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1.
The water adsorption on the bare and H-terminated Si(1 0 0) surfaces has been studied by the BML-IRRAS technique. It is found that H-terminated surfaces are much less reactive compared to the bare silicon surfaces. The (1 × 1)-H and (3 × 1)-H surfaces show similar and less reactivity pattern compared to the (2 × 1)-H surface. At higher exposures, the water reaction with coupled monohydride species provides an effective channel for oxygen insertion into the back bonds of dihydride species. It is not attributed to the H–Si–Si–H + H 2O → H–S–Si–OH + H 2, which could give rise to the characteristic Si–H and Si–OH modes, respectively at 2081 and 921 cm −1. A more suitable reaction mechanism involving a metastable species, H–Si–Si–H + H 2O → H 2Si HO–Si–H (metastable) explains well the bending modes of oxygen inserted silicon dihydride species which are observed relatively strongly in the reaction of water with H-terminated Si(1 0 0) surfaces. 相似文献
2.
The mechanism and energetics are presented of the dimerization of two adsorbed surface SiH 2 groups on the H-terminated Si(0 0 1)-(2 × 1) surface to form Si 2H 4 species during the initial stages of growth in plasma deposition of hydrogenated amorphous silicon (a-Si:H) films. The reactions are observed during classical molecular-dynamics (MD) simulations of a-Si:H film deposition from SiH 2 radical precursors impinging on an initially H-terminated Si(0 0 1)-(2 × 1) surface and substrate temperature, T, over the range 500 T700 K. The Si 2H 4 species resulting from the surface SiH 2 dimerization reactions undergo surface conformational changes resulting in either a non-rotated (NRD) or a rotated dimer (RD) configuration. The RD configuration is found to be the energetically favorable one. The MD simulation results for the structure of the NRD and RD surface Si 2H 4 configurations corroborate with ab initio calculations of optimized adsorption configurations of SiH 2 radicals on crystalline Si surfaces, as well as results of STM imaging of the thermal decomposition of disilane on Si(0 0 1). 相似文献
3.
Scanning tunneling microscopy experiments on a clean, reduced SnO 2(1 0 0)-(1 × 1) surface reveal surface defects with zero-, one-, and two-dimensions. Point defects consist of missing SnO/SnO 2 units. Line defects are probably crystallographic shear planes that extend to the surface and manifest themselves as rows of atoms, shifted half a unit cell along the [0 1 0] direction. Their ends act as preferential nucleation sites for the formation of Pd clusters upon vapor deposition. Areas of a more reduced surface phase, still with a (1 × 1) structure and a half-unit cell deep, form at [0 0 1]-oriented step edges. 相似文献
4.
A theoretical analysis based on the Hartree–Fock pseudopotential method and a density-functional theory calculation using a hybrid combination of general gradient approximation with pseudopotential procedure has been carried out to study the adsorption and dissociation of methanol on the stoichiometric SnO 2(110) surface. The dependence of the results upon model system and computing method is discussed. An optimization procedure of adsorbate and substrate atom positions on a six-layer slab model has been selected to characterize the corresponding geometric parameters, adsorption energy and charge-transfer processes related with the molecularly adsorbed CH 3OH and dissociative channels to yield methoxy or methyl fragments. In the high-coverage limit ( θ=1), we find that dissociation of the methanol molecule via the heterolytic cleavage of the C---O bond is favoured. At lower coverage ( θ=1/2), this channel and the molecularly adsorbed methanol present similar adsorption energies. 相似文献
5.
The adsorption behavior and thermal activation of carbon dioxide on the Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces have been investigated by means of density functional theory calculations and cluster models and periodic slabs. According to the cluster models, the optimized results indicate that the basis set of C and O atoms has a distinct effect on the adsorption energy, but an indistinct one on the equilibrium geometry. For the CO 2/Cu( hkl) adsorption systems studied here, the final structure of adsorbed CO 2 is near linear and the preferred modes for the adsorption of CO 2 onto the Cu(1 1 1), Cu(1 0 0), and Cu(1 1 0) surfaces are the side-on adsorption at the cross bridge site with an adsorption energy of 13.06 kJ/mol, the side-on adsorption at the short bridge site (13.54 kJ/mol), and the end-on adsorption on the on-top site with C–O bonds located along the short bridge site (26.01 kJ/mol), respectively. However, the calculated adsorption energies from periodic slabs are lower as compared to the experimental data as well as the cluster model data, indicating that the periodic slab approach of generalized gradient approximation in the density function theory may be not suitable to obtain quantitative information on the interaction of CO 2 with Cu( hkl) surfaces. 相似文献
6.
