Applied Energy 86 (2009) 2283–2297

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Applied Energy

journal homepage: www.elsevier .com/locate /apenergy

Catalysis for NOx abatement

Sounak Roy a, M.S. Hegde a, Giridhar Madras a,b,*

a Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, Indiab Department of Chemical Engineering, Indian Institute of Science, Bangalore 560 012, India

a r t i c l e i n f o

Article history:Received 4 December 2008Received in revised form 17 March 2009Accepted 18 March 2009Available online 25 April 2009

Keywords:NOx reductionMetal ion substitutionEnvironmental catalysis

0306-2619/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.apenergy.2009.03.022

* Corresponding author. Address: Department of CInstitute of Science, Bangalore 560 012, India. Tel.: +92360 1310.

E-mail address: giridhar@chemeng.iisc.ernet.in (G.

a b s t r a c t

Research in the field of NOx abatement has grown significantly in the past two decades. The general trendhas been to develop new catalysts with complex materials in order to meet the stringent environmentalregulations. This review discusses briefly about the different sources of NOx and its adverse effect on theecosystem. The main portion of the review discusses the progress and development of various catalystsfor NOx removal from exhaust by NO decomposition, NO reduction by CO or H2 or NH3 or hydrocarbons.The importance of understanding the mechanism of NO decomposition and reduction in presence ofmetal ion substituted catalysts is emphasized. Some conclusions are made on the various catalyticapproaches to NOx abatement.

� 2009 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2283

1.1. NOx sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22841.2. Noxious effect of NOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22841.3. Legislations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2284

2. Catalytic deNOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2285

2.1. NOx decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2285

2.1.1. Photocatalytic decomposition and reduction of NOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2286

2.2. NOx reduction by CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22872.3. deNOx by H2 and NH3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22892.4. Selective catalytic reduction (SCR) of NO by H2 or NH3 or ‘HC’ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22892.5. Other methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2292

2.5.1. NSR (NOx storage and reduction). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22922.5.2. Selective NOx recirculation (SNR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22932.5.3. Selective non-catalytic reduction (SNCR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22932.5.4. Ozone injection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2293

3. Conclusions and future perspectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2293References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2294

1. Introduction

Environmental catalysis can be defined as technologies usingcatalysts to reduce the emission of environmentally unacceptable

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hemical Engineering, Indian1 80 2293 2321; fax: +91 80

Madras).

compounds [1,2]. The exhausts from automobiles and stationarysources such as power plants contain CO, NOx and hydrocarbons.The conversion of these pollutants to CO2, N2 and H2O using cata-lysts is a challenge. In the last two decades, significant develop-ments have occurred in this field leading to a betterunderstanding of the catalytic NOx abatement. A number of re-views regarding various aspects of NOx abatement have been pub-lished, as discussed in the following sections. This review, however,is a compendium of all aspects of NOx abatement that include NO

2284 S. Roy et al. / Applied Energy 86 (2009) 2283–2297

decomposition, NO reduction by CO, H2, NO storage and selectivecatalytic reduction of NO.

1.1. NOx sources

The major source of nitrogen oxides is the combustion of fossilfuels such as petroleum in the engines of vehicles or coke in theelectrical power plants. The origin of NOx is generally categorizedinto mobile and stationary sources. Fig. 1 describes the differentsources of NOx in US and in European countries [3,4].

NOx is a generic term for mono-nitrogen oxides namely NO andNO2, which are produced during combustion at high temperatures.At ambient temperatures, oxygen and nitrogen do not react witheach other. However, in an internal combustion engine, high tem-peratures lead to reactions between nitrogen and oxygen to yieldnitrogen oxides. In the presence of excess oxygen, nitric oxide willbe converted to nitrogen dioxide.

Bosch and Janssen [5] categorize three types of NOx formed dur-ing the combustion process. NOx from engine exhaust typicallyconsists of a mixture of 95% NO and 5% NO2. The first category,thermal NOx, is formed by the oxidation of N2 at high temperatures.

N2 þ O2 () 2NO; DH�

298 ¼ 180:6 kJ=mol ð1Þ

This reaction takes place above 1300 K and follows the Zeldo-vich mechanism of chain reactions involving N* and O* activatedatoms:

N2 þ O� ! NOþ N� ð2ÞN� þ O2 ! NOþ O� ð3Þ

The rate of NO formation is essentially controlled by reaction(2) and increases exponentially with temperature. The Zeldovichmechanism dominates NO formation under most engine condi-tions [6]. The NOx emission from the engine can be controlled bylowering the combustion temperature by operating the engine un-der excess air (fuel-lean) conditions but most these approaches arenot very effective [6], though recent approaches based on hightemperature air combustion (HiTAC) are effective.

The second category of NOx is called fuel NOx and is formed fromthe oxidation of nitrogen present in fuels such as coal and heavyoils. In contrast to thermal NOx, fuel, NOx formation is relativelyindependent of temperature at normal combustion temperatures[6].

Power Industry Res. Fuel Comb. Agriculture Transportation0

10

20

30

40

50

60

% o

f NO

x fro

m d

iffer

ent s

ourc

es

USA Europe

Fig. 1. Illustration of emission of NOx by source category in USA and Europeancountries.

The third category of NOx is called ‘prompt NO0x (also termed asFenimore NO) which is formed by the reaction of hydrocarbonfragments with atmospheric nitrogen to yield products such asHCN and H2CN. These can be subsequently oxidized to NO in thelean zone of the flame. NO can further react with oxygen to NO2

or N2O.

NOþ 1=2O2 () NO2; DH�

298 ¼ �113 kJ=mol ð4Þ2NO() N2Oþ 1=2O2; DH

�

298 ¼ �99 kJ=mol ð5Þ

Prompt NOx formation is proportional to the number of carbonatoms present per unit volume and is independent of the identityof the parent hydrocarbon. The quantity of HCN formed increaseswith the concentration of hydrocarbon radicals. Prompt NOx canbe formed in a significant quantity at low-temperature, fuel-richconditions and where residence times are short.

Another route of formation of NO is via nitrous oxide. In thismechanism, O-atom attacks molecular nitrogen in presence of athird molecule that results in the formation of N2O. This subse-quently reacts with O atom to form NO, N2O + O ? 2 NO withan activation energy of 97 kJ/mol. This reaction route is over-looked because the total NO formed by this reaction is not signif-icant. However, lean conditions suppress Fenimore NO and lowtemperatures suppress Zeldovich NO. As high pressures promotethis reaction, the formation of NO by this route occurs primarilyin lean premixed combustion in high pressure gas turbineengines.

1.2. Noxious effect of NOx

The oxides of nitrogen play a major role in the photochemistryof the troposphere and stratosphere. NOx catalyzed ozone destruc-tion occurs via the following reactions:

NOþ O3 ! NO2 þ O2 ð6ÞNO2 þ O! NOþ O2 ð7Þ

These reactions are largely responsible for the ozone decline inmiddle to high latitudes from spring to fall [7,8]. Another adverseeffect of NOx is acid rain, which can perturb the ecosystems andcan cause biological death of lakes and rivers. Peroxyacetylene ni-trates (PAN) can also be formed from nitric oxide and contributesignificantly to global photo-oxidation pollution [9].

Some biological studies have shown NO as an essential messen-ger, which transmits the necessary information to the white bloodcells within the bloodstream to destroy tumor cells and to theneurotransmitters to dilate the blood vessels [9,10]. However, thebiologically active NO is a poisonous product of the in vivoenzyme-catalyzed transformation of the amino acid, arginine. NOdiffuses through the alveolar-cells and capillary vessels of thelungs and damages the alveolar structures and their functionsthroughout the lungs provoking both lung infections and respira-tory allergies like bronchitis, pneumonia, etc. [11,12].

1.3. Legislations

Because of the ecological and health hazards due to the pres-ence of NOx in the environment, regulations have been proposedto control NOx emissions. There is a wide variation among coun-tries with respect to both the type and the level of regulation em-ployed. The Gothenburg protocol establishes reductions of fourmain pollutants to reduce acidification, eutrophication and the ef-fect of ozone. Other than Canada and USA, 29 European countrieshave signed this protocol and these countries have estimated the

Table 1Emission standards for India [14].

Year Emission standards for 3-wheelgasoline vehicles (g/km)

Emission standards for 2-wheelgasoline vehicles (g/km)

CO HC HC + NOx CO HC HC + NOx

1991 12–30 8–12 – 12–30 8–12 –1996 6.75 – 5.40 4.50 – 3.602000 1.00 – 2.00 2.00 – 2.00

Engine Power (P) Date Emission standards for diesel engines 6800 kWfor generator sets

CO(g/kWh)

HC(g/kWh)

NOx

(g/kWh)PM(g/kWh)

Smoke(1/m)

P 6 19 kW 2004.01 5.0 1.3 9.2 0.6 0.72005.07 3.5 1.3 9.2 0.3 0.7

19 kW < P 6 50 kW 2004.01 5.0 1.3 9.2 0.5 0.72004.07 3.5 1.3 9.2 0.3 0.7

50 kW < P 6 176 kW 2004.01 3.5 1.3 9.2 0.3 0.7176 kW < P 6 800 kW 2004.11 3.5 1.3 9.2 0.3 0.7

Date Emission limits for diesel engines >800 kW for generatorsets

CO(mg/Nm3)

NMHC(mg/Nm3)

NOx

ppm(v)PM(mg/Nm3)

Until 2003.06 150 150 1100 752003.07–2005.06 150 100 970 752005.07 150 100 710 75

S. Roy et al. / Applied Energy 86 (2009) 2283–2297 2285

critical loads themselves. An overview of the different targets [13]is available.

The ongoing emission standards in Europe in Euro IV while EuroV will be effective from September 2009. The latter regulates thatthe emission will be less than 0.18 g/km for diesel and 0.06 for pet-rol driven engines, respectively. Since the year 2000, India is adopt-ing Euro I for four-wheeled light-duty and for heavy-duty vehicles.For 2- and 3-wheelers, Bharat Stage II (Euro II) was applicable fromApril, 2005 and Stage III (Euro III) standards have come in forcefrom April, 2008 [14]. Table 1 shows the emission standards fordifferent Indian vehicles and from new diesel engines used in gen-erator sets.

2. Catalytic deNOx

NO molecule has the electron configuration ðr2gÞðr2

uÞðrg ;puÞðp1gÞ.

Due to the unpaired p antibonding electron the molecule is para-magnetic and partly cancels the effect of p bonding electrons. Thebond order is 2.5, consistent with an inter-atomic distance of1.15 Å that is intermediate between triple bond distance in NO+

of 1.06 Å and double bond �1.20 Å. NO has an unpaired electronin its 2p* orbital and this has led to the notion that amphotericbonding for NO on a surface is a useful consideration. NO can eitherdonate its electron to the surface (like CO) or it can accept electrondensity from the surface into the half-filled 2p* orbital, and there-fore show a very wide variety of chemistry on surface.

NOx is thermodynamically unstable. However, it does notdecompose because its high activation energy (364 kJ/mol). There-fore, a catalyst is needed to lower the activation energy in order tofacilitate the decomposition. Research in this domain is extensive,and can be divided into four major paths of deNOx activities basedon automobile and stationary sources.

i. NO decomposition

The direct decomposition of nitric oxide to nitrogen and oxygenis one of the most attractive methods because this reaction is ther-modynamically favorable and does not need any reductants.

NO! 1=2N2 þ 1=2O2; DH�

298 ¼ �86:6 kJ=mol ð8Þ

ii. NO reduction by CO

NO reduction by CO is an important reaction in three-way catal-ysis (TWC).

NOþ CO! 1=2N2 þ CO2; DH�

298 ¼ �328 kJ=mol ð9Þ

iii. NO reduction by H2 and NH3

NO can be reduced in presence of hydrogen or ammonia.

NOþH2 ! 1=2N2 þH2O; DH�

298 ¼ �287 kJ=mol ð10Þ6NOþ 4NH3 ! 5N2 þ 6H2O ð11Þ

iv. SCR (selective catalytic reduction) of NO

While NO can be reduced with ammonia based on Eq. (11), SCRprocesses are usually carried out in presence of oxygen. Thus theequations for the reduction of ammonia and hydrocarbons in pres-ence of oxygen are given below,

4NOþ 4NH3 þ O2 ! 4N2 þ 6H2O; DH�

298 ¼ �1627 kJ=mol ð12ÞNOþ CH4 þ 3=2O2 ! 1=2N2 þ CO2 þ 2H2O;

DH�

298 ¼ �847 kJ=mol ð13Þ

The above four sections can be reclassified as two major paths,one in which the catalytic decomposition of NOx occurs in absenceof a reducing agent (i) and the second in which the catalytic reduc-tion of NOx occurs in the presence of a reducing agent (ii–iv). Thesecond pathway can be sub-classified to cases where the composi-tions of the exhaust gas are near stoichiometric conditions (ii, iii) orunder fuel-lean conditions (iv). We discuss the above four methodsof deNOx in the following sections (Sections 2.1–2.4) and also dis-cuss other methods of deNOx in a separate section (Section 2.5).

2.1. NOx decomposition

NO is a molecule that can adsorb either dissociatively or molec-ularly depending on the metal. NO normally dissociates on thebase metals and show molecular adsorption at room temperatureon noble metals [15]. However, NO dissociation often depends onsurface temperature, surface coverage, crystal plane and surfacedefects. The structure of adsorbed NO molecule on metal surfaceshas been discussed employing experimental techniques like elec-tron energy loss spectroscopy (EELS), low energy electron diffrac-tion (LEED), photo electron diffracion (PED), and theoreticalsimulation using density-functional theory (DFT) calculations [16].

Brown and King compiled N–O vibrational frequencies and as-signed sites for NO adsorption on various single crystal metals[17]. NO can be adsorbed on metal surfaces in different geometrieslike linear (atop), bent, bridge etc, as shown in Fig. 2. A generalobservation from those vibrational frequencies is that the N–Ostretching frequency in the ‘‘atop” NO is more than that of the‘‘bent” NO. Thus, the NO dissociation will be facile if the NO is ad-sorbed on the surface in bent geometry than atop geometry. NOhas an unpaired electron in its 2p* orbital. In a metal–NO bonding,NO makes a 5r–d bond with metal atoms as in M–CO, and back-bonding takes place from metal d orbital to 2p* orbital of NO. Thus,the metal–N bond will be stronger and N–O bond will be weaker. IfNO obtains an electron in its antibonding orbital it becomes NO�,which is isoelectronic to O2. O2 adsorption is always on a ‘‘side-on” geometry, which dissociates to form M@O. Similarly, when

2286 S. Roy et al. / Applied Energy 86 (2009) 2283–2297

the ‘‘bent” or linearly (‘‘atop”) adsorbed NO molecule becomesNO�, it changes its geometry to a ‘‘side-on” NO intermediate anddissociates. Rh{1 0 0} or Ni {1 0 0} shows this kind of behavior,which is absent for Pt surface [18,19]. Fig. 3 describes the geome-try. Lambert et al. showed that Rh surfaces are more efficient thanPt surfaces to dissociate NO [20].

