Insights into adsorption performances and direct decomposition mechanisms of NO on [FeO]1+-ZSM-5: A density functional theory study

Journal Pre-proofs Full Length Article Insights into adsorption performances and direct decomposition mechanisms of NO on [FeO]1+-ZSM-5: A density functional theory study Sheng Shi, Miaoting Li, Chao Ge, Jianjun Lu, Pan Chen, Peide Han, Zhifeng Yan PII: DOI: Reference:

S0169-4332(19)34029-2 https://doi.org/10.1016/j.apsusc.2019.145212 APSUSC 145212

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Applied Surface Science

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30 October 2019 2 December 2019 26 December 2019

Please cite this article as: S. Shi, M. Li, C. Ge, J. Lu, P. Chen, P. Han, Z. Yan, Insights into adsorption performances and direct decomposition mechanisms of NO on [FeO]1+-ZSM-5: A density functional theory study, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.145212

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Insights into adsorption performances and direct decomposition mechanisms of NO on [FeO]1+-ZSM-5: A density functional theory study Sheng Shia, Miaoting Lia,b, Chao Gea, Jianjun Lua,c, Pan Chenc, Peide Hanb, Zhifeng Yana aCollege

of Textile Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China bCollege of Material Science and Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China cKey Laboratory of Coal Science and Technology of Ministry of Education and Shanxi Province, Taiyuan University of Technology, Taiyuan 030024, Shanxi, China

Abstract The density functional theory (DFT) has been employed to systematically investigate the successive adsorption performances of two NO molecules onto [FeO]1+-ZSM-5 surface and subsequent direct decomposition mechanisms by using cluster model in this paper. The adsorption results indicate that there are four stable adsorption configurations of the first NO molecule on [FeO]1+-ZSM-5, all of which are exothermic and strong chemisorption. Beginning with different adsorption configurations of first NO molecule on [FeO]1+-ZSM-5 followed by the adsorption of the second NO molecule, all four direct decomposition mechanisms are studied in which N2O molecule acts as the key reaction intermediate, N2 and O2 act as the target products. Based on thermodynamic analysis, η2-NO path that starts with adsorption of the

first NO as O-down and follows the Eley-Rideal mechanism is proposed, in which ONNO species and N2O are the key catalytic intermediates. The full catalytic reaction is exothermic by 43.76 kcal/mol. The rate-determining step is the formation of N2(ads), i.e. N2O(ads)→N2(ads) +O(ads) and its barrier energy is 39.09 kcal/mol. Keywords: [FeO]1+-ZSM-5, NO adsorption, direct decomposition mechanism, density functional theory 1. Introduction Nitric oxide, one of the serious pollutants, has been receiving much attention and several methods have been developed for its removal such as selective catalytic reduction (SCR)[1], storage reduction (NSR)[2] and direct decomposition (DD)[3, 4]. Currently, SCR of NO is considered as one of the most efficient techniques with a commercial catalyst V2O5–MoO3 (WO3)/TiO2. However, the disadvantages including high operational costs, leakage problems and secondary pollution, together with the the high oxidation ability of SO2 to SO3 and the toxicity of V2O5[1], limit the widespread use of this method and related catalysts. Over the past two decades, due to the development of alkali metal-based and transition metal-based catalysts, the catalytic efficiencies for DD-NO to N2 and O2 by employing these catalysts were significantly enhanced. Therefore, this ideal route for NO removal with no additional reductant were focused again . The main issue for DD-NO is its energy barrier up to 87.00 kcal/mol [5]. Many efforts have been devoted to develop catalysts to reduce the high energy barrier. At present, several types of catalysts have been reported such as perovskite-type oxides of ABO3 and/or A2BO4 structure[6], rare earth oxides[7], novel metals supported on metal oxides[8], Cu-ZSM-5[9] and Fe-ZSM-5[10]. Many experimental studies and theoretical calculations have been performed to understand both the nature of active sites and DD-NO mechanism, especially for

Cu-ZSM-5. It has been observed that the existence of single Cu2+ and pairs of Cu2+ [11-19],

