Improved processes for activating a catalyst system used for the reduction of nitrogen oxides are provided. In one embodiment, the catalyst system is activated by passing an activation gas stream having an amount of each of oxygen, water vapor, nitrogen oxides, and hydrogen over the catalyst system and increasing a temperature of the catalyst system to a temperature of at least 180° C. at a heating rate of from 1-20°/min. Use of activation processes described herein leads to a catalyst system with superior NOx reduction capabilities.