eprints.iisc.ac.ineprints.iisc.ac.in/43306/1/Recent_advances.pdf · 1Surface Engineering Division,...

1Surface EngineeringDivision, NationalAerospace Laboratories,Bangalore 560017, India2Solid State and StructuralChemistry Unit, IndianInstitute of Science,Bangalore 560012, [email protected]

REVIEWS

Recentadvances inautoexhaustcatalysis

Parthasarathi Bera1 AND M. S. Hegde2

Abstract | In last 30 years innovative research in the area of auto exhaust catalysis is being

developed and CeO2 has been found to play a major role in this area due to its unique

redox properties. In this review, auto exhaust emission and its impact on earth’s

environment, global concern and recent advances in science and technology in automotive

exhaust catalysis have been documented. A new preparative method of dispersing metal

ions by solution combustion technique over CeO2 and TiO2 resulting mainly Ce1−xMxO2−δ,

Ti1−xMxO2−δ and Ce1−x−yTixMyO2−δ (M = Pd, Rh and Pt) catalysts, structure of these

materials, their catalytic properties towards auto exhaust catalysis, structure–property

relation and mechanism of catalytic reactions are accounted here. In these materials, metal

ions are incorporated into substrate matrix to a certain limit in the solid solution form and

we have established a new direction in heterogeneous catalysis by turning to the concept

of dispersed metal ions as catalytically active sites from the conventionally nurtured idea of

metal particles as active centers for catalysis.

1. IntroductionThe industrial development in the world is mainlyaccompanied by the consumption of fossil fuelssuch as coal, natural gas and oil. Most of thefossil fuels constitute carbon, nitrogen, hydrogenand to some extent sulfur containing compounds.The energy stored in the fossil fuels, biomass,industrial and domestic waste is released mostlyby complete as well as partial combustion of theseconstituents leading to emission of nitrogen oxides(NOx), carbon monoxide (CO), sulfur oxides (SOx),NH3 and unburned hydrocarbons (HC) to theatmosphere which cause environmental pollution.These pollutants are emitted everyday with theeffluents produced by mobile and stationary sources.The mobile sources of emission include automotiveemissions and emissions from aeroplanes, whereasthe stationary sources are industrial processes,power plants, combustion of waste and biomass anddomestic burning.

Practically, in the last decade, mammoth increasein the number of road and air traffics as well asindustrial activities especially around the thicklypopulated mega cities in the developed as well asdeveloping countries add a substantial contributionto environmental pollution and hence, this wholeman made situation causes a big threat to the livingbeing on earth. Therefore, it is necessary to controlthese pollutants for sake of our environment. Thereare several technologies for controlling exhaustemissions. Among available technologies one ofthe best ways of controlling exhaust emissioncould be to reduce or convert these pollutantgases by using catalyst at the source itself. Theconversion of these environmentally unacceptablegases to N2, CO2 and H2O using catalysts is achallenging task. Therefore, environmental catalysisor exhaust catalysis has been growing as a new andbroad area of research in heterogeneous catalysisin the last three decades. However, environmental

Journal of the Indian Institute of Science VOL 90:2 Apr–Jun 2010 journal.library.iisc.ernet.in 299

REVIEW P. Bera and M. S. Hegde

catalysis related to auto exhaust has received muchmore attention as exhaust from automobiles hassignificant contribution to the global environmentpollution. Extensive studies on the scientific researchand technological development in this area has beencarried out over the last several years. A numberof reviews and articles regarding various aspectsof exhaust catalysis have been published to updatethe status of catalytic science and technologicaldevelopment in this area from time to time.1–28

Kaspar and coworkers have enlighted the scenarioof automotive catalytic converters for the 21stcentury.23An assessment of the developments inautomotive catalysis and its future directions as weenter 21st century have been briefed by Twigg.29–32

In a recent comprehensive review, Roy and Baikerhave presented several aspects of NOx storageand reduction (NSR) catalysis where they havediscussed mechanism of NSR catalysts, role playedby noble metals, storage components and supportoxides.33 Roy et al. has briefed NOx abatementcatalysis in a recent review where they have providedupdated information about several NOx removaltechniques such as photocatalytic decompositionand reduction of NOx , selective catalytic reduction(SCR) of NO and NSR catalysis.34 Keeping severaltype of catalysts for auto exhaust catalysis inour mind, new generation catalysts have beendeveloped in our laboratory in last one decadein order to understand the metal ion–reduciblesupport interaction, relation between structure andoxygen storage capacity (OSC), synergistic effect andcatalytic activity.35 In this review, we would account,the global concern on auto exhaust emission, currentstatus as well as recent developments in advancedmaterials for auto exhaust catalysis. Finally, futuredirection of auto exhaust catalysis in the 21st centuryview point would also be discussed.

2. Formation of pollutants and theiradverse effects

The atmosphere is a chemically complex anddynamic system that interacts significantly withthe land, oceans and ecosystems. The atmosphereis the recipient of a vast array of chemicalcompounds emitted by natural processes andin recent times, increasingly emitted by humanactivities. The oxidation of this wide variety ofchemical compounds is a key process proceedingin our oxygen-rich atmosphere. Most trace gasesemitted into the atmosphere are removed byoxidation reactions involving ozone and thehydroxyl free radical. Hence, oxidation reactionsare sometimes referred to as nature’s atmospheric‘cleansing’ process. Without an efficient oxidationprocess, levels of many emitted gases could rise to

such high levels, relative to natural compositionlevel, that they would radically change the chemicalnature of our atmosphere and biosphere. Onthe other hand, the industrial revolution in thepast 150 years, together with increased biomassburning associated with human land use, has led tosignificant increases since preindustrial times in theglobal emissions of NOx , SOx , CO and HCs thatvaries from country to country.36 Also, the observedtemperature increase of 0.6 ± 0.2 ◦C in this timeperiod should have led to increases in lower levelatmospheric H2O and possibly changes in globalcloud cover. Therefore, all these human relatedactivities have affected O3 and OH concentrations ofthe natural (preindustrial) state, thereby disturbingoxidation processes that occur in the nature andhence, abrupt changes in our environment haveaugmented.37

Exhaust gas or flue gas is emitted as a result ofthe combustion of fuels such as natural gas, gasoline(petrol), diesel, fuel oil or coal. It is discharged intothe atmosphere through an exhaust pipe, flue gasstack or propelling nozzle. The principal pollutantspresent in auto exhaust emission gases are NOx ,CO and unburned HC. Other gases like H2, N2O,NH3, CO2, and SOx are also present to someextent. In case of diesel engine, exhaust, however,contains particulate matters (PM) composed ofsolid dry carbon or soot and unburned liquidfuel and lubricating oil called the soluble organicfraction (SOF) or volatile organic fraction (VOF).Furthermore, the gaseous exhaust from diesel enginecontains significant amounts of SO2. Over mostoxidation catalysts, SO2 would be converted to SO3

that eventually turned into H2SO4. This unfavorablesecondary emission obviously contributes to thetotal particulates.

Automobile engine exhaust gas composition is acomplex mixture that depends on variety of factorssuch as type of engine, driving condition, vehiclespeed and so on. There are two types of enginessuch as gasoline powered engine and diesel poweredengine. Generally, gasoline engines are referred to asinternal combustion engine (ICE) where initiationof the combustion process of air–fuel mixture isignited within the combustion chamber and theignition is done either by spark ignition (SI) orcompression ignition (CI). Gasoline powered two-stroke and four-stroke engines are such kind of ICEswhere combustion is intermittent. A two-strokeengine completes the thermodynamic cycle in twomovements of the piston, whereas engines based onthe four-stroke have one power stroke for every fourstrokes (up–down–up–down) and employ sparkplug ignition. A four-stroke spark-ignition engine isalso called Otto cycle engine. The two-stroke engine

300 Journal of the Indian Institute of Science VOL 90:2 Apr–Jun 2010 journal.library.iisc.ernet.in

Recent advances in auto exhaust catalysis REVIEW

Table 1: Typical exhaust gas composition of a gasolinepowered spark ignition internal combustion engine.

Gases Vol. (%)N2 71.0CO2 18.0H2O 9.2O2 and noble gases 0.7Pollutants

(a) CO 0.85(b) NOx 0.08(c) Unburned HCs 0.05(d) Particulate matters 0.005Others 0.115

Total 100

was popular throughout most of the 20th century inmotorcycles and some cars. This was due to theirsimple design leading to low cost and higher power-to-weight ratios. Its overall light weight and light-weight spinning parts give important operationaland even safety advantages. Most designs usedtotal-loss lubrication with the oil being burntin the combustion chamber causing visible andother pollution. This is the major reason for two-stroke engines being gradually replaced with four-stroke engines in most applications. In four-strokeengines, combustion occurs rapidly with more orless constant volume. They are used in cars, largerboats, some motorcycles and many light aircrafts.They are generally quieter, more efficient andlarger than their two-stroke counterparts. On theother hand, diesel fueled engines are compression-ignition engine (CIE) where the heat generatedfrom the compression is enough to initiate thecombustion process. Most truck and automotivediesel engines use a cycle reminiscent of a four-stroke cycle, but with a compression heating ignitionsystem, rather than needing a separate ignitionsystem. This variation is called the diesel cycle. Inthe diesel cycle, diesel fuel is injected directly intothe cylinder so that combustion occurs at constantpressure as the piston moves, rather than with thefour-stroke with the piston essentially stationary.However, amount of NOx produced from dieselengine is less due to its cooler combustion naturecompared with gasoline engine. Typical exhaust gascomposition of a gasoline powered spark ignitioninternal combustion engine is given in Table 1.26

The main atmospheric oxides of nitrogeninclude nitric oxide (NO), nitrogen dioxide (NO2)

and nitrous oxide (N2O). Collectively, NO and NO2

are commonly referred to as NOx . For more than acentury, atmospheric levels of nitrogen oxides havebeen increasing steadily. While a significant amountof this increase has been due to human activity,natural sources also make important contributionsto the overall NOx levels. Relative contributions

Table 2: Estimated global sources of NOx .

Sources NOx (109 kg N yr−1) Percentage (%)I. Human sourcesStationary 13 24Mobile 8 15Biomass burning 12 (4–24) 22Agriculture 5 (1–10) 9II. Normal sourcesSoils 8 (4–16) 15Oceans <1 -Lightening 8 (2–20) 15Others ∼ 0.5 -Total 55 (25–100) 100

of global NOx emission from different sources aregiven in Table 2.38 But it varies from country tocountry and year to year. Table 3 shows a typicalamount of NOx emission from various sources inUS for the year 2005.39 The exhaust hydrocarbonsare a mixture of paraffins, olefins and aromaticsranging from C1 to C8. Various aldehydes, alcohols,esters and ketones are also found in exhaust gasesat concentrations upto several hundred parts permillion. Several volatile organic compounds (VOC)are also emitted from a variety of stationary sourcessuch as solvent evaporation, organic chemicalmanufacturing and catalyst regeneration. Flue gasfrom waste incinerators is composed of NOx , CO,HCl, HF and SO2. In general, auto exhaust gaspollutant concentration depends mainly on air tofuel (A/F) ratios. A/F is generally defined as:

A/F = mass of air consumed by the engine

mass of fuel consumed by the engine.

The engine out exhaust gas composition is alsocommonly classified in terms of l and it is definedas:

l = actual engine A/F

stoichiometric engine A/F.

For gasoline engines, the stoichiometric value of A/Fratio occurs at 14.7. At this stoichiometric point,amount of oxidants is equal to that of reducingagents. If the A/F ratio is below this value, then itis called fuel-rich condition and the exhaust gascontains more reducing agents like CO and HC. Onthe other hand, if A/F ratio exceeds this value, thenit is called fuel-lean condition and the exhaust gascontains more oxidizing agents like NO and O2.

Pollutant gases emitted from exhausts haveharmful effects on earth’s atmosphere, ecologicalsystem and also on human health. NOx plays amajor role in the photochemistry of troposphereand stratosphere, where it catalyzes the formationof ground level ozone and ozone layer depletion,respectively. The troposphere is the lowest layer



Table 3: Amount of NOx emission from various sources inUS for the year 2005.

