Studies of the activity of catalysts based on heteropolyacids

The catalytic activity of samples such as PPy(H₄SiW₁₂O₄₀), PPy(H₅PMo₁₀V₂O₄₀), PPy(H₂Fe(III)PMo₁₀V₂O₄₀), PPy(H₃Cu(II)PMo₁₀V₂O₄₀) has been examined in two different test reactions. The acid-base and oxidation-reduction properties were studied using the conversion of isopropyl alcohol to propene and acetone. Redox ability of catalysts was examined in the reaction of oxidation of allyl alcohol to glycidol. It was found that the activity of catalysts in the oxidation of allyl alcohol increases as the oxidation properties determined from the conversion of isopropyl alcohol increase. It was also observed that stronger oxidation-reduction properties of the catalyst result in a high rate of the consecutive reaction of glycidol to 3-hydroxypropanone.The phase composition of catalysts was determined by means of X-ray diffraction (XRD).     

Synthesis,characterization and electrochemical impedance spectroscopy of VOx-NTs/PPy composites

Composites consisting of vanadium oxide nanotubes (VOx-NTs) and polypyrrole (PPy) were synthesized by a two-steps method. VOx-NTs were firstly prepared by a combined sol–gel reaction and hydrothermal treatment procedure, in which V ₂O₅ powder and H₂O₂ were used as raw materials and hexadecylamine as a structure-directing template. Then VOx-NTs/PPy composites were fabricated by a cationic exchange reaction between hexadecylamine and polypyrrole. The structure and morphology of the samples were investigated by SEM, TEM, XRD and FTIR techniques. The results confirmed that the template molecules were successfully substituted by the conducting polymers PPy without destroying the previous tubular structure. Electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the electrochemical kinetics of the samples. The results indicated that VOx-NTs/PPy composites had a lower charge transfer resistance and a faster lithium-ion diffusion speed than those of VOx-NTs, and the enhanced electrochemical kinetics could be attributed to the excellent electronic conductivity of polypyrrole.     

Dehydration of isopropyl alcohol used as an indicator of the type and strength of catalyst acid centres

The subject of our studies was determination of the kind and strength of the catalyst acid centres on isopropyl alcohol conversion. The investigations were carried out for two groups of catalysts: typical Lewis acids γ-Al₂O₃ and ZrO₂ and Brønsted acids: H₃PW₁₂O₄₀ and H₃PMo₁₂O₄₀. Considerable differences between Lewis and Brønsted acids in the conversion of isopropyl alcohol were observed. The influence of Brønsted acid centres was studied for a group of catalysts with different strength: mixtures of two different heteropolyacids. It was observed that the increasing strength of Brønsted acids centres leads to higher catalytic activity but it does not significantly affect the activation energy of dehydration to propene.     

Acrolein oxidation over 12-molybdophosphates

Previously published work [1–3] done at the University of Oxford in collaboration with Imperial Chemical Industries (ICI) PLC Mond Division is summarized. Various compositions in the catalyst system K_xH_3−xPMo₁₂O₄₀·nH₂O were prepared and characterized by thermal analysis, X-ray powder diffraction, MASNMR, ESR, electron microscopy, ESCA, BET surface-area measurements, and FTIR. No significant solid-solution range between H₃PMo₁₂O₄₀·nH₂O and K₃PMo₁₂O₄₀ was detected, but evidence is presented for an epitaxial H₃PMo₁₂O₄₀ layer on particles of K₃PMo₁₂O₄₀ that does not decompose on calcination at 400°C. The other H₃PMo₁₂O₄₀·nH₂O phases do decompose at 400°C, first reversibly and then irreversibly to MoO₃ and other products. In the compositional range 2.5≤x≤3, the exposed surface is essentially the Keggin unit, which made possible a detailed study of the reaction mechanism and its correlation with reactor studies of activity and selectivity. The epitaxial layer provides a Brønsted acidity at the surface that enhances the selectivity.     

