Special features of synthesis of a heat-resistant glass-ceramic coating

In order to protect Ni–Cr alloys from high-temperature corrosion, a new heat-resistant glass-ceramic coating was developed with a glass matrix synthesized on the basis of a composite R _x O–Al₂O₃–SiO₂–TiO₂ (R–Li^–, Na⁺, K⁺, Mg²⁺, Ca²⁺, Ba²⁺) system. The special features of the formation of crystalline phases in the glasses in heat treatment and the optimum regime for the formation of a glass ceramic structure are described.Translated from Steklo i Keramika, No. 3, pp. 30–32, March, 1996.     

Novel Supramolecular Square Grid-Like Structure: Synthesis and Characterization of [(Me3Sn)2CrO4]

Reactions of K₂Cr₂O₇ and K₂CrO₄ with Me₃SnCl yielded [(Me₃Sn)₂CrO₄] (1) and [(Me₃Sn)₂CrO₄(Me₃SnOH)] (2), respectively, which were characterized by elemental analysis, ¹H-NMR spectroscopy, and an X-ray diffraction study. Compound 1 shows a novel square grid-like structure consisting of CrO₄ ^2– and Me₃Sn⁺ ions, whereas 2 exhibits a three-dimensional structure composed of CrO₄ ^2–, Me₃Sn⁺, and [(Me₃Sn)₂SnOH]⁺ ions.     

Electrochemical studies of titanium in fluoride-chloride molten salts

The interaction between titanium and Ti⁴⁺ ions (K₂TiF₆), the electroreduction reaction of Ti⁴⁺ ions and the anodic reaction of Ti in KCl–NaCl–KF melts with K₂TiF₆ at 973 K were studied by means of electrochemical and physical measurements. It was found that the fluoride ions played a very important role in these reactionsIn KCl–NaCl-3 wt % K₂TiF₆ molten salts with less than 3 wt % KF, the interaction reaction was considered to proceed as Ti⁴⁺+Ti=2Ti²⁺. If the bath contained more than 10 wt% KF, the reaction 3Ti⁴⁺+Ti=4Ti³⁺ occurred.The electrochemical reduction of Ti⁴⁺ (K₂TiF₆) ions in the molten salts with less fluoride ions was observed to proceed according to three reaction steps, i.e. Ti⁴⁺+e=Ti³⁺, Ti³⁺+e=Ti²⁺, Ti²⁺+2e=Ti. In the case of the fluoride ion concentration being higher, two reduction steps, i.e. Ti⁴⁺+e=Ti³⁺, Ti³⁺+3e=Ti were suggested.     

Detection of Reduced W n+ Sites on WO3–ZrO2 and Pt/WO3–ZrO2 Catalysts by Infrared Spectroscopy of Adsorbed NO

Adsorption of NO on oxidized WO₃–ZrO₂ catalysts leads to formation of adsorbed N₂O, surface nitrates, NO⁺, and Zr⁴⁺(NO^- ₃)–NO species. When NO is adsorbed on a sample reduced at 523 K, W⁵⁺ –NO (1855 cm^-1) and W⁴⁺(NO)₂ (1785 and 1700 cm^-1) species are formed in low concentration. Reduction at 573 K increases the density of W⁴⁺ sites and this density remains unchanged after reduction up to 673 K. The W⁴⁺(NO)₂ species are stable towards evacuation and in the presence of oxygen; however, they quickly disappear in the simultaneous presence of NO and O₂ as a result of the oxidation of the W⁴⁺ cations to W⁶⁺. The density of the W⁴⁺ sites on a Pt/WO₃–ZrO₂ sample is significant even after reduction at 523 K. This is explained by the promotion effect of platinum on the support reduction. The use of NO as a probe molecule for detection of reduced W ⁿ⁺ sites on tungstated zirconia is discussed.     

Glass Formation and Glass-Forming Ability of Melts in A IIO–B2O3 Binary Systems (A IIO = BeO, MgO, CaO, SrO, BaO, ZnO, CdO)

The data on the sizes of glass formation regions in the A ^IIO–B₂O₃ binary systems (where A ^II is a Group IIA or IIB element of the periodic table) and the possibility of calculating the glass-forming ability of simple oxides with the use of the Sun and Rawson criteria are analyzed. The disadvantages and limitations of these criteria are discussed. The modified Sun–Rawson criterion is described, and the glass-forming ability of melts in the A ^IIO–B₂O₃ systems is calculated according to this criterion. The possibility of forming glasses in the BaO–B₂O₃, SrO–B₂O₃, and CdO–B₂O₃ systems is predicted in the composition regions for which data on the glass formation are not available.     

