Detailed Kinetic Modeling of NH3 and H2O Adsorption, and NH3

J. Phys. Chem. C 2009, 113, 1393–1405

1393

Detailed Kinetic Modeling of NH3 and H2O Adsorption, and NH3 Oxidation over Cu-ZSM-5 Hanna Sjo¨vall,† Richard J. Blint,‡ and Louise Olsson*,† Competence Centre for Catalysis, Chemical Reaction Engineering, Chalmers UniVersity of Technology, 412 96 Go¨teborg, Sweden, and Chemical and EnVironmental Sciences Laboratory, General Motors R&D Center, 30500 Mound Road, Warren, Michigan 48090-9055 ReceiVed: March 20, 2008; ReVised Manuscript ReceiVed: NoVember 25, 2008

The NH3 and H2O adsorption and desorption, and the NH3 oxidation was studied using detailed kinetic modeling and flow reactor experiments. Ammonia storage and ammonia oxidation are important for the NH3 SCR application. In this study, both ammonia storage and oxidation are investigated, with and without the presence of water. Four sites were included in the model. On each copper atom was one active site introduced, denoted S1a, where NH3, H2O, NO2 and O2 can adsorb. However, electron paramagnetic resonance studies (EPR) and also DFT calculations in the literature suggest that [Cu(NH3)4]2+complex are formed in copper zeolites. We therefore introduced three additional sites (S1b) that ammonia can adsorb on in order to add up to the four ammonia adsorbed per copper atom. It was important to separate between S1a and S1b since it is not possible for four NO2 to adsorb per copper and also in order to describe the ammonia TPD and SCR reactions simultaneously. The Cu-ZSM-5 catalyst also contains Bro¨nsted acid sites (S2), and in order to account for the large amount stored at ambient temperature, sites for weakly bound species (S3) were included as well. The Bro¨nsted sites were investigated using NH3 and H2O TPD experiments on H-ZSM-5. Water and ammonia TPD experiments on Cu-ZSM-5 were also used in the model development and the model was able to describe the experiment well. An NH3 TPD experiment with storage performed in the presence of water was used for model validation, and the model was able to predict the experimental results adequately. The model was then extended to include steps for oxygen adsorption, desorption, dissociation, recombination and two summary steps for ammonia oxidation. Ammonia oxidation in both the presence and absence of water was used in the model development. The resulting model was able to predict ammonia storage, desorption and oxidation accurately, both in the presence and absence of water. 1. Introduction Nitrogen oxides are formed during the combustion of fossil fuels. There are several possible approaches to reducing nitrogen oxides in the presence of excess oxygen, and one efficient approach for the reduction of NOx to N2 is the selective catalytic reduction (SCR) with urea or ammonia. The reaction between NH3 and NOx occurs continuously on the surface of a catalyst and there are several catalysts investigated for this application. Catalysts composed of vandia supported on titania are commonly used especially for stationary-source emissions control,1-7 but several zeolites exchanged with metal ions, such as Fe-ZSM-5 and Cu-ZSM-5, have also been tested for ammonia SCR application.8-17 The Cu-ZSM-5 has been widely studied, not only for SCR with ammonia, but also for applications such as NO decomposition and selective catalytic reduction by hydrocarbons.18-21 The typical NOx conversion in the presence of excess oxygen over a Cu-ZSM-5 catalyst initially increases with temperature, reaches a maximum between 200 and 400 °C, and then declines.9-11,22 The declining activity at high temperature is due to the competition between ammonia oxidation and NOx reduction, since the oxidation of ammonia results in a smaller amount of ammonia available for the NOx reduction.8,11,22-24 * Corresponding author. Telephone: +46-31 772 4390. Fax: +46-31 772 3035. E-mail: [email protected]. † Competence Centre for Catalysis, Chemical Reaction Engineering, Chalmers University of Technology. ‡ Chemical and Environmental Sciences Laboratory, General Motors R&D Center.

The SCR activity is influenced by the ammonia concentration in the feed, and varies with both temperature and relative amount in comparison to nitrogen oxides. An inhibiting effect of ammonia has been observed over various catalyst formulations, including ZSM-5 and V-based catalysts.13,25,26 Stevenson et al.27 observed an inhibiting effect over H-ZSM-5, which decreased with increasing temperature. They found that the effect was most likely due to competitive adsorption, with ammonia blocking sites needed for the SCR reaction to take place. A similar inhibition was observed by Eng and Bartholomew,28 who suggested that gas phase NH3 may prevent NOx diffusion to the active adsorbed NH3 species. Wallin et al.26 also investigated the ammonia blocking effect and concluded that the NO oxidation is hindered by adsorbed NH4+ ions. They also found that physisorbed or weakly adsorbed NH3 is likely to be of importance. Information about the adsorption and desorption of ammonia is thus important for the understanding of the SCR activity. In addition, in order to simulate transient experiments it is crucial to have a good description of the ammonia storage and release characteristics which play an important role in determining the transient performance of SCR catalysts. Also, since ammonia oxidation causes a decreased NOx reduction activity at high temperatures, it is important to further investigate the ammonia oxidation. Reaction steps that describe the storage and oxidation of ammonia are often included in SCR modeling. Global models that describe steady-state conditions have been developed for various catalyst formations2,4,27,29-31 and such models do not need a detailed description of the ammonia storage. Other

10.1021/jp802449s CCC: $40.75  2009 American Chemical Society Published on Web 01/07/2009

1394 J. Phys. Chem. C, Vol. 113, No. 4, 2009

Sjo¨vall et al.

TABLE 1: Weight and Ion-Exchange Level of the Catalyst Samples

sample

SiO2/ Al2O3 ratio

weight alumina layer (mg)

weight zeolite layer (mg)

