Dynamic Methods in Catalytic Reaction Engineering: Applications to

10374

Ind. Eng. Chem. Res. 2010, 49, 10374–10385

Dynamic Methods in Catalytic Reaction Engineering: Applications to the Investigation of the NH3 Selective Catalytic Reduction Reactions for Diesel Emission Control Enrico Tronconi,* Isabella Nova, and Massimo Colombo Laboratory of Catalysis and Catalytic Processes, Dipartimento di Energia, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133, Italy

For the case of the NH3 selective catalytic reduction (NH3-SCR) reactions over V-based and metal-exchanged zeolite catalysts, we have used step response and temperature-programmed experiments in a fixed-bed flow microreactor in order to accomplish the following. (i) Identify the adspecies and evaluate their storage capacities: it was found that, in addition to ammonia, NO2 as well is adsorbed in comparable amounts onto SCR catalysts in the form of nitrites and nitrates. (ii) Investigate the individual steps of mechanistic schemes: the key role of surface nitrates in the NO2-related SCR chemistry was demonstrated by dedicated transient experiments. (iii) Discriminate rival mechanistic hypotheses: the differences in the temporal evolutions of NO and NO2 during transient runs unambiguously indicated that they participate in different steps of the fast SCR mechanism. (iv) Analyze the red-ox features of the catalytic mechanism: specifically designed transient tests highlighted the superior efficiency of surface nitrates in reoxidizing the V-related redox sites. (v) Evaluate surface kinetics: chemically consistent transient kinetic models were developed from systematic sets of transient runs and applied to design and optimization of commercial SCR converters for vehicles. The results of the overall SCR investigation prove that transient response techniques can yield a superior level of mechanistic and kinetic information under process relevant conditions with a limited experimental effort. 2NH3 + NO + NO2 f 2N2 + 3H2O

Introduction Transient response techniques, or dynamic methods, consist of imposing a change in a state variable of a reacting system (e.g., temperature, concentration, flow rate) while following its temporal evolution (e.g., by monitoring the outlet concentrations). Abundant kinetic and mechanistic information is in principle available from such dynamic runs; furthermore their use enables the cover of a wide range of operating conditions with only a limited number of experiments and thus affords a reduction of time and costs of the experiments.1 In spite of this clear potential, however, transient methods are still used in catalytic reaction engineering research only to a limited extent. The scope of the present paper is to show how transient reaction analysis can be applied in a variety of different ways in order to reach a detailed, comprehensive, and quantitative understanding of a complex reacting system. We use for this purpose our recent investigation of the NH3-SCR process for the aftertreatment of diesel exhausts. Diesel engines, being inherently more thermodynamically efficient than gasoline engines, offer the prospect of reducing fuel consumption and emissions of carbon dioxide as well. However, they are also responsible for significant polluting emissions of nitrogen oxides (NOx) and particulate matter (PM).2 Concerning NOx reduction, the NH3/urea selective catalytic reduction (SCR) process was the European motor industry’s main technology of choice to meet euro 4 and euro 5 emissions requirements for heavy-duty diesel engines; recently the process was selected for passenger cars by some manufacturers in the US and in Europe, as well.2 Usually, a diesel oxidation catalyst is also present in the system configuration, upstream of the SCR converter, to partially convert NO to NO2; this enables the occurrence of the fast SCR reaction R.1: * To whom correspondence should be addressed. Tel.: +39 02 2399 3264. Fax: +39 02 2399 3318. E-mail: [email protected].

(R.1)

known to be very active already at low temperature,3–5 in comparison to the case when most part of NOx is made of NO alone and the slower Standard SCR reaction R.2 is prevalent 1 2NH3 + 2NO + O2 f 2N2 + 3H2O 2

(R.2)

Since diesel emission limits are expected to be further lowered in the near future, the automotive industry is focusing worldwide on the development of enhanced on-board SCR DeNOx systems, a task which necessarily calls for improved understanding of the SCR catalytic mechanisms and of the related kinetic aspects. The operating conditions of exhaust aftertreatment devices are intrinsically transient, involving continuous changes in temperature and flow rate over extended ranges: a kinetic description of the process rates is therefore much more demanding than in stationary SCR applications and naturally calls for transient reaction analysis.6–9 Reviewing our recent investigation of the NH3-SCR reactions over V-based10–20 and metal-exchanged zeolite catalysts21–25 as an example, it is the purpose of the present paper to demonstrate however that dynamic methods are powerful tools not only to study transient catalytic kinetics,1,10,11,13,14,16–19,21,25 but also to identify and demonstrate mechanistic features of catalytic reactions under process relevant conditions.1,12,13,15,17,19,20,22–24 Experimental and Modeling Methods Experimental Setup. For our study of the NH3-SCR reactivity we used an experimental setup specifically designed with respect to transient kinetic studies.15,20 Dynamic NH3-SCR reaction runs were performed at T ) 150-550 °C over state-of-the-art V2O5-WO3/TiO2 and metalexchanged (Fe and Cu) zeolite commercial catalysts loaded in

10.1021/ie100507z  2010 American Chemical Society Published on Web 06/02/2010

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010

a quartz tubular microflow reactor (i.d. ) 7 mm) in the form of powders. The powdered catalyst was obtained by crushing and sieving the original honeycomb monoliths (bulk, for the V-based system; washcoated, for the zeolite-based systems) to a size (dp ≈ 90 µm) suitable to rule out mass transfer limitations. The catalyst load was minimized (Wc ) 60-100 mg) in order to afford operation of fast transients under isothermal conditions. Typical feed concentrations of NOx and NH3 were 500-1000 ppm, along with 2-10% O2 and 1-10% H2O v/v, Ar as a tracer and balance He. To secure partial conversions at all temperatures, the gas hourly space velocities (GHSV) were set to high values, ranging from 45 000 up to 1 300 000 cm3/(h gcat.) (STP). Note that gcat refers to the mass of active phase in the case of washcoated zeolite-based catalysts. Two four-port fast valves were used to perform stepwise switches between different gaseous feedstreams: this assures that the overall flow rate remains constant as an inert flow entering the reactor is replaced by an equal flow containing the reactants. Furthermore, a backpressure regulator was positioned at the reactor outlet to minimize pressure changes upon switching of the feed gases. Care was taken also to avoid all possible dead volumes in the lines before and after the reactor. The overall dead time measured upon stepwise injection of an inert tracer (Ar) was in the order of 2 s, which is negligible with respect to the characteristic times of the measured responses, typically in the order of minutes and governed by ammonia adsorption-desorption reaction processes. The microreactor was inserted into an electric furnace whose temperature was controlled by a K-type thermocouple directly immersed in the catalyst bed. Under the adopted experimental conditions, associated with diluted feed streams, the investigated SCR reactive processes were accompanied by no significant thermal effects, that is, they occurred under essentially isothermal conditions. Continuous analysis of outlet gases was performed by a MS (Balzer QMS 200) for NH3, NO, NO2, N2, and N2O, thus permitting the evaluation of N-balances. The mass-spectrometer data were quantitatively analyzed using the fragmentation patterns and the response factors determined experimentally from calibration gases. Relevant interferences in the mass-tocharge signals were taken into account in determining the products composition. To improve the time resolution for the continuous analysis of reactants and products, a UV analyzer, (ABB Limas11 HW) able to detect simultaneously NH3, NO, and NO2 was coupled with the MS in a parallel arrangement.26 Further details on the experimental rig can be found elsewhere.15,20 Experimental Procedures. Before each test the catalyst sample was conditioned for 3 h at 600 °C in a stream containing oxygen (2% v/v) and water (10% v/v). Different kinds of transient experiments are presented in this work, including isothermal step concentration (ISC) runs, temperature programmed surface reaction (TPSR) runs, and Temperature programmed reaction (TPR) runs. In a typical ISC run, the reactor was kept at constant temperature under a flow of He + 1% H2O, and step changes of feed NH3 or NO or NO2 concentrations were imposed. At the end, a temperature programmed desorption ramp (15-20 °C/min, Tend ) 550 °C) in an inert atmosphere was typically performed in order to clean up the catalyst surface. TPSR runs included a preliminary phase in which an SCR reactant, for example, NO2, was fed and adsorbed onto the catalyst at constant temperature; subsequently a temperature programmed ramp (20 °C/min) was performed in the presence

