Songjian Zhao

Different crystal-forms of one-dimensional MnO2 nanomaterials for the catalytic oxidation and adsorption of elemental mercury

MnO2 has been found to be a promising material to capture elemental mercury (Hg(0)) from waste gases. To investigate the structure effect on Hg(0) uptake, three types of one-dimensional (1D) MnO2 nano-particles, α-, β- and γ-MnO2, were successfully prepared and tested. The structures of α-, β- and γ-MnO2 were characterized by XRD, BET, TEM and SEM. The results indicate that α-, β- and γ-MnO2 were present in the morphologies of belt-, rod- and spindle-like 1D materials, respectively. These findings demonstrated noticeably different activities in capturing Hg(0), depending on the surface area and crystalline structure. The performance enhancement is in the order of: β-MnO2<γ-MnO2<α-MnO2 at 150°C. The mechanism for Hg(0) removal using MnO2 was discussed with the help of results from H2-TPR, XPS and Hg(0) removal experiments in the absence of O2. It was determined that the oxidizability of three forms of MnO2 increased as follows: β-MnO2<γ-MnO2<α-MnO2. The mechanism for Hg(0) capture was ascribed to the Hg(0) catalytic oxidation with the reduction of Mn(4+)→Mn(3+)→Mn(2+). Furthermore, the interaction forces between mercury and manganese oxide sites are demonstrated to increase in the following order: β-MnO2<γ-MnO2<α-MnO2 based on the desorption tests.

Sn–Mn binary metal oxides as non-carbon sorbent for mercury removal in a wide-temperature window

A series of Sn-Mn binary metal oxides were prepared through co-precipitation method. The sorbents were characterized by powder X-ray diffraction (powder XRD), transmission electronic microscopy (TEM), H2-temperature-programmed reduction (H2-TPR) and NH3-temperature-programmed desorption (NH3-TPD) methods. The capability of the prepared sorbents for mercury adsorption from simulated flue gas was investigated by fixed-bed experiments. Results showed that mercury adsorption on pure SnO2 particles was negligible in the test temperature range, comparatively, mercury capacity on MnOx at low temperature was relative high, but the capacity would decrease significantly when the temperature was elevated. Interestingly, for Sn-Mn binary metal oxide, mercury capacity increased not only at low temperature but also at high temperature. Furthermore, the impact of SO2 on mercury adsorption capability of Sn-Mn binary metal oxides was also investigated and it was noted that the effect at low temperature was different comparing with that of high temperature. The mechanism was investigated by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs). Moreover, a mathematic model was built to calculate mercury desorption activation energy from Sn to Mn binary metal oxides.

Gaseous Heterogeneous Catalytic Reactions over Mn-Based Oxides for Environmental Applications: A Critical Review

Manganese oxide has been recognized as one of the most promising gaseous heterogeneous catalysts due to its low cost, environmental friendliness, and high catalytic oxidation performance. Mn-based oxides can be classified into four types: (1) single manganese oxide (MnOx), (2) supported manganese oxide (MnOx/support), (3) composite manganese oxides (MnOx-X), and (4) special crystalline manganese oxides (S-MnOx). These Mn-based oxides have been widely used as catalysts for the elimination of gaseous pollutants. This review aims to describe the environmental applications of these manganese oxides and provide perspectives. It gives detailed descriptions of environmental applications of the selective catalytic reduction of NOx with NH3, the catalytic combustion of volatile organic compounds, Hg0 oxidation and adsorption, and soot oxidation, in addition to some other environmental applications. Furthermore, this review mainly focuses on the effects of structure, morphology, and modified elements and on the role of catalyst supports in gaseous heterogeneous catalytic reactions. Finally, future research directions for developing manganese oxide catalysts are proposed.

