A DFT study of hydrogen chemisorption on V (100) surfaces

In this study, hydrogen adsorption on the surface of vanadium at various positions (top, bridge, and central sites) was studied, and the binding energies of hydrogen species adsorbed on vanadium were calculated using density functional theory (DFT) within the generalized gradient approximation (GGA). The potential of the adsorption of hydrogen on vanadium was examined as a function of both surface coverage and adsorption site. Our results were in excellent agreement with the experimental values reported in the literature. The relative stabilities of hydrogen chemisorption were independent of both the transition metal surface and surface coverage. That is, hydrogen exhibited insignificant selectivity with respect to positions on the metal surface. Our data on H₂/V surface chemisorption revealed that the stablest model for hydrogen adsorption was that on the vertical bridge site. The adsorption energy for this model was lower than for the other sites. However, adsorption on bridge-hydrogen vacancies was strong.     

Dissolution and adsorption of hydrogen at smooth Pd wires at potentials of the alpha phase. Influence of electrolyte

The dissolution and adsorption of hydrogen atoms was determined as a function of potential in the region of the alpha phase of a smooth Pd wire in 0.5 M H₂SO₄, 0.5 M KOH, and 0.5 M HCl by a pulse technique. Specifically adsorbable chloride ions were found to decrease the hydrogen coverage at constant potential as on platinum. In contrast to platinum hydroxyl ions did not lead to an increase of the hydrogen bond strength. The isotherms of hydrogen adsorption can be approximated by a Temkin type relationship in a range of medium coverage. The logarithm of the bulk concentration of dissolved hydrogen atoms directly below the surface was a linear function of potential between about 0.25 V and 0.1 V, interpretable in terms of the non-ideal behaviour of the atoms in the first layer below the surface.     

The effect of electric current on chemical bonding of hydrogen adsorption on an aluminum nanowire

The effect of electric current on hydrogen adsorption on an aluminum nanowire surface is studied by using nonequilibrium Green's function method. We choose the models studied in the previous work of one of the authors as an aluminum nanowire model and a hydrogen-adsorbed one. These nanowire models have conductive ability, because the aluminum part of these models is metallic. It is confirmed that electric current affects the strength of the adsorption of hydrogen atoms, and the change of the bonding of hydrogen to aluminum nanowire surface is larger for larger current. However, the change of the chemical bonding is negligibly small within the bias voltage ≤0.5 V.     

Positrons as probes of Si(100) surface with adsorbed hydrogen and oxygen

Positron annihilation induced Auger spectra from the Si(100) surface exposed to hydrogen and oxygen are analyzed by performing calculations of positron surface states and annihilation characteristics of surface trapped positrons. Positron binding energies and work functions are also computed. It is found that the adsorption of hydrogen and oxygen on the Si(100) surface leads to a displacement of the positron surface state wave function away from the substrate Si atoms. As a result of this displacement, the overlap of the positron wave function with Si core electrons and, consequently, the annihilation probability of Si core electrons reduce, in agreement with experimental data.     

Dependence of the number of hydrogen bonds per water molecule on its distance to a hydrophobic surface and a thereupon-based model for hydrophobic attraction

A water molecule in the vicinity of a hydrophobic surface forms fewer hydrogen bonds than a bulk molecule because the surface restricts the space available for other water molecules necessary for its hydrogen-bonding. In this vicinity, the number of hydrogen bonds per water molecule depends on its distance to the surface. Considering the number of hydrogen bonds per bulk water molecule (available experimentally) as the only reference quantity, we propose an improved probabilistic approach to water hydrogen-bonding that allows one to obtain an analytic expression for this dependence. (The original version of this approach [Y. S. Djikaev and E. Ruckenstein, J. Chem. Phys. 130, 124713 (2009)] provides the number of hydrogen bonds per water molecule in the vicinity of a hydrophobic surface as an average over all possible locations and orientations of the molecule.) This function (the number of hydrogen bonds per water molecule versus its distance to a hydrophobic surface) can be used to develop analytic models for the effect of hydrogen-bonding on the hydration of hydrophobic particles and their solvent-mediated interaction. Presenting a model for the latter, we also examine the temperature effect on the solvent-mediated interaction of two parallel hydrophobic plates.     

