Sulfur-Tolerant Mn-Ce-Ti Sorbents for Elemental Mercury Removal from Flue Gas: Mechanistic Investigation by XPS

Abstract

The influence of structural interaction between Mn, Ce, and Ti on the elemental mercury adsorption properties of Mn-Ce-Ti sorbents has been investigated. For this purpose, Mn-Ce-Ti sorbents have been synthesized by coprecipitation, codeposition-precipitation, coimpregnation, sequential impregnation, and deposition-impregnation to get different structural properties. All the sorbents were evaluated for elemental mercury capture in the absence and presence of SO2. Among the sorbents tested, the Mn-Ce-Ti sorbent synthesized by coimpregnation exhibits better sulfur resistance. X-ray diffraction measurements show that the samples prepared using impregnation methods exhibit intense reflections due to the cubic fluorite structure of cerium. Temperature-programmed desorption measurements reveal that the type and strength of acid sites depend on the synthesis method. Temperature-programmed reduction measurements suggest that precipitating Mn and Ce together over the surface of titania promotes surface ceria and manganese oxide reduction. However, the simultaneous precipitation of Mn, Ce, and Ti inhibits surface ceria and manganese oxide reduction. X-ray photoelectron spectroscopy (XPS) measurements suggest that the mechanism for Hg adsorption over Mn-Ce-Ti sorbents is the same regardless of the presence or absence of SO2. The Ce 3d XPS spectra show that all the samples exhibit peaks due to both Ce4+ and Ce3+ oxidation states. Surface atomic ratios determined from XPS measurements show that the sorbent synthesized by coimpregnation yields a higher concentration of cerium atoms on the surface and exhibits better sulfur resistance than the other sorbents. The sorbent synthesized by coprecipitation yields more titanium atoms on the surface and exhibits the least sulfur resistance.