Superior activity of MnOx-CeO2/TiO2 catalyst for catalytic oxidation of elemental mercury at low flue gas temperatures

TiO2 supported Mn-Ce mixed oxides (Mn-Ce/Ti) synthesized by an ultrasound-assisted impregnation method were employed to oxidize elemental mercury (Hg0) at low temperatures in simulated low-rank (sub-bituminous and lignite) coal combustion flue gas and corresponding selective catalytic reduction (SCR) flue gas. The catalysts were characterized by BET surface area analysis, X-ray diffraction (XRD) measurement and X-ray photoelectron spectroscopy (XPS) analysis. The combination of MnOx and CeO2 resulted in significant synergy for Hg0 oxidation. The Mn-Ce/Ti catalyst was highly active for Hg0 oxidation at low temperatures (150–250 °C) under both simulated flue gas and SCR flue gas. The dominance of Mn4+ and the presence of Ce3+ on the Mn-Ce/Ti catalyst were responsible for its excellent catalytic performance. Hg0 oxidation on the Mn-Ce/Ti catalyst likely followed the Langmuir–Hinshelwood mechanism, where reactive species on catalyst surface react with adjacently adsorbed Hg0 to form Hg2+. NH3 consumed the surface oxygen and limited the adsorption of Hg0, hence inhibiting Hg0 oxidation over Mn-Ce/Ti catalyst. However, once NH3 was cut off, the inhibited mercury oxidation activity could be completely recovered in the presence of O2. This study revealed the possibility of simultaneously oxidizing Hg0 and reducing NOx at low flue gas temperatures. Such knowledge is of fundamental importance in developing effective and economical mercury and NOx control technologies for coal-fired power plants.