MetalâPorphyrin: A Potential Catalyst for Direct Decomposition of N2O

Article pubs.acs.org/est

Metal−Porphyrin: A Potential Catalyst for Direct Decomposition of N2O by Theoretical Reaction Mechanism Investigation Phornphimon Maitarad,† Supawadee Namuangruk,‡ Dengsong Zhang,*,† Liyi Shi,† Hongrui Li,† Lei Huang,† Bundet Boekfa,§ and Masahiro Ehara*,§ †

Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, P. R. China National Nanotechnology Center (NANOTEC), NSTDA, 111 Thailand Science Park, Pahonyothin Road, Klong Luang, Pathum Thani 12120, Thailand § Institute for Molecular Science and Research Center for Computational Science, 38 Nishigo-naka, Myodaiji, Okazaki 444-8585, Japan

Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on July 16, 2018 at 06:36:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: The adsorption of nitrous oxide (N2O) on metal−porphyrins (metal: Ti, Cr, Fe, Co, Ni, Cu, or Zn) has been theoretically investigated using density functional theory with the M06L functional to explore their use as potential catalysts for the direct decomposition of N2O. Among these metal− porphyrins, Ti−porphyrin is the most active for N2O adsorption in the triplet ground state with the strongest adsorption energy (−13.32 kcal/mol). Ti− porphyrin was then assessed for the direct decomposition of N2O. For the overall reaction mechanism of three N2O molecules on Ti−porphyrin, two plausible catalytic cycles are proposed. Cycle 1 involves the consecutive decomposition of the first two N2O molecules, while cycle 2 is the decomposition of the third N2O molecule. For cycle 1, the activation energies of the first and second N2O decompositions are computed to be 3.77 and 49.99 kcal/mol, respectively. The activation energy for the third N2O decomposition in cycle 2 is 47.79 kcal/mol, which is slightly lower than that of the second activation energy of the first cycle. O2 molecules are released in cycles 1 and 2 as the products of the reaction, which requires endothermic energies of 102.96 and 3.63 kcal/mol, respectively. Therefore, the O2 desorption is mainly released in catalytic cycle 2 of a TiO3−porphyrin intermediate catalyst. In conclusion, regarding the O2 desorption step for the direct decomposition of N2O, the findings would be very useful to guide the search for potential N2O decomposition catalysts in new directions.

1. INTRODUCTION Nitrous oxide (N2O) emitted from industrial processes and vehicle engines is an environmentally polluting gas. Moreover, it has been recognized as a strong greenhouse gas contributing to the destruction of ozone in the stratosphere.1−3 Because of the continuous increase in its concentration in the atmosphere, the discovery and development of efficient catalysts for the reduction of N2O have become important issues in the field of environmental research. A direct catalytic decomposition of N2O into N2 and O2 is thought to be the most convenient and economical option to reduce N2O emissions.4 Therefore, several catalytic materials have been reported for direct decomposition of N2O gas, such as M-zeolites (M = Cu, Co, Fe, etc.),5−9 carbon nanotubes,10,11 perovskite-like mixed oxides,12−14 alumina-supported precious metals (Pd, Rh, etc.),15,16 and metal alloys.17 Among these catalysts, transition-metal ion-exchanged ZSM-5 zeolites, especially for the Fe-, Co-, and Cu-exchanged zeolites, have been widely used because they show high catalytic activity.9,18−23 However, these catalysts suffer from oxygen inhibition and the low reaction rate of the N2O decomposition. © 2014 American Chemical Society

In general, the full reaction mechanism of direct N2O decomposition has been reported as the following:24−28 [Cat] + N2O (g) → [Cat]···ON2

[Cat]···ON2 → [Cat]O + N2 (g)

[Cat]O + N2O (g) → [Cat]O···ON2 [Cat]O···ON2 → [Cat]O2 + N2 (g)

[Cat]O2 → [Cat] + O2 (g)

Both theoretical and experimental studies have reported that the recombination of two oxygen atoms into the O2 molecule or the O2 desorption is the rate-limiting step in the overall direct N2O decomposition reaction.9,25,28 Thus, the development of new catalysts for direct N2O decomposition will be Received: Revised: Accepted: Published: 7101

December 28, 2013 April 23, 2014 May 23, 2014 May 23, 2014 dx.doi.org/10.1021/es405767d | Environ. Sci. Technol. 2014, 48, 7101−7110

Environmental Science & Technology

Article

Figure 1. Model systems in this work.

2. METHODS As the model of catalysts, single metal−porphyrins in which the metal is Ti, Cr, Fe, Co, Ni, Cu, or Zn were examined without a support or assembly. Neutral metal−porphyrins, the core system of the catalyst, were assessed in some spin states without considering the charge transfer from the support or surroundings when locating the energetically lowest geometric structure. The most stable spin state of each metal−porphyrin was selected to examine the N2O adsorption ability. The spin state of the adsorption complex (N2O−metal−porphyrin) was set to the same spin state as the metal−porphyrin without considering spin crossing. For the initial structure of the geometry optimization, the N2O molecule was laid along the perpendicular axis to the plane of the porphyrin ring with the oxygen atom of N2O pointed to the metal atom of the metal− porphyrin (Figure 1). The adsorption energy (Ead) of N2O was calculated by

