Effects Of Yttrium Doping On Oxygen Storage Capacity Of Ce_0.33Zr_0.67O₂Mixed Oxides

Zr-rich CeO2-ZrO2mixed oxides have been widely used in automobile emissioncatalytic converter. In the present work, a series of yttrium doped Ce_0.33Zr_0.67O₂（CZ）materials were prepared by citric acid sol-gel method. The oxygen storage/releaseprocess and the effects of yttrium on the oxygen storage capacities of Ce_0.33Zr_0.67O₂were studied by OSC measurement, XRD, BET, XPS, and in-situ CO-IR, etc.It was found that the activated oxygen on the surface and subsurface werepreferentially involved into the oxygen release process of CZ, which was similar withthat of Ce-rich CeO₂-ZrO₂. In this stage, the adsorption property of the material wasthe key factor to the oxygen storage capacity. Then, with the process of the oxygenrelease, the lattice oxygen gradually diffused from the bulk to the surface. Thediffusion of the bulk oxygen was confirmed to be the rate-determining step.Yttrium could improve the structural thermal stability of CZ. For the freshsamples, the addition of Y enhanced the surface area, so as to improve the OSC of theCZ. However, for the aged sample, the OSC decrease obviously with the increasingof Y doping ratio. Confirming by XPS and in-situ CO-FTIR, Y substituted the Zrsites on the surface of CZ. The doped Y ions had a counteractive to OSC because ofreducing the adsorption properties of CZ and inhibiting the redox process of Ce ions.The adsorption model on CZ and Y doped CZ was established for further studyon the effect of Y. For CZ, the bridge-type carbonate and carboxylate species werefirstly formed by CO adsorption on Ce sites, and then the bidentate and polydentatecarbonates were formed. With the increasing of sintering temperature, the amount ofthe adsorption species decreased sharply because of the deactivation of the adsorptionsites. The high-thermal-stable polydentate carbonates became the main species on thesurface. For Y doped CZ, although the type of the adsorption species was almost thesample with those on CZ, the amount of the adsorption species of Y doped CZ wasmuch less than that of CZ. The degradation of the adsorption property was ascribed tothe inhibiting effect of Y on the transfer path of the electron between Ce ions and CO.