Dong Yujung, Ebrahimi Maryam, Tillekaratne Aashani, Zaera Francisco
Department of Chemistry and UCR Center for Catalysis, University of California , Riverside, California 92521, United States.
J Phys Chem Lett. 2016 Jul 7;7(13):2439-43. doi: 10.1021/acs.jpclett.6b01103. Epub 2016 Jun 16.
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.
通过使用同位素标记结合高通量喷射分子束装置，在可控的超高真空（UHV）条件下维持稳态转化，对金属表面催化烯烃氢化的机理进行了探究。该仪器提供的独特条件，即单碰撞模式和与毫托范围内压力相当的撞击频率，使得能够明确识别出两条竞争途径：一条是由著名的堀内-波兰尼机理解释的多重H-D同位素交换通道，但从烷基表面中间体进行β-氢化物消除（相对于其还原消除生成烷烃）的概率异常高；另一条是直接加成途径，选择性地生成双氘代烷烃。后者可能遵循埃利-里德机理，涉及一种吸附质（烯烃或由H2/D2解离吸附产生的氢/氘原子）和一个气相分子（另一种反应物），或者，它可能反映了在催化条件下，由于催化过程中存在的强键合碳质（亚烷基）层岛对位点的阻塞，氢原子在表面的扩散受限。无论如何，我们的数据清楚地表明，由烯烃与氘转化得到的烷烃同位素异构体的分布可能会显著偏离统计预期。
