A potentiometric investigation on the system (Ni)CO2, O2/CO32? was carried out at 507–637 K in the (Na, K)NO3 equimolar mixture containing carbonate ions in the range 10?5≤[CO32?]≤10?2 mol kg?1 and under a mixture of O2 and CO2 at variable partial pressure. The potential behaviour of the nickel electrodes was found largely dependent on the working temperature. At the highest tested temperature the system behaves irreversibly under potentiometric conditions and the potential was found to be independent of the oxygen concentration. At lower temperatures a large irreversibility of the system was still found, while a certain dependence (different from the theoretical one) of the potential on both oxygen and carbon dioxide partial pressure was experimentally demonstrated. The entire set of results was interpreted on the basis of the two following mechanistic models involving, in the potential determining step, solid species formed on the electrode surface by contact with the melt. Mechanism 1 (high temperature)Mechanism 2 (low temperature)相似文献
The oxygen reduction reaction on smooth gold electrodes on Li/K (53: 47 at.%) carbonate melt has been examined in the temperature range 700–800°C. As in previous work on the Na/K melt [2], two chemically produced species (O22? O2?) are reduced in parallel steps. The waves for the reduction of these species are close together in this melt, as distinct from those in the Na/K eutectic [2]. Exchange currents are somewhat higher than in the Na/K melt. Again, neutralization of the O2? ion by CO2 in rather slow, and may be rate-determining in porous electrodes. 相似文献
Minimum energy pathways of propane oxidative dehydrogenation to propene and propanol on supported vanadium oxide catalyst VOx/TiO2 were studied by periodic discrete Fourier transform (DFT) using a surface oxygen radical as the active site. The propene formation pathway was shown to consist of two consecutive hydrogen abstraction steps. The first step includes Cβ–H bond activation of propane followed by the formation of a surface hydroxyl group V–OtH and a propyl radical n-C3H7. This step with the activation energy E* = 0.56 eV (54.1 kJ/mol) appears to be rate-determining. The second step involves the reaction of the bridging Ob oxygen atom with the methylene C–H bond of propyl radical n-C3H7 followed by the formation of a hydroxylated surface site HOt–V4+–ObH and propene. The initial steps of the C–H bond activation during propane conversion to propanol and propene by ODH on V5+–(OtOb)? active sites are identical. The obtained results demonstrate that participation of surface oxygen radicals as the active sites of propane ODH makes it possible to explain relatively low activation energies observed for this reaction on the most active catalysts. The presence of very active radical species in low concentration seems to be the key factor for obtaining high selectivity. 相似文献
Variations of potential E in time , observed during the carbon monoxide interaction with preliminarily-adsorbed oxygen Oads on smooth and platinized platinum electrodes under open-circuit conditions (supporting electrolyte 0.5 M H2SO4), are measured. The potential decay rate on smooth Pt is more than ten times that on Pt/Pt; there are some differences in the transients as well. The obtained data suggest that CO interacts with Oads on smooth Pt and Pt/Pt via different mechanisms. Two models for the process on smooth platinum are considered. In one model, the interaction of Oads with CO from solution is accepted as the rate-determining step; in the other, the interaction of Oads with COads. A comparison of theoretical E vs. dependences with experimental data using the MathCad program suggests that CO interacts with Oads via both mechanisms. 相似文献
The indirect destructive electrooxidation of benzene, phenol, and N-methyl-p-aminophenol with active oxygen species generated in situ from O2, H2O2, and H2O in aqueous solutions with various pH values has been carried out using different cell designs with Pt, Pb/PbO2, and Ru-Ti oxide anodes. The indirect oxidation of the aromatic compounds mineralizes them into CO2 and H2O or converts them into simple monocarboxylic or dicarboxylic acids, which can be utilized by microorganisms in subsequent biotreatment. 相似文献
