Recent Advances in the Development of Urea-SCR for Automotive Applications

In the recent years, considerable progress has been achieved in the development of urea-SCR for nonstationary, mobile applications. Main challenges are the reduction of the catalyst volumes and the optimization of the dosing strategy to minimize transient ammonia emissions during load changes. Catalysts with increased cell density and enhanced intrinsic activity are one way to obtain smaller catalyst volumes. Another approach is the introduction of additional catalysts upstream and downstream of the SCR catalyst itself. Most effective is an oxidizing precatalyst that converts a part of NO to NO₂, thus allowing a faster SCR reaction to occur. However, such a system requires the use of fuel with reduced sulfur content.

Work performed at the Paul Scherrer Institute comprises of fundamental experiments in a laboratory scale reactor with synthetic exhaust gas and applied experiments on a diesel test stand. In addition to the evaluation of various commercial and proprietary catalysts, important problems of the process have been investigated as, for example, the fast SCR reaction, ammonia adsorption on the catalysts, the decomposition of urea, and catalyst aging.