Density functional and dynamics study of the dissociative adsorption of hydrogen on Mg (0 0 0 1) surface D. Kecik, M.K. Aydinol * Department of Metallurgical and Materials Engineering, Middle East Technical University, 06531 Ankara, Turkey article info Article history: Received 8 July 2008 Accepted for publication 17 November 2008 Available online 30 November 2008 Keywords: Density functional calculations Molecular dynamics Magnesium hydride Hydrogen adsorption abstract A first principles study is performed to investigate the adsorption characteristics of hydrogen on magne- sium surface. Substitutional and on-surface adsorption energies are calculated for Mg (0 0 0 1) surface alloyed with the selected elements. To further analyze the hydrogen–magnesium interaction, first prin- ciples molecular dynamics method is used which simulates the behavior of H 2 at the surface. Also, charge density differences of substitutionally doped surface configurations were illustrated. Accordingly, Mo and Ni are among the elements yielding lower adsorption energies, which are found to be 9.2626 and 5.2995 eV for substitutionally alloyed surfaces, respectively. In light of the dynamic calculations, Co as an alloying element is found to have a splitting effect on H 2 in 50 fs, where the first hydrogen atom is taken inside the Mg substrate right after the decomposition and the other after 1300 fs. An interesting remark is that, elements which acquire higher chances of adsorption are also seen to be competent at dis- sociating the hydrogen molecule. Furthermore, charge density distributions support the results of molec- ular dynamics simulations, by verifying the distinguished effects of most of the 3d and 4d transition metals. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The fact that fossil fuel reserves are expected to diminish in the near future and cause serious environmental pollution highlights the significance of hydrogen, as being an abundant, environmen- tally benign and efficient energy carrier. The reversible storage of hydrogen for both stationary and mobile applications can be in several forms, such as gaseous, liquid, carbon nanotubes, metal and complex hydrides [1]. Besides the advantage of metals being highly reactive with hydrogen, in the hydride form they are capa- ble of storing hydrogen in large amounts on a reversible basis [2]. The generally searched criteria for an ideal metal hydride are high storage capacity, fast hydrogenation/dehydrogenation kinetics, low desorption temperatures as well as low cost. Magnesium hy- dride (MgH 2 ), in this respect is a promising material being eco- nomic, light weight and storing high amounts of hydrogen (7.6 wt.%), despite its high reactivity towards air and oxygen and high thermodynamical stability, which leads to poor hydriding/ dehydriding kinetics [3]. In order to decrease the hydrogen desorp- tion temperature which is above 300 °C, there are several methods such as altering the chemical composition, mechanical alloying and addition of catalysts. Zaluska et al. [4] pointed out two major reasons for poor dehydrogenation kinetics as, oxide layer formation and slow dissociation rate of hydrogen on magnesium surface. It was also mentioned that since the MgO layer impedes hydrogen penetration inside the surface, the oxide layer needs to be cracked by annealing at a temperature above 400 °C, so that the metal surface would be exposed to hydrogen. Furthermore, in their study, the conclusion has been raised that small amounts of catalysts (like Pd or Fe) would induce the improvement of hydro- genation kinetics. Besides palladium, Baer et al. [5] claimed that nickel is also an outstanding catalyst since it holds the ability to dissociate and adsorb the hydrogen molecule. Titanium and vana- dium, as stated by Liang et al. [6] are as well favored catalysts for hydrogen absorption. Although their strong tendency towards oxi- dization makes the catalytic effect disappear. Moreover, addition of oxide catalysts, such as Cr 2 O 3 ,V 2 O 5 and Fe 3 O 4 [7], were found use- ful in achieving improved hydriding properties at lower tempera- tures. Dornheim et al. [8], have stressed the effect of 3d transition metals in their study, in terms of the reduction of hy- dride formation enthalpies. Nevertheless, alloying with 3d ele- ments was seen to give rise to a decline in the hydrogen storage capacity below 3.6 wt.%. Sprunger and Plummer [9], on the other hand, executed a study based on the comparison of experimental and theoretical results. Regarding the interaction of H 2 with Mg (0 0 0 1) surface, the tools used were electron energy loss spectroscopy (EELS), thermal desorption spectrometry (TDS), core level spectroscopy and work-function measurements. In their study, as a result of the 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.11.017 * Corresponding author. Tel.: +90 312 210 2523; fax: +90 312 210 2518. E-mail address: kadri@metu.edu.tr (M.K. Aydinol). Surface Science 603 (2009) 304–310 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc