铜负载量对LNT催化剂CuO-K2CO3/TiO2结构与性能的影响

采用连续浸渍法制备了一系列非贵金属稀燃NO_x阱(LNT)催化剂CuO-K₂CO₃/TiO₂,考察了Cu负载量对催化剂结构和NO_x储存还原性能的影响.发现8% (w) CuO-K₂CO₃/TiO₂催化剂的催化性能最佳,其对NO_x的储存量达到1.559 mmol·g^-1,对NO_x的还原效率高达99%,且在NO_x还原过程中无副产物N₂O产生.应用粉末X射线衍射(XRD),高分辩透射电子显微镜(HR-TEM), CO₂程序升温脱附(CO₂-TPD),扩展X射线吸收精细结构(EXAFS),氢气程序升温还原(H₂-TPR)和原位漫反射红外光谱(in-situ DRIFTS)等技术详细表征了催化剂的结构.结果表明,不同Cu负载量的催化剂中,铜物种均主要以CuO相存在.铜的负载量直接影响铜物种、钾物种的存在状态,高分散的CuO相与表面K₂CO₃之间存在较强相互作用,这种相互作用不仅有利于NO_x的储存,而且有利于增强催化剂的稳定性. in-situ DRIFTS结果表明, NO_x储存过程中产生的两个负峰(1436和1563 cm^-1)缘于碳酸盐的分解,这间接证明了碳酸盐作为储存介质参与到NO_x储存反应中. EXAFS结果表明,经过15个稀燃/富燃循环测试,催化剂中的CuO相仍保持稳定.基于以上表征结果,提出了CuO和K₂CO₃在催化剂表面的分布模型,并探讨了NO_x储存还原的可能机理.

Effect of Cu Loading on the Structure and Catalytic Performance of the LNT Catalyst CuO-K2CO3/TiO2

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.