Hydrogen Surface Coverage Research Articles

In situ monitoring of hydrogen desorption from silicon nanoparticles dispersed in a nonthermal plasma

In this paper, the authors discuss the use of Fourier transform infrared absorption spectroscopy to monitor the hydrogen surface coverage of silicon nanoparticles suspended in an argon-hydrogen nonthermal plasma. The absorption from surface silicon hydride groups is measured by growing nanoparticles from silane in a first plasma reactor and by passing them through a second plasma reactor intersected by an infrared beam. Using this setup, the authors obtain an in situ, in-flight measurement of the surface termination. They have found that hydrogen surface coverage declines at increasing plasma power. Control experiments performed on particles collected onto a substrate and exposed to the same plasma indicate that the loss of hydrogen is the result of a thermally induced desorption process. By using well-established kinetic rates for hydrogen interactions with silicon surfaces, the authors estimate the nanoparticle temperature to be in the 650–750 K range. This work provides additional experimental evidence that dust suspended in a low-pressure partially ionized gas is heated to a high temperature, enabling the production of high-quality nanocrystals.

The H2 Pressure Dependence of Hydrodeoxygenation Selectivities for Furfural Over Pt/C Catalysts

Hydrodeoxygenation of furfural was studied over a 10-wt% Pt/C catalyst at 453 K, under both low- and high-pressure conditions. With vapor-phase furfural as the feed and H2 pressures below 1 bar, decarbonylation to furan is a major product, with the selectivity to furfuryl alcohol and dimethylfuran increasing with increasing H2 pressure. When the reaction is performed at 33 bar, using 1-wt % furfural in 1-propanol solvent and high-pressure H2, no evidence for decarbonylation was observed. At high pressures, the reaction is sequential, with all the furfural proceeding to methylfuran, which in turn reacts to over-hydrogenated products, including 2-methyltetrahydrofuran and 2-pentanone. It is suggested that the hydrogen surface coverage is responsible for the apparent differences in the reaction network at high and low pressures.

Experimental Characterization of CCH(ads) and CCH2(ads) during the Thermal Decomposition of Methane and Ethylene on Ru(0 0 0 1)

AbstractCatalytic dissociation of hydrocarbons on transition‐metal substrates plays a critical role in a variety of industrially important chemistry. Despite this importance, knowledge of the dissociation pathways of even the simplest hydrocarbons on single‐metal crystal surfaces in ultrahigh vacuum (UHV) is incomplete. In this study the temperature‐induced decomposition of CH4 and C2H4 on Ru(0 0 0 1) is characterized using high‐resolution vibrational spectroscopy of all interfacial CH stretch modes. By investigating both decomposition pathways concurrently we are able to unambiguously identify for the first time CCH2(ads) during the decomposition of C2H4, and CH2(ads), CCH3(ads), and CCH(ads) during the decomposition of CH4. Following this identification, the thermal stability of all species is described and also how this stability depends on carbon and hydrogen surface coverage. Although collected in UHV, these observations suggest under‐appreciated constraints on the chemistry of CH4 and C2H4 in high‐pressure applications.

Transient analysis of carbon monoxide transport phenomena and adsorption kinetics in HT-PEMFC during dynamic current extraction

This paper investigates the transport phenomena of carbon monoxide (CO) and adsorption kinetics, in a high-temperature proton exchange membrane fuel cell (HT-PEMFC) during step-wise current extraction (step-change in current extraction). Step-wise current extraction is a common process done to accommodate a sudden power surge during an operation. Since HT-PEMFCs are capable of handling high impurity of CO, hydrogen fuel that is contaminated with trace amount of CO is usually considered for commercial benefits. Thus, a transient three-dimensional isothermal anodic electro-kinetic numerical model is developed to determine the effect of operating parameters such as fuel cell temperature, CO inlet (initial) concentration, step-change of current density and dwell time on the transport phenomena of CO and adsorption kinetics. In addition, geometrical factors such as the catalyst layer (CL) and gas diffusion layer (GDL) porosity are also varied as well. The results show that the above-mentioned operating parameters can affect the maximum CO concentration at the CL, especially at the outlet of the channel. Specifically, a reduction of fuel cell temperature can significantly increase the CO concentration near the outlet, while increasing CO inlet (initial) concentration, step-change amplitude of current density and current density dwell time can cause an increase in CO concentration at the outlet, albeit to different extent. In addition, the increase in the porosity of CL and GDL, results in the reduction of the maximum CO concentration at the outlet, albeit to different extent. In addition, the CO and hydrogen surface coverage fractions are also affected by the variance of the above mentioned parameters.

