Development of the 2010 Ford Diesel Truck Catalyst System

The choice to pursue Selective Catalytic Reduction (SCR) using aqueous urea as a NOx reductant for lean NOx control on diesel vehicles was not an easy one. It was difficult to imagine an infrastructure for delivery of aqueous urea to diesel vehicles. In the 1990s, SCR technology was best known for its use in stationary source control of NOx. There were published studies on its potential effectiveness for diesel vehicle NOx control at steady state, most notably by Degussa [13], Volkswagen [23], and Hug Engineering [25]. Earlier, interesting work was also being done at the Paul Scherrer Institute in Switzerland [28]. Ford applied urea SCR to a light-duty truck and tested it with success on transient cycles [43]. The SCR catalysts available at that time included vanadia/titania, and base metal/zeolite formulations using copper or iron, usually using ZSM-5. It was unknown if these catalyst types would ever be durable enough for a vehicle application, not only hydrothermally but with diesel fuel sulfur contents on the order of 500 ppm, or approximately 50 times the current diesel sulfur level in the U.S. and Europe today. There was also uncertainty about integration of particulate traps into SCR systems, low temperature/high space velocity applications, and aqueous urea handling issues, including freeze point and replenishment.