Titanium dioxide (TiO2) is considered one of the most attractive materials during the last decades due to its scientific and technological importance as photocatalyst. There are many reports dealing with several ways of processing TiO2 powders (coatings, tapes, and bulk sinters) and their photocatalytic characterization. However, very little research effort has been devoted to the preparation of macroporous TiO2 ceramics. This work deals with the colloidal processing of TiO2 powders to fabricate porous materials with tailored open macroporosity by direct foaming through gelcasting with polysaccharides. The obtained samples have been characterized through density, microstructure, porosimetry and photocatalytic studies. The effect of the processing parameters: suspension solid loading (20–65 wt%), viscosity, initial particle size (submicrometer and nano‐sized), tensoactive type (cationic, anionic, and non‐ionic), tensoactive content (0.1–1 wt%), and sintering temperature (1000°C–1400°C) onto the properties of the final samples is reported and discussed. Tailored macroporous samples with open porosity (up to 88 vol%) were obtained and they could be used as multifunctional filters with photocatalytic activity for water treatment under UV irradiation. The photocatalytic activity of the final samples depends on the porosity properties, tensoactive content and grain size.
Vanadium oxide (V2O5) species has been supported on different porous clay heterostructures (with silica pillars, silica-zirconia with a molar ratio Si/Zr = 5 and silica-titania with a molar ratio Si/Ti = 5) by wetness incipient method. All catalysts were characterized by X-ray diffraction (XRD), N2 adsorption-desorption at −196 °C, NH3 thermoprogrammed desorption (NH3-TPD), Raman spectroscopy, diffuse reflectance UV-Vis and X-ray photoelectron spectroscopy (XPS). After that, the catalytic activity of the vanadium-based catalysts was evaluated in the selective oxidation of H2S to elemental sulfur. The catalytic data show that both the activity and the catalytic stability increase with the vanadium content, obtaining the highest conversion values and sulfur yield for the catalysts with vanadium content of 16 wt.%. The comparison among all supports reveals that the incorporation of TiO2 species in the pillars of the PCH improves the resistance to the deactivation, attaining as best results a H2S conversion of 89% for SiTi-PCH-16V catalyst and elemental sulfur is the only compound detected by gas chromatography.
Manganese-substituted 5 mol.% yttria-stabilized zirconia (5YSZ) was explored as a prospective material for protective interlayers between electrolyte and oxygen electrodes in reversible solid oxide fuel/electrolysis cells. [(ZrO2)0.95(Y2O3)0.05]1−x[MnOy]x (x = 0.05, 0.10 and 0.15) ceramics with cubic fluorite structure were sintered in air at 1600 °C. The characterization included X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetry and dilatometry in controlled atmospheres, electrical conductivity measurements, and determination of oxygen-ion transference numbers by the electromotive force (EMF) technique. Mn-substituted 5YSZ solid solutions exhibit variable oxygen nonstoichiometry with manganese cations in a mixed 2+/3+ oxidation state under oxidizing conditions. Substitution by manganese gradually increases the extent of oxygen content variation on thermal/redox cycling, chemical contribution to thermal expansion and dimensional changes on reduction. It also deteriorates oxygen-ionic conductivity and improves p-type electronic conductivity under oxidizing conditions, leading to a gradual transformation from predominantly ionic to prevailing electronic transport with increasing x. Mn2+/3+→Mn2+ transformation under reducing atmospheres is accompanied by the suppression of electronic transport and an increase in ionic conductivity. All Mn-substituted 5YSZ ceramics are solid electrolytes under reducing conditions. Prolonged treatments in reducing atmospheres, however, promote microstructural changes at the surface of bulk ceramics and Mn exsolution. Mn-substituted 5YSZ with 0.05 ≤ x < 0.10 is considered the most suitable for the interlayer application, due to the best combination of relevant factors, including oxygen content variations, levels of ionic/electronic conductivity and thermochemical expansion.
Bulk Ni-based catalysts
derived from hydrotalcite-type (HT) compounds
show enhanced stability in the hydrogenation of CO2 into
CH4, in comparison to their supported counterparts. The
composition of the parent hydrotalcite determines the Ni loading and
particle size, as well as the basicity of the catalysts. In this work,
a comparison between the effect of Fe and La on the performance of
NiMgAl HT compounds (Ni/Mg/Al = 25/50/25 atomic ratio) calcined at
600 °C is investigated. After a reduction pretreatment at 750
°C, both promoters enhance the activity in the low-temperature
range; however, the La-containing catalyst reaches a better CH4 productivity rate at high oven temperature (i.e., 200 L gNi
–1 h–1 at 325 °C).
Although the formation of the NiFe alloy improves the activity, lanthanum
keeps smaller Ni0 particles and provides a higher basicity
to the mixed oxide than Fe. The activity of the NiMgLaAl catalyst
can be further tailored by increasing the Ni loading from 25.6 wt
% (Ni25Mg50La5Al20) to 44.6 wt % (Ni50Mg25La5Al20). Lastly, the feasibility
of HT-derived catalysts in the direct methanation of clean biogas
is proved.
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