Kinetics of the Selective Catalytic Reduction of NO with NH3 overCuO/γ-Al2O3

Avinash R. Sirdeshpande* and JoAnn S. Lighty

Department of Chemical and Fuels Engineering, 3290 Merrill Engineering Building, University of Utah,Salt Lake City, Utah 84112

A laboratory investigation of NOx reduction by NH3 over sulfated and unsulfated CuO/γ-Al2O3was carried out. The rate of the selective catalytic reduction (SCR) reaction with excess O2 overpresulfated sorbent between 325 and 425 °C in the absence of H2O and HCl was quantified bya first-order expression with a pre-exponential factor of 3.5 × 109 cm3 gm-1 s-1 and an activationenergy of 84.5 kJ/mol. HCl and H2O were shown to be detrimental to the SCR process. Thebehavior of NO and NH3 over the fresh sorbent has an important bearing on reactor design.When NO is not present, NH3 is rapidly oxidized to NO and N2O above 250 °C. When both NOand NH3 are present, the SCR reaction still occurs, but the catalyst activity is much lower.However, above 425 °C the selectivity of fresh sorbent is poor with a significant amount of NH3being oxidized. Some NH3 is also reduced to N2.

Introduction

The emission of NOx gases from anthropologic sourceshas received tremendous attention because they areproven precursors of acid rain, the greenhouse effect,and photochemical smog. Legislation by federal authori-ties requires NOx levels in the stack gases of fossil fuel-based power plants to be controlled to levels that cannotbe achieved by mere combustion modifications. In-process NOx control measures can reliably reduce emis-sions by about 50%, but higher removal efficienciesrequire external or add-on techniques. Selective cata-lytic reduction of NOx by NH3 is a well-tested technologythat is capable of achieving efficiencies in excess of 90%.The conventional selective catalytic reduction (SCR)processes employ alumina- or titania-supported V2O5,Cr2O3, or Fe2O3 catalysts that are selective in the sensethat NH3 preferentially reduces NO to N2 rather thanbeing oxidized to NO in the presence of O2. Thesecatalysts have been widely studied by several research-ers and have been covered in an exhaustive review byBosch and Janssen.1 The rate of NOx reduction over allthese catalysts has been successfully correlated by apower law expression:

The reported values of the observed apparent orders a,b, and c range from 0.6 to 1.0, 0 to 0.2, and 0 to 0.25,respectively, with pre-exponential factors between 3.10× 104 and 54 × 1017 s-1 (mol/cm3)-c and activationenergies between 24 and 115 kJ/mol. These results comefrom a comparison and analysis of 27 different catalystsby Marangozis2 in 1992.

On one hand, processes that use conventional SCRcatalysts to remove NOx must use a different processto get rid of the SO2. On the other hand, the copper oxideprocess can simultaneously remove SO2 and NOx. The

copper oxide process is a generic term used for a dry,regenerative flue gas cleanup system that employsalumina pellets impregnated with CuO as a sorbent/catalyst.3 SO2 is removed by reactive adsorption to formsulfates. The primary reaction involves CuSO4 forma-tion, but some of the alumina may also react to formAl2SO4. The CuO/CuSO4 combination can act as acatalyst for the selective reduction of NO with externallyinjected NH3. After the capacity of the sorbent for SO2has been exhausted, the sorbent may be regeneratedusing H2 or CH4. During the regeneration step, CuSO4is first reduced to elemental Cu that is subsequentlyoxidized to CuO in the presence of oxygen. The processhas been practiced in fixed-, moving-, and fluidized-bedreactors. Simultaneous SO2/NOx removal in the sametemperature range in one reactor has been shown to bemore economical than alternative techniques that treatSO2 and NOx separately.4 Further, the problem ofconventional SCR catalysts being poisoned by SO2 is notan issue.

