Characterization of urea decomposition products in selective catalytic reduction catalyst systems

Urea-based selective catalytic reduction (SCR) systems are potentially a highly-effective means of NOx reduction for vehicles and engines with lean, or oxygen-rich, exhaust. However, use of urea-SCR technologies at exhaust temperatures < 250 °C presents unique technical challenges due to the formation of urea decomposition products. This paper reports on studies undertaken on the chemistry and properties of urea decomposition products in our laboratory for the past ten years. Analytical methods have been developed to address the separations challenges posed by these compounds. Evidence for the storage of urea decomposition compounds on zeolite-based SCRs is described; this storage may represent the real-world equivalent of ammonia storage on catalysts observed in bench studies. Additional data on the particulate emissions of urea decomposition products downstream of a diesel particulate filter (DPF) and SCR system are also presented.