Density Functional Theory Study of Mechanism of N2O Decomposition

Article pubs.acs.org/JPCC

Density Functional Theory Study of Mechanism of N2O Decomposition over Cu-ZSM‑5 Zeolites Xin Liu,† Zuoyin Yang,† Runduo Zhang,† Qianshu Li,‡ and Yaping Li*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China School of Chemistry and Environment, South China Normal University, Guang Zhou 510006, China

‡

ABSTRACT: The N2O decomposition mechanism is investigated over Cu-ZSM-5 using density functional theory (DFT). Though the mechanism is extended from Fe/Co-ZSM-5, the results show that a different step may be rate-determining over Cu-ZSM-5 compared to the Fe/Co-ZSM-5 system. In the beginning, Z[Cu] as active center decomposes the first N2O and generates Z[CuO] (process 1), and the energy barrier of N2O dissociation is 35.18 kcal/mol. Then Z[CuO] could decompose the second N2O and generate Z[CuOO] (process 2), and the energy barrier of N2O dissociation is 28.07 kcal/mol. In process 2, oxygen could desorb from Z[CuOO], and the desorption energy is 39.48 kcal/mol, which is only higher 4.30 kcal/mol than 35.18 kcal/mol in the process 1. However the corresponding rate constants show approximately that the rate-limiting step is O2 desorption in process 2 and not the N2O dissociation in process 1. Next, if Z[CuOO] could not desorb O2, it could decompose the third N2O and generate Z[CuO(O2)] (process 3). In this process, the energy barrier for N2O dissociation and the O2 desorption energy from Z[CuO(O2)] are 42.10 and 63.42 kcal/mol, respectively, which are much higher than the former processes. It indicates the presence of O2 could inhibit the N2O decomposition over Cu-ZSM-5, which is in line with the kinetic experiment. The results suggest the process 1 and 2 are the main catalytic cycle in N2O decomposition. Importantly, O2 desorption from Z[CuOO] shows that the mechanism over CuZSM-5 is different from that over Fe/Co-ZSM-5 system.

1. INTRODUCTION Nitrous oxides (N2O) are detrimental greenhouse gases emitted by combustion processes in industrial boilers and vehicle engines and are responsible for smog formation, acid rain, and global warming.1 These hazardous effects have led to the studies on the catalytic decomposition and reduction of nitrogen monoxide into N2 and O2.2−4 Metals cationexchanged into ZSM-5 zeolites have been shown to decompose nitrous oxide with high catalytic activity.5−13 It is reported that Cu-supported zeolites have attracted much attention for their catalytic activity in selective oxidation14−16 and NOx decomposition.9−13,17−20 Jocono et al. adapted the electron paramagnetic resonance (EPR) to measure the concentration of Cu2+ present in Cu-ZSM-5.18 While according to the Cu+ emission spectra by Dědeček, the emission bands at 500 and 540 nm are attributed to the single Cu+ ion, the band at 500 nm to the Cu+ located in the eight-membered ring, and the band at 540 nm to the Cu+ located in the regular six-membered ring.19 Recently, Schoonheydt et al. studied the formation of the bis(μoxo)dicopper core and gave the experimental evidence for its existence by means of the UV−vis and extended X-ray absorption fine structure (EXAFS) spectroscopes in NO decomposition over Cu-ZSM-5.9 They illustrated clearly that all Cu2+ is present as isolated ions up to Cu/Al = 0.2 by UV, inductivel coupled plasma (ICP), and EPR, and (CuO)x cluster © 2012 American Chemical Society

is expected for overchanged samples, above Cu/Al = 0.2 in the selective oxidation of methane.16 Meanwhile, they proposed that the recombination of two oxygen atoms into O2 is the ratelimiting step in N2O decomposition and discussed the influence of O2, NO, and H2O on the recombination of oxygen over ZSM-5 with higher Cu loading.10,11 The studies by Kaptinjn et al. pointed out that the Cu-ZSM-5 has the high activity and low energy barriers, but it suffers from O2 inhibition in the N2O decomposition over Cu-ZSM-5.20 In Fe-ZSM-5, Lund’s group pointed out that the presence of NO molecule has a significant influence on N2O decomposition.8 The density functional theory (DFT) is used to study the mechanism of N2O decomposition over Cu-, Fe-, and Co-ZSM5 from a perspective of a molecule.15,21−26 In the selective oxidation of methane into methanol over Cu-ZSM-5 by DFT, Schoonheydt et al. regarded binuclear copper as the active site.15 While in N2O decomposition over Fe/Co-ZSM-5,27 Ryder et al. viewed mononuclear [FeO]+ and [CoO]+ as the active center. The electron spin resonance (ESR) studied by Kucherov on Fe-ZSM-528 and magnetic susceptibility studied by Armor on Co-ZSM-529 showed that the state of Fe or Co Received: January 17, 2012 Revised: August 13, 2012 Published: August 14, 2012 20262

