Development of nanosized oxide composites for catalytic CO oxidation

Abstract

Effect of composition and morphology on the catalytic activity in CO oxidation of Co-, Cu-, Ce-containing oxide materials prepared by citrate method has been studied. It was shown that composites consist of well-dispersed oxides Со₃O₄, CuO and CeO₂ with average crystallite sizes 13–24?nm, 14–26?nm and 3–7?nm, respectively. It was established that Co-, Cu-, Ce-oxide nanocomposites demonstrated higher catalytic activity in CO oxidation in comparison with individual oxides. The ternary oxide system Со₃O₄–CuO–СеO₂ (Со:Cu:Се=10:1:1) was found to possess the highest catalytic activity due to formation finely dispersed particles of CuO and СеO₂ oxides simultaneously with nanosized Со₃O₄ species and formation of large quantity of mobile oxygen at the interfaces between the oxide nanoparticles.     

Structural characterization of ionomer glasses by multinuclear solid state MAS-NMR spectroscopy

In the present study we report the results of ²⁹Si, ²⁷Al, ³¹P and ¹⁹F magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) of 4.5SiO₂-3Al₂O₃-1.5P₂O₅-(5−z)CaO-zCaF₂ glasses with z = 0-3 to elucidate the effect of fluoride content on the glass structure. The ²⁹Si MAS-NMR spectra gave a chemical shift of about −90 ppm corresponding to Q³(3Al) and Q⁴(3Al). The ²⁷Al MAS-NMR showed a large broad central peak around 50 ppm, which is assigned to four-coordinated Al linked via oxygen to P. A shoulder around 30 ppm and a small peak at about 0 to −10 ppm appeared in the ²⁷Al MAS-NMR spectra of the glasses on increasing the fluoride content assigned to five-coordinated and six-coordinated Al species, respectively. The ³¹P MAS-NMR spectra indicated the presence of Al-O-P bonds. The ³¹P chemical shift decreased with increasing fluoride content as a result of calcium being complexed with fluoride. This resulted in a reduction of the number of available cations to charge balance non bridging oxygens in phosphorus and an increase in the number of Al-O-P bonds being formed, instead. The ¹⁹F spectra indicated the presence of Al-F-Ca(n) and F-Ca(n) species in all the glasses containing fluoride as well as an additional Si-F-Ca(n) species in the glasses with higher fluoride content.     

Evaluation of glass ionomer cements properties obtained from niobium silicate glasses prepared by chemical process

Glass ionomer cements (GICs) are glass and polymer composite materials. These materials currently find use in the dental field. The purpose of this work is to obtain systems based on composition 4.5SiO₂–3Al₂O₃–XNb₂O₅–2CaO to be used in Dentistry. The systems were prepared by chemical route at 700 °C. The results obtained by XRD and DTA showed that all systems prepared are glasses. The structures of the obtained glasses were compared to commercial material using ²⁷Al and ²⁹Si MAS NMR. The analysis of MAS NMR spectra indicated that the systems developed and commercial material are formed by SiO₄ and AlO₄ linked tetrahedra. The properties of glass ionomer cements based on the glasses prepared with several niobium contents were studied. Setting and working times of the cement pastes, microhardness and diametral tensile strength were evaluated for the experimental GICs and commercial luting cements. It was concluded that setting time of the cement pastes increased with increasing niobium content of the glasses (X). The properties to the GICs such as setting time and microhardness were influenced by niobium content.     

Chemical interactions between glass fibres and cement

The rate of removal of ions from a zircono-silicate glass fibre by aqueous extracts of Portland cement has been experimentally determined. To accelerate the reaction, digestions have been carried out at elevated temperatures. The reaction rate for the glass composition investigated favours a log-time dependence. The reacted fibres show a relative enrichment of Zr on the surface, although no definite zirconia-containing phase has been identified. Soda-lime and borosilicate glass fibres which have been similarly examined for comparison purposes, required considerably shorter periods of digestion to produce suitable concentrations of glass breakdown products. The reaction rate for soda-lime glass appears to be largely controlled by Na⁺ diffusion. With borosilicate glass, no simple interpretation can be applied to the breakdown mechanism because of the complex nature of the reaction products. The significance of these results in the interpretation of the durability of zircono-silicate glass fibre cement composites is discussed.     

Prediction of piezoelectric properties of crystal-ceramic composites

In the framework of the model of a piezoelectrically active ferroelectric crystal-ceramic composite of the 0–3 type, the concentration dependences of the effective piezoelectric properties of this composite are determined. Two examples of such composites are discussed: the single-domain PbTiO₃ crystal (Pb_{1 ? x}Ca _itx)TiO₃ ceramic and the polydomain PbTiO₃ crystal (Pb_{1 ? x}Ca_x)TiO₃ ceramic. Based on these examples, we analyzed the effect of the 90° domain structure, the form of crystalline inclusions, the remanent polarization of the ceramics, molar Ca concentration (x), and other factors of the effective piezoelectric coefficients e _3j ^* and d _3j ^* and and their anisotropy. The relation between the piezoelectric polarization of the composite to single-domain inclusions and the anisotropy of its piezoelectric coefficients is also studied.     

Metallurgy of bronze-process A-15 superconducting composites

Several aspects of the metallurgy of multifilamentary composites of Nb₃Sn, V₃Ga, and other A-15 superconductors produced by the bronze process are reviewed. Among the factors discussed are phase diagrams, reaction kinetics, alloying effects, and microstructure-properties relations.     

Reaction sintering of gel derived ceramic composites

Fibrous SiC/SiO₂ matrix monolithic composites which have previously been fabricated using the sol-gel technique showed difficulty in densification. To enhance the sinterability of the SiC/SiO₂ composites, reactive ceramic species including furfuryl alcohol, alumina, and chromia were added to SiO₂ sol which was obtained by hydrolyzing tetraethoxysilane (TEOS). Physical and mechanical properties of these composites as a function of the reactive additives and of temperatures are presented.     

Effect of charge memory in organic composites

The effect of charge memory in composites based on polymer molecules has been investigated. Resistive switchings in sandwich samples prepared by lamination from commercially available polymers (polystyrene and poly(2,3-dihydrothieno-1,4-dioxine)-poly(styrene sulphonate) are analyzed. It is shown that the characteristic switching times in the composite samples reach several nanoseconds and the number of switchings exceeds 10⁶. Switchings are observed in electric fields much below the breakdown threshold, which indicates the absence of destructive processes in the polymer.     

Formation of Co nanoparticles in metal-carbon composites

The effect of the IR-pyrolysis intensity on the phase formation and structure of Co nanoparticles in a matrix of IR-pyrolyzed polyacrylonitrile has been investigated.     

Photo-chargeable titanium/vanadium oxide composites

Titanium and vanadium oxides were prepared by a sol-gel technique and supercritical drying and evaluated for light energy conversion and storage in a wet-type photoelectrochemical cell. Ultraviolet light irradiation to the nanocrystalline titania aerogel (anatase) produced significant photocurrents, and the maximum incident photon-to-current conversion efficiency (IPCE) attained 37.1%. From cyclic voltammetry, the vanadium oxide gel was shown capacitive for charge storage, associated with the shcherbinaite structure and V⁴⁺/V⁵⁺ redox pair. Photopotential responses revealed that coupled TiO₂/V₂O₅ composites not only were photo-chargeable but also exhibited a greater discharging capacity than the TiO₂ or V₂O₅ alone. The discharging capacity was remarkably reduced in the presence of dissolved oxygen. Results demonstrate that in the hybrid TiO₂/V₂O₅ system the V₂O₅ serves to accumulate photoelectrons generated by the illuminated TiO₂ during the photo-charging process. A schematic energy diagram that describes the band structure of the composite semiconductor is proposed.     

