Dynamics of dissociative chemisorption of O2 on Cu(100) surface: A theoretical study

Surface Science 688 (2019) 45–50

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Dynamics of dissociative chemisorption of O2 on Cu(100) surface: A theoretical study

T

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L. Martin-Gondre ,a, C. Cresposb, P. Larrégarayb a b

Institut UTINAM UMR 6213, CNRS/Univ. Bourgogne Franche-Comté, Besançon, France Institut des Sciences Moléculaires UMR 5255, CNRS/Univ. Bordeaux, Talence, France

A R T I C LE I N FO

A B S T R A C T

Keywords: Density functional theory Gas/surface dynamics Oxidation processes Spin adiabaticity

The dynamics of O2 dissociative chemisorption on Cu(100) is studied by means of quasi-classical dynamics simulation making use of a 6-dimensional potential energy surface based on density functional theory (DFT) calculations. The sticking probability is found in reasonable agreement with experiment above 0.3 eV collision energy. However, theory fails at capturing the activated behaviour experimentally evidenced for lower energies. While molecular beam experiments exhibit an energetic threshold in the sticking curve, simulations show a high dissociation probability at collision energies below this threshold. Present calculations suggest that this discrepancy is due to dynamics governed by a direct dissociation mechanism steming from several barrierless reaction paths associated with an indirect dissociation mechanism governed by dynamic trapping. These direct and indirect components are strongly related to the structure of the PES questioning the reliability of the DFT calculations and the choice of the functional used. Beyond the question of the DFT accuracy, this theoretical work addresses the still open question of experiments/theory comparison for systems involving O2 and metal surfaces such as Cu(100).

1. Introduction Dissociative chemisorption of molecular oxygen on transition metal surfaces is a key step in heterogeneous catalytic oxidation. On metals, it is responsible, among other things, for oxide film formation and corrosion processes [1–3]. Despite this major issue in experimental surface science, the O2 dissociation mechanisms on metallic surfaces is still not fully understood [3,4]. A relevant example concerns the Al(111) surface. The experimental sticking curve exhibits the typical S-shape for an activated process [5] where dissociation probabilities starts to be non-zero when translational energy of the molecule gets higher than a threshold value corresponding to the lowest activation barrier. However, density functional theory (DFT) calculations, within the generalized gradient approximation (GGA), predict adiabatic dissociation paths without energy barriers [6] that lead to a high probability for dissociation at low collision energy resulting from dynamic trapping [7–9] and nonmonotonous sticking curve characterized by a minimum value of the dissociation probability for some intermediate energies. To solve the apparent contradiction between experiment and theory, Belher et al. have invoked spin non-adiabatic effects upon approach of the triplet O2 molecule towards the metallic surface [10–13]. They developed a spin-

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constrained model which was able, through dynamics simulation, to reconcile theory and experiments. This interpretation has been, however, recently challenged by studies based on embedded correlated wave function (ECW) calculations [14,15]. These studies point out the inaccuracy of DFT, implemented with semi-local exchange-correlation (XC) functionals, in describing the abrupt charge transfer into play in the system, responsible for energy barriers on the ground state Potential Energy Surface (PES). The latest adiabatic dynamics simulation based on a full-dimensional (6 dimensions) global PES relying on the ECW data points led to a near quantitative agreement with experiments, giving credit to this alternative interpretation [16]. For O2 dissociating on Al(111), it is thus not clear, to date, whether spin non-adiabatic effects or DFT inaccuracies in modelling charge transfer have to be invoked to reconcile theory and experiments. The dissociative chemisorption of O2 on copper surfaces is another puzzling example. Copper metals, of industrial and technological interest [3,17], are known to undergo catalytic oxidation following the dissociation of O2, thus leading to the formation of copper oxides such as CuO and Cu2O [3]. As Copper oxides exhibit appealing properties in material science, they are widely used as, for examples, photovoltaic solar cells, catalysts or supra-conductors [18–21]. O2 dissociative chemisorption dynamics critically depends on the considered

Corresponding author. E-mail address: [email protected] (L. Martin-Gondre).

