Efficient and selective cleavage of C-aromatic-C linkages in lignin to enhance the productivity of monocyclic aromatic hydrocarbons is an extremely challenging and attractive task. Previously, we reported a Ru/NbOPO4 catalyst that realized the catalytic cleavage of both interunit C-C and C-O bonds in one-pot lignin conversions [Chem, 2019, 5, 1521-1536]. However, the roles of NbOx species in the selective C-aromatic-C cleavage are not clear and need to be extensively studied. Herein, we focus on the exploration of the catalytic mechanisms of C-aromatic-C bond cleavage over NbOx-supported Ru catalyst, by comparing with those of Ru/TiO2, Ru/ZrO2, and Ru/Al2O3 catalysts. It shows that the Ru/Nb2O5 catalyst exhibits high activity and remarkable selectivity to monocyclic hydrocarbons. A combined diffuse reflectance infrared Fourier Transform spectra (DRIFTS) and density functional theory (DFT) calculation analysis unambiguously confirmed that (i) NbOx not only can strongly adsorb phenylcyclohexane via benzene ring, but also selectively activate C-aromatic-C bond between benzene ring and cyclohexane; (ii) NbOx can promote the protonation of phenylcyclohexane with the assistance of H3PO4; and (iii) NbOx can significantly reduce the barrier of C-aromatic-C bonds cleavage of the carbocation intermediate (C6H5(H)C6H11+). A detail underlying mechanism has been established, which would have further implications to promote the design of new, high efficient catalysts. (C) 2021 Elsevier Inc. All rights reserved.