Characterization of model three-way catalysts - III. Infrared study of thesurface composition of platinum-rhodium ceria-alumina catalysts

The surface composition of model and commercial bimetallic PtRh catalysts s upported on ceria-alumina was tentatively determined by FTIR spectroscopy u sing the successive adsorption of NO at 473 K and CO at 298 K method which was previously applied to the case of PtRh/Al2O3 catalysts. The study of mo nometallic model catalysts shows that the adsorption of NO at 473 K on rhod ium gives an intense band at 1912 cm(-1) which is not modified by the prese nce of ceria and therefore can be used to quantify the number of surface rh odium atoms. However, the chemisorption properties of platinum toward the s ubsequent adsorption of CO at 298 K are strongly modified and the quantific ation of the surface platinum atoms could not be directly measured, However , their number could be indirectly obtained by combining the NO adsorption for rhodium and hydrogen volumetric adsorption for the total number of surf ace metal atoms, assuming that NO adsorption does not induce an important s egregation of rhodium for bimetallic particles. Thus, for the fresh bimetal lic model catalyst, the same composition was obtained at the surface and in the bulk, After aging at 1273 K, no rhodium was detected at the surface. T his absence of surface rhodium atoms was confirmed by the data obtained fro m the direct adsorption of CO on platinum at room temperature, which gave t he same number of surface atoms as hydrogen chemisorption. Similar results have been obtained with two commercial three-way catalysts. In particular, after aging at 1273 K, rhodium was practically not detected on the surface. (C) 1999 Academic Press.