Mechanism of selective catalytic reduction of NO over Ag/Al2O3 with the aid of non-thermal plasma

The mechanism of selective catalytic reduction (SCR) of NO by propene over Ag/Al2O3 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 NOx, 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 NO3−. Thermal C3H6-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 CxHyOzN species (such as RCN and RNO2, etc.), respectively. Nitrates reacts with electron-rich organic oxygenates effectively and generates isocyanate species. The CxHyOzN species can directly oxidized by oxygen or decomposed into isocyanate species over Ag/Al2O3. The lack of nitrates or the inability of nitrates to provide enough NO2+ in the reduction process probably leads to the low deNOx activities in NTP-assisted C3H6-SCR in presence of SO2 or at low temperatures below 250 °C.