Adsorption Of D2 Research Articles

Quantum dynamics of dissociative adsorption of H2 and D2: The time-dependent wave packet method

A time-dependent quantum wave packet method was used to study the dynamics of dissociative adsorption of H2 and D2 on a flat and static surface. The molecule-surface interaction is described using a modified London-Eyring-Polanyi-Sato (LEPS) type potential for the H2/Ni(100) system. The three-dimensional (3-D) dissociation probabilities were calculated for different initial rovibrational states as a function of initial incident energies. Our results show that the dissociation of the diatomic rotational states whose quantum numbers satisfyj+m = odd is forbidden at low energies for the homonuclear Hz and D2 due to the selection rule. The effect of the rotational orientation of diatoms on adsorption predicts that the in-plane rotation (m = j) is more favorable for dissociation than the out-of-plane rotation (m = 0). Enhanced dissociation for vibrationally excited molecules and the significant enhancement of the dissociation probability of H2 when compared to D2 were explained reasonably in terms of quantum mechanical zero-point energies, the tunneling effect and the reflection from an activation barrier.

Adsorption and desorption dynamics of H2 and D2 on Cu(111): The role of surface temperature and evidence for corrugation of the dissociation barrier

We report the effect of surface temperature on the state resolved translational energy distributions for H2 and D2 recombinatively desorbed from Cu(111). Sticking functions S(v,J,E) can be obtained by applying detailed balance arguments and follow the familiar error function form at high energy, consistent with previous permeation measurements [Rettner et al., J. Chem. Phys. 102, 4625 (1995)]. The widths of the sticking functions are identical for both isotopes and are independent of rotational state. S(E) broadens rapidly with increasing surface temperature, with a low energy component which is slightly larger than represented by an error function form. This is similar to the behavior seen on Ag(111) [Murphy et al., Phys. Rev. Lett. 78, 4458 (1997)] but on Cu(111) the low energy component remains a minor desorption channel. The broadening of S(E) can be explained in terms of a change in the distribution of barriers caused by local thermal displacement of the surface atoms, thermal activation of the surface producing sites where molecules can dissociate, or desorb, with a reduced translational activation barrier. At low energy sticking increases rapidly with surface temperature, with an activation energy of 0.54 and 0.60 eV for H2 and D2, respectively. These values are similar to the thermal activation energies calculated for translational excitation of H2/D2 and imply that thermal excitation of the surface is just as efficient as translational energy in promoting dissociation. The influence of surface temperature decreases with increasing translational energy as molecules become able to dissociate even on the static Cu(111) surface. By comparing the energy distributions for desorption with existing angular distributions we determine how the effective energy, Ee=E cosn(E) θ which contributes to adsorption–desorption, scales with translational energy. At translational energies near the threshold for sticking n(E)≈2, sticking scales with the normal component of the translational energy and is not influenced by motion parallel to the surface. At lower energy n(E) drops towards zero, indicating that motion parallel to the surface aids dissociation, consistent with dissociation at a corrugated barrier.

Poisoning of the adsorption of CO, O2 and D2 on Pt(111) by preadsorbed H2O

We have measured the influence of adsorbed H2O on the sticking coefficients and saturation coverages of CO, O2 and D2 on Pt(111) at ∼100 K. Strong poisoning is observed for all three gases. For O2 and D2, the surface is essentially totally poisoned at 1 monolayer (ML) water coverage. For CO, the effect is weaker, with some CO adsorption still occurring at 2–3 ML H2O. The influence of these results on the kinetics of the CO and H2 oxidation reactions are discussed briefly. It is concluded that the influence of water must be included in kinetics simulations, at least at low temperatures, when significant humidity levels are present in inlet gas mixtures, or produced by the reactions themselves.

Characterization of the acid properties of [Al]-, [Ga]- and [Fe]-HZSM-5 by low-temperature FTIR spectroscopy of adsorbed dihydrogen and ethylbenzene disproportionation

[Al]-, [Ga]- and [Fe]-HZSM-5 having closely similar Bronsted acid site densities were prepared. The low-temperature adsorption of H2 and D2 was studied by FTIR spectroscopy and the frequency shifts ΔνOH and ΔνHH (ΔνDD) were measured and compared with corresponding frequency shifts observed for CO and N2 probes. A linear correlation between |ΔνOH|1/2 which is proportional to the heat of formation &/Delta;HB of the H-bonded complex O-H···B (B representing the H-bond acceptor), and the proton affinities of the three probe molecules was found for [Al]-HZSM-5. The ΔνOH-values measured for CO and H2 (D2) on the three isomorphously substituted zeolites suggested the following acid strength ranking: [Al]-HZSM-5 > [Ga]-HZSM-5 ≈ [Fe]-HZSM-5. This sequence is clearly reflected in the relative activities of the materials for the acid-catalyzed disproportionation of ethylbenzene.

