© BALTZER SCIENCE PUBLISHERS 125DECEMBER 1997

Nan-Yu TopsøeHaldor Topsøe Research LaboratoriesDK-2800 Lyngby, DenmarkPhone: 45-45-272000E-mail: nat@topsoe.dk

Catalysis for NOx abatementSelective catalytic reduction of NOx by ammonia:

fundamental and industrial aspectsEmission of NOx is a major global pollution problem. Themost efficient commercially proven method of removing NOxfrom flue gases today is still the Selective Catalytic Reduction(SCR) of NOx by ammonia over vanadia/titania catalysts. Whatare the future challenges of SCR and can we today under-stand why such catalysts are especially efficient and do wehave sufficient fundamental insight into the nature of the ac-tive sites and the mechanism such that we can bridge our fun-damental knowledge with actual industrial application? Thesequestions will be addressed presently. The substantial progresswhich has been made in the molecular understanding of SCRwill be discussed and it will be shown that the microkineticapproach is a useful framework for incorporating the funda-mental insight.

Introduction

Nitrogen oxides (NOx) belong to some of the most troublesomepollutants and their damaging effects on our health and environ-ment are substantial. For example, NOx plays an important rolein the Earth’s atmospheric chemistry: A high concentration ofNOx increases the tropospheric ozone levels via photochemicaloxidation of NOx, while depleting the stratospheric ozone throughthermal reaction of nitrous oxide with oxygen atoms in the upperatmosphere. This may have many serious consequences for glo-bal warming. NOx also contributes substantially to acid rain(formed via reaction involving HO

2 or OH radicals) leading to

acid deposits which contribute to forest dying around us, de-struction of fresh and coastal water life, deterioration of buildingmaterials, and causing the reddish brown smog in urban areas(due to NO

2 formed by the photochemical oxidation of NO). The

direct health hazards relating to NOx range from bronchitis andpneumonia, to effects on the immune system resulting in increasedsusceptibility to viral infection and hay fever[1]. NOx is producednot only in all combustion processes involving fossil fuels and,burning of biomass (including field burning and forest fires), but

also by lightning, microbial decomposition of proteins in thesoil, and volcanic activity[2]. However, the first two man-madesources contribute to almost three-quarters of the total amount ofNOx emitted into the atmosphere[3]. The total amount of NOxemissions from the European Community, Japan, and the UnitedStates is about 35 million metric tons per year (based on NO

2).

The United States alone emit about 60% of this amount[4].Guidelines for maximum allowable NOx emissions from bothmobile and stationary sources were already introduced in theearly 1970’s, and these guidelines have since then been continu-ously revised and tightened. This situation has provided signifi-cant incentive for both research and commercial developmentsin the area of NOx abatement. However, emission guidelines arenot always sufficient driving forces for actual installation of thequite costly NOx control technologies and as a result regulatinglaws usually have to be passed first. Furthermore, if such laws arenot global, the production costs and the competitive situation ofthe producers will vary from country to country. Thus, solutionsto the environmental problems are complicated by local as wellas global political issues.Various technologies have been developed to control the emis-sion of nitrogen oxides; and these may be classified either as pri-mary or secondary measures. The primary technologies (“cleantechnologies”) are aimed at reducing the initial formation of ni-trogen oxides, for example, by modifications of the fuel qualityor the combustion processes. The secondary measures are aimedat removing the produced nitrogen oxides from the flue or ex-haust gases (“clean-up techniques” or “flue-gas treatments”). Al-though the primary measures are in principle more attractive than

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the NOx removal bysecondary measures, the progress in thedevelopment of primary measures has notbeen sufficient to meet the more stringent emission regulations.Thus, NOx reduction has mainly been carried out by use of sec-ondary or clean-up technology.Secondary control measures include both wet and dry methods.So far the wet methods are limited to small waste stream cleanupand are not suitable for NOx removal from large volumes of fluegases. The dry methods consist of catalytic and non-catalytictypes. Details of the various methods have been described in areview by Bosch and Janssen[3]. Among the various secondaryprocesses, the Selective Catalytic Reduction (SCR) of NOx byammonia over vanadia/titania catalysts is by far the most impor-tant commercial process today for removing NOx from flue gas.The SCR process has been commercially implemented in Japansince 1977[5] and the first installations of such units in the U.S.,West Germany and Austria came about ten years later[6,7]. Nowa-days, SCR installations for over hundred thousand MWe capacityhave been made worldwide. The market for SCR catalysts andsystems is expected to increase as stricter regulations are beingenforced in an increasing number of countries.The present article focuses on the removal of NOx from flue gasby the SCR process. First, a short description of the industrialaspects of this process and the catalysts will be given, followedby a discussion of some fundamental aspects of the catalyst sys-tem, in particular with regard to the nature of the active sites andthe SCR reaction mechanism. It will be shown that it is possibleto translate this insight to the industrial operation.

