V2O5负载量对V2O5/Al2O3氧化活化正庚烷催化裂解反应的影响

采用X射线衍射, 低温氮气吸脱附, 氨气程序升温脱附和吡啶吸附红外光谱分析方法对不同活性组分负载量V2O5/Al2O3的性质进行了表征. 根据表面VOx单元密度, 推测V2O5负载量为20%-25% (w)对应着V2O5/Al2O3表面达单层覆盖状态; V2O5的负载使Al2O3表面Lewis酸量减少, 并出现Br?nsted酸, 对应着氧化态VOx单元中的V―OH; 随着负载量的增加, Brφnsted酸量增加至负载量为20%时达到最大值. 对V2O5/Al2O3中活性组分负载量对其氧化活化正庚烷催化裂解反应的影响进行了考察. 结果表明, 在V2O5负载量为20%-25%时,V2O5/Al2O3的引入对正庚烷在HZSM-5平衡剂上催化裂解反应的促进作用最明显, 此时VOx单元在V2O5/Al2O3表面形成单层覆盖状态, 可提供最大量的表面晶格氧, 因而对正庚烷具有最强的氧化活化作用; V2O5负载量继续增加形成体相的V2O5和AlVO4, 不利于晶格氧参与正庚烷的转化, 因而反应性能有所下降.     

Epitaxial growth of single-crystalline Al(2)O(3) films on Cr(2)O(3)(0001)

Thin, crystallographically oriented single-crystalline Al2O3 films can be grown epitaxially on Cr2O3(0001) by codeposition of Al vapor and O2 at a substrate temperature of 825 K. The properties and growth of these films were monitored by Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), low-energy ion scattering (LEIS), and X-ray photoelectron spectroscopy (XPS). Two routes of preparation were investigated: (i) stepwise growth by alternating deposition of Al at room temperature and subsequent exposure to O2 at elevated temperatures; (ii) codeposition of Al and O2 at T > 800 K. The first route was consistently found to result in the growth of a complex interfacial oxide followed by the growth of polycrystalline Al2O3. The second mode of preparation provided homogeneous and ordered, probably (0001)-oriented, films of Al2O3 that maintained a LEED pattern up to a thickness around 10 A. The surface sensitive Cr MVV Auger transition at 34 eV was completely attenuated once the Al2O3 layer had reached a thickness of 6 A, pointing to film homogeneity at an early stage. This was confirmed by the absence of a significant Cr signal in LEIS spectra.     

Synthesis and characterization of new Mg(2)Al-paratungstate layered double hydroxides

Layered double hydroxides (LDHs, or hydrotalcites) with Mg(2+) and Al(3+) cations in the mixed metal hydroxide layer and paratungstate anions in the interlayer have been prepared. Different methods have been followed: anion exchange with Mg,Al LDHs originally containing nitrate or adipate, reconstruction of the LDH structure from a mildly calcined Mg(2)Al-CO(3) LDH, and coprecipitation. In all cases, the tungsten precursor salt was (NH(4))(10)H(2)W(12)O(42). The prepared solids have been characterized by elemental chemical analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy, thermogravimetric (TG) and differential thermal (DTA) analyses, scanning electron microscopy (SEM) with EDX (energy-dispersive X-ray analysis), and nitrogen adsorption at -196 degrees C for surface area and surface texture. Most of the synthesis methods used, especially anion exchange starting from a Mg(2)Al-NO(3) precursor at low temperature and short reaction times, lead to formation of a hydrotalcite with a gallery height of 9.8 A; increasing the reaction temperature to 70-100 degrees C and maintaining short contact times leads to a solid with a gallery height of 7.8 A. Both phases have been identified as a result of the intercalation of W(7)O(24)(6)(-) species in different orientations in the interlayer space. If the time of synthesis or the temperature is increased, a more stable phase, with a gallery height of 5.2 A corresponding to a solid with intercalated W(7)O(24)(6)(-), is formed, probably with grafting of the interlayer anion on the brucite-like layers. All systems are microporous. Calcination at 300 degrees C leads to amorphous species, and crystallized MgWO(4) is observed at 700 degrees C.     

