# How Surface Reconstruction Affects Hydrogen Dissociation on the N-Precovered W(110): A Theoretical Perspective

Norhan Omar, Oihana Galparsoro, Lionel Truflandier, Cedric Crespos, Pascal Larregaray

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Norhan Omar, Oihana Galparsoro, Lionel Truflandier, Cedric Crespos, Pascal Larregaray. How Surface Reconstruction Affects Hydrogen Dissociation on the N-Precovered W(110): A Theoretical Perspective. Journal of Physical Chemistry C, 2025, 129 (21), pp.9706-9716. 10.1021/acs.jpcc.5c00732. hal-05374340

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# How Surface Reconstruction Affects Hydrogen Dissociation on the N-precovered W(110): A theoretical Perspective

Norhan Omar,*,† Oihana Galparsoro,‡ Lionel Truflandier,† Cedric Crespos,† and Pascal Larregaray†

†Université de Bordeaux, CNRS, Bordeaux INP, ISM, UMR5255, F-33400 Talence, France

‡Kimika Fakultatea, Euskal Herriko Unibertsitatea UPV/EHU, Donostia-San Sebastián
20018, Spain

E-mail: norhan.omar@u-bordeaux.fr

## Abstract

We make use of density functional theory and ab-initio molecular dynamics to eluate, at the atomic scale, the impact of adsorbed nitrogen (N) impurities on W(110) surface on H₂ dissociative chemisorption. Our primary focus is on a nitrogen coverage of 0.25 ML, for which two surface structures have been postulated previously in the literature. The first structure involves N adsorption on the surface, while the second involves N absorption between the first two tungsten layers. The present study demonstrates that dissociative chemisorption of hydrogen is critically sensitive to surface structure. The sublayer absorption of nitrogen involves a surface reconstruction that renders one displaced W atom highly reactive in terms of H₂ dissociation. The dynamics are analysed in view of previous results on clean and O-precovered W(110) surfaces.
Introduction

Gas-surface interactions, which are of primary importance for a wide range of applications
such as hydrogen storage,¹,² heterogeneous catalysis³⁻⁶ or controlled nuclear fusion,⁷,⁸ are
the focus of intense fundamental and applied research. As far as nuclear fusion is concerned,
metallic surfaces, and in particular tungsten (W), are used as plasma facing materials in the
divertors of magnetic confinement devices, expected to be a sustainable and clean source of
energy for the future, as exemplified by the international thermonuclear experimental reac-
tor (ITER) project.⁹,¹⁰ In fusion reactors, the fuel is composed of hydrogen (H) isotopes.
Impurities and heat are extracted at the divertor, which protects parts of the walls facing
the plasma. The interaction of the fuel with the surface and sub-surface region determines
the fuel recycling coefficient,¹¹ which represents the proportion of species that returns to the
plasma after interacting with the reactor walls and may affect the reactor’s plasma behavior
on the short term. In that context, a detailed understanding of the elementary processes
involving hydrogen atoms, molecules, and possible impurities is essential. Such processes
include molecular adsorption (sticking), diffusion, recombination reactions, dissociative ad-
sorption, absorption into the bulk and desorption processes from the surface.

Numerous experimental<sup>12-16</sup> and theoretical<sup>17-22</sup> studies have been carried out to inves-
tigate the behavior of hydrogen on clean tungsten surfaces such as W(110) and W(100).
The surface structure has a strong influence on H<sub>2</sub> chemisorption, which significantly differs
on both surfaces, particularly at low energies.<sup>23,24</sup> Although both reaction pathways do not
involve potential energy barrier<sup>25</sup> within the framework of Density Functional Theory (DFT)
calculations based on the Perdew Wang (PW) 91 functional, the monotonously increasing
sticking curve for H<sub>2</sub> dissociation on W(110) is typical of an activated process while the non-
monotonic shape for W(100) is a known feature for a non-activated process. For H<sub>2</sub>/W(110),
the dynamic trapping is significantly reduced compared to H<sub>2</sub>/W(100), and the probability
of the direct process is also noticeably lower. More recently, Rodríguez-Fernández et al.<sup>26</sup>
studied H<sub>2</sub> dissociation on W(110) using Ab-Initio Molecular Dynamics (AIMD) method
with the Perdew-Burke-Ernzerhof (PBE)²⁷ exchange-correlation functional and the nonlocal
van der Waals density functionnal (vdW-DF).²⁸ The latter functional significantly improves
the agreement between theoretical results and experimental data, mainly because of more
repulsive regions at distances close to the surface (lower than 2 Å).²⁶

However, tungsten surfaces can be clean or covered with impurities that can alter the
surface's behavior and reactivity. Within the context of nuclear fusion, intrinsic impuri-
ties²⁹ naturally exist within a material, while extrinsic ones are externally added and aim
at modifying or enhancing the material's properties. The most common intrinsic impurity
is oxygen (O), and the extrinsic one is nitrogen (N). The N₂ molecule, which has good ra-
diative properties, is injected³⁰ close to the divertor plasma to distribute the exhaust power
radiatively (emitted through radiation) and avoid the appearance of thermal hot spots on
the W divertor.

