Origin of Steric Effects in Methane Dissociative Chemisorption - The

Article pubs.acs.org/JPCC

Origin of Steric Effects in Methane Dissociative Chemisorption Bin Jiang*,†,‡ and Hua Guo*,‡ †

Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China Department of Chemistry and Chemical Biology, University of New Mexico, Albuquerque, New Mexico 87131, United States

‡

ABSTRACT: Steric effects in chemical reactions, namely, the dependence of reactivity on reactant spatial orientation and/or alignment, provide valuable information on the anisotropic potential energy surface and access to the transition state. However, a theoretical interpretation of steric effect is often not straightforward because of convolution of the dynamics by many other factors. Here, we present a theoretical study on the dissociative chemisorption of methane using a quasi-classical trajectory method on a density functional theory based global potential energy surface and offer a theoretical interpretation of the interesting but surprising steric effect in this prototypical and industrially relevant gas−surface reaction.

I. INTRODUCTION The reactivity of a chemical reaction involving a gas phase molecule is often an average over many important properties of the reactant, including its kinetic energy, internal energy, impact parameter, and orientation. With the advent of molecular beam and laser techniques, the relative speed and internal excitation of reactants can now be exquisitely controlled,1−4 thus eliminating the energy averaging. Such control has revealed important information on how translational, vibrational, and rotational energies of the reactants affect the reactivity through the reaction transition state.5−8 Significant advances have also been made in studying reaction dynamics involving oriented or aligned molecules,9−16 which promise to further revolutionize our knowledge on reactivity because they (partially) remove the orientational averaging,4,11 thus offering insights into the steric access of the anisotropic transition state along the reaction path.17 Very recently, Beck and co-workers reported steric effects in a prototypical gas−surface reaction.13,14 Specifically, these authors observed that the dissociative sticking probability changes with the initial alignment of the rovibrationally excited CH4 and CHD3 with respect to the surface, prepared by a linearly polarized infrared laser.13,14 Different from bimolecular reactions in the gas phase, gas−surface reactions have the distinct advantage that the transition state is well-defined in the laboratory frame. As a result, reactant molecule can in principle be oriented or aligned in such a frame to gain better access to the reaction channel. Interestingly, the reactivity was found to be the largest (smallest) when the laser polarization is parallel (perpendicular) to the surface plane, while the transition state features the cleaving C−H bond with a ∼45° angle with the surface plane. The observed steric effects thus appear to be inconsistent with the notion that the polarization angle dependence of the reactivity should reflect the transition state geometry. So far, the experimental data have defied any © XXXX American Chemical Society

theoretical interpretation. In this publication, we present a theoretical analysis that offers an explanation of the experimental observation, thus shedding light on the dynamical origin of the surprising steric effects. Thanks to its importance in industrial steam reforming,18 the dissociative chemisorption (DC) of methane on transitionmetal surfaces has become one of the most studied gas−surface reactions.3,19 By use of advanced laser, molecular beam, and high vacuum techniques, quantum state resolved experiments have been performed. These state-of-the-art measurements demonstrated strong influence of translational energy and vibrational excitations on the DC of CH4 and its isotopologues on various metal surfaces.20−34 In general, stretching excitations of methane were found to be more effective than their bending counterparts and, in some cases, more than the translational energy in promoting the DC. These observations underscored the nonstatistical nature of this gas−surface reaction, in which energies in different degrees of freedom (DOFs) are not equal in promoting the reaction.6 These pioneering experiments on methane DC provided unprecedented details for this gas− surface reaction but also stringent tests for theoretical models. Even with a rigid-surface model, 15 DOFs are involved, making this system very challenging for theoretical characterization. The involvement of hydrogens in the dissociation in principle requires a high-dimensional quantum mechanical treatment of the dynamics, particularly at low collision energies. In the past decade, extensive theoretical efforts have been devoted to understand the multidimensional interaction potential energy surface (PES) and dissociation dynamics.35−50 These studies have qualitatively or semiquantitatively reproduced the experimentally observed mode specificity and bond selectivity Received: February 25, 2016 Revised: March 28, 2016

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DOI: 10.1021/acs.jpcc.6b01951 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C as well as surface effects.8,51,52 Furthermore, it has been shown that the mode specific and bond selective DC reactions can be rationalized by a simple transition-state-based model.7 The key to understanding the surprising steric effects observed experimentally is a proper treatment of the methane rotational DOFs because they dictate the spatial distribution of the reactant. So far, there have been only a few theoretical studies on the influence of rotational excitations on the DC process and most earlier models treated CH 4 as a pseudodiatom.35,36,39 This is because performing explicit quantum dynamical (QD) calculations for rotationally excited states of a polyatomic molecule is very difficult due to the large basis size and the (2J + 1) degeneracy.53 Very recently, Nattino et al. reported full-dimensional ab initio molecular dynamics (AIMD) calculations on the steric effect for the CHD3/Pt(111) system at a single collision energy,50 but no strong steric effect was found. However, this absence of strong steric effect could be due to the high energies or the insufficient number of the AIMD trajectories. More recently, Füchsel et al. reported a reduced dimensional quantum dynamical study of the rotational effect in the same system with five DOFs, and the results suggested that the bending/rotational effects are important in the dissociation dynamics and these DOFs should be included in a dynamical model.54 Here, we present a quasi-classical trajectory (QCT) study of the steric effects in the DC of CHD3 on Ni(111) using our recently developed 12-dimensional analytical PES.41 To this end, the rotational angular momentum and its projections onto both the space-fixed (SF) and bodyfixed (BF) frames are approximately treated within the QCT framework, while the dependence on the laser polarization angle is expressed in a quantum mechanical formula. Our results provide a theoretical reproduction and explanation of the steric effects experimentally observed by Beck and co-workers.13,14

