X-ray photoelectron spectroscopy is used to analyse systematically a group of La1−xSrx-based mixed-metal perovskite-type oxides of interest in the development of oxygen separation membranes. Angle- and temperature-resolved studies reveal compositional variations that correlate with the segregation of various elements during annealing. The complex O 1s, Sr 3d5/2 and La 3d5/2 spectra could be curve-fitted systematically with two La 3d5/2, three Sr 3d5/2 and five O 1s contributions, respectively, using fixed binding energies and full widths at half-maximum. The trends are discussed and assignments are made on the basis of peak intensity and composition variations noted during the angle- and temperature-resolved studies, literature data and nearest-neighbor electronegativities. In all cases, elements present at the outermost surface revealed higher binding energies than their bulk-bound counterparts. Copyright © 2002 John Wiley & Sons, Ltd.

Systematic x‐ray photoelectron spectroscopic study of La1−xSrx‐based perovskite‐type oxides

Preparation, surface state and band structure studies of SrTi(1 − x)Fe(x)O(3 − δ) (x = 0–1) perovskite-type nano structure by X-ray and ultraviolet photoelectron spectroscopy

Doping of LaCoO3 with copper to add reduction functionality in addition to the known oxidation properties has been investigated, aiming at three-way catalysis (TWC) applications. Nanoscale perovskites with nominal composition LaCo1−xCuxO3, (x = 0, 0.1, 0.3, and 0.5) have been synthesized by means of the citrate method. A stable perovskitic phase with rhombohedral geometry up to an unprecedented x = 0.5 has been obtained and characterized by BET, X-ray diffraction (XRD), Temperature Programmed Reduction (TPR), X-ray Photoelectron Spectroscopy (XPS), and Scanning Electron Microscopy (SEM). The crystallite size decreases with increasing the copper amount and also the morphology differs; moreover the surface reactivity with respect to atmospheric moisture and carbon dioxide is more evident in the sample with x = 0.5. Reactivity has been measured in simple NO + CO and CO + O2 model reactions, as well as with complex mixtures approaching automotive exhaust composition, at both stoichiometric and O2 limiting conditions. The catalysts have been characterized after reaction as well. The perovskite with the highest Cu amount, LaCo0.5Cu0.5O3, exhibits an interesting compromise of oxidation and NO reduction functionality at interesting, low temperatures with very short contact time (GHSV = 1,000,000 h−1). Still, activity for NO reduction in real mixtures requires substoichiometric O2.

Largely Cu-doped LaCo1−xCuxO3 perovskites for TWC: Toward new PGM-free catalysts

La0.6Sr0.4Co1−yFeyO3−δ (y = 0.2, 0.5, 0.8) perovskite-type oxide powders were prepared by the citrate gel method with the aim of investigating the influence of cobalt/iron atomic ratio on the chemical and structural properties as well as on the catalytic activity. The samples were characterized by means of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermal analysis, Kr physisorption surface area, scanning electron microscopy (SEM), and temperature programmed reduction (TPR). XRD outcomes reveal the formation of perovskite phase after calcination at 873 K. Depending on the Co/Fe atomic ratio, the perovskite exhibits two different structure: rhombohedral for y < 0.5, orthorhombic for y ≥ 0.5. However, other phases are also observed. The crystallite size increases with the calcination temperature, while decreases with the iron content. The specific surface area of the La0.6Sr0.4Co1−yFeyO3-δ perovskites (calcined at 1073 K) is low, but increases with the iron amount: it varies between 4....

La0.6Sr0.4Co1−yFeyO3−δ Perovskites: Influence of the Co/Fe Atomic Ratio on Properties and Catalytic Activity toward Alcohol Steam-Reforming

Surface properties of Sr- and Co-doped LaFeO3

Surface behavior of Sr-doped lanthanide perovskites

Laboratory for Inorganic Materials Science, Faculty of Chemical Technology, University of Twente, 7500 AE Enschede, The Netherlands High oxygen fluxes through mixed-conducting oxi- des with fluoride and perovskite structure are well known [1-3]. According to [4] the selective feeding of oxygen into high temperature electrochemical reactors generally comprises: (i) providing an electrochemical cell comprising a first zone and a second zone separated from the first by a solid multi- component membrane; (ii) raising the temperature of the electrochemical cell from about 300 °C to about 1400 °C; (iii) passing the oxygen-containing gas in surface contact with the membrance, and (iv) passing the methane in contact with the membrane surface in the second zone. It is evident that under a high temperamre working regime, the permeability of oxygen, as well as all of the other physical and chemical parameters, are very important for the realization of a highly effective chemical process at a large oxygen partial pressure gradient. The catalytic activity of such an oxidation process, e.g. of CO, methane, etc., is a function of the chemical state of the surface elements and decreases monotonically with the decrease of the surface atomic ratio of the 3-rd elements. In the work reported here the chemical state of the surface of an Sr-doped lanthanide cobalt perovskite (La0.3Sr0.7CoO3_~, 3LSC) was the object of an investigation carried out with the help of electron spectroscopy for chemical analysis (ESCA), includ- ing the X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) methods. In our work the method of preparation of perovskite ceramic samples developed by van Doorn and Bouwmeester [5] was accepted. The materials used were La(NO3)3.6H20,

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M. Machkova

Papers

Surface behavior of Sr-doped lanthanide perovskites

Surface analysis of doped lanthanide cobalt perovskites by X-ray photoelectron spectroscopy