Nature, density, and catalytic role of exposed species on dispersed VOx/CrOx/Al2O3 catalysts

The structure and surface composition of binary oxides consisting of CrO(x) and VO(x) dispersed on alumina and their effects on the rate and selectivity of oxidative dehydrogenation (ODH) of propane were examined and compared with those for CrO(x) and VO(x) dispersed on alumina. VO(x) deposition on an equivalent CrO(x) monolayer on alumina and deposition of CrO(x) on an equivalent monolayer of VO(x) deposited on alumina led to CrVO(4) species during thermal treatment with concomitant reduction of Cr(6+) to Cr(3+). Autoreduction of Cr(6+) to Cr(3+) is also detected for CrO(x), even without the presence of VO(x). Infrared spectroscopy of NO adsorbed at 153 K probes the relative abundance of alumina and of V(5+), Cr(3+), and Cr(6+) at surfaces. This technique detects differences in the surface composition of VO(x)/CrO(x)()/Al(2)O(3) and CrO(x)/VO(x)/Al(2)O(3). The first of these samples is enriched in VO(x) relative to CrO(x) compared with the second sample. Consistent with this finding, VO(x)/CrO(x)/Al(2)O(3) and CrO(x)/VO(x)/Al(2)O(3) are distinguishable in their ODH activities and propene selectivities. The highest ODH activity and propene selectivity is observed for VO(x)/CrO(x)/Al(2)O(3), which exhibits a surface enriched in VO(x) and having a low surface concentration of Cr(6+).     

纳米V2O5／ZnO光催化剂对壬基酚聚氧乙烯醚降解的催化活性

采用氨浸法制备了不同V2O5含量的纳米V2O5/ZnO光催化剂,并用X射线衍射、比表面积测定、透射电镜、X射线光电子能谱和漫反射紫外-可见光谱测定了催化剂的晶型、比表面积、形貌尺寸、表面组成和光谱特征.以壬基酚聚氧乙烯醚(NPE-10)为模型污染物,分别在紫外光和可见光照射下考察了光催化剂的催化活性.结果表明,随着V2O5含量的增加,V2O5/ZnO的粒径逐渐减小,比表面积逐渐增大.与纳米ZnO样品相比,V2O5/ZnO中V2p的结合能减小,而Zn2p和O1s的结合能增大,V2O5/ZnO表面的羟基氧和吸附氧含量增加.n(V)/n(Zn)=2·5%的V2O5/ZnO光催化剂样品的催化活性最高(在紫外光和可见光照射3h后,NPE-10降解率分别约为79%和62%).     

Synthesis, structure, and catalytic reactivity of isolated V5+-Oxo species prepared by sublimation of VOCl3 onto H-ZSM5

Isolated and uniform V(5+)-oxo species were grafted onto H-ZSM5 at V/Al(f) ratios of 0.2-1 via sublimation of VOCl(3) precursors. These methods avoid the restricted diffusion of solvated oligomers in aqueous exchange, which leads to poorly dispersed V(2)O(5) at external zeolite surfaces. Sublimation methods led to stable and active V-ZSM5 catalysts for oxidative dehydrogenation (ODH) reactions; they led to an order of magnitude increase in primary C(2)H(6) ODH rates compared with impregnated ZSM5 catalysts at similar V/Al(f) ratios and showed similar activity to impregnated VO(x)/Al(2)O(3). The structure of grafted V(5+)-oxo species was probed using spectroscopic and titration methods. Infrared spectra in the OH region and isotopic exchange of D(2) with residual OH groups showed that exposure to VOCl(3(g)) at 473 K led to stoichiometric replacement of H(+) by each (VOCl(2))(+) species. Raman spectra supported by Density Functional Theory electronic structure and frequency calculations showed that, at V/Al(f) < 0.5, hydrolysis and subsequent dehydration led to the predominant formation of (VO(2))(+) species coordinated to one Al site with single-site catalytic behavior (0.7-0.9 x 10(-3) mol C(2)H(4) V(-1) s(-1), 673 K). At higher V/Al(f) ratios, simulation of extended X-ray absorption fine structure spectra indicated that V(2)O(4)(2+) dimers coexisted with VO(2)(+) monomers and led to an enhancement in ODH rates as a result of bridging V-O-V (1.3 x 10(-3) mol C(2)H(4) V(-1) s(-1)). These V(5+)-oxo species form via initial reactions between VOCl(3(g)) and OH groups to form HCl((g)), hydrolysis of grafted (VOCl(2))(+) to form HCl((g)) and (VO(OH)(2))(+), and intramolecular and intermolecular condensation to form monomers and dimers, respective with the concurrent evolution of H(2)O. Raman and X-ray spectroscopies did not detect crystalline V(2)O(5) at V/Al(f) ratios of 0.2-1, but V(2)O(5) crystals were apparent in samples prepared by impregnation or physical mixtures of V(2)O(5)/H-ZSM5. Framework Al atoms and zeolite crystal structures are maintained during VOCl(3) treatment and subsequent hydrolysis; (27)Al and (29)Si MAS NMR showed that these synthetic protocols removes <10% of the framework Al atoms (Al(f)).     

