M.V.C. Sastri

Studies on the reduction of iron oxide with hydrogen

Abstract Kinetic studies have been carried out on the hydrogen reduction of pure α-Fe 2 O 3 doped with foreign metal oxides employing a sensitive micro-gravimetric technique. The results show that the reduction of pure Fe 2 O 3 proceeds by a consecutive two-step mechanism via Fe 3 O 4 , the overall rate being controlled by the topochemical reduction of Fe 3 O 4 while that of doped oxides and hematite ore takes place by a different mechanism involving the mixed ferrite formed. In addition, the reduction of pure Fe 2 O 3 is catalysed by metal additives in the presence of water vapour. This enhancement in reduction rate is attributed to a “hydrogen spill-over” effect.

Kinetics of thermal decomposition of some metal oxalates

The thermal decomposition kinetics of oxalates of Zn”, Ki” and Th” have been studied in air by isothermal and non-isothermal thermogravimetry- The isothermal kinetic results suggest that the mechanism of decomposition of the zinc compound invoives rapid nucIeation followed by two dimensionai growth in the acceleratory region, while in the case of thorium oxaiate, the initial nucieation occurs by a chain mechanism on the surface of the reactant foilowed by the growth of the product from the surface towards the interior. The results on nickel oxalate could not be interpreted in an unambiguous manner_ The activation ener,ay and the frequency factor obtained from TG curves compare we11 with those obtained from the isothermal method. The activation energies for the dehydration of these oxalates have also been evaluated from the thermogravimetric curves_

Studies on magnesium- and titanium-substituted LaCoO3

Abstract Crystallographic, electrical and magnetic studies have been carried out on the Mg 2+ - and Ti 4+ -substituted LaCoO 3 systems, LaCo 1− x Mg x O 3 and LaCo 1− x Ti x 03. In the magnesium system, substitution occurs up to x = 0.1, retaining the rhombohedral perovskite structure of LaCoO 3 . Magnesium substitution results in a lowering of the activation energy for electrical conduction, an enhancement of the positive Seebeck coefficient and a decrease of the magnetic moment per cobalt ion, owing to the creation of Co IV ions (holes). In the titanium system, isomorphous substitution of titanium occurs in LaCoO 3 up to x = 0.2. For x ⩽ 0.1, the rhombohedral perovskite structure is retained. For x = 0.2, the composition has the cubic perovskite structure. For x > 0.2, the compounds have a distorted perovskite structure exhibiting oxidative non-stoichiometry. The resistivity and its activation energy increase with x beyond x = 0.05. Ti 4+ acts as an n-type dope up to x = 0.05, beyond which the samples revert to p-type semiconductivity.

Some ABO3 oxides with defect pyrochlore structure

Abstract Studies on the formation, structures and properties of the ternary oxides TlNbO 3 , TlTaO 3 and BiVO 3+ x occurring in A 2 B 2 O 7- y defect pyrochlore structure are reported. The reaction of Tl 2 CO 3 with Nb 2 O 5 /Ta 2 O 5 initially leads to the formation of TlNbO 3+ x [ x ∼ 0.3] and TlTaO 3+ x [ x ∼ 0.4]. On further heating, the phases lose oxygen forming TlTaO 3 and TlNbO 3 . The occurrence of AB O 3 oxides having Tl + , Pb 2+ or Bi 3+ as the A cation in defect pyrochlore structure in preference to the perovskite structure is attributed to the formation of a “trap-mediated” A-A bond through 6 s 2 electrons.

Interaction of hydrogen and carbon monoxide on cobalt catalysts. Part II

The simultaneous adsorption of CO and H2 from their 1:1 mixture on a cobalt Fischer-Tropsch catalyst has been studied by kinetic and quasi-static measurements in the temperature range 0–132 °C. Mutual enhancements indicating interactive adsorption have been observed in both rates and amounts of adsorption. Breaks appear in the Elovich equation plots for the rates of adsorption of both gases from the mixture. The ratios of the post-break slopes of these plots check fairly closely with the H2CO ratios in the mixed adsorbed phase found under quasi-static conditions. Both types of measurements indicate the formation of a surface compound of the composition H2CO in the temperature interval 85–95 °C. The kinetics of interaction of hydrogen with the composite adsorbate formed at various temperatures have also been studied.

