Coverage Dependence Research Articles

Coverage dependent non-adiabaticity of CO on a copper surface.

We have studied the coverage-dependent energy transfer dynamics between hot electrons and CO on Cu(110) with femtosecond visible pump, sum frequency probe spectroscopy. We find that transients of the C-O stretch frequency display a red shift, which increases from 3 cm(-1) at 0.1 ML to 9 cm(-1) at 0.77 ML. Analysis of the transients reveals that the non-adiabatic coupling between the adsorbate vibrational motion and the electrons becomes stronger with increasing coverage. This trend requires the frustrated rotational mode to be the cause of the non-adiabatic behavior, even for relatively weak laser excitation of the adsorbate. We attribute the coverage dependence to both an increase in the adsorbate electronic density of states and an increasingly anharmonic potential energy surface caused by repulsive interactions between neighboring CO adsorbates. This work thus reveals adsorbate-adsorbate interactions as a new way to control adsorbate non-adiabaticity.

Engineering the work function of buckled boron α-sheet by lithium adsorption: a first-principles investigation.

First-principles density functional theory calculations were performed to study the effect of Li adsorption on the structural and electronic properties, particularly the work function, of boron α-sheet. The calculated binding energies indicated that boron α-sheet could be well stabilized by the adsorption of Li atoms. Furthermore, the work functions of Li-adsorbed boron α-sheets were observed to decrease drastically with increasing Li coverage. The work functions are lower than that of Mg and even, for some of them, lower than that of Ca, indicating a considerable potential application of Li-adsorbed boron α-sheets as field-emission and electrode materials. Based on the calculated geometric and electronic structures, we discuss in details some possible aspects affecting the work function. The Li coverage dependence of the work functions of Li-adsorbed boron α-sheets was further confirmed by electrostatic potential analyses. The relationship between the work function variation and the Fermi and vacuum energy level shifts was also discussed, and we observed that the variation of the work function is primarily associated with the shift of the Fermi energy level. It is the surface dipole formed by the interaction between adatoms and substrate that should be responsible for the observed variation of the work function, whereas the increasing negative charge and rumpling for boron α-sheet only play minor roles. Additionally, the effect of Li adatoms on the work function of boron α-sheet was confirmed to be much stronger than that of graphene or a graphene double layer.

DFT comparison of intrinsic WGS kinetics over Pd and Pt

We report density functional theory (DFT) results for five competing water–gas shift (WGS) pathways on identical Pd and Pt(111) surface models. We develop analytical kinetic models to compare intrinsic catalytic activity under conditions relevant to low-temperature WGS. The baseline model predicts that WGS proceeds preferentially through a carboxyl intermediate, that water dissociation is rate limiting, and that the intrinsic rate on Pt is greater than on Pd. Absolute rates are unphysically low because of unphysically high CO coverage. Systematically decreasing the CO binding energy in keeping with its known coverage dependence yields more realistic coverages and absolute rates, shifts the rate limiting step to carboxyl formation, and retains the same ordering of activity. Predicted rate orders and activation energies are not consistent with those observed for supported Pt and Pd catalysts, suggesting that even on nominally inert supports, the support plays an active role in WGS activity.

Probing the Coverage Dependence of Site and Adsorbate Configurational Correlations on (111) Surfaces of Late Transition Metals

Correlations between adsorption energies allow for the prediction of a large set of adsorption energies from a few quantum-chemical calculations, enabling quick evaluations of the catalytic activity of surfaces for multistep reactions. A critical question is whether these correlations—initially established at low coverage on geometrically similar surfaces—are useful at high coverage and in various geometric environments, which are the relevant industrial conditions of real catalysts. To answer this question, we use density functional theory to calculate almost 900 adsorption energies of several atomic adsorbates on late transition metal fcc (111) surfaces at a variety of coverages and configurations. We observe strict requirements of the geometric similarity between adsorption sites for the existence of correlations in adsorption energies. We also find that the introduction of higher coverages generally weakens and changes the correlations. We use our results to assess a simple model constructed from previously discovered scaling relationships and discuss their ramifications on catalyst discovery as well as strategies to incorporate this coverage dependence.