accelerated by the preliminary study of the energetics along the reaction pathway by using, for example, density functional theory (DFT) calculations prior to the experiment. Porphyrin readily forms ordered monolayers by self-assembly and possesses two axial coordination sites that are available as centers of catalytic activity or sensor functionality. Metal− porphyrins are well suited for anchoring on solid substrates as assemblies in various types of applications, such as photovoltaic materials, field-responsive materials, catalytic materials, etc.29−33 It has been generally recognized that the metal− porphyrins are highly active toward oxygen, nitric oxide, carbon monoxide, etc.34−39 Therefore, one important application of metal−porphyrins is the sensing of gases, in which the coordination of gas molecules to the metal center causes measurable changes of electronic properties, color, etc.30,40−43 In addition, the metal−porphyrins can be synthesized as microporous solid frameworks that have a selective sorption of small molecules and size- or shape-selective heterogeneous catalysis.44−48 Metal−porphyrins have been considered as potential catalysts for many commercially important reactions such as the reduction of carbon dioxide49−51 and nitrogen oxides52−54 and the oxidation of hydrocarbons and alcohols.55−58 Although there are some reports on the ability of metal− porphyrins with respect to N2O adsorption,35,59 their catalytic activity for direct N2O decomposition has not yet been demonstrated. Therefore, this motivated us to apply DFT calculations to the N2O adsorption ability of metal−porphyrins, where the metal is Ti, Cr, Fe, Co, Ni, Cu, or Zn, to find potential catalysts for the N2O decomposition. The DFT calculations suggest that among these metal−porphyrins, Ti− porphyrin is the most active to N2O adsorption in its triplet ground state. Therefore, the possible direct N2O decomposition over a Ti−porphyrin catalyst has been examined and three N2O decomposition mechanisms have been proposed. O2 desorption, which is a key step for the N2O decomposition, has also been considered to determine a favorable pathway. The characteristics and performance of the present Ti−porphyrin are compared with those of the potential catalysts of Fe-, Co-, and Cu-ZSM-5 zeolites.9,19,28

Ead = Ecomplex − (EM − por + E N2O)

(1)

where Ecomplex, EM−por, and EN2O are the total energies of the metal−porphyrin···N2O complex, metal−porphyrin, and N2O molecule, respectively. The direct N2O decomposition over Ti−porphyrin was examined. During the geometry optimizations, all atoms of the Ti−porphyrin complexes were fully relaxed. The transition states were confirmed as the real saddle points with only one imaginary frequency by vibrational analysis. The reaction energy profiles of each step were presented in the relative energy, which is defined as ΔE = Ecomplex − (Ecatalyst + Eabsorbate)

(2)

where Ecomplex, Ecatalyst, and Eabsorbate are the energies of the Ti− porphyrin−gas complexes, the Ti−porphyrin at each step, and the small gas molecules, e.g., N2O, O2, and N2. For the reaction pathway, we examined the possibility of the spin crossing or intersystem crossing and found that the singlet states of some intermediates and transition states were more stable than their triplet counterparts. All of the electronic structure calculations and the geometry optimizations were performed using the DFT method with the M06L functional60 without any restriction on the symmetry. 7102

dx.doi.org/10.1021/es405767d | Environ. Sci. Technol. 2014, 48, 7101−7110


Article

Table 1. Structural Parameters (Bond Lengths in Å, Angles in deg), Adsorption Energy (kcal/mol), and Partial Charges (e) on the Selected Atoms of the Metal−Porphyrin and N2O···Metal−Porphyrin Complex Systems, Where the Metal Is Ti, Cr, Fe, Co, Ni, Cu, or Zn

N2O

Ti

Cr

Fe

Co

Ni

Cu

Zn

structural parameters M-por system spin multiplicity M−Npor ∠Npor−M−Npor N2O···M-por system N2O adsorption energy M−Npor ∠Npor−M−Npor M−O1 O1−N2 N2−N3 ∠M−O1−N2 ∠O1−N2−N3 M-por system M Npor N2O···M-por system M Npor O1 N2 N3 total charge of N2O (qdiff) Mdiff (MN2O‑Mpor) Npor‑diff (NN2O‑Npor)

1.19 1.14 180.0

−0.30 0.38 −0.08 0.00

3 2.06 180

5 2.04 180

3 2.00 180

2 1.99 180

1 1.97 180

2 2.02 180

1 2.06 180

−13.32 2.06 172.4 2.25 1.20 1.14 120.3 178.3

−6.81 −7.81 2.04 2.00 178.5 179.4 2.74 2.74 1.19 1.19 1.14 1.14 111.1 108.4 179.6 179.9 partial charges

−6.84 1.99 179.2 2.64 1.19 1.14 108.7 179.9

−4.65 1.97 179.7 3.14 1.19 1.14 92.1 179.9

−5.48 2.02 179.7 2.81 1.19 1.14 106.4 179.8

−6.92 2.06 176.9 2.54 1.19 1.14 114.3 179.4

1.46 −0.70

0.96 −0.62

0.79 −0.59

0.82 −0.58

0.71 −0.56

0.99 −0.63

1.39 −0.72

1.29 −0.67 −0.29 0.40 0.02 0.12 −0.18 0.03

0.92 −0.61 −0.30 0.39 −0.06 0.04 −0.04 0.00

0.84 −0.59 −0.29 0.39 −0.06 0.04 0.05 0.00

0.78 −0.58 −0.29 0.39 −0.06 0.05 −0.04 0.00

0.69 −0.57 −0.29 0.39 −0.08 0.02 −0.02 −0.00

0.97 −0.63 −0.29 0.39 −0.07 0.03 −0.03 0.00

1.36 −0.71 −0.31 0.39 −0.04 0.04 −0.03 0.00

The 6-31G* basis set61 was used for the C, O, N, and H atoms, and the scalar relativistic effective core potential of LANL2DZ62 was adopted for the transition metal elements. All calculations in the present work were carried out using the Gaussian 09 suite of programs, revision B01.63 The charge analyses of the systems were performed using the natural bond orbital (NBO) analysis.64

