Global Kinetic Modeling of Reactions Occurring during Selective

Ind. Eng. Chem. Res. 1998, 37, 4577-4590

4577

Global Kinetic Modeling of Reactions Occurring during Selective Catalytic Reduction of NO by NH3 over Vanadia/Titania-Based Catalysts B. Roduit, A. Wokaun, and A. Baiker* Laboratory of Technical Chemistry, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland

The global kinetics of the reactions occurring during the selective catalytic reduction (SCR) of NO by NH3 over vanadia-based commercial DeNOx catalysts has been studied. The global kinetic model developed accounts for three chemical reaction pathways (Eley-Rideal and LangmuirHinshelwood type SCR and ammonia oxidation) occurring in this system and considers the effects of intraparticle diffusion phenomena. The model is applied to describe the global kinetics in the broad temperature range from 140 to 475 °C. New insight in the interplay between the processes occurring during SCR has been gained by considering the influence of the concentration of oxygen and water, the weak adsorption of NO at low temperatures (T e ca. 200 °C), the decrease of the NH3 coverage at high temperatures (T e ca. 350 °C), the relevance of the Langmuir-Hinshelwood type SCR at temperatures below ca. 200 °C, the occurrence of NH3 oxidation as a side reaction at temperatures above ca. 300 °C, and the role of intraparticle diffusion. Introduction Vanadia-based catalysts are the most widely used catalysts for the SCR process, because they exhibit high efficiency and resistance to SO2 poisoning. The environmentally important selective catalytic reduction (SCR) of NO with NH3 has been the focus of many investigations in recent years.1-19 Despite the great number of studies aimed at investigating the reaction mechanism and kinetics, many questions concerning these aspects still remain open. Several authors2,20-27 have described the reduction of NO by NH3 in terms of some power law kinetics. First-order kinetics with respect to nitric oxide is postulated for simplification in several investigations.21-26 Although SCR kinetics has been successfully described for confined operating conditions and fixed oxygen concentration, the determination and modeling of kinetics becomes considerably more demanding when an extended range of the experimental conditions is considered. Various models have been applied for this purpose including EleyRideal (ER) kinetic expressions based on the nonnegligible effects of ammonia adsorption,28-37 LangmuirHinshelwood (LH) kinetics,38 and other types of reaction rate expressions.39,40 Some of these models contain four or even more adjusted parameters.39,40 Only a few studies report values for the adsorption enthalpy of NH3.29,32,37,41 Several aspects have to be considered when attempting to describe the kinetics in a broad range of experimental conditions. The influences of low oxygen concentration on catalytic activity,42 direct ammonia oxidation by lattice oxygen at high temperatures,9,18,43-44 and the changes of the number of active sites for large temperature variations indicated the necessity of intro* Correspondence concerning this article should be addressed to A. Baiker. Fax: (+41 1) 632 11 63. E-mail: [email protected].

ducing an oxygen surface coverage term in the kinetic expressions. Furthermore, a simplified approach consisting of kinetic expressions of the rate of NO conversion only is not completely satisfactory. Kinetic expressions for the NH3 consumption rate that account for the occurrence of side reactions have been reported only in few cases,45,46 in spite of the fact that the experimental results clearly indicate non-negligible effects of ammonia oxidation.47,48 The SCR reaction rate is likely to become diffusion limited at higher temperatures, and depending on the complexity of kinetic expressions used, the determination of the kinetic constants can give rise to several problems.49 Models that do not account for diffusion restriction inside the catalyst particle,45,46 allow considerable simplification of the mathematical analysis but can result in deficiencies and misinterpretation of the calculated kinetic parameters, when applied without due care. All the above factors have contributed to some discrepancies concerning the kinetic modeling of SCR. This prompted us to develop a model that considers all the above aspects and describes the global kinetics of SCR in a broad range of experimental conditions (140 °C < T < 475 °C, 1 vol % < O2 < 10 vol %, 1 vol % < H2O < 10 vol %, 0 ppm < NO < 1200 ppm, 0 ppm < NH3 < 2000 ppm). The experimental data base was collected using commercial vanadia based catalysts. 2. Experimental Section Kinetic measurements were performed using commercial vanadia-based SCR catalysts. A first set of experimental data was obtained with a ZERONOX catalyst (Hu¨ls GmbH) containing mainly V2O5 (5 wt %) supported on titania (anatase) and a second set of data with commercial catalysts containing 1 and 3 wt % V2O5, respectively. The catalysts were crushed to a sieve fraction of 180-250 µm and calcined for 2 h at 350 °C in a flow of 20% oxygen (99.995%, PanGas) in a nitrogen

10.1021/ie980310e CCC: $15.00 © 1998 American Chemical Society Published on Web 11/12/1998

4578 Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998

balance (99.995%, PanGas) before use. This pretreatment was performed at the beginning of each test series to ensure reproducible catalytic activity. The catalytic studies were carried out in a continuous tubular fixedbed microreactor. A detailed description of the apparatus and the experimental procedures has been given in previous investigations.42,50-51 The steady state was established for all measurements. The reaction gas mixture consisted of 100-1000 ppm NO (99.0%), 202500 ppm NH3 (99.98%), 0-7.5% H2O, and 0-11.5% O2 (99.995%) in an N2 balance (99.995%). Standard experimental conditions were 0.12-4.4 g of catalyst and 43 000 to 555 000 h-1 (NTP) GHSV at a constant pressure of ca. 1.1 bar. Effective diffusivities of the reactants inside the catalyst have been estimated experimentally by applying the same method as previously used by Beckmann.52 The technique of measurement involves the passage of a gas stream inside and outside of a particular monolith channel and measuring the net flux across its walls. The values obtained for DeNO at different temperatures were between 4.47 × 10-7 m2 s-1 (T ) 200 °C) and 9.20 × 10-7 m2 s-1 (T ) 450 °C). 3. Development of the Global Kinetic Model 3.1. Limitation of a Kinetic Treatment Based on First-Order Dependence of the Reaction Rate on NO. Catalytic tests have been performed to elucidate the effect of the concentrations of NO, NH3, H2O, and O2 on NO conversion as a function of temperature. In a first step, in order to gain a general overview of the effects of the reactants, it was checked whether all experimental data obey first-order kinetics with respect to NO (RNO ) kintrcNO), in accordance with the following reaction stoichiometry:

4NH3 + 4NO + O2 f 4N2 + 6H2O

(1)

For this purpose, the relationship given by eq 2, which accounts for possible intraparticle diffusion influences, has been derived:49

( )

pNOout F ln ηk ) Wtot pNOout

(2a,b,c)

where

η)

(

)

1 1 1 ΦS tanh(3ΦS) 3ΦS

and

ΦS )

Rs 3

x

Fck De

Equation 2 assumes steady-state, isothermal conditions and spherical shape of catalyst particles of radius Rs and is derived from the mass balance over a differential shell of thickness dr and radius r with the boundary conditions (bc):

|

dpNO dr

r)0

) 0,

pNO(r ) Rs) ) pNO

(3a,b)