CuO-

and

Cu2O-like

clusters[20-23],

([Cu-O-Cu]2+)

and

(bis-[Cu-µ-(O)2-Cu]2+) dimers[24-26], and (-Cu-O-Cu-O-) chain structures [27, 28] in CuZSM-5 by using different spectroscopic techniques, each of which has been suggested as the active site in DD-NO or SCR by hydrocarbons[29]. Zakharov et al. [29] assumed -O-(Cu2+-O-Cu+) structure as the active site of Cu-ZSM-5 for DD-NO reaction by DFT method. However, only the terminal Cu+ is directly involved in the reaction. The same conclusion was drawn by the DFT studies from Morpurgo et al. [30] and Moretti et al. [31] that the DD-NO mechanism over single Cu2+ is similar to that of a pair of Cu2+. With respect to the role of α-O in reaction, it is still quite in debate. By using tracer techniques (18O2,

15N18O,

N18O, and N218O), Hall et al. [32]

proposed that N18O and N218O suffer exchange with α-O. On the contrary, the isotope exchange experiments of 15N18O and N218O from Chang and McCarty [33] suggested the α-O is not involved in the reaction. To date, it has been generally recognized that DD-NO to N2 and O2 can occur in two steps: the formation of N2O by the adsorption and reaction of two NO molecules followed by N2O decomposition to N2 and adsorbed O atom or O2 molecule [24, 34-36]. The first step that adsorption of two NO molecules on catalyst is of great importance for DD-NO mechanism. Molecular orbital analysis of NO molecule shows that both LUMO and HOMO of NO are antibonding 2π* orbitals and contributed more by N atom than by O atom, which indicates NO molecule prefers to adsorb as N-down whether NO reacts as electrophile or nucleophile[37]. Similar results are also found for adsorption of NO on Cu-ZSM-5 [34-36, 38]. Some researchers [15, 39] considered the first NO adsorbs as N-down on Cu active site, who stress that the conformation of N-down is actually more favorable than that of O-down. However, most of researchers supported the activated O-down adsorption conformation of NO [9, 34-36, 40]. Using quantum mechanical/molecular mechanical (QM/MM) method, Sajith et al.[9] showed that production of N2O from N-down conformation is energetically unfavorable. By employing DFT calculations, Izquierdo et al. [38] presented that Cu-NO and Cu-ON species coexist in dynamic equilibrium, in which

an intermediate species of Cu−κ2-NO plays an important role. In regard to the formation of N-N bond, a key step in DD-NO reaction, it was widely accepted that ONNO species acts as the catalytic intermediate[15, 29, 30, 34-36, 39]. Indeed, the ONNO species adsorbed on Ag(111) could decompose directly to adsorbed N2O molecule and adsorbed O atom with a low energy barrier of 2.0 kcal/mol[41, 42]. In addition, Cu(NO2)(NO) and Cu(N2O3) species have been proposed to be the key intermediates for the NOx decomposition on Cu-ZSM-5[21, 43, 44]. However, the theoretical study from Solans-Monfort et al. demonstrated that Cu(NO2)(NO) and Cu(N2O3) species are found impractical because of their high energy barriers of decomposition reaction[40]. Fe-based catalyst, especially Fe-ZSM-5, has been reported excellent catalytic activity for NOx decomposition [45, 46]. However, most of researches focus on the decomposition of N2O. Kawakami and Ogura [10] proposed a two-step mechanism for DD-NO reaction in the presence of excess oxygen over Fe2+ ion-exchanged zeolites, which begins with adsorption of the first NO as N-down on Fe atom followed by adsorption of the second NO on N atom of first NO to form N-N bond directly. In the proposed mechanism, the rate-determining step is 2NO→ N2O+O with the barrier energy of 38.24 kcal/mol, which is higher than that of 32.27 kcal/mol for DD-NO on Cu-ZSM-5[47]. To date, to the best of our knowledge, limited attempts have been made to reveal the adsorption performances of two NO molecules and DD-NO mechanism over Fe-ZSM-5 catalyst. Aiming to shed light on adsorption performances and subsequent DD mechanisms of NO on Fe-ZSM-5, a cluster model Si17AlFeO50H26 was first built to represent [FeO]1+-ZSM-5. And then all possible DD-NO mechanisms were studied in which stable adsorption conformations of the first NO molecule on [FeO]1+-ZSM-5 acts as initial reaction conformation respectively, adsorption of the second NO molecule acts as the initial reaction step, N2O molecule acts as the key reaction intermediate, N2 and O2 act as the target products. After DD-NO reaction finished, [FeO]1+-ZSM-5 catalyst model is regenerated and the catalyst cycle is completed.

