Gas-phase Atomic Hydrogen Research Articles

Gas-Phase Hydrogen-Atom Measurement above Catalytic and Noncatalytic Materials during Ethane Dehydrogenation

Gas-Phase Hydrogen-Atom Measurement above Catalytic and Noncatalytic Materials during Ethane Dehydrogenation

Avoiding Sabatier’s conflict in bifunctional heterogeneous catalysts for the WGS reaction

Summary A new strategy for designing efficient bifunctional heterogeneous catalysts for the water-gas-shift (WGS) reaction (CO + H2O → CO2 + H2) is presented. We avoid conflicting tasks that require ∗OH or ∗O from dissociated water to adsorb on and then desorb from the substrate of a catalyst. Instead, the CO on metal directly obtains OH from water that is connected by weak hydrogen bonds to the substrate. The task of the substrate is to efficiently channel ∗H on its surface and release them as H2 gas. Experimental and theoretical results show that bifunctional catalysts with weakly reactive substrates have significantly higher CO conversion rates compared with highly reactive substrates that follow either the LH-associative or LH-redox process.

Atomic Hydrogen Reactions of Alkanethiols on Au(111): Phase Transitions at Elevated Temperatures

Scanning tunneling microscopy (STM) was used to observe the chemical transformations of an octanethiolate monolayer on Au(111) exposed to gas-phase atomic hydrogen at 27 ○C. Reaction begins with th...

Hydrogenation of CO to Methanol on Ni(110) through Subsurface Hydrogen.

We present a combined theoretical and experimental study of CO hydrogenation on a Ni(110) surface, including studies of the role of gas-phase atomic hydrogen, surface hydrogen, and subsurface hydrogen reacting with adsorbed CO. Reaction mechanisms leading both to methane and methanol are considered. In the reaction involving surface or subsurface hydrogen, we investigate four possible pathways, using density functional theory to characterize the relative energetics of each intermediate, including the importance of further hydrogenation versus C-O bond breaking, where the latter may lead to methane production. The most energetically favorable outcome is the production of methanol along a pathway involving the sequential hydrogenation of CO to a H3CO* intermediate, followed by a final hydrogenation to give methanol. In addition, we find that subsurface hydrogen noticeably alters reaction barriers, both passively and through the energy released by diffusion to the surface. Indeed, the effective reaction barriers are even lower than for CO methanolation on Cu(211) and Cu(111) than for Ni(110). In studies of gas-phase H atoms impinging on a CO-adsorbed Ni(110) surface, Born-Oppenheimer molecular dynamics simulations show that direct impact of H is unlikely to result in hydrogenation of CO. This means that Eley-Rideal or hot-atom mechanisms are not important; thus, thermal reactions involving subsurface hydrogen are the primary reaction mechanisms leading to methanol. Finally, we demonstrate experimentally for the first time the production of methanol and formaldehyde from CO hydrogenation on Ni(110) and confirm the role of subsurface hydrogen in the mechanism of this reaction.

A computational study on the adsorption configurations and reactions of SiHx(x = 1-4) on clean and H-covered Si(100) surfaces

Possible adsorption configurations of H and SiHx (x=1−4) on clean and H-covered Si(100) surfaces are determined by using spin-polarized DFT calculations. The results show that, on the clean surface, the gas-phase hydrogen atom and SiH3 radicals effectively adsorb on the top sites, while SiH and SiH2 prefer the bridge sites of the first layer. Another possibility for SiH is to reside on the hollow sites with a triple-bond configuration. For a partially H-coverd Si(100) surface, the mechanism is similar but with higher adsorption energies in most cases. This suggests that the surface species become more stable in the presence of surface hydrogens. The minimum energy paths for the adsorption/migration and reactions of H/SiHx species on the surfaces are explored using the climbing image-nudged elastic band method. The competitive surface processes for Si thin-film formation from SiHx precursors are also predicted. The study reveals that the migration of hydrogen adatom is unimportant with respect to leaving open surface sites because of its high barriers (>29.0kcal/mol). Alternatively, the abstraction of hydrogen adatoms by H/SiHx radicals is more favorable. Moreover, the removal of hydrogen atoms from adsorbed SiHx, an essential step for forming Si layers, is dominated by abstraction rather than the decomposition processes.

