Efeito do método de preparo e da adição de magnésio nas propriedades de catalisadores de lantânio e níquel obtidos a partir de perovskitas.

Abstract

Due to the increase in oil consumption and its contribution to the greenhouse effect, the development of new routes for generating clean and renewable energy is much needed The use of biogas, obtained by anaerobic digestion of biomass, is an attractive alternative because it is a renewable resource. Among the various uses of biogas (mixture of methane and carbon dioxide), one of the most important is its catalytic transformation to generate renewable hydrogen, considered the main energy vector of the future. This process can be conveniently carried out through dry reforming of biogas, which has the advantage of consuming two gases responsible for the greenhouse effect. The main catalysts used in this process are based on platinum, iridium, rhodium, nickel and cobalt, among others. To reduce the costs and increase activity, selectivity and resistance to coke, the properties of metals can be combined. In this work, the effect of magnesium addition and preparation methods on the properties of precursors, perovskite oxides (La1-xMgxNiO3; x = 0.0; 0.1; 0.5; 0.9) and of nickel catalysts supported on lanthanum oxide for dry reforming of methane, as a model reaction of biogas. The catalysts were obtained by reducing the perovskites, in order to obtain solids with highly dispersed active phase (nickel). The precursors were prepared by the amorphous citrate, coprecipitation/impregnation and combustion methods. The samples were characterized by thermogravimetry, X-ray diffraction, temperature programmed reduction, nitrogen adsorption and desorption and temperature programmed desorption. The catalysts were evaluated in dry methane reforming, at 750 ° C, for 6 h, using a methane to carbon dioxide ratio of 1. The amount of carbon deposited on the catalysts, during reaction, was quantified by thermogravimetry, elemental analysis (C, H and N) and temperature programmed oxidation. Different crystalline phases were observed, depending on the preparation method and on the presence and magnesium content in the solids. Pure perovskite (LaNiO3) was only obtained by the amorphous citrate method, regardless of the magnesium content. In the other cases, other phases were detected coexisting with perovskites. Regardless of the preparation method and the magnesium content, the samples showed low specific surface areas, which is characteristic of the nature of these solids. On the other hand, the interactions between metal and support varied according to the method of preparation and the magnesium content. The total acidity of the solids varied with the magnesium content, in different ways, depending on the method of preparation, but the formation of strong and moderate acid sites was always favored by magnesium. The catalysts prepared by the citrate method led to high conversions of methane and carbon dioxide and hydrogen yield, beyond low yields of carbon monoxide, resulting in H2/CO ratios above the stoichiometric one. The H2/CO ratios varied between 7.0 and 11.0 during reaction, the richest magnesium catalyst leading to the highest value, while the magnesium-free catalyst leading to the lowest value, among all solids. These results show that the catalysts prepared by the citrate method are promising for the production of hydrogen and synthesis gas rich in hydrogen, being able to control the content of hydrogen or carbon monoxide by magnesium content.