Theoretical study of propene oxidation on Bi2O3 surfaces

The role of bismuth in the selective oxidation of propene has long been debated. We performed density functional calculations to study the dehydrogenation reaction of propene on Bi2O3 surfaces. Our calculated thermodynamic data reveal that the first dehydrogenation of propene on the most stable(010) surface and the(100) surface are difficult. Our calculations indicate that the barrier of the first hydrogen abstraction on the high Miller index surface(211) is much lower than those on the(100) and(010) surfaces, and is close to the experimental one. Further dehydrogenation is shown to be difficult and production of 1,5-hexadiene through dimerization of allyl is likely, in agreement with the experimental observations.     

A surface potential study of adsorbed oxygen species on a Bi2Mo3O12 catalyst

Bi₂Mo₃O₁₂ is a selective oxidation catalyst of propylene into acrolein. Surface potential measurements proved the presence of an equilibrium between gaseous oxygen and the O^2– species adsorbed on the surface of the catalyst, under oxygen atmosphere, in the range 538K–673K. This is in accordance with the fact that O^2– is a selective oxidation species. An oxygen adsorption mechanism is also discussed.

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Bi₂Mo₃O₁₂ . O^–2, , 538–613 . , O^–2 . .

     

Bi2O3 catalyzed asymmetric oxidation of sulfides

Bismuth based catalytic system for the asymmetric oxidation of alkyl and aryl benzyl sulfides using t-BuOOH as the oxidant has been investigated. This method affords sulfoxides with high enantioselectivities (up to 98% ee) and good yields in reasonable time. The over oxidized product namely the sulfone was not observed. The described method has wide range of applications, exhibits chemoselectivity/enantioselectivity, and proceeds under mild and environmentally friendly reaction conditions.     

Polymorphism in yttrium molybdate Y2Mo3O12

Yttrium molybdate (Y₂Mo₃O₁₂) has been prepared by non-hydrolytic sol–gel chemistry. The phase evolution upon heating was investigated using in situ and ex situ heat treatments combined with powder X-ray diffraction. This method has led to the isolation of two orthorhombic phases with different atomic connectivity. Yttrium adopts 6- and 7-coordinate sites in the Pbcn and Pba2 structures, respectively. Cocrystallization of both phases was observed in a narrow temperature range, suggesting that crystallization kinetics play a major role in phase formation. It was found that the Pba2 phase is the stable polymorph below 550 °C, and converts to Pbcn at higher temperatures.     

A new 2212-type stair like structure: Bi14Sr21Fe12O61, m=5 member of the generic [Bi2Sr3Fe2O9]m[Bi4Sr6Fe2O16] family

A new oxide, Bi₁₄Sr₂₁Fe₁₂O_61, with a layered structure derived from the 2212 modulated type structure Bi₂Sr₃Fe₂O₉, was isolated. It crystallizes in the I2 space group, with the following parameters: a=16.58(3) Å, b=5.496(1) Å, c=35.27(2) Å and β=90.62°. The single crystal X-ray structure determination, coupled with electron microscopy, shows that this ferrite is the m=5 member of the [Bi₂Sr₃Fe₂O₉]_m[Bi₄Sr₆Fe₂O₁₆] collapsed family. This new collapsed structure can be described as slices of 2212 structure of five bismuth polyhedra thick along , shifted with respect to each other and interconnected by means of [Bi₄Sr₆Fe₂O₁₆] slices. The latter are the place of numerous defects like iron or strontium for bismuth substitution; they can be correlated to intergrowth defects with other members of the family.     

Mild hydrothermal synthesis and ferrimagnetism of Pr3Fe5O12 and Nd3Fe5O12 garnets

Two pure light rare earth iron garnets Pr₃Fe₅O₁₂ and Nd₃Fe₅O₁₂ single crystals were synthesized under mild hydrothermal conditions and structurally characterized by single crystal and powder X-ray diffraction methods. Both compounds crystallize in cubic space group Ia3?d with lattice parameters a=12.670(2) Å for Pr₃Fe₅O₁₂ and a=12.633(2) Å for Nd₃Fe₅O₁₂, respectively. The synthesis of compounds was studied with regard to phase evolution and morphology development with hydrothermal conditions. We proposed the formation mechanisms and formulated a reasonable explanation for their growth habits. Ferrimagnetic Curie temperatures which have been inferred from thermo-magnetization curves were 580 K for Pr₃Fe₅O₁₂ and 565 K for Nd₃Fe₅O₁₂, and the transitions of long range order were also evidenced by differential scanning calorimetry method. The result of magnetic properties has shown that moments of the large radius Pr³⁺ and Nd³⁺ ions are parallelly coupled with net moments of iron ions.     

