Insights into the promotion role of phosphorus doping on carbon as a metal-free catalyst for low-temperature selective catalytic reduction of NO with NH3†
The catalytic reduction of NO with NH3 (NH3-SCR) on phosphorus-doped carbon aerogels (P-CAs) was studied in the temperature range of 100–200 °C. The P-CAs were prepared by a one-pot sol–gel method by using phosphoric acid as a phosphorus source followed by carbonization at 600–900 °C. A correlation between catalytic activity and surface P content is observed. The P-CA-800vac sample obtained via carbonization at 800 °C and vacuum treatment at 380 °C shows the highest NO conversion of 45.6–76.8% at 100–200 °C under a gas hourly space velocity of 500 h−1 for the inlet gas mixture of 500 ppm NO, 500 ppm NH3 and 5.0 vol% O2. The coexistence of NH3 and O2 is essential for the high conversion of NO on the P-CA carbon catalysts, which can decrease the spillover of NO2 and N2O. The main Brønsted acid sites derived from P-doping and contributed by the C–OH group at edges of carbon sheets are beneficial for NH3 adsorption. In addition, the C3–PO configuration seems to have the most active sites for favorable adsorption and dissociation of O2 and facilitates the formation of NO2. Therefore, the simultaneous presence of acidic groups for NH3 adsorption and the C3–PO active sites for NO2 generation due to the activation of O2 molecules is likely responsible for the significant increase in the NH3-SCR activity over the P–CAs. The transformation of C3–PO to C–O–P functional groups after the reaction is found, which could be assigned to the oxidation of C3–PO by the dissociated O*, resulting in an apparent decrease of catalytic activity for P-CAs. The C–O–P based functional groups are also active in the NH3-SCR reaction.