CuCo Alloying-Induced Electronic Structure Modulation and Its Impact on Lignin β-O-4 Bond Cleavage Performance

Co-based bimetallic alloys have attracted increasing attention because of their oxidative splitting β-O-4 bond in lignin, but the understanding of the roles Co remains lacking. This study designed and synthesized a CuCo/C bimetallic alloy catalyst, elucidated the key intrinsic mechanism of d-orbital electron re-excitation in enhancing cobalt-site catalytic activity through electronic structure analysis, and revealed its underlying mechanism. Cu species-induced electron rearrangement in the d-orbitals of the Co species causes a change in the Co spin state to an intermediate spin state. The alteration of the Co spin state increases the bonding energy between the Co metal and oxygen atoms, which is primarily responsible for the good catalytic performance of the bimetallic CuCo/C alloy catalysts. Specifically, in the β-O-4 bond oxidative cleavage experiment of 2-phenoxy-1-phenylethanol (lignin model compound), the yield of phenol for the Co/C catalyst was 39.72%, while the CuCo/C catalyst achieved a higher yield of 49.20%. To further validate this finding, several transition metals MCo/C (M = Mn, Ni, Pd, Ru) were prepared and used in the model reaction. More interesting, the catalyst has a positive effect only when Co is maintained at the intermediate spin state by introducing Cu. This phenomenon is attributed to the fact that in the intermediate spin state of Co(t_2g⁵e_g¹), its 3d orbitals hybridize appropriately with the O 2p orbitals to accelerate breaking the β-O-4 bond. Additionally, the bimetallic CuCo/C alloy catalysts exhibited good stability during the reaction. Furthermore, present CuCo/C alloy catalysts could be applied to the highly catalytic performance in the nature lignin.