A first-principles study of C + O reaction on NiCo(1 1 1) surface

A density-functional theory method has been conducted to investigate the association of C + O on (1 1 1) facets of ordered NiCo alloy and the results have been compared with those obtained on pure Ni(1 1 1) surface. In reaction of C + O, the favorable reaction path is that C adsorbed on HCP-1 site moves to the nearest Ni-Co bridge site, and associates with O migrating from FCC-1 site to result in CO adsorbed on the bridge site of Ni-Co. However, the reaction barrier is higher by 0.35 eV than that on pure Ni(1 1 1), which indicates that the incorporation of Co into the Ni crystal is not in favor of the reaction of carbon delimination.     

Kinetics of the H + NCO reaction

Ab initio transition state theory (TST) based master equation simulations are used to predict the temperature and pressure dependence of the H + NCO reaction rate and product branching. The barrierless entrance channels to form singlet HNCO and NCOH are studied with variable reaction coordinate TST employing a potential energy surface obtained from multi-reference configuration interaction ab initio calculations. The remaining channels, including reactions on the triplet surface, are studied with standard TST methods employing high level electronic structure results. The energy transfer parameters for the master equation simulations arise from a fit to the experimentally observed HNCO dissociation rate. The lowest energy threshold to formation of bimolecular products, ³NH + CO, lies well below the reactants. The bottleneck for intersystem crossing, which precedes the formation of ³NH + CO from the singlet adducts, becomes the dominant bottleneck for that channel at quite low energies relative to reactants. The effect of this bottleneck is studied with model calculations designed to reproduce detailed experimental observations of photolysis branching ratios. This bottleneck greatly reduces the flux from H + NCO to ³NH + CO via the singlet adducts. As a result, stabilization and reaction on solely the triplet surface are significant components of the overall rate. The present predictions for the high pressure and collisionless limit rate coefficients are accurately reproduced over the 200-2500 K range by the expressions, 1.53 × 10⁻⁵T^−1.86exp(−399/T) + 1.07 × 10³T^−3.15exp(−15219/T) and 5.62 × 10⁻¹²T^0.493exp(148/T) cm³ molecule⁻¹ s⁻¹, respectively, where T is in K. These predictions are in reasonably satisfactory agreement with the somewhat discordant experimental rate measurements.     

Selective surface chemistry of allyl alcohol and allyl aldehyde on Si(1 0 0)2×1: Competition of [2 + 2] CC cycloaddition with O-H dissociation and with [2 + 2] CO cycloaddition in bifunctional molecules

Competition between the CC functional group with the OH group in allyl alcohol and with the CO group in allyl aldehyde in the adsorption and thermal chemistry on Si(1 0 0)2×1 has been studied by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD), as well as density-functional theory (DFT) calculations. The similarities found in the C 1s and O 1s spectra for both molecules indicate that the O-H dissociation product for allyl alcohol and [2 + 2] CO cycloaddition product for allyl aldehyde are preferred over the corresponding [2 + 2] CC cycloaddition products. Temperature-dependent XPS and TPD studies further show that thermal evolution of these molecules gives rise to the formation of ethylene, acetylene, and propene on Si(1 0 0)2×1, with additional CO evolution only from allyl alcohol. The formation of these desorption products also supports that the [2 + 2] CC cycloaddition reaction does not occur. In addition, the formation of SiC at 1090 K is observed for both allyl alcohol and allyl aldehyde. We propose plausible surface-mediated reaction pathways for the formation of these thermal evolution products. The present work illustrates the crucial role of the Si(1 0 0)2×1 surface in selective reactions of the Si dimers with the O−H group in allyl alcohol and with the CO group in allyl aldehyde over the CC functional group common to both molecules.     

