Active Sites in Fe/ZSM-5 Zeolite

Gang Yang • Jing Guan • Lijun Zhou •

Xiuwen Han • Xinhe Bao

Published online: 18 May 2010

� Springer Science+Business Media, LLC 2010

Abstract Fe/ZSM-5 zeolite has shown great potential in

the selective oxidations of hydrocarbons such as methane

and benzene. The various competing active sites of Fe/

ZSM-5 zeolite are reviewed, including the mono-iron,

oxygen-bridged [Fe, X] (X = Fe, Al) and peroxide species;

in addition, the influences of H2 pretreatment are consid-

ered. For the mono-iron species, the H2 molecules are

chemisorbed on the Fe(III) sites via the g2-binding mode.

Both high- and low-spin Fe(III) ions play an important role

during the H2 reduction process whereas the former pre-

dominates in the N2O decomposition process. As the cal-

culated energy barriers indicate, the Fe(III) ions are facile to

be reduced by H2 pretreatment and therefore the active site

of the mono-iron species should be in the FeO(OH) form.

Instead, the oxygen-bridged [Fe, X] and peroxide species

remain stable by H2 pretreatment. The suitable oxygen-

bridged [Fe, X] structures are screened out by comparisons

with the experimental data and energy considerations from

computational aspects. The geometries are in good agree-

ment with the experimental data; meanwhile, it provides

sound explanations to the distribution of the iron valence

states, the thermodynamic facilitation of the ‘‘alfa-oxygen’’

generation by the introduction of extra-lattice Al ions as

well as the shift of the Fe–Fe distances from ca. 3.06 to

2.53 A. The superoxide species exists in Fe/ZSM-5 zeolite

but not with the presence of extra-lattice Al ions. As the

temperature increases, it gradually converts into the per-

oxide species and probably is the precursor of the peroxide

species, one of the competing active sites in Fe/ZSM-5

zeolite. The clarification of active sites lays a solid foun-

dation on the understanding of the catalytic processes and

improvement of the Fe/ZSM-5 catalyst, one of the prom-

ising candidates to meet the industry challenges.

Keywords Active sites � Fe/ZSM-5 zeolite �Theoretical calculations � H2 pretreatment �N2O decomposition

1 Introduction

Both methane and benzene are thermally stable and rather

difficult to activate. The selective oxidation and function-

alization of them into valuable feedstocks have long rep-

resented challenges to us [1–5]. The recent advent Fe/

ZSM-5 catalyst seems to be one of the promising candi-

dates that meet the challenges [6–11]. Fe/ZSM-5 zeolite

shows remarkable redox properties and catalytic activities

under mild conditions. Especially, it can convert benzene

to phenol in a single step, with high yield and selectivity

(not less than 90%) [12].

Usually, N2O is used as the oxidant for the direct benzene

hydroxylation reaction on Fe/ZSM-5 zeolite. N2O may

decompose via the Eley–Rideal (Eqs. 1 and 2) or Langmiur–

Hinshelwood (Eqs. 1 and 3) mechanism [13, 14].

N2O! a-Oþ N2 ð1Þa-Oþ N2O! N2 þ O2 ð2Þ2a-O! 2Feþ O2 ð3Þ

G. Yang � L. Zhou

Key Laboratory of Forest Plant Ecology, Northeast Forestry

University, Ministry of Education, Harbin 150040,

People’s Republic of China

G. Yang (&) � J. Guan � X. Han � X. Bao (&)

State Key Laboratory of Catalysis, Dalian Institute of Chemical

Physics, Chinese Academy of Sciences, Dalian 116023, China

e-mail: dicpgy@yahoo.com

X. Bao

e-mail: baoxh@dicp.ac.cn

123

Catal Surv Asia (2010) 14:85–94

DOI 10.1007/s10563-010-9090-8

Where ‘‘a-O’’ refers to the so-called ‘‘alfa-oxygen’’

species. It is generated by N2O decomposition on Fe/

ZSM-5 zeolite, see the above Eq. 1. The 18O-isotope

labeling studies [15, 16] supported the Eley–Rideal

mechanism. Various experimental techniques have been

used to characterize the ‘‘a-O’’ species [17–32]. As the IR,

EXAFS, TPD and TPR results [17–20] indicated, the

mono-iron species does exist in the Fe/ZSM-5 zeolite,

especially at low Fe loadings (Fe/Al \ 1.0). However, the

active sites of Fe/ZSM-5 zeolite are far more complex than

the experimentalists had expected. How it mimics the

behavior of the enzymatic methane monooxygenase

(MMO) is still an enigma to us. It is generally considered

that the oxygen-bridged binuclear iron species in Fe/ZSM-

5 zeolite features the active site of MMO [21–27].

