Dissociative Adsorption Of H2 Research Articles

H2 Dissociative Adsorption at the Zigzag Edges of Graphite

H2 Dissociative Adsorption at the Zigzag Edges of Graphite

Core level spectroscopy and reactivity of coadsorbed K+O layers on reconstructed Rh(110) surfaces

The bonding character of oxygen and potassium and the interactions in K+O coadsorbed layers on a Rh(110) surface have been studied by means of high resolution x-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). The Rh 3d5/2, K 2p, and O 1s spectra and LEED patterns were used as fingerprints for the interfacial reactions and the structural changes. Dramatic changes in the chemical state of the substrate occur in the presence of dense K+O adlayers, when the oxygen coverage exceeds one monolayer. The effect of coadsorbed potassium on the “reactivity” of oxygen was probed by following the evolution of the O 1s spectra during titration with H2. The enhanced surface capacity for oxygen adsorption and the reduced rate of H2O formation with increasing K coverage were discussed considering the influence of K on the dissociative adsorption of O2 and H2 and on the bonding of the coadsorbed species.

Role of dynamic trapping in H2 dissociation and reflection on Pd surfaces

We present results of classical trajectory calculations on dissociative adsorption and reflection of H2 impinging on a frozen Pd(110) surface. The six-dimensional potential energy surface is obtained by interpolation of density functional theory (DFT) calculations using the corrugation reducing procedure. The comparison with earlier work for H2/Pd(111) shows that DFT calculations predict that the Pd(110) surface, though more open, is less reactive than the Pd(111) one. This lower reactivity is associated with a more prominent role of dynamic trapping, at low energies, and with the fact that trapping may end up in reflection as well as in dissociation.

Dissociative hydrogen adsorption on palladium requires aggregates of three or more vacancies.

During reaction, a catalyst surface usually interacts with a constantly fluctuating mix of reactants, products, 'spectators' that do not participate in the reaction, and species that either promote or inhibit the activity of the catalyst. How molecules adsorb and dissociate under such dynamic conditions is often poorly understood. For example, the dissociative adsorption of the diatomic molecule H2--a central step in many industrially important catalytic processes--is generally assumed to require at least two adjacent and empty atomic adsorption sites (or vacancies). The creation of active sites for H2 dissociation will thus involve the formation of individual vacancies and their subsequent diffusion and aggregation, with the coupling between these events determining the activity of the catalyst surface. But even though active sites are the central component of most reaction models, the processes controlling their formation, and hence the activity of a catalyst surface, have never been captured experimentally. Here we report scanning tunnelling microscopy observations of the transient formation of active sites for the dissociative adsorption of H2 molecules on a palladium (111) surface. We find, contrary to conventional thinking, that two-vacancy sites seem inactive, and that aggregates of three or more hydrogen vacancies are required for efficient H2 dissociation.

First principles studies for the dissociative adsorption of H2 on graphene

We investigate and discuss the interaction of H2 with graphene based on density functional (DFT) theory. We calculate the potential energy surfaces for the dissociative adsorption of H2 on highly symmetric sites on graphene. Our calculation results show that reconstructions of the carbon atoms play an important role in the H2 -graphene interactions. Activation barrier for H2 dissociation on an unrelaxed graphene is considerably high, ∼4.3 eV for a T–H–T geometry and ∼4.7 eV for a T–B–T geometry. The T–H–T(T–B–T) geometry means that the center of mass position of H2 is at the hollow(bridge) site, and the two H atoms are directed towards the top sites on the graphene. On the other hand, when the carbon atoms are allowed to relax, the activation barrier decreases, and becoming 3.3 eV for the T–H–T geometry and 3.9 eV for the T–B–T geometry. In this case, the two carbon atoms near the hydrogen atoms move 0.33 Å towards the gas phase for the T–H–T geometry and 0.26 Å for the T–B–T geometry.

Quantum Monte Carlo calculations of H2 dissociation on Si(001).

