Neighboring Adsorption Sites Research Articles

Unraveling the Origin of the Repulsive Interaction between Hydrogen Adsorbates on Platinum Single-Crystal Electrodes.

Hydrogen adsorption on platinum (Pt) single-crystal electrodes has been studied intensively in both experiments and computations. Yet, the precise origin and nature of the repulsive interactions observed between hydrogen adsorbates (Hads) have remained elusive. Here, we use first-principles density functional theory calculations to investigate in detail the interactions between Hads on Pt(111), Pt(100), and Pt(110) surfaces. The repulsive interaction between Hads on Pt(111) is deconvoluted into three different physical contributions, namely, (i) electrostatic interactions, (ii) surface distortion effect, and (iii) surface coordination effect. The long-range electrostatic interaction, which is generally considered the most important source of repulsive interactions in surface adsorption, was found to contribute less than 30% of the overall repulsive interaction. The remaining >70% arises from the other two contributions, underscoring the critical influence of surface-mediated interactions on the adsorption process. Surface distortion and coordination effects are found to strongly depend on the coverage and adsorption geometry: the effect of surface distortion dominates when adsorbates reside two or more Pt atoms apart; the effect of surface coordination dominates if hydrogen is adsorbed on neighboring adsorption sites. The above effects are considerably less pronounced on Pt(100) and Pt(110), therefore resulting in weaker interactions between Hads on these two surfaces. Overall, the study highlights the relevance of surface-mediated effects on adsorbate-adsorbate interactions, such as the often-overlooked surface distortion. The effect of these interactions on the hotly debated adsorption site for the adsorbed hydrogen intermediate in the hydrogen evolution reaction is also discussed.

Optimal doping elements for inhibiting surface-diffusion of adatoms on Cu3Sn

Downward scaling of the micro-bumps in three-dimensional integrated circuits (3D-ICs) aggravates the formation of lateral Cu3Sn causing serious reliability issues. Inhibiting the continuous surface diffusion of Cu and Sn during solid-state aging is critical to suppress the growth of lateral Cu3Sn. Herein, through comprehensive density functional theory calculations, we found Ti and Si are the ideal surface doping elements for suppressing surface diffusion. The doping Ti and Si not only significantly increase the diffusion barriers of Cu and Sn adatoms in their vicinity but also reduce the mobility of the surface atoms. Through electronic structure and crystal orbital Hamilton population analysis, we reveal the underlying mechanism is the enhanced binding of Cu and Sn adatoms in neighboring adsorption sites and strong covalent bonding of Ti and Si with the surface. Moreover, it is found doping Ti impedes the surface diffusion of Sn adatoms much more greatly than Cu adatoms while doping Si hinders the surface diffusion of Cu adatoms rather than Sn adatoms. This work sheds light on the possibility of utilizing doping elements on intermetallic compound surface to effectively inhibit surface diffusion and reduce reliability risks in 3D-ICs.

(Invited) Pd Monolayer Catalyst As Transformative Concept for Electrolytic Hydrogen Isotope Separation

The effect of strain in Pd monolayer catalyst is explored for electrolytic hydrogen isotope separation. Positive/negative strain increases/decreases strength of adsorbed hydrogen bond in overpotential region where recombination of hydrogen atoms is the rate determining step in hydrogen evolution reaction. As a result, the increased/decreased hydrogen isotope separation efficiency of Pd monolayers is expected as compared to bulk Pd. The positive/negative strain rises/lowers diffusion barrier for adsorbed hydrogen atoms. This effect favors/retards recombination of isotopes with smaller mass. As the surface is stretched/compressed, the neighboring adsorption sites separate/approach each other which affects heavier hydrogen isotopes more/less and result in their lower/higher probability for recombination. All these fundamental consideration indicate that Pd monolayers with different level of strain should have much different separation factors than corresponding bulk electrodes. To study described effects, Pd monolayers we synthesized electrochemically on Au(111) and Ru(0001) electrodes each yielding qualitatively different strain levels (Au-positive, Ru-negative). The adsorption strength of hydrogen isotopes is studied by infra-red spectroscopy. These results are input in classical models for isotope separation[1] T calculations. The calculated ratio between the rates of hydrogen and deuterium recombination and separation factors for Pd monolayers and corresponding bulk electrodes are compared to experimentally measured ones. The following discussion focuses on understanding and quantification of strain effects on separation efficiency of Pd monolayers. [1] B. E. Conway, Proceedings of The Royal Society of London, A. Mathematical and Physical Sciences, 247, 400 (1958).