Hydrogen spillover mechanism on covalent organic frameworks as investigated by ab initio density functional calculation

The hydrogen spillover mechanism, including the H chemisorption, diffusion, and H(2) associative desorption on the surface of COFs and H atoms migration from metal catalyst to COFs, have been studied via density functional theory (DFT) calculation. The results described herein show that each sp(2) C atom on COFs' surface can adsorb one H atom with the bond length d(C-H) between 1.11 and 1.14 Å, and the up-down arrangement of the adsorbed H atoms is the most stable configuration. By counting the chemisorption binding sites for these COFs, we can predict the saturation storage densities. High hydrogen storage densities show that the gravimetric uptakes of COFs are in the range of 5.13-6.06 wt%. The CI-NEB calculations reveal that one H atom diffusing along the C-C path on HHTP surface should overcome the 1.41-2.16 eV energy barrier. We chose tetrahedral Pt(4) cluster and HHTP as the representative catalyst and substrate, respectively, to study the H migration from metal cluster to COFs. At most, two H atoms can migrate from Pt(4) cluster to HHTP substrate. The migration reaction is an endothermic process, undergoing an activation barrier of 1.87 eV and 0.57 eV for the first and second H migration process, respectively. Three types of H(2) associative desorption from hydrogenated COFs were studied: (I) the two H adatoms recombining to one H(2) molecule with a recombination barrier of 4.28 eV, (II) the abstraction of adsorbed H atoms by gas-phase hydrogen atoms through ER type recombination reactions with a recombination barrier of 1.05 eV, (III) the H(2) desorption through the reverse spillover mechanism with an energy barrier of 2.90 eV.

HYDROGENATION OF PAH CATIONS: A FIRST STEP TOWARD H 2 FORMATION

Molecular hydrogen is the most abundant molecule in the universe. A large fraction of H2 forms by association of hydrogen atoms adsorbed on polycyclic aromatic hydrocarbons (PAHs), where formation rates depend crucially on the H sticking probability. We have experimentally studied PAH hydrogenation by exposing coronene cations, confined in a radiofrequency ion trap, to gas phase atomic hydrogen. A systematic increase of the number of H atoms adsorbed on the coronene with the time of exposure is observed. Odd coronene hydrogenation states dominate the mass spectrum up to 11 H atoms attached. This indicates the presence of a barrier preventing H attachment to these molecular systems. For the second and fourth hydrogenation, barrier heights of 72 +- 6 meV and 40 +- 10 meV, respectively are found which is in good agreement with theoretical predictions for the hydrogenation of neutral PAHs. Our experiments however prove that the barrier does not vanish for higher hydrogenation states. These results imply that PAH cations, as their neutral counterparts, exist in highly hydrogenated forms in the interstellar medium. Due to this catalytic activity, PAH cations and neutrals seem to contribute similarly to the formation of H2.

Adatoms at the Sulfur–Gold Interface in 1-Adamantanethiolate Monolayers, Studied Using Reaction with Hydrogen Atoms and Scanning Tunneling Microscopy

Scanning tunneling microscopy is used to study monolayers of 1-adamantanethiolate as they are exposed to gas-phase atomic hydrogen. H-atom reaction results in complete removal of the organic monolayer. The relaxation of the reconstruction present at the gold–sulfur interface results in the formation of gold-atom islands, as well as the addition of gold atoms to extant surface defects such as steps and pits. Characterization of these changes shows that for 1-adamantanethiolate monolayers, 0.18 ± 0.033 monolayers of gold adatoms participate in bonding with thiolate sulfur atoms. This results in a 1:1 Au:S ratio, in contrast to the 1:2 Au:S ratio reported for n-alkanethiolate monolayers. The difference in adatom density implies a qualitative difference in binding between n-alkanethiols and 1-adamantanethiols.

On multiple adsorptions of hydrogen atoms on graphene

First-principles calculations using the Vienna Ab Initio Simulation Package (VASP) have been performed in order to scrutinize the hydrogen–graphene interaction. Emphasis has been placed on how surface relaxation and the prior chemisorption of one, two and three hydrogen atoms on graphene affect the adsorption properties of an encroaching gas phase hydrogen atom. Chemisorption at the para site was found to be barrierless, while it has been shown that a stable ortho adsorbate can form directly from the gas phase. Adsorption of the third and fourth H atoms was found in all cases to have a significant barrier. The resultant minimum energy structures show a reasonable agreement with scanning tunneling microscopy images obtained by Hornekær et al (2007 Chem. Phys. Lett.446 237).

