Abstract EN:

In the present work, we undertook a challenging task, i.e., construction a full-dimension potential energy surface (PES) for a benchmark poly-atomic molecular surface reaction, CH4/Ni(111), by using a reactive force field. Careful appraisal of the PES was made in order to establish the validity of the PES. The differences between the results for the transition state (dissociation barriers and structures) given by our PES and those by DFT calculations do not exceed 15%. The molecular dynamics simulation results obtained by using our PES are compared to experimental results for CH4 dissociation probability on Ni(111). For the vibrationally excited state, v3 (v=1, J=0), the agreement between our simulation results and the experimental ones is excellent. For the ground state, the sticking coefficient is somehow over-estimated because of the under-estimation of the dissociation barrier by about 150 meV with our reactive force field. Nevertheless, the overall agreement between simulation and experiment is pretty good. Within the help of the full-dimensional PES, we have extensively studied some important aspects of reaction dynamics, e.g., the effects of surface impact position, surface temperature, vibrationally excited state, rotationally excited states etc. For CH4 in ground state (v=0, J=0), the investigation of the effect of CH4 impact position shows that the top site is the most reactive one. The surface temperature strongly affects the reactivity of methane, especially in the region of the low incident energy near to the dissociation threshold, while in the high incident energy region, the effect is less important. For CH4 v3 (v=1, J=0), an important coupling between rotation and vibration is found. The rotation of CH4 can enhance its reactivity in the following way. In its ground state (v=0, J=0), CH4 does not rotate during its flight to the surface. In this case, only one of the two lowest C-H bonds pointing initially toward the surface can be cleaved while the two other bonds never break. In v3 (v=1, J=0) vibrational state, due to the rotation induced by vibration-rotation coupling, any of the four H atoms can be dissociated even if it forms a C-H bond which has an unfavorable initial orientation (i.e., pointing away from the substrate). The rotation of CH4 induced by vibration-rotation coupling near the substrate allows for bringing an unfavorable initial orientation of C-H bond to the right one required by a transition state (TS) during the adsorbate’s approaching to the substrate. As the enhanced reactivity of vibrationally excited molecules is concerned, the intuitively limpid and overwhelmingly accepted explanation is that the vibration-induced bond stretching helps bond breaking. Our simulation results show clearly that the vibration-induced CH4 rotation contributes an important part to the enhanced reactivity of a v3 (v=1, J=0) vibrationally excited CH4. A series of simulations to determine the sticking curves for CH4 in the vibrational ground state (ν=0) but excited to higher rotational levels (J=0-12) have also been performed. Due to its small level spacing, the lowest rotational excited states (J=1-3) of CH4 do not affect its reactivity on Ni(111) as observed experimentally. We found that rotation enhances significantly CH4 reactivity on Ni(111) with a deposited rotational energy amounting only to 12% of the dissociation barrier. Moreover, in a hypothetic simulation, we found also very striking evidences that rotation can even promote better dissociation of CH4 on Ni(111) than vibration. In a vibrationally excited CH4, its C-H bonds undergo alternate stretching and compressing and the latter hinders dissociation. In this case, the reactivity is inevitably modulated by vibration phase. However, the centrifugal force due to rotation tends always to stretch the C-H bonds for CH4 in rotationally excited states.