Structure and Electronic Properties of Solid Acids Based on Tungsten Oxide Nanostructures

Abstract

UV-visible diffuse reflectance spectroscopy was used to probe the electronic structure and domain size of tungsten oxide species in crystalline isopolytungstates, monoclinic WO 3 , and dispersed WO x species on ZrO 2 surfaces. UV-visible absorption edge analysis, CO 2 chemisorption, and Raman spectroscopic results show that three distinct regions of WO x coverage on ZrO 2 supports appear with increasing WO x surface density: submonolayer region (0-4 W nm -2 ), polytungstate growth region (4-8 W nm -2 ), and polytungstate/crystalline WO 3 coexistence region (>8 W nm -2 ). The structure and catalytic activity of WO x species on ZrO 2 is controlled only by WO x surface density (W nm -2 ), irrespective of the WO x concentration, oxidation temperature, and ZrO 2 surface area used to obtain a particular density. The submonolayer region is characterized by distorted octahedral WO x species that are well dispersed on the ZrO 2 surface. These species show a constant absorption edge energy, they are difficult to reduce, and contain few acid sites where o-xylene isomerization can occur at 523 K. At intermediate WO x surface densities, the absorption edge energy decreases, WO x domain size increases, WO x species become easier to reduce, and o-xylene isomerization turnover rates (per W atom) increase with increasing WO x surface density. At high WO x surface densities, a polytungstate monolayer coexists with monoclinic WO 3 crystallites. The growth of monoclinic WO 3 crystallites results in lower o-xylene isomerization turnover rates because WO x species become inaccessible to reactants. In the presence of H 2 at typical catalytic reaction temperatures (∼523 K), strong acid sites form on WO x -ZrO 2 catalysts with polytungstate domains by a slight reduction of the cluster and delocalization of an electron from an H atom resulting in H +δ (Brønsted acid site).

Spectroscopic characterization of tungstated zirconia prepared by equilibrium adsorption from hydrogen peroxide solutions of tungsten(VI) precursors

“…Weber [30] proposed an empirical linear correlation between the number of next nearest Mo neighbors (N Mo ) for Mo-O compounds and supported MoO x species and the value of the absorption edge energy (E edge ). Similar correlation between N W and E edge was found for zirconiasupported WO x species [3,12]. The absorption edge energy can be determined by a linear extrapolation to absorption equal zero in (ahm) 2 versus hm plots, where a is the absorbance and hm is the energy of the incident photon [3,12,30].…”

“…3. All samples exhibit absorption band at around 278-293 nm corresponding to charge-transfer (CT) transitions in W-O-W units of oligomeric tungstates [3,4,10,12,29]. The growth in intensity of this band (particularly for the cWZ-1-383) indicates increasing degree of condensation with increasing W loading.…”

“…The WO x /ZrO 2 catalysts are usually prepared by impregnating hydrous zirconium oxyhydroxide, ZrO x (OH) 4-2x , with aqueous solutions of ammonium metatungstate (AMT) [2][3][4][6][7][8][9][10][11][12][13] or by coprecipitation [5] followed by calcination in air. In general, the equilibrium adsorption procedures allow better dispersion of the active phase and this method has been used by Valigi and coworkers [13][14][15][16] to prepare various zirconia-based catalysts starting from amorphous ZrO x (OH) 4-2x .…”

Oxidative Dehydrogenation of Propane over V2O5/MoO3/Al2O3 and V2O5/Cr2O3/Al2O3: Structural Characterization and Catalytic Function

“…The Kubelka-Munk function (F(R ∞ )) was used to convert reflectance data into pseudoabsorbance using MgO as a reflective standard. 52,53 Absorption-edge energies were calculated from the x-intercept of a linear regression of [(F(R ∞ ))hν] 1/2 data versus hν. 53 2.3.…”

“…52,53 Absorption-edge energies were calculated from the x-intercept of a linear regression of [(F(R ∞ ))hν] 1/2 data versus hν. 53 2.3. Catalytic Rates and Selectivity Measurements.…”