Interatomic distances, platinum 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 review of 7( Pt, Pt) coupling constants [67] as well as other reports [68,69] indicate that they do not correlate with the interatomic distance between the platinum atoms. Surprising is the permutation of the values of y( Pt, Pt) in similar clusters (see Table 8, refs. [68] and [70]). 

We begin by considering the iridium EXAFS of a reference material such as metallic iridium or a catalyst containing pure iridium clusters. An EXAFS function for the iridium in the platinum-iridium catalyst is then generated from the function for the reference material by introducing adjustments for differences in interatomic distances, amplitude functions, and phase shifts. In making such adjustments, we are aided by the fact that the amplitude functions and phase shift functions of platinum are not very different from those of iridium, as shown in Figures 4.27 and 4.28. 

To obtain structural information on platinum-iridium clusters from EXAFS data, we concentrate primarily on the determination of interatomic distances. To obtain accurate values of interatomic distances, we need to have precise information on phase shifts. In this regard, we are fortunate that the phase shift functions of platinum and iridium are not very different. 

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 nuclearity of platinum clusters in Y-type zeolite has been studied by comparing the distribution of interatomic distances obtained experimentally with the Radial Electron Distribution (RED) technique [6,7] and various distributions calculated from model clusters with an f.c.c. structure. Fig. 2 shows that the experimental distribution... 

Figure 2. Distribution of interatomic distances in platinum clusters encaged in Y-zeolites. Curve a, distribution calculated for a AO-atom truncated f.c.c. tetrahedron (see Fig. 3). Curve b, experimental distribution obtained with the RED technique from X-ray scattering data. Curve c, calculated distribution for a mixture of AO-atom truncated tetrahedra and of six-atom octahedra.

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