Issue Date: May 21, 2012
Ever-Cleaner Auto Exhaust
Touch football games and automobile catalytic converters share little in common, as far as most people are concerned. Not so for Galen B. Fisher.
As a college student in Southern California in the 1960s, Fisher remembers all too well the choking dark clouds that drifted 30 miles eastward from the traffic-congested highways of Los Angeles and frequently descended onto the grounds of Pomona College, in Claremont, where he often played afternoon pickup games.
“Around 4 o’clock when we were out playing football, we’d often see this big cloud roll in,” Fisher recalls. “Before you knew it, all these healthy college kids would lie down on the ground gasping for air and struggling to catch their breath.”
As Fisher would come to learn, he and his football buddies of the ’60s were experiencing the noxious effects of untreated engine emissions. He would go on to spend decades researching catalytic cleanup of automobile emissions, first at General Motors, later at Delphi Automotive, and now as an adjunct professor at the University of Michigan.
Hydrocarbons, including unoxidized and partially oxidized fuel, and oxides of nitrogen (NOx) are two of the main classes of products found in gasoline and diesel engine exhaust. They can react under the influence of sunlight to form ozone-laced smog. In the pre-catalytic-converter days of the 1960s, conditions near Los Angeles were just right for forming smog; cars and trucks freely spewed high concentrations of those pollutants and others, such as the fine particulate matter typically produced from combustion of organic material. The tailpipe emissions that accumulated near busy roadways formed dense clouds that wafted eastward on the ocean breeze and forced Fisher and his friends to take a time-out. Nowadays, thanks in large part to automobile catalytic converters, that scenario is a thing of the past.
Emissions cleanup systems on today’s cars and trucks scrub engine exhaust of nearly all pollutants. Even so, carmakers continue to search for catalytic chemistry methods to further reduce emissions levels—especially diesel emissions, which until recently were not regulated—to comply with ever-tightening engine emissions laws.
In the U.S., emissions standards follow from the 1970 Clean Air Act and its amendments. This body of regulations is managed at a national level by the Environmental Protection Agency, which established federal test procedures to ensure that new vehicles comply with the regulations. California’s Air Resources Board established its own low-emissions vehicle (LEV) program in 1990 and since then has generally remained more stringent than EPA in the emissions levels it allows.
Successively more restrictive emissions standards from both organizations continue to come into force every few years. As a result, automobile manufacturers continue to search for ever-more-effective catalytic methods to rid exhaust streams of pollutants before they can escape from a vehicle’s tailpipe. This process has been driving steady advances in catalytic cleanup work for gasoline engine exhaust for decades. Hydrocarbon emissions limits, for example, have steadily declined from 1.5 g per mile in the 1970s to 0.01 g per mile under current standards. Limits on NOx emissions have followed a similar path.
For diesel emissions, the story is quite different. With few exceptions, prior to 2007, emissions from diesel vehicles were not treated catalytically. Rather, manufacturers complied with emissions regulations through clever engineering of the engine itself, according to Aleksey Yezerets, director of catalyst technology at Cummins, a Columbus, Ind.-based manufacturer of diesel engines.
But EPA and LEV standards that took effect in 2007 and became stricter in 2010 drove catalyst suppliers, engine and vehicle manufacturers (especially in the medium- and heavy-duty diesel truck business), and others to sharply focus their R&D efforts during the past several years on developing new technology to strip diesel exhaust of NOx, hydrocarbons, carbon monoxide, and particulate matter. Standards covering medium-duty diesel trucks, for example, now limit NOx emissions to 0.2 g per mile and hydrocarbon emissions to nearly 0.1 g per mile.
“We went from not having any catalysts on diesel engines to developing and broadly commercializing complex catalytic systems in a remarkably short period,” Yezerets says. In his view, the past two years marked the transition in the area of diesel emissions control “from infancy to the initial stages of maturity.”
Reducing NOx in diesel engine exhaust presents a special chemistry challenge because of the oxidizing nature of the exhaust stream. Diesel engines are designed to combust fuel in a large excess of oxygen—a fuel-lean mixture. That fuel-air composition gets part of the credit for the high fuel efficiency typical of this type of engine. But it also leaves a relatively small number of NOx molecules “searching” for reaction partners and catalytic sites for reduction in the presence of orders-of-magnitude more oxygen molecules. “It’s not a fair fight,” Fisher says.
The same type of NOx-control challenge plagues so-called lean-burn gasoline engines. These kinds of engines, like diesel engines, also operate at large air-to-fuel ratios and provide high fuel efficiency, but they are not common in the U.S. In contrast to lean-burn systems, standard gasoline engines combust fuel most efficiently at stoichiometric air-to-fuel ratios. Standard catalytic converters have been able to reduce NOx under those conditions for decades. Overall, the engine design differences that make lean-burn vehicles (gasoline and diesel) so fuel efficient and consequently attractive to manufacturers and consumers have made NOx control difficult.
In light of the lean-NOx challenge, as the problem is known, the run-up to the 2010 emissions regulation phase-in saw researchers and manufacturers scrambling to come up with effective lean-NOx treatment methods. Much of the news in diesel exhaust control nowadays is centered on the recent commercialization of one of those solutions. The system features a zeolite-based material that mediates selective catalytic reduction (SCR) of NOx to nitrogen and water through reaction with ammonia. The catalyst is a chabazite zeolite that has been subjected to an ion-exchange treatment to incorporate copper in its lattice. Ford Motor Co. and GM introduced medium-duty pickup trucks in 2010 (2011 model year) that feature SCR NOx-control technology based on this catalyst.
Ammonia, which serves as the NOx reductant, comes from an onboard supply of a temperature-controlled aqueous urea solution that’s delivered as needed to the SCR unit via a pump and injector system. The urea solution is a consumable that must be replenished periodically—on the order of the frequency of oil changes. The refill process on some truck models is similar to refilling a windshield washer reservoir.
Although the catalysis community is abuzz with talk of ammonia SCR, diesel exhaust treatment systems also include a diesel oxidation catalyst unit to remove hydrocarbons and CO, as well as a diesel particulate filter that traps and catalytically oxidizes fine particles of diesel soot.
All of the components typically take the form of porous ceramic bricks featuring long narrow channels in a honeycomb pattern. The bricks (or monoliths) vary in size depending on application and are often made of a magnesium oxide-alumina-silica material known as cordierite. According to BASF’s Sanath V. Kumar, global marketing director for mobile emissions catalysts, manufacturers coat the monoliths via a slurry deposition process with a micrometers-thick layer of various metals, supports, and stabilizer materials.
SCR units, for example, are coated with a copper-chabazite precursor and then heat-treated to form the active catalyst phase. Diesel oxidation catalysts and particulate filters are made with common supports such as alumina and silica and well-dispersed particles of platinum, palladium, or both. The same description applies to catalytic converter monoliths for gasoline emissions.
Designs of the individual catalyst units as well as the overall design of these new multicomponent diesel “aftertreatment” systems vary from manufacturer to manufacturer. In general, however, the performance of these complex, mobile chemical reactors is monitored by a set of sensors that measure the composition, pressure, and temperature of the exhaust. The sensors are tied into a sophisticated onboard diagnostic and engine-feedback system that tailors engine operation to meet the vehicle’s exhaust treatment needs. “It’s like the tail wagging the dog,” Yezerets says.
Despite the commercial status of these diesel vehicle aftertreatment systems, the technology, especially for NOx abatement, is new and therefore remains the subject of intense research, says Charles H. F. Peden, a laboratory fellow at Pacific Northwest National Laboratory. He notes that few journal papers have been published to date on the copper-chabazite SCR material. “We’re just beginning to find out why this fascinating material functions the way it does,” Peden says.
Compared with other zeolites, Cu-chabazite (also known as Cu-SSZ-13) is robust and exhibits high thermal stability under SCR conditions. As Christine K. Lambert, a technical leader in Ford’s chemical engineering department explains, the material for this application must tolerate fast and wide temperature swings of hundreds of degrees, as encountered, for example, in the first few seconds after a cold engine is turned on. The material must also be able to control NOx at 200 °C—which is a low and challenging temperature for catalysis to occur—yet remain durable at over 600 °C.
Another requirement is that the material not be a magnet for hydrocarbons. While evaluating other zeolites such as ZSM-5, Lambert found that in tests that simulated prolonged low-speed driving quickly followed by high-speed driving or the high-temperature process used to burn off soot, the temperature swing touched off a reaction of the hydrocarbons adsorbed in the zeolite pores that generated enough heat to melt portions of the ceramic brick.
Cu-chabazite’s structure endows it with smaller pores than other zeolites commonly used in catalysis, such as ZSM-5 and zeolite-beta, Kumar says. The shape and small size of the pores allow NOx and ammonia in, where they can undergo SCR, but help keep hydrocarbons out.
“The science community is scrambling to understand this material in greater detail,” Peden says. A sampling of the small number of papers on this topic was just published in Catalysis Today. In one of those studies, a team led by William F. Schneider of the University of Notre Dame and Fabio H. Ribeiro of Purdue University analyzed Cu-SSZ-13’s exchange sites and copper’s oxidation state and other properties through a combination of X-ray absorption spectroscopy and computational methods. The analysis helps clarify the role of copper’s redox chemistry in SCR: It identified SCR reaction conditions that lead copper to adopt the Cu(II) oxidation state and fourfold coordination and other conditions that favor a mixture of Cu(I) and Cu(II) and reduced coordination (Catal. Today, DOI: 10.1016/j.cattod.2011.11.037).
In another study, GM’s Se H. Oh and Steven J. Schmieg worked together with Peden and others to determine the manner and extent to which the catalytic performance of laboratory-aged Cu-chabazite samples deteriorates. On the basis of nuclear mass resonance and microscopy analysis and other techniques, the group found that catalyst degradation follows from destruction of the crystal structure and copper agglomeration. They also found that higher aging temperatures lead to faster collapse of the crystal lattice. The group notes that the results from catalyst-aging simulations compare well with those obtained from a 135,000-mile-vehicle-aged catalyst (Catal. Today, DOI: 10.1016/j.cattod.2011.10.034).
Another approach to addressing the lean-NOx problem calls for trapping and storing NOx chemically in a potassium- or barium-rich material such as barium carbonate that can react under lean conditions (excess oxygen) to form nitrates or nitrites. When the material is saturated with NOx, the engine can run briefly in the rich mode to regenerate the trap by producing reductants (CO and hydrocarbons) that desorb NOx. The desorbates can go on to be reduced to nitrogen at a nearby precious-metal site.
Many teams have studied lean-NOx traps (LNTs) in search of ways to enhance performance, reduce costs, and extend durability. The topic remains at the forefront of research in lean-burn emissions control. Two years ago, for example, Wei Li, who heads GM’s emissions catalysis group, along with Gongshin Qi and coworkers reported that strontium-doped perovskite materials work well as Pt substitutes and potential cost reducers in LNTs and diesel oxidation catalysts. They showed that a precious-metal-free La0.9Sr0.1MnO3-based LNT catalyst matched the NOx-reduction performance of a commercial Pt-based catalyst. They also showed that La1-xSrxCoO3 catalysts facilitate NO oxidation, a critical step in the NOx-trapping process, more effectively than commercial Pt-based diesel oxidation catalysts (Science, DOI: 10.1126/science.1184087).
Just recently, the GM team followed up with another study, this time evaluating the performance of a LaMnO3-based catalyst in all of the LNT catalyst roles. They found that the precious-metal-free material does a good job of mediating NO oxidation and NOx storage under lean conditions. Under rich conditions, however, it does not exhibit sufficient NOx-reduction activity unless it is doped with a small amount of Pd and rhodium. Yet, even the Pd-Rh-doped material has potential to significantly reduce catalyst costs, they say (Catal. Today, DOI: 10.1016/j.cattod.2011.11.012).
In related work, Michael D. Amiridis and coworkers at the University of South Carolina used vibrational spectroscopy to examine surface species present on Pt- and Rh-doped BaO-based LNTs. CO and propylene served as reductants in a simulated fuel-rich engine cycle. They determined that surface isocyanate (NCO) species, which remained present during all phases of the experiment, are reactive intermediates that may play a direct role in producing up to 30% of the measured N2. Amiridis and coworkers note that developing ways to increase NCO concentration could enhance overall NOx reduction (Catal. Today, DOI: 10.1016/j.cattod.2011.11.022).
The long narrow-channel configuration of monolithic emissions catalysts opens up the possibility that reactions can evolve in complex ways as reactants and products travel along the channel length. Making measurements only at a monolith inlet and outlet may lead researchers to miss that reaction complexity and form inaccurate pictures of reaction profiles. For that reason, a team led by William P. Partridge and Jae-Soon Choi of Oak Ridge National Laboratory developed a method to record spatiotemporally resolved mass spectra by sampling along the channel lengths via a set of finely translatable capillary probes. The technique, dubbed SpaciMS, generated considerable interest as word reached the catalysis community, and one instrument maker, Hiden Analytical, now offers a commercial unit.
In one recent application of the method to LNT chemistry, the Oak Ridge team found that during rich engine cycles, stored NOx is released and readsorbed downstream. In addition, sulfur poisoning of NOx storage sites near the inlet pushes the active NOx storage region toward the outlet, which in turn affects the dynamics of rich-cycle NOx desorption and re-adsorption (Catal. Today, DOI: 10.1016/j.cattod.2011.11.007).
Researchers at Ford recently used this method to evaluate an experimental combined LNT-SCR system that does not rely on urea and efficiently reduces NOx. In the test system, an LNT stores NOx and converts most of it to N2 but also makes some ammonia. SpaciMS measurements show that the ammonia travels downstream, where it is stored on an SCR catalyst. There, ammonia serves as a reductant and converts NOx that leaks through the LNT.
Regulations mandating improvements in fuel efficiency are also driving innovations in emissions research. As GM’s Oh explains, the higher the fuel efficiency of the engine, the less waste heat is available to warm up the catalyst from a cold start and stimulate cleanup reactions. Turbocharged engines suffer from the same problem because the mass of the turbocharger draws heat from the exhaust stream. As a result of the catalyst warm-up delay, unacceptably high levels of untreated hydrocarbons can be emitted in just seconds.
One approach to solving the cold-start problem is to trap the hydrocarbons in a zeolite layer below the cool precious-metal layer. As the exhaust temperature climbs, the heat warms up the catalyst and desorbs the hydrocarbons from the zeolite. The hydrocarbons rediffuse through the now-warmed precious-metal layer, where they can be scrubbed from the exhaust stream.
As longtime emissions catalysis people are fond of saying, it’s amazing that catalytic converter technology works as well as it does and continues to meet ever-tightening regulations. Ford Technical Leader Robert W. McCabe points out that unlike carefully controlled and finely optimized stationary catalytic reactors, these mobile units experience wild and frequent temperature excursions, rapid and large fluctuations in feed stream composition and pressure, and constant jostling from road vibrations. And unlike sophisticated chemical plants that are operated by teams of highly trained engineers, automobile catalytic converters can be turned on and turned off by anyone who can handle a car key.
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