Kinetics of oxygen exchange over CeO2-ZrO2 fluorite-based catalysts

The kinetics of 18O/16O isotopic exchange over CeO2-ZrO2-La2O3 and Pt/CeO2-ZrO2 catalysts have been investigated under the conditions of dynamic adsorption-desorption equilibrium at atmospheric pressure and a temperature range of 650-850 degrees C. The rates of oxygen adsorption-desorption on Pt sites, support surface, oxygen transfer (spillover) from Pt to the support as well as the amount of oxygen accumulated in the oxide bulk, and oxygen diffusion coefficient were estimated. The nanocrystalline structure of lanthana-doped ceria-zirconia prepared via the Pechini route with a developed network of domain boundaries and specific defects guarantees a high oxygen mobility in the oxide bulk (D = (1.5 / 2.0).10-18 m2 s-1 at 650 degrees C) and allows accumulation of over-stoichiometric/excess oxygen. For Pt/CeO2-ZrO2, oxygen transfer from Pt to support (characteristic time < 10-2 s) was shown to be responsible for the fast exchange between the gas-phase oxygen and oxygen adsorbed on the mixed oxide surface. The rate of direct exchange between the gas phase and surface oxygen is increased as well due to the increased concentration (up to 2 monolayers) of surface/near subsurface oxygen species accumulated on the oxygen vacancies (originated from the incorporation of highly dispersed Pt atoms). The characteristic time of diffusion of the oxygen localized in the subsurface layers is about 1 s. The overall quantity of over-stoichiometric oxygen and/or hydroxyl groups accumulated in the bulk can reach the equivalent of 10 monolayers, and characteristic time of oxygen diffusion within the bulk is about 20 s. All these kinetic data are required for the further step of modeling partial oxidation of hydrocarbons under steady- and unsteady-state conditions.     

Oxygen nonstoichiometry and ionic transport properties of La0.4Sr0.6CoO3-delta

Homogeneous samples of La0.4Sr0.6CoO3-delta were obtained by the glycine nitrate process. The oxygen nonstoichiometry was determined from oxygen exchange measurements as a function of oxygen partial pressure (10(-4) bar < PO2 < 10(-2) bar) and temperature (300 degrees C < T< 900 degrees C). The chemical diffusion coefficient D was obtained from oxygen exchange measurements applying a stepwise variation of the oxygen partial pressure of the ambient atmosphere of a disk-shaped sample. The amount of oxygen absorbed or desorbed by the perovskite was analyzed as a function of time. Chemical diffusion data were evaluated using simplified and exact fitting procedures taking into account the surface exchange coefficient. Alternatively, galvanostatic polarization measurements were performed in a PO2-range between 10(-4) and 10(-2) bar to yield D and the ionic conductivity sigma(i) from the long time solution of the diffusion equation. Values for D from polarization measurements at T= 775 degrees C and from oxygen exchange measurements at T= 725 degrees C are in good agreement with each other. D and sigma(1) increase with increasing PO2 (10(-4) to 10(-2) bar). The ionic conductivity shows a maximum at 3-delta approximately 2.82 and decreases with decreasing oxygen content indicating the possible formation of vacancy ordered structures.     

Orientation dependent oxygen exchange kinetics on single crystal SrTiO(3) surfaces

The perovskite SrTiO(3) is arguably one of the most important oxide systems in condensed matter research. In this study, we report measurement of the orientation dependence of oxygen exchange on SrTiO(3) single crystal surfaces by dynamic conductivity measurements under electrochemical perturbations. Activation energy for electrical conduction in the 923-1223 K range at an oxygen partial pressure of ～10(-11) Pa of (100), (111), and (110) single crystals was found to be 2.6 eV, 2.7 eV, and 3.1 eV, respectively. The equilibration kinetics show profound dependence on the surface orientation and are modelled using a heterogeneous relaxation process. All surfaces show similar cationic sub-lattice limited rate behavior with (111), (100), and (110) having the fastest, intermediate, and slowest rates, respectively. We discuss the orientation dependence and its relation to local atomic structure in light of previous experimental and theoretical studies.     