Adsorption and diffusion of oxygen atom on UN2(100) surface and subsurface: a density functional theory study (DFT and DFT + U)

In the present work, adsorption and dif fusion of oxygen (O) atom on uranium dinitride (UN2) is studied to map out the preferential UN2(100) surface site. The first principle method based on density functional theory (DFT) within the generalized gradient approximation PBE and the covariant version energy functional PBE + U correction were used. The supercell approach and a coverage dependence of the adsorption structures and energetic were studied in detail for several monolayers’ (ML) range. Potential energy surfaces (PES) corresponding to the interaction between O atom and UN2(100) on surface and subsurface for several sites and layers (Top U and Top N slabs) were calculated and favorable sites were identified with their maxima energy stable positions, which were then analyzed. For all positions, the PES show the same system behavior, when the O atom is sufficiently far from the UN2 surface, and the energy of the system tends to the sum of free UN2 slab and free oxygen atom energies. In return, when the distances decrease, strong interactions appear with presence of important potential wells. Calculation results showed that favored on-surface site for O atom adsorption were found to be near the bridge one for the UN (Top U slab) corresponding to five layers, uranium terminated and top one for (Top N slab) corresponding to six layers nitrogen terminated, the maximum system energy is situated at a position of about 1.2 and 1.5 A from the surface for the two layers types calculations respectively. For subsurface results, only Top N presents a favorable incorporation site at the hollow position and the penetration of O atom is about −0.5 A from the surface. DFT + U study confirms all the results obtained by DFT calculations; that is, the maxima site positions for oxygen atom and the adhesion energy values per atom are of the same order of magnitudes. The adsorption energy per oxygen atom and the mean distance from the top surface gradually decrease with the coverage of O atoms for both on-surface cases, Top U and Top N slabs, with oxygen occupying the favorable site. For the Top N slab hollow site, the incorporation of oxygen through the surface becomes effective from a coverage of 3/8 ML with an encrustation of about −0.3 A.

Theoretical study of the ammonia nitridation rate on an Fe (100) surface: a combined density functional theory and kinetic Monte Carlo study.

Ammonia (NH3) nitridation on an Fe surface was studied by combining density functional theory (DFT) and kinetic Monte Carlo (kMC) calculations. A DFT calculation was performed to obtain the energy barriers (Eb) of the relevant elementary processes. The full mechanism of the exact reaction path was divided into five steps (adsorption, dissociation, surface migration, penetration, and diffusion) on an Fe (100) surface pre-covered with nitrogen. The energy barrier (Eb) depended on the N surface coverage. The DFT results were subsequently employed as a database for the kMC simulations. We then evaluated the NH3 nitridation rate on the N pre-covered Fe surface. To determine the conditions necessary for a rapid NH3 nitridation rate, the eight reaction events were considered in the kMC simulations: adsorption, desorption, dissociation, reverse dissociation, surface migration, penetration, reverse penetration, and diffusion. This study provides a real-time-scale simulation of NH3 nitridation influenced by nitrogen surface coverage that allowed us to theoretically determine a nitrogen coverage (0.56 ML) suitable for rapid NH3 nitridation. In this way, we were able to reveal the coverage dependence of the nitridation reaction using the combined DFT and kMC simulations.

Physico–chemical Modelling of Adlayer Phase Formation via Surface–limited Reactions of Copper in Relation to Sequential Electrodeposition of Multilayered Platinum on Crystalline Gold

Electrochemical physico-chemical models, describing isothermically surface coverage dependency with electrode potential of underpotentially deposited Cu adlayers (CuUPD) as well as successive surface-limited redox replacement (SLRR) reactions between CuUPD and PtCl62–, to form multilayered Pt deposits on crystalline Au, have been explored. Modelling of such phase formation phenomena take into account heterogeneity effects and extent of adatom interactions within the adlayers on the base Au substrate and gradually formed Pt multilayered deposits. Insights on exchanged electrons, phenomenological reaction kinetics and Gibbs free energy variations during various deposition stages involving SLRR reactions are divulged.

