J. Phys. Chem. B 2001, 105, 821-830
821
Opposite Roles of O2 in NO- and N2O-Carbon Reactions: An Ab Initio Study Z. H. Zhu,† J. Finnerty,‡ G. Q. Lu,*,† and R. T. Yang§ Department of Chemical Engineering, The UniVersity of Queensland, Brisbane, 4072, Australia, Department of Chemistry, UniVersity of Queensland, Brisbane, 4072, Australia, and Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136 ReceiVed: August 23, 2000; In Final Form: NoVember 17, 2000
Previous experimental studies showed that the presence of O2 greatly enhances NO-carbon reaction while it depresses N2O-carbon reaction on carbon surfaces. A popular explanation for the rate increase is that the addition of O2 results in a large number of reactive carbon-oxygen complexes, and decomposition of these complexes produces many more active sites. The explanation for the latter is that excess O2 simply blocks the active sites, thus reducing the rate of N2O-carbon reaction. The contradiction is that O2 can also occupy active sites in NO-carbon reaction and produce active sites in N2O-carbon reduction. By using ab initio calculation, we find that the opposite roles of O2 are caused by the different manners of N2O and NO adsorption on the carbon surface. In the presence of excess O2, most of the active sites are occupied by oxygen groups. In the competition for the remaining active sites, NO is more likely to chemisorb in the form of NO2 and NO chemisorption is more thermodynamically favorable than O2 chemisorption. By contrast, the presence of excess O2 makes N2O chemisorption much less thermally stable either on the consecutive edge sites or edge sites isolated by semiquinone oxygen. A detailed analysis and discussion of the reaction mechanism of N2 formation from NO- and N2O-carbon reaction in the presence of O2 is presented in this paper.
Introduction It is well known that NO reduction with carbon can be significantly enhanced by the presence of O2. In the classic work of Smith and co-workers,1 O2 pretreatment increased the NOcarbon reaction rate whereas H2 treatment played the opposite role. They postulated that the first step in the reaction be the formation of C-O surface complexes, and these complexes were very important intermediates in the reaction. They thought the C-O surface complexes formed by O2 seemed to be greater in quantity and different in nature from those produced by NO. Their theory has been accepted widely since then. Kunii2 found that the addition of O2 increased the rate of NO destruction significantly over a range of lower temperatures, in which the desorption of C-O surface complexes was controlling the overall rate. They thought the reaction with the coadsorbed O atoms seemed to keep the carbon surface continuously rich in C-O complexes thus maintaining the constant high reaction rate. This mechanism has been furthered by Radovic and colleagues.3-5 They divided the C-O surface complexes into reactive C(O) intermediates and stable C-O complexes, both of which would be increased by the presence of O2, and the desorption of oxygen from the former would produce highly reactive (nascent) Cf sites (also sometimes referred to as dangling C atoms). Although they thought the produced active sites were attacked more easily by NO, they did not elaborate the reason. The mechanism of production of active sites by O2 has been clarified by Yang et al.6,7 They found that oxygen groups created by O2 increase the net charge of the neighboring carbons, resulting in the weakening of neighboring C-C bonds. * Corresponding author: Professor G. Q. (Max) Lu. E-mail: maxlu@ cheque.uq.edu.au † Department of Chemical Engineering, The University of Queensland. ‡ Department of Chemistry, University of Queensland. § University of Michigan.
According to their unified mechanism the rate-limiting step is the breakage of C-C bonds. This was their reason why O2 can improve the carbon gasification reaction. Yang et al. classified the oxygen groups into the in-plane and off-plane groups. The off-plane groups, (which were later proven to be epoxides) are even more reactive than the former ones. Another plausible role of O2 in NO-carbon reaction is the formation of NO2 as an intermediate. It has been reported that NO oxidization into NO2 is well catalyzed on the carbon surface.8,9 It was also reported that NO2 compared to NO is much more readily chemisorbed on the carbon surface10,11 and has much higher reactivity toward carbon than do either NO or O2.12 However, in separate experiments of NO2 and NO reaction with carbon (without O2), Suzuki13 found that although the reactivity of NO2 was very high, the major product was NO not N2. The conversion rate of NO2 to N2 was essentially the same as that of NO to N2. They thus suggested that the transformation of NO to NO2 is of little benefit for the conversion of NO removal in the presence of O2. Another opinion developed by Suzuki et al.13 was that the role of O2 is not only in the formation of C-O complexes, but also in the removal of nitrogen complexes, which they later proved to be more stable than C-O complexes. By contrast, O2 has been shown to play an opposite role in N2O reduction with carbon or carbon-based catalysts as compared to that in NO-C reactions.14-16 The common explanation is that the active sites are blocked by O2 in either the presence or absence of metal catalysts, thus depressing the N2O reduction. A contradiction arises because excess O2 occupies active sites in the NO-carbon reaction which limit reaction rate yet produces reactive C(O) complexes in the N2Ocarbon reaction. Obviously, these reported explanations fail to reconcile such an inconsistency. The recently reported work of Kyotani and Tomita17 studied different manners of NO and N2O adsorption on the carbon
10.1021/jp003036m CCC: $20.00 © 2001 American Chemical Society Published on Web 01/05/2001
822 J. Phys. Chem. B, Vol. 105, No. 4, 2001
Zhu et al. energetically stable state is selected. We calculated the heat of adsorption (∆H), according to eq 1,
∆H298 ) ∆Ee0 + ∆Hcorrec298
(1)
and the Gibbs free energy (∆G) was calculated using eq 2,
∆G298 ) ∆H298 - 298*∆S298
(2)
where ∆H298 is the enthalphy of reaction at temperature 298 K, ∆Ee0 is the energy difference between products and reactants at 0 K, ∆Hcorrec298 is the difference of thermal energy correction to enthalphy between products and reactants at 298 K, ∆G298 is the Gibbs free energies difference of products and reactants at 298 K, ∆S298 is the entropies difference of products and reactants at 298 K. More details about the calculations can be found in the literature.21 Results
Figure 1. Basic models for investigation.