Oxygen surface exchange kinetics of SrTi(1-x)Fe(x)O(3-δ) mixed conducting oxides

The oxygen surface exchange kinetics of mixed conducting perovskite oxides SrTi(1-x)Fe(x)O(3-δ) (x = 0, 0.01, 0.05, 0.35, 0.5) has been investigated as a function of temperature and oxygen partial pressure using the pulse-response (18)O-(16)O isotope exchange (PIE) technique. Arrhenius activation energies range from 140 kJ mol(-1) for x = 0 to 86 kJ mol(-1) for x = 0.5. Extrapolating the temperature dependence to the intermediate temperature range, 500-600 °C, indicates that the rate of oxygen exchange, in air, increases with increasing iron mole fraction, but saturates at the highest iron mole fraction for the given series. The observed behavior is concomitant with corresponding increases in both electronic and ionic conductivity with increasing x in SrTi(1-x)Fe(x)O(3-δ). Including literature data of related perovskite-type oxides Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ), La(0.6)Sr(0.4)Co(0.2)Fe(0.8)O(3-δ), La(0.6)Sr(0.4)CoO(3-δ), and Sm(0.5)Sr(0.5)CoO(3-δ), a linear relationship is observed in the log-log plot between oxygen exchange rate and oxide ionic conductivity with a slope fairly close to unity, suggesting that it is the magnitude of the oxide ionic conductivity that governs the rate of oxygen exchange in these solids. The distribution of oxygen isotopomers ((16)O(2), (16)O(18)O, (18)O(2)) in the effluent pulse can be interpreted on the basis of a two-step exchange mechanism for the isotopic exchange reaction. Accordingly, the observed power law dependence of the overall surface exchange rate on oxygen partial pressure turns out to be an apparent one, depending on the relative rates of both steps involved in the adopted two-step scheme. Supplementary research is, however, required to elucidate which of the two possible reaction schemes better reflects the actual kinetics of oxygen surface exchange on SrTi(1-x)Fe(x)O(3-δ).     

Potentiometric investigations of carbonate ion stability in molten cesium and sodium iodides

The reaction of carbonate ion dissociation in molten CsI and NaI was studied in the temperature range from the melting point to 800 (CsI) and 830 degrees C (NaI) by a potentiometric method with the use of a membrane oxygen electrode as an indicator one. The dissociation constant of CO(3)(2-) in the molten iodides was found to increase with the temperature. pK values for CsI are 4.47 (650 degrees C), 4.23 (700 degrees C), 3.89 (750 degrees C), 3.44 (800 degrees C); those for NaI are 4.68 (700 degrees C), 4.39 (750 degrees C), 4.17 (800 degrees C), 3.92 (830 degrees C). The plots of pK vs reverse temperature are linear. Carbonate stability in molten CsI is lower than that in the NaI due to lower stability of Cs(2)CO(3) compared with Na(2)CO(3).     

基于密度泛函理论计算的CO2在SrTiO3(100)表面的吸附

通过密度泛函理论的第一性原理,模拟了CO2分子在SrTiO3(100)表面TiO2-和SrO-位点上的吸附行为,获得了CO2在几种不同吸附模型下的结构参数及表面吸附能,进而研究了吸附机理和结构稳定性.计算结果表明,当CO2的C原子吸附在SrTiO3(100)表面SrO-及TiO2-位点的氧原子上时,吸附结构较稳定,尤其是C、O原子共吸附在TiO2-位点时最稳定,而其余吸附模型则不稳定.对吸附稳定模型的Mulliken布局数及态密度分析显示:CO2分子在SrTiO3(100)表面吸附主要是由于SrTiO3(100)面的电子跃迁至CO2分子,CO2分子得到电子形成弯曲的CO2-阴离子结构,并伴随着C-O键的伸长,从而达到吸附活化CO2的目的.     

Using (18)O/(16)O exchange to probe an equilibrium space-charge layer at the surface of a crystalline oxide: method and application

The use of an (18)O/(16)O exchange experiment as a means for probing surface space-charge layers in oxides is examined theoretically and experimentally. On the basis of a theoretical treatment, isotope penetration profiles are calculated for (18)O/(16)O exchange across a gas-solid interface and subsequent diffusion of the labelled isotope through an equilibrium space-charge layer depleted of mobile oxygen vacancies and into a homogeneous bulk phase. Profiles calculated for a range of conditions all have a characteristic shape: a sharp drop in isotope fraction close to the surface followed by a normal bulk diffusion profile. Experimental (18)O profiles in an exchanged (001) oriented single crystal of Fe-doped SrTiO(3) were measured by time-of-flight secondary ion mass spectrometry (ToF-SIMS). By extracting the space-charge potential from such profiles, we demonstrate that this method allows the spatially resolved characterization of space-charge layers at the surfaces of crystalline oxides under thermodynamically well-defined conditions.     

