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Ind. Eng. Chem. Res. 2010, 49, 10386–10391

New “Enhanced NH3-SCR” Reaction for NOx Emission Control Pio Forzatti, Isabella Nova, and Enrico Tronconi* Laboratory of Catalysis and Catalytic Processes, Dipartimento di Energia, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

NH3-selective catalytic reduction (SCR) is applied worldwide to DeNOx of combustion exhausts of stationary and mobile sources, but a greater DeNOx activity at low temperatures is desired to meet forthcoming restrictive legislations and to increase energy efficiency in specific applications. We have obtained high NO reduction efficiencies at 200-300 °C over commercial V-based and Fe-exchanged zeolite catalysts by reacting NO with both ammonia and nitrates (e.g., ammonium nitrate) according to a novel “enhanced SCR” reaction, 2NH3 + 2NO + NH4NO3 f 3N2 + 5H2O. This provides a new route for low-temperature NOx reduction wherein the DeNOx activity is maximized by dosing an aqueous solution of nitrates to the SCR reactor feed stream. Introduction The selective catalytic reduction (SCR) technology is well established and used worldwide to control NOx emissions from power plants and other stationary sources, based on extruded honeycomb monolith catalysts consisting of V2O5-WO3/TiO2 or V2O5-MoO3/TiO2;1–5 it can be broadly described as passing an exhaust gas over a catalyst at 300-400 °C in the presence of ammonia or urea (an ammonia carrier), which converts NOx to nitrogen according to the so-called “standard SCR” reaction 1, 1 2NH3 + 2NO + O2 f 2N2 + 3H2O 2

(1)

Currently NH3- or urea-SCR is also being more and more extensively employed to reduce NOx in the exhaust gases of internal combustion engines operated with excess air, such as diesel engines installed on heavy-duty vehicles and passenger cars; zeolite-based washcoated monolith catalysts promoted by transition metals, such as Fe and Cu, are considered for this application.6 One problem of SCR systems for vehicles however is the poor activity at low temperatures, e.g., during cold startup and on short traveling distances, when most of the NOx is produced. The chosen method to boost the DeNOx activity of SCR catalysts for mobile applications at low temperature is to increase the NO2/NO feed molar ratio (NO2 accounts only for a few percent of total NOx in the exhaust gases), thus promoting the occurrence of the “fast SCR” reaction 27 2NH3 + NO + NO2 f 2N2 + 3H2O

(2)

This is realized in practice by installing a diesel oxidation catalyst (DOC) to convert a fraction of NO to NO2 upstream of the SCR converter, the oxidation catalyst typically consisting of precious metals carried on a flow-through honeycomb support. In this case, considerable improvements in NOx conversion are achieved, the highest DeNOx efficiencies generally corresponding to the NO2/NO ) 1/1 molar ratio associated with reaction 2.7,8 However, the oxidation activity of the DOC is strongly dependent on temperature and flow rate of the exhaust gases so that the optimal NO2/NO unit feed ratio cannot be assured for all possible engine operating conditions. The need * To whom correspondence should be addressed. Tel.: +39 02 2399 3264. Fax: +39 02 2399 3318. E-mail: [email protected].

for low-temperature SCR activity is also intrinsic to a number of modern stationary SCR applications, like incinerators with energy efficient configurations, which call for operation at temperatures as low as 200 °C. The purpose of the present paper is to communicate to the chemical engineering community the existence of a new, effective reaction for the selective catalytic reduction of NOx with ammonia/urea (NH3-SCR), based on the use of nitrate species as oxidizing agents.9 Specifically, we present herein steady-state and transient data showing that addition of aqueous solutions of NH4NO3 to a NO-NH3 feed results in the occurrence of the “enhanced SCR” reaction over both Fe-ZSM-5 and V2O5-WO3/TiO2 commercial catalysts. Such a reaction involves the selective reduction of NO by NH3 in the presence of nitrate species (e.g., ammonium nitrate) to N2 as exemplified by 2NH3 + 2NO + NH4NO3 f 3N2 + 5H2O

