Experimental Study on the Elemental Mercury Removal Performance and Regeneration Ability of CoOx–FeOx-Modified ZSM-5 Adsorbents

Herein, a series of Co-Fe mixed oxide modified ZSM-5 adsorbents were synthesized using the ultrasonic-assisted impregnation method for the capture of elemental mercury. In comparison with other samples, Co4Fe1-ZSM-5 produced a relatively better performance, with the removal efficiency of around 96.6% Hg0 and the adsorption capacity of around 901.63 ug/mg Hg0 achieved at 120 °C. The interaction between CoOx and FeOx improved the reducibility of oxygen species, thus promoting the oxidation of Hg0. Among a variety of other gas components, O2 and NO exerted a positive effect on Hg0, which improved its removal to a certain extent. By contrast, SO2 caused an adverse effect on the capture of Hg0, which could be reversed to some degree by the introduction of 5% O2. After five cycles, the mercury removal efficiency of Co4Fe1-ZSM-5 remained above 90%, suggesting excellent recyclability. Finally, XPS analysis was conducted to reveal that Mars–Maessen mechanisms are dominant in the process of mercury adsorption.

Co3O4 Nanosheets Preferentially Growing (220) Facet with a Large Amount of Surface Chemisorbed Oxygen for Efficient Oxidation of Elemental Mercury from Flue Gas

Oxygen vacancies can capture and activate gaseous oxygen, forming surface chemisorbed oxygen, which plays an important role in the Hg⁰ oxidation process. Fine control of oxygen vacancies is necessary and a major challenge in this field. A novel method for facet control combined with morphology control was used to synthesize Co₃O₄ nanosheets preferentially growing (220) facet to give more oxygen vacancies. X-ray photoelectron spectroscopy (XPS) results show that the (220) facet has a higher Co³⁺/Co²⁺ ratio, leading to more oxygen vacancies via the Co³⁺ reduction process. Density functional theory (DFT) calculations confirm that the (220) facet has a lower oxygen vacancy formation energy. Furthermore, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results suggest that Co₃O₄ nanosheets yield more defects during the synthesis process. These results are the reasons for the greater number of oxygen vacancies in Co₃O₄ nanosheets, which is confirmed by electron energy loss spectroscopy (EELS), Raman spectroscopy, and photoluminescence (PL) spectroscopy. Therefore, Co₃O₄ nanosheets show excellent Hg⁰ removal efficiency over a wide temperature range of 100-350 °C at a high gas hourly space velocity (GHSV) of 180 000 h^-1. Additionally, the catalytic efficiency of Co₃O₄ nanosheets is still greater than 83%, even after 80 h of testing, and it recovers to its original level after 2 h of in situ thermal treatment at 500 °C.

AMn2O4 (A=Cu, Ni and Zn) sorbents coupling high adsorption and regeneration performance for elemental mercury removal from syngas

AMn₂O₄ (A= Cu, Ni and Zn) spinel sorbents synthesized by a low-temperature sol-gel auto-combustion method were for the first time used to eliminate elemental mercury (Hg⁰) from syngas. CuMn₂O₄ sorbent exhibits the highest Hg⁰ adsorption performance under simulated syngas, higher than 95 % Hg⁰ capture efficiency is obtained at 200 °C. Adsorption-regeneration experiments demonstrate that the regenerability of CuMn₂O₄ is excellent. The influences of syngas compositions on Hg⁰ elimination over CuMn₂O₄ were examined. H₂S plays the most important role in Hg⁰ removal from syngas, which can be adsorbed on CuMn₂O₄ surface and transformed into active sulfur species to react with Hg⁰ to form surface-bonded HgS. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) experiments prove that HgS is formed on the spent sorbent. The surface Cu²⁺ cations and chemisorbed/lattice oxygens participate in Hg⁰ adsorption and transformation. HCl promotes Hg⁰ removal by forming surface active chlorine species. H₂, CO, and H₂O inhibit Hg⁰ removal under N₂ atmosphere. However, they exhibit no obvious effect on Hg⁰ removal with the assistance of H₂S. The excellent Hg⁰ capture performance, good regenerability and H₂O resistance of CuMn₂O₄ make it to be a very promising sorbent for Hg⁰ removal from syngas at higher temperature.