Abstract:

A series of non-platinic lean NO_x trap (LNT) CuO-K₂CO₃/TiO₂ catalysts with different Cu loadings were prepared by sequential impregnation, and they showed relatively good performance for lean NO_x storage and reduction. The catalyst containing 8% (w) CuO showed not only the largest NO_x storage capacity of 1.559 mmol·g^-1 under lean conditions, but also the highest NO_x reduction percentage of 99% in cyclic lean/rich atmospheres. Additionally, zero selectivity of NO_x to N₂O was achieved over this catalyst during NO_x reduction. Multiple techniques, including X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), temperature-programmed desorption of CO₂ (CO₂-TPD), extended X-ray absorption fine structure (EXAFS), temperature-programmed reduction of H₂ (H₂-TPR), and in-situ diffuse reflectance Fourier-transform infrared spectroscopy (DRIFTS), were used for catalyst characterization. The results indicate that highly dispersed CuO is the main active phase for oxidation of NO to NO₂ and reduction of NO_x to N₂. The strong interaction between K₂CO₃ and CuO was clearly revealed, which favors NO_x adsorption and storage. The appearance of negative bands at around 1436 and 1563 cm^-1, corresponding to CO₂ asymmetric stretching in bicarbonates and -C=O stretching in bidentate carbonates, showed the involvement of carbonates in NO_x storage. After using the catalysts for 15 cycles of NO_x storage and reduction in alternative lean/rich atmospheres, the CuO species in the catalysts showed little change, indicating high catalytic stability. Based on the results of in-situ DRIFTS and the other characterizations, a model describing the NO_x storage processes and the distribution of CuO and K₂CO₃ species is proposed.

Key words: NO_x, Storage, Reduction, Copper oxide, Potassium carbonate