In situ ultraviolet Raman spectroscopy of the reduction of chromia on alumina catalysts

A fluidized bed in situ cell is used to examine the chromia on alumina catalyst surface. The loading is varied from 1 to 20% chromium oxide on alumina. The structure of the surface is monitored as a function of temperature, reduction, and hydration. This is the first study to use ultraviolet excitation to study a reduced chromium catalyst surface. Reduction of the sample in hydrogen gas leads to the formation of noncrystalline Cr(3+) species. Addition of potassium to the surface reduces the size of the chromia clusters.     

Scanning electrochemical microscopy of hydrogen electro-oxidation. Rate constant measurements and carbon monoxide poisoning on platinum

We describe an application of the scanning electrochemical microscope that uses tip–sample feedback to characterize the electro-oxidation of hydrogen on a polycrystalline platinum electrode in sulfuric acid solutions in the presence and absence of adsorbed carbon monoxide. The hydrogen oxidation reaction is probed by reducing protons at a diffusion-limited rate at the microscope's tip electrode while it is positioned near a platinum substrate. A series of approach curves measured as a function of the substrate potential provides hydrogen oxidation rate constant values over a wide range of substrate conditions. In the absence of CO, the rate of hydrogen oxidation exceeds 1 cm s⁻¹ at potentials within the hydrogen adsorption and double layer charging regions. A Tafel slope of 30 mV per decade is determined near the reversible potential. At increasingly positive substrate potentials, the hydrogen oxidation rate decreases exponentially with increasing potential as the surface is covered with an oxide layer. The adsorption of solution-phase carbon monoxide completely deactivates the platinum substrate towards steady-state hydrogen oxidation over a large range of substrate potentials. Approach curves indicate a near-zero rate constant for hydrogen oxidation on CO-covered platinum at potentials below oxide formation. An increase in the hydrogen oxidation rate is seen at potentials sufficiently positive that CO fails to adsorb and the platinum oxide forms. In comparison, dynamic tip–substrate voltammetry depicts a complex substrate response whereby the adsorbed carbon monoxide layer transforms from a weakly adsorbed state at low potentials to a strongly adsorbed state at high potentials. Although steady-state approach curve measurements depict the complete deactivation of catalytic activity at these potentials, a significant hydrogen oxidation current is observed during the potential-induced transformation between these weakly and strongly adsorbed CO states. The rate of hydrogen oxidation approaches that of a pristine platinum surface during this surface transformation before returning to the poisoned state.     

氢原子吸附的Cu(100)表面原子结构和电子态

用密度泛函理论的总能计算研究了金属铜(100)面的表面原子结构以及在不同覆盖度时氢原子的吸附状态. 研究结果表明, 在Cu(100)c(2×2)/H表面体系中, 氢原子吸附的位置是在空洞位置, 距最外层Cu原子层的距离为0.052 nm, 相应的Cu—H键长为0.189 nm, 并通过计算结构参数优化否定了其它的吸附位置模型. 总能计算得出Cu(100)c(2×2)/H表面的功函数为4.47 eV, 氢原子在这一体系的吸附能为2.37 eV(以孤立氢原子为能量参考点). 通过与衬底原子的杂化, 氢原子形成了具有二维特征的氢能带结构, 在费米能级以下约0.8 eV处出现的表面局域态是Cu(S)-H-Cu(S-1)型杂化的结果. 采用Cu(100)表面p(1×1)、p(2×2)和p(3×3)的三种氢吸附结构分别模拟1, 1/4, 1/9的原子单层覆盖度, 计算结果表明, 随着覆盖度的增加, 被吸附的氢原子之间的距离变短, 使得它们之间的静电排斥和静电能增大, 从而导致表面吸附能和吸附H原子与最外层Cu原子间垂直距离(ZH-Cu)逐渐减小. 在较低的覆盖度下, 氢原子对Cu(100)表面的影响主要表现为单个原子吸附作用的形式. 通过总能计算还排除了Cu(100)表面(根号2×2根号2)R45°-2H缺列再构吸附模型的可能性.     