Controlling CH2 dissociation on Ru(0001) through surface site blocking by adsorbed hydrogen

Understanding the relative stability of CHx species on surfaces is necessary for mechanistic description of much important catalytic chemistry. Here, we experimentally quantify the barrier of the reaction CH2→CH+H on Ru(0001) in UHV and find an activation energy, 65±6kJ/mol, that is >4× higher than previous computational results with 0, 1, or 2 coadsorbed H atoms per CH2, i.e. 16kJ/mol. Employing density functional theory calculations, we show that this disagreement can be reconciled if 3 coadsorbed H atoms per CH2 are present in our experiment. We further demonstrate, by calculating the surface phase diagram for one carbon species on Ru(0001) as a function of H2 chemical potential, that the additional hydrogen surface coverage requires non-equilibrium conditions. Such conditions may be important at the high temperatures and pressures of real catalytic systems.

First-principles study of hydrogen adsorption and permeation in reconstructed cubic erbium oxide surfaces

Understanding surface properties of Er2O3, especially in relation to adsorption and permeation of atomic hydrogen, is of considerable importance to the study of tritium permeation barriers. In this work, hydrogen diffusion pathways through the low-index (100), (110), and (111) surfaces of cubic Er2O3 have been calculated using density functional theory within the GGA (PBE)+U approach. The dependence of the effective U parameter on lattice constants, bulk moduli, and formation energies of Er2O3 has been investigated in detail. The energetics of hydrogen penetration from the surfaces to the solution site in bulk Er2O3 were defined using the optimum effective U value of 5.5eV. For a low surface coverage of hydrogen (0.89×1014H/cm2), a penetration energy of at least 1.7eV was found for all the low-index erbium oxide surfaces considered. The results of the present study will provide useful guidance for future studies on modeling defects, such as grain boundaries and vacancies, in tritium permeation barriers.

The effect of hydrogen on the surface segregation of phosphorus in epitaxially grown relaxed Si0.7Ge0.3 films by rapid thermal chemical vapor deposition

The surface segregation of phosphorus in relaxed Si0.7Ge0.3 epitaxial films grown on Si (100) substrates by rapid thermal chemical vapor deposition was investigated in this letter. The effect of the growth temperature on phosphorus segregation was studied experimentally and examined using a two-state model. As the growth temperature is reduced, phosphorus segregation is greatly suppressed, and we report an extremely sharp phosphorus turn-off slope of 6 nm/dec at 500 °C. The sharper slopes at low temperatures are explained by a modified two-state model which includes the effect of increased surface coverage of hydrogen at low temperatures.

Kinetic Modeling of Pt-Catalyzed Glycolaldehyde Decomposition to Syngas

Fundamental knowledge of the elementary reaction mechanisms involved in oxygenate decomposition on transition metal catalysts can facilitate the optimization of future catalyst and reactor systems for biomass upgrade to fuels and chemicals. Pt-catalyzed decomposition of glycolaldehyde, as the smallest oxygenate with alcohol and aldehyde functionality, was studied via a DFT-based microkinetic model. It was found that two decomposition pathways exist. Under conditions of low hydrogen surface coverage, the initial C-H bond breaking reaction to HOCH(2)CO* is prevalent, while under conditions of high hydrogen coverage, the rather unexpected O-H bond forming reaction to HOCH(2)CHOH* is more active (subsequent decomposition is energetically favorable from HOCH(2)CHOH*). Our results indicate the possibility that (de)hydrogenation chemistry is rate-controlling in many small polyoxygenate biomass derivatives, and suitable catalysts are needed. Finally, DFT was used to understand the increased decomposition activity observed on the surface segregated Ni-Pt-Pt bimetallic catalyst. It was found that the initial O-H bond breaking of glycolaldehyde to OCH(2)CHO* has an activation barrier of just 0.21 eV. This barrier is lower than that of any glycolaldehyde consuming reaction on Pt. These computational predictions are in qualitative agreement with experimental results.

