Insight into the formation mechanism of N, P co-doped mesoporous biochar from H3PO4 activation and NH3 modification of biomass

Understanding the formation mechanism of N, P co-doped biochar is critical for preparation of ideal N, P co-doped biochar materials. In this study, N, P co-doped mesoporous biochar was prepared by H₃PO₄ activation and NH₃ modification of biomass. Results showed that, during H₃PO₄ activation, H₃PO₄ preferentially reacted with active O-containing groups in biomass. With increase of H₃PO₄ amount, H₃PO₄ mainly reacted with carbon fragments. This process removed large amounts of oxygen and carbon of biomass, meanwhile formed lots of vacancies, and then H₃PO₄ introduced abundant phosphorus (8.95 wt%) and new oxygen into P-doped biochar, accompanying generation of developed mesoporous structure (1089.33 m²/g). P-containing groups introduced were mainly C₃-P-O, C-P-O and C-O-P groups. During NH₃ modification, NH₃ preferentially reacted with PO groups to form amine/amide-N, and then most of them converted into stable pyridinic-N, pyrrolic-N, or NP groups, meanwhile some N-containing groups would further convert into more stable quaternary-N. This process promoted decrease of C₃-P-O and C-P-O groups, and formed more PN groups. It introduced abundant nitrogen (12.21 wt%), accompanying generation of more mesopores. At last, possible chemical reaction pathways of H₃PO₄ activation, and chemical reaction pathways between NH₃ and P-containing groups during NH₃ modification were proposed for the first time.