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1.
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
2.
The interaction of CO 2 with Cs-promoted Fe(110) at 85 K as well as temperature-dependent reactions between 100 and 700 K have been studied by means of ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Several surface species could be detected at 85 K, i.e. carbon monoxide (CO), carbonate (CO n−3), physisorbed linear carbon dioxide (CO lin2) and very small amounts of oxidic oxygen (O ox). An oxalate species (C 2O m−4) could not be identified definitively, but from comparison with the literature there is evidence that C 2O m−4 is present. Increasing the temperature after saturation with CO 2 leads to a complicated reaction behaviour. CO 2 either desorbs or dissociates into CO and CO n−3 or forms C 2O m−4 at temperatures between 85 and 160 K. Above 160 K C 2O m−4, decomposes in parallel reactions into CO 2, CO n−3 and CO. Above 320 K, adsorbed CO either desorbs into the gas phase or dissociates into C and O. In the temperature region between 500 and 700 K a recombination of C and O to CO and the desorption of Cs take place. As in the case of Fe(110)/K+CO 2, at high alkali coverages two carbonate species could be detected which dissociate upon heating at different temperatures. The system Fe(110)/Cs+CO 2 is proved to be very similar to the system Fe(110)/K+CO 2. 相似文献
3.
The reaction of CO 2 and H 2O to form bicarbonate (HCO −3) was examined on the nearly perfect and vacuum annealed surfaces of TiO 2(110) with temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and high resolution electron energy loss spectrometry (HREELS). The vacuum annealed TiO 2(110) surface possesses oxygen vacancy sites that are manifested in electronic EELS by a loss feature at 0.75 V. These oxygen vacancy sites bind CO 2 only slightly more strongly (TPD peak at 166 K) than do the five-coordinated Ti 4+ sites (TPD peak at 137 K) typical of the nearly perfect TiO 2(110) surface. Vibrational HREELS indicates that CO 2 is linearly bound at the latter sites with a νa(OCO) frequency similar to the gas phase value. In contrast, oxygen vacancies dissociate H 2O to bridging OH groups which recombine to liberate H 2O in TPD at 490 K. No evidence for a reaction between CO 2 and H 2O is detected on the nearly perfect surface. In sequentially dosed experiments on the vacuum annealed surface at 110 K, CO 2 adsorption is blocked by the presence of preadsorbed H 2O, adsorbed CO 2 is displaced by postdosed H 2O, and there is little or no evidence for bicarbonate formation in either case. However, when CO 2 and H 2O are simultaneously dosed, a new CO 2 TPD state is observed at 213 K, and the 166 K state associated with CO 2 at the vacancies is absent. SSIMS was used to tentatively assign the 213 K CO 2 TPD state to a bicarbonate species. The 213 K CO 2 TPD state is not formed if the vacancy sites are filled with OH groups prior to simultaneous CO 2+H 2O exposure. Sticking coefficient measurements suggest that CO 2 adsorption at 110 K is precursor-mediated, as is known to be the case for H 2O adsorption on TiO 2(110). A model explaining the circumstances under which the proposed bicarbonate species is formed involves the surface catalyzed conversion of a precursor-bound H 2O–CO 2 van der Waals complex to carbonic acid, which then reacts at unoccupied oxygen vacancies to generate bicarbonate, but falls apart to CO 2 and H 2O in the absence of these sites. This model is consistent with the conditions under which bicarbonate is formed on powdered TiO 2, and is similar to the mechanism by which water catalyzes carbonic acid formation in aqueous solution. 相似文献
4.
We study the adsorption and reaction of CO 2 as a function of temperature between 100 and 700 K in the presence of Na on a Pd(111) surface using high resolution electron energy loss spectroscopy. While CO 2 does not react with a clean Pd(111) surface, we find various reaction channels on the Na precovered Pd(111) surface depending on the Na coverage. At intermediate coverage a bent CO 28− species with characteristic vibrational bands can be unambiguously identified. This species is stable up to 200 K, and dissociates into CO and oxygen similar to its behaviour on other surfaces, and as reported in a previous photoemission study [Wambach et al., Surface Sci. 209 (1989) 159]. In case the surface has been oxygen contaminated before Na and CO 2 exposure surface carbonates can be observed. 相似文献
5.
