Article pubs.acs.org/JPCC
Isotope Effects in Eley−Rideal and Hot-Atom Abstraction Dynamics of Hydrogen from Tungsten (100) and (110) Surfaces R. Pétuya,*,†,‡,§ M. A. Nosir,∥ C. Crespos,†,‡ R. Díez Muiño,§,∥ and P. Larrégaray†,‡ †
Université de Bordeaux, ISM, CNRS UMR 5255, Talence 33405 Cedex, France CNRS, ISM, UMR5255, F-33400 Talence, France § Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018 Donostia-San Sebastián, Spain ∥ Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU), Paseo Manuel de Lardizabal 5, 20018 Donostia-San Sebastián, Spain ‡
S Supporting Information *
ABSTRACT: The influence of isotopic substitutions on the recombination dynamics of molecular hydrogen under normal incidence scattering of hydrogen isotopes on H(D,T)-precovered W(100) and W(110) surfaces is investigated. Quasiclassical trajectory simulations on density functional theory based potential energy surfaces are first performed within the single adsorbate limit, thus focusing on the Eley−Rideal abstraction. Significant isotope effects show up regardless of surface symmetry. Homonuclear recombinations (H-on-H, D-on-D, and T-on-T) lead to very similar cross sections, whereas for heteronuclear processes (H-on-D, D-on-H, ...), cross sections are ordered by the mass ratio between the impinging atom and the adsorbate. The diatom energy partitioning is also affected by isotopic substitution. Similar effects, though less pronounced, appear for hot-atom abstraction on W(110) at θ = 0.25 ML surface coverage.
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energy ( 0.75 Å. For the other, the projectile bounces below the target Z < 0.75 Å. The contribution of the former subset is significantly lower for the H-on-D combination (0.042 Å2) than for H-on-H (0.112 Å2), D-on-D (0.123 Å2), or D-on-H (0.157 Å2) combinations. Therefore, the low reactivity of the H-on-D combination mainly stems from an important lowering of ER abstraction via rebound of the projectile at the same altitude as the target. This suggests that for a light projectile (H) to efficiently transfer normal energy to a heavy target (D) it has to collide from below via a push-up mechanism. Besides, the efficiency of sideway collisions is strongly affected by the mass ratio between projectile and target. In that case, unreactive trajectories mainly end up in the reflection channel (see Supporting Information), as observed in reduced dimensional models.3,14 The higher reactivity of the D-on-H combination, compared to the homonuclear cases, seems to result in a greater global efficiency of the projectile−target collision to displace the target, independent of Z. Similar rebound opacity maps are shown in Figure 5 for the W(110) reticular plane. As for W(100) symmetry, all isotopic combinations exhibit similar (X,Y) rebound areas suggesting that the dynamical pathways governing H2 ER abstraction on W(110)30 are similar regardless of the isotopic combination. Consistently with the W(100) symmetry, the ER reactivity is low for trajectories with small impact parameters, and ER pathways proceed via collision of the projectile off a W surface atom prior to recombination with the target. However, the density of the reactive areas evolves depending on isotopic combination. For the H-on-D combination, of lower reactivity, some areas almost disappear, whereas they significantly increase for the more reactive D-on-H combination. One clear illustration is that the contribution of the reactive area originating from projectiles
Figure 5. Rebound opacity maps for the W(110) reticular plane: (X,Y) rebound positions of the projectiles for the trajectories leading to ER recombination at initial energy of the projectile Ep = 1.0 eV. For sake of clarity, only 1/5 of the trajectories are displayed.
