Electrocatalytic activity of transition metal substituted heteropolytungstates

The electrochemical and electrocatalytic behavior of a series of heteropolytungstate anions in which a tungsten atom in the well known Keggin structure has been replaced by an iron atom is described. All of the iron substituted ions exhibit a one electron reversible couple associated with the Fe³⁺ center and a pair of two electron waves attributed to electron addition and removal from the tungsten oxo framework. The pH and ionic strength effects upon the various electrochemical processes are discussed and interpreted in terms of a competition between protonation and ion pairing of the highly negatively charged ions.

The anions are efficient catalysts for the electroreduction of hydrogen peroxide. A catalytic mechanism involving a formally Fe(IV) intermediate is proposed. Pulse radiolysis experiments were employed to detect the intermediate and evaluate the rate constants for the reactions in which it is formed and decomposed. A chain mechanism for the catalytic decomposition of hydrogen peroxide in which the Fe center shuttles between the +2, +3, and +4 oxidation states is proposed to explain the non-integral stoichiometry observed for the iron substituted polytungstate catalyzed electroreduction of hydrogen peroxide.

The anions are also efficient electrocatalyst for the electrochemical conversion of nitric oxide to ammonia. The catalyzed reduction does not produce hydroxylamine as an intermediate and appears to depend upon the ability of the multiply reduced heteropolytungstates to deliver several electrons to the bound NO group in a concerted step. A valuable feature of the heteropolytungstates is the ease at which the formal potentials of the several redox couples they exhibit may be shifted by changing the identity of the central heteroatom. Exploitation of this feature provided diagnostic information that was decisive in establishing the mechanism of electrocatalytic reduction.

The iron substituted heteropolytungstates are not degraded by repeated cycling between their oxidized and reduced states. They also show superior activity compared to their unsubstituted analogues, indicating that the Fe center acts as a binding site that facilitates inner-sphere electron transfer processes. The basic electrochemistry of several other transition metal substituted Keggin ions is also described.

相转移条件下磷钨杂多化合物在苯乙烯氧化反应中的催化作用

制备了具有keggin结构及其过渡金属离子取代的杂多化合物PW_11, PW_(11)Mn, PW_(11)Co和具有Dawson结构及其过渡金属取代的杂多化合物P_2W_(18), P_2W_(17), P_2W_(17)M_n, P_2W_(17)Co，并表征了它们的热差热重、红外、X射线衍射等性质。热分析结果表明，杂多酸和杂多酸盐的脱水行为不同，杂多酸的脱水分三个阶段，依次脱出结晶水，结合水和结构水。杂多酸盐保有结晶水和少量的结构水，结构水的脱除在热差曲线上产生一个放热峰。这个峰也是热差曲线上的第一个放热峰。它可作为杂多化合物热稳定性的一个判据。据此，Dawson结构的杂多化合物的热稳定性比kggin结构的好。P_2W_(18)、PW_(12)比相应的缺位的P_2W_(17), PW_(11)的热稳定性好。过渡金属取代不能改善热稳定性。杂多化合物的红外光谱表明，keggin结构区别于Dawson结构的是Dawson结构的杂多化合物要多一个P-P振动吸收峰（约1010cm~(-1)）缺位杂多化合物的P=0键会产生劈裂，这是它区别于非缺位杂多化合的一个明显特征。根据这些特征，可以初步鉴定杂多化合物对以上杂多化合物及PW_(12)共八种催化剂催化的H_2O_2分解行为进行了考察，keggin结构杂多化合物的催化活性比Dawson结构的好，缺位杂化合物比不缺位杂多化合物的催化活性要好得多，催化活性是由整个杂多阴离子决定的。以上八种杂多化合物催化的苯乙烯H_2O_2氧化反应实验结果表明，其催化活性基本上与H_2O_2分解反应中表现出的催化活性一致，易氧化的Mn~(2+)取代的杂多化合物PW_(11)Mn，P_2W_(17)Mn的催化活性很差。Na_2HPW_(12)O_(40)催化的H_2O_2分解反应动力学结果表明，反应开始有一个反期、催化剂、H_2O_2反应级数均为零。之后，反应线性加速，反应速度与催化剂浓度成线性关系，其表现活化能求得的为E_=161KJ/mol。Na_2HPW_(12)O_(40)催化的苯乙烯氧化反应动力学结果表明，反应明显地分为两个阶段。前阶段为诱导期，反应速度较慢，苯乙烯转化率不高。反阶段为反应加速，催化剂的反应级数是1，与苯乙烯的浓度无关。E_=15.9KJ/mol。合成并分离得到了PW_(12), H_2O和相转移剂作用生成的过氧化物，碘量法滴定结果知每分子过氧化合物含三个活性氧。在红外区域的951cm~(-1)处有一特征振动吸收峰，在紫外范围350mm处有一特征电子吸收峰。这种过氧化合物中的活性氧具有很高的活性，常温下就能与苯乙烯迅速反应。根据两者反应的特点，设计了反应过程，结合实际现象和结果提出了反应机理，反应前期的速度控制步骤反应在水相中进行。反阶段的速度控制步骤反应在有机相中进行。推导了反应动力学方程。验证了动力学推导结果与实验结果的一致性。得到了令人满意的结果。

