Vanadyl Groups Research Articles

V 2O 3(0 0 0 1) surface terminations: from oxygen- to vanadium-rich

Well-ordered epitaxial V 2O 3(0 0 0 1) films (thickness > 80 Å) have been prepared on the clean Rh(1 1 1) surface and investigated with scanning tunnelling microscopy, low-energy electron diffraction, high-resolution electron energy loss spectroscopy and high-resolution X-ray photoelectron spectroscopy with synchrotron radiation. Atomically flat V 2O 3(0 0 0 1) surfaces, terminated by vanadyl (VO) groups, have been obtained after the reactive oxide deposition and subsequent annealing in vacuum to 600 °C. Annealing the VO termination in oxygen atmosphere (500 °C, 5 × 10 −6 mbar) results in the partial removal of the vanadyl groups and in the formation of an oxygen-richer surface termination, exhibiting a (√3×√3)R30° structure. Structure models for this oxygen-rich termination are proposed, which are characterised by (OV) 0.66–O 3–V–V⋯ and (OV) 0.33–O 3–V–V⋯ stacking sequences. Conversely, deposition of submonolayer V coverages onto the VO surface leads to the formation of a metal-rich V 2O 3(0 0 0 1) surface, terminated by a close-packed V 3 layer on top of the bulk-type O 3 plane. Such a VO(1 1 1)-layer is only metastable and oxidises back readily upon annealing in vacuum to the vanadyl-terminated V 2O 3(0 0 0 1) surface.

Adsorption of VO2+ by poly[(tetrafluoroethylene)-co-(perfluorovinyl ether)] copolymer grafted with acrylic acid usingγ-irradiation

Grafted films were prepared by the reaction of acrylic acid (AAc) onto poly[(tetrafluoroethylene)-co-(perfluorovinyl ether)] copolymer (PFA) using γ-irradiation by the mutual technique. The grafted copolymer was complexed with the vanadyl group, VO2+, in aqueous solution. The grafted copolymer–metal complexes were examined by infrared and ultraviolet spectrometry, energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). The amount of vanadium in the grafted films was estimated using EDS. The thermal stability of the films was investigated through thermogravimetric and differential scanning calorimetry measurements. The degree of crystallinity of the grafted and complexed films decreased by treatment with VO2+ ions and also by heating at 300 °C. When heated at a temperature above 300 °C, the grafted chains degraded till they disappeared and the original polymer was almost completely separated. XRD investigation revealed that the metal oxide may be formed as a separate phase with subsequent decrease in the crystallinity of the copolymer. Furthermore, scanning electron microscope (SEM) investigation of the grafted and modified films, both unheated and heated (300 °C), showed changes in the structure and morphology. The tendency of the graft copolymer to adsorb and/or bind to VO2+ from aqueous solution is of promising use in the field of waste treatment of rare metals in the environment. Copyright © 2004 Society of Chemical Industry

Building block syntheses of site-isolated vanadyl groups in silicate oxidesElectronic supplementary information (ESI) available: summary of experimental procedures, 51V NMR collection parameters, BET, IR and Raman data. See http://www.rsc.org/suppdata/cc/b3/b316184f/

Cross-linking of the cubic silicate building block, Si8O20 with vanadyl chloride leads to porous solids in which a homogeneous dispersion of isolated vanadyl groups is maintained throughout the matrix even at high loadings of vanadium.

