Mercury chemistry on brominated activated carbon Erdem Sasmaz a,⇑ , Abby Kirchofer a , Adam D. Jew b , Arindom Saha c , David Abram c , Thomas F. Jaramillo c , Jennifer Wilcox a a Department of Energy Resources Engineering, Stanford University, Stanford, CA 94305, United States b Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, United States c Department of Chemical Engineering, Stanford University, Stanford, CA 94305, United States highlights " Hg is adsorbed on brominated AC as Hg 2+ at both 30 °C and 140 °C. " Chemisorption is the likely adsorption mechanism of Hg. " Hg interacts with two Br atoms at a distance of 2.55 ± 0.01 Å inside the C matrix. " Hg s- and Hg p-states hybridize with Br and C p-states to form stable complexes. article info Article history: Received 20 October 2011 Received in revised form 26 March 2012 Accepted 20 April 2012 Available online 7 May 2012 Keywords: Mercury Activated carbon Bromine XPS EXAFS abstract Activated carbon-based sorbents are the most widely tested sorbents for mercury removal in coal-fired power plants. A major problem in mercury removal is the limited understanding of the mechanism asso- ciated with elemental mercury (Hg 0 ) oxidation and its subsequent adsorption. This work investigates the possible binding mechanism of Hg 0 onto brominated fiber and powder activated carbon sorbents through packed-bed experiments in a stream of air. To better understand the mechanisms involved, a combina- tion of spectroscopy and quantum mechanical modeling were used to characterize the sorption process. X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectros- copy were used to analyze the surface and bulk chemical compositions of brominated activated carbon sorbents reacted with Hg 0 . It was found that Hg 0 is oxidized at the brominated carbon surfaces at both 30 °C and 140 °C. The oxidation state of adsorbed Hg is found to be Hg 2+ , and coordinated to two Br atoms with no detectable bonding between Hg and O. Though plane-wave density functional theory (DFT) and density of states (DOSs) calculations indicate that Hg is more stable when it is bound to the edge C atom interacting with a single Br bound atop of Hg, a model that includes an interaction between the Hg and an additional Br atom matches best with experimental data obtained from EXAFS spectroscopy. Because the most stable structures optimized in the DFT simulations were not found on the samples analyzed using EXAFS spectroscopy, Hg surface reactions on the carbon surface are thought to be kinetically controlled. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The US is putting significant effort into decreasing harmful emissions into the environment through regulations such as the EPA’s Clean Air Interstate Rule (CAIR) and the Clean Air Mercury Rule (CAMR), which place caps on NO x , SO x , and Hg emissions [1]. Recently, on March 2011 the EPA stated that all hazardous air pollutants must have emission standards and proposed that for existing sources in the category, that the standards are at least as stringent as the emission reductions achieved by the average of the top 12% best controlled sources for source categories with 30 or more sources. With this new rule, a reduction of mercury from coal emissions of approximately 90% is anticipated [1]. Recent studies indicate that mercury content in coal varies between 0.01 and 1.5 g per ton of coal, with world coal consumption in 2006 esti- mated at 6118 million tons per year [2]. In 1999, the EPA estimated that US coal combustion emits approximately 50 tons Hg/year into the air [1], while global emissions are approximately 810 tons Hg/ year [2]. The need for effective sorbent materials to capture harmful pollutants of flue gases continues to increase as coal consumption increases worldwide. Typically, activated carbon injection is used to capture oxidized mercury [3]. A major limitation with using activated carbon is that in flue gases with low halogen concentrations a large amount of activated carbon needs to be added to the system to effectively 0016-2361/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fuel.2012.04.036 ⇑ Corresponding author. E-mail address: sasmaz@cec.sc.edu (E. Sasmaz). Fuel 99 (2012) 188–196 Contents lists available at SciVerse ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel