Computational Study On The Coupling Of Carbon Dioxide And Methane To Acetic Acid Over Transition Metals Modified ZnO Catalysts

Direct transformation of CH₄ and CO₂ to value-added products not only brings in economic benefit but also contributes to environment improvement.Nevertheless,this type of reactions is practically challenging.In this work,we conducted a computational study of transition metal doped ZnO catalysts（M/ZnO,M=Fe,Co,Ni,Cu,Pd）for acetic acid synthesis from simultaneous conversion of CH₄ and CO₂ using density functional theory（DFT）based calcualtion approach.The Fe-and Co-doped ZnO surfaces were found to be highly active towards CH₄ activation/dissociation with activation barriers less than 0.4 eV.The C-C coupling occurred via a Langmuir-Hinshelwood mechanism in which the activated CO₂ inserted into theσ-bond of M-CH₃*group to form an acetate species.The location and stability of the dissociated H*had an appreciable impact on the C-C coupling kinetics and thus a H migration step was necessary prior to the C-C coupling on Fe/ZnO.C-C coupling barriers were lower on Fe-and Co-doped ZnO than Ni-,Cu-and Pd-doped surfaces.Another H migration from the metal site to surface O site can significantly promote acetic acid formation which was found to be barrierless and mildly endothermic over M/ZnO surfaces.The formed acetic acid adsorbed strongly over the active sites of Fe-,Co-,Ni-doped ZnO and thus the final desorption step was likely kinetically more relevant on these surfaces.Through systematic studies of reaction pathways and energy diagrams over M/ZnO surfaces,the Fe/ZnO catalyst was identified as a promising candidate that showing outstanding bimetallic synergetic effect in promoting CH₄activation/dissociation,surface H migration,as well as C-C coupling steps involved in acetic acid production.By studying the formation path of formic acid on the Fe/ZnO surface,it was found that Fe/ZnO catalyst supressed the side reaction of formic acid formation to a great extent.In-situ diffuse infrared Fourier transform spectroscopy（DRIFTS）experiments were used to characterize the main surface species of acetate on the surface of Fe/ZnO,which proved the rationality of the DFT calculations.The present study demonstrated the importance of DFT calculations to reveal the mechanisms,electronic features,energy pathwaysand kinetic-controlling steps for acetic acid synthesis directly from CH₄ and CO₂.The computational results are useful for optimizing and designing high-efficient catalysts.