Osmium interatomic distance

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. 

Table 18 Interatomic distances (A) in BaMg2RuDg and osmium analog...

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. 

The amplitude function A,(Af) is derived from the values of the maxima and minima of the function K x,(/0. For values of K other than those corresponding to maxima and minima, values of A, (/0 are obtained by interpolation. The values of the interatomic distance for metallic osmium and copper are 2.705 and 2.556 A, respectively. The value of 2.705 A for metallic osmium, which has the hexagonal close-packed structure, is the average of the interatomic distance (2.735 A) in a hexagonal layer and the distance of... 

Plots of the function K xUO vs. K at 100°K for the extended fine structure beyond the osmium edge for pure metallic osmium, and for the osmium-copper clusters in the catalyst containing 2 wt% Os and 0.66 wt% Cu, are shown in the left-hand sections of Figure 4.13. The associated Fourier transforms of the functions are shown in the right-hand sections of the figure. As previously noted, the Fourier transform yields the function n(R), the peaks of which are displaced from the true interatomic distances because of the phase shifts. Similar plots for the extended fine structure beyond the copper K edge for pure metallic copper and for the osmium-copper catalyst are given in our 1981 paper (32). 

From Figure 4.15 we obtain for the osmium-copper interatomic distance in the catalyst a value of 2.675 A, which is about 0.05 A larger than the sum of the metallic radii of osmium and copper (36). The values for the osmium-osmium and copper-copper nearest neighbor distances in the catalyst, which are not shown in the figure, are 2.680 A and 2.550 A, respectively. The uncertainty is estimated to be 0.01-0.02 A in the determination of these interatomic distances (19,32). 

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. 

Reference to the bond angles and interatomic distances for these molecules shows that as far as the carbon atoms are concerned their values are little different from those of the olefin molecule. It would be expected, therefore, that catalysts which were effective for hydrogenation of the latter would also function with the heterocyclic molecules. This is found to be so, in that nickel catalysts are known to give tetrahydrofuran and pyrrolidine by the hydrogenation of furan and pyrrole at 180° (Padoa, 25). Tetrahydrofuran is also formed by the use of platinum (Starr and Hixon, 26), osmium, or palladium (Shuikin, Nikiforov, and Stolyarova, 27) as the catalyst, and pyrrolidine is similarly produced by palladium or rhodium catalysts (Zelinskii and Yurev, 28). In all these metals there are spacings of the atoms very similar to those in metallic nickel, the hexagonal osmium lattice having a equal to 2.71 A. 

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