Novel method of ammonium nitrate quantification in SCR catalysts

Abstract

NH4NO3 or Ammonium nitrate (AN) can be formed onto NH3-SCR catalysts in the low-T range typical of urban or low-load operation, resulting in self-inhibition of the catalysts. Also, subsequent decomposition of AN during high-temperature transients may lead to the undesired formation of N2O, a potent greenhouse gas. Quantification of AN formation and decomposition has been not adequately addressed so far due to experimental difficulties.We propose an original method for measuring the amount of AN formed on SCR catalysts, and validate it in bench-flow runs over a commercial Cu-CHA formulation. In order to quantify the AN on the catalyst, we rely on its reaction with NO according to NO+NH4NO3→NO2+N2+2H2O, further investigated via DRIFTs in this work. Based on this stoichiometry, the amount of deposited AN can be estimated from the integral molecular balances of reliably and precisely measurable species, such as NO and NO2. This approach is initially applied over catalyst powder in a rig where N-balances could be evaluated directly, and subsequently validated over more realistic coated monolith catalyst samples in a bench-flow reactor in which N2 was used as carrier gas. In the validation tests, known amounts of AN were initially deposited on the catalyst by controlled pre-dosing of an AN aqueous solution: such amounts were found in satisfactory agreement with the estimates obtained by our method. We expect the proposed methodology for AN quantification to be generally applicable to other types of SCR catalysts as well, including V-based systems and Fe-zeolites.