Theoretical Study On The Electrooxidation Of Biomass-derived Alcohols Over Cobalt-based Catalysts

Driven by renewable energy,the conversion of biomass to high value-added chemicals is a clean and pollution-free synthesis method,which is conducive to optimizing energy structure and mitigating global warming.Cobalt-based catalysts are efficient catalysts for biomass electrooxidation.Understanding reaction mechanism and establishing structure-activity correlation are the prerequisites for the rational design of cobalt-based catalysts with high catalytic performance.Therefore,the electrooxidation of biomass-derived alcohols（glycerol,glucose）to formic acid（FA）involving C-C cleavage was used as the model reaction.Co OOH,transition metal（TM）-doped Co OOH（TM=Ti,Mn,Fe,Ni,Cu,Zn,Ru,Rh）,Co₃O₄,and main group metal Bi-doped Co₃O₄were used as catalysts.Based on density functional theory（DFT）,the key intermediates for the production of FA via C-C cleavage and the active metal-oxo species for oxidizing biomass-derived alcohols over above cobalt-based catalysts were systematically investigated,and thus the dominant reaction pathways were identified and the reaction mechanisms were revealed.Furthermore,the structure-activity correlations were established through analysis of the electronic structure of the various cobalt-based catalysts.The major contents and conclusions in this work are listed as follows:（1）For electrooxidation of glycerol to FA,Gibbs free energy diagrams of possible reaction pathways involving aldehyde or acid compounds as the key intermediates to produce FA via C-C cleavage were calculated.Based on these diagrams,the key intermediates for Co OOH and TM-doped Co OOH were determined as aldehyde compounds,while that for Co₃O₄and Bi-doped Co₃O₄were determined as acid compounds.（2）For electrooxidation of glycerol or glucose to FA,Gibbs free energy diagrams of reaction pathways involving various types of metal-oxo species over cobalt-based catalysts were calculated.Based on these diagrams,it is found that adsorbed hydroxyl（μ₁-OH）at different metal sites mainly act as the active metal-oxo species over the above catalysts,and the generation of these metal-oxo species is the potential-determining step（PDS）for corresponding dominant reaction pathway.The specific active metal-oxo species are as follows:For Co OOH containing Co³⁺,it is adsorbed hydroxyl at Co³⁺(i.e.,μ₁-OH-Co³⁺).For TM-doped Co OOH containing Co³⁺and TM³⁺,it is determined by the occupation of t_2gorbitals of TM³⁺（six-coordinate octahedral structure）.When the t_2gorbitals of TM³⁺are partially filled,it isμ₁-OH-TM³⁺;when the t_2gorbitals of TM³⁺are filled,it isμ₁-OH-Co³⁺.For Co₃O₄containing Co_Td²⁺and Co_Oh³⁺,it isμ₁-OH-Co_Td^2+δ.For Bi-doped Co₃O₄containing Co_Td²⁺,Co_Oh³⁺,and Bi³⁺,it isμ₁-OH-Co_Td^2+δadjacent to Bi³⁺.（3）By analyzing the electronic structures（band structure,density of states）of various cobalt-based catalysts,it is found that there are positive linear relationships between the charge-transfer energies（Δ）of these catalysts and Gibbs free energy barriers of PDS for corresponding dominant reaction pathways.The reducedΔcauses increased covalency of metal-oxygen bonds in catalysts,and thus the covalent bond between activeμ₁-OH with the metal site can be formed more easily,in other words,the Gibbs free energy barrier of PDS is reduced.In this work,the mechanisms of electrooxidation of glycerol and glucose to FA over various cobalt-based catalysts were revealed,and the structure-activity correlations were established.This work provides a theoretical basis for further synthesis of highly efficient cobalt-based catalysts in biomass electrorefining.