Perovskite surfaces

The 100 face of a perovskite ABO3 crystal is the most stable surface, although it cannot be prepared by cleavage. There exist two non-equivalent terminations with respective stoichiometries AO and BO2 - for example, SrO and Ti02 for SrTiOs. In the surface layer, the transition metal cations B are five-fold coordinated, instead of six-fold in the bulk, and the cations A are surrounded by eight oxygens, instead of twelve in the bulk. The 100 surfaces of both SrTiOs and BaTiOs have been studied, but a quantitative surface structure determination has only been performed on the first compound a contraction of about 10% on the SrO face and a weak expansion of the order of 2%, at the limit of the error bar, on the Ti02 face, have been found by Bickel et al (1989 1990). Hikita et al (1993), on the other hand, deduce an outward relaxation on both faces. 

The SrTiOs face may be prepared by sputtering and annealing. Depending upon the preparation mode, it remains atomically flat or 

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Isupova, LA Budneva, AA Paukshtis, EA Sadykov, VA. Nature of the perovskites surface centers as studied by the Infrared spectroscopy of adsorbed NO test molecule. J. Mol. Catal, A Chem., 2000, Volume 158, Issue 1, 275-280. 

Myhra, S., Delogu, P., Giorgi, R. Riviere, J. C. 1988c. Scanning and high-resolution Auger analysis of zirconolite/perovskite surfaces following hydrothermal treatment. Journal of Materials Science, 23, 1514-1520. 

Nishihata et al. (2002) reported the re-dispersion of Pd in a Perovskite-type oxide. They investigated the oxidation state and the local structure of Pd by using X-ray absorption analysis. Pd occupies the -site in La2PdCo06 in the oxidized sample. For the reduced catalyst, the XAD and XANES measurements suggested the segregation of metallic Pd from the perovskite crystal. They imply that Pd also moves back and forth between the -site in the perovskite structure and sites within the lattice of Pd metal clusters dispersed on perovskite surface when the catalyst is exposed to fluctuations in the redox characteristics of the emission exhaust. 

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]. 

The relative constancy of NO adsorption with temperature and the strength of its bond with perovskite surfaces have suggested the use of this molecule over CO for determining surface metallic centers (107b,... 

Fig. 22. Schematic details of the perovskite surface in electrocatalysis (a) model for the active surface (b) d-electron configuration of M3+ ions (c) MO diagrams for the M +-OH (M = Mn, Ni) bonding. (Reprinted by permission of the publisher, The Electrochemical Society, Inc., from Ref. 247.)...

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. 

A large amount of N2O was formed from the initial stage over LaM03 (M = Co, Mn, Fe, Cr, Ni) at 573 K. The time course of the NO+CO reaction (performed in a batch recirculation system) reflects this situation. These results support a two-step reaction pathway in which N2O is an intermediate for nitrogen formation, deal et al. (1994) confirm the role of N2O as intermediate in this reaction over perovskite oxides. They used steady-state isotopic transient kinetic analysis to study the mechanism of NO + CO reaction over LaCo03. They concluded that N2O was an intermediate in the formation of N2 at T < 873 K. They also concluded that at high temperature CO2 desorption became the rate-limiting step of the overall reaction. This is likely due to the rapid formation and slow decomposition of very stable carbonates on the perovskite surface as reported by Milt et al. (1996). 

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. 

Kotomin EA, Heifets E, Dorfinan S, Fuks D, Gordon A, Maier J (2004) Comparative study of polar perovskite surfaces. Surf Sci 566 231-235... 

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