Surface Temperature Dependence of Methane Activation on Ni(111

20618

J. Phys. Chem. C 2009, 113, 20618–20622

Surface Temperature Dependence of Methane Activation on Ni(111) D. R. Killelea,† V. L. Campbell, N. S. Shuman,‡ R. R. Smith,§ and A. L. Utz* Tufts UniVersity, Department of Chemistry, and W. M. Keck Foundation Laboratory of Materials Science, 62 Talbot AVenue, Medford, Massachusetts, 02155 ReceiVed: July 10, 2009; ReVised Manuscript ReceiVed: September 10, 2009

Vibrational state resolved measurements of methane’s dissociation on Ni(111) show a strong surface temperature dependence near the translational energy threshold for reaction. The reactivity of molecules excited to V ) 1 of the ν3 C-H stretching vibration and incident on the surface with a translational energy of 40 kJ mol-1 increased 8-fold as the surface temperature increased from 90 to 475 K. This enhancement is much larger than that reported for earlier studies at higher incident energies. These results support recent calculations that predict an important role for lattice deformation in transition state access. At higher surface temperatures, surface phonon excitation allows substrate atoms to sample lattice geometries with more favorable transition state energetics. We have also measured the coverage-dependent reactivity of these molecules at both surface temperatures and report a simple model that quantitatively predicts the observed coverage-dependent reactivity. Introduction Methane’s dissociative chemisorption into surface-bound CH3 and H fragments is a prototypical system for the study of surface reaction dynamics.1-7 It is rate-limiting in the industrially important steam reforming reaction that transforms methane and water into CO and H2 (syngas),8 a benchmark for theoretical models of gas-surface reactions and a model for alkane reactivity. The system offers a rich energetic complexity for dynamical study. As a polyatomic molecule, methane has four distinct stretching and bending vibrations that may differ in their ability to promote transition state access. Many experimental studies have explored how methane’s energetic coordinates promote its dissociative chemisorption (see ref 2 and citations therein). Early molecular beam experiments established that translational (Etrans) and vibrational (Evib) energy activate dissociation, but they relied on thermal excitation of methane’s vibrations and were unable to distinguish the reactivity of individual vibrational states.9-15 More recently, experiments that combine molecular beam-scattering and stateselective optical excitation of the methane reagent prior to surface impact have provided clear evidence for mode- and bond-selective behavior in which the identity of the vibrationally excited state, and not just its energy, dictates reactivity.16-21 Taken together, these results contradict assumptions underlying a purely statistical model of reactivity,22 and they indicate that energy exchange among the translational and vibrational coordinates of the methane molecule is incomplete on the reaction time scale. In contrast to the many experiments that have focused on the energetic coordinates of the gas-phase methane reagent, far fewer studies have explored how vibrational (phonon) energy in the surface affects reactivity. Internal state averaged molecular beam studies of methane activation showed a modest surface temperature (Ts) dependence on Ni(100)23,24 and Ni(111)12 and * Corresponding author. E-mail: [email protected]. † Current Address: The James Franck Institute, The University of Chicago. ‡ Current Address: Air Force Research Laboratory, Space Vehicles Directorate, Hanscom Air Force Base. § Current Address: Technical Consultant, [email protected].