dx.doi.org/10.1021/jp300548p | J. Phys. Chem. C 2012, 116, 20262−20268

The Journal of Physical Chemistry C

Article

dangling bonds of the terminal silicon and aluminum atoms are terminated with H atoms to obtain a neutral cluster. To present the effects of exchange and correlation, Becke’s 3-parameter exchange with correlation functional of Lee, Yang, and Parr (B3LYP) are used.41,42 The basis set 6-31G** is used for the N, H, O, Si, and Al atoms, while the Cu atom is treated with the effective core potential (ECP) of LANL2DZ.24,27,30 Every atom in the cluster, the reactant, and the product is relaxed in the calculations. Equilibrium geometry (EG), transition state (TS), and energy profiles are performed to determine the energy barrier of the reaction. All of the work is carried out by the Gaussian 03 programs.43 The Cu-ZSM-5 cluster and the adsorbed molecules are fully optimized geometrically by means of EG.30,44 The relative energy is defined as the following:

seemed unlikely. During N2O decomposition over Fe/CoZSM-5 by the ONIOM method, Fellah’s group pointed out that two oxygen atoms are deposited on the same iron or cobalt site and then recombine to O2.30 For N2O decomposition on hydrated and dehydrated mononuclear iron sites over Fe-ZSM5, Heyden’s group studied metal-oxo species Z[FeO] serving as active center adsorbing N2O and generating Z[FeO2] or Z[OFeO] and then oxygen desorbing from Z[FeO2] rather than from Z[FeO(O)2].31 By use of first-principles DFT, Sengupta et al. investigated the N2O decomposition over CuZSM-5 and thought the reaction pathway (ZCu + N2O + NO → ZCuN2O + NO → ZCuN2 + NO2) is the most favorable. It indicates that the presence of NO can help to convert N2O to N2 and NOx and help to reduce the oxidized Cu sites. Meanwhile, they discussed the surface crossing about spin multiple.32 These studies indicate that lots of experimental and theoretical analyses focus on the NOx decomposition over metal-exchanged ZSM-5,11,33−37 but the decomposition mechanism over Cu-ZSM-5 is not entirely clear. Experimentally, it has been reported that isotopically labeled oxygen scrambles into the products of nitrous oxide dissociation in the case of Cu-ZSM-5 but not in the case of either Fe/Co-ZSM-5.4,7,38 It means that the mechanisms of N2O dissociation are similar on Fe- and Co-ZSM-5 and may be different on Cu-ZSM-5. In the present work, our motivation is to investigate the decomposition mechanism of three N2O molecules over CuZSM-5 by means of DFT calculations. The Cu-ZSM-5 zeolites adapted are modeled as the complex [(SiH3)4AlO4Cu]. The process of the first N2O decomposition is viewed as process 1 and the second and the third as processes 2 and 3. The energy barrier of each step in three processes are calculated and compared with the previous results in theory and experiment. The reaction rate constants of the key elementary step are also computed to obtain some kinetic information.

ΔE = Esystem − (Ecluster + Eabsorbate)

(1)

where Esystem is the calculated energy of the given geometry containing the cluster and the absorbing molecule, Ecluster the energy of the cluster, and Eabsorbate the energy of the absorbing molecule, e.g., N2O, O2, and so on.44

3. RESULTS AND DISCUSSIONS Because of different spin multiplicity leading to different energy and structure,32 the correct spin multiplicity (SM) of the system is determined by the single point energy (SPE) calculation as shown in Table 1. It is found that there is the Table 1. Energy and Spin Multiplicities for Cu-ZSM-5(5T)

2. MOLECULAR MODELS AND COMPUTATIONAL METHODS The ZSM-5 zeolites has 12 crystallographically distinct tetrahedral sites (T sites), which are described as T1 to T12, and the T12 site are known as the primary Al substituted site.39 Lund’s group adapted a five tetrahedral cluster and a lager cluster to investigate the interactions with the zeolite channel, and the results showed that the channel walls do not significantly affect the energy of mechanistic steps in N2O decomposition over Fe-ZSM-5.40 In the present work, the 5T model [(SiH3)4AlO4Cu] is adapted as shown in Figure 1. In the [(SiH3)4AlO4Cu] model, the Al atom is at the T12 site, and the

SM

energy (Hartree)

energy (Hartree)

