Nb Sites Research Articles

Manganese‐Octacyanidoniobate‐Based Ferrimagnet Possessing Bridging Ligands with Disulfide Bonds

AbstractWe synthesized a cyanido‐bridged Mn−Nb metal assembly possessing large‐sized bridging ligands with disulfide bonds, [Mn(dpds)2]2[Nb(CN)8] ⋅ 6H2O (MnNb, dpds: 4,4’‐dipyridyldisulfide). MnNb has a three‐dimensional coordination framework in which the Mn and Nb sites are bridged by cyanides, while the Mn sites are combined by dpds ligands. The two crystallographically independent dpds ligands are bent around the disulfide bond with large C(pyridine)−S−S−C(pyridine) dihedral angles of 105.4(5)° and 102.2(5)°, respectively. For molecule‐based magnets, dpds is a relatively large bridging ligand, but it is compatible with the limited space of the cyanido‐bridged Mn−Nb coordination framework due to the bent structure. Additionally, antiferromagnetic coupling in MnNb of −13 cm−1 between MnII (S=5/2, g=2) and NbIV (S=1/2, g=2) induces ferrimagnetism with a critical temperature of 46 K.

CuPd alloy decorated SnNb2O6 nanosheets as a multifunctional photocatalyst for semihydrogenation of phenylacetylene under visible light

Bimetallic alloy of CuPd nanoclusters decorated SnNb2O6 nanosheet (CuPd/SN) as a multifunctional photocatalyst has been constructed successfully. Compared with SN, CuPd/SN achieves an excellent performance for semihydrogenation of phenylacetylene under visible light irradiation. Especially, CuPd/SN with the Cu:Pd molar ratio of 3:1 exhibits the highest catalytic conversion efficiency for phenylacetylene (99.6%) with high selectivity of phenylethylene (99.4%). In situ FTIR result reveals that SN could activate phenylacetylene via a C≡C → Nb π-bonded coordination which improves the selectivity of phenylethylene. Experiment results indicate that atom Pd in alloy is responsible for dissociation H2, while Cu represses the overhydrogenation. Electrochemical measurements suggest that CuPd alloy on SN surface could greatly minimize the recombination of photogenerated electron − hole pairs. The separated electrons further facilitate the production of active H. Hydrogen spillover would transfer active H to Nb site for phenylacetylene semihydrogenation. Finally, a synergetic catalytic mechanism is deeply discussed at molecule scale. This work would provide a deep insight for designing a multifunctional photocatalyst.

Twin‐Peak Electrocaloric Effect in Ferroelectric Phase of Diffusive SrBi1.98La0.02Nb2−xTaxO9 Materials

Dual‐peak electrocaloric effect (ECE) exists over a broad range significantly below Curie temperature in co‐mixed SrBi1.98La0.02Nb2–x Ta x O9 (SBLNT) compositions exhibiting diffused phase transition. The indirect measurement method is used to study the influence of defect‐modified potential barrier distributions on the ECE of lead‐free SBLNT ceramics. Variation in isothermal entropy change and adiabatic temperature change with varying tantalum (Ta) content signify effects of morphology and impact of altered potential barriers in the parent lattice without involving structural phase transition. A unique appearance of twin‐peak positive–negative ECE in equally doped SrBi1.98La0.02Nb‐TaO9 matrix is attributed to doping‐induced formation of polar nanoregions and distinct dipole switching mechanisms controlled by evenly distributed Nb–Ta potential barriers in SBLNT lattice. Higher doping concentration reduces porosity and promotes electric polarization, thereby the relative cooling power gradually increases with increasing Ta at Nb sites.

