The Influence Of Acid/Rare Earth Elements （Ce,La）-modification On The Hydrothermal Stability Of Cu-SAPO-34 For NH₃-SCR

With the shortage of oil resources, diesel engines have attracted extensive attention because of its high thermal efficiency, good fuel economy and low greenhouse emission. The lean-burn technology is applied in diesel engines. High oxygen content in the diesel exhausts contributes to the low emissions of hydrocarbons （HC） and carbon monoxide （CO）. However, the emissions of NOx from internal combustion diesel engines are obviously increase, as they may cause a host of environmental issues, such as acid rain, photochemical smog and greenhouse effect. Selective catalytic reduction of NOx with ammonia or with urea as the reducing agent are commonly used for diesel vehicle emissions due to its high activity. At present, The V2O5-WO3/TiO2 catalyst has been commonly used in commercial. However, the catalyst performs narrow temperature window （300～400℃）, poor hydrothermal stability and the bio-toxicity of vanadium. Therefore, it is essential to develop novel, remarkable SCR activity, high hydrothermal stability, environmental-friendly and vanadium-free SCR catalysts.In recent years, Copper-based chabazite （CHA） small pores molecular sieves have received widespread attention as SCR catalyst with their superior NH3-SCR activity, hydrothermal stability and N2 selectivity. The addition of acid could greatly enhance the acid sites of catalysts surface, and the amount of acid sites could promote the adsorption of NH3 on the catalytic surface. Cerium has been studied extensively for its impressive store/release oxygen ability. In this paper, it concentrates on exploring the hydrothermal stability of Cu-SAPO-34 catalysts modified by acid/rare earth elements （Ce, La）. Firstly, acid doping Cu-SAPO-34 catalysts were prepared by wet impregnation method. Secondly, rare earth elements （Ce, La） modified Cu-SAPO-34 catalyst were prepared by ion-exchange method and wet impregnation method. In order to investigate the hydrothermal stability of the catalysts, fresh samples were hydrothermally aging at 700℃ for 10 h. In addition. various techniques （BET,XRD,TEM,27Al MAS NMR, H2-TPR,UV-vis, XPS and Py-IR） were introduced to investigate the influence of acid/rare earth elements （Ce, La）-modification on the structure and physicochemical performance of the catalysts. Some specific conclusions from this work are drawn as follows:（1） The NH₃-SCR catalytic performance of Cu-SAPO-34 catalyst modified by silicotungstic acid was investigated. The results indicated that the addition of silicotungstic acid greatly improved the high temperature SCR activity and hydrothermal stability of the catalysts. BET and XRD results reveal that the addition of silicotungstic acid could make a abundance of acid sites formed, and could inhibit the reduction of BET surface areas and total pore volume during hydrothermal aging treatment. Therefore, the hydrothermal stability of catalysts was obviously promoted Furthermore, the interaction between silicotungstic acid and molecular sieves could promote the dispersion of copper species and increase the amount of isolated Cu2+ ions during hydrothermal aging.（2） The hydrothermal stability of Cu-SAPO-34 catalyst modified by Ce/La was concluded. The results indicated that the catalytic performance and hydrothermal stability of Cu-SAPO-34 catalysts were dramatically enhanced by the addition of Ce/La, and the samples showed no degraded NH3-SCR activity even the catalysts were aging at 700℃ for 10 h in 10 vol% H2O. Moreover, In particular, the SCR catalytic performance was even enhanced at 150℃ of the aged samples.（3） BET XRD and 27A1 MAS NMR results showed that the addition of Ce/La could inhibit the collapse of pore structures, maintained their surface areas and prevent dealumination from the structure of SAPO-34. The TEM, TPR, UV-vis and XPS results implied that the interaction between Cu and Ce/La existed, and the interaction could increase the amount of isolated Cu2+ions and promote the dispersion of copper species. In addition, the addition of Ce/La could inhibit the degradation of acid density during hydrothermal aging.