Studies on ferric oxide electrodes for the photo-assisted electrolysis of water

Abstract Results of the photoelectrochemical studies carried out with sintered discs of pure Fe 2 O 3 and of Fe 2 O 3 doped with TiO 2 , SnO 2 , ZrO 2 and Ta 2 O 5 are compared and discussed. Flat band potential, saturation current and minority charge carrier diffusion length are higher for TiO 2 -doped Fe 2 O 3 samples than for other doped specimens. Ta 2 O 5 -doped Fe 2 O 3 samples and SrFe 2 O 4 have been found to be photo-inactive. The results are analysed in terms of depletion layer theory.

Studies on the formation of zinc ferrite

The reactions of zinc oxide with α-Fe2O3 and with γ-Fe2O3 leading to the formation of zinc ferrite (ZnFe2O4) have been studied at different partial pressures of oxygen in the temperature range 500–800°C. The ferrite spinel is formed more readily when γ-Fe2O3 is used as the reactant. The DTA and X-ray diffraction results indicate that the γ-Fe2O3 phase is stabilized in presence of ZnO. The lowering of partial pressure of oxygen in the ambient atmosphere favours the formation of zinc ferrite, indicating a cyclic redox process featuring Fe3+, Fe2+ and oxygen at the phase boundaries. At very low oxygen pressures, conversion to zinc ferrite is not facilitated, apparently due to the formation of an intermediate species, Fe3−xZnxO4, which, under these conditions, is not easily converted to ZnFe2O4.

THERMAL DECOMPOSITION OF HYDRATED IRON(II) OXALATE AND MANGANESE(II) OXALATE IN VACUUM

The thermal decomposition of hydrated iron(II) oxalate and manganese(II) oxalate under high vacuum conditions (∼10−5 mm Hg) has been studied by differential thermal analysis. The decomposition in vacuum of iron(II) oxalate is exothermic, while that of manganese(II) oxalate is endothermic. An explanation is offered for this behaviour.

Studies on the formation of manganese molybdate

Abstract The reaction of MoO3 with various oxides of manganese (MnO, Mn2O3, Mn3O4 and MnO2) and with MnCO3 has been studied in air and nitrogen atmospheres employing DTA, TG and X-ray diffraction methods, with a view to elucidating the conditions for the formation of MnMoO4. Thermal decomposition of MnCO3 has also been studied in air and nitrogen atmospheres to help understand the mechanism of the reaction between MnCO3 and MoO3. The studies reveal that, whereas MnO, Mn2O3 and MnO2 react smoothly with MoO3 to form MnMoO4, Mn3O4 does not react with MoO3 in the temperature range investigated (48O–6OO°C). An equimolar mixture of MnCO3 and MoO3 reacts in air to yield MnMoO4, while only a mixture of Mn3O4 and MoO3 remains as final product when the same reaction is carried out in nitrogen. Marker studies reveal that manganese ions are the main diffusing species in the reaction between MoO3 and manganese oxides that result in MnMoO4.

Theoretical considerations on the oxidation of propylene on transition metal molybdates

Molecular orbital calculations of charge distributions for the π-allyl complex with transition metal ions in molybdates show that the activated π-allyl species is formed by transfer of charge density to the metal ion. The relative charge distributions on the three carbon atoms of the adsorbed allyl species govern the course of oxidation reaction.AbstractРасчеты МО распределения заряда для π-аллильного комплекса с ионами переходных металлов в молибдатах показали, что активированные π-аллильные частицы образуются путем переноса плотности заряда на металлический ион. Относительное распределение заряда на трех атомах углерода адсорбированных аллильных частиц руководит ходом реакции окисления.