The hydrogen desorption properties of vacant MgH2 structure were
investigated both experimentally and theoretically. Ion irradiation by Ar8+
and Xe8+ ions were used to induce structural changes in MgH2. Hydrogen
desorption properties were investigated by temperature programmed
desorption (TPD). To obtained deeper insight in structural changes during
desorption theoretical calculations were performed using DFT approach
within Abinit code. Results showed that there are several mechanisms
involved in desorption process, which depend on defect concentration, their
position and their interaction and ordering. It has been demonstrated that the
changes in near-surface area play the crucial role in desorption kinetics

Grbović-Novaković, Jasmina

Kurko, Sandra V.

Novaković, Nikola

Paskaš Mamula, Bojana

Vujasin, Radojka

Đukić, Anđelka B.

Repository of the Vinča Nuclear Institute (VinaR)

     PHYSICAL CHEMISTRY 2014  12th International Conference on Fundamental and Applied Aspects of Physical Chemistry          The Conference is dedicated to the 25. Anniversary of the Society of Physical Chemists of Serbia            September 22-26, 2014  Belgrade, Serbia   II   ISBN 978-86-82475-30-9 Title: PHYSICAL CHEMISTRY 2014 (Proceedings) Editors: Ž. Čupić and S. Anić Published by: Society of Physical Chemists of Serbia, Studenski trg 12-16, 11158, Belgrade, Serbia Publisher: Society of Physical Chemists of Serbia For Publisher: S. Anić, President of Society of Physical Chemists of Serbia Printed by: “Jovan” Priting and Publishing Company; 200 Copies; Number of pages: 6+ 441; Format: B5; Printing finished in September 2014.  Text an Layout: “Jovan”  Neither this book nor any part may be reproduced or transmitted in any form or by any means, including photocopying, or by any information storage and retrieval system, without permission in writing from the publisher.  200 - Coppy priting    IV  PHYSICAL CHEMISTRY 2014  12th International Conference on  Fundamental and Applied Aspects of Physical Chemistry     Organized by The Society of Physical Chemists of Serbia  in co-operation with   Institute of Catalysis Bulgarian Academy of Sciences   Boreskov Institute of Catalysis of Siberian Branch of the Russian Academy of Sciences  Faculty of Physical Chemistry, University of Belgrade, Serbia  Institute of Chemistry Technology and Metallurgy,  University of Belgrade, Serbia  Vinča Institute, University of Belgrade, Serbia  Institute of General and Physical Chemistry, Serbia    Faculty of Pharmacy, University of Belgrade, Serbia    VInternational Organizing Committee  Chairman:  S. Anić (Serbia) Vice-chairman:  M. Gabrovska (Bulgaria)                             V. A. Sadykov  (Russia)  Members: N. Cvjetičanin (Serbia), S. N. Blagojević (Serbia), M. Daković (Serbia), T. Grozdić (Serbia), D. Jovanović (Serbia), M. Kuzmanović (Serbia), D. Marković (Serbia), J. Marković-Dimitrić (Serbia), B. Milosavljević (USA), M. Mojović (Serbia), N. Ostrovski (Serbia), I. Pašti (Serbia), M. Petković (Serbia), A. Popović-Bjelić (Serbia),  B. Simonović (Serbia), D. Stanisavljev (Serbia),  B. Šljukić (Serbia), N. Vukelić (Serbia), V. Vukojević (Sweden)  International Scientific Committee  Chairman:  Ž. Čupić (Serbia) Vice-chairmans: V. N. Parmon (Russia)                                   S. Rakovsky (Bulgaria) Members: B. Adnađević (Serbia), S. Anić (Serbia), A. Antić-Jovanović (Serbia), G. Bačić (Serbia), A. Kelarakis (Greece), R. Cervellati (Italy), V. Dondur (Serbia), Ivan Gutman (Serbia), S. D. Furrow (USA), K. Hedrih (Serbia), M. Jeremić (Serbia), A. V. Knyazev (Russia), Lj. Kolar-Anić (Serbia), V. Kuntić (Serbia), Z. Marković (Serbia),  S. Mentus (Serbia), Š. Miljanić (Serbia), M. Perić (Serbia), M. Plavšić (Serbia), D. M. F. Santos (Portugal), G. Schmitz (Belgium), I. Schreiber (Czech), P. Sevčik (Slovakia), B. C. Simionescu (Romania), N. Stepanov (Russia), D. Todorović (Serbia), M. Trtica (Serbia), V. Vasić (Serbia), D. Veselinović (Serbia)   Local Executive Committee  Chairman:              S. Blagojević Vice-chairmans:    A. Ivanović-Šašić  Members: P. Banković,  N. Bošnjaković, J. Dostanić,  A. Đerić, A. Ignjatović, A. Jović, N. Jović-Jovičić, D. Lončarević, J. Krstić, J. Maksimović, V. Marković, M. Milenković, S. Maćešić, V. Marković, B. Nedić, N. Potkonjak, D. Ranković, M. Stević, M. Žunić, M. Ristić,          PHYSICAL CHEMISTRY 2014  H-26-P   695HYDROGEN DESORPTION