Adsorption properties of hydrogen on (10,0) single-walled carbon nanotube through density functional theory

The density functional theory (DFT) has been used to simultaneously investigate physi-/chemi- sorption properties of hydrogen on the (10,0) single-walled carbon nanotube (SWCNT) walls. Physisorption of H-2 outside the CNT with a vertical orientation to the tube axis above the center of a hexagon surface is the most stable state of physisorption and its binding energy is very weak, -0.792 kcal/mol. In the chemisorp ion of two hydrogen atoms the most stable state is above two adjacent carbon atoms of a hexagon with a C-H bond length of 1.10 Angstrom and one C-H bond energy of -45.761 kcal/mol. Based on these results, we have also investigated the transition state and the reaction pathway from physisorption to chernisorption of hydrogen on the CNT. The energy barrier of the reaction from physisorption to chemisorption is about 78.837 kcal/mol and the reaction is not spontaneous at 0 K. Through the calculations of the Gibbs free energy change from physisorption to chemisorption with temperatures, we learned that it is not easy for the reaction to occur, which is a major obstacle for the practical use of the CNT as a hydrogen storage medium. (C) 2004 Elsevier Ltd. All rights reserved.