Fine-Tuning of Ni/NiO over H-NbOx for Enhanced Eugenol Hydrogenation through Enhanced Oxygen Vacancies and Synergistic Participation of Active Sites

The hydrogenation of lignin-derived phenolics to produce valuable chemicals is a promising but challenging task. This study successfully demonstrates the use of sustainable transition metal-based catalysts to hydrogenate lignin-derived phenolics. A defect-induced oxygen vacancy containing H-NbO_x prepared from commercial Nb₂O₅ was employed as a catalyst. H-NbO_x exhibited higher oxygen vacancies (158.21 μmol/g) than commercial Nb₂O₅ (39.01 μmol/g), evaluated from O₂-TPD. Upon supporting 10 wt % Ni, a Ni/NiO interface was formed over H-NbO_x, which was intrinsically active for the hydrogenation of phenolics. 10Ni/H-NbO_x exhibited a two-fold increase in activity than 10Ni/Nb₂O₅, achieving >99% eugenol conversion and affording ∼94% 4-propyl cyclohexanol selectivity, wherein ∼63% eugenol conversion and ∼73% 4-propyl cyclohexanol selectivity were obtained over 10Ni/Nb₂O₅. The Ni/NiO formation was confirmed by X-ray photoelectron spectroscopy, HR-TEM, and H₂-TPR analysis, while the oxygen vacancies were verified by Raman spectroscopy and O₂-TPD analysis. The resulting interface enhanced the synergy between Ni and H-NbO_x and facilitated hydrogen dissociation, confirmed by H₂-TPD. Remarkably, 10Ni/H-NbO_x maintained its catalytic activity for five tested cycles and demonstrated exceptional activity with various phenolics. Our findings highlight the potential of a sustainable transition metal catalyst for the hydrogenation of lignin-derived phenolic compounds, which could pave the path to producing valuable chemicals in an environmentally friendly manner.