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Reduction of NOx in synthetic diesel exhaust via two-step plasma-catalysis treatment
Applied Catalysis B: Environmental 40 (2003) 207–217
Reduction of NOx in synthetic diesel exhaust via two-step plasma-catalysis treatment R.G. Tonkyn a,∗ , S.E. Barlow a , John W. Hoard b a
b
Pacific Northwest National Laboratory, P.O. Box 999, MS K8-8, Richland, WA 99352, USA Chemical Engineering Department, Ford Motor Company Research Laboratory, 2101 Village Road, MS 3179SRL, Dearborn, MI 48121, USA Received 12 April 2002; received in revised form 20 June 2002; accepted 21 June 2002
Abstract Significant reduction of NOx in synthetic light duty diesel exhaust has been achieved over a broad temperature window by combining atmospheric plasma with appropriate catalysts. The technique relies on the addition of hydrocarbon reductant prior to passing the simulated exhaust through a non-thermal plasma and a catalyst bed. The observed chemistry in the plasma includes conversion of NO to NO2 as well as the partial oxidation of the hydrocarbon. The overall NOx reduction has a maximum of less than 80%, with this maximum obtained only at high-energy input into the plasma, high concentration of hydrocarbon reductant and low space velocity. We present data in this paper illustrating that a multiple-step treatment strategy, whereby two or more plasma-catalyst reactors are utilized in series, can increase the maximum NOx conversion obtainable. Alternatively, this technique can reduce the energy and/or hydrocarbon requirements for a fixed conversion efficiency. When propene is used as the reductant, the limiting reagent for the overall process is most likely acetaldehyde. The data suggest that acetaldehyde is formed in concert with NO oxidation to NO2 in the plasma stage. The limited NOx reduction efficiency attained in a single step, even with excess energy, oxygen content and/or hydrocarbon-to-NOx ratio is well explained by this hypothesis, as is the effectiveness of the multiple-step treatment strategy. We present the data here illustrating the advantage of this approach under a wide variety of conditions. © 2002 Elsevier Science B.V. All rights reserved. Keywords: NOx reduction; Non-thermal plasma; Plasma catalysis; Synthetic diesel exhaust; Lean burn; Zeolite catalysts
1. Introduction The goal of reducing carbon dioxide emissions from mobile sources has led to an interest in introducing light duty diesel vehicles into the US fleet. However, the overwhelming excess of oxygen in diesel exhaust results in a difficult challenge for meeting stringent NOx emissions standards. In the near future these standards will require diesel aftertreatment ∗ Corresponding author. E-mail address: [email protected] (R.G. Tonkyn).
to reduce NOx in exhaust by 90% or more over the FTP cycle. At the present time there is no demonstrated technology capable of satisfactorily reaching this standard under the extremely lean (i.e. oxygen rich) conditions found in diesel exhaust. Several treatment strategies are under active consideration, including NOx absorbers with periodic rich excursions [1,2], selective catalytic reduction utilizing added urea [3,4], lean NOx catalysis with added hydrocarbon reductant [5–7], and non-thermal plasma-catalysis, also requiring added hydrocarbon reductant [8–11]. Each of these technologies has advantages and disadvantages, which
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have been discussed elsewhere [12]. In our group, we have been investigating the feasibility of plasma catalysis for light duty diesel exhaust NOx reduction. Non-thermal plasma-catalysis treatment of a gas stream consists of the sequential (or possibly simultaneous) action of a non-thermal plasma and heterogeneous catalysis. The technique is useful only insofar as the non-thermal plasma initiates unique chemistry required by the heterogeneous catalysis stage. The most common reported strategy for lean NOx treatment is to separate the plasma from the catalyst, as it is difficult to create a robust plasma inside of a catalyst bed. Fortunately, the active gas phase species important for optimum heterogeneous catalysis are long-lived. Both the additional reductant and the electrical energy required will impact the overall fuel efficiency of a vehicle. In general, our target is to achieve 90% NOx conversion with no more than a 5% fuel penalty. In practical terms, this means the deposited plasma energy density needs to be less than 15 J/l or so, depending somewhat on the quantity of reductant required [13]. A non-thermal plasma, in which the electrons are heated far beyond the gas temperature, can be created in a number of ways, including barrier discharge, corona discharge and pulsed high-voltage discharge. The gas phase chemistry is initiated by fast electrons, which collide with the background gas creating active radicals and ions. In our group, we typically use a double dielectric barrier discharge driven by high ac voltage, but other sources of plasma would produce very similar chemistry [14,15]. The broad outline of how the plasma-catalysis treatment of NOx works is reasonably well understood [8,16]. Under very lean conditions, the plasma promotes oxidaton. The observed chemistry includes the conversion of NO to NO2 as well as the partial oxidation of the added hydrocarbon reductant. This result has been theoretically predicted [17–19], and experimentally confirmed [4,18,20–22]. When the resulting gas mixture is passed over barium zeolite Y, NO2 is reduced to a mixture of N2 , N2 O, HCN and NO. Small amounts of nitrogen containing acids formed in the plasma do not exit the catalyst. Since much of the added hydrocarbon survives both the plasma and catalyst, the use of a clean-up oxidation catalyst will be necessary. Fortunately, hydrogen cyanide does not survive a platinum-alumina oxidation cata-
lyst, being apparently converted to either N2 O or N2 [23]. Plasma catalysis has achieved fairly high conversion of NOx in synthetic diesel exhaust over a broad temperature range. Sodium and barium zeolite Y (Na-Y and Ba-Y) show good conversion from 150 to 300 ◦ C, while gamma alumina is an effective catalyst above 300 ◦ C [22,24]. Panov et al. [24] demonstrated that a mechanical mixture of the two converted greater than 60% of the input NOx between 150 and 500 ◦ C. Nevertheless, the 90% conversion goal has remained elusive, even in synthetic exhaust. Under a variety of conditions, we find that the NOx conversion exponentially approaches an upper limit, which is well below 90%. The same functional form describes the dependence of the percent NOx conversion on energy input into the plasma, the oxygen concentration and the propene concentration. These results parallel the dependence of the NO to NO2 conversion in the plasma. This fact, along with the observation that many lean NOx catalysts reduce NO2 but not NO, suggested that the main purpose of the plasma was to convert NO to NO2 , which was then reduced on the catalyst. However, comparisons of the thermal and plasma activated reactivity of NO2 over Na zeolite Y have established that the plasma driven chemistry also produces a significantly more active reductant than propene [22]. Recent experiments have identified that active species as acetaldehyde, which is a major product in the plasma-initiated oxidation of propene [25,26]. When either Na-Y or Ba-Y catalysts are used, most of the unconverted NOx is released as NO. This is observed even at plasma energies known to oxidize all of the NO to NO2 upstream of the catalyst. At the same time, the plasma is only weakly oxidizing towards hydrocarbons. Within our typical operating temperature range (∼150–300 C), our catalysts are also very weakly oxidizing towards hydrocarbons. As a result, much of the added hydrocarbon reductant survives plasma-catalyst treatment intact. We find that the ratio of hydrocarbon reductant to NOx exiting the catalyst is typically similar to, or even greater than that entering the system, although both quantities are reduced in absolute concentration. Given this fact, it is not surprising that the application of a second plasma-catalyst treatment can improve the overall NOx conversion [27,28].
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In this paper, we demonstrate that this technique is effective over a wide range of temperatures and reductant concentrations. By using a second plasma region, one can reach higher final NOx conversion than is possible with a single plasma. Alternatively, one can reach the same percent conversion utilizing less energy or less hydrocarbon. We will present evidence that the conversion of NO to NO2 is vital to the plasma-catalysis reduction of NOx by propene, in part because it occurs during the production of acetaldehyde in the plasma. The limits on the maximum NOx conversion as a function of energy or hydrocarbon concentration are a natural consequence of the stoichiometry observed for aldehyde production in the plasma. A second stage of plasma is useful because it replenishes the active reductant supply, using the same NO oxidation chemistry as the first stage.
2. Experimental For the experiments reported here, our simplified simulated exhaust mixture contained 6% oxygen, 2% water, 0 or 200 ppm nitric oxide, and between 0 and 600 ppm propene.1 The balance was made up by nitrogen, which was adjusted to maintain the overall flow rate at 2 standard liters per minute (slm). The mixture was prepared from compressed gas cylinders using mass flow controllers. We left out CO and CO2 so that we could more easily observe hydrocarbon oxidation as well as look for a carbon balance. Experiments in our laboratory have determined that similar NOx conversion results are obtained with and without CO and CO2 [29]. In order to add water, the nitrogen plus oxygen flow was bubbled through a column of water held at room temperature. This limited the water concentration to approximately 2%, but allowed us to operate without heated lines. Experiments on a separate test bench at PNNL operated with variable water content, with heated lines and a heated NOx analyzer have shown that no change in the overall NOx conversion is observed between 2 and 7% water content. 1 Propene is a common test reductant utilized in lean NO catalx ysis studies, although it is not a practical on-board hydrocarbon. Preliminary work utilizing diesel fuel reductant suggests that the overall NOx conversion behavior is comparable. More work on realistic hydrocarbon reductant is planned.
