CeO2-TiO2 Catalysts for Catalytic Oxidation of Elemental Mercury in Low-Rank Coal Combustion Flue Gas

CeO2-TiO2 (CeTi) catalysts synthesized by an ultrasound-assisted impregnation method were employed to oxidize elemental mercury (Hg-0) in simulated low-rank (sub-bituminous and lignite) coal combustion flue gas. The CeTi catalysts with a CeO2/TiO2 weight ratio of 1-2 exhibited high Hg-0 oxidation activity from 150 to 250 degrees C. The high concentrations of surface cerium and oxygen were responsible for their superior performance. Hg-0 oxidation over CeTi catalysts was proposed to follow the Langmuir-Hinshelwood mechanism whereby reactive species from adsorbed flue gas components react with adjacently adsorbed Hg-0. In the presence of O-2, a promotional effect of HCl, NO, and SO2 on Hg-0 oxidation was observed. Without O-2, HCl and NO still (promoted Hg-0 oxidation due to the surface oxygen, while SO2 inhibited Hg-0 adsorption and subsequent oxidation. Water vapor also inhibited Hg-0 oxidation. HCl was the most effective flue gas component responsible for Hg-0 oxidation. However, the combination of SO2 and NO without HCl also resulted in high Hg-0 oxidation efficiency. This superior oxidation capability is advantageous to Hg oxidation in low-rank coal combustion flue gas with low HCl concentration.