state for the Cr−porphyrin. The metal−nitrogen (M−Npor) bond distance of the metal−porphyrin is about 2 Å, and the spin state is correlated to the M−Npor bond distance, reflecting the coordination field of porphyrin. All metal−porphyrins were found to be planar as represented by a ∠Npor−M−Npor angle of 180°. The present results are consistent with the previous works.65,66 Comparing the structures of the metal−porphyrins and the N2O-adsorbed complexes, the adsorption of N2O over the metal−porphyrins does not affect the M−Npor distances in almost all systems except for Ti−porphyrin, which shows slight structure reorganization (∠Npor−M−Npor = 172°). Ti− porphyrin provided a considerable adsorption energy of −13.32 kcal/mol, while other metal porphyrins exhibited low adsorption energies in the range of −4.6 to −7.8 kcal/mol. We also examined the adsorption of the N-binding mode of N2O (see Table S2) and obtained the adsorption energy of −22.27 kcal/mol, which indicates that N-binding adsorption also competes with O-binding over Ti−porphyrin. In the present work, we focus on the O-binding adsorption, as in other

3. RESULTS AND DISCUSSION 3.1. N2O Adsorption over M (Ti, Cr, Fe, Co, Ni, Cu, or Zn)−Porphyrins. The adsorption energy of N2O, partial charges, and selected structural parameters for the Ti−, Cr−, Fe−, Co−, Ni−, Cu−, and Zn−porphyrins are summarized in Table 1. The relative energies of the metal−porphyrins with various spin multiplicities are compared in Table S1. Based on the unrestricted DFT calculations, the Ni− and Zn−porphyrins are found to have a closed-shell singlet ground state, and the Co− and Cu−porphyrins prefer the low-spin doublet state. The Ti− and Fe−porphyrins, on the other hand, result in the triplet ground state, whereas the high-spin quintet state is the lowest 7103



Article

Figure 2. Mechanistic cycles of direct decomposition of N2O over Ti−porphyrin while the inner cycle shows the recycling of the active TiO− porphyrin catalytic intermediate for the third N2O decomposition.

Figure 3. First N2O decomposition over Ti−porphyrin (step 1).

works,9,19,28 because TiO−porphyrin forms a very stable intermediate, as shown later. In addition, an adsorbed distance between Ti and the O1 atom of the N2O molecule was 2.25 Å, which is the shortest metal−oxygen (M−O1) distance among the complexes. The adsorption M−O1 distance depends on both the M−O1 interaction and the metal radius. Because of its relatively strong interaction, the N2O molecule over the Ti− porphyrin catalyst resulted in the slightly elongated O1−N2 bond. The NBO partial charges of the metal−porphyrins and N2O···metal−porphyrin complexes are also listed in Table 1. The charge difference (qdiff) between N2O gas (q = 0) and adsorbed N2O shows the amount of charge reorganization or transfer from N2O to the metal−porphyrins upon N2O

adsorption. We found that qdiff increases in the order Ni− (0.02) < Cu− (0.03) < Zn−, Cr−, Fe− (0.04) < Co− (0.05) < Ti−porphyrin (0.12). This charge reorganization contributes to the adsorption strength. Therefore, the Ti−porphyrin was chosen to examine its catalytic activity of the direct N2O decomposition in the following sections. 3.2. N2O Decomposition over Ti−Porphyrin. For the direct N2O decomposition, the reaction mechanism was investigated over Ti−porphyrin, which exhibited the strongest adsorption of N2O. The assessment of the reaction pathways follows the structures and the spin states of the N2O···Ti− porphyrin complex. The reaction mechanism of direct decomposition of three N2O molecules can be decomposed into four elementary reaction steps as follows: 7104



Article

Figure 4. Second N2O decomposition over TiO−porphyrin (step 2).

from 1.19 to 1.20 Å. The calculated adsorption energy (Ead1) is −13.32 kcal/mol. At the transition state (TS1), the ∠Ti−O−N angle changes from 120° to 156° and the Ti−O bond is contracted to 2.06 Å. The first transition state (TS1) has a relative energy of −9.55 kcal/mol with one imaginary frequency of 418i cm−1, which corresponds to the reaction coordinate. The Ti active site abstracts the O1 atom from the absorbed N2O to form the TiO−porphyrin intermediate and the N2 molecule (IM1). Spin crossing occurs between TS1 and IM1, assuming that the spin−orbit interaction is large enough. Otherwise, the reaction may proceed in the triplet state, as shown in the Supporting Information (Figure S1). This step proceeds in a highly exothermic manner with the relative energy of IM1 being −116.53 kcal/mol, indicating that IM1 is very stable. Finally, the N2 molecule is easily desorbed from the TiO−porphyrin intermediate (IM2), which requires an energy of only 3.73 kcal/mol. Therefore, in the first N 2 O decomposition process, the N2O molecule readily adsorbs onto the Ti−porphyrin, the N−O bond scission has a low activation energy barrier (Ea1) of 3.77 kcal/mol, and the TiO− porphyrin is a thermodynamically stable intermediate. 3.2.2. The Second N2O Decomposition over TiO− Porphyrin (Step 2). For the second N2O decomposition (Figure 4), although all steps are assumed to be similar to the first N2O decomposition, the active site is based on the TiO− porphyrin intermediate instead of Ti−porphyrin. The second N2O molecule adsorbs over the TiO−porphyrin with the adsorption energy (Ead2) of −2.36 kcal/mol. It is seen that the second N2O molecule weakly adsorbs on the TiO−porphyrin relative to the first one. Regarding the adsorption geometry (AD2), N2O retains a linear structure with unchanged bond distances. The transition state (TS2) was confirmed with an imaginary frequency of 838i cm−1 representing the reaction coordinate. At TS2, the O2 atom of N2O approaches the TiO− porphyrin, with the Ti···O2 distance being 2.05 Å. The adsorbed N2O shows a bent structure with ∠O2−N1−N2 of 148° and undergoes the predominant dissociation of the N1− O2 bond. This step requires the activation barrier energy (Ea2) of 49.99 kcal/mol, which is much higher than the first N2O

