Adsorption dynamics of CO2 on Zn-ZnO(0001): a molecular beam study

Presented are initial S(0) and coverage Theta dependent, S(Theta), adsorption probability measurements, respectively, of CO(2) adsorption on the polar Zn-terminated surface of ZnO, parametric in the impact energy E(i), the surface temperature T(s), the impact angle alpha(i), varied along the [001] azimuth, the CO(2) flux, and the density of defects, chi(Ar(+)), as varied by rare gas ion sputtering. S(0) decreases linearly from 0.72 to 0.25 within E(i)=0.12-1.33 eV and is independent of T(s). Above E(i)=0.56 eV, S(0) decreases by approximately 0.2 with increasing alpha(i). The shape of S(Theta) curves is consistent with precursor-mediated adsorption (Kisliuk shape, i.e., S approximately const) for low E(i); above E(i)=0.56 eV, however, a turnover to adsorbate-assisted adsorption (S increases with Theta) has been observed. The initial slope of S(Theta) curves decreases thereby with increasing alpha(i), chi(Ar(+)), and T(s), i.e., the adsorbate-assisted adsorption is most distinct for normal impact on the pristine surface at low T(s) and is independent of the CO(2) flux. The S(Theta) curves have been parametrized by analytic precursor models and Monte Carlo simulations have been conducted as well. The temperature dependence of the saturation coverage shows two structures which could be assigned to adsorption on pristine and intrinsic defect sites, respectively, in agreement with a prior thermal desorption spectroscopy study. The heat of adsorption E(d) for the pristine sites amounts to 34.0-5.4Theta, whereas for adsorption on the intrinsic defect sites E(d) of approximately 43.6 kJ/mol could be estimated. Thus, a kinetic structure-activity relationship was present.     

Adsorption of CO on the copper-precovered ZnO(0001) surface: a molecular-beam scattering study

Auger electron spectroscopy (AES), thermal-desorption spectroscopy (TDS), and, in particular, molecular-beam scattering techniques have been combined to address particle size effects in the adsorption of CO on Cu-on-ZnO(0001). AES and TDS lead to a Cu coverage, theta(Cu), calibration. The TDS curves, as a function of theta(Cu) and CO exposure, revealed three structures at 150, 220, and 260-280 K, in agreement with prior studies. However, a unique assignment of the TDS structures to a particular Cu face was not possible. An enhancement of the initial adsorption probability, S0, with respect to the support indicates the effect of the Cu nanoparticles. Despite that the shape of S0 versus impact energy curves was independent of theta(Cu) and agreed with Cu single-crystal reference systems, distinct particle size effects were present with regard to the adsorption mechanism. It was possible to observe a crossover from Langmuir-type adsorption dynamics to more precursor-assisted adsorption dynamics with increasing theta(Cu). Thus, a dynamic structure-activity relationship was evident, i.e., the energy-transfer mechanism depends on the Cu morphology.     

Adsorption dynamics of CO2 on copper-precovered ZnO(0001)-Zn: a molecular-beam scattering and thermal-desorption spectroscopy study

Initial, S(0), as well as coverage-dependent adsorption probability measurements, S(Theta), have been conducted at normal impact angle and as a function of the impact energy of CO(2), E(i), adsorption temperature, T(s), and copper precoverage, Theta(Cu) (at 300 K). S(0), which decreased from approximately 0.4 exponentially to approximately 0.05 with E(i) was independent of Theta(Cu). Astonishingly, S(0) for Cu on ZnO(0001)-Zn is smaller than for the clean support which indicates a chemical modification of the support by the Cu deposits. S(Theta) curves consist of two regimes, a Kisliuk-type and Langmuirian-type section. The first is consistent with capture zone models; the second may indicate direct adsorption of CO(2) on the Cu cluster. The thermal-desorption (TDS) curves for Cu on ZnO(0001)-Zn consist of two structures with binding energies of 26 and approximately 40 kJmol (nu=1 x 10(13) ls). The TDS results indicate that CO(2) populates predominantly the Cu deposits and the rim along the Cu nanoparticles. No indications for CO(2) dissociation could be obtained with Auger electron spectroscopy.     

Structure and dynamics of CO overlayers on a hydroxylated metal oxide: the polar ZnO(0001) surface

The adsorption and desorption of CO on the hydroxylated, O-terminated polar ZnO(0001) surface has been studied using He-atom scattering. The experimental results reveal the formation of a physisorbed ordered CO overlayer. In addition to recording angular distributions of elastically scattered He atoms, also the dynamical properties of the CO overlayer have been investigated using inelastic He-atom scattering. With the aid of electronic structure calculations a loss peak with an energy transfer of 7.2 meV is assigned to the frustrated translation of the CO molecule normal to the surface.     

