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

Highlights: ► LDHs MII–Al–Cr (M = Zn, Cd) with Cr in the layer or interlayer have been prepared. ► LDHs Zn–Al or Zn–Cr decompose by heating forming ZnO and ZnAl2O4 or ZnO and ZnCr2O4. ► LDHs Zn–Al–Cr give rise to the formation of ZnO and the mixed spinel ZnAl2−xCrxO4. ► LDH Cd–Al–Cr shows the formation of CdO, CdCr2−xAlxO4, and (Al, Cr)2O3 mixed oxide. ► Calcination of the CdAl–CrO4 give rise to (Al, Cr)2O3 as the majority phase. - Abstract: 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.