Na(V3−xNbx)Nb6O14 – ein neues Oxoniobat mit [Nb6O12]- und [M2O9]-Clustern

Na(V_3?xNb_x)Nb₆O₁₄ — A Novel Oxoniobate with [Nb₆O₁₂] and [M₂O₉] Clusters Goldcolored single crystals and black powders of Na(V_3?xNb_x)Nb₆O₁₄ have been prepared by heating a pellet containing a mixture of NaNbO₃, NbO₂, NbO, VO₂ and NaF or Na₂B₄O₇ (as mineralizers) at 900°C in a sealed gold capsule. The analytically determined Nb : V ratio is 5 : 1 and means that x is about 1.5. The compound crystallizes in P6₃/m with a = 603.4(1), c = 1807.9(5) pm and Z = 3. The crystal structure can be described in terms of common close packing of sheets of O and Na atoms together with Nb₆ octahedra. Characteristic building groups of the new structure type are [Nb₆O₁₂] clusters, [M₂O₉] clusters and NbO₅ bipyramids. V atoms are distributed only on the positions of the Nb atoms within the trigonal bipyramids or the [M₂O₉] clusters. The [Nb₆O₁₂] clusters show characteristicaly short distances d_Nb-Nb = 279.4 and 281.3 pm, respectively. In the [M₂O₉] units, which are built from two MO₆ octahedra that share a common face, V or Nb atoms form M–M dumbbells with d_M–M = 255.9 pm. The electronic structure is discussed using Extended Hückel calculations.     

Synthesis,structure, and X-ray absorption spectra of LixNbO2 and NaxNbO2 (x ≤ 1)

The two series of compounds, Li_xNbO₂ and Na_xNbO₂, were synthesized from LiNbO₂ and NaNbO₂ with oxidizing agents. From neutron diffraction, it is concluded that the Li position is fully occupied for x = 1. Even though the long-range crystal symmetry does not seem to change for x < 1, as monitored by Guinier X-ray powder patterns, the Nb? L_III XANES spectra exhibit significant spectral variations with x which reflect changes in the unoccupied d-density of states.     

Na3Al2Nb34O64 und Na (Si,Nb) Nb10O19: Clusterverbindungen mit isolierten Nb6-Oktaedern

Na₃Al₂Nb₃₄O₆₄ and Na (Si, Nb) Nb₁₀O₁₉. Cluster Compounds with Isolated Nb₆-Octahedra Hexagonal ormolu coloured plates of the new compounds Na₃Al₂Nb₃₄O₆₄ ( I ) and Na(Si, Nb)Nb₁₀O₁₉ ( II ) were prepared by heating pellets of NaF, Al₂O₃, NbO₂ and NbO (3:1:8:2) and NaF, NbO₂ and NbO (1:4:2), respectively, at approx. 850°C. I was contained in a sealed gold capsule, II in a silica tube. The Si incorporated in II originates from the container material. Both compounds crystallize in R 3 , I with a = 784.4(1), c = 7065(1) pm, Z = 3 and II with a = 784.1(1), c = 4221.8(5) pm, Z = 6. I and II represent new structure types. They contain the same characteristic structural units, namely discrete Nb₆O₁₂ clusters (d_Nb–Nb = 283 ± 4 pm) and Nb₂O₁₀ units with Nb–Nb dumbells (d_Nb–Nb ≈? 269 pm) in edgesharing coordination octahedra. In addition NbO₆ octahedra containing Nb in the oxidation state + 5 and NaO₁₂ cube-octahedra occur in both compounds besides AlO₄ and SiO₄ tetrahedra in I and II , respectively. The structures can be described in terms of a common closepacking of O and Na atoms together with Nb₆ octahedra.     

