Removal Of Hydrogen Atom Research Articles

DFT studies for cleavage of CC and CO bonds in surface species derived from ethanol on Pt(111)

Results from self-consistent periodic DFT calculations were used to study the relative stabilities and reactivities of surface species on Pt(111) derived by subsequent removal of hydrogen atoms from ethanol. Within each C 2OH x isomeric set, the lowest energy surface species (with respect to gaseous ethanol and clean Pt(111) slabs) are ethanol, 1-hydroxyethyl (CH 3CHOH), 1-hydroxyethylidene (CH 3COH), acetyl (CH 3CO), ketene (CH 2CO), ketenyl (CHCO), and CCO species. The energies of these species are −27, −28, −55, −84, −82, −88, and −53 kJ/mol, respectively, where the corresponding H atoms removed from ethanol are adsorbed on separate Pt(111) slabs. Transition states for CC and CO bond cleavage reactions were calculated for the most stable intermediates and for intermediates leading to exothermic bond cleavage reactions. A linear correlation between the energies of transition state and the energies of corresponding surface species was used to estimate transition-state energies of remaining reaction intermediates. The 1-hydroxyethylidene (CH 3COH) species has the lowest energy transition state (42 kJ/mol) for CO bond cleavage, and the adsorbed ethylidyne (CCH 3) and hydroxyl product species lead to a favorable energy change for this CO bond cleavage reaction (−38 kJ/mol). The ketenyl (CHCO) species has the lowest energy transition state (4 kJ/mol) for CC bond cleavage, and the adsorbed CO and methylidyne (CH) product species lead to a very exothermic energy change for this reaction (−144 kJ/mol). Results from DFT calculations, combined with transition state theory, predict that the rate constant for CC bond cleavage in ethanol is faster than for CO bond cleavage on Pt(111) at temperatures higher than about 550 K. In addition, the calculated value of the rate constant for CC bond cleavage in ethanol is predicted to be much higher than for CC bond cleavage in ethane on Pt(111). Similarly, the rate of CO bond cleavage in ethanol is predicted to be much higher than for CO bond cleavage in carbon monoxide on Pt(111).

The role of submicrometer aerosols and macromolecules in H 2 formation in the titan haze

Previous studies of the photochemistry of small molecules in Titan’s atmosphere found it difficult to have hydrogen atoms removed at a rate sufficient to explain the observed abundance of unsaturated hydrocarbons. One qualitative explanation of the discrepancy nominated catalytic aerosol surface chemistry as an efficient sink of hydrogen atoms, although no quantitative study of this mechanism was attempted. In this paper, we quantify how haze aerosols and macromolecules may efficiently catalyze the formation of hydrogen atoms into H 2. We describe the prompt reaction model for the formation of H 2 on aerosol surfaces and compare this with the catalytic formation of H 2 using negatively charged hydrogenated aromatic macromolecules. We conclude that the PRM is an efficient mechanism for the removal of hydrogen atoms from the atmosphere to form H 2 with a peak formation rate of ∼ 70 cm −3 s −1 at 420 km. We also conclude that catalytic H 2 formation via hydrogenated anionic macromolecules is viable but much less productive (a maximum of ∼ 0.1 cm −3 s −1 at 210 km) than microphysical aerosols.

Kinetics and Reaction Pathways for Propane Dehydrogenation and Aromatization on Co/H-ZSM5 and H-ZSM5

Co/H-ZSM5 catalyzes propane dehydrogenation and aromatization reactions. Initial product selectivities, product site-yields, and the 13C content and distribution in the products of 2-13C-propane show that propane undergoes two primary reactionsdehydrogenation to propene and H2 and cracking to methane and ethene. Propene and ethene then form aromatics via oligomerization−cracking reactions and both ethene and propene hydrogenate to form ethane and propane, respectively, via both hydrogen transfer from coadsorbed intermediates and dissociative adsorption of H2. These reaction pathways resemble those occurring on H-ZSM5, but Co cations provide an alternate pathway for the removal of hydrogen atoms in adsorbed intermediates as H2. A kinetic model was used to describe experimental rates based on these observations and to obtain rate constants for individual reaction steps as a function of Co content and H2 concentration. H2 inhibits propane dehydrogenation to propene and alkene conversion to aromatics, and inc...