surface and the possible mechanism of N2 formation using ab initio simulation. Unfortunately, their work did not take into account the presence of O2. In the present paper, we investigate the role of O2 in the NO- and N2O-carbon reactions using ab intio methods. Calculation Details The Gaussian 98 package18 was used in the present ab initio calculations. The selected graphite models in the calculation are shown in Figure 1. Only substrates with zigzag edges were used, and armchair models were not considered. Indeed, in either Yang’s work of the unified mechanism of gasification6,7 or in the work of Tomita17 about the NO or N2O adsorption, the mechanism obtained from the zigzag models is not significantly different from that obtained from armchair models. In another work on catalyzed NO-carbon reaction, Yang et al.19 did not include armchair models either. The unrestricted Hartree-Fock (UHF) method with the basis set of 3-21G(d) was employed for the geometric optimization of the carbon models. The self-consistent field (SCF) energies were calculated at the higher level, B3LYP/6-31G (d). The above selection of the molecular system and model chemistry has proved to yield the best balance between the final results and computation cost in Chen and Yang’s pioneering work.20 With respect to the gaseous molecules such as NO, O2, and N2O, we used the higher level B3LYP/HF-6-31G (d) for the calculations of both geometric optimization and single point energy. All calculations include the following steps: geometric optimization for optimizing the molecular system to a minimal on the potential energy surface, checking the stability of wave functions, frequency calculation for thermo-chemical properties and higher level single point calculations with B3LYP/6-31G(d) model chemistry for more accurate total energies according to well-established conventional procedures. The spin multiplicity of each compound including the chemisorbed species was determined by the same method introduced by Kyotani and Tomita.17 Briefly, the selected multiplicity should lead to a chemically reasonable molecular structure and spin species of edge carbon atoms, with the smallest spin contamination. When these conditions are satisfied, the multiplicity giving the most
Comparison of Separate NO, O2, and N2O Chemisorption on Clean Carbon Edge Sites. The work of Kyotani and Tomita17 has shown that the approach of either NO or N2O adsorption with bond axes parallel to the edge line of carbon is more thermally favorable than that with bond axes vertical to the edge line. So the vertical bond axes are not considered here. Figure 2 shows the chemisorption models of NO (model B1), O2 (model B2), and N2O (model B3) with a parallel approach pathway. The changes of enthalpy (∆H) and Gibbs free energy (∆G) during the chemisorption are displayed in Table 1. The ∆H of NO chemisorption is more negative than that of O2, and N2O chemisorption has the least negative value of ∆H. The same order was observed in ∆G. This means that NO chemisorption is more thermodynamically favorable than that of O2, and N2O is the least thermodynamically stable on the carbon surface. The bond length and atomic bond population are given in Table 2. In the present study, the total overlap population is used to show the relative strength of the chemical bonds. It can be seen that the C-C bonds adjacent to N and O are greatly weakened (the bond population changing from above 0.8 to under 0.60) by the chemisorption of gases. This is consistent with Yang’s results.6,7 For NO adsorption (model B1), the C-C bonds neighboring the N atom are weaker than those neighboring the O atom and the C-N bond is stronger than the C-O bond. This is consistent with the experimental results of Suzuki et al.,13 which showed that surface nitrogen complexes were more stable than surface oxygen complexes. The bonds of the gas molecules are also significantly weakened on chemisorption whereas the bond length increases. According to the bond population of the gas molecules after chemisorption, the order of the dissociation tendency is N2O > O2 > NO. It is noteworthy that the calculated data of chemical bonds of NO and N2O chemisorption in the present study are the same as that in the work of Kyotani and Tomita,17 but the values of ∆H are slightly different from those reported by Kyotani and Tomita.17 The reasons are 2-fold. First, Kyotani and Tomita calculated ∆H as a difference of the total energy of the products and reactants, the thus obtained heat of reaction was based upon 0 K (our values based upon 298 K). Second, they used the lower level HF/3-21G for the geometry of small gas molecules, whereas we used the higher level B3LYP/6-31G(d) for the geometry of these gas molecules. Our calculations showed that the calculation level of geometry has relatively greater influence on small gas molecules than on big graphite groups.
Effect of O2 on NO- and N2O-Carbon Reactions
Figure 2. Models for separate chemisorption.