Oxygen covered W(110) surfaces have been experimentally well characterised.<sup>31-33</sup> Re-
cently, Rodriguez et al.<sup>34</sup> theoretically studied the effect of oxygen adatoms on H<sub>2</sub> dissociation
dynamics on W(110) surfaces. It was found that adsorbed O atoms have an inhibiting effect
by blocking H<sub>2</sub> dissociation on top of the neighboring W atoms on which H<sub>2</sub> dissociates for
the clean surface. As a consequence, at 0.5 monolayer (ML), dissociative adsorption does
not occur anymore. The findings reported on clean and oxygen covered tungsten surfaces
align with existing experiments,<sup>35</sup> where the presence of O weakens the H binding to the
substrate, as shown by a lower H<sub>2</sub> desorption temperature.

In contrast, the impact of nitrogen has not been thoroughly explored. To the best of our
knowledge, the dynamics of hydrogen dissociation on N-covered W(110) surfaces has been
neither theoretically nor experimentally investigated. Previous studies of N₂ sticking on the
W(110) surface have shown that the molecules adsorb and desorb as molecules below 200
K.<sup>36,37</sup> Moreover, Molecular Dynamics (MD) simulations for dissociation of the N₂ molecule
on the W(110) surface reveal non-activated dissociation pathways. Dissociation remains
unlikely at low energies but increases with energy, through two distinct mechanisms: a
direct mechanism at high energies and an indirect mechanism involving dynamic trapping
at low to moderate energies.³⁸ Alducin et al.³⁹ also demonstrated that N₂ molecules readily
undergo dissociation on W(100) surfaces but not on W(110) surfaces at thermal energies.
This difference in reactivity between the two surfaces is attributed to the features of the PES
at distances far from the surface (> 3 Å). In addition, an experimental study⁴⁰ of nitrogen
molecule retention on polycrystalline tungsten revealed the formation of deuterated ammonia
(ND₃) through the implantation of N and D ions at varying densities when a near saturated
N layer was present. It was suggested that ammonia precursors could be also created at the
surface at room temperature through the dissociative chemisorption of thermal D₂ catalysed
by nitrogen present at the tungsten surface.⁴⁰

Previous theoretical studies suggest that nitrogen adsorb in a three-fold hollow site on
the close-packed tungsten W(110) plane following atomic adsorption⁴¹ or dissociative ad-
sorption.⁴² Besides, nitrogen subsurface chemisorption on W(110) has also been observed
previously. Singh-Boparai et al.⁴³ extended a kinetic model to study N₂ adsorption on
stepped tungsten planes, illustrating that a gaseous molecule which hits the surface at a site
on the (110) terrace, can hop to a step (100) where it is dissociatively adsorbed. Furthermore,
the first Low-Energy Electron Diffraction (LEED) investigation reported on the N/W(110)
system⁴⁴ proposed a model for the underlayer structure, which aligns with both the LEED
pattern and surface stoichiometry by showing nitrogen penetration into the surface. More-
over, the results from angle-resolved photoemission do not conclusively suggest the presence
of N 2p levels, unlike findings in overlayer systems.⁴⁴ Changes in the difference spectra across
the valence band's width indicate a disruption in the bulk tungsten transitions, likely caused
by the formation of a thin "WN" layer on the surface, with a band structure distinct from
that of tungsten.⁴⁴ Evidence from ion scattering spectroscopy and variable incidence angle
Auger electron spectroscopy strongly suggests that nitrogen adatoms, when chemisorbed on
W(110), do not occupy conventional layer sites on the undisturbed substrate. Instead, they
seem to be sandwiched between the top two layers of tungsten atoms, partially beneath tung-
sten atoms that lie out of the plane. Pfnür et al.⁴⁵ also investigated the nitrogen molecule chemisorption on a W(110) surface, and assumed a reconstructed surface based on LEED analyses, identical to the one observed by Somerton and King.⁴⁴ Rettner et al.⁴⁶ showed that the dissociative chemisorption probability of N₂ on W(100) depends on kinetic energy (Eᵢ). At low energies (Eᵢ < 0.45 eV), a precursor-mediated mechanism dominates, where molecules are trapped before dissociating. At higher energies, dissociation occurs directly, with Eᵢ overcoming an energy barrier. For N₂/W(110), the precursor-mediated mechanism is less effective, with only 1% of trapped molecules dissociating at 300 K. The key difference between the two surfaces is the slightly higher dissociation barrier for W(110). A significant difference in the dissociation probability was observed between the experimental data of Pfnür et al.⁴⁵ and Rettner et al.,⁴⁶ particularly at high collision energies (Eᵢ = 2.3 eV). The dissociation probability of N₂ measured by Rettner et al. is 0.1 lower than that obtained by Pfnür et al., with no further explanation provided by Rettner et al. for this discrepancy.

