Al2O3 Monolith

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Ind. Eng. Chem. Res. 2008, 47, 9006–9017

Kinetic Modeling of NOx Storage/Reduction on Pt/BaO/Al2O3 Monolith Catalysts L. Cao, J. L. Ratts, A. Yezerets,† N. W. Currier,† J. M. Caruthers, F. H. Ribeiro, and W. N. Delgass* School of Chemical Engineering, Purdue UniVersity, 480 Stadium Mall DriVe, West Lafayette, Indiana 47907-2100, and Cummins Inc., 1900 McKinley AVenue, Columbus, Indiana 47201

A balance of the complexity of the reactor model and the chemical reaction mechanism has to be made in order to predict the dynamic nature of NOx storage/reduction processes in real time. In this work, a onedimensional, two-phase model is used to simulate the transient behavior of a monolithic Pt/BaO/Al2O3 catalyst for NOx storage/reduction. The following aspects of the process are discussed: (i) kinetics of NO and NO2 adsorption on BaO sites, (ii) effects of CO2 and H2O on NO/NO2 adsorption, and (iii) reduction of surface nitrates using H2. NOx adsorption with excess oxygen involves two kinetic routes, namely, NO2 disproportionation and direct NO adsorption, both of which form nitrates on the catalyst at 300 °C. A model with two time scales was found to be necessary to describe NO2 adsorption on the 20 wt % BaO catalyst. The model and parameters required to fit the NOx breakthrough curves suggest that CO2 and H2O in the feed reduce the number of sites for NO adsorption by changing the surface morphology of the Ba phase. The rate constants for both fast and slow NO2 uptake are decreased in the presence of CO2 and H2O, but the total capacity remains the same. Under reaction conditions, H2 reduction of surface NOx is limited by the supply of the reductant; that is, the rate of surface NOx removal is limited by the flux of inlet H2. NH3 serves as the reducing intermediate/H carrier during the H2 reduction process. The confined reduction front moving along the channel localizes the heat generation, thus leading to a surface temperature in the reduction front about 35 °C higher than the inlet gas temperature for our reaction conditions. 1. Introduction Diesel engines and lean-burn gasoline engines are widely used because of their high fuel efficiency and correspondingly low CO2 emissions. However, excess amounts of air in the fuel mixture result in high O2 levels in the exhaust gas, thus making NOx abatement difficult. As an alternative solution, the NOx storage/reduction (NSR) process1 comprises a long lean period in which the emitted NOx can be absorbed on the catalyst surface and a relatively short fuel-rich period in which the surface NOx can be reduced to N2. An accurate and robust NSR model is needed for the purpose of exhaust gas aftertreatment design.2 Furthermore, as indicated by existing experimental results,3,4 the NSR process is a highly transient, high-thermal-gradient/ high-mass-gradient reaction system. As a result, experimental methods are limited in interpolating the data and carrying out mechanistic studies. Mathematical modeling of this periodic NSR process is desirable and thus is widely discussed in the literature. To be able to model this NOx storage and reduction process on monolith catalysts, attention must be paid to the complexity/ accuracy levels of both monolith reactor modeling and chemical kinetics. NOx adsorption on the storage component of the NSR catalyst is a complex process,5 and the generally accepted route involves the following steps: (1) NO oxidation to NO2 on Pt sites; (2) NO2 adsorption onto BaO sites to form nitrites and nitrates and release NO; and (3) either the oxidation of in situ generated NO back to NO2 or its direct adsorption. Other components in the exhaust gas, such as CO2 and H2O, can also compete for the adsorption sites with NO and NO2. It is also observed that the NOx adsorption capacity is a function of * Address for correspondence: W.N. Delgass School of Chemical Engineering, Purdue University, Forney Hall of Chemical Engineering, 480 Stadium Mall Drive, West Lafayette, IN 47907-2100. Tel.: (765) 494-4059. Fax: (765) 494-0805. E-mail: [email protected]. † Cummins Inc.

temperature with a maximum capacity at 300 °C. At higher temperatures, the stored NOx will decompose. Epling et al.4 observed that NOx capacity is zero when the temperature is above 550 °C. As recently reviewed by Tomasic,6 monolithic catalysts with active components coated on the channel walls are often used for NOx storage/reduction catalysts. The monolith has cordierite as the substrate with open rectangular or round channels. Monolith catalysts have been used extensively for three-way catalytic converters, and a fair amount of modeling has been reported in the literature. Pioneering work in this area was performed by Young and Finlayson7–9 and Aris.10 Following Aris’ classification, Zygourakis used one- and two-dimensional models solved by the collocation method to simulate the transient behavior of a catalytic converter.11 Crocoll et al.12 and Sharma et al.13,14 used a one-dimensional model that is similar to that of Zygourakis and a finite-difference method with a semiimplicit time integrator to solve the resulting equations. Comparing the results of these various numerical methods used for modeling the catalytic converters, one finds, as expected, that the finite-element and collocation methods are more accurate and reliable than finite-difference methods, given similar model equation systems. As further discussed in section 4.1, we chose to use the finite-element method to solve the resulting equations with a commercial solver called COMSOL Multiphysics. Several different modeling approaches have been used in the literature to describe NOx storage/reduction reactions. Depending on the complexity level of the kinetic mechanism, these works can be divided into detailed microkinetic approaches and global kinetic approaches. The detailed microkinetic models include adsorption, surface reaction, and desorption reactions for NO oxidation and NOx reduction on Pt sites and NO/NO2 adsorption on BaO sites.15–22 In contrast, many authors have also used socalled global kinetic models involving rate expressions that