Battiston et al. [21] used the XAFS technique and

identified a single Fe–Fe shell, which remains quite

stable under the CO-reduction or O2-oxidation conditions.

On basis of the in situ Mossbauer spectroscopic results,

Dubkov et al. [22] proposed the presence of the binuclear

iron species in Fe/ZSM-5 zeolite, where the two extra-

lattice Fe ions are bridged by the O atoms. The contained

Fe ions can be divalent or trivalent; i.e., the oxygen-

bridged binuclear iron species is in the form of FeII–O–FeII,

FeII–O–FeIII or/and FeIII–O–FeIII. Besides, the peroxide

species (O22-) was also suggested to be the active site in

Fe/ZSM-5 zeolite [27, 28]. With the introduction of extra-

lattice Al ions, Mul et al. [29] observed an infrared

absorption peak at 1635 cm-1 and assigned it to the Fe–O–

Al linkage. The presence of the Fe–O–Al species in Fe/

ZSM-5 zeolite was confirmed by the latter studies of van

Santen et al. [30–32]. It is believed that the Fe–O–Al

species is one of the competing active sites in Fe/ZSM-5

zeolite, because the concentrations of the ‘‘a-O’’ species

increase linearly with the amounts of the extra-lattice Al

ions.

The clarification of active sites is a crucial step to

understand the catalytic processes and improve the

catalysts. As a powerful tool to study the microstructural

properties and changes, theoretical calculations have

played an important role in the studies of active sites [33].

In this review, the various competing active sites of Fe/

ZSM-5 zeolite will be discussed on basis of the published

and ongoing researches, especially those from computa-

tional aspects.

2 The Mono-Iron Species

Among the numerous competing active sites in Fe/ZSM-5

zeolite, the mono-iron species has been the most exten-

sively studied [34–42]. It is probably due to two reasons.

The mono-iron species does exist in Fe/ZSM-5 zeolite,

especially at low iron loadings (Fe/Al \ 1); in addition, it

is a good computational model for Fe/ZSM-5 zeolite owing

to the simple structure. The Fe(III) ions in Fe(III)/ZSM-5

zeolite can be the sextet (high-spin) or quartet (low-spin)

state. The low-spin state is less stable and absent in the

studies of other groups [36–39, 41]. However, both of the

high- and low-spin Fe(III)/ZSM-5 zeolites have been

considered by us, in order to understand the influences of

the iron spin states to the active sites [40, 43]. The 5-T and

25-T clusters are chosen to model the Fe(III)/ZSM-5 zeo-

lite, see Fig. 1. Their Fe–Oe distances are close to each

other and also in good agreement with the values of other

groups [36–39]. The Fe-containing structures are severely

spin-contaminated and it is rather difficult to reach the

equilibrium geometries. Accordingly, the cluster models of

relatively small molecular sizes have been widely used

[36, 37, 39, 40, 43–45]. To the best of our knowledge, the

N2O decomposition on the high-spin Fe(III)/ZSM-5 zeolite

was also studied with the 46-T cluster models by the

ONIOM theoretical approach [38]. For any of the adsorp-

tion, transition state and desorption structures, the Fe–Ob

distances (the most representative geometric parameter, see

Fig. 2) differ less than 0.1 A for the 5-T and 46-T cluster

Fe

Oe

OaOb

Al

1.6621.690

Fe

Oe

Al

1.659

O O

Si

Si

Fig. 1 Representations of the

Fe(III)/ZSM-5 zeolite by the

5-T and ONIOM (B3LYP:

6-31G*:UFF) cluster models.

