【作者】 邢斌；

【导师】 王贵昌；

【作者基本信息】 南开大学 ， 物理化学， 2012， 博士

【摘要】 本文在第一原理量子力学计算基础上，运用DFT GGA方法，对过渡金属表面上X-H（X=C，N，O，S）键解离反应、苯乙烯环氧化反应以及表面预吸附氧原子对反应机理的影响进行了研究。对于不同X-H键解离反应，本论文中主要计算了不同物种的吸附性质、反应的活化能以及预吸附氧原子的影响；苯乙烯环氧化反应中主要研究了不同银表面（Ag（110）和Ag（111））的反应活性和选择性，同时也对低氧覆盖度下氧原子及助剂原子的作用进行了研究。通过本文的研究得到以下结论：1.本论文中选取了几种典型的包含X-H键结构的分子进行研究，即氨，水，甲醇，甲烷，硫化氢，乙烯。通过计算发现清洁过渡金属表面上，X-H键解离的活化能顺序为N-H> C-H≈O-H> S-H，与X-H键的键能大致吻合（H-OH（498kJ/mol）> H-NH₂（460kJ/mol）> H-CHCH₂（452kJ/mol）> H-CH₃（440kJ/mol）>H-OCH₃（437kJ/mol）> H-SH（378kJ/mol）），其中由于O-H键解离过程中氢键的作用使得反应的过渡态更加稳定，因此水的解离活化能要低于氨的解离活化能。不同的过渡金属也表现出不同的反应活性，第一副族金属（Cu, Ag, Au）相比其他过渡金属，活性较低。在预吸附氧原子的过渡金属表面，X-H的解离活化能相比清洁表面有着明显的变化。对于难以在清洁表面直接解离的氨，水，甲醇来说，氧原子的存在促进了解离反应的进行，而对于较易分解的甲烷，乙烯，硫化氢则根据金属活性的不同表现出了不同的影响，金属越活泼，氧原子的促进作用越弱。计算结果表明，X-H解离反应符合BEP关系，即反应吸热越多，活化能越大，反应越难以进行。同时我们还发现氧原子吸附前后的活化能变化与氧原子的吸附能也存在着很好的线性关系，说明氧原子与金属表面原子和吸附物种之间存在竞争作用，与表面的结合越强，对X-H解离反应的影响越弱。2.本论文还研究了苯乙烯在金属银表面的环氧化反应，考察了不同催化剂表面结构和不同氧原子覆盖度对反应活性以及选择性的影响。由于苯乙烯分子的不对称性，环氧化反应过程中存在两种不同的中间体，相应就有两种反应路径，计算结果表明，通过直链中间体（氧原子与端基CH₂基团结合（C₆H₅-C¹H=C₂H₂···O））的反应路径容易进行。苯乙烯环氧化反应是结构敏感性反应，从计算得到的活化能结果来看，在Ag（110）表面，苯乙醛和燃烧中间体是主要产物，而在Ag（111）表面，环氧苯乙烷是主要产物，与实验结论相符合。通过动力学模拟方法，我们计算出了Ag（110）和Ag（111）表面上环氧苯乙烷生成反应的选择性，Ag（111）表面为0.38，明显高于Ag（110）表面的0.003。本论文中选取了低覆盖度氧原子覆盖的Ag（111）表面进行了比较计算，计算结果表明低氧原子覆盖度下，不同反应步骤的活化能均高于高氧原子覆盖度的情况。本论文也对Ag（111）表面预吸附助剂原子（Cl，Cs，Ru）以及Ag（111）表面的负载金属氧化物结构(Ag₁₁O₆)对苯乙烯环氧化反应的影响做了比较研究，计算结果表明在不同的助剂原子对不同物种的吸附性质没有明显的影响，但是对反应的活化能有较大的影响。更多还原

【Abstract】 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.更多还原