Source sectors Total emission (Ton)On road vehicles 6,491,821Non road equipment 4,162,872Electricity generation 3,783,659Fossil fuel combustion 2,384,297Industrial processes 1,163,635Waste disposal 155,415Fires 94,372Residential wood combustion 38,324Solvent use 6,400Fertilizer and livestock 2,098Miscellaneous 3,644

of the atmosphere, extending from the ground toroughly 10–15 km altitude, whereas the stratosphereis the layer above the troposphere that extendsto 45–50 km altitude from the tropopause whichseparates troposphere and stratosphere. Ozonelayer means that compared to troposphere belowand mesosphere above a higher fraction of ozonemolecules is found in the stratosphere between15–30 km altitude. Since ozone in troposphere isvery less and 90% of all atmospheric ozone is instratosphere, this altitude region of stratosphereis called ozone layer. Ground level ozone ortropospheric ozone is a greenhouse gas, a pollutant,a health hazard and harmful to plants and materialsand therefore, increase in ozone in troposphere is acause of concern. On the other hand, stratosphericozone is very much necessary for life on earth,because, here it filters out photons with shorterwavelengths (less than 320 nm) of ultraviolet (UV)rays (270–400 nm) from the sun that would beharmful to most forms of life in large doses. Thesesame wavelengths are also among those responsiblefor the production of vitamin D in humans. Introposphere or lower atmosphere, when NOx andVOCs react in the presence of sunlight, they formphotochemical smog, a significant form of airpollution, especially in the summer. NOx , COand VOCs are called ozone precursors and itis known that motor vehicle exhaust, industrialemissions and chemical solvents are the majoranthropogenic sources of these chemicals. Whenstagnant air masses linger over urban areas, thepollutants are held in the place for long time.Sunlight interacts with these pollutants that formsairborne particles called particulate matters andground level ozone which is a major component ofsmog. It is established that at lower altitudes, ozoneis produced via photochemical decomposition ofNO2 followed by three body reaction of atomicoxygen with O2 as shown below:

NO2 +hν → NO+O

O+O2 +M → O3 +M

where M is any third body such as N2 or O2 thatremoves the energy of the reaction and stabilizesO3.37,40 The nitric oxide produced in this reactionreacts rapidly with ozone to regenerate NO2 whichis given below:

NO+O3 → NO2 +O2.

In final stage, NO2 is regenerated and therefore,it is available to repeat the process. The abovereactions occur rapidly establishing a steady stateozone concentration. The second and third reactionsare comparatively fast and slower photolysis ofNO2 reaction is the rate determining step andthat is why ozone is not formed appreciably atnight. It is also one of the reasons why ozoneconcentration is high in the lower atmosphereduring summer months, when temperature arehigh and solar radiation is intense. An increase intemperature also accelerates photochemical reactionrates. With higher temperatures, we can expect alarger number of ‘bad ozone’ days. Recent studiesin southern California shows that amount of NOx ,CO, VOCs and PM are less at weekends comparedto weekdays during ozone season from 1995 to 2001and consequently, ozone concentration decreasesat weekends.41 In an investigation on ground levelozone concentration in Kathmandu valley in Nepalproves that the ozone concentration is stronglyinfluenced by meteorological conditions such astemperature, solar radiation and prevailing levelsof NOx and also it varies in different days suchas weekdays, weekends, clear days, cloudy days,general strike days, premonsoon and rainy season.42

However, smoggy days, in general have bad effectson human health. People with lung diseases such asasthma and people who work or exercise outsideare susceptible to adverse effects of smog suchas damage to lung tissue and reduction in lungfunction. NO2 could also be generated from thereaction of NO with peroxy radicals producedfrom the reaction of hydroxyl radical (OH) withhydrocarbons, CO and VOCs. From scientificview point, air pollutant ozone in troposphereshould not be confused with the ozone layer inthe stratosphere. In the stratosphere, ozone formsthrough the splitting of O2 molecules by sunlight asit does in the troposphere near the earth’s surface.However in the troposphere, nitrogen dioxide, notmolecular oxygen, provides the primary source ofthe oxygen atoms required for ozone formation.Here, sunlight splits nitrogen dioxide into nitricoxide and an oxygen atom. In stratosphere, ozone isformed and consumed naturally by photochemicalreactions involving UV radiation. Here, molecularoxygen interacts with UV radiation and splits into



oxygen atoms which either recombine to form O2

or combine with O2 in presence of a third body toform O3 as written below:

O2 +hν → O+O

O+O2 +M → O3 +M

where M is any third body such as N2 or O2 thatremoves the energy of the reaction and stabilizes O3.Most of the ozone in the atmosphere is formed overthe equator where amount of sunshine is highestthroughout the year. Large scale air circulationpatterns in the lower stratosphere move ozonetowards the north and south poles, where itsconcentration builds up. Hence, highest amountsof ozone in the upper atmosphere (stratosphere)usually occur at high latitudes. Once formed,destruction of ozone in the stratosphere takes placeas quickly as formation of ozone, because it is soreactive. Sunlight can readily split ozone into anoxygen molecule and an individual oxygen atomfollowed by the formation of two O2 molecules asgiven below:

O3 +hν → O2 +O

O+O3 → 2O2.

Thus, the ozone formation–destruction processin the stratosphere occurs rapidly and constantly,maintaining an ozone layer. The first of thesetwo reactions serves to regenerate atomic oxygenfor the second reaction which converts the ozoneback to molecular oxygen. This second reactionis very slow. It can be enormously accelerated,however, by catalytic reactions. In the absenceof such catalytic reactions, ozone could survivefor 1–10 years in the stratosphere region. Laterreaction could be catalyzed by trace gases such asnitric oxide (NO), nitrogen dioxide (NO2) andnitrous oxide (N2O) or simple molecules or freeradicals of the halogens, including fluorine, chlorineand bromine. At any time, the formation andconsumption of ozone proceed simultaneously. Theactual concentration of ozone is a net function ofthe rates of reactions by which it is formed andthe rates of the reactions that consume this gas.All the above molecules or free radicals catalyzethe O3 dissociation reaction rate leading to ozonelayer depletion in stratosphere.43 It has beendiscovered that man made chlorofluorocarbons(CFCs) producing chlorine free radical in presenceof UV radiation are primarily responsible forthe rapid catalytic destruction of ozone in thestratosphere.37,44−46 CFCs also known as freonshave been used in refrigeration, coolants and

aerosols since 1930s and have a lifetime of about 20–100 years and could therefore continue to destroyozone for a long period of time, so it pose a majorproblem. However, NOx which is known as asignificant auto exhaust pollutant also catalyzesozone layer depletion in the stratospheric regionthrough the reactions as given below:

NO+O3 → NO2 +O2

NO2 +O → NO+O2.

These reactions are largely responsible for ozonedecline in middle to high altitudes from spring tofall. NOx eventually form nitric acid when dissolvedin atmospheric moisture forming a componentof acid rain particularly in urban and industrialareas. The following chemical reaction occurs whennitrogen dioxide reacts with water:

2NO2 +H2O → HNO2 +HNO3.

Nitrous acid then decomposes as follows:

3HNO2 → HNO3 +2NO+H2O

where nitric oxide will oxidize to form nitrogendioxide that again reacts with water, ultimatelyforming nitric acid as follows:

4NO+3O2 +2H2O → 4HNO3.

NOx react with ammonia, moisture and othercompounds to form nitric acid vapor and relatedparticles. Small particles can penetrate deeplyinto sensitive lung tissue and damage it, causingpremature death in extreme cases. Inhalation ofsuch particles for long time sometimes causesrespiratory diseases such as emphysema, bronchitisand it may also aggravate existing heart disease.Peroxyacetyl nitrates (PAN) could also be generatedfrom NO that contribute significantly to globalphoto oxidation pollution.14 Biologically active NOis a poisonous product of in-vivo enzyme catalyzedtransformation of amino acid, arginine. NO difusesthrough the alveolar cells and capillary vessels ofthe lungs and damages the alveolar structures andtheir functions throughout the lungs provokingboth lung infections and respiratory allergies likebronchitis and pneumonia.

CO is a colorless, odorless gas and generallyknown for its extreme toxicity. It combines withhemoglobin to produce carboxyhemoglobin that isineffective for delivering oxygen to tissues present inour body. Concentration as low as 667 ppm mightcause upto 50% of the body’s hemoglobin to convertto carboxyhemoglobin. In the US, National Institute



for Occupational Safety and Health (NIOSH) hasestablished a recommended exposure limit (REL)for CO of 35 ppm as an 8 h time-weighted average(TWA) concentration. Situation could be dangerousbeyond this exposure level.

Sulfur present in fuels gets oxidized to SO2

and further oxidation of SO2 followed by thereaction with H2O eventually forms H2SO4 thatalso produces acid rain which is written as follows:

SO2 +H2O → H2SO3

SO2 +O3 → SO3 +O2

SO3 +H2O → H2SO4.

This acid generated from acid rain may be carriedto the ground in rain or snow, but often particlescontaining sulfuric acid settle out of dry air. Sothe problem of the acid rain is really one of aciddeposition in dry weather as well as wet. Acid rainhas been held responsible for damaging buildingsand statues made of limestone, damaging aquaticlife in lakes, causing a decline of the plant growth inUS and European forests and it might harm humanhealth.

Particulates, alternatively known as particulatematters (PM) or fine particles, are tiny subdivisionsof solid or liquid matter suspended in a gas or liquid.Size ranges of particulate matters are from less than10 nm to more than 10 μm. PM10 is used to describeparticles of 10 μm or less and PM2.5 representsparticles of 2.5 μm or less. Particles emitted frommodern diesel engines that is commonly referredto as diesel particulate matters (DPM) or soot aretypically in the size range of 100 nm. Larger particlesare generally filtered in the nose and throat anddo not cause problems, but particulate matterssmaller than about 10 μm can settle in the bronchiand lungs leading to serious health problems. Thesmallest particles, less than 100 nm, may be evenmore damaging to the cardiovascular system. Thereis evidence that particles smaller than 100 nm canpass through cell membranes and migrate into otherorgans, including the brain. In addition, these sootparticles also carry carcinogenic components likebenzopyrenes adsorbed on their surface.

3. Global concern and legislationsIn the early 1950’s, air pollution and cars werefirst correlated by California researchers whoestablished that huge traffics were to blame forthe smoggy skies over Los Angeles area.47–51 Sincethen, international concerns over the aspects ofenvironmental pollution caused by polluting sourceshave been increasing and therefore, several globalleaders mainly from developed countries have

been meeting at different timeframes to executethe regulations for controlling pollutants thoseare big threats for earth’s air and ecology andhealth of living beings. Because pollutants canbe carried many hundreds of kilometres by wind,thereby pollutants emitted in one country maybe deposited in other countries.52 Depositionof pollutants in a country can far exceed theamount of such pollution produced domesticallydue to pollution arriving from one or moreupwind countries. In 1976, the Nordic environmentministers proposed a European convention ontransboundary air pollution that emphasizedsulphur compounds which is called Convention onLong-range Transboundary Air Pollution (CLRTAP).After negotiations, 34 countries and the EuropeanCommission signed the Geneva Convention in 1979.The convention came into force in 1983 and has nowbeen ratified by 47 European countries along withUS and Canada. In 1999, Gothenburg Protocol issigned in Gothenburg, Sweden to support CLRTAPsetting emission limits for NOx , SOx , VOC andNH3 to be met by 2010. In the similar way, KyotoProtocol (1997) and Copenhagen Summit (2009)under United Nations Framework Convention onClimate Change (UNFCCC) have been effectiveto combat environment pollution as well as globalwarming. Implementations and status reports ofthese UNFCCC projects have been published timeto time by Intergovernmental Panel on ClimateChange (IPCC) that is a scientific body tasked withevaluating the risk of climate change caused byhuman activity. IPCC was established in 1988 bythe World Meteorological Organization (WMO)and the United Nations Environment Programme(UNEP). IPCC got Nobel Prize for peace in 2007.

Keeping reduction of pollutant gases intoaccount, several developed and later, developingcountries have adopted legislation for the control ofexhaust emissions to reduce air pollution. Each ofthese countries has set particular emission standardsby law for different kinds of vehicles according totheir own country’s situation. Emission standards setspecific limits to the amount of pollutants that couldbe released into the environment. It is a limit thatsets threshold amounts above which different typesof emission control technology must be necessary.Standards generally regulate the emissions of NOx ,SOx , PM or soot, CO and HCs.