Equation 2 takes into account possible deviation from first-order kinetics with respect to NO caused by

Figure 1. Temperature dependence (Arrhenius plot) of rate constant k derived by assuming first-order kinetics in NO for wet feed (R ) NH3in/NOin). Feed composition: 10% O2, 5% H2O, 1001000 ppm NO, 20-2000 ppm NH3, balance N2. Cat.: 1 wt % V2O5. Key: (O) 100 ppm NO (0.2 < R < 20); (4) 1000 ppm NO (0.5 < R < 1.4); (0) 1000 ppm NO (R ) 1.2); (b) 1000 ppm NO (1.1 < R < 1.4); (2) 250 ppm NO (R ) 1.0). The influence of the oxygen concentration is shown in the inset. Feed composition: 1-10% O2 , 5% H2O, 100-1000 ppm NO, 20-2000 ppm NH3, balance N2. Cat.: Hu¨ls. Key: (]) 1000 ppm NO (R ) 1.0), 1 vol % < O2 < 10 vol %; ([) 1000 ppm NO (R ) 1.0), O2 ) 10 vol %.

internal mass transfer. The plots of the natural logarithm of each calculated reaction rate constants k vs reciprocal temperature, summarized in Figure 1, allow us to confirm whether the first-order kinetics assumption holds true (straight line) or has to be discarded for the considered experimental conditions. It indicates the divergence from the assumed first-order kinetics in NO, i.e., the temperature range and experimental conditions for which the assumption is not valid. The influence of mass transfer is not apparent from Figure 1 due to the fact that mass transfer has been included in the kinetic expression for the data evaluation (eq 2). For the catalyst particles sizes used in the experiments (180-250 µm) strong effects of internal mass transfer (η < 1) occurred at temperatures above 250 °C. The following conclusions can be drawn from the analysis made (Figure 1): (i) SCR kinetics can be well described by first-order kinetics in NO for temperatures between ca. 200 and 300 °C, whereby the apparent activation energy and the pre-exponential factor differ depending on the O2 concentration. (ii) The presence of oxygen increases the reaction rate constant k; however, the increase of the catalytic activity observed for low oxygen concentrations levels off for O2 concentrations between 5% and 10%. (iii) For temperatures lower than 300 °C, the variation of the NH3 concentration has no perceptible influence on the reaction rate constant. (iv) For temperatures below ca. 200 °C, the calculated constants k indicate a decrease of the apparent activation energy and pre-exponential factor. The calculated data can no more be presented by the straight line that describes the dependence calculated for the first-order kinetics in NO for temperatures between ca. 200 and 300 °C. This finding suggests a contribution to NO conversion by an additional reaction pathway occurring at low temperatures only. (v) For temperatures above ca. 300 °C, the calculated constants k become dependent on NH3 concentration.

Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4579

This indicates a deviation from the first-order kinetics in NO, predicted for temperatures between 200 and 300 °C and suggests the occurrence of NH 3 oxidation, which diminishes the amount of reductant for the main SCR reaction (eq 1). In the present work, the ammonia oxidation products were essentially N2 and H2O. The influence of water on the activity of the tested catalysts has already been studied in previous works.42,50 Without water in the feed, the catalyst showed a higher activity for the reduction of NO. Addition of 2.5% water significantly reduced the activity. Further increase of the amount of H2O up to 7.5% resulted in no measurable further loss of activity, indicating that NO conversion is almost independent of the H2O concentration under practical SCR conditions (5-15% H2O) for the examined catalysts. 3.2. Mechanistic Justification for Proposed Global Kinetic Model. Several proposals concerning the mechanisms of the SCR reaction over vanadia-based catalysts1-19,53,54 have been reported in the literature. They agree on the suggestion that the DeNOx reaction involves a strongly adsorbed NH3 species and a gaseous or weakly adsorbed NO species but differ in the nature of the adsorbed reactive ammonia species and of the associated reaction intermediates. The kinetic rate expressions and rate-limiting steps imposed on the global kinetic model have been based on previous findings, which are considered in sequence. (1) The SCR rate depends on the concentration of surface vanadyl species and adsorbed ammonia.2,5-6,10,11,16 (2) SCR proceeds between strongly adsorbed ammonia and gaseous, or weakly adsorbed, nitric oxide.4,6,16,53 (3) Points 1 and 2 have been further developed by a kinetic analysis of Dumesic et al.54 showing that a twostep Eley-Rideal mechanism involving reaction between adsorbed NH3 and gaseous NO is not sufficient. These authors have proposed a three-step mechanism that consists of an equilibrated ammonia adsorption step, an activation of adsorbed ammonia and a subsequent reaction between the activated ammonia species and NO to form the products. The three-step mechanism has been written as follows:54

step 1 (fast):

NH3 + M S NH3 - M

step 2 (slow): NH3 - M + S S NH3 - S + M step 3 (slow): NO + NH3 - S w N2 + H2O + S where M represents an ammonia adsorption site and S is a reactive site. The M sites are associated with the V5+sOH species, and the S sites denote the V5+dO sites.6,11 The sequences of steps consists of a rapid NH3 adsorption on the V5+sOH Brønsted site, the NH3 adsorbed is then activated by the transfer of an H-atom to the adjacent V5+dO site which is reduced to V4+s OH. The ensuing activated ammonia reacts with gaseous or weakly adsorbed NO and leads to the formation of the products. The reduced V4+sOH sites are finally reoxidized to V5+dO. (4) The reaction represented by eq 1 is governing; however, as previously confirmed for several different SCR conditions, it cannot account for the observed overall stoichiometry of the SCR process. At temperatures above 300 °C, the appropriate dosing of ammonia is not equimolar to the amount of NO to be removed due to the occurrence of the undesired ammonia oxidation by oxygen:45,46,55

4NH3 + 3O2 f 2N2 + 6H2O

(4)

(5) The direct oxidation of NH3 occurs between strongly adsorbed ammonia and lattice oxygen, leading to NO as well as N2, N2O, and H2O9,18,43 or mainly to N2 and H2O.44 Considering points 1-5, which depict the principal characteristics of these reactions, the following kinetic rate expressions can be written:

{

dpNO ) RER + RLH dW dpNH3 ) RNO + RNH3-ox. RNH3 ) F dW

RNO ) F

bc: pNO(W ) 0) ) pNOin,

(5a,b)

pNH3(W ) 0) ) pNH3

in

with

RER ) -kERpNOΘNH3ΘO2

(ER-SCR)

(6)

RLH ) -kLHΘNOΘNH3ΘO2

(LH-SCR)

(7)

RNH3-ox. ) -kNH3-ox.ΘNH3ΘO2

(NH3-ox.)

