H2S-Modified Natural Ilmenite: A Recyclable Magnetic Sorbent for Recovering Gaseous Elemental Mercury from Flue Gas

H2S-modified natural ilmenite (i.e., ilmenite-C-S) was developed as a recyclable magnetic sorbent to recover Hg-0 in the flue gas as a cobenefit of the wet electrostatic precipitators (WESPs). Ilmenite-C-S showed an excellent Hg-0 capture performance at 40-100 C, and the chemical adsorption of Hg-0 on ilmenite-C-S was hardly inhibited by H2O and SO2. The chemical adsorption of Hg-0 on ilmenite-C-S mainly followed the Mars-Maessen mechanism (i.e., gaseous Hg-0 was first adsorbed, and it was then oxidized to HgS by S-2(2-) on ilmenite-C-S). As the formed Hg species was HgS, the leaching of Hg species during the wet dust collection of the WESP was negligible. Ilmenite-C-S after the five cycles of Hg-0 capture, Hg-0 recovery, and sorbent regeneration still had an excellent superparamagnetism with a saturation magnetization of 14.1 emu g(-1) and a superior performance for Hg-0 capture with Hg-0 removal efficiency of approximately 100%. Meanwhile, the ultralow concentration of Hg-0 in the flue gas was recovered as a high concentration of gaseous Hg-0 (>10 mg m(-3)), which can be condensed to liquid Hg-0. Therefore, Hg-0 recovery by ilmenite-C-S as a cobenefit of the WESP was a cost-effective technology for the centralized control of Hg-0 emission from coal-fired plants

Reverse oxygen spillover triggered by CO adsorption on Sn-doped Pt/TiO2 for low-temperature CO oxidation

Abstract The spillover of oxygen species is fundamentally important in redox reactions, but the spillover mechanism has been less understood compared to that of hydrogen spillover. Herein Sn is doped into TiO 2 to activate low-temperature (<100 °C) reverse oxygen spillover in Pt/TiO 2 catalyst, leading to CO oxidation activity much higher than that of most oxide-supported Pt catalysts. A combination of near-ambient-pressure X-ray photoelectron spectroscopy, in situ Raman/Infrared spectroscopies, and ab initio molecular dynamics simulations reveal that the reverse oxygen spillover is triggered by CO adsorption at Pt 2+ sites, followed by bond cleavage of Ti-O-Sn moieties nearby and the appearance of Pt 4+ species. The O in the catalytically indispensable Pt-O species is energetically more favourable to be originated from Ti-O-Sn. This work clearly depicts the interfacial chemistry of reverse oxygen spillover that is triggered by CO adsorption, and the understanding is helpful for the design of platinum/titania catalysts suitable for reactions of various reactants

Global Kinetic Study of NO Reduction by NH₃ over V₂O₅–WO₃/TiO₂: Relationship between the SCR Performance and the Key Factors

The nonselective catalytic reduction (NSCR) reaction and the catalytic oxidation of NH₃ to NO (C–O reaction) simultaneously happened during the selective catalytic reduction (SCR) of NO with NH₃ over V₂O₅–WO₃/TiO₂, especially at higher temperatures. There was an excellent linear relationship between the SCR reaction rate and gaseous NO concentration, and the intercept and slope can be used to describe the rate constant of NO reduction through the Langmuir–Hinshelwood mechanism and that through the Eley–Rideal mechanism, respectively. However, the NSCR reaction rate was nearly independent of gaseous NO concentration, and the reaction order of the C–O reaction with respect to gaseous NO concentration was much less than zero. According to the kinetic study, the relationship of the SCR performance (i.e., SCR activity and N₂ selectivity) with the key factors (for example V₂O₅ content, H₂O effect, and reactant concentration) was built, which can be used to predict the SCR performance

H₂S‑Modified Fe–Ti Spinel: A Recyclable Magnetic Sorbent for Recovering Gaseous Elemental Mercury from Flue Gas as a Co-Benefit of Wet Electrostatic Precipitators

The nonrecyclability of the sorbents used to capture Hg⁰ 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 H₂S-modified Fe–Ti spinel was developed as a recyclable magnetic sorbent to recover Hg⁰ from flue gas as a co-benefit of the WESP. Although the Fe–Ti spinel exhibited poor Hg⁰ capture activity in the temperature range of flue gas downstream of flue gas desulfurization, the H₂S-modified Fe–Ti spinel exhibited excellent Hg⁰ capture performance with an average adsorption rate of 1.92 μg g^–1 min^–1 at 60 °C and a capacity of 0.69 mg g^–1 (5% of the breakthrough threshold) due to the presence of S₂^2– on its surface. The five cycles of Hg⁰ capture, Hg⁰ recovery, and sorbent regeneration demonstrated that the ability of the modified Fe–Ti spinel to capture Hg⁰ did not degrade remarkably. Meanwhile, the ultralow concentration of Hg⁰ in flue gas was increased to a high concentration of Hg⁰, which facilitated the centralized control of Hg pollution

H₃PW₁₂O₄₀ Grafted on CeO₂: A High-Performance Catalyst for the Selective Catalytic Reduction of NO_<i>x</i> with NH₃