Interaction of water with ordered theta-Al(2)O(3) ultrathin films grown on NiAl(100)

The structure of an ordered, ultrathin theta-Al(2)O(3) film grown on a NiAl(100) single-crystal surface was studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED), and its interaction with water was investigated with temperature programmed desorption (TPD) and XPS. Our results indicate that H(2)O adsorption on the theta-Al(2)O(3)/NiAl(100) surface is predominantly molecular rather than dissociative. For theta(H)()2(O) < 1 ML (ML = monolayer), H(2)O molecules were found to populate Al(3+) cation sites to form isolated H(2)O species aligned in a row along the cation sites on the oxide surface with a repulsive interaction between them. For theta(H)()2(O) > 1 ML, three-dimensional ice multilayers were observed to form, which then desorb during TPD with approximate zero-order kinetics as expected. A small extent of H(2)O dissociation was observed to occur on the theta-Al(2)O(3)/NiAl(100) surface, which was attributed to the presence of a low concentration of oxygen atom vacancies. Titration of these defect sites with adsorbed H(2)O molecules revealed an estimated defect density of 0.05 ML for the theta-Al(2)O(3)/NiAl(100) system consistent with the ordered nature of the synthesized oxide film.     

Chemistry of borate in salt lake brine (XXXIV) ——Phase diagram of thermodynamic nonequilibrium state of MgO-B_2O_3-18%MgCl_2-H_2O system at 20℃

The crystallization processes of hydrated Mg-borates, boric, magnesium hydroxide and Mg-oxychloride from MgO-B2O3-18%MgCl2-H2O supersaturated solution at 20℃ have been studied by kinetic method. The crystallization solid phases were characterized by X-ray powder diffraction, IR spectra, thermal analysis and chemical analysis. The liquid-solid phase diagram of ther-modynamic nonequilibrium state has been given. In this phase diagram, there exist eight crystallization fields, boric acid(H3BO3), trigomagneborite(MgO · 3B2O3 · 7.5H2O, MgO · 3B2O3 · 7H2O), hungchaoite(MgO ·2B2O3 ·9H2O), inderite(2MgO ·3B2O3 · 15H2O), chloropinnoite(2MgO ·2B2O3 · MgCl2 · 14H2O), magnesium hydroxide(Mg(OH)2) and magnesium oxychloride (5Mg(OH)2 · MgCl2·8H2O).     

制备纳米V2O5的新方法及其电化学性能的研究

目前对锂离子电池正极材料的研究主要为达到如下目标而进行:高的比容量和能量密度,良好的循环性和寿命,资源丰富、价格便宜以及环境友好。已进行了广泛研究的LiCoO_2、LiNiO_2、LiMn_2O_4等锂离子电池正极材料,虽然具有较高的电压平台、良好的稳定性和寿命,但其容量相对较低和价格较高限制了其进一步的应用和发展。     

Fe/Co alloys for the catalytic chemical vapor deposition synthesis of single- and double-walled carbon nanotubes (CNTs). 2. The CNT-Fe/Co-MgAl2O4 system

