State selective study of the effect of molecular vibrations on the chemisorption probability on transition metal surfaces

(In collaboration with S. L. Bernasek)




The details of the mechanism of the dissociative chemisorption of methane on transition metal surfaces have been extensively studied both experimentally and theoretically due to the relevance of this process to catalysis, and in particular, as a model for C-H bond activation on metal surfaces. Methane dissociation is a direct, activated process on both platinum and nickel
surfaces in which the sticking probability increases with increasing incident translational energy.

An original qualitative model for the reactivity that was proposed involved thermally assisted tunneling through the reaction barrier which was based on the positive dependence of the sticking coefficient with increasing surface temperature and the measured kinetic isotope effect. The chemisorption mechanism is, however, not yet fully understood with respect to the dependence of the sticking coefficient on the different forms of energy involved in the reaction, including translational energy, thermal surface energy, and the internal energy of the methane molecule. For instance, since it is known that the barrier for reaction occurs on the exit channel, vibrational excitation of the incident methane should significantly increase the sticking probabilities.

What is not yet known is the relative efficacy of stretching or bending vibrational modes in the promotion of the dissociative chemisorption. To gain additional insight into the reaction mechanism and the state selective dependence on vibrational energy, we perform experiments that measure the dissociative chemisorption of methane on transition metal surfaces (Pt and Ni) prepared in single ro-vibrational states with well defined translational energy.

How do we do it?

To measure the quantity of methane that sticks to a metal surface and hence determine the sticking coefficient at zero coverage, we employ the technique of thermal energy atomic scattering (TEAS). The specular reflection of a thermal energy helium beam from a clean metal surface is very high and can be monitored using a mass spectrometer. As the methane
molecules stick to the surface, the specular intensity decays because some of the He atoms are scattered off the specular direction by the surface irregularities generated by the chemisorbed molecules. Since helium/methane mixtures are used in the experiment, the coverage of methane on the platinum surface can be monitored continuously throughout the experiment by tuning the mass spectrometer to m/z=4 as the crystal surface is exposed to the helium/methane from the molecular beam. TEAS is very sensitive to small coverages due to the large cross section for surface scattering of helium from absorbed molecules. In this way, we are assured that the sticking coefficients that are measured are a true representation of S0, since only the earliest fraction of the He specular decay which corresponds to very low coverages are used in the determination of S0.

The experimental apparatus consists of a UHV chamber coupled with a molecular beam source (a schematic of the experimental apparatus is shown in Figure 1). The UHV chamber where the platinum crystal is located is pumped by a 370 l/s turbomolecular pump and has a typical base pressure of 3x10-11 torr. The molecular beam source is pumped by a 3000 l/s
diffusion pump and consists of a supersonic expansion that is produced by expanding a high pressure gas mixture through a 23 micron quartz nozzle. The nozzle can be heated from 300K to 1100K, which along with changing the ratio of methane to helium, allows the adjustment of the translational energy of the methane in the beam. The chopped molecular beam is skimmed and passes through two turbomolecular-pumped chambers before entering the UHV chamber where it impinges on the crystal that is mounted on a six axis manipulator in the center of the chamber. A quadrupole mass spectrometer with an axial ionizer is mounted perpendicular to the molecular beam direction and monitors the He specular reflection at thetai=thetaf=45 degrees. After cleaning the crystal surface, the specular intensity of pure He is monitored for several hours to insure that the surface remains clean.

The methane molecules in the beam are vibrationally excited through the use of a continuous wave (CW) 1.5 micron Burleigh color center laser that is operated on a single longitudinal mode. The laser is actively stabilized to a 150 MHz external etalon using an intracavity electro-optic crystal. The linewidth of the laser is determined to be on the order of 1 MHz. The laser
radiation is transported to the molecular beam apparatus by way of a single mode optical fiber. The experiments consist of measuring the slope of the specular decay produced by the sticking of the excited methane and comparing it to the slope obtained when unexcited methane is adsorbed.

What have we done?

The translational energy dependence of the sticking coefficient is depicted in Figure 2 with a comparison to the data of Luntz and Bethune [1]. The platinum surface was maintained at 575 K in our experiments while those of Luntz and Bethune were obtained at Tsurf=800 K. If our measured surface temperature dependence of the sticking coefficient is taken into account, our data shows good quantitative agreement with that of Luntz and Bethune in the translational energy range of 20-55 kJ/mol.