杂多酸的固载化研究

杂多酸（HPA）是一种新型多功能催化材料，但只有将杂多酸稳定地固载在载体表面上才能有效地防止HPA在气固相反应中的溶脱流失，实现连续生产，以及在液固相条件下HPA与产物的简易分离，才能实现高价格HPA的多次重复使用。本工作在详细总结了文献中有关HPA的组成、性质、催化应用和固载研究结果的基础上，研究和探讨了具有Keggin结构的HPA在不同结构载体上的吸附规律，作用机制，和固载后应用的可能性。

Hydrothermal synthesis, crystal structure and electrochemical properties of a novel 3D Dawson-type polyoxometalate supramolecular network

A novel Dawson-type polyoxometalate supramolecular architecture of the formula [4,4'-H(2)bipy](2.5)center dot[4,4'-Hbipy]center dot[P2W18O62]center dot 6.25H(2)O (4,4'-bipy = 4,4'-bipyridine) has been hydrothermally synthesised and characterised by means of elemental analysis, IR, CV and X-ray single-crystal diffraction. X-ray crystallography indicates that the title compound consists of Dawson-type polyoxoaions [P2W18O62](6-), water molecules and 4,4'-bipy units. The polyoxoanion clusters together with 4,4'-bipy units and water molecules to construct the three-dimensional supramolecular network through hydrogen bonds. The crystal structure analyses reveal that water molecules and 4,4'-bipy units play the important role on the packing arrangements of crystals. Cyclic voltammetry shows that the title compound exhibits three chemically reversible steps

Assembly of 12-tungstosilicic acid and 4-aminobenzo-15-crown-5 ether based on the electrostatic attraction through bridging of oxonium ions on different substrates

Based on the electrostatic attraction Keggin-type polyoxometalate H4SiW12O40 (SiW12) and small molecule 4-aminobenzo-15-crown-5 ether (4-AB15C5) were alternately deposited on poly (allylamine hydrochloride) (PAH)-derived indium tin oxide (ITO) substrate through a layer-by-layer (LBL) self-assembly, forming a supramolecular multilayer film (film-A). SiW12 was also deposited on a glassy carbon electrode (GCE) derived by 4-AB15C5 via covalent bonding in 0.1 M NaCl aqueous solution and formed a composite monolayer film (film-B). UV-vis absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy measurements demonstrated that the interactions between SiW12 and 4-AB15C5 in both two film electrodes were the same and caused by the bridging action of oxonium ions. But, the nanostructure in the two film electrodes was different. 4-AB15C5 in film-A was oriented horizontally to ITO substrate, however, that in film-B was oriented vertically to GCE. Namely film-A corresponded to a layer structure, and film-B corresponded to an intercalation structure.

Carbon deposition behavior over copper salts of molybdovanadophosphoric acid catalyst supported by magnesia

MgO supported copper salt of molybdovanadophosphoric acid H4PMo11VO40 catalysts were prepared in alcohol by impregnation and the carbon deposition over these catalysts during the n-hexanol oxidation reaction was studied. The coke predominantly deposited on the catalyst surface in the form of CH., and it was not found that it caused the deactivation of the catalyst. The XRD, IR, XPS characterizations reveal that the Keggin structure of the CPMV was unaffected by carbon deposition. Moreover, it was shown that the supported CPMVs over the MgO surface can be beneficial to eliminate the coke. The temperature programmed oxidation (TPO) study showed that coke was formed over the catalyst on two different sites: (1) deposited on the CPMVs which can be burn off at a low temperature; (2) deposited on the MgO which could only be removed at higher temperature. The coke content reached constant with the reaction time increasing.