The Fe–Oe distance of the low-

spin state is given in parenthesis

86 G. Yang et al.

123

models. In addition, the energy barriers without zero-point

energy (ZPE) corrections are equal to 157.06 and

164.69 kJ mol-1 by our 5-T and Limtrakul’s 46-T cluster

models [38, 43], respectively, which are also in good

agreement with each other.

As shown in Fig. 2, the adsorbed N2O molecules form

direct bonds with the iron sites of Fe(III)/ZSM-5 zeolite,

whether of the high- or low-spin state. The adsorption

energies on the high- and low-spin Fe(III)/ZSM-5 zeolites

amount to -25.87 and -11.86 kJ mol-1, respectively,

indicating the stronger interactions of N2O with the former

(Fig. 3). In addition, all the high-spin structures of N2O

decomposition are more stable than the corresponding low-

spin ones. Accordingly, no crossing point will be observed

on the energy profiles of the high- and low-spin Fe(III)/

ZSM-5 zeolites. That is, the spin state does not change

Fig. 2 Cluster models of N2O

decompositions on Fe(III)/

ZSM-5 zeolite. The high- and

low-spin states of Fe(III) are

differed by the suffixes

-A and -B, respectively

Active Sites in Fe/ZSM-5 Zeolite 87

123

during the N2O decomposition process. The energy barriers

of N2O decompositions on the high- and low-spin

Fe(III)/ZSM-5 zeolites are calculated to be 148.14 and

128.11 kJ mol-1 (with ZPE corrections), respectively [43].

The results of the high-spin state are in excellent agreement

with those of Bell, Limtrakul and their co-workers [36, 38].

The energy barrier of N2O decomposition is somewhat

lower in the case of the low-spin state; however, the N2O

decomposition and ‘‘a-O’’ generation processes will still

predominate on the high-spin Fe(III)/ZSM-5 zeolite due to

the energy preferences of all the related structures.

3 The Oxygen-Bridged [Fe, X] Species (X 5 Fe, Al)

To the best of our knowledge, there are three computational

reports on the oxygen-bridged binuclear iron species in Fe/

ZSM-5 zeolite [42, 44, 45]. The results of van Santen’s and

Bell’s groups [44, 45] offer valuable insights into the oxy-

gen-bridged binuclear iron species; however, the optimized

geometric parameters do not satisfactorily match the

experimental data [14–16]. It is probably caused by the

introduction of two nearby lattice Al ions in their cluster

models. ZSM-5 zeolite has a high Si/Al ratio; in addition, Fe

has a smaller radius and shorter bond distance with the O

atoms compared with other elements such as La [46].

Accordingly, there is very little likelihood to find in ZSM-5

zeolite two nearby lattice-Al ions with short enough dis-

tances that provide two charge-balancing protons for the

exchanged binuclear iron species [47]. Accordingly, all

the oxygen-bridged [Fe, X] species (X = Fe, Al) consid-

ered by us are exchanged to the local sites with only one

lattice-Al ion.

Various oxygen-bridged [Fe, X] structures (X = Fe, Al)

