heterogeneous ammonia storage model for nh3−scr modeling

Heterogeneous Ammonia Storage Model for NH3−SCR ModelingJian Gong,*,†,‡ Kushal Narayanaswamy,⊥ and Christopher J. Rutland†

†Engine Research Center, University of WisconsinMadison, 1500 Engineering Drive, Madison, Wisconsin 53706, United States⊥General Motors, Global Research and Development, 30500 Mound Road, Warren, Michigan 48092, United States

*S Supporting Information

ABSTRACT: Single- and dual-site ammonia storage modelsare initially developed and calibrated from ammonia TPDexperiments. The dual-site model gives better agreement withthe experimental measurements, but neither of these models isable to adequately represent the observed ammonia storagetrends over a wide range of temperatures. An alternativeheterogeneous single-site model considering the heterogeneityof ammonia storage sites is proposed that more accuratelypredicts ammonia storage in the temperature window of 150°C−400 °C. When the heterogeneous storage site model iscombined with kinetics for NH3 and NO oxidation forsimulating standard SCR, the NH3 and DeNOx trends as wellas the NH3 inventories during reaction conditions reported forlab reactor studies are replicated.

■ INTRODUCTIONLean-burn exhaust aftertreatment systems are typically requiredto provide high nitrogen oxides (NOx) conversion efficiency(>90%) over a wide temperature window (150−550 °C) tomeet stringent NOx regulations. Urea/ammonia (NH3)selective catalyst reduction (SCR) systems have beensuccessfully applied on lean-burn engines for NOx emissioncontrol.1,2 Non-noble metal catalysts like vanadium (V), iron(Fe), and copper (Cu) supported zeolites are among the mostactive catalysts for the urea/NH3 SCR process. In the past fewyears, significant efforts have been put into Fe and Cuexchanged zeolites, which are more active than V-based systemsat low temperatures.3−5 It was also found that Cu-zeolites showhigher low temperature activity due to superior ammoniastorage at low temperature and lower sensitivity to the NO2/NOx ratio as compared to Fe-zeolites.4,6−8 Recently, Cu-zeolites with a chabazite (CHA) structure, like Cu-SAPO-349,10

and Cu-SSZ-13,11−14 have exhibited superior performance dueto their high thermal stabilities.13,15,16 Also, there is growinginterest in metal exchanged SCR catalysts for NOx control inlean-burn gasoline engines.17−21 Recently, a passive selectivecatalytic reduction (passive SCR) system, including close-coupled three-way catalysts (TWCs) and SCR catalysts, wasproposed and demonstrated by Li et al.19,22 to control NOxemission on a light-duty gasoline engine. Effective NH3generation23−25 in the TWCs and high NH3 storage capabilityin the SCR at medium temperatures are the keys to successfullyapply the passive SCR system. For diesel and gasoline engines,understanding NH3 storage characteristics is critical to achievehigh DeNOx performance by using SCR catalysts.Until recently, the understanding of the NH3 storage has

been improved to a good extent by applying advanced

diagnostic tools and well-designed experimental proto-cols.11,26−29 In situ diffuse reflectance infrared Fourier trans-form spectroscopy (DRIFTS) and temperature-programmedexperimental protocols were utilized to probe the active sitesand reaction pathway on Cu-zeolites with a chabazitestructure.26,28,29 It was found that NH3 can adsorb in theform of NH4

+ species on a high degree of heterogeneityBrønsted acid sites.9,15 Also, NH3 was observed to absorb in theform of molecular NH3 on Lewis acid sites, includingexchanged Cu ion in the zeolite framework and some extra-framework Al.15,26 A high variety of adsorption sites creates agreat challenge to accurately model NH3 adsorption.Parallel to the improved experimental understanding of the

nature of a catalyst, catalyst models have been extensivelydeveloped in the past decade.30 Mathematic models such asdiesel oxidation catalyst31,32 and diesel particulate filter33−36

have been successfully applied on aftertreatment controlstrategies development37−40 and system integration41 forhydrocarbons and particulate emissions control. Similarly,robust and accurate SCR models are needed for SCR catalystdevelopment to control NOx emissions. A variety of detailedmodels5,42−44 as well as global kinetic models45−49 for NH3−SCR are available in the literature. As discussed, one of themain challenges for SCR model development is accuratedescription of ammonia storage in a wide range of operatingconditions. The accurate description of NH3 adsorption anddesorption phenomenon is the basis for correct prediction of

Received: March 19, 2016Revised: May 5, 2016Accepted: May 6, 2016Published: May 6, 2016

Article

pubs.acs.org/IECR

the NH3−SCR catalytic performance. NH3 adsorption anddesorption has previously been described by single-sitemodels49,50 and detailed multiple-sites models.5,44 A Temkin-type adsorption isotherm kinetic model was widely used overV-based and zeolite-based SCR catalysts.46,50 In this approach,it is common to assume that the adsorption is a nonactivatedprocess, and the activation energy of the desorption process is alinear function of the adsorbed species surface coverage.Recently, a dual-site approach was developed by Colombo etal.51 on a Fe-zeolite. The dual-site storage model resulted in theaccurate description of NH3 adsorption and desorption instudied temperatures.In the present work, a single-site NH3 storage model and a

dual-site NH3 storage model are developed and comparedbased on the NH3 TPD experiments. An improvedheterogeneous single-site NH3 storage model with a temper-ature dependent heterogeneous desorption energy is presented.This heterogeneous single-site storage model is furthervalidated at standard SCR conditions.