Ag-modified AgI–TiO2 as an excellent and durable catalyst for catalytic oxidation of elemental mercury

AgI–TiO2 was employed for the removal of elemental mercury (Hg0) from flue gas, and extra elemental silver (Ag) was introduced to enhance the catalytic activity and stability. AgI–TiO2 displayed an excellent effect on Hg0 catalytic oxidation, and the Hg0 oxidation efficiency was almost 100% with only 5 ppm HCl at 350 °C, which was better than that of KI–Ti. Adding Ag to AgI–TiO2 can notably prolong the period of high efficiency, and the Hg0 oxidation efficiency was still above 90% after 10 h with only 2% Ag added. Doping with silver could suppress the decomposition of AgI and the loss of iodine, which maintains the stability of the catalyst performance. In addition, HCl was readily adsorbed and activated by the silver. The iodine in Ag(2%)–AgI–Ti mainly acted as an accelerant for Hg0 oxidation by facilitating formation of the intermediate Hg–I*; then, chlorine can further convert the intermediate to HgCl2 as the final product. In addition, the thermogravimetric (TG) analysis proved that Ag(2%)–AgI–Ti showed a good stability at high temperature. Furthermore, the ion chromatogram tests also showed the chemical stability of AgI–Ti in the presence of Ag.

The performance of Ag doped V2O5–TiO2 catalyst on the catalytic oxidation of gaseous elemental mercury

To improve the catalytic oxidation ability for gaseous elemental mercury (Hg0), silver was introduced to V2O5–TiO2 catalysts. The catalysts were prepared by an impregnation method with various additives to obtain well distributed silver nanoparticles on the carrier. It was found that doping silver onto V2O5–TiO2 can significantly improve the catalytic oxidation efficiency of Hg0, and the redox temperature range for Hg0 oxidation was enlarged markedly (150–450 °C). The addition of polyvinylpyrrolidone (PVP) during the preparation of the catalysts can improve the dispersion of silver nanoparticles more effectively, which resulted in a higher Hg0 oxidation efficiency up to 90%. However, the oxidation of Hg0 on the catalyst was slightly inhibited due to the larger silver nanoparticles when the ionic liquid (IL) [bmim][BF4] was used as the additive. The characterization results indicated that V can be induced to a higher oxidation state in the presence of silver nanoparticles, and the transformation trend of TiO2 from the anatase to rutile phase caused by Ag can be minimized in the presence of PVP or ILs. Meanwhile, the mechanisms of the elemental mercury oxidation at various temperature ranges were discussed.

Stabilization of mercury over Mn-based oxides: Speciation and reactivity by temperature programmed desorption analysis.

Mercury temperature-programmed desorption (Hg-TPD) method was employed to clarify mercury species over Mn-based oxides. The elemental mercury (Hg0) removal mechanism over MnOx was ascribed to chemical-adsorption. HgO was the primary mercury chemical compound adsorbed on the surface of MnOx. Rare earth element (Ce), main group element (Sn) and transition metal elements (Zr and Fe) were chosen for the modification of MnOx. Hg-TPD results indicated that the binding strength of mercury on these binary oxides followed the order of Sn-MnOx<Ce-MnOx∼MnOx<Fe-MnOx<Zr-MnOx. The activation energies for desorption were calculated and they were 64.34, 101.85, 46.32, 117.14, and 106.92eV corresponding to MnOx, Ce-MnOx, Sn-MnOx, Zr-MnOx and Fe-MnOx, respectively. Sn-MnOx had a weak bond of mercury (Hg-O), while Zr-MnOx had a strong bond (HgO). Ce-MnOx and Fe-MnOx had similar bonds compared with pure MnOx. Moreover, the effects of SO2 and NO were investigated based on Hg-TPD analysis. SO2 had a poison effect on Hg0 removal, and the weak bond of mercury can be easily destroyed by SO2. NO was favorable for Hg0 removal, and the bond strength of mercury was enhanced.

Research of mercury removal from sintering flue gas of iron and steel by the open metal site of Mil-101(Cr)

Metal-organic frameworks (MOFs) adsorbent Mil-101(Cr) was introduced for the removal of elemental mercury from sintering flue gas. Physical and chemical characterization of the adsorbents showed that MIL-101(Cr) had the largest BET surface area, high thermal stability and oxidation capacity. Hg0 removal performance analysis indicated that the Hg0 removal efficiency of MIL-101(Cr) increased with the increasing temperature and oxygen content. Besides, MIL-101(Cr) had the highest Hg0 removal performance compared with Cu-BTC, UiO-66 and activated carbon, which can reach about 88% at 250 °C. The XPS and Hg-TPD methods were used to analyze the Hg0 removal mechanism; the results show that Hg0 was first adsorbed on the surface of Mil-101(Cr), and then oxidized by the open metal site Cr3+. The generated Hg2+ was then combined surface adsorbed oxygen of adsorbent to form HgO, and the open metal site Cr2+ was oxidized to Cr3+ by surface active oxygen again. Furthermore, MIL-101(Cr) had good chemical and thermal stability.

