M. Schiavello

Surface configurations and infra-red studies on nickel oxide supported on η- and γ-Al2O3

An infra-red investigation has been carried out of CO adsorbed onto Ni2+/η-Al2O3 and Ni2+/γ-Al2O3 catalysts. These contain a “surface spinel” phase in which nickel ions are present both in octahedral and tetrahedral sites. Their relative concentrations are primarily determined by the atmosphere (damp or dry) in which the preparation at 600°C has been carried out. Upon CO adsorption four bands appear in the 2250–2150 cm–1 region due to molecules linked to coordinatively unsaturated surface cations. The relative intensities are related to the preliminary treatment of the sample and are therefore discussed in terms of the possible surface configurations and of the relative amounts of tetrahedral and octahedral sites occupied. The infra-red spectrum of CO adsorbed on hydrogen reduced samples has also been investigated and the importance of the preliminary treatment on the reducibility has been confirmed.

The Catalytic Activity of Several Tungsten Oxides for the Oxidation of Propene

A study has been made of the catalytic oxidation of propene over the oxides WO3, WO2.95, WO2.90, WO2.72 and WO2, which were selected because they possess specific features of chemical and structural interest rather than for their catalytic ability. It was found that the oxides WO2.95, WO2.90 and WO2.72 all selectively produce acrolein in small amounts. The oxides WO3 and WO2 were non-selective and rather inactive. The results are discussed in terms of a mechanism involving both variable valence in the crystal and the specific structural geometry of these compounds.

A study of the selective oxidation of propene over a series of ternary tungsten oxides

Abstract The oxidation of propene over a series of MWO ternary systems, with M = Ti, Ta, Nb, Ge, Sb, Sn, has been studied in the temperature range 623–763 °K. Samples were examined by recording their reflectance spectra and also by transmission electron microscopy and powder X-ray diffraction in order to gain information on the original structures of the catalysts and on their changes during reaction. Only Sb- and Sn-containing specimens were found to be selective for acraldehyde production. They were also the only systems having bronze-like properties and reasonable stability during catalytic tests. The other systems were easily oxidized and nonselective. Attempts are made to correlate structural and electronic properties with the selective behavior of the Sb- and Sn- containing systems.

The Influence of Preparation Methods on Structural and Catalytic Properties of Transition Metal Ions Supported on Alumina

Summary It is shown that when transition metal ions (Mn, Co, Ni and Cu ions) are supported on γ- and η-Al 2 O 3 , at 600°C, two reactions occur concurrently: one leads to the formation of a spinel and the other to a segregation of an oxide. The factors which effects both reactions are discussed. It is also shown how the addition of small amounts of foreign ions, such as Zn 2+ , Ga 3+ and Ge 4+ , affects the interaction between the transition ions and the aluminas. Simple catalytic tests show how the catalytic activity is influenced by the type of compound which is formed on the aluminas surface.

Structure and catalytic activity of iron oxide and magnesium oxide solid solutions. Part 2.—Catalytic activity for N2O decomposition

The catalytic activity of magnesium oxide-iron oxide and magnesium oxide-iron oxide-lithium oxide systems has been investigated for N2O decomposition. The magnesium oxide-iron oxide samples, fired in air and containing MgFe2O4, have low activity. Similar samples, fired under reducing atmospheres and containing Fe2+ in solid solution, are active. The ions Fe2+ are oxidized during the catalytic experiments to Fe3+. The latter ions are reduced to Fe2+, during outgassing, thus restoring the active sites on the surface via a mechanism of electron migration which eventually yields the phase MgFe2O4 in the bulk of the particles.The samples containing lithium oxide are active only when [Li]/[Fe] < 1. The nature of the active sites in these samples is discussed taking into account the role of lithium oxide.

Catalytic activity of CoO–ZnO–MgO solid solutions for the decomposition of N2O. Role of the matrix

The catalytic activity of CoO–ZnO–MgO solid solutions, MZCo x:y(x= 0.1–30 Zn atoms per 100 Mg atoms and y= 1–10 Co atoms per 100 Mg atoms), has been investigated for N2O decomposition in the temperature range 490–750 K at a pressure of ca. 8 kPa (60 Torr). The turnover frequency, Nion(molecules decomposed per Co ion per second), decreases with increasing zinc content (matrix effect). It also decreases with increasing cobalt concentration, more markedly at low zinc content (dilution effect). The apparent activation energy Ea increases with increasing zinc content. A comparison of the present data with earlier data on CoO–MgO, Mg1–xCoxAl2O4 and CoO–ZnO systems leads to an interpretation of the catalytic behaviour. The detrimental effect of zinc on Nion values is explained by the setting up of reaction steps which involve the formation of strongly-bound oxygen species not suitable for desorption at low temperature, and responsible for the higher Ea values.

Structure and catalytic activity of iron oxide and magnesium oxide solid solutions. Part 1.—Structural and magnetic investigations

Magnesium oxide containing up to 8 atom % iron oxide, with and without lithium oxide, prepared at 1273 K in reducing (CO + CO2) and oxidizing (air) atmospheres, has been investigated by means of lattice parameter measurements, chemical analyses and magnetic susceptibility measurements (98–293 K).Iron oxide and magnesium oxide form MgFe2O4 in air. Addition of lithium enables iron ions to enter into solid solution as Fe3+. In a reducing atmosphere (oxygen partial pressure = 1.013 × 10–7 N m–2= 10–12 atm) the iron is incorporated in MgO as Fe2+. The MgO lattice stabilizes the 2+ oxidation state of iron in solid solution. The effect of the combined action of outgassing and N2O decomposition is discussed.

Oxidation of propene over various doped tungsten oxides

The oxidation of propene was studied on several tungsten oxides which contained small amounts of Ti, Ta, Nb and Sn. Only the Sn-containing specimen was found to be selective in the conversion of propene to acrolein. The catalytic results are correlated with crystal structures determined by electron microscopy.AbstractОкисление прорена было исследовано на некоторых окислах вольфрама, содержащего примеси Ti, Ta, Nb и Sn. Было найдено, что лишь образцы, содержащие Sn, приводят к селективному превращению пропена в акролеин. Результаты каталитических исследований находятся в корреляции с кристаллической структурой, определенной с помощью электронного микроскопа.

Structure and catalytic activity of iron oxide and magnesium oxide solid solutions. Part 3.—E.s.r. characterization

Magnesium oxide + iron oxide, fired both in air and in a reducing atmosphere, and magnesium oxide + iron oxide + lithium oxide (up to 1 % atomic Fe) were investigated by e.s.r. spectroscopy.The effects of outgassing at various temperatures and of N2O decomposition on the e.s.r. spectra are discussed in terms of surface redox processes. The incipient formation of the spinel phase, MgeFe2O4, and its precipitation, at the highest outgassing temperatures adopted, were readily studied by the e.s.r. technique.The hypotheses previously proposed on the modification of the catalyst solid state, occurring during N2O decomposition and/or in the vacuum treatment, are confirmed by the present study. Further details are also given.

The Oxidation of Propene over W—O and Sn—W—O Catalysts: A Pulse Study

— 763 K. The reduced W-oxides are unstable during the course of the reaction, combined with a weak selectivity for the production of acrylaldehyde. This result is attributed to the low ability of these oxides to perform the step of allyl radical formation. The Sn —W—O system is stable under the reacting conditions and fairly active and selective for acraldehyde. This behaviour is discussed on the basis of the bronze-like properties of the Sn