Investigations of the adsorption dynamics of D2 on Si(100)

Adsorption of D2 on Si(100) has been investigated using molecular beam techniques. The dependence of the D2 sticking coefficient, S, on surface temperature, Ts, and nozzle temperature, Tn, has been characterized. The sticking coefficient increases gradually in the range 300 ≤ Tn ≤ 1040 K. With an incident translational energy of ∼ 65 meV, S rises with increasing Ts from a value insignificantly different from the null level to a value of (1.5 ± 0.1) × 10−5 at Ts = 630 K. Having established that S increases with both increasing molecular energy and increasing Ts, we demonstrate directly that the adsorption of molecular hydrogen on Si is activated and that lattice excitations play an important role in the adsorption process. The implications of these results for the calculation of the Si-H bond strength are discussed.

Beam investigations of D2 adsorption on Si(100): On the importance of lattice excitations in the reaction dynamics

The adsorption of D2 on Si(100) has been investigated by means of supersonic molecular beam techniques. We have succeeded in measuring the dependence of the molecular D2 sticking coefficient S on surface temperature Ts and nozzle temperature Tn. The sticking coefficient increases gradually in the range 300≤Tn≤1040 K. The influence of increased v=1 population has not been deconvoluted from the effects of translational energy alone. The dependence on Ts is more interesting. With an incident translational energy of 65 meV, S rises from a value insignificantly different from the background level to a maximum value of (1.5±0.1)×10−5 at Ts=630 K. The decrease in the effective sticking coefficient beyond this Ts is the result of desorption during the experiment. Having established that S increases with both increasing molecular energy and increasing sample temperature, we have demonstrated directly for the first time that the adsorption of molecular hydrogen on Si is activated and that lattice vibrational excitations play an important role in the adsorption process.

Hydrogen adsorption on alkali metal surfaces: Wetting, prewetting and triple-point wetting

We report the results of a quartz microbalance study of the multilayer adsorption of H2 and D2 on sodium and on rubidium. On sodium H2 and D2 display the well known phenomenon of triple-point wetting. Interestingly low temperature adsorption isotherms of H2 on the evaporated Na surface show sharp layering transitions. On rubidium a weaker substrate H2 and D2 undergo a wetting transition in the liquid phase followed by sharp presetting transitions away from liquid-vapor coexistence. The prewetting phase diagram of H2 on Rb has been quantitatively mapped out.

Dissociative adsorption of H2 using the close-coupling wave packet method

A time-dependent quantum mechanical wave packet method was used to study the dynamics of the dissociative adsorption of H2 and D2 on a flat, static surface. A three-dimensional (3D) model was used in which the molecular rotational, vibrational, and center-of-mass translational motion normal to the surface are treated. In the close-coupling wave packet method the wave function is represented using a combination of a basis set expansion for the rotational degrees of freedom and a 2D L-shaped grid for the vibrational and translational coordinates. The time propagation is carried out by expanding the time-evolution operator in a series of Chebyshev polynomials. The molecule–surface interaction is described using a modified London–Eyring–Polanyi–Sato (LEPS) potential with parameters chosen to represent the H2/Ni(100) system. The dissociation probability was calculated for different incident energies and initial rotational and vibrational states and compared to the results of other theoretical calculations. Higher incident energies are required for D2 dissociation than for H2. The barrier height and zero-point energies at the saddle point can be determined from the energy dependence of the dissociation probabilities for H2 and D2.

Adsorption and desorption kinetics of D2 and H2 on metallic reconstructed surfaces: a Monte Carlo simulation

The authors study dissociative adsorption kinetics of H2 and D2 on W(001).

Effect of rotation on the translational and vibrational energy dependence of the dissociative adsorption of D2 on Cu(111)

We have investigated the dependence on the rotational and vibrational states of the translational energy of D2(v,J) formed in recombinative desorption from Cu(111). These results provide information about the effect of rotational energy relative to that of vibrational and translational energy on the dissociative chemisorption of D2 on Cu(111). The range of rovibrational states measured includes rotational states J=0–14 for vibrational state v=0, J=0–12 for v=1, and J=0–8 for v=2. D2 molecules were detected in a quantum-state-specific manner using three-photon resonance-enhanced multiphoton ionization (2+1 REMPI). Kinetic energies of desorbed molecules were obtained by measuring the flight time of D2+ ions in a field-free region. The mean kinetic energies determined from these measurements depend strongly on the rotational and vibrational states. Analyzing these results using the principle of detailed balance confirms previous observations that vibrational energy is effective, though not as effective as translational energy, in promoting adsorption. Rotational motion is found to hinder adsorption for low rotational states (J≤5) and enhance adsorption for high rotational states (J≥5). Even for high J states, however, rotational energy is less effective than either vibrational energy, which is 30%–70% more effective than rotational energy, or translational energy, which is 2.5–3 times more effective than rotational energy in promoting adsorption. The measured internal state distributions for the rovibrational states listed above are consistent with the observed dependence of the kinetic energy of the de- sorbed molecules with the rotational state. In addition, the analysis performed yields the dependence of the adsorption probability on kinetic energy separately for each rovibrational state. These functions have very similar sigmoidal shapes for all states examined. Changing the quantum state is primarily associated with a shift in the position, or threshold energy, for the curves. The level at which these functions saturate or level off at high energy is independent of rotational state but varies nonmonotonically with the vibrational state.