Although several processes for simultaneous SO2/NOxremoval using CuO/γ-Al2O3 sorbents are practiced, thereduction of NOx over this catalyst has received verylittle attention. The open literature has only a fewinvestigations on the SCR reaction using this catalyst,a fact confirmed by a more recent investigation by Jeonget al.5 Kiel et al.6 studied the reduction over silica-supported copper oxide produced by impregnation orcoprecipitation with varying degrees of sulfation. Theyfound the rate of reduction over the catalyst subjectedto several cycles of sulfation-regeneration to follow afirst-order behavior with A ) 1.6 × 107 s-1 and E ) 73kJ/mol for T e 375 °C. The freshly sulfated catalyst wasslightly less active with A ) 9.2 × 107 s-1 and E ) 62kJ/mol for T e 330 °C.

Centi et al.7 studied the DeNOx behavior of sulfatedCuO/γ-Al2O3 as a function of the surface coverage withsulfate species. Although they did not prescribe a rateexpression for NOx removal, their work led to severaluseful observations:

(1) NH3 is adsorbed on the sulfated sorbent to form asurface ammonium sulfate. During this period, NH3 isnot available for NOx reduction.

* Corresponding author. Present address: Department ofChemical and Biochemical Engineering, College of Engineer-ing, 98 Brett Road, Busch Campus, Piscataway, NJ 08854-8058. E-mail: asirdesh@sol.rutgers.edu. Tel.: (732) 445 7061.Fax: (732) 445 2421.

r ) kpNOa pNH3

b pO2

c (1)

1781Ind. Eng. Chem. Res. 2000, 39, 1781-1787

10.1021/ie990795m CCC: $19.00 © 2000 American Chemical SocietyPublished on Web 05/09/2000

(2) The adsorption of NH3 may be strong enough toinhibit the reaction between gas-phase NO and ad-sorbed-phase NH3 via the Eley-Rideal mechanism.

(3) According to a mechanism proposed by Anderssonet al.,8 NO is first adsorbed to form a nitrate-like speciesthat reacts with ammonium ions to form N2O. N2O isfinally decomposed to form N2. Hence, when ammoniais not available to reduce NO, it may desorb as N2O.

(4) When the temperature is increased from 300 to450 °C, the ammonium sulfate complex decomposes toyield NH3 and SO2. Simultaneously, NH3 may beoxidized to NO, further reducing the efficiency.

The objective of this work was to prescribe a rate lawfor the SCR reaction over a sulfated catalyst and studythe behavior of NH3, NO, and NO2 under aerobic andanaerobic conditions over the sulfated and unsulfated(fresh) sorbent/catalyst.

Process Chemistry

The chemistry of the system NH3-NO-O2 participat-ing in a SCR process is complex, especially in thepresence of SO2, HCl, N2, CO2, and H2O. The reactionsmay be broadly classified as main reactions responsiblefor the reduction of NOx and side reactions consumingammonia or nitrogen to produce unwanted products.

The main reactions in the presence of oxygen are

while, in the absence of oxygen, nitrogen oxides arereduced by ammonia according to

The undesired reactions consuming ammonia involvethe formation of sulfates, nitrates, chlorides, carbonates,and nitrogen oxides. Direct oxidation of ammonia mayproduce NO, N2O, or N2:

Nitrous oxide may also be formed by the interaction ofnitric oxide and ammonia:

In the presence of nitrogen dioxide, ammonium nitriteand ammonium nitrate may be produced by

The presence of SO2 induces the formation of am-monium bisulfite, ammonium sulfite, and ammoniumsulfate:

Low-temperature (below 250 °C) SCR catalysts are oftenpoisoned by ammonium sulfates formed by the reactionof NH3 with SO3 produced by catalytic oxidation of SO2over vanadia. Ammonia tends to form white deposits ofammonium chloride when HCl is present, which foulboiler tubes and downstream ductwork:

Ammonium carbonate may be produced in the presenceof carbon dioxide and water:

At high temperatures, nitrogen in the feed may beoxidized to nitric oxide. However, this reaction occursto a negligible extent at the temperatures involved inSCR processes. Nitric oxide is itself oxidized to formnitrogen dioxide above 400 °C. Nitrogen dioxide canrapidly equilibrate with nitrogen tetraoxide.