a stronger dependence on Pt(111),13 Ir(111),25 and Ru(0001).26 Bulb experiments, in which the methane gas and the metal surface are in thermal equilibrium, suggested that Evib in methane is more effective at promoting reactivity than Evib in the metal surface, but again, there was no clear separation in the contribution of each type of energy to the observed reactivity.26-28 Recent transition state calculations have renewed interest in exploring the role of surface phonon excitation in methane activation.29-33 They predict that the dissociation of CH4 into surface-bound CH3 and H occurs over an on-top site on the Ni(111) surface and that the Ni atom beneath the active C-H bond puckers up by 0.25 Å at the minimum transition state energy. Thus, surface phonon excitation may provide access to reaction geometries with more favorable energetic requirements. Here, we describe state-resolved beam surface scattering measurements that compare the reactivity of methane at surface temperatures of 90 and 475 K. Our approach benefits from precise control over reagent energetics. Laser excitation prepares gas-phase methane in a single rotational and vibrational quantum state with precisely defined internal energy, while the molecular beam defines translational energy. By eliminating averaging over methane’s energetic degrees of freedom, our experiments accentuate the effects of surface temperature on methane’s dissociative chemisorption. We focus on methane molecules prepared with a total energy (Etrans + Evib) near the threshold energy for reaction. In this way, the experiments more clearly reveal the importance of surface atom motion in promoting methane’s dissociative chemisorption. During the course of these studies, we employed and validated a temperature-programmed desorption (TPD) method for quantifying methane’s dissociative chemisorption products, and we monitored the coverage dependence of reactivity to extract values of the reaction probability in the limit of zero coverage, S0. We found that a simple Langmuirian site-blocking model quantitatively predicts this surface coverage dependence, and we describe this model. Experimental Approach Prior publications describe our experimental apparatus and methods in detail.2,19,34,35 Here, we summarize key details and

10.1021/jp9065339 CCC: $40.75  2009 American Chemical Society Published on Web 11/04/2009

Surface Temperature Activation highlight new methods. Mixtures of methane (1% and 5%) seeded in helium 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) sample housed in an ultrahigh vacuum (UHV) chamber. Numerous control experiments, detailed in ref 2, verified that our results were not influenced by the adsorption of background gases or impurities in the beam or main chamber. For example, when the beam impinged on a 90 K surface at a glancing angle for 3 h (where the normal energy was insufficient to activate methane dissociation but reactive impurities and background gases could still adsorb), AES measurements indicated C and O accumulation less than our 0.01 ML detection limit. All surface temperatures investigated were well above the temperature at which CH4 desorbs from the surface, so molecularly adsorbed methane does not interfere with the measurements either. Methane molecules in the beam had a narrow Etrans distribution (∆Etrans/Etrans < 5% as determined by time-of-flight measurements) and were rotationally cold (as determined by rotationally resolved IR absorption measurements in the beam) but had a vibrational state distribution related to a Boltzmann distribution at the nozzle source temperature (550, 600, and 830 K for these studies).36 The UHV chamber contained standard surface-science instruments including a quadrupole mass spectrometer for time-of-flight beam characterization and temperature programmed desorption detection and an Auger electron spectrometer (AES) for quantifying carbon coverage and verifying surface cleanliness. In the state-resolved experiments, 3-5 mW of narrow bandwidth infrared light from a continuous wave color center laser excited 9.6% of the methane molecules in the beam from V ) 0 to ν3 (V ) 1, J ) 2) via the R(1) transition at 3038.495 cm-1. A computer-controlled feedback loop locked the laser frequency to the transmission fringe of a temperaturestabilized high-resolution Fabry-Perot etalon, and we verified that the laser maintained resonance with the methane absorption feature in the beam by inserting a room temperature pyroelectric bolometer into the beam to quantify absorption before and after each experiment. On ) and without We measured reaction probability with (SLaser 0 Laser Off ) laser excitation by exposing a beam of known flux to (S0 the surface for a fixed time. We then quantified the surface bound reaction products using one of two independent methods and divided the yield of surface bound products by the integrated incident flux to obtain S0. One method used AES to measure C coverage (θC), as the methyl dissociative chemisorption product is the only C-containing species that remains bound to the surface under our experimental conditions. In the second method, we used a temperature-programmed desorption (TPD) detection method to titrate surface bound methyls with H atoms embedded beneath the Ni surface. This approach was originally reported by Johnson et al.37 At about 190 K, the subsurface H reacted with surface methyls to form methane, which promptly desorbed. Desorption is quantitative, with all methane desorbing in a sharp peak only several kelvin wide. In addition, the method is selective and sensitive; it only detects surface-bound methyls and has a detection limit of ca. 0.001 ML. A detailed discussion of our implementation of this method, including representative TPD traces, appears in refs 1, 2, 19, and 35. We integrated the resulting methane desorption peak to quantify methyl coverage. We found this second method to be a much more sensitive and precise measure of reactivity. It is important to note that for the Ts ) 90 K data set only a limited range of Etrans is accessible using He as a carrier gas. Unlike the case for higher surface temperatures, H2 recombinative desorption is slow, and H2