Biodegradation behavior of chitosan/calcium phosphate composites

A variety of biomimetic materials with structural and mechanical equivalence to bone have been developed to repair bone defects. Chitosan/calcium phosphate composites composed of bioactive calcium phosphate and flexible chitosan were made by a simple mixing-and-heating method. Mechanical properties, morphology, phase composition, and weight change after immersion in Hanks’ solution were evaluated. Experimental results showed that the formation of pores/cracks on immersed sample surface obviously depended on the calcium phosphate content and immersion time. The immersion time imposed in this study did have a statistically significant effect on mechanical properties. When immersed for 90 days in Hanks’ solution, the strength of immersed composites containing 10 wt/v% calcium phosphate with the initial strength of 27 MPa was about 2 MPa, having a reduction of 92%. Based on the above results, the organic–inorganic hybrid composites with high initial strength might be an acceptable material candidate for bone tissue repair.     

Polarization characteristics of polydimethylsiloxane-based composites during first-order phase transitions

Crystallization of polydimethylsiloxane (PDMS) filled with insulating carbon has been studied in an inhomogeneous temperature field. It is shown that the material polarization during the melt-crystal transitions depends on the concentration of the filler. This results in a decrease of the temperature coefficient of polarization with the concentration of dielectric carbon. The temperature dependences of thermodynamic functions are calculated for the crystallization range of the crystals under study.     

Mössbauer study of multiphase iron oxide composites

A representative nanocomposite made of ferrimagnetic γ-Fe₂O₃ and antiferromagnetic α-Fe₂O₃ nanoparticles with estimated weight fractions of ～90 and ～10%, respectively, and grown in transparent silica xerogels has been characterized by Mössbauer spectroscopy with respect to the temperature. The changes of the hyperfine parameters of the different subspectral components have been compared with other macroscopic magnetic and structural measurements. This comparison has allowed us to identify two different superparamagnetic transitions, located at ～50 K and at ～250 K, as well as the phases involved in each transition.     

Investigation of the hydrogen capacity of composites based on ZnOCu

The composites ZnOCuH(D) saturated with hydrogen (deuterium) to a content of ～1 wt % are investigated by the neutron scattering methods. Upon cooling of the samples (the ZnO matrix containing Cu crystals ～10 nm in size) from 300 to 4 K, hydrogen (deuterium) is condensed on the cluster surface and penetrates inside the clusters in which the atomic hydrogen content with respect to copper can be as high as 30% at 20 K. Simultaneously, hydrogen fills nanopores of the ZnO matrix. It is revealed that, at temperatures of 90–300 K, approximately one-third of the hydrogen amount participates in the fast diffusion (the diffusion constant is approximately equal to 8 × 10^?5 cm²/s) and the other two-thirds are immobilized. At 20 K, the fraction of mobile hydrogen decreases to ～10%. An analysis of the results obtained demonstrates that the energy barriers retaining hydrogen in defect regions are relatively low.     

Interfacial polarization in piezoelectric fibre–polymer composites

Polymer composites of an epoxy resin matrix filled with PZT fibres were studied by means of dielectric spectroscopy in the frequency range 0.1 Hz to 100 kHz and temperature interval from 80 °C to 170 °C. An interfacial or a Maxwell–Wagner–Sillars relaxation process was revealed in the frequency range between 0.1 Hz and 10 Hz and temperatures above the glass transition. This interfacial relaxation was found to follow the Debye law for the distribution of relaxation times.     

Optical excitations in C60/PPV composites

Charge transfer (CT) and subsequent exciton dissociation are essential for organic donor/acceptor solar cells. We analyze these processes by calculating optical intrachain and CT excitations in a model donor/acceptor system. It is found that the increased electron affinity of low-gap polymers may seriously inhibit the CT efficiency. The consequences for exciton dissociation via tunneling are computed.     