According to Bosch and Janssen,1 reaction (2) is thepredominant reaction under flue gas conditions, that is,dilute mixtures of NO and NH3 with O2 in large excess.The formation of solid compounds is a more seriousconcern in the pipes leading to and from the reactor,which may be relatively cooler (see Table 1).

Experimental Section

Apparatus and Conditions. The experiments wereperformed in a fixed-bed reactor operated in the integralmode. The reactor is a quartz tube, 8.1 mm in diameter,with a porous quartz frit to support the catalyst. Thesorbent used in all tests was ALCOA 25351-35-5 ob-tained from the ALCOA Technical Center in Pittsburgh,PA with the following properties: 4.52 wt % Cu, particledensity ) 1175 kg/m3, BET surface area ) 167.8 m2/g,pore volume ) 0.34 cm3/gm, and average pore diameter) 80 Å. The catalyst was prepared by sulfating thesorbent at 375 °C with 2500 ppm SO2 and excess O2until the bed was saturated (as indicated by an exit SO2concentration equal to the feed SO2 concentration). Atthis point, the sorbent/catalyst was assumed to becompletely sulfated. The tests were carried out with

Table 1. Stability of Ammonium Compounds

compound T (°C) action

(NH4)2CO3 58 decomposesNH4Cl 340 sublimesNH4HSO3 150 sublimes(NH4)2SO3 60-70 decomposes

150 sublimes(NH4)2SO4 235 decomposesNH4NO2 60-70 explodesNH4NO3 169.6 melts

4NO + 4NH3 + O2 f 4N2 + 6H2O (2)

4NO2 + 4NH3 + O2 f 3N2 + 6H2O (3)

6NO + 4NH3 f 5N2 + 6H2O (4)

6NO2 + 8NH3 f 7N2 + 12H2O (5)

NO2 + NO + 2NH3 f 2N2 + 3H2O (6)

4NH3 + 5O2 f 4NO + 6H2O (7)

2NH3 + 2O2 f N2O + 3H2O (8)

4NH3 + 3O2 f 2N2 + 6H2O (9)

4NO + 4NH3 + 3O2 f 4N2O + 6H2O (10)

8NO + 2NH3 f 5N2O + 3H2O (11)

2NO2 + 2NH3 + H2O f NH4NO2 + NH4NO3 (12)

2NO2 + 2NH3 f N2 + H2O + NH4NO3 (13)

SO2 + NH3 + H2O f NH4HSO3 (14)

NH4HSO3 + NH3 f (NH4)2SO3 (15)

2NO2 + 4(NH4)2SO3 f N2 + 4(NH4)2SO4 (16)

NH3 + HCl f NH4Cl (17)

2NH3 + CO2 + H2O f (NH4)2CO3 (18)

N2 + O2 f 2NO (19)

2NO + O2 f 2NO2 (20)

2NO2 T N2O4 (21)

1782 Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000

0.025-0.1 g of catalyst particles in the size range of210-250 µm. The catalyst was diluted with 0.4 g ofquartz beads that were previously determined to beinert under the operating conditions. The length of thecatalyst bed under these circumstances was 0.5-0.8 cm.These conditions ensure a low-pressure drop and a ratethat is neither too low nor too high to result inunacceptably small or large conversions. The inlet andexit concentrations of the gas mixture (NO, NO2, N2O,and NH3) were measured with a Magna-IR 550 FTIRfrom Nicolet with a Foxboro model LV7 low-volume (236mL) multipath (up to 7.25 m) gas cell. The regions usedfor quantification and sample spectra are as follows:NO, 1873-1877 cm-1; NO2, 1580-1634 cm-1; N2O,1230-1325 cm-1; NH3, 906-910 cm-1. O2 was used inlarge excess in all tests except in those where it wasdeliberately absent. A schematic representation of theapparatus is given in Figure 1.

Experimental Results. Tests with Fresh Sorbent.Preliminary tests were carried out to study the behaviorof NO and NH3 over fresh sorbent in the presence ofoxygen. Figure 2 shows the results of NO oxidation inthe absence of NH3. When 1100 ppm of NO, correspond-ing to a space velocity of ≈4700 g min mol-1 was passedover the sorbent with no NH3 in the feed, the amountof NO2 in the exit stream increased steadily from 0 ppmat 200 °C to 270 ppm at 500 °C, indicating that NO wasoxidized to NO2 over the sorbent. NO shows no tendencyof being adsorbed on CuO/γ-Al2O3. N2O was not pro-duced at any temperature.

