Adsorption Of H₂ On MgO Clusters Studied By Ab Initio Methods

There is a general realization that hydrogen will be an important energy. Realizing hydrogen energy application in reality is confronted with some technical problem. The key task of resolving the problem is to design or find out ideally hydrogen storage materials. Research in hydrogen storage materials needs to focus on (1) understanding the fundamental factors governing bond strength, kinetics, absorption and desorption behavior, and degradation with cycling; (2) applying these principles to modify the performance of known hydrogen storage materials; and (3) identifying new materials and new classes of materials whose properties can be tailored to meet the transportation and other demands of the hydrogen economy. Clusters have the size between those of atoms and macroscopical systems, and have many unique properties. So many experimental and theoretical researches have been done on them. Researching into the adsorption properties of H₂ on the surface of cluster is also one essential aspect of scientific researches.In this paper, we have studied the adsorption of H₂ on the surface of the rock-salt-structure (MgO)₉,12 and the tube-structure (MgO)₁₂ at the DFT/B3LYP level. First,we fully optimized designed adsorption structures and calculated their frequency. For some adsorption structures with negative frequency, stable structures were gotten by analyzing their normal modes and eliminating their negative frequency. The adsorption properties of all stable adsorption structures also were analyzed. Second, we studied molecular orbital properties of H₂ on the surface of (MgO)_n cluster. Finally, we investigated the process of H₂ desorption on the (MgO)₁₂ cluster.The results show that physical adsorption can be formed on Mg^+q or O^-q ions at different sites of the clusters. On top of the Mg^+q ions, H₂ is adsorbed in side-on manners and donates electrons to the ions. On top of the O^-q ions, H₂ is adsorbed in end-on manners and the molecule is polarized. The strength of the adsorption depends mainly on the location of the Mg/O ions, the smaller of the coordination number of the ions, the stronger of the adsorption of H₂. For the Mg/O ions with the same coordination number, the adsorption formed onto the Mg^+q is stronger. According to the Mg or O ions and their locations, the adsorption energy is from 0.03eV to 0.08eV.