Abstract
Magnesium based niobium oxides (Mg–Nb–O) were prepared by solid-state reactions owing to understand the function of transition metal oxides as promoters/catalysts for practical application. Magnesium niobate (Mg3Nb6O11) was synthesized for the first time in nearly pure form reported in this context. MgNb2O6 and Mg4Nb2O9 were prepared in oxidizing conditions; on the contrary, Mg3Nb6O11 preferred reducing environment. Stoichiometric mixtures of the precursor materials MgO, Nb2O5 and/or metallic Nb were annealed for the syntheses which revealed the effect of temperature on phase formation, reaction kinetics and heat of reaction. The products were examined by ex-situ, in-situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Crystallographic parameters of various binary and ternary compounds (Mg/Nb/O) formed in different calcination conditions, were extracted by Rietveld method. In-situ experiment results in single step reaction for the MgNb2O6 synthesis and the heat of formation of the solid-state reaction obtained to be minimum (93 kJ/mol). In contrast, the formation of Mg4Nb2O9 and Mg3Nb6O11 compounds towards pure phases rather complicated due to multistep reactions and corresponding heat of formation were estimated to be 140 and 190 kJ/mol. Experimental results have been discussed based on kinetic and thermodynamic constrains.
Highlights
Hydrogen storage represents an important step in the development of hydrogen economy and various storage systems were reported in the literatures
The X-ray diffraction (XRD) patterns monitored along the process of formation of M gNb2O6, Mg4Nb2O9 and Mg3Nb6O11 are reported in the Figs. 1, 2 and 3, respectively and the phase abundance estimated by Rietveld refinement has been inserted in Tables 1, 2 and 3
In this work three Magnesium based niobium oxides (Mg–Nb–O) compounds of MgNb2O6, Mg4Nb2O9 and Mg3Nb6O11 were successfully prepared by solid-state reactions and the formation mechanism was characterized by means of XRD performed both in-situ and ex-situ
Summary
Hydrogen storage represents an important step in the development of hydrogen economy and various storage systems were reported in the literatures Among these systems, magnesium hydride (MgH2) is an interesting material for H2 storage[1,2] owing to high abundance in the lithosphere, cost-effective and less toxic properties[3,4]. During preparation of MgNb2O6 compound from the starting materials (MgO and Nb2O5), MgNb2O6 phase of columbite structure is usually obtained including corundum-like Mg4Nb2O929. Crystallographic, microstructural and morphological features of MgNb2O6 and M g4Nb2O9 compounds were reported elsewhere[30]. They are stable phases at room temperature explored by You et al.[31]. Proper selection of starting materials, calcination conditions to be optimized and reaction mechanism aiming to have pure phases not yet studied in details
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