The activation mechanism of NH3 in the selective catalytic reduction of NO by NH3 on a V2O7H4 cluster was investigated using a complete active space self-consistent field method. Because of the easy bond dissociation of NH4+ adsorbed on Brønsted acid sites of the V2O5 configuration, the radical species NH3+ can occur with an activation energy of only 26.7 kcal/mol. The highly active intermediate NH3+ is stabilized by forming a very strong hydrogen bond of approximately 29.2 kcal/mol to the vanadyl oxygen. This stabilization mechanism is very similar to the low-barrier hydrogen bond in the transition state, or in an unstable intermediate state, which has been reported for some enzymatic reactions.