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Science and Technology of Advanced Materials 8 (2007) 93–100
www.elsevier.com/locate/stam
Diesel emission control: Catalytic filters for particulate removal
Debora Fino
Department of Materials Science and Chemical Engineering, Politecnico di Torino, C. so Duca degli Abruzzi 24, 10129 Torino, Italy
Received 7 August 2006; accepted 13 November 2006
Available online 9 January 2007
Abstract
The European diesel engine industry represents a vital sector across the Continent, with more than 2 million direct work positions and
a turnover of over 400 billion Euro. Diesel engines provide large paybacks to society since they are extensively used to transport goods,
services and people. In recent years increasing attention has been paid to the emissions from diesel engines which, like gasoline engine
emissions, include carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx). Diesel engines also produce significant
levels of particulate matter (PM), which consists mostly of carbonaceous soot and a soluble organic fraction (SOF) of hydrocarbons that
have condensed on the soot.
Meeting the emission levels imposed for NOx and PM by legislation (Euro IV in 2005 and, in the 2008 perspective, Euro V) requires the
development of a number of critical technologies to fulfill these very stringent emission limits (e.g. 0.005 g/km for PM). This review is
focused on these innovative technologies with special reference to catalytic traps for diesel particulate removal.
r 2006 NIMS and Elsevier Ltd. All rights reserved.
Keywords: Diesel particulate; Soot combustion; Catalytic filter; Diesel emissions; Catalytic oxidation
1. Introduction
The high efficiency of diesel engines, their low-operatingcosts, high durability and reliability have provided themwith a leadership role in the heavy-duty vehicle market.Recently, diesel engines also achieved a growing share ofthe light-duty vehicle market (60% of all commercial vansare equipped with these engines), especially in those areaswhere fuel costs are high. In Asia and Europe particularly,sales are expected to grow considerably over the nextyears. This trend of growth of the diesel market requires acareful evaluation of the related environmental effects. Inthis context, diesel particulate rises serious health concernsdue to its carcinogenity [1], owing to the presence ofpolycyclic aromatic hydrocarbons (PAH) and nitro-PAHin its soluble organic fraction (SOF), as well as to aparticulate size falling into the lung-damaging range(10–200 nm).
During the 1980s and the early 1990s there have beensignificant advancements in the development of technolo-
e front matter r 2006 NIMS and Elsevier Ltd. All rights rese
am.2006.11.012
ess: [email protected].
gies for the control of diesel particulate emissions.Attention has mainly been paid to improvements in enginedesign [2] (e.g., as high pressure fuel injection, smallinjection nozzle hole area, high swirl ratio, large volumeratio of the piston cavity, improvement of combustionchamber shape and high response turbo-charger), or fuelpre-treatments [3], or more simply by better tuning of thecombustion process [4], or modification of fuel formulationor use of alternative non-fossil fuel such as natural gas,alcohols or esters [5] and, finally, the use of filtering or non-filtering after-treatment devices. In this last context, thenon-filtering devices use honeycomb monolithic oxidationcatalysts (also called diesel oxidation catalysts—DOCs)with the aim to reduce at least the soluble organic fractionof particulate. Conversely, the filtering systems consist of atrap capable of collecting the particulate matter (PM).Such filters can be ceramic wall-flow monoliths, ceramic ormetallic yarns, or ceramic or metallic foams. All thesedevices operate mostly through inertial impaction, inter-ception or diffusion mechanisms [6]. Wall-flow filters act as‘‘cake filters’’, while foams and fibre yarns act as deep bedfilters.
rved.
ARTICLE IN PRESSD. Fino / Science and Technology of Advanced Materials 8 (2007) 93–10094
Once the particulate is collected, it is necessary to burn itoff. Diesel particulate spontaneously burns in air at about600–625 1C. This temperature range is not regularlyachieved in the typical diesel vehicle operations forsufficient periods of time to enable self-regeneration. Ifan excess of soot is collected on the filter, the exhaust gastemperature raises due to the increased back-pressure, andleads to a sudden burn off, which might occasionally causethe filter temperature to raise above the melting point ofthe filter itself [7].
