Laura E. Briand

The state of the art on Wells–Dawson heteropoly-compounds: A review of their properties and applications

The scientific literature concerning the structure, hydrolytic stability in solution, thermal stability in the solid state, redox-acid properties and applications of heteropoly-compounds (HPCs) with Wells–Dawson structure is summarized in the present work. Wells–Dawson heteropoly-anions possess the formula [(X n+ )2M18O62] (16−2n)− where X n+ represents a central atom (phosphorous(V), arsenic(V), sulfur(VI), fluorine) surrounded by a cage of M addenda atoms, such as tungsten(VI), molybdenum(VI) or a mixture of elements, each of them composing MO6 (M-oxygen) octahedral units. The addenda atoms are partially substituted by other elements, such as vanadium, transition metals, lanthanides, halogens and inorganic radicals. The Wells–Dawson heteropoly-anion is associated with inorganic (H + , alkaline elements, etc.) or organic countercations forming hybrid compounds. Wells–Dawson acids (phospho-tungstic H6P2W18O62·24H2O, phospho-molybdic H6P2Mo18O62·nH2O and arsenicmolybdic H6As2Mo18O62·nH2O) possess super-acidity and a remarkably stability both in solution and in the solid state. These properties make them suitable catalytic materials in homogeneous and heterogeneous liquid-phase reactions replacing the conventional liquid acids (HF, HCl, H2SO4, etc.). Although, the application of the acids in heterogeneous gas-phase reactions is less developed, there is a patented method to oxidize alkanes to carboxylic acids on a supported Wells–Dawson catalyst that combines acid and redox properties. Wells–Dawson anions possess the ability to accept or release electrons through an external potential or upon exposure to visible and UV radiation (electro and photochemical reactions). Additionally, Wells–Dawson HPCs catalyze the oxidation of organic molecules with molecular oxygen, hydrogen peroxide and iodosylarenes; epoxidation and hydrogenation in homogeneous and heterogeneous liquid-phase conditions. The ability of transition metal substituted Wells–Dawson HPCs to be reduced and re-oxidized without degradation of the structure is promising in the application of those HPCs replacing metalloporphyrins catalysts in redox and electrochemical reactions.

Quantitative determination of the number of active surface sites and the turnover frequencies for methanol oxidation over metal oxide catalysts: I. Fundamentals of the methanol chemisorption technique and application to monolayer supported molybdenum oxide catalysts

Abstract A detailed study of the methanol chemisorption and oxidation processes on oxide surfaces allowed the development of a method to quantify the number of surface active sites (Ns) of metal oxide catalysts. In situ infrared analysis during methanol adsorption showed that molecular methanol and surface methoxy species are co-adsorbed on an oxide surface at room temperature, but only surface methoxy species are formed at 100°C. Thermal stability and products of decomposition of the adsorbed species were determined with temperature programmed reaction spectroscopy (TPRS) experiments. Controlled adsorption with methanol doses resulted in a stable monolayer of surface methoxy species on the oxide surfaces. The stoichiometry of methanol chemisorption resulted in one surface methoxy adsorbed per three Mo atoms for polymerized surface molybdenum oxide structures, regardless of surface molybdenum oxide coordination. The activity of the catalysts per surface active sites (turnover frequencies — TOF) was calculated in order to quantitatively compare the reactivity of a series of monolayer supported molybdenum oxide catalysts. The TOF value trends reflect the influence of the bridging Mo–O–Support bond and the electronegativity of the metal cation of the oxide support.

Vanadium(V) reduction in Thiobacillus thiooxidans cultures on elemental sulfur

SummaryWe describe the reduction of vanadium (V) to vanadium (IV) in cultures of Thiobacillus thiooxidans on elemental sulfur, for initial vanadium (V) concentrations up to 5 mM. The vanadium (V) is reduced by intermediate compounds generated by bacterial oxidation of elemental sulfur. The limit of initial vanadium (V) allowing bacterial action seems to be related to the inhibition caused by such vanadium species, rather than connected to the vanadium (IV) species, which did not show inhibitory effects up to concentrations of about 0.1 M. This reduction mechanism of vanadium (V) is potentially applicable in the recovery of vanadium from spent solid catalysts, by a low-cost methodology.