Metal-doped KNbO3 for visible light photocatalytic water splitting: A first principles investigation

Materials suitable for visible light photocatalytic water splitting provide sustainable green energy production and environmental pollution solution. Potassium niobate, KNbO3, is not widely used in photocatalytic applications because of its large bandgap, which is not appropriate for the visible range of the solar spectrum. However, doping of semiconductors may help reduce their bandgaps by pinning a dopant level near the top/bottom of the valence/conduction band. We employ first-principles calculations to gain insight into the electronic and optical properties of KNbO3 doped with a number of 3d and 4d transition metals to design and enhance its photocatalytic behavior. We demonstrate the substitutional doping with these elements at the Nb site decreases the bandgap and improves the optical and photocatalytic activities of KNbO3. Our calculations prove that the best candidates for water splitting and CO2 gas reduction are Ag- and Mn-doped KNbO3, respectively. Computational outcomes are compared and discussed with existing experimental ones for doped KNbO3 and KTaO3 structures. Meanwhile, we found out Tc-doped KNbO3 can be beneficial for spintronic applications. The results achieved in this study will initiate a number of experimental investigations for the full exploration of the cubic perovskites, especially in green energy production.

Role of Organic Fluoride Salts in Stabilizing Niobium Oxo-Clusters Catalyzing Epoxidation.

We present here that easily available organic salts can stabilize/modify niobium (Nb) oxo-clusters. The as-synthesized Nb oxo-clusters have been characterized by various methods. These Nb oxo-clusters were catalytically active for the epoxidation of allylic alcohols and olefins with H2O2 as an oxidant. Notably, Nb-OC@TBAF-0.5 appeared as highly dispersed nanosized particles and showed the highest catalytic activity, which can be attributed to the following reasons on the basis of characterization. First, the strong coordination of fluorine ions with Nb sites and the steric protection with bulky organic cations led to high stabilization and dispersion of the oxo-clusters in the course of the reaction. Second, a hydrogen-bond interaction between the coordinated fluorine atom and the -OH group of allylic alcohol favored the epoxidation reaction. Third, the electron density of Nb sites decreased due to the strong electron-withdrawing ability of F- adjacent to Nb sites, thus promoting the electrophilic oxygen transfer to the C═C bond.

Effects of Transition Elements on the Structural, Elastic Properties and Relative Phase Stability of L12 γ′-Co3Nb from First-Principles Calculations

In order to explore novel light-weight Co-Nb-based superalloys with excellent performance, we studied the effects of alloying elements including Sc, Ti, V, Cr, Mn, Fe, Ni, Y, Zr, Mo, Tc, Ru, Rh, Pd, Hf, Ta, W, Re, Os, Ir and Pt on the structural stability, elastic and thermodynamic properties of γ′-Co3Nb through first-principles calculations. The results of transfer energy indicate that Y, Zr, Hf and Ta have a strong preference for Nb sites, while Ni, Rh, Pd, Ir and Pt have a strong tendency to occupy the Co sites. In the ground state, the addition of alloying elements plays a positive role in improving the stability of γ′-Co3Nb compound. The order of stabilizing effect is as follows: Ti > Ta > Hf > Pt > Ir > Zr > Rh > V > Ni > W > Sc > Mo > Pd > Re > Ru. Combining the calculation results of elastic properties and electronic structure, we found that the addition of alloying elements can strengthen the mechanical properties of γ′-Co3Nb, and the higher spatial symmetry of electrons accounts for improving the shear modulus of γ′-Co3Nb compound. At finite temperatures, Ti, Ta, Hf, Pt, Ir, Zr and V significantly expand the stabilization temperature range of the γ′ phase and are potential alloying elements to improve the high-temperature stability of the γ′-Co3Nb compounds.

