Ceria sintering

At the end of the seventies, scientists at Exxon discovered that metal particles supported on titania, alumina, ceria and a range of other oxides, lose their ability to chemisorb gases such as H2 or CO after reduction at temperatures of about 500 °C. Electron microscopy revealed that the decreased adsorption capacity was not caused by particle sintering. Oxidation, followed by reduction at moderate temperatures restored the adsorption properties of the metal in full. The suppression of adsorption after high temperature reduction was attributed to a strong metal-support interaction, abbreviated as SMSI [2]. 

Most of the literature focuses on the aspects of sinterability and microstructure, but limited data on the electrical properties is available. Tok [152] reported a conductivity of 18.3 x 10-3 Scm-1 at 600°C for Gd0 jCeo.gOj 95, and we measured a high conductivity of 22 x 10-3 scm-1 for Sm0 2Cc08O 9 at the same temperature. Their activation energies are relatively low—less than 0.7 eV. Although conductivity data reported for doped ceria prepared with carbonate precipitation is varied from different authors [153-155], the conductivity is generally high and the activation energy is usually low for ceria electrolytes fabricated with this method. 

During the cathode sintering phase at high temperatures, these materials tend to react with zirconia electrolytes to give La2Zr207, which has low ion conductivity and hence will result in low performance cells. The reaction is generally prevented by sandwiching a thin ceria layer between the electrolyte and the cathode. 

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

B. Sintering Inhibition Mechanism of Platinum Supported on Ceria-based Oxide... 

As for the complete oxidation of propene, propane and methane, Nieuwenhuys and coworkers studied the influence of metal oxides additives on the catalytic activity of Au/Al203 [109-115], The addition of 3d transition metal oxides (MnOx, CoOx or FeOx), which were active by themselves, or ceria that was poorly active by itself promoted the catalytic activity of Au/Al203 in the total oxidation of propene [112]. The most active catalyst was Au/Ce0x/Al203, with a T95 at 497 K and with a high stability. In these cases, ceria and the transition metal oxides may act as co-catalysts and the role is twofold it stabilizes the Au NPs against sintering (ceria)... 

Ceria-zirconia nanophases were synthesied by a surfactant-assisted method. The refined structural data concerning the crystallite size, lattice parameters, structural microstrain, cationic occupy number and cationic defect concentration are reported. Zirconium addition into the cubic structure of ceria inhibits crystal sintering but leads to structure distortion. Different CO-metal bonds are formed when CO chemisorbs on Pd-loaded CesZr. x02 catalysts. Catalytic tests reveal that the lower zirconium content benefits the CO oxidation. 

Nanosized ceria-zirconia materials with improved thermal stability can be prepared by using the surfactant-assisted method. Structural refinements confirm that the nanocrystals contain structural microstrain and cationic lattice defects. Zirconium addition to ceria supresses the crystal sintering and imporves the thermal stability but leads to structure distortion. Both catalytic tests and CO-chemisorption show that Pd supported ceria-zirconia nanoparticles are active for CO oxidation. 

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