Mechanism of Ethanol Synthesis from Syngas on Rh(111)
Mechanism of Ethanol Synthesis from Syngas on Rh(111)
复制标题
DOI：
10.1021/ja903013x
复制
发表时间：
2009-09-16
影响因子：
15
通讯作者：
Liu, Ping
中科院分区：
文献类型：
--
作者：
Choi, YongMan;Liu, Ping
铑基催化剂在催化合成气合成乙醇（2CO +4 H（2）-> C2 H5 OH + H2O）中表现出独特的效率和选择性。从分子水平上理解反应机理是合理设计乙醇合成催化剂的关键，也是乙醇能源化面临的主要挑战之一。本工作采用密度泛函理论（DFT）对Rh（111）上乙醇的合成进行了研究。结果表明，Rh（111）催化剂上乙醇的合成是从CO加氢生成甲酰基开始的，随后是加氢反应和CO插入反应。该过程涉及三种主要产物：甲烷、甲醇和乙醇，其中乙醇产率低，并且Rh（111）对甲烷而不是乙醇或甲醇具有高度选择性。反应的限速步骤是CO加氢生成甲酰基，而生成乙醇的选择性受甲烷生成和甲基与CO之间C-C键的形成控制，Rh-CO间强相互作用阻碍了CO加氢反应，从而减慢了反应的速度;然而，其对甲基、氧和乙酰基物质的高亲和力确实有助于甲氧基物质的C-O键断裂，并因此有助于通过CO插入的直接乙醇合成。我们的研究结果表明，为了实现高的生产率和乙醇的选择性，Rh必须得到促进剂的帮助，这应该能够抑制甲烷的形成和/或促进C-C键的形成。本研究为认识和开发新型铑基乙醇合成催化剂奠定了基础。
Rh-based catalysts display unique efficiency and selectivity in catalyzing ethanol synthesis from syngas (2CO + 4H(2) -> C2H5OH + H2O). Understanding the reaction mechanism at the molecular level is the key to rational design of better catalysts for ethanol synthesis, which is one of major challenges for ethanol application in energy. In this work, extensive calculations based on density functional theory (DFT) were carried out to investigate the complex ethanol synthesis on Rh(111). Our results show that ethanol synthesis on Rh(111) starts with formyl formation from CO hydrogenation, followed by subsequent hydrogenation reactions and CO insertion. Three major products are involved in this process: methane; methanol, and ethanol, where the ethanol productivity is low and Rh(111) is highly selective to methane rather than ethanol or methanol. The rate-limiting step of the overall conversion is the hydrogenation of CO to formyl species, while the selectivity to ethanol is controlled by methane formation and C-C bond formation between methyl species and CO. The strong Rh-CO interaction impedes the CO hydrogenation and therefore slows down the overall reaction; however, its high affinity to methyl, oxygen, and acetyl species indeed helps the C-O bond breaking of methoxy species and therefore the direct ethanol synthesis via CO insertion. Our results show that to achieve high productivity and selectivity for ethanol, Rh has to get help from the promoters, which should be able to suppress methane formation and/or boost C-C bond formation. The present study provides the basis to understand and develop novel Rh-based catalysts for ethanol synthesis.