High Hydrogen Coverage Research Articles

Adsorption Geometry Determines Catalytic Selectivity in Highly Chemoselective Hydrogenation of Crotonaldehyde on Ag(111)

The chemoselective hydrogenation of crotonaldehyde to crotyl alcohol was studied by temperature programmed desorption/reaction, high resolution XPS and NEXAFS. The organic molecule adsorbed without decomposition, all three possible hydrogenation products were formed and desorbed, and the clean overall reaction led to no carbon deposition. Selectivities up to 95% were found under TPR conditions. The observed behavior corresponded well with selectivity trends previously reported for Ag/SiO2 catalysts and the present findings permit a rationalization of the catalytic performance in terms of pronounced coverage-dependent changes in adsorption geometries of the reactant and the products. Thus at low coverages the C=O bond in crotonaldehyde lay almost parallel to the metal surface whereas the C=C was appreciably tilted, favoring hydrogenation of the former and disfavoring hydrogenation of the latter. With increasing coverage of reactants, the C=C bond was forced almost parallel to the surface, rendering it vulnerable to hydrogenation, thus markedly decreasing selectivity towards formation of crotyl alcohol. Butanol formation was the result of an overall two-step process: crotonaldehyde → crotyl alcohol → butanol, further hydrogenation of the desired product crotyl alcohol being promoted at high hydrogen coverage due to the C=C bond in the unsaturated alcohol being driven from a tilted to a flat-lying geometry. Finally, an explanation is offered for the strikingly different behavior of Ag(111) and Cu(111) in the chemoselective hydrogenation of crotonaldehyde in terms of the different degrees of charge transfer from metal to C=O π bond, as suggested by C 1s XPS binding energies.

Two-dimensional superstructures and softened C–H stretching vibrations of cyclohexane on Rh(111): Effects of preadsorbed hydrogen

Adsorption structures and interaction of cyclohexane molecules on the clean and hydrogen-preadsorbed Rh(111) surfaces were investigated using scanning tunneling microscopy, spot-profile-analysis low-energy electron diffraction, temperature-programmed desorption, and infrared reflection absorption spectroscopy (IRAS). Various ordered structures of adsorbed cyclohexane were observed as a function of hydrogen and cyclohexane coverages. When the fractional coverage (θ(H)) of preadsorbed hydrogen was below 0.8, four different commensurate or higher-order commensurate superstructures were found as a function of θ(H); whereas more densely packed incommensurate overlayers became dominant at higher θ(H). IRAS measurements showed sharp softened C-H vibrational peaks at 20 K, which originate from the electronic interaction between adsorbed cyclohexane and the Rh surface. The multiple softened C-H stretching peaks in each phase are due to the variation in the adsorption distance from the substrate. At high hydrogen coverages they became attenuated in intensity and eventually diminished at θ(H) = 1. The gradual disappearance of the soft mode correlates well with the structural phase transition from commensurate structures to incommensurate structures with increasing hydrogen coverage. The superstructure of adsorbed cyclohexane is controlled by the delicate balance between adsorbate-adsorbate and adsorbate-substrate interactions which are affected by preadsorbed hydrogen.

A density-functional theory study of water on clean and hydrogen preadsorbed Rh(111) surfaces

We study the water bilayer on clean and hydrogen preadsorbed Rh(111) surfaces by means of density-functional theory with the generalized gradient approximation and the van der Waals density functional, to investigate the influence of adsorbed hydrogen on the adsorption state of water. We found that adsorbed hydrogen interacts repulsively with water through its 1b(1) and 4a(1) orbitals. The repulsion dominates at high hydrogen coverage, resulting in a hydrophobic Rh(111)-H surface.

Electronic Structures and Structural Evolution of Hydrogenated Graphene Probed by Raman Spectroscopy

The electronic structures and structural evolution of hydrogenated graphene are investigated by Raman spectroscopy with multiple excitations. The excitation energy dependent saturation effect on the ratio of integrated intensities of D and G modes (ID/IG) is revealed and interpreted by a D band active model with D band Raman relaxation length and photoexcited electron/hole wavelength as critical length scales. At low hydrogen coverage, the chemisorbed H atoms behave like defects in sp2 C═C matrix; for a high hydrogen coverage, the sp3 C−H bonds become coalescent clusters resulting in confinement effect on the sp2 C domains. Electronic structure changes caused by varying hydrogen coverage are evidenced by excitation energy dependent red shift of D and 2D bands. Our results provide a useful guide for developing applications of hydrogenated graphene as well as for using Raman spectroscopy for quick characterization in further exploring other kinds of graphene derivatives.