Fig. 1. Schematic of a simple tube array reactor. The alumina tubes have one end sealed. Ground and high voltage tubes are oriented in opposite directions.
The plasma reactors consisted of two sets of electrodes suspended transverse to the gas flow. A sketch is shown in Fig. 1. The electrodes were simply metal rods placed inside of single ended alumina tubes, which were held in place and sealed from atmosphere with high temperature silicone sealant. The two sets of electrodes were oriented in opposite directions to keep the input power lines well separated. One set of electrodes was connected to the high voltage supply while the other set was held at ground potential. The electrode grid was designed such that four ground electrodes surrounded each high-voltage electrode. Gas was forced to travel between the high-voltage and ground electrodes by locating the outermost ground electrodes against the wall of the reactor. This geometry ensured that all flow paths traversed two regions of plasma. We call this type of double dielectric barrier reactor a tube array reactor, or TAR. The first TAR had three high-voltage electrodes surrounded by eight ground electrodes while the second TAR had two high-voltage electrodes surrounded by six ground electrodes. Other dimensions were similar, including a gap between dielectrics of 2 mm. The TARs were driven by sinusoidal high voltage at 400 Hz. Voltages of up to 7 kV rms were generated from a small
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Fig. 2. Schematic of our two stage cascade reactor. The two plasma devices are controlled by independent power supplies (HV). The power delivered is measured independently by comparing the high voltage in to the current in the ground leg of the circuit. Bypass valves were used to sample the input gas, as well as for studying the plasma chemistry alone.
signal generator by combining audio amplifiers with high-voltage transformers. The input power was varied by adjusting the voltage, and was measured by a capacitive circuit that has been previously described [30,31]. For the comparison of NOx conversion by single versus double plasma-catalyst treatment the gas was passed through two plasma-catalyst reactors in series. A simple schematic is shown in Fig. 2. Data were taken with the second plasma voltage either off or on. The flow through the catalyst beds was unchanged. In order to measure plasma chemistry alone, the first reactor was bypassed entirely, as was the second catalyst stage. The plasma and catalyst stages were separately heated. Because of limitations imposed by their size and shape as well as the tendency of high voltage to arc to heater wire, the TARs were heated only to between 100 and 115 ◦ C. This was done by heating the input gas and carefully insulating the reactors. The catalyst sections were placed inside of furnaces and heated from 170 to 290 ◦ C. Barium Y zeolite was prepared from sodium Y (obtained in extrudate form from Zeolyst) by solution ion exchange with Ba(NO3 )2 . Details of the process have
been published [25]. Each catalyst bed held 10 cm3 of extrudates. At a flow of 2 slm, the space velocity was 12,000 h−1 over each bed, or 6000 h−1 overall. Although this is quite low, we wanted to make sure that most of the unconverted NOx was released as NO. Our previous experience suggests that at low space velocity the output NOx is predominantly NO. Although we did not sample the NOx after the first plasma-catalyst reactor we did find the output of the second to be predominantly NO in almost every case. The only exception was at low (e.g. 3:1) hydrocarbon-to-NOx ratio, where the percentage of NO2 in the output NOx reached as high as 50%. The NOx conversion data was obtained using either a chemiluminescent NOx analyzer (California Analytical model 400-HCLD) or an FTIR (Nicolet Magna-IR 560) equipped with a 2 m gas cell and operated at 0.5 cm−1 resolution. We found good agreement between the NOx analyzer and FTIR for NO and NO2 measurements. The FTIR was also used to monitor hydrogen cyanide, nitrous oxide, carbon monoxide, carbon dioxide, methanol, formaldehyde, acetaldehyde and propene. Calibration spectra either taken here or supplied by Nicolet with the instrument were used for all of the above except hydrogen cyanide. For that compound we used a calibration curve based on a series of spectra taken at the same temperature, path length and resolution at General Motors [26].
3. Results and discussion A plot of the percent NOx conversion versus plasma energy is typically similar in form to the solid trace in Fig. 3. This form, which can be fit analytically by assuming an exponential approach to a limiting value, has also been observed for the percent NOx conversion dependence on oxygen and propene concentration [20]. We have seen this behavior not only for Na-Y and Ba-Y, but for a host of less efficient catalysts as well. While we continue to search for more efficient catalysts, the implication is that there is an upper bound to the possible NOx conversion regardless of the energy input or gas composition. Unfortunately, for all the materials we have tested, that limit is below our 90% target. The other traces in Fig. 3 illustrate what happens if a second plasma stage is added between the catalyst beds. Note that the data are plotted versus
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Fig. 3. Comparison of the percentage NOx conversion vs. total energy deposited by one (solid line) or two (dashed lines) plasmas. Gas composition was 6% O2 , 2% H2 O, 600 ppm C3 H6 , 200 ppm NO with balance N2 . The flow rate was 2 slm, and the catalysts were held at 170 ◦ C.
total energy deposited, whether by one or two plasma regions. The target of 90% is shown for comparison, as is our nominal goal of 15 J/l deposited plasma energy. For the two-plasma data, the power into the first plasma was fixed while the power into the second was varied. At moderate to high power into the first plasma, the additional plasma stage enhances the overall NOx removal performance. In Fig. 3, the two-plasma data was also fit by an exponential approach to a final value. The only adjustment required was to allow a non-zero intercept. This result is consistent with our expectation that the chemistry in the second reactor should be very similar to that in the first one. At relatively high power levels, the use of a second plasma reactor improves the overall NOx performance. However, at low power the use of a second plasma is counterproductive. This can be seen by comparing the solid and dashed line in Fig. 3 for the case where the first plasma was set to deliver only 3 J/l. Under these conditions, using the second plasma is less effective than increasing the power to the first. Presumably, this results from underutilization of the catalyst. The limit of zero power into the first plasma and all the power into the second one is equivalent to simply doubling the space velocity, and would therefore be expected to lower the NOx conversion. Above 10 J/l or so, de-
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Fig. 4. NOx conversion vs. catalyst temperature and C1:NO ratio for single and double plasma-catalyst treatment. Open symbols represent data with two plasma regions. Closed symbols indicate single plasma data.
livering extra electrical power is best done using the second plasma. Judging from Fig. 3, it seems possible that 90% conversion could be reached by a third plasma-catalysis reactor. In order to confirm the general utility of the twoplasma strategy, we tested it under a variety of conditions. Using Fig. 3 as a guide, we chose to operate at high powers into the both plasmas. The data shown in Fig. 4 illustrate that the two-step treatment of NOx can lead to significantly enhanced removal over a wide range of temperature and hydrocarbon concentrations. The data were taken in a single experiment that ran continuously for 7 days. The total plasma power was held at 31 J/l, either using one or both the plasma reactors. When both were used, approximately half the power was delivered by each. The temperature and the hydrocarbon concentration were changed as necessary to fill out the matrix of data shown in Fig. 4. For each of the sets of data the NOx conversion versus temperature is similar in form to previously reported temperature profiles for plasma-catalyst NOx conversion by Ba-Y [25]. The efficiency drops below 200 ◦ C, and starts to fall off slowly above 250 ◦ C or so. As expected, a higher concentration of hydrocarbon improves NOx removal at all temperatures, for either single or double plasma treatment. The improvement is greatest between 3:1 and 6:1 C1:NOx ratios. A further increase is possible by adding more hydrocarbon, but
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consistent with earlier reported results [22], the data suggest that increasing the hydrocarbon concentration beyond 9:1 C1:NOx will only incrementally improve the NOx conversion. As can be seen in Fig. 4, using two plasmas is advantageous between 170 and 290 ◦ C, particularly at higher propene concentrations. For 6:1 and 9:1 C1:NO ratios, the increase in the conversion was roughly 10% across the board. At 3:1 C1:NO, the overall NOx conversion efficiency is down considerably, and the improvement is less obvious. Interestingly, the results at 6:1 C1:NO indicate that the addition of a second plasma region is more effective than the introduction of another NOx -equivalent of propene. Both, hydrocarbon use and electrical energy use, impact the vehicle fuel consumption, so this could turn out to be an important tradeoff. For these experiments, no detection of nitrogen was possible due to the huge concentration in the bath gas. Previous experiments with helium as the bath gas and Na-Y catalyst have shown that N2 is formed [22,29]. In those experiments, more N2 than N2 O was formed, but combined they added up to only 70–80% of the missing NOx . We now know that hydrogen cyanide, which was undetected, would have accounted for most, if not all, of the missing nitrogen. Figs. 5 and 6 trace the effect of temperature, hydrocarbon concentration and number of plasma stages on the N2 O and outlet HCN concentrations. In the
Fig. 5. Change in the output N2 O concentration vs. catalyst temperature for single and double plasma treatment at 31 J/l deposited plasma energy.