The effect of SO2 on CaO capture selenium in the flue gas: Density functional theory and experimental study

• SO 2 inhibits CaO capture selenium by surface inhibition. • SO 2 promotes CaO capture selenium through gaseous selenium conversion. • SeO 2 was hardly adsorbed by the sulfide product CaSO 4 from CaO. • The intensity of CaO capture selenium species is: Se > SeO > SeO 2 . In this work, the mechanism of SO 2 on CaO capture selenium was deeply investigated combined density functional theory (DFT) with adsorption experiments. The fixed-bed experiments result showed that SO 2 inhibited CaO capture selenium from 0 to 1000 while promoted in the range of 1000–2000 ppm, and CaSeO 3 was the primary adsorption product at 850 °C. Freundlich equation accurately predicted the adsorption capacity of CaSO 4 for SeO 2 with the concentration from 0 to 800 ppm, indicating weak adsorption activities for SeO 2 . Furthermore, the SO 2 and selenium oxides adsorption mechanism was studied by DFT. It demonstrated that SO 2 and SeO 2 were adsorbed on CaO (0 0 1) surface involved chemical adsorption with oxygen atoms as adsorption sites, which led to the competition of SO 2 and SeO 2 for adsorption sites on the CaO surface. SO 2 adsorbed on the CaO surface inhibited the activity of neighboring adsorption sites for SeO 2 . Moreover, compared with SeO 2 , SeO and Se 0 reduced by SO 2 were more beneficial adsorbed by CaO, the conversion of SeO 2 to selenium and SeO was firstly proposed in selenium adsorption. In detail, SO 2 promoted CaO capture selenium through homogeneous selenium conversion. Finally, it was identified that the CaSO 4 (0 0 1) surface was inert for selenium adsorption in the flue gas, indicating CaO transition to CaSO 4 inhibited selenium adsorption.

Strain Effect on Electrolytic Hydrogen Isotope Separation

The effect of strain in Pt and Pd monolayer catalyst is explored for electrolytic hydrogen isotope separation. Positive/negative strain increases/decreases strength of adsorbed hydrogen bond in overpotential region where recombination of hydrogen atoms is the rate determining step in hydrogen evolution reaction. As a result, the increased/decreased hydrogen isotope separation efficiency of Pt and Pd monolayers is expected as compared to bulk Pt and Pd. The positive/negative strain rises/lowers diffusion barrier for adsorbed hydrogen atoms. This effect favors/retards recombination of isotopes with smaller mass. As the surface is stretched/compressed, the neighboring adsorption sites separate/approach each other which affects heavier hydrogen isotopes more/less and results in their lower/higher probability for recombination. All these fundamental consideration indicate that Pt and Pd monolayers with different level of strain should have much different separation factors than corresponding bulk electrodes. To study described effects, Pt and Pd monolayers we synthesized electrochemically on Au(111) and Ru(0001) electrodes each yielding qualitatively different strain levels (Au-positive, Ru-negative). Pd monolayers are synthesized using electrodeposition[1] while Pt monolayers were synthesized using deposition via surface limited redox replacement reaction[2]. The adsorption strength of hydrogen isotopes is studied by infra-red spectroscopy. These results will are input in our classical models for isotope separation[3] and to our DFT calculations. The calculated ratio between the rates of hydrogen and deuterium recombination and separation factors for Pt and Pd monolayers and corresponding bulk electrodes are compared to experimentally measured ones. The following discussion is focused on understanding and quantification of strain effects on separation efficiency of Pt and Pd monolayers. [1] H. –F. Waibel, M. Kleinert, L. A. Kibler, and D. M. Kolb, Electrochim. Acta, 47, 1461 (2002). [2] S. R. Brankovic, J. X. Wang and R.R. Adzic, Surface Science, 474, L173 (2001). [3] B. E. Conway, Proceedings of The Royal Society of London, A. Mathematical and Physical Sciences, 247, 400 (1958).