understand the intriguing steric effects, we take the CHD3 molecule as a prototype. In the laser polarization frame, the R(0) transition excites the rovibrationally ground state of CHD3 to the v1 = 1, |JKM⟩ = |100⟩ state, following the ΔJ = 1, ΔM = 0, ΔK = 0 selection rules.12 The rotational angular momentum vector (J)⃗ is thus aligned perpendicular to the polarization axis Z of the laser. On the other hand, the R(1) transition prepares equal populations in the J = 2, M = 0 and ±1 levels, leading to a broader distribution of the angular momentum. C. Quasi-Classical Treatment of the Rotational Motion. Here, only the R(0) transition for the v1 = 1 band is considered because it yields a single rotational state |JKM⟩ = |100⟩ in the laser polarization frame and showed the strongest steric effects in the experiment. To correctly describe the specific rotational state of a symmetric top with respect to a quantization axis, we follow Hase60 by sampling the three components (Jx, Jy, Jz) of the classical angular momentum vector J ⃗ in the BF frame, where Jz along the principal axis (z in BF frame) takes the value of ℏK and the length of J ⃗ is [J(J + 1)]1/2ℏ. The other two components Jx and Jy are selected randomly in the plane perpendicular to the principal axis, with their sum of squares equaling [J(J + 1) − K2]ℏ2. This quasiclassical treatment has been successfully used to study rotational effects in the Cl + CHD3 reaction.61 The resultant angular momentum vector is subsequently transformed to the SF frame. To specify M, we have to rotate the molecule allowing the J ⃗ vector to form an angle of arccos(M / J(J + 1) ) with the SF Z axis, followed by a random rotation with respect to the J ⃗ vector and about the SF Z axis, respectively. In our case, the SF Z axis is conveniently defined as the surface normal. Figure 1a illustrates the relationship of J, K, and M in the SF and BF frames.

II. COMPUTATIONAL DETAILS A. Potential Energy Surface. Our 12D global PES was constructed for the DC of methane on the rigid Ni(111) surface within the flat surface approximation, which neglects the lateral coordinates (X, Y) and azimuthal angle (ϕ) dependence of the PES. The PES was fit to over 36 000 density functional theory (DFT) points using the permutationally invariant polynomials method55 modified for gas−surface interaction,41 in which the projection of the molecular center on the surface is fixed at that of the transition state, as in our earlier work on the DC PES for water.56−58 Although full-dimensional global PESs for polyatomic DC processes are now available,44,48,49,59 we do not believe that the additional DOFs will change the physics in a fundamental way. This 12D PES preserves the permutation symmetry of the four H atoms, vital for studying mode specificity and bond selectivity in this reaction. It has been used in both QD and QCT studies of the methane DC reaction, yielding results that are in good agreement with observations.41,42 Because of its polynomial form, this PES is very smooth and fast to evaluate, allowing efficient and accurate QCT calculations. We refer the reader to refs 41 and 42 for more details of the PES. B. Alignment Due to Rovibrational Transitions. In the experiments of Beck and co-workers,13,14 a linearly polarized infrared laser was used to pump the impinging methane molecule to a specific rovibrationally excited state determined by the selection rules. As a result, the spatial distribution of the methane molecule is no longer random but aligned depending on the angular momentum on the excited state. In order to

Figure 1. (a, left) Vectorial representation of the angular momentum J and its projection (K, M) of a symmetric top (CHD3 here) in spacefixed (X, Y, Z) and body-fixed (x, y, z) frame. The schematic diagrams for the |JKM⟩ = |100⟩ and |JKM⟩ = |101⟩ states are given in (b) (middle) and (c) (right), respectively. The red arrows point to possible directions of J ⃗ vector.

