Mark C. McMaster

Dissociative chemisorption of methane on Pt(111)

Abstract Dissociative chemisorption of methane on clean Pt(111) was studied with a supersonic molecular beam. Initial dissociative sticking probabilities from 0.01 to 0.19 were measured directly with incident total beam energies from 68 to 95 kJ/mol, surface temperatures from 500 to 1250 K, and angles of incidence from 0° to 45° measured from the surface normal. The nozzle temperature and stagnation pressure were both fixed so that the effect of translational energy at a fixed incident vibrational energy could be probed. The initial dissociative sticking probability of methane on clean Pt(111) equalled 0.06 ± 0.02 and was independent of surface temperature between 500 and 1250 K for a fixed normal incident kinetic energy of 68 kJ/mol, implying that dissociation proceeded via direct collisional activation rather than via trapping or precursor-mediated processes in this energy range. The initial dissociative sticking probability of methane on clean Pt(111) increased exponentially with increasing normal kinetic energy. The barrier height for C-H bond rupture by kinetic energy is 121 kJ/mol. The exponential dependence is consistent with a model for dissociative methane adsorption that involves quantum mechanical tunneling of a hydrogen atom through a one-dimensional, parabolic barrier of this height with a thickness at half height of 0.13 ± 0.01 A. Differences in the initial dissociative sticking probabilities observed on Pt(111) versus Ni(111) and W(110) in studies from different laboratories can be reconciled on the basis of the different vibrational energies employed, but this explanation does not account for the high reactivity on Ir(110)-(1 × 2). The activation barriers predicted by the bond order conservation theory of Shustorovich agree closely with the barrier heights estimated from the tunneling model for Pt(111), W(110), and Ir(110) if no correction for energy dissipation to the lattice is made. The activation barriers predicted by the molecular orbital analysis of Anderson and Maloney are much lower than the barrier heights estimated from the tunneling model. The barrier thicknesses show qualitative agreement with the C-H bond elongations in the transition state predicted by their molecular orbital theory, but do not correlate with the crystalline radii of the metal atoms or the stiffness of the metal lattices. Furthermore, the values of the tunneling parameters may be dependent on the vibrational and translational energies employed in the studies.

Trapping dynamics of xenon on Pt(111)

The dynamics of Xe trapping on Pt(111) was studied using supersonic atomic beam techniques. Initial trapping probabilities (S0) were measured directly as a function of incident translational energy (EinT) and angle of incidence (θi) at a surface temperature (Tins) 95 K. The initial trapping probability decreases smoothly with increasing ET cosθi;, rather than ET cos2θi, suggesting participation of parallel momentum in the trapping process. Accordingly, the measured initial trapping probability falls off more slowly with increasing incident translational energy than predicted by one-dimensional theories. This finding is in near agreement with previous mean translational energy measurements for Xe desorbing near the Pt(111) surface normal, assuming detailed balance applies. Three-dimensional stochastic classical trajectory calculations presented herein also exhibit the importance of tangential momentum in trapping and satisfactorily reproduce the experimental initial trapping probabilities.

Dynamics of molecular CH4 adsorption on Pt(111)

Abstract The dynamics of molecular methane adsorption on Pt(111) were probed with supersonic molecular beam techniques. Initial trapping probabilities were directly measured between 0.94 and 0.16 for incident total translational energies between 3.4 and 20.2 kJ/mol and angles of incidence (with respect to the surface normal) between 0° and 45° at a surface temperature ( T s ) of 100 K. The incident methane molecules were rotationally and vibrationally cold. The initial trapping probability decreases with increasing incident translational energy ( E T ) and decreasing angle of incidence (θ i ) and varies smoothly with incident normal energy ( E n = E T cos 2 θ i ), indicating a low corrugation of the molecule-surf ace interaction potential. The dependence of the initial trapping probability on incident normal translational energy agrees quantitatively with both a modified hard cube model and Leuthaussers theory at incident normal translational energies below 8 kJ/mol. At higher incident normal translational energies the observed initial trapping probabilities are higher than the values predicted by both models. Energy loss mechanisms other than surface phonon excitations may partially account for this discrepancy. A rapid decrease in the apparent adsorption probability as the surface temperature approaches 140 K is caused by the competitive influence of desorption. The temperature at which the apparent adsorption probability goes to zero agrees well with the desorption temperature measured independently by temperature programmed desorption. In accordance with the aforementioned models, the measured in-plane angular distributions suggest that the trapping probability is relatively independent of surface temperature in the range of 160 to 500 K. The relatively low intensity of methane found near the surface normal in the angular distributions may be partially explained by a wider than cosine angular distribution for the trapped-desorbed channel, which is consistent with the observation that the trapping probability increases with angle of incidence. Comparison of our initial trapping probability versus normal translational energy data to previous mean translational energy measurements of methane molecules desorbing from Pt(111) at the surface normal suggests that detailed balance applies for the non-equilibrium situation involving a collimated monoenergetic molecular beam of methane incident on a Pt(111) surface.