Interaction of Gas-phase Atomic Hydrogen with Chemisorbed Oxygen Atoms on a Silicon Surface

The reaction of gas-phase atomic hydrogen with oxygen atoms chemisorbed on a silicon surface is studied by use of the classical trajectory approach. We have calculated the probability of the OH formation and energy deposit of the reaction exothermicity in the newly formed OH in the gas-surface reaction H(g) + O(ad)/Si → OH(g) + Si. All reactive events occur in a single impact collision on a subpicosecond scale, following the EleyRideal mechanism. These events occur in a localized region around the adatom site on the surface. The reaction probability is dependent upon the gas temperature and shows the maximum near 1000 K, but it is essentially independent of the surface temperature. The reaction probability is also independent upon the initial excitation of the O-Si vibration. The reaction energy available for the product state is carried away by the desorbing OH in its translational and vibrational motions. When the initial excitation of the O-Si vibration increases, translational and vibrational energies of OH rise accordingly, while the energy shared by rotational motion varies only slightly. Flow of energy between the reaction zone and the solid has been incorporated in trajectory calculations, but the amount of energy propagated into the solid is only a few percent of the available energy released in the OH formation.

Alkanethiol Monolayers Contain Gold Adatoms, and Adatom Coverage Is Independent of Chain Length

The nature of the sulfur−gold bond in alkanethiol films self-assembled on Au(111) surfaces is widely contested, despite significant interest in these systems over the last 25 years. Recent theoretical and experimental studies have suggested gold adatoms are incorporated into the alkanethiol−gold interface. We have exposed alkanethiol self-assembled monolayers (SAMs) to gas-phase hydrogen atoms to remove the monolayer; the gold adatoms remain on the surface and form features that we observe using scanning tunneling microscopy (STM). The features include the formation of single-atom-thick gold islands, decreasing size of surface vacancy pits, and faceting of terrace step edges; compared to the alkanethiol-terminated surface, these features indicate a net increase in the amount of gold present on the surface, compared to the alkanethiol-terminated bulk structure. Varying the length of the alkane chain does not affect the total adatom coverage, as the adatom coverages for ethanethiol (0.172 ± 0.039), octaneth...

Quantum Dynamics of the Eley−Rideal Hydrogen Formation Reaction on Graphite at Typical Interstellar Cloud Conditions

Eley-Rideal formation of hydrogen molecules on graphite, as well as competing collision induced processes, are investigated quantum dynamically at typical interstellar cloud conditions, focusing in particular on gas-phase temperatures below 100 K, where much of the chemistry of the so-called diffuse clouds takes place on the surface of bare carbonaceous dust grains. Collisions of gas-phase hydrogen atoms with both chemisorbed and physisorbed species are considered using available potential energy surfaces (Sha et al., J. Chem. Phys.2002 116, 7158), and state-to-state, energy-resolved cross sections are computed for a number of initial vibrational states of the hydrogen atoms bound to the surface. Results show that (i) product molecules are internally hot in both cases, with vibrational distributions sharply peaked around few (one or two) vibrational levels, and (ii) cross sections for chemisorbed species are 2-3x smaller than those for physisorbed ones. In particular, we find that H(2) formation cross sections out of chemically bound species decrease steadily when the temperature drops below approximately 1000 K, and this is likely due to a quantum reflection phenomenon. This suggests that such Eley-Rideal reaction is all but efficient in the relevant gas-phase temperature range, even when gas-phase H atoms happen to chemisorb barrierless to the surface as observed, e.g., for forming so-called para dimers. Comparison with results from classical trajectory calculations highlights the need of a quantum description of the dynamics in the astrophysically relevant energy range, whereas preliminary results of an extensive first-principles investigation of the reaction energetics reveal the importance of the adopted substrate model.

