Reaction Mechanisms of H2 Reduction and N2O Decomposition on

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J. Phys. Chem. C 2009, 113, 18184–18190

Reaction Mechanisms of H2 Reduction and N2O Decomposition on Fe/ZSM-5 Zeolite: A Density Functional Theoretical Study Gang Yang,*,†,‡ Lijun Zhou,‡ Xianchun Liu,† Xiuwen Han,† and Xinhe Bao*,† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China and Key Laboratory of Forest Plant Ecology, Northeast Forestry UniVersity, Ministry of Education, Harbin 150040, People’s Republic of China ReceiVed: January 26, 2009; ReVised Manuscript ReceiVed: September 6, 2009

The Fe/ZSM-5 catalysts are usually prepared with H2 pretreatment. In this work, the reaction mechanisms of H2 reduction and N2O decomposition on Fe/ZSM-5 zeolite were studied with B3LYP density functional calculations. Before reduction, the vertical H2 adsorption mode on the high-spin Fe(III)/ZSM-5 zeolite must transform into the parallel mode, which is almost barrierless. Both high- and low-spin Fe(III)/ZSM-5 zeolites play an important role during the H2 reduction processes; in addition, Fe(III)/ZSM-5 zeolite is readily reduced by H2 pretreatment at moderate temperatures, because the ZPE-corrected energy barriers of the high- and low-spin states equal 46.22 and 18.61 kJ mol-1, respectively. Albeit with large H2 adsorption energies, Fe(III)/ ZSM-5 zeolite may not be suitable for H2 storage due to the difficulty of releasing the H2 molecules. On the H2-reduced Fe(II)/ZSM-5 zeolite, the energy barrier of N2O decomposition was calculated at 117.77 kJ mol-1, which is less than those of the high- and low-spin Fe(III)/ZSM-5 zeolites. In Fe(III)/ZSM-5 zeolite, the highspin state predominates the N2O decomposition process due to the higher structural stabilities, where the ZPE-corrected energy barrier is equal to 148.14 kJ mol-1 and in good agreement with the previous data. Accordingly, the N2O decomposition reactions are facilitated by H2 pretreatment. In addition, the “R-oxygen” species produced by N2O decomposition over the reduced Fe(II)/ZSM-5 zeolite should be responsible for the reaction activities of the monoiron species. 1. Introduction With the advent of Fe/ZSM-5 catalysts, the oxidation of benzene to phenol can be accomplished in one step and with high selectivity, at not less than 90%.1-4 This process has been acknowledged as one of the few successful examples in the field of selective oxidation. N2O is the most frequently used oxidant for the Fe/ZSM-5 catalysts, producing the so-called “R-oxygen” species at the extra-lattice iron sites. Recently, agreement has been reached that the “R-oxygen” species produced from N2O decomposition is responsible for selective oxidation processes, such as methane to form methanol and benzene to form phenol.5-8 As is known, H2 pretreatment is a conventional means to prepare Fe/ZSM-5 catalysts, which largely determines the chemical states of the extra-lattice iron species and further the performance of the oxidation processes.3,9,10 In contrast to the intensive studies on N2O adsorption and decomposition,11-23 very little research has been performed regarding H2 adsorption and reduction on Fe/ZSM-5 zeolite.24-26 As the inelastic neutron scattering (INS) technique indicated, the H2 molecules are strongly bound at the extra-lattice iron sites in Fe/ZSM-5 zeolite.24 The latter density functional calculations of our previous work26 revealed that the H2 molecules are chemisorbed on Fe/ZSM-5 zeolite via three different modes. The adsorption configurations with the iron species at the sextet spin state predominate and cause the main peak in the INS spectroscopy.24 In addition, the calculated frequencies at ca. 4000 cm-1 are in * To whom correspondence should be addressed. E-mail: dicpyanggang@ yahoo.com.cn (G.Y.); [email protected] (X.B.). † Chinese Academy of Sciences. ‡ Northeast Forestry University.