Phase states in the Bi4Ti3O12-BiFeO3 section in the Bi2O3-TiO2-Fe2O3 system

Specific features of the thermal behavior of Bi _{m + 1}Fe _m−3Ti₃O_{3m + 3} layered perovskite-like compounds (where m takes integer and some fractional values between 3 and 9) were considered, and the temperature limits of stability of these compounds were determined. The phase diagram of the Bi₄Ti₃O₁₂-BiFeO₃ section through the Bi₂O₃-TiO₂-Fe₂O₃ system was constructed.     

Bi2Ti2O7/TiO2/Bi4Ti3O12多异质结的构筑及其增强的可见光催化性能

以静电纺丝技术制备的TiO₂纳米纤维为基质,硝酸铋为铋源,KOH为矿化剂,成功制备了多异质结Bi₄Ti₃O₁₂/TiO₂/Bi₂Ti₂O₇复合纳米纤维光催化剂。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、紫外可见漫反射光谱(UV-Vis DRS)等一系列表征,对其物相组成、微观形貌和光学性质等进行分析。结果表明： TiO₂纳米纤维的介入,将Ⅰ型异质结Bi₂Ti₂O₇/Bi₄Ti₃O₁₂分离为2个Ⅱ型异质结Bi₂Ti₂O₇/TiO₂和Bi₄Ti₃O₁₂/TiO₂。Bi₂Ti₂O₇、Bi₄Ti₃O₁₂和TiO₂三者的协同作用,有效提高了可见光吸收能力,改变了光生载流子的传输路径,降低了光生电子与空穴的复合几率,从而获得高效的光催化降解CH₃CHO性能。可见光照8 h,Bi₄Ti₃O₁₂/TiO₂/Bi₂Ti₂O₇复合纳米纤维对CH₃CHO的降解率达到87.1%。     

Bi2Ti2O7/TiO2/Bi4Ti3O12多异质结的构筑及其增强的可见光催化性能

以静电纺丝技术制备的TiO₂纳米纤维为基质,硝酸铋为铋源,KOH为矿化剂,成功制备了多异质结Bi₂Ti₂O₇/TiO₂/Bi₄Ti₃O₁₂复合纳米纤维光催化剂。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、紫外可见漫反射光谱(UV-Vis DRS)等一系列表征,对其物相组成、微观形貌和光学性质等进行分析。结果表明：TiO₂纳米纤维的介入,将Ⅰ型异质结Bi₂Ti₂O₇/Bi₄Ti₃O₁₂分离为2个Ⅱ型异质结Bi₂Ti₂O₇/TiO₂和Bi₄Ti₃O₁₂/TiO₂。Bi₂     

Structural and magnetic characterisation of Aurivillius material Bi2Sr2Nb2.5Fe0.5O12

The n=3 Aurivillius material Bi₂Sr₂Nb_2.5Fe_0.5O₁₂ is investigated and combined structural refinements using neutron powder diffraction (NPD) and X-ray powder diffraction data (XRPD) data reveal that the material adopts a disordered, tetragonal (I4/mmm) structure at temperatures down to 2 K. Significant ordering of Fe³⁺ and Nb⁵⁺ over the two B sites is observed and possible driving forces for this ordering are discussed. Some disorder of Sr²⁺ and Bi³⁺ over the M and A sites is found and is consistent with relieving strain due to size mismatch. Highly anisotropic thermal parameters for some oxygen sites suggest that the local structure may be slightly distorted with some rotation of the octahedra. Magnetic measurements show that the material behaves as a Curie-Weiss paramagnet in the temperature range studied with no evidence of any long-range magnetic interactions. Solid solutions including Bi_3−xSr_xNb₂FeO₁₂, Bi₂Sr_2−xLa_xNb₂FeO₁₂ and Bi₂Sr₂Nb_3−xFe_xO₁₂ were investigated but single-phase materials were only successfully synthesised for a narrow composition range in the Bi₂Sr₂Nb_3−xFe_xO₁₂ system.     

