of the alkene in the gold-catalyzed solvent-free autoxidation of methylcyclohexane, evidencing the catalytic potential of gold nanoparticles for low temperature C-H activation.

Design of hybrid titania nanocrystallites as supports for gold catalysts.

Citrate-functionalized titania nanocrystallites are successfully synthesized from a heteroleptic titanium alkoxide precursor in a low temperature, hydrolytic process and used as gold catalyst supports for CO oxidation and aerobic stilbene epoxidation.