When 1190 ppm NH3 was passed over 0.1 g of freshsorbent at a total flow rate of 0.4 L/min (STP), both NOand N2O were produced in increasing amounts as thetemperature was increased. Between 200 and 275 °C,there were no nitrogen oxides in the exit stream andall the ammonia passed through the reactor uncon-verted. Above 275 °C, N2O began to appear in theeffluent but NO was absent. However, the decrease in

NH3 concentration was more than that correspondingto the stoichiometric amount required for N2O forma-tion. This indicates that ammonia was reduced to N2at temperatures above 275 °C. At 375 °C, the exitstream showed traces of NO which continued to increasesteadily as the temperature was increased up to 500 °Cat which point 210 ppm NO was present. However, theN2O concentration at this point had risen from 8 ppmat 275 °C to 50 ppm, which shows that the formation ofN2O occurs much more slowly than that of NO fromNH3. In these tests, NO2 was not observed in theeffluent. By ≈425 °C, practically all the NH3 wasconsumed. Hence, even at higher temperatures, amajority of the NH3 goes to N2. In practice, this meansthat any NH3 slip from the bed will end up as some NOand N2O at the optimum operating temperature rangeof 325-450 °C for the sulfation reaction. Figure 3 showsthe results of ammonia reduction and oxidation over anunsulfated sorbent.

If both NO and NH3 escape the sulfated section of thereactor, it is of interest to know whether the reductiontakes place in the absence of CuSO4. Stelman9 hasreported that CuSO4 is necessary for the SCR reaction,although earlier work by Blanco et al.10 and the recentresults of Centi et al.7 clearly demonstrate the activityof fresh CuO/γ-Al2O3 toward SCR.

Figure 4 shows the observations made when ≈800ppm NO and 835-1235 ppm NH3 were passed overunsulfated CuO/γ-Al2O3. The NO conversion was 17%

Figure 1. Schematic representation of experimental apparatus.

Figure 2. Behavior of NO over fresh CuO/γ-Al2O3 as a functionof temperature in the absence of NH3 and with excess oxygen inthe feed. Conditions: 0.1 g of sorbent, 0.4 L min-1 (STP), and≈1130 ppm NO in feed.

Figure 3. Behavior of ammonia over fresh CuO/γ-Al2O3 as afunction of temperature in the absence of NH3 and with excessoxygen in the feed. Conditions: 0.1 g of sorbent, 0.4 L min-1 (STP),and 1190 ppm NH3 in feed.

Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1783

at 200 °C and rose steadily to 77% at 425 °C. Asobserved previously, the amount of N2O produced grewsteadily with temperature, rising from 10 ppm at 300°C to 60 ppm at 425 °C. Again, more NH3 was consumedthan that required for NO reduction or N2O formation,which indicates that some NH3 went to N2. However,the relatively large NO reduction for the given feed ratioof NH3:NO shows that the catalyst has a greaterselectivity toward the SCR reaction. Temperaturesgreater than 425 °C were not investigated because NOand NH3 were previously shown to be oxidized to NOand NO2, respectively, under these conditions.

The SCR tests over unsulfated sorbent were repeatedwithout oxygen in the feed. The results are shown inFigure 5 and are similar to those obtained under aerobicconditions except that after reaching a peak of ≈75%at 400 °C, the conversion drops to ≈68% at 425 °C. Atthis point, NH3 is a limiting reagent in the process.

Some further tests were carried out with glass beadsand pure γ-Al2O3 instead of CuO/γ-Al2O3 in the reactor.The results obtained for NH3 and NO oxidation weresimilar but the SCR reaction did not take place. Hence,NH3 and NO oxidation may take place homogeneously.This implies that the lines carrying these gases mustnot be heated to temperatures higher than 300 °C forpreheating purposes. For the same reasons, contact ofthe feed mixture with heated stainless steel was mini-mized. Because pure γ-Al2O3 does not catalyze thereduction of NO with NH3, its role in the SCR process

is truly that of a support to disperse the active phase,CuO, and provide a large surface area for reaction.