Fig. 6. Change in the output HCN concentration vs. catalyst temperature for single and double plasma treatment at 31 J/l deposited plasma energy.
figures, solid lines through open symbols represent two-plasma data. Dashed lines through closed symbols represent single TAR data. We have left out the 3:1 C1:NOx data for clarity. At higher catalyst temperatures, the system naturally produced more HCN and less N2 O. The tradeoff appears to be close to quantitative, implying that the N2 production was relatively constant. Note that although the 10% increase in efficiency seen in Fig. 4 is a significant fraction of the distance towards our target of 90% conversion, in absolute numbers it amounts to only 20 ppm NOx lost. Due to the small absolute difference observed, it is difficult to say with confidence where the NOx went. However, only a small increase in HCN concentration can be traced to the addition of the second plasma. The effect of adding the second plasma on N2 O is not so clear. At 9:1 C1:NOx there is a consistent increase in N2 O with the addition of a second plasma. At 6:1, there is a consistent decrease in N2 O out. All we can say is that the additional NOx conversion promoted by the second plasma results in a mix of HCN, N2 O and probably N2 , which is likely very similar to the single plasma case. Separate experiments in helium will be necessary to confirm the production of nitrogen. Note that for these, it may also be necessary to increase the NOx concentration in order to increase the absolute values of the concentration changes. In principle, the plasma chemistry of N2 O and HCN could be useful. Neither compound is desirable, and if they were re-oxidized back to some form of NOx
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by the second plasma we would have another chance at reducing them to N2 . However, experiments that included sampling before and after the second plasma found no detectable change in either concentration at 24 J/l plasma energy. Both compounds are apparently quite inert in an atmospheric plasma. The presence of hydrogen cyanide is problematic, as it is clearly not a desirable exhaust gas species. Work carried out at Ford on the plasma-catalysis treatment of synthetic exhaust using Na-Y has shown that hydrogen cyanide will not escape a platinum (on alumina) oxidation catalyst. For these experiments, the gas mix contained 235 ppm NO, 530 ppm propene, 6% water, and 8% oxygen. The catalysts were held at 180 ◦ C. Complete experimental details have been published elsewhere [23]. Data in Table 1 show the concentrations of various nitrogen containing species at the input, after the plasma, after the Na-Y catalyst bed and after the oxidation catalyst. The calculated N2 concentration reflects the quantity of undetected nitrogen. The plasma oxidation of NO is apparent, as is the production of HCN over the catalyst. Both, the measured N2 O and calculated N2 concentrations, increased after passing through the oxidation catalyst, while HCN went to zero. Both, modeling [19,32] and experimental [21,25,26] work, indicate that the oxidation of propene in a dielectric barrier discharge produces relatively large amounts of formaldehyde and acetaldehyde. Only acetaldehyde appears to react with NOx on Ba-Y, and in fact two recent papers have identified acetaldehyde as an important reductant for NOx over both Na-Y [26] and Ba-Y [25]. In thermal catalysis experiments on Ba-Y, NO and NO2 reacted quite similarly with acetaldehyde at 250 ◦ C, with increasing conversion Table 1 Concentrations of nitrogen species as detected by FTIR for plasma catalysis over Na-Y
Input Plasma Na-Y Pt
NO (ppm)
NO2 (ppm)
NOx (ppm)
N2 O (ppm)
HCN (ppm)
235 0 104 41
6 221 24 16
241 221 128 57
0 5 10 45
0 2 18 0
N2 (ppm) 0 −1 38 47
The C1:NOx ratio was ∼7:1. The first column indicates the sampling port. The last column is calculated by mass balance. Note that 10 ppm CH3 ONO2 is observed after the plasma.
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up to a limit of ∼80% at high acetaldehyde concentration. The temperature dependence of the reactivity of 200 ppm NOx with 500 ppm acetaldehyde was also reported, and is very similar to the temperature dependence indicated in Fig. 3. When we decided to try a second plasma region, we believed that the conversion of NO to NO2 was required for successful reduction on the catalyst. In that context, the second plasma was a logical step because of the re-appearance of NO over Ba-Y. To the extent that the thermal reactivity of NO2 with propene contributes to the overall NOx conversion, our initial premise makes good sense [22,25]. However, in view of the results mentioned above, we need to re-examine the basic process. A large fraction of the observed NOx conversion in plasma catalysis is apparently due to the production of acetaldehyde in the plasma. Since, as has been reported, acetaldehyde reacts equally well with NO or NO2 on Ba-Y [25], is the re-oxidation of NO to NO2 crucial? Furthermore, why is there such a similarity between the NOx conversion chemistry and the NO oxidation chemistry in the plasma? We believe both questions can be answered as follows. The NO to NO2 conversion is important for that fraction of NOx conversion due to NO2 reactions with propene. Nitrogen balance experiments have confirmed the production of N2 and suggest as much as 25% NOx conversion is due to these reactions [22]. However, the majority of the NOx conversion observed (with Na-Y and Ba-Y and propene reductant) is due to reactions of NO and/or NO2 with acetaldehyde. We believe the plasma oxidation of NO to NO2 is equally important for these reactions because it is directly coupled to acetaldehyde formation. The plasma chemistry model of Dorai and Kushner directly supports this proposition [19], as does the work of Penetrante et al. [17]. Dorai and Kushner performed extensive kinetics calculations on a mixture containing 8% O2 , 6% H2 O, 260 ppm NO, and propene as the hydrocarbon reductant. They also added CO2 , CO and H2 , but these have little bearing on the relevant results. In their model, they predict that NO oxidation occurs mainly after OH attack on propene, through two separate pathways. Interestingly, both reaction pathways proceed through peroxides and eventually result in the production of one molecule of formaldehyde, one molecule of acetaldehyde, and one molecule of HO2 . The net
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reaction along either reaction path is OH + C3 H6 + 2O2 + NO → CH2 O + CH3 CHO + HO2 + NO2
(1)
Another molecule of NO will readily be oxidized by HO2 , leaving behind OH. HO2 + NO → NO2 + OH
(2)
The reaction scheme can be summarized by adding Eqs. (1) and (2). C3 H6 + 2O2 + 2NO → CH2 O + CH3 CHO + 2NO2 (3) It is the catalytic reactivity of OH that accounts for the remarkably low energy required for NO oxidation in the plasma [17]. A third route to NO oxidation also exists, namely reaction of NO with O3 to form NO2 and O2 . This route is minimized in the presence of propene, but it will oxidize NO without producing acetaldehyde. In the same model, direct oxygen attack on propene occurs, but does not lead to the formation of acetaldehyde or the production of NO2 . However, this step may still be a crucial source of the initial OH. According to the model, some propionaldehyde could be produced by O atom reaction, but experiments have shown that very little is actually formed [8]. The usefulness of the second plasma is seen to be two-fold. First, it does reconvert NO to NO2 , permitting further reduction by propene. More importantly though, a second plasma replenishes the acetaldehyde concentration in the second catalyst stage. The fact that NO is released by the catalyst is vital because it leads to facile production of acetaldehyde in the second plasma stage. The overall similarity between the NO oxidation and NOx conversion is actually a consequence of the fact that NO oxidation is required to produce acetaldehyde. In this model, the poor performance of our system at low hydrocarbon concentration suggests low acetaldehyde production in the plasma. In Fig. 3, the plasma energy was high enough that even at the lowest hydrocarbon concentration used the NOx out of the plasma consisted of NO2 only. As we will show below, at low propene concentration much of the NO oxidation proceeds by another mechanism, most likely through O3 .
Acetaldehyde production was therefore limited, which significantly reduced the NOx conversion. The ubiquitous limiting behavior of the NOx conversion (well below 100%) can also be explained by this hypothesis. In the ideal of Eq. (3), only one molecule of acetaldehyde is produced for every two NO oxidations. On the catalyst, we require two NOx molecules per acetaldehyde in order to produce N2 (or N2 O), meaning that if every NO oxidation occurred by this path, and if no other acetaldehyde production occurred, there would be only exactly enough acetaldehyde formed. Unfortunately, a significant fraction of the heterogeneous reduction of NOx converts NO2 to NO. These reductions presumably consume acetaldehyde while simply converting one form of NOx to another. Although some surplus acetaldehyde (and/or propionaldehyde) could be produced without NO oxidation, it is offset by the oxidation of NO by O3 . Our hypothesis is that under any conditions, both the limited production of acetaldehyde and the partial reduction back to NO lead to maximum NOx conversions well below 100%. Given this limitation, in order to increase the NOx conversion we need to find another viable reductant or a means to increase the acetaldehyde production. The only realistic alternative candidate for reductant among those present after plasma treatment is unreacted propene. Propene does have some NOx conversion activity over Ba-Y with NO2 . However, this reaction does not occur at all for NO. The net release of NOx as NO from our catalysts means that this pathway will not result in high net NOx conversion. The other possibility is to increase the acetaldehyde production, which is precisely what we believe the second plasma stage does. We have examined the plasma chemistry, looking for evidence that the production of acetaldehyde is dependent on NO oxidation in the plasma. Fig. 7 shows the NO to NO2 conversion as a function of energy for gas mixtures initially containing 200 ppm of NO with from 0 to 600 ppm of propene added. The other components were identical to those used for the experiments illustrated in Figs. 3–5. In agreement with earlier theoretical and experimental results [20,33], the energetic requirements for NO oxidation depend strongly on the hydrocarbon concentration. This is expected from the model due to the catalytic behavior of OH in the NO oxidation. As mentioned earlier, for a
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Table 2 Comparison of the total propene oxidized at 15 J/l plasma energy, with and without NO present Input C3 H6 (ppm)
600 400 200 100
Propene oxidized (ppm) 200 ppm NO
0 ppm NO
112 88 56 31
50 45 28 24
C1:NOx ratio of 3:1 (200 ppm C3 H6 ), the oxidation of NO is complete at 32 J/l. Therefore, the concentration of NO2 entering the catalyst for the single plasma experiments illustrated in Fig. 3 was identical at all three hydrocarbon concentrations. Any variation in the subsequent NOx reactivity was due to other factors. Fig. 8 shows similar data for the destruction of propene versus energy with and without NO present. Only the 600 ppm propene data are shown, but the trend is similar at all concentrations, as shown in Table 2. Of course, the rate of loss of propene depends
directly on the initial propene concentration. However, in every case the presence of NO enhanced the propene oxidation. This is consistent with Eqs. (1)–(3). When NO is present, HO2 oxidation recycles OH for repeated attacks on propene. At high energy, the oxidation of propene slows down as NO is depleted through oxidation to NO2 . Fig. 9 shows the correlation between the amount of NO2 formed and the amount of propene oxidized for four hydrocarbon to NOx ratios. Plotting the data this way removes any effect of the change in the energy required at the various ratios and highlights the reaction stoichiometry. According to Eq. (3), each OH attack on propene should produce two NO oxidations—one by direct reaction with an organo-peroxide and one by reaction with the resulting breakdown product, HO2 . At higher propene concentrations, the data follow this prediction remarkably well. The data for 400 and
Fig. 8. Effect of NO on the rate of propene oxidation in the plasma. At 15 J/l, the NO is completely consumed (see Fig. 5).