Surface complexes of monomethyl phosphate stabilized by hydrogen bonding on goethite (α-FeOOH) nanoparticles

Typically, a significant fraction of phosphorus in soils is composed of organic phosphates, and this fraction thus plays an important role in the global phosphorus cycle. Here we have studied adsorption of monomethyl phosphate (MMP) to goethite (α-FeOOH) as a model system in order to better understand the mechanisms behind adsorption of organic phosphates to soil minerals, and how adsorption affects the stability of these molecules. The adsorption reactions and stability of MMP on goethite were studied at room temperature as a function of pH, time and total concentration of MMP by means of quantitative batch experiments, potentiometry and infrared spectroscopy. MMP was found to be stable at the water-goethite interface within the pH region 3-9 and over extended periods of time, as well as in solution. The infrared spectra indicated that MMP formed three predominating pH-dependent surface complexes on goethite, and that these interacted monodentately with surface Fe. The complexes differed in hydrogen bonding interactions via the auxiliary oxygens of the phosphate group. The presented surface complexation model was based on the collective spectroscopic and macroscopic results, using the Basic Stern approach to describe the interfacial region. The model consisted of three monodentate inner sphere surface complexes where the MMP complexes were stabilized by hydrogen bonding to a neighboring surface site. The three complexes, which had equal proton content and thus could be defined as surface isomers, were distinguished by the distribution of charge over the 0-plane and β-plane. In the high pH-range, MMP acted as a hydrogen bond acceptor whereas it was a hydrogen bond donor at low pH.     

A study of the effect of some pollutant gases on cathodes used in membrane-covered amperometric oxygen detectors

The effects of various gaseous pollutants on the electrochemical activity of materials used as cathodes for membrane-covered amperometric oxygen detectors are described. It is shown that gold and platinum cathodes are not unduly affected by concentrations of sulphur dioxide and chlorine many times greater than those likely to be encountered in test solutions, and that a gold cathode is also unaffected by hydrogen sulphide. A platinum cathode is rapidly and significantly poisoned on contact with hydrogen sulphide; an analysis of the fall in the rate of oxygen reduction as a function of time indicates that the poisoning occurs by the blocking of surface sites by sulphide. The effect of hydrogen sulphide on silver, nickel and nickel sulphide electrodes is also reported. Of these materials, only nickel sulphide is an effective electrocatalyst for oxygen reduction in the presence of sulphide.     

Quantum chemical studies on hydrogen adsorption in carbon-based model systems: role of charged surface and the electronic induction effect

Quantum chemical studies on the molecular hydrogen adsorption in a six-membered carbon ring has been undertaken to mimic the adsorption process in carbon nanotubes, considering the fact that the six-membered carbon ring is found to be one of the basic units of the carbon nanotubes and fullerenes. Our results reveal that the carbon surface as such is not a good candidate for hydrogen adsorption but a charged surface created by doping of an alkali metal atom can play an important role for the improvement in adsorption of molecular hydrogen. The strength of hydrogen interaction as well as the number of hydrogen molecules that can be adsorbed on the system is found to depend on the nature of the cation doped in the system. We have also studied the role of electronic induction by substituting different functional groups in the model system on the hydrogen adsorption energy. The results demonstrate that the binding energy of the cation with the carbon surface as well as the hydrogen adsorption energy can be tuned significantly through the use of suitable substituents. In addition, we have shown that the extended planar or the curved carbon surface of the coronene system alone may not be suitable for an effective molecular hydrogen adsorption. In essence, our results reveal that the ionic surface with a significant degree of curvature will enhance the hydrogen adsorption effectively.     

