Estudo da reação de redução do NO pelo CO em catalisadores modelo de cobre e paládio suportados alumina e ceria-zircônia

Abstract

The objective of this work was to evaluate the performance of copper and palladium catalysts supported on alumina or cerium-zirconium mixed oxide (CZ) in the reaction of NO reduction by CO, in absence of oxygen. Pure supports and respective catalysts were named as CZ, CuCZ, PdCZ, Al2O3, Cu/Al2O3 and Pd/Al2O3. A commercial γ-alumina and a CZ prepared by co-precipitation were used as carriers. Metals were introduced by wetness impregnation with a metal content of 8 wt.% of Cu and 0,5 wt.% of Pd. The catalysts were characterized by ICP-OES, N2 physisorption, XRD, Raman spectroscopy, TGA-DTA and TPR. Catalyst evaluation was carried out with a 1% equimolar mixture of NO and CO in He. The reactants conversions and products yields were measured in temperature programmed test from ambient temperature up to 500 ºC (light-off curves). Moreover, this reaction was also studied using a temperature step protocol until a stable conversion was achieved in each condition. The effect of a pre-reduction treatment on activity and selectivity was investigated for some catalysts. Characterization results indicated that metal impregnation did not impart significant changes in the support structure; some bulk copper oxide was observed on CZ and alumina supported catalysts. CZ supported catalysts were more active in low reaction temperatures (up to 300 ºC) indicating that metal-support interactions may be affecting the activity in this temperature range. CuCZ catalyst formed N2O up to 250 ºC and was selective to N2 at higher temperatures. In contrast, CuAl2O3 catalyst was less active than CuCZ. PdCZ formed N2O over a broad range of temperatures and was selective to N2 at temperatures above 400 ºC. Although less active, PdAl2O3 also formed N2O up to high temperatures. Tests at constant temperatures showed that on Pd catalysts and, particularly PdCZ, there was a pronounced increase in N2O formation when reaction temperature was raised from 100 to 200 ºC followed by a slow decrease in the catalyst activity at this later temperature. This behavior as associated to a maximum on NO conversion at intermediate temperatures observed in the light off curves for CZ supported catalysts. Previous catalyst reduction in presence of hydrogen promoted the activity of copper catalysts but had little effect on Pd ones. It is proposed that the more pronounced formation of N2O on CZ supported catalysts is associated with metal sites with intermediate oxidation state, favored by the mobility of oxygen atoms of the carrier structure