(8)

where

ΘNH3 )

KNH3pNH3 1 + KNH3pNH3

,

ΘNO ) and

KNOpNO , 1 + KO2pO2 + KNOpNO

ΘO2 )

KO2pO2 1 + KO2pO2 + KNOpNO

(9a,b,c)

The proposed reaction rates (eq 6-8) assume the active sites to be directly dependent on the concentration of vanadia and its state of oxidation. Equations 9 imply that NO and oxygen compete for the same adsorption site, whereas ammonia is bound to a different site (V5+s OH). Spectroscopic studies have not indicated significant amounts of adsorbed NO during SCR reaction.3,56,57 However, Bosch et al.58 proposed a redox mechanism, which later has been supported by Janssen et al.,59 where the oxygen vacancies after reduction can be reoxidized by gaseous O2, by lattice oxygen from underlying layers, or by NO, depending on the reaction conditions. Went et al.53,60,61 included an adsorbed NO species in the proposed reaction mechanism, although they did not explicitly specify the adsorption sites. Ozkan et al.9 and Kumthekar et al.18 demonstrated that an interaction between NO molecules and the vanadia lattice occurs only in the form of fast oxygen exchange. A strong ammonia and weak NO adsorption over the same vanadia/titania-based catalyst as used in the present work has been confirmed very recently using thermoanalytic methods combined with mass spectroscopy.44 Consequently, weak adsorption of NO, in the lower temperature range eq 9b leading to a LangmuirHinshelwood type SCR has been taken into account in the global kinetic model. 4. Structure of Global Kinetic Model 4.1. Derivation of Kinetic Rate Expressions and Estimation of Kinetic Constants. The calculation of the reaction rate constants is based on two different procedures:


Figure 3. (A) Temperature dependence (Arrhenius plot) of kaER (s, linear regression, first-order kinetics in NO). Wet feed: 10% O2, 5% H2O, 1000 ppm NO, 1000 ppm NH3, balance N2. Dry feed: 11.5% O2, 1000 ppm NO, 1000 ppm NH3, balance N2. (B) Temperature dependence of KO2 (Hu¨ls) (s, linear regression). Wet feed: 1-10% O2, 5% H2O, 1000 ppm NO, 1000 ppm NH3, balance N2. Dry feed: 2-11.5% O2, 1000 ppm NO, 1000 ppm NH3, balance N2. (C) Temperature dependence of Ka′()kaLHKNO) (s, linear regression). Feed: 10% O2, 5% H2O, 1000 ppm NO, 1000 ppm NH3, balance N2.

Figure 2. Relevance of different reaction pathways and intraparticle diffusion limitation in the global SCR process (cat.: 1 wt % V2O5, dp ) 180 µm). 1: LH-SCR prevalent (kinetically controlled). 2: ER prevalent (kinetics first order in NO) (kinetically controlled). 3: ER prevalent (kinetics first order in NO) (diffusion controlled). 4: ER prevalent, ammonia side oxidation (NH3-ox.) (diffusion controlled). 5: ER prevalent, desorption of ammonia and ammonia side oxidation (NH3-ox.) (diffusion controlled). The estimation of each parameter is done by choosing suitable operating conditions such as partial pressures of the reactants and temperature. The results are plotted as Arrheniustype plots (see Figure 3 and 4).

(i) Under certain experimental conditions some terms in the kinetic expressions (eqs 6-8) can be simplified, allowing us to extract the effect of others. This strategy facilitates the calculation of the kinetic constants (kER, KO2, kLH and KNO, K′ ()kLHKNO), kNH3-ox., and KNH3) in a gradual manner. The procedure applied in the different experimental domains to minimize the statistical correlation between the estimated model parameters is illustrated schematically in Figure 2. The domains in Figure 2 have been chosen as a consequence of the findings presented in Figure 1 and the mass transfer limitation. For temperatures between ca. 200 and 300 °C (2, 3) the SCR rate can be well described by first-order kinetics in NO, which in fact does not reflect the ERmechanism type, because ammonia coverage is maximal (ΘNH3 = 1). The effect of the concentration of lattice oxygen involved in the reaction is taken into account by insertion of an oxygen coverage term. The variation of the oxygen concentration between 1 and 10% allows the estimation of the O2 “coverage” expression term (2,3). At low temperatures (below ca. 200 °C), a dominance of LH-SCR is observed 1. At temperatures above ca. 300 °C the amount of ammonia remaining at the catalyst surface declines due to NH3-ox. reaction (4, 5) and ammonia desorption (5) (ΘNH3 < 1). The strong decrease of the NH3 coverage at higher temperature can also limit the reaction rates. The investigation of transport phenomena (η < 1) has revealed a strong

influence of internal mass transfer processes, especially at high temperatures (3, 4, 5). (ii) The estimation of the kinetic parameters is performed for each isothermal data set by minimizing the total sum of residual squares between the calculated and experimental partial pressures at the outlet of the reactor: n

(pNO ,exp - pNO ,cal)2 ∑ i)1 i

i

(10)

Kinetic parameters are differently dependent on the particular experimental conditions. Compared to the above method (i-ii), conventional multivariable optimization would lead to a nonlinear regression, which consists of adjusting several parameters simultaneously. In that case, the estimated kinetic constants are usually strongly correlated and can contain a high level of uncertainty. The common source of mistakes by applying a multiple nonlinear regression without due care can result in overfitting of the experimental data set; i.e., constants can be estimated in a domain where they are not relevant. The method used in this work provides global estimates of the parameters, because in this system the kinetic constants or terms are not put to zero but are likely to become zero (or one) depending on the experimental domain where they are considered. The method is therefore well suited for the examined system. Next the rate expression applied for the parameter estimation in the different domains (Figure 2) are presented. The results are plotted as a function of the reciprocal temperature in Figures 3 and 4 and all calculated constants are listed in Table 1. 4.2. Medium Temperature Range (200-300 °C, Domains 2 and 3): Determination of kER and KO2. In this temperature range, we can assume: η e 1, ΘNH3 = 1, ΘNO = 0, KO2pO2 . KNOpNO (pO2 . pNO), LH-SCR = 0 because ΘNO = 0, NH3-ox. = 0 (kNH3-ox. = 0). This

Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4581 Table 1. Calculated Kinetic Constants with Linear Regression (ln(kI) ) ln(AI, Koi) - (EI, ∆HI)/RT)a a AER , m3 kg-1 s-1

a σ[ln(AER , m3 kg-1 s-1)]

Hu¨ls 1 wt % V2O5 3 wt % V2O5

1.20 × 2.17 × 107 9.94 × 106 KoO2 , Pa-1 6.01 × 109 AaLH, Pa m3 kg-1 s-1

1 wt % V2O5 3 wt % V2O5

1.86 × 1012 ( 2.1 × 1012 1.19 × 1011 ( 5.9 × 1010 Aa′ () AaLHKoNO), m3 kg-1 s-1 4.85 × 10-8 (2.35 × 10-8*) 2.08 × 10-8 (3.77 × 10-9*) ANH3-ox., Pa m3 kg-1 s-1 7.22 × 106 KoNH3, Pa-1 3.78 × 10-12

