Increased SCR performance of Cu-CHA due to ammonium nitrate buffer: Experiments with oscillating NO/NO2 ratios and application to real driving cycles

Currently, the literature on ammonium nitrate formation across SCR catalysts mainly sees ammonium nitrate formation as an undesired problem because it can inhibit the catalytic activity. This paper shows that, besides the inhibition effects, ammonium nitrate formation on the catalyst can also positively influence the catalytic activity because it acts as an NO2 buffer that stores NO2 under NO2-excess conditions and releases NO2 under NO2-deficient conditions. To demonstrate the working principle of the ammonium nitrate buffer, in a first step the ammonium nitrate loading is determined as a function of the NO2/NOx ratio in the feed, using a titration scheme recently proposed in the literature. Then, step tests are performed where at otherwise constant feed conditions, the NO2/NOx ratio is switched between 75 % NO2/NOx and 25 % NO2/NOx. During the slow frequency switching (3 min per phase), an increased NOx conversion is observed after both the transition to NO2-rich and NO2-lean. This can be explained by a build-up of ammonium nitrate during NO2-rich and the consumption of ammonium nitrate during NO2-lean conditions owing to the enhanced SCR reaction. When the frequency of the NO2/NOx switches is increased to 30 s, the ammonium nitrate storage loading can no longer follow the fluctuations and stays at an intermediate level. During this scenario, the catalyst shows a constant high conversion close to the Fast SCR reaction. Finally, it is demonstrated that the ammonium nitrate buffer contributes to the overall NOx conversion during standard driving cycles. In particular, the NO2 emissions are predicted significantly better by a model that accounts for the ammonium nitrate storage equilibrium.