Effects of Rapid Thermal Annealing on the Electrical Properties of Cobalt Contact to p-GaN

The effects of rapid thermal annealing (RTA) on Co/p-GaN contacts in an O2/N2 atmosphere without intermediate metal were investigated. It was observed that the contact resistance (ρc) decreased with increasing RTA temperature and that the resistance was reduced by a factor of about 30 after RTA at 600°C in the O2/N2 atmosphere. The specific minimum contact resistance was in the 10-2 Ω·cm2 range. Comparison of the R0 and ρc values revealed that the rapid thermal annealing in the O2/N2 was more effective for reducing the contact resistance than conventional furnace annealing. The reason for the reduction of resistance was expected to be the increase of hole concentration due to the highly reactivated removal of hydrogen atoms in the p-GaN by RTA in the O2/N2 atmosphere.

Preparation of ferromagnetic carbon electrode from charcoal blocks

A ferromagnetic carbon electrode, if it really exists, will have an interesting role in the electrochemistry because it attracts the paramagnetic species such as free radicals or ion radicals and still it may keep the stabilities that carbon has. Since ferromagnetic carbon has been prepared in a powdery form, to extend that technique for the preparation of a solid block of ferromagnetic carbon, have been expected in the present work. A wood charcoal piece was selected as the skelton of the ferromagnetic electrode. An aromatic hydrocarbon was adsorbed by the charcoal piece. Hydrogen reduction and vacuum heating was applied to this charcoal piece. Removing hydrogen atoms from both charcoal and the hydrocarbon were carried out in a CCl4 atmosphere at 550°C. The results have shown that mechanically strong ferromagnetic carbon electrode can be obtained by the method described in this report.

Influence of molecular flexibility on DNA radiosensitivity: a simulation study.

Radiation damage in DNA is caused mainly by hydroxyl radicals which are generated by ionizing radiation in water and removing hydrogen atoms from the DNA chain. This damage affects certain nucleotide sequences more than others due to differences in the local structure of the DNA chains. This sequence dependence has been analyzed experimentally and calculated theoretically for a rigid DNA model. In this paper we take into account the flexibility of the DNA chain and show how it modifies the strand breakage probabilities. We use a simple harmonic model for DNA flexibility which permits the study of a long (68 base pair) fragment with modest computational effort. The essential influence of flexibility is an increased breakage probability towards the ends of the fragment, which can also be identified in the experimental data.

Chemical Reaction of Intercalated Atoms at the Edge of Nano-Graphene Cluster

The chemical reactions of halogen molecules on a nano-graphite cluster are calculated using a semi-empirical calculational method. A real time calculation of the chemical reaction of halogen molecules up to 200 fs which is performed by the dynamic reaction coordinates (DRC) method of the quantum chemistry library shows that the iodine molecule exhibits a special reaction involving the removal of hydrogen atoms from edge carbon atoms. This result might be relevant to recent experiments on the graphitization of pitch at a low temperature of 400°C. We discuss the deformation of the nano-graphite cluster upon the intercalation of halogen atoms.

A computational study of bond dissociation enthalpies and hydrogen abstraction energy barriers in model urethanes

Ab initio energies obtained at the MP2/6-311+G(3df,2p)//MP2/6-31G(d) level of theory were employed using isodesmic reactions to compute C–H and N–H room temperature bond dissocation enthalpies (BDEs), for the various hydrogens in a series of model alkyl urethanes (carbamates). It was found that the BDEs [in kJ/mol] vary in the order α-C(N) [402]<α-C(O) [417]<β-C(N) [425]≈β-C(O)≪N–H [465] (where α-C(N) refers to the α-carbon on the nitrogen side of the functional group, etc.). The unusually high N–H BDE was found to result from the loss of stabilization via conjugation with the carbonyl's π-electrons in the radical. The low C–H bond strengths on both the α-carbons were attributed to resonance stabilization of the radical center by the nitrogen or oxygen lone-pair electrons. These results, in which it is predicted that the C–H on the α-C(N) is the weakest bond, are in accordance with the results of a recent experimental study on photodecomposition of polyurethanes, where it was concluded that initial radical attack is at this site. It was also found that C–H BDEs on the α-carbons increase when a methyl group is added at this position. This trend was rationalized on the basis of lessened resonance stabilization of the branched radicals.Transition state structures and energies, and reaction energy barriers, ΔE0‡, were computed for hydrogen abstraction reactions by a methyl radical, RH+CH3→R+CH4, for the C–H and N–H bonds in the four model urethane. There was an excellent correlation between ΔE0‡ and BDE for all hydrogens on the linear alkyl urethanes. However, energy barriers for the removal of hydrogen atoms from the α-carbons of the two branched urethanes lay significantly below the curve established by the linear species. This observation was explained on the basis of comparative structures of the parent compounds, transition states and radicals. As ΔE0‡[α-C(N)] was lower for the branched than the linear urethane, it was concluded that one should not expect any significant enhancement of the photochemical stability of branched aliphatic polyurethane coatings relative to their linear counterparts.