Growth of Ir overlayers on a Mo(110) surface

Abstract Ir overlayers on a Mo(110) surface were studied by LEED, AES and TPD of D2. At room temperature, Ir grows layer-by-layer, with LEED indicating a distorted Ir(111) layer in the Nishiyama-Wasserman (NW) orientation for coverages from one to four monolayers when bulk Ir appears. At 880 K, a different growth mode is found, with alloy formation taking place when the coverage exceeds one monolayer. Changes in LEED and AES on annealing two monolayers at 900 K indicate formation of a multilayer of IrMo alloy. TPD of D2 from the annealed layers confirms their alloy character, with a D2 adsorption state intermediate between those for pure Ir and Mo.

Translation-to-vibrational excitation in the dissociative adsorption of D2

In this work we have studied the dissociation dynamics of deuterium on two potential energy surfaces. In each case, there is significant translational-to-vibrational coupling that results in molecules that emerge vibrationally excited. In each case, the translational energy dependence of the onset of inelastic scattering is similar. The surfaces also result in dissociative adsorption but the thresholds in this case are well separated. The particular topological differences between the two surfaces that result in these findings are discussed.

Local adlayer order and sticking probability: D2 adsorption on p(2 × 2)O/Pt(111)

The influence of the order of an oxygen adlayer on Pt(111) on the adsorption of D2 has been investigated using a combination of He diffraction and reactive scattering techniques. A close correlation was found between the onset of the order/ disorder phase transition of the p(2 × 2)O structure and an increase of the D2 sticking probability. A detailed analysis shows that the adsorption process responsible for this phenomenon is nonactivated and that it is sensitive to local order on a scale of 4–7 atoms. An explanation is proposed which considers the influence of the local symmetry on the dynamics of the adsorption process.

Dynamics of hydrogen adsorption on clean and alkali-metal covered Cu(110)

The activated dissociative adsorption of H2 and D2 on a clean Cu(110) and a Cu(110)-K(1 × 2) reconstructed surface has been studied using supersonic molecular beams. Enhanced sticking of H2 over D2 at any particular normal energy in the pure beam experiment on Cu(110) is interpreted as an indication of the importance of the vibrational coordinate in the dissociation dynamics. The relative contributions of vibrational and translational energy in accessing the barrier is separated in vibrationally hot, translationally cold seeded beams. Results for hydrogen are presented which indicate a translational onset for the first vibrationally excited state H2(v= 1) at 130 meV. This result is consistent with a theoretically predicted barrier to dissociation of ca. 700 meV which lies somewhat in the exit channel of a two-dimensional potential-energy surface (2D-PES). The effect of alkali-metal adsorption on the (1 × 2) reconstructed surface is shown to enhance hydrogen adsorption, increase the activation energy to desorption and reduce the activation energy to adsorption. This result is consistent with a reduction in the height of a late barrier caused by a deeper atomic potential in the ‘product’ region of the 2D-PES.

Surface chemistry of monolayer metallic films on Re(0001) and Mo(110)

The overlayer growth of Fe, Ni, and Cu on a Mo(110) surface and Fe and Cu on Re(0001) as well as adsorption of D2 and CO on the prepared bimetallic surfaces have been studied in the sample temperature (Ts ) range 115–1500 K, using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS). Fe, Ni, and Cu on Mo(110) and Fe and Cu on Re(0001) were found to grow layer by layer upon deposition at Ts =115 K whereas multilayer deposits, upon annealing to Ts >330 K, nucleate into three-dimensional (3D) clusters. The 3D clusters of Fe and Cu are shown by LEED to be epitaxial with respect to the Mo(110) and Re(0001) substrates. Fe at submonolayer coverages is believed to form a strained pseudomorphic superlattice on Mo(110) while similar coverages of Ni on Mo(110) and of Cu on Re(0001) form a superstructure. The results of chemisorption of D2 and CO on the prepared bimetallic surfaces Fe/Mo(110), Ni/Mo(110), and Cu/Re(0001) are presented and discussed. It is shown that these ultrathin metal films exhibit chemisorptive properties quite different from those of the bulk metal.