2. Computational models and details Similar to the active sites of Cu-ZSM-5, both Fe3+ and Fe-Ox site have been suggested to be the active sites of Fe-ZSM-5 for NO decomposition[10, 46]. Considering the controversial role of α-O of Cu-ZSM-5 in DD-NO reaction [32, 33], [FeO] was chosen as the active center of Fe-ZSM-5 model to check its impact just as suggested by others. A 18T cluster including the full 12-membered ring, was cut from ZSM-5 framework for investigation. The Si atom at T12 site was substituted by an Al atom as reported the most likely site for Si/Al substitution [48-50], resulting in a negative charge in the zeolite framework. This negative charge was neutralized by [FeO] species and a cluster model Si17AlFeO50H26 was built to represent [FeO]1+-ZSM-5 (Fig. 1). The second shell framework O of framework Si atom was saturated by H atom with O-H length at 1.00 Å, which oriented along the bond direction to what would otherwise have been the next framework atoms. In all the calculations, O-H bonds of the external two layers of [FeO]1+-ZSM-5 model were fixed in their original crystallographic positions to retain the zeolite structure while other atoms were allowed to relax. All DFT calculations were carried out with the program package Dmol3 implemented in Materials Studio [51, 52]. The generalized gradient approximation (GGA) of Perdew–Burke–Ernzerhof (PBE) functional was selected to deal with exchange-correlation potential [53]. Double-numeric quality basis set with polarization function (DNP) was used for all the atoms. The basis sets of DFT Semi-core Pseudopots (DSPP), which introduces some degree of relativistic correction into the core, was employed to treat of core electrons for H, O, Si, Al and Fe atoms. Open shells were performed based on spin-unrestricted wavefunctions. The convergence criterion for energy, maximum force and maximum displacement changes were specified at 2.0 × 10−5 Ha, 4.0 × 10−3 Ha/Å and 5.0 × 10−3 Å, respectively, while the self-consistent-field (SCF) density convergence threshold value of 10−5 Ha was set.

Adsorption energy Eads was defined as the following equation:

E ads = E molecule/cluster -E cluster -E molecule (1) Where Emolecule/cluster, Ecluster and Emolecule represent the total energy of the substrate cluster with adsorbed molecule, the substrate cluster and the free molecule, respectively. By this definition, a more negative Eads indicates a stronger interaction. The electronic properties of adsorption structures were examined by the Mulliken population analysis [54-57] and Mayer bond order indices [58, 59]. The transition states (TS) structures were located by employing the complete LST/QST method [42], which began with a linear synchronous transit (LST) maximization, and followed by repeated conjugate gradient minimizations and quadratic synchronous transit (QST) maximization until a stationary saddle point (transition state) was located. Furthermore, every transition state was confirmed by the presence of single imaginary frequency, whose eigenvector corresponds to the motion along the reaction path. Vibrational analysis was also performed to confirm that the other configurations have no imaginary frequency.

3. Results and discussion The optimized [FeO]1+-ZSM-5 model was shown in Fig. 1, in which the loaded [FeO] species bonds with ZSM-5 framework by the formation of Fe-Oa and Fe-Ob bonds. Bond lengths of both Fe-Oa and Fe-Ob are 1.960 Å and corresponding bond orders are 0.509 and 0.494, respectively. This little difference in bond order is attributed to the incompletely symmetric structure of ZSM-5 framework and the set of no symmetry constraint in calculations. In addition, the calculated bond length of free NO molecule (1.164 Å) agrees well with the experimental values of 1.151 Å[60].