The initiation and characterization of single bimolecular reactions with a scanning tunneling microscope.

A scanning tunneling microscope (STM) operating at 9 K in ultrahigh vacuum was used to initiate a bimolecular reaction between isolated hydrogen sulfide and dicarbon molecules on the Cu(001) surface. The reaction products ethynyl (CCH) and sulfhydryl (SH) were identified by inelastic electron tunneling spectroscopy (STM-IETS) and by sequentially removing hydrogen atoms from an H2S molecule using energetic tunneling electrons. For comparison, the thermal diffusion and reaction of H2S and CC at 45 K and H2O and CC at 9 K were also observed.

Link between hot electrons and interface damage in n-MOSFETs: A Monte Carlo analysis

Typical measurements on n-channel MOSFETs show that interface damage, causing threshold and transconductance shifts, tends to peak when V GS≈0.5 V DS. In such conditions, more impact ionization (substrate current) is measured. At higher gate voltage, one expects that channel hot electrons would impact the interface at increased energy and numbers. Therefore, it has normally been assumed that hot carriers in the channel cannot directly contribute to damage, and impact-generated holes must be injected into the oxide for damage to take place. However, there is increasing experimental evidence that hot electrons should be able to remove hydrogen atoms which passivate interface dangling bonds, even through a multiple collision process, at energies below oxide-injection threshold (typically 3.0 eV). A detailed full-band Monte Carlo analysis shows that in a typical 0.25 μm gate device there are, indeed, more energetic electrons hitting the surface when V GS= V DS than at the condition of maximum impact damage V GS≈0.5 V DS. However, the electrons impact the surface at much shallower angles, with lower chance of perpendicular momentum transfer to the interface. This analysis reconciles damage measurements with the physical picture suggesting direct damage generation by energetic electrons.

Conductance of a finite missing hydrogen atomic line on Si(001)-(2×1)-H

We present the results of a calculation of zero-temperature elastic conductance through a finite ``atomic wire'' between Au pads, all supported by a Si(001)-(2\ifmmode\times\else\texttimes\fi{}1)-H surface. The atomic wire consists of a line of dangling bonds which can be fabricated by removing hydrogen atoms by applying voltage pulses to a scanning tunneling microscopy (STM) tip along one side of a row of H-passivated silicon dimers. Two different line configurations, without and with Peierls distortion, have been considered. We find that the nondistorted line behaves like a single ballistic transmission channel. Conversely, with Peierls distortion present, tunneling occurs through the small resulting energy gap $(0.2 \mathrm{eV}),$ leading to inverse decay length of the current of $0.09 {\AA{}}^{\ensuremath{-}1}.$ The conductance of the substrate between the pads without the defect line has also been calculated. In this case, tunneling occurs through a much wider energy gap and a larger inverse decay length of $0.41 {\AA{}}^{\ensuremath{-}1}.$ These fully three-dimensional atomistic computations represent an application of the electron-scattering quantum-chemistry method which was previously used to calculate the conductance of ``molecular wires'' and of STM junctions with various adsorbates. Compared to molecular wires previously investigated by the same method, the atomic wire studied here exhibits the smallest inverse decay length.

