A Study of the N2 Formation Mechanism in Carbon− N2O Reaction by

Energy & Fuels 1999, 13, 941-946

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A Study of the N2 Formation Mechanism in Carbon-N2O Reaction by Using Isotope Gases Kenshi Noda, Philippe Chambrion, Takashi Kyotani, and Akira Tomita* Institute for Chemical Reaction Science, Tohoku University, Katahira, Sendai 980-8577, Japan Received January 26, 1999. Revised Manuscript Received May 5, 1999

To clarify the N2 formation mechanism during the C-N2O reaction, the detailed nitrogen mass balance was established. It was also attempted to reveal the role of surface nitrogen species in this reaction by using isotope gases. In contrast to the case of the C-NO reaction, the accumulation of nitrogen on the carbon surface was very small during this reaction. Despite this accumulation, the trapped surface nitrogen species hardly contribute to the N2 formation process. The main reaction path for the N2 formation is the attack of N2O to carbon free site, forming N2 with no NtN splitting and leaving oxygen on the carbon surface.

Introduction N2O is a key substance that affects the depletion of the ozone layer as well as global warming as a result of greenhouse effect. Since fluidized bed coal combustion is operated at relatively low temperature, it has a demerit of producing more N2O than conventional pulverized coal combustion. Therefore, the formation and consumption of N2O during coal combustion has become an important issue to be fully understood. Although the gas phase reactions mainly determine the overall emission of N2O, it is now recognized that the heterogeneous reaction of N2O with carbon cannot be neglected. Thus, it would be necessary to understand the C-N2O reaction in more detail. One of the pioneering studies on this reaction system has been done by the group of Strickland-Constanble et al.1 They established detailed kinetics at around 300 °C and proposed the following mechanism:

C( ) + N2O f C(O) + N2

(1)

C(O) + N2O f C( ) + CO2 + N2

(2)

C(O) + CO f C( ) + CO2

(3)

Here, C( ) and C(O) denote the surface free site and surface oxygen containing complex on carbon, respectively. Since they employed low temperature for the reaction, CO2 was the main oxygen-containing product gas. They noticed the formation of stable and labile surface oxygen complexes during the reaction and also clarified the catalytic effect of mineral matter on this reaction. Another important contribution was that by Smith et al.2 Their reaction temperature was somewhat higher, 400-650 °C, and they derived essentially the same reaction mechanism as above. They examined the (1) Madley, D. G.; Strickland-Constanble, R. F. Trans. Faraday Soc. 1953, 49, 1312-1324. (2) Smith R. N.; Lesnini D.; Mooi J. J. Phys. Chem. 1957, 61, 8186.

effect of surface oxygen species on the reaction kinetics using carbon black and charcoal. They found that the effect of surface treatment on the kinetics was quite different for these two carbons. Recently, more attention has been paid to this reaction in relation to the fluidized bed combustion of coal. Many researchers have studied the formation of N2O during N-containing coal combustion,3-11 whereas rather few studies have been done on the C-N2O reaction, which is also important to understand the overall N2O emission during coal combustion. De Soete3,4 has first pointed out the importance of C-N2O heterogeneous reaction in relation to the coal combustion. He carried out detailed experiments to reveal the various aspects of this reaction system. He confirmed the abovementioned reaction scheme (eq 1) and reported that N2O was more readily reduced by carbon than NO. DeGroot and Richards12 have compared the carbon gasification reactivities against various oxidizing gases including N2O. Rodriguez-Mirasol et al.9 examined the effect of coexisting CO and O2 on this reaction. They emphasized the importance of eq 2 for regenerating active C( ) site in the low-temperature reaction and speculated the rate enhancement by CO being the result of a similar regeneration (eq 3). More recently, Teng et al.13 investigated the global kinetics on the C-N2O reaction and (3) De Soete, G. G. 23rd International Symposium on Combustion 1990, 1257-1264. (4) De Soete, G. G. 5th International Workshop on Nitrous Oxide Emissions Tsukuba, 1992, 199-222. (5) Moritomi, H.; Suzuki, Y.; Kido, N.; Ogisu, Y. Proceedings of the 1991 International Conference on Fluidized Bed Combustion ASME, 1991, 1005-1011. (6) Tullin, C. J.; Goel, S.; Morihara, A.; Sarofim, A. F.; Beer, J. M. Energy Fuels 1993, 7, 796-802. (7) Pels, J. R.; Wo´jtowicz, M. A.; Moulijn, J. A. Fuel 1993, 72, 373379. (8) Horio, M.; Mochizuki, M.; Koike, J. 4th Japan-China Symposium on Coal and C1 Chemistry Suita, 1993, 249-252. (9) Rodriguez-Mirasol, J.; Ooms, A. C.; Pels, J. R.; Kapteijn, F.; Moulijn, J. A. Combust. Flame 1994, 99, 499-507. (10) Miettinen, H. Energy Fuels 1996, 10, 197-202. (11) Miettinen, H.; Abul-Milh, M. Energy Fuels 1996, 10, 421-424. (12) DeGroot, W. F.; Richards, G. N. Carbon 1991, 29, 179-183.