The task of a ‘‘controlled’’ regeneration can be facedfollowing two different approaches: either the exhaust gasand/or the filter is heated to the particulate ignitiontemperature, or the ignition temperature is lowered withthe aid of substances which catalyse soot oxidation. Asystem incorporating both of these approaches has beendeveloped by the Peugeot-Citroen Societe d’Automobiles(PSA) group and been installed on more than 1 millionvehicles produced since 2000 [8].
In the first case the exhaust gas temperature can beraised by the occasional post-injection of some fuel thatgets burned in a specific honeycomb oxidation catalystplaced upstream of the trap [9] or using an external heatingsystem. The filter can be heated by means of electricallydriven devices, such as microwave heating [10], when usinga conducting material for the filters [11], or by equippingthe filter with heating wires [12]. Alternatively, the filter canbe heated by a burner [13]: for this purpose, automatic dualfilter systems have been developed, in which one clean filteris loaded by the exhaust gas and simultaneously the secondfilter is regenerated off-line by the burner [14] (Table 1).
Concerning the second approach (i.e. the use ofcatalysts), there are two different alternatives: the additionof the catalyst to the fuel in the form of organic derivativesof active metals and the deposition of a catalytic coatingonto the filter surface.
EURO V regulations will force, from 2008 on, to adoptthe latter solution. Most car manufacturers admit that timehas come for combined use of catalysts and particulatetraps. This paper reviews this technology developmentfield.
Table 1
Possible options for the regeneration of soot-laden traps
Measure Alternatives
Exhaust gas heating Fuel post injection+oxidation precatalyst
External heating devices
Filter trap heating Microwaves
Heating wires
Dual filter heated via burner
Catalytic soot oxidation Metal organic compounds in fuel (additives)
Catalyst coated traps
Combined measures Combined heating and
catalytic soot oxidation
2. Traps and catalysts: features and operating procedures
Fig. 1 shows the three major trap types available in themarket [15]. Wall-flow filters (Fig. 1a; materials: partiallysintered SiC or cordierite (Mg2Al4Si5O18) grains; pore size:10 mm) are very efficient (490%) since they are based on ashallow-bed filtration mechanism, whereas foam (Fig. 1b;materials: zirconia-toughened-alumina or—mullite, SiC;pore size: 100–400 mm) and fibre filters (Fig. 1c; materials:doped-alumina; fibre size: 10 mm) are somehow less efficientas a consequence of the deep filtration mechanism theyenable. The traps, located in the under-floor exhaust line,get laden by particulate. This increases their pressure dropup to a level which entails such a fuel penalty that trapregeneration is required. This is accomplished by burningthe trapped particulate off, an operation which can beassisted by the use of catalysts.Fig. 2 shows commercial and R&D-level catalytic
systems used in this context. Beyond non-trapping catalyticconverters (i.e. DOCs), the use of fuel additives [16] hasbeen employed commercially, as well as the production ofNO2 by upstream catalytic oxidation on NO (NO2 is amuch stronger particulate oxidizer than O2 [17]). Con-versely, directly catalysed traps based on either deep- orshallow-bed filtration are currently being developed. Acrucial issue in this context is the contact between catalystand carbon which can be improved by the use of liquid-phase-generating catalysts (Fig. 2.1, tailored for deep-filtration systems [18–20]), of oxygen-spillover-promotingcatalysts (Fig. 2.2 [21,22]) or of multifunctional catalystsenabling particulate direct- (via contact points) andindirect- (via NO2 formation) particulate oxidation(Fig. 2.3 [23]).
3. Commercial catalytic system
3.1. Diesel oxidation catalysts
The first contribution to the reduction of particulatemass emission has been the use of the so-called DOCs [24],become mandatory for new diesel-engined cars in 1996 in
Fig. 1. Sketches of the major trap types: (a) wall-flow monolith, (b)
ceramic foam traps, and (c) alumina–ceramic fiber filter.
