Fundamental ageing studies of metal-exchanged zeolites for selective catalytic reduction of nitrogen oxides in oxygen excess
Paper in proceedings, 2016
Selective catalytic reduction with ammonia (NH3-SCR) is today a well-established and effective method to eliminate nitrogen oxides (NOx) under lean conditions for stationary and mobile applications. For the latter case vanadia supported on titania was the first NH3-SCR catalyst that was commercialized. This catalyst is highly effective around 350-450°C, however at lower or higher temperatures, the efficiency of the catalyst to reduce NOx decreases. Furthermore, problems like toxicity of volatile vanadium compounds and high activity to oxidize SO2 have promoted the development of alternative catalysts. Zeolite systems are in this connection interesting candidates. Presently, iron- and copper-exchanged zeolite structures are the most attractive alternatives to the traditional vanadia-based SCR catalyst. However, several challenges arise when using metal-exchanged zeolites in exhaust gas after-treatment systems for vehicles. Two of the more important issues are the hydrothermal stability and the tolerance against chemical poisoning. Furthermore, the possibility to control the distribution of the metal species in the zeolite by thermal treatment during the preparation or after deactivation is another important aspect of metal-exchanged zeolite structures. The lecture will focus on thermal and chemical ageing mechanisms of iron-exchanged zeolite beta, Fe-BEA, as NH3-SCR catalyst. The deactivation of Fe-BEA after hydrothermal treatment, and phosphorous and potassium exposure has been studied experimentally and by kinetic modeling as well as activation and regeneration of the catalyst using hydrogen treatment. The fundamental mechanisms for thermal and chemical degradation of Fe-BEA will be presented and discussed. Furthermore, the activation and regeneration of the catalyst by hydrogen exposure will be discussed.