The Quantum Chemistry Studies On The Electronic Structure Of Cu₂O（111） Surface And The Adsorption And Activation Of CO₂on The Surface

The reduction of CO2into organic fuels with photocatalytic or electrochemical methods is a challenging subject. Cu2O, the visible-induced photo-catalyst, shows certain selectivity to alcohols in the hydrogenation of CO2. Recently, it was used as the electrocatalyst for CO2reduction. Cu2O is a promising catalyst for the photo-catalytic reduction of CO2. However, the properties of the material surface and the interactions between the CO2species and the surface are still poorly understood. In this thesis, we investigated the properties of Cu2O (111) surface and CO2adsorption and activation on the surface by using quantum chemical methods.Firstly, we investigated the geometric and electronic structures of Cu2O (111) perfect and oxygen vacancy surfaces by using first-principles method based on density functional theory (SIESTA program) with periodic slab models. We generated the first-principles pseudopotentials for Cu and O atoms, which then were well tested. The calculations indicate that:(1) The relaxation on the defect-free surface does not exceed three TL (nine atomic layers. TL denotes the minimal periodic unit of O-Cu-O three atomic layer) from the outermost.(2) The surface reconstructions caused by the oxygen vacancies are very small and localized in top two TLs, especially around the vacancy sites.(3) The electronic structures of atoms around the vacancy are changed apparently. The reaction activities in these regions are increased.(4) The formation energies of oxygen vacancies are rather low, indicating that the vacancies are easy to create.Then, we investigated the adsorption of various CO2species, CO2, H2CO3, HCO3-and CO32-, on Cu2O (111) perfect surface at DFT/B3LYP level (Gaussian03program) with two embedded cluster models, Cu28O14+824PCs+10AIMPs and Cu52O26+780PCs+18AIMPs. PC denotes point charges, and AIMP denotes ab initio model potential. The calculations indicate that:(1) CO2molecules are non-activated adsorbed on the surface. The effective adsorption of CO32-ions cannot be formed.(2) H2CO3molecules are dissociated into adsorbed H+and HCO3-ions on the surface.(3) HCO3-ions are adsorbed on the surface with bond length of C-Oads (CuCUS) increased. The charge densities on C atom decreased, which made the nucleophilic attack on this site easily. The results implied CO2activated adsorbed on Cu2O (111) perfect surface is via the form of HCO3-ion.Lastly, we studied the adsorption of CO2on Ou2O (111) oxygen vacancy surface by using the first-principles calculations based on density functional theory with the periodic slab model (SIESTA code). The results show that:(1) The total energy of the system would be increased by+82.4kJ/mol or+176.1kJ/mol if the dissociation adsorption processed, which means that the dissociation reaction is thermodynamically unfeasible.(2) The oxygen vacancy weakened the adsorption at Cucus, and Ocus sites.(3) The adsorbed molecule can be converted into a reactive radical when it horizontally adsorbed at oxygen vacancy site. The adsorption energy is only-0.2kJ/mol, which implies the radical is easy to migrate on the surface or leave away.