Influence of divalent metal on the decomposition products of hydrotalcite-like ternary systems M II –Al–Cr (M II =Zn, Cd)

Layered double hydroxides (LDHs) containing MII, AlIII, and CrIII in the brucite-like layers (M=Cd, Zn) with different starting Al/Cr molar ratios and nitrate/carbonate as the interlayer anion have been prepared following the coprecipitation method at a constant pH: ZnII–AlIII–CrIII–CO32− at pH=10, and CdII–AlIII–CrIII–NO3− at pH=8. Two additional MII,AlIII–LDH samples (M=Cd, Zn) with chromate ions (CrO42−) in the interlayer have been prepared by ionic exchange at pH=9 and 8, respectively, starting from MII–AlIII–NO3−. The samples have been characterised by absorption atomic spectrometry, powder X-ray diffraction (PXRD), FT-IR spectroscopy and transmission electron microscopy (TEM). Their thermal stability has been assessed by DTA-TG and mass spectrometric analysis of the evolved gases. The PXRD patterns of the solids calcined at 800°C show diffraction lines corresponding to ZnO and ZnAl2−xCrxO4 for the Zn-containing samples, and diffraction lines attributed to CdO and CdCr2O4 and (Al,Cr)2O3 for the Cd-containing ones. Additionally a minority oxide, Cd2CrO5, is observed to CdII–AlIII–LDH samples with chromate ions in the interlayer.