The Selective Catalytic Reduction Process

In the SCR process, NOx is reduced by ammonia over a catalystto form harmless nitrogen and water vapor without creating anysecondary pollutants. This process has the following overall sto-ichiometry[3]:

4NH3 + 4NO + O

2 ➔ 4N

2 + 6H

2O

Figure 1 shows a schematic diagram of the SCR process. Themain components of the SCR process basically consist of a reac-tor with the catalyst and an ammonia storage and injection sys-tem. The reducing agent can be either liquid, water-free ammoniaunder pressure or it can be a 25% aqueous ammonia solution atatmospheric pressure. A solution of urea can be used as well. Theammonia (e.g. aqueous NH

3) is volatilized in an electrically steam

or hot water heated evaporator and is subsequently diluted withair before it is injected into the exhaust gas duct. The injection ofthe mixture normally takes place through a system of nozzles in

Figure 1

Schematic process dia-

gram for Selective Cata-

lytic Reduction (SCR)

of NOx .

Figure 2

Schematic diagram of the deNOx process for high dust and low dust/tail end

operations for coal fired boilers.

SCR = reactor for SCR, APH = air preheater, ESP = electrostatic precipitator

(dust filter), FGD = flue gas desulfurization, H = heater.

The numbers give the gas temperature in °C.

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order to achieve a uniform mixing of the ammonia with the ex-haust gas. A static mixer may be placed in the exhaust gas duct tofurther improve mixing. In order to ensure efficient NOx removaland minimum NH

3 slip (“leakage”) from the SCR reactor, it is

very important that the flue gas-ammonia mixture has a uniformNH

3/NOx ratio.

In practice, the degree of conversion of the nitrogen oxides de-pends on the amount of ammonia added and it increases with in-creasing NH

3/NOx ratio. One should not, however, add NH

3 above

the stoichiometrically required amount in order to avoid the slipof unreacted ammonia.

The SCR process arrangement depends onthe particular industrial application[3,8].For example, for coal-fired boilers, thereare the so-called high dust and low dust/tail end processes (Fig. 2). In the high dustoperation, the SCR reactor is placed im-mediately after the boiler and upstreamof the electrostatic precipitator (or the dustfilter), whereas in the low dust/tail endoperation the SCR reactor is placed down-stream of the flue gas desulfurization(FGD). The high dust position is preferredfrom an economical point of view. How-ever, if flue gases contain a significantamount of catalyst poisons (particularlyin molten slag coal fired boilers), or if util-ity plants offer inadequate space aroundthe boiler, the low dust/tail end process isadopted.Other important applications of SCR in-clude NOx emission control from gas- andoil-fired boilers, gas turbines, chemicalplants (e.g. HNO3 plants), and internalcombustion engines. The growing impor-tance of the last application which removesNOx from diesel engines is demonstratedin installations of reciprocating engineson several marine vessels in California andSweden (an example is illustrated in Fig.3). Besides the applications mentionedabove, SCR is also used in connection withthe treatment of emission from industrialplants and waste incinerators.SCR may also be integrated in a process(e.g. SNOX from Haldor Topsoe andDESONOX from Degussa) for the com-bined removal of nitrogen and sulfur ox-ides from flue gases with the productionof H

2SO

4. The typical flue gas composi-

tion from a coal-fired boiler is as follows:Figure 3 Installation of an SCR system on board of a marine vessel

NO 400-700 ppmNO2 2-5 ppmSO2 500-2000 ppmSO3 2-20 ppmH2O 6-8 %

O2 4-5 %CO2 10-12 %

Dust 5-20 mg/Nm3

(low dust)

Dust 10-20 g/Nm3

(high dust)

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In the SNOX process[9] (shown in Fig. 4), the flue gas after dustremoval enters the SCR reactor where 99% of the NOx is reducedby NH

3 in the presence of O

2. The sulfur is converted to concen-

trated commercial grade sulfuric acid. The conversion involvesthe catalytic oxidation of SO

2 into SO

3, which is subsequently

hydrated to sulfuric acid and condensed as 95% sulfuric acid inthe air-cooled multi-tube WSA (Wet Sulfuric Acid) condenser.The SNOX process has the advantage that it can operate at NH

3/

NOx ratios slightly above 1.0 without ammonia slip, because allof the surplus ammonia will be oxidized to nitrogen and NOx inthe downstream SO

2 oxidation reactor[10]. This

combined process gives an overalldeNOx degree of about 97%,and the higher NOx re-moval efficiency achiev-ed by the higher NH

3/

NOx allows the use ofsmaller catalyst vol-ume as compared toconventional SCR.A zeolite based cata-lyst is used in theDESONOX process[11]

which produces about70% H

2SO

4.

