MetalâPorphyrin: A Potential Catalyst for Direct Decomposition of N2O

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Metal-porphyrin: A Potential Catalyst for N2O Direct Decomposition by Theoretical Reaction Mechanism Investigation Phornphimon Maitarad, Supawadee Namuangruk, Dengsong Zhang, Liyi Shi, Hongrui Li, Lei Huang, Bundet Boekfa, and Masahiro Ehara Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 23 May 2014 Downloaded from http://pubs.acs.org on May 25, 2014

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Environmental Science & Technology

1

Metal–porphyrin: A Potential Catalyst for Direct

2

Decomposition of N2O by Theoretical Reaction

3

Mechanism Investigation

4

Phornphimon Maitarad,† Supawadee Namuangruk,‡ Dengsong Zhang,†* Liyi Shi,†

5

Hongrui Li,† Lei Huang,† Bundet Boekfa§ and Masahiro Ehara§* †

6

Research Center of Nano Science and Technology, Shanghai University, Shanghai

7

8

200444, P. R. China ‡

National Nanotechnology Center (NANOTEC), NSTDA, 111 Thailand Science Park,

9

10

Pahonyothin Road, Klong Luang, Pathum Thani 12120, Thailand §

Institute for Molecular Science and Research Center for Computational Science, 38

11

Nishigo-naka, Myodaiji, Okazaki, 444-8585, Japan

12

ABSTRACT

13

The adsorption of nitrous oxide (N2O) on metal–porphyrins (metal: Ti, Cr, Fe, Co, Ni,

14

Cu, or Zn), has been theoretically investigated using density functional theory with the

15

M06L functional to explore their use as potential catalysts for the direct decomposition of

16

N2O. Among these metal–porphyrins, Ti–porphyrin is the most active for N2O adsorption

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in the triplet ground state with the strongest adsorption energy (–13.32 kcal/mol). Ti–

18

porphyrin was then assessed for the direct decomposition of N2O. For the overall reaction

19

mechanism of three N2O molecules on Ti–porphyrin, two plausible catalytic cycles are

20

proposed. Cycle 1 involves the consecutive decomposition of the first two N2O molecules,

21

while cycle 2 is the decomposition of the third N2O molecule. For cycle 1, the activation

22

energies of the first and second N2O decompositions are computed to be 3.77 and 49.99

23

kcal/mol, respectively. The activation energy for the third N2O decomposition in cycle 2

24

is 47.79 kcal/mol, which is slightly lower than that of the second activation energy of the

25

first cycle. O2 molecules are released in cycles 1 and 2 as the products of the reaction,

26

which requires endothermic energies of 102.96 and 3.63 kcal/mol, respectively.

27

Therefore, the O2 desorption is mainly released in catalytic cycle 2 of a TiO3–porphyrin

28

intermediate catalyst. In conclusion, regarding the O2 desorption step for the direct

29

decomposition of N2O, the findings would be very useful to guide the search for potential

30

N2O decomposition catalysts in new directions.

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Keywords: Porphyrin; N2O decomposition; Density Functional Theory; Reaction

32

mechanism

33


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1. INTRODUCTION

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Nitrous oxide (N2O) emitted from industrial processes and vehicle engines is an

36

environmentally polluting gas. Moreover, it has been recognized as a strong greenhouse

37

gas contributing to the destruction of ozone in the stratosphere.1–3 Because of the

38

continuous increase in its concentration in the atmosphere, the discovery and

39

development of efficient catalysts for the reduction of N2O have become important issues

40

in the field of environmental research. A direct catalytic decomposition of N2O into N2

41

and O2 is thought to be the most convenient and economical option to reduce N2O

42

emissions.4 Therefore, several catalytic materials have been reported for direct

43

decomposition of N2O gas, such as, M-zeolites (M = Cu, Co, Fe, etc.),5–9 carbon

44

nanotubes,10, 11 perovskite-like mixed oxides,12–14 alumina-supported precious metals (Pd,

45

Rh, etc.),15, 16 and metal alloys.17 Among these catalysts, transition-metal ion-exchanged

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ZSM-5 zeolites, especially for the Fe-, Co-, and Cu-exchanged zeolites, have been widely

47

used because they show high catalytic activity.9,

48

from oxygen inhibition and the low reaction rate of the N2O decomposition.

