NH3 pulsing adsorption and SCR reactions over a Cu-CHA SCR catalyst

In passive selective catalytic reduction (SCR) applications, NH3 is generated on an upstream catalyst to reduce NOx on a downstream SCR catalyst. Therefore, there is no continuous input of reductant, and in most cases the NH3 comes to the SCR catalyst as a pulse. In this paper NH3 adsorption profiles on a Cu chabazite (CHA) SCR catalyst were measured as a function of different NH3 pulse parameters, NH3 migration after adsorption was investigated and standard and fast SCR reactions on the catalyst pre-covered with different levels of NH3 were characterized. The data show that when pulsing times are short, but the upstream portion of the catalyst was still far from saturated, the amount of NH3 adsorbed decreased from inlet to outlet. As the quantity of input ammonia increased, the adsorption profiles leveled out in the front as that part of catalyst approached saturation. Also, the NH3 inlet concentration had a significant effect on the NH3 adsorption profile, when the catalyst was far from saturated. For the same amount of input NH3, a high NH3 inlet concentration with shorter pulsing time would result in steeper NH3 adsorption profiles. Catalyst aging and higher temperature reduced the catalyst's ability to adsorb NH3. Ammonia was observed migrating after NH3 was removed from the inflow. There were at least two kinds of adsorbed NH3: weakly adsorbed NH3 and strongly adsorbed NH3. Weakly adsorbed NH3 migrated out of the catalyst channel within several seconds, while strongly adsorbed NH3 did not, at least at 250 °C and 300 °C. Catalyst aging had little effect on strongly adsorbed NH3, but reduced the catalyst's ability in adsorbing weakly adsorbed NH3 and enhanced weakly adsorbed NH3 migration. The effect of temperature on NH3 migration was as expected; higher temperature enhanced NH3 migration but had less effect on strongly adsorbed NH3. NH3 adsorption profiles were compared with NOx consumption profiles during the fast SCR reaction. NOx consumption profiles were found to be flatter than their corresponding NH3 adsorption profiles due to the effect of NH3 migration after an NH3 pulse. The standard and fast SCR reactions on the SCR catalyst pre-covered with NH3 of similar distribution were compared at 250 °C and it was found that there was no significant difference between NO and NOx consumption profiles.

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► NH3 adsorption profiles were dependent on both exposure time and inlet NH3 concentration.
► NH3 migration as a function of time was evident.
► Fast and standard SCR reactions were identical on NH3 pre-covered catalysts.