A comparativein situRietveld refinement study: thermal decomposition and transformation of CoAl and CoZnAl layered double hydroxides

Abstract

Rietveld refinement based onin situX-ray powder diffraction (XRPD) data was combined with thermogravimetric analysis (TGA) and mass spectrometry (MS) to study and compare the phase transformations, thermal stability, microstructural and structural changes of two cobalt-containing nitrate-based layered double hydroxides (LDHs) upon heating in a controlled inert atmosphere of nitrogen. The XRPD data were collected, using synchrotron X-ray radiation, with a time resolution of 107 s, which made it possible to carry out detailed structural studies of the initial layered double hydroxides as well as their decomposition products: spinel for a CoAl–NO3LDH and spinel/zincite for a CoZnAl–NO3LDH. Correlating these data with those from the TGA–MS analyses gives us information about the transformation mechanisms. Rietveld refinements of the two spinel phases reveal remarkable differences. Theaaxis of the spinel formed by decomposition of the CoAl–NO3LDH increases almost linearly from approximately 598 to 1163 K, mainly due to the dominating thermal expansion, whereas theaaxis of the spinel formed by decomposition of the CoZnAl–NO3shows a more complex temperature dependency. Between approximately 698 and 1073 K, theaaxis is almost constant due to pronounced chemical interaction with an additional amorphous phase and the zincite phase, whereas from 1073 up to 1163 K it increases linearly. Calculations, based on the results of the Rietveld refinements, of the size of the octahedral and tetrahedral coordination polyhedra in the spinel show that the octahedra shrink and the tetrahedra expand with increasing temperature. The unusual thermal behaviour of the octahedra is discussed and attributed to the low formation temperature of the cobalt aluminium spinel phase. Finally, the intensity of a low-angle scattering (LAS) signal observed in the XRPD patterns was correlated with the decomposition of the LDH, and determination of the specific surface areas gave the temperature-dependent BET surface areas.