Tests with Sulfated Sorbent. The preliminary testswith a sulfated sorbent were directed toward thebehavior of NH3 and NO as a function of temperaturein the absence of oxygen. When NO was passed oversulfated CuO/γ-Al2O3 under anaerobic conditions, noadsorption was observed between 200 and 500 °C: theentire feed NO passed unconverted through the reactor.However, when NH3 was passed over sulfated sorbentunder similar conditions, no ammonia slip was detected,indicating that NH3 was adsorbed as a surface sulfate.When the temperature was raised and the surfacecoverage by the sulfate complex was complete, thebehavior of the excess ammonia was similar to that overunsulfated sorbent: NH3 was oxidized to NO and N2Oor reduced to N2. Hence, even over sulfated sorbent, theNH3 may be consumed by separate paths.

Figure 6 shows the NO reduction data plotted in theform of conversion against the space time, W/FNO

0 ,representing the weight of catalyst per unit molar feedrate of NO at the reactor entrance, with temperatureas a parameter. The results show that the reaction rateincreases with temperature at a fixed value of spacetime and the conversion at any temperature increaseswith space time.

The results with the sulfated sorbent were obtainedunder dry conditions. A series of runs were also carriedout with varying amounts of water in the feed butotherwise constant operating conditions (space velocityand temperature). At 375 °C, it was observed that theNO reduction was ≈100% with no water, 97% with 10%water, and 91% with 25% water in the feed. When theamount of water was increased to 55%, the conversiondropped to 83%, and with 75% water, the highestachievable water concentration with the current experi-mental setup, the conversion of NO was 72%. Hence, a30% drop in conversion may be expected with a verywet flue gas. When similar tests were performed with≈250 ppm HCl in the feed, the conversion again droppedby about 25%. However, in this case the results aredifficult to assess because a number of side reactionscausing the formation of chlorides and sulfates in thecolder regions of the apparatus complicated the mea-surements.

Theoretical Section

Rate Modeling. The experimental results show thatthe reaction proceeds between NO in the gas phase and

Figure 4. Selective catalytic reduction of NO with NH3 over freshCuO/γ-Al2O3 in the presence of oxygen. Conditions: 0.1 g ofsorbent, 0.6 L min-1 (STP), and 800 ppm NO and 835-1235 ppmNH3 in feed.

Figure 5. Selective catalytic reduction of NO with NH3 over freshCuO/γ-Al2O3 in the absence of oxygen. Conditions: 0.1 g of sorbent,0.4 L min-1 (STP), and 1237 ppm NO and 1237 ppm NH3 in feed.

Figure 6. Selective catalytic reduction of NO with NH3 oversulfated CuO/γ-Al2O3 in the presence of oxygen: conversion versusspace time with temperature as a parameter.

1784 Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000

adsorbed NH3 on the catalyst. Following the Eley-Rideal mechanism and the Langmuir-Hinshelwood-Hougen-Watson hypothesis, the rate of reaction isproportional to the gas-phase concentration of NO andthe fraction of the surface covered by adsorbed NH3

where k is the specific reaction rate constant. Thefractional coverage of NH3 according to the Langmuirisotherm is given by

where K is the adsorption equilibrium constant. Usingthe reparametrized version of Arrhenius’ law, the tem-perature dependency of the parameters k and K maybe described by

where k0 and K0 represent the natural logarithms of kand K, respectively, evaluated at the reference temper-ature, T0. The pre-exponential factor of Arrhenius’ lawmay be obtained from k0 and K0 once the activationenergy is known. Thus, the Eley-Rideal rate expressionfor the SCR reaction becomes

Two limiting cases of this expression occur whenammonia is strongly adsorbed and when the adsorptionis weak. In the former case, (26) reduces to the first-order form

while in the latter case a second-order rate law isobtained:

It may be noted that (27) and (28) are special cases ofthe general form

Reactor Model. The experimental fixed-bed reactoris operated in the integral mode. Assuming plug flowof gas, no volume change with reaction, negligible axialand radial dispersion of mass, isothermal conditions,and steady-state operation, a differential mole balanceon the packed bed yields

subject to xNO ) 0 at w ) 0. In (30), w represents theweight of the catalyst at a distance z from the reactorentrance, xNO is the conversion of NO, FNO

0 is the molar

feed rate of NO, and rNO is the reaction rate per unit ofcatalyst weight. Depending on the form of rNO, differentintegrated forms of (30) may be derived. The expressionscorresponding to (26), (27), and (28) are given by (31),(32), and (33) below:

When the rate is given by (29) for arbitrary values of aand b, the final conversion must be obtained by inte-grating the reactor model

In (31)-(34), CNO0 is the molar concentration of NO in

the feed, v0 is the volumetric flow rate at the bed inlet,and M ) CNO

0 /CNH3

0 is the molar feed ratio of NO to NH3in the gas phase prior to the bed.

Data Correlation and Analysis. The data on theselective catalytic reduction of NO over sulfated CuO/γ-Al2O3 were fitted to determine k and K at eachtemperature by using multiple linear regression on (31).Linear regression performed on the natural logarithmsof these parameters as a function of the inverse of theabsolute temperature gave values of the pre-exponentialfactors and activation energies. All 45 data points werethen simultaneously fit to determine the parameters in(24) and (25) using the Levenberg-Marquardt methodof nonlinear regression.11 The parameter estimates andregression summaries are shown in Table 2.

To obtain the above results, 7 points had to beexcluded because they were outliers. The parity plot forthe fit is shown in Figure 7.

rNO ) kCNOθNH3(22)

θNH3)

KCNH3

1 + KCNH3

(23)

k ) exp[k0 -Ea

Rg(1T

- 1T0

)] (24)

K ) exp[K0 --∆Hads

Rg(1T

- 1T0

)] (25)

rNO )kKCNOCNH3

1 + KCNH3

(26)

rNO ) k′CNO (27)

rNO ) k′′CNOCNH3(28)

rNO ) k′′′CNOa CNH3

a (29)

FNO0 dxNO

dw) rNO (30)

Table 2. Parameters in the Eley-Rideal Model for NOReduction with NH3 over Sulfated CuO/γ-Al2O3 for 325 eT e 425 °C

parameterestimate

T0 ) 650 K

95%confidenceintervals

k0 6.5627 (4.017 × 10-4

Ea/(RgT0) 18.5966 (410 × 10-3

K0 0.3675 (3.346 × 10-3

-∆Hads/(RgT0) -38.7787 (3.359 × 10-2

residual mean sum of squares) 0.0038 on 34 degrees of freedomA ) 8.4439 × 101 cm3/(g s)Ea ) 100.5 kJ/molAads ) 2.081 × 10-7 cm3/mol-∆Hads ) 209.564 kJ/mol

ln 11 - xNO

) kWv0

- 1K

1(M - 1)CNO

0ln

1 - xNO/M1 - xNO

;

M * 1, M > xNO

) kWv0

- 1KCNO

0

xNO

1 - xNO; M ) 1 (31)

k′ )v0

Wln 1

1 - xNO(32)

k′′ )v0

WCNO0

1(M - 1)

ln1 - xNO/M

1 - xNO; M * 1, M > xNO

)v0

WCNO0

xNO

1 - xNO; M ) 1 (33)

WFNO

0) 1

k′′′(CNO0 )a+b ∫0

xNO dxNO

(1 - xNO)a(M - xNO)b(34)

Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1785

Table 2 also shows that the adsorption equilibriumconstant K is extremely small in magnitude. Hence, thedata were also fit to the first-order model according to(27) and (32). The Arrhenius plot for the data is shownin Figure 8. The pre-exponential factor and activationenergy obtained in this way were A ) 3.508 × 109 cm3/(mol s) and Ea ) 84.5 kJ/mol. These values fall withinthe range of previously determined experimental valuesfor other catalysts.2

