Performance Characterization of Cu/Zeolite and Fe/Zeolite Catalysts for the SCR of NOx

Giovanni  Cavataio, Ford Motor Company

Reduction of nitrogen oxides (NOx) in the lean exhaust gas of diesel engines is not trivial. The well-known reaction chemistry of a typical precious metal three-way catalyst (TWC) is not effective for NOx conversion in lean exhaust. Urea-based Selective Catalytic Reduction (SCR) has the potential to meet U.S. Diesel Tier 2 Bin 5 emission standards for NOx in 2010 [1]. The reduction of NOx with either ammonia (NH3) or urea has been used extensively for stationary source emission control. NOx reduction is possible due to the high selectivity of the NH3 and NOx reaction to form elemental N2. Urea decomposes to NH3 following a well-known two-step decomposition process involving thermolysis and hydrolysis.
The U.S. Tier 2 Bin 5 NOx standard represents a challenging 90+% reduction from previous standards. The varying operating and driving conditions of light-duty and heavy-duty vehicles make it necessary to customize catalyst features to the application. Generally, light-duty applications require SCR catalysts to function well at low temperatures (<350°C) whereas heavy-duty applications require functionality at higher temperature (>350°C) and high space velocity. One major thermal durability requirement of SCR formulations involves withstanding the high temperature process of regenerating particulate filters from accumulated soot. On the basis of performance, three important parameters for the selecting the proper SCR formulation involves the exhaust temperature profile, the NO2/NOx ratio, and the amount of NH3 stored on the catalyst surface. In addition to thermal durability, this presentation reviews a detailed comparison between the basic catalytic activity of Cu/Zeolite and Fe/Zeolite formulations, and their resistance to poisons such as sulfur and hydrocarbons.

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