If the metal d orbital is promoted by an extra electron from al-kali metal or alkaline-earth metal, then the charge transfer frommetal d to 2p* orbital of NO should increase and dissociate NO.Lambert and co-workers have shown that sodium or potassiumpromoted Rh surface dissociates NO more than the clean surface[21]. However, the problem in using a single crystal metal to disso-ciate NO is that the dissociated oxygen either oxidizes the metalsurface to metal oxides or makes an oxygen adlayer over the metalsurface that hinders the dissociation process.

In one study [22], the catalytic activities of 40 metallic (Mg, Zn,Ni, etc.) oxides were examined in the temperature range of 500–800 �C for the decomposition of NO to nitrogen and oxygen. Underconditions where the O2 formed was continuously removed asNO2, the reaction was first order with respect to NO pressure. Ina closed reaction system, O2 retards the decomposition of NO bycompeting for the same surface centers. The oxygen ‘‘self-poison-ing” effect with respect to NO decomposition is slower on ceria-supported catalysts, which may be attributed to ‘‘oxygen spillover”from the noble metal to the reduced ceria [23]. Cobalt oxide(Co3O4) is one of the most active single-component metal oxidesfor NO decomposition [24]. The high activity of Co3O4 seems tobe due to the relatively weak Co–O bond, which leads to an easydesorption of lattice oxygen in the lower temperature range.Co3O4 has a spinel structure with Co(II) and Co(III). During NOdecomposition, Co(II) may oxidize to Co(III) and form Co2O3 likephase on the surface. But Co2O3 is not stable and decomposes toCo3O4 thus facilitating NO dissociation. With the addition of Na[25] or Ag [26], the activity enhances primarily due to excess elec-tron density on the surface. Fang and White [27] studied thechemisorption of NO on platinised TiO2 by Fourier transform infra-red spectroscopy. Two adsorption bands at about 1760 and1700 cm�1, which were assigned as NO adsorbed on Pt open sitesand on closed packed sites, respectively, were found to vary inintensity with reduction and O2 pretreatment. NO uptake alsodiminished on reduced Pt/TiO2 [27]. A similar observation has beenmade with Rh/TiO2 [28,29]. The room-temperature chemisorptionof NO was observed to decrease with increasing catalyst reduction

N

M

:O:

N

O:

M: M

:O:

:N

M

N

M

:O:

Linear(~sp) Bent(~sp2)

Bridging(~sp2)

Fig. 2. Schematic representation of the bonding in NO complexes [17].

N

O:

M: M

:O:

:NOr MM

N

M M

NO O

M

Fig. 3. Schematic representation of NO dissociation [20].

temperature. Thus, reduced Pt and Rh on TiO2 are less active for NOdissociation.

The adhesive and catalytic properties of rhodium, palladium,and platinum on a-Al2O3 have been examined and it has been ob-served that the variation of the Fermi level as a function of metal-ceramic interface indicate that the ceramic is not inert. The oxy-gen–metal interface is unstable for rhodium and was predictedto be particularly favorable both for dissociative adsorption ofNO as well as for a coupling reaction [30].

The decomposition of NO over Cu-exchanged zeolites has beenstudied extensively since the pioneering work of Iwamoto et al.[31–34]. NOx decomposition over Cu–MFI zeolites (ZSM-5 has beennamed as MFI with silica/alumina = 23.3) proceeds via a redox typemechanism. NO could be adsorbed as NO+, NO� and NO�2 species onthe Cu–zeolite. Cu+ ions generated through pretreatment, at ele-vated temperature, are oxidized to Cu2+ ions by oxygen and a partof the resulting Cu2+ ions acts as adsorption sites for NO+. The NOmolecule has an unpaired electron in a degenerate antibondingorbital and the electron transfer of this unpaired electron fromthe antibonding orbital of NO to empty or partially filled 3d orbi-tals of transition metal ions can then occur easily. This is followedby lone pair donation from NO and p back-bonding to the NO orbi-tals, generating a nitrosyl complex. Although the molecular orbitalstructure of the nitrosyl is rather complex, the process can be con-veniently schematized as: Cu2þ þ NO() ½CuþNOþ�. A fraction ofthe Cu2+ ion becomes reduced to Cu+ through spontaneous desorp-tion of oxygen. Reoxidation with NO regenerates the oxidized Cu2+

sites and N2 is formed. From the IR studies, Giamello et al. sup-ported the above data and showed the formation of the nitrosylicadduct [35]. Cheung et al. concluded that the catalytic performanceof Cu in a zeolite matrix for the decomposition of NO was linked tothe Cuþ () Cu2þ redox cycle, which is essential for the formationof the ON—Cu2þ—NO�2 complex, and thus dependent on the relativestability of the Cu+ and Cu2+ species [36]. However, the low hydro-thermal stability of these materials for gasoline engines is a seriousdrawback.

Many of the theoretical calculations provide an insight on themechanism of NOx decomposition on zeolites. Schneider and co-workers have proposed a mechanism pathway involving two suc-cessive O-atom transfers to an isolated, zeolite-bound Cu+ center,initiated by formation of a short-lived and difficult to detect isonitro-syl intermediate, and yielding sequentially N2O and Cu-bound O fol-lowed by N2 and Cu-bound O2 [37]. A recent density functionaltheory (DFT) calculated electronic structure and excitation energyspectra for the model system (HO)3Al–O–Cu–O–Cu, show that an an-ionic NO dimer (ONNO)� is formed on (–O–Cu–O–Cu–) chain [38].

However, NO decomposition over metal surfaces, oxides or zeo-lites has some serious practical difficulties in practice. NO decom-position process is a high temperature phenomenon, which isundesirable. NO dissociation over single crystal metal surface oxi-dizes the metal surface, which in turn hinders the NO dissociationprocess. So, a reductant is required to scavenge the dissociatedoxygen. From an exhaust catalysis view, it is not sufficient to dis-sociate NO, other pollutants like CO and hydrocarbons (HC) shouldalso be oxidized. Therefore, NO reduction by CO, HC, H2, and NH3 ismore attractive for NOx abatement.

2.1.1. Photocatalytic decomposition and reduction of NOx

An alternative method of dissociating and reducing NOx is bythe use of photocatalysis at room temperature. Direct photocata-lytic decomposition of NO would yield N2 and O2. Anpo and hisco-workers have studied photocatalytic decomposition of NO onTiO2 and found anatase TiO2 exhibit a high efficiency for thedecomposition of NO in a flow system [39]. Lim et al. have investi-gated the photocatalytic decomposition of NO on Degussa P-25TiO2 in an annular flow type reactor and showed the conversion

S. Roy et al. / Applied Energy 86 (2009) 2283–2297 2287

of NO to NO2, N2O and N2 by photocatalysis increases with lightintensity, residence time and decreasing initial NO concentration[40]. Bowering et al. have showed that the photocatalytic activityof Degussa P-25 TiO2 decreases with the increasing pretreatmenttemperature [41]. We have shown the reduction of NO over thecatalyst Ti1�xPdxO2�d was two orders of magnitude higher thanunsubstituted TiO2 [42]. Direct NO decomposition into N2 andN2O was also studied and it was observed that NO dissociates inpresence of UV light at room temperature. The products foundwere N2, N2O and O2. The O2 evolved reacts with NO to give NO2.NO2 formed is adsorbed by catalyst. Prolonged NO2 adsorptionthus makes the surface inactive for NO dissociation [42]. However,NO dissociation starts again when a reductant like CO is passed toscavenge the evolved dissociated O2.

2.2. NOx reduction by CO

NO reduction by CO is a primary reaction in three-way catalysis(TWC). The introduction of catalytic treatment of automotive ex-hausts began with the removal of the incomplete combustionproducts, CO and residual HC. These reactions are

2COþ O2 ! 2CO2 ð14Þ2CxHy þ 2xþ y=2O2 ! 2xCO2 þ yH2O ð15Þ

Simple oxidation catalysts, referred to as ‘two-way Catalysis’,accomplished this. The regulations necessitating the catalytic re-moval of nitrogen oxides, formed in the combustion chamber, be-gan an essential part of exhaust treatment and thus simultaneousoxidation as well as reduction was required. A combustion exhaustof a slightly rich mixture of fuel and air was fed to the upstreamcatalyst bed, and reduction of NOx was fast and nearly complete.Secondary air was then injected ahead of a downstream oxidationcatalyst to remove the CO and HCs. However, due to the presenceof hydrogen in the exhaust, the reduction of NOx in the upstreamcatalyst resulted mainly in ammonia. When re-oxidized on thedownstream catalyst, the ammonia converted back to NOx, violat-ing the whole approach [43,44]. It was subsequently proposed thatif one could equilibrate the combustion exhaust of an exactly stoi-chiometric combustion mixture, it was thermodynamically possi-ble to remove all the three pollutants, namely NOx, CO and ‘HC’leaving only water, CO2 and N2 [45]. This is known ‘three-waycatalysis’ and three-way catalytic converters have been at the heartof vehicle emission control systems since the 1980s. The air–fuelratio is the mass ratio of air to fuel present during combustionand when all the fuel is combined with all the free oxygen, thisis known as the stoichiometric ratio and designated as k = 1. Forgasoline, the stoichiometric air/fuel mixture is approximately14.7 times the mass of air to fuel. If A/F ratio is below 14.7(k < 1), then it is called fuel-rich condition and if A/F ratio exceedsthis value (k > 1), then it is called fuel-lean condition. This requirestight stoichiometry constraints and in a randomly oscillating dy-namic combustion, this is not easily attained.

Before 1975, research on automotive catalysts was devoted tonon-noble metals. However, a catalyst in the rigorous automotiveexhaust condition should have (a) intrinsic reactivity, (b) poisonresistance ability and (c) durability and it soon became apparentthat the base metal oxides of Ni, Co, Mn, Cr do not have these prop-erties [46,47]. The noble metals in typical three-way catalysts(Pt, Pd, Rh) are dispersed as individual atoms or small clusters ofatoms over an oxide support. In the early TWCs, Rh became the cat-alytic element of choice for NOx control [48]. In the case of Rh/Al2O3 catalysts, presence of the metal-support interaction has beenshown to be present between zero valent Rh atoms at the interfaceand the oxygen ions of the support [49]. The oxidation and reduc-tion of Rh affects dynamic behavior because oxidation of Rh se-

verely decreases its NO reduction activity [50]. Hecker and Bellsubsequently studied NO + CO reaction over Rh/SiO2 and showedthat conversion of NO in presence of oxidized Rh is higher com-pared to that over reduced Rh [51]. The product selectivity is oneof the very important factors in NO + CO reaction. With the forma-tion of N2, the unselective product N2O is also formed. Hecker andBell [51] also showed that the pretreatment of the catalyst (i.e. oxi-dation) increases N2 selectivity in NO + CO reaction. Further insighton the mechanism of NO + CO reaction was given by Oh et al. [52],who studied this reaction over alumina-supported Rh catalyst andover Rh(1 1 1) single crystal. It was shown that the NO dissociationis the key step in the reaction. In late 1980s and in early 1990s,more emphasis was placed on the reaction mechanism, order ofreaction and determining the intermediate products. Dictor ob-served isocyanate species when NO + CO was reacted over Rh/Al2O3 [53]. Cho derived a detailed mechanism of NO reduction byCO and showed that the reaction N2O + CO is an important inter-mediate reaction in overall NO + CO reaction and the intermediateN2O + CO reaction in NO + CO reaction is faster by an order of mag-nitude than the isolated N2O + CO reaction over Rh/Al2O3 [54,55].

This theoretical comparative mechanistic investigation pro-vided by Cho [54,55] was the starting point with successive re-views made by Zhdanov [56] and Cho [57]. In a subsequent work[58], it was studied extensively and theoretical calculations per-formed over monometallic and bimetallic Pt and Rh clusters [59],showed that N2O adsorbs via the terminal N atom, whereas theconfiguration with N2O bound to the metal via the oxygen is ther-modynamically unstable over noble metals. Hence, both theoreti-cal and kinetic results suggest molecular N2O adsorption andsubsequent dissociation of N2O involving active sites.

The major problem with Rh/Al2O3 catalyst is ageing and loss ofRh reducibility at 600 �C and due to the formation of a solid solu-tion of the type Al2�xRhxO3 [60]. Since oxygen is balanced to bothAl3+ and Rh3+ ion in solid solution, Rh3+ ion is difficult to reduceand loses its catalytic activity.

As discussed earlier, different configurations of adsorbed NOmolecules exhibit a different reactivity towards dissociation [17].Parallel comparisons can be made on polycrystalline catalysts thatcontain Rh on the nature of active NO species towards the disso-ciation with nitrosyl and/or dinitrosyl species that are involved inthe formation of the reaction products. In this respect, in situinfrared spectroscopy was used to examine the intermediatesand determine the influence of the oxidation state of rhodium inthree-way catalysts [61]. Different spectroscopic features havebeen observed on Rh and Pt after NO exposure related to the buildup of positively, neutral and negatively charged NO species on Rh,with neutral NO species predominating on Pt. The formation ofRh–NO+ and gem-dicarbonyl species RhI(CO)2 during the adsorp-tion of NO and the CO + NO reaction has been associated mainlywith surface Rh oxidation at low-temperature [62]. In this context,an excellent overview on the mechanistic aspects of the NO reduc-tion with CO in presence of Rh based catalysts has been provided[63].

Several studies [51–56] on deNOx processes in TWC make use ofnano metal clusters supported on Al2O3 or SiO2. Associating noblemetals with a reducible support has been investigated in detail. Forexample, the influence of Ce additive on the catalytic performancesof three-way bimetallic Pt–Rh/Al2O3 in the CO + NO reactionshowed a beneficial effect of ceria on the conversion of NO mainlyat low temperatures and had no effect at temperatures above280 �C [64]. A bi-functional mechanism involving reaction of oxy-gen from ceria with adsorbed CO molecules on the metals followedby dissociation of adsorbed NO molecules on anionic vacanciessuccessfully predicted the experimental data. At high tempera-tures, ceria is extensively reduced into Ce3+, leading to the forma-tion of anionic vacancies in the vicinity of metal particles. The

100 200 300 400 500 600 700 800 900 10000

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150 200 250 300 350 400 450 5000

20

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N2 s

elec

tivity

(%)

Temperature (0C)

% N

O c

onve

rsio

n

Temperature (0C)

Ce0.98Pt0.02O2-δ Ce0.98Rh0.02O2-δ Ce0.98Pd0.02O2-δ

Fig. 4. TPR profile of NO in NO + CO reaction over of Ce0.98Pt0.02O2�d,Ce0.98Rh0.02O2�d and Ce0.98Pd0.02O2 [74]. The N2 selectivity is shown in the inset.

2288 S. Roy et al. / Applied Energy 86 (2009) 2283–2297

strong interaction generated with noble metals alters their elec-tronic properties and adsorption properties [64].

