(68b) Mercury Oxidation Over Cupric Chloride-Impregnated Alumina Catalyst for Mercury Emissions Control

CuCl₂ has demonstrated excellent mercury oxidation performance in our previous studies. This work aims to advance the fundamental mechanistic understanding of the heterogeneous catalytic oxidation by using the reaction between Hg(0) vapor and CuCl₂ impregnated on both α-Al₂O₃ and γ-Al₂O₃. The CuCl₂/Al₂O₃ catalysts (both fresh and spent) were characterized by SEM-EDX, TGA-MS, TPR, XRD, and XPS. 

The CuCl₂ crystallites formed onto α-Al₂O₃ were found to be very stable up to 300 °C, and undergo the thermal reduction process from Cu(II) to Cu(0) via Cu(I).  In the absence of HCl and O₂ gases, CuCl₂ was found to follow a Mars-Maessen mechanism by consuming lattice chlorine of CuCl₂ for Hg(0) oxidation and to be reduced to CuCl.  In the presence of 10 ppmv HCl, 2,000 ppmv SO₂, and 6% O₂ gases, the CuCl₂/α-Al₂O₃ sample works as an Hg(0) oxidation catalyst exhibiting >90% conversion with good resistance to SO₂ at 140 °C.  The reduced CuCl was found to be re-chlorinated to CuCl₂ under HCl and O₂ gases by following the Deacon reaction.  CuCl₂ is expected to be able to be used as a catalyst by impregnating onto non-carbonaceous substrates in a temperature window after the air preheater.

Multiple copper species were found to be formed when γ-Al₂O₃ is used as a substrate as opposed to one Cu(II) species on α-Al₂O₃.  At low CuCl₂ loading, strong interaction exists between CuCl₂ and γ-Al₂O₃ surface, resulting in a thermally-stable copper aluminate phase.  At higher loadings (e.g. 10% CuCl₂/γ-Al₂O₃), amorphous CuCl₂ overlaps the surface aluminate and exists in the forms of highly dispersed CuCl₂ and Cu₂(OH)₃Cl phases .  The CuCl₂/γ-Al₂O₃ catalysts with low CuCl₂ loadings showed low catalytic performances in mercury oxidation.  In contrast, high loading CuCl₂/γ-Al₂O₃ catalysts showed almost complete Hg(0) oxidization in the presence of 10 ppmv HCl and 6%(v) O₂ gas balanced with N₂ gas, regardless of the presence of 2,000 ppmv SO₂ gas over 140 hrs of the performance evaluations.