Kinetic Modeling of Dynamic Aspects of the Standard NH 3-SCR Reaction Over V 2 O 5-WO 3 / TiO 2 and Fe-Zeolite Commercial Catalysts for the Aftertreatment of Diesel Engines Exhausts

Kinetic Modeling of Dynamic Aspects of the Standard NH3-SCR Reaction Over V2O5-WO3/TiO2 and Fe-Zeolite Commercial Catalysts for the Aftertreatment of Diesel Engines Exhausts — Dynamic kinetic data collected over commercial V2O5-WO3/TiO2 and Fe-zeolite catalysts analyzing the standard NH3-SCR of NO for the abatement of NOx from vehicle exhausts showed transient effects at low temperatures related to the inhibiting action of excess ammonia. In order to describe such transient effects, a specific rate expression accounting for NH3 inhibition of the standard SCR reaction is needed in mathematical models of real SCR converters. We present herein a dual-site Mars-Ven Krevelen-type rate equation suitable to describe dynamic features associated with ammonia injection and shut-off over both V-based and Fe-zeolite catalysts. Oil & Gas Science and Technology – Rev. IFP Energies nouvelles, Vol. 66 (2011), No. 4, pp. 681-691 Copyright © 2011, IFP Energies nouvelles DOI: 10.2516/ogst/2011132 E-COSM'09 IFAC Workshop on Engine and Powertrain Control, Simulation and Modeling E-COSM'09 Colloque IFAC sur le contrôle, la simulation et la modélisation des moteurs et groupes motopropulseurs IFP Energies nouvelles International Conference Rencontres Scientifiques d’IFP Energies nouvelles ogst10005_Nova 14/10/11 13:01 Page 681


NOTATIONS
E°d es Activation energy for NH 3 desorption at zero-coverage (J/mol) E NO Activation energy for the standard SCR reaction (J/mol) k ads Rate constant for NH 3 adsorption rate (1/s) k°d es Pre-exponential factor for NH 3 desorption rate constant (mole/m 3 /s) k°N O Pre-exponential factor for standard SCR reaction rate constant (1/s) k O 2 Rate parameter in Equation (4) (-) K NH 3 Rate parameter in Equation (5) (-) k sp Rate constant for NH 3 spillover rate (mole/m 3 /s) p O 2 Partial pressure of O 2 (-) r ads Rate of NH 3 adsorption (mol/m 3 /s) r des Rate of NH 3 desorption (mol/m 3 /s) r NO Rate of standard SCR reaction (mol/m 3 /s) r Spill Rate of ammonia spillover reaction (mol/m 3 /s) T Temperature (K) α Parameter for surface coverage dependence, Equation (2) (-) θ NH 3 NH 3 surface coverage on S 2 sites (-) σ NH 3 surface coverage on S 1 sites (-)