The adsorption and desorption of in situ deposited glycine on Pt(111) were investigated with thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). Glycine adsorbs intact on Pt(111) at all coverages at temperatures below 250 K. The collected results suggest that the glycine molecules adsorb predominantly in the zwitterionic state both in the first monolayer and in multilayers. Upon heating, intact molecules start to desorb from multilayers around 325 K. The second (and possibly third) layer(s) are somewhat more strongly bound than the subsequent layers. The multilayer desorption follows zero order kinetics with an activation energy of 0.87 eV molecule −1. From the first saturated monolayer approximately half of the molecules desorbs intact with a desorption peak at 360 K, while the other half dissociates before desorption. Below 0.25 monolayer all molecules dissociate upon heating. The dissociation reactions lead to H 2, CO 2, and H 2O desorption around 375 K and CO desorption around 450 K. This is well below the reported gas phase decomposition temperature of glycine, but well above the thermal desorption temperatures of the individual H 2, CO 2, and H 2O species on Pt(111), i.e. the dissociation is catalyzed by the surface and H 2, CO 2, and H 2O immediately desorb upon dissociation. For temperatures above 500 K the remaining residues of the dissociated molecules undergo a series of reactions leading to desorption of, for example, H 2CN, N 2 and C 2N 2, leaving only carbon left on the surface at 900 K. Comparison with previously reported studies of this system show substantial agreement but also distinct differences. 相似文献
6.
The electrical conductivity of single crystal lithium niobate (LiNbO 3) was determined as a function of temperature for various oxygen partial pressures. The electrical conductivity is proportional to Po2−1/4 which can be explained by a defect equilibrium involving singly ionized oxygen vacancies and electrons. Measurements of electrical transport numbers at 1000°K show the electrical conductivity of LiNbO3 to be ionic at one atmosphere of oxygen and electronic at low oxygen partial pressures. Thermoelectric measurements indicate that LiNbO3 at low oxygen partial pressures is n-type and that the concentration of electrons at 1000°K and in an atmosphere of 50% C0/50% CO2a is 4 × 1017cm3 with a mobility of 1.7 cm2V sec. The diffusion of oxygen in LiNbO3 was determined as a function of temperature at an oxygen partial pressure of 70 Torr. by measuring O18/O16 isotope exchange with the gas phase as a function of time. The diffusion data may be represented by D = 3.03 × 10−6 exp (−29.4 kcal mole−1/RT)cm2sec. Consideration of the Nernst-Einstein relation for oxygen and the variation in conductivity with Li2O activity indicate that the ionic conduction is caused by transport of lithium ions. 相似文献
7.
The influence of deposited potassium on the oxidation and NO reactivity of a Co(0 0 0 1) surface was studied using X-ray photoelectron spectroscopy. The formation of surface CoO was observed when the clean Co(0 0 0 1) surface was exposed to O 2 at 500 K. In contrast, the Co atoms on the K-deposited Co(0 0 0 1) surface remained at a lower oxidation state, CoO x (0 < x < 1). No adsorption or dissociation of NO occurred on the CoO/Co(0 0 0 1) surface at 320 K, whereas a NO 2− species formed on the oxidized K/Co(0 0 0 1) surface. This species is considered to be an intermediate in NO decomposition. It was concluded that the role of potassium was (i) to form the NO 2− intermediate, and (ii) to keep the Co surface partially oxidized (CoO x) as the active site for the dissociation of the NO 2− species. 相似文献
8.
Quasi-elastic neutron scattering has been used to characterize the diffusivity of CH 4 molecules condensed in single-wall carbon nanotubes. It is shown that the two sites of adsorption, previously observed by adsorption volumetry and calorimetry measurements, correspond to a solid-like phase for the more strongly bound site at T<120 K and to a liquid-like component for the more weakly bound site at 70< T<120 K. The diffusion coefficients of the mobile molecules range between 3×10 −7 to 15×10 −7 cm 2 s −1. The fraction of this viscous liquid diminishes as the temperature is decreased; the adsorbate is fully solidified at 50 K and below. 相似文献
9.