NO molecule needs to be dissociated for its conversion to nitro-gen. NO, when dissociatively chemisorbed, produces N2 whilemolecularly adsorbed NO leads to N2O. Therefore, the NO reductionby CO should be a site-specific reaction wherein CO should bemolecularly adsorbed and NO should be dissociatively chemi-sorbed. Thus a catalyst should be designed in such a fashion thatthere should be sites for CO adsorption and NO dissociation. To-wards this objective, an entirely alternative approach towardsTWC was performed in our laboratory where noble metals in theirionic form were substituted in the reducible support like CeO2 orTiO2 by a novel solution combustion synthesis method leading toCe1�xMxO2�d or Ti1�xMxO2�d, where M = Rh, Ru, Pd, Pt, Cu, Ag, Au.The idea here was to have higher metal dispersion in the form ofions and to create oxide ion vacancies due to lower valent ionssubstituted in CeO2 or TiO2. The solid solution Ce1�xRhxO2�d, whereRh in 3+ ionic state substituted for Ce4+, showed higher rates andmore product selectivity in NO + CO reaction compared to Rh me-tal-supported CeO2. The noble metal ion acts an adsorbent for COwhile the oxide ion vacancy dissociates NO to nitrogen. In this cat-alyst, the Rh3+–O–Ce4+ interaction plays a key role towards thehigher activity of Ce1�xRhxO2�d [65]. This catalyst also showed highrates of oxidation of CO and propane.

Several studies investigated the use of Rh-free catalysts and itwas found that Pt showed good three-way catalysis properties[66]. Several metal oxides were added to Pt/SiO2 and their effectson CO oxidation and NO reduction reactions were studied. It wasobserved that Pt/CoOx/SiO2 showed remarkably good low-tempera-ture CO oxidation activity, while Pt/MnOx/SiO2 showed good perfor-mance in the NO reduction by CO [66]. Granger et al. have shownthat at low-temperature the overall conversion of NO over Pt is bet-ter than the other noble metals [67]. Pt supported on CeO2 synthe-sized by solution combustion method forms a solid solution wherePt is in ionic state and creates an oxide ion vacancy. The NO reduc-tion by CO over Ce1�xPtxO2�d showed orders of magnitude higherrate than Pt/Al2O3 catalyst, and that enhanced rate has been attrib-uted to the dissociation of NO in the oxide ion vacancy [68]. How-ever, the selectivity of N2 and N2O was not measured. Further itwas proved that the rate of the reaction increases with the increasein oxide ion vacancies by substituting La or Y in CeO2 with Pt [69].

Supported bimetallic catalysts often exhibit certain desirableproperties (e.g. improved activity, selectivity, thermal stability,and poison resistance), which are absent in each of the individualmetals. Pt–Rh bimetallic catalysts exhibit synergism [70] and showsubstantially higher catalytic activity than the physical mixture ofthe Pt and Rh catalysts. Similarly, the catalytic synergy effects be-tween cobalt phases and noble metals have been observed in Co–Pt(Pd,Rh)/Ce–Al�O catalysts [71].

Recently we have shown that the enhanced catalytic activity ofCe1�xPtx/2Rhx/2O2�d (x = 0.01) bimetal ionic catalysts compared tomono-metal ionic catalysts, Ce1�xPtxO2�d and Ce1�xRhxO2�d. The en-hanced catalytic activity of these bimetal ionic catalysts is attrib-uted to synergism due to the easy reduction of Rh3+ ion by Pt2+ ion[72]. Other than this, palladium can also be substituted in the cata-lyst. Palladium, among all other noble metals, is not only more plen-tiful, but also it has been found to be more durable at higher reactiontemperature [73]. Thus a catalytic converter utilizing a Pd-only cat-alyst can be positioned nearer the engine than a Pt–Rh catalyst thatcould potentially decrease the cold-start emission [44]. Pd-basedcatalyst exhibits a high rate of deNOx activity at low-temperatureand also shows higher product selectivity than Pt or Rh based cata-lysts [74]. Fig. 4 shows the conversion of NO and N2 selectivity overthe three catalysts, Ce1�xMxO2�d (M = Rh, Pt, Pd).

Goodman and co-workers have contributed significantly to theunderstanding of NO reduction over Pd metal [75,76]. They found

that NO reduction by CO over Pd catalyst is not only a structuresensitive reaction, but also it depends on the surface coverage ofthe gaseous molecules, surface temperature and on crystal orienta-tion [75] and concluded that the formation of the stable inactiveatomic nitrogen species plays an important role. NO reduction byCO over Pd clusters supported by MoO3 (Pd8Mo) showed NOadsorption on different oxidation state of Pd and Mo forms stableisocyanate species [77]. Recently, NO + CO reaction over Pd(111)was investigated by molecular beam method and molecular as wellas dissociative chemisorption of NO was observed over Pd(111)[78]. In many of the above studies, the metal is in the zero valentstate or move into higher valent state during reaction.

Significant observations have been provided by in situ and oper-ando spectroscopic studies that justify this conclusion. Particularly,the dependency of the activity and selectivity of palladium parti-cles in the production of N2O during the overall NO/CO reactionhas been discussed extensively [79,80]. The nitrogen speciesformed during the reaction was analyzed by determining the sur-face changes on the catalyst and this was dependent on the oxida-tion state of Pd.

Ionically substituted Pd in CeO2 or TiO2 showed an order ofmagnitude higher rate of NO reduction by CO and also higherselectivity towards N2 [81,82]. The catalysts, Ce1�xPd1�xO2�d orCe1�xTixPdyO2�d, were synthesized by solution combustion meth-od. Pd was in 2+ and more ionic than PdO. Due to lower valent io-nic substitution in CeO2, oxide ion vacancies are created. Theseoxides showed high CO oxidation activity primarily due to COadsorption on Pd2+ site and O2 dissociation in oxide ion vacancysite. With a similar analogy, the mechanism of NO reduction byCO has been found to be very much site specific. A bi-functionalmechanism based on Pd2+ being the adsorbent of NO and CO andthe oxide ion vacancies for NO dissociative chemisorption was pro-posed and this model fits the experimental data well. The ionic cat-alysts not only have the higher TOF at lower temperature but alsohigher selectivity of N2 compared to the other catalysts reported inthe literature [83–85].

The oxide support in the catalysts plays a crucial role in the cat-alytic activity. Non-reducible oxide supports like Al2O3 and SiO2

disperse the precious metals and enhance the surface area. How-ever, reducible rare-earth oxides like CeO2 can release oxygen cre-ating oxide ion vacancies. This is commonly known as ‘oxygenstorage capacity’ (OSC) and is a necessary step in the incorporationof oxygen ions from diatomic gaseous oxygen into a solid. The main

S. Roy et al. / Applied Energy 86 (2009) 2283–2297 2289

role of this is to extend the three-way window on the lean side ofstoichiometry by acting as a sink for gas-phase oxygen duringrich-to-lean transients. Further, the oxygen storage componentcan also promote oxidation of reductants, like CO and hydrocarbonsduring lean-to-rich transients. CeO2 can be reduced to Ce2O3 andoxidized back to CeO2 [86]. Attempts to stabilize the interactionsbetween the noble metals and ceria were successful since mid1990s. Ceria–zirconia mixed oxide systems have been consideredas a substitute for ceria on the basis of their greater oxygen stor-age/release. This could potentially decrease the cold-start emission,mainly by allowing the catalyst to be located in position closer toengine manifold [87,88]. It was shown that in NO reduction byCO, the turnover frequency of NO over Pd/Ce0.6Zr0.4O2 is abouttwo orders of magnitude higher than that of Pd/CeO2/Al2O3

[89,90]. Substitution of Zr in CeO2 to the extent of 30%, i.e. Ce0.7Z-r0.3O2, helps Ce4+ to reduce to 3+ state more easily than CeO2 itselfenhancing OSC. Enhancement of OSC due to Zr substitution in CeO2

is shown to be due to destabilization of oxide ion sublattices in CeO2

leading to 4 + 4 coordination around both Ce and Zr ions [91]. Thisleads to creation of long Ce–O and short Ce–O bonds and oxygenfrom the longer (weaker) Ce–O bond can be easily extracted byCO forming CO2. Thus, the creation of oxide ion vacancy is facili-tated by the substitution of Zr in CeO2. It is not yet clear if La3+

ion substitution leads to such enhancing catalytic property, thoughsome work showed that La3+ ions can be directly incorporated intothe ceria promoter to increase its oxygen storage capacity throughextrinsic defects [92]. Pd supported Ce–Ti solid solution has alsoshowed a promising TWC activity due to its enhanced reducibility.The labile lattice oxygen was attributed to play an important role inlow-temperature catalytic activity of NO reduction by CO [93].

2.3. deNOx by H2 and NH3

As H2 is present in the exhaust, it can act as a TWC reductant toNO. In fact, NO + H2 is a comparatively low-temperature reactioncompared to that of NO + CO. The main products of this reactionare N2 and H2O. However, unselective products like N2O and NH3

are also formed. The NO reduction by H2 over oxidized and reducedRh supported catalysts has been investigated but the selectivity ofN2 to N2O was low [94]. A small enhancement of selectivity wasobserved when Rh was used with Sn. No significant change was ob-served in the reaction rate and the product selectivity with otherGroup VIII metals.

NO reduction by H2 and the complete reaction mechanism overthe Pt–Rh(100) alloy single crystal surface was also studied [95]and found that Rh exhibits an excellent activity for the selectivereduction of NO towards N2, because of more NO dissociationand more probability of pairing-up of N-adatoms on Rh surface.Pt, on the other hand, is more selective in the reduction of NO to-wards NH3 and N2O under net-reducing conditions. Nieuwenhuysand co-workers used Pt as a catalyst for the reaction, but the for-mation of ammonia was significant at high concentrations of H2

[96]. Unlike the NO + CO reaction where only NO dissociates formore N2 formation, in this case, both NO and H2 dissociate. Wehave shown that over Ru, Rh, Pd, Pt ion substituted TiO2, rates ofNO reduction by H2 depends the reducibility of the catalysts. ByH2-uptake and electrochemical analysis, it was found that the cat-alyst, which has better reducibility at lower temperature or at low-er potential, shows more NO + H2 reaction activity. Among the fourcatalysts, Ti0.99Pd0.01O2�d showed better H2 adsorption/dissociationproperty and consequently better catalytic activity [97]. The reduc-tion of NO by hydrogen on Pt/Al2O3 as a function of temperaturehas been examined. N2O is formed from the NO chemisorbed onPt metal at low temperatures. However, at higher temperatures,the formation of N2 is predominant because the reaction rate forN2 formation is higher than that for N2O formation indicating that

reductive conditions and appropriate reaction temperatures areimportant factors in determining the selective formation of nitro-gen from the reduction of NO.

A detailed overview on the reduction of NO with hydrogen inpresence of supported Pt-, Rh- and Pd-based catalysts has been re-ported and the activity follows the order: Pt/Al2O3 > Pd/Al2O3 > Rh/Al2O3 [98]. The rate enhancement observed for the NO + H2 reac-tion has been mainly related to the involvement of a dissociationstep of chemisorbed NO molecules assisted by adjacent chemi-sorbed H atoms. The kinetics mechanisms confirm that Pd andRh are predominantly covered by chemisorbed NO molecules andthe lack of ammonia formation on Rh/Al2O3 during the reaction. In-deed the generation of chemisorbed H atoms may assist the disso-ciation of NO that explains significant shifts of the light-off curvestowards lower temperatures in comparison with those obtainedwhen CO is used as a reducing agent.

NO reduction by NH3 in absence of oxygen has also been inves-tigated [99] and the reaction was over Ce0.95Cu0.05O2�d, thusachieving a N2 to N2O ratio of 7.

10NH3 þ 10NO! 7N2 þ N2Oþ 9H2Oþ 4NH3 ð16Þ

2.4. Selective catalytic reduction (SCR) of NO by H2 or NH3 or ‘HC’

DeNOxing from stationary sources and mobile sources can beefficiently achieved by using the SCR process in which NO is re-duced by hydrogen or ammonia or hydrocarbons in presence of ex-cess oxygen. Perovskite-type oxides of general formula ABO3

(where A is usually a rare-earth metal coordinated by 12 oxygenatoms and B is usually a transition metal surrounded by six oxygenatoms in octahedral coordination) have been investigated for cata-lytic converter applications since the early 1970s [100]. The oxida-tion state of B cations and the structural defects can be changed bypartially substituting A and/or B with metals (A0, B0 correspond-ingly) of different oxidation states. Perovskites like LaCoO3,LaCo1�xCuxO3�d, have been suggested as potential NOx abatementcatalyst for automobile exhaust control [101,102]. The partial sub-stitution of lanthanum and manganese to form mixed oxidesLa1�xAxMn1�yByO3 and its effect on the catalytic activity was inves-tigated. La3+ has been partially replaced by A+, A2+, or A4+ ions in or-der to obtain Mn ions in various oxidation states or to create O2�

vacancies in the lattice, which can dissociate NO. La0.8K0.2MnO3

showed more selective NO reduction than LaMnO3 [103]. Mn3+

was also partially substituted by other catalytically active transi-tion metals, such as Cu2+, leading to much higher activity for theCO + NO reaction [104,105].

Incorporating noble metals into a perovskite structure can sta-bilize the noble metal against sintering, reaction with the support,or volatilization. It was found that the perovskite La0.5CexSr0.5xM-nO3 exhibited a relatively low catalytic activity but the addition of0.1 wt% Pt resulted in a catalyst showing high activity and N2 selec-tivity for the SCR [106]. This was attributed to the oxygen vacancysites of the support located next to the small Pt clusters that couldprovide the means for the formation of adsorbed NO with the Natom located on the Pt metal and the O atom on the oxygen va-cancy. In addition, adsorbed oxygen formed on the oxygen vacan-cies of the support located at the metal–support interface is moreactive in removing adsorbed hydrogen from the Pt surface [107].Rh containing LaMn1�xRhxO3 shows better TWC activity than LaM-nO3 [108]. However, the rate of the reactions and the selectivity ofproducts are not better than ceria-supported noble metal catalysts.

Recently, another perovskite-type catalyst containing Pd, La-Fe0.57Co0.38Pd0.05O3, was reported, which had high catalytic activityand high metal dispersion due to the structural responses to redoxatmospheres at 800 �C [109]. Structural analysis showed that cat-ionic Pd occupied the B-site of the ABO3 type perovskite crystal

2290 S. Roy et al. / Applied Energy 86 (2009) 2283–2297

structure and that PdO segregated out to form a metallic alloy ofPd–Co in the reductive atmosphere. This movement of Pd sup-pressed the agglomeration and growth of the metal particles andwas termed as the ‘‘self-regeneration” of Pd. The mobility of Pdin and out of the perovskite lattice requires a high energy of acti-vation and indeed this catalyst has been operated at very high tem-perature (�900 �C).

However, additional fundamental approaches [110,111] showthat these processes may occur at relative low-temperaturecorresponding to the usual temperature conditions of three-waycatalysts. Because Co is a carcinogenic, another perovskite,LaFe0.95Pd0.05O3 was synthesized [112,113], which also showedself-regeneration and is a commercial automotive catalyst in Japan.

