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 NH, the catalytic combustion of volatile organic compounds, Hg 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.

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).

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The flue gases with high concentration of mercury are often encountered in the nonferrous smelting industries and the treatment of mercury-containing wastes. To recover mercury from such flue gases, sorbents with enough large adsorption capacity are required to capture and enrich mercury. ZnS is a cheap and readily prepared material, and even can be obtained from its natural ores. In this work, a simple controllable oxidation methodsoaking in cupric solutionwas developed to improve the interfacial activity of ZnS and its natural ores for Hg0 adsorption. The gaseous Hg0 adsorption capacity of ZnS was enhanced from 0.3 to 3.6 mg·g–1 after such treatment. Further analysis indicated that a new interface rich in S1– ions was formed and provided sufficient active sites for the chemical adsorption of Hg0. In addition, the cyclic Hg0 adsorption and recovery experiments demonstrated that the adsorption performance of spent activated-ZnS was recovered after reactivating sorbents with Cu2+, indicating the recovery of activated interface. Meanwhile, the high concentration of adsorbed mercury at the surface can be collected using a thermal treatment method. Utilization of raw materials from a zinc production process provides a promising and cost-effective method for removing and recovering mercury from nonferrous smelting flue gas.

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[MoS] clusters were bridged between CoFe layered double hydroxide (LDH) layers using the ion-exchange method. [MoS]/CoFe-LDH showed excellent Hg removal performance under low and high concentrations of SO, highlighting the potential for such material in S-Hg mixed flue gas purification. The maximum mercury capacity was as high as 16.39 mg/g. The structure and physical-chemical properties of [MoS]/CoFe-LDH composites were characterized with FT-IR, XRD, TEM&SEM, XPS, and H-TPR. [MoS] clusters intercalated into the CoFe-LDH layered sheets; then, we enlarged the layer-to-layer spacing (from 0.622 to 0.880 nm) and enlarged the surface area (from 41.4 m/g to 112.1 m/g) of the composite. During the adsorption process, the interlayer [MoS] cluster was the primary active site for mercury uptake. The adsorbed mercury existed as HgS on the material surface. The absence of active oxygen results in a composite with high sulfur resistance. Due to its high efficiency and SO resistance, [MoS]/CoFe-LDH is a promising adsorbent for mercury uptake from S-Hg mixed flue gas.