Adsorption states of amphipatic solutes at the surface of naturally hydrophobic minerals: a molecular dynamics simulation study

An initial molecular dynamics simulation study regarding interfacial phenomena at selected naturally hydrophobic surfaces is reported. Simulation results show that, due to the natural hydrophobicity of graphite and talc basal planes, the cationic surfactant dodecyltrimethylammonium bromide preferentially adsorbs at these surfaces through hydrophobic interactions. When a model dextrin molecule is considered, the simulation results suggest that the hydrophobic interaction between the naturally hydrophobic surfaces of graphite, talc basal plane, and sulfur and the hydrophobic moieties (C-H and methylene groups) in the dextrin molecule plays a significant role in dextrin adsorption at these surfaces. The hydroxyl group in the dextrin molecule also contributes to its adsorption at the talc basal plane surface. In contrast, dextrin was not found to adsorb at talc edge surfaces.     

Adsorption of a single polymer chain on a surface: A molecular dynamics simulation study

We performed simulations of the physical adsorption of a single globular chain on a surface of hemispherical shape by means of molecular dynamics simulations. For the chain, we took advantage of a united atom model. Interactions within the chain were limited to stretching, bending, and torsional as well as nonbonded interactions between the nonadjacent atoms. The interaction between each chain element and the surface formation are reigned by a Lennard–Jones potential. In this article, we focused on differences in the behavior of the adsorbed globule to the free unadsorbed one particularly in two different zones of the immediate vicinity of the surface. There were strong indications for a localized acceleration of the dynamics as compared with the bulk that appears in an increase of trans–gauche switches. For explanation we came up with an adsorption scenario. Special attention was given to the shift of the percentage of trans and gauche conformations within the globule in dependence on the strength of the adsorption potential that might be related to crystallization or glass transition. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2333–2339, 2001     

Modification of the ZnO(0001)-Zn surface under reducing conditions

The ZnO(0001)-Zn terminated crystal face was studied after reduction at high temperatures by combination of STM, STS, XPS and TDS. The clean ZnO(0001)-Zn surface exhibits triangular reconstruction in UHV, while after exposure to 10(-5) mbar H(2) at RT this reconstruction is lifted and a rough surface has formed. The roughness as well as the metallic character of the surface increased with the applied low-pressure reduction temperature up to 800 K. XPS revealed that exposure to 1 bar H(2) at RT led to the formation of OH groups; at higher temperatures progressive metallization of the ZnO surface was found to occur. Analysis of the thermal desorption results showed that huge amounts of H(2) dissolved into the ZnO crystal. The results obtained under these conditions were in good accordance with thermodynamic calculations. The experimental ratio between the absorbed amount of H(2) at RT and 800 K amounts to 1000. The ratio calculated from increasing diffusion coefficients with temperature only amounts to 6. This emphasizes the importance of ZnO as a H supplier by spillover, and proves that metallic Zn boosts dissociative adsorption of H(2). This surface modification of the ZnO structure during the reduction promotes an enhanced activity of the Cu/ZnO catalyst at elevated temperatures.     

Investigation of Formaldehyde Adsorption on ZnO(0001) Surface by Density Functional Theory

Russian Journal of Physical Chemistry A - The interaction between formaldehyde and ZnO (0001) was investigated by density functional theory (DFT) to evaluate the detection of formaldehyde. The...     

Hydrogen adsorption on graphite (0001) surface: a combined spectroscopy-density-functional-theory study

The adsorption of H/D atoms on the graphite (0001) surface is investigated by means of both high-resolution electron-energy loss spectroscopy (HREELS) and periodic first-principle density-functional theory. The two methods converge towards two modes of adsorption: adsorption in clusters of about four hydrogen atoms and adsorption in pairs of atoms on contiguous carbon sites. The desorption energies estimated from the calculated dissociation energies range from 8 to 185 kJ mol(-1) leading to an estimated surface coverage at saturations of 30-44 at. %. These results are compared with previous thermal desorption spectroscopy results. New HREEL signal assignments are proposed based on quantum calculations.     

