共查询到20条相似文献,搜索用时 31 毫秒
1.
Spin polarisation effects of labile manganese–oxygen bonds in the X-ray diffraction structure of the oxygen-evolving complex (OEC) of photosystem II (PSII) at 1.9 Å resolution have been investigated by the UB3LYP computations on the basis of three different theoretical models with and without hydrogen bonds: quantum-mechanical (QM) Model I, QM(Model II)/MM and QM Model III. The spin densities on the manganese and oxygen atoms of the CaMn 4O 5 cluster revealed by these computations have elucidated internal, semi-internal and external reductions of high-valent manganese ions in the CaMn 4O 5 cluster in OEC of PSII. The internal reduction of Mn(IV) ions by the back charge transfer from oxygen dianions is remarkable in the small QM Model I, whereas it is significantly reduced in the case of more realistic QM Model III including hydrogen bonding stabilisations of oxygen dianions. However, semi-internal reduction of the CaMn 4O 5 cluster with remote amino acid residues such as Asp61 anion occurs even in QM Model III, indicating the necessity of large QM parts for redox-active systems such as OEC of PSII. The computational results have clearly demonstrated important roles of confinement effects of the CaMn 4O 5 cluster with labile Mn–O bonds with protein. These computational results have been applied to molecular design of artificial robust catalysts for water oxidation by use of sunlight. 相似文献
2.
Redox kinetics of oxygen carrier in chemical looping is an important component for material preparation, reactor design and process demonstration. How to bridge the gap between the microscale density functional theory (DFT) and the macroscale redox kinetics and develop a first-principle-based theoretical model is still a challenge in the field of chemical looping. This study addresses this challenge and proposes a DFT-based microkinetic rate equation theory to calculate the heterogeneous kinetics of Fe 2O 3 reduction by CO in chemical looping. Firstly, the DFT calculation is adopted to search the reaction pathways and to obtain the energy barriers of elementary reactions. Secondly, the DFT results are introduced into the transition state theory (TST) to calculate the reaction rate constants and build the rate equations of elementary surface reactions. Finally, by considering the bulk diffusion, a rate equation is developed to bridge the gap between the elementary surface reactions and the grain conversion. In the theory, the reaction mechanism obtained from DFT and kinetic rate constants obtained from TST are directly implemented into the rate equation to predict the reduction kinetics of oxygen carriers without fitting experimental data. The accuracy of the developed theory is validated by experimental data of two Fe 2O 3 oxygen carriers obtained from the thermogravimetric analyzer (TGA). The microkinetic rate equation theory is based on the first principles calculation and can predict directly the redox kinetics of oxygen carriers without depending on the experimental kinetic data, therefore, it provides a powerful theoretical tool to screen the oxygen carrier materials and optimize the microstructure of oxygen carriers. 相似文献
3.
Chemical looping combustion (CLC) is an advanced oxyfuel process that enables CO 2 capture with low efficiency penalty. CLC of gaseous fuels has successfully been demonstrated in several pilots up to 150 kW th. Numerous oxygen carriers have been tested regarding fuel conversion performance and lifetime. This work is a scale-up study of gaseous fuel CLC to MW th scale. A Ca-Mn-based oxygen carrier has been developed and manufactured in ton-scale prior to the present test. Investigations were conducted in a 1 MW th CLC unit that was adapted to utilize natural gas as fuel. Stable CLC conditions were reached during tests with Ca-Mn-based material, and the transition to operation with ilmenite was studied. The fuel conversion was in the range of 80%. During operation, 99% of the unburned methane was converted in the post oxidation chamber. The solids circulation rate and the lifetime of solids were determined by means of solids samples from the process, which were investigated in terms of attrition and degree of oxidation. The solids circulation rate was 17 tons h ?1 MW ?1 which is higher than in former tests but lower compared to other units. The most important limiting factors of the fuel conversion are the low solids inventory of the fuel reactor and the oxygen carrier to fuel ratio that corresponds to the solids circulation. 相似文献
4.
57Fe Mössbauer spectroscopy has provided precise and accurate iron redox ratios Fe 2+/Fe 3+ in ilmenite, FeTiO 3, found within kimberlite samples from the Catoca and Camatxia kimberlite pipes from N.E. Angola. Ilmenite is one of the key indicator minerals for diamond survival and it is also one of the iron-bearing minerals with iron naturally occurring in one or both of the oxidation states Fe 3+ and Fe 2+. For this reason it is a good indicator for studying oxygen fugacities ( fO 2) in mineral samples, which can then be related to iron redox ratios, Fe 2+/Fe 3+. In this paper we demonstrate that the oxidation state of the ilmenite mineral inclusion from sampled kimberlite rock is a key indicator of the oxidation state of the host kimberlite assemblage, which in turn determines the genesis of diamond, grade variation and diamond quality. Ilmenite samples from the two different diamondiferous kimberlite localities (Catoca and Camatxia) in the Lucapa graben, N.E. Angola, were studied using Mössbauer spectroscopy and X-Ray Diffractometry, in order to infer the oxidation state of their source regions in the mantle, oxygen partial pressure and diamond preservation conditions. The iron redox ratios, obtained using Mössbauer spectroscopy, show that the Catoca diamond kimberlite is more oxidised than kimberlite found in the Camatxia pipe, which is associated within the same geological tectonic structure. Here we demonstrate that 57Fe Mössbauer spectroscopy can assist geologists and mining engineers to effectively evaluate and determine whether kimberlite deposits are economically feasible for diamond mining. 相似文献
5.
The magnetic and structural characterization of Ti 1−xFe xO 2 ( x=0.025, 0.05, 0.07, 0.125, and 0.15) samples prepared by mechano-synthesis using TiO 2 and Fe 2O 3 as starting materials are reported. XANES measurements performed at the Fe K-edge show that Fe ions are in 3+ oxidation state in the 7 at% Fe-doped sample and in a mixture of 2+ and 3+ oxidation states in the other samples. EXAFS results show the incorporation of Fe ions substituting Ti ones in the rutile TiO 2 structure. They also reveal a strong correlation between the number of oxygen nearest neighbours and the Fe 2+ fraction, i.e the number of oxygen near neighbours decreases when the Fe 2+ fraction increases. All samples present ferromagnetic-like behaviour at room temperature. We found a clear dependence between saturation magnetization and coercivity with the fraction of Fe 2+ and/or the number of Fe near neighbour oxygen vacancies. 相似文献
6.
Spinel CuFe 2O 4 is a promising oxygen carrier due to its synergistic enhanced performance. A fundamental understanding of the reaction mechanism between oxygen carrier and fuels is important for a rational design of highly efficient oxygen carrier. The reaction mechanism of spinel CuFe 2O 4 with CO during chemical-looping combustion (CLC) was studied based on thermogravimetric analyses (TGA) and density functional theory (DFT) calculations. Two distinct reaction stages were clearly observed. CuFe 2O 4 was mainly transformed into Cu and Fe 3O 4 with a rapid reaction rate in the initial stage, and then product Fe 3O 4 was slowly reduced to FeO or even to Fe. The reactivity of CuFe 2O 4 is much higher than that of Fe 2O 3, which is ascribed to the existence of Cu. The enhanced oxygen evolution activity of CuFe 2O 4 at low temperature is validated by both the experimental and theoretical methods. Three types of surface oxygen coordinated with different metal atoms show different reactivity. Two kinds of reaction pathways are involved in CO oxidation over CuFe 2O 4. In the one-step reaction pathway, CO directly reacts with the oxygen bonding to two octahedral Cu and one octahedral Fe atoms to form a CO 2 molecule without an energy barrier, which corresponds to the surface oxygen consumption observed in TGA experiments. In the possible two-step reaction pathway, CO first adsorbs on the surface, and then reacts with the oxygen bound to one octahedral Cu and two octahedral Fe atoms to generate CO 2 by surmounting an energy barrier of 10.84 kJ/mol, which is the most kinetically and thermodynamically favorable pathway. 相似文献
7.
Perovskite-related phases derived from SrFeO 3-δ are among known mixed conductors with highest oxygen permeability and are thus of interest as the ceramic membrane materials for oxygen separation and partial oxidation of light hydrocarbons. Dense ceramics of SrFe 1-xAl xO 3-δ (x=0.1–0.5) were prepared via the glycine-nitrate process. The cubic solid solution formation was found to occur in the concentration range x=0–0.35. Increasing aluminum content leads to decreasing thermal expansion coefficients (TECs), relative fraction of Fe 4+ under oxidizing conditions, and also the total conductivity, predominantly p-type electronic at oxygen pressures close to atmospheric. The TECs vary in the range (13.5–16.4)×10 ?6 K ?1 at 373–923 K and increase up to (18.6–31.9)×10 ?6 K ?1 at 923–1273 K. The oxygen permeation fluxes decrease moderately with aluminum additions. The Mössbauer spectroscopy data and p(O 2) dependencies of electrical properties indicate a small-polaron mechanism of electronic transport in SrFe 1-xAl xO 3-δ. Reducing oxygen partial pressure results in transition from dominant p- to n-type electronic conduction. The low-p(O 2) stability limit of SrFe 1-xAl xO 3-δ perovskites lies between that of LaFeO 3-δ and Fe/Fe 1-γO boundary. 相似文献
8.