An embedded cluster study of the formation of water on interstellar dust grains

The formation of water in the interstellar medium from hydrogen and oxygen atoms on a pristine olivine surface (forsterite (010)) is investigated with an embedded cluster approach. The 55-atom quantum cluster is described at the density functional level while the remaining 1629 atoms of the surface cluster are described with atomistic potentials. Transition states are most easily calculated with our modified implementation of the climbing-image nudged elastic band method in ChemShell. With these computational techniques, we find that gas phase hydrogen atoms can chemisorb (-102 kJ mol(-1)) without an activation barrier on the forsterite (010) surface, concomitantly creating a surface electron at the adjacent magnesium atom site. Subsequently, an oxygen atom chemisorbs strongly to this surface electron site (-432 kJ mol(-1)). The rearrangement of the adjacently chemisorbed O and H to a chemisorbed OH-radical is endothermic by 4 kJ mol(-1) and activated by 27 kJ mol(-1). This chemisorbed OH can then react barrierlessly with a second hydrogen atom to yield adsorbed water (-511 kJ mol(-1)). Alternatively, if O and H do not recombine to form OH, but instead thermally equilibrate, a second hydrogen atom can react with the chemisorbed oxygen atom (-501 kJ mol(-1)) to yield dissociatively adsorbed water (OH(-) and H(+)), which then can rearrange to associatively adsorbed water (-5 kJ mol(-1), DeltaE(double dagger) = 18 kJ mol(-1)) or gas phase water (+91 kJ mol(-1)). The formation of water on a bare dust grain from hydrogen and oxygen atoms is thus catalysed by an olivine surface by stabilising the reaction intermediates and product. Since the reaction proceeds via three chemisorbed intermediates, thermal equilibration is facilitated and back-dissociation of the freshly formed reaction products OH and H(2)O would not occur as frequently as it would in the gas phase or when the reactants are physisorbed on a surface.

Reaction between Gas-phase Hydrogen Atom and Chemisorbed Bromine Atoms on a Silicon(001)-(2X1) Surface

Electron transfer of a redox protein at a bare gold electrode is too slow to observe the redox peaks. A novel Nafion-riboflavin functional membrane was constructed during this study and electron transfer of cytochrome c, superoxide dismutase, and hemoglobin were carried out on the functional membrane-modified gold electrode with good stability and repeatability. The immobilized protein-modified electrodes showed quasireversible electrochemical redox behaviors with formal potentials of 0.150, 0.175, and 0.202 V versus Ag/AgCl for the cytochrome c, superoxide dismutase and hemoglobin, respectively. Whole experiment was carried out in the 50 mM MOPS buffer solution with pH 6.0 at 25 oC. For the immobilized protein, the cathodic transfer coefficients were 0.67, 0.68 and 0.67 and electron transfer-rate constants were evaluated to be 2.25, 2.23 and 2.5 s?1, respectively. Hydrogen peroxide concentration was measured by the peroxidase activity of hemoglobin and our experiment revealed that the enzyme was fully functional while immobilized on the Nafion-riboflavin membrane.

Degradation of octanethiol self-assembled monolayers from hydrogen-atom exposure: A molecular-scale study using scanning tunneling microscopy

Octanethiol self-assembled monolayers were exposed to gas-phase hydrogen atoms, and the resulting changes in the order and chemical structure of the surface were monitored using scanning tunneling microscopy (STM). Extensive damage to the monolayer was observed in the form of both dark and bright features in STM images. These changes began along domain boundaries and moved into close-packed regions of the monolayer as hydrogen-atom exposure time increased. Increasing exposure also results in an accelerated rate of observed surface changes, indicating that the reactivity of the surface increases as a result of initial gas-surface reactions. Complex restructuring of the alkanethiol monolayer is observed, including defect formation and the disordering of the alkanethiol monolayer. However, in some cases the monolayer demonstrates the capability of self-healing, with local annealing and reordering of close-packed domains. This annealing and reordering likely results from increased mobility of surface-bound alkanethiolates in the vicinity of monolayer defects, or from diffusion and readsorption of transiently formed alkanethiol molecules.

Reactions of Gas-Phase Atomic Hydrogen with Chemisorbed Hydrogen on a Graphite Surface

The reaction of gas-phase hydrogen atoms H with H atoms chemisorbed on a graphite surface has been studied by the classical dynamics. The graphite surface is composed of the surface and 10 inner layers at various gas and surface temperatures (Tg, T s ). Three chains in the surface layer and 13 chains through the inner layers are considered to surround the adatom site. Four reaction pathways are found: H 2 formation, H-H exchange, H desorption, and H adsorption. At (1500 K, 300 K), the probabilities of H 2 formation and H desorption are 0.28 and 0.24, respectively, whereas those of the other two pathways are in the order of 10 -2 . Half the reaction energy deposits in the vibrational motion of H 2 , thus leading to a highly excited state. The majority of the H 2 formation results from the chemisorption-type H(g)-surface interaction. Vibrational excitation is found to be strong for H 2 formed on a cold surface (∼10 K), exhibiting a pronounced vibrational population inversion. Over the temperature range (10-100 K, 10 K), the probabilities of H 2 formation and H-H exchange vary from 0 to ∼0.1, but the other two probabilities are in the order of 10 -3 .

