Thermodynamic Properties of Niobium Oxides

Thermodynamic properties of three oxides of niobium have been measured using solid state electrochemical cells incorporating yttria-doped thoria (YDT) as the electrolyte in the temperature range T = (1000 to 1300) K. The standard Gibbs energies of formation of NbO, NbO2, and NbO2.422 from the elements can be expressed as: Delta(f)G(NbO)(o) +/- 547/J . mol(-1) = -414 986 + 86.861(T/K) Delta(f)G(NbO2)(o) +/- 548/J . mol(-1) = -779 864 + 164.438(T/K) Delta(f)G(NbO2.422)(o) +/- 775/J . mol(-1) = -911 045 + 197.932(T/K) The results are discussed in comparison with thermodynamic data reported in the literature. The new results refine data for NbO and NbO2 presented in standard data compilations. There are no data in thermodynamic compilations for NbO2.422 (Nb12O29). In the absence of the heat capacity and enthalpy of formation measurements, only the Gibbs energy of formation of NbO2.422 can be assessed. The free energy of formation of stoichiometric Nb2O5 is evaluated on the basis of measurements on NbO2.422 and information available in the literature on phase boundary compositions and isothermal variation of nonstoichiometric parameter with oxygen potential for Nb2O5-x. The results suggest a minor revision of data for Nb2O5. A minimum in the Gibbs energy of mixing for the system Nb-O occurs in the nonstoichiometric domain of Nb2O5-x with x = 0.036.

Niobium oxides and niobates physical properties

Niobium oxides have been pointed as an alternative to tantalum in the production of solid electrolytic capacitors, with advantages regarding the dielectric constant, density and price. In this work, it is intended to create a new family of niobium oxides based capacitors, adapting the technology and production line currently used with tantalum. Despite the known potentialities of niobium oxides, and many types of niobates, in several technological applications, the understanding of these oxide systems is still noticeably insufficient. Hence, a careful bibliographic review is shown, which evidences the complexity of these materials, the difficulty in identifying of their different phases and polymorphs, as well as in the interpretation of their properties. In this context, several fundamental studies on niobium oxides are presented, namely structural, microstructural, optical and electrical characterizations, which allow not only to contribute in an important way for the general knowledge of the physical properties of these materials, but also to advance to a sustained development of the niobium oxides based solid electrolytic capacitors. Several processing parameters were studied, clearing the way towards the creation of a prototype. It was also decided to perform a preliminary study on the synthesis and characterization of other oxide systems based in niobium, namely rare-earth orthoniobates (RENbO4), which interest has been related to their optical properties and protonic conductivity. Hence, single and polycrystalline samples of RENbO4 were synthesized and characterized structural, optical and electrically, leaving open an interesting future work.

The Growing Impact of Niobium in Organic Synthesis and Catalysis

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Raman scattering, differential scanning calorimetry and Nd3+ spectroscopy in alkali niobium tellurite glasses

Alkali niobium tellurite glasses have been prepared and some of their properties measured by differential scanning calorimetry and Raman scattering. The vitreous domain was established in the pseudo ternary phases diagram for the system TeO2-Nb2O5-(0.5K(2)O-0.5Li(2)O). Raman scattering shows that for samples in the TeO2 rich part of the phase diagram the vitreous structure is composed essentially of (TeO4) units connected by the vertices, as in the alpha-TeO2 crystal. The addition of alkali and niobium oxides causes depolymerization to occur with structures composed essentially of (TeO3) and (NbO6) units. Samples with the composition (mol%) 80TeO(2)-10Nb(2)O(5)-5K(2)O-5Li(2)O, stable against crystallization, were prepared containing up to 10% mol Nd3+. The addition of this oxide increases the rigidity of the vitreous network shifting characteristic temperatures to higher temperatures. For the 10% Nd3+ sample amorphous phase separation is assumed to exist from the observation of two glass transition temperatures. Spectroscopic properties such as Judd-Ofelt Omega(lambda) intensity parameters, radiative emission probabilities, and induced emission cross sections were calculated. From these results and also from the emission quenching observed as a function of Nd3+ concentration, we suggest that these glasses could be utilized in optical amplifying devices. (C) 1999 Elsevier B.V. B.V. All rights reserved.

SEM/EDX and XPS studies of niobium after electropolishing

The studies of niobium after electrochemical polishing EP in sulfuric-methanesulfonic acid mixture were performed. The NbOx/Nb surface was studied by SEM/EDX and XPS methods to find out the chemical composition of the oxygen-induced structures. Specifically the XPS results obtained after EP treatment indicate prevailing part of oxygen with niobium oxides on the sample surface. In order to correctly interpret these structures the photoelectron spectra of main niobium oxides were analyzed, and the spectra of internal Nb 3d and O 1s electronic states and valence band spectra were measured for them. (C) 2012 Elsevier B. V. All rights reserved.

