Platinum-iridium clusters chemisorption data

Data on the chemisorption of hydrogen at room temperature on platinum-iridium clusters dispersed on alumina and silica are shown in Figures 4.19 and 4.20 as a function of the amount of platinum and iridium in the catalyst (4). The data are for catalysts containing equal fractions by weight of platinum... 

When the platinum-iridium clusters are still more highly dispersed, and are supported on an alumina carrier instead of silica, the results shown in the right-hand sections of Figure 4.30 are obtained (48). The metal dispersion of the clusters as determined by hydrogen chemisorption is 0.93. If the clusters were spherical, the average diameter calculated from the chemisorption data would be about 12 A. Again the clusters are too small to give a satisfactory X-ray diffraction pattern. As with the previous two catalysts, the values... 

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 Mossbauer data on sample D-600, coupled with chemisorption data on this sample, show that calcination of alumina-supported platinum-iridium in air at 600°C is unsatisfactory for the formation of highly dispersed bimetallic clusters of platinum and iridium. 

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