In Situ Spectroscopic Investigation of Molecular Structures of Highly ...

10844 J. Phys. Chem. B, Vol. 102, No. 52, 1998 Gao et al.TABLE 1: Surface Areas and Compositions of V 2 O 5 /SiO 2Catalystscatalystsurface area(m 2 /g)wt %V 2O 5asurface density(V atom/nm 2 )SiO 2 332 0.0 0.01% V 2O 5/SiO 2 306 0.9 0.25% V 2O 5/SiO 2 274 4.3 0.910% V 2O 5/SiO 2 234 8.7 1.912% V 2O 5/SiO 2 254 11.7 2.615% V 2O 5/SiO 2 233 14.2 3.3aActual V 2O 5 concentration obtained by atomic absorption.the Kubelka-Munk function F(R ∞ ) from the absorbance. Theband-gap energy (E g ) for allowed transitions was determinedby finding the intercept of the straight line in the low-energyrise of a plot of [F(R ∞ ) × hν] 2 against hν, where hν is theincident photon energy. 22 Samples were loaded in a quartz flowcell with a Suprasil window. After each treatment, the quartzcell was quickly sealed off and cooled to room temperature forDRS measurements. The hydrated spectra were obtained underambient conditions. The dehydrated spectra were obtained aftersamples were calcined at 500 °C in flowing O 2 /He for 1 h. TheDRS spectra for methanol chemisorption were recorded afterthe dehydrated samples were exposed to a gaseous mixture ofCH 3 OH/O 2 /He (4 mol % CH 3 OH in the saturated gaseousmixture) at 120 or 230 °C for 30 min.4. X-ray Absorption Spectroscopy (XANES). The X-rayabsorption experiments at the V K edge were performed onbeam line X19A at the National Synchrotron Light Source,Brookhaven National Laboratory. The storage ring operatedat 2.5 GeV with a current between 200 and 300 mA. A quartzin situ XAFS cell with Kapton windows was used for XANESmeasurements. XANES spectra were initially acquired at roomtemperature in a He purge and after heating to 500 °C at10°C/min in O 2 /He (20/80), holding for 30 min at 500 °C, andthen cooling to room temperature. The experimental detailsare described elsewhere. 19The XANES spectra were processed using the BAN softwarepackage. The energy scale for vanadium oxide was establishedby setting the first inflection point of the vanadium metal inthe derivative spectrum at 5465.0 eV to zero. The spectra werenormalized to unity absorption by dividing by a least-squaresfit of the absorption between 50 and 250 eV above theabsorption edge.Results1. Bulk Compositions and Surface Areas of the V 2 O 5 /SiO 2 Catalysts. The nominal and actual bulk compositions andsurface areas of V 2 O 5 /SiO 2 catalysts are presented in Table 1.The actual V 2 O 5 concentrations of the V 2 O 5 /SiO 2 samples areonly slightly in variance with the expected values. Unlessotherwise notified, all vanadia loadings mentioned in the paperare referred to the nominal values. The surface area of the V 2 O 5 /SiO 2 samples decreases systematically with increasing vanadialoading.2. Raman Spectroscopy. The Raman spectra of thedehydrated 0-15% V 2 O 5 /SiO 2 samples are shown in Figure 1.The silica support possesses Raman features at ∼410, ∼487,607, 802, and ∼976 cm -1 . The ∼976 cm -1 band, which isassociated with Si-OH stretching mode of the surface hydroxyls,23 decreases with increasing vanadia loading. The bandsat ∼802 and 410-430 cm -1 , which have been assigned to thesymmetrical Si-O-Si stretching mode and the Si-O-Sibending mode, respectively, 24 decrease with increasing vanadiaFigure 1. Raman spectra of the dehydrated V 2O 5/SiO 2 samples.loading, suggesting the breaking of Si-O-Si bridges. Thebroad bands at 607 and 487 cm -1 are assigned to D2 and D1defect modes, which have been attributed to tri- and tetracyclosiloxanerings produced via the condensation of surfacehydroxyls, respectively. 20,24 A sharp band at ∼1040 cm -1 wasobserved for all the dehydrated V 2 O 5 /SiO 2 samples, which hasbeen assigned to the VdO stretching vibration of isolated VO 4species. 12,13 The Raman band at 607 cm -1 due to the threememberedrings of the silica support significantly decreases withincreasing vanadia loading, suggesting that the number of threemembersiloxane rings decreases upon dispersion of the surfacevanadium oxide species. In addition, two broad Raman bandsappear at ∼1075 and ∼915 cm -1 , and their intensity does notchange noticeably with increasing vanadia loading. These twobands, which are also observed in the dehydrated, highlydispersed TiO 2 /SiO 2 system, 19 are characteristic of Si-O - andSi(-O - ) 2 functionalities 25 and can be assigned to perturbed silicavibrations that are indicative of the formation of V-O-Sibonds. These results reveal that in addition to the consumptionof Si-OH hydroxyls, the deposition of the surface vanadiumoxide species also breaks some Si-O-Si siloxane bridges forthe formation of V-O-Si bridging bonds. When the loadingreaches 12% V 2 O 5 on silica, a very weak Raman band appearsat ∼144 cm -1 , which is due to the formation of a trace amountof crystalline V 2 O 5 . This result demonstrates that the maximumcoverage of surface vanadium oxide species on this SiO 2 supportis achieved at ∼2.6 V atoms/nm 2 (see Table 1). Even at thismaximum coverage, no extra Raman bands, which might berelated to V-O-V polymeric species, are observable. At 15%V 2 O 5 loading, strong Raman bands due to V 2 O 5 crystallites areshown at 994, 697, 518, 404, 303, 284, and 144 cm -1 .However, the broad bands between 400 and 600 cm -1 seemmuch stronger than the combined bands due to both silica andcrystalline V 2 O 5 vibrations, which suggest the possible coexistenceof an additional unknown vanadium oxide species.The Raman spectra of the fully hydrated 1-12% V 2 O 5 /SiO 2samples are provided in Figure 2. These spectra are very similar