Physico-Chemical nanomaterials science

Gas-phase bromination of carbon materials for the production of surface precursors
Diyuk V. E.1, Vakaliuk A. V.1, Tsapyuk G. G.1, Yatsymyrskyi A. V.1, Mariychuk R.2, Boldyrieva O. Yu.1, Lisnyak V. V.1,2
1 Faculty of Chemistry, Taras Shevchenko National University of Kyiv, Volodymyrska Street, 64/13, Kyiv 01601, Ukraine.
E-mail: vdiyuk@gmail.com; lisnyak@chem.univ.kiev.ua
2 Presov Universirty in Presov, Ul. 17 novembra, 1, Presov 15 080 01, Slovakia.

Carbon nanomaterials are traditionally used as sorbents, carriers, and catalysts. The functional properties of nanoporous materials depend significantly on surface chemistry. The formation of functional groups on the carbon surface is possible only as a result of chemical modification. The most versatile method of modifying is halogenation and the subsequent halogen replacement by other functional groups. This approach allows obtaining the required functional O-, N-, and S-groups at a given concentration on the carbon surface. The treatment with bromine vapor prevents unwanted parallel oxidation and more actively involves the active centers in bromine addition. Here gas-phase bromination of various nanoporous carbons, e.g., NACs, NACFs, soot, and MWCNTs, was performed. Bromination was carried out by Br2 vapors in an argon stream. The studies were carried out under isothermal and nonisothermal conditions at 30–800 °C, under gravimetric control. Miscellaneous methods, e.g., adsorption of N2, SEM, thermal analysis, and FTIR methods, were used to characterize the samples. Depending on the carbon material and the surface temperature, 0.4–2.7 mmol g–1 of chemisorbed bromine is attached to the surface. The rate of bromination depends on the texture of nanocarbons. The highest and the lowest contents of grafted bromine were obtained for NACs and MWCNTs, correspondingly. The kinetic analysis of the bromination reaction shows that this process passes in two kinetic modes in the range of 200–500 °C. The rapid process involves chemisorption of 65–80% bromine in the first 30–35 min. The slow process takes place over the next 100–250 min, depending on the bromination temperature. A reaction model is proposed that describes the interaction of bromine with a system of isolated and coupled C=C bonds. The increase in the surface reactivity is determined by the number of removed oxygen-containing functional groups and does not depend on their nature. Thus, the proposed bromination techniques are effective in creating bromine-containing surface precursors to new surface functionalities.