Cu loading (wt-%)12

ion-exchange level (Cu/2 Al)12

H-ZSM512 Cu-ZSM-5-a12 Cu-ZSM-5-b

27 27 27

92 73 78

1002 1038 1010

2.03 2.03

0.70 0.70

authors include one adsorption and desorption reaction in order to account for transient conditions. A step for ammonia adsorption and desorption has been included in kinetic models for ammonia SCR over several catalysts such as Cu-ZSM-5,32 Fe-exchanged zeolites33 and vandia based catalysts.34-36 However, there are no detailed kinetic models that describe the adsorption and desorption of ammonia on the different sites which are available for ammonia storage on copper zeolites. The objective with this study is to perform experiments and develop a detailed kinetic model that describes the ammonia storage as well as the competition between ammonia and water for storage sites. The ammonia oxidation is also investigated, both with and without water in the feed. 2. Experimental Section 2.1. Catalyst Preparation. The catalysts were prepared using H-ZSM-5 powder with silica to alumina ratio of 27 obtained from Alsi-Penta. The Cu-ZSM-5 powder was ion-exchanged in two steps. The H-ZSM-5 powder was initially ion exchanged using a NaNO3 solution at ambient temperature. The ion exchange to sodium was performed twice before the Na-ZSM-5 obtained was placed in an oven and dried. The copper was introduced into the zeolite by exchange in a Cu(CH3COO)2 solution at ambient temperature. The slurry was stirred for several hours, followed by two additional ion exchange steps. After the last exchange, the powder was filtered and washed with distilled water and then the Cu-ZSM-5 was placed in an oven and dried. The aluminum and copper contents were determined by using inductively coupled plasma and atomic emission spectrometry (ICP-AES). The zeolite powder was added to the cordierite monoliths using incipient wetness technique. The monoliths had a cell density of 400 cpsi. Each slurry mixture was composed of a liquid phase containing distilled water and ethanol and a solid phase containing the binder boehmite and Cu-ZSM-5 (or HZSM5). A thin layer of alumina was coated on the catalyst to generate an improved surface for the zeolite attachment. The sample was then calcined before introducing the zeolite layer. The monolith was coated with the zeolite slurry by immersing the monolith in the slurry, blowing away the excess slurry, and drying and heating in air. This procedure was repeated until the monolith was coated with the desired amount of washcoat, followed by the final calcination of the catalyst. Two Cu-ZSM-5 monoliths were prepared at the same time, using the same slurries. They are denoted Cu-ZSM-5-a and Cu-ZSM-5-b.The length and the diameter of the samples were 30 mm and 22 mm, respectively. Details about the ion-exchange and the monolith coating can be found in.12 The catalysts weight and the ion-exchange level are shown in Table 1. 2.2. Flow Reactor Experiments. A flow reactor was used for the storage experiments and the activity experiments. The feed gas composition was controlled by an Environics 2000 gas mixer and the flow reactor consisted of a horizontal quartz tube supplied with a heating coil and insulation. A thermocouple was

placed in front of the catalyst to measure the gas phase temperature; a second thermocouple was placed inside the sample to measure the catalyst temperature. A Bio-Rad FTS 3000 Excalibur FTIR spectrometer was used to measure the concentration of NH3 and H2O. The gas flow rate and the corresponding space velocity were 3500 mL/min and 18 400 h-1, respectively, in all experiments. Prior to each experiment, the catalyst was exposed to 8% O2 in argon for 20 min at 500 °C and cooled in Ar. All experiments were performed at atmospheric pressure and the inert balance was argon. A detailed description of the reactor setup is available elsewhere.12 Temperature programmed desorption (TPD) experiments were carried out using both the H-ZSM-5 and the Cu-ZSM-5-b catalyst. The samples were exposed to either 500 ppm NH3 or 5% H2O for 60 min, followed by 80 min of argon only and a temperature ramp (10 °C/min) in argon. Ammonia was stored at two temperatures, 27 and 150 °C, but water was stored at 150 °C only. A corresponding experiment was made exposing the catalyst to 500 ppm NH3 and 5% H2O simultaneously at 150 °C, followed by a 63 min period of argon only and a temperature ramp (10 °C/min) in argon. Prior to the introduction of ammonia was the catalyst exposed to water for 10 min. Ammonia oxidation experiments were made using the CuZSM-5-a sample and a feed composition of 500 ppm NH3 and 8% O2, with or without 5% H2O. The temperature was increased stepwise from 100 to 400 °C. The temperature was held constant at 100 °C for 50 min, at 150 °C for 20 min and for 10 min at each of 200, 250, 300, 350, and 400 °C. 3. Catalyst Model 3.1. The Reactor Model. In the calculations presented in this study, the monolith is described as a series of continuously stirred tank reactors. The dispersion model in combination with the tanks-in-series model37 were used to estimate the number of tanks needed, which resulted in about 40. However, the simulation time would be very long using 40 elements and the model is only sensitive to changing the number of elements when using few tanks. Ten elements were used by Westerberg et al. 38 in the HC SCR model, instead of the theoretical number of 40-140. We also chose to use 10 elements in this study. The error between using 10 and 15 elements was less than 0.3%. This control was conducted using 12 experiments, including ammonia adsorption, water adsorption, ammonia oxidation and SCR experiments. The assumptions made for the reactor model are: • No gas-phase accumulation • No radial concentration gradients • The energy balance is not solved, since only small difference between inlet gas temperature and measured catalyst temperature was observed. The catalyst bed temperature was used in the simulations. The mass-transfer in the washcoat has been studied experimentally and the results indicated that there were no diffusion limitations in a similar Cu-ZSM-5 catalyst,32 which is the reason for not including it into the model. The film model is used to describe the mass-transfer from the gas to the surface. The mass balance for each gas component (i) in each tank (n) is

Fi,n-1 - Fi,n - kc,i,nAn(cg,i,n - cs,i,n) ) 0

(1)

and the surface balances are presented by reactions 2 and 3.

Detailed Kinetic Modeling

J. Phys. Chem. C, Vol. 113, No. 4, 2009 1395

kc,i,nAn(cg,i,n - cs,i,n) )

∑ νi,jrj,nmwc,n

(2)

TABLE 2: Number of Sites in the Model

j

Ncat

∂Θi,n ) ∂t

∑ νi,jrj,n

(3)

j

The range for the coverage for each specie, Θi, is between 0 and 1, where a coverage of 1 means one molecule per site. 3.2. The Surface Description. The amount of copper exchanged into the zeolite was determined using ICP-AES. The result was used to calculate the theoretical number of Cu sites (S1) in the zeolite. Williamson et al. 39,40 investigated ammonia adsorption on CuY using infrared spectroscopy and Electron paramagnetic resonance studies (EPR). On the basis of their experimental findings, they suggest that [CuII(NH3)4]2+ ions are formed. These results are supported by density functional theory (DFT) calculations by Delabie et al.,41 who found [Cu(NH3)4]2+ complexes coordinated to oxygen atoms in the lattice after saturation of CuY with ammonia. In addition, Komastu et al.8 propose that each copper ion can coordinate four ammonia molecules in their mechanism of ammonia SCR. Based on these experimental and theoretical findings four ammonia molecules can bind to each copper in our model. NO2 temperature programmed desorption experiments (TPD)42 showed that the NO2 storage is much smaller than the ammonia storage on CuZSM-5. About one NOx adsorbed per copper atom fits the experiments well.42 In the model we therefore use one active site on each copper atom, denoted S1a. On this site can all the molecules adsorb (NH3, O2, NO2, H2O) and the reaction always occurs from this site. In order to add up to the four NH3 molecules per copper, which was suggested experimentally and theoretically in the literature we add three additional sites per copper (denoted S1b), where ammonia can adsorb. Also several water molecules can bind to each copper, according to DFT simulations.41 In the model is therefore also H2O allowed to adsorb on S1b. The silica to alumina ratio was 27 which was used to calculate the number of acid sites (S2) in the HZSM-5 catalyst. For low loading and low Si/Al ratio the copper atom (Cu2+) may replace two hydrogen ions (H+), since the sites are sufficiently close.19 Since the ion exchange level is only 70% (Cu/2 Al) and the Si/Al ratio is low (27) we assume that each copper ion in the Cu-ZSM-5 catalyst replace two hydrogen ions, thus only 30% of the initial number of Bro¨nsted acid sites were present after the ion exchange. This was used to calculate the number of S2 sites in the Cu-ZSM-5 catalysts. Ammonia adsorbs very easily, and may adsorb at additional positions in the catalyst. The ammonia TPD conducted at room temperature revealed that the ammonia storage was much larger than the theoretical number of sites. This is likely due to loosely bound NH3, but can also be ammonia adsorbed on alumina, which was used as a binder. A third site (S3), was introduced, which could account for the additional bound ammonia. A site for physisorbed molecules were also proposed by Grossale et al.,43 in their study of NOx adsorption onto Fe zeolites. The total number of sites (S1a + S1b + S2 + S3) was equal to the amount of ammonia adsorbed at ambient temperature and this was used to estimate the number of S3 sites. Since the same zeolite powder was used for Cu-ZSM-5-a and Cu-ZSM-5-b they have the same number of sites per wash-coat weight. These catalysts are prepared using the same slurries and they contain very similar amounts of Cu-ZSM-5, 1038 and 1010 mg, respectively. Further, they are prepared the same time, and after the initial stabilization the samples gave the same conversion (only about 5 ppm difference). The