A detailed 57Fe M?ssbauer study of the Mg(0.8)Fe(0.2-y)Co(y)Al2O4 (y = 0, 0.05, 0.1, 0.15, 0.2) solid solutions and of the CNT-Fe/Co-MgAl2O4 nanocomposite powders prepared by reduction in H2-CH4 has allowed characterization of the different iron phases involved in the catalytic process of carbon nanotube (CNT) formation and to correlate these results with the carbon and CNT contents. The oxide precursors consist of defective spinels of general formulas (Mg(1-x-y)(2+)Fe(x-3alpha)(2+)Fe(2alpha)(3+)[symbol: see text](alpha)Co(y)(2+)Al2(3+))O4(2-) . The metallic phase in the CNT-Fe/Co-MgAl2O4 nanocomposite powders is mostly in the form of the ferromagnetic alpha-Fe/Co alloy with the desired composition. For high iron initial proportions, the additional formation of Fe3C and gamma-Fe-C is observed while for high cobalt initial proportions, the additional formation of a gamma-Fe/Co-C phase is favored. The higher yield of CNTs is observed for postreaction alpha-Fe(0.50)Co(0.50) catalytic particles, which form no carbide and have a narrow size distribution. Alloying is beneficial for this system with respect to the formation of CNTs.     

Synthesis, structure, negative thermal expansion, and photocatalytic property of Mo doped ZrV2O7

A new series of compounds identified in the phase diagram of ZrO(2)-V(2)O(5)-MoO(3) have been synthesized via the solution combustion method. Single crystals of one of the compounds in the series, ZrV(1.50)Mo(0.50)O(7.25), were grown by the melt-cool technique from the starting materials with double the MoO(3) quantity. The room temperature average crystal structure of the grown crystals was solved using the single crystal X-ray diffraction technique. The crystals belong to the cubic crystal system, space group Pa3 (No. 205) with a = 8.8969 (4) ?, V = 704.24 (6) ?(3), and Z = 4. The final R(1) value of 0.0213 was achieved for 288 independent reflections during the structure refinement. The Zr(4+) occupies the special position (4a) whereas V(5+) and Mo(6+) occupy two unique (8c) Wyckoff positions. Two fully occupied O atoms, (24d) and (4b), one partially occupied O atom (8c) have been identified for this molybdovanadate, which is a unique feature for these crystals. The structure is related to both ZrV(2)O(7) and cubic ZrMo(2)O(8). The temperature dependent single crystal studies show negative thermal expansion above 370 K. The compounds have been characterized by powder X-ray diffraction, solid-state UV-vis diffuse reflectance spectra, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The photocatalytic activity of these compounds has been investigated for the degradation of various dyes, and these compounds show specificity toward the degradation of non-azoic dyes.     

New SrAl2B2O7:Eu2+, Mn2+ phosphors for white light-emitting diodes

A series of Eu(2+) and Mn(2+) co-doped SrAl(2)B(2)O(7) phosphors were prepared by solid-state reaction method. X-ray powder diffraction (XRD) and photoluminescence excitation and emission were employed to characterize the phosphors. The results show that energy transfers between Eu(2+)and Mn(2+) ions. As the content of Ca(2+) ions in Ca(x)Sr(0.92-x)Al(2)B(2)O(7):Eu(2+)(0.06), Mn(2+)(0.02) phosphors increased, the CIE coordinates decreased and close to the white color standard mandated by the National Television Standard Committee (NTSC). Meanwhile, a white LED (light-emitting diode) was fabricated by combining the Ca(0.5)Sr(0.42)Al(2)B(2)O(7):Eu(2+)(0.06), Mn(2+)(0.02) phosphors with a 370 nm InGaN chip. The color coordinate of the fabricated white LED was also close to the white color standard, indicating that the Ca(0.5)Sr(0.42)Al(2)B(2)O(7):Eu(2+)(0.06), Mn(2+)(0.02) phosphor is a promising single-host phosphor that can be used in white LEDs.     

