Surface Processes Induced by Collisions

The desorption of CH4 physisorbed on Ni(111) is observed to be induced by collision with Ar atoms incident with energies less than 2 eV. The absolute cross section for collision-induced desorption of CH4 in the low coverage limit of an isolated CH4 molecule and from a saturated CH4 monolayer is measured as a function of the kinetic energy and incident angle of the Ar beam. The dominant mechanism for collision-induced desorption is determined to involve the direct collision of the incident Ar with the physisorbed CH4. Indirect, surface mediated desorption processes and multiple desorptions are found to be unimportant. Three-dimensional, classical molecular dynamics simulations based upon a hard sphere/hard cube model of the direct collision mechanism show that the complicated dependence of the desorption cross section at low CH4 coverage on the Ar energy and incident angle is the result of two competing dynamical effects: the increase in the geometrical collision cross section and the decrease in the Ar kinetic energy that can be transferred to CH4 motion normal to the surface as the Ar incident angle increases. Multiple Ar–CH4 collisions and mirror collisions are found to make relatively minor contributions to the cross section for collision induced desorption. Normal energy accommodation during the CH4-surface collision plays a significant role in determining the threshold energy for desorption. At high CH4 coverage, the obstruction of small impact parameter, head-on Ar–CH4 collisions by neighboring CH4 molecules at large angles of incidence is the origin of the difference in the cross section observed for low and high CH4 coverage.

: Semi-empirical calculations were performed for small clusters modelling the adsorption of CO on nickel (100), (110), and (111) surfaces at low and high coverage levels and on nickel-copper alloy (100) surfaces at low coverage. The bearing of the results upon the understanding of experimentally observed faceting and magnetization (Hedvall) effects is discussed. A preference for adsorption at multifold sites is observed at low coverage, and the (100) and (111) surfaces are predicted to be energetically smooth at high coverages. The results for alloy surfaces are qualitatively in agreement with experiment. (Author)

Furfural adsorption on the g-C3N4 monolayer: A DFT analysis

The growth of Au on the stable, high-index Si(5 5 12) surface has been studied using scanning tunneling microscopy (STM). At very low coverages and moderate annealing temperatures (≤0.1 ML, 400–500°C), Au appears to decorate the underlying Si rows and form an array of rows that maintains the underlying (5 5 12) periodicity of 5.4 nm. For higher annealing temperatures and coverages, however, Au causes faceting to a number of nearby planes. The two primary facets formed at lower (∼0.15 ML) and higher (0.5–2 ML) coverages are the (337) and (225) planes, which are tilted 0.7° down [towards (111)] and 1.1° up from (5 5 12), respectively. Both orientations are in fact subunits of the (5 5 12) unit cell, so their presence is not surprising. In addition to these facets, two types of sawtooth morphologies composed of planes oriented further from (5 5 12) are found at very high annealing temperatures (800–900°C). These include (113)+(7 7 15) planes at very low coverage (0.05 ml) and (113)+(5 5 11) planes at higher coverage (∼1 ML), where (113) is tilted up by 5.3° and (7 7 15) and (5 5 11) are tilted down by 2.9° and 2.2°, respectively. Au adsorption on Si(5 5 12) therefore results in the formation of five possible facet planes: (113), (225), (337), (5 5 11), and (7 7 15).

The chemisorbed system tellurium on Ni(111) has been studied using a lattice gas model with pair interactions as long range as sixth neighbor. We discover three distinct (2√3×2√3)R30 phases: low coverage (θ= (1)/(12) ), centered (θ= (1)/(6) ), and high coverage (θ= (1)/(4) ), when interactions are adjusted to match qualitatively the other features of the experimental phase diagram. Each pair of neighboring ordered phases is separated by a coexistence region at low temperatures. The low-coverage 2√3 phase disorders via a first-order transition at higher temperature. At higher coverage, a (√3×√3)R30 phase occurs at intermediate temperature. This phase melts by a first-order transition at low coverages and a continuous, three-state Potts transition at higher coverages. Thus, the simulations suggest that this chemisorbed system may provide an experimental realization of Potts tricritical behavior. Alternate sets of interactions in this long-range lattice gas model produce the (√7×√7)R19, (3×3), and (2×2) phases, which have been reported for other chemisorbed systems on triangular lattices.

Collision-induced desorption (CID) and Eley–Rideal (ER) type abstraction have been investigated in the reaction system, H(g)+Dad/Si(100)→D2,HD, at surface temperature of 573 K where dihydride phase is unstable. Rate equations for CID were obtained based on a second-order kinetics with respect to doubly occupied Si dimers DSi–SiD, DSi–SiH, and HSi–SiH, which are considered as parent species of transiently formed dihydrides, DSiD, DSiH, and HSiH, respectively. The coverages of the doubly occupied dimers were determined as a function of H exposure time by means of temperature-programmed-desorption (TPD), and then the rate equations were calculated. As a result, the experimental D2 and HD rate curves were reasonably fit with the rate equations formulated. This fact indicates that CID is induced when two dihydride species encounter during their propagation via dihydride–monohydride isomerization reactions. It was found that HSi–SiH does not play a role in CID of both D2 and HD. Possible origins of this isotope effect were discussed in terms of quantum effects on associative desorption and diffusion processes. The HD rate curve due to direct ER abstraction channel was fit with a first-order kinetics in Dad coverage for a low coverage regime where dangling bonds exist. However, for a high coverage regime where the surface is saturated, the HD rate curve was fit with DSi–SiD coverages, suggesting that Dad that are paired up with Had in the same Si dimers are not abstracted.