1 3

Genecp(6-31G**) −1905.0539635 −1904.9689296

6-31G** −3349.1726116 −3349.1363273

lowest SPE when the spin multiplicity is 1, which is accepted for the present work. In fact, there will be surface crossing between singlet and triplet.32 To compare the present result with the result of Fe/Co-ZSM-5, the same method is used, as well as Bell’s group27 and Fellah’s group.30 Figure 1 shows the optimized geometry of the Cu-ZSM-5 cluster and the N2O molecule with a neutral charge and with a singlet spin multiplicity. The Si−O distances for the cluster are 1.628 and 1.666 Å, and the two Cu−O bonds distance are 2.004 and 2.006 Å, respectively, which are very close to the experimental value of 1.98 Å.45 For EG of N2O molecule as a reactant, the optimized linear N2O molecules have a distance of 1.191 Å for N−O bond length and 1.133 Å for N−N bond length, respectively, which are in good agreement with the experiment data of 1.185 and 1.128 Å.30 3.1. Mechanism of N2O Decomposition over Cu-ZSM5. According to the reaction mechanism previously reported by Bell’s group,27,31 the cycle processes of N2O decomposition on a mononuclear site of Cu-ZSM-5 are proposed as shown in the following:

Figure 1. The structure diagram of the computational model. (A1) Cu-ZSM-5 zeolites, (A2) optimized 5T model, and (A3) optimized N2O model. 20263



Article

Process 1 (First N2O Decomposition) Z[Cu] + N2O → Z[Cu‐ON2)] TS1

Z[Cu‐ON2] ⎯⎯⎯→ Z[CuO] + N2 Process 2 (Second N2O Decomposition) Z[CuO] + N2O → Z[CuO‐ON2] TS2

Z[CuO‐ON2] ⎯⎯⎯→ Z[CuOO] + N2 path A

Z[CuOO] ⎯⎯⎯⎯⎯→ Z[Cu] + O2 Process 3 (Third N2O Decomposition) path B

Z[CuOO] + N2O ⎯⎯⎯⎯⎯→ Z[CuOO‐ON2] TS3

Z[CuOO‐ON2] ⎯⎯⎯→ Z[CuO(O2 )] + N2 Z[CuO(O2 )] → Z[CuO] + O2

Figure 2. Energy vs reaction coordinates for the first N2O decomposition over Z[Cu].

Here, Z represents ZSM-5 zeolites. TS1, TS2, and TS3 represent three transition states of N2 dissociation in three processes, respectively (their key geometrical parameters are list in Table 2). In process 1, Z[Cu] as the active center adsorbs

1.749 and 1.795 Å (in Table 2), respectively. It illustrates that N1−O1 bond would break and Cu−O1 bond would form in the coming. Additionally, the N2−N1−O1 angle changes from 178° in the adsorbed state a2 to 143° in the TS1 (a3), and the imaginary frequency of the TS1 is −294 cm−1. Furthermore, it is found that N2 would desorb from the a4 structure, and the desorption energy is only −0.04 kcal/mol. It implies the N2 could easily desorb from the structure a4. The results of the process show that the first N2O molecule is easily adsorbed over Cu-ZSM-5 and the weak interaction exists between the Z[CuO] (a5) and N2. The products of process 1 are Z[CuO] and N2. (All discussions are located in part 3.5). 3.3. The Second N2O Decomposition over the CuZSM-5 Cluster. On the basis of Z[CuO] (a5), the product of process 1, it could adsorb the second N2O molecule as shown in Figure 3. It is found that the absorption energy of the second

Table 2. Selected Geometrical Parameters (Distances in Ångstroms, Angles in Degrees, and Frequencies in cm−1) of Three TS Structures r(N1−N2) r(N1−O1) r(Cu−O1) ∠Cu−O1−N1 ∠N2−N1−O1 frequency

TS1

TS2

TS3

1.110 1.795 1.749 125.2 143.6 −294

1.120 1.466 1.900 122.4 149.5 −799

1.116 1.590 2.028 113.6 146.2 −766

and decomposes the first N2O and generates Z[CuO]. Then Z[CuO] adsorbs and decomposes the second N2O and generates Z[CuOO] (process 2). For Z[CuOO], in path A it could desorb O2, and in path B it could adsorb and decompose the third N2O and generate Z[CuO(O2)] and release O2 and N2 (process 3). In the present work, to reveal the quite unique characteristics of the copper, the two possible reaction pathways for O2 desorption from Z[CuOO] and Z[CuO(O2)] have been considered. Processes 1 and 2 or processes 1−3 constitute a completely catalytic cycle. Though the present mechanism is extended from Fe/Co-ZSM-5, different reaction mechanisms might be occurring within the copper center. For example, the studies by Schoonheydt’s group showed the existence of the bis(μ-oxo)dicopper.9 They point out that Omigration is not necessary when two O atoms are present in the bis(μ-oxo)dicopper core, which greatly facilitates oxygen recombination.10,11 3.2. The First N2O Decomposition over Cu-ZSM-5 Cluster. The possible cycle begins with the adsorption of the first N2O molecule on the Z[Cu] as shown in Figure 2. It is found that the adsorption energy is only −10.07 kcal/mol. It suggests that the first N2O molecule is easily adsorbed on CuZSM-5. Then it is seen that the energy barrier of the N2O dissociation is 35.18 kcal/mol in Figure 2. In the absorbed structure Z[Cu-ON2] (a2), the Cu−O1 and N1−O1 bond distances are 2.054 and 1.210 Å, while in the TS1 (a3), they are

