ELSEV IER
`
`Catalysis Today 62 (2000) 35 -50
`
`CATALYSIS
`TODAY
`
`www.elsevier.com/locate/cattod
`
`Twenty -five years after introduction
`of automotive catalysts: what next?
`M. Shele`, R.W. McCabe
`Chemical and Physical Sciences Laboratory, Ford Research Laboratory,
`Ford Motor Company, MD -3179, SRL, PO Box 2053, Dearborn, MI 48121, USA
`
`Abstract
`
`The union of catalysts and the automobile has been one of the greatest successes of heterogeneous catalysis over the last 25
`years. Here, the history of automotive catalysis is briefly reviewed, followed by an assessment of where automotive catalysis
`stands today and where it is headed in the future. A key distinction between past automotive catalysis experience and that
`projected for the future is an increased focus on catalysts in upstream of power plant applications, such as on -board fuel
`processing units for fuel cell vehicles. Driven by ever tighter regulations, there will be continued research and development
`activity focused also on downstream applications (i.e. exhaust emission aftertreatment), especially for fuel -efficient, lean -burn
`vehicles, both diesel and spark- ignited. C 2000 Elsevier Science B.V. All rights reserved.
`
`Keywords: Automotive catalysts; Heterogeneous catalysis; On -board fuel processing units
`
`1. Introduction
`
`Automotive catalysts designed to detoxify the ex-
`haust were implemented in production in US on
`vehicles of the model year 1975 and, as we are reach-
`ing a full quarter century of their use, there is ample
`information available to allow us to declare that these
`devices, which are the principal emission control
`tools, have proved to be an unqualified success. Fol-
`lowing the positive experience in US, in short order
`Japan and thereafter Europe, in 1986, adopted the
`use of automotive catalysts. Less affluent developing
`societies have come to the realization that emission
`control in heavily populated areas is not a costly frill
`but a tangible benefit for the quality of life and the use
`of automotive catalysts is rapidly spreading around
`the globe. Even a subjective, casual visitor to urban
`
`* Corresponding author. Tel.: +1- 313- 337 -4041;
`fax: +1- 313 -248 -5627.
`E -mail address: mshelef @ford.com (M. Shelef).
`
`centers where these devices have not yet been widely
`implemented will quickly notice the difference in air
`quality. The ubiquity of automobiles, and by exten-
`sion of catalysts, has made catalysts and their function
`much more familiar to the population at large.
`This paper, touches briefly on the development this
`scientific /technological effort has traversed, where it
`stands now, and what can be discerned on the horizon.
`The field is driven by environmental issues whose aim
`is to mitigate the undesirable side effects of modern
`lifestyle. Personal mobility is considered an essential
`part of this lifestyle and has come to be viewed as
`almost an inalienable right. The national and interna-
`tional regulatory bodies serve diligently as "enforcers"
`by promulgating ever more stringent emission rules so
`that the field of automotive catalysis is perpetually at
`the very edge of technology.
`In the past, vehicular emission abatement was con-
`cerned mainly with (1) the residual uncombusted hy-
`drocarbons (HC), (2) the partial combustion product
`
`- carbon monoxide (CO), (3) nitrogen oxides (NOi)
`
`0920 -5861/00/$ - see front matter CO 2000 Elsevier Science B.V. All rights reserved.
`PII: S0920- 5861(00)00407 -7
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`EXHIBIT
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`M Shelef, R.W. McCabe /Catalysis Today 62 (2000) 35 -50
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`formed from atmospheric nitrogen during combustion,
`and (4) particulate matter (PM), especially carbona-
`ceous particulate formed in diesel engines. Presently
`the overriding concern is shifting to the unavoidable
`product of energy generation from carbonaceous fos-
`
`sil fuels - carbon dioxide (CO2), a molecule whose
`
`concentration in the atmosphere has continuously risen
`since the beginning of the fossil fuels era and which,
`allegedly, is implicated in the "global warming" trend.