Structural studies of some body-centered cubic phases of mixed oxides involving Bi2O3: The structures of Bi25FeO40 and Bi38ZnO60

The structures of the compounds initially reported to be 7·Bi₂O₃·ZnO and 96·Bi₂O₃·4Fe₂O₃, have been determined by X ray methods. Three dimensional, absorption corrected diffractometer data were used and atomic parameters were refined by least-squares procedures. The structures are isomorphous, cubic witha = 10.194(3)and10.179(3)A?, respectively, and space group I23. Each Bi³⁺ ion is surrounded by five oxygen atoms that form an incomplete octahedral arrangement with BiO distances ranging from 2.07–2.60A?. The6s²inert electron pair completes the octahedron. The Bi³⁺ ions are vibrating anisotropically. Tetrahedral sites in the structures contain 61 and 46 electrons, respectively. These values are consistent with a statistical distribution of Zn²⁺ and Bi⁵⁺ ions or Fe³⁺ and Bi⁵⁺ ions on these sites. Molar ratios are derived that agree with the observed distributions of electron density and give rise to perfectly stoichiometric systems, devoid of cationic or anionic vacancies. The compositions studied correspond to Bi³⁺₂₄Bi⁵⁺Fe³⁺O₄₀ and Bi³⁺₃₆Bi⁵⁺₂ZnO₆₀ and they are optical enantiomorphs.It is proposed that a reduction in the percentage composition of Bi₂O₅ leads to metastable phases, in which all atomic positions remain fully occupied but some tetrahedral sites contain Bi³⁺ ions. The end product of the series is γ-Bi₂O₃ in which 50% of these sites contain Bi³⁺ and the remainder Bi⁵⁺ ions. We believe that γ-Bi₂O₃ is Bi³⁺₂₅Bi⁵⁺O₄₀.     

Following the crystallisation of Bi2Mo2O9 catalyst by combined XRD/QuEXAFS

The formation ofβ-phase Bi₂Mo₂O₉ catalyst from a precursor precipitate has been studied using thein situ combined XRD/QuEXAFS technique and DSC during calcination. Accordingly the precursor was observed to undergo a number of changes in both the molybdenum (VI) coordination and long-range ordering during this heating. Initially the two other forms of bismuth molybdate (α-andγ-phases) were observed to form from the poorly crystalline precursor at about 230°C, however, theβ-phase eventually crystallised after prolonged heating at 560°C. Dedicated to Professor C N R Rao on his 70th birthday     

化合物Al2Mo3-xWxO12的合成与结构

合成并表征了一系列化合物Al     

Control-Growth and Photocatalytic Activities of Low-Dimensional Bi2Fe4O9 Crystals

Low-dimensional Bi₂Fe₄O₉ nanosheets and microrods have been selectively prepared by a solvothermal method, from which the growth of the Bi₂Fe₄O₉ crystals can be controlled by the variation of reaction conditions. Structure determination showed that the nanosheets are mainly exposed by {001} facets while the microrods are exposed by {110} facets. Ab- sorption spectra revealed that there are two bandgaps observed for both nanosheets (at 1.9 and 1.55 eV) and microrods (1.7 and 1.45 eV), and they both would be available for the sunlight photocatalysis e ciently due to the intensive absorption ability in a wide region. Photocatalytic investigation demonstrated that the overall photocatalytic performance of the microrods is prior to that of the nanosheets due to the variation of bandgaps and exposed facets. The present report provides a useful alternative strategy for the controlling growth of nanostructures and/or microcrystals besides the present demonstration of the Bi₂Fe₄O₉ crystals with diflerent bandgaps and facets that would be able to tune the corresponding photocatalytic ability selectively.     

The standard free energy of formation of YbFe2O4, Yb2Fe3O7, YbFeO3, and Yb3Fe5O12 at 1200°C

The standard free energy of formation of YbFe₂O₄, Yb₂Fe₃O₇, YbFeO₃, and Yb₃Fe₅O₁₂ from metallic iron, Yb₂O₃, and oxygen was determined to be ?100.38, ?158.38, ?58.17, and ?283.40 kcal/mole, respectively, at 1200°C on the basis of the phase equilibria in the FeFe₂O₃Yb₂O₃ system. The FeFe₂O₃-Lanthanoid sesquioxide systems were classified into four types with respect to the assemblage of the ternary compounds in stable existence at 1200°C, and the standard free energy of formation of YbFeO₃ was compared with those of the other lanthanoid-iron perovskites.     

PdCl2-CuCl2/Al2O3催化剂低温催化CO氧化的失活机理

采用X射线衍射、N₂吸-脱附、X射线光电子能谱分析、氢气-程序升温还原和原位红外漫反射等方法对新鲜和失活的PdCl₂-CuCl₂/Al₂O₃低温催化CO氧化催化剂进行表征,研究了高相对湿度(100%)下催化剂的失活机理.结果表明,催化剂表面沉积的水使得活性铜物种容易从催化剂表面向载体孔道内部迁移,由于Pd、Cu相互作用弱化从而减弱了Pd与Cu物种间的相互作用,使得催化剂的氧化还原性能受到影响,抑制了Pd⁰再氧化为Pd²⁺的过程,从而因CO氧化反应中催化剂氧化还原循环受阻而导致失活.     

