State-Resolved Reactivity of Methane - ACS Publications - American

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State-Resolved Reactivity of Methane (ν2 + ν4) on Ni(111) Nan Chen, Yongli Huang, and Arthur L. Utz* Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, United States ABSTRACT: Vibrational state-resolved experiments that probe methane dissociation on Ni(111) quantify the reactivity of CH4 excited to the ν2 + ν4 bending vibration. A comparison of these data and previous state-resolved measurements reveals that this bending vibration is significantly less reactive than the ν3 C−H stretching vibration in the same polyad of vibrations. Comparison with the 3ν4 bend overtone also suggests that the doubly degenerate bending state, ν2, is less effective than the triply degenerate bend (ν4) in promoting methane dissociative chemisorption on Ni(111). This observation of vibrational mode selectivity contradicts thermal statistical theories of gas−surface reactivity and provides direct experimental evidence of roles that different vibrational states can play in activating this gas−surface reaction.

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INTRODUCTION Heterogeneously catalyzed reactions are central to the modern chemical industry. In many of these reactions, gas-phase reagents adsorb on the surface of a metal or supported metal particle, undergo chemical transformation to form the desired reaction product, and then desorb from the surface. Even though the catalyst may significantly lower the energy requirement for reaction, there are a number of industrially important reactions where a significant energetic barrier to reaction persists, even in the presence of catalyst. The steam reforming reaction, in which methane and water react on a nickel catalyst to form CO and H2, is one example. The rate-limiting step in this reaction, which is the chief industrial process for H2 production, is the dissociative chemisorption of a gas-phase methane molecule into surface-bound methyl and H fragments.1 Exploring the dynamics of gas−surface reactions provides detailed insight into the mechanistic origin of catalytic activity.2 Supersonic molecular beams prepare reagents with a welldefined translational energy (Etrans). Monitoring the yield of surface-bound reaction products and the energetics of scattered species reveals how Etrans affects the incident molecule’s reactivity and interaction with the surface. Beam-surface scattering studies have explored the role of translational and internal energy in promoting methane dissociation on a range of transition metal surfaces, including W(110),3 Pt(111),4 Pt(110) (1×2),5 Ni(111),6 Ni(100),7 Rh(111),8 Ir(111),9 Ir(110) (2×1),10 Ru(0001),6e and Pt(553).11 Because vibrations do not cool well during supersonic expansion, one can vary seeding conditions and the nozzle temperature of the molecular beam source to investigate how the reagent’s vibrational energy (Evib) influences reactivity. Studies of D2 dissociation on Cu(111)12 and methane dissociation on W(110),3b Ni(111),6a and Ni(100)7b all provided clear evidence that methane’s thermally excited vibrations promote reactivity. In another experiment, thermally © XXXX American Chemical Society

excited vibrational states of CCl4 and SF6 were found to inhibit condensation on a cryogenically cooled surface.13 Experiments that combine a supersonic molecular beam with state-resolved laser excitation of the gas-phase molecule prior to surface impact reveal how Evib influences the probability and products of reaction.14 These studies are particularly revealing in the case of polyatomic reagents such as methane, where both the energy and the nature of the vibrational state (bend vs stretch, localized vs distributed excitation, ...) can influence reactivity. The earliest vibrational state-resolved measurements of methane activation examined the reactivity of the ν3 (C−H stretch) fundamental on Ni(111)15 and its first overtone (v = 2) on the Pt(111)16 and Ni(100)17 surfaces. Since then, studies have been extended to additional vibrational states, surfaces, and isotopologues of methane. Mode-selective chemistry, in which the identity of the vibrational state and not just its energy determines the reaction probability, and bond-selective chemistry, in which vibrational excitation dictates the products of reaction, appear to be widespread. Statistical models of reactivity, in which internal energy is rapidly scrambled prior to reaction, are not consistent with these observations. Several recent reviews summarize these state-resolved studies of methane activation.18 Methane’s four vibrational states include the ν3 triply degenerate antisymmetric C−H stretching vibration, the ν1 symmetric C−H stretch, the ν2 doubly degenerate bend, and the ν4 triply degenerate bend. State-resolved studies show that methane molecules prepared in vibrational fundamentals, combinations, and overtones of these states are more reactive on Ni(111) than those in the ground vibrational state. Up to Special Issue: Prof. John C. Wright Festschrift Received: January 17, 2013 Revised: April 25, 2013