Hydrogen-bonded assembly of methanol on Cu(111)

Investigation of methanol's surface chemistry on metals is a crucial step towards understanding the reactivity of this important chemical feedstock. Cu is a relevant metal for methanol synthesis and reforming, but due to the weak interaction of methanol with Cu, an atomic scale view of methanol's coverage-dependent ordering and self-assembly on Cu(111), the most abundant facet of most nanoparticles, has not yet been possible. Low and variable temperature scanning tunneling microscopy coupled with density functional theory reveal a coverage-dependent range of highly ordered structures stabilized by two hydrogen bonds per molecule. While extended chains that resemble the hydrogen-bonded zigzag structures reported for solid methanol are an efficient way to pack methanol at higher coverages, lower surface coverages yield isolated hexamer units. These hexamers form the same number of hydrogen bonds as the chains but appear to repel one another on the surface. Annealing treatments lead to the desorption of methanol with almost no decomposition. This data serves as a useful guide to both the preferred adsorption geometries and energies of a variety of methanol structures on Cu(111) surfaces as a function of surface coverage.     

Determination of the surface pK of carboxylic- and amine-terminated alkanethiols using surface plasmon resonance spectroscopy

When using self-assembled monolayers (SAMs) with ionizable functional groups, such as COOH and NH2, the dissociation constant (pKd) of the surface is an important property to know, since it defines the charge density of the surface for a given bulk solution pH. In this study, we developed a method using surface plasmon resonance (SPR) spectroscopy for the direct measurement of the pKd of a SAM surface by combining the ability of SPR to detect the change in mass concentration close to a surface and the shift in ion concentration over the surface as a function of surface charge density. This method was then applied to measure the pKd values of both COOH- and NH2-functionalized SAM surfaces using solutions of CsCl and NaBr salts, respectively, which provided pKd values of 7.4 and 6.5, respectively, based on the bulk solution pH. An analytical study was also performed to theoretically predict the shape of the SPR plots by calculating the excess mass of salt ions over a surface as a function of the difference between the solution pH and surface pKd. The analytical relationships show that the state of surface charge also influences the local hydrogen ion concentration, thus resulting in a substantial local shift in pH at the surface compared to the bulk solution as a function of the difference between the bulk solution pH and the pKd of the surface.     

Adsorption of hydrogen peroxide on functionalized mesoporous silica surfaces

MCM-41 and MSU-H mesoporous silicas were successfully functionalized with hydrogen bonds forming organic moieties, which have been proven by elemental analysis. Both moieties, based on oxygen and nitrogen containing groups, were introduced with high efficiency—the amount of carbon in all cases exceeded 10 % and the elemental ratios suggest binding to the surface through two or three Si–O–Si bonds. Hydrogen peroxide adsorption was conducted in its aqueous solutions and the amount adsorbed was determined using the ferric thiocyanate method. Results are presented as a function of hydrogen peroxide concentration in aqueous solution from 5 to 30 %. Both functionalized silicas show increased adsorption capacity when compared with that of their unfunctionalized analogues. The surface modified with nitrogen-based organic moiety revealed better adsorption properties as well as higher resistance against oxidation. MSU-H silica, due to its larger pore diameter, provides more space to bind hydrogen peroxide molecules and thus was found to have higher adsorption capacity: it adsorbed up to four times more hydrogen peroxide than MCM-41.     

Photodecomposition of water with methane over titanium oxide photocatalysts modified with metal

Metal-loaded titanium oxide photocatalysts produced hydrogen in the photodecomposition of water vapor with methane in the flow reactor. Ag/TiO₂ has the highest activity in comparison with other metal-loaded catalysts. The experiment in the absence of methane indicated that methane could effectively function as a reducing reagent of water. The significant decrease in the hydrogen formation rate with the time on stream was observed under all reaction conditions. The recalcination and the hydrogen rereduction of the used catalyst led to the restoration of the activity of the hydrogen formation. The adsorption of products and/or reactants on the catalyst surface seemed to cause these deactivations of the hydrogen formation.     

