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«Minerals 2014, 4, 388-398; doi:10.3390/min4020388 OPEN ACCESS minerals ISSN 2075-163X Article Precious Metals in ...»

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Minerals 2014, 4, 388-398; doi:10.3390/min4020388

OPEN ACCESS

minerals

ISSN 2075-163X

www.mdpi.com/journal/minerals

Article

Precious Metals in Automotive Technology: An Unsolvable

Depletion Problem?

Ugo Bardi 1,* and Stefano Caporali 2,3

Dipartimento di Scienze della Terra, Università di Firenze, Via G. La Pira 4, 50121 Firenze, Italy

Dipartimento di Chimica, Università di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino,

Italy; E-Mail: stefano.caporali@unifi.it

Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali, Via Giusti 9, 50123 Firenze, Italy * Author to whom correspondence should be addressed; E-Mail: ugo.bardi@unifi.it;

Tel.: +39-055-457-3118; Fax: +39-055-457-3120.

Received: 4 November 2013; in revised form: 24 April 2014 / Accepted: 25 April 2014 / Published: 30 April 2014 Abstract: Since the second half of the 20th century, various devices have been developed in order to reduce the emissions of harmful substances at the exhaust pipe of combustion engines. In the automotive field, the most diffuse and best known device of this kind is the “three way” catalytic converter for engines using the Otto cycle designed to abate the emissions of carbon monoxide, nitrogen oxides and unburnt hydrocarbons. These catalytic converters can function only by means of precious metals (mainly platinum, rhodium and palladium) which exist in a limited supply in economically exploitable ores. The recent increase in prices of all mineral commodities is already making these converters significantly expensive and it is not impossible that the progressive depletion of precious metals will make them too expensive for the market of private cars. The present paper examines how this potential scarcity could affect the technology of road transportation worldwide. We argue that the supply of precious metals for automotive converters is not at risk in the short term, but that in the future it will not be possible to continue using this technology as a result of increasing prices generated by progressive depletion. Mitigation methods such as reducing the amounts of precious metals in catalysts, or recycling them can help but cannot be considered as a definitive solution. We argue that precious metal scarcity is a critical factor that may determine the future development of road transportation in the world. As the problem is basically unsolvable in the long run, we must Minerals 2014, 4 389 explore new technologies for road transportation and we conclude that it is likely that the clean engine of the future will be electric and powered by batteries.

Keywords: platinum; platinum group metals (PGM); automotive; catalytic converter

1. Introduction Road transportation today is mostly based on vehicles powered by internal combustion engines.

These engines need fuels which can be easily gasified and which can provide a large amount of energy per unit weight and volume. In practice, all the engines commonly available on the market use hydrocarbons as fuels. In particular, liquid hydrocarbons such as gasoline and diesel fuel are the most commonly used even though, in recent times, gas phase fuels such as methane and liquefied petroleum gas (LPG) have become popular due to their lower cost. The combustion of hydrocarbons in these engines creates a number of polluting substances, including unburnt hydrocarbons, particulate matter, and harmful chemicals such as carbon monoxide (CO) and nitrogen oxides (NOx). In engines operating with the Otto cycle and using gasoline as fuel, these chemicals are removed using catalytic converters at the exhaust. These devices can substantially reduce the amount of toxic substances emitted, but they are also expensive because of the need of using platinum group metals (PGM) as active catalytic substrates. On average, an automotive catalytic converter can store 1–3 × 10−3 kg of platinum and smaller amounts of rhodium and palladium. As a consequence, nowadays, automotive converters use more than half of the world’s mineral production of platinum [1]. That raises the question of whether there exist sufficient PGM mineral resources extractable at reasonable prices in order to satisfy the future demand.

This subject has been studied in previous papers and a popularized discussion of the platinum depletion problem was reported by Cohen in 2007 in “The New Scientist” [2]. Several academic papers discussing the issue of PGM supply were published during the past few years, such as by Glaister et al. [3] and Mudd et al. [4], while a paper specifically dedicated to the problem of PGM depletion in view of the needs of the automotive industry was published by Yang in 2009 [5]. In the present paper, we update the previous results and we discuss the issue in view of what appears to be a “production peak” for PGMs observed in recent years. We discuss how the depletion of PGM may affect the world’s road transportation system and we arrive to the conclusion that high costs of platinum group metals is a problem destined to get worse with time. That creates a critical problem for a large sector of the world’s road transportation system which cannot run without PGM-based catalysts, unless we were to return to unacceptable levels of pollution. This situation is a strong incentive for developing radically different alternatives, in particular battery powered vehicles which are inherently cleaner and appear to suffer from less important depletion problems.





2. Pollution Removal from Combustion Engines by Means of Catalytic Converters

–  –  –

ammonia), could be also be used, but at present they find no market applications. The combustion of hydrocarbons in internal combustion engines generates mainly water (H2O) and carbon dioxide (CO2).

Neither is considered a harmful substance even though CO2 is toxic for human beings at very high concentrations [6]. Both water and carbon dioxide are greenhouse gases, but only carbon dioxide creates global warming because, unlike water, it remains in the atmosphere for times of the order of tens of thousands of years [7]. Then, the untreated emissions of an internal combustion engine normally contain substances which are toxic for human beings even at low concentrations. The most important ones are: (1) unburnt hydrocarbons, especially if aromatic, (2) carbon monoxide (CO), (3) nitrogen oxides (NOx) and (4) particulate matter, typically in the form of carbon micro and nano-particles (much debate is ongoing about the harmful effect of these particles but it is generally agreed that they are a major health problem [8]). Additives to fuels may also create dangerous materials at the exhaust and, until not long ago, tetra-ethyl lead and ethyl bromide were common additives to gasoline, fortunately today forbidden by law in most (although not all) countries of the world [9].

