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Critical Materials in Catalysis

A major use of critical materials is in catalysis, which is the subject of both this chapter and the next chapter. Catalysts can be used on a large scale, as is the case with many heterogeneous catalysts, or on a small scale, as is the case with most homogeneous catalysts. A critical element may not be the most costly component of a catalyst, but the availability of that element or sudden changes in cost can disrupt its use. For example, as described in this chapter, platinum-based catalysis and electrocatalysis hold particular promise in several energy technologies, but platinum falls into one of the highest levels of criticality, as described in the previous chapter. Major research efforts are therefore under way to reduce the use of platinum in catalytic processes and to develop replacements for platinum in various applications.

CRITICAL MATERIALS IN CATALYSIS

“Catalysis is a broad topic, one that covers many orders of magnitude, from the 5-million-pound-per-hour scale down to the gram scale,” explained James Stevens. However, catalysts can be divided into two main divisions, homogenous and heterogeneous catalysis, each with its own issues for critical materials.

In terms of volume, most catalysts are heterogeneous catalysts. Heterogeneous catalysts are easy to separate from product, and they work at very high temperatures, with alumina and silica catalysts capable of operating at temperatures of 1000°C. Heterogeneous catalysts are challenging to study and afford a poor degree of synthetic control. However, if heterogeneous catalysts could be rationally designed, they could offer tremendous advantages—for example, in emissions control applications.

Homogenous catalysts are more sophisticated chemically and can perform more useful chemical reactions. They are more selective and produce higher reaction rates than heterogeneous catalysts, and it is possible to design complex structures for specific tasks and use high-throughput, combinatorial methodologies to discover new homogenous catalysts and new reaction schemes that use new homogenous catalysts. However, homogenous catalysts are limited to use in lower-temperature chemical reactions.

Homogenous catalysts are by their nature used in very small amounts and produce many moles of product per mole of catalyst, with very high reaction rates. For example, a chiral ferrocynal iridium phosphine catalyst, used in the largest application of asymmetric synthesis to perform an enantioselective hydrogenation as part of an herbicide synthesis, produces over 2 million turnovers per catalyst molecule at a rate of 600,000 per hour (Blaser et al., 1999). Producing 10 million kilograms of product per year requires about 5 kilograms of iridium, assuming that no recycling occurs. This catalyst accounted for about 0.1 percent of U.S. imports of iridium in 2010.

Costs and the Supply Chain

While the 5 kilograms of iridium used per year in this catalyst would cost about $100,000, again assuming that no recycling occurred, that would account for less than 30 percent of the cost of the catalyst (Blaser, 2002). The most expensive components of this catalyst are the ligands surrounding the metal, which is often the case with homogenous catalysts. In addition, although the metal can usually be recycled, the ligands cannot. Other examples showing that ligand costs can dominate catalyst costs include a constrained-geometry titanium catalyst for ethylene co-polymerization; a zirconium catalyst for isotactic polypropylene synthesis (for which the metal accounts for about only 0.5 percent of the total catalyst cost); and a rhodium catalyst for enantioselective hydrogenation (for which the



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3 Critical Materials in Catalysis A major use of critical materials is in catalysis, which heterogeneous catalysts, and it is possible to design com- is the subject of both this chapter and the next chapter. plex structures for specific tasks and use high-throughput, Catalysts can be used on a large scale, as is the case with combinatorial methodologies to discover new homogenous many heterogeneous catalysts, or on a small scale, as is the catalysts and new reaction schemes that use new homog- case with most homogeneous catalysts. A critical element enous catalysts. However, homogenous catalysts are limited may not be the most costly component of a catalyst, but the to use in lower-temperature chemical reactions. availability of that element or sudden changes in cost can Homogenous catalysts are by their nature used in very disrupt its use. For example, as described in this chapter, small amounts and produce many moles of product per platinum-based catalysis and electrocatalysis hold particular mole of catalyst, with very high reaction rates. For example, promise in several energy technologies, but platinum falls a chiral ferrocynal iridium phosphine catalyst, used in the into one of the highest levels of criticality, as described in the largest application of asymmetric synthesis to perform previous chapter. Major research efforts are therefore under an enantioselective hydrogenation as part of an herbicide way to reduce the use of platinum in catalytic processes and synthesis, produces over 2 million turnovers per catalyst to develop replacements for platinum in various applications. molecule at a rate of 600,000 per hour (Blaser et al., 1999). Producing 10 million kilograms of product per year requires about 5 kilograms of iridium, assuming that no recycling CRITICAL MATERIALS IN CATALYSIS occurs. This catalyst accounted for about 0.1 percent of U.S. “Catalysis is a broad topic, one that covers many orders imports of iridium in 2010. of magnitude, from the 5-million-pound-per-hour scale down to the gram scale,” explained James Stevens. However, cata- Costs and the Supply Chain lysts can be divided into two main divisions, homogenous and heterogeneous catalysis, each with its own issues for While the 5 kilograms of iridium used per year in this critical materials. catalyst would cost about $100,000, again assuming that In terms of volume, most catalysts are heterogeneous no recycling occurred, that would account for less than catalysts. Heterogeneous catalysts are easy to separate from 30 percent of the cost of the catalyst (Blaser, 2002). The product, and they work at very high temperatures, with most expensive components of this catalyst are the ligands alumina and silica catalysts capable of operating at tempera- surrounding the metal, which is often the case with homog- tures of 1000°C. Heterogeneous catalysts are challenging to enous catalysts. In addition, although the metal can usually study and afford a poor degree of synthetic control. However, be recycled, the ligands cannot. Other examples show- if heterogeneous catalysts could be rationally designed, they ing that ligand costs can dominate catalyst costs include could offer tremendous advantages—for example, in emis- a c onstrained-geometry titanium catalyst for e t hylene sions control applications. co-polymerization; a zirconium catalyst for isotactic poly- Homogenous catalysts are more sophisticated chemi- propylene synthesis (for which the metal accounts for about cally and can perform more useful chemical reactions. They only 0.5 percent of the total catalyst cost); and a rhodium are more selective and produce higher reaction rates than catalyst for enantioselective hydrogenation (for which the 13

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15 CRITICAL MATERIALS IN CATALYSIS The development of lower-cost catalysts that meet critical The latest version of this catalyst, introduced a decade performance characteristics could have significant economic ago by Celanese, uses rhodium again but offers better iodine benefits. However, the cost of the metal is not the only factor and water management and allows for larger plant construc- that must be considered. A catalyst that uses an inexpensive tion. This, in turn, lowered the cost per pound of the final metal but expensive ligands could end up costing the same. product. Though rhodium is a critical material, the amount Stevens elaborated to say that the current platinum-based used by the chemical industry to manufacture acetic acid is catalyst is used because it has the desirable cure kinetics that relatively small. enable the polymer to remain liquid for the time necessary to fill a mold. It also produces regioselectivity and chemo- Emissions Catalysts selectivity, resulting in high-molecular-weight polymers, which is a critical parameter. Emissions catalysts, particularly for use in cleaning Potential approaches to meeting these requirements with a up diesel exhaust, are significant users of platinum-group lower-cost catalyst include identifying new silane and olefin metals. Diesel is growing as a percentage of vehicle pro- activation chemistries and new reaction mechanisms that duction worldwide, largely because of the higher fuel effi- do not require the two-electron redox processes catalyzed ciency of diesel engines. However, diesel exhaust also has so effectively by platinum. The application of new high- significant emissions problems. Removing NOx from diesel throughput catalyst discovery methodologies holds promise exhaust requires a reduction catalyst and is difficult using for making such advances. existing catalyst technology, whereas removal of carbon Another promising area is the development of new monoxide and unburned hydrocarbons requires an oxidation catalysts for acetic acid production, the second largest use catalyst (Figure 3-2). Particulates must also be removed from of homogeneous catalysis. In the 1960s, BASF launched diesel exhaust, and filters contain metal catalysts. an acetic acid process that used a cobalt catalyst, but this Today, emissions catalysts account for 81 percent of U.S. was replaced in 1970 with Monsanto’s greatly improved platinum-group metal imports. Emissions catalysts also use rhodium-based catalyst. The new catalyst was more highly significant quantities of cerium, which acts as an oxygen selective and required lower pressures, reducing overall buffer in NOx reduction. Looking ahead, new filter struc- costs per pound of acetic acid. Rhodium recovery was also tures will require new catalysts, and there are opportunities very high. to develop catalysts that do not use platinum-group metals. This catalyst, however, was replaced in the 1990s by an Nickel-based catalysts may prove useful, as may copper- iridium-based catalyst developed by BP. This catalyst, which based catalysts now that concern over the potential to pro- could be used in the same plant as Monsanto’s rhodium- duce dioxins as a byproduct has been alleviated. based catalyst, had higher selectivity still and afforded better water use, thereby lowering the capital costs associated with Hydroformylation and Enantioselective Catalysis drying columns. The lower capital costs more than made up for the increased cost of the metal used. Iridium, unlike The biggest application of homogeneous catalysis, in rhodium, is not at the moment considered a critical mate- terms of the amount of product made, is in hydroformylation. rial because it is not used in significant quantities today. The first catalysts were rolled out in the 1940s and used If demand for iridium increases significantly, it could be cobalt, a first-row transition element. Cobalt catalysis suf- considered a critical element. fered from the need for high pressures and temperatures, FIGURE 3-2 Removing pollutants from diesel emissions requires several catalysts and filters. SCR: selective catalyst reduction; LNT: lean NOx trap. SOURCE: Stevens (2011).

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17 CRITICAL MATERIALS IN CATALYSIS turnover rates and under mild conditions. There is still room, however, to improve the chemoselectivity and functional group tolerance. “The key is to find out the critical perfor- mance characteristics needed for commercial viability,” said Stevens. Finally, Stevens noted that there is real need for high- performance catalysts that can provide enantioselectivity around a carbonyl bond. Catalysts now available achieve turnover numbers on the order of 10 to 1,000 s–1and function at very low rates. ALTERNATIVE MATERIALS TO REPLACE PLATINUM IN CATALYTIC AND ELECTROCATALYTIC APPLICATIONS In the quest for secure sources of energy, the conversion of renewable resources into liquid fuels—biomass into diesel and gasoline, for example, and water into hydrogen—is among the most active areas of research. Platinum-based FIGURE 3-5 Thin-film and monolayer catalysts can greatly reduce catalysis and electrocatalysis hold particular promise in the amount of platinum needed for catalysis. ML: mono-layer SOURCE: Esposito and Chen (2011). producing renewable, domestic sources of transportation fuels and other basic chemicals, but platinum is a strate- gic, imported metal. In the case of electrocatalysis, said Jingguang Chen, efforts are ongoing to reduce the amount num catalysis occurs only on the very top layer of a platinum of platinum needed by a factor of 10 to produce hydrogen particle (Figure 3-5). via the electrolysis of water. For the catalytic conversion of biomass into liquid oxygenates, research is focusing on Hydrogen Production from Water Electrolysis m pp ma e completely replacing platinum with tungsten in the neces- sary catalysts. One rationale for developing efficient methods of generat- As has already been highlighted, platinum-group metals ing hydrogen via water electrolysis is that hydrogen could are used widely in the chemical and refining industries. In the then be used to store intermittent solar or wind energy. area of clean energy technologies, platinum is the most prom- Electrons would be harvested during production and used to ising catalyst for low-temperature fuel cells, electrolyzers, electrolyze water, producing hydrogen that would be stored and photoelectrochemical cells, which use sunlight to power and converted back into electricity using a fuel cell at night electrolysis. These clean-energy catalysts, however, require or when winds are calm. The major obstacle to this scenario larger platinum nanoparticles than are needed in traditional is that large-scale commercialization is impossible because heterogeneous catalysts, which would boost platinum use of the demand for and cost of the platinum catalysts now dramatically. Indeed, recent studies, said Chen, suggest that used in electrolysis and fuel cells. there is not enough platinum in the world to use such cata- There are two ways to use sunlight to power electrolysis. lysts on the scale envisioned to power industrial processes. The indirect method is to use photovoltaic panels to convert The major challenge in replacing platinum with an early sunlight into electricity that then is used to electrolyze water. transition metal is that the electronic properties are different. The direct method is to use a catalyst that generates electrons Many research teams are attempting to use carbon to modify directly from sunlight and uses those electrons to produce the electronic properties of tungsten and other metals that, hydrogen and oxygen from water. Both processes require like platinum, are in the 5d group of metals. In particular, platinum in the cathode. metal carbides have many properties desired for industrial As part of a concerted effort to develop a monolayer heterogeneous catalysts. They have high hardness and wear platinum cathode for electrolysis, Chen and his collaborators resistance, are stable at high temperatures, and are excel- asked the following questions: lent electrical conductors. Using theory and model systems, researchers have developed design principles that have • What is the descriptor responsible for making plati- enabled them to create new metal carbide catalysts that have num the optimal catalyst for the hydrogen evolution platinum-like activity and stability. reaction? Efforts to reduce, rather than eliminate, platinum use • Does monolayer platinum on a tungsten carbide par- are focusing on creating thin-layer or monolayer supported ticle meet such a descriptor for high activity in the catalysts. This approach takes advantage of the fact that plati- hydrogen evolution reaction?

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18 THE ROLE OF THE CHEMICAL SCIENCES IN FINDING ALTERNATIVES TO CRITICAL RESOURCES Platinum-Free Catalysts for Biomass Conversion • Is monolayer platinum on a tungsten carbide particle stable under the relatively harsh conditions of the In addition to his work on electrocatalysis, Chen and his hydrogen evolution reaction? collaborators have been developing platinum-free catalysts for converting biomass into oxygenated chemicals. These The key feature of the descriptor comes from research studies use quantum calculations and model systems to on the binding energy of hydrogen to various metals. These rationally design potential catalysts. Such studies enabled his experiments show that the hydrogen binding energy with group to develop a nickel-on-tungsten carbide catalyst that platinum is −0.46 electron volts (eV). Quantum calculations performs better and is more stable than a nickel-on-platinum are used to predict which carbides and monolayer systems catalyst for converting glycoaldehyde into hydrogen and would produce a hydrogen binding energy similar to that carbon monoxide, which then can be combined using stan- seen with platinum. Tungsten carbide alone has a predicted dard chemistry to produce a wide range of chemicals. hydrogen binding energy of −0.99 eV, but when combined Though these examples show that it is possible to reduce with a monolayer of platinum, the predicted hydrogen bind- or even eliminate platinum in industrially important cata- ing energy drops to −0.43 eV. lysts, several challenges remain for these efforts to have a Chen and his collaborators confirmed these predictions major impact. One is to increase the surface area of the plati- experimentally. Tungsten carbide alone, which costs four num on metal carbide materials. Currently, high temperatures orders of magnitude less than platinum, is inactive electro- are needed to make stable metal carbides, but the resulting chemically. However, electrocatalytic activity increased materials have low surface areas. as the density of platinum on a tungsten carbide electrode A related challenge is to develop new synthetic methods increased. When the platinum density reached one mono- that create metal carbide with high surface areas, which layer, electrocatalytic production of hydrogen was virtually would result in catalysts with much higher activity. Similarly, identical to that seen with pure platinum. there is a need for better methods of depositing monolayers Chen noted that platinum binds very tightly and stably of platinum on metal hydrides while inhibiting carbon depo- to tungsten carbide. Furthermore, extended electrolysis sition on the particles, which reduces catalytic activity. A tests showed that monolayer platinum on tungsten carbide challenge in the electrocatalysis area is to develop platinum- structures was stable. Hydrogen production remained con- sparing metal carbide materials that have long-term stability stant, and physical characterization using scanning electron over a range of pH values. microscopy and x-ray photoelectron spectroscopy confirmed that the platinum monolayer was stable on tungsten carbide DISCUSSION under the harsh electrolysis conditions. In fact, said Chen, the platinum monolayer on tungsten carbide material is more In response to a question about whether there might be a stable than the commercial catalyst used today. balance between complete coverage of a metal carbide parti- Thin films such as these are useful for experimentation cle with platinum and somewhere between 25 and 75 percent and model calculations, but platinum monolayers on support- coverage in terms of cost and stability, Chen said further tests ing particles would perform better in commercial applica- may demonstrate that reduced coverage may be possible to tions. Chen’s group is using atomic layer deposition to form achieve the same results. He explained that his group does not platinum monolayers on tungsten carbide particles and to have the facilities to perform hundreds of hours of stability create a commercially viable material after 10 deposition tests to determine the optimal coverage, and he hopes that cycles. Electrolysis measurements show that the platinum- a commercial vendor will be interested in conducting such on-tungsten carbide particles produced after 10 atomic-layer tests. Chen also noted that recovering the platinum from these deposition cycles are just as active as commercial platinum metal carbide materials appears to be easy. catalyst. However, elemental analysis reveals that these Ernest Chamot, of Chamot Labs, asked if biomimetic particles contain 10-fold less platinum than the commercial approaches might be better than the rational design approach catalyst. for creating new catalysts for biomass conversion. Chen This is not the end of the story. The same binding energy replied that a biomimetic approach would likely work well model suggests, as Chen said, that “instead of just using for homogeneous catalysis. Such an approach would center tungsten carbides as a base material, we can actually think on developing better ligands. For heterogeneous catalysts, about other carbides, such as molybdenum carbides. We can however, a biomimetic approach is not likely to be successful. even think about using palladium on metal carbides.” He also Levi Thompson, Richard Balzhiser Professor of Chemical noted that this approach can be used to design platinum on Engineering at the University of Michigan, and, with Thiel metal carbide catalysts for other electrochemical devices, and Eggert, the third member of the workshop organizing including their use as the anode material in direct alcohol fuel committee, noted that his group has observed that the under- cells or the cathode material for oxygen reduction reactions. lying metal carbide can actually dissolve under some condi-

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19 CRITICAL MATERIALS IN CATALYSIS tions. He wondered if the highly oxygenated conditions in miles of sugar cane. “The net primary productivity of any a biomass reactor would be conducive for dissolution of the plant-based photosynthetic process is a real limitation when underlying carbide, and he asked if Chen had observed that you talk about the scale that’s required to make chemicals in his experiments. Chen replied that there is some tungsten on a big scale.” dissolution, but only a small amount. The trick to suppress- Stevens also was asked what advice he would give a ing dissolution, he explained, is to use very high pressures 22-year-old thinking about picking a research advisor, and to compensate for oxidation. he replied that industry is currently having trouble finding In response to a question about the efficiency of biomass- chemical engineers with the skills industry needs. He also to-chemical processes, Stevens noted that photosynthesis thought that because the efficiency of photovoltaics is so is actually very inefficient, using less than 1 percent of the much greater than the biomass conversion processes, photo- incident energy. Even with sugar cane in the tropics, which is voltaics would be the more promising field. Finally, over the one of the best plants in terms of turning sunlight into useable next few decades basic research will remain essential for materials, efficiency is less than 1 percent. As a result, mak- energy issues in areas such as the manipulation of matter on ing 700 million pounds a year of ethylene requires 450 square a small scale.

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