Structural characterization and catalytic evaluation of transition and rare earth metal doped ceria-based solid solutions for elemental mercury oxidation

Abstract

The catalytic behavior of various CeO₂-based solid solutions, namely, Ce_1−xTM_xO_2−δ (TM = Mn, Fe, or Zr) and Ce_1−xRE_xO_2−δ (RE = Pr, La, or Sm) was studied for the removal of elemental mercury (Hg⁰) from coal-derived flue gas by catalytic oxidation (Hg⁰ → Hg²⁺). The investigated catalysts were synthesized by a coprecipitation method and characterized by various techniques, namely, X-ray diffraction (XRD), Raman spectroscopy (RS), high-resolution electron microscopy (HREM), Brunauer–Emmett–Teller (BET) surface area, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), and diffuse reflectance spectroscopy (UV-DRS). The XRD results confirmed the incorporation of Mn, Fe, Zr, La, Pr, and Sm cations into the CeO₂ lattice and the formation of nanocrystalline solid solutions. The TEM measurements established the nanocrystalline nature of the solid solutions. The RS measurements suggested that the substitution process promotes the formation of oxygen vacancies, which hastens the diffusion rate of oxygen and improves the Hg oxidation. UV-vis DRS studies demonstrated the presence of the charge transfer transitions O²⁻ → Ce³⁺ and O²⁻ → Ce⁴⁺. The XPS and H₂-TPR results suggested that the reduction of Ce⁴⁺ → Ce³⁺ is the foremost reason for the increase in oxygen vacancies, which are beneficial for Hg⁰ removal. The order of mercury oxidation activity over various doped catalysts is as follows: CM > CL > CZ > CF > CS > C > CP.