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

Abstract

Rietveld refinement based on in situ X‐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–NO3 LDH and spinel/zincite for a CoZnAl–NO3 LDH. 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. The a axis of the spinel formed by decomposition of the CoAl–NO3 LDH increases almost linearly from approximately 598 to 1163 K, mainly due to the dominating thermal expansion, whereas the a axis of the spinel formed by decomposition of the CoZnAl–NO3 shows a more complex temperature dependency. Between approximately 698 and 1073 K, the a axis 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.

Oxidative Thermal Conversion of Hydrothermal Derived Precursors toward the Mixed-Metal Cobaltite Spinel Oxides (ZnCo2O4 and NiCo2O4): In-Situ Investigation by Synchrotron-Radiation XRD and XAS Techniques

“…The emerging temperature for ZnCo 2 O 4 and NiCo 2 O 4 spinels is in a good agreement to the required temperature reported by other studies [19,24]. Furthermore, our study is in agreement with one from Johnsen and Norby, suggesting that the incorporated Zn 2+ content helps the crystalline spinel to form at a lower temperature [51]. The significantly larger ionic radius Zn 2+ readily occupies T d site, as the elevated temperature gradually enlarges the site.…”

The fast Z-scan method for studying working catalytic reactors with high energy X-ray diffraction: ZSM-5 in the methanol to gasoline process

“…[3][4][5][18][19][20][21][78][79][80] Helium bubbled through methanol at 20 1C was fed over the catalyst at a rate of 20 ml min À1 . Diffraction data were collected at a wavelength of 0.6978 Å on a MAR345 image plate detector and integrated into 2D powder patterns using FIT2D.…”

“…Data for validation of the behaviour of the catalyst were collected on beamline BM01A of the ESRF (The Swiss Norwegian beamline) using the capillary reactor described by Norby et al and used in numerous PXRD studies of catalysts and other systems. [3][4][5][18][19][20][21][78][79][80] Helium bubbled through methanol at 20 1C was fed over the catalyst at a rate of 20 ml min À1 . Diffraction data were collected at a wavelength of 0.6978 Å on a MAR345 image plate detector and integrated into 2D powder patterns using FIT2D.…”