Highly-efficient Cerium-based Catalysts For Selective Catalytic Reduction Of NO_x With NH₃

Selective catalytic reduction of NOx with NH3 (NH3-SCR) is considered to be one of the most promising after-treatment technologies for NOx abatement from diesel exhaust. The catalyst plays a key role in NH3-SCR system. Since the operating conditions of mobile source (i.e. diesel vehicle) were generally unstable, and the space on board for catalyst is limited, so the catalyst applied for diesel exhaust must meet some requirements, such as a wide temperature window, high space velocity resistant properties and excellent low-temperature activity. However, there are still some inevitable problems with practical applications of existing NH3-SCR catalyst. There are continuing efforts to develop environmentally-benign NH3-SCR catalyst with excellent activity and stability for controlling the NOx emission from diesel engines.In this study, a CeO2/WO3-TiO2 catalyst was prepared by a facile homogeneous precipitation method. The influences of preparation process on the catalyst have been investigated, the resistance to high space velocity condition as well as H2O and SO2 have also been investigated. N2 physisorption, XRD, SEM-EDS、XPS、H2-TPR and NH3/NOx-TPD, etc., were used to investigated the relationship between structure and activity, in addition, we explored the promotion mechanisms for a novel stepwise precipitation approach and Ce species on SCR activity over CeO2/WO3-TiO2 catalyst. The main research results as follows:We prepared a highly-efficient NH3-SCR catalyst, Ce0.2W0.1TiOx. This catalyst showed excellent catalytic performance with outstanding SCR activity, high N2 selectivity, and over 80% NOx conversion being achieved in a wide temperature range from 175℃ to 425℃ under a GHSV of 200,000 h-1. In addition, the SCR activity under high space velocity conditions was still high, and the catalyst had excellent anti-poisoning performance for H2O and/ or SO2, indicating that the CeO2/WO3-TiO2 catalyst is a very promising catralyst for the removal of NOx from diesel engine exhaust.Combined with the pH variation of the mixed solution during the preparation of the catalyst, XPS analysis and EDS results, the formation process of the CeO2/WO3-TiO2 catalyst can be proposed. When the pH of the mixed solution first increased, the Ti and W species were firstly co-precipitated, and then the Ce species was precipitated uniformly onto the precipitated WTiOx species following the further pH increase. This homogenous precipitation process is very important for the production of highly dispersed CeO2 on WTiOx, which is helpful to improve the SCR performance of the catalyst.Combinded with all kinds of characterization methods can know the relationship between structure and activity, the samples showed a gradual increase of surface Ce species with precipitation times, then the interaction between Ce species and WTiOx species was enhanced, as result the BET surface areas of the the catalyst. Introducing Ce species suppressed recrystallization of TiO2 when CeO2/WO3-TiO2 catalyst was calcined, so that the active Ce species are better dispersed. The extension of the precipitation time could increase the amount of surface oxygen vacancies and enhance the redox property of the catalyst, which can benefit the low temperature activity by facilitating the "fast SCR" reaction. In addition, the extension of the precipitation time could enhance both the high temperature activity and N2 selectivity by inhibiting the unselective oxidation of NH3 at high temperatures.