Interactions of oxalic acid and ice on Cu surface

The interactions between oxalic acid (C 2H 2O 4) and H 2O on a polycrystalline Cu surface have been investigated by reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) methods. The desorption of H 2O and C 2H 2O 4 was studied; we found that the ice desorption temperature increases with the ice-film thickness. Desorption of the C 2H 2O 4 layer involves a structural modification and sublimation. The H 2O/C 2H 2O 4 and C 2H 2O 4/H 2O interfaces and the codeposited C 2H 2O 4+H 2O were prepared on the Cu surface by varying deposition sequences of gaseous C 2H 2O 4 and H 2O at 155 K. We found that the interaction between ice and C 2H 2O 4 does not lead to the H 2O-induced deprotonation of C 2H 2O 4 in a temperature range 155-283 K. However, H-bonding interactions between H 2O and C 2H 2O 4 can lead to the formation of a metastable oxalic acid-ice complex in the C 2H 2O 4/H 2O and C 2H 2O 4+H 2O systems during the TPD process. Desorption of H 2O from the C 2H 2O 4/H 2O/Cu system is suggested to involve the diffusion of H 2O through the top C 2H 2O 4 layer. H 2O desorption is followed by a rearrangement of C 2H 2O 4 to form a C 2H 2O 4 adlayer on Cu in the C 2H 2O 4+H 2O system. These experimental findings suggest that C 2H 2O 4 is not ionized on snow and ice in the polar boundary layer and at upper tropospheric temperatures ( approximately 240 K).     

偏硼酸锶系列发光材料的制备及其发光性能研究

利用液相共沉淀法制备了SrB2O4 4H2O和SrB2O4 4H2O∶Eu3+,利用高温焙烧前驱体法制备了SrB2O4和SrB2O4∶Eu3+发光材料,通过X射线粉晶衍射(XRD)、X射线能谱分析(EDS)和扫描电子显微镜(SEM)等手段对产物进行了表征.通过荧光光谱研究了其发光性质,并考察了反应时间及Eu3+掺杂浓度对发光强度的影响.结果表明,基质SrB2O4 4H2O和SrB2O4在紫外区具有较强的发光性能,SrB2O4 4H2O∶Eu3+和SrB2O4∶Eu3+均在613 nm有最强发射峰.通过调整反应时间和提高掺杂量,可以克服结构水的猝灭作用的影响,大大提高SrB2O4 4H2O∶Eu3+发光性能,且具有更高的红橙比,是一种良好的新型发光基质.     

Synthesis, characterization, and reactivity of ferrous and ferric oxo/peroxo pivalate complexes in relation to Gif-type oxygenation of substrates