In US, emission standards are managed ona national level by Environmental ProtectionAgency (EPA), whereas state governments andlocal governmental bodies play a subsidiary role.California state has passed its own more stringentvehicle emission standards set by California AirResources Board (CARB). Automotive market in



California is one of the largest in the world andparticularly Los Angeles metropolitan areas havesevere automobile-driven air pollution problemsand CARB imposes strict regulations over emissionrequirements that major automakers must meet ifthey want to sell their cars into California market.Therefore, it is regarded as one of the stricteststandards in the world and other states in USalso choose to follow either federal standard orthe stricter California standard. States followingCalifornia standards include Maine, Vermont,Massachusetts, Rhode Island, Connecticut, NewYork, New Jersy, Pennsylvania, Maryland, Oregon,Washington, Arizona, Washington DC and NewMexico and these states are frequently referred toas CARB states. Regulations of CARB have alsoinfluenced European Union emission standards. InUS, clean air legislation was first enacted in 1955,followed by Air Quality Act 1967 and Clean AirAct Amendments 1970, 1977 and 1990. Severalcategories of new engines as well as vehicles suchas car and light trucks, heavy duty truck and busengines, mobile non road diesel engines, railwaylocomotives, marine engines, small spark ignited(SSI) engines, large spark ignited (LSI) engines andstationary diesel engines are subject to emissionstandards in US. Two sets of emission standards(Tier I and Tier II) for light duty vehicles in US weredefined in Clean Air Act Amendments 1990.53 TierI standard was adopted in 1991 and was effectivefrom 1994 until 2003, whereas Tier II standards arebeing implemented from 2004 to 2009. There isa subranking ranging from BIN 1-10 within TierII. BIN 1 is defined as cleanest emission standardand the vehicle having BIN 1 is called zero emissionvehicle (ZEV), whereas BIN 10 is regarded as dirtiestemission standard. Automotive emission standardsfor low emission vehicle program in California aremore stringent than national Tier regulations. Themajor emission categories for low emission vehicleprogram include transitional low emission vehicle(TLEV), low emission vehicle (LEV), ultra lowemission vehicle (ULEV), super ultra low emissionvehicle (SULEV) and ZEV. New categories for 2010to 2016 are inherently low emission vehicle (ILEV),partial zero emission vehicle (PZEV), advancedtechnology partial zero emission vehicle (ATPZEV)and national low emission vehicle (NLEV). Thesestandards specifically restrict emissions of CO, SOx ,PM, HCHO, non methane hydrocarbons (NMHC)and non methane organic gases (NMOG). ZEV isstrictly restricted to electric vehicles and hydrogencars. Tier II regulations also defined restrictionsfor the amount of sulfur allowed in gasoline anddiesel fuel, since sulfur can interfere with operationof advanced exhaust treatment systems such as

selective catalytic converters and particulate filters.Future emission regulations will be more stringentimposed by CARB moving towards a ZEV. Heavyduty vehicles must be regulated with Tier III andTier IV within 2014.

European Union has its own policies foremission standards that all new vehicles must meet.Emission standards are applicable for all roadvehicles, trains, barges and mobile non road enginessuch as tractors. No standards are set for seagoingships or aeroplanes. European emission standardsdefine the acceptable limits for exhaust emissionsof new vehicles sold in European Union memberstates. Currently, emissions of NOx , NMHC, PM,CO and total hydrocarbon (THC) are regulatedfor most vehicles. The toxic emission standardsare defined as Euro 1, Euro 2, Euro 3, Euro 4 andEuro 5 fuels for light duty vehicles, whereas thecorresponding series of standards for heavy dutyvehicles are Euro I, Euro II, Euro III, Euro IV andEuro V. The European Union is to introduce Euro 4on 1st January 2008, Euro 5 and Euro 6 standards areeffective on 1st January 2010 and 1st January 2014.But these dates have been postponed for two yearsto give oil refineries the opportunities to modernizetheir plants. A typical European Union emissionstandards is given in Table 4.54

Australia started certification for new motorvehicle emissions with Euro categories in early 2000s.Euro III came into effect on 1st January 2006 andfollowing Euro standards are being introduced withEuropean introduction dates.

In 1992, Japan adopted Motor Vehicle NOx Lawto curb NOx . The regulation was imposed for Tokyo,Saitama, Kanagawa, Osaka and Hyogo as these areaswere affected with significant air pollution due toNOx emitted from motor vehicles. This regulationwas amended in 2001 to add PM into the regulationalong with stricter NOx removal requirements.

In China, emission standards are governed byState Environmental Protection Administration(SEPA).55 China started to regulate its first emissioncontrols on automobiles in 1990s that was similarto Euro I and it was modified in 2004 to Euro IIstandard. Similarly, more strict National StandardIII, equivalent to Euro III was effective in 2007.Beijing introduced Euro IV in 2008, whereas the restof the country would adopt this in 2010. Legislationwas adopted for all new passenger cars with SIengines in Hong Kong to meet Euro IV standard in2006. To meet increasingly strict emission standards,manufacturers are developing low pollution vehiclesusing alternative power, such as compressed naturalgas vehicles (CNGV), liquefied petroleum gasvehicles (LPGV), hybrid electric vehicles (HEV),fuel cell vehicles (FCV) and electric vehicles (EV).



Table 4: European Union emission standards for gasoline and diesel powered (within bracket) passenger cars (g km−1).

Tier Timeframe CO THC NMHC NOx HC + NOx PM

Euro 1 July 1992 2.72(2.72)

−(−)

−(−)

−(−)

0.97(0.97)

−(0.14)

Euro 2 January 1996 2.2(1.0)

−(−)

−(−)

−(−)

0.5(0.7)

−(0.08)

Euro 3 January 2000 2.3(0.64)

0.20(−)

−(−)

0.15(0.50)

−(0.56)

−(0.05)

Euro 4 January 2005 1.0(0.50)

0.10(−)

−(−)

0.08(0.25)

−(0.30)

−(0.025)

Euro 5 September 2009 1.000(0.500)

0.100(−)

0.068(−)

0.060(0.180)

−(0.230)

0.005(0.005)

Euro 6 September 2014 1.000(0.500)

0.100(−)

0.068(−)

0.060(0.080)

−(0.170)

0.005(0.005)

In China, CNGVs and LPGVs are currently themost practical and popular clean vehicles. In thefuture, the government plans to develop EVs drivenby storage batteries for buses and other publictransportation vehicles, to encourage the productionof HEVs for buses, to promote the innovation ofFCVs as a longterm strategy, to strive for commercialuse of FCVs by 2010 and to establish a methanolproduction and supply system to facilitate theintroduction of FCVs.

India passed regulations for controllingemissions in 1991 and since 2000 it has startedto adopt European emission standard for fourwheeler light duty and heavy duty vehicles. Indianown emission standards will apply to two andthree wheeled vehicles. National Auto Fuel Policyannounced in 2003 envisaged a phased programfor introducing Bharat Stage II to IV standardswhich are similar to Euro 2 to Euro 4 emissionstandards by 2010. These standards are applicable toall four wheelers sold and registered in the areas likeNational Capital Region (Delhi, Gurgaon, Faridabad,Noida and Ghaziabad), Mumbai, Kolkata, Chennai,Bangalore, Hyderabad, Ahmedabad, Pune, Surat,Kanpur and Agra. Bharat Stage III (Euro 3) standardfor 2 and 3 wheelers would come into effect between2008 and 2010. A typical Indian emission standardsis summarized in Table 5.56

4. Technologies developed for controllingauto exhaust emission

4.1. Processes for controlling pollution fromauto exhaust

As the concern about global and regionalenvironmental problems continues to grow,enormous efforts have been made in last severalyears to advance the scientific and technological

development for controlling auto exhaust emission.Some of invented technologies are briefly discussedbelow:

(a) Improvement in the engine design —Engine efficiency could be steadily improved withmodified engine design, more precise ignitiontiming, electronic ignition and computerized enginemanagement. Improvements in engine and vehicletechnology continually reduce the toxicity of exhaustleaving the engine, but these alone have generallybeen proved insufficient to meet emission standards.

(b) Air injection — One of the first exhaustemission control systems is secondary air injectioninto each of the exhaust ports of an engine. Thefresh air entering the hot exhaust manifold providesoxygen to burn any remaining fuel before it can exitthe tailpipe. It was popular in the early 1990s beforethe extensive use of catalytic converters and now itis employed to reduce start up emissions.

(c) Fuel injection — The primary emissioncontrol is fuel injection. Fuel injection offers precisefuel control over a wide range of conditions.Feedback supplied by exhaust gas oxygen sensor andvarious engine sensors allows the injection system tocompensate for changes in atmospheric conditions(temperature, altitude) as well as mechanicalconditions (such as state of tune).

(d) Exhaust gas recirculation — Many enginesproduced after the 1973 model year have an exhaustgas recirculation (EGR) valve between the exhaustand intake manifolds. The valve opens under certainconditions to admit exhaust into the intake tract.Exhaust is largely inert, it neither burns nor supportscombustion, so it dilutes the incoming A/F mixtureto reduce peak combustion chamber temperatures.This, in turn, reduces the formation of NOx . TheEGR valve is the main control device in this system.



Table 5: Indian emission standards for four-wheeler vehicles.

Standard Reference Timeframe RegionIndia 2000 Euro 1 2000 Nationwide

Bharat Stage II Euro 2 2001 NCR*, Mumbai, Kolkata and Chennai

April 2003 NCR, Mumbai, Kolkata, Chennai, Bangalore, Hyderabad,Ahmedabad, Pune, Surat, Kanpur and Agra

April 2005 Nationwide

Bharat Stage III Euro 3 April 2005 NCR*, Mumbai, Kolkata, Chennai, Bangalore,Hyderabad, Ahmedabad, Pune, Surat, Kanpur and Agra

April 2010 Nationawide

Bharat Stage IV Euro 4 April 2010 NCR*, Mumbai, Kolkata, Chennai, Bangalore,Hyderabad, Ahmedabad, Pune, Surat, Kanpur and Agra

*NCR stands for National Capital Region (Delhi, Gurgaon, Faridabad, Noida, and Ghaziabad).

(e) Catalytic converters — The catalyticconverter is a device placed in the exhaust pipethat converts NOx , CO and unburned HCs intobenign gases by using different kind of catalysts.

(f) Evaporative emission control — Evaporativeemissions are the result of raw fuel vapors escapingfrom the vehicle’s fuel system. The emissionsgenerated from the evaporation of raw fuel suchas gasoline are controlled by the use of a closedventilation system for the fuel tank or carburetorbowl vent on carbureted vehicles and a canister filledwith activated charcoal to capture the fuel vapors.The charcoal canister captures and stores the vaporsas they expand. When the engine is started, a purgecontrol valve is opened allowing the engine to drawfresh air through the charcoal canister, pulling thevapor into engine, where it is burned. Since 1970,all US vehicles have had fully sealed fuel systemsthat do not vent directly to the atmosphere.

(g) Improvement in road condition andtraffic network — Driving style can have animportant influence on exhaust emission andfuel consumption. Good roads and changes intraffic network configurations improve overallflow of vehicles. This could lead to the reductionin emissions per vehicle kilometer traveledby decreasing the number of acceleration anddeceleration events. Therefore, traffic signals ortraffic jams due to congested roads should bereplaced by roundabouts or flyovers. It is alsoobserved that NOx emission increases and CO, PMand HC emissions decreases with increase in enginetemperature or increase in vehicle speed. Hence,there must be speed limit on the road, especiallyon the highway where numbers of vehicles aremore. Some researchers argue that smooth flow ofvehicles might generate extra traffic in that sectioncausing more pollution and therefore, they claimthat pollution impacts of traffic signals are highlysituation specific.

4.2. Catalytic converterAmong all the types of technologies developed sofar, use of catalytic converters is the best way tocontrol auto exhaust emission. A catalytic converter(colloquially called catcon) is a device that usuallyreduces the toxicity of exhaust emissions generallyfrom gasoline engines. A catalyst housed insidea catalytic converter performs chemical reactionsby which pollutant combustion by-products areconverted to less-pollutant substances. It wasfirst widely introduced in automobiles in the USmarket in 1975 to comply with tightening EPAregulations on auto exhaust and catalytic convertersare still most commonly used in motor vehicleexhaust systems for controlling emission. Catalyticconverters are also used on trucks, buses, trains,mining equipments, generators and other engine-equipped machines. The catalytic converter wasinvented by Eugene Houdry, a French mechanicalengineer who lived in the US. When the results ofearly studies of smog in Los Angeles were publishedin early 1950s,47–51 Houdry became concernedabout the role of automobile exhaust in air pollutionand established a company, Oxy-Catalyst, to developcatalytic converters for gasoline engines and heobtained a patent (US2742437) for his inventionof catalytic converter. But it was not that muchuseful until the extremely effective anti-knockagent tetra-ethyl lead was eliminated from mostgasoline over environmental concerns that wouldpoison the converter by forming a coating onthe catalyst’s surface, thereby effectively disablingit. The catalytic converter was later on furtherdeveloped by John J. Mooney and Carl D. Keithat the Engelhard Corporation, starting the firstproduction of catalytic converter in 1973.

Since then catalytic converters have undergoneseveral stages of development over the years. Therehave been two types of catalytic converters:



(a) Two-way catalytic converters are widely usedon diesel engines to reduce hydrocarbon and carbonmonoxide emissions. They were also used on sparkignited gasoline engines in US automobile marketthrough 1981. But this two-way converter is unableto control NOx and three-way catalytic converterscome into the scenario. It has two simultaneoustasks such as (i) oxidation of CO to CO2 and (ii)oxidation of unburned hydrocarbons to CO2 andH2O.

