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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.

It is likely that use of urea-based selective catalytic reduction (SCR) will be needed to meet U.S. Tier 2 diesel emission standards for oxides of nitrogen (NOx). The ideal ratio of ammonia (NH3) molecules to NOx molecules (known as alpha) is 1:1 based on urea consumption and having NH3 available for reaction of all of the exhaust NOx. However, SCR efficiency can be less than 100% at low temperatures in general, and at higher temperatures with high exhaust SCR catalyst space velocities. At the low temperatures where NOx conversion efficiency is low, it may be advantageous to reduce the alpha ratio to values less than one (less NH3 than is needed to convert 100% of the NOx emissions) to avoid NH3 slip. At higher space velocities and high temperatures, the NOx conversion efficiency may be higher with alpha ratios greater than 1. There is however concern that the additional NH3 will be slipped under these conditions.

Selective catalytic reduction (SCR) is a technology capable of meeting Tier 2 Bin 5 emissions levels of oxides of nitrogen (NOX) for diesel engines. Base metal zeolite catalysts show the best combination of thermal durability and NOX conversion activity. It is shown in this work that some base metal zeolite catalysts can store high levels of hydrocarbons (HCs). Also, base metal zeolite catalysts can catalyze oxidation of HCs under certain conditions. Oxidation of stored hydrocarbons can lead to permanent catalyst deactivation due to the exotherm generated in the SCR catalyst (over-temperature condition leading to SCR catalyst damage). This paper discusses a laboratory bench test to characterize hydrocarbon storage and burn-off characteristics of several SCR catalyst formulations, as well as engine dynamometer tests showing hydrocarbon storage and exotherm generation.