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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Hot Atom Mediated Dynamical Displacement of CO Adsorbed on Cu(111) by Incident H Atoms: an Ab Initio Molecular Dynamics Study Xueyao Zhou, Liang Zhang, and Bin Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04123 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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The Journal of Physical Chemistry

Hot Atom Mediated Dynamical Displacement of CO Adsorbed on Cu(111) by Incident H Atoms: an Ab Initio Molecular Dynamics Study Xueyao Zhou, Liang Zhang, Bin Jiang* Hefei National Laboratory for Physical Science at the Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China

*: corresponding author: [email protected]

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Abstract Non-thermal reactions between gaseous particles and adsorbates on a solid surface commonly exist at gas-surface interface. Besides the well-known “Eley-Rideal” abstraction reaction, the adsorbate can also be exchanged by the gaseous species via dynamic displacement. We present here the first ab initio molecular dynamics study on the dynamical displacement of CO adsorbed on Cu(111) by incident H atoms. Our results agree reasonably well with the measured displacement cross section, product energy, and angular distributions. More importantly, we explicitly demonstrate that this displacement takes place via a hot atom mechanism. The energetic H atom, accelerated by its chemisorption energy near the surface, could cause CO desorption in various ways accompanied by a surprisingly large energy dissipation to the surface phonons. As the CO-Cu binding is weakened, the metastable HCO intermediate is possibly formed. This scenario provides a common picture for similar processes with a strong repulsion between the pre-adsorbed and impinging species.

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I.

INTRODUCTION Chemical reactions at the gas-surface interface are essential in many processes

such as corrosion, semiconductor processing, and heterogeneous catalysis.1 Such reactions induced by collisions of the gas-phase species on solid surfaces could proceed with a variety of pathways. A majority of gas-surface reactions take place with adsorbed reagents in thermal equilibrium on the surface with the so-called Langmuir-Hinschelwood (LH) mechanism. However, there are certain non-thermal reactions that occur between an energetic projectile from the gas phase and an adsorbate more or less directly. Such reactions are indeed in many ways similar to the already well-studied A + BC type reaction in gas phase (assuming C is a surface). For example, the Eley–Rideal (ER) mechanism proposed long time ago2 can be actually attributed to the direct abstraction of an adsorbate by a gas phase particle and denoted as A+BC→AB+C. Indeed, ER reactions have been commonly observed for H/D + D*/H*,3-7 H + Cl*,8-10 and N/O + N*,11, 12 on various metal surfaces, and theoretical characterizations of the ER dynamics have been quite successful.13-18 It has been argued LH and ER mechanisms represent two limiting cases and in practice the impinging atom (or molecule) may not catch the adsorbed species by a single collision, but keep hopping on the surface for some time before finally abstracting the adsorbate away, which is known as the hot atom (HA) mechanism.19 On the other hand, analogous to the exchange reaction in gas phase, i.e. A+BC→B+AC, the adsorbate can also be replaced by the incident atom (or molecule), as first demonstrated by Rettner and Lee by H/N/O atoms colliding with the O2 pre-covered 3



Pt(111) surface.20 Although similar observations have been later confirmed for systems like H + CO/Cu(111),21 H + N2/Ru(0001),22 CO/O + O2/Pt(111),23, 24 this so-called dynamic displacement (DD) mechanism severely lacks a theoretical interpretation and is much less understood. The hydrogenation of CO adsorbed on metal surfaces is of great importance in the conversion of syngas (the mixture of H2 and CO) into hydrocarbons and oxygenated compounds via various metal-based catalysts. For example, Co and Ru are active catalysts for the Fisher-Tropsch synthesis to produce light olefins25 while methanol production is favored with Cu catalysts.26 However, the mechanism of these complicated catalytic processes remains poorly understood. There was spectroscopic evidence that atomic H could drive the chemisorbed CO to form the HCO intermediate via the ER mechanism on Ru(0001) at surface temperature of 100 K.27, 28 In a joint theoretical and experimental study, however, Yates and coworkers observed no HCO intermediate but only the displacement of CO by atomic H on the same surface at slightly higher surface temperature.29 On Cu(111), Rettner and Auerbach investigated experimentally the dynamics of the displacement of pre-covered CO by incident H atoms, in which the product energy and angle distributions of desorbed CO have been measured.21 They speculated that DD is most likely induced electronically by the change of electronic structure of the metal substrate with the adsorption of the incident atom. By contrast, Luntz and coworkers22 later suggested a phonon-mediated process in the N2 displacement from Ru(0001) by incident H(D) atoms, which is supposed to be similar to the H+CO/Cu(111) system. In this work, theoretically, we 4