The structure of the (0 0 0 1) surface of the -MgCl 2 crystal has been investigated using DFT-GGA periodic calculations. The calculated surface relaxation is in agreement with LEED measurements. Motivated for the use of MgCl 2 as support for the Ziegler–Natta reaction, we have studied the adsorption of the catalyst (titanium chlorides as monomers or dimers) on the (1 0 0) and (1 1 0) MgCl 2 surfaces. The structures of adsorbed species are close to those previously found on cluster models: bridging chlorine atoms connect the Ti to the Mg atoms and the systems remain in high spin states. The (0 0 0 1) surface is the most stable face of the -MgCl 2 crystal; however it is Cl-terminated and henceforth poorly reactive; it had been suggested to deposit metallic Mg in order to improve its reactivity. Our modelling explains the failure of this tentative; the interaction between the deposited metal and the surface is repulsive and uncharged Mg atom does not bind. 相似文献
7.
First-principles calculations based on density functional theory and the pseudopotential method have been used to investigate the energetics of H 2O adsorption on the (110) surface of TiO 2 and SnO 2. Full relaxation of all atomic positions is performed on slab systems with periodic boundary conditions, and cases of full and half coverage are studied. Both molecular and dissociative (H 2O→OH −+H −) adsorption are treated, and allowance is made for relaxation of the adsorbed species to unsymmetrica configurations. It is found that for both TiO 2 and SnO 2 an unsymmetrical dissociated configuration is the most stable. The symmetrical molecularly adsorbed configuration is unstable with respect to lowering of symmetry, and is separated from the fully dissociated configuration by at most a very small energy barrier. The calculated dissociative adsorption energies for TiO 2 and SnO 2 are in reasonable agreement with the results of thermal desorption experiments. Calculated total and local electronic densities of states for dissociatively and molecularly adsorbed configurations are presented, and their relation with experimental UPS spectra is discussed. 相似文献
8.
The unique ability of the vanadyl pyrophosphate (1 0 0) surface to activate n-butane and then selectively oxidise the hydrocarbon to maleic anhydride was investigated using modern quantum chemical methods. Bulk (VO) 2P 2O 7, together with stoichiometric and phosphorus-enriched (1 0 0) surfaces, were analysed using periodic density functional theory calculations. Also simulated was surface ionic relaxation from bulk geometry, and surface hydration. Density of states (DOS) plots show that, whether stoichiometric or phosphorus-enriched, bulk terminated or relaxed, bare or hydrated, local covalent reactivity at the (1 0 0) surface is controlled by vanadium species. Terminal P-O oxygen species are the most nucleophilic surface oxygens, as indicated by their predominance of sub-vanadium high-lying valence band levels. A periodic treatment of (VO) 2P 2O 7(1 0 0) hence gives results qualitatively identical to those obtained from earlier cluster calculations. Simulation of surface ionic relaxation shows that in-plane P-O-V oxygens may also be involved in rupture of substrate C-H bonds for mild oxidation, while surface hydration calculations indicate that dissociative chemisorption of water may play a key role in perpetuation of the selective oxidation cycle. 相似文献
9.
Mo(CO) 6 can be useful as a precursor for the preparation of Mo and MoS x nanoparticles on a Au(1 1 1) substrate. On this surface the carbonyl adsorbs intact at 100 K and desorbs at temperatures lower than 300 K. Under these conditions, the dissociation of the Mo(CO) 6 molecule is negligible and a desorption channel clearly dominates. An efficient dissociation channel was found after dosing Mo(CO) 6 at high temperatures (>400 K). The decomposition of Mo(CO) 6 yields the small coverages of pure Mo that are necessary for the formation of Mo nanoclusters on the Au(1 1 1) substrate. At large coverages of Mo (>0.15 ML), the dissociation of Mo(CO) 6 produces also C and O adatoms. Mo nanoclusters bonded to Au(1 1 1) exhibit a surprising low reactivity towards CO. Mo/Au(1 1 1) surfaces with Mo coverages below 0.1 ML adsorb the CO molecule weakly (desorption temperature<400 K) and do not induce C–O bond cleavage. These systems, however, are able to induce the dissociation of thiophene at temperatures below 300 K and react with sulfur probably to form MoS x nanoparticles. The formed MoS x species are more reactive towards thiophene than extended MoS 2(0 0 0 2) surfaces, MoS x films or MoS x/Al 2O 3 catalysts. This could be a consequence of special adsorption sites and/or distinctive electronic properties that favor bonding interactions with sulfur-containing molecules. 相似文献
10.
