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

摘要

In this article, we present a new method for the obtaining of ZnCr_2O_4 and MgCr_2O_4 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(NO_3)_3·9H_2O, Zn(NO_3)_2·6H_2O and Cr(NO_3)_3·9H_2O, Mg(NO_3)-2·6H2O with tetraethyl orthosilicate and 1,3-propanediol for final compositions 50% ZnCr_2O_4/50% SiO_2 and 50% MgCr_2O_4/50% SiO_2. 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 (Cr_2O_3 χ x and ZnO) and (Cr_2O_3 χ x and MgO), respectively. At 400 °C, Cr_2O_3 χ x turn to Cr2O3 which reacts with ZnO forming ZnCr2O4/SiO_2. Starting with 400 °C, Cr_2O_3 reacts with MgO to an intermediary phase MgCrO4, which decomposes with the formation of MgCr_2O_4/SiO_2. 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.