Jean Luc Rehspringer

Electrochemical study of nickel-aluminium-manganese spinel NixAl1-xMn2O4. Electrocatalytical properties for the oxygen evolution reaction and oxygen reduction reaction in alkaline media

The oxygen reduction reaction and the oxygen evolution reaction have been studied on mixed oxides NixAl1−xMn2O4 (1≥x≥0) crystallising in the spinel structure. Two different preparation methods have been used: coprecipitation of metal hydroxides and sol–gel. The different activity that is observed is related to the solid-state couple in octahedral sites. AlMn2O4 shows the poorest activity due to intrinsic resistivity. The substitution of Al by Ni increases the catalytic activity, the maximum being that exhibited by NiMn2O4. These electrochemical reactions can be used to detect the nature of the electrodes by way of the electrocatalytic activity.

Preparation of ZnFe2O4/SiO2 and CdFe2O4/SiO2 nanocomposites by sol–gel method

Abstract The purpose of this paper is to characterise the phase relations in the system ZnFe2O4/SiO2 and CdFe2O4/SiO2 by dependence on the thermal treatment. Both ferrites have the structure of normal spinels. The samples were prepared by the sol–gel method, heated at a temperature of 800–1100 °C and characterised by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction, and Mossbauer spectroscopy. HRTEM revealed nanocrystals of the size of about 5–10 nm, depending on the heat treatment temperature. The spinel structure of ZnFe2O4 in the amorphous silica matrix proved to be very stable up to 1100 °C without decomposition to the zinc silicate and iron (III) oxide. On the other hand, CdFe2O4 already starts to decompose at 900 °C and complete decomposition to the α-Fe2O3 and cadmium silicate can be observed at 1100 °C. This decomposition is also accompanied with the recrystallisation of the silica matrix to quartz and cristobalite.

Synthesis of glass “precursor” BaO,TiO2,nH2O by gel processing

Abstract Sol-gel methods are currently applied for binary systems such as BaOTiO 2 using the alkoxide hydrolysis method. The control of hydrolysis kinetics with the use of weak concentration and low temperature leads to monolithic gels. The drying process stage leads to a gel-glass ceramic displaying small egg-shaped crystals of BaTiO 3 in a glass matrix at 725°C. The TGA, DTA and dilatometric curves of this glass are given with SEM studies.

Optical properties of Co2+-doped silica gel monoliths

Silica gel monoliths containing Co2+ with a Co/Si molar ratio up to 120 were prepared by acid-catalyzed hydrolysis of tetraethoxysilane in the presence of cobalt nitrate and formamide. UV-visible spectra recorded at different steps of the preparation showed important changes in the Co2+ environment. A change from octahedral to tetrahedral environment was observed during aging. It was attributed to the formation of bonds between cobalt ions and the gel skeleton induced by an increase of pH. In xerogels, Co2+ was in tetrahedral coordination and the calcination temperature in the 300 to 700°C range had no influence on the spectra. A partial reverse evolution from tetrahedral to octahedral was observed when calcinated gels were exposed to moisture.