The high oxygen permeability combined with reasonable structural stability of perovskite-type ABO3−δ compounds is vital for their potential applications in gas separation, solid oxide fuel cells, sensors, etc. Hence, an attempt is made to develop SrCo0.8Fe0.2O3−δ-based dense membranes with sol-gel-derived oxalates and study their phase stability and oxygen permeation. While X-ray diffraction confirms the presence of a perovskite-type cubic phase above 800 °C, X-ray photoelectron spectroscopy reveals the presence of cobalt and iron in 3+ and 4+ oxidation states with O2
2−, O2
− and O− species. The electrical conductivity increases up to a characteristic temperature and decreases slowly thereafter via pronounced carrier scattering. A 1.5-mm-thick membrane displays reasonable oxygen permeability of 1.05 × 10−6 mol cm−2 s−1 at 900 °C but has inadequate stability. Partial substitution of iron with zirconium is shown to improve permeability and stability significantly. Thus, SrCo0.8Fe0.15Zr0.05O3−δ membrane shows promise for oxygen permeation purposes. 相似文献
9.
The cation distribution in spinel-related titanium-substituted lithium ferrite, Li 0.5+0.5xFe 2.5−1.5xTi xO 4 has been explored using interatomic potential and ab initio calculations. The results suggest that the cation distribution with Ti 4+ substituting for Fe 3+ on octahedral B sites and excess Li + substituting for Fe 3+ on tetrahedral A sites is stabilised by the formation of clusters of two octahedrally coordinated Ti 4+ ions and one tetrahedrally coordinated Li + ion linked through a common oxygen. 相似文献
10.
High temperature X-ray diffraction (HT-XRD), temperature programmed desorption (TPD), thermogravimetric analysis–differential thermal analysis (TGA/DTA) and neutron diffraction were combined to determine the structure and oxygen stoichiometry of SrCo 0.8Fe 0.2O 3−δ (SCF) and Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3−δ (BSCF) up to 1273 K in the pO 2 range of 1 to 10 − 5 atm. Formation of the vacancy-ordered brownmillerite phase, SrCo 0.8Fe 0.2O 2.5, was observed as a region of zero oxygen release in the TPD measurements and confirmed by HT-XRD and TGA/DTA. No ordering was observed in the BSCF system by any of the techniques utilized in this work. The oxygen vacancy concentration of BSCF was found to be considerably higher than that of SCF and always higher than that of the ordered brownmillerite phase of SCF, δ = 0.5. The combination of a high vacancy concentration and absence of ordering leads to higher oxygen permeation fluxes through BSCF membranes in comparison to SCF. 相似文献
11.
In the present work, the structural, electronic, elastic and mechanical properties of Ti 2AlC and Ti 2Al(C 1-xO x) solid solutions were investigated using first-principles calculations for varied O content incorporation (x = 0, 0.125, 0.25, 0.375, 0.5). According to the calculation results, all Ti 2Al(C 1-xO x) solid solutions with various x values are stable, and the bonding strength of the Ti–Al bond increases with the doping of O element. In addition, the shear modulus G and C44 elastic constant of Ti 2Al(C 1-xO x) solid solutions are both lower than the bulk modulus B, indicating that the phase has good damage tolerance. Not only that, compared with Ti 2AlC, the plasticity and toughness of Ti 2Al(C 1-xO x) solid solutions are improved with the increase of O atom doping and doping ratio. Simultaneously, the doping of O atom is also beneficial to reduce the generalized stacking fault energy of Ti 2AlC, making the Ti 2Al(C 1-xO x) solid solutions more prone to shear deformation, thereby further enhancing plasticity. 相似文献
12.