`It so happens that every remedy proposed to mitigate
`the amount of CO2 produced by automotive vehicles
`is associated with a catalytic process, either on -board
`or off -board the vehicle. Hence, when discussing the
`developments on the horizon, we include these under
`the overall umbrella of automotive catalysts.
`
`2. Early developments
`
`The reader is referred to a number of previous
`reviews that treat in more detail developments during
`the early stages of automotive exhaust catalysis [1 -6].
`Here we confine ourselves to a brief recap of major is-
`sues and developments that shaped the course of tech-
`nological development. The decision to put catalysts
`on vehicles was preceded by a lengthy gestation period
`during which a series of hurdles had to be overcome
`and there were also a few false starts. It had long been
`recognized, given a hot enough exhaust stream and
`an excess of oxygen, that several materials, especially
`noble metals and even some base metal combinations,
`might be sufficiently active to afford the oxidation of
`the unburned HC and CO on a catalyst placed in the
`exhaust downstream of the engine. To begin with, all
`the parties on whose business the implementation of
`catalytic exhaust after treatment impinged were con-
`cerned with the economic consequences. The overall
`perception was that the consumer would be saddled
`with excessive costs for measures whose actual value
`was not altogether clear. This perception was equally
`shared by the automotive industry and by the fuel pro-
`ducers, both of whose businesses would be affected.
`The apprehensions were many, the most prominent
`among these being: (a) can a catalyst survive the harsh
`environment of the exhaust ?; (b) if not, because of
`the poisoning by lead, could the use of antiknock lead
`compounds in gasoline be dispensed with? (note that
`in its time the introduction of lead antiknock corn-
`
`pounds was hailed as a tremendous step forward which
`inter alía allowed the allied air superiority in WWII);
`(c) if base metals were not up to the task, lacking suf-
`ficient activity or being deactivated by sulfur, would
`there be enough affordable noble metals available?
`Faced with the passage of laws stipulating defi-
`nite limits on exhaust pollution, research efforts were
`redoubled and solutions to these problems gradually
`emerged. Lead was virtually legislated out of auto-
`motive fuel, this being based on its incompatibility
`with catalytic converters (although one can surmise
`that it would have been, in time, removed because
`of its adverse health effects). The octane quality of
`automotive fuels did not deteriorate as improved re-
`fining methods were implemented to compensate for
`the removal of lead. The list of catalytic elements
`having sufficient activity was indeed narrowed down
`to the noble metals and new mining and metal refin-
`ing facilities were contracted for at affordable costs.
`The worries about irretrievably squandering a unique
`natural resource were, to a large extent, mitigated by
`the prospect of recycling the metals from converters
`removed from cars consigned to scrap. Finally, the
`survivability of the catalysts in the field was ascer-
`tained by field tests, including a 450 -car field trial
`carried out by Ford in California in 1974 [7]. As is
`the case with every new technology, the early years
`of use witnessed catalyst failures due to misfueling,
`upstream engine and system malfunctions, etc., which
`were corrected as the experience with catalyst use was
`accumulated. At present, the reliability of catalysts
`equals that of other automotive components. This is
`an important issue since governmental regulations
`require proper functionality of emission systems for
`100 000 miles of use (120 000 miles for trucks).
`The combined requirements of compactness, high
`volumetric flow rates and low back pressure led to the
`adoption of a monolithic embodiment for automotive
`catalysts, quite different from the packed -bed forms
`that prevailed in almost all industrial and petroleum
`catalysis. The monoliths were multi -channeled ce-
`ramic catalyst bodies (square, triangular or honey-
`comb channel configurations), with the exhaust gas
`flowing through the channels on whose walls there is a
`coated high surface area porous layer with finely dis-
`persed noble metal catalytic particles. (At the dawn of
`the implementation period, some manufacturers stuck
`to the more familiar granular catalysts. In use, these
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`37
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`catalyst granules or beads suffered from attrition and
`ultimately were abandoned in favor of the monoliths
`because of this and also poorer warm -up characteris-
`tics.) Presently, monolithic catalysts (mostly ceramic
`but also metallic in some special instances) are uni-
`versally used in automotive catalysis. Moreover, the
`"monolith" catalytic technology is migrating into the
`realm of industrial catalysis, most often in processes
`for treatment of industrial effluents.