Synthesis of Complex Networks Regular Fractals

The article is sacred to the questions of design of topology of complex networks with the use of new methods of fractal geometry. A complex network structure is inprocess synthesized, through replacement of tops of count by a countfuse, and athe simplest, including affine transformations. The distinguishing feature of prefractal counts is pos sibility of receipt of numeral descriptions of network, coming from the analysis of properties of his countsfuses. For the got prefractal count properties o f fractal and scale invariance are investigational. An example of calculation of fractal dimension of prefracta l count of complex network is made at the use of square and threecornered blocks for coverage of network. It is shown that for the effective analysis of properties selfsimilarity of prefractal counts it is necessary to use V� coverage Dependence of fractal dimension of prefra ctal count is investigational on the linear size of grate e � coverage with the purpose of determining the amount of factors, influencing on the system. They got results are actual at the decision of intricate problems.

Theoretical study of heavy metal Cd, Cu, Hg, and Ni(II) adsorption on the kaolinite(0 0 1) surface

Heavy metal pollution is currently of great concern because it has been recognized as a potential threat to air, water, and soil. Adsorption was one of the most popular methods for the removal of heavy metal. The adsorption of heavy metal Cd, Cu, Hg, and Ni(II) atoms on the hydroxylated (001) surface of kaolinite was investigated using density-functional theory within the generalized gradient approximation and a supercell approach. The coverage dependence of the adsorption structures and energetics were systematically studied for a wide range of coverage Θ [from 0.11 to 1.0 monolayers (ML)] and adsorption sites. The most stable among all possible adsorption sites for Cd(II) atom was the two-fold bridge site followed by the one-fold top site, and the top site was the most favorite adsorption site for Cu and Ni(II) atoms, while the three-fold hollow site was the most stable adsorption site for Hg(II) atom followed by the two-fold bridge site. The adsorption energy increases with the coverage for Cd, Cu, and Hg(II) atoms, thus indicating the higher stability of surface adsorption and a tendency to the formation of adsorbate islands (clusters) with increasing the coverage. However, the adsorption energy of Ni(II) atoms decreases when increasing the coverage. The adsorption capabilities of the kaolinite clay for the heavy metal atoms were in the order of Ni>Cu>Cd>Hg(II). The other properties of the Cd, Cu, Hg, and Ni(II)/kaolinite(001) system including the different charge distribution, the lattice relaxation, and the electronic density of states were also studied and discussed in detail.

Femtosecond Trapping of Free Electrons in Ultrathin Films of NaCl on Ag(100).

We report the excited-state electron dynamics for ultrathin films of NaCl on Ag(100). The first three image potential states (IPSs) were initially observed following excitation. The electrons in the spatially delocalized n = 1 IPS decayed on the ultrafast time scale into multiple spatially localized states lower in energy. The localized electronic states are proposed to correspond to electrons trapped at defects in the NaCl islands. Coverage and temperature dependence of the localized states support the assignment to surface trap states existing at the NaCl/vacuum interface. These results highlight the importance of electron trapping in ultrathin insulating layers.

First Principle Based Simulation of Cyclic Voltammogram: Bromide Adsorption on Pt(111) Surface