It is perhaps worth pointing out that the quasi-classical orientational probability densities (ρQC(θ)) described above are not identical to their quantum mechanical counterparts (ρQM(θ) = |JKM⟩⟨JKM|), particularly for low rotational states (θ is the angle between BF and SF Z axis, shown in Figure 1a). As discussed in detail by Choi and Bernstein,62 the two share roughly the same angular domain, but the latter often has nodal structures and extends into classically forbidden regions. Nonetheless, the quasi-classical quantization conditions are required in our classical description of the DC dynamics, as it B


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angles have been integrated out. A similar equation was given by Zare and co-workers for steric effect in the Cl + CHD3 3 1 = 4π cos2 θ reaction.12 Quantum mechanically, we have ρ0,0

properly quantizes the angular momenta and energy in the classical phase space. All QCT calculations reported here assume the Born− Oppenheimer approximation, in which the influence of surface electron−hole pairs is ignored. This adiabatic approximation seems to be appropriate for DC processes, as suggested by several recent friction model calculations.63,64 Other details of the QCT calculations are given briefly below. The methane molecule was initiated at 6.0 Å above the metal surface with the initial coordinates and momenta prepared by the standard normal mode sampling,60,65 assigning a quantized energy of the v1 mode for CHD3, followed by the procedure above to describe the |JKM⟩ state in the SF frame. The propagation time step of 0.10 fs was sufficient to converge the energy better than 10−3 kcal/mol. A trajectory was terminated if the C−H or C−D bond reached a separation of 2.2 Å, which was counted as reactive. Otherwise, a trajectory was considered nonreactive if the molecule was scattered back beyond Z = 6.1 Å. Only normal incidence was considered. For each quantum state, up to 80 000 trajectories were ran, which are orders of magnitude more than the recent AIMD calculations.50 The statistical errors in our calculations are typically less than 3%. D. Dependence on Laser Polarization Angle. In the experiment of Beck and co-workers,13,14 the R(0) transition excites CHD3 exclusively to the |100⟩ state within the laser polarization reference frame while to a linear combination of multiple M states when the reference frame is rotated. Following Nattino et al.,50 we always set up the surface normal as the space-fixed Z axis, which forms an angle χ with the laser polarization Z′ axis. This angle differs from that polarization angle ϑ defined in the experiment of Beck and co-workers by 90° (ϑ = 90° − χ) because they chose ϑ as the angle between the laser polarization direction and surface plane.13 Note that the other two Euler angles involved in the rotation are irrelevant as the angles between the X and Y axes and specific parallel lattice vectors are undetermined. As a result, by use of the reference frame with the surface normal as quantization axis, the R(0) transition prepares only the M = 0 state for a laser polarization direction perpendicular to the surface (χ = 0°, ϑ = 90°), while a coherent superposition of the M = −1 and M = 1 states is obtained for a laser polarization direction parallel to the surface (χ = 90°, ϑ = 0°). According to quantum mechanics, the initial rotational state |Ψ0⟩ in the R(0) excitation with respect to an arbitrary laser polarization direction can thus be expressed as66 |Ψ0⟩ = −

3

and ρ0,1 ±1 = 8π sin 2 θ . However, in this work, we use the quasiclassical probability distributions for these two states, as detailed above. The reaction probability at an arbitrary polarization angle also essentially follows eq 2.

III. RESULTS AND DISCUSSION Before presenting our results, it is worthwhile to recall the rotational alignment effects in the DC of H2, which have been studied in the associative desorption experiment on Cu(111)67 and theoretically for several activated systems.68−71 From these studies, it was concluded that rotationally aligned states with M = J, for which the molecule rotates classically like a “helicopter”, are more reactive than those with M = 0, for which the molecule behaves classically like a “cartwheel”. This steric effect has a straightforward origin in the geometry of the DC transition state, which features H2 parallel to the surface plane over the bridge site. The effect becomes weaker as the translational energy increases,67,70 suggesting the absence of dynamical steering which would force the molecule to move along the minimum energy path and thus weaken the steric effect. Now let us consider the DC of CHD3 in the |JKM⟩ = |100⟩ or |JKM⟩ = |101⟩ state, as depicted in Figure 1b and Figure 1c. Since the projection of the angular momentum (K) on the BF z axis, which points from the center of mass of CD3 group to the H atom or approximately corresponds to C−H bond, is zero, this symmetric top behaves approximatively like a diatomic molecule with J ⃗ being perpendicular to the C−H bond. As a result, the |J00⟩ state, for which J ⃗ lies in the plane perpendicular to the SF Z axis, rotates classically like a “cartwheel”, while the | J0J⟩ state corresponds to a titled “helicopter”. Unlike the DC of hydrogen, in which the transition state features H2 symmetric and parallel to the surface, the polar angle of the BF z axis with respect to the surface normal (θ) at the transition state of the methane DC is about 130°, with the dissociating H atom pointing to the surface. In addition, the transition state is very tight, as shown in the contour plot of the PES in Figure 2 as a function of θ and the angle φ rotating the CD3 group with respect to the BF z axis, with other coordinates fixed at the transition state values. In other words, the potential energy increases dramatically as the geometry distorts away from the transition state. It is therefore interesting to investigate first the θ dependence of the reactivity. To this end, we performed QCT calculations, simply fixing the initial orientation of the CD3-H vector with respect to the surface normal with zero angular momentum. Note that the other Euler angles have been sampled randomly. Figure 3 presents the corresponding results for incident energy Ei of 18 and 20 kcal/mol. It is clear that the molecule with the CD3−H vector initially aligned in such a way that matches the transition-state geometry is much more reactive than others and the reactivity decreases quickly as the polar angle (θ) deviates from ∼130°. This straightforward correspondence between the reactivity and transition-state geometry indicates the importance of the transition state in dictating the stereodynamics, much the same way as in the H2 DC case. This observation also suggests the validity of the rotational sudden approximation and a marginal effect of the dynamical steering

