Alejandro Natoli

Ceramics samples with the nominal composition [(ZrO₂)_0.95(Y₂O₃)_0.05]_1-x[PrO_y]_x and praseodymia contents of x = 0.05–0.15 were prepared by the direct firing of compacted 5YSZ + PrO_y mixtures at 1450–1550 °C for 1–9 h and characterized for prospective applicability in reversible solid oxide cells. XRD and SEM/EDS analysis revealed that the dissolution of praseodymium oxide in 5YSZ occurs via the formation of pyrochlore-type Pr₂Zr₂O₇ intermediate. Increasing PrO_y additions results in a larger fraction of low-conducting pyrochlore phase and larger porosity, which limit the total electrical conductivity to 2.0–4.6 S/m at 900 °C and 0.28–0.68 S/m at 700 °C in air. A longer time and higher temperature of firing promotes the phase and microstructural homogenization of the ceramics but with comparatively low effect on density and conductivity. High-temperature processing leads to the prevailing 3+ oxidation state of praseodymium cations in fluorite and pyrochlore structures. The fraction of Pr⁴⁺ at 600–1000 °C in air is ≤2% and is nearly independent of temperature. 5YSZ ceramics with praseodymia additions remain predominantly oxygen ionic conductors, with p-type electronic contribution increasing with Pr content but not exceeding 2% for x = 0.15 at 700–900 °C. The average thermal expansion coefficients of prepared ceramics are in the range of 10.4–10.7 ppm/K.

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[MnO_y]_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. Mn^2+/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.

In this study, 5Y-PSZ-based ceramics with 15 mol.% of manganese oxide were obtained from PSZ + MnO₂ powders mixtures by pressing and direct firing. The resulting materials show a stable cubic fluorite structure with only minor traces of segregated manganese oxides and relative density from 90% to 98%. The linear thermal expansion coefficient is in the order of 10⁻⁵ K⁻¹ at 500 K and increases gradually with temperature, due to the onset of a contribution of chemical expansion, reaching about 13 × 10⁻⁶ K⁻¹ at 1100 K. These results are suitable for prospective applicability as buffer layers to minimize degradation and delamination of electrolyte/oxygen electrode interfaces in solid electrolyte cells. The electrical conductivity remains close to 1 S/m at 973 K and close to 7 S/m at 1273 K, suggesting mixed conductivity with a prospective contribution to electrode processes occurring at electrode/electrolyte interfaces. Guidelines for further improvement were also established by a detailed analysis of the impact of heating/cooling rate, firing temperature, and time on those properties, based on Taguchi planning.

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.

Titanium dioxide ( TiO ₂ ) 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 TiO ₂ powders (coatings, tapes, and bulk sinters) and their photocatalytic characterization. However, very little research effort has been devoted to the preparation of macroporous TiO ₂ ceramics. This work deals with the colloidal processing of TiO ₂ 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.