Mechanism of selective catalytic reduction of NO over Ag/Al 2O 3 with the aid of non-thermal plasma

Abstract

The mechanism of selective catalytic reduction (SCR) of NO by propene over Ag/Al 2O 3 was investigated with the aid of non-thermal plasma (NTP) using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The HC-SCR reactions were proposed to be divided into two main processes, viz. nitration and reduction, which were aimed to generate isocyanate species (–NCO) and reduce NO x , respectively. The nitration process of thermal HC-SCR was suggested to be effective only in presence of abundant electron-rich organic species. The efficiency of the reduction process was determined by the balance between the oxidation of isocyanate species and the reaction of isocyanate species with NO 3 −. Thermal C 3H 6-SCR was found to be ineffective below 400 °C, mainly owing to the inability of propene oxidation to provide oxygenates for nitration. With the assistance of NTP, the nitration process could be promoted by activating propene in an O-predominant or N-predominant environment to electron-rich organic oxygenates or C x H y O z N species (such as RCN and RNO 2, etc.), respectively. Nitrates reacts with electron-rich organic oxygenates effectively and generates isocyanate species. The C x H y O z N species can directly oxidized by oxygen or decomposed into isocyanate species over Ag/Al 2O 3. The lack of nitrates or the inability of nitrates to provide enough NO 2 + in the reduction process probably leads to the low deNO x activities in NTP-assisted C 3H 6-SCR in presence of SO 2 or at low temperatures below 250 °C.