■ EXPERIMENTAL SECTION

Catalyst. The catalyst for this study is a commercial smallpore Cu-chabazite SCR catalyst. The catalyst is a 100% ionexchanged zeolite containing 2.8% Cu.52 The exchanged zeolitewas coated on a cordierite ceramic honeycomb substrate. Thespecification of the Cu SCR catalyst is shown in Table S1. Thecore sample was hydrothermally degreened at 700 °C for 4 h inhumidified air (∼10% H2O, 20% O2, balance N2) in alaboratory furnace.Experimental Methodology. In order to study the SCR

performance, a small core sample was extracted from SCRcatalyst brick. Experiments were performed at Oak RidgeNational Laboratory (ORNL) in a fixed flow bench reactorsystem. The catalyst was located in a position inside the heatedzone of the furnace so as to achieve a near-isothermalcondition.Two test protocols were utilized in this study. Temperature-

programmed desorption (TPD) experiments were carried outto quantify NH3 adsorption and desorption. In ammonia TPDexperiments, a SCR catalyst was exposed to 350 or 420 ppm ofNH3 and 5% H2O for about 70 min at 150 °C and followed bya temperature ramp for about 80 min. The TPD experimentsbegan with a NH3 adsorption step, which consists of a stepwiseincrease in NH3 concentration (from 0 to 350 or 420 ppm),while the catalyst temperature was held constant at 150 °C.After the catalyst was saturated with NH3 (as evidenced by asteady-state outlet concentration being the same as the inletconcentration), the flow of NH3 was shut off. At this point, thecatalyst was held at the 150 °C while NH3 isothermallydesorbed. When the outlet NH3 concentration dropped below5 ppm (indicating the end of isothermal desorption), thecatalyst temperature was increased to 550 °C at a rate of 5 °Cmin−1.An experimental protocol to characterize the key reactions

that control SCR NOx conversion performance was used tostudy the kinetics. The isothermal protocol consists of a seriesof step changes in inlet gas composition designed to measurereaction rates and NH3 inventories under SCR operatingconditions. Each step in the protocol was allowed to run untilthe outlet gas concentrations reached a steady state. Standardspace velocity (at 1 atom and 273 K) is 60,000 h−1, and 5%H2O was maintained in all the tests. However, only part of the

experimental data obtained from this protocol was describedand used in this study.The isothermal SCR protocol was conducted over a range of

temperatures (150 to 400 °C) at ORNL. Table 1 shows the

detailed gas composition for each step in the protocol. TheNH3 storage started in step 1.1 with NH3 uptake measured inthe absence of O2 and NOx. This step yielded a measurementof the total NH3 storage capacity while avoiding thecomplications introduced by NH3 oxidation. The rate of NH3oxidation was measured in step 1.2 after O2 was turned on.After step 1.2 reached steady state, NO was turned on in step1.3. Integrating the NOx reduced by the NH3 stored on thecatalyst during this step yielded a measure of the NH3inventory. Standard SCR was conducted in step 1.4. In step1.5, NH3 inventory measurement was repeated after standardSCR reaction by feeding NO. The protocol was repeated attemperatures of 400 °C, 350 °C, 300 °C, 250 °C, 200 °C, and150 °C.

■ KINETIC MODELINGReactor Model. A single-channel, one-dimensional (1D)

model used to describe the monolith converter has beendeveloped with the following assumptions: 1) steady state; 2)axisymmetric geometry; 3) neglect axial diffusion of mass andheat (axial Peclet number, defined as the ratio of axial diffusiontime to the axial convection time, is about 1000 for heat andmass transport, which indicates the dominance of theconvective heat and mass transfer17,53).The mass conservation equation of bulk gas species is

described by

∂∂

+∂∂

= − −c

xk G c c( )

j g j gm j sa j g j s

, ,, , , (1)

where cj,g is the concentration of the jth bulk gas species, whilecj,s is the concentration of the jth surface species. In eq 1, Gsa isthe geometric surface area to catalyst volume ratio. The widelyused film model is applied to account for mass transfer betweenbulk gas and washcoat by assuming that the reaction rate isequal to the diffusion rate

− =k G c c G R( )m j sa j g j s ca j, , , (2)

In eq 2, Gca is the specific catalyst surface area, and km,j is themass transfer coefficient of jth species. Also, stored NH3 at thekth storage sites is described by

∑θΩ

∂∂

k3,(3)

Table 1. Isothermal SCR Protocol

step descriptionNH3(ppm)

NO(ppm)

NO2(ppm)

O2(%) T (°C)

1.0 cool 0 0 0 10 600→T1.1 NH3 storage 350 0 0 0 T1.2 NH3 oxidation 350 0 0 10 T1.3 NO oxidation/NH3

inventory0 350 0 10 T

1.4 NO SCR 350 350 0 10 T1.5 NO SCR NH3

inventory0 350 0 10 T

Industrial & Engineering Chemistry Research Article

DOI: 10.1021/acs.iecr.6b01097Ind. Eng. Chem. Res. 2016, 55, 5874−5884

where Ωk is the storage capacity of kth storage sites, and θk is the

NH3 coverage at site k. The energy equation of the bulk gas iswritten as

ρ∂∂

+∂∂

= − −⎛⎝⎜

⎞⎠⎟Cp

xh G T T( )g g g sa s

Here, hg is the heat transfer coefficient between the catalystsurface and gas. To complete the 1D model, the monolith orsurface energy equation is included which lumps the washcoatand substrate

∑ρ λ∂∂

=∂∂

+ − + −Δ=

h G T T G R H( ) ( )s p ss

g sa s caj

The mass transfer coefficient is determined by the

correlations with the Sherwood number =kjShD

h. Similarly,

the heat transfer coefficient is calculated from the Nusselt

number =λ

h. Other symbols and notations used in the

above equations are given in the nomenclature. Also, thewashcoat mass transfer is not considered in this study, whichalso has been done in several other NH3−SCR modelingstudies.44,50,54 There are several reasons for neglecting thewashcoat mass transfer in this work: 1) the washcoat masstransfer will not likely affect the NH3 storage characteristics andeventually the NH3 storage model development; 2) the NH3−SCR performance data were collected in the temperature from150 to 400 °C, at which the washcoat mass transfer may beminor; 3) the catalyst we studied is small-pore zeolite, on whichthe washcoat mass transfer has less effects on the SCRperformance compared to large-pore zeolite catalysts.

■ RESULTS AND DISCUSSIONNH3 Storage Modeling. Temkin isotherm is widely used

to describe the NH3 adsorption and desorption process.Assuming a nonactivated adsorption rate constant,50,55 anadsorption rate expression can be described by eq 6

θ= · · −R A c (1 )NH ad ad NH g, ,3 3 (6)

In the Temkin isotherm model, indirect adsorbate−adsorbateinteractions on adsorption isotherms have been taken intoaccount by observing that the heats of adsorption woulddecrease with increasing coverage during the desorptionprocess.3 In other words, the heterogeneity of the catalystsurface is modeled by considering the desorption energy as alinear function of surface coverage, which has been used inseveral other models to simulate desorption.55−57 The NH3desorption rate equation is shown in eq 7, and ϵcat is a modelconstant, which describes the dependence of desorptionactivation energy Eade on NH3 coverage θ

θθ= · −

− ϵ·

⎡⎣⎢⎢

⎤⎦⎥⎥R A

(1 )NH de de

Single-site and multiple-sites NH3 storage models are bothwidely used to model the dynamic NH3 adsorption anddesorption process. Generally, it is ideal to use less adsorptionsites to simplify the model, and therefore less calibration effortis needed. However, adding additional adsorption sites may be

necessary to describe multiple NH3 desorption peaks duringNH3 desorption.

Single-Site Storage Model. A single-site storage model hasbeen found to be sufficient for kinetic models valid from 150°C and higher temperatures in previous studies.50 Also, it wasobserved that NH3 desorbing from Brønsted acid sites andLewis acid sites showed analogous profiles at temperatureabove 200 °C.26 This indicates that one adsorption site may beadequate to describe desorption dynamics since both types ofsites have similar acid strengths. In order to make the modelsimple to be used, a single-site ammonia storage model isinitially developed to model the NH3 adsorption anddesorption characteristics. The catalyst active site density aswell as the kinetic parameters is calibrated by minimizing thedifference of outlet NH3 concentration between the model andTPD experiments using the least-squares regression algorithm.The active site density of this catalyst, which is calibrated fromammonia TPD experiments, turns out to be 250 mol/m3.Estimated kinetic parameters are given in Table 2.

The outlet NH3 concentrations from the NH3 TPDexperiment (inlet NH3 = 350 ppm) and simulation arepresented in Figure 1. There is a total uptake of ammonia for

about 2 min, and thereafter the ammonia starts to breakthrough. During isothermal desorption, the single-site modelgives slightly lower ammonia desorption. When the temper-ature is increasing, the model seems to overpredict ammoniadesorption with a slightly higher but flat desorption peak. Itseems that there is a large overlap between the NH3 desorptionfrom Brønsted acid sites and Lewis acid sites around 200 °C,which is consistent with other studies of NH3 adsorption onsmall pore Cu-CHA SCR catalysts.26 Desorption of ammonia iscomplete at 450 °C, which is correctly predicted by the model.

Dual-Site Storage Model. In order to compare to the single-site storage model, a dual-site ammonia storage model isdeveloped and calibrated. The first site is used to model weakly

Table 2. Kinetic Parameters of a Single-Site AmmoniaStorage Model

parameters value units

Aad 1.5 1/sAde 1.00 × 1011 mol/m3/sEade 162.18 kJ/mol

ϵcat 0.73 -Ωsite 250.00 mol/m3

Figure 1. Single-site and dual-site NH3 storage models from the NH3TPD experiment (NH3 = 350 ppm, T = 150 °C, SV = 60K h−1).

adsorbed ammonia and physisorbed ammonia on acid sites fora correct description of NH3 desorption at low temperatures.Similar to the single-site model, the Temkin isotherm model isapplied on the first site considering there is a high degree ofheterogeneity of acid sites of weakly adsorbed NH3. Thesecond site is used to model strongly bonded NH3 in order todescribe NH3 desorption at relatively high temperatures.However, a simple Langmuir isotherm model is utilized onthe second site by realizing that most of the NH3 adsorbed athigh temperature is from the Brønsted acid site of bridging Al−OH−Si and Lewis acid sites of exchanged Cu ion, which mayhave similar acid strengths.The rate expressions for NH3 adsorption and desorption on

each site are described by eq 8−eq 11. Estimated kineticparameters are summarized in Table 3. The desorption energy

at the first site (weak adsorption) is 43 kJ/mol at zero coverage,which is much lower compared to 88 kJ/mol of the second site(strong adsorption). The heterogeneity constant ϵcat,1 iscalibrated to be 0.89 on the first site, which is consistent withour initial assumption of a high degree of heterogeneity ofadsorption sites of weakly adsorbed NH3. Also, it is interestingto see that the total number of storage site density of this dual-site model is very comparable to that of the single-site model(235 vs 250 mol/m3)

θ= · · −R A c (1 )NH ad ad NH g, ,1 ,1 , 13 3 (8)

θθ= · −

− ϵ·

⎡⎣⎢⎢

⎤⎦⎥⎥R A

(1 )NH de de

cat, ,1 ,1

a ,1 11

θ= · · −R A c (1 )NH ad ad NH g, ,2 ,2 , 23 3 (10)

θ= · − ·⎡⎣⎢

⎤⎦⎥R A

RTexpNH de de, ,2 ,2

Ammonia TPD comparison between the single-site and dual-site model is shown in Figure 1. The dual-site model accuratelydescribes ammonia adsorption as well as the single-site model.During isothermal desorption, the dual-site model showsslightly higher desorption of ammonia, which is attributed tothe lower desorption energy of the dual-site model compared tothat of the single-site model. When temperature is ramping up,the dual-site model captures desorption much better than thesingle-site model. This is probably due to a low storage sitedensity of the second site in the dual-site model.Ammonia Storage at Different Temperatures. The single-

site and dual-site ammonia storage models were calibrated andcompared with the ammonia TPD experiments at 150 °C in theprevious section. In order to further evaluate the storagemodels, the ammonia storage step (step 1.1) of the isothermal

protocol experiment shown in Table 1 is simulated using thesingle-site and dual-site models at temperatures from 150 to400 °C with an inlet ammonia concentration of 350 ppm.At a specific temperature, the amount of NH3 adsorption can

be calculated from the model as below

θ= Ω ·CNH ab cal cat eq, ,3 (12)

where the equilibrium NH3 coverage θeq is obtained by setting

= − =θ

RR RR 0d

dt ad deeq at equilibrium. Here Ωcat is the active

site density of the catalyst. From the experiments, the amountof NH3 adsorption can be calculated from eq 13

∫= − · ·C x x SVP

R Tdt( )NH ab NH in NH out

u, ,exp , ,3 3 3 (13)

In Figure 2, ammonia storages at different temperatures arecompared between the single-site and dual-site models under

steady-state conditions. Surprisingly, none of these two modelsgive acceptable predictions in the amount of NH3 storage atdifferent temperatures. The single-site model significantlyunderpredicts ammonia storage at temperatures greater than150 °C, and the differences between model and experimentsincrease with temperature. The differences indicate that thesingle-site model gives a higher desorption rate (or a lowerdesorption activation energy) at a higher temperature, whichleads to less amount of ammonia adsorbed. The dual-site modeloverpredicts with a constant bias at temperature higher than150 °C. The single-site model shows a linear relationshipbetween the adsorbed ammonia and temperature, which isconsistent with the experimental observations.3 The dual-sitemodel shows a slight deviation from the linearity whentemperature is higher than 150 °C. Through these compar-isons, it can be seen that ammonia storage models calibratedfrom TPD experiments are not adequate to accurately predictthe ammonia storage over a wide range of temperatures. It isnecessary to validate the storage model to different temper-atures after TPD calibrations.

Heterogeneous Single-Site Storage Model. Theoretically,the dynamic ammonia adsorption and desorption can beaddressed and modeled by using a large number of sites.However, this approach will definitely result in an increase incomplexity, a higher computation cost, and a significantcalibration effort. To keep the storage mode as simple aspossible, an alternative approach using a single storage site ispresented.

Table 3. Kinetic Parameters of a Dual-Site Ammonia StorageModel

site 1 site 2

parameters value units parameters value units

Aad,1 0.75 1/s Aad,2 21.5 1/sAde,1 13.25 mol/m3/s Ade,2 1.97E7 mol/m3/sEade,1 43 kJ/mol Eade,2 88 kJ/mol

ϵcat,1 0.89 - ϵcat,2 - -Ωsite,1 207 mol/m3 Ωsite,2 28 mol/m3

Figure 2. Comparison of ammonia storage capacities at differentadsorption temperatures between single-site and dual-site models.

In the previous single-site model, classic Temkin isothermwas used by considering the adsorbent−adsorbate interactionto model the coverage dependence desorption energy. On theother hand, based on the recent experimental studies ofammonia storage,9,15,28 there are different groups of adsorptionsites (weak acid sites like P−OH and Si−OH groups and extra-framework Al, physisorbed NH3 molecules) with distinctadsorption strengths at low temperatures. At high temperatures,only NH3 adsorbed on Brønsted acid sites and Lewis acid siteswith similar acid strengths stays. This indicates the hetero-geneity of ammonia adsorption is temperature dependent. Thistemperature dependent heterogeneity of adsorption sites ismissed in the classic Temkin isotherm.In order to capture the temperature dependent heterogeneity

of ammonia adsorption sites in a single-site model, atemperature dependent ϵcat is introduced rather than using aconstant value. A higher value of ϵcat indicates moreheterogeneous NH3 adsorption sites. The ammonia desorptionrate equation from the previous single-site model is thenmodified as

θθ= · −

− ϵ ··

⎡⎣⎢⎢

⎤⎦⎥⎥R A

(1 ( ) )NH de de

a cat,

The temperature dependent term ϵcat(T) is calibrated basedon the ammonia adsorption step in the isothermal test protocolat temperatures of 150−400 °C, while the kinetics ofadsorption and desorption and the total storage site densityare the same as the previous single-site model. The results ofammonia storage capacity at different temperatures are given inFigure 3. With the heterogeneous single-site storage model,

predicted cumulative ammonia adsorption profiles are in goodagreement with the experiments at different temperatures,which are described in Figure 4.The relationship between ϵcat and temperature can be readily

described by a quadratic function as shown in Figure 5. In thisquadratic function, the value of ϵcat decreases with temperature.The calibrated ϵcat results in a low value of 0.07 at 400 °C and ahigh value of 0.73 at 150 °C. This is consistent with theexperimental observations15,26,28 that the adsorption sites (onlystrong Brønsted acid sites and Lewis acid sites with similar acidstrengths) are less heterogeneous at high temperatures, whilethe adsorption sites are more heterogeneous with distinct

groups of adsorption sites with different adsorption strengths atrelatively low temperatures.Figure 6 shows the variation of the desorption energy with

NH3 surface coverage for the three different approaches. The

single-site model shows that the desorption energy is linear tothe NH3 surface coverage, which results from the classicTemkin isotherm model (see eq 7). This linear dependence isnecessary by ignoring the heterogeneity of various families ofadsorption sites. The dual-site model shows significantlydifferent desorption energy between the two sites. Site 1representing the weakly adsorbed NH3 site shows a relativelylower desorption energy (<50 kJ/mol) and a similar linearrelationship with NH3 surface coverage compared to the single-

Figure 3. Comparison of ammonia storage capacities at differentadsorption temperatures for single-site, dual-site, and heterogeneoussingle-site with temperature dependent ϵcat(T) ammonia storagemodels.

Figure 4. Cumulative adsorbed ammonia at temperatures from 150 to400 °C (solid line: experiments; dash line: heterogeneous single-sitemodel).

Figure 5. Calibrated values of temperature dependent term ϵcat(T).

Figure 6. Comparison of desorption energy as a function of NH3coverage.

site model. While site 2 shows a constant desorption energy byconsidering the desorption energy of strongly adsorbed NH3sites (Brønsted acid sites) is well-defined.58,59 By introducingthe temperature dependent ϵcat(T), the heterogeneous single-site model exhibits a nonlinear relationship to the NH3 surfacecoverage. This nonlinear relationship is in line with the recentexperimental ammonia isotherm studies, which reported asimilar nonlinear relationship between adsorption enthalpy andNH3 coverage.

60 As it can be seen from Figure 6, at low NH3coverage (<0.25), NH3 desorption energy is high and tends tobe a constant. At mediate NH3 coverage (0.25−0.7), thedesorption energy exhibits a linear relationship to NH3coverage. As NH3 coverage further increases, the desorptionenergy decreases with a slow rate. The nonlinear relationship ofthis heterogeneous single-site model essentially describes theeffects of dynamic change of heterogeneity of variousadsorption sites at different NH3 surface coverage (or differenttemperatures) on the desorption energy.Ammonia TPD experiment is revisited by applying the

heterogeneous single-site model. Figure 7 shows the compar-

ison between the two single-site models. The differencesbetween the previous single-site model and heterogeneoussingle-site model are indistinguishable during the adsorptionand isothermal desorption steps. In the nonisothermaldesorption step, a second ammonia desorption peak is observedaround 380 °C in the heterogeneous single-site model, while aflat ammonia desorption peak is found in the previous single-site model. The second ammonia desorption peak was widelyreported in the ammonia TPD studies of Cu-chabazite NH3−SCR catalysts.26,28 However, the second ammonia desorptionpeak is not readily distinguished from the first ammoniadesorption peak in the experimental data. This secondammonia desorption peak physically corresponds to the NH3desorbed from Brønsted acid site. The two ammoniadesorption peaks are overlapped due to the similarity of theacid strengths for these two types of sites. In Figure 7, theheterogeneous model shows a better prediction of ammoniadesorption when the temperature is below 300 °C. Attemperature above 300 °C, the heterogeneous single-sitemodel overpredicts the ammonia desorbed. Also, the activesite densities of NH3 storage between the two single-sitemodels are given the same value in the model (250 mol/m3).Therefore, the heterogeneous single-site model overpredicts thesecond desorption peak with a higher magnitude compared tothe original single-site model since the total amount of

desorbed ammonia is mathematically balanced. This may alsoindicate that the calibrated active site density is slightly higher.Another NH3 TPD experiment, which was conducted at thesame temperature of 150 °C but with a different NH3 feeding(420 ppm), is simulated by using the two single-site models andthe dual-site model. The simulated NH3 TPD profiles, whichare shown in Figure S1, are similar to the TPD curves in Figure7.