Characterization of oxygen dynamics in ZrW2O8

The dynamics of oxygen motion in ZrW(2)O(8) have been characterized using (17)O solid-state NMR. Rates of dynamic exchange have been extracted from magnetization transfer experiments over a temperature range of 40 to 226 degrees C, and distinct values for the associated activation barrier have been observed on either side of the order/disorder phase transition at approximately 175 degrees C. A detailed model for the dynamical process is proposed, which reconciles the observation of continuing oxygen dynamics in the low-temperature phase with the static order implied by earlier X-ray diffraction studies.     

Effect of defect structure of lanthanum manganite on oxygen exchange kinetics and diffusion

The method of oxygen isotopic exchange was used to study the oxygen exchange kinetics and diffusion in the LaMnO_{3 + δ} oxide at the temperatures of 600–850°C and in the range of oxygen pressures of 133.3–9332.4 Pa. The rate of interface exchange and diffusion coefficient of oxygen are much lower in the case of LaMnO_{3 + δ} as compared to LaCoO_{3 − δ}, which may be due to the different defect structure of these oxides. It is shown that the first exchange type prevails in LaMnO_{3 + δ} occurring without participation of oxygen from the oxide surface. At the same time, in the case of LaCoO_{3 - δ}, an increase in the temperature results in a significant contribution of both the second and third exchange types with participation of one and two oxygen atoms of the oxide surface, accordingly. The rate determining exchange stage is the process of dissociative oxygen adsorption/desorption on the oxide surface.     

A new route of oxygen isotope exchange in the solid phase: demonstration in CuSO4.5H2O

Temperature-programmed desorption mass spectrometry (TPD-MS) measurements on [(18)O]water-enriched copper sulfate pentahydrate (CuSO(4).5H(2)(18)O) reveal an unambiguous occurrence of efficient oxygen isotope exchange between the water of crystallization and the sulfate in its CuSO(4) solid phase. To the best of our knowledge, the occurrence of such an exchange was never observed in a solid phase. The exchange process was observed during the stepwise dehydration (50-300 degrees C) of the compound. Specifically, the exchange promptly occurs somewhere between 160 and 250 degrees C; however, the exact temperature could not be resolved conclusively. It is shown that only the fifth, sulfate-associated, anionic H(2)O molecule participates in the exchange process and that the exchange seems to occur in a preferable fashion with, at the most, one oxygen atom in SO(4). Such an exchange, occurring below 250 degrees C, questions the common conviction of unfeasible oxygen exchange under geothermic conditions. This new oxygen exchange phenomenon is not exclusive to copper sulfate but is unambiguously observed also in other sulfate- and nitrate-containing minerals.     

Oxygen isotope exchange in praseodymium nickelate

Oxygen surface exchange kinetics and diffusion were studied in Pr₂NiO_4?+?δ (PNO) by the isotope exchange method with gas phase equilibration in the temperature range of 600–800 °C and oxygen pressure range of 0.33–1.62 kPa. The oxygen heterogeneous exchange rate (r_H), oxygen diffusion coefficient (D), rates of oxygen dissociative adsorption (r_a), and oxygen incorporation (r_i) were calculated along with the apparent activation energies of oxygen surface exchange and diffusion processes. The temperature dependence of r_H was found to benon-linear in Arrhenius coordinates. The apparent activation energy changed from 1.4?±?0.2 eV at T?>?700 °C to 2.0?±?0.1 eV. This might be attributed to the change in the rate-determining stage of oxygen exchange for Pr₂NiO_4?+?δ at T ~?700 °C, because of a shift in the ratio between r_a and r_i caused by the difference in their activation energies. Possible reasons for the observed changes in the rate-determining stage are discussed.     