1 wt % V2O5 3 wt % V2O5 1 wt % V2O5 1 wt % V2O5 a

109

(1.01 (0.18 ( 0.29 σ[ln(KoO2, Pa-1) (0.59 KoNO, Pa-1 (Tref ) 500 K) σ[ln(Aa′, m3 kg-1 s-1)] (0.13 (0.82 σ[ln(ANH3-ox., Pa m3 kg-1 s-1)] (1.18 σ[ln(KoNH3, Pa-1)] (0.6

a EER , kJ mol-1

a σ(EER , kJ mol-1)

101.9 89.9 75.6 ∆HO2, kJ mol-1 -47.4 ∆HNO, kJ mol-1 1.21 × 105 ( 1.24 × 105 2.63 × 105 ( 1.18 × 105 Ha′ ()EaLH + ∆HNO), kJ mol-1 -42.9 (-53.5*) -52.8 (-67.2*) ENH3-ox., kJ mol-1 76.4 ∆HNH3, kJ mol-1 -139.2

correl coeff R

(4.13 (0.92 (1.30 σ(∆HO2, kJ mol-1) (2.92

-0.989 -0.999 -0.999 correl coeff R 0.996

σ (Ha′, kJ mol-1) (0.47 (2.87 σ(ENH3-ox., kJ mol-1) (8 σ(∆HNH3, kJ mol-1) (3.73

-143.4 ( 0.8 -142.8 ( 0.8 correl coeff R 0.999 0.995 correl coeff R -0.984 correl coeff R 0.986

σ ) standard deviation. * denotes the same calculated constants derived from the nonlinear optimization.

with

KO2pO2|10% kER ) kaER/ 1 + KO2pO2|10% Due to the fact that η e 1, the internal diffusion in the catalyst particle must be considered:

d2pNO 2

dr

|

dpNO dr

bc:

r)0

+

2 dpNO Fc a ) k p r dr De ER NO

) 0,

(13)

pNO(r ) RS) ) pNO

With Figure 4. (A) Temperature dependence of kNH3-ox. (s, linear regression). (B) Temperature dependence of KNH3 (1% V2O5).(s, linear regression). Cat.: 1% V2O5. Feed: 10% O2, 5% H2O, 100 ppm NO, 20-2000 ppm NH3, balance N2.

{


ηa )

In the following part, the determination of the parameters kER and KO2 is carried out in two steps as depicted in Figure 2. 4.3. Oxygen Concentration ) 10%: Determination of kER. The influence of oxygen is normalized at 10% in order to suppress the correlation between kER and KO2, and eq 11a,b become

{

RNO ) F


{

(11a,b)

pNH3(W ) 0) ) pNH3in

dpNO ) -kaERpNO dW dpNH3 RNH3 ) RNO ) F dW

x

RS 3

FckaER De

and

allows to simplify eqs 5a,b to the form:

KO2pO2 dpNO RNO ) F ) -kERpNO dW 1 + KO2pO2 dpNH3 RNH3 ) RNO ) F dW

Φas )

(12a,b)


(

1 1 1 a a Φs tanh(3Φs ) 3Φas

)

dpNO ) -ηakaERpNO dW dpNH3 RNH3 ) RNO ) F dW

RNO ) F


{

(

(14)


)

Wtot F w Wtot pNH3,out ) pNH3,in exp -ηakaER F pNO,out ) pNO,in exp -ηakaER

(

)

(15a,b)

The estimated kinetic constants kaER for different temperatures are summarized in Figure 3A and Table 1. 4.4. Oxygen Concentration e 10%: Determination of KO2. A low concentration of oxygen is sufficient to maintain the activity of the catalyst. The influence of the oxygen concentration on the reaction rates (eqs 6-8) is especially perceptible below 5%, whereas for variation in the range 5-10% the influence of the oxygen concentration on the catalytic activity is rela-


tively small (Figure 1). However, for great temperature changes (140 °C e T e 475 °C; see Figure 5 later) the oxygen “coverage” expressed by eq 9c has to be considered for a proper description of all three reaction rates (eqs 6-8). We can write

{

KO2pO2 dpNO ) -kERpNO dW 1 + KO2pO2 dpNH3 RNH3 ) RNO ) F dW

RNO ) F


(16a,b)


KO2pO2|10% kER ) kaER/ 1 + KO2pO2|10%

(17)

dr2 bc:

KO2pO2 2 dpNO Fc ) kERpNO r dr De 1 + KO2pO2

dpNO | ) 0, dr r)0

(18)

pNO(r ) RS) ) pNO

)

RNH3 ) RNO ) F

dpNO dW

(22a,b)



dpNO ) (-kaERpNO) + dW KNOpNO -kaLH 1 + KO2pO2 + KNOpNO

(

)

RNH3 ) RNO ) F

(23a,b)

dpNH3 dW pNH3(W ) 0) ) pNH3in

where

RS Φs,O2 ) 3 and

ηO2 )

x

FckER

KO2pO2|10% kER ) kaER/ 1 + KO2pO2|10% + KNOpNO

KO2pO2 1 + KO2pO2 De

and

(

1 1 1 Φs,O2 tanh(3Φs,O ) 3Φs,O2 2

KO2pO2 dpNO ) -ηO2kERpNO dW 1 + KO2pO2 dpNH3 RNH3 ) RNO ) F dW

)

KO2pO2|10% kLH ) kaLH/ (24a,b) 1 + KO2pO2|10% + KNOpNO (19a,b)

RNO ) F

{

)


With

{

(

(

KO2pO2 dpNO + ) -kERpNO dW 1 + KO2pO2 + KNOpNO

KO2pO2 KNOpNO -kLH 1 + KO2pO2 + KNOpNO 1 + KO2pO2 + KNOpNO

RNO ) F

Due to the fact that η < 1, the internal diffusion in the catalyst particle must be considered:

+

RNO ) F

4.6. Oxygen Concentration ) 10%: Determination of kLH, KNO. If the oxygen concentration amounts to 10%, eqs 22a,b can be written as

with

d2pNO

{ {

> ER-SCR, NH3-ox. = 0 because kNH3-ox. = 0. This allows us to simplify eqs 5a,b to the form:


(

(20a,b) EaER

pNH3(W ) 0) ) pNH3in KO2pO2

)

Wtot pNO,out ) pNO,in exp -ηO2kER 1 + KO2pO2 F w KO2pO2 Wtot pNH3,out ) pNH3,in exp -ηO2kER 1 + KO2pO2 F

(

Snark et al.4 concluded from TPR/TPD studies that for the present system reactions proceeding according to ER-SCR and LH-SCR mechanisms have the same activation energy. Hence it follows:

)

(21a,b)

The estimated adsorption equilibrium constants KO2 for different temperatures are summarized in Figure 3B and Table 1. 4.5. Low Temperature Range (e200 °C, Domain 1): Determination of kLH, KNO, K′. In this temperature range, we can assume: η = 1, ΘNH3 = 1, LH-SCR

=

EaLH

kaLH

and

)

AaLH

( )

EaER exp RT (25a,b)

In order to reduce the correlation between KoNO and the reaction enthalpy ∆HNO, the adsorption equilibrium constant for NO is expressed as62

(

KNO ) KoNO exp -

(

))

∆HNO 1 1 R T Tref()500 K)

(26)

The outlet NO and NH3 partial pressures are obtained by numerical integration of eqs 23a,b over the mass of the catalyst. The determination of each constant ALH, KoNO, and ∆HNO is performed by minimizing the total sum of residual squares between the calculated and experimental partial pressures (eq 10). The kinetic constants and correlation matrices obtained after optimization are listed in Tables 1 and 2, respectively.

Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4583 Table 2. Correlation Matrices for the Determination of the Adsorption Enthalpy of NO after Nonlinear Optimization 1 wt % V2O5

AaLH

KoNO

∆HNO

AaLH KoNO

1.000 -0.9608 0.6930

1.000 -0.4897

1.000

∆HNO

3 wt % V2O5

AaLH

KoNO

∆HNO

AaLH KoNO

1.000 -0.7163 0.6814

1.000 -1.131 × 10-2

1.000

∆HNO

4.7. Oxygen Concentration ) 10%: Determination of K′ ) kLH KNO. The assumption KO2pO2 . KNOpNO (pO2 . pNO), allows us to write eqs 22a,b in the form

{

(

)

dpNO 1 p ) - kaER + Ka′ dW 1 + KO2pO2 NO dpNH3 RNH3 ) RNO ) F dW

RNO ) F


(27a,b)


With

( ) (( ((

Ka′ ) Aa′ exp -

{

Ha′ ) kaLHKNO RT

(28)

) ) ) ) (29a,b) Ka′

Finally, the determination of each kinetic constant (see Figure 3C and Table 1) is carried out using eq 30:

(

( ) )

pNO,out F ln + kaER (1 + KO2pO2) K ′) Wtot pNO,in

{

(30)

If the oxygen concentration is lower than 10%, instead of eqs 29a,b we obtain

pNO,out ) pNO,in × KO2pO2 KO2pO2 exp - kER + K′ 1 + KO2pO2 (1 + KO2pO2)2 pNH3,out ) pNH3,in × KO2pO2 KO2pO2 exp - kER + K′ 1 + KO2pO2 (1 + KO2pO2)2

(( ((

In the high temperature range, we can make the following assumptions: η < 1, ΘNH3 < 1, LH-SCR = 0 because ΘNO = 0. This allows us to simplify eqs 5a,b to the form

{

RNO ) F

Wtot 1 a pNO,out ) pNO,in exp - kER + Ka′ 1 + KO2pO2 F w Wtot 1 a pNH3,out ) pNH3,in exp - kER + Ka′ 1 + KO2pO2 F

a

4.8. High Temperature Range (300-475 °C, Domains 4 and 5): Determination of kNH3-ox. and KNH3. In this temperature range, the solutions of the reaction rate expressions require solving a system of two first-order nonlinear differential equations (33a,b) which express the reaction progress of both reactants (NO and NH3) along the length of the microreactor. The influence of diffusion on the reaction rate expression is determined by solving a system of two second-order nonlinear equations (eqs 34a,b) in the variables pNH3 and pNO. Due to the nonlinearity of the system of differential equations and the distribution of the boundary conditions (at center and surface of the particle), analytical solutions of these equations and the effectiveness factor cannot be carried out. However, the consideration of the arithmetical average of the NH3 Langmuir isotherm expression (eq 40) between the center and surface of the particle permits an approximate determination of both NO and NH3 partial pressure gradients (eqs 41a,b). Once these gradients have been determined, the numerical integration of eqs 42 a,b over the catalyst mass of the microreactor yields the partial pressures of NO and NH3 at the outlet of the reactor.

) ) ) )

( (

dpNO ) RER + RLH ) dW KO2pO2

-kERpNO -kLH

dpNH3

-kNH3-ox.

KO2pO2 1 + KO2pO2 + KNOpNO

(32)

)

1 + KNH3pNH3

)

(33a,b) pNH3(W ) 0) ) pNH3in

Considering the influence of the internal diffusion (η < 1) on the reaction rates, eqs 33a,b become:

{

d2pNO

bc:

|

dpNO dr

r)0

2 dpNO Fc ) R r dr De NO dr d2pNH3 2 dpNH3 Fc + R ) r dr De NH3 dr2 +

2

) 0,

(34a,b)

pNO(r ) RS) ) pNO,

|

dpNH3 dr

KO2pO2|10% K′ ) Ka′/ 1 + KO2pO2|10%

+


Wtot F

with

KNH3pNH3

) RNO + RNH3-ox. ) RNO +

dW

Wtot F

(31a,b)

+

KNH3pNH3 KO2pO2 KNOpNO + 1 + KO2pO2 + KNOpNO 1 + KO2pO2 + KNOpNO 1 + KNH3pNH3

RNH3 ) F

(

)

KNH3pNH3

1 + KO2pO2 + KNOpNO 1 + KNH3pNH3

r)0

) 0,

pNH3(r ) RS) ) pNH3

The above system of differential equations can be simplified using Rs/3 as the characteristic length and


{

neglecting the first derivate of the partial pressure:49

d2pNO

|

dpNO bc: dx

x)0

2

)

dx d2pNH3 dx2

) 0,

Fc R De NO

(35a,b)

Fc ) R De NH3

pNO(x ) RS) ) pNO,

|

dpNH3 dx

x)0

) 0,

pNH3(x ) RS) ) pNH3

Fc k , De ER

kd,LH )

Fc k , De LH

1 + KO2pO2 + KNOpNO

kd,ER,O2 ) kd,ER

=

KO2pO2 1 + KO2pO2

KO2pO2

(36)

1 + KO2pO2

kd,LH,NO,O2 ) kd,LHKNO

KO2pO2

(37)

(1 + KO2pO2)2

kNH3-ox.,O2 ) kd,NH3-ox.

