Dispersion metal load effect

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

The scheme implies that in the presence of a metal which establishes the olefin-paraffin equilibrium, the carbonium ion concentration on the surface depends on the hydrogen partial pressure. The stabilizing effect of a given metal load will depend on its dispersion and distribution and on the prevailing hydrogen pressure. Similar experiments show that for zeolite Y based catalysts the reaction mechanism is identical with that discussed above for mordenite. 

Metal dispersion has a positive influence on both conversion and p-picoline selectivity, whereas at equal metal dispersion the metal loading has no effect on the deactivation pattern. Increasing the metal loading, however, prolongs the catalyst life. 

The effect of metal loading on the reducibility was examined with PdNaY (71). For samples calcined at 500°C, the TPR peak maximum shifts from 190 to 150°C the Pd loading increases from 2.0 to 6.7 wt%. This has been attributed to the formation of ion pairs in sodalite cages. The reduction conditions are important for the resultant metal dispersion. TEM and radial electron distribution (RED) evidence shows that reduction of Ir ions in an H2 flow results in much smaller Ir aggregates than reduction under static H2 (173). 

NMR adsorption isotherms for Ru/SiOi catalysts have been obtained using explicit calibration (89). Although the pressure over the sample could be adjusted in situ, no volumetric data were taken simultaneously, probably because of the important spillover effects in this catalytic system (see Section III.A). The NMR study was performed at pressures between 10 and 760 Torr and at temperatures between 323 and 473 K (only the 323-K results are reviewed here). The dispersion of the catalyst was determined from the irreversible H NMR signal as 0.29. The metal loading was 8 wt% so that a monolayer coverage on 1 g of catalyst corresponds to 2.8 cm of H2 under standard conditions. It is typical for an NMR sample to contain 0.5 g of material in a 1-cm sample volume, and the pores in the powder make up about half the volume. If such a sample of this catalyst is under 760 Torr of hydrogen, the gas phase corresponds to one-third of a mono-layer, and it can make a detectable contribution to the NMR signal. 

As already pointed out, the first particle size effect in skeletal rearrangement was found for the hydrogenolysis of methylcyclopentane (55). Nonselective hydrogenolysis takes place on highly dispersed catalysts, with a metal loading smaller than 0.6%, while selective rupture of bisecondary C-C bonds occurs on heavily loaded catalysts (more than 6% platinum on alumina). 

Rhodium catalysts were prepared by hydrogen reduction at atmospheric pressure of a cationic organometallic rhodium complex and anchored onto lamellar and zeolitic products. The effect of the structure and characteristics of the support on metal load and dispersion was studied in the heterogeneous catalysts prepared. The new rhodium catalysts were applied in the hydrogenation of acetone. The reaction was carried out under milder conditions. 

Works about catalysts with a similar content in active metals have shown the different behaviour of the catalysts depending on the characteristics of the support over which the metal is anchored, and also on the accessibility of the active centres [14], The values of conversion decreases along with the fall in the temperature of the reaction (Table 2). The slight increase in conversion in Rh/BENa, when the temperature decreases, indicates difflisional effects, that can be attributed to the location of the active centres in the interlamellar space and not only in the external surfece as in zeolites. The differences of the conversion between the catalysts prepared on zeolitic supports (higher in ZE- -P than in ZE- -X) are caused by the zeolitic structure rather than the metal loading. The difference in the results obtained in quite similar conditions (rhodium loading and metal dispersion)... 

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