This study examines structural features and aspects of reactivity of Gif-type reagents, which depend on O2/Zn to mediate oxidation of hydrocarbons. The reagents investigated derive from the use of iron complexes with the anion of the weak carboxylic acid Me3CCO2H (pivalic acid (PivH)) in pyridine/PivH. In these solutions, the known compound [Fe3O(O2CCMe3)6(py)3] is reduced by Zn to generate yellow-green [FeII(O2CCMe3)2(py)4], which readily reverts to [Fe3O(O2CCMe3)6(py)3], and eventually to [Fe3O(O2CCMe3)6(py)3]+, upon exposure to dioxygen. All three species are equally well suited to mediate Gif-like oxygenation of substrates supported by O2/Zn. [FeIII3O(O2CCMe3)6(L)3]+ (L = H2O, py) is converted by H2O2 to afford the hexairon(III) peroxo compounds [Fe6(O2)(O)2(O2CCMe3)12(L)2] (L = Me3CCO2H, py), which feature a [Fe6(eta 2-mu 4-O2)(mu 3-O)2] core previously documented in the closely related [Fe6(O2)(O)2(O2CPh)12(H2O)2]. A similar peroxo species, [Fe6(O2)(O)2(O2CCMe3)2(O2CCF3)10(H2O)2], is obtained upon replacing all pivalate ligands by trifluoroacetate groups with the exception of those pivalates that bridge between the two [Fe3O(O2CCF3)5(H2O)]2+ units. The structure of the [Fe6(O2)(O)2] core in these peroxo species is found to range from a recliner to a butterfly-type conformation. Reduction of [Fe6(O2)(O)2(O2CCMe3)12(HO2CCMe3)2] with NaBH4 generates [Na2Fe4(O)2(O2CCMe3)10(L)(L')] (L = CH3CN, L' = Me2CO; L = L' = Me3CCO2H), which feature a [Na2Fe4(O)2] core possessing a bent butterfly conformation of the [Fe4(O)2] unit. Oxidation of the same peroxo complex by CeIV or NOBF4 regenerates the oxo-bridged [Fe3O(O2CCMe3)6(solv)3]+ (solv = EtOH, H2O, thf). Employment of the sterically encumbered 2-Me-5-Etpyridine provides the tetrairon compound [Fe4(O)2(O2CCMe3)8(2-Me-5-Etpy)2], which can be readily transformed upon treatment with H2O2 to the asymmetric peroxo complex [Fe6(O2)(O)2(O2CCMe3)12(2-Me-5-Etpy)2]. The peroxo-containing complexes oxidize both cis-stilbene and adamantane in either benzene or py/PivH, but only under forceful conditions and at very low yields. The low reactivity and high selectivity (tert/sec = 8) obtained in the oxidation of adamantane suggests that the present type of peroxo species is not directly involved in catalytic Gif-type oxygenations of adamantane.     

CH4,CO2和O2制合成气反应中载体对Ni催化剂抗氧化性能的影响

在ＣＨ４、ＣＯ２ 催化氧化制合成气反应中, Ｎｉ／Ａｌ２Ｏ３ 催化剂在高温下生成ＮｉＡｌ２Ｏ４ 尖晶石,是导致催化剂失活的一个重要因素． 通过向载体（Ａｌ２Ｏ３）中添加各种氧化物, 使得催化剂的抗氧化性能得到改善． 并运用ＴＰＲ、ＸＲＤ对催化剂进行表征, 发现催化剂的抗氧化性顺序为： Ｎｉ／ＣａＯ－Ａｌ２Ｏ３ ＞ Ｎｉ／ＭｇＯ－Ａｌ２Ｏ３ ＞ Ｎｉ／ＣｅＯ２－Ａｌ２Ｏ３ ＞ Ｎｉ／Ｌａ２Ｏ３－Ａｌ２Ｏ３ ＞ Ｎｉ／Ｙ２Ｏ３－Ａｌ２Ｏ３ ＞ Ｎｉ／ＴｉＯ２－Ａｌ２Ｏ３＞ Ｎｉ／Ａｌ２Ｏ３＞ Ｎｉ／Ｆｅ２Ｏ３－Ａｌ２Ｏ３．     

Solid state coordination chemistry of oxomolybdenum organoarsonate materials

The oxomolybdenum-arsonate system was investigated under hydrothermal conditions in the presence of charge-compensating copper(II)-organonitrogen complex cations as secondary building blocks. A series of materials of the Mo/O/As/Cu/ligand family has been prepared and structurally characterized. The architectures of the products reflect the identity of the arsonate component and the organonitrogen ligand, as well as the reaction conditions. The structural versatility of this emerging class of compounds is manifested by the one-dimensional structures of [[Cu(o-phen)(H(2)O)(2)](2)Mo(6)O(18)(O(3)AsOH)(2)] (1), [[Cu(terpy)](2)Mo(4)O(13)H(AsO(4))(2)].2H(2)O (2.2H(2)O), [[Cu(2,2'-bpy)(H(2)O)](2)Mo(6)O(18)(O(3)AsC(6)H(5))(2)].2H(2)O (4.2H(2)O), and [[Cu(o-phen)(H(2)O)](2)[Mo(6)O(18)(O(3)AsC(6)H(5))(2)]].4H(2)O (5.4H(2)O), by the two-dimensional materials [[Cu(2)(tpyprz)(H(2)O)(2)]Mo(6)O(18)(O(3)AsOH)(2)].2H(2)O (3.2H(2)O), [[Cu(terpy)](2)Mo(6)O(18)(O(3)AsC(6)H(5))(2)].H(2)O (6.H(2)O), and [[Cu(2)(tpyprz)]Mo(6)O(18)(O(3)AsC(6)H(5))(2)].2H(2)O (7.2H(2)O), and the molecular clusters [[Cu(2,2'-bpy)(2)](2)Mo(12)O(34)(O(3)AsC(6)H(5))(4)].2.35H(2)O (8.2.35H(2)O) and [Cu(o-phen)(H(2)O)(3)][Cu(o-phen)(2)Mo(12)O(34) (O(3)AsC(6)H(5))(4)].3H(2)O (9.3H(2)O).     