3.1 Adsorption of the first NO molecule Fig. 2 presents the optimized adsorption configurations of the first NO on [FeO]1+-ZSM-5 and corresponding bond lengths, Mayer order, Mulliken charges and adsorption energies are shown in Table 1. The interaction between NO and [FeO] is highlighted and ZSM-5 framework is omitted for clarity in Fig. 2. As displayed in Fig. 2, NO molecule can adsorb on [FeO]1+-ZSM-5 in four ways: (a) NO adsorbs on

Fe atom as N-down and Fe-N bond forms (denoted as η1-NO); (b) NO adsorbs on Fe atom as O-down and Fe-ON bond forms ( denoted as η2-NO); (c) both N and ON atoms of NO molecule bond with Fe atom and Fe-N and Fe-ON bond form (denoted as η3-NO); (d) N atom bonds with OFe atom and ON atom bonds with Fe atom leading to the formation of N-OFe and O-Fe bond (denoted as η4-NO). When NO adsorbs on Fe atom as N-down, the formed Fe-N bond is 1.629 Å with the bond order of 1.785, indicating its strong covalent character. Compared with free NO molecule before adsorption, the length of N-O bond almost keeps unchanged but its bond order reduces from 2.279 to 1.744 after adsorption, which reveals the activation of NO after adsorption. Similar change trend is observed on Fe-OFe bond. Adsorption of NO leads to the increase of coordination of Fe atom from three to four, which results in the weakness of interaction between [FeO] and framework just as the increase of bond length and reduction of bond order for Fe-Oa and Fe-Ob bonds. The Mulliken population analysis in Table 1 shows that NO donates 0.292 e to [FeO]1+-ZSM-5. For η2-NO, the bond length of 1.708 Å and bond order of 1.008 for the formed Fe-ON bond shows its single bond character. After adsorption, the change trend of N-ON bond is similar to that of η1-NO and 0.146 e charge is transferred from NO to [FeO]1+-ZSM-5. For η3-NO, the bond lengths of formed Fe-N and Fe-ON bonds are 1.779 Å and 2.041 Å with the bond orders of 1.275 and 0.684, respectively. The adsorbed NO shows more activation degree compared with that of η1-NO and η2-NO according to its increase of bond length and reduction of bond order. There is 0.015 e charge transfer from NO to [FeO]1+-ZSM-5. For η4-NO, similar bond lengths and bond orders of N-ON (1.297 Å and 1.380) and N-OFe (1.288 Å and 1.398) bonds exhibits the formation of adsorbed NO2. The Mulliken population analysis indicates that [FeO]1+-ZSM-5 donates 0.018 e charge to NO and the adsorbed NO2 carries 0.345 e charge. In these four adsorption configurations, all the adsorbed NO are activated to a certain extent and the interaction between [FeO] and framework are weaken by the fact of the increase of bond length and reduction of bond order for Fe-Oa and Fe-Ob bonds.

All four adsorption configurations have exothermic adsorption energies, which are -60.43, -19.89, -36.65 and -41.54 kcal/mol for η1-NO, η2-NO, η3-NO and η4-NO respectively, indicating the strong chemsorption of NO on [FeO]1+-ZSM-5. According to the adsorption energies data, the stabilities for these four adsorption configurations decrease in the following order: η1-NO> η4-NO> η3-NO> η2-NO. It is seen that this order is not linearly correlated with the charge change of adsorbed NO. Our previous work expressed that N atom offers more contribution to both HOMO and LUMO of NO molecule than O atom[37]. Further insight into this fact here, the stability of adsorption configuration for NO molecule is strictly related to the contribution of N atom to bonding interaction between NO and [FeO]1+-ZSM-5. A larger contribution to bonding of N atom will lead to a stronger bonding interaction and a more stable adsorption configuration, which is consistent with the adsorption results from other studies[30, 34-38].

3.2 Reaction pathways for DD-NO reaction Following the proposed two step mechanism that NO→N2O→N2 [39-80], all possible reaction pathways for DD-NO reaction have been studied beginning with the adsorption configuration of first NO molecule (η1-NO, η2-NO, η3-NO and η4-NO), followed by the adsorption of a second NO molecule. Optimized geometric configurations of reactant, intermediates, transient states (TS) and product are given in Figs. S1–S8 (Supporting information) while corresponding potential energy profiles are depicted in Figs. 3–7 and calculated reaction energies and barrier energies of elementary steps are showed in Table 2-3 and Table S1-S3 (Supporting information). 3.2.1 DD-NO reaction along the η1-NO reaction pathway As displayed in Fig.S1, the second NO can adsorbs on η1-NO configuration in four ways: (a) NⅡOⅡ adsorbs on the top of adsorbed NⅠOⅠ and α-OFe while the distances of NⅡ-NⅠ and OⅡ-OFe are 2.152 and 2.294 Å, respectively (denoted as η1-NO-NO). The adsorbed NⅠ-OⅠ is activated with the length of 1.184 Å but the adsorbed NⅡ-OⅡ is inactivated a little according its length of 1.152 Å. (b) NⅡOⅡ

