FIGURE 4-3 U.S. diesel exhaust treatment systems have become progressively more sophisticated to reduce emissions.
SOURCE: Ford (2011).
At Ford, Lambert and her colleagues found that a zeolite known as chabazite, which has a very small average pore size, combined with copper is a suitable diesel exhaust catalyst (Kwak et al., 2010; McEwen et al., in press) in an SCR system. Numerous academic studies suggest that copper is located inside the cage of the zeolite structure and that ammonia and NOx enter the cage, where they react in the presence of copper and oxygen to produce nitrogen gas and water.
There were a number of challenges to overcome to commercialize the SCR system that Ford now uses with its diesel engines. First, Lambert and her colleagues had to stabilize the platinum-based oxidation catalyst that sits in front of the SCR system. They accomplished this task by adding palladium to the catalyst mixture. After working with various ratios of platinum to palladium, they found that a 1:4 platinum-to-palladium mixture resulted in the best combination of hydrocarbon oxidation at cold-start temperatures and stability. The latter is important because volatilized platinum interferes with the NOx reduction process by poisoning the copper catalyst in the SCR system (Cavataio et al., 2009). Lambert noted that it is important enough to avoid preciousmetal contamination of the copper catalyst that the two components are made in separate buildings. Having even a small amount of platinum on the copper-zeolite catalyst turns the latter into an ammonia oxidation catalyst instead of a NOx reduction catalyst.
Lambert noted that Ford researchers first worked with a beta-type zeolite as the copper support, but that this combination was poisoned by hydrocarbons to some extent. The catalyst could be regenerated by heating it to 500°C, but treatment at this temperature was found to produce melting that destroyed the zeolite’s structure. Moving to chabazite solved this problem. She added that chabazite’s small pore size prevents larger hydrocarbon molecules from reaching the active copper catalyst, preventing the formation of dioxins in diesel exhaust.
Sulfur can negatively impact copper-zeolite catalyst activity, particularly at temperatures below 300°C. Sulfur can be removed from the catalyst at flter regeneration temperatures, however. In fact, research found that at the now-mandated level of sulfur allowed in diesel fuel—15 ppm or less—the copper-chabazite catalyst can tolerate the amount of sulfur absorbed between 500-mile regenerations and still reduce NOx levels enough to meet exhaust standards.
The use of aqueous urea filtration is possible because manufacturers and suppliers worked together under the aus-