U.B. Mioč

Thermally induced phase transformations of 12-tungstophosphoric acid 29-hydrate : synthesis and characterization of PW8O26-type bronzes

The phase transformations of 12-tungstophosphoric H3PW12O40-29H2O (29-WPA) acid in the temperature range from ambient temperature to 1150°C were investigated and characterized by differential thermal analysis (DTA), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), scanning electron microscopy (SEM), infrared (i.r.) and Raman spectroscopies. From room temperature to 550°C, 29-WPA passes through a dehydration process, which characterizes the formation of different crystallohydrates, in anhydrous form as well as “denuded” Keggins anions, the D-phase (PW12O38). During these processes, Keggins anions are not disturbed too much and they are preserved up to about 550°C. The “D” phase is transformed by solid-solid recrystallization at about 600°C in a new monophosphate bronze type compound PW8O26. Unit cell dimensions were calculated from XRPD data (ao=0.7515 nm). With the temperature increasing up to 1150°C, novel synthesized cubic bronze passed through three polymorphous phase transitions. According to a general formula for monophosphate tungsten bronzes (WO3)2m (PO4)4 all four polymorphs have m=16.

Equilibrium of the protonic species in hydrates of some heteropolyacids at elevated temperatures

Abstract Hydrates of three heteropolyacids: 12-molybdophosphoric H3PMo12O40·nH2O (MoPA·nH2O), 12-tungstophosphoric H3PW12O40·nH2O (WPA·nH2O) and 12-tungstosilisilic acid H4SiW12O40·nH2O (WSiA·nH2O) were examined by using thermal analysis, infrared spectroscopy and impedance measurements. These compounds are known as superionic proton conductors and their behaviour was examined as a function of temperature. At the TGA and DSC curves, in the 300–500 K temperature range, transformations were observed at several temperatures. These transformations are partially due to the dehydration process and partially to the change of equilibrium of protonic species (H3O+, H2O and OH- and their possible association: H5O+2, H7O+3, ...), as shown by thermal analysis and infrared spectroscopy.

Infrared and Raman study of some heteropolyacid hydrates

Abstract Infrared and Raman spectra of hydrates od 12-molybdophosphonic, 12-tungstophosphonic and 12-silicotungstenic acid have been investigated over a wide temperature range. Three types of protonic entities H 3 O + , H 2 O and OH groups attached to the skeleton have been shown to exist and there is a temperature dependent dynamic equilibrium between various species. The structural disorder and the conductivity mechanism in these superionic protonic conductors are discussed.

Structure and proton conductivity of 12-tungstophosphoric acid doped silica

Abstract Sol–gel syntheses offer a wide range of possibilities for introduction of materials with specific properties (electronic, optic and ion-conductive, etc.) into optically clear matrices. Preparation of silica gel bulk, containing 12-tungstophosphoric acid (WPA) in mesopores, as well as its structural and conduction characteristics are reported. Characteristics of the obtained doped gels depend on the gel preparation, gelation process and the WPA content. According to the obtained results, especially for the high conductivity ( σ ∼0.1 S/cm), WPA doped silica gel is a promising material for solid electrolytes.

Inelastic neutron scattering study of protonic species during the thermal dehydration of 12-tungstophosphoric hexahydrate

Abstract 12-Tungstophosphoric acid hexahydrate and its derivatives dehydrated at 473, 573 and 873 K have been studied by incoherent inelastic neutron scattering. Comparison is made with the sodium salts of this compound. Sharp spectra assigned to H 5 O + 2 species (hexahydrate at 25 K), H 3 O + (473 K dehydrated) and a lone proton (573 K) have been obtained. Protons fixed to the host lattice are possible minor species in the hexahydrate and in the material dehydrated at 573 K. The strong background above 300 cm −1 in the hexahydrate and above 50 cm −1 in the dehydrated materials may indicate the presence of mobile protons (proton gas).