New host–guest supramolecular coordination polymers based on [(Me3Sn)3Fe(CN)6]n with alkali metal iodides and their applications as electrode materials in batteries

The metal iodides reduce partially the host coordination polymer of the type $ ^{ 3}_{\infty } \left[ {\left( {{\text{Me}}_{ 3} {\text{Sn}}} \right)_{ 3} {\text{Fe}}\left( {\text{CN}} \right)_{ 6} } \right] $ , I, to give new host–guest supramolecular coordination polymers (SCP). The physical and chemical characteristics of the new products were studied by elemental analyses, X-ray powder diffraction, IR, UV/Vis, and solid state NMR spectra. The host–guest SCP are [M_x(Me₃Sn)₃Fe_(1–x)^IIIFe _x ^II (CN)₆]_n M = Li⁺·2H₂O, 1; Li⁺, 2; Na⁺, 3; K⁺, 4; Cu⁺, 5, [Li(Me₃Sn)₃Fe^II(CN)₆]_n, 6 and [(LiDEE)_0.9(Me₃Sn)₃Fe _o.1 ^III Fe _o.9 ^II (CN)₆]_n, 7. The stoichiometry and nature of the guest depend on the type of the metal iodide and the reaction conditions. The polymeric nature of these SCP is due to the presence of trigonal bipyramidal configured structure which bridges between the single d-transition metal ions. The host–guest SCP containing the Li ions have been tested as electrodes to construct four different lithium-ion batteries.     

Comparative study of Li(Li1/3Ti5/3)O4 and Li(Ni1/2−xLi2x/3Tix/3)Ti3/2O4 (x = 1/3) anodes for Li rechargeable batteries

A solid solution of spinel (2/3)Li(Li_1/3Ti_5/3)O₄–(1/3)Li(Ni_1/2Ti_3/2)O₄ was prepared, and its structural/electrochemical properties were compared with Li(Li_1/3Ti_5/3)O₄ to identify the effect of doping to the structural invariance of Li(Li_1/3Ti_5/3)O₄. The solid solution retained the zero strain characteristic of Li(Li_1/3Ti_5/3)O₄ during discharge/charge with an excellent cycle stability, while the rate capability was notably improved. However, a reversible broadening of the XRD peak was observed at the end of discharge, indicating some structural changes. XANES measurements showed that the oxidation state of Ti was +4 and that of Ni was +2 in the solid solution.     

Li+ Extraction from Spinel-Type LiMn2O4 in Different Eluents and Li+ Insertion in the Aqueous Phase

In order to afford a possible way to avoid manganese dissolution during Li⁺ extraction/insertion of spinel-type LiMn₂O₄, the elution properties of HCl, (NH₄)₂S₂O_8, and Na₂S₂O₈ were studied and the adsorption performance of Li⁺-extracted samples was characterized in Li⁺-containing solution. The results showed that Li⁺ was extracted by two different pathways: with manganese loss and without manganese loss. In the Li⁺ extraction process with manganese loss, ionic sieve was obtained after extracting Li⁺ from its precursor LiMn₂O₄, with accompanying partial transformation of Mn³⁺ to Mn⁴⁺ and Mn²⁺ by disproportionation reaction. This change caused the destruction of the framework and weight loss of ionic sieve due to the dissolution of Mn²⁺ in the solution. In the Li⁺ extraction process without manganese loss, Li⁺ was extracted with the reaction that part of Mn³⁺ was oxidized to Mn⁴⁺ by S₂O₈^2-. The Li⁺-extracted sample contained a small number of H⁺ which should exchange Li⁺ during this type of elution. The two Li⁺ extraction pathways also indicated that Li⁺ could not be completely eluted from the ionic sieve precursor. The uptake of Li⁺ on the ionic sieve was incomplete.     

