The heterogeneous mercury reaction mechanism, reactions among elemental mercury (Hg-0) and simulated flue gas across laboratory-scale selective catalytic reduction (SCR) reactor system was studied. The surface of SCR catalysts used in this study was analyzed to verify the proposed reaction pathways using transmission electron microscopy with energy dispersive X-ray analyses (TEM-EDX) and X-ray photoelectron spectroscopy (XPS). The Langmuir-Hinshelwood mechanism was proven to be most suitable explaining first-layer reaction of Hg-0 and HCl on the SCR catalyst. Once the first layer is formed, successive layers of oxidized mercury (HgCl2) are formed, making a multi-layer structure.