In the ‘bad old days’, when diesel lorries pro-duced clouds of black smoke as they accelerated orclimbed hills, a diesel engine in a car was a rarity,but during the last few years Western Europe hasseen a huge increase in the production of dieselpassenger cars. Today more than 50% of all newEuropean cars have a diesel engine (1). Thisincreased demand results from the introduction ofthe powerful turbocharged high-speed dieselengine that provides excellent driving characteris-tics with high torque at low speed, and very goodfuel economy. Modern passenger car dieselengines produce much less soot or PM than didtheir older counterparts, because of improvedfuelling and enhanced combustion characteristics.For instance, fuel pumps operating at very highpressure enable injection via several very fine noz-zles into the cylinder and these injection systemspermit multiple injections of fuel. In spite of theimprovements in PM emissions from diesel pow-ered vehicles, there are still concerns about the

environmental consequences of these emissions.Legislation is being introduced that will demandfitment of PM filters to all diesel car models sold inWestern Europe, with the implementation of theEuropean Stage 5 emissions requirements (2) start-ing in 2009. In fact a growing number of newdiesel passenger cars have PM filters, even thoughthey may not be necessary to meet current legisla-tive requirements.

The Origin of Particulate Matter The operation of a diesel engine involves com-

pressing air in the cylinder producing heat via theJoule-Thomson effect, and then injecting finely‘atomised’ fuel under very high pressure (up to2000 bar) directly into the hot gas that causes it toexplosively combust. The exact details of the com-bustion process are the subject of active research,and it is clear that the atomised fuel droplets evap-orate and burn in a fuel-rich region limited byingress of oxygen into the burning flame front. In

27Platinum Metals Rev., 2009, 53, (1), 27–34

Cleaning the Air We Breathe – ControllingDiesel Particulate Emissions fromPassenger CarsBy Martyn V. Twigg*Johnson Matthey PLC, Orchard Road, Royston, Hertfordshire SG8 5HE, U.K.; *E-mail: twiggm@matthey.com

and Paul R. PhillipsEmission Control Technologies, Johnson Matthey PLC, Orchard Road, Royston, Hertfordshire SG8 5HE, U.K.

The mechanism of formation of particulate matter (PM) in the diesel engine combustion processis outlined, and the increasingly stringent PM emissions limits in current and projectedenvironmental legislation are noted in the context of the increasing use of fuel-efficienthigh-performance diesel engines in passenger cars. The types of filter systems for abatingdiesel particulates are described, as are the principles of filter regeneration – the controlledoxidation of PM retained in the filter, to prevent an accumulation which would ultimately blockthe filter and degrade engine performance. PM is characterised in terms of both particlesize (coarse, accumulation mode, and nucleation mode nanoparticles) and chemical composition,and the filtration issues specific to the various PM types are outlined. Likely future trends infilter design are projected, including multifunctional systems combining PM filtration withNOx control catalysts to meet yet more stringent legislative requirements, including EuropeanStage 5 and 6, and the so called ‘Bin 5’ levels in the U.S.A.

DOI: 10.1595/147106709X390977

the fuel-rich zone, carbon forms from reactiveintermediates. Subsequently when excess oxygen ispresent the carbon that has formed may be burnt,and if this is not completed when the combustedmixture is discharged from the cylinder throughthe exhaust ports, a residue of fine carbon coresremains in the exhaust gas. As the gas cools duringpassage into the exhaust manifold, turbochargerand the associated pipework, the carbon particlesagglomerate forming high surface area materialonto which uncombusted and partially combustedproducts adsorb, as well as sulfur oxides and nitro-gen oxides (NOx) formed during the hightemperature combustion in the cylinder.

When inhaled the scale of some of the smallestnanosized particles enables them to pass almostunheeded into the lungs and then even into thebloodstream. It is this mobility, coupled with thecomposition of the cocktail of adsorbed species,which gives rise to environmental health con-cerns. Figure 1 illustrates schematically the natureof diesel PM, and Figure 2 shows a chromato-graph trace that indicates the very large number ofdifferent species that are adsorbed on typicaldiesel car PM.