https://doi.org/10.1016/j.susc.2019.05.006 Received 20 March 2019; Received in revised form 7 May 2019; Accepted 28 May 2019 Available online 28 May 2019 0039-6028/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1). The 6D global PES was obtained from the FPLEPS model [34–36] using more than 600 DFT points. By construction, this analytical model allows to parametrize separately the 1D potential for the isolated O2 molecule, the O/Cu(100) atom-surface 3D potential and the O2/Cu (100) molecule-surface 6D potential. The FPLEPS model is an extended version of the standard periodic LEPS function initially developed for gas-phase triatomic systems [45–48] and adapted for the first time to the gas-surface reactions by Sato [49,50] and later by McCreery and Wolken [51,52]. The 6D PES V (rOA, rOB) can be written as a function of the two vectors rOA (XA , YA, ZA) and rOB (XB , YB, ZB ) referring the position of the OA and OB atoms (see Fig. 1):

crystallographic plane. If the process is experimentally found non-activated on Cu(110) [22], it is activated on Cu(111) [23]. Molecular beam experiments have also shown that the dissociation of O2 on Cu (100) is activated [24–26]. For both (111) and (110) planes, such results have been theoretically rationalized in terms of adiabatic processes on the ground state electronic states computed via DFT implemented with GGA XC functionals [27,28]. For O2/Cu(100), electronic structure calculations [29–32] and ab initio molecular dynamics (AIMD) [33] simulations but with a limited number of trajectories have suggested the process to be non-activated. Despite the disagreement between those experimental and theoretical works for O2 dissociative chemisorption on Cu(100), the simulation of the dynamics using a full-dimensional (6D) PES based on DFT calculations is still lacking. We here implement such a study making use of the flexible periodic LEPS model (FPLEPS) [34–36] for fitting the PES and implementing Quasi-Classical Trajectories (QCT). We directly compare the dissociation probability obtained in our simulations with the experimental sticking curve and investigate the details of O2 chemisorption. We present the PES in Section 2, study the dissociation dynamics in Section 3 and concludes in Section 4.

V (rOA, rOB) = UOA Cu (rOA) + UOB Cu (rOB) + UO2 (∥rOB − rOA ∥) − {QO22 (∥rOB − rOA ∥) + (QOA Cu (rOA) + QOB Cu (rOB) )2 − QO2 (∥rOB − rOA ∥) (QOA Cu (rOA) + QOB Cu (rOB) ) }1/2 + A exp ⎡− ⎢ ⎣

(Z − z 0)2 ⎤ ⎥ σ2 ⎦

(1)

where Ui and Qi (i = O2, OACu, OBCu) represent the Coulomb and exchange integrals for two-body system, respectively: 2. Potential energy surface

Ui

The interaction energy between the O2 molecule and the Cu(100) surface is computed from spin-polarized DFT calculations within the generalized gradient approximation (GGA) and using the Perdew-Wang energy functional (PW91) [37]. All DFT calculations were carried out with the VASP code [38–40] that uses a plane-wave basis set for the electronic orbitals and the projector augmented wave (PAW) method [41] to describe the interaction of valence electrons with the atomic cores. The Cu lattice constant, obtained from a fcc bulk calculation, is a = 3.63 Å close to the experimental value of a = 3.615 Å [42]. A periodic supercell models the adsorbate/substrate system with a threelayer slab of 0.484a interlayer distance. The nearest neighbor distance on the (100) surface is δ = 2.57 Å (see Fig. 1) and the repetition distance along Z is 25.7 Å (=10 δ) to avoid non-physical interaction between neighboring slabs. O2/Cu(100) calculations were performed for a (2x2) unit cell using a (7x7x1) Monkhorst-Pack grid [43] of k-points to sample the Brillouin zone and a cutoff energy of 500 eV for the planewave basis set. An electron smearing of σ = 0.3 eV was also introduced within the Methfessel and Paxton approach [44]. The position and orientation of the O2 molecule with respect to Cu(100) surface is depicted by six coordinates: X, Y, and Z for the position of the molecular center of mass, r for the interatomic distance, and (θ, ϕ) respectively the polar and azimuthal angles for the orientation of the molecular axis (see