Ab initio density functional theory studies on oxygen stabilization at the V 2O 3(0 0 0 1) surface

Geometric and electronic details of the V ′OV and VV ′O terminated V 2O 3(0 0 0 1) surface after oxygen chemisorption are studied theoretically by density-functional theory using large cluster models. Adsorption of oxygen above vanadium centers of the V ′OV terminated surface (O tV ′O termination) results in very strongly bound vanadyl groups removing part of the relaxation of the initial adsorbate-free surface. In addition, it affects the oxygen 2p dominated valence band region of V 2O 3(0 0 0 1) which increases its width by 1.5 eV due to the adsorbate, as shown in partial densities-of-states curves. Oxygen adsorption between vanadium centers of the VV ′O terminated surface is considered for coverages Θ(O b)=1/3 and Θ(O b)=2/3 yielding terminations O bVV ′ and O b 2VV ′. For both coverages the adsorbate stabilizes above the surface and forms V ′–O b–V bridges. As before, the adsorption removes part of the perpendicular relaxation of the topmost layers at the initial VV ′O terminated surface. In addition, the V–O b–V ′ bridges lead to decreased lateral distances between the corresponding V/V ′ centers. As a result, the lateral honeycomb geometry of the combined V/V ′ layers of the VV ′O termination is distorted by adsorbed oxygen where the distortion leads to different surface patterns for the O bVV ′ and O b 2VV ′ termination. The theoretical analysis suggests that only the O b 2VV ′ termination, corresponding to an oxygen coverage Θ(O b)=2/3, is compatible with the geometry obtained from STM images of V 2O 3(0 0 0 1) films on Cu 3Au(0 0 1).

Model Catalyst Studies on Vanadia Particles Deposited onto a Thin-Film Alumina Support. 2. Interaction with Carbon Monoxide

The interaction of CO molecules with a model catalyst system formed by alumina-supported vanadia particles was studied utilizing infrared (IR) spectroscopy. A thin, well-ordered alumina film grown on the NiAl(110) alloy surface was used as a support oxide. The structural characterization of this model system has been performed previously by applying scanning tunneling microscopy, X-ray photoelectron spectroscopy, and IR spectroscopy. Exposure to CO at 90 K results in a complex adsorption behavior as a function of vanadia particle size. At high vanadia coverages, only one CO species can be detected that interacts with vanadyl groups (VO) present at the surface of the particles. At lower coverages, however, three further species were observed. Their existence seems to be correlated with either defect sites or with special sites provided at the vanadia−alumina interface. Furthermore, particles in this coverage regime are capable of adsorbing a maximum amount of CO molecules. Regarding the reactivity of our model system, we found only molecular desorption of CO upon thermal treatment. However, exposing the CO covered particles to low-energy electrons renders a reaction possible. A portion of the adsorbed CO gets oxidized to CO2 via a redox mechanism, i.e., by a transfer of lattice oxygen. According to our experiments, the vanadyl oxygen does not seem to be the species that is consumed during the course of the reaction.

V 2O 3( [formula omitted]) on Au( [formula omitted]) and W( [formula omitted]): growth, termination and electronic structure

We have prepared well-ordered V 2O 3(0 0 0 1) films with thicknesses between 60 and 160 Å on W(1 1 0) and Au(1 1 1). These films have been studied with respect to the surface electronic and geometric structure and the interaction with oxygen. We find that after preparation with oxygen the surface is terminated by vanadyl groups. The surface may be reduced by electron bombardment. Oxygen adsorption with subsequent annealing restores the vanadyl groups. The vanadyl groups are thermally stable on the surface until at least 1000 K under UHV conditions. In XPS spectra of the vanadyl terminated surface a V2p induced surface state becomes visible under surface sensitive conditions. The binding energy shift of this state is compatible with a higher oxidation state of the vanadium atoms on the surface as compared to the bulk atoms. The V2p surface state induces additional intensity also in NEXAFS spectra as concluded from a set of spectra recorded with two different methods.