Interaction of gas phase atomic hydrogen with Pt(111): Direct evidence for the formation of bulk hydrogen species

Employing hot tungsten filament to thermal dissociate molecular hydrogen, we generated gas phase atomic hydrogen under ultra-high vacuum (UHV) conditions and investigated its interaction with Pt(111) surface. Thermal desorption spectroscopy (TDS) results demonstrate that adsorption of molecular hydrogen on Pt(111) forms surface Had species whereas adsorption of atomic hydrogen forms not only surface Had species but also bulk Had species. Bulk Had species is more thermal-unstable than surface Had species on Pt(111), suggesting that bulk Had species is more energetic. This kind of weakly-adsorbed bulk Had species might be the active hydrogen species in the Pt-catalyzed hydrogenation reactions.

FTIR and computational studies of gas-phase hydrogen atom abstraction kinetics by t-butoxy radical

By using Fourier-Transform Infrared (FTIR) absorption spectroscopy, rate coefficients in the range of 10 −16 to 10 −14 cm 3 molecule −1 s −1 have been determined for the hydrogen atom abstraction reactions of several substrates including halogenated organic compounds and amines by t-butoxy radical generated from the uv photolysis of t-butyl nitrite in the gas phase. Arrhenius parameters for selected reactions have been measured in the temperature range 299–318 K. Transition states and activation barriers for such reactions have been computed with the help of G aussian 03 software and found to match very well with the experimental values.

Displacement of metal atoms from salts by hydrogen and the role of this reaction in chain processes

The interaction of hydrogen atoms with a variety of alkali metal and alkaline-earth metal salts results not only in the recombination of these atoms but also in the displacement, into the gas phase, of free radicals (CaCl·(A 1 P 1/2, B 2 S +) and CaF·(A 2 P)) and metal atoms, including their excited species, which are detected spectroscopically. Transmission spectra indicate that the NaCl surface undergoes metallization when treated with a high-frequency discharge and a rarefied hydrogen flame. Combustion is affected by the gas-phase hydrogen atoms involved in the chain reaction and by the varying composition and properties of the surface. The concentration of Na atoms over the NaCl surface at 770 K is 109−1011 cm−3 in a stream of H atoms at 1 Torr and in the 2H2 + O2 flame at 4 Torr. The concentration of sodium atoms in the 2 P 3/2 and 2 P 1/2 excited states is ∼5 × 106−5 × 108 cm−3. The role of the discovered reactions in combustion, pyrolysis, and plasma chemistry is discussed.

Laser Spectroscopic Excitation Function and Reaction Threshold Studies of the H + DCl → HCl + D Gas-Phase Isotope Exchange Reaction

The dynamics of the gas-phase hydrogen atom exchange reaction H + DCl --> HCl + D were studied using the pulsed laser photolysis/laser induced fluorescence "pump-and-probe" method. Laser photolysis of H2S at 222 nm was used to generate nonequilibrium distributions of translationally excited hydrogen atoms at high dilution in a flowing moderator gas (Ar)/reagent (DCl) mixture. H and D atoms were detected with sub-Doppler resolution via Lyman-alpha laser induced fluorescence spectroscopy, which allowed the measurement of the line shapes of the moderated H atom Doppler profiles as well as the concentration of the D atoms produced in the H + DCl --> HCl + D reaction. From the measured H atom Doppler profiles, the time evolution of the initially generated nascent nonequilibrium H atom speed distribution toward its room-temperature thermal equilibrium form was determined. In this way, the excitation function and the reaction threshold (E0 = 0.65 +/- 0.13 eV) for the H + DCl --> HCl + D reaction could be determined from the measured nonequilibrium D atom formation rates and single collision absolute reaction cross-section values of 0.12 +/- 0.04 A2 and 0.45 +/- 0.11 A2 measured at reagent collision energies of 1.0 and 1.4 eV, respectively.