49 50

18–23

However, these catalysts suffer

In general, the full reaction mechanism of direct N2O decomposition has been reported as the following:24–28

51

[Cat] + N2O(g)

→

[Cat]…ON2

52

[Cat]…ON2

→

[Cat]O + N2(g)

53

[Cat]O + N2O(g) →

[Cat]O…ON2

54

[Cat]O…ON2

→

[Cat]O2 + N2(g)

55

[Cat]O2

→

[Cat] + O2(g)



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Both theoretical and experimental studies have reported that the recombination of

57

two oxygen atoms into the O2 molecule or the O2 desorption is the rate-limiting step in

58

the overall direct N2O decomposition reaction.9,

59

catalysts for direct N2O decomposition will be accelerated by the preliminary study of the

60

energetics along the reaction pathway by using, for example, density functional theory

61

(DFT) calculations prior to the experiment.

25, 28

Thus, the development of new

62

Porphyrin readily forms ordered monolayers by self-assembly and possesses two

63

axial coordination sites that are available as centers of catalytic activity or sensor

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functionality. Metal–porphyrins are well suited for anchoring on solid substrates as

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assemblies in various types of applications, such as photovoltaic materials, field-

66

responsive materials, catalytic materials, etc.29–33 It has been generally recognized that the

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metal–porphyrins are highly active toward oxygen, nitric oxide, carbon monoxide, etc.34–

68

39

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which the coordination of gas molecules to the metal center causes measurable changes

70

of electronic properties, color, etc.30,

71

synthesized as microporous solid frameworks that have a selective sorption of small

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molecules and size- or shape-selective heterogeneous catalysis.44–48 Metal–porphyrins

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have been considered as potential catalysts for many commercially important reactions

74

such as the reduction of carbon dioxide49–51 and nitrogen oxides,52–54 and the oxidation of

75

hydrocarbons and alcohols.55–58

Therefore, one important application of metal–porphyrins is the sensing of gases, in

40–43

In addition, the metal–porphyrins can be

76

Although there are some reports on the ability of metal–porphyrins with respect to

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N2O adsorption,35, 59 their catalytic activity for direct N2O decomposition has not yet been

78

demonstrated. Therefore, this motivated us to apply DFT calculations to the N2O


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adsorption ability of metal–porphyrins, where the metal is Ti, Cr, Fe, Co, Ni, Cu, or Zn,

80

to find potential catalysts for the N2O decomposition. The DFT calculations suggest that

81

among these metal–porphyrins, Ti–porphyrin is the most active to N2O adsorption in its

82

triplet ground state. Therefore, the possible direct N2O decomposition over a Ti–

83

porphyrin catalyst has been examined and three N2O decomposition mechanisms have

84

been proposed. O2 desorption, which is a key step for the N2O decomposition, has also

85

been considered to determine a favorable pathway. The characteristics and performance

86

of the present Ti–porphyrin are compared with those of the potential catalysts of Fe-, Co-,

87

and Cu-ZSM-5 zeolites.9, 19, 28

88 89

2. METHODS

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As the model of catalysts, single metal–porphyrins in which the metal is Ti, Cr, Fe, Co,

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Ni, Cu, or Zn, were examined without a support or assembly. Neutral metal–porphyrins,

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core system of the catalyst, were assessed in some spin states without considering the

93

charge transfer from the support or surroundings when locating the energetically lowest

94

geometric structure. The most stable spin state of each metal–porphyrin was selected to

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examine the N2O adsorption ability. The spin state of the adsorption complex (N2O–

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metal–porphyrin) was set to the same spin state as the metal–porphyrin without

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considering spin crossing. For the initial structure of the geometry optimization, the N2O

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molecule was laid along the perpendicular axis to the plane of the porphyrin ring with the

99

oxygen atom of N2O pointed to the metal atom of the metal–porphyrin (Figure 1). The

100 101

adsorption energy (Ead) of N2O was calculated by Ead = Ecomplex – (EM–por + EN2O),

(1)



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where Ecomplex, EM–por, and EN2O are the total energies of the metal–porphyrin···N2O

103

complex, metal–porphyrin, and N2O molecule, respectively.