Fig. 9. Production of NO2 at various initial propene concentrations vs. propene oxidized. The ratios correspond to 100, 200, 400 and 600 ppm propene with 200 ppm NO.
Fig. 7. Effect of propene on the energy required to oxidize NO.
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600 ppm propene are very similar, with initial slopes of 2.1 and 2.2 NO2 produced per C3 H6 oxidized, respectively. At 200 ppm propene, slightly more NO2 is formed per C3 H6 oxidation, with an initial slope of 2.5. At 100 ppm, the initial slope is even higher at 2.7. This is consistent with the oxidation of NO by O3 becoming increasingly important at low hydrocarbon concentration. The ozone reaction probably also explains why the initial NO oxidation rate was always higher than 2. All of the data sets deviate from a straight line, tending towards less NO2 produced per propene oxidation. This is a result of the fact that the NO is depleted at twice the rate of propene. As the NO disappears, other propene oxidation routes become more likely. The type of curvature seen means that the alternative reactions which oxidize propene without NO (e.g. O+C3 H6 → products) eventually become faster than the alternative reactions that oxidize NO without propene (e.g. O3 + NO → NO2 + O2 ). In Fig. 10, we show the acetaldehyde concentration exiting the plasma under various conditions. Note that acetaldehyde is formed in the plasma without NO present. However, adding NO significantly increases the amount formed while at the same time decreasing the energy required. The increase was much greater at high propene concentration. Without NO present,
Fig. 10. Effect of NO on the production of acetaldehyde in the plasma.
increasing the propene concentration had a minor effect on the final acetaldehyde concentration. With this result, we can easily explain the large difference in the single plasma results for 3:1 and 9:1 HC:NOx ratios. Although the NO2 concentrations entering the catalyst were identical, the acetaldehyde concentration was much higher in the latter case.
4. Conclusions In the plasma-catalysis treatment of synthetic diesel exhaust, the “discharge” portion of the reaction can be thought of as a substitute for a highly specific catalyst, in that it selectively converts NO to NO2 and partially oxidizes propene to acetaldehyde. Both products appear to be active species in the subsequent SCR activity of Na-Y and Ba-Y zeolite. While a single application of plasma-catalyst treatment is reasonably effective at NOx conversion, the results inevitably fall short of the desired 90% conversion limit. The fact that some of the heterogeneous reactions on Ba-Y return unconverted NOx mainly as NO, and that most of the propene reductant survives both the plasma and catalyst suggests that further plasma-catalyst treatment could be beneficial. We have shown that multiple plasma regions can lead to higher NOx conversion efficiencies under a wide range of conditions. Not only does a second plasma stage result in higher maximum NOx conversion, but it can also produce energy savings, either by reducing the plasma energy required or by reducing the amount of hydrocarbon required. These results were obtained with propene as the reductant hydrocarbon, and we must be cautious in inferring behavior for other hydrocarbons. However, preliminary data give us reasons to be hopeful. The increased complexity of the reactor design is more than offset by the increased NOx conversion and reduced energy requirements. Further, as discussed elsewhere, the cascade reactor is more robust than a single plasma-catalyst reactor [27]. Our data support a model that links the plasma oxidation of NO to NO2 to the production of acetaldehyde from propene. Measurements of the acetaldehyde production in the plasma confirm the importance of NO in this regard. We suggest that the limited NOx conversion of single-step plasma catalysis is due in part to the limited acetaldehyde produced for a given
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amount of NO in the exhaust. The cascade reactor utilizes subsequent plasma stages to replenish the acetaldehyde while reconverting NO to NO2 . This model is consistent with earlier speculation that the importance of the plasma step is to oxidize NO to NO2 . Acknowledgements This work was performed under a CRADA with the Low Emission Technologies Research and Development Partnership. The authors gratefully acknowledge the DOE Energy Efficiency Office of Advanced Automotive Technology and Laboratory Technology Research for support of this program. Much of the research described in this paper was performed at the W.R Wiley Environmental Molecular Sciences Laboratory, a national scientific facility sponsored by the DOE Office of Biological and Environmental Research and located at PNNL. A portion of the research was carried out at Ford Research Laboratory. The Pacific Northwest National Laboratory is operated for the US DOE by Battelle Memorial Institute under contract no. DE-AC0676RLO1831. References [1] H. Mahzoul, J.F. Brilhac, P. Gilot, Appl. Catal. B: Environ. 20 (1999) 47. [2] T. Nakatsuji, R. Yasukawa, K. Tabata, K. Ueda, M. Niwa, Appl. Catal. B: Environ. 21 (1999) 121. [3] M. Koebel, M. Elsener, M. Kleeman, Catal. Today 59 (2000) 335. [4] S. Broer, T. Hammer, Appl. Catal. B: Environ. 28 (2000) 101. [5] P. Ciambilli, P. Corbo, F. Migliardini, Catal. Today 59 (2000) 279. [6] J.M. Garcia-Cortes, M.J. Illan-Gomez, A.L. Solano, C.S.-M.d. Lecea, Appl. Catal. B: Environ. 25 (2000) 39. [7] S. Kameoka, Y. Ukisu, T. Miyadera, Phys. Chem. Chem. Phys. 2 (2000) 367. [8] J.W. Hoard, SAE Technical Paper, 2001-01-0185, 2001. [9] M.L. Balmer, G.B. Fisher, J.W. Hoard (Eds.), Plasma Exhaust After Treatment, Vol. SP-1395, Society of Automotive Engineers, 1998. [10] M.L. Balmer, G.B. Fisher, J.W. Hoard (Eds.), Non-Thermal Plasma for Exhaust Emission Control, Vol. SP-1483, Society of Automotive Engineers, 1999.