Hydrothermal Syntheses and Structural Characterization of Layered Vanadium Oxides Incorporating Organic Cations: alpha-, beta-(H(3)N(CH(2))(2)NH(3))[V(4)O(10)] and alpha-, beta-(H(2)N(C(2)H(4))(2)NH(2))[V(4)O(10)

Four new layered mixed-valence vanadium oxides, which contain interlamellar organic cations, alpha-(H(3)N(CH(2))(2)NH(3))[V(4)O(10)] (1a), beta-(H(3)N(CH(2))(2)NH(3))[V(4)O(10)] (1b), alpha-(H(2)N(C(2)H(4))(2)NH(2))[V(4)O(10)] (2a), and beta-(H(2)N(C(2)H(4))(2)NH(2))[V(4)O(10)] (2b), have been prepared under hydrothermal conditions and their single-crystal structures determined: 1a, triclinic, space group P&onemacr;, a = 6.602(2) ?, b = 7.638(2) ?, c = 5.984(2) ?, alpha = 109.55(3) degrees, beta = 104.749(2) degrees, gamma = 82.31(3) degrees, Z = 1; 1b, triclinic, P&onemacr;, a = 6.387(1) ?, b = 7.456(2) ?, c = 6.244(2) ?, alpha = 99.89(2) degrees, beta = 102.91(2) degrees, gamma = 78.74(2) degrees, Z = 1; 2a, triclinic, P&onemacr;, a = 6.3958(5) ?, b = 8.182(1) ?, c = 6.3715(7) ?, alpha = 105.913(9) degrees, beta = 104.030(8) degrees, gamma = 94.495(8) degrees, Z = 1; 2b, monoclinic, space group P2(1)/n, a = 9.360(2) ?, b = 6.425(3) ?, c = 10.391(2) ?, beta = 105.83(1) degrees, Z = 2. All four of the compounds contain mixed-valence V(5+)/V(4+) vanadium oxide layers constructed from V(5+)O(4) tetrahedra and pairs of edge-sharing V(4+)O(5) square pyramids with protonated organic amines occupying the interlayer space.     

Intercalation of [Cr(C2O4)3]3 - complex in mg,al layered double hydroxides

A Mg,Al layered double hydroxide (LDH) with [Cr(C(2)O(4))(3)](3)(-) anions in the interlayer has been synthesized following two different routes: reconstruction from a mildly calcined Mg,Al-carbonate LDH, and anion exchange from a Mg,Al-nitrate LDH. The solids prepared have been characterized by elemental chemical analysis, powder X-ray diffraction, FT-IR and UV-vis/DR (diffuse reflectance) spectroscopies, thermal methods, nitrogen adsorption at -196 degrees C, and FT-IR monitoring of pyridine adsorption. The results obtained indicate that the most appropriate method is anion exchange, leading to a well crystallized LDH with an interlayer spacing of 10 A. Due to the high pH value (>8) of the solution in the reconstruction method, however, a polyphasic system is obtained, where, in addition to a phase with the LDH structure, amorphous magnesium oxalate and chromium oxohydroxides are also formed due to hydrolysis of the complex. The interlayer complex is stable up to 200 degrees C, but the layered structure is stable up to 330 degrees C, probably because of the presence of interlayer oxalate anions formed during decomposition of the complex. Calcination leads to oxidation of Cr(3+) ions to the six-valent state, which reverts to Cr(3+) when the calcination temperature is further increased.     

氟插层水滑石催化烯烃环氧化反应

制备了一系列不同Mg/Al比氟插层的水滑石(LDH-F),并用X射线衍射、傅里叶变换红外光谱、扫描电镜和BET比表面积检测法对催化剂进行了表征,考察了不同Mg/Al比的LDH-F在苯乙烯环氧化反应中的催化活性.结果表明,Mg/Al=5的LDH-F具有最佳催化活性.通过以乙腈和苯甲腈分别作为反应用腈的对比实验发现,苯甲腈的效果优于乙腈.以Mg/Al=5的LDH-F为催化剂,H2O2和苯甲腈为氧化剂对不同官能团化烯烃进行环氧化反应,结果表明,此催化体系具有良好的催化性能,在苯乙烯、茚、丙烯醇和烯丙基氯环氧化反应中转化率和选择性均在95%以上。     

Investigations of V(2)O(5)-based LPG sensors

Sensor characteristics of V(2)O(5) dispersed on oxide supports such as Al(2)O(3), TiO(2) and ZrO(2) with respect to various gases and vapours including liquefied petroleum gas (LPG) have been investigated. Of all the systems studied, 20 mol% V(2)O(5) dispersed on ZrO(2) shows the highest sensitivity for LPG, the log sensitivity-log concentration (in ppm) plots being linear up to 1000 ppm or more. The sensitivity is not affected by humidity or recycling. Addition of P(2)O(5) to V(2)O(5) however destroys the sensitivity. Considering all aspects, 20 mol% V(2)O(5)/ZrO(2) is suggested for use as a practical LPG sensor. ESR spectroscopy indicates the formation of V(4+) species on exposure of V(2)O(5)/ZrO(2) or TiO(2) to LPG. In-situ high-temperature x-ray diffraction measurements show the formation of an unusual monoclinic form of VO(2) on exposure to LPG at 625 K which gets oxidized back to V(2)O(5) on exposure to air.     

