Thermal Evolution of Chromium(III) Ions in Hydrotalcite-like Compounds

Abstract

Hydrotalcite-like compounds containing Mg(II) and Cr(III) or Ni(II) and Cr(III) cations in the brucite-like layers have been synthesized by the coprecipitation method. The layered structure has been confirmed by Powder X-ray diffraction, and characterization has been completed by elemental chemical analysis, FT-IR and vis−UV/diffuse reflectance spectroscopies, and thermal analysis (differential thermal analysis and thermogravimetric analysis). The behavior shown during the thermal studies depends on the atmosphere used to carry out the study; a weak endothermic effect, at temperatures coinciding with a small weight loss, is recorded when the analysis is performed in air but not when performed in nitrogen, and so it is ascribed to oxidation processes. Formation of chromate-like species in the samples calcined at intermediate temperatures has been confirmed by powder X-ray diffraction for sample MgCr-HT and is suggested by FT-IR spectroscopy for sample NiCr-HT; the lack of success in identifying chromate-like species for sample NiCr-HT calcined at different temperatures by X-ray diffraction suggests that these species are very well dispersed. Their presence has been undoubtedly established by temperature-programmed reduction analysis of the samples, showing a maximum oxidation state of Cr ions (+6) when the sample is calcined at 623 K. Calcination at high temperatures leads to formation of MCr 2 O 4 (M = Mg, Cr) spinel in addition to rock salt oxide of the divalent cation. Formation of amorphous materials at intermediate calcination temperature accounts for specific surface area development for these samples.