Jean Naud

Characterization of Bismuth-manganese Oxide Catalysts for Methane Oxidative Coupling

This paper reports a finding, namely a cooperation between separate phases, which may help understand the mechanism of oxidative coupling of methane. The activity of co-precipitated bismuth-manganese oxides in the oxidative coupling of methane, measured in the continuous flow regime, is maximum for Bi:Mn = 1:2. The catalysts have been investigated by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy and microanalysis; catalytic pulse experiments were performed. The Bi2Mn4O10 phase is present in these catalysts. But, surprisingly, although it corresponds to the Bi:Mn ratio of 1:2 giving maximum activity, it decomposes to alpha-Bi2O3 and alpha-Mn2O3 in a few hours during the catalytic test. The higher catalytic activity observed at Bi:Mn = 1:2 thus does not seem due to the Bi2Mn4O10 phase. The high activity catalyst is actually composed of Bi2O3 in contact with an other phase. This might be a Bi-depleted Bi2Mn4O10-like phase; it is speculated that this phase could donate oxygen to Bi2O3 and make it more active. This would suggest that oxygen mobility or spill-over between phases. observed to be beneficial in other reactions occurring at 350-450-degrees-C, also promotes the selective oxidative coupling of methane above 700-degrees-C.

Geochemical evaluation of the Lueshe niobium deposit (Zaire) by Rietveld quantitative X-ray diffraction

The Lueshe Nb deposit is used as an example to show that a modal analysis of fine-grained samples can readily be obtained by X-ray powder diffraction and the Rietveld method. This permits applications such as the visualisation of the weathering profile, the calculation of enrichment factors and estimation of the amount of nondiffracting material. This quantification method is based on the knowledge of the structural parameters of all the phases present in the sample and, in the case of geological material (especially secondary minerals), it sometimes requires structure refinement to optimize the results

Synergy in the Fe-Mo-Sb-O Multiphase System

Abstract The calcination of a mixture of Fe 2 (MoO 4 ) 3 and α-Sb 2 O 4 yields a complex mixture containing, in addition to the starting phases, FeSbO 4 , MoO 3 and possibly some contaminated phases. We give an overview of the results obtained in attempting to analyse the various synergetic cooperations occurring between these phases in the oxidation of isobutene to methacrolein. The work is based on results of catalytic activities with phases contaminated artificially by impregnation and with mixtures of two pure or contaminated phases. The remote control seems to play a major role in the effects observed. The overall result of calcination is the obtention of catalysts with higher activities and selectivities.

Solid state reaction in the iron-molybdenum-antimony oxide system

This work deals with the evolution of a mechanical mixture of Fe2(MoO4)3 and alpha-Sb2O4 (prepared separately) during calcination in air at 500-degrees-C over a period of six days. The samples were studied by X-ray diffraction, X-ray photoelectron spectroscopy, BET surface area, CTEM, analytical electron microscopy and selected area electron diffraction. A reaction occurs between the two phases, leading to a mixture of Fe2 (MoO4)3, Sb2O4, FeSbO4 and MoO3 with a significant increase of the surface area. Taking into account previous results, the present work suggests that the ability of alpha-Sb2O4 to produce oxygen spillover facilitates the oxidation of Sb3+ to Sb5+ allowing the solid state transformation to FeSbO4 to take place.

Role of Tellurium Oxide in the Selective Oxidation of Isobutene to Methacrolein: α-Sb2O4—TeO2 Catalysts

Abstract Antimony and tellurium oxide catalysts were synthesized by (i) mechanically mixing the oxides (ii) coprecipitation and (iii) impregnation of one of the oxides by the metallic element of the other. Some oxides or mixtures were subjected to further thermal treatments. Samples were characterized before and after catalytic test by BET surface area measurement, X-ray diffraction, XPS, CTEM and AEM, carbon content and isoelectric point measurements. Catalysts were tested in the oxidation of isobutene to methacrolein. Neither the formation of Sb 2 Te 2 O 9 nor contamination explain the synergy observed. The synergy seems to be due to a remote control where TeO 2 plays the role of a relatively weak oxygen spill-over acceptor.

Distribution de la densite des electrons de valence dans Cu2As

The distribution of valency electron density in Cu/sub 2/As ( /b C/38) has been determined by Fourier synthesis using as coefficients the values Delta /b F/=/b F//sub obs/-/b F/ /sub core/ (/b F//sub core/ corresponds to the structure factors of the inner orbitals). The bonding between the pyramidal-site copper atoms and the arsenic atoms is exposed, as well as the bonding between tetrahedral-site and pyramidal-site copper atoms. The structural evolution of the unit cell from the Cu/sub 2/Sb-type (/b C/38) to the Fe/sub 2/P-type (/b C/22) and Co/sub 2/P-type (/b C /23) can be related to the metal-metalloid interaction. This interaction mainly involves the pyramidal-site metal atoms in the Cu/sub 2/Sb-type, and the tetrahedral-site atoms in the Fe/sub 2 /P- and Co/sub 2/P-types.