C. Crespos

Analysis of H2 dissociation dynamics on the Pd(111) surface

We perform a detailed analysis of the dynamics of the dissociative adsorption of H2 molecules on a Pd(111) surface using ab initio data for the molecule–surface interaction and classical trajectory methods. We show that the reaction probability is completely determined by the molecule–surface interaction in the approach toward the surface before it reaches a critical distance of 1.5 A. The corresponding dynamics can be reduced to a 2D one, involving only the translational and rotational degrees of freedom, except in the lower energy range where an important role is played by dynamic trapping. We establish the relation between the dissociation probability and the shape of 2D cuts of the potential energy surface using a simple model of the evolution of orientational forces as the molecule approaches the surface. Whereas above 1.5 A the molecule evolves “as a whole,” below 1.5 A the dynamics has the character of independent atom–surface interactions which explains why it dissociates with a probability close ...

Classical dynamics of dissociative adsorption for a nonactivated system: The role of zero point energy

We present dissociative adsorption probabilities of H2 on Pd(111) computed with the classical trajectory method. We perform both classical (C) and quasiclassical (QC) calculations, the latter including, by contrast with the former, the initial zero point energy (ZPE) of H2. We analyze in detail the role played by the ZPE and demonstrate the strong and weak points of both C and QC calculations. We show that ZPE is crucial in accelerating the molecules toward the surface through vibrational softening. However, at low energies, dynamic trapping is quenched in QC calculations by processes of vibration to rotation energy transfer that would be associated with closed channels in a quantum approach. In this study we use a new representation of the H2/Pd(111) potential energy surface (obtained by interpolation of ab initio data) with a significantly better accuracy in the entrance channel region which plays a decisive role in the dissociation dynamics.

DFT Study of Dissociative Adsorption of Hydrogen Sulfide on Cu(111) and Au(111)

Density functional theory (DFT) is used to investigate the reaction pathways for H2S adsorption on Au(111) and Cu(111) at low coverage as well as the full decomposition of H2S on Cu(111). On both surfaces, a weakly bonded molecular state is found with the S atom bond on top sites being molecular adsorption, a nonactivated process. The H-SH dissociation process is endothermic on Au(111), and all reaction pathways present high activation energy barriers which explains the extremely low dissociation probability of H2S on defect-free Au(111) estimated from experiments. This scenario slightly changes for H2S/Cu(111): (i) dissociated configurations are energetically more favorable than the molecular state and (ii) the H-SH bond cleavage process presents a relatively small activation energy barrier. This is not inconsistent with low but nonzero reactive sticking probability of thermal H2S molecules reported in experiments. The complete energy profile for the H2S adsorption and full decomposition is compatible with the accumulation of S-adatoms observed experimentally.

Dynamics simulation of N2 scattering onto W(100,110) surfaces: A stringent test for the recently developed flexible periodic London-Eyring-Polanyi-Sato potential energy surface

An efficient method to construct the six dimensional global potential energy surface (PES) for two atoms interacting with a periodic rigid surface, the flexible periodic London-Eyring-Polanyi-Sato model, has been proposed recently. The main advantages of this model, compared to state-of-the-art interpolated ab initio PESs developed in the past, reside in its global nature along with the small number of electronic structure calculations required for its construction. In this work, we investigate to which extent this global representation is able to reproduce the fine details of the scattering dynamics of N(2) onto W(100,110) surfaces reported in previous dynamics simulations based on locally interpolated PESs. The N(2)/W(100) and N(2)/W(110) systems are chosen as benchmarks as they exhibit very unusual and distinct dissociative adsorption dynamics although chemically similar. The reaction pathways as well as the role of dynamic trapping are scrutinized. Besides, elastic/inelastic scattering dynamics including internal state and angular distributions of reflected molecules are also investigated. The results are shown to be in fair agreement with previous theoretical predictions.

Multiconfiguration time-dependent hartree method applied to molecular dissociation on surfaces : H2+Pt(111)

Four-dimensional quantum dynamics calculations are performed on the dissociative chemisorption of H(2) on Pt(111) using the multiconfiguration time-dependent Hartree method. The aim of this work is to study the performance of the multiconfiguration time-dependent Hartree method for a gas-surface reaction by comparison with the standard time-dependent wave-packet propagation method. The initial-state resolved dissociation probability of H(2) is calculated within two four-dimensional models. The first four-dimensional model treats explicitly the rotational motion of the molecule and the H(2) dissociation is studied above two different sites of the Pt(111). For this model, only a potential-energy surface of general form was available. This potential was refitted to a sum of product form to allow efficient calculations with the multiconfiguration time-dependent Hartree method. The second model focuses on the description of the center-of-mass motion parallel to the surface, the rotational motion of the molecule being frozen. These four-dimensional quantum dynamics calculations yield important insights which can help with performing full six-dimensional calculations on H(2) dissociating on Pt(111). The multiconfiguration time-dependent Hartree method is shown to be particularly efficient for computing initial-state selective dissociation probabilities for the system studied, with a good accuracy and a reduced amount of memory and computational time when compared to the standard time-dependent wave-packet method.

