Department of Chemistry, Northwestern University , Evanston, Illinois 60208, United States.
J Am Chem Soc. 2017 Dec 6;139(48):17582-17589. doi: 10.1021/jacs.7b09914. Epub 2017 Nov 21.
We present a combined theoretical and experimental study of CO hydrogenation on a Ni(110) surface, including studies of the role of gas-phase atomic hydrogen, surface hydrogen, and subsurface hydrogen reacting with adsorbed CO. Reaction mechanisms leading both to methane and methanol are considered. In the reaction involving surface or subsurface hydrogen, we investigate four possible pathways, using density functional theory to characterize the relative energetics of each intermediate, including the importance of further hydrogenation versus C-O bond breaking, where the latter may lead to methane production. The most energetically favorable outcome is the production of methanol along a pathway involving the sequential hydrogenation of CO to a HCO* intermediate, followed by a final hydrogenation to give methanol. In addition, we find that subsurface hydrogen noticeably alters reaction barriers, both passively and through the energy released by diffusion to the surface. Indeed, the effective reaction barriers are even lower than for CO methanolation on Cu(211) and Cu(111) than for Ni(110). In studies of gas-phase H atoms impinging on a CO-adsorbed Ni(110) surface, Born-Oppenheimer molecular dynamics simulations show that direct impact of H is unlikely to result in hydrogenation of CO. This means that Eley-Rideal or hot-atom mechanisms are not important; thus, thermal reactions involving subsurface hydrogen are the primary reaction mechanisms leading to methanol. Finally, we demonstrate experimentally for the first time the production of methanol and formaldehyde from CO hydrogenation on Ni(110) and confirm the role of subsurface hydrogen in the mechanism of this reaction.
我们提出了一个关于 Ni(110)表面上 CO 加氢的理论和实验综合研究，包括研究气相原子氢、表面氢和亚表面氢与吸附 CO 反应的作用。考虑了导致甲烷和甲醇生成的反应机制。在涉及表面或亚表面氢的反应中，我们使用密度泛函理论来研究了四个可能的途径，以描述每个中间体的相对能量，包括进一步加氢与 C-O 键断裂的重要性，后者可能导致甲烷的生成。最有利的结果是沿着涉及 CO 依次加氢到 HCO*中间体，然后最后加氢生成甲醇的途径生成甲醇。此外，我们发现亚表面氢明显改变了反应势垒，无论是通过扩散到表面释放能量的被动方式，还是通过扩散到表面释放能量的主动方式。事实上，有效反应势垒甚至比 Cu(211)和 Cu(111)上的 CO 甲醇化反应的 Ni(110)还要低。在研究气相 H 原子撞击 CO 吸附的 Ni(110)表面的实验中，Born-Oppenheimer 分子动力学模拟表明，H 的直接冲击不太可能导致 CO 的加氢。这意味着 Eley-Rideal 或热原子机制不重要；因此，涉及亚表面氢的热反应是导致甲醇生成的主要反应机制。最后，我们首次在 Ni(110)上通过 CO 加氢实验证明了甲醇和甲醛的生成，并证实了亚表面氢在该反应机制中的作用。
