Abstract :
A combination of differential scanning calorimetry (DSC) and high-resolution differential thermogravimetric analysis (DTGA) coupled to a gas evolution mass spectrometer has been used to study the thermal properties of a chromium based series of Cu/Zn hydrotalcites of formulae CuxZn6−xCr2(OH)16(CO3)·4H2O where x varied from 6 to 0.
The spacing for the Cu6Cr2(OH)16(CO3)·4H2O hydrotalcite is 6.95 Å which decreases to 6.69 Å with two moles of Zn substitution. The effect of cation substitution is small on the interlayer space. The variation was 0.26 Å. Crystallite size was found to be cation dependent. The larger sizes were observed for the zinc substituted hydrotalcites. It is observed that that the sum of the mass spectrometric curves of water and carbon dioxide matches precisely the differential thermogravimetric (DTG) curve. The effect of increased Zn composition results in the increase of the endotherms and weight loss steps to higher temperatures. Evolved gas mass spectrometry shows that water is lost in a number of steps and that the interlayer carbonate anion is lost simultaneously with hydroxyl units. Hydrotalcites in which M2+ consist of Cu, Ni or Co form important precursors for mixed metal-oxide catalysts. The application of these mixed metal oxides is in the wet catalytic oxidation of low concentrations of retractable organics in water. Therefore, the thermal behavior of synthetic hydrotalcites, CuxZn6−xCr2(OH)16CO3·nH2O was studied by thermal analysis techniques in order to determine the correct temperatures for the synthesis of the mixed metal oxides.
