Investigation of the Poisoning Mechanism of Lead on the CeO_2-WO_3 Catalyst for the NH_3-SCR Reaction via in Situ IR and Raman Spectroscopy Measurement

摘要

The in situ IR and Raman spectroscopy measurements were conducted to investigate lead poisoning on the CeO_2-WO_3 catalysts. The deactivation mechanisms were studied with respect to the changes of surface acidity, redox property, nitrateitrite adsorption behaviors, and key active sites (note that the results of structure-activity relationship of CeO_2-WO_3 were based on our previous research). (1) Lewis acid sites originated from CeO_2 and crystalline WO_3, whereas Bronsted acid sites originated from Ce_2(WO_4)_3. The poisoned catalysts exhibited a lower surface acidity than the fresh catalysts: the number of acid sites decreased, and their thermal stability weakened (2) The reducibility of catalysts and the amount of active oxygen exhibited a smaller influence after poisoning because lead preferred to bond with surface WO_x species rather than CeO_2. (3) The quantity of active nitrate species decreased due to the lead coverage on the catalyst and the partial bridged-nitrate species induced by lead exhibited a low degree of activity at 200 ℃. (4) Crystalline WO_3 and Ce_2(WO_4)_3 originated from the transformation of polytungstate sites. These sites were the key active sites during the SCR process. The formation temperatures of polytungstate on the poisoned catalysts were higher than those on the fresh catalysts.