=

Di [(3 + Δi)exp(−2αi (ri − rieq)) − (2 + 6Δi)exp 4(1 + Δi) (−αi (ri − rieq))]

(2)

Qi =

Di [(1 + 3Δi)exp(−2αi (ri − rieq)) − (6 + 2Δi)exp 4(1 + Δi) (−αi (ri − rieq))]

(3)

where ri = rO2 for i = O2 and ri = ZA or ZB for i = OA Cu or OBCu. Di, αi and rieq are the Morse parameters, namely the potential energy well depth, the range of the interaction and the equilibrium distance between two bodies O2, OACu, or OBCu. The (X, Y) periodic dependence of the full 6D-potential is taken into account in the atom-surface Morse parameters by a Fourier expansion representing the fcc(100) symmetry. Six different positions (top, bridge, hollow, top-bridge, top-hollow, bridge-hollow) of the oxygen atom over the copper surface have been considered, as shown in Fig. 1, involving a fifth-order Fourier expansion. According to our previous work, modified Morse potentials are used to better describe the 1D O − O and O − Cu interactions by including a ri dependence in the αi parameters. For αOCu, a separate description is made between the attractive and the repulsive parts of the Morse potential giving a ri dependence for each branch [35]. Δi are the Sato parameters allowing to describe correctly the location and barrier heights along reaction pathways. The last term in Eq. (1) is a Gaussian function added to the global potential to improve the description of barriers and molecular wells (located in the entrance channel) where A, z0 and σ are the amplitude, width and position of the Gaussian function, respectively. Similarly to the Morse parameters, the Sato and Gaussian parameters are site-dependent. Moreover, a (θ, ϕ) angular dependence (see Fig. 1) is introduced for these molecule-surface parameters allowing improved flexibility. Three high-symmetry sites (top, bridge, hollow), three θ angles (0°, 45°, 90°) and three ϕ angles (0°, 45°, 90°) have been considered leading to 17 two-dimensional (r, Z) cuts (2D-cuts) used to optimize the Sato and Gaussian parameters, r defining the O-O distance and Z the molecule-surface distance. All the FPLEPS parameters are given in the Supplementary Material. DFT calculations for the O2 molecule are known to overestimate the binding energy [53,54]. Consequently, FPLEPS parameters for intramolecular O2 potential have been determined via coupled cluster ab initio calculations [55,56]. The computed binding energy (5.22) eV

Fig. 1. (Color online) (left panel) Definition of the coordinates: (X, Y, Z) define the center-of-mass coordinates of the O2 molecule and (r, θ, ϕ) are used to describe the ro-vibrational motion. (right panel) Cu(100) surface unit cell with δ representing the distance between nearest neighbor atoms. Blue squares and green dots indicate the higher and lower symmetry sites respectively used for the FPLEPS PES construction. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 46

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Fig. 2. (Color online) Contour plot of (r, Z) 2D-cuts of the O2/Cu(100) PES. Left panel: top site with θ = 90∘ and ϕ = 0∘ ; central panel: bridge site with θ = 90∘ and ϕ = 90∘ ; right panel: hollow site with θ = 90∘ and ϕ = 0∘ . The black thick solid line corresponds to zero potential energy. Blue thin solid (red dashed) contour lines correspond to positive (negative) values of potential energy (eV). Isovalues are plotted every 0.2 eV. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

adsorption takes place when r distance reaches the value r = 2.5 Å and the radial momentum is positive. Atomic absorption (or molecular absorption) is considered when an oxygen atom (or the center-of-mass of O2 molecule) reaches negative Z values with a negative momentum vector according to Z. After 50 ps, O2 is considered as trapped even if molecular adsorption process should not be observed as no molecular adsorption well is present in the 6D FPLEPS PES. Energy dissipation through phonon or electron-hole pair excitations is not considered in this work, according to the so-called static surface approximation.