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vibrational ground state.25 Direct infrared absorption measurements of molecules in the beam showed that rotational cooling was nearly complete for nozzle temperatures below 570 K. Frequency-stabilized infrared (IR) light from a continuous wave, narrow band laser intersected the molecular beam and excited from 6 to 10% of the molecules to a single internal quantum state, J = 2 of the ν2 + ν4 vibration, via the R(1) transition at 2868.72 cm−1. Anharmonic force field calculations show that the zero-order composition of this eigenstate is very near that of a pure bending state, containing 98% ν2 + ν4 character and only 2% of the ν3 C−H stretch.26 A pyroelectric detector translated into the molecular beam quantified IR absorption for both room temperature and heated nozzle expansions. The IR radiative lifetime of methane is much longer than the flight time from the optical excitation region to the surface,27 so molecules prepared in a rovibrational eigenstate of the gas-phase molecule impinged on the 475 K Ni(111) surface in their initially prepared state. Intramolecular vibrational redistribution (IVR) did not occur until methane began to experience the gas−surface interaction potential, which typically occurred a few hundred femtoseconds prior to reaction or nonreactive scattering. The UHV chamber contained standard surface-science instruments including a quadrupole mass spectrometer for time-of-flight beam characterization and temperature programmed desorption (TPD) detection, and Auger electron spectroscopy and argon ion sputtering for surface characterization and cleaning. Radiative heating provided better than ±1 K control over the surface temperature. We measured reaction probability with (SLaserOn ) and without 0 LaserOff (S0 ) laser excitation by exposing a beam of known flux to the surface for a fixed time. Following the methane dose, we quantified the surface bound C reaction products using oxygen titration.6e Mass spectrometry monitored the partial pressure of desorbing CO whereas the 550 K surface was held in a 1.4 × 10−8 Torr background pressure of O2. The monolayer (ML) coverage (between 6 and 9% ML) of carbon on the surface was calibrated with saturated ethylene (4% C2H4 in Ar) doses, which leads to a well-defined and self-limiting saturation coverage (43% ML) of carbon on Ni(111).6d Our measurements of the sticking probability with and without laser excitation allowed us to calculate a state-resolved reactivity for the particular laser-excited state ν2 + ν4. Equation 1 relates the state-resolved reactivity (Sv02+v4), to the reactivity with (SLaserOn ), without (SLaserOff ) laser excitation, the fraction of 0 0 molecules excited (fexc), which we measure directly, and the 15,18c vibrational ground state reactivity (Sv=0 0 )

now, publications report state-resolved reactivity of methane (CH4) on Ni(111) when excited to the ν3,19 2ν3,20 and 3ν421 vibrational states. No measurements reveal the efficacy of the ν2 doubly degenerate bending state on this surface, and a direct comparison between bending and stretching states of comparable internal energy is missing. Methane dissociation is an ideal system for study as it provides much of the complexity inherent to larger molecular systems while remaining accessible to high-level theory and detailed experimental study. As a polyatomic reagent, methane provides distinct internal modes and energy redistribution channels characteristic of larger molecules, but its size remains small enough to permit high-level quantum dynamics calculations that provide detailed insight into the microscopic details of reactivity. A recent 15-D reaction path calculation reveals significant vibrational mode selectivity for the reaction, and it suggests that the ν2 bending state may be particularly poorly coupled to the reaction coordinate for dissociative chemisorption on a Ni surface.22 The experiments described here are ideally suited for comparison with these calculations. Here, we report state-resolved beam−surface reactivity measurements of methane in the (ν2 + ν4) combination vibration and incident on a clean Ni(111) surface. The data allow us to quantify reaction probability (S0) over a wide range of Etrans. We compare our measurements with previously published studies of methane incident on Ni(111) and excited to antisymmetric C−H stretch (ν3)19 and the second overtone of the triply degenerate bend (3ν4)21 to assess the reactivity of this combination vibration and the efficacy of ν2 excitation in promoting methane dissociation on Ni(111). State-resolved studies of methane activation on Pt(110) (1×2) provide another point of comparison for our work.23

