Replacing critical materials with abundant materials, particularly in applications that use large amounts of catalysts, would have many benefits. Abundant materials are cheaper, less susceptible to supply fluctuations, and more environmentally benign. Cheap and abundant metals also can be less selective, less tolerant of functional groups, and use more expensive ligands than rare and expensive metals, but research gradually is reducing these shortcomings.
A particular application discussed in this chapter is the use of precious metals in automotive catalytic converters. The automotive industry is a major user of platinum, palladium, and rhodium in catalytic converters, which has spurred research on the use of other types of materials as catalysts. Although no good alternatives to the use of these materials yet exist, promising approaches are being investigated.
In many important processes powered by a homogeneous catalyst, the cost of the catalyst’s metal component is a small part of the overall expense. Nonetheless, chemists are developing novel reaction schemes that use homogeneous catalysts made with “cheap metals,” said Morris Bullock, Laboratory Fellow and Director of the Center for Molecular Electrocatalysis at the Pacific Northwest National Laboratory (PNNL). These efforts are centered on using abundant, inexpensive metals—mostly first-row metals, but also molybdenum and tungsten—to replace precious metals.
Even in cases where an expensive metal is a fraction of a catalyst’s total cost, creating efficient catalysts from inexpensive metals is likely to produce significant savings, said Bullock. Platinum, on a per mole basis, is approximately 4,000 times more expensive than nickel and 10,000 times more expensive than iron. Similarly, palladium is 3,000 times more expensive than copper, while ruthenium is 2,000 times more expensive than iron.
Palladium-based homogenous catalysis, in particular, is of critical importance in the pharmaceutical and agricultural industries for forming carbon-carbon bonds. The 2010 Nobel Prize in Chemistry was awarded for palladium-catalyzed cross-coupling reactions, which can be used to make virtually any type of carbon-carbon bond needed. The powerful Buckwald-Hartwig carbon-nitrogen bond-forming reactions are another class of palladium-catalyzed chemistries used widely in the pharmaceutical and agricultural industries (Hartwig, 1998; Wolfe et al., 1998). This latter set of reactions, Bullock noted, uses palladium loadings as low as 10 parts per million (ppm), so the expense of the precious metal in this case is not a significant factor.
It is possible, though, to substitute less expensive metals for palladium. A copper iodide/L-proline catalyst, for example, can be used to form carbon-carbon and carbon-nitrogen bonds (Ma et al., 2003). A nickel catalyst can be used to make carbon-carbon bonds with some stereoselectivity, which enables the assembly of fairly complex organic molecules (Harath and Montgomery, 2008). Chemists also have developed iron catalysts in carbon-carbon bond-forming reactions, although the results are not always what they seem. In one case, researchers made the observation that 98 percent pure iron chloride, compared to 99.99 percent pure material, produced higher yields of the desired product. Further study found that the reaction was actually catalyzed by a 10 ppm copper oxide impurity (Buchwald and Bolm, 2009). “There are plenty of other reactions that do get catalyzed by iron, but it highlights something that you have to be careful about in making sure that you can identify the real catalysts in these reactions,” said Bullock.
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4 Replacing Critical Materials with Abundant Materials Replacing critical materials with abundant materials, and 10,000 times more expensive than iron. Similarly, particularly in applications that use large amounts of cata- palladium is 3,000 times more expensive than copper, while lysts, would have many benefits. Abundant materials are ruthenium is 2,000 times more expensive than iron. cheaper, less susceptible to supply fluctuations, and more Palladium-based homogenous catalysis, in particular, is environmentally benign. Cheap and abundant metals also can of critical importance in the pharmaceutical and agricultural be less selective, less tolerant of functional groups, and use industries for forming carbon-carbon bonds. The 2010 Nobel more expensive ligands than rare and expensive metals, but Prize in Chemistry was awarded for palladium-catalyzed research gradually is reducing these shortcomings. cross-coupling reactions, which can be used to make virtu- A particular application discussed in this chapter is the use ally any type of carbon-carbon bond needed. The powerful of precious metals in automotive catalytic converters. The Buckwald-Hartwig carbon-nitrogen bond-forming reactions automotive industry is a major user of platinum, palladium, are another class of palladium-catalyzed chemistries used and rhodium in catalytic converters, which has spurred widely in the pharmaceutical and agricultural industries research on the use of other types of materials as catalysts. (Hartwig, 1998; Wolfe et al., 1998). This latter set of reac- Although no good alternatives to the use of these materials tions, Bullock noted, uses palladium loadings as low as 10 yet exist, promising approaches are being investigated. parts per million (ppm), so the expense of the precious metal in this case is not a significant factor. It is possible, though, to substitute less expensive metals MOLECULAR ELECTROCATALYSTS FOR ENERGY for palladium. A copper iodide/L-proline catalyst, for exam- CONVERSIONS USING ABUNDANT METALS ple, can be used to form carbon-carbon and carbon-nitrogen In many important processes powered by a homogeneous bonds (Ma et al., 2003). A nickel catalyst can be used to make catalyst, the cost of the catalyst’s metal component is a carbon-carbon bonds with some stereoselectivity, which small part of the overall expense. Nonetheless, chemists are enables the assembly of fairly complex organic molecules developing novel reaction schemes that use homogeneous (Harath and Montgomery, 2008). Chemists also have devel- catalysts made with “cheap metals,” said Morris Bullock, oped iron catalysts in carbon-carbon bond-forming reactions, Laboratory Fellow and Director of the Center for Molecular although the results are not always what they seem. In one Electrocatalysis at the Pacific Northwest National Labora- case, researchers made the observation that 98 percent pure tory (PNNL). These efforts are centered on using abundant, iron chloride, compared to 99.99 percent pure material, inexpensive metals—mostly first-row metals, but also produced higher yields of the desired product. Further study molybdenum and tungsten—to replace precious metals. found that the reaction was actually catalyzed by a 10 ppm Even in cases where an expensive metal is a frac- copper oxide impurity (Buchwald and Bolm, 2009). “There tion of a catalyst’s total cost, creating efficient catalysts are plenty of other reactions that do get catalyzed by iron, but from inexpensive metals is likely to produce significant it highlights something that you have to be careful about in savings, said Bullock. Platinum, on a per mole basis, is making sure that you can identify the real catalysts in these approximately 4,000 times more expensive than nickel reactions,” said Bullock. 21
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22 THE ROLE OF THE CHEMICAL SCIENCES IN FINDING ALTERNATIVES TO CRITICAL RESOURCES The Pros and Cons of Cheap Metals is required. Instead, the reaction occurs through a hydride ion on the ruthenium and a proton from the ligand-attached In addition to the large price advantage that comes with nitrogen coordinated to the ruthenium. The end result is substituting a prevalent, cheap metal for a rare, expensive the same, but the mechanism is completely different than metal, cheap metals are often environmentally more benign. expected (Noyori et al., 2001). Losses of metal are more easily tolerated in an industrial pro- “The overall point I want to make is that if you’re trying cess, which can reduce or eliminate the recycling steps that to develop a new type of catalyst with a different metal, it is are almost mandatory with expensive metal catalysts. In the going to look a lot different,” said Bullock. “You don’t want pharmaceutical industry, the Food and Drug Administration to replace platinum or palladium with iron or copper and try may or may not allow trace levels of residual catalyst in a to use the same ligand set. The ligands will almost certainly final drug product. As Bullock stated, “How much palladium change.” The idea, he explained, is to not try to emulate what can you have in a pharmaceutical body compared to how precious metals are doing as catalysts. Instead, the intention much iron?” is to look at the reactivity characteristics of the cheap metals, The reasons that more cheap metal catalysts are not understand the electronics of the reactions and the energy widely used today are many, and Bullock listed several of states, and then build a catalyst around those metals from them. One reason is that reactions catalyzed by cheap metals the ground up using fundamental principles. have not been widely studied to date, though they are receiv- As an example, Bullock discussed work done in his ing more attention now. Another reason is that the selectivity laboratory developing a molybdenum-based catalyst for of cheap metal catalysts is not as good as is obtained with hydrogenating ketones to make alcohols at low tempera- palladium catalysts, and the scope of the reactions is not as ture and hydrogen pressure and under mild conditions broad. Boosting the activity of cheap metal catalysts can (Bullock and Voges, 2000). This reaction occurs by a differ- mean using more expensive ligands; for example, catalysts ent mechanism, one that capitalizes on the reactivity patterns based on aryl iodides are more reactive, but more expensive, of molybdenum hydrides and involves delivering a proton to than aryl chlorides. the oxygen atom in the ketone first, leaving a metal hydride Cheap metal catalysts are often less tolerant of functional that then delivers hydride to the carbon atom, creating the groups on the reactants. A reaction that works with an ester saturated alcohol. Fundamental research on the acidity of moiety present may not work when an alcohol or carboxylic metal hydrides and both the kinetics and thermodynamics acid functional group is present. In contrast, palladium-based of metal hydride behavior made the development of this catalysts often work with a wide range of modified starting catalyst possible. materials. In addition, cheap metals may require a higher The same types of basic research studies were done by catalyst loading than when palladium is used, negating some other researchers to develop an iron-based catalyst that also of the cost advantage. Bullock added, though, that this may performs a heterolytic cleavage of hydrogen as the key step be a result of the fact that cheap metal catalysis has not been in the hydrogenation of carbon-oxygen double bonds (Casey studied as exhaustively as has palladium-based catalysis, and Guan, 2009). But equally important is the fact that the and that additional research is likely to make headway on catalyst is regenerated under low-pressure hydrogen condi- this problem. tions. More recently, another group created an iron-based The final problem facing cheap metal catalysts is one of catalyst that under similarly mild conditions works at very motivation. For a pharmaceutical company making a high- low catalyst loadings of 0.05 mole percent (Langer et al., value-added drug at small scale, and for which catalyst cost 2011). is not a major factor in the final price of the drug, there is Iron-based catalysts also can be used to hydrogenate often little motivation to expend research dollars solving a carbon-carbon double bonds. Again, this work was based on relatively small problem. solid fundamental chemistry research to create redox-active To illustrate some of the challenges in developing cheap ligands that help drive the reaction. One of these catalysts metal catalysts, Bullock discussed the fact that reaction achieves turnover frequencies of up to 1,800 per hour in the mechanisms may not be universal, making the search for new conversion of 1-hexene to hexane (Bart et al., 2004). catalysts difficult. For example, an important class of chemi- cal reactions hydrogenate carbon-oxygen double bonds. These carbonyl hydrogenation reactions use ruthenium- and High-Volume Applications of Cheap Metal Catalysts rhodium-based catalysts to convert ketones and aldehydes Although the examples cited above show that it is possible into alcohols. One such ruthenium catalyst, for which the to create potent catalysts for the production of the type of Nobel Prize was awarded, does not operate via the traditional low-volume specialty chemicals used in the pharmaceutical mechanism for ketone hydrogenation. Normally, the reacting and agricultural industries, the impact on the overall demand ketone would first coordinate with the metal, after which for expensive and rare metals is not likely to be substantial. oxygen inserts itself into a metal-hydrogen bond. With this An area where a real impact could be had is in the area of particular ruthenium catalyst, no coordination or insertion
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23 REPLACING CRITICAL MATERIALS WITH ABUNDANT MATERIALS renewable energy production, which would require massive proton relays also lower the barrier for heterolytic cleavage amounts of catalyst, as was discussed in the previous chapter. of hydrogen (DuBois and Bullock, 2011) and facilitate “We hope that there is going to be a much higher use the coupled proton-electron transfers that are important in of solar energy and other types of renewable energy in the reduction of oxygen and nitrogen. the future,” said Bullock. “What we want to do is store The inspiration for using pendant amines to design nickel that energy in the form of the chemical bonds in a fuel.” hydrogenation catalysts comes from nature, which does not Making this conversion, he added, requires “developing use rare and precious metals in its catalysts. Protein crystal- electrocatalysts that will convert electrical energy to chemi- lography studies of the structure of an iron-iron hydrogenase cal bonds, largely in the form of molecular hydrogen. Then, enzyme revealed the presence of pendant amines in the when you need the electricity, you can run hydrogen in a fuel coordination sphere surrounding the metal atoms. But rather cell and get your electricity back.” than building structural mimics of nature’s catalyst, the Research at PNNL is focusing on developing catalysts PNNL team is designing functional models that re-create that do not require platinum for hydrogen oxidation, which the electronic and energy environment of the enzyme’s releases electrons, and the reverse reaction, proton reduc- catalytic center. tion, that stores electrons. This work is also germane to Bullock discussed a few early examples of the catalysts the broader topic of oxygen and nitrogen reduction, which developed from this effort. As shown in Figure 4-1, adding are more complex reactions given that oxygen reduction to two pendant amines to nickel’s coordination sphere produced water is a four-electron and four-proton event and nitrogen huge positive changes in the hydrogen oxidation activity of reduction to ammonia is a six-proton and six-electron event. the resulting nickel catalyst by reducing the overpotential The theoretical framework for this research is based on of the system, which increases the catalyst’s energy effi- understanding the first and second coordination spheres ciency. Taking this approach one step further, the PNNL team of nickel, the region where the electronic properties of the reduced the flexibility of the pendant amines, essentially ligands surrounding a metal atom have the biggest influence locking them into place around the nickel atom. This cut the on the metal’s catalytic properties (Rakowski DuBois and activation energy nearly in half and increased the turnover DuBois, 2009). In particular, Bullock and his colleagues frequency from less than 0.5 per second to 10 per second. are focusing on the role that phosphine ligands bearing Developing a clearer understanding of the mechanism pendant amines play in proton relays when these ligands involved in catalytic oxidation of hydrogen played an are built into the second coordination sphere around nickel. important role in taking these initial results and creating Research at PNNL has shown that proton transfer into or far superior nickel hydrogen oxidation catalysts. These away from the metal plays a key role in accelerating intra- studies, which relied heavily on nuclear magnetic resonance and intermolecular proton transfers and stabilizing binding spectroscopy, showed that moving protons onto pendant of hydrogen to the metal. These pendant amine-facilitated amines avoids having the reaction pass through a nickel(III) FIGURE 4-1 Nickel catalysts can oxidize hydrogen. SOURCE: Bullock (2011).
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24 THE ROLE OF THE CHEMICAL SCIENCES IN FINDING ALTERNATIVES TO CRITICAL RESOURCES intermediate, which lowers the energy barrier considerably. The heart of the automotive catalyst system is the cata- Based on this mechanistic understanding, the PNNL team lytic converter, explained Christine Lambert. The catalytic created a nickel-based catalyst that produces 50 turnovers per converter is essentially a ceramic honeycomb encased in a second at one atmosphere of hydrogen (Yang et al., 2010), metal can attached to the vehicle’s exhaust pipe. Within the and another one that was not as good a catalyst for hydrogen ceramic honeycomb is a supported metal catalyst that has oxidation but that was able to mimic the natural hydrogenase been wash-coated onto the ceramic support, as shown in enzyme and catalyze both hydrogen oxidation and reduction Figure 4-2. (Kilgore et al., 2011). This catalyst is the first reported to The standard catalytic converter on a gasoline-powered carry out both the forward and reverse reaction. vehicle uses a three-way catalyst engineered over many years One issue that arose in these studies was that protons can to be extremely durable, said Lambert. Current versions are become trapped between two pendant amines in the same designed to meet emissions standards for 120,000 miles. ligand, reducing catalytic activity. Switching to a ligand that They operate between 350°C and 650°C, the normal operat- had two phosphorous atoms to coordinate to the nickel core ing range of a gasoline engine, but they are durable to over but only one nitrogen, a so-called P2N1 ligand, produced 1000°C. The catalysts operate in near-stoichiometric exhaust a dramatic increase in catalytic activity and resulted in gas, which means that the air and fuel fed into the engine is at turnovers of over 100,000 per second, which is more than 10 the stoichiometric ratio needed to burn that fuel and that there times faster than the iron-based hydrogenase that served as is no excess oxygen. Contact time with the catalyst ranges the inspiration for this work (Helm et al., 2011). However, from 60 to 300 milliseconds (Heck and Farrauto, 2001). this catalyst still has a high overpotential that needs to be As engine control has improved over the years, the pol- addressed to improve its energy efficiency. lutant content of the exhaust has fallen into a tighter range of The PNNL also has used rational design principles to concentrations. As a result, catalysts are now able to handle create the first iron-based catalyst for hydrogen oxidation. all three gaseous pollutants simultaneously using what is Turnover rates for this catalyst are only about two per second, called a three-way catalyst. Particulates are an issue only but this research is still in its infancy. with diesel engines, and they are dealt with separately. In addition, working with a team at the University of Today’s three-way catalyst is a complex, multicomponent California, San Diego, Bullock and his colleagues have system. The bulk of the catalyst is made of cordierite, a mag- created a nickel-based electrochemical catalyst that oxi- nesium iron aluminum cyclosilicate that forms the ceramic honeycomb. γ-Alumina forms the support base, and cerium dizes formate. This is the first reported instance of a homog- enous formate oxidation catalyst, and it is the first example oxide (ceria) and zirconium oxide (zirconia) are oxygen- of a formate oxidation catalyst of any kind that does not use storage components to help the catalyst when it is in the platinum-group metals (Galan et al., 2011). These studies high-efficiency stoichiometric window. are ongoing. In addition to the rare earth mineral ceria, two other rare Bullock concluded his presentation by noting that perform- earth minerals, lanthanum oxide and neodymium oxide, ing homogenous catalysis without precious metals has many maintain the alumina in its gamma form and play an impor- advantages. In particular, iron, nickel, and other abundant tant role in durability. Barium oxide and strontium oxide also metals are much less expensive and are often more environ- serve as stabilizers and can store NOx if the engine is run- mentally benign. He added that, while an increasing amount of ning lean, though that is not their primary purpose. A small research is being done in this area, more fundamental research amount of nickel oxide suppresses the formation of hydrogen is needed to drive catalyst design efforts. The notable suc- sulfide from the relatively high amount of sulfur that is still cesses that the field has achieved in finding replacements for present in gasoline. Finally, there is a small amount, about palladium in organic synthesis and for platinum in fuel cell and 0.5 percent, of the precious metals platinum, palladium, and energy applications were all made possible by fundamental rhodium (Gandhi and McCabe, 2004). organometallic chemistry research, he observed. Summarizing the 30-plus-year history of gasoline catalyst development, Lambert explained that early fundamental research focused on identifying which elements could be NOVEL METALS AND BASE METALS IN good candidates for further development. Gold and silver AUTOMOTIVE CATALYST SYSTEMS were eliminated as candidates because of their limited dura- The auto industry is a major user of both precious- and bility and activity. Ruthenium, iridium, and osmium had base-metal catalysts. In automotive applications, catalysts suitable activity profiles, but they form volatile oxides at are used to reduce the amount of regulated pollutants in the elevated temperatures, ruling them out as automotive cata- exhaust from gasoline and diesel vehicles. These pollutants lysts. “At that point, the choices boiled down to platinum and include hydrocarbons, carbon monoxide, nitrogen oxides palladium,” said Lambert. (NOx), and particulate matter. Catalysts convert these pollut- Platinum and palladium were incorporated into the first ants into nitrogen gas, carbon dioxide, and water. catalytic converters when regulations mandated reductions
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25 REPLACING CRITICAL MATERIALS WITH ABUNDANT MATERIALS FIGURE 4-2 An automotive catalytic converter contains a catalyst on a ceramic substrate. SOURCE: Heck and Farrauto (2001). in hydrocarbon and carbon monoxide emissions. Later, catalyst market, there is no well-developed internal recycling when NOx standards were added to the regulations, rhodium infrastructure in the automotive market. was incorporated into the catalyst mix. Over the years, the Most of the world’s platinum comes from South Africa, stabilizers were added, as were ceria and zirconia as oxygen which supplied over 75 percent of the 6.06 million ounces stabilizers. When lead was removed from gasoline, platinum produced in 2010. Russia supplied just under 14 percent of was eliminated from the catalyst because palladium then the total, with the rest coming from other countries. Total could replace all platinum-based activity. Today’s catalytic demand for platinum in 2010 was 7.88 million ounces, with converters still contain some rhodium because, as Lambert automobile catalysts requiring 3.125 million ounces, or said, there is no acceptable substitute for rhodium when 40 percent of the total. Recycling was able to make up for it comes to NOx reduction. Improvements in the physical the difference between supply and demand. design of the catalyst, with platinum-group loadings chang- Russia is the biggest supplier of palladium, produc- ing from the front to the back of the converter, have reduced ing 3.72 million ounces in 2010, while South Africa pro- the amount of metal needed in the converter (JM, 2011). duced 2.575 million ounces. Other countries added just under 1 million ounces to the world’s available stores of palladium. Recycling added another 1.845 million ounces, Supply and Demand but taken together, the world’s demand for palladium—some The automotive catalyst market is not isolated but sits 9.625 million ounces—outstripped supplies by nearly a half within a supply chain that serves industrial catalyst manu- million ounces. Automotive catalysts accounted for 57 per- facturers, jewelry makers, and electrical equipment manufac- cent of the demand for palladium (JM, 2011). Lambert added turers. The catalytic converter supply chain starts with the that the automotive catalyst market accounts for most of the substrate manufacturers, who send their product to the cata- world demand for rhodium. lyst coaters who purchase the metals. From there, the coated Because the automotive industry is a major user, if not ceramic goes to the canners, who assemble the finished cata- the major user, of these metals, the industry is impacted lytic converter and ship it to the auto manufacturer. Currently, significantly by the price volatility for these metals. Since the automotive catalytic converter market consumes more 1992, platinum, palladium, and rhodium have all seen one platinum-group metals than it recycles, a situation that needs or more price spikes. As a result, catalyst manufacturers to be improved, according to Lambert. Unlike the industrial have developed several designs that use varying amounts
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26 THE ROLE OF THE CHEMICAL SCIENCES IN FINDING ALTERNATIVES TO CRITICAL RESOURCES of these three metals in an attempt to mitigate dramatic metal diesel oxidation catalyst, an aqueous urea tank that is shifts in costs. refilled during oil changes, a mixing system that injects the Price volatility also has contributed to the drive to urea into the exhaust stream, a selective catalytic reduction develop catalysts based on less expensive metals such as chamber in which a copper-zeolite catalyst converts NOx and copper. Though copper prices can spike as well, prices urea into nitrogen gas and water, and finally a particulate may go from $1 per pound to $4 per pound, compared to a filter. Approximately every 500 miles, the temperature in spike from $1,000 per ounce to as high as $12,000 per ounce the filter is raised to 500°C to burn off the trapped carbon for rhodium (Kitco, 2011). particles, producing carbon dioxide. However, a comparison of platinum-group metals with Lambert noted that there is a huge exhaust temperature other metals shows that the former are hard to beat when difference between gasoline and diesel engines. Once a it comes to carbon monoxide and hydrocarbon oxidation diesel engine is at full operating temperature, exhaust tem- activity, particularly in the presence of sulfur at tempera- peratures average about 200°C, whereas a gasoline engine’s tures under 500°C (Kummer, 1980). In the early 1990s, exhaust runs about 500°C. “It is hard to do catalysis at 200°C researchers at Ford tested a variety of copper and copper- under the high space and velocity conditions characteristic chromium combinations in catalytic converters installed in of engine exhaust,” she said. an actual car exhaust system. For NOx conversion, a catalyst An added complication for diesel engine exhaust is that its comprising 4 weight-percent copper and 2 weight-percent oxygen content jumps significantly when the engine deceler- chromium performed the best, but only when operated under ates. During deceleration, no fuel goes into the engine, so rich conditions, which reduced carbon monoxide and hydro- the exhaust stream is just air with about 20 percent oxygen carbon conversion and fuel economy. This study showed, too, content, making NOx control under conditions that range that copper catalysts needed to be close-coupled to the engine from rich to lean challenging. Manufacturers have responded in order to avoid sulfur poisoning (Theis and Labarge, 1992). with several systems, including urea-based systems that have After more than 30 years of research, Lambert concluded, a wide temperature window. Other approaches to achieving there still are no good options for creating catalysts to treat NOx control under lean conditions include a hydrocarbon gasoline engine exhaust that do not use platinum-group selective catalyst reduction (SCR) system that uses platinum metals. The only real advance, she said, was the transition but does not require a regenerating filtration system, lean NOx to palladium-rhodium and palladium-only catalysts enabled traps that use barium to absorb NOx during lean conditions, by the elimination of lead from gasoline. and a recently developed system that uses an ethanol-in-silver catalyst that is most useful for off-road diesel engines. The Importance of Diesel Urea and Zeolite Systems As a percentage share of the U.S. market, diesel accounts for about 1 percent of new car sales and under 10 percent of Urea is a nontoxic commodity chemical produced by light truck sales. For heavier trucks, such as the Ford F-250 fertilizer manufacturers. When heated with water, it produces and Dodge Ram, diesel models account for close to 90 per- carbon dioxide and ammonia. It is injected into the exhaust cent of new vehicle sales. While diesel car sales are increas- system upstream of the catalyst as an aqueous solution at ing, so, too, are the regulatory demands on diesel emissions. 32.5 weight-percent, a eutectic mixture with the lowest pos- Engine makers have responded by improving catalysts, sible freezing point. In the presence of a suitable catalyst, resulting in a substantial drop in diesel emissions since 1990. ammonia reacts with the various nitrogen oxides in the pres- In the mid-1990s, diesel exhaust treatment systems con- ence of oxygen to produce nitrogen gas and water. sisted solely of oxidation catalysts. Some diesel exhaust cata- There are a number of non-precious-metal catalysts lysts did not use any platinum-group metals, with ceria and suitable for NOx reduction. Copper zeolites are best at low alumina providing just enough catalytic activity to oxidize temperatures, whereas iron zeolites perform best at high particulate matter enough to meet emission standards then temperatures. Vanadium-based catalysts, which are very in force. The introduction of ultra-low-sulfur diesel fuel in inexpensive, are also effective but are not appropriate for 2007 opened new opportunities for diesel exhaust emissions U.S. diesels equipped with particulate filters because of the control systems. Particulate filters became standard, as did high temperatures at which those filter systems must operate catalytic converters with high precious-metal content. These (Cavataio et al., 2007). new systems operated under lean conditions to control NOx Zeolites, Lambert explained, are made of aluminum oxide emissions, and some engine manufacturers added NOx traps. and silicon oxides with a crystalline structure. They are found In 2010, engine manufacturers added an extra reductant in nature but in most cases are synthesized to achieve the high to the vehicles in the form of aqueous urea and turned to a purity needed for industrial purposes. Zeolites are in wide- copper catalyst for NOx control (Figure 4-3). The complete spread use as water softeners, absorbents, and desiccants exhaust treatment system now includes a platinum-group and in oil refining.
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27 REPLACING CRITICAL MATERIALS WITH ABUNDANT MATERIALS Line Heater Diesel Exhaust Fluid Tank Unit No x /Temperature Sensor Pressure and Temperature Sensors Exhaust Gas Temperature Sensor DEF Injector Diesel Oxidation Catalyst Selective Catalytic Reduction Catalyst Diesel Particulate Filter 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 amount of platinum on the copper-zeolite catalyst turns the known as chabazite, which has a very small average pore latter into an ammonia oxidation catalyst instead of a NOx size, combined with copper is a suitable diesel exhaust cata- reduction catalyst. lyst (Kwak et al., 2010; McEwen et al., in press) in gure 4 F an SCR Lambert noted that Ford researchers first worked with a system. Numerous academic studies om originalcopper is f suggest that sour e beta-type zeolite as the copper support, but that this combi- d 2011) located inside the cage of the zeolite structure and that ammo- nation was poisoned by hydrocarbons to some extent. The ve tor edit nia and NOx enter the cage, where they react in the presence catalyst could be regenerated by heating it to 500°C, but of copper and oxygen to produce nitrogen gas and water. treatment at this temperature was found to produce melting There were a number of challenges to overcome to com- that destroyed the zeolite’s structure. Moving to chabazite mercialize the SCR system that Ford now uses with its diesel solved this problem. She added that chabazite’s small pore engines. First, Lambert and her colleagues had to stabilize size prevents larger hydrocarbon molecules from reach- the platinum-based oxidation catalyst that sits in front of ing the active copper catalyst, preventing the formation of the SCR system. They accomplished this task by adding dioxins in diesel exhaust. palladium to the catalyst mixture. After working with vari- Sulfur can negatively impact copper-zeolite catalyst activ- ous ratios of platinum to palladium, they found that a 1:4 ity, particularly at temperatures below 300°C. Sulfur can be platinum-to-palladium mixture resulted in the best combina- removed from the catalyst at filter regeneration temperatures, tion of hydrocarbon oxidation at cold-start temperatures and however. In fact, research found that at the now-mandated stability. The latter is important because volatilized platinum level of sulfur allowed in diesel fuel—15 ppm or less—the interferes with the NOx reduction process by poisoning the copper-chabazite catalyst can tolerate the amount of sulfur copper catalyst in the SCR system (Cavataio et al., 2009). absorbed between 500-mile regenerations and still reduce Lambert noted that it is important enough to avoid precious- NOx levels enough to meet exhaust standards. metal contamination of the copper catalyst that the two com- The use of aqueous urea filtration is possible because ponents are made in separate buildings. Having even a small manufacturers and suppliers worked together under the aus-
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28 THE ROLE OF THE CHEMICAL SCIENCES IN FINDING ALTERNATIVES TO CRITICAL RESOURCES pices of the U.S. Council for Automotive Research to define has been very helpful in understanding energy potentials, product specifications. Aqueous urea is now sold as Diesel reaction intermediates, and step-by-step mechanisms. Exhaust Fluid™ (DEF™) in amounts ranging from gallon With regard to the synthetic complexity and cost of the bottles to drums. The cost of DEF ranges from $2.79 per gal- ligands used with cheap metal catalysts, Bullock acknowl- lon in bulk to $4.65 per gallon in bottles. The Department of edged that these are important issues. He added, though, Energy created a website, www.finddef.com, to point users that the ligands developed at PNNL, despite their apparent to retail locations that sell DEF. complexity, are easy to make in a two-step synthesis from The use of urea-based SCR conversion systems not only simple starting materials. has a direct effect on NOx emissions, but data from the He also noted that the cheap metal catalysts developed Environmental Protection Agency suggest that it has had so far for organic synthesis have not yet achieved turnover an indirect effect on carbon dioxide emissions. Because the rates comparable to palladium-based catalysts. He hoped that SCR systems are performing the bulk of NOx reduction, industry would now step in and take the catalysts developed diesel engines can now run at higher fuel efficiencies, which in academia and make the improvements needed to create reduces carbon dioxide emissions. At light and moderate commercially viable catalysts based on cheap, abundant engine loads, engines equipped with urea-based NOx control metals. systems will have a greater than 10 percent advantage over Finally, Bullock pointed to the importance of being open engines equipped with other NOx control systems. Urea- to unusual and unexpected results. Past examples of such based systems are now used on the majority of diesels sold advances should be kept in mind so that novel and important starting in 2010. results are not ignored because they are so different from Summarizing the impact of medium-duty diesel vehicles on past findings. platinum-group metal utilization, Lambert said that volumes Lambert was asked about the high-sulfur marine have increased since 2005 because of more stringent emissions diesel used in many parts of the world, and she noted that standards, but the move in 2010 from an all-platinum catalyst vanadium-based catalysts are highly tolerant of sulfur, but to a palladium-rich catalyst has dropped platinum use close to currently they lack the necessary temperature stability. 2007 levels. Diesel engines still use more platinum than in I mproving that stability could be a fruitful avenue of comparable gasoline-powered medium-duty trucks, but that research, and in fact a number of catalysts have been devel- gap is shrinking. oped. She added that there is little commercial pressure Looking to the future, Lambert noted that adding an SCR for such a catalyst because regions of the world that use to other lean NOx catalyst components may enable further high-sulfur diesel fuel do not have stringent standards for advances in the development and adoption of lean-burn tech- particulate matter. nologies for gasoline engines that lower or eliminate the need In response to a question about what happens when a for platinum-group metal catalysts (Xu et al., 2010). However, vehicle runs out of urea, Lambert replied that the vehicles adopting SCR technologies for use with gasoline engines will are designed to run at slower speeds, which frustrates drivers require lowering the sulfur content of gasoline from its current and provides an incentive to refill the urea containers on the 80 ppm to 15 ppm or developing technology that removes vehicles. sulfur from the exhaust stream prior to the SCR unit. Lambert pointed to an increase in research on batteries Research is also aimed at boosting NOx oxidation activ- and electric vehicles at Ford and elsewhere. But she also said ity of palladium to continue the trend of replacing platinum that “the internal combustion engine is not dead.” Funda- with palladium as a cost-reduction strategy. Toward this end, mental research still needs to be done on exhaust gas emis- researchers at General Motors have developed a platinum- sions even as electric vehicle technologies advance. free perovskite catalyst that rivals the performance of a platinum-based catalyst at reducing NOx levels under lean- burn conditions (Kim et al., 2010). DISCUSSION In response to a question about the role of theory in his work on electrocatalysts, Bullock said that theory and com- putational 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 pre- dict which molecules to make from the ground up, but theory