Vanadium Clusters Research Articles

High-Visible-Light Photoactivity of Plasma-Promoted Vanadium Clusters on Nanozeolites for Partial Photooxidation of Methanol.

Cold VCl3-plasma is employed for the preparation of highly dispersed vanadium oxide clusters on nanosized zeolite. Different types of zeolites, such as EMT, FAU (z.X), and Beta, are used. The activity of the prepared catalysts is studied in the selective photooxidation of methanol under polychromatic visible and UV irradiations. The physicochemical properties and catalytic performance of plasma-treated zeolite Beta (P-V2O5@Beta) catalyst is compared with zeolite Beta (V2O5@Beta) and amorphous silica (V2O5@SiO2) impregnated vanadium oxide catalysts. Pure V2O5 is used as a reference material. The set of catalytic data shows that plasma-prepared zeolite Beta based catalyst displays the highest activity. Complementary characterization techniques including XRD, N2-sorption, FTIR, ionic exchange, pyridine adsorption, Raman, NMR, TPR, and EDX-TEM are used to study the impact of the preparation approach on the physicochemical properties and catalytic performance of photocatalysts.

Structural and Electronic Properties of Hydrated VnH2O and Vn+H2O, n ≤ 13, Systems

Water adsorption on small vanadium clusters, Vn–H2O and Vn+–H2O, n ≤ 13, is crucial to understanding the details of the metal-solvation process, very difficult to determine experimentally. Therefore, by means of density functional theory, BPW91-D2 all-electron calculations with dispersion corrections, the low-lying states of Vn and Vn+ are studied in this work, where dimer motifs, acting as basic building blocks, give origin to distorted structures. The results for Vn and Vn+ are consistent with experimental determinations for ionization energies (IE), dissociation energies, and IR spectra. We found that multiple isomers are involved in the spectrum of some Vn+ ions and that the IEs are reduced by water attachment in Vn–H2O. By means of metal–oxygen bonding, V–O, water adsorption is done on atop sites of the Vn+ cations. This is confirmed by the results of vibrational analysis, which are in agreement with reported experimental findings, revealing small blue and red shifts of the bending frequency of the a...

Adsorption and dehydrogenation of ammonia on vanadium and niobium nitride cluster cations

Reactions of vanadium and niobium nitride cluster cations, VnNm+ and NbnNm+ (n=3–6; m=n, n–1), with NH3 were studied using a guided ion beam tandem mass spectrometer. Two main reactions are observed at a near-thermal energy under single collision conditions: simple adsorption of NH3 and adsorption of decomposed NH (i.e., dehydrogenation of NH3). The total reaction cross section and the branching fraction between these two reactions change significantly with the number of atoms and the composition of the cluster. The specific equiatomic vanadium nitride clusters, V4N4+ and V6N6+, are relatively inert and exhibit only simple NH3 adsorption with low probabilities, which can be understood by considering the stable structures (cube and cuboid) resembling subunits of bulk VN. In contrast, most of the other clusters including Nb4N4+ are found to be highly reactive and give rise to the NH3 dehydrogenation. The multiple-collision reactions of NbnNm+ with (n,m)=(3,2), (4,3), and (4,4) result in the complete dehydrogenation of ammonia to give the product ions of NbnNmN2+.

Structural influences of arsenic–vanadium clusters and transition metal complexes on final structures of arsenic–vanadium-based hybrids

Three new compounds based on arsenic–vanadium clusters {[Cu(im)2]4[H2As8V14O42(SO4)]}·H2O (1) and [Cu(phen)(im)]3[Cu(phen)2][As8V14O42(H2O)] (2, 3) (im=imidazole, phen=phenanthroline) have been synthesized and characterized by IR, UV–Vis, XRD, TG analysis, elemental analysis, single-crystal X-ray diffraction analysis and catalytic properties. Crystal analysis reveals that compound 1 shows an unprecedented 3-D twofold interpenetrating network structure with each of the two interpenetrating frameworks exhibiting a novel diamondoid framework structure based on POMs and transition metal complexes. Compounds 2 and 3 are based on identical polyanions and identical transition metal complexes. However, the two compounds are not isomorphous and isostructural with each other. That is to say, the two compounds have an identical chemical formula with different structures: the two are polymorphs of each other. In addition, catalytic properties of compounds 1–3 have been studied.

New self-assembly hybrid compounds based on arsenic–vanadium clusters and transition metal mixed-organic-ligand complexes

Seven compounds exhibiting novel structures constructed from arsenic–vanadium polyoxoanions and metal mixed-organic-ligand complexes have been synthesized and characterized.