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EXPERIMENTAL METHODS Prior publications provide a detailed description of our experimental apparatus and methods.18c,24 Here, we summarize key details. Mixtures of methane seeded in hydrogen (1%, 3%, and 5% from Airgas, Inc.) expanded continuously and supersonically through a temperature-controlled nozzle and into a high vacuum chamber. The triply differentially pumped beam impinged on a Ni(111) crystal housed in an ultrahigh vacuum (UHV) chamber. Control experiments verified that our results were not influenced by the adsorption of background gases or impurities in the beam or main chamber.18c Experiments were performed at a surface temperature of 475 K. At this temperature, molecularly adsorbed CH4 and any H adsorbed following dissociation of the carrier gas desorb rapidly. The methyl reaction products of dissociative chemisorption decompose to chemisorbed C and H, and all H resulting from methane dissociation recombine and desorbs promptly, leaving adsorbed C on the surface as a signature of methane’s dissociation. Performing the measurements in the limit of low surface coverage minimizes site blocking by adsorbed reaction products. Our experimental approach provides precise control over reagent energy. Laser excitation prepares gas-phase methane in a single rotational and vibrational quantum state with precisely defined internal energy. The narrow Etrans distribution (ΔEtrans/ Etrans < 5%) in the molecular beam, which was verified by timeof-flight measurements, provided tight control over translational energy. Methane molecules in the beam had a vibrational state distribution related to a Boltzmann distribution at the nozzle source temperature and the vast majority were in the

S0v2 + v4 =

S0LaserOn − S0LaserOff + S0v = 0 fexc

When Etrans is relatively low, and can be neglected.

Sv=0 0

contributes negligibly to

(1) v +v S02 4

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RESULTS We plot Sν02+ν4 and SLaserOff in Figure 1 as a function of 0 translational energy, Etrans. Each point in Figure 1 represents an average of at least three individual measurements. Error bars are 95% confidence limits and include standard deviations observed in replicate measurements as well as our estimates of uncertainty in fexc. The laser-off data provide an upper limit on the reactivity of the vibrational ground state. As Etrans ν +ν increases, the difference between S02 4 and SLaserOff decreases. 0 B

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Article v=0 4 curves for Sν03, S3ν from previous reports appear for 0 , and S0 19,21 comparison. The curves passing through the data for each state are error functions with identical shapes. They only differ by their shift along the Etrans axis. When this is the case, the Etrans shift between any two vibrational states is constant at all reaction probabilities. The shift of these curves along the Etrans axis relative to the v = 0 state’s reactivity, ΔEtrans, provides a measure of how much vibrational excitation reduces the Etrans requirement for reaction.28 We calculate the vibrational efficacy for each vibrational state, ηv = Δ Etrans/Evib and show our results in Table 1.

Table 1. Summary of Vibrational Efficacies Measured for CH4 Activation on Ni(111) Figure 1. Reaction probability for methane in the ν2 + ν4 excited state (red solid circles) and without laser excitation (open circles) on Ni(111). The solid black line denotes the reactivity of the vibrational ground state. Between 75 and 130 kJ/mol, thermally populated vibrational states in the molecular beam cause the laser off data to exceed the vibrational ground state reactivity. The ν 2 + ν 4 measurements are well described by shifting the reactivity curve for the v = 0 state by 15 kJ/mol along the horizontal axis. If (ν2 + ν4) had the same efficacy as 3ν4, its reactivity would fall on the dashed red line.

vibrational states

Evib (kJ/mol)

ΔEtrans (kJ/mol)

η

ν3 2ν3 3ν4 ν2 + ν4

36 72 47 34

45 68 34 15

1.2519 0.920 0.7221 0.44

(open We first note that our measurements of SLaserOff 0 squares) are consistent with previous published data (open circles).19 Second, the 34 kJ/mol of Evib in ν2 + ν4 reduces the Etrans threshold for reaction by 15 kJ/mol. This leads to an efficacy (ηνvib2+ν4 = ΔEtrans/Evib) of 0.44, which is much lower than that of the ν3 antisymmetric C−H stretch (ηνvib3 = 1.25) and the 21 4 3ν4 triply degenerate bend (η3ν vib = 0.74). In the statistical limit where Etrans and Evib are completely scrambled prior to reaction, the efficacy for all states would be unity. If rapid IVR processes scrambled Evib prior to reaction, the states would at least have the same efficacy. Instead, the data point to nonstatistical reactivity patterns that likely arise from differences in transition state access and the extent of vibrational energy flow that might occur during the brief period of molecule−surface interaction that precedes reaction.