In situ video STM studies of the hydrogen-induced reconstruction of Cu(100): potential and pH dependence

The surface structure of Cu(100) electrodes in perchloric acid solutions of pH 1 to 3 was studied in the potential range of hydrogen evolution by video-rate scanning tunneling microscopy, focusing on the recently reported hydrogen-induced surface reconstruction [H. Matsushima et al., J. Am. Chem. Soc. 2009, 131, 10362]. Potential-dependent measurements reveal a two step formation process: at potentials close to the onset of hydrogen evolution a p(1×8) phase emerges, where Cu surface atoms in stripe-like structures are laterally and vertically displaced; at ≈30 mV more negative potentials a transition to a c(p×8) structure with an expanded Cu surface lattice occurs. Correlation of these observations with electrochemical data and studies on hydrogen interactions with Cu(100) surfaces under vacuum conditions support that these phases are induced by hydrogen in subsurface sites, pointing towards a high hydrogen coverage on this electrode surface under reaction conditions.

Ab initio investigation of hydrogenation of (BN) 16: A comparison with that of (BN) 12

The hydrogenation of B 16N 16 cage has been studied using ab initio molecular orbital theory with B3LYP/6-31G(d) method. The structure characters of the most stable B 16N 16H n ( n = 2–32) isomers are discussed in detail. The results show that the average binding energies of hydrogenated B 16N 16 cage are smaller than that of B 12N 12 cage especially in high hydrogen coverage. The smaller angle distortion and shorter average B–N bond length of B 16N 16 are the main reason for the smaller average binding energy per H 2 of B 16N 16H n comparing with B 12N 12H n . Gibbs free energy calculation shows the reaction of B 16N 16 + 16H 2 → B 16N 16H 32 will reverse at about 110 K, which is lower than the reversing temperature 320 K for the reaction of B 12N 12 + 12H 2 → B 12N 12H 24.

Effects of H 2S and process conditions in the synthesis of mixed alcohols from syngas over alkali promoted cobalt-molybdenum sulfide

The present work is an investigation of how the process conditions influence the synthesis of mixed alcohols from syngas over a K 2CO 3/Co/MoS 2/C catalyst. The emphasis in the investigations is upon the effects of H 2S in the syngas feed. However the effects of the temperature and of the partial pressures of H 2 and CO are also investigated. With or without H 2S in the feed the pre-sulfided catalyst requires an initiation period to reach a stabilized behavior, but the duration of this period depends upon the H 2S level. Operation with a feed containing more than 103 ppmv H 2S leads to a fairly rapid stabilization of the product distribution and ensures that higher alcohols are the dominant reaction products. With less than 57 ppmv H 2S in the feed the stabilization of the product distribution is much slower, and methanol is the dominant product. An investigation of the reaction kinetics indicates a high CO coverage and low hydrogen coverage. Hydrogen sulfide in the syngas feed generally promotes chain growth for both alcohols and hydrocarbons, but lowers the alcohol selectivity by enhancing the hydrocarbon formation. The highest alcohol productivity reached in these investigations was 0.276 g/g cat./h, and this was achieved at 350 °C, 100 bar, GHSV = 5244 h −1, Feed: 49.9 vol% H 2, 50.1 vol% CO. Finally it is found that sulfur fed to the reactor as H 2S is incorporated into the condensed alcohol product, and the incorporation of sulfur species into the product continues for some time after H 2S has been removed from the feed. When the catalyst is operated with an S-free syngas feed, the amount of sulfur in the condensed liquid product decreases over time, but after 35 h of operation with an S-free syngas the alcohol product still contains 340 ppmw of sulfur. Thiols appear to be the dominant sulfur compounds in the product.

Geometric and electronic structures of hydrogenated transition metal (Sc, Ti, Zr) clusters