J. Phys. Chem. C, Vol. 113, No. 48, 2009 20619

Figure 1. Integrated area of TPD desorption peak resulting from bulk H addition to surface bound methyls (left axis, filled squares) and MLs of C (θC) measured by AES (right axis, open circles) plotted vs dose time.

dissociation products would promptly passivate the surface toward methane dissociation at 90 K.2 Calibration experiments verified that both methods for quantifying reaction products yielded comparable results. We measured the reactivity of a 5% mixture of CH4 in He expanded through a 830 K nozzle (Etrans ) 62 kJ mol-1, flux ≈ 2.1 ML CH4 s-1) without laser excitation to provide a highly reproducible and spatially uniform reactive flux of methane molecules for dose-time-dependent calibration experiments. Figure 1 shows both TPD peak integrals for our bulk-H titration method (left axis) and AES measurements of C coverage (θC, right axis) plotted as a function of methane beam exposure time. The surface was held at 90 K during the methane dose. The figure shows that the TPD integrals and AES-based measurements of CH3 and C coverage, respectively, correlate well. Two other points are noteworthy. Error bars, which correspond to (2σ for replicate measurements, are generally smaller for the TPD method at a given dose time, and the TPD method is a more sensitive method for detecting low methyl coverage. While these experiments were performed without laser excitation, we observe analogous saturation behavior for our laser excitation studies. Our initial measurements on a 90 K surface showed that S0 decreased significantly upon lowering the surface temperature from 475 to 90 K. Before ascribing this decrease to surface activation, we examined the coverage dependence of S0 to assess whether site blocking by surface-bound reaction products could account for the observed reactivity difference. HREELS studies have shown that the methyl and H products of methane’s dissociative chemisorption accumulate on the surface at 90 K, but at 475 K, CH3 dehydrogenates and the surface-bound H atoms recombinatively desorb leaving only C on the surface.38 Therefore, it is reasonable to suspect that site-blocking effects may differ as a function of surface temperature. Figure 1 shows that methane dissociation on a 90 K surface results in an asymptotic carbon saturation coverage slightly more than 0.20 ML. We compare AES-based S0 measurements on a 475 K surface with those of a 90 K surface in Figure 2. The limiting C coverage on a 475 K surface is 0.43 ML, which is consistent with prior published reports6,39 but about twice that observed at 90 K. A simple Langmuirian site-blocking model reproduces these data and also accounts for the small, but measureable, site blocking effects of residual H that remains on the surface following our bulk H preparation step. We use the results of this model, which we describe in the following paragraphs, to identify the coverage range where reactivity

20620

J. Phys. Chem. C, Vol. 113, No. 48, 2009

Killelea et al.

Figure 2. Carbon accumulation as a function of dose time at surface temperatures of 90 and 475 K. The data show the effects of site blocking on reactivity. The solid lines are predictions of a site-blocking model described in the text.

averaged over dose time properly reflects the zero-coverage reactivity, S0, and to assess the impact of residual H on reactivity. We derive an expression for the dose-time-dependent C coverage, θC(t), as follows. If one assumes that dissociative chemisorption requires two adjacent surface vacancies, then eq 1 relates the coverage-dependent reaction probability for an incident methane molecule, S(θC), to the reaction probability on a clean surface, S0. The factor ζ is the number of surface sites that each dissociating methane molecule blocks, and it relates the fractional coverage of C atoms, θC(t), to the effective surface coverage of dissociation products at time t.