10.1021/ie8001809 CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008

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include only gas-phase concentrations in order to simplify the model.13,14,23–26 Olsson et al. used a mean-field model for NOx storage15,16 and a global kinetic model for both NOx storage and regeneration.23,27 The proposed mean-field model consisted of a reaction network with a large number of reaction rate parameters. Their global kinetic model used reaction rates derived from mechanisms proposed in the literature. Diffusion within the washcoat was handled through the use of a shrinkingcore model on the barium particles. However, this radial dependency made the model two-dimensional and, thus, more difficult to solve. The reaction kinetics published in this work were also implemented by Sharma et al. to study lean NOx traps.19 Laurent et al.28 developed a NOx storage model with the purpose of predicting the amount of NOx stored on the barium component of the catalyst, as well as the emissions of NO and NO2. Adsorption of NO, NO2, and O2 was described by equilibrium reactions. Additional reactions were then used to describe the transformation of adsorbed NOx compounds to barium nitrates on the surface. The model showed the amount of stored NOx at multiple temperatures and was compared to experimental data on powder catalysts used in a fixed-bed reactor. This model, however, was not capable of describing both NO and NO2 breakthrough profiles together and incorporated only surface diffusion of NOx species from Pt sites to BaO sites. Tuttlies and co-workers29,30 developed a single-particle model to study in detail the effects of diffusion and the formation of dense nitrate layers in the barium phase of the catalyst using an isothermal flat-bed reactor. The model was solved using a tank-in-series approach. However, the simplified NOx adsorption and reduction kinetics made the model unable to predict the catalyst performance under different inlet conditions. Previous work in our group has focused on developing a model that is complex enough to describe both the transport effects and the reaction mechanisms while still being simple enough to be solved in real time. Kromer et al.31 used a onedimensional model to describe NO oxidation and NOx adsorption reactions on Pt/BaO catalysts. A two-time-scale process was found to be necessary to describe the total NOx adsorption, but details of the reaction network were not included. In this work, NO/NO2 adsorption on Pt/BaO catalysts is described in more detail, and the effects of CO2 and H2O are included. Although the NO/O2 gas mixture is closer to practical application, we chose to use NO2/O2 as the inlet for NOx storage study in this work because NO2 is believed to be responsible for the majority of NOx adsorption. The coupling of heat and mass transfer with chemical reactions during NOx reduction is also included to allow modeling of the full storage/regeneration cycle. 2. Experimental Methods 2.1. Catalysts. The Pt/BaO/Al2O3 catalyst used in the NOx storage study was supplied by Johnson Matthey Inc. in monolithic form. The catalyst has a cell density of 400 cells/ (in.2 of monolith cross section), with a washcoat loading of 1.25 g/in.3 and a square cell geometry. The Pt loading is 2.13 wt %, and the Ba loading is 20 wt %, all based on the washcoat weight. Pt dispersion, defined as the ratio of the number of surface Pt atoms to the total number of Pt atoms, was measured by the H2-O2 titration method32 and was found to be 31% for the catalyst used in this work. Table 1 lists the dimensions and physical properties of the monolith used.

Table 1. Physical Properties of the Monolith Catalyst Used in This Work loading (with respect to washcoat) cell density length, L diameter, D open frontal area, washcoat frontal area, R washcoat porosity, s gas-solid contact area, av washcoat thickness, dw bulk density, Fs gas thermal conductivity, kg (at 300 °C) gas heat capacity, Cpg solid thermal conductivity, λs solid heat capacity, Cps

2.13 wt % Pt, 20 wt % Ba 400 cells/in.2 1.5 in. 0.7 in. 0.62 0.12 0.50 2210 m2/m3 50 µm 2500 kg/m3 0.043 W/(m °C) 1 kJ/(kg °C) 0.921 W/(m °C) 1 kJ/(kg °C)

Another Pt/BaO/Al2O3 monolith provided by EmeraChem LLC was used for NOx reduction experiments, with details given elsewhere.33 This catalyst had a cell density of 200 cells/in.2 with a Pt dispersion of about 60%. The barium loading with respect to the washcoat was 20 wt %. 2.2. Gas-Phase Reactions. The experimental apparatus used for this study was described in detail elsewhere34 and is summarized briefly here. NO2/O2 storage experiments were carried out on a 1.5-in.-long, 0.7-in.-diameter Pt/BaO sample at 300 °C. The flow rate was 3.75 L/min, giving a space velocity (SV) of 30 000 h-1. NO/O2 adsorption was done on the samediameter sample but with a length of 3 in.; the corresponding flow rate was 7.5 L/min in order to maintain the space velocity as in the NO2/O2 case. Subsequently, 8.35% CO2 and 8.35% H2O were added to the NO2/O2 gas mixture, with all other conditions unchanged. For experiments containing water in the feed, deionized water was metered using a high-precision liquid metering pump (Fluid Metering, Inc., model QVG50). To avoid fluctuations in the water partial pressure, a 1.6-mm- (0.0625in.-) diameter stainless steel capillary tube with an internal diameter of 0.254 mm (0.01 in.) was used to deliver a continuous flow of water in the gas mixture after it had passed through a preheater held at 140 °C. The NO, NO2, CO2, and H2O concentrations in the outlet gas stream were measured with an Fourier transform infrared (FTIR) gas analyzer (MKS MultiGas Analyzer, model 2030). During regeneration, the 3-in.-long EmeraChem Pt/BaO catalyst was first saturated with 350 ppm NO/10% O2 in Ar at 300 °C. Then, after an inert purge, 0.75% H2 in Ar with a space velocity of 30 000 h-1 was used to reduce the catalyst. The NO, NO2, N2O, NH3, and H2O concentrations in the outlet gas stream were again measured with the FTIR gas analyzer, whereas the N2 concentration was measured with a quadrupole mass spectrometer (SRS RGA 200). The mass spectrometer was calibrated to measure N2 concentrations in the 0-6500 ppm range either by the injection of pulses of known volumes of N2 or by sampling of calibrated N2/Ar mixtures. The gas-phase temperature was monitored at the inlet and outlet of the catalyst, with one thermocouple placed each 0.5 cm before and 0.5 cm after the monolith. 2.3. In Situ DRIFTS Study. All diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) data were collected using a Spectra-Tech Collector II accessory with the optional high-temperature/-pressure chamber. A Thermo Magna 550 FTIR spectrometer with a liquid-nitrogen-cooled MCT/A detector and a KBr beam splitter was used to collect the data at a resolution of 4 cm-1, along with the data collection/manipulation software Omnic v7.2a. All spectra presented here were averaged over 32 scans and collected with a mirror velocity of 1.8988 cm/s.

9008 Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008

The monolith used in the DRIFTS study was the same monolith as used in the adsorption experiments (2.13 wt % Pt/ 20 wt % BaO/Al2O3). After the reactor experiments had been completed, a piece was cut from the center of the monolith core and then filed down to a ∼4.5-mm-wide circle with the top layer’s walls removed so that the sample could be placed in the sample cup with a washcoat layer exposed to the infrared radiation. The sample was one channel thick, and a K-type thermocouple was placed in direct contact with the top of the monolith to measure the temperature. The sample was pretreated by first being heated to 500 °C in N2 and then being held at that temperature for 20 min. The temperature was subsequently reduced to 400 °C, and the catalyst was exposed to pure H2 for 30 min. This procedure was used to decompose the carbonates on the catalyst in order to achieve a stable background. Once the H2 had been shut off, the temperature was decreased to 300 °C, and the experiments were completed. The spectra presented here are the results of many capture/regeneration cycles until they were reproducible.

The superficial linear velocity in the gas phase, u, was calculated by the equation

3. Development of the Mathematical Model 3.1. Equations and Assumptions. In this work, we used a transient, one-dimensional model to study the reaction/diffusion problem of NOx storage/reduction in a single channel of the monolith. The following balances were considered in gas phase as well as in the washcoat: (1) mass balance in the flowing gas along the channel, including accumulation, convection, and external mass transfer between the gas phase and the washcoat layer; (2) mass balance in the washcoat layer, including accumulation, external mass transfer, and catalytic reactions; (3) mass balance of the surface species, including accumulation and chemical reactions; (4) enthalpy balance of the flowing gas, including accumulation, convection, and gas-solid heat transfer; and (5) enthalpy balance of the solid phase, including accumulation, axial conduction, gas-solid heat transfer, and a heat source from the catalytic reactions. Because the heat- and mass-transfer Peclet numbers in the gas phase were above 1000, the axial gradient was much higher than that in the transverse direction. Taking advantage of this difference, the model neglects radial variations of the gas-phase temperature, concentration, and velocity within the individual channels, so that these variables are to be interpreted as cross-sectional averages. The mass balance in the gas phase is given by ∂Cgi ∂Cgi +u ) kmav(Cwi - Cgi) (1) ∂t ∂x where Cgi and Cwi (i ) 1, 2,..., M) are the dimensionless concentrations of the ith species in the gas phase and in the washcoat layer, respectively. The reference concentrations of each species were chosen to be their inlet values. As illustrated in Figure 1, represents the fraction of channel open area relative to the total cross-sectional area of the monolith and was determined by ε

ε)

aw2 ap2

(2)

Here, the term “monolith” represents the bulk catalyst including both the gas and solid phases. The symbol km is the gas-solid mass-transfer coefficient determined by the Sherwood number (Sh). The ratio, av, is defined as the gas-solid contact area (channel wall area) per unit bulk monolith volume av )

4aw ap2

Figure 1. Schematic of one monolith channel. ap ) 1.43 mm, aw ) 1.13 mm, dw ≈ 50 µm. According to the definitions in section 3.1, ε ) aw2/ap2, R ) 4dw (aw + dw)/ap2, av ) 4aw/ap2, with values shown in Table 1.

(3)

u)

Vinlet(T) As

(4)

where Vinlet(T) is the inlet gas flow rate at the reaction temperature and As is the total cross-sectional area of the whole monolith. The mass balance for the ith component in the washcoat is Rεs

k ∂Cwi ) kmav(Cgi - Cwi) + νi,jRj(Cwj,Ts) ∂t j)1

∑

(5)

where R is the volume fraction of washcoat in the entire monolith and εs is the porosity of the washcoat. Here, the reaction rate for the ith component is defined in units of moles per total monolith volume per time. All reaction rate expressions used in this work are listed in Table 2. The mass balance for surface species i is given by Ls

k ∂θi ) νi,jRj(θj,Cwj,Ts) ∂t j)1

∑

(6)

where Ls represents the total available BaO site density in units of moles per unit catalyst volume and θi represents the surface coverage of species i. Because the solid walls are assumed to be impermeable to matter, a simple one-dimensional problem must be solved for the gas-phase temperature

(

)

∂Tg ∂Tg +u ) hav(Ts - Tg) (7) ∂t ∂x where h is the heat-transfer coefficient determined from the Nusselt number (Nu). Because we assume adiabatic boundary conditions for the whole monolith and identical flows in each open channel, the solid temperatures for all channels are equivalent, and there is no heat flux across the channels. The solid enthalpy balance is given by FgCpg ε

(1 - ε)FsCps

n ∂Ts ∂2Ts ) (1 - ε)λs 2 + hav(Tg-Ts) + (-∆Hi)Ri ∂t ∂x i)1 (8)

∑

where λs represents the thermal conductivity of the cordierite. Because the washcoat loading is relatively small compared to

Ind. Eng. Chem. Res., Vol. 47, No. 23, 2008 9009 Table 2. Reaction Mechanisms and the Rate Law for NOx Storage/Reduction reaction

rate law NO oxidation rf,oxi ) kf,oxi[NO]1.09[O2]0.86[NO2]-0.85 roverall,oxi ) rf,oxi(1 - β)

2NO + O2 a 2NO2 NO/NO2 adsorption 3NO2 + BaO f Ba(NO3)2 + NO, fast 3NO2 + BaO f Ba(NO3)2 + NO, slow 2NO + 3/2O2 + BaO f Ba(NO3)2

r1f ) k1fLs,f(1 - θs,f)CwNO2 r1s ) k1sLs,s(1 - θs,s)CwNO2 rNO ) kNOLs,NO(1 - θs,NO)CwNOCwO2 NOx reduction

8H2 + Ba(NO3)2 f BaO + 2NH3 + 5H2O 10 /3NH3 + Ba(NO3)2 f BaO + 8/3N2 + 5H2O

r1,red ) k1,redLs(1 - θs)CwH2 r2,red ) k2,redLs(1 - θs)CwNH3

the amount of cordierite, the thermal properties of the solid phase, such as the heat capacity Cps, are determined by the cordierite substrate. The values of the gas-solid heat- and mass-transfer coefficients, km and h, respectively, were estimated from the equations

4. Results and Discussion

h)

Nukg DH

(9)

4.1. Choice of Solution Methods. As stated earlier, in this work, we chose to use the finite-element method (FEM) to solve the reaction/diffusion model. This choice was made by comparing the simulation results for a simple NOx adsorption process using the forward difference method (FDM) and the finiteelement method (FEM). The reaction was as follows

km )

ShDg DH

(10)

NOx + Ls f NOx-Ls

and

Here, we neglect the effects of the mass and thermal entrance region by assuming a fully developed flow pattern. Correlations used to calculate the Nu and Sh were taken from ref 35. These correlations were used to predict average values for the entire length of the monolith channel. The system of equations consisting of eqs 1 and 3–6 had to be solved simultaneously subject to the following boundary conditions at the inlet (x ) 0) and outlet (x ) L) Tg ) 0 ) Tin λs

fitted values. It uses the Gauss-Newton algorithm with Levenberg-Marquardt modifications for global convergence.38

∂Ts ∂Ts ) λs )0 ∂x x)0 ∂x x)L Cgi ) 0 ) Cgi,in

(11) (12) (13)

An exponential, Arrhenius-type temperature dependence of the kinetic rate constants was considered

( )

kj ) Aj exp

-Eaj RgTs

(14)

where Aj is the pre-exponential factor, Eaj is the activation energy for the jth reaction, and Rg is the gas constant. The units of the pre-exponential factors depend on the reaction expressions, so that the rates of reactions always have units of moles per unit volume of monolith per unit time. 3.2. Solution Methods. This transient, one-dimensional, twophase model was solved using the finite-element method implemented in a COMSOL Multiphysics time-dependent solver.36 The finite-element method (FEM) discretization was performed using the so-called method of lines. The resulting ordinary differential equation (ODE) or differential-algebraic equation (DAE) system was solved using a version of the DAE solver DASPK37 with the variable-order variable-stepsize backward differentiation formula (BDF). The nonlinear model fitting was done by using the MATLAB Statistics Toolbox with the function NLINFIT. NLINFIT returns the least-squares parameter estimates by minimizing the sum of the squared error between the observed responses and their

The outlet NOx based on this simple, irreversible adsorption mechanism shows a symmetric breakthrough nature. This is due to the single time scale for adsorption.31 Usually, for finitedifference and finite-element methods, the solution converges as the number of finite-difference elements increases. This is called a “mesh-independent solution”. A series of FDM solutions was obtained using mesh numbers of 15, 100, and 500. By comparing the outlet NOx breakthrough curves, we found that the 15-mesh solution was about 10% different from the 100mesh solution and increasing the mesh to 500 yielded no apparent improvement in the solution. Therefore, for this single adsorption model, which represents a typical type of reaction of the NSR process, 15-grid meshing is not adequate. The grid has to be finer than 100 to ensure the accuracy of the solution. Further comparison of the 30-element-mesh FEM results with the 500-mesh FDM performance showed a difference of less than 2%. This result indicates that FEM can reach the same accuracy while using a significantly smaller mesh number, thus saving computational effort. Therefore, in this work, FEM implemented in COMSOL Multiphysics was used to solve the model equations. 4.2. NO Oxidation. Because the adsorption of NO2 on the NSR catalyst is more favorable than that of NO, the oxidation of NO to NO2 becomes an important step in NOx storage.5 Figure 2 shows NO and NO2 breakthrough curves for 318 ppm NO and 10% O2 storage on the Pt/BaO catalyst. About 600 s passes before the breakthrough of any NOx species. Then, NO and NO2 breakthrough were observed and reached a constant level, which indicates that uptake was close to complete. Apparently, with NO and O2 as the feed, NO is converted to NO2 with the help of Pt during NOx storage. Detailed kinetic modeling of NO oxidation on Pt using Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) models has been reported in the literature.16,17,31 However, in this work, we emphasize a global kinetics model that is simple enough to be integrated into the NOx storage model. Previous experimental studies34,39,40 of the global NO oxidation kinetics on Pt/Al2O3 monolith catalysts have shown that the turnover rate (moles of


Figure 2. NO and NO2 breakthrough curves on the Pt/BaO/Al2O3 sample. Inlet: 318 ppm NO/10% O2 in N2. T ) 300 °C, SV ) 30 000 h-1.

NO converted per mole of surface Pt per unit time) for NO oxidation on a Pt/Al2O3 monolith can be represented by the rate equation rf,oxi ) kf,oxi[NO]1.09[O2]0.86[NO2]-0.85 with an apparent activation energy of 82 kJ/mol. To account for the contribution of NO oxidation to NOx adsorption, an expression for the overall net reversible NO oxidation must be used. This is obtained by using the β factor derived from NO oxidation equilibrium, defined as β)

2 1 [NO2] Keq [NO]2[O ] 2

where Keq is the equilibrium constant for the NO oxidation reaction and is a function of temperature.41 As a result, the overall rate for NO oxidation used in this work has the form roverall,oxi ) rf,oxi(1 - β) It should be noted that, depending on the local concentrations of NO and NO2, this overall rate can be positive or negative. A positive value means that NO is being oxidized to NO2, whereas a negative value means that NO2 is being decomposed into NO and O2 on Pt sites. This kinetic expression was integrated into the NOx storage model with fixed reaction orders for all reactants. The 60 kJ/mol apparent activation energy value used in the work was adopted from a measurement of NO oxidation kinetics on a Pt/K2O/Al2O3 catalyst.34 The reason for the difference in activation energies for NO oxidation on Pt/Al2O3 and Pt/KNO3/Al2O3 remains unclear. On the other hand, the pre-exponential factor for NO oxidation on the Pt/BaO/Al2O3 catalyst was determined by tuning the NO oxidation model to match the “pseudo-steady-state” NO and NO2 breakthrough curves in Figure 2 with the assumption that, after a sufficient time, the flat NO and NO2 concentrations are determined solely by NO oxidation kinetics. The change in NO oxidation kinetics due to the presence of storage components has also been observed by other researchers. By comparing Pt/Al2O3 and Pt/ BaO/Al2O3 samples, Olsson et al. observed a decrease in NO conversion during a temperature-programmed NO oxidation experiment with BaO present.16 During that work, this change in activity was attributed to a decrease in Pt dispersion. 4.3. NOx Storage. 4.3.1. CO2 and H2O Free Case. In this work, we define “adsorption” and “storage” in the following way: NOx adsorption represents the chemical reaction of NO and NO2 with BaO sites to form nitrites or nitrates, whereas NOx storage as opposed to NOx reduction/regeneration refers to the process whereby NO and NO2 are captured under lean

