High Hydrogen Coverage Research Articles

Chemistry of Methoxonium on (2 × 1)Pt(110)

In a previous paper, (Chen, N.; Blowers, P.; Masel, R. I. Surf. Sci. 1998, 418, 329−341.) we found that methoxonium ions (CH3OH2)+ can form when methanol and hydrogen coadsorb on Pt(110). In this paper, we characterize the chemistry of methoxonium on (2 × 1)Pt(110) using mainly TPD. We observe three reaction pathways: a simple dehydrogenation to CO and hydrogen; an SN1 pathway to form water, methane, and adsorbed CHx groups; and an SN2 pathway yielding dimethyl ether. The SN1 pathway shows a strong secondary isotope effect implying that the transition state is ionic. The SN2 pathway is seen at high hydrogen coverages and shows kinetics typical for an SN2 process. This work demonstrates for the first time that cationic species on surfaces react via the same SN1 and SN2 pathways seen during reactions of cations in solution.

The identification of hydrogen trapping states in an Al-Li-Cu-Zr alloy using thermal desorption spectroscopy

Thermal desorption spectroscopy (TDS) was utilized to identify several metallurgical states in an Al -2Li - 2Cu-0.1Zr (wt pct) alloy, which trap absorbed hydrogen. Six distinct metallurgical desorption states for hydrogen were observed for tempers varying from the T3 to peakaged condition. Lower energy thermal desorption states were correlated with interstitial sites, lithium in solid solution, and δ′ (Al3Li) precipitates. These states have trap-binding energies ≤25.2 kJ/mol. Under the charging conditions utilized, approximately 4 pct of the total (e.g., trapped and lattice) hydrogen content was associated with interstitial sites, consistent with the view that the intrinsic lattice solubility of hydrogen in aluminum is very low. In contrast, dislocations, grain boundaries, and T1 (Al2CuLi) particles were found to be higher energy-trap states with trap-binding energies ≥31.7 kJ/mol. Approximately 78 pct of all absorbed hydrogen occupied these states. Moreover, greater than 13 pct of the available trap sites at grain boundaries were occupied. Such a high hydrogen coverage at grain-boundary sites supports the notion that hydrogen contributes to grain-boundary environmental cracking in Al-Li-Cu-Zr alloys. Also, it points out the error in assuming that hydrogen cannot play a major role in cracking of Al-based alloys due to the low lattice solubility.

Annealing experiments on supercritical Si 1– xGe x layers grown by RPCVD

This paper presents a study of the relaxation mechanisms for SiGe epitaxy performed by reduced pressure chemical vapor deposition (RPCVD) in a commercially available single wafer epitaxial reactor. The samples were grown at 500°C and an overall pressure of 40 Torr, with SiH 2Cl 2 and GeH 4 as precursor gasses. In contrast to the situation for molecular beam epitaxy (MBE), we found that under RPCVD conditions the relaxation by 3D-growth was strongly suppressed. Relaxation then occurred by the development of misfit dislocations even in the case of high Ge-concentrations (>40%). This behavior is attributed to the high hydrogen coverage of the substrate surface during the RPCVD, which reduces the surface migration of the adsorbed species. Furthermore, the relaxation mechanism has been studied in annealing experiments where wave and island formation was observed. The annealing experiments clearly confirm that surface migration plays a significant role for 3D-growth. Using the annealing procedure layers of high island density and strong photoluminescence have been produced.

Ring opening of methylcyclopentane over Al2O3 and SiO2 supported Rh catalysts

Hydrogenolytic ring opening of methylcyclopentane (MCP) was investigated on Rh/Al2O3 and Rh/SiO2 catalysts, prepared by the incipient wetness method. Strong dependence can be seen in the yield and distribution of ring opening products as a function of temperature and hydrogen pressure. They depended also on the support used. The ring opening reaction required high hydrogen coverage, and was not random (hindered in the vicinity of the methyl group), thus, mainly 2-methylpentane (2MP) and 3-methylpentane (3MP) were formed. The fragments consisted of C1–C5 alkanes, with methane andi-pentane as main fragments. This means the possibility of breaking two C−C bonds during one sojourn of the reactant on the catalyst, both taking place far from the substituent. The loose positive correlation between the ratios ofi-pentane/n-pentane and 3MP/n-hexane seems to support this conclusion.

