Platinum alumina-supported

Reduction of the aromatic nuclei contained in catalytic C-9 resins has also been accomplished in the molten state (66). Continuous downward concurrent feeding of molten resin (120°C softening point) and hydrogen to a fixed bed of an alumina supported platinum—mthenium (1.75% Pt—0.25% Ru) catalyst has been shown to reduce approximately 100% of the aromatic nuclei present in the resin. The temperature and pressure required for this process are 295—300°C and 9.8 MPa (lOO kg/cni2), respectively. The extent of hydrogenation was monitored by the percent reduction in the uv absorbance at 274.5 nm. 

In addition to this work on charcoal- and silica-supported catalysts and on evaporated platinum films, a number of studies have been made on alumina-supported platinum catalysts (e.g., 111-114, 81,115) in which the aim has been the study of reactions at the platinum alone. In these cases, one cannot automatically dismiss the possibility of participation of the alumina support (i.e., of dual function behavior of the catalyst) because it is known that alumina may have acidic properties, particularly when retained halogen is present. In general terms, there is no immediate answer to this problem because the nature of this sort of catalyst wall be much dependent on the details of catalyst history, preparation, and use. However, there can be little doubt that in many experimental studies using plati-num/alumina, and in which the assumption has been made that the alumina support is inert, this assumption is essentially valid. For instance, one may note the inert alumina used by Davis and Venuto (111) and the justification provided by Gault et al. (116) for the inertness of the alumina used in a substantial body of previous work irrespective of whether the catalyst was... 

Gopalan, U., Hydrodenitrogenation of Pyridine over Alumina Supported Platinum Catalyst. Master of Applied Science Thesis. University of Otawa, Chemical Engineering. 1998, p. 69. 

Shabaker, J. W. Davda, R. R. Huber, G. W. Cortright, R. D. Dumesic, J. A., Aqueous-phase reforming of methanol and ethylene glycol over alumina-supported platinum catalysts. Journal of Catalysis 2003, 215, 344. 

We describe in some detail the techniques of nuclear magnetic resonance which are used for studying alumina-supported platinum catalysts. In particular, we describe the spin-echo technique from which the Pt lineshape can be obtained. We also discuss spin echo double resonance between surface Pt and chemisorbed molecules and show how the NMR resonance of the surface Pt can be separately studied. We present examples of experimental data and discuss their interpretation. 

Figure 4 in Ref. 216, reproduced on the right, displays Pt 4d5/2 XPS spectra from calcined alumina-supported platinum catalysts, pure (Pt/A) and doped with lanthanum (Pt/A-L), cerium... 

Garin and Gault 82) reported a hydrogen order of - 3.4 for position isomerization of n-pentane over alumina-supported platinum between 240° and 300°C. They concluded therefore that a surface species that lost 3-4 hydrogen atoms would cyclize. It is not clear to what extent this discrepancy (cf. 77 and 82) may be attributed to different experimental conditions, to different catalysts, or actually to different mechanisms that is, C5 cyclization with participation of unsaturated intermediates. 

Although the mechanism of the platinum catalysis is by no means completely understood, chemists do know a lot about how it works. It is an example of a dual catalyst platinum metal on an alumina support. Platinum, a transition metal, is one of many metals known for its hydrogenation and dehydrogenation catalytic effects. Recently bimetallic platinum/rhenium catalysts are now the industry standard because they are more stable and have higher activity than platinum alone. Alumina is a good Lewis acid and as such easily isomerizes one carbocation to another through methyl shifts. 

Promotion and deactivation of unsupported and alumina-supported platinum catalysts were studied in the selective oxidation of 1-phenyl-ethanol to acetophenone, as a model reaction. The oxidation was performed with atmospheric air in an aqueous alkaline solution. The oxidation state of the catalyst was followed by measuring the open circuit potential of the slurry during reaction. It is proposed that the primary reason for deactivation is the destructive adsorption of alcohol substrate on the platinum surface at the very beginning of the reaction, leading to irreversibly adsorbed species. Over-oxidation of Pt active sites occurs after a substantial reduction in the number of free sites. Deactivation could be efficiently suppressed by partial blocking of surface platinum atoms with a submonolayer of bismuth promoter. At optimum Bi/Ptj ratio the yield increased from 18 to 99 %. 

Recently Pliskin and Eischens (167) have observed bands at 4.74 and 4.86 y for hydrogen chemisorbed on alumina-supported platinum. These are associated with weakly and strongly bonded H atoms, respectively, and it is suggested that the former are adsorbed on single surface Pt atoms while the latter are adsorbed by interaction with two Pt atoms. No evidence was found for adsorbed molecular species such as Hj and no absorption bands could be detected for hydrogen chemisorbed on silica-supported nickel. 