{

KO2pO2

(38)

1 + KO2pO2

KNH3pNH3 ) (kd,ER,O2 + kd,LH,NO,O2) p 1 + KNH3pNH3 NO

bc:

d2pNO 2

dx

dx

|

dpNO dx

x)0

+ kd,NH3-ox.,O2

) 0,

KNH3pNH3 1 + KNH3pNH3

|

x)0

) 0,

pNH3(x ) RS/3) ) pNH3

Because of the nonlinearity of the system of differential equations (eqs 39a,b), we apply the arithmetic average for ΘNH3 (eq 40) between the center and the surface of the particle in order to get an analytically solvable system of linear differential equations of second order in pNO and pNH3:

1 pNH3

)

x)Rs/3

∫0p

NH3

KNH3p′NH3 1 + KNH3p′NH3

dp′NH3 )

2

pNO

-Q1(1 - Q22)

xQ1(1 + Q2 ) 2

pNO +

Rs Θ h k 3 d,NH3-ox.,O2 NH3

(41a,b)

where

(

Q2 ) exp -

)

Rs xQ1 3

{

RNO ) F

|

dpNO 3De dpNO )dW RsFc dx

)-

x)Rs/3

|

3De dpNH3 RNH3 ) F )dW RsFc dx dpNH3

x)Rs/3

3De -Q1(1 - Q22) p RsFc Q (1 + Q 2) NO x 1 2

)-

(

3De -Q1(1 - Q22) p + 2 NO RsFc xQ1(1 + Q2 )

Rs 1 (K p - ln(1 + KNH3pNH3))) k 3 d,NH3-ox.,O2KNH3pNH3 NH3 NH3

(42a,b) pNH3(W ) 0) ) pNH3in

The numerical integration over the catalyst mass W along the length of the catalyst bed provides the outlet partial pressures of both NO and NH3. As in the previous parameter estimations, the objective function is defined as the sum of residual squares between the calculated and experimental partial pressures at the outlet of the reactor. The adjustment of the two reaction rate constants kNH3-ox. and KNH3 (Figure 4 and Table 1) in eq 42 a,b is made for each isothermal data set and NH3 partial pressure changes between 20 and 2000 ppm.

(39a,b)

pNO(x ) RS/3) ) pNO,

dpNH3 dx

Θ h NH3 )

dx

xQ1(1 + Q2 )


Finally

2

dpNH3

-Q1(1 - Q22)

Knowing the partial pressure gradients at the surface of the particle (eqs 41a,b), the total reaction rate for a given distance along the microreactor can be obtained by applying eqs 42a,b:

because KO2pO2 . KNOpNO for pO2 . pNO, we define

)

x)Rs/3

Fc kd,NH3-ox. ) kNH3-ox. De

KO2pO2

dx2 d2pNH3

)

(KNH3pNH3 - ln(1 + KNH3pNH3))

and

d2pNO

{

| |

dpNO dx

1 × Q1 ) (kd,ER,O2 + kd,LH,NO,O2) KNH3pNH3

With

kd,ER )

The system of differential equations of second order (eqs 39a,b) is solved for both partial pressures pNH3 and pNO. After differentiation, the partial pressure gradients at the surface of the catalyst are obtained:

1 KNH3pNH3

(KNH3pNH3 - ln(1 + KNH3p′NH3)) (40)

5. Results and Discussion 5.1. Operating Domains. The global kinetic model developed in this study takes into account the following important phenomena: (i) occurrence of two competitive reactions: nitrogen oxide reduction by ammonia (eqs 6 and7) and ammonia oxidation by oxygen (eq 8), (ii) strong NH3 adsorption (eq 9a), (iii) weak NO adsorption at low temperature (eq 9b), (iv) influence of oxidation state of the vanadia catalyst (eq 9c) on the activity, and (v) internal mass transfer. The process features relevant for the different temperature regions are presented in Figure 2. Five domains are discernible for the description of the results obtained in the broad field of operating conditions, which can be divided into two temperature regions. In the region up to 250 °C, the chemical reaction dominates (η ) 1, domains 1 and 2), whereas for temperatures exceeding 250 °C intraparticle

Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4585 Table 3. Operating Domains and the Equations Required for the Calculation of the Outlet Partial Pressures of NH3 and NO domain

eqs for the calcn of the outlet partial pressures

1 2 3 4 5

(eqs 31a,b) or numerical integration of (eqs 22a,b) (eqs 21a,b) (eqs 21a,b) numerical integration of (eqs 42a,b) numerical integration of (eqs 42a,b)

diffusion (η < 1, domains 3, 4, and 5) becomes increasingly important. The equations used to calculate the outlet partial pressures in the respective domains are listed in Table 3. They are different due to (i) simplification of some reaction terms depending on the operating conditions (e.g., ΘNH3 = 1, ΘNO = 0, ΘO2 = 1, kNH3-ox. = 0, ...) and (ii) changes in the mathematical treatment of the diffusion problems by applying different systems of differential equations. 5.2. Significance of Estimated Kinetic Parameters. The comparison of the estimated parameters with corresponding literature data can only be rigorously made for the activation energy EER and the adsorption enthalpy of ammonia ∆HNH3. The activation energy obtained in the present study for the 3 wt % V2O5/TiO2 catalyst (75.6 kJ/mol, Table 1) is comparable to the activation energy reported by Inomata et al.63 (69.5 kJ/mol) and Odenbrand et al.32 (73.5 kJ/mol), who considered a confined temperature range (225 °C < T < 325 °C) where intraparticle diffusion phenomena were apparently absent. The activation energy of 89.9 kJ/mol for the 1 wt % V2O5/ TiO2 catalyst (Table 1) is slightly higher than that for the 3 wt % V2O5/TiO2 catalyst. The influence of the vanadia content on the activation energy has been investigated in several studies.37,64,65 By systematically varying the vanadia loading in a series of catalysts (V2O5/TiO2, V2O5/TiO2/SiO2, and V2O5-WO3/TiO2), Amiridis et al.65 showed that the activation energy decreases significantly from ca. 85 to 40 kJ/mol as the V2O5 content is increased from ca. 0.5 to 16 wt %. The resulting increase of the reaction rate was, however, less than 1 order of magnitude due to a concomitant decrease of the pre-exponential factor. This behavior also emerges from the kinetic parameters determined in the present work. An increase of the vanadia content from 1 to 3 wt % resulted in a decrease of the activation energy from 89.9 to 75.6 kJ/mol (Table 1). Based on the application of the kinetic expression

RER ) -kERpNOΘNH3 Ruppel et al.41 calculated an adsorption enthalpy for NH3 of ca. -140 kJ/mol for the broad temperature range 200-500 °C. This value is in good agreement with the value of -139.2 kJ/mol obtained in this study, indicating the necessity of temperatures higher than ca. 350 °C for the determination of the NH3 adsorption coverage term (see Figure 2, domain 5). Recently, a heat of adsorption of -105 kJ/mol over the same vanadia/ titania-based catalyst as used in the present work has been determined using differential scanning calorimetry.44 The adsorption enthalpy for NH3 of -52.5 kJ/ mol reported by Odenbrand et al.32 is significantly lower than our calculated enthalpy of -139.2 kJ/mol (Table 1). The difference may be attributed to the disregard of mass transfer phenomena. Indeed, Odenbrand et

Figure 5. Coverages of NO (inset), O2, and NH3 as a function of temperature.