As a consequence of these previous works, we decided to examine H₂ dissociation dynamics on two N-covered W(110) surfaces in which nitrogen is either adsorbed on the three-fold hollow surface site (W:N<sub>on</sub> in the following)⁴¹,⁴² or N inside the W slab (W:N<sub>in</sub> in the following).⁴⁴ The results are analyzed in view of the previous studies concerning the effects of oxygen impurities on H₂ dissociation.³⁴

Methodology

AIMD simulations are here carried out to investigate the dynamics of hydrogen sticking on N-covered W(110) surfaces. DFT calculations are conducted using the Vienna Ab initio Simulation Package (VASP) code⁴⁷⁻⁵¹. Two adsorption structures of nitrogen on W(110) are considered, whose DFT-optimized configurations are illustrated in Figure 1, each comprising a 5-layer slab of W(110), with a (2×2) surface unit cell. The ratio of N (1 atom) to W per
layer (4 atoms), denoted as the Θ coverage, is maintained at 0.25 ML for both surfaces across all calculations. To avoid artificial interactions between periodic supercells, a vacuum gap of 18 Å was introduced in the direction normal to the surface. Projector-Augmented Wave (PAW) pseudopotentials<sup>52,53</sup> represent the ionic cores, and a plane wave cutoff energy of 600 eV was used in all the calculations. The vdW-DF non-local function developed by Dion et al.<sup>28</sup> with the GGA approximation is used for the exchange-correlation functional<sup>54,55</sup> to be consistent with previous studies on the clean and O-covered surfaces.<sup>26,34</sup> We selected this functional based on its proven accuracy in modeling dissociation processes, as demonstrated in the studies of H<sub>2</sub> dissociation on Ru(0001)<sup>56</sup> and N<sub>2</sub> dissociation on W(110) at low energies and normal incidence.<sup>57</sup> Additionally, Rodríguez et al.<sup>26</sup> showed that the vdW-DF functional provided results in closer agreement with experimental data<sup>23</sup> for H<sub>2</sub> dissociation on W(110), which further justified this specific choice. The convergence criteria were set to 10<sup>-5</sup> eV/Å for ionic relaxation forces and 10<sup>-6</sup> eV for electronic relaxation energies. The Brillouin zone is sampled with a (5×5×1) k-points grid within the Monkhorst-Pack approach<sup>58</sup> and a Fermi broadening of 0.4 eV using the Methfessel-Paxton scheme.<sup>59</sup> The electronic charge optimization uses the Residual minimization method direct inversion in the iterative subspace (RMM-DIIS).<sup>60,61</sup> Further calculation details can be found in previous studies on a clean tungsten W(110) surface.<sup>26</sup>

The bulk optimization of W resulted in an equilibrium lattice constant $a_W$ of 3.203 Å. This value is slightly higher than the experimentally measured lattice constant at low temperature, which is 3.163 Å.<sup>62</sup> This overestimation is consistent with the tendency of vdW-DF to overestimate lattice constants and bond lengths of weakly bound compounds, due to the relatively repulsive nature of the original revPBE exchange interaction.<sup>57</sup> The first N-bearing relaxed structure corresponds to the adsorption of the nitrogen on tungsten (W:N<sub>on</sub>) in the three-fold hollow site at a distance Z=1.11 Å from the surface, with the average Z position of the four topmost layer tungsten atoms used as a reference point for the surface (Z=0), as shown in Fig.1. During relaxation, the bottom two layer atoms are fixed.
<figure><img src="image_2.png" /><figcaption>Figure 1: Top (upper panels) and side (lower panels) views of the W:N<sub>on</sub> (left panels) and W:N<sub>in</sub> (right panels) surface atomic optimized structures. The black lines indicate the 2x2 unit cell. N atoms are depicted in blue and W atoms in gray. The first and second layers of the W slab in top views are shown in gray and light gray respectively.</figcaption></figure>

The second relaxed structure comprises N inside the W slab (W:N<sub>in</sub>).<sup>44,45</sup> DFT relaxation resulted in the out-of-plane displacement of the closest tungsten surface atom to the nitrogen, which is excluded from the average to set the Z=0 reference for the surface. The N atom is situated beneath the displaced W atom located at Z=0.54 Å, thus nitrogen absorbs between the top two layers of the W(110) surface, at approximately Z=-1.04 Å. DFT calculations show that experimentally observed structure W:N<sub>in</sub> is less stable than W:N<sub>on</sub>, difference in energy between the two configurations being 2.63 eV but, as mentioned above, this structure has been suggested in several experimental studies.<sup>44,45</sup> In the following, the three W atoms from topmost layer which are the closest to the N atom in the two configurations are referred to as N-adsorbed W atoms, while the remaining W atom will be called N-free W atom.