The Adsorption-Desorption Cycle. Reversibility of the BSA-Silica System

The reversibility of the adsorption-desorption cycle was established by comparing the thermostability (determined by differential scanning calorimetry) and secondary structure (obtained by circular dichroism spectroscopy) of BSA before adsorption, adsorbed on, and exchanged from silica particles. Circular dichroism was also measured as a function of temperature at a given wavelength. Adsorbed BSA presents a higher thermostability and a lower alpha-helix content than the native protein while it regains its conformation when released from the surface back into the solution; the homomolecular exchange is reversible.The changes in ellipticity (at a given wavelength) as a function of the temperature show that the thermal denaturation of native, adsorbed, and exchanged BSA proceeds in two steps. For the dissolved protein, the first step up to 50 degrees C involves a slight change in the structure while in the 50-90 degrees C temperature range the actual unfolding takes place. For the adsorbed BSA, the first step proceeds up to 60 degrees C and includes some intermolecular association between the adsorbed protein molecules, which may be responsible for the increased thermostability. The unfolding occurs in the 60-90 degrees C range; it is less cooperative and involves a lower enthalpy change than the native protein. Adsorbed BSA presents the same secondary structure as that observed for dissolved BSA that has passed a heating-cooling cycle. Copyright 2001 Academic Press.     

The Influence of pH and Temperature on the Equilibrium and Dynamic Surface Tension of Aqueous Solutions of Sodium Oleate

For aqueous solutions of sodium oleate, the dynamic surface tension (using the maximum bubble pressure technique) and the equilibrium surface tensions (using the du Noüy ring method) were studied. Experiments were carried out within the pH range 7 to 13 and at temperatures from 20 to 60 degrees C. From the equilibrium surface tension studies at 25 degrees C, a wide variation in surface activity was found to occur in the premicellar concentration range, depending on pH and oleate concentration, and this was explained on the basis of the formation of strongly surface-active premicellar species. It was also shown that the critical micelle concentration (cmc) of the system increased with pH within the pH range 7-12. Dynamic surface tension experiments were carried out from 20 to 60 degrees C at concentrations beyond the cmc in alkaline solutions, and the adsorption kinetics at the air/solution interface were analyzed using established theoretical diffusion models. From these data, the effective diffusion coefficients (D(eff)) for the oleate species were determined and the presence of an interfacial activation barrier in the diffusion process was confirmed. It was found that the D(eff) values obtained within the temperature range from 40 to 60 degrees C at 2-3 mM were in general agreement with previously reported values for the oleate dimer obtained by pulse-gradient FT-NMR. Furthermore, D(eff) significantly increased with temperature and also increased with concentration at higher temperatures (>40 degrees C). Copyright 2001 Academic Press.     

An optical thermometer based on the delayed fluorescence of C70

A sensitive and broad-ranged optical thermometer, based on the thermally activated delayed fluorescence of fullerene C70, is presented. It consists of C70 molecularly dispersed in a polymer film. Several polymer matrices were investigated. In the absence of oxygen the fluorescence intensity increases markedly with temperature. At 25 degrees C the fluorescence intensity of C70 increases maximally by a factor of between 17 and 22, depending on the polymer, whereas at 100 degrees C the fluorescence intensity can be 79 times higher. In the absence of oxygen and for temperatures above 20 degrees C, the red fluorescence of C70 in the films is so intense that it is easily perceived by the naked eye. For the systems studied, the fluorescence intensity is very sensitive to temperature. This results in a working range from -80 to at least 140 degrees C in the case of C70 in poly(tert-butyl methacrylate) (PtBMA). Perylene was incorporated into the film as an internal reference in order to enable ratiometric measurements. The sensitivity of the lifetime of the delayed fluorescence to temperature is also high and results in an even wider working range. The performance of the C70/PtBMA film was measured against a well-known optical temperature probe, [Ru(phen)3] (phen=phenanthroline). The results show that the C70/PtBMA film is a very good system for optical temperature-sensing over a wide range of temperatures, outperforming known standards.     

Purification and characterization of a cyclomaltodextrin glucanotransferase from Paenibacillus campinasensis strain H69-3

A cyclomaltodextrin glucanotransferase (E.C. 2.4.1.19) from a newly isolated alkalophilic and moderately thermophilic Paenibacillus campinasensis strain H69-3 was purified as a homogeneous protein from culture supernatant. Cyclomaltodextrin glucanotransferase was produced during submerged fermentation at 45 degrees C and purified by gel filtration on Sephadex G50 ion exchange using a Q-Sepharose column and ion exchange using a Mono-Q column. The molecular weight of the purified enzyme was 70 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the pI was 5.3. The optimum pH for enzyme activity was 6.5, and it was stable in the pH range 6.0-11.5. The optimum temperature was 65 degrees C at pH 6.5, and it was thermally stable up to 60 degrees C without substrate during 1 h in the presence of 10 mM CaCl(2). The enzyme activity increased in the presence of Co(2+), Ba(2+), and Mn(2+). Using maltodextrin as substrate, the K(m) and K(cat) were 1.65 mg/mL and 347.9 micromol/mg x min, respectively.     