INTRODUCTION
Diesel engines, being inherently more thermodynamically efficient than petrol engines, offer the prospect of reducing fuel consumption and emissions of carbon dioxide as well.Indeed, they are fated to significantly increase their worldwide penetration on light duty vehicles [1], while they already monopolize the market for heavy duty ones.On the other hand, Diesel engines produce higher emissions of nitrogen oxides (NO x ) and particulates.Improvements in combustion and/or alternative fuels can lead to lower NO x emissions, but it is widely accepted that, in order to meet the future legislative standards (Euro 6 for light duty and Euro VI for heavy duty vehicles, as well as US 2010 Bin5), the adoption of aftertreatment systems will be required to reduce both NO x and particulate emissions [1].
Concerning NO x reduction, there are different major aftertreatment technologies presently under consideration: the NO x adsorbers, which are used with direct injection gasoline and Diesel engines, the HC-SCR catalysts for light-duty Diesel engines, and the Selective Catalytic Reduction (SCR) process [1].
SCR was the European motor industry's main technology choice to meet Euro 4 and Euro 5 emissions requirements for heavy-duty Diesel engines, and recently it has been announced by some manufacturers for passenger cars in the US and in Europe, as well [1].An SCR system is designed to catalytically reduce NO x emissions in the oxygen rich environment of Diesel exhausts.To reduce NO x emissions, the SCR system needs a chemical reagent, or reductant, to help convert the NO x to nitrogen: in mobile source applications the preferred reductant is typically an aqueous solution of non-toxic urea that is used as ammonia source.In current technologies, the so-called Adblue ® solution is injected into the exhaust system, where it is decomposed to NH 3 .
Most of the NO x emitted by the engine consist of nitric oxide (NO).However, upstream of the SCR converter, a Diesel Oxidation Catalyst is typically present in the system configuration, that allows the partial conversion of NO to NO 2 ; this permits the occurrence of the fast SCR reaction: very active already at low temperature, in addition to the well known standard SCR reaction: The SCR technology was first brought to the market in 2005 for heavy duty vehicle applications by Daimler under the trade mark BLUETEC ® , based on the use of extruded honeycomb monolith catalysts consisting of V 2 O 5 -WO 3 /TiO 2 .The application of such a new technology was optimized by using an unsteady kinetic model of the SCR process specifically developed for mobile applications: indeed, simulation tools able to describe the performance of catalytic converters as a function of the several operating parameters have been proven very useful in the development and design of aftertreatment systems.Nevertheless, the capability of such models in predicting accurately NO x and ammonia emissions mainly relies on the close adherence of kinetic schemes to the real catalytic process.
More recently, new catalysts are being considered for the NH 3 -SCR reactions for mobile applications, such as metalpromoted zeolites.Among these, the Fe-promoted zeolite system is known for its very high activity in the presence of NO 2 , high selectivity to dinitrogen also at high temperatures, and high thermal durability.
In this paper we present the development of unsteady kinetic models of the NH 3 -SCR process for both vanadiumbased and Fe-zeolite catalysts, specifically addressing the transient effects observed at low temperatures in the case of the simpler NO/NH 3 -O 2 reacting system.Indeed, although SCR systems for mobile applications are typically designed to promote the fast SCR reactivity, the NO 2 concentration at SCR inlet can also be very low (e.g.low activity of the upstream DOC at low temperature); under these conditions the dynamics related to the standard SCR reaction can become dramatically important.

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The presented models implement kinetic schemes derived from an extensive investigation of reactivity, chemistry and catalytic mechanism of all the main SCR reactions over commercial V 2 O 5 -WO 3 /TiO 2 and Fe-zeolite catalysts performed in our laboratories during the last few years [2][3][4][5][6][7][8][9][10][11][12][13][14].The related rate expressions were fitted to transient data collected over powdered catalysts in the chemical regime, and in their global form allow the kinetic analysis of the full NH 3 -NO/NO 2 SCR reacting system.
The intrinsic kinetic models, as well as the relevant geometrical and morphological characteristics of the monolith commercial catalysts, were then incorporated into a fully transient heterogeneous 1D+1D mathematical model of SCR monolith reactors, which was validated against transient SCR runs performed at different scales, eventually including test bench data collected on full scale SCR monolith converters, using real Diesel engine exhaust gases [2-4, 10, 11, 13].

