Evolution of Phosphorus-Containing Groups on Activated Carbons during Heat Treatment

Two types of activated carbons have been prepared by H₃PO₄ activation of lignocellulose and by H₃PO₄ modification of activated carbon, and then heat-treated at temperatures from 400 to 900 °C in an atmosphere of N₂ or H₂ to investigate the evolution of phosphorus-containing groups. Elemental analysis, X-ray photoelectron spectroscopy, ³¹P nuclear magnetic resonance, nitrogen adsorption, and scanning electron microscopy have been used to analyze the physicochemical properties of the activated carbons. The results show that C-O-P linkages of phosphorus-containing groups can progressively evolve into C-P-O, C₃-P═O, C₃-P, and eventually elemental phosphorus as a result of heat treatment. Phosphate-like groups are much more thermally stable in an N₂ than in an H₂ atmosphere. In N₂, C-O-P linkages significantly evolve into C-P-O and C₃-P═O at up to 800 °C, whereas C₃-P linkages are not formed even at 900 °C. In H₂, the corresponding evolution remarkably occurs at 500 °C, forming C₃-P linkages and eventually elemental phosphorus. Moreover, the two activated carbons exhibit different evolution trends, suggesting that the evolution happens more easily for phosphorus-containing groups located on the edges of graphite-like crystallites than those in the lattice. Finally, we propose different evolution pathways of phosphorus-containing groups upon heat treatment in N₂ and H₂ atmospheres.