Supported Tantalum Oxide Catalysts: Synthesis, Physical Characterization, and Methanol Oxidation Chemical Probe Reaction

Fabrication of Smart Tantalum Carbide MXene Quantum Dots with Intrinsic Immunomodulatory Properties for Treatment of Allograft Vasculopathy

MXene nanomaterials have sparked significant interest among interdisciplinary researchers to tackle today's medical challenges. In particular, colloidal MXene quantum dots (MQDs) offer the high specific surface area and compositional flexibility of MXene while providing improvements to aqueous stability and material-cell interactions. The current study for the first time reports the development and application of immunoengineered tantalum-carbide (Ta4C3T x ) MQDs for in vivo treatment of transplant vasculopathy. This report comes at a critical juncture in the field as poor long-term safety of other MXene compositions challenge the eventual clinical translatability of these materials. Using rational design and synthesis strategies, the Ta4C3T x MQDs leverage the intrinsic anti-inflammatory and antiapoptotic properties of tantalum to provide a novel nanoplatform for biomedical engineering. In particular, these MQDs are synthesized with high efficiency and purity using a facile hydrofluoric acid-free protocol and are enriched with different bioactive functional groups and stable surface TaO2 and Ta2O5. Furthermore, MQDs are spontaneously uptaken into antigen-presenting endothelial cells and alter surface receptor expression to reduce their activation of allogeneic T-lymphocytes. Finally, when applied in vivo, Ta4C3T x MQDs ameliorate the cellular and structural changes of early allograft vasculopathy. These findings highlight the robust potential of tailored Ta4C3T x MQDs for future applications in medicine.

A Review and Experimental Revisit of Alternative Catalysts for Selective Oxidation of Methanol to Formaldehyde

The selective oxidation of methanol to formaldehyde is a growing million-dollar industry, and has been commercial for close to a century. The Formox process, which is the largest production process today, utilizes an iron molybdate catalyst, which is highly selective, but has a short lifetime of 6 months due to volatilization of the active molybdenum oxide. Improvements of the process’s lifetime is, thus, desirable. This paper provides an overview of the efforts reported in the scientific literature to find alternative catalysts for the Formox process and critically assess these alternatives for their industrial potential. The catalysts can be grouped into three main categories: Mo containing, V containing, and those not containing Mo or V. Furthermore, selected interesting catalysts were synthesized, tested for their performance in the title reaction, and the results critically compared with previously published results. Lastly, an outlook on the progress for finding new catalytic materials is provided as well as suggestions for the future focus of Formox catalyst research.

In situ total scattering experiments of nucleation and crystallisation of tantalum-based oxides: from highly dilute solutions via cluster formation to nanoparticles

The exact formation mechanism of tantalum oxides (and in general, metal/mixed metal oxides) from alkoxide precursors is still not fully understood, particularly when forming cluster-like or amorphous materials. The structural evolution of Ta-based oxides was studied in detail using X-ray total scattering experiments along with subsequent pair distribution function (PDF) analyses. Starting from a tantalum alkoxide precursor (Ta₂(OEt)₁₀), the formation of hydrolysed Ta_xO_yH_z clusters in highly diluted aqueous solution was analysed. From the PDF data, the connectivity and arrangement of Ta_xO_y octahedra in the cluster could be deduced as well as the approximate size of the clusters (<1 nm). Construction of cluster models allowed for identification of common structural motifs in the Ta_xO_yH_z clusters, ruling out the formation of chain- or ring-like clusters. More likely, bulky clusters with a high number of corner-sharing octahedra are formed. After separation of the amorphous solid from the liquid, temperature-induced crystallisation processes were monitored via in situ total scattering experiments. Between room temperature and 600 °C, only small rearrangements of the amorphous structure are observed. At about 610 °C, amorphous Ta_xO_yH_z transforms directly into crystalline orthorhombic L-Ta₂O₅ without formation of any crystalline intermediate structures.

Characterization of K6Ta10.8O30 Microrods with Tetragonal Tungsten Bronze-Like Structure Obtained from Tailings from the Penouta Sn-Ta-Nb Deposit

In this work, a deep characterization of the properties of K6Ta10.8O30 microrods has been performed. The starting material used to grow the microrods has been recovered from mining tailings coming from the Penouta Sn-Ta-Nb deposit, located in the north of Spain. The recovered material has been submitted to a thermal treatment to grow the microrods. Then, they have been characterized by scanning electron microscopy, X-ray diffraction, micro-Raman and micro-photoluminescence. The results of our study confirm that the K6Ta10.8O30 microrods have a tetragonal tungsten bronze-like crystal structure, which can be useful for ion-batteries and photocatalysis.

