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Vibrational Mode-Specific Reaction of Methane on a Nickel Surface

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Science  03 Oct 2003:
Vol. 302, Issue 5642, pp. 98-100
DOI: 10.1126/science.1088996

Abstract

The dissociation of methane on a nickel catalyst is a key step in steam reforming of natural gas for hydrogen production. Despite substantial effort in both experiment and theory, there is still no atomic-scale description of this important gas-surface reaction. We report quantum state–resolved studies, using pulsed laser and molecular beam techniques, of vibrationally excited methane reacting on the nickel (100) surface. For doubly deuterated methane (CD2H2), we observed that the reaction probability with two quanta of excitation in one C-H bond was greater (by as much as a factor of 5) than with one quantum in each of two C-H bonds. These results clearly exclude the possibility of statistical models correctly describing the mechanism of this process and attest to the importance of full-dimensional calculations of the reaction dynamics.

The reaction of methane on a nickel catalyst to form surface-bound methyl and hydrogen is the rate-limiting step in steam reforming, which is the principal process for industrial hydrogen production as well as the starting point for the large-scale synthesis of many important chemicals such as ammonia, methanol, and higher hydrocarbons (1). Because of its importance, the dissociation of methane on nickel has been considered a prototype for chemical bond formation between a polyatomic molecule and a solid surface, with many experimental and theoretical studies directed at elucidating its mechanism (214). In view of the enormous economic importance of this process (15), it would be desirable to have a reliable theoretical description that could guide the development of improved catalysts (2, 3). Despite intense effort, there is still no atomic-scale picture of the dynamics of this important gas-surface reaction.

Molecular beam experiments (46) have firmly established that methane chemisorption is a direct process that can be activated with about equal efficiency by both incident kinetic energy normal to the surface and thermal vibrational energy of the incident methane. State-resolved reactivity measurements that used laser excitation of the asymmetric stretch fundamental vibration (ν3) (7) and first overtone (2ν3) (8) of CH4 incident on Ni(100) have confirmed the notion that vibrational energy is similar to translational excitation in its efficiency in promoting this reaction. Theoretical treatments of methane chemisorption have included wave packet simulations with up to nine vibrational degrees of freedom (9), reduced-dimensionality dynamical models with only a single C-H stretch vibration (1012), and a greatly simplified statistical model (13). Despite having diametrically opposed presuppositions, both dynamical and statistical approaches claim to reproduce existing experimental data (10, 16), although they make different predictions about the role of methane vibrational excitation in promoting the reaction. Some dynamical calculations suggest that the reactivity of vibrationally excited methane on nickel should depend on the precise nature of the vibrational mode (9, 17), whereas statistical models predict the complete absence of such effects (16). Although the reverse process— the associative desorption of methane from transition metal surfaces—seems to deviate somewhat from a purely statistical model (18, 19), the experimental results reported thus far do not exclude either approach, because there is no reported evidence for mode specificity in the surface reaction of methane.

In contrast, mode-specific reactivity of methane in the gas phase has been observed. Yoon et al. (20) have found that when methane is excited to the symmetric stretch-bend combination ν1 + ν4, it is more reactive with atomic chlorine (by a factor of 1.9) than when it is promoted to the nearly isoenergetic antisymmetric combination ν3 + ν4. In a similar study, Kim et al. (21) observed that the product state distribution for the reaction of CD2H2 with chlorine depends on the initially prepared reactant vibrational state. Two states with similar energy but different C-H bond stretching amplitudes produce a CD2H methyl fragment in completely different vibrational states. Both studies show that vibrational energy put into specific modes of methane is not redistributed internally by the reactive encounter, but instead contributes in a mode-specific way to promoting the chemical reaction. It remains unknown whether similar nonstatistical dynamics would occur in the encounter of a gas-phase molecule with a solid surface.

To test for vibrational mode–specific behavior in gas-surface reactions, we performed quantum state–resolved measurements in a molecular beam surface science apparatus, using pulsed laser preparation of vibrationally excited methane molecules incident on a Ni(100) single-crystal surface (22). Figure 1 shows a schematic illustration of our experiment. After a deposition time adjusted to produce a surface coverage of about 5% of a monolayer (ML), the products of the dissociative chemisorption of methane were detected as surface-bound carbon by Auger electron spectroscopy. We quantify the amount of deposited carbon by recording C and Ni Auger spectra in a line scan across the surface and calibrate it by comparison with the C/Ni Auger ratio for a known 0.5-ML saturation coverage obtained by extended exposure to a high-energy beam of methane. We use the doubly deuterated isotopomer of methane (CD2H2) rather than CH4 for these experiments because of its spectroscopic properties (23). In CD2H2, combinations of the two infrared (IR) active C-H stretch fundamentals (ν1 and ν6) form nearly isoenergetic states with comparable absorption strength but different nuclear motion, labeled |20〉 and |11〉 in local mode notation (24).

Fig. 1.

Schematic of experimental setup. We prepared methane under collision-free conditions by expanding it in a mixture with hydrogen gas through a solenoid-actuated pulsed valve operating at 20 Hz. The resulting supersonic jet expansion accelerates the methane molecules to a kinetic energy controlled by the methane/H2 seed ratio and the valve temperature, as determined by time-of-flight measurements. The free-jet expansion was skimmed, and the resulting molecular beam was sliced into 30-μs pulses by a chopper wheel. Before the molecular beam impinged on the sample surface (Ni single crystal 10 mm in diameter, cut within 0.1° of the 100 plane), it was irradiated by pulsed, tunable IR light to prepare a fraction (1 to 2%) in a specific ro-vibrationally excited quantum state. Sticking coefficients averaged over all vibrational states of methane populated in the beam were obtained from the ratio of the carbon coverage to the incident methane dose. Sticking coefficients for the state-selected methane molecules were calculated from the difference obtained with the laser on and off, because contributions from the small amount of thermally excited vibrational states in the beam drop out of this difference (7).

