Fluorine and Bromine Dual-Doped Nanoporous Carbons:\nPreparation and Surface Chemistry Studies

A novel method for\nthe concurrent introduction of fluorine and\nbromine into the surface of nanoporous activated carbon (NAC) is evaluated.\nAccording to the method, the preheated NAC was treated with 1,2-dibromotetrafluoroethane\nat elevated temperatures (400–800 °C). Potentiometric\nand elemental analysis, nitrogen adsorption–desorption, scanning\nelectron microscopy-energy-dispersive X-ray spectroscopy, X-ray photoelectron\nspectroscopy (XPS), and <sup>19</sup>F solid-state NMR were used to\nstudy the NAC microstructure and changes in surface chemistry. The\nspecific modification temperature was found to have a decisive influence\non the resulting halogen content of the NAC surface. About 1.5 mmol\ng<sup>–1</sup> of bromine and only 0.5 mmol g<sup>–1</sup> of fluorine are chemisorbed on the NAC surface when dual-doped at\n400 °C. The fluorination efficiency increases dramatically to\n1.84–2.22 mmol g<sup>–1</sup> when the process temperature\nis increased to 500–700 °C. Under the same conditions,\nthe bromination efficiency unexpectedly decreases to 0.66–1.32\nmmol g<sup>–1</sup>. Since halogen-containing groups undergo\nsignificant thermal decomposition around 800 °C, the overall\nhalogenation efficiency decreases, accordingly. Both the volume and\nsurface area of the micropores decrease moderately when halogen-containing\ngroups are introduced into the carbon surface layer. Fluorine and\nbromine are unevenly distributed in the porous structure of the dual-doped\nNACs, and the outer surface is more halogen-rich than the inner surface\nof the micropores. XPS and <sup>19</sup>F solid-state NMR revealed\nthe selective formation of CF<sub>2</sub> groups in the NAC surface\nlayer independent of the temperature. In contrast, the percentage\nof semi-ionic fluorine in the form of CF groups directly bonded to\nthe π-electron system of the carbon matrix increases significantly\nwith temperature.