11. Platinum Group Metals The Platinum Group Metals (PGMs) consist of a group of 6 chemically very similar elements which are further classified as the light platinum metals ruthenium (Ru) rhodium (Rh) palladium (Pd) and the heavy platinum metals iridium (Ir), osmium (Os), and platinum (Pt). In general, PGMs exhibit high density, high electrical conductivity, high melting points and low reactivity. Other properties typical of transition metals are very marked such as catalytic activity due to their inclination to change valence, formation of intermediate compounds, color, paramagnetism and a strong tendency to form complexes. Platinum is the metal that is the commercially most important of all the PGMs, having the largest range of applications from jewelry to autocatalysts to electronics. Because of the unique properties of PGMs, substitutes for these elements are practically nonexistent, although they can in some situations substitute each other.1,2 PGMs are generally found together in nature, with the concentration of platinum and palladium being higher than that of the other PGMs. Therefore, platinum and palladium also dominate the production figures for PGMs. PGMs are extracted either as main metals (with co-production of all PGMs) or as a by-product of nickel mining. The former is the case for deposits mined South Africa, Zimbabwe and USA while the latter case applies to Russia and Canada. Deposits in Russia and North America have high palladium contents while the deposits in South Africa are richer in platinum 3. By far the largest primary (mine) producers of PGMs are South Africa and Russia, together accounting for more than 85% of world primary supply.

Figure 1: Distribution of PGM production4 and corresponding scores of the producing countries in the Human Development Index (HDI)5, Environmental Performance Index (EPI)6, and World Governance Indicators (WGI)7. Both the EPI and WGI are used to assess supply risks with the EU methodology for determining critical raw materials.8 ZAF = South Africa; RUS = Russia. Global production of platinum has been declining from its peak of 320 tons in 2006, and in 2012 was down to 300 tons. The supply situation is extremely tight, with 72% of production located in South Africa. World primary production of palladium stands at about 230 tons per year, with 44% of supplies coming from Russia and 35% from South Africa 9. Canada, the US and Zimbabwe are minor suppliers of palladium.

Traded volumes have been higher than production during recent years, due to the sale of Russian stockpiles, but experts consider that this source is largely depleted now.10 The high prices of PGMs have encouraged the establishment of efficient recycling chains (see Table 1) for both pre-consumer scrap (e.g. ruthenium sputter targets in the electronic industry) and post-consumer scrap (in particular, catalytic converters from motor vehicles; recycling from smaller applications such as hard disk drives is not yet economical).11 The sale of residues containing platinum and palladium is in many cases an important source of income (or cost reduction) for companies employing them in their processes despite strong and daily price fluctuations. Table 1: Snapshot of individual PGM prices (12 July 2013; excluding osmium) and associated recycling rates.12,13 PGM Metal Price $/oz % of supplies from recycling Platinum

1521

26

Palladium

740

26

Rhodium

1010

26

Iridium

800

25-50 (post-consumer scrap)

Ruthenium

70

10-25 (post-consumer scrap)

Unit value (1000 USD/t)

30000 25000 20000 15000

10000 5000 0 1980

1985

1990

1995

2000

2005

2010

Year Figure 2: PGM price development during 1980 – 2011. The unit value of PGM reports the value of 1 metric ton (t) of PGM apparent consumption (estimated).14 The main driver for the demand for platinum, palladium and rhodium is the need for emission reduction from motorized transport and the related legislation obliging the automotive industry to equip gasoline and diesel engines with catalytic converters. The high costs and price volatility are potential drivers for

developing substitutes, particularly for platinum and palladium.* Any successful substitution strategy in the field of autocatalysts would considerably ease the strain on the platinum, palladium and rhodium markets (but would hardly affect the minor PGM metals). The expected extension of emission limits to China and other emerging economies, and to diesel motors used for non-road purposes, is expected to lead to further (upward) pressure on prices even if new deposits can be developed (due to the long lead times of 8-10 years).10

