Hydrogen Surface Coverage Research Articles

Development of a New Electrodeposition Process for Plating of Zn‐Ni‐X (X = Cd, P) Alloys Permeation Characteristics of Zn‐Ni‐Cd Ternary Alloys

It is shown that an electrodeposited Zn‐Ni‐Cd alloy coating produced from sulfate electrolyte inhibits the discharge of hydrogen on carbon steel. The newly developed ternary alloys have approximately ten times higher corrosion resistance when compared to a Zn‐Ni alloy. Hydrogen permeation characteristics of Zn‐Ni‐Cd alloy coatings were studied and compared with those of a bare and a Zn‐Ni alloy coated steel. The transfer coefficient, α, exchange current density, , thickness dependent adsorption‐absorption rate constant, k″, recombination rate constant, , surface hydrogen coverage, , were obtained by applying a mathematical model to experimental results. Alloys obtained from baths containing higher concentration than 3 g/L of in the sulfate plating bath are seen to have superior permeation inhibition properties compared to the Zn‐Ni alloy coating and bare steel. The hydrogen permeation current was zero under normal corroding conditions for Zn‐Ni‐Cd alloy and it increased to at a cathodic overpotential of 250 mV. The hydrogen permeation current density for steel and Zn‐Ni alloy under similar conditions were 62.1 and , respectively. © 2000 The Electrochemical Society. All rights reserved.

Investigation of Hydrogen Chemisorption on GaAs (111)A Ga Surface by In Situ Monitoring and Ab Initio Calculation

Hydrogen chemisorption on the GaAs (111)A Ga surface is investigated under atmospheric pressure using an in situ optical monitoring system which consists of GaAs halogen transport atomic layer epitaxy (ALE) and surface photoabsorption (SPA) systems. The results of the in situ monitoring indicate the existence of a stable hydrogen-terminated surface. It is shown that hydrogen in the carrier gas reacts dissociatively with the (111)A Ga surface, and the relationship between the surface hydrogen coverage and hydrogen partial pressure is explained well by the Langmuir isotherm with dissociation. Atomic configuration on the surface is examined further by means of ab initio molecular dynamics calculation. The total energy change due to the calculated process is very close to the standard enthalpy of adsorption obtained by the experiment.

Determination of the Rate Constants of Hydrogen Absorption into Metals

Hydrogen absorption into iron (steel) membranes was studied using an electrochemical hydrogen permeation technique. The Iyer, Pickering, and Zamanzaden analysis of the measured steady‐state hydrogen permeation rates, obtained for different membrane thicknesses, gives the rate constants of hydrogen absorption, , and desorption, , in addition to those quantities which can be obtained from data for a single membrane thickness: i.e., the hydrogen surface coverage, , exchange current density, , rate constants, and , of the hydrogen evolution reaction, and the important composite quantity called the kinetic‐diffusion constant, , where L is the membrane thickness and D is the diffusivity of hydrogen within the metal. The product of k and determines the steady‐state permeability of hydrogen within the metal. For large D and/or small L, , and the hydrogen permeation process is under surface control. From the analysis, we obtained and for iron in acidic sulfate solution. © 2000 The Electrochemical Society. All rights reserved.

Calculation of the Hydrogen Surface Coverage and Rate Constants of the Hydrogen Evolution Reaction from Polarization Data

We show in this communication that the polarization data of the hydrogen evolution reaction (HER) can be analyzed to calculate the hydrogen surface coverage and the rate constants of the hydrogen discharge and recombination reactions for metals which have very low permeabilities of hydrogen and on which the HER proceeds through a coupled Volmer discharge‐Tafel recombination mechanism. The analysis is applied to the results of the HER on copper and iron. For both metals, this polarization analysis yields exchange current densities that are comparable to the literature values and degrees of surface coverage which vary with potential in a manner that is well documented in the literature. The results on iron yield rate constants that are in a good agreement with those predicted in other studies using the Iyer, Pickering, and Zamanzadeh analysis. © 2000 The Electrochemical Society. All rights reserved.