Photoluminescence Study of the Introduction of V in Si-MCM-41: Role of Surface Defects and Their Associated SiO- and SiOH Groups

The photoluminescent properties of highly ordered siliceous and vanadium-substituted mesoporous molecular sieves (MCM-41) are examined. Both Si-MCM-41 and V-loaded MCM-41 show green/blue/violet photoluminescence at 490−400 nm, likely due to surface defects associated with SiO- and/or SiOH groups. The intensity of the green/blue/violet emission strongly decreases when MCM-41 materials contain well-dispersed tetrahedral VV species. This intensity decrease is related to the consumption of surface defects and their associated SiO- and/or SiOH groups. Photoluminescence spectroscopy distinguishes only one kind of tetrahedral VV species, characterized by a vibrational fine structure and energy corresponding to a short VO bond. The intensity of photoluminescence from vanadyl groups in V-loaded MCM-41 increases as a function of V content. The addition of an anti-foaming agent into the synthesis solution enhances reproducibility of V-MCM-41 materials and causes a highly ordered hexagonal MCM-41 structure. The local symmetry of vanadium ions dispersed in highly ordered MCM-41 structure is closely related to the pore size of the mesoporous matrix.

The one dimensional chain structures of vanadyl glycolate and vanadyl acetateElectronic supplementary information (ESI) available: difference plots and reflections lists for VO(CH3COO)2 and VO(OCH2CH2O). See http://www.rsc.org/suppdata/jm/b2/b208100h/

The solvothermal reaction, at 200 °C, of vanadium pentoxide and lithium hydroxide in acetic acid or ethylene glycol leads to the formation of vanadyl acetate and vanadyl glycolate respectively. The structure of the acetate contains vanadium in octahedral coordination whereas the glycolate contains VO5 square pyramids. The VO6 octahedra in the acetate, VO(CH3COO)2, are joined through the vanadyl groups, giving a rather long VO bond of 1.684(7) Å and a short trans V-O bond of 2.131(7) Å, and by bridging acetate groups. The vanadium atoms interact along the ⋯VO⋯VO⋯ chain giving one-dimensional antiferromagnetic behavior. In contrast in the glycolate, the apical VO bond is shorter, 1.58(1) Å, and the square pyramids share edges in a two up–two down fashion to give chains of formula VO(OCH2CH2O). Magnetic susceptibility of vanadyl glycolate is consistent with an isolated spin dimers model.

Magnetic properties of phosphate-bridged copper(ii) organic–inorganic hybrid compounds

Hydrothermal synthesis has allowed us to isolate and characterize three new inorganic–organic hybrid materials, the compounds [Cu2(phen)2(H2O)2(H2PO4)2](NO3)2·2H2O (1), [(Cu2bipy2)2(V4O9)(PO4)2(HPO4)(H2P2O7)]n·nH2O (2) and [Cu2(dpa)2(V2P2O12)]n, (4), which have been magneto-structurally characterized. In addition, the magnetic behaviour of the related compound [Cu2(bipy)2(VO2)2(PO4)2]n (3), has been measured. Compound 1 has a dimeric structure, while compound 2 and 4 have dimer chain structures. The dependence of the magnetic susceptibility on temperature suggests antiferromagnetic coupling for 1, 2 and 3, while compound 4 can be well understood in terms of the Curie–Weiss law with θ = −23 K. The fitting of the magnetic behaviour to the Bleaney–Bowers spin–dimer coupling expression leads to J = −8.0 cm−1 for 1, J = −28.8 cm−1 for 2 and J = −29.0 cm−1 for 3. The use of alternating chain expressions leads to J = −28.8 cm−1, α = −0.03 for 2 and J = −29.4 cm−1, α = −0.027 for 3. Although a variety of coordination geometries are exhibited by the compounds in the series, all of them have O–P–O bridges connecting the cupric centres. It was possible to identify the role of the vanadyl groups of the phosphovanadate framework as an important factor affecting the ability of the phosphate groups to transmit magnetic interactions.

