A heterogeneous model of a reverse-flow NOx-SCR (selective catalytic reduction) reactor was developed to demonstrate ammonia trapping and efficient nitric oxide reduction with side-stream ammonia introduction. The flue gas containing nitric oxide is cycled between the reactor ends while the ammonia flux is distributed among one or more side streams. Similar to energy trapping, a pseudo-steady state is achieved after several how reversals when the spatiotemporal ammonia adsorbate profiles become invariant, displaying regenerative mass exchange and mass trapping characteristics. The nitric oxide conversion occurs predominantly at the trailing edge of the adsorbate front. Analogous to energy trapping, more adsorbate is trapped at higher reversal frequencies. However, unlike energy trapping wherein increasing the power input increases temperature-peak heights, increasing the gas-phase ammonia concentration broadens the adsorbate profile due to the equilibrium limitations on ammonia adsorption. For stoichiometric amounts of ammonia introduced, sufficient adsorbate is trapped in the catalyst at a pseudo-steady state to convert nitric oxide for tens of minutes. Thus, transient increases in nitric oxide concentration can be easily accommodated with virtually complete nitric oxide conversion and little ammonia slippage from the reactor. Periodic flow reversal with side-stream feed addition is thus an attractive concept for maximizing the utilization of reactive species that adsorb strongly on catalysts.