When subsequent H₂ scattering dynamics is investigated, the methodology employs the
same computational approach as in previous investigations of H₂ dynamics on clean²⁶ and
oxygen (O)<sup>34</sup> covered surfaces. During the dynamics, the W atoms in the top three layers and the N atom, initially positioned at equilibrium, are allowed to move. Normal-incidence collisions are simulated at energies of 30, 80, 100, and 200 meV. The calculations do not take into account the zero point energy of the hydrogen molecule. For each incident energy, 400 trajectories are computed with random initial positions and orientations. 400 trajectories were found to be adequate for achieving reasonable convergence. The initial projections of the center-of-mass positions of the H<sub>2</sub> molecule on the surface are randomly sampled from a uniform distribution of the points in the surface unit cell at Z<sub>cm</sub>=9 Å. The polar θ (with respect to the surface normal Z [110] direction) and the azimuthal φ (with respect to the X [001] direction) internal angles are also randomly chosen. The bond length between the two hydrogen atoms is initially fixed at 0.74 Å, corresponding to the equilibrium distance of the molecule calculated at DFT level. The simulation runs for a duration of 1.5 ps in the case of W:N<sub>on</sub> and extends up to 7.0 ps for W:N<sub>in</sub>, with a time step of 1.5 fs. Trajectories are stopped according to the following criteria: i) dissociation occurs when the interatomic distance, r, exceeds twice the equilibrium bond length (1.5 Å) with a positive radial velocity (dr/dt > 0); ii) the H<sub>2</sub> molecule is considered to be reflected if its center of mass momentum is positive and its position exceeds a distance of Z=4.5 Å for W:N<sub>on</sub> and Z=6 Å for W:N<sub>in</sub>. Otherwise, the molecule is assumed to be molecularly adsorbed on the surface.

Results and discussion

As specified above, AIMD simulations have been performed to determine the dissociative
sticking curves of H<sub>2</sub> on the W:N<sub>on</sub> and W:N<sub>in</sub> surfaces as a function of collision energy for
Θ=0.25ML. Figure 2 displays the H<sub>2</sub> dissociative sticking probability for both surfaces along
with previous results for the clean tungsten surface,²⁶ W<sub>clean</sub>(Θ=0 ML) and the oxygen-
covered one W:O<sub>on</sub>(Θ=0.25 ML),³⁴ as well as experimental results for clean W(110) from
molecular beam experiments.²³ The error bars for the dissociative sticking probability corre-
spond to the standard error for the binomial distribution $\sigma_p = \sqrt{\frac{p(1-p)}{N}}$, with $p$ is a dissociation probability calculated from $N$ trajectories. For the clean W(110) surface,²⁶ dissociation of H₂ increases with collision energy. Rodríguez et al.’s²⁶ results using the vdW-DF functional are closer to those from molecular beam experiments²³ than those obtained with other functionals. However, according to the authors, the absence of factors like the initial surface temperature at 300 K and the zero-point energy (ZPE) of H₂ may account for the remaining quantitative difference, possibly underestimating the sticking coefficient by less than 0.1.²⁵ As shown in a previous work,³⁴ the presence of oxygen (W:O<sub>on</sub>) significantly decreases H₂ chemisorption. An even more drastic effect is observed when the tungsten surface is contaminated with N atoms. For the W:N<sub>on</sub> surface, H₂ dissociation is totally suppressed over the entire energy range. Conversely, for the reconstructed W:N<sub>in</sub> surface, a significant value of H₂ chemisorption shows up, especially for low energy values. At 30 meV, this probability is 0.41, which is more than one order of magnitude greater than the other surfaces, especially the clean one (0.03 at 50 meV). This probability is almost constant up to 80 meV and reaches 0.57 at 200 meV, which is higher than those of W:O<sub>on</sub> and W:N<sub>on</sub> surfaces as well as W<sub>clean</sub> surface. The large dissociation probability at low collision energy suggests a non-activated path in a large part of the W:N<sub>in</sub> surface unit cell, whereas the shape of the sticking curves are typical from activated systems for the W:N<sub>on</sub> and W:O<sub>on</sub> surfaces.

Figure 3 shows the evolution of the H₂ molecules center of mass altitude, $Z_{cm}$, as a function of time for 30 and 200 meV collision energies for the 400 trajectories. Figure 3 (a) and (b) correspond to W:N<sub>on</sub> surface. Regardless of the collision energy, no dissociation of H₂ occurs. Instead, H₂ either reflects or is molecularly adsorbed onto the surface. Molecular adsorption is a quite rare event as its probability decreases with collision energy, ranging from 2.5% at 30 meV to 1.25% at 200 meV. Some of the molecules (5% at 200 meV) are trapped on the surface and bounce several times before they desorb from the surface. This aspect, which is less important when the collision energy is lower, has already been observed
<figure><img src="image_3.png" /></figure>

Figure 2: H₂ dissociative sticking probabilities on clean and covered W(110) surfaces as a function of collision energy. The H₂ sticking probabilities on W(110), derived from molecular beam experiments, were obtained from Berger et al.²³.

when investigating the dynamics of the H₂ molecule on W:O<sub>on</sub> surface.³⁴ Approximately 20–25% of the molecules trapped on W:O<sub>on</sub> at 0.4 ps were observed to desorb to the vacuum afterwards. The average Z<sub>cm</sub> of the first rebound on W:N<sub>on</sub> is 2.69 Å (3.13 Å) for an energy of 200 meV (30 meV).

