NH3-SCR over Cu-zeolite catalysts: activity and deactivation studies

Sammanfattning: NOx emission is one of the major contributors to air pollution especially in the urban environment. The most frequently applied technique to reduce NOx from diesel engine is to employ ammonia selective catalytic reduction (NH3-SCR) using Cu‑zeolite as the catalyst.

The major challenge for zeolite-based materials is their susceptibility towards hydrothermal aging. The impact of hydrothermal treatment was investigated using Cu‑Beta aged at 500-900 ⁰C. Although the zeolite structure was still intact after aging up to 800 ⁰C, the SCR activity was readily affected by the aging; however, this effect was less than that of the oxidation reactions. Further, a structure collapse was observed after aging at 900 ⁰C.

A comparison study between the activity of the large-pore zeolite, Cu‑Beta and the small-pore zeolite/ silicoaluminophospate, Cu‑SAPO‑34 and Cu‑SSZ‑13, was performed.  In general, Cu‑CHA catalysts showed a higher activity at 150 ⁰C and a lower N2O production than Cu‑Beta did. During low temperature SCR in the presence of NO2, more stable ammonium nitrate species were observed in Cu‑CHA catalysts resulting in a lower conversion of NOx than in Cu‑Beta. 

Sulfur poisoning has been known to degrade the activity of Cu‑zeolite catalysts which in this study were investigated using Cu‑SAPO‑34 and Cu‑SSZ‑13. The standard SCR reaction especially at low temperature was severely altered by SO2 poisoning, whereas a lower level of impact was noticeable on SCR reactions in the presence of NO2. A lowering of the reducibility of copper species was found in the sulfated samples, implying that the sites on which the reactions occurred were influenced. A greater effect at low temperature was observed when the poisoning was performed under SCR conditions than in the presence of O2 and H2O only. Moreover, some of the activities could be recovered by performing repeated SCR reactions. In addition, the SO2 treatment under SCR conditions also suppressed the oxidation reactions at low temperature. The temperature programmed desorption (TPD) experiment demonstrated that a larger amount of copper sulfate species was observed when the SO2 exposure was conducted in the presence of H2O than in dry environment. When the SO2 treatment was performed in the presence of NH3, ammonium sulfate species were formed and their formation was not affected by the presence of NO and NO2. Copper sulfate species were found to be more strongly attached to the surface than ammonium sulfate species were. In addition, hydrothermal aging at 800 ⁰C lowered the formation of ammonium sulfate species and the aging facilitated the formation of copper sulfate species.

The internal mass transfer limitation in the washcoat was studied by performing experiments in constant ratio of total flow to weight of washcoat using Cu‑SSZ‑13 monoliths with different washcoat thickness. The effect of internal diffusion was noticeable in SCR reaction; however, it was not visible in NH3 oxidation owing to the low conversion of NH3 during the reaction.