sample H-ZSM5 Cu-ZSM5-a Cu-ZSM5-b

weak Cu-sites, Cu-sites, Bro¨nsted adsorption S1a S1b acid sites, sites, S3 (mol/kgzeolite) (mol/kgzeolite) S2 (mol/kgzeolite) (mol/kgzeolite) 0.320 0.320

0.959 0.959

0.928 0.288 0.288

1.227 1.227 1.227

TABLE 3: Reactions and Rate Expressions for NH3 and H2O Adsorption on and Desorption from H-ZSM-5 reactions rI,f

NH3 + S2 798 NH3-S2

reaction rate expressions rI,f ) kI,f cNH3θS2-vacant rI,b ) kI,bθNH3-S2

rI,b

rII,f

H2O + S2 798 H2O-S2

rII,f ) kII,f cH2OθS2-vacant rII,b ) kII,bθH2O-S2

rII,b

rIII,f

NH3 + S3 798 NH3-S3

rIII,f ) kIII,f cNH3θS3-vacant rIII,b ) kIII,bθNH3-S3

rIII,b

rIV,f

H2O + S3 798 H2O-S3

rIV,f ) kIV,f cH2OθS3-vacant rIV,b ) kIV,bθH2O-S3

rIV,b

resulting numbers of S1a, S1b, S2, and S3 sites in the three catalysts are shown in Table 2. 3.3. The Kinetic Model. 3.3.1. The Kinetic Model for NH3 and H2O Adsorption oWer H-ZSM-5. It was observed in a previous study14 that ammonia is adsorbed on both Bro¨nsted and Lewis sites in the Cu-ZSM-5 catalyst. Since there are residual Bro¨nsted acid sites in the Cu-ZSM-5 catalyst, NH3 and H2O TPD experiments over the H-ZSM-5 catalyst were conducted in order to differentiate between copper sites and Bro¨nsted sites. In the model for the HZSM-5 catalyst we have used S2 (Acid sites) and S3 (sites for physisorbed species). The ammonia was adsorbed and desorbed from S2 and S3 and one ammonia TPD experiment was used to investigate these steps. In the same way, the water was adsorbed and desorbed from the two sites and one H2O TPD was used to develop the model. The reactions and reaction rates are presented in Table 3. 3.3.2. The Kinetic Model for NH3 and H2O Adsorption and Desorption oWer Cu-ZSM-5. The model contains reactions steps for adsorption on and desorption from all sites available. The Cu-ZSM-5 catalysts are modeled using copper sites (S1a and S1b), Bro¨nsted acid sites (S2) and additional sites for weakly bound species (S3). As described in section 3.2 four ammonia can adsorb on each copper, according to EPR studies39,40 and DFT calculations.41 This is the reason for the addition of ammonia adsorption on both S1a and S1b (one mole S1a and 3 mol S1b per mole copper). It was important to divide the copper sites into S1a and S1b, due to that much lower adsorption of NOx compared to NH3 was observed. We also observed a large ammonia adsorption on the acid sites (S2), which was investigated using H-ZSM-5. Finally, ammonia adsorption on S3 (loosely bound) is crucial at lower temperatures. Grossale et al.43 also propose sites for physisorbed NOx species based on NO2 TPD on Fe zeolites. Zeolites are known to adsorb a lot of water, and DFT simulations41 showed that several water

1396 J. Phys. Chem. C, Vol. 113, No. 4, 2009 TABLE 4: Reaction and Rate Expressions for NH3 and H2O Adsorption on and Desorption from Cu-ZSM-5 reactions r1,f

NH3 + S1a 798 NH3-S1a

reaction rate expressions r1,f ) k1,f cNH3θS1a-vacant r1,b ) k1,bθNH3-S1a

r1,b

r2,f

NH3 + S1b 798 NH3-S1b

r2,f ) k2,f cNH3θS1b-vacant r2,b ) k2,bθNH3-S1b

r2,b

r3,f

NH3 + S2 798 NH3-S2

r3,f ) k3,f cNH3θS2-vacant r3,b ) k3,bθNH3-S2

r3,b

r4,f

NH3 + S3 798 NH3-S3

r4,f ) k4,f cNH3θS3-vacant r4,b ) k4,bθNH3-S3

r4,b

r5,f

H2O + S1a 798 H2O-S1a

r5,f ) k5,f cH2OθS1a-vacant r5,b ) k5,bθH2O-S1a

r5,b

r6,f

H2O + S1b 798 H2O-S1b

r6,f ) k6,f cH2OθS1b-vacant r6,b ) k6,bθH2O-S1b

r6,b

r7,f

H2O + S2 798 H2O-S2

r7,f ) k7,f cH2OθS2-vacant r7,b ) k7,bθH2O-S2

r7,b

r8,f

H2O + S3 798 H2O - S3 r8,b

r8,f ) k8,f cH2OθS3-vacant r8,b ) k8,bθH2O-S3

molecules can bind to each copper. Water was therefore chosen to be able to adsorb on all sites available. The reaction rates for adsorption and desorption of both ammonia and water are presented in Table 4. 3.3.3. The Kinetic Model for Ammonia Oxidation. The detailed mechanism for ammonia oxidation is not well-known. It has been suggested that ammonia is oxidized directly to nitrogen.44 Some authors instead suggest that ammonia is first oxidized to NO, which subsequently reacts with additional ammonia to form nitrogen and water.23,45,46 However, we do not observe NO production during NH3 oxidation and we therefore produce nitrogen directly in our model. Further, in our detailed model, the ammonia oxidation is represented by a summary step, in order to introduce as few parameters as possible (Table 5, reaction 11). The model also includes reaction steps for adsorption and desorption of oxygen on copper sites (S1a). The oxygen is assumed to adsorb molecularly and then dissociate to oxygen atoms on the surface.42 The reaction steps for O2 adsorption, dissociation and desorption, and ammonia oxidation are shown in Table 5. Ammonia is available at four different sites, but the oxidation is modeled as a reaction between oxygen on S1a and ammonia on S1b. It is likely that a reaction will occur between species positioned nearby and, since S1a and S1b represents sites on the same copper, the model include a reaction between species on these sites. Since the ammonia oxidation is a summary step it was not possible to use the

