51V Nuclear Magnetic Resonance Research Articles

Dynamic signature of orbital selective Mott transition in the metallic phase of VO2

An orbital-selective technique, nuclear magnetic resonance (NMR) spectroscopy, which could simultaneously probe the atomic structure and electronic properties, was employed to address the ‘bad metal’ behaviors, i.e., negligible electron thermal conductivity in the metallic phase of VO2. The measured electric field gradients at V sites by 51V NMR were found to evolve significantly in the pure metallic phase within the vicinity of structural phase transition temperatures, unexpected in a simple metal within this temperature region according to structural analysis by diffraction. This abnormal temperature-dependent local symmetry evolution can be well explained by a simple phenomenological model of orbital selective Mott transition in this multiorbital system.

Lanthanum vanadate catalysts for selective and stable methane oxybromination

A series of lanthanum vanadium oxides, LaVxO2.5x+1.5, with molar V:La ratios in the range of x = 0–1.2 have been synthesized using the citrate method and evaluated in the oxybromination of methane. This reaction aims at valorizing natural gas by its selective conversion into bromomethanes (CH3Br and CH2Br2) that can be used as a platform for the production of fuels and chemicals. In order to establish relationships between the composition, structure, and catalytic performance, the materials in fresh and equilibrated forms have been characterized by X-ray diffraction, N2 sorption, scanning transmission electron microscopy, elemental mapping, and Raman, 51V nuclear magnetic resonance, and X-ray photoelectron spectroscopies. At x = 1, the catalyst comprises nanocrystals of monoclinic-LaVO4, which are accompanied by crystalline hexagonal-La2O3 or amorphous vanadium oxide at sub-stoichiometric (x < 1) or over-stoichiometric (x > 1) ratios, respectively. Pure LaVO4 displays a pretty high selectivity to bromomethanes (86–78% at methane conversions of 15–25%) and stable structural and catalytic behavior. The selectivity is increased to 92–95% when a certain excess of lanthanum is used (e.g., LaV0.5O2.75) due to synergistic effects of the mixture of LaVO4 and tetragonal-LaOBr nanocrystals, formed by partial bromination of La2O3. In this catalyst, LaOBr moderates the oxidation potential of LaVO4 and minimizes the formation of carbon oxides, while LaVO4 avoids agglomeration of the LaOBr phase and secures stable operation. Catalysts outside the compositional window of x = 0.2–1 deactivate due to sticking of LaOBr particles (x < 0.2) or the volatilization of free vanadium oxide (x > 1). These results emphasize that the tuning the composition of LaVxO2.5x+1.5 enables to attain catalysts competing with the best reported oxybromination systems.

Frustration-driven C4 symmetric order in a naturally-heterostructured superconductor Sr2VO3FeAs

A subtle balance between competing interactions in iron-based superconductors (FeSCs) can be tipped by additional interfacial interactions in a heterostructure, often inducing exotic phases with unprecedented properties. Particularly when the proximity-coupled layer is magnetically active, rich phase diagrams are expected in FeSCs, but this has not been explored yet. Here, using high-accuracy 75As and 51V nuclear magnetic resonance measurements, we investigate an electronic phase that emerges in the FeAs layer below T0 ~ 155 K of Sr2VO3FeAs, a naturally assembled heterostructure of an FeSC and a Mott-insulating vanadium oxide. We find that frustration of the otherwise dominant Fe stripe and V Neel fluctuations via interfacial coupling induces a charge/orbital order in the FeAs layers, without either static magnetism or broken C4 symmetry, while suppressing the Neel antiferromagnetism in the SrVO3 layers. These findings demonstrate that the magnetic proximity coupling stabilizes a hidden order in FeSCs, which may also apply to other strongly correlated heterostructures.

Nuclear Magnetic Resonance Signature of the Spin-Nematic Phase in LiCuVO_{4} at High Magnetic Fields.

We report a ^{51}V nuclear magnetic resonance investigation of the frustrated spin-1/2 chain compound LiCuVO_{4}, performed in pulsed magnetic fields and focused on high-field phases up to 56T. For the crystal orientations H∥c and H∥b, we find a narrow field region just below the magnetic saturation where the local magnetization remains uniform and homogeneous, while its value is field dependent. This behavior is the first microscopic signature of the spin-nematic state, breaking spin-rotation symmetry without generating any transverse dipolar order, and is consistent with theoretical predictions for the LiCuVO_{4} compound.