The dissociative adsorption of H2 on the Si(001) surface is theoretically investigated for several reaction pathways using quantum Monte Carlo methods. Our reaction energies and barriers are at large variance with those obtained with commonly used approximate exchange-correlation density functionals. Our results for adsorption support recent experimental findings, while, for desorption, the calculations give barriers in excess of the presently accepted experimental value, pinpointing the role of coverage effects and desorption from steps.

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...

High H2 sensing performance of anodically oxidized TiO2 film contacted with Pd

TiO2 films with nanoholes have been prepared by anodic oxidation of a Ti plate in a H2SO4 solution. The TiO2 film sensor equipped with Pd and Ti electrodes exhibited a diode-type current–voltage characteristic in air, but nearly ohmic behavior in H2 balanced with dry air. The sensor showed quick response to H2 in dry air with high sensitivity, especially at 250°C under a reverse bias voltage of 0.1V. Reversible response to H2 was also achieved in N2, while the sensitivity decreased from that in air. The high H2 sensitivity in air is suggested to arise partly from the change in the height of the Schottky barrier at the interface between Pd and TiO2, and H atoms formed by dissociative adsorption of H2 at the Pd surface are assumed to play an important role in the H2 sensing properties. The sensor was less sensitive to CO, but water vapor interfered with the H2 sensitivity.

1H MAS NMR characterization of hydrogen over silica-supported rhodium catalyst

Hydrogen species in both SiO2 and Rh/SiO2catalysts pretreated in different atmospheres (H2, O2, helium or air) at different temperatures (773 or 973 K) were investigated by means of1H MAS NMR. In SiO2 and O2-pretreated catalysts, a series of downfield signals at ∼7.0, 3.8–4.0, 2.0 and 1.5–1.0 were detected. The first two signals can be attributed to strongly adsorbed and physisorbed water and the others to terminal silanol (SiOH) and SiOH under the screening of oxygen vacancies in SiO2lattice, respectively. Besides the above signals, both upfield signal at ∼−110 and downfield signals at 3.0 and 0.0 were also detected in H2-pretreated catalyst, respectively. The upfield signal at ∼−110 originated from the dissociative adsorption of H2 over rhodium and was found to consist of both the contributions of reversible and irreversible hydrogen. There also probably existed another dissociatively adsorbed hydrogen over rhodium, which was known to be β hydrogen and in a unique form of “delocalized hydrogen”. It was presumed that the β hydrogen had an upfield shift of ca. −20–−50, though its1H NMR signals, which, having been masked by the spinning sidebands of Si-OH, failed to be directly detected out. The downfield signal at 3.0 was assigned to spillover hydrogen weakly bound by the bridge oxygen of SiO2. Another downfield signal at 0.0 was assigned to hydrogen held in the oxygen vacancies of SiO2 (Si-H species), suffering from the screening of trapped electrons. Both the spillover hydrogen and the Si-H resulted from the migration of the reversible hydrogen and the β hydrogen from rhodium to SiO2 in the close vicinity. It was proved that the above migration of hydrogen was preferred to occur at higher temperature than at lower temperature.

A theoretical study of Si4H2 cluster with ab initio and density functional theory methods

Various isomers of Si4H2 cluster have been investigated with ab initio molecular orbital and density functional theory (DFT) calculations. Nine local minimum isomers on the potential energy surface have been obtained with both Mo/ller–Plesset perturbation theory (MP2) and DFT methods. The Si4 frame is slightly distorted by the dissociative adsorption of H2 on it. The most stable isomer of Si4H2 is a classical structure with both hydrogen atoms bonded to a single silicon atom. The nonclassical H-bridged structures are also found in the calculations, but predicted to be less stable than the nonbridged structures energetically. The formation of the most stable isomer of Si4H2 from Si4 and H2 is proven to be a two-step process and exothermic. The first step is the dissociative adsorption of H2 on Si4 cluster by overcoming an energy barrier of 19.27 kcal/mol, and the second step of conversion from the intermediate to the product will readily proceed with a barrier of only 0.53 kcal/mol.

