Sulfur tolerance of SCR catalysts : Development of a lab testing method

NOx emissions from diesel vehicles are a currently well-discussed matter due to the related health and environmental issues. At the same time, diesel vehicles are an important part of transportation in society as diesel engines are more fuel efficient than gasoline engines. NOx emissions are regulated by European standards, this put demands on a well-functioning exhaust gas aftertreatment system that lower the NOx emissions for the whole lifetime of the vehicle. One of the most effective solutions is by reduction of NOx with ammonia over a SCR catalyst. Cu/SSZ-13 zeolite has shown to be a promising SCR catalyst. SCR catalysts are however deactivated by several different mechanisms, where sulfur poisoning is a significant mechanism. There are many different factors that will influence how sulfur impacts the catalyst. Different catalyst materials are more or less sulfur tolerant. The form of sulfur is important as well as both temperature and availability of ammonia. It is thus important to investigate how sulfur affects the SCR catalyst. The aim of this master’s thesis project was to develop a method for measuring sulfur tolerance of different SCR catalysts and to gain understanding of sulfur poisoning of Cu/SSZ-13 catalysts. A literature study on sulfur poisoning of SCR catalysts with a focus on Cu/SSZ-13 was first carried out. Followed by an experimental part in a rig at Scania CV AB in Södertälje, where the method was developed. A method that can be used for testing SCR catalysts on sulfur tolerance after SO2 exposure was successfully achieved. The experimental procedure was designed to consist of eight steps, including de-greening, sulfur poisoning, regeneration at two temperatures and catalyst activity test after each step. Clear differences on NOx conversion between fresh, sulfated and regenerated catalysts are observed when using the method. Three different sulfation temperatures were evaluated where the lower sulfation temperatures, 220 ⁰C and 280 ⁰C, caused a larger impact on NOx conversion compared to sulfation at 350 ⁰C. Two different catalysts were compared on their NOx conversion over fresh, sulfated and regenerated catalysts. The method was correlated to engine aging with respect to sulfur exposure, sulfur capture, gas and temperature conditions and activity loss. The method corresponds to approximately 430 000 km driving based on sulfur exposure over the catalyst. The lab method is best comparable to a SCR catalyst located first in the exhaust gas aftertreatment system in a vehicle, based on two conclusions. (1) Poisoning is performed with only SO2. (2) Standard SCR conditions occur in the activity test. The method should be further developed by enabling testing of NOx conversion at fast and slow SCR conditions. More work should be carried out on the correlation to vehicle aging by comparision with engine aged catalysts