Selective adsorption of ethylene on bimetallic CuVn+/0 (n = 1–5) clusters: A theoretical study

The adsorption of ethylene on bimetallic CuVn+/0 (n=1–5) clusters were studied employing density functional theory calculations. The most stable structures of [CuVn(C2H4)]+/0 (n=1–5) clusters were predicted. Ethylene is adsorbed on bimetallic CuVn+/0 (n=1–5) in both the π and di-σ bonded modes. The di-σ bonded clusters are favorable energetically to the π bonded species. Ethylene interacts with the doped Cu only in the π bonded mode. The binding energies, ionization potentials and adsorption energies of [CuVn(C2H4)] (n=1–5) clusters were calculated. The results indicate that the doped Cu improves the adsorption selectivity of ethylene on small size vanadium clusters.

Design and synthesis of purely inorganic 3D frameworks composed of reduced vanadium clusters and manganese linkers

Two purely inorganic three-dimensional (3D) frameworks [Mn4(H2O)11V(IV)(18)O42(PO4)](7-) (1) and [Mn2(H2O)7V(IV)(18)O42(PO4)](11-) (2) were synthesized under hydrothermal conditions and fully characterized by single-crystal X-ray structural analysis, IR spectroscopy, thermogravimetric analysis and PXRD. Structural analysis revealed that these two compounds contained a similar all reduced polyoxoanion [V(IV)(18)O42(PO4)](15-) linked by different amounts of manganese centers to form 3D framework materials. The V centers in these two compounds were all reduced to the +IV oxidation state, resulting in an all reduced polyoxoanion, which was firstly used as the building block for constructing 3D framework materials. The all reduced typical polyoxoanion [V(IV)(18)O42(PO4)](15-) with 15 negative charges supplied enough charge amount to accept TM cations. In these two structures, the anions were surrounded by 12 and 5 Mn(2+) ions, respectively, adjusted by varying the feeding amount of MnCl2·4H2O. An electrocatalytic study revealed that compound 1 exhibits electrocatalytic activity for reduction of H2O2.

Gas-phase reaction of CeV2O7+ with C2H4: activation of C-C and C-H bonds.

The reactivity of metal oxide clusters toward hydrocarbon molecules can be changed, tuned, or controlled by doping. Cerium-doped vanadium cluster cations CeV2O7(+) are generated by laser ablation, mass-selected by a quadrupole mass filter, and then reacted with C2H4 in a linear ion trap reactor. The reaction is characterized by a reflectron time-of-flight mass spectrometer. Three types of reaction channels are observed: 1) single oxygen-atom transfer , 2) double oxygen-atom transfer , and 3) C=C bond cleavage. This study provides the first bimetallic oxide cluster ion, CeV2O7(+), which gives rise to C=C bond cleavage of ethene. Neither Ce(x)O(y)(±) nor V(x)O(y)(±) alone possess the necessary topological and electronic properties to bring about such a reaction.

The Use of a Vanadium Species As a Catalyst in Photoinduced Water Oxidation

The first water oxidation catalyst containing only vanadium atoms as metal centers is reported. The compound is the mixed-valence [(V(IV)5V(V)1)O7(OCH3)12](-) species, 1. Photoinduced water oxidation catalyzed by 1, in the presence of Ru(bpy)3(2+) (bpy = 2,2'-bipyridine) and Na2S2O8, in acetonitrile/aqueous phosphate buffer takes place with a quantum yield of 0.20. A hole scavenging reaction between the photochemically generated Ru(bpy)3(3+) and 1 occurs with a bimolecular rate constant of 2.5 × 10(8) M(-1) s(-1). The time-resolved formation of the oxidized molecular catalyst 1(+) in bimolecular reactions is also evidenced for the first time by transient absorption spectroscopy. This result opens the way to the use of less expensive vanadium clusters as water oxidation catalysts in artificial photosynthesis schemes.

A Novel Octanuclear Vanadium(V) Oxide Cluster Complex Having an Unprecedented Neutral V8O20 Core Functionalized with 4,4′-Di-tert-butyl-2,2′-bipyridine

A novel octanuclear vanadium(V) oxide cluster complex, [V8O20(4,4'-(t)Bubpy)4] (1), was synthesized and characterized by single-crystal X-ray structure analysis to reveal that 1 has an unprecedented neutral V8O20 core. An unexpected interconversion between 1 and the methoxo(oxo)vanadium(V) cluster complex, [V4O8(OMe)4(4,4'-(t)Bubpy)2] (2), was observed upon changes in the solvent systems.

Synthesis and characterization of a tetraradical-containing octanuclear vanadium cluster formed via ligand C–N bond breaking and C–O bond making

A tetraradical-containing octanuclear vanadium cluster (V8, complex 2) was synthesized by reacting N(2-methoxyanilino)-3,5-di-tert-butylphenol and VOSO4·5H2O in the presence of Et3N in MeOH. The reaction proceeded via a ligand centre C–N bond cleavage and C–O bond formation. The V8 cluster was comprised of two identical V4 clusters where one V4 cluster was replicated by the other by an inversion centre. A strong antiferromagnetic coupling between the two V4 clusters, having S = 1/2 spin state each, provided a diamagnetic ground state.