Morphological and chemical analysis of surface interaction of irrigant-endosequence root repair material

This study aimed to evaluate the characterization of chemical interaction of root canal irrigants on the surface of EndoSequence root repair materials using spectroscopy analysis. Round discs of putty and paste were obtained and immersed in saline, sodium hypochlorite (NaOCl), or chlorhexidine. On the surface of chlorhexidine treated putty, diffuse red pigmentation was detected by Raman analysis and diffuse black pigmentation having unusual needle-like shaped crystals was detected by scanning electron microscopy. This pigmentation formed from nitrogen-rich compounds detected by Raman, infrared spectroscopy, and X-ray diffraction analysis. Mineral/amide ratios were increased by NaOCl and significantly decreased by chlorhexidine. Carbonate/phosphate ratios showed no significant changes by NaOCl, while a significant decrease by chlorhexidine. A full half of maximum width_Raman of the phosphate band was significantly increased by NaOCl, while decreased by chlorhexidine. In conclusion; nitrogen-rich compounds produced by chlorhexidine altered the surface morphology of the putty without crystallinity reduction. However, NaOCl reduced the organic fillers that affected its binding to phosphate ions. This reaction affects the chemical properties and perfection of root repair material. The clinicians should be aware of different irrigants to be used after repairing the perforation.

Micro and Macro-Tribology Behavior of a Hierarchical Architecture of a Multilayer TaN/Ta Hard Coating

The micro- and macro-tribological behaviors of a novel hierarchical TaN/Ta coating deposited on Ti6Al4V biomedical alloy by direct current magnetron sputtering were analyzed in the present work. This analysis was associated with the morphological, structural, and mechanical properties, as well as the roughness changes during and after the tribological tests. The wear track of the coating after the macro-tribology tests was evaluated by Raman spectroscopy in order to detect the compounds formed as a result of the tribo-reactions that occurred during the tests. Micro- and macro-tribology behaviors showed a significant wear rate reduction of the hierarchical coating in comparison to the Ti6Al4V substrate. For the case of the micro-tribology tests, this reduction was attributed to the high hardness of the coating (31.4 GPa); however, this hardness caused a considerable increment in the friction coefficient. On the other hand, the macro-tribology performance was associated with the hardness and the ability of the hierarchical architecture to prevent the propagation of cracks. Moreover, the friction coefficient increased considerably at the end of the test; this increment was associated with the tantalum oxides in the wear track detected by Raman spectroscopy.

Ethanol-to-butadiene: the reaction and its catalysts

Catalytic conversion of ethanol is a promising technology for producing sustainable butadiene. This paper reviews the reaction and its catalysts, and discusses the challenges their development faces.

Direct Visualization of Independent Ta Centers Supported on Two-Dimensional TiO2 Nanosheets

Highly dispersed, supported oxides are ubiquitous solid catalysts but can be challenging to characterize with atomic precision. Here, it is shown that crystalline anatase TiO₂ nanosheets (∼5 nm thick) are ideal supports for imaging highly dispersed active sites. Ta cations were deposited by several routes, and high-resolution high angle annular dark-field scanning transmission electron microscopy was used to determine the location of Ta with respect to the TiO₂ lattice and quantify Ta-Ta distances. In the best case, it is shown that >80% of Ta atoms are isolated from one another, whereas other techniques are blind to this critical catalytic property or give only qualitative estimates. TiO₂ nanosheets may prove to be a useful platform for other types of catalysis studies.

Morphological and Spectroscopic Study of an Apatite Layer Induced by Fast-Set Versus Regular-Set EndoSequence Root Repair Materials

This study aimed to evaluate the morphology and chemistry of an apatite layer induced by fast-set versus regular-set EndoSequence root repair materials using spectroscopic analysis. Holes of a 4 mm diameter were created in the root canal dentin, which were filled with the test material. Fetal calf serum was used as the incubation medium, and the samples incubated in deionized water were used as controls. The material-surface and material-dentin interfaces were analyzed after 28 days using Raman and infrared spectroscopy, scanning electron microscopy/energy dispersive X-ray, and X-ray diffraction. After incubation in fetal calf serum, both materials formed a uniform layer of calcium phosphate precipitate on their surfaces, with the dentinal interface. This precipitated layer was a combination of hydroxyapatite and calcite or aragonite, and had a high mineral maturity with the regular-set paste. However, its crystallinity index was high with the fast-set putty. Typically, both consistencies (putty and paste) of root repair material have an apatite formation ability when they are incubated in fetal calf serum. This property could be beneficial in improving their sealing ability for root canal dentin.

