Copper mobility in zeolite-based SCR catalysts
Paper in proceeding, 2017
Selective catalytic reduction with ammonia (NH3-SCR) is an effective, well-established method to eliminate nitrogen oxides (NOx) in oxygen excess for stationary and mobile applications. Titania-supported vanadia catalysts are traditionally used for NH3-SCR. This type of catalyst is effective in the range 300-450°C, but the NOx reduction efficiency decreases at both lower and higher temperatures. The efficiency of the NH3-SCR process can be improved significantly by using catalysts based on copper-exchanged zeolites and zeotypes, due to their high activity around 200°C. Solid-state ion-exchange in a mixture of copper oxide and zeolite is an efficient way to prepare such catalysts, but this process usually requires high (>700°C) temperatures. The ion-exchange can be considerably affected by appropriate choice of atmosphere during the process. It is shown that the copper-exchange is possible at unprecedented low temperatures, as low as 250°C, in presence of ammonia. The influence of the treatment conditions on the copper-exchange and the mechanism of the reaction-driven 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 Cu2O or CuO and zeolite with either CHA, MFI or *BEA framework structure were exposed to well-defined gas atmospheres at constant temperature. After the treatment, the SCR activity of the samples was determined by steady state and transient flow reactor experiments, and the physicochemical properties of the samples were characterized with bulk and surface sensitive characterization techniques. Furthermore, first-principles calculations were used to investigate the energetic conditions for the ion-exchange process.
We show that in the presence of ammonia, copper 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, active for NH3-SCR, on a time scale below 5−10 h, irrespective of zeolite framework structure. For CuO as copper precursor, the migration rate of copper can be enhanced in the presence of nitric oxide during the ion-exchange process. Furthermore, the time window of the copper migration can be further extended by using Cu2O, in combination with ammonia, as the copper precursor. In fact, starting from Cu2O eliminates the need for nitric oxide, 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 copper diamine complex into the zeolite is found to be exothermic, whereas in the absence of ammonia, the ion-exchange process is endothermic. Furthermore, we find that the copper diamine complex can diffuse easily in the microporous structure of the zeolite with a slight barrier. 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 applications.
Selective Catalytic Reduction