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The formation of defects over a crystal surface could significantly improve the elemental mercury (Hg0) removal performance of pyrite (FeS2). Herein, the role of surface defects in Hg0immobilization over FeS2(100) was investigated by adopting quantum chemistry. The contribution of surface defects to mercury immobilization over pyrite was mainly manifested in facilitating Hg0adsorption. Compared with a perfect pyrite surface, a defective pyrite surface exhibited a higher affinity to Hg0, whose adsorption energy was up to −53.03 kJ/mol. In addition, surface defects could induce the dissociation of HCl, a common component in coal combustion flue gas, which provided active chlorine (Cl*) for subsequent Hg0oxidation. The Langmuir–Hinshelwood mechanism is responsible for Hg0oxidation over the defective FeS2(100) surface. First, Hg0and HCl were simultaneously adsorbed over the defective surface, and the presence of defects could promote Hg0adsorption and HCl dissociation. Subsequently, adsorbed Hg0would combine with the adjacent active Cl* site, whose energy barrier was 92.60 kJ/mol. Eventually, HgCl could spontaneously react with another Cl* site to form a HgCl2molecule. Thus, the formation of a HgCl intermediate was the rate-determining step for Hg0oxidation over a defective pyrite surface. This work reveals the role of defects in Hg0immobilization over a pyrite surface, which provides a scientific basis for the design and modification of natural mineral sulfide sorbents.

Mercury chloride (HgCl2) is considered as the dominant form of mercury in coal combustion flue gas. Abundant studies were conducted to investigate how to stimulate the oxidation of elemental mercury (Hg0) to HgCl2over selective catalytic reduction (SCR) catalysts, which could be easily removed in a wet flue gas desulfurization system. However, the reverse reaction of Hg0oxidation, i.e., the reduction of HgCl2, was rarely studied. In this work, the entire mercury conversion process, including both Hg0oxidation and HgCl2reduction, was investigated over a low-temperature SCR catalyst. Nitric oxide (NO) alone did not inhibit Hg0oxidation to HgCl2. However, NO with the assistance of ammonia (NH3) gas unexpectedly inhibited Hg0conversion, even in the presence of hydrogen chloride (HCl), which is generally recognized to be a great promotor of Hg0oxidation. The coexistence of NO and NH3induced the reduction of HgCl2, which partly counteracted the forward Hg0oxidation reaction, thus leading to a lower observed Hg0oxidation efficiency. HgCl2could not directly react with NO and NH3to generate Hg0at low temperatures, while it could react with reduced species such as hydroxyl on the catalyst surface to produce Hg0. This knowledge is of fundamental importance in accurately predicting Hg0conversion efficiency over SCR catalysts and developing efficient Hg0oxidation technologies.

Magnetic biochar (MBC) has been proven to be an efficient sorbent for elemental mercury (Hg0) removal from coal-fired flue gas in our previous studies. However, SO3is an acidic component of flue gas, and its role in Hg0removal by MBC has not been determined. In this work, the Hg0removal behaviors and the mechanisms involved were investigated in the presence of SO3. The results showed that SO3notably impeded the Hg0adsorption process, whereas the oxidation efficiency of Hg0increased as the SO3concentration increased. The inhibitory effect of SO3on Hg0adsorption was attributed to competitive adsorption between SO3and Hg0as well as the formation of metal sulfates on the MBC surface. Moreover, the addition of SO3into the atmosphere resulted in the generation of HgSO4, especially when H2O was also present. Additionally, the deposition of HgSO4and H2SO4further hindered the removal of Hg0by MBC. Nevertheless, SO3promoted the formation of C═O groups, resulting in the higher Hg0oxidation efficiency observed. In addition, more mercury that was adsorbed on the MBC was transformed into HgO in the presence of SO3. This work illustrated the effect of SO3on Hg0removal by MBC and explained the mechanism involved, which will help with the further development of an efficient sorbent for Hg0removal.

The LaMnO3, CeMnO3, and PrMnO3perovskite oxides were synthesized by the sol–gel method and employed as the catalyst of selective catalytic reduction (SCR) for NO removal and synergistic mercury removal from coal combustion flue gas. The experimental results indicated that CeMnO3exhibited the best NO and Hg0removal activity among the three catalysts. NO conversion over CeMnO3could reach a maximum value of 89.2% in the atmosphere of 4% O2+ 400 ppm NO + 400 ppm NH3, and the optimal reaction temperature for the NO removal was 200–250 °C, demonstrating good low-temperature catalytic activity. Hg0removal efficiency of CeMnO3decreased with the rise of the reaction temperature. The individual flue gas components of O2, HCl, NO, and CO2had promotions on the Hg0removal over CeMnO3, while SO2, NH3, and H2O displayed inhibitory actions on the efficiency. The performance of CeMnO3on simultaneous NO and Hg0removal was the best at 200–250 °C in simulated coal-fired flue gas. Specifically, the NO conversion and Hg0removal efficiency were, respectively, 73.7% and 80.9% at 200 °C, and both the efficiencies remained excellent stability during a long experimental period. A series of characterization analyses were also carried out to identify the physicochemical properties of the catalysts. The results demonstrated that the superior catalytic performance of CeMnO3mainly derived from the abundance of chemisorbed oxygen and the great activity of Mn3+besides Mn4+in the catalyst.

CuCl2-modified carbonaceous materials have been regarded as a kind of mercury sorbent, but the Hg0reaction mechanism over CuCl2-impregnated sorbent surface is still unclear. In this work, the binding mechanism of Hg0on CuCl2-impregnated carbonaceous material surface was investigated using hybrid density functional theory (DFT). The results indicate that the dissociation mechanism is responsible for CuCl2adsorption over carbonaceous material sorbent surface. The active chlorine species generated from CuCl2adsorption can significantly enhance mercury adsorption over carbonaceous material sorbent surface. Hg0adsorption over CuCl2-impregnated carbonaceous material surface is dominated by a chemisorption mechanism. Surface Cl and C atoms are identified as the active sites for Hg0adsorption on CuCl2-impregnated carbonaceous material surface. CuCl2plays an important role in mercury adsorption on CuCl2-impregnated carbonaceous materials. CuCl2includes the following roles: (1) CuCl2can increase the reactivity of its neighbor adsorption sites on carbonaceous material surface; (2) CuCl2can provide additional active sites for Hg0adsorption; (3) CuCl2can provide Cl atoms for the oxidation of Hg0into HgCl. Heterogeneous mercury oxidation over CuCl2-impregnated carbonaceous material surface includes four steps: Hg0adsorption, Cl migration, HgCl2formation, and HgCl2desorption. HgCl2formation is identified as the rate-limiting step of Hg0oxidation on CuCl2-impregnated carbonaceous material surface.