Theoretical Study Of Photocatalytie Decomposition Of H₂O And Co₂Conversion Coupling Based On Zn₂GeO₄

In this work, we choose the Zn22GeO4as a photocatalytic material, around the solar photocatalytic decomposition of H2O and CO2conversion coupling reaction, systematically studied the structure and nature of the surface, the interactions way of CO2and H2O with different of Zn2GeO4surface by means of density functional theory calculations. We also examined CO2and H2O adsorption on different surface active sites, and analyzed the surface geometry and electronic structure before and after the adsorption, and determined the surface activity and photocatalytic mechanism. In addition, we studied the interaction between the co-catalyst and the different Zn2Ge04surfaces; and examined the impact of the co-catalyst to the surface catalytic activity. Then, we discussed coadsorption of CO2and H2O on Zn2GeO4surface and studied the reactions between them, and thus to clarify the reaction mechanism for coupling of CO2with H2O on Zn2GeO4surface. These results may have significant implications for the design of more efficient photocatalysts and for atomistic-level understanding of the other photocatalytic hydrogenation reaction. Based on this, we had carried out the following aspects research work.In the first chapter, we reviewed the research background and the research progress in the related filed. Including the research photocatalytic reduction of CO2, the mechanism of photocatalytic reduction of CO2and the impact factors of photocatalytic reduction of CO2. Finally, we summarized the research motivation and contents of this dissertation.In the second chapter, we have introduced the density functional theory, the intrinsic reaction coordinate theory, and the software packages used in this work.In the third chapter, we focused on the interaction of CO2and H2O with Zn2GeO4surfaces, expounds the intrinsic link between the microstructure and properties of the exposed crystal surface. The major finding is that the interaction of CO2and H2O with Zn2GeO4surfaces is structure dependent, and the surface states at the top of the valence band is the important factor that affects H2O dissociation. Additionally, the oxygen vacancies are the active sites; CO2adsorbs directly at the Vo site can be dissociated into CO and O and the Vo defect can be healed by the oxygen atom released during the dissociation process. Further analysis of the dissociative adsorption mechanism of CO2on the surface oxygen defect site, we concluded that dissociative adsorption of CO2favors the stepwise dissociation mechanism and the dissociation process can be described as:CO2+VOâ†'CO2δ-/Voâ†'COadsorbed+Osurface.In the forth chapter, we studied the configurations and electronic structures of the (RuO2)x (x=2,3) and Pt4cluster deposited on the Zn2GeO4surfaces, and studied the interaction of H2O with the (RuO2)x/Zn2GeO4and Pt4/Zn2GeO4surfaces. According to the analysis of the adsorption energies and the interaction forms, we found that the interaction of (RuO2)X (x=2,3) and Pt4cluster with Zn2GeO4surfaces and the interaction of H2O with the (RuO2)x/Zn2GeO4and Pt4/Zn2GeO4surfaces are structure dependent, and H2O adsorbs at the supported (RuO2)x (x=2,3) cluster can spontaneously dissociates into an H atom and an OH group, while the interface between the Zn2GeO4(010) surface and Pt4clusters are the most favorable position for H2O spontaneous dissociation.In the fifth chapter, on the basis of the results of the above studies, we presented an atomic-level description of the initial process of CO2hydrogenation on the Zn2GeO4(010) surface. We systematically studied CO2hydrogenation with coadsorbed H2O on different active sites of the perfect and defective Zn2GeO4(010) surfaces and examined two possible pathways:(1) CO2is hydrogenated at its C atom, forming a HCOO*species;(2) CO2is hydrogenated at its O atom leading to a COOH*species. On the perfect Zn2GeO4(010) surface, the results revealed that COOH*formation is more favorable both kinetically and thermodynamically than HCOO*formation, indicating that the dominant product of the initial steps of CO2hydrogenation is COOH*. On the defective Zn2GeO4(010) surface, COOH*formation becomes exothermic and with a small activation barrier, while HCOO*formation is still kinetically difficult and thermodynamically unfavorable, suggesting that COOH*formation is greatly facilitated by the presence of surface oxygen vacancy. Additionally, the calculations also show that different surface sites have different activities for coadsorption of CO2and H2O, as well as for hydrogenation of CO2to COOH*. Further analysis shows that these differences in the activity of the surface sites originate from the differences in the surface geometric and electronic structure.At the end of this dissertation, in the seventh chapter, we summarized the main conelusions of this dissertation, and proposed some possible subjects for the further research in the related fields.