ARTICLE IN PRESS
Fig. 2. Commercial and R&D level alternatives for coupling catalysts and traps for trap regeneration purposes. Legend: 1 ¼ mobile catalyst; 2 ¼ catalyst
promoting oxygen spill-over; 3 ¼ catalyst coupling a NO-NO2 functionality (indirect catalyst).
D. Fino / Science and Technology of Advanced Materials 8 (2007) 93–100 95
the US and in 1998 in Europe. These converters resemblethe conventional catalytic converters for gasoline engineswith some significant variation to the catalyst composition(in any case based on noble metals) so as to optimize thecatalyst activity under lean conditions. Obviously, sootparticles are not trapped by these systems but CO andhydrocarbons (including those of the SOF of the particu-late) are burned out from 200 1C on [25]. The particulatemass abatement efficiency in the flow-through DOC ishowever lower than 5% at gas hourly space velocities of50,000–300,000 h�1 [26]. Finally, some problems mightarise due to the SO2 oxidation activity of the catalyst thatcan lead at temperatures above 300 1C to significantformation of SO3 (which dissolves in condensed waterdroplet to give H2SO4 after engine switch off) and even apossible increase of the total particulate mass [26].
3.2. The Peugeot-Citroen Societe d’Automobiles system
The key components of the Peugeot-Citroen Societed’Automobiles (PSA) system (Fig. 3), are [8]:
�
SiC wall-flow monolith: selected for its high filtrationefficiency and superior physical properties (high-tem-perature and thermal-shock resistance). � Active regeneration strategy: when trap regeneration isneeded owing to the high-pressure drops detected by aproper sensor, fuel post-injection, enabled by multi-jetcommon-rail engines, provides unburned hydrocarbonsto the pre-oxidiser, resulting in an increase of the exhustgas temperature.
� Pre-oxidiser: a catalytic converter that burns out thepost-injected hydrocarbons thereby enhancing furtherthe flue gas temperature and igniting the trappedparticulate.
�
Ce-fuel additive: this fuel additive leads to formation ofCeO2 particles well embedded in the structure of dieselparticulate and thus in very good contact with the soot,which lowers ignition temperatures by catalytic meanswith the benefit of post-injected fuel savings [27].As above anticipated, this system is now running onmore than 1,000,000 cars with apparently no majorproblems. However, it has some drawbacks: � CeO2 deposits: periodic cleaning or trap over-sizing isneeded to cope with the cerium oxide that remains in thetraps,
� High investment costs: they are entailed by the presenceof many components (additives, additive-storage tankand dosing pump, pre-oxidiser, pressure and tempera-ture sensors, control electronics, common-rail dieselsystem),
� High operating costs: post-injected fuel does notcontribute to driving power and thus corresponds to acost (fuel penalty). A trade-off with the fuel penaltycaused by the increased trap pressure drop due toparticulate loading leads to the determination of anoptimal interval between subsequent regenerations of300–400 km, which results in an overall fuel penalty ofabout 4%.
3.3. The continuously-regenerating-trap (CRT) system
Another patented and tested technology is the continu-ously regenerating-trap (CRT) system by Johnson Mattheybased on the early discoveries by Cooper and Roth [24]concerning the role of NO2 in diesel particulate combustion(Fig. 2); it consists of a wall-flow trap with an up-streamflow-through diesel oxidation catalytic monolith called pre-oxidiser (Fig. 4).
ARTICLE IN PRESS
Fig. 3. Schematic of the PSA fuel additives based system (courtesy of the PSA group).
Fig. 4. The continuously regenerable trap system by Johnson Matthey (courtesy of JM Ltd.).