Photoluminescence studies on nanocomposite graphene decorated with tantalum oxide

Graphene-tantalum oxide (Ta₂O₅) hybrid material is synthesized using a simple hydrothermal method to elucidate its optical properties. The prepared sample is characterized by X-ray diffraction, scanning electron microscope (SEM), high-resolution-transmission electron microscope (HR-TEM), thermo-gravimetric and differential thermal analysis (TG-DTA), Fourier transform-Raman spectra, and photoluminescence (PL) studies. SEM and HR-TEM analysis revealed that the Ta₂O₅ particles are embedded on the surface of thin sheets of well-defined graphene structure. Thermogravimetric analysis has provided substantial evidence for the thermal stability of the material with minimal percentage of weight loss at 700°C. Further, the excitation of the nanocomposite at a wavelength of 280 nm leading to emission spectra at 567 nm using PL studies, which clearly indicates the emission of light, occurs in the visible green region.

Supported sub-nanometer Ta oxide clusters as model catalysts for the selective epoxidation of cyclooctene

The preparation of organic ligand-free, isolated and catalytically active tantalum oxide complexes (Ta₁) and small clusters (Ta_n>1) on flat silicate support was accomplished by ultra-high vacuum (UHV) techniques followed by oxidation in air.

Selective Electrodeposition of Tantalum(V) Oxide Electrodes

Thin films of TaO_xH_y were cathodically electrodeposited from an aqueous solution containing Ta-IPA precursor and KNO₃ as a sacrificial agent. It was shown that the deposition resulted from a precipitation reaction triggered by the local change of pH at the surface of working electrode. Combined structural and compositional analysis revealed that during the electrodeposition the oxidation state of tantalum remained constant, Ta(V). The as-deposited films are mesoporous amorphous tantalum oxide hydrate films, which can be converted to either pure Ta₂O₅ or Ta₃N₅ by high-temperature annealing in either air (or Ar) or ammonia, respectively. The Ta₃N₅ electrodes exhibited promising PEC activity for water oxidation. These results open the door for the reduced temperature synthesis of Ta₃N₅ electrodes on TCO substrates which would allow for efficient overall solar water splitting.

Influence of lithium concentration on the structure and Li+ transport properties of cubic phase lithium garnets

To understand the influence of lithium concentration on the structure and Li(+) transport properties of cubic lithium garnets, systematic AC impedance, Raman and powder X-ray diffraction (PXRD) investigations have been carried out on lithium garnets with nominal compositions LixA3B2O12 (A = Y(3+), La(3+), Sr(2+), Ba(2+); B = Te(6+), Ta(5+), Zr(4+) and 3 ≤x≤ 7.5). The size of the three dimensional A3B2O12 frame is found to be an important factor in determining the capacity of housing and the nature of distribution of lithium atoms among available tetrahedral and octahedral sites in the cubic LixA3B2O12 lithium garnets. The Li(+) conduction in a cubic phase lithium garnet is primarily dependent upon the concentration of lithium and the nature of distribution of lithium atoms among tetrahedral and octahedral sites. AC impedance analysis of cubic phase LixA3B2O12 revealed an increase in the hopping of Li(+) with the increase in Li(+) concentration, reaches a maximized value for the nominal composition Li7La3Zr2O12 and then decreases with the further increase of lithium concentration.