Raman spectroscopy studies of concentrated vanadium redox battery positive electrolytes

Spectroscopic changes in highly concentrated vanadium(V)-sulfate solutions to be used in the vanadium redox battery are consistent with the presence of more than one V(V)-sulfate species. The results of Raman spectroscopy indicate that the major species in highly acidic conditions are VO₂SO₄ ^–, VO₂(SO₄)₂ ^3–, VO₂(HSO₄)₂ ^–, VO₃ ^–, V(V) dimers with V₂O₃ ⁴⁺ and V₂O₄ ²⁺ central units. The nature and amount of these species depends upon the V(V) and total sulfate concentrations as well as on S to V and H⁺ to V ratios in the positive half-cell electrolyte. V(V) forms V₂O₃ ⁴⁺, VO₂(SO₄)₂ ^3– and their copolymer species at higher total sulfate concentrations, which tends to stabilize the vanadium (V) positive electrolyte in the vanadium redox battery. The V(V) and V(IV) species show the least interaction with each other. Ageing of concentrated V(V) solutions at elevated temperature (50 °C) produces decomposition of species causing formation of V₂O₅ precipitates with a decrease in the amount of vanadium polymer.     

The role of co-dopants on the luminescent properties of α-Al2O3:Mn4+ and BaMgAl10O17:Mn4+

Mn⁴⁺ doped aluminate materials with efficient red emission are promising components for warmer white light-emitting diodes. However, it still remains as a challenge on increasing its luminous efficiency. For Mn⁴⁺ doped aluminate phosphors, co-dopants such as Li⁺, Mg²⁺, Na⁺, Si⁴⁺, or Ge⁴⁺ ions are often added to tailor the photoluminescence properties of phosphors during preparing process. However, the role of the ions is still in debate. In this work we took BaMgAl₁₀O₁₇:Mn (BMA:Mn) and α-Al₂O₃:Mn as examples to study the effects of Li⁺, Mg²⁺, Na⁺, and Si⁴⁺ on their luminescent properties. The energy levels induced by the co-dopants and some possible intrinsic defects of hosts (Al₂O₃) were calculated using the first-principles method. It is found that the Mg²⁺ and Na⁺ ions, compared with Li⁺ and Si⁴⁺, can prefer to form hole-type defects which enhance the valence stability of Mn⁴⁺ and thus enhance the emission intensity of the as-prepared phosphors.     

Electrical Properties and the Structure of Glasses in the xNa2O–(1 – x)2PbO · B2O3 System

The temperature–concentration dependences of the electrical conductivity and the activation energy for electrical conduction of glasses in the Na₂O–B₂O₃ and Na₂O–2PbO · B₂O₃ systems are studied. The investigation into the nature of the electrical conduction in these glasses reveals that the contribution from the electronic component (10^–3%) of the conductivity is within the sensitivity of the Liang–Wagner technique. A considerable alkali conductivity is observed upon introduction of more than 12 mol % Na₂O. The true transport number of sodium _Na is as large as unity at [Na₂O] 15 mol %. It is shown that the observed temperature–concentration dependences of the electrical and transport properties are governed by the ratio between the concentrations of polar and nonpolar structural–chemical units of the Na⁺[BO_4/2]^–, Na⁺[O^–BO_2/2] Na⁺[O^–BO_2/2], Pb²⁺ _1/2[BO_4/2]^–, Pb²⁺ _1/2[O^–BO_2/2], and [BO_3/2] types.     

Red,green and blue-violet emission coexistence in white-light-emitting Ca3SiO4Cl2:Bi3+, Li+, Eu2+, Eu3+ phosphor

A series of emission-tunable Ca₃SiO₄Cl₂:Bi³⁺, Li⁺, Euⁿ⁺(n =2, 3) (CSC:Bi³⁺, Li⁺, Euⁿ⁺) phosphors have been synthesized via sol-gel method. The X-ray diffraction results indicate that the as-synthesized phosphors crystallize in a low temperature phase with the space group of P21/c. Energy transfer from Bi³⁺ to Eu³⁺/Eu²⁺ exists in CSC:Bi³⁺, Li⁺, Euⁿ⁺ phosphors. Under the excitation of 327 or 365 nm, the Ca_2.98−ySiO₄Cl₂:0.01Bi³⁺, 0.01Li⁺, yEuⁿ⁺(y=0.0001–0.002) phosphors show an intense green emission band around 505 nm, while under the excitation of 264 nm, three emission bands centered around 396 nm (Bi³⁺), 505 nm (Eu²⁺) and 614 nm (Eu³⁺) are observed and tunable colors from blue-violet to green or white are achieved in these phosphors by varying the content of Eu. White-light emission with the color coordinate (0.312, 0.328) is obtained in Ca_2.978SiO₄Cl₂:0.01Bi³⁺, 0.01Li⁺, 0.002Euⁿ⁺(n =2, 3). Based on these results, the as-prepared CSC:Bi³⁺, Li⁺, Eu²⁺, Eu³⁺ phosphors can act as color-tunable and single-phase white emission phosphors for potential applications in UV-excited white LEDs.     