`
`3. Which precious metals?
`
`The choice of precious metals (synonymous to no-
`ble metals or platinum group metals) as the active cat-
`alytic materials in automotive materials was the result
`of three factors: (a) only the precious metals had the
`required activity needed for the removal of the pollu-
`tants in the very short residence times dictated by the
`large volumetric flows of the exhaust in relation to the
`size of catalyst which could be accommodated in the
`available space; (b) the precious metals were the only
`catalytic materials with the requisite resistance to poi-
`soning by residual amounts of sulfur oxides in the ex-
`haust; (c) the precious metals were less prone (but not
`entirely immune) to deactivation by high -temperature
`interaction with the insulator oxides of Al, Ce, Zr,
`etc., which constitute the so- called high surface area
`"washcoat" on which the active catalytic components
`are dispersed. While initially Pt and Pd in various
`proportions were used as oxidation catalysts, Rh was
`introduced with the advent of the three -way catalysts,
`having considerably better activity than Pt or Pd for the
`catalytic reduction of the oxides of nitrogen [8 -11].
`In a short time span, the automotive use of Rh
`accounted for the bulk of its production in the world
`and, since it is produced along with Pt at a more or
`less constant ratio, market demand created sharp price
`spikes. Pd has historically traded at much lower prices
`than Pt and Rh owing to sources outside South Africa,
`such as Russia, Canada, and US. Efforts to substitute
`Pd for Pt and/or Rh on a large scale were thwarted,
`however, by technical limitations, namely increased
`sensitivity of Pd relative to Pt and Rh to poisoning by
`lead and sulfur. By the late 1980s, residual lead lev-
`els in US unleaded. gasoline had dropped to levels at
`which Pd could be implemented as a substitute for Pt.
`Ford introduced Pd/Rh catalysts in some of its models
`
`in California in 1989, replacing the long -standing use
`of Pt /Rh in three -way catalysis (TWC) formulations.
`The use of Pd /Rh catalyst technology quickly spread
`within US, spurred on by improvements in techniques
`for segregating Pd and Rh into separate washcoat lay-
`ers, this being to prevent the deleterious formation of
`bimetallic Pd -Rh particles. Introduction of Pd /Rh cat-
`alysts in Europe and other markets has been much
`slower due to a much more gradual process of elimi-
`nating the sale of leaded fuels.
`A resurgence in environmental awareness in the
`late 1980s led US to the Clean Air Act of 1990 and
`a corresponding enactment of stricter emission reg-
`ulations in Europe. These initiatives, in turn, greatly
`increased the worldwide demand for noble metals.
`While increased use of Pd as a substitute for Pt took
`some of the pressure off of Pt, Rh was still in very
`great demand and its price became unstable, peaking
`at $7000 /troy -oz in July of 1992 (compared to histori-
`cal levels of ca. $1000- $1500). While the high prices
`were only temporary and to a large extent specula-
`tive, there was growing concern over the ability of
`the automotive industry to sustain long -term use of
`Rh at levels higher than the mine ratio. Clearly, an
`alternative to Rh was needed.