Modification of optoelectronic properties of porous silicon produced in an electrolyte based on heavy water

The methods of infrared spectroscopy, electron spin resonance, and photoluminescence were used to study the porous-silicon layers formed by electrochemical treatment of Si in an HF: D2O solution. In contrast with the samples prepared in a conventional electrolyte (HF: H2O), a steady increase in the photoluminescence intensity in the course of routine oxidation of the sample was observed, with the hydrogen coverage of the silicon-skeleton surface retained. A mechanism for anomalous oxidation of the layers of porous silicon obtained in a mixture of HF and heavy water is suggested.

Nature of the photographic diamond surface phenomenon on boron-doped diamond

We have recently reported the photo-memory effect on boron-doped semiconducting diamond surfaces, which involves the photoelectrochemical pretreatment and subsequent sensitized deposition of copper and which we have referred to as the ‘photographic diamond surface phenomenon’ [S. Yoshihara, K. Shinozaki, T. Shirakashi, K. Hashimoto, D.A. Tryk, A. Fujishima, Electrochim. Acta 44 (1999) 2711]. In the present work, the origin of this phenomenon was investigated by AC impedance and contact angle methods. After polarization at −0.25 V versus SCE in aqueous H 2SO 4 solution under UV illumination, the surface exhibited the photographic effect. The polarized surface showed higher surface conductivity and increased hydrophilicity. Also, surface analysis suggested that the polarized surface has a higher surface coverage of hydrogen. These results strongly suggest that this phenomenon is related to changes of the surfacial functional groups induced by photoelectrochemical hydrogen evolution.

The Effect of Iodide Ions on the Kinetics of Hydrogen Absorption by Iron

The effect of iodide ions on the kinetics of both the hydrogen evolution reaction. HER, and the hydrogen absorption reaction, HAR, on iron (steel) membranes was investigated using an electrochemical hydrogen permeation cell. Iodide ions inhibit the HER by decreasing the rate constant of proton discharge. Iodide enhances the HAR, and in turn increases the concentration of hydrogen inside the metal, although it decreases the hydrogen surface coverage. The increased HAR rate is due to an increase in the composite rate constant of the hydrogen absorption step at the metal surface. © 2000 The Electrochemical Society. All rights reserved.

Effect of Surface Hydrogen Coverage on Field Emission Properties of Diamond Films Investigated by High-Resolution Electron Energy Loss Spectroscopy

The influence of surface hydrogen coverage on the electron field emission of diamond films was investigated by high-resolution electron energy loss spectroscopy. It was found that hydrogen plasma treatment increased the surface hydrogen coverage while annealing caused hydrogen desorption and induced surface reconstruction. Field electron emission measurements manifested that increase of surface hydrogen coverage could improve the field emission properties, due to the decrease of electron affinity of the diamond surface by hydrogen adsorption.

Annealing experiments on supercritical Si 1– xGe x layers grown by RPCVD

This paper presents a study of the relaxation mechanisms for SiGe epitaxy performed by reduced pressure chemical vapor deposition (RPCVD) in a commercially available single wafer epitaxial reactor. The samples were grown at 500°C and an overall pressure of 40 Torr, with SiH 2Cl 2 and GeH 4 as precursor gasses. In contrast to the situation for molecular beam epitaxy (MBE), we found that under RPCVD conditions the relaxation by 3D-growth was strongly suppressed. Relaxation then occurred by the development of misfit dislocations even in the case of high Ge-concentrations (>40%). This behavior is attributed to the high hydrogen coverage of the substrate surface during the RPCVD, which reduces the surface migration of the adsorbed species. Furthermore, the relaxation mechanism has been studied in annealing experiments where wave and island formation was observed. The annealing experiments clearly confirm that surface migration plays a significant role for 3D-growth. Using the annealing procedure layers of high island density and strong photoluminescence have been produced.

