Titration by Oxygen of the Spill-over Hydrogen Adsorbed on Ceria–Zirconia Supported Palladium–Rhodium Catalysts

Abstract

To find a method for estimating the surface area of ceria–zirconia in composite three-way catalysts, successive O2/H2/O2 chemisorption measurements were performed at room temperature on model Pd and/or Rh/Ce0.63Zr0.37O2 catalysts previously reduced at 573 K by hydrogen, according to a methodology developed for ceria or ceria–alumina supported metal catalysts. The quantity adsorbed during the first oxygen chemisorption was found to correspond to the reoxidation of several layers of the support. The number of layers increases when the catalysts are aged at 1323 or 1423 K, which clearly indicates the great mobility of bulk oxygen in CeO2–ZrO2 supports, as already shown in the literature. After a subsequent H2 chemisorption, the titration by oxygen of the hydrogen adsorbed on metal particles and on the support leads to a lower quantity of chemisorbed oxygen which is closely related to the surface area of ceria–zirconia. This relationship was observed in fresh and aged catalysts and was used to calculate a mean calibration coefficient of 5.1 μatom O m−2. Thus, contrary to temperature-programmed reduction measurements, this method of quantification by oxygen of the spill-over hydrogen can be used to estimate the surface area of ceria–zirconia in composite catalysts in which CeO2–ZrO2 is one of the components of the support. This latter point is illustrated with model Pd and PdRh catalysts deposited on a ‘ceria–zirconia + alumina’ support.