The temperature dependence of the electrochemistry of the lithium-sulphur dioxide system

Value for the activation energy, U ^act, and the entropy change, S, for the reaction 2Li + S₂O ₄ ^2– Li₂S₂O₄+2e in acetonitrile have been found to be 72 kJ mol^–11 and — 0.3 kJ mol^–1 K^–1, respectively, by a combination of impedance techniques and the use of a temperature-controlled environment on commercially manufactured cells which acted as constant volume containers.     

Synthetic ionophores for the separation of Li, Na, K, Ca, Mg metal ions using liquid membrane technology

Carrier-assisted transport through liquid membranes is one of the important applications of supramolecular chemistry. This work investigates the use of synthetic carrier (ionophore) for the separation of metal ions. We have tested the effect of structure of ionophore on the separation of metal ions. For this purpose, we have used a new series of non-cyclic ionophores having different end groups and chain length (R₁–R₅). 1,5 bis (2-naphthyloxy)-3-oxapentane (R₁), 1,8 bis (2-naphthyloxy)-3,6-di-oxaoctane (R₂), 1,11 bis (2-naphthyloxy)-3,6,9-tri-oxaundecane (R₃) 1,11-(dianthraquinonyloxy) 3,6,9-trioxaundecane (R₄), 1,8 (dianthraquinonyloxy) 3, 6-dioxaoctane (R₅) have been used in extraction, bulk liquid membrane (BLM) and supported liquid membrane (SLM) transport of alkali (Li⁺, Na⁺, K⁺) and alkaline earth metal cations (Ca²⁺, Mg²⁺). The supported liquid membrane consisted of a porous cellulose nitrate and and an onion membrane support impregnated with ionophore using chloroform as a solvent. The results reveal that ionophores R₁, R₂ and R₃ are better extractants for K⁺ while R₄ and R₅ are better extractants for Ca²⁺. Among these ionophores R₃ and R₄ are best extractants for K⁺ and Ca²⁺ ions. The results of BLM reveal that ionophores R₁, R₂ and R₃ transport Na⁺ at a greater extent, while R₄ and R₅ transport Ca²⁺ and K⁺ at a greater extent. In SLM experiments using a cellulose nitrate membrane support, it was observed that naphthyl end group bearing ionophores (R₁–R₃) transports Na⁺ > K⁺, and anthraquinone bearing ionophores (R₄ and R₅) transport K⁺ > Na⁺ > Ca²⁺ respectively. In the onion membrane support R₄ transports Ca²⁺ and Na⁺ equally and R₅ transports K⁺ selectively. On comparing the membrane support, the cellulose nitrate membrane is found better support for the transport of metal ions. The results suggest that due to the presence of different end groups and chain lengths the selectivity of non-cyclic ionophores towards metal ions is enhanced. Thus selectivity of ionophores may have fruitful application in ion selective electrodes and separation of metal ions.     

Characterization and use of aqueous caesium chloride as an ultra-concentrated salt bridge

Five strong aqueous binary electrolytes — one symmetrical (CsCl) and four unsymmetrical (Li₂SO₄, K₂SO₄, Rb₂SO₄, Cs₂SO₄) — have been examined, for possible use as salt bridges for the minimization of liquid junction potentials (E _L), up to the highest concentrations practicable, by the method of homoionic transference cells: Pt–Ir | Cl₂ | CsCl (m ₂) CsCl (m ₁) | Cl₂ | Pt–Ir and Hg | Hg₂Cl₂ | CsCl (m ₂) CsCl (m ₁) | Hg₂Cl₂ | Hg for CsCl, and Hg | Hg₂SO₄ | Me₂SO₄ (m ₂) Me₂SO₄ (m ₁) | Hg₂SO₄ | Hg for the Me₂SO₄ sulphates where Me=Li, K, Rb and Cs. CsCl, K₂SO₄, Rb₂SO₄, and Cs₂SO₄, prove to belong to the class obeying close equality of transference numbers for their ions, that is,t ₊= |t _–|=0.5, over the whole concentration range (namely, from infinite dilution up to saturation). This result qualifies aqueous CsCl as an unrivalled salt bridge, whose equitransference is obeyed more stringently than any other salt. This is now demonstrated experimentally over the whole molality range, the saturation molality being as high as 11.30 mol kg^–1 at 25°C. The observed propertyt ₊=|t _–|=0.5 excludes K₂SO₄, Rb₂SO₄, and Cs₂SO₄, as possible salt bridges because the equitransference conditions for minimization ofE _L's are ₊ = |_–| = l/(z ₊ + |z _–|) = 0.333, i.e.,t ₊=0.333 andt _–=2t ₊=0.667. Finally, Li₂SO₄, though behaving quite differently from the other three sulphates studied, does not sufficiently approach the required conditions, contrary to what one might have hoped from its known infinite-dilution transference numbers.     