al.32 observed that the reaction rates obtained from an integral reactor experiment showed significant deviations compared to the rates determined in a differential reactor. They attributed these differences to the presence of mass transfer limitations, although they did not consider it in the determination of their parameters. As emerges from Figure 2, the consideration of mass transfer phenomena is imperative for temperatures higher than ca. 250 °C. The estimation of the adsorption enthalpy of NO is impaired by the fact that both the reaction rate and the temperature range of the LH-SCR pathway are small, giving rise to significant uncertainty in the estimated value. Thus no physical significance should be ascribed to this parameter. 5.3. Coverages of NO, O2, and NH3 and Reaction Rates. In the proposed kinetic model adsorbed ammonia reacts mainly with gaseous NO and lattice oxygen from V2O5, leading to the formation of nitrogen and water. Reaction with gaseous oxygen leads to regeneration of the surface site. The dependence of the surface coverage of the reactants as a function of temperature and concentration is illustrated in Figure 5 for NO, NH3, and O2 (eq 9a,b,c). The calculated very low nitric oxide and large ammonia coverages, which decrease with increasing temperature, are in line with observations reported for vanadia/titania-based catalysts,6 particularly with those made by Maciejewski et al.44 on the same catalyst as used in the present study. The change of the catalytic activity (eq 9c) with increasing temperature corroborates results obtained by Dumesic et al.,54 who also found that a three-step mechanism better describes the SCR behavior in the case of a wide range of experimental conditions than a two-step mechanism. A two step-mechanism involves the reaction between adsorbed NH3 and gaseous (or weakly adsorbed) NO only and does not consider the activation of ammonia by an S site,54 defined later as the V5+dO site by Topsoe.6 To examine the relevance of each reaction pathway as a function of temperature, calculations of the different reaction rates have been performed for NH3 concentrations of 10, 100, 1000, and 2000 ppm, respectively. The reaction rates of NO reduction and ammonia oxidation with oxygen are presented in Figure 6. Except for the LH-SCR reaction (Figure 6A), reaction rates are appreciable only at temperatures higher than ca. 300 °C. The influence of the LH-SCR reaction on the NO conversion is relevant only at temperatures below ca.


Figure 6. Reaction rates of LH-SCR (A), ER-SCR (B), and NH3ox. (C) versus temperature. The NH3 concentrations in ppm are marked on the curves. Gas mixture: 10% O2, 5% H2O, 1000 ppm NO, NH3 (marked on curves), balance N2.

Figure 8. (A) Conversion of NO versus NH3 feed concentrations for different temperatures (s, calculated progress). Feed: 10% O2, 5% H2O, 100 ppm NO, 20-2500 ppm NH3, balance N2. W ) 0.125 g, dp ) 180 µm, Fo ) 250 L/h (NTP). (B) Conversion of NO versus temperature for different R-ratios (NH3/NO ratio in the feed): (s ) calculated progress; (‚‚‚) kinetics first order in NO. Feed: 10% O2, 5% H2O, 100 ppm NO, 100-250 ppm NH3, balance N2. W ) 0.125 g, dp ) 180 µm, Fo ) 250 L/h (NTP).

Figure 7. (A) Conversion of NO versus temperature, LH-SCR and ER-SCR paths. Feed: 10% O2, 5% H2O, 1000 ppm NO, 1000 ppm NH3, balance N2. W ) 4.41 g, dp ) 250 µm, Fo ) 260 L/h (NTP). (B) Conversion of NO and effectiveness factor η versus temperature, influence of the diffusion on NO conversion. Feed: 10% O2, 5% H2O, 1000 ppm NO, 1000 ppm NH3, balance N2. W ) 0.121 g, dp ) 180 µm, Fo ) 258 L/h (NTP).

200 °C (see Figure 7A later) because for higher temperatures the NO coverage falls asymptotically to zero (Figure 5, inset) and consequently the contribution of the LH-SCR reaction rate also approaches zero (Figure 6A). The reaction rate of ER-SCR (Figure 6B) is much faster than the net rate of ammonia oxidation by lattice oxygen (Figure 6C); however, low NO concentration gives rise to growing relevance of this undesired side reaction. 5.4. Influence of Diffusion and Contribution of LH and ER Mechanisms. Figure 7A shows the conversion of NO versus the temperature and the

contribution of the LH-SCR and ER-SCR paths to the overall NO conversion. For temperatures below ca. 200 °C the reaction occurs mainly via the LH-SCR reaction pathway. In the higher temperature range, NO adsorption ceases and as a consequence the LH-SCR dies off. The calculation scheme (Figure 2) based on the considered kinetics allows us to estimate the effects of the influence of the intraparticle diffusion for all three reaction pathways. For temperatures lower than 250 °C the role of intraparticle diffusion may be neglected; however, for higher temperatures, only a proper consideration of the mass transfer processes permits a good fitting of the experimental data, as emerges from Figure 8B. It can be seen that the NO removal efficiency decreases considerably with increasing temperatures due to the growing influence of the diffusion limitation. 5.5. Influence of NH3 Desorption and NH3 Oxidation. For a given temperature, the SCR rates are maximal when the relative ammonia coverage ΘNH3 over the catalyst reaches its highest value, i.e., about 1. Isothermal conversions for NO are plotted in Figure 8A as a function of the NH3 feed concentration. The asymptotic course of the NO conversion curve corresponds to the maximal ammonia coverage (ΘNH3 = 1), where, for a constant O2 concentration, the catalytic behavior can be described well with a kinetic expression of first order with respect to NO and zeroth order with respect to NH3. In that case, the value of the maximum of NO conversion depends only on temperature, flow rate, and mass of catalyst used (catalyst bed length).

Ind. Eng. Chem. Res., Vol. 37, No. 12, 1998 4587

Figure 10. Changes of ΘNH3 and XNO along the length of the microreactor for different R-ratios. Feed: 10% O2, 5% H2O, 100 ppm NO, 50-2465 ppm NH3, balance N2. W ) 0.125 g, dp ) 180 µm, Fo ) 250 L/h (NTP), T ) 425 °C. Key: (2) experimental data; (s) calculated progress of XNO; (- - -) calculated progress of ΘNH3.

Figure 9. ΘNH3 and changes of NH3 and NO concentration along the length of the catalyst bed for R ) 1.2 (A) and 1.0 (B). Note the relevance of the NH3-ox. reaction for low NO concentrations. Feed: 10% O2, 5% H2O, 1000 ppm NO, 1000-1200 ppm NH3, balance N2. W ) 4.41 g, dp ) 250 µm, Fo ) 250 L/h (NTP), T ) 350 °C.