conditions. This storage process includes chemical reactions such as NO oxidation and NO/NO2 adsorption. During NOx storage, once NO is converted to NO2 on Pt, both NO and NO2 are capable of being absorbed on the storage sites. NO2 is favored over NO for adsorption on BaO sites, as evidenced by the fact that NO adsorption on BaO/Al2O3 in the absence of Pt is much lower than NO2 adsorption.42 As a result, we chose NO2/O2 as the inlet mixture to study NOx storage on the Pt/BaO/Al2O3 catalyst, so that the dominating kinetic process responsible for NO/NO2 breakthrough curves would be NO/ NO2 adsorption rather than NO oxidation. On the other hand, because the goal of this work was to develop a global kinetic model to capture the main physics and chemistry of the NSR process, we chose not to implement the elementary mechanisms for NO/NO2 adsorption in order to reduce the number of kinetic parameters. To develop such a global kinetic model, NOx adsorption on a Pt/BaO monolith was first studied in the absence of CO2 and H2O using an in situ DRIFTS cell and was reported earlier.31 The major species present in the spectra were ionic barium nitrates, which exhibit multiple peaks in the range of 1480-1330 cm-1.43,44 Negligible amounts of nitrates were formed on alumina sites for the 20 wt % BaO catalyst. A study by Szanyi et al. on Ba/Al2O3 powder catalysts using temperature-programmed desorption (TPD), transmission IR spectroscopy, and 15N solid-state NMR spectroscopy44 showed that these ionic barium nitrates have a bulk structure. For simplicity, these species will be referred to as bulk barium nitrates throughout this work. Our IR spectra also showed that, at 300 °C, NO2 + O2, NO2, and NO all eventually formed nitrates on the Pt/BaO catalyst. Examination of the time evolution of the spectra showed further that nitrites were formed initially and then oxidized to nitrates. On the basis of the above surface observations, we proposed the following mechanism for NOx adsorption at 300 °C k1f

3NO2 + BaO 98 Ba(NO3)2 + NO k1s

(R1f)

3NO2 + BaO 98 Ba(NO3)2 + NO

(R1s)

2NO + O2 f 2NO2

(R2)

kNO 3 2NO + O2 + BaO 98 Ba(NO3)2 2

(R3)

As discussed in our previous work,31 a two-time-scale process is required to explain the asymmetric NO2 breakthrough curve. In the above mechanism, reactions R1f and R1s represent NO2 adsorption on the fast and slow surface sites, respectively, through the disproportionation reaction. R2 refers to the NO oxidation reaction, and R3 is the direct NO reaction with BaO in an atmosphere of O2 to form nitrates. The corresponding rate expressions of the above reactions are listed in Table 2. As shown in Figure 3A, the model can explain the experimental data well, with the model parameters determined by fitting both NO and NO2 breakthrough curves to the data. The values of the parameters are listed in Table 3. The nitrate formation due to NO2 and NO adsorption is plotted in Figure 3B. Fast ads and slow ads represent nitrate formation on the fast and slow BaO sites, respectively. NO ads is the amount of nitrate formation due to direct NO reaction with BaO. The initial NO2 adsorption mainly occurs on the fast uptake sites; then, as these sites become saturated after some time, the slow sites start to take effect, corresponding to the slowly


Figure 4. Surface IR spectra on Pt/BaO monolith with 376 ppm NO2/12% O2/5% CO2/5% H2O at T ) 300 °C.

Figure 3. (A) Experimental and simulated NO and NO2 breakthrough curves on Pt/BaO monolith with 300 ppm NO2/10% O2 inlet at 300 °C, SV ) 30 000 h-1. (B) Model-predicted surface NOx accumulation as a function of time. Table 3. Model Parameters and 95% Confidence Intervals for NOx Adsorption with and without CO2 and H2O parameter

without CO2 and H2O

with CO2 and H2O

k1f, m3/(mol s) Ls,f, mol/m3 k1s, m3/(mol s) Ls,s, mol/m3 kNO, m3/(mol s) Ls,NO, mol/m3

3.5 ( 0.3 20.0 ( 0.6 0.4 ( 0.02 15.3 ( 0.6 7.1 ( 0.5 29.7 ( 0.4

2.4 ( 0.2 13.6 ( 0.5 0.18 ( 0.01 24.6 ( 0.5 6.3 ( 1.0 12.6 ( 0.8

increasing portion of the NO2 breakthrough curve. From Figure 3B, the amount of nitrates stored on the fast sites until the end of storage is slightly larger than that stored on the slow sites. The NO2 disproportionation stoichiometry used in reactions R1f and R1s is an overall rate expression and has been widely used in the literature as the NO2 adsorption mechanism. It can be obtained by several possible combinations of more elementary steps. For example, Kabin et al.45 showed a nitrite pathway and a nitrate pathway for NOx adsorption involving BaO2 or BaONO2 as an adsorption intermediate. As opposed to gas-phase oxidation of NO to NO2, reaction R3 is sometimes referred to as the “surface oxidation” pathway,46,47 in which, in the presence of O2, NO is directly adsorbed to form BaO nitrites that are progressively oxidized to nitrates. Several groups have reported the importance of two time scales in modeling NOx storage process. One possible reason for the multiple adsorption time scales might be the existence of more than one distinct BaO site in a Pt/BaO system. Piacentini et al.48,49 observed three BaO phases with different thermal stabilities for barium loadings higher than 16 wt % of washcoat. Olsson et al.23,24 implemented a shrinking-core model to account for bulk diffusion in the barium particles. Tuttlies et al.30 also included mass transfer in the particles in their model. Other reasons for this two-time-scale process might include the proximity between Pt and BaO sites. The effects of the proximity between Pt and BaO on NOx uptake were discussed by comparing Pt/BaO/Al2O3 and physically mixed Pt/Al2O3 and

