Source Of Hydrogen Atoms Research Articles

Hydrogen Storage in Polymer-Functionalized Pd-Decorated Single Wall Carbon Nanotubes

Palladium is usually supported on porous materials in the form of nanoparticles. The hydrogen storage capacity of such a system is usually much higher than the separated capacity of the metal (approximately 0.7 H/Pd) and the support. Pd nanoparticles provide a source of hydrogen atoms by dissociation. The atomic hydrogen spills over from the Pd structure to the support via surface diffusion and this phenomenon is known as hydrogen spillover. In this study commercial SWNTs were dispersed in PEG 200 solution. Then the precursor PdCl2 in PEG 200 was added and the whole left to react under stirring with reflux at 200 degrees C for 1 h. Succeeding washings with ethanol and centrifugation followed for several times and finally the sample was dried at 60 degrees C. Through this procedure a 3 wt% Pd loading was achieved whereas the TEM derived nanoparticle size distribution indicated a 50% percentage of Pd nanoparticles with diameter less than 8 nm. Hydrogen isotherms up to 2 MPa were carried out with the gravimetric method. The defined storage capacity of 1.2 wt% at 0.2 MPa was quite satisfactory. However, a 0.2 wt% portion of this storage capacity was attributed to the formation of water molecules through reaction of H atoms with the dissociatively adsorbed oxygen atoms on the Pd nanoparticles. This conclusion was educed from a series of thermal desorption experiments following the H2 adsorption/desorption cycles and regeneration. Through this set of experiments several other important parameters were defined as the temperature for complete hydrogen desorption and the optimum conditions for PEG removal.

ChemInform Abstract: Radical Deoxygenation and Dehalogenations with Dialkyl Phosphites as Hydrogen Atom Source.

ChemInform Abstract: Radical Deoxygenation and Dehalogenations with Dialkyl Phosphites as Hydrogen Atom Source.

Understanding the Exceptional Hydrogen-Atom Donor Characteristics of Water in TiIII-Mediated Free-Radical Chemistry

In recent years solid evidence of HAT reactions involving water as hydrogen atom source have been presented. In this work we demonstrate that the efficiency of titanocene(III) aqua complexes as an unique class of HAT reagents is based on two key features: (a) excellent binding capabilities of water toward titanocene(III) complexes and (b) a low activation energy for the HAT step. The theory has predictive capabilities fitting well with the experimental results and may aid to find more examples of this remarkable radical reaction.

Low-Temperature Etching of Cu by Hydrogen-Based Plasmas

A simple, plasma-based, low-temperature etch process was developed for the subtractive etching of copper (Cu) films. Hydrogen (H2) plasma etching of Cu thin films was performed in an inductively coupled plasma (ICP) reactor at temperatures below room temperature. This process achieved anisotropic Cu features and an etch rate of ∼13 nm/min. Cu etch rate and patterning results were consistent with an etch process that involved both chemical and physical characteristics. This conclusion was reached by consideration of the plasma as a source of ultraviolet photons, ions, and hydrogen atoms, which promote Cu etching.

Suprathermal hydrogen produced by the dissociation of molecular hydrogen in the extended atmosphere of exoplanet HD 209458b

The ionization and dissociation of molecular hydrogen by the ultraviolet (UV) radiation of the parent star lead to the formation of hydrogen atoms with an excess of kinetic energy and, thus, are an important source of suprathermal hydrogen atoms in the upper atmosphere of exoplanet HD 209458b. Contemporary aeronomical models did not investigate these processes because they assumed the fast local thermalization of the hot atoms of hydrogen by elastic collisions. However, the kinetics and transfer of these atoms were not calculated in detail, because they require the solving of the Boltzmann equation for a nonthermal atom population. This work estimates the effect of the UV radiation of the parent star and the accompanying photocleacton flux on the production of the suprathermal fraction of atomic hydrogen in the H2 → H transition region. We also consider the formation of the escaping flux of Hatoms created by this effect in the upper atmosphere of HD 209458b. We calculate the production rate and energy spectrum of the hydrogen atoms with excess kinetic energy during the dissociation of H2. Using the numerical stochastic model created by Shematovich (2004) for a hot planetary corona, we investigate the molecular-scale kinetics and transfer of suprathermal hydrogen atoms in the upper atmosphere and the emergent flux of atoms evaporating from the atmosphere. The latter is estimated as 3.4 × 1012 cm−2 s−1 for a moderate stellar activity level of UV radiation, which leads to a planetary atmosphere evaporation rate of 3.4 × 109 g s−1 due to the process of the dissociation of H2. This estimate is close to the observational value of ∼1010 g s−1 for the rate of atmospheric loss of HD 209458b.