We report first-principles electronic structure calculations of hydrogen adsorption and saturation on ${M}_{13}$ $(M=\text{Sc},\text{Ti},\text{Zr})$ clusters of icosahedral $({I}_{h})$ and cuboctahedral $({O}_{h})$ symmetries. Hydrogen saturation of the ${I}_{h}$ metal clusters yields energetically stable ${M}_{13}{\text{H}}_{20}$ and ${M}_{13}{\text{H}}_{30}$ systems compared to ${M}_{13}{\text{H}}_{14}$ and ${M}_{13}{\text{H}}_{24}$ systems for ${O}_{h}$ clusters. In all these clusters, the hydrogen adsorption involves dissociative chemisorption of ${\text{H}}_{2}$ molecules. Upon initial hydrogenation, the dissociated hydrogen atoms lie above either the triangular or quadrangular face of the metal cluster. A further increase in hydrogen saturation leads to the formation of bridged hydrogen bond between adjacent metal atoms. The role of the unfilled $d$ orbitals in imparting stability to the hydrogenated clusters is explored by examining their density of states (DOS) near the Fermi level. It has been found that the inverse correlation between the $d$-band center and the chemisorption energies is valid only at low hydrogen concentrations. At higher hydrogen coverages, the trend is reversed. The results illustrate that at high adsorbate concentrations or surface coverage, the correlation of the chemisorption energies with the $d$-band center of the pure metal cluster or nanoparticle may not be realistic but one has to take into account the changes in the $d$-orbital DOS due to the presence of coadsorbed molecules. To obtain further insights into the initial steps involved in hydrogen adsorption and dissociation, the transition states and activation energies of the ${M}_{13}{\text{H}}_{2}$ system have been determined and found to conform with the empirical Br\o{}nsted-Evans-Polanyi relationship. A highly intense infrared band in the $1000--1500\text{ }{\text{cm}}^{\ensuremath{-}1}$ region, associated with the adsorbed hydrogens in these hydrogenated metal clusters, may be used to experimentally monitor hydrogen coverage.

On the Origin of the Cobalt Particle Size Effects in Fischer−Tropsch Catalysis

The effects of metal particle size in catalysis are of prime scientific and industrial importance and call for a better understanding. In this paper the origin of the cobalt particle size effects in Fischer-Tropsch (FT) catalysis was studied. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) was applied to provide surface residence times and coverages of reaction intermediates as a function of Co particle size (2.6-16 nm). For carbon nanofiber supported cobalt catalysts at 210 degrees C and H(2)/CO = 10 v/v, it appeared that the surface residence times of reversibly bonded CH(x) and OH(x) intermediates increased, whereas that of CO decreased for small (<6 nm) Co particles. A higher coverage of irreversibly bonded CO was found for small Co particles that was ascribed to a larger fraction of low-coordinated surface sites. The coverages and residence times obtained from SSITKA were used to describe the surface-specific activity (TOF) quantitatively and the CH(4) selectivity qualitatively as a function of Co particle size for the FT reaction (220 degrees C, H(2)/CO = 2). The lower TOF of Co particles <6 nm is caused by both blocking of edge/corner sites and a lower intrinsic activity at the small terraces. The higher methane selectivity of small Co particles is mainly brought about by their higher hydrogen coverages.

The sticking probability for H2 on some transition metals at a hydrogen pressure of 1bar

The sticking probability for hydrogen on films of Co, Ni, Cu, Ru, Rh, Pd, Ir, and Pt supported on graphite has been measured at a hydrogen pressure of 1 bar in the temperature range 40-200 degrees C. The sticking probability is found to increase in the order Ni, Co, Ir, Pd, Pt, Rh, and Ru at temperatures below 150 degrees C, whereas at higher temperatures, the sticking probability for Pd is higher than for Pt. The sticking probability for Cu is below the detection limit of the measurement. The measured sticking probabilities are slightly lower than those obtained at high hydrogen coverage under ultrahigh vacuum conditions. This could be a consequence of the higher hydrogen pressure used here. The apparent desorption energies extracted from the steady-state desorption rate are found to agree reasonably well with published values for the heat of adsorption at high coverage. However, the sticking probability is not related in a simple way to published values for the heat of adsorption at low coverage, with Ru and Rh giving exceptionally high values for the sticking probability. It is suggested that this is due to the presence of adsorption sites with very low desorption energy on Ru and Rh.

Long range orientation of meta-stable atomic hydrogen adsorbate clusters on the graphite(0 0 0 1) surface

We present scanning tunneling microscopy (STM) images of meta-stable hydrogen adsorbate structures on the highly oriented pyrolytic graphite (HOPG) (0 0 0 1) surface and identify two unique and stable hydrogen structures. One of these is observed after thermal anneals to 525 K at both high and low hydrogen coverage and is identified as a hydrogen dimer structure. The other, a novel, more complex structure not previously observed, appears only after thermal anneals at high hydrogen coverage and is observed to exhibit long range orientation within each micro-crystallite region on the HOPG surface.