The role of steps in the dynamics of hydrogen dissociation on Pt(533)

The dissociative adsorption of H2 and D2 on Pt(533) (Pt{4(111)×(100)}) has been investigated using temperature programmed desorption and supersonic molecular beams. Associative desorption of D2 from (100) step sites is observed at lowest exposures in TPD (assigned β3) at 375 K. Saturation of this peak at ΘH=0.14 corresponds to the filling of half of the available four-fold sites at the (100) step edge. At higher coverages, additional desorption takes place from the (111) terraces in a broad peak below 300 K similar to that observed (assigned β1 and β2) for the Pt(111) surface. The incident kinetic energy (Ei), surface temperature (Ts), coverage (ΘD), and incident angle (Φ) dependence of the dissociative sticking probability (S) was also measured. The initial dissociative sticking probability (S0) first decreases with increasing kinetic energy over the range 0<Ei(meV)<150 (low energy component), and subsequently increases (high energy component). Comparison with D2 dissociation on Pt(111), where (S0) increases linearly with Ei, leads to the conclusion that it is the step sites that are responsible for the low energy component to dissociation on Pt(533). The high energy component is a result of a direct dissociation channel on (111) terraces of the Pt(533) surface. The probability of dissociation through the direct channel on the (111) terraces is found to be independent of Ts. The probability of dissociation through the low energy component associated with the (100) steps, over most of the range of Ei where it contributes, is also shown to be independent of Ts. Only at the very lowest value (6.6 meV) of Ei investigated does S0 exhibit a (negative) temperature dependence. A (0.8-ΘD)2 dependence (where 0.8 is the measured saturation coverage) of S with ΘD is observed at Ei=180 meV where the direct channel dominates. However, the dependence of S on ΘD exhibits characteristics similar to those expected for precursor mediated dissociation at Ei=16 meV and Ei=6.6 meV where the low energy channel dominates. The angular dependence S0(Φ) scattering in a plane perpendicular to the step direction is asymmetric about the Pt(533) surface normal at both Ei=6.6 meV and Ei=180 meV. At 180 meV S0(Φ) can be understood by considering direct dissociation at the (111) terrace and (100) step plane. At 6.6 meV, S0 tends to scale with total energy. The observed characteristics of the low energy channel is discussed in the light of models [specifically the role steps and defects, precursors (accommodated and dynamical), and steering] suggested to account for the low energy component for H2/D2 dissociation and exchange on metal surfaces presenting low activation barriers. At lowest energies (Ei=6.6 meV) dissociation through a conventional accommodated precursor takes place. In addition, more significant proportion of sticking in the range 0<Ei(meV)<150 takes place through an indirect channel involving an unaccommodated precursor dissociating at step sites, and is unlikely to be accounted for through a steering mechanism.

Kinetic simulation of metal chemical-vapor deposition on high aspect ratio features in modern very-large-scale-integrated processing

Chemical-vapor deposition of tungsten is extensively used for very-large-scalae-integrated metallization because of its ability to adequately coat the bottom of high aspect ratio features. Despite this, a detailed model of the surface kinetics is not yet widely accepted. Such a model is essential for predicting film coverage over deep topography where fluxes and adsorbate coverage can be very different from those on flat surfaces. By considering the dissociative adsorption of H2 and WF6 and the desorption of H2 and HF molecules, a new surface kinetic model for tungsten deposition is presented. The model includes temperature- and coverage-dependent sticking coefficients of adsorbing reactions, the inhibiting effects of F on H2 adsorption, and multiple reaction pathways. Predictions of the model show reasonable agreement with experimental measurements of H2 partial pressure dependence of tungsten deposition rate over a wide pressure range. Particularly, the model explains the recently observed effect of reduced deposition rate when the H2 pressure becomes comparable to the WF6 pressure. This kinetic model is used by a kinetic thin-film simulator, GROFILMS, to study the W film deposition over high aspect ratio topography. The film growth profile, the coverage of F and H, and the impingement fluxes along the film surface are analyzed.

