Initial Trapping Probability Research Articles

N-Butane, iso-Butane and 1-Butene Adsorption on Imidazolium-Based Ionic Liquids Studied with Molecular Beam Techniques.

The interaction of molecules, especially hydrocarbons, at the gas/ionic liquid (IL) surface plays a crucial role in supported IL catalysis. The dynamics of this process is investigated by measuring the trapping probabilities of n‐butane, iso‐butane and 1‐butene on a set of frozen 1‐alkyl‐3‐methylimidazolium‐based ILs [C n C1Im]X, where n=4, 8 and X−=Cl−, Br−, [PF6]− and [Tf2N]−. The decrease of the initial trapping probability with increasing surface temperature is used to determine the desorption energy of the hydrocarbons at the IL surfaces. It increases with increasing alkyl chain length n and decreasing anion size for the ILs studied. We attribute these effects to different degrees of alkyl chain surface enrichment, while interactions between the adsorbate and the anion do not play a significant role. The adsorption energy also depends on the adsorbing molecule: It decreases in the order n‐butane>1‐butene>iso‐butane, which can be explained by different dispersion interactions.

On the adsorption of n-butane on alkyl imidazolium ionic liquids with different anions using a new molecular beam setup.

The adsorption of reactants is an elementary step in the interaction of molecules with liquid or solid surfaces. We recently reported on the trapping of n-butane on the frozen surfaces of ionic liquids (ILs), namely, 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ILs ([CnC1Im][Tf2N]; n = 1, 2, 3, and 8). To study the influence of the anion, we now present results concerning the trapping of n-butane on 1-alkyl-3-methylimidazolium hexafluorophosphate ILs ([CnC1Im][PF6]; n = 2, 4, and 8), that is, ILs with a smaller anion. The adsorption energies close to zero coverage are determined from the temperature dependence of the initial trapping probability using a novel approach. For both groups of ILs, the binding energy is dominated by the interaction of n-butane with the alkyl chain of the cation, whereas the ionic headgroups contribute only weakly. Comparing ILs with different alkyl chains at the IL cation, we find that the adsorption strength of n-butane increases with increasing length of the alkyl chain. In addition, detailed information on the new setup and the data analysis is provided.

On the Dynamic Interaction of n-Butane with Imidazolium-Based Ionic Liquids.

The impact of a reactant from the gas phase on the surface of a liquid and its transfer through this gas/liquid interface are crucial for various concepts applying ionic liquids (ILs) in catalysis. We investigated the first step of the adsorption dynamics of n‐butane on a series of 1‐alkyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide ILs ([CnC1Im][Tf2N]; n=1, 2, 3, 8). Using a supersonic molecular beam in ultra‐high vacuum, the trapping of n‐butane on the frozen ILs was determined as a function of surface temperature, between 90 and 125 K. On the C8‐ and C3‐ILs, n‐butane adsorbs at 90 K with an initial trapping probability of ≈0.89. The adsorption energy increases with increasing length of the IL alkyl chain, whereas the ionic headgroups seem to interact only weakly with n‐butane. The absence of adsorption on the C1‐ and C2‐ILs is attributed to a too short residence time on the IL surface to form nuclei for condensation even at 90 K.

The Interaction of Water with the Oxygen-Terminated, Polar Surface of ZnO

The adsorption of H2O on the oxygen-terminated polar surface of ZnO, ZnO(000-1), has been studied by He atom scattering (HAS), low-energy electron diffraction (LEED), adsorption probability measurements, He atom reflectivity measurements as a function of exposure and surface temperature (He atom thermal desorption measurements, “He-TDS”), and X-ray photoelectron spectroscopy (XPS). The clean O−ZnO(000-1) surface is characterized by an ordered (1 × 3) oxygen vacancy structure which converts to a (1 × 1) hydrogen (OH)-terminated structure upon dissociative H2O adsorption, even at adsorption temperatures as low as TS = 200 K. The formation of the OH-species is accompanied by the formation of a shoulder in the XPS O 1s line. A detailed investigation of the coverage dependence of the H2O adsorption probability indicates the presence of a distinct precursor state. The initial trapping probability is S0 = 0.8 ± 0.1. The most probable microscopic adsorption mechanism which is consistent with the obtained data is ...