金纳米粒子和杂多酸的多层组装及其对氧的催化还原

通过层层自组装方法,将荷正电的线性聚乙烯亚胺稳定的金溶胶纳米粒子和荷负电的Keggin结构的磷钨十二酸通过静电作用交替沉积在透明氧化锡(ITO)导电玻璃基底,石英玻璃基底和4 氨基苯甲酸(4 ABA)功能化的玻碳电极表面.用紫外 可见吸收光谱(UV/vis)、X 射线光电子能谱(XPS)以及循环伏安法(CV)监测了这种多层膜的生长过程,对其生长的均匀性、多层膜的组成和稳定性进行了表征,结果表明该多层膜不仅均匀稳定,而且对氧还原有较好的催化活性.

一种新型直接甲醇燃料电池阳极添加剂的电化学研究

将磷钼酸 (H4PMo12 O40 ·xH2 O ,PMo12 )作为一种添加剂 ,制备了直接甲醇燃料电池阳极Pt Ru/C PMo12 复合催化剂 ,并对甲醇在含有此复合催化剂的阳极上的氧化进行了电化学研究 .测试表明该添加剂降低了甲醇及其电氧化中间产物转化的活化能 ,改善了电极内部的质子传输状况 ,对甲醇的电化学氧化过程具有明显的促进作用 ,该复合催化剂与常规的Pt Ru/C催化剂相比 ,甲醇的阳极氧化电流提高了 46% .添加剂的这一效应可能与磷钼酸的Keggin结构有关

New proton exchange membranes based on poly (vinyl alcohol) for DMFCs

A series of new composite proton exchange membranes for direct methanol fuel cells (DMFCs) based on poly (vinyl alcohol) (PVA), phosphotungstic acid (PWA) and silica were prepared. The highest proton conductivity (a) of these membranes is 0.017 S/cm at ambient temperature. The methanol permeability (D) of these composite membranes ranges from 10(-7) to 10(-8) cm(2)/S. From the ratios of sigma/D, it was found that the optimal weight composition of the PVA/PWA/SiO2 membrane is PVA/PWA/SiO2=0.40:0.40:0.20 wt. Infrared (IR) spectrographic measurements indicate that the Keggin structure characteristics of the PW12O403- anion is present in the composite membranes. Cyclic voltammetry shows that the electrochemical stability window of the complex membrane is from -0.5 to 1.5 V vs. Ag/AgCl electrode. The results of differential scanning calorimetry (DSC) show that silica can improve the thermal stability of the complexes and the single Tg of the membrane indicates that the membrane is homogeneous. The complexes behave as X-ray amorphous.

Stable multilayer films based on photoinduced interaction between polyoxometalates and diazo resin

Ultrathin multilayers films consisting of Keggin anion [PMo12O40](3-) and diazo resin were first prepared by the electrostatic layer-by-layer self-assembly method. This film material could be stabilized by the photoinduced interaction between Keggin anion and diazo resin. IR spectra and X-ray photoelectron spectra revealed the occurrence of the partial transformation from ionic bond to covalent bond between layers of the film under irradiation by UV light. Such transformation increases the stability of the film, which was demonstrated by AFM images and the etching experiments with organic solvent.

Structure and catalytic properties of magnesia-supported copper salts of molybdovanadophosphoric acid

The structure and stability of magnesia-supported copper salts of molybdovanadophosphoric acid (Cu2PMo11VO40) were characterized by different techniques. The catalyst was prepared in ethanol by impregnation because this solvent does not hurt texture of the water-sensitive MgO and Cu2PMo11VO40. The Keggin-type structure compound may be degraded partially to form oligomerized polyoxometalate when supported on MgO. However, the oligomers can rebuild as the Keggin structure again after thermal treatment in air or during the reaction. Meanwhile, the V atoms migrate out of the Keggin structure to form a lacunary structure, as observed by Fourier transform IR spectroscopy. Moreover, the presence of Cu2+ as a countercation showed an affirmative influence on the migration of V atoms, and the active sites derived from the lacunary species generated after release of V from the Keggin anion. The electron paramagnetic resonance data imply that V5+ autoreduces to V4+ in the fresh catalyst, and during the catalytic reaction a large number of V4+ ions are produced, which enhance the formation of O2- vacancies around the metal atoms. These oxygen vacancies may also improve the reoxidation function of the catalyst. This behavior is correlated to higher catalytic properties of this catalyst. The oxidative dehydrogenation of hexanol to hexanal was studied over this catalyst.