S(θ) ) S0(1 - ζθC)2

(1)

In experiments using the subsurface H titration method, a very small coverage of H atoms remains on the surface following preparation of the subsurface H titrant. These H atoms can also block methane dissociation, and we account for their site-blocking effects by introducing their fractional coverage at the start of the methane dose, θH,i in eq 2. Our best fits to TPDbased measurements where this small coverage is present have θH,i ) 0.03 ML, which is in excellent agreement with the HREELS-based measurement of residual surface H coverage reported by Maynard et al.40

Figure 3. Reaction probability for CH4 on Ni(111) as a function of Etrans. The solid squares are ν3 state-resolved sticking, and the open squares are laser-off sticking at Ts ) 90 K. Error bars are 2σ confidence intervals. The solid lines represent the ν3 and laser-off data from ref 22 at Ts ) 475 K. Both data sets show a decrease in reactivity as Ts decreases from 475 to 90 K.

In Figure 2, the carbon uptake curve at Ts ) 475 K is best fit with ζ ) 2 and that at 90 K with ζ ) 4, indicating the CH3 and H fragments present at 90 K block twice as many sites as the C atom that remains on the surface at 475 K. This difference has a significant effect on the asymptotic C coverage at the two surface temperatures. The model suggests that while site blocking becomes significant at coverages above θC ≈ 0.10 ML, experiments performed at θC < 0.10 ML will closely approximate the low-coverage limit. Furthermore, we find that the presence of residual H on the surface following bulk H preparation has a reproducible, but minor, impact on S0 measurements performed at low coverage. On the basis of these results, all S0 values reported here were measured for θC < 0.10 ML where the number of blocked sites was small and corrections for site blocking were unnecessary. Our measurements of the average reaction probability with and without laser excitation allow us to calculate a state-resolved reaction probability for the laser-excited state. Equation 3 relates On Off and SLaser , the reactivity of the laser-excited state, Sν03, to SLaser 0 0 the fraction of molecules the laser excites (fexc ) 9.6% in these 2,41 experiments), and the vibrational ground state reactivity, Sν)0 0 .

Sν03

S(θ) ) S0(1 - θH,i - ζθC)2

(2)

The change in coverage that occurs during an interval dt centered at time t for an incident methane flux, f (ML s-1), is

dθC ) f S0(1 - θH,i - ζθC(t))2 dt

(4)

Finally, we integrate eq 4 to obtain the total C coverage deposited during the dose

On Off SLaser - SLaser 0 0 ) + Sν)0 0 fexc

(3)

When Etrans is relatively low, as is the case for all data reported here, S0ν)0 contributes negligibly to S0ν3 and can be neglected. Finally, we note that the reaction probability for methane incident on a Ni(111) surface is not significantly altered by the presence of subsurface H. Reaction probabilities measured on 90 K Ni(111) surfaces with and without subsurface H differ only by the slight site-blocking effect of the ca. 0.03 ML of H that remains on the surface after preparation of the subsurface H is complete. Results and Discussion

θC(t) )

[

(1 - θH,i) (1 - θH,i)(ζ · f · S0 · t) ζ 1 + (1 - θH,i)(ζ · f · S0 · t)

]

(5)

We measured the state-resolVed reactivity for CH4 molecules incident along the surface normal on a 90 K Ni(111) surface. Figure 3 shows internal state-averaged (S0Laser Off, circles) and state-resolved (Sν03, (ν3, V ) 1, J ) 2), squares) data. The symbols