Density functional theory studies of mechanistic aspects of the SCR reaction on vanadium oxide catalysts

Density functional theory (DFT) calculations were carried out on a vanadium oxide cluster containing four vanadium atoms to probe the mechanism of the selective catalytic reduction (SCR) of NO with ammonia. The interaction of ammonia with Brønsted acid sites on this V 4-cluster leads to the formation of NH 4 species bonded to two vanadyl (VO) groups, with a bonding energy of −110 kJ/mol. This adsorbed NH 4 species reacts with NO in a series of steps to form an adsorbed NH 2NO species, which subsequently undergoes decomposition to form N 2, H 2O, and a reduced vanadium oxide cluster (V 4H). The latter reaction occurs via a series of hydrogen-transfer steps by a “push–pull” mechanism with adjacent VO and VOH groups on the vanadium oxide cluster. The rate limiting process in this conversion of NO and NH 3 to give N 2, H 2O, and V 4H involves the reaction of an adsorbed NH 3NHO adduct to form NH 2NO species. The transition state of this step may be stabilized through hydrogen bonding with surrounding vanadia and/or titania moieties.

Model Catalyst Studies on Vanadia Particles Deposited onto a Thin-Film Alumina Support. 1. Structural Characterization

A model system of alumina-supported vanadia particles, representing catalysts of the type oxide1/oxide2, was prepared under ultrahigh vacuum (UHV) conditions and characterized regarding its structural and electronic properties. As supporting oxide we used a thin, well-ordered alumina film grown on NiAl(110), which allows the application of scanning tunneling microscopy (STM), infrared reflection−absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS) without charging effects. Vanadium oxide particles were prepared via metal evaporation in an oxygen ambient, leading to the growth of small, roundish oxide particles with vanadium in the +3 oxidation state. The particles are shown to interact strongly with the alumina support, resulting in an increased alumina film thickness and a distortion of the alumina film structure. IR absorption signals of the deposits could be successfully assigned to specific V-containing species, thus providing insight into the inner structure of the particles. The species identified are surface-localized vanadyl groups (VO), interface-localized vibrations involving V, O, and Al ions, and lattice structures typical of bulk V2O3.

Crystal structure of a new sodium vanadyl(IV) fluoride phosphate Na3{V2O2F[PO4]2}

Abstract The title compound has been prepared by hydrothermal synthesis and its crystal structure was determined by single crystal X-ray diffraction: space group I4/mmm, a=6.3856(2), c=10.6119(9) A, wR2=0.039, R=0.016. The crystal structure of Na3{V2O2F[PO4]2} is formed by layers of alternating [VO5F] octahedra and [PO4] tetrahedra sharing O vertices parallel to the ab plane. Along the c direction the V octahedra are joined pairwise through common F vertices in the inversion centres. The sixth O vertex of the V octahedron is the terminal oxo ligand of a vanadyl group. Thus, the structure is described by a mixed paraframework of octahedra and tetrahedra {V2O2F[PO4]2} ∞ ∞ ∞ with disordered Na atoms in the interstices. The new phase is discussed as a member in the row of compounds structurally related to the mineral natisite and VOPO4·2H2O.

Long chain alkyl amine templated synthesis of a mesostructured lamellar vanadium phosphate phase

n-Hexadecylamine (HDA) has been used as a template to synthesize a mesolamellar vanadium phosphate (VPO) phase. FTIR studies indicate covalent interaction of the amine head with the vanadyl groups of the VPO matrix, in contrast to the other reported syntheses of mesostructured VPO phases which involve ionic interaction between a cationic surfactant and the inorganic matrix. XRD, TEM and solid state 13C MAS NMR studies show that the incorporated HDA induces the formation of a mesolamellar phase with a basal spacing of 44.5 A. On ageing however, a restructuring of the incorporated HDA is observed resulting in a curvature of the inorganic layers and a decrease of the basal spacing to 38.04 A.