Self-trapped excitons at the quartz(0001) surface

We have studied self-trapped excitons in alpha-quartz using density functional theory (DFT), both in the crystal and at the (0001) surface. The excitons are triplet excited states that distort the crystal locally. They have a long lifetime, of the order of a millisecond, and become thermally equilibrated. We have calculated the drop in the exciton energy as it approaches the surface from the interior of the crystal. In the subsurface layer of the -OH terminated (0001) surface, the energy has dropped by 0.7 eV. Another 0.4 eV drop occurs as the exciton enters the surface layer, where it breaks off an OH radical. The drop in energy can be understood from the greater ease of structural distortion at the surface. These calculations illustrate that excitons formed in the bulk could migrate out to the surface and form chemically active surface species. Molecules adsorbed at the surface could also serve as traps for the excitons and could, in principle, be induced to undergo structural or chemical transitions.     

Adsorption and solvation of ethanol at the water liquid-vapor interface: a molecular dynamics study

The free energy profiles of methanol and ethanol at the water liquid-vapor interface at 310K were calculated using molecular dynamics computer simulations. Both alcohols exhibit a pronounced free energy minimum at the interface and, therefore, have positive adsorption at this interface. The surface excess was computed from the Gibbs adsorption isotherm and was found to be in good agreement with experimental results. Neither compound exhibits a free energy barrier between the bulk and the surface adsorbed state. Scattering calculations of ethanol molecules from a gas phase thermal distribution indicate that the mass accommodation coefficient is 0.98, and the molecules become thermalized within 10 ps of striking the interface. It was determined that the formation of the solvation structure around the ethanol molecule at the interface is not the rate-determining step in its uptake into water droplets. The motion of an ethanol molecule in a water lamella was followed for 30 ns. The time evolution of the probability distribution of finding an ethanol molecule that was initially located at the interface is very well described by the diffusion equation on the free energy surface.     

Adsorption of bisulfate on the Ru(0001) single crystal electrode surface

Adsorption of anions from sulfuric acid solutions has been studied on Ru(0001) single crystal and polycrystalline surfaces by electrochemical techniques and in-situ Fourier transform infrared spectroscopy. Infrared spectroscopy shows that bisulfate is the anion adsorbed on the Ru(0001) surface. The bisulfate adsorption is detected at the H₂ evolution potential and extends into the potential region where the Ru surface is oxidized. A method for extracting unipolar bands from bipolar bands has been presented. The tuning rate of adsorbed bisulfate in the double layer potential region of Ru(0001) was found to be significantly smaller than those observed for other platinum metals. This has been ascribed to a small change in bisulfate coverage on Ru(0001) in this potential range. Bisulfate vibration frequencies are higher on this surface than at any face-centered cubic metal with the (111) orientation. Oxidation of the Ru(0001) surface is limited to one electron per Ru atom, distinctly different from the high degree of oxidation seen in polycrystalline surfaces. For oxidized polycrystalline Ru, only solution phase sulfates and bisulfates are observed in the IR spectra.     

Impact of defects on the surface chemistry of ZnO(0001 macro)-O

Scanning tunneling microscopy and core level photoelectron spectroscopy measurements have been used to investigate the morphology of ZnO(0001 macro)-O, and its reactivity with carbon monoxide and carbon dioxide, as a function of surface preparation. Real space images of the surface indicate that increasing the substrate anneal temperature during preparation significantly reduces the surface step density. Surface defect concentration is also monitored by employing formic acid as a chemical probe, which is shown to adsorb dissociatively (HCOOH --> [HCOO](-) + H(+)) only on zinc cations at step edges. Carbon 1s X-ray photoelectron spectra show that carbon monoxide and carbon dioxide both react to form surface carbonate species. Spectra, recorded both as a function of surface preparation and following coadsorption, demonstrate that the carbonate formed from either reactant molecule is located at oxygen vacancies at step edges, evidencing the significant role that defects can play in the surface chemistry of ZnO(0001 macro)-O.     

Adsorption of water molecules inside a Au nanotube: a molecular dynamics study

A molecular dynamics simulation of water molecules through a Au nanotube with a diameter of 20 A at bulk densities 0.8, 1, and 1.2 gcm(3) has been carried out. The water molecules inside a nanoscale tube, unlike those inside a bulk tube, have a confined effect. The interaction energy of the Au nanotube wall has a direct influence on the distribution of water molecules inside the Au tube in that the adsorption of the water molecules creates shell-like formations of water. Moreover, the high number of adsorbed molecules has already achieved saturation at the wall of the Au nanotube at three bulk densities. This work compares the distribution percentage profiles of hydrogen bonds for different regions inside the tube. The structural characteristics of water molecules inside the tube have also been studied. The results reveal that the numbers of hydrogen bonds per water molecule influence the orientational order parameter q. In addition, the phenomenon of a group of molecules bonded inside the tube can be observed as the number of hydrogen bonds increase.     

