Tungsten Oxide Species Research Articles

Structural and Catalytic Characterization of Solid Acids Based on Zirconia Modified by Tungsten Oxide

Tungsten oxide species form strong acid sites on ZrO2supports and inhibit ZrO2crystallite sintering and tetragonal to monoclinic structural transformations. W-LIX-ray absorption near-edge spectra suggest that the W centers are in a distorted octahedral oxygen environment, even after dehydration at 673 K, in all WOx–ZrO2samples (2–21 wt.% W) oxidized at 1073 K. Maximumo-xylene isomerization turnover rates (per W atom) on WOx–ZrO2solids occur at WOxsurface densities (10 W nm−2) that exceed the theoretical monolayer capacity of ZrO2. Similar turnover rates are obtained on WOx–ZrO2samples with similar WOxsurface densities (W nm−2) over a large range of oxidation temperatures (773–1223 K) and WOxconcentrations (5–21 wt.% W). UV–visible spectra suggest an increase in WOxdomain size with increasing surface density. High isomerization turnover rates appear to require the presence of WOxdomains of intermediate size on ZrO2surfaces. WOxdomains of intermediate size appear to provide a compromise between reducibility and accessibility of WOxcenters. These domains are necessary to delocalize a temporary charge imbalance that forms Brønsted acid sites in the presence of H2and stabilizes carbocation intermediates. The presence of H2duringo-xylene isomerization increases turnover rates and prevents rapid deactivation. Slow D2/o-xylene exchange reactions indicate that H atoms from H2are not frequently involved in the activation or desorption of xylenes. H2is required, however, in order to reverse the occasional desorption of H atoms duringo-xylene isomerization reactions. These desorption processes lead to the destruction of Brønsted acid sites by the formation of strongly adsorbed unsaturated species in the absence of H2. After promotion with Pt (0.3 wt.%), WOx–ZrO2solids catalyzen-heptane isomerization in the presence of H2at 400–500 K with much higher selectivity than sulfated oxides or zeolitic acids at similar turnover rates. On Pt/WOx–ZrO2, efficient hydrogen transfer steps prevent extensive cracking of adsorbed carbocations by limiting their surface lifetimes.

Oxidation of sulfur dioxide to sulfur trioxide over supported vanadia catalysts

The objectives of this research are to establish the fundamental kinetics and mechanism of sulfur dioxide oxidation over supported vanadia catalysts and use these insights to facilitate the design of SCR DeNO x catalysts with minimal sulfur dioxide oxidation activity. A series of supported vanadia catalysts were prepared on various metal-oxide supports: ceria, zirconia, titania, alumina and silica. Raman spectroscopy was used to determine the coordination of surface species. At low vanadia loadings, vanadia preferentially exists on oxide support surfaces as isolated tetrahedrally coordinated (M–O) 3V +5=O species. At higher vanadia loadings, the isolated (M–O) 3V +5=O species polymerize on the oxide support surface breaking two V–O–M bonds and forming two V–O–V bridging bonds. The turnover frequency for sulfur dioxide oxidation was very low, 10 −4 to 10 −6 s −1 at 400°C, and was independent of vanadia coverage suggesting that only one vanadia site is required for the oxidation reaction. As the support was varied, sulfur dioxide oxidation activity of the supported vanadia catalysts varied by one order of magnitude (Ce>Zr, Ti>Al>Si). The basicity of the bridging V–O–M oxygen appears to be responsible for influencing the adsorption and subsequent oxidation of the acidic sulfur dioxide molecule. Over the range of conditions studied, the rate of sulfur dioxide oxidation is zero-order in oxygen, first-order in sulfur dioxide and inhibited by sulfur trioxide. The turnover frequency for sulfur dioxide oxidation over WO 3/TiO 2 was an order of magnitude lower than that found for V 2O 5/TiO 2, and no redox synergism between the surface vanadia and tungsten oxide species was evident for a ternary V 2O 5/WO 3/TiO 2 catalyst. This suggests that WO 3 promoted catalysts may be suitable for low-temperature SCR where minimal sulfur dioxide oxidation activity is required.

