Platinum-iridium clusters

The sole example of a silicon-platinum cluster is the compound in entry 24 its structure has been noted in Section IV,A. It seems very likely that many further cluster systems await discovery, particularly with iridium, platinum, and gold, and that this represents an important future area of research. One obvious application is as precursors to metal silicides with high metal silicon ratios using c.v.d. techniques (compare Section V,A). 

Chemistry similar to that described above for iridium clusters has also been observed for rhodium clusters. Several authors [16-18] have prepared [Rh6(CO)i6] in NaY zeolite [R1i4(CO)i2] has also been formed [18], and each of these has been decarbonylated with minimal changes in the metal frame, as shown by EXAFS spectroscopy [18]. Thus there appears to be some generality to the method of forming small clusters in zeolite cages by synthesis of stable metal carbonyl precursors followed by decarbonylation. However, the method is limited. Attempts to use it to prepare zeolite-supported platinum clusters that are structurally simple and uniform have apparently not been successfiil. The literature of platinum carbonyl clusters in zeolites is not considered here because it is still contradictory. 

Alternatively, extremely small clusters of platinum have been formed in zeolite LTL without a calcination step. Vaarkamp et al. [25] prepared platinum clusters of about 5 or 6 atoms each, on average, from Pt(NH3)4(N03)2 in zeolite BaKLTL simply by reduction at 500°C. Triantafillou et al [26] similarly prepared iridium clusters from [Ir(NH3)5Cl]Cl2 in zeolite KLTL by reduction in hydrogen at 300 or 500°C, also without calcination, the average cluster contained about 5-6 iridium atoms. 

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. 

Iridium clusters in zeolite KLTL, like the platinum clusters, consisting of 4 to 6 atoms on average, have also been prepared by hydrogen reduction of [Ir(NH3)5Cl]Cl2 in the pores at temperatures >300°C [26]. Even though the iridium clusters were as small as the selective platinum clusters in the same basic zeolite support, they were found to be unselective catalysts, being similar to other iridium catalysts for conversion of n-hexane and hydrogen principally into hydrogenolysis products. It is inferred that the combination of cluster size, electronic... 

Noble Metal (Platinum, Palladium, Rhodium, Ruthenium, Iridium, Osmium) Clusters... 

If there were no interaction between the platinum and iridium in the catalysts, that is, if the catalysts consisted simply of mixtures of platinum clusters and iridium clusters, it would be reasonable to expect the hydrogenolysis activity per iridium atom to remain constant as platinum is incorporated in the catalysts. This expectation would be based on the fact that the activity of platinum for hydrogenolysis is negligible compared to that of iridium, about five orders of magnitude lower for ethane hydrogenolysis (44), so that the added platinum would presumably behave as an inert diluent. 

In the diffraction patterns in Figure 4.23, the platinum-iridium bimetallic clusters exhibit a single diffraction line (middle field of figure) about midway between the lines (upper field of figure) for bulk platinum and bulk iridium. The line for the clusters is broader than the lines for the bulk metals because... 

The diffraction pattern of the physical mixture of silica-supported platinum clusters and iridium clusters in the lower field of Figure 4.23 consists of overlapping lines for the two individual types of clusters, and is clearly different from the pattern for the platinum-iridium bimetallic clusters. The overlapping is due to the broadened nature of the individual lines resulting from the small sizes of the individual clusters of platinum and iridium. 

Figure 4.23 X-ray diffraction patterns for a platinum-iridium bimetallic cluster catalyst and for reference materials consisting of physical mixtures of platinum and iridium in the form of large crystals or dispersed monometallic clusters (4). (Reprinted with permission from Academic Press, Inc.)...

In contrast to the metal clusters in the Pt/Si02 and Ir/Si02 reference catalysts (19), those in the Pt/Al203 and lr/Al203 reference catalysts exhibit interatomic distances lower than the distances in the corresponding pure metals, which are 2.775 A and 2.714 A (33), respectively, for platinum and iridium. The contraction observed when the clusters are dispersed on alumina indicates an interaction with the carrier that is not apparent in the silica-supported clusters. The finding that the distance contractions are more pronounced for the bimetallic platinum-iridium catalyst than for the monometallic reference catalysts provides additional evidence that the bimetallic catalyst is. not simply a mixture of platinum clusters and iridium clusters. 