Step 1 [Ti] + 1st N2O (g) → [Ti]···ON2 → [Ti]O + N2 (g) Step 2 [Ti]O + 2ndN2O (g) → [Ti]O···ON2 → [Ti]O2 + N2 (g) → [Ti] + O2 (g)

Step 3 [Ti]O2 + 3rd N2O (g) → [Ti]O2 ···ON2 → [Ti]O3 + N2 (g) Step 4 [Ti]O3 → [Ti]O···O2 → [Ti]O + O2 (g)

Following the previous reports on the full direct N2O decomposition mechanism,9,24−28 the present system, namely, N2O decomposition over Ti−porphyrin, is schematized in Figure 2. The products of the N2O decomposition are N2 and O2 molecules. We assume that the overall reaction mechanism consists of two cycles. Cycle 1 has two steps: steps 1 and 2 (see equations above), which involve the consecutive decomposition of the first and second N2O molecules, respectively. Cycle 2 consists of the third N2O decomposition (step 3) and the depletion of O2 from the TiO3−porphyrin intermediate (step 4). The calculated total energy of all of the intermediates and transition states are listed in Table S3, and the coordinates of the transition states are given in SI. 3.2.1. The First N2O Decomposition over Ti−Porphyrin (Step 1). In step 1, the reaction energy profile of the first N2O molecule over Ti−porphyrin consists of N2O adsorption, N−O bond scission, and N2 desorption (Figure 3). At the adsorption complex (AD1), the N2O molecule is located on the Ti atom in a tilted structure with a ∠Ti−O−N angle of about 120°. The Ti···O distance is 2.25 Å and the O−N1 bond slightly elongates 7105



Article

Figure 5. Third N2O decomposition over TiO2−porphyrin (step 3).

Figure 6. O2 depletion and desorption processes from the TiO3−porphyrin intermediate catalyst (step 4).

(Figure 5). The adsorption energy (Ead3) of the third N2O is −5.22 kcal/mol (AD3), which is slightly stronger than that of the second N2O adsorption (−2.36 kcal/mol). Reflecting the weak adsorption, the bond distance of the TiO2−porphyrin catalyst and N2O molecule is nearly unaltered at the adsorption step. Thus, to activate an N2O molecule over the TiO2− porphyrin via TS3, it needs the relatively high activation energy (Ea3) of 47.79 kcal/mol. At TS3 with an imaginary frequency of 747i cm−1, the oxygen atom (O3) of the N2O molecule points to the Ti active site with the interaction distance of Ti···O3 = 2.08 Å. The O1···O2 interaction distance is shortened from 1.44 to1.38 Å, while the Ti−O1 and Ti−O2 bond distances are elongated. At TS3, the N2O molecule is distorted from linear to a bent structure of about ∠O3−N1−N2 = 142°. In addition, the O3−N1 bond is significantly lengthened, which implies that the O3−N1 bond dissociates and then O3−Ti or O3−O2 may

decomposition (Ea1). In the N2 generation (IM21), the O atom from the N2O molecule is abstracted by the TiO−porphyrin to form the TiO2−porphyrin intermediate. The Ti−O bond and O1···O2 distance are 1.83 and 1.44 Å, respectively. The calculated relative energy is −3.65 kcal/mol. The N2 molecule simultaneously desorbs from the TiO2−porphyrin intermediate (IM22) with desorption energy of 3.70 kcal/mol. It is possible that the O2 molecule is generated in this step. However, it was found that the O2 desorption energy (Ede1) is very high (102.96 kcal/mol) (IM23), implying that the TiO2−porphyrin would not easily release the O2 molecule. Thus, an alternative route for the TiO2−porphyrin catalytic intermediate is considered, and it could decompose another N2O molecule. 3.2.3. The Third N2O Decomposition over TiO2−Porphyrin (Step 3). The energetic profile for the third N2O decomposition over the TiO2−porphyrin intermediate (IM22) was calculated 7106



Article

Figure 7. Energy profile of the full reaction pathway for the direct decomposition of three N2O molecules on the Ti−porphyrin catalyst.

form a new bond on the catalyst surface. Thus, the TiO2− porphyrin abstracts the oxygen atom from the N2O molecule, then the N2 molecule and TiO3−porphyrin intermediate are produced (IM31). In step 3, the TiO2−porphyrin intermediate shows a stronger N2O adsorption ability than that of the TiO− porphyrin of step 2. The activation energy barrier of the third N2O insertion on the TiO2−porphyrin is also slightly lower than that of the second N2O insertion by about 2 kcal/mol. These can imply that the third N2O desorption over the TiO2− porphyrin is a prominent reaction pathway as compared with O2 desorption of the TiO2−porphyrin. Desorption of the N2 molecule is calculated to be 4.72 kcal/mol (IM32), which is slightly higher than those of the first and second N2 molecules. This is because of the less stable nature of the TiO3−porphyrin intermediate. 3.2.4. Oxygen Depletion from TiO3−Porphyrin (Step 4). The intermediate obtained via the three N2O decompositions is the TiO3−porphyrin (IM32). The four-membered-ring structure (Ti−O1−O2−O3) of IM32 is very strained; the O2 molecule is therefore easily lost to release the strain (Figure 6). The optimized structure of TiO3−porphyrin shows that the Ti−O1 and Ti−O2 have equal bond lengths of 1.91 Å. At TS4 (imaginary frequency = 188i cm−1), the obtained geometry is very similar to the IM32; therefore, it is not surprising that the activation energy (Ea4) of the O2 depletion from IM32 is only 5.97 kcal/mol because of the unstable four-membered-ring structure in the TiO3−porphyrin intermediate. Thus, this observation can imply that the TiO3−porphyrin catalytic intermediate would spontaneously generate an O2 molecule. Spin crossing again occurs to produce the stable triplet TiO··· O2 intermediate (IM41) located at −59.69 kcal/mol. The

