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Silica-Alumina-Supported Acidic Molybdenum Catalysts -TPR and XRD Characterization.-The role of the composition of various silica-alumina supports on the stability of Mo oxide catalysts and of solid state reaction products of Mo oxides and silica-alumina is investigated by XRD and TPR. The reducibility increases with increasing Mo loading regardless of the support composition. Low loading (≤ 4 wt.% MoO3) favors tetrahedral Mo species of low reducibility and high loading promotes octahedral, multilayered Mo species having high reducibility and a low-temp. TPR peak. Generally, the reduction of alumina-rich catalysts is much less than that of silica-rich ones. Crystalline phases of Mo are only formed on Mo-rich and silica-rich catalyst, e.g. orthorhombic MoO3 on 100% SiO2 and MoO3 and Al2 (MoO4)3 on Al2O3-containing silica. -(RAJAGOPAL, S.; MARINI, H. J.; MARZARI, J. A.; MIRANDA, R.; J. Catal. 147 (1994) 2, 417-428; Dep. Chem. Eng., Univ. Louisville, Louisville, KY 40292, USA; EN)

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The optimal conditions for the development of complexation reactions between Cr(III) and 2-(5-chloro-2-pyridylazo)-5-dimethylaminophenol (5.CIDMPAP) were studied. When a mixture of a hydroethanolic solution of the reagent and an aqueous solution of Cr(III) were heated, three different complex species were formed, depending on the composition and pH of the medium (E=1.7X1051 mol-1 cm-1). All of them were hardly soluble when the proportion of organic solvent was low. The stoichiometry of the different complexes was determined, the formation mechanisms were elucidated and the respective constants were calculated. A useful absorptiometric method is proposed for Cr(III) in concentrations ranging from 15 to 400 ppb. The proper ways to reduce interferences produced by Fe(III), Nb(V), Ta(V), Ti(IV), V(V), Co(II), Zr(IV), Sn(II), Al(III), Mn(II) and Cu(II) are described. The methods were applied to chromium determination in water samples with very satisfactory results.