TABLE 1: ∆H and ∆G for NO, O2, and N2O Chemisorbed on Model B B1 B2 B3
∆H (kJ/mol)
∆G (kJ/mol)
-540.0 -450.0 -385.0
-530.0 -440.0 -325.0
Interestingly, the thermochemistry data is in line with the change of the chemical bonds. Because N2O is not thermally stable on the carbon surface it tends to dissociate readily, whereas NO is relatively stable, thus it possesses a weaker tendency to decompose. Agreement can be found between reported experimental results and our calculations. Degroot et al.22 compared the chemisorption of O2, NO, and N2O on a char surface in the temperature range 100-200 °C. They reported that NO is much more reactive than O2 toward chemisorption, whereas N2O chemisorption is negligible in this temperature range. After analysis of the surface species by TPD, they suggested that NO appeared less likely to dissociate on the surface of carbon after chemisorption than O2. Their further analysis showed that NO has a bond order of 2.5 and a single unpaired electron in an antibonding (π*) orbital. O2 has a bond order of 2.0 and two unpaired electrons in two antibonding (π*) orbitals. The higher bond order of NO was given as one of the reasons for the lesser
J. Phys. Chem. B, Vol. 105, No. 4, 2001 823 tendency of NO to dissociate. They thought that for chemisorption of either gas to lead to reaction, the available molecular orbitals must first interact with those of the carbon and subsequently, the chemisorbed gas would dissociate. Disproportionation of the chemisorbed reactant gas molecule is aided if the multiple bond order of the reactant gas molecules is reduced (e.g., to a bond order of one). This can be achieved if one or more electrons are transferred from the carbon into the antibonding orbitals of the reactant molecule. They thus concluded that the transfer of electrons to oxygen may be easier because there are more modes available for electron transfer to oxygen (viz., the π* orbitals in the y and z directions), whereas NO has only one such orbital. In contrast, we show that the electron transfer depends mainly on the thermodynamics of chemisorption instead of the number of modes for electron transfer, and NO chemisorption has shown to be more thermodynamically favorable than O2 chemisorption. Therefore, the second reason given by Degroot et al.22 is contradictory to both their own experimental results and our theoritcal calculation. We propose that with two π* orbitals, one O2 molecule can accept two electrons, while NO with one π* orbital can only accept one electron. This should be the second reason for the lower tendency of dissociation of NO. As for N2O, Degroot22 did not give any analysis, but the weak bond order of O-N being just 1.61 may indicate why N2O is so easy to dissociate. NO Chemisorption on Carbon Edge Sites in the Presence of O2. The TPD and TPR experimental results of Tomita et al.23 showed that during the NO-carbon reaction with the presence of excess O2 (typically ca. 500 ppm NO in 5% O2), most of the surface complexes formed are C-O complexes instead of C-N complexes. So the very lower concentration of NO has to compete for the remaining active sites with the excess O2. Smith and co-workers24 studied the reaction of oxygen with carbon at 25 °C and 450 °C. They found that at 25 °C, the produced groups involved lactones together with lesser amount of carbonyls. But at 450 °C, carbonyl groups (including carbonyl and semiquinone oxygen) were the major oxygen groups. It is not easy to study chemisorption at 450 °C because of the accompanying vigorous gasification. To model the chemisorption of NO and N2O in the presence of O2 under practical combustion conditions, we assume that graphite carbon surfaces have been put in O2 at 450 °C then cool to 25 °C with the major groups present. In this way, the modeling process is simplified by “separating” chemisorption from gasification and having only carbonyl groups on the carbon surface. For the sake of ease, only semiquinone oxygen is selected in the present study. The models used are shown as model C1 of Figure 3. Among the five carbon edge sites, we assume that four sites have been occupied by semiquinone oxygen and only one site - C,37 is available for further adsorption of NOx or O2. The values of ∆H and ∆G are given in Table 3. The vertical approach of NO chemisorption on C37 with N down (model C4) has negative values of ∆H (-130 kJ/mol) and ∆G (-92 kJ/mol). We failed to locate a stable wave function for the vertical approach with O down (model C5). However, it has been known17 that, on the clean carbon surface, the vertical approach of NO chemisorption with O down is less thermodynamically favorable than that with N down. As there is no connection or reaction between NO and the adjacent semiquinone oxygen if NO goes to C37 in the vertical pathway, model C4 is still more likely to happen than model C5. That is, if NO vertically approaches C,37 the ∆H and ∆G values are less negative with O down than with N down. There are also two possible side-on modes of NO chemisorption:1 O of NO
824 J. Phys. Chem. B, Vol. 105, No. 4, 2001
Zhu et al.
TABLE 2: Bond Length and Atomic Bond Population for Model B1-B3 model
bond
length (Å)
bond population
B1
N(35)-O(36) N(35)-C(30) O(36)-C(32) C(31)-C(30) C(31)-C(32) C(29)-C(30) C(32)-C(33) C(28)-C(29) O(35)-O(36) O(35)-C(30) O(36)-C(32) C(31)-C(30)
1.47 1.36 1.39 1.40 1.37 1.44 1.41 1.39 1.50 1.42 1.42 1.38
0.27 0.72 0.41 0.47 0.60 0.26 0.30 0.92 0.14 0.33 0.33 0.52
B2
connected to the carbon edge sites with N to a neighboring semiquinone oxygen (model C6);2 N to the carbon edge site and O to the neighboring semiquinone oxygen (model C7). The first side-on approach (with ∆H of -212 kJ/mol and ∆G of -161 kJ/mol) is more stable than the second one (with ∆H of -145 kJ/mol and ∆G of -95 kJ/mol). In addition, these two side-on approches both are more stable than the vertical pathways. In the absence of O2, the side-on approach of NO chemisorption was also reported to be more stable than the vertical mode.17 There is another tilted approach (model C8) with N of NO connected to both the carbon site and the neighboring semiquinone oxygen and the O of NO free in the space. This tilted mode (with ∆H of -268 kJ/mol and ∆G of -218 kJ/mol) is even more stable than the first side-on one (model C6). It can be seen that the two most stable modes, the first side-on approach (model C6) and the tilted mode (model C8), actually result in NO2, thus are in very good agreement with the previous experimental results. In a study of NO chemisorption on char in the presence of 5% O2, Rubel et al.11 studied the mechanisms and kinetics of NOx adsorption/ desorption on activated carbon under conditions typical of a combustor using TG/MS. They found that presaturation of carbon with O2 followed by NO adsorption in He, increased adsorption from 1 wt % to 6 wt %, suggesting a carbon surface reaction mechanism. They thus suggested that NO must be converted into a surface species similar to NO2 during the adsorption step. Simultaneous differential thermal analysis (DTA) conducted during these experiments supported such a conversion. A significant exotherm accompanied NO adsorption while no heat of reaction was associated with NO2 adsorption. Their suggestion was confirmed by the detection of NO2 during the desorption of NOx from the O2-presaturated carbon surface. Kong et al.8 found that NO could be partially homogeneously oxidized into NO2 by O2 even at room temperature with carbon acting as a catalyst for this conversion. Kong et al.8 also suggested that NO is adsorbed on the carbon surface in the form of NO2 in the presence of O2. Similar results were previously reported by Rao and Hougen.9 If O2 attaches to the carbon edge site C,37 there are two possible modes of chemisorption, a vertical one (model C2) and a side-on one (model C3). One can see from Table 3 that, just like NO, the side-on mode of O2 chemisorption (with ∆H of -168 kJ/mol and ∆G of -121 kJ/mol) is more stable than the vertical pathway (with ∆H of -64.2 kJ/mol and ∆G of 32.3 kJ/mol). However the heat released by O2 chemisorbing on the isolated edge site is less than that by NO if NO is chemisorbed in the form of NO2 as per models C6 and C8. Therefore it is reasonable to conclude that NO chemisorption is more thermodynamically favorable than O2 chemisorption even in the presence of excess O2. This makes NO more favored in the competition for the carbon sites surrounded by oxygen groups.