Investigation of Mechanisms About Effects of Promoters P_2O_5,K_2O and A1_2O_3 on Oxidation of o-Xylene to Phthalic Anhydride

We succeeded in designing an effective catalyst, V2O5-P2O5-K2O/Al2O3. SiO2, by which a high yield of PA,105wt% can be gained in middle-sized industrial fluidized bed apparatus without addition of any promoting gas.The mechanisms of effects of P2O5, K2O and Al2O3 on the surface properties of V2O5 were investigated by means of TPD and XRD. And the selectivity of oxidation are explained.Addition of a great deal of P2O5 restrains the activity of donating surface oxygen from V2O5, but increases the number of sites which donate surface oxygen. Addition of K2O promotes donation of surface oxygen from V2O6, and decreases the number of sites of donating oxygen, on the other hand, addition of K|O makes the surface structure of V2O5 catalysts more stable. Coating a small amount of Al2O2 onto support, SiO2, restrains the activity of donating oxygen and increases the number of sites of donating surface oxygen from V2O5.     

Polyoxometalate-supported transition metal complexes and their charge complementarity: synthesis and characterization of [M(OH)6Mo6O18[Cu(Phen)(H2O)2]2][M(OH)6Mo6O18[Cu(Phen)(H2O)Cl]2].5H2O (M = Al(+, Cr3+)

Two Anderson-type heteropolyanion-supported copper phenanthroline complexes, [Al(OH)6Mo6O18[Cu(phen)(H2O)2]2]1+ (1c) and [Al(OH)6Mo6O18[Cu(phen)(H2O)Cl]2]1- (1a) complement their charges in one of the title compounds [Al(OH)6Mo6O18[Cu(phen)(H2O)2]2][Al(OH)6Mo6O18[Cu(phen)(H2O)Cl]2].5H2O [1c][1a].5 H2O 1. Similar charge complementarity exists in the chromium analogue, [Cr(OH)6Mo6O18[Cu(phen)(H2O)2]2][Cr(OH)6Mo6O18[Cu(phen)(H2O)Cl]2].5 H2O [2c][2a].5 H2O 2. The chloride coordination to copper centers of 1a and 2a makes the charge difference. In both compounds, the geometries around copper centers are distorted square pyramidal and those around aluminum/chromium centers are distorted octahedral. Three lattice waters, from the formation of intermolecular O-H.....O hydrogen bonds, have been shown to self-assemble into an "acyclic water trimer" in the crystals of both 1 and 2. The title compounds have been synthesized in a simple one pot aqueous wet-synthesis consisting of aluminum/chromium chloride, sodium molybdate, copper nitrate, phenanthroline, and hydrochloric acid, and characterized by elemental analyses, EDAX, IR, diffuse reflectance, EPR, TGA, and single-crystal X-ray diffraction. Both compounds crystallize in the triclinic space group P. Crystal data for 1: a = 10.7618(6), b = 15.0238(8), c = 15.6648(8) angstroms, alpha = 65.4570(10), beta = 83.4420(10), gamma = 71.3230(10), V = 2182.1(2) angstroms3. Crystal data for 2: a = 10.8867(5), b = 15.2504(7), c = 15.7022(7) angstroms, alpha = 64.9850(10), beta = 83.0430(10), gamma = 71.1570(10), V = 2235.47(18) angstroms3. In the electronic reflectance spectra, compounds 1 and 2 exhibit a broad d-d band at approximately 700 nm, which is a considerable shift with respect to the value of 650-660 nm for a square-pyramidal [Cu(phen)2L] complex, indicating the coordination of [M(OH)6Mo6O18]3- POM anions (as a ligand) to the monophenanthroline copper complexes to form POM-supported copper complexes 1c, 1a, 2c, and 2a. The ESR spectrum of compound 1 shows a typical axial signal for a Cu2+ (d9) system, and that of compound 2, containing both chromium(III) and copper(II) ions, may reveal a zero-field-splitting of the central Cr3+ ion of the Anderson anion, [Cr(OH)6Mo6O18]3-, with an intense peak for the Cu2+ ion.     