have been proposed, where the contained Fe ions are of

different valence states and spin states [42]. The univalent

Fe ion is absolutely non-existent in Fe/ZSM-5 zeolite, since

the extra-lattice Al ion turns to divalent in the assumed

[FeI, Al] cluster model. As to the oxygen-bridged binuclear

iron species, the valence states of Fe are likely to be (?II,

?II), (?II, ?III) or (?III, ?III). It indicates that the mixed

valence states of ?II and ?III can exist in a single cluster

model, consistent with the in situ Mossbauer spectroscopic

observations [22].Two steps are taken in order to screen out

the suitable oxygen-bridged [Fe, X] structures (X = Fe,

Al). (1) Comparisons with the experimental data available

to us. It indicates that the Fe–Fe distances approximate

2.53 or 3.06 A and meanwhile the Fe–O distances fall

within the ranges 1.6–1.7 A (double bond) or 1.8–2.3 A

(single bond) [19, 22, 48]. Many distances of Santen’s and

Bell’s groups [44, 45] conflict with the experimental data,

which is probably caused by the presence of two close

lattice-Al ions in their cluster models. The spin density

states exert influences to the geometries, albeit not so

observable as the valence states. The Fe–Fe distances in

Fe3IIIFe3

III (Fig. 4c, where both Fe(III) ions are of the low-

spin state) and its corresponding high-spin structure are

equal to 3.186 and 3.302 A, and the former rather than the

latter matches finer with the experimental data. (2) Energy

considerations from the computational aspects. Besides the

geometric aspects, the corresponding high-spin structure of

Fe3IIIFe3

III has a higher energy of 24.6 kJ mol-1. An isomer

of Fe4IIAl (Fig. 4e) also exists but is more unstable by

64.2 kJ mol-1. These two structures are not energy-

preferred and therefore abandoned from the further con-

siderations. In addition, the thermodynamics of N2O

decompositions on all the oxygen-bridged [Fe, X] struc-

tures (X = Fe, Al) are calculated and the structure of the

mixed-valence states (?II, ?III) is excluded from the

active sites. It is found that the generation of the ‘‘a-O’’

species is thermodynamically facilitated by the introduc-

tion of extra-lattice Al ions, in agreement with the exper-

imental results [30, 32].

With the above two steps, the suitable oxygen-bridged

[Fe, X] structures (X = Fe, Al) are presented, as Fe4IIFe4

II,

Fe3IIIFe3

III and Fe4IIAl (Fig. 4). The N2O molecules are

decomposed on these structures and generate the corre-

sponding ‘‘a-O’’ species, see Fe3IIIOFe4

II, Fe2IVOFe3

III and

Fe3IIIOAl in Fig. 4. The similar structures can be found in

the catalytic cycles of Santen et al. [44, 45], by omitting

the extra-lattice protons used to balance the charges of the

exchanged species. By sharing one dangling Fe=O bond

with the other Fe site, Fe3IIIFe3

III (Fig. 4c) transforms

smoothly to Fe3IIIOFe4

II (Fig. 4b) and causes the shift of

the Fe–Fe distances from 3.186 to 2.658 A. It thus pro-

vides a sound explanation to the experiment-inexplicable

observations of the Fe–Fe shift from ca. 3.06 to 2.53 A

[48].

-40

0

40

80

120

160 FeII

FeIII

-A

FeIII

-C

148.14

128.11117.77

-17.44-20.33-35.18

Reaction coordinate

Ene

rgy

barr

ier

(kJ

mol

-1)

Fig. 3 Energy profiles of N2O decompositions on Fe(III) and Fe(II)/

ZSM-5 zeolites. The high- and low-spin states of Fe(III) are differed

by the suffixes -A and -B, respectively

88 G. Yang et al.

123

4 The Peroxide and Superoxide Species

With the aid of density functional calculations, the struc-

tures of the superoxide (O2-) and peroxide (O2

2-) species

are determined in Fe/ZSM-5 zeolite [49] and shown in

Fig. 5. These two species are validated by comparing their

O–O distances, Mulliken charge, spin densities and O–O

vibrations with the isolated O2, O2- and H2O2 structures

(Table 1). The parameters of the superoxide and peroxide

species in Fe/ZSM-5 zeolite are close to those of the O2-

and H2O2 structures, respectively. In Fe/ZSM-5 zeolite,

only one Fe ion in the superoxide species whereas both Fe

ions in the peroxide species form direct bonds with the

bridged O atoms. According to the theoretical results, the

superoxide rather than peroxide species should be active in

the ESR spectra, consistent with the recent experimental

observations [27]. However, no superoxide structures rep-

resent to be the local energy minima with the introduction

of extra-lattice Al ions [50].

The mO–O Raman frequencies of the superoxide and

peroxide species in Fe/ZSM-5 zeolite are equal to 1007.3

and 807.4 cm-1, in good agreement with the experimental

approximations at 1015–1180 and 800 cm-1, respectively.

