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For a typical monolithic sample, the lower ... of the NOx storage-reduction catalyst, i.e., NOx adsorption to a basic site. 1. ..... For example, with...
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Ind. Eng. Chem. Res. 2001, 40, 52-59

Reaction Pathways in the Selective Catalytic Reduction Process with NO and NO2 at Low Temperatures M. Koebel,* M. Elsener, and G. Madia Combustion Research, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

The low-temperature behavior of the selective catalytic reduction (SCR) process with feed gases containing both NO and NO2 was investigated. The two main reactions are 4NH3 + 2NO + 2NO2 f 4N2 + 6H2O and 2NH3 + 2NO2 f NH4NO3 + N2 + H2O. The “fast SCR reaction” exhibits a reaction rate at least 10 times higher than that of the well-known standard SCR reaction with pure NO and dominates at temperatures above 200 °C. At lower temperatures, the “ammonium nitrate route” becomes increasingly important. Under extreme conditions, e.g., a powder catalyst at T ≈ 140 °C, the ammonium nitrate route may be responsible for the whole NOx conversion observed. This reaction leads to the formation of ammonium nitrate within the pores of the catalyst and a temporary deactivation. For a typical monolithic sample, the lower threshold temperature at which no degradation of catalyst activity with time is observed is around 180 °C. The ammonium nitrate route is interesting from a standpoint of general DeNOx mechanisms: This reaction combines the features typical to the SCR catalyst with the features of the NOx storage-reduction catalyst, i.e., NOx adsorption to a basic site. 1. Introduction Forthcoming European legislation pertaining to heavyduty diesel engines aims at the simultaneous reduction of the emissions of particles and NOx. It is generally assumed that the EURO 4 emission standards proposed for the year 2005 will be no longer feasible by primary measures alone but will require additional exhaust gas aftertreatment techniques. Figure 1 shows the large gap between feasible raw emissions of heavy-duty diesel engines and forthcoming emission standards. There exist two basic strategies to attain the new standards: (a) Optimize the combustion with respect to a low emission of NOx, leading to a high emission of unburned material (soot, CO, and hydrocarbons). Use a particulate filter in the aftertreatment. (b) Optimize the combustion with respect to a low emission of unburned material, leading to a high emission of NOx. Use a DeNOx process in the aftertreatment. Because a better fuel economy is attained with route b, various processes have been studied in recent years for selectively reducing NOx in competition to O2 in lean exhaust gases. These include HC-SCR (selective catalytic reduction with hydrocarbons),1,2 SCR with Ncontaining reducing agents,3,4 and the NOx storagereduction (NSR) catalyst.5,6 For the heavy-duty engines, the SCR process using urea as the reducing agent looks particularly attractive and has been studied intensively by us and others in the last years.4,7,8 Urea may be considered as a storage compound for ammonia and is usually applied in the form of an aqueous solution of 30-40%. The main problems and challenges in the application of urea SCR to mobile engines stem from the demand of a small system volume and have been discussed recently.9 The main demand is that for a reduced * To whom correspondence should be addressed. Tel: +4156-310 26 04. Fax: +41-56-310 21 99. E-mail: Manfred.Koebel@ psi.ch.

Figure 1. Tradeoff between particulate matter and NOx for heavy-duty truck engines and prospected EURO emission standards.

catalyst volume (compared to a stationary engine), thus implying the use of a catalyst with increased volumetric activity. A diesel engine has a widely varying exhaust temperature depending mainly on the load and the revolutions per minute. The values may typically be below 200 °C at 10% load and go up to ≈550 °C at full load. Because of the diminishing activity of the SCR catalyst with decreasing temperature, a minimum load/temperature will exist where the degree of NO reduction is no longer sufficient with a given catalyst. This problem of a minimum lower temperature is even more pronounced in the case of diesel passenger cars. Because of the tendency toward a generous engine sizing in passenger cars, the average exhaust gas will reach 200 °C only in the highway part of the test cycle but hardly in the city test cycle. Therefore, besides boosting the volumetric activity of the SCR catalyst, a new process has recently been proposed to increase the rate of the SCR reaction.9,10 It relies on the fact that the SCR reaction will be much faster in a mixture of NO-NO2 than in pure NO. We shall discuss the details in the following section. The fact that SCR DeNOx may now be principally achieved at temperatures below 220 °C (a typical

10.1021/ie000551y CCC: $20.00 © 2001 American Chemical Society Published on Web 11/18/2000

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 53

previous lower limit) has shown to us that new chemical reactions can become important at temperatures below 200 °C. It is the aim of this paper to give some insight into this highly interesting chemistry. 2. DeNOx and Side Reactions with Ammonia 2.1. Without the Formation of a New Solid Phase. To elucidate the main points, we will restrict the present discussion to ammonia as the selective reducing agent. The use of urea would only complicate the following equations without changing the basic statements. NOx in diesel exhaust is usually composed of >90% NO. Therefore, the main reaction of SCR with ammonia will be