Effects of surface step on molecular propane adsorption

We have studied the effects of surface step on molecular propane adsorption using molecular-dynamics simulations and a model stepped surface, Pt(6 5 5). Incidences along the step edge (smooth azimuth) and perpendicular to the step edge with upstairs momentum (upstairs azimuth) and downstairs momentum (downstairs azimuth) are considered. In general, the surface step enhances the initial trapping probability of propane except for the downstairs incidences. The most efficient zone in facilitating adsorption is near the bottom of the surface step on the lower terrace where incident molecules experience stronger attraction and an “additional-layer” effect when crossing the step. The least efficient zone is the top of the surface step on the upper terrace due to an opposite “missing-layer” effect. Surface step also creates steric effects such that more incident molecules along the upstairs azimuth but significantly less molecules along the downstairs azimuth impact the step-bottom zone. The latter steric effect, a shadowing effect, undermines the high trapping efficiency of the step-bottom zone to cause the downstairs incidences to have the lowest trapping probabilities. While the shadowing effect can be enhanced by larger incident angles and lower incident energies, the other steric effect on the upstairs incidences is relatively insensitive to the incident energy. Overall, the influence of surface step on molecular adsorption diminishes at low incident energies and large incident angles because longer contact times and less normal momenta result in high trapping probability across the entire stepped surface.

Adsorption Dynamics and Desorption Kinetics of Argon and Methane on MgO(100)

The adsorption dynamics and desorption kinetics of Ar and CH4 on MgO(100) are studied using a combination of molecular beam scattering and temperature-programmed desorption (TPD). Both Ar and CH4 exhibit an initial trapping probability that decreases dramatically with increasing kinetic energy and is independent of incident angle indicating a barrierless process obeying total energy scaling. The trapping probability for both systems increases roughly linearly with increasing adsorbate coverage in the first layer. Analysis of the TPD spectra yields desorption energies of 8.5 and 13 kJ/mol for Ar and CH4, respectively.

The adsorption dynamics of molecular carbon dioxide on Pt( [formula omitted]) and Pd( [formula omitted

The dynamics of the nondissociative adsorption of carbon dioxide on Pt(1 1 1) and Pd(1 1 1) were investigated by supersonic molecular beam techniques at a surface temperature of 80 and 60 K, well below the desorption temperatures of 95 and 88 K, respectively. The initial trapping probability of CO 2 on both Pt(1 1 1) and Pd(1 1 1) scale with normal and near-normal energy scaling functions, indicating smooth gas–surface potentials. On the other hand, trapping on the carbon dioxide-saturated Pt(1 1 1) exhibits a more corrugated gas–surface potential, on which trapping scales as E Tcos 0.7 θ. The probability of carbon dioxide trapping increases with self-coverage and is successfully described by the modified Kisliuk model, indicating that trapping into the extrinsic precursor state is more efficient than trapping on the clean surface. The coverage of carbon dioxide into Pt(1 1 1) saturates at about 0.275 ML at a surface temperature of 80 K as a result of beam exposure at normal incidence at 8.5 kJ/mol. The stochastic, three-dimensional classical dynamics for the simulation of CO 2 on Pt(1 1 1) indicates that the excitation of surface phonons on the first bounce determines trapping on Pt(1 1 1). The same set of Morse potentials were employed to predict the CO 2 trapping on Pd(1 1 1). In general, the prediction agrees reasonably with the experimental results. The difference between the calculated trapping probabilities from simulations and the experimental values is within 20%.