Thermal processing and native oxidation of silicon nanoparticles

In this study, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) were used to investigate in-air oxidation of silicon nanoparticles ca. 11 nm in diameter. Particle samples were prepared first by extracting them from an RF plasma synthesis reactor, and then heating them in an inert carrier gas stream. The resulting particles had varying surface hydrogen coverages and relative amounts of SiH x (x = 1, 2, and 3), depending on the temperature to which they had been heated. The particles were allowed to oxidize in-air for several weeks. FTIR, XPS, and EELS analyses that were performed during this period clearly establish that adsorbed hydrogen retards oxidation, although in complex ways. In particular, particles that have been heated to intermediate hydrogen coverages oxidize more slowly in air than do freshly generated particles that have a much higher hydrogen content. In addition, the loss of surface hydride species at high processing temperatures results in fast initial oxidation and the formation of a self-limiting oxide layer. Analogous measurements made on deuterium-covered particles show broadly similar behavior; i.e., that oxidation is the slowest at some intermediate coverage of adsorbed deuterium.

Surface and Electronic Properties of Hydrogen Terminated Si [001] Nanowires

The calculated band gaps reported previously for silicon nanowires (SiNW) have disagreed with the experimental values both in magnitude and in the behavior with radius. We resolve this discrepancy here. We report ab initio quantum mechanical calculations of hydrogen terminated Si (001) nanowires (HSiNWs) asafunctionofdiameter(d) andhydrogencoverage using theB3LYP density functional. For smaller diameters (d e 1.9 nm) we find that the most stable surface is fully saturated with hydrogen leading to directbandgaps.Forlargerdiameters, thesurfacedanglingbonds are notsaturated,leading tosurface LUMO and HOMO states thatlower the gap andlead toan indirectband gap.Thistransition from direct to indirect gap resolves the previous disagreement in the scaling of band gap with diameter. We conclude that the electronic properties of Si NW depend sensitively on controlling the diameter and surface hydrogen coverage.

Textural and morphological studies on zinc–iron alloy electrodeposits

Zinc–iron alloy electrodeposits have industrial significance, since they provide better corrosion resistance and with improved mechanical properties when compared to pure zinc coatings. This is due to the unique phase structure of the alloy formed. But this deposition belongs to anomalous deposition, where the electrochemically less noble zinc deposits more preferentially, than the more noble iron. So the industrial process control over the deposition becomes difficult. So, this study correlates the effect of various deposition parameters over the deposition kinetics and deposited alloy characteristics. Transition and partial current densities were computed. Effect of hydrogen overpotential and surface coverage due to the adsorbed intermediates over anomalous deposition were explored. Plausible deposition mechanism and mathematical model was proposed to predict the anomalous electrodeposition characteristics. Textural, morphological and phase structural characteristics of the alloy was investigated. By the substitution of iron in the hcp lattice, c/a ratio was lowered and the lattice geometry was distorted. Intermetallic compounds of variable composition such as FeZn14, Fe5Zn33, Fe3Zn13 and FeZn3 with ‘η’ and ‘Γ’ phase structures were noted. Electrodeposition parameters were optimized and smooth, adherent, strain-free deposits with required iron content and hardness were obtained. The role of charge and mass transfer control on the texture and morphology of zinc–iron alloy electrodeposition was studied. Zinc-rich alloy with distorted ‘η’ hcp phase was predominant under kinetic limitations, by the iron substitution along ‘c’ axis. The ‘Γ’ phase with relatively iron-rich alloy was formed under diffusion control.

First‐principles study on the effect of surface hydrogen coverage on the adsorption process of ammonia on InN(0001) surfaces

AbstractTo examine the effect of hydrogen on the initial growth process of InN, the adsorption processes of group‐V sources commonly used in vapor phase epitaxy (VPE) were investigated using first‐principle calculations based on the density functional theory (DFT). The ideal and reconstructed surfaces with an adsorbed 0.75 monolayer of hydrogen were used as the initial surface structures to examine the influence of surface hydrogen coverage of InN(0001) surfaces on the adsorption processes of group‐V sources including NH3 and NH2 generated by the decomposition of NH3. From the calculations of the adsorption energies for each case, it was revealed that surface hydrogen coverage did not have an effect on the adsorption of NH3, whereas NH2 was significantly affected by the presence of hydrogen on the surface. In comparison with our previous results, it was also shown that the adsorption energies for NH3 and NH2 on InN(0001) surfaces are smaller than those on GaN and AlN(0001) surfaces (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Production of vibrationally excited hydrogen molecules by atom recombination on Cu and W materials