Vanadyl naphthalocyanine and vanadyl porphine phenyl substituted macrocycles: SERS and thin film organisation studies

Infrared (middle and far) and Raman spectroscopy were used to study the adsorption of vanadylnaphthalocyanine and vanadylporphine tetraphenyl substituted macrocycles adsorbed onto metallic surfaces. The IR spectra of the compounds deposited onto a KBr monocrystal and onto a smooth copper surface suggest a weak adsorbate–substrate interaction and no significant structural modifications imposed by surface effect. The reflection–absorption IR (IRRAS) spectra allowed to propose a preferential molecular orientation of the molecules deposited onto the copper surface. Surface-enhanced Raman spectral data obtained on colloidal Ag as well as Ag island films suggest a weak macrocycle interaction and small structural modifications of the naphthalocyanine complex on the surface. The whole spectral data indicate that the naphthalocyanine complex is orientated with the naphthalocyanine plane face-on to the surface. In both complexes the vanadyl group is perpendicular to the coordination site and opposed to the surface. In both molecular systems the phenyl substituents, oriented perpendicular to the macrocycle plane, are responsible of the weak adsorbate–substrate interaction. Porphine complex has no interaction with the surface. This different behaviour of the above complexes is related to the different extent of the π-electronic size.

Adsorption of molecular and atomic hydrogen on vacuum-cleaved V 2O 5( [formula omitted

The (0 0 1) surface of a vacuum-cleaved V 2O 5 single crystal has been studied using high-resolution electron energy loss spectroscopy (HREELS), UV-photoelectron spectroscopy angular resolved photoemission spectra (ARUPS) and X-ray photoelectron spectroscopy (XPS). Special emphasis was put onto the interaction with molecular and atomic hydrogen and the existence of hydroxyl groups on the surface. Dosage of atomic hydrogen induces reduction of the V 2O 5(0 0 1) surface as is obvious from the evolution of intensity near to the Fermi edge in the photoelectron spectra. Even after strong reduction no indications of OH groups could be observed in the spectra. Detailed inspection of the ARUPS and HREELS data and comparison with theory leads to the conclusion that the hydrogen induced defects on the surface are most likely missing bridging oxygen atoms whereas the terminal oxygen atoms of the vanadyl groups appear to be stable with respect to removal by atomic hydrogen.

Surface Characterisation of V2O5/TiO2 Catalytic System

Samples of the V 2 O 5 /TiO 2 system were prepared by the sol-gel method and calcined at different temperatures. Surface species of vanadium, their dispersion, as well as the structural evolution of the system were analysed by XRD, Raman, EPR. and XPS techniques. The results of XRD showed the evolution of TiO 2 from anatase phase to rutile phase. The Raman spectra for calcination temperatures up to 500 °C showed a good dispersion of vanadium over titania in the form of monomeric vanadyl groups (V 4+ ) and polymeric vanadates (V 5+ ). At least three families of V 4+ ions were identified by EPR investigations. Two kinds of isolated V 4+ species are placed in sites of octahedral symmetry, substituting Ti 4+ in the rutile phase. The third is formed by pairs of V 4+ species on the surface of titania. Above 500 °C part of superficial V 4+ is inserted into the matrix of titania and part is oxidized to V 5+ . The XPS results showed that the V/Ti ratio rises with increasing calcination temperature, indicating a smaller dispersion of vanadium.

(V2O5)n Gas-Phase Clusters (n = 1−12) Compared to V2O5 Crystal: DFT Calculations

Stable structures of neutral (V2O5)n clusters (n = 1−5, 8, 10, and 12) are determined by density functional calculations (BP86 functional with a double-ζ (V)/triple-ζ (O) valence basis set augmented by polarization functions). Comparison is made with calculations for the periodic structure of solid V2O5. The most stable structure of the smallest cluster is doubly O-bridged, OV−O2−VO2, and by 184 kJ/mol VO2.5 less stable than the periodic bulk structure. From the tetrahedral V4O10 structure on (41 kJ/mol VO2.5 above the crystal energy) polyhedral cage structures are the most stable isomers: trigonal prism (V6O12), cube (V8O20), pentagonal prism (V10O25), 16-hedron (V16O40), dodecahedron (V20O50), and truncated octahedron (V24O60). The polyhedra have vanadyl groups at the apexes and bridging oxygen atoms on the edges. Differently from the crystal structure, vanadium is 4-fold coordinated and 3-fold coordinated oxygen is avoided. The energies relative to the periodic solid are 22.1, 12.4, 9.4, 5.5, 3.3, and...