New high-dimensional networks based on polyoxometalate and crown ether building blocks

Three novel supramolecular assemblies constructed from polyoxometalate and crown ether building blocks, [(DB18C6)Na(H2O)(1.5)](2)Mo6O19.CH3CN, 1, and [{Na(DB18C6)(H2O)(2)}(3)(H2O)(2)]XMo12O40.6DMF.CH3CN (X = P, 2, and As, 3; DB18C6 = dibenzo-18-crown-6; DMF = N,N-dimethylfomamide), have been synthesized and characterized by elemental analyses, IR, UV-vis, EPR, TG, and single crystal X-ray diffraction. Compound 1 crystallizes in the tetragonal space group P4/mbm with a = 16.9701(6) Angstrom, c = 14.2676(4) Angstrom, and Z = 2. Compound 2 crystallizes in the hexagonal space group P6(3)/m with a = 15,7435(17) Angstrom, c = 30.042(7) Angstrom, gamma = 120degrees, and Z = 2. Compound 3 crystallizes in the hexagonal space group P6(3)/m with a = 15.6882(5) Angstrom, c = 29.9778(18) Angstrom, gamma = 120degrees, and Z = 2. Compound 1 exhibits an unusual three-dimensional network with one-dimensional sandglasslike channels based on the extensive weak forces between the oxygen atoms on the [Mo6O19](2-) polyoxoanions and the CH2 groups of crown ether molecules, Compounds 2 and 3 are isostructural, and both contain a novel semiopen cagelike trimeric cation [{Na(DB18C6)(H2O)(2)}(3)(H2O)(2)](3+). In their packing arrangement, an interesting 2-D "honeycomblike" "host" network is formed, in which the [XMo12O40](3-) (X = As and P) polyoxoanion "guests" resided.

Synthesis and crystal structure of the dimeric, Ti-O-Ti bridged hydrid form polyoxoanion [alpha-1, 2-PW10Ti2O39](2)(10-)

The reaction of trivacant precursor Nag [A-PW9O34] . 19H(2)O with Ti(SO4)(2) affords the novel dimeric, di-Ti-IV-substituted tungstophosphate K4Na6[alpha-1,2-PW10Ti2O39](2) . 14H(2)O. The X-ray structural determination shows the dimeric, anhydride structure was formed by two Ti-O-Ti bonds linking two di-titanium-substituted Keggin anion [alpha-1,2-PW10Ti2O40]. It was also characterized by elemental analysis, TGA, FT-IR and U-V-vis spectroscopies.

Hydrothermal synthesis and crystal structure of a hybrid material based on [Co-4(phen)(8)(H2O)(2)(HPO3)(2)](4+) and a highly reduced polyoxoanion

An unusual composite hybrid material [Co-4 (phen)(8) (H2O)(2) (HPO3)(2)](H3O)(3) [PMo8VI V-4(IV) O-40 ((VO)-O-IV) 2] 1 (phen = 1,10-phenanthroline) has been hydrothermally synthesized from a mixture of NH4VO3, Na2MoO4.2H(2)O, CoCl2.6H(2)O, phen, H3PO3 and water. It was characterized by elemental analysis, IR, UV-vis, XPS, EPR, TG and single crystal X-ray diffraction. The title compound is constructed from the organic-inorganic hybrid [Co-4(phen)(8)(H2O)(2) (HPO3)(2)](4+) and highly reduced bi-capped pseudo-Keggin [(PMo8V4O40)-V-VI-O-IV ((VO)-O-IV)(2)](7-) polyoxoanions The structure exhibits an extended 2D network through hydrogen bonds among cations, anions and H2O, combining polyoxometalates with metal phosphonates for the first time.