adsorbs on the α-OFe by the formation of NⅡ-OFe bond while NⅡOⅡ oriented along the bond direction to NⅠ (denoted as η1-O-NO). The lengths of NⅡ-OFe, NⅠ-OⅠ and NⅡ-OⅡ are 1.398, 1.162 and 1.202 Å. In addition, the distance of NⅡ-NⅠ is 2.885 Å. (c) NⅡOⅡ adsorbs on Fe atom as N-down with the formation of Fe-NⅠ bond (1.785 Å) while the distance of NⅡ-NⅠ is 2.367 Å (denoted as η1-Fe-NO). (d) NⅡOⅡ adsorbs on the side of adsorbed NⅠOⅠ while the distance of NⅡ-NⅠ is 2.174 Å (denoted as η1-N-NO). The adsorption energies of four adsorption configurations are -5.15, -32.69, -11.04 and -3.01 kcal/mol for η1-NO-NO, η1-O-NO, η1-Fe-NO and η1-N-NO, respectively, which indicates the strong chemsorption of NⅡOⅡ for η1-NO-NO and η1-N-NO and the weak physisorption of NⅡOⅡ for η1-O-NO and η1-Fe-NO. The adsorption stabilities can also be seen obviously from the bonding characters between NⅡOⅡ and η1-NO configuration. In the pathway of η1-NO-NO, the adsorbed NⅠOⅠ and NⅡOⅡ can produce intermediate 3 including ONNO species by overcoming the energy barrier of 19.92 kcal/mol. The ONNO species bonds to Fe atom by Fe-OⅡ (1.292 Å) and Fe-NⅠ (1.881 Å) bonds while the length of NⅡ-NⅠ is 1.374 Å. Subsequently, Fe-NⅠ bond weakens and is enlarged and leads to the formation of intermediate 4 including N2O, which has an energy barrier of 30.38 kcal/mol and an exothermic energy of 29.40 kcal/mol. The adsorbed OⅡ and α-OFe atoms bond and produce adsorbed O2 with an energy barrier of 36.28 kcal/mol and an endothermic energy of 14.37 kcal/mol accompanied by the reorientation of N2O. The distance of Fe-O between Fe atom and N2O for intermediate 5 is 2.524 Å. Afterward, transformation from 5 to 6 is exothermic and a high activation barrier of 39.09 kcal/mol must be conquered in order to break N-O bond and produce N2. The formed Fe-O bond is 1.599 Å while N2 weakly adsorbs on this O atom. Subsequent complete desorption of N2 requires an endothermic energy of 4.20 kcal/mol and no energy barrier. The complete desorption of O2 consists two elementary steps (7→TS5→8 and 8→R) that the successive breaking of two Fe-O bonds. The breaking of first Fe-O bond has an energy barrier of 15.23 kcal/mol and an endothermic energy of 9.94 kcal/mol while the second breaking step is the desorption step of O2 in fact with an endothermic energy of 29.12

kcal/mol and no energy barrier. Finally, the catalyst [FeO]1+-ZSM-5 is regenerated along the pathway of η1-NO-NO, which can be described as NO(g)→ NO(ads)→ NO(ads)+NO(ads)