A DFT Study of the Structures of Aux Clusters on a CeO2(111) Surface

Studying the structures of metal clusters on oxide supports is challenging due to their various structural possibilities. In the present work, a simple rule in which the number of Au atoms in different layers of Au_x clusters is changed successively is used to systematically investigate the structures of Au_x (x=1–10) clusters on stoichiometric and partially reduced CeO₂(111) surface by DFT calculations. The calculations indicate that the adsorption energy of a single Au atom on the surface, the surface structure, as well as the Au? Au bond strength and arrangement play the key roles in determining Au_x structures on CeO₂(111). The most stable Au₂ and Au₃ clusters on CeO₂(111) are 2D vertical structures, while the most stable structures of Au_x clusters (x>3) are generally 3D structures, except for Au₇. The 3D structures of large Au_x clusters in which the Au number in the bottom layer does not exceed that in the top layer are not stable. The differences between Au_x on CeO₂(111) and Mg(100) were also studied. The stabilizing effect of surface oxygen vacancies on Au_x cluster structures depends on the size of Au_x cluster and the relative positions of Au_x cluster and oxygen vacancy. The present work will be helpful in improving the understanding of metal cluster structures on oxide supports.     

Oxygen Atom Adsorption and Diffusion on Pd Low-index Surfaces and （311） Stepped Surface

Introduction Atom adsorption on transition metal surfaces has attracted special attention as a base for understanding the fundamental processes of oxidative catalysis. Particularly interesting is the adsorption and diffusion of oxygen on well-defined metal surfaces. An oxygen covered palladium surface, for example, plays a central role in several important reactions such as oxidation of carbon monoxide and ammonia. In particular, the (100), (111), (110) surfaces and the interactions with oxyge…     

Thermochemische Untersuchungen im System V/Nb/O. II Zum chemischen Transportverhalten im Bereich V2O5/Nb2O5/VO2/NbO2

Thermochemical Investigations in the System V/Nb/O. II. Chemical Transport in the Region V₂O₅/Nb₂O₅/VO₂/NbO₂ Transport experiments were used to support the phase relationships of the V₂O₅/Nb₂O₅/VO₂/NbO₂ system, which were established by annealing experiments of powder mixtures. The phase relations were studied in the NbO₂-rich region of the system by means of X-ray and ESMA methods. The NbO₂-rich section is characterized by the following two phase and three phase regions: Two phase region: V₃Nb₉O₂₉/rutile mixed crystal V_1?xNb_xO₂ Two phase region: BI-mixed crystal/V_xNb_1?xO₂ Three phase region: V₃Nb₉O₂₉/solubility limit LG1 (V_1?xNb_xO₂)/BI-mixed crystal Three phase region: solubility limit LG1 (V_1?xNb_xO₂)/BI-mixed crystal/solubility limit LG2 (V_xNb_1?xO₂). The composition of the solubility limits LG1 and LG2 was ascertained by means of ESMA-investigation: LG1: 57.5 ± 5 mol% NbO₂/43.5 ± 5 mol% VO₂ LG2: 22.5 ± 5 mol% NbO₂/78.5 ± 5 mol% VO_2?     

Nitridation of niobium oxide films by rapid thermal processing

The possibility of forming niobium oxynitride through the nitridation of niobium oxide films in molecular nitrogen by rapid thermal processing (RTP) was investigated. Niobium films 200 and 500 nm thick were deposited via sputtering onto Si(100) wafers covered with a thermally grown SiO₂ layer 100 nm thick. These as-deposited films exhibited distinct texture effects. They were processed in two steps using an RTP system. The as-deposited niobium films were first oxidized under an oxygen atmosphere at 450 °C for various periods of time and subsequently nitridated under a nitrogen atmosphere at temperatures ranging from 600 to 1000 °C for 1 min. Investigations of the oxidized films showed that samples where the start of niobium pentoxide formation was detected at the surface and the film bulk still consisted of a substoichiometric NbO_x phase exhibited distinctly lower surface roughness and microcrack densities than samples where complete oxidation of the film to Nb₂O₅ had occurred. The niobium oxide phases formed at the Nb/substrate interface also showed distinct texture. Zones of niobium oxide phases like NbO and NbO₂, which did not exist in the initial oxidized films, were formed during the nitridation. This is attributed to a “snow-plough effect” produced by the diffusion of nitrogen into the film, which pushes the oxygen deeper into the film bulk. These oxide phases, in particular the NbO₂ zone, act as barriers to the in-diffusion of nitrogen and also inhibit the outdiffusion of oxygen from the SiO₂ substrate layer. Nitridation of the partially oxidized niobium films in molecular nitrogen leads to the formation of various niobium oxide and nitride phases, but no indication of niobium oxynitride formation was found. Figure Schematic representation of the phase distribution in 200 nm Nb film on SiO₂/Si substrate after two steps annealing using an RTP system. The plot below represents the SIMS depth profiles of the nitridated sample with the phase assignment     

