Catalytic oxidation and adsorption of elemental mercury over nanostructured CeO2–MnOx catalyst

Abstract

A nanostructured CeO₂–MnO_x catalyst was synthesized by a coprecipitation method and subjected to different calcination temperatures at 773 and 1073 K to understand the surface structure and the thermal stability. The structural and redox properties were deeply investigated by various techniques, namely, X-ray diffraction (XRD), inductively coupled plasma-optical emission spectroscopy (ICP-OES), Brunauer–Emmett–Teller (BET) surface area, transmission electron microscopy (TEM), Raman spectroscopy (RS), hydrogen-temperature programmed reduction (H₂-TPR), and X-ray photoelectron spectroscopy (XPS). The CeO₂–MnO_x catalyst calcined at 773 K was tested towards elemental mercury (Hg⁰) oxidation and the achieved results are compared with the pure CeO₂ and MnO_x. The XRD and TEM results confirmed the incorporation of Mn ions into the ceria lattice and the formation of a nanostructured solid solution, respectively. The RS and TPR results showed that the CeO₂–MnO_x catalyst exhibits more oxygen vacancies with superior redox ability over CeO₂ and MnO_x. XPS analysis indicates that Ce and Mn existed in multiple oxidation states. Compared to pure CeO₂ and MnO_x, the CeO₂–MnO_x catalyst exhibited greater Hg⁰ oxidation efficiency (E_oxi) of 11.7, 33.5, and 89.6% in the presence of HCl, O₂, and HCl/O₂-mix conditions, respectively. The results clearly indicated that the HCl/O₂-mix had a promotional effect on the catalytic Hg⁰ oxidation. This was most likely due to the presence of surface oxygen species and oxygen vacancies being generated by a synergetic effect between CeO₂ and MnO_x. In the presence of HCl, the CeO₂–MnO_x catalyst exhibited good adsorption efficiency (E_ads) of 92.4% over pure CeO₂ (46.5%) and MnO_x (80.6%). It was found that increasing the operating temperature from 423 to 573 K resulted in considerable increase of E_oxi and a decrease in the sorption of Hg⁰ on the catalyst.