Ethane dissociation dynamics on Pt(111)

Abstract Supersonic molecular beam techniques were used to measure the initial dissociative sticking probability, S 0 , of ethane on clean Pt(111) as a function of its translational energy, vibrational energy in excited states, incident angle with respect to the surface normal and surface temperature. At incident total translational energies greater than 81 kJ/mol, ethane dissociates on Pt(111) via direct collisional activation. Over the range of energies studied the initial dissociative sticking probability on the clean surface increases smoothly with increasing normal translational energy. Neither changes in the vibrational energy of the incident ethane nor the surface temperature affect ethane dissociation on Pt(111) within the limits of experimental error. The initial dissociative sticking probabilities for the direct collisional activation of methane and ethane on Pt(111) are nearly equal over a similar range of translational energies suggesting that the initial step for ethane dissociation involves C|H bond cleavage.

Molecular propane adsorption dynamics on Pt(110)−(1 × 2)

Abstract The molecular adsorption probability of propane on clean and propane-covered Pt(110)-(l × 2) was measured as a function of incident translational energy, E i , incident polar angle, θ i , propane surface coverage, θ cov , and azimuthal angle at a surface temperature of 95 K. The dependence of the adsorption probability on E i , and θ i at all propane coverages reflects significant participation of parallel momentum in the adsorption process due to corrugation in the propane/Pt(110)-(1 × 2) interaction. When the tangential velocity component of the molecular beam is directed along the close-packed atomic rows (the [110] direction), the initial adsorption probability increases with increasing θ i ; however, the initial adsorption probability decreases with θ i , when the crystal azimuthal angle is rotated 90°, indicating parallel momentum exchange dominates the adsorption process on this azimuth. Adsorbed propane facilitates trapping; S 0 increases linearly with propane coverage up to 0.55 saturation coverage. As θ cov increases, corrugation in the adsorbed layer dominates the adsorption process for all angles of incidence, and the azimuthal dependence of the adsorption probability becomes negligible except at the most glancing angles of incidence. A sudden change in the coverage dependence of the adsorption probability is observed at surface coverages above 0.55 saturation coverage.

Adsorbate‐assisted adsorption: Trapping dynamics of Xe on Pt(111) at nonzero coverages

The trappingdynamics of Xe on Pt(111) has been probed as a function of Xe coverage with supersonic molecular‐beam techniques. Adsorption probabilities were directly measured at a surface temperature of 95 K at coverages ranging from zero to monolayer saturation at incident translational energies between 6 and 63 kJ/mol and incident angles between 0° and 60°. In apparent agreement with the predictions of the original Kisliuk model, the adsorption probability at the lowest incident translational energy (6 kJ/mol) remains almost constant with coverage up to near monolayer saturation. However, in contradiction to the original Kisliuk model, at higher incident translational energies, the trapping probability increases nearly linearly with xenon coverage up to near monolayer coverage. For example, the trapping probability increases from 0.06 to 0.42 for an incident translational energy of 63 kJ/mol at normal incidence as the coverage is increased from zero to saturation monolayer coverage. This behavior can be explained adequately by a model that incorporates enhanced trapping onto the monolayer compared to the clean surface, a property of the model that is confirmed directly by experiments presented herein. The angular dependence of the adsorption probability shows progressive deviation from normal energy scaling with increasing Xe surface coverage, proving that the degree to which parallel momentum participates in the adsorption process increases with adsorbate coverage. The initial trapping probability of Xe onto the monolayer is independent of incident angle indicating total‐energy scaling. The above findings are qualitatively identical to our previous results for the molecular adsorption of ethane on the same surface, suggesting that these phenomena occur, in general, for weak molecular adsorption regardless of molecular shape and internal degrees of freedom, at least for small molecules.

