A First-principles Investigation On Adsorption And Conversion Of Small Molecules Over Functionalized Two-Dimensional Materials

The controlled fabrication and applications of 2D-materials has drawn considerable attention in recent years. Due to their layered structures, two-dimensional materials exhibit properties different from those found in conventional materials, such as special optical and electronic properties, large surface area, outstanding mechanical strength, unique chemical reactivities and etc. Functionalization, especially via doping with heteroatoms and functional groups, has been shown to be feasible to further tune the electronic structures of 2D-materials to exhibit desired chemical and physical properties for application in various fields, including gas adsorption and separation, heterogeneous catalysis and etc. In this dissertation, with extensive first-principles based calculations, we addressed the impact of N-doping on CO₂ adsorption performance in carbon materials, the possibility for CO₂ reduction with H₂ over Re-doped hexagonal boron nitride ?ReBSV?, and the reaction mechanism for ethylene oxidation on Pt-doped hexagonal boron nitride ?PtBSV?. The key findings are:Firstly, we investigated the impact of N-doping on CO₂ adsorption performance in carbon materials. We studied CO₂ adsorption at five different types of adsorption sites over a series of carbon structures with different size and doped by different N-containing groups. We found that N-doped carbon materials mainly adsorbing CO₂ by three ways:?1?through hydrogen bond between the C-H on the edge of the materials and the O of CO₂ ?CHO site?; ?2?through the interaction between nitrogen groups and CO₂ ?NHO site/O-C site?; ?3?through electrostatic interaction between CO₂ and the edge part of the substrate ?OE site?. The interactions at CHO site is weak and the adsorption energy is lower than-5.07 kJ/mol. N-containing functional groups could interact with CO₂ through N or O atom, and the more Mulliken charges on the atom, the stronger the CO₂ adsorption. Adsorption at OE site could be enhanced by the quaternary N, pyridine and -NH₂ groups, as they can increase the charge localization at the edge of the carbon flakes. Due to the large population of OE site, the introduction of N-containing group of this type can not only enhance the interaction, but also significantly increase the CO₂ adsorption quantity.Then, we investigated the mechanism of CO₂ reduction with H₂ over ReBSV with first-principles based calculations. The results show that ReBSV can not only activate adsorbed CO₂ and H₂, but also promote their reactions. The reactions take place through the Langmuir-Hinshelwood mechanism. The activated CO₂ reacts with three H₂ successively and get a methanol in the final step. The reaction starts with the coadsorption of CO₂ and H₂, and then the two molecules decompose and form CO and H₂O. After the desorption of H₂O, another H₂ adsorbs and reacts with CO for formation of a HCHO. Finally, a third H₂ comes and reacts with the formaldehyde and forms a CH3OH as the final product. The catalytic cycle finishes with the desorption of the CH3OH. The reaction barriers are all less than 1.0 eV, showing that ReBSV is a potential catalyst for CO₂ reduction. The results also show that the reaction thermodynamics can be further tuned by controlling the affinity of the transition metal ensemble to oxygen to achieve a better CO₂ reduction performance.Finally, we performed extensive theoretical calculations and investigated the reaction mechanisms of ethylene oxidation on Pt-doped hexagonal boron nitride. Due to the limited coordinate sites at the mono-dispersed Pt atoms, they cannot influence the hydride transfer for formation of CH3CHO, leaving the total selectivity to ethylene epoxide. The reaction initiates through the Langmuir-Hinshelwood mechanism with the formation of a cyclic peroxide intermediate by the reaction among coadsorbed ethylene and O₂, by the dissociation of which an ethylene epoxide and a coadsorbed O atom are formed. After the desorption of ethylene epoxide, another ethylene reacts with the remnant O atom and forms another ethylene epoxide. The energy barriers of the formation and dissociation of the cyclic intermediate and the regeneration of PtBSV are 0.29 eV,0.70 eV and 0.17 eV, respectively, showing that PtBSV is a potential superior catalyst for ethylene oxidation.