(b) Three-way catalytic converters have beenused in vehicle emission control systems in NorthAmerica and many other countries on roadgoingvehicles since 1981. A three-way catalytic converterhas three simultaneous tasks, namely (i) oxidationof CO to CO2 and (ii) oxidation of unburnedhydrocarbons to CO2 and H2O and (iii) reductionof nitrogen oxides to nitrogen and oxygen.

The essential requirement for the catalystemployed in the present-days catalytic converterfor gasoline engine is to cater the high conversionof NOx , CO and HCs present in the auto exhaustgases to inert and harmless N2, CO2 and H2O. Theoverall catalytic reactions which are important forcontrolling auto exhaust emissions are given by thefollowing stoichiometric equations:

a) reduction reactions

2NO+2CO → N2 +2CO2

2NO+2H2 → N2 +2H2O(2x+ y

2

)NO+CxHy →

(x+ y

4

)N2

+xCO2 + y

2H2O

b) oxidation reactions

2CO+O2 → 2CO2

2H2 +O2 → 2H2O

CxHy +(

x+ y

4

)O2 → xCO2 + y

2H2O

c) water gas shift reaction (WGSR)

CO+H2O → CO2 +H2

d) steam reforming reaction

CxHy +xH2O → xCO+(

x+ y

2

)H2.

However, depending on the operating conditions ofthe catalyst a number of secondary reactions mightoccur. The most common of these are:

a) reactions with SO2

2SO2 +O2 → 2SO3

SO2 +3H2 → H2S+2H2O

b) reactions with NO

2NO+5H2 → 2NH3 +2H2O

6NO+4NH3 → 5N2 +6H2O

2NO+CO → N2O+CO2

2NO+O2 → 2NO2.

Moreover, removal of NO from auto exhaustemission could also be achieved by selective catalyticreduction (SCR) of NO to N2 using NH3. Otherreductants such as H2, CO and hydrocarbons arealso used for SCR of NO. The main reactionsinvolved in SCR are:

4NO+4NH3 +O2 → 4N2 +6H2O

6NO2 +8NH3 → 7N2 +12H2O.

All the above reactions required some heator temperature on the catalyst surface for thereaction to occur. When automobile first starts,both the engine and catalyst are cold. After startup, heat of combustion is transferred from theengine and the exhaust piping begins to heat up.Finally, a temperature is reached within the catalystthat initiates the catalytic reactions. This light-off temperature depends on the chemistry of thecatalyst. Typically CO reaction begins first followedby HC and NOx reactions.

The sale of diesel vehicles is increasingdramatically due to its cost effectiveness anddecrease in emission of green house gases, especiallyCO2. In traditional stoichiometric gasoline enginesthe combustion mixture always contains sufficientoxygen to combine with the fuel. On the otherhand, in a diesel engine, oxygen is always in excess,since only sufficient diesel fuel is injected into ahighly compressed hot air in the cylinder to producethe power at a particular instant. Thus, unlikegasoline engine, diesel engine is a compression-ignition process. Its lean nature results in a coolercombustion with less NOx formation. Here role ofcatalyst is to reduce soluble organic fraction (SOF)and gaseous CO and HC emission, but specificallyselective to minimize the oxidation of SO2 to SO3

leading to H2SO4. Thus, the challenges for catalystsare quite demanding and different from that forits gasoline counterpart, because catalysts withhigh activity for oxidizing SOF would likely begood catalysts for SO2 to SO3. For diesel engines,



the most commonly used catalyst in the catalyticconverter is the diesel oxidation catalyst. This usesexcess O2 in the exhaust gas stream to oxidize SOFto CO2 and H2O, CO to CO2 and hydrocarbonsto H2O and CO2. In the early 1990s the dieselengine manufacturers, being aware of the successesof the catalytic converter for the gasoline engine,considered a catalyzed flow-through monolith forcontrolling emissions from their engines. Dieselengines run in lean conditions and thereforeare much cooler than modern stoichiometricallyoperated gasoline engines and thus, even if SOFfrom diesel engine are gaseous or aerosols theyare quickly cooled and condensed in this idleor low power output conditions. The catalystwould therefore have to function at much lowertemperatures than in the gasoline counterpartand would be required to treat liquids as well asgases. The catalyst also must have high selectivitytowards the oxidation of gaseous CO and HC whilenot oxidizing SO2. These converters often reach90% efficiency, virtually eliminating diesel odorand helping to reduce visible particulates (soot).However they are incapable of reducing NOx inlean-burn condition as chemical reactions alwaysoccur in the simplest possible way and the existingO2 in the exhaust gas stream would react first.To reduce NOx on a diesel engine, the chemicalcomposition of the exhaust must first be changed.Two main techniques such as SCR of NO and NOx

traps or NOx absorbers are used. Diesel engineexhaust contains relatively high levels of particulatematter (PM) or soot, consisting in large part ofelemental carbon. Catalytic converters cannot cleanup elemental carbon, so particulates are cleanedup by a soot trap or diesel particulate filter (DPF).In US, all on road heavy duty vehicles poweredby diesel and built after 1st January 2007 must beequipped with a catalytic converter and a DPF. Themost satisfactory way of removing trapped PM is tooxidize it to CO2 and H2O. The PM can be almostcompletely removed from the exhaust stream usinga DPF. This trapped PM can then be combustedusing either NO2 (at low temperatures, 250–300 ◦Cand above), or O2 (at high temperatures, 550–600◦C). On heavy duty diesel vehicles, such as trucksand buses, the engine is often working at high loadand the exhaust temperature is in the range 250–400◦C. Under these conditions it is possible to use thealready present NO in the exhaust gas in a processthat continually oxidizes trapped PM. An oxidationcatalyst upstream of the filter oxidizes HCs and COto CO2 and H2O and also converts NO to NO2 thatis very powerful oxidant and continually removesPM. Details of modern-days catalytic converters andits scientific and technological aspects have beenreviewed in several articles.22,30–32,57–61

4.3. Controlling pollution from cold-startemission

It has been established that 60–80% of unburnedHCs and CO are emitted from a motor vehicleequipped with a three way catalyst (TWC) withinfirst few minutes or approximately first fewkilometers after vehicle starts.62–64 This periodis called cold-start condition and the emissionoccurred during this period is called cold-startemission. Under cold-start conditions, the enginehas to overcome larger frictional forces in its movingparts that requires to introduce a substantially largeramount of fuel than the stoichiometric proportionin order to guarantee prompt vaporization andstarting and the TWC is totally inactive, because thecatalytic converter has not yet warmed up. Together,these two important factors are responsible for thehigher tailpipe emission during cold-start phase.The extent of this cold-start phase is also dependenton the outside temperature and characteristics of thevehicle. TWCs in catalytic converters used in motorvehicles presently are able to achieve the reductionsof CO, NOx and unburned HCs by up to 95% whenthey are full warmed up. TWC will not be able tofunction effectively until it reaches the light-off

(operating) temperature as the conversion efficiencydepends strongly on the working temperature and ispractically zero during the starting and warming upperiod. The process of catalyst warm up is typicallyreferred to as light-off. Some researchers refer to thecatalyst warm up time from almost zero efficiencyat cold-start to 50% conversion efficiency as thelight-off time. Typically it is necessary to reach350 ◦C for light-off to occur. The delay until itreaches operating temperature leads to high levelsof tailpipe emissions especially hydrocarbons. Manydistances driven are so short that the end of thecold-start phase is either not reached or just aboutreached. Short journeys, mostly consisting of hometo work place, home to various service units inthe city such as post office and bank, home toschool, home to local shops, home to doctor’schamber have large average fuel consumption andemission values (g km−1) due, essentially to cold-start. Average trip length in Europe is only about5 km and about 50% of city driving is 2 km oreven less. On average the cold distance defined asthe distance necessary for cold tailpipe emissionsto reach hot emission levels is typically within 5km. That indicates that a large amount of urbandriving occur in cold condition. This scenario iseven worse at low ambient temperature.65 Cold-startemission of pollutant gases from vehicles is thus oneof the most severe problems in large cities, wherethe number of vehicles and daily engine startingsper populated area is high. Hence, minimizing



emission of pollutants mainly, HCs in the cold-startperiod is very critical for getting very low tailpipeemission. Therefore, in recent years, there are effortsto reduce tailpipe emission during cold-start inorder to achieve low emission vehicles. As a result,both US and European emission standards includecold-start operation tests using US 1975 FederalTest Procedure (FTP 75), Economic Commissionfor Europe (ECE) and New European Driving Cycle(NEDC) at ambient temperatures of 20–30 ◦C (68–86 ◦F), before the catalytic converter is warmedup. Especially, the Euro III and Euro IV emissionstandards have included a subambient cold-starttest at a temperature of −7 ◦C, during the first 780s of the urban driving cycle, limiting HC and COtailpipe emissions in such conditions.

The technology to develop suitable methods tocontrol cold-start emission having both catalytic aswell as some unique techniques includes (a) close-coupled catalyst, (b) electrically heated catalyst, (c)hydrocarbon trap, (d) chemically heated catalyst, (e)employing partial oxidation catalyst, (f) exhaust gasignition, (g) pre-heat burners, (h) cold-start sparkretard or post manifold combustion, (i) variablevalve combustion chamber (j) double walled exhaustpipe and (k) electronic ignition system.66–72 Allthese approaches contain underfloor catalysts ofvarious compositions.73 It has been established thatdevelopment of high temperature close-coupledcatalyst is the leading technology for most LEV,ULEV and SULEV applications. Close-coupledcatalyst near the engine manifold has been usedfor removal of HCs, whereas the underfloorcatalyst removes remaining CO and NOx . Anotherprominent approach is the use of hydrocarbon trapin which the cold HCs are adsorbed and retainedon an adsorbent until the catalyst reaches the light-off temperature. Cold-start phase could also beovercome by using electrically heated catalyst (EHC).During start up period exhaust gas or catalystsurface could be heated by resistive materials and acurrent/voltage source that could reduce the timerequired for catalyst light-off. EHC is placed in frontof light-off catalyst that receives the pre-heated gasand thus, provides very efficient reaction in thecold-start period.

5. Catalysts for controlling auto exhaustpollutants

5.1. Present conventional catalysts for theabatement of auto exhaust emission

Generally, metal, alloy and metal oxide catalystswere used for exhaust emission catalytic reactions inearly days. Basic studies on metal catalysts involvesingle crystal or polycrystalline forms of bulk metalsuch as powder, wire, foil or ribbons. Prominent

metals used are Pt, Pd, Rh, Ru and Ir. CuO, Fe2O3,TiO2, V2O5, MoO3, WO3, Cr2O3 and Al2O3 areemployed as oxide catalysts.74

Transition metal ion exchanged zeolites havealso been developed for SCR of NO. Copper is oneof the promising elements which has the ability toincrease the SCR activity of NO with NH3. Cu2+ion exchanged Y zeolite, mordenite, ZSM-5 andMFI-ferrisilicate have been found to be effectiveSCR catalysts for NO. Other metal ions such as Fe3+,Co2+, Ce4+ and Ag+ exchanged zeolites also showgood SCR activity for NO. Noble metal exchangedzeolites are active for the removal of NO.74

The catalytic reduction of NO, oxidation ofCO and hydrocarbons have also been carried outover pure and cation substituted perovskite suchas LaCoO3, LaMnO3, Ln1−xPbxMnO3 (Ln = La,Pr and Nd), LaMn1−xCuxO3 and La1−xSrxMnO3.They have different cationic sites in the crystalstructure which are highly amenable to substitution.Usually metals of high oxidation states can bestabilized in perovskite structure. A large number ofspinels, AB2O4 (A = Mg2+, Co2+, Ni2+, Cu2+ andZn2+ and B = Al3+, Cr3+, Mn3+, Fe3+ and Co3+)

are also employed for NO reduction and CO andhydrocarbon oxidation.74

In early 1980s, supported metal catalysts havecome in the scenario of auto exhaust catalysis.75–79

Metal catalysts with low metal loading are usuallydispersed on the surface of a support. Highlydispersed catalysts provide a maximum surfacearea. As metal loading increases the crystallites comecloser together causing larger particle size. Generally,the more expensive precious metal catalysts havelow metal loading and are highly dispersed, whereascatalysts containing less expensive base metals havehigher metal loadings, usually 20–40% by weightor atom. In particular, present-days automotiveexhaust catalysts revolves around three noble metalssuch as Pt, Pd and Rh that have been dispersed,stabilized, promoted, alloyed and segregated insophisticated ways over the years to fulfill therequired performance and γ-Al2O3 has been foundto be the main support.24 Current auto exhaustcatalyst consists of noble metal particles, mainlyPd to the extent of 2% by weight along with Ptand Rh possible to the limit of 0.5% each dispersedover high surface area γ-Al2O3 mixed with 30% byweight of Ce0.7Zr0.3O2 as oxygen storage material.24