Introduction: Adsorption of halide anions on the electrode surface is fundamental phenomena and important in the various applications, such as the synthesis of the shape-controlled nano-particles. Cyclic voltammetry is the most important experiment for understanding the surface reactions. However it is sometimes difficult to assign the origin of peak positions and/or shapes of the voltammogram. Comparison of the experimental results with the simulated voltammograms might be helpful for assigning and clarifying of the atomistic behavior. In this study, we focus on the bromide adsorption on the Pt(111) surface, which is complicated process because Br- adsorbs simultaneously with the desorption of H+. The transition of Br-adsorption structure during voltammetry has been discussed, but is still unclear. We simulated voltammogram based on the density functional theory (DFT) calculations and discussed the peak positions, shape, and adsorption/desorption processes. Method: In this study, following two redox reactions were considered,H+ (aq) + * + e -<--> H* (1),Br - (aq) + * <--> Br* + e - (2),where * indicates the Pt(111) surface. The adsorption free energies (ΔGads) for these reactions were calculated by DFT. (√3x√3)-Br, (3x3)-Br, and (√3x√3)-H adosprtion structures were considered by using the slab models. The surface coverage (θ) dependence of the ΔGads are also calculated. Methodology of the voltammogram simulation has been disscussed by some research groups [1-3]. We adopted a slimilar approach to their works. The θBr and θH were calculated at each potential steps from the ΔGads(θ) under the steady-state approximation. The currents were evaluated by the variation of the θBr and θHduring potential sweep.All DFT calculations were performed by the “interface” program package [4]. The RPBE exchange-correlation functional, the norm-conserving pseudopotential, and numerical basis sets were used. Results and discussion: Figure 1 shows the calculated θH and θBr dependence of the ΔGads. This result describes the repulsive interaction between adsorbed atoms, for example, the ΔGads increases with the increase of θBr in the case of θH=0.33. The simulated voltammograms by using these ΔGads values are shown in Fig. 2. Although the peak shape is broad, the peak positions are in good agreements with the experimental one [5]. Figure 3 shows the calculated surface coverage θH and θBr. The θBr gradually increases from 0.1V, which corresponds the (√3x√3)-Br- adsorption. The θBr exceeds 0.33 around 0.3V, which means that the adsorption structure of Br-gradually changes from (√3x√3) to (3x3). Therefore, it is indicated that the shoulder peak around 0.3V in Fig. 2 is originated from the transition to (3x3)-Br adsorption structure.

Polarization complex-plane plot of impedance for two-stage charge-transfer reaction complicated with an intermediate adsorption (by example of benzene oxidation at boron-doped diamond electrode)

Polarization complex-plane plot of impedance for two-stage anodic reaction with partial charge transfer, complicated with an intermediate adsorption, is theoretically analyzed. The case of linear coverage dependence of the oxidation rate and linear potential dependence of the effective number of electrons involved in each stage, hence, quadratic potential dependence of the activation energies is considered. The result of the modeling agrees qualitatively with experimental data on benzene oxidation at boron-doped diamond electrode. The using of the partial charge transfer concept allowed revealing fine effects of the organics oxidation, which are ascribed to adsorption, in particular, emergence of a loop in the polarization complex-plane plot of impedance.

Adsorption preference reversal phenomenon from multisite-occupancy theory for two-dimensional lattices

The statistical thermodynamics of polyatomic species mixtures adsorbed on two-dimensional substrates was developed on a generalization in the spirit of the lattice-gas model and the classical Guggenheim-DiMarzio approximation. In this scheme, the coverage and temperature dependence of the Helmholtz free energy and chemical potential are given. The formalism leads to the exact statistical thermodynamics of binary mixtures adsorbed in one dimension, provides a close approximation for two-dimensional systems accounting multisite occupancy and allows to discuss the dimensionality and lattice structure effects on the known phenomenon of adsorption preference reversal.

Influence of surface coverage on the chemical desorption process.

In cold astrophysical environments, some molecules are observed in the gas phase whereas they should have been depleted, frozen on dust grains. In order to solve this problem, astrochemists have proposed that a fraction of molecules synthesized on the surface of dust grains could desorb just after their formation. Recently the chemical desorption process has been demonstrated experimentally, but the key parameters at play have not yet been fully understood. In this article, we propose a new procedure to analyze the ratio of di-oxygen and ozone synthesized after O atoms adsorption on oxidized graphite. We demonstrate that the chemical desorption efficiency of the two reaction paths (O+O and O+O2) is different by one order of magnitude. We show the importance of the surface coverage: for the O+O reaction, the chemical desorption efficiency is close to 80% at zero coverage and tends to zero at one monolayer coverage. The coverage dependence of O+O chemical desorption is proved by varying the amount of pre-adsorbed N2 on the substrate from 0 to 1.5 ML. Finally, we discuss the relevance of the different physical parameters that could play a role in the chemical desorption process: binding energy, enthalpy of formation, and energy transfer from the new molecule to the surface or to other adsorbates.