This is a common feature of state-resolved reaction probability curves. As the total energy available to the reagent increases, Evib becomes less critical for the system to surmount the ν +ν reaction barrier. We were unable to quantify S02 4 at Etrans > 105 kJ/mol. Higher translational energies require elevated nozzle temperatures. Under those expansion conditions, rotational cooling becomes increasingly incomplete, the vibrational ground state population is diluted over an increasing number of rotational states, and the excitation fraction is reduced to a point where the contrast between SLaserOn and 0 SLaserOff is vanishingly small. 0 The data show that ν2 + ν4 excitation increases S0 at all Etrans studied. As expected, the enhancement in S0 is largest at the lowest Etrans where the system is most “starved” for the energy that vibrational excitation provides. At Etrans > 90 kJ/mol, molecules in the ν2 + ν4 state are about 2 times more reactive than are molecules in v = 0, but at Etrans < 50 kJ/mol, the enhancement increases to a factor of 11. Figure 2 provides direct evidence for vibrational mode specificity in this gas−surface reaction. Experimental data and

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DISCUSSION Calculations of the transition state for methane dissociation on Ni(111) and other transition metals show significant distortions from methane’s gas-phase geometry.29 These distortions result in a “late” barrier for reaction and introduce a bias in which certain vibrational excitations in the gas-phase reagent may enjoy enhanced transition state access. Theoretical calculations based on density functional theory (DFT)29e,30 and experimental data21,31 suggest and show mode-selective reactivity in which different vibrational modes promote S0 differently. On the Ni(100) surface, dissociative chemisorption efficacies for methane’s symmetric (ν1)31b and antisymmetric (ν3)15 C− H stretching vibrations show that the symmetric ν1 state has a significantly higher efficacy. This observation is consistent both with a vibrationally adiabatic model of reactivity32 that has been invoked to explain mode-selectivity in bimolecular gas-phase methane reactions33 and with quantum wavepacket scattering calculations.34 In general, these calculations point to a relative ranking of vibrational efficacy for dissociative chemisorption of ν1 > ν3 > ν2 ≈ ν4. A more recent reaction path Hamiltonian calculation from Jackson and co-workers35 provides further insight into the origin of mode-selective behavior. If vibrationally excited molecules can undergo a transition to a lower-lying state (or the vibrational ground state) during a gas−surface encounter, energy freed by vibrational relaxation can convert into motion along the reaction path, which results in a large

Figure 2. Reaction probability curves for methane dissociation on the Ni(111) surface. The states shown are ν3 (black squares),19 3ν4 (blue diamonds),21 ν2 + ν4 (red circles), and the v = 0 ground vibrational state (open symbols)19 of methane on Ni(111). C

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CONCLUSION Data reported here deepen our understanding of vibrational activation in CH4 dissociation on a nickel surface. We have measured the state-resolved reaction probability for methane molecules excited to the ν2 + ν4 combination vibration and incident on a Ni(111) surface with translational energies ranging from 45 to 100 kJ/mol. Relative to the vibrational ground state, ν2 + ν4 decreases the Etrans threshold for reaction by 15 kJ/mol, corresponding to a vibrational efficacy of ηνvib2+ν4 of 0.44, which is significantly less that that of the ν3 antisymmetric C−H stretch or the 3ν4 overtone of the triply degenerate bending state. We estimate the efficacy of the ν2 quantum in the vibration to be much less than that of all other vibrational fundamentals in methane on the basis of our prior measurements of the 3ν4 state on this surface. The wide range of vibrational efficacies observed for methane dissociation on Ni(111) suggest that purely statistical theories are unreliable predictors of reactivity at the quantum-state-resolved level. Instead, it is likely that the dynamics of vibrational energy flow during the brief gas−surface encounter are the key to understanding the origin and generality of vibrational modeand bond-selective reactivity on metal surfaces. Finally, the large difference in efficacy for the ν3 and ν2 + ν4 states, both of which belong to the same polyad, suggest that energy transfer between bending and stretching vibrations in the same polyad are not fast enough to influence reactivity.

