Mixed bonds in SiOCs lead to a more disordered/defect-free carbon phase with high capacity for reversible storage of Li ions.

In light of the ongoing debate on the structure of supported vanadia, we report on a spectral marker enabling the direct identification of oligomeric surface structures. A series of VO x /SiO2/Si­(100) planar samples with chemical complexity was synthesized by spin-coating and investigated in detail by UV resonance Raman spectroscopy at 256.7 nm excitation as well as by X-ray photoelectron spectroscopy. The enhanced Raman sensitivity allows vibrational spectra to be recorded as a function of vanadium loading (0 ≤ L V ≤ 20.2 V nm–2) despite the small surface area of the planar model samples. At low loadings (L V < 7.3 V nm–2) the spectra are dominated by dispersed vanadia species, whereas at higher loadings the presence of crystalline V2O5 is also observed. We identify new spectral features at 492, 562, and 676 cm–1, which are attributed to V–O–V-related modes of oligomeric vanadia surface species. The vanadia surface species show a linear increase with vanadium loading, saturating at a loading of L V = 7.3 V nm–2, at which V2O5 crystallite formation is observed to increase significantly. Our spectroscopic results are consistent with a growth model that includes oligomerization of vanadia surface species to increase the packing density, thereby reducing the number of V–O–Si linkages to the support.

A novel operando spectroscopic approach combining multiwavelength and time-resolved Raman spectroscopy with gasphase Fourier transform infrared (FTIR) spectroscopy is presented, supported by in situ UV−vis diffuse reflectance (DR) spectroscopy. The potential of this approach is demonstrated in a case study of the oxidative dehydrogenation (ODH) of ethanol over a silica-supported vanadia catalyst. The structural dynamics of the catalyst upon switching from oxidative to reactive conditions was extensively studied by Raman spectroscopy with different excitation wavelengths in the visible and UV, exploiting resonance effects in a targeted manner. Time-dependent correlation of Raman and IR spectra over several reaction cycles allows identification of active vanadia surface structures. Detailed Raman spectroscopic analysis reveals that the adsorption of ethoxy species onto dispersed VO x structures occurs via opening of both V−O−Si and V−O−V bonds. During reaction, large oligomeric VO x structures are decomposed into smaller units. Combined Raman and UV−vis results show that these structural changes cannot be completely regenerated.

Polymer-derived silicon oxycarbides exhibiting¯1 and 10 vol.% of segregated carbon finely dispersed within a glassy Si x O y C z matrix have been investigated by UV Raman spectroscopy using a laser excitation of 4.8 eV ( = 256.7 nm). Carbon exists as amorphous sp 2 sp 3 bonded component in SiOC/C (¯1 vol.%) pyrolyzed at 1100°C in H 2 , including CC single bonds, polymeric chains and small polycyclic aromatic hydrocarbons (PAHs). The formation of nanocrystalline carbon at T > 1400°C is seen in the Raman spectra of SiOC/C (¯1 vol.%) and SiOC/C (10 vol.%) by the appearance of the G band of graphite. Tempering at 1600°C increases the degree of order within the carbon phase. However, the slight narrowing of the G peak with processing temperature (by about 5%) indicates still not well-crystallized carbon: the Raman results can be best explained by turbostratic carbon (with a lateral size L a of µ2 nm) and do not support the model description in literature as a network of single layer graphene.