Prediction for Two Spatially Modulated Superfluids: $$^4\hbox {He}$$ 4 He on Fluorographene and on Hexagonal BN

We have derived the adsorption potential of $^4$He atoms on fluorographene (GF), on graphane and on hexagonal boron nitride (hBN) by a recently developed ab initio method that incorporates the van der Waals interaction. The $^4$He monolayer on GF and on hBN is studied by state-of-the-art quantum simulations at T=0 K. With our adsorption potentials we find that in both cases the ground state of $^4$He monolayer is a fluid and not an ordered state with localized atoms as on graphite and on graphene. In the case of GF the present result is in qualitative agreement with the superfluid phase that was obtained using an empirical adsorption potential [M. Nava et al., Phys. Rev. B 86, 174509 (2012)]. This fluid state of $^4$He on GF and on hBN is characterized by a very large density modulation and at the equilibrium density the ratio $\Gamma$ between the largest and the smallest local density along the direction of two neighboring adsorption sites and averaged over the perpendicular direction is $\Gamma$ = 1.91 for GF and $\Gamma$ = 1.65 for hBN. Recent experiments [J. Nyeki et al., Nature Physics 13, 455 (2017)] have discovered a superfluid phase in the second layer $^4$He. This is a spatially modulated superfluid that turns out to have anomalous thermal properties. This gives a strong motivation for an experimental study of monolayer $^4$He on GF and on hBN that we predict to be a superfluid with a much stronger spatial modulation.

Influence of Step and Island Edges on Local Adsorption Properties: Hydrogen Adsorption on Pt Monolayer Island Modified Ru(0001) Electrodes

The influence of steps and island edges on the local electronic structure of a (bi-)metallic single crystalline electrode surface and on the local, site-specific adsorption energy of adsorbed species, the so-called structural effects, was studied by periodic density functional theory based calculations, focusing on longer-range effects. Using hydrogen adsorption energies as a local probe, calculations were performed both for partly Pt monolayer covered planar Ru(0001) surfaces and for a stepped Ru( $10\bar {19}$ ) surface decorated with a row of Pt atoms. The calculations demonstrate that the steps/island edges affect not only the nearest neighbor adsorption sites but also more distant ones with the extent depending on the particular structure. This longer-range effect is in excellent agreement with recent temperature-programmed desorption and spectroscopy experiments (Hartmann et al. Phys. Chem. Chem. Phys. 14, 10919, 2012). For the interaction of water molecules with partly Pt monolayer covered Ru(0001), similar trends as in the hydrogen adsorption have been found. In addition, hydrogen adsorption energies as a function of coverage have been used to derive the hydrogen coverage as a function of the electrode potential, exhibiting a broad range of stable hydrogen adsorption structures.

Influence of Coadsorbed Water and Alcohol Molecules on Isopropyl Alcohol Dehydration on γ-Alumina: Multiscale Modeling of Experimental Kinetic Profiles

Successfully modeling the behavior of catalytic systems at different scales is a matter of importance not only for a fundamental understanding but also for a more rational design of catalysts and a more precise definition of the kinetic laws used as inputs in chemical engineering. We have developed here a multiscale modeling of the dehydration of isopropyl alcohol to propene and diisopropyl ether on γ-alumina catalysts, which clearly evidences and explains the central character of cooperative effects between coadsorbates in the kinetic network. The evolution of partial pressures with contact time was simulated using an original DFT-based microkinetic model based on a “macro site” centered on the main active site located on the (100) planes of alumina and comprising several neighboring adsorption sites. The formation of isopropyl alcohol–isopropyl alcohol or water–isopropyl alcohol dimers on the surface was required to correctly simulate the production of the minor product, diisopropyl ether, and the evolu...

Strong Selective Adsorption of Polymers.