Transient responses of the local electronic and geometric structures of vanado-molybdo-phoshate catalysts H 3+ nPV nMo 12− nO 40 in selective oxidation

The thermal stability of the series of title compounds was investigated with thermogravimetry (TG), differential thermal analysis (DTA), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and in situ UV–VIS–NIR difference reflectance spectroscopy (UV–VIS–NIR–DRS). The results were combined with temperature programmed reaction experiments (TPR) of dehydration and catalytic tests involving methanol oxidation. The HPA are fully dehydrated and autoreduced at the temperature of maximum conversion. The presence of gas phase oxygen and of the organic substrate have only a limited influence on the autoreduced state. The oxygen storage function was probed by pulse reduction experiments which gave evidence for the existence of two kinetically different supply processes. The active state of the HPA is a partially reduced oligomer of polyoxoanions bridged by vanadyl groups. During catalytic action a steady-state exists between the precursor monomer and the oligomerised state. This steady-state forms as consequence of the thermal load in water and oxygen atmospheres and is qualitatively not altered upon addition of the organic substrate. Above 673 K the steady-state turns irreversibly into defective oxides associated with the loss of selective oxidation activity. A consistent picture is concluded of the solid state chemistry occurring during response of the precursor HPA to thermal and chemical conditions of selective oxidation catalysis. The HPA solid appears as inherently unstable material during operation as gas phase oxidation catalyst.

Density Functional Theory Calculations of the Oxidative Dehydrogenation of Propane on the (010) Surface of V2O5†

Density functional theory and the calculations of oxygen nucleophilicity have been applied to an analysis of the oxidative dehydrogenation (ODH) of propane on the (010) surface of V2O5. These calculations show that the energetically preferred initial step is the dissociative adsorption of propane to form i-propoxide and hydroxyl species. Two VO groups [O(1)] bonded by a V−O−V bridge are required. One of the vanadyl groups attacks the β-C atom of propane and is converted to a V−OCH2(CH3)2 species, whereas the other vanadyl group is converted into a V−OH group. The activation barrier for this process is 9.4 kcal/mol. Dissociative adsorption to form an n-propoxide can also occur, but the activation barrier for this process is 14.5 kcal/mol. Propene and water are formed via a concerted process in which an H atom of one of the methyl groups of i-propoxide reacts with an O(3)H group. Exploration of alternative pathways for this step reveals that neither O(1, 2, 3), O(1)H, nor O(2)H are sufficiently reactive. These findings are in good qualitative agreement with experimental observations concerning the mechanism and kinetics of propane ODH.

Hybrid open-frameworks. Hydrothermal synthesis and structure determination of MIL-40 or V IV O(H 2 O){HO 3 P–CH 2 –(C 6 H 4 )–CH 2 –PO 3 H}: a lamellar vanadodiphosphonate prepared with p -xylendiphosphonic acid

Abstract VIVO(H2O){HO3P–CH2–(C6H4)–CH2–PO3H} labelled MIL-40 is a hybrid organic–inorganic compound whose bidimensional structure is built up from (VIV=O)2+ vanadyl groups and p-xylendiphosphonate units. MIL-40 was hydrothermally prepared at 443 K and its structure solved from single crystal X-ray diffraction data. It is triclinic (space group P 1 , n° 2) with lattice parameters a = 7.255 8(8) A, b = 7.573 9(8) A, c = 11.493(1) A, α = 93.230(2) °, β = 103.018(2) °, γ = 96.871(2) °, V = 608.7(1) A3, Z = 2, R1(F0) = 0.091 1, wR2(F02) = 0.189 6 for 2 843 unique reflections with I ≥ 2 σ(I). Its thermal decomposition is investigated.