model
B3
bond
length (Å)
bond population
C(31)-C(32) C(29)-C(30) C(32)-C(33) C(28)-C(29) N(36)-O(37) N(36)-N(35) C(30)-N(35) C(32)-O(37) C(30)-C(31) C(32)-C(33) C(29)-C(30)
1.38 1.40 1.40 1.40 1.47 1.32 1.35 1.39 1.41 1.40 1.43
0.52 0.40 0.40 0.81 0.05 0.50 0.66 0.47 0.22 0.48 0.38
The data of chemical bonds are listed in Table 4. As mentioned above, the vertical approch of NO chemisorption with N down (model C4) is more thermodynamically favorable than with O down (model C5). The former is not really useful for the dissociation of NO as the N46-O47 bond is still strong with a bond population of 0.58. Thus, model C4 is not beneficial to the further chemical reaction. By contrast, the side-on approch of model C6, with N to a neighboring semiquinone oxygen, is not only thermodynamically favorable but also leads to the breakage of the N-O bond of NO due to the weak bond populations of N47-O46 and N47-O.44 This pathway is useful for the further formation of N2 according to our later discussion. Compared to model C6, model C7 is less thermally favorable. But model C7 also leads to the breakage of the O-N bond of NO with N left on a carbon edge site. The thus produced C(N) surface complexes can react with another NO to produce N2 according to the previous work of Tomita et al.17 One can see that, in models B1, C4, and C7, the C-N bonds have a high bond population. In contrast, in the tilted approach of model C8, the C37-N46 bond (with a bond population of 0.096) and the bond N46-O44 (with a bond population of 0.22) have much lower bond populations compared to the bond population 0.54 of N46-O.47 It is noteworthy that the temperature considered here is comparatively low (298 K). If temperature increases, the adsorbed N46O47 will break away from the carbon surface and go back to the gas-phase. This is also in good agreement with the experimental results reported by Degroot,22 who found that NO chemisorption decreased with increasing temperature. On the other hand, if NO has been oxidized to NO2 by O2 before chemisorption and the NO2 is adsorbed on carbon sites as in model C8, this approach is thermodynamically favorable at low temperature. But NO2 will be easily reduced to NO again by carbon when the temperature is increased. This explains why NO2 is reduced to NO instead of N2. It has been shown that the side-on approach of O2 chemisorption (model C3) is more thermodynamically favorable than the vertical one. This side-on mode also weakens the O-O bond of O2, thus resulting in the decomposition of O2. As O47 is weakly connected to both O46 and O,44 an NO molecule may easily capture O47 to form NO2. We therefore have two possible pathways in which NO is catalytically converted to NO2 in the presence of O2. The first is that NO reacts with oxygen complexes on the surface of carbon to form NO2 (models C6 and C8). Second, NO may capture O from O2 decomposition on the carbon surface (which is also catalyzed by carbon surface) before NO itself reaches the carbon surface. N2O Chemisorption in the Presence of O2. The possible approaches of N2O chemisorption in the presence of O2 are shown in Figure 4 and the themochemistry and bond data are listed in Tables 3 and 4. We found that the vertical approach of N2O chemisorption with O down (model C9) is not thermody-
Effect of O2 on NO- and N2O-Carbon Reactions
J. Phys. Chem. B, Vol. 105, No. 4, 2001 825
Figure 3. Models for NO chemisorption in the presence of O2.
namically favorable (with a positive ∆H (248.0 kJ/mol) and ∆G (292.0 kJ/mol)). Another vertical pathway with N down (model C10) is even more unstable (with ∆H 529.8 kJ/mol and ∆G 559.0 kJ/mol). Kyotani and Tomita17 also concluded that, on clean carbon surface, N2O adsorption with O down was more thermodynamically favorable than that with N down. However,
they calculated the thermochemistry of the vertical approach with O down using directly dissociative chemisorption, but did that with N down using nondissociative chemisorption. In their result, the ∆H for the former is -500.0 kJ/mol and -20.0 kJ/ mol for the latter. Our results are based upon nondissociative chemisorption in both cases. and thus more credible as chemi-
826 J. Phys. Chem. B, Vol. 105, No. 4, 2001
Zhu et al.