Experimental Procedures
Unsteady kinetic SCR experiments of various nature were performed in the 50-550°C T-range over commercial V 2 O 5 -WO 3 /TiO 2 and Fe-zeolite catalysts, originally supplied by Daimler as extruded and washcoated honeycomb monoliths, respectively.The monolith catalysts were crushed and ground to powder (140-200 mesh), a sample of powder (160 mg for vanadium-based and 80 mg for Fe-zeolite catalysts) was diluted with 80 mg of quartz and loaded in a flowmicroreactor consisting of a quartz tube (6 mm i.d.) directly connected both to a UV analyzer (ABB Limas 11-HW) and to a quadrupole mass spectrometer (Balzers QMS 200), which operated in parallel [14].
The dynamics of the SCR reactions were investigated using Transient Response Methods (TRM), that consist in performing stepwise changes of concentration of one species in the feed mixture by means of fast response pulse valves at constant temperature.TRM runs were typically carried out in the presence of oxygen (2% v/v) and water vapor (1 or 3% v/v) in the temperature range 150-500°C, while the feed concentrations of NH 3 and NO x were varied between 0 and 500 or 1000 ppm.A Gas Hourly Space Velocity (GHSV) between 194 000 and 890 000 cm 3 /(h g cat ) was used in all the experiments.Further experimental details can be found elsewhere [10,11,15].
Catalyst conditioning consisted in a temperature ramp of 5 K/min from room temperature up to 600°C in 10% O 2 v/v + 10% v/v H 2 O, followed by hold at 600°C for 5 h.
Intraparticle gradients and gas-solid mass and heat transfer limitations in the powdered catalyst bed were ruled out by diagnostic criteria [16].An analysis of the mass transport limitations in microreactor experiments over the V 2 O 5 -WO 3 /TiO 2 catalyst can be found in [11].For the Fe-zeolite catalyst the same procedure was adopted, based on estimates of the species effective diffusivities of the order of 10 -7 m 2 /s at 200°C.
The SCR reactions were also investigated over core drilled monolith samples (with square section 11.5 × 11.5 mm, and 50 mm length) under isothermal steady-state conditions within the 150-550°C T-range, using a different rig.The catalyst conditioning procedure was the same as for the powder samples.Typical feed concentrations of NO x and NH 3 were 500-1 000 ppm, O 2 2-10% v/v and water 1-10% v/v, with balance N 2 .Analysis of the outlet gases was performed using a UV-analyzer (ABB LIMAS 11HV) directly connected to the reactor outlet via a heated line.

Kinetic Analysis
The transient data collected over the powdered SCR catalysts were analyzed according to a dynamic one-dimensional isothermal isobaric heterogeneous plug-flow model of the test microreactor in order to obtain estimates of the relevant SCR rate parameters [2-4].
The complete models [4,11] included several kinetic steps, in order to fully describe the NH 3 -NO-NO 2 /O 2 reacting system in the whole T-range; Table 1 shows the steps necessary to analyse the simpler NH 3 -NO/O 2 reacting system, while in Table 2 the corresponding rate expressions are reported.The orders of magnitude of the estimated rate parameters for both the V 2 O 5 -WO 3 /TiO 2 catalyst and the Fe-zeolite one can be found respectively in [11] and [4].
The rate expression of step R.4 has been derived assuming that two different types of sites are available on the catalyst surface: one Redox site for O 2 and NO adsorption/reaction (S1), likely associated with V or Fe, and one acidic site for NH 3 adsorption (S2) associated with the W or Ti and zeolite material.

TABLE 1
List of reactions included in the kinetic model Briefly, a weak NO adsorption occurs onto S 1 sites, while on adjacent S 2 sites NH 3 is strongly adsorbed; then, the DeNO x step (NO + NH 3 ) involves reduction of the S 1 sites, followed by reoxidation of reduced S 1 sites by gaseous oxygen [10].
In addition, a reversible "NH 3 spillover" step R.5 from S 2to S 1 -sites, which results in blocking the active redox sites, was included in the scheme in order to account for ammonia inhibition effects (vide infra).
Starting from this Redox scheme, and using a Mars-Van Krevelen formalism, the reaction rate expression of the standard SCR reaction Equation (4), also referred to as Modified Redox rate law (MR), has been derived [10].Also a kinetic approach was assumed for NH 3 spillover, which eventually leads to Equation (5), rate expression for step (R.5).
Indeed, the evaluation of the SCR rate r NO according to Equation (4) requires time integration of Equation (5).
It is important to notice that the complex rate expression Equation (4) can be simplified.E.g. at low ammonia coverages, as typical of temperatures above 250-300°C, it reduces to the well known Eley Rideal rate law (ER): in which one molecule of adsorbed ammonia species reacts with one molecule of gaseous NO; such an expression is commonly used in the kinetic literature for SCR stationary applications [17], which indeed typically work under steady conditions in the 250-400°C T-range.

Test Reactor Model
The test reactor model comprises the transient 1D mass balance equations of adsorbed and of gaseous species.The set of Partial Differential Equations, with obvious initial and boundary conditions, was solved numerically according to the method of lines, based on axial discretization with backward finite differences and on time integration by Gear's algorithm.
The kinetic parameters for ammonia adsorption-desorption and for ammonia oxidation were estimated on the basis of kinetic runs performed by feeding only NH 3 in the presence of O 2 and water.As an example, Figure 1 shows a typical adsorption/temperature programmed desorption test over the Fe-zeolite catalyst used for the estimation of NH 3 adsorption/ desorption rate parameters, in comparison with the corresponding kinetic fit.
Concerning the estimation of the rate parameters in the remaining rate expressions for the SCR reactions, the temporal evolutions of the outlet NH 3 , NO, N 2 concentrations were used as fitted responses in a global multiresponse nonlinear regression of different TRM experiments performed varying temperature, the NO 2 /NO x feed ratio, ammonia, NO x and O 2 concentrations, using a robust multimethod regression routine [18].

Transport Model
The model of the honeycomb monolith SCR converter is an heterogeneous dynamic 1D+1D model of a single monolith channel: it includes the unsteady differential mass balance equations of gaseous species and of adsorbed species.Rate equations are exactly those determined from kinetic runs at the microreactor scale.Enthalpy balances for the gas and for the solid phase are included to account also for thermal effects.In addition, intra-phase diffusional limitations are accounted for by equations for diffusion-reaction of the gaseous reactants in the porous catalytic monolith wall or washcoat, with effective diffusivities evaluated from the   In all the experiments, upon NH 3 step feed the NH 3 outlet concentration trace exhibited a dead time and then slowly grew with time on stream, eventually approaching a steadystate value.In parallel, a fast decrease in the NO outlet concentration together with a mirror-like increment of the N 2 concentration, associated with the startup of the SCR reaction were observed.The levels of NH 3 , NO and N 2 at steady-state were in all cases consistent with the stoichiometry of the standard SCR reaction (2) [2, 3, 5, 10,12,13,20,21].Results obtained in the presence of different oxygen contents were qualitatively very similar.The main effect of the higher oxygen concentration was a slight enhancement of the SCR conversion, as measured by the steady state levels of ammonia, NO and nitrogen: for example, at 250°C a conversion less than 80% was measured at steady state when 2% oxygen was fed to the reactor, while in the presence of 6% of oxygen 500 1 000 1 500 2 000 2 500 3 000 3 500 0 500 1 000 1 500 2 000 2 500 3 000 3 500 such a value was increased to almost 85%.On the other hand, a significant effect of temperature was evident: above 225°C the temperature affected the SCR reaction only by increasing the NO and ammonia conversions.As opposite, in the low T-range a different dynamic behavior of NO and N 2 was observed during both the NH 3 start-up phase and the NH 3 shut-off.Figure 4a focuses on the experiment run at 200°C with 2% oxygen: when ammonia was added to the feed flow (t = 0 s) the NO concentration (paralleled by the N 2 formation) decreased, went through a minimum, and then reached an higher steady state value; at ammonia shutdown the NO concentration (and symmetrically the N 2 production) decreased significantly passing through a minimum, then began to increase and approached its inlet value.Such an effect was observed in the runs with both 2 and 6% oxygen, indicating that oxygen content did not alter the dynamic features of the transient experiments at low temperature.
Further experiments were performed with high frequency NH 3 feed pulses in a stream of 1 000 ppm of NO, 2% v/v O 2 and 1% v/v H 2 O (Fig. 4b).The results (symbols) showed again the characteristic transient behaviour associated with ammonia addition and removal from the feed flow: the greatest NO conversion was always reached during each pulse after the NH 3 shut down.
Thus it appears the DeNO x activity of the system was temporarily hindered by excess ammonia, as already pointed out by other authors on different systems [22][23][24][25][26], suggesting the existence of an optimal ammonia surface concentration, which is lower than the coverage established at steady state.This behavior was attributed to an ammonia inhibition effect, e.g.due to electronic interaction or possibly via direct blocking of the redox sites.
TRM run performed at 200°C presented in Figure 3a.The NH 3 surface concentration was calculated from the integral difference between inlet and outlet concentrations, taking into account that a part of the NH 3 fed to the reactor was converted to nitrogen according to the stoichiometry of the Standard SCR reaction.Looking at the solid line obtained during the NH 3 feed transient, it can be seen that the NO conversion increased up to about 30% for an ammonia surface concentration of 0.15-0.2mmol/g and then remained more or less stable.Interestingly, the NO evolution during the following NH 3 shut-off transient (thin red dashed line) exhibited a different behaviour, with maximum NO conversion close to 40% for an ammonia surface concentration of about 0.23 mmol/g pointing out that different NO x conversions could be associated with the same NH 3 surface concentration during transients, thus resulting in a hysteresis behaviour.
Solid lines in Figures 2-5 (MR thick lines) represent the data fit obtained using the MR kinetics, Equation (4).A satisfactory agreement was achieved between experimental and calculated data in the experiments run in the presence of both 2 and 6% oxygen.It is worth of note that the MR rate expression (Eq.4), relying on the assumption that the SCR reaction is governed by a redox mechanism, could nicely predict the kinetic dependence on oxygen.Furthermore, as more evident from Figures 4 and 5, the MR rate law captured nicely the complex maxima-minima features of the experimental NO and N 2 traces at low T at both NH 3 startup and shutdown.Focusing on Figure 5, it appears that the MR rate expression (thick lines) is able to qualitatively and quantitatively reproduce the observed NO conversion coverage dependence during both NH 3 feed and shut-off transients.In accordance with the experimental data, the model predicts a hysteresis, with a higher NO conversion during the NH 3 shut off transient (red thick dashed line).For comparison purposes, Figure 4 also shows the results of a fit performed adopting an Eley Rideal rate expression (ER thin lines, Eq. 6) to describe the NO consumption.It appears that in this case a satisfactory agreement was achieved between experimental data and model fit at steady state, but not in the case of the transient associated with the ammonia inhibition effect, which indeed is not taken into account in Equation (6).

Transient Experiments Over the Fe-Zeolite Catalyst
In analogy with what done over the V-based catalyst, different sets of isothermal transient experiments were performed at different temperatures over the Fe-zeolite catalyst by feeding in a stepwise manner 500 ppm of NH 3 while continuously flowing 500 ppm of NO in the presence of oxygen (2%), water (3%) and balance helium.
The NH 3 , NO, N 2 and N 2 O (symbols) concentration profiles versus time collected in transient reaction experiments with ammonia pulses of different lengths at 200 and 250°C are shown in Figures 6 and 7, respectively.
In the run at 200°C (NH 3 step feed at 2 700 s in a flow of NO, oxygen and water, Figure 6a), as soon as ammonia was fed to the reactor the outlet concentration trace exhibited about 25 s of dead time and then slowly grew with time on stream, eventually approaching a steady-state value of about 480 ppm.At the same time, the NO signal rapidly went through a minimum before eventually approaching the steady state value of about 480 ppm; the nitrogen signal mirrored that of NO.During both the dynamic phase and the steady state condition the concentrations of NO, NH 3 and N 2 were consistent with the stoichiometry of the Standard SCR reaction (2), indicating a NO x conversion of ~4%.A maximum in the DeNO x activity, with a simultaneous nitrogen peak, were also noticed at NH 3 shut-off, while the NH 3 concentration rapidly dropped to zero.As already pointed out in the case of the V-based catalyst, the temporary enhancements in the DeNO x activity observed over the Fe-zeolite catalyst both at NH 3 opening and shut off are likely associated with an inhibiting effect played by excess NH 3 present on the catalyst surface or in the gas phase [4, 6-8, 26, 27].
In Figures 6b, c, the results collected at 200°C by adding and removing NH 3 from the feed stream every 300 and 150 s, respectively, are reported.Data shows that the transient features observed at NH 3 opening and shut off were still present and comparable with those already discussed in the case of a longer NH 3 feed: a maximum in the DeNO x activity was always reached when NH 3 entered the reactor and also when it was removed from the feed flow.Also, the steady state conversion achieved at the end of each transients remained unaltered.However, the increase of DeNO x activity during the dynamics leads to an increase of the time-averaged NO conversion.-c (symbols) show similar experiments performed at 250°C.The temperature did not affect qualitatively the transient effects in the outlet concentration profiles of NO and nitrogen.However, such dynamics became less pronounced as the temperature increased.In parallel, the temperature positively affected the NO x conversion, which increased from 4% at 200°C to 10% at 250°C.
The similarities between the dynamics observed in the case of vanadium based catalysts and over the present Fe-zeolite system suggest that the NH 3 inhibition invoked for V 2 O 5 -WO 3 /TiO 2 can apply to Fe-promoted zeolites as well.Indeed, it is well known in the literature that the presence of suitable metals (Fe, Cu) in the zeolite framework greatly enhances the SCR activity [30], while it is well established that NH 3 easily adsorbs onto the acidic sites of the zeolite [31,32].However, it has been reported in the literature that NH 3 is able to adsorb on the Fe sites, too [33].We can thus speculate that, in analogy with vanadium based catalysts, the Standard SCR rate-limiting step occurs on Fe-sites involving a redox catalyst function.The adsorption/interaction of NH 3 with these sites may result in blocking of the redox function, thus causing a decrease of the DeNO x activity.Accordingly, the same Modified redox kinetic scheme originally developed for the vanadium based catalyst has been applied to the Fe-zeolite system.
The solid lines in Figure 6 and 7 show the corresponding model fits.The model was indeed able to reproduce qualitatively and quantitatively the different transient effects in the NO and N 2 concentration profiles observed both at NH 3 opening and shutoff over the Fe-zeolite catalyst as well.The effects both of the NH 3 pulse length and of temperature on the system dynamics were nicely captured by the Modified Redox kinetic model.

