Abstract: The effect of alkali metal modification on g-C₃N₄ photocatalyst was systematically studied. The density functional theory is used to explain the microscopic mechanism of different alkali metal modification on improving photocatalytic activity from the various angles of geometry and electronic structure. The results show that the binding energies of alkali metals and g-C₃N₄ become weaker and weaker with the increase of the number of extra-nuclear electronic layers of Li, Na, K and Rb atoms, and the influence on the structure of g-C₃N₄ is weaker and weaker. The electronic structure analysis shows that Li, Na, and K atoms have an activation effect on g-C₃N₄, and Rb atoms have a passivation effect on the surface of g-C₃N₄. At the same time, the adsorption of N₂ molecules on M-g-C₃N₄ (M=Li, Na, K, Rb) is simulated, and the interaction mechanism between N₂ molecules and alkali metals is understood. By analyzing the adsorption energy, structural parameters and electronic properties of N₂ adsorption, it is found that the influence of Li and Na atoms on N₂ adsorption is greater. The N—N bond of N₂ is elongated, and the K and Rb atoms have little effect on N₂. Alkali metal modified g-C₃N₄ is more beneficial for the adsorption of N₂ molecules than pure g-C₃N₄, but as the atomic radius of the alkali metal increases, the adsorption capacity becomes weaker and weaker, and the electron transfer becomes less and less. That is to say, the ability of activating N₂ molecule decreases in the order of Li, Na, K and Rb.

Key words: Density functional theory, Graphite phase carbon nitride, Alkali metal modification, N₂ molecular adsorption

摘要： 研究了碱金属改性对g-C₃N₄光催化剂的影响，利用密度泛函理论从几何和电子结构的角度阐述了不同碱金属改性对提高光催化活性的微观机理。结果表明，Li、Na、K、Rb原子随着核外电子层数的增加，碱金属和g-C₃N₄的结合能越来越弱，对g-C₃N₄结构的影响也越来越弱。电子结构分析可知，Li、Na、K原子对g-C₃N₄具有活化作用，Rb原子对g-C₃N₄表面有钝化作用。同时模拟了N₂分子在M-g-C₃N₄ (M=Li、Na、K、Rb)上的吸附，理解N₂分子与碱金属之间的作用机理，通过分析N₂吸附的吸附能、结构参数和电子特性，发现Li、Na原子对N₂吸附的影响更大，N₂的N—N键长伸长，K和Rb原子对N₂基本没有作用。碱金属改性g-C₃N₄比纯g-C₃N₄更有利于N₂分子的吸附，但是随着碱金属原子半径的增加，吸附能力越来越弱，电子的转移也越来越少，即活化N₂分子的能力按照Li、Na、K、Rb的顺序降低。

关键词: 密度泛函理论, g-C₃N₄, 碱金属改性, 氮气吸附