Hydrogen Surface Coverage Research Articles

Hydrogen Sulfide Effect on Hydrogen Entry into Iron—A Mechanistic Study

Abstract The recently developed I-P-Z model is modified in order to analyze the observed enhanced permeation of hydrogen that occurs in the presence of hydrogen sulfide during cathodic hydrogen charging of iron. The modification accounts for the fact that the energy of adsorption becomes coverage dependent at the higher coverages and affects the hydrogen evolution reaction (HER) in the presence of H2S. Charging experiments were performed on Ferrovac E-iron membranes 0.5 mm thick using a Devanathan-Stachurski cell in deaerated, pre-electrolyzed solutions made from 0.1 M HClO4 and 0.1 M NaClO4 with pH values of 1 and 2. The transfer coefficient, α, exchange current density, i0, thickness-dependent absorption-adsorption rate constant, k″, recombination rate constant, k3, surface hydrogen coverage, θH, and discharge rate constant, k1o, were obtained by application of the model to the experimental results. As a result, the role of H2S has been clarified. While θH is increased in the presence of H2S, the overpo...

Equilibrium surface hydrogen coverage during silicon epitaxy using SiH4

Epitaxial silicon has been grown on Si(100) wafers using SiH4 in a rapid thermal chemical vapor deposition reactor in the temperature regime from 450–700 °C. Gas analysis during growth and thermal desorption spectra (TDS) after growth were measured with a differentially pumped mass spectrometer. We have attempted to estimate the surface population of hydrogen during epitaxial growth by ‘‘freezing out’’ the surface hydrogen with a rapid cool down and pump down followed by a temperature programmed desorption taken in the reactor. SiH is found as the majority species in equilibrium during growth, with the surface population decreasing from one monolayer around 550–600 °C, at a pressure of three mTorr SiH4. Molecular hydrogen does not interfere with silane adsorption in this pressure regime.

Kinetics of silicon epitaxy using SiH4 in a rapid thermal chemical vapor deposition reactor

The equilibrium hydrogen surface coverage on Si(100) during silicon epitaxy using SiH4 has been measured in a rapid thermal chemical vapor deposition reactor. The hydrogen coverage could be ‘‘frozen out’’ completely on the surface by a rapid cool-down and pump-down of the reactor up to temperatures of ≂575 °C; at temperatures above 575 °C only partial ‘‘freeze-out’’ is achieved. Surface hydrogen was titrated in situ using the reactor as a thermal desorption spectrometer. Epitaxial silicon films were grown in the temperature range 450–700 °C and the film growth kinetics was correlated with the equilibrium hydrogen coverage. The growth mechanism changes from the low-temperature regime, where the surface is hydrogen covered, to the high-temperature regime, where the surface is essentially clean.

Hydrogen surface coverage: Raising the silicon epitaxial growth temperature

The chemisorption of molecular hydrogen onto the Si (100) surface is shown to disrupt the epitaxial growth of silicon and silicon/germanium alloys grown by molecular beam epitaxy. It is only after the substrate temperature is raised above the hydrogen desorption temperature, or the deposition rate is lowered, that high quality single-crystal films can be grown. The results also suggest the surface segregation of hydrogen during growth.

Analysis of Hydrogen Evolution and Entry into Metals for the Discharge‐Recombination Process

A mechanistic model has been developed which for the first time considers the effect of hydrogen entry into a metal on the kinetics of the hydrogen evolution reaction (h.e.r.). The model enables computation of (i) the hydrogen surface coverage and surface concentration; (ii) the hydrogen adsorption, absorption, discharge and recombination rate constants; and (iii) the h.e.r. coverage‐dependent transfer coefficient, α, and the exchange current density , from a knowledge of the steady‐state hydrogen permeation current, cathodic charging current, hydrogen overvoltage, and hydrogen diffusivity. The model predicts a linear relationship between the permeation flux and the square root of the hydrogen recombination flux, and provides an analytical method to determine the cathodic potential range for operation of a coupled discharge‐recombination mechanism of the h.e.r. With modifications the model can treat permeation data for which (i) the mechanics of the discharge step involve a (proposed) selvedge reaction, and (ii) surface hydrogen coverages are relatively high as in the presence of poisons (e.g., or ). Some of the existing literature data for hydrogen permeation in iron and nickel in acid and alkaline solutions are successfully analyzed.

