Three alternative approaches for the development of heterogeneous photocatalysts are comparatively evaluated, namely (i) the use of molecular imprinting concept for the development of heterogeneous catalysts employing rhodamine B as template and sol–gel as synthesis route; (ii) the impregnation of TiCl4 on mixed nano- and micro-metric silicas, followed by calcination; (iii) the use of industrial and academic chemical residues as source of potential photocatalyst species impregnated on supports. All tests were carried on with rhodamine B as target molecule. For comparative reasons, photocatalytic tests were carried out with commercial titania (P25). The solids were characterized by nitrogen porosimetry, small-angle X-ray scattering (SAXS), zeta potential (ZP), diffuse reflectance spectroscopy in the ultraviolet region (DRS-UV), diffuse reflectance infrared Fourier transmission spectroscopy (DRIFTS), and Rutherford backscattering spectrometry (RBS). The supported catalysts resulting from silica nanoparticles and residue of the petrochemical industry achieved higher percentage of the dye degradation under ultraviolet (68.0 and 66.8%, respectively) radiation. The industrial waste reached the highest photocatalytic activity under visible (61%) radiation, while the commercial P25 achieved 82.0and 12.3% for ultraviolet and visible radiation, respectively. The textural and structural characteristics of the supported catalyst prepared with fumed silica and petrochemical waste (SiPe), namely the low-energy bandgap (1.8 eV), large surface area (280 m2 g−1), high pore volume (1.9 cm3 g−1), and high zeta potential value (−36.4 mV), may have been responsible for their high activity.
Part of the book: Semiconductor Photocatalysis
Fourier Transform Infrared and Raman are powerful techniques to evaluate silica and hybrid silica structure. It is possible to evaluate the silica network formation along the hydrolysis and condensation reactions in terms of siloxane rings formation and Si–O(–Si) angle deformation due to the introduction of organic groups, the employed synthetic route or encapsulated species interaction. The siloxane four- or six-membered rings imply in a more rigid or flexible network, respectively, in order to accommodate the organic groups. A structural analysis of the materials is of high importance, since interactions between the encapsulated molecules and the matrix are critical for the device performance, such as sensors. This type of device needs the permeation of an analyte to activate the encapsulated receptor molecules inside the silica structure. Fourier transform infrared spectrometry can be also used to determine parameters of the silica network as a function of the hydrophilicity/hydrophobicity degree and the siloxane ring structure with respect to thin film porosity. This silica structural analysis is reviewed along the text in a tentative of better exploring the data resulting from these powerful techniques. In addition, the functionalization of silica structures by the use of organoalkoxysilanes, which is important to the creation of high-specific materials, can be well described by these two complementary techniques. The Si–C bonds and the maintenance of the organic substituents such as methyl, octyl, octadecyl, vinyl, phenyl, aminopropyl, mercaptopropyl, isocyanatopropyl, iodopropyl, chloropropyl and glicydoxypropyl could be evaluated after the sol-gel synthesis process. The literature regarding silica vibrational spectroscopy is also explored creating a data bank of wave numbers for the most important bonds for different types of silica and hybrid silica materials obtained by different synthetic routes.