Exploring adsorption behavior and oxidation mechanism of mercury on monolayer Ti2CO2 (MXenes) from first principles

Abstract

Developing low-cost, high adsorption capacity, and recyclable mercury sorbents is an important step for control of mercury in coal-fired flue gas. However, few sorbents meet all the requirements. Herein, we explore the adsorption behavior and oxidation mechanisms of mercury on 2D Ti2CO2 (MXenes) monolayer and defective Ti2CO2 monolayer with an Ov (Ov-Ti2CO2) through first-principles computations based on density functional theory (DFT). Our calculation results show that Hg0 adsorption on Ti2CO2 and Ov-Ti2CO2 monolayers are mainly physisorption and chemisorption mechanisms, respectively. Moreover, the presence of a single oxygen vacancy on Ti2CO2 monolayer can improve the interaction between the mercury and Ov-Ti2CO2 monolayer, thereby enhancing the adsorption energy of mercury by 66 kJ/mol. Hg0 has a strong interaction with Ti atom on Ov-Ti2CO2 monolayer by the atomic orbital hybridization and overlap. The adsorption of HgO molecule on Ti2CO2 and Ov-Ti2CO2 monolayers are chemisorption processes. Furthermore, electron density difference analysis indicates that the remarkable charge accumulation across the interface is closely related to the strong interaction between HgO molecule and Ti2CO2 or Ov-Ti2CO2 monolayers. The three-step reaction processes (Hg0 → Hg(ads) → HgO(ads) → HgO) are taken place on Ti2CO2 and Ov-Ti2CO2 monolayers, resulting in the formation of gaseous HgO molecule. The energy barrier for the Hg0 oxidation reaction step on Ov-Ti2CO2 monolayer (54.85 kJ/mol) is about half of that on Ti2CO2 monolayer (104.73 kJ/mol). The desorption steps of the resultant HgO molecule from Ti2CO2 and Ov-Ti2CO2 monolayers are the rate-determining steps, which need external energies of about 119.3 and 183.2 kJ/mol, respectively.