Showing papers by "José Maria C. Bueno published in 2002"

Characterization of ceria-coated alumina carrier

Abstract: CeO2–Al2O3 mixed oxides with different CeO2 loading (in the range of 0.5–12 wt.%) were prepared by wetness impregnation of alumina with aqueous solution of di-ammonium hexanitrate cerate (NH4)2[Ce(NO3)6]. The samples after calcination at 773 and 1073 K were characterized by different techniques, using X-ray diffraction (XRD), UV–VIS diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction (TPR). It is shown that different types of cerium oxide species are formed on the surface depending on the amount of CeO2 and temperature of calcination of the samples. XRD showed the formation of nanocrystallites of ceria on alumina surface when the amount of CeO2 is higher than 6 wt.%; at lower concentrations ceria was found to be amorphous. The ceria loading lower than 6 wt.% stabilizes the textural properties of alumina. At loading of 1 wt.% of CeO2 XPS spectra reveals the presence of a strong interaction between ceria and alumina leading to a formation of superficial CeAlO3-like phase. TPR results show that well dispersed CeO2 particles present on the surface of alumina form CeAlO3 at temperature of reduction in the range of 873–993 K, while for the reduction of CeO2 crystallites, a higher temperature of reduction of 1190 K is needed.

Co/SiO2 catalysts for selective hydrogenation of crotonaldehyde II: influence of the Co surface structure on selectivity

Abstract: The effect of interaction of silica-supported CoO x on catalytic properties of Co/SiO 2 catalysts was investigated. The CoO x precursors supported on silica were obtained by impregnation of the support with cobalt nitrate solution. Cobalt precursors, with different interactions with support, were obtained by changes of impregnation solvent, drying time and temperature; and calcination temperature. The Co surface structure of the catalysts was characterized by diffuse reflectance FTIR spectroscopy (DRIFTS) of adsorbed CO and temperature-programmed desorption of hydrogen (TPD-H 2 ). The TPD-H 2 and DRIFT of CO spectra indicate the presence of at least four different Co surface sites, labeled α , β , γ , and σ . The relative amount of these species varied depending on the degree of CoO x interaction with SiO 2 in precursors. For crotonaldehyde (CROALD) hydrogenation in gas phase, selectivity to crotyl alcohol and butanol depended strongly on the β /( γ + σ ) ratio. Interestingly, selectivity to butyraldehyde does not depend on β , γ , or σ sites present.

CO2 reforming of CH4 over Rh-containing catalysts

Abstract: Two series Rh catalysts were prepared depending on the kind of the support: (i) Rh supported on zeolite and (ii) on oxide carriers. Rh/NaY zeolite catalysts were prepared by ion-exchange from an aqueous solution of [Rh(NH 3 ) 5 ]Cl 3 . Rh catalysts supported on γ-Al 2 O 3 , Nb 2 O 5 and TiO 2 were prepared by incipient wetness impregnation method of the carrier with an aqueous solution of RhCl 3 ·2H 2 O. The catalysts were characterized by X-ray diffraction (XRD) spectroscopy, nuclear magnetic resonance (NMR), temperature-programmed desorption of hydrogen (TPD-H 2 ). Test reaction for Rh catalysts was CO 2 reforming of methane at different reaction temperatures. TPD data showed that the dispersion of metal particles depends on the pretreatment activation procedure; highest dispersion was observed for samples previously calcined and then activated in H 2 . Catalysts activated directly in H 2 , steam or submitted directly to the reaction conditions showed large metal particles. The effect of the protons on the destabilization of the zeolite framework during reaction was revealed by the higher Si/Al ratios, due to a dealumination of the zeolite. Neutralization of protons with solution of NaOH leaded to a stabilization of the dispersion and zeolite structure. Correlation between the dispersion and specific activity in CO 2 reforming was found for zeolite-supported Rh catalysts. When the activities are compared by turnover frequencies, oxide-supported Rh catalysts are significantly more active compared to zeolite-supported ones due to a higher degree of participation of the reverse water–gas shift reaction (WGSR). The difference in activity and thermal stability was related to the nature of the support.