D. Fino / Science and Technology of Advanced Materials 8 (2007) 93–10096
The pre-oxidiser converts about 90% of the HC and COpresent in the exhaust gas and promotes the abatement ofat least 3% of nitrogen oxides. The most interesting featureof the CRT system is its ability to promote a continuoustrap regeneration provided its operating temperature iskept in the range 200–450 1C [28]. Above 200 1C the pre-oxidiser activity is sufficient to oxidize the HC and CO aswell as to convert NO to NO2 which can rapidly react withdiesel particulate leading to its combustion to give CO2 andNO. Above 450 1C thermodynamics becomes unfavourablefor NO2 formation in the pre-oxidizer. Because ofcontinuous regeneration, extreme temperature gradientswithin the trap are avoided which prolongs the trap lifeexpectation. A satisfactory performance over 600,000 kmhas been reported by Hawker and coworkers [29]. The
major drawback of the CRT system lies in the sensitivity ofthe pre-oxidiser to the presence of sulphur compoundswhich has hampered significant introduction in the market[29]. However, since 2005 low fuel sulphur levels (50 ppmw)have been mandatory in Europe and this may lead tomassive introduction of this technology. Another weakpoint of the system is its dependency on the presence ofNOx, as it is uncertain whether future diesel engines willproduce high-enough NOx-to-soot ratios.
3.4. The Toyota motors system
In the diesel particulate and NOx reduction (DPNR)Toyota Motors system [9,30,31] a layer of an ‘‘activeoxygen’’ storage alkali metal oxide hosting Pt is deposited
ARTICLE IN PRESSD. Fino / Science and Technology of Advanced Materials 8 (2007) 93–100 97
over the diesel soot filtration device. The ‘‘active oxygen’’ iscreated by the conversion of gas-phase NO over theplatinum into surface nitrate species. These surface nitratesare decomposed at the interface between soot and activeoxygen layer producing very reactive adsorbed oxygenatom and NO. The NO can be reoxidized to surface nitrate,while the reactive adsorbed oxygen atoms can oxidize thesoot at temperatures and 300 1C and above. If the systemcannot convert all the deposited soot the back pressureover the filter will increase and trigger the regeneration ofthe trapped soot due to a temperature rise in the filter. Theincrease in temperature is accomplished, once again by fuelpost-injection.
The active storage material acts at the same time as aNOx trap. When the NOx trap has reached its maximalallowable buffer capacity for retaining NOx as nitrates,then the NOx trap needs to be regenerated. CO and HCscan then reduce these surface nitrates to N2. These CO andHCs are generated by running the engine rich or by fueladdition into the exhaust stream at a temperature ofaround 450 1C. The newly generated CO and HCs areconverted into CO2 by surface nitrates while the nitratesthemselves convert mainly to N2 and, to some extent, toNO. Fig. 5 illustrates the chemical processes of the Toyotasystem.
Total reduction of diesel exhaust emissions (CO, HCs,PM, and NOx) is preferably achieved in a single filtersystem such as the DPNR system. However, this systemmay encounter several problems such as engine ashdeposit, complexity of data logging, and the effectivenessof engine-out NOx concentration. It is reported that thefresh DPNR system reduces 80% of NOx and PMemissions and might meet the US tier 2 bin 5 or 6emissions standards using low-sulfur diesel fuel. Fleet testshould however, demonstrate the efficiency and robustnessof this system [32].
3.5. R&D systems developed at Politecnico di Torino
Finally, some more innovative technologies still at theR&D stage at Politecnico di Torino and in many other
Fig. 5. Schematic of the diesel particulate and NOx abatement technology
by Toyota Motors operating under lean or rich conditions (adapted from
Gulati et al. [32]).
