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

Abstract

CeO₂–TiO₂ (CeTi) catalysts synthesized by an ultrasound-assisted impregnation method were employed to oxidize elemental mercury (Hg⁰) 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 Hg⁰ oxidation activity from 150 to 250 °C. The high concentrations of surface cerium and oxygen were responsible for their superior performance. Hg⁰ oxidation over CeTi catalysts was proposed to follow the Langmuir–Hinshelwood mechanism whereby reactive species from adsorbed flue gas components react with adjacently adsorbed Hg⁰. In the presence of O₂, a promotional effect of HCl, NO, and SO₂ on Hg⁰ oxidation was observed. Without O₂, HCl and NO still promoted Hg⁰ oxidation due to the surface oxygen, while SO₂ inhibited Hg⁰ adsorption and subsequent oxidation. Water vapor also inhibited Hg⁰ oxidation. HCl was the most effective flue gas component responsible for Hg⁰ oxidation. However, the combination of SO₂ and NO without HCl also resulted in high Hg⁰ oxidation efficiency. This superior oxidation capability is advantageous to Hg⁰ oxidation in low-rank coal combustion flue gas with low HCl concentration.