→ONNO(ads)→

N2O(ads)→

N2O(ads)+O2(ads)→

N2(ads)+O2(ads)→ N2(g)+O2(ads)→ N2(g)+O2(g), as shown in Fig. 3 and Table 2. The full catalytic reaction is exothermic by 43.77 kcal/mol and its rate-determining step is N2O(ads)→N2(ads) with the barrier energy of 39.09 kcal/mol. In the pathway of η1-O-NO, 2 transforms via TS1 into 3 with a high barrier of 37.85 kcal/mol in which two N atoms tend to approach, bond and produce OONNO species. The relatively weak Fe-OⅠ and Fe-OⅡ bonds break via TS2 into 4 with an energy barrier of 22.76 kcal/mol. The configuration of intermediate 4 is almost identical to 4 in η1-NO-NO pathway and subsequent reaction pathway after 4 are considered be the same as that along η1-NO-NO pathway. Therefore, subsequent reaction pathways from 4 are not given repeatedly here and corresponding configurations can be seen in Fig. S2. In the pathway of η1-Fe-NO, the adsorbed NⅡOⅡ approaches NⅠ atom and the distance of NⅡ-NⅠ decreases, which gives rise to the formation of N-N bond and 3. This step is endothermic by 17.93 kcal/mol and needs an energy barrier of 35.72 kcal/mol. The weakness and break of Fe-NⅠ bond leads to the formation of 4 via TS2, which is also almost identical to 4 in η1-NO-NO pathway. The energy barrier for this step is 30.30 kcal/mol and the process is exothermic by 29.35 kcal/mol. In the pathway of η1-N-NO, bonding of NⅡ-NⅠ results in the formation of 3 including ONNO species, which bonds with Fe atom by Fe-NⅠ and Fe-OⅡ. The energy barrier for this step is 18.90 kcal/mol and the process is endothermic by 9.97 kcal/mol. Afterward, Fe-NⅠ bond breaks and produces 4 via TS1, which is also almost the same to 4 in η1-NO-NO pathway. The corresponding reaction energy and energy barrier are -29.41 and 30.57 kcal/mol, respectively. Based on the analysis of all four possible reaction pathways along η1- NO, it is seen clearly that the same intermediate 4 including adsorbed N2O will be produced and the same rate-determining step is N2O(ads)→N2(ads) no matter which adsorption way the second NO is. Potential energy profiles of the reactions from the first

adsorbed NO to N2O along η1-NO pathway are presented in Fig. 4 and corresponding thermodynamic data are listed in Table S1 (Supporting information). In the pathways of η1-NO-NO and η1-N-NO, the formations of intermediate 4 have lower energy barrier, less energy change of elementary reactions and therefore are somewhat easier than that along η1-O-NO and η1-Fe-NO. 3.2.2 DD-NO reaction along the η2-NO reaction pathway As displayed in Fig.S6, the second NO can be adsorbed on η2-NO configuration by the interaction of NⅠ-NⅡ and OⅠ-OⅡ with an exothermic energy of 20.46 kcal/mol. NⅡ atom of NⅡOⅡ approaches NⅠ atom of NⅠOⅠ to form intermediate 3 through the transition state TS2. This step is exothermic by 35.63 kcal/mol, and the energy barrier is 10.60 kcal/mol. The next step is the formation of N2O by the breaking of Fe-OⅠ and NⅡ-OⅡ bond, leading to the formation of 4 via TS2. This process is exothermic by 6.90 kcal/mol with an energy barrier of 17.92 kcal/mol. The formed intermediate 4 along η2-NO pathway is nearly identical to 4 in η1-NO-NO pathway. Subsequent reaction pathways from 4 are not given repeatedly here. 3.2.3 DD-NO reaction along the η3-NO reaction pathway As displayed in Fig.S7, the second NⅡOⅡ can inserts the top site of adsorbed NⅠ-OⅠ and OFe to form 2. The adsorption energy for this step is calculated to be −10.98 kcal/mol. The distances of NⅠ-NⅡ and OⅡ-OFe decrease and NⅠ-NⅡ and OⅠ-OⅡ bonds form to produce 3 via TS1. Meanwhile, NⅠ-OⅠ bond breaks and Fe-OⅠ bond forms. This step is endothermic by 16.00 kcal/mol and the energy barrier is 25.73 kcal/mol. Subsequent step (3→TS2→4) is the breaking of NⅡ-OⅡ bond of OONN species which directly produces N2 without the participation of N2O. This step has a low energy barrier of 7.93 kcal/mol and a large exothermic energy of 54.31 kcal/mol. Afterward, the desorption of N2 leads to the formation of 5 from 4 with a endothermic energy of 3.14 kcal/mol. The configuration of intermediate 5 along η3-NO pathway is nearly the same as 7 in η1-NO-NO pathway and therefore, subsequent reaction pathway after 7 are regarded as the same as that along η1-NO-NO pathway and not discussed repeatedly here. Corresponding configurations can be seen in Fig. S2.