1 1 sin χ |10−1⟩ + cos χ |100⟩ + sin χ |101⟩ 2 2 (1)

where |JKM⟩ denotes the rotational eigenstates of this symmetric top defined in the SF frame. Since the |10−1⟩ and |101⟩ states yield actually the same distribution for BF z axis (the same alignment of C−H bond), they are expected to have the same reactivity, representing the reactivity for the polarization angle being ϑ = 0°. From eq 1, it can be shown that at any polarization angle χ with respect to the surface normal, the θ distribution of the molecule can be expressed as 1 1 ρχ (θ ) = cos2 χρ0,0 (θ ) + sin 2 χρ0,1 (θ )

(2)

J where ρK,M are the polar angle distributions of specific rotational states. In deriving this result, the other two Euler

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Figure 2. PES as a function of θ and φ with degrees of freedom fixed at those of the transition state, which is inserted in the corner. The potential energy cutoff is 92 kcal/mol.

Figure 3. Dependence of reactivity on the initial polar angle of CD3− H vector and surface normal for CHD3(v1 = 1, J = 0) at Ei of 18 and 20 kcal/mol.

in methane DC, consistent with the recent AIMD calculations.46 In Figure 4a, the dissociation probabilities of the |100⟩ and |101⟩ states of CHD3 (v1 = 1) are compared as a function of incident energy. The |101⟩ state is found to be always more reactive than the |100⟩ state in the energy range studied here, suggesting that its DC favors the “helicopter” rotation over the “cartwheel” rotation. This difference in reactivity is readily understood in terms of the angular (θ) distributions of the two states. As shown in Figure 4b, the |101⟩ state corresponds to a quasi-classical angular distribution that peaks near θ = 45° and 135°, in better overlapping with the transition-state geometry, while the |100⟩ state corresponds to a uniform angular distribution in the quasi-classical framework, thus sampling most orientations with low reactivity.50 Our results qualitatively agree with the reduced-dimensional QD calculations of Carre and Jackson35 and Xiang et al.,36 where CH4 was treated as a pseudodiatom. These authors both found higher reactivity for larger mj state at high energies for which the C−H bond is closer to the favorable alignment to the transition state.

Figure 4. (a, top) Dissociation probabilities corresponding to the | 101⟩ and |100⟩ states of CHD3(v1 = 1). (b, middle) Initial polar angle distributions of the |101⟩ and |100⟩ states of CHD3(v1 = 1) sampled in QCT calculations (histograms) are compared with those analytical probability distribution functions in classical (dotted curves) and quantum mechanical regime (solid curves). (c, bottom) The quantum mechanical polar angle distribution of CHD3(v1 = 1, J = 1) as a function of the laser polarization angle (χ).

Quantitatively, the calculated dissociation probability of the | 101⟩ state is twice as that of the |101⟩ state at Ei = 14 kcal/mol, while only ∼20% larger at 24 kcal/mol. In other words, the difference in dissociation probabilities becomes much weaker at high energies. At low energies near the threshold, only those molecules aligned to match the transition-state configuration are able to dissociate, while with increasing energy, molecules D


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polarization angle dependence and renders it difficult to rationalize based on simple geometric factors. To illustrate this point, we plot in Figure 4c the quantum mechanical θ distribution for different laser polarization angles, according to eq 2. It is clear from the figure that the θ distribution changes 3 3 1 = 4π cos2 θ and ρ0,1 ±1 = 8π sin 2 θ , without smoothly from ρ0,0 any peak between the two extreme angles. Since the reactivity is strongly correlated with the CHD3 orientation angle θ, so is the dependence on the polarization angle. We also note in passing that the quantum state specific reaction probabilities (P10,1 and P10,0) can be replaced by those computed using quantum dynamical methods, once they become available. Quantitatively, the experimentally observed alignment effect can be quantified by the alignment contrast Δp, defined as13

with a broader range of alignments are able to overcome the barrier, thus weakening the steric effect, as discussed in earlier studies the DC of hydrogen.62,65 This is consistent with the conclusion in the recent AIMD studies of Nattino et al.,50 who found negligible difference between the reactivity of the |100⟩ and |101⟩ states for CHD3 dissociation on Pt(111) at Ei = 18.9 kcal/mol, which is also much higher than the corresponding adiabatic barrier height. Due to the high expense of AIMD, only a small number of trajectories was computed, which results in a relatively large statistical uncertainty. To compare with the dependence on the experimental polarization angle (ϑ), the quantum state resolved dissociative probabilities are assembled according to eq 2 and presented in Figure 5 for normalized dissociative probabilities at several

Δp =

S0 − S0⊥ S0 + S0⊥

(3)

where the S∥0 and S⊥0 correspond to the sticking probability with laser polarization parallel and perpendicular to the surface plane, which are approximately taken in the present work as P10,1 and P10,0, respectively. Table 1 lists our calculated alignment Table 1. Calculated Alignment Contrast Δp for CHD3(v1 = 1) on Ni(111) with R(0) Transition Defined in the Text incident energy (kcal/mol) Δp