The stabilization of well-dispersed PdOx entities in strong inter-action with the perovskite structure could be attractive for pro-moting the decomposition of N2O and may represent aninteresting practical issue for the replacement of Rh based cata-lysts. An alternative strategy than that previously developed byUenishi et al. [114] was proposed for the promotion of well-dis-persed Pd/LaCoO3 with lower Pd content and higher surface Pdconcentration at the surface. This consists of a two-step procedurewith a conventional wet impregnation of Pd precursor and appro-priate successive reductive and oxidative thermal treatments[115]. Those thermal treatments are accompanied with surfacestructural modifications that favor the re-dispersion of oxidic pal-ladium species. According to this procedure [115], a significant rateenhancement is observed which cannot be related to weak interac-tions between PdO and LaCoO3 but mainly to the occurrence of re-dispersion processes which could be governed by surface recon-structions of the perovskite structure during those successive ther-mal ageing. In that case, oxidic Pd species strongly interacting withLaCoO3 would be stabilized in unusual oxidation states. The depo-sition of palladium on reducible supports such as LaCoO3 leads tohigher activity from temperature-programed experiments in com-parison with conventional supports such as alumina. Interestingly,successive reductive and oxidative thermal treatments in the reac-tant mixture at high temperature enhance the conversion of N2Oparticularly on perovskite support and show a re-dispersion andthe stabilization of palladium species in unusual oxidation stateswhich would originate a rate enhancement in the decompositionof N2O [116].

The industrial catalysts for SCR from stationary sources arebased on TiO2 supported V2O5–WO3 and/or V2O5–MoO3 oxides[117–120] while mobile systems use zeolite based catalysts. Ana-tase form of TiO2 is the support of choice mainly because SO2 poi-

O V O Vs O

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O

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Reaction (2) O2

H2O

Fig. 5. Mechanism of SCR over va

soning does not take place on TiO2 [121,122]. Generally the activesites on the V2O5–WO3/TiO2 and V2O5–MoO3/TiO2 industrial SCRcatalysts are vanadium oxide species [121–123]. It is interestingto note that V2O5/TiO2 (anatase) is unstable where TiO2 (anatase)is a metastable polymorph that converts into thermodynamicallystable form rutile at higher temperature and pressure. V2O5 favorsthis transformation and the anatase sintering and loss of surfacearea. However, with WO3 and MoO3, both the reduction of surfacearea and transformation of anatase to rutile are lower [124,125].Further these catalysts act as inhibitors for SO2 oxidation[125,126]. SCR reaction is a redox process that occurs with a redoxor Mars–van Krevelen-type mechanism on vanadium-based cata-lysts. NH3 adsorbs on pure V2O5, on V2O5–TiO2, on V2O5/SiO2–TiO2, on V2O5–WO3/TiO2 and on V2O5–MoO3/TiO2 in two differentstrongly held species: (i) molecularly adsorbed ammonia, througha Lewis-type interaction and (ii) ammonia observed as ammoniumions, over Bronsted acidic –OH surface hydroxyl groups [127–129].15N NMR experiments showed that the adsorption on Lewis site ispredominant in V2O5–TiO2 [130]. The TiO2-anatase supports, onlyshows Lewis acidity [131], whereas ammonium ions are formedon V–OH sites. The adsorption of NO (the other SCR reactant) hasalso been extensively investigated in the literature. It has beenshown that the interaction of NO is very weak over many V2O5-based catalysts. By adsorption of NO over V2O5–TiO2, Ramis et al.observed the formation of a surface nitrosyl species, coordinatedto Ti4+ sites [132]. However, NO does not adsorb on an ammonia-covered surface because NH3 has greater basicity and blocks theTi4+ adsorption sites. This data is in good agreement with the ki-netic observations on V2O5-based catalysts where zero and first or-der kinetics with respect to NH3 and NO, respectively [133–136] isobserved. In vanadia-based catalysts, NH3 adsorbs mainly on theBronsted acid sites while insignificant NO adsorption is observed.Thus an Eley–Rideal mechanism has been suggested for SCR. Tak-agi et al. proposed one of the first reaction schemes for SCR overV2O5-based catalysts [137]. Subsequently, Inomata and his groupproposed a popular reaction mechanism [138–140] in which thereaction of NHþ4 species occurs with gaseous NO through an ‘acti-vated complex’ (Fig. 5). According to Janssen et al., the polyvana-date species, O@VAOAV@O adsorbs ammonia over and leads tothe intermediate species V–ONH2 [141]. In 1990s, Ramis et al. pro-posed the reaction pathway [132] in which ammonia is adsorbedover a Lewis acid site that activates ammonia to an amide NH2 spe-cies resulting in catalyst reduction. This amide species then reactswith gas-phase NO to nitrosamide as an intermediate, whichdecomposes to nitrogen and water. The reduced catalyst sites are

O V O Vs O

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OHOH N

N HH

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nadium oxide catalysts [138].

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Fig. 6. NH3-SCR over Ti0.9M0.1O2�d (M = Cr, Mn, Fe, Co, Cu) [158].

S. Roy et al. / Applied Energy 86 (2009) 2283–2297 2291

then regenerated by gas-phase oxygen. This is the first mechanismproposed for vanadia-based catalyst that suggested activation ofammonia on Lewis acid cites.

This concept was popularized and proven by the theoreticaldensity-functional theory (DFT) calculations [142–144]. A recentdensity-functional theoretical calculation shows that on V2O9H8

cluster surface, SCR reaction is initiated more favorably by theammonia activation on Brønsted acidic V–OH site, where NH4+

ion is formed. Then with interaction with gaseous NO, NH2NO ad-duct is formed, which decomposes into N2 and H2O [145].

Despite its problems like accounting for band-gaps and long-range forces [146], it is a versatile tool that can be used for verify-ing the kinetic mechanisms. We discuss a couple of exampleswhere DFT has been successfully used for determining the mecha-nisms in SCR. Among the various catalysts for SCR, Ag/Al2O3 is oneof the superior catalysts. In the SCR of NOx, the adsorbed NO�3 spe-cies on the Al2O3 and the Ag/Al2O3 play an important role in thekinetics. The adsorbed NO�3 could be bonded to metal cations onthe surface via the following adsorption structure: bridging, biden-tate and monodentate [147]. The interpretations of the infraredspectra proposed by researchers can be different. For example,the band observed around 1300 cm�1 has been attributed to mono-dentate nitrate [148] bidentate nitrate [147] or solvated nitrate[149]. Thus the assignment and structures of the nitrates specieson Ag/Al2O3 and Al2O3 were still not clear. The vibrational modesof nitrate species were simulated by DFT and analyzed by Gaussian98 and Hyperchem 7.0 program. Based on the density-functionaltheory calculations, the band at 1304 cm�1 was assigned to an iso-lated bidentate nitrates species, whose formation could involve lat-tice oxygen of Al2O3 [150]. A possible mechanism for NOx reductionby C2H5OH over Ag/Al2O3 was proposed to be similar to that ofC3H6 [151] However, this mechanism cannot explain whyC2H5OH has a higher efficiency for the SCR of NOx over Ag/Al2O3

than hydrocarbons such as C3H6. Thus a different mechanismbased on surface enolic species on Ag/Al2O3 was proposed. DFT cal-culations were used to confirm the structure of adsorbed enolicspecies on Ag/Al2O3. Thirty-two models of adsorbed species werecalculated and different kinds of adsorbed species and the interac-tion of the surface with the adsorbed species were considered.However, DFT calculations were in good agreement with theexperimental value for only the enolic species confirming themechanism of NOx reduction with ethanol in presence of Ag/Al2O3 [152] Thus, DFT can be successfully used to test and confirmmechanisms of NOx reduction.

The three major aspects of SCR reaction are to reduce the reac-tion temperature, enhance the SCR window and increase the prod-uct selectivity. The V2O5-based catalysts do operate atcomparatively higher temperature. Extensive studies on SCR ofNO have been carried out over other first row transition metaloxide than vanadium. Mn oxides based catalysts are becomingpopular for SCR reactions because of its low-temperature activity.Kapteijn and his group did extensive work on Mn-based catalystsfor SCR process [153–155]. In unsupported Mn-oxide studies itwas found that SCR activity in the following order: MnO2 > M-n5O8 > Mn2O3 > Mn3O4. In alumina-supported manganese oxide,they found two Lewis-type coordinatively unsaturated Mn3+ ionsas the active species. A comparatively recent work based on Mn-oxide-supported hombicat TiO2 has shown NH3 adsorption ontothe Lewis acid sites of Mn4+, and has given a conclusive mechanismfor unselective N2O formation [156,157]. Though the Mn-basedcatalysts operate at lower temperature, the product selectivity ispoor. In our work on first row transition metal substituted anataseTiO2, SCR catalytic activity of the materials was tested.Ti0.9Mn0.1O2�d showed low-temperature activity whereasTi0.9Fe0.1O2�d showed the widest SCR window and better productselectivity (Fig. 6). The active phase for the catalytic activity isthe substituted anatase phase, not the illmenite phase. NH3-TPDstudies showed that Ti0.9Mn0.1O2�d has the highest Bronsted acid-ity and Ti0.9Fe0.1O2�d has the highest Lewis acidity among all othercatalysts [158]. Therefore, a solid solution of Ti0.9Mn0.05Fe0.05O2�d

was synthesized and found to be the best catalyst in terms oflow-temperature reactivity and product selectivity. Several papers[159,160] have investigated the kinetics over conventional V2O5–WO3/TiO2 catalysts and shown that NH3 is stored in the Ti or Wsurface and when the feed NH3 concentration is lowered, thestored ammonia reacts with the NO to form N2.

Cu2+-exchanged ZSM-5 zeolites are active catalysts for thereduction of nitric oxide with ammonia in the presence of oxygen.It was shown that NO reduction by NH3 over Cu(II) ion-exchangedY-type zeolites [Cu(II)NaY] followed Langmuir–Hinshelwoodkinetics [161]. Many studies suggest that the ammonia SCR reac-tion takes place by the strong adsorption of ammonia on the cata-lyst, and that the molecularly adsorbed ammonia species interactswith NO from the gas phase or from a weakly adsorbed statethrough an Eley–Rideal type mechanism [162]. It has been re-ported that Fe2O3 [163], Fe containing mixed oxides [164] andFe-exchanged materials [165], and in particular Fe–ZSM-5 [166–169] show significant SCR activity.

A comparison of different metal-exchanged zeolites showed ironbeta as having good activity, nitrogen selectivity and ageing charac-teristics [170], however studies of low-temperature SCR of NO withammonia over zeolite based catalysts are few [171]. The reactionshows considerable sensitivity to the nature of the oxide supportand a comparative study of iron oxide on different oxide supportsincluding MgAl2O4, SiO2, TiO2 and ZrO2 has been performed [162].Besides using zeolites ZSM-5 and zeolite beta (BEA) in these reac-tions, different synthetic and natural zeolitic materials such asmordenite (MOR), heulandite–clinoptilolite (HEU), ferrierite (FER)and chabazite (CHA) have also been investigated [172]. Among

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them, Fe–mordenite and Fe–clinoptilolite have shown catalyticproperties for the SCR of NO with ammonia, similar to that of Fe–ZSM-5 [172]. Natural zeolites [173] have also been used becausethey have a good combination of their crystalline structures and ex-hibit interesting physicochemical properties with various metaloxy-hydroxide phases that are naturally embedded inside theirpores [174]. However, potential commercialization of natural zeo-lites as catalysts is not feasible due to the cost of homogenisationand purification of the zeolite-bearing tuffs [175].

Unreacted ammonia can escape in the exhaust (called slip) andthus the emission controls also restrict ammonia in the exhaust.Further, the transportation and storage of ammonia is not verycost-effective. This led to research on the NOx reduction by hydro-carbons as reducing agents.

The first catalyst that was found to have a good hydrocarbon NOx

reduction capability in oxygen rich conditions was Cu/ZSM-5[26,34]. Subsequently, this reaction has been extensively studied[176,177] over different cation exchanged zeolites. It has also beenextensively studied over different base oxides and these oxides pro-moted by Co, Ni, etc. as supports [178]. However, many of these cat-alysts are deactivated in presence of water vapor [179].Subsequently, another system – Ag/c-Al2O3 catalyst was found tobe active for HC-SCR [180]. The optimal silver content was reportedto be 2 wt% [181,182]. Thus a catalytic system based on silver sup-ported alumina was developed by Klingstedt et al. [183] andshowed the optimum silver content, the correct choice of the alu-mina support, the ratio of hydrocarbon and NOx on the high perfor-mance of the HC-SCR deNOx [184]. While a maximum conversion of90% was observed at 450 �C, improved conversion could be obtainedif the catalyst was divided into a few layers [185]. However, one ofthe drawbacks of this catalyst is its poor activity below 300 �C andlarge formation of CO. But the presence of a small amount of hydro-gen in the feed improved the catalytic activity significantly[186,187]. This has given added impetus to this area of research[188,189]. Hydrogen promotes the NOx reduction over Ag/c-Al2O3

catalysts when using a range of lower alkanes and alkenes and high-er alkanes [190] as reductants. Silver has also been used in zeolitesfor HC-SCR of NOx with various reducing agents [191–193].

SOx is detrimental to activity at low temperatures but that athigher temperatures the effect of SOx is minimal or may even en-hance NOx conversion. Meunier and Ross [194] found that for a1.2 wt% Ag/c-Al2O3 catalyst the activity decreased rapidly in thepresence of 100 ppm SO2 in the reaction feed and found that thesulfation of Ag rather than c-Al2O3 was primarily responsible forthe deactivation. Abe et al. [195] noted that silver sulfate decom-poses at a lower temperature (427 �C) than aluminium sulfate(727 �C) and explained why catalysts tested in the literature main-tain their activity in the presence of SOx at higher temperatures(>427 �C) but tend to be inactive at temperatures lower than this.

The effect of SOx is also influenced by the nature of the reduc-tant. For example, SOx enhanced the activity of the 5 wt% Ag/c-Al2O3 catalyst [196] and a pre-sulfated Pt/Al2O3 catalyst [197]when propene was used as a reductant but caused severe deactiva-tion when propane was used. This is because the reaction occursmainly on the support for propane and is hindered by stronglybound sulfates on the support. However, the reaction takes placepredominantly on the Pt sites which do not adsorb sulfur stronglywhen using propene as a reductant. Reactive R–SOx species seem tobe key intermediates in the SCR reaction with propene over Ag/Al2O3 catalyst [198]. The effect of SOx on Ag/c-Al2O3 catalyst forthe NOx-SCR with hydrogen and hydrocarbon was investigated[199] and that the effect of ageing was a function of the gas mixand temperature of ageing. The effect of sulfur on the deactivationwas dependent on the temperature and was explained by theactivity of the catalyst for the oxidation of SO2 to SO3 and the rel-ative stability of silver and aluminium sulfates.

