Cobalt doped ceria for abundant storage of surface active oxygen and efficient elemental mercury oxidation in coal combustion flue gas

Abstract

Cobalt doped CeO2 (Co-CeO2) prepared by a single-step hydrothermal method was used for Hg0 catalytic oxidation. The catalysts were characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance, X-ray photoelectron spectroscopy, H2-temperature programmed reduction test, and thermogravimetric measurement. The results show that the exposed planes for the flake-shaped Co-CeO2 mainly were (110) and (100) with low oxygen vacancy formation energies. Abundant oxygen vacancy defects were identified on the Co-CeO2, which further induced plentiful chemisorbed oxygen on the surface. The oxygen storage capacity for the Co-CeO2 was up to 1.43 μmol O2 m−2 at 200 °C, and a large portion of this capacity was for chemisorbed oxygen. Without the introduction of gas-phase O2, Hg0 oxidation probably occurred through a Mars-Maessen mechanism, where active chemisorbed oxygen originated from oxygen vacancy defects reacts with adsorbed Hg0 to form HgO. Active chlorine species was generated from the interactions of chemisorbed oxygen with trace amount gaseous HCl. The active chlorine compensated the slight inhibitive effects of SO2 and H2O, leading to above 90% Hg0 oxidation under simulated low-rank coal burning flue gas atmosphere at a gas hourly space velocity of 160,000 h−1. The abundant active chemisorbed oxygen played important role in and guaranteed an efficient Hg0 oxidation in extremely adverse application environment. These results reveal the role of doping metals on structure-catalytic property relations for CeO2, which opened a new strategy for controlling mercury emission from coal-fired flue gas using CeO2 based catalysts by tuning the morphology and exposed planes.