Iridium/alumina catalysts, hydrogen chemisorption

Figure 4.19 Hydrogen chemisorption, at room temperature and 10 cm Hg pressure, as a function of the total content of platinum and iridium in catalysts comprising equal weights of platinum and iridium dispersed on alumina or silica (4). (Reprinted with permission from Academic Press, Inc.)...

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... 

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... 

For both total and strong chemisorption, H/M increases as the metal content of the catalyst decreases and is consistently higher for catalysts in which the platinum-iridium clusters are dispersed on alumina. As shown in Figure 4.19, the H/M values for total chemisorption frequently exceed unity. Values of H/M approaching 2 are observed at the lowest metal contents when the platinum-iridium clusters are dispersed on alumina. For strongly chemisorbed hydrogen, H/M appears to approach a limiting value near unity as the metal content is decreased to about 1 wt% or lower. 

In Table 4.1 chemisorption data on alumina-supported platinum-iridium catalysts and related catalysts containing platinum or iridium alone show the effect of varying the temperature of calcination of the catalyst (in air or oxygen-helium mixture) on the metal dispersion (40,41). Data are presented for chemisorption of carbon monoxide, hydrogen, and oxygen. The final three catalysts in the table contained more metal than the first three. They also contained 0.1 wt% Fe (enriched with 57Fe) incorporated as a probe for Moss-bauer spectroscopy experiments (41). The presence of the iron is ignored in the discussion of the chemisorption results. 

For iridium dispersed on alumina, the data on the catalysts calcined at 260-270°C indicate that more than one atom of hydrogen or one molecule of carbon monoxide is adsorbed per surface iridium atom (40,43). However, the data on the quantity O/M show no evidence of values in excess of unity. In contrast with platinum catalysts, increasing the temperature of calcination of iridium catalysts to 500-600°C decreases chemisorption capacities markedly. Calcination in air or oxygen at 500-600°C leads to formation of large Ir02 crystallites that yield large iridium crystallites on reduction (3,4,41). The fraction of the iridium atoms present as surface atoms thus decreases markedly, and this, in turn, is reflected in marked decreases in the chemisorption capacity for all three gases. 

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