Oxygen Electroreduction Performance of Ultrasmall Gold Nanoclusters

Ultrasmall gold nanoclusters consisting of 2-4 Au atoms were synthesized and their performance in electrocatalytic oxygen reduction reactions (ORR) was examined. These clusters were synthesized by exposing AuPPh₃Cl to the aqueous ammonia medium for one week. Electrospray ionization mass spectrometry (ESI-MS), X-ray absorption fine structure (XAFS), and X-ray photoelectron spectroscopy (XPS) analyses indicate that the as-synthesized gold clusters (abbreviated as Au_x) consist of 2-4 Au atoms coordinated by the triphenylphosphine, hydroxyl, and adsorbed oxygen ligands. A glassy carbon disk electrode loaded with the Au_x clusters (Au_x/GC) was characterized by the cyclic and linear-sweep voltammetry for ORR. The cyclic voltammogram vs. RHE shows the onset potential of 0.87 V, and the kinetic parameters of J_K at 0.47 V and the electron-transfer number per oxygen molecule were calculated to be 14.28 mA/cm² and 3.96 via the Koutecky-Levich equations, respectively.     

Effect of Doping on Rutile TiO2 Surface Stability and Crystal Shapes

Transition-metal-(TM-)doped TiO₂ has been considered as promising electrode material for the oxygen evolution reaction (OER). OER activity is expected to depend on the coordination of the surface atoms. In this study, we theoretically investigate the stability of low-index surfaces of TM-doped rutile, (110), (100), (101) and (001), with 50 % of the Ti atoms substituted by Sc, Y, V, Nb or Ta. For Sc and Y, we also consider models with O vacancies providing the most stable oxidation state of Sc and Y. Surface energies are calculated with DFT(+U). Based on the Gibbs-Wulff theorem, the shape of the single crystals is predicted. It is observed that p-doping leads to spontaneous oxygen loss and O vacancies cause surface reconstruction. The Wulff shapes of n-doped TiO₂ have smaller contributions of the (110) facet and, for Nb and Ta, larger contributions of other facets. Given the higher coordinative unsaturation of the TM atoms in the latter, a higher catalytic activity is expected.     

Single crystal RuO2 (110): Surface structure

The surface structure of RuO₂ (110) has been studied with LEED, AES and XPS. The “as-grown” surface shows no LEED patterns and both AES and XPS indicate that the surface is depleted in oxygen in high vacuum. After extensive annealing in an O₂ atmosphere reproducible LEED patterns characteristic of the (110) surface were obtained. For the well-ordered surface the oxygen XPS results revealed oxygen associated with the bulk RuO₂, the presence of RuO₃ and oxygen bound to surface atoms.     

The Structural and Electronic Properties of Tin-Based Heteroatom Clusters Studied by the Density Functional Theory

The focus of this study is on compound clusters and, due to the existence of many phases with different structural properties, tin-based materials have been chosen as the reference case. The clusters considered below are of two types: in the first case the clusters have the skeleton of the pure tin clusters and are doped with oxygen and aluminum atoms with composition Sn _x Y _y with Y = Al, O, x = 1, 10 and y = 1, 2. In the second case the clusters have a rutile lattice with a columnar or a spherical shape and a size up to 80 atoms and are doped with a number of aluminum atoms up to 20. The calculations are based on the Density Functional Theory (DFT) and the results describe the cluster structure, its binding energy and the density of states (DOS). The general indication of the calculations is that the additive coordinates outside, rather than inside, the pristine skeleton with the formation of hybrid bonds with properties similar to the ones of the host atoms. Also conspicuous effects of hybridization are observed in the electronic structure and, due to these effects, the binding energy may decrease with respect to the one in the undoped clusters.     