`The alternative to Rh came in the form of Pd but
`Pd deployed in ways different than before, using con-
`siderably higher loadings. At one point, Allied Signal
`researchers claimed equivalence between a Pd -only
`catalyst at 56.7 g /ft3 loading and a 5:1 Pt/Rh catalyst
`at 20 g /ft3 [12]. New ways of promoting Pd were also
`developed, mostly involving rare earth oxides such
`as lanthana and ceria [13 -15]. To a large extent, the
`use of Pd has benefited from the significant advances
`made in "oxygen storage" materials during the 1990s,
`as discussed below. However, it should also be noted
`that, in contrast to Pt (and Rh at levels normally em-
`ployed), Pd itself contributes to oxygen storage by its
`ability to undergo a redox cycle under exhaust condi-
`tions. Our own work [ 16] has shown that this source of
`oxygen storage persists even under severe aging con-
`ditions in which the oxygen storage due to rare earth
`oxides is largely lost. This is shown in Fig. 1, in which
`the solid curve represents oxygen storage on Pd cat-
`alysts which contain no other oxygen storage agents.
`After 120 h of dynamometer aging, the Pd catalysts
`which contain rare earth oxides show no more oxy-
`gen storage than their ceria -free counterparts. Finally,
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`M. Shelef R.W. McCabe /Catalysis Today 62 (2000) 35 -50
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`10
`
`30
`20
`50
`60
`40
`Bulk oxygen capacity (as PdO)
`
`70
`
`80
`
`Fig. 1. 0-atoms consumed (limo! O- atoms /g cat) in a pulse of CO flowed over a variety of TWCs containing different amounts of Pd.
`Filled squares: catalysts that contain no oxygen storage agents; triangles: dynamometer -aged catalyst containing oxygen storage agents
`(adapted from Ref. f16]).
`
`given the high sensitivity of Pd to poisoning by sul-
`fur [ 17 -18], reduction of fuel sulfur to levels below
`50 ppm in California reformulated fuels was another
`important factor in opening the door to widespread
`use of Pd -only catalysts.
`Currently, TWC formulations containing Pt/Rh,
`Pt /Pd /Rh (trimetal), Pd -only, and Pd /Rh noble metal
`combinations are all in commercial use. It is not
`uncommon to find two formulations in use on the
`same vehicle (e.g. Pd -only as a light -off catalyst and
`Pd /Rh as downstream underbody catalyst). Issues of
`fuel quality (residual Pb levels, sulfur concentration)
`still play an important role in the choice of catalyst
`formulation for a particular market. As all of the
`formulations have improved over time, however, de-
`cisions regarding the choice of the noble metal and
`loading are becoming more and more based on cost
`factors. Indeed, with fluctuations in noble metal prices
`likely to persist into the future, one strategy for vehi-
`cles producers may be to have a number of available
`formulations "on the shelf" that can be deployed in
`response to changing market conditions.
`
`4. Chemistry and electronics
`
`That the catalyst performance is only as good as the
`"quality" of the exhaust gas mixture supplied to it is a
`truism of which all automotive catalyst researchers are
`
`keenly aware. The earliest catalytic converters were
`designed solely for the oxidation of CO and HC and
`were generally used in conjunction with an air pump
`that ensured that, no matter how the engine was run-
`ning, enough air was added to maintain excess oxygen
`required for efficient conversion. By the early 1980s,
`however, emission standards for NO
`in US had been
`tightened to the point where, for most vehicles, efforts
`to reduce NO emissions by lowering compression
`ratios or by exhaust gas recirculation were not suffi-
`cient and an NO catalyst was needed in addition for
`the CO and HC removal. Thus the era of TWC was
`ushered in. With TWC came a coupling of electron-
`ics and chemistry that, unlike many unions, has only
`strengthened with time.
`As noted, CO and HC are converted under oxidizing
`conditions (i.e. with excess air) while NO
`is reduced
`to N2, requiring excess fuel. This is readily seen in
`a standard plot of conversion efficiency for the three
`species as a function of the mass air -fuel ratio (or al-
`ternatively, the X value, which is the actual air -fuel
`ratio divided by the stoichiometric air -fuel ratio) as
`shown in Fig. 2a. Originally, these opposing condi-
`tions were met by dual -bed converters. Here the ve-
`hicle was operated to the rich side of the stoichio-
`metric point and the front catalyst converted primarily
`NO. Air was then injected behind the front converter
`and the exhaust was led over a second catalyst to re-
`move CO and HC. Dual -bed systems were far from
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`M Shelef, R.W. McCabe /Catalysis Today 62 (2000) 35 -50
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`39
`
`100
`
`80
`
`60
`
`40
`
`20
`
`NOx
`
`H
`
`c
`
`co
`
`.