Formation of protonic defects in perovskite-type oxides with redox-active acceptors: case study on Fe-doped SrTiO(3)

The thermodynamics of water incorporation into Fe-doped SrTiO(3) was investigated by thermogravimetric measurements. Changes in valence states of redox-active dopant ions (Fe(3+)/Fe(4+)) with water vapor pressure were taken into account in the defect chemical analysis. The proton solubility was significantly enhanced by the presence of the redox centers. The hydration enthalpies and entropies were -60 kJ mol(-1) and -122 J mol(-1) K(-1). The defect chemical model was applied to describe the water vapor dependence of the electrical conductivity in mixed ionic and electronic conducting Fe-doped SrTiO(3) single crystals.     

Ion-selective electrode for the determination of trazodone in tablets

A coated wire trazodone-selective electrode based on incorporation of trazodone-tetraphenylborate ion pair in a poly(vinylchloride) coating membrane was constructed. The influences of membrane composition, temperature, pH of the test solution, and foreign ions on the electrode performance were investigated. The electrode showed a Nernstian response over a trazodone concentration range from 1.41 x 10(-5) to 0.89 x 10(-2) M, at 25 degrees C, and was found to be very selective, precise, and usable within the pH range 2.4-9.0. The standard electrode potentials, E degree, were determined at 20, 25, 30, 35, 40 and 45 degrees C and used to calculate the isothermal temperature coefficient (dE degree/dT) of the electrode. Temperatures higher than 45 degrees C seriously affected the electrode performance. The electrode was successfully used for potentiometric determination of trazodone hydrochloride both in pure solutions and in pharmaceutical preparations.     

掺杂钙钛矿型氧化物的固体结构及其可交换氧

钙钛矿型氧化物具有稳定的立方结构，以不同价态元素（Ｖ＾５＋，Ａｌ＾３＋，Ｍｇ＾２＋，Ｌｉ＾＋）对ＳｒＴｉＯ３中的Ｔｉ＾４＋进行取代，ＸＲＤ表明基基本结构保持不变，同时其谱线有不同程度的宽化。这种宽化归因于取代所引起的晶体不完整性。取代元素的价态及量将在不同程度上破坏晶体的完成性。这种不完整性导至氧空位Ｖｏ的形成，从而可使晶格氧与气相氧在较低温度进行交换，ＴＰＤ显示３００～６００℃的可交换氧量决定于     

Transport numbers from hydrogen concentration cells over different oxides under oxidising and reducing conditions

Partial hydrogen ion conductivity in acceptor doped CaTiO(3), BaCeO(3) and ZrO(2) and nominally undoped BaTiO(3) and TiO(2) has been investigated by transport number measurements using the hydrogen concentration cell/EMF method in wet atmospheres as a function of pO(2)(10(-20)-1 atm) at 800 or 1000 [degree]C. All oxides investigated, except ZrO(2), show minor proton conductivity in oxidising atmospheres. Earlier indications of apparent negative charge transport by hydrogen under reducing conditions and high temperatures in SrTiO(3) samples have been reproduced for all the investigated oxides including ZrO(2).     

Kinetics of interaction of gas phase oxygen with cerium-gadolinium oxide

The method of isotopic exchange with gas phase analysis was used to study the kinetics of oxygen interaction with the Ce_0.80Gd_0.20O_{1.90 ? ??} oxide at the oxygen pressure of 0.27?C1.33 kPa in the temperature range of 700?C800°C. The values of oxygen interphase exchange rate and diffusion coefficient, and also effective activation energies of the processes of oxygen exchange and diffusion were determined. The contributions of three exchange types and amounts of equivalent exchangeable oxygen were calculated. It was shown that the limiting exchange stage is the process of dissociative oxygen adsorption/desorption on the surface of the Ce_0.80Gd_0.20O_{1.90 - ??} oxide.