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explore for the first time the mechanism and dynamics of this reaction, i.e., H(g) + CO(a)/Cu(111)→CO(g) + H(a)/Cu(111). We take advantage of the ab initio molecular dynamics (AIMD) approach that evaluates the forces and energies on the fly bypassing the need to build a high-dimensional potential energy surface. In addition, the surface motion is explicitly included to account for the energy exchange between the adsorbate and surface phonons. AIMD calculations have been successfully applied to explore the reaction mechanism30, 31 and dynamics in variety of gas-surface reactions.32-36 A small number of AIMD trajectories with electronic friction (AIMDEF)34, 37 have also been carried out to approximately account for the influence of surface electron-hole pairs (EHPs). Our goal here is to understand the energy exchange and the mechanism of the DD process. II.

METHODS Total energy calculations were performed with the spin-polarized DFT in Vienna

Ab initio Simulation Package (VASP).38, 39 The Cu(111) surface was constructed by a 4 layers of 2 × 2 unit cell with a vacuum spacing of 15 Å. The top three layers metal atoms were allowed to relax. Generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional was used40 and the ion-electron interactions were described by using the projector-augmented wave (PAW) method.41 The kinetic energy cutoff for a plane wave basis set was 400 eV. The k-points was generated via the Monkhorst-Pack procedure with the 5 × 5 × 1 mesh.42 Saddle points were determined by the climbing image nudged elastic band (CI-NEB)43 method. AIMD simulations were run in the conditions as close to the experimental ones21 5



as possible. To that end, the CO-precovered Cu(111) surface (1/4 ML) were equilibrated at surface temperature (Ts) of 90 K by running the surface only simulations in the NVE ensemble prior to the incidence of the H atom, and the snapshots after equilibration were randomly taken as initial configurations representing the surface. The position of H atom was initiated at 6 Å above the surface and randomly distributed in the unit cell for lateral coordinates. The incidence angle was set at 15° off the surface normal according to the experimental condition and the azimuthal angle is randomly sampled from 0° to 360°. The mean incidence energy was 0.07 eV and the experimental velocity distribution44 was considered. One thousand trajectories were computed up to 2 ps with a time step of 0.5 fs. The total energy was well conserved within ~30 meV for most trajectories. A trajectory was considered as: (I) “CO displacement” if the CO molecule desorbed and reached 6.1 Å above the surface. (II) “H scattered”, if the H atom was reflected back with the CO molecule remained adsorbed on the surface. (III) “Trapped”, if no product flied out when the maximum propagation time was reached. For the displaced CO in the gas phase, the vibrational action number was determined via the Einstein-Brillouin-Keller (EBK) semiclassical quantization method.45 The traditional histogram binning (HB) was used to extract the vibrational quantum number. Additional two hundred trajectories with an incidence energy of 0.6 eV were performed with other setups kept the same as above to account for the energy dependence of reaction mechanism. The influence of electronic excitation was approximately considered by AIMD with electronic friction (AIMDEF) calculations 6


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carried out using a modified VASP program, in which the local density friction approximation (LDFA) combined with the independent atom approximation (IAA) is implemented.34, 37, 46 In the electronic friction theory,47 the nonadiabatic effect on the nuclei dynamics is approximated as a friction force that is responsible for the energy dissipation to electron hole pairs. Specifically, the following generalized Langevin equation48 is used:

mi

d 2Ri dR ∂V (R ) =− − ηi (R i ) i + Fi , 2 dt ∂R i dt

(1)

where Ri are the coordinates of the ith atom in the molecule with mi as its mass, and V (R) is the adiabatic potential energy which is calculated on the fly in this work,