`
`NQx
`
`'
`
`0
`13.5
`
`(a)
`
`14
`
`14.5
`AIR -FUEL RATIO
`
`15
`
`15.5
`
`.8
`
`.6
`
`.4
`
`.2
`
`o
`13.5
`
`(b)
`
`14
`
`14.5
`AIR -FUEL RATIO
`
`15
`
`15.5
`
`Fig. 2. (a) Typical TWC catalyst efficiency plot vs. air -fuel ratio.
`(b) The corresponding output voltage from the zirconia -based EGO
`sensor.
`
`an optimal solution because a rich engine calibration
`adversely affected fuel economy and constrained the
`operating range of the engine. Moreover, under rich
`conditions, the reducing catalyst promoted significant
`conversion of NO, to ammonia (NH3) rather than the
`desired product, N2. This in turn resulted in reconver-
`sion of ammonia to NO over the oxidizing catalyst
`limiting the effectiveness of the combined system [2].
`It was recognized early that if the air -fuel ratio
`could be controlled sufficiently close to the stoichio-
`metric value, all three pollutants could be converted
`(essentially equilibrated to CO2, H2O and N2) with
`
`high efficiency over a single catalyst [19]. Concurrent
`rapid developments in electronic processors and in
`gas sensors ushered in the era of closed -loop engine
`control. The basic scheme is to utilize an exhaust
`gas oxygen (EGO) sensor which senses at any given
`time whether the exhaust gas mixture is net- oxidizing
`(lean) or net reducing (rich) and sends a signal back
`to the electronic engine control module. This module,
`in turn, signals the fuel injectors to increase or de-
`crease the fueling rate as needed to drive the exhaust
`gas mixture back toward the stoichiometric point.
`Fig. 2b shows the typical switching characteristic of
`a zirconia -based exhaust gas sensor as a function of
`air -fuel ratio.
`Throughout the 1980s and continuing today, huge
`advances have been made in emission control through
`the coupling of electronics and catalytic chemistry.
`Both hardware and software (i.e. calibration strat-
`egy) advances have narrowed the range of air -fuel
`ratio oscillations. At the same time, catalyst for-
`mulations have improved, through the addition of
`so- called "oxygen storage" components mentioned
`above, countervailing the residual excursions from
`the stoichiometric point. Fig. 3 shows typical air -fuel
`traces for a 1986 vehicle compared to a 1990 vehicle,
`with the typical range of air -fuel excursions in each
`case, mapped onto a plot of catalyst conversion effi-
`ciency vs. Air -fuel ratio. One notes the "tightening
`of the air -fuel control which occurred over the four
`year period when carburetors and central fuel injector
`systems were replaced by multi -point fuel injectors,
`in some cases operated sequentially to coordinate fuel
`injection with the opening of cylinder intake valves.
`The aim is to supply just the right mix of air and
`fuel to each cylinder at just the right time to ensure
`stoichiometric combustion in each cylinder. These
`advances in fuel injection and air metering hardware
`have been matched by advances in control strategy,
`specifically improvements to the control algorithms
`introducing aspects of anticipatory control. By know-
`ing the characteristic response times of fuel injectors
`and air flow rates, as induced for example by a sudden
`acceleration, it is now possible to compensate for these
`factors and maintain nearly stoichiometric exhaust
`gas mixtures even during highly transient driving
`modes.
