Elucidation of redox mechanism is vital to develop highly active catalysts for selective catalytic reduction (SCR) of NOx. Here, we apply an integrated experimental and theoretical approach to investigate low-temperature SCR (LT-SCR) mechanism over VOx/CeO2, a model system containing industrially-relevant vanadia active-species and an eye-catching redox support, i.e. ceria, frequently documented in SCR studies. We show that NO oxidative activation to a gaseous nitrite-precursor intermediate, which was trapped by BaO/Al2O3 and further spectroscopically and computationally validated, serves as a key step in LT-SCR. This scheme involves paired contributions from vanadia and ceria, in which vanadium stabilizes at +5 while Ce3+/Ce4+ varies in the redox cascade, and coupling of V5+-OH and proximal Ce-4(+)-O reduces the NO oxidative activation barrier. These findings progress the understanding of LT-SCR mechanism and deliver a specific perspective on the synergy of surface active-sites and supports, which are essential for the design of further improved SCR catalysts. (C) 2020 Elsevier Inc. All rights reserved.