Characterization of Zirconia-Supported Tungsten Oxide Catalyst

Tungsten oxide−zirconia catalysts were prepared by drying powdered Zr(OH)4 with ammonium metatungstate aqueous solution, followed by calcining in air at high temperature. Characterization of prepared catalysts was performed by using Fourier transform infrared (FTIR), Raman, and X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) and by measuring surface area. The addition of tungsten oxide up to 20 wt % to zirconia shifted the phase transition of ZrO2 from amorphous to tetragonal to higher temperature due to the interaction between tungsten oxide and zirconia, and the specific surface area and acidity of catalysts increased in proportion to the tungsten oxide content. Since the ZrO2 stabilizes the tungsten oxide species, for the samples equal to or less than 5 wt %, tungsten oxide was well dispersed on the surface of zirconia, but for the samples containing 13 wt % or above 13 wt %, the triclinic phase of WO3 was observed at any calcination temperat...

Partial oxidation of methane to formaldehyde over W/HZSM-5 catalysts

The partial oxidation of methane to formaldehyde using molecular oxygen as oxidant was studied over W/HZSM-5 catalysts with tungsten loading varying from 0.3% to 11.9%. The catalysts were characterized by means of XRD, UV diffuse reflectance spectroscopy, BET, TPR and TPD of ammonia. Three different types of supported species were detected: tetrahedral monomeric species, octahedral polymeric species, octahedral polymeric species and WO 3 crystallites. The relative amount of each one varies significantly with the tungsten loading. At high contents, a decrease in BET areas due to the partial blocking up of the channels of the zeolite and a lessening of the number of acid sites were observed. It was found that supported tungsten oxide phase possesses active sites to partial oxidation of methane to formaldehyde. HCOH selectivity seems to be dependent on the dispersion and the nature of the tungsten oxide species in the catalyst.

The influence of metal oxide additives on the molecular structures of surface tungsten oxide species on alumina: I. Ambient conditions

The molecular structures of WO 3/Al 2O 3 catalysts, with and without the presence of secondary metal oxide additives (P, Sn, Fe, Ni, Zn, Ce, Co, La, Ca, Mg, K and Na), were determined with Raman spectroscopy under ambient conditions where the surface is hydrated. The hydrated surface tungsten oxide species found on the alumina support (WO 4 −2, HW 6O 21 −5, and U-metatungstate) were related to the net pH at the point of zero charge (pzc) of the thin aqueous layer. The isolated tungstate species was dominant at high pH values and the polytungstate species were dominant at lower pH values. The net pH at pzc was a function of tungsten oxide loading (decreasing from 8.9 to 4.4 for 1 to 25% WO 3/Al 2O 3, respectively) and the specific secondary metal oxide additive (Na>K>Mg>Ca>La>Co>Ce>Zn>Ni>Fe>Sn>P). Below monolayer coverages, the secondary metal oxide additives preferentially interacted with the alumina support to form surface metal oxide species rather than with the tungsten oxide species to form mixed tungsten oxide crystalline compounds. Above monolayer coverages, both surface metal oxide species and mixed tungsten oxide crystalline compounds were formed. The more basic secondary metal oxide additives (Ca, La) formed mixed tungsten oxide crystalline compounds. In the absence of the formation of mixed tungsten oxide crystalline compounds, the alumina support was able to accommodate approximately two monolayer-equivalents of the surface metal oxides during calcination because the surface tungsten oxide species and the secondary surface metal oxide species occupied different sites on the alumina support. The surface tungsten oxide species preferentially interacted with the surface hydroxyls of the alumina support and the secondary surface metal oxide species preferentially interacted with coordinately unsaturated Al +3 sites.

