Niobium Compounds: Preparation, Characterization, and Application in Heterogeneous Catalysis

Design of Nickel Supported on Water-Tolerant Nb2O5 Catalysts for the Hydrotreating of Lignin Streams Obtained from Lignin-First Biorefining

In biomass conversion, Nb2O5 has attracted increasing attention as a catalyst support presenting water-tolerant Lewis acid sites. Herein, we address the design of Ni/Nb2O5 catalysts for hydrotreating of lignin to hydrocarbons. To optimize the balance between acidic and hydrogenating properties, the catalysts were first evaluated in the hydrotreating of diphenyl ether. The best catalyst candidate was further explored in the conversion of lignin oil obtained by catalytic upstream biorefining of poplar. As primary products, cycloalkanes were obtained, demonstrating the potential of Ni/Nb2O5 catalysts for the lignin-to-fuels route. However, the Lewis acidity of Nb2O5 also catalyzes coke formation via lignin species condensation. Thereby, an acidity threshold should be found so that dehydration reactions essential to the hydrotreatment are not affected, but the condensation of lignin species prevented. This article provides a critical "beginning-to-end" analysis of aspects crucial to the catalyst design to produce lignin biofuels.

Application of metal oxide semiconductors in light-driven organic transformations

Herein, we provide an up-to-date overview of metal oxide semiconductors (MOS) as versatile and inexpensive photocatalysts to enable light-driven organic transformations.

Ambient electrochemical N2-to-NH3 fixation enabled by Nb2O5 nanowire array

A Nb₂O₅ nanowire array on carbon cloth (Nb₂O₅/CC) is efficient for ambient electrocatalytic N₂-to-NH₃ fixation with excellent selectivity in 0.1 M Na₂SO₄.

Nb2O5 supported on mixed oxides catalyzed oxidative and photochemical conversion of anilines to azoxybenzenes

Supported niobium oxide as an efficient heterogeneous catalyst for chemoselective preparation of azoxybenzenes from anilines.

Multifunctional Nb–Cu nanostructured materials as potential adsorbents and oxidation catalysts for real wastewater decontamination

Hybrid oxidation catalysts and adsorbent materials containing niobium(v)–copper(ii) for the decontamination of real wastewater containing organic pollutants are reported herein.

Low-Temperature Continuous-Flow Dehydration of Xylose Over Water-Tolerant Niobia-Titania Heterogeneous Catalysts

The sustainable conversion of vegetable biomass-derived feeds to useful chemicals requires innovative routes meeting environmental and economical criteria. The approach herein pursued is the synthesis of water-tolerant, unconventional solid acid monolithic catalysts based on a mixed niobia-titania skeleton building up a hierarchical open-cell network of meso- and macropores, and tailored for use under continuous-flow conditions. The materials were characterized by spectroscopic, microscopy, and diffraction techniques, showing a reproducible isotropic structure and an increasing Lewis/Brønsted acid sites ratio with increasing Nb content. The catalytic dehydration reaction of xylose to furfural was investigated as a representative application. The efficiency of the catalyst was found to be dramatically affected by the niobia content in the titania lattice. The presence of as low as 2 wt % niobium resulted in the highest furfural yield at 140 °C under continuous-flow conditions, by using H₂ O/γ-valerolactone as a safe monophasic solvent system. The interception of a transient 2,5-anhydroxylose species suggested the dehydration process occurs via a cyclic intermediates mechanism. The catalytic activity and the formation of the anhydro intermediate were related to the Lewis acid sites (LAS)/Brønsted acid sites (BAS) ratio and indicated a significant contribution of xylose-xylulose isomerization. No significant catalyst deactivation was observed over 4 days usage.