agreement with the FT-IR results of Berlier et al.25 In this work, first-principles density functional calculations were employed to study the H2 reduction mechanisms on Fe(III)/ZSM-5 zeolite. All three adsorption modes were taken into calculation and compared with each other. The N2O decomposition mechanism on the H2-reduced Fe(II)/ZSM-5 zeolite was then explored. The results were compared with those on Fe(III)/ZSM-5 zeolite without H2 reduction, by performing parallel calculations at the same level of theory. In addition, the potential of Fe/ZSM-5 zeolite to be utilized as H2-storage materials was evaluated, due to the large adsorption energies observed in the previous work.26-29 Finally, the effects of H2 pretreatment on the “Roxygen” generation were discussed, which will aid in understanding the active sites of Fe/ZSM-5 catalysts and further oxidation processes. 2. Computational Details 2.1. Cluster Models. As shown in Figure 1, the local structures of ZSM-5 zeolite were represented by 5-T clusters; i.e., those in the form of [Si(OH)3]2(SiH3)2AlO4.26,30,31 The present cluster models terminated with the Si-OH groups are somewhat larger than the usual ones terminated with the Si-H groups.17,20,32-35 With the exception of the boundary Si and O atoms, which were fixed in the crystallographic positions,36 all of the other atoms were allowed to fully relax. For convenience, ZFeII and ZFeIII were used to refer to the clusters of Fe(II)/ZSM-5 and Fe(III)/ZSM-5 zeolites, respectively. As to the Fe(II)/ZSM-5 clusters (ZFeII), the spin multiplicity was expected to equal 5 (Ms ) 5). As to the Fe(III)/ ZSM-5 clusters (ZFeIII), the spin multiplicities can be either 6 (high-spin, Ms ) 6) or 4 (low-spin, Ms ) 4), and the corresponding clusters were differed by suffixing -A or -B. That

10.1021/jp906426j CCC: $40.75  2009 American Chemical Society Published on Web 09/24/2009

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Figure 1. Cluster models representing the high-spin (a) and low-spin (b) Fe(III)/ZSM-5 zeolites as well as Fe(II)/ZSM-5 zeolite (c).

is, ZFeIII-A and ZFeIII-B represent the high- and low-spin Fe(III)/ ZSM-5 clusters, respectively. In case of the high-spin Fe(III)/ ZSM-5 zeolite, two modes of H2 adsorption were determined,26 and the latter reported (i.e., the vertical adsorption mode) was represented to be ZFeIII-a, see Figures 1-4. 2.2. Theoretical Methods. All of the calculations were run with the B3LYP density functional37,38 implemented in Gaussian98 software packages.39 With the exception of Fe, the H2 molecule, and the H species obtained from H2 reduction, all of the elements were treated with the commonly used 6-31G(d) basis set. The core and valence electrons of Fe were represented by the LANL2DZ effective core potential (ECP) and LANL2DZ basis set supplemented with one f-function, respectively. As to the H2 molecule and the related H species, they were described with the 6-311++G(d, p) basis set. For convenience, the above combination of basis sets and ECP was referred to as B1 and used throughout this work, except when otherwise specified. Frequency calculations at the same level of theory were performed, making sure that the local energy minima and transition states have no and one imaginary frequency, respectively. The zero-point energy (ZPE) corrections were then made. The N2O dissociation process on Fe(II)/ZSM-5 zeolite was also studied with the larger B2 basis set. B2 was defined as follows. The 6-31+G(d, p) basis set was used to treat all of the elements except Fe, whose core and valence electrons were described with the LANL2DZ effective core potential (ECP) and LANL2DZ basis set, respectively. In addition, the use of the B1 basis set to treat the H2-related structures was also validated by additional calculations, see the details in ref 40. The natural bond orbital (NBO) program41 was used to obtain the Wiberg bond indices (bond orders), which are a measure of bond strengths.42 The 〈S2〉 values were computed for all of the clusters, confirming that the spin contaminations are small and can be neglected.

Figure 2. Cluster models of the parallel H2 adsorption mode as well as H2-reduction and N2O-decomposition reactions on the high-spin Fe(III)/ZSM-5 zeolite.

Figure 3. Cluster models of the vertical H2 adsorption mode (left panel) and transition state between the vertical and parallel adsorption modes (right panel) on the high-spin Fe(III)/ZSM-5 zeolite.