We have measured vibrational population of H(2) and D(2) molecules produced by atom (H or D) recombination on tungsten and copper material. The vibrational spectroscopy, based on the properties of dissociative electron attachment to hydrogen molecule, was used. The vibrationally excited molecules were produced by atom recombination in a cell where the studied sample is exposed to hydrogen atoms, from hot tungsten filament. Vibrational populations were obtained for the studied materials, which can be well described by the Boltzmann distribution, with specific vibrational temperatures for each material. The experimentally obtained vibrational populations for copper approximately agree with the theoretical predictions, whereas the experimentally obtained vibrational temperature for tungsten is higher and thus showing a considerable overpopulation of highly excited vibrational states than predicted. We propose that the origin of this higher excitation is related to the existence of high hydrogen surface coverage on tungsten, where hydrogen is occupying binding sites with different desorption energies. In order to obtain an insight into the recombination mechanism with more than one binding site per unit cell, a Monte Carlo simulation was performed, where it was assumed that the main production of molecules proceeds through the hot-atom recombination with an adsorbed atom. The results show that the recombination proceeds mainly through the weak binding sites, once they are occupied.

Theoretical study on the influence of surface hydrogen coverage on the initial growth process of AlN(0001) surfaces

AbstractThe adsorption processes of group‐V sources used in metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) were investigated using first‐principle calculations based on the density functional theory (DFT) to understand the initial growth process of AlN(0001). To examine the influence of surface hydrogen coverage on the initial growth process, the ideal surface and a reconstructed surface with an adsorbed 0.75 monolayer of hydrogen were used as the initial surface structures. NH3 and NH2 generated by the decomposition of NH3 were considered as group‐V sources. From the calculations, the influence of surface hydrogen coverage on the initial growth process of AlN(0001) surfaces in vapor phase epitaxy was clarified. It was revealed that surface hydrogen coverage does not have an effect on adsorption of NH3, whereas NH2 was affected by hydrogen on the surface. It was also demonstrated that the adsorption energies for NH3 and NH2 on the ideal and reconstructed GaN(0001) surfaces are smaller than those on the ideal and reconstructed AlN(0001) surfaces (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evaluation of the Propensity of Niobium to Absorb Hydrogen During Fabrication of Superconducting Radio Frequency Cavities for Particle Accelerators.

During the fabrication of niobium superconducting radio frequency (SRF) particle accelerator cavities procedures are used that chemically or mechanically remove the passivating surface film of niobium pentoxide (Nb2O5). Removal of this film will expose the underlying niobium metal and allow it to react with the processing environment. If these reactions produce hydrogen at sufficient concentrations and rates, then hydrogen will be absorbed and diffuse into the metal. High hydrogen activities could result in supersaturation and the nucleation of hydride phases. If the metal repassivates at the conclusion of the processing step and the passive film blocks hydrogen egress, then the absorbed hydrogen or hydrides could be retained and alter the performance of the metal during subsequent processing steps or in-service. This report examines the feasibility of this hypothesis by first identifying the postulated events, conditions, and reactions and then determining if each is consistent with accepted scientific principles, literature, and data. Established precedent for similar events in other systems was found in the scientific literature and thermodynamic analysis found that the postulated reactions were not only energetically favorable, but produced large driving forces. The hydrogen activity or fugacity required for the reactions to be at equilibrium was determined to indicate the propensity for hydrogen evolution, absorption, and hydride nucleation. The influence of processing conditions and kinetics on the proximity of hydrogen surface coverage to these theoretical values is discussed. This examination found that the hypothesis of hydrogen absorption during SRF processing is consistent with published scientific literature and thermodynamic principles.

