The effect of temperature and oxygen partial pressure on the reduction mechanism in the Pr2CuO4/Ce0.9Gd0.1O1.95 system

The electrochemical behavior of a porous electrode based on Pr₂CuO₄ (PCO) screen printed on the surface of Ce_0.9Gd_0.1O_1.95 (CGO) solid electrolyte is studied by impedance spectroscopy. The rate-determining stages of the oxygen reduction reaction at the PCO/CGO interface are found for the oxygen partial pressure interval of 30–10₅ Pa and temperatures of 773–1173 K. Changeover of the rate-determining stage of electrode reaction is shown to occur depending on the temperature and the oxygen partial pressure. The PCO electrode polarization resistance is 1.7 Ω cm² at 973 K in air and remains constant at thermocycling of the electrochemical cell in the temperature range of 773–1173 K. Based on the found data, PCO can be considered as the promising cathodic material for solid-oxide fuel cells operating at moderate temperatures (773–973 K).     

Enhanced reactivity of Pt nanoparticles supported on ceria thin films during ethylene dehydrogenation

The adsorption and reaction of ethylene on Pt/CeO(2-x)/Cu(111) model catalysts were studied by means of high resolution photoelectron spectroscopy (HR-PES) in conjunction with resonant photoemission spectroscopy (RPES). The dehydrogenation mechanism is compared to the HR-PES data obtained on a Pt(111) single crystal under identical conditions. It was found that the Pt nanoparticle system shows a substantially enhanced reactivity and several additional reaction pathways. In sharp contrast to Pt(111), partial dehydrogenation of ethylene on the supported Pt nanoparticles already starts at temperatures as low as 100 K. Similar to the single crystal surface, dehydrogenation occurs via the isomer ethylidene (CHCH(3)) and then mainly via ethylidyne (CCH(3)). In the temperature region between 100 and 250 K there is strong evidence for spillover of hydrocarbon fragments to the ceria support. In addition, splitting of ethylene to C(1) fragments is more facile than on Pt(111), giving rise to the formation of CH species and CO in the temperature region between 250 and 400 K. Upon further annealing, carbonaceous deposits are formed at 450 K. By heating to 700 K, these carbon deposits are completely removed from the surface by reaction with oxygen, provided by reverse spillover of oxygen from the ceria support.     

Selective oxidation of crotyl alcohol and crotonaldehyde on V2O5/MgO: In situ FTIR study

The adsorption of crotyl alcohol, crotonaldehyde and crotonic acid on MgO, V₂O₅ and V₂O₅/MgO mixed oxide was studied under oxidizing conditions by in situ FTIR spectroscopy. The results were further analyzed in order to investigate the surface intermediates of crotonaldehyde oxidative dehydrogenation to maleic anhydride. A stable surface compound, alkoxide, was found during the crotyl alcohol adsorption for all oxides at room temperature (RT). The reaction between the adsorbed crotonaldehyde and the oxygen atom of the catalyst resulted in the formation of carboxylate at higher temperatures (T>373 K). The formation of two different reaction products (furane and maleic anhydride) was detected from IR spectra following the crotonaldehyde adsorption at various oxygen pressures. It can be deduced that crotonic acid easily dissociates to form the surface carboxylate-crotonate, which is further oxidized to maleate and consequently to maleic anhydride at 573 K.     

An experimental study of the intersystem crossing and reactions of C2(X1Sigmag+) and C2(a3Pi(u)) with O2 and NO at very low temperature (24-300 K)

A low-temperature gas-phase kinetics study of the reactions and collisional relaxation processes involving C2(X1Sigma(g)+) and C2(a3Pi(u)) in collision with O2 and NO partners at temperatures from 300 to 24 K is reported. The experiments employed a CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) apparatus to attain low temperatures. The C2 species were created using pulsed laser photolysis at 193 nm of mixtures containing C2Cl4 diluted in N2, Ar, or He carrier gas. C2(X1Sigma(g)+) molecules were detected via pulsed laser-induced fluorescence in the (D1Sigma(u)+ <-- X1Sigma(g)+) system, and C2(a3Pi(u)) molecules were detected via pulsed laser-induced fluorescence in the (d 3Pi(g) <-- a 3Pi(u)) system. Relaxation of 3C2 by intersystem crossing induced by oxygen was measured at temperatures below 200 K, and it was found that this process remains very efficient in the temperature range 50-200 K. Reactivity of C2(X1Sigma(g)+) with oxygen became very inefficient below room temperature. Using these two observations, it was found to be possible to obtain the C2(X1Sigma(g)+) state alone at low temperatures by addition of a suitable concentration of O2 and then study its reactivity with NO without any interference coming from the possible relaxation of C2(a3Pi(u)) to C2(X1Sigma(g)+) induced by this reagent. The rate coefficient for reaction of C2(X1Sigma(g)+) with NO was found to be essentially constant over the temperature range 36-300 K with an average value of (1.6 +/- 0.1) x 10(-10) cm3 molecule(-1) s(-1). Reactivity of C2(a3Pi(u)) with NO was found to possess a slight negative temperature dependence over the temperature range 50-300 K, which is in very good agreement with data obtained at higher temperatures.     