Direct and precursor dynamics in dissociative hydrogen chemisorption on Ni(100)

We report evidence for the coexistence of direct and precursor dynamics in the dissociative chemisorption of H2 on Ni(l00). Hydrogen and deuterium uptakes on Ni(l00) were measured at various surface temperatures by following the secondary ion ratios, Ni2 H+/Ni+ and Ni2 D+/Ni+ , which are proportional to surface hydrogen and deuterium coverage on both clean and carbon-covered Ni(100). Between 100 and 200 K on clean Ni(l00), the initial sticking coefficient of hydrogen decreases, but only slightly, as the surface temperature increases. The decrease is more pronounced both in the presence of predosed carbon and for deuterium adsorption on Ni(100). This is interpreted as due to the involvement at low temperatures of a molecular precursor which mediates dissociative adsorption at low temperatures. The precursor probably involves surface defects and sites formed in the presence of carbon. In addition to the precursor channel, a direct dissociation channel also operates, and dominates for T≥200 K.

Silicon hydride etch products from the reaction of atomic hydrogen with Si(100)

The formation of silane, SiH 4, on the Si(100) surface following atomic hydrogen chemisorption has been investigated using temperature programmed desorption (TPD) mass spectrometry and static secondary ion mass spectrometry (SSIMS). The yield of SiH 3 + ions in SSIMS is correlated with trends observed in the SiH 4 TPD yield, as the hydrogen surface coverage (θ H) and the surface temperature during hydrogen exposure are varied. A mixture of silicon hydrides is formed on the Si(100)-(2×1) surface by adsorption of H atoms, including SiH 3(a) which yields SiH 4(g) during the temperature program. The peak temperature ( T p) for SiH 4 in TPD occurs at 375 °C, which is 50 °C below the H 2 β 2 desorption peak. The maximum yield of SiH 4 is observed at θ H = 1.5 monolayers, with roughly 4% of all the surface hydrogen desorbing as SiH 4, resulting in removal of about 1% of a monolayer of silicon atoms. The minimum hydrogen coverage needed for detectable SiH 4 formation is 0.25–0.3 monolayers. The SiH 4 TPD yield and the SiH 3 + intensity in SSIMS are proportional to θ H between θ H = 0.25 and 0.5. As θ H is increased beyond 0.5, the SiH 4 TPD yield gradually saturates at the maximum value. Desorption of polysilicon hydrides, Si x H y ( x = 2,3,4) is also observed. These higher silicon hydride species desorb in TPD with T p coincident with the β 2 desorption peak for H 2 at 425 °C. Molecular species Si 2H 6, Si 3H 8 and Si 4H 10 desorb. and mass spectrometry fragmentation patterns indicate that hydrogen deficient radical species such as Si 2H 2, Si 2H, or Si 2 are thermally desorbed. The silicon surface temperature during hydrogen adsorption dramatically affects the yields of all the silicon hydride products.

Butane hydrogenolysis over single-crystal rhodium catalysts

Hydrogenolysis of n-butane has been studied over the (110) and (111) surfaces of Rhodium. On Rh(110), the products of the hydrogenolysis consist of methane > ethane > propane. The hydrogenolysis reaction exhibits a good fit to Arrhenius behaviour for reaction temperatures up to 500 K. At higher temperatures the reaction rate tends to ‘roll over’ due to insufficient surface coverage of hydrogen. The ‘roll over’ affects product distribution, yielding more complete hydrogenolysis to methane. The behaviour is qualitatively similar on Rh(111), with ‘roll over’ occurring at a lower temperature, namely 475 K. However, the hydrogenolysis is more selective on Rh(111), yielding 50 mol % ethane. The high ethane selectivity seen in previous work on Ir(110) is not seen on the Rh(110) surface presumably becasue the Rh surface does not exhibit the (1 × 2)‘missing-row’ reconstruction that is stable on Ir(110) surfaces under reaction conditions. The hydrogenolysis selectivity of the Rh single-crystal surfaces correlates well with supported Rh metal particles subjected to oxidation-reduction cycling.