3.2.4 DD-NO reaction along the η4-NO reaction pathway Beginning with the adsorption configuration of η4-NO, the second NⅡOⅡ can adsorb on Fe atom as O-down to form 2. This adsorption process is exothermic by 27.67 kcal/mol. Afterward, NⅡ-OⅡ approaches and inserts into the Fe-OFe bond via TS1 to form 3 including ONNO species. This step is exothermic by 6.71 kcal/mol with a large the energy barrier of 46.68 kcal/mol. The next step is the formation of N2O by the weakness and further break of Fe-OFe and NⅡ-OⅡ bond, leading to the formation of 4 via TS2. The energy barrier for this step is 17.35 kcal/mol and the process is exothermic by 6.99 kcal/mol. The formed intermediate 4 along η4-NO pathway is also almost identical to 4 in η1-NO-NO pathway. Subsequent reaction pathways after 4 are also not repeatedly discussed here.

3.3 Comparison of reaction pathways for DD-NO reaction Izquierdo et al. [38] indicated that Cu-NO and Cu-ON species could coexist on Cu–ZSM-5. The adsorption results this paper further demonstrate that three types of adsorption ways of NO can coexist including N-down, O-down and both N and O atoms coadsorb on Fe atom of [FeO]1+-ZSM-5. Beginning with different adsorption configuration of first NO molecule, four main pathways (η1-NO, η2-NO, η3-NO and η4-NO) are obtained. However, comparing the desorption energies of the second NO and energy barriers of reaction between two adsorbed NO species following adsorption of the second NO (1→2 vs 2→TS1→3), the desorption energy is smaller than that of the energy barrier along η1-NO, η3-NO and η4-NO pathways while the opposite comparison result is obtained along η2-NO pathway (20.46 kcal/mol vs 10.60 kcal/mol). The results show that the second NO tends to desorb along η1-NO, η3-NO and η4-NO pathways but prefers to react with the first adsorbed NO along η2-NO pathway. Therefore, the pathway along η2-NO via the Eley-Rideal mechanism is proposed in spite of the lower energy barrier along η3-NO pathway. The proposed η2-NO pathway is described as follows: O-Fe + NO  O-Fe-ON (2)

O-Fe-ON + NO  O-Fe-  ON  NO (3)

O-Fe-  ON  NO  O-Fe-ONNO (4) O-Fe-ONNO  O-Fe-O + N 2 O  ads (5) O-Fe-O + N 2 O  ads  Fe-O 2 + N 2 O  ads (6) Fe-O 2 + N 2 O  ads  O-Fe-O 2 +N 2  ads (7) O-Fe-O 2  O-Fe + O 2  ads (8) The proposed mechanism starts with the adsorption of NO as O-down, which is consistent with that suggested by previous reports [9, 34-36, 40]. ONNO species is suggested as the key catalytic intermediate, which is in accordance with the generally accepted view [15, 29, 30, 34-36, 39, 41, 42, 61]. In addition, our calculations along η4-NO pathway also displays a high energy barrier for the formation of (NO2)(NO) species. The DD-NO pathway via (NO2)(NO) species has also been considered a unfavorable pathway by Solans-Monfort et al. whereas the energy barrier of decomposition of Cu(NO2)(NO) is high to 74.5 kcal/mol [40].

4. Conclusions In this paper, the successive adsorption performances of two NO molecules and subsequent DD-NO mechanisms are explored on [FeO]1+-ZSM-5 using DFT method. Four stable configurations are obtained for adsorption of the first NO including N-down, O-down and both N and O atoms coadsorb on Fe atom. The stabilities for these four adsorption configurations decrease in the following order: η1-NO> η4-NO> η3-NO> η2-NO. According to the investigation on all possible DD-NO reaction beginning with different adsorption configurations of the first NO, the η2-NO reaction pathway is proposed, in which the first NO adsorbs as O-down on Fe-atom and the second NO adsorbs on NO following Eley-Rideal mechanism. The proposed η2-NO pathway can be described as NO(g)→ NO(ads)→ (ON)(NO)(ads)→ ONNO(ads)→ N2O(ads)→ N2O(ads)+O2(ads)→ N2(ads)+O2(ads)→ N2(g)+O2(ads)→ N2(g)+O2(g). The rate-determining step for DD-NO reaction is the formation of adsorbed N2, which requires an energy barrier of 39.09 kcal/mol. The α-O of [FeO] plays a crucial role in two ways: influences on adsorption performances of NO and participation in DD-NO