The spectrum of sample D-600 resembles that of sample B in Figure 4.32 more closely than it resembles the spectrum of sample D. The Mossbauer parameters in Table 4.2, especially the ratios A 2M, and W2IWU show substantial differences between samples D and D-600. The iridium in sample D-600 is largely present in the same poorly dispersed form as the iridium in sample C-600, as has been found from X-ray diffraction studies of similar samples. Sample D-600 therefore can be characterized approximately as consisting of highly dispersed platinum clusters incorporating iron atoms and separate iridium crystallites of much lower dispersion that are not significantly associated with iron atoms. This characterization is consistent with chemisorption data (41). 

The data indicate that platinum clusters associated with iron are altered by addition of iridium to the sample. We conclude that the added iridium is incorporated in the platinum clusters to give Ptlr clusters containing the iron probe atoms. If the added iridium was present as separate iridium clusters... 

At low temperature, platinum clusters 20 react with [Ir(CO)4] to give, via GO loss, butterfly complex 55. One carbonyl ligand in 55 can be displaced by triphenylphosphite giving 56 (Scheme 12), the X-ray structure of which clearly shows a butterfly structure with iridium at a wingtip position. Complex 55 is thermally unstable, and is shown... 

Yang et al. [236] succeeded in preparing platinum-iridium clusters by coexchange of Pt(NH3) and IrfNHjlsCF followed by calcination in O2 and reduction in H2 at 573 K. The Xe NMR data were sensitive to the number of clusters and to the surface composition. In a subsequent study, Hwang and Woo [237] prepared Pt-Ir clusters by ion-exchanging a PtY zeolite containing Pt-clusters with Ir(NH3)5CF followed by calcination and reduction. From Xe NMR and FTIR of CO adsorbed on metal clusters, it was concluded that iridium atoms were located on the surface of platinum clusters. 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

We plan to make studies on palladium-copper, iridium-copper, and platinum-copper catalysts to extend our investigation of the effect of varying miscibility of the components on the structural features of the bimetallic clusters present. With these additional systems, the whole range from complete immiscibility to total miscibility of copper with the Group VIII metal will be encompassed. 

Bimetallic clusters of platinum and iridium can be prepared by coimpregnating a carrier such as silica or alumina with an aqueous solution of chloroplatinic and chloroiridic acids (22,34). After the Impregnated carrier is dried and possibly calcined at mild conditions (250°-270 C), subsequent treatment in flowing hydrogen at elevated temperatures (300 -500°C) leads to formation of the bimetallic clusters. 

Because of- the similarity in the backscattering properties of platinum and iridium, we were not able to distinguish between neighboring platinum and iridium atoms in the analysis of the EXAFS associated with either component of platinum-iridium alloys or clusters. In this respect, the situation is very different from that for systems like ruthenium-copper, osmium-copper, or rhodium-copper. Therefore, we concentrated on the determination of interatomic distances. To obtain accurate values of interatomic distances, it is necessary to have precise information on phase shifts. For the platinum-iridium system, there is no problem in this regard, since the phase shifts of platinum and iridium are not very different. Hence the uncertainty in the phase shift of a platinum-iridium atom pair is very small. 

From results on interatomic distances derived from analysis of EXAFS data, one can draw some conclusions about the structure of platinum-iridium clusters (13,17). If the clusters were truly homogeneous, the interatomic distance characteristic of the platinum EXAFS should be identical to that characteristic of the iridium EXAFS. When we analyze EXAFS data on the clusters, however, we do not find this simple result. We find in general that the distances are not equal. The data indicate that the clusters are not homogeneous in other words,the environments about the platinum and iridium are different. We conclude that the platinum concentrates at the surface or boundary of the clusters. In the case of very highly dispersed platinum-iridium clusters on alumina, the clusters may well have "raft-like" two dimensional structures, with platinum... 

Recently we reported EXAFS results on bimetallic clusters of iridium and rhodium, supported on silica and on alumina (15). The components of this system both possess the fee structure in Efie metallic state, as do the components of the platinum-iridium system. The nearest neighbor interatomic distances in metallic iridium and rhodium are not very different (2.714A vs. 2.690A). From the results of the EXAFS measurements, we concluded that the interatomic distances corresponding to the various atomic pairs (i.e., iridium-iridium, rhodium-rhodium, and iridium-rhodium) in the clusters supported on either silica or alumina were equal within experimental error. Since the Interatomic distances of the pure metals differ by only 0.024A, the conclusion is not surprising. 

A stabilization of low valencies of metals and an induction time before cluster formation have been observed as well in the case of iridium [105], platinum [53], palladium [147], copper [94], or nickel [115]. 

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