The First Principle Study Of Propylamine, Ethanol, Ethylene Adsorption And Reaction On Precious Metal Surfaces

In this thesis, the reaction mechanism for selective oxidation of propylamine and ethanol on clean and oxygen-covered gold surface has been studied by the first principle quantum mechinism calculations at the level of generalized gradient approximation (GGA) with slab model. In addition, adsorption and oxidation of ethylene on clean and M-Promoted (M=Cs, Ru, Rh, Pd, Pt, O) Ag(111) surfaces have been investigated by the same way. On the basis of above systemical studies, the configuration of possible intermediates, the most stable adsorption and transition state structure in the reaction, adsorption energy, and the activation energy of reaction have been obtained. By analyzing the above date, the electronic strecture of configurations and decompositing adsorption energy, the conclusions are summarized as follows:(1) The calculation results on reaction mechanism for selective oxidation of propylamine on oxygen-covered gold have been indicated that the adsorption energy of propylamine decrease with the increasing oxygen coverage, that is,-0.38,-0.20and-0.10eV on clean,2/9monolayer (ML) and2/3monolayer (ML) oxygen, respectively. The adsorption energies of the intermediates also have the trend of the gradual lower. The present work also indicated that the final product distribution depends on the oxygen coverage:Propylamine undergoes N-H bond and C-H bond cleavage to produce propionitrile and water at low-oxygen-coverage (θ0=2/9ML), and to yield propionitrile, propionaldehyde and water at high-oxygen-coverage (θ0=2/3ML).The energy barrier of first step of propyamine oxidation (CH3CH2CH2NH2�'CH3CH2CH2NH) is0.16eV (θ0=2/9ML) and0.38eV (θ0=2/3ML). On the second step, the barrier energy is0.16eV and0.25eV of CH3CH2CH2NH�'CH3CH2CH2N, the next both of C-H breakage and the barrier energy is0.20eV (CH3CH2CH2N�'CH3CH2CHN) and0.25eV (CH3CH2CHN�'CH3CH2CN) eV on low oxygen coverage, and0.15eV (CH3CH2CH2N�'CH3CH2CHN) and0.26eV eV(CH3CH2CHN�'CH3CH2CN) on the high oxygen coverage. The additional reaction step of CH3CH2CHN�'CH3CH2CHO is occurred on the high oxygen coverage, and the associated barrier is0.41eV. The calculation results show that the oxidation of propylamine can occur at room temperature due to the lower energy barrier. Furthermore, it was found that the energy barrier for the possible reaction steps at the low oxygen coverage is general smaller than that on high oxygen coverage, which agrees with the experimental results.(2) The mechanism of ethanol oxidation on O-adsorbed Au(111) surface is study. The oxygen coverage on Au(111) surface is zero,1/9monolayer (ML). On clean,1/9ML oxygen coverage, the adsorption energy of ethanol respectively are-0.12,-0.02eV. The chance rule of the adsorption energy with the increase of oxygen coverage is meet with the conclusion above:the adsorption energy of adsorption species reduce. Ethanol has three hydrogen bonding:O-H bond(CH3CH2OH), C-H bond(CH3CH2OH), C-H bond(CH3CH2OH). So ethanol oxidation on Au(111) surface of oxygen coverage, the way of dehydrogenation have three on the frist step. So there are three pathway designed.(1), reaction of ethanol and oxygen atom produce ethoxyl (CH3CH2OH+O�'CH3CH2O+OH). The energy barrier is0.03eV.(2) The another way is Cα-H bond (CH3CH2OH) breaking of ethanol (CH3CH2OH+O�'CH3CHOH+OH) and the energy barrier is0.39eV.(3), Cβ-H bond(CH3CH2OH) cleavage of ethanol lead to CH2CH2OH and OH (CH3CH2OH+O�'CH2CH2OH+OH) and the activation barrier is1.04eV. From the calculation results that the energy barrier of CH3CH2OH+O�'CH3CH2O+OH is lowest, so it is best reaction pathway. The second step is oxidation of the produced ethoxyl also include two C-H (Ca-H bond CH3CH2O, Cβ-H bond of CH3CH2O). Two pathway is designed here.(1), CH3CH2O+OH�'CH3CHO+H2O need the energy barrier of0.40eV.(2), the path is breakage of Cβ-H bond of ethoxyl (CH3CH2O+OH�'CH2CH2O+H2O) and the activation barrier is1.2eV. Generation of acetaldehyde need minimum energy, so reaction tend to break Ca-H bond. Ethanol undergoes O-H bond and Ca-H bond cleavage to produce acetaldehyde and water at oxygen-coverage (θ0=1/9ML) Au(111) surfaces.(3) On clean and M-promoted Ag(111) surfaces, the oxidation reaction of ethylene (C2H4) and oxygen (O) produce ethylene oxide (C2H4O) and acetaldehyde (CH3CHO). The reactants, transition state, product geometric configurations and the stablest state adsorption site are established on clean and M-promoted Ag(111) surfaces. Further the analysis, as compared to clean Au(111) surface, the presence of Cs、Rh、 Pb、Pt and O on Au(111) surfaces improved selectivity of C2H4O. The Cs atom is adsorbed on the Au(111) surface form electric field, then stabilize transition state, leading to C2H4O. The Ru、Rh、Pd、Pt atoms respectively adsorbed on Ag(111) surfaces affect acid and alkaline of the oxygen, so the selectivity enhancement of ethylene oxide. When the oxygen atom promote in ethylene epoxidation on Ag(111), yield of the ethylene oxide improved. The results proof that the increase of oxygen coverage is beneficial to product ethylene oxide.