5.2. Importance of oxygen storage in autoexhaust catalysis and role of CeO2

Since early 1980s catalytic converter with threeway catalyst (TWC) has been regarded as a majorbreakthrough in the development of device forthe controlling of auto exhaust emission. TWCs



are capable of converting CO, HC and NOx intobenign CO2, H2O and N2 simultaneously andefficiently, provided that A/F ratio is constantly keptin the exhaust at the stoichiometric point (14.7)where concentrations of oxidizing and reducinggases are equal. High efficiency of conversionis achieved over TWC in a very narrow A/Fwindow that requires a highly efficient controlof exhaust composition, particularly in terms ofresidual oxygen concentration. Strong deviationsor oscillations of A/F from the stoichiometric valuelead to widening of operating window resultingin an average poor performance of TWC. Theengines with earliest catalytic emission controlsystems were fuelled through carburetor or fuelinjection device, but precise control of the amountof fuel that mixed with intake air could not bedone by this system. Therefore, for widespreadapplication of TWC, controlling of A/F ratio hasbeen required further development on the basisof engineering as well as chemical control. Presentintegrated TWC system contains (a) an oxygensensor in the exhaust to provide an electrical signalindicating the engine is running rich or lean, (b)electronic fuel injection (EFI) that could meterprecise amounts of fuel providing a stoichiometricA/F mixture and (c) a microprocessor to controla feedback-loop using oxygen sensor signals todetermine the amount of fuel injected underspecific engine operating conditions to maintainstoichiometric conditions. In recent years, startingin California, vehicles are also required to have on-board diagnostic (OBD) systems for monitoring theperformance of a number of emission related vehiclecomponents and systems that contain differentsensors for exhaust gas constituents (CO, HCsand NOx).80 However, the exhaust gases fluctuatebetween slightly rich to slightly lean conditions, withan inevitable time lag in the system as it adjusts theratio electronically. This can decrease the conversionrate, especially when the difference between thedegree of enrichment is large and the alternatingperiod becomes long and hence, the range of A/Fratios should be widened to take dynamic drivinginto account. In this context, chemical control ofA/F ratio comes into the picture. To widen theoperating window for the catalyst, the exhaust gascomposition on the catalyst must be controlled bystoring oxygen in the oxygen-rich condition andby releasing oxygen in the oxygen-lean condition.This could be achieved by adding oxygen storagematerial in TWC which shows high oxygen storagecapacity (OSC). This oxygen storage material readilyundergoes reduction–oxidation cycles that provideoxygen for CO and HC oxidation in rich region andthe reduced state can remove oxygen from gas phase

when the exhaust gas runs into lean region. Thus,an oxygen storage material not only widens A/Fratio window, but also promotes oxidation activity.

OSC is the ability to attenuate the negativeeffects of rich and lean fluctuation of exhaust gascomposition upon pollutant conversion. It is acrucial property of TWC directly linked to itsefficiency operating under fluctuating conditions.Generally, OSC is classified as total OSC anddynamic OSC.81–83 Total OSC refers to the amountof oxygen thermodynamically available that has beeninvestigated employing temperature programmedreduction (TPR), temperature programmedoxidation (TPO) and thermogravimetric (TG)studies.84−87 TPR by H2 or CO and oxidation byO2 gives an estimate of OSC of an oxide. In thisexperiment, the rate of oxygen released from theoxide is measured as a function of temperature,under flowing hydrogen or carbon monoxide. Thereleased oxygen can be estimated by the rate ofconsumption of hydrogen or carbon monoxidedetected from the mass-spectral peak of water orcarbon dioxide or by oxygen pump or by thermalconductivity detector (TCD).88–90 On the otherhand, dynamic OSC accounts for the amount ofoxygen kinetically available during fast transitionsbetween reduction and oxidation environmentsand it has been investigated using oxygen isotopicexchange, CO pulse and CO/O2 pulse alternatingat a given frequency measurement. Dynamic OSCprovides better simulations of fluctuations thatexhaust gas might undergo during real operationand is therefore, considerably more useful in theevaluation oxygen storage activity of the material.

Materials for TWC applications must fitvery demanding kinetic and thermodynamicrequirements. They should have redox behaviorcapable to release/uptake rapidly as much oxygenas possible leading to have high OSC. Utilization oflattice oxygen from reducible oxides under reducingcondition and thereafter replenishment of oxygenunder oxidizing atmosphere has played a criticalrole in various oxidation reactions.91 Therefore, areducible oxide with redox characteristics could actas both oxidizing and reducing agent to variousreactants depending on the reaction condition. Inthis context, CeO2 has been found as an efficientlysuitable oxygen storage material for the wideapplication in TWC according to the followingreversible reactions:

CeO2 → CeO2−δ + δ/2O2

CeO2−δ + δ/2O2 → CeO2

where first reaction is related to oxygen-lean regionand second one is for oxygen-rich region and value



for δ is in between 0 and 0.25. Experimentallyachieved δ is ∼0.05 in the temperature range upto600 ◦C.

CeO2 is usually treated as an active support forprecious metal as well as base metal catalysts inheterogeneous catalysis.92–94 It has been widely usedas a promoter for TWC such as Al2O3 employedfor the control of toxic emission from automobileexhausts for many years.95,96 It is observed thataddition of ceria increases the activity of Al2O3

catalyst towards TWC. The promoting effect ofCeO2 is largely attributed to the enhancement ofmetal dispersions, OSC, increase in oxygen mobilitydue to oxide ion defects and thermal stability.81,82,97

The ability of ceria to store/release oxygen isprimarily caused by the fluorite crystal structurethat it adopts and the presence of mixed valence ofcerium, Ce4+ and Ce3+ leading to redox properties.The fluorite lattice having a metal ion of variablevalency is flexible to release oxygen to accommodatesignificant amount of oxygen vacancies underreducing conditions. These vacancies will be refilledwith oxygen atoms under oxidizing conditions.Hence, Ce4+ is reduced to Ce3+ in reducingatmosphere in CeO2 and it is reoxidized toCe4+ in oxidizing atmosphere. Therefore, CeO2

is able to act as an efficient oxygen buffer byreleasing/storing oxygen because of its capability toundergo effective reduction and reoxidation underfuel-rich (A/F<14.7) and fuel-lean (A/F>14.7)conditions. This reversible oxygen storage/releasefeature coupled with chemical stability in adverseconditions is a unique feature that offers ceriathe prime position in TWC design. Generally,higher conversion efficiencies are obtained withhigher OSC of the promoter. The process of oxygenstorage and transport in CeO2 occurs through thelattice oxygen defects. These defects could be eitherintrinsic or extrinsic depending on the mode ofcreation of oxygen vacancies. These oxygen defectsplay an important role in promoting the catalyticactivity of CeO2-based materials. In recent studies,it has been shown that incorporation of Zr, Y,La, Tb, Gd, Pb and Pr in CeO2 leads to createoxygen vacancies improving the catalytic activityand redox properties to a considerable extent.74

The enhancement in the activity is attributed tohigh oxygen ion mobility in the fluorite lattice.It has also been shown that activity of CeO2 inredox reactions is enhanced not only by noblemetals but also by base metal like copper. Martınez-Arias et al. and Flytzani-Stephanopoulos et al. haveextensively investigated the promoting effect ofCeO2 in CuO/CeO2, Cu/CeO2/Al2O3, CeO2/Al2O3

and Cu/CeO2 regarding their catalytic behavior andredox properties.98,99 The promoting effect in these

catalysts has been correlated with the synergism ofthe redox properties of the system which is achievedby metal–ceria interaction.

The discovery of CeO2 as oxygen storagematerial and support for heterogeneous catalystsled to enormous investigation since the early 1980s.However, OSC of CeO2 for oxidation/reductionreaction was first time estimated in a H2-TPRexperiment on CeO2 by Yao and Yao.81 Simplemechanism of OSC from H2-TPR could be writtenas:

CeO2 + δH2 → CeO2−δ + δH2O

CeO2−δ + δ/2O2 → CeO2.

Unique redox property of ceria has made it aspecial component of three-ways catalysts, since itcan store oxygen during lean (net oxidizing, excessof O2) conditions and release it during rich (netreducing, deficient of O2) conditions as given below:

CeO2 + δCO → CeO2−δ + δCO2

CeO2 +CxHy → CeO2−(2x+1/2y)

+xCO2 +1/2yH2O

CeO2 + δH2 → CeO2−δ + δH2O

CeO2−δ + δNO → CeO2 + δ/2N2

CeO2−δ + δH2O → CeO2 + δH2

CeO2−δ +1/2δO2 → CeO2

H2-TPR of CeO2 shows primarily two peaksat approximately 500 and 825 ◦C.81 The lowtemperature peak is due to the reduction of mosteasily reducible surface oxygen of CeO2 whileremoval of bulk oxygen is suggested as the causeof high temperature at 825 ◦C. The first peak isdependent on the surface area and responsible forhigher catalytic activity at lower temperature. Thereis a good correlation between BET surface area ofCeO2 and H2 consumption. Higher surface areamaterial means more surface oxygen can be removedby H2 or CO gas in that material leading to higherOSC. In another kind of investigations, substitutionof isovalent cations like Ti4+, Zr4+, Hf4+ andTh4+ ions forming Ce1−xMxO2 solid solutionsmight be much more important in enhancing OSC.Substitution of Zr ion in CeO2 showed much higherOSC as well as redox behavior.82,83,100−102 It maybe noted that ZrO2 is not reducible by H2 or CO butZr substitution in Ce site in Ce1−xZrxO2 showedenhanced reduction of Ce4+ thus increasing OSC.In general, high surface area, oxygen vacancy andmovement of oxide ions from tetrahedral sites



to vacant octahedral sites are the possibilities forenhancing the OSC. In another sense, Ce1−xZrxO2

and such oxygen storage materials are not thecatalysts themselves for real applications. Noblemetals such as Pt, Pd Rh, bimetallic Pt–Rh aredispersed mostly in the form of nanocrystallinemetal particles on such OSC materials and they areused as catalysts. Often, Ce0.7Zr0.3O2 is dispersedover γ-Al2O3 and noble metals are dispersedover this for catalytic use. Thus, real catalyst isquite complex and understanding their catalyticactivity would involve structural study of suchcomplex materials. Realizing the immense potentialof substituted CeO2 based materials regarding OSCas well as catalytic activity we have synthesizeda series of doped CeO2 compounds by solutioncombustion method and examined their OSC andseveral catalytic reactions related to auto exhaustcatalysis which would be discussed in the followingsections.103,104

5.3. New approach for developing novel catalystsFrom basic view point of TWC it is necessary todevelop such kind of materials that would provideunique OSC and active sites for redox type ofexhaust catalytic reactions as both the propertiesare correlated. Over the years, it is established thatmetals are in general active sites for adsorptionand catalysis and hence, most exhaust catalystscontain nanocrystalline noble metals (Pt, Pd and Rh)dispersed on supports like Al2O3 or SiO2 promotedby CeO2.24 However, in conventional catalysts,noble metal atoms on the surfaces of the nano metalparticles in their zero valent state serve as active sitesfor both oxidizing and reducing molecules. Even in4–6 nm metal particles, only 1/4 or 1/5 of the totalnoble metals are utilized for catalytic conversion.If 6 nm Pt particle is considered, then number of Ptatoms would be ∼14800, where number of surfacePt atom would be only ∼2870. This means 1/5 of Ptatoms are available for adsorption. If all Pt atomsare utilized, then reaction rate would be higher by 5times. Thus, catalytic activity increases with increasein metal dispersion. Atomic dispersion of noblemetals on traditional supports like Al2O3 is difficultbecause metal atoms sinter into metal particles dueto metal-metal bonding. The complete dispersion ofnoble metals could be achieved only as ions withina reducible oxide support.

Understanding the importance of CeO2 asoxygen storage material as well as active catalysts forredox type exhaust catalytic reactions, an entirelynew approach of synthesizing uniform solid catalystscomprising noble metal ions and CeO2 supportcould be developed. Noble metal oxides such as PtO,PtO2, PdO and Rh2O3 are known and therefore,

it should be possible to synthesize solid solutionbetween CeO2 and noble metal oxides. Noble metalloading in exhaust catalysts is only to the extentof 1–2 wt.% and hence substitution of only 1–2% noble metal ion in CeO2 would be sufficientto develop a catalyst. The underlying principleof doping aliovalent metal ions into CeO2 is toretain its parent structure. Thus, synthesis of singlephase oxide, Ce1−xMxO2−δ (M = noble metals)is a new concept which is the basis of metal ioncatalysts. Similarly, substitution of noble metals inTiO2 forming Ti1−xMxO2−δ (M = noble metals)retaining anatase structure of TiO2 is basic ideabehind the noble metal ion dispersion in a reduciblesupport oxide. It has been more than 10 yearswe have been following this novel approach tosynthesize noble metal ionic catalysts by solutioncombustion method. Promoting action of CeO2

and TiO2, metal–support interaction, high OSC,hydrogen spillover and synergistic interaction couldall be attributed to the interaction of M0/Mn+,Ce4+/Ce3+ and Ti4+/Ti3+ redox couples in thenoble metal ion substituted in a reducible oxides.