Non-isothermal thermoanalytical studies on the salt roasting of chalcopyrite using KCl

In an earlier study [1], the isothermal kinetics of salt roasting of chalcopyrite under an oxidizing atmosphere using KCl was studied in the temperature range 523–773 K. The salt roasting reaction was found to be chemically controlled at temperatures below 600 K both under static air and oxygen atmosphere. At higher temperatures, the process was not thermally activated because of a change in the chemistry of the process. In the present study, the salt roasting of chalcopyrite using KCl under oxygen and static air atmosphere was studied by non-isothermal thermoanalytical studies up to 723 K. The effect of salt content, heating rate and particle size on the salt roasting behavior was studied using TG/DTA techniques at a programmed linear heating rate. The TG and DTA studies reveal two distinct chemical processes, one operative up to 620 K and the other from 620 to 723 K. The integral method of Coats and Redfern was used for the treatment of non-isothermal kinetic data. The non-isothermal analysis confirmed the chemical control mechanism at temperatures below 620 K. However, the activation energy for the process derived from non-isothermal thermogravimetric analysis is almost twice as that deduced from isothermal measurements. In the temperature range 620–723 K, the kinetic data still obeys the interfacial reaction control model although the activation energy in this temperature range is very low.     

On the Pace–Datyner theory of diffusion of small molecules through polymers

The Pace–Datyner theory for diffusion of penetrant molecules in polymers has been analyzed. It has been found that the correct solution of the problem they pose is possible only at 0 K, since then the separation of two chains at x = ∞ is equal to the minimum of the DiBenedetto potential for their interaction. Otherwise the energy of symmetrical separation is infinite. By using the linearization method to solve the differential equation, Pace and Datyner neglected the problem of unnatural boundary conditions at x = ∞ for temperatures above 0 K. The exact numerical solutions of differential equations at temperature 0 K were therefore compared with the results of the Pace–Datyner linear approximation. For temperatures different from 0 K the solution of the problem is possible only when the proper cutoff is imposed. The analytical expression for the coeffients in the DiBenedetto potential has been found, and the potential can be written as     

Excited-state proton transfer in methanol-doped ice in the presence of KF

Steady-state and time-resolved emission techniques were employed to study the photoprotolytic cycle of an excited photoacid in ice in the presence of a low concentration of a weak base-like F(-). In previous studies we found that the photoprotolytic cycle in methanol-doped ice (1% mol fraction) is too slow to be observed at temperatures below 190 K. In this study we found that at temperatures below 240 K an additional proton-transfer process occurs in ice doped with 10 mM KF. We attributed this reaction to the creation of a mobile L-defect by F(-) ions. We used a diffusion-assisted reaction model, based on the Debye-Smoluchowski equation, to account for the direct reaction of the L-defect with the excited photoacid at temperatures below T < 240 K. Below 160 K the spectroscopic properties as well as the photoprotolytic cycle change dramatically. We propose that below 160 K the sample enters a new phase. The excited-state proton-transfer (ESPT) process was observed and followed down to a liquid nitrogen temperature of approximately 78 K. In the low-temperature phase the ESPT rate is almost twice as much as at 180 K and the temperature dependence of the rate is very small. The kinetic isotope effect of the ESPT at the low-temperature phase is small of about 1.3.     

