EVOLUTION OF THE CERIA SURFACE-AREA OF PT-RH CERIA-ALUMINA CATALYSTS SUBMITTED TO HYDROTHERMAL AGING AT 1273 K

The accessible surface area of cerium oxide was measured on model ceri a-alumina supported Pt and/or Rh catalysts submitted to an hydrotherma l treatment at 1273 K (10% H2O in N-2). Two distinct methods, develope d for fresh catalysts, were utilized. They are based respectively on t he hydrogen temperature programmed reduction (TPR) profiles and the in frared spectra of the specific adsorption of CO2 on the hydroxyl group s of alumina. After aging, the TPR curves have become flattened and it is difficult to have a good reliability in the deconvolution of the p eak corresponding to the ceria surface reduction. On the support alone , the results of both methods are in agreement and indicate a decrease of the ceria surface area by a factor close to 4, whereas the total s urface area is only divided by 2. It appears that cerium oxide sinters more rapidly than alumina. For the metallic catalysts, the calculated ceria surface areas values depend on the hypothesis done on the oxida tion state of the precious metals before the TPR. If reliable results can be obtained in the case of rhodium with O/Rh = 1.5, in presence of platinum, the mean oxidation state of platinum O/Pt = 1 results in un derestimated values. The ceria surface areas deduced from the infrared results are higher than those deduced from TPR. Although the former a re uncertain, they are in good agreement with a lower coverage of the alumina surface by sintered cerium oxide particles.