H adsorption and the formation of alane oligomers on Al(111)

Complementary scanning tunneling microscopy and surface infrared measurements show that H reacts strongly with Al(111), producing a variety of new alane (aluminum hydride) surface species. Alane oligomers, ranging in size from the monomer to 30-mers, form through a sequence of surface etching and condensation reactions. Atomic hydrogen initiates production by extracting aluminum atoms from the surface lattice to create mobile monohydride monomers (ad-AlH), which predominate in the low-coverage regime. At higher hydrogen coverages, multihydride oligomers form in coexistence with the ad-AlH. These alane oligomers are more thermally stable, remaining on the surface at room temperature, where they are directly imaged. The mass transfer of aluminum to surface alanes is discussed in relationship to alane stoichiometry.

Morphology changes of thin Pd films grown on SiO2: influence of adsorbates and temperature

Under certain conditions morphology changes occur when thin Pd films, grown on SiO2 at room temperature, are subject to elevated temperatures. First holes in the metal are observed, followed by network formation and finally isolation of metal islands. This process is known as agglomeration. The influence of gas exposures on this restructuring process has been studied by following variations in the capacitance of the structure and by atomic force microscopy, transmission electron microscopy and ultraviolet photoelectron spectroscopy. The capacitance measurements show that carbonaceous species have an impeding influence on the rate of agglomeration and may lock the film structure in a thermodynamic non-equilibrium state. By removing these species with oxygen exposure, i.e. by forming volatile CO and CO2, a clean surface is obtained and the agglomeration process can proceed. High oxygen or hydrogen coverages also lower the rate of restructuring, compared to the case of a clean surface. For the clean Pd surface, an apparent activation energy of 0.64 eV is found for the restructuring process.

Gas source homoepitaxy on Si(113) – the interrelation of H-induced reconstructions and growth morphology

High-temperature, time-lapsed STM-studies are used to investigate the surface morphology and the growth mode of gas source MBE of Si on Si(113) with disilane (Si 2H 6) as a precursor. A varying hydrogen coverage on the growing surface caused by the dynamic equilibrium between hydrogen supply from the precursor and thermal H desorption at 480°C results in several different reconstructions. Phases with (2×7) and (2×5) at a low flux of disilane and a (2×2) at high flux were distinguished that consist of only two basic elements: a low-H-coverage double-row structure and the gradual insertion of high-H-coverage (2×2)-like heavy domain walls. Above H coverages resulting in the (2×5) larger domains of (2×2) separate, for which a model is suggested. Besides an anisotropic shape of the Si islands in all stages of growth, a strong influence of the H-induced reconstructions on the silicon growth mode is found. Whereas step flow dominates for the lower H-coverage structure (2×7) and (2×5), the formation of the (2×2) leads to a massive nucleation of Si islands arranged in a quasi-periodic way. This difference in nucleation behaviour results in a mechanism for a layer-by-layer-like growth mode, even on a surface with a very high hydrogen coverage in contrast to Si(111).

Atomic and electronic structure of the diamond (100) surface: Reconstructions and rearrangements at high hydrogen coverage

The atomic and electronic structure of the diamond (100) surface has been investigated theoretically via a semiempirical tight-binding model for a range of hydrogen coverages. Model parameters for C-C interactions have been taken from the work of Goodwin [J. Phys. Condens. Matter 3, 3869 (1993)], while new parameter sets have been determined for C-H and H-H interactions. The model gives results for the clean and monohydrogenated surfaces in good agreement with previous studies, but different features have been identified for higher H coverages. When the H coverage is sufficiently high, the substrate lattice is found to distort in order to reduce steric repulsions between dihydride units. As an important example, we obtain two structures for the dihydrogenated surface that are significantly more stable than those proposed previously. For H coverages intermediate between the monohydrogenated and dihydrogenated surfaces, stable geometries consisting of monohydrogenated dimer units and dihydride units are found. In contrast, geometries that include isolated monohydride units, such as have been previously investigated, are found to be thermodynamically and kinetically unstable. Tight-binding molecular dynamics is used to illustrate a mechanism for the rapid removal of isolated monohydride units. The electronic structures of the surfaces are described via the total and partial electronic densities of state, which are obtained directly from the tight-binding Hamiltonian. For the monohydrogenated surface and higher coverages, the stable geometries are found to yield no states in the bulk band gap.

