High coverage CO adsorption and dissociation on the Co(0001) and Co(100) surfaces from DFT and thermodynamics

摘要

The adsorption, dissociation and desorption of CO at different coverage over two Co surfaces and the corresponding equilibrium phase diagrams under different temperatures and partial pressures have been systematically investigated; here, the results are obtained using density functional theory calculations and atomistic thermodynamics together with the periodic slab model. Our results show that the saturated adsorption on both Co(0001) and (100) surface are 7/9 ML; meanwhile, on these two surfaces, the lateral repulsive interaction has an effect on the adsorption structures and the corresponding stepwise adsorption energies of these adsorbed CO molecules, and"become stronger with the increasing of CO coverage, which further leads to CO migration over Co surface; moreover, the adsorption energies decrease gradually with the increasing of CO coverage until to the saturated adsorption. According to the stepwise adsorption configurations, further calculations on CO dissociation show that only molecular adsorption CO is favored over (0001) surface; whereas on Co(100) surface, when the coverage is 1/9 ML, CO dissociation is more facile than its desorption; when the coverage is from 2/9 to 5/9 ML, only the first CO dissociation is preferred, while desorption is more favored for the left CO molecules; when the coverage is equal to or greater than 6/9 ML, CO desorption will be more favorable than its dissociation. Further, the adsorption and activation of CO under different temperatures and partial pressures from atomistic thermodynamics method well illustrate the relationship between the stable CO adsorption with the temperatures and CO partial pressure on Co(0001) and Co(100) surface, respectively. Therefore, on the basis of above results, we can obtain that the surface structures mainly affect the adsorption form of CO due to the different activity towards CO, while CO coverage will affect its adsorption energies and configurations, as well as CO dissociation on Co surfaces. The calculated stretching frequencies of CO molecule at different coverage agree with the available experimental data. This study provides a more accurate and detailed information for the process of CO adsorption and activation on Co surface. (C) 2016 Elsevier B.V. All rights reserved.