(3)

and is associated with superior NO reduction efficiencies in the 200-350 °C temperature range, similar to those observed in the fast SCR reaction 2; in this case, however, nitrate species rather than gaseous NO2 are included in the feed stream as the boosting agent. Preliminary data on the enhanced SCR reaction 3 have been recently reported in ref 10. Herein we present more extensive results addressing other experimental conditions, and we provide an interpretation of the related catalytic chemistry. Experimental Section The NO/NH3/O2 SCR reactions were investigated over core samples (about 5 cm3) from commercial V2O5-WO3/TiO2 (300 cpsi, extruded sample) and Fe-zeolite (400 cpsi cell density, washcoat thickness around 0.150 mm) SCR honeycomb catalysts under isothermal steady-state and transient conditions within the 200-450 °C temperature range. The monolithic catalyst samples were loaded in a stainless steel reactor tube placed in an oven with air recirculation. Prior to the activity tests, the catalysts were conditioned in a temperature-ramp at 10 °C/min up to 550 °C, followed by a hold at 550 °C for 1 h, in continuous flow of 2% O2 (v/v) and 10% (v/v) H2O + N2. Typical feed concentrations of NO and NH3 during the activity runs were 1000 ppm, with 0-2% O2, 1% H2O v/v, and balance N2. In addition, aqueous solutions of ammonium nitrate

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At t ) 1150 s, 1000 ppm of NO were also admitted to the reactor; the ammonia outlet concentration decreased, passed through a minimum; and then leveled off to a steady state value close to 920 ppm. Simultaneously the NO outlet concentration increased up to 900 ppm. The steady-state conversions of NO, about 10%, and of ammonia (somewhat lower) were consistent with the stoichiometry of the “slow SCR” reaction 4 5 3NO + 2 NH3 f N2 + 3H2O 2

Figure 1. Transient experiment over the V2O5-WO3/TiO2 catalyst at T ) 210 °C, GHSV ) 33 000 h-1. Temporal evolution of NO, NH3, and NO2 outlet concentrations upon subsequent addition of (A) 1000 ppm NH3, 1000 ppm NO, 2% v/v O2, 390 ppm NH4NO3 + 1% H2O in N2; (B) 1000 ppm NH3, 1000 ppm NO, 390 ppm NH4NO3 + 1% H2O in N2.

were also dosed to the reactor by means of a peristaltic pump; such solutions were vaporized and mixed with the gaseous feed upstream of the SCR catalysts. The solution concentrations and the pump flow rates were selected to result in NH4NO3 feed concentrations in the 300-400 ppm range. The space velocity was set to 33 000-36 000 h-1 in most of the experiments. Operating at such GHSV values and with substoichiometric amounts of ammonium nitrate with respect to reaction 3, complete conversion of the ammonium nitrate additive was always achieved. Continuous analysis of NO, NH3, and NO2 in the outlet gases was performed using a UV-analyzer (ABB LIMAS 11HV). Further details on the experimental apparatus and procedures can be found in refs 11 and 12. Results and Discussion Enhanced SCR Activity and Oxygen Effect. Figure 1A shows the temporal evolution of the NO, NH3, and NO2 outlet concentrations measured during a transient run at about 210 °C over the V2O5-WO3/TiO2 extruded catalyst, during which several reactants, namely, ammonia, NO, O2, and NH4NO3 + H2O, were added to the feed stream in subsequent steps. At the beginning of the run, N2 only was flown through the reactor; then, at around 250s, 1000 ppm of ammonia were instantaneously added to the feed stream: the NH3 outlet concentration showed a dead time, during which NH3 was completely adsorbed onto the catalyst surface, and then it started to grow, approaching the feed value of 1000 ppm after ∼500 s.