Figure 3. Energy vs reaction coordinates for the second N2O decomposition over Z[CuO]. 20264



Article

Table 3. Rate Coefficients for Key Elementary Step with Activation Energies T, K reaction TS1

Z[Cu(N2O)] ⎯⎯⎯→ Z[CuO] + N2 TS2

Z[CuO(N2O)] ⎯⎯⎯→ Z[Cu(O2 )] + N2 Z[Cu(O + )] → Z[Cu] + O2

constant

600

k1, s−1 k−1, s−1 bar−1 k2, s−1 k−2, s−1bar−1 k3, s−1 k−3, s−1 bar−1

1.88 1.31 7.34 4.49 5.09 1.25

N2O is −1.50 kcal/mol, which is higher than that in process 1. It is the reason that steric hindrance enhances the barriers for the absorption reaction. Then from the adsorbed state b1 to the TS2 (b2), it is seen that Cu−O1, Cu−O2, and O1−O2 distances are from 3.363, 1.708, and 2.288 Å to 1.900, 1.732, and 2.000 Å, respectively. Moreover, the N2−N1−O1 angle changes from 180° in structure b1 to 122° in structure b2, and the imaginary frequency of the TS2 is −799 cm−1. In structure b3, Cu−O1, Cu−O2, and O1−O2 distances are 1.865, 1.862, and 1.332 Å, respectively, and it is seen that there is a threemembered ring formation among Cu, O1, and O2. More importantly, it is calculated that the energy barrier of N2O dissociation is 28.07 kcal/mol, which is lower than that in process 1. The formation of three-membered ring and the lower energy barrier indicated that N2O dissociation is easier on Z[CuO] than that on Z[Cu]. Then the N2 desorption energy is calculated to be −1.07 kcal/mol, which suggests that N2 could easily desorb from structure b3. For Z[CuOO] (b4), in path A, O2 could desorb and product Z[Cu] (a1); thus processes 1 and 2 could comprise a whole catalytic cycle. Additionally, it is found that O1and O2 atoms are closing from 2.000 Å (b2) to 1.332 Å (b3) and then to 1.331 Å (b4), which is the tendency to O2 desorption. It is calculated that the energy of O2 desorption from Z[CuOO] (b4) is 39.48 kcal/mol, which is much lower than the theoretical value 67.30 kcal/mol for Z[FeOO] and 64.95 kcal/mol for Z[CoOO], respectively,30 and experimental value 45.70 kcal/mol for Z[FeOO].31 Bell et al. calculated that the O2 desorption energies are 94.9 kcal/mol from Z[FeOO] and 85.6 kcal/mol from Z[CoOO] in the presence of water vapor, respectively.27 In contrast to the previous findings, the present results show that O2 desorption from Cu-ZSM-5 is easier than that from Fe/Co-ZSM-5 system. Then, comparing the O2 desorption energy (39.48 kcal/mol) in process 2 with the activation energy of N2O dissociation (35.18 kcal/mol) in process 1, they are almost close to 32.5 kcal/mol (experiment value), the apparent activation energy.20 Which step is the rate-limiting step in the catalytic cycle? Some studies reported the O2 desorption step is the rate-limiting step in the N2O decomposition cycle.11,46 To clarify the question, based on the standard statistical mechanics and absolute rate theory,47 the reaction rate constants are computed as the following31 k=

⎛ ΔE ⎞ kBT ⎟ exp⎜ − ⎝ RT ⎠ h

× × × × ×

700 12

10 102 10−16 10−2 1013

1.49 2.11 2.48 6.07 6.76 1.46

× × × ×

102 1012 104 10−12

× 1013

800 4.03 3.07 3.54 7.74 2.69 1.67

× × × × × ×

103 1012 105 10−9 103 1013

and k2) for the N2O decomposition are higher than that (k3) for the O2 desorption, and the rate parameters (k−1 and k−2) for the N2O decomposition are lower than that (k−3) for the O2 desorption. It means that the O2 desorption step of process 2 is the rate-limiting step in the former cycle (processes 1 and 2). Smeets’s group recognized the recombination of oxygen atoms into O2 as the rate-limiting step in N2O decomposition in Cucontaining zeolites.10,11 The result of the present work is in more reasonable agreement with the studied experiment.10,11,46 3.4. The Third N2O Decomposition over the Cu-ZSM-5 Cluster. Furthermore, if Z[CuOO] (b4) could not desorb O2, it could adsorb the third N2O molecule in path B. Figure 4

Figure 4. Energy vs reaction coordinates for the third N2O decomposition over Z[CuOO].