ChemInform Abstract: Review of NMR Studies of Nanoscale Molecular Magnets Composed of Geometrically Frustrated Antiferromagnetic Triangles

AbstractReview: 68 refs.

Characterization of aqueous formulations of tetra- and pentavalent forms of vanadium in support of test article selection in toxicology studies.

Tetravalent (VIV) and pentavalent (VV) forms of vanadium were selected for testing by the National Toxicology Program via drinking water exposure due to potential human exposure. To aid in the test article selection, drinking water formulations (125-2000mg/L) of vanadyl sulfate (VIV), sodium orthovanadate, and sodium metavanadate (VV) were characterized by ultraviolet/visible (UV/VIS) spectroscopy, mass spectrometry (MS), or 51V nuclear magnetic resonance (NMR) spectroscopy. Aqueous formulations of orthovanadate, metavanadate, and vanadyl sulfate in general were basic, neutral, and acidic, respectively. Changes in vanadium speciation were investigated by adjusting formulation pH to acidic, neutral, or basic. There was no visible difference in UV/VIS spectra of pentavalent forms. NMR and MS analyses showed that the predominant oxidovanadate species in both ortho- and metavanadate formulations at basic and acidic pH, respectively, were the monomer and decamer, while, a mixture of oxidovanadates were present at neutral pH. Oxidovanadate species were not observed in vanadyl sulfate formulations at acidic pH but were observed at basic pH suggesting conversion of VIV to VV. These data suggest that formulations of both ortho- and metavanadate form similar oxidovanadate species in acidic, neutral and basic pH and exist mainly in the VV form while vanadyl sulfate exists mainly as VIV in acidic pH. Therefore, the formulation stability overtime was investigated only for sodium metavanadate and vanadyl sulfate. Drinking water formulations (50 and 2000mg/L) of metavanadate (~pH 7) and vanadyl sulfate (~pH 3.5) were ≥92% of target concentration up to 42days at ~5°C and ambient temperature demonstrating the utility in toxicology studies.

Colloidal Counterpart of the TiO2-Supported V2O5 System: A Case Study of Oxide-on-Oxide Deposition by Wet Chemical Techniques. Synthesis, Vanadium Speciation, and Gas-Sensing Enhancement

TiO2 anatase nanocrystals were surface modified by deposition of V(V) species. The starting amorphous TiO2 nanoparticles were prepared by hydrolytic processing of TiCl4-derived solutions. A V-containing solution, prepared from methanolysis of VCl4, was added to the TiO2 suspension before a solvothermal crystallization step in oleic acid. The resulting materials were characterized by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared, Raman, and magic angle spinning solid-state 51V nuclear magnetic resonance spectroscopy (MAS NMR). It was shown that in the as-prepared nanocrystals V was deposited onto the surface, forming Ti–O–V bonds. After heat treatment at 400 °C, TEM/electron energy loss spectroscopy and MAS NMR showed that V was partially inserted in the anatase lattice, while the surface was covered with a denser V–O–V network. After heating at 500 °C, V2O5 phase separation occurred, further evidenced by thermal analyses. The 400 °C nanocrystals had a mean size of a...

Towards understanding the poor thermal stability of V 5+ electrolyte solution in Vanadium Redox Flow Batteries

The V 5+ electrolyte solution from Vanadium Redox Flow Batteries was studied by variable temperature 17O and 51V Nuclear Magnetic Resonance (NMR) spectroscopy and density functional theory (DFT) based computational modeling. It was found that the V 5+ species exist as hydrated penta co-ordinated vanadate ion, i.e. [VO 2(H 2O) 3] 1+. This hydrated structure is not stable at elevated temperature and change into neutral H 3VO 4 molecule via a deprotonation process and subsequently leading to the observed V 2O 5 precipitation in V 5+ electrolyte solutions.

51V nuclear magnetic resonance study of vanadium oxide nanotubes

We report on a 51VNMR study of vanadium oxide - decylamine nanotubes. 51V NMR spectra show three lines resulting from a diamagnetic V5 + and two paramagnetic V4 + ions, respectively. The V4 + /V5 + ratio is estimated to be ~0.47. 51V spin-lattice relaxation comprises both magnetic and quadrupolar contributions.