Effect of simultaneous substitution of Y and Ta on the stabilization of cubic phase, microstructure, and Li(+) conductivity of Li7La3Zr2O12 lithium garnet

Garnet-type lithium stuffed oxide Li7La3Zr2O12 (LLZ) in the cubic phase has received significant attention because of its high Li(+) conductivity at room temperature and excellent stability against lithium metal anodes. In addition to the high Li(+) conductivity, the dense microstructure is also a critical issue for the successful application of LLZ as a solid electrolyte membrane in all-solid-state lithium and lithium-air batteries. The stabilization of LLZ in the cubic phase with dopants indicated a reduction in sintering temperature with La(3+) site doping and improved conductivity by doping the Zr(4+) site. However, there are only a few reports regarding the simultaneous substitution on the La(3+) and on the Zr(4+) site in LLZ. In the present study, systematic investigations have been carried out on Li7-xLa3-yYyZr2-xTaxO12 (x = 0.4, y = 0, 0.125, 0.25, and 0.5) to understand the effect of simultaneous substitution of Y(3+) for La(3+) and Ta(5+) for Zr(4+) in LLZ on the stabilization of the high conductive cubic phase, microstructure, and Li(+) conduction behavior. Powder X-ray diffraction (PXRD) revealed the stabilization of a cubic-like garnet structure for the entire selected compositional range of Li7-xLa3-yYyZr2-xTaxO12 (x = 0.4, y = 0, 0.125, 0.25, and 0.5) samples sintered at 750 °C. However, the Raman spectra revealed that the cubic phase stabilized at around 750 °C for the Li7-xLa3-yYyZr2-xTaxO12 (x = 0.4, y = 0, 0.125, 0.25, and 0.5) samples is different from the high Li(+) conductive cubic phase (Ia3̅d), and the transformation to the high Li(+) conductive cubic phase with a distorted lithium sublattice (Ia3̅d) is observed only for the samples sintered at elevated temperature. Preliminary thermogravimetric (TG), Raman, and Fourier transform infrared (FTIR) studies indicated that the observed low temperature cubic phase of the investigated samples sintered at 750 °C might result from insertion of water vapor from the humid atmosphere into the crystal lattice and subsequent replacement of the lithium ions by protons to form O-H bonds. The AC impedance analysis indicated that the optimal Y substitution in Li7-xLa3-yYyZr2-xTaxO12 (x = 0.4, y = 0.125 and 0.25) helps to reduce the grain boundary resistance in a major way and also helps to reduce the bulk resistance slightly. Among the investigated compositions, Li6.6La2.75Y0.25Zr1.6Ta0.4O12 sintered at 1200 °C exhibits a maximized room temperature total (bulk + grain boundary) Li(+) conductivity of 4.36 × 10(-4) S cm(-1) along with the improved ceramic density.

Effect of Al2O3 Addition on Microstructure, Thermal Expansion and Mechanical Properties of Ta2O5 Ceramics

The Ta2O5-based ceramics were prepared by dry pressing/sintering technique using Ta2O5and Al2O3as the starting materials. The present work investigated the effect of alumina (Al2O3) additions on the composition, microstructure, thermal expansion coefficient (TEC) and bending strength of Ta2O5ceramics. The thermal expansion of the samples was measured by the dilatometry method. It was found that Al2O3additions can effectively inhibit the β to α phase transformation in Ta2O5ceramics. Orthorhombic AlTaO4as the minor phase formed when 2.5 and 7.0 wt% Al2O3was added. The addition of Al2O3results in obvious change of TEC and an increase of bending strength. This work demonstrated that the addition of Al2O3is an effective way to modify the TEC and mechanical strength of Ta2O5ceramics.

Enhancement of epoxidation efficiencies for Ta-SBA15 catalysts. The influence of modification with -EMe3 (E = Si, Ge, Sn) groups

Site-isolated Ta(V) centers were introduced onto the surface of a mesoporous SBA-15 support via the thermolytic molecular precursor method. After thermal treatment under oxygen, the resulting Si-OH and Ta-OH sites of TaSBA15-O(2)were modified with a series of trimethyl group 14 species, Me(3)E-, by treatment with Me(3)E-NMe(2) (E = Si, Ge, Sn) reagents. The resulting surface-modified catalysts (Me(3)E)(cap)TaSBA15 exhibit a significantly increased rate of cyclohexene epoxidation with H(2)O(2) as an oxidant, and provided a decreased amount of allylic oxidation products with respect to the unmodified material, TaSBA15-O(2). The rate of nonproductive H(2)O(2) decomposition, as monitored via (1)H NMR spectroscopy, significantly decreased after the surface modification. The structure of the TaSBA15 catalysts and potential Ta(V) epoxidation intermediates (formed upon treatment of Ta(V) materials with H(2)O(2)) were probed using UV-visible absorbance and diffuse-reflectance UV-visible spectroscopy. A Ta(V)(η(2)-O(2)) intermediate species is proposed for the TaSBA15-O(2), (Me(3)Si)(cap)TaSBA15, and (Me(3)Ge)(cap)TaSBA15 catalysts, while intermediate species for the (Me(3)Sn)(cap)TaSBA15 catalysts could not be characterized.