`Recent advances include control systems which
`add a second EGO sensor (or a heated EGO sensor)
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`M Shelef, R.W. McCabe /Catalysis Today 62 (2000) 35 -50
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`AF
`
`14
`
`12
`
`16
`
`12
`
`NF
`
`14
`
`16
`
`100
`
`80
`
`60
`
`- HC,
`
`40
`
`20
`
`15
`14
`Air /Fuel Ratio
`
`Fig. 3. Typical TWC conversion efficiency plot for HC, CO, and
`as a function of air -fuel ratio. Also shown are representative
`NO
`air -fuel ratio vs. time traces for 1986 and 1990 vehicles with the
`control bandwidth mapped onto the catalyst efficiency plot.
`
`behind the catalyst in addition to the main controlling
`sensor upstream of the catalyst [20]. The downstream
`sensor is less prone to thermal aging or contamina-
`tion, and also senses a more fully equilibrated exhaust
`
`the upstream sensor. Hence, it
`gas mixture than
`switches at a gas composition more representative of
`the true stoichiometric mixture and is used to correct
`the switch point of the main controlling sensor, par-
`ticularly as that sensor ages. A rough block diagram
`for such a fore -aft oxygen sensor system is shown in
`Fig. 4.
`A similar front and rear oxygen sensor config-
`uration is also used to satisfy one of the on -board
`diagnostic requirements (OBD) specified by US EPA
`
`- that of catalyst efficiency monitoring. The strategy
`
`relies on the fact that a highly efficient catalyst has a
`large damping effect on the switching characteristics
`of a downstream sensor, whereas a deactivated cata-
`lyst causes relatively little damping. It is thus possible
`to compare the responses of upstream and down-
`stream sensors and infer the activity of the catalyst.
`Such a system can be used to activate a dashboard
`light which notifies the driver of potential problems
`with the catalyst.
`The coupling of electronics with chemistry for
`optimum emissions control is one research direction
`which will remain a fertile ground far into the future.
`As on -board computational capabilities continue to
`increase, more sophisticated control strategies will
`be implemented. As noted below, the cold -start pe-
`riod and periods of highly transient driving will be
`the focus of further work. In addition, greater capa-
`bility will be developed to incorporate learning and
`self -diagnostic features into the control strategy which
`will allow the system to compensate for mileage in-
`duced changes in catalyst activity and in the sensor
`response characteristics.
`
`Tailpipe
`Emissions
`
`Hego
`Sensor
`
`Engine
`
`Catalyst
`
`Hego
`Sensor
`
`Post -Cat
`Hego Sensor
`Feedback
`Controller
`
`Base Fuel
`Controller
`
`A/F
`Feedback
`Controller
`
`4141+
`EEC -IV
`A/F Bias
`Table
`
`Fig. 4. Block diagram of a modern engine control system employing both pre- and post- catalyst heated EGO (HEGO) sensors.
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`41
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`5. Advances in "oxygen storage"
`
`The concept of storing oxygen
`to buffer the
`lean -rich swings in exhaust gas composition during
`vehicle operation has been actively pursued since the
`earliest days of "three -way" automotive catalyst re-
`search, pre -dating the first commercial application of
`TWCs in the early 1980s [21,22]. Early on, ceria was
`recognized as a promising storage material because
`of its combination of facile redox cycling between
`the trivalent and tetravalent oxidation states of the
`Ce ions, good thermal stability, ease of impregnation
`onto alumina, and compatibility with noble metals.
`For many years, ceria has been the chief oxygen stor-
`age component for three -way catalysts and the mech-
`anisms by which it works have been the subject of
`many studies and excellent review papers, including
`a recent detailed survey by Trovarelli [23]. Given the
`large body of information that exists, we confine our
`discussion to a brief summary of the most important
`commercial advances in oxygen storage technology
`over the years as reflected in three main eras, each
`associated with a distinct technological advance and
`a step -function improvement in TWC performance.