FROM VACANT MgH2  S. Kurko, R. Vujasin, A. Đukić, B. Paskaš Mamula, J. Grbović Novaković and N. Novaković  Vinča Institute for Nuclear Sciences, University of Belgrade, P.O. Box 522, 11000 Belgrade, Serbia (radojka.vujasin@vinca.rs)  ABSTRACT The hydrogen desorption properties of vacant MgH2 structure were investigated both experimentally and theoretically. Ion irradiation by Ar8+ and Xe8+ ions were used to induce structural changes in MgH2. Hydrogen desorption properties were investigated by temperature programmed desorption (TPD). To obtained deeper insight in structural changes during desorption theoretical calculations were performed using DFT approach within Abinit code. Results showed that there are several mechanisms involved in desorption process, which depend on defect concentration, their position and their interaction and ordering. It has been demonstrated that the changes in near-surface area play the crucial role in desorption kinetics.   INTRODUCTION Magnesium and magnesium based alloys have a great potential as rechargeable hydrogen storage materials since its high hydrogen storage capacity (7.6 wt %) and reversible, relatively simple, endothermic hydrogen desorption reaction. However, slow kinetics and relatively high dehydrogenation temperature still limit its practical application [1]. Severe attempts have been made to overcome these drawbacks. The most used method is nanostructuring of material by mechanical milling with different types of dopants like transition metals, intermetallics, oxides, carbides [2-5], etc. Metal oxides such as VO2 and CeO2 that have vacant structure were proved as very effective catalysts [4, 5].  The other way to destabilize the MgH2 structure and improve its hydrogen desorption kinetics is ion irradiation. By inert, heavy ion irradiation it is possible to create a considerable amount of defects, particularly vacancies and to control their quantity and depth distribution through sample [6]. It is especially important for the surface layer modification which condition is crucial for the hydrogen desorption properties of material. On the other hand, DFT calculations can give an insight in the stability of the system and consequently, the possible mechanism of desorption process. From these calculations one can also        PHYSICAL CHEMISTRY 2014  H-26-P   696determine the influence of vacancy position and concentration on the hydrogen desorption energy [7].  In this paper, mechanism of hydrogen desorption from MgH2 irradiated with Ar8+ and Xe8+ ions is investigated and correlated with DFT calculation findings.  EXPERIMENTAL Commercial (Alfa Aeser, (AA), of purity 98%, with initial mean particle diameter of 38 μm) MgH2 powder was prepared as explained in our previous work and homogeneously irradiated using 120 keV and Ar+8 (sample A16) and Xe+8 (sample S16) ions with fluence of 1016 ions/cm2 [6]. Interaction of incident ions with material was calculated using Monte Carlo method as implemented in SRIM package. X-ray analysis was by Siemens KRISTALLOFLEX D-500 device, with Cu-Kα Ni filtrated radiation (λ=1.5406 Å). Malvern 2000SM Mastersizer laser scattering particle size analysis system was used to obtain the quantitative MgH2 particle size distributions. Homemade apparatus with a quadruple mass spectrometer EXTORR XT300 were used for TPD measurements. The kinetic parameters were deduced using non-isothermal approach.  For DFT calculations Abinit code and Troulliere Martins pseudopotentials were used. (110) plane was exposed and the supercell consists of 12 parallel atomic planes with total of 96 atoms, with H two-fold bonded surface atoms. The surface of the supercell was separated from its periodic image by 1,5 nm of vacuum.  RESULTS AND DISCUSSION In the table 1 are given number of produced vacancies per incident ion and range of maximum ions in the sample. It can be seen that the Xe8+ ions produce about 50 % more vacancies than Ar8+ which are deposited nearer to                                        Table 1. SRIM, XRD, laser scattering and kinetic data for commercial (AA) and samples irradiated with Xe8+ (S16) and Ar8+ (A16) ions with fluence of 1016 ions/cm2 Sample Produced vacancies per ion Range maximum position (nm) Crystallite size (nm) Mean particle size (μm) Avrami-parameter Eades (kJ/mol) AA - - 83 38 3 372 S16 1818 85 45 21.7 4 97 A16 1244 175 76 22.6 2 215         PHYSICAL CHEMISTRY 