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.     

Influence of support on resistance to carbon-deposition of catalyst for CH4, CO2 with O2 to synthesis gas

In the reaction of catalytic oxidation of CH4,CO2 with O2 to synthesis gas, carbon-deposition is an important factor for deactivation. By adding different oxides to Ni/AI2O3 catalyst, its resistance to carbon-deposition was improved. The experimental results indicate that the order of resistance to carbon-deposition is as follows: Ni/CaO-AI2O3>Ni/MgO-AI2O3>Ni/ TiO2-AI2O3>Ni/CeO2-AI2O3>Ni/La2O3-AI2O3>Ni/Y2O3-AI2O3>Ni/Fe2O3-AI2O3>Ni/AI2O3. The catalysts were characterized by CO2-TPD, O2-TPD and XPS methods. Here the relation between the order of resistance to carbon-deposition and performance of catalyst is discussed.     

H_2O_2氧化N_2生成N_2O和H_2O机理的研究

采用量子化学计算方法研究了Ｈ２Ｏ２ 氧化Ｎ２ 生成Ｎ２Ｏ 和Ｈ２Ｏ 的机理．结果发现, Ｈ２Ｏ２ 氧化Ｎ２ 先通过１ 个四元环过渡态形成中间体Ｈ２Ｎ２Ｏ２ 分子,Ｈ２Ｎ２Ｏ２ 再通过一个五元环过渡态形成Ｎ２Ｏ和Ｈ２Ｏ．根据计算得到的每步反应的活化能,得知Ｈ２Ｏ２ 氧化Ｎ２ 生成中间体Ｈ２Ｎ２Ｏ２ 分子是整个反应的控制步骤．     

p-Hydroquinone-metal compounds: synthesis and crystal structure of two novel VV-p-hydroquinonate and VIV-p-semiquinonate species

Reaction of the p-hydroquinone derivative H2Na4bicah.4H2O with either VIVOSO(4).3H2O and NaVVO3 in equivalent quantities or with NaVVo3 yields the tetranuclear VIVO2+ macrocycle-semiquinonate compound Na6[(VIVO)4-(mu2-O)2[mu2-bicas.(-5)-N,O,O,O]2].Na2SO(4).20H2O (1.Na2SO(4).20H2O) and the dinuclear cis-VVO2(+)-hydroquinone species Na4[(VVO2)2[mu2-bicah(-6)-N,O,O,O]].11H2O (2.11H2O) respectively. Compounds 1.Na2SO(4).20H2O and 2.11H2O were characterized by X-ray structure analysis and ab initio calculations.     