Fi , represents the temperature-dependent random fluctuation. In the LDFA, the atomic friction constants ηi (Ri ) are approximately regarded as the friction coefficients when these atoms are embedded in a homogeneous free-electron gas.49, 50 In the IAA, in which the friction coefficient of each atom in the molecule is calculated independently to be that of this atom embedded in the homogeneous free electron gas with the electron density equivalent to that on the bare surface.51 This AIMDEF treatment has been successfully applied to previous studies on relaxation of hot atoms and molecules34 and Eley-Rideal reactions.36 In this work, 200 AIMDEF trajectories were carried out under the same experimental condition. III.

RESULTS AND DISCUSSION Figure 1 depicts the static reaction energy profile associated with the incidence

of an H atom on a CO-covered Cu(111) surface. More detailed geometric parameters and energetics of stationary points are presented in the Table 1. Both CO and H prefer 7



to absorb at fcc site, with binding energies of -0.85 eV and -2.45 eV (relative to atomic H in gas phase), respectively. As a result, the displacement of CO by H is thermodynamically favorable with a high exothermicity of ~1.60 eV. There are two HCO* adsorption states (where * represents an adsorbed state) with close structure and energy, and a COH* isomer with higher energy. The two HCO* intermediates are connected via a very low barrier, however, the isomerization barrier from HCO* to COH* is quite high (~1.54 eV), making the H shift from C to O kinetically difficult. Interestingly, we are not able to find a reaction pathway to form HCO* by the gaseous hydrogen directly approaching the adsorbed CO* because of the strong repulsion between the two species. Instead, H* and CO* have to overcome a notable barrier of 0.87 eV to form HCO*-1 (one of the two HCO* adsorbed configuration) and 2.33 eV to form COH*, while the HCO* intermediate itself easily decomposes back to co-adsorbed H* + CO* via a barrier of only ~0.08 eV. It should be noted that the desorption of HCO* which competes with the decomposition is highly endothermic and thus expected to be very unfavorable. This explains the displacement of CO rather than the desorption of HCO (ER or HA product) with the incidence of the H atom observed in experiment21 and our simulations (see below). Also highly endothermic is breaking the C-O bond leading to the CH* + O* product, which has to go through a very high barrier of 1.69 eV. Overall, our results are in good accord with previous DFT results for H + CO interacting with Cu surfaces.52-54 We note in passing that DFT energies for the reactants and products using revised PBE (RPBE) functional55 are shown in parentheses in Figure 1. We find the relative energy differences of various 8


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products are similar using both functionals, consistent with the conclusion of Su et al. in Ref. 53. One thousand trajectories was calculated for up to 2 ps at Ts=90 K and Ei=0.07 eV. Figure 2a displays the initial lateral positions of H atoms in the 2×2 unit cell corresponding to different outcomes under this condition. Almost half of the incident hydrogen (453 trajectories) are reflected back to the vacuum and most of them are focused on an area centered at the adsorbed CO* with a radius of ~2.0 Å. This can be attributed to the tiny mean kinetic energy of the H atom compared the extremely high barrier for the incident H approaching the surface nearby CO*, as shown in Figure 3. Lin et al. found a similar phenomenon in the low energy hydrogen scattering on the CO2 adsorbed Ni(110) where three-fourths of H atoms are reflected.30 Once the H atoms are able to reach the surface bypassing the CO*, they can be vastly accelerated by the gas-surface interaction.30 These hot H atoms with excess energy can attack the CO* on the surface leading to its desorption. Indeed, 51 trajectories with CO displaced by H are observed, yielding a 5% displacement probability. Considering the 22.92 Å2 surface area in our model, this corresponds to a cross section of (1.17 ± 0.16) Å2 per H atom, which is in excellent agreement with the experimental value (~1 Å2 / H atom).21 Consistent with our static DFT results, neither the desorbed HCO nor C-O dissociative products are found. The remaining trajectories are thus considered to be trapped within the 2 ps timescale, although a minor fraction of adsorbed H atoms may not be completely thermalized yet. Let us next turn to the dynamics of the displacement process. In the experiment 9



