Design of the synthesis of Pd/Ce0.8Zr0.2O2: influence on H2 spillover

F. C. GENNARI; A. CONDÓ; J. ANDRADE GAMBOA; T. MONTINI; P. FORNASIERO

Reykjavik, Iceland

Simposio; International Symposium on Metal-Hydrogen Systems; 2008

Metal-Hydrogen Systems Association

Ceria-based materials have received a great attention because of their applications in different fields such as high-temperature ceramics, catalysts and solid oxide fuel cells. The reactivity of ceria is connected to Ce4+/Ce3+ redox couple, resulting in its high capacity for storing/releasing oxygen under oxidizing/reducing conditions. Under hydrogen atmosphere, the reduction of ceria mixed oxides leads to the elimination of lattice oxygen and the creation of anionic vacancies. In the presence of a noble metal, the reduction rates are significantly increased due to extensive hydrogen spillover onto the support, even at room temperature. In particular, the hydrogen interaction with  Pd/CexZr1-xO2 is a complex phenomenon, involving reduction of Pd2+/Pd0, Pd hydride formation, hydrogen activation, interaction between noble-metal and support, etc.  The reactivity of hydrogen with Pd/CexZr1-xO2 can be affected  by several factors such as structure of the mixed oxides, surface area, pore size distributions, structural defects, metal dispersion, etc., which are mainly determined by the actual synthesis procedure. Although various methods have recently been described for the preparation of ceria-based solutions (such as high energy milling, sol-gel techniques and hydrothermal synthesis under supercritical water), they rarely provide a monophasic material with high surface area and desired textural properties.  In the present study, nanoparticles of ceria-zirconia mixed oxides with the composition Ce0.8Zr0.2O2 were synthesized by co-precipitation and microemulsion routes. Nitrogen adsorption isotherms, x-ray diffraction, scanning electron microscopy and transmission electron microscopy were used to study the effects of synthesis procedure on the surface and bulk properties of the catalyst. The calcined material shows the formation of fluorite type structure of CeO2 independently of the synthesis route. XRD and TEM characterizations evidence the presence of highly dispersed crystallites with diameters of <6 nm. Oxygen storage capacity (OSC) and hydrogen adsorption kinetics of Pd/Ce0.8Zr0.2O2 were investigated as a function of temperature by quantitative dynamic-OSC and volumetric measurements, respectively. Remarkably, the materials show high surface area, mesoporous network and nanometric characteristics even after high temperature treatments and consistently they possess also optimal and stable redox properties. The rate and the amount of oxygen storage/release capacity are greatly enhanced by Pd addition The hydrogen uptake rate is strongly dependent on the temperature and involve PdO reduction as well as complete surface reduction. A mechanism describing the interaction of hydrogen with the surface and bulk catalyst structure is proposed.