Photoelectron imaging study of the effect of monohydration on O2 - photodetachment

The photodetachment of the O(2)(-).H(2)O cluster anion at 780 and 390 nm is investigated in comparison with O(2)(-) using photoelectron imaging spectroscopy. Despite the pronounced shift in the photoelectron spectra, the monohydration has little effect on the photoelectron angular distributions: for a given wavelength and electron kinetic energy (eKE) range, the O(2)(-).H(2)O angular distributions are quantitatively similar to those for bare O(2)(-). This observation confirms that the excess electron in O(2)(-).H(2)O retains the overall character of the 2ppi(g) HOMO of O(2)(-). The presence of H(2)O does not affect significantly the partial wave composition of the photodetached electrons at a given eKE. An exception is observed for slow electrons, where O(2)(-).H(2)O exhibits a faster rise in the photodetachment signal with increasing eKE, as compared to O(2)(-). The possible causes of this anomaly are (i) the long-range charge-dipole interaction between the departing electron and the neutral O(2).H(2)O skeleton affecting the slow-electron dynamics; and (ii) the s wave contributions to the photodetachment, which are dipole-forbidden for pi(g)(-1) transitions in O(2)(-), but formally allowed in O(2)(-).H(2)O due to lower symmetry of the cluster anion and the corresponding HOMO.     

水热条件硼酸镁盐过饱和水溶液的相转化

针对中国盐湖硼资源特点,模拟合成MgO•3B2O3-H2O过饱和溶液,在120 ℃水热条件下对固相析出的过程进行跟踪.析出固相采用化学分析方法、X射线衍射、红外光谱进行物相鉴定.对析出固相的相转化反应机理进行了探讨,结合有关实验分析了MgSO4对固相析出的影响.     

Atmospheric chemistry of a model biodiesel fuel, CH3C(O)O(CH2)2OC(O)CH3: kinetics, mechanisms, and products of Cl atom and OH radical initiated oxidation in the presence and absence of NOx

Relative rate techniques were used to study the kinetics of the reactions of Cl atoms and OH radicals with ethylene glycol diacetate, CH3C(O)O(CH2)2OC(O)CH3, in 700 Torr of N2/O2 diluent at 296 K. The rate constants measured were k(Cl + CH3C(O)O(CH2)2OC(O)CH3) = (5.7 +/- 1.1) x 10(-12) and k(OH + CH3C(O)O(CH2)2OC(O)CH3) = (2.36 +/- 0.34) x 10(-12) cm3 molecule-1 s-1. Product studies of the Cl atom initiated oxidation of ethylene glycol diacetate in the absence of NO in 700 Torr of O2/N2 diluent at 296 K show the primary products to be CH3C(O)OC(O)CH2OC(O)CH3, CH3C(O)OC(O)H, and CH3C(O)OH. Product studies of the Cl atom initiated oxidation of ethylene glycol diacetate in the presence of NO in 700 Torr of O2/N2 diluent at 296 K show the primary products to be CH3C(O)OC(O)H and CH3C(O)OH. The CH3C(O)OCH2O* radical is formed during the Cl atom initiated oxidation of ethylene glycol diacetate, and two loss mechanisms were identified: reaction with O2 to give CH3C(O)OC(O)H and alpha-ester rearrangement to give CH3C(O)OH and HC(O) radicals. The reaction of CH3C(O)OCH2O2* with NO gives chemically activated CH3C(O)OCH2O* radicals which are more likely to undergo decomposition via the alpha-ester rearrangement than CH3C(O)OCH2O* radicals produced in the peroxy radical self-reaction.     

丙烯选择氧化反应中钼基催化剂动态结构的研究

 用原位共焦显微拉曼光谱技术考察了丙烷选择氧化反应中Ag－M\r\no－P－O催化剂的结构，讨论了催化剂动态结构的成因及其对催化剂性\r\n能的影响．实验结果表明，在773K和n（C3H8）∶n（O2）∶n（N2）＝\r\n3∶1∶4的反应条件下，Ag－Mo－P－O催化剂中的Mo－O物种可转化为A\r\ngMoO2PO4中的Mo－O物种（多钼酸根），此时催化剂对丙烷选择氧化具\r\n有较高的催化活性．催化剂中Mo－O物种的转化是由MoO3中Mo－O物种和\r\nAgMoO2PO4中Mo－O物种的结构特性决定的．AgMoO2PO4中的Mo－O物种具\r\n有较强的参与MarsvanKrevelen氧化－还原循环的能力，可能是丙烷选\r\n择氧化反应的活性物种．     

