A First-principles Investigation On Sub-nanometer Sized Transition Metal Structures Supported On Two-Dimensional Materials

In recent years, controlled preparation and application of novel nano-materials with well-defined properties has become one of the hotest fields in nano-research. One of the key applications of transition metal (TM) nanoparticles is as the catalyst for a chemical reaction. As the reactivity of TM is determined by the energy levels of TM-d states that are sensitive to the local coordination environments, unsaturated TM atoms are the reaction sites, and the size effect observed in experiments is mainly due to variation of the number of exposed reaction centers with the particle size. Downsizing the TM nanoparticle to sub-nano scale to achieve a higher density of reaction centers with superior catalytic performance together and a high utilization of precious TM resources is one of the frontier fields in nanocatalysis. But the decrease of nanoparticle size is also accompanied with the increase of the surface energy, which reduces the stability of TM nanostrctures. By deposition onto a support material, the interfacial interaction can not only improve the stability of the nanoparticles, but also synergistically promote the performance of the composites as catalysts. Due to their large specific surface area and other superior properties, two-dimensional layered materials, such as hexagonal boron nitride and molybdenum disulfide, are eligible as the support materials for TM nanoparticles. In this thesis, we investigated the catalytic performance of single transition metal atom (Cu, Pt) embedded in hexagonal boron nitride nanosheet in CO oxidation, and the growth rules of Pt nanoparticles on molybdenum disulfide nanosheet by first-principles-based calculations. The main contents are as the following:Firstly, we investigated the electronic structure of Cu atoms embedded in hexagonal boron nitride nanosheet and the mechanisms of CO catalytic oxidation to CO2 on it by first-principles-based calculations. We showed that Cu atoms prefer to bind directly with the localized defective structures on the boron nitride nanosheet, which act as strong trapping sites for Cu atoms and inhibit their aggregation and clustering. The strong binding of Cu atoms at boron vacancy also up-shifts the energy level of Cu-d states to the Fermi level and the adsorbed oxygen species can be efficiently activated. We found that the CO oxidation would like to proceed following Langmuir-Hinshelwood mechanism with the formation of a peroxide-like complex by reaction of coadsorbed CO and O2, with the dissociation of which the a CO2 molecule and an adsorbed O atom are formed. Then, the embedded Cu atom is regenerated by the reaction of another gaseous CO with the remnant O atom. The calculated energy barriers for the formation and dissociation of peroxide complex and the regeneration of embedded Cu atoms are as low as 0.26,0.11 and 0.03 eV, respectively, showing the superioity of trapped Cu atoms for low temperature CO oxidation.Then, we also investigated the electronic structure of Pt atoms embedded in hexagonal boron nitride and the mechanisms of CO catalytic oxidation. Different from those of the case of Cu, spin is rised by the unpaired electrons and the reactively is significantly enhanced. The coadsorption of one CO and one O2 is an exothermic process and the corresponding adsorption energies is -3.04 eV, which not only overwhelms the coadsorption 2 CO (-2.92 eV), but is also stronger than the individual adsorption of either CO and O2. In this sense, there is no CO poisoning problem in the Pt embedded system. We found that the CO oxidation over these stabilized Pt atoms would also proceed through Langmuir-Hinshelwood mechanism, and the catalytic activity is similar as compared with those of trapped Cu atoms.Finally, we studied the growth rules and electronic properties of Pt nanoparticles over molybdenum disulfide nanosheet. We showed that a single Pt atom prefers to adsorb at the top site of a Mo atom, instead of the top site of a S atom. Due to the domination of Pt-Pt over Pt-S interactions, the active site for Pt nanoparticles growth on the molybdenum disulfide surface shifts from the top site of Mo atom for Pt and Pt2 nanoparticle to the top site of S atom for Pt3-Ptio. With the increase of the number of deposited Pt atoms, the corresponding average binding energies grows gradually and the Pt structures will prefer to grow in a 3-Dimension manner, with the Pt-S interface structure matching that of the (001) surface of molybdenum disulfide.