Sjo¨vall et al. stoichiometric coefficients to determine the reaction orders, and they were therefore put to the simplest form which is 1. The reaction order for ammonia was also set to 1 for NH3 oxidation simulations on Cu-ZSM-5,32 HZSM-5,27 Fe ZSM-5,47 commercial Fe zeolites48 and vanadia.49,50 Water was found to inhibit the NH3 oxidation to a quite small extent.12 However, water had a major impact on the NO oxidation.12 Based on detailed kinetic simulations of NO oxidation both with51 and without42 water present we propose that this inhibition is due to large formations of OH groups on the copper. The OH groups are formed from a reaction between oxygen and water on the copper sites and this reaction is showed in Table 5 (reaction 12). In order to simulate the poisoning of the NO oxidation of water, large coverages of stable OH groups are needed.51 However, the NH3 oxidation is only affected very little by the OH groups and we therefore suggest that it is possible for the ammonia to react with these OH groups on the surface and propose a second summary step where ammonia reacts with OH and oxygen to form nitrogen and water (Table 5, reaction 13). In an earlier study we observed SCR activity on HZSM-5.12 At higher temperatures the consumption of NH3 was larger than NO implying NH3 oxidation activity.12 However, the activity was much smaller on H-ZSM-5 compared to Cu-ZSM-5. For example at 350 °C we observed a conversion of 75% of NO and 100% of NH3 on Cu-ZSM-5, but only 10% NO conversion and 25% NH3 conversion on H-ZSM-5. Further, the ion exchange level was 0.70 thus there are less acid sites available compared to copper sites in the catalyst. In order to simplify the model we have therefore chosen not to add reaction steps for NH3 oxidation on S2. 3.4. The Rate Parameters. The rate constants are described by the Arrhenius expression

kj ) Aje-Ea,j/RT

(4)

For reactions where both the pre-exponential factor and the activation energy were fitted, a centered expression was used to describe the rate constant. This expression was used to reduce the correlation between the parameters. The reference temperature, Tref was set to 600 K.

kj ) Aje-(Ea,j/R)(1/T-1/Tref)

(5)

In the Results and Discussion section all parameters are given in the standard form of the Arrhenius expression, eq 4. There is a very large storage at room temperature of ammonia and it increases when decreasing the temperature, which implies that the storage of ammonia is nonactivated or has a very low value. Further, adsorption reactions are often nonactivated or associated with low activation energies and the activation energies for the adsorption reactions were therefore set to 0 kJ/ mol in this model. This was also done for ammonia SCR simulations over Fe zeolites,48 Cu-ZSM-5,32 and vanadia.50 Calorimetric measurements of ammonia adsorption on HZSM-5 showed that the heat of adsorption for ammonia varied between 120-180 kJ/mol.52 According to Topsoe et al.53 and Hunger and Hoffman54 there are three adsorption states for ammonia in HZSM-5 denoted R, β, and γ, with activation barriers of 87, 97 and 163 kJ/mol, respectively.53 Martins et al.55 also propose three different Bro¨nsted sites, based on FTIR and NH3 TPD. Thus it is not possible to model the ammonia adsorption on the Bro¨nsted acid sites (S2) with a single value. In addition, the ammonia TPD experiment over H-ZSM5 showed that the desorption peak was broad. In order to simplify the model we



TABLE 5: Reaction and Rate Expressions for O2 Adsorption, Dissociation, and Desorption,42 OH Production, and Ammonia Oxidation reactions

reaction rate expressions r9,f ) k9,f cO2θS1a-vacant r9,b ) k9,bθO2-S1a

r9,f

O2 + S1a 798 O2-S1a r9,b

r10,f ) k10,f θO2-S1aθS1a-vacant r10,b ) k10,bθO-S1a2

r10,f

O2-S1a + S1a 798 2O-S1a r10,b

r11 ) k11θNH3-S1bθO-S1a

r11

2NH3-S1b + 3O-S1a 98 N2(g) + 2H2O-S1b + H2O-S1a + 2S1a r12,f ) k12,fθO-S1aθH2O-S1a r12,b ) k12,bθOH-S1a2

r12,f

H2O-S1a + O-S1a 9 7 8 r12,b

2OH-S1a r13 ) k13θNH3-S1bθO-S1aθOH-S1a

r13

2NH3-S1b + 2O-S1a + 2OH-S1a 98 N2(g) + 2H2O-S1b + 2H2O-S1a + 2S1a TABLE 6: Entropy Change (J/(mol K)) for Adsorption (Calculated at 600 K) component

NH3

H2O

S ) R ln(q) + RT O242

entropy loss, due to adsorption (J/(mol K)) -167.0 -166.8 -188.3

(6)

Also the high temperature peak, denoted γ, was by Hunger and Hoffmann54 described with a coverage dependent activation barrier. We also found it necessary to introduce, coverage dependence for the desorption from the copper site (S1b) in order to describe the broadness of the high temperature peak. It is likely that there are different strengths of the sites in the same way as for ammonia on the Bro¨nsted acid sites. The same strategy needed to be adopted also for ammonia on S3 and for water adsorption in order to describe the broad peaks. The values for the coverage constant Ri,j for all reactions are shown in Tables 7-11. The pre-exponential factors in most of the desorption rates and surface reaction rates are fixed to 1013 s-1, to avoid correlation between parameters. Since the channels in the zeolite are narrow, the pre-exponential factors for adsorption were not calculated according to the kinetic gas theory. Instead, an estimated entropy change for adsorption was used.

( )

∆S ) R ln

Aads Ades

(7)

The entropy was calculated with the use of partition functions, q.

(8)

The partition function (q) for rotation and translation can be calculated according to reactions 9-11.

use one site (S2) for the Bro¨nsted acid sites and one for loosely bound species (S3) and use coverage dependent activation energies:

Ea,j ) Ea,j(0)(1 - Ri,jθi)

d(ln(q)) dT

qtr,3D ) qrot )

(2πmkBT)3/2

8π2√8π3IxIyIz(kBT)3/2 σrh3

h3

V

(9)

(non-linear molecule)

(10) 2

qrot )

(8π IxkBT) σrh2

(linear molecule)

(11)

According to Sharma et al.56 ammonia adsorbed on H-ZSM-5 loses the translational entropy and approximately one-third of the local entropy (rotation and vibration). The compounds were assumed to adsorb as localized species, and therefore lose all translational entropy and, in addition, one-third of the rotational entropy. The vibrational contribution to the entropy is much smaller compared to the translational and rotational contribution. For example, for ammonia in the gas phase the total entropy is 220.6 J/(mol K) at 600 K,57 and the translation part is 150.3 J/(mol K), the rotation 55.5 J/(mol K). The remaining entropy originates from the vibration and this is only 14.8 J/(mol K). Further, according to Zhdanov58 the partition function for vibration can be neglected for adsorption due to that it is close to unity and he gives one example of CO which has a partition function for two-dimensional translation of about 460, for rotation 180 and for vibration 1.0.58 The loss of vibrational entropy was therefore neglected in our estimation of


Sjo¨vall et al.