104

The direct N2O decomposition over Ti–porphyrin was examined. During the

105

geometry optimizations, all atoms of the Ti–porphyrin complexes were fully relaxed. The

106

transition states were confirmed as the real saddle points with only one imaginary

107

frequency by vibrational analysis. The reaction energy profiles of each step were

108

presented in the relative energy, which is defined as,

109

∆E = Ecomplex – (Ecatalyst + Eabsorbate),

(2)

110

where Ecomplex, Ecatalyst, and Eabsorbate are the energies of the Ti–porphyrin–gas complexes,

111

the Ti–porphyrin at each step, and the small gas molecules, e.g., N2O, O2, and N2. For the

112

reaction pathway, we examined the possibility of the spin crossing or intersystem

113

crossing and found that the singlet state of some intermediates and transition states were

114

more stable than their triplet counterparts.

115

All of the electronic structure calculations and the geometry optimizations were

116

performed using the DFT method with the M06L functional60 without any restriction on

117

the symmetry. The 6-31G* basis set61 was used for the C, O, N, and H atoms and the

118

scalar relativistic effective core potential of LANL2DZ62 was adopted for the transition

119

metal elements. All calculations in the present work were carried out using the

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Gaussian09 suite of programs, revision B01.63 The charge analyses of the systems were

121

performed using the natural bond orbital (NBO) analysis.64

122 123

3. RESULTS AND DISCUSSION


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3.1. N2O adsorption over M (Ti, Cr, Fe, Co, Ni, Cu, or Zn) – porphyrins. The

125

adsorption energy of N2O, partial charges, and selected structural parameters for the Ti–,

126

Cr–, Fe–, Co–, Ni–, Cu–, and Zn–porphyrins are summarized in Table 1. The relative

127

energies of the metal–porphyrins with various spin multiplicities are compared in Table

128

S1. Based on the unrestricted DFT calculations, the Ni– and Zn–porphyrins are found to

129

have a closed-shell singlet ground state, and the Co– and Cu–porphyrins prefer the low-

130

spin doublet state. The Ti– and Fe–porphyrins, on the other hand, result in the triplet

131

ground state, whereas the high-spin quintet state is the lowest state for the Cr–porphyrin.

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The metal–nitrogen (M–Npor) bond distance of the metal–porphyrin is about 2 Å and the

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spin state is correlated to the M–Npor bond distance, reflecting the coordination field of

134

porphyrin. All metal–porphyrins were found to be planar as represented by a ∠Npor–M–

135

Npor angle of 180°. The present results are consistent with the previous works.65, 66

136

Comparing the structures of the metal–porphyrins and the N2O-adsorbed complexes,

137

the adsorption of N2O over the metal–porphyrins does not affect the M–Npor distances in

138

almost all systems except for Ti–porphyrin, which shows slight structure reorganization

139

(∠Npor–M–Npor = 172°). Ti–porphyrin provided a considerable adsorption energy of –

140

13.32 kcal/mol, while other metal porphyrins exhibited low adsorption energies in the

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range of –4.6 to –7.8 kcal/mol. We also examined the adsorption of the N-binding mode

142

of N2O (see Table S2) and obtained the adsorption energy of –22.27 kcal/mol, which

143

indicates that N-binding adsorption also competes with O-binding over Ti–porphyrin. In

144

the present work, we focus on the O-binding adsorption, as in other works,9, 19, 28 because

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TiO–porphyrin forms a very stable intermediate, as shown later. In addition, an adsorbed

146

distance between Ti and the O1 atom of the N2O molecule was 2.25 Å, which is the



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shortest metal–oxygen (M–O1) distance among the complexes. The adsorption M–O1

148

distance depends on both the M–O1 interaction and the metal radius. Because of its

149

relatively strong interaction, the N2O molecule over the Ti–porphyrin catalyst resulted in

150

the most elongated O1–N2 bond and the shortest N2–N3 bond, as compared with N2O

151

over the other porphyrins.

