Abstract
ABSTRACT Cu-Mn and Cu-Mn-Ce oxide-incorporated mesoporous silica was formed by hydrothermally exfoliating silicate, and the physicochemical properties and NO/Hg0 removal efficiency were investigated. The exfoliation induced structural reformation, resulting in a large specific surface area and the uniform dispersion of metal oxides on the surface. The transfer of valences between Cu2+ and Mn3+ in the Cu-Mn silica contributed to the single reduction peak displayed in the H2 temperature-programmed reduction profiles and the high Mn4+/Mn and Cu+/Cu ratios observed via X-ray photoelectron spectroscopy (XPS). The high oxygen lability of the Cu-Mn silica may have inhibited its ability to remove NO. By contrast, when SO2 was present, incorporating Ce enhanced the NO removal efficiency due to the increased number of Bronsted acid sites. Hg0 removal tests indicated that adsorption was the primary removal mechanism for both the Cu-Mn and the Cu-Mn-Ce silica samples. Cu2Mn8 exhibited the highest Hg removal efficiency, suggesting that Ce’s enhancing effect on Hg0 adsorption was diminished when a large amount of Mn was present. Of the gaseous components, the adsorbed HCl was mainly responsible for the oxidation and subsequent adsorption of Hg0. Furthermore, with the addition of SO2, the competitive adsorption of SO2 and the resulting HgCl2 did not decrease the Cu-Mn silica’s efficiency in oxidizing Hg0, but the oxidized Hg was less adsorptive.
Highlights
Coal-fired power plants (CFPPs) have been reported as one of the major anthropogenic emission sources of NOx, SO2, and heavy metals, such as mercury (U.S EPA, 2012; UNEP, 2013)
The transfer of valences between Cu2+ and Mn3+ in the Cu-Mn silica contributed to the single reduction peak displayed in the H2 temperature-programmed reduction profiles and the high Mn4+/Mn and Cu+/Cu ratios observed via X-ray photoelectron spectroscopy (XPS)
Catalyst activity primarily depended on the calcination temperature which resulted in increasing pore volume and metal species in high oxidation state
Summary
Coal-fired power plants (CFPPs) have been reported as one of the major anthropogenic emission sources of NOx, SO2, and heavy metals, such as mercury (U.S EPA, 2012; UNEP, 2013). The difference in crystallinity, oxidation state, and the amount of specific surface area and pore volume of metal oxides supported by porous materials could influence the NO and Hg0 removal performance. The surface reformation leads to (1) uniform dispersion of metal oxides on the silica surface, (2) a high Cu-Mn and Cu-Mn-Ce loading, and (3) large surface area and pore volume, all of which could be highly beneficial for Hg0 adsorption and NO reduction by NH3, but not thoroughly understood by previous research. The CuOx-MnOx/SiO2 samples were prepared by the silicate exfoliation method with a mole ratio of the precursor metal nitrate set at (Cu + Mn)/Si = 1. The metal oxides were expected to completely incorporate in the SiO2 samples through the silicate exfoliation method. X-ray photoelectron spectroscopy (XPS; ESCALAB 250, VG Scientific) was employed to understand the surface chemical compositions and valence states of metal oxides on the samples.
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