Local structure of aluminum in zeolite mordenite as affected by temperature

The local aluminum structure in zeolite mordenite was studied at temperatures up to 1000 K in a vacuum by Al K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The interatomic aluminum-oxygen distances and the number of coordinating oxygen atoms were determined by Fourier transform analyses of experimental Al K-edge XANES spectra and the fits of the nearest oxygen atoms contributions, using a limited number of variables. The values of fixed parameters for Fourier transform and fit are established from the spectrum of Na-mordenite, considered the reference compound for the studied zeolites H-mordenites, which was also used to test the accuracy and the stability of the determined structural parameters. To reveal the aluminum coordination in H-mordenite at various temperatures, the Fourier transform peak of the coordinating oxygen polyhedron was fitted first with a single-shell model, and the obtained structural information was refined by the fits, on the basis of the most plausible models for the aluminum coordination environment. The choice of such models for each temperature was performed according to the qualitative predictions on the aluminum local atomic structure provided by the preedge data analysis and 27Al magic angle spinning (MAS) NMR experiments. By this method, the presence of sixfold aluminum atoms, aside from the fourfold ones, in H-mordenite at room temperature was revealed quantitatively, and the concentrations of these mixed coordinations were determined; the structural distortion of the oxygen tetrahedron around aluminum in dehydrated H-mordenite at T = 575 K was found to be strong, and the corresponding Al-O distances for this distortion were obtained; for H-mordenite at 985 K, the presence of threefold coordinated aluminum atoms, aside from the fourfold ones, was revealed, and an estimate of the amount of threefold aluminum was given.     

Oxygen Selectivity on Partially K Exchanged Na-A Type Zeolite at Low Temperature

In previous work by the authors on Na-A zeolite (Izumi et al., Japan Patent Toku-Kou-Shou 63-058614, 1988), it was shown that the combination of high-temperature calcination and operation of the adsorption step at low temperatures improved the selectivity for oxygen over nitrogen from air (Izumi et al., CATS J Meeting Abstracts, 31(2A), 10, 1989; Izumi and Suzuki, Adsorption, 6, 2000). Berlin discloses in his U.S. Patent 3282028 (1966) that the partial exchange of potassium ions for sodium ions in the Na-A type zeolite also improved selectivity for oxygen by reducing the uptake rate of nitrogen. It was therefore expected that the oxygen selectivity of Na-K-A with high-temperature calcination and low-temperature adsorption might be enhanced. For the confirmation of optimum conditions for the appearance of oxygen selectivity on Na-K-A, samples were prepared with a K exchange ratio varied from 0–20 mol% (0–2.4 K ions/unit cell), and a calcination temperature varied from 923 to 1073 K, and an experiment concerning oxygen and nitrogen adsorption on Na-K-A was undertaken with a small adsorbent column under pressure swing adsorption (PSA) conditions at adsorption temperatures from room temperature to 213 K. It was found that (a) the K exchange ratio of 7 mol% (0.84 K ions/unit cell), and (b) the calcination temperature of 993 K, resulted in a remarkable increase in oxygen selectivity. Under optimum conditions for Na-K-A, the oxygen separation factor was about 8. Na-K-A has the potential to effectively separate oxygen and nitrogen from air by means of PSA.     

磷酸二氢铵水溶液构型能删和径向分布函数的分子动力学模拟

采用分子动力学模拟对不同温度下磷酸二氢铵水溶液的构型能和径向分布函数进行了研究．磷酸二氢根被看作七节点模型,铵离子被看作五节点模型,而水分子则被看作简单点电荷模型．在饱和温度附近,体系局域粒子数密度有波动．373-400K的溶液势能增长缓慢表明磷酸二氧铵部分分解．磷酸二氢根中的氧原子与铵离子中氢原子的径向分布函数在三种不同温度下呈现明显不同,表明溶液中平均氢键数目随温度的变化明显改变．温度对磷酸二氢根中的氧原子和氧原子的结合有一定的影响,而在饱和溶液中有更多的生长基无产生．     

Molecular beam measurements and Monte Carlo simulations of the kinetics of N2O decomposition on Rh(111) single-crystal surfaces

The kinetics of N2O decomposition on Rh(111) single-crystal surfaces were investigated both experimentally by isothermal molecular beam measurements and theoretically using a Monte Carlo algorithm. The present work was directed to the understanding of two unusual observations derived from our previous work on this system, namely, (1) the lower rates for N2O decomposition seen at higher reaction temperatures, and (2) the lower total nitrogen yields and final oxygen surface coverages that accompany that behavior. Experimentally, it was determined here that after the rhodium surface is rendered inactive by N2O decomposition at high (520 K) temperatures, significant activity is still possible at lower (350 K) temperatures. The Monte Carlo simulations explain these observations by assuming that the surface sites required for the activation of adsorbed N2O increase in size with increasing reaction temperature.     