The prediction of trapping probabilities for ethane by molecular dynamics simulations: scaling from Pt [formula omitted] to Pd [formula omitted

The dynamics of ethane trapping on Pd(1 1 1) were investigated by supersonic molecular beam techniques and three-dimensional stochastic trajectory simulations. The initial trapping probability was measured over the range of incident energy, E T, from 10 to 34 kJ/mol and incident angles, θ, from 0° to 45° at a surface temperature of 95 K. The trapping probability scales with E Tcos 0.9 θ, indicating a corrugated gas–surface potential. Stochastic trajectory simulations utilizing a Morse potential whose parameters were obtained experimentally from ethane trapping on Pt(1 1 1) [J. Chem. Phys. 104 (1996) 3134] predict the corresponding experimental values of the initial trapping probability of ethane trapping on Pd(1 1 1) to within 30%. Calculations of energy transfer for ethane after the first bounce on Pd(1 1 1) clearly indicate that vibrational excitation of the lattice phonons account primarily for the increase in trapping probabilities of ethane on Pd(1 1 1) compared with Pt(1 1 1). The enhancement in phonon excitation results from increasing the mass ratio of ethane to the metal atoms.

The dynamics of krypton trapping on Pt(1 1 1) at 50 K: experiment and molecular dynamics simulation

The dynamics of krypton trapping on clean Pt(1 1 1) were investigated using supersonic molecular beam techniques at a surface temperature of 50 K. The initial trapping probability of Kr scales with normal incident energy (E T cos 2θ) , indicating a smooth gas–surface potential. The Kr trapping probability decreases from 0.8 to 0 as the incident normal energy is increased from 2 to 23 kJ/mol. Trapping on the krypton-saturated monolayer is greatly enhanced at all incident energies and angles compared to the clean surface, and exhibits near total energy scaling (E T cos 0.3θ) , indicating a corrugated gas–surface potential. Stochastic trajectory simulations [J. Chem. Phys. 105 (1996) 1609] using a Morse potential with parameters based on simulations of Ar trapping [J. Chem. Phys. 94 (1991) 1516] agree with the experimentally measured energy and angular dependence of the trapping probability of Kr.

Trapping of Ar on well ordered Ar, Kr, and Xe overlayers on Pt(1 1 1) at 30 K

The dynamics of Ar trapping on Ar, Kr, and Xe covered Pt(1 1 1) were investigated using supersonic molecular beam techniques at a surface temperature of 30 K. The initial trapping probability of Ar on the clean surface decreases from 0.8 to 0 as the normal incident energy is increased from 1.75 to 28 kJ mol −1, and scales with normal incident energy (E T cos 2θ) , indicating a smooth gas–surface potential [A.F. Carisson R.J. Madix, submitted for publication]. In contrast, the trapping probability on the Ar, Kr and Xe saturated surfaces decreases from 0.9 to only 0.3 in the same energy range and is nearly independent of the angle of incidence. The relative trapping probability on the Ar and Xe covered surfaces is in general accord with the relative energy exchange expected from the simple Baule model, but the trapping probability on the Kr covered surface appears further enhanced by potential corrugation. Experimental results for Ar trapping on Pt(1 1 1)–Ar are in qualitative agreement with a recoil effect predicted by Head-Gordon and Tully for Ar trapping on Ar covered Ru(0 0 1) [Surf. Sci. 268 (1992) 113]. At high incident energies on the gas covered Pt(1 1 1) surface, the adsorption probability is nearly independent of angle of incidence between 0° and 30°, but it then increases rapidly between 30° and 60°. Similar effects are observed for Ar trapping on Pt(1 1 1) saturated by Kr and Xe. The dependence of the trapping probability on coverage is quantitatively described by the modified Kisliuk model [J. Chem. Phys. 92 (1990) 1397; J. Phys. Chem. 95 (1991) 2461], which allows for an extrinsic precursor to adsorption as well as a direct channel. The probability of Ar trapping increases with Ar uptake to different degrees on the Ar, Kr and Xe covered surfaces.