A Kinetic Study on the Adsorption and Reaction of Hydrogen over Silica-Supported Ruthenium and Silver–Ruthenium Catalysts during the Hydrogenation of Carbon Monoxide

The simultaneous adsorption and reaction of hydrogen with preadsorbed carbon monoxide was investigated over a series of silica-supported ruthenium and silver–ruthenium bimetallic catalysts to elucidate the role of site sensitivity on hydrogen adsorption and methane formation. The specific rate of methane synthesis, which is a direct measure of the catalyst activity toward the Fischer–Tropsch synthesis, measured at 460 Torr and temperatures from 400 to 500 K, varied from 0.01×10−3 to 3×10−3 s−1. As little as 3% Ag reduced the methanation rate of Ru by 80%. The temperature dependence of the turnover frequency showed that the apparent activation energy for methanation dropped from 24 kcal mol−1 for the monometallic ruthenium case to 18 kcal mol−1 for the bimetallic case. Since silver does not adsorb or react with either hydrogen or carbon monoxide, these results show that silver does not act merely as a dilutant in the bimetallic system. These observations are elucidated in terms of a new model for surface-sensitive hydrogen adsorption, termed “portal site mediated adsorption,” where low-coordination edge and corner sites on the catalyst surface act as sinks for rapid, dissociative adsorption of weakly bound, highly mobile surface hydrogen. In situ1H-NMR measurements of surface hydrogen coverages during reaction were used to confirm the kinetic model treatment of the reaction data.

Extending surface Raman spectroscopic studies to transition metals for practical applications: II. Hydrogen adsorption at platinum electrodes

Hydrogen adsorption at platinum electrodes in acidic solution is investigated by confocal microprobe Raman spectroscopy. Vibrations for on-top adsorbed hydrogen against platinum surface at c. 2088 cm −1 are observed in the overpotential deposition (OPD) region of hydrogen evolution. With the negative change in potential, there is a red shift of Pt–H vibrational frequency. In the underpotential deposition (UPD) region, only a very weak and broad band is discernible when the surface hydrogens are saturated. This indicates that the nature and the bonding configuration of the surface hydrogen are remarkably different in the UPD and OPD regions. Several factors, including the applied potential, hydrogen surface coverage, interactions with electrolyte ions and water molecules, are briefly discussed in terms of their influence on the observed spectroscopic properties.

Kinetic and thermodynamic analysis of liquid‐phase benzene hydrogenation

AbstractLiquid‐phase benzene hydrogenation on Pd/η‐Al2O3 catalysts was studied between 358 and 488 K and over an H2 pressure range of 7 to 54 atm. Kinetic data were obtained under differential reaction conditions that were free of all transport limitations, thus allowing the development of a kinetic model that could be compared to that for the vapor‐phase reaction. In contrast to vapor‐phase studies at 1 atm, no activity maximum was observed during liquid‐phase hydrogenation between 358 and 493 K and at 54‐atm H2 pressure. A model that invokes dissociative, noncompetitive H2 adsorption, addition of the first H atom as the rate‐determining step, and a concurrent dehydrogenated benzene surface species was consistent with experimental observations. The fitted parameters were thermodynamically consistent and agreed with those from previous vapor‐phase studies. In addition, thermodynamic analysis of the quasi‐equilibrated hydrogen adsorption process indicates that in the presence of solvent effects, increasing hydrogen solubility in the liquid phase can increase the surface coverage of hydrogen. In the absence of any solvent effect, however, surface coverage depends only on the partial pressure of hydrogen and is independent of the nature of the solvent. Furthermore, when solvent effects exist, use of the liquid‐phase hydrogen concentration in the rate expression yields a solvent‐independent adsorption equilibrium constant, while in the absence of solvent effects, P should be used in the rate expression to acquire this parameter.

