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A typical diesel After-Treatment (AT) system to meet 2010 North American regulations consists of a Diesel Oxidation Catalyst (DOC) for oxidizing Hydrocarbons (HCs) and Carbon Monoxide (CO), followed by a Selective Catalytic Reduction (SCR) catalyst for NOx reduction, and a diesel particulate filter. DOCs are also used to oxidize NO in the exhaust to NO2 that substantially improves low-temperature SCR NOx reduction performance. Vehicle and engine dynamometer data under typical operating conditions show (1) engine-out NO2/NOx can be as high as 40%, (2) DOC can reduce NO2 to NO in the presence of reductants, and (3) an aged DOC with a low CO/HC conversion can be a net consumer of NO2 over a wide range of temperatures and space velocities. Flow reactor experiments simulating diesel exhaust conditions show that DOC-out NO2 is independent of the inlet NO2/NOx in the feed.

This paper presents the development of an analytical model that complements laboratory based experiments to provide a tool for Selective Catalyst Reduction (SCR) applications. The model calibration is based on measured data from NOx reduction performance tests as well as ammonia (NH3) adsorption/desorption tests over select SCR catalyst formulations in a laboratory flow reactor. Only base metal/zeolite SCR samples were evaluated. Limited validations are presented that show the model agrees well with vehicle data from Environmental Protection Agency Federal Test Procedure (EPA FTP) emission assessments. The model includes energy and mass balances, several different NH3 reactions with NOx, NH3 adsorption and desorption algorithms, and NH3 oxidation.

This paper presents a study of urea SCR catalyst aging characteristics and implementation into an analytical model that complements laboratory based experiments for a dynamometer-aged SCR brick. The model calibration is based on measured data taken from a 120k-mile simulated dynamometer-aged base metal/zeolite SCR. Dynamometer aging led to non-uniform axial deterioration with more severe deactivation toward the front of the SCR brick compared to the rear. Data from a 120k-mile simulated hydrothermally oven-aged SCR (uniform axial aging) is used to establish baseline aged NOx performance and NH3 adsorption/desorption behavior. An axial deterioration factor is applied to the baseline model to account for differences between oven and dynamometer aging. The model is exercised using engine out vehicle data to examine how different aging processes (oven vs. dynamometer) affect overall NOx performance during the EPA FTP (Environmental Protection Agency Federal Test Procedure).

Urea-based Selective Catalytic Reduction (SCR) has the potential to meet U.S. Diesel Tier 2 Bin 5 emission standards for NOx in 2010. The operating and driving conditions of light-duty and heavy-duty vehicles make it necessary to customize catalyst features to the application. This paper reviews the selection of SCR catalyst technology for the U.S. and the appropriate aging and poisoning protocols for current supplier SCR catalysts. Generally, light-duty applications require SCR catalysts to function well at low temperature whereas heavy-duty applications require functionality at high temperature and high space velocity. One main durability requirement of SCR formulations involves withstanding the high temperature process of regenerating particulate filters from accumulated soot. Unrefined engine exhaust temperature control coupled with the inexact temperature measurement may also expose SCR catalysts to additional over-temperature conditions.