Very recently, Li et al. [51] has detected the characteristic

Raman band of the peroxide species at 867 cm-1, which

further confirms its presence in Fe/ZSM-5 zeolite and the

rationality of our cluster models. At low temperatures (e.g.,

77 K [27, 28]), the superoxide species is more stable than

Fig. 4 Configurations of the

oxygen-bridged [Fe, X] species

in Fe/ZSM-5 zeolite (X = Fe,

Al). The valence and spin states

of Fe, which have been

corrected due to the spin

contaminations, are given in the

superscripts (Roman numerals)

and subscripts (Arabic

numerals), respectively

Active Sites in Fe/ZSM-5 Zeolite 89

123

the peroxide species, and its relative energy with the per-

oxide species at 77 K equals -5.0 kJ mol-1 [49]. The

relative stability of the peroxide species increases with

the increase of temperatures. At normal temperature

(298.15 K), the peroxide species becomes more stable with

the energy difference calculated to be 8.0 kJ mol-1. It

indicates that the superoxide species gradually converts

into the peroxide species as the temperature goes up, which

agrees well with the experimental results [52]. Accord-

ingly, the superoxide species is probably the precursor of

the peroxide species, one of the competing active sites in

Fe/ZSM-5 zeolite.

5 The Influence of Hydrogen Pretreatment

Hydrogen pretreatment is an essential step to prepare the

Fe/ZSM-5 catalyst, which may change the chemical states

of the Fe ions and further influence the catalytic activities.

Accordingly, it is of high significance to study the

adsorption and reduction of H2 on Fe/ZSM-5 zeolite. As

the density functional results [40] indicate, three H2

adsorption modes co-exist in Fe(III)/ZSM-5 zeolite con-

taining the mono-iron species as shown in Fig. 6. The H2

molecules cannot adsorb directly on the Fe sites of the

oxygen-bridged [Fe, X] (X = Fe, Al) or the peroxide

species (unpublished results); instead, they will shift to

interact with the bridging or dangling O atoms. It is

probably due to the more coordinations with the neigh-

bouring O atoms than in the mono-iron species, which

causes the vacant space not suitable to accommodate the

H2 molecules. It is consistent with the experimental results

that the binuclear iron species remain stable under the CO

reduction atmosphere [21].

