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.
Catalytic combustion technology is
one of the effective methods
to remove VOCs such as toluene from industrial emissions. The decomposition
of an aromatic ring via catalyst oxygen vacancies is usually the rate-determining
step of toluene oxidation into CO2. Series of CeO2 probe models were synthesized with different ratios of surface-to-bulk
oxygen vacancies. Besides the devotion of the surface vacancies, a
part of the bulk vacancies promotes the redox property of CeO2 in toluene catalytic combustion: surface vacancies tend to
adsorb and activate gaseous O2 to form adsorbed oxygen
species, whereas bulk vacancies improve the mobility and activity
of lattice oxygen species via their transmission effect. Adsorbed
oxygen mainly participates in the chemical adsorption and partial
oxidation of toluene (mostly to phenolate). With the elevated temperatures,
lattice oxygen of the catalysts facilitates the decomposition of aromatic
rings and further improves the oxidation of toluene to CO2.
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