Iridium enhancement factor

Fig. 24. (a) Enhancement factor R(t) as a function of the energy relative to the Fermi level for field emission from oxygen covered iridium (100) and (110) surfaces, (b) Schematic representation of the oxygen derived levels at the surface (51). 

Nonetheless, the effectiveness of PC02H as a precursor to a stable iridium(III) hydride is limited since on a preparative scale the product of the reaction of equimolar quantities of 1 and PCO2H consists of a mixture which contains considerable quantities of the iridium(I) complex (v o = 1960 cm-1) wherein the carboxylic acid group is pendant. The chelating phenol, POH, is comparable to PC02H in its R value the enhancement factor, RpOH PhOH is significantly greater than that for the benzoic acids. 

These results confirmed that the presence of metal particles clearly enhance the OSC of these materials. OSC measured at 400X could be increased by a factor of 3 to 7. With both supports, iridium and ruthenium were shown to be the most effective metal to enlarge OSC. 

Figure 24 Concentration profiles of siderophile elements in a radially zoned Fe,Ni grain in the CBb chondrite, QUE 94411 (a) electron microprobe data (b) and (c) trace element data from laser ablation ICPMS (Campbell et ai, 2001). The nickel, cobalt, and chromium profiles can be matched by nonequilibrium nebular condensation assuming an enhanced dust-gas ratio of —36 X solar, partial condensation of chromium into silicates, and isolation of 4% of condensates per degree of cooling (Petaev etal, 2001). Concentrations of the refractory siderophile elements, osmium, iridium, platinum, ruthenium, and rhodium, are enriched at the center of the grain by factors of 2.5-3 relative to edge concentrations, which are near Cl levels after normalization to iron (reproduced by permission of University of Arizona on behalf of The Meteoritical Society from Meteorit. Planet. ScL, 2002, 37, pp. 1451-1490).

Relative rates of O2 uptake by tra/is-[Ir(X)(CO)(PPh2R)2] ( = Cl, Br, I R = Ph, Et, Me) in CHjCh follow the order R = Me > Et > Ph and X = I > Br > Cl > F. If dioxygen addition is considered as an acid-base reaction (metal complex = Lewis base), then factors which increase the electron density at the metal centre should (in the absence of unfevourable steric effects) enhance the addition of O2 (reaction 90). The results suggest that as the basicity of the phosphine ligand increases and the electronegatively of X decreases, the rate of O2 addition also increases. The X-ray crystal structure of [Ir(Cl)(C0)(02)(PPh2Et)2] reveals a side-on coordination with the O2 centre, CO and Cl moieties defining the equatorial plane, or a distorted octahedral iridium(III) system with... 

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