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.     

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.     

CO oxidation over Ru(0 0 0 1) at near-atmospheric pressures: From chemisorbed oxygen to RuO2

RuO₂(1 1 0) was formed on Ru(0 0 0 1) under oxygen-rich reaction conditions at 550 K and high pressures. This phase was also synthesized using pure O₂ and high reaction temperatures. Subsequently the RuO₂ was subjected to CO oxidation reaction at stoichiometric and net reducing conditions at near-atmospheric pressures. Both in situ polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and post-reaction Auger electron spectroscopy (AES) measurements indicate that RuO₂ gradually converts to a surface oxide and then to a chemisorbed oxygen phase. Reaction kinetics shows that the chemisorbed oxygen phase has the highest reactivity due to a smaller CO binding energy to this surface. These results also show that a chemisorbed oxygen phase is the thermodynamically stable phase under stoichiometric and reducing reaction conditions. Under net oxidizing conditions, RuO₂ displays high reactivity at relatively low temperatures (?450 K). We propose that this high reactivity involves a very reactive surface oxygen species, possibly a weakly bound, atomic oxygen or an active molecular O₂ species. RuO₂ deactivates gradually under oxidizing reaction conditions. Post-reaction AES measurements reveal that this deactivation is caused by a surface carbonaceous species, most likely carbonate, that dissociates above 500 K.     

Double counting in LDA + DMFT—The example of NiO

An intrinsic issue of the LDA + DMFT approach is the so called double counting of interaction terms. How to choose the double-counting potential in a manner that is both physically sound and consistent is unknown. We have conducted an extensive study of the charge-transfer system NiO in the LDA + DMFT framework using quantum Monte Carlo and exact diagonalization as impurity solvers. By explicitly treating the double-counting correction as an adjustable parameter we systematically investigated the effects of different choices for the double counting on the spectral function. Different methods for fixing the double counting can drive the result from Mott insulating to almost metallic. We propose a reasonable scheme for the determination of double-counting corrections for insulating systems.     

Controlling the micropore size of activated carbons for the treatment of fuels and combustion gases

Exclusively microporous activated carbons have been prepared from cork by physical and chemical activation under different conditions. The results show that it is possible to control the pore size of the activated carbons and to obtain materials with narrow micropore size (≥0.69 nm) and high micropore volume (≤0.64 cm³ g⁻¹) equal to or better than the best activated carbon fibres. Higher micropore volumes are generally obtained by chemical activation at higher temperature using dry or potassium hydroxide impregnation. On the other hand, wet or carbonate impregnation, as well as high temperature, or physical activation with CO₂ or H₂O under appropriate conditions, favours low mean pore widths.     

Characterization of diamond (1 0 0) surface with oxygen termination

Ab initio density functional theory (DFT) was employed to study reconstructions of diamond (1 0 0) surfaces in the presence of hydrogen, oxygen and hydroxyl. Clean and (2 × 1):1H surfaces are taken as reference. The properties of oxidization diamond surfaces with several adsorption structures, namely, O-on-top (OT) site, O-bridge (BR) site, hydroxyl (-OH), hydroxyl/hydroxyl, OT/hydroxyl, BR/hydroxyl have been considered. The calculated results indicate that the BR model is much more stable than the OT model, and the most energetically favorable structures of oxygenated surfaces are those with chemisorbed hydroxyl (-OH) group. Furthermore, the stability of the structures is also discussed from the point of HOMO-LUMO gap. Analysis of electronic structures shows that the presence of hydrogen induces surface conductivity whereas oxygen weakens it.     

On the formation of OH ordered layers by dissociation of H2O on an oxygen covered Ag(1 1 0) surface: An STM investigation

We present results of an STM investigation of water interaction with an oxygen covered Ag(1 1 0) on the example of the O(4 × 1) reconstructed surface. In agreement with numerous previous experimental works, using diffraction techniques, we found that a structure of OH(1 × 2) type, displaying rows in the [1 −1 0] direction, is formed. The new features revealed by this local probe study, is the presence of quasi rectangular islands evenly distributed across the terraces, with a density of 0.22 ± 0.03 and a mean area of 90 ± 15 nm² at 220 K. They are imaged at an apparent height of 0.14 nm. It is remarkable that the same OH row structure is present on the whole terrace “on top” and “in between” the islands. These features are attributed to silver islands of mono-atomic height, formed by clustering of silver ad-atoms released during reaction of the O atoms with the water molecules. These findings point to a more complex behaviour of the reaction dynamics than previously described. They emphasise the key role of the silver ad-atoms, present in the added rows of the initial Ag(1 1 0)-O(4 × 1) surface, in the formation of the nanostructures. In turn it is concluded that the rows evidenced by this STM and previous diffraction studies, are formed by OH chains.     

