Formation Of Mixed Oxides Research Articles

From compounds to materials: heterodinuclear complexes as precursors in the synthesis of mixed oxides; crystal structures of [Cu(H2LA)] and [{CuY(LA)(NO3)(dmso)}2]·2dmso [H4LA=N,N′-ethylenebis(3-hydroxysalicylideneimine), dmso = dimethyl sulphoxide

Mononuclear copper(II) and nickel(II) complexes [M(H2LA)] have been obtained by reaction of the appropriate metal acetate with the potentially hexadentate compartmental ligand H4LA, prepared by condensation of 2,3-dihydroxybenzaldehyde with ethylenediamine. Physicochemical data suggest the metal ion is in the inner N2O2 compartment. An X-ray investigation on [Cu(H2LA)] showed that the crystals are orthorhombic, space group Pnca, with a= 19.381(5), b= 15.327(4), c= 10.158(4)A for Z= 8 and confirms the inner occupancy of copper(II) which exhibits an almost square-planar configuration. The bond lengths to CuII[Cu–N 1.93(1), Cu–O 1.89(1)A(mean)] are within the usual range. The complexes [M(H2LA)] act as ligands in the formation of heterodinuclear [M1M2LA(X)(solv)n](M1= Cu or Ni, M2= La or Y, X = NO3 or Cl) or [M1M2LA(solv)n](M = Cu or Ni, M2= UO2 or Ba) species [solv = H2O, methanol or dimethyl sulphoxide (dmso)]. Crystals of [{CuY(LA)(NO3)(dmso)}2]·2dmso, grown from dmso–MeOH are triclinic, space group P, with a= 9.919(5), b= 11.357(5), c= 13.789(6)A for Z= 1 (the asymmetric unit is half of the molecule). The X-ray structure shows that the copper(II) is four-co-ordinated in the inner N2O2 compartment while the yttrium(III) is eight-co-ordinated in the outer O2O2. The complex is a tetranuclear asymmetric unit, two yttrium(III) ions being held together by phenolic oxygen bridges. The Y–O(ligand) bonds are in the range 2.28–2.40 A, Y–O(dmso) is 2.31(1)A and Y–O(nitrate) are 2.48 A(mean). The Cu ⋯ Y and Y ⋯ Y distances are 3.397(3) and 3.781(2)A respectively. The co-ordination of each yttrium(III) ion is completed by the oxygens of a bidentate nitrate ion and a dmso molecule. The geometry around the metal ion can be described as pentagonal bipyramidal. Two dmso molecules are present in the cell as clathrate solvent. The thermal decomposition of copper(II)–yttrium(III) and –lanthanum(III) complexes and the consequent formation of mixed oxides have also been studied and the preliminary results are reported.

Silica coatings on strongly passivated substrates

Silicon dioxide can be deposited by pyrolysis of silicon alkoxide onto highly passivating substrates such as stainless steels ( e.g. AISI 316L), superalloys (H34CT), and titanium and its alloys. Before silicon dioxide deposition the passivating layer has to be removed, and a strongly adherent, dense oxide layer has to be re-formed (pre-oxidation). The substrate composition, in particular the carbon content, is the determining factor for the type of surface preconditioning used; this pretreatment is necessary for the complete removal of the chromium-rich passivating layer. Mechanical grinding, honing and polishing, chemical wet etching, electrochemical polishing and gas phase etching are typical treatments considered for surface preparation; these treatments replace partially or completely the reduction of the passivating oxide layer by means of the carbon in the substrate, which occurs above 780 °C. For the final mechanical operation only diamond should be considered (tool, abrasive); the use of other materials tends to contaminate the surface and leads to the formation of mixed oxides during the pre-oxidation. The pre-oxidation itself is performed under pure water vapour, moist nitrogen or isopropanol; in some cases (titanium and its alloys) preconditioning under pure nitrogen must be carried out. Internal stresses must be relieved before the pre-oxidation and silicon oxide deposition treatments. It was also found that, during the pyrolysis of silicon alkoxide, the reactor atmosphere must have some water vapour, which can be produced by an adequate evaporator or by dehydration of isopropanol.

Preparation and characterization of supported mixed metal oxides: Thermal decomposition of heteropoly metal complexes immobilized on silica

Supported mixed metal oxides were prepared by the temperature-programmed decomposition of a highly characterized system of silica-immobilized heteropoly complexes containing Cu(II) and M(III) ( M(III) = Al, Cr, Fe). The investigation focused on two main aspects of the thermal degradation: a study of the decomposition path and characterization of the resultant samples. The decomposition process, monitored in situ by thermogravimetric analysis, infrared spectroscopy, and mass spectrometry, appeared to lead to the formation of mixed oxides with a stoichiometry of MCu 6O 7.5. Electron microscopy showed sintering of the oxides into particles which contain Cu and M. The population of these particles increased with increasing temperature. Selective chemisorptions of NH 3, CO, and NO indicated that temperature of decomposition affects active site densities. In particular, it was observed that conditions for the decomposition could be selected to maximize the activation of metal oxide sites by complex decomposition yet minimize the deactivation by sintering of supported oxide particles.

