A Low-Temperature Route Triggered by Water Vapor during the Ethanol-SCR of NOx over Ag/Al2O3

A negative temperature dependence was found for the selective catalytic reduction of NOx by ethanol (ethanol-SCR) over Ag/Al2O3 in the absence of water vapor. Activation energy measurements for this process confirmed that two reaction routes occurred in different temperature ranges. In situ DRIFTS experiments revealed that these temperature dependent reactions were closely related to the process of the partial oxidation of ethanol. During the partial oxidation of ethanol at low temperatures below 400 degrees C, enolic species and acetates were produced, the former of which exhibited much higher activity for NOx reduction than the latter. Therefore, the formation of enolic species and their further transformation to produce N-2 governs the low-temperature route for ethanol-SCR. At temperatures above 400 degrees C, only acetate appeared during the partial oxidation of ethanol, and its further reaction with NOx accounts for the high-temperature route. More importantly, introduction of water vapor significantly enhanced the deNOx activity of Ag/Al2O3 for ethanol-SCR, especially in the low-temperature region. On pure Al2O3, however, the ethanol-SCR process was suppressed by the presence of water vapor, indicating that the promotion effect of water vapor is closely related to silver. Within the low temperature region, water addition promoted the partial oxidation of ethanol to produce enolic species, the occurrence of which also enhanced the formation of NO2 during the ethanol-SCR over Ag/Al2O3. The produced NO2 in turn accelerated the formation of enolic species and also exhibited a higher reactivity toward enolic species compared with NO. Such synergistic effects of NO2 and enolic species induced by water vapor addition thus triggered a cyclic reaction pathway for NOx reduction with high efficiency.