Constant rate thermal analysis of a dehydrogenation reaction
Summary (2 min read)
1. Introduction
- Solid-state hydrides, including metal, intermetallic and complex hydrides present the highest volumetric capacities of hydrogen storage, and have recently attracted interest for thermal energy storage applications.
- Thermogravimetry is one of the most used techniques to study the kinetics of absorption and desorption of hydrogen from Mg related compounds.20-23 Authors normally employ conventional constant heating rate or isothermal experiments to collect the data.
- It has been demonstrated that constant rate thermal analysis (CRTA) presents a higher resolution power for the discrimination of the kinetic model followed by solid state reactions, because the shape of CRTA curves is related to the kinetic model.24-25.
- For these reasons, it has been used for the kinetic study of different types of solid-state processes.26-28 CRTA implies controlling the temperature in such a way that the decomposition rate is maintained constant all over the process at a value previously selected by the user, employing an experimental signal proportional to the reaction rate or reaction fraction as control parameter.
- The combined kinetic analysis procedure will be used to obtain the kinetic parameters.
2. Experimental
- Magnesium hydride was purchased from Aldrich, product number 683043.
- The samples were studied as received, no activation procedures were carried out to avoid possible modification of the samples.
- The CRTA control system is constituted by a Eurotherm programmer that received the analog output of the thermocouple and controls the temperature of the sample placed in the thermobalance, at the heating rate previously selected.
3. Theoretical
- The kinetic analysis has been carried out from the following general kinetic equation: 2 where dα/dt is the reaction rate, A is the preexponential factor of Arrhenius, E is the activation energy, T is the absolute temperature and f(α) is a function representing the kinetic model obeyed by the reaction.
- Values of the activation energy of 150 kJ mol-1 and the pre-exponential factor of 5×1015 min-1 were employed for the simulation, and a constant reaction rate of 2×10-3 min-1.
- Thus, the shape of the α-T plots permits to have an idea of the kinetic model obeyed by the process before performing any numerical analysis.
- The values of the parameters n and m that provide the best linearity to the straight line obtained are determined by and optimization procedure.
4. Results and discussion
- It is stablished in a literature review that during the synthesis of bulk micron sized particles a shell of magnesium hydride is formed that prevents the hydrogenation of the remaining metal core.
- Thus, the values of (dα/dt)/(1-α)nαm determined as a function of the temperature from these curves were substituted into equation (5) and the left-hand side of the equation versus the inverse of temperature was plotted .
- 27, 45 The proposed kinetic mechanism was further checked by comparing the calculated f(α) with the most used conversion functions in literature,46 which are normalized at α = 0.5 to better distinguish between the different models .
- The high resolution micrograph shows that the particle is constituted by elongated crystals welded in a mosaic structure.
- 49 Different authors have correlated the morphology of MgH2 samples with their dehydrogenation properties.
5. Conclusions
- Constant rate thermal analysis procedure has been applied to the thermal dehydrogenation of MgH2 under high vacuum for the first time.
- The higher resolution power of the CRTA curves for discriminating the kinetic model obeyed by the reaction has been proven in comparison with conventional heating rate curves.
- The thermal dehydrogenation of MgH2 under high vacuum follows first-order kinetics (F1) with activation energy of 108 kJ mol-1.
- These results have been interpreted taking into account the planar morphology of the starting MgH2 particles and crystallites, according to SEM and TEM micrographs.
- An alternative explanation of the kinetic mechanism, based on the broad particle size distribution of the MgH2 particles, would be also plausible.
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Frequently Asked Questions (16)
Q2. What is the commonly used technique for the study of the kinetics of absorption and?
Thermogravimetry is one of the most used techniques to study the kinetics of absorption and desorption of hydrogen from Mg related compounds.20-23 Authors normally employ conventional constant heating rate or isothermal experiments to collect the data.
Q3. What are the popular solid-state hydrides?
Solid-state hydrides, including metal, intermetallic and complex hydrides present the highest volumetric capacities of hydrogen storage, and have recently attracted interest for thermal energy storage applications.