Research Centres are described (Fig. 2). For instance,catalytic foam traps were developed in the framework ofthe activities of a just-ended EU project (CATATRAP) bydepositing through tailored techniques Cs-V-based cata-lysts over the pore walls of zirconia-toughened aluminafoams developed by Centro Ricerche FIAT and SaintGobain. The catalysts employed in this context were of themobile type either by liquid-phase formation(CsVO3+KCl [33]; Cs2O �V2O5 [34]) or via oxygen spil-lover (Cs4V2O7 [35]) so as to improve the contactconditions between catalyst and carbon. With catalyticfoams EURO IV legislation limits were achieved instandard driving cycles, with no need of active regenerationmeans. However, the filtration efficiency of foams is toolow (about 50%) to provide them with future perspectives.Conversely, wall-flow catalytic filters enabled efficiencieshigher than 95% at the price of the need of activeregeneration means like those employed by the PSAsystem. However, this catalyst cannot be employed withwall-flow filters as their pores are so fine that the liquidcatalyst phase would be rapidly sucked into them pluggingthe trap. For this reason, mixed-oxide catalysts (e.g.LiCrO2) were deposited on the walls of the inlet channelsof wall-flow traps via an ad-hoc-developed technique, thein situ combustion synthesis [36–38] (Fig. 6). The foamymicrostructure of nanosized catalyst particles (Fig. 6b)thereby obtained is particularly suitable to achieve
Fig 6. View of LiCrO2 catalyzed wall-flow at different magnification levels
the a traps developed at Politecnico di Torino [37] (trap material:
cordierite by Corning).
ARTICLE IN PRESSD. Fino / Science and Technology of Advanced Materials 8 (2007) 93–10098
intensive contact conditions between catalyst and soot, aprerequisite to achieve required combustion kinetics.
The development of the common-rail engines in last1990s has deeply changed the perspectives in the diesel-exhaust treatment field, by enabling a momentary rise ofthe exhaust gas temperature for trap regeneration pur-poses, even if this implies a certain fuel penalty. On thegrounds of good thermal and chemical stability, severalperovskite, delafossite and spinel type oxide catalysts wereprepared by solution combustion synthesis, characterizedand tested as catalysts for both NOx and soot removal[39–41] or only particulate combustion [37,38,42–46].
Nanostructured spinel-type oxides catalysts AB2O4
(where A ¼ Co and Mn, and B ¼ Cr and Fe), preparedby the solution combustion synthesis method, proved to beeffective in the simultaneous abatement of soot and NOx.The activity order for soot combustion was found to beCoCr2O44MnCr2O44CoFe2O4, while the activity orderfor NOx reduction was found to be CoFe2O44Co-Cr2O44MnCr2O4 [39]. The best compromise between sootand nitrogen oxide abatement was therefore shown by
Fig. 7. Concentration plots of the outlet gaseous species in a TPR run per
concentrations: O2 ¼ 10 vol%, NO ¼ 1000ppmv, He ¼ balance; catalyst—soo
Fig. 8. Results of the temperature-programmed desorption tests on all selected
oxygen species b [44].
CoCr2O4 catalyst; it could promote soot combustion andappreciable NOx reduction below 400 1C (see Fig. 7), themaximum temperature reached in the exhaust line of adiesel engine. The activity of the chromite catalysts couldbe explained by their higher concentration of suprafacial,weakly chemisorbed oxygen, which contributes actively tosoot combustion by spillover in the temperature range300–500 1C.As previously discussed in a review by Seyama [47] and
in some papers by the Authors [42–46], the most recentcatalysts developed at Politecnico di Torino can desorb twodifferent types of oxygen species accompanied by relateddesorption peaks (Fig. 8): oxygen desorbed in the low-temperature range of 300–600 1C named either a species orsuprafacial species or weakly chemisorbed species, andoxygen desorbed at high-temperature (600–900 1C), namedeither b, or intrafacial species, more bound to theperovskite structure and therefore less easily desorbed[42]. The main reason for the superior activity of themtowards carbon oxidation should indeed lie in a radicallyhigher specific surface concentration of active oxygen
formed with the CoCr2O4 catalyst under tight contact conditions (feed
t mass ratio ¼ 9:1; W/F ¼ 27.1 kgNm�3) [41].
