Efeitos dos métodos de preparação de catalisadores Cu/Nb2O5/Al2O3: caracterização e reatividade catalítica na produção de H2 a partir de etanol

Abstract

In this work it was tried to evaluate the influence of the preparation method of Cu/Nb2O5/Al2O3 catalysts on the generation of the copper-support interaction as well as the efficiency of that interaction in the ethanol steam reforming reaction. The catalysts were prepared by three methods: recipient wet impregnation, ion exchange and precipitation. For each method the supports Nb2O5, Nb2O5/Al2O3 and Al2O3 were used. The prepared samples were characterized by elementary analysis of atomic absorption, specific total superficial area (BET), diffuse reflectance spectroscopy (DRS), temperature programmed reduction (TPR) and metallic area of copper by N2O decomposition. The TPR analysis indicated the presence of the interaction in all the catalysts. They presented different reduction percentages and profiles, independent of the preparation method and support. By DRS, it was verified that the preparation method and the influence of the support generate differentiated phases, as well as their amounts. The absorption maxima around 360 nm in the catalysts that contain alumina evidence the presence of (Cu-O-Cu)n clusters. For the binary catalysts with Nb2O5 and ternary, the maximum band around 444 nm, shows strong Cu-Nb interaction and the bands above 800 nm suggest the presence of highly distorted NbO6 octahedra with Nb=O bonds responsible for the Lewis acid sites as well as slightly distorted NbO7 and NbO8 octahedra with Nb = O bonds responsible for the Brönsted acid sites. The analyses of metallic area evidenced SMSI effect in the Cu/Nb2O5 catalysts independent of the preparation method. The importance of the interaction as responsible for the generation of the active sites was checked by the catalytic test. The reaction used was the ethanol steam reforming and it was verified that independent of the preparation method or the alumina presence, the catalysts with smaller copper amount are more actives. The catalysts prepared by the ion exchange method are the ones that more convert the ethanol in relation to the different supports, mainly those that contain Nb2O5 showing the importance of the copper-niobia interaction in the conversion, because in those catalysts a larger dispersion of the copper on the surface of the support occurs, promoting larger exposition of the sites where the interaction and conversion take place. The largest selectivity and yield were found in the Cu/Nb2O5 catalysts prepared by wet impregnation.