14

16

18

20

22

24

0.35

0.30

0.27

0.22

0.16

0.10

contrast from 14 to 24 kcal/mol for CHD3(v1 = 1) dissociation on Ni(111), which decreases from ∼0.35 to ∼0.10. These results are in qualitative agreement with the experimental data on Ni(100), in which Δp decreases gradually from ∼0.18 to ∼0.06 as the molecular beam energy increases from 6.7 to 17.0 kcal/mol,13 given that the barrier height for methane dissociation predicted by DFT is roughly 3.7 kcal/mol lower on Ni(100) than that on Ni(111).72 As discussed above, the evidence strongly indicates the absence of significant steering effects in methane DC.13

Figure 5. Normalized reactivity of CHD3(v1 = 1) on Ni(111) prepared by the R(0) excitation with respect to the parallel polarization (ϑ = 0°, χ = 90°) as a function of polarization angle ϑ (or χ). Experimental data are extracted from Supporting Information in ref 13 on Ni(100) for qualitative comparison.

incident energies, along with experimental data.13 The theoretical curves all decay monotonically from ϑ = 0 to 90° and show the same trend as the experiment. The maximum near ϑ = 0 is apparently due to the large reactivity of the |101⟩ state, while the minimum at ϑ = 90° stems from that of the lessreactive |100⟩ state. Admittedly, the agreement between theory and experiment is not quantitative, and the errors may have several possible sources. For example, the sticking probability of CHD3 was measured on Ni(100)13 rather than Ni(111) as in this work, and neither energy transfer nor site average was accounted for in our model. More importantly, the experiment was performed at much lower energies where tunneling may play a dominant role, which was not properly taken into account here because of the classical treatment of the dynamics. Given these approximations, the theory−experiment agreement in Figure 5 is quite remarkable. We emphasize that this agreement stems directly from expressing the initial wave function as a linear combination of the two quantum states, namely, |100⟩ and |101⟩ (eq 1). Although the reactivity of each individual state can be readily understood in terms of the transition-state geometry, as discussed above, the superposition of the two convolutes the

IV. CONCLUSIONS In this work, we systematically investigated the alignment effects in the DC of CHD3 on Ni(111), using a QCT method on a 12D global PES based on a large number of DFT points. Our strategy is to compute the dissociation probabilities of specific rotational states with quasi-classical quantization of the corresponding rotational quantum numbers and to simulate the experimental dependence on the laser polarization angle with a quantum mechanical formula. The dissociation probability of individual rotational state can be readily understood in terms of a sudden approximation, in which the reactivity is strongly correlated with the overlap of the initial orientation of the molecule with that of the transition state. Perhaps more importantly, our calculated polarization angle dependence of the dissociation probability reproduces the experimental observation qualitatively, thus offering a theoretical interpretation of the mysterious steric effect. The seemingly counterintuitive steric effect is a natural consequence of the change of the probability distribution as a function of polarization angle, which should be interpreted quantum mechanically as a combined effect of the |100⟩ and |101⟩ states rather than E