BaO/Al2O3.50 From this point of view, the fast site can be attributed to the BaO that is close to Pt sites, with the slow uptake occurring on the BaO sites that are far from the Pt sites. It should be noted that both bulk diffusion and Pt-BaO proximity can play a role in the real system. 4.3.2. Effects of CO2 and H2O. Because percentage levels of CO2 and H2O are always present in engine exhaust, the effects of these two components on NOx adsorption were also studied in this work. Figure 4 shows the IR spectra when 5% CO2 and 5% H2O are both added to the 376 ppm NO2/12% O2 mixture. From the NO2/O2 adsorption experiments, we know that there is only one adsorption species on the 20 wt % Ba sample after the initial storage period and that is bulk barium nitrate. Upon addition of CO2 and H2O, we see peaks that represent the formation of three different types of barium carbonates. The large peak at ∼1550 cm-1 is the CdO stretching mode of a bidentate carbonate that is associated with barium.51,52 The peak at ∼1460 cm-1 corresponds to the antisymmetric CO3- stretching mode of a monodentate carbonate associated with barium.51 The smaller peak at ∼1440 cm-1 corresponds to the antisymmetric CO3- stretching mode of a noncoordinated carbonate associated with barium. All three of these carbonate species also have a vibrational mode in the range of ∼1090-1020 cm-1.51 However, because of the strong absorption of infrared radiation by the alumina support, these peaks cannot be identified. As NOx is stored, the bidentate barium carbonates are displaced, as evidenced by the decrease of the peak at ∼1550 cm-1. The corresponding increase of the peaks in the region 1480-1330 cm-1 indicates that bulk nitrates are formed. These observations indicate that the NOx adsorption process with CO2 and H2O in the inlet follows the same reaction mechanism (i.e., also goes through R1f, R1s, R2, and R3) but, in this case, the initial state of the catalyst surface is BaCO3 rather than BaO, as when CO2 and H2O are absent. On the basis of this postulate, we used the same mechanism that was used to fit the NO2/O2 data to predict NOx breakthrough with NO2/O2/CO2/H2O as the inlet. A comparison of the model predictions with experiments is shown in Figure 5. We found that the same NOx adsorption mechanism, i.e., NO2 disproportionation plus NO adsorption, can still explain the NO and NO2 breakthrough curves well. However, the kinetic parameters for adsorption changed upon addition of CO2 and H2O, as shown in Table 3. First, because the surface for NOx adsorption changed from BaO to BaCO3, a change in the adsorption site density as well as the adsorption rate constants is expected. The total amount of NO adsorbed decreased significantly with CO2 and H2O, as indicated by the change in the parameter Ls,NO in Table


Figure 5. (A) Experimental and simulated NO and NO2 breakthrough curves on Pt/BaO monolith with 270 ppm NO2/10% O2 with 8.35% CO2 and 8.35% H2O inlet at 300 °C, 30 000 h-1 SV. (B) Model-predicted surface NOx accumulation as a function of time.

3. The total capacity for NO2 adsorption remained almost the same (35 versus 38 mol/m3), if one sums the slow site capacity Lss and the fast site capacity Lsf for NO2 adsorption without and with CO2 and H2O. One change for NO2 adsorption is that the ratio between the slow uptake and the fast uptake for NO2 changed from 0.8 without CO2 and H2O to about 1.8 with CO2 and H2O, as indicated in Figure 5B. Also, by comparing model parameters listed in Table 3, it can be seen that the rate constants for NO (kNO) and NO2 (k1f and k1s) adsorption all decreased in the presence of CO2 and H2O. It is well-known that the presence of CO2 and H2O decreases the amount of NOx adsorbed on NOx storage/reduction catalysts during cyclic operation.53–56 These studies revealed that the negative effects of both CO2 and H2O not only decrease the time for complete capture, but also decrease the total amount stored up to saturation. These effects were all observed in our work, if one compares the NO2 and NO breakthrough curves in Figures 3A and 5A. In addition, the modeling results indicate that the decrease in NOx storage is mainly caused by a decrease in NO adsorption rather than NO2 adsorption. This can be explained by the argument that, although CO2 changes the surface from BaO to BaCO3, the total available sites should be retained. The decrease in NO adsorption can be related to the change in surface barium morphology due to the presence of H2O.57 H2O can sinter the highly dispersed barium phase to a bulk phase, thus decreasing the proximity between Pt and Ba as proposed by Szanyi et al.58 If direct NO adsorption (reaction R3) is assumed to rely on the facilitation of Pt to activate O2 for nitrite formation and subsequent oxidation, then this would provide a route by which the NO adsorption process can be suppressed by the presence of H2O. Model validation for NOx adsorption was done by comparing model predictions with independent experiments at different inlet NOx concentrations. Figure 6A shows the results for 82 and 405 ppm NO2 inlet concentrations in the absence of CO2 and H2O and gives the model predictions using the parameters obtained by fitting the data at 300 ppm NO2 inlet. The NO/ NO2 breakthrough profiles from the case of 405 ppm NO2 were