Ultrathin layers of cerium oxide have been deposited on a Rh(1 1 1) surface and their growth morphology, structure, and thermal stability have been investigated by LEED, STM, XPS, and valence band resonant photoemission. STM and LEED indicate that the ceria grows epitaxially in form of ordered CeO 2 islands at elevated substrate temperature (250–300 °C), with (1 1 1) faces parallel and orientationally aligned to the main azimuthal directions of the substrate. The ultrathin ceria films contain significant amounts of reduced Ce 3+ species, which appear to be located predominantly at the ceria–Rh interface. For thicker films (>6 equivalent monolayers) stoichiometric CeO 2 is detected in XPS. Vacuum annealing produces morphologically well-defined hexagonal islands, accompanied by partial reduction and the formation of oxygen vacancies at the ceria surface. The thermal stability and the degree of reduction is a function of the oxide layer thickness, with thinner layers being thermally less stable. At temperatures >800 °C, the ceria decomposes and Ce–Rh alloy phases are identified. 相似文献
11.
The adsorption of CO and CO 2 on K-predosed Pd{1 1 0} at room temperature has been examined via reflection–absorption infrared spectroscopy (RAIRS). CO 2 adsorbs on 0.37 ML K-predosed Pd{1 1 0} with high sticking probability and a reactive chemisorbed intermediate, CO 2−, is detected in RAIRS at room temperature. Reaction of this species ultimately yields carbonate. The same high K precoverage induces dissociation of CO at low CO exposure. Carbonate is detected at higher CO exposure and is probably produced via stepwise oxidation of molecularly adsorbed CO. In contrast at low K precoverage (0.11 ML), CO remains intact but the C–O bond is considerably weakened with respect to CO chemisorbed on clean Pd{1 1 0}. These findings illustrate a dual promoter mechanism of K in the adsorption and reaction of CO or CO 2 at high K coverage. The alkali metal induces dissociation of these molecules and directly participates in the formation of a surface compound, K 2CO 3. 相似文献
12.
First-principles calculations have been performed to investigate the adsorption of oxygen on unreconstructed and reconstructed Ni(1 1 0) surfaces. The energetics, structural, electronic and magnetic properties are given in detail. For oxygen adsorption on unreconstructed surface, ( n×1)( n=2,3) substrate with oxygen atom on short-bridge site is found to be the most stable adsorption configuration. Whereas energetically most favorable adsorption phase of reconstructed surface is p( n×1) substrate with oxygen atom located at long-bridge site. Our calculations suggest that the surface reconstruction is induced by the oxygen adsorption. We also find there are redistributions of electronic structure and electron transfer from the substrate to adsorbate. Our calculations also indicate surface magnetic moment is enhanced on clean surfaces and oxygen atoms are magnetized weakly after oxygen adsorption. Interestingly, adsorption on unreconstructed surface does not change surface magnetic moment. However, adsorbate leads to reduction of surface magnetic moment in reconstructed system remarkably. 相似文献
13.
In situ solid-state NMR methodologies have been used to investigate the photocatalytic oxidation of ethanol (CH 3CH 2OH) over a series of SnO 2-based photocatalysts. The adsorption of ethanol on commercially available SnO 2 powder was studied using both cross-polarization 13C NMR and REDOR experiments, and showed the formation of two surface ethanol species, hydrogen-bonded ethanol at surface hydroxyl groups and ethanol chemisorbed to the SnO 2 surface (Sn–OCH 2CH 3). 13C NMR of the adsorbed ethanol was used to characterize the surface of monolayer SnO 2–TiO 2 coupled photocatalysts supported on porous Vycor glass. In situ solid-state NMR studies showed that the photooxidation of ethanol over the monolayer photocatalysts was slower than that over a supported TiO 2 monolayer photocatalyst due to the build-up of reaction intermediates such as acetic acid on the catalyst surface. 119Sn NMR experiments characterized the tin species on the porous Vycor glass support. 相似文献
14.