Substitution of noble metal ions in CeO2

has been achieved for the first time by a novelsolution combustion method.105 The solutioncombustion method involves rapid heating ata particular temperature of an aqueous redoxmixture containing stoichiometric amounts ofcorresponding metal salts and hydrazine basedfuels and the product is formed within 5 min.106

Redox compounds or mixtures containing hydrazidegroups have been found useful for the combustionsynthesis of oxide materials. An ideal fuel should bewater soluble and have low ignition temperature(<500 ◦C). It should be compatible with transitionmetal nitrate so that the combustion can becontrolled, smooth and does not lead to explosion.The merits of solution combustion techniqueare: (a) being a solution process, it has allthe advantages of wet chemical process suchas control of stoichiometry, doping of desiredamount of impurity ions and forming nano sizeparticles (b) low temperature initiated process, (c)highly exothermic due to redox reaction, (d) selfpropagating, (e) transient high temperature, (f)production of huge amount of gases, (g) simple,fast and economical and (h) formation of highsurface area, voluminous and homogeneous product.The synthesis of fine noble metal ion CeO2 solidsolution by solution combustion synthesis is anaccidental discovery. When an aqueous solutioncontaining stoichiometric amounts of Al(NO3)3,H2PtCl6 and urea was heated rapidly, the solutionboils, froths and burns with flame temperatureof 1500 ◦C yielding nano sized Pt metal particles



Figure 1: Sequences of solution combustion synthesis.

Figure 2: XRD of Ce0.98Pd0.02O2−δ.

dispersed on α-Al2O3. On the other hand, Pt metalparticles were not obtained when the same attemptwas made to disperse 1–2% Pt metal in CeO2 by thecombustion of ceric ammonium nitrate, H2PtCl6and oxalyldihydrazide (C2H6N4O2, ODH). Rather,25–30 nm Ce1−xPtxO2−δ crystallites were formedwhere Pt ions are in +2 and +4 oxidation states.Figure 1 shows a typical sequence of the formationof a catalyst by solution combustion process.Combustion of TiO(NO3)2, noble metal salts andODH fuel was employed for the preparation ofTi1−xMxO2−δ catalysts and metal ion doped 5–10nm TiO2 in anatase phase was obtained. Table 6summarizes metal ion substituted oxide materialsprepared by solution combustion method.

5.4. Structure of Ce1−xMxO2−δ andTi1−xMxO2−δ catalysts

Crystal structure, morphology, electronic structure,local structure and oxide ion vacancies of thecatalysts have been evaluated by X-ray diffraction(XRD), transmission electron microscopy (TEM),X-ray photoelectron spectroscopy (XPS) and X-rayabsorption fine structure spectroscopy (XAFS).

High resolution XRD data (Rigaku-2000) withrotating anode shows that CeO2 crystallizes influorite structure where Ce4+ ions are cubic closepacked with all tetrahedral sites occupied by oxygen.In 1 and 2% Pd/CeO2, diffraction lines are indexedfor fluorite structure and lines corresponding toPd or PdO are not observed in the XRD patterns.Even a slow scan in Pd(111) region (2θ ≈40 ± 5◦) does not show any indication of Pdmetal peak. In contrast, Pd/CeO2 prepared byimpregnation method shows diffraction lines due toPd metal. Rietveld refinement shows that R factorsare less than 1%. Lattice parameter decreases from5.4113(2) A in pure CeO2 to 5.4107(3) A in 2%Pd/CeO2 confirming the substitution of Pd2+ ions(0.84 A) for Ce4+ ions (0.99 A) in CeO2. TypicalXRD pattern of 2% Pd/CeO2 is shown in Figure 2.In 1% Pt/CeO2, no impurity lines could be observedrelated to any platinum oxides, whereas a smallbroad hump at 2θ = 39◦ due to Pt(111) couldbe seen in the diffraction pattern indicating thepresence of trace amount of Pt. Comparing theintensity ratio of Pt(111) in 1% Pt/CeO2 with that in1% Pt + CeO2 physical mixture it has been inferred



Table 6: Various metal ions substituted CeO2 and TiO2 based catalysts prepared by solution combustion method.

Metalions

Support oxides

CeO2 TiO2 Ce1−xTixO2−δ Ce1−xZrxO2−δ Ce1−xSnxO2−δ

Cu2+ Ce1−xCuxO2−δ

(x = 0.01–0.1)Ti1−xCuxO2−δ

(x = 0.2)Ce0.8−xTi0.2CuxO2−δ

(x = 0.05, 0.1)Ce1−x−yZrxCuyO2−δ

(x = 0.25, y = 0.02)Ag+ Ce1−xAgxO2−δ

(x = 0.01)Ti1−xAgxO2−δ

(x = 0.01)Au3+ Ce1−xAuxO2−δ

(x = 0.01)Pd2+ Ce1−xPdxO2−δ

(x = 0.01, 0.02)Ti1−xPdxO2−δ

(x = 0.01, 0.02)Ce0.75−xTi0.25PdxO2−δ

(x = 0.01, 0.02)Ce1−x−yZrxPdyO2−δ

(x = 0.25, y = 0.02)Ce0.78Sn0.2Pd0.02O2−δ

Rh3+ Ce1−xRhxO2−δ

(x = 0.005, 0.01)Ti1−xRhxO2−δ

(x = 0.01)Pt2+ Ce1−xPtxO2−δ

(x = 0.01, 0.02)Ti1−xPtxO2−δ

(x = 0.01)Ce0.85−xTi0.15PtxO2−δ

(x = 0.01, 0.02)Pt2+,Rh3+

Ce1−xRhx/2Ptx/2O2−δ

(x = 0.01,0.02)

that at least 92 at.% of the platinum taken in thepreparation of 1% Pt/CeO2 is incorporated intoCeO2 lattice. Total oxygen content in 1% Pt/CeO2

is 1.883 and that in pure CeO2 is 1.934 and thereis a decrease in lattice parameter in 1% Pt/CeO2

compared to pure CeO2. Similarly, diffractionlines due to Rh metal, Rh2O3 and RhO2 were notdetected in the XRD of 1% Rh/CeO2 catalyst. Totaloxygen content in 1% Rh/CeO2 catalyst is 1.87,whereas it is 1.934 for pure CeO2. Therefore, adecrease in oxygen content in the catalyst comparedwith pure CeO2 signifies the creation of oxideion vacancy. Thus, formation of solid solutionphases of Ce1−xMxO2−δ (M = Pd, Rh, Pt, Ru,Cu, Ag and Au) and Ce1−x−yTiyMyO2−δ (M =Pd and Pt) have been ascertained from detailedXRD studies.107–118 Similarly, solution combustionsynthesized Ti1−xMxO2 (M = Pd and Pt) catalystscrystallize in anatase structures.119,120

High resolution TEM (FeI Technai 20) imageof all the catalysts shows cubic morphology of CeO2

crystallites with average sizes of 25–30 nm. Noblemetal particles are hardly observed on the crystallites.For example, a high resolution TEM imageand electron diffraction pattern of combustionsynthesized 2% Pd/Ce0.75Ti0.25O2 is shown inFigure 3. The fringes spacing at ∼3.12 A correspondsto (111) layers of Ce0.75Ti0.25O2. The electrondiffraction pattern clearly shows fluorite structureof the compound and it is completely crystalline.There is no lattice fringe or ring due to Pd metal andfringes due to Pd metal particles separated at 2.25 Aare absent. On the other hand, Figure 3 C showsthe ED pattern of 2% Pd/Ce0.75Ti0.25O2 preparedby impregnation method and clearly Pd(111)ring is visible as indicated by arrow. Absence ofdiffraction ring due to Pd metal in the combustionsynthesized material shows substitution of Pd inthe lattice forming Ce0.73Ti0.25Pd0.02O2−δ solidsolution.118 The average particle size obtained fromlow magnification image is ∼20 nm. Similarly,

Figure 3: TEM of Ce0.73Ti0.25Pd0.02O2−δ (A), electron diffraction of Ce0.73Ti0.25Pd0.02O2−δ (B) andImpregnated 2 at.% Pd/Ce0.75Ti0.25O2 (C).



Figure 4: XPS of Pd metal, PdO, Ce0.98Pd0.02O2−δ and PdCl2.

a high resolution image of Pt ion substitutedCe0.85Ti0.15O2 shows only 3.2 A lattice fringes of(111) plane of Ce0.85Ti0.15O2 and fringes due toPt metal particles separated at 2.30 A are absent.Energy dispersive X-xay spectroscopy (EDXS) fromlattice fringes confirms the presence of 1% Pt inthe catalyst. Therefore, absence of Pt metal fringesand presence of Pt X-ray emission confirmed Ption substitution in Ce0.85Ti0.15O2.117 In contrast,large number of nano size fine Pt metal particles canbe dispersed on α-Al2O3 by solution combustionmethod.109,121

Oxidation states of Pt, Pd, Rh, Ru, Cu, Ag,Au, Ce and Ti present in the catalysts studiedhere could be obtained from XPS. For example,XPS (ESCA-3 Mark II, VG Scientific Ltd.) of Pd3dcore level region of 2 at.% Pd/CeO2 along withPd metal, PdO and PdCl2 is given in Figure 4.Pd3d5/2 peaks are observed at 337.4, 335.2, 336.4and 338.0 eV respectively for 2% Pd/CeO2, Pd metal,PdO and PdCl2. Thus, Pd core levels in 2% Pd/CeO2

catalyst are shifted by 2.2 eV with respect to Pdmetal, whereas Pd3d shift is 1.2 eV in comparisonwith PdO. Binding energies of Pd3d peaks in 2%Pd/CeO2 are close to of Pd3d peaks in PdCl2 clearlyindicating that Pd2+ ions are much more ionic inCeO2 matrix than PdO. XP spectrum of Ce3d with

satellite features shows that Ce is present in +4oxidation state in the catalyst. Therefore, XPS of 2%Pd/CeO2 by combustion method clearly indicatesthat Pd is present in highly ionic Pd2+ state. In 1%Pt/CeO2 catalyst, Pt is present in both +2 (85%)and +4 (15%) oxidation states.109,110 Thus, XPSresults demonstrate that Rh in Ce1−xRhxO2−δ, Cuin Ce1−xCuxO2−δ, Ag in Ce1−xAgxO2−δ and Au inCe1−xAuxO2−δ are fully dispersed in +3, +2, +1and +3 oxidation states, respectively.108,112−114

Stabilization of noble metal ions in CeO2 orTiO2 could be substantiated from the relativepositions of metal valence levels with respect tovalence levels of CeO2 and TiO2. The valence bandXPS of CeO2 as well as TiO2 consists of the O2pband spread over ∼3–9 eV (Figure 5a) and theempty Ce4f level is located at ∼2 eV below the Fermilevel (EF ) where binding energy is referenced to zero.Pt, Pd, and Rh metals have high electron density atthe EF (Figure 5b) and the valence bands of thesemetals extend even up to 6 eV below EF . When thesemetals are oxidized, Mn+d bands shift to higherbinding energies with the effect that the Mn+dbands are located at about 2.5–3.5 eV below EF as inPdO (Figure 5c). In Ce1−xMxO2−δ, the Mn+d bandlies below the Ce4f level but above the O2p band andhence, Ce in the compound remains mostly in the +4state. When metal ion in Ce1−xMxO2−δ is reducedto metal in the lattice, the metal valence level movesup toward the EF , which is above the empty Ce4flevel. Hence, electron transfer from metal to Ce4+ion (M0 + 2Ce4+4f0 → M2+ + 2 Ce3+4f1) becomesfacile and the noble metals remain ionic in CeO2.35

For example, the difference of valence band XPS ofCeO2 and Ce0.98Pd0.02O1.98 shows a small densityof states at ∼3.2 eV from EF . This density of statescorresponds to Pd2+4d8 occupied state that is higherthan that of Ce4f level.122 Similarly, Pt and Pd ionsget stabilized in TiO2 because Pt2+5d and Pd2+4dbands lie below Ti3+3d and above O2p bands.

Substitution of metal ions into CeO2 latticehas been confirmed by XAFS studies (SPring-8,Japan). XAFS analysis shows unique Ce–Pd andCe–Rh and Ce–Pt distances at 3.31, 3.16 and 3.28 Arespectively, whereas Ce–Ce distance is observed at3.84 A demonstrating the metal ion substitution intoCeO2 lattice.107,108,110 Significantly, these uniquecorrelations are absent in either CeO2 or noblemetals or their oxides such as PtO2, PdO andRh2O3. Typical XAFS spectra of 1% Pd/CeO2 alongwith PdO, Pd metal is shown in Figure 6. Lowercoordination numbers around metal ions comparedto Ce ion indicate the oxide ion vacancy due tolower valent metal ion substitution.