Lone-pair electron effect induced a rapid photorefractive response in site-controlled LiNbO3:Bi,M (M = Zn, In, Zr) crystals

As a promising candidate material for holographic 3D displays, lithium niobate (LN) is limited by its low photorefractive (PR) response. Recently, it has been reported that bismuth dopants significantly improve the PR properties of LN crystals. However, the mechanism of photorefraction enhancement and whether the performance can be further optimized are not clear. In this paper, we demonstrate that Zn2+, In3+, and Zr4+ co-dopants can enhance the photorefraction of LiNbO3:Bi crystals. In particular, the PR sensitivity of LN:Bi,Zn8.0 crystal reaches 11.7 cm/J at 488 nm, with a diffraction efficiency of 16.67% and a response time of 290 ms. We propose that Bi ions occupy Nb sites, forming BiNb2−/BiNb0 in LN:Bi,Zn crystals, while still occupying Li sites, forming BiLi2+/BiLi4+ in LN:Bi,Zr crystals, when the Zn/Zr concentration exceeds the doping threshold. These occupying models are confirmed by the atomic resolution of scanning transmission electron microscopy. Additionally, we find that the lone-pair electron effect of Bi is pronounced when Bi3+ ions occupy Nb sites, forming the most highly efficient PR centers, which induce an outstanding PR response in LN:Bi,Zn8.0 crystal. Our results clarify the occupation of bismuth ions in Zn, In, or Zr co-doped LiNbO3:Bi and confirm that the PR performance can be further improved by site control.

Nb2O5‐Based Photocatalysts

Photocatalysis is one potential solution to the energy and environmental crisis and greatly relies on the development of the catalysts. Niobium pentoxide (Nb2O5), a typically nontoxic metal oxide, is eco‐friendly and exhibits strong oxidation ability, and has attracted considerable attention from researchers. Furthermore, unique Lewis acid sites (LASs) and Brønsted acid sites (BASs) are observed on Nb2O5 prepared by different methods. Herein, the recent advances in the synthesis and application of Nb2O5‐based photocatalysts, including the pure Nb2O5, doped Nb2O5, metal species supported on Nb2O5, and other composited Nb2O5 catalysts, are summarized. An overview is provided for the role of size and crystalline phase, unsaturated Nb sites and oxygen vacancies, LASs and BASs, dopants and surface metal species, and heterojunction structure on the Nb2O5‐based catalysts in photocatalysis. Finally, the challenges are also presented, which are possibly overcome by integrating the synthetic methodology, developing novel photoelectric characterization techniques, and a profound understanding of the local structure of Nb2O5.

Oxygen vacancy mediated room temperature ferromagnetism in Cu-doped LiNbO3 thin films

Room temperature ferromagnetic Cu-doped LiNbO3 thin films with Cu contents of 2–8 at.% were deposited by RF-magnetron sputtering. X-ray absorption spectroscopy together with multiple-scattering ab initio theoretical calculations at Cu K-edge revealed the incorporation of Cu ions into Nb sites in the LiNbO3 host lattice with divalent valence state. The detailed local structural analysis also confirmed that oxygen vacancies are present in the nearest coordination shell of the Cu atoms. The activated ferromagnetism in the Cu-doped LiNbO3 is intrinsic and probably originates from indirect interactions between the oxygen vacancies and dopant ions, which forms bound magnetic polarons and overlapping polaron orbitals, resulting in stable ferromagnetic ordering. These results present a practical method of triggering room temperature ferromagnetism in non-magnetic Cu element doped LiNbO3 systems with oxygen defects, which helps elucidate the origin of ferromagnetism in perovskite ferroelectric oxides.

Selective hydrogenation of 5-(hydroxymethyl)furfural to 5-methylfurfural over single atomic metals anchored on Nb2O5

