Three-way catalyst (TWC) compositions employ platinum, palladium, and/or rhodium in various combinations for the abatement of emissions from petrol/rich-burn engines. TWCs simultaneously removecarbon monoxide, hydrocarbons and nitrous oxides from exhaust emissions. Rhodium is the most effectiveelement in the conversion of nitrous oxides to nitrogen. Pal-ladium demand for TWCs in the large US petrol-vehicle market has increased in the past few years owing to palladium being significantly lower priced than platinum. Under lean-burn or diesel exhaust conditions, reductants in the emission (carbon monoxide, hydrocarbons, hydrogen) are preferentially oxidised by the high concentrations of oxygen (~10%) over diesel-oxi-dation catalysts (DOCs). The reduction of nitrous oxides under lean-burn conditions is a problem. This is in contrast to the stoichiometric (air and fuel in exact chemical balance) or slightly rich-burn conditions of normal petrol-engine exhausts, where oxygen concentrations are~1-2%, and the nitrous oxides can be reduced by the standard TWCsystem. Nitrous-oxide emissions from diesel exhausts could beremoved, for example, by lowering the temperature of combustion. However, increased emissions of carbon monoxide result under these conditions, which necessitateincreasing the platinum catalystproportions to compensate.
Palladium is readily poisoned by sulphur in diesel emissions, and has become an alternative only since the availability of low sulphurdiesel fuels. Recent developments by Umicore and Engelhard have revealed that a quarter to a third of the platinum in DOCs can be sub-stituted by palladium. The diesel light-vehicle sector in Europe continues to expand, and diesel-powered units constitute some 50% of all new light vehicles sold in Western Europe. This higherdemand, combined with the intro-duction of catalysed soot filters (CSFs) in diesel vehicles, has caused a further increase in platinumdemand. Diesel emissions contain a significant amount of particulate matter resulting from the incomplete combustion of the hydrocarbon fuel. CSFs are PGM-based catalyticexhaust after-treatment systems consisting of a ceramic framework with a platinum catalyst to oxidise particulate matter.
Platinum and palladium jewellery is popular in Asian markets.These white-metal jewellery items are hallmarked with purities of 990 and 950 fineness (example, 950 parts per 1 000 of PGM or 95% PGM by mass), which containsignificantly higher precious metalcontent compared to white-goldalloys (18 ct equals 750 fineness and 14 ct, 583 fineness). Anotheradvantage of palladium overplatinum is the density differenceof almost 50%, making palladiumjewellery lighter and, therefore,more profitable the density ofpalladium is 12 g.cm3 and platinum 21,4 g.cm3. Recent patentfilings reveal a tendency to market 14 ct and 18 ct platinum and palladium jewellery alloys. Iridium and ruthenium are used in jewelllery mainly as grain refiners, with less than 0,5% by mass generally used.
PGMs are used extensively in the chemicals sector as catalysts. The largest application for platinum catalysts is in the production ofsilicones, followed by paraxylene production. Silicones are used by the automotive and constructionindustries, while paraxylene is aprecursor for purified terephthalicacid used in the production ofplastic polyethylene terephthalate and polyester fibre. Palladium acts as catalyst in purified terephthalic-acid production. Palladium-gold catalysts are used in the productionof vinyl acetate monomer, whichis an intermediate product in the manufacture of polymers used in products such as paints, adhesives and textiles. Platinum and palla-dium gauzes are well known in the production of nitric acid. Platinum is further used as a catalyst in reforming and isomerisation reactions in the petroleum industry. Rhodium catalysts are used in the manufacturing of oxo-alcohols and acetic acid, the latter also using ruthenium and iridium as catalysts. Dimensional stable anodes, which consist of titanium coated with either ruthenium or ruthenium-iridium, are used in the chloralkali industry.
Hard disks for personal and business computers, video recorders and MP3 music players are important consumers of platinum. A potential remarkable use for platinum in the future is in fuel-cell technology. Fuel cells generateelectricity by means of an electrochemical reaction whereby hydrogen ions and oxygen combine to form water. Since water is the only by-product of the fuel-cell reaction,the technology is extremely environ-ment-friendly. The two electrodes in the fuel cell consist of a carbon-based substrate coated with a pla-tinum catalyst. If large-scale commercialisation becomes a reality, most platinum will be used in proton-exchange-membrane and direct methanol fuel cells for automotive, electronic and stationarypower applications. Commerciali-sation of these two types of fuel cells in portable electronic devices isexpected within the next 18 months. High-temperature platinum-wire thermocouples for the glass and semiconductor industries areother applications of platinum. The single largest application for palladium in the electronics sector is in multilayer ceramic capacitors(MLCC) for personal computers and mobile phones, as well as in automotive electronic components. Palladium-silver pastes are also used in MLCCs. The largest application for ruthenium is for hard-disk production, where the metal enables increased storage capacity. Ruthenium is also used in plasma-display panel, where it is applied as a conductive paste to the inner surface of the screen for improved image quality. The use of iridium in the electronics sector is dominantly in crucibles for the production ofhigh-quality single crystals.
Platinum, with its excellent resistance to corrosion and wear at high temperatures, is the material of choice in glass production, in which some 355 000 ouces are consumed a year. Major applications are in the production of liquid crystal displays for flat-screen computermonitors and televisions, especiallyin Asia and Japan, as well as in the production of glassfibre in the US. The glass industry also consumes approximately 55 000 oz/y of rhodium.
Platinum drugs for the use in the treatment of cancer have saved many lives during the past three decades. Cisplatin, first approved in 1978, was overtaken in the late 1980s by carboplatin and in the late 1990s by oxaliplatin, owing to their lower toxicityto healthy cells and higher effectivityfor specific cancer types. Platinum is radio-opaque and thus visible under X-rays. This property makes it an ideal choice in the treatment of blocked arteries and in devices such as pacemakers and implantable cardioverter defibrillators used in the treatment of cardiacarrhythmias (irregularities in the beating of the heart). Platinumalloys containing high or low gold additions are used in dentalbridges and crowns. Platinum imparts increased strength to high-gold alloy, which typically contain 75% to 99% gold, 1% to 20%platinum and small amounts ofpalladium, silver and base metals. Up to 80% of palladium is consumed in low-gold alloys.
Outside of autocatalysis, otherautomotive-sector applications are in spark plugs and oxygen sensors. All new North America vehicles are fitted with platinum-tipped spark plugs, which have an effective life of 100 000 miles or more owing to the metal’s excellent resistance tochemical and electrical erosion. Oxygen sensors control the air-fuel mixture that is fed to the combustion chambers, making platinum an indispensable part of the close-loop engine management system. The sensor detects the oxygenlevels in the exhaust gases andenables adjustment of the air to fuel ratio in order to optimise theefficiency of the autocatalyst inminimising emissions.
Platinum aluminide coatings areapplied to aeroengine turbine blades to provide oxidation resistance at temperatures of more than 1 500