Based on the extensive characterization studiesemploying XRD, TEM, XPS and XAFS we have



Figure 5: Valence band XPS of (a) CeO2 and TiO2 (b) Pt, Pd, and Rhmetals and (c) PdO and the difference of valence band spectra ofTi0.99Pd0.01O1.99 and TiO2 showing Pd2+4d band position.

demonstrated that in combustion synthesizedM/CeO2 catalysts, metal ions are incorporated inthe CeO2 matrix in the form of solid solutionphase Ce1−xMxO2−δ. In addition to the oxideion vacancies created due to aliovalent ionicsubstitutions for Ce4+, there seems to be additionaloxide ion vacancies to the extent of 3.5% in nanoCeO2.

5.5. OSC of CeO2 supported noble metal ioncatalysts

To understand the reducibility of oxygen species aswell as the metal–support interaction in CeO2 andrelated materials, H2-TPR is extensively employedwhere the volume of H2 consumed by the reductionof an oxide is measured. Pure CeO2, TiO2, PdO,Rh2O3, CuO and SnO2 show weak H2 uptakepeak at higher temperatures in comparison withmetal substituted CeO2. ZrO2 does not showredox properties as there is no peak in theirrespective H2-TPR profiles, whereas significant OSChave been observed in Ce1−xZrxO2 samples. Forspecific example, H2-TPR profiles of CeO2, Pd

doped CeO2 compounds along with pure PdO andthe corresponding oxides supports are shown inFigure 7. Pure CeO2 shows H2 uptake from 350 ◦Cwith a low temperature peak at ∼500 ◦C, which hasbeen attributed to surface cerium reduction followedby bulk cerium reduction beyond 550 ◦C. The areaunder the peak up to 700 ◦C corresponds to 5.2 cm3

of H2 per g of CeO2. Reduction of Ti substitutedcompounds starts at 300 ◦C and Ce0.75Ti0.25O2

is reduced to Ce0.75Ti0.25O1.82. This compositioncorresponds to complete reduction of Ti4+ to Ti3+and 8% of Ce4+ to Ce3+. PdO is reduced at 80 ◦Cand the area under the peak corresponds to PdO+ H2 → Pd + H2O. A small negative peak afterPdO reduction is attributed to the desorption ofH2 from the dissociation of PdHx species. H2

uptake starts at ∼30 ◦C in Ce0.98Pd0.02O2−δ witha hydrogen adsorption peak at ∼65 ◦C and asmall peak at 400 ◦C due to CeO2 reduction isseen. Taking the low temperature H2 peak at 65◦C, the ratio of H/Pd is four in Ce0.98Pd0.02O2−δ.In the case of Ce0.73Ti0.25Pd0.02O2−δ, H/Pd ratiois 17. A ratio of H/Pd greater than 2 indicatesreduction of support because, Pd2+ + 2 H can givePd0. Thus, Ce0.75Ti0.25O2 upon substitution withPd2+ ion is more easily reducible at temperatureas low as 90 ◦C. The composition of the reducedoxide corresponds to Ce0.73Ti0.25Pd0.02O1.82 andit becomes Ce0.73Ti0.25Pd0.02O1.74 when reducedup to 700 ◦C. Thus there is a huge decrease inthe reduction temperature of Ce0.75Ti0.25O2 in

Figure 6: XAFS of Pd metal, PdO and 1%Pd/CeO2.



Figure 7: H2-TPR of CeO2, Ce0.75Ti0.25O2, PdO,Ce0.98Pd0.02O2−δ and Ce0.73Pd0.25Pd0.02O2−δ.

Figure 8: CO oxidation reaction over several combustion synthesizedcatalysts.

presence of 2% Pd ion in the lattice. Thus, Ti4+to Ti3+ and Ce4+ to Ce3+ reduction temperaturedecreased in presence of Pd2+ ion. Similarly, not

only Rh3+ ion in Ce1−xRhxO2−δ gets reducedbut part of Ce4+ ion also gets reduced withH/Rh >4.8 for Ce0.99Rh0.01O2−δ and the processis reversible. In Ce1−xTixO2, the oxide ions arealready activated due to distortion of oxide ionsublattice. Pt or Pd ionic substitution in the mixedoxide further activates the lattice oxygen. In fact,hydrogen is adsorbed at room temperature overPt or Pd ion doped Ce1−xTixO2 with H/Pt andH/Pd ratios of ∼10 and ∼15, respectively. Oxygenstorage capacities of a large number of CeO2 basedmaterials studied in our laboratory are given inTable 7.108,111,115−118,122−126

From our studies with H2-TPR, XAFS, DFTcalculations, it has been found that change ofstructure at the atomic level is to be the reasonfor higher OSC in these types of mixed oxides.Because of the smaller size of substituent cation,surrounding oxygen ions are displaced from theoriginal position and the oxygens at a larger distanceare removed by reducing gas easily. In Ce1−xZrxO2,there are several long and short Ce–O and Zr–Obonds compared to pure CeO2 and ZrO2. Shortbonds are strong, whereas long bonds are weakthat could be easily removed by H2 leading tohigher OSC.116 Similarly, XAFS studies and DFTcalculations show the presence of short and longCe–O and Ti–O bonds Ce1−xTixO2 which givesstrong and weakly bound oxygens.115 The strongerbonds have a valency greater than 2 and the weakerones have a valency less than 2 and weakly boundoxygen has been found to be responsible for higherOSC in these mixed oxides. On the other hand, thiskind of situation could not be found out in pureCeO2 and TiO2, thereby these pure oxides showlower OSC compared to mixed oxides.

5.6. Exhaust catalysis on CeO2 supported metalion catalysts

The noble metal ionic catalysts were examinedfor various catalytic reactions and their activitieswere compared with the corresponding metalssupported on oxides. Among the noble Pt, Pd,and Rh metal ions, Pd ion substituted CeO2 orCe1−xTixO2 or TiO2 showed the highest rate of COconversion and lowest activation energy towardsCO oxidation. In this sense, it would be noteworthythat Pd is the cheapest among noble metals. Atypical %CO conversion profile for CO oxidationreaction over Pd substituted catalysts are shown inFigure 8. It has also been also that activation energydecreases with increase in the effective charge onPd ion in these oxides.123 There are also pointsto note that in general, rates with metal ioniccatalysts are higher by 20–30 times compared withthe same amount of metal impregnated catalysts.



Table 7: Oxygen storage capacities (OSC) of CeO2 based materials studied in our laboratory.

Materials Composition after reduction OSC (μmol g−1) ReferencesCeO2 CeO1.97 174 123Ce0.98Rh0.02O1.99 Ce0.98Rh0.02O1.95 244 108Ce0.8Zr0.2O2 Ce0.8Zr0.2O1.81 1130 122Ce0.75Zr0.25O2 Ce0.75Zr0.25O1.7 1900 116Ce0.6Zr0.4O2 Ce0.6Zr0.4O1.63 2429 123Ce0.5Zr0.5O2 Ce0.5Zr0.5O1.57 2900 116TiO2 TiO1.92 1000 117Ce0.9Ti0.1O2 Ce0.9Ti0.1O1.88 740 117Ce0.85Ti0.15O2 Ce0.85Ti0.15O1.86 880 117Ce0.8Ti0.2O2 Ce0.8Ti0.2O1.83 1110 117Ce0.75Ti0.25O2 Ce0.75Ti0.25O1.81 1340 115Ce0.7Ti0.3O2 Ce0.7Ti0.3O1.77 1590 117Ce0.6Ti0.4O2 Ce0.6Ti0.4O1.73 2000 117Ce0.99Pt0.01O1.99 Ce0.99Pt0.01O1.95 290 117Ce0.84Ti0.15Pt0.01O1.99 Ce0.84Ti0.15Pt0.01O1.75 1570 117Ce0.83Ti0.15Pt0.02O1.98 Ce0.83Ti0.15Pt0.02O1.63 2320 117Ce0.73Ti0.25Pd0.02O1.98 Ce0.73Ti0.25Pd0.02O1.79 1286 123Ce0.75Hf0.25O2 Ce0.75Hf0.25O1.87 714 123Ce0.73Hf0.25Pd0.02O1.98 Ce0.73Hf0.25Pd0.02O1.78 1121 123Ce0.58Hf0.4Pd0.02O1.98 Ce0.58Hf0.4Pd0.02O1.83 804 123Ce0.98Pd0.02O1.98 Ce0.98Pd0.02O1.94 232 123Ce0.83Zr0.15Pd0.02O1.98 Ce0.83Zr0.15Pd0.02O1.87 638 123Ce0.73Zr0.25Pd0.02O1.98 Ce0.73Zr0.25Pd0.02O1.74 1509 123Ce0.58Zr0.4Pd0.02O1.98 Ce0.58Zr0.4Pd0.02O1.73 1612 123Ce0.9Sn0.1O2 Ce0.9Sn0.1O1.83 980 124Ce0.8Sn0.2O2 Ce0.8Sn0.2O1.72 1650 124Ce0.7Sn0.3O2 Ce0.7Sn0.3O1.63 2100 124Ce0.6Sn0.4O2 Ce0.6Sn0.4O1.58 2545 124Ce0.5Sn0.5O2 Ce0.5Sn0.5O1.56 2780 124Ce0.78Sn0.2Pd0.02O1.98 Ce0.78Sn0.2Pd0.02O1.7 1650 122Ce0.5Sn0.48Pd0.02O1.98 Ce0.5Sn0.48Pd0.02O1.77 1330 122Ce0.78Zr0.2Pd0.02O1.98 Ce0.78Zr0.2Pd0.02O1.88 620 122Ce0.95Ru0.05O1.97 Ce0.95Ru0.05O1.75 1335 111Ce0.9Ru0.1O1.94 Ce0.9Ru0.1O1.52 2513 111Ce0.9Fe0.1O1.88 Ce0.9Fe0.1O1.77 665 125Ce0.89Fe0.1Pd0.01O1.84 Ce0.89Fe0.1Pd0.01O1.71 780 125Ce0.67Cr0.33O2.11 Ce0.67Cr0.33O1.75 2513 126

Pd substituted Ce1−xSnxO2 and Ce1−xFexO2 alsoshows high OSC as well as CO conversion.122,124

CO conversion rates for CO + O2 reaction overseveral catalysts studied in our laboratory are at leastone order magnitude higher than catalysts reportedin the literature (Table 8).127–131 In another type ofexperiment, CO gets oxidized to CO2 by extractingactivated lattice oxygen in the absence of feedoxygen. On exposure to oxygen, the lattice oxygen isreplenished. This means that the feed oxygen getsadsorbed and incorporated in the oxide ion vacancyindicating the presence of two independent siteson the catalyst that adsorb reducing and oxidizingmolecules respectively.111,118,124

Ce0.98Pd0.02O2−δ shows high rates of NO andN2O reduction by CO compared with Pt, Pd,and Rh metal supported catalysts. N2 selectivityis highest among all the three catalysts in the

entire temperature range. In the temperaturerange of 125–350 ◦C, N2 selectivity is more than70% over Ce0.98Pd0.02O2−δ, whereas at the sametemperature range N2 selectivity is only 40–50%with Ce0.98Rh0.02O2−δ catalyst and it is much less(∼20%) with Ce0.98Pt0.02O2−δ. But N2 selectivitiesare more or less same over all catalyst at highertemperature (>300 ◦C).132,133 A comparison of Pt,Pd and Rh ion substituted CeO2 showed highercatalytic activity with Pd ion than both Pt and Rhions (Figure 9). Ti0.99Pd0.01O1.99 also shows highrates of NO conversion and high N2 selectivity andspecifically, rates of N2O reduction by CO are veryhigh.134 NO conversion rates and N2 selectivitiesfor NO + CO reaction over several catalysts studiedin our laboratory are much higher than the catalystsreported in the literature (Table 9).135−140 Morethan three hydrogen adsorptions per Pd2+ ion



Table 8: Comparison of CO conversion rates and activation energies (Ea) for CO + O2 reaction over several reportedcatalysts.

