NH₃-SCR Performance Over Cu-Ce Modified USY Zeolites At Low Temperature

Nitrogen oxides (NOx) are responsible for the cause of a host of environmental issues, such as photochemical smog, acid rain and the greenhouse effect. With the increasing number of motor vehicles as well as the rapid industrial development, the emissions are much more serious. Therefore, it is imminent to eliminate NOx pollution problems. Currently,ammonia selective catalytic reduction of nitrogen oxides(NH3-SCR) technology has become one of the most important approaches of NOx elimination. Cu-based zeolite with excellent low-temperature SCR activity, N2selectivity and wide temperature window,are widely considered to be the most practical SCR catalyst.In this paper, five different Cu-zeolite catalysts were prepared by an incipient wetness impregnation method to study the influence of different zeolite on the catalytic performance for NH3-SCR. Based on the above results, further investigation of the effect of different Cu loding and various Ce addition on the NH3-SCR performance was evaluated. Moreover, the hydrothermal stability and its resistance to SO2were also studied. Furthermore, all the catalysts were characterized by N2adsorption/desorption, XRD,H2-TPR,XPS,CO-DRIFTs and In-situ FTIR techniques to study the physicochemical performance of the catalysts. Some specific conclusions from this work are drawn as follows:1. The influence of different zeolite on the NH3-SCR performance for Cu-zeolite catalysts was investigated. The results indicated that5Cu-ZSM-5and5Cu-beta(40) exhibited excellent low temperature activity and the temperature window was rather wide within the range of170～400℃, yet the N2selectivity was relatively poor.And the low-temperature SCR activity followed the sequence of5Cu-ZSM-5、5Cu-beta(40)>5Cu-USY>5Cu-beta(40)>5Cu-SAPO-34.However,5Cu-USY catalyst showed higher SCR activity over the temperature range of100～140℃compared with other catalysts.Meanwhile,it exhibited favorable N2selectivity and the temperature window ranged from200℃to420℃. Futher studies showed that the the low-temperature SCR activity of the Cu-USY catalyst apparently improved with the increasing Cu loading while the N2selectivity decreased obviously. Moreover, the addition of ceria could effectively modified the the low-temperature activity of Cu-USY catalyst and had little impact of the N2selectivity at the same time. When the Ce loading is8wt.%, the initial reaction temperature is the lowest, and the window temperature is the widest(160～410℃).2. The studies of H2-TPR and CO-DRIFTs indicated that three types of copper species, including isolated Cu2+and Cu+ions, CuOx clusters,existed in the Cu-USY and Cu-Ce-USY catalysts, and the isolated cupric ions and cuprous ions,located at the supercages,were proposed to be the active sites for low temperature SCR reaction on the catalysts. When the Cu loading was excessive, the amount of CuOx clusters, dispersing on the external surface,was markedly increased,thus resulting in a poor N2selectivity. By doping a small amount of ceria, the reducibility of copper species significantly improved because of the the strong interaction between copper and ceria species,which was in favor of the oxidation of NO to NO2and the reduction of NOx at low temperature. The results of in-situ FTIR studies showed that ammonia can strongly adsorbed onto both Lewis and Br(?)nsted acid sites, thus participating in the SCR reaction, and Lewis acid site-adsorbed ammonia species were much more active than Bransted sites-bound NH3at low temperature.3. After hydrothermal treatment, the low-temperature SCR activity of the Cu-USY catalyst was found to be markedly improved whereas the Ce modified Cu-USY catalyst was slightly improved.The thermodynamic state,provide by the hydrothermal treatment,not only promoted the isolated Cu2+species migration from the sodalite cavities into the supercages,but also contributed to the generation of Cu+species in the supercages,thus facilitating the low-temperature SCR activity.However, with the increasing aging temperature,the catalysts became less selective toward the formation of N2,yielding higher amounts of undesied by-products such as NO2and N2O.All the Cu-USY and Cu-Ce-USY catalysts suffered from the serious SO2poisoning effect on the activity after presulfation.This was probablely due to the sulfation of the isolated cupric ions (or cuprous ions) or the external CuOx species of the catalysts,resulting in the great loss of Lewis acid sites,which played a significant role in the low-temperature SCR reaction.