Coexistence of ferromagnetism and superconductivity in YBCO nanoparticles

Nanoparticles of superconducting YBa(2)Cu(3)O(7-δ) were synthesized via a citrate pyrolysis technique. Room temperature ferromagnetism was revealed in the samples by a vibrating sample magnetometer. Electron spin resonance spectra at selected temperatures indicated that there is a transition from the normal to the superconducting state at temperatures below 100 K. The M-T curves with various applied magnetic fields showed that the superconducting transition temperatures are 92 K and 55 K for the air-annealed and the post-annealed samples, respectively. Compared to the air-annealed sample, the saturation magnetization of the sample by reheating the air-annealed one in argon atmosphere is enhanced but its superconductivity is weakened, which implies that the ferromagnetism maybe originates from the surface oxygen defects. By superconducting quantum interference device measurements, we further confirmed the ferromagnetic behavior at high temperatures and interesting upturns in field cooling magnetization curves within the superconducting region are found. We attributed the upturn phenomena to the coexistence of ferromagnetism and superconductivity at low temperatures. Room temperature ferromagnetism of superconducting YBa(2)Cu(3)O(7-δ) nanoparticles has been observed in some previous related studies, but the issue of the coexistence of ferromagnetism and superconductivity within the superconducting region is still unclear. In the present work, it will be addressed in detail. The cooperation phenomena found in the spin-singlet superconductors will help us to understand the nature of superconductivity and ferromagnetism in more depth.     

Hydrogen desorption of reduced Pt/TiO2 catalyst

The temperature-programmed desorption of hydrogen from a Pt/TiO₂ catalyst reduced in a wide temperature range (RT-773 K) has been studied. It is found that the presence of labile surface oxygen species increases the amount of hydrogen species formed at room temperature, and greatly decreases the quantities of adsorbed hydrogen species at medium temperatures. After the catalyst was reduced at high temperature, it is observed that two strong hydrogen desorption peaks appear at 450–600 K and above 600 K, which are ascribed to surface titanium hydride and the hydrogen species stored in the sublayer and bulk of the support, respectively.     

镁皂石的水热合成与表征研究

采用优化的水热合成法,在473K反应2h合成了具有优良长程有序性的2：1型三八面体蒙皂石(皂石)。XRD分析表明合成产物具有典型的皂石层状结构。TEM测试发现,不同温度下合成的皂石具有类似的形态,但其层状片大小随着合成温度的提高而增大。不同温度(423K,473K,523K,573K)下水热合成实验结果表明,473K是合成具有良好层状有序结构皂石的临界温度。不同层电荷合成皂石对于活性染料(MB)的等温吸附实验说明吸附过程包含离子交换和絮凝机理。相对于天然蒙脱石,合成皂石对于离子性染料具有更大的吸附容量和作用力。     

Growth of cerium tungstate epitaxial layers: influence of temperature

Thin films of cerium tungstate prepared in situ by cerium deposition in oxygen atmosphere onto the W(100) single‐crystal were investigated by means of photoelectron spectroscopy and low‐energy electron diffraction (LEED). The studied temperature range was 173–1073 K. It was found that the temperature necessary for the oriented growth of Ce₆WO₁₂(100) was 673 K, and at higher temperatures, the LEED pattern improved. Photoemission data revealed the partial formation of CeO₂ on the surface at preparation temperatures below 473 K due to limited diffusion of tungsten atoms from the substrate. Copyright © 2015 John Wiley & Sons, Ltd.     

The temperature dependence of the exchange reaction between oxygen atoms and dioxygen molecules studied by means of isotopes and spectroscopy

The temperature dependence of the rate constant for the exchange reaction between oxygen atoms and dioxygen molecules has been studied using the oxygen isotopes ¹⁶O and ¹⁸O. The reaction was studied by VIS-photolysis of ozone in the presence of isotopic dioxygen and with nitrogen as a bath gas at five different temperatures, 143 K, 173 K, 203 K, 253 K, and 295 K. High-resolution microwave spectroscopy was used to measure the composition of the ozone isotopomer mixtures and mass-spectrometry was used to determine the abundances of the isotopomeric dioxygen species formed during the reaction. The rate constant was determined to be k_exchange=(2.66±0.78)×¹⁰⁻¹² (T/300 K)^{−(0.88±0.26)} cm³s⁻¹ (±2σ ) or as the ratio between rate constants for exchange and for ozone formation, (4.67±1.3)×10²¹ (T/300 K)^(1.74±0.19) cm⁻³ (±2σ). © 1997 John Wiley & Sons, Inc.     

