H₂ Chemisorption on W(100) and W(110) Surfaces

Abstract

Chemisorption of H and H 2 on clean W(100) and W(110) surfaces is investigated from extensive density functional theory (DFT) calculations within the generalized gradient approximation (GGA). We obtain properties of the clean surfaces [e.g., the well-known reconstructed structure of W(100) below 200 K] as well as adsorption energies and geometries of H atoms chemisorbed on both faces of W in very good agreement with available experimental data. From our DFT-GGA results, we build accurate six-dimensional potential energy surfaces (PESs) which are used to compute dissociative sticking probabilities of H 2 on both faces of W through classical trajectory calculations. For both systems, our theoretical results agree with the available experimental data at low impact energies and are larger than experiments at higher energies. The main signatures of the sticking probabilities found in molecular beam experiments are reproduced, at least qualitatively, by our calculations: for instance, (i) the larger values for W(100) than for W(110), (ii) the nonmonotonic and monotonic energy variation for W(100) and W(110), respectively, and (iii) the dependence on incidence angle as a function of W face and impact energy. Our calculations show that dissociative adsorption of H 2 on both W(100) and W( 110) is a nonactivated process. In contrast with the widely accepted assumption that H 2 chemisorption on W( 110) is a purely direct process, we obtain that, at very low energies, adsorption takes place through an indirect (dynamic trapping) and a direct mechanism on both surfaces. Thus, the qualitatively different behavior of the sticking probability at low energies arises from a smaller contribution of dynamic trapping in the case of H 2 /W(110), due to slight differences between the corresponding PESs far from the surface in the entrance channel.

Dynamics of H2 Eley-Rideal abstraction from W(110): Sensitivity to the representation of the molecule-surface potential

“…11,37 More recently, Rutigliano and Cacciatore 38 investigated the ER abstraction process for H+H/W(100) by using a tight binding approximation for the PES 39,40 that allowed them to consider explicitly not only the six degrees of freedom of the H 2 but also the dynamical coupling with tungsten phonons. Still, a word of caution must be given about the accuracy of the PES employed in the latter study since it predicts the fourfold hollow site as the most stable for H adsorption, in contrast with experiments 41 and Density Functional Theory (DFT) calculations 42 for which the lowest energy adsorption site at low H coverage is the bridge site. To what extent such an error in the PES might affect the outcome of the ER dynamics and in particular, the rovibrational-state distribution of the nascent H 2 molecules, is unclear.…”

“…42 They were carried out with the Vienna Ab initio Simulation Package (VASP) [95][96][97][98][99] within the slab supercell approach and using the generalized gradient approximation (GGA) proposed by Perdew and Wang (PW91) for the exchange-correlation functional. 100 Plane wave basis set is used for the description of electronic wave functions with a cut-off energy of 230 eV.…”

“…To extend the CRP PES of Ref. 42, both non-spin polarized (NSP) and spin polarized (SP) DFT calculations were required. Indeed, the description of the ER entrance channel (the target atom adsorbed in its equilibrium position and the projectile atom approaching from the gas phase) requires taking into account the spin magnetization of the impinging H atom.…”

“…The angular interpolation over (θ , φ) is performed using a symmetry adapted expansion of trigonometric functions and a Fourier series is employed to describe the (X, Y) dependence of the molecular parameters (Gaussian an Sato parameters). The molecular parameters were fitted on 2D-(Z CM , r) DFT cuts computed by Busnengo et al 42 The details for the implementation of the FPLEPS can be found in Ref. 56.…”

“…One of the PESs (hereafter referred to as FPLEPS) is a result of a global analytical fitting 52, 53 using a generalization [54][55][56] of the LondonEyring-Polanyi-Sato (LEPS) function [57][58][59] widely used in the past to investigate H+H ER abstraction processes on metal surfaces. 12,14,20,21,25,43,44,46 The other PES (hereafter referred to as CRP) has been obtained by interpolation using the corrugation reducing procedure 60 which has been widely used 42,49,[60][61][62][63][64][65][66][67][68][69][70][71][72][73][74] and successfully gauged against AIMD results 49,75 of dissociative adsorption probabilities but scarcely used to investigate ER abstraction processes. It has to be mentioned that CRP has already been used in the context of recombination process but for modeling the LangmuirHinshelwood mechanism (LH), 76 which can be seen as the reverse mechanism of dissociative adsorption.…”