Combined Fe-Cu SCR Systems with Optimized Ammonia to NOx Ratio for Diesel NOx Control 2008-01-1185

Selective catalytic reduction (SCR) is a viable option for control of oxides of nitrogen (NOx) from diesel engines. Currently, copper zeolite (Cu-zeolite) SCR catalysts are favored for configurations where the exhaust gas temperature is below 450°C for the majority of operating conditions, while iron zeolite (Fe-zeolite) SCR catalysts are preferred where NOx conversion is needed at temperatures above 450°C. The selection of Cu-zeolite or Fe-zeolite SCR catalysts is based on the different performance characteristics of these two catalyst types. Cu-zeolite catalysts are generally known for having efficient NOx reduction at low temperatures with little or no NO₂, and they tend to selectively oxidize ammonia (NH₃) to N₂ at temperatures above 400°C, leading to poor NOx conversion at elevated temperatures. Fe-zeolite catalysts are very efficient at NOx conversion at temperatures as high as 600°C or higher, but they are not as efficient as Cu-zeolite catalysts at lower temperatures in the absence of NO₂.

In this work, a combined SCR system consisting of an Fe-zeolite catalyst in front of a Cu-zeolite catalyst is tested and compared to a Cu-only system. The experiments are performed on a flow reactor with simulated diesel exhaust gas. It is seen that the operating temperature range of the SCR catalyst can be widened for a combined Fe-Cu system. At low temperatures, the Cu-zeolite improves NOx conversion efficiency vs. an Fe-only system. At elevated temperatures, the Fe-zeolite is more active. Also, one can overdose NH₃ at elevated temperatures with the combined Fe-Cu system without NH₃ slip, while the Feonly system leads to substantial NH₃ slip when overdosing. A simple optimization of NH₃ to NOx ratio (alpha) is performed for the combined Fe-Cu system and compared to a Cu-only system. Also, the ratio of Fe-zeolite to Cu-zeolite is evaluated. Transient test results are also shown for these systems to determine the impact of a combined system on transient response at low temperatures.