One‐Pot Synthesis of Algae‐Like MoS2/PPy Nanocomposite: A Synergistic Catalyst with Superior Peroxidase‐Like Catalytic Activity for H2O2 Detection

A facile one‐pot synthetic route is reported to prepare algae‐like molybdenum disulfide/polypyrrole (MoS₂/PPy) nanocomposite through a redox reaction between ammonium tetrathiomolybdate and pyrrole monomer under a hydrothermal condition without any other templates. The as‐prepared unique algae‐like MoS₂/PPy nanocomposites are composed of few layer MoS₂ nanosheets, which are covered with PPy. Structural and morphological characterizations of this unique nanocomposite are investigated by Fourier‐transform infrared spectra, Raman spectra, X‐ray diffraction pattern, X‐ray photoelectron spectra, energy‐dispersive X‐ray spectroscopy, and transmission electron microscopy. The as‐prepared MoS₂/PPy nanocomposites exhibit an excellent peroxidase‐like catalytic activity toward the oxidation of 3,3,5,5‐tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H₂O₂) in acetate buffer solution (pH 4.0), which provides a facile strategy for the colorimetric detection of H₂O₂ with a high sensitivity.     

Study of alternating current conduction mechanism in polypyrrole-magnesium ferrite hybrid nanocomposite through correlated barrier hopping model

In the present work, both polypyrrole (PPy) and optimized polypyrrole–magnesium ferrite (PPy-MgFe₂O₄) hybrid nanocomposite were synthesized separately by simple oxidative chemical polymerization method and then structurally characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. The FTIR spectrum of the composite showed the presence of characteristic absorption bands of both PPy and MgFe₂O₄ in the composite confirming interfacial interaction of PPy with MgFe₂O₄. That this interaction is not affected by crystalline behaviour of predominant MgFe₂O₄ particles but that MgFe₂O₄ has embedded in PPy matrix was confirmed by XRD studies. Agglomerated granular spherical morphology of the composite was confirmed by SEM studies. Decrease in AC conductivity of the composite as compared to PPy due to the formation of interfacial heterojunction barrier between p-type PPy and n-type MgFe₂O₄ was confirmed experimentally and well supported theoretically by calculating binding energy, hopping distance and density of states at Fermi level of PPy and the composite as per CBH model.     

EPR investigations of oxidation effects in (V1−xMox)2−δO3

Electron paramagnetic resonance (EPR) investigations has been carried out on the new family of molybdenum doped vanadium sesquioxides (V_1−xMo_x)_2−δO₃. The oxidation effects were monitored from the rate of paramagnetic V⁴⁺ created when the sample is exposed to the air. The effects of the oxidation time, sample temperature, and annealing at 1000 °C under a diluted hydrogen atmosphere on the EPR signal features are analyzed. The V⁴⁺ concentration in the oxidized samples is determined and the relaxation effects driven by the conduction electrons are pointed out from the thermal behaviour of the EPR line features. EPR spectra of all the oxidized samples also reveal a small ferromagnetic contribution strongly correlated with the V⁴⁺ content.     

Absorption spectra of vanadyl ion doped in MgNH4PO4·6H2O (struvite) crystal

Results of Electron Paramagnetic Resonance (EPR) and optical absorption studies of VO²⁺ ion doped in struvite at room liquid nitrogen temperatures are reported. Three preferential V=O bond directions in the crystal have been identified. The optical and EPR data have shown the formation of NH₄(PO₄VO(H₂O)₅ complex in the crystal as a result of VO²⁺ doping. Correlating the optical and EPR data the molecular orbital coefficients are also obtained and discussed.     

Impedance of nonuniform distributed solid protonic electrolyte - polymer systems

The conductivity of mixtures of the solid electrolytes H₃[PMo₁₂O₄₀]·13H₂O and Cs₂H[PW₁₂O₄₀] · 13H₂O follows the predictions of the percolation theory, whereas the electrode impedance in the high frequency range fits well the model of double layer adsorption relaxation. It is concluded that both the adsorption relaxation and the reversibility of the electrodes are related to oxygen transfer, the oxygen ions being the minority charge carriers. Fast oxygen dispersion may occur at the polymer-solid electrolyte interface. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Effect on the EPR and local lattice structure distortion of V3+ ion doping corundum crystal: three models studied