Roles of SiH 4 and SiF 4 in growth and structural changes of poly-Si films

The structural properties of polycrystalline silicon films, prepared by plasma enhanced chemical vapor deposition system, with different flow rates of SiH 4/SiF 4 mixtures at 300 °C were investigated. This study indicates that the low hydrogen coverage on the growing surface, under optimum fluorine radicals, will be leaded to an improvement of crystallized area as compared with case of high hydrogen coverage surface. Moreover, the studies of the role of SiH 4 and SiF 4 radicals show that the SiH 4 radicals are important in the nucleation and growth of grains. However, SiF 4 radicals are effective in the structural change of grain boundaries regions and by this way, in the present system, establish the growth of grains under the dominant 〈1 1 0〉 direction. The stress investigation indicates that addition of high flow rate of SiF 4 in amorphous film, results in the nearly stress free films. Finally, we found that the changes in g-value reflect the changes in the intrinsic compressive and tensile stress in the both polycrystalline and amorphous silicon films.

Experimental and theoretical evidence for a hydrogen stabilizedc(2×2)reconstruction of the P-rich InP(001) surface

The formation of hydrogen bonds was investigated on the P-rich InP(001) surface employing attenuated total-reflection Fourier-transform infrared spectroscopy, low-energy electron diffraction, and total-energy density-functional theory calculations. Strong evidence was found for a $c(2\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}2\mathrm{P}\text{\ensuremath{-}}3\mathrm{H}$ reconstruction with a higher hydrogen coverage than is characteristic for the metal-organic chemical-vapor deposition prepared hydrogen-stabilized $(2\ifmmode\times\else\texttimes\fi{}2)\text{\ensuremath{-}}2\mathrm{D}\text{\ensuremath{-}}2\mathrm{H}$ surface. The new surface reconstruction was formed upon exposure to atomic hydrogen. Complete transformation of all the metastable atomic configurations to form the new surface reconstruction was not achieved, since prior to this the surface began to deteriorate. The latter effect was monitored as the formation of In-H bonds. Two observations, i.e., nearly complete screening of the infrared peaks for excitation with $p$-polarized light and a pronounced redshift of P-H peaks with increasing hydrogen coverage were attributed to dipole-dipole interaction between the vibrating adsorbates.

MICROSCOPIC MODEL OF ASSOCIATIVE DESORPTION FOR HYDROGEN ON Mo(110)

Adsorbed hydrogen layers on the Mo (110) surface have been investigated both experimentally by temperature programmed desorption (TPD) method and theoretically by means of DFT-based optimization of surface structures. We suggest a novel microscopic model of the associative hydrogen desorption, which explains essential features of the process. In this model, the process of hydrogen desorption can be described as association of hydrogen atoms on the surface, but molecular formation is actually accomplished while the molecule moves away from the surface. We also suggest a new algorithm for realistic Monte Carlo simulations of associative desorption, which implements the microscopic description of the association of hydrogen adatoms into a molecule with activation energy, found from the DFT calculations. Good agreement between simulated and experimental TPD spectra gives insight into different behavior of the spectra, obtained for low and high hydrogen coverages on the Mo (110) surface.

Roles of SiH 4 and SiF 4 in growth and structural changes of poly-Si films

The structural properties of polycrystalline silicon films, prepared by plasma enhanced chemical vapor deposition system, with different flow rates of SiH 4/SiF 4 mixtures at 300 °C were investigated. This study indicates that the low hydrogen coverage on the growing surface, under optimum fluorine radicals, will be leaded to an improvement of crystallized area as compared with case of high hydrogen coverage surface. Moreover, the studies of the role of SiH 4 and SiF 4 radicals show that the SiH 4 radicals are important in the nucleation and growth of grains. However, SiF 4 radicals are effective in the structural change of grain boundaries regions and by this way, in the present system, establish the growth of grains under the dominant 〈1 1 0〉 direction. The stress investigation indicates that addition of high flow rate of SiF 4 in amorphous film, results in the nearly stress free films. Finally, we found that the changes in g-value reflect the changes in the intrinsic compressive and tensile stress in the both polycrystalline and amorphous silicon films.

Ab initio surface reaction energetics of SiH4 and Si2H6 on Si(001)-(2×2)

First-principles pseudopotential calculations, within a simple dynamically constrained scheme, have been performed to investigate the reaction of 0.25 ML coverage of SiH4 and Si2H6 with the Si(001)-(2 x 2) surface. The silane molecule (SiH4) is adsorbed on to the surface at a number of different sites (on dimer, interrow, or intrarow) with varying barrier heights. Two distinct structures, which are similar in energy, arise from the initial dissociative reaction SiH4-->SiH3(silyl) + H, where the dissociated species are adsorbed either on the same dimer components or on adjacent dimer components. Several further decays of silyl from SiH4 are presented in two separate regimes of high and low ambient hydrogen coverages. The decomposition of silyl can form two different bridging structures: an on top or an intrarow bridging structure in both of the two hydrogen coverage regimes. The disilane molecule (Si2H6) is also adsorbed upon this surface with varying energy barriers, resulting in a dissociation reaction where two SiH3 species are adsorbed on one dimer or in an adjacent dimer configuration. Plausible energy reaction paths for the above models are presented. The stability of the SiH2 species is also discussed.