For mobile SCR application urea can be used instead of NH3

[200]. Urea is usually applied as an aqueous solution in urea-SCR.When this solution is atomized into the hot exhaust gas stream,water evaporates from the droplets, thus leading to solid or moltenurea [201]. Urea then subsequently decomposes to ammonia andisocyanic acid. The rate of HNCO hydrolysis is much higher thanthat of the SCR reaction and thus ammonia is the active reducingagent even when urea is used [201]. Due to compactness of the de-NOx systems, only short residence times are realized and this leadsto considerable mixing problems for NOx with the reducing agent.

2.5. Other methods

In addition to the above methods, a few other techniques areused for NOx reduction. The simplest approach, of course, is tomodify the operational conditions such that NOx formation is de-creased. Thus furnaces with low production of NOx are devisedsuch that the NOx burners minimize thermal formation of NOx byreducing combustion temperatures and controlling flame stoichi-ometry. However, this increases formation of CO and also has prob-lems of corrosion and slags because of a local reducingenvironment [202]. Another method is to inject methanol into fluegases so that NO is converted to NO2. This is subsequently removedusing limestone scrubber. In the 1990s, two techniques namely theNOx storage and reduction (NSR) and selective NOx recirculation(SNR) were developed by the car industry. In both techniques,NOx is adsorbed on a sorbent but the subsequent decompositionprocedure is different. In general, this material should have a highNOx trapping capacity, a high selectivity and a high resistance toSO2 poisoning. Finally, we discuss another method, namely thenon-catalytic selective catalytic reduction.

2.5.1. NSR (NOx storage and reduction)The reduction of NOx takes place by a two-stage operation. Dur-

ing the fuel-lean stage, the NOx is trapped on an adsorbent in theform of a nitrate. Then, the engine is switched to a fuel-rich condi-tion where the hydrocarbons, hydrogen and CO react with the ni-trate to yield nitrogen, water and carbon dioxide.

During the lean-burn stage in presence of excess oxygen, NOx isoxidized by oxygen to NO2 over the platinum site and stored on thebarium oxide as barium nitrate. In this stage, hydrocarbons, H2,and CO are readily oxidized into water and carbon dioxide. Whenthe engine is switched to operation with the normal air–fuel mix-ture, HC, H2, and CO do not oxidize. So these reductants react withthe NO�3 stored in the catalyst and forms nitrogen, water, and car-bon dioxide. Adopting this mode of operation, NSR catalysts weredeveloped and commercialized by Matsumoto from Toyota in1994 [203–205]. NOx storage materials consist of alkaline-earthmetals or alkaline metals and noble metals such as platinum andrhodium dispersed on the support, e.g. Pt–Ba/Al2O3. The supportof NSR catalysts has been changed from Al2O3 to CeO2 to CeO2–ZrO2. However, formation of BaAl2O4 and BaCeO3 has been ob-served around 850 �C [206,207].

A study on Pt/Ba/Al2O3 catalyst for a NSR reaction with H2,showed that at relatively low-temperature, the NOx storage capac-ity is sufficient, but that the rate of NOx release and reduction (NOx

regeneration) is slow and insufficient. Further, with a large amountof H2 injection, a large amount of NH3 was detected as product[208,209]. One other comparative study between Pt/Ba/Al2O3 andPt/K/Al2O3 catalysts for NSR reaction, showed that the under flowconditions during a complete NSR cycle, barium containing cata-lyst shows better NSR performance. However, no ammonia wasproduced [210] for the reaction over Pt/K/Al2O3. A main problemof the NSR catalysts is sulfur poisoning. Sulfur oxides (SOx) in ex-haust gas react on the catalyst in the same way as NOx. Therecan be two types of sulfur poisoning [204] wherein SO2 reacts with

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alumina to form aluminium sulfate that hinders reactivity andwhere SO2 competes with the NOx for BaO to form barium sulfate(BaSO4). The presence of TiO2 suppresses the sulfate formationwithout reducing the NOx storage capacity significantly [211].

2.5.2. Selective NOx recirculation (SNR)SNR technique deals with the treatment of exhaust gases from a

fuel-lean or diesel and was developed by Daimler–Chrysler in1994. In this method [212], two adsorbers are arranged in paralleland operate in the adsorption and desorption modes alternately.The principle of the concept consists in the concentration andrecirculation of NOx into the combustion chamber of the enginewhere they are decomposed thermally [213]. Because thermaldecomposition depends on NOx concentration, a high performanceof the adsorbent material is the key for the success of the SNR con-cept [202,214].

It is implied from the information presented above that theamount of sorbed NOx strongly depends on the sorption mecha-nism as well as on the physical properties of sorbents and sorbates.Thus, for future practical applications, the sorbents have to retaintheir adsorption capability for prolonged periods in presence ofwater, CO2 and SO2.

2.5.3. Selective non-catalytic reduction (SNCR)NOx reduction by NH3 or urea in presence of excess oxygen

without catalysts is also practiced and is called selective non-cata-lytic reduction (SNCR). Processes using ammonia, urea, or cyanuricacid for the SNCR of nitric oxides from engine exhaust gases werecompared. Ammonia, urea, and cyanuric acid were found to bemost effective at low, intermediate, and high oxygen concentra-tions, respectively [215]. The reaction of NO and ammonia only oc-curs after a minimum temperature, while ammonia decomposes toNO at higher temperatures. Thus the reaction needs a specific tem-perature window to be efficient. Further, the reaction also needssufficient reaction time in that temperature window and need tobe sufficiently mixed to avoid ammonia slip [216]. It has been re-ported that injection of some additives together with the reducingagents in SNCR processes can lower and widen the optimum reac-tion temperature window for NOx reduction [217]. Though selec-tive non-catalytic reduction can achieve the same efficiency ofabout 90 % as SCR without a catalyst, practical constraints of tem-perature, time, and mixing often lead to worse results in practice.One possible way of using SNCR, at lower molar ratios and mini-mum NH3 slip, may be the combination of SNCR processes with aback-end NO oxidation process [216].

2.5.4. Ozone injectionOzone injection is an attractive technology that is more energy

efficient than plasma and electron beam processes directly appliedto the exhaust gas for the oxidization of NO [218,219]. In this tech-nique, small amounts of oxygen or air can be discharged to produceozone, which can then be injected into the flue gas. The key step inthis method is the oxidization property of NO in flue gas. Both kinet-ics and mixing play a key role in the application of this technologyand a good understanding the ozone�NOx reaction jet is fundamen-tal to the design and optimization of this technology [220]. The effec-tiveness of ozone injection on the selective catalytic reduction ofnitrogen oxides has been investigated [221]. Nitric oxide in the ex-haust gas is initially oxidized to nitrogen dioxide by ozone, and thenthe exhaust gas containing the mixture of NO and NO2 is reduced tonitrogen in a catalytic reactor. The ozone injection method wasdetermined to be very efficient for the oxidation of NO to NO2 in awide range of temperatures, and the increase in the content of NO2

by the ozone injection remarkably improved the performance ofthe catalytic reactor for the reduction to nitrogen [222].

3. Conclusions and future perspectives

There has been a significant research in the area of NOx abate-ment recently. This review initially briefly discussed the differentNOx sources, the different legislations existing in various countries.The synthesis of various catalysts and the reaction mechanisms forNOx removal by NO decomposition, NO reduction by CO or H2 orNH3 or hydrocarbons was discussed.

Currently, two main methods for the removal of NOx from emis-sion gases are employed namely the TWC developed for mobilesources that use gasoline and the SCR, which is applied for station-ary sources such as power plants and for lean-burn engines. Theuse of TWC is a much more established technology for the catalyticreduction of NOx produced by gasoline engines than compared tothe catalysts for vehicles with diesel and lean-burn gasoline en-gines. The industrial catalysts for SCR from stationary sources arebased on TiO2 supported V2O5–WO3 and/or V2O5–MoO3 oxideswhile mobile systems for SCR use zeolite based catalysts.

The TWC has three functions, namely the oxidation of CO, theoxidation of unburnt hydrocarbons (HC’s) and the reduction ofNOx to nitrogen. They work well during operation at or close tostoichiometric conditions. However, when the engine runs atfuel-rich conditions, the overall exhaust gas composition is reduc-ing in nature, and it is difficult to carry out oxidation reactions onthe catalyst surface. TWC’s have been developed to incorporateceria that stores oxygen during leaner periods of the operating cy-cle, and releases oxygen during richer periods of the operating cy-cle. However, ceria, when doped with metal such as Pd losessurface area at high temperatures of 800 �C. Thus mixed oxidesof ceria–zirconia are often used instead of ceria as the oxygen stor-age component. Ceria itself is a rare-earth metal with restrictedsuppliers and ceria–zirconia is a relatively expensive materialwhen available commercially, and it would be desirable to find amaterial having at least as good oxygen storage performance asceria–zirconia, but utilizing less expensive materials.

Although selective catalytic reduction by ammonia is currentlythe most widespread method for the clean up of flue gas from sta-tionary sources, many problems still exist. The advantages of NH3

as the reducing agent are high selectivities toward the reactionwith NO in the presence of O2 and the promoting effect of oxygenon the reaction kinetics of NO with NH3. The most significant tech-nical parameters are the positioning of catalyst from the source ofgas and the design of ammonia injection. Typically, during a com-mercial SCR process over V2O5–WO3–TiO2 systems, stoichiometriccontrol of the ammonia must be maintained to avoid emissions ofunreacted ammonia. In addition, there are difficulties of transportand storage of ammonia. It would be necessary to develop new cat-alysts operating with low or high temperature; and it would bedesirable to substitute ammonia by another reductant because ofthe dangers of storage, leakage, and transport of liquid ammonia.

For mobile applications, the problems are more complicated.The first generation of commercial SCR catalysts for mobile applica-tions were monoliths made of anatase TiO2 supporting V2O5 orWO3, similar to the vanadium-based catalysts used for stationarySCR applications. However, the stringent legislation on NOx emis-sions, the necessity of catalysts to be active up to higher tempera-tures and the toxicity of vanadium have driven the research focustowards hydrogen- and metal-exchanged zeolites. Since the discov-ery of HC-SCR technology, various types of catalysts have been re-ported, such as ion-exchanged zeolites, supported preciousmetals, and metal oxide-based catalysts. Ion-exchanged zeolitesshowed very high performance on HC-SCR. However, they areunstable in hydrothermal conditions that cause deactivation of cat-alysts. Supported precious metal catalysts, especially supported Ptcatalysts, show high stability and high tolerance to sulfur oxides

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(SOx) and water vapor. They show very high activity at lower tem-peratures, but the non-selectivity to nitrogen and a narrow temper-ature range for NOx reduction are problems. Metal oxide-basedcatalysts show high stability and moderate tolerance to SOx andwater vapor and in this context, Ag/c-Al2O3 has been developed.However, one of the drawbacks of silver supported alumina catalystis its poor activity below 300 �C and large formation of CO. Thus ithas been combined with a suitable catalyst to oxidize the formedCO and unburned hydrocarbons and thus an dual-catalytic systemhas been developed wherein the platinum catalyst is placed afterthe Ag/alumina catalyst. However, the selectivity to nitrogen of thisdual system is significantly lower and thus further investigationsare needed to obtain a good dual system without loss of activityand selectivity. In this context, the self-regenerating Pd perovskiteoxides seem promising. However, the activation of oxygen in theperovskite support is not significant and needs to be improved.

An alternative to the catalytic approach is based on NOx adsorp-tion. NOx is stored in the catalyst under lean conditions and is regen-erated for a short period under fuel-rich conditions. However, thistechnique fails when fuels contain high levels of sulfur. Overcomingthe problems of preserving adsorbents catalytic activities in pres-ence of water, CO2, and SO2 and the selectivity of noble metals toN2O and NH3 formation are some of the keys for the future practicalapplication of sorbing catalytic material in NOx depollution.

New approaches to develop novel catalysts may also be investi-gated. One can modify the surface of an oxide by substituting afraction of metal atoms with another metal. The presence of an-other metal disrupts the bonding in the oxide. This leads to twophenomena: the oxygen atoms whose proximity is close to the me-tal become chemically reactive and aid oxidation while the metalalso becomes reactive and is able to adsorb/activate oxygen. Sev-eral methods have been developed for synthesizing these typesof compounds. However, one can further the OSC if one is able tosubstitute lower valent metal ions in the reducible matrix. Thus,there would be two distinct sites for reducing and oxidizing mole-cules in the ionic catalysts unlike metal surfaces. Since the adsorp-tion sites are next to each other, the electron transfer fromreducing molecules to oxygen would be facilitated by the latticeleading to very high catalytic activity. In such a compound, the me-tal is fully dispersed as ions and they cannot sinter due to ionicrepulsion. Thus one could have a compound in which higher metaldispersions in the form of ions are observed and oxide ion vacan-cies are created due to lower valent ions substituted in CeO2 orTiO2. These catalysts may yield high rates of reaction for SCR.

Several challenges have to be faced when trying to solve theproblem of NOx pollution with a catalytic system: selectivity, oper-ational temperature, and poisoning. From the studies highlightedin this article, it is clear that more work needs to be carried outto understand the mechanism of NO decomposition and reductionunder various operating conditions.

References

[1] Armor JN. Environmental catalysis. Appl Catal B: Environ 1992;1:221–56.[2] Centi G, Ciambelli P, Perathoner S, Russo P. Environmental catalysis: trends

and outlook. Catal Today 2002;75:3–15.[3] Klingstedt F, Arve K, Eranen K, Murzin DU. Toward improved catalytic low

temperature NOx removal in diesel powered vehicles. Acc Chem Res2006;39:273–82.

[4] Helms GT, Vitas JB, Nikbakht PN. Regulatory options under the US clean airact: the federal view. Water Air Soil Pollut 1993;67:207–16.

[5] Bosch H, Janssen F. Formation and control of nitrogen oxides. Catal Today1988;2:369–79.

[6] Garin F. Mechanism of NOx decomposition. Appl Catal A: Gen2001;222:183–219.

[7] Crutzen PJ, Bruh C. Catalysis by NOx as the main cause of the spring to fallstratosphere ozone decline in the northern hemisphere. J Phys Chem A2001;105:1579–82.

[8] Ravishankara AR. Introduction: atmospheric chemistry long-term issues.Chem Rev 2003;103:4505–8.

[9] Fritz A, Pitchon V. The current state of research on automotive lean NOx

catalysis. Appl Catal B: Environ 1997;13:1–25.[10] Snyder S, Bredt D. Les fonctions biologiques du monoxyde d’azote. Pour La Sci

1992;177:70–7.[11] Chiron M. Air pollution by automobile exhaust and public health. Stud Surf

Sci Catal 1987;30:1–10.[12] Mauzerall DL, Sultan B, Kim N, Bradford DF. NOx emissions from large point

sources: variability in ozone production, resulting health damages andeconomic costs. Atmos Environ 2005;39:2851–66.

[13] Erisman JW, Greenfelt P, Sutton M. The European perspective on nitrogenemission and deposition. Environ Int 2003;29:311–25.

[14] <http://www.dieselnet.com/standards/in>.[15] Broden G, Rhodin TN, Bruker C, Benbow R, Hurych Z. Synchrotron radiation

study of chemisorptive bonding of CO on transition metals – polarizationeffect on Ir(100). Surf Sci 1976;59:593–611.

