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 degrees C in an atmosphere of N-2 or H-2 to investigate the evolution of phosphorus-containing groups. Elemental analysis, X-ray photoelectron spectroscopy, P-31 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, C-3-P=O, C-3-P, and eventually elemental phosphorus as a result of heat treatment. Phosphate-like groups are much more thermally stable in an N-2 than in an H-2 atmosphere. In N-2, C-O-P linkages significantly evolve into C-P-O and C-3-P=0 at up to 800 degrees C, whereas C-3-P linkages are not formed even at 900 degrees C. In H-2, the corresponding evolution remarkably occurs at 500 degrees C, forming C-3-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 N-2 and H-2 atmospheres.