5-Methylfurfural (MF) is a very useful chemical. Selective hydrogenation of biomass platform molecule 5-(hydroxymethyl)furfural (HMF) to MF using H2 as the reducing agent is very attractive, but challenging because hydrogenation of C=O bond in HMF is more favourable than C–OH both kinetically and thermodynamically, and this route has not been realized. In this work, we prepare isolated single atomic catalysts (SACs) Pt1/Nb2O5-Ov, Pd1/Nb2O5-Ov, and Au1/Nb2O5-Ov, in which single metal atoms are supported on oxygen defective Nb2O5 (Nb2O5-Ov). It is discovered that the SACs can efficiently catalyze the hydrogenation of HMF to MF using H2 as the reducing agent with MF selectivity of >99% at complete conversion, while the selectivities of the metal nanocatalysts supported on Nb2O5 are very poor. A combination of experimental and density function theory (DFT) studies show that the unique features of the SACs for the reaction result from the cooperation of the Nb and Pt sites near the interface in the Pt1/Nb2O5-Ov. The Pt atoms are responsible for the activation of H2 and the Nb sites activate C-OH in the reaction. This work opens the way for producing MF by direct hydrogenation of biomass-derived HMF using H2 as the reductant.

Diffusion in A15 Nb <sub>3</sub>Sn: An Atomistic Study

Detailed diffusion mechanism in the A15 Nb3Sn superconducting compound has been investigated using an atomistic simulation based on a newly developed interatomic potential for the Nb-Sn binary alloy system. In terms of volume diffusion, Sn atoms diffuse via Nb sites due to the structural feature of A15 Nb3Sn, and the calculated energetics shows that the diffusion of SnNb anti-site is easy whereas the formation of SnNb is rate determining. However, the calculated diffusivity shows that the experimentally observed growth kinetics of the Nb3Sn compound layer cannot be fully explained by volume diffusion. Grain boundary diffusion is predicted to be three to four orders of magnitude faster than the volume diffusion, which can sufficiently explain the known growth kinetics of the Nb3Sn layer. Accordingly, grain boundary diffusion is suggested as the governing diffusion mechanism in Nb3Sn. The fundamental understanding and the details of diffusion revealed in the present work will contribute to future studies for detailed analyses of the alloying effects on the growth kinetics of the Nb3Sn compound.

High-Temperature Synthesis of Small-Sized Pt/Nb Alloy Catalysts on Carbon Supports for Hydrothermal Reactions.

Catalytic biomass conversions are sustainable processes to produce value-added fuels and chemicals but need stable catalysts that can tolerate harsh hydrothermal conditions. Herein, we report a hydrothermally stable catalyst by alloying Pt with a high-melting-point metal Nb. The Pt/Nb alloy catalysts are prepared by H2 reduction at a high temperature of 900 °C with a high-surface-area carbon black support, which can suppress metal sintering at high temperatures and thus lead to small-sized alloyed Pt/Nb particles of only 2.2 nm. Taking the advantages of surface acid property provided by the Nb sites and the size effect, the prepared C-supported small-sized Pt/Nb alloy catalysts exhibit attractive activities for the hydrogenation of levulinic acid into γ-valerolactone and the water-gas shift reaction. More significantly, benefiting from the inherent stability of high-melting-point Nb, the Pt/Nb alloy catalysts show much enhanced hydrothermal stability compared to commercial Pt/C and Ru/C catalysts.

High Oxygen Evolution Activity of Tungsten Bronze Oxides Boosted by Anchoring of Co2+ at Nb5+ Sites Accompanied by Substantial Oxygen Vacancy.

The participation of lattice oxygen in the oxygen evolution reaction (OER) process has been proved to be faster in kinetics than the mechanisms where only metal is involved, although activating the lattice oxygen in the traditional rigid structures remains a big challenge. In this work, efforts are devoted to exploring a new flexible structure that is competent in providing large amounts of oxygen vacancies as well as offering the freedom to manipulate the electronic structure of metal cations. This is demonstrated by anchoring low valence state Co at high valence state Nb sites in the tetragonal tungsten bronze (TTB)‐structured Sr0.5Ba0.5Nb2‐ xCoxO6‐δ, with different ratios of Co to Nb to optimize the Co substitution proportion. It is found that the occupation of Co in the Nb5+ sites gives rise to the generation of massive surface oxygen vacancies (Ovac), while Co itself is stabilized in Co2+ by adjacent Ovac. The coexistence of Ovac and LS Co2+ enables an oxygen intercalation mechanism in the optimal SBNC45 with specific activity at 1.7 V versus reversible hydrogen electrode that is 20 times higher than for the commercial IrO2. This work illuminates an entirely new avenue to rationally design OER electrocatalysts with ultrafast kinetics.