Organic-inorganic hybrid materials: hydrothermal syntheses and structural characterization of bimetallic organophosphonate oxides of the type Mo/Cu/O/RPO(3)(2-)/organoimine

The hydrothermal reactions of a Cu(II) starting material, a molybdate source, 2,2'-bipyridine or terpyridine, and the appropriate alkyldiphosphonate ligand yield two series of bimetallic organophosphonate hybrid materials of the general types [Cu(n)(bpy)(m)Mo(x)O(y)(H(2)O)(p)[O(3)P(CH(2))(n)PO(3)](z)] and [Cu(n)(terpy)(m)Mo(x)O(y)(H(2)O)(p)[O(3)P(CH(2))(n)PO(3)](z)]. The bipyridyl series includes the one-dimensional materials [Cu(bpy)(MoO(2))(H(2)O)(O(3)PCH(2)PO(3))] (1) and [[Cu(bpy)(2)][Cu(bpy)(H(2)O)](Mo(5)O(15))(O(3)PCH(2)CH(2)CH(2)CH(2)PO(3))].H(2)O (5.H(2)O) and the two-dimensional hybrids [Cu(bpy)(Mo(2)O(5))(H(2)O)(O(3)PCH(2)PO(3))].H(2)O (2.H(2)O), [[Cu(bpy)](2)(Mo(4)O(12))(H(2)O)(2)(O(3)PCH(2)CH(2)PO(3))].2H(2)O (3.2H(2)O), and [Cu(bpy)(Mo(2)O(5))(O(3)PCH(2)CH(2)CH(2)PO(3))](4). The terpyridyl series is represented by the one-dimensional [[Cu(terpy)(H(2)O)](2)(Mo(5)O(15))(O(3)PCH(2)CH(2)PO(3))].3H(2)O (7.3H(2)O) and the two-dimensional composite materials [Cu(terpy)(Mo(2)O(5))(O(3)PCH(2)PO(3))] (6) and [[Cu(terpy)](2)(Mo(5)O(15))(O(3)PCH(2)CH(2)CH(2)PO(3))] (8). The structures exhibit a variety of molybdate building blocks including isolated [MoO(6)] octahedra in 1, binuclear subunits in 2, 4, and 6, tetranuclear embedded clusters in 3, and the prototypical [Mo(5)O(15)(O(3)PR)(2)](4-) cluster type in 5, 7, and 8. These latter materials exemplify the building block approach to the preparation of extended structures.     

Computational study on the existence of organic peroxy radical-water complexes (RO2.H2O)

The existence of a series of organic peroxy radical-water complexes [CH3O2.H2O (methyl peroxy); CH3CH2O2.H2O (ethyl peroxy); CH3C(O)O2.H2O (acetyl peroxy); CH3C(O)CH2O2.H2O (acetonyl peroxy); CH2(OH)O2.H2O (hydroxyl methyl peroxy); CH2(OH)CH2O2.H2O (2-hydroxy ethyl peroxy); CH2(F)O2.H2O (fluoro methyl peroxy); CH2(F)CH2O2.H2O (2-fluoro ethyl peroxy)] is evaluated using high level ab initio calculations. A wide range of binding energies is predicted for these complexes, in which the difference in binding energies can be explained by examination of the composition of the R group attached to the peroxy moiety. The general trend in binding energies has been determined to be as follows: fluorine approximately alkyl < carbonyl < alcohol. The weakest bound complex, CH3O2.H2O, is calculated to be bound by 2.3 kcal mol-1, and the strongest, the CH2(OH)O2.H2O complex, is bound by 5.1 kcal mol-1. The binding energy of the peroxy radical-water complexes which contain carbonyl and alcohol groups indicates that these complexes may perturb the kinetics and product branching ratios of reactions involving these complexes.     