fit very well. This model gives a slight overestimate of the NO2 adsorption (lower NO2 breakthrough curves) for the case of 82 ppm NO2. In the presence of CO2 and H2O, as shown in Figure 6B, the model predicts a small delay in the NO2 breakthrough at high concentration (380 ppm NO2 inlet) and a slight acceleration at low inlet concentration (82 ppm NO2 inlet). The overall error is smaller than in the CO2- and H2O-free case. We take this performance as a validation of the model in this operating region. Furthermore, it is noteworthy to point out that the NOx adsorption model containing R1f, R1s, R2, and R3 is a minimum set to describe the physical processes. Eliminating any of the above four steps would result in a lack of fit to the experimental data. 4.4. NOx Reduction. Previous experimental work33 has shown that, when NH3 is used as a regenerating gas instead of H2, the process is equivalent to that with H2. Based on this finding, we proposed that the regeneration of the partially saturated trap catalyst using H2 as the reductant involves the formation of NH3 as an intermediate from the stored NOx and that NH3 acts as a carrier of H atoms.33,59 The NH3 intermediate mechanism and the moving reduction front are further discussed in this work in relation to a reduction model. Regeneration of stored surface NOx was studied with 0.75% H2 in Ar as the model reductant at SV ) 30 000 h-1. Figure 7A and C shows the respective temporal profiles of the outlet species and the outlet gas-phase temperature under reaction conditions. Starting from the above argument, the following two sequential reactions were used for the NOx reduction process 8H2 + Ba(NO3)2 ) BaO + 2NH3 + 5H2O

(R4)

-∆Hrxn ) 900 kJ/mol 10 8 NH3 + Ba(NO3)2 ) BaO + N2 + 5H2O 3 3

(R5)

-∆Hrxn ) 625 kJ/mol where the heat of reaction was obtained from HSC Chemistry 4.1 (Chemistry Software Ltd.), assuming H2O to be in the gaseous state. In the reduction model, the rate constants for reactions R4 and R5 are set high relative to the mass-transfer rate and the supply rate of the reductant as characterized by the residence time. This is done as a consequence of assuming a reductant supply limitation as has been found previously.33,59 The model simulation results are also shown in Figure 8. First, the gasphase H2 concentration inside the monolith at different times during regeneration is plotted in Figure 8A. The model reveals that, when H2 enters the monolith with an inlet concentration, it is consumed completely within a short length of the front part of the catalyst and reaches zero concentration in the downstream gas phase. The region of the catalyst where the H2 concentration drops from the inlet level to zero is referred to as the reduction zone. Then, as time goes on, this reduction zone moves toward the end of the catalyst. In other words, the surface NOx is gradually “eaten up” by this reduction zone. According to reaction R4, consumption of H2 results in the production of NH3. In Figure 8B, the gas-phase NH3 concentration is plotted as a function of the catalyst length at the same time intervals as in Figure 8A. It is clear that NH3 is produced within the H2 reduction zone by reaction R4. However, this in situ generated NH3 reacts with the residual surface NOx according to reaction R5 when it passes through the downstream catalyst. This explains why, at the beginning of reduction, no NH3 is seen at the outlet of the monolith, as shown in Figure 7A. When the H2 reduction zone approaches the end of the


Figure 6. (A) NO and NO2 breakthrough curves on Pt/BaO monolith for (left) 82 and (right) 405 ppm NO2 inlet with 10% O2 at 300 °C, 30 000 h-1 SV. (B) NO and NO2 breakthrough curves on Pt/BaO monolith for (left) 82 and (right) 380 ppm NO2 inlet with 10% O2/8.35% CO2/8.35% H2O. Open squares, experimental data; solid lines, simulation.

catalyst, there is not enough downstream surface NOx to react with the NH3, and NH3 breakthrough starts at the outlet. At the end of reduction, when the surface NOx is almost completely converted, H2 breakthrough starts, and the NH3 level decreases because of the shutdown of reaction R4. As a validation of the model, the above-proposed mechanism is also capable of explaining the outlet gas breakthrough curves of N2, H2O, H2, and NH3 when one compares the model-predicted (Figure 7B) and experimentally observed (Figure 7A) outlet breakthrough curves. Nova et al.60 and Larson et al.22 very recently observed similar experimental results for the selectivities of N2 and NH3 during the regeneration of saturated Pt/BaO/Al2O3 catalysts. These authors also explained the data by assuming a reduction front mechanism. It should be noted that the model presented here is qualitative and is used only to help understand the consequence of reductant supply limitation and the role of NH3 as an intermediate. The time scale in the model is shorter than that in the experiments because the amount of previously stored NOx is less in the model than in the experiments, so that the reduction process is completed earlier. The fluctuations of temperature in the gas phase and in the solid washcoat during reduction were also studied. As shown in Figure 7C, during catalyst regeneration, we observed a 33 °C temperature rise in the outlet gas phase. More interestingly, this thermal spike had a delay in time; that is, the gas outlet temperature increased almost at the end of regeneration and experienced a long dissipation process until it decreased back to the inlet gas temperature, which was 300 °C. This gas-phase temperature rise indicates that there is a local “hot” zone on

the surface, where the temperature is higher than the operating inlet gas temperature. By solving the coupled mass/enthalpy balance model associated with reactions R4 and R5, we simulated the outlet gasphase temperature profile shown in Figure 7D. Comparing the simulated profile to the experimental results, the model predicts both the general shape of the gas outlet temperature and the maximum value of the spike, which is slightly above 33 °C. This further indicates that the reduction model captures the critical physics and chemistry of the NOx reduction process. According to the model, a “thermal zone” exists inside the catalyst, because of the moving front of the exothermic reduction reactions R4 and R5. Figure 9A shows the calculated solid temperature profile along the channel at different times. The maximum surface temperature was calculated to be about 35 °C higher than the operating temperature. This is consistent with the assumption that reduction is not kinetically controlled, because the reaction rate is more temperature sensitive than the mass-transfer rate. The model also explains the delay in outlet gas temperature rise. First, the heat flux from the gas phase to the solid phase is defined by heat flux ) h(Tg - Ts)

(15)

This heat flux has units of watts per square meter and is plotted along the catalyst at different times in Figure 9B. According to the definition, a negative sign of the heat flux means that the surface temperature is higher than that of the gas phase and heat is being transferred from the surface to the gas phase. A positive sign indicates the opposite. The simulation results show


Figure 7. (A) Experimental species breakthrough curves for NOx reduction using 0.75% H2 at 300 °C with SV ) 30 000 h-1 (adapted from ref 33). (B) Model-predicted species breakthrough for NOx reduction under the same conditions. (C) Outlet gas temperature profile during reduction. (D) Model-predicted gas temperature profile during reduction.