3. Results and Discussion The cluster models of Fe(III) and Fe(II)/ZSM-5 zeolites were shown in Figure 1. The Fe-Oa distances and bond orders were calculated to be 1.662 Å and 0.67 for ZFeIII-A, 1.690 Å and 0.55 for ZFeIII-B as well as 1.791 Å and 0.25 for ZFeII, respectively.17,26,30 Accordingly, the FedO double bonds were considered to exist in the high- and low-spin Fe(III)/ZSM-5 zeolites, whereas the Fe-O single bond was considered to exist in Fe(II)/ZSM-5 zeolite. As the previous work26 indicated, three H2 adsorption structures were determined on Fe(III)/ZSM-5 zeolite. The two

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Figure 5. Energy profiles of the H2-reduction reactions on Fe(III)/ ZSM-5 zeolite. The energies with zero-point energy corrections are differed by suffixing (Z).

Figure 4. Cluster models representing the H2-reduction and N2Odecomposition reactions on the low-spin Fe(III)/ZSM-5 zeolite.

structures with the Fe(III) ions of the high-spin state (H2/ZFeIII-A in Figure 2a and H2/ZFeIII-a in Figure 3a, Ms ) 6) are of close energy and predominate over the one with the low-spin state (H2/ZFeIII-B in Figure 4a, Ms ) 4). The H2 (Ha-Hb) molecule in H2/ZFeIII-A is oriented nearly parallel to the Fe-Oa bond, whereas nearly vertical in H2/ZFeIII-a and H2/ZFeIII-B, which can be observed from the Ψ(OaFeHbHa) dihedral values in Table 1. 3.1. H2 Reduction. 3.1.1. H2/ZFeIII-A. The reduction of H2/ ZFeIII-A by H2 pretreatment causes the formation of H-H/ ZFeIII-A in Figure 2c, where the Ha-Hb, Oa-Ha, and Fe-Hb distances were optimized at 2.943, 0.966, and 1.623 Å, respectively (Table 1). The Ha and Hb atoms are no longer bonded to each other but form direct bonds with the Oa and Fe atoms, respectively. Meanwhile, the Fe and Oa interactions are somewhat weakened, and the Fe-Oa distance is stretched from 1.669 Å in H2/ZFeIII-A to 1.801 Å in H-H/ZFeIII-A. The transition state structure of the H2 reduction process was obtained and shown in Figure 2b, which was characterized by the imaginary frequency at 1475.9i cm-1. In TS1/ZFeIII-A, the Ha-Hb, Fe-Ha, Fe-Hb, and Oa-Ha distances were optimized at 0.965, 1.833, 1.925, and 1.295 Å, respectively. Compared with H2/ZFeIII-A, the Ha-Hb distance in TS1/ZFeIII-A is elongated somewhat, in contradistinction to the Fe-Hb and Oa-Ha distances. The energy profiles of H2 reduction reactions on Fe(III)/ ZSM-5 zeolite were shown in Figure 5. In the case of H2/ZFeIIIA, the energy barrier was calculated to be 50.65 (46.22) kJ mol-1; in addition, the reaction process is exothermic, with the reaction heat equal to -65.63 (-72.77) kJ mol-1. Note that the values in parentheses were corrected by zero-point energies

Figure 6. Intrinsic reaction coordinates (IRC) of the H2 reduction reaction from the vertical adsorption mode on the high-spin Fe(III)/ ZSM-5 zeolite. The region with the Oa-Ha distance of 3.20-2.75 Å was enlarged, corresponding to the transformation process from the vertical adsorption mode to the parallel one.

(ZPE). It indicated that the extra-lattice Fe(III) ions of the highspin state are readily reduced by H2 pretreatment. 3.1.2. H2/ZFeIII-a. Different from the parallel adsorption in H2/ZFeIII-A, the H2 (Ha-Hb) molecule in H2/ZFeIII-a (Figure 3a) is almost vertical to the Fe-Oa bond. The reduction of H2/ ZFeIII-a by H2 pretreatment is divided into two steps. Step 1 is the transformation to the parallel adsorption mode (i.e., H2/ ZFeIII-A). The transition state structure of this step is TS/ZFeIIIaA (Figure 3b), which has the characteristic imaginary frequency of 122.2i cm-1. In TS/ZFeIII-aA, the Ψ(OaFeHbHa) dihedral was optimized to be -44.40°, at the midway of -79.25° in H2/ZFeIII-a and 1.92° in H2/ZFeIII-A (Table 1). As shown in Figure 6, the intrinsic reaction coordinate (IRC) computations were also performed and confirmed TS/ZFeIII-aA as the transition state. Step 2 is the reduction of H2/ZFeIII-A by the adsorbed H2 molecules, which leads to the formation of H-H/ZFeIII-A (Figure 2). The details of Step 2 can be found in Section 3.1.1. The energy barrier of Step 1 was calculated to be 0.87 (-0.05) kJ mol-1, see Figure 6. The value is so small that it can be considered almost barrierless. Accordingly, Step 1 will exert no observable influences on the H2-reduction activities. Step 2 has an energy barrier of 50.65 (46.22) kJ mol-1 and therefore is rate-determining for the reduction of H2/ZFeIII-a. Accordingly,