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of Rettner and Auerbach,21 the internal and translational energy distributions of the desorbed CO were found to be far from thermal equilibrium (i.e. Ts=90 K). This implies that the displacement occurs concurrently with the adsorption of H and is therefore “dynamic” rather than “thermodynamic”.22 This scenario is reasonably reproduced by AIMD simulations, yielding translationally and internally hot products. Figure 4a-d compares available experimental data with these calculated translational, vibrational, rotational energy, and angular distributions of CO, respectively. It is found that the calculated mean translational energy of CO (0.31 eV) is even higher than the experimental value (0.22 eV). The energy distribution also shifts to higher energies, compared to the experimental one estimated by a translational temperature of 1300 K.21 On the other hand, a broad vibrational energy distribution is seen in Figure 4b, implying some vibrational excitation. Indeed, 7 out of 51 displacement trajectories produce CO(ν=1), resulting in the CO(ν=0)/CO(ν=1) ratio of 6.3 ± 2.5. This value is close to the experimental value ~6.0 at low CO* coverage (0.05 ML) corresponding a vibrational temperature greater than 1300 K.21 However, the experimental ratio increases rapidly with the coverage and goes up to ~50 at saturation (~0.44 ML).21 The experimentally estimated rotational temperature for CO(ν=0) is relatively low at Trot=390±20 K,21 corresponding to the mean rotational energy good agreement with the calculated

Erot

Erot

of 0.07 eV, in

value of 0.09 eV. The rotationally cold

and translationally hot product, as observed in experiment, indicates that the repulsive forces that drive the CO fly out from the surface is largely used to accelerate the center of mass of the linear molecule.21 Figure 4d shows the angular distribution of 10


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CO where θf is the polar angle of the CO velocity off the surface normal. Within the statistical errors, the experiment-theory agreement is fairly good. The distribution is relatively sharp, consistent with the “dynamic” nature of this process. The remaining theory-experiment discrepancies may stem from the uncertainties of the DFT energies. To further uncover the reaction mechanisms, we analyze the time-dependent features for “CO displacement” trajectories. The displacement of CO by H occurs in a broad timescale ranging from ~500 fs to 1.8 ps (Figure 5a). However, even in the fastest DD process, the incident H atom follows neither the ideal ER nor the direct collision induced desorption (CID) mechanism,56 due apparently to the strong repulsive force around CO as discussed above. Instead, while approaching the surface, the H atom acquires a large kinetic energy converted from its adsorption energy making itself highly mobile. This hot H atom thus triggers the desorption of CO via a HA-like mechanism prior to being thermalized. Specifically, both geometric and energetic information for several representative trajectories are traced as a function of time in Figure 6, including the C-H distance (rCH), the vertical heights of H (ZH) and CO (ZCO), as well as the kinetic energies of H, CO, and Cu atoms, respectively. In Figure 6a-b, for example, the hot H atom approaches quickly as close as ~1 Å to the adsorbed CO* before thermalizing with the surface. Taking advantage of the strong repulsion between the two species, this contact induces a fast energy transfer from the energetic H* to CO* and drives CO* out of the surface concurrently at ~200 fs. Alternatively, the hot H atom may 11