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(16) Wang, F.; Liu, K.; Rakitzis, T. P. Revealing the stereospecific chemistry of the reaction of Cl with aligned CHD3(v1 = 1). Nat. Chem. 2012, 4, 636−641. (17) Levine, R. D. The chemical shape of molecules: an introduction to dynamic stereochemistry. J. Phys. Chem. 1990, 94, 8872−8880. (18) Rostrup-Nielsen, J. R. Catalytic Steam Reforming. In Catalysis, Science and Technology; Anderson, J. R., Boudart, M., Eds.; SpringerVerlag: Berlin, 1984; Vol. 5. (19) Beck, R. D.; Utz, A. L. Quantum-state resolved gas/surface reaction dynamics experiments. In Dynamics of Gas-Surface Interactions; Díez Muiño, R., Busnengo, H. F., Eds.; Springer: Heidelberg, Germany, 2013. (20) Juurlink, L. B. F.; McCabe, P. R.; Smith, R. R.; DiCologero, C. L.; Utz, A. L. Eigenstate-resolved studies of gas surface reactivity: CH4(v3) dissociation on Ni(100). Phys. Rev. Lett. 1999, 83, 868−871. (21) Schmid, M. P.; Maroni, P.; Beck, R. D.; Rizzo, T. R. Surface reactivity of highly vibrationally excited molecules prepared by pulsed laser excitation: CH4(2v3) on Ni(100). J. Chem. Phys. 2002, 117, 8603−8606. (22) Beck, R. D.; Maroni, P.; Papageorgopoulos, D. C.; Dang, T. T.; Schmid, M. P.; Rizzo, T. R. Vibrational mode-specific reaction of methane on a nickel surface. Science 2003, 302, 98−100. (23) Smith, R. R.; Killelea, D. R.; DelSesto, D. F.; Utz, A. L. Preference for vibrational over translational energy in a gas-surface reaction. Science 2004, 304, 992−995. (24) Maroni, P.; Papageorgopoulos, D. C.; Sacchi, M.; Dang, T. T.; Beck, R. D.; Rizzo, T. R. State-resolved gas-surface reactivity of methane in the symmetric C-H stretch vibration on Ni(100). Phys. Rev. Lett. 2005, 94, 246104. (25) Juurlink, L. B. F.; Smith, R. R.; Killelea, D. R.; Utz, A. L. Comparative study of C-H stretch and bend vibrations in methane activation on Ni(100) and Ni(111). Phys. Rev. Lett. 2005, 94, 208303. (26) Bisson, R.; Sacchi, M.; Dang, T. T.; Yoder, B.; Maroni, P.; Beck, R. D. State-resolved reactivity of CH4(2v3) on Pt(111) and Ni(111): Effects of barrier height and transition state location. J. Phys. Chem. A 2007, 111, 12679−12683. (27) Killelea, D. R.; Campbell, V. L.; Shuman, N. S.; Utz, A. L. Bondselective control of a heterogeneously catalyzed reaction. Science 2008, 319, 790−793. (28) Killelea, D. R.; Campbell, V. L.; Shuman, N. S.; Utz, A. L. Surface temperature dependence of methane activation on Ni(111). J. Phys. Chem. C 2009, 113, 20618−20622. (29) Bisson, R.; Sacchi, M.; Beck, R. D. State-resolved reactivity of CH4 on Pt(110)-(1 × 2): The role of surface orientation and impact site. J. Chem. Phys. 2010, 132, 094702. (30) Chen, L.; Ueta, H.; Bisson, R.; Beck, R. D. Vibrationally bondselected chemisorption of methane isotopologues on Pt(111) studied by reflection absorption infrared spectroscopy. Faraday Discuss. 2012, 157, 285−295. (31) Chen, N.; Huang, Y.; Utz, A. L. State-resolved reactivity of methane (ν2 + ν4) on Ni(111). J. Phys. Chem. A 2013, 117, 6250− 6255. (32) Hundt, P. M.; van Reijzen, M. E.; Ueta, H.; Beck, R. D. Vibrational activation of methane chemisorption: The role of symmetry. J. Phys. Chem. Lett. 2014, 5, 1963−1967. (33) Dombrowski, E.; Peterson, E.; Del Sesto, D.; Utz, A. L. Precursor-mediated reactivity of vibrationally hot molecules: Methane activation on Ir(111). Catal. Today 2015, 244, 10−18. (34) Hundt, P. M.; Ueta, H.; van Reijzen, M. E.; Jiang, B.; Guo, H.; Beck, R. D. Bond-selective and mode-specific dissociation of CH3D and CH2D2 on Pt(111). J. Phys. Chem. A 2015, 119, 12442−12448. (35) Carre, M.-N.; Jackson, B. Dissociative chemisorption of CH4 on Ni: The role of molecular orientation. J. Chem. Phys. 1998, 108, 3722− 3730. (36) Xiang, Y.; Zhang, J. Z. H.; Wang, D. Y. Semirigid vibrating rotor target model for CH4 dissociation on a Ni(111) surface. J. Chem. Phys. 2002, 117, 7698−7704. (37) Nave, S.; Jackson, B. Methane dissociation on Ni(111): The role of lattice reconstruction. Phys. Rev. Lett. 2007, 98, 173003.

classically as a continuous rotation of the molecular principle axis with respect to the space-fixed axis. These results provide a deeper understanding of the influence of molecular rotation and alignment in methane DC. A quantitative understanding of the experimental results may further benefit from a future quantum mechanical treatment of the rotational DOFs and the inclusion of other effects ignored in our model, such as the mechanical and electronic couplings with the surface and site dependence.

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AUTHOR INFORMATION

Corresponding Authors

*B.J.: phone, +86 551 63600922; e-mail, [email protected]. *H.G.: phone, 505-277-1716; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank National Natural Science Foundation of China (Grant 21573203 to B.J.), U.S. National Science Foundation (Grant CHE-1462019 to H.G.). We gratefully acknowledge many useful discussions with Dr. Helen Chadwick and Prof. Rainer Beck. Parts of the calculations have been done on the supercomputing system in the Supercomputing Center of University of Science and Technology of China.

