HF在α-AlF3(0001)表面吸附的密度泛函理论研究

利用密度泛函理论系统研究了不同覆盖度下HF在3F、2F、1F与Al 终端的α-AlF₃(0001)表面的吸附行为, 分析了HF与不同终端表面相互作用的电子机制. 计算结果表明: HF在3F终端的α-AlF₃(0001)表面物理吸附; 在2F及1F终端表面化学吸附, 形成Al-F键和FHF结构, 使HF分子活化, 可以参加下一步的氟化反应; 在Al 终端表面解离吸附形成Al-F与Al-H键. 3F、2F、1F及Al 终端表面配位不饱和数目分别为0、1、2与3配位.不同覆盖度研究表明, 在2F终端表面上, 吸附一个HF分子使表面Al 配位达到饱和, 后续吸附的HF为物理吸附; 而在1F与Al 终端表面仍可化学吸附. 因此, 推测α-AlF₃暴露不同终端表面中Al 原子配位不饱和数越高, 其对HF吸附与活化能力越强, 可能的氟化催化反应活性越高. 差分电荷密度与电子态密度分析表明, HF与3F终端α-AlF₃(0001)表面发生弱相互作用, 而与2F、1F与Al 终端表面形成较强的电子相互作用.

A Density Functional Theory Study of HF Adsorption on the α-AlF3(0001) Surface

Using density functional theory, the adsorption behaviors of HF at α-AlF₃(0001) surfaces with different coverages of 3F, 2F, 1F, and Al terminations were studied systematically. The electronic interactions between HF and the α-AlF₃(0001) surfaces were also analyzed. Our results indicated that physisorption occurs when HF adsorbs at the 3F-terminated surface. Strong chemisorption occurs, and Al-F and FHF structures form when HF adsorbs at surfaces with 2F and 1F terminations. Under these conditions, the HF molecule is activated, and might take part in the subsequent fluorination reactions. Dissociated adsorption occurs, and Al-F and Al-H bonds form when HF is adsorbed on the Al-terminated surface. The unsaturated coordination numbers for surface Al with 3F, 2F, 1F, and Al-terminated surfaces are 0, 1, 2, and 3, respectively. The coordination number of the AlF2 surface is saturated when one HF molecule adsorbs; then, only physical adsorption occurs for any subsequently adsorbed HF molecules. However, it can still chemisorb at the 1F and Al-terminated surfaces. It is therefore reasonable to deduce that the higher the unsaturated coordination number of the surface, the higher the amount of activated HF, and possibly the higher the catalytic activities in the fluorination reactions. Charge density difference and density of states indicated that weak interactions occur between the HF and the 3F-terminated surface, while strong interactions occur between the HF and the 2F, 1F, Al-terminated surfaces.