Anomalous surface compositions of stoichiometric mixed oxide compounds.

Surface-oxide films are present in many types of oxidecontaining materials, such as grain boundaries in ceramics, interfaces in ceramic-ceramic and metal-oxide systems, and affect their materials and transport properties. In heterogeneous catalysis, the properties of the outermost surface layer are of prime importance because they control the catalytic performance. Although bulk mixed-metal oxide catalysts are widely used in industrial selective oxidation processes, not much is known about their outermost surface composition. Models based on surfaces derived from a truncation of the bulk structure have dominated discussion on catalytic reaction mechanisms and active sites (reviewed, for example, in Ref. [6]). This view has been questioned by several recent studies reporting the surface enrichment and depletion phenomena in solid-oxide solutions (e.g., CoxNi1 xO ), the identification of TiO2-rich overlayers on reconstructed SrTiO3(001) model surfaces, [8] and evidence for the formation of amorphous oxide overlayers in which there is surface enrichment of one of the components under selective oxidation reaction conditions. However, the development of realistic concepts on reactant activation, surface reaction mechanisms, and the design of advanced catalytic materials are still hampered by the lack of detailed knowledge of the surface composition and structure of bulk mixed-metal oxides. For such studies, X-ray photoelectron spectroscopy (XPS) with laboratory sources is of limited value because its average sampling depth of 1–3 nm results in a signal where the outermost surface layer only contributes on the order of 30%. Synchrotron radiation allows for increasing the surface sensitivity of XPS by decreasing excitation and, hence, photoelectron kinetic energies. Exclusive information on the outermost surface layer, however, is only given by low-energy ion scattering (LEIS) because ions penetrating below the surface become largely neutralized. The surfaces of stoichiometric bulk mixed-metal molybdates and vanadates have also been characterized through their interactions with probe molecules, for example, CH3OH, [12–15] which allows CH3O* and intact CH3OH* intermediates on different surface cations to be discriminated by IR spectroscopy. For such materials, combined methanol chemisorption and oxidation kinetic studies suggested a strong surface enrichment of MoOx or VOx. [12,14,15] In methanol oxidation studies, similar catalytic turnover frequencies were found over bulk mixed-metal oxides and related supported metal oxides (e.g., Fe2(MoO4)3 and MoO3/Fe2O3), which supports the idea of surface MoOx enrichment of the bulk phases. These observations, however, are qualitative as exposed metal oxide ions of low catalytic activity would not be detected by the test reaction. Thus, we have undertaken a study of the outermost surface compositions of such compounds by LEIS and excitation-energy resolved XPS (ERXPS). The LEIS was applied in sputter series taking advantage of its destructive character, the ERXPS is a version utilizing information from different sampling depths. LEIS sputter series from stoichiometric bulk mixed oxides and related supported metal oxides are given in Figure 1 and [*] Dr. S. V. Merzlikin, Prof. Dr. W. Gr nert Lehrstuhl f r Technische Chemie, Ruhr-Universit t Bochum Postfach 102148, 44780 Bochum (Germany) Fax: (+49)234-32-14115 E-mail: w.gruenert@techem.rub.de Homepage: http://www.techem.rub.de Dr. N. N. Tolkachev N. D. Zelinsky Institute of Organic Chemistry Russian Academy of Sciences, Moscow (Russia)

Structural modelling of coprecipitated VTiO catalysts

Abstract A statistical study of the particle shape and size of pure V2O5 and TiO2, and samples of coprecipitated V2O5TiO2 catalysts with different V Ti ratios, has been performed. They were also characterized by XRD, EDAX, SEM and XPS. The results showed that pure vanadium pentoxide is compose by large square or needle-shaped particles, while pure titanium dioxide has small and rounded ones. VTiO samples presented an area and shape, depending on the V Ti ratio. These results and the spectroscopical characterization conducted to a particle model of the catalysts. Those VOTi samples with high V Ti ratio would have large V2O5 crystals acting as support of a V Ti O 2 solid solution. In contrast, those samples with a low V Ti ratio, would have the solid solution supporting vanadium pentoxide crystals.