Oxygen dissolution in polycrystalline palladium Pd(poly) at O2 pressures ($P_{O_2 } $) of 0.1 to 100 Pa and a temperature of 600 K has been investigated by temperature-programmed desorption. The dissolution process under these conditions includes O2 chemisorption on the oxide film surface, the insertion of Oads atoms under the oxide layer, and their diffusion into the subsurface layers of palladium. During chemisorption, a structure ensuring that the Oads coverage of the surface increases with increasing $P_{O_2 } $ forms on the surface of the oxide film. This is favorable for Oads penetration through the oxide film and increases the amount of absorbed oxygen. The Oads coverage of the surface calculated via the Langmuir equation at an O2 desorption activation energy of Edes = 125 kJ/mol correlates with the number of absorbed oxygen monolayers (n). At n ≥ 1, oxygen absorption by Pd(poly) is due to the diffusion of O atoms in the palladium lattice. After the accumulation of 14–18 oxygen monolayers in the subsurface layers of palladium, oxygen absorption practically stops depending on $P_{O_2 } $. Thus, the acceleration of oxygen dissolution in palladium is due to the formation of the surface oxide film and the increase in the Oads coverage of this film, which facilitates the insertion of Oads atoms into the subsurface layers of palladium. 相似文献
The characteristics of porous Pt/YSZ (ZrO2 + 10 mol % Y2O3) electrodes activated with small amounts of either oxides of rare-earth elements (REE) of the cerium subgroup (CeO2, PrOx, TbOx) or a mixed oxide with the Сe2Tb4O11 composition by the procedure of impregnating the electrodes with ethanol solutions of REE nitrates and subsequent heating at 850°С are studied by the impedance method. The studies are carried out for those cases where the REE oxides after thermal treatment form a film on the electrolyte and also where no activator film is formed. The characteristics of films and activated electrodes are compared. Film-activated Pt/YSZ electrodes are discussed within the framework of the model of compact oxide electrodes. 相似文献
The mechanism of catalytic CO oxidation on Pt(100) and Pd(110) single-crystal surfaces and on Pt and Pd sharp tip (~103 Å) surfaces has been studied experimentally by temperature-programmed reaction, temperature desorption spectroscopy, field electron microscopy, and molecular beam techniques. Using the density functional theory the equilibrium states and stretching vibrations of oxygen atoms adsorbed on the Pt(100) surface have been calculated. The character of the mixed adsorption layer was established by high resolution electron energy loss spectroscopy—molecular adsorption (O2ads, COads) on Pt(100)-hex and dissociative adsorption (Oads, COads) on Pt(100)-(1×1). The origin of kinetic self-oscillations for the isothermal oxidation of CO in situ was studied in detail on the Pt and Pd tips by field electron microscopy. The initiating role of the reversible phase transition (hex) ? (1 × 1) of the Pt(100) nanoplane in the generation of regular chemical waves was established. The origination of self-oscillations and waves on the Pt(100) nanoplane was shown to be caused by the spontaneous periodical transition of the metal from the low-active state (hex) to the highly active catalytic state (1 × 1). A relationship between the reactivity of oxygen atoms (Oads) and the concentration of COads molecules was revealed for the Pd(110) surface. Studies using the isotope label 18Oads demonstrated that the low-temperature formation of CO2 at 150 K is a result of the reaction of CO with the highly reactive state of atomic oxygen (Oads). The possibility of the low-temperature oxidation of CO via interaction with the so-called “hot” oxygen atoms (Ohot) appearing on the surface at the instant of dissociation of O2ads molecules was studied by the molecular beam techniques. 相似文献
Gold oxide films obtained on the surface of polycrystalline gold foil upon oxidation by oxygen activated by a high-frequency discharge have been studied by X-ray photoelectron spectroscopy. High-frequency O2 activation affords oxide films more than 3–5 nm thick. As follows from Au4f spectra, the surface gold atoms are oxidized to the oxidation state +3. The O1s spectra have a composite shape and are decomposed into four components that characterize nonequivalent states of oxygen in the resulting oxide films. It is assumed that the two major oxygen states (Eb(O1s) = 529.0 and 530.0 eV) correspond to the oxygen atoms in two-and three-dimensional gold oxide Au2O3, respectively. The oxygen states characterized by the higher binding energies (Eb(O1s) = 531.8 and 535.2 eV) likely correspond to molecular oxygen in peroxide and superoxide groups, respectively. 相似文献
The structural and electronic properties of two series of Group VB transition metal oxide clusters, M4On? and M4On (M = Nb, Ta; n = 8–11), are investigated using density functional theory calculations. Generalized Koopmans’ theorem is applied to predict the vertical detachment energies and simulate the photoelectron spectra. Large highest occupied molecular orbital–lowest unoccupied molecular orbital gaps are observed for these two stoichiometric M4O10 clusters and estimated to be 3.98 and 4.38 eV for M = Nb and Ta, respectively. The M4O10?/0 (M = Nb, Ta) clusters are polyhedral cage structures with high symmetry (Td for the neutral and D2d for the anion) in which each metal atom joints three bridging and one terminal O atoms. For the Nb oxide species, Nb4O8?