The effects of alkylidyne and atomic oxygen overlayers on the dynamics of adsorption of methane on Pt(111)

The dynamics of the trapping of methane on p(2×2)-oxygen, -ethylidyne, -butylidyne and -isobutylidyne overlayers on Pt(111) at 50 K was investigated using supersonic molecular-beam techniques. Since each of these adsorbates forms the same structure, trapping dynamics could be studied as a function of the internal structure of the adsorbate. Trapping probabilities on these surfaces at a fixed incident energy and angle increase in the order oxygen<ethylidyne<isobutylidyne<butylidyne, and each of the adsorbate-covered surfaces enhances trapping compared with the clean surface. On the butylidyne-covered surface the initial molecular trapping probability decreases from 0.88 to 0.38 as the incident energy is increased from 8 to 24 kJ mol −1, compared with a decrease in trapping probability from 0.3 to zero over the same energy range on the clean surface. The angular dependence of the trapping probability indicates that the overlayers lead to an increase in the static corrugation in gas–surface potential. The generally higher values of the trapping probabilities on the alkylidyne-covered surfaces and the weaker angular dependence they exhibit suggest that they have a more corrugated potential than the clean surface. The further increase in trapping probabilities for methane on Pt(111) covered by the analogous alkanes suggests that the additional frustrated translations of the alkanes may also enhance trapping into the extrinsic precursor. The change in the adsorption probability with methane coverage on the oxygen-covered surface is described by the modified Kisliuk model [J. Phys. Chem. 95 (1991) 2461; J. Chem. Phys. 92 (1990) 1397]. The trapping probability increases with methane coverage, indicating that trapping into the extrinsic precursor state is more efficient in the presence of methane.

The dynamics of argon and methane trapping on Pt(111) at 30 and 50 K: energy scaling and coverage dependence

The dynamics of argon and methane trapping on clean Pt(111) were investigated using supersonic molecular beam techniques at a surface temperatures of 30 and 50 K, well below the desorption temperatures of 46 and 67 K, respectively. The initial trapping probabilities for both Ar and methane scale with normal incident energy ( E T cos 2 θ), indicating a smooth gas–surface potential. The trapping probability for Ar decreases from 0.8 to zero as the incident normal energy is increased from 2 to 29 kJ mol −1. Below 10 kJ mol −1 incident energy, the results agree with previous experiments [C.B. Mullins et al., Chem. Phys. Lett. 163 (1989) 111] and molecular dynamics simulations [M. Head-Gordon et al., J. Chem. Phys. 94 (1991) 1516] which were conducted above the desorption temperature. The trapping probability for methane decreases from 0.7 to zero as the incident normal energy is increased from 3 to 20 kJ mol −1. Trapping on the argon- or methane-saturated surface is greatly enhanced compared to the clean surface at all incident energies and angles, and exhibits near total energy scaling ( E T cos 0.3 θ), indicating a corrugated gas–surface potential. The kinetics of trapping as a function of coverage are quantitatively described by the modified Kisliuk model [H.C. Kang et al., J. Chem. Phys. 92 (1990) 1397; C.R. Arumainayagam et al., J. Chem. Phys. 94 (1991) 1516], which allows for an extrinsic precursor to adsorption. The trapping probabilities of both argon and methane increase with coverage, indicating that trapping into an extrinsic precursor state is more efficient than trapping onto the bare Pt(111) surface.

Molecular ethane adsorption dynamics on oxygen-covered Pt(111)

The dynamics of ethane trapping on Pt(111)-p(2×2)-O were investigated by supersonic molecular beam techniques at a surface temperature of 100 K. The initial trapping probability was measured in the range of incident energy from 10 to 45 kJ/mol and incident angles from 0° to 60°. A broad angular distribution of scattered ethane and total energy scaling (ET cos 0.2θ) for ethane trapping indicated a corrugated gas–surface potential. Stochastic trajectory simulations employing a potential developed from the trapping of ethane on Pt(111) gives quantitative agreement of the measured initial trapping probabilities over entire ranges of incident energies and angles. Calculations of energy transfer for ethane after the first bounce on Pt(111) and Pt(111)-p(2×2)-O clearly indicate that interconversion of parallel and perpendicular momentum and energy transfer to lattice vibrations account primarily for the differences in trapping probabilities between ethane on the two surfaces. At glancing incidence trapping is not significantly reduced on the oxygen-covered Pt(111) because the parallel momentum appears to be transferred partially to phonons.

