DeNOx activity–TPD correlations of NH3-SCR catalysts

A series of Mn-exchanged zeolite catalysts was tested for its DeNOx activity in ammonia selective catalytic reduction (NH3-SCR) reactions. Activity was found to depend strongly on metal loading such that at low Mn content, NOx conversion is limited to 50% at 700K. However, above a mass percentage of Mn between 1.6 and 1.9%, near complete NOx conversion is achieved at 525K. Ammonia – temperature programmed desorption (NH3-TPD) was used in an attempt to explain the findings of these experiments. Specifically, we have used NH3-TPD to determine the population and strength of the acid sites present in the catalysts as a function of Mn content. Acid sites with adsorption energies ranging from 45 to 68.7kJ/mole have been detected. At temperatures where DeNOx activity differs significantly among the catalysts, corresponding differences in their acid site distributions, and hence surface NH3 populations, are observed.