Molecularly resolved studies of the role of basicity in the reaction of amines with oxygen at a Cu(1 1 0) surface

The role of basicity in the surface reactions of amines has been investigated in an STM/XPS study of the oxidation of methylamine, ethylamine and 2,2,2-trifluoroethylamine at a Cu(1 1 0) surface. Methylamine and ethylamine are strongly basic molecules and for sub monolayer concentrations of oxygen form an amine/oxygen complex characterised by a (3 × 1) structure. The complex rapidly decomposes at room temperature to form water and an imide which desorbs via β-elimination. In contrast, 2,2,2-trifluoroethylamine has a relatively low basicity and its reactions with surface oxygen do not involve a stable intermediate.     

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.     

Kinetics of the N2O-CO reaction on Rh(1 1 0)

Our recent studies of the steady-state kinetics of the N₂O-CO reaction on Rh(1 1 0) indicate that at CO excess, the reaction rate increases with increasing temperature. At N₂O excess, the reaction rate is nearly independent of temperature at T < 520 K and rapidly decreases with increasing temperature at T > 520 K. Our present analysis of the relevant data indicates that the latter feature seems to be related to surface-oxide formation. Following this line, we propose a mean-field kinetic model making it possible to describe and clarify the experiment.     

A theoretical study of the water gas shift reaction mechanism on Cu(1 1 1) model system

The water gas shift (WGS) reaction is an important reaction system and has wide applications in several processes. However, the mechanism of the reaction is still in dispute. In this paper we have investigated the reaction mechanism on the model Cu(1 1 1) system using the density functional method and slab models. We have characterized the kinetics and the thermodynamics of the four reaction pathways containing 24 elementary steps and computed the reaction potential energy surfaces. Calculations show that the formate (HCOO) intermediate mechanism (CO + OH → HCOO → CO₂ + H) and the associative mechanism (CO + OH → CO₂ + H) are kinetically unlikely because of the high formation barrier. On the other hand, the carboxyl (HOCO) intermediate mechanism (CO + OH → HOCO → CO₂ + H) and the redox mechanism (CO + O → CO₂) are demonstrated to be feasible. Our calculations also indicate that surface oxygen atoms can reduce the barriers of both water dissociation and HOCO decomposition significantly. The calculated potential energy surfaces show that the water dissociation which produces OH groups is the rate-determining step at the initial stage of the reaction or in the absence of surface oxygen atoms. With the development of the reaction or in the presence of oxygen atoms on the surface, CO + OH → HOCO and CO + O → CO₂ become the rate-limiting step for the carboxyl and redox mechanisms, respectively.     

The reaction of vinyl acetate with Pd(1 1 0) studied with TPD and molecular beams

The adsorption and reaction of vinyl acetate with the clean Pd(1 1 0) surface has been investigated using temperature programmed desorption and molecular beam reaction measurements. These show that, under low pressure conditions, the main reaction pathway above 400 K is total dehydrogenation to yield hydrogen and carbon dioxide in the gas phase, and surface carbon. This occurs at a steady state, notwithstanding the fact that carbon is being deposited continuously onto the surface. The reaction continues because the vast majority of this carbon is lost from the surface to the bulk of the sample. Between about 320-380 K the reaction profile is somewhat different; the molecule dissociates at the CH₃COOCHCH₂ bond, producing the most stable intermediate, the acetate, and the reaction stops after the build-up of adsorbed acetate and surface carbonaceous species. At ∼300 K, the products are very similar to those for acetaldehyde adsorption (namely, methane, CO and some surface carbon), and they evolve in a non-steady state manner due to the build up of adsorbed CO on the surface. Thus the mechanism is dominated here by dissociation at the CH₃COOCHCH₂ bond, and formation of the acetyl intermediate. Consideration is given to the connection between these data and vinyl acetate synthesis.     