Promoting effect of Nb doping on catalytic performance for deep oxidation of 1, 2-dichloroethane over (Ce,Cr)xO2-Nb2O5 catalysts

A series of Nb-doped (Ce,Cr)_xO₂-Nb₂O₅ mixed oxides with varying (Ce,Cr)_xO_2/Nb₂O₅ mass ratio were prepared by a co-precipitation method and evaluated for the catalytic performance of eliminating 1,2-dichloroethane (DCE). The results indicate that there exists a strong synergistic effect between acid sites and redox species in (Ce,Cr)_xO₂-Nb₂O₅ improving the catalytic activity for DCE oxidation. Appropriate Nb doping could promote the high dispersion and the interaction of metal oxides in the (Ce,Cr)_xO₂-Nb₂O₅ catalysts, resulting in the formation of more Cr⁶⁺ species with strong oxidizing ability and excellent mobility of oxygen species from bulk to surface to create more active sites for DCE deep oxidation. The (Ce,Cr)_xO₂-Nb₂O₅ catalysts with (Ce,Cr)_xO₂/Nb₂O₅ ratios of 2/1~1/2 exhibit excellent catalytic activity and durability for DCE degradation in dry air as well as benzene or water vapor, and less chlorinated byproduct is produced during the degradation of DCE.

Defect-Enhanced Charge Separation and Transfer within Protection Layer/Semiconductor Structure of Photoanodes

Silicon (Si) requires a protection layer to maintain stable and long-time photoanodic reaction. However, poor charge separation and transfer are key constraint factors in protection layer/Si photoanodes that reduce their water-splitting efficiency. Here, a simultaneous enhancement of charge separation and transfer in Nb-doped NiO_x /Ni/black-Si photoanodes induced by plasma treatment is reported. The optimized photoanodes yield the highest charge-separation efficiency (η_sep ) of ≈81% at 1.23 V versus reversible hydrogen electrode, corresponding to the photocurrent density of ≈29.1 mA cm^-2 . On the basis of detailed characterizations, the concentration and species of oxygen defects in the NiO_x -based layer are adjusted by synergistic effect of Nb doping and plasma treatment, which are the dominating factors for forming suitable band structure and providing a favorable hole-migration channel. This work elucidates the important role of oxygen defects on charge separation and transfer in the protection layer/Si-based photoelectrochemical systems and is encouraging for application of this synergistic strategy to other candidate photoanodes.

Nb2O5 nanowires in-situ grown on carbon fiber: A high-efficiency material for the photocatalytic reduction of Cr(VI)

Niobium oxide nanowire-deposited carbon fiber (CF) samples were prepared using a hydrothermal method with amorphous Nb₂O₅·nH₂O as precursor. The physical properties of the samples were characterized by means of numerous techniques, including X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), UV-visible spectroscopy (UV-vis), N₂ adsorption-desorption, Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy. The efficiency for the removal of Cr(VI) was determined. Parameters such as pH value and initial Cr(VI) concentration could influence the Cr(VI) removal efficiency or adsorption capacity of the Nb₂O₅/carbon fiber sample obtained after hydrothermal treatment at 160°C for 14hr. The maximal Cr(VI) adsorption capacity of the Nb₂O₅ nanowire/CF sample was 115mg/g. This Nb₂O₅/CF sample also showed excellent photocatalytic activity and stability for the reduction of Cr(VI) under UV-light irradiation: the Cr(VI) removal efficiency reached 99.9% after UV-light irradiation for 1hr and there was no significant decrease in photocatalytic performance after the use of the sample for 10 repeated cycles. Such excellent Cr(VI) adsorption capacity and photocatalytic performance was related to its high surface area, abundant surface hydroxyl groups, and good UV-light absorption ability.

Iron-Catalyzed Selective Etherification and Transetherification Reactions Using Alcohols

Simple and readily available iron(III) triflate turned out to be a cheap, environmentally benign, and efficient catalyst for the direct etherification of alcohols. The use of ammonium chloride as an additive (5 mol %, 1 equiv relative to catalyst) suppressed the side reactions completely and ensured the selective ether formation even on challenging substrates containing electron-donating substituents. This method allows the selective synthesis of symmetrical ethers from benzylic secondary alcohols and unsymmetrical ethers directly from secondary and primary alcohols. Moreover, transetherification of symmetrical ethers using primary alcohols is attained. The reaction progress of symmetrical ether and unsymmetrical ether formation followed zero-order and first-order kinetics, respectively. Electron paramagnetic resonance (EPR) measurements of the reaction mixture and simple iron(III) triflate clearly indicated that oxidation state of the metal center remains same throughout the catalysis. Mechanistic studies confirmed that the unsymmetrical ether formation occurs via the in situ formed symmetrical ethers.