shows the reaction profiles for the possible process 3. It is found that the adsorption energy of the third N2O is 9.38 kcal/ mol, which is much higher than the former two processes. It is reason that the steric hindrance becomes the main effects on adsorption reaction. From the structure c1 to c2, it is seen that the Cu−O1 distance decreases from 2.610 to 2.028 Å, that the N1−O1 distance increases from 1.200 to 1.590 Å, and that the N2−N1−O1 angle changes from 179° in the adsorbed state (c1) to 114° at the TS3 (c2), which tends to formation a fourmembered ring among the Cu, O1, O2, and O3. In the TS3, the Cu−O1 and N1−O1 distance are 2.028 and 1.590 Å, and the corresponding imaginary frequency is −766 cm−1 (Table 2). Then, it is calculated that the activation energy of N2O dissociation is 42.10 kcal/mol, which is higher than the former two processes. And the dissociation product is Z[CuO(O2)] (c4). Next, O2 could desorb possibly from Z[CuO(O2)],

(2)

Here, k, kB, h, R, and T are the reaction rate constant, Boltzmann constant, Plank constant, universal gas constant, and temperature, respectively. ΔE is the energy barriers or desorption energy, the values are computed using the barriers calculated for the N2O decomposition over Cu-ZSM-5 from the DFT calculations. The reaction rate constants of the key elementary step are shown in Table 3. The rate parameters (k1 20265



Article

Figure 5. Summary energy diagram showing all processes for three N2O molecule decomposition on the Cu-ZSM-5 cluster.

Table 4. Comparison of the Adsorption Energy and the Activation Energy of N2O Decomposition over Cu-ZSM-5 with Literature Values energy barrier (kcal/mol) Cu-ZSM-5 process 1 process 2

process 3

a

N2O adsorption activation energy N2O adsorption activation energy O2 desorption N2O adsorption activation energy O2 desorption

−10.07a 35.18a −1.50a 28.07a 39.48a 9.38a 42.10a 63.42a

Fe-ZSM-5 4.41b −6.6c 58.13b 67.3b −4.9c 44.6c 51.9c

37.6c 94.9c

Co-ZSM-5 −13.3d 2.8d −10.1d 30.4d 54.2d −2.3d 20.1d 6.9d

6.28b −5.2c 48.56b 64.95b −5.2c 40.2c 52.8c

32.9c 85.6c

Present work. bFellah et al.31 cRyder.28 dHeyden.32 The data are O2 desorption from Z[FeO2], not Z[OFeO] when M = 6.

the second N2O decomposition over Cu-ZSM-5 is 28.07 kcal/ mol, lower than that on Fe/Co-ZSM-5. Importantly, for the rate-limiting step, the O2 desorption energy over Cu-ZSM-5 is much lower than that on Fe/Co-ZSM-5. In process 3, the adsorption energy of the third N2O is 9.38 kcal/mol, which is higher than −4.9 kcal/mol for Fe-ZSM-5 or −5.2 kcal/mol for Co-ZSM-5, respectively. The activation energy of N2O decomposition is 42.10 kcal/mol, which is almost equal to that on Fe/Co-ZSM-5. O2 desorption energy is 63.42 kcal/mol, which is higher than that of Fe/Co-ZSM-5. It implies that in the rate-limiting step O2 desorbs from Z[CuOO] on the CuZSM-5 system, while it is from Z[FeO(O2)] or Z[CoO(O2)] on the Fe/Co-ZSM-5 system. In summary, it could be concluded from the present work and from other experimental and theoretical works that: (1) only from the view of adsorption, the N2O adsorption on Fe/ Co-ZSM-5 is easier than that on Cu-ZSM-5. However, from the view of reaction and its active energy or the key desorption energy, N2O decomposition and O2 desorption over the CuZSM-5 system are easier than those over the Fe/Co-ZSM-5 system, which are very significant in a reaction. (2) The mechanism of N2O decomposition over Cu-ZSM-5 is different from that over Fe/Co-ZSM-5, particularly about the O2 desorption. Ryder’s group pointed out that the O2 desorption energy with 94.9 kcal/mol for Z[FeO2] and 85.6 kcal/mol for Z[CoO2] would result in the prohibition of the reaction even

produce Z[CuO] (a5), and release O2. The O2 desorption energy is 63.42 kcal/mol, much higher than that in process 2. From the view of energy, the energy barrier of the step is so high that the reaction of the process is hard to carry out. It is to say that the presence of O2 inhibits the N2O decomposition on Cu-ZSM-5, which is consistent with the TPD experiments by Kapteijn et al.20 They found that the peak maximum for O2 desorption occurs around 673 K, which indicates that oxygen desorption is a difficult step, and the reversibility of this step can inhibit the N2O decomposition.20 Thus, processes 1 and 2 could comprise the main catalytic cycle. Figure 5 summarizes the energy diagram of reaction processes of N2O decomposition cycle over the Cu-ZSM-5 cluster. 3.5. Discussions. The adsorption energy and the activation energy of the N2O decomposition cycle processes are compared with available theoretical literature values, as shown in Table 4. In process 1, the N2O adsorption energy on CuZSM-5 is −10.07 kcal/mol, which is higher than that on FeZSM-5. The activation energy of N2O decomposition on CuZSM-5 is 35.18 kcal/mol, which is higher than that on Fe/CoZSM-5. It suggests that the first N2O decomposition on Fe/CoZSM-5 is easier than that on Cu-ZSM-5. However in process 2, it is found that the N2O decomposition on Cu-ZSM-5 is easier than that on Fe/Co-ZSM-5. The energy of N2O absorption is −1.50 kcal/mol over Cu-ZSM-5; it is to say there is little difference with that on Fe/Co-ZSM-5. The activation energy of 20266



Article

4. CONCLUSIONS The catalytic cycles of three N2O molecules decomposition over Cu-ZSM-5 cluster modeled as [(SiH3)4AlO4Cu] are investigated using DFT. The present work considerers the two possible reaction pathways for O2 desorption. According to the results, it is found that O2 desorption occurs from the Z[CuOO] species, rather than from the Z[CuO(O2)] species, and that the O2 desorption energy of from Z[CuOO] is lower than that from Z[FeOO] or Z[CoOO]. Moreover, the O2 desorption in process 2 is the rate-limiting step with an energy barrier of 39.48 kcal/mol. Resulting from the much higher energy barriers of the process 3 than other processes, processes 1 and 2 comprise the main catalytic cycle in N 2 O decomposition over Cu-ZSM-5, which are favorable pathway as shown in the following:

by an entropic gain, and thought that O2 desorbs from Z[FeO(O2)] and Z[CoO(O2)], corresponding desorption energy are 51.9 and 52.8 kcal/mol, respectively.27 While in the present work, O2 desorption mainly comes from Z[CuOO], rather than from Z[CuO(O2)], because the energy barrier of process 3 is so high that the reaction is hard to carry out. Heyden et al. discovered that the main pathway of O2 desorption is the Z[FeO(O2)] yielding Z[FeO], which is the fast O2 desorption process with a relatively low energy barrier in N2O decomposition on hydrated and dehydrated Fe-ZSM5.31 Their results indicate that the N2O decomposition over FeZSM-5 is absent of O2 inhibition. Thus, the mechanism of N2O decomposition over Cu-ZSM-5 is different from that over Fe/ Co-ZSM-5, especially in the O2 desorption step. Additionally, the adopt mechanism exhibits the monocopper pathway. In fact, there are two different pathways: one from monocopper and the other from dicopper or copper pairs.9−12 H2 temperature-programmed reduction monitored by mass spectrometry and in situ X-ray absorption spectroscopy showed that the Cu on ZSM-5 occurs predominantly as isolated exchanged Cu2+ and as exchanged Cu dimers.13 It suggests that it is very likely that both kinds of species are present in the zeolite.9−13 For the single copper on the ZSM-5, Yamashita reported that the isolated Cu+ monomers and Cu+-Cu+ dimers are found to exist within the zeolites, their relative concentrations strongly depends on the type of zeolite used. With ZSM-5 and mordenite zeolites, most of the copper cations are found to exist as isolated Cu+ monomers with low coordinate numbers (planar 3 or linear 2 coordinate geometry) in contrast to the case of the Y-zeolite having dimers and/or clusters.48,49 The EPR spectra show the presence of a single copper type with g⊥ = 2.06, g∥ = 2.38, and A∥ = 410 MHz.50 The EXAFS data also suggested contribution of structure of Cu−Cu bond in Cu-ZSM-5 with high Cu loading.51 The work from Schoonheydt et al. applied the UV−vis and EXAFS techniques to identify the dimer copper core based on the high Cu loading, i.e., Cu/Al > 0.2.9,16 Simultaneously, it also could be concluded from Figure 5 and Table 3 that the O2 desorption step is the rate-limiting step for the N2O decomposition over Cu-ZSM-5. While the O2 desorption step is a reversible equation, and the partial pressure of O2 has significant effect on the desorption equation. Because of the high partial pressure of O2, the Z[CuOO] hardly absorb the third N2O according to process 3. It results in the active center of Cu-ZSM-5 loss of catalytic activity. The result clearly shows that the Cu-ZSM-5 is prone to suffer from O2 inhibition, which is consistent with the TPD experiments by Kapteijn’s group.20 Though it is mentioned in section 3.3 that the energy barrier (28.07 kcal/mol) of N2O dissociation is lower than that (35.18 kcal/mol) in process 1, it only suggests the N2O dissociation is easier on Z[CuO] than that on Z[Cu]. While from the above analysis of kinetic study, it is known that the O2 desorption step of process 2 is the rate-limiting step for the N2O decomposition over Cu-ZSM-5. It displays that the whole catalytic cycle in process 1 and 2, in which Z[Cu] is active center, is more favorable than that in process 2 and 3, in which Z[CuO] is active center. Therefore, processes 1 and 2 form the main catalytic cycle for N2O decomposition over Cu-ZSM-5. In contrast to Fe/Co-ZSM-5, Cu-ZSM-5 exhibits higher activity with the lower energy barrier of O2 desorption step which is the rate-limiting step. It indicates that Cu-ZSM-5 is predicted to be significant application in catalyzing N2O decomposition.