The performance and mechanism for the catalytic oxidation of dibromomethane (CH2Br2) over Co3O4/TiO2 catalysts

Brominated hydrocarbons are a typical pollutant in exhaust gas from the synthesis process of Purified Terephthalic Acid (PTA), and may cause various environmental problems once emitted into the atmosphere. Dibromomethane (DBM) was employed as the model compound in this study, and a series of Co3O4/TiO2 catalysts with various Co contents were prepared for the catalytic oxidation of DBM. The prepared catalysts were characterized by XRD, BET, SEM, TEM, XPS, H2-TPR and NH3-TPD. Among the prepared catalysts, CoTi-5 (5 wt% Co/TiO2) showed the highest catalytic activity, with T90 at about 346 °C, which was mainly attributed to the enrichment of well-dispersed Co3O4 and the high surface Co3+/Co2+ ratio, as it could provide more surface active sites and active oxygen species. The kinetic study showed that the reaction order of DBM was pseudo first-order and the reaction order of oxygen was approximately zero-order. A plausible DBM reaction mechanism over Co3O4/TiO2 catalysts was also proposed based on the results of in situ FTIR and the analysis of gas products by GC-MS. The reaction process started with the adsorption on surface oxygen vacancies, breakage of C–Br bonds and partial dissociation of C–H bonds with the formation of intermediate species, and then the intermediate species were further oxidized to form CO and CO2.

Mn-Promoted Co3O4/TiO2 as an efficient catalyst for catalytic oxidation of dibromomethane (CH2Br2)

Brominated hydrocarbon is the typical pollutant in the exhaust gas from the synthesis process of Purified Terephthalic Acid (PTA), which may cause various environmental problems once emitted into atmosphere. Dibromomethane (DBM) was employed as the model compound in this study, and a series of TiO2-supported manganese and cobalt oxide catalysts with different Mn/Co molar ratio were prepared by the impregnation method and used for catalytic oxidation of DBM. It was found that the addition of Mn significantly enhanced the catalytic performance of Co/TiO2 catalyst. Among all the prepared catalysts, Mn(1)-Co/TiO2 (Mn/Co molar ratio was 1) catalyst exhibited the highest activity with T90 at about 325°C and good stability maintained for at least 30h at 500ppm DBM and 10% O2 at GHSV=60,000h(-1), and the final products in the reaction were COx, HBr and Br2, without the formation of Br-containing organics. The high activity and high stability might be attributed to the redox cycle (Co(2+)+Mn(4+)↔Co(3+)+Mn(3+)) over Mn-promoted Co3O4/TiO2 catalyst. Based on the results of in situ DRIFT studies and analysis of products, a plausible reaction mechanism for catalytic oxidation of DBM over Mn-Co/TiO2 catalysts was also proposed.

The performance and mechanism of Ag-doped CeO2/TiO2 catalysts in the catalytic oxidation of gaseous elemental mercury

To improve the ability of CeO2/TiO2 catalysts to catalyze the oxidation of gaseous elemental mercury, silver was introduced. Doping with Ag can significantly enhance the Hg0 oxidation ability of CeO2/TiO2. In addition, the temperature window was widened (from 150 to 450 °C). The catalysts were characterized by TEM, XRD, XPS and H2-TPR. The results indicated that silver nanoparticles can be loaded on the TiO2 support. The catalysts had better crystallization and higher redox ability after addition of silver. Silver existed mostly in its metallic state, which can keep Ce in a higher Ce(IV) state. HCl was oxidized into active Cl by CeO2 and then was adsorbed on the silver nanoparticles. In addition to the HCl and Hg0 breakthrough experiments, a Hg0 desorption experiment and a Cl2 yield experiment were conducted to study the catalytic mechanisms of elemental mercury oxidation over various temperature ranges; these experiments indicated that the reaction followed the Langmuir–Hinshelwood mechanism at low temperature, and the Eley–Rideal mechanism and homogeneous gas-phase reaction at high temperature. Furthermore, a mercury valence state change experiment was performed, which indicated that HCl was the major catalytic oxidization component.