Surface Temperature Activation were measured for a 90 K surface, and the solid lines represent reported measurements for CH4 dissociation on the same surface at 475 K.20,21 The ν3 antisymmetric C-H stretch quantum has Evib ) 36 kJ mol-1, so at the translational energies studied Etrans + Evib ranges from 76 to 86 kJ mol-1, which is comparable to experimentally measured activation energies on the Ni(111) surface. We find that as Etrans decreases over this narrow energy range S0ν3 for the two surface temperatures diverges rather dramatically. Although the data reported here span a narrow range of Etrans, they provide clear evidence that surface temperature can have a significant impact on reactivity. At Etrans ) 40 kJ mol-1, S0ν3 increases 8-fold as the surface temperature increases from 90 to 475 K. When we increase Etrans by 9 to 49 kJ mol-1, this enhancement decreases to a factor of 2. The Ts enhancement of S0Laser Off is weaker and does not show the same strong dependence on Etrans. When Etrans increases from 40 to 49 kJ mol-1, the reactivity difference between the 90 and 475 K surfaces decreases from 2.5 to 1.5. The difference in reactivity with respect to surface temperature shows a strong dependence on Etrans for the S0ν3 data set, Off data are much more weakly dependent on Etrans. but the SLaser 0 This difference likely results from vibrational state ensemble averaging inherent to laser-off experiments. The use of He as a carrier gas required us to use elevated nozzle temperatures to access Etrans g 40 kJ mol-1. When Etrans ) 40 kJ mol-1, Etrans alone is not sufficient to surmount the energetic threshold for reaction, but the internal energy of thermally populated vibrational states in the beam can activate reaction. The relatively broad thermal spread of vibrational energies at Tn ) 600 K blurs the energy threshold for reaction along the Etrans coordinate and lessens the impact of Ts on reactivity. In contrast, the laser-excited molecules have a well-defined total energy. When the total energy of the methane reagent (Evib + Etrans) is very near the threshold for reaction, small changes in the surface phonon energy or surface geometry spell the difference between reactive and nonreactive scattering because the surface is the only remaining source of energy for activation. The data clearly reveal this effect. At Etrans ) 40 kJ mol-1, increasing the surface temperature from 90 to 475 K increases reactivity by nearly an order of magnitude. Increasing Etrans to 49 kJ mol-1 increases the quantity of Etrans available for reaction and decreases the necessity for surface energy to surmount the reaction barrier. Therefore, these data provide direct experimental evidence that methane activation on Ni(111) is not only sensitive to the vibrational energy and mode of the incident methane molecule but also extremely sensitive to the extent of vibrational excitation of the surface near the energy threshold for reaction. Since reaction paths near the threshold dominate thermal reactivity in activated reactions, the data also indicate that lattice excitation is likely a key factor in methane activation under thermal conditions. Recent theoretical calculations on the CH4/Ni(111) system shed further light on our results.30-33 While there has been general agreement for some time that methane dissociation occurs over an on-top site on the surface, with the H and CH3 fragments moving toward adjacent 3-fold hollows,42 recent studies have focused on the role of surface atom position by permitting lattice relaxation during the optimization of transition state geometry. The results suggest that the position of the Ni atom beneath the active C-H bond plays a very important role in transition state energy. For example, Nave and Jackson found that when the Ni atom puckers 0.2 Å above the surface the transition state energy decreases by 21 kJ mol-1, but when the