Uses and Substitutability Autocatalysts PGM use for catalysts in the automotive industry—mainly platinum and palladium—has been the prime driver for demand growth in recent years.15 40% of platinum demand, 67% of palladium and 81% of rhodium (data referring to 2012) is related to the production of different types of autocatalysts. An autocatalyst, or catalytic converter, is a cylinder made from ceramic or metal and coated with a solution of chemicals, including PGMs. It is installed in the exhaust line of the vehicle, and converts over 90% of pollutants including hydrocarbons, carbon monoxide and nitrogen oxides into carbon dioxide, nitrogen and water vapour.9 Therefore, autocatalysts reduce the environmental impact and toxicity of vehicle engine emissions. The PGM loading in a catalytic converter depends on the engine type and emission standards. It ranges from one to two grams for a small car in a lightly regulated environment to 12-15 g for a large truck with stringent regulation. PGMs in catalytic converters have proven extremely difficult to substitute, although they are eminently suitable for recycling. Currently, the only viable substitution option is replacement of platinum with palladium and vice versa.2 Substituting PGM content in catalytic converters in cars is the most promising market in terms of business volume. Research into substitutes has been ongoing for many years, but as of yet, no appropriate substitute with the same catalytic power and durability has been commercialized. Several promising research approaches are briefly described here. 



*

Research funded by the Renewable Fuels Association (2002-2007) examined the potential for replacing platinum in catalytic converters with transition metal carbides and oxycarbide nanoparticles.16 Although this research did not lead to a suitable substitute, it led to significant advances in nanoparticle synthesis. Recent results in the field of combustion synthesis of nanoparticles show some promise for the substitution of palladium in catalysts with nanoparticles containing copper and chromium.17 Production costs for combustion synthesis compare very favourably to prices for mined metals, even at laboratory scale. This is particularly so in the case of the highly priced platinum and palladium.† A similar strategy is pursued by the NextGenCat project (FP7), which aims to fully or partially replace PGMs in catalytic converters using comparable transition metal nanotechnology. The project strategy is to tightly control the incorporation of transition metal nanoparticles into the catalyst substrate precursor using adsorption and ion-exchange. In principle, this should allow for highly efficient catalysis that could ultimately phase out PGM use in converters.

Iridium prices have also shown some oscillations in recent years but prices for ruthenium and osmium have been stable. † The cost of producing a potential substitute material derived from Cu metal oxides by combustion synthesis on laboratory scale is estimated at around 10,000 to 11,000 €/ton based on Tecnalia's personnel and equipment costs; this estimate does not accounting for the saving potential of industrial-scale processes.



Research into perovskite oxides La1–xSrxCoO3 and La1–xSrxMnO3 by the GM Research and Development Team (2010) yielded the creation of a strontium-doped catalyst that is as effective as some current platinum designs. However, this converter still relies on palladium in order to catalyse the oxidation of hydrocarbons and carbon monoxide, which although being a cheaper option than platinum, still does not resolve the dependence on PGMs.

Since, attempts to substitute PGMs in catalytic converters have so far been unsuccessful, research efforts have also focussed on the reduction of PGM content in existing converters and more commercial success has been achieved against this target.18,19

Jewellery Jewellery accounts for 20% of PGM use, mainly employing platinum with at least 85% purity. Other elements that make up the remaining 15% include palladium, iridium, ruthenium, copper and cobalt. Rhodium is mainly used in jewellery as plating for decorative and protective purposes. Platinum can be found in most jewellery applications. Its strength and resistance to tarnish in addition to the fact that it can be heated and cooled without hardening and oxidation effects, all the while retaining its shape, allows for the secure setting of precious stones and its flexibility in usage for a wide range of jewellery applications.9 Trends in PGM consumption by the jewellery sector point to an increasing acceptance of palladium in China and India, so that stronger demand is expected in this business, possibly substituting platinum use in jewellery.9

