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.