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)₃·9H₂O, Zn(NO₃)₂·6H₂O and Cr(NO₃)₃·9H₂O, Mg(NO₃)₂·6H₂O 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?°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?°C, with formation of oxides mixtures (Cr₂O_3?+?x and ZnO) and (Cr₂O_3?+?x and MgO), respectively. At 400?°C, Cr₂O_3?+?x turn to Cr₂O₃ which reacts with ZnO forming ZnCr₂O₄/SiO₂. Starting with 400?°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.