Significance of the HO2 + CO reaction during the combustion of CO + H2 mixtures at high pressures

This paper constitutes an experimental and numerical study, using uncertainty analysis of the most important parameters, to evaluate the mechanism for the combustion of CO + H₂ mixtures at high pressures in the range 15-50 bar and temperatures from 950 to 1100 K. Experiments were performed in a rapid compression machine. Autoignition delays were measured for stoichiometric compositions of CO + H₂ containing between 0 and 80% CO in the total fuel mixture. The experimental results showed an unequivocal monotonic increase as the proportion of CO in the mixture was raised. Comparisons were made also with the measured ignition delays in mixtures of H₂ with increasing dilution by N₂, corresponding to the proportions of CO present. These times also increased monotonically, albeit with a greater sensitivity to the extent of dilution than those measured in the CO + H₂ mixtures. By contrast, numerical simulations for the same mixtures, based on a kinetic model derived by Davis et al. displayed a qualitative discrepancy as there was virtually no sensitivity of the ignition delay to the changing ratio of CO + H₂, certainly up to 80% replacement. No exceptions to this trend were found, despite tests being made using seven other kinetic models for CO + H₂ combustion. Global uncertainty analyses were then applied to the Davis et al. model in order to trace the origins of this discrepancy. The analyses took into account the uncertainties in all rate parameters in the model, which is a pre-requisite for evaluation against ignition delay data. It is shown that the reaction rate constant recommended by Baulch et al. for the HO₂ + CO reaction, at T ∼ 1000 K, could be up to a factor of 10 too high and that lowering this rate corrected the qualitative anomaly between experiment and numerical prediction.     

Layer-resolved photoelectron diffraction from Si(0 0 1) and GaAs(0 0 1)

Photoelectron diffraction in the layer-resolved mode brings more detailed information about local atomic arrangement than is obtained in the standard mode. This is demonstrated in crystals with diamond and zinc-blende structures, both for unpolarized photon excitation as well as for circularly polarized excitation. The full angular distributions of photoemission intensities are evaluated for large atomic clusters representing ideally truncated surfaces of Si(0 0 1) and GaAs(0 0 1). Highly structured layer-resolved patterns enable a more detailed understanding of the standard mode outcomes. Photoelectron intensities from atomic layers placed at different depths under the crystal surface provide direct evidence about electron attenuation and its anisotropy in crystals.     

STM study of Bi-on-Cu(1 0 0)

Scanning tunneling microscopy (STM) has been used to study the various possible structures of adsorbed Bi on the Cu(1 0 0) surface, after equilibration at a temperature of 520 K. All of the structures previously identified by X-ray diffraction (lattice gas, c(2 × 2), c(9√2 × √2)R45°, and p(10 × 10), in order of increasing Bi-coverage) were found to be present on a single sample produced by diffusing Bi onto the Cu(1 0 0) surface from a 3-d source. By investigating the possible coexistence of various pairs of phases, it was demonstrated that the c(2 × 2) phase transforms to the c(9√2 × √2)R45° phase by a first order transition, whereas the transition from c(9√2 × √2)R45° to p(10 × 10) is continuous. In addition, the structure of surface steps was studied as a function of Bi-coverage. The results showed that the presence of Bi changes the nature of the step-step interactions at the Cu(1 0 0) surface from repulsive to attractive. The attractive step-step interactions transform any small deviations from the nominal (1 0 0) orientation of the Cu substrate into (3 1 0) microfacets. When compared with the known equilibrium crystal shape (ECS) of Bi-saturated Cu, the observed microfaceting may imply that the ECS of Cu-Bi alloys is temperature dependent.     

Vibrationally-assisted dissociative adsorption of oxygen on Ru(0 0 0 1)-p(2 × 1)-O

Supersonic molecular beam technique combined with high resolution X-ray photoelectron spectroscopy using synchrotron radiation was applied to the study of the dynamics of dissociative adsorption of oxygen on Ru(0 0 0 1) surface in high coverage region. The Ru(0 0 0 1) surface pre-covered with oxygen atoms of 0.5 monolayer, which corresponds to the p(2 × 1)-O structure, was dosed to oxygen molecules with translational energy of 0.5 eV. Oxygen uptake was compared between the cases with and without the beam source heated in order to verify the effects of internal energy of oxygen. We found drastic enhancement in initial sticking probability of oxygen when the beam source was heated to 1400 K. We concluded that the enhancement of sticking probability is mainly caused by molecular vibrational excitation, indicating that dissociation barrier is located in the exit channel on potential energy surface.     