152

The NBO partial charges of the metal–porphyrins and N2O···metal–porphyrin

153

complexes are also listed in Table 1. The charge difference (qdiff) between N2O gas (q =

154

0) and adsorbed N2O shows the amount of charge reorganization or transfer from N2O to

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the metal–porphyrins upon N2O adsorption. We found that qdiff increases in the order Ni–

156

(0.02) < Cu– (0.03) < Zn–, Cr–, Fe– (0.04) < Co– (0.05) < Ti–porphyrin (0.12). This

157

charge reorganization contributes to the adsorption strength. Therefore, the Ti–porphyrin

158

was chosen to examine its catalytic activity of the direct N2O decomposition in the

159

following sections.

160 161

3.2. N2O decomposition over Ti–porphyrin. For the direct N2O decomposition, the

162

reaction mechanism was investigated over Ti–porphyrin, which exhibited the strongest

163

adsorption of N2O. The assessment of the reaction pathways follows the structures and

164

the spin states of the N2O···Ti–porphyrin complex. The reaction mechanism of direct

165

decomposition of three N2O molecules can be decomposed into four elementary reaction

166

steps as follows:

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

168

Step 2 [Ti]O + 2nd N2O(g) → [Ti]O···ON2 → [Ti]O2 + N2(g) → [Ti] + O2(g),

169

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


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Step 4 [Ti]O3 → [Ti]O···O2 → [Ti]O + O2(g).

171

Following the previous reports on the full direct N2O decomposition mechanism,9, 24–28

172

the present system, namely, N2O decomposition over Ti–porphyrin, is schematized in

173

Figure 2. The products of the N2O decomposition are N2 and O2 molecules. We assume

174

that the overall reaction mechanism consists of two cycles. Cycle 1 has two steps: steps 1

175

and 2 (see equations above), which involve the consecutive decomposition of the first and

176

second N2O molecules, respectively. Cycle 2 consists of the third N2O decomposition

177

(step 3) and the depletion of O2 from the TiO3–porphyrin intermediate (step 4). The

178

calculated total energy of all of the intermediates and transition states are listed in Table

179

S3, and the coordinates of the transition states are given in SI.

180 181

3.2.1. The first N2O decomposition over Ti–porphyrin (Step 1). In step 1, the reaction

182

energy profile of the first N2O molecule over Ti–porphyrin consists of N2O adsorption,

183

N–O bond scission, and N2 desorption (Figure 3). At the adsorption complex (AD1), the

184

N2O molecule is located on the Ti atom in a tilted structure with a ∠Ti–O–N angle of

185

about 120°. The Ti···O distance is 2.25 Å and the O–N1 bond slightly elongates from

186

1.19 to 1.20 Å. The calculated adsorption energy (Ead1) is –13.32 kcal/mol. At the

187

transition state (TS1), the ∠Ti–O–N angle changes from 120° to 156° and the Ti–O bond

188

is contracted to 2.06 Å. The first transition state (TS1) has a relative energy of –9.55

189

kcal/mol with one imaginary frequency of 418i cm–1, which corresponds to the reaction

190

coordinate. The Ti active site abstracts the O1 atom from the absorbed N2O to form the

191

TiO–porphyrin intermediate and the N2 molecule (IM1). Spin crossing occurs between

192

TS1 and IM1, assuming that the spin–orbit interaction is large enough. Otherwise, the



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193

reaction may proceed in the triplet state, as shown in the supporting information (Figure

194

S1). This step proceeds in a highly exothermic manner with the relative energy of IM1

195

being –116.53 kcal/mol, indicating that IM1 is very stable. Finally, the N2 molecule is

196

easily desorbed from the TiO–porphyrin intermediate (IM2), which requires an energy of

197

only 3.73 kcal/mol. Therefore, in the first N2O decomposition process, the N2O molecule

198

readily adsorbs onto the Ti–porphyrin, the N–O bond scission has a low activation energy

199

barrier (Ea1) of 3.77 kcal/mol, and the TiO–porphyrin is a thermodynamically stable

200

intermediate.