This paper reports on the application of conventional models (the displacement model and the elongation model) and the four-parameter model in detecting the local molecular structure of a coordination complex. By diagonalizing the complete energy matrices, the EPR parameters and optical spectra as well as the local distortion structure for V³⁺ ion in the Al₂O₃ : V³⁺ system have been studied using the three models. The four-parameter model is shown to provide a significant improvement over the conventional models. Based on these calculations, it was found that when V³⁺ is doped in Al₂O₃, the distance between upper and lower ligand oxygen plane increases by 0.16097 Å and the shifts of O^2? ions in the upper and lower ligand oxygen planes expand by 0.07076 Å and 0.004066 Å, respectively. To understand the detailed physical and chemical properties of the doped Al₂O₃, the contributions of the spin-orbit coupling to the zero-field splitting for V³⁺ ion are investigated.     

Study of DC conductivity and relative magnetic permeability of nanoparticle NiZnFe2O4/PPy composites

The DC conductivity and the relative magnetic permeability have been measured as functions of temperature for five powder samples of nanoparticle ferrites (Ni_xZn_1−xFe₂O₄; x=0, 0.25, 0.5, 0.75 and 1), a pure polypyrrole (PPy) powder sample and many composite samples prepared by mixing different ratios of the ferrites to PPy. By comparing the results it is found that there is an obvious increase in DC conductivity of the ferrite/PPy composite samples compared to the corresponding pure ferrite samples, whereas compared to the pure PPy sample there is a decrease in DC conductivity. On the contrary, the magnetic permeability of the composites is higher than that of the pure PPy sample and lower than that of the pure ferrite samples as was expected.     

XPS study of vanadium surface oxidation by oxygen ion bombardment

Oxidation of vanadium metal surfaces at room temperature by low-energy oxygen ion beams is investigated by X-ray photoelectron spectroscopy (XPS). It is observed that ion-beam irradiation of clean V results in formation of thin oxide layer containing vanadium in oxidation states corresponding to VO, V₂O₃, VO₂ and V₂O₅ oxides. The composition of the products of ion-beam oxidation depends markedly on oxygen ion fluence. The results of angle-resolved XPS measurements are consistent with a structure of oxide film with the outermost part enriched in V₂O₅ and VO₂ oxides and with V₂O₃ and VO oxides located in the inner region of the oxide layer.     

X-ray photoelectron spectroscopy of Sm-doped CaO-MgO-Al2O3-SiO2 glasses and glass ceramics

Sm³⁺ doped CaO-MgO-Al₂O₃-SiO₂ glass and glass ceramics have been prepared. The diopside crystal (CaMgSi₂O₆) was identified in the glass ceramics by X-ray diffraction analysis. X-ray photoelectron spectra of the glass and glass ceramics were measured by a monochromatised Al-Kα XPS instrument. Sm 3d core level spectra for the Sm doped samples showed that Sm ions are predominantly in the Sm (III) state in glass and glass ceramics. The O 1s core spectra could be fitted by summing the contributions from bridging oxygen (BO) and non bridging oxygen (NBO) for samarium undoped glass, BO, NBO and Si-O-Sm for the doped glass. The O 1s XPS spectrum of undoped glass ceramics was curve fitted with BO and NBO in glass phase, as well as SiOSi, SiOMg and SiOCa in diopside. In addition to the five components above mentioned, SiOSm in diopside also appeared in O 1s XPS spectra of samarium doped glass ceramics. According to the fitting results, we demonstrate that the Sm₂O₃ exist in glass network as a glass modifier. After heat treatment, nearly all the Sm³⁺ existed in diopside phase as the substitution for Ca²⁺.     

