Insights into the mechanism of ethanol formation from syngas on Cu and an expanded prediction of improved Cu-based catalyst

Abstract

Cu-based catalysts have been widely used and produced promising results for ethanol formation from syngas; understanding the underlying mechanism at the molecular level is the key to rational design of catalytically selective and inexpensive Cu-based catalysts. In this study, the thermochemistry and activation barriers for all possible elementary steps involved in ethanol formation from syngas on Cu catalyst have been systematically investigated. Then, on the basis of the results on Cu catalyst, an extended prediction for the selective modification of Cu-based catalyst has been proposed to improve catalytic performance toward ethanol formation from syngas. Our results show that an optimal route of ethanol formation on Cu catalyst starts with the first process of CO+H→CHO→CH2O→CH3O+H→CH3+OH to produce CH3; subsequently, CO insertion into CH3 leads to CH3CO, followed by successive hydrogenation to form ethanol. Meanwhile, an optimal route of methanol formation via CO→CHO→CH2O→CH3O→CH3OH is found to be favored compared with CH3 formation; thus, the productivity and selectivity of ethanol is low due to the preferable methanol formation rather than CH3. Interestingly, the hydrogenation, dissociation, and coupling of CH3 leading to CH4, CH2, and C2H6 are all very difficult to occur due to the high activation barrier in comparison with CO insertion into CH3, and this result suggests that starting from CH3, Cu catalyst is favorable for CO insertion into CH3 to CH3CO, and further hydrogenates to ethanol rather than CH2, CH4, and C2H6. The overall process of ethanol formation is controlled by CH3 formation and CO insertion into CH3, and the productivity and selectivity of ethanol is controlled by methanol and CH3 formations; therefore, to achieve high productivity and selectivity of ethanol, Cu has to get help from the promoters, which should be able to boost CH3 formation and/or decrease methanol formation. As a result, an expanded prediction strategy of fabricating an inexpensive Rh-decorated Cu catalyst is demonstrated to tune the relative activity of the key elementary steps that determine the productivity toward ethanol, and our results indicate that the promoter Rh can facilitate CH3 formation and CO insertion into CH3; moreover, it can effectively decrease methanol formation to achieve high productivity and selectivity for ethanol. The present study provides the basis to understand and develop novel Cu-based catalysts for ethanol formation from syngas.