The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited

Ultrathin ordered CeO2 overlayers on Pt(111): interaction with NO2, NO, H2O and CO

This review traces the development of oxide solid solutions as catalysts from their first use in the 1960s to their current application in basic and applied research. Oxide solid solutions provide the means to control the properties of catalytically active ions in defined surface environments. When applied to transition metal (TM) ions, interaction with neighbors can be suppressed or progressively developed, depending on the concentration chosen for the active solute and the structure of the insulating matrix selected as solvent oxide. Simple examples are nickel, cobalt and chromium ions in MgO and MgAl 2 O 4 . The successful preparation of solid solutions demands a knowledge of the reactivity of solids and the behavior of crystal defects. This is exemplified in the methods described for preparing solid solutions of low and high specific surface area, respectively. Characterization receives detailed attention and the methods specific to oxide solid solutions are illustrated. Emphasis is placed on quantitative determination of surface composition for which X-ray photo-electron spectroscopy is the most widely applicable technique. The acidity and basicity of oxide solid solution surfaces is linked with coordinative unsaturation and this aspect of characterization involves adsorption calorimetry and infra-red spectroscopy. The account of oxide solid solutions as catalysts is divided into two parts. The first comprises studies where solid solutions have been used as model catalysts to identify and compare the catalytic properties of individual TM ions. For this purpose the catalysis of N 2 0 decomposition, CO oxidation and H 2 D 2 equilibration have long served as prototypical test reactions. These simple reactions enable issues such as the distinctive behavior of isolated ions, pairs and chains to be addressed and matrix effects to be explored. The motivation here is detailed understanding of catalysis on highly characterized microcrystalline oxides. The second catalytic part is broader in scope and focuses to a greater extent on the application of oxide solid solutions as catalysts for reactions of industrial interest. Combustion of hydrocarbons is a high-temperature reaction for which perovskite-structured solid solution catalysts are especially attractive since they accommodate a wide range of TM and main group ions in solid solution. A second sector covered is selective oxidation of hydrocarbons. Oxide solid solutions containing TM ions made an early entry as catalysts for alkene conversion and remained when interest switched to alkanes. The solid solution approach featured strongly in the search for methane coupling catalysts and currently contributes in a new guise in titanium silicalite. The acidity developed at solute sites is the source of activity for carbenium ion catalysis. SAPOs and McAPOs fall within the solid solution domain as covalently-bonded counterparts of ionic solid solutions. Finally, reduced solid solutions with phase-separated transition metal clusters are effective catalysts for reforming of alkanes.

Oxide solid solutions as catalysts

Core-level Mn and Co 2p x-ray photoelectron spectroscopic studies give evidence for different spin states of Mn and Co in the two different phases of the ferromagnetic compound LaMn0.5Co0.5O3. Ferromagnetism in the high-Tc (Tc?230?K) phase of the compound arises from Mn3+-O-Mn3+ superexchange interactions, whereas in the low-Tc (Tc?150?K) phase Mn4+-O-Co2+ superexchange interactions are responsible for the ferromagnetism.

The origin of ferromagnetism in the two different phases of LaMn0.5Co0.5O3: evidence from x-ray photoelectron spectroscopic studies

The selective propane oxidation catalyst MoVTeNb oxide M1 was investigated by microwave conductivity, synchrotron X-ray photoelectron, soft X-ray absorption and resonant photoelectron spectroscopy under reaction conditions to identify the influence of steam on the electronic bulk and surface properties. Steam significantly increases both the conversion of propane and the selectivity to the target product acrylic acid. The increased catalytic performance comes along with a decreased conductivity, a modification of the surface chemical and electronic structure with an enrichment of covalently bonded V(5+) species to the extent of Mo(6+), a decreased work function and hence polarity of the surface and a modified valence band structure. The higher degree of covalency in metal oxide bonds affects the mobility of the free charge carriers, and hence explains the decrease of the conductivity with steam. Furthermore we could prove that a subsurface space charge region depleted in electrons and thus an upward bending of the electronic band structure are induced by the reaction mixture, which is however not dependent on the steam content.

/pdf/the-impact-of-steam-on-the-electronic-structure-of-the-4y89s0ekpo.pdf

The impact of steam on the electronic structure of the selective propane oxidation catalyst MoVTeNb oxide (orthorhombic M1 phase).

Resonant photoemission performed at the SRS Daresbury is used to investigate the temperature- and doping-dependent spin transitions in single crystal samples of the La1-xSrxCoO3 system (x=0, x=0.1). For LaCoO3, the measurements taken over a wide temperature range are interpreted in the light of current models for the spin transitions. The Sr-doped compound is found to be in the same spin state at all temperatures. The temperature dependence of low binding energy features primarily associated with Co in the low spin state is investigated in detail. In particular, the intensity and onset energy of the Co3p→3d resonance associated with these features is measured at small binding energy intervals. This is used to locate the separate contributions of the low spin and higher spin states to these signals. In addition, the surface reactivity of the LaCoO3 (111) surface is investigated using H2O as a probe molecule, and the results are used to interpret changes which may occur in the spectra as a function of time in UHV.

Electronic structure and surface reactivity of La1-xSrxCoO3

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Electronic structure and surface reactivity of La1-xSrxCoO3