Figure 3 (c) and (d) correspond to the W:N<sub>in</sub> surface. The trajectories that end at altitudes lower than 2 Å from the surface before the 1.5 ps have undergone dissociation. No H₂ molecular adsorption on the surface is predicted. Some H₂ molecules (2.5%) are temporarily trapped at 4-4.5 Å from the W:N<sub>in</sub> surface up to 1.5 ps. Subsequently, throughout the extended simulation lasting until 7 ps, 60% of these trapped molecules progressively dissociate on the surface, while 40% are gradually reflected back into the vacuum. Such a trapping is less important at 200 meV (0.5%), with both H₂ molecules eventually reflecting back into the vacuum at about 4 ps. The average Z position of Z<sub>cm</sub> at which they bounce off the surface is 2.77 Å for an energy of 30 meV. This distance decreases slightly as the collision energy increases, becoming 2.45 Å at 200 meV. At high collision energies, similar to
that found for H₂ dissociation on the clean W(110) surface by Rodríguez et al.,²⁶ molecular adsorption is absent on W:Nᵢₙ and dissociative adsorption increases with collision energy. It has to be noticed that no trapping at 4-4.5 Å is observed on the W:Nᵦₐ surface, but low levels of trapping occur at lower altitudes, around 2 Å. Overall, these findings show that the presence of nitrogen on the tungsten surface has a significant impact on the adsorption and dissociation of the H₂ molecule. While the presence of N atoms on the surface completely inhibit dissociation, their presence beneath the surface promotes substantial dissociation of H₂ molecules within the entire collision energy range studied here.

<figure><img src="image_4.png" /><figcaption>Figure 3: Time evolution of the H₂ center-of-mass altitude, Z cm for all trajectories. Left panels (a and c) correspond to 30 meV collision energy. Right panels (b and d) correspond to 200 meV collision energy. Upper panels (a and b) correspond to the W:N on surface. Lower panels (c and d) correspond to the W:N in surface.</figcaption></figure>

In Figure 4, first rebound positions of the reflected H₂ molecules at 200 meV collision
energy for the two N covered surfaces are compared. Left panels display the XY positions of the center of mass of the molecule at its first rebound, while right panels display the $Z_{cm}$ altitude distributions. Bottom panels correspond to the W:N<sub>in</sub> surface, while the top ones correspond to the W:N<sub>on</sub> surface. The dots in green, black and red represent reflections at $Z_{cm}$ below 2.4 Å, 2.4 Å < $Z_{cm}$ < 2.8 Å and above 2.8 Å, respectively. The altitude range between 2.4 and 2.8 Å ($Z_{clean}$) approximates the contribution of free W atoms as it corresponds to the ones for clean tungsten surface<sup>26</sup> with a mean rebound altitude, $\bar{Z}_{clean}$, of 2.59 Å. On the W:N<sub>on</sub> surface, all molecules that bounce on the nitrogen side bounce at $Z_{cm}$ greater than $Z_{clean}$. In contrast, molecules bouncing at altitudes within the $Z_{clean}$ range reflect on the nitrogen-free side. Similar observation were done when examining the hydrogen molecule on O-covered W surface. A few molecules are reflected at distances closer to the surface (< 2.4 Å), which are mainly located on top of the N-free W atom. This was not observed for oxygen covered surfaces<sup>26</sup> as such close collisions lead to dissociation.

On the other hand, for the W:N<sub>in</sub> surface, all molecules bouncing on the reconstructed tungsten atom located at $Z=0.54$ Å above the other tungsten surface atoms, bounce at altitudes lower than $Z_{clean}$. Few molecules come also close to the surface and reflect just on top a N-free W atom. Almost all the other molecules bouncing elsewhere on the surface exhibit rebounds at $Z_{cm}$ higher or within the $Z_{clean}$ range. The presence of nitrogen on or within the surface creates a new reflection pathway in which molecules get closer to the surface, with respect to the clean tungsten surface<sup>26</sup> or the one covered with oxygen.<sup>34</sup>

It is important to highlight the discernible impact of nitrogen on hydrogen molecules, depending on the surface structure. Figure 5 illustrates the evolution of the H<sub>2</sub> molecules center of mass position in the XY plane at 0, 135, 162 et 189 fs, for W:N<sub>on</sub> (upper panels) and W:N<sub>in</sub> (lower panels) at a collision energy of 200 meV. Various colors have been employed to depict the mechanism occurring on the surface: incident molecules before the first rebound are represented by green dots, molecules scattering off the surface are represented by red dots,
<figure><img src="image_5.png" /><figcaption>Figure 4: Left: First rebound in-plane XY positions of the H₂ center-of-mass for the W:N<sub>on</sub> (a) and W:N<sub>in</sub> (c) surfaces. Gray and blue larger circles indicate the tungsten and nitrogen atoms, respectively. The green, black and red dots represent the position of the H₂ first rebound at Z<sub>cm</sub> below, within and above Z<sub>clean</sub> range, respectively (see text). Right: First rebound altitude distribution of the H₂ center-of-mass for all reflected trajectories, at 200 meV collision energy for the W:N<sub>on</sub> (a) and W:N<sub>in</sub> (c) surfaces.</figcaption></figure>