Effect of beam energy and surface temperature on the dissociative adsorption of H2 on Si(001)

Dissociative adsorption of H2 from a high-flux supersonic molecular beam on flat and vicinal Si(001) surfaces was investigated by means of optical second harmonic generation (SHG). The initial sticking coefficients for terrace adsorption varied between 10−8 and 10−4. They revealed a strongly activated dissociation process, both with respect to the kinetic energy of the incident molecules (70 meV⩽Ekin⩽380 meV) and the surface temperature (440 K⩽Ts⩽670 K). The results indicate that dynamical distortions of Si surface atoms can lower the effective adsorption barriers from 0.8±0.2 eV to almost negligible values. Previously proposed defect-mediated processes can be ruled out as a major adsorption channel.

Quantum dynamics of dissociative adsorption of H2 on Ni(100) using the dynamical Lie algebraic method

A dynamical Lie algebraic method has been applied to treating the quantum dynamics of dissociative adsorption of H2 on a static flat metal surface. An LEPS potential energy surface has been used to describe the interaction of H2 with Ni(100) surface. The dependence of the initial state-selected dissociation probability was obtained analytically on the initial kinetic energy and time. A comparison with other theoretical calculations and experiments is made. The results show that the method can be effectively used to describe the dynamics of reactive gas-sdace scattering.

Electronic Effects in the Activation of Supported Metal Clusters: Density Functional Theory Study of H2 Dissociation on Cu/SiO2

We have performed density functional theory calculations on the dissociation of molecular hydrogen on small gas-phase and silica-supported Cu clusters and on the Cu(111) surface. The Cu surface and the silica support have been represented by cluster models. The supported Cu clusters interact with a nonbridging oxygen, a paramagnetic defect present on the surface of dehydroxylated silica, giving rise to a partial charge transfer to the substrate. While free Cu clusters and the Cu metal are rather unreactive toward H2 dissociative adsorption, the supported clusters exhibit a much higher reactivity connected with a strong geometrical rearrangement of the metal structure. As a consequence of the interaction with the substrate, the Cu clusters become active catalysts in H2 dissociation.

Rotational effects in six-dimensional quantum dynamics for reaction of H2 on Cu(100)

We present results of six-dimensional (6D) quantum wave-packet calculations for the dissociative adsorption of (ν=0,j=4,mj) H2 on Cu(100). The potential-energy surface is a fit to points calculated using density-functional theory (DFT), with the generalized gradient approximation (GGA), and a slab representation for the surface. New aspects of the methodology we use to adapt the wave function to the symmetry of the surface, which relate to calculations for initial rotational states with odd mj (the magnetic quantum number), are explained. Invoking detailed balance, we calculate the quadrupole alignment for H2 as it would be measured in an associative desorption experiment. The reaction of the helicopter (ν=0,j=4,mj=4) state is preferred over that of the (ν=0,j=4,mj=0) cartwheel state for all but the lowest collision energies considered here. The energy dependence of the quadrupole alignment that we predict for (ν=0,j=4) H2 desorbing from Cu(100) is in good qualitative agreement with velocity-resolved associative desorption experiments for D2+Cu(111). The vibrational excitation probability P(ν=0,j→ν=1) is much larger for j=4 than for j=0, and the mj-dependence of P(ν=0,j=4,mj→ν=1) is markedly different from that of the initial-state-resolved reaction probability. For all but the highest collision energies, vibrational excitation from the (ν=0,j=4) state is accompanied by loss of rotational energy, in agreement with results of molecular beam experiments on scattering of H2 and D2 from Cu(111).

