Jacques, J., Pauly, T., Zammit, M., Ahari, H. et al., "Robust SCR Design Against Environmental Impacts," SAE Technical Paper 2016-01-0954, 2016, doi:10.4271/2016-01-0954.
Significant reduction in Nitrogen Oxide (NOx) emissions will be required to meet LEV III Emissions Standards for Light Duty Diesel passenger vehicles (LDD). As such, Original Equipment Manufacturers (OEMs) are exploring all possible aftertreatment options to find the best balance between performance, robustness and cost. The primary technology adopted by OEMs in North America to achieve low NOx levels is Selective Catalytic Reduction (SCR) catalyst. The critical parameters needed for SCR to work properly are: an appropriate reductant such as ammonia (NH3) typically provided as urea, adequate operating temperatures, and optimum Nitrogen Dioxide (NO2) to NOx ratios (NO2/NOx). The NO2/NOx ratio is mostly influenced by Precious Group Metals (PGM) containing catalysts located upstream of the SCR catalyst. Different versions of zeolite based SCR technologies are available on the market today and these vary in their active metal type (iron, copper, vanadium), and/or zeolite type. To select an appropriate SCR type, the application’s operating conditions as well as environmental factors must be considered. To fundamentally understand these differences, a study was conducted where various SCR catalysts are evaluated in a laboratory environment in regards to, 1) PGM contamination 2) operating temperature and resulting thermal aging, 3) NO2 levels from the system architecture, 4) the extent of reductant usage and overexposure, and 5) the impact of oxygen concentrations during the aging of catalysts. In this study, various types of copper SCR catalysts are evaluated using several unique and standard testing methods to expose them to conditions simulating lifetime exposure. Some non-ideal or “worst case scenarios” were explored regarding Diesel Emissions Fluid (DEF) usage, dosing quality and thermal exposure. Results highlight the advantages and disadvantages of various SCR and zeolite types available in the marketplace.