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

Abstract

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.