The chemistry and preparation of tantalum complexes with 2,3-dihydroxy benzoic acid: experimental and theoretical investigation

The effect of 2,3-dihydroxybenzoic acid (2,3DHBA, pyrocatechuic acid) on the chloro-alkoxo-species [TaCl(5-x)(OMe)(x)], formed by dissolving TaCl(5) in MeOH, has been studied. The coordination of 2,3DHBA-H(2)(-) on Ta (V) replacing MeO-terminal groups was monitored via NMR spectroscopy. The yellow solid 1 was isolated from the mixture of TaCl(5), with neutral 2,3-DHBA, in MeOH. From this solid the elemental (C, H and Ta), the thermogravimetric analyses, the IR, NMR, ESR and electronic spectra support the formula Ta(2)(2,3DHBA)(2)(O)(2)Cl(4)(MeO)(4). The ESR spectrum of solid 1, at 4.2 K, shows a half-field signal apart from a multiline signal around g=2, supporting evidence for semiquinone and Ta (IV) presence. The occurrence of superoxide radical, in the low temperature of ESR spectrum recording, cannot be ruled out. By heating the solid 1 at 500°C, an oxide phase showing porous character (SEM) and retaining CO(2) (IR), is evident. Solid 1 heated at 900°C, leads to the formation of β-Ta(2)O(5) orthorhombic phase, as the XRD pattern indicates. The hydrolytic process of solid 1, in aqueous solutions, has been studied; the presence of paramagnetic species generated in situ upon addition of base and the consequent degradative process of 2,3-DHBA, under aerobic conditions is obvious. In order to gain information for the structure of solid 1, DFT calculations have been performed for some theoretical models, based on the empirical formula of solid 1. The calculated structural and spectroscopic parameters have been correlated to experimental results. The energy optimized structures may give an idea about the way of MeCl and MeOMe formation as well some possible intermediates of the hydrolytic mechanism.

Low-temperature phase transformation and phonon confinement in one-dimensional Ta2O5nanorods

The thermochromic phase transformations of one-dimensional Ta2O5nanorods have been analyzed at elevated temperatures ranging from 80 to 300 K. The nanorods, grown in a large-area high-density array, are 14–22 nm wide and approximately 500 nm long. The array contained ∼93.5% of the orthorhombic (β) phase and ∼6.5% of the tetragonal (α) phase. Low-temperature X-ray diffraction results showed complex and polymorphic thermochromic phase transformations of the β(001), α(101) and α(103) lattice planes of the nanorods, which incorporate (i) α-to-α (α–α), (ii) α–α–β and (iii) α–β phase transitions. In comparison with the Raman scattering of three-dimensional bulk powder and two-dimensional thin films of Ta2O5, there were concurrent Raman blue- and redshifts in the one-dimensional Ta2O5nanorods, indicating that the molecular vibrations of the nanorods were confined owing to the reduction of size and dimension.

Synthesis of Hexagonal BaTa2O6 Nanorods and Influence of Defects on the Photocatalytic Activity

Hexagonal barium tantalate (BaTa2O6) nanorods were synthesized by a hydrothermal method based on the reaction of concentrated Ba(OH)2 solution and Ta2O5. BaTa2O6 samples show a uniform cylindrical structure with diameters of 5-30 nm and the lengths of 50-200 nm. The formation of BaTa2O6 nanorods follows a dissolution-recrystallization mechanism and is governed by hydrothermal temperature and time. BaTa2O6 nanorod samples prepared at 270 degrees C for 72 h have exhibited the highest photocatalytic activity in the degradation of Rhodamine B (RhB) in aqueous solution under UV radiation. Hydrogen-related defects were detected in BaTa2O6 nanorods, which originate from the oxygen octahedron. The number of defects was dependent on the hydrothermal temperature, and the photocatalytic activities of BaTa2O6 nanorods increase with the decrease of defect amounts. On the basis of the experiment results, the difference in photocatalytic activities for samples is mainly caused by lattice defects, which can act as inactivation centers.