Heat- and Mass-Transfer Limitations. The tem-perature and concentration at the reaction sites on thesurface of the catalyst may be different from the valuesin the bulk of the gas phase because of transportphenomena intrusions. Hence, it is necessary to verifythat the measured rate is indeed the intrinsic rate, thatis, the rate unfalsified by external and internal mass-and heat-transfer resistances. Four dimensionless cri-teria based on the work of Carberry12 were evaluatedat all the experimental points to validate the claim thatthe fitted values of A and E describe the intrinsic rate.The highest values of these parameters are given in(35)-(38) below and show that all the criteria forneglecting transport resistances (less than 5% deviationbetween true and global rates) are satisfied.

Because the experiments were performed under con-ditions where L/dp ) 32 (>30) and dt/dp ) 16.2 (>10),the axial and radial dispersion of mass and heat mayalso be neglected. Hence, the plug-flow assumption forthe gas phase flowing through the interstices of thepacked bed is also valid.

Summary and Conclusions

The selective catalytic reduction of NO on sulfatedand unsulfated CuO/γ-Al2O3 was investigated with the

aim of ascertaining the behavior of NO and NH3 on thesorbent in the presence and absence of oxygen andobtaining a rate expression for the activity of thesulfated catalyst. Assuming the validity of the Eley-Rideal mechanism which postulates that the reactionproceeds between NO in the gas phase and adsorbedNH3 on the catalyst, the rate could be described by atwo-parameter model involving a specific reaction rateconstant and the adsorption equilibrium constant of theLangmuir isotherm. Nonlinear regression of the dataobtained between 325 and 425 °C showed that theadsorption term was small enough to be neglected. Therate was adequately described with a first-order expres-sion. The fitted values of the pre-exponential factor andactivation energy of Arrhenius’ law were 3.508 × 109

cm3/(g s) and 84.5 kJ/mol, respectively, which areconsistent with the range of values reported in theliterature for other SCR catalysts.

A number of interesting observations were made onthe behavior of NO and NH3 on the sulfated and freshsorbents, both in the presence and absence of oxygen:

(1) NO is not adsorbed on sulfated or unsulfated CuO/γ-Al2O3. NH3 is strongly adsorbed on a sulfated sorbent,probably as a surface ammonium sulfate, but not on theunsulfated sorbent.

(2) On one hand, when oxygen is present, NO isoxidized to NO2 over CuO/γ-Al2O3 above 425 °C but N2Ois not formed. On the other hand, NH3 is either oxidizedto NO and N2O or reduced to N2. The reduction of NH3and formation of N2O begin at ≈275 °C whereas theoxidation to NO proceeds at 375 °C. The rate ofoxidation to NO is much higher than that of N2O butconsiderably lower than N2 generation.

(3) When NO and NH3 are passed under aerobic oranaerobic conditions over fresh CuO/γ-Al2O3, the SCRreaction does proceed but the selectivity of the catalysttoward NO reduction is considerably lower than thatof sulfated CuO/γ-Al2O3.

Thus, the optimum temperature for the sulfationreaction (350-400 °C)6 is also an initiation point for anumber of side reactions involving NH3 and limits theefficiency of the catalyst. Another limiting factor is thepresence of water and HCl, which reduce the conversionby about 30%. The effect of water and HCl needs to bestudied in greater detail. From a practical standpoint,these side reactions dictate the need for strict controlof the NH3:NO ratio at the reactor entrance. The rateexpression developed for the activity of the sulfatedcatalyst may be used to determine the control strategyfor the NH3 injection system.

Figure 7. Parity plot for the regression results of the Eley-Ridealmodel.

Ca )robs

kmamcb) 0.036 (<0.05) (35)

ωCa )(-∆Hr)cb

FgcpgTbLe2/3Ca ) 6.42 × 10-4 (<0.05)

(36)

WW )robs

Decb(Vp

Ap)2

) 0.109 (<0.1) (37)

â )(-∆Hr)Decb(1 - Ca)

keTb(1 + ωCa)6.51 × 10-4 (<0.05) (38)

Figure 8. Arrhenius plot for k′ in the first-order model.