The removal of carbon dioxide from the exhaust gases of a mobile engine is normally considered impossible, although it can be contemplated in the case of large, stationary engines. However, most of the toxic substances emitted can be strongly reduced in concentration by a combination of suitable operating parameters and catalytic chemical filters at the exhaust. At present, there exist two main approaches in this field. For diesel engines, the “lean” mixture of fuel and air reduces the problem of carbon monoxide and hydrocarbons, so that the main problem is to eliminate particulate matter and nitrogen oxides. The abatement of these pollutants is normally obtained by means of selective catalytic reactions (SCR), that is by a combination of an oxidation catalyst based on cerium oxide (to remove particulate) and by reaction with ammonia to remove nitrogen oxides. Ammonia, in turn, is generated by the injection of urea into the exhaust gas. Exhaust gas recirculation can also be used to reduce NOx production when starting a cold engine.

For gasoline engines, instead, the problem of particulate matter is less important and the exhaust filter must address the problem of eliminating three different harmful gases: CO, NOx and unburnt hydrocarbons. This is accomplished by means of “three way” catalysts based on noble metals (Pt, Pd and Rh, collectively referred to as “PGM” or “platinum group metals”. Of these three metals, rhodium catalyzes reduction while palladium catalyzes oxidation; platinum, is active for both. The task of the catalyst is complex because it must perform several tasks at the same time: oxidize CO and unburnt hydrocarbons, while reducing NOx. In order to optimize the yield of these reactions, the exhaust gas must contain a specific fraction of oxygen. The correct gas composition is obtained by controlling the air/fuel mix by means of oxygen sensors at the exhaust. In general, when in good conditions and operated properly, the converter can remove up to about 90% of the three gases; as described, for instance, by Kummer [10].

Considerable efforts have been dedicated to developing non-PGM materials that can catalyze these three reactions, but the task has turned out to be very difficult and a practical solution has not been found [11,12]. A catalyst which does not use PGMs called “Noxicat™” has been recently developed, but it is designed mainly for the abatement of NOx in diesel engines. Other solutions based on oxides such as perovskites [13] and boehmites [14] as catalysts have been proposed but they seem to be far from industrial applications. In the end, the electronic structure of the platinum group metals is unique Minerals 2014, 4 391 and it generates chemical properties that are not matched by any other element of the periodic table nor by compounds which can remain stable for a long time in the conditions of high temperature of automotive catalytic converters. Therefore, although it is not possible to exclude an unexpected breakthrough, the present situation raises a serious problem of future availability of PGMs in sufficient amounts, as it will be discussed in the next section.

3. Platinum Group Metals Abundance and Production

PGMs are rare in the Earth’s crust, with typical average abundances of the order of a few parts per billion (ppb) at most. Of the three PGMs used in automotive catalysts, the most abundant is palladium (average 15 ppb), followed by platinum with 5 ppb and rhodium with about 1 ppb (data from [15]).

PGMs are often found in sulphide minerals [16] and are also known to be siderophilic (iron-loving).

The latter property accounts for their scarcity on the Earth’s crust, since they were efficiently extracted by the metallic Fe-Ni phases in the Earth’s core during planetary accretion. PGMs may occur in native form associated with gold, iron, copper and chromium and, due to their high weight and chemical inertness, can also be found in placer deposits. The production of PGMs is concentrated in a few mines: the main ones are the Bushveld igneous complex (South Africa), the sulphide deposits of Norilsk in Russia, placer deposits in the Ural mountains (Russia), the Sudbury mine (Ontario, Canada), the Hartley mine (Zimbabwe), the Still-water complex (Montana, USA), Northern Territory (Australia) and the Zechstein copper deposit in Poland. South Africa produces about 85% of the total world PGM production has 82% of the world’s resources [17].

According to the United States Geological Survey [18], the total reserves of platinum group metals (PGMs) amount to 66 million tonnes, to be compared to a total combined use of platinum and palladium in 2011 of 400,000 metric tons. Hence, the ratio of reserves to production (R/P, with production assumed to be constant and equal to the present value), is of about 130 years. This result may appear comforting but the question here is not for how long we can produce PGMs in the unlikely hypothesis of constant future production, but how and if it will be possible to keep a sufficiently large production at costs compatible with the needs of road vehicles—i.e., at costs which would not destroy the demand for these elements.

This question is related to a well known effect in economics, that of “diminishing returns”. As less and less concentrated ores are exploited, the energy needed for extraction increases. As a consequence, production costs increase and, ultimately, market prices must increase since, obviously, nobody can produce at a loss for a long time. This effect had been described for the first time for mineral resources by William Stanley Jevons in his “The Coal Question” of 1866 [19]. Today, it is known that in many cases, the mineral industry is forced to access resources from lower and lower grade ores, as it has been shown, for instance, for the case of copper in a recent paper by Mudd and Weng [20]. This effect is surely a factor in the observed increasing prices of most mineral resources worldwide (see, e.g., Valero [21]). However, the increasing prices of all mineral commodities are also directly related to the increased prices of fossil fuels which provide most of the energy needed for extraction. As shown by Hall et al. [22], the progressive depletion of fossil fuels causes a reduction of the energy return on energy invested (EROI) which in turn generates a rise in prices as the result of the decreasing economic returns on extraction. In the case of platinum group metals, all these factors are at play and Minerals 2014, 4 392 Mudd [4] clearly shows that the industry is progressively forced to exploit lower grade, more expensive PGE metal ores.



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