In situ Fourier transform infrared and laser Raman spectroscopic study of the thermal decomposition of Co–Al and Ni–Al hydrotalcites

Abstract

In situ infrared spectra and in situ laser Raman spectra of Co–Al and Ni–Al hydrotalcites (HTlcs) have revealed several novel aspects of the structure, and physico-chemical transformations upon thermal decomposition of these materials in air. The infrared spectra of the Co–Al-HTlc at room temperature show the presence of a fraction of the carbonate in a low symmetry, which was not observed for Ni–Al-HTlc. Furthermore, compared to Co–Al-HTlc, a relatively smaller ratio of carbonate over hydroxyls is present in the as-synthesized Ni–Al-HTlc, as revealed by the infrared and Raman spectra. This is in agreement with a mass spectrometric analysis of the as-synthesized materials upon thermal decomposition, and the relatively small c-parameter determined from the X-ray diffraction (XRD) pattern of the as-synthesized Ni–Al-HTlc. An extraordinary stability of interlayer water in Ni–Al-HTlc was found, which was only completely removed above 300°C, a temperature of 100°C higher than found for Co–Al-HTlc. Dehydroxylation of the octahedral layers and carbonate reorganization and decomposition also require higher temperatures for Ni–Al-HTlc (300–350 and >550°C) than for Co–Al-HTlc (150–200 and 450–500°C). Raman spectra indicate the presence of an intermediate cobalt oxide (CoO x ) phase upon decomposition of Co–Al-HTlc in the temperature range of 175–250°C, before formation of a solid solution of cobalt spinels (Co(Co, Al) 2O 4) occurs. Highly dispersed NiO strongly interacting with the Al 2O 3 support was identified after decomposition of Ni–Al-HTlc at 550°C. The spectroscopically determined transition temperatures are in excellent agreement with XRD, thermogravimetric, and mass spectrometric analyses.