Insights into the mechanism of carboxylic acid hydrogenation into alcohols at the MnO/Cu (111) interface: a combined DFT and kinetic study

Abstract

Hydrodeoxygenation (HDO) process of carboxylic acids (CAs) is a promising way to utilize biomass resources and refine high-grade fuels. Mn addition could improve the performance of Cu-based catalysts, but there is no clear understanding at the atomic level of this process. In this study, a MnO/Cu interface model was built to reveal the specific role of the Mn promoter and the reaction mechanism based on combined DFT and kinetic studies with acetic acid as the model reactant. The results showed that the acetic acid was transformed into acetaldehyde following a nucleophilic addition–elimination mechanism first and then continued generating ethanol through a proton-transfer process with the help of the interfacial hydroxyls formed under the reducible reaction atmosphere. Though the interfacial sites were set as low (less than 1%), the activity of the MnO/Cu surface was still enhanced by about an order of magnitude compared to monometallic copper based on the microkinetics results. In addition, the selectivity to ethanol was about 92%, which well matched the experimental results, implying the improved catalytic performance in terms of both the activity and selectivity for the HDO process of acetic acid. According to energetic and charge analysis, the generation of key CH₃CHOOH* species was promoted due to the strong interaction between the interfacial Mn–O and O–H atomic pair in the transition state. The reaction mechanism and the role of the Mn promoter toward Cu-based catalysts were thus revealed to give a deeper understanding of the metal/oxide interface and to contribute to the development of more efficient catalysts for similar HDO processes of carboxylic acids and their derivatives.