compares very well with the experimental value (5.21) eV [57]. Regarding the O/Cu(100) potential, an analysis of the atom/surface potential topology shows an atomic adsorption occurring at the hollow site. This adsorption site is characterized by a minimum energy of − 4.94 eV at a distance of 0.85 Å from the copper surface (the zero potential energy is defined for the oxygen atom infinitely far from the metal surface). These results are consistent with experimental results giving an oxygen atom adsorbed at 0.8 Å above the first Cu layer [58] and with theoretical calculations leading to a distance of 0.81 Å along with an adsorption energy of − 5.13 eV [59]. To illustrate the topology of the full-6D global PES, contour plots of selected (r, Z) 2D-cuts are shown in Fig. 2 for different surface sites with the O2 molecule oriented parallel to the Cu(100) surface (θ = 90 °). From these selected configurations, no entrance barrier is found for the top site but a barrier in the dissociation path of about 0.5 eV shows up. Contrariwise, small entrance barriers are found for bridge and hollow sites (respectively 65 and 20 meV) but no dissociation barriers. From these 2D elbow plots, non-activated O2 dissociation pathways may be anticipated. For instance the approach of the O2 molecule above the top site, with parallel orientation, down to an altitude of Z = 2 Å from the surface followed by a displacement towards the bridge or hollow sites leads to direct dissociation. A thorough analysis of the 6D PES show the existence of several similar paths characterizing non-activated dissociation. Clearly, the DFT-PW91 based PES is attractive for approach and dissociation of O2 on the Cu(100) confirming previous works [30–32]. Nevertheless dissociation being a multidimensional process, a static analysis of the PES is not fully relevant [60,61] and molecular dynamics simulations are required in order to examine possible dynamical effects such as dynamic trapping mediated dissociation [7–9]. The O2/Cu(100) dynamics are here studied by means of quasiclassical trajectory (QCT) calculations. The zero-point energy of O2 (0.097 eV) is accounted for by semi-classically quantizing the molecule vibrational action [62]. The dissociation probability has been determined from 5000 trajectories with a conventional Monte-Carlo procedure to sample the initial (X, Y) position of the molecule over the unit cell and its internal coordinates (r, θ, ϕ). Normal and off-normal incidence are considered in this work. Each trajectory starts at Zini = 6 Å from the surface and is propagated until a total integration time of 50 ps unless the following events are observed: reflection, dissociation and absorption. Reflection process occurs whenever the molecule goes back towards vacuum and reaches its initial distance to the surface Zini with a momentum vector pointing to vacuum. Dissociative

3. Dynamics of O2 dissociation on Cu(100) As mentioned above, molecular beam techniques have been used to study the O2 interaction with Cu(100) for a surface temperature of 200K [26] and 300K under normal [24] and 15° incidence [25] with respect to surface normal. Experimental sticking probabilities as a function of normal incidence energy (Ez) are displayed in Fig. 3 along with the present QCT results. As anticipated from the PES topology, calculations predict a high sticking probability at vanishing collision energy and a non monotonous sticking curve typical of a non-activated behavior, with a minimum around 0.3 eV. Conversely, the experimental sticking probabilities increase monotonously after a 0.03 eV threshold up to a plateau. The activated character observed experimentally might involve the existence of energy barriers in the entrance channel that are not reproduced with DFT-PW91 calculations as shown elsewhere for the Cu(111) surface [28]. At higher energies (Ez > 0.25 eV), the agreement between molecular dynamics results and experimental data of Junell et al. [26] is satisfactory even if our simulations are performed within the static surface approximation.The recent work of Ramos et al. [28] for Cu(111) shows that dissociation probability barely changes with surface temperature, thus suggesting a weak coupling between the O2 molecule and copper surface. Moreover, experiments show that the increase of surface temperature leads only to a slight increase of the sticking probability [24,26]. Therefore, the theoretical results for O2 dissociation on Cu(100) above Ez = 0.25 eV are expected to be valid. The low collision energy discrepancies between theory and experiments are not expected to stem from the static surface approximation because, to the best of our knowledge, temperature has not been shown to induce such drastic change in the shape of the sticking curve in the past [63,64]. Previous theoretical studies based on DFT-PW91 calculations have not be able to reproduce early energy barriers [30–32]. The existence of 47

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Fig. 4. (Color online) Probability for O2 to reach a given Z altitude as a function of normal incidence energy EZ. The insets represent the position of the molecule center of mass over the Cu(100) surface unit cell for these specific Z distances at collision energy of 0.5 eV. The θ polar angle distribution is also displayed.