Electronics 11% of PGMs are destined for use in the electronics and electric sector. Palladium, rhodium and platinum are used in electrical contacts for their resistance to sparking, erosion, corrosion and because they do not become welded together.20 Platinum and, especially, palladium are utilized in electronics because of their electrical conductivity and durability. Palladium is used in almost every type of electronic device, from basic consumer products to specialized hardware. Its main applications include multi layer ceramic (chip) capacitors (MLCC); conductive tracks in hybrid integrated circuits (HIC); plating connectors and lead frames. The electronics industry started to search for substitute materials at the end of the last century, after a major palladium shipment crisis in 1997, and the consequences of this strategy are now apparent in the palladium market. Particularly for MLCC (an important use of palladium in this sector), market observers have already confirmed the impact of substitution strategies based on nickel and copper together with a steady reduction of the palladium content of multi-layer ceramic capacitors. The improving performance and reliability of base metal capacitors has enabled manufacturers of electronic systems to employ them in applications where previously only the performance of precious metals was acceptable. For example, palladium capacitors have been displaced from many automotive electronics and their use is increasingly confined to more demanding applications such as military aircraft systems. Even though the demand for MLCCs is still increasing (driven by smart phones, tablets and automotive electronics), palladium consumption in this application is now decreasing.9,12 Current research aims at developing advanced capacitors—or supercapacitors—and batteries based on graphene for use in smart phones, but no estimates of substitution effects of PGM metals have been found so far. A second main application of PGMs in electronic equipment are hard disks, in which data storage density is increased substantially by applying small amounts of this metals to chip resistors and electrical contacts. In hard disks, platinum is found in the magnetic sublayer together with cobalt and chromium and is preferred due to its thermal stability. Ruthenium is also needed because it aids in orienting the magnetic grains and

reduces interference between layers. 9 For this application, an interesting example for substitution through product innovation can be observed: demand for ruthenium for computer hard disks, fell by just over 9% in 2012, partially due to a technology shift towards tablets and smart phones. Platinum and rhodium are applied in thermocouples due to their high thermal stability and in resistance thermometry because of its exceptional electrical resistivity.20 Thermocouples for the semiconductor industry are important due to their potential importance for energy harvesting through thermoelectric devices. For this purpose, both platinum and rhodium, as well as other materials combinations are being researched.21. Alternatives to PGM materials for energy-harvesting are the so-called semiconductor thermoelectric generator TEG22 with one promising candidate material being silicon nanowires. Potential uses of ruthenium for the energy sector are solar energy technologies, due to the metal’s ability to absorb light throughout the visible spectrum, as well as superconducting materials. Materials combinations including ruthenium are widely researched for the fabrication of dye-sensitized solar cells.23

Catalysts: chemicals, petroleum and other fuel production The share of PGMs which is used by the chemical industry as catalysts is 6%. Many chemical processes employ PGMs to improve the efficiency of various reactions. Rhodium, palladium and platinum are utilized as homogeneous catalysts, their properties that make them ideal for these applications are their high activity (leads to low concentration), high selectivity, and mild reaction conditions.20 For this sub-sector, silicone production represents the major use of platinum, followed by bulk chemical production (nitric acid for fertilizer production and paraxylene, a building block in PET manufacture), petroleum reforming and speciality chemical production (notably pharmaceuticals). The predominant uses of palladium in the sub-sector are PET and nitric acid production. Rhodium is utilised in the production of acetic acid and oxo-alcohols, supplying the paint, solvent and polymer industries. The petroleum sector is responsible for 1% use of PGMs as catalysts for cracking. Platinum, palladium, rhodium, iridium, and ruthenium are all used as heterogeneous catalysts (and are recovered from spent catalysts) for their efficiency of reaction. Platinum is also used in the production of high-octane gasoline for automobiles and piston-engine aircraft.20 An important field of research employing ruthenium is its use as catalyst for CO2 hydrogenation and related strategies for using CO2 as prime material for industry and transport, although there is also strong research activities in alternative methods for CO 2 conversion, which do not rely on scarce materials or try to avoid the need for catalysts. Palladium is still being used in innovative technologies, for example in membranes for separation of hydrogen from CO2. However, due to its very high price, research naturally aims at reducing palladium content of an application to a minimum. In the mentioned membranes, palladium sheets are reduced to 600 - 800 nanometers, so that only a very small part of the costs of a membrane is actually related to palladium use. As is the case for other applications, substitution options for PGM-containing industrial catalysts are extremely limited; substitution is currently either not possible or results in a significant loss of performance. Nickel, rhodium, ruthenium are substitutes for platinum in theory, but in practice reactions are very substrate specific. Often the only option available is substitution of one PGM for another, and even this conservative approach often involves compromise. For many catalytic applications, despite the high materials cost, there is little drive to substitute. Catalysts pay their way: small amounts are required relative to total production, they are often efficiently recycled and an effective catalyst can often confer additional efficiency gains (for example reduced water usage or lower temperatures and pressures).