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TABLE 1: Geometric Parameters and Relative Energies (RE) for the Structures of H2 Reduction Processes over Fe(III)/ZSM-5 Zeolitesa H2/ZFeIII-A TS1/ZFeIII-A H-H/ZFeIII-A H2/ZFeIII-B TS1/ZFeIII-B H-H/ZFeIII-B H2/ZFeIII-a TS/ZFeIII-aA

d(Ha-Hb)

d(Oa-Ha)

d(Fe-Ha)

d(Fe-Hb)

d(Fe-Oa)

Ψ(OaFeHbHa)

0.753 0.965 2.943 0.751 0.900 2.716 0.751 0.753

2.767 1.295 0.966 3.095 1.427 0.973 3.188 2.861

2.243 1.833 2.410 2.382 1.708 2.261 2.387 2.271

2.304 1.925 1.623 2.346 1.702 1.484 2.331 2.292

1.669 1.749 1.801 1.703 1.655 1.746 1.666 1.667

1.92 1.23 2.82 -77.08 1.86 23.65 -79.25 -44.40

RE

b

0.00 (0.00) 50.65 (46.22) -72.77 (-65.63) 0.00 (0.00) 28.71 (18.61) -129.88 (-127.83) 0.00 (0.00) 0.87 (-0.05)

a The distances are in angstrom, dihedrals in degrees, and energies in kJ mol-1, respectively. b The energies with zero-point energy (ZPE) corrections are in parentheses.

the H2 reduction processes on the high-spin Fe(III)/ZSM-5 zeolite have energy barriers not more than 50.65 (46.22) kJ mol-1, which easily take place even at moderate temperatures. 3.1.3. H2/ZFeIII-B. In H2/ZFeIII-B (Figure 4a), the Ha-Hb (H2) molecule is nearly vertical to the Fe-Oa bond. The reduction of H2/ZFeIII-B by H2 pretreatment forms H-H/ZFeIII-B given in Figure 4c. In H-H/ZFeIII-B, the Ha-Hb, Oa-Ha, Fe-Hb, and Fe-Oa distances were calculated to be 2.716, 0.973, 1.484, and 1.746 Å, respectively (Table 1). This indicated that the Ha-Hb bond has been broken and that, meanwhile, Oa-Ha and Fe-Hb bonds have formed, which are identical to the situation in the case of H2/ZFeIII-A. The transition state structure of H2 reduction over the low-spin Fe(III)/ZSM-5 zeolite was shown in Figure 4b and characterized with the imaginary frequency of 1008.8i cm-1. In TS1/ZFeIII-B, the Ha-Hb, Fe-Ha, Fe-Hb, and Oa-Ha distances are equal to 0.900, 1.708, 1.702, and 1.427 Å, respectively. The Ha-Hb distance in TS1/ZFeIII-B is somewhat less than that in TS1/ZFeIII-A, which may be responsible for the lower energy barrier. As indicated in Figure 5, the energy barrier of H2 reduction over the low-spin Fe(III)/ ZSM-5 zeolite equals 28.71 (18.61) kJ mol-1. In addition, the Mulliken charges on the Ha and Hb atoms were calculated to be 0.063 and 0.044 |e| in H2/ZFeIII-A, 0.058 and 0.011 |e| in H2/ZFeIII-a, as well as 0.057 and -0.008 |e| in H2/ZFeIII-B, respectively. During the H2 reduction processes, the Ha and Hb atoms gradually move toward the negatively charged Oa atom and the positively charged Fe atom, respectively. Accordingly, the slight positive charge on the Ha atom and slight negative charge on the Hb atom in H2/ZFeIII-B will facilitate the reduction process. 3.1.4. Potential as H2 Storage Materials. The adsorption energies of H2 on Fe(III)/ZSM-5 zeolite were calculated up to -18.1 kJ mol-1, indicating that Fe(III)/ZSM-5 zeolite should be a good candidate for H2 storage.26-29 The energy barriers of the H2 reduction reactions are not more than 50.65 (46.22) kJ mol-1, whether the iron species are of the high- or low-spin state. This indicated that the adsorbed H2 molecules are ready to be dissociated into the H species. As Figure 5 indicated, the reverse reactions of H2 reduction; i.e., the combinations of two H atoms into the H2 molecule, have to cross over relatively large energy barriers, 123.42 (111.85) and 158.59 (146.44) kJ mol-1 for the high- and low-spin Fe(III)/ZSM-5 zeolites, respectively. That is, the dissociated H species are difficult to restore into H2 molecules on Fe(III)/ZSM-5 zeolite, which obviously becomes the main obstacle of using Fe(III)/ZSM-5 zeolite as an H2-storing material. 3.2. N2O Dissociation. 3.2.1. N2O/ZFeII. The reduction of Fe(III)/ZSM-5 zeolite by H2 pretreatment produces Fe(II)/ ZSM-5 zeolite, see Figure 1c. On the reduced Fe(II)/ZSM-5 zeolite, the N2O adsorption and decomposition were studied at