SCR CatalystsThe SCR reactor usually consists of one or more layers of cata-lysts. The catalyst volume, and consequently the size of the reac-tor, depends among other things on the activity of the catalyst,the desired degree of NOx reduction, the NOx concentration, theflue gas pressure, the acceptable level of NH

3 slip and the dust

content.Typically, very large volumes of gas have to be treated (e.g.1,000,000 Nm3/h for a 300 MWe power plant) and reliable opera-tion of the SCR unit is a key issue, in order not to interfere withthe operation of the power plant itself. To minimize problems inhigh dust plants arising from the deposition of dust contained inthe exhaust gases, monolithic catalysts are necessary in combina-tion with regular “soot-blowing”.SCR catalysts may be made in several ways[8]. The catalyst mate-rial can either be extruded into honeycomb monoliths or be de-

posited onto perforated metal plates or nets.Another procedure involves wash coat-

ing a layer of catalyst onto a mono-lith matrix consisting of cordier-ite or corrugated ceramic paper[10].Figure 5 shows a schematic sketchof a SCR reactor loaded with cata-

Figure 4 Schematic diagram of the SNOXTM process.

Figure 5

Schematic sketch of an SCR reactor

loaded with monolithic catalysts.

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lysts of the latter type. The catalyst may be manufactured withdifferent channel diameters since this will influence the volumeactivity of the catalyst, the pressure drop and the dust deposition(plugging of channels). Thus, the choice of the channel diameterdepends on the dust content of the exhaust gas, the characteris-tics of the dust and the allowable pressure drop across the SCRreactor.Many different types of catalysts have been investigated for use inthe deNOx process. (For a review, see e.g. Bosch and Janssen[3]).The active components may be based on, for example, iron, chro-mium or vanadium, while support materials tested include TiO

2,

Al2O

3, SiO

2, zeolites and various combinations of these oxides[12].

However, the most commonly used catalyst we have today for thereduction of NOx from flue-gas is titania-supported vanadia[3]. Thiscatalyst has high SCR activity and is resistant to poisoning by SO

2[4,7]. The titania used is predominately in the anatase form, sinceanatase is easier to produce with high surface areas than rutile.Figure 6 shows some typical NOx removal behavior[10] for vanadia/titania catalysts at two different space velocities, plotted as theNOx conversion against the NH

3/NOx ratio. The curves show

that NOx conversions above 95% can be achieved at space ve-locities (i.e. NHSV, expressed as normal cubic meter of flue gasper hour per cubic meter of catalyst) below 10,000 Nm3/m3h withNH

3/NO xratios ~1. However, high NH

3/NOx ratios are not desir-

able, since as discussed above and shown in the figure, a largeincrease in the ammonia slip occurs above NH

3/NOx ~1. In indus-

try, the maximum allowable ammonia slip is often the decisivefactor for determining the required minimum catalyst volume. Asmentioned previously, it is also very important to avoid an un-even flow distribution and poor NH

3 mixing, since such effects

may also lead to large increases in the NH3 slip.

Process Constraints and CatalystDeactivation in SCR

Normally, an ammonia slip above 5-10 ppm cannot be toleratedsince it can react with SO

3 to give ammonium bisulfate which

causes corrosion and fouling problems in downstream equipment.Furthermore, in the high dust application, a too high ammoniaslip contaminates the fly ash, making it unsuitable for cement pro-duction. In power plants with wet bottom boilers and ash recircu-lation, diarsenic trioxide is a severe poison[13] for the deNOx cata-lysts. Other causes of deactivation include poisoning by alkalimetals present in the flue gas, thermal sintering, and plugging ofcatalyst pores and/or channels by dust deposit. The alkali metalproblem is especially severe in connection with the utilization ofbiomass.

SCR Reaction KineticsThe very large volumes of flue gas to be treated require the instal-lation of a huge amount of catalyst (e.g. about 250 m3 of high dustcatalyst in a 300 MWe power plant). Hence, it is important to beable to describe in detail the degree of NOx removal for a giveninstalled catalyst volume. Thus, to design industrial deNOx reac-tors, it is essential to have detailed kinetic information. For thispurpose power law kinetic expressions are often used in industry.However, the simple power rate law has severe inherent limita-tions and seldomly it describes the reaction kinetics adequatelyover wide range of reaction conditions[14]. This is especially so inenvironmental processes (e.g., ultrapurification or zero emission

processes) where one or several of the components have to beremoved down to the ppm-level[15,16]. The above problems arealso important for the SCR reaction and in fact, apparent reactionorders ranging from 0.5 to 1 and -0.15 to 1 have been reported fornitric oxide and ammonia, respectively. Reaction kinetics has thusbeen one of the controversial issues in SCR. In the subsequentsection, we show that the microkinetic approach[14] is a usefulframework for introducing fundamental knowledge on the SCRreaction and, consequently, many of the controversies could beresolved.