reaction. It is expected that the proposed novel mechanism can provide a deeper understanding of the DD-NO mechanism on Fe-ZSM-5. We also hope the design of catalysts on DD-NO reaction could also be further developed based on the theoretical predictions of adsorption performances and reaction mechanisms in this paper.

Acknowledgements The authors gratefully acknowledge the National Natural Science Foundation of China (No. 51703153 and No. 21802101) and Shanxi Province Science Foundation for Youths (Grant No. 201801D221365) for the financial support.

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Fig. 1. The optimized configuration of [FeO]1+-ZSM-5 model. Bond distances in Å are also given. Fig. 2. The optimized configuration s of NO molecule adsorption on [FeO]1+-ZSM-5: (a) η1-NO; (b) η2-NO; (c) η3-NO; (d) η4-NO. ZSM-5 framework is omitted for clarity. Bond distances in Å are also given. Fig. 3. Potential energy profile along the η1-NO-NO reaction pathway Fig. 4. Partial potential energy profile along the η1-NO reaction pathway Fig. 5. Partial potential energy profile along the η2-NO reaction pathway Fig. 6. Potential energy profile along the η3-NO reaction pathway Fig. 7. Partial potential energy profile along the η4-NO reaction pathway

Table 1. Bond lengths (Å, previous line), Mayer order (next line), Mulliken charges (q, e) and adsorption energies (Eads, kcal/mol) of stable geometries for NO before and after adsorption on Fe-ZSM-5 NO molecule and ZSM-5

η1-NO

η2-NO

η3-NO

η4-NO

length: 1.164

1.167

1.168

1.220

1.288

order: 2.279

1.744

1.725

1.530

1.398

1.629

1.708

1.779

1.913

1.785

1.008

1.275

0.706

length: 1.613

1.617

1.619

1.612

1.941

order: 2.168

2.012

2.006

1.923

0.615

length: 1.960

1.995

2.005

1.995

1.983

order: 0.509

0.474

0.455

0.464

0.464

length: 1.960

2.007

2.017

1.938

1.986

order: 0.494

0.458

0.437

0.537

0.518

qN

0.046

0.323

0.065

0.121

0.247

qON

-0.046

-0.031

0.081

-0.106

-0.265

qFe

0.751

0.461

0.635

0.611

0.718

qOFe

-0.394

-0.387

-0.394

-0.315

-0.327

qOa

-0.876

-0.874

-0.833

-0.881

-0.886

qOb

-0.893

-0.885

-0.884

-0.878

-0.867

Eads

-

-60.43

-19.89

-36.65

-41.54

N-ON

Fe-N(ON)

Fe-OFe

Fe-Oa

Fe-Ob

-

Table 2. Calculated reaction energies (kcal/mol) and barrier energies (kcal/mol) for all the elementary steps along the η1-NO-NO reaction pathway Elementary step

Reaction energy

Barrier energy

R+NO(g)→1

-60.43

-

1+NO(g)→2

-5.15

-

2→TS1→3

12.09

19.92

3→TS2→4

-29.40

30.38

4→TS3→5

14.37

36.28

5→TS4→6

-18.51

39.09

6→7+N2(g)

4.20

7→TS5→8

9.94

8→R+O2(g)

29.12

2 NO(g) →N2(g) +O2(g)

-43.77

15.23

Table 3. Calculated reaction energies (kcal/mol) and barrier energies (kcal/mol) for the elementary steps along the η2-NO reaction pathway Elementary step

Reaction energy

Barrier energy

R+NO(g)→1

-19.89

-

1+NO(g)→2

-20.46

-

2→TS1→3

-35.63

10.60

3→TS2→4

-6.90

17.92

R+2 NO(g) →4

-82.88