Iridium/alumina catalysts, hydrogen

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.)...

Conditions cited for Rh on alumina hydrogenation of MDA are much less severe, 117 °C and 760 kPA (110 psi) (26). With 550 kPa (80 psi) ammonia partial pressure present ia the hydrogenation of twice-distilled MDA employing 2-propanol solvent at 121°C and 1.3 MPa (190 psi) total pressure, the supported Rh catalyst could be extensively reused (27). Medium pressure (3.9 MPa = 566 psi) and temperature (80°C) hydrogenation usiag iridium yields low trans trans isomer MDCHA (28). Improved selectivity to aUcychc diamine from MDA has been claimed (29) for alumina-supported iridium and rhodium by iatroduciag the tertiary amines l,4-diazabicyclo[2.2.2]octane [280-57-9] and quiaucHdine [100-76-5]. 

The catalyst powders were compressed to thin disks under a pressure of about 50 kg/cm2, with the exception of the alumina-supported catalysts which required a pressure of 1500 kg/cm2 to obtain reasonable transmittance. The samples were reduced in a stream of hydrogen supplied at a rate of 10 1 hr-1 (SV 30,000 hr-1). The temperatures of reduction were 350°-450°C for the nickel samples, 475°C for the palladium samples, and 425°C for the iridium catalysts. 

The co-existence of at least two modes of ethylene adsorption has been clearly demonstrated in studies of 14C-ethylene adsorption on nickel films [62] and various alumina- and silica-supported metals [53,63—65] at ambient temperature and above. When 14C-ethylene is adsorbed on to alumina-supported palladium, platinum, ruthenium, rhodium, nickel and iridium catalysts [63], it is observed that only a fraction of the initially adsorbed ethylene can be removed by molecular exchange with non-radioactive ethylene, by evacuation or during the subsequent hydrogenation of ethylene—hydrogen mixtures (Fig. 6). While the adsorptive capacity of the catalysts decreases in the order Ni > Rh > Ru > Ir > Pt > Pd, the percentage of the initially adsorbed ethylene retained by the surface which was the same for each of the processes, decreased in the order... 

The metal-catalysed hydrogenation of cyclopropane has been extensively studied. Although the reaction was first reported in 1907 [242], it was not until some 50 years later that the first kinetic studies were reported by Bond et al. [26,243—245] who used pumice-supported nickel, rhodium, palladium, iridium and platinum, by Hayes and Taylor [246] who used K20-promoted iron catalysts, and by Benson and Kwan [247] who used nickel on silica—alumina. From these studies, it was concluded that the behaviour of cyclopropane was intermediate between that of alkenes and alkanes. With iron and nickel catalysts, the initial rate law is... 

Al-Ammar AS, Webb G (1978) Hydrogenation of acetylene over supported metal catalysts Part 1 - Adsorption of [ C] Acetylene and [ C] ethylene on silica supported rhodium, iridium and palladium and alumina supported palladium. J Chem Soc Earaday Trans 74 195 Al-Ammar AS, Webb G (1979) Hydrogenation of acetylene over supported metal catalysts Part 3 - [ C] tracer studies of the effect of added ethylene and carbon monoxide on the reaction catalyzed by silica-supported palladium, rhodium and iridium. J Chem Soc Faraday Trans 75 1900... 

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. 

While a value of H/M from Figure 4.20 may be taken as an approximate measure of the metal dispersion, inspection of the data on the alumina-supported platinum-iridium catalyst with the lowest metal content indicates that the value of H/M may be as high as 1.3. Consequently, there is slightly more than one hydrogen atom per surface metal atom in the chemisorbed layer remaining after the adsorption cell is evacuated at room temperature. 

If the alternative procedure of extrapolating the nearly pressure-independent region of the original adsorption isotherm back to zero pressure is employed, as discussed in Chapter 2, it is observed that the value of H/M for the alumina-supported platinum-iridium clusters in the catalyst containing 0.6 wt% metal is about 1.7. The strongly chemisorbed hydrogen determined by the method involving room temperature evacuation is approximately 75% of this value. 

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... 

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