Energy & Fuels 1993, 7, 85-89
85
Influence of Char Surface Chemistry on the Reduction of Nitric Oxide with Chars Hiromi Yamashita and Akira Tomita' Institute for Chemical Reaction Science, Tohoku University, Katahira, Sendai, 980 Japan
Hajime Yamada, Takashi Kyotani, and Ljubisa R. Radovic Department of Materials Science and Engineering, Fuel Science Program, The Pennsylvania State University, University Park, Pennsylvania 16802 Received August 5, 1992. Revised Manuscript Received October 21, 1992
The reactions of brown coal chars with nitric oxide, oxygen, and their mixture were carried out at 300 "C. The copper-catalyzed char-NO reaction was remarkably enhanced by the presence of 0 2 . The formation of reactive surface intermediates (C(0)) and stable carbon-oxygen complexes ((2-0) during these reactions was investigated by the combination of transient kinetics and temperatureprogrammed desorption techniques. The majority of the carbon-oxygen complexes generated during the reactions were the stable C-0 species. The concentration of reactive C(0) was increased by the presence of both 02 and Cu catalyst. To further investigate the influence of carbon surface chemistry, the reduction of NO was carried out with partially gasified char samples subjected to various surface treatments. A mechanism is proposed to account for the observed kinetic behavior.
Introduction The removal of nitric oxide (NO) from the exhaust stream of various combustion sources has become increasingly important. The catalytic decomposition of NO into Nz and 02 is preferred.' However, sufficient catalytic activity for decomposition of NO in oxygen-rich atmosphere has not been realized because of the deactivation of catalyst by oxygen. For this reason, the reduction of NO has been attempted using reducing gases like NH3, CO, Hz, and hydrocarbon g a s e ~ . ~ Carbon J would be another candidate as a reducing agent.4-7 Chars obtained from brown coal are suitable because of their high reactivity in oxidizing gases and low cost. Previously, we have demonstrated that the effective removal of dilute NO can be realized by using carbon as a reducing agent.8 This C/NO reaction was catalyzed by metals loaded on coal char and was remarkably promoted in the presence of oxygen a t temperatures as low as 300 "C. A high conversion of NO was achieved with a copperloaded brown coal char. Three possible explanations have been proposed for the enhancement effect by oxygen: (1) oxidation of metallic Cu by 0 2 is beneficial for the C/NO reaction; (2) NO reacts with 02,yielding NOz, the reactivity of which is higher than that of NO; (3) the C/O2 reaction
increases the concentration of surface sites, which can react easily with NO. Although each one of these factors may be important, in the present studywe paid special attention to the last hypothesis. We have attempted to clarify how the C/O2 reaction modifies the surface state of the coal char. Significant new insights about the role of carbon surface chemistry in carbodchar gasification have recently been obtained using the techniques of transient kinetics ( T K P and temperature-programmed desorption (TPD).12-15 When combined, they can make a distinction between reactive intermediates (C(0)) and stable complexes (C0) on the char surface. Although several studies have demonstrated the usefulness of the TK technique for both uncatalyzed and catalyzed char gasification,1618 these measurements were carried out at high reaction temperatures (usually > 700 "C). We have recently demonstrated that the TK technique is also useful for the evaluation of OC).19 char gasification with 0 2 at low temperatures (~300
(9)Radovic, L. R.; Lizzio, A. A.; Jiang, H. In Fundamental Issues in Control of Carbon Gasification Reactiuity; Lahaye, J., Ehrburger, P., Eds.; Kluwer: Dordrecht, 1991;p 235-255. (10)Adschiri, T.; Nozaki, T.; Furusawa, T.; Zhu, Z-B. AIChE J. 1991, 37,897-904. (ll),Kapteijn,F.; Meijer,R.;vanEck,B.;Moulijn,J. A. InFundamental Issues rn Control of Carbon GasrficationReactioity; Lahaye, J., Ehrburger, P., Eds.; Kluwer: Dordrecht, 1991;p 221-230. (12)Cazorla-Amoros, D.; Linares-Solano, A.; Joly, J. P.; Salinas(1)Iwamoto, M.; Yahiro, H.; Mine, Y.; Kagawa, S. Chem. Lett. 1989, Martinez de Lecea, C. Catal. Today 1991,9,219-226. 213-216. (2)Sato,S.;Yu-u,Y.;Yahiro,H.;Mizuno,N.;Iwamoto,M.Appl.Catal. (13)Zhang, Z.-G.; Kyotani, T.; Tomita, A. Energy Fuels 1989,3,566571. 1991,70,L1-5. (14)Lizzio, A. A,; Jiang, H.; Radovic, L. R. Carbon 1990,28,7-19. (3)Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. (15)Calo, J. M.; Hall, P. J. In Fundamental Issures in Control of Catal. 1990,64,L1-4. Carbon Gasification Reactiuity; Lahaye, J., Ehrburger, P., Eds., Kluwer: (4)Inui, T.; Otowa, T.; Takegami, Y. Ind. Eng. Chem. Prod. Res. Deu. Dordrecht, 1991;p 329-368. 1982,21,56-59. (16)Radovic, L. R.;Jiang, H.; Lizzio, A. A. Energy Fuels 1991,568(5)Teng,H.;Suuberg,E. M.;Calo, J. M.;Hall,P. J.Proc. 19thBiennial 74. Conf. Carbon 1989,514-515. (17)Nozaki, T.; Adschiri, T.; Fujimoto, K. Energy Fuels 1991,5,610(6)Mochida, I.; Sun, Y. N.; Fujitau, H.; Kisamori, S.;Kawano, S. J. Chem. SOC.Jpn. 1991,885-890. 611. (18)Lizzio, A. A.; Radovic, L. R. Ind. Eng. Chem. Res. 1991,30,1735(7)Kapteijn, F.; Mierop, A. J. C.; Abbel, G.;Moulijn, J. A. J. Chem. 1744. SOC.,Chem. Commun. 1984,1085-1086. (19)Kyotani, T.; Yamada, H.; Yamashita, H.; Tomita, A.; Radovic, L. (8)Yamashita, H.; Yamada, H.; Tomita, A. Appl. Catal. 1991,78,L1R. Energy Fuels, in press. 6.
