Analysis Of Oxidation Kinetics Research Articles

Effects of high-temperature oxidation and reduction on the structure and activity of [formula omitted] catalysts

The effects of high-temperature oxidizing and reducing treatments on the structure and activity of Rh Al 2O 3 and Rh SiO 2 catalysts were examined using the techniques of temperature-programmed reduction, H 2 chemisorption, infrared spectroscopy, transmission electron microscopy, and CO oxidation kinetic analysis. Sequential treatments in O 2 and H 2 at 1075 K had little effect on the Rh SiO 2 catalysts beyond increasing the mean particle size from ≈4 nm to greater than 12 nm. No interactions between the Rh and the support were observed for the Rh SiO 2 catalysts. In contrast, 1075 K oxidation of the Rh Al 2O 3 catalysts caused most of the Rh to diffuse into the bulk of the alumina and resulted in a five- to tenfold decrease in CO oxidation activity. Nearly all of the Rh could be restored to the surface of the alumina by a subsequent 1075-K 1-h H 2 reduction followed by a reoxidation at 775 K for 1 h. The reactivated catalyst oxidized CO at greater rates than the fresh catalyst and with an apparent activation energy of ≈22 kcal/g mol compared to ≈30 kcal/g mol for the fresh catalyst. The data suggest that the most active form of Rh in the reactivated catalyst is small clusters whereas most of the active Rh in the fresh catalyst is present in larger Rh particles which are more bulk-like in character.

Étude de la chimisorption de O 2 À 77 K à l'aide d'isothermes d'adsorption physique à marches : Systèmes CdOKr et CdOCF 4

The evolution of stepwise isotherms of CF 4 and Kr on cadmium at 77.4 K, when increasing oxygen quantities are prechemisorbed at the same temperature, brings out some information about that chemisorption. It can be deduced from the splitting of steps that, at 77.4K, oxygen chemisorbs as rather homogeneous patches. It seems to be islands spreading in proportion to the chemisorbed quantities. As a mean, 4 oxygen atoms seem to be adsorbed for every surface cadmium atom. Such a number involves a reaction with slightly deeper Cd atoms. Over these oxidized patches, oxygen seems to physisorb (precursor state of oxidation) more readily than upon the bare part of the metal. This suggests one modification in the set of hypotheses usually acknowledged for the analysis of oxidation kinetics in that temperature range. A brief comparison is made with the somewhat different islands that are known to grow on metals when oxidization, or sulfurization, occurs at higher temperatures. Some remarks are aimed at the effects of “thermal regeneration” under vacuum.