The amount of exhaust PM that Europeandiesel cars are permitted to emit has decreasedconsiderably over the last couple of decades, and

this is illustrated graphically in Figure 3. TheEuropean PM emissions limits have decreased bymore than an order of magnitude since 1983.Although the test conditions for each of the emis-sion levels are not exactly the same, the overalldownward trend is clear. The very low passengercar PM emissions limits for the European Stage 5legislation, due to be phased in during 2009, canonly be achieved through the fitment of filters, andlegislation in other parts of the world will meanthat filters will also be fitted to diesel cars else-where in the future.

Diesel Particulate Filter TypesSeveral types of ceramic and sintered metal

diesel particulate filters (DPFs) have been devel-oped. The most successful and the mostcommonly used commercially, are porous ceramicwall-flow filters, as shown schematically in Figure 4. Refractory materials used to make theminclude cordierite, silicon carbide and aluminiumtitanate. Alternate channels are plugged, so theexhaust gas is forced through the channel walls.The exhaust gases pass through the walls but thePM does not and it is trapped in the filter. As PMaccumulates in the filter, the backpressure across itincreases, and if this continues it will becomeexcessive, and significantly degrade engine

Platinum Metals Rev., 2009, 53, (1) 28

Vapour phase

hydrocarbons

Soluble organic

fraction (SOF)/

particle phase

hydrocarbons

Adsorbed

hydrocarbons

Hydrated sulfate

species

Solid

carbon

cores

Solid carbon cores

(0.01–0.08 mm),

agglomerate (0.05–

1.0 mm) and adsorbed

vapour phase species

Adsorbed

hydrocarbons

Liquid condensed

hydrocarbon

particles

Hydrated sulfate

species

Fig. 1 Schematic representation of diesel particulate matter (PM) formed during combustion of atomised fuel droplets.The resulting carbon cores agglomerate and adsorb species from the gas phase

performance – ultimately the engine will stop! It istherefore essential that the backpressure across thefilter is not allowed to rise above a predeterminedlimit, so PM must periodically be removed fromthe filter to prevent this from happening. The bestway of removing PM from the filter is to oxidise itto carbon dioxide (CO2) and water.

Filter RegenerationThe process of oxidising retained PM in a diesel

filter is called regeneration (3, 4). The temperatureof diesel passenger car exhaust gas rarely exceeds250ºC during urban driving, so the use of nitrogendioxide (NO2) as shown in Equations (i) and (ii)for combustion of trapped PM (written as “CH”)that takes place at temperatures in the range 250 to400ºC can only remove some of the accumulated

soot when suitable temperature conditions areachieved:

2NO + O2 → 2NO2 (i)

5NO2 + 2“CH” → 5NO + 2CO2 + H2O (ii)In contrast, heavy-duty trucks and buses operateat higher temperatures and therefore the regener-ation with NO2 is very effective and continuouslycleans the filter. Whilst the exhaust gas tempera-ture for cars is too low for this regenerationmethod when driving in urban conditions, atspeeds of around 100 km h–1, the exhaust gas temperature can be sufficiently high for nitricoxide (NO) in the exhaust gas to be oxidised overa platinum catalyst, producing NO2 which can inturn oxidise retained PM in the filter, as inEquation (ii). This type of regeneration is called

Platinum Metals Rev., 2009, 53, (1) 29

mV

400

300

200

100

0

5 10 15 20 25

Time, minutes

Fig. 2 A gaschromatogram showingthat a large number oforganic species areadsorbed on diesel PM.Each peak correspondsto a specific compound

PM

, g k

m–1

0.3

0.2

0.1

0

1980 1985 1990 1995 2000 2005 2010

Year

Fig. 3 The decrease inEuropean legislated PMpassenger car emissionlimits. Since 1983 thepermitted emissionshave been reduced by anorder of magnitude, andfuture stringentlegislation will demandthe fitment of filters