Role of catechol in the radical reduction of B-alkylcatecholboranes in presence of methanol

Mechanistic investigations on the previously reported reduction of B-alkylcatecholboranes in the presence of methanol led to the disclosure of a new mechanism involving catechol as a reducing agent. More than just revising the mechanism of this reaction, we disclose here the surprising role of catechol, a chain breaking antioxidant, which becomes a source of hydrogen atoms in an efficient radical chain process.

Study of analyte dissociation and diffusion in laser-induced plasmas: implications for laser-induced breakdown spectroscopy

Plasma–particle interactions are explored through the introduction of single microdroplets into laser-induced plasmas. Both spectroscopic analysis and direct imaging of analyte atomic emission are used to provide insight into the various fundamental processes, namely desolvation, atomization, and atomic diffusion. By doping the 50 µm droplets with Lu, Mg or Ca, the analyte excitation temperature and the ion-to-neutral emission ratio are explored as a function of plasma residence time following breakdown. The data suggest a change in the local plasma conditions about the analyte atoms around 15–20 µs following breakdown, which may be interpreted as an overall transition from localized (i.e. perturbed) plasma conditions to the global (i.e. bulk) plasma conditions. A direct assessment of the hydrogen atomic diffusion coefficient following analyte desolvation reveals a value of 1.7 m2 s−1 in the first 250–500 ns. This value is in good overall agreement with a theoretical analysis and with an analytical treatment of a surface source of hydrogen atoms. In contrast, calcium emission is only observed beyond about 1 µs, with a diffusion coefficient at least an order of magnitude below the hydrogen value. The temporal H and Ca emission data suggest that water vaporizes first, leaving an ever increasing Ca analyte concentration until finally, with nearly all water desorbed, the Ca fraction is vaporized. Overall, the data support the conclusion that finite time-scales of heat and mass transfer play an important role in localized plasma perturbations in the vicinity of the analyte, which has important implications for the LIBS analyte signal.

Commercializable power source from forming new states of hydrogen

Commercializable power source from forming new states of hydrogen

Understanding the Effect of Hydrogen Surface Passivation and Etching on the Shape of Silicon Nanocrystals

One of the significant challenges in the use of nanocrystals, is the control of crystal shape when grown from the gas-phase. Recently, the Kortshagen group has succeeded in generating cubic Si nanocrystals in a nonequilibrium plasma. In this paper we consider the energetics of various shaped Si nanocrystals, and the role that hydrogen surface termination plays. We consider cube, truncated octahedron, icosahedron, and spherical shapes for both bare and hydrogen coated silicon nanocrystals for sizes between 2 and 10 nm. From our molecular dynamics (MD) simulations, show that for bare Si crystals, icosahedron crystals are the most energetically stable, and cubic the least. On the other hand, when hydrogenated, the cubic structure comes about because 1) the cubic structure is energetically favored when hydrogen terminated and 2) the plasma that operates with hydrogen also provides a steady source of hydrogen atoms for etching.

Design and characterization of a thermal hydrogen atom source

The hydrogen atom source considered here incorporates a hot capillary fed by hydrogen gas. Our earlier measurements on a source heated by electron bombardment are interpreted in terms of a simple model which encourages us to design a source heated by the radiation from a filament. The radiatively heated source is much simpler, more reliable, and easier to run than the electronically heated source. Furthermore, the radiatively heated source is free of any energetic particles. In order to obtain quantitative data on the intensity, an apparatus is constructed revealing the angular distribution of the hydrogen atoms and molecules by means of a quadrupole mass analyzer. The intensity of the source is controlled by the mass flow rate of the feed gas and the electric power to the filament. The flux density of hydrogen atoms at a substrate 6 cm away from the source is variable over two orders of magnitude and extends up to some 1015 atoms/cm2 s.

Halohydrination of epoxy resins using sodium halides as cationizing agents in MALDI‐MS and DIOS‐MS

Halohydrination of epoxy resins using sodium halides as cationizing agents in matrix-assisted laser desorption/ionization (MALDI) and desorption ionization on porous silicon mass spectrometry (DIOS-MS) were investigated. Different mass spectra were observed when NaClO(4) and NaI were used as the cationizing agents at the highest concentration of 10.0 mM, which is much higher than that normally used in MALDI-MS. MALDI mass spectra of epoxy resins using NaI revealed iodohydrination to occur as epoxy functions of the polymers. The halohydrination also occurred using NaBr, but not NaCl, due to the differences in their nucleophilicities. On the basis of the results of experiments using deuterated CD(3)OD as the solvent, the hydrogen atom source was probably ambient water or residual solvent, rather than being derived from matrices. Halohydrination also occurred with DIOS-MS in which no organic matrix was used; in addition, reduction of epoxy functions was observed with DIOS. NaI is a useful cationizing agent for changing the chemical form of epoxy resins due to iodohydrination and, thus, for identifying the presence of epoxy functions.

