25 Aug 2016-RSC Advances (The Royal Society of Chemistry)-Vol. 6, Iss: 84, pp 81454-81460
Abstract: The Constant Rate Thermal Analysis (CRTA) procedure has been employed for the first time to study the kinetics of MgH2 dehydrogenation by thermogravimetry under high vacuum. CRTA implies controlling the temperature in such a way that the decomposition rate is maintained constant all over the process, employing the mass change as the experimental signal proportional to the reaction rate. The CRTA curves present a higher resolution power to discriminate the kinetic model obeyed by the reaction in comparison with conventional heating rate curves. The combined kinetic analysis has been applied to obtain the kinetic parameters, which show that MgH2 decomposition under high vacuum obeys first-order kinetics (F1). It has been proposed that the dehydrogenation of MgH2 under high vacuum takes place by instantaneous nucleation in the border line of the bidimensional crystallites followed by the growth of the nuclei.
Abstract: Thermodynamics and kinetics of hydrogenation-dehydrogenation of nanometric iron (nFe) doped Mg-MgH 2 system have been studied. The nFe-doped Mg could be hydrogenated even at 0 °C up to 45% of the theoretical hydrogen storage capacity within an hour. The dehydrogenation of nFe doped MgH 2 starts below 150 °C. The remarkably improved hydrogenation-dehydrogenation kinetics could be attributed to the nano-engineered surface of MgH 2 by nFe. The enthalpies of hydrogenation-dehydrogenation were found to be 76 kJ/mol, and 77 kJ/mol respectively. The activation energy of hydrogenation was evaluated as 41 ± 2 kJ/mol which is same as the diffusion barrier of hydrogen in Mg matrix. The apparent activation energy of dehydrogenation of nFe-doped MgH 2 was found to be 74 ± 1 kJ/mol which is same as the enthalpy of dehydrogenation. The nFe-doped Mg-MgH 2 system has shown cyclic stability up to 50 cycles without significant changes in the kinetics and hydrogen storage capacity. Three-dimensional diffusion seems to be controlling the dehydrogenation process.
Abstract: Automatic lay-up and in-situ consolidation with thermoplastic composite materials is a technology under research for its expected use in the profitable manufacturing of structural aeronautical parts. This study is devoted to analysing the possible effects of thermal degradation produced by this manufacturing technique. Rheological measurements showed that there is negligible degradation in PEEK for the temperatures reached during the process. Thermogravimetric analysis under linear heating and constant rate conditions show that thermal degradation is a complex process with a number of overlapping steps. A general kinetic equation that describes the degradation of the material with temperature has been proposed and validated. Attenuated total reflectance Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy confirmed that there is no remarkable degradation. The use of a combination of in-situ and ex-situ experimental techniques, including kinetic modelling, not only provides reliable information about degradation but also allows setting optimal processing conditions.
Abstract: Linz-Donawitz (LD) or BOF (Basic Oxygen Furnace) Slag produced in the LD Process during steelmaking is one of the major threats to the steel producers worldwide. In general, BOF Slag undergoes phys...
Abstract: Hydrogen is an ideal energy carrier which is considered for future transport, such as automotive applications. In this context storage of hydrogen is one of the key challenges in developing hydrogen economy. The relatively advanced storage methods such as high-pressure gas or liquid cannot fulfill future storage goals. Chemical or physically combined storage of hydrogen in other materials has potential advantages over other storage methods. Intensive research has been done on metal hydrides recently for improvement of hydrogenation properties. The present review reports recent developments of metal hydrides on properties including hydrogen-storage capacity, kinetics, cyclic behavior, toxicity, pressure and thermal response. A group of Mg-based hydrides stand as promising candidate for competitive hydrogen storage with reversible hydrogen capacity up to 7.6 wt% for on-board applications. Efforts have been devoted to these materials to decrease their desorption temperature, enhance the kinetics and cycle life. The kinetics has been improved by adding an appropriate catalyst into the system and as well as by ball-milling that introduces defects with improved surface properties. The studies reported promising results, such as improved kinetics and lower decomposition temperatures, however, the state-of-the-art materials are still far from meeting the aimed target for their transport applications. Therefore, further research work is needed to achieve the goal by improving development on hydrogenation, thermal and cyclic behavior of metal hydrides.
TL;DR: The future of a particularly promising class of materials for hydrogen storage, namely the catalytically enhanced complex metal hydrides, is discussed and the predictions are supported by thermodynamics considerations, calculations derived from molecular orbital (MO) theory and backed up by simple chemical insights and intuition.
Abstract: This review focuses on key aspects of the thermal decomposition of multinary or mixed hydride materials, with a particular emphasis on the rational control and chemical tuning of the strategically important thermal decomposition temperature of such hydrides, Tdec. An attempt is also made to predict the thermal stability of as-yet unknown, elusive or even unknown hydrides. The future of a particularly promising class of materials for hydrogen storage, namely the catalytically enhanced complex metal hydrides, is discussed. The predictions are supported by thermodynamics considerations, calculations derived from molecular orbital (MO) theory and backed up by simple chemical insights and intuition.
TL;DR: This work summarizes commonly employed models and presents their mathematical development as nucleation, geometrical contraction, diffusion, and reaction order.
Abstract: Many solid-state kinetic models have been developed in the past century. Some models were based on mechanistic grounds while others lacked theoretical justification and some were theoretically incorrect. Models currently used in solid-state kinetic studies are classified according to their mechanistic basis as nucleation, geometrical contraction, diffusion, and reaction order. This work summarizes commonly employed models and presents their mathematical development.
Abstract: The hydrogen storage properties of MgH 2 are significantly enhanced by a proper engineering of the microstructure and surface. Magnesium powders are produced in a nanocrystalline form, which gives remarkable improvement of absorption/desorption kinetics. Ball milling, which is used for fabrication of nanocrystalline magnesium, improves both the morphology of the powders and the surface activity for hydrogenation. The hydriding properties are further enhanced by catalysis through nano-particles of Pd located on magnesium surface. Nanocrystalline magnesium with such a catalyst exhibits an outstanding hydrogenation performance: very fast kinetics, operation at lower temperatures than conventional magnesium and no need for activation.
Abstract: It has recently been discovered that energetic ball milling of hydrides can improve their hydrogen sorption properties significantly. In this work, we present a systematic study of structural modifications and hydrogen absorption–desorption kinetics of ball-milled magnesium hydride. Structural investigations showed that after only 2 h of milling, a metastable orthorhombic (γ) magnesium hydride phase is formed. A Rietveld analysis of the X-ray diffraction spectrum of the 20 h milled sample gave a proportion of 74 wt.% MgH2, 18 wt.% γ MgH2 and 8 wt.% MgO. The hydrogen capacity and sorption kinetics were measured before and after milling. We found that the sorption kinetics are much faster for the milled sample compared to the unmilled one. This explains the fact that the hydrogen desorption temperature of the ball-milled sample as measured by pressured differential scanning calorimetry (PDSC), is reduced by 64 K compared to the unmilled sample. There is no significant change of the storage capacity upon milling and the absorption plateau pressure does not change. From the desorption curves, the activation energy was deduced. The milling also increased the specific surface area. This was confirmed by SEM micrographs and BET measurements. Possible mechanisms explaining the improved kinetics are presented.