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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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Metal–porphyrin: A Potential Catalyst for Direct
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Decomposition of N2O by Theoretical Reaction
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Mechanism Investigation
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Phornphimon Maitarad,† Supawadee Namuangruk,‡ Dengsong Zhang,†* Liyi Shi,†
5
Hongrui Li,† Lei Huang,† Bundet Boekfa§ and Masahiro Ehara§* †
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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
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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–
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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
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N2O decomposition catalysts in new directions.
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Keywords: Porphyrin; N2O decomposition; Density Functional Theory; Reaction
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mechanism
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1. INTRODUCTION
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Nitrous oxide (N2O) emitted from industrial processes and vehicle engines is an
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environmentally polluting gas. Moreover, it has been recognized as a strong greenhouse
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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
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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
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[Cat]…ON2
→
[Cat]O + N2(g)
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[Cat]O + N2O(g) →
[Cat]O…ON2
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[Cat]O…ON2
→
[Cat]O2 + N2(g)
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[Cat]O2
→
[Cat] + O2(g)
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Both theoretical and experimental studies have reported that the recombination of
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two oxygen atoms into the O2 molecule or the O2 desorption is the rate-limiting step in
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the overall direct N2O decomposition reaction.9,
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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
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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
64
functionality. Metal–porphyrins are well suited for anchoring on solid substrates as
65
assemblies in various types of applications, such as photovoltaic materials, field-
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responsive materials, catalytic materials, etc.29–33 It has been generally recognized that the
67
metal–porphyrins are highly active toward oxygen, nitric oxide, carbon monoxide, etc.34–
68
39
69
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
72
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
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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,
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to find potential catalysts for the N2O decomposition. The DFT calculations suggest that
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among these metal–porphyrins, Ti–porphyrin is the most active to N2O adsorption in its
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triplet ground state. Therefore, the possible direct N2O decomposition over a Ti–
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porphyrin catalyst has been examined and three N2O decomposition mechanisms have
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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-,
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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,
91
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
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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
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complex, metal–porphyrin, and N2O molecule, respectively.
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The direct N2O decomposition over Ti–porphyrin was examined. During the
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geometry optimizations, all atoms of the Ti–porphyrin complexes were fully relaxed. The
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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
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presented in the relative energy, which is defined as,
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∆E = Ecomplex – (Ecatalyst + Eabsorbate),
(2)
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where Ecomplex, Ecatalyst, and Eabsorbate are the energies of the Ti–porphyrin–gas complexes,
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the Ti–porphyrin at each step, and the small gas molecules, e.g., N2O, O2, and N2. For the
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reaction pathway, we examined the possibility of the spin crossing or intersystem
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crossing and found that the singlet state of some intermediates and transition states were
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more stable than their triplet counterparts.
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All of the electronic structure calculations and the geometry optimizations were
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performed using the DFT method with the M06L functional60 without any restriction on
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the symmetry. The 6-31G* basis set61 was used for the C, O, N, and H atoms and the
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scalar relativistic effective core potential of LANL2DZ62 was adopted for the transition
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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
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performed using the natural bond orbital (NBO) analysis.64
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3. RESULTS AND DISCUSSION
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3.1. N2O adsorption over M (Ti, Cr, Fe, Co, Ni, Cu, or Zn) – porphyrins. The
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adsorption energy of N2O, partial charges, and selected structural parameters for the Ti–,
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Cr–, Fe–, Co–, Ni–, Cu–, and Zn–porphyrins are summarized in Table 1. The relative
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energies of the metal–porphyrins with various spin multiplicities are compared in Table
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S1. Based on the unrestricted DFT calculations, the Ni– and Zn–porphyrins are found to
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have a closed-shell singlet ground state, and the Co– and Cu–porphyrins prefer the low-
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spin doublet state. The Ti– and Fe–porphyrins, on the other hand, result in the triplet
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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
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porphyrin. All metal–porphyrins were found to be planar as represented by a ∠Npor–M–
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Npor angle of 180°. The present results are consistent with the previous works.65, 66
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Comparing the structures of the metal–porphyrins and the N2O-adsorbed complexes,
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the adsorption of N2O over the metal–porphyrins does not affect the M–Npor distances in
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almost all systems except for Ti–porphyrin, which shows slight structure reorganization
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(∠Npor–M–Npor = 172°). Ti–porphyrin provided a considerable adsorption energy of –
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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
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of N2O (see Table S2) and obtained the adsorption energy of –22.27 kcal/mol, which
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indicates that N-binding adsorption also competes with O-binding over Ti–porphyrin. In
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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
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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
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distance depends on both the M–O1 interaction and the metal radius. Because of its
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relatively strong interaction, the N2O molecule over the Ti–porphyrin catalyst resulted in
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the most elongated O1–N2 bond and the shortest N2–N3 bond, as compared with N2O
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over the other porphyrins.