10.1021/ef9900132 CCC: $18.00 © 1999 American Chemical Society Published on Web 06/16/1999

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found that the reaction mechanism is different below and above 475 °C. In our previous studies on the C-NO reaction, we tried to clarify the reaction mechanism by using isotope gases, and it was revealed that surface nitrogen and oxygen complexes play important roles in this reaction.14-17 However, for the C-N2O reaction, most researchers have accepted the reaction mechanism shown in eq 1, and no further detailed examination of reaction mechanism has been done so far. The main objective of the present study is to clarify the N2 formation mechanism by using isotope gases and by analyzing the reaction products in more detail. Special attention has been paid to the behavior of accumulated surface nitrogen species during the reaction. Experimental Section Samples. Phenol formaldehyde resin (PF) char was used throughout this study. The resin was prepared as described in the previous paper.15 Carbonization was carried out in He at 950 °C for 30 min. The particle size was less than 0.5 mm in diameter and the specific surface area determined by N2 adsorption (BET method) was 0.3 m2/g. Since the surface area of PF char develops rapidly with burnoff, it is better to activate to some extent prior to all experiments. Thus, the above green char was activated by 5% O2 at 600 °C for 30 min and heattreated again in He at 950 °C for 30 min, and then cooled to a desired temperature. This char is denoted as “heat-treated char” in this paper. Upon this treatment, the specific surface area increased to about 170 m2/g. In most cases, this sample was used for the subsequent reaction. After this activation, the variation of surface area during the N2O reaction is rather modest. In some experiments, to introduce different amounts of surface oxygen complexes, the sample was activated again with 0.1-5% O2 at 600 °C for 6-30 min. In this paper, this doubly activated char is termed as “O2-activated char”. Isotope gases, 15N2O and 18O2, were purchased from Isotec Inc. The isotopic purity was 99% for 15N and 97% for 18O. Temperature Programmed Reactions and Step Response Experiments. The reaction between PF char and N2O was carried out in a fixed bed reactor at an ambient pressure.15 The gas residence time in the sample bed was 0.05 s, while that in the hot reactor was 1.5 s. The sample was pretreated as described above and cooled to 300 °C. Then the temperatureprogrammed reaction (TPR) was carried out by heating the sample from 300 to 1000 °C at a rate of 10 °C/min. The reactant gas was 100 ppm N2O diluted with He. Step response experiment was performed at 850 °C by switching the gas feed from 15N2O to 14N2O. Gaseous products were analyzed by a mass spectrometer (MS: Anelva AQA 200) and high-speed gas chromatograph (GC: Aera M200) equipped with a MS5A column for O2, N2 and CO analysis and PPQ column for CO2 and N2O. The MS and the GC were calibrated with commercially available standard gases.

Results Reaction Profile. First, the gas-phase decomposition of N2O was examined in the absence of carbon. The (13) Teng, H.; Lin, H.-C.; Hsieh, Y.-S. Ind. Eng. Chem. Res. 1997, 36, 523-529. (14) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 33, 2840-2845. (15) Chambrion, Ph.; Orikasa, H.; Suzuki, T.; Kyotani, T.; Tomita, A. Fuel 1997, 76, 493-498. (16) Chambrion, Ph.; Kyotani, T.; Tomita, A. Energy Fuels 1998, 12, 416-421. (17) Chambrion, Ph.; Kyotani, T.; Tomita, A. Twenty-Seventh Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1998; pp 3053-3059.

Figure 1. Gas-phase decomposition of N2O upon heating to 1000 °C and the subsequent isothermal condition in the absence of carbon.

Figure 2. Product gas evolution during TPR of heat-treated PF char. N2O concentration: 96 ppm. Heating rate: 10 °C/ min.

equilibrium constant for this reaction is fairly large for the temperature range examined: 1.8 × 1018 atm at 25 °C and 2.9 × 107 atm at 927 °C. Under the present reaction conditions, the decomposition started at 800 °C as can be seen in Figure 1. When the temperature reached to 1000 °C, the temperature was kept at this temperature for 20 min. At 1000 °C, about 75% of N2O was decomposed to N2 and O2. Therefore, the contribution of this gas-phase reaction cannot be neglected at high temperatures. Figure 2 illustrates the TPR profile for the reaction of PF char with N2O. N2O consumption started at much lower temperature than in the absence of carbon. Initially the ratio of CO/CO2 was almost unity, but with increasing temperature it significantly increased and, at 1000 °C, CO became essentially the sole oxygencontaining product. The mass balance was checked at each stage of TPR. Nitrogen consumed as N2O was slightly larger than nitrogen produced as N2. Small amount of nitrogen was captured on carbon. Similarly, oxygen consumed as N2O was larger than oxygen evolved as CO and CO2, and it is clear that some oxygen was also accumulated on carbon.

N2 Formation Mechanism in C-N2O

Figure 3. Isothermal reaction of heat-treated PF char with N2O at 850 °C. N2O concentration: 250 ppm.

Figure 4. TPR of O2-activated PF char with N2O. N2O concentration: 90 ppm. Heating rate: 10 °C/min.

Isothermal reaction was carried out at 850 °C and the results are shown in Figure 3. A slight decrease in the N2O conversion rate with time was observed, and almost all of the N2O consumed was converted to N2. However, the amount of N2 produced was slightly lower than N2O consumed as in the isothermal reaction at 1000 °C (right-hand side of Figure 2). Some nitrogen might be accumulated on carbon, but the amount was too small to be accurately estimated from the above difference. This accumulation was confirmed from the experiments described below. At 850 °C, the ratio of CO/CO2 was about 10, which is much smaller than at 1000 °C. Effect of Surface Oxygen Complex and Coexisting O2. It is well-known that the presence of surface oxygen species, C(O), affects the reaction profile in many carbon gasification systems. Our previous study on the C-NO reaction also showed that the presence of C(O) enhances the rate of C-NO reaction.14,15,18 In the present study, C(O) was introduced by treating PF char with O2 at 600 °C for different periods of time. One example of TPR result is shown in Figure 4. The evolution of larger amounts of CO and CO2 in compari(18) Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Energy Fuels 1993, 7, 85-89.

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Figure 5. Effect of the amount of C(O) on the N2O consumption over PF char O2-activated at various conditions. N2O concentration: 90 ppm.

Figure 6. Evolution of various CO and CO2 during TPR of 18O -activated PF char with N O. 18O pretreatment: 600 °C, 2 2 2 30 min. N2O concentration: 90 ppm.

son with those in Figure 2 is due to the decomposition of C(O). The starting temperature of the N2O decomposition was almost the same as in Figure 2, but N2O consumption and N2 formation in the range of 600-800 °C were much larger. In the isothermal region at 1000 °C, N2O was completely converted to N2 and CO. From the mass balance calculation, it was clearly seen that more nitrogen was accumulated in this O2-activated sample compared with the heat-treated char. The effect of the extent of activation was examined, and the results are shown in Figure 5. With an increase in the amount of C(O), N2O was consumed more quickly. Thus it can be concluded that the presence of C(O) enhanced the C-N2O reaction. It is of interest to distinguish the COx formation via decomposition of C(O) and that via C-N2O reaction. The use of isotope gases makes it very easy to separate these contributions. Figure 6 shows the TPR result using PF char that had been previously treated with 18O2. The main oxygen species in the reactant N2O was 16O. Therefore, the C18O formation can be attributed to the decomposition of C(18O), while the C16O formation was due to the reaction with N216O. The evolution of both CO and CO2 started at 600 °C. In the initial stage, the

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Figure 7. N2O consumption during TPR of O2-activated PF char with a mixture of N2O and O2. N2O concentration: 9097 ppm.

