Catalisadores bimetálicos de Co e Ni aplicados à reforma à vapor do etanol: propriedades de oxiredução na resistência à deposição de carbono

Abstract

The effect of variation of cobalt (Co)/nickel (Ni) composition in catalysts based on these transition metals was evaluated focusing mainly in the oxi-reduction properties and their relation to carbon deposition in the Steam Reforming of Ethanol (SRE) reaction. The catalysts, with metal loading of 15 and 8 wt%, were prepared by conventional impregnation of the Co and Ni salts on the sol-gel synthesized support MgAl2O4. The characterization was performed by N2 adsorption-desorption isotherms; X-ray diffraction (XRD); temperature programed reduction (H2-TPR); thermogravimetric analysis (TGA); scanning electron microscopy (SEM) and transmission electron microscopy (TEM); X-ray absorption near edge spectroscopy (H2-XANES), in situ extended X-ray absorption on fine structure spectroscopy (SRE-EXAFS) and Raman spectroscopy. The catalytic tests were done in the temperature range of 250 to 700°C and the conversion and products distribution dependence on the metal loading and composition were evaluated. Reduction profiles and diffraction patterns lead to believe that occur the formation of a spinel NiCo2O4 in the weight ratio 1Co:1Ni during calcination. Such spinel once reduced yields an alloy Co-Ni, accordinto to TEM images and EXAFS data. The alloy formation, although it is hard to characterize, seems to be determining factor on carbon deposition resistance on the catalyst surface during SRE. Carbon deposition, as addressed by TGA and SEM, is strongly sensitive to metal loading, being severe on monometallic samples with higher metal loading; independent of the loading, the bimetallic catalysts in the proportion 1:1 are resistant to carbon deposition. SRE-EXAFS showed that these catalysts didn t oxidize during reaction, differing from Co catalysts, where the equilibrium between oxide and metallic sites controls the ethanol activation and carbon oxidation rates.