Structure and properties of Y5Mo2O12 and Gd5Mo2O12: Mixed valence oxides with structurally equivalent molybdenum atoms

Crystals of Ln₅Mo₂O₁₂ (Ln = Y, Gd) were grown by electrochemical reduction of alkali-molybdate/rare-earth oxide melts at 1075–1100°C. A single crystal of Y₅Mo₂O₁₂, used for structure determination, was found to be monoclinic with a = 12.2376(7) Å, b = 5.7177(8) Å, c = 7.4835(5) Å, β = 108.034(5)°, and Z = 2. Although the structure was refined in space group C2/m, the true space group appears to be P2₁/m. In Y₅Mo₂O₁₂, rutile-like sheets of edge-shared MoO₆ chains linked by YO₆ octahedra are interconnected with YO₇ monocapped trigonal prisms. The Mo atoms in the chains have alternating distances of 2.496 and 3.221 Å and in that respect are similar to MoO₂. However, in contrast to metallic MoO₂ both the Y and Gd compounds are n-type semiconductors with room temperature resistivities of the order of 10³ ohm-cm. Magnetic susceptibility measurements confirm the presence of one unpaired electron per Mo₂ unit. The semiconducting behavior can be explained in terms of an unfavorable bridging oxygen coordination which prevents electron delocalization through metal-oxygen pi bonding as in MoO₂.     

On the deactivation of Fe2O3?MoO3/SiO2 catalysts in the oxidation of methanol to formaldehyde

The deactivation behavior of Fe₂O₃–MoO₃/SiO₂ catalysts with different Fe₂O₃+MoO₃ content in the oxidation of methanol to formaldehyde is investigated. A simplified reaction-deactivation kinetic model is presented and used to compare and discuss the different behaviors.

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Fe₂O₃–MoO₃/SiO₂ Fe₂O₃+MoO₃ . : -.

     

浸渍-分解法制备高可见光催化活性的Bi2O3/TiO2纳米管阵列

用浸渍-分解法将Bi₂O₃纳米颗粒沉积在TiO₂纳米管壁上, 制备了Bi₂O₃/TiO₂纳米管阵列. 用电感耦合等离子体发射光谱(ICP-AES)测定了Bi₂O₃/TiO₂ 纳米管阵列的化学组分, 利用X 射线衍射(XRD)、扫描电镜(SEM)、透射电镜(TEM)和紫外-可见(UV-Vis)吸收光谱表征了所制备的样品. 通过在可见光下(λ>400 nm)降解甲基橙(MO)水溶液来评价样品的光催化活性. 结果表明, Bi₂O₃纳米颗粒均匀地沉积在TiO₂纳米管中. Bi₂O₃/TiO₂纳米管阵列具有比纯Bi₂O₃膜和N-TiO₂纳米管阵列高得多的可见光催化活性. Bi₂O₃/TiO₂纳米管阵列活性的增强是其强可见光吸收和Bi₂O₃与TiO₂之间形成的异质结的协同作用的结果.     

The study of Bi3B5O12: synthesis, crystal structure and thermal expansion of oxoborate Bi3B5O12

The paper presents a new data on the crystal structure, thermal expansion and IR spectra of Bi₃B₅O₁₂. The Bi₃B₅O₁₂ single crystals were grown from the melt of the same stoichiometry by Czochralski technique. The crystal structure of Bi₃B₅O₁₂ was refined in anisotropic approximation using single-crystal X-ray diffraction data. It is orthorhombic, Pnma, a=6.530(4), b=7.726(5), c=18.578(5) Å, V=937.2(5) Å³, Z=4, R=3.45%. Bi³⁺ atoms have irregular coordination polyhedra, Bi(1)O₆ (d(B-O)=2.09-2.75 Å) and Bi(2)O₇ (d(B-O)=2.108-2.804 Å). Taking into account the shortest bonds only, these polyhedra are considered here as trigonal Bi(1)O₃ (2.09-2.20 Å) and tetragonal Bi(2)O₄ (2.108-2.331 Å) irregular pyramids with Bi atoms in the tops of both pyramids. The BiO₄ polyhedra form zigzag chains along b-axis. These chains alternate with isolated anions [B₂^IVB₃^IIIO₁₁]⁷⁻ through the common oxygen atoms to form thick layers extended in ab plane. A perfect cleavage of the compound corresponds to these layers and an imperfect one is parallel to the Bi-O chains. The Bi₃B₅O₁₂ thermal expansion is sharply anisotropic (α₁₁≈α₂₂=12, α₃₃=3×10⁻⁶ °C⁻¹) likely due to a straightening of the flexible zigzag chains along b-axis and decreasing of their zigzag along c-axis. Thus the properties like cleavage and thermal expansion correlate to these chains.