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The NBO partial charges of the metal–porphyrins and N2O···metal–porphyrin
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complexes are also listed in Table 1. The charge difference (qdiff) between N2O gas (q =
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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–
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(0.02) < Cu– (0.03) < Zn–, Cr–, Fe– (0.04) < Co– (0.05) < Ti–porphyrin (0.12). This
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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.
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3.2. N2O decomposition over Ti–porphyrin. For the direct N2O decomposition, the
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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
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decomposition of three N2O molecules can be decomposed into four elementary reaction
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steps as follows:
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Step 1 [Ti] + 1st N2O(g) → [Ti]···ON2 → [Ti]O + N2(g),
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Step 2 [Ti]O + 2nd N2O(g) → [Ti]O···ON2 → [Ti]O2 + N2(g) → [Ti] + O2(g),
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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).
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Following the previous reports on the full direct N2O decomposition mechanism,9, 24–28
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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
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(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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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
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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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3.3. Comparison of Ti–porphyrin and zeolites. The comparison of theoretical
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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
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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
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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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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
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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
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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
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562
563 564
Figure 1. Model systems in this work.
565
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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
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572 573
Figure 3. The first N2O decomposition over Ti–porphyrin (Step 1).
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575 576
Figure 4. The second N2O decomposition over TiO–porphyrin (Step 2).
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578 579
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
583
catalyst (Step 4).
584
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585 586
Figure 7. Energy profile of the full reaction pathway for the direct decomposition of
587
three N2O molecules on the Ti–porphyrin catalyst.
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Article
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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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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17
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.
31
Keywords: Porphyrin; N2O decomposition; Density Functional Theory; Reaction
32
mechanism
33
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1. INTRODUCTION
35
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
46
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
64
functionality. Metal–porphyrins are well suited for anchoring on solid substrates as
65
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
67
metal–porphyrins are highly active toward oxygen, nitric oxide, carbon monoxide, etc.34–
68
39
69
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
72
molecules and size- or shape-selective heterogeneous catalysis.44–48 Metal–porphyrins
73
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
77
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
90
As the model of catalysts, single metal–porphyrins in which the metal is Ti, Cr, Fe, Co,
91
Ni, Cu, or Zn, were examined without a support or assembly. Neutral metal–porphyrins,
92
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
95
examine the N2O adsorption ability. The spin state of the adsorption complex (N2O–
96
metal–porphyrin) was set to the same spin state as the metal–porphyrin without
97
considering spin crossing. For the initial structure of the geometry optimization, the N2O
98
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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102
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
120
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.
132
The metal–nitrogen (M–Npor) bond distance of the metal–porphyrin is about 2 Å and the
133
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
141
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
145
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
155
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:
167
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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239
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
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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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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
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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
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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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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
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Table 2. Comparison of the adsorption energy, activation energy, and desorption energy
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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
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Figure Captions
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Figure 1. Model systems in this work.
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Figure 2. Mechanistic cycles of direct decomposition of N2O over Ti–porphyrin while
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the inner cycle shows the recycling of the active TiO–porphyrin catalytic intermediate for
553
the third N2O decomposition.
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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
560
three N2O molecules on the Ti–porphyrin catalyst.
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563 564
Figure 1. Model systems in this work.
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567 568
Figure 2. Mechanistic cycles of direct decomposition of N2O over Ti–porphyrin while
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the inner cycle shows the recycling of the active TiO–porphyrin catalytic intermediate for
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the third N2O decomposition.
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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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585 586
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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