and molecules undergoing dissociation (when $r > 1.5$ Å and $dr/dt > 0$) are represented by blue dots. For W:N<sub>on</sub> surface, the repulsive effect of nitrogen is clear very early on, starting from 135 fs, as indicated by the appearance of red dots. This effect becomes significantly more important over time, indicating that the blocking effect is due to the nitrogen, which pushes the hydrogen away from the surface. This repulsive effect is similar to that observed for W:O<sub>on</sub>.<sup>34</sup> No hydrogen dissociation is observed. In contrast, for the reconstructed tungsten atom on W:N<sub>in</sub> surface, hydrogen dissociation is significant on the displaced tungsten atom, partially on top of N atom. Almost all molecules that dissociate have also undergone their first rebounds on top or close to this atom.

On the clean W surface,<sup>26</sup> all H<sub>2</sub> molecules dissociate on top of W atoms. In the case of W:O<sub>on</sub>,<sup>34</sup> W atoms constituting the three fold hollow site in which oxygen is adsorbed do not lead to H<sub>2</sub> dissociation. Conversely, W atoms for which all neighboring three fold hollow
site are free of oxygen remain reactive for H₂ dissociation, regardless of the presence of O on the surface. Here, we observe that, for W:N<sub>on</sub>, all W surface atoms become non reactive for H₂ dissociation. Conversely, for the W:N<sub>in</sub> surface structure, the reconstructed W atom is a very active site for H₂ dissociation. Meanwhile, other W atoms either exhibit no reactivity at low collision energies or display minimal reactivity at high energies.

<figure><img src="image_6.png" /><figcaption>Figure 5: Time evolution of H₂ center-of-mass position in the XY plane for the W:N<sub>on</sub> (upper panels) and W:N<sub>in</sub> (lower panels) surfaces at 200 meV collision energy. The specific times are displayed on top of the figure. Gray and blue larger circles indicate tungsten and nitrogen atoms, respectively. The green and red dots represent the center of mass position of the incident and reflecting H₂ molecules respectively. The blue ones represent the dissociated ones. The scheme to the right sketches these three sets of molecules.</figcaption></figure>

To get more insights into the collision dynamics, the interatomic distance *r* has been monitored in the course of collision. Figure 6 shows the evolution of the center of mass position of the H₂ molecule in the XY plane at 144, 150, 162 and 171 fs, for W:N<sub>on</sub> (upper panels) and W:N<sub>in</sub> (lower panels) at a collision energy of 200 meV. The different colors are used to represent specified ranges for *r* (see plot caption). The molecules begin to bounce off the surface at 144 fs. For the W:N<sub>on</sub> surface, all molecules remain close to their equilibrium distance (*r*<sub>eq</sub> = 0.74 Å). Gradually, the interatomic distance increases slightly (red dots) for molecules located on top of N-free W atoms but as mentioned before, do not lead to dissociation. As a result, the molecules are either adsorbed onto the surface or returned back into the vacuum. Conversely, for the W:N<sub>in</sub> surface, the interatomic distance begins to increase before 144 fs for H₂ molecules bouncing off the reconstructed atom. Molecules are
also able to dissociate above the N-free W atoms. However, those require longer reaction
times, suggesting more complex dynamics to reach a position or orientation where disso-
ciation is possible. At low collision energies (not shown here), only dissociation above the
reconstructed tungsten atoms are observed.

<figure><img src="image_7.png" /><figcaption>Figure 6: Same as Figure 5 but the red and blue dots represent the center-of-mass position of the H₂ that have : 0.74 Å < *r* ≤ 0.90 Å and 0.90 Å < *r* ≤ 1.50 Å, respectively. Molecules with *r* close to equilibrium distance *r*<sub>eq</sub> = 0.74 Å are represented by green dots. The scheme to the right sketches these three sets of molecules.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Figure 7: θ polar angle distribution of dissociating molecules, P(θ), for the W:N<sub>in</sub> surface for different altitudes. The top (lower) panels correspond to 30 meV (200 meV) collision energy.</figcaption></figure>

Figure 7 displays the distributions of the polar angle of the dissociating H₂ molecules
as a function of the Z<sub>cm</sub> altitude for the W:N<sub>in</sub> surface at energies of 30 and 200 meV. The
initial distributions ($Z_{cm}=9$Å) approximately reproduce the cosine distribution, the difference observed between 30 and 200 meV is due to the initial Montecarlo sampling of the angle, which is different in both cases. As the hydrogen molecules approach the surface to dissociate, this distribution tends to center around 90° to align parallel to the surface. Moreover, we have computed the average value of $P_2(\cos\theta)$ for each distribution, using the second-order Legendre polynomial $P_2(\cos\theta)=(3\cos^2\theta-1)/2$ (displayed in each panel of Fig. 7). The obtained values confirm our observation that, for the initial distributions at $Z_{cm} = 9$Å, the values of $\langle P_2(\cos\theta) \rangle$ are close to zero, indicating an isotropic distribution with no preferred orientation. As $Z_{cm}$ decreases and $H_2$ approaches the surface (i.e., for $Z_{cm} \le 2.5$Å), the values become negative, indicating a preference for angles close to 90°, corresponding to in-plane alignment.