Negative ion formation at a barium surface exposed to an intense positive-hydrogen ion beam

A fundamental study of the formation of negative hydrogen ions via surface conversion is presented. Employed is a novel type of converter, namely a pure barium metal surface. In spite of the high work function of barium compared to more conventional cesiated converters, considerable yields of negative ions were produced. Conversion efficiencies up of 4% are obtained, which is of the same order as for cesiated converters. The high negative-ion yield is probably related to the electron density of barium, which is almost twice that of cesiuim. This is confirmed by model calculations and by UHV scattering experiments under well-defined conditions. Furthermore, calculations showed that the hydrogen coverage of the converter increases with increasing flux of positive hydrogen ions to the surface. This behavior is confirmed experimentally. Seeding the hydrogen plasma with argon has no significant effect on the conversion efficiency. This is believed to be related to the competition between the lowering of the surface hydrogen coverage and the increase of the hydrogen desorption rate, both due to the higher sputter coefficient of argon compared to hydrogen.

Desorption kinetics of hydrogen and deuterium from Si(111) 7×7 studied using laser-induced thermal desorption

The desorption of hydrogen and deuterium from Si(111) 7×7 was studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) mass spectrometry. Isothermal LITD measurements enabled the surface coverage of hydrogen and deuterium to be monitored as a function of time. These isothermal results were used to obtain accurate desorption kinetics of hydrogen and deuterium from the high-temperature β1 state on Si(111) 7×7. The desorption of hydrogen displayed second-order kinetics with an activation barrier of 61±4 kcal/mol and a preexponential factor of 1.2×101±1.3 cm2/s. Likewise, the desorption kinetics of deuterium displayed second-order kinetics with an activation barrier of 59±3 kcal/mol and a preexponential factor of 2.8×100±1.0 cm2/s. These desorption activation barriers yield upper limits of 82.6 and 81.6 kcal/mol for the Si–H and Si–D chemical bond energies, respectively, on Si(111) 7×7. TPD results obtained as a function of hydrogen coverage were consistent with second-order desorption kinetics. The TPD experiments were also used to measure hydrogen coverages and to calibrate the LITD signals. In addition, LITD techniques were used to study the surface diffusion of hydrogen on Si(111) 7×7. No evidence of significant hydrogen surface mobility (D≤10−9 cm2/s) was found for surface temperatures as high as 740 K.

Kinetics of hydrogen absorption in alpha titanium

Hydrogen absorption and desorption often limits a material’s application. For titanium, hydrogen absorption kinetics determine its suitability for tritium storage, tritium gettering, and vacuum pump applications. This study examines the absolute rate theory energy surface which molecular hydrogen gas encounters as it is absorbed into alpha-phase titanium. This results in useful new predictions for hydrogen absorption rates, desorption rates, and surface coverages on titanium. The only energy surface which is consistent with observed activation and absorption enthalpies, while predicting all absorption/desorption rate data, is found to contain a new activation barrier. Accuracy is within a factor of 3.6 for two surface preparations and temperatures between 250 and 500 °C.

