Ce in car cats for oxygen storage

Introduction

Ce is the most common rare earth element and usually seen as non-critical (3), but has also been classed as “near critical” in the short term by the US Department of Energy due to its importance to clean energy technologies in combination with projected supply risks (7) (citing (8)). Its abundance is similar to that of copper (5). However, it is still one of the eight rare earth elements of highest economic interest (alongside lanthanum, neodymium, praseodymium, samarium, dysprosium, europium and terbium) (5) Common applications include lens polishes, petroleum refining, metal alloys and automotive catalysts (6, 7).

Ce used in automotive catalysts

Automotive catalysts convert toxic exhaust fumes into less harmful fumes. Hydrocarbons (CmHn) carbon monoxide (CO) and nitrous oxides  (NOx)are converted into CO2; H2O and N2 via redox reactions (2).

In order for the process to work efficiently, a certain operating temperature, a large surface area and a certain level of oxygen are necessary. In the first few minutes of the engine warm-up phase, the operating temperatures are still too low for the process to work well, which results in most emissions being released in this phase (1). The required surface area is often achieved by a honeycomb structure made from ceramic or stainless steel and coated with aluminum oxide, rare earth oxides and platinum group metals which act as catalysts (2). Three-way catalysts use ceria compounds for oxygen storage (4). The compounds act as a buffer by absorbing and releasing oxygen, thereby helping to generate the required stoichiometric conditions for the redox transformations to work (4). The cerium is oxidized “by default”, but capable of absorbing further oxygen in an oxygen-rich atmosphere, thereby helping to increase the efficiency of the nitrogen oxide reduction process (2). The stored oxygen is released again when needed, which again helps the oxidation of carbon monoxide to work more efficiently (2). The process is supported by sensors which measure the oxygen content and the air-to-fuel ratio is adjusted accordingly (10, 4). It may be interesting to mention that it has been suggested to utilize the oxygen buffer capacity of cerium oxides in medical applications, too. Scientists at Rice University found that the use of cerium oxide as an antioxidant aimed at damaging reactive oxygen species could help treat traumatic brain injuries, cardiac arrest and Alzheimer’s patients and help with radiation-induced side effects suffered by cancer patients (9).

Furthermore, organic cerium compounds added to diesel fuel help promote soot combustion and thereby help avoid the clogging of diesel particulate filters (4).

The conversion efficiency rates of cats (for hydrocarbons, carbon monoxide and nitrogen oxides) lie at around 98% (10). The use of rare earths in automotive catalyst applications has helped to improve their performance greatly and may help further enhance the exhaust fume control and fuel efficiency in future (4).

Cat recycling activities

Monoliths (the previously described honeycomb structures in automotive catalysts) are recycled, which is primarily due to the fact that they contain platinum group metals. BASF, Umicore and Johnson Matthey, for example, refine precious metals from cats (11, 12, 13). Used cats provide a valuable recycling stock at 75-250 $ per piece (2010 figures) (2). Existing commercial-scale recycling processes do not currently recover cerium compounds, which are disposed of with the slag (2). However, proactive research on the recovery of cerium compounds from cats is being undertaken to prepare for potential future cerium supply shortages and/or price increases, and 70% recovery rates have been achieved in small scale experiments (2).

References

1)    GSF & Flugs (2004): Katalysatoren in Kraftfahrzeugen – Freund oder Feind für Umwelt und Gesundheit?

2)    Bleiwas, D.I., USGS (2013):  Potential for recovery of cerium contained in automotive catalytic converters; Open-File Report 2013–1037; U.S. Department of the Interior, U.S. Geological Survey.

3)    Hatch, G. P. (TMR, LCC) (2011): Critical Rare Earths Global supply & demand projections and the leading contenders for new sources of supply.

4)    Shinjoh, H. (2006): Rare earth metals for automotive exhaust catalysts, Journal of Alloys and Compounds 408–412, 1061–1064

5)    Massari, S.; Ruberti, M. (2013): Rare earth elements as critical raw materials: Focus on international markets and future strategies, Resources Policy 38, 36–43

6)    Hayes-Labruto, L,  Schillebeeck, S.; Workman, M., Shah, N. (2013): Contrasting perspectives on China’s rare earths policies: Reframing the debate through a stakeholder lens, Energy Policy 63, 55–68

7)    Weber, R.J., Reisman, D.J. (2012):  Rare Earth Elements: A Review of Production, Processing, Recycling, and Associated Environmental Issues

8)    US Department of Energy (2011): Critical Materials Strategy – Summary

9)    Phys.org (2013): Scientists create a super antioxidant: Common catalyst cerium oxide opens door to nanochemistry for medicine Oct 15, 2013 http://phys.org/news/2013-10-scientists-super-antioxidant-common-catalyst.html, accessed 09th April 2014

10) BASF (2014): How Catalytic Converters Work http://www.catalysts.basf.com/p02/USWeb-Internet/catalysts/en/content/microsites/catalysts/prods-inds/mobile-emissions/how-it-works , accessed 09th April 2014

11) BASF (2014): Autocatalyst Recycling http://www.catalysts.basf.com/p02/USWeb-Internet/catalysts/en/content/microsites/catalysts/prods-inds/prec-metal-svcs/autocat-recycling?mid=0, accessed 16th April 2014

12) Johnson Matthey (2014): Refining and Precious Metal Management Services http://matthey.com/whatwedo/productsandtechnologies/refiningandmanagement, accessed 23rd April 2014

13) Umicore (2014): Excellence in Recyclinghttp://www.preciousmetals.umicore.com/PMR/; accessed 23rd April 2014

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