New synthesis method for M(II) chromites/silica nanocomposites by thermal decomposition of some precursors formed inside the silica gels

In this article, we present a new method for the obtaining of ZnCr2O4 and MgCr2O4 embedded in silica matrix. This method consists in the formation of Cr(III), Zn(II) and Cr(III), Mg(II) hydroxycarboxylate/carboxylate compounds, during the redox reaction between the nitrate ion and diol (1,3-propanediol), uniformly dispersed in the pores of hybrid gels. The thermal decomposition of these precursors leads to a mixture of corresponding metal oxides. The gels were synthesized starting from mixtures of Cr(NO3)(3)center dot 9H(2)O, Zn(NO3)(2)center dot 6H(2)O and Cr(NO3)(3)center dot 9H(2)O, Mg(NO3)(2)center dot 6H(2)O with tetraethyl orthosilicate and 1,3-propanediol for final compositions 50% ZnCr2O4/50% SiO2 and 50% MgCr2O4/50% SiO2. The obtained gels have been thermally treated at 140 A degrees C, when the redox reaction nitrates-diol took place with formation of the precursors within the xerogels pores. The thermal decomposition of all precursors took place up to 300 A degrees C, with formation of oxides mixtures (Cr2O3 + x and ZnO) and (Cr2O3 + x and MgO), respectively. At 400 A degrees C, Cr2O3 + x turn to Cr2O3 which reacts with ZnO forming ZnCr2O4/SiO2. Starting with 400 A degrees C, Cr2O3 reacts with MgO to an intermediary phase MgCrO4, which decomposes with the formation of MgCr2O4/SiO2. The formation of the precursors inside the silica matrix and the evolution of the crystalline phases were studied by thermal analysis, FT-IR spectrometry, XRD, and TEM.