Earlier proposals regarding SCR mechanismand active sites:

Many different types of reaction mechanisms have been pro-posed in the literature for the SCR reaction over vanadia/titaniacatalysts and still today no general consensus has been reached.Proposals have ranged from Eley-Rideal type mechanisms in-volving gas phase NO, surface-mediated mechanisms involvingweakly adsorbed NO to Langmuir-Hinshelwood type mechanisms,with different types of adsorbed NH

3 and NO species. For ex-

ample, based on separate results on TiO2 and V

2O

5 samples,

Odriozola et al.[17] considered V2O

5/TiO

2 as a bifunctional cata-

lyst and suggested a Langmuir-Hinshelwood mechanism involv-ing reaction between adsorbed NO on TiO

2 and adsorbed NH

3 on

V5+. Went et al.[18] proposed a similar mechanism involving areaction between adsorbed NH

3 on Lewis acid sites and adsorbed

NO on other, non-specified sites. Such proposals are however notin agreement with the failure to observe significant amounts ofadsorbed NO under reaction conditions[19,20]. Recent TPD studies[21] have also shown that the adsorption of NO is weak.Takagi et al.[22,23] proposed a reaction of the Langmuir-Hinshelwoodtype between adsorbed NO

2 and adsorbed NH

4+. However, Inomata

et al.[24] did not observe oxidation of NO to NO2 by O

2 in a flow of

dilute gases with concentrations (1000 ppm NO in 1% O2) resem-

bling those encountered under industrial conditions. Also, in ourown work[25] we did not observe NO

2 on the vanadia/titania cata-

lyst surface under typical SCR conditions.Eley-Rideal mechanisms between adsorbed NH

4+ and gas phase

NO were proposed by Inomata et al.[24,26] and later by Gasior et

0,6 0,7 0,8 0,9 1,0 1,1

80

100

60 60

40

20

40

20

0

NO

x R

educ

tion

(%)

NH3 / NOx Ratio

NH

3 Sl

ip (

ppm

)

NHSV = 5.000 Nm3/m3 h NHSV = 10.000 Nm3/m3 h

NOx Reduction and NH3 Slip

Figure 6

NOx conversion and NH3 slip over vanadia-titania catalyst vs. NH3/NOx

ratio at space velocities of 5,000 and 10,000 Nm3/m3h.

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Microkinetic analysisOur earlier kinetic analysis of the deNOx reaction[33], made toevaluate the various mechanisms, showed that a simple two-stepEley-Rideal mechanism involving reaction between adsorbed NH

3

and gaseous (or weakly adsorbed) NO is not consistent with theavailable information. Rather, a three-step mechanism could givean adequate representation of the data[33]. This mechanism con-sists of an equilibrated ammonia adsorption step, followed by anactivation step and a subsequent reaction between the activatedammonia species and NO to form the products. The three-stepmechanism was written as below:

Step 1 NH3 + M NH3-M

Step 2 NH3-M + S NH3-S + M

Step 3 NO + NH3-S Products + S

where M represents an ammonia adsorption site, and S representsa reactive site on which ammonia subsequently becomes activated.It was not possible to distinguish kinetically whether Step 3 was atrue Eley-Rideal step, involving reaction with gaseous NO, or areaction with weakly adsorbed NO. The three-step mechanismwas able to quantitatively describe the NO conversion data[33] un-der industrially relevant conditions for operating the vanadia/tita-nia catalysts. Furthermore, it gave a very good description of theammonia slip[33] behavior, which previously has been found moredifficult to describe than the conversion of NOx. However, de-spite of the good fit of the kinetic data, the mechanism was stillsemi-empirical in nature, since direct information concerning thenature of the M and S sites not provided.

al.[20]. Bosch et al.[27] proposed a redox mechanism where V5+ isfirst reduced by NH

3 and subsequently reoxidized by NO via an

Eley-Rideal step, producing nitrogen. This mechanism was sup-ported by Janssen et al.[28]. Later, Ramis et al.[29] also proposed aredox mechanism in which the reaction takes place between astrongly adsorbed form of ammonia and gaseous, or weaklybonded NO.Despite the apparent similarities in some of the Eley-Ridealmechanisms, different adsorption sites and mechanistic detailswere suggested. For example, based on their studies using la-beled molecules, Janssen et al.[28] found that surface V=O groupsare the active sites where NH

3 adsorbs, is activated to become

NH2, and subsequently reacts with gaseous NO. However, they

neither studied fully oxidized catalysts nor did they use a largeexcess of oxygen in the reaction mixture, rendering uncertainwhether the experimental observations are relevant to typicalSCR reaction conditions.Lewis acid sites (VO2+) were proposed by Ramis et al.[29] as theactive sites which coordinate molecular ammonia. Inomata etal.[24,26] proposed that NH

