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The surface composition of alumina-supported Pd, Rh, and Pd–Rh catalysts fresh or after high temperature treatments under oxidizing (O2 in N2) or reducing (H2 in N2) mixture has been characterized using various complementary techniques: chemisorption and titration of H2 and O2 probe molecules, 18O2/16O2 isotopic equilibration (OIE), FTIR spectroscopy of adsorbed CO and NO, and electron microscopy with X analysis. From both techniques it is concluded that the surface composition is close to the bulk composition for all the fresh bimetallics, whatever the Rh content. The surface state of the bimetallics treated at 1173 K is strongly dependent on the nature of the gas mixture. There is a relative increase in surface Pd after treatment in an oxidative medium, because of a Rh3+ migration into alumina, and in surface Rh after treatment in a reducing medium. The specific role of alumina can explain these results, in contradiction with thermodynamic models predicting the reverse situation (enrichment in Rh in O2 and in Pd in H2). Activity for propane oxidation at 573 K and for propane steam reforming at 673 K was also determined over fresh and sintered catalysts. Because Pd is more active than Rh in oxidation and Rh is more active than Pd in steam reforming, activity changes with the Rh content, before and after sintering, may reflect modifications of surface composition. The results obtained with the series of Pd–Rh catalysts are in agreement with the measurements made by OIE and FTIR spectroscopy. The hypothesis and the conditions of applications of the methods are discussed. |
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