‘passive regeneration’. But to provide a regenera-tion method for all driving conditions, an ‘active’form of regeneration must be employed that periodically increases the exhaust gas temperatureto burn PM in the filter with oxygen (typically 550to 600ºC) every 400 to 2000 km, depending on theactual driving conditions. Three commercial filtersystems developed for cars using active periodicoxygen regeneration are illustrated in Figure 5.‘Generation 1’ employs one or two platinum-based oxidation catalysts in front of the filter tocontrol hydrocarbons (HCs) and carbon monoxide (CO) emissions. The catalyst also oxidises extra partially burnt fuel when it is

injected into the engine, to raise the exhaust temperature for active PM combustion with oxygen (5). This system was introduced in 1999 (6)and uses a base metal fuel additive to lower thetemperature for PM combustion with oxygen. Thefirst fuel additive was based on ceria, and othersnow in use contain base metals such as iron orstrontium, and one based on platinum has beendescribed. These multicomponent systems workwell, although they are costly and fuel additiveresidues are retained in the filter as inorganic ash(see below), and this contributes to a higher back-pressure across the filter than would be the case ifno fuel additive were used.

Platinum Metals Rev., 2009, 53, (1) 30

Fig. 4 A schematicrepresentation of a ceramic wall-flow filter. The arrows indicatethe gas flow through the walls.PM is trapped in the upstreamside of the filter, and periodicallythis has to be removed to preventunacceptable pressure-dropacross the filter (Courtesy ofCorning Inc)

Generation 1: Fuel additive type

Generation 3: CSF-only (integrated oxidation catalyst)

Generation 2: DOC + CSF

FLOW

FLOW

FLOW

CSF

DOC CSF

DOC DPF

Fig. 5 Three filter systems used ondiesel cars: Generation 1: there is a platinumoxidation catalyst before the filter toperiodically burn partially combustedfuel to achieve high temperatures, anda fuel additive is used to lower the PMcombustion temperature; Generation 2: no additive isemployed, the filter contains catalystto accelerate PM combustion;Generation 3: all of the requiredcatalyst functionality is incorporatedin a single filter.

DOC = platinum-only orplatinum/palladium diesel oxidationcatalyst; DPF = diesel particulatefilter; CSF = platinum-only orplatinum/palladium catalysed sootfilter

‘Generation 2’ has the advantage of using nofuel additive. As well as one or two upstream oxidation catalysts, the filter has catalyst in thewalls to promote PM combustion, and today manycars use this configuration. The more recentlyintroduced (2005) ‘Generation 3’ by JohnsonMatthey requires neither a fuel additive nor anupstream catalyst. It comprises a single catalysedfilter, incorporating all of the oxidation catalystfunctionality to oxidise HC and CO during normaldriving, and to periodically oxidise extra partiallyburnt fuel to raise the temperature sufficiently tocombust PM with oxygen during active regenera-tions. Under some conditions, the catalyst mightalso oxidise some NO to NO2 to provide somepassive PM removal during high-speed driving.This system is thermally the most efficient, becauseduring active regeneration there is only the filter toheat, which is mounted actually on the engine turbocharger so as to minimise heat losses. Theoxidation reactions used to boost the temperatureactually take place in the filter, in the same locationas the retained PM (7, 8). In contrast, systems witha separate upstream catalyst lose some of the heat

provided during regenerations to the surroundingsvia the pipework between the turbocharger and thefilter and so are less thermally efficient.

Particulate Matter NanoparticlesFigure 6 shows the range of particle sizes

typically present in diesel exhaust gas. Filters canremove the larger, coarse, micron-sized PM and‘accumulation mode’ particles above 100 nm insize that together account for almost all of the PMmass. Very small nanoparticles, about 10 nm andeven smaller in size, are now being addressedbecause when they are inhaled they can passthrough the bronchial tissue into the bloodstream.Although collectively they have very little mass,they can be present in huge numbers. Recentresearch (9) indicates that most of this ‘nucleationmode’ PM comes from volatile organic or inorganic precursors that are formed as theexhaust gas cools. Laboratory and on-road studieson heavy-duty diesel engines show that when hot,the platinum oxidation catalyst can effectivelyremove all the HCs in the exhaust gas. Then, mostof the nucleation mode PM is inorganic ‘sulfate’,

Platinum Metals Rev., 2009, 53, (1) 31

Nuclei mode:

Usually forms from

volatile precursors

as exhaust dilutes

and cools

Nanoparticles,

Dp < 50 nm

Ultrafine particles,

Dp < 100 nm

Fine particles,

Dp < 2.5 μm

PM10,

Dp < 10 μm

These modeseliminated byfiltration

Coarse mode: Usually

consists of reentrained

accumulation mode

particles, crankcase fumes

Accumulation mode: Usually

consists of carbonaceous

agglomerates and adsorbed

material

In some casesthis mode mayconsist of verysmall particlesbelow the rangeof conventionalinstruments, Dp < 10 nm

1 10 100 1000 10,000

Diameter, nm

Number Mass

0.25

0.2

0.15

0.1

0.05

0Norm

alised c

oncentr

ation, (1

/Cto

tal)d

C/d

log(D

p/n

m)

Fig. 6 Classification of diesel engine PM according to size. Most of the PM mass (dashed line) is associated with theaccumulation-mode (~ 100 nm) and coarse-mode particles, but there are many more nanoparticles (solid line) in thenucleation mode (~ 10 nm) that are so small they can penetrate the human bronchial system (Courtesy of ProfessorDavid B. Kittelson, Center for Diesel Research, University of Minnesota, U.S.A.)

probably as sulfuric acid, ammonium sulfate((NH4)2SO4) or ammonium hydrogen sulfate(NH4HSO4) derived from sulfur compounds originally present in the diesel fuel and lubricationoil and traces of ammonia (NH3) present in the air.The lifetime of this PM is expected to be short,because such very fine particles coalesce andundergo other processes that take them out of theair (10). As expected, they are not formed if thesulfur concentration in the fuel and oil is reducedto below a critical level. Research in this area isvery active, and more work is needed to obtain afull understanding of the nature and reactivity ofnanoparticles from diesel exhaust gas. However,recently it was shown that careful chemical designof catalytic filter systems can control emissions ofnanoparticles, as well as the coarser types of PM(11), and work in this area is continuing.

Inorganic AshInorganic compounds are added to lubrication

oils as viscosity modifiers and to provide antiwearand antioxidant properties, and to keep solid matter, especially soot, in suspension. The morecommonly used compounds contain elementssuch as phosphorus, calcium, zinc, magnesium andsulfur (12, 13). These elements can be present inthe exhaust gas, having originated from the smallamounts of oil burnt in the cylinder, and they areretained as stable compounds in the filter.Similarly, inorganic species derived from the fuelare also trapped in the filter. As mentioned above,PM combustion aids are used as fuel additives inthe first generation of filters on passenger cars, andthese can include compounds of elements such ascerium, iron or strontium.

Because of the very high temperatures duringcombustion in the engine, the nature of thespecies present in the exhaust gas is determined bytheir thermodynamics (14), as is the compositionof the ultimate deposit in the filter. Typically, zincphosphate and calcium sulfate, together withmaterial resulting from engine wear, are found infilters after a car has travelled large distances.Although the rate of ash accumulation in the filteris gradual, its presence does cause the backpressure across the filter with no PM present

to increase over the lifetime of the vehicle. Thegradual backpressure increase caused by accumu-lating inorganic ash can be minimised in threemain ways: using a larger filter, using lubricationoils with reduced concentrations of inorganicadditives (‘ashless oil’), and using filters withasymmetric channel structures that provide a larger inlet volume compared to that in the outletside. The last approach may result in slightly higher backpressure for an asymmetric filter thanfor a symmetrical one, when fresh, but this relativedifference decreases as ash accumulates in the filter and the asymmetric structure then has thelower backpressure (15). The advantage of asymmetric channels in the long term is significant, and filters of this type are likely to beused increasingly in the future. During the devel-opment of modern emissions control systems, it isessential that the performance be maintained oververy many miles of use. Durability is tested bothby real-world driving trials and by laboratorywork, as illustrated in Figure 7.