TABLE 7: Kinetic Parameters for NH3 Adsorption on and Desorption from H-ZSM-5 rate NH3 NH3 NH3 NH3

adsorption desorption adsorption desorption

(S2 (S2 (S3 (S3

sites) sites) sites) sites)

rate constants

pre-exponential factor

activation energy (kJ/mol)

kI,f kI,b kII,f kII,b

8.67 × 10 9.28 × 1012 a,c 1.15 × 103 b,d 1.23 × 1013 a,c

0 162.59 ( 1.39e 0 117.92 ( 0.31e

2 b,d

coverage dependence (R) -0.398 ( 0.0106 ( · θNH3-S2)e -0.638 ( 0.0100 ( · θNH3-S3)e

a mol/(s kgzeolite). b m3/(s kgzeolite). c Fixed to 1013 s-1, values in table are given in mol/(s kgzeolite). d Calculated from entropy loss. e 13 experiments was used when fitting the parameters, including NH3 TPD at 150 °C on HZSM-5 and NH3 TPD at 30 and 150 °C, H2O TPD at 150 °C, NH3 oxidation with and without H2O, NO oxidation with water and six SCR experiments on Cu-ZSM-5.

TABLE 8: Kinetic Parameters for H2O Adsorption on and Desorption from H-ZSM-5 rate H2O H2O H2O H2O

adsorption desorption adsorption desorption

(S2 (S2 (S3 (S3

sites) sites) sites) sites)

rate constants



kII,f kII,b kIII,f kIII,b

8.90 × 10 9.28 × 1012 a,c 1.18 × 103 b,d 1.23 × 1013 a,c

0 180.70 ( 3.50e 0 119.07 ( 1.87e

2 b,d

coverage dependence (R) -0.404 ( 0.013 ( · θH2O-S2)e -0.897 ( 0.067 ( · θH2O-S3)e

a mol/(s kgzeolite). b m3/(s kgzeolite). c Fixed to 1013 s-1, values in table are given in mol/(s kgzeolite). d Calculated from entropy loss. e 13 experiments was used when fitting the parameters, including NH3 TPD at 150 °C on HZSM-5 and NH3 TPD at 30 and 150 °C, H2O TPD at 150 °C, NH3 oxidation with and without H2O, NO oxidation with water and five SCR experiments on Cu-ZSM-5.

TABLE 9: Kinetic Parameters for NH3 Adsorption on and Desorption from Cu-ZSM-5 rate NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3

adsorption desorption adsorption desorption adsorption desorption adsorption desorption

(S1a) (S1a) (S1b) (S1b) (S2) (S2) (S3) (S3)

rate constants



k1,f k1,b k2,f k2,b k3,f k3,b k4,f k4,b

2.99 × 10 3.20 × 1012 a,c 8.96 × 102 b,d 9.59 × 1012 a,c 2.69 × 102 b,d 2.88 × 1012 a,c 1.15 × 103 b,d 1.23 × 1013 a,c

0 97.78 ( 0.19e 0 229.69 ( 1.39e 0 162.59 ( 1.39e 0 117.92 ( 0.31e

2 b,d

coverage dependence (R) -0.133 ( 0.0062 ( · θNH3-S1a)e -0.907 ( 0.0188 ( · θNH3-S1b)e -0.398 ( 0.0106 ( · θNH3-S2)e -0.638 ( 0.0100 ( · θNH3-S3)e

a mol/(s kgzeolite). b m3/(s kgzeolite). c Fixed to 1013 s-1, values in table are given in mol/(s kgzeolite). d Calculated from entropy loss. e 13 experiments was used when fitting the parameters, including NH3 TPD at 150 °C on HZSM-5 and NH3 TPD at 30 and 150 °C, H2O TPD at 150 °C, NH3 oxidation with and without H2O, NO oxidation with water and six SCR experiments on Cu-ZSM-5.

TABLE 10: Kinetic Parameters for H2O Adsorption on and Desorption from Cu-ZSM-5 rate H2O H2O H2O H2O H2O H2O H2O H2O

adsorption desorption adsorption desorption adsorption desorption adsorption desorption

(S1a) (S1a) (S1b) (S1b) (S2) (S2) (S3) (S3)

rate constants



k5,f k5,b k6,f k6,b k7,f k7,b k8,f k8,b

3.07 × 10 3.20 × 1012 a,c 9.20 × 102 b,d 9.59 × 1012 a,c 2.04 × 103 b,d 2.13 × 1013 a,c 1.18 × 103 b,d 1.23 × 1013 a,c

0 67.70 ( 0.44f 0 184.61 ( 1.49f 0 180.70e 0 119.07e

2 b,d

coverage dependence (R)

-0.638 ( 0.0070 (θH2O-S1b)f -0.404 (θH2O-S2)e -0.897 (θH2O-S3)e

a mol/(s kgzeolite). b m3/(s kgzeolite). c Fixed to 1013 s-1, values in table are given in mol/(s kgzeolite). d Calculated from entropy loss. e Determined from TPD over H-ZSM-5. f 13 experiments was used when fitting the parameters, including NH3 TPD at 150 °C on HZSM-5 and NH3 TPD at 30 and 150 °C, H2O TPD at 150 °C, NH3 oxidation with and without H2O, NO oxidation with water and six SCR experiments on Cu-ZSM-5.

the entropy loss due to adsorption. The calculated entropy changes are shown in Table 6. The parameters not fixed were then fitted to the experiments using the least-squares method. The parameters for water adsorption and desorption on S2 and S3 was fitted to the H2O TPD on HZSM-5. It was important to fit the parameters for the loosely ammonia species to the NH3 TPD at the lowest temperature, which was 30 °C for the Cu-ZSM-5. Since S3 also plays a role in the HZSM-5 catalyst the parameters for these experiments needed to be fitted simultaneously. In addition, when developing the SCR model the values for the ammonia and water adsorption and desorption were crucial in order to describe the activity and NH3 inhibition properly.51 Further, the OH groups formed during water adsorption in the presence of water were very important when simulating the inhibition effect of water on the NO oxidation and NH3 oxidation. Therefore was in total 13 experiments used when tuning the parameters. The experiments used was NH3 TPD at 150 °C on HZSM-5,

NH3 TPD at 30 and 150 °C on Cu-ZSM-5, H2O TPD at 150 °C on CuZSM-5, NH3 oxidation with and without water, NO oxidation with water51 and six SCR experiments.51 4. Results and Discussion 4.1. The Kinetic Model for NH3 and H2O Adsorption on and Desorption from H-ZSM-5. In order to differentiate between the copper sites and the Bro¨nsted acid sites in the CuZSM-5 catalysts, a TPD experiment was made using the H-ZSM-5 catalyst. The catalyst was exposed to 500 ppm NH3 ammonia for 60 min at 150 °C, followed by a period of argon flush, and finally a heating ramp 10 °C/min. The kinetic model contains reaction steps for adsorption and desorption of ammonia over S2 (Bro¨nsted acid sites) and S3 (loosely bound species) (Table 3). The result is shown in Figure 1. At the start of the ammonia exposure there is a total uptake of ammonia for about 13 min, which the model describes well. The experimental TPD peak is a combination of two peaks, one at 230 °C and a second