J. Phys. Chem. C, Vol. 113, No. 48, 2009 20621 Ni atom sinks beneath the surface plane by 0.2 Å, the transition state energy increases by 25 kJ mol-1.31 Thus, Ni atom position modulates the threshold energy for reaction by a chemically significant amount. Phonon excitation can enhance reactivity in two ways.31 In the first, vibrational energy in a phonon mode increases the energy available to the reaction complex. Reaction rate theories predict that additional energy in the reaction complex will increase the reaction rate. The second effect arises from geometric factors. At low surface temperature, surface phonons are in their vibrational ground state, and the lattice is relatively flat. As temperature increases, phonon excitation will cause some Ni atoms to move above the surface. Transition state calculations indicate that this type of distortion provides access to reactive geometries with lower threshold energies. Thus, surface temperature can simultaneously increase the energy available for reaction and decrease the energy threshold for reaction. Under these conditions, the vibrational efficacy for promoting reaction, i.e., the relative efficacy for Evib relative to Etrans for promoting reaction, can exceed unity. In our experiments, the classical vibrational period of a surface phonon mode is long relative to the subpicosecond interaction time of a hyperthermal CH4 molecule and the surface. At 90 K, over 93% of the surface phonons are in their ground vibrational state and are not significantly displaced from their equilibrium position in the surface plane. Therefore, most methane molecules sample a surface atom geometry that while not optimal is relatively homogeneous. As surface temperature increases, Ni atoms begin to experience displacements above and below the surface plane. Since there is little surface atom motion during the duration of the methane-Ni collision, some methane molecules will impact a Ni atom when it is displaced well above the surface plane and experience a reduced Etrans threshold for reaction. Thus, surface phonon excitation opens up new channels for reaction. Nave and Jackson predict that even 2 quanta of surface phonon excitationsa degree of surface activation readily accessible at a surface temperature of 475 Kscan lead to a significant enhancement in S0.31 At total energies very near the threshold for reaction, molecules whose incident Etrans is below the threshold for reaction on a flat surface may react if they encounter a Ni atom momentarily displaced above the surface. This can result in a large absolute increase in reaction probability for those members of the incident ensemble of methane molecules. Alternatively, an incident CH4 may impinge on a Ni atom that has receded beneath the surface plane and experience a higher barrier to reaction. Since that molecule’s reactivity on a flat surface was already vanishingly small, any increase in the energy threshold to reaction can only decrease an already low reactivity. When one calculates the (algebraic) ensemble average over surface atom displacements, the absolute reactivity increase resulting from a Ni atom displacement above the surface more than compensates for the decrease in reactivity of a nearly nonreactive trajectory. Thus, the average reactivity increases with increasing excitation of the surface phonon oscillator. We note that this picture differs from the thermally assisted tunneling model proposed by Luntz and Harris 43,44 in two ways. First, it suggests that phonon excitation not only alters the effective collision energy of the gas-surface encounter but also modulates the energy threshold for reaction. Second, it suggests a difference in the surface atom location at the point of highest reactivity. If barrier height modulation dominates, one would expect a reactivity bias for reactive trajectories to occur near the outer turning point of the surface atom’s vibration above the surface.

20622

J. Phys. Chem. C, Vol. 113, No. 48, 2009

In contrast, thermally assisted tunneling reaction probabilities peak at the maximum relative velocity of the surface atom and the incident methane; that model would predict that reaction occurs when the Ni atom is moving toward vacuum and passing through the surface plane. Our work and the calculations of Nave and Jackson address a long-standing question surrounding methane activation on lowindex surfaces of catalytically active metals. Prior beam-surface experiments have uncovered a pronounced increase in CH4 reactivity with increasing Ts on Pt13 and Ir25. Prior studies suggested that this effect is significantly less pronounced on Ni surfaces even though transition state calculations suggest a prominent role for surface lattice distortion on all three metals. Electronic structure and dynamics calculations reveal the likely cause for this apparent discrepancy.31 The necessary lattice distortion required for transition state access on Ni requires less energy due to the Ni atom’s lighter mass relative to Pt or Ir. Even at relatively low surface temperatures (300-500 K) thermal activation of the surface phonons is sufficient to access the critical geometry required to minimize transition state energy. Increasing Ts further provides little additional reactivity. On Pt and Ir surfaces, dislocating the heavy surface atom far enough to minimize transition state energy takes more energy, and much higher surface temperatures are needed to activate the reaction. Therefore, Ts effects remain pronounced over the temperature range typically examined experimentally on both Pt and Ir surfaces, but the effect saturates at temperatures below those previously examined on Ni surfaces. By extending measurements of surface temperature dependence to lower Ts and to translational energies near the threshold for reaction, we have succeeded in observing the important role that phonon excitation plays in direct dissociative chemisorption of methane on Ni. Conclusions Internal state-resolved measurements of methane dissociation on Ni(111) near the threshold energy for translational activation reveal a prominent role for surface atom motion in promoting methane activation. Reactivity measurements at Etrans ) 40 kJ mol-1 reveal an 8-fold increase in dissociative chemisorption probability as Ts increased from 90 to 475 K. These results are consistent with recent theoretical predictions of a significant surface atom distortion at the transition state. Increasing Ts both increases the energy available to reaction and also results in surface phonon excitation that allows surface atoms to sample more energetically favorable transition state geometries. In the course of making these measurements, we investigated the coverage-dependent reactivity of methane on Ni(111) at two different surface temperatures and report a simple Langmuirian site-blocking model consistent with our data. Finally, we note that these results establish the utility of state-selected reagents as probes of gas-surface reaction energetics in a wide range of energetic degrees of freedom. We are currently performing experiments in our lab over an expanded range of Etrans and Ts to better assess the efficacy of vibrational phonons in promoting reaction and to identify the relative importance of barrier height modulation on this prototypical gas-surface reaction. Acknowledgment. We thank the National Science Foundation for financial support of this work (CHE-0415574 and CHE0809802). V.L.C. thanks the U.S. Department of Education for a GAANN fellowship. References and Notes (1) Utz, A. L. Curr. Opin. Solid State Mater. Sci. 2009, 13, 4.

