Selective catalytic reduction of NO at low temperature using a (ethanol+ammonia) mixture over a Ag/Al2O3 + WO3/Cex-ZryO2 dual-bed catalytic system: Reactivity insight of WO3/Cex-ZryO2

Abstract

The Selective Catalytic Reduction (SCR) is one of the most efficient process for NOx removal from Diesel exhaust gas. However, the urea/NH3-SCR process implemented in recent vehicles still suffers from a poor activity in the low temperature range (T < 250°C). One main reason is its dependency against the NO2/NOx ratio, limiting the expected fast-SCR reaction in this temperature range. Recently, we shown that the addition of ethanol to ammonia led to a significant increase of the activity of a Ag/Al2O3 catalyst in this low temperature range. Moreover, in a dual-bed configuration (Ag/Al2O3+WO3/Cex-ZryO2), a remarkable improvement was achieved at low temperature using only NO as NOx. The present work aims to highlight the DeNOx chemistry encountered over the WO3/Cex-ZryO2 catalyst in a bifunctional (EtOH+NH3) mixture. In addition to the fact that this process takes advantage of the low temperature NO2 formation over the upstream Ag/Al2O3 catalyst, this work also puts in evidenced unexpected interactions between NO2 and CH3CHO (resulting from ethanol oxidation over Ag/Al2O3) thus leading to NO emission.

Effect of Ammonia on the Intermediates and Chemical Pathway of Selective Catalytic Reduction of NO by Ethanol over a Pt/Al2O3 Catalyst

“…Therefore, the study of high deNO x performances using ethanol and NH 3 as reductants in the process of SCR under low- and medium-temperature conditions has a significant impact on the reduction of the emitted exhaust gas. Barreau et al added the ethanol + NH 3 mixture to the dual-bed catalyst (Ag/Al 2 O 3 + WO 3 /Ce x –Zr y O 2 ) to remove NO and found that the NO 2 generated from the oxidation of NO by ethanol would promote the reaction of NH 3 with NO, which means that the reaction rate of NH 3 and NO + NO 2 in the (EtOH + NH 3 )-SCR process is faster than that of NH 3 and NO + O 2 in the NH 3 -SCR process. Yu et al found that the addition of NH 3 promoted the conversion of NO x and the partial oxidation of ethanol in ethanol-SCR and used DRIFT analysis for the first time to reveal that NH 3 reacts with olefin species over the Ag/Al 2 O 3 catalyst to generate −NCO, which provides important evidence for NH 3 to promote the deNO x process in ethanol-SCR.…”

Tungsten-Based Catalysts for Environmental Applications

“…The respective roles of Ag/Al2O3 and WO3/CeZrO2 in this complex reaction were summarized in the review by Barreau et al [279]. Figure 25 illustrates the reaction pathway identified in the ethanol-NH3 SCR reaction over the Ag/W dual bed.…”

“…Fortunately, this reaction is totally selective for N 2 over the tungsten catalyst. From Barreau et al [279] with permission from Elsevier.…”

Emissions from Light-Duty Vehicles—From Statistics to Emission Regulations and Vehicle Testing in the European Union

“…Oxidation of carbon monoxide and hydrocarbons in the DOC catalytic reactor proceeds effectively at a temperature of 250-300 • C [110]. In the case of nitrogen oxides, the literature indicates the optimal operating temperature of SCR and LNT in the range from 250-300 • C to 450-500 • C [101,111]. However, a 90% reduction of NO x can already occur at 200 • C [112].…”