3 adsorption occurs on dual sites involv-

ing V-OH and an adjacent V=O, which assists in the activation ofNH

3. They found that the rate of the NH

3-NO reaction is propor-

tional to the amount of V=O. Other studies[25,30,31], however, couldnot find a correlation between V5+=O and SCR activity. Gasior etal.[20] proposed V-OH to be the active site. This suggestion hassince been supported by several investigators[25,30,31]. The interac-tion of ammonia and V-OH was confirmed by our IR studies[32]

which revealed a direct correlation between the concentration ofV-OH groups and the Brønsted acidity.Not surprisingly, it has been difficult to use any of the proposalson the SCR reaction mechanism and the nature of the active sitesto explain the industrial SCR operation. It is likely that the dis-agreement in the literature relates to the fact that many of thesestudies were done under conditions which are not representativefor the industrial application.

Figure 8

On-line mass spectrometry results on the 6% V2O5 /TiO2 during TPSR in

(a) O2 /Ar,

(b) NO + O2 /Ar, and

(c) NO/Ar, shown as concentrations vs. temperatures. (Reprinted with permis-

sion from ref. [32]).

Figure 7bFigure 7a

Figure 8a

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In view of these ambiguities, we decided to perform in situ infra-red spectroscopic studies[25,32] to elucidate the nature of the sur-face interactions of the various reactant molecules such as ammo-nia, nitric oxide, and oxygen. Information about the reaction in-termediates and the reactivity of the surface sites was also ob-tained by combining the infrared studies with simultaneous analy-sis of reactants and products. Adsorption of ammonia has shownthat both Brønsted and Lewis acid sites are present on the surfaceof vanadia/titania catalysts as reflected by the infrared absorptionbands of ammonium species and coordinatively bonded ammo-nia species, respectively[25,32,34-37]. The relative concentrations ofthese sites depend on the concentration of vanadia and the state ofoxidation. Specifically, more Brønsted acid sites are found on oxi-dized vanadia catalysts with a high loading[25]. Only Lewis acidsites are observed on the titania support itself[25,38]. The questionthat has been frequently raised in the literature is: which of thesesites is responsible for the SCR activity?This issue was addressed by the use of in situ on-line Fouriertransform infrared spectroscopy[32,39,40] and by monitoring simul-taneously the changes in catalytic activity and the changes in theconcentration of surface sites and adsorbed species. The resultsshow no simple correlation for the Lewis acid sites, but they doreveal a direct correlation between the concentration of Brønstedacid sites (as indicated by the intensity of a band at 1430 cm-1)and NOx conversion[39]. Clearly, the Brønsted acid sites appear tobe the catalytically important sites for NH

3 adsorption. Separate

experiments have shown that the V-OH groups on the vanadia

Figure 7 In situ IR spectra of 6% V2O5 /TiO2 during TPSR in

(a) O2 /Ar,

(b) NO + O2 /Ar, and

(c) NO/Ar.

The first and the second spectra from the rear are obtained at room tempera-

ture in flowing O2 /Ar prior to adsorption, and in flowing NH3 /Ar, respectively.

The subsequent spectra are obtained in the various gas streams from 325 K to

625 K (the front spectra) at 50 K increments. (Adapted from ref. [32]).

Figure 7c

surface are the Brønsted acid sites[32]. Thus, the M sites in theabove semi-empirical mechanism are V-OH groups.Further information on the surface sites and their catalytic sig-nificance was obtained by looking at the changes in the nature ofadsorbed species and the changes in gas composition over thecatalyst during temperature-programmed surface reaction (TPSR).For example, Figure 7 compares the in situ infrared results ob-tained during the course of such TPSR studies over a preoxidized6% vanadia/titania catalyst pre-saturated with ammonia, in O

2,

N2, and a mixture of NO+O

2. The initial spectra of the catalyst

saturated with ammonia show absorption bands due to NH4+ spe-

cies (bands at 3020, 2810, 1670, and 1430 cm-1) and coordinatedNH

3 (bands at 3364, 3334, 3256, 3170, and 1602 cm-1). Clear

differences are seen in the manner that these spectra change whenthe temperature is raised in the different gases. In general, theammonia absorption bands disappear gradually with increasingtemperature, but at different rates. The results show that thepreadsorbed ammonia species are removed fast in the mixtureNO and O

2 (Fig. 7b), and slowly in O

2 alone (Fig. 7a). The corre-

sponding gas compositions during TPSR are shown in Figure 8.The production of N

2 parallels that of water during the initial ex-

posure of the ammonia-saturated catalyst surface to the mixtureof NO and O

2 (Fig. 8b), indicating that the SCR reaction pro-

ceeds. In contrast, mainly gas phase ammonia is seen when theTPSR is carried out in O