Two new noncentrosymmetric (NCS) polar oxides: syntheses, characterization, and structure-property relationships in BaMTe2O7 (M = Mg2+ or Zn2+)

Two new noncentrosymmetric (NCS) polar oxides, BaMgTe(2)O(7) and BaZnTe(2)O(7), have been synthesized and characterized, with their crystal structures determined by single crystal X-ray diffraction. The iso-structural materials exhibit structures consisting of layers of corner-shared MgO(5) or ZnO(5), Te(6+)O(6), and Te(4+)O(4) polyhedra that are separated by Ba(2+) cations. The Te(4+) cation is found in a highly asymmetric and polar coordination environment attributable to its stereoactive lone-pair. The alignment of the individual TeO(4) polar polyhedra results in macroscopic polarity for BaMgTe(2)O(7) and BaZnTe(2)O(7). Powder second-harmonic generation (SHG) measurements revealed a moderate SHG efficiency of approximately 5 × KDP (or 200 × α-SiO(2)) for both materials. Piezoelectric charge constants of 70 and 57 pm/V, and pyroelectric coefficients of -18 and -10 μC·m(-2)·K(-1) were obtained for BaMgTe(2)O(7) and BaZnTe(2)O(7), respectively. Although the materials are polar, frequency dependent polarization measurements indicated that the materials are not ferroelectric, that is, the observed macroscopic polarization cannot be reversed. Infrared, UV-vis diffuse spectroscopy, and thermal properties were also measured. Crystal data: BaMgTe(2)O(7), orthorhombic, space group Ama2 (No. 40), a = 5.558(2) ?, b = 15.215(6) ?, c = 7.307(3) ?, V = 617.9(4) ?(3), and Z = 4; BaZnTe(2)O(7), orthorhombic, space group Ama2 (No. 40), a = 5.5498(4) ?, b = 15.3161(11) ?, c = 7.3098(5) ?, V = 621.34(8) ?(3), and Z = 4.     

Unimolecular reactions of dihydrated alkaline earth metal dications M2+(H2O)2, M = Be, Mg, Ca, Sr, and Ba: salt-bridge mechanism in the proton-transfer reaction M2+(H2O)2 --> MOH+ + H3O

The unimolecular reactivity of M(2+)(H(2)O)(2), M = Be, Mg, Ca, Sr, and Ba, is investigated by density functional theory. Dissociation of the complex occurs either by proton transfer to form singly charged metal hydroxide, MOH(+), and protonated water, H(3)O(+), or by loss of water to form M(2+)(H(2)O) and H(2)O. Charge transfer from water to the metal forming H(2)O(+) and M(+)(H(2)O) is not favorable for any of the metal complexes. The relative energetics of these processes are dominated by the metal dication size. Formation of MOH(+) proceeds first by one water ligand moving to the second solvation shell followed by proton transfer to this second-shell water molecule and subsequent Coulomb explosion. These hydroxide formation reactions are exothermic with activation energies that are comparable to the water binding energy for the larger metals. This results in a competition between proton transfer and loss of a water molecule. The arrangement with one water ligand in the second solvation shell is a local minimum on the potential energy surface for all metals except Be. The two transition states separating this intermediate from the reactant and the products are identified. The second transition state determines the height of the activation barrier and corresponds to a M(2+)-OH(-)-H(3)O(+) "salt-bridge" structure. The computed B3LYP energy of this structure can be quantitatively reproduced by a simple ionic model in which Lewis charges are localized on individual atoms. This salt-bridge arrangement lowers the activation energy of the proton-transfer reaction by providing a loophole on the potential energy surface for the escape of H(3)O(+). Similar salt-bridge mechanisms may be involved in a number of proton-transfer reactions in small solvated metal ion complexes, as well as in other ionic reactions.