The adsorption of CO 2 on the NaCl(100) surface was studied with a high-resolution LEED-system. Measurements without charging up at low electron energies and without damage by the e-beam could be performed by using ultrathin epitaxial films on a conducting Ge(100) substrate. The adsorption behavior was recorded as a function of time and pressure at constant substrate temperatures of 78 and 83 K and CO 2 partial pressures from 4 × 10 −8−2 × 10 −3 Pa. The adsorption system shows a first-order two-dimensional phase transition to a (2 × 1) superstructure including glide planes (herringbone-like structure) at p = 7.2 × 10 −8Pa ( T = 78 K). The condensation of the CO 2 solid is starting at p = 1.5 × 10 −4 Pa ( T = 78 K). The LEED-pattern shows in this c(2 × 2) superstructure, which corresponds to the pyrite-like structure of the CO 2 solid. Both observed superstructures are commensurable with the NaCl(100) surface. Observation of island growth shows that the domains of the (2 × 1) superstructures have already at coverage of 5% of a monolayer an average lateral size of at least 200 A. 相似文献
10.
The decomposition reactions of formaldehyde on clean and oxygen dosed Pt(110) have been studied by LEED, XPS and TPRS. Formaldehyde is adsorbed in two states, a monolayer phase and a multilayer phase which were distinguishable by both TPRS and XPS. The saturated monolayer (corresponding to 8.06 × 10 14 molecules cm −2) desorbed at 134 K and the multilayer phase (which could not be saturated) desorbed at 112 K. The only other reaction products observed at higher temperatures were CO and H 2 produced in desorption limited processes and these reached a maximum upon saturation of the formaldehyde monolayer. The desorption spectrum of hydrogen was found to be perturbed by the presence of CO as reported by Weinberg and coworkers. It is proposed that local lifting of the clean surface (1 × 2) reconstruction is responsible for this behaviour. Analysis of the TPRS and XPS peak areas demonstrated that on the clean surface approximately 50% of the adsorbed monolayer dissociated with the remainder desorbing intact. Reaction of formaldehyde with preadsorbed oxygen resulted in the formation of H 2O (hydroxyl recombination) and CO 2 (decomposition of formate) desorbing at 200 and 262 K, respectively. The CO and H 2 desorption peaks were both smaller relative to formaldehyde decomposition on the clean surface and in particular, H 2 desorbed in a reaction limited process associated with decomposition of the formate species. No evidence was found for methane or hydrocarbon evolution in the present study under any circumstances. The results of this investigation are discussed in the light of our earlier work on the decomposition of methanol on the same platinum surface. 相似文献
11.
Infrared spectroscopic evidence is presented for the formation of linearly bonded CO species, as a result of surface interaction between H 2 and CO 2 at room temperature over silicalite-supported Pt. Comparison with direct CO adsorption results suggests that the active sites for this CO 2 reaction are the corner or step sites on platinum particles. The CO formed on these active sites then migrates to other sites on the surface of Pt particles. Co-adsorbed hydrogen and water make the linearly bonded CO species more strongly adsorbed on Pt particles. However, exposure to oxygen or air at room temperature effectively removes these CO species. 相似文献
12.
The adsorption of SO 2 on a CaO surface derived from Ca(OH) 2 has been studied by X-ray photoelectron Spectroscopy. It is shown that SO 2 adsorbs molecularly on the CaO surface forming SO 32− and SO 42−. The adsorbate SO 32− species shift the Ca2 p core levels about 2 eV toward higher binding energy. However, the formation of SO 42− species does not induce any further observable changes in the Ca core line spectra. These results are explained and discussed in terms of an electronic charge transfer from the substrate metal atoms to the adsorbed molecules and vice versa. 相似文献
13.
The study by X-ray diffraction, calorimetry, vibrational and impedance spectroscopy of CsH(SO 4) 0.76(SeO 4) 0.24 new solid solution is presented. Crystals of this composition undergo two phase transitions at T = 333 and 408 K. The last one at 408 K is a superionic-protonic transition (SPT) related to a rapid [HS(Se)O 4−] reorientation and fast H + diffusion. A sudden jump in the conductivity plot confirms the presence of this transition. Above 408 K, this high temperature phase is characterized by high electrical conductivity (7 × 10 t-3 Ω − cm −1) and low activation energy ( Ea < 0.3 eV). 相似文献
14.
