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1.
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.  相似文献   

2.
The surface reaction and desorption of sulfur on Rh(1 0 0) induced by O2 and H2O are investigated with X-ray photoelectron spectroscopy (XPS) technique. The Rh(1 0 0) sample covered with atomic sulfur is prepared by means of the exposure to H2S gas, and subsequently the sample is annealed under O2 or H2O atmosphere. The XPS results show that atomic sulfur adsorbed on Rh(1 0 0) reacts with O2 and desorbs from the surface at 473 K or more. On the other hand, atomic sulfur can not be removed from Rh(1 0 0) surface by H2O at any temperature.  相似文献   

3.
Density functional theory has been employed to investigate the adsorption and the dissociation of an N2O at different sites on perfect and defective Cu2O(1 1 1) surfaces. The calculations are performed on periodic systems using slab model. The Lewis acid site, CuCUS, and Lewis base site, OSUF are considered for adsorption. Adsorption energies and the energies of the dissociation reaction N2O → N2 + O(s) at different sites are calculated. The calculations show that adsorption of N2O is more favorable on CuCUS adsorption site energetically. CuCUS site exhibits a very high activity. The CuCUS-N2O reaction is exothermic with a reaction energy of 77.45 kJ mol−1 and an activation energy of 88.82 kJ mol−1, whereas the OSUF-N2O reaction is endothermic with a reaction energy of 205.21 kJ mol−1 and an activation energy of 256.19 kJ mol−1. The calculations for defective surface indicate that O vacancy cannot obviously improve the catalytic activity of Cu2O.  相似文献   

4.
The angular distribution of desorbing N2 was studied in the decomposition of N2O(a) on Rh(1 0 0) at 60-140 K by means of angle-resolved temperature-programmed desorption. N2 desorption shows two peaks at around 80 K and 110 K. At low N2O coverage, the former collimates far from the surface normal toward the [0 0 1] direction, whereas at high coverage, the desorption sharply collimates along the surface normal. The adsorption form of N2O and its dissociation were also examined by DFT-GGA calculations. Dissociating N2O is proposed to be lying along the [0 0 1] direction at low coverage and to change to an upright form bonding through the terminal oxygen at high coverage.  相似文献   

5.
V2O3(0 0 0 1) films have been grown epitaxially on Au(1 1 1) and W(1 1 0). Under typical UHV conditions these films are terminated by a layer of vanadyl groups as has been shown previously [A.-C. Dupuis, M. Abu Haija, B. Richter, H. Kuhlenbeck, H.-J. Freund, V2O3(0 0 0 1) on Au(1 1 1) and W(1 1 0): growth, termination and electronic structure, Surf. Sci. 539 (2003) 99]. Electron irradiation may remove the oxygen atoms of this layer. H2O adsorption on the vanadyl terminated surface and on the reduced surface has been studied with thermal desorption spectroscopy (TDS), vibrational spectroscopy (IRAS) and electron spectroscopy (XPS) using light from the BESSY II electron storage ring in Berlin. It is shown that water molecules interact only weakly with the vanadyl terminated surface: water is adsorbed molecularly and desorbs below room temperature. On the reduced surface water partially dissociates and forms a layer of hydroxyl groups which may be detected on the surface up to T ∼ 600 K. Below ∼330 K also co-adsorbed molecular water is detected. The water dissociation products desorb as molecular water which means that they recombine before desorption. No sign of surface re-oxidation could be detected after desorption, indicating that the dissociation products desorb completely.  相似文献   

6.
S. Funk 《Applied Surface Science》2007,253(17):7108-7114
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 Ei = 0.1-1.1 eV and adsorption temperatures of Ts = 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 Ed = 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 (Ed = 39.5 and 30.5 kJ/mol). The initial adsorption probability, S0, is independent of Ts for both systems and decreases exponentially from 0.69 to 0.22 for Cr(1 1 0) with increasing Ei, with S0 smaller by ∼0.15 for the surface oxide. The coverage dependence of the adsorption probability, S(Θ), at low Ei is approx. independent of coverage (Kisliuk-shape) and increases initially at large Ei 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 Ed has been obtained by means of a Redhead analysis of the TDS curves.  相似文献   