Model Validation
The derived kinetic model was eventually validated by performing additional runs both over a powdered catalyst at the microreactor scale and over monolith catalysts, and comparing them with the corresponding simulations.The same isothermal Plug Flow reactor model employed for the kinetic fit was used for the simulations of the microreactor experiments, while a fully transient heterogeneous 1D+1D model of SCR monolith reactors [2-4, 13,15] was employed for simulation of the experiments performed over structured systems, accounting also for mass transfer effects in the monolith catalyst.
Figure 8 shows as an example part of a validation run over a small core sample of the monolith Fe-zeolite catalyst: pulses of 750 ppm NH 3 (5 min on/5 min off) were added to a continuous feed stream containing 500 ppm NO, 8% v/v H 2 O, 8% v/v O 2 and balance N 2 at 250°C.A good agreement is apparent between the experimental traces (symbols) and the model predictions (solid lines) of NO and ammonia outlet concentrations.Overall, the very demanding transient features associated with the instantaneous changes in the NH 3 feed concentration were well reproduced by the model.
Transient test bench validation runs were carried out over the same commercial SCR catalysts used for the kinetic study, but in the shape of full scale honeycomb monoliths, feeding real engine exhausts [2-4, 13,15].In particular, it has to be noticed that both over the small core samples and over the full scale monolith catalysts the transient effects related to ammonia injection and shut off, associated with NH 3 inhibition effects, were quite evident.The Modified Redox rate expression developed to take into account such ammonia inhibition effect was indeed able to simulate on a pure predictive basis also the dynamic features observed at the real scale.
In all cases and over both the vanadium based and the Fe-zeolite catalyst, a general good correlation between simulation and experiment was obtained, being the mean deviation in NO x and ammonia concentrations always below 5%, thus validating the model in the explored temperature and concentration ranges and proving the effectiveness of the scale-up procedure.