rebounds close to the W atom in the middle of the upper W(110) unitary cell, 1.085 Å < X < 2.085 and 1.0 Å < Y < 2.0 Å. This area, representing 0.011 Å2 (0.014 Å2) for the homonuclear combination H2 (D2), highly decreases (increases) to contribute for only (up to) 0.002 Å2 (0.034 Å2) for the H-on-D (D-on-H) isotopic combination. Figure 6 displays the distributions of the altitudes of the projectiles and targets (Z), at first projectile’s rebound for ER reactive trajectories at Ep = 1.0 eV for the W(110) symmetry. Comparison with Figure 4 reveals that the shape of the distributions is highly similar on both planes. As for the W(100) symmetry, the contribution of reaction involving sideway collisions (0.015 Å2) for the H-on-D combination is sensibly lower than for the homonuclear combination (0.057 Å2 and 0.047 Å2, respectively, for H-on-H and D-on-D). This confirms that, for both surface symmetries, a light projectile must attack a heavy target from below to efficiently displace it. Unlike for the W(100) plane, this contribution is here significantly higher (0.110 Å2) for the D-on-H combination than for homonuclear recombination. Looking at the opacity maps (Figure 5) and at the distributions of the Y coordinate of the projectiles at the rebound position (see Supporting Information), the higher reactivity of the D-on-H combination mostly stems from an increase of reactivity in the upper (Y > 0.0 Å) W(110) unitary cell. A similar investigation has confirmed that previous rationalization of the recombination mechanisms also applies to isotopic combinations involving hydrogen and tritium atoms. 15328
DOI: 10.1021/acs.jpcc.5b03693 J. Phys. Chem. C 2015, 119, 15325−15332
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than on W(110), is thus system dependent and appears in contradiction with experimental results at very low collision energy (0.075 eV) for H2 recombination on Cu(111).47 Conversely, rotational energies seem almost unaffected. However, such detailed observables might be rather sensitive to the details of the PES.5 Abstraction Dynamics from W(110) at Finite Coverage. To investigate the effect of isotopic substitutions on the HA mechanism, simulations have been performed at θ = 0.25 ML coverage for the W(110) reticular plane. The cross sections for the above-defined exit channels are plotted in Figure 8 as a function of Ep. Competition between primary HA process and ER abstraction appears independent of the isotopic combination. At low collision energy, abstraction is dominated by the primary HA process, about four times more efficient than ER abstraction. As Ep increases, primary HA abstraction decreases rapidly, whereas ER abstraction remains almost constant up to Ep = 2.0 eV, before slightly decreasing. Both ER and HA processes then involve similar reactivity for collision energies higher than 2.5 eV. As apparent from Figure 8, ER cross sections for θ = 0.25 ML coverage are higher than that of the previous subsection. Such a feature stems from two origins. First, at finite coverage, ER recombinations with targets initially adsorbed outside the projectile sampling area are available.31 In addition, the single adsorbate study has been here performed with FPLEPS PES, whereas the multiadsorbate potential was developed from a CRP PES, which leads to higher ER reactivity as previously investigated.29 The influence of isotope effects observed in single adsorbate studies is also present here. Homonuclear combinations of isotopes lead to similar cross sections, whereas the reactivity for heteronuclear combinations is ordered by the (mp/mt) ratio. In agreement with previous studies,7,16,23 total abstraction is thus higher in the case of a heavy projectile and a light target than the reverse combination. However, in agreement with refs 23 and 43, but on the contrary to refs 7, 16, and 35, proportions between primary abstraction (ER + primary HA) and secondary HA for the H-on-D and D-on-H combinations are very close at low collision energies. For instance, at Ep = 0.1 eV (1.0 eV), secondary HA represents 5.7% (11.2%) of the recombination for H-on-D and 6.3% (11.0%) for D-on-H, whereas it contributes to 5% for H-on-D and 22% for D-on-H on Cu(111).16 Figure 9 displays the vibrational distributions of D2 resulting from experiments46 and the present simulations. In the experiments, a room-temperature polycrystalline tungsten sample is exposed to hydrogen atoms previously produced by dissociation of hydrogen molecules on a tungsten filament at 2000 K. Single adsorbate simulations for W(100) and W(110) surfaces at 300 K have been performed within the GLO,78−81 whereas finite coverage simulations for W(110) used the BOSS model. In every case, a thermal averaging87 of the simulated vibrational population has been performed as detailed in refs 30 and 87. Populations for levels higher than v = 5 are almost negligible. The results are in reasonable agreement with experiments, in particular for W(110) when taking into account all abstraction processes with θ = 0.25 ML surface coverage as previously observed for H2 recombination.31 Nevertheless, the experimental distributions are slighlty colder than the theoretical ones. The difference could stem from energy dissipation to e−h pairs upon abstraction. Investigations on that issue are underway.