B3LYP/B1 and B3LYP/B2 levels of theory. The related structures and geometric parameters are given in Figure 7 and Table 2. In N2O/ZFeII (Figure 7a), the Fe-Oa, Fe-Ob, Ob-Na, and Na-Nb distances were optimized at 1.802, 2.356, 1.206, and 1.127 Å at B3LYP/B1 level and 1.807, 2.369, 1.207, and 1.127 Å at B3LYP/B2 level, respectively. In the free N2O molecule, the Ob-Na and Na-Nb distances are equal to 1.194 and 1.135 Å at B3LYP/B1 level and 1.197 and 1.134 Å at B3LYP/B2 level,43 in good agreement with the experimental values of 1.184 and 1.128 Å44 as well as the computational values of 1.185 and 1.157 Å at MP2/6-311++G(2d,2p) level and 1.178 and 1.154 Å at MP2/6-311++G(3df,3pd) level.45 The slight elongation of the Ob-Na bond and shortening of the Na-Nb bond indicated that the N2O molecule is slightly activated by the adsorption of on Fe(II)/ZSM-5 zeolite. The dissociation of N2O generates the “R-oxygen” species, see N2-O/ZFeII in Figure 7c. In N2-O/ ZFeII, the Fe-Oa, Fe-Ob, Ob-Na, and Na-Nb distances were calculated to be 1.761, 1.606, 3.299, and 1.104 Å at B3LYP/ B1 level and 1.765, 1.603, 3.433, and 1.105 Å at B3LYP/B2 level, respectively. The Na-Nb distance in N2-O/ZFeII is almost equivalent to the value of the free N2 molecule, 1.105 Å at both B3LYP/B1 and B3LYP/B2 theoretical levels. The transition state of the N2O decomposition reaction was shown in Figure 7b. In TS2/ZFeII, the Fe-Oa, Fe-Ob, Ob-Na, and Na-Nb distances were optimized at 1.795, 1.819, 1.511, and 1.132 Å at B3LYP/ B1 level and 1.800, 1.851, 1.505, and 1.129 Å at B3LYP/B2 level, respectively. The characteristic negative frequency at B3LYP/B1 level was at 626.9i cm-1. From the above discussions, it was found that the geometries of the B3LYP/B1 and B3LYP/B2 methods are very close to each other. As Figure 8 indicated, the energy barrier at B3LYP/ B1 level was calculated to be 131.15 (117.77) kJ mol-1, which is also in good agreement with the B3LYP/B2 value of 128.35 kJ mol-1. Accordingly, the default B3LYP/B1 method is qualified to treat the present reaction systems. 3.2.2. N2O/ZFeIII. In contrast to the reduced Fe(II)/ZSM-5 zeolite, the “R-oxygen” generation by N2O decomposition on Fe(III)/ZSM-5 zeolite has been extensively studied, especially with the high-spin state (ZFeIII-A).14-17,20 The N2O adsorption dissociated and the transition state structures on Fe(III)/ZSM-5 zeolite were given in Figures 2d-f and 3d-f, where the geometries are in agreement with the previous results.14-17,20 As the data in Figures 2d, 3d, and Table 2 indicated, the N2O (ObNaNb) geometries of N2O/ZFeIII-A, N2O/ZFeIII-B resemble that of N2O/ZFeII; in addition, the Fe-Ob distances in the three adsorbed structures are all approximated at 2.35 Å. However, the N2O (ObNaNb) geometries of the transition state structures and dissociated structures may differ from each other, mainly in the ∠ObNaNb angles. For example, the ∠ObNaNb angles were