4NH3 + 4NO + O2 f 4N2 + 6H2O

(1)

formation of solid ammonium nitrate in SCR catalysts.13-15 It is evident that this reaction may also be considered as a selective DeNOx reaction and will lead to a NOx reduction of at least 50% because of the direct formation of N2. However, the fate of ammonium nitrate formed has to be cleared up further. Reaction 7 is the sum of several individual steps, some of them being well-known from nitric acid production:

2NO2 H N2O4

(8)

N2O4 + H2O f HNO3 + HNO2

(9)

HNO3 + NH3 f NH4NO3

(10)

HNO2 + NH3 f NH4NO2

(11)

This reaction implies a 1:1 stoichiometry for NH3 and NO and the consumption of some oxygen and will be further denoted as “standard SCR”. The reaction consuming no oxygen is much slower and is therefore not relevant in lean combustion gases:

Ammonium nitrite is known to be a very unstable compound. decomposing explosively at temperatures above 60 °C.16 It may also be considered as the hydrate of nitrosoamine (NH2-NO), which is known to be unstable with respect to nitrogen and water:

4NH3 + 6NO f 5N2 + 6H2O

NH4NO2 f NH2-NO + H2O

(12)

NH2-NO f N2 + H2O

(13)

(2)

On the other hand, the reaction rate with equimolar amounts of NO and NO2 is much faster than that of the main reaction 1:

4NH3 + 2NO + 2NO2 f 4N2 + 6H2O

(3)

This reaction will be further denoted as “fast SCR”. The increase in the reaction rate has long been known11,12 and is now proposed as a practical measure to increase the performance of an automotive DeNOx system. To increase the fraction of NO2 in the exhaust, a strong oxidation catalyst (Pt-based) is placed upstream of the SCR catalyst. It should be mentioned that the reaction with pure NO2 is again slower than reactions 1 and 3:

4NH3 + 3NO2 f 31/2N2 + 6H2O

(4)

The conversion of NO to NO2 in the oxidation reactor should therefore not exceed 50%. At high temperatures (>400 °C), the commonly used catalysts based on TiO2WO3-V2O5 tend to form nitrous oxide. One of the possible reactions leading to nitrous oxide is

4NH3 + 4NO + 3O2 f 4N2O + 6H2O

(5)

At still higher temperatures the oxidizing character of the reaction system gets even more pronounced. This will cause the direct oxidation of ammonia to NO, thus limiting the maximum NOx conversion:

4NH3 + 5O2 f 4NO + 6H2O

(6)

2.2. With the Formation of a New Solid Phase: Ammonium Nitrate. Reverting again to the situation of low temperatures, we must envisage the formation of ammonium nitrate at T < 200 °C. This reaction requires NO2 as the oxidizing reactant and has the following overall stoichiometry:

2NH3 + 2NO2 f NH4NO3 + N2 + H2O

(7)

This reaction is well-known in the cleanup process of nitric acid production and may lead to the disturbing

The sum of reactions 8-13 will therefore yield eq 7. The ammonium nitrate formed in reaction 7 will deposit as a solid or liquid (melting point ) 170 °C) if the product of the partial pressures of NH3 and HNO3 exceeds the equilibrium constant Kp of the decomposition reaction:

NH4NO3(s) H NH3 + HNO3

(14)

Kp ) pNH3pHNO3

(15)

Conversely, solid ammonium nitrate can decompose according to eq 14 with rising temperature into NH3 and HNO3. However, usually the decomposition for temperatures up to 260 °C yields water and nitrous oxide:17

NH4NO3 f N2O + 2H2O

(16)

The explosive decomposition under initial ignition is also possible and yields nitrogen, oxygen, and water:17

2NH4NO3 f 2N2 + O2 + 4H2O

(17)

Which of the above decomposition reactions contributes the most depends on the exact conditions, especially on temperature and heating rate.17 A main goal of the present work is to determine the contribution of the various reactions under realistic low-temperature SCR conditions. 3. Experimental Section 3.1. Catalyst Samples and Reactors. The preparation of the catalysts has been described in a previous paper.18 The ternary catalyst contains 3% V2O5 and 8% WO3 on TiO2 as the support. The BET surface is ≈70 m2/g. Two sample types of this catalyst were used in the experiments: (a) A powder sample of 150-200 mg with grain sizes in the range of 160-200 µm. This sample was tested in a microreactor consisting of stainless steel. Further details on the reactor have been given previously.18

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Figure 2. Experimental setup: 1, water reservoir; 2, liquid massflow controller; 3, mass-flow controller; 4, water evaporator; 5, reactor; 6, catalyst sample; 7, filter; 8, flowmeter; 9, diaphragm pump; 10, gas cell.