On the other hand, substitutes for catalysts used in dissipative applications where there is no possibility of recycling are highly desirable. However, finding these replacements represents an enormous technical challenge. QID Nanotechnologies is a small European company dedicated to developing nanomaterials to replace PGMs in catalysis. Dow Corning reported that extensive R&D efforts have failed to identify a viable alternative to platinum catalysts for pressure sensitive release coating applications used widely as the backing for labels and envelopes. Instead the company focussed its innovation efforts on technologies which delivered reduced platinum content of 50-80%. Academics have also recognised the opportunity24–26. Further applications are electrodes in fuel cells (discussed in detail in the next paragraph), and oil refining. Platinum catalysts are also used in the growing biomass conversion sector, which includes bioethanol, hydrogen and platform chemical production. Some substitution efforts are in progress including research into nickel catalysts for hydrogen production from biomass and increased use of biocatalysts. Platinum is further used as a catalyst in fuel cells mainly because of its high catalytic activity and selectivity.9 Future demand curves for platinum for mobility will be largely determined by the composition of the vehicle fleets, i.e. the global uptake of plug-in hybrid electric vehicles (PHEVs), battery electric vehicles (BEVs), and fuel cell vehicles (FCVs). The 2012 International Energy Agency (IEA) case study27 predicts an uptake of 27 million PHEVs and BEVs by 2020 in their “improve” scenario, although they acknowledge that this is dependent on a fast rate of development for these vehicles. PHEVs continue to require an autocatalyst (see separate section above), however BEVs and FCVs do not, although FCVs still require a significant quantity of platinum used as catalyst. This dependence on platinum could severely compromise the deployment of fuel cell technologies, which presently require 46 g for a 50 kWh fuel cell, costing approximately €2000. Even accounting for technological advances to reduce Pt content in fuel cells to 5 g—already achieved on laboratory scale due to better deposition techniques—between 17,300 and 20,500 tons of platinum are expected to be employed in fuel cell vehicles between 2005 and 2050. In order to overcome this potential bottleneck, minimization strategies are pursued through new deposition techniques, which aim at maintaining the catalytic effect of platinum, while reducing costs substantially. For fuel cells as well as for advanced metal-air batteries strong research efforts are ongoing to develop metal-free electrocatalysts approaches, and include use of graphene and of biobased materials combined with nanoparticles (see Cao et al.28,29 for a recent example).

Dental alloys With a share of 6%, PGMs are employed in dental alloys known as “standard” alloys as well as in alloys with ceramic veneering. Platinum, palladium and silver are used for standard alloys mainly due to their oral stability, for the whitening of the alloy and to increase the melting range.20. However, in dental implants, palladium has by now been successfully substituted by base metals and advanced ceramics.

