AUTHOR

DR. KLAUS HARTHis Vice President Environmental

Catalysis Research at theBASF Corporation in Ludwigshafen

(Germany),

INCREASING REQUIREMENTS FORTHE CATALYTIC CONVERTER SYSTEM

Over the last several decades, advancesin environmental catalyst technologiesbave contributed significantly to reduc-ing tailpipe emissions from combustionengines. At present, a modern catalyticsystem is capable of converting morethan 95 % of the carbon monoxide (CO),hydrocarbons (HC), nitrogen oxides(NO~) and soot preseut in the exhaustgas to carbon dioxide, water and nitro-gen gas. While future environmentalreguIations will require further reduc-tions of these harmful emissions, com-bustion engine development is driven bythe need for higher fuel efficiency aodless production of carbon dioxide. Thesetrends will demand further continuousperformance improvements of the cata-lytic exhaust gas treatment system. Inthis article, the development of catalyticsystems is explained by the example ofdiesel passenger cars.

CATALYTIC COMPONENTS FORDIESEL EXHAUST GAS TREATMENT

The primary function of the diesel oxida-tion catalyst (DOC), ~, is to completelyoxidize hydrocarbons and carbon mon-oxide in the exhaust gas to carbon diox-ide and water. In specific applications,the DOC is also expected to partiallyconvert nitrogen oxide (NO) to nitrogendioxide (NO~). A stable concentration ofNOx can be used to oxidize soot on a cat-alytic soot filter (CSF) or to promote NO~

HC, COComplete oxidation ofcarbonmonoxide (CO) andhydrocarbons (HC)

Stable NO; formation

Precious metal particles(Pt or Pt-Pd) withhigh thermal stability

Ptonly Fresh

!800 °C

Aged

Diesel oxidation catalyst (DOC)

COVER STORY EMISSIONS

HC, CO Particulate matter (PM)

Filtration of particulate matter

Active or passive reseneration

Complete oxidation of CO and HC

Stable NO2 formation

Trapped sootparticles~ -. Plu88ed

PM ..~:~’~’-_~ cellsHC co,NO, ,,~ Hz0~ 0o,

System of DOC and CSF

conversion over the selective catalyticreduction (SCR).

The active components of a DOC coat-ing are small precious metal particles ofPt and Pd supported on high surface areainorganic oxides (e.g. alumina). The Doewashcoat may also contain componentslike zeolites to better manage the conver-sion of hydrocarbons during cold start.

In addition to the architecture of thewashcoat, the size, composition andmatrix of the precious metal particlesplay a crucial role in reliable DOC perfor-mance under real driving conditions.Utilizing the broad experience in cataly-sis and material science, BASF has devel-oped a broad portfolio of high perfor-mance DOCs for different applications.These DOCs can be further tailored tospecific customer applications.

Early emission regulations for lightduty diesel vehicles could be met with asingle DOC plus engine control adjust-

lnents. At that time, the volume of theDOC was comparable to the engine dis-placement volume. Recently, filter ele-ments (CSF) have been added to dieselvehicles to prevent soot-particles fromgetting to the atmosphere, ~). In contrastto flow-through substrates of conven-tional vehicle catalysts, the channels of afilter substrate are blocked at alternatingends. This forces the exhaust gas to flowthrough the porous wall of the monolith.Soot particles are retained and accumu-lated in the filter until a critical pressuredrop across the filter element triggers anactive regeneration. Regeneration occurswhen extra fuel is combusted over theDOC and the resultant heat ignites of sootin the filter. This extra fuel is injectedeither into the combustion chamber ordirectly into the exhaust gas upstream ofthe DOC.

In addition to this active regenerationprinciple, passive regeneration systems

can currently be found in heavy dutydiesel vehicles. The soot retained in pas-sive systems undergoes continuous oxi-dation by the NOa produced by theupstream DOC. In addition, the filteritself may contain catalytic components.Common coatings comprise preciousmetals, which - in analogy to DOC -ensure complete oxidation of CO and HCas well as a stable formation of NO~.

The two leading technologies for con-trolling NO~ emissions are lean NO~ traps(LNT} or selective catalytic reduction cat-alysts {SCR), O. Each uses a reducingagent for the conversion of NO~ to nitro-gen gas. The LNT uses partially corn-busted diesel fuel and the SCR usesammonia as the reducing agent. Ammo-nia is usually produced by the decompo-sition of urea on board the vehicle. Incor-poration of these NO., abatement compo-nents into the exhaust gas treatmentsystem adds significant vohune and corn-

CSF

"-Reduction of NO, by ammonia (NH~):NO + NO.~ + 2NH~ -) 2N~ + 3H~O

Ammonia produced by on-boardhydrolysis of urea

: Metal-zeolites (e.g. Cu-chabazite)with high activity and stability System of DOC, CSF and SCR

~) DeNO~ performance and dependence of backpressureon filter-polosity and SCR material loading [] 220 °C [] 300 °C [] Back-lxessure

100

8O

6O

4O

2O

0

Filter porosity 57 %Reference SCR loading

Filter porosity 63 %SCR Ioeading + 15 %

Filter porosity 63 %SCR loading + 30 %

150

100

5O

plexity to the emission control system.For light duty diesel applications the

SCR catalyst consists of a Cu or Fe con-taining zeolite. Typical zeolites are Fe-beta and Cu-chabazite. Cu-chabaziteexhibits an excellent low temperatureactivity, a broad temperature window ofactivity and superior high temperaturestability. In addition, an a]nmonia oxida-tion catalyst may be used downstream ofthe SCR to prevent ammonia slip. Systemsof this type are already in use for heavyduty applications. They require sophisti-cated control of timing of CSF regenera-tion cycles and an active urea dosingstrategy. For light duty applications, sys-tems with smaller volume requirementsand less complexity are highly desired.