Synthesis of RGO/Cu8S5/PPy Composite Nanosheets with Enhanced Peroxidase‐Like Activity for Sensitive Colorimetric Detection of H2O2 and Phenol

In this paper, a novel strategy for the fabrication of reduced graphene oxide (rGO)/Cu₈S₅/polypyrrole (PPy) composite nanosheets with Cu₈S₅ nanoparticles and PPy layer anchored on the surface of rGO as peroxidase‐like nanocatalyst is reported. During the synthesis, graphene oxide (GO)/CuO composite nanosheets are prepared first and used as templates, then the sulfuration of CuO and polymerization of pyrrole are accompanied with the reduction of GO, resulting in ternary rGO/Cu₈S₅/PPy composite nanosheets. The synthesized Cu₈S₅ nanoparticles with a diameter in the range from tens to hundreds of nanometers are dispersed within PPy decorated rGO nanosheets. The resultant ternary rGO/Cu₈S₅/PPy composite nanosheets exhibit a higher peroxidase‐like catalytic activity toward the oxidation of 3,3′,5,5′‐tetramethylbenzidine in the presence of H₂O₂ than GO/CuO and rGO/CuS composite nanosheets, revealing a synergistic effect on their activity. The as‐prepared rGO/Cu₈S₅/PPy platform provides a simple colorimetric approach for the detection of H₂O₂ and phenol with a high sensitivity. This work offers a new way for the fabrication of rGO‐based nanocomposite with superior enzyme‐like activity, which displays great potential applications in biocatalysis and environmental monitoring.     

Characterization of Cu1.4Mn1.6O4/PPy composite electrodes

In this work, we have studied the multilayered polypyrrole(PPy)/oxide composite electrode on glassy carbon (GC) having the structure GC/PPy/PPy(Cu_1.4Mn_1.6O₄)/PPy using X-ray Photoelectron Spectroscopy and Mn K-edge and Cu K-edge XANES and EXAFS. The mixed oxide particles have been incorporated into the PPy matrix simultaneously to the electropolymerization of Py from a solution containing 0.1 M Py + 0.15 M KCl + Cu_1.4Mn_1.6O₄. The XPS data have shown that, prior to the incorporation of the oxide into the PPy matrix, it contains Cu⁺, Cu²⁺, Mn³⁺ and Mn⁴⁺. The XPS, XANES and EXAFS results have shown that when the oxide is incorporated into the PPy matrix, the Cu⁺ present in the original oxide suffers dismutation to give Cu²⁺ and metallic Cu. The metallic Cu is segregated out of the spinel structure. The Mn K-edge XANES and EXAFS data show that, after the incorporation into the PPy matrix, Mn is present as Mn³⁺ and Mn⁴⁺ occupying octahedral sites in a spinel-related structure while the Cu K-edge XANES and EXAFS data indicate that copper occupies tetrahedral sites predominantly in that structure but having a large degree of disorder in the second and higher coordination shells.     

Monodisperse Polypyrrole Nanoparticles Prepared via γ‐Ray Radiolysis of Water: An Efficient Near‐Infrared Photothermal Agent for Cancer Therapy

Biosafe nanoparticles with strong near‐infrared (NIR) light photothermal conversion effect can bring effective hyperthermia as one of the promising approaches in cancer therapy. In this work, a new facile and green preparation method of polypyrrole (PPy) nanoparticles based on ⁶⁰Co γ‐ray radiation on a simple air‐saturated strong acidic aqueous solution of pyrrole (pH ≤ 1) is studied. According to the MCAP‐FACSIMILE simulation on the concentrations of the radiolysis products of water at the presence of H⁺ and O₂, the main strong oxidative radiolysis products · OH and H₂O₂ rapidly induce the polymerization of pyrrole. The size of the prepared PPy nanoparticles is about several tens of nanometers and can be controlled by the pH, the concentration of the stabilizer poly(vinyl alcohol), and the absorbed dose rate (the amount of energy absorbed per unit mass of the irradiated material within per unit of time). The PPy nanoparticles show rapid and remarkable NIR (808 nm) photothermal conversion efficiency up to 40.1% in water. Furthermore, the in vitro and in vivo experiments confirm that the prepared PPy nanoparticles exhibit enough strong NIR photothermal effect in tumor cells (4T1 and HeLa) and show a promising prospect as the NIR photothermal agent for the future cancer therapy.     