NH3−SCR Modeling. The heterogeneous single-siteammonia storage model has been developed and validated inthe previous section. In this section, the heterogeneous single-site storage model is further validated under SCR reactionconditions. In order to model the SCR performance as well aspredict the NH3 and NO concentration, it is crucial to examinethe NH3 and NO oxidation separately. Therefore, kinetics ofNH3 oxidation, NO oxidation, and standard SCR are obtainedthrough the isothermal protocol in Table 1. With an ammoniato NOx ratio (ANR) of 1.0, the isothermal protocol wasrepeated at different temperatures from 150 to 400 °C in orderto calibrate the kinetics over the whole temperature window.The calibrated model is then validated at the same temperaturewindow with a different ANR of 1.2.Due to the complexity of the SCR reaction network, only the

standard SCR performance is modeled, and the heterogeneoussingle-site NH3 storage model is evaluated at the standard SCRconditions. “Fast” SCR as part of the complex NH3−SCRreaction networks is critical to DeNOx performance at lowtemperatures. However, the “fast” SCR mechanism is far morecomplicated than the well-established standard SCR mecha-nism. Recently, it was found that surface ammonia nitrates playa significant role in “fast” SCR reaction. Multiple reaction stepsincluding ammonia nitrate formation, ammonia nitrate titrationby NO, and ammonia nitrate decomposition are involved in the“fast” SCR mechanism.61−63 In order to accurately describe thefast and NO2 SCR reactions, significant effort involvingmodeling of NO2 adsorption, desorption. and interactionbetween ammonia nitrates and NO is needed. Therefore, thefast SCR and NO2 SCR are not considered in the current study.

NH3 Oxidation. After the ammonia storage (step 1.1) in theisothermal protocol, which has been presented in Figure 4, 350ppm of NH3 is flowing to the catalyst in the presence of 10% ofO2. This step is simulated by considering NH3 oxidation, whichis modeled in eq 15

θ= · ·R k cNH oxi NH oxi O g, , ,3 3 2 (15)

It was reported that Cu-zeolites selectively oxidize NH3 tonitrogen when the temperature is below 500 °C.5 At highertemperatures (T > 500 °C), NO was observed from oxidationof NH3.

64 However, selective NH3 oxidation is not consideredin this SCR model as the temperature window of SCRoperation is below 500 °C in this study. The activation energyof NH3 oxidation of 68 kJ/mol is obtained, which is shown inTable 4. This value is within the range of NH3 oxidationactivation energy (64−78 kJ/mol) reported by Kamasamudramet al.4 on Fe-zeolite SCR catalysts. Also, it is comparable to 74kJ/mol obtained on a Cu-exchanged zeolite54 below 400 °C. Acomparison of the NH3 concentration between the model andexperiment is shown in Figure 8. As indicated in Figure 8, NH3starts to significantly decrease when the temperature is above250 °C, but 100% of NH3 conversion due to oxidation is notreached at a temperature of 400 °C, which is well captured bythe model.

Figure 7. Comparison of ammonia TPD simulation between twosingle-site storage models (NH3= 350 ppm, T = 150 °C, SV = 60Kh−1).

NO Oxidation. Description of the influence of changing theNO/NO2 ratio on NH3−SCR performance is important.Therefore, a NO oxidation step (step 1.3) with an inlet gasof 350 ppm of NO and 10% of O2 was taken after the NH3oxidation step. It is widely accepted that NO oxidation is areversible reaction. At low temperatures, NO to NO2conversion is kinetically limited, and NO2 increases astemperature increases. As temperature increases, NO2 reachesits maximum at about 450 °C and starts to decrease due tothermodynamic constraints. The rate expression of NOoxidation is then described by eq 16

= · −⎛⎝⎜⎜

⎞⎠⎟⎟R k c c

kNO oxi NO oxi NO g O gNO g

eq, , , ,

0.5 ,2

The equilibrium constant based on molar concentration keq isrelated to the equilibrium constant in the unit of pressure

through = ·( )k keq p eqRTP,

0.5. The equilibrium constant based on

partial pressure is calculated from = − Δ( )k expp eqG

RT, , where

ΔG is the Gibbs function with a definition of ΔG = ΔH −TΔS. The NO oxidation activation energy from calibration is48 kJ/mol, which is very consistent to the values (48 kJ/mol49

39.6 kJ/mol65) reported on Cu exchanged zeolites. As seenfrom Figure 9, the NO2 conversion increases to 12% as thetemperature increases to 400 °C, while the calculated NO2conversion at 400 °C with the same composition at equilibriumis about 20%. This implies that NO to NO2 conversion doesnot reach the equilibrium limit over the temperature windowbetween 150 and 400 °C. The model correctly predicts theNO2 concentration in this temperature window.Standard SCR. The standard SCR step (step 1.4) was

conducted with 350 ppm of NO and 350 ppm of NH3 in thepresence of 10% O2. FTIR studies have shown that thestandard SCR cannot be explained by reactions between surfacespecies alone and that gas phase NO is likely important.66

