甲胺在清洁及磷改性Mo(100)表面的解离

采用广义梯度近似(GGA)的密度泛函理论(DFT)(DFT-GGA)并结合平板模型, 研究了甲胺在清洁及磷(P)改性的Mo(100)表面(P-Mo(100))发生C—N键断裂的反应历程(CH3NH2→CH3+NH2). 优化了裂解过程中反应物、过渡态和产物的几何构型, 获得了反应路径上各物种的吸附能及反应的活化能数据. 计算结果表明, 在清洁和磷改性的Mo(100)表面, 甲胺均稳定吸附在顶位, 甲基和氨基最稳定的吸附位置均为桥位. 甲胺的C—N键在P-Mo(100)表面裂解的活化能为2.39 eV, 高于其在清洁表面的活化能(1.99 eV). 这表明Mo(100)表面被预吸附的P原子钝化了. 电子结构分析表明, 改性P原子使得金属Mo的供电子能力减弱, 导致它的d带中心下移, 从而降低了该表面的反应活性, 提高了甲胺的C—N键裂解的活化能. 活化能的分解表明, C—N键在P-Mo(100)与Mo(100)表面裂解的活化能的差异主要体现在初态到过渡态时甲胺的结构变化引起的能量变化(△EdefCH3NH2)、过渡态仅有甲基存在时的吸附能(ETSCH3)和过渡态甲基和氨基的相互作用(EintCH3…NH2). △EdefCH3NH2和ETSCH3使活化能升高幅度大于EintCH3…NH2使活化能降低幅度, 最终导致甲胺的C—N键在P-Mo(100)表面裂解的活化能要高于在Mo(100)表面裂解的活化能.

Decomposition of Methylamine on the Clean and P Modified Mo(100) Surfaces

The reaction pathways for methylamine decomposition (CH_3NH_2→CH_3+NH_2) on a clean Mo(100) surface and on a phosphorus (P) modified Mo(100) surface (P-Mo(100)) were investigated using first-principles (density functional theory based on generalized gradient approximation (DFT-GGA)) calculations with the slab model. Geometries of reactants, transition states, and products were calculated. Adsorption energies of possible species and activation energy barriers of the reaction were obtained. Calculated results show that methylamine is adsorbed in the top site while the methyl and amino groups are adsorbed in the bridge site on the clean and phosphorus modified Mo(100) surfaces. The activation energy of methylamine C-N cleavage was found to be 2.39 eV on the phosphorus modified Mo (100) surface, which is higher than that on the clean Mo(100) surface (1.99 eV). This indicates that the Mo(100) surface is passivated by phosphorus atoms. An electronic structure analysis shows that a modified phosphorus atom reduces the electron donation ability of the molybdenum which results in a downshift of the surface metal atom d-band center. Thus, the reactivity of the Mo(100) surface decreases and the activation energy for methylamine C-N cleavage increases. The decomposition of activation energy indicates that the difference in methylamine C-N cleavage activation energy for the two surfaces is caused by the structural deformation of methylamine (△E_(CH_3NH_2)~(def)) from the initial state to the transition state, the adsorption energy of the methyl (without an amino group) in the transition state configuration (E_(CH_3)~(TS)) and the interaction energy between methyl group and amino group in the transition state (E_(CH_3…NH_2)~(int)). Compared with Mo(100), the increase in activation energy induced by △E_(CH_3NH_2)~(def) and E_(CH_3)~(TS) is higher than the decrease in activation energy induced by E_(CH_3…NH_2)~(int) on the phosphorus modified Mo(100) surface, which results in the methylamine C-N cleavage activation energy on the phosphorus modified Mo(100) surface being higher than the that on clean Mo(100) surface.