TABLE 11: Kinetic Parameters for O2 Adsorption, Dissociation and Desorption, and Ammonia oxidation. rate O2 adsorption O2 desorption O2 dissociation O2 recombination NH3 oxidation OH production H2O production NH3 oxidation with OH and oxygen

rate constants



k9,f k9,b k10,f k10,b k11 k12,f k12,b k13

1.23 × 10 3.20 × 1012 a,c 3.21 × 1010 a,c 3.20 × 1012 a,c 4.10 × 105 ( 3.34× 104 a,d,e 3.20 × 1012 a 3.20 × 1012 a 3.52 × 109 ( 2.35× 108 a,d,e

0 48.77c 83.25c 161.39c 97.36 ( 6.20e 84.37 ( 0.45e 182.56 ( 0.43e 103.93 ( 2.51e

2 b,c

a mol/(s kgzeolite). b m3/(s kgzeolite). c Reference 42. d Fitted using a centered pre-exponent. e 13 experiments was used when fitting the parameters, including NH3 TPD at 150 °C on HZSM-5 and NH3 TPD at 30 and 150 °C, H2O TPD at 150 °C, NH3 oxidation with and without H2O, NO oxidation with water and six SCR experiments on Cu-ZSM-5.

Figure 1. (a) Measured and calculated concentrations during an ammonia TPD experiment using H-ZMS-5. Storage performed at 150 °C, the temperature was raised 10 °C/min during the heating ramp. (b) Calculated mean coverage on the Bro¨nsted acid sites.

peak at 350 °C. The peak at low temperature is small and may be related to ammonia desorption from sites for weakly bound ammonia, and the peak at the higher temperature, is likely desorption from Bro¨nsted acid sites. The model also predicts two desorption peaks, but the first peak is larger in the simulation and the second somewhat to small. However, the total amount of stored and released ammonia agrees well between the experiment and the model. Integration of the stored and released ammonia in the experiment result in 1.2 and 1.1 mmol, respectively. This shows that the ammonia stored on the HZSM-5 catalyst is desorbed and there are no or very small amounts of ammonia left on the surface. The difference of 0.1 mmol is within the error margins, when integrating over such a long time period. The model also predicts ammonia coverages close to zero at the end of the ramp. The kinetic parameters and their linearized 95% confidence regions are shown in Table 7. The heat of adsorption for ammonia on H-ZSM-5 varied between 120-180 kJ/mol52 according to calorimetric measurements. Topsoe et al.53 and Hunger and Hoffman54 propose that there are states for ammonia desorption in H-ZSM-5, which were denoted R, β, and γ. The high temperature state (γ) had a barrier of 163 kJ/mol according to Topsoe et al.53 and 159 kJ/mol for low coverages. We obtained 160 kJ/mol for zero coverage, which is in good agreement with the reported literature values. Further, we model the acid sites as one site using a coverage dependent activation energy in order to simplify the model. We obtain a range of 160 to 99 kJ/mol when changing to coverage between 0 and 1. This is in the same range as the values for the three desorption states, 87 kJ/mol (R), 97 kJ/mol (β), and 163 kJ/mol (γ).

A similar experiment was carried out to investigate the adsorption and desorption of water. In the same way as for the ammonia, water was adsorbed and desorbed from both S2 and sites for weakly bound species (S3). It was difficult in the experiments to start the water feed precisely and this in combination with the high water concentration (5%) made it difficult to observe the storage of water. The release of water during the temperature ramp was therefore used in the model development The result is shown in Figure 2. It can be seen that water desorb from the surface throughout the entire temperature range. Some water is still left on the surface at the end of the heating ramp. There are two desorption peaks observed in both the model and the experiment. The first one is in the model described by a release of water from both S2 and S3 and the second peak originates from water desorbing from S3. There is a good agreement between the experiment and the model. The kinetic parameters and their linearized 95% confidence regions are shown in Table 8. 4.2. The Kinetic Model for NH3 and H2O Adsorption on and Desorption from Cu-ZSM-5. Two TPD experiments were carried out using the Cu-ZSM-5-b catalyst. Ammonia was stored at 30 or 150 °C, followed by a period of argon flush and a heating ramp. The result from the low temperature experiment is shown in Figure 3 and the result from the high temperature experiment is shown in Figure 4. A large amount of ammonia is stored at 30 °C and there is a complete uptake of ammonia for about 28 min, which the model describes adequately. Thereafter, the outlet concentration increases slowly, and has not reached the feed concentration level when ammonia is turned off after 80 min. The reason might be that the adsorption of


Figure 2. (a) Measured and calculated concentrations during water TPD experiment using H-ZSM-5. Storage performed at 150 °C, the temperature was raised 10 °C/min during the heating ramp. (b) Calculated mean coverage on the Bro¨nsted acid sites (S2) and sites for weakly bound species (S3).

ammonia is hindered when the zeolite is close to saturation. After ammonia is shut off, weakly bound ammonia desorbs during the argon flush from the catalyst. As soon as the temperature is raised, ammonia starts to desorb from the catalyst. A large peak is observed at 150 °C, followed by several small peaks at higher temperatures. The ammonia from S3 (loosely bound site) starts to desorb at a low temperature and the release is finalized at 300 °C. The desorption of ammonia at higher temperatures occurs in the model from Bro¨nsted acid sites (S2) and copper sites (S1b), where the ammonia on the copper is the one most strongly bound. However, the ammonia on the active site S1a is desorbed at a quite low temperature. These features are crucial in order to describe the ammonia TPD and SCR activity simultaneously.51 The ammonia on S1b needed to be strongly bound in order to describe the release of ammonia at high temperatures in the ammonia TPD. However, if the same parameters would have been applied on the S1a, which is the site we use for reactions, there would have been a very much too large ammonia blockage of the SCR reaction. Thus, since a small blocking of ammonia is observed experimentally for the SCR reaction12 it is important to have NH3 adsorbing on S1a, but the ammonia must be bound quite loosely in order to describe the experiments.51 The model is able to describe the desorption well. There is still ammonia left on the surface after the TPD. Thus some ammonia is very strongly bound. Integration of the amount of stored and released NH3 shows that 2.8 mmol ammonia is stored and 2.2 mmol desorbed. Also in the model there are ammonia left on the copper (S1b) in the end of the ramp. The result from the experiment and simulation of the NH3 TPD conducted at 150 °C is presented in Figure 4. The catalyst (Cu-ZSM-5-b) adsorbs ammonia completely for 15 min, which is captured by the model. The amount of stored ammonia is 1.5 mmol and the released ammonia is 1.1 mmol. Thus, there is ammonia left on the surface at the end of the ramp. In the model, the NH3 storage is 1.4 mmol and in the desorption 1.2 mmol, which agrees very well with the experiment. The storage of ammonia at 150 °C is about 50% of the storage at 30 °C, which shows the importance of loosely bound species in order to explain ammonia storage at low temperature. The adsorption period is followed by an Ar only period, where ammonia is released. During the proceeding temperature ramp there is

