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 uptake, three types of one-dimensional (1D) MnO2 nano-particles, alpha-, beta- and gamma-MnO2, were successfully prepared and tested. The structures of alpha-, beta- and gamma-MnO2 were characterized by XRD, BET, TEM and SEM. The results indicate that alpha-, beta- and gamma-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: beta-MnO2 < gamma-MnO2 < alpha-MnO2 at 150 degrees C. The mechanism for Hg-0 removal using MnO2 was discussed with the help of results from H-2-TPR, XPS and Hg-0 removal experiments in the absence of O-2. It was determined that the oxidizability of three forms of MnO2 increased as follows: beta-MnO2 < gamma-MnO2 < alpha-MnO2. The mechanism for Hg-0 capture was ascribed to the Hg-0 catalytic oxidation with the reduction of Mn4+ -> Mn3+ -> Mn2+. Furthermore, the interaction forces between mercury and manganese oxide sites are demonstrated to increase in the following order: beta-MnO2 < gamma-MnO2 < alpha-MnO2 based on the desorption tests. (C) 2015 Elsevier B.V. All rights reserved.