[16] Egawa C, Onawa K, Iwai H, Oki S. Interaction of CO and NO with Fe thin filmsgrown on Rh(100) surface. Surf Sci 2004;557:31–40.

[17] Brown WA, King DA. NO chemisorption and reactions on metal surfaces: anew perspective. J Phys Chem B 2000;104:2578–95.

[18] Whitman LJ, Ho W. The kinetics and mechanisms of alkali metal-promoteddissociation: a time resolved study of NO adsorption and reaction onpotassium-precovered Rh(100). J Chem Phys 1988;89:7621–46.

[19] Ho W. Time resolved electron energy loss spectroscopy of surface kinetics.Electron Spectrosc Relat Phenom 1987;45:1–18.

[20] Palermo A, Lambert RM, Harkness IR, Yentekaksi IV, Marina O, Vayenas CG.Electrochemical promotion by Na of the platinum-catalyzed reactionbetween CO and NO. J Catal 1996;161:471–9.

[21] Goddard PJ, West J, Lambert RM. Adsorption, coadsorption, and reactivity ofsodium and nitric oxide on Ag(111). Surf Sci 1978;71:447–61.

[22] Winter ERS. The catalytic decomposition of nitric oxide by metallic oxides. JCatal 1971;22:158–70.

[23] Oh SH, Eichel CC. Effects of cerium addition on CO oxidation kinetics overalumina-supported rhodium catalysts. J Catal 1988;112:543–55.

[24] Boreskov GK. Forms of oxygen bonds on the surface of oxidation catalysts.Discuss Faraday Soc 1968;41:263–9.

[25] Park PW, Kil JK, Kung HH, Kung MC. NO decomposition over sodium-promoted cobalt oxide. Catal Today 1998;42:51–60.

[26] Iwamoto M, Hamada H. Removal of nitrogen monoxide from exhaust gasesthrough novel catalytic processes. Catal Today 1991;10:57–61.

[27] Fang SM, White JM. Chemisorption of nitric oxide on platinized titania. J Catal1983;83:1–8.

[28] Pande NK, Bell AT. A study of the interactions of NO with Rh/TiO2 and TiO2-promoted Rh/TiO2. J Catal 1986;97:137–49.

[29] Chin AA, Bell AT. Kinetics of nitric oxide decomposition on silica-supportedrhodium. J Phys Chem 1983;87:3700–6.

[30] Ward TR, Alemany P, Hoffman R. Adhesion of rhodium, palladium, andplatinum to alumina and the reduction of nitric oxide on the resultingsurfaces: a theoretical analysis. J Phys Chem 1993;97:7691–9.

[31] Iwamoto M, Yokoo S, Sakai K, Kagawa S. The carbonylation of alcoholscatalyzed by Cu (I) carbonyl. J Chem Soc Faraday Trans 1981;77:1629–36.

[32] Iwamoto M, Furukawa H, Mine Y, Uemura F, Mikuriya S, Kagawa S. Copper(II)ion-exchanged ZSM-5 zeolites as highly active catalysts for direct andcontinuous decomposition of nitrogen monoxide. J Chem Soc, Chem Commun1986;11:1272–3.

[33] Iwamoto M, Yahiro H, Mine Y, Kagawa S. Excessively copper ion-exchangedZSM-5 zeolites as highly active catalysts for direct decomposition of nitrogenmonoxide. Chem Lett 1989;19:213–7.

[34] Iwamoto M, Yahiro H, Tanda K, Mizuno N, Mine Y, Kagawa S. Removal ofnitrogen monoxide through a novel catalytic process. 1. Decomposition onexcessively copper–ion-exchanged ZSM-5 zeolites. J Phys Chem1991;95:3727–30.

[35] Giamello E, Murphy D, Magnacca G, Morterra C, Shioya Y, Nomura T, et al. Theinteraction of NO with copper ions in ZSM5: an EPR and IR investigation. JCatal 1992;136:510–20.

[36] Cheung T, Bhargava SK, Hobday M, Foger K. Adsorption of NO on Cuexchanged zeolites, an FTIR study: effects of Cu levels, NO pressure, andcatalyst pretreatment. J Catal 1996;158:301–10.

[37] Schneider WF, Hass KC, Ramprasad R, Adams JB. First-Principles analysis ofelementary steps in the catalytic decomposition of NO by Cu-exchangedzeolites. J Phys Chem B 1997;101:4353–7.

[38] Zakharov II, Ismagilov ZR, Ruzankin SP, Anufrienko VF, Yashnik SA, ZakharovaOI. Density functional theory molecular cluster study of copper interactionwith nitric oxide dimer in Cu–ZSM-5 catalysts. J Phys Chem C2007;111:3080–9.

[39] Zhang J, Ayusawa T, Minagawa M, Kinugawa K, Yamashita H, Matsuoka M,et al. Investigations of TiO2 photocatalysts for the decomposition of NO in theflow system: the role of pretreatment and reaction conditions in thephotocatalytic efficiency. J Catal 2001;198:1–8.

[40] Lim TH, Jeong SM, Kim SD, Gyenis J. Photocatalytic decomposition of NO byTiO2 particles. J Photochem Photobio A: Chem 2000;134:209–17.

[41] Bowering N, Walker GS, Harrison PG. Photocatalytic decomposition andreduction reactions of nitric oxide over Degussa P25. Appl Catal B: Environ2006;62:208–16.

S. Roy et al. / Applied Energy 86 (2009) 2283–2297 2295

[42] Roy S, Hegde MS, Ravishankar N, Madras G. Creation of redox adsorption sitesby Pd2+ ion substitution in nanoTiO2 for high photocatalytic activity of COoxidation, NO reduction, and NO decomposition. J Phys Chem C2007;111:8153–60.

[43] Shelef M. Nitric oxide: surface reactions and removal from auto exhaust. CatalRev Sci Eng 1975;11:1–40.

[44] Trovarelli A. Catalysis by ceria and related materials. Imperial College Press;2002.

[45] Gross H. US patent 3,370,914.[46] Kummer JT. Catalysts for automobile emission control. Prog Energy Combus

1980;6:177–99.[47] Gandhi HS, Graham GW, McCabe RW. Automotive exhaust catalysis. J Catal

2003;216:433–42.[48] Taylor KC, Schlatter JC. Selective reduction of nitric oxide over noble metals. J

Catal 1980;63:53–71.[49] Van Zon JBAD, Koningsberger DC, Van’t Blick HFJ, Sayers DE. An EXAFS study

of the structure of the metal–support interface in highly dispersed Rh/Al2O3

catalysts. J Chem Phys 1985;82:5742–55.[50] Schlatter JC, Taylor KC, Sinkevltch RM. General Motors Research Pub. GMR-

3112; 1979.[51] Hecker WC, Bell AT. Reduction of NO by CO over silica-supported rhodium:

infrared and kinetic studies. J Catal 1983;84:200–15.[52] Oh SH, Fisher GB, Carpenter JE, Goodman DW. Comparative kinetic studies of

CO–O2 and CO–NO reactions over single crystal and supported rhodiumcatalysts. J Catal 1986;100:360–76.

[53] Dictor R. An infrared study of the behavior of CO, NO, and CO + NO over Rh/Al2O3 catalysts. J Catal 1988;109:89–99.

[54] Cho BK. Mechanistic importance of intermediate N2O + CO reaction inoverall NO + CO reaction system: I. Kinetic analysis. J Catal 1992;138:255–66.

[55] Cho BK. Mechanistic importance of intermediate N2O + CO reaction in overallNO + CO reaction system. II. Further analysis and experimental observations. JCatal 1994;148:697–708.

[56] Zhdanov VP. Does the N2O–CO subreaction play an important role in the NO–CO reaction on Rh? J Catal 1996;162:147–9.

[57] Cho BK. Emphasizing the Mechanistic Importance of Intermediate N2O + COreaction in overall NO + CO reaction system. J Catal 1996;162:149–51.

[58] Granger P, Malfoy P, Leclercq L, Leclercq G. Kinetics of the CO + N2O reactionover noble metals: II – Rh/Al2O3 and Pt–Rh/Al2O3. J Catal 2004;223:142–51.

[59] Granger P, Malfoy P, Esteves P, Paul J-F, Leclercq L, Leclercq G. Europacat IV,Book of abstracts, Rimini; 1999.

[60] Duprez D, Delahay G, Abderrahim H, Grimblot J. Characterization of Rh/Al2O3

catalysts by gas adsorption and X-ray photoelectron spectroscopy. J ChimPhys 1986;83:465–77.

[61] Dujardin C, Mamede A-S, Payen E, Sombret B, Huvenne J-P, Granger P.Influence of the oxidation state of rhodium in three-way catalysts on theircatalytic performances: an in situ FTIR and MS study. Top Catal 2004;30–31:347–52.

[62] Granger P, Praliaud H, Billy J, Leclercq L, Leclercq G. An infrared investigationof the transformation of NO over supported Pt and Rh based three-waycatalysts. Surf Interf Anal 2002;34:92–6.

[63] Granger P, Dujardin C, Paul JF, Leclercq G. An overview of kinetic andspectroscopic investigations of three-way catalysts: Mechanistic aspectsof the CO+NO and the CO + N2O reactions. J Mol Catal 2005;228:241–53.

[64] Granger P, Delannoy L, Lecomte JJ, Dathy C, Praliaud H, Leclercq L, et al.Kinetics of the CO + NO reaction over bimetallic platinum–rhodium onalumina: effect of ceria incorporation into noble metals. J Catal2002;207:202–12.

[65] Gayen A, Priolkar KR, Sarode PR, Jayaram V, Hegde MS, Subbana GN, et al.Ce1�xRhxO2�d solid solution formation in combustion-synthesized Rh/CeO2

catalyst studied by XRD, TEM, XPS, and EXAFS. Chem Mater2004;16:2317–28.

[66] Mergler YZ, Aalst AV, Delft JV, Nieuwenhuys BE. Promoted Pt catalysts forautomotive pollution control: characterization of Pt/SiO2, Pt/CoOx/SiO2, andPt/MnOx/SiO2 catalysts. J Catal 1996;161:310–8.

[67] Granger P, Dathy C, Lecomte JJ, Leclercq L, Prignet M, Mabilon G, et al. Kineticsof the NO and CO reaction over platinum catalysts: I. Influence of the support.J Catal 1996;173:304–14.

[68] Bera P, Patil KC, Jayaram V, Subbana GN, Hegde MS. Ionic dispersion of Pt andPd on CeO2 by combustion method: effect of metal–ceria interaction oncatalytic activities for NO reduction and CO and hydrocarbon oxidation. JCatal 2000;196:293–301.

[69] Gayen A, Baidya T, Ramesh GS, Srihari R, Hegde MS. Design and fabrication ofan automated temperature programmed reaction system to evaluate 3-waycatalysts Ce1�x�y(La/Y)x PtyO2�d. J Chem Sci 2006;118:47–55.

[70] Oh SH, Carpenter JE. Platinum–rhodium synergism in three-way automotivecatalysts. J Catal 1986;98:178–90.

[71] Meng M, Lin P-Y, Fu Y-L, Yu S-M. The structures, catalytic properties andspillover effects of the catalysts Co-Pt(Pd,Rh)/Ce-Al-O. In: Guerrero-Ruiz A,Rodriguez-Ramos I, editors. Studies in surface science and catalysis, vol.138. Elsevier; 2001. p. 387–94.

[72] Gayen A, Baidya T, Biswas K, Roy S, Hegde MS. Synthesis, structure and threeway catalytic activity of Ce1�xPtx/2Rhx/2O2�d (x = 0. 01 and 0.02) nano-crystallites: synergistic effect in bimetal ionic catalysts. Appl Catal A: Gen2006;315:135–46.

[73] Gaspar AB, Dieguez LC. Dispersion stability and methylcyclopentanehydrogenolysis in Pd/Al2O3 catalysts. Appl Catal A: Gen 2000;201:241–51.

[74] Roy S, Hegde MS. Pd ion substituted CeO2: a superior de-NOx catalyst to Pt orRh metal ion doped ceria. Catal Commun 2008;9:811–5.

[75] Xu X, Chen P, Goodman DW. A comparative study of the coadsorption ofcarbon monoxide and nitric oxide on Pd(100), Pd(111), and silica-supportedpalladium particles with infrared reflection–absorption spectroscopy. J PhysChem 1994;98:9242–6.

[76] Rainer DR, Koranne M, Vesecky SM, Goodman DW. CO + O2 and CO + NOreactions over Pd/Al2O3 catalysts. J Phys Chem 1997;101:10769–74.

[77] Schmal M, Baldanza MAS, Vannice MA. Pdx Mo/Al2O3 catalysts for NOreduction by CO. J Catal 1999;185:138–51.

[78] Thirunavukkarasu K, Thirumoorthy K, Libuda L, Gopinath CS. A molecularbeam study of the NO + CO reaction on Pd(111) surfaces. J Phys Chem B2005;109:13272–82.

[79] Mamede A-S, Leclercq G, Payen E, Grimblot G, Granger P. Surface resonanceRaman spectroscopy study of NO transformation over Pd-based catalysts.Phys Chem Chem Phys 2003;5:4402–6.

[80] Mamede A-S, Gengembre L, Payen E, Granger P, Leclercq G, Grimblot J. XPScharacterization of adsorbed reaction intermediates on automotive exhaustgas catalysts. The NO and NO + CO interaction with Pd. Surf Interf Anal2002;34:105–11.

[81] Roy S, Marimuthu A, Hegde MS, Madras G. High rates of NO and N2Oreduction by CO, CO and hydrocarbon oxidation by O2 over nano crystallineCe0.98Pd0.02O2�d: catalytic and kinetic studies. Appl Catal B: Environ2007;71:23–31.

[82] Roy S, Marimuthu A, Hegde MS, Madras G. High rates of CO and hydrocarbonoxidation and NO reduction by CO over Ti0.99Pd0.01O1.99. Appl Catal B: Environ2007;73:300–10.

[83] Granger P, Lecomte JJ, Dathy C, Leclercq L, Leclercq G. Kinetics of the CO + NOreaction over rhodium and platinum–rhodium on alumina: II. Effect of Rhincorporation to Pt. J Catal 2007;175:194–203.

[84] Holles JH, Switzer MA, Davis RJ. Influence of ceria and lanthana promoters onthe kinetics of NO and N2O reduction by CO over alumina-supportedpalladium and rhodium. J Catal 2000;190:247–60.

[85] Schmal M, Baldanza MAS, Vannice MA. PdxMo/Al2O3 catalysts for NOreduction by CO. J Catal 1999;185:138–51.

[86] Herz RK. Dynamic behavior of automotive catalysts. 1. Catalyst oxidation andreduction. Ind Eng Chem Prod Res Dev 1981;20:451–7.

[87] Balducci G, Kasper J, Fornasiero P, Graziani M, Islam MS. Surface andreduction energetics of the CeO2–ZrO2 catalysts. J Phys Chem B1998;102:557–61.