Methyl radical chemical reactions on a tungsten surface

In this paper an interaction of the inner dome surfaces of the International Thermonuclear Experimental Reactor with a gas mixture containing hydrocarbon radicals has been modelled. To study the processes of transport and deposition of methyl radicals in a C x H y –H2–H (x=1−3, y=1−6) flow, a stream technique has been used. A radiofrequency inductive discharge initiated in methane at the inlet of a quartz tube was a source of methyl and hydrogen atoms. Methyl radicals were transported by a gas flow that passed through cylindrical tungsten inserts and were deposited on the tube wall, on silicon samples and on tungsten samples, forming polymer-like hydrogenated amorphous carbon films. It has been shown that at the tungsten insert temperature increases to 420 K, and carbon deposition rate decreased significantly. However, in the gaseous phase the secondary hydrocarbon species accumulated and condensed on the tungsten surface at a temperature lower than 340 K.

Study of the hydride forming process of in-situ grown MgH 2 thin films by activated reactive evaporation

MgH x thin films are grown by activated reactive evaporation in a Molecular Beam Epitaxy system fitted with an atomic hydrogen source. During deposition the electrical and optical properties are measured in-situ. The structural properties are determined ex-situ by Atomic Force Microscopy. These measurements confirm the growth of the MgH 2 phase, however the presence of 10 vol.% of metallic Mg cannot be prevented. The metallic Mg grains cause an optical absorption edge at 2.0 eV, which has a completely different origin than the observed band gap of MgH 2 at 5.6 eV. The observed optical spectra can be modelled using an effective medium theory. The Mg hydride films are electrically insulating despite the presence of metallic Mg particles. Upon re-hydrogenation of a de-hydrogenated in-situ grown MgH x thin film, the absorption edge at 2.0 eV disappears and the resistivity decreases to values normally observed for ex-situ hydrogenated films.

Photochemical Degradation of PCBs in Snow

This work represents the first laboratory study known to the authors describing photochemical behavior of persistent organic pollutants in snow at environmentally relevant concentrations. The snow samples were prepared by shock freezing of the corresponding aqueous solutions in liquid nitrogen and were UV-irradiated in a photochemical cold chamber reactor at -25 degrees C, in which simultaneous monitoring of snow-air exchange processeswas also possible. The main photodegradation pathway of two model snow contaminants, PCB-7 and PCB-153 (c approximately 100 ng kg(-1)), was found to be reductive dehalogenation. Possible involvement of the water molecules of snow in this reaction has been excluded by performing the photolyses in D2O snow. Instead, trace amounts of volatile organic compounds have been proposed to be the major source of hydrogen atom in the reduction, and this hypothesis was confirmed by the experiments with deuterated organic cocontaminants, such as d6-ethanol or d8-tetrahydrofuran. It is argued that bimolecular photoreduction of PCBs was more efficient or feasible than any other phototransformations under the experimental conditions used, including the coupling reactions. The photodegradation of PCBs, however, competed with a desorption process responsible for the pollutant loss from the snow samples, especially in case of lower molecular-mass congeners. Organic compounds, apparently largely located or photoproduced on the surface of snow crystals, had a predisposition to be released to the air but, at the same time, to react with other species in the gas phase. It is concluded that physicochemical properties of the contaminants and trace co-contaminants, their location and local concentrations in the matrix, and the wavelength and intensity of radiation are the most important factors in the evaluation of organic contaminants' lifetime in snow. Based on the results, it has been estimated that the average lifetime of PCBs in surface snow, connected exclusively to the photoreductive dechlorination process, is 1-2 orders of magnitude longer than that in surface waters when subjected to the equivalent solar radiation. However, in case that the concentration of the hydrogen peroxide in natural snow is sufficient, the photoinduced oxidation process could succeed the photoreductive dechlorination and evaporative fluxes as the major sink.

Chemical methods for the hydrogen termination of silicon dangling bonds

Highly ordered hydrogen-terminated silicon surfaces are ideal testing grounds for molecular electronics. However, upon formation of these surfaces it is inevitable that some surface sites are not capped by hydrogen. These remaining dangling bonds can interfere with the chemical and electronic properties of nanostructures formed on the silicon surface. In this work, using scanning tunneling microscopy, high resolution electron energy loss spectroscopy and ab initio computational methods, we explore two chemical approaches to refining the hydrogen termination process. We investigate the utility of diimide (N 2H 2) and N, N-diethylhydroxylamine (DEHA) as hydrogen atom sources that have the ability to cap dangling bonds.

