Mobility of copper in zeolite-based SCR catalysts

Paper in proceeding, 2017

Selective catalytic reduction with ammonia (NH3-SCR) is a well-established and effective method to eliminate nitrogen oxides (NOx) in oxygen excess for stationary and mobile applications. Vanadia supported on titania was the first NH3-SCR catalyst that was commercialized. This type of catalyst is effective around 300-450°C, however at lower or higher temperatures, the efficiency of the catalyst to reduce NOx decreases. To increase the overall NOx reduction, high SCR activity around 200°C is required and copper-exchanged zeolites are interesting candidates in this respect. Solid-state ion-exchange in a mixture of copper oxide and zeolite is an efficient method to prepare such catalysts, but the process usually requires high (>700°C) temperatures. The ion-exchange process with copper oxides and zeolites can be considerably affected in presence of reactive atmospheres. It is shown that the copper-exchange is possible at unprecedented low temperatures, as low as 250°C, when facilitated by ammonia. The influence of the treatment conditions on the copper-exchange and the mechanism of the ion-exchange process will be presented and discussed. Such copper-exchanged zeolite structures with high copper loading are potentially interesting catalysts for a number of technical applications. Powder mixtures of CuO or Cu2O and zeolite with either the MFI, *BEA or CHA framework structure were exposed to well-defined gas atmospheres at constant temperature. After the treatment the SCR activity was determined by steady state and transient flow reactor experiments, and the physico-chemical properties of the samples were characterized with bulk and surface sensitive characterization techniques. Furthermore, density functional theory calculations were used to investigate the energetic conditions for the ion-exchange process. We show that copper in the presence of ammonia becomes mobile at considerably lower temperatures, <250°C, than conventionally used in solid-state ion-exchange (typically 700-800°C). The treatment in ammonia can be used to prepare copper-exchanged zeolites that are active for NH3-SCR on a time scale below 5−10 h, irrespective of zeolite framework structure. For CuO as the copper precursor, the migration rate of copper can be enhanced by adding NO to the treatment atmosphere. Furthermore, the time window of the copper migration can be even further extended by using Cu2O, in combination with NH3, as the copper precursor. In fact, starting from Cu2O eliminates the need for NO, which simplifies the ion-exchange process. The mobility of copper at low temperatures is proposed to be related to the ability of ammonia to form linear Cu(NH3)2+ complexes. The ion-exchange of the diamine complex into the zeolite is found to be exothermic, whereas the exchange process is endothermic in the absence of ammonia. Furthermore, we find that the diamine complex can diffuse easily in the zeolite. The charge neutrality of the system is maintained via exchange of protons from the zeolite to the Cu2O surface, where water can be formed. We suggest that the protons are transported from the zeolite in the form of NH4+. By conventional ion-exchange methods using aqueous Cu2+ solutions, an Exchange level around Cu/Al = 0.5 is typically the maximum exchange level that can be achieved. However, by using the reaction-driven solid-state ion-exchange method, copper-exchanged zeolites with a Cu/Al ratio of 1.0 could in principle be prepared, because copper is in oxidation state +1. Such Cu-zeolites with high copper loading are potentially interesting catalysts for a number of technical interesting catalysts for a number of technical applications.