Understanding the synergistic catalysis of balanced Cu0-Cu+ sites and oxygen vacancies in Cu/ZrO2 catalysts for the efficient hydrogenation of furfural

Identifying the active sites and synergistic catalytic effect for Cu-based catalysts remains challenging due to the evolving structures of Cu⁰ and Cu⁺ species during the hydrogenation reaction process. In this study, Cu/t-ZrO₂ catalysts prepared by the oxalic acid complex precipitation method were explored for the hydrogenation of furfural to understand the catalytic functions of Cu⁰ and Cu⁺ during the reaction. The catalyst calcined at 300°C (Cu/t-ZrO₂-300) exhibited a superior catalytic efficiency in furfural hydrogenation to furfuryl alcohol. A 100 % yield of furfuryl alcohol together with a high TOF of 24.2 h⁻¹ was achieved at 120°C. Extensive characterization, including in-situ FTIR, in-situ XPS, EPR, and Raman, substantiated that the excellent catalytic performance of the catalysts was assigned to the synergistic catalysis Cu⁺, Cu⁰ and oxygen vacancies. Among them, Cu⁺ and oxygen vacancies in the catalyst were favorable to the adsorption and activation of the carbonyl group, where sites were more preferred to adsorb CO and formed a stable complex compared to Cu⁰ active sites. Furthermore, the DFT calculations verified that these abundant oxygen vacancies in the Cu-ZrO₂ interface induced a reverse of charge transfer from Zr to Cu atoms in the catalyst, resulting in a downshift of the d-band center for Cu, which was beneficial to the adsorption and activation of furfural and the desorption of active H*. Meanwhile, the results corroborated that more oxygen vacancies in the catalyst can regulate the adsorption configuration of furfural on the catalyst surface. This study elucidated the catalytic mechanism of complex active sites in Cu-based catalysts for furfural hydrogenation, and which will offer a valuable reference for the design of efficient catalysts for biomass platform conversion.