<figure><img src="image_9.png" /><figcaption>Figure 8: 2D (r, $Z_{cm}$) cuts of the $H_2$/W:$N_{on}$ PES for the $H_2$ center-of-mass on-top of a W atom and $\theta = 90^\circ$ and $\phi = 80^\circ$. $H_2$ is located on-top a N-free W atom (left panel) or a N-adsorbed W atom (right panel). The dark blue lines indicate the 0 energy level, taken as the isolated $H_2$ energy at the equilibrium distance ($r_{eq} = 0.74$ Å). Solid blue lines represent positive values, while dashed ones represent negative values, with a level spacing of 0.1 eV. The inset illustrates different configurations corresponding to each PES cut. Gray and blue circles represent W and N atoms, respectively, while black circles denote H atoms.</figcaption></figure>

As shown in Figure 2, the dissociation probabilities vary significantly between the two types of nitrogen-covered tungsten surfaces, even though the molecules can approach relatively close to the surface. Figures 8 and 9 show selected 2D contour plots ($r, Z_{cm}$) for
<figure><img src="image_10.png" /><figcaption>Figure 9: Same as Figure 8 for the H₂/W:Nᵢₙ PES.</figcaption></figure>

H₂/W:Non and H₂/W:Nin PES, respectively for θ=90° and φ=80° over different W atoms. The specific orientation was chosen to approximately minimize the energy barrier for dissociation, for H₂ molecules located on top of tungsten atoms. For H₂/W:Non, whether the H₂ center of mass is positioned on-top of a N-adsorbed W atom (right panel in Fig. 8) or N-free W atom (left panel in Fig. 8), the dissociative adsorption of H₂ involved an energy barrier located at Zcm ≈ 1.5 Å and r ≈ 1.5 Å and higher than 0.4 eV. In the right panel, the repulsive effect of nitrogen is stronger, preventing H₂ molecules from approaching the surface at low collision energy. In contrast, in the left panel, the molecules can get closer to the surface and might even be trapped at an altitude of 2 Å from the surface, on top of a N-free W atom. This observation is illustrated in Figure 3 (a) and (b), where some molecules are trapped on the surface at a distance of 2 Å. For the W:Nin, H₂ can spontaneously dissociate on the reconstructed atom (right panel in Fig. 9). Therefore, as indicated by the dissociation probability curve, H₂ dissociation on the W:Nin surface is a non-activated process. The trapping at altitudes of 4-4.5 Å (Fig. 3(c)) stems from the van der Waals well showing up in the PES. The ones that are trapped at an altitude of ≈ 2 Å probably stem from the well showing up above the N-free W atom, which can later dissociate at another site following diffusion across the surface. The 2D-cuts clearly illustrate previous observations that the molecule dissociates more readily above the reconstructed atom at low energies, and for higher energies, dissociation can also occur above other W atoms at the surface. To
assess the influence of the surface unit cell size on the energy landscape, we computed a 2D PES cut using the larger (3x3) surface unit cell, as shown in Fig. 10. This figure reproduces the same (r, Zcm) configuration space as in Fig. 9b, but with the (3x3) cell instead of the (2x2). The critical points were first identified on the (2x2) PES cut (Fig. 9b), and the corresponding energies were then extracted from the (3x3) PES at the same (r, Zcm) coordinates for a direct comparison. Table 1 summarizes the energies of the two local minima and the saddle point at those positions. The results reveal substantial energy differences of more than 1 eV between the (2x2) and (3x3) evaluations, with the (2x2) values appearing markedly more stable. These deviations likely arise from artificial constraints introduced by the smaller cell size, including limited atomic relaxation and enhanced interactions between periodic images. While the overall topography of the potential energy surface is preserved, the absolute energy values differ. These observations underscore that unit cell size must be carefully considered, as it can strongly influence the stability of adsorbed species.

<figure><img src="image_11.png" /><figcaption>Figure 10: Same as Figure 9 (right panel) for the H₂/W:Nᵢₙ PES with a surface unit cell of (3x3).</figcaption></figure>

Figure 11 shows a similar 2D PES cut (r, Zcm) of H₂/Wclean system which reveals a late barrier located at Zcm ≈ 1.45 Å and r ≈ 1.25 Å, followed by a local minimum. The energy profile reveals that, while dissociation requires overcoming this barrier of approximately 0.2 eV, the process becomes thermodynamically favorable when the total energy exceeds this
<table><caption>Table 1: Comparison of energy values at critical points computed using the (2×2) and (3×3) surface unit cells. The same (r, Z<sub>cm</sub>) coordinates were used to extract single-point energies from the (3×3) PES.</caption><thead><tr><th>Point</th><th>r (Å)</th><th>Z<sub>cm</sub> (Å)</th><th>E (2×2) (eV)</th><th>E (3×3) (eV)</th><th>ΔE (3×3 - 2×2) (eV)</th></tr></thead><tbody><tr><td>Minimum A</td><td>0.60</td><td>2.26</td><td>-0.60</td><td>0.55</td><td>+1.15</td></tr><tr><td>Saddle Point</td><td>1.29</td><td>2.00</td><td></td><td>0.95</td><td></td></tr><tr><td>Minimum B</td><td>3.19</td><td>1.37</td><td>-0.70</td><td>0.40</td><td>+1.10</td></tr></tbody></table>

value, thereby increasing the probability of dissociation.