Trapping dynamics of disilane on bare and monohydride-terminated Si(100)-2×1

The initial trapping probability of disilane on bare and monohydride-terminated Si(100)-2×1 surfaces was measured using molecular beam techniques. On the bare surface, trapping is shown to be very efficient, with near unit trapping probability at incident kinetic energies up to 0.2 eV, and nearly 50% trapping at 1 eV kinetic energy. The monohydride-terminated surface displays significantly less trapping efficiency than the bare surface. Interestingly, trapping probabilities decrease as the angle of incidence is increased, with the decrease becoming more pronounced at higher kinetic energies; this effect is discussed in terms of parallel momentum accommodation dynamics.

Trapping dynamics of isobutane, n-butane, and neopentane on Pt(111): Effects of molecular weight and structure

The molecular adsorption dynamics of isobutane, n-butane, and neopentane on Pt(111) was investigated using supersonic molecular beam techniques and stochastic trajectory calculations. Using the united atom approach, a single, pairwise-additive Morse Potential for methyl (methylene)–plantinum interactions quantitatively simulates the dependence of the initial trapping probability, α, on the initial translational energy, ET, and angle of incidence, θi, for each alkane. For both isobutane and n-butane, the dependence of α on ET and θi best scales with ET cos0.8θi, which is similar to that found previously for ethane and propane trapping on Pt(111). The initial trapping probability of neopentane exhibits a more pronounced dependence on θi, which scales according to ET cos1.3θi. The simulations suggest that the enhanced angular dependence of α for neopentane is related to its molecular weight. As the mass of the incident species is increased, momentum transfer to the surface becomes more efficient than the interconversion of incident parallel and normal momentum due to corrugation of the surface potential. The net effect is an increase in the trapping probability at glancing incidence compared to lighter molecules, and a resulting shift in the angular dependence of α towards normal energy scaling. The calculations also predict that collisional energy transfer to rotation is important in promoting adsorption. For each molecule, rotational excitation is determined to be the most effective energy transfer process that discriminates trapping from scattering. In addition, translational energy transfer to torsional vibration about the central C–C bond is highly efficient for n-butane, and greatly facilitates adsorption. Less excitation is predicted for C–C–C bending modes for all of these molecules.

Surface corrugation effects: molecular ethane adsorption dynamics on rigid adsorbate-covered surfaces of Pt(111)

Angular distributions of ethane molecules scattered from sulfur- and ethylidyne-covered Pt(111), and clean Pt(111) were measured at surface temperatures of 135 and 210 K. Direct-inelastic scattering from the adsorbate-covered surfaces displays dramatic, broad back-scattering of ethane, while the clean surface produces maximum scattering intensity near the specular angle. Stochastic trajectory simulations for ethane on Pt(111)S employing a potential developed from the trapping of ethane on Pt(111) gives a quantitative account of the broad angular distributions and the measured initial trapping probabilities at all energies and polar angles of incidence studied. The simulations clearly suggest facile interconversion of the incident parallel and perpendicular momenta of ethane upon the first collision with Pt(111)S, which enhances trapping relative to clean Pt(111) at normal incidence, but increases scattering at more glancing angles. The simulations indicate the trapping probabilities for ethane on Pt(111)S are determined to within 20% by the fate of the first bounce. Analysis of a large number of trajectories illustrates that at all angles of incidence, ethane is steered by the corrugation of the surface toward the bare platinum regions, producing both the broad angular distributions of the scattered species and the weak dependence of the trapping probability on the angle of incidence.

