Estudo de catalisadores à base de cobre e alumínio, oriundos de precursores do tipo MOF, aplicados na conversão de óxidos de nitrogênio

Abstract

As an alternative to noble metal-based catalysts, copper-containing catalysts are very promising in NOx conversion. In this work, two precursors were prepared by the co-precipitation method, one bimetallic (Cu, Al) and the other monometallic (Cu) in order to compare the effect of Al on the performance and thermal stability of catalysts in the reduction reactions of NO and N2O by CO (NO-CO and N2O-CO reactions, respectively). The precursors were prepared by the coprecipitation method with the terephthalate ligand, calcined at 600 °C for 4 h (CuAlc600 and Cuc600) and later thermally aged at 900 °C for 12 h (CuAlenv900 and Cuenv900). The precursors presented a mixture of MOFs (Metal-Organic Framework) with impurities. No significant differences were observed in the EXAFS spectra of both precursors, indicating that neighboring atoms and interatomic distances are probably not very different. After calcination, the materials showed the crystalline phase CuO. In addition, in the CuAlenv900 catalyst the spinel phase stands out. Cuenv900 catalyst presented negligible surface area, probably due to the sintering phenomenon. This result was evident by SEM results, where the presence of particle agglomerates was seen. In general, catalysts with aluminum had good catalytic activity, but catalysts without aluminum showed a loss of activity above 400 °C. Among the catalysts containing Al, CuAlenv900 showed the best activity in both reactions, probably due to the presence of the spinel phase. CuAlc600 presented the lowest N2O yield during the NO-CO reaction, as a result of the efficiency in converting N2O on this catalyst. In particular, it was found that CuAlc600 and CuAlenv900 catalysts presented the highest adsorbed oxygen/network oxygen ratio on their surface when compared to free-aluminum catalysts, indicating that these catalysts probably have more adsorption sites on their surfaces, which may justify their higher activity. Despite the differences in the configuration of the in-situ experiments carried out at LNLS (reactor, flow, catalyst mass, pre-treatment time), the results contributed to the analysis of conventional catalytic tests. In the N2O-CO reaction accompanied by in situ XRD, it was observed that the size of metallic copper crystallites present in the catalysts with aluminum are more thermally stable than those present in free-aluminum catalysts, which may explain their better performance throughout the studied temperature range. The results of NO and/or N2O reduction reactions accompanied by XANES and in situ XRD indicate that the Cu2O and metallic copper crystalline phases and Cu+ and Cu0 species play a determinant role in both reactions. Apparently there is no specific specie that is acting in these reactions, but pairs of species that operate in a redox cycle, Cu2+ ↔ Cu+ at low temperatures (< 300°C), and Cu+ ↔ Cu0 at high temperatures (> 300 °C).