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No "silver bullet" for platinum
Brendan Ryan
Posted: Fri, 23 May 2008
[miningmx.com] -- MITSUI MINING’S announcement that it has developed technology that could replace platinum with silver in vehicle autocatalysts isn’t being viewed as a “silver bullet” by the platinum industry.
Johnson Matthey (JM) – the world’s largest manufacturer of autocatalysts, which help clean vehicle exhaust emissions – says there’s a “possibility” silver could find limited use but there are a number of major technical challenges that must first be addressed.
The Mitsui development does highlight one of the platinum mining industry’s greatest fears: that sky-high prices for the metal are revving up attempts to find viable, cheaper substitutes for platinum in autocatalysts.
The automobile industry is now by far the greatest consumer of platinum, accounting for 56% of total demand in 2007, says GFMS’s 2008 Platinum and Palladium Survey.
The threat is a real one but, so far, the only metal successfully used as a substitute for platinum has been its cheaper sibling – palladium. There are six metals collectively referred to as “platinum group metals” – or PGMS – and three of them – platinum, palladium and rhodium – are used in autocatalysts.
Petrol-driven vehicles typically use a “three way” catalyst that converts carbon monoxide,
hydrocarbons and oxides of nitrogen to carbon dioxide, water and nitrogen. Since the early Nineties palladium has been increasingly used to substitute platinum in such catalysts.
But the real demand growth for platinum is currently coming from diesel engines. More diesel-powered cars are being sold because they’re more fuel efficient than petrol engines and therefore generate less carbon dioxide pollution.
However, unlike petrol engines, diesel engines generate soot – technically called particulate matter – which causes huge problems for autocatalysts. Diesels can have two kinds of catalysts fitted: a diesel oxidisation catalyst (DOC), which removes carbon monoxide, hydrocarbons and some soot – and a diesel particulate filter (DPF), which removes even more soot. The trapped soot has to be burnt off the DPF periodically – a process called regeneration.
So far, only platinum can deal with the soot in DPFs, although palladium used in conjunction with
platinum has made some inroads in DOCs on diesel-driven cars.
Mitsui now wants to use silver in DPFs. JM says: “Unlike platinum, silver is a poor catalyst for oxidising nitric oxide to nitrogen dioxide, which is essential for regenerating the filter in circumstances where the vehicle operating temperature is low, as in city driving. Silver is also not a good catalyst for oxidisation of hydrocarbons and carbon monoxide thus can’t be used instead of platinum and palladium in the oxidisation catalyst or DOC.
“Moreover, a second DOC containing platinum would be needed to oxidise any residual hydrocarbon and carbon monoxide, which would pass through a silver-based DPF unconverted.
“Any use of silver in a DPF is thus likely to be in conjunction with a PGM catalyst while any PGM replaced by silver in a DPF would, to a greater or lesser extent, need to be added back into the system elsewhere.”
Other problems with silver are that, unlike platinum, it
can’t take elevated temperatures – at which it may “vaporise” – while silver is also susceptible to poisoning by sulphur. “Stabilisation of silver is essential in order to provide the thermal durability required for commercial use,” says JM.
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