Structural Variety of Niobium(V) Polyoxo Clusters Obtained from the Reaction with Aromatic Monocarboxylic Acids: Isolation of {Nb2O}, {Nb4O4} and {Nb8O12} Cores

The reactivity of aryl monocarboxylic acids (benzoic, 1- or 2-naphtoic, 4’-methylbiphenyl-4-carboxylic, and anthracene-9-carboxylic acids) as complexing agents for the ethoxide niobium(V) (Nb(OEt)₅ precursor has been investigated. A total of eight coordination complexes were isolated with distinct niobium(V) nuclearities as well as carboxylate complexation states. The use of benzoic acid gives a tetranuclear core Nb₄(μ₂-O)₄(L)₄(OEt)₈] (L=benzoate ( 1 )) with four Nb−(μ₂-O)−Nb linkages in a square plane configuration. A similar tetramer, 7 , was obtained with 2-naphtoic acid by using a 55 % humid atmosphere synthetic route. Two types of dinuclear brick were identified with one central Nb−(μ₂-O)−Nb linkage; they differ in their complexation state, with one bridging carboxylate ([Nb₂(μ₂-O)(μ₂-OEt)(L)(OEt)₆], with L=1-naphtoate ( 3 ) or anthracene-9-carboxylate ( 5 )) or two bridging carboxylate groups ([Nb₂(μ₂-O)(L)₂(OEt)₆], with L=4’-methylbiphenyl-4-carboxylic ( 4 ) or anthracene-9-carboxylate ( 6 )). An octanuclear moiety [Nb₈(μ₂-O)₁₂(L)₈(η₁-L)_4−x(OEt)_4+x] (with L=2-naphtoate, x=0 or 2; 8 ) was obtained by using a solvothermal route in acetonitrile; it has a cubic configuration with niobium centers at each node, linked by 12 μ₂-O groups. The formation of the niobium oxo clusters was characterized by infrared and liquid ¹H NMR spectroscopy in order to analyze the esterification reaction, which induces the release of water molecules that further react through oxolation with niobium atoms, in different {Nb₂O}, {Nb₄O₄} and {Nb₈O₁₂} nuclearities.     

Electronic structure and Chemical bonding in Sr4Nb17O26

The electronic structure of oxoniobate Sr₄Nb₁₇O₂₆ is studied by the linear muffintin orbital (LMTO) method. It is shown that the highenergy conduction band consists of the Nb4d states and the hybridized valence band is formed by the Nb4d and O2p states. The band structure of this compound is characterized by superposition of the bands of the 2p states of perovskite oxygen atoms and the 4d states of monoxide niobium atoms. The degree of oxidation of the perovskite and monoxide niobium atoms is +5 and +2.56, respectively. Chemical bonding is analyzed using the electron localization function and model Hückel calculations. The niobiumoxygen bond is shown to be the strongest. The Fermi level is localized in the vicinity of the bottom of the niobium antibonding state band, which explains the existence of Sr_4−xNb₁₇O₂₆ in the homogeneity region corresponding to 0 < x < 0.3. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 5, pp. 771–780, September–October, 1998. This work was supported by RFFR grant No. 96-03-32015a.     

State of atoms of the transition group elements in dilute solid solutions based on LiMO2 (M = Sc, Ga, Al): III. State of iron and lithium atoms in the LiFe x Sc1−x O2 solid solutions

LiFe _x c_1?x O₂ solid solutions (0.005 < x < 0.10) were studied by the NMR and magnetic susceptibility methods. The anomalies of temperature and concentration dependences of magnetic susceptibility are accounted for by the contributions of single iron atoms and clusters containing iron, oxygen, and lithium atoms. According to the NMR spectroscopy data, lithium atoms also are in two states and enter the cluster composition.     

