Direct Comparison of Urea SCR and NH3 SCR Activities Over Acidic Oxide and Exchanged Zeolite Prototype Powdered Catalysts
In order to develop new NOx selective catalytic reduction (SCR) catalysts for automotive application, the DeNOx catalytic activity is commonly evaluate at the laboratory scale using NH3 as reductant. However, NH3 is not directly used on board: an ammonia precursor based on urea aqueous solution is injected in the exhaust pipe upstream the SCR catalyst. It is admitted that ammonia is then obtained by two successive reactions: the thermal decomposition of urea, leading to HNCO and NH3, and the HNCO hydrolysis, providing the second molecule of NH3. However, the complete availability of ammonia from urea could be not achieved before the SCR catalyst. Then, the influence of the SCR catalyst on these reactions may impact the NOx reduction efficiency. With the aim to study the possible role of the SCR catalyst on the ammonia availability, an innovative synthetic gas bench adjusted to powdered material was developed, allowing the direct comparison of the use of gaseous NH3 or urea (injected aqueous solution) for the NOx conversion, depending on the temperature (200 500 degC) This work presents results obtained with an oxide based prototype SCR catalyst in comparison with a patented Fe exchanged zeolite, evaluated in both standard and fast SCR stoichiometry. This study points out that, in contrast with the exchanged zeolite, the evaluated oxide based catalyst may not allow an optimal NOx conversion because of a lack in ammonia availability, attributed to insufficient activity in HNCO hydrolysis.
Topics in Catalysis