/0 and Nb4O9?/0 can be viewed as removing two and one terminal O atoms from Nb4O10?/0, respectively. The Ta species follow the same rule to the Nb species, except that the anionic Ta4O8? is formed by removing one terminal and one bridging O atoms from Ta4O10?. The Ta4O9 containing a localized Ta3+ site can readily react with O2 to form the Ta4O11 which can also be viewed as replacing a terminal oxygen atom in Ta4O10 by a peroxo O2 unit, whereas the added oxygen atom is found to be a bridging one in the O-rich clusters Nb4O11?/0 and the anionic Ta4O11?. Molecular orbital analyses are performed to analyze the chemical bonding in the tetra-nuclear metal oxide clusters and to elucidate their structural and electronic evolution. 相似文献
The voltammetry method with a linear potential scan is used for investigating the effect the electrode material (Ni, Co, electrodes on the basis of cobalt oxides modified with carbon) exerts on the reduction of gaseous oxygen at interfaces solid fluoride-conducting electrode LaF3:Eu2+/electrode, O2, and conjugated processes. Properties of the modified electrodes are characterized by the impedance spectroscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy methods. The oxygen reaction is irreversible at the LaF3:Eu2+|Ni (or Co) interfaces. At the interface of LaF3:Eu2+ with modified electrodes Co (C n at %), where n = 5 and 9, mobile forms of oxygen are reversible and the reduction of gaseous and chemisorbed oxygen is controlled by diffusion with different effective kinetic parameters. 相似文献
Using crossed atomic, molecular cluster, and cw laser beams in conjunction with mass spectrometric ion detection, we have obtained for the first time results for electron transfer fromstate-selected Rydberg atoms to molecular clusters. We report negative ion mass spectra for (CO2)k? (4≦k≦25) and (O2)k? (1≦k≦13) cluster ions, resulting from collisions of Ar** (nd) Rydberg atoms (12≦n≦40) with (CO2)m and (O2)m clusters at relative velocities around 830 m/s, and, for comparison, positive ion mass spectra due to Ne(3s3P2, 0) Penning ionization. For both CO2? and O2-clusters, the negative and the positive ion mass spectra are very different. For (CO2)kEmphasis>/? cluster ions, the mass spectra show distinct variations with principal quantum number of the Rydberg atom, corresponding to differentn-dependences of the effective rate constant for selected cluster ions, as measured relative to the knownn-dependence for SF6? formation in collisions with SF6. For (O2)k? cluster ions, on the other hand, the mass spectra are almost independent ofn with ion intensities, which clearly reflect their thermochemical stabilities (O4? as dominant species). 相似文献
Kinetics of chemiluminescence reactions of 2-methyl-6-phenylimidazo[1,2-a]pyrazin-3(7H)-one (1c, Cypridina luciferin analogue) and substituent effects of the 6-aryl group of derivatives 1 strongly suggest that the rate-determining step is a single electron transfer from an anion derived from 1 to a triplet molecular oxygen (O2) in the oxygenation process. 相似文献
The synthesis of hydrolytically active heteroligand precursors of the composition [M(O2C5H7)x(iOC5H11)y] (M = Zr4+ and Y3+) using zirconium and yttrium acetylacetonates was investigated. It was shown that the reactivity of the obtained precursors in hydrolysis and polycondensation depends on the composition of the coordination sphere. It was studied how thin films of their solutions are applied by dip coating to the surface of Al2O3 substrates with platinum interdigital electrodes and a microheater. A study was made of the effect of the crystallization conditions and thickness of the oxide coatings of the composition 8 mol % Y2O3–ZrO2 on their microstructure and sensor characteristics in oxygen detection. 相似文献
The interaction between oxygen and polycrystalline palladium (Pd(poly)) at \(P_{O_2 } \) = 2.6 × 10?6–10 Pa and T = 300–1300 K was studied by the thermal desorption (TD) method. The interaction between O2 and Pd(poly) is governed by the O2 pressure and the sample temperature. At low pressures of \(P_{O_2 } \) (≤1.3 × 10?5 Pa), O2 is chemisorbed dissociatively on the Pd(poly) surface. During chemisorption, the Oads-surface bond energy and the O2 sticking coefficient gradually decrease as the surface coverage θ increases. At \(P_{O_2 } \) ≥ 10?2 Pa and T ≤ 500 K, after the saturation of the Oads layer (θ ~ 0.5), Oads atoms penetrate under the surface layer of the metal to form surface palladium oxide. At \(P_{O_2 } \) ≥ 1 Pa and T > 500 K, after the saturation of the surface oxide film 2 ML in thickness (n ~ 2), Oads atoms penetrate into the oxide film and then into the subsurface palladium layer and diffuse deep into the metal bulk. As a result, the oxygen uptake at 700 K is n ~ 50. Upon heating, the surface oxides decompose, desorbing O2, which gives rise to a low-temperature TD peak with Tmax = 715 K. The release of oxygen inserted in the subsurface layers of palladium shows itself as a distinct high-temperature TD peak with Tmax ≥ 750 K. 相似文献