2 alone (Fig. 8a). Thus, the disappearance

of ammonia absorption bands in mixture of NO and O2 in the

infrared spectra (Fig. 7) is attributed to the SCR reaction, while

NO

NH3

Co

ncen

trat

ion,

ppm

Temperature, K

100

50

300 400 500 6000

H2O

N2

NO2

Figure 8b Figure 8c

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their disappearance in O2 is mainly due to desorption of ammo-

nia. Figure 8c shows that some SCR reaction also occurs whenthe ammonia pre-saturated catalyst surface is exposed to NO alone,but to a lower extent than in the NO - O

2 mixture.

A closer examination of the infrared spectra reveals importantchanges in both the V-OH and the V=O groups during the reac-tion cycle[32,39]. It is seen that after the initial intensity decrease ofthe V-OH band (at 3640 cm-1) due to interaction with ammonia, anew, more reduced V-OH is formed (at 3660 cm-1). At the sametime, a downward shift is seen in the V=O band reflecting a weak-ening of the V=O bond. This observation indicates that the V cat-ions have undergone reduction, most probably by the transfer (per-haps partial) of H from the adsorbed NH

3 to the surface vanadyl

groups to form an activated ammonia species. The formation ofnew reduced V-OH is consistent with this. Thus, the S sites in themechanism appear to be V=O groups. Upon further increase intemperature during the TPSR, V=O groups reappear with the si-multaneous appearance of the original V-OH.These experimental observations have been summarized in a cata-lytic cycle for the SCR reaction over vanadia/titania catalysts(Scheme 1)[39,40]. Basically, the reaction involves both acid-baseand redox catalytic functions. The reaction is initiated by the ad-sorption of ammonia on Brønsted acid sites (V5+-OH), followedby “activation” of ammonia via reaction with redox sites (V=O).This “activated” form of ammonia then reacts with gaseous orweakly adsorbed NO, producing N

2 and H

2O while releasing V4+-

OH. To complete the catalytic cycle, the V4+-OH species is oxi-dized by either NO or O

2 to regenerate the original V=O species.

Our studies[40] also show that the type of activity correlationchanges at high and low O

2 pressures. At high oxygen pressure

(8%), a rather good correlation is seen between the NOx conver-

sion and the product of the concentration of the Brønsted acidsites and the partial pressure of NO at the reactor outlet, whereasat low O

2 pressure, the activity appears to correlate better with the

product of the concentrations of the Brønsted acid sites and vanadylgroups. These results suggest that the relative importance of thedifferent steps in the catalytic cycle changes depending on the

Scheme 1 Catalytic cycle for the SCR reaction over vanadia/titania. (Adapted from ref. [40]).

Figure 9 Comparison of experimentally measured NO conversion (XNOx%

(exp)) and calculated NO conversion (XNOx% (fit)) based on a proposed cata-

lytic cycle for SCR.

O=V5+

NO

O2

H2O

V5+-O-H

NH3

N2 + H2OH-O-V4+

V5+-O-...+H3N...H-O-V4+

V5+-O-...+H3N-N=O...H-O-V4+

V5+-O-...H-N+H3

V5+-O-...+H3N-N=O

Acid Redox

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oxygen concentration. Changes in the significance of the reac-tion steps may also be expected upon other variations in reactionand catalyst parameters. Such effects have been recently observedby Amiridis and Solar[41] for V

2O

5/TiO

2, V

2O

5/TiO

2/SiO

2 and V

2O

5-

WO3/TiO

2 catalysts with varying vanadia loadings.

The two examples above illustrate why simple power law kineticexpressions will not always apply over a wide range of reactionconditions. However, a recent microkinetic analysis[42] using thecatalytic cycle proposed in Scheme 1 showed that one can quan-titatively describe both the laboratory (Fig. 9) and industrial ki-netic data for the SCR reaction over a wide range of conditions.

Questions and answersWhat are the major future challenges

for the Selective Catalytic Reduction process?The main challenges will continue to depend on politicsand on environmental legislation. We still need to find sat-isfactory solutions to NOx removal from mobile dieselengines. This will no doubt require an integration and cou-pling of the operation of the engine and the SCR process.Also, in response to the environmental debate over CO2

emissions, burning of biomass has in some countries beenconsidered as an alternative to the combustion of fossilfuel. This poses new challenges for SCR since, for example,alkali metals, which are serious catalyst poisons, are presentin fairly large quantities in bio-fuels. Separate alkali metalsremoval procedures may thus have to be introduced. Theuse of V-containing fuels like heavy oil or Orimulsion alsopresents a challenge, as accumulation of V2O5 is a problemwith these fuels.

