A Low-Temperature Route Triggered by Water Vapor during the Ethanol-SCR of NO<i>x</i> over Ag/Al₂O₃

Abstract

A negative temperature dependence was found for the selective catalytic reduction of NO<i>x</i> 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 °C, enolic species and acetates were produced, the former of which exhibited much higher activity for NO<i>x</i> reduction than the latter. Therefore, the formation of enolic species and their further transformation to produce N₂ governs the low-temperature route for ethanol-SCR. At temperatures above 400 °C, only acetate appeared during the partial oxidation of ethanol, and its further reaction with NO<i>x</i> accounts for the high-temperature route. More importantly, introduction of water vapor significantly enhanced the deNO<i>x</i> 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 NO<i>x</i> reduction with high efficiency