Oxide-solid interfaces

One of the most active current research areas in metal oxide surface science is oxide-solid interfaces. Work to date falls into three broad categories. The earliest work was the growth of metal films on single-crystal oxide substrates. One of the motivations there was to produce model systems to study the basic properties of oxide-supported metal catalysts. Quite a few groups have been involved in that work, and several papers have reviewed parts of the field [35-37]. An example of this approach is presented in Chap. 9 here. 

There has been a general updating of the material in all the chapters the treatment of films at the liquid-air and liquid-solid interfaces has been expanded, particularly in the area of contemporary techniques and that of macromolecular films. The scanning microscopies (tunneling and atomic force) now contribute more prominently. The topic of heterogeneous catalysis has been expanded to include the well-studied case of oxidation of carbon monoxide on metals, and there is now more emphasis on the flexible surface, that is, the restructuring of surfaces when adsorption occurs. New calculational methods are discussed. 

Equations la and lb are for a simple two-phase system such as the air-bulk solid interface. Real materials aren t so simple. They have natural oxides and surface roughness, and consist of deposited or grown multilayered structures in many cases. In these cases each layer and interface can be represented by a 2 x 2 matrix (for isotropic materials), and the overall reflection properties can be calculated by matrix multiplication. The resulting algebraic equations are too complex to invert, and a major consequence is that regression analysis must be used to determine the system s physical parameters. ... 

To describe hypergolic heating, Anderson and Brown (A10) proposed a theoretical model based upon spontaneous exothermic heterogeneous reactions between the reactive oxidizer and a condensed phase at the gas-solid interface. In these studies, the least complex case was considered, i.e., the one in which the solid phase is instantaneously exposed to a stagnant (nonflowing) gaseous oxidizer environment. This situation can be achieved experimentally provided the sample to be tested is suddenly injected into the desired environment in a manner designed to minimize gas flow. 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

Debye phonon velocity) and lower in the case of very dissimilar materials. For example, the estimated Kapitza resistance is smaller by about an order of magnitude due to the great difference in the characteristics of helium and any solid. On the other hand, for a solid-solid interface, the estimated resistance is quite close (30%) to the value given by the mismatch model. The agreement with experimental data is not the best in many cases. This is probably due to many phenomena such as surface irregularities, presence of oxides and bulk disorder close to the surfaces. Since the physical condition of a contact is hardly reproducible, measurements give, in the best case, the temperature dependence of Rc. 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

Other successful but limited surface modifying trials have been done such as Pt dispersed onto Ti02, Pt-Co and Mn perovskite , Pt-transition metal oxides , Pt-Au-Fe203 or Ru02- These materials are known as gas-solid interface carbon monoxide catalysts and have been... 

Since the solid—solid interface and bulk of the mixed conductor remain in chemical and electrical equilibrium, the measured overpotential t] is related directly to the spatially uniform oxidation state of the film through the Nemst equation 4Ft] = RTf d — (3o). Solving for d( and recognizing that the impedance Z = rjU, one obtains... 

Figure 13. Schematic representation of the setup used for the infrared characterization of liquid-solid interfaces [63], The main cell consists of a platinum disk used for adsorption and reaction, a Cap2 prism for guidance of the infrared beam, and a liquid solution trapped between those two elements. The overall arrangement includes gas and liquid sample introduction stages as well as the electronics used for the electrochemical oxidation-reduction cycles needed to preclean the platinum surface.

Williams and coworkers preliminarily reported that CO oxidation on Pt/Al203 is faster in the presence of water solvent than in the presence of ethanol [141]. We then studied CO oxidation on platinum surface in the presence of different solvents, and identified obvious solvent effects, namely, CO oxidation takes place the most easily with water solvent, the least easily with carbon tetrachloride solvent, and follows the overall trend of water > ethanol > methanol > cyclohexane > benzene carbon tetrachloride [67]. We subsequently took advantage of the solvent effect to design a diagnosing tool to pin down low-coverage CO at the liquid-solid interface, by flushing the liquid-solid interface with water and carbon tetrachloride individually [67]. 

Metal catalytic activity may be expected to be a function of the solubility of the active species and/or the ease of electron transfer to the catalyst. The results given in Table IV show conclusively that the suggestion that catalysis occurs at a gas-solid interface (13) does not hold in these systems. Preliminary experiments showed that copper ion- and haemin-catalyzed systems oxidized rapidly with no trace of solid precipitation, and that cobalt and nickel catalysis were characterized by the production of colored solutions and precipitates. Filtration experiments showed these precipitates played only a small part in catalysis (Table IV). 

Barbara J. Finlayson-Pitts is Professor of Chemistry at the University of California, Irvine. Her research program focuses on laboratory studies of the kinetics and mechanisms of reactions in the atmosphere, especially those involving gases with liquids or solids of relevance in the troposphere. Reactions of sea salt particles to produce photochemically active halogen compounds and the subsequent fates of halogen atoms in the troposphere are particular areas of interest, as are reactions of oxides of nitrogen at aqueous and solid interfaces. Her research is currently supported by the National Science Foundation, the Department of Energy, the California Air Resources Board, the Dreyfus Foundation, and NATO. She has authored or coauthored more than 80 publications in this area, as well as a previous book, Atmospheric Chemistry Fundamentals and Experimental Techniques. 

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