TABLE 3: ∆H and ∆G for NO, O2, and N2O Chemisorbed on Model C in Presence of Excess O2 model C2 C3 C4 C6 C7 C8 C9 C10 C11 C12
∆H (kJ/mol)
∆G (kJ/mol)
-64.2 -168.0 -130.0 -212.0 -145.0 -268.0 248.0 529.8 37.0 -63.0
-32.3 -121.0 -92.0 -161.0 -95.0 -218.0 292.0 559.0 97.0 -10.0
sorption occurs before dissociation in the case of dissociative chemisorption. As for the side-on approaches of N2O chemisorption, the mode with the end N toward the carbon edge site and the O toward a neighboring semiquinone oxygen (model C11) is not stable (with a positive ∆H (37 kJ/mol) and ∆G (97kJ/mol)). But the reverse side-on mode with O down toward the carbon edge site and the end N connected to a neighboring semiquinone oxygen (model C12) is relatively more thermodynamically favorable (with negative ∆H (-63.0 kJ/mol) and ∆G (-10.0 kJ/mol)). The highly positive ∆H and ∆G values of models C9 and C10 indicate that N2O chemisorption on the isolated edge sites in vertical approaches are impossible. N2O is mostly likely to chemisorb in the side-on approach as model C12. The data of chemical bonds in Table 4 also shows that, in model C12, the N47-N48 bond (bond population 0.36) is more tightly connected compared to the other two adjacent bonds N48-O44 (bond population 0.12) and N47-O46 (bond population 0.10). Therefore model 12 is also very beneficial to the further formation of N2. According to Table 3, the stable pathway of O2 chemisorption on the isolated edge site C37 (model C3) has a ∆H -168.0 kJ/ mol, which is much less negative than that of N2O chemisorption (-63.0 kJ/mol) (model C12). Therefore, in the presence of excess O2, N2O is much less thermodynamically favorable in the competition with O2 for the carbon active sites. On the basis of the above analysis, we found that three points should be taken into account during the analysis of chemisorption. The first is whether the chemisorption pathway is thermodynamically favorable. If it is not, then the following discussion is not really necessary, because this is not going to happen. The second point is the decomposition of gas molecule. Provided the adsorption path is thermodynamically favorable, but no decomposition occurs, then NOx conversion will decrease quickly once the adsorption is getting saturated just like model C4. The last point is whether the decomposition will lead to the preferable product. For instance, the tilted approach of NO chemisorption (model C8) is thermally stable at low temperatures, but produced NO instead of N2, thus making no contribution to the whole reaction. The Mechanism of N2 Formation during NOx-Carbon Reaction in the Presence of O2. As discussed above, N2O is more likely to chemisorb in the side-on pathway either on consective (model B3) or isolated edge sites (model C12); the O-N bond of N2O in turn breaks easily, resulting in the release of N2. Therefore, the mechanism for N2 formation in N2Ocarbon reaction is quite clear either in the presence or absence of excess O2. However, the mechanism of N2 formation in NO-C reaction, especially in the presence of O2, is still subject to debate. Two different mechanisms of N2 formation have been typically suggested by Suuberg et al.25-27 and Tomita et al.,23,28-30 respectively. The mechanism by Suuberg et al. is as follows.
The process of NO chemisorption can be split into two separate pathways: one is a reversible chemisorptive pathway forming NO surface complexes C(NO), and the other is an irreversible chemisorption pathway yielding N2 and carbon surface oxides C(O) and C(O2). The processes of formation of the surface oxides both appear to involve dual-site mechanisms, and all of these processes are kinetically of second order with respect to NO. The latter has been hypothesized to be a consequence of the tendency of NO to form a dimer species within microporous carbon, i.e.,
2C* + (NO)2 f C(O2) + N2
(3)
This mechanism was supported by the work of Ruckenstein and Hu,31 which showed that NO-carbon reaction proceeds more rapidly in micropores than in mesopores. In the early work of Tomita et al. about the mechanism of N2 formation,32 they thought that even in steady-state gasification in pure NO at 873 K, the formation of N2 appears to involve no surface nitrogen intermediate. Later on, they found that the reason they did not observe N2 in the early work was due to the treatment with He prior to the reaction with NO; the treatment stabilized the nitrogen species to some extent.28 They used a transient kinetic method, in which the reactant gas was switched from 14N16O to 15N18O.28-30 The surface N-containing species produced and accumulated in the first stage, C(14N), was found to react with 15N18O, yielding 14N15N either in the presence or absence of O2. They thus concluded that N2 formation is the reaction between C(N) complexes and NO (either in the presence or absence of O2), i.e.,
2C() + N f C(O) + C(N)
(4)
C(O) f C() + CO
(5)
C(N) + NO f C(O) + N2
(6)