ChemInform Abstract: Hydrogenation of Carbon Monoxide over Rhodium/Silica Catalysts Promoted with Molybdenum Oxide and Thorium Oxide.

The promotion of silica-supported rhodium catalysts in the hydrogenation of carbon monoxide by molybdenum oxide and thorium oxide has been examined. Temperature programmed reduction studies indicated the formation of rhodium molybdates, while no evidence was found for the formation of such mixed oxides in the thorium oxide-promoted catalysts. Hydrogen and carbon monoxide chemisorption were suppressed by the presence of molybdenum oxide, pointing to a coverage of the rhodium particles by this promoter oxide. The catalysts with MO: Rh ratios exceeding one even exhibited an almost complete suppression of the rhodium chemisorption capacity. In the thorium oxide-promoted catalysts the chemisorption of hydrogen and carbon monoxide were not suppressed. Infrared spectroscopy of adsorbed carbon monoxide showed that molybdenum oxide completely suppressed bridge-bonded and linearly bonded carbon monoxide, as well as the gemdicarbonyl species. Thorium oxide addition resulted in a minor decrease of the linearly bonded carbon monoxide, while the bridge-bonded carbon monoxide was suppressed to a greater extent. The IR spectra of the thorium oxide-promoted catalysts also exhibited a broad absorption band between 1300 and 1750 cm-‘, which is thought to be due to carbon monoxide bonded with the carbon atom to the metal and with the oxygen atom to the promoter ion. Carbon monoxide hydrogenation was greatly enhanced by the presence of both molybdenum oxide and thorium oxide. Thorium oxide-promoted catalysts had a high selectivity to C,-oxygenates, while the molybdenum oxide-promoted catalysts exhibited a high methanol selectivity. Ethylene addition to a working catalyst showed that the carbon monoxide insertion reaction, which is thought to be responsible for the formation of oxygenates, was not enhanced by molybdenum oxide, nor by thorium oxide. The ethylene addition experiments indicated that the role of the promoter is to enhance carbon monoxide dissociation. The results can be understood by assuming that side-bonded carbon monoxide, with its weakened C-O bond, is responsible for the higher carbon monoxide dissociation activity.

Hydrogenation of carbon monoxide over rhodium/silica catalysts promoted with molybdenum oxide and thorium oxide

The promotion of silica-supported rhodium catalysts in the hydrogenation of carbon monoxide by molybdenum oxide and thorium oxide has been examined. Temperature programmed reduction studies indicated the formation of rhodium molybdates, while no evidence was found for the formation of such mixed oxides in the thorium oxide-promoted catalysts. Hydrogen and carbon monoxide chemisorption were suppressed by the presence of molybdenum oxide, pointing to a coverage of the rhodium particles by this promoter oxide. The catalysts with Mo:Rh ratios exceeding one even exhibited an almost complete suppression of the rhodium chemisorption capacity. In the thorium oxide-promoted catalysts the chemisorption of hydrogen and carbon monoxide were not suppressed. Infrared spectroscopy of adsorbed carbon monoxide showed that molybdenum oxide completely suppressed bridge-bonded and linearly bonded carbon monoxide, as well as the gemdicarbonyl species. Thorium oxide addition resulted in a minor decrease of the linearly bonded carbon monoxide, while the bridge-bonded carbon monoxide was suppressed to a greater extent. The IR spectra of the thorium oxide-promoted catalysts also exhibited a broad absorption band between 1300 and 1750 cm −1, which is thought to be due to carbon monoxide bonded with the carbon atom to the metal and with the oxygen atom to the promoter ion. Carbon monoxide hydrogenation was greatly enhanced by the presence of both molybdenum oxide and thorium oxide. Thorium oxide-promoted catalysts had a high selectivity to C 2-oxygenates, while the molybdenum oxide-promoted catalysts exhibited a high methanol selectivity. Ethylene addition to a working catalyst showed that the carbon monoxide insertion reaction, which is thought to be responsible for the formation of oxygenates, was not enhanced by molybdenum oxide, nor by thorium oxide. The ethylene addition experiments indicated that the role of the promoter is to enhance carbon monoxide dissociation. The results can be understood by assuming that side-bonded carbon monoxide, with its weakened C O bond, is responsible for the higher carbon monoxide dissociation activity.

