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The mechanism of N2O formation during the low-temperature selective catalytic reduction reaction (SCR) over Mn-Fe spinel was studied. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and transient reaction studies demonstrated that the Eley-Rideal mechanism (i.e., the reaction of adsorbed NH3 species with gaseous NO) and the Langmuir-Hinshelwood mechanism (i.e., the reaction of adsorbed NH3 species with adsorbed NOx species) both contributed to N2O formation. However, N2O selectivity of NO reduction over Mn-Fe spinel through the Langmuir-Hinshelwood mechanism was much less than that through the Eley-Rideal mechanism. The ratio of NO reduction over Mn-Fe spinel through the Langmuir-Hinshelwood mechanism remarkably increased; therefore, N2O selectivity of NO reduction over Mn-Fe spinel decreased with the decrease of the gas hourly space velocity (GHSV). As the gaseous NH3 concentration increased, N2O selectivity of NO reduction over Mn-Fe spinel increased because of the promotion of NO reduction through the Eley-Rideal mechanism. Meanwhile, N2O selectivity of NO reduction over Mn-Fe spinel decreased with the increase of the gaseous NO concentration because the formation of NH on Mn-Fe spinel was restrained. Therefore, N2O selectivity of NO reduction over Mn-Fe spinel was related to the GHSV and concentrations of reactants.

Magnetic pyrrhotite, derived from the thermal treatment of natural pyrite, was developed as a recyclable sorbent to recover elemental mercury (Hg) from the flue gas as a cobenefit of wet electrostatic precipitators (WESP). The performance of naturally derived pyrrhotite for Hg capture from the flue gas was much better than those of other reported magnetic sorbents, for example Mn-Fe spinel and Mn-Fe-Ti spinel. The rate of pyrrhotite for gaseous Hg capture at 60 °C was 0.28 μg g min and its capacity was 0.22 mg g with the breakthrough threshold of 4%. After the magnetic separation from the mixture collected by the WESP, the spent pyrrhotite can be thermally regenerated for recycle. The experiment of 5 cycles of Hg capture and regeneration demonstrated that both the adsorption efficiency and the magnetization were not notably degraded. Meanwhile, the ultralow concentration of gaseous Hg in the flue gas was concentrated to high concentrations of gaseous Hg and Hg during the regeneration process, which facilitated the centralized control of mercury pollution. Therefore, the control of Hg emission from coal-fired plants by the recyclable pyrrhotite was cost-effective and did not have secondary pollution.

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A series of nanosized (Fe3-xMnx)1-δO4 (x = 0, 0.2, 0.5, and 0.8) were synthesized for elemental mercury capture from the flue gas. Cation vacancies on (Fe3-xMnx)1-δO4 can provide the active sites for elemental mercury adsorption, and Mn(4+) cations on (Fe3-xMnx)1-δO4 may be the oxidizing agents for elemental mercury oxidization. With the increase of Mn content in the spinel structure, the percents of Mn(4+) cations and cation vacancies on the surface increased. As a result, elemental mercury capture by (Fe3-xMnx)1-δO4 was obviously promoted with the increase of Mn content. (Fe2.2Mn0.8)1-δO4 showed an excellent capacity for elemental mercury capture (>1.5 mg g(-1) at 100-300 °C) in the presence of SO2 and HCl. Furthermore, (Fe2.2Mn0.8)1-δO4 with the saturation magnetization of 45.6 emu g(-1) can be separated from the fly ash using magnetic separation, leaving the fly ash essentially free of sorbent and adsorbed Hg. Therefore, nanosized (Fe2.2Mn0.8)1-δO4 may be a promising sorbent for the control of elemental mercury emission.

Nonstoichiometric Fe-Ti spinel

Hg0 capture by sorbents was a promising technology to control Hg0 emission from coal-fired power plants and smelters. However, the design of a high performance sorbent and the predicting of the extent of Hg0 adsorption were both extremely limited due to the lack of adsorption kinetics and structure–activity relationship. In this work, the adsorption kinetics of gaseous Hg0 onto MoS3/TiO2 was investigated and kinetic parameters were obtained by fitting breakthrough curves. According to the kinetic parameters, the removal efficiency, the adsorption rate and the capacity for Hg0 capture were accurately predicted. Meanwhile, the structure–activity relationship of metal sulfides for gaseous Hg0 adsorption was built. The chemical adsorption rate of gaseous Hg0 was found to mainly depend on the amount of surface adsorption sites available for the physical adsorption of Hg0, the amount of surface S2 2– available for Hg0 oxidation and gaseous Hg0 concentration. As MoS3/TiO2 showed a superior performance for capturing high concentrations of Hg0 due to the large number of surface adsorption sites for the physical adsorption of gaseous Hg0, it has promising applications in recovering Hg0 from smelting flue gas.