Alkane dissociation dynamics on Pt(110)–(1×2)

Supersonic molecular beam techniques were used to study the reactive adsorption dynamics of methane and ethane on Pt(110)–(1×2). The initial dissociative sticking probability, S0, was measured as a function of surface temperature, incident translational energy, incident total vibrational energy, and incident polar angle at two azimuthal orientations. Under all experimental conditions, both alkanes dissociated via direct collisional activation. Over the range of translational energies studied here neither S0(CH4) nor S0(C2H6) exhibited a dependence on nozzle temperature in these experiments suggesting that excitation of the normal vibrational motions of methyl deformation, methyl rocking, C–C stretching, and torsional vibrational modes do not play a significant role in the direct dissociation of either alkane on Pt(110)–(1×2) under these experimental conditions. The C–H stretching modes were not sufficiently populated to determine the extent of their participation. Methane and ethane displayed almost ident...

Molecular propane adsorption dynamics on Pt(111)

Abstract Supersonic molecular beam techniques and temperature programmed desorption (TPD) were used to study the adsorption dynamics of propane onto clean and propane-covered Pt(111). The propane sticking probability was measured directly as a function of incident translational energy, E i , incident angle, θ i , and propane coverage, θ cov , at a surface temperature of 95 K. Under these experimental conditions, propane adsorbs molecularly onto Pt(111) terrace sites. Non-normal energy scaling is observed at all propane coverages indicating the importance of parallel momentum in the adsorption process. At all incident translational energies and angles studied, the sticking probability on a propane covered surface, S (θ cov ), increases with increasing propane coverage. Upon saturation of the Pt(111) terrace sites, spontaneous desorption is observed in the direct adsorption probability experiments.

Surface microstructure effects: molecular ethane adsorption dynamics on Pt(110)-(1 × 2)

Abstract The molecular adsorption probability of ethane on clean Pt(110)-(1 × 2) at a surface temperature of 95 K was measured as a function of incident translational energy, E T , incident polar angle, θ i , and azimuthal angle, o. At normal incidence the adsorption probability decreases with incident translational energy from near unity at an incident kinetic energy of 10 kJ/mol to 0.5 at 40 kJ/mol. For molecules incident with the tangential velocity component directed along the [110] (smooth) direction, the initial adsorption probability increases with increasing θ i , scaling with E T cos 0.6 θ i ; however, the adsorption probability decreases with θ i for molecular beams directed along the [100] (rough) direction, indicating the effects of corrugation. Stochastic trajectory simulations employing ethane-Pt Morse potential parameters previously developed from measurements of the adsorption probabilities of ethane on Pt(111) give quantitative predictions of the initial trapping probability of ethane on Pt(110)-(1 × 2) for both azimuthal angles at all energies and polar angles of incidence. The simulations suggest that interconversion of normal and parallel momenta due to the surface corrugation governs the adsorption process and serves as an effective mechanism which facilitates trapping. Excessive parallel momentum, however, can cause ethane to scatter on subsequent bounces because of the reconversion of large amounts of parallel energy into normal energy. Analysis of a large number of trajectories illustrated that collisions on the ridges of Pt(110)-(1 × 2) mitigate against trapping of ethane while collisions within the troughs facilitate trapping. Finally, the simulations show that the trapping probability is determined to within 12% by the fate of the first bounce.

Kinetic isotope effect in direct ethane dissociation on Pt(111)

Supersonic molecular beam techniques were used to study the dynamics of direct C2H6 and C2D6 dissociation on Pt(111). The initial dissociation probabilities for both isotopes, S0(C2H6 and S0(C2D6), increased with normal translational energy, En, over the entire range of En studied. A significant normal kinetic isotope effect was observed; the ratio S0(C2H6/S0(C2D6) decreased from 2.7 to 1.6 as En was increased from 80 to 118 kJmol. A one-dimensional quantum mechanical tunneling model based on an Eckart potential barrier quantitatively accounts for these observations. After correcting for translational energy dissipation to the lattice, the extracted barrier heights, V0, and widths, L, are 123 kJmol, 1.1 A and 130 kJmol, 1.1 A for C2H6, and C2D6, respectively. The larger barrier height for the direct dissociation of C2D6 relative to C2H6 can be attributed to differences in zero point energy for C-H(D) stretching motion. Neither. S0(C2H6) nor S0(C2D6) exhibited a dependence on nozzle temperature in these experiments suggesting that excitation of the normal vibrational motions of methyl rocking and deformation, C-C stretching and torsion do not promote direct ethane dissociation on Pt(111) under these experimental conditions.