Lang Sandra M, Bernhardt Thorsten M, Barnett Robert N, Yoon Bokwon, Landman Uzi
Institute for Surface Chemistry and Catalysis, University of Ulm, Albert-Einstein-Allee 47, 89069 Ulm, Germany.
J Am Chem Soc. 2009 Jul 1;131(25):8939-51. doi: 10.1021/ja9022368.
Small gas-phase gold cluster cations are essentially inert toward molecular oxygen. Preadsorption of molecular hydrogen, however, is found to cooperatively activate the binding of O(2) to even-size Au(x)(+) (x = 2, 4, 6) clusters. Measured temperature- and reaction-time-dependent ion intensities, obtained by ion trap mass spectrometry, in conjunction with first-principles density-functional theory calculations, reveal promotion and activation of molecular oxygen by preadsorbed hydrogen. These processes lead to the formation of a hydroperoxo intermediate on Au(4)(+) and Au(6)(+) and culminate in the dissociation of O(2) via the release of H(2)O. Langmuir-Hinshelwood reaction mechanisms involving the coadsorption of both of the reactant molecules are discussed for both cluster sizes, and an alternative Eley-Rideal mechanism involving hydrogen molecules adsorbed on a Au(6)(+) cluster reacting with an impinging gaseous oxygen molecule is analyzed. Structural fluctionality of the gold hexamer cation, induced by the adsorption of hydrogen molecules, and resulting in structural isomerization from a ground-state triangular structure to an incomplete hexagonal one, is theoretically predicted. Bonding of H(2) on cationic gold clusters is shown to involve charge transfer to the clusters. This serves to promote the bonding of coadsorbed oxygen through occupation of the antibonding 2pi* orbitals, resulting in excess electronic charge accumulation on the adsorbed molecule and weakening of the O-O bond. The theoretical results for hydrogen saturation coverages and reaction characteristics between the coadsorbed hydrogen and oxygen molecules are found to agree with the experimental findings. The joint investigations provide insights regarding hydrogen and oxygen cooperative adsorption effects and consequent reaction mechanisms.
小尺寸气相金团簇阳离子对分子氧基本呈惰性。然而，发现预先吸附分子氢能协同激活O₂与偶数尺寸Au(x⁺)（x = 2、4、6）团簇的结合。通过离子阱质谱法获得的随温度和反应时间变化的离子强度测量值，结合第一性原理密度泛函理论计算，揭示了预先吸附的氢对分子氧的促进和激活作用。这些过程导致在Au(4⁺)和Au(6⁺)上形成氢过氧中间体，并最终通过释放H₂O使O₂解离。针对两种团簇尺寸，讨论了涉及两种反应物分子共吸附的朗缪尔 - 欣谢尔伍德反应机理，并分析了一种替代的埃利 - 里德机理，即吸附在Au(6⁺)团簇上的氢分子与撞击的气态氧分子反应。理论上预测了氢分子吸附引起的金六聚体阳离子的结构灵活性，导致其从基态三角形结构异构化为不完全六边形结构。结果表明，H₂在阳离子金团簇上的键合涉及向团簇的电荷转移。这通过占据反键2π*轨道促进了共吸附氧的键合，导致吸附分子上电子电荷积累过剩，O - O键减弱。发现氢饱和覆盖率以及共吸附氢与氧分子之间反应特性的理论结果与实验结果一致。联合研究提供了关于氢和氧协同吸附效应及相应反应机理的见解。