Surface diffusion of H and CO on Cu/Ru(001): evidence for long-range trapping by copper islands

Abstract The surface diffusion of H and Co on Cu/Ru(001) was studied using laser-induced thermal desorption (LITD) techniques. The surface mobilities of H and CO on the Ru(001) surface were both dramatically attenuated by submonolayer copper coverages. At 300 K and Θ H =0.25 ML, the H diffusion coefficient decreased by a factor of ∼30 from D H =8.7 × 10 −7 cm 2 /s on clean Ru(001) to D H =3.0 × 10 −8 cm 2 /s at Θ Cu = 0.09 ML. At fixed copper coverages of Θ Cu = 0.10 ML and Θ Cu = 0.25 ML, the hydrogen diffusion coefficient was negligible at low hydrogen coverages and then increased at higher hydrogen coverages. The copper coverages also increased the hydrogen diffusion activation barrier from E H = 2.8 ± 0.2 kcal mol on clean Ru(001) to E H = 9.8 ± 1.0 kcal mol at Θ Cu = 0.15 ML . The dependence of hydrogen mobility on hydrogen coverage and the increase in diffusion activation barrier at higher copper coverages suggest that hydrogen is trapped by copper on Ru(001). The copper coverage on Ru(001) also reduced the CO diffusion coefficient. At 300 K and Θ CO = 0.41 ML, the CO diffusion coefficient decreased by a factor of 2400 as the copper coverage increased from Θ Cu = 0.00 ML to 0.10 ML. Recent scanning tunneling microscope (STM) studies have revealed that copper forms islands on Ru(001). Monte Carlo simulations of these surface diffusion results are consistent with a trapping interaction between H and the copper islands. The Monte Carlo results also argue for a very long-ranged perturbation of ∼200 A induced by the copper islands. These experimental and theoretical results for dramatic and long-ranged trapping indicate that copper seriously perturbs the surface electronic properties of Ru(001).

Adsorption of hydrogen and disilane on Si(100) and SiGe surfaces

Temperature-programmed desorption has been used to study the adsorption of hydrogen and disilane on Si(100), and epitaxial Ge and SiGe alloy film surfaces. Dissociative adsorption of disilane on Si(100) exhibits no deuterium kinetic isotope effect and occurs with high probability at 300 K ( S 0 ∼ 0.5), but decreases with increasing substrate temperature ( E a≈−9 kJ mol −1) and is strongly suppressed at high surface hydrogen coverages. Complete removal of surface hydrogen for the Si 2H 6 Si(100) system only occurs at T> 800 K, but in the presence of surface Ge, hydrogen desorption from the surface is much more facile.

Evidence for non-hydrogen desorption limited growth of Si from disilane at very low temperatures in gas source molecular beam epitaxy?

We have studied the growth of Si from disilane on Si(001) in gas source molecular beam epitaxy (GSMBE). The growth rates were measured in situ from reflection high energy electron diffraction (RHEED) intensity oscillations. Various experiments at different substrate temperature and disilane fluxes were carried out. At low temperatures the rate limiting step during the deposition of Si from disilane is the desorption of hydrogen from the Si surface. The hydrogen stems from the dissociation of the disilane molecule and ties up surface dangling bonds, which in turn block adsorption sites for incoming disilane molecules. Therefore, when measuring the growth rate at a given temperature against the supplied disilane flux, one expects to observe saturation of the rate for high absolute surface hydrogen coverages (θ ∗≈1). This behaviour was indeed found. Interestingly, even at growth temperatures as low as 468°C, the growth rate still increases slightly with increasing flux. According to the literature, at such low temperatures, the steady state absolute surface hydrogen coverage is already larger than 1 for the applied fluxed. It has been suggested that an additional reaction path for the growth from disilane on hydrogen terminated surfaces must exist, albeit that it contributes only weakly to the overall growth rate. For example, a possible alternative reaction path, proposed by Kulkarni et al. for the decomposition of disilane on a Si(111) surface: Si 2H 6(g)+H(s)→SiH 4(g)+SiH 3(s), could occur for Si(001) as well (s and g denote species at the surface and in the gas phase, respectively). However, it will be shown in this paper that an additional reaction mechanism is not at all necessary to describe the growth rate dependence on the flux even at low temperatures.

Electrochemical behaviour of platinum surfaces containing (110) sites and the problem of the third oxidation peak

The voltammetric behaviour of flame-treated Pt(110) electrodes has been studied for different cooling atmospheres. It can be stated that avoiding thermally adsorbed oxygen during the cooling step gives rise to samples with the lowest density of surface defects. Electrochemical criteria for checking the cleanliness of this orientation have been proposed. They involve the voltammetric characteristics of both hydrogen and oxygen adsorption—desorption processes. The monitoring of surface cleanliness by the two adspecies is due to the ambiguous interpretation of hydrogen adsorption—desorption only, whose voltammetric profile is not affected significantly by surface contaminants, while the change of the hydrogen electrical charge could be ascribed to two independent origins, either contamination or reconstruction lifting. Then, the problem of the so-called peak III is treated by considering Pt(110) prepared with different surface conditions and the stepped surfaces containing (110) sites on terraces or steps, respectively. It may be concluded that formation of peak III occurs if surface domains of (110) sites with a minimum size are present. Conversely, (110) sites far apart on the surface cannot give rise to peak III. A mechanism for the formation of a well defined type of site at high hydrogen coverage is discussed which accounts for the irreversibility of the adsorption desorption process ascribed to peak III where hydrogen is the desorbing species.

