Theoretical studies of mercury-bromine species adsorption mechanism on carbonaceous surface

Brominated activated carbon sorbent has been demonstrated to be quite effective in removing mercury from coal-fired flue gas, but the Hg-sorbent binding mechanism is still unclear. In this paper, a systematic theoretical study on the mechanism of mercury-bromine species adsorption on carbonaceous surface is carried out by means of first principles quantum mechanical methods based on density functional theory to provide molecular-level understanding of the adsorption mechanism. The adsorption complex structure, adsorption energy, atomic bond population and atomic charge were calculated. In addition, the schematic energy profiles of different pathways of mercury-bromine species adsorption on surface were given in order to elucidate the mechanism of adsorption process. The results indicate that bromine has a positive effect on Hg-0 adsorption on carbonaceous surface because of the transfer of charge. The adsorption of HgBr on carbonaceous surface is highly thermally favorable and belongs to chemisorption. The preferred adsorption mode of HgBr on surface is Hg-down mode. HgBr can exist stably on the surface since the desorption of HgBr are highly endothermic, yet there is still a possibility that HgBr dissociates and Hg desorbs from the surface. For HgBr2 adsorption, it is not very stable on the surface and is likely to adsorb dissociatively. The preferred pathway is that HgBr2 dissociates and adsorbs as HgBr on the surface. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.