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• Oxidative activation of NO to a mobile nitrite-precursor over VOx/CeO 2 is proposed. • A mechanism for the reduction half-cycle of low-temperature SCR is established. • Synergy of VO x and CeO 2 surface is revealed in electronic and kinetic manners. • A combined experimental and theoretical method is reported for SCR mechanism study. 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/CeO 2 , 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/Al 2 O 3 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 Ce4+−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. [ABSTRACT FROM AUTHOR]