Fig. 3. (Color online) Sticking probability of O2 on Cu(100) as a function of normal incidence energy EZ. The total (black solid line), direct (green dashed line) and indirect (red dashed-dotted line) components are represented (see text). Open symbols correspond to experimental results: blue circles [24], cyan diamond [25], orange squares [26]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Molecules, that have lost too much normal kinetic energy to go back into vacuum, are trapped near the surface and undergo rebounds before encountering a dissociation pathway. The non monotonous behavior of the sticking curve then originates from dynamic trapping. Furthermore, the direct contribution is significant even at very low energies attesting the existence of reactive pathways that not require significant reorientation of the molecule (see Fig. 2). A more comprehensive analysis is revealed by studying the probability for O2 to reach a given Z altitude as a function of normal incidence energy Ez (see Fig. 4). As apparent from the figure, all O2 molecules reach the Z = 2.5Å altitude, whatever the incident energy Ez. Between 2.5 and 2Å, a non negligible number of molecules are reflected for intermediate collision energies (around 0.25 eV). The probability of O2 to reach the Z = 2 Å distance has a non-monotonous behavior similar to the final probability showing that indirect process (see Fig. 3) linked to the dynamic trapping play a role in the entrance channel. Finally, at Z = 1.5Å from the surface, molecules with low energies will be systematically dissociated whereas at higher energies, molecules need to reach a distance between 1 and 1.5Å before dissociating. Insets representing the (X, Y) distributions of the center-of-mass of O2 for various Z distances and θ polar angle distributions are also depicted in Fig. 4 for 0.5 eV collision energy. These data show that, except around the top sites, a large part of the molecules can access to all regions over the unit cell for Z < 2.5Å. Moreover, the O2 approach toward the surface involves a preferred orientation with θ values around 90° before accessing to the dissociation pathways. These insets confirm the direct character of the dissociative dynamics observed at this collision energy (see Fig. 3). A similar analysis has been performed for a lower collision energy (0.05 eV) for which the indirect contribution is significant (not shown). In that case, a parallel orientation of the molecule is observed from 3.5Å down. Nevertheless, even for this low collision energy, O2 molecule can approach most surface sites down to Z = 2 Å before undergoing drifting to specific sites allowing dissociation. The results, reported in Fig. 3 and Fig. 4, reveal a relative simplicity of the dissociative dynamics of O2 on Cu(100) reflecting the FPLEPS PES topology that shows open entrance channels and attractive dissociation channels. Influence of incidence angle Θs (defined as the angle between the velocity vector of the center-of-mass of the molecule and a direction normal to the copper surface) on sticking probability is also highlighted in Fig. 5 as a function of total incidence energy Etot. Results indicate a small variation of the dissociation rate for angles lower than 45°. For higher Θs incidence angles, the sticking probability becomes different at