Atomic Layer Deposition of Lithium Niobium Oxides as Potential Solid-State Electrolytes for Lithium-Ion Batteries

The development of solid-state electrolytes by atomic layer deposition (ALD) holds unparalleled advantages toward the fabrication of next-generation solid-state batteries. Lithium niobium oxide (LNO) thin films with well-controlled film thickness and composition were successfully deposited by ALD at a deposition temperature of 235 °C using lithium tert-butoxide and niobium ethoxide as Li and Nb sources, respectively. Furthermore, incorporation of higher Li content was achieved by increasing the Li-to-Nb subcycle ratio. In addition, detailed X-ray absorption near edge structure studies of the amorphous LNO thin films on the Nb L-edge revealed the existence of Nb as Nb⁵⁺ in a distorted octahedral structure. The octahedrons in niobium oxide thin films experienced severe distortions, which could be gradually alleviated upon the introduction of Li atoms into the thin films. The ionic conductivities of the as-prepared LNO thin films were also measured, with the highest value achieving 6.39 × 10^-8 S cm^-1 at 303 K with an activation energy of 0.62 eV.

Light-activated Ullmann homocoupling of aryl halides catalyzed using gold nanoparticle-functionalized potassium niobium oxides

Lamellar, or layered, potassium niobium oxide perovskites are a class of underdeveloped semiconductors in organic photocatalysis that offer the inherent advantages of larger particle size and ease of recoverability as compared to traditional semiconductor materials.

Hydrothermal synthesis of ZnO quantum dot/KNb3O8 nanosheet photocatalysts for reducing carbon dioxide to methanol

ZnO quantum dots and KNb₃O₈ nanosheets were synthesized by a two-step hydrothermal method for the photocatalytic reduction of CO₂ to methanol where isopropanol is simultaneously oxidized to acetone . The as-prepared photocatalysts were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and UV-vis absorption spectroscopy (UV-vis). The photocatalytic activity of the materials was evaluated by formation rate of methanol under UV light irradiation at ambient temperature and pressure. The methanol formation rate of pure KNb₃O₈ nanosheets was found to be 1257.21 μmol/g/h, and after deposition of 2 wt % ZnO quantum dots on the surface of KNb₃O₈ nanosheets, the methanol production rate was found to increase to 1539.77 μmol/g/h. Thus, the ZnO quantum dots obviously prompted separation of charge carriers, which was explained by a proposed mechanism for this photocatalytic reaction.

Electromagnetic and optical characteristics of Nb5+-doped double-crossover and salmon DNA thin films

We report the fabrication and physical characteristics of niobium ion (Nb⁵⁺)-doped double-crossover DNA (DX-DNA) and salmon DNA (SDNA) thin films. Different concentrations of Nb⁵⁺ ([Nb⁵⁺]) are coordinated into the DNA molecules, and the thin films are fabricated via substrate-assisted growth (DX-DNA) and drop-casting (SDNA) on oxygen plasma treated substrates. We conducted atomic force microscopy to estimate the optimum concentration of Nb⁵⁺ ([Nb⁵⁺]_O = 0.08 mM) in Nb⁵⁺-doped DX-DNA thin films, up to which the DX-DNA lattices maintain their structures without deformation. X-ray photoelectron spectroscopy (XPS) was performed to probe the chemical nature of the intercalated Nb⁵⁺ in the SDNA thin films. The change in peak intensities and the shift in binding energy were witnessed in XPS spectra to explicate the binding and charge transfer mechanisms between Nb⁵⁺ and SDNA molecules. UV-visible, Raman, and photoluminescence (PL) spectra were measured to determine the optical properties and thus investigate the binding modes, Nb⁵⁺ coordination sites in Nb⁵⁺-doped SDNA thin films, and energy transfer mechanisms, respectively. As [Nb⁵⁺] increases, the absorbance peak intensities monotonically increase until ∼[Nb⁵⁺]_O and then decrease. However, from the Raman measurements, the peak intensities gradually decrease with an increase in [Nb⁵⁺] to reveal the binding mechanism and binding sites of metal ions in the SDNA molecules. From the PL, we observe the emission intensities to reduce them at up to ∼[Nb⁵⁺]_O and then increase after that, expecting the energy transfer between the Nb⁵⁺ and SDNA molecules. The current-voltage measurement shows a significant increase in the current observed as [Nb⁵⁺] increases in the SDNA thin films when compared to that of pristine SDNA thin films. Finally, we investigate the temperature dependent magnetization in which the Nb⁵⁺-doped SDNA thin films reveal weak ferromagnetism due to the existence of tiny magnetic dipoles in the Nb⁵⁺-doped SDNA complex.