Palladium overlayers deposited on TiO 2(110) by metal vapour deposition have been investigated using LEED, XPS and FT-RAIRS of adsorbed CO. Low coverages of palladium (<3 ML) deposited at 300 K adsorb CO exclusively in a bridged configuration with a band (B 1 at 1990 cm −1) characteristic of CO adsorption on Pd(110) and Pd(100) surfaces. When annealed to 500 K, XPS and LEED indicate the nucleation of Pd particles on which CO adsorbs predominantly as a strongly bound linear species which we associate with edge sites on the Pd particles (L* band at 2085 cm −1). Both bridged and linear CO bands are exhibited as increases in reflectivity at the resonant frequency, indicating the retention of small particle size during the annealing process. Palladium overlayers of intermediate coverages (10–20 ML) deposited at 300 K undergo some nucleation during growth, and adsorbed CO exhibits both absorption and transmission bands in the B 1 (1990 cm −1) and B 2 (1940 cm −1) regions. The latter is associated with the formation of Pd(111) facets. Highly dispersed Pd particles are produced on annealing at 500 K. This is evidenced by the dominance of transmission bands for adsorbed CO and a significant concentration of edge sites, which accommodate the strongly bound linear species at 300 K. Adsorption of CO at low temperature also allows the identification of the constituent faces of Pd and the conversion of Pd(110)/(100) facets to Pd(111) facets during the annealing process. High coverages of palladium (100 ML) produce only absorption bands in FT-RAIRS of adsorbed CO associated with the Pd facets, but annealing these surfaces also shows a conversion to Pd(111) facets. LEED indicates that at coverages above 10 ML, the palladium particles exhibit (111) facets parallel to the substrate and aligned with the TiO 2(110) unit cell, and that this ordering in the particles is enhanced by annealing. 相似文献
15.
A series of polyacrylonitrile–dimethylsulfoxide–CuX 2 (X=CF 3SO 3−, Cl −, Br −), films (foils) were prepared by means of the solution cast technique. The thickness of the foils was between 0.04 and 0.09 cm and they contained 70–80 wt.% of the solvent. Conductivities of the solid electrolytes were obtained from impedance measurements. The conductivity increases with the increase of the salt content up to 8 wt.%; at higher concentrations (>8 wt.%) the conductivity is more or less stable, and reaches, in the case of Cu(CF 3SO 3) 2 and CuCl 2, the level of ca. 10 −3 Ω −1 cm −1 at room temperature. The foils based on the CuBr 2 show even higher conductivity, close to 10 −2 Ω −1 cm −1 at room temperature, a value comparable to that characteristic for liquid solutions. The temperature variation of the conductivity for all the systems studied is of the Arrhenius type. The activation energy, determined from linear plots ln σ=f( T−1), is of the order ca. 14 kJ mol −1 for the PAN/CuBr 2/DMSO and of ca. 21 kJ mol −1 for the PAN/CuCl 2/DMSO and the PAN/Cu(CF 3SO 3) 2/DMSO systems. 相似文献
16.
The behavior of oxygen on Pd 1/W(110) has been investigated from 25 to 200 K by thermal desorption, UPS, XPS, and work function measurements. At 25 K only dioxygen species are present. A weakly bound O 2layer, containing O2/ Pd = 0.31 or 4.4 × 10 14 O2 molecules/cm 2 is desorbed at 35 K, leaving a coverage of O2/ Pd = 0.35 or 5 × 10 14 O2 molecules/cm 2. Heating to 200 K results in desorption of molecular O 2 as well as conversion to O, with O/Pd = 0.3. The molecular states, except the very weakly bound one, have high dipole moments with electron transfer to O 2, and thus correspond to Superoxide and peroxide species. These have UPS spectra quite different from physisorbed O 2. At 90 K adsorption is still mostly molecular with a sticking coefficient s near unity. At 200 K, adsorption is atomic with an initial s0 = 0.8. This must be contrasted with Cu 1/W(110) where s0 is unity even at 300 K. The difference can be explained by the much better size match of Pd and W, than Cu and W which makes it easier for Cu to take up momentum of impinging O 2 molecules. The behavior of oxygen on Pd1/W(110) is very similar to that on bulk Pd(111), suggesting that for oxygen the former surface resembles bulk Pd. This is not so for CO adsorption which is much weaker on Pd1/W(110) than on bulk Pd. The reasons for this difference are not presently understood. 相似文献
17.
