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.

In this work, the novel relationships of N 2 selectivity of NO reduction over MnO x −CeO 2 with the gas hourly space velocity (i.e., GHSV) and the reactants' concentrations were discovered. Meanwhile, the mechanism of N 2 O formation during the low temperature selective catalytic reduction reaction (SCR) over MnO x −CeO 2 was studied using in situ DRIFTS study and the transient reaction study. N 2 O formation over MnO x −CeO 2 mainly resulted from the Eley−Rideal mechanism (i.e., the reaction between overactivated NH 3 and gaseous NO), and the Langmuir−Hinshelwood mechanism (i.e., the reaction between adsorbed NH 3 species and adsorbed NO x ) did not contribute to N 2 O formation. There was an excellent linear relationship of NO reduction and N 2 formation with gaseous NO concentration. Meanwhile, the reaction order of N 2 O formation with respect to gaseous NO concentration was nearly 1. However, the reaction orders of NO reduction, N 2 O formation, and N 2 formation over MnO x −CeO 2 with respect to gaseous NH 3 concentration were all higher than 0 due to the adsorption competition between NH 3 and NO+O 2 . Therefore, N 2 selectivity of NO reduction over MnO x −CeO 2 remarkably increased with the increase of gaseous NO concentration, and it slightly decreased with the increase of gaseous NH 3 concentration.

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The nonrecyclability of the sorbents used to capture Hg from flue gas causes a high operation cost and the potential risk of exposure to Hg. The installation of wet electrostatic precipitators (WESPs) in coal-fired plants makes possible the recovery of spent sorbents for recycling and the centralized control of Hg pollution. In this work, a HS-modified Fe-Ti spinel was developed as a recyclable magnetic sorbent to recover Hg from flue gas as a co-benefit of the WESP. Although the Fe-Ti spinel exhibited poor Hg capture activity in the temperature range of flue gas downstream of flue gas desulfurization, the HS-modified Fe-Ti spinel exhibited excellent Hg capture performance with an average adsorption rate of 1.92 μg g min at 60 °C and a capacity of 0.69 mg g (5% of the breakthrough threshold) due to the presence of S on its surface. The five cycles of Hg capture, Hg recovery, and sorbent regeneration demonstrated that the ability of the modified Fe-Ti spinel to capture Hg did not degrade remarkably. Meanwhile, the ultralow concentration of Hg in flue gas was increased to a high concentration of Hg, which facilitated the centralized control of Hg pollution.

In this work, a novel phenomenon was discovered that N 2 O selectivity of NO reduction over MnO x /TiO 2 was related to the concentration of gaseous NO and that lower concentration of gaseous NO would cause higher N 2 O selectivity. In situ DRIFTS and transient reaction studies demonstrated that both the Eley-Rideal mechanism (the reaction of over-activated NH 3 with gaseous NO) and the Langmuir-Hinshelwood mechanism (the reaction of adsorbed NO 3 − with adsorbed NH 3 on the adjacent sites) could contribute to the formation of N 2 O. Kinetic study demonstrated that N 2 O selectivity would be independent of gaseous NO concentration if NO reduction over MnO x /TiO 2 mainly followed the Langmuir-Hinshelwood mechanism.If NO reduction over MnO x /TiO 2 mainly followed the Eley-Rideal mechanism, there was competition between the selective catalytic reduction (SCR) reaction and non selective catalytic reduction (NSCR) reaction. As gaseous NO concentration increased, more -NH 2 was used to reduce gaseous NO to form N 2 and the further oxidization of -NH 2 to -NH was restrained, resulting in an obvious decrease of N 2 O selectivity. The Eley-Rideal mechanism played an important role in NO reduction over MnO x /TiO 2 , especially at higher temperatures. Therefore, N 2 O selectivity of the low temperature SCR reaction over MnO x /TiO 2 decreased especially at higher temperatures after the increase of gaseous NO concentration.

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H2O effect on NO reduction over Mn–Fe spinel was related to the competition adsorption and the decrease in oxidation ability.