Dispersion and solubility of In, Tl, Ta and Nb in the aquatic environment and intertidal sediments of the Scheldt estuary (Flanders, Belgium)

Certain specialty elements are indispensable in modern technologies for their particular properties. Yet, potential risks associated to the release of these elements at any stage, remains unknown. Therefore, the dispersion of indium (In), thallium (Tl), tantalum (Ta) and niobium (Nb) in the aquatic environment of the Scheldt estuary (Flanders, Belgium) was studied. Maximum concentrations in intertidal sediments of 101 ± 15 μg kg^-1 for In, 481 ± 37 μg kg^-1 for Tl, 88 ± 19 μg kg^-1 for Ta and 1162 ± 4 μg kg^-1 for Nb appeared on the sampling location closest to the river mouth, i.e. 57.5 km upstream. Their distribution in the intertidal sediments depends on the physicochemical sediment characteristics along the flow of the river Scheldt. The same was the case for most other metals and aluminum as their occurrence also correlated (p < 0.05) with the occurrence of In, Tl and Nb. While in general, studied elements correlate to the OM content and sulfur and phosphorus herein included, a relative enrichment of In, Tl and Nb was seen at Rupelmonde (92.0 km from the river mouth). Mainly the intertidal sediment silt fraction is capable of retaining the elements by exchanging with other ions in the mineral interlayer. Increasing salinity towards the river mouth can furthermore induce the formation of insoluble chloride species. Overall, the solubility of In, Tl, Ta and Nb appeared extremely low upon extraction of pore water from intertidal sediments saturated to 100% field capacity.

Niobium-Catalyzed Intramolecular Addition of O-H and N-H Bonds to Alkenes: A Tool for Hydrofunctionalization

A convenient, versatile, and easy to handle intramolecular hydrofunctionalization of alkenes (C-O and C-N bonds formation) is reported using a novel niobium-based catalytic system. This atom economic and eco-friendly methodology provides an additional synthetic tool for the straightforward formation of valuable building blocks enabling molecular complexity. Various pyran, furan, pyrrolidine, piperidine, lactone, and lactam derivatives as well as spirocyclic compounds are produced in high yields and selectivities.

Structural, Electronic, and Thermodynamic Properties of the T and B Phases of Niobia: First-Principle Calculations

Different polymorphs of Nb₂O₅ can be obtained depending on the pressure and temperature of calcination leading to different catalytic properties. Two polymorphs of niobia, T-Nb₂O₅ and B-Nb₂O₅, have been investigated by means of density functional/plane waves method. The equation of state predicted that B-Nb₂O₅ phase is more stable than the T-Nb₂O₅ at low temperature; however at high pressure both phases are stable. These results are in good agreement with the available experimental data. The calculated cohesive energies of 6.63 and 6.59 eV·atom^-1 for the B-Nb₂O₅ and T-Nb₂O₅, respectively, also corroborate this conclusion, and it can be compared to the experimental value of 9.56 eV atom^-1 estimated for the most thermodynamically stable phase. The topological analyses based on quantum theory of atoms in molecules (QTAIM) and electron localization function (ELF) were applied and reveal bonds with large ionic character for both phases. The B-Nb₂O₅ presented larger stiffness than T-Nb₂O₅, and the oxygen sites in the T-Nb₂O₅ are more compressible. The density of states comparison for both structures indicates that B-Nb₂O₅ has lower concentration of acid sites compared to T-Nb₂O₅. This result is consistent with the experimental observations that the concentration of Lewis acid sites decreases with the temperature.