0887-0624/93/2507-0085$04.00/00 1993 American Chemical Society
Yamashita et a1.
86 Energy & Fuels, Vol. 7, No. 1, 1993
In the present work, the Cu-catalyzed C/NO reaction was investigated in the absence and presence of 0 2 at 300 OC. The quantities of reactive intermediates and stable carbon-oxygen complexes formed during the reaction were determined by TK and TPD techniques, respectively. The role of reactive intermediates in the C/NO reaction were clarified.
.C
NO/OZ (Culchar)
P
'.- - - - - - - - - -- - - - - -- -
.-
NOiOz (Nonichar)
Experimental Section C h a r Preparation. The Loy Yang brown coal from Victoria, Australia, was used as the char precursor. Catalyst impregnation was performed by immersing coal particles (100-200 mesh) in an aqueous solution of copper acetate. The dried sample was then devolatilized in N2 at 650 "C in a small fluidized-bed reactor. The copper loading after this treatment was 4 w t %. Gasification. In a typical kinetic experiment, a 1-g sample was placed in a conventional fixed-bed reactor made of quartz (id. 10 mm) and heated under Ar flow a t 600 OC for 1h before reaction. The reactions with the following three gas mixtures were carried out in the same reactor a t 300 "C: (a) NO, 2% ; 02, 5%; Ar, balance, (b) NO, 2%; Ar, balance, and (c) 02, 5 % ; Ar, balance. The gas flow rate was 100 mL min-1, and the pressure was atmospheric pressure. The outlet gas was monitored with a quadrupole mass spectrometer (Dycor, Model M100). The sensitivity of each gas and the contribution of fragments were taken into consideration in the calculation of gas concentrations. Since the side reaction 2 N 0 O2 2N02 took place in the presence of 02, the reactant gas contains both NO and NOn. Therefore, the NO decomposition rate was estimated from the total concentration of NO and NO2 a t the reactor outlet. Thus, the results in this paper will be presented as NO, conversion. The determination of NO2 concentration by mass spectrometry is generally somewhat difficult. However, it caused little problem in the present study, because the actual amount of NO2 in the outlet gas was very small. The carbon conversion (to be distinguished from char conversion) was obtained from the CO and CO2 concentration determined by the mass spectrometer, and the results were in good agreements with those determined from the weight changes during the reaction. The high exothermicity of the C/O2 reaction can be a source of concern about the sample temperature isothermality in the reaction zone, especially in the presence of the Cu catalyst. However, as reported in our previous study,s the effect on NO conversion is small in the temperature range investigated here. Characterization of C h a r Surface. In the T K experiments, the flow of reactant gas was switched to that of pure Ar at the reaction temperature. The transient response of the product gas was analyzed by the mass spectrometer. The integrated area under the transient CO and COz concentration vs time curves was used to determine the quantity of reactive C(0) intermediates on the char surface. The integration was performed by considering the decay curve observed in a blank run14 (without the sample in the reactor), which has demonstrated that the gasphase replacement in the reactor occurred within a very short time frame. After the T K measurement a t 300 "C, the sample was heated under Ar flow a t a constant heating rate of 10 K/min to the final temperature of 600 "C. The CO and COz evolved during this T P D measurement were used to evaluate the quantity of stable carbon-oxygen complexes, C-0, generated during gasification. It is impossible to remove all surface complexes a t this temperature. However, since we selected this temperature as the pretreatment temperature to avoid the sintering of catalyst, this was also the final temperature for T P D experiments. Both in T K and T P D experiments, the oxidation state of Cu changed, and therefore apart of gas evolution is due to this change. This has not been taken into consideration, because we think that there is no essential distinction between oxygen from the carbon surface and that from the metal surface in the catalytic gasification reaction.