Q4. What are the important aspects of the kinetics of Mg-based materials?
Magnesium needs temperatures above 573 K to absorb hydrogen, the dehydrogenation temperature of MgH2 is even higher because of its high thermodynamic stability, and finally, MgH2 presents a high reactivity towards air and oxygen.3, 7, 12-13 Desorption temperature has been reduced and the hydrogenationdehydrogenation reactions have been fasten by mechanical milling and alloying, doping with catalytic additives and employing cycles of hydrogenation-dehydrogenation.11, 14-19 However, the mechanism and kinetic parameters of these reactions, which are of the most interest for practical applications, have been less thoroughly studied.
Q5. What is the purpose of this work?
The objective of this work is the application of the CRTA methodology for the first time to study the dehydrogenation kinetics of MgH2 in conditions far from equilibrium.
Q6. What is the kinetics of the CRTA procedure?
The use of the CRTA procedure is not limited to first order reactions, but it can be employed independently of the kinetic model followed by the reaction, and for this reason has been used to study the kinetics of thermal decomposition ofdifferent materials.
Q7. What was the kinetic analysis used to determine the morphology of MgH2?
The kinetic parameters obtained from the combined kinetic analysis were tested simulating the CRTA curves and the curve registered at 2.5 K min-1 heating rate, assuming such kinetic parameters.
Q8. What is the kinetic energy of the dehydrogenation of magnesium hydride?
61-62 The results here reported have been obtained under high vacuum in order to assure that the dehydrogenation of magnesium hydride is taking place very far from equilibrium and thus the activation energy obtained is representative of the forward reaction.
Q9. What are the main issues that have to be overcome for the use of Mg-based materials?
Among all the solid-state hydrides, Mg-based is the most studied family, due to the large hydrogen content of MgH2 (7.6 mass%), the high hydrogenationdehydrogenation enthalpy and the ample abundance of magnesium in earth.7-11 Nevertheless, the kinetic and thermodynamic properties of Mg-based materials present several issues that have to be overcome for its use in practical applications.
Q10. What is the kinetic analysis of a reaction plot?
If the α-T (or α-t) plot is obtained at a constant decomposition rate (C = dα/dt), equation (2) can be rearranged, after taking logarithms, in the form:ln ln 3It has been previously shown that CRTA permits to discriminate the kinetic model obeyed by the reaction from the analysis of a single α-T plot, which is not possible if this plot is obtained from conventional rising temperature experiments.
Q11. What is the kinetic analysis of MgH2?
The combined kinetic analysis, which allows calculating the kinetic parameters without any assumption about the kinetic model followed by the reaction, has been applied to the curves registered under CRTA conditions together with a curve registered under linear heating rate conditions, and the validity of the kinetic parameters obtained has been checked comparing the experimental curves with simulated curves.
Q12. What changed the morphology of the MgH2 particles?
as the synthesis medium evolved from inert atmosphere of argon to hydrogen pressure, the morphology changed from rod like to small particles, with sizes in the range of 25-170 nm.
Q13. What is the kinetics of the dehydrogenation of MgH2?
Constant rate thermal analysis procedure has been applied to the thermal dehydrogenation of MgH2 under high vacuum for the first time.
Q14. What is the kinetic energy of the dehydrogenation of MgH2?
in high vacuum, the dehydrogenation of MgH2 would take place through instantaneous nucleation in the border line of the crystallites followed by growth of the magnesium particles by diffusion into the bidimensional crystallites.
Q15. What is the kinetic mechanism of the dehydrogenation of MgH2?
the dehydrogenation of MgH2 would be explained by assuming that the reaction takes place through a mechanism that implies instantaneous nucleation followed by a growth of nuclei by diffusion in two directions.
Q16. What temperature does the intensity of MgH2 increase?
From this temperature, the intensity of the MgH2 peaks decreases steadily, while the intensity of the Mg peaks increases in the temperature range 459-567 K.