Figure 8. Model-simulated NOx reduction obtained by first saturating the catalyst with 350 ppm NO/10% O2 and then using 0.75% H2 for reduction at 300 °C: gas-phase (A) H2 and (B) NH3 concentrations along the monolith at different times.

Figure 9. (A) Model-predicted washcoat temperature profiles at different times. (B) Heat flux from gas phase to solid along the monolith at different times during reduction.

that, during reduction, the heat flux actually changed sign along the catalyst. During the first 10 s of reduction, the heat flux has a negative sign in the first one-third of the catalyst and switches to positive in the rest of the catalyst. This means that the surface is hotter than the gas phase in the first one-third catalyst length

and cooler in the rest of the catalyst. This is because the reaction front is confined to the first one-third of the catalyst at that time. In other words, the incoming gas is first heated by the front portion of the catalyst where the reduction takes place. Then, when the gas travels downstream after passing through the


reaction front, it gives the heat back to the cold solid material that has not yet seen the front. This is the reason for the induction period for outlet gas-phase temperature. As the reduction front moves toward the end of the catalyst, as other curves in Figure 9B indicate, a greater portion of the channel is “ignited” by reduction. Eventually, toward the end of reduction, the whole solid phase has a temperature rise. What happens afterward is the process of cooling of the solid phase by contact with the flowing gas. In general, this phenomenon is present when an exothermic reaction front exists in the reactor. Oh and Cavendish61 observed a similar change in the heattransfer direction in a study of the exothermic CO oxidation reaction on a monolith catalyst. 5. Conclusions A mathematical model for a periodically operated NOx storage/ reduction (NSR) monolith has been developed. We incorporated in the model a simplified kinetic mechanism to capture the main effects of NSR chemistry. The reactor model helps to elucidate the complex spatial and temporal features during NOx storage and reduction. The experimental NOx breakthrough curves are wellpredicted by the model simulation, and the two time scales that the adsorption model requires to explain the NOx breakthrough curves are consistent with NOx adsorption/reaction on highly dispersed BaO sites near Pt (fast uptake) and NOx adsorption/ reaction on bulk BaO particles (slow uptake). The simulation also shows the importance of understanding the coupling between the chemical and transport processes. In particular, the existence of a reduction front that is moving along the catalyst is critical to understand the breakthrough of NH3 when H2 is the reductant and the temperature rise in the outlet stream. An interesting consequence of the moving reaction front is a changing direction of the heat flux between the gas and solid phases. In contrast to the model catalyst used in this work, commercial catalysts often contain an oxygen storage component (OSC, such as CeO2). In that case, the currently proposed mechanism for both NOx adsorption and reduction might need to be revised to account for the addition of the oxygen source/sink. Nomenclature Ai ) Arrhenius preexponential factor for the ith reaction As ) cross-sectional area of the whole monolith, m2 av ) geometric gas-solid contact area per unit passage volume, m2/m3 Cgi ) gas-phase concentration of species i, mol/m3 Cwi ) washcoat concentration of species i, mol/m3 Cpg ) gas-phase heat capacity, J/(kg °C) Cps ) solid heat capacity, J/(kg °C) dw ) washcoat thickness, m DH ) hydraulic diameter of cell, m Di ) diffusion coefficient of species i in the gas phase, m2/s Eai ) activation energy for the ith reaction, J/mol h ) external heat-transfer coefficient between the gas and solid phases, J/(m2 s) (-∆H)i ) heat of the ith reaction, J/mol kf,oxi ) rate constant for the forward NO oxidation reaction, s-1 k1f ) rate constant for the fast NO2 adsorption reaction, m3/(mol s) k1s ) rate constant for the slow NO2 adsorption reaction, m3/(mol s) kNO ) rate constant for the NO adsorption reaction, m3/(mol s) k1,red ) rate constant for H2 reduction by surface nitrates, m3/(mol s)

k2,red ) rate constant for NH3 reduction by surface nitrates, m3/ (mol s) kg ) gas thermal conductivity, W/(m °C) km ) external mass-transfer coefficient between the gas and solid phases, mol/(m2 s) L ) length of the monolith channel, m Ls ) total barium site density per unit monolith volume, mol/m3 Ls,f ) fast barium site density for NO2 adsorption per unit monolith volume, mol/m3 Ls,s ) slow barium site density for NO2 adsorption per unit monolith volume, mol/m3 Ls,NO ) barium site density for NO adsorption per unit monolith volume, mol/m3 Nu ) Nusselt number R ) radius of the monolith core, m Ri ) reaction rate for the ith component per unit monolith volume, mol/(m3 s) rf,oxi ) turnover rate of forward NO oxidation, s-1 roverall,oxi ) overall turnover rate of NO oxidation to NO2, s-1 r1f ) NO2 adsorption rate on fast barium sites, mol/(m3 s) r1s ) NO2 adsorption rate on slow barium sites, mol/(m3 s) rNO ) NO adsorption rate on barium sites, mol/(m3 s) r1,red ) rate of H2 reduction by surface nitrates, mol/(m3 s) r2,red ) rate of NH3 reduction by surface nitrates, mol/(m3 s) Sh ) Sherwood number Tg ) gas-phase temperature, K Ts ) monolith wall temperature, K Tin ) gas-phase inlet temperature, K T0 ) ambient temperature, K u ) gas superficial velocity, m/s Greek Letters ) open frontal area of the monolith s ) washcoat void fraction R ) washcoat frontal area β ) equilibrium factor for the NO oxidation reaction Fg ) density of the gas, kg/m3 Fs ) density of the solid, kg/m3 λs ) solid thermal conductivity, W/(m °C) θs ) fraction of barium that is covered by nitrates θs,f ) surface coverage of fast NO2 adsorption sites θs,s ) surface coverage of slow NO2 adsorption sites θs,NO ) surface coverage of NO adsorption sites

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ReceiVed for reView January 31, 2008 ReVised manuscript receiVed May 22, 2008 Accepted May 29, 2008 IE8001809

Al2O3 Monolith

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