The influence of the R-ratio (NH3/NO ratio in the feed) on NO conversion as a function of temperature is presented in Figure 8B. Depending on the R-ratio, maximal conversion is reached for temperatures between 400 and 450 °C. The decrease of NO conversion at higher temperatures is caused by a decrease of the NH3 coverage and the increasing relevance of NH3 oxidation. If the whole amount of NO contained in the feed is reduced and the temperature is higher than ca. 300 °C, the remaining NH3 fraction (R > 1) begins to be oxidized by lattice oxygen, leading to the formation of nitrogen and water. The significance of the NH3-ox. reaction is presented in Figure 9A. The relevance of the direct ammonia oxidation as compared to the main SCR reaction (ER-SCR) increases with decreasing NO concentration as emerges from Figures 9A,B. Near the entrance of the microreactor (see Figures 9A,B), we denote a weak decline of the ammonia coverage with a significant decrease of both NO and NH3 concentration due to the ER-SCR reaction. However, as soon as the concentration of NH3 passes below ca. 100 ppm, a marked decrease of the NH3 coverage is observable (ca. 60% and 18% of the reactor length in Figures 9A,B, respectively). The essential change of the amount of adsorbed ammonia on the catalyst surface occurs only if the NO concentration is sufficiently high to maintain the ER-SCR reaction rate (eq 6). If the NO concentration drops to zero before reaching the end of the reactor, we can conclude that the diminishing of remaining ammonia depends only on the ammonia oxidation reaction as it emerges from Figure 9A. This reflects the importance of the ammonia oxidation and indicates that

kinetic modeling of concentration profiles based on NO reduction only can lead to serious inaccuracies. An example of the variations of NH3 coverage and corresponding NO conversion for different R-ratios as a function of the catalyst bed length is depicted in Figure 10. The influence of changes of the R-ratio on NO conversion is much higher at low R-values. As it emerges from the presented data and calculations, the change of R from 0.51 to 1.2 leads to increase of NO conversion from ca. 25% to 40%, whereas for the change of R from 5.72 to 11.63 the increase of NO conversion amounts to ca. 3% only. Finally, we can state that the global kinetic modeling of the processes occurring during SCR of NO by NH3 provided interesting insight into the interplay between these processes and can be used as a potent tool to optimize the global SCR reaction, as it has been demonstrated in another study66 dealing with the 3D modeling of a honeycomb SCR reactor. Conclusions The present study aimed at a global kinetic treatment of the processes occurring during the selective catalytic reduction of NO by NH3 (SCR) on vanadia-based catalysts. We discussed the difficulties encountered by the kinetic description of the SCR reaction for a wide range of experimental conditions. The kinetic model described accounts for three different chemical reactions occurring: the main SCR path proceeding via an ER-type mechanism, an SCR path occurring via an LH-type mechanism in the low temperature region (T e 200 °C), and the direct NH3 oxidation at high temperatures (T g 300 °C). These reaction pathways differ in the role played by nitric oxide, either reacting as gaseous or adsorbed species, and ammonia, either reacting with nitric oxide (in the form of gaseous or adsorbed species) or lattice oxygen. The kinetic expressions have been validated in a broad temperature range (140-475 °C). The kinetic results indicate that the oxidation state of the catalyst is decisive for the amount of ammonia that can be adsorbed on the catalyst surface and that a reaction between adsorbed ammonia and oxygen from the gas phase does not occur. The experimental data revealed the necessity of introducing an oxygen surface coverage term in the kinetic expressions. The relevance of the ammonia oxidation depends strongly on the NO


concentration in the gas phase. With the exception of the LH-SCR reaction, sufficiently high reaction rates are obtained at temperatures higher than ca. 300 °C. The influence of the LH-SCR reaction rate on the NO conversion is relevant only at temperatures below 200 °C. In the proposed kinetic model, the presence of intraparticle mass transfer is taken into account. Appropriate changes of the experimental conditions allowed us to simplify some terms in the kinetic expression in such a way that the determination of the kinetic parameters could be accomplished separately and in a gradual manner. A global consideration of the processes occurring during SCR of NO by NH3 is of paramount importance for the proper design of technical SCR reactors. The global kinetic model presented here has been successfully applied in a 3D model describing SCR on a technical honeycomb structure.66 Acknowledgment Financial support of this work by the “Nationaler Energie-Forschungs-Fonds” (NEFF-Project 569) is gratefully acknowledged. Thanks are due to Katalysatorwerke Hu¨ls GmbH, Germany, for providing the ZERONOX catalyst. The authors wish to thank Dr. M. Koebel (Paul Scherrer Institut, PSI, Switzerland) for providing some experimental data concerning the DeNOx-SCR applications. Nomenclature De ) effective diffusion coefficient in the bulk, m2 s-1 ds ) pellet diameter, m Ei ) activation energy, J mol-1 F ) gas flow rate, m3 s-1 ∆Hi ) adsorption enthalpy, J mol-1 a kER ) apparent reaction rate constant for the EleyRideal type reaction between adsorbed ammonia and gaseous nitric oxide, m3 kg-1 s-1 kER ) reaction rate constant for the Eley-Rideal type reaction between adsorbed ammonia and gaseous nitric oxide, m3 kg-1 s-1 kERO2 ) kERΘO2, simplified reaction rate term for the EleyRideal type reaction between adsorbed ammonia and gaseous nitric oxide, m3 kg-1 s-1 Ki ) adsorption equilibrium constant, Pa kintr ) intrinsic chemical reaction rate constant, m s-1 ki ) kintrSp, modified reaction rate constant Koi, Ai ) pre-exponential factors of Van’t Hoff and Arrhenius equations kLH ) reaction rate constant for the Langmuir-Hinshelwood type reaction between adsorbed ammonia and adsorbed nitric oxide, Pa m3 kg-1 s-1 K′ ) kLHKNO, m3 kg-1 s-1 kLHNOO2 ) kLHΘNOΘO2, simplified reaction rate term for the Langmuir-Hinshelwood type reaction between adsorbed ammonia and adsorbed nitric oxide, Pa m3 kg-1 s-1 kNH3-ox.O2 ) kNH3-ox.ΘO2 simplified reaction rate term for the reaction between adsorbed ammonia and lattice oxygen (direct ammonia oxidation), Pa m3 kg-1 s-1 p ) partial pressure, Pa RER ) reaction rate for Eley-Rideal type reaction between adsorbed ammonia and gaseous nitric oxide, Pa m3 kg-1 s-1 RLH ) reaction rate for Langmuir-Hinshelwood type reaction between adsorbed ammonia and adsorbed nitric oxide, Pa m3 kg-1 s-1 RNH3-ox. ) reaction rate for the reaction between adsorbed ammonia and lattice oxygen (direct ammonia oxidation), Pa m3 kg-1 s-1

RNH3 ) total reaction rate for NH3, Pa m3 kg-1 s-1 RNO ) total reaction rate for NO, Pa m3 kg-1 s-1 Rs ) pellet radius, m Sp ) BET surface area, m2 kg-1 T ) temperature, K W ) mass of catalyst, kg Wtot ) total mass of catalyst used, kg Greek Symbols R ) inlet reactant ratio NH3in/NOin Φ ) Thiele modulus η ) effectiveness factor Θi ) surface coverage of component i Fc ) density of catalyst, kg m-3

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Received for review May 19, 1998 Revised manuscript received September 28, 1998 Accepted October 8, 1998 IE980310E

Global Kinetic Modeling of Reactions Occurring during Selective

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