(b) A monolithic sample of 7.3 cm3. The monolith was a metallic carrier with 600 cpsi (cells per square inch) and had a total coating mass of 1.4 g of catalyst. This sample was tested in a glass reactor. Further details on the reactor have been given previously.18 3.2. Analysis of the Gaseous Components. The gases at the reactor outlet were continuously analyzed by means of a FTIR spectrometer (Nicolet Magna IR 560, OMNIC QuantPad software) equipped with a heated multiple-pass gas cell (Graseby Specac G-2-4BA-AU, path length 2 m) and a liquid-nitrogen-cooled mercury cadmium telluride detector. The method developed allowed one to quantify the concentrations of NO, NO2, N2O, NH3, and H2O. The detection limits were 1 ppm for NO2, N2O, and NH3; 5 ppm for NO; and 500 ppm for H2O. 3.3. Analysis of Nitrate and Nitrite. These measurements have been made on an ion chromatograph (Waters ILC-1) using UV detection and an anionexchange column (Waters IC-Pak HC, No. 26770). More details have been given in ref 19. 3.4. Experimental Setup. The experimental setup is shown in Figure 2. The composition of the base feed gas was adapted to a typical diesel exhaust gas, containing 10% O2 and 5% H2O with balance N2. NH3 was used as a reducing agent. To obtain the feed gas, gas mixtures of higher concentrations (5% NO, NO2, or NH3 in N2; 100% O2 and 100% N2) obtained from Carbagas were diluted. Flow rates were regulated using mass-flow controllers (Brooks 5850S). Water was dosed through a microcapillary into an electrically heated evaporator, controlled by means of a liquid mass-flow controller (Brooks 5881). 4. Results and Discussion 4.1. Standard and Fast SCR at T > 200 °C. The influence of varying the ratio of NO2/NOx has a dramatic effect on the performance of an SCR catalyst. Figure 3 was obtained at a fixed temperature of 200 °C by varying the stoichiometric ratio R ) NH3/NOx and plotting the observed values of ammonia slip as a function of NOx conversion DeNOx. The monolithic catalyst was tested at fixed gas hourly space velocity (GHSV) ) 52 000 h-1 for 1000 ppm NOx in the feed at

Figure 3. Performance of monolithic catalyst sample at T ) 200 °C for varying ratios of NO2/NOx at GHSV ) 52 000 h-1. Feed: 1000 ppm NOx, 5% H2O, 10% O2, balance N2.

Figure 4. Performance of monolithic catalyst sample at T ) 300 °C for varying ratios of NO2/NOx at GHSV ) 52 000 h-1. Feed: 1000 ppm NOx, 5% H2O, 10% O2, balance N2.

different ratios NO2/NOx. It can be seen that the DeNOx for an ammonia slip of 10 ppm increases from ≈20% for pure NO to ≈95% with the 1:1 mixture of NO + NO2. Expressed in terms of a first-order rate law, the ratio of the two rate constants amounts to ≈13. This beneficial effect of NO2 addition is to be attributed to reaction 3, i.e. fast SCR. The effect of increased performance in mixtures containing NO2 may also be observed at higher temperatures, although in a less sensational manner because of the increasing rate of the standard SCR reaction. The case of still lower temperatures will be presented below. However, if the gas feed contains more than 50% NO2, the conversions will diminish again. The results of such experiments are shown in Figure 4 for a temperature of 300 °C. This temperature was chosen from the viewpoint of a realistic SCR system with preceding oxidation catalyst. NO2 fractions over 50% may only be expected if both the rate of NO oxidation is high enough and the temperature is below ≈400 °C. Because of the temperature dependence of the thermodynamic equilibrium NO + 1/2O2 H NO2, the maximum possible NO2 fraction decreases with rising temperature and attains an equilibrium value of ≈50% at 400 °C. Figure 4 shows that practically 100% DeNOx can be obtained with 50% of NOx being NO2 in the feed at the conditions of the experiment. This is not much more than the 94% (at 10 ppm NH3) reached with pure NO. However, NO2 in excess of 50% will cause a strong depression of the SCR

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 55

Figure 5. Efflux of NH3 and NO2 behind the powder sample. Feed: NO2(in) ) 490 ppm, NH3(in) ) 530 ppm, T ) 150 °C.

performance. For example, with 80% NO2 in the feed, the DeNOx (at 10 ppm NH3) will be suppressed to ≈50%. This is because in this case only 40% of the total NOx will react according to the fast SCR reaction 3. The remaining 60% is pure NO2 and reacts according to the slow reaction 4. We should mention that blank experiments were made to check for the decomposition of NO2 into NO and O2 at higher temperatures. When the base feed + 500 ppm NO2 was fed to the reactor containing the catalyst, the conversion to NO was less than 4% at 400 °C and 10% at 450 °C. This proves that at temperatures below 400 °C the influence of NO2 decomposition may be neglected. Summarizing, we may say that the utilization of the fast SCR reaction 3 is a very effective means to increase the performance of a given SCR catalyst at low temperatures. With increasing temperature its influence will be less pronounced because the rate of the standard SCR reaction also increases. The preferred way to generate the desired fraction of NO2 is to use a strong oxidation catalyst upstream of the SCR catalyst. In any case, NO2 fractions higher than 50% should be avoided because excessive NO2 will react according to reaction 4, having much slower kinetics. 4.2. Basic Experiments at T ) 150 °C. Below about 200 °C the possible formation of ammonium nitrate (reaction 7) consuming NO2 only must be taken into consideration. The reactions of NO and NO2 with ammonia at 150 °C were tested first with the powder sample (a) at a high GHSV of ≈ 500 000 h-1 (200 mg catalyst, 150 LN/h). A first experiment with the base feed + 490 ppm NO + 530 ppm NH3 revealed that the stationary outlet concentrations of NO and NH3 are practically equal to the feed concentrations. A subsequent temperatureprogrammed desorption (TPD) experiment showed no ammonia in excess to the usual blank value of this catalyst. Therefore, these experiments prove that NO and NH3 do not react to any substantial degree under these conditions (T, GHSV). Figure 5 shows the same experiment with NO2 (instead of NO) and ammonia. The stationary outlet concentrations up to ≈55 min are obviously lower than the inlet concentrations by about 70 ppm. This proves that some low-temperature reaction between NO2 and NH3 takes place. We should mention that no significant N2O concentration could be detected in the efflux. The addition of NO2 and NH3 to the feed was stopped after ≈55 min and the temperature kept at 150 °C up