Initial stages of atomic hydrogen chemisorption on GaAs(110): a high resolution photoemission study

We analyzed atomic hydrogen chemisorption on GaAs(110) surfaces through synchrotron radiation photoemission studies of core level and valence band emission in the 2200–8800 L exposure range. The variation of the surface shifted and hydrogen induced core components as a function of exposure indicates that hydrogen chemisorption involves the formation of both GaH and AsH bonds during the early stages of chemisorption, as observed at higher hydrogen exposures, although not all of the dangling bonds are saturated by hydrogen, and there is no evidence of preferential As desorption that takes place at higher hydrogen coverages. The simple electronegativity picture of HGaAs bonding that would suggest a partially ionic character of the chemisorption bond with charge transfer from surface anions and cations to the hydrogen appears inconsistent with the observed variation in surface work function and electron affinity with hydrogen exposure.

Microcrystalline Silicon by Dc Magnetron Sputtering: Growth Mechanisms

ABSTRACTWe have used real-time, in situ spectroscopie ellipsometry (SE) and infrared reflectance (IR) to study microcrystalline silicon (μc-Si) formation in reactive magnetron sputtering (RMS). μc-Si growth occurs at high hydrogen partial pressures and moderate substrate temperatures. We use IR studies and SE studies of film growth on rough surfaces, respectively, to show that these conditions lead to high hydrogen coverage of the film surface and high effective surface diffusivity. The interface region is amorphous and its thickness decreases with deposition rate. For a fixed growth flux, we observe a 30% decrease in the deposition rate of μc-Si relative to the amorphous interface region. This could be due to increased etching or decreased sticking coefficients during microcrystalline growth.

Hydrogen-carbon monoxide interactions on metal surfaces at high hydrogen coverages

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTHydrogen-carbon monoxide interactions on metal surfaces at high hydrogen coveragesJohn L. Gland, Daniel A. Fischer, Deborah Holmes Parker, and Shikong ShenCite this: Langmuir 1991, 7, 11, 2574–2579Publication Date (Print):November 1, 1991Publication History Published online1 May 2002Published inissue 1 November 1991https://doi.org/10.1021/la00059a030RIGHTS & PERMISSIONSArticle Views62Altmetric-Citations9LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (623 KB) Get e-Alerts Get e-Alerts

Theoretical investigation of the mechanisms for olefinic hydrogenation on platinum(110) and platinum(111) surfaces

Calculations using the crystalline extension of the extended Hueckel theory were performed to study the mechanism for olefinic hydrogenation on platinum single-crystal surfaces. Ethene (C{sub 2}H{sub 4}) has been chosen as a model olefin. The results obtained for the C{sub 2}H{sub 4} + H/Pt coadsorbed system extends the authors previous work on C{sub 2}H{sub 4}/Pt adsorption. Hydrogenation mechanisms involving either C{sub 2}H{sub 4}(ads) and H(ads) (Langmuir-Hinshelwood (LH) type) or C{sub 2}H{sub 4}/Pt adsorption. Hydrogenation mechanisms involving either C{sub 2}H{sub 4}(ads) and H(ads) (Langmuir-Hinshelwood (LH) type) or C{sub 2}H{sub 4}(g) and H(ads) (Eley-Rideal (ER) type) have been investigated. The influence of the hydrogen coverage ({theta}{sub H}) and of the crystallographic orientation ((110) vs (111)) on the energetic of the mechanisms is discussed. On both Pt(110) and Pt(111) surfaces, the most favorable reaction pathway (involving the lowest energy barriers) corresponds to an ER dihydrogenation mechanism leading to C{sub 2}H{sub 6}(g). This mechanism involves weakly bonding atop chemisorbed H atoms at high hydrogen coverages. The energy barriers for this mechanism are about the same on Pt(110) and on Pt(111).