A scaling theory is developed for selective adsorption of polymers induced by the strong binding between specific monomers and complementary surface adsorption sites. By "selective" we mean specific attraction between a subset of all monomers, called "sticky", and a subset of surface sites, called "adsorption sites". We demonstrate that, in addition to the expected dependence on the polymer volume fraction ϕbulk in the bulk solution, selective adsorption strongly depends on the ratio between two characteristic length scales, the root-mean-square distance l between neighboring sticky monomers along the polymer, and the average distance d between neighboring surface adsorption sites. The role of the ratio l/d arises from the fact that a polymer needs to deform to enable the spatial commensurability between its sticky monomers and the surface adsorption sites for selective adsorption. We study strong selective adsorption of both telechelic polymers with two end monomers being sticky and multisticker polymers with many sticky monomers between sticky ends. For telechelic polymers, we identify four adsorption regimes at l/d < 1 that are characterized by the fraction of occupied adsorption sites and whether the dominant conformation of adsorbed chains is a single-end-adsorbed "mushroom" or double-end-adsorbed loop. For l/d > 1, we expect that the adsorption layer at exponentially low ϕbulk consists of separated unstretched loops, while as ϕbulk increases the layer crosses over to a brush of extended loops with a second layer of weakly overlapping tails. For multisticker chains, in the limit of exponentially low ϕbulk, adsorbed polymers are well separated from each other. As l/d increases, the conformation of an individual polymer changes from a single-end-adsorbed "mushroom" to a random walk of loops. For high ϕbulk, adsorbed polymers at small l/d are mushrooms that cover all the adsorption sites. At sufficiently large l/d, adsorbed multisticker polymers strongly overlap. We anticipate the formation of a self-similar carpet and with increasing l/d a two-layer structure with a brush of loops covered by a self-similar carpet. As l/d exceeds the threshold determined by the adsorption energy, the brush of loops under the carpet reaches a saturated state, resulting in a l/d-independent brush-under-carpet structure, which can also be applied to describe adsorbed multisticker polymers in nonselective adsorption where a sticker can strongly bind to any place on the adsorption surface. We examine the adsorbed amount Γ of multisticker polymers in different regimes for selective adsorption. If the adsorbed multisticker polymers are nonoverlapping mushrooms, the adsorbed amount Γ increases linearly with the surface density of adsorption sites Σ ≈ 1/d2. In the self-similar carpet regime, Γ increases sublinearly as Σ0.15 in a good solvent, while only logarithmically in a theta solvent. Formation of a brush layer under the carpet contributes an additional adsorbed amount. This additional amount increases linearly with Σ and eventually dominates the overall adsorbed amount Γ before saturating at a plateau value controlled by the adsorption energy.

Heterodimerization via the Covalent Bonding of Ta@Si16 Nanoclusters and C60 Molecules

Initial products prepared via the surface immobilization of Ta-atom-encapsulated Si16 cage (Ta@Si16) nanoclusters on solid surfaces terminated with monolayer films of C60 molecules were investigated using scanning tunneling microscopy (STM). The STM results indicated that marked aggregation and desorption of surface-immobilized Ta@Si16 nanoclusters were not induced, even after thermal annealing at ∼500 K, whereas the local vertical and lateral positions of the Ta@Si16 nanoclusters with respect to neighboring adsorption sites in the C60 film were modified. This local positional transition occurred on C60 monolayer films weakly bonded (via van der Waals forces) to substrates such as highly oriented pyrolytic graphite (HOPG) but did not occur on C60 monolayer films covalently bonded to substrates such as Si(111)7 × 7. These results indicated that the heterodimer consisting of a Ta@Si16 nanocluster and a C60 molecule, Ta@Si16–C60, was formed as an initial product via covalent bonding, which inhibited wide-ran...

Investigation of the spin-lattice relaxation of 13CO and 13CO2 adsorbed in the metal-organic frameworks Cu3(btc)2 and Cu3−xZnx(btc)2