Catalysts Rate (μmol g−1 s−1) Ea (kJ mol−1) References5 wt.% Ru/SiO2 1.0 (110 ◦C) 94 1275 wt.% Rh/SiO2 0.0251 (110 ◦C) 103 1275 wt.% Pd/SiO2 0.316 (143 ◦C) 103 1275 wt.% Ir/SiO2 0.26 (180 ◦C) 105 1275 wt.% Pt/SiO2 0.32 (115 ◦C) 56 127Pd/CeO2/Al2O3 38.0 (250 ◦C) 84 1280.014 wt.% Rh/Al2O3 0.6 (196 ◦C) 115.5 1290.014 wt.% Rh/9 wt.% Ce/Al2O3 0.95 (196 ◦C) 90.3 1290.5 wt.% Pd/CeO2−ZrO2 0.7 (220 ◦C) 175 130Pd/SiO2 0.155 (140 ◦C) - 13125Au75Pd/SiO2 0.128 (140 ◦C) - 13150Au50Pd/SiO2 0.06 (140 ◦C) - 131Ce0.98Pd0.02O2−δ 7.0 (130 ◦C) 67.2 Our work (118)1 at.% Pd/Al2O3 (Impregnated) 2.73 (200 ◦C) 86.88 Our work (134)1 at.% Pd/TiO2 (Impregnated) 0.758 (100 ◦C) 75.25 Our work (134)Ce0.99Pt0.01O2−δ 0.6 (155 ◦C) 55.4 Our work (117)Ce0.84Ti0.15Pt0.01O2−δ 28.0 (155 ◦C) 82.3 Our work (117)Ce0.73Ti0.25Pd0.02O2−δ 18.0 (120 ◦C) 54.6 Our work (118)2% Pd/Ce0.75Ti0.25O2−δ - 132.3 Our work (118)Ce0.78Sn0.2Pd0.02O2−δ 1.9 (50 ◦C) 84 Our work (124)Ce0.95Ru0.05O2−δ 2.05 (100 ◦C) 94.5 Our work (111)Ce0.9Ru0.1O2−δ 3.3 (100 ◦C) 44.8 Our work (111)Ce0.89Fe0.1Pd0.01O2−δ - 52.5 Our work (125)Ce0.9Fe0.1O2−δ - 46.2 Our work (125)Ce0.99Pd0.01O2−δ - 63 Our work (125)

Figure 9: NO + CO reaction over Pt, Pd and Rh doped CeO2.

in Ce1−xPdxO2−δ makes it a better catalyst forNO reduction by H2 and 100% N2 selectivity isobserved at low temperature.141 NO and N2Oreduction by H2 over Ti0.99M0.01O2−δ (M = Pt,

Pd, Rh and Ru) have been studied and again,Pd ion is found to be better than other noblemetal ions.120 High rates of NO to N2 conversionwere observed over the Pd2+ ion substituted CeO2

catalyst during SCR in presence of excess oxygen. OnCe0.73Ti0.25Pd0.02O2−δ, NO conversion rates andN2 selectivity are higher than on Ce0.98Pd0.02O2−δ.SCR of NO by NH3 over Ti1−xMxO2−δ (M = Cr,Mn, Fe, Co or Cu; x = 0.1) shows high selectivitywith Fe ion.142 With noble metal ions in TiO2,dissociation rate of NH3 is low, while Mn or Fe ionin TiO2 shows higher dissociation.143

5.7. Reaction mechanism on metal ion dopedCeO2 catalysts

Earlier we have demonstrated that there are twoindependent sites on the metal ionic catalystwhich adsorb reducing and oxidizing molecules,respectively during redox type of reactions. Adual site mechanism of CO oxidation based onthe structure of Ce1−xPtxO2−δ is compared withLangmuir–Hinshelwood mechanism on Pt metalsurface in Figure 10. Dissociation of oxygen occursdue to the expansion of O–O bond over Pt metalsurface. On the Pt ionic catalyst surface, CO isadsorbed on the electron-deficient metal ions,whereas O2 is adsorbed on the oxide ion vacancy



Table 9: Comparison of NO conversion rates, activation energies (Ea) and nitrogen selectivities (SN2 ) for NO + CO reaction over several reportedcatalysts.

Catalysts Rate (μmol g−1 s−1) Ea (kJ mol−1) SN2 SN2 O References1 wt.% Pt/Al2O3 0.1 (300 ◦C) 92.4 27 73 1350.2 wt.% Rh/Al2O3 7.2 (300 ◦C) 184.8 32 68 1361 wt.% Pt-0.02 wt.% Rh/Al2O3 0.58 (300 ◦C) 126 39 61 136Pt-Rh/Al2O3-CeO2 0.54 (300 ◦C) 84 33 67 137Rh/Al2O3 0.58 (227 ◦C) 86.6 45 55 138Pd/Rh/Al2O3 0.11 (227 ◦C) 100 43 57 138Pd/Al2O3 0.83 (227 ◦C) 59.8 36 64 138Pd/La2O3/Al2O3 4.3 (227 ◦C) 100 30 70 1381% Pd/γ-Al2O3 0.75 (287 ◦C) 158 34 66 139Pd8Mo/Al2O3 2.0 (300 ◦C) 84 39 61 140Ce0.98Pd0.02O2−δ 0.242 (175 ◦C)

1.51 (200 ◦C)2.67 (225 ◦C)

70.2 758177

251923

Our work (132)

Ti0.99Pd0.01O2−δ 1.47 (175 ◦C)3.09 (200 ◦C)5.13 (225 ◦C)

64.13 705162

304938

Our work (134)

1% Pd/Al2O3 (Impregnated) 0.05 (240 ◦C) 86.6 59 41 Our work (132)Ce0.99Pt0.01O2−δ 0.2 (135 ◦C) 100.8 - - Our work (117)Ce0.84Ti0.15Pt0.01O2−δ 0.2 (135 ◦C) 96.6 - - Our work (117)Ce0.73Ti0.25Pd0.02O2−δ 2.0 (180 ◦C) 52.9 - - Our work (118)Ce0.78Sn0.2Pd0.02O2−δ 5.0 (150 ◦C) 105.4 100 - Our work (124)Ce0.95Ru0.05O2−δ 3.8 (200 ◦C) 41 100 - Our work (111)Ce0.9Ru0.1O2−δ 3.4 (200 ◦C) 53.2 100 - Our work (111)

because the vacancy site is activated by the electron-rich environment. The size of the oxide ion vacancyis ∼2.8 A, which can accommodate the oxygenmolecule of diameter 2.42 A. Thus, there are twodistinct sites, one for reducing and another foroxidizing molecules, in the ionic catalysts unlike Ptmetal. Electron transfer from reducing moleculesto oxygen is facilitated by the lattice via couplingbetween accessible Pt2+/Pt0 and Ce4+/Ce3+ redoxcouples. The enhanced activity is due to the creationof redox sites leading to site-specific adsorptionand electronic interaction between the noble metalions and the lattice. In addition to this, the latticeoxygen is activated in the catalyst with long M–Oand Ce–O bonds and CO can get oxidized through aMars–van Krevelen mechanism whereby oxide ion iscontinuously consumed and formed. The differencein the rate of CO oxidation over different metal ionsubstituted catalysts should be due to differences intheir redox properties. Similar explanation couldbe given for NO + CO reaction. NO is an electrondonor as well as an acceptor molecule. Therefore,on the ionic catalysts depicted in Figure 10, NO canbe molecularly adsorbed on noble metal ions anddissociatively chemisorbed on the oxide ion vacantsites. CO is specifically adsorbed on metal ion sites.NO adsorbed molecularly on the metal ion siteswould lead to N2O formation and on the oxide ionvacant site to N2 formation.

5.8. Exhaust catalysis with CeO2 based metalion catalysts coated on cordierite monolith

In recent days, monolith supports are attractivealternative carriers for catalysts in heterogeneouscatalysis. Monolith supports could either be metallicor ceramic containing single blocks of small (0.5–4 mm) parallel channels with catalytic wall andthe structure looks like honeycomb. The monolithcould be thought of as a series of parallel tubes witha cell density ranging from 300 to 1200 cpsi. Atpresent, monoliths have been extensively used ascatalyst supports inside the catalytic convertersfor exhaust gas clean up in the automobileindustry.30−32,144−146 Generally, a catalyst is coatedon a ceramic monolith support of honeycombstructure. Monolith is first coated with high surfacearea oxide like γ-Al2O3 and the process is calledwashcoating. After washcoating, catalyst active phaseis dispersed over the monolith support. Thereare several methods for washcoating and coatingof catalyst active phase on monoliths which hasbeen described in several articles.147,148 Monolithicsupport dominates the entire automotive marketas the preferred catalyst support due to advancesin monolith technology, simple catalyst mountingmethods, flexibility in reactor design, low pressuredrop and high heat and mass transfer rates. Amongmonolith supports ceramic monolith materials areleaders in the market and the preferred material iscordierite.



Figure 10: (a) Langmuir–Hinshelwood mechanism of CO + O2 reaction on Pt metal surface and (b)mechanism of CO + O2 reaction on Ce1−xPtxO2−δ.

In order to examine the reactivity for exhaustcatalysis over ionic catalysts supported on ceramicmonolith, these catalysts are coated on cordieritehoneycombs by a single-step combustion method.The process consists of growing γ-Al2O3 oncordierite by solution combustion method andactive catalyst phase, Ce0.98Pd0.02O2−δ on γ-Al2O3

coated cordierite again by the same method. About100% conversion of CO is achieved below 80 ◦Cat a space velocity of 880 h−1. At much higher

space velocity of 21,000 h−1, 100% conversion isobtained below 245 ◦C.149,150 At a space velocity of880 h−1, 100% NO conversion is attained below 185◦C and 100% conversion of HC (C2H2) below 220◦C. Typical CO + O2 and NO + CO reactions overCe0.98Pd0.02O2−δ coated on cordierite monolith aregiven in Figure 11. At same space velocity three-way catalytic performance over Ce0.98Pd0.02O2−δ

coated monolith shows 100% conversion of all thepollutants below 220 ◦C with 15% of excess oxygen.



Figure 11: CO + O2 and NO + CO reactions over Ce0.98Pd0.02O2−δ coatedon cordierite monolith.

Thus, Pd2+ incorporated CeO2 catalyst coated oncordierite shows significant activity toward thereactions related to auto exhaust catalysis.

6. Conclusions and future directionsA new approach of dispersing metal ions overCeO2 and TiO2 by solution combustion techniqueresulting in Ce1−xMxO2−δ and Ti1−xMxO2−δ

(M = Pd, Rh and Pt) catalysts, structure ofthese materials and their catalytic properties aredocumented here. As the solution combustionmethod is a redox reaction type of preparationprocedure, in these materials, noble metal ions areincorporated to a certain limit into the reducibleoxide substrate matrices like CeO2 and TiO2. Inthis point of view, this preparation method for thesupported metal catalysts is unique and differentfrom other conventional techniques. Extensiveworks done in the last 10 years in our laboratoryhave established that these ionically dispersedmetal catalysts show very high OSC and they arecatalytically much more active than usual dispersedfine metal particles toward several auto exhaust

catalytic reactions such as CO and hydrocarbonoxidation and NO reduction by CO, NH3 and H2.This special preparation technique for dispersingionic metals could be extended to the other reducibletransition metal oxide supports such as SnO2, V2O5,WO3 and MoO3 and these materials could be probedfor several reactions related to auto exhaust catalysisexploring more insights into the metal–supportinteraction, redox properties and synergistic effects.

In last 30 years, pollutions from automotiveexhaust are a major concern for our environmentand numerous studies are being carried outto abate the pollutants through catalysis. Wehope, ionically dispersed noble metal catalystssynthesized by solution combustion method couldprovide a new direction to achieve best performingcatalysts for several abatement reactions for autoexhaust clean up. In broad sense, extensive studieson these materials for several heterogeneouscatalytic reactions of present-days’ importance otherthan auto exhaust catalytic reactions would beworthwhile area to carry out in catalysis coupledwith solid state chemistry view point.

AcknowledgmentsWe thank all our past and present lab membersand collaborators for their significant contributionswhich are reflected in the references. We thank DST,Govt. of India for financial support. PB thanks Dr.K. S. Rajam, National Aerospace Laboratories forher constant encouragement.

Received 20 June 2010.

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Parthasarathi Bera is a native of Moynain Purba Medinipur district of WestBengal, India. He obtained his B. Sc.and M. Sc. in Chemistry from JadavpurUniversity, Kolkata and earned his Ph. D.in Heterogeneous Catalysis from IndianInstitute of Science, Bangalore under thesupervision of Professor M. S. Hegde.He worked as a Post Doctoral Fellow atUniversity of Washington, Seattle, USA

with Professor Charles T. Campbell and University of Pennsylvania,Philadelphia, USA with Professor John M. Vohs. He was a MarieCurie Fellow at Instituto de Catalisis y Petroleoquımica, CSIC,Madrid, Spain under Professor Arturo Martınez-Arias. He joinedNational Aerospace Laboratories, Bangalore as a Scientist Fellow inJanuary 2010. His main research interests include heterogeneouscatalysis for green energy and environment, materials chemistryand surface science and engineering.

M. S. Hegde received his Ph.D. fromIndian Institute of Technology, Kanpurin 1976 and joined the Indian Institute ofScience in 1977. He is a professor in SolidState and Structural Chemistry Unit andDean of Science. His research interestsare in the areas of surface science, solidstate chemistry and catalysis.


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