201 202

3.2.2. The second N2O decomposition over TiO–porphyrin (Step 2). For the second N2O

203

decomposition (Figure 4), although all steps are assumed to be similar to the first N2O

204

decomposition, the active site is based on the TiO–porphyrin intermediate instead of Ti–

205

porphyrin. The second N2O molecule adsorbs over the TiO–porphyrin with the

206

adsorption energy (Ead2) of –2.36 kcal/mol. It is seen that the second N2O molecule

207

weakly adsorbs on the TiO–porphyrin relative to the first one. Regarding the adsorption

208

geometry (AD2), N2O retains a linear structure with unchanged bond distances. The

209

transition state (TS2) was confirmed with an imaginary frequency of 838i cm–1

210

representing the reaction coordinate. At TS2, the O2 atom of N2O approaches the TiO–

211

porphyrin, with the Ti···O2 distance being 2.05 Å. The adsorbed N2O shows a bent

212

structure with ∠O2–N1–N2 of 148° and undergoes the predominant dissociation of the

213

N1–O2 bond. This step requires the activation barrier energy (Ea2) of 49.99 kcal/mol,

214

which is much higher than the first N2O decomposition (Ea1). In the N2 generation

215

(IM21), the O atom from the N2O molecule is abstracted by the TiO–porphyrin to form


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the TiO2–porphyrin intermediate. The Ti–O bond and O1···O2 distance are 1.83 Å and

217

1.44 Å, respectively. The calculated relative energy is –3.65 kcal/mol. The N2 molecule

218

simultaneously desorbs from the TiO2–porphyrin intermediate (IM22) with desorption

219

energy of 3.70 kcal/mol. It is possible that the O2 molecule is generated in this step.

220

However, it was found that the O2 desorption energy (Ede1) is very high (102.96 kcal/mol)

221

(IM23), implying that the TiO2–porphyrin would not easily release the O2 molecule.

222

Thus, an alternative route for the TiO2–porphyrin catalytic intermediate is considered,

223

and it could decompose another N2O molecule.

224 225

3.2.3. The third N2O decomposition over TiO2–porphyrin (Step 3). The energetic profile

226

for the third N2O decomposition over the TiO2–porphyrin intermediate (IM22) was

227

calculated (Figure 5). The adsorption energy (Ead3) of the third N2O is –5.22 kcal/mol

228

(AD3), which is slightly stronger than that of the second N2O adsorption (–2.36

229

kcal/mol). Reflecting the weak adsorption, the bond distance of the TiO2–porphyrin

230

catalyst and N2O molecule is nearly unaltered at the adsorption step. Thus, to activate an

231

N2O molecule over the TiO2–porphyrin via TS3, it needs the relatively high activation

232

energy (Ea3) of 47.79 kcal/mol. At TS3 with an imaginary frequency of 747i cm–1, the

233

oxygen atom (O3) of the N2O molecule points to the Ti active site with the interaction

234

distance of Ti···O3 = 2.08 Å. The O1···O2 interaction distance is shortened from 1.44 Å

235

to 1.38 Å, while the Ti–O1 and Ti–O2 bond distances are elongated. At TS3, the N2O

236

molecule is distorted from linear to a bent structure of about ∠O3–N1–N2 = 142°. In

237

addition, the O3–N2 bond is significantly lengthened, which implies that the O3–N2

238

bond dissociates and then O3–Ti or O3–O2 may form a new bond on the catalyst surface.