2014  H-26-P   697the surface. XRD data showed small decrease in crystallite size for irradiated samples and shift of peaks towards higher angles as a consequence of material disordering. Besides, there are visible peaks of Mg(OH)2 and MgO phases in samples S16 and A16 as a consequence of high reactivity of MgH2 towards oxygen an moisture from air. Mean particle size of irradiated samples is also reduced, but with no significant difference between samples A16 and S16. Hydrogen desorption temperatures are 720, 643 and 665 K for samples AA, S16 and A16 consequently. The best fitting of thermal data was achieved by using Avrami-Erofeev nucleation models and results given in table 1 show significant decrease in apparent activation energy for hydrogen desorption for both irradiated samples, but more pronounced for the sample S16. There can also be seen change in the dimensionality of nucleus growth from 3D in sample AA to 2D for A16 and random in S16. It can be assumed that the reduction of temperature and acti- vation energies are caused by the increase of diffusi-on coefficient of hydrogen in subsurface layers of de-stabilized MgH2 and in-crease in the number of active sites for hydrogen molecule recombination, both results of the vacant surface structure. This explanation is supported by Park et al. on type and behavior of defects in the magnesium hydride in conditions of extremely poor hydrogen, existing during dehydrogenation process [8]. These effects are dominant for the sa-mple S16 that has more vacancies closer to the sample surface. So, the concentration of defects has effect on desorption temperature and activation energy, while their position influences the reaction mechanism. Results of DFT calculation (Fig. 1) showed that H-desorption energy strongly depends on number of surface H atoms and their configuration. Surface vacancies considerably lower the H-vacancy formation energies as well in the sub-surface layers. These results suggest that the role of surface  Figure 1.  The H-vacancy formation energies in the (110) MgH2 sub-surface layers as a function of surface coverage with H atoms. Configurations with 1 and 3H vacancies in 3rd and 6th layer are connected with dashed line.        PHYSICAL CHEMISTRY 2014  H-26-P   698hydrogen concentration and distribution is decisive for the H-desorption kinetics, not only because of lowering the potential barrier for further H-desorption, but an increased number of surface hydrogen vacancies supports creation of the sub-surface vacancies network and make diffusion of bulk hydrogen atoms toward the surface easier.  CONCLUSION The influence of structural changes induced by heavy ion irradiation using Ar and Xe ions on desorption properties of MgH2. To obtain deeper insight in structural changes DFT calculations using Abint code has been performed. It has been shown that the large concentration of the surface and near-surface vacancies promotes diffusion of H atoms toward the surfaces of grains, which lowers temperature and activation energy of H2 desorption. Not only that concentration, but also the depth distribution of vacancies have strong influence on the desorption process.  ACKNOWLEDGEMENT  This research was financially supported by Ministry of Education, Science and Technological Development of the Republic of Serbia under grant III 45012.   REFERENCES [1] L. Schlapbach, A. Züttel, Nature, 2001, 414, 353-358. [2] J. Huot, G. Liang, S. Boily, A. Van Neste, R. Schulz, J. Alloys Compd., 1999, 293, 495-500. [3] C. Suryanarayana, Prog. Mater. Sci., 2001, 46, 1-184. [4] S. Milošević, Ž. Rašković-Lovre, S. Kurko, R. Vujasin, N. Cvjetićanin, Lj. Matović, J. Grbović Novaković, Ceram. Int., 2013, 39, 51-56. [5] J. Gulicovski, Ž. Rašković-Lovre, S. Kurko, R. Vujasin, Z. Jovanović, Lj. Matović, J. Grbović Novaković, Ceram. Int., 2012, 38, 1181-1186. [6] J. Grbović Novaković, Lj. Matović, S. Milovanović, M. Drvendžija, N. Novaković, D. Rajnović, M. Šiljegović, Z. Kačarević Popović, N. Ivanović, Int. J. Hydrogen Energ., 2008, 33, 1876-1879. [7] A. J. Du, S. C. Smith, G. Q. Lu, J. Phys. Chem. C, 2007, 111, 8360-8365. 2008, 42, 337-351. [8] M. S. Park, A. Janotti, C. G. Van de Walle, Phys Rev B, 2009, 80, 064102.  

Hydrogen desorption from vacant MgH2

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Hydrogen desorption from vacant MgH2

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