The importance of the gas-solid heterogeneous reaction was supported by the reaction kinetics that was of the first order in NO and C(14N) in the absence of O2.30 However, according to our calculations, we found that neither Suuberg nor Tomita’s hypothesis was complete. Suuberg et al. did postulate on the existence of nitrogen surface complexes but did not take these surface nitrogen complexes into account during the analysis of N2 formation. They did not include the role of O2 in their work either. Finally, they did not consider the intermediacy of N2O, which was shown to exist as an intermediate during NO-carbon reaction either in the presence33,34 or absence of O2.35 We do agree with the analysis of Tomita et al.28-30 about the mechanism of N2 formation in the absence of O2. However, in the presence of excess O2, they neglected two important phenomena. First, they did not take the formation of NO2 into account. Although Suzuki et al.13 thought that NO2 was of little benefit to NO conversion in the presence of O2, our calculation shows that the intermediacy of NO2 is important to the whole reaction, and a more detailed discussion follows. Another important point was actually observed by them. When only O2 was used in either isothermal13 or temperature-programmed reaction (TPR)28 following 14N16O reaction with carbon, 14N2 was still observed. This indicates that the reaction between C(N) surface complexes and gas NO suggested by Tomita et al. is not the only pathway for N2 formation in the presence of O2, as N2 could form without the added NO gas. Essentially, even in the transient kinetic experiments, O2 could also partially remove C(14N) as 14N16O, and 14N15N could form from 14N16O and 15N18O. Therefore the
Effect of O2 on NO- and N2O-Carbon Reactions
J. Phys. Chem. B, Vol. 105, No. 4, 2001 827
TABLE 4: Bond Length and Bond Population for NO, O2 and N2O Chemisorbed on Model C in Presence of Excess O2 model C1
C2
C3
C4
C6
C7
bond
length (Å)
bond population
C(39)-C(40) C(38)-C(39) C(39)-O(44) C(36)-C(37) C(37)-C(38) C(37)-C(38) C(38)-C(39) C(39)-O(44) O(46)-O(47) O(46)-C(37) O(46)-O(47) O(44)-O(47) C(37)-O(46) C(39)-C(44) C(36)-C(37) C(37)-C(38) C(38)-C(39) C(39)-C(40) N(46)-O(47) C(37)-N(46) C(37)-C(38) C(38)-C(39) C(39)-O(44) O(46)-N(47) O(44)-N(47) O(46)-C(37) O(44)-C(39) C(37)-C(38) C(38)-C(39) C(39)-C(40) C(36)-C(37) N(46)-O(47) N(46)-C(37) O(44)-O(47) O(44)-C(39) C(37)-C(38) C(38)-C(39) C(36)-C(37) C(39)-C(40)
1.45 1.45 1.27 1.38 1.38 1.45 1.41 1.38 2.82 1.28 1.47 1.47 1.39 1.39 1.41 1.40 1.40 1.41 1.22 1.32 1.45 1.45 1.28 1.42 1.42 1.38 1.38 1.40 1.40 1.40 1.41 1.42 1.34 1.48 1.38 1.43 1.40 1.45 1.41
0.39 0.31 1.05 0.60 0.60 0.37 0.41 0.66 0.006 1.00 0.014 0.016 0.42 0.42 0.59 0.36 0.34 0.59 0.58 0.56 0.40 0.36 1.0 0.13 0.14 0.42 0.42 0.38 0.34 0.56 0.55 0.07 0.62 0.054 0.42 0.26 0.20 0.53 0.60
experimental resutls of Tomita et al. could not rule out the mechanism of Suuberg et al. in the presence of O2. In the absence of O2, the experimental results of Tomita et al. cannot clarify if the mechanism involves the direct participation of gassolid heterogeneous reaction or if the newly chemisorbed NO is responsible for the formation of N2 by interaction with C(N).30 But the simulation of Tomita et al.17 proved that N2 is formed mainly through the latter and the former is more likely to result in N2O. Our results clearly show that the mechanisms should be a combination of Tomita and Suuberg’s hypotheses. As mentioned above, in the competition for the carbon edge sites isolated by oxygen groups, NO will be less competitive than O2 if NO adsorbed in the form of C-NO or C-ON (i.e., the vertical way of models C4 and C5). If this case was true, O2 would depress NO conversion to N2. This is apparently contradictory to the experimental results, thus impossible. Only in the form of NO2 including model C6 and model C8 can NO chemisorption be more thermally favorable than that of O2. The chemisortion approach of model C7 is also a side-on approach, but not in the form of NO2, and this chemisorption is less thermally stable than O2 chemisorption. Therefore, NO2 is a key intermediate during the NO-carbon reaction in the presence of O2. Unfortunately, although the tilted approach of model C8 is thermally very stable, when temperature goes up, the chemisorbed NO will be released again and the product is still NO instead of N2. The side-on approach of model C6 in the form of NO2 is not only thermally favorable but also makes the N atom very weakly connected to the two adjacent O atoms. This pathway would
model C8
C9
C10
C11
C12
bond
length (Å)
bond population
N(46)-O(47) N(46)-O(44) N(46)-C(37) O(44)-C(39) C(37)-C(38) C(39)-C(38) C(39)-C(40) C(36)-C(37) N(48)-N(47) N(47)-O(46) O(46)-C(37) C(37)-C(38) C(38)-C(39) C(39)-C(44) C(39)-C(40) N(47)-O(48) N(47)-N(46) N(46)-C(37) C(37)-C(38) C(38)-C(39) C(39)-C(40) C(39)-O(44) N(46)-N(47) N(47)-O(48) O(48)-O(44) C(39)-O(44) N(46)-C(37) C(37)-C(38) C(38)-C(39) C(39)-C(40) C(36)-C(37) O(46)-N(47) N(47)-N(48) N(48)-O(44) C(37)-O(46) C(39)-O(44) C(37)-C(38) C(38)-C(39) C(39)-C(40)