`The first era of ceria -containing TWCs spanned
`roughly the years 1981 -1985. At that time the TWC
`formulation was relatively simple. The catalysts con-
`tained noble metals impregnated onto either alumina
`pellets or an alumina washcoat on a monolithic sub-
`strate. Ceria was co- impregnated onto the alumina (at
`a few wt. %) along with the noble metals. Even this
`rudimentary addition of ceria dramatically improved
`TWC performance. Laboratory data of Kim [24], for
`example, comparing Pt-Pd-Rh/A1203 catalysts with
`and without 3 wt.% ceria after 24 h exposure to air
`at 932 °C gave a 15 -20% increase in the CO -NO
`cross -over efficiency' with the addition of ceria. Kim's
`data also showed little benefit of ceria in promoting
`HC conversion, except under rich conditions in which
`steam reforming is promoted by the ceria.
`The second period of ceria deployment was that of
`the so- called "high- tech" three -way catalysts, roughly
`between 1986 and 1992. Although "high- tech" was
`a designation that was inclusive of a number of ad-
`vances in the TWC, the main one was the use of more
`ceria. The effectiveness of ceria as an oxygen stor-
`age component lies in its ability to store and supply
`oxygen to the noble metals on which the oxidation
`
`of CO proceeds. For pure ceria and ceria supported
`on alumina, this transfer is primarily via oxygen from
`the surface or near- surface region of the ceria. Thus,
`much of the emphasis in the design of the "high- tech"
`TWCs was on increasing the surface area of ceria in
`contact with the noble metals. Although surface im-
`pregnation onto alumina was still the main mode of
`deploying the ceria, loadings of ceria in some cata-
`lysts were increased to levels approaching 50% that
`of alumina by using physical mixtures of alumina and
`ceria particles. Again, the improvements in emission
`control were dramatic. Data of Williamson et al. [25],
`reproduced in Fig. 5, are representative of the levels of
`NO conversion that could be achieved by increasing
`the ceria content by a factor of 3 over base levels.
`The success of the high -tech formulations could be
`weighed against an unintended consequence they pro-
`duced for automobile manufacturers: the rotten egg
`odor associated with hydrogen sulfide (H2S) forma-
`tion. Here the problem was one of sulfate storage on
`ceria under lean operating conditions and subsequent
`reduction to H2S under rich conditions. In US, the
`answer to the H2S odor problem has been to incor-
`porate nickel into the three -way catalyst formulation.
`The nickel acts as a scavenger for the sulfur (believed
`to be stored as NiS under rich conditions), and then
`releases it as SO2 under lean conditions [26]. In Eu-
`rope, concerns over the potential formation of poi-
`sonous nickel carbonyl, although never substantiated
`in practice, have led either to the use of other H2S
`scavengers or to thermal pretreatment of the catalyst
`to reduce the ceria surface area.
`The formation of H2S was a problem of a "fresh"
`catalyst. The H2S odor would go away with use as the
`ceria lost surface area through thermal sintering. The
`tendency toward thermal sintering, however, revealed
`a fundamental limitation of the "high- tech" catalyst
`formulations; they deactivated rapidly when exposed
`to exhaust gas temperatures in excess of about 800 °C.
`The primary deactivation mechanism involved sinter-
`ing of both the noble metals and the ceria, with con-
`comitant loss of contact area between the two [27].
`Fig. 6, for example, shows the growth of large ceria
`particles (as tracked by X -ray diffraction) for high -tech
`Pd/Rh and Pt/Rh catalyst formulations as a function
`of accumulated aging time in the lab (Fig. 6a) or
`on a vehicle (Fig. 6b). Transmission electron micro-
`scope characterization of noble metal particle sizes
`
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`42
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`M Shelef, R.W. McCabe /Catalysis Todaa, 62 (2000) 35 -50
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`4
`
`A
`
`100
`
`90
`
`80
`
`70-1
`
`60
`
`50
`420
`
`3XCé
`
`2X Ce
`
`1X Ce
`
`0.7X Ce
`
`460
`500
`540
`EVALUATION TEMPERATURE (°C)
`
`580
`
`Fig. 5. NOx conversion efficiency as a function of evaluation temperature for Pt/Rh TWCs differing in their amount of ceria relative to a
`base level (IX Ce) (adapted from Ref. [25]).