CONCLUSIONS
Dynamic kinetic data collected over commercial V 2 O 5 -WO 3 /TiO 2 and Fe-zeolite catalysts showed transient effects related to inhibition by excess ammonia.Accordingly, a specific rate expression accounting for NH 3 inhibition of the Standard SCR reaction is needed in mathematical models of real SCR converters.
For this purpose, an original, dual-site Mars-Van Krevelen unsteady kinetic model of the standard SCR reaction over both vanadium based and Fe-zeolite catalysts for the abatement of NO x emissions from the exhausts of Diesel engines was derived, based on large experimental data sets collected under chemical regime over powdered catalysts and validated against runs performed over small and full-scale monolith catalyst samples.The standard SCR kinetic model was able to reproduce successfully not only the steady state behaviours, but also the detailed and complex transient features observed during dynamic runs at low temperatures.
The model herein reported for the standard SCR reaction only is part of a more complex kinetic model, chemically consistent with a redox catalytic mechanism proposed on the basis of an extensive investigation of reactivity, chemistry and catalytic mechanism of the full NH 3 -NO/NO 2 SCR reacting system over both V 2 O 5 -WO 3 /TiO 2 and Fe-zeolite catalysts performed in our laboratories: it fully accounts for stoichiometry, selectivity and kinetics of the global NH 3 -NO/NO 2 SCR process.The complete model was also successfully validated at the real converter scale, taking into account mass and heat transfer effects, and is currently applied to the development of industrial SCR technologies for vehicles.

Figures 7a
Figures7a-c(symbols) show similar experiments performed at 250°C.The temperature did not affect qualitatively the transient effects in the outlet concentration profiles of NO and nitrogen.However, such dynamics became less pronounced as the temperature increased.In parallel, the temperature positively affected the NO x conversion, which increased from 4% at 200°C to 10% at 250°C.The similarities between the dynamics observed in the case of vanadium based catalysts and over the present Fe-zeolite system suggest that the NH 3 inhibition invoked for V 2 O 5 -WO 3 /TiO 2 can apply to Fe-promoted zeolites as well.Indeed, it is well known in the literature that the presence of suitable metals (Fe, Cu) in the zeolite framework greatly enhances the SCR activity[30], while it is well established that NH 3 easily adsorbs onto the acidic sites of the zeolite[31,32].However, it has been reported in the literature that NH 3 is able to adsorb on the Fe sites, too[33].We can thus speculate that, in analogy with vanadium based catalysts, the Standard SCR rate-limiting step occurs on Fe-sites involving a redox catalyst function.The adsorption/interaction of NH 3 with these sites may result in blocking of the redox function, thus causing a decrease of the DeNO x activity.Accordingly, the same Modified redox kinetic scheme originally developed for the vanadium based catalyst has been applied to the Fe-zeolite system.The solid lines in Figure6and 7 show the corresponding model fits.The model was indeed able to reproduce qualitatively and quantitatively the different transient effects in the NO and N 2 concentration profiles observed both at NH 3 opening and shutoff over the Fe-zeolite catalyst as well.The effects both of the NH 3 pulse length and of temperature on the system dynamics were nicely captured by the Modified Redox kinetic model.

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Nova et al. / Kinetic Modeling of Dynamic Aspects of the Standard NH 3 -SCR Reaction Over V 2 O 5 -WO 3 /TiO 2 and Fe-Zeolite Commercial Catalysts for the Aftertreatment of Diesel Engines Exhausts

TABLE 2
List of rate expressions (NH 3 -NO/O 2 reacting system) I Nova et al. / Kinetic Modeling of Dynamic Aspects of the Standard NH 3 -SCR Reaction Over V 2 O 5 -WO 3 /TiO 2 and Fe-Zeolite Commercial Catalysts for the Aftertreatment of Diesel Engines Exhausts 685 catalyst morphological data according to a modified Wakao-Smith random pore model [19].The resulting set of model PDEs was solved numerically according to the method of lines, applying orthogonal collocation techniques to the discretization of the unknown variables along both the z (axial) and x (intraporous) coordinates, and integrating the resulting ODEs in time by Gear's algorithm for stiff systems.Transient Experiments Over the V-based Catalyst The NH 3 , NO and N 2 concentration profiles versus time collected in transient reaction experiments at different temperatures feeding NO and NH 3 in the presence of different oxygen feed contents are shown as symbols in Figures 2 (O 2 = 2% v/v) and 3 (O 2 = 6% v/v).In all cases, 1 000 ppm ammonia were fed to the reactor in a stepwise manner while flowing 1 000 ppm of NO + O 2 + 1% v/v H 2 O.