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

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 temperatur e 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 re latively smaller ratio of carbonate over hydroxyls is present in the as-syn thesized Ni-Al-HTlc, as revealed by the infrared and Raman spectra. This is in agreement with a mass spectrometric analysis of the as-synthesized mate rials upon thermal decomposition, and the relatively small c-parameter dete rmined 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 fou nd, which was only completely removed above 300 degreesC, a temperature of 100 degreesC higher than found for Co-Al-HTlc. Dehydroxylation of the octah edral layers and carbonate reorganization and decomposition also require hi gher temperatures for Ni-Al-HTlc (300-350 and >550 degreesC) than for Co-Al -HTlc (150-200 and 450-500 degreesC). Raman spectra indicate the presence o f an intermediate cobalt oxide (CoOx) phase upon decomposition of Co-Al-HTl c in the temperature range of 175-250 degreesC, before formation of a solid solution of cobalt spinels (Co(Co, Al)(2)O-4) occurs. Highly dispersed NiO strongly interacting with the Al2O3 support was identified after decomposi tion of Ni-Al-HTlc at 550 degreesC. The spectroscopically determined transi tion temperatures are in excellent agreement with XRD, thermogravimetric, a nd mass spectrometric analyses. (C) 2001 Elsevier Science B.V. All rights r eserved.