7.
K. Ozawa  Y. Oba 《Surface science》2009,603(13):2163-1659
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.  相似文献   

8.
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.  相似文献   

9.
Well ordered V2O3(0 0 0 1) films were prepared on Au(1 1 1) and W(1 1 0) substrates. These films are terminated by a layer of vanadyl groups under typical UHV conditions. Reduction by electron bombardment may remove the oxygen atoms of the vanadyl layer, leading to a surface terminated by vanadium atoms. The interaction of oxygen with the reduced V2O3(0 0 0 1) surface has been studied in the temperature range from 80 to 610 K. Thermal desorption spectroscopy (TDS), infrared reflection absorption spectroscopy (IRAS), high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) were used to study the adsorbed oxygen species. Low temperature adsorption of oxygen on reduced V2O3(0 0 0 1) occurs both dissociatively and molecularly. At 90 K a negatively charged molecular oxygen species is observed. Upon annealing the adsorbed oxygen species dissociates, re-oxidizing the reduced surface by the formation of vanadyl species. Density functional theory was employed to calculate the structure and the vibrational frequencies of the O2 species on the surface. Using both cluster and periodic models, the surface species could be identified as η2-peroxo () lying flat on surface, bonded to the surface vanadium atoms. Although the O-O vibrational normal mode involves motions almost parallel to the surface, it can be detected by infrared spectroscopy because it is connected with a change of the dipole moment perpendicular to the surface.  相似文献   

10.
Adsorption probability measurements (molecular beam scattering) have been conducted to examine the adsorption dynamics (i.e. the gas-surface energy transfer processes) of CO2 adsorption on the Zn-on-Cu(1 1 0) bimetallic system. The results indicate surface alloy formation, which is in agreement with prior studies. Depositing Zn at 300 K on Cu(1 1 0), above the condensation temperature of CO2, leads to a “blocking” of CO2 adsorption sites by Zn which is incorporated in the Cu(1 1 0) surface. This apparent site blocking effect indicates a lowering of the CO2 binding energy on the alloyed surface as compared with the clean Cu(1 1 0) support. The Zn coverage has been calibrated by Auger electron spectroscopy and thermal desorption spectroscopy.  相似文献   

11.
László Bugyi 《Surface science》2009,603(19):2958-2963
The investigation of Rh, Mo and Rh-Mo nanosized clusters formed by physical vapor deposition on TiO2(1 1 0) single crystal was performed by X-ray Photoelectron Spectroscopy (XPS), Low Energy Ion Scattering (LEIS) and Auger Electron Spectroscopy (AES). There was no sign for site-exchange between Mo and Rh atoms during deposition of Mo onto Rh particles at 330 K. Mixing between Ti and Mo ions was facilitated at the Mo particle-titania interface due to reaction at 550-700 K. The redox process between titania and Mo deposit was hindered at 330 K by forming predeposited rhodium layer (ΘRh = 2.0 ML), but reached nearly the same extent as without Rh after moderate heating to 600 K. The encapsulation of Rh by titania was complete by about 700 K in the presence of 1.2 ML Mo, in case of Mo-predeposition and Mo-postdeposition as well. Elevating the temperature of TiO2/Rh-Mo layers above 700 K, these metals form alloy at the Mo-Rh interface irrespective of deposition sequences.  相似文献   

12.
J.M.R. Muir  H. Idriss 《Surface science》2009,603(19):2986-2990
The reaction of formamide over the (0 1 1) faceted TiO2(0 0 1) surface has been studied by Temperature Programmed Desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS). Two main reactions were observed: dehydration to HCN and H2O and decomposition to NH3 and CO. The dehydration reaction was found to be three to four times larger than the decomposition at all coverages. Each of these reactions is found to occur in two temperature domains which are dependent upon surface coverage. The low temperature pathway (at about 400 K) is largely insensitive to surface coverage while the high temperature pathway (at about 500 K) shifts to lower temperatures with increasing surface coverage. These two temperature pathways may indicate two adsorption modes of formamide: molecular (via an η1(O) mode of adsorption) and dissociative (via an η2(O,N) mode of adsorption). C1s and N1s XPS scans indicated the presence of multiple species after formamide absorption at 300 K. These occurred at ca. 288.5 eV (-CONH-) and 285 eV (sp3/sp2 C) for the C1s and 400 eV-(NH2), 398 eV (-NH) and 396 eV (N) for the N1s and result from further reaction of formamide with the surface.  相似文献   