Within the frame of possible precursory photoreactions in the generation of humic substances, the visible-light promoted interaction between riboflavin (Rf), a native photosensitizer in aqueous systems, and gallic acid (GA), a polyphenol naturally formed after lignin degradation, was investigated. A systematic kinetic and mechanistic study was conducted under aerobic conditions in aqueous media, through visible-light continuous photolysis, polarographic detection of oxygen uptake, stationary and time resolved fluorescence spectroscopy, time resolved near-IR phosphorescence detection and laser flash photolysis techniques. GA is degraded relatively fast in pH 7 aqueous solutions, where singlet molecular oxygen (O2(1Δg)), superoxide radical anion (O2?) and hydrogen peroxide (H2O2) – all three species photogenerated from triplet excited Rf – participate in the photoprocess. The general conclusion is that in natural waters GA can undergo spontaneous phototodegradation under environmental conditions. Radical species generated in the presence of Rf can participate in condensation or polymerization reactions promoting the natural synthesis of humic products. 相似文献
On the basis of consideration of dissociation, hydration, association, and ligand exchange, the assignment of absorption bands in the electronic spectra of aqueous solutions of the Na4[UO2(O2)CO3)2] complex has been performed. It has been demonstrated that the absorption in the range 190–400 nm is caused by the oxygen atoms of the O22- and CO32- groups and hydration water molecules of dissociated and neutral complex species Na3[UO2(O2)(CO3)2]–, Na2[UO2(O2)(CO3)2]2–, and Na4[UO2(O2)(CO3)2]. 相似文献
The decomposition of thin surface oxide films on polycrystalline palladium Pd(poly) at 500–1300 K was investigated by mathematical modeling. This process was analyzed in terms of a model including O2 desorption from the chemisorbed oxygen layer (Oads) and the passage of oxygen inserted under the surface layer of the metal (Oabs) and oxygen dissolved in metal subsurface layers (Odis) to the surface. O2 desorption was modeled on a surface with a square lattice of adsorption sites, with account taken of the energy of the lateral repulsive interactions between adjacent Oads atoms (εaa). At εaa = 10 kJ/mol and when the activation energy of O2 desorption for a chemisorbed-oxygen surface coverage of θ ≈ 0 is Edes0 = 230 kJ/mol, the calculated spectra are in agreement with the oxygen temperature-programmed desorption (TPD) spectra obtained for Pd(poly) at θ ≤ 0.5. The passage of Oabs and Odis atoms to the surface was calculated using a first-order equation, with account taken of the activation energy for these atoms coming out to the surface (E2 and E3, respectively). As the oxide film is heated, O2 desorption is accompanied by the passage of Oabs and then Odis to the surface, which leads to an increase in the Oads surface coverage and, accordingly, to a buildup of lateral surroundings in the adsorbed layer. Owing to this fact and to the repulsive interactions between Oads atoms, the bonds between Oads and the surface weaken and Edes decreases. As a consequence, the O2 desorption rate increases and a low-temperature peak with Tmax ≈ 710 K, which is due to the passage of Oabs atoms to the surface, and then a high-temperature peak with Tmax ≈ 770 K, which is due to the passage of Odis atoms to the surface, appear in the TPD spectrum. At εaa = 10 kJ/mol, Edes0 = 230 kJ/mol, E2 = 145 kJ/mol, and E3 = 160 kJ/mol and when the number of inserted oxygen monolayers is θabs ≤ 0.3 and the number of oxygen monolayers dissolved in subsurface layers is θdis ≤ 10, the TPD spectra calculated for the given model are in agreement with the O2 TPD spectra that are observed for Pd(poly) and are due to the decomposition of surface oxide films. 相似文献
SbxOy clusters are produced by using a gas aggregation technique. Antimony vapor is mixed with He/O2 or He/N2O and cooled in a reaction channel. After photoionisation with a KrF (248 nm) or ArF (193 nm) excimer laser the products are mass analyzed in a time of flight mass spectrometer. In the presence of N2O no oxide clusters besides SbO+ can be detected, while with oxygen under similar experimental conditions dramatic changes can be observed. At low oxygen partial pressure the obtained spectra are dominated by the pure Sbx+ clusters with low intensity of SbxOy+, whereas at high oxygen partial pressure antimony oxides following the general sequence SbO+(Sb2O3)n are most abundant. The same stable species can furthermore be produced via aggregation of vaporised solid antimony oxide (Sb2O3). Within these experiments another new Series of antimony oxides tentatively assigned to (Sb2O3)n+ appeared in the mass spectra. 相似文献