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Thus, the TiO2–porphyrin abstracts the oxygen atom from the N2O molecule, then the N2

240

molecule and TiO3–porphyrin intermediate are produced (IM31). In step 3, the TiO2–

241

porphyrin intermediate shows a stronger N2O adsorption ability than that of the TiO–

242

porphyrin of step 2. The activation energy barrier of the third N2O insertion on the TiO2–

243

porphyrin is also slightly lower than that of the second N2O insertion by about 2

244

kcal/mol. These can imply that the third N2O desorption over the TiO2–porphyrin is a

245

prominent reaction pathway as compared with O2 desorption of the TiO2–porphyrin.

246

Desorption of the N2 molecule is calculated to be 4.72 kcal/mol (IM32), which is slightly

247

higher than those of the first and second N2 molecules. This is because of the less stable

248

nature of the TiO3–porphyrin intermediate.

249 250

3.2.4. Oxygen depletion from TiO3–porphyrin (Step 4). The intermediate obtained via the

251

three N2O decompositions is the TiO3–porphyrin (IM32). The four-membered-ring

252

structure (Ti–O1–O2–O3) of IM32 is very strained; the O2 molecule is therefore easily

253

lost to release the strain (Figure 6). The optimized structure of TiO3–porphyrin shows that

254

the Ti–O1 and Ti–O2 have equal bond lengths of 1.91 Å. At TS4 (imaginary frequency =

255

188i cm–1), the obtained geometry is very similar to the IM32, therefore, it is not

256

surprising that the activation energy (Ea4) of the O2 depletion from IM32 is only 5.97

257

kcal/mol because of the unstable four-membered-ring structure in the TiO3–porphyrin

258

intermediate. Thus, this observation can imply that the TiO3–porphyrin catalytic

259

intermediate would spontaneously generate an O2 molecule. Spin crossing again occurs to

260

produce the stable triplet TiO···O2 intermediate (IM41) located at –59.69 kcal/mol. The


Page 13 of 37


261

product of this step is the TiO–porphyrin (IM2), which is a key intermediate catalyst for

262

the N2O decomposition of cycle 2.

263

The O2 desorption is an important process for the direct N2O decomposition reaction

264

because it is usually the rate-limiting step.9, 25, 26 In the present case, considering the O2

265

desorption (Figure 2), there are two possible intermediates to recombine two oxygen

266

atoms into an O2 molecule. The first one is the TiO2–porphyrin (IM22), an intermediate

267

from the second N2O decomposition. The TiO2–porphyrin needs a very high desorption

268

energy (Ede1) of about 103 kcal/mol to release the O2 molecule; therefore, this process

269

hardly occurs. Another possible route for the O2 desorption exists in step 4. Herein, the

270

O2 desorption energy (Ede2) was calculated to be only 3.63 kcal/mol, which is much lower

271

than that of the O2 desorption in step 2. Therefore, the O2 molecule desorbs from the

272

TiO–porphyrin···O2 intermediate in a spontaneous route that requires a very low

273

endothermic energy.

274

A full reaction mechanism for the direct N2O decomposition over the Ti–porphyrin

275

catalyst is summarized in Figure 7. As mentioned above, there are two catalytic cycles for

276

the three N2O decompositions. Cycle 1 consists of steps 1 and 2 based on Ti–porphyrin

277

recycling catalyst form; this step shows an O2 desorption barrier of about 103 kcal/mol.

278

For steps 2, 3, and 4 in Cycle 2, the TiO–porphyrin is a recycling catalyst for this cycle,

279

and the O2 desorption barrier requires only 3.63 kcal/mol. Therefore, based on the key

280

step of O2 desorption, Cycle 2 is the main catalytic pathway for N2O decomposition over

281

Ti–porphyrin.