Modeling of oxygen adsorption on silver

Different adsorption forms of oxygen on silver are discussed. Four main types of oxygen forming at different temperatures and oxygen pressures have been distinguished. A kinetic model describing the formation and transformations of the oxygen forms and taking into account the surface amorphization has been proposed. Numerical modeling of stationary concentrations using this model gives evidence for a temperature window ΔT=500–800 K, where a quasimolecular oxygen state (E=530.5 eV, T_des=800–900 K) can exist at high oxygen pressures.     

交联SU-8光刻胶与Ni基底结合性的分子动力学模拟

应用分子动力学模拟软件Materials Studio构建SU-8光刻胶与Ni基底的界面结构,研究后烘温度对界面结合性的影响.结合工艺中所采用的后烘温度,模拟计算了338～368K时Ni基底上SU-8胶的交联反应,在经过反复的能量最小化和分子动力学模拟后,对最终得到的平衡结构进行了界面结合能的计算.计算结果表明界面结合能随着后烘温度的升高而增大,在368K时结合能达到最大值,说明此时界面结合最好.对分子体系进行了能量分析,结果表明界面分子间的范德华力作用能是影响界面结合的主要因素.对体系界面原子间进行了径向分布函数分析,发现范德华力作用范围内(0.31～0.60nm)出现两组Ni—O的强峰,也证实了上述结论。     

The thermodynamic method for selecting oxygen carriers used for chemical looping air separation

Chemical looping air separation (CLAS) has been suggested as a new and energy saving method for producing oxygen from air. The selection of suitable oxygen carriers is the key issue for CLAS system. This paper shows a comprehensive thermodynamic method for selecting oxygen carriers used for CLAS through studying the properties of 34 different oxygen releasing reactions referring to 18 elements at different temperatures. The research mainly includes analysis of oxygen releasing capacity by calculating the Gibbs free energy change (ΔG) and the equilibrium partial pressure of oxygen of the reduction or oxidation reaction at different temperatures. Oxygen content and transport capacity were calculated. The spontaneous reaction temperatures for oxygen releasing reactions were presented to determine the operating temperatures. Also, the minimum demand of the steam for the reduction reaction was discussed. On the basis of the comprehensive thermodynamic study, the oxide systems of CrO₂/Cr₂O₃, PbO₂/Pb₃O₄, PbO₂/PbO, Pb₃O₄/PbO, MnO₂/Mn₂O₃, and Ag₂O/Ag have been found suitable for the CLAS process in low temperatures (500–800 K). The systems of PdO₂/PdO, PdO₂/Pd, PdO/Pd, MnO₂/MnO, and MnO₂/Mn₃O₄ were suitable for medium temperatures (800–1100 K) CLAS process. And Co₃O₄/CoO, CuO/Cu₂O, Mn₂O₃/Mn₃O₄, and OsO₂/Os systems only worked successfully in high temperatures (1100–1400 K). In addition, the CaO₂/CaO system was not suitable for CLAS because of the reaction with steam. The various binders such as SiO₂, TiO₂, Al₂O₃, Y₂O₃, ZrO₂, and YSZ which have been used for CLC could also be the supports for CLAS oxygen carriers.     

ZrO2催化无溶剂条件下醇的液相氧化反应

采用一种环境友好的方法,以分子氧为氧化剂,以ZrO2为催化剂,在无溶剂的条件下实现了醇的选择性氧化.苯甲醛、环己酮和辛醛等是相应醇的主要氧化产物.考察了不同反应条件(搅拌速度、反应时间和温度)、催化剂制备参数(焙烧温度和负载量)及氧分压等的影响.结果发现,对于醇氧化生成相应羰基化合物,1 223 K焙烧的ZrO2比723 K焙烧的ZrO2显示出更高的催化活性.催化剂在反应混合物中不溶解,可以通过简单的过滤使其分离并重复使用.当搅拌速度大于900 r/min时,对醇的转化速率无明显的影响.