Speciation of vanadium (V) extracted from acidic sulfate media by trioctylamine in n-dodecane modified with 1-tridecanol

The speciation of vanadium (V) extracted from acidic sulfate media by protonated trioctylamine in n-dodecane modified with 5% (wt) 1-tridecanol has been investigated by Fourier transformed infrared spectroscopy (FTIR) and 51V nuclear magnetic resonance spectroscopy ( 51V NMR). In aqueous sulfate solutions, vanadium (V) exists both as VO 2 + and VO 2SO 4 − ions. The FTIR spectra of 0.2 mol kg − 1 protonated trioctylamine in n-dodecane modified with 5% (wt) 1-tridecanol, loaded with various concentrations of vanadium (V) by extraction from 1 mol kg − 1 H 2SO 4, indicate that vanadium (V) exists in organic phases as polyvanadates, likely as decavanadates. The condensed nature of the extracted form of vanadium (V) was neither confirmed nor precluded by 51V NMR as the micellar structure of these organic phases imposes local conditions which allow the transformation of VO 2 + and VO 2SO 4 − into polyvanadates, but also modify the chemical shifts compared to the ones observed in bulk aqueous solutions for mononuclear and polynuclear vanadium (V) species.

Rotation of hyperfine fields at the V site in the multiband metal K2V8O6

51V nuclear magnetic resonance measurements are conducted on a single crystal of the mixed valence compound K2V8016 with 1.25 3d electrons per one V site. We determine the 51V Knight shift tensor, 51K, in the metallic state. 51K has the small anisotropy despite the conduction electrons in the anisotropic 3d orbitals. The principal axis of 51K rotates continuously on cooling temperature in the metallic state without breaking the lattice symmetry, I4/m. Taking into account the isotropic spin susceptibility, the thermal variation is attributed to a change in the hyperfine field tensor reflecting the 3d orbital shape.

Identification of Mixed Valence Vanadium in ETS-10 Using Electron Paramagnetic Resonance, 51V Solid-State Nuclear Magnetic Resonance, and Density Functional Theory Studies

Microporous vanadium-substituted titanosilicate ETS-10 solids are promising photocatalysts for decomposition of organic molecules. The dopant vanadium metal modulates the electronic environment of the titanosilicate matrix and plays a major role in the enhancement of the photocatalytic activity. However, the local electronic and geometric structure of the vanadium sites in these materials is a subject of controversy. Using vanadium electron paramagnetic resonance (EPR) and 51V nuclear magnetic resonance (NMR) spectroscopy, we have characterized the local environments of the vanadium sites in vanadium-substituted ETS-10 samples with different vanadium loadings. The measurements reveal clearly the presence of V(IV) and V(V) oxidation states. The EPR results suggest that V(IV) is in octahedral sites and, therefore, must substitute for Ti in the framework. 51V NMR studies indicate that the V(V) species are adjacent to the V(IV) species in most cases on the basis of significant electron−nuclear dipolar interaction between the V(V) nuclei and the unpaired electron on V(IV). The NMR chemical shift and electric field gradient parameters estimated from the NMR spectra are used in conjunction with density functional theory calculations to propose a model where the V(V) species preferentially occupy sites at the ends of the octahedral chains.

Vanadium-modified molecular sieves: preparation, characterization and catalysis

Vanadium-containing molecular sieves are redox catalysts and are good candidates as substitutes for oxide-supported V2O5 in a number of reactions. These materials have the advantage of presenting better dispersion of vanadium species, as well as shape-selective properties and controllable acidities. They may be prepared by one-pot synthesis or by post-synthesis methods and a number of techniques such as diffuse reflectance UV-visible spectroscopy, 51V nuclear magnetic resonance and electron paramagnetic resonance, to name but a few, have been used to characterize these materials. In this review, methods of preparation of vanadium-modified molecular sieves, their characterization and applications in catalysis are discussed.

NMR study of the quasi-one-dimensional compound BaCo 2V 2O 8

We report the results of a 51V nuclear magnetic resonance (NMR) study on the single crystal BaCo 2V 2O 8 at temperatures between 3 and 300 K. The NMR features for fields both parallel and perpendicular to the c-axis have been identified. We found no substantial changes in the NMR shift and line width at low temperatures as a constant field of 7.06 T was applied along the c-axis. The NMR observations thus indicate the absence of magnetic long-range ordering under this field, consistent with the specific heat result which showed the disappearance of the magnetic transition beyond an external field of 4 T in BaCo 2V 2O 8.