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[11] M.L. Balmer, G.B. Fisher, J.W. Hoard (Eds.), Non-Thermal Plasma, Vol. SP-1566, Society of Automotive Engineers, 2000. [12] T.V. Johnson, SAE Technical Paper, 2001-01-0184, 2001. [13] J.W. Hoard, P. Laing, M.L. Balmer, R.G. Tonkyn, SAE Techical Paper, 2000-01-2895, 2000. [14] B.M. Penetrante, M.C. Hsiao, B.T. Merritt, G.E. Vogtlin, P.H. Wallman, M. Neiger, O. Wolf, T. Hammer, S. Broer, Appl. Phys. Lett. 68 (1996) 3719. [15] C.R. McClarnon, B.M. Penetrante, SAE Technical Paper, 982434, 1998. [16] B.M. Penetrante, R.M. Brusasco, B.T. Merritt, W.J. Pitz, G.E. Vogtlin, M.C. Kung, H.H. Kung, C.Z. Wan, K.E. Voss, in: Proceedings of the 1998 DOE Diesel Engine Emission Reduction Workshop, Castine, Maine, 6–9 July 1998, p. 177. [17] B.M. Penetrante, W.J. Pitz, M.C. Hsiao, B.T. Merritt, G.E. Vogtlin, in: Proceedings of the 1997 Diesel Engine Emissions Reduction Workshop, La Jolla, CA, 28–31 July 1997, p. 123. [18] B.M. Penetrante, R.M. Brusasco, B.T. Merritt, W.J. Pitz, G.E. Vogtlin, M.C. Kung, H.H. Kung, C.Z. Wan, K.E. Voss, SAE Technical Paper, 982508, 1998. [19] R. Dorai, M.J. Kushner, J. Appl. Phys. 88 (2000) 3739. [20] M.L. Balmer, R.G. Tonkyn, S. Yoon, A. Kolwaite, S.E. Barlow, G. Maupin, SAE Technical Paper, 1999-01-3640, 1999. [21] G.B. Fisher, C.L. DiMaggio, A. Yezerets, M.C. Kung, H.H. Kung, S. Baskaran, J.G. Frye, M.R. Smith, D.R. Herling, W.J. LeBarge, J. Kupe, SAE Paper 2000-01-2965, 2000. [22] R.G. Tonkyn, S. Yoon, S.E. Barlow, A.G. Panov, A. Kolwaite, M.L. Balmer, SAE Technical Paper, 2000-01-2896, 2000. [23] J.W. Hoard, A.G. Panov, SAE Technical Paper, 2001-01-3512, 2001. [24] A.G. Panov, R.G. Tonkyn, S. Yoon, A. Kolwaite, S.E. Barlow, M.L. Balmer, SAE Technical Paper, 2000-01-2964, 2000. [25] A.G. Panov, R.G. Tonkyn, M.L. Balmer, C.H.F. Peden, A. Malkin, J.W. Hoard, SAE Technical Paper, 2001-01-3513, 2001. [26] S.J. Schmieg, B.K. Cho, S.H. Oh, SAE Technical Paper, 2001-01-3565, 2001. [27] S.E. Barlow, R.G. Tonkyn, J.W. Hoard, W. Follmer, SAE Paper, 2001-01-3509, 2001. [28] R.G. Tonkyn, S.E. Barlow, SAE Technical Paper, 2001-013510, 2001. [29] M.L. Balmer, R.G. Tonkyn, A. Kim, S. Yoon, D. Jimenez, T.M. Orlando, S.E. Barlow, J.W. Hoard, SAE Technical Paper, 982511, 1998. [30] L.A. Rosenthal, D.A. Davis, IEEE Trans. Ind. Appl. IA 11 (1973) 328. [31] R.G. Tonkyn, S.E. Barlow, T.M. Orlando, J. Appl. Phys. 80 (1996) 4877. [32] H.-H. Shin, W.-S. Yoon, SAE Technical Paper, 2000-01-2969, 2000. [33] B.M. Penetrante, R.M. Brusasco, B.T. Merritt, G.E. Vogtlin, in: Proceedings of the1999 Diesel Engine Emissions Reduction Workshop, Castine, Maine, 5–9 July 1999, p. 79.
Reduction of NOx in synthetic diesel exhaust via two-step plasma-catalysis treatment R.G. Tonkyn a,∗ , S.E. Barlow a , John W. Hoard b a
b
Pacific Northwest National Laboratory, P.O. Box 999, MS K8-8, Richland, WA 99352, USA Chemical Engineering Department, Ford Motor Company Research Laboratory, 2101 Village Road, MS 3179SRL, Dearborn, MI 48121, USA Received 12 April 2002; received in revised form 20 June 2002; accepted 21 June 2002
Abstract Significant reduction of NOx in synthetic light duty diesel exhaust has been achieved over a broad temperature window by combining atmospheric plasma with appropriate catalysts. The technique relies on the addition of hydrocarbon reductant prior to passing the simulated exhaust through a non-thermal plasma and a catalyst bed. The observed chemistry in the plasma includes conversion of NO to NO2 as well as the partial oxidation of the hydrocarbon. The overall NOx reduction has a maximum of less than 80%, with this maximum obtained only at high-energy input into the plasma, high concentration of hydrocarbon reductant and low space velocity. We present data in this paper illustrating that a multiple-step treatment strategy, whereby two or more plasma-catalyst reactors are utilized in series, can increase the maximum NOx conversion obtainable. Alternatively, this technique can reduce the energy and/or hydrocarbon requirements for a fixed conversion efficiency. When propene is used as the reductant, the limiting reagent for the overall process is most likely acetaldehyde. The data suggest that acetaldehyde is formed in concert with NO oxidation to NO2 in the plasma stage. The limited NOx reduction efficiency attained in a single step, even with excess energy, oxygen content and/or hydrocarbon-to-NOx ratio is well explained by this hypothesis, as is the effectiveness of the multiple-step treatment strategy. We present the data here illustrating the advantage of this approach under a wide variety of conditions. © 2002 Elsevier Science B.V. All rights reserved. Keywords: NOx reduction; Non-thermal plasma; Plasma catalysis; Synthetic diesel exhaust; Lean burn; Zeolite catalysts
1. Introduction The goal of reducing carbon dioxide emissions from mobile sources has led to an interest in introducing light duty diesel vehicles into the US fleet. However, the overwhelming excess of oxygen in diesel exhaust results in a difficult challenge for meeting stringent NOx emissions standards. In the near future these standards will require diesel aftertreatment ∗ Corresponding author. E-mail address: [email protected] (R.G. Tonkyn).
to reduce NOx in exhaust by 90% or more over the FTP cycle. At the present time there is no demonstrated technology capable of satisfactorily reaching this standard under the extremely lean (i.e. oxygen rich) conditions found in diesel exhaust. Several treatment strategies are under active consideration, including NOx absorbers with periodic rich excursions [1,2], selective catalytic reduction utilizing added urea [3,4], lean NOx catalysis with added hydrocarbon reductant [5–7], and non-thermal plasma-catalysis, also requiring added hydrocarbon reductant [8–11]. Each of these technologies has advantages and disadvantages, which
0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 1 5 0 - 9
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have been discussed elsewhere [12]. In our group, we have been investigating the feasibility of plasma catalysis for light duty diesel exhaust NOx reduction. Non-thermal plasma-catalysis treatment of a gas stream consists of the sequential (or possibly simultaneous) action of a non-thermal plasma and heterogeneous catalysis. The technique is useful only insofar as the non-thermal plasma initiates unique chemistry required by the heterogeneous catalysis stage. The most common reported strategy for lean NOx treatment is to separate the plasma from the catalyst, as it is difficult to create a robust plasma inside of a catalyst bed. Fortunately, the active gas phase species important for optimum heterogeneous catalysis are long-lived. Both the additional reductant and the electrical energy required will impact the overall fuel efficiency of a vehicle. In general, our target is to achieve 90% NOx conversion with no more than a 5% fuel penalty. In practical terms, this means the deposited plasma energy density needs to be less than 15 J/l or so, depending somewhat on the quantity of reductant required [13]. A non-thermal plasma, in which the electrons are heated far beyond the gas temperature, can be created in a number of ways, including barrier discharge, corona discharge and pulsed high-voltage discharge. The gas phase chemistry is initiated by fast electrons, which collide with the background gas creating active radicals and ions. In our group, we typically use a double dielectric barrier discharge driven by high ac voltage, but other sources of plasma would produce very similar chemistry [14,15]. The broad outline of how the plasma-catalysis treatment of NOx works is reasonably well understood [8,16]. Under very lean conditions, the plasma promotes oxidaton. The observed chemistry includes the conversion of NO to NO2 as well as the partial oxidation of the added hydrocarbon reductant. This result has been theoretically predicted [17–19], and experimentally confirmed [4,18,20–22]. When the resulting gas mixture is passed over barium zeolite Y, NO2 is reduced to a mixture of N2 , N2 O, HCN and NO. Small amounts of nitrogen containing acids formed in the plasma do not exit the catalyst. Since much of the added hydrocarbon survives both the plasma and catalyst, the use of a clean-up oxidation catalyst will be necessary. Fortunately, hydrogen cyanide does not survive a platinum-alumina oxidation cata-
lyst, being apparently converted to either N2 O or N2 [23]. Plasma catalysis has achieved fairly high conversion of NOx in synthetic diesel exhaust over a broad temperature range. Sodium and barium zeolite Y (Na-Y and Ba-Y) show good conversion from 150 to 300 ◦ C, while gamma alumina is an effective catalyst above 300 ◦ C [22,24]. Panov et al. [24] demonstrated that a mechanical mixture of the two converted greater than 60% of the input NOx between 150 and 500 ◦ C. Nevertheless, the 90% conversion goal has remained elusive, even in synthetic exhaust. Under a variety of conditions, we find that the NOx conversion exponentially approaches an upper limit, which is well below 90%. The same functional form describes the dependence of the percent NOx conversion on energy input into the plasma, the oxygen concentration and the propene concentration. These results parallel the dependence of the NO to NO2 conversion in the plasma. This fact, along with the observation that many lean NOx catalysts reduce NO2 but not NO, suggested that the main purpose of the plasma was to convert NO to NO2 , which was then reduced on the catalyst. However, comparisons of the thermal and plasma activated reactivity of NO2 over Na zeolite Y have established that the plasma driven chemistry also produces a significantly more active reductant than propene [22]. Recent experiments have identified that active species as acetaldehyde, which is a major product in the plasma-initiated oxidation of propene [25,26]. When either Na-Y or Ba-Y catalysts are used, most of the unconverted NOx is released as NO. This is observed even at plasma energies known to oxidize all of the NO to NO2 upstream of the catalyst. At the same time, the plasma is only weakly oxidizing towards hydrocarbons. Within our typical operating temperature range (∼150–300 C), our catalysts are also very weakly oxidizing towards hydrocarbons. As a result, much of the added hydrocarbon reductant survives plasma-catalyst treatment intact. We find that the ratio of hydrocarbon reductant to NOx exiting the catalyst is typically similar to, or even greater than that entering the system, although both quantities are reduced in absolute concentration. Given this fact, it is not surprising that the application of a second plasma-catalyst treatment can improve the overall NOx conversion [27,28].