reactivity enhancement. The gas-phase vibrational symmetries of the doubly degenerate ν2 (E) and the triply degenerate ν4 (F2) bending vibrations differ in their ability to couple to lowerenergy vibrations and therefore have different efficacies for promoting dissociative chemisorption. In fact, ν2 practically behaves as a spectator to dissociative chemisorption on Ni(100) in these studies. To date, measured reaction probabilities for the ν3 vibrational fundamental (v = 1) and the second overtone of ν4 (3ν4) on Ni(111) and for the ν1, ν3, and 3ν4 levels on Ni(100) are consistent with these trends, but the data presented here are the first measurements for a mode containing significant ν2 vibrational character and incident on a Ni surface. If we were to assume that ν2 + ν4 has the same efficacy for promoting reaction as does 3ν4 (η = 0.72), we would expect ΔEtrans for ν2 + ν4 to be 72% of the state’s 34 kJ/mol of vibrational energy, or ΔEtrans = 25 kJ/mol. The dashed red line in Figure 1 shows the S0 curve we would expect if ν2 + ν4 and 3ν4 shared the same efficacy. The ν2 + ν4 state is clearly less reactive than that prediction. Instead, Figure 1 shows ΔEtrans for ν2 + ν4 is only 15 kJ/mol, which is 60%, or just over half of the ΔEtrans predicted on the basis of the 3ν4 overtone. Though it is not clear that the ΔEtrans shifts for vibrations in an overtone or combination state are always additive,23 it is likely that ΔEtrans for the ν4 fundamental is at least 1/3 that of the 3ν4 overtone, or about 11 kJ/mol. That is because each additional quantum in a given vibrational coordinate will likely provide incrementally less additional transition state access.15,17,36 Therefore, if we assume the ΔEtrans shifts for the ν2 and ν4 coordinates to be additive, we would conclude that 18 kJ/mol of Evib in ν2 contributes 4 kJ/ mol or less to the ΔEtrans = 15 kJ/mol we observe for ν2 + ν4. That leads to an inferred efficacy for ν2 in this combination vibration of ηνvib2 ≤ 0.22, the lowest observed for any vibrational mode in methane’s dissociative chemisorption and nearly a factor of 6 less than that reported for the ν3 antisymmetric C− H stretch (ηνvib3 = 1.25). The particularly low efficacy for ν2 is consistent with the predictions of Jackson’s reaction path calculations for the Ni(111) surface. In contrast to our results on Ni(111), Bisson et al. report an efficacy for the (2ν2 + ν4) combination vibration on Pt(110) (1×2) of 0.40, which is quite similar to the efficacy they report for the 2ν3 C−H stretching overtone state (0.47). It therefore appears that the chemical and structural identity of the surface can impact the ability of a particular methane vibration to promote dissociative chemisorption.23 Our ability to quantify S0 for ν2 + ν4 allows us to consider another aspect of energy flow during methane’s dissociative chemisorption. Methane’s vibrational states group in polyads of similar total energy. This structure arises from the near 2:1 resonance between the bend and stretch vibrational frequencies. In general, the energy gap between polyads in methane is much greater than that between states in a given polyad. In studies of collisional energy transfer, intrapolyad energy transfer is much more efficient than interpolyad relaxation due to propensities that favor relatively small changes in Evib.25 The reactivity data presented here are the first for a bending state that belongs to the pentad polyad in CH4. The pentad also contains the previously studied ν1 and ν3 C−H stretching fundamentals, along with the 2ν2 and 2ν4 bending overtones. The fact that the efficacy for ν2 + ν4 is much less than that for the ν3 C−H stretch state shows that the gas−surface collision is not sufficient to significantly scramble excitation between stretching and bending states in the same polyad.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 617-627-3473. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under Grant number CHE-1111702. REFERENCES

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