INTEGRATED CATALYST

The increasing complexity of catalyticsystems for vehicles with diesel enginesis a driving force to develop smart andless complex systems for the future.Here, the trade-off between complexityand cost reduction on one side and therequirements to meet current and futureemission regulations on the other sidemust be balanced.

A possible approach to simplificationis to integrate CSF and SCR function inone component and place the activemass of the SCR catalyst on the filtersubstrate of the CSF. This integrated cat-alyst is in the following called SCR onFilter or just SCRoF. For such an SCRoFcomponent there are complex require-ments. To achieve on the one hand ahigh NO.~ conversion level, particularly

in the aged state, SCR-active materialswith very high intrinsic activity arerequired. At the same time it is desirableto accommodate the highest possibleamount of these active compositions inthe pores of the filter substrate. However,here limits are set by the maximum pres-sure loss, a filter component may have.On the other hand, filters with lowporosity are preferred for the safe controlof all particulate matter emissions andthe compliance of small so-called sootmass limits.

Therefore SCRoF applications requirefilter substrates with tailor-made porosi-ties which overcome this confiict. Inaddition to the filter porosity, the cata-lytic material and the coating process areof critical importance. For use as an inte-grated system, such as a close coupledposition, the dosage and the thernaolysisof urea and the regeneration conceptmust also be addressed.

~ shows the possibilities that open upthe use of filters with optimized porosity.Shown are the measured NO~ conversionlevels in stationary engine tests for cata-lysts with different porosity and activemass loading. This evaluation was carriedout at BASF’s engine laboratory in Hano-ver. A filter substrate with a mediumporosity of 57 % and a standard SCR cata-lyst loading served as reference. The useof filters with bigher porosity (63 %) leadsto a lower initial back-pressure and ena-bles to increase the SCR loading by 15 %.This results in an increase of at least 10% points versus the reference. A furtherincrease of SCR catalyst mass on the highporosity filter does not lead to an addi-

tional increase in NO., conversion underthe chosen stationary conditions.

The filtration efficiency was also eval-uated. Because of the close-coupleddesign of the system, there was no sig-nificant adverse effect observed with theSCRoF. Even with the higher porosity ofthe filter, the soot-mass regulation (4.5mg/km in the Euro cycle) and the limitfor the soot particle nnmber (6*10n/kinin the Euro cycle) could be met.

An important method for evaluation ofcatalyst systems remains the transientevaluation on the vehicle. Several tran-sient cycles were studied for SCRoF. Theperformance in the European DrivingCycle (NEDC), the U.S. light duty cycle(FTP72 and US06), and other cycles (e.g.WLTP) was evaluated. It turns out thatthe exact system design is an importantelement for performance optimization. Itis important that a high-performanceDOC has a very fast light-off and ensurescomplete oxidation of CO and HC as wellas stable oxidation of NO. In addition,because of the extremely close-coupledlocation of the DOC, it has to be ther-mally stable. The DOC developed byBASF met the limit for CO (500 mg/kmin the Euro cycle) after aging for 16 h at800 °C. A significant improvement wasachieved for the NO oxidation behavioras well. The deterioration of the NO oxi-dation could be significantly reduced.

The Euro 6 limit values for NO~(80 mg/km in the Euro driving cycle)can be met with a SCRoF system on avehicle even under difficult testing con-ditions, ~}. In this example, the averageNO to NO2 ratio was 25 % (as opposed to

MTZ 0912012 Volume 73 5

COVER STORY EMISSIONS

2,5CSF Urea .... Raw emissions

/ DOC+CSF+SCR

Euro 6 m t .... ,’°"" ~---~~ :DOCFSCRoF

~.j"~’-~ "~~ " ,’ , ~ ! ,’ ! I ’ ; 1~ DOC and SCRoF system can meet Euro 6 NO~ limit200 400 600 800 1000 1200

Time [$]

the optimum of about 50 %), This con-straint was deliberately chosen to pushthe boundaries of the system and also todifferentiate the ability of next genera-tion DOC to work with SCRoF catalysts.Only an optimized DOC and an opti-mized SCRoF were abIe to meet theselimits. With this optimized system, a sig-nificant improvement was observedcompared to the classical system com-posed of DOC, CSF and SCR.

The new generation DOC and SCRoFcatalyst system was also evaluated on theU.S. cycle. The NOx conversion observedwas > 85 % and the requirements of Tier2Bin5 were met. Additional performanceimprovement can be achieved when asmall SCR catalytic converter is usedunder tile floor (downstream of tileSCRoF). The catalyst volume of this SCRpart was 50 % of the SCRoF component.

This additional SCR increased NOx con-version to more than 90 %.

DURABILITY

An important and crucial developmentgoal for SCRoF catalyst systems is thedurability, O. The U.S. test cycle (FTP72)was chosen. Similar results wereobtained under steady state and in othertransient cycles (e.g. NEDC). In this test,the components were loaded with sootand regenerated under engine condi-tions. This process was repeated morethan 220 times to mimic realistic end-of-life conditions for the catalysts. Over anequivalent mileage of 180,000 km, nosignificant deterioration of the NO~ con-version was observed, This result clearlyunderlines the robustness of the SCRoFsystem developed by BASR

CONCLUSION

BASF has developed a new smart cata-lyst technology based on the broadknow-how in catalysis and materialsresearch. This new compact system com-prising an SCRoF is able to meet strin-gent emission requirements.

[-ISCRoF ~SCRoF + SCR

8O

Fresh = 0 km 50.000 km 90.000 km 140.000 km 180.000 km

!~ DOC and SCRoF thermal durability