perovskite catalysts; low-temperature oxygen species a; high temperature
ARTICLE IN PRESS
Table 2
Effect of the ageing treatments on the BET area, the activity and
selectivity of LiCrO2 catalyst
Ageing conditions BET area (m2/g) Tp (1C) ZCO2(%)
LiCrO2 (fresh) 24 387 99
96 h 300 1C air 18 387 98
96 h 300 1C wet 18 385 96
96 h 300 1C air+SO2 18 392 98
24 h 650 1C air 4 389 98
24 h 650 1C wet 4 410 96
24 h 650 1C air+SO2 17 395 98
D. Fino / Science and Technology of Advanced Materials 8 (2007) 93–100 99
species compared to other catalytic materials developed inthe late 1990s. The Li-substituted chromite catalysts(La0.8Cr0.9Li0.1O3, La0.8Cr0.8Li0.2O3) exhibit the highestactivity as a consequence of their superior amount ofweakly chemisorbed O� species (a-oxygen), that werepointed out as the key player in the soot oxidation state.In subsequent investigations the LiCrO2 catalyst waspointed out to be even more active than the abovecounterparts [37]. This catalyst was lined by in situcombustion synthesis over ceramic wall-flow monolith trapsand tested according to a standard loading and regenera-tion cycle. Fig. 9 shows that the LiCrO2 catalyzed trapganea more complete regeneration compared to a non-catalyzedtrap and in ca. half the time.
In a parallel effort, catalytic materials characterized bythe highest possible a-oxygen type concentration weredeveloped, with careful attention to their compatibilitywith the substrate material or the poisoning componentspresent in the diesel exhaust gas (e.g. sulfur oxides). Thedurability tests performed on LiCrO2 delafossite catalystpowders under accelerated ageing conditions in thepresence of some of the most critical compounds in dieselexhaust gases, showed a comparatively good behaviour,with limited loss of catalytic activity, as shown in Table 2[37]. No serious deactivation was found after any of theageing test protocols. The highest deactivation (limited,however, to an increase of about 30 1C of the Tp value) wasnoticed after high temperature hydrothermal ageing, whichmostly affected the specific surface area of the catalyst andslightly reduced the selectivity to CO2 (ZCO2
).Finally, an innovative multifunctional catalyst
(La0.9K0.1Cr0.9O3�d +1wt%Pt [48]), with a very lowcontent of precious metal (widely used in this kind ofcatalysis), was developed for diesel soot combustioncombining direct and indirect (NO–NO2–NO cycle) oxida-tion mechanisms (Fig. 2.3). This catalyst was applied over awall-flow SiC trap and tested according to a standardprotocol on an engine bench. It was found that thepresence of the catalyst not only accelerates soot combus-
Fig. 9. Results of loading and regeneration runs for a catalyzed (LiCrO2)
and a noncatalytic wall-flow trap [37].
tion on occasional trap heating (regeneration phase) butalso prolongs the trap-loading phase (soot accumulationduring normal operation) due to the indirect oxidationmechanism. This should allow a more than 50% reductionin a post-injected fuel consumption.
4. Conclusions
The current scenario of catalytic traps for dieselparticulate abatement has been briefly reviewed, enlighten-ing the most promising commercial technologies availableand the most interesting opportunities that might outper-form in a few years the nowadays available systems.The 2010 European law limits (EURO V) will almost
certainly been met by wall-flow type traps using either fueladditives or NOx-aided regeneration or catalysts depositedover the trap itself. In any case, the use of active means tooccasionally rise the trap temperature would likely berequired, accompanied by the related fuel penalty.The next frontier for further progress in the field will be
to find the technological means to reduce or even eliminatethis fuel penalty. Both catalyst and trap development willbe needed for this sake. Targets will be lower soot ignitiontemperature but also lower trap pressure drop, efficiencyremaining excellent. Oppurtunities for innovation willprovide many R&D challenges for this field in the future.
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