Ambient electrosynthesis of ammonia with efficient denitration

The NH 3 electrosynthesis under ambient conditions represents an attractive alternative to the traditional Haber−Bosch industrial process, and NO is one of the major air pollutants. Here, with single-atom-site Nb catalyst, the ambient electrochemical denitration of NO exhibited a record high NH 3 yield rate of 8.2 × 10 −8 mol cm −2 s −1 , which is two orders of magnitude larger than those achieved by the best-reported catalysts for the ambient NH 3 electrosynthesis and approaches the US Department of Energy target. Theoretical calculations reveal that the single-atomic Nb sites not only facilitate the NO adsorption but also lower the energy barrier of the potential-determining step. High-efficiency electrocatalytic NO-to-NH 3 conversion was realized by Nb single-atom-site catalysis under ambient conditions. The Nb single-site catalyst achieved a record high NH 3 yield rate of 8.2 × 10 −8 mol cm −2 s −1 , approaching the DOE target. • A series of single metal atoms supported on B,N co-doped carbon nanotubes have been synthesized for NH 3 electrosynthesis. • The Nb catalyst achieves a record high NH 3 yield rate of 8.2 × 10 −8 mol cm −2 s −1 , which is approacheing the US DOE target. • This electrochemical NO reaction strategy well integrates the NH 3 production as well as environmental protection.

Structural, defect, transport and dopant properties of AgNbO3

AbstractSilver niobate (AgNbO3) is a candidate lead‐free piezoelectric materials with potential applications in electronic technology and catalysis. Atomistic simulation techniques are used to examine the defects, diffusion of Ag+ and O2− ions, solution of dopants and electronic structures of pristine and doped configurations in AgNbO3. The Ag Frenkel is the most favourable intrinsic defect leading to the formation of Ag vacancies that can vehicle self‐diffusion of Ag+ ions in this material. The calculated activation energy for the diffusion of O2− ions (1.07 eV) is significantly lower than that calculated for the diffusion of Ag+ ions (2.44 eV). The prominent isovalent dopants on the Ag and the Nb sites are found to be Na+ and Ta5+ respectively. Doping of Ge on the Nb site can facilitate the formation of oxygen vacancies required for the oxygen diffusion. Additional Ag vacancies required for the self‐diffusion of silver can be introduced by doping of Ca on the Ag site. Electronic structures of non‐defective and defective AgNbO3 are discussed using density functional theory calculations.

The key role of photoisomerisation for the highly selective photocatalytic hydrogenation of azobenzene to hydrazobenzene over NaNbO3 fibre photocatalyst

NaNbO3 fibre catalyst can efficiently hydrogenate azobenzene to hydrazobenzene with 100 % selectivity under Xe light irradiation. The catalytic activity was closely dependent on the trans-cis photoisomerisation of azobenzene and the subsequent moderate activation of cis NN bond on Nb site. The adsorption of azobenzene with cis or trans conformation on the Nb active site was investigated by the computational methods. The apparent increase of the cis NN bond length from 1.254 to 1.348 Å, suggested an effective activation. The important role of the cis isomer in the reaction was also reflected in the quenching experiment, in which a silver quencher was used to inhibit the generation of the cis isomer. The weak interaction between Nb site and NN of hydrazobenzene was the key to inhibit the over reduction, which was confirmed by the in situ diffuse-reflectance IR spectra.