Structural analysis of the c(4 × 2) reconstruction in Si(0 0 1) and Ge(0 0 1) surfaces by low-energy electron diffraction

The c(4 × 2) structures in (0 0 1) surfaces of Si and Ge have been studied by low-energy electron diffraction (LEED). Using a proper cleaning method for the Si surface, we were able to observe clear c(4 × 2) LEED patterns up to incident energy of ∼400 eV as well as the Ge surface. Extensive experimental intensity-voltage curves allowed us to optimize the asymmetric dimer model up to the eighth layer (including the dimer layer) in depth in the dynamical LEED calculation. Optimized structural parameters are almost the same for the Si and Ge except for the height of the buckled-up atom of the asymmetric dimer. For the Ge surface, the structural parameters are in excellent agreement with those obtained by a previous theoretical calculation. The tilt angle and bond length of the dimer are 18 ± 1 (19 ± 1)° and 2.4 ± 0.1 (2.5 ± 0.1) Å for the Si(0 0 1) (Ge(0 0 1)), respectively.     

Olefin bonding mechanism and growth to (1 0 0)(2 × 1):H diamond

The bonding and growth mechanism of photochemically attached olefin molecules to (1 0 0)(2 × 1):H diamond is characterized using atomic force (AFM) and scanning tunneling microscopy (STM) experiments in combination with molecular orbital calculations. To identify growth schemas, diamond surfaces after 10, 40 and 90 min of photo-chemically stimulated growth have been characterized. These data show clearly island formation which is discussed taking into account a growth model from silicon. The island growth shows no directional properties which are attributed to arrangement and geometrical properties of hydrogen terminated carbon bonds at the surface of (1 0 0) oriented (2 × 1) reconstructed diamond.     

Surface phase transition and electronic structure of c(5√2 × √2)R45°-Pb/Cu(1 0 0)

The c(5√2 × √2)R45°-Pb/Cu(1 0 0) surface phase is investigated by means of angle resolved ultraviolet photoemission and low energy electron diffraction in the temperature range between 300 and 550 K. We identify and characterize a temperature-induced surface phase transition at 440 K from the room temperature c(5√2 × √2) R45° phase to a (√2 × √2)R45° structure with split superstructure spots. The phase transition is fully reversible and takes place before the two-dimensional melting of the structure at 520 K. The electronic structure of the split (√2 × √2)R45° phase is characterized by a metallic free-electron like surface band. This surface band is backfolded with c(5√2 × √2)R45° periodicity phase at room temperature, giving rise to a surface band gap at the Fermi energy. We propose that a gain in electronic energy explains in part the stability of the c(5√2 × √2)R45° phase.     

Electronic structure study of reconstructed Au-SiC(0 0 0 1) surfaces

A study of surface and interface properties of reconstructed Au-SiC(0 0 0 1) surfaces is reported. Two reconstructions were prepared on SiC(0 0 0 1), a √3 × √3R30° and a Si-rich 3 × 3, before Au deposition and subsequent annealing at different temperatures. For the Si-rich 3 × 3 surface the existence of three stable reconstructions 2√3 × 2√3R30°, 3 × 3 and 5 × 5 are revealed after deposition of Au layers, 4-8 Å thick, and annealing at progressively higher temperatures between 500 and 950 °C. For the 2√3 surface two surface shifted Si 2p components are revealed and the Au 4f spectra clearly indicate silicide formation. The variation in relative intensity for the different core level components with photon energy suggests formation of an ordered silicide layer with some excess Si on top. Similar core level spectra and variations in relative intensity with photon energy are obtained for the 3 × 3 and 5 × 5 phases but the amount of excess Si on top is observed to be smaller and an additional weak Si 2p component becomes discernable.For the √3 surface the evolution of the core level spectra after Au deposition and annealing is shown to be distinctly different than for the Si-rich 3 × 3 surface and only one stable reconstruction, a 3 × 3 phase, is observed at similar annealing temperatures.     

Structural study of Si(1 1 1)-6 × 1-Ag surface using surface X-ray diffraction

The surface structure of Si(1 1 1)-6 × 1-Ag was investigated using surface X-ray diffraction techniques. By analyzing the CTR scattering intensities along 00 rod, the positions of the Ag and reconstructed Si atoms perpendicular to the surface were determined. The results agreed well with the HCC model proposed for a 3 × 1 structure induced by alkali-metals on a Si(1 1 1) substrate. The heights of the surface Ag and Si atoms did not move when the surface structure changed from Si(1 1 1)-√3 × √3-Ag to Si(1 1 1)-6 × 1-Ag by the desorption of the Ag atoms. From the GIXD measurement, the in-plane arrangement of the surface Ag atoms was determined. The results indicate that the Ag atoms move large distances at the phase transition between the 6 × 1 and 3 × 1 structures.     