282



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283

3.3. Comparison of Ti–porphyrin and zeolites. The comparison of theoretical

284

activation energies and O2 desorption energies of three direct N2O decompositions over

285

the Fe-, Cu-, and Co-ZSM5 zeolites9,

286

summarized in Table 2. For the first N2O decomposition, the Ti–porphyrin and Fe/Co-

287

ZSM-5 zeolites show similar activation energy barriers in the range of 4–6 kcal/mol,

288

which is much lower than that of the Cu-ZSM-5 zeolite. This suggests that the first N2O

289

decomposition over Ti–porphyrin and Fe/Co-ZSM-5 zeolites is easier than that of the Cu-

290

ZSM-5 zeolite. On the other hand, the second N2O decomposition is feasible over the Cu-

291

ZSM-5 zeolite. It is noted that the third N2O decomposition over all catalysts has similar

292

energy barriers, and it is important to mention that Ti–porphyrin results in a slightly

293

higher activation energy. In particular, the activation energy for the third N2O

294

decomposition is higher than that for the first and second decompositions in all catalysts

295

because of the greater steric effect of oxygen atoms bonded to the Ti active site.

296

Considering the rate-limiting step of the O2 desorption in the second and third N2O

297

decompositions, the Cu-ZSM-5 zeolite prefers to desorb the O2 molecule from the second

298

N2O decomposition or in the form of [Cu]-O2-ZSM-5 intermediate catalyst. In contrast,

299

O2 desorption over Ti–porphyrin prefers to release from the third N2O decomposition or

300

in the form of [TiO]O2–porphyrin intermediate catalyst, which corresponds well with the

301

Fe/Co-ZSM-5 zeolite. Significantly, for the O2 desorption from the third N2O

302

decomposition, the present Ti–porphyrin catalyst shows the lowest desorption energy

303

barrier compared with those of O2 desorption over the Fe-, Cu-, and Co-ZSM5 zeolites,

304

which implies that the O2 desorption seems to be a spontaneous pathway over the

305

[TiO]O2–porphyrin intermediate catalyst. Therefore, based on the energy comparison, we

19, 28

and the present Ti–porphyrin catalyst are


Page 15 of 37


306

propose that the Ti–porphyrin catalyst is one of the candidates for the direct

307

decomposition of N2O.

308

Hence, the present theoretical study demonstrates the potential use of Ti–porphyrin

309

as a catalyst for a direct N2O decomposition that comparably enhances the N2O

310

decomposition and effectively produces O2 depletion and desorption compared with the

311

reaction pathways over conventional catalysts such as zeolites. Therefore, the synthesis

312

of this catalyst and reaction kinetics are very interesting, and it is underway in our

313

laboratory. Theoretical analysis of the extended model system of this catalyst is also

314

underway taking account of the surroundings.

315 316

ASSOCIATED CONTENT

317

Supporting Information

318

(1) Total energies of isolated metal–porphyrins based on M06L/6-31G* (C, N, O, and H)

319

LANL2DZ (Ti, Cr, Fe, Co, Ni, Cu, and Zn). (2) The N2O adsorption energies for O- and

320

N-binding modes over the metal–porphyrins. (3) Total energies of the adsorption

321

complexes (AD1, AD2, AD3), intermediates (IM1, IM2, IM21, IM22, IM31, IM32,

322

IM41, IM42), and transition states (TS1, TS2, TS3, TS4). (4) Cartesian coordinates of

323

the transition states. (5) Reaction pathway of N2O decomposition in the singlet and triplet

324

states of Ti–porphyrin. This information is available free of charge via the Internet at

325

http://pubs.acs.org.

326 327

AUTHOR INFORMATION



Page 16 of 37

328

Corresponding Author

329

*Ph: +86- 21-66136079. E-mail: [email protected] (D.Z.)

330

*Ph: +81-564-55-7461. E-mail: [email protected] (M.E.)

331

Notes

332

The authors declare no competing financial interest.

