Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts

Abstract

The molecular structure of silica-supported vanadium oxide (VO x ) catalysts over wide range of surface VO x density (0.0002−8 V/nm2) has been investigated in detail under dehydrated conditions by in situ multiwavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV−vis diffuse reflectance spectroscopy. Resonance Raman scattering is clearly observed using 244 and 325 nm excitations, whereas normal Raman scattering occurs using excitation at the three visible wavelengths. The observation of strong fundamentals, overtones, and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VO x Raman bands (920, 1032, and 1060 cm−1) whose assignments have been controversial. The resonance Raman spectra of dehydrated VO x /SiO2 show a VO band at a smaller Raman shift than that in visible Raman spectra, an indication of the presence of two different surface VO x species on dehydrated SiO2 even at submonolayer VO x loading. Quantitative estimation shows that the two different monomeric VO x species coexist on silica surface from very low VO x loadings and transform to crystalline V2O5 at VO x loadings above the monolayer. It is postulated that one of the two monomeric VO x species has pyramidal structure and the other is in the partially hydroxylated pyramidal mode. The two VO x species show similar reduction−oxidation behavior and may both participate in redox reactions catalyzed by VO x /SiO2 catalysts. This study demonstrates the advantages of multiwavelength Raman spectroscopy over conventional single-wavelength Raman spectroscopy in structural characterization of supported metal-oxide catalysts.

Resonance Raman and surface- and tip-enhanced Raman spectroscopy methods to study solid catalysts and heterogeneous catalytic reactions

“…First, UV excitation of the heme-cyanide complex causes strong, selective enhancement of the vinyl CQC stretching Raman intensity, while little enhancement occurs for the heme vibrational modes. Quite recently, a similar RR result has been reported 52 showing the selective enhancement of the VQO stretching mode for silica supported vanadium oxide with 244 nm excitation. 172 Second, the (totally-symmetric) porphyrin C-N stretching Raman band of deoxy-hemoglobin 173 is selectively enhanced when the laser excitation energy approaches the Soret electronic absorption band.…”

“…Various cell designs applied to heterogeneous catalysis have been described in reviews. 52,72 The laser beam focused onto the top surface of a solid sample (Fig. 60,70,71 More recently, commercially-available Raman cells have been applied to in situ or operando measurements.…”

Phthalocyanine- and Calixarene-Templating Effect on the Catalytic Performance of Solid Supported Vanadates

“…However, assignment of this band in literature is controversial. While originally this band was attributed to the V-O-V stretching vibration [40], recent research [41,42] supports its assignment to the V-O-support group.…”

Role of defects and metal coordination on adsorption of acid gases in MOFs and metal oxides: An in situ IR spectroscopic study

“…The Raman measurements were performed with a catalytic Raman reactor (Linkam CCR1000) using in-house built multiple-wavelengths Raman system [50] that includes laser excitations at λ=325 and 532 nm. Raman scattering was collected using fiber optics connected directly to the spectrograph stage of the triple Raman spectrometer (Princeton Instruments Acton Trivista 555).…”