Hydrothermal synthesis, structural, and spectroscopic studies of vanadium substituted ETS-4

Titano-vanadium silicate with the structure of ETS-4 has been hydrothermally synthesized to produce V-ETS-4. Direct comparison between ETS-4 and V-ETS-4 showed successful isomorphous substitution of titanium by vanadium. The existence of vanadium is confirmed by energy dispersive X-ray spectrometry (EDS). 51V and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy revealed that the vanadium is present as V 4+ and prefers the octahedral site in the ETS-4 structure. Diffuse reflectance ultraviolet–visible (DR UV–Vis) and Raman spectroscopy analyses showed the existence of five- and six-coordinated V 4+, which partly substitute TiO 5 square-pyramids and interrupts the chains of TiO 6 octahedra in the ETS-4 structure. This isomorphous substitution also impacts the thermal stability of V-ETS-4 and it was observed by differential scanning calorimetry (DSC).

Spin structure of the nanoscale molecular magnet V15 cluster revealed by [formula omitted

51V nuclear magnetic resonance (NMR) measurements below 100 mK using a He 3 – He 4 dilution refrigerator have been carried out to investigate the magnetic states of V ions in the V15 cluster. We report a direct evidence for spin structure of the V15 cluster where the V 4 + ( s = 1 2 ) spins on the outer hexagons are in a singlet state and V ions on the inner triangle have s = 1 2 spin moments at low temperature.

Detecting magnetic order in [formula omitted] via 51V NMR

We present 51V nuclear magnetic resonance (NMR) measurements in the magnetically ordered state of the Haldane compound PbNi 2 V 2 O 8 doped with Co impurities. The two-peak NMR spectra provide an order parameter and imply partial field-induced spin reorientation. The possible mechanisms leading to the observed splitting of the NMR spectra are discussed.

Structural and electrical properties of tellurovanadate glasses containing Li 2O

Glassy materials are promising intercalation compounds, due to their open network structure and absence of grain boundaries. Some glasses containing alkali ions and a high concentration of transition metal ions can present mixed ionic-electronic conductivity and are therefore potential candidates for application as cathode material in Li-ion batteries. The present work is devoted to the ternary system xLi 2O–(1 − x)[0.3V 2O 5–0.7TeO 2] with 0 ≤ x ≤ 0.4. These compounds were prepared by heat treatment in air at 800 °C followed by traditional quenching. Raman spectroscopy and 51V nuclear magnetic resonance measurements were performed in order to highlight the structural short range order modifications induced by the introduction of the Li 2O network modifier. These structural effects can be related to the electrical behaviour, as studied by complex impedance spectroscopy measurements.

Incorporation of mixed valence vanadium in the microporous titanosilicate AM-2

Titanium and silicon have been partly substituted by vanadium in the microporous titanosilicate AM-2 to produce V-AM-2 via direct hydrothermal method. The presence of vanadium was confirmed by energy dispersive X-ray spectrometry (EDS) analysis. Its structural incorporation leads to slight extension of the unit cell volume. This fact suggests an isomorphous substitution of tetrahedral silicon by vanadium, which is bigger than the former one. Over 500 °C the TG curve of V-AM-2 indicates increasing mass due to the oxidation of V 4+–V 5+. The presence of vanadium and its oxidation state are also investigated by diffuse reflectance ultraviolet–visible (DR UV–Vis) spectroscopy, 51V and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy.

A new class of highly dispersed VO x catalysts on mesoporous silica: Synthesis, characterization, and catalytic activity in the partial oxidation of ethanol

The morphology of vanadium oxide supported on a titania-modified mesoporous silica (MCM-41), obtained by means of a careful grafting process through atomic layer deposition, was studied using a variety of characterization techniques. The X-ray diffraction (XRD) together with transmission electron microscopy (TEM), 51V nuclear magnetic resonance ( 51V-NMR), Raman, FTIR and DRS-UV/Vis results showed that the vanadia species are extremely well dispersed onto the surface of the mesoporous support; the dispersion being stable upon thermal treatments up to 400 °C. Studies of the catalytic activity of these materials were performed using the partial oxidation of ethanol as a probe reaction. The results indicate an intrinsic relationship between dispersion, the presence of a TiO 2–VO x phase, and catalytic activity for oxidation and dehydration.