The Mechanism of Hydrogen Gas Evolution on GaAs Cathodes Elucidated by In Situ Infrared Spectroscopy

The microscopic mechanism of hydrogen gas evolution on GaAs(100) cathodes is elucidated by studying the reaction intermediate, adsorbed hydrogen, using in situ infrared spectroscopy in real time. The spectra show that hydrogen adsorbs cathodically at arsenic sites only and replaces As−OH groups present in the anodic range. Absolute submonolayer coverages by adsorbed hydrogen and variations in surface dipole potential due to hydrogen adsorption are both determined quantitatively. Molecular hydrogen is formed upon proton reduction at As−H sites. The microscopic reaction mechanism at GaAs is found to be pH-independent; the reason is that rate-limiting electron transfer is not to solution species but to As−H surface sites. A simple mechanistic model accounts for the infrared spectroscopic results as well as for the electrochemistry, explaining both the approximately linear potential dependence of hydrogen surface coverage and the hysteresis loop in cyclic voltammograms.

Ｈａｌｌ Ｅｆｆｅｃｔ Ｇｅｎｅｒａｔｏｒ回路を用いた磁気測定によるニッケル微粒子触媒のガス吸着の研究

The effects of chemisorbed H2, CO2 and CO on the saturation magnetization of nickel have been investigated on fine particles of nickel supported and not supported.Apparatus provided with "Hall Effect Generator" circuit and the permanent magnet of the intensity of 0.5T, was designed for in situ measurements of magnetization after gas adsorption.Change of magnetization with gas amount adsorbed at the room temperaure was measured. The reduction of magnetization by adsorbed gas causes solely from pairing of electron spins. The value of ε is numerically the number of spins cancelled per the hydrogen atom adsorbed on nickel by pairing in the d band. The values of ε for the various nickel particles have been determined. The obvious influence of particle size over the range 20 to 360Å in radius on ε for coprecipitates was not observed. But the magnetic data showed clear examples of support influence in changing the value of ε. The effect of chemisorbed hydrogen on ε appears to be almost independent of the surface coverage for supported nickel coprecipitate exhibiting the superparamagnetism. However, the values of ε decrease with the surface coverage of hydrogen for the samples contained larger metallic particles than the catalysts showing the superparamagnetic behavior. The ε value for nickel-zeolite impregnate is 0.85 and very close to that of massive nickel, Ni-Al2O3, or Ni-kaolin-supported samples in the range of low hydrogen coverage. A possible reason for the low value of ε in Ni-SiO2-Al2O3 is explained by chemical interaction in the interfaces between the support and nickel metal.

Role of hydrogen surface coverage during anodic plasma deposition of hydrogenated nanocrystalline germanium

Nanocrystalline Ge:H films were deposited on the anode of a conventional plasma-enhanced chemical vapor deposition system using highly H2-diluted GeH4 as source gas. The structural changes with substrate temperature, hydrogen dilution, and rf power were investigated by x-ray diffraction and Raman scattering. The hydrogen content and H bonding configuration were determined by infrared absorption spectroscopy. It was found that increased rf power mainly favors nucleation of grains while increased H2 dilution mainly favors the subsequent growth of grains. Transitions from an amorphous to a crystalline and then back to an amorphous phase are observed as the substrate temperature is increased. The amorphous phase at high temperatures and variations in the preferred crystallite orientations are explained by changes in the diffusion length of adsorbed precursors due to changing hydrogen coverage on the growing surface.

The Role of Coke in Acetylene Hydrogenation on Pd/α-Al2O3

The formation of coke and the influence of the coke on selectivity were investigated during hydrogenation of acetylene on supported palladium catalysts. It was found that the total amount of coke was not directly related to the increase in formation of undesired ethane. Instead, the surface coverage of hydrogen during the deactivation was found to be a crucial parameter. A catalyst deactivated at low hydrogen surface coverage showed a higher ethane selectivity than a sample deactivated at higher surface coverage of hydrogen when compared under the same reaction conditions. In contrast, the coke formation rate was found to increase with increased hydrogen surface coverage. The role of carbon monoxide was also investigated, and the impact on selectivity and coke formation was explained by the reduced surface coverage of hydrogen in the presence of carbon monoxide. The coke was characterized by temperature-programmed oxidation, and deconvolution of the obtained peaks was carried out using a power-law model.

