物理化学学报
物理化學學報
물이화학학보
ACTA PHYSICO-CHIMICA SINICA
2009年
11期
2336-2342
,共7页
密度泛函理论%甲胺%解离%Mo(100)%磷改性的Mo(100)%广义梯度近似%平板模型
密度汎函理論%甲胺%解離%Mo(100)%燐改性的Mo(100)%廣義梯度近似%平闆模型
밀도범함이론%갑알%해리%Mo(100)%린개성적Mo(100)%엄의제도근사%평판모형
Density functional theory%Methylamine%Decomposition%Mo(100)%Phosphorus modified Mo(100)%Generalized gradient approximation%Slab model
采用广义梯度近似(GGA)的密度泛函理论(DFT)(DFT-GGA)并结合平板模型,研究了甲胺在清沽及磷(P)改性的Mo(100)表面(P-Mo(100))发生C-N键断裂的反应历程(CH_3NH_2→CH_3+NH_2).优化了裂解过程中反应物、过渡态和产物的几何构型,获得了反应路径上各物种的吸附能及反应的活化能数据.计算结果表明,在清洁和磷改性的Mo(100)表而,甲胺均稳定吸附在顶位,甲基和氨基最稳定的吸附位置均为桥位.甲胺的C-N键在P-Mo(100)表面裂解的活化能为2.39 eV,高于其在清洁表面的活化能(1.99 eV).这表明Mo(100)表面被预吸附的P原子钝化了.电子结构分析表明,改性P原子使得金属Mo的供电子能力减弱,导致它的d带中心下移,从而降低了该表面的反应活性,提高了甲胺的C-N键裂解的活化能.活化能的分解表明,C-N键在P-Mo(100)与Mo(100)表面裂解的活化能的差异主要体现在初态到过渡态时甲胺的结构变化引起的能量变化(△E_(CH_3NH_2)~(def))、过渡态仪有甲基存在时的吸附能(E_(CH_3)~(TS))和过渡态甲基和氨基的相互作用(E_(CH_3…NH_2)~(int)).△E_(CH_3NH_2)~(def)和E_(CH_3)~(TS),使活化能升高幅度大于E_(CH_3…NH_2)~(int)使活化能降低幅度,最终导致甲胺的C-N键在P-Mo(100)表面裂解的活化能要高于在Mo(100)表面裂解的活化能.
採用廣義梯度近似(GGA)的密度汎函理論(DFT)(DFT-GGA)併結閤平闆模型,研究瞭甲胺在清沽及燐(P)改性的Mo(100)錶麵(P-Mo(100))髮生C-N鍵斷裂的反應歷程(CH_3NH_2→CH_3+NH_2).優化瞭裂解過程中反應物、過渡態和產物的幾何構型,穫得瞭反應路徑上各物種的吸附能及反應的活化能數據.計算結果錶明,在清潔和燐改性的Mo(100)錶而,甲胺均穩定吸附在頂位,甲基和氨基最穩定的吸附位置均為橋位.甲胺的C-N鍵在P-Mo(100)錶麵裂解的活化能為2.39 eV,高于其在清潔錶麵的活化能(1.99 eV).這錶明Mo(100)錶麵被預吸附的P原子鈍化瞭.電子結構分析錶明,改性P原子使得金屬Mo的供電子能力減弱,導緻它的d帶中心下移,從而降低瞭該錶麵的反應活性,提高瞭甲胺的C-N鍵裂解的活化能.活化能的分解錶明,C-N鍵在P-Mo(100)與Mo(100)錶麵裂解的活化能的差異主要體現在初態到過渡態時甲胺的結構變化引起的能量變化(△E_(CH_3NH_2)~(def))、過渡態儀有甲基存在時的吸附能(E_(CH_3)~(TS))和過渡態甲基和氨基的相互作用(E_(CH_3…NH_2)~(int)).△E_(CH_3NH_2)~(def)和E_(CH_3)~(TS),使活化能升高幅度大于E_(CH_3…NH_2)~(int)使活化能降低幅度,最終導緻甲胺的C-N鍵在P-Mo(100)錶麵裂解的活化能要高于在Mo(100)錶麵裂解的活化能.
채용엄의제도근사(GGA)적밀도범함이론(DFT)(DFT-GGA)병결합평판모형,연구료갑알재청고급린(P)개성적Mo(100)표면(P-Mo(100))발생C-N건단렬적반응역정(CH_3NH_2→CH_3+NH_2).우화료렬해과정중반응물、과도태화산물적궤하구형,획득료반응로경상각물충적흡부능급반응적활화능수거.계산결과표명,재청길화린개성적Mo(100)표이,갑알균은정흡부재정위,갑기화안기최은정적흡부위치균위교위.갑알적C-N건재P-Mo(100)표면렬해적활화능위2.39 eV,고우기재청길표면적활화능(1.99 eV).저표명Mo(100)표면피예흡부적P원자둔화료.전자결구분석표명,개성P원자사득금속Mo적공전자능력감약,도치타적d대중심하이,종이강저료해표면적반응활성,제고료갑알적C-N건렬해적활화능.활화능적분해표명,C-N건재P-Mo(100)여Mo(100)표면렬해적활화능적차이주요체현재초태도과도태시갑알적결구변화인기적능량변화(△E_(CH_3NH_2)~(def))、과도태의유갑기존재시적흡부능(E_(CH_3)~(TS))화과도태갑기화안기적상호작용(E_(CH_3…NH_2)~(int)).△E_(CH_3NH_2)~(def)화E_(CH_3)~(TS),사활화능승고폭도대우E_(CH_3…NH_2)~(int)사활화능강저폭도,최종도치갑알적C-N건재P-Mo(100)표면렬해적활화능요고우재Mo(100)표면렬해적활화능.
The reaction pathways for methylamine decomposition (CH_3NH_2→CH_3+NH_2) on a clean Mo(100) surface and on a phosphorus (P) modified Mo(100) surface (P-Mo(100)) were investigated using first-principles (density functional theory based on generalized gradient approximation (DFT-GGA)) calculations with the slab model. Geometries of reactants, transition states, and products were calculated. Adsorption energies of possible species and activation energy barriers of the reaction were obtained. Calculated results show that methylamine is adsorbed in the top site while the methyl and amino groups are adsorbed in the bridge site on the clean and phosphorus modified Mo(100) surfaces. The activation energy of methylamine C-N cleavage was found to be 2.39 eV on the phosphorus modified Mo (100) surface, which is higher than that on the clean Mo(100) surface (1.99 eV). This indicates that the Mo(100) surface is passivated by phosphorus atoms. An electronic structure analysis shows that a modified phosphorus atom reduces the electron donation ability of the molybdenum which results in a downshift of the surface metal atom d-band center. Thus, the reactivity of the Mo(100) surface decreases and the activation energy for methylamine C-N cleavage increases. The decomposition of activation energy indicates that the difference in methylamine C-N cleavage activation energy for the two surfaces is caused by the structural deformation of methylamine (△E_(CH_3NH_2)~(def)) from the initial state to the transition state, the adsorption energy of the methyl (without an amino group) in the transition state configuration (E_(CH_3)~(TS)) and the interaction energy between methyl group and amino group in the transition state (E_(CH_3…NH_2)~(int)). Compared with Mo(100), the increase in activation energy induced by △E_(CH_3NH_2)~(def) and E_(CH_3)~(TS) is higher than the decrease in activation energy induced by E_(CH_3…NH_2)~(int) on the phosphorus modified Mo(100) surface, which results in the methylamine C-N cleavage activation energy on the phosphorus modified Mo(100) surface being higher than the that on clean Mo(100) surface.