The First-principles Calculations of H2S Adsorption and Decomposition on the ZnO（0001） Surface

The adsorption and decomposition of H2S on the ZnO(0001) surface have been investigated with first-principles calculations.The results reveal that H2S is dissociatively adsorbed on the clean ZnO(0001) surface to generate HS-and hydrogen species.To our interest,as indicated by Mulliken charge and density of states of the configuration calculation,the bonding mechanism of H2S on the ZnO(0001) surface can involve the donation of charge from the "s lone pairs" into the surface and the back donation of surface electrons to H2S.Therefore,the electrons should play an important role in decomposition.Furthermore,the reactivity of H2S adsorption and further thermal decomposition reactions on the ZnO(0001) surface have also been discussed by calculating the possible reaction pathways.As expected,H2 will be easily generated during the decomposition process.     

Adsorption of atomic hydrogen on ZnO(1010): STM study

The adsorption of atomic hydrogen on a single crystal ZnO(1010) surface has been studied by scanning tunneling microscopy (STM) under ultrahigh vacuum conditions at room temperature and at elevated temperatures. High resolution STM images indicate that a well-ordered (1x1) H adlayer is formed on the ZnO(1010) surface. The STM data strongly indicate that the hydrogen adsorbs on top of the oxygen atoms forming hydroxyl species. Scanning tunneling spectroscopy (STS) studies reveal a H atom induced metallization at room temperature. In contrast to the clean surface for the hydrogen-covered surface distinct defects structures consisting of missing O and Zn atoms could be identified.     

Quantum molecular dynamics study of water on TiO2(110) surface

The adsorption of water on perfect TiO(2)(110) surface is studied by quantum molecular dynamics simulation adopting a periodic model formed by five water molecules on a (5 x 1) surface unit cell of a five layer slab of TiO(2). The total simulation time is 3.2 ps. At about 1.3 ps, one water molecule dissociates with the help of other adsorbed waters and surface bridging oxygens. During the remaining 1.9 ps, the waters and OH groups vibrate, but no more dissociation or recombination is observed. By comparing recent experimental O1s photoemission (x-ray photoelectron spectroscopy) spectra of H(2)O/TiO(2)(110) to the computed spectrum of the adsorbate in the configurations supplied by the molecular dynamics simulation, the observed peaks can be attributed to different oxygen species. The proposed assignment of the main spectral features supports the occurrence of partial water dissociation (approximately 20%) also on a perfect TiO(2) surface.     

Collisional dynamics of Ag19 on Pd(100): a molecular dynamics study

By means of molecular dynamics simulations based on realistic n-body potentials we investigate structural and dynamical features inherent to the energetic collision of a silver cluster (Ag₁₉) on the Pd(100) substrate. Both the system and the impact energy (E_i = 95 eV) adopted have been chosen to parallel an experimental study of size selected Ag cluster deposition on Pd(100). Our results indicate that the experimental cross section obtained via thermal energy atom scattering at the same collision energy is well reproduced by the simulations.The modeling allows to rationalize the collision outcome in terms of defect production and cluster atoms implantation. The adsorbed structures have an heterogenous nature and are mostly two-dimensional.     

Six-dimensional potential energy surface for H2 at Ru(0001)

The six-dimensional (6D) potential energy surface (PES) for the H(2) molecule interacting with a clean Ru(0001) surface has been computed accurately for the first time. Density functional theory (DFT) and a pseudopotential based periodic plane-wave approach have been used to calculate the electronic interactions between the molecule and the surface. Two different generalized gradient approximation (GGA) exchange-correlation functionals, PW91 and RPBE, have been adopted. Based on the DFT/GGA calculated potential energies, an analytical 6D PES has been constructed using the corrugation reducing procedure. A very accurate representation of the DFT/GGA data has been achieved, with an average error in the interpolation of about 3 meV and a maximum error not larger than about 30 meV. The top site is found to be the most reactive site for both functionals used, but PW91 predicts a higher reactivity than RPBE, with lower-energy and earlier-located dissociation barriers. The energetic corrugation displayed by the RPBE PES is larger than the PW91 PES while the geometric corrugation is smaller. The differences between the two PESs increase as the distance of the molecular center of mass to the surface decreases. A direct comparison with experimental investigations on H(2)/Ru(0001) could shed light on the suitability of these XC potentials often used in DFT calculations.