Controllable Synthesis of a Porous Flower-like Ni-NiO Heterojunction Catalyst for the Hydrogenation of Maleic Anhydride: Synergistic Effects of Metal and Oxygen Vacancies

A series of Ni-NiO heterojunction catalysts containing many defects are synthesized by finely tuning the reduction temperature of NiO, which is produced via calcination of Ni(OH)2 with a porous flower-like structure. The catalytic evaluation demonstrates that the Ni-NiO/450 catalyst (after reduction at 450 °C) exhibited the best catalytic performance which can achieve the efficient hydrogenation of maleic anhydride (MA) to γ-butyrolactone (GBL) under mild reaction conditions. The structure–activity relationship of the catalyst was systematically investigated through comprehensive experimental and characterization studies. The results demonstrated that the formation of Ni-NiO heterojunctions and the generation of oxygen vacancies are key factors that significantly enhance the catalytic performance. A Ni-NiO heterojunction containing oxygen vacancy model (denoted as Ni-NiO/OV) was constructed via density functional theory (DFT) theory to investigate the specific reaction pathways for the hydrogenation of maleic anhydride (MA) to γ-butyrolactone (GBL). Compared with the pure Ni(111) surface, the transition state energy barriers on the Ni-NiO/OV surface were significantly reduced. DFT results revealed that the Ni-NiO/OV surface led to both reactants and intermediates preferentially adopting a parallel adsorption configuration in the vacancy regions, while the adsorption and dissociation of H2 predominantly occurred at the Ni sites. Along with the DFT results, the formation of the Ni-NiO heterojunction enhanced the synergistic catalytic effect between Ni and oxygen vacancies. Additionally, the catalysts were also suitable for the efficient hydrogenation of various unsaturated compounds, which is theoretically significant for designing and developing future high-efficiency Ni-based catalysts.