to ≈100 min. As expected, some weakly bound NH3 is desorbed during this period, but no NO2 may be detected in the efflux. After ≈100 min a TPD experiment is started that leads to the desorption of substantial amounts of ammonia stored on the catalyst. These amounts are much higher than the quantity of ammonia adsorbed/desorbed in a blank experiment, i.e., when the previous adsorption experiment is carried out with ammonia only in the base feed. A trace of N2O could also be detected in the desorption experiment but neither NO nor NO2. On the other hand, new adsorption bands appeared in the FTIR spectra around 1200-1300 and 1700 cm-1. Furthermore, the formation of an irreversible band at ≈1250 cm-1 in the baseline of the FTIR instrument was noticed, and this is attributed to the superficial transformation of the KBr windows into KNO3. All of these observations prove that nitric acid is desorbed together with ammonia in the desorption experiment. Ammonium nitrate has therefore been formed under these conditions according to reaction 7. According to this equation, 50% of NH3(in) should be found in the desorption experiment, i.e., from the decomposition of ammonium nitrate. Our calculations gave ≈30% NH3 recovered in the desorption experiment. In view of the limited precision of the FTIR measuring technique, long integration times, and additional adsorption effects in the apparatus, we believe that the difference between theory and experiment is acceptable. It is also probable that some of the ammonium nitrate formed leaves the reactor already during the reaction as NH3 + HNO3 because of the vapor pressure of solid ammonium nitrate (eq 14). This would only be perceptible by a slightly higher amount of NO2 consumed compared to the amount of NH3 consumed and will again be hidden by the limited precision of the measuring technique. Another experiment was performed with the goal to determine the amount of nitrate and possibly nitrite stored in the catalyst. NO2 and NH3 (500 ppm each) in standard feed were fed to 200 mg of powder catalyst (a) at 150 °C for 1 h. The reactor was then cooled to room temperature and the catalyst taken out and eluted in deionized water. The filtrate was then analyzed for nitrite and nitrate by ion chromatography. The amount of nitrate detected amounted to ≈30% of NO2 supplied in the feed instead of 50% according to eq 7. No nitrite could be detected which proves the instability of this compound under these conditions. Summarizing, we may say that at very low temperatures pure NO will no longer react with NH3. On the other hand, NO2 may still react according to reaction 7. In this reaction, half of the NO2 will form ammonium nitrate and the other half elementary nitrogen. The decisive reaction step is a simple disproportioning reaction of dimerized NO2 and its hydration according to reactions 8 and 9. The reaction of nitric and nitrous acid with ammonia will then yield ammonium nitrate and ammonium nitrite according to eqs 10 and 11. Ammonium nitrite is unstable and will decompose according to eqs 12 and 13 into nitrogen and water. The overall reaction is again reaction 7. It is interesting to realize that the ammonium nitrate route becomes so significant at low temperatures. This is certainly because the equilibrium 8 will shift to the right side with decreasing temperature. In the presence of water, N2O4 can then easily disproportionate into N3+ and N5+. This reaction is also well-known from the

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Table 1. Conversion of NO2 Due to Reactions 3 and 7 on a Powdered Catalyst Sample (151 mg)a DeNOx due to NH4NO3 T NO(out) NO2(out) NH3(out) fast SCR formation [°C] [ppm] [ppm] [ppm] [%] [%] total[%] 200 190 180 170 160 150 140

169 192 210 226 238 245 250

161 176 193 202 212 217 224

358 400 438 462 484 495 506

32.4 23.2 16 9.6 4.8 2 0

1.6 3.2 3.4 4.8 5.2 5.6 5.2

34 26.4 19.4 14.4 10 7.6 5.2

a Conditions: NO ) 250 ppm, NO ) 250 ppm, NH ) 530 ppm, 2 3 O2 ) 10%, H2O ) 5%, balance N2. V* ) 150 LN/h.

