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.