Catalysts, bimetallic Platinum-iridium clusters

While the X-ray diffraction data considered earlier for the catalyst containing 10 wt% each of platinum and iridium do not eliminate the possibility of differences in the average environments about the two types of atoms in the clusters, they also do not provide evidence for it. The diffraction data are entirely consistent with the platinum and iridium being present as homogeneous bimetallic clusters. This interpretation is reasonable in the absence of other information. The results of the EXAFS studies, however, provide evidence that the catalyst consists of platinum-iridium clusters that are not homogeneous. Hence the EXAFS data provide more detailed structural information than can be obtained from a diffraction pattern. 

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 selective production of methanol and of ethanol by carbon monoxide hydrogenation involving pyrolysed rhodium carbonyl clusters supported on basic or amphoteric oxides, respectively, has been discussed. The nature of the support clearly plays the major role in influencing the ratio of oxygenated products to hydrocarbon products, whereas the nuclearity and charge of the starting rhodium cluster compound are of minor importance. Ichikawa has now extended this work to a study of (CO 4- Hj) reactions in the presence of alkenes and to reactions over catalysts derived from platinum and iridium clusters. Rhodium, bimetallic Rh-Co, and cobalt carbonyl clusters supported on zinc oxide and other basic oxides are active catalysts for the hydro-formylation of ethene and propene at one atm and 90-180°C. Various rhodium carbonyl cluster precursors have been used catalytic activities at about 160vary in the order Rh4(CO)i2 > Rh6(CO)ig > [Rh7(CO)i6] >... 

X-Ray Diffraction Studies. The dependence of the lattice parameter of bulk platinum-iridium alloys on composition is shown in Figure 4.22 (4,45). Lattice parameters are commonly obtained from X-ray diffraction measurements. For platinum-iridium catalysts, X-ray diffraction measurements provide a way of demonstrating the presence of bimetallic clusters of platinum and iridium, if the metal dispersion is not too high. 

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.)...

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. 

The inclusion of platinum with iridium in bimetallic clusters (7) provides a way of moderating the formation of low molecular weight alkanes while still retaining activity and activity maintenance that are much superior to those of a platinum catalyst. Our work has repeatedly indicated the desirability of this interaction between platinum and iridium in obtaining a satisfactory product distribution. 

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