+
-
Time on stream / min
Figure 1. NO, conversion in the reaction of chars with NO (2 % ) and NO (2%)-02 (5%)a t 300 "C.
._
-
401
9
~ 0 1 0 2 (Cuichar)
i
8 c
20
0
0 2 (Cuichar)
_ _ _ _ - _ _ _-_---
Nolo2 (Nonlchar)
0
./---- _ _ - 0
30
NO (Cuichar)
60
Time / min
F i g u r e 2. Carbon conversion profiles for the reaction of chars with NO (2%), NO (2%)-02 ( 5 % ) , and 0 2 (5%) a t 300 "c. Reaction between NO and C h a r s w i t h Different S u r f a c e Properties. In order to determine the effect of char surface chemistry on the reactivity of NO, several chars with different surface properties were prepared and they were allowed to react with NO at 300 "C. The first char was prepared by gasifying the raw char with 0 2 a t 300 "C for 2 h, followed by the T K measurement a t 300 "C. This sample contains no C(0) complexes but has C-0 complexes. The procedure for the second one was as follows: the gasification and the T K measurement as above, the T P D up to 600 "C in Ar, and then cooling down in Ar down to 300 "C. In this case, only C-0 complexes, which could not be removed a t 600 "C, were present on the surface.
Results Steady-State Kinetics. The variation of NO, conversion during the C/NO reaction with Cu-loaded coal char (Cu/char) or as-receivedcoal char (Nodchar) is shown in Figure 1. In the absence of 02, NO, reduction with both samples drastically decreased in the initial stage and became less than 5 % . The rate enhancement by adding the Cu catalyst is hardly observed a t this temperature. Figure 1 also shows the NO, conversion in the presence of 5 % 02.Without the Cu catalyst, conversion at 2 h was about 20%. The conversion with Cu/char was greatly increased, maintaining a value of more than 60% for 2 h. The reactivity enhancement in the presence of 02 for Cu/ char was larger than for Nodchar. During the reaction, carbon was consumed not only by the C/NO reaction, which is the reaction of interest, but also by the C/O2 reaction, which is the main reaction under the conditions used here. Figure 2 shows the extent of carbon consumption; the presence of both 02 and Cu catalyst increased the reactivity. The NO, conversion and carbon conversion rate after 2 h are summarized in Table I. Transient Kinetics. After the UNO, C/NO-02, or C/O2 reactions at 300 OC for 2 h, the TK response was measured. Figure 3 shows typical responses from the
Energy & Fuels, Vol. 7, No. 1, 1993 87
Reduction of Nitric Oxide with Chars 3
c) Nodchar after TK
E u
b) Nodchar
3
b) Cdchar aRer TPD
a) Nodchar after TPD
1 d) Cdchar after TK
1)
1
0.4-
' 0
-1
mle.28
mle.44
L.L--*--L __.._--* 20 40 Time I min
0
20
40
0 60
Time 1 min
Figure 5. Gas evolution profiles for the char/NO reaction at 300 O C : (a) Non/char after TPD, (b) Cu/char after TPD, (c) Non/ char after TK, (d) Cu/char after TK. Time / min
Figure 3. Transient kinetics response after the reaction of chars with NO (2%)-02 (5%) at 300 O C : (a) Cu/char, (b) Nodchar.