reaction sequence of nitric acid production. Once the intermediate of nitrous acid with ammonia, i.e., ammonium nitrite, has been formed, its decomposition into nitrogen and water is fast. Another important conclusion from the experiments at 150 °C concerns the products formed when the sample is heated in the subsequent TPD experiment: Ammonium nitrate decomposes mainly into ammonia and nitric acid according to eq 14 and not into nitrous oxide and water (eq 16). Only traces of nitrous oxide could be observed. This is quite different from the wellknown decomposition of bulk ammonium nitrate yielding mainly nitrous oxide and water (N2 only as a minor byproduct).17 4.3. Reaction with NO + NO2 between 140 and 200 °C. The contributions of the fast SCR reaction 3 and reaction 7, i.e., formation of ammonium nitrate, to the total NOx conversion were investigated in the temperature range of 140-200 °C. These experiments were made with a feed gas containing NO and NO2 in a 1:1 ratio. The behavior of powder and monolithic catalyst samples proved to be quite different. Table 1 shows the results obtained for the powder sample. If NO(in) ) NO2(in), the following simplified equations are valid to calculate the contributions of reactions 3 (fast SCR) and 7 (ammonium nitrate route) to NOx conversion:

Fast SCR DeNOx(3) )

NO(in) - NO(out) × 100 NO(in)

(18)

Ammonium nitrate DeNOx(7) )

NO(out) - NO2(out) 2NO(in)

× 100

(19)

Total DeNOx DeNOx(tot) ) DeNOx(3) + DeNOx(7)

(20)

It may be seen that the fast SCR reaction 3 no longer contributes to the DeNOx at temperatures below 150 °C; i.e., its rate becomes insignificant compared to the rate of reaction 7. On the other hand, the fast SCR reaction is dominating in the conversion of NOx at 200 °C: 32.4% of DeNOx are due to the fast SCR reaction, but only 1.6% are due to reaction 7. (In terms of NO2 consumption, the respective values are 32.4% and 3.2%.) We may generalize our observations that reaction 3 has a positive and reaction 7 a negative temperature dependence.

Figure 6. NO2 converted because of fast SCR (reaction 3) and ammonium nitrate formation (reaction 7) and the resulting total DeNOx.

Figure 7. Poisoning of monolithic catalyst at 150 °C due to the formation of ammonium nitrate. NO(in) ) NO2(in) ) 500 ppm, NH3(in) ) 1000 ppm, GHSV ) 52 000 h-1.

This is illustrated by Figure 6 where the results of Table 1 have been plotted. These experiments have also been repeated with the monolithic catalyst sample. An experiment with the same concentrations as those used above for the powder sample showed a very small contribution from the ammonium nitrate reaction 7. The experiment was therefore repeated with a base feed containing 500 ppm of NO and NO2 each and 1000 ppm of NH3. The results are also included in Figure 6 for stationary conditions. The total DeNOx is much higher and the reaction to ammonium nitrate is almost negligible over the whole temperature range. Figure 7 shows the behavior of the monolith as a function of time. Both contributions from fast SCR and the ammonium nitrate route are much higher at the beginning of the experiment and decrease with time. This decrease may be ascribed to the deposition of ammonium nitrate within the pores of the catalyst. Practically stationary values are reached after some time because of the dependence of the condensation temperature on the pore size, small pores being filled first. The practically stationary values reached after ≈60 min were used in drawing Figure 6. Figure 8 shows the total DeNOx as a function of time for various temperatures between 150 and 200 °C. These experiments demonstrate that the critical temperature for solid or liquid ammonium nitrate deposition (melting point ) 170 °C) is somewhere between 170 and 180 °C

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 57

Figure 8. Poisoning of monolithic catalyst at various temperatures due to the formation of ammonium nitrate. NO(in) ) NO2(in) ) 500 ppm, NH3(in) ) 1000 ppm, GHSV ) 52 000 h-1.

under these conditions. Higher temperatures are required if the condensation of ammonium nitrate is to be avoided. A. Powder vs Monolithic Sample. Figure 6 shows the different behavior of the powder and the monolithic catalyst sample, especially at the lowest temperatures of 140-150 °C. Practically no NOx conversion due to the fast SCR reaction is observed on the powder sample at 150 °C, but this contribution is still ≈13% on the monolithic sample. On the other hand, over the whole temperature range (140-200 °C), the contribution from the ammonium nitrate reaction is higher on the powder sample than on the monolithic sample. The monolith shows a significant contribution from the ammonium nitrate route only at a temperature of 150 °C (4.1% vs 5.6% for the powder), although the concentrations of NOx and NH3 were twice as high than those in the case of the powder (≈1000 instead of ≈500 ppm). Thus, it can be concluded that the fast SCR reaction proceeds at a higher rate on the monolithic catalyst. The inverse is true for the ammonium nitrate reaction which proceeds faster on the powder sample. Because of this inverse relationship, these differences cannot be ascribed to the different “load” of the catalyst samples expressed as volume flow/grams of catalyst (V*/w ) 275 cm3/(g‚s) for the powder sample vs 75 cm3/(g‚s) for the monolithic sample). The results thus suggest an increased rate of the ammonium nitrate reaction in the case of the powder sample. Experiments performed with a powder sample without vanadia gave results identical to those of the powder sample with vanadia. A fully convincing explanation for the higher rate of the ammonium nitrate reaction in the powder sample cannot be given at the present time. Our main impression is that the ammonium nitrate reaction is not catalyzed by the BET surface but by the geometric surface, which is higher in the case of the powder sample. This view is supported by experiments without catalyst, showing that the ammonium nitrate reaction does not proceed to any measurable extent in the gas phase at these conditions. Further differences between the powder and monolith are as follows: (1) For the powder, the mass transfer from the gas stream to the outer particle surface is much more intense, and this would help to accelerate the ammonium nitrate route which is supposed to depend on intense interaction of the gas and geometric surface. (2) For the monolith, the rate of the fast SCR reaction is enhanced because of the lower “load” V*/w cited above and a higher effectiveness factor due to a smaller

Figure 9. Thermodynamic stability region of solid ammonium nitrate (above and to the right of the limiting curves).

diffusion length within the catalyst (layer thickness of monolith ≈ 50 µm, mean radius of powder particles ≈ 90 µm). Because the ammonium nitrate reaction is faster in the case of the powder sample, ammonium nitrate can deposit faster than in the case of the monolith. This will lead to an increased blocking of the pores and thus slow the fast SCR reaction which definitely needs the pores of the catalyst. In the case of the monolithic sample, the production of ammonium nitrate is slower, and thus the fast SCR reaction will be less inhibited by ammonium nitrate deposition. B. Inhibition by the Deposition of Ammonium Nitrate. The thermodynamic condition for the formation of solid or liquid NH4NO3 is given by the condition

pNH3pHNO3 > Kp

(15a)

The following equation of Kp has been derived from vapor pressure measurements of ammonium nitrate:13

Kp ) 1015.0636-9340/T [bar2]

(21)

We have calculated and plotted the stability region of solid ammonium nitrate in Figure 9. We see that for an NH3 concentration of 500 ppm the threshold partial pressure of HNO3 amounts to ≈200 ppm at 150 °C. This is quite a high value and is hardly present in the bulk gas phase of a monolithic channel. However, capillary condensation effects within the pores will lower further the partial pressures necessary for condensation. Because the partial pressure of nitric acid within the catalyst is not really known, no safe prediction about the threshold temperature of ammonium nitrate deposition can be made. The prediction is further complicated by the capillary condensation effects, and these are dependent on the pore diameter. Practical experiments are therefore the only way to obtain information at which temperature ammonium nitrate deposition starts. The results in Figure 8 suggest that ammonium nitrate will no longer deposit within the pores of the present monolith above 180 °C. We believe that the value of 180 °C is useful as a rough reference. However, this value will depend on the experimental conditions, especially the ammonia concentration in the feed and the actual conversions of the two reactions. The inhibition by ammonium nitrate deposition will lead to a slowly decreasing performance of the catalyst as shown in Figures 7 and 8. This kind of catalyst

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standard SCR reaction 1, where either NO or NH3 has to be oxidized by O2. However, studying the thermodynamic stability of N2O3 gives little support to such a simple reaction scheme. The equilibrium for gaseous nitrogen trioxide lies completely to the left side of eq 23, so that the necessary equilibrium concentrations for the direct reaction of NH3 + N2O3 according to eq 3 are nonexistent at temperatures above 200 °C:20

NO + NO2 f N2O3

Figure 10. SCR reaction with pure NO2 in the presence and absence of water. Powder sample (a). NO2(in) ) NH3(in) ) 500 ppm in base feed.

poisoning is reversible by heating to temperatures above ≈200 °C. This is in full analogy to the well-known reversible deposition of ammonium sulfates ((NH4)2SO4 and NH4HSO4) in SCR catalysts. Because of capillary condensation effects, small pores will block first and larger pores only later or not at all. This is the main cause of the observed time dependence of activity. 4.4. Dependence of the NO2 Conversion on Temperature and Water. Figure 10 shows the results of an experiment with pure NO2 over the temperature range 150-400 °C in the presence and absence of water. Evidently, in the case of the water-containing feed, two reactions with opposed temperature coefficients must exist, leading to a minimum DeNOx value. These experiments give further insight into the reaction mechanism of reaction 7, i.e., the ammonium nitrate route. At temperatures above ≈200 °C, the NO2 conversion is higher for the water-free feed, and this is due to the inhibiting action of water on the “slow” SCR reaction 4 with pure NO2. In the case of the water-containing feed, the conversion reflects the influence of two reactions leading to a minimum at ≈270 °C: At higher temperatures the “slow” SCR reaction 4 is dominating (although inhibited by the presence of water). At temperatures below 270 °C, the reaction leading to ammonium nitrate contributes increasingly to the NO2 conversion, and this effect leads to even higher conversions than in the case of the water-free feed. (The lower point at 150 °C is probably due to an experimental artifact: formation of ammonium nitrate in the pores leading to a decreased reaction rate as the points were taken going from high to low temperatures.) Therefore, water accelerates the overall reaction 7. It is probable that reaction 9 is the rate-limiting step. This is in accordance with a rate law determined experimentally by Mearns and Ofosu-Asiedu14 which describes the disappearance of NO2 in the gas-phase system H2O-NO2-NH3: 2