Effects of annealing in an oxygen ambient on electrical properties of ohmic contacts to p-type GaN

Effects of annealing ambient of an oxygen and nitrogen mixed gas on the electrical properties were studied for Au-based ohmic contacts (NiAu, CoAu, CuAu, PdAu, and PtAu) to p-type GaN. Addition of oxygen to the nitrogen gas reduced the specific contact resistances (ρc) and resitivities of the p-GaN epilayers (ρs) of the contacts after annealing at temperatures of 500–600°C. The microstructural analysis at the p-GaN/metal interfaces did not detect the heterostructural intermediate semiconductor layer at the GaN/metal interfaces. The reason for reduction of both the ρc and ρs values by the oxygen gas addition was believed to be due to formation of the p-GaN epilayer with high hole concentrations, caused by removal of hydrogen atoms which bonded with Mg atoms.

Nanolithography by selective chemical vapor deposition with an atomic hydrogen resist

We report the fabrication of Al nanostructures using selective chemical vapor deposition (CVD) growth and an atomic hydrogen resist. A scanning tunneling microscope is used to pattern the hydrogen terminated surface by local removal of hydrogen atoms. The high selectivity of the CVD process limits Al growth to the uncovered regions. We demonstrate the fabrication of Al features as small as 2 nm.

Stable anionic sites on hydrogenated (111) surfaces of cubic boron nitride resulting from hydrogen atom removal under chemical vapor deposition conditions

In plasma or hot-filament assisted chemical vapor deposition of cubic boron nitride (cBN), the formation of a surface radical site by hydrogen atom removal from a hydrogenated (111) surface was expected to be followed by the formation of an anionic vacant site (AVS) by capturing an electron, since this yields a closed shell structure of the site atom. This possibility was investigated by ab initio as well as by semiempirical molecular orbital calculations using large cluster models. The stabilization energy of the AVS on cBN(111) with a boron top layer, namely (111) B, was 1.8–2.3 eV; this value is comparable with that of diamond. On the other hand, that of cBN(111) on a nitrogen surface, namely (111) N, amounted to 5.3–5.9 eV. The nucleophilic nature of the AVS with a lone pair of electrons suggests electrophilic reagents for related reactions. This helped us to propose SN2 growth reactions for diamond growth both on diamond and cBN(111) surfaces. The difference in chemical reactivity between (111) N and (111) B could be interpreted in terms of the “hard” and “soft” acid base concept that has been renewed by the frontier orbital theory.

