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

摘要

Layered double hydroxides (LDHs) containing M~Ⅱ, Al~Ⅲ, and Cr~Ⅲ 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: Zn~Ⅱ-Al~Ⅲ-Cr~Ⅲ-CO_3~(2-) at pH = 10, and Cd~Ⅱ-Al~Ⅲ-Cr~Ⅲ-NO_3- at pH = 8. Two additional M~Ⅱ,Al~Ⅱ-LDH samples (M = Cd, Zn) with chromate ions (CrO_4~(2-)) in the interlayer have been prepared by ionic exchange at pH = 9 and 8, respectively, starting from M~Ⅱ-Al~Ⅲ-NO_3-. 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 ℃ show diffraction lines corresponding to ZnO and ZnAl_(2-x)Cr_xO_4 for the Zn-containing samples, and diffraction lines attributed to CdO and CdCr_2O_4 and (Al,Cr)2O_3 for the Cd-containing ones. Additionally a minority oxide, Cd_2CrO_5, is observed to Cd~Ⅱ-Al~Ⅲ-LDH samples with chromate ions in the interlayer.