In situ DRIFTS study of the adsorption–oxidation of CH3OH on V2O5

Abstract The oxidation of CH3OH on V2O5 has been studied from room temperature to 250°C. The reaction products were analyzed by on-line gas chromatography (GC) and the adsorbed species were characterized by “in situ” diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Both isothermal and non-isothermal experiments were carried out in order to identify adsorbed species in the oxidation of CH3OH on V2O5 catalysts. The interaction of CH3OH with theV2O5 surface results in the formation of methoxy groups and V–OH species as stated by DRIFTS. The infrared spectra suggest that more than one catalytic site are involved in the adsorption process. The formation of V–OH and the resulting reduction of the vanadia catalyst leading to V4+⎓O and V3+ groups is inferred from DRIFTS data and X-ray diffraction patterns of the used catalysts showing the presence of reduced oxide phases. As the reaction proceeds, the adsorbed methoxy species are oxidized to H2CO, formate species, partial oxidation products, CO and CO2.

Thermal stability and catalytic activity of Wells-Dawson tungsten heteropoly salts

Potassium-tungsten Dawson type salts, pure and monosubstituted with vanadium, are presented in this study. These compounds were thoroughly characterized by 31 P-NMR, 31 P MAS-NMR, scanning electron microscopy, EDX microprobe, X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy and thermal analysis. Both Dawson salts are thermally stable up to 400C. Above that temperature they recrystallize in a potassium‐tungsten Keggin type salt and P2O5-WO3(-K2O) compounds. Vanadium that was initially part of the tungsten framework is expelled out of the structure and covers the surface of the new phases. Dawson salts showed activity in methanol selective oxidation. Formaldehyde is the only product detected during methanol reaction on intact Dawson salts, which indicates the presence of redox sites. After decomposition, dimethyl ether along with formaldehyde is produced, due to the exposure of acidic phosphorous sites. The selectivity of the decomposed vanadium containing Dawson salt towards dimethyl ether is lower than the pure potassium‐tungsten salt, since vanadium is covering acid sites.

Stability of phospho-molybdic Wells–Dawson-type ion P2Mo18O626− in organic media

Abstract The stability of the ammonium phospho-molybdic Wells–Dawson-type salt (NH 4 ) 6 P 2 Mo 18 O 62 ·12H 2 O in various organic media was investigated at room temperature and 50 °C through infrared spectroscopy. The heteropoly-anion P 2 Mo 18 O 62 6− is soluble and stable in methanol, ethanol, 2-propanol, ethyl ether, acetone and acetonitrile. The alcohols dissociatively chemisorb on the anion forming alkoxy (methoxy, ethoxy and isopropoxy) species. The interaction with acetone and acetonitrile is nondissociative.

Bulk and surface characterization of crystalline and plastic sulphur oxidized by two Thiobacillus species

Abstract This work studies the surface interaction between Thiobacillus ferrooxidans and Thiobacillus thiooxidans with crystalline and plastic elemental sulphur. The interaction mechanisms were analysed by fractal geometry which describes textural modifications of the substrate caused by bacterial action. The results demonstrated that the bacteria are able to produce two different effects depending on the substrates. Only surface smoothing (decrease on fractal dimension values) was detected on crystalline sulphur (this effect being stronger with T . ferrooxidans than with T . thiooxidans), but, perforation of the bulk was also observed in plastic sulphur

Formation of a solid solution of vanadium in TiO2(anatase) on vanadium–titanium solids with high vanadium content

Vanadium–titanium solids, with V/Ti atomic ratios ranging from 0.14 to 7.99, have been prepared by coprecipitation followed by calcination in air. These solids were then washed with a basic medium to obtain pure solid solutions. The products were characterized by means of EDAX, XRD, FTIR, XPS, SEM and laser Raman spectroscopy. The results provide evidence for the formation of a substitutional solid solution of vanadium in TiO2(anatase).