Oxide catalysts are frequently used to convert toxic species to environmentally benign molecules, and to prevent the formation of toxic species in the first place. In this paper, growth and characterization of model oxide systems employed in both approaches is discussed. An example of the former approach is the selective catalytic reduction (SCR) of NO emitted from power plants by NH 3, which employs tungsten and vanadium oxides supported on the anatase polymorph of TiO 2. To model SCR catalysts, epitaxial titanium, vanadium and tungsten oxide films were grown using molecular beam epitaxy and magnetron sputtering. Two different anatase orientations were grown on LaAlO 3 substrates and their interactions with vanadia were characterized. On LaAlO 3 (0 0 1), anatase exposed a (4 × 1) reconstructed (0 0 1) surface. Vanadia lifted the reconstruction and at 1 ML a (1 × 1) surface with mostly V 5+ was observed. Continued V 2O 5 growth led to loss of order, but at high temperatures epitaxial VO 2 could be grown; vanadia behaved similarly on anatase films on LaAlO 3 (1 1 0). Results suggested that the monolayer is pseudomorphic with O adsorption oxidizing the surface V to 5+, since the anatase structure cannot accommodate more bulk oxygen, only a monolayer can be pseudomorphic and have only V 5+. Thus the vanadia monolayer has unique structural and chemical properties that can help explain why vanadia monolayers on TiO 2 are much more active than bulk V 2O 5. For WO 3, a series of added row reconstructions were observed as the epitaxial films were reduced. The effect of these structures on surface chemistry was characterized by studying 1-propanol adsorption. The results indicated that the structure of the WO 3 surface did not alter its catalytic function but had a strong effect on reaction kinetics. As an example of a system where catalysts prevent the formation of toxic species, the reactivity of oxidized Pd surfaces used in CH 4 catalytic combustion were studied. An ordered PdO-like monolayer was found to be less reactive towards CO than adsorbed O on Pd. On the other hand, the PdO layer favored a lower activation energy C 3H 6 oxidation pathway. The results indicated that Pd oxidation reduces the sticking coefficient of reactive species but once molecules adsorb, the oxide surface can reduce the activation energy for subsequent reaction. 相似文献
15.
The adsorption and dissociation of CH 2I 2 were studied at 110 K with the aim of generating CH 2 species on the Ru(001) surface. The methods used included X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), temperature programmed desorption (TPD), Auger electron spectroscopy (AES) and work function measurements. Adsorption of CH 2I 2 is characterized by a work function decrease (0.96 eV at monolayer), indicating that adsorbed CH 2I 2 has a positive outward dipole moment. Three adsorption states were distinguished: a multilayer ( Tp=200 K), a weakly bonded state ( Tp=220 K) and an irreversibly adsorbed state. A new feature is the formation of CH 3I, which desorbs with Tp=160 K. The adsorption of CH 2I 2 at 110 K is dissociative at submonolayer, but molecular at higher coverages. Dissociation of the monolayer to CH 2 and I proceeded at 198–230 K, as indicated by a shift in the I(3d 5/2) binding energy from 620.6 eV to 619.9 eV. A fraction of adsorbed CH 2 is self-hydrogenated into CH 4 ( Tp=220 K), and another one is coupled to di-σ-bonded ethylene, which — instead of desorption — is converted to ethylidyne at 220–300 K. Illumination of the adsorbed CH 2I 2 initiated the dissociation of CH 2I 2 monolayer even at 110 K, and affected the reaction pathways of CH 2. 相似文献
16.
The interaction of SO 2 with oxygen-sputtered Au(1 1 1) ( θoxygen 0.35 ML) was studied by monitoring the oxygen and sulfur coverages as a function of SO 2 exposure. The morphology of the sputtered Au is relatively smooth on a long length scale, but rough on a finer scale with islands averaging 15 nm. The rough surface is not stable to scanning with the STM. Two reaction regimes were observed: oxygen depletion followed by sulfur deposition. An enhanced, transient sulfur deposition rate is observed at the oxygen depletion point. This effect is specifically pronounced if the Au surface is continuously exposed to SO 2. The enhanced reactivity towards S deposition seems to be linked to the presence of highly reactive, under-coordinated Au atoms. Adsorbed oxygen appears to stabilize, but also to block these sites. In absence of the stabilization effect of adsorbed oxygen, i.e. at the oxygen depletion point, the enhanced reactivity decays on a timescale of a few minutes. These observations shed a new light on the catalytic reactivity of highly dispersed gold nanoparticles. 相似文献
17.
The adsorption of oxygen and carbon dioxide on cesium-reconstructed Ag(1 1 0) surface has been studied with scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). At 0.1 ML Cs coverage the whole surface exhibits a mixture of (1 × 2) and (1 × 3) reconstructed structures, indicating that Cs atoms exert a cooperative effect on the surface structures. Real-time STM observation shows that silver atoms on the Cs-covered surface are highly mobile on the nanometer scale at 300 K. The Cs-reconstructed Ag(1 1 0) surface alters the structure formed by dissociative adsorption of oxygen from p(2 × 1) or c(6 × 2) to a p(3 × 5) structure which incorporates 1/3 ML Ag atoms, resulting in the formation of nanometer-sized (10–20 nm) islands. The Cs-induced reconstruction facilitates the adsorption of CO 2, which does not adsorb on unreconstructed, clean Ag(1 1 0). CO 2 adsorption leads to the formation of locally ordered (2 × 1) structures and linear (2 × 2) structures distributed inhomogeneously on the surface. Adsorbed CO 2 desorbs from the Cs-covered surface without accompanied O 2 desorption, ruling out carbonate as an intermediate. As a possible alternative, an oxalate-type surface complex [OOC–COO] is suggested, supported by the occurrence of extensive isotope exchange between oxygen atoms among CO 2(a). Direct interaction between CO 2 and Cs may become significant at higher Cs coverage (>0.3 ML). 相似文献
18.