Probing the effect of local structure on the thermodynamic redox properties of V(+5): a comparison of V2O5 and Mg3(VO4)2

Coulometric titration, an electrochemical method for measuring oxidation isotherms, has been used to characterize the redox properties of V2O5 and Mg3(VO4)2 between 823 and 973 K. V2O5 shows distinct regions in the isotherms corresponding to equilibrium with mixtures of V2O3 and V2O4 and of V2O4 and V2O5. From this data, the enthalpies for oxidation of V2O3 to V2O4 and for V2O4 to V2O5 are shown to be -380 +/- 10 and -285 +/- 20 kJ mol-1 O2, respectively. Oxidation isotherms for Mg3(VO4)2 exhibit a single step between the oxidized sample (all V+5) and a completely reduced sample (all V+3). The enthalpy of oxidation is found to increase with the oxidation state of the sample, from -370 +/- 30 kJ mol-1 O2 at an O:V ratio of 1.5 to -460 +/- 10 kJ mol-1 O2 at an O:V ratio of 2.5.     

Synthesis and characterization of octa- and hexanuclear polyoxomolybdate wheels: role of the inorganic template and of the counterion

Eight new compounds based on [O3PCH2PO3]4- ligands and {MoV2O4} dimeric units have been synthesized and structurally characterized. Octanuclear wheels encapsulating various guests have been isolated with different counterions. With NH4+, a single wheel was obtained, as expected, with the planar CO32- guest, (NH4)12[(MoV2O4)4(O3PCH2PO3)4(CO3)2].24H2O (1a), while with the pyramidal SO32- guest, only the syn isomer (NH4)12[(MoV2O4)4(O3PCH2PO3)4(SO3)2].26H2O (2a) was characterized. The corresponding anti isomer was obtained with Na+ as counterions, Na12[(MoV2O4)4(O3PCH2PO3)4(SO3)2]39H2O (2b), and with mixed Na+ and NH4(+) counterions, Na+(NH4)11[(MoV2O4)4(O3PCH2PO3)4(SO3)2].13H2O (2d). With [O3PCH2PO3]4- extra ligands, the octanuclear wheel Li12(NH4)2[(MoV2O4)4(O3PCH2PO3)4(HO3PCH2PO3)2].31H2O (4a) was isolated with Li+ and NH4+ counterions and Li14[(MoV2O4)4(O3PCH2PO3)4(HO3PCH2PO3)2].34H2O (4c) as a pure Li+ salt. A new rectangular anion, formed by connecting two MoV dimers and two MoVI octahedra via methylenediphosphonato ligands with NH4+ as counterions, (NH4)10[(MoV2O4)2(MoVIO3)2(O3PCH2PO3)2(HO3PCH2PO3)2].15H2)O (3a), and Li9(NH4)2Cl[(MoV2O4)2(MoVIO3)2(O3PCH2PO3)2]. 22H2O (3d) as a mixed NH4+ and Li+ salt have also been synthesized. The structural characterization of the compounds, combined with a study of their behavior in solution, investigated by 31P NMR, has allowed a discussion on the influence of the counterions on the structure of the anions and their stability. Density functional theory calculations carried out on both isomers of the [(MoV2O4)4(O3PCH2PO3)4(SO3)2]12- anion (2), either assumed isolated or embedded in a continuum solvent model, suggest that the anti form is favored by approximately 2 kcal mol(-1). Explicit insertion of two solvated counterions in the molecular cavity reverses this energy difference and reduces it to less than 1 kcal mol(-1), therefore accounting for the observed structural versatility.     

Ti,Na与O2反应机理的理论研究

基于MP2/6-311+G(d)水平, 分别对过渡金属Ti和碱金属Na与O2的反应机理进行了研究. 比较了Ti和Na分别以垂直O—O键和沿着O—O键的方向逼近O2, 以及中性体系Ti/Na+O2和带1个负电荷的体系(Ti/Na+O2)-的情况. 详细分析了不同反应路径的结合能和电荷变化的曲线, 预测了最佳反应方式. 结果表明, 垂直接近方式要比水平接近方式更具有优势; 体系带一个负电荷(Ti/Na+O2)-有利于金属与O2的结合. 同时, 计算结果表明在Ti+O2和(Ti+O2)-体系中Ti容易与单态的O2结合; 在中性体系中Na也容易与单态O2结合, 而在(Na+O2)-体系中Na与三态O2的结合更稳定. 在CCSD(T)/6-311++G(3df)//MP2/6-311+G(d)水平下, 计算了Ti+O2和(Ti+O2)-的反应势能面.     