The Influence of Strain on Hydrogen Entry and Transport in a High Strength Steel in Sodium Chloride Solution

The influences of mechanical strain on hydrogen entry and transport are reported for a high strength steel alloy (AISI 4340) under cathodic polarization in aqueous sodium chloride solution. The overall rate of hydrogen permeation and the quantity of hydrogen absorbed are shown to be particularly low in alkaline chloride at low cathodic current densities if the charging surface has been contaminated by slight corrosion. Evidence is presented that illustrates the role of mechanical strain in promoting enhanced hydrogen absorption as a result of film rupture. In addition to the conventional Devanathan‐Stachurski method, results from plastic straining of a cylindrical annular Devanathan‐Stachurski sample confirm the film rupture concept. Film removal is shown to enhance the overall rate of hydrogen entry by increasing the hydrogen surface coverage and possibly the adsorption‐absorption rate constant on the bare surfaces formed. This increases the mobile hydrogen concentration in the metal lattice. The presence of the intact metastable film modifies the kinetics of the hydrogen evolution reaction, lowering the cathodic hydrogen overpotential at a given cathodic current density. This, consequently, is shown to lower hydrogen absorption for a given cathodic current density. The net effect of film rupture is to promote an increase in the hydrogen permeation rate independent of changes in the diffusion coefficient. Additional information indicates that long range enhanced diffusion of hydrogen by dislocation transport is not observed. Further evidence suggests that for sharp cracks, the rate of hydrogen accumulation at a position a distance from the crack tip where the crack initiates is an absorption controlled transport process. These findings provide additional insight on the rate limiting steps controlling the “time dependent” hydrogen assisted failure process of high strength steels in sodium chloride solution.

Summary Abstract: Static secondary ion mass spectroscopy as a real-time monitor of surface hydrogen coverage on Ni(100)

Summary Abstract: Static secondary ion mass spectroscopy as a real-time monitor of surface hydrogen coverage on Ni(100)

Coverage dependence of the surface diffusion coefficient for hydrogen on Ru(001)

The coverage dependence of the surface diffusion coefficient for hydrogen on Ru(001) was studied using laser-induced thermal desorption(LITD) techniques. The LITD measurements were performed for a wide range of initial hydrogen surface coverages in the temperature range 230–270 K. At a given temperature, the surface diffusion coefficient was found to be constant as a function of coverage. The absence of coverage dependence in the surface diffusion coefficient indicates that adsorbate-adsorbate interactions between adsorbed hydrogen atoms on Ru(001) are negligible. This observation suggests that the coverage-dependent features observed in work function, high resolution electron energy loss spectroscopy and possibly temperature programmed desorption studies of hydrogen on Ru(001) are associated with two inequivalent hydrogen sites rather than repulsive lateral hydrogen interactions. Monte Carlo simulations reveal that the surface diffusion coefficient should be coverage independent on a surface with two inequivalent sites in the absence of adsorbate-adsorbate interactions.

Relationship between surface hydrogen and fracture stress of 4340 steel

The surface hydrogen coverage of 4340 steel has been measured directly using electron-stimulated desorption. Coverage varies with sulfur coverage. A maximum hydrogen coverage was observed for a sulfur coverage of 0.3 with smaller hydrogen coverages for greater and less sulfur coverages. This result was related to the probable effect of sulfur on hydrogen adsorption, dissociation and recombination. The fracture stress of 4340 steel was measured using an in-situ tensile test, and a maximum decrease in the fracture stress was found to coincide with the maximum hydrogen coverage. A linear relationship between the fracture stress and hydrogen coverage. It was proposed that this relationship reflected the fracture stress-bulk hydrogen concentration and that these results support a linear relationship between fracture stress and bulk hydrogen concentration.

A theoretical study of hydrogen surface coverage over Ni(100)

AbstractWe have used the MINDO/SR molecular orbital method in order to model the migration of hydrogen atoms over a Ni(100) surface. The present calculations indicate the existence of two different states for adsorbed hydrogen, a result which is in agreement with experimental thermal desorption data and LEED. A detailed analysis of the electronic factors involved in this process is presented.

ChemInform Abstract: NMR STUDIES ON HYDROGEN CHEMISORPTION AND WATER FORMATION ON PLATINUM BLACK CATALYSTS

AbstractDie FT‐NMR‐Methode erlaubt eine Unterscheidung und Bestimmung von adsorbierten H2O und Hbei der Reduktion von Sauerstoff‐bedecktem Platinschwarz mit H2.

