Perovskites surface composition

J.L.G. Fierro, Structure and composition of perovskite surface in relation to adsorption and catalytic properties, CataL Today 5 153 (19%). 

Determination of stoichiometric effects (mainly controlled by cation substitution [82, 100]) was an important achievement of electrocatalytic studies of perovskites. These served to widen views on the adsorption properties of these materials, and to test assumptions on the composition of adsorption layers on perovskites made on the basis of the analysis of kinetic data of oxygen reactions [85,101,102]. The probability of the formation of various oxygen-containing adsorbates in certain sites on perovskite surfaces was estimated by theoretical analysis [83]. 

Under this heading all the reactions involving H2 as one of the reactants will be reviewed. They include hydrocarbon hydrogenation and hydrogenolysis, COx hydrogenation and olefin hydroformylation. The common denominator in all these systems is that the perovskite surface is reduced to varying extents depending on the reactant mixture composition. 

Perovskite-type oxides with A and/or B sites partially substituted present properties such as structural defects and reactivity of adsorbed and lattice oxygen that play a central role in catalytic combustion. However, preparation methods as well as temperature of calcination could affect the surface area, and most important, changes on the surface composition that will be reviewed in the following section. 

Fierro, J.L.G. Composition and structure of perovskite surfaces. In Properties and Application of Perovskite Type Oxides, Tejuea, L and Fierro, J.L.G. Eds. Chemical Industries Dekker, New-York. Vol. 50, 1993, pp. 195-214. 

Table 11.1 Surface composition of the substituted perovskites promoted by Ce [5].

Table 11.1 for the modified perovskites. The results indicate that the surface of the synthesized perovskite is quite enriched with lanthanum and the La/Co ratio differs substantially from the bulk. The promoted isostructural substitutions did not change the surface composition relative to the cobalt, independent of the substituent. 

The surface composition was determined on the perovskite-type sample. Figure 11.6 displays the Co 2p, La 3d, and O Is spectra. 

The surface composition of the LaCoOa perovskite is presented in Table 11.2. The Co/La ratio was 0.47, lower than the stoichiometric value (1.0) for the LaCoOa structure, probably due to the segregation of La ions at the surface and the formation of La(OH)3. 

Surface Composition and Chemical State of Elements in Complex Perovskites... 

The heats of oxygen absorption in the surface layers of perovskites and composites reduced by high-temperature pretreatment in He were estimated using a SEN SYS TG-DSC flow microcalorimeter and pulses of O2 in He at 450 °C. 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

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