steering effect that leads molecule out of reach of sites allowing direct dissociation has been suggested to explain the experimental threshold [32]. Clearly, the dynamics results from the present work are not in favor of this interpretation. The DFT PES seems to be too open in the entrance channels to prevent dissociation process at low energies. As suggested by Ramos et al. [28], the shortcomings at low collision energy could originate from the choice of the XC functional. A study on the N2/W(100) and N2/W(110) systems has also highlighted the variability of DFT energies according to the choice of the XC functional [65]. By comparing PESs built within PW91 and RPBE [66] functionals, the authors showed that the DFT-RPBE calculations might lead to a more repulsive PES in the entrance channels, corresponding to the approach of the molecule toward the metal surface. Therefore, dynamics results might significantly differ according to the XC functional chosen for the PES construction. Following that line, the use of a PES constructed from RPBE-DFT calculations yielded an activated behavior for the dissociation of O2 on Cu(111) [28]. In this context, preliminary DFT calculations relying on the RPBE functional have been performed for the O2/Cu(100) system. The results are in line with previous works. As an example, for the configuration at the bridge site in Fig. 2, an energy barrier of 280 meV located at Z = 2 Å has been obtained with the RPBE functional whereas this barrier is of 65 meV with the PW91 functional. Further study, which is beyond the scope of the present work, would be necessary to build a global RPBE-DFT-based 6D PES and perform dynamics calculation. A thorough analysis of the dynamics can be achieved by decomposing the total sticking probability into direct and indirect contributions (Fig. 3). The separation between both contributions is based on the number of rebounds of the molecule on the surface in the course of the collision. A rebound is defined as a change of the center-of-mass momentum in Z from pointing toward the surface to pointing toward the vacuum. The number of rebounds to make this decomposition, which is arbitrary and has been discussed elsewhere [67], is chosen to be 4 as in our previous work [36]. The decomposition indicates that the indirect contribution, which originates from dynamic trapping of the O2 molecule, is significant at low energies (Ez < 0.30 eV) but decreases very quickly with increasing collision energy. The so-called dynamic trapping mechanism is associated with the PES anisotropy and corrugation that produce an energy transfer from translational kinetic energy normal to the surface into other molecular degrees of freedom.

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4. Conclusions The dynamics of the dissociative adsorption of O2 on the Cu(100) surface has been studied theoretically with the use of a full six dimensional DFT-PES based on PW91 functional. Within this framework, sticking probability is representative of a non-activated system in contradiction with experiment. A detailed analysis of the dynamics shows that the dissociative molecular adsorption is dominated by a direct process and that dynamic trapping only play a role for low collision energies. This direct process is linked to the topology of the FPLEPS PES characterized by very open entrance channels. The lack of early energy barriers, which is suggested by the experimental activated behavior, could stem from the specific choice of the XC functional, non adiabatic spin effects, or inaccuracies of DFT to account for charge transfers. Further work is required to solve this still open question. Acknowledgments Fig. 5. (Color online) Sticking probabilities of O2 on Cu(100) as a function of total incidence energy Etot for different incidence angles Θs.