Electrochemical scanning tunneling microscopy examination of the structures of benzenethiol molecules adsorbed on Au(1 0 0) and Pt(1 0 0) electrodes

In situ electrochemical scanning tunneling microscopy (STM) has been used to examine the structures of benzenethiol adlayers on Au(1 0 0) and Pt(1 0 0) electrodes in 0.1 M HClO₄, revealing the formation of well-ordered adlattices of Au(1 0 0)-(√2 × √5) between 0.2 and 0.9 V and Pt(1 0 0)-(√2 × √2)R45° between 0 and 0.5 V (versus reversible hydrogen electrode), respectively. The coverage of Au(1 0 0)-(√2 × √5) is 0.33, which is identical to those observed for upright alkanethiol admolecules on Au(1 1 1). In comparison, the coverage of Pt(1 0 0)-(√2 × √2)R45° - benzenethiol is 0.5, much higher than those of thiol molecules on gold surfaces. This result suggests that benzenethiol admolecules on Pt(1 0 0) could stand even more upright than those on Au(1 0 0). All benzenethiol admolecules were imaged by the STM as protrusions with equal corrugation heights, suggesting identical molecular registries on Au(1 0 0) and Pt(1 0 0) electrodes, respectively. Modulation of the potential of a benzenethiol-coated Au(1 0 0) electrode resulted in irreversible desorption of admolecules at E ? 0.1 V (vs. reversible hydrogen electrode) and oxidation of admolecules at E ? 0.9 V. In contrast, benzenethiol admolecule was not desorbed from Pt(1 0 0) at potentials as negative as the onset of hydrogen evolution. Raising the potential rendered deposition of more benzenethiol molecules before oxidation of admolecules commenced at E > 0.9 V.     

Growth and morphology of the epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) Trilayers

Growth and surface morphology of epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) trilayers constituting a magnetic tunnel junction were investigated by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). STM reveals a grain-like growth mode of MgO on Fe(1 1 0) resulting in dense MgO(1 1 1) films at room temperature as well as at 250 °C. As observed by STM, initial deposition of MgO leads to a partial oxidation of the Fe(1 1 0) surface which is confirmed by Auger electron spectroscopy. The top Fe layer deposited on MgO(1 1 1) at room temperature is relatively rough consisting of clusters which can be transformed by annealing to an atomically flat epitaxial Fe(1 1 0) film.     

Study of vacancy on diamond (1 0 0) (2 × 1) surface from first-principles

Structure and energy related properties of neutral and charged vacancies on relaxed diamond (1 0 0) (2 × 1) surface were investigated by means of density functional theory. Calculations indicate that the diffusion of a single vacancy from the top surface layer to the second layer is not energetically favored. Analysis of energies in charged system shows that neutral state is most stable on diamond (1 0 0) (2 × 1) surface. The multiplicity of possible states can exist on diamond (1 0 0) surface in dependence on the surface Fermi level, which supports that surface diffusion of a vacancy is mediated by the change of vacancy charge states. Analysis of density of states shows surface vacancy can be effectively measured by photoelectricity technology.     

Adsorption and reaction of methanethiol on the Ru(0 0 0 1)-p(2 × 2)O surface: A TPD and XPS study

Methanethiol adsorbed on Ru(0 0 0 1)-p(2 × 2)O has been studied by TPD and XPS. The dissociation of methanethiol to methylthiolate and hydrogen at 90 K is evidenced by the observation of hydroxyl and water. The saturation coverage of methylthiolate is ∼0.15 ML, measured by both XPS and TPD. A detailed analysis suggests that only the hcp-hollow sites have been occupied. Upon annealing the surface, water and hydroxyl desorb from the surface at ∼210 K. Methylthiolate decomposes to methyl radical and atomic sulphur via C-S cleavage between 350 and 450 K. Some methyl radicals (0.05 ML) have been transferred to Ru atoms before they decompose to carbon and hydrogen. The rest of methyl radicals desorb as gaseous phase. No evidence for the transfer of methyl radical to surface oxygen has been found.     