Effect of the cluster size in modeling the H2 desorption and dissociative adsorption on Si(001)

Three different clusters, Si9H12, Si15H16, and Si21H20, are used in density-functional theory calculations in conjunction with ab initio pseudopotentials to study how the energetics of H2 dissociative adsorption on and associative desorption from Si(001) depends on the cluster size. The results are compared to five-layer slab calculations using the same pseudopotentials and high quality plane-wave basis set. Several exchange-correlation functionals are employed. Our analysis suggests that the smaller clusters generally overestimate the activation barriers and reaction energy. The Si21H20 cluster, however, is found to predict reaction energetics, with Eades=56±3kcal/mol (2.4±0.1eV), reasonably close (though still different) to that obtained from the slab calculations. Differences in the calculated activation energies are discussed in relation to the efficiency of clusters to describe the properties of the clean Si(001)-2×1 surface.

Erratum: “Dissociative adsorption of H2 on Cu(100): A four-dimensional study of the effect of rotational motion on the reaction dynamics” [J. Chem. Phys. 106, 4248 (1997)

First Page

THE ENHANCED CATALYTIC DISSOCIATION OF ADSORBED HYDROGEN CONTAINING MOLECULES

Some general conceptions and mechanisms of dissociative adsorption and catalysis are analyzed. The role of such factors as electronic shell configurations of isolated atoms, crystalline structure of the surface, local structural irregularities and defects, orientation of the surface bonds, dimensional effects, the presence of foreign atoms in the local atomic environment as well as the local symmetry of adsorption center are discussed. These considerations were used in developing the program for computer simulation of the process. The catalytic properties of PdCux surface alloys are analyzed using computer modelling and thus the enhancement of dissociative adsorption of H2 for such a system is predicted. The experimental data, demonstrating the enhancement of sensitivity of MIS sensor with CuPd electrode in comparison with the pure Pd electrode, are presented.

In Situ Infrared Study of Methanol Synthesis from H2/CO over Cu/SiO2and Cu/ZrO2/SiO2

The interactions of CO and H2/CO with Cu/SiO2, ZrO2/SiO2, and Cu/ZrO2/SiO2have been investigated by in situ infrared spectroscopy with the aim of understanding the nature of the species involved in methanol synthesis and the dynamics of the formation and consumption of these species. In the case of Cu/SiO2, carbonate species adsorbed on Cu and methoxide species adsorbed on Cu and silica are observed at 523 K, H2/CO=3, and a total pressure of 0.65 MPa. When ZrO2/SiO2or Cu/ZrO2/SiO2is exposed to H2/CO, the majority of the species observed are associated with ZrO2. CO adsorption on either ZrO2/SiO2or Cu/ZrO2/SiO2results in the formation of carboxylate, bicarbonate, and formate species on zirconia. In the presence of H2, formate species are hydrogenated to methoxide species adsorbed on ZrO2. The presence of Cu greatly accelerates the rate of formate hydrogenation to methoxide species, a process in which methylenebisoxy species are observed as intermediates. Cu also significantly promotes the reductive elimination of methoxide species as methanol. Thus, methanol synthesis over Cu/ZrO2/SiO2is envisioned to occur on ZrO2, with the primary role of Cu being the dissociative adsorption of H2. The spillover of atomic H onto ZrO2provides the source of hydrogen needed to hydrogenate the carbon-containing species. Spillover of absorbed CO from Cu to zirconia facilitates formate formation on zirconia at lower temperatures than in the absence of Cu. The reductive elimination of methoxide species appears to be the slow step in methanol formation by CO hydrogenation. The lower rate of methanol synthesis over Cu/ZrO2/SiO2from CO as compared to CO2hydrogenation is attributed to the lack of water formation in the former reaction, preventing facile release of methoxide by hydrolysis. The enhanced rate of methanol synthesis from CO over Cu/ZrO2/SiO2as compared to Cu/SiO2is attributed to the lower energy-barrier (formate) pathway available on Cu/ZrO2/SiO2.