(4)

i.e., the poorly active DeNOx reaction between NO and NH3 prevailing on SCR catalysts when oxygen is not included in the feed mixture.1,3,13–15 At t ) 2300 s, O2 (2% v/v) was added to the reactor feed; both the NO and ammonia concentrations dropped within a few seconds of to a new steady state value about 610 ppm. As expected, in fact, the addition of oxygen resulted in the onset of a significant conversion of NO and NH3 associated with the standard SCR reaction 1. Finally, at t ) 3400 s, the pump was switched on and started to inject an aqueous solution of ammonium nitrate in the reactor feed stream, resulting in feed concentrations of 390 ppm NH4NO3 and 1% H2O v/v. The NO and ammonia outlet concentrations decreased rapidly again, both approaching a value of roughly 220 ppm. Such concentrations are in line with the occurrence of reaction 3, with an overall NO conversion of about 80%, indicating the complete consumption of the limiting reactant NH4NO3. Notably, only minor amounts of NO2 were observed throughout the whole transient run, as also evident in Figure 1A. Figure 1B shows a similar experiment wherein NH4NO3 was again added to the feed, whereas oxygen was not. It is apparent that injection of the ammonium nitrate solution (at t ) 2700 s) resulted in the same enhanced NO and ammonia conversions as displayed in Figure 1A for the case of oxygen-containing feed. Thus, the data in Figure 1 clearly point out that the addition of ammonium nitrate to the NO-NH3-O2 reacting system dramatically increased the NO reduction activity at low temperature over the V2O5-WO3/TiO2 extruded catalyst; furthermore, the added NH4NO3 was totally converted according to reaction 3. It is also worth emphasizing that the extent of the enhanced SCR reaction 3 was essentially unaffected by the presence of oxygen; notably, the oxygen effect is negligible for the fast SCR reaction 2 as well but not for the standard SCR reaction 1, wherein oxygen acts as a reactant. As discussed in more detail below, a likely explanation is that in the SCR redox catalytic mechanism the role of the oxidizer is played by gaseous oxygen in the case of the standard SCR reaction but (more effectively) by NO2 and by nitrates in the fast and in the enhanced SCR reactions, respectively. The beneficial effect of the NH4NO3 addition on the NOx removal efficiency as compared to the standard SCR deNOx activity was confirmed also by experiments at higher temperatures. Figure 2 shows steady state NO and ammonia conversions measured between 200 and 350 °C over the V2O5-WO3/ TiO2 extruded catalyst when feeding 1000 ppm NO, 1000 ppm NH3, and 340 ppm of NH4NO3 + 1% H2O both in N2 in the presence (solid lines) and in the absence (dashed lines) of 2% O2 v/v. In the absence of oxygen, the steady-state NO and ammonia conversions remained more or less stable over the whole explored temperature range, and the data were in line with the limit imposed by the substoichiometric feed concentrations of

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Figure 2. Effect of the addition of 2% O2 to the feed stream on the steadystate NO, NO2, and ammonia concentrations over the V2O5-WO3/TiO2 catalyst as a function of temperature. GHSV ) 33 000 h-1. Feed ) 1000 ppm NO, 1000 ppm NH3, 1% H2O, 2% O2, 300 ppm of NH4NO3 in N2.

Figure 3. Transient experiment over the Fe-ZSM-5 catalyst at T ) 200 °C, GHSV ) 33 000 h-1. Temporal evolution of NO, NH3, and NO2 outlet concentrations upon addition and removal of 420 ppm NH4NO3 to 1000 ppm NH3 in N2, followed by addition of 1000 ppm NO to 1000 ppm NH3 in N2.

ammonium nitrate according to reaction 3. On the other hand, Figure 2 shows that in the presence of 2% oxygen the measured NO and NH3 conversions were similar to those without O2 at the lowest temperature but progressively increased with growing temperature, likely due to the growing contribution of the standard SCR reaction 1, which of course did not proceed in the absence of oxygen. Such an effect is evident due to the substoichiometric ammonium nitrate feed contents, which indeed limited the contribution of the enhanced SCR reaction at all temperatures. Transient Behavior. Figure 3 shows a different experiment performed over the Fe-zeolite washcoated catalyst at 200 °C under transient conditions. In this case the pump was turned on at t ) 350 s and turned off at t ) 1320 s, dosing 420 ppm of NH4NO3 to a gaseous feed stream consisting of 1000 NH3 in N2; afterward, at t ) 1650 s, 1000 ppm of NO were also added to the feed stream. In the first part of the experiment, when the ammonium nitrate aqueous solution was added to the NH3 feed mixture, no significant effects were evident in the outlet concentration profiles, apart from some desorption and oscillations in the ammonia concentration and a small evolution of NO2. This is ascribed to ammonium nitrate which, once fed to the system at