Surface properties, textural features and catalytic performance for NO + CO abatement of spinels MAl2O4 (M = Mg, Co and Zn) developed by reverse and bicontinuous microemulsion method

Three typical spinels of general formula MAl₂O₄ (M = Mg, Co and Zn) have been successfully prepared via a microemulsion method both in the reverse and bicontinuous state. The final solids were characterized by X-ray diffraction followed by Rietveld analysis, N₂ adsorption-desorption porosimetry and SEM. Pore connectivity (c) was also calculated with Seaton's method. The analysis of all these properties shown that spinels prepared via reverse microemulsion route have better surface and textural properties than bicontinuous ones. The spinels were tested for NO + CO reaction and reverse spinels shown better catalytic activity than bicontinuous ones while the full sequence of catalytic activity is: ZnAl₂O₄-r > ZnAl₂O₄-b > MgAl₂O₄-r > MgAl₂O₄-b > CoAl₂O₄-r > CoAl₂O₄-b. The Rietveld analysis helped us to give an explanation about the catalytic activity and shown that the configuration of inverse spinel phase is the critical factor for the catalytic behavior of final solids. The reactants NO and CO react in a 2:1 ratio at low temperature but they convert in a 1:1 ratio at high temperatures. From the kinetic analysis the heats of NO adsorption are estimated and are in full agreement with the results of catalytic activity.     

Effects of electronic confinement and substrate on the low-temperature growth of Pb islands on Si(1 0 0)-2 × 1 surfaces

The growth of Pb films on the Si(1 0 0)-2 × 1 surface has been investigated at low temperature using scanning tunneling microscopy. Although the orientation of the substrate is (1 0 0), flat-top Pb islands with (1 1 1) surface can be observed. The island thickness is confined within four to nine atomic layers at low coverage. Among these islands, those with a thickness of six layers are most abundant. Quantum-well states in Pb(1 1 1) islands of different thickness are acquired by scanning tunneling spectroscopy. They are found to be identical to those taken on the Pb(1 1 1) islands grown on the Si(1 1 1)7 × 7 surface. Besides Pb(1 1 1) islands, two additional types of Pb islands are formed: rectangular flat-top Pb(1 0 0) islands and rectangular three-dimensional (3D) Pb islands, and both their orientations rotate by 90° from a terrace to the adjacent one. This phenomenon implies that the structures of Pb(1 0 0) and 3D islands are influenced by the Si(1 0 0)-2 × 1 substrate.     

CO2 adsorption on Cr(1 1 0) and Cr2O3(0 0 0 1)/Cr(1 1 0)

We attempt to correlate qualitatively the surface structure with the chemical activity for a metal surface, Cr(1 1 0), and one of its surface oxides, Cr₂O₃(0 0 0 1)/Cr(1 1 0). The kinetics and dynamics of CO₂ adsorption have been studied by low energy electron diffraction (LEED), Aug er electron spectroscopy (AES), and thermal desorption spectroscopy (TDS), as well as adsorption probability measurements conducted for impact energies of E_i = 0.1-1.1 eV and adsorption temperatures of T_s = 92-135 K. The Cr(1 1 0) surface is characterized by a square shaped LEED pattern, contamination free Cr AES, and a single dominant TDS peak (binding energy E_d = 33.3 kJ/mol, first order pre-exponential 1 × 10¹³ s⁻¹). The oxide exhibits a hexagonal shaped LEED pattern, Cr AES with an additional O-line, and two TDS peaks (E_d = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S₀, is independent of T_s for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing E_i, with S₀ smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low E_i is approx. independent of coverage (Kisliuk-shape) and increases initially at large E_i with coverage (adsorbate-assisted adsorption). CO₂ physisorbs on both systems and the adsorption is non-activated and precursor mediated. Monte Carlo simulations (MCS) have been used to parameterize the beam scattering data. The coverage dependence of E_d has been obtained by means of a Redhead analysis of the TDS curves.     

Characteristic energy band values and electron attenuation length of a chemical-vapor-deposition diamond (0 0 1)2 × 1 surface

The characteristic energy band values such as the Fermi-level position with respect to valence band top for a boron-doped p-type hydrogen-terminated chemical-vapor-deposition (CVD) diamond (0 0 1)2 × 1 surface and for a clean CVD diamond (0 0 1)2 × 1 surface have been determined by a new method with an accuracy of ±0.02 eV. The electron attenuation length for the clean diamond (0 0 1)2 × 1 surface for the electron kinetic energy of C 1s X-ray photoemission peak by Mg Kα excitation is experimentally determined to be 2.1-2.2 nm. These values are compared and discussed with the previously reported experimental and simulation values.