Venus Lyα: A Morphological and Radiative Transfer Analysis

The Venus Lya corona is caused by resonance scattering of the solar 1215.67 A Lya line by hydrogen atoms in the Venus upper atmosphere. The atmospheric atomic hydrogen content is probed remotely via Lya observations. On 1990 February 10 the Galileo spacecraft flew by Venus, obtaining a series of Venus scans with the Ultraviolet Spectrometer Experiment. The Pioneer Venus (PV) Orbiter Ultraviolet Spectrometer obtained Venus Lya images approximately weekly throughout its 14 year mission (1978–1992), spanning the 11 year solar cycle. I develop a morphology of the behavior of the Venus Lya signal with respect to location on Venus and solar activity level. The predawn hydrogen density enhancement extends to high latitudes (160 ). I find an equatorial minimum of hydrogen and evidence for a polar hood of enhanced hydrogen abundance. This hood may be produced by a hydrogen influx from the predawn region that is not removed as efficiently as hydrogen in the subsolar region. All features examined persist throughout the solar cycle and increase in hydrogen abundance with solar activity. A search for small-scale (1000 km) features produced a null result. However, the timescale for ballistic transport of atomic hydrogen in the exosphere is ∼8 minutes, too short for the detection of nonequilibrium density features. I analyze the data using a two-dimensional nonisothermal complete-frequency-redistribution multiple-scattering code modified from the LYAB code provided by James Bishop for the geocorona. I employ the VTS3 neutral thermosphere model (A. Hedin, H. Niemann, W. Kasprzak, & A. Seiff, J. Geophys. Res., 88, 73 [1983]) and calculate diffusive profiles for the vertical distribution of atomic hydrogen, characterized by the hydrogen number density and vertical flux at the exobase n f 0 0 (L. Paxton, D. Anderson, & A. Stewart, J. Geophys. Res., 93, 1766 [1988]). The flux parameter regulates the amount of hydrogen in the lower thermosphere, and the exobase density controls the amount in the upper thermosphere and exosphere. The almost linear relationship between the PV Langmuir probe photoelectron current and the observed solar Lya output provides a daily proxy for solar illumination of Venus hydrogen. I determine the parameter values that best fit the data for selected segments of the sunlit disk and examine their behavior with respect to location and solar activity. Numerical results are limited to the region of spatially uniform temperature in the exosphere and thermosphere. For high solar activity ( ), subsolar values are F ∼ 200 n (5.6 10.7 0 cm 3 and cm 2 s , cor4 7 0.8) # 10 f (5.8 0.9) # 10 0 responding to hydrogen column densities of (2.0 0.3) # cm 2 above 200 km and between 135 12 13 10 (1.0 0.2) # 10 km and the CO2 Lya absorbing layer at 112 km. These values agree with the subsolar work of Paxton et al. and with densities derived from in situ measurements by H. Brinton, H. Taylor, H. Niemann, H. Mayr, A. Nagy, T. Cravens, & D. Strobel (Geophys. Res. Lett., 7, 865 [1980]). Exobase density, , den0 creases from a local solar time (LST) of 10 hr to 15 hr by to cm . At 12 hr LST, increases from the 4 4 7 # 10 4 # 10 n0 equator to 60 latitude by to cm . This exo4 4 4 # 10 7 # 10 base density minimum at low latitudes on the dayside is consistent with solar EUV heating driving the transport removal of hydrogen atoms from the dayside upper atmosphere. The hydrogen abundance in the lower thermosphere, , does not f0 vary significantly. However, because of the opacity of the overlying upper thermosphere and exosphere, the variation of hydrogen column abundance for the values observed would n0 not be detectable in . Both and increase with solar f n f 0 0 0 activity, and there is evidence suggesting solar cycle phase dependence. A detailed interpretation of these quantitative results requires a photochemical model of the Venus upper atmosphere. The details of suprathermal hydrogen atom production are essential to understanding the transport behavior of hydrogen in the upper thermosphere and exosphere.

Bifunctional Condensation Reactions of Alcohols on Basic Oxides Modified by Copper and Potassium

Alcohol dehydrogenation and condensation reactions are involved in chain growth pathways on Cu/MgCeOxpromoted with potassium. These pathways lead to the formation of isobutanol with high selectivity via reactions of higher alcohols with methanol-derived C1species in reaction steps also relevant to higher alcohol synthesis from CO/H2mixtures at higher pressures on K–Cu/MgCeOxcatalysts. Ethanol reactions on K–CuyMg5CeOxshow that both Cu and basic sites participate in alcohol dehydrogenation and aldol condensation steps leading to n-butyraldehyde and acetone. Chain growth occurs by condensation reactions involving a metal-base bifunctional aldol-type coupling of alcohols. Reactions of12C2H5OH–13C2H4O mixtures show that direct condensation reactions of ethanol can occur without requiring the intermediate formation of gas phase acetaldehyde. Reactions of C2H5OH/D2mixtures show that Cu sites increase the rate of aldol condensation by introducing recombinative desorption sites that remove hydrogen atoms formed in C–H activation steps leading to the unsaturated aldol-type species required for chain growth. Reactions of acetaldehyde and13C-labeled methanol lead predominantly to 1-13C-propionaldehyde and 2-13C-isobutyraldehyde, both of which lead to isobutanol during CO/H2reactions. Mixtures of propionaldehyde and13C-labeled methanol lead to singly-labeled isobutyraldehyde. Chain growth to C2+alcohols occurs via addition of a methanol-derived C1species to adsorbed oxygen-containing intermediates. The gradual appearance of13C in the unlabeled reactant within these mixtures shows that aldol coupling reactions are reversible. Reverse aldol condensation reactions after intramolecular hydride transfer lead to the formation of acetone from ethanol. Isobutyraldehyde is a preferred end-product of aldol-type chain growth reactions of alcohols because it lacks the two α-hydrogens required for subsequent chain growth.

