Nanoparticles iridium

The isolated Ru(0) nanoparticles were used as solids (heterogeneous catalyst) or re-dispersed in BMI PP6 (biphasic liquid-liquid system) for benzene hydrogenation studies at 75 °C and under 4 bar H2. As previously described for rhodium or iridium nanoparticles, these nanoparticles (heterogeneous catalysts) are efficient for the complete hydrogenation of benzene (TOP = 125 h ) under solventless conditions. Moreover, steric substituent effects of the arene influenced the reaction time and the decrease in the catalytic TOP 45, 39 and 18h for the toluene, iPr-benzene, tBu-benzene hydrogenation, respectively, finally. The hydrogenation was not total in BMI PPg, a poor TOE of 20 h at 73% of conversion is obtained in the benzene hydrogenation. 

Catalysts other than homogeneous (molecular) compounds such as nanoparticles have been used in ionic liquids. For example, iridium nanoparticles prepared from the reduction of [IrCl(cod)2] (cod = cyclooctadiene) with H2 in [bmim][PF6] catalyses the hydrogenation of a number of alkenes under bipha-sic conditions [27], The catalytic activity of these nanoparticles is significantly more effective than many molecular transition metal catalysts operating under similar conditions. 

Iridium nanoparticles generated in l-n-butyl-3-methylimidazolium (BMI)-based ionic liquids were found to be excellent recyclable catalytic systems for the hydrogenation of a variety of substrates, including ketones such as simple ketones. The Ir nanoparticles were prepared by simple reduchon of [Ir(cod)Cl]2 dispersed in BMI-PFis at 75 °C under 4 atm of H2. Benzaldehyde, cyclopentanone, methyl butanone and derivatives were hydrogenated with almost complete conversion, with TOFs ranging from 17 to 96h under solventless conditions (substrate Ir ratio = 250, 75 °C, 4 atm FI2) [102]. 

Soluble and stable iridium nanoparticles (3.0 0.4nm diameter) have been prepared by reduction of the polyoxoanion-supported lr(l) complex (n-Bu4N)sNa3 [(C0D)lr(P2WisNb3062)] (COD = 1,5-cyclo-octadiene) with molecular hydrogen in... 

A simple and general method for the preparation of surfactant-free, thiol-functionalized iridium nanoparticles was reported by Ulman and coworkers in 1999 [11], The synthesis consisted of a reduction of the dihydrogen hexachloroiri-date (IV) H2lrCl6 H20 precursor by lithium triethylborohydride ( super-hydride ) in the presence of octadecanethiol (C18H37SH) in tetrahydrofuran (THF) (Scheme 15.1). The obtained iridium nanoparticles were crystaUine with fee (face-centered cubic) packing, and showed a wider size distribution with diameters ranging from 2.25 to 4.25 nm. 

Scheme 15.1 One-phase synthesis of thiol-functionalized iridium nanoparticles as proposed by Ulman and coworkers.

Figure 15.1 Transmission electron microscopy images showing iridium nanoparticles prepared in four different capping ligands, (a) Oleic acid/oleylamine (b) TOAB ...

An interesting method to produce water-soluble iridium nanoparticles was proposed by Chaudret and coworkers [13]. Here, aqueous soluble iridium nanoparticles were synthesized by the chemical reduction of iridium trichloride with sodium borohydride in an aqueous solution of the surfactant N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium chloride (Scheme 15.2). The precursor reduction was assisted by sonication, while the gradual conversion of Ir(lll) ions to lr(0) nanoparticles was followed using UV spectroscopy. The use of a molar surfactant Ir ratio of 10 proved sufficient to obtain stable aqueous soluble iridium nanoparticles however, if the molar surfactant Ir ratio used was <10 then agglomeration was observed in solution after several days. TEM analysis of the iridium nanoparticles revealed a monodispersed size distribution and a mean diameter of 1.9 0.7nm (Figure 15.2). 

I 75 Catalytic Properties of Soluble Iridium Nanoparticles NaBH4, surfactant, ultrasound irradiation... 

Scheme 15.2 System employed by Chaudret and coworkers to prepare soluble iridium nanoparticles in an aqueous medium.

Figure 15.2 Transmission electron microscopy image of iridium nanoparticles of 1.9 0.7nm in diameter (400 particles counted) prepared in the presence of the surfactant N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium chloride. (Reproduced with permission from Ref [13] 2004 Wiley-VCH).

Scheme 15.3 Preparation of soluble iridium nanoparticles from in situ reduction of the organometallic precursor [ir(COD)Cl]2 in imidazolium ionic liquids.

Figure 15.4 Transmission electron microscopy images and size-distribution histograms (300 particles counted) for iridium nanoparticles prepared in (a) BMI BF (b) BMI PFs and (c) BMI CF3SO3. (Reproduced with permission from Ref [25] 2006 Elsevier).

The typical in situ reduction of the precursor [lr(COD)Cl]2 by molecular hydrogen under the same reaction conditions have been also performed in 1-n-butyl-3-methylimidazolium trifluoromethanesulfonate (BMl-CFsSOs) and 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMl-BF [25]. The iridium nanoparticles prepared in BMTCF3SO3 and BM1-BF4 ILs, as previously observed with BM1-PF6, display irregular shapes with a monomodal size distribution (Figure 15.4). Mean diameters in the range of 2-3 nm were estimated with in situ TEM and small-angle X-ray scattering (SAXS) analyses of the lr(0) nanoparticles soluble in the ionic hquids, and by X-ray diffraction (XRD) of the isolated material. The mean diameters of iridium nanoparticles synthesized in the three ILs, as estimated by TEM, SAXS and XRD, are summarized in Table 15.1. 

Table 15.1 Comparison of the mean diameters of iridium nanoparticles as determined by TEM, SAXS and XRD techniques [25],...

XPS measurements showed clearly the interachons of the IL with the metal surface, that occurs through F (for BF4 and PF i) or O (for CF3SO1) of the anions, demonstrating the formation of an IL protechve layer surrounding the iridium nanoparticles. Additional extended X-ray absorption fine structure (EXAFS) analyses also provided evidence for interaction of the IL liquid with the metal surface. 

Kinetic Studies of Iridium Nanoparticle Formation The Autocatalytic Mechanism... 

Using the well-defined system of polyoxoanion/Bu4N -stabilized iridium nanoparticles [9, 29] as a model for the studies, Finke and coworkers [30] proposed a method that attempted to explain the formation and growth of transition-metal nanoparticles. This indirect method is based on an autocatalytic mechanism that considers a nudcation step in which a precursor A is converted to a zero-valent nuclei B with a rate constant fej, and a second step that considers the autocatalytic surface growth of the metal nanoparticles where species B catalyzes its own formation with a rate constant tc2 (Scheme 15.5). 

An interesting additional experiment to follow the iridium nanoparticles formation was demonstrated by Watzky and Finke ]30], who used a direct method of monitoring by gas-hquid chromatography (GLC) the evolution of cyclo-octane... 

In the same context, the autocatalytic mechanism was successfully applied to the formation of iridium nanoparticles dispersed in imidazolium-based ILs [25,... 

Indeed, in many cases the creation of soluble nanoparhcles has provided singular catalyhc activities/selectivities that differ from those expected for both molecular (single-site) and heterogeneous (mulh-site) catalysts [40, 41]. As a result, iridium nanoparticles have attracted much interest in terms of their catalyhc performance in the hydrogenation of olefins, ketones and aromahc compounds. 

Interestingly, the authors noted an increasing catalytic activity during a recycling reaction conducted with TOAB-coated iridium nanoparticles, over four cycles, with... 

---