Sjo¨vall et al. ammonia desorption observed from multiple overlapping peaks. The model can also describe the desorption of ammonia adequately. The kinetic parameters and their linearized 95% confidence regions are shown in Table 9. The activation barriers we obtained for ammonia desorption on S1b and S1a were 219 and 99 kJ/mol, respectively, for zero coverage (both decreasing when increasing the coverage). A high barrier for desorption of ammonia from S1b was important in order to describe the residues of ammonia at 500 °C. As discussed above it is central to have a low value on S1a in order to describe the SCR experiments and ammonia blockage adequately. In an earlier more globalized model we obtained an activation barrier of 182 kJ/mol for the ammonia desorption.32 The storage of water on Cu-ZSM-5 was investigated using a H2O TPD at 150 °C (Cu-ZSM-5-b), which results in a broad peak, similar to the peak produced by desorption from H-ZSM5. This can be seen in Figure 5, where a comparison of the water TPD over HZSM-5 and Cu-ZSM-5 is shown simultaneously. The result is shown in Figure 6. Two desorption peaks are observed. In the model the first desorption peak origins from S3 and S1b. The same amount of water adsorbed on Cu-ZSM-5 and H-ZSM-5, but there is much less Bro¨nsted acid sites on Cu-ZSM-5. Thus water needs to be adsorbed also on the copper and we obtained large coverages on S1b. However, in the same way as for the ammonia model, it is not possible to adopt the same parameters for the water adsorption and desorption for S1a, since that would block the actives sites needed for the SCR reaction.51 The water on S1a is loosely bound, which results in a low coverage in the TPD. However, the step must be introduced since the water formed in the SCR reaction must be able to desorb from the surface.51 The kinetic parameters and their linearized 95% confidence regions are shown in Table 10. 4.2.1. Validation of the NH3 and H2O Adsorption on and Desorption from Cu-ZSM-5. A validation experiment not included in the fitting procedure was performed exposing the catalyst (Cu-ZSM-5-b) to a mixture of H2O and NH3 at 150 °C, followed by a period of argon flush and a heating ramp. The result can be seen in Figure 7. Prior to the introduction of ammonia the catalyst was saturated with water (5%) for 10 min and this procedure was also used in the simulations (not shown). The ammonia in the inlet feed is completely stored the first 11 min, which is well captured by the model. The total uptake is less when water is present, 11 min compared to 15 min without water. This is due to that water blocks some sites for ammonia adsorption. After the adsorptioin period the ammonia is shut off and desorption of ammonia is observed. The peak during the desorption is also lowered due to the smaller storage when water is present. The release of ammonia decreases from 1.1 to 0.8 mmol when water is present. During the temperature a release of ammonia is observed, which is slightly underestimated in the model. 4.3. The Kinetic Model for Ammonia Oxidation. An ammonia oxidation experiment was made exposing the CuZSM-5-a catalyst to 500 ppm NH3 and 8% O2, and increasing the temperature in steps from 100 to 400 °C. The result is shown in Figure 8. At 100 °C ammonia is totally stored for 20 min, which the model describes adequately by adsorption of ammonia on copper, Bro¨nsted acites and loosely bound sites. The total adsorption corresponds to 1.9 mmol, which is between the 2.8 and 1.5 mmol obtained for the ammonia storage at 30 and 150 °C respectively. Desorption of ammonia is observed at each temperature increase, and at 250 °C ammonia starts to oxidize. All feed ammonia is converted to N2 at 350 °C (N2 was not measured directly, but NO, NO2



Figure 3. (a) Measured and calculated concentrations during an ammonia TPD experiment using Cu-ZSM-5-b. Storage performed at 30 °C, the temperature was raised 10 °C/min during the heating ramp. (b) Calculated mean coverage at each site.

Figure 4. (a) Measured and calculated concentrations during an ammonia TPD experiment using Cu-ZSM-5-b. Storage performed at 150 °C, the temperature was raised 10 °C/min during the heating ramp. (b) Calculated mean coverage at each site.

Figure 5. Comparison between desorption of water from the H-ZSM-5 and the Cu-ZSM-5-b catalysts. The catalysts were heated 10 °C/min in argon after water exposure.

and N2O could not be detected). A good agreement is found between the measured and calculated concentrations. The model predicts high initial coverage of ammonia and also some oxygen on Cu-S1a. When the temperature is raised, the surface coverage of ammonia deceases, freeing up vacant sites for oxygen to adsorb and dissociate which results in an increased coverage of S1a-O. As ammonia starts to oxidize, water is formed and is observed on the surface. On the active copper sites S1a the water reacts with oxygen to produce OH groups on the S1a. At 400 °C the coverages of OH and H2O starts to decrease since the desorption of water increases

Figure 6. (a) Measured and calculated concentrations during a water TPD experiment using Cu-ZSM-5-b. Storage performed at 150 °C, the temperature was raised 10 °C/min during the heating ramp. (b) Calculated mean coverage at each site.

with the temperature. In the lower panel of Figure 8 the coverages on the copper sites are plotted, since they are the active sites in the model. Adsorption and desorption of


Sjo¨vall et al.

Figure 7. (a) Measured and calculated concentrations during an ammonia+water TPD experiment using Cu-ZSM-5-b (validation). Storage performed at 150 °C, the temperature was raised 10 °C/min during the heating ramp. (b) Calculated mean coverage of ammonia. (c) Calculated mean coverage of water.

Figure 8. (a) Measured and calculated concentrations during an ammonia oxidation experiment over Cu-ZSM-5-a. The temperature was raised step-wised from 100 to 400 °C. (b) Calculated mean coverage of NH3, H2O, O and O2 on S1a and S1b.

ammonia and water on the S2 and S3 sites occur, but are not shown in the figure. One additional ammonia oxidation experiment in presence of water was used to develop the model. This experiment was conducted over the Cu-ZSM-5-a catalyst and the result is displayed in Figure 9. There is a total uptake of ammonia for about 14 min, which is significantly less than the 20 min without the presence of water. The total storage decreases from 1.9 to 1.4 mmol, which is due to water blocking some of the storage sites for ammonia. Also the desorption of ammonia when increasing the temperature is decreased. The model predicts the reduced ammonia storage well, and also the decreased release of ammonia at each temperature increase. The reason is the competition for surface sites

between ammonia and water. The model predicts high initial coverage of water due to high gas phase concentration (5%) The ammonia competes and the coverages of ammonia are increasing slowly, but the saturation level is lower compared to in the dry experiment. Formation of large amount of OH groups is also observed in the model. These are crucial in order to describe the large poisoning effect that water has on the NO oxidation.51 The coverage of OH groups in the NH3 oxidation experiment in the presence of water is about 0.9 at 250 °C. This high coverage reduces the rate of reaction 11 very much. It was therefore crucial to add reaction 13 where NH3 can react with OH and O on the copper. At 300 °C the NH3 oxidation start (reaction 13) and the coverages of OH drops and now also reaction 11 becomes important.