D2 on Pd(110): Surface and subsurface phases, absolute coverages, and interconversion

The adsorption of deuterium on Pd(110) has been studied by means of low‐energy electron diffraction, work function measurements (Δφ), thermal desorption spectroscopy, nuclear reaction analysis (NRA), and Rutherford backscattering (RBS). These measurements show that, upon D2 adsorption at low temperature, (2×1) and (1×2) phases are formed in sequence. The RBS data show that the (2×1) structure is unreconstructed, while the (1×2) is reconstructed, with an entire monolayer (ML) of Pd atoms displaced laterally from their bulklike locations by ≳0.01 nm. The NRA data demonstrate that depending on the method of preparation, the (2×1) phase can be associated with a total coverage of 1 or 1.5 ML, while the (1×2) phase can be produced with total coverages of 1.5 or 2 ML. The additional 0.5 ML, associated with the high coverage form of each phase, is due to subsurface adsorbed species.

Bending vibrations of OH groups resulting from H2 dissociation on ZnO

Dissociative adsorption of H2 on ZnO, apart from the bands due to OH and ZnH stretching vibrations, gives rise to bands at ca. 840 and 810 cm–1 which can be attributed to the bending modes of these surface structures. Analysis of the integrated intensities of the bands arising after H2 and HD adsorption at 100 and 300 K shows that both the 840 and 810 cm–1 bands are due to ZnOH group vibrations. Their shift after 18O isotope substitution of surface oxygen, only 1 and 2 cm–1, respectively, implies that they cannot be associated with the Zn—O vibration or its overtone enhanced by Fermi resonance. They should be attributed to two bending modes of surface hydroxyls. After HD and D2 adsorption a band for the OD bending vibration was detected at 637 cm–1. In accordance with these data, νOH(D)+δOH(D) combinations were observed at 4297 and 3195 cm–1, which give the values of 807 and 611 cm–1 for δOH and δOD, corresponding, apparently, to the more intense low-frequency component.

Summary Abstract: The adsorption of H2 and D2 on Fe(110): Kinetics and energetics

Summary Abstract: The adsorption of H₂ and D₂ on Fe(110): Kinetics and energetics

Isotope effects in the interaction of hydrogen and deuterium with a rhodium (110) surface

The adsorption of H2 and D2 on a Rh (110) surface at 100 K leads to a sequence of ordered phases, among others 1×2 phases atθ H =0.5 and atθ H =1.5 which likely involve a partial surface reconstruction consisting of a small perpendicular displacement of Rh surface atoms. The structure of the adsorbate phases is strongly correlated with the binding energy of the adsorbed phases. Three H (D) binding states (α1,α2 andβ) are populated at saturation as determined by thermal desorption spectroscopy (TDS). Whereas the peak temperature of theβ state is invariant with the hydrogen isotope, the D α1 state appears at a ∼8 Klower and theD α2 state at a ∼5 Khigher temperature than the respective H states. Generally the D phases exhibit a better long-range order than the H phases. The rate of adsorption is identical for the first three adsorbed phases but D2 adsorbs appreciably faster in the 1×2–3H and the final l×1–2H phases. Zero point energy effects as well as a H coverage dependent local interaction model could account for the observed effects.

Hydrogenation of conjugated dienes and isomerization of butenes on CdO, Co3O4 and Cr2O3

Deuteration of buta-1,3-diene, 2-methylbuta-1,3-diene and penta-1,3-diene selectively yielded but-2-ene, 2-methylbut-2-ene and 1,4-[2H2]pent-2-ene on CdO and Co3O4, and but-1-ene, 2-methylbut-1-ene and 4,5-[2H2]pent-2-ene on Cr2O3, respectively. These results imply that the selectivity for 1,2- and 1,4-hydrogen addition is maintained in hydrogenation of conjugated dienes, i.e.1,4-addition on CdO and Co3O4, and 1,2-addition on Cr2O3. The hydrogenation of buta-1,3-diene using a 1:1 mixture of H2 and D2 gave [2H2]but-1-ene in larger amounts than the [2H0]isomer on Cr2O3. An isotope effect smaller than unity was interpreted by preferential adsorption of D2 on the Cr2O3 surface. Formation of [2H1]butene was dominant in deuteration of buta-1,3-diene, indicating that residual hydrogen participates in the reaction; the amount corresponded to 7 times as much as that of chemisorbed hydrogen at room temperature. Co-isomerization of [2H0] and [2H8]but-1-ene yielded, selectively, [2H0] but-2-ene on CdO. Hydrogenation of buta-1,3-diene with a 1:1:1 mixture of H2, HD and D2 also gave [2H0] butene isomers selectively on CdO. These results infer that one intermediate species for hydrogenation of buta-1,3-diene is closely related to that for isomerization of butenes, and these reactions occur through a π-allylic anion intermediate on CdO.