Surface area changes during reduction

Fig. 5.18 Changes of special surface area along with reduction degree during reduction...

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. 

For pure nonionic EO adducts, increase in the number of oxyethylene groups in the molecule results in a decrease in the tendency to form micelles and an increase in the surface tension of the solution at the critical micelle concentration (1 ) (l. ) This change in surface activity is due to the greater surface area of the molecules in the adsorption layer and at the micellar surface as a result of the presence there of the highly hydrated polyoxyethylene chain. The reduction in the tendency to form micelles is due to the increase in the free energy of micelle formation as a result of partial dehydration of the polyoxyethylene chain during incorporation into the micelle ( 1 6) (17). 

Fig. 5 (b ) and (c) indicated that the particle sizes of iron species in Al-Fe-oxide(urea) and Cr-Fe-oxide(urea) were kept small even after the third oxidation. The changes of particle sizes during redox cycles could be also suggested from the changes in specific surface areas of the samples. Fig. 6 shows specific surface areas of the iron oxide samples before the first reduction and after the third oxidation. The specific surface areas before the first reduction for Al-Fe-oxide(urea) and Cr-Fe-oxide(urea) were larger than that for Fe-oxide(urea), which was consistent with the results of SEM images shown in Fig. 5. The redox cycles decreased the specific surface areas of all the samples. However, the surface areas after the third cycle for AI-Fe-oxide(urea) and Cr-Fe-oxide(urea) were...

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