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In this paper, we demonstrate that this technique is effective over a wide range of temperatures and reductant concentrations. By using a second plasma region, one can reach higher final NOx conversion than is possible with a single plasma. Alternatively, one can reach the same percent conversion utilizing less energy or less hydrocarbon. We will present evidence that the conversion of NO to NO2 is vital to the plasma-catalysis reduction of NOx by propene, in part because it occurs during the production of acetaldehyde in the plasma. The limits on the maximum NOx conversion as a function of energy or hydrocarbon concentration are a natural consequence of the stoichiometry observed for aldehyde production in the plasma. A second stage of plasma is useful because it replenishes the active reductant supply, using the same NO oxidation chemistry as the first stage.
2. Experimental For the experiments reported here, our simplified simulated exhaust mixture contained 6% oxygen, 2% water, 0 or 200 ppm nitric oxide, and between 0 and 600 ppm propene.1 The balance was made up by nitrogen, which was adjusted to maintain the overall flow rate at 2 standard liters per minute (slm). The mixture was prepared from compressed gas cylinders using mass flow controllers. We left out CO and CO2 so that we could more easily observe hydrocarbon oxidation as well as look for a carbon balance. Experiments in our laboratory have determined that similar NOx conversion results are obtained with and without CO and CO2 [29]. In order to add water, the nitrogen plus oxygen flow was bubbled through a column of water held at room temperature. This limited the water concentration to approximately 2%, but allowed us to operate without heated lines. Experiments on a separate test bench at PNNL operated with variable water content, with heated lines and a heated NOx analyzer have shown that no change in the overall NOx conversion is observed between 2 and 7% water content. 1 Propene is a common test reductant utilized in lean NO catalx ysis studies, although it is not a practical on-board hydrocarbon. Preliminary work utilizing diesel fuel reductant suggests that the overall NOx conversion behavior is comparable. More work on realistic hydrocarbon reductant is planned.
Fig. 1. Schematic of a simple tube array reactor. The alumina tubes have one end sealed. Ground and high voltage tubes are oriented in opposite directions.
The plasma reactors consisted of two sets of electrodes suspended transverse to the gas flow. A sketch is shown in Fig. 1. The electrodes were simply metal rods placed inside of single ended alumina tubes, which were held in place and sealed from atmosphere with high temperature silicone sealant. The two sets of electrodes were oriented in opposite directions to keep the input power lines well separated. One set of electrodes was connected to the high voltage supply while the other set was held at ground potential. The electrode grid was designed such that four ground electrodes surrounded each high-voltage electrode. Gas was forced to travel between the high-voltage and ground electrodes by locating the outermost ground electrodes against the wall of the reactor. This geometry ensured that all flow paths traversed two regions of plasma. We call this type of double dielectric barrier reactor a tube array reactor, or TAR. The first TAR had three high-voltage electrodes surrounded by eight ground electrodes while the second TAR had two high-voltage electrodes surrounded by six ground electrodes. Other dimensions were similar, including a gap between dielectrics of 2 mm. The TARs were driven by sinusoidal high voltage at 400 Hz. Voltages of up to 7 kV rms were generated from a small
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Fig. 2. Schematic of our two stage cascade reactor. The two plasma devices are controlled by independent power supplies (HV). The power delivered is measured independently by comparing the high voltage in to the current in the ground leg of the circuit. Bypass valves were used to sample the input gas, as well as for studying the plasma chemistry alone.
signal generator by combining audio amplifiers with high-voltage transformers. The input power was varied by adjusting the voltage, and was measured by a capacitive circuit that has been previously described [30,31]. For the comparison of NOx conversion by single versus double plasma-catalyst treatment the gas was passed through two plasma-catalyst reactors in series. A simple schematic is shown in Fig. 2. Data were taken with the second plasma voltage either off or on. The flow through the catalyst beds was unchanged. In order to measure plasma chemistry alone, the first reactor was bypassed entirely, as was the second catalyst stage. The plasma and catalyst stages were separately heated. Because of limitations imposed by their size and shape as well as the tendency of high voltage to arc to heater wire, the TARs were heated only to between 100 and 115 ◦ C. This was done by heating the input gas and carefully insulating the reactors. The catalyst sections were placed inside of furnaces and heated from 170 to 290 ◦ C. Barium Y zeolite was prepared from sodium Y (obtained in extrudate form from Zeolyst) by solution ion exchange with Ba(NO3 )2 . Details of the process have
been published [25]. Each catalyst bed held 10 cm3 of extrudates. At a flow of 2 slm, the space velocity was 12,000 h−1 over each bed, or 6000 h−1 overall. Although this is quite low, we wanted to make sure that most of the unconverted NOx was released as NO. Our previous experience suggests that at low space velocity the output NOx is predominantly NO. Although we did not sample the NOx after the first plasma-catalyst reactor we did find the output of the second to be predominantly NO in almost every case. The only exception was at low (e.g. 3:1) hydrocarbon-to-NOx ratio, where the percentage of NO2 in the output NOx reached as high as 50%. The NOx conversion data was obtained using either a chemiluminescent NOx analyzer (California Analytical model 400-HCLD) or an FTIR (Nicolet Magna-IR 560) equipped with a 2 m gas cell and operated at 0.5 cm−1 resolution. We found good agreement between the NOx analyzer and FTIR for NO and NO2 measurements. The FTIR was also used to monitor hydrogen cyanide, nitrous oxide, carbon monoxide, carbon dioxide, methanol, formaldehyde, acetaldehyde and propene. Calibration spectra either taken here or supplied by Nicolet with the instrument were used for all of the above except hydrogen cyanide. For that compound we used a calibration curve based on a series of spectra taken at the same temperature, path length and resolution at General Motors [26].
3. Results and discussion A plot of the percent NOx conversion versus plasma energy is typically similar in form to the solid trace in Fig. 3. This form, which can be fit analytically by assuming an exponential approach to a limiting value, has also been observed for the percent NOx conversion dependence on oxygen and propene concentration [20]. We have seen this behavior not only for Na-Y and Ba-Y, but for a host of less efficient catalysts as well. While we continue to search for more efficient catalysts, the implication is that there is an upper bound to the possible NOx conversion regardless of the energy input or gas composition. Unfortunately, for all the materials we have tested, that limit is below our 90% target. The other traces in Fig. 3 illustrate what happens if a second plasma stage is added between the catalyst beds. Note that the data are plotted versus
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Fig. 3. Comparison of the percentage NOx conversion vs. total energy deposited by one (solid line) or two (dashed lines) plasmas. Gas composition was 6% O2 , 2% H2 O, 600 ppm C3 H6 , 200 ppm NO with balance N2 . The flow rate was 2 slm, and the catalysts were held at 170 ◦ C.
total energy deposited, whether by one or two plasma regions. The target of 90% is shown for comparison, as is our nominal goal of 15 J/l deposited plasma energy. For the two-plasma data, the power into the first plasma was fixed while the power into the second was varied. At moderate to high power into the first plasma, the additional plasma stage enhances the overall NOx removal performance. In Fig. 3, the two-plasma data was also fit by an exponential approach to a final value. The only adjustment required was to allow a non-zero intercept. This result is consistent with our expectation that the chemistry in the second reactor should be very similar to that in the first one. At relatively high power levels, the use of a second plasma reactor improves the overall NOx performance. However, at low power the use of a second plasma is counterproductive. This can be seen by comparing the solid and dashed line in Fig. 3 for the case where the first plasma was set to deliver only 3 J/l. Under these conditions, using the second plasma is less effective than increasing the power to the first. Presumably, this results from underutilization of the catalyst. The limit of zero power into the first plasma and all the power into the second one is equivalent to simply doubling the space velocity, and would therefore be expected to lower the NOx conversion. Above 10 J/l or so, de-
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Fig. 4. NOx conversion vs. catalyst temperature and C1:NO ratio for single and double plasma-catalyst treatment. Open symbols represent data with two plasma regions. Closed symbols indicate single plasma data.