Development of a multi-active center catalyst in mediating the catalytic destruction of chloroaromatic pollutants: A combined experimental and theoretical study

In this article, we unveiled the feasibility of designing a multi-active center catalyst system for the efficient destruction of chlorinated organics. Cu-HZSM-5 catalysts modified by Nb were developed, which was subsequently utilized for the catalytic elimination of chlorobenzene (CB). The underlying reaction mechanisms, particularly H2O activation and Cl adsorption/desorption behaviors, were explored using in situ DRIFTS, H2O-TPD and Density Functional Theory (DFT) calculations. Experimental results indicated that Brønsted acidic HZSM-5 and Nb-OH provided sufficient H protons to react with Cl, forming the HCl at low temperature and facilitating the Cl desorption. H2O molecule preferentially adsorbed on Cu-Nb interfaces, followed by dissociation on Cu sites, which generated enriched hydroxyls to form HCl. Dissociated Cl preferentially adsorbed on Nb sites that protected the active Cu species and maintained the activation of gaseous O2, thereby hindering electrophilic chlorination to form more toxic polychlorinated byproducts.

Responses of the seagrass Halophila stipulacea to depth and spatial gradients in its native region (Red Sea): Morphology, in situ growth and biomass production

The tropical seagrass Halophila stipulacea is the most dominant and widespread seagrass species in the Gulf of Aqaba (GoA, northern Red Sea). In light of the ecological services provided by H. stipulacea in its native regions, and its reported colonization and dispersion in non-native areas, there is a growing global interest in understanding its vegetative development and growth strategies under varying environmental gradients. Aiming to compare growth and biomass production rates in native vs. invasive regions in the near future, it is rather surprising that such baseline data hardly exists for H. stipulacea. Focusing on the northern GoA, we investigated morphological characteristics, in situ growth rates, and biomass production at three different sites (NB, TY and SB), and at each site, at two depths (5 and 14 m) during the summer months (June-July 2019). Significantly larger (11 %) rhizome internodes and longer (19 %) and wider (15 %) leaves were observed in deeper plants compared to those inhabiting shallow meadows. On the contrary, shoot and internode formation rates in shallow plants were markedly higher than in deep-adapted plants, with production values of one shoot and one internode every 5.9 ± 0.8 and 6.9 ± 0.6 days, respectively. Rhizome elongation rates did not vary significantly across sites and depth with an average value of 0.2 cm d−1. In addition, larger leaf area, number of leaves, and higher above and below-ground biomass were observed in the NB site, compared with TY and SB, suggesting that more favourable growth conditions for H. stipulacea existed at NB which is exposed to high anthropogenic pressures. Results highlight how the species is highly responsive to in situ conditions, and thus contribute to an improved understanding of its vegetative dynamics and growth strategy as a basis for future comparisons and temporal assessments.

Realizing p-type NbCoSn half-Heusler compounds with enhanced thermoelectric performance via Sc substitution

ABSTRACT N-type half-Heusler NbCoSn is a promising thermoelectric material due to favourable electronic properties. It has attracted much attention for thermoelectric applications while the desired p-type NbCoSn counterpart shows poor thermoelectric performance. In this work, p-type NbCoSn has been obtained using Sc substitution at the Nb site, and their thermoelectric properties were investigated. Of all samples, Nb0.95Sc0.05CoSn compound shows a maximum power factor of 0.54 mW/mK2 which is the highest among the previously reported values of p-type NbCoSn. With the suppression of thermal conductivity, p-type Nb0.95Sc0.05CoSn compound shows the highest measured figure of merit ZT = 0.13 at 879 K.

Electric field gradients in 2H–NbSe2: 93Nb NMR measurements and first-principles calculations

Accurate atomic scale structure is of importance for revealing the still mysterious electronic phase transitions in a famous 2D metal, 2H–NbSe2. In this work, the electric field gradients (EFGs) of 2H–NbSe2 at Nb sites in the normal state were investigated by 93Nb nuclear magnetic resonance spectroscopy in combination with first-principles computations. The previous T 3/2 and linear T models for describing the temperature dependent EFGs were tested and discussed according to our measured and theoretically computed EFG data in this two-dimensional metal.