The recombination of hydrogen atoms with nitric oxide at high temperatures

The rate constant for the H+NO+N2 reaction (R1,N2) has been determined in the temperature range 1000–1170K from flow-reactor experiments on the CO/O2/H2O/N2 system perturbed with different amounts of NO. The initiation temperature of this system is highly sensitive to reaction R1, which is the rate-controlling step in the nitric oxide catalyzed removal of hydrogen atoms. Based on the flow-reactor results and the limited amount of data reported in literature, a rate constant for the H+NO+N2 reaction of 4.0×1020T−1.75 cm6/(mol2s) was determined. This value is in good agreement with the recent result of Allen and Dryer at 1000K but significantly lower at high temperatures than the recommendation of Tsang and Herron. With the recently determined value of ΔHf,298 (HNO) of 26.0 kcal/mol, which is 2 kcal/mol higher than previous estimates, our results correspond to a rate constant of 1.7×1019T−1.5 exp(−23,400/T) cm3/(mol s) for the HNO+N2 dissociation reaction in the 1000–2500 K range. The sharp drop-off in the rate constant for H+NO+M at high temperatures suggested by the flow-reactor results are supported by reinterpretation of data reported in literature on H2/O2/N2 flames doped with NO. Theoretical considerations suggest that the effect can be attributed to weak-collision effects.

Stable anionic site on hydrogenated (111) surface of diamond resulting from hydrogen atom removal under chemical vapor deposition conditions

The stability of a deprotonated site on the hydrogenated (111) surface of diamond under chemical vapor deposition (CVD) conditions was studied by using ab initio and semiempirical molecular orbital (MO) methods. The formation of this sort of anionic vacant site (AVS) was expected to occur when a surface hydrogen atom is removed and the resultant radical site captures an electron; this yields a closed shell structure of the anionic carbon atom. The effects of model cluster size as well as basis set selection in the MO studies were also examined. The stabilization energy approached about 1.5–1.9 eV at the limit of the calculation; this surface localized state lies within the band gap of diamond as was expected. The spatial distribution of the highest occupied molecular orbital confirmed that the captured electron was localized around the AVS. The AVS formed on the hydrogenated (111) surface of diamond is worth taking into account in studying the CVD growth mechanism of diamond.

Unusual DAhydrations in anaerobic bacteria: considering ketyls (radical anions) as reactive intermediates in enzymatic reactions

DAhydratases have been DAtected in anaerobic bacteria which use 2-, 4- or 5-hydroxyacyl-CoA as substrates and are involved in the removal of hydrogen atoms from the unactivated β- or γ-positions. In addition there are bacterial DAhydratases acting on 1,2-diols which are substrates lacking any activating group. These enzymes contain either FAD, or flavins + iron-sulfur clusters or coenzyme B 12. It has been proposed that the overall DAhydrations are actually reductions followed by oxidations or vice versa mediated by these prosthetic groups. Whereas the γ-hydrogen of 5-hydroxyvaleryl-CoA is activated by a transient two-electron α,β-oxidation, the other substrates are proposed to require either a transient one-electron reduction or an oxidation to a ketyl (radical anion).

An experimental and theoretical study of Auger lineshapes in hydrogenated amorphous silicon structures

Simple models of hydrogenated amorphous silicon (a-Si : H) consisting of hypothetical silane molecules with diamond or similar lattices were studied by the semi-empirical AMI method. Valence band densities of states (VBDOS) were calculated for the silane molecules with and without the removal of hydrogen atoms. Silicon L 1L 2.3V Auger spectra have also been obtained using X-ray excitation from disordered crystalline silicon (c-Si), and glow discharge produced a-Si : H. Comparisons have been made between the experimental L 1L 1.2V Auger spectra of samples with different amounts of hydrogen on the surface and the corresponding VBDOS calculations. Ignoring differences due to matrix elements, good agreement is obtained between the experimental and theoretical spectra and it is shown how the theoretical curves can help interpret changes in the experimental spectra after various treatments. The results provide confirmation that disordering of c-Si leads to an increase in the number of Si p-like defect states at the top of the VB. Furthermore, it is shown that hydrogenation plays an important role in stabilising defects in a-Si by tying off dangling bonds, removing the Si p-like states at the VB edge. This leads to the establishment of new electron energy levels in the VB, with different methods of hydrogenation leading to different bonding configurations. It is shown how the success of these methods provides the basis for subsequent studies of annealing, hydrogenation and photodegradation of a-Si : H alloys.