2

2

-dpNO2/dt ) k1pNH3pNO2 + k2pH2OpNO2 - k3pO2pNO

(22) 4.5. Possible Mechanism of the Fast SCR Reaction. Providing the mixed oxide of nitrogen (NO + NO2 ≈ N2O3) in the feed mixture increases the reaction rate drastically. Therefore, the first idea was that N3+ in N2O3 can react directly with N3- in NH3. No further redox reaction is required like in the case of the

(23)

It seems also improbable that the large molecule N2O3 reacts in a single reaction step with two NH3 molecules. We therefore suggest a mechanism based on one of the well-known reaction schemes for the standard SCR reaction 1: According to this idea, NO would react with activated NH3 (N2-). NO2 would just play the role of reoxidizing the vanadia reduced in the activation step of NH3 more easily. The oxidizing properties of NO2 are very similar to powerful ozone and thus much superior to molecular oxygen. The most probable reaction mechanisms for standard SCR are presently the nitrosamidic (NI) reaction mechanism first proposed by Ramis et al.21 and the Brønsted acid-redox mechanism proposed by Topsoe.22 In both mechanisms, ammonia is first oxidatively activated to a N2- species by V5+, which is simultaneously reduced to V4+. In the NI mechanism, this species is (-N•H2)+; in the Brønsted acid-redox mechanism, this is (N•H3)+. A recent review from Lietti et al.23 resumes the current debate on the two mechanisms. On the basis of the NI mechanism, we propose the following reaction sequence for the fast SCR reaction:

V5+dO + NH3 f HO-V4+-N•H2

(24)

HO-V4+-N•H2 + •NO f HO-V4+-(NH2)-NO (25) HO-V4+-(NH2)-NO f HO-V4+ + N2 + H2O

(26)

HO-V4+ + •NO2 f OdV5+ + HNO2

(27)

HNO2 + NH3ads. f NH4NO2ads.

(28)

NH4NO2ads. f N2 + 2H2O

(29)

The steps 24-26 are identical with the first three steps in the NI mechanism, whereas eqs 27-29 are different. Step 27 is the reoxidation of V4+ to V5+, which in fast SCR is performed with NO2 yielding nitrous acid HNO2. We propose that nitrous acid subsequently reacts with adsorbed NH3. The resulting intermediate ammonium nitrite is unstable and will decompose rapidly into nitrogen and water. 5. Conclusions 1. The partial conversion of NO into NO2 with the help of an oxidation catalyst may be useful to increase the performance of a mobile SCR system, especially in the range of lower temperatures. The optimum ratio of NO/ NO2 is 1:1, allowing all NOx to react by the fast SCR reaction. NO2 fractions in excess of 50% should be avoided because of the lower reaction rate of NO2 with NH3. 2. At temperatures below 200 °C, a second reaction becomes important, thus competing with the fast SCR reaction. This reaction consumes NH3 and NO2 and

Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 59

leads to the formation of ammonium nitrate. Depending on temperature and the partial pressures of ammonia and nitric acid, ammonium nitrate may deposit within the pores of the catalyst and thus cause its reversible inhibition. The experiments suggest that a rough value of the threshold temperature where no disturbing amounts of ammonium nitrate form within the catalyst lies around 180 °C. The ammonium nitrate route consumes only NO2 and will thus leave NO unreacted in the efflux. At lower temperatures, the formation of ammonium nitrate becomes important and may help to reach higher values of NOx conversion. However, considerable amounts of ammonium nitrate will be formed in the pores, which will be released in the form of ammonia and nitric acid when the catalyst is later brought to higher temperatures. An additional emission of nitric acid is therefore to be expected. 3. We suggest 180 °C as a lower temperature limit also for an SCR system equipped with a strong oxidation precatalyst furnishing a NO-NO2 mixture. Lower temperatures are allowable for a short, but not for an extended period of time. 4. Finally, a short general comment seems appropriate on the reaction of NO2 with NH3 by the ammonium nitrate route. This reaction combines typical features of two well-known techniques used to reduce NOx in lean exhaust gases: The primary reaction is the disproportionation of 2NO2 and its hydration, leading to nitric acid and nitrous acid (reaction 9). The latter will react with ammonia, forming ammonium nitrite, which is equivalent to the hydrate of nitrosamine NH2-NO. Both ammonium nitrite and nitrosamine are unstable and are probable intermediates in various proposed SCR mechanisms. The part of NO2 leading to ammonium nitrite as an intermediate thus decomposes according to the main lines of typical SCR mechanisms. HNO3 formed in eq 9 will react with ammonia, and thus yield ammonium nitrate. This reaction may be considered as the equivalent of the nitrate adsorption on a NSR catalyst.5 In the case of the NSR catalyst, a basic oxide like, for example, BaO acts as the adsorbing site for HNO3. In the case of the ammonium nitrate route, NH3 represents the basic adsorbent and after reacting with HNO3 leads to the formation of solid NH4NO3. Unlike with an NSR catalyst, adsorbed nitrate is not reduced in a subsequent cycle with rich reducing exhaust but decomposed when higher temperatures are again attained. In conclusion, the ammonium nitrate route of NO2 combines the features typical to the SCR and the NSR catalyst. Acknowledgment The financial support of the Swiss Federal Office of Energy (BFE) is gratefully acknowledged. We thank the working group “GD-KAT” of the “VDMA-Gesellschaft fu¨r Forschung und Innovation” (VFI) for suggestions and valuable discussions. Literature Cited (1) Shelef, M. Selective Catalytic Reduction of NOx with N-Free Reductants. Chem. Rev. 1995, 95, 209.

