Preparation of Model Ceria Supports

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.  

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 deseribed 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 

Another method for production of ceria films is by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE). This method has been reviewed by Chambers. The advantage of the oxygen assisted plasma technique is that for certain films the dissociation of O2 is limiting in the growth of fully oxidized films. By use of a plasma source of oxygen, O atoms and ions are provided to the 

Polycrystalline films can be prepared by spray pyrolysis. In this technique a spray of an aqueous solution of cerium salt is nebulized and directed by a stream of compressed gas onto a heated substrate. Various parameters are important for determining the resulting structure of the ceria film. The effects of the spray solution and of substrate temperature for films deposited upon silica substrates have 

Thin films of Ce02 have been obtained using molecule based metal-organic chemical vapor deposition (MO-CVD). This method is attractive because of the possibility to coat complex shapes and to use lower growth temperatures. The actual application of this technique was accomplished for the first time using a newly 

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Following deposition of an active metal upon a ceria surface, it is possible to study chemisorption on a surface that models many of the important aspects expected for actual ceria supported catalysts. Surface techniques offer the possibility to identify where the adsorbates are located and to identify intermediates that are formed in their interaction. By comparison of ceria surfaces, with and without metal, the synergisms between metal and support can be deduced. By controlled metal deposition, it is possible to study the effects of loading and particle size. By selected preparation of the ceria substrate it is possible to vary factors which may affect the interaction between the metal particle and the ceria, sueh as structure, defeet concentration or oxidation state of the ceria. The goal of chemisorption studies, summarized below, is to relate all these factors to the interaction of the model catalyst with particular adsorbates. 

The further step of monitoring a dynamic catalytic reaction on a surface is barely approachable with existing surface science method by using molecular beam techniques, or by isolating a model catalysts prepared and analysed in UHV in a reactor appended to the UHV system. Although such studies apparently have not been performed for ceria supported model catalysts, it is appropriate to mention reactor studies of CO oxidation and water gas shift d reactions performed... 

Fig. 4.38. The effects of various pretreatments (oxidative and reductive) on CO oxidation on a 40-nm Pt/ceria model catalyst prepared by colloidal lithography as measured by the temperature of 50% of CO conversion and the apparent activation energy from the Arrhenius plot. CO reduction was made in 0.5% CO for Ih at 573K, H2 oxidation (a-treatment) was done at a = Ph2/(.Ph.2 + P02) = 0.33 at 573 K for 1 h, and finally /3 = CO oxidation (/3-treatment) was done in the O-rich regime (oxidative conditions), /3 = Pco/ Pco + P02) = 0.2 with 0.3% CO and 1.2% O2 at temperatures between 300 and 673 K. It is seen that reduction leads to a lower Tbo and activation energy, while sustained CO oxidation leads to an increase of the activation energy, which is not recovered by reductive treatments. The latter is explained in terms of strong-metal-support interactions (SMSI) and particle reshaping (see text)...

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. 

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