+N2O

−N2

+N2O

Z[Cu] ⎯⎯⎯⎯⎯→ Z[CuON2] ⎯⎯⎯→ Z[CuO] ⎯⎯⎯⎯⎯→ Z[CuOON2] −N2

−O2

⎯⎯⎯→ Z[CuOO] ⎯⎯⎯→ Z[Cu]

These results show that the mechanism of N2O decomposition over the Cu-ZSM-5 system is different from that over the Fe/ Co-ZSM-5 system, though the adapted mechanism is similar with Fe/Co-ZSM-5. Meanwhile, the mechanism of three processes could easily illustrate the effects of O2 inhibition on N2O decomposition over Cu-ZSM-5, which is in agreement with both the experiment and theoretical literature. In our future work, the mechanism about the dicopper pathway would be discussed and calculated by the method of DFT. We expect that the present calculated results could provide some clues for the further experimental research.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone:+86-10-64446649. Fax: +86-10-64446649. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The present work is supported by the National Natural Science Foundation of China (Grant Nos. 21103007, 21177008, and 21075007), by the 863 Program (Grant No. 2012AA03A609), and by the Fundamental Research Funds for the Central Universities (Grant No. ZZ1231). The project is also supported by the “Chemical Grid Project” of Beijing University of Chemical Technology.

■

REFERENCES

(1) Yamada, K.; Pophal, C.; Segawa, K. Microporous Mesoporous Mater. 1998, 21, 549−555. (2) Jiang, H.; Wang, H.; Liang, F.; Werth, S.; Schiestel, T.; Caro, J. Angew. Chem. 2009, 48, 2983−2986. (3) Pérez-Ramírez, J.; Kapteijn, F.; Schöffel, K.; Moulijn, J. A. Appl. Catal. B: Environ. 2003, 44, 117−151. (4) Kapteijn, F.; Rodriguez-Mirasol, J.; Moulijn, J. A. Appl. Catal. B: Environ. 1996, 9, 25−64. (5) Yu, J.; Xu, R. Acc. Chem. Res. 2010, 43, 1195−1204. (6) Chang, Y. F.; McCarty, J. G.; Wachsman, E. D.; Wong, V. L. Appl. Catal. B: Environ. 1994, 4, 283−299. (7) Panov, G. I.; Sobolev, V. I.; Kharitonov, A. S. J. Mol. Catal. 1990, 61, 85−97. (8) Sang, C.; Kim, B. H.; Lund, C. R. F. J. Phys. Chem. B 2005, 109, 2295−2301.