CD2H2 in the molecular beam was excited into either the |20〉 or the |11〉 C-H stretch local mode state (25). A comparison of the surface carbon Auger signals for incident CD2H2 excited to these two states is shown in Fig. 2. To obtain these data, we directed a molecular beam containing 20% CD2H2 in H2 at normal incidence for 15 min at two different positions on the initially clean Ni(100) surface. For the first deposition (left-hand peak), the |20〉 state of CD2H2 with one unit of angular momentum (J = 1) was excited by 120-mJ pulses from an IR laser tuned to the Δ J = 0 transition at 5879.8 cm–1, and for the second deposition (right-hand peak), the J = 1 level of the |11〉 state was prepared using the same IR pulse energy to excite the corresponding transition at 6000.2 cm–1. We used cavity ring down spectroscopy (CRDS) (26) in a separate free-jet expansion to ensure that the excitation laser remained resonant with the selected transition throughout the deposition. CRDS was also used to determine an upper limit for the rotational temperature of 9 K for the CD2H2 in the beam and to measure the absolute transition strength. Although the transition we used to prepare the |20〉 level is weaker than that used to excite the |11〉 level by a factor of 1.5 ± 0.1, the former leads to a carbon signal at least three times as large, indicating clear mode-specific behavior. Control experiments such as reversing the order and surface location of the deposition did not change the result. Experiments under identical beam conditions but without laser excitation showed no detectable carbon signal above the background. To measure the reactivity of the unexcited methane beam, we increased the flux by a factor of 80 and used a deposition time of up to 110 min. Our laser-off measurements represent only an upper limit for the reactivity of CD2H2 in the vibrational ground state because of a small fraction of thermally excited CD2H2 in the molecular beam. Both laser-on and laser-off measurements were made for a series of incident kinetic energies. At each energy, the experiment was repeated up to 10 times. Figure 3 shows the state-resolved sticking coefficients for CD2H2 determined from these measurements.

Fig. 2.

Surface carbon Auger signal for identical doses of CD2H2 excited to the |20〉 and |11〉 vibrational states incident on a Ni(100) surface at kinetic energy of 41 kJ/mol. The dotted line indicates the background level of carbon accumulated during the deposition and analysis time.

Fig. 3.

State-resolved sticking coefficients for CD2H2 in (from top to bottom) the |20〉 (♦), |11〉 (⚫), and ground (◼) vibrational states on Ni(100) as a function of incident kinetic energy normal to the surface. The surface temperature is 473 K.

At 41 kJ/mol, the lowest incident kinetic energy investigated so far, we find that CD2H2 is 5.4 times as reactive when promoted to the |20〉 state relative to the |11〉 state. The reactivity for both states is enhanced by several orders of magnitude with respect to incident molecules in the ground vibrational state with the same kinetic energy. The relative reactivity of the |20〉 and |11〉 states decreases with increasing kinetic energy, reaching a factor of 2 at a kinetic energy of 80 kJ/mol. At still higher kinetic energy we observe a continuation of this trend, although an accurate determination of the absolute reactivity becomes increasingly difficult as a result of the higher reactivity of the ground-state molecules. This decrease in mode specificity is likely due to the increase of the total amount of available energy relative to the reaction barrier. As the reaction probability approaches its asymptotic value, the difference between the two vibrational modes is expected to decrease. On the other hand, the mode selectivity should be even larger at lower kinetic energy.

The increased reactivity of the |20〉 level relative to |11〉 can be rationalized in terms of their different vibrational amplitudes: The former contains two quanta of stretch vibration in a single C-H bond, whereas the latter contains one quantum in each of two C-H bonds. In the gas-phase reaction of CD2H2 with chlorine, the product state distributions observed by Kim et al. (21) confirm this local mode description by demonstrating that one of the two bonds acts as a spectator during the reaction. The same description also rationalizes our observation of higher reactivity of the |20〉 state in terms of the larger vibrational amplitude along the C-H bond relative to |11〉. This difference in reactivity implies that the C-H bond stretch has a substantial projection on the reaction coordinate, in agreement with ab initio calculations of the transition-state structure (27).

Our observation of vibrational mode–specific gas-surface reactivity has important implications for theoretical treatments of this process. Mode-specific reactivity is inconsistent with the existence of a transient physisorbed complex, as has been suggested in the statistical model proposed by Harrison (13). His model assumes complete intramolecular redistribution of the initial vibrational energy in methane as it transiently resides in a local “hot spot” and weakly interacts with a limited number of surface atoms, and it determines rates for desorption and dissociation according to microcanonical rate theory. As a result, it predicts a reactivity that scales only with the total available energy independent of vibrational mode, which is inconsistent with our experimental results. In contrast to the assumptions of this (or any) statistical model, our observation that CH2D2 retains a clear memory of the initially prepared quantum state indicates that its interaction with the metal surface does not induce extensive intramolecular energy redistribution before the reaction occurs.

In addition to excluding statistical assumptions, the observation of mode specificity in the reaction probability provides guidance for dynamical models. It suggests that a realistic description of the chemical dynamics will need to go beyond low dimensionality. Calculations including all nine vibrational degrees of freedom of the incident molecule are now starting to become feasible (9, 28), and our experimental results provide stringent tests for such calculations as well as reassurance that efforts in this direction are warranted.

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