For the mono-iron species, the two structures of the

high-spin rather than the low-spin state are energy-pre-

ferred and predominate during the H2 adsorption process,

consistent with the Inelastic Neutron Scattering results

[53]. The mH–H vibrations of the three structures are cal-

culated at 4015.7, 4047.2 and 4052.5 cm-1, respectively,

which are comparable to the FT-IR peaks at 3990 and

3960 cm-1 for H2 adsorption on Fe-silicalite [54]. In

addition, both of the two H atoms in the H2 molecules form

direct bonds with the Fe sites in the three adsorption

structures, because all the Fe–H distances fall within the

range 2.24–2.39 A [40]. That is, the H2 molecules are

chemisorbed on Fe(III)/ZSM-5 zeolite via the g2-binding

mode. As to the parallel adsorption mode of the high-spin

state (H2/ZFeIII-A in Fig. 6a), the H2 reduction process

finishes in one step, with the proton (Ha) transferred from

the Fe site to the Oa atom. However, it is not the case for

the vertical adsorption mode (H2/ZFeIII-a in Fig. 6b),

where the reduction of H2/ZFeIII-a by H2 is divided into

two steps. Step 1 is the transformation to the parallel

Al12

O32

O36

O14

O32

Oc

OaFe2

Fe12.219

2.171 2.255

2.067

2.107

1.3521.670

Ob Od

2.131

1.809 2.039

Al12

O32

O36

O14

O32

Oc

OaFe2

Fe1 2.273

2.193

2.256

2.042

2.235

1.480 1.672

ObOd

2.059

1.817 2.047

1.909

(a) Fe-O2- (b) Fe-O2

2-

Si

Si

Fig. 5 Configurations of

superoxide and peroxide species

in Fe/ZSM-5 zeolite

Table 1 Superoxide and peroxide species in Fe/ZSM-5 zeolite

O2 O2- H2O2 O2

- in Fe–O2- O2

2- in Fe–O22-

O–O distance A 1.215 1.353 1.456 1.352 1.480

Mulliken charge 0.000 -1.000 -2.000 -1.317 -1.981

Spin density 2.000 1.000 0.000 1.135 0.691

mO–O/ cm-1 1509.4 1103.1 875 1007.3 807.4

90 G. Yang et al.

123

adsorption mode, where the energy barrier equals

-0.05 kJ mol-1 (with ZPE corrections) and can be regar-

ded as barrierless. That is, this step shows almost no

influences on the H2-reduction activities. Accordingly, the

decisive steps of the high-spin Fe(III)/ZSM-5 zeolites are

the same for both parallel and vertical adsorption modes,

with the energy barriers calculated to be 46.22 kJ mol-1

(with ZPE corrections), see Fig. 7. For the low-spin Fe(III)/

ZSM-5 zeolite, only the vertical adsorption mode (H2/

ZFeIII-B in Fig. 6c) exists and the H2 reduction has an

energy barrier of 18.61 kJ mol-1 (with ZPE corrections).

Accordingly, both of the high- and low-spin Fe(III)/ZSM-5

zeolites are facile to be reduced during hydrogen pre-

treatment. In addition, the low-spin rather than high-spin

state facilitates the H2 reduction process. Different from

N2O decompositions discussed above, not all the high-spin

structures of H2 reductions are more stable than the cor-

responding low-spin ones. Accordingly, a crossing point

occurs on the energy profiles of the high- and low-spin

Fe(III)/ZSM-5 zeolites. The H2 reduction process initiates

preferentially on the high-spin state due to the energy

preference. During the transformation towards the transi-

tion states the spin state will be altered and changed, since

the transition state of the low spin is more stable than the

high spin by energy of -33.18 kJ mol-1 (with ZPE cor-

rections). Therefore, both high- and low-spin states play an

important role during the H2 reduction process.

On the reduced Fe(II)/ZSM-5 zeolite, the N2O decom-

position reaction takes place, with the energy profile and

related structures given in Figs. 8 and 3, respectively.

(a) H2/ZFeIII-A (b) H2/ZFeIII-a

Fe

O32

Al12

0.753

Si12 O36

Oa

Si3

Ha

Hb

1.669

(c) H 2/ZFeIII-B

Fe

O32

Al12

0.751

Si12 O36

Oa

Si3

Ha

Hb1.666

Fe

O32

Al12

0.751

Si12 O36

Oa

Si3

Ha

Hb1.703

Fig. 6 Cluster models of H2

adsorption on Fe(III)/ZSM-5

zeolite. The high- and low-spin

states of Fe(III) are differed by

the suffixes -A (-a) and -B,

respectively

-130

-90

-50

-10

30

70

-127.83

18.61

-65.63

46.22

Reaction coordinate

Ene

rgy

barr

ier

(kJ

mol

-1)

FeIII

-A

FeIII

-B

Fig. 7 Energy profiles of H2 reductions on the Fe(III)/ZSM-5 zeolite.

The high- and low-spin states of Fe(III) are differed by the suffixes

-A and -B, respectively

Active Sites in Fe/ZSM-5 Zeolite 91

123

The energy barrier of N2O decomposition on Fe(II)/ZSM-5

zeolite amounts to 117.77 kJ mol-1 and is 30.37 kJ mol-1

less than that of the high-spin Fe(III)/ZSM-5 zeolite (with

ZPE corrections). It indicates the N2O decomposition

process is facilitated by the pre-reduction of the Fe(III) ions

into the Fe(II) ions. In addition, the energy barriers of H2

reduction over Fe(III)/ZSM-5 zeolite are below one-third

of those of N2O decomposition, whether at high- or low-

spin state. Accordingly, the Fe(III) ions of Fe(III)/ZSM-5

zeolite are more likely to be reduced by H2 pretreatment

before reactions with N2O. The active site of the mono-iron

species should be generated from the reduced Fe(II)/ZSM-

5 zeolite and the active site is in the form of FeO(OH)

(Fig. 8c).

6 Conclusions

Fe/ZSM-5 zeolite is a heterogeneous catalyst that mimics

the behavior of the enzymatic methane monooxygenase

(MMO). It has shown great potential in the selective oxi-

dations of methane to methanol and benzene to phenol.

