Rare earth elements possess 4f orbitals without full electron occupancy and lanthanide contraction.This characteristic results in their unique catalytic performance when they are used as active components or as catalyst supports.Research into and the development of rare earth catalytic materials will significantly promote the high-efficiency utilization of abundant rare earth elements,such as lanthanum and cerium.Currently,rare earth catalytic materials play an important role in such areas as the petroleum chemical industry,the catalytic combustion of fossil fuels,automotive emissions control,the purification of industrial waste air,and solid solution fuel cells.In this paper,we review the application of and recent research progress that has been made on rare earth catalytic materials,including relative theoretical research.The effects of rare earth elements on the structure,activity,and stability of the catalysts of interest are described.
Wangcheng ZhanYun GuoXueqing GongYanglong GuoYanqing WangGuanzhong Lu
In order to investigate the influence of MnO2 modification methods on the catalytic performance of CuO/CeO2 catalyst for NO reduction by CO, two series of catalysts （xCuyMn/Ce and xCu/yMn/Ce） were prepared by co-impregnation and step- wise-impregnation methods, and characterized by means of X-ray diffraction （XRD）, Raman spectra, H2-temperature programmed reduction （H2-TPR）, in situ diffuse reflectance infrared Fourier transform spectra （in situ DRIFTS） techniques. Furthermore, the cata- lytic performances of these catalysts were evaluated by NO＋CO model reaction. The obtained results indicated that： （1） The catalysts acquired by co-impregnation method exhibited stronger interaction owing to the more sufficient contact among each component of the catalysts compared with the catalysts obtained by stepwise-impregnation method, which was beneficial to the improvement of the reduction behavior; （2） The excellent reduction behavior was conducive to the formation of low valence state copper species （Cu＋/Cu0） and more oxygen vacancies （especially the surface synergetic oxygen vacancies （SSOV, Cu＋-n-Mn（4-x）-）） during the reaction process, which were beneficial to the adsorption of CO species and the dissociation of NO species, respectively, and further promoted the en- hancement of the catalytic performance. Finally, in order to further understand the difference between the catalytic performances of these catalysts prepared by co-impregnation and stepwise-impregnation methods, a possible reaction mechanism （schematic diagram） was tentatively proposed.
The CuO/CeO2 catalysts were investigated by means of X-ray diffraction （XRD）, laser Raman spectroscopy （LRS）, X-ray photoelectronic spectroscopy （XPS）, temperature-programmed reduction （TPR）, in situ Fourier transform infrared spectroscopy （FTIR） and NO＋CO reaction. The results revealed that the low temperature （〈150℃） catalytic performances were enhanced for CO pretreated samples. During CO pretreatment, the surface Cu＋/Cu0 and oxygen vacancies on ceria surface were present. The low va- lence copper species activated the adsorbed CO and surface oxygen vacancies facilitated the NO dissociation. These effects in turn led to higher activities of CuO/CeO2 for NO reduction. The current study provided helpful understandings of active sites and reaction mechanism in NO＋CO reaction.
