B. Gillot

Nonstoichiometry-activity relationship in perovskite-like manganites

Abstract High surface area LaMnO3+δ and La1−xBaxMnO3+δ catalysts were prepared through decomposition of citrate precursors. Thermogravimetric analyses of lanthanum manganite revealed the existence of a reversible non-stoichiometry dependent on thermal treatment conditions. When heated in air, a portion of the Mn4+ manganese cations underwent a reduction to Mn3+. During cooling reoxidation occurred, with the final Mn 3+ Mn 4+ ratio dependent on the cooling rate. Electrical conductivity measurements suggest that this ratio is close to unity for a cooling rate of 20 °C h−1. We have found that varying the cooling rate did not modify this ratio by more than 10%. The catalytic activity for the total oxidation of carbon monoxide was found to be independent of such a small variation in Mn 3+ Mn 4+ ratio. To obtain a large change in this ratio, barium was substituted for lanthanum. Reoxidation during cooling could only be observed for lower amounts of barium (x 0.5. Lastly, the catalytic activity was shown to vary with barium content, reaching a maximum for 0.2

Intermediate valencies of vanadium cations appearing during oxidation of vanadium-iron spinels

Abstract The oxidation state of vanadium cations in nanosized vanadium–iron spinels at stoichiometry and at oxidative nonstoichiometry has been investigated by associating X-ray diffraction, thermal analysis (DSC), infrared and X-ray photoelectron spectroscopies. It has been established that, during oxidation, the V3+ cations present several intermediate valencies before to realize the final oxidation state +5. A comparison was made with the oxidation of V2O3.

Valence states of copper in copper ferrite spinels CuxFe3−xO4 (0 < x ≤ 1) fine powders: Evidence of copper insertion

Abstract The oxidation in cation deficient spinels of copper iron spinels Cu x Fe 3− x O 4 (0 x ≤ 1) synthesized by soft chemistry with a grain size x per mole of ferrite, three oxidation phenomena corresponding to Cu + at B-sites (130°C), Fe 2+ at B-sites (180°C) and Cu + at A-sites (240°C) have been found in close relation with the cation-oxygen distance of each oxidizable cation. For nonstoichiometric synthesized spinels Cu x Fe 3− x O 4+ δ with δ x , the presence of additional interstitial Cu + ions has been envisaged. The oxidation temperature of these interstitial ions (> 300°C) is higher than that for Cu + ions at Asites suggesting that interstitial copper ions also essentially reside in A-sites.

Analysis of the oxidation process and mechanical evolution in nanosized copper spinel ferrites. Role of stresses on the coercivity

Isothermal oxidations of the iron(II) and copper(I) cations in nanosized copper spinel ferrites which were oxidized in cation-deficient spinels have been studied by thermogravimetric analysis. The low temperature kinetics of oxidation of each oxidizable cation has been explained on the basis of a diffusion-induced stress effect, these stresses being generated in the ferrite particles by the chemical gradient induced during the oxidation process where the cations at the surface are more oxidized than those in the bulk. A reaction mechanism coupling the chemical and the mechanical effects has been proposed. When the Cu-ferrites contain some Co-cations, it has also been revealed that the stresses have a significant influence on the enhancement of the coercivity.

Mechanical activation conditions of the Fe2O3 and V2O3 mixture powders in order to obtain a nanometric vanadium spinel ferrite

Abstract Co-milling of iron and vanadium oxides allows to obtain an intimate oxides mixture at a nanoscale, similar to a coprecipitate elaborated by soft chemistry. Reduction of such a mixture in the same temperature and oxygen partial pressure conditions (500°C and 10−25 Pa) as the soft chemistry products leads to a nanometric vanadium ferrite with the only spinel phase. The characterization of the powders is achieved by X-ray diffraction (XRD), scanning electron microscopy, infrared (IR) spectrometry, thermogravimetry and calorimetry. Homogeneity of grain size and chemical composition is reached if the initial oxides have similar grain size.

Effect of the preparation method and grinding time of some mixed valency ferrite spinels on their cationic distribution and thermal stability toward oxygen

Abstract The reactivity in oxygen of several mixed valency ferrite spinels, namely Fe3O4, FeCr2O4, Fe3 − xTixO4 and Fe3 − xMoxO4 was investigated by derivative thermogravimetry (DTG) analysis as a function of the conditions of preparation and grinding. For all these compounds, low temperature preparation or prolonged grinding time enables small particles ( 0.5 μm), it was observed that oxidation was starting at higher temperature and that the defect spinel phases could not be retained during oxidation. This behaviour was attributed to the presence of stresses induced by the lattice parameter gradient and promoting the formation of nuclei of the α-rhombohedral phase from the superficial γ-defect phase.