Surface temperature effects on the dynamics of N2 Eley-Rideal recombination on W(100)

Quasiclassical trajectories simulations are performed to study the influence of surface temperature on the dynamics of a N atom colliding a N-preadsorbed W(100) surface under normal incidence. A generalized Langevin surface oscillator scheme is used to allow energy transfer between the nitrogen atoms and the surface. The influence of the surface temperature on the N(2) formed molecules via Eley-Rideal recombination is analyzed at T = 300, 800, and 1500 K. Ro-vibrational distributions of the N(2) molecules are only slightly affected by the presence of the thermal bath whereas kinetic energy is rather strongly decreased when going from a static surface model to a moving surface one. In terms of reactivity, the moving surface model leads to an increase of atomic trapping cross section yielding to an increase of the so-called hot atoms population and a decrease of the direct Eley-Rideal cross section. The energy exchange between the surface and the nitrogen atoms is semi-quantitatively interpreted by a simple binary collision model.

Dynamical reaction pathways in Eley-Rideal recombination of nitrogen from W(100)

The scattering of atomic nitrogen over a N-pre-adsorbed W(100) surface is theoretically described in the case of normal incidence off a single adsorbate. Dynamical reaction mechanisms, in particular Eley-Rideal (ER) abstraction, are scrutinized in the 0.1-3.0 eV collision energy range and the influence of temperature on reactivity is considered between 300  and 1500 K. Dynamics simulations suggest that, though non-activated reaction pathways exist, the abstraction process exhibits a significant collision energy threshold (0.5 eV). Such a feature, which has not been reported so far in the literature, is the consequence of a repulsive interaction between the impinging and the pre-adsorbed nitrogens along with a strong attraction towards the tungsten atoms. Above threshold, the cross section for ER reaction is found one order of magnitude lower than the one for hot-atoms formation. The abstraction process involves the collision of the impinging atom with the surface prior to reaction but temperature effects, when modeled via a generalized Langevin oscillator model, do not affect significantly reactivity.

Recombination and chemical energy accommodation coefficients from chemical dynamics simulations: O/O2 mixtures reacting over a β-cristobalite (001) surface

A microkinetic model is developed to study the reactivity of an O/O(2) gas mixture over a β-cristobalite (001) surface. The thermal rate constants for the relevant elementary processes are either inferred from quasiclassical trajectory calculations or using some statistical approaches, resting on a recently developed interpolated multidimensional potential energy surface based on density functional theory. The kinetic model predicts a large molecular coverage at temperatures lower than 1000 K, in contrary to a large atomic coverage at higher temperatures. The computed atomic oxygen recombination coefficient, mainly involving atomic adsorption and Eley-Rideal recombination, is small and increases with temperature in the 700-1700 K range (0.01 < γ(O) < 0.02) in good agreement with experiments. In the same temperature range, the estimated chemical energy accommodation coefficient, the main contribution to which is the atomic adsorption process is almost constant and differs from unity (0.75 < β(O) < 0.80).

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

Dynamics of the Eley-Rideal (ER) abstraction of H2 from W(110) is analyzed by means of quasi-classical trajectory calculations. Simulations are based on two different molecule-surface potential energy surfaces (PES) constructed from Density Functional Theory results. One PES is obtained by fitting, using a Flexible Periodic London-Eyring-Polanyi-Sato (FPLEPS) functional form, and the other by interpolation through the corrugation reducing procedure (CRP). Then, the present study allows us to elucidate the ER dynamics sensitivity on the PES representation. Despite some sizable discrepancies between both H+H/W(110) PESs, the obtained projectile-energy dependence of the total ER cross sections are qualitatively very similar ensuring that the main physical ingredients are captured in both PES models. The obtained distributions of the final energy among the different molecular degrees of freedom barely depend on the PES model, being most likely determined by the reaction exothermicity. Therefore, a reasonably good agreement with the measured final vibrational state distribution is observed in spite of the pressure and material gaps between theoretical and experimental conditions.

Hydrogen abstraction from metal surfaces: when electron–hole pair excitations strongly affect hot-atom recombination

Using molecular dynamics simulations, we predict that the inclusion of nonadiabatic electronic excitations influences the dynamics of preadsorbed hydrogen abstraction from the W(110) surface by hydrogen scattering. The hot-atom recombination, which involves hyperthermal diffusion of the impinging atom on the surface, is significantly affected by the dissipation of energy mediated by electron-hole pair excitations at low coverage and low incidence energy. This issue is of importance as this abstraction mechanism is thought to largely contribute to molecular hydrogen formation from metal surfaces.