`
`also revealed extensive sintering of the noble metal
`particles, and the temperature -programmed reduction
`(TPR) trace showed loss of features associated with
`"interactions" between the noble metal and ceria.
`The thermal deactivation problems of the TWCs
`having high ceria loadings, together with a new push
`at the beginning of the 1990s for lower emissions,
`and an increase in US durability requirements from
`50 000 to 100 000 miles, all called for new "oxygen
`storage" technology. The lower emissions standards
`brought about by new Federal, California, and Euro-
`pean legislation in the early 1990s also amplified the
`importance of mounting the catalyst close to the en-
`gine to improve cold start emissions. This exacerbated
`the durability problems with the high ceria formula-
`tions as they were now required to operate in a hotter
`environment close to the exhaust manifold.
`The solution to this problem was achieved by ceria
`stabilization. The ceria is stabilized by another metal
`oxide forming a solid solution with it. This is done
`through the use of pre -formed powders and, in some
`cases, by co- impregnation of the oxide precursors onto
`alumina. Like many advances in automotive catalysis,
`the exact source of this invention is difficult to iden-
`tify. The first published paper showing improvements
`in engine dynamometer catalyst performance linked
`with direct evidence for improved oxygen adsorption
`characteristics and smaller CeO2 crystallite sizes is
`
`Reactor Ageing (hours)
`
`cç
`
`20
`
`60
`40
`Miles (1000)
`
`80
`
`100
`
`Fig. 6. CeO2 particle size analysis (by X -ray diffraction) of (a)
`reactor aged and (b) vehicle aged high -tech Pd /Rh and Pt/Rh
`TWCs (adapted from Ref. [27]).
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`M Shelef, R.W. McCabe /Catalysis Today 62 (2000) 35 -50
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`43
`
`probably that of Funabiki and Yamada [28]. The in-
`corporation of stabilized cerias into automotive cata-
`lysts was a major advance in the ability of TWCs to
`perform at high efficiency even after severe thermal
`aging. Bartley et al. [29], for example, tested cata-
`lyst formulations containing stabilized ceria vs. a high
`standard ceria TWC after aging for 45 h at 1050 °C. In
`federal test procedure (FTP) tests, catalysts contain-
`ing stabilized ceria decreased the amount of uncon-
`to 15% of the engine -out level (compared
`verted NO
`to 22% of the engine -out level for the standard cata-
`lyst). The corresponding reductions for CO were 12
`and 20 %, respectively. Consistent with this, they also
`reported smaller ceria particle sizes in the case of the
`stabilized ceria (12.5 nm) compared to the standard
`catalyst (20 nm).
`Although stabilization of ceria particle size (i.e.
`dispersion) by the addition of secondary oxides is
`important, it is not the only effect associated with the
`formation of solid- solution oxygen storage agents.
`Recent work in our laboratory, in collaboration with
`Rhodia, has shown that solid solutions of ceria and
`zirconia provide greatly enhanced participation by
`oxygen from the bulk of the ceria -zirconia particles
`compared to pure ceria [30,31]. For a model Pd cat-
`alyst supported on 70 wt. %- ceria -30 wt. %- zirconia,
`
`a TPR scan after 1050 °C (and even 1150 °C) aging
`(Fig. 7) shows the presence of a new peak between
`100 and 200 °C, not seen in the corresponding TPR
`of Pd on pure ceria. The new peak is due to oxygen
`supplied from the bulk of the ceria -zirconia particles.
`This relaxes the constraint of maintaining high sur-
`face area. Current research on further improving the
`thermal stability of "oxygen storage" components is
`directed at optimizing such factors as synthesis proce-
`dures, the composition of the mixed oxide materials
`and the role of additional surface and bulk promot-
`ers. Ultimately, the goal is to develop oxygèn storage
`materials which are so stable that they can be used
`in any exhaust system location without deactivation
`over the life of the vehicle and without any