However, the active sites of Fe/ZSM-5 zeolite are far more

complex than expected. With the aid of theoretical

calculations, the various competing active sites in Fe/ZSM-

5 zeolite have been presented to us. On Fe(III)/ZSM-5

zeolite with the mono-iron species, three H2 adsorption

modes co-exist and the two of the high-spin state pre-

dominate, consistent with the Inelastic Neutron Scattering

results. All the H2 molecules are chemisorbed on the

Fe(III) sites via the g2-binding mode. Both high- and low-

spin states play an important role during the H2 reduction

process whereas the former predominates in the N2O

decomposition process. Owing to the low energy barriers,

the Fe(III) ions in Fe(III)/ZSM-5 zeolite are facile to be

reduced into the Fe(II) ions by H2 pretreatment. In addi-

tion, the H2 reduction of Fe(III)/ZSM-5 zeolite facilitates

the N2O decomposition process. Accordingly, the active

site of the mono-iron species in Fe/ZSM-5 zeolite should

be in the FeO(OH) form (Fig. 8c).

Unlike the mono-iron species, the oxygen-bridged

[Fe, X] (X = Fe, Al) and peroxide species seem to be

uninfluenced by H2 pretreatment, because the H2 molecules

cannot adsorb directly on the contained Fe sites. It is

consistent with the experimental results that the oxygen-

bridged binuclear iron species remain stable under the

CO reduction atmosphere. The univalent Fe ion is abso-

lutely non-existent in Fe/ZSM-5 zeolite. The suitable

2O/ZFeII II

1.206(1.207)

Al12

Oa

1.127(1.127) Fe

Ob

NbNa

O32 Si12 O36 Si3

Ha

1.802 (1.807)

1.132 (1.129)

Al12

Oa

Fe

ObNb

Na

Ha

1.511 (1.505)

1.795(1.800)

1.104(1.105)

Al12

Oa

Fe

Ob

Nb Na

Ha

1.761(1.765)

(a) N (b) TS2/ZFe

(c) N 2-O/ZFeII

Fig. 8 Cluster models

representing the N2O

decomposition on Fe(II)/ZSM-5

zeolite

92 G. Yang et al.

123

oxygen-bridged [Fe, X] structures (X = Fe, Al) are

screened out by two steps: (1) Comparisons with the

experimental data available to us; (2) Energy consider-

ations from the computational aspects. It is found that the

spin density states exert influences to the geometries, albeit

not so observable as the valence states. The selected

geometries of ours are in good agreement with the exper-

imental data, see Fe4IIFe4

II, Fe3IIIFe3

III and Fe4IIAl as well as

their oxidized forms (the active sites) Fe3IIIOFe4

II, Fe2IVOFe3

III

and Fe3IIIOAl in Fig. 4. The distribution of the Fe valence

states in the oxygen-bridged binuclear iron species is

consistent with the in situ Mossbauer spectroscopic

observations. Meanwhile, it is found that the generation of

the ‘‘a-O’’ species is thermodynamically facilitated by the

introduction of extra-lattice Al ions, in agreement with

the experimental results. In addition, it provides a sound

explanation to the experiment-inexplicable shift of the

Fe–Fe distances from ca. 3.06 to 2.53 A. The structures of

the superoxide (O2-) and peroxide (O2

2-) species in

Fe/ZSM-5 zeolite are shown in Fig. 5, which are validated

by comparisons with the O–O distances, Mulliken charge,

spin densities and O–O vibrations with the O2, O2- and

H2O2 structures. No superoxide structures represent as the

local energy minima with the presence of extra-lattice Al

ions. The superoxide rather than peroxide species is more

stable at low temperatures; however, as the temperature

goes up, the relative stability of the peroxide species

increases, and the superoxide species gradually converts

into the peroxide species. Accordingly, the superoxide

species is probably the precursor of the peroxide species,

one of the competing active sites in Fe/ZSM-5 zeolite.

Acknowledgment We gratefully acknowledged the financial

supports from the National Natural Science Foundation (No. 20903019)

and Ministry of Science and Technology of the Peoples’ Republic of

China (2003CB615806).

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