Oxygen-17 NMR study of aqueous peroxotungstates

Aqueous peroxotungstates have been studied from pH 0.5 to 9.0, over a range of peroxide concentrations. Although equilibria are not always established, many anions can be identified by 17O NMR because the ratio of deltaO(W anion)/deltaO(known Mo anion) is consistently 79 +/- 3%. They are [WO3(HO2)]-; [WO(OH)(O2)2]-; [WO(OH2)(O2)2]0; [W2O3(O2)4]2-; [W2O3(OH)(O2)4]3-; [W4O12(O2)2]4-; [W7O23(O2)]6- and [W7O22(O2)2]6-. Their pKa values, where measurable, are about 2 units lower than the corresponding peroxomolybdates, e.g. 0.0 for [WO(OH2)(O2)2]0 and 8.0 for [W2O3(O2)4]2-. Other peroxotungstate species are also present but can only be broadly identified. These include Keggin structures with relatively low peroxo content, and a very unsymmetrical anion appearing at pH ca. 7 that bears no obvious structural relationship to any species previously reported. The main product from the reaction of powdered W metal with 30% aqueous peroxide is provisionally identified as the symmetrical anion [W6O13(OH)2(OH2)2(O2)5]2-, although other minor species are also formed, probably having fewer peroxo substituents.     

A study of the redox properties of MoOx/SiO2

A sample of MoOx/SiO2, in which all of the Mo cations are present as isolated mono-oxo molybdate moieties, was prepared and investigated to understand the redox chemistry of such molybdate species and their ability to exchange oxygen with O2 and H2O. Raman spectroscopy was used to monitor the exchange of 18O for 16O in the Mo=O bond of isolated molybdate species, whereas mass spectrometry was used to follow the isotopic composition of the gaseous species, i.e., O2 and H2O. Reduction in H2 at 920 K results in the loss of one O atom per Mo atom, and consistent with this, all of the Mo(VI) cations are reduced to Mo(IV) cations. Raman spectroscopy shows that virtually all Mo=O bonds of the original molybdate species are lost upon reduction. While reoxidation of Mo(IV) cations by O2 is quantitative, studies using 18O2 reveal that only a small part of the newly formed Mo=O bonds are 18O labeled, and that the balance are 16O labeled, indicating that O-atom exchange between the support, SiO2, and the supported MoOx species occurs during reoxidation. Rapid exchange of O atoms was observed upon exposure of both bare SiO2 and MoOx/SiO2 to H2(18)O at 920 K, and the presence of MoOx species was found to enhance the rate of exchange. By contrast, very slow exchange of O atoms was observed when the oxidized catalyst was exposed to 18O2 at 920 K. In situ observations of the catalyst during exposure to a mixture of H2 and 18O2 at 920 K showed that all of the Mo cations remained in the VI oxidation state and that O atom exchange occurred at a rate comparable to that observed upon exposure to H2(18)O. The results of this investigation suggest that reoxidation of Mo(IV) cations following H2 reduction involves the formation of a Mo-peroxide species and subsequent O atom migration from such a species to the SiO2 support. It is proposed that the steady-state oxidation of H2 also involves the formation of Mo-peroxide species by interaction of O2 with a small number of Mo(IV) centers. The Mo-peroxide species are then rapidly reduced by H2 to form H2O and a Mo=O bond. The rapid exchange of O atoms between the gas phase and the catalyst observed during steady-state oxidation of H2 is attributed to interactions of the product H2O with the catalyst, rather than to O atom migration originating from the Mo-peroxide species formed on the catalyst surface.