The authors gratefully acknowledge A. Salin and G. Volpilhac for providing us with the DFT potential energies. Computer time was provided by the Pole Modelisation computing facilities of the Institut des Sciences Moléculaires (UMR5255, CNRS/U. Bordeaux), partly funded by the Nouvelle Aquitaine Region and the Mésocentre de Calcul, a regional computing center at the Université de Franche-Comté. Supplementary material Supplementary material associated with this article can be found, in the online version, at https://doi.org/10.1016/j.susc.2019.05.006 References [1] T. Zambelli, J. Barth, J. Winterlin, G. Ertl, Complex pathways in dissociative adsorption of oxygen on platinum, Nature 390 (1997) 495–497. [2] C. Stampfl, M.V. Ganduglia-Pirovano, K. Reuter, M. Scheffler, Catalysis and corrosion: the theoretical surface-science context, Surf. Sci. 500 (2002) 368–394. [3] M. Montemore, M.A. van Spronsen, R. Madix, C. Friend, O2 activation by metal surfaces: implications for bonding and reactivity on heterogeneous catalysts, Chem. Rev. 118 (2816–2862) (2018). [4] H. Over, A.P. Seitsonen, Oxidation of metal surfaces, Science 297 (2002) 2003. [5] L. Österlund, I. Zoric, B. Kasemo, Dissociative sticking of O2 on Al(111), Phys. Rev. B: Condens. Matter Mater. Phys. 55 (1997) 15452–15455. [6] K. Honkala, K. Laasonen, Oxygen molecule dissociation on the Al(111) surface, Phys. Rev. Lett. 84 (2000) 705–708. [7] C. Crespos, H.F. Busnengo, W. Dong, A. Salin, Analysis of H2 dissociation dynamics on the Pd(111) surface, J. Chem. Phys. 114 (2001) 10954. [8] H.F. Busnengo, C. Crespos, W. Dong, A. Salin, J.C. Rayez, Role of orientational forces in nonactivated molecular dissociation on a metal surface, Phys. Rev. B 63 (2001) 041402. [9] H.F. Busnengo, C. Crespos, W. Dong, J.C. Rayez, A. Salin, Classical dynamics of dissociative adsorption for a nonactivated system: the role of zero point energy, J. Chem. Phys. 116 (2002) 9005. [10] J. Behler, B. Delley, S. Lorenz, K. Reuter, M. Scheffler, Dissociation of O2 at Al(111): The role of spin selection rules, Phys. Rev. Lett. 94 (2005) 036104. [11] J. Behler, K. Reuter, M. Scheffler, Nonadiabatic effects in the dissociation of oxygen molecules at the Al(111) surface, Phys. Rev. B 77 (2008) 115421. [12] C. Carbogno, J. Behler, A. Gross, K. Reuter, Fingerprints for spin-selection rules in the interaction dynamics of O2 at Al(111), Phys. Rev. Lett. 101 (2008) 096104. [13] C. Carbogno, J. Behler, K. Reuter, A. Gross, Signatures of nonadiabatic O2 dissociation at Al(111): first-principles fewest-switches study, Phys. Rev. B 81 (2010) 035410. [14] F. Libisch, C. Huang, P. Liao, M. Pavone, E.A. Carter, Origin of the energy barrier to chemical reactions of O2 on Al(111): evidence for charge transfer, not spin selection, Phys. Rev. Lett. 109 (2012) 198303. [15] F. Libisch, C. Huang, E.A. Carter, Embedded correlated wavefunction schemes: theory and applications, Acc. Chem. Res. 47 (2014) 2768–2775. [16] R. Yin, Y. Zhang, F. Libisch, E.A. Carter, H. Guo, B. Jiang, Dissociative chemisorption of O2 on Al(111): dynamics on a correlated wave-function-based potential energy surface, J. Phys. Chem. Lett. 9 (2018) 3271–3277. [17] R.J. Madix, J.T. Roberts, The Problem of Heterogeneously Catalyzed Partial Oxidation: Model Studies on Single Crystal Surfaces, Surfaces Reactions 34 Springer-Verlag, 1994. [18] L.C. Olsen, F.W. Addis, W. Miller, Experimental and theoretical studies of Cu2O solar cells, Solar Cells 7 (1982) 247. [19] B. Cure, B. Blau, D. Campi, L.F. Goodrich, I.L. Horvath, F. Kircher, R. Liikamaa, J. seppala, R.P. Smith, J. Teuho, L. Vieillard, The superconducting strand for the

Fig. 6. (Color online) Sticking probabilities of O2 on Cu(100) as a function of normal incidence energy EZ for O2 molecule initially in various ro-vibrational states. Comparison between direct and indirect contributions for the ro-vibrational ground state (v = 0, j = 0 ) and a ro-vibrational excited state (v = 0, j = 30 ) is shown in the inset.

medium and high energies. These findings point out that dissociative dynamics does not follow a total energy scaling (TES) except at lower energies (Etot < 0.2 eV). At these lower energies, indirect contribution due to dynamic trapping involves a redistribution of the total incidence energy leading to a loss memory of the initial conditions. The only factor thus influencing the dynamics is the initial total energy of the molecule. Finally, the effect of the initial ro-vibrational state of the O2 molecule on sticking probability is depicted in Fig. 6. For lower rotational excitation (j ≤ 10), effect on dissociation probability is negligible. This is not the case for high rotational excitation (j ≥ 30) where sticking probability strongly decreases for lower energies due to a significant drop of the indirect process as shown in the inset of Fig. 6. This effect, already noticed for H2/Pd(111) system [68], is related to a less efficient redistribution of normal collision energy to rotational degrees of freedom that already have a significant amount of energy ( j = 30 corresponds to a rotational energy of about 160 meV). O2 molecule then keeps enough translational energy along Z coordinate to escape the PES attraction and goes back into vacuum. Therefore, for highly excited rotational states, dissociation dynamics mainly involves direct dissociation mechanism.

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