Why is the vanadia/titania system the most widely usedcatalyst for SCR?Today vanadia/titania is still the most active and economi-cal SCR catalyst and it exhibits the best resistance to wa-ter vapor and to poisoning by sulfur. Other catalysts basedon noble metals, metal oxides and zeolites have been widelyinvestigated. However, none of these systems can meet theperformance of vanadia/titania yet.

How do we reduce the “ammonia slip” from SCR pro-cesses?The “ammonia slip” or the unreacted ammonia from theSCR process can be reduced by installing either more orimproved catalysts. Furthermore, it is very important tooptimize the design of the ammonia injection system toensure good mixing of the ammonia with the exhaust gassuch that a uniform distribution of NH3/NOx is achievedthroughout the catalyst bed. Ammonia slip is eliminatedin the SNOX process[9] which combines SCR with SO2

oxidation to produce sulfuric acid. In this process, surplusammonia is oxidized to nitrogen and NOx in the down-stream SO2 oxidation reactor.

Can one use other reducing agents than ammonia?Yes, there has been an interest to replace ammonia withother reducing agents in order to avoid possible hazardsassociated with the storage and transportation of liquidammonia. Other nitrogen containing compounds such asurea, melamine and cyanuric acid have been found to givealmost similar catalytic performance as ammonia[12]. Theycan be considered as convenient ammonia storage mediaand are decomposed to ammonia prior to the SCR reac-tion. Hydrocarbons have also been tried as reductants forNOx[43], but the performance is still insufficient for com-mercialization. Although direct catalytic decomposition ofNOx seems to be an attractive alternative, and extensiveresearch has been done in this area, it has not yet movedbeyond the laboratory phase.

How relevant is microkinetic analysis for process design?Since microkinetic models are based on actual molecularinsight, they have the potential of guiding both catalyst andprocess research and developments. One could add thatin catalysis, empirical kinetic models are often sufficient ifthe goal is mainly to provide the basis for design. However,such empirical models often break down when one has todescribe environmental processes like the SCR[15,16], wherelarge concentration changes are encountered. This in factis seen to be the situation in SCR and it was one of thereasons for obtaining the molecular basis for a microkineticanalysis.

SummarySome industrial aspects of selective catalytic reduction of NOxhave been addressed with respect to process layout, catalyst con-figuration and problems encountered in industrial operation. Fur-thermore, we discussed the difficulties one often encounters whentraditional kinetics is used to describe environmental processes.One source of complication is the necessity to cover large con-centration changes, since it is desirable to remove the pollutantsto the ppm-range. The widely used power rate laws are often in-adequate in these “ppm” processes and the use of such expres-sions may lead to large errors in the recommended catalyst vol-ume[15]. The problem can be avoided by using the so-calledmicrokinetic analysis, which is based on a good fundamental un-derstanding of the nature of the reaction and the active sites. Wealso discussed how the use of combined in situ on-line spectro-scopic and catalytic activity studies establish the catalytic cycle,which forms the basis for a microkinetic analysis, which is able toprovide an adequate description of the kinetic data from both labo-ratory and industry over a wide range of conditions. Such de-tailed understanding of the fundamental aspects of the catalystsystem and the reaction mechanism under realistic industrial con-ditions will aid in guiding future reactor design and in developingimproved catalysts for the removal of NOx.

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[1] White Paper: Selective Catalytic Reduction Controls to AbateNOx Emission, Industrial Gas Cleaning Institute, Inc., July 1989,Washington, D. C.

[2] J.H. Seinfeld, Science 243 (1989) 745.[3] H. Bosch, and F. Janssen, Catal. Today 2, (1988) 369.[4] U.S. Environmental Protection Agency. “National Air Quality

and Emissions Trends Report, 1988” U.S. Govt. Rept. No. EPA-450/4-91-002 (1990).

[5] N. Nojiri, Y. Sakai, and Y. Watanabe, Catal. Rev. -Sci. Eng. 31,(1995) 145.

[6] F. Nakajima, Catal. Today, 10 (1991) 1.[7] J. Baker, Eur. Chem. News, 57, (1991) 16.[8] P. Forzatti, and L. Lietti, Heterogeneous Chemistry Review, 3,

(1996) 33.[9] S.M. Duranni, Environ. Sci. Technol. 28 (1994) 88A.[10] J. Andreasen, and P. Morsing, International Power Generation

Magazine, 1990.[11] J.N. Armor, Appl. Catal. 1 (1992) 221.[12] F. Nakajima, I. Hamada, Catal. Today, 29 (1996) 109.[13] H. Gutberlet, VGB Kraftwerkstech. 68 (1988) 287.[14] J.A. Dumesic, D.F. Rudd, L.M. Aparicio, J.E. Rekoske, and A.A.