penetrate into the surface and dissipate much of its energy to the surface phonons creating a local lattice “hot spot”. This strong local phonon excitation then causes the desorption of CO*, as illustrated in Figure 6c-d. Interestingly, these two trajectories present two limiting cases of the hot precursor mechanism suggested by Luntz and coworkers in explaining their experimental results for DD of N2 from Ru(0001) by incident H(D) atoms.22 In a more general case, e.g. in Figure 6e-f, the atomic H collides with CO* in a relatively mild way, i.e., rCH is oscillating about ~2 Å with a minimum greater than ~1.6 Å. As a result, the adsorbed CO* is gradually pushed away from the most favorable adsorption site with its geometry tilted or parallel to the surface, giving rise to a weaker binding with the surface. Meanwhile, hydrogen loses its excess energy to the CO-lattice system, which thermalizes itself and thermally desorbs CO*. It is found that a majority of displacement trajectories undergo these three reaction pathways. More interestingly, the hot H* with high kinetic energy can also overcome the remarkable barrier to combine with CO* yielding the HCO* intermediate. Figure 6g-h presents an example in which H attempts to bind with CO for a few times and starts to form HCO* at ~600 fs. However, HCO* produced in this way is not stable, which decomposes back to H* + CO* after 700 fs, in which the released energy becomes the driving force for the subsequent CO desorption. Four trajectories with CO displaced are found to encounter HCO* intermediate with a 50 fs or longer lifetime. In addition, we also observed that many trapped trajectories involve the recombination and decomposition of HCO*. The lifetime of HCO* can be as large as 1.8 ps within the 12


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longest time scale of our calculations. The formation of temporary HCO* intermediate was speculated by Rettner and Auerbach21 and is now confirmed. Given the HA mechanism discussed above, significant energy exchange from the adsorbate to the substrate is expected. Indeed, since this process involves the exchange reaction of adsorbed and gaseous species, unlike the atomic/molecular scattering on surface, the energy transfer in a DD process is less well understood. The total energy transfer to the lattice can be evaluated by the static exothermicity for H + CO/Cu(111)→CO + H/Cu(111) ( ∆Er = 1.60eV ) plus the initial incidence energy of H ZPE atom (Ei) and the zero point energy (ZPE) of the pre-adsorbed CO ( ECO* , 0.18 eV),

and minus the total energy for the desorbed CO (ECO(g)) and the ZPE of the product ZPE ZPE ZPE − ECO( g ) − EH* H* ( EH* , 0.17 eV), i.e., Eloss = ∆Er + Ei + ECO* . In Figure 7a, we plot

the total energy transfer distribution for mean Ei=0.07 eV, which is rather broad ranging from 0.4 eV to 1.5 eV, consistent with the reaction time distribution with both fast and slow processes. There is on average a large amount of energy (~1.15 eV), which is ~68% of the available energy (the sum of the exothermicity and incidence energy), dissipated into the substrate phonons. The remaining energy flows into the CO molecule, leading to the largely translational and modestly internal excitations. At the first glance, this is very surprising that such a substantial energy transfer way exceeds the limit estimated by the Baule model.57 In this model, the estimated energy loss is Eloss =

4 µ ( Ei + Eads )

(1 + µ )

2

, where µ is the mass ratio of H and Cu atoms and Eads

is the adsorption energy of atomic H. Nevertheless, one should note that the Baule model assumes the binary collision of two hard spheres, which is certainly incapable 13



of describing the complicated DD process. As observed in Figure 6, the hot H atom could repeatedly strike on and transfer its energy to CO* or lattice along with the energy exchange between CO* and lattice as well, resulting in a complex trinary system. As a result, the large energy transfer to surface phonons reflects the non-binary nature of this process. In order to consider the dependence of the reaction mechanism on the incidence energy, we ran additional 200 AIMD trajectories with the incidence energy of H atom increased to 0.6 eV. Figure 2b shows in this case that the H atom can access more closely to the adsorbed CO*. Increasing the kinetic energy of the incident projectile significantly facilitates the DD process and enhances the displacement cross section (σ=(2.87 ± 0.54) Å2) by three times. As shown in Figures 7b, compared to the results of Ei=0.07 eV, more than half of the increased incidence energy (0.35 eV) is dissipated to the surface phonons and the rest mainly flows to the vibrational excitation (0.13 eV) of CO (Figure 8). Compared to the results at Ei=0.07 eV, the mean translational, vibrational, and rotational energies of the desorbed CO increase to 0.36, 0.26, and 0.09 eV, which are about 16.4%, 11.6% and 3.9% of the available energy, respectively, while the angular distribution still looks similar. Even the available energy is now higher than the ER channel by ~1.0 eV, no HCO desorption is seen. H atoms remain adsorb on the surface first and then displace the CO* adsorbate via the several reaction pathways discussed above (Figure 9). This comparative set of AIMD trajectories confirms the dominant role of the HA mechanism in this process. Our results present direct evidence for the hot precursor mechanism proposed by 14