main CO species was C18O, and the main CO2 species were C18O2 and C16O18O. In the isothermal reaction stage, the principal products were C16O and C16O2, both of which are the gasification products. Since the presence of C(O) accelerated the N2O consumption, the presence of O2 was expected to enhance the C-N2O reaction rate. In fact, in the case of C-NO reaction, it was found that the reaction rate increased with O2 concentration.14,15,17-19 Figure 7 shows the effect of O2 concentration on the C-N2O reaction. Contrary to the above expectation, the N2O consumption rate decreased with increasing O2 concentration. The reason for this will be discussed later. Nitrogen Accumulation on Char. In the C-NO reaction, we have found that a considerable amount of nitrogen was accumulated on carbon, and this surface nitrogen complex, C(N), plays an important role in the N2 formation during this reaction.15-17 Therefore it is of interest to investigate the nitrogen accumulation in the C-N2O system. Some nitrogen accumulation was suggested from the mass balance during TPR, but the amount was quite small and thus the reliability was not high. More clearly, the presence of accumulated nitrogen can be ascertained by gasifying the residual char. If the formation of N2 and NO was observed upon the gasification of carbon with O2, the presence of C(N) can clearly be confirmed. One example is as follows. After the TPR of PF char (200 mg) with N2O (400 ppm), the nitrogen accumulation was checked by gasifying the residue. About 10 mmol of nitrogen was detected as N2. Unfortunately this value is not reliable, because of the following two reasons. NO evolved during the O2gasification could not be detected, and the N2 formation could not be monitored after a certain period of time. Both of these difficulties come from the experimental limitation; analyses of NO (by MS) and N2 (by GC) are both quite difficult in the presence of a large amount of O2. Thus the actual amount of nitrogen, accumulated during the above TPR, would be much more than the observed amount, 10 mmol. In fact, the accumulated nitrogen estimated from the mass balance was 39 mmol. (19) Yamashita, H.; Yamada, H.; Tomita, A. Appl. Catal. 1991, 78, L1-L6.

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Figure 8. N2 formation during TPR of O2-activated PF char with a mixture of 14N2O and 15N2O. 14N2O and 15N2O concentration: 52 ppm and 60-70 ppm, respectively.

Despite such an uncertainty, these observations, one from mass balance and one from O2 gasification, are quite important, because the presence of C(N), even in a very small amount, was definitely confirmed. Reactions Using Isotope Gases. First, the reaction of activated PF char was carried out with a mixture of 14N O and 15N O in order to check the possibility of 2 2 mixing of 14N and 15N in the product N2. Figure 8 shows the concentration of N2O and total N2 determined by GC during TPR and the subsequent isothermal reaction at 1000 °C. The other three lines in this figure represent the concentration of 14N2, 15N2, and 14N15N determined by MS. The concentration of 14N2 was determined as a difference in concentrations between m/e 28 species determined by MS and CO determined by GC, and therefore the accuracy is very poor. On the other hand, the data for 15N2 and 14N15N are quite reliable. Almost no 14N15N was formed during this reaction. This result clearly implies that the N2 was formed from N2O without bond breaking between NtN. To distinguish the two kinds of CO, one that desorbed from C(O) and one produced from the N2O reaction, the experiments using isotope gases were designed. First, TPD was carried out in He by using the PF char that had been pretreated with 18O2, and in the second experiment the same char was subjected to TPR with 14N 16O. The evolution profiles of C18O and C18O in both 2 2 experiments are illustrated in Figure 9. The profiles of C18O and C18O2 evolution from C(18O) during TPR are almost the same as those in TPD. In other words, the decomposition of surface C(18O) was not affected irrespective of the simultaneous occurrence of C-N2O reaction. The step response experiment using isotope gases is a very powerful technique to reveal the function of surface species during gasification.15-17 This technique was applied to the present C-N2O reaction system. The result is shown in Figure 10, where the feed gas was 15N O for the first 90 min and it was 14N O in the 2 2 subsequent 90 min. N2 gas analysis was mainly made by MS, and in addition the total amount of N2 gas was determined by GC. If C(15N) is involved in the N2 formation reaction, the product gas immediately after the feed gas switching would contain a significant

N2 Formation Mechanism in C-N2O

Figure 9. Evolution of C18O and C18O2 during TPD of 18O2activated PF char, together with the results on TPR of the same sample with N2O. PF char activation: 0.1% 18O2, 600 °C, 30 min. N2O concentration in TPR: 90 ppm.

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Since C(O) accelerated the N2O consumption, the presence of O2 was expected to enhance the C-N2O reaction rate as well. However, contrary to this expectation, Figure 7 shows the retardation effect of O2 on C-N2O reaction. In the temperature range between 600 and 800 °C, the N2O consumption became less with increasing O2 concentration. The reason for this may be speculated as follows. If the C( ) site is more easily occupied by O2 rather than by N2O in the presence of excess O2, the N2O consumption rate would decrease with increasing O2 concentration. It should be noted that the sample used in a series of experiments in Figure 7 was the activated char. This char already had many active sites, and thus further activation by adding O2 may have little effect on the rate enhancement. The negative effect mentioned above would be predominant. Reaction Mechanism. To elucidate the reaction mechanism, the nitrogen accumulation was investigated. Some nitrogen accumulation was confirmed from the mass balance during the C-N2O reaction (Figure 3). This was further confirmed by the fact that nitrogencontaining gas was formed upon O2 gasification of the residual char. In the step response experiment, small amounts of 14N15N and 15N2 (about one thirtieth of total N2) were observed in the second stage (Figure 10). Therefore there is no doubt about the nitrogen accumulation on carbon surfaces during C-N2O reaction. However, there is no clear evidence that the nitrogen containing surface species, C(N), plays an important role in the C-N2O reaction. In the reaction with a gas mixture (Figure 8), almost no 14N15N was observed. Also in the step response experiment, the formation of 14N15N was quite small. Therefore, a reaction path such as

C(N) + C(N) f N2

Figure 10. Step response experiment at 850 °C with switching feed gas from 15N2O to 14N2O. Sample: Heat-treated PF char. 15N2O concentration: 240 ppm. 14N2O concentration: 250 ppm.

amount of 14N15N. However, the amount of 14N15N after switching was very small (∼2% of the total N2). This is in great contrast with the result for the C-NO system.15,16 However, it should be noted that the amount of 14N15N was not zero, suggesting that the NtN bond breaking in N2O takes place, even though the extent is very small. Discussion Effect of C(O) and O2. The PF char containing more C(O) has a larger activity to reduce N2O in the temperature range between 600 and 800 °C (Figure 5). This tendency was also observed in the C-NO reaction.14,15,18 The role of C(O) is likely to produce a free C( ) site which is the active site for the N2O reaction. Figure 6 shows a significant CO and CO2 desorption in the same temperature range, and many C( ) sites would be created as a result of the decomposition of C(O).

(4)

would be a very minor path in the present system. The present results support that the principal mechanism is the generally accepted one (eq 1). The splitting of NtN bond is not involved in this mechanism. This is in remarkable contrast with the C-NO reaction, where the following reaction is thought to be a main reaction path to produce N2:15-17

C(N) + NO f N2

(5)

Of course, this is not surprising, because the splitting of the NtN bond in N2O would be quite difficult. A preliminary computational study has been done on the C-N2O reaction by using ab initio molecular orbital method,20 and the calculation strongly supports the above reaction mechanism, that is, N2 is produced without the splitting of the NtN bond in N2O. There is no direct evidence to reveal the relative importance between eqs 1 and 2. However, the decomposition of the C(O) complex would readily take place in the present system, because the reaction temperature is much higher than that used by Strickland-Constanble et al. Therefore, the principal reaction path under the present condition is likely eq 1 followed by eq 6:

C(O) f C( ) + CO

(6)

(20) Kyotani, T.; Tomita, A. J. Phys. Chem. B 1999, 103, 3434-3441.

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High CO/CO2 ratios observed in Figures 2-4, 6, and 9 may support this speculation.

Noda et al.

lated on the char. However, these species hardly contribute as a main reaction intermediate during the N2 formation process.

Conclusion The present study presented direct evidence for the reaction mechanism proposed so far (eq 1). There is little NtN splitting during C-N2O reaction. Even though this is the main reaction path, there is solid evidence that some nitrogen from N2O was definitely accumu-

Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research No. 08455369 and 07242209 from the Japanese Ministry of Education, Science, Sports and Culture. EF9900132