Wenjun Huang

MnOx/Graphene for the Catalytic Oxidation and Adsorption of Elemental Mercury.

MnOx/graphene composites were prepared and employed to enhance the performance of manganese oxide (MnOx) for the capture of elemental mercury (Hg(0)) in flue gas. The composites were characterized using FT-IR, XPS, XRD, and TEM, and the results showed that the highly dispersed MnOx particles could be readily deposited on graphene nanosheets via hydrothermal process described here. Graphene appeared to be an ideal support for MnOx particles and electron transfer channels in the catalytic oxidation of Hg(0) at a high efficiency. Thus, MnOx/graphene-30% sorbents exhibited an Hg(0) removal efficiency of greater than 90% at 150 °C under 4% O2, compared with the 50% removal efficiency of pure MnOx. The mechanism of Hg(0) capture is discussed, and the main Hg(0) capture mechanisms of MnOx/graphene were catalytic oxidation and adsorption. Mn is the main active site for Hg(0) catalytic oxidation, during which high valence Mn (Mn(4+) or Mn(3+)) is converted to low valence Mn (Mn(3+) or Mn(2+)). Graphene enhanced the electrical conductivity of MnOx, which is beneficial for catalytic oxidation. Furthermore, MnOx/graphene exhibited an excellent regenerative ability, and is a promising sorbent for capturing Hg(0).

Novel regenerable sorbent based on Zr–Mn binary metal oxides for flue gas mercury retention and recovery

To capture and recover mercury from coal-fired flue gas, a series of novel regenerable sorbents based on Zr-Mn binary metal oxides were prepared and employed at a relatively low temperature. PXRD, TEM, TPR, XPS, and N2-adsorption methods were employed to characterize the sorbents. The Hg(0) adsorption performance of the sorbents was tested, and the effects of the main operation parameters and the gas components on the adsorption were investigated. Zr significantly improved the sorbents mercury capacity, which was nearly 5mg/g for Zr0.5Mn0.5Oy. Furthermore, the spent sorbent could be regenerated by heating to 350°C, and the highly concentrated elemental mercury released could be facilely recycled. Therefore, a much greener process for mercury capture and recovery could be anticipated based on this regenerable sorbent.

Mechanism of the Selective Catalytic Oxidation of Slip Ammonia over Ru-Modified Ce–Zr Complexes Determined by in Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy

The slip ammonia from selective catalytic reduction (SCR) of NOx in coal-fired flue gas can result in deterioration of the utilities or even the environmental issues. To achieve selective catalytic oxidation (SCO) of slip ammonia, Ru-modified Ce-Zr solid solution catalysts were prepared and evaluated under various conditions. It was found that the Ru/Ce(0.6)Zr(0.4)O2(polyvinylpyrrolidone (PVP)) catalyst displayed significant catalytic activity and the slip ammonia was almost completely removed with the coexistence of NOx and SO2. Interestingly, the effect of SO2 on NH3 oxidation was bifacial, and the N2 selectivity of the resulting products was as high as 100% in the presence of SO2 and NH3. The mechanism of the SCO of NH3 over Ru/Ce(0.6)Zr(0.4)O2(PVP) was studied using various techniques, and the results showed that NH3 oxidation follows an internal SCR (iSCR) mechanism. The adsorbed ammonia was first activated and reacted with lattice oxygen atoms to form an -HNO intermediate. Then, the -HNO mainly reacted with atomic oxygen from O2 to form NO. Meanwhile, the formed NO interacted with -NH2 to N2 with N2O as the byproduct, but the presence of SO2 can effectively inhibit the production of N2O.

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.

