Avaliação de catalisadores derivados de hidrotalcita NiCuAl-Tereftalato na reação de redução do NO pelo CO

Abstract

Most of the catalysts used in the NOx reduction are based on noble metals. They are expensive and scarce, besides that, in some situations, they promote the formation of nitrous oxide. In the present work, alternative catalysts based on copper and nickel were evaluated in the NO reduction by CO, in order, to evaluate the activity and selectivity toward N2. In the preparation of the precursor, the coprecipitation method was used to obtain hydrotalcite-like compound (HTLC) with terephthalate as a compensation anion, and the metals, copper, nickel and aluminum constituting the layers. Different ratios y = Cu/(Cu+Ni) (0; 0,3; 0,5; 0,7 e 1) were used. It was used the ratio Al/(Cu+Ni+Al) equal to 0,5. The chemical analysis showed that the metals were incorporated in the structure of the solids in the desired proportion. The formation of layered compound was observed at y values smaller than or equal to 0,7. The calcined precursors, i.e., the catalysts, are constituted of CuO and NiO binaries oxides. In addition to these binary oxides, it was also identified the NiAl2O4 e CuAl2O4 spinels for the aged catalysts. It was observed that the catalysts with higher nickel content have high surface area, which it is in agreement with the results of the NiO particle size. The X-ray absorption spectroscopy analysis showed that the catalysts with copper have Cu2+ species, manly. It was observed that catalysts with nickel and copper together have greatest reduction capacity when compared to the catalysts with nickel or copper separated, suggesting that, probably there was the formation of mixed oxide between the components. It was found that the catalyst containing only nickel is not active for the reaction in this study, but catalysts containing copper and nickel showed significant activities. It was also studied the behavior of these catalysts toward a reducing pretreatment, and the effect of a thermal aging. The reducing pretreatment showed to improve the catalytic activity. However, the thermal aging promoted a decline in the activity of the catalysts, except the catalyst with y = 0,5, which, practically, maintained its activity after thermal aging, indicating that it has excellent thermal and catalytic stability. Regard to the N2O formation, the results showed to be promising because, despite of its presence, in general the levels are considerably smaller than those levels observed in the catalysts based on noble metals.