Electrical conductivity of new zinc phosphate glass/metal composites

Zinc phosphate–glass/metal composites have been successfully prepared. Glass with composition of 45 mol% ZnO–55 mol% P₂O₅ (ZP) has been filled with metallic powders (nickel and cobalt). The glass matrix thermal stability has been assessed by differential thermal analysis technique. The morphology has been examined by scanning electronic microscopy, showing almost homogenous composites. Comparison between the measured and calculated densities as a function of metallic content exhibits a good coherence and allows the estimation of porosity inside the composites. X-ray diffraction analysis has revealed that the ZP-matrix phase is amorphous when the temperature treatment is below the glass transition temperature T_g. However, the principal peaks observed in the case of the composites have been assigned to the metallic crystals of nickel or cobalt fillers. It has been found that the phosphate glass phase is not affected by the growing of the metallic network. The electronic conductivity measurements versus filler volume fraction have been investigated for the first time on phosphate–glass/metal composites. This study has shown the occurrence of a conducting transition at around 30% filler volume fraction. The obtained result has been interpreted on the basis of the statistical percolation theory frame.     

Hydrothermal synthesis of MoO3 nanobelt‐graphene composites

A composite of graphene sheets decorated with molybdenum trioxide (MoO₃) nanobelts has been fabricated via a facile and efficient hydrothermal route in the presence of NaCl. The structure, morphology of these promising composites were investigated by means of field‐emission scanning electron microscopy (FE‐SEM), transmission electron microscopy (TEM), X‐ray diffraction (XRD), Raman spectroscopy and thermogravimetric (TG) analysis. FESEM and TEM studies suggest the presence of uniform crystalline MoO₃ nanobelts and graphene sheets in as‐prepared hybrid materials. XRD and Raman results confirm the reduction of graphite oxide (GO) sheets to graphene sheets accompanying by the formation of MoO₃ nanobelts. Moreover, thermal properties of GO and MoO₃ nanobelt‐graphene composites reveal that thermal stability of the obtained MoO₃ nanobelt‐graphene composites is obviously higher than that of GO due to the transformation of GO sheets to highly stable graphene sheets in the hybrids. This work could provide new insights into the fabrication of high quality MoO₃‐graphene hybrid nanomaterials and facilitate their potential applications in different fields. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Formation of low density polyethylene/hollow glass microspheres composites

Hydrogen is used to absorb heavy particle radiation, which is the most damaging radiation in space for humans. Low density polyethylene/hollow glass microsphere composites have been suggested as a possible radiation shield because of the high concentration of hydrogen and the low gravimetric density of the microspheres. Composites pressed under 3.90 MPa (566 psi) and 120 °C have the highest probability of success thus far compared to polymers pressed at higher pressures and lower temperatures. Hollow glass microspheres made of borosilicate glasses do not break as easily as hollow glass microspheres made of aluminosilicate glasses. A smooth microsphere surface is better than a rough surface because it distributes the force more evenly, resulting in a more hydrostatic stress environment.     

Synthesis and characterization of mesoporous,tungsten-containing molecular sieve composites

Mesoporous, tungsten-containing molecular sieve (W-SBA-15) composites were successfully synthesized via one-step hydrothermal processing using tetraethyl orthosilicate (TEOS) as the silica precursor, sodium tungstate as the tungsten precursor, and pluronic P123 triblock polymer (EO₂₀PO₇₀EO₂₀, M_av = 5800) as a structure-directing reagent. The influence of various synthesis factors, such as TEOS/sodium tungstate (Si/W) molar ratios, stirring solution temperatures, TEOS pre-hydrolysis time, and crystallization temperatures, on the structure of the W-SBA-15 composite were investigated. The prepared materials were characterized by using X-ray diffraction (XRD), infrared spectroscopy (IR), diffuse reflectance ultraviolet–visible spectroscopy (DR UV–vis), scanning electron microscopy (SEM), and nitrogen adsorption–desorption measurements. The results showed that all the W-SBA-15 composite materials retained the mesopore structure of SBA-15 and the tungsten oxide species successfully substituted silica in the framework.