Nuclear Dynamics and Electronic Effects of Hydrogen on Solid Surfaces

Hydrogen is involved in a variety of chemical processes on surfaces. While hydrogen exhibits vibrational and rotational dynamics in its adsorption state, it in some cases undergoes diffusion into the substrate as well as on the surface, and participates in chemical reactions. Furthermore, hydrogen exchanges an electron with surfaces having a significant effect on the surface electronic structure. In this personal account, we review our recent studies on surface nuclear dynamics of hydrogen, hydrogen transport across surfaces, catalytic hydrogenation/isotope exchange reactions, and charge transfer between the surface and hydrogen by using a depth‐resolved technique of nuclear reaction analysis and a quantum‐state‐selective detection of resonance enhanced multiphoton ionization in combination with surface science techniques. As a future prospect, we refer to ultraslow μ spin rotation spectroscopy for a direct probe of the hydrogen charge state at surfaces.     

Surface structure of (10(-)10) and (11(-)20) surfaces of ZnO with density functional theory and atomistic simulation

We have calculated the stability of two of the low-index surfaces known to dominate the morphology of ZnO as a function of stoichiometry. These two surfaces are (10(-)10) and (11(-)20). In each case, two terminations only are stable for a significant range of oxygen and hydrogen chemical potential: the pure stoichiometric surface and a surface covered in a monolayer of water. The mode by which the water adsorbs is however different for the two surfaces considered. On the (10(-)10) surface the close proximity of the water molecules means hydrogen bonding can occur between adjacent chemiabsorbed water molecules and hence there is little difference in the stability of the hydrated and hydroxylated surface, and in fact the most stable surface occurs with a combination of dissociated and undissociated water adsorption. In the case of the (11(-)20) surface, it is only when full dissociation has occurred that a hydrogen-bonding network can form. Our results also show good agreement between DFT and atomistic simulations, suggesting that potential based methods can usefully be applied to ZnO.     

Two-dimensional superstructures and softened C-H stretching vibrations of cyclohexane on Rh(111): effects of preadsorbed hydrogen

Adsorption structures and interaction of cyclohexane molecules on the clean and hydrogen-preadsorbed Rh(111) surfaces were investigated using scanning tunneling microscopy, spot-profile-analysis low-energy electron diffraction, temperature-programmed desorption, and infrared reflection absorption spectroscopy (IRAS). Various ordered structures of adsorbed cyclohexane were observed as a function of hydrogen and cyclohexane coverages. When the fractional coverage (θ(H)) of preadsorbed hydrogen was below 0.8, four different commensurate or higher-order commensurate superstructures were found as a function of θ(H); whereas more densely packed incommensurate overlayers became dominant at higher θ(H). IRAS measurements showed sharp softened C-H vibrational peaks at 20 K, which originate from the electronic interaction between adsorbed cyclohexane and the Rh surface. The multiple softened C-H stretching peaks in each phase are due to the variation in the adsorption distance from the substrate. At high hydrogen coverages they became attenuated in intensity and eventually diminished at θ(H) = 1. The gradual disappearance of the soft mode correlates well with the structural phase transition from commensurate structures to incommensurate structures with increasing hydrogen coverage. The superstructure of adsorbed cyclohexane is controlled by the delicate balance between adsorbate-adsorbate and adsorbate-substrate interactions which are affected by preadsorbed hydrogen.     

Structural Peculiarities of Anomalous Electrolytic Palladium Deposits Prepared under Hydride Formation Conditions

The peculiarities of bulk and surface structures of electrolytic palladium deposits formed under hydride formation conditions are studied by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy techniques. For anomalous deposits with high hydrogen capacities, the evolution of the XRD behavior is traced as a function of thermal treatment. The material is shown to comprise two cubic phases with different lattice parameters.