product of this step is the TiO−porphyrin (IM2), which is a key intermediate catalyst for the N2O decomposition of cycle 2. The O2 desorption is an important process for the direct N2O decomposition reaction because it is usually the ratelimiting step.9,25,26 In the present case, considering the O2 desorption (Figure 2), there are two possible intermediates to recombine two oxygen atoms into an O2 molecule. The first one is the TiO2−porphyrin (IM22), an intermediate from the second N2O decomposition. The TiO2−porphyrin needs a very high desorption energy (Ede1) of about 103 kcal/mol to release the O2 molecule; therefore, this process hardly occurs. Another possible route for the O2 desorption exists in step 4. Herein, the O2 desorption energy (Ede2) was calculated to be only 3.63 kcal/mol, which is much lower than that of the O2 desorption in step 2. Therefore, the O2 molecule desorbs from the TiO− porphyrin···O2 intermediate in a spontaneous route that requires a very low endothermic energy. A full reaction mechanism for the direct N2O decomposition over the Ti−porphyrin catalyst is summarized in Figure 7. As mentioned above, there are two catalytic cycles for the three N2O decompositions. Cycle 1 consists of steps 1 and 2 based on Ti−porphyrin recycling catalyst form; this step shows an O2 desorption barrier of about 103 kcal/mol. For steps 2, 3, and 4 in cycle 2, the TiO−porphyrin is a recycling catalyst for this cycle, and the O2 desorption barrier requires only 3.63 kcal/ mol. Therefore, based on the key step of O2 desorption, cycle 2 is the main catalytic pathway for N2O decomposition over Ti− porphyrin. 3.3. Comparison of Ti−Porphyrin and Zeolites. The comparison of theoretical activation energies and O2 desorption energies of three direct N2O decompositions over the Fe-, Cu-, and Co-ZSM5 zeolites9,19,28 and the present Ti−porphyrin 7107



■

catalyst are summarized in Table 2. For the first N2O decomposition, the Ti−porphyrin and Fe/Co-ZSM-5 zeolites

(1) Total energies of isolated metal−porphyrins based on M06L/6-31G* (C, N, O, and H) LANL2DZ (Ti, Cr, Fe, Co, Ni, Cu, and Zn). (2) The N2O adsorption energies for O- and N-binding modes over the metal−porphyrins. (3) Total energies of the adsorption complexes (AD1, AD2, AD3), intermediates (IM1, IM2, IM21, IM22, IM31, IM32, IM41, IM42), and transition states (TS1, TS2, TS3, TS4). (4) Cartesian coordinates of the transition states. (5) Reaction pathway of N2O decomposition in the singlet and triplet states of Ti−porphyrin. This material is available free of charge via the Internet at http://pubs.acs.org.

energy (kcal/mol)

first N2O second N2O

third N2O

a

CuZSM-5

FeZSM-5

CoZSM-5

3.77

35.18a

4.41b

6.28b

49.99

28.07a

58.13b

48.56b

O2 desorption

102.96

39.48

a

activation barrier O2 desorption

47.79

42.10a

37.6c 67.30b 94.9c 44.6c

32.9c 64.95b 85.6c 40.2c

3.63

63.42a

51.9c

52.8c

activation barrier activation barrier

ASSOCIATED CONTENT

S Supporting Information *

Table 2. Comparison of the Activation Energy, and Desorption Energy for the Direct Decomposition of N2O over Potential Zeolites and Ti−Porphyrin Catalyst Ti− porphyrin

Article

■

AUTHOR INFORMATION

Corresponding Authors

*Phone: +86- 21-66136079. E-mail: [email protected]. *Phone: +81-564-55-7461. E-mail: [email protected].

Liu et al.9 bFellah et al.28 cRyder et al.19

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the supports of STCSM postdoctoral foundation (12R21413300) and National Natural Science Foundation of China (51108258). The work was also supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We thank Research Center for Computational Science in Okazaki, Japan and National Nanotechnology Center (NANOTEC) in Thailand for computing resources.

show similar activation energy barriers in the range of 4−6 kcal/mol, which is much lower than that of the Cu-ZSM-5 zeolite. This suggests that the first N2O decomposition over Ti−porphyrin and Fe/Co-ZSM-5 zeolites is easier than that of the Cu-ZSM-5 zeolite. On the other hand, the second N2O decomposition is feasible over the Cu-ZSM-5 zeolite. It is noted that the third N2O decomposition over all catalysts has similar energy barriers, and it is important to mention that Ti− porphyrin results in a slightly higher activation energy. In particular, the activation energy for the third N2O decomposition is higher than that for the first and second decompositions in all catalysts because of the greater steric effect of oxygen atoms bonded to the metal active site. Considering the rate-limiting step of the O2 desorption in the second and third N2O decompositions, the Cu-ZSM-5 zeolite prefers to desorb the O2 molecule from the second N2O decomposition or in the form of [Cu]-O2-ZSM-5 intermediate catalyst. In contrast, O2 desorption over Ti−porphyrin prefers to release from the third N2O decomposition or in the form of [TiO]O2−porphyrin intermediate catalyst, which corresponds well with the Fe/Co-ZSM-5 zeolite. Significantly, for the O2 desorption from the third N2O decomposition, the present Ti− porphyrin catalyst shows the lowest desorption energy barrier compared with those of O2 desorption over the Fe-, Cu-, and Co-ZSM5 zeolites, which implies that the O2 desorption seems to be a spontaneous pathway over the [TiO]O2−porphyrin intermediate catalyst. Therefore, based on the energy comparison, we propose that the Ti−porphyrin catalyst is one of the candidates for the direct decomposition of N2O. Hence, the present theoretical study demonstrates the potential use of Ti−porphyrin as a catalyst for a direct N2O decomposition that comparably enhances the N2O decomposition and effectively produces O2 depletion and desorption compared with the reaction pathways over conventional catalysts such as zeolites. Therefore, the synthesis of this catalyst and reaction kinetics are very interesting, and it is underway in our laboratory. Theoretical analysis of the extended model system of this catalyst is also underway taking account of the surroundings.

