Effect of Cu Loading on the Structure and Catalytic Performance of the LNT Catalyst CuO-K<sub>2</sub>CO<sub>3</sub>/TiO<sub>2</sub>

Abstract

A series of non-platinic lean NOx trap (LNT) CuO-K2CO3/TiO2 catalysts with different Cu loadings were prepared by sequential impregnation, and they showed relatively good performance for lean NOx storage and reduction. The catalyst containing 8% (w) CuO showed not only the largest NOx, storage capacity of 1.559 mmol.g(-1) under lean conditions, but also the highest NOx, reduction percentage of 99% in cyclic lean/rich atmospheres. Additionally, zero selectivity of NOx to N2O was achieved over this catalyst during NOx 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 H-2 (H-2-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 NOx to N-2. The strong interaction between K2CO3 and CuO was clearly revealed, which favors NOx 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 storage. After using the catalysts for 15 cycles of NOx 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 NOx storage processes and the distribution of CuO and K2CO3 species is proposed.