The adsorption of NO on single gold atoms and Au 2 dimers deposited on regular O 2− sites and neutral oxygen vacancies (F s sites) of the MgO(1 0 0) surface have been studied by means of DFT calculations. For Au 1/MgO the adsorption of NO is stronger when the Au atom is supported on an anionic site than when it is on a F s site, with adsorption binding energies of 1.1 and 0.5 eV, respectively. In the first case the spin density is mainly concentrated on the metal atom and protruding from the surface. In such a way, an active site against radicals such as NO is generated. On the F s site, the presence of the vacancy delocalizes the spin into the substrate, weakening its coupling with NO. For Au 2/MgO, as this system has a closed-shell configuration, the NO molecules bonds weakly with Au 2. Regarding the N–O stretching frequencies, a very strong shift of 340–400 cm −1 to lower frequencies is observed for Au 1/MgO in comparison with free NO. 相似文献
19.
Adsorption of thermal (2000 K) D (H) atoms on HOPG surfaces prior to and after bombardment with 500 eV Ar ions was studied with thermal desorption and vibrational spectroscopies. Ion bombardment of HOPG generates vacancy (VD, displaced surface C atoms) and interstitial (ID, Ar captured between 1st and 2nd C plane) defects. These defects remove the ability of the surface to adsorb D like on virgin HOPG surfaces and to form C gr–D bonds. After a dose of 0.1 Ar per C surface atom, D adsorption is markedly suppressed. Annealing of bombarded surfaces at 1350 K, connected with desorption of trapped Ar and removal of ID, recovers a large fraction of the adsorption capacity for D. Therefore, the long range stress in the surface plane introduced by ID must be responsible for a significant fraction of D adsorption blocking. It is suggested that ID prevent reconstruction of the C surface which is required for the formation of C gr–D bonds. For ion doses above 0.5 Ar/C, adsorption of D on the surface is negligible. After annealing at 1350 K, D can be adsorbed in quantities comparable to the virgin HOPG surface, however forming C–D bonds which are similar to those observed in hydrogenated amorphous carbon instead of those which are normally formed on HOPG. Instationary etching via release of deuterocarbon species occurs primarily in the C 1 and C 2 channels. It is only observed at bombarded HOPG prior to annealing and probably due to the presence of isolated C 1 and C 2 species on the surface generated upon VD formation. 相似文献
20.
This paper extends a previous study (Pehrsson and Mercer, submitted to Surf. Sci.) on unheated, hydrogenated, natural diamond (100) surfaces oxidized with thermally activated oxygen (O *2). In this paper, the oxidation is performed at substrate temperatures from Tsub=24 to 670°C. The diamond surface composition and structure were then investigated with high resolution electron energy loss spectroscopy (HREELS), Auger electron spectroscopy (AES), electron loss spectroscopy (ELS) and low energy electron diffraction (LEED). The oxygen coverage (θ) increased in two stages, as it did during oxidation at T<80°C. However, there are fundamental differences between the oxidation of nominally unheated and heated diamond surfaces. This difference is attributed to simultaneous adsorption and rapid desorption of oxygen species at higher temperatures; the desorption step is much slower without heating. The initial oxidation rates were similar regardless of the substrate temperatures, but the peak coverage (θ) was lower at higher temperatures. For example, θ plateaued at 0.4±0.1 ML at 600°C. The lower saturation coverage is again attributed to oxygen desorption during oxidation. Consistent results were obtained on fully oxidized surfaces, which when heated in vacuum to Tsub=600°C, lost 60% of their adsorbed oxygen. ELS revealed few C=C dimers on the oxidized surfaces, and more graphitization than on unheated surfaces. Oxidation at elevated temperatures also increased the carbonyl to ether ratio, reflecting etching-induced changes in the types of surface sites. The carbonyl and C–H stretch frequencies increased with oxygen dose due to formation of higher oxidation states and/or hydrogen bonding between adjacent groups. The oxygen types did not interconvert when the oxidized surfaces were heated in vacuum. Oxygen desorption generated a much more reactive surface than heating-induced dehydrogenation of the smooth, hydrogenated surface. 相似文献
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