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Figure 7. Cluster models representing the N2O-decomposition reactions on Fe(II)/ZSM-5 zeolite.

TABLE 2: Geometric Parameters and Relative Energies (RE) for the Structures of N2O Decomposition Processes over Fe(II)/ and Fe(III)/ZSM-5 Zeolitesa N2O/ZFeII TS2/ZFeII N2-O/ZFeII N2O/ZFeIII-A TS2/ZFeIII-A N2-O/ZFeIII-A N2O/ZFeIII-B TS2/ZFeIII-B N2-O/ZFeIII-B

d(Na-Nb)

d(Ob-Na)

d(Fe-Ob)

d(Fe-Oa)

θ(NaNbOb)

1.127 1.132 1.104 1.126 1.137 1.102 1.127 1.110 1.105

1.206 1.511 3.299 1.208 1.517 2.795 1.205 1.531 3.249

2.356 1.819 1.606 2.318 1.819 1.671 2.386 1.896 1.577

1.802 1.795 1.761 1.668 1.866 1.660 1.715 1.604 1.582

178.16 136.27 131.01 178.24 139.58 141.75 178.50 167.83 105.25

RE

b

0.00 (0.00) 128.35 (117.77) -15.99 (-20.33) 0.00 (0.00) 157.06 (148.14) -11.27 (-17.44) 0.00 (0.00) 133.15 (128.11) -30.20 (-35.18)

a The distances are in angstrom, dihedrals in degrees, and energies in kJ mol-1, respectively. b The energies with zero-point energy (ZPE) corrections are in parentheses.

optimized at 131.01° in N2-O/ZFeII, 141.75° in N2-O/ZFeIII-A and 105.25° in N2-O/ZFeIII-B, respectively (Table 2). The imaginary frequencies associated with TS2/ZFeIII-A and TS2/ ZFeIII-B fall at 882.9i and 835.5i cm-1, respectively. As indicated in Figure 8, the energy barrier of N2O decomposition on the high-spin Fe(III)/ZSM-5 zeolite was calculated to be 157.06 (148.14) kJ mol-1, in good agreement with the previous data of 164.69 kJ mol-1 by Limtrakul et al.16 and 157.17 kJ mol-1 by Bell et al.17 The energy barrier of N2O decomposition on the low-spin Fe(III)/ZSM-5 zeolite is somewhat lower and equals 133.15 (128.11) kJ mol-1. The lower energy barrier of N2O decomposition in the case of the lowspin state is similar to that of H2 reduction on Fe(III)/ZSM-5 zeolite discussed above. 3.3. Active Sites of Fe/ZSM-5 Zeolite. As to the N2O decomposition reactions over Fe(III)/ZSM-5 zeolite, the energy

barrier is lower for the low-spin state rather than the high-spin state, see the energy profile in Figure 8. However, all of the structures of the high-spin state are more stable in geometry. For example, the ZPE-corrected energy difference between the low- and high-spin adsorbed structures was calculated to be 22.87 kJ mol-1. There is no crossing point on the energy profiles of the high- and low-spin Fe(III)/ZSM-5 zeolites, and accordingly, the spin-state changes will not take place (Figure 8). Owing to the energy preference, the previous theoretical studies16,17 only considered the high-spin state during the generation of the “R-oxygen” species by N2O decomposition. However, the energy barrier of H2 reduction over Fe(III)/ ZSM-5 zeolite is less than one-third that of the N2O decomposition on the high-spin Fe(III)/ZSM-5 zeolite, see Figures 5 and 8. It is consistent with the facile reduction of Fe(III)/ZSM-5 zeolite by H2 pretreatment as discussed earlier. Apart from the

