pices of the U.S. Council for Automotive Research to define product specifications. Aqueous urea is now sold as Diesel Exhaust Fluid™ (DEF™) in amounts ranging from gallon bottles to drums. The cost of DEF ranges from $2.79 per gallon in bulk to $4.65 per gallon in bottles. The Department of Energy created a website, www.fnddef.com, to point users to retail locations that sell DEF.
The use of urea-based SCR conversion systems not only has a direct effect on NOx emissions, but data from the Environmental Protection Agency suggest that it has had an indirect effect on carbon dioxide emissions. Because the SCR systems are performing the bulk of NOx reduction, diesel engines can now run at higher fuel efficiencies, which reduces carbon dioxide emissions. At light and moderate engine loads, engines equipped with urea-based NOx control systems will have a greater than 10 percent advantage over engines equipped with other NOx control systems. Urea-based systems are now used on the majority of diesels sold starting in 2010.
Summarizing the impact of medium-duty diesel vehicles on platinum-group metal utilization, Lambert said that volumes have increased since 2005 because of more stringent emissions standards, but the move in 2010 from an all-platinum catalyst to a palladium-rich catalyst has dropped platinum use close to 2007 levels. Diesel engines still use more platinum than in comparable gasoline-powered medium-duty trucks, but that gap is shrinking.
Looking to the future, Lambert noted that adding an SCR to other lean NOx catalyst components may enable further advances in the development and adoption of lean-burn technologies for gasoline engines that lower or eliminate the need for platinum-group metal catalysts (Xu et al., 2010). However, adopting SCR technologies for use with gasoline engines will require lowering the sulfur content of gasoline from its current 80 ppm to 15 ppm or developing technology that removes sulfur from the exhaust stream prior to the SCR unit.
Research is also aimed at boosting NOx oxidation activity of palladium to continue the trend of replacing platinum with palladium as a cost-reduction strategy. Toward this end, researchers at General Motors have developed a platinum-free perovskite catalyst that rivals the performance of a platinum-based catalyst at reducing NOx levels under lean-burn conditions (Kim et al., 2010).
In response to a question about the role of theory in his work on electrocatalysts, Bullock said that theory and computational models were very helpful in providing insights into how these reactions work and increasing confidence that postulated intermediates in proposed mechanisms really existed. Theory has not yet reached a point where it can predict which molecules to make from the ground up, but theory has been very helpful in understanding energy potentials, reaction intermediates, and step-by-step mechanisms.
With regard to the synthetic complexity and cost of the ligands used with cheap metal catalysts, Bullock acknowledged that these are important issues. He added, though, that the ligands developed at PNNL, despite their apparent complexity, are easy to make in a two-step synthesis from simple starting materials.
He also noted that the cheap metal catalysts developed so far for organic synthesis have not yet achieved turnover rates comparable to palladium-based catalysts. He hoped that industry would now step in and take the catalysts developed in academia and make the improvements needed to create commercially viable catalysts based on cheap, abundant metals.
Finally, Bullock pointed to the importance of being open to unusual and unexpected results. Past examples of such advances should be kept in mind so that novel and important results are not ignored because they are so different from past findings.
Lambert was asked about the high-sulfur marine diesel used in many parts of the world, and she noted that vanadium-based catalysts are highly tolerant of sulfur, but currently they lack the necessary temperature stability. Improving that stability could be a fruitful avenue of research, and in fact a number of catalysts have been developed. She added that there is little commercial pressure for such a catalyst because regions of the world that use high-sulfur diesel fuel do not have stringent standards for particulate matter.
In response to a question about what happens when a vehicle runs out of urea, Lambert replied that the vehicles are designed to run at slower speeds, which frustrates drivers and provides an incentive to refill the urea containers on the vehicles.
Lambert pointed to an increase in research on batteries and electric vehicles at Ford and elsewhere. But she also said that “the internal combustion engine is not dead.” Fundamental research still needs to be done on exhaust gas emissions even as electric vehicle technologies advance.