Platinum Metals Rev., 2013, 57, (2), 157
Final Analysis: NOx Emissions Control for Euro 6
The control of oxides of nitrogen (NOx) emissions to meet more stringent motor vehicle emission legislation has been enabled by the development of various exhaust gas aftertreatment technologies, notably those that employ platinum group metals (pgms).
For gasoline engines the most common aftertreatment for the control of NOx, as well as the other major regulated pollutants, carbon monoxide (CO) and unburnt hydrocarbons (HCs), is the three-way catalyst (TWC). This technology was developed in the late 1970s (1). It allows the oxidation of CO and HC over platinum-palladium or just palladium during lean (excess oxygen) conditions to form carbon dioxide and water, while rhodium performs the reduction of NOx to N2 under rich (oxygen depleted) conditions. This technology relies on the engine operating around the stoichiometric point (air:fuel ratio of 14.7:1) where maximum simultaneous reduction of NOx and oxidation of CO and HCs can take place. Emissions standards for European gasoline vehicles which have been in force since 2009 (2) specify NOx emissions must not exceed 0.06 g km−1 (Table I), a limit that is met by TWC technology.
For diesel engines, which operate under lean conditions, NOx is harder to deal with. Previous diesel vehicles used advanced engine technologies to significantly lower NOx emissions. For example, exhaust gas recirculation (EGR) is used to recirculate a proportion of the exhaust gas back into the engine cylinders to reduce the cylinder temperature during combustion and thereby reduce formation of NOx. A disadvantage of this method is that it increases emissions of particulate matter (PM). Tighter PM limits have now been enforced across many jurisdictions and are met by using a pgm-coated diesel particulate filter (also known as a catalysed soot filter (CSF)).
New Legislation Challenges
New legislation in force for European heavy-duty diesel vehicles from 2013, light-duty diesels from 2014 and some non-road diesel engines from 2014 requires a further reduction of NOx emissions. As shown in Table I, NOx emissions for light-duty diesel passenger cars reduce from the current Euro 5 limit of 0.18 g km−1 to the Euro 6 limit of 0.08 g km−1 from 2014. PM emissions are already regulated to the extremely low level of 0.005 g km−1 by the current Euro 5 legislation. The development of fuel efficient lean-burn gasoline engines also presents new challenges – NOx levels typically generated in the engine cylinder, whilst lower than conventional gasoline engines, are nevertheless still well above the Euro 6 limits and therefore some form of catalytic aftertreatment is required.
The two leading catalyst technologies used to remove NOx in a lean-burn engine to meet the above legislation are lean NOx trap (LNT) or selective catalytic reduction (SCR). LNT catalysts remove NOx from a lean exhaust stream by oxidation of NO to NO2 over a platinum catalyst, followed by adsorption of NO2 onto the catalyst surface and further oxidation and reaction with metal species incorporated in the catalyst, for example barium, to form a solid nitrate phase. Once the catalyst is filled with the solid nitrate phase, the engine is then run rich for a short period to release the NOx from its adsorbed state. The released NOx is then converted during the rich period to N2 over a rhodium catalyst. SCR systems use a platinum-based diesel oxidation catalyst (DOC) or a combination of a DOC and a platinum-based CSF to oxidise a proportion of the NOx into NO2 and remove HC/CO. A NOx reductant, usually in the form of aqueous urea, is then injected into the exhaust gas after the oxidation catalyst and the NO/NO2 mixture is then selectively reduced over the downstream SCR catalyst.
The decision whether to use LNT or SCR on a vehicle involves many factors. SCR requires space on the vehicle to fit the urea tank and dosing system, which is less of a constraint on heavy-duty and larger light-duty vehicles. Furthermore, the need to run the engine rich for LNT systems is more technically demanding for larger engines so LNT systems are more suited to smaller light-duty vehicles. SCR systems are impractical for use on gasoline vehicles as their NOx output is significantly higher than from diesel, and hence unfeasibly large urea tanks would be required.
NOx and other pollutant levels emitted from vehicles are assessed by use of a standardised driving cycle for Europe. The current driving cycle which is used to measure emissions from light-duty vehicles may be changed in the future to include an even wider range of driving conditions, for example further extended low speed driving conditions such as common in congested city driving or much higher speed driving conditions than used in the current drive cycle.
For diesel LNTs the future challenge is to maximise NOx conversion at low speed driving conditions as well as providing high NOx conversion during high speed driving. For diesel SCR systems, the future challenge is also to boost NOx conversion when the engine is operating at very low speeds. This low speed challenge may be helped by moving the SCR closer to the engine where it can benefit from higher temperatures, but there are space and system layout considerations. There is currently a good deal of research ongoing into diesel powertrain optimisation for a wide range of driving scenarios.
The proposed enforcement of a particulate number limit (3) for gasoline engines in Europe also presents challenges by requiring control of PM to extremely low levels in addition to keeping emissions of other pollutants at minimal levels. One possibility is to use a filter coated with similar material to a TWC as part of the overall aftertreatment system.
For gasoline engines, new on-board diagnostic limits that come into force at Euro 6 part 2 in 2017 (3) reduce by 70% the threshold amount of NOx emitted before the driver is notified of a problem with the catalyst. Some manufacturers are therefore looking at ways of further improving the durability of catalysts, including by increasing the relative loadings of rhodium. Due to the excellent NOx reduction capability of rhodium, it may be possible to substitute palladium with small quantities of rhodium to give a cost- and performance-optimised system.
There remains a good deal that can be done on controlling NOx emissions from vehicles using pgms. As regulations tighten, cover more vehicle types and are adopted by more jurisdictions around the world, greater use of pgm-containing emissions control systems can be anticipated. Good progress has been made on the control of NOx from gasoline engines and developments are being made on lowering NOx emissions from diesels to meet upcoming emissions limits.
- B. Harrison, B. J. Cooper and A. J. J. Wilkins, Platinum Metals Rev., 1981, 25, (1), 14 LINK https://www.technology.matthey.com/article/25/1/14-22/
- ‘Regulation (EC) No 715/2007 of the European Parliament and of the Council of 20 June 2007 on type approval of motor vehicles with respect to emissions from light passenger and commercial vehicles (Euro 5 and Euro 6) and on access to vehicle repair and maintenance information (Text with EEA relevance)’, The European Parliament and the Council of the European Union, Official Journal of the European Union, L 171/1, 29th June, 2007
- ‘Commission Regulation (EU) No 459/2012 of 29 May 2012 amending Regulation (EC) No 715/2007 of the European Parliament and of the Council and Commission Regulation (EC) No 692/2008 as regards emissions from light passenger and commercial vehicles (Euro 6) (Text with EEA relevance)’, The European Commission, Official Journal of the European Union, L 142/16, 1st June, 2012
An interactive PDF showing global light-duty diesel NOx and PM limits was published with Johnson Matthey’s “Platinum 2012 Interim Review”. It is available to download by clicking the link below.
Global Light Duty Diesel Emission Control — NOx and PM: LINK http://www.platinum.matthey.com/media/1591743/platinum_2012_interim_review_sf_qr_code.pdf
Jonathan Cooper is Gasoline Development Manager at Johnson Matthey Emission Control Technologies and has over 13 years’ experience in global gasoline aftertreatment systems research at Johnson Matthey. He holds a degree and DPhil in Chemistry from the University of Oxford, UK.
Paul Phillips is European Diesel Development Director at Johnson Matthey Emission Control Technologies. He has 17 years’ experience at Johnson Matthey aiding the development of emission control systems. Paul has a BSc in Chemistry and a PhD in Organometallic Chemistry from the University of Warwick, UK.