Fine Tuning of Metallaborane Geometries: Chemistry of Metallaboranes of Early Transition Metals Derived from Metal Halides and Monoborane Reagents

Reaction of [Cp_nMCl_4?x] (M=V: n=x=2; M=Nb: n=1, x=0) or [Cp*TaCl₄] (Cp=η⁵‐C₅H₅, Cp*=η⁵‐C₅Me₅), with [LiBH₄?thf] at ?70 °C followed by thermolysis at 85 °C in the presence of [BH₃?thf] yielded the hydrogen‐rich metallaboranes [(CpM)₂(B₂H₆)₂] ( 1 : M=V; 2 : M = Nb) and [(Cp*Ta)₂(B₂H₆)₂] ( 3 ) in modest to high yields. Complexes 1 and 3 are the first structurally characterized compounds with a metal–metal bond bridged by two hexahydroborate (B₂H₆) groups forming a symmetrical complex. Addition of [BH₃?thf] to 3 results in formation of a metallaborane [(Cp*Ta)₂B₄H₈(μ‐BH₄)] ( 4 ) containing a tetrahydroborate ligand, [BH₄]^?, bound exo to the bicapped tetrahedral cage [(Cp*Ta)₂B₄H₈] by two Ta‐H‐B bridge bonds. The interesting structural feature of 4 is the coordination of the bridging tetrahydroborate group, which has two B? H bonds coordinated to the tantalum atoms. All these new metallaboranes have been characterized by mass, ¹H, ¹¹B, and ¹³C NMR spectroscopy and elemental analysis and the structural types were established unequivocally by crystallographic analysis of 1 – 4 .     

Nitrile Cluster Compounds [(M6X12)X2(RCN)4] (M=Nb, Ta; X=Cl, Br; R=Et, n-Pr, n-Bu)

Three new series of mixed-ligand clusters of the [(M₆X₁₂)X₂(RCN)₄] (M=Nb, Ta; X=Cl, Br; R=Et, n-Pr, n-Bu) composition have been prepared. It is supposed that four nitrile molecules and two halogen atoms are coordinated to the terminal octahedral coordination sites of the [M₆X₁₂]²⁺ unit.     

Deposition of polysiloxane‐like nanofilms onto an aluminium alloy by plasma polymerized hexamethyldisiloxane: characterization by XPS and contact angle measurements

In this work, aluminium (Alclad 2024‐T3) substrates were cleaned by an r.f. (13.56 MHz) plasma, using argon (Ar), oxygen (O₂) and a mixture of O₂/Ar (50:50) gases. The effectiveness of plasma cleaning was checked in situ using X‐ray photoelectron spectroscopy (XPS) and ex situ using water contact angle measurements. XPS O/Al surface atomic ratios are in excellent agreement with those of the crystalline boehmite and the pseudoboehmite. Oxygen O 1s peak‐fitting was used to quantify the proportion of hydroxyl ions and the functional composition on the aluminium surface: the surface cleaned with O₂ plasma contains 50% of aluminium hydroxides, the ones cleaned with Ar plasma and with Ar/O₂ plasma contain, respectively, 25 and 37% hydroxyl ions. The binding energy separation between Al 2p and O 1s is characteristic of AlO(OH). Thin SiO_x films were subsequently deposited from a mixture of hexamethyldisiloxane (HMDSO) and oxygen. In the absence of oxygen, a hydrophobic (Θ≥ 100° ) film characteristic of polydimethylsiloxane (PDMS) is formed: polysiloxane‐like thinner films (SiO_x) are obtained with the introduction of oxygen. XPS and contact angle measurements confirmed both the composition and the structure of these films. More importantly, contact angle measurements using different liquids and interpreted with the van Oss‐Good‐Chaudhury theory allowed determination of the surface free energy of the deposited films: the calculated values of surface tension of the film formed from HMDSO/O₂: (50/50) are in excellent agreement with those of reference silica‐based materials such as a silicon wafer and cleaned glass. Copyright © 2007 John Wiley & Sons, Ltd.     