Figure 4. Transient NO, NH3, and NO2 outlet concentrations over the FeZSM-5 catalyst at 205 °C. GHSV ) 33 000 h-1. Feed ) 1000 ppm NO, 680 ppm NH3, 340 ppm NH4NO3, 1% H2O, 2% O2 in N2.

such a low temperature, was stored onto the catalyst surface and partially decomposed to ammonia and NO2. Accordingly, when NH4NO3 was removed from the feed flow, both the NO2 and ammonia concentrations decreased slowly. Then, around 1650 s, NO was also fed to the reactor while flowing ammonia and in the presence of ammonium nitrate prestored on the catalyst; the slow transients apparent in the concentration profiles of ammonia and NO in Figure 3 are associated with the consumption of such (ammonium) nitrate species preloaded on the catalyst, since no fresh NH4NO3 was being fed. This result is also of practical importance, as it shows that proper dosing of the ammonium nitrate solution can warrant enhanced NO reduction efficiencies also during transient operation typical of SCR applications to vehicles or during start up and shut down operations in stationary applications. Control of the Ammonia Slip. Figure 4 illustrates the measured outlet concentrations of NO, NH3, and NO2 during a transient run at 205 °C over the Fe-zeolite catalyst involving substoichiometric feed contents of both NH3 (680 ppm) and NH4NO3 (340 ppm) with respect to NO (1000 ppm) relative to reaction 3. A t ) 0 s, the ammonium nitrate solution was added to the feed flow of NO and ammonia; the concentrations dropped from the feed values to those corresponding roughly to total ammonia conversion and a mean value of about 68% for NO conversion. This confirms that the NO conversion was set by the stoichiometric limit of reaction 3 and was associated with the total depletion of both limiting reactants NH3 and NH4NO3. This result is again of practical importance, as it shows that, by a proper selection of the reactants feed concentrations, reaction 3 can warrant enhanced NO reduction efficiencies with negligible ammonia slip. Notably, considerable oscillations of the NOx conversion were observed during NH4NO3 injection. At this low temperature, such a behavior is likely related to the buildup, decomposition, and reaction of ammonium nitrate on the catalyst. Indeed, it tends to disappear at increasing temperatures (results not shown). Comparison with Standard and Fast SCR. Figure 5 compares steady state NOx and ammonia conversions in standard (feed, 1000 ppm NO; 1000 ppm NH3; 2% O2; 1% H2O in N2) and fast SCR (feed, 500 ppm NO, 500 ppm NO2, 1000 ppm NH3, 2% O2, 1% H2O in N2) over the Fe-zeolite catalyst

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Figure 5. Effect of the addition of 340 ppm of NH4NO3 to the feed stream on the steady-state NOx and ammonia conversions over the Fe-ZSM5 catalyst as a function of temperature in comparison to standard and fast SCR. GHSV ) 33 000 h-1. Feed ) 1000 ppm NO, 1000 ppm NH3, 1% H2O, 2% O2 in N2. Standard SCR runs, NO2/NOx ) 0; fast SCR runs, NO2/NOx ) 1/2.

