Fluorite-Type Catalysts

Ce02 with fluorite-type structure is a component in N02 dissociation catalysts [57] and biomass gasification catalysts [58], 

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Ceria is another type of mixed conducting oxide which has been shown already to induce electrochemical promotion.71 Ceria is a catalyst support of increasing technological importance.73 Due to its nonstoichiometry and significant oxygen storage capacity it is also often used as a promoting additive on other supports (e.g. y-A Cb) in automobile exhaust catalysts.79 It is a fluorite type oxide with predominant n-type semiconductivity. The contribution of its ionic conductivity has been estimated to be 1-3% at 350°C.71... 

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. 

Ceria-based catalysts are intensively used because of their high chemical and physical stability, high oxygen mobility and high oxygen vacancy concentrations, which are characteristic of fluorite-type oxides. The possibility of cycling easily between reduced and oxidized states (Ce Ce" ) permits the reversible addition... 

Vargas, J.C., Libs, S., Roger, A.-C., and Kiennemann, A. Study of Ce-Zr-Co fluorite-type oxide as catalysts for hydrogen production by steam reforming of bioethanol. Catalysis Today, 2005, 107-108, 417. 

We prepared by sol-gel some thermally stable Zro.io(Cei-xPrx)o.9o02 mixed oxides (x between 0 and 0.75) with a fluorite-type structure. This structure was confirmed by the presence, in the Raman spectrum, of a single band ca. 460 cm characteristic of the M-0 vibration in the fluorite-type structure. Moreover, the band position and the shoulder at 570 cm indicate the presence of oxygen vacancies probably associated with praseodymium cations. Consequently, high OSCs appears to be the result of the presence of both cerium and praseodymium atoms. Thus, addition of praseodymium atoms into zirconia-ceria oxides appears to be very promising for the design of new automotive catalysts. 

A certain amount of Ca is incorporated into the Ce02 matrix to form a fluorite-type solid solution with a solubility limit around 20%. According to XPS results the surface composition was approximately equal to the bulk. There is a small enrichment of Ca on the surface for solids with low Ca content. The surface area of the catalysts prepared did not differ significantly. 

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