Platinum-alumina catalysts adsorption

Nevertheless, the picture was not uniform. Loc et al. (1996) reported a retarding effect of steam over platinum-alumina catalysts. The different effects of steam on Pt activity could be related to the properties of steam adsorption on different supports. These results also indicate that the effect of steam on the reaction rate is related to the catalysts used for the investigations. 

As it is generally the case with bifunctional catalysis processes, the balance between hydrogenating and acid functions determines for a large part the catalyst activity. This was quantitatively shown for series of bifunctional catalysts constituted by mechanical mixtures of a well dispersed Pt/Alumina catalyst and of mordenite samples differing by their acidity and their porosity (25). The balance between hydrogenating and acid functions was taken as nPt/nH+ the ratio between the number of accessible platinum atoms and the number of protonic sites determined by pyridine adsorption. 

The presence of other materials in the impregnating solution can have a marked effect on the location of the metal within the support particle. These additives have been conveniently divided into three classes. Class 1 additives consist of simple inorganic electrolytes which influence the electrostatic interactions at the solution-support interface. Simple salts such as sodium nitrate, sodium chloride, or calcium chloride do not adsorb strongly enough on alumina to compete with platinum salts for adsorption. Fig. 13.9a 0 shows the concentration profile of platinum on an alumina particle when the impregnation of chloroplatinic acid was done in the absence of any additives. This a somewhat diffused egg shell profile. Fig. 13.9b shows the adsorption profile for the catalyst prepared by impregnation in the presence of an amount of sodium nitrate equimolar to the chloroplatinic acid. Here the amount of platinum adsorbed decreases while the adsorption profile approaches a uniform distribution. It is... 

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 increasing accessibility of platinum sites for adsorption and catalytic reaction in a series Pt/SZ < R/SZA < Pt/A can be attributed both to a decrease in the content of sulfur compotmds in the catalyst upon dilution of sulfated zirconia with alumina, and to a higher resistance to poisoning of more disperse supported platinum crystallites produced by chemisorption anchoring of a precursor (J.R. Chang et al., 1997). 

Rare earths have also been included as desirable SOx catalyst components in early patents but the catalytic behavior of cerium, in particular, had not been clarified. This paper has presented evidence that cerium catalyzes the oxidative adsorption of SO2 on high alumina cracking catalyst, alumina, and magnesia. We also have shown the catalytic character of platinum. The details of the catalysis especially by cerium, however, remain unexplained. 

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 %. 

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 relationship between the two catalytic components is quite complex. Interactions between the support and the hydrogenation component can alter this relationship. For example, Larson et- al. (6) showed that, with platinum on silica-alumina, a selective adsorption of platinum by acid sites causes a reduction in catalyst acidity. Similarly, nickel reacts with the acid sites on silica-alumina forming nickel salts of the silica-alumina acid sites. It has been suggested (J) that one of the effects of sulfiding a nickel on... 

Type III is relatively rare—a recent example of nitrogen on ice and seems to be characterised by a heat of adsorption equal to or less than heat of liquifaction of the adsorbate. The example of Type II isotherms are furnished by adsorption of nitrogen on an iron or a platinum catalyst at -195 C and those Type III by adsorption of bromine on silica or alumina gas at 80 C. 

The TPD apparatus consisted of a stainless steel flow system connected to a thermal conductivity cell. Catalyst samples of 0.1 g were placed in one arm of an L-shaped, 6 mm Vycor tube. A dual adsorption bed containing alumina and Oxy-Trap (Alltech) was placed in the other arm to prevent contamination by water and respectively. Frequent regeneration in and He was required. This in-situ adsorption bed was found necessary despite purification traps on all gas lines coming into the flow system. Pulses of 0.25 cc of a 10% mixture of CO in He were injected into the He carrier gas and passed over the pretreated catalyst at room temperature. All runs were programmed heated at a rate of 20 K min . The Pt catalysts, either commercial or laboratory produced, were prepared by the impregnation of chloroplatinic acid on Cyanamid s Aero 1000 alumina, except for two catalysts which were prepared by platinum diamino dinitrite impregnation. 

In addition to the catalyst itself, there may be a wish to study using NMR the phases adsorbed on this solid. The variations in the spectrum of the chemisorbed probe molecule compared to that of the isolated molecule, can be used to investigate how the adsorption takes place. It has thus been possible to show that, in the reaction of ethylene on platinum supported on alumina, if the adsorption takes place at low temperature (77 K), the ethylene is fixed by a re bond (a single signal in carbon NMR at 70 ppm) whilst if the temperature is increased to 343 K the adsorbed form is ethylidene, CH3=CH-M (d 200 and 90 ppm) which, above this temperature, converts into methane. 

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