以理論計算方式研究以下反應機構： I.水煤氣轉換反應在M/TiO2(110) (M=Pt, Au, Cu)表面之反應路徑探討 II.硫化氫在金屬表面W(111)的分解與氧化

Theoretical Studies of the Following Reaction Mechanisms: I.The Water-Gas Shift Reaction on M/TiO2(110) (M= Pt, Au, Cu) Surfaces II. The H2S Dissociation and Sulfur Oxidation on W(111) Surface

彭士峰(Shih-Feng Peng)

指導教授 ： 何嘉仁

國立臺灣師範大學/理學院/化學系/博士(2011年)

摘要

我們使用週期性密度泛函理論來研究燃料電池系統中可能的催化反應，包括水煤氣轉換反應和硫化氫的分解與硫原子的氧化反應。在水煤氣轉換反應中，我們研究了在pure-TiO2(110)和M/TiO2(110)(M = Pt, Au, Cu8)表面上可能的反應途徑，其中以Cu8/TiO2(110)表面的反應活性最好，我們另外也計算了在Pt/TiO2(110)表面上的分子動態模擬反應。在Cu8/TiO2(110)表面上，我們計算了carboxyl和redox兩種反應機制的反應位能曲面，結果顯示在Cu8/TiO2(110)表面較容易進行redox反應路徑。而CO氧化反應的能障，從26 kcal/mol (pure-TiO2(110)) 降低到5.38 kcal/mol (經Cu cluster協助)；另外，H2O的分解反應也從12.08 kcal/mol (pure TiO2(110)) 降低到 8.35 kcal/mol (經O-vacancy協助)。然而在Pt/TiO2(110)的表面上，因Pt和CO分子的吸附能太大(約44 kcal/mol)，導致過度穩定中間態存在，發生毒化現象，不利於水煤氣轉換反應的進行。我們也計算了H2S在W(111)表面的分解反應，斷去其中兩個H-S鍵各需要跨越能障5.06 kcal/mol和15.68 kcal/mol，最終形成表面S原子和氣態H2分子，整個反應放熱51.80 kcal/mol。該S原子可繼續通入氧氣去除，形成SO和SO2的反應能障分別為14.44 kcal/mol和34.46 kcal/mol，最終形成氣態SO2分子放熱12.52 kcal/mol。

關鍵字

DFT計算 ； WGSR ； TiO2(110) ； metal cluster ； W(111) ； H2S

並列摘要

The mechanism of the water-gas shift reaction, involving the adsorption of CO followed by desorption of CO2 and dehydrogenation of H2O, on pure-TiO2(110) and M/TiO2(110)(M= Pt, Au, Cu8) surfaces was investigated with periodic density functional theory. The result of MD (molecular-dynamic simulation) of H2O decomposition on Pt/TiO2 surface (with O-vacancy) at 300 K was also presented. The Cu/TiO2- (110) surface displays an extremely high catalytic activity toward water- gas shift reaction, in which Cu was considered to be the most active metal on TiO2(110) supported surface. We reported the possible potential energy surfaces for the carboxyl and redox mechanisms of WGSR on the interface between the Cu cluster and TiO2 support. Our results show that the redox mechanism would be the dominant path, in which the resided Cu cluster greatly reduces the barrier of CO oxidation process (5.38, and 26 kcal/mol, with and without Cu cluster, respectively). When the adsorbed CO was catalytically oxidized by the bridging oxygen of Cu/TiO2(110) surface to form CO2, the release of CO2 from the surface would result in the formation of an O-vacancy on the surface to facilitate the followed water splitting reaction (barrier 8.35 vs. 12.08 kcal/mol, with and without the aid of surface vacancy). Density functional theory calculations were also employed to investigate the dissociative adsorption of molecular H2S on a W(111) surface. The energy minimum of the adsorbed H2S was identified to bind preferentially at the top site. The adsorption sites of other S moieties (SH and S) were also examined, and they were found predominately at the bridge sites between first and second layers and the bridge sites between second and third layers, respectively. The binding of H2S and its S-containing species is stronger on the W(111) surface than on other metal surfaces, such as Pd, Ni, Cu, Au, Ag, and Ir. The elementary reactions of successive abstraction of H from H2S on the surface were examined. We also extend our study to the oxidation reaction of the adsorbed S by adding gaseous oxygen to the surface, which will react with S and eventually form SO2 and then desorb from the surface. Our results show that the above H2S dissociation and sulfur oxidation reactions do not bear high energy barriers and the overall reactions are exothermic on the W(111) surface.

並列關鍵字

DFT calculaton ； WGSR ； TiO2(110) ； metal cluster ； W(111) ； H2S

參考文獻

1. Milewski, J.; Miller, A.; Sałacinski, J. Int. J. Hydrogen Energy 2007, 32, 687.

3. Biographical Memoirs of Fellows of the Royal Society, vol. 39 (Feb. 1994), pp. 3-18.

7. Ralph, T. R.; Hogarth, M. P. Platinum Met. Rev. 2002, 46, 117

8. (a) Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; VCH: New York, 1997.; (b) Burch, R. Phys. Chem. Chem. Phys. 2006, 8, 5483.

Perez, M.; Evans, J. Catal. Today 2009, 143, 45.