M. Asomoza

Chlorobenzene, chloroform, and carbon tetrachloride adsorption on undoped and metal-doped sol-gel substrates (SiO2, Ag/SiO2, Cu/SiO2 and Fe/SiO2).

Adsorption isotherms of chlorobenzene, chloroform and carbon tetrachloride vapors on undoped SiO(2), and metal-doped Ag/SiO(2), Cu/SiO(2) and Fe/SiO(2) substrates were measured in the temperature range of 398-593K. These substrates were prepared from a typical sol-gel technique in the presence of metal dopants that rendered an assortment of microporous-mesoporous solids. The relevant characteristic of these materials was the different porosities and micropore to mesopore volume ratios that were displayed; this was due to the effect that the cationic metal valence exerts on the size of the sol-gel globules that compose the porous solid. The texture of these SiO(2) materials was analyzed by X-ray diffraction (XRD), FTIR, and diverse adsorption methods. The pore-size distributions of the adsorbents confirmed the existence of mesopores and supermicropores, while ultramicropores were absent. The Freundlich adsorption model approximately fitted the chlorinated compounds adsorption data on the silica substrates by reason of a heterogeneous energy distribution of adsorption sites. The intensity of the interaction between these organic vapors and the surface of the SiO(2) samples was analyzed through evaluation of the isosteric heat of adsorption and standard adsorption energy; from these last results it was evident that the presence of metal species within the silica structure greatly affected the values of both the amounts adsorbed as well as of the isosteric heats of adsorption.

Trapping of BTX compounds by SiO2, Ag-SiO2, Cu-SiO2, and Fe-SiO2 porous substrates.

Adsorption isotherms of BTX aromatic hydrocarbons (benzene, toluene, and p-xylene) on pristine (SiO2) and metal-doped (Ag-SiO2, Cu-SiO2 and Fe-SiO2) mesoporous and microporous substrates were measured and interpreted. These adsorbents were synthesized by the sol-gel procedure and their BTX sorption isotherms were obtained by the gas chromatographic technique (GC) at several temperatures in the range 423-593 K. The uptake amount of these hydrocarbon adsorptives on SiO2, Ag-SiO2, Cu-SiO2 and Fe-SiO2 mesoporous and microporous substrates was temperature-dependent. Additionally, the interaction of BTX molecules with the pore walls was evaluated by means of the corresponding isosteric heat of adsorption (qst), which was found to follow the next increasing sequence: qst (benzene)<qst (toluene)<qst (p-xylene). In general, the isosteric heat of adsorption of aromatic BTX compounds on microporous silica depicted an increasing tendency when the amount adsorbed was raised. This was a consequence of the existence of cohesive interactions (adsorbate-adsorbate) besides of the adhesive ones (adsorbate-adsorbent). The inclusion of silver or iron atoms within the SiO2 structure leads to an increased adsorbed amount of BTX molecules on the solid surface if compared with the Cu-SiO2 adsorbent. The adsorption of benzene, but not of toluene and p-xylene, molecules on pristine SiO2 is facilitated by the pore size of this substrate since this is the highest of all materials.

Preparation and characterization of phosphate-modified mesoporous TiO 2 incorporated in a silica matrix and their photocatalytic properties in the photodegradation of Congo red

This study describes the development of mesostructured TiO2 photocatalysts modified with PO43- to improve its specific surface area and reduce the recombination rate of the electron—hole pairs. The mesoporous photocatalyst was successfully incorporated into a high specific surface area silica matrix by the hydrolysis reaction of tetraethyl orthosilicate (TEOS). Pluronic 123 and phosphoric acid were used as the directing agent for the structure of the mesoporous TiO2 and as a source of phosphorus, respectively. TiO2, P/TiO2, TiO2-SiO2 and P/TiO2-SiO2 materials were characterized by BET, XRD, TEM-EDS, FTIR and UV-vis DRS measurements. The photoactivity of TiO2-SiO2 nanocomposites containing 15 wt.% photocatalyst/silica was evaluated in the degradation reaction of anionic dyes with UV radiation. The proposed nanomaterials showed high potential for applications in the remediation of wastewater, being able to reuse in several cycles of reaction, maintaining its photoactivity and stability. The separation and recovery time of the material is reduced between cycles since no centrifugation or filtration processes are required after the photooxidation reaction.

Microporosity in Silica Synthesized by the Sol-Gel Method. Adsorption of Nitrogen at 76 K

Microporous SiO2 solids were synthesized by the sol-gel method and analyzed by adsorption methods. The shapes of nitrogen adsorption isotherms at 76 K in these materials (Type Ib) indicated high micropore contents and an important interaction between the nitrogen molecules, i.e.a cooperative filling process between adsorbate molecules was evident within the porous structures. The BET, Langmuir and external surface areas of the materials under study were evaluated from the analysis of nitrogen adsorption data at 76 K at relative vapor pressures \( p/{p^0} \) ranging from 0.04 to 0.2. The total pore volume, \( {V_{\sum }} \), was estimated on the basis of the volume adsorbed at a relative pressure, \( p/{p^0} = ca \) 0.95, by the Gursvitch Rule. The materials under study were purely mesoporous, i.e. these adsorbents depicted an almost total absence of pores of great sizes (i.e. mesopores and macropores). The negative values of the BET constant, C B , indicated that adsorption could be better described as a volume filling process rather than in terms of a multilayer formation mechanism. The microporosity developed in these samples was studied through different approaches such as: Sing αs-plots, Lee and Newnham direct comparison method, the difference isotherm scheme, and the classical Dubinin-Raduskevich (DR) equation. The results of these studies indicated that the structure of these Si02 sol-gel materials was made of two types of micropores: a certain amount of micropores with sizes of nearly molecular dimensions, called ultramicropores (um), and a dominant proportion of larger micropores, identified as supermicropores (sm).