We have used temperature-programmed desorption with isotopically labeled gases to study O exchange between gas phase NO and adsorbed atomic O on Pt(335). We find two distinct types of adsorbed O, one of which exchanges at least 40 times faster than the other, at room temperature. Based on their relative concentrations, we tentatively identify the more active species as O at the step edge and the less active one as O at terrace sites. The temperature dependence of the faster exchange rate implies two parallel reaction pathways. Above 240 K, the exchange rate increases with temperature with an apparent activation energy of 3.8 kcal mol −1. At lower temperatures the exchange rate is nearly temperature-independent, with an apparent activation energy near zero but a very low pre-exponential factor. These results are interpreted in terms of a competition between oxygen exchange and NO desorption. The low-temperature process probably requires special sites or adsorbate configurations. The room temperature exchange rates of O 2 gas with preadsorbed atomic O, and with NO at edge sites, are more than 100 times slower than for NO gas and adsorbed O. 相似文献
18.
Atomic oxygen coverages of up to 1.2 ML may be cleanly adsorbed on the Au(111) surface by exposure to O 3 at 300 K. We have studied the adsorbed oxygen layer by AES, XPS, HREELS, LEED, work function measurements and TPD. A plot of the O(519 eV)/Au(239 eV) AES ratio versus coverage is nearly linear, but a small change in slope occurs at ΘO=0.9 ML. LEED observations show no ordered superlattice for the oxygen overlayer for any coverage studied. One-dimensional ordering of the adlayer occurs at low coverages, and disordering of the substrate occurs at higher coverages. Adsorption of 1.0 ML of oxygen on Au(111) increases the work function by +0.80 eV, indicating electron transfer from the Au substrate into an oxygen adlayer. The O(1s) peak in XPS has a binding energy of 530.1 eV, showing only a small (0.3 eV) shift to a higher binding energy with increasing oxygen coverage. No shift was detected for the Au 4f 7/2 peak due to adsorption. All oxygen is removed by thermal desorption of O 2 to leave a clean Au(111) surface after heating to 600 K. TPD spectra initially show an O 2 desorption peak at 520 K at low ΘO, and the peak shifts to higher temperatures for increasing oxygen coverages up to ΘO=0.22 ML. Above this coverage, the peak shifts very slightly to higher temperatures, resulting in a peak at 550 K at ΘO=1.2 ML. Analysis of the TPD data indicates that the desorption of O 2 from Au(111) can be described by first-order kinetics with an activation energy for O 2 desorption of 30 kcal mol −1 near saturation coverage. We estimate a value for the Au–O bond dissociation energy D(Au–O) to be 56 kcal mol −1. 相似文献
19.
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
20.
A 35Cl nuclear quadrupole resonance ( 35Cl-NQR) investigation of polycrystalline Ca(ClO 3) 2·2H 2O is described. The 35Cl-NQR frequencies (ν Q) for two resonance lines (ν Q1 and ν Q2), the spin lattice relaxation time ( T1Q) for ν Q2 only and the line width δν Q2 were measured in the temperature range 292–345 K, except for the frequency measured up to 455 K. The observed decrease in the resonance frequencies with increasing temperature permitted the determination of the frequencies of librations of the ClO 3− ion about two axes perpendicular to the three-fold axis of the ion mainly responsible for this effect. The temperature dependence of the relaxation time T1Q proved the occurrence of water diffusion and hindered rotation of ClO 3− ions. The activation energies of these two molecular motions were determined, and their effect on the electric field gradient at the site of a chlorine nucleus was discussed. Temperature measurements of the line width δν Q2 confirmed the conclusions drawn from the analysis of T1Q( T). 相似文献
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