The influence of metal oxide additives on the molecular structures of surface tungsten oxide species on alumina. II. In situ conditions

The dehydrated Raman spectra of unpromoted WO 3/Al 2O 3 indicate that the tungsten oxide surface species have the mono-oxo structure and are present as tetrahedrally coordinated monomers at low loadings and as a mixture of tetrahedrally and octahedrally coordinated surface polymers at high loadings. The effect of introducing secondary metal oxide additives to WO 3/Al 2O 3 catalysts, where tungsten oxide is present as a two-dimensional surface metal oxide overlayer, was investigated with in situ Raman spectroscopy under dehydrated conditions. The secondary metal oxide additives P 2O 5, SnO 2, Fe 2O 3, NiO, ZnO, CoO, CeO 2 and MgO coordinate directly to the oxide support without significantly interacting with the surface tungsten oxide phase. The secondary metal oxide additives La 2O 3, CaO, K 2O and Na 2O directly interact with surface tungsten oxide species to form both a surface mixed metal oxide complex and, for La 2O 3 and CaO, a crystalline tungstate compound. The noninteracting additives tend to be acidic (P 2O 5 and SnO 2) or mildly basic (Fe 2O 3, NiO, ZnO, CoO, CeO 2 and MgO) and the interacting additives tend to be more basic (La 2O 3, CaO, K 2O and Na 2O).

The structure and surface acidity of zirconia-supported tungsten oxides

The structure and surface acidity of zirconia-supported tungsten oxide are studied by using BET, XRD, FT Raman, Tian–Calvet microcalorimetry as well as XPS. Three different crystal forms of ZrO2 and their hydroxide precursors were prepared in a controllable way. It was found that the starting material and preparing conditions used have a significant effect on the structure of ZrO2 obtained. The tungstate species on zirconium hydroxide generally promote the transformation of the hydroxide precursor into tetragonal ZrO2, while the same species on the pre-calcined ZrO2 has less effect during calcination. A bulk-phase WO3 existed in the samples prepared by initially impregnating the monoclinic, tetragonal and cubic ZrO2 or the hydroxide precursors of m- and t-ZrO2, while no crystalline WO3 was present in the samples prepared by initially impregnating the hydroxide precursor of c-ZrO2. Raman results revealed that the surface tungstate(s) on three different zirconium hydroxides or their respective zirconia have similar structural features before calcination. The surface tungsten oxides and some WO3 bulk phase are the detectable tungsten species in the final samples, the ratio of each component is strongly dependent on the preparation history and the nature of the support. Creation of very strong acidic sites on the zirconia-supported tungsten oxide is related to the crystal form of zirconia itself, the type of tungsten oxide species, and the cooperation between the surface tungsten oxide overlayer and zirconia. The strong acidic sites can be completely poisoned by the remaining sodium ion impurities, but have not been appreciably influenced by added yttrium component. The crystalline WO3 seems of little importance in building up the strong surface acidity.

The dynamic states of silica-supported metal oxide catalysts during methanol oxidation

The in situ Raman spectra of silica-supported metal oxide catalysts (containing surface metal oxide species of V, Nb, Cr, Mo, W and Re) were measured during methanol oxidation. Stable surface methoxy species were found to form via reaction with the surface SiOH groups for all the catalysts investigated. The surface SiOH groups were also titrated by the surface metal oxide species and the surface concentration of SiOH groups decreases with increasing metal oxide loading. The stable surface MOCH 3 were only found for the V 2O 5/SiO 2 catalysts. The surface metal oxide species were all influenced by the methanol oxidation reaction. The surface rhenium oxide species were removed from the silica surface and the surface molybdenum oxide species were partially agglomerated to crystalline β-MoO 3 particles by the formation of Re-methoxy and Mo-methoxy complexes. The surface niobium oxide and tungsten oxide species were partially reducing by the net reducing methanol oxidation environment. In situ Raman spectra for the CrO 3/SiO 2 catalysts could not be obtained due to the formation of reduced chromium oxide species during methanol oxidation which gave rise to sample fluorescence. The in situ Raman observations provided a fundamental basis for understanding the selectivity patterns of the silica-supported metal oxide catalysts during methanol oxidation. However, the mechanism by which the silica support ligands activate the redox properties of the surface metal oxide species is not completely understood.