1.27 1.47 1.40 1.39 1.40 1.38 1.40 1.42 1.21 1.27 1.34 1.41 1.45 1.28 1.45 1.29 1.14 1.32 1.44 1.45 1.41 1.30 1.24 1.48 1.45 1.37 1.35 1.44 1.41 1.43 1.45 1.39 1.26 1.40 1.38 1.39 1.41 1.41 1.42
0.54 0.22 0.096 0.376 0.32 0.42 0.50 0.32 0.56 0.45 0.40 0.43 0.36 1.01 0.42 0.69 -0.25 -0.001 0.42 0.42 0.45 0.90 0.70 0.06 0.036 0.24 0.56 0.33 0.36 0.64 0.64 0.10 0.36 0.12 0.43 0.44 0.37 0.29 0.60
lead to the formation of N2. However, until a complete pathway model is constructed, the mechanism of N2 formation is still far from clear. The models in Figure 1 have been idealized. In reality, the carbon rings cannot be so flat and smooth. Smith and coworkers24 proposed a layer plane arrangement of carbon atoms with a variety of peripheral carbon fragments and ring positions available for gas molecules. To investigate the formation of N2, we referred to Smith’s model and proposed the models in Figure 5, and the corresponding thermochemistry and bond data are tabulated in Tables 5 and 6, respectively. In model A1, in the atmosphere NO/excess O2, two edge sites C17 and C19 are taken by O27 and O,25 and N26 bridged over the two adsorbed oxygen atoms forms a structure like -C-ONOC- (i.e., NO chemisorbed in the same approach as model C6). On the right-hand side of -C-ONO-C-, the structure -C22C23- is the peripheral carbon fragments proposed by Smith et al.24 Obviously, C23 is still available for O2 or NO. If one NO goes to C,23 then two pathways are most possible, i.e., with O to C23 and N to N26 (model A2) or with N to C23 and O to N26 (model A3). The model A2 has very negative ∆H (-324.2 kJ/ mol) and ∆G (-275.3kJ/mol), while model A3 has relatively less negative ∆H (-222.0 kJ/mol) and ∆G (-175.1 kJ/mol). Obviously the former is much more thermodynamically favorable, and thus should be focused. A further analysis of the chemical bonds shows that after NO adsorption in the former pathway, the N29 from NO is tightly connected with the N26 from the intermediate -C-ONO-C-, whereas both N29 and N26 are weakly connected to the adjacent oxygen atoms. This
828 J. Phys. Chem. B, Vol. 105, No. 4, 2001
Zhu et al.
Figure 4. Models for N2O chemisorption in the presence of O2.
TABLE 5: ∆H and ∆G for the First Pathway of N2 Formation A2 A3
Figure 5. Models for N2 formation during NO-carbon reaction with the participation of -C-ONO-C in the presence of O2.
will be obviously followed by the release of N2, leaving three semiquinone oxygen on the graphite edge sites. It is noteworthy that the peripheral carbon fragments could also be on the lefthand side or the top side of the structure -C-ONO-C- and N2 can be formed in the same mechanism. It is known that the graphite crystal is composed of layers of fused hexagons held approximately 3.35 Å apart by weak van der Waal forces.36 If a -C-ONO-C- is located in one plane
∆H (kJ/mol)
∆G (kJ/mol)
-324.2 -222.0
-275.3 -175.1
and an NO attaches to an adjacent plan, an N2 molecule can also form. A similar pathway was suggested by Smith.24 There is another pathway, which can also lead to the formation of N2. If two -C-ONO-C- groups get close enough with N to N together, then N2 will also form from oxygen groups. This mechanism is actually similar to Suuberg’s hypothesis and would be more possible to occur in micropores. If an NO molecule gets close to the -C-ONO-C- without being chemisorbed, the weakly connected N of -C-ONOC- will go to the NO molecule resulting in one N2O molecule. However, as mentioned before, even in the presence of O2, it is impossible that all of the carbon edge sites are isolated by oxygen groups and NO is chemisorbed as in model C6 or C8. Some consective carbon edge sites can also exist. In this case, NO would be chemisorbed in the parallel approach as model B1 (Figure 2), resulting in the formation of C(N) complexes. Even for the isolated carbon edge sites, a small number of C(N) complexes can also form in the side approach of model C7, although model C7 is not as favorable as models C6 and C8. The formation of nitrogen surface complexes in the presence of O2 has been evidenced by the XPS experiments of Tomita et al.23 The thus produced C(N) can reaction with NO chemisorbed on a adjacent carbon site to form N2, and this mechansim has be clarified by the simulation work of Kyotani and Tomita.17 In conclusion, our mechanism for NO reduction to N2 in the presence of O2 includes two pathways. The first one is through the intermediate of NO2 (in the form of -C-ONO-C-)
Effect of O2 on NO- and N2O-Carbon Reactions
J. Phys. Chem. B, Vol. 105, No. 4, 2001 829
TABLE 6: Bond Length and Atomic Bond Population for the First Pathway of N2 Formation model A1
A2
bond
length (Å)
bond population
C(17)-O(27) C(17)-C(18) C(18)-C(19) C(19)-O(25) C(19)-C(20) C(20)-C(21) C(21)-C(22) C(22)-C(23) N(26)-O(27) N(26)-O(25) C(17)-C(18) C(17)-O(27) C(18)-C(19) C(19)-O(25) C(19)-C(20) C(20)-C(21) C(21)-C(22) C(22)-C(23)
1.39 1.39 1.41 1.39 1.36 1.50 1.36 1.41 1.41 1.44 1.40 1.39 1.40 1.39 1.41 1.44 1.46 1.31
0.34 0.69 0.50 0.26 0.58 0.72 0.91 0.83 0.16 0.12 0.65 0.35 0.55 0.32 0.64 0.73 0.80 0.88