■

REFERENCES

(1) Ravishankara, A. R.; Daniel, J. S.; Portmann, R. W. Nitrous oxide (N2O): the dominant ozone-depleting substance emitted in the 21st century. Science 2009, 326 (5949), 123−125. (2) Hall, S. J.; Matson, P. A. Nitrogen oxide emissions after nitrogen additions in tropical forests. Nature 1999, 400, 152−155. (3) Becker, K. H.; Lörzer, J. C.; Kurtenbach, R.; Wiesen, P. Nitrous oxide (N2O) emissions from vehicles. Environ. Sci. Technol. 1999, 33, 4134−4139. (4) Kapteijn, F.; Rodriguez-Mirasol, J.; Moulijn, J. A. Heterogeneous catalytic decomposition of nitrous oxide. Appl. Catal., B 1996, 9, 25− 64. (5) Abu-Zied, B. M.; Schwieger, W.; Unger, A. Nitrous oxide decomposition over transition metal exchanged ZSM-5 zeolites prepared by the solid-state ion-exchange method. Appl. Catal., B 2008, 84 (1−2), 277−288. (6) Li, L. D.; Shen, Q. Y. J. J.; Hao, Z. P.; Xu, Z. P.; Lu, G. Q. M. FeUSY zeolite catalyst for effective decomposition of nitrous oxide. Environ. Sci. Technol. 2007, 41, 7901−7906. (7) Pieterse, J. A. Z.; Booneveld, S.; van den Brink, R. W. Evaluation of Fe-zeolite catalysts prepared by different methods for the decomposition of N2O. Appl. Catal., B 2004, 51 (4), 215−228. (8) Pérez-Ramírez, J.; Kapteijn, F.; Mul, G.; Moulijn, J. A. NOassisted N2O decomposition over Fe-based catalysts: effects of gasphase composition and catalyst constitution. J. Catal. 2002, 208 (1), 211−223. (9) Liu, X.; Yang, Z.; Zhang, R.; Li, Q.; Li, Y. Density functional theory study of mechanism of N2O decomposition over Cu-ZSM-5 zeolites. J. Phys. Chem. C 2012, 116 (38), 20262−20268. (10) Namuangruk, S.; Khongpracha, P.; Tantirungrotechai, Y.; Limtrakul, J. Decomposition of nitrous oxide on carbon nanotubes. J. Mol. Graphics Modell. 2007, 26 (1), 179−186. 7108



