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.