Iridium, chemisorption

However, it will at any rate be clear now that the palladium, nickel, and iridium catalysts used in our experiments differ widely in surface characteristics, as is evident from the variations in chemisorptive behavior. An obvious question that may be asked now is whether the catalysts differ also in catalytic behavior. This induced us to study the reaction of benzene with deuterium on the nickel and iridium catalysts. 

Whereas determination of chemisorption isotherms, e.g., of hydrogen on metals, is a means for calculating the size of the metallic surface area, our results clearly demonstrate that IR studies on the adsorption of nitrogen and carbon monoxide can give valuable information about the structure of the metal surface. The adsorption of nitrogen enables us to determine the number of B5 sites per unit of metal surface area, not only on nickel, but also on palladium, platinum, and iridium. Once the number of B5 sites is known, it is possible to look for other phenomena that require the presence of these sites. One has already been found, viz, the dissociative chemisorption of carbon dioxide on nickel. 

The surface areas of the iridium and palladium catalysts were determined by chemisorption of hydrogen and carbon monoxide, respectively, the monolayer volume being determined from an adsorption isotherm taken at 20°C. 

The difference in reactivity of metal clusters and metal surfaces has also been well illustrated in these iridium-based systems [205]. A lack of reactivity of alkyli-dyne species on Ir4/y-Al203 with H2 is observed meanwhile, the chemisorption of H2 is not hindered. This behavior contrasts with that of metallic surfaces, which allow the reaction between alkylidyne species and H 2. It is inferred that over metallic clusters the reaction of H2 with alkyklidyne is not allowed because of the lack of available adjacent metal sites, which are necessary for the formation of the intermediates [205]. 

IV. Chemisorption of Hydrocarbons on Low and High Miller Index Surfaces of Platinum, Iridium, and Gold... 

D. The Chemisorption of Hydrocarbons on Gold and Iridium Crystal Surfaces... 

The chemisorption of acetylene, ethylene, benzene, and cyclohexane were also studied on the Ir(lll) and stepped Ir[6(111) x (100)] crystal surfaces (30). Chemisorption characteristics of the Ir(lll) and Pt(lll) surface are markedly different. Also, the chemisorption characteristics of the low Miller index Ir(l 11) surface and the stepped Ir[6(l 11) x (100)] surface are markedly different for each of the molecules studied. The hydrocarbon molecules form only poorly ordered surface structures on either the Ir(l 11) or stepped iridium surfaces. Acetylene and ethylene (C2H2 and C2H4) form surface structures that are somewhat better ordered on the stepped iridium than on the low Miller index Ir(l 11) metal surface. The lack of ordering on iridium surfaces as compared to the excellent ordering characteristics of these molecules on... 

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. 

Titania-supported Metals. - After reduction at 473 K, platinum-group metals supported on Ti02 chemisorbed both hydrogen and carbon monoxide in quantities indicative of moderate-to-high dispersion, but following reduction at 773 K chemisorption was drastically lowered e.g., H/Mt <0.01 for Pt, Ir, and Rh, 0.05-0.06 for Pd and Ru, and 0.11 for Os). Agglomeration, encapsulation, and impurities were eliminated as possible causes and a strong metal-support interaction (SMSI) was proposed. Titania is not unique in its SMSI properties and 11 oxides used to support iridium were classified as follows ... 

The addition of iridium to platinum on ij-AlaOa causes a marked increase in the H/M ratio of hydrogen uptake assuming that H/Ms = 1.69.70 value of two was not taken to be conclusive proof of spillover so the authors turned to the chemisorption of carbon monoxide and compared the ratios H/CO for uptake at saturation on the supported and unsupported bimetallic catalysts. The following results were obtained for the quotient of the two H/CO ratios at various catalyst compositions —... 

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... 

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.)...

The amount of hydrogen adsorbed is expressed in terms of the quantity H/M, which represents the ratio of the number H of adsorbed hydrogen atoms to the number M of metal atoms (platinum and iridium) in the catalyst. In Figure 4.19 the data are for total hydrogen chemisorption at a pressure of 10 cm Hg. The data in Figure 4.20 represent strongly chemisorbed hydrogen, that is, the amount retained by the catalyst when the adsorption cell is evacuated at room temperature for 10 minutes (to a pressure of approximately 10 6 torr) after completion of the adsorption isotherm. As noted previously, in Chapters 2 and 3, this quantity is the difference between the iso-... 

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. 

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... 

Mossbauer spectra at 25°C are shown in Figure 4.32 for alumina-supported platinum, iridium, and bimetallic platinum-iridium catajysts containing 57Fe (samples B, C, and D, respectively) (3,4 V. The platinum-iridium catalyst contained 1.75 wt% each of platinum and iridium, while the other two catalysts contained 1.75 wt% of either platinum or iridium. All of the catalysts had metal dispersions (as determined by chemisorption) in the range 0.7-1. Also... 

However, sample C-600 gives a spectrum which is very different from that presented for sample C in Figure 4.32. The spectrum for sample C-600 is nearly identical to that of sample A in Figure 4.32, in which the iron exists in the ferrous state. The iridium in sample C-600 is not present as highly dispersed clusters, as shown also by chemisorption data (41). It appears that the poorly dispersed iridium crystallites (dispersion <0.1) are not associated to a significant extent with the iron in the sample. Hence sample C-600 behaves like a sample containing no iridium. The Mossbauer parameters in Table 4.2 are consistent with this statement. 

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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