The (13)C nuclear spin-lattice relaxation time of (13)CO and (13)CO2 molecules adsorbed in the metal-organic frameworks (MOFs) Cu2.97Zn0.03(btc)2 and Cu3(btc)2 is investigated over a wide range of temperatures at resonance frequencies of 75.468 and 188.62 MHz. In all cases a mono-exponential relaxation is observed, and the (13)C spin-lattice relaxation times (T1) reveal minima within the temperature range of the measurements and both frequencies. This allows us to carry out a more detailed analysis of the (13)C spin relaxation data and to consider the influence due to the spectral functions of the thermal motion. In a model-free discussion of the temperature dependence of the ratios T1 (T)∕T1,min we observe a motional mechanism that can be described by a single correlation time. In relation to the discussion of the relaxation mechanisms this can be understood in terms of dominating translational motion with mean jump distance being larger than the minimum distances between neighboring adsorption sites in the MOFs. A more detailed discussion of the jump-like motion observed here might be carried out on the basis of self-diffusion coefficients. From the present spin relaxation measurements activation energies for the local motion of the adsorbed molecules in the MOFs can be estimated to be 3.3 kJ∕mol and 2.2 kJ∕mol, for CO and CO2 molecules, respectively. Finally, our findings are compared with our recent results derived from the (13)C line shape analysis.

Atomistic and Coarse-Grained Modeling Strategies for Thin Film Nucleation and Growth on Quasicrystalline Surfaces

ABSTRACTStrategies are described for modeling the kinetics of non-equilibrium film growth during deposition of metals on quasicrystalline substrates. We review previous atomistic-level lattice-gas modeling and Kinetic Monte Carlo simulation for pseudomorphic (or commensurate) submonolayer growth based on a “disordered or irregular bond-network” (DBN) of neighboring adsorption sites. We describe extensions to treat strain effects and multilayer growth, and discuss a type of commensurate-incommensurate transition expected around 2-3 layers. We also describe a coarse-grained “step dynamics” modeling which tracks the dynamics of island edges in each layer rather than individual atoms. Step dynamics models can also include key aspects of the physics such as layer-dependent energetics, including quantum size effects, and strain effects.

Hydrogen adsorption in the metal–organic frameworks Fe2(dobdc) and Fe2(O2)(dobdc)

The hydrogen storage properties of Fe(2)(dobdc) (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) and an oxidized analog, Fe(2)(O(2))(dobdc), have been examined using several complementary techniques, including low-pressure gas adsorption, neutron powder diffraction, and inelastic neutron scattering. These two metal-organic frameworks, which possess one-dimensional hexagonal channels decorated with unsaturated iron coordination sites, exhibit high initial isosteric heats of adsorption of -9.7(1) and -10.0(1) kJ mol(-1), respectively. Neutron powder diffraction has allowed the identification of three D(2) binding sites within the two frameworks, with the closest contacts corresponding to Fe-D(2) separations of 2.47(3) and 2.53(5) Å, respectively. Inelastic neutron scattering spectra, obtained from p-H(2) (para-H(2)) and D(2)-p-H(2) mixtures adsorbed in Fe(2)(dobdc), reveal weak interactions between two neighboring adsorption sites, a finding that is in opposition to a previous report of possible 'pairing' between neighboring H(2) molecules.

Local Reaction Kinetics by Imaging: CO Oxidation on Polycrystalline Platinum

Local Reaction Kinetics by Imaging: CO Oxidation on Polycrystalline Platinum

Inhibiting Adatom Diffusion through Surface Alloying

Abinitio and kinetic MonteCarlo calculations elucidate the electronic nature of surface Sn alloying on the stability and mobility of a Cu adatom on the Cu-Sn (111) alloy surface. Sn atoms segregate on the surface and introduce forbidden areas around them within which adatom adsorption is strictly prohibited. In addition they reduce dramatically both the binding and the mobility of Cu adatoms in neighboring adsorption sites outside the forbidden areas, in contrast to experimental suggestions. Thus, Sn atoms act as blocking sites inhibiting the Cu adatom diffusion. The underlying mechanisms are the structural deformation associated with the oversized Sn atoms and the enhancement of the adatom-surface interaction in the vicinity of Sn atoms.

Adsorption Species of Ethyl Benzoate in MgCl2-Supported Ziegler−Natta Catalysts. A Density Functional Theory Study

Adsorption species of ethyl benzoate (EB) on the (104) and (110) MgCl2 surfaces have been studied within DFT. As a result, monodentate and bidentate complexes of EB were obtained on both the MgCl2 surfaces. The bidentate structures on the (104) MgCl2 surface proved to be stabilized by the decreased distance between neighboring adsorption sites (surface Mg cations). The different affinity of EB for the five- and four-coordinated Mg cations predicted was suggested to be the cause of changing the equilibrium shape of MgCl2 crystals upon growing in the presence of EB: EB chemisorption seems to stabilize the (110) MgCl2 surface to a greater degree as compared to the (104) MgCl2 surface. The influence of EB coordination mode on active site stereoselectivity is discussed.

