Abstract:

The reaction pathways for methylamine decomposition (CH3NH2→CH3+NH2) 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 (△EdefCH3NH2) from the initial state to the transition state, the adsorption energy of the methyl (without an amino group) in the transition state configuration (ETSCH3) and the interaction energy between methyl group and amino group in the transition state (EintCH3…NH2). Compared with Mo(100), the increase in activation energy induced by △EdefCH3NH2 and ETSCH3 is higher than the decrease in activation energy induced by EintCH3…NH2 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.

Key words: Density functional theory, Methylamine, Decomposition, Mo(100), Phosphorus modified Mo(100), Generalized gradient approximation, Slab model