Elemental mercury oxidation over manganese oxide octahedral molecular sieve catalyst at low flue gas temperature

Abstract

Manganese oxide octahedral molecular sieve (OMS-2) with cryptomelane structure synthesized by a solvent-free method was employed to oxidize gaseous elemental mercury (Hg0) in coal combustion flue gas for the first time. Brunauer-Emmett-Teller (BET) surface area analysis, X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectra, H2 temperature-programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS) measurement were employed to characterize the catalyst. The OMS-2 with large surface area and abundant pores presented excellent Hg0 oxidation performance at a wide temperature range of 100–250 °C. At the optimal operating temperature of 150 °C, 93% Hg0 oxidation efficiency was obtained under a gas hourly space velocity as high as 1,350,000 h−1. Hg0 oxidation on OMS-2 catalyst was supposed to follow the Mars-Maessen mechanism, where the gaseous Hg0 was firstly adsorbed on the OMS-2 catalyst surface to form adsorbed Hg0, and then the adsorbed Hg0 was oxidized by the lattice oxygen over catalyst surface to form mercury oxide (HgO). Both HCl and NO in the flue gas promoted Hg0 oxidation mainly due to the formation of active species like Cl∗, NO2, which reacted with Hg0 to form volatile mercury species. SO2 inhibited the Hg0 oxidation by reducing the adsorbed HgO into gaseous Hg0 or generating sulfate on the catalyst surface. Water vapor also played an inhibitive role in Hg0 oxidation. However, the SO2 and H2O resistance of OMS-2 catalyst was much superior compared to other commercial catalysts, which makes it promising for industrial application. This knowledge is beneficial for developing economical and efficient Hg0 oxidation catalysts for coal-fired power plants.