Particles iridium

Iridium and rhodium nanoparticles have also been studied in the hydrogenation of various aromatic compoimds. In all cases, total conversions were not observed in BMI PF6. TOFs based on mol of cyclohexane formed were 44 h for toluene hydrogenation with Ir (0) and 24 h and 5 h for p-xylene reduction with lr(0) or Rh(0) nanoparticles, respectively. The cis-1,4-dimethylcyclohexane is the major product and the cisitrans ratio depends on the nature of the metal 5 1 for lr(0) and 2 1 for Rh(0). TEM experiments show a mean diameter of 2.3 nm and 2.1 nm for rhodium and iridium particles, respectively. The same nanoparticle size distribution is observed after catalysis (Fig. 4). 

In the absence of cyclohexene the same procedure yields larger (Ir(0) 9oo) nanoclusters (size 3 0.4nm). Besides zerovalent Iridium- [167,288,290], Rh(0)-nanocluster of the Finke-type have been prepared [290-292]. Finke s nanoclusters have been carefully examined using a combination of modern instrumental analysis methods [167]. It was revealed that the lr(0) core is uncharged and that the iridium particles exhibit an extremely clean, fully exposed, and chemically very reactive metallic surface. 

Figure 8 shows the appearance of iridium particles on alumina, after first reducing and then calcining in oxygen at 600 C for 0.5 hr. Inspection of this micrograph reveals some major differences in behavior from that encountered with the... 

Although the Ira and Ir clusters catalyze the same reactions as metallic iridium particles, their catalytic character is different, even for structure-insensitive hydrogenation reactions. It is inferred [15] that the clusters are metal-like but not metallic consistent with the structural inferences stated above, we refer to them as quasi molecular. Thus these data show the limit of the concept of structure insensitivity it pertains to catalysis by surfaces of structures that might be described as metallic, i.e., present in three-dimensional particles about 1 nm in diameter or larger. This conclusion suggests that supported metal clusters may be found to have catalytic properties superior to those of conventional supported metals for some reactions. The suggestion finds some support in the results observed for platinum clusters in zeolite LTL, as summarized below. 

Kotseva K, Popov T (1998) Study of the cardiovascular effects of occupational exposure to organic solvents. Int Arch Occup Environ Health 71(suppl) S87-S91 Kreyling WG, Semmler M, Erbe F, Mayer P, Takenaka S, Schulz H, Oberdorster G, Ziesenis A (2002) Translocation of ultrafine insoluble iridium particles from lung epithelium to extrapulmonary organs is size dependent but very low. J Toxicol Environ Health A 65 1513-1530... 

Iridium particles were obtained [242, 243] from microemulsions in the system PEGDE / hexane / aqueous solution of IrCl. The reduction was achieved through hydrogen. The particles had an average diameter of 2.5 nm. 

Somerville et al. [221] employed similar preparation techniques to prepare encapsulated Ir4(CO)i2 clusters inside NaY zeolite they also prepared lr4(CO)i2 clusters on the external surface by impregnation of the zeoUte with a slurry of Ir4(CO)i2 in cyclohexane. Upon thermal decomposition under hydrogen the carbonyl in NaY yielded 4-6 Ir atom clusters, whereas the carbonyl on NaY sintered into 2 nm iridium particles. 

CO oxidation, an important step in automotive exhaust catalysis, is relatively simple and has been the subject of numerous fundamental studies. The reaction is catalyzed by noble metals such as platinum, palladium, rhodium, iridium, and even by gold, provided the gold particles are very small. We will assume that the oxidation on such catalysts proceeds through a mechanism in which adsorbed CO, O and CO2 are equilibrated with the gas phase, i.e. that we can use the quasi-equilibrium approximation. 

In the early work on the thermolysis of metal complexes for the synthesis of metal nanoparticles, the precursor carbonyl complex of transition metals, e.g., Co2(CO)8, in organic solvent functions as a metal source of nanoparticles and thermally decomposes in the presence of various polymers to afford polymer-protected metal nanoparticles under relatively mild conditions [1-3]. Particle sizes depend on the kind of polymers, ranging from 5 to >100 nm. The particle size distribution sometimes became wide. Other cobalt, iron [4], nickel [5], rhodium, iridium, rutheniuim, osmium, palladium, and platinum nanoparticles stabilized by polymers have been prepared by similar thermolysis procedures. Besides carbonyl complexes, palladium acetate, palladium acetylacetonate, and platinum acetylac-etonate were also used as a precursor complex in organic solvents like methyl-wo-butylketone [6-9]. These results proposed facile preparative method of metal nanoparticles. However, it may be considered that the size-regulated preparation of metal nanoparticles by thermolysis procedure should be conducted under the limited condition. 

MeV a-particles and used the Au/Ir source after annealing without any further chemical or physical treatment. Commercially available sources are produced via Pt(p, n) Au. The most popular source matrix into which Au is diffused is platinum metal although it has the disadvantage of being a resonant matrix - natural platinum contains 33.6% of Pt. Using copper and iridium foils as host matrices for the Au parent nuclide, Buym et al. [327] observed natural line widths and reasonable resonance absorption of a few percent at 4.2 K. 

Other metals on silica supports have been investigated less extensively than platinum and nickel, and average particle diameters have only been estimated by gas adsorption methods, supported in a few cases by X-ray line broadening data. Thus, rhodium, iridium, osmium, and ruthenium (44, 45) and palladium (46) have all been prepared with average metal particle diameters <40 A or so, after hydrogen reduction at 400°-500°C. 

These spectra show even fewer features than those of the palladium catalysts. Absorption takes place almost exclusively in the region 2000-2100 cm-1. There are some weak bands below 2000 cm-1, but our experimental method did not allow us to determine their frequencies with reasonable accuracy. It is clear that also with the iridium catalysts the particle size has an effect on the spectra. The spectrum of Ir-8 shows only one intense band at 2048 cm-1, whereas the other two have additional bands at higher frequencies. There is also a marked dependence of the intensity of the 2048 cm-1 band on the CO pressure, especially in the case of Ir-37 and Ir-100. We shall not try to interpret the CO spectra of the iridium samples, as we consider the data available insufficient for the purpose. 

The results obtained with nickel raised the question whether the relation found between rate of exchange and particle size holds also for other metals of group VIII. We therefore carried out the benzene-D2 reaction on some iridium catalysts widely differing in particle size. We chose iridium because we knew from earlier experiments that iridium black gives a very characteristic cyclohexane isotopic distribution pattern with a maximum for C6H4Ds, whereas the patterns of Ni, Ru, Pd, and Pt show a maximum for the d6 compound. 

The outcome of the experiments on nickel and on iridium shows that there is a marked contrast between these metals as regards the influence of particle size upon the course of the reaction of benzene with deuterium. Clearly, further experiments should be done along the same lines with other metals of group VIII of the periodic system. 

Pt catalysts was the dehydrogenation to toluene, which was independent of catalyst particle size. Meanwhile, the selectivity for RO of MCH on iridium and ruthenium was 50% and RO was found to occur only at unsubstituted C-C bonds. 

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