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The Adsorption of Mercury-Species on Fixed and Rumpled Calcium Oxide (0 0 1) Surfaces Investigated by Density Functional Theory

Bo Gyeong Kim and Paul Blowers. Chemical and Environmental Engineering, The University of Arizona, PO Box 210011, Tucson, AZ 85721-0011

PWDS (Paper Waste Derived Sorbent), which contains CaO 23%, Al2O3昐iO2 29%, CaCO3 41 %, inert 6% and Ca(OH)2 1%, is a novel sorbent to remove mercury from coal combustion flue gases. This effective and inexpensive sorbent was newly-developed in order to reduce mercury treatment costs. However, the mechanism of mercury control in this sorbent has not been studied. The CaO surface was studied first due to the characteristic rock-salt structure modeled more easily using computational chemistry than the other constituents.

The adsorption of Hg, HgCl, and HgCl2 to fixed and rumpled CaO (0 0 1) surfaces were investigated with Density Functional Theory (DFT) methods. The CaO surface was modeled with a 4󫶖 cluster, 5󬊂 cluster and a periodic slab designed with a 5󬊂 unit cell. Structures were optimized with the Perdew-Wang (PWC) functional with the local density approximation (LDA) using the Harris approximation in order to reduce the computational time. Adsorption energies of Hg, HgCl, and HgCl2 binding to the CaO surface were calculated with the gradient corrected (GGA) method with the BLYP functional using the lower level optimized geometries. The spin polarization effect was considered for the mercury-species on the CaO surface when it was necessary for better quality results. Predicted adsorption energies for Hg between the rumpled surface and the fixed surface are very similar. However, the relaxation and rumpling effects lead to shorter bond lengths of Hg O bond and stronger adsorption energies for HgCl and HgCl2 species compared with a fixed surface.