Figure 6. Effect of the addition of 390 ppm of NH4NO3 to the feed stream on the steady-state NOx and ammonia conversions over the V2O5-WO3/ TiO2 catalyst as a function of temperature in comparison to standard and fast SCR. GHSV ) 33 000 h-1. Feed ) 1000 ppm NO, 1000 ppm NH3, 1% H2O, 2% O2 in N2. Standard SCR runs, NO2/NOx ) 0; fast SCR runs, NO2/NOx ) 1/2.

with those measured when feeding 1000 ppm NO, 1000 ppm NH3, 2% O2 in N2 along with 340 ppm of NH4NO3 + 1% H2O. When only NO and ammonia were fed to the system while flowing oxygen and water, the activity was poor, conversion being still far from complete even at 350 °C. The outlet concentration levels of NO and ammonia were in line with the occurrence of the standard SCR reaction 1; however, at temperatures above 300 °C a growing negative deviation from the expected 1/1 NO/NH3 molar consumption ratio was observed, possibly related to the onset of the ammonia oxidation, which is known to be active on Fe-zeolite catalysts in this temperature range.12,14,17,18 When an equimolar mixture of NO and NO2 (500 ppm each) was fed to the Fe-zeolite catalyst along with ammonia, though, 100% conversions were observed already at 200 °C, in line with the strong sensitivity of Fe-zeolite catalysts to the NO2 feed content and with the corresponding high activity of the fast SCR reaction 2.12,18–20 In the case of the runs in the presence of NH4NO3, the steadystate NO and ammonia conversions were as high as 68% (i.e., limited by the feed concentration of ammonium nitrate) already at 200 °C, and they remained more or less stable up to 250 °C when they began to grow slowly with temperature. The outlet concentrations measured in the range 200-250 °C are consistent with to the stoichiometry of reaction 3; actually they correspond to the stoichiometric limit imposed by the feed concentration of ammonium nitrate and further originate from the poor activity of the standard SCR reaction over this catalytic system. At temperatures in excess of 250 °C, the situation was modified. Both the NO and the ammonia conversion increased, eventually approaching total conversion at the highest temperatures. Notably, NO2 was not detected in significant amounts at any investigated temperature. The incremented activity is due to the increasing contributions of the standard SCR reaction and of the ammonia oxidation with growing temperature. At these temperatures also the “NO2SCR” reaction should be taken into account,

by oxygen or by nitrates, is responsible for the reduced NOx/ NH3 conversion ratio. Notably ammonia oxidation was found selective to nitrogen over similar Fe-zeolite catalysts.12,17 Thus, also over the Fe-zeolite catalyst addition of ammonium nitrate to the feed stream resulted in promoting the DeNOx activity with respect to the standard SCR reaction. Because of the substoichiometric feed amount of NH4NO3, the NOx conversion of the fast SCR reaction was not reached. N2O formation was checked in a few cases by a dedicated IR analyzer (ABB Uras) and found comparable to what was observed under the fast SCR conditions. Figure 6 shows that over the V2O5-WO3/TiO2 extruded catalyst the promoting effect of ammonium nitrate on the steadystate DeNOx activity as compared to the standard SCR case, already delineated at 210 °C in the experiment of Figure 1, was clearly visible also in a wide range of higher temperatures. Over the vanadium-based system, however, the 1/1 molar conversion ratio of NO and ammonia, which is common to both the standard and the enhanced SCR reactions, was maintained up to the highest temperatures. Notably, the injection of the substoichiometric amount of NH4NO3 (390 ppm < 500 ppm) resulted here in a NOx removal efficiency approaching the optimal DeNOx activity of the fast SCR reaction. Other experiments with lower NH4NO3 feed contents (see, for example, Figure 2) confirmed that the NOx removal efficiency increased upon increasing the NH4NO3 feed concentration: the observed effect was in fact quite similar to the well-known effect of increasing the NO2/NOx feed ratio from 0 to 1/2 over similar catalysts.13,16 Analysis of the Catalytic Chemistry. It is well-known that ammonium nitrate participates in the dissociation equilibrium (reaction 6) with nitric acid and ammonia,

7 3NO2 + 4NH3 f N2 + 6H2O 2

(5)

whose mechanism is likely associated with the oxidation of ammonia by surface nitrates;17 thus, ammonia oxidation, either

NH4NO3 T HNO3 + NH3

(6)