20267



Article

(9) Groothaert, M. H.; van Bokhven, J. A.; Battiston, A. A.; Weckhuysen, B. M.; Schoonheydt, R. A. J. Am. Chem. Soc. 2003, 125, 7629−7640. (10) Smeets, P. J.; Groothaert, M. H.; van Teeffelen, R. M.; Leeman, H.; Hensen, E. J. M.; Schoonheydt, R. A. J. Catal. 2007, 245, 358−368. (11) Smeets, P. J.; Sels, B. F.; Van Teeddelen, R. M.; Leeman, H.; Hensen, E. J. M.; Schoonheydt, R. A. J. Catal. 2008, 256, 183−191. (12) Lisi, L.; Pironea, R.; Russoc, G.; Santamariac, N.; Stanzionea, V. Appl. Catal., A 2012, 413- 414, 117−131. (13) Da Costa, P.; Modén, B.; Meitzner, G. D.; Ki Leeza, D.; Iglesia, E. Phys. Chem. Chem. Phys. 2002, 4, 4590−4601. (14) Groothaer, M. H.; Smeets, P. J.; Sels, B. F.; Jacobs, P. A.; Schoonheydt, R. A. J. Am. Chem. Soc. 2005, 127, 1394−1395. (15) Woertink, J. S.; Smeets, P. J.; Groothaert, M. H.; Vance, M. A.; Sels, B. F.; Schoonheydt, R. A.; Solomon, E. I. Proc. Natl. Acad. Sci. 2009, 106, 18908−18913. (16) Vanelderen, P.; Hadt, R. G.; Smeets, P. J.; Solomon, E. I.; Schoonheydt, R. A.; Sels, B. F. J. Catal. 2011, 284, 157−164. (17) Wood, B. R.; Reimer, J. A.; Bell, A. T.; Janicke, M. T.; Ott, K. C. J. Catal. 2004, 224, 148−155. (18) Jocono, M. L.; Fierro, G.; Dragone, R.; Feng, X.; Itri, J.; Hall, W. K. J. Phys. Chem. B 1997, 101, 1979−1984. (19) Dědeček, J.; Wichterlovǎ, B.; Kubát, P. Microporous Mesoporous Mater. 1999, 32, 63−74. (20) Kapteijn, F.; Marban, G.; Mirasol, J. R.; Moulijn, J. A. J. Catal. 1997, 167, 256−265. (21) Malek, K.; Li, C.; van Santen, R. A. J. Mol. Catal. A: Chem. 2007, 271, 98−104. (22) Pápai, I.; Goursot, A.; Fajula, F.; Weber, J. J. Phys. Chem. 1994, 98, 4654−4659. (23) Yang, H.; Asplund, M. C.; Kotz, K. T.; Wilken, M. J.; Frei, H.; Harris, C. B. J. Am. Chem. Soc. 1998, 120, 10154−10165. (24) Hansen, N.; Brüggemann, T.; Bell, A. T.; Keil, F. J. J. Phys. Chem. C 2008, 112, 15402−15411. (25) Fellah, M. F.; Onal, I. J. Phys. Chem. C 2010, 114, 3042−3051. (26) Fellah, M. F.; Pidko, E. A.; van Santen, R. A.; Onal, I. J. Phys. Chem. C 2011, 115, 9668−9680. (27) Ryder, J. A.; Chakraborty, A. K.; Bell, A. T. J. Phys. Chem. B 2002, 106, 7059−7064. (28) Kucherov, A. V.; Shelef, M. J. Catal. 2000, 195, 106−112. (29) Armor, J. N.; Farris, T. S. Appl. Catal. B: Environ. 1994, 4, L11− L17. (30) Fellah, M. F.; Onal, I. Catal Today. 2008, 137, 410−417. (31) Heyden, A.; Peters, B.; Bell, A. T.; Keil, F. J. J. Phys. Chem. B 2005, 109, 1857−1873. (32) D. Sengupta, D.; Adams, J. B.; Schneider, W. F.; Hass, K. C. Catal. Lett. 2001, 74, 193−199. (33) Despres, J.; Koebel, M.; Krövher, O.; Elsener, M.; Wokaun, A. Microporous Mesoporous Mater. 2003, 58, 175−183. (34) Zecchina, A.; Bordiga, S.; Palomino, G. T.; Scarano, D.; Lamberti, C.; Salvalaggio, M. J. Phys. Chem. B 1999, 103, 3833−3844. (35) Moretti, G.; Ferraris, G.; Fierro, G.; Jacono, M. L.; Morpurgo, S.; Faticanti, M. J. Catal. 2005, 232, 476−487. (36) Beutel, T.; Adelman, B. J.; Sachtler, W. M. H. Appl. Catal. B: Environ. 1996, 9, L1−L10. (37) Zakharov, I. I.; Ismagilov, Z. R.; Ruzankin, S. P.; Anufrienko, V. F.; Yashnik, S. A.; Zakharova, O. I. J. Phys. Chem. C 2007, 111, 3080− 3089. (38) da Cruz, R. S.; Mascarenhas, A. J. S.; Andrade, H. M. C. Appl. Catal. B: Environ. 1998, 18, 223−231. (39) Redondo, A.; Hay, P. J. J. Phys. Chem. 1993, 97, 11754−11761. (40) Lund, C. R. F. J. Catal. 2006, 243, 438−441. (41) Becke, A. D. Phys. Rev. B 1988, 38, 3098−3100. (42) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789. (43) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;

Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.02; Gaussian, Inc.: Wallingford, CT, 2004. (44) Fellah, M. F.; van Santen, R. A.; Onal, I. J. Phys. Chem. C 2009, 113, 15307−15313. (45) Zhang, Y.; Drake, I. J.; Bell, A. T. Chem. Mater. 2006, 18, 2347− 2356. (46) Bulushev, D. A.; Kiwi-Minsker, L.; Renken, A. J. Catal. 2004, 222, 389−396. (47) McQuarrie, D. A. Statistical Mechanics; Harper Collins Publisher Inc.: New York, 1973. (48) Yamashita, H.; Matsuoka, M.; Tsuji, K.; Shiya, Y.; Anpo, M. J. Phys. Chem. 1996, 100, 397−402. (49) Yamashita, H.; Matsuoka, M.; Anpo, M.; Che, M. J. Phys. IV 1997, 7, C2−941- 942. (50) Carl, P. J.; Vaughan, D. E. W.; Goldfarb, D. J. Am. Chem. Soc. 2006, 128, 7160−7161. (51) Hamada, H.; Matsubayashi, N.; Shimada, H.; Kintaichi, Y.; Ito, T.; Nishjima, A. Catal. Lett. 1990, 5, 189−196.

20268


Density Functional Theory Study of Mechanism of N2O Decomposition

Recommend Documents