13.
First-principles calculation on the basis of the density functional theory (DFT) and generalized gradient approximation have been applied to study the adsorption of H2 on the stoichiometric O-terminated Cu2O (1 1 1), Cu2O (1 1 1)-CuCUS and Cu-terminated Cu2O (1 1 1) surfaces. The optimal adsorption position and orientation of H2 on the stoichiometric O-terminated Cu2O (1 1 1) surface and Cu-terminated Cu2O (1 1 1) surface were determined and electronic structural changes upon adsorption were investigated by calculating the Local Density of States (LDOS) of the CuCUS 3d and CuCUS 4s of stoichiometric O-terminated Cu2O (1 1 1) surface. These results showed that H2 molecule adsorption on CuCUS site parallel to stoichiometric O-terminated Cu2O (1 1 1) surface and H2 molecule adsorption on Cu2 site parallel to Cu-terminated Cu2O (1 1 1) surface were the most favored, respectively. The presence of surface copper vacancy has a little influence on the structures when H2 molecule adsorbs on CuCSA, OCUS and OCSA atoms and the H2 molecule is only very weakly bound to the Cu2O (1 1 1)-CuCUS surface. From the analysis of stoichiometric O-terminated Cu2O (1 1 1) Local Density of States, it is observed that CuCUS 3d orbital has moved to a lower energy and the sharp band of CuCUS 4s is delocalized when compared to that before H2 molecule adsorption, and overlapped substantially with bands due to adsorbed H2 molecule. The Mulliken charges of H2 adsorption on CuCUS site showed that H2 molecule obtained electron from CuCUS which was consistent with the calculated electronic structural changes upon H2 adsorption.  相似文献   

14.
M.S. Zei 《Surface science》2006,600(9):1942-1951
The growth and structures of aluminum oxides on NiAl(1 0 0) have been investigated by RHEED (reflection high energy electron diffraction), complemented by LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and STM (scanning tunneling microscopy). Crystalline θ-Al2O3 phase grows through gas-phase oxidation on the NiAl(1 0 0) substrate with its a and b-axes parallel to [0 −1 0] and [0 0 1] direction of the substrate, respectively, forming a (2 × 1) unit cell. Whilst, three-dimensional nano-sized NiAl(1 0 0) protrusions and Al2O3, NiAl(0 1 1) clusters were found to co-exit at the surface, evidenced by extraordinary transmission spots superposed to the substrate reflection rods in the RHEED patterns. Particularly, the NiAl(0 1 1) clusters develop with their (0 1 1) plane parallel to the NiAl(1 0 0) surface, and [1 0 0] axis parallel to the [0 −1 1] direction of the substrate surface. STM observation combined with information from AES and TPD (temperature programmed desorption) suggest the formation of these 3D structures is closely associated with partial decomposition of the crystalline oxides during annealing. On the other hand, smoother (2 × 1) oxide islands with thickness close to a complete monolayer of θ-Al2O3 can be formed on NiAl(1 0 0) by electro-oxidation, in contrast with the large crystalline films formed by gas-oxidation.  相似文献   

15.
To investigate solvent effects, CO and H2 adsorption on Cu2O (1 1 1) surface in vacuum, liquid paraffin, methanol and water are studied by using density functional theory (DFT) combined with the conductor-like solvent model (COSMO). When H2 and CO adsorb on Cucus of Cu2O (1 1 1) surface, solvent effects can improve CO and H2 activation. The H-H bond increases with dielectric constant increasing as H2 adsorption on Osuf of Cu2O (1 1 1) surface, and the H-H bond breaks in methanol and water. It is also found that both the structural parameters and Mulliken charges are very sensitive to the COSMO solvent model. In summary, the solvent effects have obvious influence on the clean surface of Cu2O (1 1 1) and the adsorptive behavior.  相似文献   

16.
The absorption spectrum of nitrous oxide, N2O, in natural isotopic abundance has been recorded by CW-Cavity Ring Down Spectroscopy between 6000 and 6833 cm−1. The spectra were obtained at Doppler limited resolution by using a CW-CRDS spectrometer based on a series of fibered DFB lasers. The typical sensitivity of 2 × 10−10 cm−1, allowed for the detection of lines with intensity as weak as 2 × 10−29 cm/molecule while the minimum intensity value provided by HITRAN in the considered spectral region is 2 × 10−25 cm/molecule. More than 6000 line positions of five isotopologues contributing to the spectra (14N216O, 15N14N16O, 14N15N16O, 14N218O and 14N217O), were measured with a typical accuracy of 1.5 × 10−3 cm−1 and rovibrationally assigned on the basis of their respective global effective Hamiltonian models. Highly excited rovibrational levels corresponding to J values larger than 80 could be detected for the stronger vibrational bands. The band by band analysis led to the determination of the rovibrational parameters of a total of 68 bands, 49 of them being newly reported. The rms value of the deviations of the predictions of the effective Hamiltonian models from the observed line positions is 0.010 cm−1. As expected, the quality of the predictions degrades for the minor isotopologues for which important deviations up to a few wavenumbers were evidenced. Most of the bands were found unperturbed but in a few cases, local rovibrational perturbations were evidenced. The interaction mechanisms and the perturbers were univocally assigned on the basis of the effective Hamiltonian model. In particular, interpolyad couplings were evidenced indicating that the polyad version of the effective Hamiltonian has to be extended to include Coriolis and interpolyad anharmonic interactions.  相似文献   

