Platinum interatomic distance

If we consider a set of platinum atoms (for example) along a line, so that the 1-dimensional Bravais lattice vector is R na, where a is the platinum interatomic distance and n an integer, the harmonic potential energy has the form [2]... 

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. 

Fig. 6.86. Oxygen-oxygen pair correlation function obtained from molecular dynamic simulations on the adsorbed layer of a Pt(100) surface. Ax and Ay are the projections of the interatomic distances in the x- and indirections, respectively. They reflect the positions of the oxygen atoms on the top site of the platinum lattice, and the pronounced form of the peaks refers to their relatively small displacement. (Reprinted from E. Spohr, G. Toth, and K. Heinzinger, Electrochim. Acta 41 2131, copyright 1996, Fig. 10a, with peimission from Elsevier Science.)...

Other Covalent Radii.—Bipositive nickel, palladium, and platinum and tripositive gold form four coplanar dsp bonds, directed to the comers of a square, with attached atoms. Examination of the observed values of interatomic distances reveals that square dsp radii of atoms have the same values as the corresponding octahedral d sp radii, as given in Table 7-15. This is shown by the comparisons on the following page. 

The rubidium salt Rbii67[Pt(C204)2]1.5H20 (RbOP) is a compound exhibiting a commensurate 2kF structure and, therefore, the distortion of the Pt chain has been determined precisely.66 The unit cell contains a distorted nonlinear sixfold Pt atom chain and involves six fPt(C204)2]1-67" units in one repeating unit of 17.11 A. There are three independent Pt—Pt distances of 2.716, 2.830 and 3.015 A. The Pt—Pt distance of 2.717 A is the shortest spacing so far observed in partially oxidized platinum complexes and it is shorter than the 2.77 A interatomic distance in Pt metal. 

Platinum-based bi-metallics (Pt M, M = Ti, Cr, V, Mn, Fe, Co, Ni, Cu, etc.) have been shown to exhibit enhanced activity toward the OER. Several rationales have been proposed including (1) enhanced chemisorption of intermediates (2) a lattice change of Pt that results in the shortening of Pt-Pt interatomic distances by alloying (3) the formation of skin Pt which has increased d-electron vacancy of the thin Pt surface layer caused by the underlying alloy and the anchor effect of alloy metals on a carbon carrier.93,94... 

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]). 

Osmium, iridium, and platinum catalysts with dispersions (ratio of surface metal atoms to total metal atoms) in the range 0.7 to 1 have been studied by Via, Sinfelt, and Lytle.As expected for small particles, the average co-ordination numbers were between 7 and 10, significantly lower than the value of 12 for the bulk metals. This result is in agreement with gas chemisorption data. Also, the disorder of the metal atoms, represented by the r.m.s. deviation of interatomic distance about its equilibrium value, was found to be greater by factors of 1.4—2.0 than for atoms in the bulk metals. Information on such disorder has not been available previously. 

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. 

Similar considerations apply to the phase shifts of interest in the analysis of the iridium ZH, EXAFS for platinum-iridium catalysts. Therefore, for simplicity, the phase shift functions for PtPt and Irlr are used in the analysis of the EXAFS associated with the platinum and iridium edges, respectively. This simplifying assumption introduces an uncertainty of only about 0.001 A in the interatomic distances derived from the data. 

To determine values of interatomic distances we return to Eq. 4.13. The platinum EXAFS function is given by the equation... 

The quantity D is a scaling factor related to coordination numbers, and Adifference between the value of cr2 characteristic of the platinum L m EXAFS of the platinum-iridium catalyst and the corresponding value of cr2 for a platinum reference material (48). The reference material is pure metallic platinum or a platinum catalyst. The quantity A (K) is the amplitude function of the platinum reference material. It is emphasized that the parameter A different from the analogous parameter in Eq. 4.14. Also, the values of the interatomic distance R in Eqs. 4.14 and 4.17 will not in general be equivalent. 

The interatomic distance determined from X-ray diffraction data on the alloy is 2.751 A, which is very close to the average of the two distances derived from Figure 4.29. If one assumes a linear relation between interatomic distance and alloy composition (Vegard s law), the value of 2.751 A would correspond to an alloy composition of 60% platinum, 40% iridium. The alloy composition as determined by X-ray fluorescence is 58% platinum, 42% iridium (48). 

Figure 4.30 Determination of interatomic distances in platinum-iridium catalysts by evaluation of the quality of fit of the expression for the sum of the platinum L EXAFS function and the function I,(K) arising from the iridium L m EXAFS to the corresponding sum derived from experimental data (48). (Reprinted with permission from the American Institute of Physics.)...

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. 

By applying the method described here to account for the overlap of the iridium and platinum EXAFS in the extended fine structure associated with the L in edge of platinum, it has been possible to obtain information on interatomic distances in a series of platinum-iridium catalysts. The ability of Eq. 4.13 for Xi+(A0 to fit the corresponding function derived from the experimental data, with the use of Eqs. 4.16 and 4.17 for the functions I,(K) and Xi(/0, respectively, is illustrated in Figure 4.31 for the various catalysts (48). 

Third, the doublet and, especially, sextet models require very precise superimposing of the molecule on the catalyst lattice. We have found that the cyclohexane derivatives, in accordance with the sextet model, smoothly dehydrogenate only on the following metals nickel, cobalt, iridium, palladium, platinum, ruthenium, osmium, and rhenium, all of which crystallize in Al, A3 lattices with certain interatomic distances. These results extend to the alloys of these metals. The catalytic activity of rhenium for this reaction was predicted by the multiplet theory as this metal maintains the square of activity this prediction was realized experimentally in the laboratory of the author. Similar correlations take place in the exchange of cyclanes with deuterium. 

The constitution of the platinum complex has been elucidated by crystal structure investigation 82), and the molecule (XXXVa-e) has been found to be completely planar. The interatomic distances and bond angles are given in Fig. 2. As would be expected, the nickel compound is diamagnetic, and the cobalt and iron compounds show paramagnetism, corresponding with one and two unpaired d-electrons, respectively 41). These... 

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