Ceria single crystal

The first challenge to surface studies of model ceria surfaces is to produce a suitable Ce02 surface that can be mounted and manipulated within a UHV system. Typically bulk polished or cleaved single crystals, wafers or polycrystalline foils of the desired material are used for surface studies. This approach has been used in the case of Ce02. Ceria single crystals can be obtained commercially, and can be cut and polished to a desired orientation. Several studies on the (111) surface of bulk single crystal Ce02 have been reported." ... 

Cerium oxide (CeO ) was deposited onto a (100) LaAlOj single-crystal substrate with a single cube texture and good in- and out-ofplane alignment. A cp scan of the (220) plane of the CeO film yielded a FWHM of 2.45 ° 0.05 ° the rocking curve of the (200) plane indicated a FWHM of 1.688° 0.006°. A layer of YSZ was then deposited by CCVD on top of this ceria layer. The majority of the YSZ deposited as one cube texture (Figure 3) with only a minor 45° film rotation (<1% intensity) present. The... 

Figure 2.16. Left oxygen diffusion data of various ceria-coniaining materials 1 CeOj. single crystal 2 CeOj poly crystalline sample 3 CeogYojOiv 4 CCo Yo Oig 5 CeO,g 6 Ceo9Cao,0,/ 7 (0.3%)Od doped CcOj 8 Ce02. shaded area corresponds to oxygen diffusion values calculated from ionic conductivity of rare-earth doped ceria CC. M 02.o,. with x 0.2 and 0.3 using Eq. 2.29. Right details of oxygen diffusion coefficients of rare-earth doped ceria with x=0,3 from top to the bottom (M=Eu,Nd.Ho,Gd.Er.Sm,Y.La.Yb,Dy).

Some interesting fundamental studies have also been performed on model systems prepared by depositing ceria onto Pt(111) (136,140), and Rh(M 1) (213) single crystals. Finally, the structural nature and catalytic behavior of Pt/CeO systems generated by the oxidation with different chemical agents of some Pt-Ce (Ce 2 and CctPIb) crystalline alloys (123) have also been investigated. 

Growth by vapor deposition and oxidation (VDO) of Ce onto a substrate has been used successfully. The simplicity of this approach and its ability to be integrated into UHV systems designed for multiple surface diagnostic methods makes this a common technique for surface studies of chemisorption and surface reaction studies on model catalytic surfaces. Many of the ceria films used in work described below were produced in this way. Ce deposition and oxygen exposure (oxidation) may be performed simultaneously or sequentially. - Single crystal metals (Pt, Cu, Pd, Ni, and Ru ) and oxides, including yttrium-stabilized zirconia (YSZ), and sapphire, have been used as substrates for this approach. Such films have been... 

Adsorption of methanol has been studied on various ceria surfaces, including single crystal Ce02( 111), and ceria films grown on YSZ, sapphire and Cu(l 11). The results are sensitive to structure or more probably the amount of anion vacancies on the surface. Ferrizz et al find that annealed single crystal Ce02(l 11) is relatively unreactive to methanol. ... 

The adsoqjtion of NO on metal loaded ceria has been examined for Pt, and Pd, As known from work on single crystals, NO dissociates to some extent on each of these metals. The amount of dissociation is dependent upon the structure of the metal surface. Gorte considered Pt and Pd particles deposited on rough, poly crystal line ceria films grown by spray pyrolysis.For Pt they found variation in the TPD results (amount of NO uptake and shape of N2 desorption profile) that varied with the size of the Pt particles. However, the results were comparable to NO TPD results from Pt grown on sapphire. It was concluded that no unusual interaction existed between Pt and the (oxidized) ceria. For Pd it was found that a pronounced difference in the TPD product ratio, NO/N2, occurred for Pd on ceria compared to Pd on sapphire. They attributed the difference to NO adsorption on reduced ceria. 

Recently, it has been reported by Chiang et al/ that nanocrystalline (ca. lOnm crystallite size) undoped and doped ceria are very easily reduced. The heat of reduction was found to be less than one-half of the value for conventional polycrystallinc and single crystal samples. 

Another approach to preparing model catalysts is the preparation of inverse supported catalysts . In this approach, the catalytically active metal (usually single crystal) is used as a substrate upon which an oxide is deposited, presumably leaving patches of exposed metal. This approach has been used to study reduction of ceria, and methanation kinetics on Rh as promoted by deposited ceria, and chemisorption of various molecules. As stated above, it is generally assumed that thick enough ceria layers will continuously cover the metal substrate, placing a limit on the thickness of the ceria islands that can be achieved for an inverse supported catalyst. The different procedures used for the inverse and metal particle on bulk oxide model catalysts is expected to produce differences in thermal stability, morphology and surface structure which may have consequences for the reactivity of the model catalyst. 

In order to understand the reactivity of ceria surfaces and the interaction of it with metal particles or adsorbates, it is of fundamental interest to know its surface structure and the extent or type of defects present. Even though the film may be an oriented single crystal, there is still the question of whether the surface is terminated in oxygen anions, Ce cations, a mixture or in defects associated with the termination. Charge neutrality, interfacial relaxation and dielectric discontinuities may modify the properties of an oxide surface. Also the ability of the surface to adsorb or give up oxygen, as well as the structure, clustering and reactivity of defects may be expected to depend upon the surface orientation and structure. 

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