Hydrogen adsorption and hydrogen-deuterium equilibration on sulfided ruthenium and bulk ruthenium sulfide catalysts

The effect of presulfidization on hydrogen adsorption over ruthenium catalysts has been studied using volumetric adsorption and H 2D 2 equilibration. On reduced ruthenium catalysts (101 kPa of H 2 at 673 K for 8 h) 10.2 μmol of H 2/m 2 of ruthenium (measured at 373 K) was irreversibly adsorbed, the activation energy for adsorption was negligible and the initial heat of adsorption was estimated to be between −109 and −126 kJ/mole H 2. On “mildly sulfided” ruthenium catalysts (101 of kPa 10% H 2S H 2 at 673 K for 2 h, S a Ru (s) = 0.25 ) both reversible and irreversible hydrogen adsorption were suppressed at temperatures below 473 K. Results of H 2D 2 equilibration kinetics on these catalysts (508 ≤ T <- 623 K) revealed an activation energy of 71 ± 2 kJ/mole. On “extensively sulfided” ruthenium catalysts (101 kPa of 100% H 2S at 673 K for 2 h, i.e., crystalline RuS 2 at the surface) partial restoration of the H 2 adsorption capacity was observed (6.5 μmol/m 2 at 373 K). H 2D 2 exchange rates at 573 K were ca. two orders of magnitude faster than those over mildly sulfided samples. The activation energy for hydrogen adsorption was 32 ± 2 kJ/mole (on RuS 2-like surfaces) and the heat of adsorption was calculated to be −269 kJ/mole H 2. It is suspected that this large heat of adsorption may result in higher hydrogen coverages at HDS reaction conditions. The above noted differences in hydrogen accommodation provide a unique means for understanding the selectivity toward hydrogenated products which extensively sulfided ruthenium Catalysts possess.

The co-adsorption of H 2 and CO on Ni(110)

The effect of co-adsorption of hydrogen and CO on Ni(110) has been examined with HREELS. In the mixed adlayer at low CO and hydrogen coverage, the spectra indicate that no strong interaction between H (a) and CO (a) occurs. At high hydrogen coverage the surface reconstructs and consequently the adsorption site for the CO is modified. The reconstruction and site modification is reversible and depends on the hydrogen coverage.

Reactions of azomethane on a clean, H-covered, and O-covered Pt(111) surface

The reactions of azomethane were studied on a clean, H-covered, and O-covered Pt(111) surface at different coverages of azomethane, and ratios of hydrogen or oxygen to azomethane, by temperature programmed desorption. On a clean surface, dehydrogenetion, together with breaking of the NN bond to produce HCN and H 2 were the major reactions. Small amounts of methylamine and cyanogen were also observed. On an H-covered surface at high hydrogen coverage, methane was the dominant product, in addition to H 2. This adsorbed hydrogen promoted breaking of the CN bond. In addition, the dehydrogenation product HCN was also observed, as was methylamine. On the O-covered surface, CO, CO 2, H 2O, and small amounts of NO were observed together with the products typical for a clean surface. Comparison of the results with those for acetylene, ethylene, and diazomethane is made.

Electrochemical induction of changes in the distribution of the hydrogen adsorption states on Pt (100) and Pt (111) surfaces in contact with sulphuric acid solution

Electrochemical adsorption-desorption of hydrogen has been shown to be a structure-sensitive reaction. The influence of the crystallographic surface structure on this process is typically represented by the voltammogram of such a surface in contact with 0.5 M H 2SO 4 solution. For a given crystallographic plane, the voltammetric profile shows several peaks which are ascribed to various H adsorption states. Defined polarization programmes, applied to such electrodes, changed in a controlled way the population of each state with the possibility of inducing new ones. These transformations in the shape of the Pt (100) voltammogram in the hydrogen adsorption region suggest that the surface undergoes reconstruction due to hydrogen adsorption above 0.3 of a monolayer. The combined effect of low oxygen coverage with high hydrogen coverage induces a new type of voltammogram. At least four types of voltammogram have been observed for this orientation, each related to a particular surface structure which remains unknown in the present state of knowledge. This behaviour is in agreement with the known behaviour of this surface in contact with the gas phase at low pressure whose surface structure depends on the presence or not of an adsorbate such as hydrogen and on thermal treatment. This work gives preliminary evidence for the existence of possibilities of passing reversibly, by an electrochemical route, from one surface structure to another. For the Pt (111) surface, only one ordered state and a disturbed state with a different degree of disorder are observed. There is no known electrochemical possibility of passing reversibly from one state to the other; the transformation always occurs from the ordered to the disordered state. Comparison of these various voltammograms with that of a polyoriented model surface represented by a sherical Pt single crystal suggest that for both orientations the most strongly bonded hydrogen states are directly connected with the presence of two-dimensional long-range ordered domains on these low index surfaces.