UV Photon-Induced Water Decomposition on Zirconia Nanoparticles

Formation of H2 from photolysis of water adsorbed on zirconia (ZrO2) nanoparticles using 254 nm (4.9 eV) and 185 nm (6.7 eV) photon irradiation was examined. The H2 yield was approximately an order of magnitude higher using monoclinic versus cubic phase nanoparticles. For monoclinic particles containing 2 monolayers (ML) of water, the maximum H2 production rate was ∼0.4 μmole h–1 m–2 using 185 + 254 nm excitation and a factor of 10 lower using only 254 nm. UV reflectance reveals that monoclinic nanoparticles contain fewer defects than cubic nanoparticles. An H2O coverage dependence study of the H2 yield is best fit by a sum of interactions involving at least two types of adsorbate–surface complexes. The first dominates up to ∼0.06 ML and is attributed to H2O chemisorbed at surface defect sites. The second dominates at coverages up to a bilayer. H2 formation is maximum within this bilayer and likely results from efficient energy transfer from the particle to the interface. Energy transfer is more efficient for the monoclinic ZrO2 nanoparticles and likely involves mobile excitons.

Adsorption structure of water molecules on the Be(0001) surface

By using density functional theory calculations, we systematically investigate the adsorption of water molecules at different coverages on the Be(0001) surface. The coverage dependence of the prototype water structures and energetics for water adlayer growth are systematically studied. The structures, energetics, and electronic properties are calculated and compared with other available studies. Through our systematic investigations, we find that water molecules form clusters or chains on the Be(0001) surface at low coverages. When increasing the water coverage, water molecules tend to form a 2 × 2 hexagonal network on the Be(0001) surface.

Coverage-dependent magnetic properties of Ni ultrathin films on Pd(001) investigated using X-ray magnetic circular dichroism

Coverage-dependent magnetic properties of Ni ultrathin films on Pd(001) was investigated using X-ray magnetic circular dichroism (XMCD). Magnetization curves and Ni L2,3 XMCD spectra were measured as a function of Ni coverage. These results revealed a spin reorientation transition from in-plane to out-of-plane as the Ni coverage increased. Ni 3d spin and orbital magnetic moments were evaluated using the magneto-optical sum rules. The presence of a magnetic dead layer inside the Ni film explains the Ni coverage dependence of spin magnetic moment.

Dissociative adsorption of O2on unreconstructed metal (100) surfaces: Pathways, energetics, and sticking kinetics

An accurate description of oxygen dissociation pathways and kinetics for various local adlayer environments is key for an understanding not just of the coverage dependence of oxygen sticking, but also of reactive steady states in oxidation reactions. Density functional theory analysis for M(100) surfaces with $M=\text{Pd}$, Rh, and Ni, where O prefers the fourfold hollow adsorption site, does not support the traditional Brundle-Behm-Barker picture of dissociative adsorption onto second-nearest-neighbor hollow sites with an additional blocking constraint. Rather adsorption via neighboring vicinal bridge sites dominates, although other pathways can be active. The same conclusion also applies for $M=\text{Pt}$ and Ir, where oxygen prefers the bridge adsorption site. Statistical mechanical analysis is performed based on kinetic Monte Carlo simulation of a multisite lattice-gas model consistent with our revised picture of adsorption. This analysis determines the coverage and temperature dependence of sticking for a realistic treatment of the oxygen adlayer structure.

Desorption Kinetics of Methanol, Ethanol, and Water from Graphene

The desorption kinetics of methanol, ethanol, and water from graphene covered Pt(111) are investigated. The temperature programmed desorption (TPD) spectra for both methanol and ethanol have well-resolved first, second, third, and multilayer layer desorption peaks. The alignment of the leading edges is consistent with zero-order desorption kinetics from all layers. In contrast, for water, the first and second layers are not resolved. At low water coverages (<1 monolayer (ML)) the initial desorption leading edges are aligned but then fall out of alignment at higher temperatures. For thicker water layers (10-100 ML), the desorption leading edges are in alignment throughout the desorption of the film. The coverage dependence of the desorption behavoir suggests that at low water coverages the nonalignment of the desorption leading edges is due to water dewetting from the graphene substrate. Kinetic simulations reveal that the experimental results are consistent with zero-order desorption. The simulations also show that fractional order desorption kinetics would be readily apparent in the experimental TPD spectra.