Novel Effective Catalyst for Elemental Mercury Removal from Coal-Fired Flue Gas and the Mechanism Investigation

Mercury pollution from coal-fired power plants has drawn attention worldwide. To achieve efficient catalytic oxidation of Hg(0) at both high and low temperatures, we prepared and tested novel IrO2 modified Ce-Zr solid solution catalysts under various conditions. It was found that the IrO2/Ce0.6Zr0.4O2 catalyst, which was prepared using the polyvinylpyrrolidone-assisted sol-gel method, displayed significantly higher catalytic activity for Hg(0) oxidation. The mechanism of Hg(0) removal over IrO2/Ce0.6Zr0.4O2 was studied using various methods, and the Hg(0) oxidation reaction was found to follow two possible pathways. For the new chemisorption-regeneration mechanism proposed in this study, the adsorbed Hg(0) was first oxidized with surface chemisorbed oxygen species to form HgO; the HgO could desorb from the surface of catalysts by itself or react with adsorbed HCl to be release in the form of gaseous HgCl2. O2 is indispensable for the chemisorption process, and the doping of IrO2 could facilitate the chemisorption process. In addition, the Deacon reaction mechanism was also feasible for Hg(0) oxidation: this reaction would involve first oxidizing the adsorbed HCl to active Cl species, after which the Hg(0) could react with Cl to form HgCl2. Additionally, doping IrO2 could significantly improve the Cl yield process. In summary, the novel IrO2 modified catalyst displayed excellent catalytic activity for elemental mercury oxidation, and the proposed reaction mechanisms were determined reasonably.

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.

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.

Enhancement of Ce1−xSnxO2 support in LaMnO3 for the catalytic oxidation and adsorption of elemental mercury

Mn-based perovskite oxide was used as the active site for elemental mercury (Hg0) removal from coal-fired flue gas. Ce1−xSnxO2 binary oxides were selected as the catalyst supports for LaMnO3 to enhance the catalytic oxidation and adsorption performance. Ce0.7Sn0.3O2 had the best Hg0 removal performance among the as-prepared Ce1−xSnxO2 binary oxides; the Hg0 removal efficiency was 95.2% at 350 °C. LaMnO3 had better performance at low temperatures (<200 °C). LaMnO3/Ce0.7Sn0.3O2 enlarged the reaction temperature window and enhanced the Hg0 removal efficiencies. The correlation between the physicochemical properties and the catalytic removal performance was investigated by XRD, BET surface area measurements, Raman spectroscopy, H2-TPR and XPS analysis. With the addition of Ce–Sn binary oxides as catalyst support, the surface areas of LaMnO3 was enlarged, the reducibility was enhanced and the oxygen mobility was improved. In addition, the Hg0 removal mechanism was illustrated on the basis of the experimental results. The roles of Ce, Sn and LaMnO3 were also discussed in this study.

Catalytic oxidation of dibromomethane over Ti-modified Co3O4 catalysts: Structure, activity and mechanism

Ti-modified Co3O4 catalysts with various Co/Ti ratios were synthesized using the co-precipitation method and were used in catalytic oxidation of dibromomethane (CH2Br2), which was selected as the model molecule for brominated volatile organic compounds (BVOCs). Addition of Ti distorted the crystal structure and led to the formation of a Co-O-Ti solid solution. Co4Ti1 (Co/Ti molar ratio was 4) achieved higher catalytic activity with a T90 (the temperature needed for 90% conversion) of approximately 245°C for CH2Br2 oxidation and higher selectivity to CO2 at a low temperature than the other investigated catalysts. In addition, Co4Ti1 was stable for at least 30h at 500ppm CH2Br2, 0 or 2vol% H2O, 0 or 500ppm p-xylene (PX), and 10% O2 at a gas hourly space velocity of 60,000h-1. The final products were COx, Br2, and HBr, without the formation of other Br-containing organic byproducts. The high catalytic activity was attributed to the high Co3+/Co2+ ratio and high surface acidity. Additionally, the synergistic effect of Co and Ti made it superior for CH2Br2 oxidation. Furthermore, based on the analysis of products and in situ DRIFTs studies, a receivable reaction mechanism for CH2Br2 oxidation over Ti-modified Co3O4 catalysts was proposed.