Monolayer coverage effects on size and ordering of self-organized InAs islands grown on (1 1 3)B InP substrates

Analyst Coverage and the Glamour Discount

Theoretical study of Sn adsorbed on the Au(1 1 1) surface

Ag-doped Pt nanoparticles (NP) were tested for continuous-flow C2H2 hydrogenation reactivity and CO adsorption–desorption from 100 to 300 °C to investigate how submonolayer levels of an insoluble metal dopant can distribute on steps and terraces of a NP surface and modify reactivity. High Ag coverage (0.5 monolayer, ML) on Pt promoted selective hydrogenation of C2H2 at T < 200 °C; at higher temperatures, both low (0.06 ML) and high (0.5 ML) Ag coverages were seen to increase selective C2H4 formation. Ag incorporation onto Pt step and terrace sites, and resultant temperature-dependent desorption behavior, were probed using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) with CO. At low coverage, Ag appeared to occupy Pt step sites; for higher coverage, Ag occupied both step and terrace sites, which lowered the CO desorption temperature. Highly correlated reactivity and DRIFTS data show the importance of step site blocking and desorption from Pt during selective C2H2 hydrogenation. Multiple reaction cycles from 100 to 300 to 100 °C demonstrate how immiscible metal dopants can preferentially modify nanoparticle surface compositions and promote active, selective, and stable catalysts over large temperature operating windows.

Results of a retrospective database analysis of adherence to statin therapy and risk of nonfatal ischemic heart disease in daily clinical practice in Italy

Quantitative Analysis of Water Dynamics in and near Proteins

Molecular dynamics (MD) simulations were carried out to study the hydrate structure and diffusion of water and cations in the clay interlayer region. The goal of this work was to examine the mechanisms by which the clay counterions influence the swelling and dispersion of shale. Three series of MD simulations were performed to study Wyoming montmorillonites containing either Na+, K+ or Li+ counterions and 100, 200 or 300 mg of adsorbed water per gram of clay. Here, the TIP4P interaction model for liquid water is used in the MD simulations. The TIP4P results are compared with simulations using the MCY interaction model, particularly for the K-clay hydrates. For the one- and two-layer K-hydrates, the counterion speciation and water structure observed are in agreement with the MCY model. However, we observe that the two-layer hydrate persists up to high water contents and that – contrary to the MCY model – the hypothetical three-layer hydrate is unstable, in agreement with experimental observations. If one notes that the one- and two-layer hydrates are only saturated at 56 and 96 water molecules, respectively, then the observed equilibrium layer spacings are in excellent agreement with experiment. Integration of the ion density profiles shows that the surface complexes represent 4.4 out of 6 ions for the K-hydrate and only 2 out of 6 ions for Na-hydrate. This means that K+ ions adsorb more strongly and screen the negatively charged surface more effectively than Na+ ions. Self-diffusion coefficients were calculated from mean-square displacement analysis of ions and water molecules. For the Na+ and Li+ cases, satisfactory agreement with experimental quasi-elastic neutron scattering diffusion coefficients was found. No experimental values are available for the K+ case, and therefore predictions are made in this report. Diffusion coefficients for water in K-smectite increase from 10−10 to 10−9 m2 s−1 with water contents increasing from 100 to 300 mg g−1 of clay. We find that water diffusion rates in K-smectites are roughly twice as small as in Na- and Li-clays with the same water content. It appears that the microscopic diffusion coefficients calculated for interlayer diffusion of Na+ and K+ ions are two orders of magnitude smaller than the macroscopic values calculated from experimental work on hindered motion of water and ions through a clay membrane. This implies that the experimental clay film has a tortuous structure, with clay platelets aligned orthogonally to the main direction of mass transport, and a tortuosity factor of . The results show that the MD simulation technique is a valuable tool to study transport properties of water and ions in hydrated smectite clays.

A combined experimental (mid- and far-infrared FTIR spectroscopy and THz time domain spectroscopy (TTDS) (0.3-1.6 THz)) and molecular dynamics (MD) simulation technique are used to understand the evolution of the structure and dynamics of water in its binary mixture with 1,2-dimethoxy ethane (DME) over the entire concentration range. The cooperative hydrogen bond dynamics of water obtained from Debye relaxation of TTDS data reveals a non-monotonous behaviour in which the collective dynamics is much faster in the low Xw region (where Xw is the mole fraction of water in the mixture), whereas in Xw ∼ 0.8 region, the dynamics gets slower than that of pure water. The concentration dependence of the reorientation times of water, calculated from the MD simulations, also captures this non-monotonous character. The MD simulation trajectories reveal presence of large amplitude angular jumps, which dominate the orientational relaxation. We rationalize the non-monotonous, concentration dependent orientational dynamics by identifying two different physical mechanisms which operate at high and low water concentration regimes.

Mechanisms of bulk and surface diffusion in metallic glasses determined from molecular dynamics simulations