0 300
400
500
tjuu
Temperature / OC Figure 4. TPD patterns of char after gasification with NO (2 % )02 (5%) and subsequent TK measurement at 300 O C : (a) Cu/ char, (b) Nodchar. partially gasified chars. The m/e = 28signal includes both CO and N2, and the mle = 44 signal can be both C02 and N20. However, based on the following observations, they are considered to be only due to CO and C02, respectively: (1) a chromatographic analysis described in our previous study8 indicated that N20 was not produced during reaction; (2)the desorption of Nz from the carbon surface would occur without delay; and (3)the m/e = 14 signal, which is a fragment of either N2 and NzO, was not observed.
Thus, the content of N2 and N2O in the decay gas, if present, is considered to be negligible. The quantities of C(0) determined from the integrated area under the transient CO and C02 curves are shown in Table I. Here the assumption waa made that 1 mol of both CO and C02 corresponds to one C(0). For the C/NO reaction, the amount of C(0) was small and there was little difference between Culchar and Nonlchar. With both samples, the amount of C(0) increased in the presence of 0 2 . The Cu catalyst is very effective in promoting the formation of C(0) species during the C-NO-02 and C-02 reactions. Temperature-Programmed Desorption. After the transient kinetics experiments at 300 "C, TPD measurementa were carried out from 300 to 600 "C to obtain information about stable C-0 complexes on the char surface. Figure 4 shows TPD patterns for Culchar and Nonlchar. The mle = 44 signal appeared above 330 "C and the mle = 28 signal appeared above 380 "C. They represent the production of CO and C02 from the decomposition of C-0 complexes, because the mle = 14 fragment was not observed a t all. The final temperature for the TPD experiments was 600"C, the same temperature used for char pretreatment. The amount of CO and C02 evolved during this TPD corresponds, therefore, to a partial quantity of C-0 complexes. The data are listed in Table I. Under the present reaction conditions, the quantity of C-0 was much greater than that of C(0). It was also increased in the presence of 0 2 . In every reactant gas, the formation of stable complexes was found to be more extensive on Non/ char than on Culchar. Reduction of NO with Surface-Modified Chars. Nodchar and Cu/char were pretreated in two different ways before the reaction with NO. The concentrations of surface carbon-oxygen complexes were different among these samples. After TPD, all C(0) intermediates and a part of C-0 complexes are removed from the char surface; are thus available for reaction. many free surface sites (Cf) After TK, all of the C ( 0 ) species are removed but all of the C-0 species still remain on the surface. Figure 5 shows the gas formation profiles during the reaction of chars with NO at 300 "C. In case of the chars that were subjected to TPD (Figure 5a,b), sharp mle = 28 signal was observed in the very early stage. This is due to N2 evolution, since only a sharp mle = 14 fragment, with no sharp m/e = 12 fragment, was observed. Thereafter a significant amount of NO (mle = 30)was consumed,
Yamashita et al.
88 Energy & Fuels, Vol. 7, No. 1, 1993
Table I. NO. Conversion. Carbon Conversion Rate, and Amounts of C ( 0 ) and C-0 for Chars Gasified for 2 h at 300 OC reactant gas sample NO, conversion/.~% C conversion rate/h-l amount of C(O)/(mg of C/g of char) amount of C-O/(mg of C/g of char)
Cu/char Nonlchar Culchar Non/char Cu/char Nodchar Cu/char Nodchar
Table 11. Amount of Gas Products (m/e = 28 and m/e = 44) during NO Reduction at 300 O C following Different Char Treatments amount of gas produced (au) sample
Nz or COG
COz or NzOa
Nodchar after TK Non/char after TPD Cu/char after TK Cuichar after TPD
0.002 0.4 0.2 0.5
0.03 0.3 0.3 0.5
a m / e = 28 and m / e = 44 correspond to NZ (or CO) and COz (or NzO), respectively. However, from the stoichiometry, the combination should be either N:, + CO:, or CO + Nz0.
accompanied by the appearance of mle = 28 and mle = 44 signals. The reaction products must be a combination
of either CO + N20, N2 + CO2, or their mixture. It should be noted that this reaction does not reach steady state but almost stops after a certain period of time (about 20 min). With the chars that are subjected to only TK (Figure 5c,d), there was no sharp N2 evolution. Furthermore, Non/char showed very small NO consumption (Figure 5c),indicating the inactive nature of the surface. The amounts of gas produced during this transient reaction at 300 "C are summarized in Table 11. Among the char samples tested, the char with the highest concentration of free (Cf) sites (after TPD) exhibited the highest NO reduction reactivity, although it did not last for a long time. The small reactivity difference between Culchar and Non/char (in the absence of 02)is also consistent with the NO, conversion data shown in Figure 1 and Table I. A third char was also prepared by gasifying char in 5 5% 0 2 and then cooling down to room temperature in the same atmosphere. The surface of this char was covered with both C(0) and C-O. The reaction with NO at room temperature (not at 300 "C) was examined, and it was found that thereactivity of the third char was much smaller than that of the other two chars, as expected. Discussion The copper catalyst is effective in enhancing the rates of both the NO conversion reaction (Figure 1) and the
carbon conversion reaction (Figure 2). The enhancement of the C-NO reaction in the presence of 02,reported previously,*is analyzed in more detail below. This effect is contrary to the inhibiting effect of oxygen observed by Stegenga20in NO reduction by CO on carbon-supported Cu-Cr oxide catalysts. It is seen in Table I that the quantity of C(0) species on the char surface after every reaction is much smaller than that of C-0 species. Taking into consideration that only a fraction of C-0 species was detected in our TPD experiments, it is concluded that only a very small portion (20)Stegenga,S. AutomotiveExhaust Catalystawithout Noble Metals. Ph.D. Thesis, University of Amsterdam, 1991;Chapter 3.
NO (2%) 1 1
0.002 0.002 0.02 0.02 1
2
02 ( 5 % ) 0.16
NO (2%)/02(5%) 61 19 0.19 0.02 0.3 0.1 4 16
0.02 0.4 0.1
8 22
of the carbon surface participates directly in the gasification reaction. This is particularly true for the reaction with NO, where only about 2 out of every lo5carbon atoms form the reactive C(0) intermediates. Table I also shows that, irrespective of the nature of the reactant gas, the concentration of stable C-O species on Nodchar was larger than that on Culchar, even though the extent of carbon conversion is greater for the catalyzed reaction. This finding is very interesting, but there is no definitive explanation for it a t the present time; its implications are discussed e1~ewhere.l~ In the presence of 0 2 , the overall oxygen surface coverage increased. In the absence of the catalyst, this increase is accounted primarily by the formation of stable C-0 complexes. The increase in the concentration of C(0) species was very small. When the copper catalyst was added, however, the quantity of C(0) species increased to a larger extent as a consequence of the C/O2 reaction, both in the absence and presence of NO. The presence of both 02 and Cu catalyst is thus essential for the formation of C(0) species. The quantity of C(0) species correlates well with the char gasification reactivity, in agreement with previous studies.+l1 The sample with a higher concentration of reactive C(0) intermediates also exhibited a higher NO reduction capability. Theoe results suggest that the formation of C(0) sites is important for NO reduction. On the basis of these results, discussed above, as well as the proposed analogy to other gasification reactions involving oxygen transfer, the following mechanism can be postulated for the reduction of NO by carbon in the presence of oxygen: 2Cf + 2N0
-
2C(O) (or 2C-0)
-
+ N2
2Cf + 0, 2C(O) (or 2C-0) 2C(O)
C(0)
- + - + -c-0 - + + - +
2CO + 2N0
(2)
C02 nCf
(3)
CO
(4)
nCf
C(0) 2C(O) + 2N0
(1)
2C0,
2C0,
N2 nCf N,
(5) (6)
(7)
Since a sharp peak for nitrogen evolution was observed in the beginning of the reaction of NO with carbon (Figure 5a,b), the initial step is the chemisorption of NO on very active Cf sites, reaction 1,followed by the regeneration of Cf sites according to reactions 3 and 4. Although the activation of NO on oxygen-containing sites on the char surface, e.g., C(0) (reaction 6), as proposed by Mochida et