Selective Catalytic Reduction of NO with NH3over Supported Vanadia Catalysts

The selective catalytic reduction (SCR) of NO with NH3was systematically investigated over a series of supported vanadia catalysts to obtain additional insight into this important industrial reaction. The influence of surface vanadia coverage, promoters (surface tungsten oxide, niobium oxide, and sulfate species), and the specific oxide support (TiO2, Al2O3, and SiO2) was examined. The molecular structures of the surface metal oxide species were determined byin situRaman spectroscopy, and the corresponding surface acidity properties were monitored with infrared spectroscopy employing pyridine adsorption. The redox properties of the surface metal oxide species were probed with the sensitive methanol oxidation reaction and temperature-programmed reduction. The SCR reactivity of the various catalysts was determined over a wide temperature range. The current findings suggest that a dual-site (a surface vanadia redox site and an adjacent nonreducible metal oxide site) mechanism is required for the efficient selective catalytic reduction of NO with NH3over supported vanadia catalysts. The SCR reaction is sensitive to the immediate environment of the surface vanadia species: overall surface coverage of the metal oxide overlayer (factor of 5 in turnover frequency), nature of adjacent surface metal oxide species (factor of 10 in turnover frequency) and oxide support ligands (factor of 3 in turnover frequency). The SCR reaction, however, does not appear to depend on the specific structure of the surface vanadia species. The SCR selectivity toward N2formation also varies with the immediate environment of the surface vanadia species. The selectivity depends on the specific oxide support (TiO2>Al2O3>SiO2), temperature (decreases at higher temperature due to oxidation of NH3and NO to N2O), and surface concentration of redox sites (decreases with the concentration of pairs of redox sites). The SCR reaction is not related to the properties of the terminal V=O bond sincein situRaman studies during SCR, employing V=18O, demonstrate that this bond is relatively stable under reaction conditions (possessing a lifetime that is ∼10 times the characteristic reaction time). Thus, the bridging V–O–support bond appears to be involved in the rate-determining step.

Surface structures of supported tungsten oxide catalysts under dehydrated conditions

The molecular structures of the WO 3/support (Al 2O 3, TiO 2, Nb 2O 5, ZrO 2, SiO 2, and MgO) catalysts under in situ dehydrated conditions have been investigated by Raman spectroscopy. The series of catalysts was synthesized by the aqueous incipient wetness method. The WO 3/support catalysts, with the exception of the WO 3 SiO 2 and WO 3 MgO catalysts, possess a highly distorted, octahedrally coordinated surface tungsten oxide species with one short WO bond (mono-oxo tungsten oxide species) at high surface coverages. The WO 3 SiO 2 catalysts exhibit strong Raman features of crystalline WO 3 particles due to the relative low density and reactivity of the surface hydroxyl groups. The WO 3 MgO catalysts possess non-stoichiometric compounds, Mg x (WO 4) y and Ca x (WO 4) y , at low tungsten oxide contents and crystalline MgWO 4 and CaWO 4 at high tungsten oxide contents. This result is attributed to the high aqueous solubility of MgO as well as the CaO impurity and the strong acid-base interaction between WO 2− 4 with Mg(OH) 2 and Ca(OH) 2. The current findings for supported tungsten oxide catalysts parallel the previous findings for supported molybdenum oxide catalysts and reflect the similar surface structural chemistry of these two oxides.