[88] Fernandez-Garcia M, Martinez-Arias A, Iglesis-Juez A, Hungria AB, AndersonJA, Conesa JC, et al. Behavior of bimetallic Pd–Cr/Al2O3 and Pd–Cr/(Ce,Zr)Ox/Al2O3 catalysts for CO and NO elimination. J Catal 2003;196:214. and 220–33.

[89] Di Monte R, Kaspar J, Fornasiero P, Graziani M, Paze C, Gubitosa G. NOreduction by CO over Pd/Ce0.6Zr0.4O2–Al2O3 catalysts: in situ FT-IR studies ofNO and CO adsorption. Inorg Chim Acta 2002;334:318–26.

[90] Martinez-Arias A, Fernandez-Garcia M, Iglesis-Juez A, Hungria AB, AndersonJA, Conesa JC, et al. New Pd/CexZr1�xO2/Al2O3 three-way catalysts prepared bymicroemulsion: Part 2. In situ analysis of CO oxidation and NO reductionunder stoichiometric CO + NO + O2. Appl Catal B: Environ 2001;31:51–60.

[91] Dutta G, Waghmare UV, Baidya T, Hegde MS, Priolkar KR, Sarode PR.Reducibility of Ce1�xZrxO2: origin of enhanced oxygen storage capacity. CatalLett 2006;108:165–72.

[92] Trovarelli A, de Leitenburg C, Dolcettis G. Design better cerium-basedoxidation catalysts. Chemtech 2006;32:11–4.

[93] Baidya T, Marimuthu A, Hegde MS, Ravishankar N, Madras G. Higher catalyticactivity of nano-Ce1�x�yTixPdyO2�d compared to nano-Ce1�xPdxO2�d for COoxidation and N2O and NO reduction by CO: role of oxide ion vacancy. J PhysChem C 2007;111:830–9.

[94] Hecker WC, Bell AT. Reduction of NO by H2 over silica-supported rhodium:infrared and kinetic studies. J Catal 1985;92:247–59.

[95] Hirano H, Yamada T, Tanaka KI, Siera J, Cobden P, Nieuwenhuys BE.Mechanisms of the various nitric oxide reduction reactions on a platinum–rhodium (100) alloy single crystal surface. Surf Sci 1992;262:97–112.

[96] Mergler YZ, Nieuwenhuys BE. NO reduction by H2 over promoted Pt catalysts.Appl Catal B: Environ 1997;12:95–110.

[97] Roy S, Hegde MS, Sharma S, Lalla NP, Marimuthu A, Madras G. Appl Catal B:Environ 2008;84:341–50.

[98] Granger P, Dhainaut F, Pietrzik S, Malfoy P, Mamede A-S, Leclercq L, et al. Anoverview: comparative kinetic behaviour of Pt, Rh and Pd in the NO + CO andNO + H2 reactions. Top Catal 2006;39:65–76.

[99] Bera P, Aruna ST, Patil KC, Hegde MS. Studies on Cu/CeO2: a new NO reductioncatalyst. J Catal 1999;186:36–44.

[100] Libby WF. Promising catalyst for auto exhaust. Science 1971;171:499–500.[101] Tanaka H, Tan I, Uenishi M, Kimura M, Dohmae K. In: Kruse N, Frennet A,

Bastin JM, editors. Top catal, vols. 16/17. New York: Kluwer AcademicPublishers; 2001. p. 63–70.

[102] Sorenson SC, Wronkiewicz JA, Sis LB, Wirtz GP. Properties of LaCoO3

as a catalyst in engine exhaust gases. Bull Am Ceram Soc 1974;53:446–52.

[103] Voorhoeve RJH, Remeika JP, Trimble LE. Perovskites containing ruthenium ascatalysts for nitric oxide reduction mater. Res Bull 1974;9:1393–8.

[104] Nitadori T, Kurihara S, Misono M. Catalytic properties of La1�xA0xMnO3

(A0 = Sr, Ce, Hf). J Catal 1986;98:221–8.

2296 S. Roy et al. / Applied Energy 86 (2009) 2283–2297

[105] Mizuno N, Fujiwara Y, Misono M. Pronounced synergetic effect in thecatalytic properties of LaMn1�xCuxO3. J Chem Soc, Chem Commun1989:316–8.

[106] Costa CN, Stathopoulos VN, Belessi VC, Efstathiou AM. An Investigation of theNO/H2/O2 (lean-deNOx) reaction on a highly active and selective Pt/La0.5Ce0.5MnO3 catalyst. J Catal 2001;197:350–4.

[107] Costa CN, Savva PG, Andronikou C, Lambrou PS, Polychronopoulou K, BelessiVC, et al. An investigation of the NO/H2/O2 (lean de-NOx) reaction on a highlyactive and selective Pt/La0.7Sr0.2Ce0.1FeO3 catalyst at low temperatures. J Catal2002;209:456–71.

[108] Guilhaume N, Primet M. Three-way catalytic activity and oxygen storagecapacity of perovskite LaMn0.976Rh0.024O3+d. J Catal 1997;165:197–204.

[109] Nishihata Y, Mizuki J, Akao T, Tanaka H, Uenishi M, Kimura M, et al. Self-regeneration of a Pd-perovskite catalyst for automotive emissions control.Nature 2002;418:164–7.

[110] Twagirashema I, Frere SM, Gengembre L, Dujardin C, Granger P. Structuralregeneration of LaCoO3 perovskite-based catalyst during the NO + H2 + O2

reactions. Top Catal 2002;42–43:171–6.[111] Dhainaut F, Pietrzyk S, Granger O. Kinetics of the NO/H2 reaction on Pt/

LaCoO3: a combined theoretical and experimental study. J Catal2008;258:296–305.

[112] Nishihata Y, Mizuki J, Tanaka H, Uenishi M, Kimura M. Self-regeneration ofpalladium-perovskite catalysts in modern automobiles. J Phys Chem Solids2005;66:274–82.

[113] Tanaka H, Taniguchi M, Kajita N, Uenishi M, Tan I, Sato N, et al. Top catal, vols.30/31. New York: Kluwer Academic Publishers; 2004. p. 389–96.

[114] Uenishi M, Tanigushi M, Tanaka H, Kimura M, Nishihata Y, Mizuki J, et al. Thereducing capability of palladium segregated from perovskite-type LaFePdOx

automotive catalysts. Appl Catal B: Environ 2005;57:267–73.[115] Twagisrashema I, Engelmann-Pirez M, Frère M, Gengembre L, Burylo L,

Dujardin C, et al. An in situ study of the NO + H2 + (O2) reaction on Pd/LaCoO3

based catalysts. Catal Today 2007;119:100–5.[116] Dacquin JP, Dujardin C, Granger P. Surface reconstruction of supported Pd on

LaCoO3: consequences on the catalytic properties in the decomposition ofN2O. J Catal 2008;253:37–49.

[117] Wood SC. Select the right Nox control technology – consider the degree ofemission reduction needed, the type of fuel, combustion device design, andoperational factors. Chem Eng Prog 1994;90:32–8.

[118] Bosch H, Janssen FJ. Preface. Catal Today 1998;2:5–8.[119] Cho SM. Properly apply selective catalytic reduction for NOx removal. Chem

Eng Prog 1994;90:39–44.[120] Forzatti P, Lietti L. Recent advances in de-NOxing catalysis for stationary

applications. Heterogen Chem Rev 1996;3:33–51.[121] Alemany LJ, Berti F, Busca G, Ramis G, Robba D, Toledo GP, et al.

Characterization and composition of commercial V2O5–WO3–TiO2 SCRcatalysts. Appl Catal B: Environ 1996;248:299–311.

[122] Went GT, Leu LJ, Bell AT. Quantitative structural analysis of dispersed vanadiaspecies in TiO2 (anatase)-supported V2O5. J Catal 1992;134:479–91.

[123] Orlik SN, Ostapyuk VA, Martsenyuk-Kurkharuk MG. Selective reduction ofnitrogen-oxides with ammonia on V2O5/TiO2 catalysts. Kinet Katal1995;36:284–9.

[124] Oliveri G, Busca G, Lorenzelli V. Structure and surface area evolution ofvanadia-on-titania powders upon heat treatment. Mater Chem Phys1989;22:511–21.

[125] Cristiani C, Bellotto M, Forzatti P, Bregani F. On the morphological propertiesof tungsten–titania de-NOxing catalysts. J Mater Res 1993;8:2019–26.

[126] Hums E, Burzlaff H, Rothammel W. Evidence of the interaction betweenarsenious oxide and molybdenum oxide in deNOx catalysts by structureanalysis of MoAs2O7. Appl Catal 1991;73:L19–24.

[127] Inomata M, Mori K, Miyamoto A, Ui T, Murakami Y. Structures of supportedvanadium oxide catalysts. 1. Vanadium(V) oxide/titanium dioxide (anatase),vanadium(V) oxide/titanium dioxide (rutile), and vanadium(V) oxide/titanium dioxide (mixture of anatase with rutile). J Phys Chem1983;87:754–61.

[128] Topsoe NY. Characterization of the nature of surface sites on vanadia–titaniacatalysts by FTIR. J Catal 1991;128:499–511.

[129] Dines TJ, Rochester CH, Ward AM. Infrared and Raman study of the surfaceacidity of titania-supported vanadia catalysts. J Chem Soc Faraday Trans1991;87:1611–7.

[130] Hu S, Harris SP. 15N NMR study of the adsorption of NO and NH3 on titania-supported vanadia catalysts. J Catal 1996;158:199–204.

[131] Busca G, Saussey H, Saur O, Lavalley JC, Lorenzelli V. FT-IR characterization ofthe surface acidity of different titanium dioxide anatase preparations. ApplCatal 1985;14:245–60.

[132] Ramis G, Busca G, Bregani F, Forzatti P. Fourier transform-infrared study ofthe adsorption and coadsorption of nitric oxide, nitrogen dioxide andammonia on vanadia–titania and mechanism of selective catalyticreduction. Appl Catal 1990;64:259–78.

[133] Wong WC, Nobe K. Kinetics of nitric oxide reduction with ammonia on‘‘chemical mixed” and impregnated vanadium(V) oxide-titanium(IV) oxidecatalysts. Ind Eng Chem Prod Res Dev 1984;23:564–8.

[134] Nam I, Eldridge JW, Kittrell IR. Model of temperature dependence of avanadia–alumina catalyst for nitric oxide reduction by ammonia: freshcatalyst. Ind Eng Chem Prod Res Dev 1986;25:186–92.

[135] Beekman JW, Hegedus LL. Design of monolith catalysts for power plantnitrogen oxide (NO) emission control. Ind Eng Chem Res 1991;30:969–78.

[136] Pinoy LJ, Hosten LH. Experimental and kinetic modelling study of DeNOx onan industrial V2O5–WO3/TiO2 catalyst. Catal Today 1993;17:151–8.

[137] Takagi M, Kowai T, Soma M, Onishi I, Tamaru K. The mechanism of thereaction between NOx and NH3 on V2O5 in the presence of oxygen. J Catal1977;50:441–6.

[138] Inomata M, Miyamoto A, Murakami Y. Mechanism of the reaction of NO andNH3 on vanadium oxide catalyst in the presence of oxygen under the dilutegas condition. J Catal 1980;62:140–8.

[139] Miyamoto A, Kobayashi K, Inomata M, Murakami Y. Nitrogen-15 tracerinvestigation of the mechanism of the reaction of nitric oxide with ammoniaon vanadium oxide catalysts. J Phys Chem 1982;86:2945–50.

[140] Inomata M, Miyamoto A, Ui T, Kobayashi K, Murakami Y. Activities ofvanadium pentoxide/titanium dioxide and vanadium pentoxide/aluminumoxide catalysts for the reaction of nitric oxide and ammonia in the presenceof oxygen. Ind Eng Chem Prod Res Dev 1982;21:424–8.

[141] Janssen F, Van den Kerkhof F, Bosch H, Ross JJ. Infrared and x-rayphotoelectron spectroscopy study of carbon monoxide and carbon dioxideon platinum/ceria. Phys Chem 1987;91:5931–7.

[142] Wolf M, Yang DL, Durant JL. A Comprehensive Study of the ReactionNH2 + NO ? products: reaction rate coefficients, product branchingfractions, and ab initio calculations. J Phys Chem A 1997;101:6243–51.

[143] Marcy TP, Heard DE, Leone SR. Product Studies of Inelastic and ReactiveCollisions of NH2 + NO: effects of vibrationally and electronically excited NH2.J Phys Chem A 2002;106:8249–55.

[144] Duan X, Page M. Theoretical characterization of structures and vibrationalfrequencies for intermediates and transition states in the reaction of NH2

with NO. J Mol Struct (Theochem) 1995;333:233–42.[145] Soyer S, Uzun A, Senkan S, Onal I. A quantum chemical study of nitric oxide

reduction by ammonia (SCR reaction) on V2O5 catalyst surface. Catal Today2006;118:268–78.

[146] von Barth U. Basic density-functional theory an overview. Phys Scripta2004;T109:939–42.

[147] Kameoka S, Ukisu Y, Miyadera T. Selective catalytic reduction of NOx

with CH3OH, C2H5OH and C3H6 in the presence of O2 over Ag/Al2O3

catalyst: role of surface nitrate species. Phys Chem Chem Phys2000;2:367–74.

[148] Shimizu K, Shibata J, Satsuma A, Hattori T. Mechanistic causes of thehydrocarbon effect on the activity of Ag–Al2O3 catalyst for the selectivereduction of NO. Phys Chem Chem Phys 2001;3:880–5.

[149] Underwood GM, Miller TM, Grassian VH. Transmission FT-IR and Knudsencell study of the heterogeneous reactivity of gaseous nitrogen dioxide onmineral oxide particles. J Phys Chem A 1982;103:6184–9.

[150] Zhang X, He H, Gao H, Yu Y. Experimental and theoretical studies of surfacenitrate species on Ag/Al2O3 using DRIFTS and DFT. Spectrochim Acta Part A:Mol Biomol Spectrosc 2008;71:1446–51.

[151] Meunier FC, Zuzaniuk V, Breen JP, Olsson M, Ross JRH. Mechanisticdifferences in the selective reduction of NO by propene over cobalt- andsilver-promoted alumina catalysts: kinetic and in situ DRIFTS study. CatalToday 2000;59:287–91.

[152] He H, Yu Y. Selective catalytic reduction of NOx over Ag/Al2O3 catalyst: fromreaction mechanism to diesel engine test. Catal Today 2005;100:37–47.

[153] Kapteijn F, Singoredjo L, Dekke NJJ, Moulijn JA. Kinetics of the selectivecatalytic reduction of nitrogen oxide (NO) with ammonia over manganeseoxide (Mn2O3)–tungsten oxide (WO3)/c-alumina. Ind Eng Chem Res1993;32:445–52.

[154] Kapteijn F, Singoredjo L, van Driel M, Andreini A, Moulijn JA, Ramis G, et al.Alumina-supported manganese oxide catalysts: II. Surface characterizationand adsorption of ammonia and nitric oxide. J Catal 1994;150:105–16.

