RESUMO
In the present study a series of Au-transition metal oxides supported on a clay mineral such as sepiolite were tested in the preferential oxidation of CO in an excess of H2 under simulated solar light irradiation and in the absence of light, at 30 °C and atmospheric pressure. Transition metal oxides (ZnO, Fe2O3, NiO, MnO2, and Co3O4) were dispersed over the sepiolite surface where, subsequently, Au nanoparticles with an average particle size between 2 and 3 nm were successfully deposited-precipitated. The obtained photocatalysts were characterized by XRD, XRF, DRUV-Vis, N2 adsorption-desorption and HRTEM in order to evaluate the optical, structural and chemical properties of the prepared samples. Despite the low amount of gold (nominal 1.0 wt%), the catalysts exhibited an outstanding behavior under light irradiation, with reaction rates between 4.5 and 5.2 mmol COox gcat-1 h-1 for the Au-NiSep, Au-CoSep and Au-ZnSep samples. These photocatalysts exhibited a high dispersion of the respective transition metal oxides over the sepiolite support and the presence of low-coordinated hemispherical gold nanoparticles. The superior photocatalytic efficiency of these samples was ascribed to the reduction of the electron-hole pair recombination of photogenerated charge carriers by the excitation of the localized surface plasmon resonance of the Au nanoparticles. The BET surface area and the gold particle size seemed to be relevant factors affecting the catalytic performance.
RESUMO
In this work, SBA-15 silica and silica-titania have been used as supports for photocatalysts based on AuCu alloy (Au:Cu = 1) to be used in the preferential oxidation of CO (CO-PROX) in excess of hydrogen at room temperature and atmospheric pressure both in the dark and under simulated solar light irradiation. To study their textural, structural, chemical and optical properties, the samples were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), adsorption-desorption of N2 at -196 °C, 13C and 29Si solid state nuclear magnetic resonance (NMR), X-ray photoelectron spectroscopy (XPS) and diffuse reflectance ultraviolet-visible (DRUV-vis) spectroscopy. Titanium was present mainly in the form of titania aggregates, but also as small particles interacting with the SBA support. In both catalysts, the metal alloy nanoparticles displayed an average size of 4 nm as demonstrated by TEM measurements. AuCu/Ti-SBA turned out to be photoactive and selective in the photo-CO-PROX reaction showing the highest activity, with conversion and selectivity towards CO2 of 80%, due both to the presence of titania incorporated in SBA-15 and to the synergistic effect of Cu when alloyed with Au.
RESUMO
The photocatalytic degradation of methylene blue (MB) dye has been performed under UV irradiation in aqueous suspension, employing photocatalysts based on Au (1.5 wt %) and AuCu (Au/Cu = 1, 2.0 wt %), and supported on SBA-15-ordered mesoporous silica, with and without titania (Si/Ti = 3), in order to evaluate the versatility of this mesoporous support in this type of reaction of great impact from the environmental point of view. Samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption-desorption at -196 °C, and X-ray photoelectron spectroscopy (XPS), so as to study their structural, optical, and chemical properties. All the prepared catalysts were found to be active in the test reaction. The bimetallic AuCu-based catalysts attained very high MB degradation values, in particular AuCu/SBA-15 titania-silica sample reached 100% of dye oxidation after the monitored reaction period (120 min).
RESUMO
Here we describe the step-wise grafting of the fluorophore dansyl chloride on the interlayer aluminol groups of kaolinite. The modified clay was characterized by powder RD, TGA, FT-IR and (27)Al, (19)Si, (13)C MAS-NMR, which confirmed the achievement of the clay functionalization. The photophysical properties of the resulting nanohybrid material were evaluated by photoluminescence excitation and emission measurements.
RESUMO
Nanostructured Au/Al(2)O(3)-CeO(2) catalysts with a low content of precious metal (0.9% wt.) were prepared immobilizing two different stabilized Au sols on a high surface area Al(2)O(3)-CeO(2) mixed oxide with a uniform pore size distribution, synthesized by a one-pot methodology. The samples were characterized by elemental analysis, N(2) physisorption, XRPD, TEM and (27)Al-MAS NMR techniques. The catalytic activity of the two samples in the preferential oxidation of CO in excess of H(2) (CO-PROX) was comparatively evaluated in the 35-110 degrees C temperature range. The Au-THPS/AlCe20 sample, prepared immobilizing a sol obtained reducing an aqueous solution of gold tetrachloroaurate salt with bis[tetrakis(hydroxymethyl)phosphonium sulfate], resulted very active and selective at low temperatures and its catalytic activity was correlated with the structural characteristics of the metal particles and of the ordered mesoporous support.
RESUMO
Two dyes (4-nitrostilbene, NST and 4-N,N-dimethylamino-4'-nitrostilbene, DANS) included in zeolites with nanometric channels and different Si : Al ratios have been photoexcited and their triplet state studied by time resolved EPR (TR-EPR). This is the first time that a TR-EPR spectrum of photoexcited triplet states of dyes in zeolites has been observed. The zeolites used were ZSM-5 and mordenite, with either protons or lithium as charge compensating ions, and the aluminium-free porosil. The ZFS parameters and the polarized spin populations have been obtained, and compared with those obtained in glassy solutions and in the organic nanostructured matrix perhydrotriphenylene (PHTP). For (3)DANS in neutral solvents and in PHTP the dimethylamino group is pi conjugated, whereas only its acid form is detected in all the zeolites. In the latter the spectra of the radical cations formed by spontaneous oxidation have also been observed both by cw-EPR and TR-EPR, the last spectra being spin polarized in emission. The mobility of the triplets is discussed, taking into account the spin polarization of the radical cations indicating a strong radical-triplet interaction.