Modeling growth in SiGe gas-source molecular beam epitaxy using Si2H6 and GeH4

We have modeled SiGe growth with gas-source molecular beam epitaxy using Si2H6 and GeH4. The model is described by a combination of Si2H6 and GeH4 adsorptions and hydrogen desorption. The Si2H6 and GeH4 adsorptions are two-site and four-site adsorption processes respectively. The hydrogen desorption is a first order reaction. The model allows predictions of substrate-temperature dependencies of SiGe growth rate and surface-hydrogen coverage in the wide temperature range of 450–700°C.

Negative electron affinity mechanism for diamond surfaces

The energy distribution of the secondary electrons for chemical vacuum deposited diamond films with negative electron affinity (NEA) was investigated. It was found that while for completely hydrogenated diamond surfaces the negative electron affinity peak in the energy spectrum of the secondary electrons is present for any energy of the primary electrons, for partially hydrogenated diamond surfaces there is a critical energy above which the peak is present in the spectrum. This critical energy increases sharply when hydrogen coverage of the diamond surface diminishes. This effect was explained by the change of the NEA from the true type for the completely hydrogenated surface to the effective type for the partially hydrogenated surfaces.

Morphological study of kinetic roughening on amorphous and microcrystalline silicon surface

An ex situ study of surface morphology of amorphous (a-Si:H) and microcrystalline silicon ( μc-Si:H) has been carried out to understand their growth mechanism using an atomic force microscope. For a-Si:H, a standard Karder, Parisi and Zhang (KPZ) model well describes the kinetics of the roughness evolution with increasing film thickness except at T s∼360°C, where a significant roughness enhancement is observed. The morphology of μc-Si:H on Si(001) substrate shows a similar roughness formation at much lower temperatures of 150 to 200°C. This roughness formation is found to be correlated with the surface hydrogen coverage.

Evolution and Entry of Hydrogen into Iron during Cathodic Charging in Alkaline Solution with Ethylenediaminetetraacetic Acid

Hydrogen permeation and electrochemical impedance were measured on iron membranes cathodically polarized at a constant current in 0.1 M NaOH solution without and with ethylenediaminetetraacetic acid (EDTA). Auger electron spectroscopy and scanning electron microscopy (SEM) were used to examine the surface. During the first few hours of charging, the hydrogen permeation rate (ip) attained a quasi‐steady low level, the electrode potential (E) shifted in the negative direction, the charge‐transfer resistance (Rct) increased, and the double‐layer capacitance (Cd) slightly decreased. Later ip strongly increased with simultaneously decreasing E and increasing Rct; Cd increased only in the absence of EDTA. The ip reached a maximum after about 3 days of an uninterrupted charging and then decreased with time, accompanied by reversed changes in E and Rct, and further increase in Cd. Electrochemical parameters were associated mainly with the hydrogen evolution reaction (HER). They indicated that the hydrogen entry into iron increased with increasing overpotential and charge‐transfer resistance of the HER. It is suggested that the rise in E and Rct resulted from the increasing hydrogen coverage of the bare metal surface. The persistent increase in Cd might result from the formation of a new surface layer which can be composed of disintegrated iron and its oxides. Surface oxides facilitated HER, but they hindered the hydrogen entry. Addition of EDTA strongly accelerated the hydrogen entry, mainly due to the removal of oxides. Variations in the hydrogen entry are explained in terms of , surface oxides, metal disintegration, enlargement of the surface area, and exposure of bare metal.