Oxygen vacancies tuning over Co3O4 nanosheet preferentially growing (2 2 0) facet for efficient Hg0 removal from flue gas

Abstract

Oxygen vacancy is very important for the Hg0 oxidation process due to it can capture and activate gaseous oxygen forming surface chemisorbed oxygen to oxidize Hg0 to HgO. Precise control of oxygen vacancies amount is a big challenge in this field. For Co3O4, oxygen vacancy can be generated by Co3+ reduction to Co2+ and Co3+ mainly exist in (220) facet. Thus, in the present work, Co3O4 nanosheets preferentially growing (220) facet with more Co3+ were synthesized and CO was used as reduction agent to reduce Co3O4 at different reduction condition. After that, Co3O4 nanosheets with different amount of oxygen vacancies were obtained. XPS, Raman and PL results show that, the amount of oxygen vacancies increase firstly and then decrease with the increase of reduction temperature and time. CO can reduce Co3+ to Co2+ forming oxygen vacancies. However, when the reduction temperature and time are at a high level, Co3O4 will be over reduced to CoO leading to a decrease of oxygen vacancies. DFT confirm that, when Co3O4 is over reduced, O2 will hard to adsorb on Co3O4 surface and lattice oxygen will be formed even after O2 adsorption. Co3O4-Te300Ti2 has an optimal reduction degree leading to higher oxygen vacancy concentration and better O2 adsorption and activation ability. Therefore, Co3O4-Te300Ti2 has a high Hg0 oxidation efficiency which is over 96 % in a wide temperature range of 150–300 °C at a high GHSV of 180,000 h−1.