CeO₂ showed a poor selective catalytic reduction (SCR) activity due to its poor ability to adsorb NH₃. To improve the SCR performance of CeO₂, tungsto­phosphoric acid (i.e., HPW, H₃PW₁₂O₄₀) with a high acidic strength was grafted on CeO₂ by the adsorption of HPW on CeO₂ in a HPW solution. The grafting of HPW on the surface of CeO₂ was demonstrated with characterizations by XPS, XRF, TG-DSC, and in situ DRIFT spectra. As HPW on HPW/CeO₂-500 still retained the Keggin structure, HPW/CeO₂-500 exhibited an excellent ability for NH₃ adsorption. Both HPW and CeO₂ on/in HPW/CeO₂-500 played their functions to the greatest limit for the SCR reaction. CeO₂ in HPW/CeO₂-500 played the role in the activation of adsorbed NH₃ and NO, and the grafted HPW on HPW/CeO₂-500 acted as the active sites for NH₃ adsorption. Therefore, HPW/CeO₂-500 showed a superior SCR performance at 200–450 °C and an excellent H₂O/SO₂ resistance above 300 °C

N₂ Selectivity of NO Reduction by NH₃ over MnO_<i>x</i>–CeO₂: Mechanism and Key Factors

In this work, the novel relationships of N₂ selectivity of NO reduction over MnO_<i>x</i>–CeO₂ with the gas hourly space velocity (i.e., GHSV) and the reactants’ concentrations were discovered. Meanwhile, the mechanism of N₂O formation during the low temperature selective catalytic reduction reaction (SCR) over MnO_<i>x</i>–CeO₂ was studied using in situ DRIFTS study and the transient reaction study. N₂O formation over MnO_<i>x</i>–CeO₂ mainly resulted from the Eley–Rideal mechanism (i.e., the reaction between overactivated NH₃ and gaseous NO), and the Langmuir–Hinshelwood mechanism (i.e., the reaction between adsorbed NH₃ species and adsorbed NO_<i>x</i>) did not contribute to N₂O formation. There was an excellent linear relationship of NO reduction and N₂ formation with gaseous NO concentration. Meanwhile, the reaction order of N₂O formation with respect to gaseous NO concentration was nearly 1. However, the reaction orders of NO reduction, N₂O formation, and N₂ formation over MnO_<i>x</i>–CeO₂ with respect to gaseous NH₃ concentration were all higher than 0 due to the adsorption competition between NH₃ and NO+O₂. Therefore, N₂ selectivity of NO reduction over MnO_<i>x</i>–CeO₂ remarkably increased with the increase of gaseous NO concentration, and it slightly decreased with the increase of gaseous NH₃ concentration

Elemental Mercury Oxidation over Fe–Ti–Mn Spinel: Performance, Mechanism, and Reaction Kinetics

The design of a high-performance catalyst for Hg⁰ oxidation and predicting the extent of Hg⁰ oxidation are both extremely limited due to the uncertainties of the reaction mechanism and the reaction kinetics. In this work, Fe–Ti–Mn spinel was developed as a high-performance catalyst for Hg⁰ oxidation, and the reaction mechanism and the reaction kinetics of Hg⁰ oxidation over Fe–Ti–Mn spinel were studied. The reaction orders of Hg⁰ oxidation over Fe–Ti–Mn spinel with respect to gaseous Hg⁰ concentration and gaseous HCl concentration were approximately 1 and 0, respectively. Therefore, Hg⁰ oxidation over Fe–Ti–Mn spinel mainly followed the Eley–Rideal mechanism (i.e., the reaction of gaseous Hg⁰ with adsorbed HCl), and the rate of Hg⁰ oxidation mainly depended on Cl^• concentration on the surface. As H₂O, SO₂, and NO not only inhibited Cl^• formation on the surface but also interfered with the interface reaction between gaseous Hg⁰ and Cl^• on the surface, Hg⁰ oxidation over Fe–Ti–Mn spinel was obviously inhibited in the presence of H₂O, SO₂, and NO. Furthermore, the extent of Hg⁰ oxidation over Fe–Ti–Mn spinel can be predicted according to the kinetic parameter <i>k</i>_E‑R, and the predicted result was consistent with the experimental result

Why the Low-Temperature Selective Catalytic Reduction Performance of Cr/TiO₂ Is Much Less than That of Mn/TiO₂: A Mechanism Study

Although Mn/TiO₂ and Cr/TiO₂ had many similar physicochemical properties, the low-temperature selective catalytic reduction (SCR) performance of Mn/TiO₂ was much better than that of Cr/TiO₂. In this work, the physicochemical properties of Mn/TiO₂ and Cr/TiO₂ were characterized. Meanwhile, the mechanism of NO reduction over Mn/TiO₂ and Cr/TiO₂ was compared using the transient reaction. Furthermore, the kinetic parameters of NO reduction over Mn/TiO₂ and Cr/TiO₂ were obtained from the steady-state kinetic study. The Eley–Rideal mechanism contributed to NO reduction over both Mn/TiO₂ and Cr/TiO₂. As the SCR reaction through the Eley–Rideal mechanism needed to overcome the binding of activated NH₃ species with the interface and the binding of activated NH₃ with Cr/TiO₂ was much stronger than that with Mn/TiO₂, the rate constant of the SCR reaction over Cr/TiO₂ through the Eley–Rideal mechanism was much lower than that over Mn/TiO₂. Meanwhile, the rate of the catalytic oxidation of NH₃ to NO (i.e., C–O reaction) over Cr/TiO₂ was much higher than that of Mn/TiO₂ due to the stronger oxidation and more Cr⁶⁺ on the surface. As a result, the low-temperature SCR activity of Cr/TiO₂ was much less than that of Mn/TiO₂