Collective diffusion in the presence of substrate inhomogeneities and particle–particle interactions

A novel variational analytic approach to collective diffusion allowing the density dependent collective diffusion coefficient to be calculated in systems of interacting particles adsorbed on a crystalline substrate is presented. The approach, based on a kinetic lattice gas model extracts the diffusion coefficient directly from the master equations which account for the microscopic kinetics of the system in which microscopic processes underlying the diffusion are particle jumps between neighboring adsorption sites. Variational parameters minimizing the evaluated diffusional eigenvalue of the microscopic rate matrix are ‘geometrical’ and ‘correlational’ phases accounting, for the local potential energy landscape experienced by the adsorbed particle and the local correlations, respectively, i.e. an instantaneous occupation pattern of adsorption sites around the particle. Selected results, collective diffusion as a function of particle coverage, for the system of interacting particles adsorbed on a one dimensional non-homogeneous substrate with steps and for a system of non-interacting particles adsorbed on a two dimensional striped–stepped surface are presented and discussed. It is demonstrated in the latter case that the mean field approach which is known in the literature overestimates the diffusion coefficient and corresponds to the variational result in the limit of infinitely fast hopping kinetics in the direction parallel to steps.

Adsorption and dissociation of water on the (0001) surface of double hexagonal close packed americium

AbstractAb initio total energy calculations within the framework of density functional theory have been performed for water molecule adsorption on the (0001) surface of double hexagonal packed americium using a full‐potential all‐electron linearized augmented plane wave plus local orbitals method (FP‐L/APW + lo). Subsequent partial dissociation (OH + H) and complete dissociation (H + O + H) of the water molecule have been examined. The completely dissociated H + O + H configuration exhibit the strongest binding with the surface (3.35 eV), followed by partially dissociated species OH + H (2.23 eV), with all molecular H2O configurations showing weak physisorption (0.366 eV). For molecular adsorptions, the flat lying orientation of the water molecule if found to be more favorable for majority of the cases. In the case of partial dissociation (OH + H), the vertical orientation of OH molecule with O facing the surface adsorbed at a h3 adsorption site and the H atom adsorbed at another neighboring h3 site is found to be the most favorable. For complete dissociation (H + O + H), the dissociated species occupy three different neighboring h3 adsorption sites. The changes in work functions and net magnetic moments are presented for all the cases studied. The adsorbate‐substrate interactions have been analyzed in detail using difference charge density distributions and the local density of states. The effects of adsorption on the Am 5f electron localization‐delocalization characteristics have been discussed. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The heterogeneous catalytic reaction 2A + B 2 → 2AB exactly solved on a small lattice

We studied a kinetic model of a generic heterogeneous catalytic reaction 2 A ∗ + B 2 ∗ → 2 AB g . This reaction includes the steps of adsorption and desorption of reactants A and B 2, where B 2 requires two neighboring adsorption sites to be adsorbed. We solved the model exactly and without any restriction on a 2 × 2 lattice. Despite the reduced number of sites employed, the solution shows the main general features observed by simulating the mechanism on a large N × N lattice. This result should encourage the search for an analytical solution as a reliable form to unravel details of a chemical reaction prior to performing numerical simulations.

Video STM Studies of Adsorbate Diffusion at Electrochemical Interfaces

Direct in situ studies of the surface diffusion of isolated adsorbates at an electrochemical interface by high-speed scanning tunneling microscopy (video STM) are presented for sulfide adsorbates on Cu(100) in HCl solution. As revealed by a quantitative statistical analysis, the adsorbate motion can be described by thermally activated hopping between neighboring adsorption sites with an activation energy that increases linearly with electrode potential by 0.50 eV per V. This can be explained by changes in the adsorbate dipole moment during the hopping process and contributions from coadsorbates.