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Luntz and coworkers in a similar system, i.e. H(g) + N2(a)/Cu(111)→N2(g) + H(a)/Ru(0001).22 In that work, it was suggested that the electronic excitation does not play an important role in their system, thanks to the observed isotope effect that D atoms are about twice as effective as H in displacing the adsorbed N2*. To explore the role of EHPs in the title system, 200 AIMDEF34, 37 trajectories were run under the same experimental condition. No displacement at all was observed within 2 ps, as the electronic friction opens an additional channel for energy dissipation, which probably brings the system below the energy available for the DD process. For those trapped trajectories, we also find similar ways for energy transfer to surface phonons via the hot atom mechanism (Figure 10). These results seem consistent with the argument of Luntz and coworkers,22 at least in the electronic friction framework. But further investigations may be necessary using a more sophisticated way to account for the charge transfer between the surface and adsorbate. This is however not straightforward in direct dynamics simulations and requires the calculations of the negative ion state and associated couplings between adiabatic states. Recent studies on vibrational deexcitation of NO on Au and Ag surfaces have been reported, using the independent-electron surface hopping approach with pre-parametrized potential energy surfaces.58-61 Our work also provides side evidence for the experiments of hydrogenation of CO on Ru surfaces,27-29 which are related to the Fischer-Tropsch synthesis. Our AIMD results lend support to the experiment of Yates et al.29 who observed no formyl intermediate but only CO desorption induced by the atomic H beam, given the 15



similarly low HCO decomposition barriers predicted on Cu(111) and Ru(0001) in Ref. 29. Interestingly, Ashwell et al. found a low possibility of forming short-lived COH*,31 in recent AIMD simulations of the translationally hot H atom impinging on the CO covered Ni(110). These results indicate that the possibility of ER or DD as well as the stability of intermediate may sensitively depend on the energetics and kinetics of the related reaction pathways. IV.

CONCLUSION To summarize, we present the first AIMD study for DD of the pre-adsorbed CO

on Cu(111) by atomic hydrogen. Our AIMD results reasonably reproduce the experimental reaction cross section, product energy, and angular distributions. More importantly, with the real time illustration of different possible pathways in this system, we identify that DD takes place exclusively via the hot atom mechanism undergoing various pathways. The release of chemisorption energy of the projectile forms the first necessary step, followed by sufficiently large energy transfer from hydrogen to either the adsorbed CO*, or Cu atoms, or the CO-lattice entirety that ultimately causes CO desorption. It is emphasized here that the repulsive interaction between the pre-adsorbed species and the hot atom plays an essential role to assist the energy transfer. In addition, no ER product (HCO desorption) is found but DD may occur via an HCO intermediate. It is therefore believed that DD and ER reactions may widely coexist and compete in gas-surface systems, whose preference is largely dependent on the energetics and kinetics. For example, the displacement of Cl* by the incident H atom on Au(111) becomes a minor channel in addition to the ER reaction 16


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desorbing HCl, owing to much larger exothermicity (~2.67 eV) of ER than that (0.08 eV) of DD.36,

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This HA mechanism, suggested experimentally in the N2

displacement from Ru(0001) by incident H(D) atom and verified theoretically in this work, is expected to be common in DD processes where a strong repulsion exists between the pre-adsorbed and impinging species.

Acknowledgements: This work was supported by National Key R&D Program of China (2017YFA0303500), National Natural Science Foundation of China (91645202, 21722306, and 21573203), and Anhui Initiative in Quantum Information Technologies. We are grateful to the Supercomputing Center of USTC and AM-HPC for offering us high-performance computing services.