Theoretical Investigation of the Gas-Phase Kinetics Active during the GaN MOVPE

Quantum chemistry investigations have been performed to study the gas-phase chemistry active during the MOVPE of GaN when Ga(CH3)(3) and NH3, diluted in a H2 carrier gas, are used as precursors. Optimized molecular geometries, energies, and transition-state structures of gas-phase species have been determined with density functional theory at the B3LYP/6-311+g(d,p) level. On the basis of the similarity with the soot formation mechanism active during hydrocarbon combustion, we propose that in this system a gas-phase chemistry is active and its reactivity is enhanced by a radical chain mechanism started from methyl radicals. Initiation reactions are surface processes or the pyrolysis of Ga(CH3)(3). A propagation mechanism composed of fast radical reactions, most of which without an activation energy, was identified, and kinetic constants were determined for each step. The proposed mechanism is able to describe the formation of large GaN adducts formed by up to three R-Ga-NH units. These molecules can give fast cyclization reactions that lead to the formation of six-membered cyclic species, which, similar to benzene for combustion, are thermodynamically stable in vast temperature and pressure ranges and can thus be considered as the first GaN nuclei. We also found that the presence of H2 as a carrier gas can greatly enhance the rate of formation of gas-phase particles because it is a major source of atomic hydrogen, a promoter of gas-phase reactivity.

Quantifying the flux and spatial distribution of atomic hydrogen generated by a thermal source using atomic force microscopy to measure the chemical erosion of highly ordered pyrolytic graphite

A new method has been developed to quantify the flux and spatial distribution of atomic hydrogen (AH) produced from a thermal source. Experimentally, a partially masked highly ordered pyrolytic graphite (HOPG) substrate was chemically etched by AH under vacuum conditions. The resultant erosion profile (step edge) created on the HOPG substrate was analyzed ex situ with atomic force microscopy. By measuring the variation in the average step height as a function of exposure time, the AH source was shown to produce an average flux of 1.3×1017Hatoms∕scm2 at a source-sample distance of 3.5cm and a hydrogen pressure PH2 of 1.0×10−4Torr. Results demonstrating the ability of this method to measure the spatial distribution of AH flux on a solid substrate as well as the influence of different operating conditions on the average AH flux are also presented.

Alcohols and Water as Reducing Agents in Radical Reactions

The radical reduction of B-alkylcatecholboranes into alkanes using alcohols and water as hydrogen atom source is described. This process involves a completely unexpected mechanism: the alcohol (or water) complexed to a borate derivative acts as a reducing agent. The complexation activates the alcohol resulting in a decrease of the O–H bond dissociation energy. This approach offers promising opportunities for the development of tin-free radical reducing agent. Closely related reports from the recent literature involving boron and titanium derivatives are also presented.

The role of atomic hydrogen in pre-epitaxial silicon substrate cleaning

The cleaning of silicon (Si) surfaces is a very important issue for the fabrication of novel semiconductor devices on the nanoscale. Established methods for the removal of organic impurities and the native or chemical oxide are often combined with high temperature desorption steps. However, devices with small feature sizes will be unfunctional if, for example, out-diffusion of dopants is not prevented. In this paper we present two possible processes for low-temperature cleaning: an atomic hydrogen source, based on dissociative adsorption of hydrogen at a heated tantalum (Ta) surface and a hydrogen DC plasma source as a part of an UHV cluster tool. The influence of atomic hydrogen on carbon and oxide removal is surveyed and the existing model for native oxide etching with an argon/hydrogen DC plasma is adapted.

Water: The Ideal Hydrogen‐Atom Source in Free‐Radical Chemistry Mediated by TiIII and Other Single‐Electron‐Transfer Metals?

Die Autoren dieser Zuschrift möchten einen weiteren Beitrag zitieren. Im Jahr 2002 berichteten Doris und Mitarbeiter über die radikalische Reduktion von α,β-ungesättigten Ketonen in Gegenwart von [Cp2TiCl] und MeOH. Diese Beobachtung ist nahe verwandt mit der von den Autoren im selben Jahr beschriebenen Reduktion von Kohlenstoffradikalen in Gegenwart von [Cp2TiCl] und Wasser, wurde aber nicht zitiert. Die Autoren entschuldigen sich für dieses Versehen. Lit. 4 sollte daher lauten: