Embed Size (px)
Citation preview
158 nature materials | VOL 10 | MARCH 2011 | www.nature.com/naturematerials
commentary
Managing the scarcity of chemical elementsThe issues associated with the supply of rare-earth metals are a vivid reminder to all of us that natural resources are limited. Japan’s Element Strategy Initiative is a good example of a long-term strategy towards the sustainable use of scarce elements.
Eiichi Nakamura and Kentaro Sato
For chemists and materials scientists, the period from the 1960s to the 1980s was an era when pioneers were
racing through the unexplored and fertile wilderness of the periodic table, searching for treasure suitable for technical applications. Indeed, a number of new materials found in this ‘element hunt’ have been successfully used in high-tech products that now play an indispensable role in our lives.
The element hunt was not only prevalent in materials science, but was also being hotly pursued in the field of synthetic organic chemistry. For example, in the palladium-catalysed organic synthesis process developed at that time by Heck, Negishi and Suzuki, for which they were awarded the 2010 Nobel prize in chemistry, the properties of palladium, zinc and boron are used to
the full1. These functions cannot easily be provided by other elements.
A few decades on, we are in a crisis, as the seeming abundance as well as availability of many elements turns out to be an illusion. One of the most imminent issues to have drawn attention is that of the rare-earth elements. The properties of these elements are similar, but their uses and outputs differ. Cerium is essential for the glass-polishing process in lens manufacturing, neodymium and dysprosium in manufacturing strong magnets, and yttrium and europium as laser light sources2.
Although, contrary to their name, rare earths are not that rare in the Earth’s crust, their supply is limited. China, which produces 97% of all rare-earth elements3, has gradually reduced its export quotas
and at the same time raised export taxes. In September 2010, the country announced a 40% reduction in rare-earth exports, and in December, an increase in tariff rates caused a shock throughout the world. In response, the United States decided to reopen the Mountain Pass mine4 and Japan, one of the largest consumers of rare-earth metals, started emergency efforts to develop mineral veins through international cooperation. Other countries are also taking various measures to address the situation.
Concerns are not just limited to these rare earths. Phosphorus, which is not generally regarded as scarce, is one of the elements considered to be in possible danger of depletion. One report5 indicates that if the population continues to increase and the global economy continues to grow at the current pace, the quantity of phosphate ore that is available for mining might become depleted in the next 50 years. A shortage of phosphorus fertilizer would cause significant damage to agriculture, resulting in an immediate food crisis. As phosphate ore is also widely used for industrial purposes such as in incombustible materials, rust-prevention agents and food additives, a wide range of industries would not be able to continue their operations if the supply stopped. From 2006 to 2008, the price of phosphorus rose sharply because of the biofuel boom and the Sichuan earthquake in China, which occurred near phosphate mines6. It seems likely that the supply of phosphorus will never be without significant economic risk in the future.
Some rare elements are not only scarce in their output but also imbalanced in their distribution among countries, making the issues more complicated. For example, the production of platinum group metals, important as catalysts, is concentrated in South Africa and Russia; niobium, which is used as a steel additive, is mostly produced in Brazil and Canada. The tantalum used in capacitors is almost exclusively produced in Australia and Brazil7.
Figure 1 | Science and Technology Future Strategies (a Materials Science division) Workshop participated in by leading Japanese materials scientists, held in April 2004 in Hakone (Japan). Front row includes the author (E.N.) (third from right), Kohei Tamao, representative coordinator of the workshop and current director of the RIKEN Advanced Science Institute (sixth) and Shinji Murai, president of the workshop and Principal Fellow in Chemistry/Materials Science at the JST (seventh); second row includes Koichi Kitazawa, the current President of JST (fourth from right), and Hideo Hosono (seventh from right).
nmat_2969_MAR11.indd 158 8/2/11 16:14:31
© 2011 Macmillan Publishers Limited. All rights reserved
nature materials | VOL 10 | MARCH 2011 | www.nature.com/naturematerials 159
commentary
These rare elements tend to be prime targets for speculation and price manipulation, and therefore destined to be extremely economically unstable. Issues involving rare metals have already been used as political and economic tools, not only exerting a negative impact on the industry, but also possibly triggering conflicts between countries.
the element strategy initiativeThis is not the first time that humans have experienced a shortage of materials relying on certain elements. About a century ago, when the advent of food shortages caused by the lack of nitrogen fertilizers became apparent, the advanced chemistry of that time averted disaster: the Haber–Bosch process saved the world from the crisis. The process enables ammonia and nitrate, which are essential for food production, to be synthesized from nitrogen in the air.
The power of science may not always be successful in overcoming crisis. It is impossible to produce scarce elements in the same way as chemical compounds can be synthesized through reactions such as the Haber–Bosch process. For example, the production of new elements by nuclear reactions is out of the question because of its risks and cost.
This reasoning led the Japanese government to develop the concept of an ‘Element Strategy Initiative’, which was first proposed by one of us (E.N.) at the 2004 Science and Technology Future Strategies Workshop (Fig. 1), sponsored by the Japan Science and Technology Agency (JST). A conclusion reached was that where resources are scarce but high technology is abundant, necessity is the mother of invention. A call was made for research into alternatives for scarce elements.
The proposal was adopted by the Japan Ministry of Education, Culture, Sports, Science and Technology and the Ministry of Economy, Trade and Industry. As a consequence, the Element Strategy Commission was established8–11. The Commission discussed in detail the concept and direction of the Element Strategy Initiative and its aim to develop “a strategy for the use of elements to build a sustainable society.” A national initiative to promote science and technology was established in 2007.
This government initiative consists of four pillars: substitution, regulation, reduction and recycling. Of these, the substitution strategy, which aims to replace rare or toxic elements with abundant and harmless elements, will be particularly important for scientists. Two main themes form the basis of scientific research necessary to achieve
substitution. First, problems associated with materials and their uses need to be identified multilaterally and systematically, and ways to control them researched in a top-down approach. Ways to achieve a certain target function of a given compound must then be studied in terms of a material’s electronic states, atomic arrangements and molecular structure. In parallel, innovative materials synthesis and growth processes are designed in a bottom-up approach.
alternative materialsCarbon is present abundantly and universally, and can be transformed into many kinds of compounds. It is therefore a very promising candidate as an ingredient in alternative materials for a range of functions. A pioneering example is the organic semiconductors that are used in OLEDs and thin-film solar cells. In that respect, the position of organic thin-film solar cells relative to silicon solar cells and compound solar cells should be assessed not merely in terms of our energy policy, but also in terms of the element strategy.
Another currently promising example is the use of alternative materials as the catalyst in fuel cells12–14. Fuel-cell vehicles are regarded as a clean and efficient means of transportation. With currently available technology, however, an impractically large amount of platinum would be needed per vehicle, which means that even if all the currently mined platinum were used for fuel cells, it would only be enough to support two million vehicles annually, or less than 5% of the global production of automobiles. In recent years, it has been found that nitrogen-doped graphene can be used as a substitute for the platinum catalyst12–14. If this is put into practical application, one of the huge obstacles to the wider use of fuel cells would be removed. Another unique approach, although apparently not yet practical, is nitrogen fixation with a complex of fullerene and cyclodextrin as catalysts at normal temperatures and pressures15.
In the same vein, croconic acid (Fig. 2), an organic compound that has been well known for many years, proves to be a
high-performance organic ferroelectric16. Croconic acid shows spontaneous polarization as strong as that of barium titanate and with sufficiently stable ferroelectricity at room temperature. The discovery that a simple low-molecular-mass organic compound consisting of carbon, hydrogen and oxygen exhibits excellent ferroelectricity has had great impact and could prompt further discoveries of high-performance compounds.
Organocatalysis17, the use of a catalyst consisting of carbon, hydrogen, oxygen and other non-metal elements instead of metal catalysts, should also be part of the wider element substitution strategy. Organocatalysis is considered to have less impact on the environment and particularly promises to reduce the risks associated with heavy metal residues, especially in pharmaceutical products.
Some may think that even the supply of carbon may not be limitless. So what about the idea of producing plastic from water (Fig. 3)? This ‘aquamaterial’ consists of 98% water and has physical properties similar to those of silicone rubber18.
When searching for alternative materials, ideally we will be able to create a material with new properties that do not just substitute for those of traditional materials and functional substances, but exceed them. To achieve this goal, we must understand the properties of each element more deeply, and we need a breakthrough supported by innovative ideas.
Take flat panel displays. Over the past two decades, the flat panel display has changed the technology landscape. Flat panel displays are used as TV screens, in cell phones and in many other applications. However, the production of flat panel displays faces restrictions and shortages that previously have attracted little attention. The issue is indium, a component of the indium tin
Figure 3 | Aquamaterial: non-flammable and environmentally friendly, easy to mould, and with silicone-rubber-like properties. Photo courtesy of T. Aida.
Figure 2 | The structure of croconic acid.
nmat_2969_MAR11.indd 159 8/2/11 16:14:31
© 2011 Macmillan Publishers Limited. All rights reserved
160 nature materials | VOL 10 | MARCH 2011 | www.nature.com/naturematerials
commentary
oxide used for the transparent electrodes that connect the elements of the display. Indium is a trace by-product of mines producing zinc, and is difficult to extract because it naturally occurs with other toxic elements such as cadmium. These environmental implications and the constraints they place on the supply of indium are not something we cannot just sit back and watch.
In 2002, Hideo Hosono and colleagues announced that they had discovered a material that could substitute for the indium tin oxide currently used19. This new material is composed of 12CaO·7Al2O3, in other words lime and alumina, more simply known as cement — a construction material that has been used since ancient Roman times. By encapsulating an electron in a three-dimensional nanocage within the crystal structure, the material can be made to show metallic conduction while maintaining its
transparency. Because this new material is composed of aluminium, calcium and oxygen, which are abundant on Earth, there is no lack of supply. This is just one of the successful examples where a rare element with unusual and useful properties can be replaced with ubiquitous elements to develop properties that cannot be found in existing transparent conductors such as indium tin oxide.
As another example, iron catalysts have recently attracted the attention of researchers in the field of organic synthetic chemistry. In the late 1990s, one of us (E.N.) started to study highly active and selective iron-catalysed reactions, where the reaction progresses under more moderate conditions than are required for palladium-catalysed reactions, and with a selectivity that cannot be achieved with rare metal catalysts20. Iron toxicity is negligible, and therefore the
treatment of iron waste is easy, which is a great advantage.
regulatory strategyFrom a regulatory perspective, it is also important to develop substitutes for elements that are expected to cause potential hazards or shortages. One such example is a new material that undergoes a phase transition from semiconductor to metal when irradiated by light21 (Fig. 4). Made of titanium oxide, which is safe and abundant, the material theoretically could become a recording medium with a larger capacity than current Blu-ray disks. Because it does not use germanium, tellurium or antimony, which are currently used for DVDs, this new material has created high expectations in terms of future applications and environmental protection. Because materials such as this help not only to avoid risks but also to grow new industries, more effort needs to be focused on this aspect in the future.
reduction strategyTo address issues associated with rare element depletion, we also need to consider reducing the use of the elements themselves. The use of dysprosium in neodymium magnets is one such example. Dysprosium is added to enhance the ferromagnetism of a material at high temperatures but is not necessarily essential to achieve high magnetism. Researchers are now developing technology to reduce the use of dysprosium by improving the crystallization process22. But this is not where this strategy stops. The aim is, of course, to create a strong magnet that uses neither dysprosium nor neodymium.
recovery strategyRecovery of rare metals from products is also of high practical importance. So far, recycling efforts have mainly focused on more sophisticated element isolation techniques and the optimization of materials recycling by improving the material flow. Another important factor is the development of technology to mine resources from low-grade sources. In Japan, for example, there has been concern over the large amounts of rare elements contained in discarded high-tech devices. This is known as ‘urban mining’, and the technology to recycle metals from these gadgets is being developed. According to Japan’s National Institute for Materials Science (NIMS), there are 6,800 tons of gold (16% of the world reserves), 60,000 tons of silver (22%) and 1,700 tons of indium (15.5%) in Japanese urban mines23. Effective recycling of these rare metals is an important issue that needs to be addressed. Included in ongoing studies are notable approaches such as the collection of rare metals using microorganisms24.
λ–Ti3O5
β
β
metallic conductor
β–Ti3O5
semiconductor
Ti(1)
Ti(1)
Ti(2)
Ti(2)
Ti(3)
Ti(3)
λ
λ
Light
Light
Figure 4 | Titanium oxide (inset photos) undergoing a phase transition induced by light. Figure courtesy of S. Ohkoshi.
nmat_2969_MAR11.indd 160 8/2/11 16:14:31
© 2011 Macmillan Publishers Limited. All rights reserved
nature materials | VOL 10 | MARCH 2011 | www.nature.com/naturematerials 161
commentary
OutlookAfter exploiting many of our planet’s resources lavishly over the past few centuries, it has become clear that there are limits. To address resource and environmental issues and to build a sustainable society, materials science must take the initiative. We must be determined to take responsibility for addressing issues not only in the fields of resources, energy and the environment, but also in the life sciences and in information technology.
The element hunt has, in many cases, been based on reductionism science, and scientists have worked on a single element as thoroughly as possible. The problem we now face forces us to deal with more complex systems — the hunt for combinations of multiple elements and for new strategies that will provide us with opportunities for discovering new properties.
The issues that we need to address are extremely complex. To solve them, a radically designed interdisciplinary and well-integrated cooperative system involving researchers from a wide range of fields is indispensible. Our journey may not be easy and the issues facing researchers
are challenging. Instead of regarding this situation merely as a crisis to overcome, we should convert it into an opportunity to create a new materials science, new materials and a new industry.
In the words of T. R. Malthus, in An Essay on the Principle of Population (1798), evil exists in the world not to create despair but activity. Although the depletion of rare elements could trigger a new conflict, it could also serve as an opportunity to make a dream come true, to create new science and technology. We hope that as many researchers as possible will get involved in creating a new world of element science and will share our dream of opening the door to the future of humankind. ❐
Eiichi Nakamura and Kentaro Sato are in the Department of Chemistry, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. e-mail: [email protected]; [email protected]
References1. de Meijere, A. & Diedrich, F. Metal-Catalyzed Cross-Coupling
Reactions (Wiley, 2004).2. http://pubs.usgs.gov/fs/2002/fs087–02/3. http://www.gao.gov/new.items/d10617r.pdf
4. Bradsher, K. Challenging China in rare earth mining. The New York Times (21 April 2010).
5. Gilbert, N. Nature 461, 716 (2009).6. Tremblay, J-F. Chem. Eng. News 86, 8 (2008).7. http://minerals.usgs.gov/minerals/pubs/mcs/2010/mcs2010.pdf8. http://crds.jst.go.jp/output/pdf/contents2010.pdf p.909. http://crds.jst.go.jp/output/pdf/10sp02.pdf10. www.mext.go.jp/component/english/__icsFiles/afieldfile/
2010/11/18/1299187_08.pdf11. www.meti.go.jp/english/press/data/20090728_01.html12. Ozaki, J., Kimura, N., Anahara, T. & Oya, A. Carbon
45, 1847–1853 (2007).13. Y. Nabae. et al. ECS Trans. 25, 463–467 (2009).14. Ozaki, J., Tanifuji, S., Furuichi, A. & Yabutsuka K. Electrochim. Acta
55, 1864–1871 (2010).15. Nishibayashi, Y. et al. Nature 428, 279–280 (2004).16. Horiuchi, S. et al. Nature 463, 789–792 (2010).17. Berkessel, A. & Groeger, H. (ed.) Asymmetric Organocatalysis
(Wiley, 2007).18. Wang, Q. et al. Nature 463, 339–343 (2010).19. Hayashi, K., Matsuishi, S., Kamiya, T., Hirano, M. & Hosono, H.
Nature 419, 462–465 (2002).20. Nakamura, E. & Yoshikai, N. J. Org. Chem. 75, 6061–6067 (2010).21. Ohkoshi, S. et al. Nature Chem. 2, 539–545 (2010).22. Nozawa, N. et al. J. Mag. Mater. 323, 115–121 (2010).23. www.nims.go.jp/jpn/news/press/pdf/press215_2.pdf24. Kuroda, K., Ueda, M. Appl. Microbiol. Biotechnol. 87, 53–60 (2010).
AcknowledgementsWe acknowledge partial support by the Global COE Program, ‘Chemistry Innovation through Cooperation of Science and Engineering’, MEXT, Japan, and Strategic Promotion of Innovative Research and Development (Japan Science and Technology Agency). We thank H. Hosono, S. Murai, K. Tamao and T. Nakayama for advice.
nmat_2969_MAR11.indd 161 8/2/11 16:14:32
© 2011 Macmillan Publishers Limited. All rights reserved