Vibrational Analysis of (bi)Sulfate Adsorption on Pt (111) Surface in Aqueous Solution from the First Principles Molecular Dynamics Simulation

Sulfuric acid is known as a typical specific adsorbent on platinum electrodes. Pt/sulfuric acid system is token as a model of the fingerprints of voltammetric profile for the single crystal electrodes [1]. Even for decades of study, the chemical nature and vibrational properties are still under controversies and remain unclear. It is important to clarify the (bi)sulfate/Pt adsorption state for both scientific research and practical applications. First principles molecular dynamics was used to simulate the interface between Pt(111) surface and sulfuric acid solution. Electric bias was applied by the ESM method [3,4]. An orthogonal unit cell of 8.38x9.67x26.04 Å3 in size was used with SO4groups, enough water layers and hydronium ions in the unit cell to mimic solution phase. A three-layer Pt slab was employed and only the bottom layer is fixed during molecular dynamic simulation. A cell with 12 atoms in each layer was used to simulate the single (bi)sulfate anion adsorption. In order to do simulation at the high surface coverage (adlayer), we have adopted the larger cell that has 20 Pt atoms each layer. Generalized gradient approximation (GGA) with PBE parameterized exchange-correlation function was employed in the MD simulation. The simulation was done by the plane wave pseudopotential package of “Simulation Tool for Atom Technology (STATE)[5]” with ultra-soft pseudopotential. A 2x2x1 k-point mesh is generated by Monkhorst Pack scheme. In order to clarify the controversies over the vibrational spectra, the important contributions from electric bias, adsorption conformation, adsorption species, and solvent are included in these simulations. We observed reversible transition between two-point and three-point binding at a certain bias. Vibrational spectra of SO4 and water molecules were also calculated by Fourier transformation of velocity auto correlation function and compared with experimental and previous simulation results [6,7]. Vibrational mode analysis shows the band at 950 cm−1 corresponds to the breathing mode of SO4, and the band at 1250 cm−1corresponds to the stretching mode between unadsorbed-O and S in the three-point adsorbed sulfate anion. We also point out the problem of simple water bilayer models during the study of Stark effect. Our simulation also shows a surface coverage dependence of Stark effect for the sulfate vibration band at 1250 cm−1. At low surface coverage the Stark effect for the band is red-shift, but at high surface coverage it is blue-shift. The Stark coefficient decreases with the increase of electrode potential as shown in the figure, which is consistent with the experimental study by IR[8] and SFG[9].Fig. Vibrational spectra showing vibration Stark effect of a SOu stretching mode in the sulfate-water coadsorption layer on Pt(111) surface with different biased surface charge (µC/cm2).[1] K. Itaya, Prog. Surf. Sci. 58, 121 (1998); J. Clavilier, K. El Achi and A. Rodes, J. Electroanal. Chem., 1989, 272, 253[2] B. Braunschweig, W. Daum , Langmuir 25, (18) 11112 (2009)[3] M. Otani and O. Sugino, Phys. Rev. B 73, 115407 (2006)[4] M. Otani, I. Hamda, O. Sugino, Y. Morikawa, Y.Okamoto, T. Ikeshoji J. Phys. Soc. Jpn 77, 024802 (2008)[5] Y. Morikawa, K. Iwata, and K. Terakura, Appl. Surf. Sci. 169, 11 (2001)[6] R. Jinnouchi, T. Hatanaka, Y. Morimoto and M. Osawa Phys. Chem. Chem. Phys., 14, 3208 (2012)[7] J.A. Santana, C. R. Cabrera and Y. Ishikawa Phys. Chem. Chem. Phys., 12, 9526 (2010)[8] V. Climent, N. García-Araez, J. M. Feliu, Electrochem. Comm. 2006, 8, 10, 1577[9] B. Braunschweig, P. Mukherjee, D. D. Dlott and A. Wieckowski, J. Am. Chem. Soc. 2010, 132 (40), 14036