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Table 1. The calculated geometric parameters, adsorption energies (Eads) and relative barrier energies (Eb) of species shown in Figure 1 in the main text. Adsorption energies are given by Eads = Etot − Emole − Eslab , where Etot is the total energy of slab with the adsorbate, Emole is molecular energy in gas phase and Eslab is the energy of clean surface. Each barrier height is defined as the energy difference between the corresponding transition and initial states. Za (Å) Species rCO (Å) rCH (Å) θHCO (°) CO* 1.182 1.491 H* 0.987 CH* 1.099 1.239 O* 1.189 HCO*-1 1.221 1.218 113.0 1.616 HCO*-2 1.264 1.116 115.8 1.734 COH* 1.345 1.923 28.5 1.318 TS1 1.199 1.511 107.9 1.574 TS2-HCO 1.243 1.136 116.8 1.670 TS2-COH 1.301 1.378 57.8 1.391 TS3b 2.032 1.095 87.8 1.303 a The closest distance of the adsorbate above the surface. b The transition state for HCO*→CH* + O*

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Eads (eV) -0.85 -2.45 -5.12 -4.98 -1.20 -1.29 -2.86 -

Eb (eV) 0.87 0.04 1.54 1.69

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Figure 1. Structures and energetics (in eV) for possible stationary points involved in the reaction: H(g) + CO(a)/Cu(111)→CO(g) + H(a)/Cu(111), where * denotes the adsorbed species. Energetics computed with RPBE functional are given in parentheses.

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Figure 2. Initial lateral positions of H atoms in the 2×2 unit cell for “CO displacement”, “H scattered” and “Trapped” trajectories at two incidence energy (a) Ei=0.07 eV, (b) Ei=0.6 eV. The circles with the radius of ~2.0 Å (a) and ~1.0 Å (b) are guides for the eyes.

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Figure 3. Relative potential energy of H as a function of height above the Cu(111) surface from top, bridge and fcc sites nearest to the adsorbed CO.

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Figure 4. Translational energy (a), vibrational energy (b), rotational energy (c) and scattering angle (d) distributions of desorbed CO compared with the experimental data when available from Ref. 21. In panel (d), the black curve is a guide to the eye taken from the experimental fit : f (θ f ) = 0.63cos5 θ f + 0.37 cos θ f .

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Figure 5. Distribution of reaction time for “CO displacement” trajectories at (a) Ei=0.07 eV and (b) Ei=0.6 eV.

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Figure 6. Geometric and kinetic energy information of four representative AIMD trajectories a function of time, including the C-H distance (rCH, black), the vertical heights of H (ZH, blue) and CO (ZCO, red), as well as the kinetic energies of H, CO, and Cu atoms, respectively. Each pair of panels, i.e., a/b, c/d, e/f, g/h corresponds to each representative trajectory. The green dashed-dotted lines mark the time of CO desorption.

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Figure 7. Total energy loss to surface phonons in the dynamic displacement: H(g) + CO(a)/Cu(111)→CO(g) + H(a)/Cu(111), at Ei=0.07 eV (a) and Ei=0.6 eV (b).

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Figure 8. Translational energy (a), vibrational energy (b), rotational energy (c) and scattering angle (d) distributions of desorbed CO at Ei=0.6 eV.

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Figure 9. Geometric and kinetic energy information of four representative AIMD trajectories a function of time, including the C-H distance (rCH, black), the vertical heights of H (ZH, blue) and CO (ZCO, red), as well as the kinetic energies of H, CO, and Cu atoms, respectively. Each pair of panels, i.e., a/b, c/d, e/f, g/h corresponds to each representative trajectory. The green dashed-dotted lines mark the time of CO desorption. Ei=0.6 eV.

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Figure 10. Geometric and kinetic energy information of four representative AIMDEF trapped trajectories (no CO desorption) a function of time, including the C-H distance (rCH, black), the vertical heights of H (ZH, blue) and CO (ZCO, red), as well as the kinetic energies of H, CO, and Cu atoms, respectively. Each pair of panels, i.e., a/b, c/d, e/f, g/h corresponds to each representative trajectory. Ei=0.07 eV.

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TOC graphic:

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by Incident H Atoms - American Chemical Society

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