Combined Raman and IR study of MOx–V2O5/Al2O3(MOx= MoO3, WO3, NiO, CoO) catalysts under dehydrated conditions

The influence of a second metal oxide (tungsten oxide, molybdenum oxide, nickel oxide, cobalt oxide) upon a V2O5/Al2O3 catalyst has been investigated by a combined Raman and IR study under dehydrated conditions. The presence of tungsten or molybdenum oxide was found to increase the concentration of polymerized surface vanadium oxide species, and this reflects the higher surface coverages of the metal oxides on the alumina support. This is probably caused by competition between vanadium and molybdenum oxide (or vanadium and tungsten oxide) species for reaction with the alumina hydroxy groups, since the IR spectra showed that on addition of these metal oxides the same type of alumina hydroxy groups are consumed. The presence of nickel or cobalt oxide on alumina increases the concentration of polymerized vanadium oxide species dramatically, which indicates that the presence of these oxides is also experienced by the surface vanadium oxide. However, the hydroxy groups are not affected as they are for the molybdenum and tungsten oxide systems.

Characterization of (V 2O 5-WO 3) on TiO x/Al 2O 3 Catalysts by 1H-, 15N-, and 51V Solid State NMR-Spectroscopy

Vanadium and tungsten oxide supported on TiO x /Al 2O 3 catalysts have been studied by solid state NMR spectroscopy. 1H-MAS NMR spectra demonstrated that basic AlOH groups are involved in anchoring TiO x and VO x species. A certain percentage of the AlOH groups are located in positions, probably in the bulk of the alumina particles, where they remain inaccessible for interaction with titania, vanadia, or tungsten oxide. Titania modification of the Al 2O 3 surface leads to formation of bridging hydroxyls, namely Ti(OH)Ti and (or) Ti(OH)Al. Hydroxyl groups coordinated to Ti atoms are involved in anchoring vanadium and tungsten oxide species. VOH (and probably WOH) groups are formed when this ternary oxide phase is dispersed on the TiO x /Al 2O 3 support. 51V-NMR spectra indicated that tungsten oxide participates in the formation of the structure of the mixed oxide surface complexes. 15N-NMR spectroscopy of adsorbed N 2O permitted the detection of strong Lewis acid sites on these multicomponent supported oxide catalysts. The acid strength of (V 2O 5-WO 3) on TiO x /Al 2O 3 catalysts turned out to be comparable to that of the TiO x /Al 2O 3 support material.

Preparation and characterization of WO3/SiO2 catalysts

Two sets of WO3/SiO2 catalysts were prepared from (NH4 6H2W12O40 (aqueous method) and W(η3-C3H5)4 (non-aqueous method). The molecular structures and dispersions of the surface tungsten oxide species for the WO3/SiO2 catalysts under ambient and in situ dehydrated conditions were investigated by Raman spectroscopy. The samples prepared from (NH4)6H2W12O40 (aqueous method) exhibit very strong Raman features due to the presence of crystalline WO3 and the samples prepared from W(η3-C3H5)4 (non-aqueous method) do not possess crystalline WO3. These results suggest that the preparation method exerts an influence on the dispersion of the surface tungsten oxide species on SiO2. The surface tungsten oxide species under ambient conditions possess polytungstate clusters, W12O 42 12− , on the silica support. Upon dehydration at elevated temperatures, the hydrated polytungstate clusters decompose and interact with the silica support via the formation of isolated, octahedrally coordinated tungsten oxide species.

Use of Nafion as a Solid Polymer Electrolyte for the Electroreduction of Tungsten (VI) Fluoride

A new method is described in which is electroreduced in a solid‐state cell configuration with a Nafion membrane serving as a solid polymer electrolyte. Cyclic voltammetry indicates a behavior similar to that of metallic tungsten for coatings obtained at dry conditions and similar to that of tungsten oxide species when water vapor is not totally expelled. Surface analysis using Auger electron spectroscopy and x‐ray photoelectron spectroscopy shows that solid‐state electro‐reduction of in dry conditions yields coatings free of fluorine, which contain much less oxygen than electrodeposits obtained from aqueous solutions. However, due to possible oxidation and reduction reactions occurring before and during the surface‐analysis process, it is not possible at this stage to determine the exact content of metallic and oxide species in the deposits obtained by the present method.