livering extra electrical power is best done using the second plasma. Judging from Fig. 3, it seems possible that 90% conversion could be reached by a third plasma-catalysis reactor. In order to confirm the general utility of the twoplasma strategy, we tested it under a variety of conditions. Using Fig. 3 as a guide, we chose to operate at high powers into the both plasmas. The data shown in Fig. 4 illustrate that the two-step treatment of NOx can lead to significantly enhanced removal over a wide range of temperature and hydrocarbon concentrations. The data were taken in a single experiment that ran continuously for 7 days. The total plasma power was held at 31 J/l, either using one or both the plasma reactors. When both were used, approximately half the power was delivered by each. The temperature and the hydrocarbon concentration were changed as necessary to fill out the matrix of data shown in Fig. 4. For each of the sets of data the NOx conversion versus temperature is similar in form to previously reported temperature profiles for plasma-catalyst NOx conversion by Ba-Y [25]. The efficiency drops below 200 ◦ C, and starts to fall off slowly above 250 ◦ C or so. As expected, a higher concentration of hydrocarbon improves NOx removal at all temperatures, for either single or double plasma treatment. The improvement is greatest between 3:1 and 6:1 C1:NOx ratios. A further increase is possible by adding more hydrocarbon, but
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consistent with earlier reported results [22], the data suggest that increasing the hydrocarbon concentration beyond 9:1 C1:NOx will only incrementally improve the NOx conversion. As can be seen in Fig. 4, using two plasmas is advantageous between 170 and 290 ◦ C, particularly at higher propene concentrations. For 6:1 and 9:1 C1:NO ratios, the increase in the conversion was roughly 10% across the board. At 3:1 C1:NO, the overall NOx conversion efficiency is down considerably, and the improvement is less obvious. Interestingly, the results at 6:1 C1:NO indicate that the addition of a second plasma region is more effective than the introduction of another NOx -equivalent of propene. Both, hydrocarbon use and electrical energy use, impact the vehicle fuel consumption, so this could turn out to be an important tradeoff. For these experiments, no detection of nitrogen was possible due to the huge concentration in the bath gas. Previous experiments with helium as the bath gas and Na-Y catalyst have shown that N2 is formed [22,29]. In those experiments, more N2 than N2 O was formed, but combined they added up to only 70–80% of the missing NOx . We now know that hydrogen cyanide, which was undetected, would have accounted for most, if not all, of the missing nitrogen. Figs. 5 and 6 trace the effect of temperature, hydrocarbon concentration and number of plasma stages on the N2 O and outlet HCN concentrations. In the
Fig. 5. Change in the output N2 O concentration vs. catalyst temperature for single and double plasma treatment at 31 J/l deposited plasma energy.
Fig. 6. Change in the output HCN concentration vs. catalyst temperature for single and double plasma treatment at 31 J/l deposited plasma energy.
figures, solid lines through open symbols represent two-plasma data. Dashed lines through closed symbols represent single TAR data. We have left out the 3:1 C1:NOx data for clarity. At higher catalyst temperatures, the system naturally produced more HCN and less N2 O. The tradeoff appears to be close to quantitative, implying that the N2 production was relatively constant. Note that although the 10% increase in efficiency seen in Fig. 4 is a significant fraction of the distance towards our target of 90% conversion, in absolute numbers it amounts to only 20 ppm NOx lost. Due to the small absolute difference observed, it is difficult to say with confidence where the NOx went. However, only a small increase in HCN concentration can be traced to the addition of the second plasma. The effect of adding the second plasma on N2 O is not so clear. At 9:1 C1:NOx there is a consistent increase in N2 O with the addition of a second plasma. At 6:1, there is a consistent decrease in N2 O out. All we can say is that the additional NOx conversion promoted by the second plasma results in a mix of HCN, N2 O and probably N2 , which is likely very similar to the single plasma case. Separate experiments in helium will be necessary to confirm the production of nitrogen. Note that for these, it may also be necessary to increase the NOx concentration in order to increase the absolute values of the concentration changes. In principle, the plasma chemistry of N2 O and HCN could be useful. Neither compound is desirable, and if they were re-oxidized back to some form of NOx
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by the second plasma we would have another chance at reducing them to N2 . However, experiments that included sampling before and after the second plasma found no detectable change in either concentration at 24 J/l plasma energy. Both compounds are apparently quite inert in an atmospheric plasma. The presence of hydrogen cyanide is problematic, as it is clearly not a desirable exhaust gas species. Work carried out at Ford on the plasma-catalysis treatment of synthetic exhaust using Na-Y has shown that hydrogen cyanide will not escape a platinum (on alumina) oxidation catalyst. For these experiments, the gas mix contained 235 ppm NO, 530 ppm propene, 6% water, and 8% oxygen. The catalysts were held at 180 ◦ C. Complete experimental details have been published elsewhere [23]. Data in Table 1 show the concentrations of various nitrogen containing species at the input, after the plasma, after the Na-Y catalyst bed and after the oxidation catalyst. The calculated N2 concentration reflects the quantity of undetected nitrogen. The plasma oxidation of NO is apparent, as is the production of HCN over the catalyst. Both, the measured N2 O and calculated N2 concentrations, increased after passing through the oxidation catalyst, while HCN went to zero. Both, modeling [19,32] and experimental [21,25,26] work, indicate that the oxidation of propene in a dielectric barrier discharge produces relatively large amounts of formaldehyde and acetaldehyde. Only acetaldehyde appears to react with NOx on Ba-Y, and in fact two recent papers have identified acetaldehyde as an important reductant for NOx over both Na-Y [26] and Ba-Y [25]. In thermal catalysis experiments on Ba-Y, NO and NO2 reacted quite similarly with acetaldehyde at 250 ◦ C, with increasing conversion Table 1 Concentrations of nitrogen species as detected by FTIR for plasma catalysis over Na-Y
Input Plasma Na-Y Pt
NO (ppm)
NO2 (ppm)
NOx (ppm)
N2 O (ppm)
HCN (ppm)
235 0 104 41
6 221 24 16
241 221 128 57
0 5 10 45
0 2 18 0
N2 (ppm) 0 −1 38 47
The C1:NOx ratio was ∼7:1. The first column indicates the sampling port. The last column is calculated by mass balance. Note that 10 ppm CH3 ONO2 is observed after the plasma.
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up to a limit of ∼80% at high acetaldehyde concentration. The temperature dependence of the reactivity of 200 ppm NOx with 500 ppm acetaldehyde was also reported, and is very similar to the temperature dependence indicated in Fig. 3. When we decided to try a second plasma region, we believed that the conversion of NO to NO2 was required for successful reduction on the catalyst. In that context, the second plasma was a logical step because of the re-appearance of NO over Ba-Y. To the extent that the thermal reactivity of NO2 with propene contributes to the overall NOx conversion, our initial premise makes good sense [22,25]. However, in view of the results mentioned above, we need to re-examine the basic process. A large fraction of the observed NOx conversion in plasma catalysis is apparently due to the production of acetaldehyde in the plasma. Since, as has been reported, acetaldehyde reacts equally well with NO or NO2 on Ba-Y [25], is the re-oxidation of NO to NO2 crucial? Furthermore, why is there such a similarity between the NOx conversion chemistry and the NO oxidation chemistry in the plasma? We believe both questions can be answered as follows. The NO to NO2 conversion is important for that fraction of NOx conversion due to NO2 reactions with propene. Nitrogen balance experiments have confirmed the production of N2 and suggest as much as 25% NOx conversion is due to these reactions [22]. However, the majority of the NOx conversion observed (with Na-Y and Ba-Y and propene reductant) is due to reactions of NO and/or NO2 with acetaldehyde. We believe the plasma oxidation of NO to NO2 is equally important for these reactions because it is directly coupled to acetaldehyde formation. The plasma chemistry model of Dorai and Kushner directly supports this proposition [19], as does the work of Penetrante et al. [17]. Dorai and Kushner performed extensive kinetics calculations on a mixture containing 8% O2 , 6% H2 O, 260 ppm NO, and propene as the hydrocarbon reductant. They also added CO2 , CO and H2 , but these have little bearing on the relevant results. In their model, they predict that NO oxidation occurs mainly after OH attack on propene, through two separate pathways. Interestingly, both reaction pathways proceed through peroxides and eventually result in the production of one molecule of formaldehyde, one molecule of acetaldehyde, and one molecule of HO2 . The net
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reaction along either reaction path is OH + C3 H6 + 2O2 + NO → CH2 O + CH3 CHO + HO2 + NO2
(1)
Another molecule of NO will readily be oxidized by HO2 , leaving behind OH. HO2 + NO → NO2 + OH
(2)
The reaction scheme can be summarized by adding Eqs. (1) and (2). C3 H6 + 2O2 + 2NO → CH2 O + CH3 CHO + 2NO2 (3) It is the catalytic reactivity of OH that accounts for the remarkably low energy required for NO oxidation in the plasma [17]. A third route to NO oxidation also exists, namely reaction of NO with O3 to form NO2 and O2 . This route is minimized in the presence of propene, but it will oxidize NO without producing acetaldehyde. In the same model, direct oxygen attack on propene occurs, but does not lead to the formation of acetaldehyde or the production of NO2 . However, this step may still be a crucial source of the initial OH. According to the model, some propionaldehyde could be produced by O atom reaction, but experiments have shown that very little is actually formed [8]. The usefulness of the second plasma is seen to be two-fold. First, it does reconvert NO to NO2 , permitting further reduction by propene. More importantly though, a second plasma replenishes the acetaldehyde concentration in the second catalyst stage. The fact that NO is released by the catalyst is vital because it leads to facile production of acetaldehyde in the second plasma stage. The overall similarity between the NO oxidation and NOx conversion is actually a consequence of the fact that NO oxidation is required to produce acetaldehyde. In this model, the poor performance of our system at low hydrocarbon concentration suggests low acetaldehyde production in the plasma. In Fig. 3, the plasma energy was high enough that even at the lowest hydrocarbon concentration used the NOx out of the plasma consisted of NO2 only. As we will show below, at low propene concentration much of the NO oxidation proceeds by another mechanism, most likely through O3 .