Article

(11) Pannopard, P.; Khongpracha, P.; Warakulwit, C.; Namuangruk, S.; Probst, M.; Limtrakul, J. Structures, energetics and reaction mechanisms of nitrous oxide on transition-metal-doped and -undoped single-wall carbon nanotubes. Chem. Phys. Chem. 2012, 13 (2), 583− 587. (12) Russo, N.; Fino, D.; Saracco, G.; Specchia, V. N2O catalytic decomposition over various spinel-type oxides. Catal. Today 2007, 119 (1−4), 228−232. (13) Jiang, H.; Wang, H.; Liang, F.; Werth, S.; Schiestel, T.; Caro, J. Direct decomposition of nitrous oxide to nitrogen by in situ oxygen removal with a perovskite membrane. Angew. Chem., Int. Ed. 2009, 48 (16), 2983−2986. (14) Xue, L.; He, H.; Liu, C.; Zhang, C.; Zhang, B. Promotion effects and mechanism of alkali metals and alkaline earth metals on cobaltcerium composite oxide catalysts for N2O decomposition. Environ. Sci. Technol. 2009, 43, 890−895. (15) Parres-Esclapez, S.; Illán-Gómez, M. J.; de Lecea, C. S.-M.; Bueno-López, A. On the importance of the catalyst redox properties in the N2O decomposition over alumina and ceria supported Rh, Pd and Pt. Appl. Catal., B 2010, 96 (3−4), 370−378. (16) Yuzaki, K.; Yarimizu, T.; Ito, S.; Kunimori, K. Catalytic decomposition of N2O over supported rhodium catalysts: high activities ofRh/USY and Rh/Al2O3 and the effect of Rh precursors. Catal. Lett. 1997, 47, 173−175. (17) Wei, X.; Yang, X.-F.; Wang, A.-Q.; Li, L.; Liu, X.-Y.; Zhang, T.; Mou, C.-Y.; Li, J. Bimetallic Au−Pd Alloy catalysts for N2O decomposition: effects of surface structures on catalytic activity. J. Phys. Chem. C 2012, 116 (10), 6222−6232. (18) Sang, C.; Kim, B. H.; Lund, C. R. F. Effect of NO upon N2O decomposiiton over Fe/ZSM-5 with low iron loading. J. Phys. Chem. B 2005, 109, 2295−2301. (19) Ryder, J. A.; Chakraborty, A. K.; Bell, A. T. Density functional theory study of nitrous oxide decomposition over Fe- and Co-ZSM-5. J. Phys. Chem. B 2002, 106, 7059−7064. (20) Groothaert, M. H.; van Bokhoven, J. A.; Battiston, A. A.; Weckhuysen, B. M.; Schoonheydt, R. A. Bis(μ-oxo)dicopper in CuZSM-5 and its role in the decomposition of NO: a combined in situ XAFS, UV-Vis-Near-IR, and kinetic study. J. Am. Chem. Soc. 2003, 125, 7629−7640. (21) Lisi, L.; Pirone, R.; Russo, G.; Santamaria, N.; Stanzione, V. Nitrates and nitrous oxide formation during the interaction of nitrogen oxides with Cu-ZSM-5 at low temperature. Appl. Catal. A, Gen. 2012, 413−414, 117−131. (22) Wood, B. R.; Reimer, J. A.; Bell, A. T.; Janicke, M. T.; Ott, K. C. Nitrous oxide decomposition and surface oxygen formation on FeZSM-5. J. Catal. 2004, 224 (1), 148−155. (23) Pantu, P.; Boekfa, B.; Sunpetch, B.; Limtrakul, J. Nanocavity effects on N2O decomposition on different types of Fe-Zeolites (FeFER, Fe-BEA, Fe-ZSM-5 and Fe-FAU): a combined theoretical and experimental study. Chem. Eng. Commun. 2008, 195 (11), 1477−1485. (24) Kapteijn, F.; Rodriguez-Mirasol, J.; Moulijn, J. A. Heterogeneous catalytic decomposition of nitrous oxide. Appl. Catal., B 1996, 9, 25− 64. (25) Liu, N.; Zhang, R.; Chen, B.; Li, Y.; Li, Y. Comparative study on the direct decomposition of nitrous oxide over M (Fe, Co, Cu)−BEA zeolites. J. Catal. 2012, 294, 99−112. (26) Heyden, A.; Peters, B.; Bell, A. T.; Keil, F. J. Comprehensive DFT study of nitrous oxide decomposition over Fe-ZSM-5. J. Phys. Chem. B 2005, 109, 1857−1873. (27) Sengupta, D.; Adams, J. B.; Schneider, W. F.; Hass, K. C. Theoretical analysis of N2O to N2 conversion during the catalytic decomposition of NO by Cu-zeolites. Catal. Lett. 2001, 74, 193−199. (28) Fellah, M. F.; Onal, I. N2O decomposition on Fe- and Co-ZSM5: A density functional study. Catal. Today 2008, 137 (2−4), 410− 417. (29) Suslick, K. S.; Rakow, N. A.; Kosal, M. E.; Chou, J.-H. The materials chemistry of porphyrins and metalloporphyrins. J. Porphyrins Phthalocyanines 2000, 4, 407−413.

(30) Campbell, W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh, P. J.; Gordon, K. C.; Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang, Q.; Grätzel, M.; Officer, D. L. Highly efficient porphyrin sensitizers for dye-sensitized solar cells. J. Phys. Chem. C. Lett. 2007, 111, 11760− 11762. (31) Harvey, J. Developments in the metal chemistry of N-confused porphyrin. Coordin. Chem. Rev. 2003, 247 (1−2), 1−19. (32) Scandola, F.; Chiorboli, C.; Prodi, A.; Iengo, E.; Alessio, E. Photophysical properties of metal-mediated assemblies of porphyrins. Coord. Chem. Rev. 2006, 250 (11−12), 1471−1496. (33) Cheng, N.; Kemna, C.; Goubert-Renaudin, S.; Wieckowski, A. Reduction reaction by porphyrin-based catalysts for fuel cells. Electrocatalysis 2012, 3 (3−4), 238−251. (34) Karpuschkin, T.; Kappes, M. M.; Hampe, O. Binding of O2 and CO to metal porphyrin anions in the gas phase. Angew. Chem., Int. Ed. 2013, 52 (39), 10374−10377. (35) Abdurahman, A.; Renger, T. Density functional studies of ironporphyrin cation with small ligands X (X: O, CO, NO, O2, N2, H2O, N2O, CO2). J. Phys. Chem. A 2009, 113, 9202−9206. (36) Chen, O.; Groh, S.; Liechty, A.; Ridge, D. P. Binding of nitric oxide to iron(ii) porphyrins: radiative association, blackbody infrared radiative dissociation, and gas-phase association equilibrium. J. Am. Chem. Soc. 1999, 121, 11910−11911. (37) Tsai, A. L.; Berka, V.; Martin, E.; Olson, J. S. A ″sliding scale rule″ for selectivity among NO, CO, and O2 by heme protein sensors. Biochemistry 2012, 51 (1), 172−186. (38) Kano, K.; Itoh, Y.; Kitagishi, H.; Hayashi, T.; Hirota, S. A supramolecular receptor of diatomic molecules (O2, CO, NO) in aqueous Solution. J. Am. Chem. Soc. 2008, 130, 8006−8015. (39) Wasser, I. M.; Huang, H.-W.; Moënne-Loccoz, P.; Karlin, K. D. Heme/non-heme di-iron(ii) complexes and O2, CO, and NO adducts as reduced and substrate-bound models for the active site of bacterial nitric oxide reductase. J. Am. Chem. Soc. 2005, 127, 3310−3320. (40) Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E. Tetrachelate porphyrin chromophores for metal oxide semiconductor sensitization: effect of the spacer length and anchoring group position. J. Am. Chem. Soc. 2007, 129, 4655−4665. (41) Li, L. L.; Diau, E. W. Porphyrin-sensitized solar cells. Chem. Soc. Rev. 2013, 42 (1), 291−304. (42) Sivalingam, Y.; Magna, G.; Pomarico, G.; Catini, A.; Martinelli, E.; Paolesse, R.; Di Natale, C. The influence of gas adsorption on photovoltage in porphyrin coated ZnO nanorods. J. Mater.Chem. 2012, 22 (37), 20032−20037. (43) Giancane, G.; Valli, L. State of art in porphyrin LangmuirBlodgett films as chemical sensors. Adv. Colloid Interface Sci. 2012, 171−172, 17−35. (44) Hu, J.-S.; Guo, Y.-G.; Liang, H.-P.; Wan, L.-J.; Jiang, L. Threedimensional self-organization of supramolecular self-assembled porphyrin hollow hexagonal nanoprisms. J. Am. Chem. Soc. 2005, 127, 17090−17095. (45) Lee, S. J.; Hupp, J. T. Porphyrin-containing molecular squares: design and applications. Coord. Chem. Rev. 2006, 250 (13−14), 1710− 1723. (46) Yang, X. L.; Xie, M. H.; Zou, C.; He, Y.; Chen, B.; O’Keeffe, M.; Wu, C. D. Porous metalloporphyrinic frameworks constructed from metal 5,10,15,20-tetrakis(3,5-biscarboxylphenyl)porphyrin for highly efficient and selective catalytic oxidation of alkylbenzenes. J. Am. Chem. Soc. 2012, 134 (25), 10638−10645. (47) Roy, S.; George, C. B.; Ratner, M. A. Catalysis by a zincporphyrin-based metal−organic framework: from theory to computational design. J. Phys. Chem. C 2012, 116 (44), 23494−23502. (48) Suslick, K. S.; Bhyrappa, P.; Chou, J.-H.; Kosal, M. E.; Nakagaki, S.; Smithenry, D. W.; Wilson, S. R. Microporous porphyrin solids. Acc. Chem. Res. 2005, 38, 283−291. (49) Finn, C.; Schnittger, S.; Yellowlees, L. J.; Love, J. B. Molecular approaches to the electrochemical reduction of carbon dioxide. Chem. Commun. 2012, 48 (10), 1392−1399. (50) Leung, K.; Nielsen, I. M. B.; Sai, N.; Medforth, C.; Shelnutt, J. A. Cobalt-porphyrin catalyzed electrochemical reduction of carbon 7109