In turn, nitric acid can be stored onto SCR catalysts as surface nitrate species; in the absence of coreactants and above a given temperature threshold, such species thermally decompose13,16,17 to form NO2, O2, and water, 1 1 HNO3 f NO2 + H2O + O2 2 4

(7)

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To document the occurrence of reactions 6 and 7, a blank experiment (not shown) was run dosing an aqueous solution of NH4NO3 to a nitrogen stream flowing over a bare cordierite honeycomb sample. A substantial fraction of ammonium nitrate was decomposed to NH3 and NO2 even in the absence of a catalyst. A small conversion of NO2 to NO was also observed, whereas only traces of N2O were detected, originating from direct thermal decomposition of ammonium nitrate NH4NO3 f N2O + 2H2O

(8)

On the other hand, the enhancement of DeNOx activity observed over the V-based and the Fe-ZSM-5 catalysts upon adding ammonium nitrate to a NO/NH3 feed mixture indicates that nitrate adspecies formed by reaction 6 directly participate in SCR reactions rather than being decomposed according to reaction 7. It should be also pointed out in this respect that a similar enhanced SCR activity was in fact observed when feeding directly nitric acid in aqueous solution rather than NH4NO3.10 Indeed, all of the results herein presented appear to be in line with several indications in the literature concerning the important role of surface nitrates in the catalytic mechanism of the fast SCR reaction 2.13–18,21,22 Some of us first showed by transient kinetic experiments21 that at temperatures as low as 150-170 °C, the fast SCR reaction proceeds via a sequential scheme in which ammonium nitrate is first formed, reaction 9, and then reacts with NO, reaction 10: 2NH3 + 2NO2 f NH4NO3 + N2 + H2O

(9)

NH4NO3 + NO f NO2 + N2 + 2H2O

(10)

The sum of reactions 9 and 10 yield the stoichiometry of the fast SCR, reaction 2. Further studies15–18 pointed out that the first step in the above sequence, i.e., ammonium nitrate formation (reaction 9), involves several reactions, namely, NO2 dimerization (reaction 11), disproportionation (reaction 12) and successive reactions between nitrous and nitric acid and NH3, with rapid decomposition of ammonium nitrite to nitrogen (reaction 13): 2NO2 T N2O4

(11)

N2O4 + H2O T HONO + HNO3

(12)

NH3 + HONO T NH4+ + NO2- T [NH4NO2] f N2 + 2H2O

(13)

NH3 + HNO3 T NH4+ + NO3- T NH4NO3

(6rev) Also the second step in the fast SCR sequential scheme, i.e., reaction 10, is actually the sum of the ammonium nitrate dissociation (reaction 6), the successive oxidation of NO to NO2 by nitric acid, which is thus reduced to nitrous acid (reaction 14), and the reaction of the latter with NH3 to form N2 via ammonium nitrite decomposition (reaction 13): NH4NO3 T NH3 + HNO3

(6a)

HNO3 + NO T NO2 + HONO

(14)

NH3 + HONO f N2 + 2H2O

(13a)

Finally, according to a Mars-Van Krevelen interpretation of

the SCR chemistry over V-based catalysts, the key global reaction 14 is likely associated with a redox cycle involving the very effective reoxidation of reduced V-sites by surface nitrates,15 which explains the higher rate of the fast SCR reaction as compared to the standard SCR chemistry, wherein the catalyst reoxidation step is carried out by gaseous oxygen. The catalytic chemistry of the enhanced SCR reaction 3 herein demonstrated appears to be consistent with our current understanding of the fast SCR mechanism as discussed above. Dissociation of the feed NH4NO3 in fact results in the formation of the very same adsorbed species which are key intermediates in the fast SCR chemistry, namely, ammonia and nitrates. Notably, in the runs shown in Figure 1B and in Figure 2, oxygen was not included in the feed mixture; thus, the result of these experiments point out as well that oxygen is unnecessary for NO conversion, in agreement with the global reaction 3 and with the more detailed reaction scheme presented above (reactions 9-14): according to the Mars-Van Krevelen mechanism, in fact, nitrates replace gaseous oxygen in the red-ox cycle. Injecting nitrate species in order to reduce NOx is apparently paradoxical. Nevertheless, our data prove that this is effective for promoting NOx removal, likely due to the extreme oxidizing properties of such species which could thus play a strong promoting role in the NH3-SCR catalytic mechanism, whose red-ox nature is well established.1,2,4,5,15,23 We have shown in fact that feeding nitrate species in aqueous solution rather than gaseous NO2 results in a DeNOx activity similar to that of the “fast SCR” reaction. It is also worth recalling that the selectivity to nitrogen associated with the enhanced SCR reaction 3 was always very high. In the case of mobile applications, the “enhanced SCR” concept could be seen as an alternative to the oxidation catalyst (DOC), currently used in order to boost NOx reduction at low temperatures; in the actual configuration, the NO2 production over the DOC catalyst positioned upstream of the SCR reactor, depending on temperature and space velocity, could limit the overall deNOx efficiency over the SCR unit. This problem can be solved in the enhanced SCR concept, as an optimal control of the amount of aqueous solution of the oxidizing additive directly added to the SCR reactor is possible in principle. Conclusions In the present paper we have presented a new, effective reaction for the selective catalytic reduction of NOx with ammonia/urea (NH3-SCR) based on the use of nitrate species as oxidizing coreactants. Such species can be fed to the SCR catalyst, e.g., in the form of an aqueous solution of ammonium nitrate, using the same injection systems currently applied to the dosage of ammonia/urea solutions and possibly by simply adding the ammonium nitrate to the existing urea solution. Both transient and steady-state data showed that the addition of aqueous solutions of either NH4NO3 (or HNO3) to a NO-NH3 feed results in the occurrence of the new “enhanced SCR” reaction (2 NH3 + 2 NO + NH4NO3 f 3 N2 + 5 H2O) over both Fe-ZSM-5 and V2O5-WO3/TiO2 commercial catalysts. Such a reaction is associated with superior NO reduction efficiencies with respect to the standard SCR reaction in the 200-350 °C temperature range. Under the adopted experimental conditions, the new reaction is accompanied by total conversion both of the nitrate additives and of ammonia when these are fed in substoichiometric amounts, and thus it is also compatible with limitations on the ammonia slip. Because of the substoichiometric amount of

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additive injected in the experiments, the limiting conversion of the fast SCR reaction was only approached. In conclusion, our data indicate the existence of a new SCR reaction that in principle could be applicable to all the existing classes of SCR commercial catalysts and both to stationary and mobile SCR applications. Indeed, the main characteristic of the new reaction is to be particularly active at low temperature; thus, its application to the improvement of the low-T DeNOx activity of new generation, energy efficient incinerators and of SCR converters for diesel vehicles seems promising. Specifically for mobile applications, adoption of nitrate species in aqueous solution rather than NO2 as a promoting agent affords independent optimized dosage of the promoter for all conditions. Indeed, NO2 formation, which in the current system configurations occurs by partial NO oxidation on a diesel oxidation catalyst (DOC) positioned upstream of the SCR converter, is strongly dependent on temperature and flow rate of the engine exhaust gases, so that the optimal NO2/NO unit feed ratio cannot be assured for all possible engine conditions. The aqueous solution of the oxidizing additive could be added to the exhaust gas stream entering the SCR reactor by means of the same injection systems currently employed for dosage of ammonia/urea solutions. As a matter of fact, a single aqueous solution containing both the reducing agent (urea) and the oxidizing additive (ammonium nitrate) could be used for this purpose. Urea-ammonium nitrate (UAN) solutions, used as fertilizers, are commercially available, with freezing points ranging between 0 and -18 °C, depending on their composition.24 The practical implementation of the enhanced SCR concept further involves of course a number of issues related to considerations of safety, corrosion, and system design aspects, which are however beyond the scope of the present report and need to be specifically addressed in relation to each envisaged application. Literature Cited (1) Forzatti, P.; Lietti, L.; Tronconi, E. Nitrogen Oxides Removal. In Encyclopedia of Catalysis; Wiley: New York, 2003; Vol. 5, pp 298-343. (2) 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. (3) Nova, I.; Beretta, A.; Groppi, G.; Lietti, L.; Tronconi, E.; Forzatti, P. Monolithic catalysts for NOx removal from stationary sources. In Structured Catalysts and Reactors, 2nd ed.; Cybulski, A., Moulijn, J. A., Eds. Taylor and Francis: Boca Raton, FL, 2006; pp 171-214. (4) 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: EnViron. 1998, 18 (1-2), 1–36. (5) Casagrande, L.; Lietti, L.; Nova, I.; Forzatti, P.; Baiker, A. SCR of NO by NH3 over TiO2-supported V2O5-MoO3 catalysts: reactivity and redox behavior. Appl. Catal., B: EnViron. 1999, 22 (1), 63–77. (6) Johnson, T. Diesel Engine Emissions and Their Control An overview. Platinum Met. ReV. 2008, 52 (1), 23–37. (7) Kato, A.; Matsuda, S.; Kamo, T.; Nakajima, F.; Kuroda, H.; Narita, T. Reaction between nitrogen oxide (NOx) and ammonia on iron oxidetitanium oxide catalyst. J. Phys. Chem. 1981, 85 (26), 4099–4102.