A3
reacting with adsorbed NO or through the reaction between two intermediate NO2 groups. The other pathway is through the mechanism proposed by Kyotani and Tomita, i.e., the reaction between C(N) and chemisorbed NO. Briefly,
-C- C(O) + NO f -C-ONO-C-
(7)
-C-ONO-C- + NO + -C- f - 3C(O) + N2 (8) 2C-ONO-C- f 4-C(O) + N2
(9)
-C-ONO-C- + NO f -C(O) + N2O
(10)
-C-C- + NO f -C(O)-C(N)
(11)
-C(O)-C(N) + NO + -C- f -C(O)-C- + -C(O)- + N2 (12) -C(O)-C(N) + NO f -C(O) -C- +N2O
model
(13)
The pathway represented by eqs 7-10 does not involve the surface C(N) complexes. This is based upon Suuberg et al.’s hypothesis but is not the same. Suuberg’s hypothesis focused on the dual-site of (NO)2 but did not involve the presence of O2. Our mechanism considers low concentration NO with the presence of excess O2 and emphasizes the importance of the intermediate of NO2 (i.e., in the form of -C-ONO-C-). The pathway represented by eqs 11-13 is nearly the same as Tomita’s mechanism, but our mechanism does not regard this way as the only pathway of N2 formation in the presence of O2. Our combined mechanism can reconcile various experimental results much better. This mechanism is consistent with the transient kinetic experiments of Tomita et al.,28-30 as both pathways can lead to 14N15N when switch from 14NO to 15NO. The formation of N2O as intermediate is interpreted by eqs 10 and 13. The roles of C(N) surface complexes are showed by eqs 11-13. More importantly, our mechanism regards NO2 as a key intermediate, which is very important not only to NO chemisorption but also to the formation of N2, thus in good agreement with the previous experiment results. Considering the importance of the intermediate NO2 based upon the previous experiments8,9,11 and the present calculation, we suggest that, in the presence of excess O2, N2 mainly forms through the first pathway. Indeed, in the XPS experiment of Tomita et al.,23 it was reported that pyrrolic and pyridinic nitrogens were produced by NO-carbon reaction with 1% O2 at 600 °C. However, in
bond
length (Å)
bond population
C(23)-O(28) N(26)-O(27) N(26)-O(25) N(26)-N(29) O(28)-N(29) C(17)-C(18) C(17)-O(27) C(18)-C(19) C(19)-O(25) C(19)-C(20) C(20)-C(21) C(21)-C(22) C(22)-C(23) C(23)-O(28) N(26)-O(27) N(26)-O(25) N(26)-O(29) N(28)-O(29)
1.16 1.43 1.43 1.40 2.89 1.40 1.39 1.40 1.38 1.40 1.46 1.43 1.39 1.16 1.42 1.42 3.38 2.87
1.22 0.21 0.22 0.27 0.001 0.66 0.36 0.51 0.37 0.63 0.69 0.92 0.50 1.65 0.14 0.15 0.003 0.01
the later simulation work,17 they found that neither pyrrolic nor pyridinic nitrogen is favorable for the formation of N2. This may further hint that N2 formation is mainly through the first pathway in the presence of O2. Discussion On the basis of the above results, we can explain why O2 plays opposite roles in NO- and N2O-carbon reactions. The side-on approaches of chemisorption of either NO or O2 on carbon surface are more thermodynamically stable than the vertical pathways in the presence of excess O2. More importantly, more negative values of both ∆H and ∆G are found for NO than O2 chemisorption either on consective edge sites or isolated carbon edge site (in the form of NO2). Thus, in the competition for the remaining active sites, NO chemisorption is more thermodynamically favorable than O2. N2 can form through either -C-ONO-C- or -C(N) complexes, and the former one is the main pathway. It is not necessary to weaken or break the adjacent C-C bonds during the formation of N2. After the release of N2, oxygen still remains on the carbon edge sites. New active sites are quickly produced through the formation of epoxy oxygen because of the presence of excess O2 as suggested by Yang.6,7 Consequently, the NO-carbon reaction is greatly improved by excess O2. By contrast, N2O chemisorption is much less thermodynamically favorable than O2 chemisorption on either consective edge sites or the isolated edge sites. Although the presence of excess O2 produces a larger amount of new active sites, the produced active sites are occupied by O2 again. Therefore, the N2Ocarbon reaction rate is depressed by the presence of excess O2. References and Notes (1) Smith, R. N.; Swinehart, J.; Lesnini, D. The Oxidation of Carbon by Nitric Oxide, J. Phys. Chem. 1959, 63, 544. (2) Kunii, D.; Wu, K. T.; Furusawa, T. NOx Emission Control from a Fluidized Bed Combustion of Coal, Effects of in Situ Formed Char on “NO” Reduction, Chem. Eng. Sci. 1980, 35, 170. (3) Yamashita, H.; Tomita, A.; Yamada, H.; Kyotani, T.; Radovic, L. R. Influence of Char Surface Chemistry on the Reduction of Nitric Oxide with Chars, Energy Fuels 1993, 7, 85-89. (4) Illa´n-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartı´nez de Lecea, C. NO Reduction by Activated Carbons. 7. Some Mechanistic Aspects of Uncatalyzed and Catalyzed Reaction, Energy Fuels 1996, 10, 158. (5) Li, Y. H.; Radovic, L. R.; Lu, G. Q.; Rudolph, V. A New Kinetic Model for the NO-Carbon Reaction, Chem. Eng. Sci. 1999, 54, 4125.
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