Iridium catalysts adsorption

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... 

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. 

The slopes of the isotherms for the various substances are approximately identical in the central part and close to the slope of the hydrogen adsorption isotherm. On iridium the methanol adsorption isotherms are satisfedby the Freundlich equation, where the power in the equation of the isotherm is close to that in the isotherm for hydrogen adsorption on iridium. The adsorption isotherms for methanol and hydrogen on rhodium are the same in nature. These results lead to the conclusion that the dependence of 0 andc is determined not by the nature of the adsorbed substance but by the nature and character of inhomogeneity in the catalyst surface. 

Et4N]2[Fe2lr2(CO)i2] cluster precursor, which exhibit a high activity in the synthesis of methanol from CO and H2, were studied by Ir and Fe Mossbauer spectroscopy. The study extends from the precursors via the fresh to the aged catalysts. The presence of iridium in the metallic state as well as the presence of trivalent, divalent and alloyed iron is detected. Representative Ir and Fe Mossbauer spectra are shown in Fig. 7.69. Information about the adsorption on the surface of MgO... 

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. 

Effect of Sulfur on the Hydrogen Adsorption Capacity and the Hydrogen Binding Energy of Platinum- and Iridium-Supported Catalysts... 

The selective partial hydrogenation of symmetric cyclic diketones was accomplished by interrupting the reaction after the consumption of one equivalent of hydrogen, which indicates a stronger mode of adsorption for the diketone than for the ketol product. 2 The hydrogenation of 1,4-cyclohexanedione (30) to the ketol (Eqn. 18.23) was catalyzed by nickel, copper, palladium, platinum, iridium, and ruthenium. Iridium was the most active and selective of these catalysts. Hydrogenations run over this catalyst in iso-propanol at 20°C and 6 atmospheres... 

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... 

Actually, rhodium and iridium can be used as well as ruthenium in mixtures with palladium. The results, however, allow only the qualitative statement that olefins are significant intermediates. The presence of more than one olefin isomer and uncertainty of the precise ratios of formation and rates of migration preclude any quantitative estimate. The experiments with mixed catalysts were terminated after 20 to 50% conversion. The mixed catalysts not only gave higher yields of (raws-decalin but the actual concentrations of the octalins present in the reaction mixture were lowered owing to preferential adsorption on the palladium component. 

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-... 

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. 

Iridium zeolites, hydrogenation catalyst 26-0-04 Isosteric heats of adsorption 17-P-I4... 

The volumetric method has very often been used with platinum catalysts for which quite satisfactory results are generally obtained it is usual to assume that the monolayer volume or amount, obtained as just described or by extrapolation corresponds to an H Ms (hydrogen atom to metal surface atom) ratio of 1 1. Some justification for this assumption is to be found, at least for particles of moderate size, in the adsorption stoichiometry shown by films and single crystals, but for very small particles and at high pressures the H/Mj ratio can exceed unity quite substantially this is especially so with rhodium" and iridium (see below). Care is however needed with palladium " " because of the risk of forming the hydride however, monolayer coverage is obtained at pressures below which dissolution starts. The base metals iron, cobalt and nickel have been... 

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