NMR Studies on Hydrogen Chemisorption and Water Formation on Platinum Black Catalysts

AbstractFourier Transform NMR spectroscopy has been used to study the reduction of oxygen covered Pt‐black and the chemisorption of hydrogen. The 1H Knight shift is dependent on hydrogen surface coverage and Pt particle size. T1, and T2 relaxation times were measured for adsorbed H2O and H as a function of temperature. The results are discussed with respect to the surface stoichiometry and the mechanism of the catalytic water reaction, isosteric heats of hydrogen chemisorption, estimation of metal surface areas of Pt catalysts and surface mobility of adsorbed H2O and H.

The low temperature water formation reaction on Pt(111): A static SIMS and TDS study

The formation of water by the reaction of preadsorbed oxygen with hydrogen on a Pt(111) surface has been characterized, using secondary ion mass spectroscopy, below the desorption temperature of H 2O (180 K). The concentration of chemisorbed water was monitored during the reaction by following the SIMS H 3O + signal. Reaction profiles were measured over a temperature range of 120 to 153 K, and an H 2 pressure range of 10 -9 to 10 -6 Torr. Under all conditions the reaction profiles were characterized by an induction time, a region of rapid reaction, and finally a steady decline in the rate. In the rapid region, an overall activation energy of 2.9 ± 0.3 kcal mol -1 and a half-order H 2 pressure dependence were observed. At low initial oxygen concentrations the induction time increased and the maximum rate decreased. The reaction was slow in the absence of gas phase hydrogen, even when the surface coverage of hydrogen was relatively high. Water and hydrogen thermal desorption spectra, measured after stopping the reaction by removal of gas phase hydrogen, were complex functions of the H 2 exposure, exhibiting several peaks between 170 and 400 K. However, after an exposure large enough to drive the reaction to completion, only one H 2O peak at 173 K and one H 2 peak at 350 K were observed. The results indicate that only a fraction of the total H(a) on the surface was readily available for reaction during H 2 exposure at T ⩽ 153 K. the remainder either recombined to form H 2 or reacted with O(a) during the thermal desorption ramp. There is good evidence for a surface rearrangement during the induction period. A model is proposed which involves the formation of water clusters that accelerate the rate.

The low temperature water formation reaction on Pt(111): A static sims and TDS study

Abstract The formation of water by the reaction of preadsorbed oxygen with hydrogen on a Pt(111) surface has been characterized, using secondary ion mass spectroscopy, below the desorption temperature of H 2 O (180 K). The concentration of chemisorbed water was monitored during the reaction by following the SIMS H 3 O + signal. Reaction profiles were measured over a temperature range of 120 to 153 K, and an H 2 pressure range of 10 -9 to 10 -6 Torr. Under all conditions the reaction profiles were characterized by an induction time, a region of rapid reaction, and finally a steady decline in the rate. In the rapid region, an overall activation energy of 2.9 ± 0.3 kcal mol -1 and a half-order H 2 pressure dependence were observed. At low initial oxygen concentrations the induction time increased and the maximum rate decreased. The reaction was slow in the absence of gas phase hydrogen, even when the surface coverage of hydrogen was relatively high. Water and hydrogen thermal desorption spectra, measured after stopping the reaction by removal of gas phase hydrogen, were complex functions of the H 2 exposure, exhibiting several peaks between 170 and 400 K. However, after an exposure large enough to drive the reaction to completion, only one H 2 O peak at 173 K and one H 2 peak at 350 K were observed. The results indicate that only a fraction of the total H(a) on the surface was readily available for reaction during H 2 exposure at T ⩽ 153 K . the remainder either recombined to form H 2 or reacted with O(a) during the thermal desorption ramp. There is good evidence for a surface rearrangement during the induction period. A model is proposed which involves the formation of water clusters that accelerate the rate.