(2) Misono, M. Catalytic reduction of nitrogen oxides by bifunctional catalysts. CATTECH 1998, 3, 53. (3) Bosch, H.; Janssen, F. Catalytic Reduction of Nitrogen Oxides. Catal. Today 1988, 2, 369. (4) Koebel, M.; Elsener, M.; Marti, T. NOx-Reduction in Diesel Exhaust Gas with Urea and Selective Catalytic Reduction. Combust. Sci. Technol. 1996, 121, 85. (5) Takahashi, N.; et al. The new concept 3-way catalyst for automotive lean-burn engine: NOx storage and reduction catalyst. Proceedings of the 1st World Congress on Environmental Catalysis, Pisa, Italy, May 1-5, 1995; EFCE Publications, Series 112, Chemical Society of Italy: Rome, Italy, pp 45-48. (6) Brandt, S.; et al. Entwicklungsfortschritte bei NOx-Adsorber-Katalysatoren fu¨r magerbetriebene Ottomotoren. Kraftfahrwesen und Verbrennungsmotoren - 3. Stuttgarter Symposium, Stuttgart, Germany, Feb 23-25, 1999; Verlag Chemie: Weinheim, Germany, p 83. (7) Fra¨nkle, G.; et al. SINOx, The Exhaust Gas Purification System for Trucks. 18th International Wiener Motorensymposium, Vienna, Austria, April 24 and 25, 1997. (8) Maurer, B.; Jacob, E.; Weisweiler, W. Modellgasuntersuchen mit NH3 und Harnstoff als Reduktionsmittel fu¨r die katalytische NOx-Reduktion. Motortech. Z. 1999, 60, 6. (9) Koebel, M.; Elsener, M.; Kleemann, M. Urea-SCR: A promising technique to reduce NOx emissions from automotive diesel engines. Catal. Today 2000, 59, 335. (10) Jacob, E.; Emmerling, G.; et al. NOx-Verminderung fu¨r Nutzfahrzeugmotoren mit Harnstoff-SCR-Kompaktsystemen (GDKAT). 19th International Wiener Motorensymposium, Vienna, Austria, May 7 and 8, 1998. (11) Kato, A.; Matsuda, S.; Nakajima, F.; Kuroda, H.; Narita, T. J. Phys. Chem. 1981, 85, 4099. (12) Tuenter, G.; Leeuwen, W.; Snepvangers, L. Kinetics and mechanism of the NOx reduction with NH3 on V2O5-WO3-TiO2 catalyst. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 633. (13) Mearns, A.; Ofosu-Asiedu, K. Ammonium nitrate formation at low concentration mixtures of oxides of nitrogen, ammonia and water vapour. J. Chem. Technol. Biotechnol. 1984, 34A, 350. (14) Mearns, A.; Ofosu-Asiedu, K. Kinetics of reaction of low concentration mixtures of oxides of nitrogen and ammonia. J. Chem. Technol. Biotechnol. 1984, 34A, 341. (15) Odenbrand, C. U. I.; Andersson, L. A. H.; Brandin, J. G. M.; Lundin, S. T. Catalytic reduction of nitrogen oxides. 2. The reduction of NO2. Appl. Catal. 1986, 27, 363. (16) Gmelin. Handbuch der anorganischen Chemie, Ammonium, 8th ed.; Lieferung 1, 1936/69; p 89. (17) Gmelin. Handbuch der anorganischen Chemie, Ammonium, 8th ed.; Lieferung 1, 1936/69; p 109. (18) Kleemann, M.; Elsener, M.; Koebel, M.; Wokaun, A. Hydrolysis of Isocyanic Acid on SCR Catalysts. Ind. Eng. Chem. Res. 2000, 39, 4120. (19) Koebel, M.; Elsener, M. Determination of Urea and its Thermal Decomposition Products by High-Performance Liquid Chromatography. J. Chromatogr., A 1995, 689, 164. (20) Gmelin. Handbuch der anorganischen Chemie, Stickstoff, 8th ed.; Lieferung 3, 1936/69; p 738. (21) Ramis, G.; Busca, G.; Bregani, F.; Forzatti, P. Fourier Transform-Infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on vanadia-titania and mechanism of selective catalytic reduction. Appl. Catal., B 1990, 64, 259. (22) Topsoe, N. Y. Mechanism of the Selective Catalytic Reduction of Nitric Oxide by Ammonia Elucidated by in Situ On-Line Fourier Transform Infrared Spectroscopy. Science 1994, 265, 1217. (23) Lietti, L.; Ramis, G.; et al. Chemical, structural and mechanistic aspects of NOx SCR over commercial and model oxide catalysts. Catal. Today 1998, 42, 101.

Received for review June 5, 2000 Revised manuscript received September 14, 2000 Accepted September 21, 2000 IE000551Y