Reduction of Nitric Oxide with Chars al.21 cannot be ruled out, the activity of Cf toward NO is much higher than that of C(O), at least in the absence of oxygen. The gradual rate decrease during the first 20 min (Figure 5a,b,d) suggests the gradual transformation of C(0) to stable C-0, according to forward reaction 5. The role of oxygen is to enhance the concentration of both reactive C(0) intermediates and stable C-0 complexes, according to reactions 2 and 5. A comparison of the degree of rate enhancement with the relative increase in C(0) and C-O concentrations suggests that these two effects reinforce each other. This is consistent with the notion that C-0 complexes, even though they are temporarily inhibiting, contribute to higher char reactivity14 through transformation to C(0) in back reaction 5. The increase in the concentration of C(0) species drives reactions 3 and 4 forward, thus creating additional active sites (Cf)for reaction 1. Although these newly formed Cf sites would be attacked mainly by 02,the probability of NO attack is also increased. This mechanism is consistent with the conclusion of Suuberg et alanthat oxide desorption plays an important role in NO gasification of chars. The proposed mechanism is also consistent with the observation of Mochida et al.,l that sulfuric acid treatment (and consequent creation of "oxidative sites") activates the carbon for NO reduction. Finally, and perhaps most significantly, the proposed mechanism of NO reduction enhancement in the presence of 02 can also be used to explain a similar effect recently reported by Treptau and Miller= for enhanced hydrogasification of carbons oxidized by HN03, for which the concept of "nascent active sites" was invoked. The highly reactive ("nascent") Cfsites (also sometimes referred to as dangling C atoms) formed upon C(0) desorption are evidently attacked much more easily by NO (or Hz). The C/O2 reaction produces a considerable amount of CO, and therefore the contribution of reaction 7 cannot be neglected. However, the enhancement of NO decomposition rate by CO is not enough to explain the observed data.8 Another plausible role of oxygen is the formation of NO2 via gas-phase reaction before entering reaction zone. It can play an important role as an intermediate in NO reduction, as suggested by reactions 8 and 9. It has been reported recently that NO2 has a much higher reactivity toward carbon than either NO or OZaz4Although a considerable portion of NO2 would regenerate NO by reaction 9, some NO2 might be converted to Nz.*
(21) Mochida, I.; Ogaki, M.; Fujitsu, H., Komataubara, Y.; Ida, S. Fuel 1985,64, 1054-1057. (22) Suuberg, E. M.; Tang, H.; Calo, J. M. Tcuenty-Third Symposium
(International)on Combustion, [Proceeding]; Thecombustion Institute; Pittsburgh, 1990; p 1199. (23) Treptau, M. H.; Miller, D. J. Carbon 1991, 29, 531-539. (24) Cooper, B. J.;Thoss, J. E. SAE Paper 1989, No.890404,612-624.
Energy & Fuels, Vol. 7,No.1, 1993 89
2 N 0 + 0, C, + NO,
-
-
2N0,
(8)
C(0) + NO
(9)
The Cu-catalyzed C/NO reaction was remarkably enhanced in the presence of 02. The straightforward explanation is that Cu catalyzes the C/O2 reaction and increases the concentration of active sites, C(0) (and thus Cf). Indeed, as mentioned previously, the quantityof C(0) species correlates with the NO, conversion reactivity. This is also consistent with the finding that the Cu-catalyzed C/O2 reaction has the same activation energy as the uncatalyzed reaction.25 (In the absence of 02,the active sites are easily deactivated according to forward reaction 5 (Figure 5).) Nevertheless, the role of 02 in activating the catalyst surface for NO, reduction cannot be ruled out. In our previous study? we found that the surface state of the Cu catalyst was a mixture of CUIand CUI*in the region where Cu/char exhibited a high NO, conversion. The oxidation of Cu by 02 may be an important factor leading to high activity. Even in this case, however, the Cf sites are likely to play an important role, as acceptors of oxygen species supplied by the catalyst. More work is needed to investigate in more detail the influence of Cu catalyst on this complicated reaction. The effect of other gases like steam, SO,, and hydrocarbons present in the actual exhaust gas should also be clarified.
Conclusion The enhancement effect of oxygen in the reduction of nitric oxide with chars can be understood in terms of the presence of carbon-oxygen complexes on the char surface. Transient kinetics experiments, which make the distinction between reactive intermediates and stable carbonoxygen complexes, provide a good quantitative measurement of this enhancement. The key feature of the proposed mechanism of both Cu-catalyzed and uncatalyzed C/NO-02 reaction is the enhanced formation of reactive C(0) intermediates, and thus free carbon sites, analogous to that proposed for other carbon gasification reactions. These active sites function either by directly reacting with NO or as acceptors of oxygen species generated on the catalyst surface.
Acknowledgment. This study was partly supported by the Monbusho International Scientific Research Program, Japan, University-to-University Cooperative Research (No. 03045013). Coal Corp. of Victoria, Australia, kindly supplied the raw brown coal. (25) Radovic, L. R.; Jenkins, R. G.; Walker, P. L., Jr. J. Catal. 1982, 62,382-394.