Spectroscopy of mixed molybdenum-vanadium oxides and catalytic oxidation of toluene

The EPR, FTIR, and XRD techniques are used to examine individual and mixed vanadiummolybdenum oxides of composition (1 ? x)V₂O₅: xMoO₃ prepared by coprecipitation. The phase and structural heterogeneity of these oxides is characterized. The oxidation of toluene with air oxygen to maleic anhydride on V₂O₅ and mixed oxides involves singlet oxygen ¹O₂. A correlation between the content of V(IV) ions in the mixed catalysts and the amount desorbed ¹O₂ is observed. It is demonstrated that the oxidation of toluene causes changes in the spatial distribution of the V(IV) ions in the matrix and the phase composition of the samples.     

Characterization and transport properties of intercalated polypyrrole-vanadium pentoxide xerogel nanocomposite

Layered vanadium pentoxide (V₂O₅) xerogel is used as host material for the synthesis of conducting polypyrrole intercalated nanocomposites. Powder X-ray diffraction spectra show the increase of interlayer space of V₂O₅ xerogel indicating the formation of polypyrrole in the interlayer space of the host. Thermal stability of intercalated polypyrrole is superior compared to bulk polypyrrole. Dc conductivity of the xerogel follows Arrhenius type temperature dependence while the nanocomposites exhibit Mott's three-dimensional variable range hopping. Optical band gap decreases with intercalation of polypyrrole.     

Sonochemical synthesis and structural characterization of an organic-inorganic nanohybrid based on a copper-dithiocarbamate complex and PMo12O403− polyanion as a novel sonocatalyst

A new organic-inorganic nanohybrid compound, ([Cu{(HOCH₂CH₂)₂NCS₂}₂]₃[PMo₁₂O₄₀] (1)), has been prepared by sonochemical technique using copper(II) dithiocarbamate complex and a Keggin-type polyoxomolybdate in this research. FT-IR, XRD, FE-SEM, TEM, EDX, UV–Vis, TGA, BET, and single crystal XRD analyses were applied to describe the properties of the composition of the nanohybrid. Compound (1) is composed of [PMo₁₂O₄₀]^3– building blocks and [Cu{(HOCH₂CH₂)₂NCS₂}₂]¹⁺ cationic moieties, and electrostatic forces and substantial hydrogen-bonding interactions were applied to pack them; and consequently, a three dimensional supramolecular framework was made based on single-crystal X-ray diffraction patterns. FE-SEM and TEM images approved the morphology of the nanohybrid sample to be extremely penetrable. Very good sonocatalytic performance is shown by this supramolecular nanohybrid in the degradation of Rhodamine B (RhB), which is a cationic organic dye. The results showed complete degradation of cationic RhB (25 mg/L) within 70 min with the rate constant of 0.039min⁻¹ in the presence of nanohybrid (1) and H₂O₂ (4 mmol/L). Also, sonocatalytic activity of the nanohybrid (1) was higher than H₃PMo₁₂O₄₀, showing that the combining Cu(DEDTC)₂ complex with H₃PMo₁₂O₄₀ could be an excellent choice to improve its sonocatalytic activity. The used nanohybrid (1) can be recycled after easily removing from the reaction media by centrifuging, and there was no considerable loss of catalytic activity and retention of the structure.     

Photo-catalytic H2 evolution over a series of Keggin-structure heteropoly blue sensitized Pt/TiO2 under visible light irradiation

We report the visible light-induced hydrogen generation over a series of Keggin-structure heteropoly blue (HPB) anions (PW₁₂O₄₀³⁻, phosphotungstic blue (PTB), GeW₁₂O₄₀⁴⁻ (GTB), SiW₁₂O₄₀⁴⁻ (STB), BW₁₂O₄₀⁵⁻ (BTB)) sensitized Pt/TiO₂ photo-catalysts. The sensitization of TiO₂ by HPB was certified using photo-electrochemical measurements and UV-vis absorption spectra. PTB showed the most pronounced sensitization effect for TiO₂ in those HPB anions and Pt/TiO₂-PTB showed the highest hydrogen generation activity. The sensitization of TiO₂ was significantly dependent on the reduction potential of HPA, which was determined by the kind of central atom in HPA.