Therefore, the standard SCR is described by a rate that includesNH3 on the surface and NO in the gas phase, which has beenwidely used to model standard SCR reactions42,55

θ= · ·R k cSCR std SCR std NO g, , , (17)

The amount of NH3 at the catalyst outlet as well as total NOxconversion efficiency is shown in Figure 10 and Figure 11. At all

the test temperatures, NO reacts with NH3 stoichiometrically.The NOx conversion efficiency increases with temperature asexpected. More than 90% conversion efficiency is observedwhen the temperature is higher than 250 °C. At ANR = 1.0,both NH3 and NOx conversions are accurately predicted overthe whole temperature window. When the ANR increases to1.2, a significant amount of NH3 is available at the temperaturesof 250 to 350 °C though the DeNOx efficiency is close to100%. The model slightly underpredicts the amount of NH3

Table 4. Kinetic Parameters of SCR Reactionsa

no. reactionrate

constantspre-exponential(m3/mol/s)

activationenergy(kJ/mol)

1 4NH3 + 3O2 → 2N2 +6H2O

kNH3,oxi 2.2 × 1002 68

2 2NO + O2 ←→ 2NO2 kNO,oxi 9.5 × 1001 483 4SNH3+ 4NO + O2 →

4N2+ 6H2O + 4SkSCR,std 2.3 × 1008 84.9

aThe reaction constant is in the form of = · −( )k A exp ERT

Figure 8. Outlet NH3 concentration at different temperatures (350and 420 ppm of NH3 with 10% O2).

Figure 9. Outlet NO2 concentration at different temperatures (350ppm of NO and 10% O2).

Figure 10. Outlet NH3 concentrations at different temperatures (350ppm of NO and 10% O2 with ANR = 1.0 and ANR = 1.2).

Figure 11. Total NOx conversion efficiencies at different temperatures(350 ppm of NO and 10% O2 with ANR = 1.0 and ANR 1.2).

slip at temperatures of 300 and 350 °C. This could be due tothe slightly high NH3 adsorption predicted by the model atthese two temperatures. As expected, the NOx conversionefficiency at ANR = 1.2 is almost the same as that of ANR =1.0. At the standard SCR conditions, no N2O was observedfrom the experiments as well. There are excellent agreementsbetween the model and data. The calibrated reaction constantsof the NH3 oxidation, NO oxidation, and standard SCRreactions are given in Table 4.NH3 Inventory during Reaction Conditions. During each

step shown in Table 1, NH3 inventory was calculated andanalyzed. This was conducted by turning off the NH3 andleaving NOx flowing after each step reached steady state. AnyNOx that was consumed during each of these steps must bereacting with NH3 stored on the catalyst surface. Therefore, theNH3 inventory or an “effective” ammonia storage capacity canbe calculated by integrating the difference between inlet NOxand outlet NOx. On the other hand, the NH3 surface coverageat each step is available from simulation. The NH3 inventoryfrom the simulation at each step can be calculated bysubtracting the NH3 surface coverage at the beginning of thestep from the NH3 surface coverage at the end of the step.Positive values represent NH3 uptake during the step, andnegative values correspond to NH3 release or consumption.The NH3 inventory storage capacity at the ammonia adsorptionstep (step 1.1) has been discussed in the section of ammoniastorage modeling. Therefore, only the NH3 inventory storagecapacities in steps 1.2−1.5 in terms of gram NH3 per litercatalyst are presented in Figure 12. As seen from Figure 12, theNH3 inventories at each step of the protocol are wellrepresented by the model.

In the NH3 oxidation step (step 1.2), almost no stored NH3is consumed until 300 °C, which is consistent with the NH3oxidation conversion discussed in Figure 8 and is correctlypredicted by the model. In the NO oxidation step (step 1.3), astemperature increases, stored NH3 is consumed due to standardSCR and released due to desorption. At 150 °C, DeNOxefficiency is about 15%; the amount of stored NH3 beingconsumed due to standard SCR is limited by the duration ofthis step. As temperature increases to 200 °C, DeNOx efficiencyincreases to about 75%, and a significant portion of stored NH3has been consumed. As temperature further increases, NH3desorption starts to play a critical role in the total NH3consumption besides the standard SCR. Therefore, a “concave”shape of the change of NH3 inventory is observed. Theoverpredicted NH3 inventory between 250 and 400 °C is

probably due to the higher NH3 desorption, which is evidencedin Figure 7.In the standard SCR step (step 1.4), there are multiple

reactions on the surface: NH3 adsorption, desorption, NH3 andNO oxidation, and standard SCR reaction. A positive NH3inventory is due to NH3 adsorption since NH3 is cofeeding tothe catalyst with NO and O2, compared to the negative one instep 1.3. This positive NH3 inventory essentially represents adynamic or effective NH3 storage capacity at standard SCRreaction conditions. The standard SCR inventory step (step1.5) is similar to the NO oxidation step from Table 1 with onlyNO and O2 in the catalyst. Also, it is found that the change ofNH3 inventory with temperature shows a symmetric relation-ship compared to the standard SCR step. However, in thetemperature window of 250 to 400 °C, the NH3 inventoryconsumption in the NO oxidation inventory step is higher thanthat in the standard SCR inventory step. This is because there ismore NH3 storage on the surface prior to the NO oxidationstep (or in the NH3 oxidation step). This indicates that thedynamic change of NH3 inventory depends on the catalystoperation history besides temperature. Therefore, in real-worldapplication of NH3−SCR catalysts, traditional engine-out NOxand temperature based SCR control strategies may not beadequate for accurate and optimal urea or NH3 dosing. Amodel-based predictive NH3 storage model is necessary andcritical.