Having established that the translational energy dependence of the sticking coefficient can be reliably measured using TEAS, we performed experiments where the laser was both on and off resonance with a transition to the 2n3 level while the molecular beam of CH4/He was impinging on the crystal. A beam of 80% CH4/20% He was used for the experiments in which laser excitation of the methane was performed. The nozzle was kept at 295 K which gave a normal translational energy of 5.4 kJ/mol. Figure 2 also displays the sticking coefficient of the molecules without laser excitation (blue triangle) providing the control experiment to verify the proper functioning of the experimental apparatus. Experiments were performed by tuning the laser to both the Q(1) line (6004.827 cm-1) and the R(1) line (6026.208 cm-1). By using these two lines, we could pump both the J=1 and J=2 rotational states of the excited vibrational level. In the analysis of the data, the effect of the rotational state will be neglected since the energy difference is small in comparison to the vibrational energy and hence a change in sticking coefficient was not detected within the errors of our experiment.

The mean sticking coefficient S0 of the control experiments without laser excitation was determined to be 6x10-6 at 5.4 kJ/mol normal translational energy. The total sticking coefficient derived from the specular decay curves of the experiments where the laser was exciting the transition was determined to be 2x10-5 included the contribution from both laser excited and unexcited methane. The state selective sticking coefficient of methane in the 2n3 J=1,2 states could be found by taking the difference between the control experiments and those with laser excitation and normalizing to the fraction of molecules excited in a given experiment. By following this procedure, S0 of the 2n3 J=1 and J=2 state of methane on Pt(111) is found to be 1.8x10-4 (Figure 2 red square). This represents an enhancement factor in the reactivity of 28-fold for methane with two quanta in the asymmetric stretch as compared to the reactivity of the ground vibrational state. Even at internal energies higher
than the barrier for dissociation, the efficacy of the vibrational energy is only 40%. This points to the fact that a truly statistical picture of the reaction mechanism is not adequate, as the total energy in the collision should be enough to overcome the barrier with near unit probability.

Figure 2. Comparison of the sticking coefficient of the 2n3 state of CH4 (red square) with the translational energy dependence obtained without laser excitation (green circle) on Pt(111) . The sticking coefficient of the control experiment at 5.4 kJ/mol is given by (blue up triangle). The error bars are 2sigma (95% confidence limits) of replicate measurements. Measurements obtained in this work using TEAS at a surface temperature of 575 K, (black down triangle)-data obtained from Luntz and Bethune at a surface temperature of 800 K [2].

What Next?

Experiments are currently being carried out in our laboratory to measure the vibrational activation of methane chemisorption on the (111), (100), and (110) faces of nickel. In the only other experiment of this kind ever reported, Utz and co-workers [2] have shown that the enhancement factor for excitation of the 1n3 vibrational level is greatest at the lowest incident translational energies. Our experiments on Ni should provide information on the dependence of the reactivity of methane on nickel as a function of the extent of the vibrational excitation.

We are also interested in pumping different modes of methane and determining how the sticking coefficient changes. In this way, we can determine if bending or stretching modes (or combinations thereof) are more effective at promoting chemisorption at energies comparable to the barrier height for dissociation. These studies can also be extended to include
other small molecules that have a low sticking coefficients on metal surfaces. Saturated hydrocarbons, both alkanes and those with functional groups, are good candidates for these studies while unsaturated hydrocarbons have been found to dissociatively chemisorb on Pt and Ni with high probabilities, irrespective of their vibrational excitation.

Finally, we are presently studying, using again He atom reflectivity as a detection method, the resurfacing of H atoms which have been buried under the surface by bombardment with 5eV Xe atoms. Subsurface hydrogen is an important and not well known factor in many surface chemical processes.

[1] A. C. Luntz and D. S. Bethune. J. Chem. Phys. 90, 1274 (1989).
[2] L. B. F. Juurlink, P. R. McCabe, R. R. Smith, C. L. DiCologero, and A. L. Utz. Phys. Rev. Lett. 83, 868 (1999).

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