Diesel engines are an attractive alternative to gasoline internal combustion engines because they operate with high compression ratios making them more fuel efficient than their gasoline stoichiometric counterparts. Furthermore, the large amount of excess air results in cooler maximum temperatures preserving the life of the engine. However, because the diesel fuel is injected as a liquid, the combustion results in solid and liquid particulates and gaseous NOx, CO, and HC. Currently CO and HC are abated using monolithic diesel oxidation catalysts and NOx is treated with selective catalytic reduction (SCR). NOx gases contribute to global warming, ground level ozone, and the formation of acid rain, and are therefore regulated in the U.S. by the Environmental Protection Agency (EPA). As NOx standards are becoming more stringent for motor vehicles under the EPA Tier 2 program, the need for NOx abatement technology is growing.

In power plants and stationary sources, SCR is already used to reduce NOx with NH3. For diesel vehicles it is proposed to use an aqueous solution of urea as the NH3 source because it is non-toxic and can be much more easily and safely transported.

After the injection of the urea solution into the hot diesel exhaust, urea ideally decomposes into ammonia and carbon dioxide. However, another possible intermediate product of the decomposition is isocyanic acid, which can react to polymeric species such as cyanuric acid, a potential catalyst poison.

This research will characterize the impact of urea and its decomposition products on the stability of zeolite candidate SCR catalysts. Ammonia adsorption capacity was used as an indicator of Y, Cu-Y, Na-Beta and Fe-Beta zeolite stability. Therefore, the zeolites were aged hydrothermally at 600°C in 10% water vapor containing different ammounts of urea. Alternatively, the zeolites were also impregnated with aqueous urea solutions. Thermogravimetric analysis was used to study the urea decomposition in the presence and absence of zeolite catalysts and to determine the differences in NH3 adsorption between fresh and aged zeolites. Work in progress will be presented.