Article

dioxide in water. 2. mechanism from first principles. J. Phys. Chem. A 2010, 114, 10174−10184. (51) Morris, A. J.; Meyer, G. J.; Fujita, E. Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels. Acc. Chem. Res. 2009, 42, 1983−1994. (52) Groves, J. T.; Roman, J. S. Nitrous oxide activation by a ruthenium porphyrin. J. Am. Chem. Soc. 1995, 117, 5594−5595. (53) Saito, S.; Ohtake, H.; Umezawa, N.; Kobayashi, Y.; Kato, N.; Hirobe, M.; Higuchi, T. Nitrous oxide reduction-coupled alkenealkene coupling catalysed by metalloporphyrins. Chem. Commun. 2013, 49 (79), 8979−8981. (54) Phougat, N.; Vasudevan, P.; Jha, N. K.; Bandhopadhyay, D. K. Metal porphyrins as electrocatalysts for commercially important reacyions. Transition Metal Chem. 2003, 28, 838−847. (55) Wang, Y. Selective oxidation of hydrocarbons catalyzed by ironcontaining heterogeneous catalysts. Res. Chem. Intermed. 2006, 32, 235−251. (56) Meunier, B. Metalloporphyrins as versatile catalysts for oxidation reactions and oxidative DNA cleavage. Chem. Rev. 1992, 92, 1411−1456. (57) Murahashi, S.-I.; Naota, T.; Komiya, N. Metalloporphyrincatalyzed oxidation of alkanes with molecular oxygen in the presence of acetaldehyde. Tetrahedron Lett. 1995, 36, 8059−8062. (58) Liu, L.; Yu, M.; Wayland, B. B.; Fu, X. Aerobic oxidation of alcohols catalyzed by rhodium(iii) porphyrin complexes in water: reactivity and mechanistic studies. Chem. Commun. 2010, 46 (34), 6353. (59) Collman, J. P.; Marrocco, M.; Elliott, C. M.; L’Her, M. Electrocatalysis of nitrous oxide reduction comparison of several porphyrins and binary ″face-to-face″ porphyrins. J. Electroanal. Chem. 1981, 124, 113−131. (60) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215−41. (61) Ditchfield, R.; Hehre, W. J.; Pople, J. A. Self-consistent molecular orbital methods. 9. Extended Gaussian-type basis for molecular-orbital studies of organic molecules. J. Chem. Phys. 1971, 54, 724. (62) Dunning Jr., T. H.; Hay, P. J. Modern Theoretical Chemistry; Schaefer, H. F., III, Eds.; Plenum: New York, 1976; vol. 3, pp 1−28. (63) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; 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.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, revision B.01; Gaussian, Inc.: Wallingford, CT, 2010. (64) Foster, J. P.; Weinhold, F. Natural hybrid orbitals. J. Am. Chem. Soc. 1980, 102, 7211−7218. (65) Feixas, F.; Sola, M.; Swart, M. Chemical bonding and aromaticity in metalloporphyrin. Can. J. Chem. 2009, 87, 1063−1073. (66) Feng, X.-T.; Yu, J.-G.; Lei, M.; Fang, W.-H.; Liu, S. Toward understanding metal-binding specificity of porphyrin: a conceptual density functional theory study. J. Phys. Chem. C 2009, 113, 13381− 13389.

7110


MetalâPorphyrin: A Potential Catalyst for Direct Decomposition of N2O

Recommend Documents

MetalâPorphyrin: A Potential Catalyst for Direct Decomposition of N2O