(Invited) A First-Principles Molecular Dynamics Study on Active Sites on Metal Nano Particles: Application of the Compressed Gas Model

Abstract

The first-principles Molecular Dynamics (FPMD) is applied to investigate gas adsorption onto surfaces of metal nano particles. Herein we focus on catalytic reactions for ammonia synthesis and decomposition. Some Ru nano clusters are prepared as a model of the catalyst, and gas adrosptions of the H2, N2, and NH3 molecules are investigated. In order to do a conceptually unbiased mechanistic study, a natural and reasonable strategy would be to make many models to be investigated by “electronic-structure simulations”. In this study, we apply the compressed gas model in which distribution of gas molecule in high pressure simulations are taken into account. The compressed gas cluster is inserted in the simulation box containing a metal nano particle. Because of the compressed nature of the “ball” of the gas molecules, they automatically diffuse into the vacuum region and hit on the nano particle surface with velocity. By repeating this simulation, we could identify some characteristics of the adsorption sites on the metal nano particle and the gas molecule dependence in the adsorption behavior. It was found that the H2 adsorption should be structure insensitive, while the N2 and NH3 adsorptions should be structure sensitive. In the simulation of the NH3 adsorption, we observed formation of the H2 molecule from NH­ 3. It was found that the kink plays an important role as a reaction center of the H2 formation. In the simulation, H2 was yielded in a mechanism similar to the Eley-Rideal mechanism.