Chemical Characterization of Complex Oxide Products on Titanium‐Enriched 310SS

The phases present in the complex oxide scales formed on titaniummodified 310 stainless steel alloys after exposure to coal gasification atmospheres are described. The significant titanium concentrations in the inner layer have been shown to arise from the substitution of titanium for chromium in the due to the increased stability of Ti3+ at low oxygen partial pressures. In the absence of manganese, the external layer after a 100 hr exposure at 1255 K consists of and a cubic spinel phase. When manganese is present in the base alloy, is found instead of . The absence of external sulfides on the surface of these alloys has been explained by the preferential formation of mixed oxides containing base metal elements.

Interference effects of different metals in the atomic absorption spectrophotometric determination of chromium

The interfering effect of thirty six metal salts on chromium absorbance in the air-acetylene flame has been studied. The interference does not depend on the boiling point of the added foreign metals or particle size as previously assumed but is postulated to result from the formation of mixed oxides or bimetallic species or from suppression of ionized gaseous chromium pressure. The effect of the alkali sulphates as releasing agents in the determination of chromium has been tested and results show that they can be used to eliminate both suppressive and enhancing interferents.

Etude des reactions du cesium avec l'acier inoxydable

In irradiated fuel elements of the mixed-oxide type, (UPu)O2±x, clad in stainless steel, the reaction zone at the clad/ oxide interface contains caesium. The role played by this element in the relevant reaction has been examined out-of-reactor by means of isothermal anneals or anneals in a thermal gradient, in sealed vessels containing the reagents. The compatibility of caesium and steel is excellent under all conditions of heat-treatment. Things are different in the presence of an oxygen source. When isothermal or gradient anneals are carried out in the presence of Cs + Cs2O or Cs + CsOH mixtures, substantial attack of the steel is observed at all temperatures with formation of mixed complex oxides (of Cr, Fe, Mn and Cs). The activation energy derived from measurements of the depth of the zone of attack in the presence of CsOH is 20 kcal/mole.

Elimination of Iron Interference on Atomic Absorption of Chromium with Air-Acetylene Flame

The interference of iron on the absorbance of chromium and its elimination in atomic absorption spectrometry was investigated by the use of air-acetylene flame.The depression of chromium absorption was observed in the spraying of chromium (III) and iron (III) mixture, while the enhancement was observed in the spraying of chromium (VI) and iron (III) mixture. However, the addition of hydroxylamine was effective to suppress iron interference on chromium absorption, presumably resulting from the reduction of iron In to iron (II). The elimination of iron interference was completed by the addition of hydroxylamine and o-phenanthroline. Possibly the organic ligands hinder the formation of mixed oxide in the condensed phase and release the chromium directly in the vapor state before a non-volatile oxide forms.This technique of interference elimination was applied to the determination of chromium in steels successfully.

Some aspects of “chemical” interferences in atomic-absorption spectroscopy

Studies on the mutual chemical interferences in the atomic-absorption spectroscopy of Ca, Mg, Ba, Sr, Ti, Zr, Hf, Fe, as simple salts and metallocenes, show that the results can be interpreted in terms of formation of mixed oxides of two elements, non-volatility of the mixed oxide compound, and the crystal structure of the mixed oxide compound.

CORROSION RESEARCH ROUND‐UP

Monocrystal formation of iron. Observations carried out at the Institute for Medical Physics of the University of Münster (Westphalia) over a number of years had shown that, during the oxidation of pure metals, alloys and metals in contact with another metal, certain needle or leaf‐shaped oxide monocrystals are formed on the oxide surface. Further research has been carried out into the nature of these oxide monocrystals. In particular, it was desired to determine the conditions which favour the formation of such oxide monocrystals, to investigate their shape, and to identify them. It was found that the formation of free oxide crystals is essentially governed by one of the metal components. Where both of two components gave rise to the formation of oxide crystals, such formation is liable to undergo changes as a function of temperature. Further research is being carried out on the possible formation of mixed oxides.— (G. Pfefferkorn and J. Vahl, Metall‐oberfläche, 1962, 16 (2), 33–37.)

Role of Minor Elements In the Oxidation of Metals★

Abstract An analysis is made of the role of impurities in the oxidation of metals. The various impurities are considered in terms of their effects on the physical and chemical structure of the oxide, oxide-metal interface and metal. Eight types of effects are discussed and where experimental data are available these data are used to illustrate the nature of the effect. The eight types of effects are: (1) formation of new oxide of impurity atoms in outer layer, (2) formation of mixed oxides or spinels, (3) formation of impurity atom oxide in inner layer, (4) change in conductivity of oxide film, (5) change in physical properties and adhesive properties of oxide and oxide-alloy interface, (6) formation of inclusions and other defects in the metal, (7) concentration of impurities at the grain boundaries of the metal and (8) surface oxide-impurity atom reaction with volatile reaction product. 3.7.2