以理論計算方式探討以下反應機構：I.Pt(111)和Ni(111)表面上的C-N鍵結合反應 II.Pt(111)表面上以CHxNO為起始物之HCN生成反應

Theoretical Studies of the Following Reaction Mechanisms:I.The C-N Coupling Reaction on Pt(111) and Ni(111) Surface II.the HCN Formation from CHxNO on Pt(111) surface

陳維家

指導教授 ： 何嘉仁

國立臺灣師範大學/理學院/化學系/碩士(2011年)

摘要

第一部分：Pt(111)和Ni(111)表面上的C-N鍵結合反應

我們使用周期性密度泛函理論來研究Pt(111)和Ni(111)表面上的C-N鍵結合反應，這是工業上用來製成氫氰酸(HCN)的重要催化反應。這個反應包含以下幾個部分：CH4和NH3的脫氫、反應物和產物(CHx、NHy和CHxNHy；x=0-3、y=0-2)的吸附、反應分子在表面上的移動以及C-N鍵結合反應。根據我們的計算結果，反應物CHx和NHy在Pt(111)/Ni(111)表面上的吸附能為7.41/6.91、6.97/6.52、4.58/4.39、2.19/2.01 eV以及5.10/5.49、4.12/4.79、2.75/2.87 eV，符合以下規律：C > CH > CH2> CH3以及N > NH > NH2；而產物的吸附能則是在Pt(111)上CNH2最佳，在Ni(111)上NCH3最佳。在C-N鍵結合的部分，不同表面上的活化能及反應熱都不盡相同，但它們的起始物、過度狀態以及產物的吸附結構都非常相似。其中，在Pt(111)表面上， CH2+NH2有最低的活化能；而在Ni(111)表面上，則是CH+NH2有最低的活化能。我們也使用了電子態密度(LDOS)、電子局域密度函數(ELF)以及電荷分析，用以佐證我們的計算結果。

第二部分：Pt(111)表面上以CHxNO為起始物之HCN生成反應

我們使用周期性密度泛函理論來研究HCN在含氧情況下的生成反應，用以模擬HCN工業製成中的Andrussow process。我們使用NO (由O2氧化NH3產生)和CHx (由CH4脫氫產生)結合成的CHxNO (x=0-3)為起始物，研究其生成HCN的反應機構。根據我們的計算結果，CHxNO吸附在Pt(111)表面上之吸附能分別為4.11、1.91、2.04和2.12 eV。其中，從CH3NO生成HCN之最可能反應路徑為：CH3NO依序斷兩個C-H鍵形成CHNO，CHNO進一步氫化成CHNOH後再斷N-OH鍵形成最終產物HCN。此步反應之速率決定步驟為CH3NO(a)→CH2NO (a) + H(a)，活化能為1.22 eV。

關鍵字

表面 ； 密度泛函裡論 ； 反應機構 ； C-N鍵結合

並列摘要

I: The C-N Coupling Reaction on Pt(111) and Ni(111) Surface.
We used the density functional theory (DFT) with the projector-augmented-wave method (PAW) to systematically investigate the C-N coupling reaction, an important catalytic process in industrial synthesis to form hydrogen cyanide (HCN), on Pt(111) and Ni(111)surface. This reaction includes several steps, such as the adsorption of reactants and products (CHx, NHy and CHxNHy x=0-3 y=0-2), dehydrogenation of methane and ammonia, movement of molecular fragments on the surface, and C-N coupling processes. From our calculation, the adsorption energy of CHx and NHy on Pt(111)/Ni(111) surfaces in the decreasing order are: C > CH > CH2> CH3,and N > NH > NH2 with the values of 7.41/6.91, 6.97/6.52, 4.58/4.39, 2.19/2.01 eV, and 5.10/5.49, 4.12/4.79, 2.75/2.87 eV, respectively. For the adsorption energy of CHxNHy, the CNH2 species is the largest on Pt(111) surface, but on Ni(111) surface, CH3N is the most stable. The C-N coupling barriers are different on the two metal surfaces while the initial, transition state and finial structures are very similar. On Pt(111) surface, the coupling reaction of CH2+NH2 has the lowest barrier, but CH+NH2 is the most favorable on Ni(111) surface. The detail local density of states (LDOS), electron localization function (ELF), and Bader-charge analysis have also been investigated to rationalize the calculated outcomes.
II: The HCN formation from CHxNO on Pt(111) surface.
We applied density functional theory (DFT) with the projector-augmented-wave method (PAW) to investigate the hydrogen cyanide synthesis in the presence of oxygen, a simulation of Andrussow process. The CHxNO (x=0-3), produced by the coupling of NO (oxidation of NH3) and CHx (dehydrogenation of CH4), which is used as the reactant in our caculatation, with adsorption energies 4.11, 1.91, 2.04 and 2.12 eV on Pt(111) surface, respectively. The most possible synthesis pathway from CH3NO to HCN is: (i) the continuous dehydrogenation of CH3NO to CHNO, (ii) the hydrogenation of CHNO to CHNOH, and (iii) the bond scission of N-OH to form the finally product, HCN. The rate determing state is CH3NO (a)→CH2NO(a) +H(a)，Ea = 1.22 eV.

並列關鍵字

Surface ； DFT Calculation ； Reaction Mechanism ； C-N Coupling

參考文獻

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