RESUMO
Caffeic acid (CA) molecules were immobilized in a layered inorganic host matrix based on zinc hydroxide structures with different starting interlayer anions, nitrate, and acetate. The chemical composition, structure, thermal stability, morphology, and surface of the host matrices and hybrid compounds were analyzed by X-ray diffraction (XRD), themogravimetric/differencial thermal analysis (TG/DTA), Fourier transform infrarred spectroscopy (FT-IR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Additionally, the surface charge of the materials was investigated using zeta potential at pH ~7. The results show an influence of the surface charge on the chemical, interaction, and structure of the resulting hybrid materials as a function of the starting layered structures. An expansion of the basal spacing to 10.20 Å for zinc hydroxide nitrate (ZHN), and a shrinkage to 10.37 Å for zinc hydroxide acetate (ZHA). These results suggest that the CA lies with a tilt angle in the interlayer region of the inorganic host matrix. The immobilization of CA is favored in ZHN, with respect to ZHA, because a single-layered phase was identified. A higher thermal stability at 65 °C was observed for ZHN-CA than for ZHA-CA. The evaluation of the release behavior showed a higher percentage of CA released from ZHN than ZHA, and the release mechanism was described by the Elovich model. The hybrid materials show potential characteristics for use as bioactive delivery systems.
RESUMO
The focus of this paper is centered on the thermal reduction of KMnO4 at controlled temperatures of 400 and 800 °C. The materials under study were characterized by atomic absorption spectroscopy, thermogravimetric analysis, average oxidation state of manganese, nitrogen adsorption-desorption, and impedance spectroscopy. The structural formulas, found as a result of these analyses, were K 0.29 + ( M n 0.84 4 + M n 0.16 3 + ) O 2.07 · 0.61 H 2 O and K 0.48 + ( M n 0.64 4 + M n 0.36 3 + ) O 2.06 · 0.50 H 2 O . The N2 adsorption-desorption isotherms show the microporous and mesoporous nature of the structure. Structural analysis showed that synthesis temperature affects the crystal size and symmetry, varying their electrical properties. Impedance spectroscopy (IS) was used to measure the electrical properties of these materials. The measurements attained, as a result of IS, show that these materials have both electronic and ionic conductivity. The conductivity values obtained at 10 Hz were 4.1250 × 10-6 and 1.6870 × 10-4 Ω-1cm-1 for Mn4 at 298 and 423 K respectively. For Mn8, the conductivity values at this frequency were 3.7074 × 10-7 (298) and 3.9866 × 10-5 Ω-1cm-1 (423 K). The electrical behavior was associated with electron hopping at high frequencies, and protonic conduction and ionic movement of the K+ species, in the interlayer region at low frequencies.
RESUMO
The electrical characterization of catalysts composed of layered manganese oxide in the form of birnessite supported on γ-Al2O3, which have been successfully used in the combustion of soot, is presented. The results indicate that the electrical conduction and ion conduction processes are influenced by the amount of the active phase. There was also evidence of Grotthuss-type proton conductivity favored by the presence of surface water on the exposed alumina surface. The above is supported by the porous nature of the catalyst in which the surface area varied between 125.2 ± 1.2 and 159.0 ± 1.1 m2/g, evidencing changes in the alumina surface. The conductivity, determined from measurements of impedance spectroscopy, at low frequency showed changes associated with the amount of the active phase. The values ranged from 2.61 × 10-8 ± 2.1 × 10-9 Ω-1·cm-1 (pure alumina) to 7.33 × 10-8 ± 5.9 × 10-9 Ω-1·cm-1, 7.21 × 10-8 ± 5.8 × 10-9 Ω-1·cm-1 and 4.51 × 10-7 ± 3.6 × 10-8 Ω-1·cm-1 at room temperature for catalysts with nominal active phase contents of 5.0, 10.0 and 20.0%, respectively. Such results indicate that it is possible to modulate the electrical properties with variations in the synthesis parameters.