Hydrogen adsorption and diffusion on the PdTa alloy surface
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DISORDER, AND PHASE TRANSITION IN CONDENSED SYSTEM
Hydrogen Adsorption and Diffusion on the PdTa Alloy Surface S. S. Kulkov, A. V. Bakulin, and S. E. Kulkova* National Research Tomsk State University, Tomsk, 634050 Russia Institute of Strength Physics and Materials Science, Siberian Branch, Russian Academy of Sciences, Tomsk, 634021 Russia *email: [email protected] Received March 27, 2014
Abstract—The hydrogen adsorption on the PdTa alloy surface is studied using a pseudopotential method with a generalized gradient approximation for an exchangecorrelation functional. The most preferable hydrogen adsorption sites on two lowindex surfaces ((001), (110)) are determined. It is shown that hydrogen adsorption at the bridge site is preferred on the PdTa(001) surface that terminates by one or two tantalum lay ers and on PdTa(110). The preference of hydrogen adsorption at tantalumrich sites is caused by partial pop ulation of its d shell. During adsorption, the electronic structure of the states involved in interaction with hydrogen is shown to change most substantially, which is accompanied by the corresponding shifts of these states and the appearance of peaks in the densities of states of the metal in the region of the hydrogen valence band. The effect of hydrogen on the electron and structural characteristics of the surfaces is analyzed. The hydrogen diffusion barriers are calculated in the bulk of the alloy and from the surface into the bulk. DOI: 10.1134/S106377611408007X
1. INTRODUCTION The interaction of hydrogen with metals has been experimentally and theoretically studied since the dis covery that palladium can reversibly absorb hydrogen [1, 2]. Constant interest in hydrogen in metals is caused by searching for alternative energy sources. Palladium and its alloys are known to be used for both hydrogen storage and hydrogenpenetrable mem branes. However, palladium has serious disadvantages, since lattice expansion and compression depending on the hydrogen concentration upon hydrogen loading and unloading results in metal failure. On the other hand, the interaction of hydrogen with palladium atoms leads to the appearance of hydrogeninduced vacancies, which also affect the strength of a material in technological cycles [3–5]. The mechanical prop erties of a material can be improved with alloying addi tions. For example, silver alloying increases the service life of palladium membranes; however, the cost of these materials is very high. Moreover, hydrogen diffu sion in fcc palladium is lower than in bcc metals, such as Ta, V, Zr, and Nb [6]. However, oxides form on the surfaces of these metals, which hinders hydrogen dif fusion from these surfaces. This problem is also chal lenging for a TiFe alloy, which is considered to be one of the promising materials for hydrogen storage. The authors of [7, 8] proposed to use a palladium coating to prevent oxide formation on the surface. As shown in [9], a palladium coating on the surface is hydrogen penetrable. A similar solution was proposed in [10]. The use
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