Acetaldehyde production was therefore limited, which significantly reduced the NOx conversion. The ubiquitous limiting behavior of the NOx conversion (well below 100%) can also be explained by this hypothesis. In the ideal of Eq. (3), only one molecule of acetaldehyde is produced for every two NO oxidations. On the catalyst, we require two NOx molecules per acetaldehyde in order to produce N2 (or N2 O), meaning that if every NO oxidation occurred by this path, and if no other acetaldehyde production occurred, there would be only exactly enough acetaldehyde formed. Unfortunately, a significant fraction of the heterogeneous reduction of NOx converts NO2 to NO. These reductions presumably consume acetaldehyde while simply converting one form of NOx to another. Although some surplus acetaldehyde (and/or propionaldehyde) could be produced without NO oxidation, it is offset by the oxidation of NO by O3 . Our hypothesis is that under any conditions, both the limited production of acetaldehyde and the partial reduction back to NO lead to maximum NOx conversions well below 100%. Given this limitation, in order to increase the NOx conversion we need to find another viable reductant or a means to increase the acetaldehyde production. The only realistic alternative candidate for reductant among those present after plasma treatment is unreacted propene. Propene does have some NOx conversion activity over Ba-Y with NO2 . However, this reaction does not occur at all for NO. The net release of NOx as NO from our catalysts means that this pathway will not result in high net NOx conversion. The other possibility is to increase the acetaldehyde production, which is precisely what we believe the second plasma stage does. We have examined the plasma chemistry, looking for evidence that the production of acetaldehyde is dependent on NO oxidation in the plasma. Fig. 7 shows the NO to NO2 conversion as a function of energy for gas mixtures initially containing 200 ppm of NO with from 0 to 600 ppm of propene added. The other components were identical to those used for the experiments illustrated in Figs. 3–5. In agreement with earlier theoretical and experimental results [20,33], the energetic requirements for NO oxidation depend strongly on the hydrocarbon concentration. This is expected from the model due to the catalytic behavior of OH in the NO oxidation. As mentioned earlier, for a
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Table 2 Comparison of the total propene oxidized at 15 J/l plasma energy, with and without NO present Input C3 H6 (ppm)
600 400 200 100
Propene oxidized (ppm) 200 ppm NO
0 ppm NO
112 88 56 31
50 45 28 24
C1:NOx ratio of 3:1 (200 ppm C3 H6 ), the oxidation of NO is complete at 32 J/l. Therefore, the concentration of NO2 entering the catalyst for the single plasma experiments illustrated in Fig. 3 was identical at all three hydrocarbon concentrations. Any variation in the subsequent NOx reactivity was due to other factors. Fig. 8 shows similar data for the destruction of propene versus energy with and without NO present. Only the 600 ppm propene data are shown, but the trend is similar at all concentrations, as shown in Table 2. Of course, the rate of loss of propene depends
directly on the initial propene concentration. However, in every case the presence of NO enhanced the propene oxidation. This is consistent with Eqs. (1)–(3). When NO is present, HO2 oxidation recycles OH for repeated attacks on propene. At high energy, the oxidation of propene slows down as NO is depleted through oxidation to NO2 . Fig. 9 shows the correlation between the amount of NO2 formed and the amount of propene oxidized for four hydrocarbon to NOx ratios. Plotting the data this way removes any effect of the change in the energy required at the various ratios and highlights the reaction stoichiometry. According to Eq. (3), each OH attack on propene should produce two NO oxidations—one by direct reaction with an organo-peroxide and one by reaction with the resulting breakdown product, HO2 . At higher propene concentrations, the data follow this prediction remarkably well. The data for 400 and
Fig. 8. Effect of NO on the rate of propene oxidation in the plasma. At 15 J/l, the NO is completely consumed (see Fig. 5).
Fig. 9. Production of NO2 at various initial propene concentrations vs. propene oxidized. The ratios correspond to 100, 200, 400 and 600 ppm propene with 200 ppm NO.
Fig. 7. Effect of propene on the energy required to oxidize NO.
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600 ppm propene are very similar, with initial slopes of 2.1 and 2.2 NO2 produced per C3 H6 oxidized, respectively. At 200 ppm propene, slightly more NO2 is formed per C3 H6 oxidation, with an initial slope of 2.5. At 100 ppm, the initial slope is even higher at 2.7. This is consistent with the oxidation of NO by O3 becoming increasingly important at low hydrocarbon concentration. The ozone reaction probably also explains why the initial NO oxidation rate was always higher than 2. All of the data sets deviate from a straight line, tending towards less NO2 produced per propene oxidation. This is a result of the fact that the NO is depleted at twice the rate of propene. As the NO disappears, other propene oxidation routes become more likely. The type of curvature seen means that the alternative reactions which oxidize propene without NO (e.g. O+C3 H6 → products) eventually become faster than the alternative reactions that oxidize NO without propene (e.g. O3 + NO → NO2 + O2 ). In Fig. 10, we show the acetaldehyde concentration exiting the plasma under various conditions. Note that acetaldehyde is formed in the plasma without NO present. However, adding NO significantly increases the amount formed while at the same time decreasing the energy required. The increase was much greater at high propene concentration. Without NO present,
Fig. 10. Effect of NO on the production of acetaldehyde in the plasma.
increasing the propene concentration had a minor effect on the final acetaldehyde concentration. With this result, we can easily explain the large difference in the single plasma results for 3:1 and 9:1 HC:NOx ratios. Although the NO2 concentrations entering the catalyst were identical, the acetaldehyde concentration was much higher in the latter case.
4. Conclusions In the plasma-catalysis treatment of synthetic diesel exhaust, the “discharge” portion of the reaction can be thought of as a substitute for a highly specific catalyst, in that it selectively converts NO to NO2 and partially oxidizes propene to acetaldehyde. Both products appear to be active species in the subsequent SCR activity of Na-Y and Ba-Y zeolite. While a single application of plasma-catalyst treatment is reasonably effective at NOx conversion, the results inevitably fall short of the desired 90% conversion limit. The fact that some of the heterogeneous reactions on Ba-Y return unconverted NOx mainly as NO, and that most of the propene reductant survives both the plasma and catalyst suggests that further plasma-catalyst treatment could be beneficial. We have shown that multiple plasma regions can lead to higher NOx conversion efficiencies under a wide range of conditions. Not only does a second plasma stage result in higher maximum NOx conversion, but it can also produce energy savings, either by reducing the plasma energy required or by reducing the amount of hydrocarbon required. These results were obtained with propene as the reductant hydrocarbon, and we must be cautious in inferring behavior for other hydrocarbons. However, preliminary data give us reasons to be hopeful. The increased complexity of the reactor design is more than offset by the increased NOx conversion and reduced energy requirements. Further, as discussed elsewhere, the cascade reactor is more robust than a single plasma-catalyst reactor [27]. Our data support a model that links the plasma oxidation of NO to NO2 to the production of acetaldehyde from propene. Measurements of the acetaldehyde production in the plasma confirm the importance of NO in this regard. We suggest that the limited NOx conversion of single-step plasma catalysis is due in part to the limited acetaldehyde produced for a given
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amount of NO in the exhaust. The cascade reactor utilizes subsequent plasma stages to replenish the acetaldehyde while reconverting NO to NO2 . This model is consistent with earlier speculation that the importance of the plasma step is to oxidize NO to NO2 . Acknowledgements This work was performed under a CRADA with the Low Emission Technologies Research and Development Partnership. The authors gratefully acknowledge the DOE Energy Efficiency Office of Advanced Automotive Technology and Laboratory Technology Research for support of this program. Much of the research described in this paper was performed at the W.R Wiley Environmental Molecular Sciences Laboratory, a national scientific facility sponsored by the DOE Office of Biological and Environmental Research and located at PNNL. A portion of the research was carried out at Ford Research Laboratory. The Pacific Northwest National Laboratory is operated for the US DOE by Battelle Memorial Institute under contract no. DE-AC0676RLO1831. References [1] H. Mahzoul, J.F. Brilhac, P. Gilot, Appl. Catal. B: Environ. 20 (1999) 47. [2] T. Nakatsuji, R. Yasukawa, K. Tabata, K. Ueda, M. Niwa, Appl. Catal. B: Environ. 21 (1999) 121. [3] M. Koebel, M. Elsener, M. Kleeman, Catal. Today 59 (2000) 335. [4] S. Broer, T. Hammer, Appl. Catal. B: Environ. 28 (2000) 101. [5] P. Ciambilli, P. Corbo, F. Migliardini, Catal. Today 59 (2000) 279. [6] J.M. Garcia-Cortes, M.J. Illan-Gomez, A.L. Solano, C.S.-M.d. Lecea, Appl. Catal. B: Environ. 25 (2000) 39. [7] S. Kameoka, Y. Ukisu, T. Miyadera, Phys. Chem. Chem. Phys. 2 (2000) 367. [8] J.W. Hoard, SAE Technical Paper, 2001-01-0185, 2001. [9] M.L. Balmer, G.B. Fisher, J.W. Hoard (Eds.), Plasma Exhaust After Treatment, Vol. SP-1395, Society of Automotive Engineers, 1998. [10] M.L. Balmer, G.B. Fisher, J.W. Hoard (Eds.), Non-Thermal Plasma for Exhaust Emission Control, Vol. SP-1483, Society of Automotive Engineers, 1999.
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