Hydrogen interactions with low-index surface orientations of tungsten

Abstract

We report on density functional theory calculations that have been performed to systematically investigate the hydrogen-surface interaction as a function of surface orientation. The interactions that were analyzed include stable atomic adsorption sites, molecular hydrogen dissociation and absorption energies, migration pathways and barriers on tungsten surfaces, and the saturation coverage limits on the (1 1 1) surface. Stable hydrogen adsorption sites were found for all surfaces. For the reconstructed W(1 0 0), there are two primary adsorption sites: namely, the long-bridge and short-bridge sites. The threefold hollow site (3F) was found to be the most stable for W(1 1 0), while the bond-centered site between the first and second layer was found to be most stable for the W(1 1 1) surface. No bound adsorption sites for H 2 molecules were found for the W surfaces. Hydrogen (H) migration on both the (1 0 0) and (1 1 0) surfaces is found to have preferred pathways for 1D motion, whereas the smallest migration barrier for net migration of H on the W(1 1 1) surface leads to 2D migration. Although weaker H interactions are predicted for the W(1 1 1) surface compared to the (1 0 0) or (1 1 0) surfaces, we observe higher H surface concentrations of Θ = 4.0 at zero K, possibly due to the corrugated surface structure. These results provide insight into H adsorption, surface saturation coverage and migration mechanisms necessary to describe the evolution from the dilute limit to concentrated coverages of H.

Predictive Atomistic Model for Hydrogen Adsorption on Metal Surfaces: Comparison with Low-Energy Ion Beam Analysis on Tungsten

“…The reconstruction of the W(100) surface makes its energetics more complex to determine as compared with the W(110) surface. Bergstrom et al 35 however established by DFT two activation barriers for the diffusion of hydrogen on the bare W(100) surface; they are 0.44 and 0.67 eV along the SB-SB and SB−LB−SB paths, respectively, on the reconstructed surface. However, the reconstruction is lost at the temperature of the experiment where hydrogen desorbs, and there is no reconstruction anymore at coverage θ = 1.00 and above.…”

“…This 1D translation is confirmed by recent DFT calculations that established an activation barriers as low as 0.07 eV for hydrogen along a 1D channel of the W(110) surface. More precisely, Bergstrom et al, 35 and Nojima et al 24 established two diffusion paths for hydrogen on the W(110) plane: one along the TF-LB-TF adsorptions sites with related activation barrier of 0.30 eV, and another one along the TF-SB-TF path with an activation barrier of 0.07 eV. In the z-direction, the activation barrier for desorption is much higher at around 0.8eV 17 .…”

“…The reconstruction of the W(100) surface makes its energetics more complex to determine as compared with the W(110) surface. Bergstrom et al 35 however established by DFT two activation barriers for the diffusion of hydrogen on the bare W(100) surface; they are 0.44 eV and 0.67 eV along SB-SB and SB-LB-SB paths, respectively on the reconstructed surface.…”

Modelling tritium adsorption and desorption from tungsten dust particles with a surface kinetic model

“…It varies between 0.5-0.7 [22] and is independent of temperature [23]. In addition, DFT calculations show that the activation energy for the dissociation of H 2 on W(100), W(110) and W(111) is close to zero [12,13,13]. Thus, in these simulations, the sticking coefficient is also assumed to be independent of the temperature and set to unity.…”

“…where ν X+Y 0 is the pre-exponential factor (s −1 ) and E X+Y des (eV) is the activation barrier for the desorption of the XY molecule, that can depend on the total coverage of the surface as shown by density functional theory (DFT) calculations [15,12,13,16] and experimental results [17,18]. The mass difference of the hydrogen isotopes is taken into account in the pre-exponential factor as:…”