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

PWDS (Paper Waste Derived Sorbent), which contains CaO 23%, Al₂O₃•SiO₂ 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 4×4×2 cluster, 5×5×2 cluster and a periodic slab designed with a 5×5×2 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.