Spectroscopic studies of SnO2 doped with Cr, Co or Nb

SnO2 ceramics doped with different amounts of Co, Cr or Nb were investigated using visible and infrared spectroscopy at room temperature. Based on the observed d-d transitions the valence states of incorporated dopants were determined. Values of the optical band-gap were calculated in all samples. The infrared spectra of the samples displayed variations in the position, relative intensity and width of the bands, which were attributed to the presence of dopants.

Ativação da ligação c-h no metano por espécies MOm n+ (M = Nb, Fe; m = 0, 1; n = 0, 1, 2): estrutura eletrônica, mecanismo de reação e reatividade

The need for renewal and a more efficient use of energy resources has provided an increased interest in studies of methane activation processes in the gas phase by transition metal oxides. In this respect, the present work is an effort to assess , by means of a computational standpoint, the reactivity of NbOm n+ and FeOm n+ (m = 1, 2, n = 0, 1, 2) oxides in the activation process of the methane C-H bond, which corresponds to the first rate limiting step in the process of converting methane to methanol. These oxides are chosen, primarily, because the iron oxides are the most experimentally studied, and iron ions are more abundant in biological mediums. The main motive for choosing niobium oxides is the abundance of natural reserves of this mineral in Brazil (98%), especially in Minas Gerais. Initially, a thorough investigation was conducted, using different theoretical methods, to analyze the structural and electronic properties of the investigated oxides. Based on these results, the most reliable methodology was selected to investigate the activation process of the methane C-H bond by the series of iron and niobium oxides, considering all possible reaction mechanisms known to activate the C-H bond of alkanes. It is worth noting that, up to this moment and to our knowledge, there are no papers, in literature , investigating and comparing all the mechanisms considered in this work. I n general, the main results obtained show different catalytic tendencies and behaviors throughout the series of monoxides and dioxides of iron and niobium. An important and common result found in the two studies is that the increase in the load on the metal center and the addition of oxygen atoms to the metal, clearly favor the initial thermodynamics of the reaction, i.e., favor the approach of the metal center to methane, distorting its electron cloud and, thereby, decreasing its inertia. Comparing the two sets of oxides, we conclude that the iron oxides are the most efficient in activating the methane C-H bond. Among the iron oxides investigated, FeO + showed better kinetic and thermodynamic performance in the reaction with methane, while from the niobium oxides and ions NbO 2+ and NbO2 2+, showed better catalytic efficiency in the activation of the methane C-H bond.

Spectroscopic, morphological and electrochromic characterization of layer-by-layer hybrid films of polyaniline and hexaniobate nanoscrolls

The combination of semiconducting oxides and polyaniline in the nanoscale range may result in hybrid materials having enhanced properties, such as electrochromism and charge capacity. This paper reports the spectroscopic, morphological and electrochromic characterization of hybrid films made up of hexaniobate one-dimensional (1D) nanoscrolls and polyaniline prepared by the layer-by-layer assembly technique (LbL). Secondary electron imaging and backscattered electron imaging techniques performed using a scanning electron microscope showed that polyaniline is adsorbed on the hexaniobate nanoscrolls, which confirms the combination of the components in the nanoscale domain. UV-VIS-NIR electronic spectra of the LbL hybrid films showed the absorption tail in the NIR region, assigned to delocalized polarons of the polyaniline. Resonance Raman spectra in the 1000-1700 cm(-1) range indicated that hybrid films present a higher relative intensity of polaron bands at 1337 and 1508 cm(-1) than pristine polyaniline in the emeraldine salt form. These results suggest that hexaniobate nanoscrolls induce a secondary doping of polyaniline. The cyclic voltammetry (CV) data for the hybrid film showed a specific capacity of 870 C cm(-3). According to CV results, the synergistic effect on charge storage properties of the hybrid material is attributed to the enhanced electroactivity of the hexaniobate component in the LbL film. Spectroelectrochemical experiments showed that the electrochromic efficiencies at 420 nm are ca. -41 and 24 cm(2) C-1 as the potential changes from 0.8 to -0.9 V and from -0.9 to -1.8 V, respectively, whereas at 800 nm the efficiencies are ca. -55 and 8 cm(2) C-1 for the same potential ranges. The electrochromic efficiencies and multi-colour character of the LbL film of hexaniobate nanoscrolls and polyaniline indicate that this novel hybrid material is an interesting modified electrode for electrochromic devices.

Hydrothermally formed functional niobium oxide doped tungsten nanorods

Nanorod forms of metal oxides is recognised as one of the most remarkable morphologies. Their structure and functionality have driven important advancements in a vast range of electronic devices and applications. In this work, we postulate a novel concept to explain how numerous localised surface states can be engineered into the bandgap of niobium oxide nanorods using tungsten. We discuss their contributions as local state surface charges for the modulation of a Schottky barrier height, relative dielectric constant and their respective conduction mechanisms. Their effect on the hydrogen gas molecule interactions mechanisms are also examined herein. We synthesised niobium tungsten oxide (Nb17W2O25) nanorods via a hydrothermal growth method and evaluated the Schottky barrier height, ideality factor, dielectric constant and trap energy level from the measured I-V vs temperature characteristics in the presence of air and hydrogen to show the validity of our postulations.

Thin films and nanostructures of niobium pentoxide : fundamental properties, synthesis methods and applications

As one of the transition metal oxides, niobium pentoxide (Nb2O5) offers a broad variety of properties that make it a potentially useful and highly applicable material in many different areas. In comparison to many other transition metal oxides, Nb2O5 has received relatively little attention, which presents a significant opportunity for future investigations aimed at fundamentally understanding this material and finding new and interesting applications for it. In this article, a general overview of Nb2O5 is presented which focuses on its fundamental properties, synthesis methods and recent applications, along with a discussion on future research directions relevant to this material.

Sol-gel synthesis and characterization of cubic bismuth zinc niobium oxide nanopowders

Bismuth zinc niobium oxide (BZN) was successfully synthesized by a diol-based sol-gel reaction utilizing metal acetate and alkoxide precursors. Thermal analysis of a liquid suspension of precursors suggests that the majority of organic precursors decompose at temperatures up to 150°C, and organic free powders form above 350°C. The experimental results indicate that a homogeneous gel is obtained at about 200°C and then converts to a mixture of intermediate oxides at 350–400°C. Finally, single-phased BZN powders are obtained between 500 and 900°C. The degree of chemical homogeneity as determined by X-ray diffraction and EDS mapping is consistent throughout the samples. Elemental analysis indicates that the atomic ratio of metals closely matches a Bi1.5ZnNb1.5O7 composition. Crystallite sizes of the BZN powders calculated from the Scherrer equation are about 33–98 nm for the samples prepared at 500–700°C, respectively. The particle and crystallite sizes increase with increased sintering temperature. The estimated band gap of the BZN nanopowders from optical analysis is about 2.60–2.75 eV at 500-600°C. The observed phase formations and measured results in this study were compared with those of previous reports.

Synthesis of Layered Perovskite Oxides, ACa2-xLaxNb3-xTix010 (A = K, Rb, Cs), and Characterization of New Solid Acids, HCa2-xLaxNb3-xTixOlo (0 < x d 2), Exhibiting Variable Bronsted Acidity?

Layered perovskite oxides of the formula ACa~,La,Nb3-,Ti,010 (A = K, Rb, Cs and 0 < x d 2) have been prepared. The members adopt the structures of the parent ACazNb3010. Interlayer alkali cations in the niobium-titanium oxide series can be ion-exchanged with Li+, Na+, NH4+, or H+ to give new derivatives. Intercalation of the protonated derivatives with organic bases reveals that the Bronsted acidity of the solid solution series, HC~ ~ , L ~ ,N~ ~ , T ~ ,dOep~eOnd, s on the titanium content. While the x = 1 member (HCaLaNbzTiOlo) is nearly as acidic as the parent HCazNb3010, the x = 2 member (HLazNbTizOlo) is a weak acid hardly intercalating organic bases with pKa - 11.3. The variation of acidity is probably due to an ordering of Nb/Ti atoms in the triple octahedral perovskite slabs, [Ca~,La,Nb~,Ti,0~0], such that protons are attached to NbO6 octahedra in the x = 1 member and to Ti06 octahedra in the x = 2 member.

Glycerol oxidehydration to acrylic acid on complex mixed-metal oxides

The project of this Ph.D. thesis is based on a co-supervised collaboration between Università di Bologna, ALMA MATER STUDIORUM (Italy) and Instituto de Tecnología Química, Universitat Politècnica de València ITQ-UPV (Spain). This Ph.D. thesis is about the synthesis, characterization and catalytic testing of complex mixed-oxide catalysts mainly related to the family of Hexagonal Tungsten Bronzes (HTBs). These materials have been little explored as catalysts, although they have a great potential as multifunctional materials. Their peculiar acid properties can be coupled to other functionalities (e.g. redox sites) by isomorphous substitution of tungsten atoms with other transition metals such as vanadium, niobium and molybdenum. In this PhD thesis, it was demonstrated how it is possible to prepare substituted-HTBs by hydrothermal synthesis; these mixed-oxide were fully characterize by a number of physicochemical techniques such as XPS, HR-TEM, XAS etc. They were also used as catalysts for the one-pot glycerol oxidehydration to acrylic acid; this reaction might represent a viable chemical route to solve the important issue related to the co-production of glycerin along the biodiesel production chain. Acrylic acid yields as high as 51% were obtained and important structure-reactivity correlations were proved to govern the catalytic performance; only fine tuning of acid and redox properties as well as the in-framework presence of vanadium are fundamental to achieve noteworthy yields into the acid monomer. The overall results reported herein might represent an important contribution for future applications of HTBs in catalysis as well as a general guideline for a multifaceted approach for their physicochemical characterization.