10375

of another reactant, for example, gaseous NH3, in the feed stream. These runs were performed, for example, to study the reactivity of NH3 with the surface species resulting from NO2 adsorption. Finally, in TPR runs feed mixtures containing different reactants were continuously fed to the reactor while the temperature was linearly increased from room temperature up to 500 at 10 °C/min. Further details on the experimental procedures can be found elsewhere.23,24 Microreactor Model for Kinetic Analysis. Altogether, the experimental configuration afforded to collect intrinsic kinetic information under isothermal conditions in a chemical regime during fast reaction transients. The kinetic analysis of the experimental runs relied on a simple isothermal one-dimensional dynamic PFR model based upon the following unsteady material balance equations for gaseous (i) and adsorbed (j) species: (1) gaseous phase: ∂Ci ∂Ci ε ) -V + (1 - ε)Ri ∂t ∂z

(1)

(2) adsorbed phase: Ωj

∂θj ) Rj ∂t

(2)

where Ci is the gas-phase concentration of species i, θj is the surface coverage of species j, ε is the void fraction of the catalyst bed, V is the gas linear velocity [m/s] and Ωj is the catalyst adsorption capacity of species j [mol/(m3 cat)]. R is the intrinsic rate of formation of species i or j [mol/(m3cat/s)], computed according to the following general expression: NR

Ri )

∑r ν

k i,k

(3)

k)1

where i is the species index, rk is the intrinsic rate of reaction k and νi,k is the stoichiometric coefficient of species i in reaction k. The system of partial differential equations (PDE) formed by the unsteady mass-balance equations was integrated numerically according to the method of lines. The discretization of the variables along the spatial coordinate z was based on robust first order backward finite differences. To integrate the resulting set of ordinary differential equations system in time a library solver, based on Gear’s method for stiff ODEs, was used. The adaptive kinetic parameters appearing in the rate expressions of rk were estimated by global multiresponse nonlinear regression based on the least-squares method, using the temporal evolutions of the five relevant species concentrations (NH3, NO, NO2, N2, N2O) as experimental responses. For this purpose the BURENL routine, developed by Professor Guido BuzziFerraris,27 was adopted. Results and Discussion Storage of Adsorbed Species. It is well established that ammonia adsorption-desorption plays a key role in controlling SCR dynamics. NH3 step feed runs followed by TPD have been traditionally used to estimate ammonia storage capacities and NH3 adsorption-desorption kinetics.8,9,14,21,28 For three of such runs performed over the V2O5-WO3/TiO2, the Fe- and the Cu-zeolite commercial catalysts are herein investigated, respectively, Figure 1 shows the evolution of the NH3 outlet concentration (solid lines) monitored upon opening

10376


Figure 1. Adsorption-desorption of NH3 (NH3 step addition at t ) 0 s, shut-off at t ) 2000 s and thermal desorption) over V2O5-WO3/TiO2, Fe-zeolite and Cu-zeolite catalysts. Tads ) 200 °C, NH3)1000 ppm, O2 ) 0%, H2O ) 1% (V2O5-WO3/TiO2), 3% (Cu- and Fe-zeolites), T-ramp ) 15 K/min.

and shutting down the NH3 feed (dotted line) at T ) 200 °C, followed by TPD. In all cases, upon the step addition to the reactor (at t ) 0 s) of 1000 ppm of ammonia, the ammonia outlet concentrations show a dead time, that is, a period during which the NH3 is completely adsorbed onto the catalyst surface, and then they increase with time, approaching slowly the feed value of 1000 ppm. The area included between the ammonia inlet and outlet concentration traces is proportional to the amount of NH3 adsorbed onto the catalyst, so the transient experiments provide direct estimates of the NH3 storage capacities associated with the different catalysts. In the case of the V-based system the catalyst was able to adsorb up to 0.15 mmol/gcat at 200 °C; the capacity increased to 0.62 mmol/gcat for the Fe-zeolite catalyst, and to 1.31 mmol/gcat for the Cu-zeolite, with gcat referring to the mass of active phase in the case of the washcoated zeolite-based systems. Upon NH3 shut-off (t ) 2000 s) the reactor outlet NH3 concentrations dropped due to the desorption of the stored NH3. However, complete desorption of NH3 was achieved only after performing a TPD (temperature programmed desorption) experiment, linearly increasing the catalyst temperature at 15 °C/min up to 550 °C. As soon as the heating started, the NH3 signals increased again, but different behaviors were evident for the three considered catalysts. In the case of the V-based and of Fe-zeolite catalysts the ammonia traces increased above 250 °C, showed a maximum close to 330-350 °C and then decreased, so that the desorption of ammonia was completed already at 500 °C. In the case of the Cu-zeolite catalyst the

ammonia desorption peak was delayed to higher temperatures (around 450 °C) and ammonia was still desorbing at 550 °C. This indicates the presence of NH3 adsorption sites on the Cu-zeolite which are stronger than those associated with the V-based and Fe-zeolite catalysts. Of course, the data in Figure 1 also provide useful kinetic information about the ammonia adsorption-desorption processes.29,30 NO adsorption experiments were carried out at different temperatures (T ) 50-200 °C), too, both in the presence and in the absence of oxygen and water. In contrast to ammonia, it was found that the storage capacity of NO was negligible for all the considered catalysts, even if it is not possible to rule out a weak NO adsorption onto the catalyst surfaces. The same experimental approach was finally applied to study the adsorption of the third SCR reactant, that is, NO2. Figure 2 compares the evolutions of NO2 (dashed lines) and NO outlet concentrations (solid lines) monitored over the V2O5-WO3/ TiO2, the Fe-zeolite, and the Cu-zeolite commercial catalyst, respectively, upon opening and shutting down the NO2 feed (dotted line) at T ) 50 °C, followed by TPD runs. During the NO2 adsorption phase, an evolution of NO was observed immediately after the step addition of 1000 ppm of NO2 (t ) 0 s) in all cases, while the NO2 outlet concentration traces exhibited a delay before slowly approaching the NO2 feed level. The concentration profiles of NO and NO2, even if different from a quantitative point of view, are similar for the three considered catalysts, and have been explained considering


10377

Figure 2. Adsorption-desorption of NO2 (NO2 step addition at t ) 0 s and thermal desorption) over V2O5-WO3/TiO2, Fe-zeolite and Cu-zeolite catalysts. Tads ) 50 °C, NO2 ) 1000 ppm, O2 ) 0%, H2O ) 1% (V2O5-WO3/TiO2), 3%(Cu- and Fe-zeolites), T-ramp ) 20 K/min.

that NO2 disproportionates20,31 according to the following overall stoichiometry: 3NO2 + H2O f 2HNO3 + NO

(R.3)

where HNO3 is representative of surface nitrates.3,24,32–34 Indeed reaction R.3 likely results from the sum of the consecutive reactions R.4 and R.5: 2NO2 + H2O T HONO + HNO3

(R.4)

HONO + NO2 T HNO3 + NO

(R.5)

Figure 2 shows in fact that during the dynamic experiments over the three catalysts the molar ratio between evolved NO and converted NO2 was indeed close to 1/3, and therefore in line with the overall stoichiometry of reaction R.3. Quantitative analysis of the transient data also provides the estimates of nitrates storage capacities for the three catalysts, which are remarkable, being in the same orders of magnitude as the corresponding ammonia storage capacities. Formation and storage of nitrates via NO2 disproportionation is confirmed by the subsequent TPD runs after catalyst saturation, also reported in Figure 2, which show that nitrates decomposition leads to evolution of mainly NO2, along with minor amounts of NO (together with oxygen, not reported in the figures).20 In the case of the Fe- and Cu-zeolite, two NO2 desorption peaks were observed, the low-T one (centered around 100 °C) being likely associated with the decomposition of nitrates/nitric acid in the liquid H2O on the catalyst surface,

whereas the high-T peak originated from decomposition of the nitrates strongly adsorbed onto the catalyst sites. The TPD data in Figure 2 further indicate that the strongly bonded nitrates formed over the three catalysts are characterized by different thermal stabilities: indeed, over the Cu-zeolite the NO2 peak temperature (390 °C) was much higher than observed in the case of both the V-based (330 °C) and the Fe-zeolite (320 °C) catalysts. This obviously suggests that nitrates formed by NO2 disproportionation on the Cu catalyst are significantly more stable than those formed over either the V-based catalyst or the Fe containing zeolite. In view of the important role of nitrates as surface intermediates in the fast SCR chemistry, this result may be relevant for explaining the activity differences of the three catalysts in the NO2-related reactions. Thus, transient experiments not only provide quantitative information about the amounts of stored species, but can also point out differences in the properties of the adsorption sites. Traditionally, this is pursued by spectroscopic characterization techniques (FTIR, laser Raman), which however only seldom can provide quantitative evaluations, and may often require operating conditions not fully representative of real operation. As opposite, transient experiments can supply such lacking pieces of information: on the other hand, of course, neither evidence about the nature of the sites nor direct identification of the species formed upon adsorption is possibly obtained. Analysis of Mechanistic Steps. A series of dedicated transient low-T reaction experiments were used to study the mechanism of the fast SCR reaction (R.1), addressing systemati-

10378


Figure 3. Comparison between activity of fast SCR and activity of NO + NH4NO3 reactions over Fe-ZSM5. Feed ) 1% H2O, 0% O2, NH3 ) 0-1000 ppm, NO ) 0-500 ppm, NO2 ) 0-500 ppm + He. T ) 170 °C (adapted from ref 24).

cally the individual and overall steps of the NH3/NO/NO2 reactivity over the three SCR commercial catalysts.18,22,24,25 A very detailed investigation of the fast SCR reaction over V2O5/WO3/TiO2 catalysts was first reported a few years ago by Koebel and co-workers.5,35,36 Analyzing the reactivity of NH3/ NO/NO2 over V-based catalysts at 150 °C, they showed that the addition of NO2 to NO/NH3 feed mixtures results in the occurrence of the well-known fast SCR reaction R.1 and of two additional side reactions, namely the formation of NH4NO3 from NH3 and NO2: 4NH3 + 4NO2 f 2NH4NO3 + 2N2 + 2H2O

(R.6)

and further the decomposition of NH4NO3 by NO, 2NH4NO3 + NO f 3NO2 + 2NH3 + H2O

(R.7)

They also showed that under specific low-T conditions ammonium nitrate salt builds up onto the catalysts, while at higher temperature it is in equilibrium with gaseous nitric acid and ammonia according to the following reaction: NH3 + HNO3 S NH4NO3

(R.8)

However, reactions R.6 and R.8 were regarded as parallel reactions, not associated with the primary fast SCR reaction R.1. To address specifically the potential role of ammonium nitrate and/or of surface nitrates in the catalytic mechanism of the fast SCR reaction, dedicated transient experiments were performed in our laboratories over both the V-based and the Fe-zeolite catalysts. Figure 3 shows a typical run performed over the Fe-zeolite at 170 °C: at the beginning 1000 ppm of NH3 were fed to the reactor, then a mixture of 500 ppm of NO and 500 ppm of NO2 were added to the feed stream, and finally NO2 only was removed from the feed stream, leaving a flow consisting of 1000 ppm of NH3 and 500 ppm of NO. We observe that at t ) 0 s, when both NO and NO2 were added to NH3 in the feed stream, a reaction transient became apparent associated with conversion of both NH3 and NOx, along with formation of N2 and of a few ppm of N2O. At steady state, the concentrations of reactants and products were in agreement with the simultaneous occurrence of the fast SCR reaction R.1, and the reaction of NH4NO3 formation R.6. Accordingly during this phase, of the low temperature experiment ammonium nitrate was being formed and accumulated onto the catalyst, but also

Figure 4. (A) Reactivity of NH3-NO-NO2-O2 as a function of temperature over Fe-zeolite: NH3 ) 1000 ppm, NO ) 500 ppm, NO2 ) 500 ppm, H2O ) 1%, O2 ) 0%. (B) NH3 + NO reactivity with prestored nitrates over Fe-zeolite: NH3:NO ) 1000 ppm; H2O ) 1%, O2 ) 0%; catalyst pretreated with 1000 ppm NO2; H2O ) 1%, O2 ) 0% at 60 °C; T-ramp ) 20 K/min (adapted from ref 24).

partially dissociated to nitric acid and ammonia; indeed, formation of NH4NO3 is confirmed by a lack in the atomic nitrogen balance at steady state and is also compatible with the observed N2 concentrations. Also, NH4NO3 accumulation is consistent with the performance degradation observed during the initial part of the transient (0 to ∼3000 s), while in the following part (3000-10000 s) equilibrium between formation, deposition, and dissociation of ammonium nitrate was likely established, resulting in the observed pseudo-steady-state behavior. At the same time the fast SCR reaction was also proceeding to an extent directly reflected by the NO conversion level. In the second and final part of the experiment (at t ) 10000 s) NO2 was removed from the feed thus stopping both the nitrate formation (reaction R.6) and the fast SCR reaction (reaction R.1). Nevertheless NH3 and NO continued to be converted (and nitrogen produced) for a while. This indicates the occurrence of reaction R.7, in which ammonium nitrate deposited on the catalyst is reduced by NO. In fact, the eventual depletion of NH4NO3 is responsible for the subsequent stop of the NO conversion observed after roughly 4000 s. It is quite important to notice that the NO conversion level remained practically unaltered before and after the NO2 removal from the feed flow: this proves that the fast SCR reaction (occurring when NO/NO2 and ammonia were all flowing into the reactor) and the reaction between NO and NH4NO3 (occurring when NO2 had been removed from the feed flow) progressed at the same rate, which clearly indicates a consecutive scheme where the fast SCR, reaction R.1, results from the sum of reactions R.6 and R.7, reaction R.7 acting as the rate determining step at the investigated low temperatures. Thus, the results of this dynamic experiment effectively point out that ammonium nitrate, or surface nitrates in equilibrium with ammonium nitrate via reaction R.8, are not terminal species, but rather behave as intermediates of the fast SCR reaction. To further analyze the role of nitrates in the fast SCR reactivity new transient experiments were performed over the Fe-zeolite catalyst. Figure 4 shows the temperature dependence of NO conversion measured during NO + NH3 TPR experiments with equal space velocities and heating rates (20 K/min). In the first one (curve A) the feed included NH3 (1000 ppm) + NO/NO2 (500 ppm each) + H2O (1% v/v) O2 (2% v/v);


10379

accordingly, curve A is representative of the T-dependence of NO conversion in the fast SCR reaction R.1. Figure 4 also shows curve B, obtained running an equal T-ramp feeding NO + NH3 (1000 ppm each) + H2O (1% v/v) only over a catalyst sample that had been pre-exposed to 1000 ppm of NO2 + 1% H2O at 60 °C; this pretreatment allowed formation and storage of surface nitrates onto the catalyst surface, as shown by the experiments displayed in Figure 2. Curve B in Figure 4 is essentially overlapped to curve A up to about 220 °C; but at higher temperatures curve B drops sharply, approaching zero NO conversion. This indicates that nitrates stored onto the catalyst effectively participate in the reaction until their complete depletion. The superposition of curves A and B in the first part of the transient experiments therefore indicates that in both runs gaseous NO and ammonia were reacting with surface nitrates, either formed directly via NO2 disproportion (curve A) or formed previously and stored on the catalyst during its pretreatment with NO2 (curve B). Considering the bulk of information gained by the analysis of ammonia and NO2 adsorption and by the transient reactive experiments above presented for the Fe-zeolite, a reaction mechanism was proposed for the fast SCR reaction, wherein (i) NO2 is responsible for the formation of surface nitrate and nitrite adspecies, reaction R.4; (ii) NO acts as a reductant toward nitrates, converting them to nitrites, HNO3 + NO S HONO + NO2

(R.9)

(iii) NH3 eventually reacts with nitrites to give harmless nitrogen and H2O via decomposition of the unstable ammonium nitrite, HONO + NH3 f N2 + 2 H2O

(R.10)

Very similar results were also obtained over the vanadium and the Cu-zeolite catalysts,18,25 confirming the mechanism proposed for the fast SCR reaction, and in particular the central role of surface nitrates and nitrites. In the following we discuss how we have used transient dynamic experiments to further scrutinize the individual steps (in particular reactions R.9 and R.10) of the overall fast SCR reaction mechanism. We analyzed first the reactivity of surface nitrites. According to the reaction mechanism above presented, nitrites are initially formed by NO2 disproportionation simultaneously with nitrates (reaction R.4); then, they can be further oxidized to more nitrates by gaseous NO2 (reaction R.5), but in the presence of ammonia they can also lead to nitrogen via decomposition of unstable ammonium nitrite (reaction R.10). To clarify the importance of the latter step, we used again transient ISC experiments performed at different temperatures, as shown in Figure 5 for the Cu-zeolite catalyst: 1000 ppm of NO2 were fed at t ) 0 s to a catalyst presaturated with ammonia in the range 50-150 °C. At 50 °C NO2 was immediately consumed with simultaneous production of NO. Quantitative analysis of the outlet concentration profiles indicates the occurrence of reaction R.3, wherein NO2 disproportionates eventually leading to formation of surface nitrates and evolution of NO, as indeed already observed in the NO2 adsorption experiments (Figure 2). Thus, at such a low temperature the disproportionation of NO2 was essentially unaffected by the presence of adsorbed ammonia. Conversely, at temperatures above 80 °C a different behavior became evident: upon NO2 addition to the reactor feed (t ) 0 s), evolution of N2 together with NO was observed, and on further increasing the temperature the nitrogen evolution

Figure 5. NO2 reactivity with preadsorbed ammonia over Cu-zeolite (NO2 step addition at t ) 0 s): T ) 50, 80, 100, 120, 150 °C, NO2 ) 1000 ppm, H2O ) 3%, O2 ) 0%. Catalyst pretreated with 1000 ppm NH3 at the same temperature.

increased, while that of NO decreased. The formation of nitrogen instead of NO, observed when NO2 was fed to the reactor in the presence of preadsorbed ammonia, is explained by the occurrence of reaction R.10, wherein nitrites react with preadsorbed ammonia to form nitrogen via formation/decomposition of unstable ammonium nitrite, rather than being further oxidized by gaseous NO2 according to reaction R.5. Thus, in all the experiments shown in Figures 2 and 5, NO2 fed to the reactor disproportionates to form nitrates and nitrites onto the catalyst (reaction R.4). Then, while in the absence of ammonia, nitrites are oxidized to nitrates by NO2 (reaction R.5). In the presence of adsorbed ammonia and at sufficiently high temperature nitrites react with ammonia to form ammonium nitrite which eventually decomposes to N2 (reaction R.10). So the dynamic experiments displayed in Figure 5 clearly indicate that under specific conditions the acid-base reaction of ammonia with nitrites is faster than the competing path of nitrites oxidation by NO2 and leads very selectively to dinitrogen already at low temperature.37,38 Dynamic experiments were also applied to investigate reaction R.9, that was proven to be the rate determining step of the fast SCR reaction. Figure 6A shows the reactivity of gaseous NO at T ) 50 °C over the Fe-zeolite catalyst with surface nitrate species, originally preadsorbed on the catalyst according to the NO2 disproportionation mechanism (reaction R.3) by feeding 1000 ppm of NO2 (at t ) 0 s) at the same temperature. In the second part of the run, after NO2 removal from the feed stream, NO was fed at t ) 4500 s to the catalyst saturated with nitrates. In spite of the low temperature (50 °C) a strong transient activity was observed: NO was initially converted while a peak of NO2 (800 ppm) was simultaneously evident. This behavior is explained by reaction R.11, that is, the reverse of R.3,

10380


2HNO3 + NO T 3NO2 + H2O

(R.11)

Accordingly, this experiment clearly proves that the activity of NO in reducing surface nitrates is very strong already at low temperature (50 °C). Apparently, however, such a strong NO reactivity with nitrates at 50 °C is not consistent with reaction R.9 being the rate determining step of the fast SCR mechanism, that was found to be active only at temperatures greater than ca. 150-160 °C over this Fe-zeolite catalyst.22 However, it should be noticed that ammonia is present in the fast SCR reacting system, while NO oxidation by surface nitrates at 50 °C (Figure 6A) was performed without ammonia in the feed stream. Accordingly, a new dynamic run was performed: the experiment (shown in Figure 6B) was identical to that reported in Figure 6A, but for the fact that the reactivity of NO toward adsorbed nitrates was herein studied in the presence of gaseous ammonia. Figure 6B shows that NO2 addition over a clean catalyst (at t ) 0 s) resulted, as expected, in an immediate NO evolution in line with the 1:3 stoichiometry expected from reaction R.3. Then, at t ) 4200 s, 1000 ppm of ammonia were added to the feed flow and no reactions were observed. Finally, at t ) 6100 s, NO was added to the reactor feed: its outlet concentration quickly recovered the feed value, indicating no reaction as well. So, while in the run of Figure 6A NO was able to reduce the nitrates on the catalyst, in the run shown in Figure 6B, where ammonia was present, NO did not react with nitrate ad-species. This is a clear indication that ammonia blocks the reaction between NO and nitrates at low temperature over the Fe-zeolite.22 Only after increasing the temperature to 140-160 °C (results not reported) ammonia and NO started to convert nitrates with production of nitrogen in line with the expected stoichiometries. Dynamic experiments presented in Figures 6 panels A and B provide evidence that nitrates are able to oxidize NO according to reaction R.11 already at 50 °C (Figure 6A); however, in the presence of NH3 and under identical experimental conditions, the same reaction R.11 is blocked by ammonia: such a blocking effect represents an intrinsic lower bound to the fast SCR activity at low temperature, and has been associated with a strong interaction between ammonia and surface nitrates.22 Interestingly, the present transient data thus suggest that the blocking effect of NH3 on the fast SCR reaction

Figure 6. (A) NO2 adsorption (NO2 step addition at t ) 0 s and shut-off at t ) 3000 s) followed by NO addition (at t ) 4500 s) over Fe-zeolite. T ) 50 °C, NO2 ) NO ) 1000 ppm; H2O ) 1%, O2 ) 0%. (B) NO2 adsorption (NO2 step addition at t ) 0 s and shut-off at t ) 3150 s) followed by NH3 addition (t ) 4200 s), by NO addition (at t ) 6150 s) at T ) 50 °C. NO2) NH3 ) NO ) 1000 ppm; H2O ) 1%, O2 ) 0% (adapted from ref 22).

Figure 7. (A) NO + NO2 addition (at t ) 0 s) and NH3 shut off (at t ) 6500 s) over V2O5-WO3/TiO2 catalyst. T ) 200 °C, NH3 ) 700 ppm, NO ) NO2 ) 500 ppm, H2O ) 1%, O2 ) 2% (adapted from ref 26). (B) NH3 pulses (350 s on, 350 s off) over V2O5-WO3/TiO2 catalyst. T ) 200 °C, NH3 ) 0-700 ppm, NO ) NO2 ) 500 ppm, H2O ) 1%, O2 ) 2%.

occurs not because of the ammonia competitive chemisorption on the catalytic sites, but because ammonia captures a key intermediate in an unreactive form. Discrimination of Alternative Mechanistic Proposals. The mechanistic investigation of the fast SCR reaction over zeolitebased catalysts was pioneered by Sachtler and co-workers, mainly by IR techniques:39,40 the main conclusion of their work was that nitrogen production in the SCR reacting system originates from a fast decomposition of ammonium nitrite, which is unstable above 100 °C. On the other hand, in the case of equimolar NO/NO2 feeds, formation of ammonium nitrite would occur via gas-phase formation of N2O3 and its subsequent reaction with water and ammonia; that is, NO + NO2 f N2O3

(R.12)

N2O3 + H2O + 2NH3 f 2NH4NO2 f 2N2 + 4H2O (R.13) Indeed the combination of reactions R.12 and R.13 results in the fast SCR reaction R.1. Furthermore, step R.13 is essentially the same as R.10. Even if this reaction scheme nicely explains the optimal 1/1 NO/NO2 feed ratio of the fast SCR reaction on the basis of well-known chemistry, it cannot explain all of the several products (N2, NH4NO3, N2O) observed in SCR reactivity experiments covering the full range of NO/NOx feed contents. Moreover, no evidence was provided concerning the compatibility of this scheme with the fast SCR kinetics. To challenge the alternative mechanistic proposals represented by reactions (R.4-5), (R.9-10), and by reactions (R.12-13), respectively, we performed transient experiments over the V-based SCR catalyst. In Figure 7A, at time ) 0 s 500 ppm of NO and 500 ppm of NO2 were instantaneously added to a feed stream containing 700 ppm of NH3, 2% O2, 1% H2O, and He (balance) at 200 °C. When NOx was admitted to the reactor the onset of a reaction was observed resulting in the instantaneous formation of nitrogen and in the almost total conversion of ammonia. The concentration levels of reactants and product at steady state in Figure 7A are consistent with the stoichiometry of the fast SCR reaction R.1 and indicate a NOx conversion of about 70%. It is worth emphasizing however the very different dynamic evolutions of the NO and NO2 concentration traces observed


after NOx admission (t ) 0 s) and removal (t ) 6500 s) from the reactor. The uncoupled transient trends of NO and NO2 clearly indicate that the two reactants play different roles in the fast SCR mechanism, otherwise their signals would be overlapped. In particular the initial peak of NO indicates that, at NOx step addition, NO2 is the first species to react with NH3. Consumption of NO is delayed by a few seconds, as evidenced by its drop after the initial peak. The differences in the dynamic evolutions of NO and NO2 are even more evident in the dual experiment presented in Figure 7B, where ammonia pulses (700 ppm) were repeated every 350 s in a flow of NO and NO2 (500 ppm each), 2% O2, 1% H2O, and He (balance) at 200 °C. Every time ammonia was added to the feed flow, the fast SCR reaction started to occur with consumption of NOx. But it clearly appears again that very different dynamic behaviors were associated with the concentration traces of NO and NO2. This is indeed consistent with the mechanistic proposal in which NO2 first reacts forming nitrates species (reaction R.3), which are consecutively reduced by NO to nitrites (reaction R.9). In the presence of ammonia, this leads to ammonium nitrites, which eventually decompose to gaseous nitrogen (reaction R.10). On the other hand, the clear and straightforward indication of the different roles of NO and NO2 in the mechanism of the fast SCR reaction provided by the transient experiments rules out the possibility for the fast SCR to proceed according to reactions 12 and 13, which would call for a direct reaction between equimolar amounts of NO and NO2 as the a first step of the fast SCR mechanism. Redox Features of the SCR Mechanism. Dynamic methods were further applied in an attempt to elucidate the role of the catalyst in the overall SCR chemistry over V2O5-WO3/TiO2. Because of its widespread application in the abatement of NOx emissions from stationary sources, the mechanism of the standard SCR reaction R.2 has been extensively investigated in the literature in the past two decades: it is generally agreed that the reaction proceeds according to a redox scheme, whose rate determining step is the reoxidation of V-sites by gaseous oxygen.41–44 The redox cycle begins with the catalyst reduction, reported by many authors as an NH3 activation process (i.e., oxidation) on vanadium sites.45–47 To close the cycle, reoxidation of the V-sites by gaseous oxygen is invoked. Concerning the fast SCR reaction R.1, the key-role of NO2 in promoting the catalyst reoxidation was addressed by Koebel and co-workers:36 on the basis of in situ Raman experiments they proposed that over V2O5/TiO2 catalysts the observed higher rates of the fast SCR R.1 reaction at low T in comparison to those of the standard SCR reaction R.2 were due to gaseous NO2 replacing oxygen as a more effective oxidizing agent, thus allowing a faster reoxidation of the vanadium sites. To analyze the redox features of the SCR reactions, dedicated transient experiments were run in our laboratories over the V2O5-WO3/TiO2 catalyst, and for comparison purposes also over a V-free WO3/TiO2 catalyst. It was found that, contrary to the NH4NO3 formation (reaction R.6), the rate limiting step (reaction R.9) as well as the global fast SCR reaction (reaction R.1) did not proceed over a V-free WO3/TiO2 catalyst, thus demonstrating the catalytic role of vanadium redox sites in the fast SCR mechanism.20 To identify the species responsible for catalyst reduction and reoxidation during the standard and fast SCR reactions, dedicated dynamic runs were performed to analyze the activity of different potential reducing or oxidizing agents participating in the SCR reacting system.

10381

Figure 8. (A) NO concentration profiles during NO + NH3 TPR over catalysts after different pretreatments: (curve A) NO (1000 ppm) at 200 °C; (curve B) NH3 (1000 ppm) at 200 °C; (curve C) reference oxidized sample; (curve D) reference reduced catalyst samples; (curve E) NO (1000 ppm) + NH3 (1000 ppm) at 200 °C. (B) NO concentration profiles during NO + NH3 TPR over catalysts after different pretreatments: (curve A) O2 (500 ppm) at T ) 150 °C; (curve B) NO2 (100 ppm) at 150 °C; (curve C) HNO3 (∼100 ppm) at 150 °C; (curve D) reference oxidized sample; (curve E) reference reduced catalyst samples. TPR conditions: feed ) 1000 ppm NO + 1000 ppm NH3 in He. Heating rate ) 20 °C/min. (adapted from ref 20).

For the catalyst reduction, NO, NH3, H2O, and combinations thereof, were considered. Figure 8A compares the NO reduction activity in TPR runs (feed stream of NH3 (1000 ppm) + NO (1000 ppm) with H2O (1% v/v), balance He and no oxygen and T-ramp from 50 up to 250 °C) performed over the catalyst samples after various pretreatments run over a catalyst originally in a reference oxidized state (exposed to O2 during a T-ramp up to 550 °C). Specifically, such reducing pretreatments involved feeding NO (1000 ppm) at 200 °C (curve A), NH3 (1000 ppm) at 200 °C (curve B), NO + NH3 (1000 + 1000 ppm) at 200 °C (curve E). In addition, NO reduction curves obtained over the reference oxidized (exposed to O2 during a T-ramp up to 550 °C, curve C) and over the reference reduced (exposed to NH3 at 550 °C, curve D) catalyst samples are also shown for comparison purposes. It clearly appears that exposure of the oxidized catalyst to atmospheres containing either NO or NH3 alone at 200 °C did not reduce the catalyst. On the other hand, the exposure of the catalyst to a mixture of both NO and NH3 at 200 °C resulted in a deNOx activity quite similar to the one observed over the

10382


reference reduced catalyst, indicating that NO + NH3 jointly were able to reduce the V-catalyst already at 200 °C. Many authors have reported that NH3 activation (i.e., oxidation) is the first step of the standard SCR catalytic mechanism on V-based catalysts.45–47 However our dynamic data do not support that at 200 °C ammonia alone is directly activated by deeply reducing the V-catalyst; rather, a cooperative action of both NO and NH3 is envisaged as the key step in the catalyst reduction stage of the redox cycle. In the second part of the work, dynamic experiments were performed to rank the activity of different oxidizers, namely O2, NO2, and HNO3, in the reoxidation step of the SCR mechanism. Figure 8B compares the DeNOx activity observed in NO + NH3 TPR runs over three catalyst samples: each sample had been first reduced according to a reference procedure (exposure to NH3 at 550 °C), then pretreated at 150 °C with a different oxidizing agent, namely O2 (curve A), NO2 (curve B), and HNO3 (curve C), and finally tested in a NO + NH3 TPR run (feed stream of NH3 (1000 ppm) + NO (1000 ppm) with H2O (1% v/v), balance He, and no oxygen and T-ramp from 50 up to 250-550 °C). Data collected in the identical TPR experiment over the reference oxidized (exposed to O2 during a T-ramp up to 550 °C, curve D) and the reference reduced (exposed to NH3 at 550 °C, curve E) catalyst samples are displayed for comparison, too. It appears that the NO evolution recorded over the catalyst sample pretreated in oxygen flow at 150 °C (curve A) is essentially overlapped with the curve obtained over the reference reduced catalyst (curve E): hence, oxygen was apparently unable to reoxidize the prereduced V-catalyst at 150 °C. On the contrary, the NO evolution recorded after exposing the reduced catalyst both to NO2 (curve B) and to HNO3 (curve C) at 150 °C show that NO conversion was initiated already at about 130 °C, a threshold temperature very similar to that typical of the reference oxidized catalyst (curve D): these data then suggest that both NO2 and HNO3 were able to reoxidize effectively the V-catalyst already at 150 °C. Most importantly, it is also noted that above 170 °C the NO conversion curve of the catalyst samples pretreated with NO2 and HNO3 (curves B and C, respectively) exhibited a marked increment of DeNOx activity, followed by a sudden drop of the NO conversion above 230 °C. Notice that such a temperature threshold corresponds to the onset of nitrates decomposition (see Figure 2). This indicates a strong promoting action of the adsorbed nitrates on the NO + NH3 SCR reactivity at low temperature. The surface nitrates would then be decomposed and eventually depleted, which explains the subsequent drop of the NO conversion. These data, coupled with the knowledge of the role of nitrates in the fast SCR mechanism secured by other dynamic experiments, strongly suggest that, like in the standard SCR, the rate determining step in the fast SCR reaction is still associated with reoxidation of the V-sites, which is however accomplished more effectively by adsorbed nitrates, generated from NO2, rather than by gaseous oxygen, as in the case of the standard SCR reaction. Accordingly, on the basis of experimental evidence gained by transient reaction analysis only, a unifying mechanistic redox scheme was proposed for both standard and fast SCR reactions, that is, for the global NH3-NO/NO2 SCR process over V-catalysts:18,20 Catalyst reduction: NO + NH*3 + V5+dO f N2 + H2O + V4+sOH (R.14)

Catalyst reoxidation: V4+sOH + 1/4O2 f V5+dO + 1/2H2O

(R.15)

+ 5+ V4+sOH + NO3 + H f V dO + NO2 + H2O (R.16)

Reactions R.14 and R.15 describe the redox cycle in the NO + NH3 standard SCR, wherein reaction R.15 is the rate limiting reoxidation step involving gaseous oxygen. In the case of the NO + NO2 + NH3 fast SCR, the reduction of the V-sites still occurs according to the same global reaction R.14, but the rate determining step in the redox process, that is, the reoxidation of V-sites, is radically changed, being carried out in this case more rapidly by nitrates according to reaction R.16. It is worth emphasizing that the unified redox scheme R.14-R.16 (i) explains the higher rate of the fast SCR as due to the faster reoxidation of V-sites by nitrates; (ii) explains why the fast SCR reaction occurred also in the absence of gaseous NO2 due to the involvement of surface nitrates; (iii) explains why this reaction does not proceed effectively on a V-free catalyst; and (iv) is consistent with the reducing action of NO on both adsorbed nitrates and ammonium nitrate observed in our experiments. Surface Kinetics. Consistently with the mechanistic evidence above presented, a dynamic Mars-van Krevelen kinetic model that unifies standard and fast SCR reactions into a single redox approach for the V2O5-WO3/TiO2 catalyst was eventually derived.18 Briefly, it was assumed that two different types of centers prevail on the SCR catalysts, namely S1 sites, related to vanadium and associated with its redox properties, and S2 nonredox sites, associated with other nonreducible oxide components, possibly tungsta and titania. Ammonia as well as nitrates are stored onto S2 sites, whereas the redox cycle occurs on S1 sites. Also it was assumed that ammonia can migrate from S2 sites to S1 sites in order to take into account the inhibition effect by ammonia on the standard SCR activity observed during transient kinetic runs at low T (see Figure 9).16,18,19 According to the Mars-van Krevelen approach the rate of S1 sites oxidation, involving both gaseous oxygen and surface nitrates, was set equal to that of the reduction step, involving simultaneously NO and ammonia adsorbed species. The following (eq 4) was thus obtained, which provides the sum of the rates of both the standard and the fast SCR reactions (R.1-R.2). rDeNOx )

(

1 + KNH3

kNOCNOθNH3 θNH3 1 - θNH3 - θHNO3

)(

1+

kNOCNOθNH3

)

kox1pO21/4 + kox2θHNO3 (4)

It is interesting to examine the asymptotic behaviors of the rate law under different conditions. In the absence of NO2 (θHNO3 ) 0) eq 4 reduces to a redox rate law, eq 5, for the standard SCR reaction only, that was reported to reproduce successfully the slight promoting action of O2 and the inhibiting action of ammonia observed in several transient runs at low temperature.16


10383

Figure 9. Step feed of NH3 in NO/NO2 containing feed stream with different NO2/NOx feed ratios and at different temperatures over V2O5-WO3/TiO2 catalyst: NH3 ) 1000 ppm, NOx ) 1000 ppm, NO2/NOx ) 0, 0.5, 1; H2O ) 1%, O2 ) 2%, T ) 200, 225, 275, 350 °C. Symbols denote the measured concentrations of NH3, NO, N2, NO2, and N2O at reactor outlet. Lines denote kinetic fit. (adapted from ref 18).

rstd )

(

kNOCNOθNH3

1 + KNH3

θNH3 1 - θNH3

)(

1 + kO2

CNOθNH3 pO21/4

)

(5)

Likewise, at low ammonia coverages, as for example at temperatures above 250-300 °C, the rate equation formally simplifies to the well-known Eley-Rideal rate law, eq 6, extensively used in the kinetic literature for SCR stationary applications.41 rstd ) kNOCNOθNH3

(6)

On the other hand, the rate law predicts that the fast SCR reactivity should exist even in the absence of oxygen, the

reoxidation of the V-sites being carried out by nitrates only, which is indeed in line with experimental evidence.20 To provide data for the estimation of the rate parameters of the unified redox model, a set of 42 transient kinetic runs was performed varying temperature (160-425 °C), NO2/NOx feed ratio (0-1), ammonia (250-1000 ppm), NOx (250-1000 ppm) and O2 (2-6% v/v) concentrations over the powdered V-based SCR catalyst. Figure 9 shows selected results of such a kinetic study: the NH3, NO, and N2 concentration profiles versus time collected in transient reaction experiments with step changes of feed concentration at different temperatures and with different NO2/NO feed ratios (symbols, experimental results; lines, kinetic fit) are displayed. In all runs 1000 ppm of NOx were fed to the reactor in a stepwise manner while flowing 1000 ppm ammonia + 1% v/v H2O + 2% O2. In all cases, the levels of NH3, NO,

10384


and N2 at steady-state were consistent with the stoichiometries of the expected SCR reactions. Worthy of note, when feeding NO and ammonia only at the lowest investigated temperatures, a particular dynamic behavior was evident in the concentration profiles of NO and N2 during the NH3 shut-off. When NH3 was removed from the feed flow the NO concentration (mirrored by the N2 evolution) went through a minimum before recovering its inlet value. Thus it appears that the steady-state DeNOx activity of the system was hindered by excess ammonia, as already pointed out by other authors on different systems,48–52 suggesting the existence of an optimal ammonia surface concentration, which is lower than the coverage established at steady state. This behavior, evidenced thanks to the dynamic nature of the experiments, was attributed to an ammonia inhibition effect, for example, due to electronic interaction or possibly via direct blocking of the redox sites. The bulk of data collected varying temperature, NO2/NOx feed ratio and NOx and ammonia concentrations discussed above were fitted by global nonlinear regression, using the temporal evolution of the NH3, NO, NO2, N2, and N2O outlet concentrations as fitted responses. For selected conditions, the goodness of fit can be evaluated by inspection of Figure 9, where the solid lines represent model predictions. In all cases a good agreement is apparent between experimental (symbols) and calculated (solid lines) traces both at steady state and during concentration step changes. This eventually confirms the adequacy of the model in predicting the reactivity of the complete NH3-NO/NO2 SCR reacting system over a wide range of operating conditions, which are similar to those of real applications.18 Also, it is worth emphasizing that the model captured satisfactorily the complex transient behaviors observed in the low-temperature runs after the step changes of the reactants concentrations. Even if goodness of fit cannot be taken as a conclusive proof in favor of a proposed kinetic mechanism, this further supports the assumptions of the unified redox mechanistic scheme on which the SCR kinetic model was grounded. Conclusions Dynamic reactive methods are a powerful tool to identify and demonstrate mechanistic and kinetic features of catalytic reactions. With reference to the specific process of NH3-SCR DeNOx for diesel emission control over three different commercial catalytic systems, we have shown herein the successful application of transient reaction analysis to the investigation of several relevant aspects, namely: (i) Identification of the surface stored species and estimation of their storage capacities. In addition to the well-known ammonia, another key reactant, NO2, is also stored in comparable amounts on SCR catalysts in the form of nitrates, which has strong implications on the mechanism of the very important fast SCR reaction. (ii) Analysis of mechanistic steps. Dedicated transient experiments have been designed and performed, whose results provide conclusive evidence in favor of the key role of surface nitrates in the SCR catalytic chemistry when NO2 is included in the NOx feed mixture. (iii) Discrimination between rival mechanisms. The differences in the temporal evolutions of NO and NO2 observed during transient runs unambiguously rule out mechanistic proposals involving their simultaneous equimolar participation in the fast SCR chemistry; (iv) Redox features of the catalytic mechanism. By suitably designed transient tests over V-based and V-free catalysts it was

possible to compare the reducing and oxidizing activities of all the species involved in the SCR chemistry and demonstrate the superior efficiency of surface nitrates (rather than gaseous NO2) in reoxidizing the V-sites, which explains the unmatched DeNOx activity associated with the fast SCR reaction; (v) Unsteady kinetics. Eventually, a systematic set of transient runs has been fitted by a comprehensive, chemically consistent kinetic model in close agreement with the redox SCR mechanism over V-based catalysts, successfully accounting for the strong promoting role of NO2. Our results prove that transient response techniques can yield a high level of mechanistic and kinetic information under process relevant conditions, with a relatively limited experimental effort. It should be recognized on the other hand that transient response methods can provide only indirect information concerning the nature of the catalyst active sites, and the speciation of the adsorbed intermediates. In this respect, they can be nicely complemented by modern operando spectroscopic techniques, combining in situ and quantitative approaches.53 Acknowledgment Financial support from Daimler AG (Germany) is gratefully acknowledged. The authors thank Dr. Bernd Krutzsch, Dr. Michel Weibel, and Dr. Volker Schmeisser, Daimler AG, for many useful discussions. Literature Cited (1) Berger, R. J.; Kapteijn, F.; Moulijn, J. A.; Marin, G. B.; De Wilde, J.; Olea, M.; Chen, D.; Holmen, A.; Lietti, L.; Tronconi, E.; Schuurman, Y. Dynamic methods for catalytic kinetics. Appl. Catal. A 2008, 342 (12), 3–28. (2) Johnson, T. Diesel engine emissions and their control: An overview. Platinum Met. ReV. 2008, 52 (1), 23–37. (3) Brandenberger, S.; Kroecher, O.; Tissler, A.; Althoff, R. The state of the art in selective catalytic reduction of NOx by ammonia using metalexchanged zeolite catalysts. Catal. ReV.: Sci. Eng. 2008, 50 (4), 492–531. (4) Koebel, M.; Madia, G.; Elsener, M. Selective catalytic reduction of NO and NO2 at low temperatures. Catal. Today 2002, 73 (3-4), 239–247. (5) Madia, G.; Koebel, M.; Elsener, M.; Wokaun, A. The effect of an oxidation precatalyst on the NOx reduction by ammonia, SCR. Ind. Eng. Chem. Res. 2002, 41 (15), 3512–3517. (6) Gu¨thenke, A.; Chatterjee, D.; Weibel, M.; Krutzsch, B.; Kocı´, P.; Marek, M.; Nova, I.; Tronconi, E.; Guy, B. M. Current Status of Modeling Lean Exhaust Gas Aftertreatment Catalysts . AdVances in Chemical Engineering; Academic Press: Amsterdam, The Netherlands, 2007; Vol. 33, pp 103-211, 280-283. (7) Malmberg, S.; Votsmeier, M.; Gieshoff, J.; Soger, N.; Mussmann, L.; Schuler, A.; Drochner, A. Dynamic phenomena of SCR-catalysts containing Fe-exchanged zeolitessExperiments and computer simulations. Top. Catal. 2007, 42-43 (1-4), 33–36. (8) Sjovall, H.; Blint, R. J.; Gopinath, A.; Olsson, L. A Kinetic model for the selective catalytic reduction of NOx with NH3 over an Fe-zeolite Catalyst. Ind. Eng. Chem. Res. 2010, 49 (1), 39–52. (9) Sjovall, H.; Blint, R. J.; Olsson, L. Detailed kinetic modeling of NH3 SCR over Cu-ZSM-5. Appl. Catal. B 2009, 92 (1-2), 138–153. (10) Chatterjee, D.; Burkhardt, T.; Bandl-Konrad, B.; Braun, T.; Tronconi, E.; Nova, I.; Ciardelli, C. Numerical simulation of ammonia SCR catalytic converters: Model development and application. SAE Tech. Pap. 2005, (01), 0965. (11) Chatterjee, D.; Burkhardt, T.; Weibel, M.; Tronconi, E.; Nova, I.; Ciardelli, C. Numerical simulation of NO/NO2/NH3 reactions on SCR catalytic converters: Model development and applications. SAE Tech. Pap. 2006, (01), 0468. (12) Ciardelli, C.; Nova, I.; Tronconi, E.; Chatterjee, D.; Bandl-Konrad, B. A “Nitrate Route” for the low temperature “Fast SCR” reaction over a V2O5-WO3/TiO2 commercial catalyst. Chem. Commun. 2004, (23), 2718– 2719. (13) Ciardelli, C.; Nova, I.; Tronconi, E.; Chatterjee, D.; Burkhardt, T.; Weibel, M. NH3SCR of NOx for diesel exhausts aftertreatment: Role of NO2 in catalytic mechanism, unsteady kinetics and monolith converter modelling. Chem. Eng. Sci. 2007, 62 (18-20), 5001–5006.

Ind. Eng. Chem. Res., Vol. 49, No. 21, 2010 (14) Ciardelli, C.; Nova, I.; Tronconi, E.; Konrad, B.; Chatterjee, D.; Ecke, K.; Weibel, M. SCR-DeNO(x) for diesel engine exhaust aftertreatment: Unsteady-state kinetic study and monolith reactor modelling. Chem. Eng. Sci. 2004, 59 (22-23), 5301–5309. (15) Nova, I.; Ciardelli, C.; Tronconi, E.; Chatterjee, D.; Bandl-Konrad, B. NH3-NO/NO2 chemistry over V-based catalysts and its role in the mechanism of the fast SCR reaction. Catal. Today 2006, 114 (1), 3–12. (16) Nova, I.; Ciardelli, C.; Tronconi, E.; Chatterjee, D.; Bandl-Konrad, B. NH3-SCR of NO over a V-based catalyst: Low-T redox kinetics with NH3 inhibition. AIChE J. 2006, 52 (9), 3222–3233. (17) Nova, I.; Ciardelli, C.; Tronconi, E.; Chatterjee, D.; Weibel, M. NH3-NO/NO2 SCR for diesel exhausts after treatment: Mechanism and modelling of a catalytic converter. Top. Catal. 2007, 42-43 (1-4), 43–46. (18) Nova, I.; Ciardelli, C.; Tronconi, E.; Chatterjee, D.; Weibel, M. Unifying redox kinetics for standard and fast NH3-SCR over a V2O5-WO3/ TiO2 Catalyst. AIChE J. 2009, 55 (6), 1514–1529. (19) Tronconi, E.; Nova, I.; Ciardelli, C.; Chatterjee, D.; Bandl-Konrad, B.; Burkhardt, T. Modelling of an SCR catalytic converter for diesel exhaust after treatment: Dynamic effects at low temperature. Catal. Today 2005, 105 (3-4), 529–536. (20) Tronconi, E.; Nova, I.; Ciardelli, C.; Chatterjee, D.; Weibel, M. Redox features in the catalytic mechanism of the “standard” and “fast” NH3SCR of NOx over a V-based catalyst investigated by dynamic methods. J. Catal. 2007, 245 (1), 1–10. (21) Chatterjee, D.; Burkhardt, T.; Weibel, M.; Nova, I.; Grossale, A.; Tronconi, E. Numerical simulation of zeolite and V-based SCR catalytic converters. SAE Tech. Pap. 2007, (01), 1136. (22) Grossale, A.; Nova, I.; Tronconi, E. Ammonia blocking of the “Fast SCR” reactivity over a commercial Fe-zeolite catalyst for diesel exhaust aftertreatment. J. Catal. 2009, 265 (2), 141–147. (23) Grossale, A.; Nova, I.; Tronconi, E. Role of nitrate species in the “NO2-SCR” mechanism over a Commercial Fe-zeolite catalyst for SCR mobile applications. Catal. Lett. 2009, 130 (3-4), 525–531. (24) Grossale, A.; Nova, I.; Tronconi, E.; Chatterjee, D.; Weibel, M. The chemistry of the NO/NO2-NH3 “fast” SCR reaction over Fe-ZSM5 investigated by transient reaction analysis. J. Catal. 2008, 256 (2), 312– 322. (25) Grossale, A.; Nova, I.; Tronconi, E.; Chatterjee, D.; Weibel, M. NH3-NO/NO2 SCR for diesel exhausts aftertreatment: Reactivity, mechanism and kinetic modelling of commercial Fe- and Cu-promoted zeolite catalysts. Top. Catal. 2009, 52 (13-20), 1837–1841. (26) Ciardelli, C.; Nova, I.; Tronconi, E.; Ascherfeld, M.; Fabinski, W. Combined use of a mass-spectrometer and a UV analyzer in the dynamic study of NH3-SCR for diesel exhaust aftertreatment. Top. Catal. 2007, 4243 (1-4), 161–164. (27) Buzzi-Ferraris, G.; Donati, G. A powerful method for HougenWatson model parameter estimation with integral conversion data. Chem. Eng. Sci. 1974, 29 (6), 1504–1509. (28) Lietti, L.; Nova, I.; Camurri, S.; Tronconi, E.; Forzatti, P. Dynamics of the SCR-DeNO(x) reaction by the transient-response method. AIChE J. 1997, 43 (10), 2559–2570. (29) Lietti, L.; Nova, I.; Tronconi, E.; Forzatti, P. Transient kinetic study of the SCR-DeNOx reaction. Catal. Today 1998, 45 (1-4), 85–92. (30) Nova, I.; Lietti, L.; Tronconi, E.; Forzatti, P. Transient response method applied to the kinetic analysis of the DeNOx-SCR reaction. Chem. Eng. Sci. 2001, 56 (4), 1229–1237. (31) Colombo, M.; Nova, I.; Tronconi, E. , A comparative study of the NH3-SCR reactions over a Cu-zeolite and a Fe-zeolite catalyst. Catal. Today, published online February 9, 2010, http://dx.doi.org/10.1016/ j.cattod.2010.01.010. (32) Devadas, M.; Krocher, O.; Elsener, M.; Wokaun, A.; Soger, N.; Pfeifer, M.; Demel, Y.; Mussmann, L. Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5. Appl. Catal. B 2006, 67 (3-4), 187–196. (33) Sanchez-Escribano, V.; Montanari, T.; Busca, G. Low temperature selective catalytic reduction of NOx by ammonia over H-ZSM-5: An IR study. Appl. Catal. B 2005, 58 (1-2), 19–23.

10385

(34) Sun, Q.; Gao, Z. X.; Wen, B.; Sachtler, W. M. H. Spectroscopic evidence for a nitrite intermediate in the catalytic reduction of NOx with ammonia on Fe/MFI. Catal. Lett. 2002, 78 (1-4), 1–5. (35) Koebel, M.; Elsener, M.; Madia, G. Reaction pathways in the selective catalytic reduction process with NO and NO2 at low temperatures. Ind. Eng. Chem. Res. 2001, 40 (1), 52–59. (36) Koebel, M.; Madia, G.; Raimondi, F.; Wokaun, A. Enhanced reoxidation of vanadia by NO2 in the fast SCR reaction. J. Catal. 2002, 209 (1), 159–165. (37) Li, M.; Yeom, Y.; Weitz, E.; Sachtler, W. An acid catalyzed step in the catalytic reduction of NOx to N2. Catal. Lett. 2006, 112 (3), 129– 132. (38) Li, M. J.; Henao, J.; Yeom, Y.; Weitz, E.; Sachtler, W. M. H. Low activation energy pathway for the catalyzed reduction of nitrogen oxides to N2 by ammonia. Catal. Lett. 2004, 98 (1), 5–9. (39) Savara, A.; Li, M.-J.; Sachtler, W. M. H.; Weitz, E. Catalytic reduction of NH4NO3 by NO. Effects of solid acids and implications for low temperature DeNOx processes. Appl. Catal. B 2008, 81 (3-4), 251– 257. (40) Yeom, Y. H.; Henao, J.; Li, M. J.; Sachtler, W. M. H.; Weitz, E. The role of NO in the mechanism of NOx reduction with ammonia over a BaNa-Y catalyst. J. Catal. 2005, 231 (1), 181–193. (41) Forzatti, P. Present status and perspectives in De-NOx SCR catalysis. Appl. Catal. A 2001, 222 (1-2), 221–236. (42) Willi, R.; Roduit, B.; Koeppel, R. A.; Wokaun, A.; Baiker, A. Selective reduction of NO by NH3 over vanadia-based commercial catalyst: Parametric sensitivity and kinetic modelling. Chem. Eng. Sci. 1996, 51 (11), 2897–2902. (43) Lietti, L.; Forzatti, P. Temperature programmed desorption/reaction of ammonia over V2O5/TiO2 De-NOxing Catalysts. J. Catal. 1994, 147 (1), 241–249. (44) Lietti, L.; Forzatti, P.; Bregani, F. Steady-State and Transient Reactivity Study of TiO2-Supported V2O5/WO3 De-NOx Catalysts: Relevance of the vanadium tungsten interaction on the catalytic activity. Ind. Eng. Chem. Res. 1996, 35 (11), 3884–3892. (45) Ramis, G.; Busca, G.; Bregani, F.; Forzatti, P. Fourier transforminfrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on vanadia-titania and mechanism of selective catalytic reduction. Appl. Catal. 1990, 64, 259–278. (46) Dumesic, J. A.; Topsøe, N. Y.; Topsøe, H.; Chen, Y.; Slabiak, T. Kinetics of selective catalytic reduction of nitric oxide by ammonia over vanadia/titania. J. Catal. 1996, 163 (2), 409–417. (47) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: A review. Appl. Catal. B 1998, 18 (1-2), 1–36. (48) Kapteijn, F.; Singoredjo, L.; Dekker, N. J. J.; Moulijn, J. A. Kinetics of the selective catalytic reduction of NO with NH3 over Mn2O3-WO3/γAl2O3. Ind. Eng. Chem. Res. 1993, 32 (3), 445–452. (49) Koebel, M.; Elsener, M. Selective catalytic reduction of NO over commercial DeNOx-catalysts: Experimental determination of kinetic and thermodynamic parameters. Chem. Eng. Sci. 1998, 53 (4), 657–669. (50) Nova, I.; Lietti, L.; Tronconi, E.; Forzatti, P. Dynamics of SCR reaction over a TiO2-supported vanadia-tungsta commercial catalyst. Catal. Today 2000, 60 (1-2), 73–82. (51) Schuler, A.; Votsmeier, M.; Kiwic, P.; Gieshoff, J.; Hautpmann, W.; Drochner, A.; Vogel, H. NH3-SCR on Fe zeolite catalystssFrom model setup to NH3 dosing. Chem. Eng. J. 2009, 154 (1-3), 333–340. (52) Willey, R. J.; Lai, H.; Peri, J. B. Investigation of iron oxidechromia-alumina aerogels for the selective catalytic reduction of nitric oxide by ammonia. J. Catal. 1991, 130 (2), 319–331. (53) Sobalik, Z.; Jisa, K.; Jirglova, H.; Bemauer, B. Simultaneous FTIR/ UV-vis study of reactions over metallo-zeolites: Approach to quantitative in situ studies. Catal. Today 2007, 126 (1-2), 73–80.

ReceiVed for reView March 6, 2010 ReVised manuscript receiVed May 15, 2010 Accepted May 19, 2010 IE100507Z

Dynamic Methods in Catalytic Reaction Engineering: Applications to

Recommend Documents