Insights into the promotion role of phosphorus doping on carbon as a metal-free catalyst for low-temperature selective catalytic reduction of NO with NH(3)

The catalytic reduction of NO with NH(3) (NH(3)-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-800(vac) 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 NH(3) and 5.0 vol% O(2). The coexistence of NH(3) and O(2) is essential for the high conversion of NO on the P-CA carbon catalysts, which can decrease the spillover of NO(2) and N(2)O. The main Brønsted acid sites derived from P-doping and contributed by the C–OH group at edges of carbon sheets are beneficial for NH(3) adsorption. In addition, the C(3)–P[double bond, length as m-dash]O configuration seems to have the most active sites for favorable adsorption and dissociation of O(2) and facilitates the formation of NO(2). Therefore, the simultaneous presence of acidic groups for NH(3) adsorption and the C(3)–P[double bond, length as m-dash]O active sites for NO(2) generation due to the activation of O(2) molecules is likely responsible for the significant increase in the NH(3)-SCR activity over the P–CAs. The transformation of C(3)–P[double bond, length as m-dash]O to C–O–P functional groups after the reaction is found, which could be assigned to the oxidation of C(3)–P[double bond, length as m-dash]O 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 NH(3)-SCR reaction.