Killelea et al. (2) Juurlink, L. B. F.; Killelea, D. R.; Utz, A. L. Prog. Surf. Sci. 2009, 84, 69. (3) Luntz, A. C. The Dynamics of Making and Breaking Bonds at Surfaces. In Chemical Bonding at Surfaces and Interfaces; Nilsson, A., Pettersson, L. G. M., Nørskov, J. K., Eds.; Elsevier: Amsterdam, 2008; p 143. (4) Beck, R. D.; Rizzo, T. R. Chimia 2004, 58, 306. (5) Weaver, J. F.; Carlsson, A. F.; Madix, R. J. Surf. Sci. Rep. 2003, 50, 107. (6) Larsen, J. H.; Chorkendorff, I. Surf. Sci. Rep. 1999, 35, 165. (7) Ceyer, S. T. Science 1990, 249, 133. (8) Rostrup-Nielsen, J. R.; Sehested, J.; Nørskov, J. K. Hydrogen and synthesis gas by steam- and CO2 reforming. In AdVances in Catalysis; Knöpffer, H., Gates, B. C., Eds.; Academic Press: San Diego, 2002; Vol. 47, p 65. (9) Rettner, C. T.; Pfnu¨r, H. E.; Auerbach, D. J. Phys. ReV. Lett. 1985, 54, 2716. (10) Rettner, C. T.; Pfnu¨r, H. E.; Auerbach, D. J. J. Chem. Phys. 1986, 84, 4163. (11) Lee, M. B.; Yang, Q. Y.; Tang, S. L.; Ceyer, S. T. J. Chem. Phys. 1986, 85, 1693. (12) Lee, M. B.; Yang, Q. Y.; Ceyer, S. T. J. Chem. Phys. 1987, 87, 2724. (13) Luntz, A. C.; Bethune, D. S. J. Chem. Phys. 1989, 90, 1274. (14) Oakes, D. J.; Newell, H. E.; Rutten, F. J. M.; McCoustra, M. R. S.; Chesters, M. A. Chem. Phys. Lett. 1996, 253, 123. (15) Schoofs, G. R.; Arumainayagam, C. R.; McMaster, M. C.; Madix, R. J. Surf. Sci. 1989, 215, 1. (16) Bisson, R.; Dang, T. T.; Sacchi, M.; Beck, R. D. J. Chem. Phys. 2008, 129, 081103. (17) Maroni, P.; Papageorgopoulos, D. C.; Sacchi, M.; Dang, T. T.; Beck, R. D.; Rizzo, T. R. Phys. ReV. Lett. 2005, 94, 246104. (18) Beck, R. D.; Maroni, P.; Papageorgopoulos, D. C.; Dang, T. T.; Schmid, M. P.; Rizzo, T. R. Science 2003, 302, 98. (19) Killelea, D. R.; Campbell, V. L.; Shuman, N. S.; Utz, A. L. Science 2008, 319, 790. (20) Juurlink, L. B. F.; Smith, R. R.; Killelea, D. R.; Utz, A. L. Phys. ReV. Lett. 2005, 94, 208303. (21) Smith, R. R.; Killelea, D. R.; DelSesto, D. F.; Utz, A. L. Science 2004, 304, 992. (22) Abbott, H. L.; Bukoski, A.; Harrison, I. J. Chem. Phys. 