Geometrical and magnetic properties of vanadium clusters supported on graphene

We report ab-initio calculations of vanadium-cluster Vn(n = 2–5) adsorption on graphene sheets. Geometrical and magnetic properties of various adsorption configurations are studied using first-principles density-functional theory with the generalized gradient approximation. The geometrical and magnetic properties of vanadium clusters are found to be size-dependent, and the supported graphene sheet could influence the formation of the vanadium clusters. Low-dimensional Vn cluster configurations could be easily formed when they are absorbed on a graphene sheet, and the combined Vn-graphene systems exhibit a nonmagnetic state, which is the most stable magnetic configuration. Our calculations for the geometrical and the magnetic moment properties of Vn-graphene systems may be of interest for some nanotechnological applications.

Three New Imidazole‐Functionalized Hexanuclear Oxidovanadium Clusters with Exceptional Catalytic Oxidation Properties for Alcohols

Cluster cat: Three new imidazole-functionalized neutral hexanuclear vanadium clusters have been prepared, one of which shows exceptional catalytic activity in the selective aerobic oxidation of alcohols (see scheme). Furthermore, these new polyoxovanadium clusters can be readily recycled and reused with unchanged catalytic activity. A plausible catalytic cycle is also proposed.

Probing the structural and electronic properties of small vanadium dioxide clusters by density functional theory and comparison with experimental photoelectron spectroscopy

The structural evolution and bonding of a series of early transition-metal dioxide clusters, V(n)O(2)(q)(n = 3-9, q = 0, -1), have been investigated using density functional theory (DFT) calculations and the results are compared with experimental literature data. For each vanadium dioxide cluster, many low-lying isomers are generated using the Saunders "Kick" global minimum stochastic search method. Theoretical electron detachment energies (both vertical and adiabatic) were compared with the experimental measurements to verify the ground states of the vanadium dioxide clusters obtained from the DFT calculations. Five kinds of dissociative adsorption configurations of ground-state structure of V(n)O(2)(q) are identified. The dissociative adsorption of O(2) on V(n)(-1, 0) is more favorable than O(2) molecular adsorption. Furthermore, the adsorption energy of O(2) is higher than that of a single atom on the bare V(n)(-1, 0) clusters, but less than twice the adsorption energy for an atom, indicating that O(2) being adsorbed on vanadium clusters are more difficult than single O atom adsorbed on vanadium clusters.

Comparative DFT study of N2 and no adsorption on vanadium clusters Vn (n = 2–13)

Using gradient-corrected density functional theory, we have comparatively studied the adsorption properties of diatomic molecules N(2) and NO on vanadium clusters up to 13 atoms. Spontaneous dissociation is found for N(2) adsorbing on V(n) with n = 4-6, 12, and for NO with n = 3-12, respectively, whereas for the rest of the clusters, N(2) (NO) molecularly adsorbs on the cluster for all the possible sites. The incoming N(2) retains the magnetism of V(n) except for V(2) and V(6) whose moments are quenched from 2 μ(B) to zero. Consequently, the moments of V(n)N(2) (n = 2-13) show even/odd oscillation between 0 and 1 μ(B). On the adsorption of NO, the magnetic moments of V(n) with closed electronic shell are raised to 1 μ(B) at n = 4, 8, and 10, and 3 μ(B) at n = 12, whereas for open shell clusters, their magnetic moments increase for n = 5 and 9 and decrease for n = 2, 3, 5-7, 11, and 13 by 1 μ(B). These findings are rationalized by combinatory analysis from several aspects, for example, the geometry and stability of bare clusters, charge transfer induced by the adsorption, feature of frontier orbitals, and spin density distribution.

Carbon monoxide adsorption on neutral and cationic vanadium doped gold clusters

The effect of a single vanadium dopant atom on the reactivity of small gold clusters is studied in the gas phase. In particular we investigated carbon monoxide adsorption on vanadium doped gold clusters using a low-pressure collision cell. Employing this technique the reactivity of both neutral and cationic clusters was studied under the same experimental conditions. Analysis of the kinetic data as a function of the pressure in the reaction cell shows that the reaction mechanism is composed of a fast adsorption and a delayed dissociation reaction. It is demonstrated that the reactivity of positively charged Au(n)V(m)(+) (n = 8-30, m = 0-3) is greatly enhanced as compared to the corresponding neutral species and that dissociation rates decrease with decreasing temperatures. While the overall magnitude of the reactivity does not change upon doping with vanadium clusters, the size dependence is significantly affected. The neutral singly vanadium doped gold clusters show a sudden drop after size Au(13)V, followed by a smooth increase, in contrast to the extended odd-even staggering for bare gold clusters. This difference can be explained by changes in the electronic structure of the clusters, related to the partly filled 3d shell of the vanadium dopant atom.