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REFERENCES

(1) Zare, R. N. Laser control of chemical reactions. Science 1998, 279, 1875−1879. (2) Yang, X. State-to-state dynamics of elementary bimolecular reactions. Annu. Rev. Phys. Chem. 2007, 58, 433−459. (3) Juurlink, L. B. F.; Killelea, D. R.; Utz, A. L. State-resolve probes of methane dissociation dynamics. Prog. Surf. Sci. 2009, 84, 69−134. (4) Liu, K. Perspective: Vibrational-induced steric effects in bimolecular reactions. J. Chem. Phys. 2015, 142, 080901. (5) Polanyi, J. C. Some concepts in reaction dynamics. Science 1987, 236, 680−690. (6) Crim, F. F. Molecular reaction dynamics across the phases: similarities and differences. Faraday Discuss. 2012, 157, 9−26. (7) Guo, H.; Jiang, B. The sudden vector projection model for reactivity: Mode specificity and bond selectivity made simple. Acc. Chem. Res. 2014, 47, 3679−3685. (8) Jiang, B.; Yang, M.; Xie, D.; Guo, H. Quantum dynamics of polyatomic dissociative chemisorption on transition metal surfaces: Mode specificity and bond selectivity. Chem. Soc. Rev. 2016, DOI: 10.1039/C5CS00360A. (9) Brooks, P. R. Reactions of oriented molecules. Science 1976, 193, 11−16. (10) Stolte, S. Reactive scattering studies on oriented molecules. Ber. Bunsenges. Phys. Chem. 1982, 86, 413−421. (11) Parker, D. H.; Bernstein, R. B. Oriented molecule beams via the electrostatic hexapole: preparation, characterization and reactive scattering. Annu. Rev. Phys. Chem. 1989, 40, 561−595. (12) Simpson, W. R.; Rakitzis, T. P.; Kandel, S. A.; Orr-Ewing, A. J.; Zare, R. N. Reaction of Cl with vibrationally excited CH4 and CHD3 State-to-state differential cross sections and steric effects for the HCl product. J. Chem. Phys. 1995, 103, 7313−7335. (13) Yoder, B. L.; Bisson, R.; Beck, R. D. Steric effects in the chemisorption of vibrationally excited methane on Ni(100). Science 2010, 329, 553−556. (14) Yoder, B. L.; Bisson, R.; Hundt, P. M.; Beck, R. D. Alignment dependent chemisorption of vibrationally excited CH4(v3) on Ni(100), Ni(110) and Ni(111). J. Chem. Phys. 2011, 135, 224703. (15) Wang, F.; Lin, J.-S.; Liu, K. Steric control of the reaction of CH stretch-excited CHD3 with chlorine atom. Science 2011, 331, 900−903. F


Article

The Journal of Physical Chemistry C (38) Tiwari, A. K.; Nave, S.; Jackson, B. Methane dissociation on Ni(111): A new understanding of the lattice effect. Phys. Rev. Lett. 2009, 103, 253201. (39) Krishnamohan, G. P.; Olsen, R. A.; Kroes, G.-J.; Gatti, F.; Woittequand, S. Quantum dynamics of dissociative chemisorption of CH4 on Ni(111): Influence of the bending vibration. J. Chem. Phys. 2010, 133, 144308. (40) Jackson, B.; Nave, S. The dissociative chemisorption of methane on Ni(100): Reaction path description of mode-selective chemistry. J. Chem. Phys. 2011, 135, 114701. (41) Jiang, B.; Liu, R.; Li, J.; Xie, D.; Yang, M.; Guo, H. Mode selectivity in methane dissociative chemisorption on Ni(111). Chem. Sci. 2013, 4, 3249−3254. (42) Jiang, B.; Guo, H. Mode and bond selectivities in methane dissociative chemisorption: quasi-classical trajectory studies on twelvedimensional potential energy surface. J. Phys. Chem. C 2013, 117, 16127−16135. (43) Jackson, B.; Nave, S. The dissociative chemisorption of methane on Ni(111): The effects of molecular vibration and lattice motion. J. Chem. Phys. 2013, 138, 174705. (44) Shen, X. J.; Lozano, A.; Dong, W.; Busnengo, H. F.; Yan, X. H. Towards bond selective chemistry from first principles: Methane on metal surfaces. Phys. Rev. Lett. 2014, 112, 046101. (45) Nattino, F.; Ueta, H.; Chadwick, H.; van Reijzen, M. E.; Beck, R. D.; Jackson, B.; van Hemert, M. C.; Kroes, G.-J. Ab initio molecular dynamics calculations versus quantum-state-resolved experiments on CHD3 + Pt(111): New insights into a prototypical gas−surface reaction. J. Phys. Chem. Lett. 2014, 5, 1294−1299. (46) Jackson, B.; Nattino, F.; Kroes, G.-J. Dissociative chemisorption of methane on metal surfaces: Tests of dynamical assumptions using quantum models and ab initio molecular dynamics. J. Chem. Phys. 2014, 141, 054102. (47) Guo, H.; Jackson, B. Mode- and bond-selective chemistry on metal surfaces: The dissociative chemisorption of CHD3 on Ni(111). J. Phys. Chem. C 2015, 119, 14769−14779. (48) Shen, X.; Chen, J.; Zhang, Z.; Shao, K.; Zhang, D. H. Methane dissociation on Ni(111): A fifteen-dimensional potential energy surface using neural network method. J. Chem. Phys. 2015, 143, 144701. (49) Shen, X.; Zhang, Z.; Zhang, D. H. CH4 dissociation on Ni(111): a quantum dynamics study of lattice thermal motion. Phys. Chem. Chem. Phys. 2015, 17, 25499−25504. (50) Nattino, F.; Migliorini, D.; Bonfanti, M.; Kroes, G.-J. Methane dissociation on Pt(111): Searching for a specific reaction parameter density functional. J. Chem. Phys. 2016, 144, 044702. (51) Kroes, G.-J. Towards chemically accurate simulation of molecule−surface reactions. Phys. Chem. Chem. Phys. 2012, 14, 14966−14981. (52) Nave, S.; Tiwari, A. K.; Jackson, B. Dissociative chemisorption of methane on Ni and Pt surfaces: Mode-specific chemistry and the effects of lattice motion. J. Phys. Chem. A 2014, 118, 9615−9631. (53) Jiang, B.; Li, J.; Xie, D.; Guo, H. Effects of reactant internal excitation and orientation on dissociative chemisorption of H2O on Cu(111): Quasi-seven-dimensional quantum dynamics on a refined potential energy surface. J. Chem. Phys. 2013, 138, 044704. (54) Füchsel, G.; Thomas, P. S.; den Uyl, J.; Ozturk, Y.; Nattino, F.; Meyer, H.-D.; Kroes, G.-J. Rotational effects on the dissociation dynamics of CHD3 on Pt(111). Phys. Chem. Chem. Phys. 2016, 18, 8174−8185. (55) Braams, B. J.; Bowman, J. M. Permutationally invariant potential energy surfaces in high dimensionality. Int. Rev. Phys. Chem. 2009, 28, 577−606. (56) Jiang, B.; Ren, X.; Xie, D.; Guo, H. Enhancing dissociative chemisorption of H2O on Cu(111) via vibrational excitation. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 10224−10227. (57) Jiang, B.; Xie, D.; Guo, H. Vibrationally mediated bond selective dissociative chemisorption of HOD on Cu(111). Chem. Sci. 2013, 4, 503−508.