[155] Singoredjo L, Korver R, Kapteijn F, Moulijn J. Alumina supported manganeseoxides for the low-temperature selective catalytic reduction of nitric oxidewith ammonia. Appl Catal B: Environ 1992;1:297–316.

[156] Kapteijn F, Singoredjo L, Andreini A, Moulijn JA. Activity and selectivity ofpure manganese oxides in the selective catalytic reduction of nitric oxidewith ammonia. Appl Catal B: Environ 1994;3:173–89.

[157] Pena DA, Uphade BS, Reddy EP, Smirniotis PG. Identification of Surfacespecies on titania-supported manganese, chromium, and copper oxide low-temperature SCR catalysts. J Phys Chem B 2004;108:9927–36.

[158] Roy S, Viswanath B, Hegde MS, Madras G. Low-temperature selectivecatalytic reduction of NO with NH3 over Ti0.9M0.1O2�d (M = Cr, Mn, Fe, Co,Cu). J Phys Chem C 2008;112:6002–12.

[159] Nova I, Lietti L, Tronconi E, Forzatti P. Dynamics of SCR reaction over a TiO2-supported vanadia–tungsta commercial catalyst. Catal Today 2000;60:73–82.

[160] Nova I, Lietti L, Tronconi E, Forzatti P. Transient response method applied tothe kinetic analysis of the deNOx-SCR reaction. Chem Eng Sci2001;56:1229–37.

[161] Adelmana BJ, Sachtler WMH. The effect of zeolitic protons on NOx reductionover Pd/ZSM-5 catalysts. Appl Catal B: Environ 1997;14:1–11.

[162] Apostolescu N, Geiger B, Hizbullah K, Jan MT, Kureti S, Reichert D, et al.Selective catalytic reduction of nitrogen oxides by ammonia on iron oxidecatalysts. Appl Catal B: Environ 2006;62:104–14.

[163] Chmielarz L, Kustrowski P, Zbroja M, Łasocha W, Dziembaj R. Selectivereduction of NO with NH3 over pillared clays modified with transition metals.Catal Today 2004;90:43–9.

[164] Willey RJ, Lai H, Peri JB. Investigation of iron oxide-chromia-alumina aerogelsfor the selective catalytic reduction of nitric oxide by ammonia. J Catal1991;130:319–31.

S. Roy et al. / Applied Energy 86 (2009) 2283–2297 2297

[165] Long RQ, Yang RT. Selective catalytic reduction of NO with ammonia overFe3+-exchanged mordenite (Fe-MOR): catalytic performance,characterization, and mechanistic study. J Catal 2002;207:274–85.

[166] Salker AV, Weisweiler W. Catalytic behaviour of metal based ZSM-5 catalystsfor NOx reduction with NH3 in dry and humid conditions. Appl Catal A: Gen2000;2:221–9.

[167] Krishna K, Seijger GBF, Van den Bleek CM, Makkee M, Mul G, Calis HPA.Selective catalytic reduction of NO with NH3 over Fe–ZSM-5 catalystsprepared by sublimation of FeCl3 at different temperatures. Catal Lett2003;86:121–32.

[168] Chen H, Sun Q, Wen B, Yeom Y, Weitz E, Sachtler WMH. Reduction overzeolite-based catalysts of nitrogen oxides in emissions containing excessoxygen: unraveling the reaction mechanism. Catal Today 2004;96:1–10.

[169] Krocher O, Devadas M, Elsener M, Wokaun A, Soger N, Pfeifer M, et al.Investigation of the selective catalytic reduction of NO by NH3 on Fe–ZSM5monolith catalysts. Appl Catal B: Environ 2006;66:208–16.

[170] Rahkamaa-Tolonen K, Maunula T, Lomma M, Huuhtanen M, Keiski RL. Theeffect of NO2 on the activity of fresh and aged zeolite catalysts in the NH3-SCRreaction. Catal Today 2005;100:217–22.

[171] Qi G, Yang RT. Low-temperature SCR of NO with NH3 over noble metalpromoted Fe–ZSM-5 catalysts. Catal Lett 2005;100:243–6.

[172] Long RQ, Yang RT. Selective catalytic oxidation (SCO) of ammonia to nitrogenover Fe-exchanged zeolites. J Catal 2001;201:145–52.

[173] Kim H, Hwang U-C, Nam I-S, Kim YG. The characteristics of a copper-exchanged natural zeolite for NO reduction by NH3 and C3H6. Catal Today1998;44:57–65.

[174] Katranas TK, Vlessidis AG, Tsiatouras VA, Triantafyllidis KS, Evmiridis NP.Dehydrogenation of propane over natural clinoptilolite zeolites. MicroporMesopor Mater 2003;61:189–98.

[175] Ates A. Characteristics of Fe-exchanged natural zeolites for thedecomposition of N2O and its selective catalytic reduction with NH3. ApplCatal B: Environ 2007;76:282–90.

[176] Lombardo EA, Sill GA, d’Itri JL, Hall WK. The Possible Role of Nitromethane inthe SCR of NOx with CH4 over M–ZSM5 (M = Co, H, Fe, Cu). J Catal1998;173:440–9.

[177] Desai AJ, Kovalchuk VI, Lombardo EA, d’Itri JL. CoZSM-5: why this catalystselectively reduces NOx with methane. J Catal 1999;184:396–405.

[178] Burch R, Breen JP, Meunier FC. A review of the selective reduction of NOx withhydrocarbons under lean-burn conditions with non-zeolite oxide andplatinum group metal catalysts. Appl Catal B: Environ 2002;39:283–303.

[179] Parvulescu VI, Granger P, Delmon B. Catalytic removal of NO. Catal Today1998;46:233–316.

[180] Miyadera T. Alumina-supported silver catalysts for the selective reduction ofnitric oxide with propene and oxygen-containing organic compoundsalumina-supported catalysts for the selective reduction of nitric oxide bypropene. Appl Catal B: Environ 1993;2:199–205.

[181] Hoost TE, Kudla RJ, Collins KM, Chattha MS. Characterization of Ag/c-Al2O3

catalysts and their lean-NOx properties. Appl Catal B: Environ1997;13:59–67.

[182] Martinez-Arias A, Fernandez-Garcia M, Iglesias-Juez A, Anderson JA, ConesaJC, Soria J. Study of the lean NO(x) reduction with C3H6 in the presence ofwater over silver/alumina catalysts prepared from inverse microemulsions.Appl Catal B: Environ 2000;28:29–41.

[183] Klingstedt F, Eranen K, Lindfors L, Andersson S, Cider L, Landberg C, et al. Ahighly active Ag/alumina catalytic converter for continuous HC-SCR duringlean-burn conditions: From laboratory to full-scale vehicle tests. Top Catal2004;30/31:27–30.

[184] Arve K, Capek L, Klingstedt F, Eranen K, Lindfors L, Murzin DY, et al.Preparation and characterisation of Ag/alumina catalysts for the removal ofNOx emissions under oxygen rich conditions. Top Catal 2004;30/31:91–6.

[185] Eranen K, Lindfors L, Klingstedt F, Murzin DY. Continuous reduction of NOwith octane over a silver/alumina catalyst in oxygen-rich exhaust gases:Combined heterogeneous and surface-mediated homogeneous reactions. JCatal 2003;219:25–40.

[186] Shibata J, Takada Y, Shichi A, Satokawa S, Satsuma A, Hattori T. Ag cluster asactive species for SCR of NO by propane in the presence of hydrogen over Ag–MFI. J Catal 2004;222:368–76.

[187] Satokawa S. Enhancing the NO/C3H8/O2 reaction by using H2 over Ag/Al2O3

catalysts under lean-exhaust conditions. Chem Lett 2000;3:294–5.[188] Shibata J, Takada Y, Shichi A, Satokawa S, Satsuma A, Hattori T. Influence of

zeolite support on activity enhancement by addition of hydrogen for SCR ofNO by propane over Ag–zeolites. Appl Catal B: Environ 2004;54:137–44.

[189] Richter M, Fricke R, Eckelt R. Unusual activity enhancement of NO conversionover Ag/Al2O3 by using a mixed NH3/H2 reductant under lean conditions.Catal Lett 2004;94:115–8.

[190] Burch R, Breen JP, Hill CJ, Krutzsch B, Konrad B, Jobson E, et al. Exceptionalactivity for NOx reduction at low temperatures using combinations ofhydrogen and higher hydrocarbons on Ag/Al2O3 catalysts. Top Catal2004;30–31:19–25.

[191] Shibata J, Shimizu K, Takada Y, Shichi A, Yoshida H, Satokawa S, et al.Structure of active Ag clusters in Ag–zeolites for SCR of NO by propane in thepresence of hydrogen. J Catal 2004;227:367–74.

[192] Shi C, Cheng M, Qu Z, Yang X, Bao X. On the selectively catalytic reduction ofNOx with methane over Ag–ZSM-5 catalysts. Appl Catal B: Environ2002;36:173–82.

[193] Konova P, Arve K, Klingstedt F, Nikolov P, Naydenov A, Kumar N, et al. Acombination of Ag/alumina and Ag modified ZSM-5 to remove NOx and COduring lean conditions. Appl Catal B: Environ 2007;70:138–45.

[194] Meunier FC, Ross JRH. Effect of ex situ treatments with SO2 on the activity of alow loading silver–alumina catalyst for the selective reduction of NO and NO2

by propene. Appl Catal B: Environ 2000;24:23–32.[195] Abe A, Aoyama N, Sumiya S, Kakuta N, Yoshida K. Effect of SO2 on NOx

reduction by ethanol over Ag/Al2O3 catalyst. Catal Lett 1998;51:5–9.[196] Angelidis TN, Kruse N. Promotional effect of SO2 on the selective catalytic

reduction of NOx with propane/propene over Ag/c-Al2O3. Appl Catal B:Environ 2001;34:201–12.

[197] Burch R, Sullivan JA, Watling TC. Mechanistic considerations for the reductionof NOx over Pt/Al2O3 and Al2O3 catalysts under lean-burn conditions. CatalToday 1998;42:13–23.

[198] Angelidis TN, Christoforou S, Bongiovanni A, Kruse N. On the promotion bySO2 of the SCR process over Ag/Al2O3: Influence of SO2 concentration withC3H6 versus C3H8 as reductant. Appl Catal B: Environ 2002;39:197–204.

[199] Breen JP, Burch R, Hardacre C, Hill CJ, Krutzsch B, Bandl-Konrad B, et al. Aninvestigation of the thermal stability and sulphur tolerance of Ag/c-Al2O3

catalysts for the SCR of NOx with hydrocarbons and hydrogen. Appl Catal B:Environ 2007;70:36–44.

[200] Gabrielsson PLT. Urea-SCR in automotive applications. Top Catal2004;28:177–84.

[201] Koebel M, Elsener M, Kleeman M. Urea-SCR: a promising technique to reduceNOx emissions from automotive diesel engines. Catal Today 2000;59:335–45.

[202] Gomez-Garcia MA, Pitchon V, Kiennemann A. Pollution by nitrogen oxides:an approach to NOx abatement by using sorbing catalytic materials. EnvironInt 2005;31:445–67.

[203] Takahashi N, Shinjoh H, Iijima T, Suzuki T, Yamazaki K, Yokota K, et al. Thenew concept 3-way catalyst for automotive lean-burn engine: NOx storageand reduction catalyst. Catal Today 1996;27:63–9.

[204] Matsumoto S. DeNOx catalyst for automotive lean-burn engine. Catal Today1996;29:43–5.

[205] Takeuchi M, Matsumoto S. NOx storage-reduction catalysts for gasolineengines. Top Catal 2004;28:151–8.

[206] Casapu M, Grunwaldt J-D, Maciejewski M, Wittrock M, Göbel U, Baiker A.Formation and stability of barium aluminate and cerate in NOx storage-reduction catalysts. Appl Catal B: Environ 2006;63:232–42.

[207] Casapu M, Grunwaldt J-D, Maciejewski M, Krumeich F, Baiker A, Wittrock M,et al. Comparative study of structural properties and NOx storage-reductionbehavior of Pt/Ba/CeO2 and Pt/Ba/Al2O3. Appl Catal B: Environ2008;78:288–300.

[208] Sjoblom J, Papadakis K, Creaser D, Odenbrand CUI. Use of experimentaldesign in development of a catalyst system. Catal Today 2005;100:243–8.

[209] Sakamoto Y, Motohiro T, Matsunaga S, Okumura K, Kayama T, Yamazaki K,et al. Transient analysis of the release and reduction of NOx using a Pt/Ba/Al2O3 catalyst. Catal Today 2007;121:217–25.

[210] Malpartida I, Guerrero-Perez MO, Herrera MC, Larrubia MA, Alemany LJ. MS–FTIR reduction stage study of NSR catalysts. Catal Today 2007;126:162–8.

[211] Takahashi N, Suda A, Hachisuka I, Sugiura M, Sobukawa H, Shinjoh H. Sulfurdurability of NOx storage and reduction catalyst with supports of TiO2, ZrO2

and ZrO2–TiO2 mixed oxides. Appl Catal B: Environ 2007;72:187–95.[212] Bögner W, Krämer M, Krutzsch B, Piscinger S, Voigtländer D, Wenniger G.

Removal of nitrogen oxides from the exhaust of a lean-tune gasoline engine.Appl Catal B: Environ 1995;7:153–71.

[213] Hodjati S, Vaezzadeh K, Petit C, Pitchon V, Kiennemann A. NOx sorption–desorption study: application to diesel and lean-burn exhaust gas (selectiveNOx recirculation technique). Catal Today 2000;59:323–334;.

[214] Trichard JM. Current tasks and challenges for exhaust after-treatmentresearch: an industrial viewpoint. Stud Surf Sci Catal 2007;171:211–33.

[215] Caton JA, Zhiyong X. The selective non-catalytic removal (SNCR) of nitricoxides from engine exhaust streams: Comparison of three processes. J EngGas Turb Power 2004;126:234–40.

[216] Javed MT, Irfan N, Gibbs BM. Control of combustion-generated nitrogen oxidesby selective non-catalytic reduction. J Environ Manage 2007;83:251–89.

[217] Bae SW, Roh SA, Kim SD. NO removal by reducing agents and additives in theselective non-catalytic reduction (SNCR) process. Chemosphere2006;65:170–5.

[218] Fua Y, Diwekar UM. Cost effective environmental control technology forutilities. Adv Environ Res 2004;8:173–96.

[219] Mok YS, Lee Heon-Ju. Removal of sulfur dioxide and nitrogen oxides by usingozone injection and absorption–reduction technique. Fuel Process Technol2006;87:591–7.

[220] Wang Z, Zhou J, Fan J, Cen K. Direct numerical simulation of ozone injectiontechnology for NOx control in flue gas. Energy Fuels 2006;20:2432–8.

[221] Mok YS. Oxidation of NO to NO2 using the ozonization method for theimprovement of selective catalytic reduction. Chem Eng Jpn 2004;37:1337–44.

[222] Mok YS, Shin DN, Koh DJ, Kim KT. Effect of ozone injection on selectivecatalytic reduction of nitrogen oxides. Appl Chem 2005;9:217–20.