A Study of First Principles on Nitrogen Reduction Mechanism at the Surface of Ti Based Two-dimensional Transition Metal Carbides

The density of states (DOS) , Gibbs free energy (ΔG)of the reaction intermediate structure, charge density difference (CDD), work function and activation energies (E_a) of the reaction intermediate structure on the surface of two-dimensional transition metal carbon nitride (MXenes) Ti₂CT₂ and Ti₃C₂T₂ (T=O* or OH*) catalysts before and after N₂ adsorption were calculated by first-principles, to explore the new mechanism of N₂ reduction reaction (NRR) on the surface of OH* terminal MXene. The results show that N₂ undergoes physical adsorption on Ti₂CO₂ and Ti₃C₂O₂ surfaces (∆q≈0e), while chemical adsorption occurs on Ti₂C(OH)₂ and Ti₃C₂(OH)₂ surfaces (∆q>0.2e), Ti₂CO₂ and Ti₃C₂O₂ are not conducive to NRR due to high η_NRR. While Ti₂C(OH)₂ and Ti₃C₂(OH)₂ surfaces can provide H atoms during the initial adsorption step and undergo NRR through the Enzymatic mechanism, with the corresponding η_NRR decreasing to 0.29 V and 0.38 V, respectively. Moreover, the calculated η_NRR can be used as a function of φ: η_NRR=0.44φ−0.71, where the correlation coefficient (R²) is 0.97, showing a strong linear relationship between overpotential and work function. The hybridization intensity of N 2p and O 2p orbitals on the surface of *OH terminal and *O terminal is different, resulting in different NRR activity of OH* MXene. Therefore, it is believed that N₂ is adsorbed on Ti₂C(OH)₂ and Ti₃C₂(OH)₂ via “N₂+2*H=*N₂H₂” , followed by NRR along the Enzymatic mechanism.