Effect of hydrogen on the structure of Si and C clusters formed in gas phase by CVD processes

Hartree Fock (HF) method and the hybrid density functional theory have been used to find the structure of Si and C clusters that are created in gas phase by Chemical Vapour Deposition (CVD). The role of hydrogen in analysed system has been examined. The results indicate that the presence of hydrogen in the CVD processes strongly affects the structure of Si, C compounds which are formed in gas phase. It has been shown that structures with high hydrogen coverage are energetically favourable. Conducting process in H 2 ambient is considered to stabilise sp 3 structure. The presence of hydrogen enhances the creation of tetrahedral Si and C clusters that are supposed to be crystal nuclei. The adsorption of gaseous SiH 3 and CH 3 species on silicon and carbon surface has been analysed. The reaction between gaseous species and third-order surface atoms seems to be favourable.

Molecular Dynamics Simulation of the Interactions of H Radicals with a Hydrogenated Si(001) Surface

In order to analyze the effects of the surface H atoms on elementary surface reactions, we repeated molecular dynamics simulations in which an H radical fell onto a hydrogen-terminated Si(001) surface. As a result of the simulation, we found that high hydrogen coverage inhibits H radicals from being adsorbed within the silicon substrate. Hydrogen radicals mainly caused adsorption or the H abstraction reaction, which generated several kinds of local surface structures with dangling bonds, and it was suggested that various local surface structures coexist on a growing surface. The local surface structure significantly affected the result, and its reactivity was high when the structure had a dangling bond, although a small difference in results was observed depending on the surrounding structures. These results indicate that the change in the local surface structure caused by H radicals may induce subsequent surface reactions.

Reactivity of the hydrocarbon C–C bonds as a function of the reaction conditions in the conversion of C 6 alkanes and methylcyclopentane over Rh catalysts

The conversion of 3-methylpentane (3MP), 2-methylpentane (2MP), n-hexane ( nH), and methylcyclopentane (MCP) was investigated on 0.3 and 10% Rh/Al 2O 3 and 5% Rh/SiO 2 as a function of hydrogen pressure and temperature. The catalysts were prepared by the incipient wetness method. Metal accessibility was 57, 18 and 36%, which corresponded to mean particle size of 1.5, 5 and 2.5 nm, respectively. The hydrogenolytic cleavage of hydrocarbon C–C bonds was the main reaction. Skeletal isomers (up to 20%) were formed from methylpentanes on 10% Rh/Al 2O 3. C 5-cyclization was a minor reaction (less than 10%) and was promoted by low p(H 2). Particle size effect was clearly observed in the non-degradative reaction path; this route was favored by larger Rh particles. Single splitting of C–C bonds was catalyzed at high hydrogen coverage. Decreasing p(H 2) caused “deepening” of the hydrogenolysis and the catalysts lost much of their activity. The hindrance in the re-hydrogenation of the surface intermediate of fragmentation was proposed to explain the positive hydrogen order. The role of further hydrogenolysis of particular fragments or ring opening intermediates was significant at low p(H 2). Reaction conditions governed the desorption or the further reactions of the surface intermediates. For instance, fragments were produced in the conversion of MCP mainly from branched ring opening intermediates. The fragmentation patterns of hexane isomers were successfully applied for modeling the fragment distribution of MCP.

Infrared spectroscopy study of adsorption of silane on Si( [formula omitted

We used infrared absorption spectroscopy in the multiple internal reflection geometry to investigate the adsorption of SiH 4 on the Si(0 0 1)(2×1) surface. Comparing infrared data with the density functional cluster calculation, we show that at low hydrogen coverage the silane molecule dissociatively adsorbs on Si(0 0 1)(2×1) to populate a dihydride (SiH 2) at the bridge site between two adjacent dimers and monohydride species; monohydrides are formed by terminating the unsaturated dangling bonds of dimers by hydrogen atoms released from the silane molecule. We suggest that at high hydrogen coverage, silane adsorbs onto a single dimer to generate monohydride and sylil groups (–SiH 3). We also demonstrate that the dihydride species that is initially generated by silane adsorption, decomposes to monohydride species even at room temperature.