Synthesis and photoluminescence properties of Sr3(PO4)2:Re3+, Li+ (Re = Eu,Sm) red phosphors for white light-emitting diodes

Sr₃(PO₄)₂:Re³⁺, Li⁺ (Re = Eu, Sm) red phosphors were prepared via a high temperature solid state reaction, and their structure and luminescence properties were investigated. X-ray diffraction patterns indicate that the phase of as-prepared samples is in good agreement with standard Sr₃(PO₄)₂ structure. Under 395 nm excitation, the emission of Sr₃(PO₄)₂:Eu³⁺ consists of a strong peak centered at 622 nm and two weak peaks centered at 598 nm and 660 nm, which correspond to ⁵D₀→⁷F₂, ⁵D₀→⁷F₁ and ⁵D₀→⁷F₃ transitions, respectively. Also, the emission spectrum of Sr₃(PO₄)₂:Sm³⁺ shows three main peaks at 568 nm, 603 nm and 651 nm, which are attributed to ⁴G_5/2→⁶H_I/2 (I = 5, 7, 9) transitions of Sm³⁺. Furthermore, luminescence properties of Sr₃(PO₄)₂:Re³⁺, Li⁺ (Re = Eu, Sm) samples are enhanced significantly by Li⁺ ions doping as charge compensator. Results indicate that as-prepared Sr₃(PO₄)₂:Re³⁺, Li⁺ (Re = Eu, Sm) could be the potential red phosphors used in white light-emitting diodes.     

Exploration of molten hydroxide electrochemistry for thermal battery applications

The electrochemistry of molten LiOH–NaOH, LiOH–KOH, and NaOH–KOH was investigated using platinum, palladium, nickel, silver, aluminum and other electrodes. The fast kinetics of the Ag⁺/Ag electrode reaction suggests its use as a reference electrode in molten hydroxides. The key equilibrium reaction in each of these melts is 2 OH^– = H₂O + O^2– where H₂O is the Lux-Flood acid (oxide ion acceptor) and O^2– is the Lux–Flood base. This reaction dictates the minimum H₂O content attainable in the melt. Extensive heating at 500 °C simply converts more of the alkali metal hydroxide into the corresponding oxide, that is, Li₂O, Na₂O or K₂O. Thermodynamic calculations suggest that Li₂O acts as a Lux–Flood acid in molten NaOH–KOH via the dissolution reaction Li₂O_(s) + 2 OH^– = 2 LiO^– + H₂O whereas Na₂O acts as a Lux–Flood base, Na₂O_(s) = 2 Na⁺ + O^2–. The dominant limiting anodic reaction on platinum in all three melts is the oxidation of OH^– to yield oxygen, that is 2 OH^– 1/2 O₂ + H₂O + 2 e^–. The limiting cathodic reaction in these melts is the reduction of water in acidic melts ([H₂O] [O^2–]) and the reduction of Na⁺ or K⁺ in basic melts. The direct reduction of OH^– to hydrogen and O^2– is thermodynamically impossible in molten hydroxides. The electrostability window for thermal battery applications in molten hydroxides at 250–300 °C is 1.5 V in acidic melts and 2.5 V in basic melts. The use of aluminum substrates could possibly extend this window to 3 V or higher. Preliminary tests of the Li–Fe (LAN) anode in molten LiOH–KOH and NaOH–KOH show that this anode is not stable in these melts at acidic conditions. The presence of superoxide ions in these acidic melts likely contributes to this instability of lithium anodes. Thermal battery development using molten hydroxides will likely require less active anode materials such as Li–Al alloys or the use of more basic melts. It is well established that sodium metal is both soluble and stable in basic NaOH–KOH melts and has been used as a reference electrode for this system.     