Glass making equipment Glass making equipment makes up 2% of total PGM use. Platinum and platinum alloys are used in the fabrication of vessels that hold, channel and form the molten glass because of platinum’s high melting point and strength. The addition of rhodium increases the strength of platinum equipment and extends their life.9 Large amounts of platinum and rhodium are used in the production of Liquid Crystal Displays (LCD). This is mainly due to their resistance to corrosion, high melting point and strength since LCDs are produced under extremely harsh conditions and demand high quality.9

PGM consumption by the glass industry is expected to increase in the coming years, due to growing “demand for increasingly sophisticated electronic displays, solar panels and lightweight, durable glass fibre composite materials”.30 No indication was found as to suitable substitutes for PGM in this use.

Summary Table 2 shows the present uses of the six PGMs in all relevant applications. Potential uses with possibly strong impacts on PGM demand, others than those listed in the table, are discussed above. Historically, there have been strong substitution trends within the group of platinum metals, taking advantage of the lower-priced palladium and rhodium to reduce platinum content. The automotive industry, for example, introduced changes to the fuel technologies to permit the use of up to 25% of palladium in catalysts for diesel-powered engines, which, before the year 2000, had to be made entirely of platinum31. The trend is still visible in the jewellery business, with an increasing acceptance of palladium by clients in India and China12, but the palladium market already shows a supply deficit, which strongly limits substitution strategies. In view of the difficulties of substituting PGM, minimization strategies can be observed in many industries and research projects. However, this approach has a serious draw-back, as it can render recycling uneconomical. Table 2: Summary of uses of individual platinum group metals. Material

End use category

Concrete applications/examples

Platinum

Autocatalysts

Catalytic converters ceramic capacitors in electronic devices; glass for fiber optics

Jewellery Electrical and electronics

High temperature thermocouples; switch contacts in automotive controls; Semiconductor crystals for lasers; alloys for magnetic disks (computer hard drives); switch contacts in substitute for gold in electronic connections

Catalysts (chemicals & fuels)

Diverse incl. manufacture of silicones and hydrogen production

Dental alloys Glass making equipment

Palladium

Others

Fuel cells (mobile, stationary), high temperature alloys for air planes; medical industry (pace-makers electrodes, aural and retinal implants, anti-cancer drugs), dental implants; high quality flutes; smoke and carbon monoxide detectors; coatings for razors; Hydrogen separation membrane

Autocatalysts

Catalytic converters

Jewellery

Material

End use category

Concrete applications/examples

Electrical and electronics

High temperature thermocouples; electrical connection; nuclear reactors

Catalysts (chemicals & fuels)

Diverse chemical processes incl. hydrogen production

Dental alloys Rhodium

Others

Hydrogen separation membranes

Autocatalysts

Three-way catalytic converters for gasoline engines

Jewellery

Iridium

Electrical and electronics

Liquid crystal displays (LCDs); Semi-conductors for LED

Catalysts (chemicals & fuels)

Production of oxo-alcohol and nitric oxide for fertilizers and explosives; refining

Others

Glass, mirrors

Jewellery Electrical and electronics

Semi-conductors for LED (mobile phones, flat panel displays and touchscreen); satellite receiver, wireless communications; military electronic systems

Catalysts (chemicals & fuels)

Production of chlorine and caustic soda

Others

Alloys for aircrafts, space launch vehicles, lasers for medical and industrial welding applications; medical scanners, surgical tools and implants; glass industry, optical coatings

Ruthenium Jewellery

Osmium

Electrical and electronics

Hard disks; chip resistors, electrical contacts

Catalysts (chemicals & fuels)

Fuel production (hydrogen, natural gas, methanol), chlorine production

Others

Solar energy; superconductors, wear resistant alloys, glass industry

Others

Hard electrical contacts; Medical: surgical implants; microscopy; pen tips; needles

Figure 3: Distribution of end-uses and corresponding substitutability assessment for PGM. The manner and scaling of the assessment is compatible with the work of the Ad-hoc Working Group on Defining Critical Raw Materials (2010).

References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

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24

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