Probing the structural and electronic properties of small vanadium monoxide clusters

The structural evolution and bonding of a series of early transition-metal oxide clusters, V(n)O(q) (n = 3-9, q = 0,-1), have been investigated with the aid of previous photoelectron spectroscopy (PES) and theoretical calculations. For each vanadium monoxide cluster, many low-lying isomers are generated using the Saunders "Kick" global minimum stochastic search method. Theoretical electron detachment energies (both vertical and adiabatic) were compared with the experimental measurements to verify the ground states of the vanadium monoxide clusters obtained from the DFT calculations. The results demonstrate that the combination of photoelectron spectroscopy experiments and DFT calculation is not only powerful for obtaining the electronic and atomic structures of size-selected clusters, but also valuable in resolving structurally and energetically close isomers. The second difference energies and adsorption energies as a function of the cluster size exhibit a pronounced even-odd alternation phenomenon. The adsorption energies of one O atom on the anionic (6.64 → 8.16 eV) and neutral (6.41 → 8.13 eV) host vanadium clusters are shown to be surprisingly high, suggesting strong capabilities to activate O by structural defects in vanadium oxides.

Fragmentation of sputtered vanadium and niobium cluster ions, kinetic release and dissociation energy

The generation and unimolecular fragmentation of Vn+ and Nbn+ clusters formed in sputtering vanadium and niobium surfaces by Xe+ ions has been studied. The method of measuring the kinetic energy of fragment ions (kinetic energy release distribution) has been used to determine the dissociation energy. Kinetic energy spectra have been measured in the field-free zone (corresponding to a time window of 10−5–10−4 sec after emission) of an ion microanalyzer with double focusing in reverse geometry. The results of spectra measurement were treated using the Rice-Ramsperge-Kassel theory of unimolecular reactions and the “evaporative ensemble”, which allowed us to calculate the dissociation energies of homonuclear Vn1 (n= 5–11) and Nbn1 (n = 3–8) clusters.

NMR study of magnetic excitation in LiVX2 (X = O, S)

Magnetic excitation in the two-dimensional triangular lattice LiVO2 and its itinerant analogue LiVS2 are investigated by 51V- and 7Li-NMR experiments. The nuclear spin-lattice relaxation rate 1/T1 on 51V and 7Li nuclei shows thermally activated behavior with different size of spin gap, Δ/kB ∼ 3400 K for LiVO2 and ∼ 1900 K for LiVS2, below the transition temperature Tc from magnetic to non-magnetic ground state. In 51(1/T1(T))/51(1/T1(Tc)) vs. T/Tc plot, the normalized relaxation rate for both materials seems to fall on the same curve. This scaling behavior indicates that the ground states of LiVO2 and LiVS2 are the same, i.e., V3+ (3d2 S = 1) spins form spin singlet with the formation of triangular vanadium clusters, although the replacement of oxygen by sulfur may broaden the band width and largely suppresses Tc from ∼ 500 K in LiVO2 to ∼ 310 K in LiVS2. However, the magnetic properties in the high temperature phase are different between LiVO2 and LiVS2. The value of 7(1/T1T) in LiVS2 becomes smaller with decreasing temperature, which suggests the development of pseudogap above Tc, in qualitatively consistent with the results from the magnetic susceptibility and the small change in entropy at Tc. This is a contrast to the case in LiVO2 where 51(1/T1T) is proportional to 1/T and it is explained by a localized spin model well above Tc.

Comparative Study of the Interaction of O2 and C2H4 with Small Vanadium Clusters from Density Functional Theory

We have studied C(2)H(4) and O(2) molecules separately or simultaneously for adsorption on V(n) (n = 2-8) clusters, and V(n) clusters catalyzed ethylene oxidation to acetaldehyde using spin-polarized density functional theory calculations. Molecular adsorption and clear size-dependent adsorption energy are predicted for C(2)H(4). O(2) is dissociately adsorbed with nearly constant adsorption energy. In the case of coadsorption, O(2) and C(2)H(4) adsorb on the V(n) surface simultaneously. Each keeps the same adsorption form, molecular or dissociative, as in separate adsorption. The noted cooperative effect is noted in C(2)H(4) and O(2) coadsorption, which activates the C-C double bond of C(2)H(4) and favors its oxidization. Furthermore, both the separate and coadsorptions result in magnetic enhancement or reduction of V(n), which is found to be dependent on the cluster size and the adsorbates. In addition, we reveal the reaction mechanism of V(2) (V(6))-catalyzed ethylene oxidation to acetaldehyde and find the overall reaction is exothermic and barrierless.