Organic–inorganic hybrid material as zwitterion: Synthesis and structure of terminal ammonium ions-containing organosilyl species supported on mono-lacunary Dawson polyoxometalate

The title compound 1 with a formula of (Bu₄N)₄[α₂-P₂W₁₇O₆₁{Me₃N⁺(CH₂)₃Si}₂O] · CH₃CN was obtained in 77.1% (4.21 g scale) yield by a 1:2 molar-ratio reaction of the mono-lacunary Dawson polyoxometalate (POM) [P₂W₁₇O₆₁]¹⁰⁻ with N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride ([Me₃N(CH₂)₃Si(OMe)₃]Cl) in mixed water/acetonitrile solution under acidic conditions and unequivocally characterized with complete elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), FTIR, solution (³¹P, ¹H, ¹³C and ²⁹Si) NMR and X-ray crystallography. Compound 1 contained two types of the ammonium cations, i.e., the counterions Bu₄N⁺ (free cations) and the terminal quaternary ammonium ions (bound cations) which are connected to the POM through the organosilyl groups.     

Synthesis,optical, and magnetic properties of six-layered Aurivillius bismuth ferrititanate

This work reports on the preparation, structure, photochemical, and magnetic properties of six-layered Aurivillius bismuth ferrititanates, that is, Bi₇Ti₃Fe₃O₂₁, Bi₇(Ti₂Nb)Fe₃O_21+δ, and Bi₇(Ti₂Mg)Fe₃O_21−δ nanoparticles. The samples were prepared through the modified citrate complexation and precursor film process. The XRD Rietveld refinements were conducted to study the phase formations and crystal structure. The morphological and chemical component characteristics were investigated using SEM, TEM, and EDX analyses. Bi₇Ti₃Fe₃O₂₁, Bi₇(Ti₂Nb)Fe₃O_21+δ, and Bi₇(Ti₂Mg)Fe₃O_21−δ nanoparticles present an indirect allowed transitions with band energies of 2.04, 2.03, and 2.02 eV, respectively. The hybridized (O2p+Fet_2g+Bi6s) formed the valence band (VB) and electronic components of (Ti–3d+Fe–e_g) formed the conduction band (CB) of this six-layered Aurivillius bismuth ferrititanate. The three samples showed efficient photocatalytic degradation of Rhodamine B (RhB) dyes with the excitation wavelength λ > 420 nm. The optical absorption, photodegradation, and magnetic abilities were improved through microstructural modification on “B” site via partial substitution of Mg²⁺ and Nb⁵⁺ for Ti⁴⁺. The photocatalytic results were discussed based on the layer structure and multivalent Fe ions. Fe^3+/2+ in the perovskite slabs (Bi₅Fe₃Ti₃O₁₉)²⁻ could act as the catalytic mediators in the photocatalysis process. As a photocatalyst, Aurivillius Bi₇(Ti₂Mg)Fe₃O_21−δ nanoparticle is advantageous due to its photocatalytic and magnetically recoverable abilities.     

Molecular Engineering of Supported Vanadium Oxide Catalysts Through Support Modification

Highly dispersed, multilayered surface metal oxide catalysts (V₂O₅/MO _x /SiO₂, M = Ti(IV), Zr(IV) or Al(III)) were successfully synthesized by taking into account various factors that govern the maximum dispersion of metal oxide species on silica. The characterization results revealed that the molecular structures of the surface vanadium oxide species on the modified supports are a strong function of environmental conditions. The surface vanadium oxide species under dehydrated conditions are predominantly isolated VO₄ units, similar to the dehydrated V₂O₅/SiO₂ catalysts. Upon hydration, the surface vanadium oxide species on the modified supports consist of polymerized VO₅/VO₆ units and/or less polymerized (VO₃) _n species, which depend on the vanadia content and the specific second metal oxide loading. The surface V cations are found to preferentially interact with the surface metal (Ti, Zr or Al) oxide species on silica. The V(V) cations in the dehydrated state appear to possess both oxygenated ligands of Si(IV)–O^– and M–O^–. Consequently, the reducibility and catalytic properties of the surface vanadium oxide species are significantly altered. The turnover frequencies of the surface VO₄ species on these modified supports for methanol oxidation to redox products (predominantly formaldehyde) increase by more than an order of magnitude relative to the unmodified V₂O₅/SiO₂ catalysts. These reactivity enhancements are associated with the substitution of Si(IV)–O^– oxygenated ligands by less electronegative M–O^– ligands in the O=V(–O–support)₃ structure, which strongly suggests that the bridging V–O–support bonds play a key role in determining the reactivity of the surface vanadium oxide species on oxide supports.