Molecular dynamics simulations for xenon adsorption on Pt(111): dynamical differences in the effects produced by the Barker-Rettner and Morse potentials

In order to gain a better insight into the effects of gas-surface potential functions and on observable quantities related to adsorption, xenon trapping on cold Pt(111) was simulated using classical three-dimensional stochastic trajectory calculations with a pairwise additive Morse potential and a potential developed by Barker and Rettner [J. Chem. Phys. 97 (1992) 5844]. Both potentials predict the same dependence of the initial trapping probability on the translational energy and angle of incidence of the colliding xenon atoms, but only the Barker-Rettner potential gives agreement with the observed in-plane scattered distributions, thus providing a basis for distinguishing the potentials. The features of the potentials that determine the differences in collisional dynamics are the potential corrugation and the steepness of the repulsive wall. Corrugation causes substantial scrambling between the components of incident gas-atom parallel and normal momentum, whereas the impulsiveness of the collisions determines the efficiency of energy transfer between the gas and surface. Long collision times computed for the softer Morse potential result in secondary surface recoil effects which cause scattering xenon atoms to gain normal momentum from the vibrationally excited surface; these effects are absent in the impulsive Barker-Rettner potential. These differences in the microscopic dynamics predicted by the potentials strongly effect the computed scattering distributions, but are compensatory in predicting the initial trapping probabilities. The Barker-Rettner potential behaves much like traditional hard cube models, whereas the Morse potential predicts that the efficacious dissipation of incident parallel momentum to the surface facilitates trapping at glancing incidence.

The dynamical origin of non-normal energy scaling and the effect of surface temperature on the trapping of low molecular weight alkanes on Pt(111)

Molecular classical dynamical simulations of alkanes trapping on platinum surfaces were performed to examine the origin of non-normal energy scaling for molecular adsorption. Conversion of normal to parallel translational energy at normal incidence and conversion of parallel translational energy into normal translational energy at glancing angles are the primary mechanisms which produce non-normal energy scaling of alkanes trapping on cold Pt(111). In addition, a tendency to convert rotational energy gained in the first gas-surface collision into normal translational energy for collisions at glancing incidence further increases the degree of non-normal energy scaling. Increasing surface temperature is shown to have little effect on energy transfer processes in the first bounce but increasing influence on subsequent bounces. Despite difficulties in defining trapping at high surface temperatures, simulations indicate that the initial trapping probability of ethane on Pt(111) does not fall by more than a factor of two over the surface temperature range of 100–700 K.

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

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, ø. 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 [11̄0] (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.

Molecular adsorption of alkanes on platinum surfaces: A predictive theoretical model

The adsorption probabilities of methane and propane on Pt(111), and propane on Pt(110)-(1×2) have been successfully predicted for a wide range of incident energies and angles with classical stochastic trajectory simulations, using a pairwise additive Morse methyl–platinum potential previously developed from the measured trapping probabilities of ethane on Pt(111). These predictions, along with those for ethane adsorption on Pt(110)–(1×2), comprise a unified model for the molecular adsorption of alkanes on platinum surfaces. The simulations show the initial trapping probabilities of methane and propane on Pt(111) are determined to within approximately 10% by the fate of the first bounce. They also indicate that at normal incidence on Pt(111) energy conversions from perpendicular translational motion to both cartwheeling rotation and lattice phonons play increasingly important roles in increasing the trapping probability as the alkane increases in size and molecular weight. For methane itself excitation of parallel translational momentum after the first bounce serves as the most effective energy storage mechanism which facilitates trapping, whereas for propane cartwheel rotational motion plays the dominant role. Excessive excitation of these modes of motion, however, can cause scattering on subsequent bounces by reconversion of the energy into perpendicular translational energy. Collisions of methane with the hollow and bridge sites on the Pt(111) surface appear less effective in trapping than do atop sites. The simulations also suggest excitation of the C–C–C bending mode of propane has little effect on the trapping of propane on platinum surfaces for beam energies below 55 kJ/mol.