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Although the urea-SCR technology exhibits high NO reduction efficiency over a wide range of temperatures among the lean NO reduction technologies, further improvement in low-temperature performance is required to meet the future emission standards and to lower the system cost. In order to improve the catalyst technologies and optimize the system performance, it is critical to understand the reaction mechanisms and catalyst behaviors with respect to operating conditions. Urea-SCR catalysts exhibit poor NO reduction performance at low-temperature operating conditions (T ≺ 150°C). We postulate that the poor performance is either due to NH₃ storage inhibition by species like hydrocarbons or due to competitive adsorption between NH₃ and other adsorbates such as H₂O and hydrocarbons in the exhaust stream. In this paper we attempt to develop one-dimensional models to characterize inhibition and competitive adsorption in Fe-zeolite-based urea-SCR catalysts based on bench reactor experiments.

Urea-selective catalytic reduction (SCR) catalysts are the leading aftertreatment technology for diesel engines, but there are major challenges associated with meeting future NOx emission standards, especially under transient drive cycle conditions that include large swings in exhaust temperatures. Here we present a simplified, transient, one-dimensional integral model of NOx reduction by NH₃ on a commercial small-pore Cu-zeolite urea-SCR catalyst for which detailed kinetic parameters have not been published. The model was developed and validated using data acquired from bench reactor experiments on a monolith core, following a transient SCR reactor protocol. The protocol incorporates NH₃ storage, NH₃ oxidation, NO oxidation and three global SCR reactions under isothermal conditions, at three space velocities and at three NH₃/NOx ratios.

The selective catalytic reduction of NO using urea (urea-SCR) is one of the promising technologies for removing NOx from diesel engine exhaust. The engine-out NOx is reduced by ammonia (NH3) derived from urea over a catalyst to environmentally benign N2. In this paper, we investigate the effect of various reactor operating conditions on the NOx reduction performance of three different catalyst formulations (e.g., Cu-zeolite, Fe-zeolite, Vanadium-based) to obtain useful guidance in the design and operation of urea-SCR lean NOx emission control systems. We examine the effects of NO:NO2 ratio on the steady-state NOx reduction activity at typical diesel engine exhaust temperatures (150-550°C). Transient measurements were also performed to determine the impact of NH3:NOx ratio and NH3 storage on catalyst performance. The impacts of hydrocarbon poisoning and sulfur poisoning/regeneration were also examined.