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(8) Koebel, M.; Elsener, M.; Kleemann, M. Urea-SCR: a promising technique to reduce NOx emissions from automotive Diesel engines. Catal. Today 2000, 59 (3-4), 335–345. (9) Forzatti, P.; Tronconi, E.; Nova, I. Apparatus and process for reducing the content of nitrogen oxides in exhaust gases of combustion system. International Patent Application WO2008/126118, 23.10.2008, 2008. (10) Forzatti, P.; Nova, I.; Tronconi, E. Enhanced NH3 Selective Catalytic Reduction for NOx Abatement. Angew. Chem., Int. Ed. 2009, 48 (44), 8366–8368. (11) Chatterjee, D.; Burkhardt, T.; Weibel, M.; Nova, I.; Grossale, A.; Tronconi, E. Numerical simulation of zeolite and V-based SCR catalytic converters. SAE Technical Paper 2007-01-1136, 2007. (12) Grossale, A.; Nova, I.; Tronconi, E. Study of a Fe-zeolite-based system as NH3-SCR catalyst for Diesel exhaust aftertreatment. Catal. Today 2008, 136 (1-2), 18–27. (13) Ciardelli, C.; Nova, I.; Tronconi, E.; Chatterjee, D.; Bandl-Konrad, B.; Weibel, M.; Krutzsch, B. Reactivity of NO/NO2-NH3 SCR system for Diesel exhaust aftertreatment: Identification of the reaction network as a function of temperature and NO2 feed content. Appl. Catal., B: EnViron. 2007, 70 (1-4), 80–90. (14) 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. (15) 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. (16) 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. (17) 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. (18) 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. (19) Brandenberger, S.; Kroecher, O.; Tissler, A.; Althoff, R. The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal-Exchanged Zeolite Catalysts. Catal. ReV.: Sci. Eng. 2008, 50 (4), 492–531. (20) 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. (21) 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. (22) Rahkamaa-Tolonen, K.; Maunula, T.; Lomma, M.; Huuhtanen, M.; Keiski, R. L. The effect of NO2 on the activity of fresh and aged zeolite catalysts in the NH3-SCR reaction. Catal. Today 2005, 100 (3-4), 217– 222. (23) Nova, I.; Ciardelli, C.; Tronconi, E.; Chatterjee, D.; Weibel, M. Unifying Redox Kinetics for Standard and Fast NH3-SCR over a V2O5WO3/TiO2 Catalyst. AIChE J. 2009, 55 (6), 1514–1529. (24) UNIDO and International Fertilizer Development Center. Fertilizer Manual; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1998 (ISBN 0-7923-5032-4).

ReceiVed for reView March 12, 2010 ReVised manuscript receiVed June 7, 2010 Accepted June 11, 2010 IE100600V