2004, 121, 3792. (23) Holmblad, P. M.; Wambach, J.; Chorkendorff, I. J. Chem. Phys. 1995, 102, 8255. (24) Luntz, A. C. J. Chem. Phys. 1995, 102, 8264. (25) Seets, D. C.; Reeves, C. T.; Ferguson, B. A.; Wheeler, M. C.; Mullins, C. B. J. Chem. Phys. 1997, 107, 10229. (26) Egeberg, R. C.; Ullmann, S.; Alstrup, I.; Mullins, C. B.; Chorkendorff, I. Surf. Sci. 2002, 497, 183. (27) Abild-Pedersen, F.; Lytken, O.; Engbaek, J.; Nielsen, G.; Chorkendorff, I.; Norskov, J. K. Surf. Sci. 2005, 590, 127. (28) Ølgaard Nielsen, B.; Luntz, A. C.; Holmblad, P. M.; Chorkendorff, I. Catal. Lett. 1995, 32, 15. (29) Henkelman, G.; Jonsson, H. Phys. ReV. Lett. 2001, 86, 664. (30) Henkelman, G.; Arnaldsson, A.; Jonsson, H. J. Chem. Phys. 2006, 124, 044706. (31) Nave, S.; Jackson, B. J. Chem. Phys. 2009, 130, 054701. (32) Nave, S.; Jackson, B. J. Chem. Phys. 2007, 127, 224702. (33) Nave, S.; Jackson, B. Phys. ReV. Lett. 2007, 98, 173003. (34) McCabe, P. R.; Juurlink, L. B. F.; Utz, A. L. ReV. Sci. Instrum. 2000, 71, 42. (35) Killelea, D. R.; Campbell, V. L.; Shuman, N. S.; Utz, A. L. J. Phys. Chem. C 2008, 112, 9822. (36) Bronnikov, D. K.; Kalinin, D. V.; Rusanov, V. D.; Filimonov, Y. G.; Selivanov, Y. G.; Hilico, J. C. J. Quant. Spectrosc. Radiat. Transfer 1998, 60, 1053. (37) Johnson, A. D.; Daley, S. P.; Utz, A. L.; Ceyer, S. T. Science 1992, 257, 223. (38) Yang, Q. Y.; Maynard, K. J.; Johnson, A. D.; Ceyer, S. T. J. Chem. Phys. 1995, 102, 7734. (39) Holmblad, P. M.; Larsen, J. H.; Chorkendorff, I. J. Chem. Phys. 1996, 104, 7289. (40) Maynard, K. J.; Johnson, A. D.; Daley, S. P.; Ceyer, S. T. Faraday Discuss. Chem. Soc. 1991, 91, 437. (41) Juurlink, L. B. F.; McCabe, P. R.; Smith, R. R.; DiCologero, C. L.; Utz, A. L. Phys. ReV. Lett. 1999, 83, 868. (42) Kratzer, P.; Hammer, B.; Nørskov, J. K. J. Chem. Phys. 1996, 105, 5595. (43) Harris, J.; Simon, J.; Luntz, A. C.; Mullins, C. B.; Rettner, C. T. Phys. ReV. Lett. 1991, 67, 652. (44) Luntz, A. C.; Harris, J. Surf. Sci. 1991, 258, 397.

JP9065339