Figure 6. Distributions of projectile (color code described in the text) and target (black) altitudes at first rebound of the projectile for ER abstraction trajectories on W(110) at the initial energy of the projectile Ep = 1.0 eV.
Figure 7. Final average translational, rotational, and vibrational energies as a function of Ep (eV). Final average translational (upper panel), rotational (middle panel), and vibrational (lower panel) energies as a function of the initial energy of the projectile, Ep (eV).
As evidenced in Figure 7, isotopic substitutions also affect the partition of final available energy among the degrees of freedom of the nascent molecule: the final average translational, rotational, and vibrational energies are displayed as a function of the initial perpendicular energy of the projectile. ER products are vibrationally and rotationally excited. At low collision energy, in agreement with Rettner and Auerbach experiments,1,2,47 more than half of the exothermicity is channeled into internal energy (see Supporting Information). However, as the initial collision energy increases, most of the final available energy is transferred to diatom translation. As for reactivity, homonuclear combinations behave similarly on both reticular planes. In the Ep = 0.1− 5.0 eV energy range, the D-on-H combination exhibits higher translational energy and a lower vibrational energy than the H-on-D combination. This effect, more pronounced on W(100)
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CONCLUSION QCT simulations of hydrogen isotope recombination under normal incidence scattering are performed for both W(100) and 15329
DOI: 10.1021/acs.jpcc.5b03693 J. Phys. Chem. C 2015, 119, 15325−15332
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The Journal of Physical Chemistry C
Figure 8. Cross sections (Å2) as a function of the initial energy of the projectile Ep (eV) for different isotopic combinations at θ = 0.25 ML coverage.
Similar isotope effects also appear for HA abstraction. Final energy partition also depends on the heavy−light/projectile− target combination. Vibrational distributions of the D 2 recombined molecules are found in reasonable agreement with recent experiments.
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ASSOCIATED CONTENT
S Supporting Information *
Effects of surface temperature on Eley−Rideal recombination of D2 and T2 on W(100) and W(110), reflection cross sections, distributions of the Y coordinate of the projectile at the rebound position for the W(110) surface, and final translational, rotational, and vibrational energies of the recombined molecules. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b03693.
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Figure 9. Relative vibrational populations of the recombined molecules. Green and blue triangles represent the ER single adsorbate results at a surface temperature of 300 K, respectively, on W(100) and W(110), and black squares represent the total abstraction results at θ = 0.25 ML within the BOSS model for W(110). Experimental results are in red dots. Lines are drawn to guide the eye. Results for excitation higher than v = 6 (population about 10−4 and lower) are not displayed.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Phone: +34943015421. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the support of France Grilles for providing computing resources on the French National Grid Infrastructure and the Mésocentre de Calcul Intensif Aquitain (MCIA). R.P., C.C., and P.L. acknowledge ECOS-sud program for funding. MAN and RDM acknowledge financial support by the Gobierno Vasco - UPV/EHU (Grant No. IT756-13) and the Spanish Ministerio de Economiá y Competitividad (Grant No. FIS2013-48286-C02-02-P).
W(110) crystallographic planes in the single adsorbate limit and with a surface coverage of θ = 0.25 ML for the W(110) plane. DFT-based potentials are used to model the atomic interactions, and the influence of collision energy is studied in the range Ep = 0.1−5.0 eV. In agreement with a previous work,30 ER abstraction proceeds via collision with a tungsten surface atom prior to recombination, regardless of the isotopic combination and surface symmetry. Nevertheless, significant isotope effects are observed on both W(100) and W(110) planes. Over the entire energy range, cross sections are ordered by the (mp/mt) ratios, with mp and mt, respectively, being the masses of the projectile and the target. This effect is connected to the higher (lower) reflection probability of a light (heavy) projectile from a heavy (light) target. In particular, the contribution of sideway collisions significantly decreases with the mass ratio, whereas the push-up mechanism becomes the main recombination mechanism.
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DOI: 10.1021/acs.jpcc.5b03693 J. Phys. Chem. C 2015, 119, 15325−15332