The sticking of H2(v = 1, J = 1) on Cu(100) measured using laser-induced thermal desorption

The sticking of vibrationally excited H2 on Cu(100) is measured using molecular beam techniques and stimulated Raman scattering for state preparation. Laser Induced Thermal Desorption (LITD) is developed to measure surface hydrogen coverage by calibration with known H-coverages. We find that LITD can be effectively used to measure adsorbed hydrogen concentrations with a sensitivity of better that 1% of a monolayer. Sticking coefficients of H2 are determined from the ratio coverage to exposure. Measurements are made for H2 incident translational energies in the range of 74–189 meV. For the first excited vibrational state, we find and an upper limit of the H2 sticking coefficient to be 0.065 at E i = 74 meV and 0.17 at E i = 189 meV. These limits imply that previous measurements of loss of H2(v = 1) upon scattering from Cu(100) cannot be predominately a result of dissociative adsorption. The results are broadly consistent with results determined from quantum-state resolved measurements for recombinatative desorption and the principle of detailed balance for H2 from Cu(111).

The effect of low concentrations of CO on H 2 adsorption and activation on Pt/C. Part 1: In the absence of humidity

The presence of CO in the H 2-rich gas used as fuel for hydrogen fuel cells has a detrimental effect on PEMFC performance and durability at conventional operating conditions. This paper reports on an investigation of the effect of CO on H 2 activation on a fuel cell Pt/C catalyst close to typical PEMFC operating conditions using H 2–D 2 exchange as a probe reaction and to measure hydrogen surface coverage. While normally limited by equilibrium in the absence of impurities on Pt at typical fuel cell operating temperatures, the presence of ppm concentrations of CO increased the apparent activation energy ( E a) of H 2–D 2 exchange reaction (representing H 2 activation) from approximately 4.5–5.3 kcal mole −1 (Bernasek and Somorjai (1975) [24], Montano et al. (2006) [25]) (in the absence of CO) to 19.3–19.7 kcal mole −1 (in the presence of 10–70 ppm CO), similar to those reported by Montano et al. (2006) [25]. Calculations based on measurements indicate a CO surface coverage of approximately 0.55 ML at 80 °C in H 2 with 70 ppm CO, which coincide very well with surface science results reported by Longwitz et al. (2004) [5]. In addition, surface coverages of hydrogen in the presence of CO suggest a limiting effect on hydrogen spillover by CO. Regeneration of Pt/C at 80 °C in H 2 after CO exposure showed only a partial recovery of Pt sites. However, enough CO-free Pt sites existed to easily achieve equilibrium conversion for H 2–D 2 exchange. This paper establishes the baseline and methodology for a series of future studies where the additional effects of Nafion and humidity will be investigated.

Influence of rare earth content on electrode kinetics in Misch metal-based AB 5 MH alloys – Cyclic voltammetric investigations

The influence of lanthanum/cerium ratio (0.42–14.14) in Misch metal (Mm)-based AB 5-type hydrogen storage alloys has been investigated. The samples were subjected to X-ray florescence (XRF) and cyclic voltammetry (CV) studies. The metal hydride electrodes were assembled, and their discharge capacity was determined. These alloys delivered discharge capacity between 118 and 266 mAh/g. CV investigations threw light on charge-transfer reactions at the electrode/electrolyte interface and hydrogen surface coverage capacity. The CV parameters in general indicate that the battery activity increases with lanthanum/cerium ratio. At low lanthanum/cerium values, the discharge reactions proceed at potentials that are more negative. Furthermore, electrochemical reversibility of the hydrogen absorption–desorption on the alloy surface is enhanced at optimum lanthanum/cerium ratio as revealed by decreasing values of peak separation. The slopes of graphical plots of I peak (anodic) vs. ν 1/2 for the fully charged samples reverse direction at very high lanthanum/cerium values. The results suggest that the discharge plateau is optimum at lanthanum/cerium ratio around 12.

Study of hydrogen adsorption on FeTi using molecular dynamics simulations

We have used molecular dynamics simulation to study the adsorption isotherms of molecular hydrogen on FeTi at several temperatures ranging from 60 to 100 K. Adsorption coverage, isosteric heat, and binding energy were calculated at different temperatures and pressures. The results indicated that FeTi can be used as an ideal hydrogen storage material. The surface coverage or total amount of hydrogen adsorbed on FeTi is between 0.28 to 0.35.

Reason of Inhibiting the Iron Anodic Dissolution in Acid Sulfate Electrolyte by Tetrabutylammonium Cations

Hydrogen surface coverage (ΘH) is calculated for iron electrode as a function of tetrabutylammonium cations surface coverage and the hydrogen concentration in the metal (CH). It is shown that the effect of the organic corrosion inhibitor on the iron dissolution rate is due to the increase in the ΘH / CH ratio.