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.