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

Abstract

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-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 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-P[double bond, length as m-dash]O 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-P[double bond, length as m-dash]O 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-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 C₃-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 NH₃-SCR reaction.

Fig. 1. The deconvoluted and the fitted results of (a) P2p and (b) O1s spectra of the P-CAs.

Fig. 2. NH₃-TPD profiles and the corresponding amounts of adsorbed NH₃ over the CA-800, P-CA-800 and P-CA-800_vac.

Fig. 3. (a) NO conversions over the CA-800 and P-CAs (reaction conditions: [NO] = 500 ppm, [NH₃] = 500 ppm, [O₂] = 5%, N₂ balance, GHSV = 500 h⁻¹) and (b) Arrhenius plots of the P-CA-800 and CA-800 samples.

Fig. 4. Relative surface concentrations of phosphorous species for the P-CA-800_vac sample after the NH₃-SCR at different reaction temperatures obtained from (a) P2p XPS spectra and (b) O1s XPS spectra. Reaction conditions: [NO] = 500 ppm, [NH₃] = 500 ppm, [O₂] = 5%, N₂ balance, GHSV = 500 h⁻¹.

Fig. 5. FTIR spectra of the P-CA-800_vac samples before and after the NH₃-SCR reaction for 1 h at different temperatures.

Fig. 6. (a) Stability test of NH₃-SCR for the P-CA-800_vac sample, (b) P2p and (c) O1s XPS spectra of the P-CA-800_vac after NH₃-SCR reaction for 24 h. Reaction conditions: [NO] = 500 ppm, [NH₃] = 500 ppm, [O₂] = 5%, N₂ balance, GHSV = 500 h⁻¹, 200 °C.

Fig. 7. NO conversions, NO₂ and N₂O concentration in the outlet gas over the P-CA-800_vac sample for the different inlet gases of NO, NO + O₂ and NO + NH₃ + O₂. Reaction conditions: [NO] = 500 ppm, [NH₃] = 500 ppm (when used), [O₂] = 5% (when used), N₂ balance, GHSV = 500 h⁻¹.

Fig. 8. Proposed mechanism for structure evolution from C₃–PO to C–O–P.