Trevino,”The Microkinetics of Heterogeneous Catalysis” Am.Chem. Soc. Washington, D.C.. 1993.

[15] J.R. Rostrup-Nielsen, Elementary Reaction Steps inHeterogeneous Catalysis, (R.W. Joyner and R.A. van Santen(eds)) p441-460, Kluwer Academic Publishers, Dordrecht, 1993.

[16] M. Boudart, Ind. Eng. Chem. Res. presented at Spring ACSMeeting, San Diego, 1994, Symposium on Catalytic ReactionEngineering for Environmentally Benign Processing.

[17] J.A. Odriozola, H. Heinemann, G.A. Somorjai, J.F. Garcia de laBanda, and P. Pereira, J. Catal. 119 (1989) 71.

[18] G.T. Went, L-J. Leu, R. R. Rosin, and A.T. Bell, J. Catal. 134(1992) 492.

[19] N.-Y. Topsøe, and H. Topsøe, Catal. Today 9 (1991) 77.[20] M. Gasior, J. Haber, T. Machej, and T. Zeppe, J. Mol. Catal. 43

(1988) 359.[21] T.Z. Srnak, J.A. Dumesic, B.S. Clausen, E. Törnqvist, and N.-Y.

Topsøe, J. Catal. 135 (1992) 46.

[22] M. Takagi, T. Kawai, M. Soma, T. Onishi, and K. Tamaru, J.Catal. 50 (1977) 441.

[23] M. Takagi-Kawai, M. Soma, T. Onishi, and K. Tamaru, Can. J.Chem. 58 (1980) 2132.

[24] M. Inomata, A. Miyamoto, Y. Murakami, J. Catal. 62 (1980) 140.[25] N.-Y. Topsøe, J. Catal. 128 (1991) 499.[26] A. Miyamoto, K. Kobayashi, M. Inomata, and Y. Murakami, J.

Phys. Chem. 86 (1982) 2945.[27] H. Bosch, F.J.J.G. Janssen, J. Oldenziel, F.M.G. van den Kerkhof,

J.G. van Ommen, and J.R.H. Ross, Appl. Catal. 25 (1986) 239.[28] F.J.J.G. Janssen, F.M.G. van den Kerkhof, H. Bosch, and J.R.H.

Ross, J. Phys. Chem. 91 (1987) 5921.[29] G. Ramis, G. Busca, V. Lorenzelli, and P. Forzatti, Appl. Catal. 64

(1990) 243.[30] R. Rajadhyaksha, and H. Knözinger, Appl. Catal. 51 (1989) 81.[31] J.P. Chen, and R.T. Yang, J. Catal. 125 (1990) 411.[32] N.-Y. Topsøe, H. Topsøe, and J.A. Dumesic, J. Catal. 151 (1995)

226.[33] J.A. Dumesic, N.-Y. Topsøe, T. Slabiak, P. Morsing, B.S. Clausen,

E. Törnqvist, and H. Topsøe, Proc. of 10th InternationalCongress on Catalysis, Budapest, 1325 (1993).

[34] G. Busca, Langmuir 2, (1986) 577.[35] M. Inomata, K. Mori, A. Miyamoto, and Y. Murakami, J.

Phys. Chem. 87 (1983) 761.[36] H. Miyata, Y. Nagakawa, T. Ono, and Y. Kubokawa, Chem.

Lett. (1983) 1141.[37] M. Takagi, T. Kawai, M. Soma, T. Onishi, and K. Tamaru, J.

Catal. 57 (1979) 528.[38] G. Busca, H. Saussey, O. Saur, J.C. Lavelley, and V. Lorenzelli,

Appl. Catal. 14 (1985) 45.[39] N.-Y. Topsøe, Science, 265 (1994) 1217.[40] N.-Y. Topsøe, J.A. Dumesic, and H. Topsøe, J. Catal. 151 (1995)

241.[41] M.D. Amiridis, and J.P. Solar, Ind. Eng. Chem. Res. 35, 3 (1996)

978.[42] J.A. Dumesic, N.-Y. Topsøe, H. Topsøe, Y. Chen, and T. Slabiak,

J.Catal.163 (1996) 409.[43] M. Iwamoto, Catal. Today, 22 (1994) 1.

Nan-Yu Topsøe is a researchscientist in the fundamentalcatalysis research group atthe Haldor Topsøe ResearchLaboratories. She has formany years been involved incatalyst research using infra-red spectroscopy.

Her work has focused on the development of new materials, newsupported metal, oxide and sulfide catalysts for the refining, envi-ronmental and petrochemical industries. She has published over40 papers.

KeywordsSelective Catalytic Reductionnitric oxidesvanadia-titaniacatalytic cycle

AcknowledgmentHelpful discussions with Henrik Topsøe, Torben Slabiak andPer Morsing are greatly appreciated.

References

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