X-ray photoelectron and ion scattering studies of tungsten oxides and tungsten oxide catalysts

X-ray photoelectron spectroscopy and ion scattering spectroscopy are employed to study well-defined mixed valence oxides of tungsten and an operational alkene isomerisation catalyst, consisting of alumina-supported tungsten trioxide which is partially reduced by preconditioning in hydrogen saturated in water vapour. Both γ-alumina and γ-alumina partially transformed to α-alumina by heating are used as catalyst supports. In both cases it is found that the tungsten trioxide interacts with the alumina to form an irreducible surface species in which the tungsten is tetrahedrally coordinated. Sodium impurities in the alumina appear to assist the reaction. Catalysis takes place on the partially reduced tungsten oxide species which is stabilised by a weak interaction with the alumina. The heat-treated alumina is a better catalyst support because, for equivalent loadings, its smaller surface area leaves more free tungsten trioxide for reduction and subsequent catalytic activity.

Structure of surface tungsten oxide species in the tungsten trioxide/alumina supported oxide system from x-ray absorption near-edge spectroscopy and Raman spectroscopy

The X-ray absorption near-edge spectra (XANES) of the L1 tungsten edge in WO3/Al2O3 samples indicate that the symmetry of the tungsten environment depends on both the surface coverage and the presence of coordinated water. At coverages of less than 1/3 monolayer, in the absence of coordinated water, the XANES spectrum indicates a distorted tetrahedral structure for the surface tungsten oxide species. Samples exposed to air at room temperature have water molecules coordinated to the surface tungsten oxide species and produce an octahedral site symmetry, but the water is removed by heating to 500 C. The Raman spectra of the W03/A1203 samples are consistent with a distorted tetrahedral tungsten oxide environment and, in addition, show features due to W=O and W-0-W bonds. These results suggest that the surface tungsten oxide is present as both isolated and dimeric tetrahedra. At coverages approaching a monolayer, in the absence of coordinated water, a significant fraction of the surface tungsten oxide sites appears to have a distorted octahedral environment in the XANES spectra. At this high coverage the effect of coordinated water molecules is much less evident than at low coverage. The Raman results, however, only provide information about the tetrahedral component because the Raman cross section of the tetrahedral tungsten oxide is much higher than the octahedral tungsten oxide. The Raman spectra show features of W4 and W-0-W bonds in the tetrahedral fraction of the surface tungsten oxide monolayer on alumina. These observations are consistent with a surface complex where the supported tungsten oxide has formed a polymeric structure on the alumina support composed of W04 and W06 units jointed in infinite chains.

Laser Raman characterization of tungsten oxide supported on alumina: Influence of calcination temperatures

The influence of calcination temperature upon the solid state chemistry of WO 3 on Al 2O 3 was examined with laser Raman spectroscopy. Laser Raman spectroscopy revealed the amorphous and crystalline structural transformations occurring in the WO 3 on Al 2O 3 oxide system. Below monolayer coverage of tungsten oxide on alumina, the tungsten oxide phase is present as a highly dispersed and amorphous surface complex on the support. A close-packed monolayer of the tungsten oxide surface complex on alumina is formed as the surface area of the alumina support decreases at high calcination temperatures. The lower the tungsten oxide loading, the more severe the calcination temperature must be to reach the close-packed monolayer. The close-packed tungsten oxide monolayer accommodates the further desurfacing at still higher temperatures by forming the bulk tungsten oxide phases WO 3 and Al 2(WO 4) 3. The Al 2(WO 4) 3 phase is formed from the reaction of WO 3 crystallites with the Al 2O 3 support. The parameter controlling the phases present in the WO 3 on Al 2O 3 system is the surface density of the tungsten oxide species on the alumina surface.