17.
Interactions of atomic and molecular hydrogen with perfect and deficient Cu2O(1 1 1) surfaces have been investigated by density functional theory. Different kinds of possible modes of H and H2 adsorbed on the Cu2O(1 1 1) surface and possible dissociation pathways were examined. The calculated results indicate that OSUF, CuCUS and Ovacancy sites are the adsorption active centers for H adsorbed on the Cu2O(1 1 1) surface, and for H2 adsorption over perfect surface, CuCUS site is the most advantageous position with the side-on type of H2. For H2 adsorption over deficient surface, two adsorption models of H2, H2 adsorbing perpendicularly over Ovacancy site and H2 lying flatly over singly-coordinate Cu-Cu short bridge, are typical of non-energy-barrier dissociative adsorption leading to one atomic H completely inserted into the crystal lattice and the other bounded to CuCUS atom, suggesting that the dissociative adsorption of H2 is the main dissociation pathway of H2 on the Cu2O(1 1 1) surface. Our calculation result is consistent with that of the experimental observation. Therefore, Cu2O(1 1 1) surface with oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of H2.  相似文献   

18.
The adsorption and reactivity of SO2 on the Ir(1 1 1) and Rh(1 1 1) surfaces were studied by surface science techniques. X-ray photoelectron spectroscopy measurements showed that SO2 was molecularly adsorbed on both the Ir(1 1 1) surface and the Rh(1 1 1) surface at 200 K. Adsorbed SO2 on the Ir(1 1 1) surface disproportionated to atomic sulfur and SO3 at 300 K, whereas adsorbed SO2 on the Rh(1 1 1) surface dissociated to atomic sulfur and oxygen above 250 K. Only atomic sulfur was present on both surfaces above 500 K, but the formation process and structure of the adsorbed atomic sulfur on Ir(1 1 1) were different from those on Rh(1 1 1). On Ir(1 1 1), atomic sulfur reacted with surface oxygen and was completely removed from the surface, whereas on Rh(1 1 1), sulfur did not react with oxygen.  相似文献   

19.
The adsorption and dissociation of O2 on the perfect and oxygen-deficient Cu2O(1 1 1) surface have been systematically studied using periodic density functional calculations. Different kinds of possible modes of atomic O and molecular O2 adsorbed on the Cu2O(1 1 1) surface are identified: atomic O is found to prefer threefold 3Cu site on the perfect surface and Ovacancy site on the deficient surface, respectively. CuCUS is the most advantageous site with molecularly adsorbed O2 lying flatly over singly coordinate CuCUS-CuCSA bridge on the perfect surface. O2 adsorbed dissociatively on the deficient surface, which is the main dissociation pathway of O2, and a small quantity of molecularly adsorbed O2 has been obtained. Further, possible dissociation pathways of molecularly adsorbed O2 on the Cu2O(1 1 1) surface are explored, the reaction energies and relevant barriers show that a small quantity of molecularly adsorbed O2 dissociation into two O atoms on the deficient surface is favorable both thermodynamically and kinetically in comparison with the dissociation of O2 on the perfect surface. The calculated results suggest that the presence of oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of O2 and can obviously improve the catalytic activity of Cu2O, which is in agreement with the experimental observation.  相似文献   

20.
A.P. Farkas 《Surface science》2007,601(1):193-200
The adsorption, desorption and dissociation of ethanol have been investigated by work function, thermal desorption (TPD) and high resolution electron energy loss (HREELS) spectroscopic measurements on Mo2C/Mo(1 0 0). Adsorption of ethanol on this sample at 100 K led to a work function decrease suggesting that the adsorbed layer has a positive outward dipole moment By means of TPD we distinguished three adsorption states, condensed layer with a Tp = 162 K, chemisorbed ethanol with Tp = 346 K and irreversibly bonded species which decomposes to different compounds. These are hydrogen, acetaldehyde, methane, ethylene and CO. From the comparison of the Tp values with those obtained following their adsorption on Mo2C it was inferred that the desorption of methane and ethylene is reaction limited, while that of hydrogen is desorption limited process. HREEL spectra obtained at 100 K indicated that at lower exposure ethanol undergoes dissociation to give ethoxy species, whereas at high exposure molecularly adsorbed ethanol also exists on the surface. Analysis of the spectral changes in HREELS observed for annealed surface assisted to ascertain the reaction pathways of the decomposition of adsorbed ethanol.  相似文献   

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