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Figure 9. The active site of ZFe(II)O formed by N2O decomposition on Fe(II)/ZSM-5 zeolite.

species will appear and dominate; when the extra-lattice aluminum ions are introduced, the [Fe, Al] species will be produced, where the extra-lattice Fe and Al ions are bridged with O atoms.30,31 In future work, the oxidation reactions of benzene to form phenol will be studied on the presently proposed monoiron species as well as the previous di-iron and [Fe, Al] species.30,31 4. Conclusions

Figure 8. Energy profiles of the N2O-decomposition reactions on Fe(III) and Fe(II)/ZSM-5 zeolites. The energies calculated with the B2 basis set and corrected with zero-point energies (ZPE) are labeled by suffixing (B2) and (Z), respectively.

N2O-decomposition structures, the H2-reduction structures of the high-spin state are not necessarily more stable in geometry. H-H/ZFeIII-B of the low-spin state is lower in energy than H-H/ZFeIII-A of the high-spin state, with the ZPE-corrected energy difference equal to -33.18 kJ mol-1. Accordingly, a crossing point occurs on the energy profiles of the high- and low-spin Fe(III)/ZSM-5 zeolites (Figure 5). The H2 reduction processes initiate preferentially on the high-spin Fe(III)/ZSM-5 zeolite and by changing the spin states, terminate in the lowspin product (i.e., H-H/ZFeIII-B instead of H-H/ZFeIII-A). It indicated that the low-spin Fe(III) ions in Fe(III)/ZSM-5 zeolite may also play an important role during the H2-reduction process. In addition, the ZPE-corrected energy barrier of N2O decomposition over the reduced Fe(II)/ZSM-5 zeolite is 30.37 kJ mol-1 less than that over the high-spin Fe(III)/ZSM-5 zeolite. It showed that the generation of the “R-oxygen” species will be facilitated when the Fe(III) ions are reduced into the Fe(II) ions. The active site produced by N2O decomposition over the reduced Fe(II)/ ZSM-5 zeolite was shown in Figure 9, which should be responsible for the oxidation activities of the monoiron species in Fe/ZSM-5 zeolite. The monoiron species is one of the most important active sites in Fe/ZSM-5 zeolite, especially at low iron loadings. When the loadings of iron increase, the di-iron

B3LYP density functional calculations were carried out to study the reaction mechanisms of H2 reduction and N2O decomposition on Fe/ZSM-5 zeolite. Both high- and low-spin states were considered for the Fe(III) ion. The main findings were summarized as follows. Structures of the H2 reduction reactions on Fe(III)/ZSM-5 zeolite were obtained and checked with frequency analysis. As to the vertical H2 adsorption mode of the high-spin state, it will first transform to the parallel adsorption mode almost without energy barrier and then undergo H2 reduction with the rupture of the H-H bonds. The entire reaction process was modeled with the additional calculations of intrinsic reaction coordinates (IRC). The ZPE-corrected energy barriers of H2 reduction on the high- and low-spin Fe(III)/ZSM-5 zeolites were calculated to be 46.22 and 18.61 kJ mol-1, respectively. It indicated that Fe(III)/ZSM-5 zeolite is readily reduced by H2 pretreatment even at moderate temperatures. Considering the structural stabilities and energy barriers, both the high- and low-spin Fe(III)/ZSM-5 zeolites play an important role during the H2 reduction processes. The potential for using Fe(III)/ZSM-5 zeolite as H2-storage materials was evaluated as well. It was found that the H2 molecules are difficult to restore due to the large energy barriers. The N2O adsorption structures on the reduced Fe(II)/ZSM5, high- and low-spin Fe(III)/ZSM-5 zeolites were found to resemble each other; however, differences were observed in the transition state and dissociated structures. As to Fe(III)/ZSM-5 zeolite, the N2O decomposition process is dominated by the high-spin rather than low-spin Fe(III) ions owing to the higher structural stabilities. The ZPE-corrected energy barrier of N2O decomposition on the reduced Fe(II)/ZSM-5 zeolite was calculated to be 117.77 kJ mol-1, less than the value 148.14 kJ

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