Laboratory Testing of Urea-SCR Formulations to Meet Tier 2 Bin 5 Emissions

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. Therefore, base metal/zeolite formulations are more appropriate for U.S. diesel applications than vanadia-containing formulations. On the basis of performance, two important parameters for selecting the proper SCR formulation involve the exhaust temperature profile and the NO₂/NOx ratio expected during typical driving conditions. Other operating parameters such as oxygen level, water level, and amount of ammonia on the catalyst surface all have influence on the system performance. A detailed comparison is made between the basic catalytic activity of Cu/zeolite and Fe/zeolite formulations, and their durability to poisons such as sulfur and hydrocarbons is investigated.