1786 Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000

Nomenclature

a, b, c ) partial orders of the SCR with respect to NO, NH3,and O2, respectively.

am ) external surface area for mass transfer per unitvolume of particle, 1/m

A ) pre-exponential factor in Arrhenius’ law, same unitsas the specific reaction rate constant

Ap ) external surface area of particle, m2

c, C ) molar concentration, kmol/m3

CNO0 , CNH3

0 ) molar concentration of NO and NH3 at theentrance of the catalyst bed, kmol/m3

Ca ) Carberry number, defined in (35)cb ) bulk concentration in the gas phase, kmol/m3

cpg ) specific heat capacity of the gas phase, J/(kmol K)dp ) diameter of the catalyst particle, mdt ) internal diameter of the reactor, mDe ) effective diffusivity of SO2 in the pellet, m2/sDm ) molecular/ordinary diffusion coefficient for the gas

phase, m2/sFNO

0 ) molar flow rate of NO at the bed inlet, kmol/sE, Ea ) activation energy, J/kmolk, k′, k′′, k′′′ ) specific reaction rate constants, units

consistent with those of the concentration-dependentterm and the rate

ke ) effective thermal conductivity of pellet, W/(m K)km ) external mass-transfer coefficient, m/skth ) thermal conductivity of gas phase, W/(m K)K ) adsorption equilibrium constant in Langmuir iso-

therm, m3/kmolL ) length of the catalyst bed, mLe ) Lewis number, )Sc/Pr, dimensionlessM ) CNO

0 /CNH3

0 , dimensionlesspi ) partial pressure of species i, PaPr ) Prandtl number, )cpgµg/kth, dimensionlessrobs ) observed rate of the reaction, kmol/(kg s)rNO ) intrinsic rate of reaction of NO per unit catalyst

weight, kmol/(kg s)Rg ) universal gas constant, )8314 (Pa m3)/(kmol K)Sc ) Schmidt number, )µg/(FgDm), dimensionlessT ) absolute temperature, KTb ) bulk temperature of gas phase, KT0 ) reference temperature in modified Arrhenius’ law, Kv0 ) total volumetric flow rate of the gas phase at the

reactor entrance under the actual operating conditions,m3/s

Vp ) volume of catalyst particle, m3

w ) weight of the catalyst at a distance z from the start ofthe catalyst bed, kg

W ) weight of catalyst, kgWW ) Weisz-Wheeler criterion, defined in (37)x ) conversion of NO, (CNO

0 - CNO)/CNO0 , dimensionless

z ) distance from the start of the catalyst bed, m

Greek Symbols

â ) Prater number, defined in (38)∆Hads ) heat of adsorption, J/gmol∆Hrxn ) standard heat of reaction, J/gmolµg ) gas-phase viscosity, Pa sθNH3 ) fractional surface coverage of catalyst surface by

adsorbed NH3, dimensionlessFg ) density of gas phase, kg/m3

ω ) defined in eq 36

Literature Cited(1) Bosch, H.; Janssen, F. J. J. G. Catalytic Reduction of

Nitrogen Oxides: A Review on the Fundamentals and Technology.Catal. Today 1988, 2, 369.

(2) Marangozis, J. Comparison and Analysis of Intrinsic Kinet-ics and Effectiveness Factors for the Catalytic Reduction of NOwith Ammonia in the Presence of Oxygen. Ind. Eng. Chem. Res.1992, 31, 987.

(3) Yeh, J. T.; Drummond, C. J.; Joubert, J. I. Process Simula-tion of the Fluidized-Bed Copper Oxide Process Sulfation Reaction.Environ. Prog. 1987, 6 (2), 44.

(4) Frey, H. C.; Rubin, E. S. Probabilistic Evaluation ofAdvanced SO2/NOx Control Technology. J. Air Waste Manage.Assoc. 1991, 41(12), 1585.

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Received for review November 1, 1999Revised manuscript received March 30, 2000

Accepted April 1, 2000

IE990795M

Ind. Eng. Chem. Res., Vol. 39, No. 6, 2000 1787