Adsorption behaviors of V2O5 nanowires on binary mixed self-assembled monolayers

The adsorption behaviors of V₂O₅ nanowires on binary mixed self-assembled monolayers (SAMs) were investigated with variation of the mixing ratio of two differently terminated thiolates on Au. Hydroxyl-covered V₂O₅ nanowires showed a preferential adsorption on amine (NH₂)-terminated thiolates over methyl (CH₃)-terminated ones. However, on the binary mixed SAM of NH₂- and CH₃-terminated thiols, the adsorption behavior did not follow a simple expectation based upon the electrostatic interaction. The total number of adsorbed V₂O₅ nanowires increased with the mole fraction of NH₂-terminated thiolates up to χNH₂∼0.5, then it decreased with further increase of χNH₂. The height distribution of adsorbed nanowires showed that the relative portion of the agglomerated wires thicker than 3.5 nm to individual wires thinner than 3.5 nm increased up to χNH₂∼0.75 and then it decreased with further increase of χNH₂. The dispersion of molecules with polar-functional groups as well as the molecular ordering of mixed SAMs is attributed to such adsorption behaviors of V₂O₅ nanowires.     

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.     

Analysis of the kinetics of N2O-CO reaction on Pd(1 1 0)

Experimental studies indicate that the N₂O-CO reaction occurring on Pd(1 1 0) under UHV conditions exhibits a first-order kinetic phase transition in the steady-state case and also transient kinetics strongly dependent on the initial state of the system. We construct a mean-field kinetic model describing these phenomena. With a minimal number of the fitting parameters, the model reasonably reproduces the special features of the reaction kinetics.     

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.     

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.     

Nanostructure formation by reactions of H2O with pre-adsorbed O on a Ag(1 1 0) surface

We present results of an STM investigation of water interaction with an oxygen covered Ag(1 1 0) in the case of the O(4 × 1) reconstructed surface. Regarding the formation of one-layer-thick silver nanostructures previously demonstrated, they point to the key role of the surface temperature at which the water dosing is made. Indeed we measure silver nanostructuring for dosing temperatures lower than 235 K. We follow, in real time during the water dosing, the modifications induced at the surface for two temperatures of 200 K and 240 K. Drastic differences are exhibited. At 200 K, after an initial stage of formation of molecular assembly strips along the [0 0 1], the reactive process leading to the conversion to an OH layer occurs clearly going along with the appearance and development of quasi-rectangular silver nanostructures. At 240 K, no such initial phase is evidenced. The complete conversion to an OH row structure of the scanned area occurs with no concomitant silver nanostructure formation. The dynamical behaviour of the reaction front allows the unravelling of the key role of the developing OH row ends intersecting the remaining Ag-O rows as particular reactive adsorption sites for the completion of the OH layer.     

Formation and characterization of the Cu2O overlayer on Zn-terminated ZnO(0 0 0 1)

Low-energy electron diffraction, X-ray photoelectron spectroscopy and synchrotron-radiation-excited angle-resolved photoelectron spectroscopy have been used to characterize Cu-oxide overlayers on the Zn-terminated ZnO(0 0 0 1) surface. Deposition of Cu on the ZnO(0 0 0 1)-Zn surface results in the formation of Cu clusters with (1 1 1) top terraces. Oxidation of these clusters by annealing at 650 K in O₂ atmosphere (1.3 × 10⁻⁴ Pa) leads to an ordered Cu₂O overlayer with (1 1 1) orientation. Good crystallinity of the Cu₂O(1 1 1) overlayer is proved by energy dispersion of one of Cu₂O valence bands. The Cu₂O(1 1 1) film exhibits a strong p-type semiconducting nature with the valence band maximum (VBM) of 0.1 eV below the Fermi level. The VBM of ZnO at the Cu₂O(1 1 1)/ZnO(0 0 0 1)-Zn interface is estimated to be 2.4 eV, yielding the valence-band offset of 2.3 eV.     

Coadsorption of CO and O on Ir(1 0 0): First principles calculations

Density functional theory (DFT) calculations with generalized gradient approximation (GGA) are performed to investigate the structural, electronic and energetic properties of Ir(1 0 0)-(mCO+nO), m=1, 2; n=1, 2 and 3, coadsorption systems. The stability of coadsorbed oxygen is not affected by the increase of CO coverage from 0.25 ML to 0.50 ML. However, the stability of CO increases by increasing O coverage from 0.25 ML to 0.50 ML and 0.75 ML as a result of the rumpling of the topmost Ir layer.