<figure><img src="image_12.png" /><figcaption>Figure 11: Same as Figure 8 for the H<sub>2</sub>/W<sub>clean</sub> PES.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Figure 12: Same as Figure 8 for H<sub>2</sub>/W:O<sub>on</sub> PES. Gray and red circles represent W and O atoms, respectively, while black circles denote H atoms.</figcaption></figure>

2D PES cuts for the O-covered tungsten surfaces H<sub>2</sub>/W:O<sub>on</sub> are also displayed in Figure 12 for the same molecular configurations. For H<sub>2</sub> on top of O-adsorbed W atom (right panel in Fig. 12), the dissociation path within the cut is energetically forbidden, more than 0.4 eV
is required to get close to the surface (<2 Å). More than 1.3 eV is required in order to double the internuclear distance of the molecule. In contrast, a late barrier for dissociation lower than 0.3 eV is observed when the molecule is on-top of O-free W atom (left panel in Fig. 12). This is in-line with Rodríguez-Fernández et al.³⁴ results, indicating a minimum energy requirement for dissociation. This is a typical characteristic of an activated process in which all reactive paths show a reaction barrier, as observed within the two 2D-cuts. This is also in agreement with their observations during the study of 0.5 ML O-covered the surface for which no such dissociation was observed in absence of O-free three-fold hollow sites.

The 2D PES cuts for H₂/W:N<sub>on</sub> and H₂/W:O<sub>on</sub> exhibit strong similarity, both adsor-bates induce a repulsive effect, and no reactivity is observed in their vicinity. For all N- or O-covered surfaces studied, when the molecule is placed on top of an adsorbate-free W atom, the 2D-cuts display a high late energy barrier. In contrast, the early barriers are lowered with respect to clean tungsten, which explains why the molecules approach closer to the surface but do not dissociate and are instead reflected. As expected, the changes introduced by the adsorbate in the PES at the W atoms close to the adsorbed impurity are more drastic. When N is inside the surface a non-activated pathway for dissociation is available. In contrast, when N or O is adsorbed on the surface, an energetically prohibited pathway is observed at the site.

It should be noted that all the dynamics simulations were carried out without considering the 0.275 eV Zero-Point Energy (ZPE) of H₂, in order to maintain a relevant comparison with previous studies on the clean tungsten surface²⁶ and the one covered with oxygen.³⁴ The comparison between purely classical dynamics with quasiclassical dynamics accounting for ZPE perfomed by Busnengo et al.²⁵ on the clean W(110) surface showed a constant shift of the probability to higher values by ≈ 0.1 for incident energies up to 150 meV. For the W:N<sub>on</sub> surface, the ZPE energy is enough to overcome the observed barriers, and therefore a non-zero dissociation probability might arise even at low collision energy. For the W:N<sub>in</sub> surface, on the other hand, a shift as found in the previous work could be expected. However, the
qualitative comparison between the two different surfaces is not expected to change, that is,
the W:N<sub>in</sub> is expected to remain highly reactive for H<sub>2</sub> dissociation in comparison to W:N<sub>on</sub>.
Work is underway to precisely assess the influence of ZPE on H<sub>2</sub> chemisorption dynamics on
such surfaces.

Conclusion

The effect of nitrogen impurities on the H₂ dissociative sticking dynamics on the tungsten (110) surface is investigated by means of ab initio molecular dynamics based on density functional theory. Two distinct surface structures, previously proposed in the literature, are studied depending on the adsorption/absorption site of nitrogen. When it is absorbed into the sub-layer site, a surface reconstruction renders one displaced tungsten atom very reactive in terms of H₂ chemisorption. This reaction is conversely inhibited when nitrogen is adsorbed on the surface. The dynamics is rationalized in terms of the topology of the potential energy surfaces and compared with previous studies for clean and O-pre-covered W(110) surfaces.

Acknowledgement

The Pôle modélisation HPC of the Institut des Sciences Moléculaires in Bordeaux and
the TGCC (Très Grand Centre de Calcul du CEA, GENCI grant: 2023-AD010814394) as
well as the Atlas EDR of Donostia International Physics Center in Donostia are aknowl-
edged for computing time. This research has been conducted within the frameworks of the
QuantumChemPhys Transborder Common Laboratory Theoretical (ANR-10-IDEX-03-02)
and the DALTON ANR project (ANR-22-CE30-0013). The authors acknowledge finan-
cial support by the Gobierno Vasco-UPV/EHU Project No. IT1569-22 and by the Spanish
MCIN/AEI/10.13039/501100011033 (PID2022-140163NB-I00).
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