Fio. 33. CD adsorption on alumina-supported platinum (40) and palladium (42) characterized by ATR-IR spectroscopy during catalysis. Top spectra taken at different CD concentrations in hydrogen-saturated CH2CI2. The CD concentration increases from bottom to top (0-3 x 10 M for platinum and 0 4.3 X 10 M for palladium). Vibrational bands indicative of various adsorption modes of CD (bottom) are marked. Bands indicative of species 2 are absent from spectra observed for CD adsorption on palladium (40,42). 

The reactions of the n -butenes with deuterium have been studied over alumina-supported platinum and iridium [103] and palladium [124]. In general, the results obtained are similar to those discussed above for ethylene—deuterium and propene—deuterium reactions. A comparison of the deuteroalkane distributions over platinum is shown in Fig. 17. 

In the presence of hydrogen the isomerization of paraffins of five or more carbon atoms over dual function catalysts, such as amorphous silica-alumina supported platinum, can be described by the following scheme ... 

Differences in the chemical behavior of the Cabosil-supported and 7-alumina-supported platinum samples are evident. The initial reduction of the chloroplatinic acid will start at 35° C. when Cabosil is used, but it requires a temperature of 200° C. before reduction is started on the alumina-supported samples. The CO chemisorbed on 7-alumina-supported platinum is much more difficult to oxidize, especially the bridged form, than is CO on Cabosil-supported platinum. 

An example of the enhanced resolution is presented in Figure 64, where the Au L3-edge XANES of a gold foil is shown both in the normal transmission mode and in the FIERFD mode (Safonova et al., 2006). This spectral sharpening was used to determine the actual adsorption site of CO on a small alumina-supported platinum cluster (Safonova et al., 2006). The measured spectra are shown in Figure 65. Clearly, the features in the XANES spectra recorded by the use of HERFD are richer than what is determined by using total fluorescence detection. 

In the gas-phase, benzene shows a single line,77 78 and can yield useful information regarding the diffusion/transport properties. Benzene trapped within pores in glasses and silica gels too yields results, about pore size and adsorbed versus liquid-phase conditions.79 Chemisorption on alumina-supported platinum catalysts leads to disclosure as to how and where the benzene molecules are located, via FT NMR.80... 

C. F. Tlrendl, G. A. Mills and C. Dybowski, Platinum-proton coupling in the NMR spectrum of benzene on an alumina-supported platinum catalyst.. Phys. Chem., 1992, 96(12), 5045-5048. 

Lu, Y. T., Reaction modeling of propylene hydrogenation oyer alumina-supported platinum, M.S. Thesis, University of Wisconsin-Madison (1988). 

This organometallic approach to the preparation of mixed metals has also been used to prepare mixed metal species containing tin.201-206 As shown in Fig. 13.21 the reaction of tetraalkyl tin with silica or alumina supported platinum. 

Bond GC, Wells PB (1965) The hydrogenation of acetylene I. The reaction of acetylene with hydrogen catalyzed by alumina-supported platinum. J Catal 4 211... 

For efficient raw material utilization as well as for fuel conservation (e.g., methane), discharge controls integral with the process absorbers have obvious advantages. Nitric oxide and nitrogen dioxide may be reduced by ammonia using alumina-supported platinum catalysts without the need to consume the residual oxygen first [54] (Eqs. 11.50 and 11.51). 

The discussion in this section pertains to alumina-supported platinum catalysts. The work by C.P. Poole and D.S. Maciver provides an extensive review of chromia-alumina catalysts... 

The key role of dehydrogenation catalysts is to accelerate the main reaction while controlling other reactions. Unmodified alumina-supported platinum catalysts are highly active but are not selective to dehydrogenation. Various by-products, as indicated in Figs. 5 and 6, can also form. In addition, the catalyst rapidly deactivates because of fouling by heavy carbonaceous materials. Therefore, the properties of platinum and the alumina support need to be modified to suppress the formation of by-products and to increase catalytic stability. 

Aben, P.C. van der Eijk, H. and Oelderik, J.M. "The characterization of the metal surface of alumina-supported platinum catalysts by temperature-programmed desorption of chemisorbed hydrogen." In Hightower, J.W., ed. Proceedings of the Fifth International Congress on Catalysis. Vol. 1. Amsterdam North-Holland 1973 p. 717-726. 

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. 

Hgure 4.21 Arrhenius plots for ethane hydrogenolysis over a series of alumina-supported platinum-iridium catalysts containing 0.3 wt% iridium and variable amounts of platinum corresponding to Pt lr atomic ratios ranging from 0 to 9. 

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... 

Figure 4.32 Mossbauer spectra at 25°C on alumina-supported platinum, iridium, and platinum-iridium catalysts (samples B, C, and D, respectively) containing a small amount of Fe (0.1 wt%) enriched with 57Fe as a Mossbauer probe (3,41). (Sample A is a reference material containing only the enriched Fe on alumina.) (Reprinted with permission from Academic Press, Inc.)...

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