Figure 9. (a) Measured and calculated concentrations during an ammonia oxidation experiment over Cu-ZSM-5-a in presence of water. The temperature was raised stepwise from 100 to 400 °C. (b) Calculated mean coverage of NH3, H2O, O and O2 on S1a and S1b.

The parameters and their linearized 95% confidence regions are shown in Table 11. The parameters for oxygen adsorption, desorption, dissociation and recombination are taken from an earlier study by Olsson et al.42 There is a range of activation energies reported in the literature. Baik et al. 30 obtained 166 kJ/mol for NH3 oxidation over Cu-ZSM-5 and Williamson et al. 39 between 148-158 k/mol for Cu-Y, Chatterjee et al. 48 178 kJ/mol for an commercial Fe zeolite and Long and Yang47 88 kJ/mol for Fe-ZSM-5. The activation barriers for the two NH3 oxidation reactions in our work were 93 and 115 kJ/mol for reaction 11 and 13, respectively. Thus our values are close to the one reported by Long and Yang for Fe-ZSM-5.47 5. Conclusions The NH3 and H2O adsorption and desorption and the NH3 oxidation was studied using detailed kinetic modeling and flow reactor experiments. The ammonia storage is important for the NH3 SCR application, since minimum ammonia slip is desired. Also, given the large storage of ammonia on the catalyst, it is crucial to describe the ammonia adsorption well in order to accurately simulate transient experiments. Also, ammonia oxidation at high temperature may cause decreased NOx reduction activity, and in this study, ammonia storage and oxidation are investigated separately. The water adsorption and desorption was also included in the study. Electron paramagnetic resonance studies (EPR) and also DFT calculations in the literature suggest that [Cu(NH3)4]2+complex are formed in copper zeolites. In our model we therefore use four sites on each copper for ammonia adsorption. Further, in other experiments we found that the adsorption of NO2 was significantly less than NH3 and it is not possible to adsorb four NO2 on each copper. We therefore introduce one S1a site per copper where both ammonia and NOx adsorb. This is the active site, where the SCR reactions occur. In addition, we also use three extra sites per copper (S1b) for ammonia adsorption in order to add up to the four ammonia adsorbed per copper. Also ammonia can be adsorbed on the Bro¨nsted acid sites and we denote them with S2. These sites were investigated separately using a H-ZSM-5 catalyst. Finally, we observed a very large adsorption of ammonia at 30 °C, which was much larger than the sum of

S1a, S1b, and S2 sites, and we therefore introduced S3 sites for loosely bound species. One ammonia TPD experiment and one water TPD experiment, both conducted at 150 °C, were modeled using the H-ZSM-5 catalyst to get information about the ammonia and water storage and release. The same experiments were also conducted using the Cu-ZSM-5 catalyst in order to describe the effect of copper on the adsorption and desorption of ammonia and water. The same experiment were also conducted over the Cu-ZSM-5 catalyst and was used to describe the effect copper had on the adsorption and desorption of ammonia and water. An ammonia TPD at 30 °C was also used. The model was able to describe the experiments adequately. An NH3 TPD experiment with storage performed in presence of water was used for model validation. The water reduced the ammonia storage, due to blocking of storage sites. The model was able to predict the experimental result satisfactorily. Elementary reaction steps for oxygen adsorption, desorption, dissociation and recombination were incorporated into the model together with two summary steps for ammonia oxidation. In the first step ammonia is reacting with oxygen on the surface to produce N2 and this step is very important for describing ammonia oxidation in dry conditions. When water is present there is a large formation of OH groups that blocks the surface, through a reaction between water and oxygen. These OH groups are necessary in order to describe the large decrease of NO oxidation in the presence of water.51 However, NH3 oxidation is only mildly affected by the presence of water and therefore was a summary reaction between ammonia oxygen and OH introduced. The model was then able to predict ammonia storage, desorption and oxidation well, both with and without water. 6. Nomenclature List parameter description unit monolith wall area in each tank, m2 An Aj pre-exponential factor for reaction j, depends on rate expression pre-exponential factor for adsorption, used for calAads culating ∆S, s-1

1404 J. Phys. Chem. C, Vol. 113, No. 4, 2009 Ades cg,i,n cs,i,n Ea,j Ea,j(0) Fi,n Fi,n-1 h Im kB kc,i,n kj M mwc,n Ncat q qtr,3D qrot rj,n R ∆S S S1a S1b S2 S3 t T Tref V Ri,j σr νi,j Θi,n θi

pre-exponential factor for desorption used for calculating ∆S, s-1 concentration in the gas phase of component i in tank number n, mol/m3 concentration at the surface of the wash-coat of component i in tank number n, mol/m3 activation energy for reaction j, J/mol activation energy for reaction j for zero coverage J/mol molar flow of component i in tank number n, mol/s molar flow of component i in tank number n - 1, mol/s Planck’s constant, J s rotational moment of inertia in direction m, where m ) x, y, or z, kgm2 Boltzmann constant, J/K mass transfer coefficient for component i in tank number n, m/s rate constant for reaction j, depends on rate expression molecule mass, kg mass of zeolite in tank n, kg number of active sites per mass zeolite, mol/kg zeolite) partition function partition function for 3 DIM translation partition function for rotation rate of reaction j in tank n, mol/(s kg zeolite) gas constant, J/(mol · K) entropy loss, due to adsorption, J/(mol · K) entropy, J/(mol · K) Cu-site Cu-site Bro¨nsted acid sites weak adsorption sites time, s temperature, K reference temperature, set to 600 K, K molecule volume, m3 coverage dependence for species i in reaction j symmetry number stoichiometric coefficient for component i and reaction j coverage of species i in tank n coverage of species i

Acknowledgment. The work was performed at the Competence Centre for Catalysis (KCK), Chalmers and at General Motors Research and Development Center. The authors would like to acknowledge helpful discussions with Se Oh, Ed Bissett, Jong-Hwan Lee, Byong Cho and Steven J. Schmieg of the General Motors Research and Development Center and Ashok Gopinath at GM ISL. We would also like to thank GM R&D Center for the financial support. L,O, would also like to acknowledge the Swedish Research Council (Contracts 621-2003-4149 and 621-2006-3706) for additional support. References and Notes (1) Topsoe, N.-Y.; Topsoe, H.; Dumesic, J. A. J. Catal. 1995, 151, 226. (2) Dumesic, J. A.; Topsoe, N.-Y.; Topsoe, H.; Chen, Y.; Slabiak, T. J. Catal. 1996, 163, 409. (3) Wachs, I. E.; Deo, G.; Weckhuysen, B. M.; Andreini, A.; Vuurman, M. A.; Boer, M. d.; Amiridis, M. D. J. Catal. 1996, 161, 211. (4) Roduit, B.; Wokaun, A.; Baiker, A. Ind. Eng. Chem. Res. 1998, 37, 4577.

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JP802449S

Detailed Kinetic Modeling of NH3 and H2O Adsorption, and NH3

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