Structural properties and proton conductivity of the 12-tungstophosphoric acid doped aluminosilicate gels

Abstract Preparation of aluminosilicate gels bulk, containing 12-tungstophosphoric acid (WPA) in mesopores, as well as their structural and conduction characteristics are reported. Doped gels are prepared from aluminum–silicon alkoxide AlSi(OR) x , iso-propanol, water and WPA·29H 2 O mixtures. Characteristics of the obtained doped gels depend on the WPA content. Results such as WPA-filled mesopores and infra red (IR) band shifts indicate differences with respect to material obtained by mechanical alloying and interaction between the WPA and aluminosilicate host framework. According to the obtained results, especially of the higher conductivity ( σ ∼10 −3 S/cm), WPA-doped aluminosilicate gel is a promising material for solid electrolytes.

Structural modifications of dodecatungstophosphoric acid hexahydrate induced by temperature in the 10–358 K range. In situ high-resolution neutron powder diffraction investigation

Abstract Dodecatungstophosphoric acid hexahydrate H 3 PW 12 O 40 ·6H 2 O crystal structure has been investigated by neutron powder diffraction (NPD) at different temperatures in the 10–358 K range. A nonconvergent reversible phase transition has been noticed at about 320 K. This transition is associated with a change in dynamic equilibrium of hydrate species and partial reduction/oxidation (redox) W 6+ ↔W 5+ . Expressive structure changes lie in the PO bonding inside Keggins anion and the H 5 O 2 + conformational angle.

Keggin’s ion structural modification and expansion of dodecatungstophosphoric acid hexahydrate induced by temperature treatment: In situ X-ray powder diffraction and raman investigations

Abstract The changes in lattice and molecular structure of the dodecatungstophosphoric acid hexahydrate (WPA-6) induced by the temperature treatment between 244 and 373 K, were studied in situ by X-ray powder diffraction and Raman spectroscopy. The increase in temperature was followed with the slight modifications of Keggin’s anion, expansion of the unit cell and an abrupt change in the crystallite microstructure of WPA-6 what was monitored by the Rietveld and Warren–Averbach methods. Those two discontinuities in the thermal expansion of 4 0 0 interplanar spacing, observed at about 320 and 340 K, are result of the non-convergent reversible phase transition, produced by partial reduction and oxidation of the tungsten, known as an ion with mixed valence. The extraordinary compression of P–Oa bond length [1.61(1) A at 298 K and 1.44(2) A at 373 K], broadening and splitting of ν1(P–Oa) bond with the increase in temperature, the abrupt change of crystallite dimension and the strain at 338 K, indicate a modification in the charge distribution followed with the partial reduction of W6+ into W5+ ions (i.e. ‘heteropoly blue’ formation). The partial reduction is closely related to the change in a dynamic equilibrium between different proton species and hydrated proton entities, such as: oxonium H3O+, dioxonium H5O2+, triaquaoxonium H7O3+, etc.

Microcalorimetry in the Identification and Characterization of the Most Reactive Active Sites of Heterogeneous Catalysts

The microcalorimetric technique has been applied to investigate the strongest active sites of zeolites, heteropolyacids and perovskites. The differential heats of adsorption of carbon monoxide, ammonia and oxygen were determined. The results imply that the method is very useful in the identification and characterization of the most reactive active sites of solid catalytic materials.

Characterization of Bronze Surface Layer Formed by Microarc Oxidation Process in 12-Tungstophosphoric Acid

This paper is a brief review of our recent research into novel uses for heteropoly compounds as precursors for thin films that can be used as catalysts and materials with good optical, conductive and other characteristics. In view of this, we have chosen thin film obtained with 12-tungsphosphoric acid on aluminum substrates. In all cases, a relatively new, microarc oxidation technique has been used to prepare oxide coatings on substrate surfaces. Advanced physicochemical methods, AFM and SEM-EDS, XRD, Raman and Micro-Raman, and luminescence spectroscopy, as the most powerful techniques, have been used for the characterization of new materials. Possible applications have been discussed as well.