(58) Hundt, P. M.; Jiang, B.; van Reijzen, M.; Guo, H.; Beck, R. D. Vibrationally promoted dissociation of water on Ni(111). Science 2014, 344, 504−507. (59) Jiang, B.; Guo, H. Dynamics of water dissociative chemisorption on Ni(111): Effects of impact sites and incident angles. Phys. Rev. Lett. 2015, 114, 166101. (60) Hase, W. L. Classical Trajectory Simulations: Initial Conditions. In Encyclopedia of Computational Chemistry,; Alinger, N. L., Ed.; Wiley: New York, 1998; Vol. 1, pp 399−402. (61) Liu, R.; Wang, F.; Jiang, B.; Czakó, G.; Yang, M.; Liu, K.; Guo, H. Rotational mode specificity in the Cl + CHD3 → HCl + CD3 reaction. J. Chem. Phys. 2014, 141, 074310. (62) Choi, S. E.; Bernstein, R. B. Theory of oriented symmetric-top molecule beams: Precession, degree of orientation, and photofragmentation of rotationally state-selected molecules. J. Chem. Phys. 1986, 85, 150−161. (63) Juaristi, J. I.; Alducin, M.; Díez Muiño, R.; Busnengo, H. F.; Salin, A. Role of electron-hole pair excitations in the dissociative adsorption of diatomic molecules on metal surfaces. Phys. Rev. Lett. 2008, 100, 116102. (64) Jiang, B.; Alducin, M.; Guo, H. Electron−hole pair effects in polyatomic dissociative chemisorption: Water on Ni(111). J. Phys. Chem. Lett. 2016, 7, 327−331. (65) Peslherbe, G. H.; Wang, H.; Hase, W. L. Monte Carlo sampling for classical trajectory simulations, a chapter in Monte Carlo methods in chemical physics. Adv. Chem. Phys. 1999, 105, 171−201. (66) Zare, R. N. Angular Momentum; Wiley: New York, 1988. (67) Hou, H.; Gulding, S. J.; Rettner, C. T.; Wodtke, A. M.; Auerbach, D. J. The stereodynamics of a gas-surface reaction. Science 1997, 277, 80−82. (68) Diaz, C.; Olsen, R. A.; Auerbach, D. J.; Kroes, G.-J. Sixdimensional dynamics study of reactive and non reactive scattering of H2 from Cu(111) using a chemically accurate potential energy surface. Phys. Chem. Chem. Phys. 2010, 12, 6499−6519. (69) Sementa, L.; Wijzenbroek, M.; van Kolck, B. J.; Somers, M. F.; Al-Halabi, A.; Busnengo, H. F.; Olsen, R. A.; Kroes, G. J.; Rutkowski, M.; Thewes, C.; et al. Reactive scattering of H2 from Cu(100): Comparison of dynamics calculations based on the specific reaction parameter approach to density functional theory with experiment. J. Chem. Phys. 2013, 138, 044708. (70) Dai, J.; Light, J. C. The steric effect in a full dimensional quantum dynamics simulation for the dissociative adsorption of H2 on Cu(111). J. Chem. Phys. 1998, 108, 7816−7820. (71) Jiang, B.; Guo, H. Six-dimensional quantum dynamics for dissociative chemisorption of H2 and D2 on Ag(111) on a permutation invariant potential energy surface. Phys. Chem. Chem. Phys. 2014, 16, 24704−24715. (72) Nave, S.; Tiwari, A. K.; Jackson, B. Methane dissociation and adsorption on Ni(111). Pt(111), Ni(100), Pt(100) and Pt(110)(1x2): Energetic study. J. Chem. Phys. 2010, 132, 054705.

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