Direct Addition Mechanism during the Catalytic Hydrogenation of Olefins over Platinum Surfaces

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• 作者：Yujung Dong ; Maryam Ebrahimi ; Aashani Tillekaratne ; Francisco Zaera
• 刊名：The Journal of Physical Chemistry Letters
• 出版年：2016
• 出版时间：July 7, 2016
• 年：2016
• 卷：7
• 期：13
• 页码：2439-2443
• 全文大小：335K
• 年卷期：0
• ISSN：1948-7185

文摘

The mechanism of the hydrogenation of olefins catalyzed by metal surfaces was probed by using isotope labeling in conjunction with a high-flux effusive molecular beam setup capable of sustaining steady-state conversion under well-controlled ultrahigh vacuum (UHV). The unique conditions afforded by this instrument, namely, a single collision regime and impinging frequencies equivalent to pressures in the mTorr range, led to the clear identification of two competing pathways: a multiple H–D isotope exchange channel explained by the well-known Horiuti–Polanyi mechanism but with an unusually high probability for β-hydride elimination from the alkyl surface intermediate (versus its reductive elimination to the alkane), and a direct addition route that produces dideuterated alkanes selectively. The latter may follow an Eley–Rideal mechanism involving an adsorbate (either the olefin or the hydrogen/deuterium atoms resulting from dissociative adsorption of H₂/D₂) and a gas-phase molecule (the other reactant), or, alternatively, it could reflect the limited diffusion of the hydrogen atoms on the surface under catalytic conditions because of site blocking by the islands of strongly bonded carbonaceous (alkylidyne) layers present during catalysis. Regardless, our data clearly show that the distribution of alkane isotopologues obtained from the conversion of olefins with deuterium can deviate significantly from statistical expectations.