Platinum Metals Rev., 2009, 53, (1) 32

Fig. 7 Durability testing of a compact diesel particulatefilter on a vehicle. The robot (upper right insert) ‘drives’the car in simulated service, and the emissions aremeasured periodically over the European test cycle toconfirm that the emissions control system is workingcorrectly

Platinum Metals Rev., 2009, 53, (1) 33

Future Filter SystemsFitment of filters to diesel engines is environ-

mentally important, and future legislation willdemand their use in Europe and elsewhere aroundthe world to reduce PM emissions. The overalltrend in diesel emissions control systems is one ofincreasing complexity. Initially, platinum-basedoxidation catalysts were used on diesel cars to control HC and CO emissions (16). More recently, PM filters were introduced, and the typesused have evolved so that now all of the catalyticoxidation and filtration functions can be incorporated in a single relatively small filter. Inthe future, additional control of NOx emissionsfrom passenger car diesel engines will be done byone of two processes. In the first, NOx is converted to nitrate species within a catalyst andthey are periodically reduced to nitrogen (N2) bypulses of enriched exhaust gas obtained by lateinjection of fuel into the engine. This approach hasthe advantage that the reductant for convertingNOx to N2 (diesel fuel) is already available on thevehicle. The second method uses ammonia as thereductant, which is derived from an aqueous urea((NH2)2CO) solution that is injected into the hotexhaust gas. Over a special catalyst, the ammoniaselectively reduces NOx to N2 (a process known asselective catalytic reduction (SCR)). To be cost-and space-effective, some of these functions willbe combined in single components. So SCR orNOx-trapping components are likely to be incorporated into future designs of filters fitted todiesel passenger cars. When CO, HC, PM andNOx emissions are controlled by a single unit, thesystems will be known as ‘four-way catalysts’(FWCs) (17).

Conclusions Sophisticated emissions control systems are

being developed for fuel-efficient (lower CO2)modern diesel engines in passenger cars. For several years platinum-based catalysts have beenfitted to diesel engine exhausts to oxidise CO andHCs, and the spotlight is now on preventing PMfrom entering into the atmosphere. This is donewith wall-flow filters, and periodically it is necessary to combust the PM retained in the filter

to prevent build-up of PM. This is done by catalytically oxidising with platinum-based catalysts extra fuel that is partially burnt in theengine to achieve the temperatures needed to burnPM with oxygen. Catalytic filter systems are capable of eliminating coarse and accumulationmode PM from diesel exhaust, and the latest andmost efficient of these used on cars is mounteddirectly on the turbocharger in the small space inthe engine compartment. The small filter containsall of the catalytic functionality to oxidise CO andHCs during normal driving, as well as to oxidiseadditional CO and HCs to provide sufficient temperature for regenerations with oxygen.Nanoparticles from diesel engines are the subjectof much research, and ways of controlling themare understood. In the future, NOx reduction systems will be needed to meet legislative require-ments, and will involve NOx-trapping technologyor SCR using ammonia derived from an aqueoussolution of urea. Once these approaches havebeen fully developed, it is likely that multifunctionfour-way catalyst systems will be developed, analogously to the use of three-way catalyst systems on traditional gasoline passenger cars.

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2006, 0614, pp. 1–2; (b) AID, Schmidt’s auto publi-cations, 1st August, 2008, 0814, pp. 1–5

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The AuthorsMartyn Twigg is the Chief Scientistof Johnson Matthey PLC andpreviously Technical Director for theEnvironmental Catalysts andTechnologies Division. Followingwork at the University of Toronto,Canada, and a fellowship at theUniversity of Cambridge, U.K., hejoined ICI where he aided thedevelopment and production ofheterogeneous catalysts used in the

production of hydrogen, ammonia and methanol. Martyn hasauthored or co-authored many research papers, writtennumerous chapters in encyclopedic works, and edited andcontributed to several books. He edits a book series onfundamental and applied catalysis.

Paul Phillips is the European DieselDevelopment Manager for theEnvironmental Technologies Divisionof Johnson Matthey PLC, Royston.He is responsible for thedevelopment of oxidation catalysts,the latest generation of catalysedsoot filters, and NOx reductiontechnologies for diesel poweredvehicles. Paul has a B.Sc. inchemistry and a Ph.D. in

organometallic chemistry of main group elements from theUniversity of Warwick, U.K.

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V. Twigg, ‘Chemistry of Inorganic Additives to Fueland Oil: In-Cylinder Reactions and Effects onEmissions Control Systems’, IMechE Conference“Tribology 2006: Surface Engineering and Tribologyfor Future Engines and Drivelines”, London, U.K.,12th–13th July, 2006

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34Platinum Metals Rev., 2009, 53, (1)