Insights on NH3 Storage Modeling. NH3 storage modelscalibrated from TPD experiments may not be able to accuratelypredict the ammonia storage over a wide range of temperatures.As seen from Figure 1 and Figure 7, the dual-site model showsthe best agreement to the experimental data for the NH3-TPDamong the original single-site, the heterogeneous single-site,and dual-site models. However, ammonia storage capacities atdifferent temperatures are not well predicted by the originalsingle-site and dual-site models (in Figure 3). It is necessary tovalidate the storage model to different temperatures after TPDcalibrations.The heterogeneous single-site model involves three param-

eters to describe a quadratic function of ϵcat, while the single-site model has a constant ϵcat. Comparing the number of kineticparameters among these three NH3 storage models, theheterogeneous single-site model introduces two more param-eters compared to the single-site model and two less parameterscompared to the dual-site model. There are some advantagesfor the heterogeneous single-site model compared to the dual-site model based on the number of kinetic parameters in themodel and calibration effort. However, the purpose of thisstudy is not to conclude which model is better than the othersbut to develop an alternative approach to model NH3 storage.Recent studies on Cu-zeolites with chabazite structure NH3−

SCR catalysts (e.g., Cu-SAPO-34 and Cu-SSZ-13) have shownsome similarities in terms of NH3 storage sites and reactionmechanisms as well.28,62 These similarities make it possible thatthe proposed approach can be used universally in the samefamily of Cu-chabazite NH3−SCR catalysts. The heterogeneityof NH3 storage sites with respect to temperature may bedifferent. However, this heterogeneity can be calibrated throughNH3 TPD and steady-state NH3 storage capacities measure-ment.

■ CONCLUSIONAn alternative ammonia storage model is developed byconsidering the heterogeneity of ammonia storage sites. The

Figure 12. NH3 inventor comparison between model and experimentsat different temperatures (solid line: experiments; dashed line: model).

results of this study indicate that NH3 adsorption anddesorption from a commercial small-pore Cu-chabazite SCRcatalyst are well represented by a heterogeneous single-sitemodel corresponding to a Temkin isotherm for which theadsorption energy is a function of both temperature and NH3coverage. This model gives good agreement with experimentalammonia TPD measurements over a temperature range of 150°C−400 °C.The kinetic parameters of NH3 and NO oxidation and

standard SCR reactions are determined through separateisothermal experiments. When the ammonia storage model iscombined with NH3 and NO oxidation kinetics, the resultingpredictions for standard SCR are in good agreement withexperimental measurements. NH3 inventories during reactionconditions at each step of the isothermal experiments can bereproduced by the model.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.iecr.6b01097.

Specification of a SCR catalyst sample, comparison ofsimulated NH3 TPD results from three storage models at150 °C with an ANR of 1.2 (PDF)

■ AUTHOR INFORMATIONCorresponding Author*Phone: 812-377-3251. E-mail: jian.gong@cummins.com.Present Address‡1900 McKinley Avenue, MC 50183, Columbus, Indiana47201, United States.Author ContributionsThe manuscript was written through contributions of allauthors. All authors have given approval to the final version ofthe manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors would like to thank General Motors Research andDevelopment for their funding and support of this researchthrough the GM-UW collaborative research laboratory (CRL)program. The authors also appreciate Josh A. Pihl, Todd J.Toops, James E. Parks II, and Stuart Daw in the Emissions &Catalysis Research Group at Oak Ridge National Laboratory forproviding experimental data and valuable discussions in thiswork.

■ ABBREVIATIONScj = concentration of gas/surface species j [mol/m3]km,j = mass transfer coefficient of species j [m/s]u = bulk gas velocity in the channel [m/s]Gsa = geometric surface area per catalyst volume [1/m]Gca = catalyst surface area per catalyst volume [1/m]Rj = reaction rate per catalyst surface area [mol/m2-s]kj = reaction rate constant [1/m-s]Cp = specific heat capacity [J/kg]T = temperature [K]Ru = universal gas constant [J/mol-K]t = time [s]x = length [m]

hg = heat transfer coefficient [W/m2-s]ΔHj = enthalpy of reaction [J/mol]Sh = Sherwood number [-]Dj = diffusion coefficient of species j [m2/s]Nu = Nusselt number [-]CNH3,ab = adsorbed ammonia density [mol/m3]SV = space velocity [1/s]xNH3

= NH3 molar fraction [-]A = pre-exponential coefficient [varies]Ea = activation energy [kJ/mol]

Greek lettersΩk = storage site density of site k [mol/m3]θk = NH3 coverage at site k [-]λ = thermal conductivity [W/m-s]ρ = density [kg/m3]

Subscripts and superscriptsg = gass = surfacein = inletout = outletad = adsorptionde = desorptioncal = calculationexp = experimenteq = equilibriumoxi = oxidationstd = standard

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heterogeneous ammonia storage model for nh3−scr modeling

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