333

ACKNOWLEDGEMENTS

334

The

335

(12R21413300) and National Natural Science Foundation of China (51108258). The

336

work was also supported by Nanotechnology Platform Program (Molecule and Material

337

Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology

338

(MEXT), Japan. We thank Research Center for Computational Science in Okazaki, Japan

339

and National Nanotechnology Center (NANOTEC) in Thailand for computing resources.

authors

acknowledge

the

supports

of

STCSM

postdoctoral

foundation

340 341

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Page 27 of 37


536

Table 1. Structural parameters (bond lengths in Å, angles in degrees), adsorption energy

537

(kcal/mol), and partial charges (e) on the selected atoms of the metal–porphyrin and

538

N2O…metal–porphyrin complex systems, where the metal is Ti, Cr, Fe, Co, Ni, Cu, or Zn. N3

539

O1

N2

540 541

M

542 543 N2O

Ti

Cr

Fe

Co

Ni

Cu

Zn

Structural Parameters M-por system Spin multiplicity M–Npor ∠Npor–M–Npor

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

N2O…M-por system N2O Adsorption Energy M–Npor ∠Npor–M–Npor M–O1 O1–N2 N2–N3 ∠M–O1–N2 ∠O1–N2–N3

-13.32 2.06 172.4 2.25 1.20 1.14 120.3 178.3

-6.81 2.04 178.5 2.74 1.19 1.14 111.1 179.6

-7.81 2.00 179.4 2.74 1.19 1.14 108.4 179.9

-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

Partial Charges M-por system M Npor

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

N2O…M-por system M Npor O1 N2 N3 Total charge of N2O (qdiff) Mdiff (MN2O-Mpor) Npor-diff (NN2O-Npor)

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

1.19 1.14 180.0

-0.30 0.38 -0.08 0.00

544 27 ACS Paragon Plus Environment


Page 28 of 37

545

Table 2. Comparison of the adsorption energy, activation energy, and desorption energy

546

for the direct decomposition of N2O over potential zeolites and Ti–porphyrin catalyst.

st

1 N2O 2nd N2O

3rd N2O

Activation barrier Activation barrier O2 desorption

Ti–porphyrin 3.77

Energy (kcal/mol) Cu-ZSM-5 Fe-ZSM-5 a 35.18 4.41b

49.99

28.07a

102.96

39.48a

Activation 47.79 barrier 3.63 O2 desorption a 9b 28 c Liu, et al. Fellah et al. Ryder et al. 19

Co-ZSM-5 6.28b

42.10a

58.13b 37.6c 67.30b 94.9c 44.6c

48.56b 32.9c 64.95b 85.6c 40.2c

63.42a

51.9c

52.8c

547 548


Page 29 of 37


549

Figure Captions

550

Figure 1. Model systems in this work.

551

Figure 2. Mechanistic cycles of direct decomposition of N2O over Ti–porphyrin while

552

the inner cycle shows the recycling of the active TiO–porphyrin catalytic intermediate for

553

the third N2O decomposition.

554

Figure 3. The first N2O decomposition over Ti–porphyrin (Step 1).

555

Figure 4. The second N2O decomposition over TiO–porphyrin (Step 2).

556

Figure 5. The third N2O decomposition over TiO2–porphyrin (Step 3).

557

Figure 6. O2 depletion and desorption processes from the TiO3–porphyrin intermediate

558

catalyst (Step 4).

559

Figure 7. Energy profile of the full reaction pathway for the direct decomposition of

560

three N2O molecules on the Ti–porphyrin catalyst.

561



Page 30 of 37

562

563 564

Figure 1. Model systems in this work.

565


Page 31 of 37


566

567 568

Figure 2. Mechanistic cycles of direct decomposition of N2O over Ti–porphyrin while

569

the inner cycle shows the recycling of the active TiO–porphyrin catalytic intermediate for

570

the third N2O decomposition.

571



Page 32 of 37

572 573

Figure 3. The first N2O decomposition over Ti–porphyrin (Step 1).

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Figure 4. The second N2O decomposition over TiO–porphyrin (Step 2).

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Figure 5. The third N2O decomposition over TiO2–porphyrin (Step 3).

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Figure 6. O2 depletion and desorption processes from the TiO3–porphyrin intermediate

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catalyst (Step 4).

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Figure 7. Energy profile of the full reaction pathway for the direct decomposition of

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three N2O molecules on the Ti–porphyrin catalyst.

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Table of Contents (TOC)

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

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MetalâPorphyrin: A Potential Catalyst for Direct Decomposition of N2O