Nb(110)表面氧原子覆盖度对氧分子解离的影响

用基于密度泛函理论的第一性原理方法研究了Nb(110)表面氧原子覆盖度分别为0.25、0.50、0.75 和1.00单层时对氧分子解离的影响. 结果表明, 在氧原子覆盖度不大于0.50单层时, 由于氧分子和表面铌原子的较强相互作用, 使它们能够自发解离. 然而在氧原子覆盖度为0.75单层时, 氧分子只能够在未占据的洞位附近解离, 同时发生严重的晶格畸变. 在形成一个氧原子单层后(1.00 单层), 氧分子只能弱吸附在Nb(100)表面上,此时氧原子向内扩散成为氧分子继续解离的速率决定步骤. 这些结果从理论上解释了在形成一个氧原子单层后, Nb(110)表面氧分子吸收速率迅速下降的原因.     

Mn负载量对nMnO_x/TiO_2催化剂NH_3-SCR催化性能的影响

MnO_x/TiO_2催化剂由于具有优异的低温脱硝性能,已成为SCR催化剂的研究热点之一.我们通过浸渍法制备了一系列不同Mn负载量的nMnO_x/TiO_2(n=2.5%, 5%, 10%, 15%)(质量分数)催化剂,考察Mn负载量对催化剂脱硝性能的影响.利用N_2物理吸附, X-Ray Diffraction (XRD), Scanning Electron Microscope(SEM),Temperature Programmed Reduction with H_2(H_2-TPR),Temperature Programmed Desorption with NH_3(NH_3-TPD)和X-Ray Photoelectron Spectroscopy (XPS)对其结构进行表征.结果表明,催化剂的脱硝性能随着Mn负载量(2.5%～15%)(质量分数)的变化呈现"火山型"曲线,当Mn负载量为10%(质量分数)时,催化剂的脱硝性能最佳. H_2-TPR和XPS结果表明nMnO_x/TiO_2催化剂上表面氧比例和表面Mn~(4+)浓度均随着Mn负载量的增大,先增大后减小,具体顺序为10MnO_x/TiO_(2 ) 15MnO_x/TiO_(2 )5MnO_x/TiO_(2 ) 2.5MnO_x/TiO_2,与脱硝性能顺序完全一致.进一步关联表面氧的比例与T_(50)发现,催化剂的表面氧的比例与T_(50)呈线性关系,即表面氧比例越高, T_(50)越小,脱硝活性越高. NH_3-TPD结果表明,弱酸酸量的增加有助于低温脱硝活性的提高.这些结果揭示了Mn负载量影响脱硝性能的作用规律,为今后开发高效的锰基低温脱硝催化剂提供了技术支撑.     

Compositional and electrical properties of Cr,Nb, Cr/Nb,CrNbN, and CrN/NbN multilayers grown using the d.c. magnetron sputtering technique

Cr, Nb, Cr/Nb, CrN_x, NbN_x, CrNbN, and (CrN/NbN)_n structures were produced on Si and glass substrates, using the d.c. magnetron sputtering technique. Compositional analysis, based on binding energies of Cr, Nb, and N, was carried out by means of X-ray photoelectron spectroscopy (XPS). Through Auger electron spectroscopy (AES), depth profiles were obtained, allowing to demonstrate the multilayers production. Surface morphological characteristics, as roughness and grain size, were evaluated by atomic force microscopy (AFM), revealing very smooth surfaces, that is a consequence of the deposition parameters used in the synthetization experiments. Finally, for different configurations, conductivity measurements were carried out, revealing the influence of nitrogen content and temperature on electron transport. It was found that substoichiometric nitrides (CrN_0.35 and NbN_0.12) exhibited the highest conductivity, because the nitrogen atoms act as donor of electrons.