Anion-deficient substituted ferrites Ca 2Fe 2 − x
N
x
O 5 ( N = Sc 3+, Al 3+) and mixed manganite CaMn 7O 12 have been investigated by 119Sn and 57Fe probe M?ssbauer spectroscopy. The mechanism of charge compensation for heterovalent impurity Sn 4+ ions in the structure of the ferrite Ca 2Fe 2O 5 has been established. The presence of nonequivalent crystallographic positions of manganese cations, caused by their charge
ordering in the structure of the manganite CaMn 7O 12, is shown. Magnetic ordering of Mn 3+ and Mn 4+ cations in the octahedral sublattice of CaMn 7O 12 at T < T
M2 ≈ 90 K is established.
Original Russian Text ? A.V. Sobolev, I.A. Presnyakov, K.V. Pokholok, V.S. Rusakov, T.V. Gubaidulina, A.V. Baranov, G. Demazeau,
2007, published in Izvestiya Rossiiskoi Akademii Nauk Seriya Fizicheskaya, 2007, Vol. 71, No. 9, pp. 1347–1354. 相似文献
13.
The redox behavior of perovskite-type La 0.90Sr 0.10Al 0.85−xFe xMg 0.15O 3−δ ( x=0.20-0.40) mixed conductors was analyzed by the Mössbauer spectroscopy and measurements of the total conductivity and Seebeck coefficient in the oxygen partial pressure range from 10 −20 to 0.5 atm at 1023-1223 K. The results combined with oxygen-ion transference numbers determined by the faradaic efficiency technique in air, were used to calculate defect concentrations, mobilities, and partial ionic and p- and n-type electronic conductivities as a function of oxygen pressure. The redox and transport processes can be adequately described in terms of oxygen intercalation and iron disproportionation reactions, with the thermodynamic functions independent of defect concentrations. No essential delocalization of the electronic charge carriers was found. The oxygen non-stoichiometry values estimated from the conductivity vs. p(O 2) dependencies, coincide with those evaluated from the Mössbauer spectra. 相似文献
14.
ABSTRACTAtmospheric oxygenation and evolution of aerobic life on our earth are a result of water oxidation by oxygenic photosynthesis in photosystem II (PSII) of plants, algae and cyanobacteria. The water oxidation in the oxygen-evolving complex (OEC) in PSII is expected to proceed through five oxidation states, known as the S i ( i = 0, 1, 2, 3 and 4) states in the Kok cycle, with the S 1 being the most stable state in the dark. The OEC in PSII involves the active catalytic site made of four Mn ions and one Ca ion, namely the CaMn 4O 5 cluster. Past decades, molecular structures of the CaMn 4O 5 cluster in OEC in PSII have been investigated by the extended X-ray absorption fine structure (EXAFS). The magneto-structural correlations were extensively investigated by electron paramagnetic resonance (EPR) spectroscopy. Recently, Kamiya and Shen groups made great breakthrough for determination of the S 1 structure of OEC of PSII by the X-ray diffraction (XRD) and X-ray free-electron laser (XFEL) experiments, providing structural foundations that are crucial for theoretical investigations of the CaMn 4O 5 cluster. Large-scale quantum mechanics/molecular mechanics calculations starting from the XRD structures elucidated geometrical, electronic and spin structures of the CaMn 4O 5 cluster, indicating an important role of the Jahn–Teller (JT) effect of Mn(III) ions. This paper presents theoretical formulas for estimation of the JT deformations of the CaMn 4O 5 cluster in OEC of PSII. Scope and applicability of the formulas are examined in relation to several different structures of the CaMn 4O 5 cluster proposed by XRD, XFEL, EXAFS and other experiments. Implications of the computational results are discussed for further refinements of geometrical parameters of the CaMn 4O 5 cluster. 相似文献
15.
Co xTi 1−xO 2−δ films have been prepared on Si(001) substrates by sol-gel method. When heat treated in air, Co xTi 1−xO 2−δ films are non-ferromagnetic at room temperature. However, after further vacuum annealing or hydrogenation, Co xTi 1−xO 2−δ films show room-temperature ferromagnetism (RTFM). When the vacuum annealed Co xTi 1−xO 2−δ films are reheated in air, the magnetic moments of the films strongly reduce. After these films are vacuum annealed once again, the magnetic moments are greatly enhanced, confirming the role of vacuum annealing in ferromagnetism of Co xTi 1−xO 2−δ films. The x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and measurements of magnetization ( M) vs temperature ( T) fail to detect Co clusters in the vacuum annealed and the hydrogenated Co xTi 1−xO 2−δ films. Oxygen vacancies are formed in Co xTi 1−xO 2−δ films after vacuum annealing and hydrogenation, determined by XRD and XPS measurements. These results indicate that oxygen vacancies created by vacuum annealing and hydrogenation play an important role in the generation of RTFM in Co xTi 1−xO 2−δ films. 相似文献
16.
