DFT Study Of The Effect Of Pre-adsorbed Atomic Oxygen On The Dehydrogenation And Epoxidation Reaction Over Metal Surface

Based on the first principle quantum mechinism calculations and DFT GGAmethod, the dissociation of X-H （X=C, N, O, S） bond on transition metal surfaces, theepoxidation of styrene and the effect of pre-adsorbed atomic oxygen on the reactionmechanism have been systemically investigated. For the dissociation X-H bond, thisthesis mainly calculated the most stable adsorption structure and the activation barrierchange affected by pre-adsorbed atomic oxygen. For the epoxidation of styrene, twodifferent Ag surfaces （Ag（110） and Ag（111））are chosen to study the reaction activityand selectivity, and the Ag（111） surface with high and low oxygen coverage also beenstudied to analyse the effect of atomic oxygen and different additive atoms.The main conclusions of this work are summarized as follow.1. Several typical X-H bond contained moleculars have been chosen to studiedin this thesis, that is NH3, H₂O, CH₃OH, CH4, H₂S and C2H4. On clean transitionmetal surfaces, the calculated results indicated that the order of activation barrier forX-H bond dissociation is N-H> C-H≈O-H> S-H, which is roughly correlated withthe respective X-H bond strength （i.e. H-OH （498kJ/mol）> H-NH₂（460kJ/mol）>H-CHCH₂（452kJ/mol）> H-CH₃（440kJ/mol）> H-OCH₃（437kJ/mol）> H-SH （378kJ/mol））. The low barrier for the O-H bond cleavage may due to the formation ofhydrogen-bond formed in transition state, which would stabilize the transition stateand thus give rise to the low barrier compared to the NH3dissociation. And dfferentmetal surfaces present different reaction activity, for example, the first IB groupmetals have lower activity compared with other transition metals. On transition metalsurfaces with pre-adsorbed atomic oxygen, the activation barrier for X-H dissociationhas obviously change. For NH3, H₂O and CH₃OH that is hard to directly dissociatedon clean metal surfaces, the existence of atomic oxygen promoted the reactionprocess, but for CH4, C2H4and H₂S that is relatively easier to be dissociated, theeffect of atomic oxygen is different depends on the activity of metals, that is, thepromotion effect is stronger on less active metal surfaces. The calculated results alsoindicate that the X-H dissociation is consistent with the BEP relationship, that is, the more endothermic the reaction is, the higher the activation energy will be, whichmeans the reaction is hard to process. Moreover, it is found that the change ofactivation energy with and without pre-adsorbed atomic oxygen is related to theadsorption energy of atomic oxygen. This result means that there is competitionbetween atomic oxygen and reactant with the surface metal atoms, if the atomicoxygen bond to the surface strongly, the effect on the dissociation process is weak.2. The selective oxidation of styrene on oxygen-covered Ag（110） and Ag（111）surface have been studied by the density functional theory calculation with theperiodic slab model. Due to the asymmetry of the styrene molecule, there are twopossible reaction intermediates in the styrene epoxidation processes. The calculatedresults is able to show that the formation of styrene epoxide via the linearoxametallacycle （i.e. the pre-adsorbed atomic oxygen bound to the methylene groupin styrene, C₆H₅-C¹H=C₂H₂···O） is the favorable reaction mechanism on both Ag（110）and Ag（111） surface. Styrene epoxidation is structure sensitive and from calculatedacitviation energies, the main product is phenyl acetaldehyde and combustionintermediate on Ag（110） surface, while on Ag（111） surface, the main product isstyrene epoxide, which is in good agreement with experimental conclusion. Theselectivity toward styrene epoxide can be calculated by kinetic simulations, and onAg（111） surface, the selectivity is0.38, which is much higher than that on Ag（110）surface （0.003）. The Ag（111） surface with lower oxygen coverage has been chosen tocompare. The calculated results indicate that the activation energies of differentreaction steps are both higher than that owith higher oxygen coverage. Differentadditive atoms and surface oxide structure based on Ag（111） surface has little effecton the adsorption properties of different species, but they have obviously effect on thereaction activities, that is, the Cl atom shows promotion effect while other additiveatoms and surface oxide structure inhibit the styrene epoxidation. But the selectivitytowards styrene epoxide has no evident change except that on the surface oxidestructure which lower the seclectivity, and this is futher confirm that styreneepoxidation is structure sensitive.