Temperature dependence of oxidation behavior and coercivity evolution in fine-grained spinel ferrites

Abstract The present paper reviews for a series of fine-grained MxCoyFe(3−x−y)O4 spinel ferrites (M=Mn, Mo, Cu, V) which oxidize into mixed-valence defect ferrites, a systematic study of the correlation between the oxidation temperature and the variation of coercivity. An increase of the coercivity is observed as often as the oxidation reactions Fe2+→Fe3+ and Mn+→M(n+m)+ (1⩽m⩽3) occur. This enhancement of coercivity has been explained by stresses generated during each specific oxidation reaction. The well-known property of cobalt to strongly magnify the magnetostrictive effects has been used to support this interpretation.

Structural analysis of submicron Ti- and Mn-ferrite spinels via their reactivity studies

Abstract The reactivity data on finely grained Ti- and Mn-substituted magnetites of size 3+ 2 λ Fe 2+ 1−2 λ ) A (Fe 3+ 2−2 λ −2 x Fe 2+ 2 λ + x Ti 4+ x ) B O 2- 4 with λ =0.5 for 0 x λ x 2+ ions determined for A and B sites, through observation of weight gain on oxidation, under isothermal conditions, agrees reasonably well with that deduced by other methods. For submicron Mn-substituted magnetites with structural formula given by (Mn 2+ 0.8 x Fe 3+ 1−0.8 x A Fe 3+ 1+0.6 x Fe 2+ 1−0.8 x Mn 3+ 0.2 x ) B O 2- 4 a three-stage-oxidation process defining the distribution of Fe 2+ and Mn 3+ ions on B-sites and Mn 2+ ions on A-sites was discerned. A quantitative analysis of thermal curves shows that octahedral Fe 2+ and Mn 3+ ions are totally oxidized while tetrahedral Mn 2+ ions exhibit only a partial oxidation, during the conditions of oxidation. Further, the Mn 4+ ions formed at about 270°C are reduced to Mn 3+ ions at higher temperature and Mn 2+ ions not oxidized at low temperature reacted at about 650°C to produce Mn 3+ ions. Thus specific reactivity of samples towards oxidation, quantitatively studied by thermogravimetric measurements may be used as a direct means to monitor and analyse their microstructural evolution.

Experimental set up for determining the temperature—oxygen partial pressure conditions during synthesis of spinel oxide nanoparticles

Abstract Nanometric spinel oxide powders, Fe 3− x M x O 4 , where M is a transition element, have been synthetized by soft chemistry. This method generally leads to a non-stoichiometric phase, Fe 3− x M x O 4+δ where δ is the deviation from stoichiometry so that further annealing at low temperatures around 450°C and low oxygen partial pressure around 10 −25 Pa given by N 2 /H 2 /H 2 O gas mixtures is required: this enables a stoichiometric compound to be obtained and a nanometric size to be maintained. The complete set up consisting of a gas mixer, a thermogravimetric apparatus and a preparative furnace is described. Some results concerning the conditions of temperature and oxygen partial pressure leading to stoichiometry are given in the case of vanadium ferrites containing or without Co.

Correlation between oxidation states of transition metal ions and variation of the coercivity in mixed-valence defect spinel ferrites

Abstract Due to the very high dispersion of mixed-valence spinel ferrites prepared by ‘soft chemistry’ it becomes possible to oxidize in the spinel lattice, not only ferrous ions but also different transition metal ions (Cr 3+ , Mn 2+ , Mn 3+ , Mo 3+ , Mo 4+ V 2+ , V 3+ , Cu + ), by ‘soft’ oxidation between 150 and 500°C, under formation of non-stoichiometric spinels cation vacancies. The oxidation state with the coordination and the oxidation temperature of the cations have been determined in coppermanganese ferrites by derivative thermogravimetry (DTG), based on the specific solid-state reactivity of cations in the redox reaction. For Co-modified non-stoichiometric spinels, which present interesting magneto-optical properties due to their exceptional high coercivity (1000–3500 Oe) and whose cation distribution has been determined by the method described above, close correlations have been found between the oxidation state of the spinel and the variation of the coercivity.