By means of 57Fe-probe Mössbauer spectroscopy the data on copper and oxygen charge states in the Cu(1)O δ — plane of YBa 2 (Cu 0.99Fe 0.01) aO σ+δ were obtained for 0≤δ≤1. These data enable to calculate the holes concentration vs oxygen content. 相似文献
17.
The oxidation kinetics of Bi 1.3Pb 0.8Sr 2Ca 0.8Y 0.2Cu 2O 8+δ solid solutions at different temperatures and \(p_{O_2 } = 0.21\) atm is investigated by thermogravimetry. The results obtained are compared with the previously studied oxidation kinetics of Bi 1.3Pb 0.8Sr 2Ca 0.8Y 0.2Cu 2O 8+δ solid solutions. It is found that the substitution of yttrium for calcium leads to an appreciable retardation of the initial oxidation stage associated with the oxygen diffusion. The phonon spectra of the solid solutions are examined using inelastic neutron scattering on a DIN-2PI direct-geometry spectrometer. The high-frequency (>50 meV) phonon densities of states for yttrium-containing and yttrium-free solid solutions are analyzed. The possible model is proposed for a correlation between the differences observed in the high-frequency phonon densities of states attributed to the vibrations of oxygen atoms and the differences in the oxidation kinetics of the solid solutions under consideration. 相似文献
18.
本文报道,通过对Y 1-xCa xBa 2Cu 3-xMxO 7-δ(M=Fe,Ni)体系样品的晶体结构、氧含量、正常态电阻率与温度的关系,以及超导转变温度等测量,并与YBa 2Cu 3-xMxO 7-δ(M=Fe,Ni)体系进行比较,发现Y 1-xCa xBa 2Cu 3-xFe xO 7-δ体系的T c显著地高于相应x值的YBa 2Cu 3-xFe xO 7-δ体系,而Y 1-xCa xBa 2Cu 3-xNi xO 7-δ体系则相反,T c低于仅Ni替代的体系,表明Ca和Fe同时替代时两者引起的载流子浓度(n H)变化相互补偿,抑制了仅Fe替代时引起的n H和T c急剧下降;而作Ca和Ni同时替代时主要的不是两者引起载流子浓度变化的相互补偿,Ca和Ni替代效应之间的关联较弱。作者认为,对Y 1-xCa xBa 2Cu 3-xFe xO 7-δ体系属于CuO 2平面外的元素替代,这时载流子浓度是决定Tc的主要因素;而对Y 1-xCa xBa 2Cu 3-xNi xO 7-δ体系,由于Ni 2+离子主要占据Cu(Ⅱ)位,它导致磁拆对效应,Ni 2+离子的拆对效应是引起T c下降的直接原因。
关键词: 相似文献
19.
Ba 0.96Sr 0.04Zr x Y 0.005Ti 0.995–xO 3–δ ceramics were prepared by conventional sintering. The dielectric properties and the Curie temperature of the ceramics were studied. The Ba 0.96Sr 0.04Zr x Y 0.005Ti 0.995–xO 3–δ ceramics ( x = 0.04) exhibit a giant dielectric constant ( εr ~ 10 5). The peak value of dielectric constant is ~250,000 at 100 Hz and ~70,000 at 1 MHz for Ba 0.96Sr 0.04Zr x Y 0.005Ti 0.995–xO 3–δ ceramics ( x = 0.04). The results show that Ba 0.96Sr 0.04Zr x Y 0.005Ti 0.995–xO 3–δ ( x = 0.04) ceramics are a promising candidate material for microelectronic applications. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
20.
The effect of Mn 2+Co 2+Ti 4+ substitution on microwave absorption has been studied for BaCo 0.5Mn 0.5Ti 1.0Fe 10O 19 ferrite-acrylic resin composites in frequency range from 12 to 20 GHz. X-ray diffraction (XRD), scanning electron microscope (SEM), vibrating sample magnetometer, AC susceptometer and vector network analyzer were used to analyze the structural, magnetic and microwave absorption properties. The results showed that the magnetoplumbite structures for all samples have been formed. Based on microwave measurement on reflectivity, BaCo 0.5Mn 0.5Ti 1.0Fe 10O 19 may be a good candidate for electromagnetic compatibility and other practical applications at high frequency. 相似文献
|