Reaction mechanism for NH3-SCR of NOx over CuMn2O4 catalyst

Accepted Manuscript Reaction mechanism for NH3-SCR of NOx over CuMn2O4 catalyst Yingju Yang, Jing Liu, Feng Liu, Zhen Wang, Junyan Ding, Hao Huang PII: DOI: Reference:

S1385-8947(18)32591-9 https://doi.org/10.1016/j.cej.2018.12.103 CEJ 20643

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

23 November 2017 26 November 2018 19 December 2018

Please cite this article as: Y. Yang, J. Liu, F. Liu, Z. Wang, J. Ding, H. Huang, Reaction mechanism for NH3-SCR of NOx over CuMn2O4 catalyst, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej. 2018.12.103

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Reaction mechanism for NH3-SCR of NOx over CuMn2O4 catalyst Yingju Yanga, Jing Liua,*, Feng Liua, Zhen Wanga, Junyan Dinga, Hao Huanga,b a

State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong

University of Science and Technology, Wuhan 430074, China b

Wuhan Senyuan Lantian Environmental Technology Engineering Co Ltd, Wuhan 430074, China

ABSTRACT: The relationship between the types of active sites and the selective catalytic reduction (SCR) activity of NO with NH3 over CuMn2O4 spinel was established through density functional theory (DFT) calculations. A skeletal reaction scheme including the possible elementary steps was proposed to understand N2, NO2 and N2O formation during NH3-SCR of NO over CuMn2O4 catalyst. DFT calculation results show that chemisorption mechanism is responsible for the adsorption of reactants, possible intermediates and products over CuMn2O4(100) surface. 2-fold coordinated surface Cu atom plays a crucial role in NH3-SCR of NO, because it is the active site for NH3 and NO adsorption. NH2 produced from NH3 dehydrogenation is identified as a key reactive intermediate of SCR reaction. NH2 easily reacts with the adsorbed NO to form N2 and H2O via NH2* + NO* → N2* + H2O* which is activated by 6.87 kJ/mol. The activation energy barrier of N2O formation over CuMn2O4 catalyst is much higher than that of N2 formation, which indicates that CuMn2O4 catalyst shows a good N2 selectivity for NO reduction. The optimal reaction pathway for NH3-SCR of NO over CuMn2O4(100) surface is a two-step process controlled by NH3* + * → NH2* + H* and NH2* + NO* → N2* + H2O*. The rate-determining step of N2 formation during NO reduction is the first dehydrogenation reaction of NH3.

1

Keywords: NH3-SCR; NO; CuMn2O4 catalyst; Reaction pathway; Density functional theory 1. Introduction Nitrogen oxides (NOx) generated from stationary and automobile combustion sources are the primary atmospheric pollutants and have caused serious environmental pollution problems such as photochemical smog, acid rain, ozone layer depletion and even global warming [1, 2]. Selective catalytic reduction (SCR) of NOx with NH3 has long been known to be the well-established and widely used technology controlling NOx emission [3, 4]. The nature of NH3-SCR is a redox reaction which achieves N2 formation from NO and NH3: 4NH3 + 4NO + O2 → 4N2 + 6H2O

(R1)

This redox reaction is promoted by SCR catalyst. To date, V2O5-WO3/TiO2 and V2O5-MoO3/TiO2 are the commercially used SCR catalysts [5]. In coal-fired power plants, the SCR devices are located between air preheater and economizer due to its optimal operation temperature window (300-400 ºC) [6, 7]. However, a large number of fly ashes in flue gas block the pores of SCR catalysts and mask the active sites, thereby accelerating the catalyst deactivation [8, 9]. Additionally, V2O5-based catalysts also show a lot of practical problems including low N2 selectivity at high temperature, high transformation of SO2 to SO3, and toxicity of vanadium species [10]. Consequently, numerous attempts have been made to develop novel and effective low-temperature SCR catalysts [11-13], which can be installed downstream of electrostatic precipitators (ESP) or fabric filters (FF) to avoid the catalyst deactivation caused by fly ash. In recent years, spinel-type transition-metal oxides represented by a general formula of AB2O4 have attracted considerable interest from scientists engaged in catalysis field, such as the removal of gaseous pollutants [14-16], steam reforming [17, 18], and water-gas shift reactions [19, 20]. Copper 2

and manganese oxides which are widely dispersed in the Earth's surface region are regarded as the promising low-temperature SCR materials, because the flexible valence of Cu1+/2+ and Mn2+/3+/4+ allows for catalytic behavior [21, 22]. CuMn2O4 spinel shows the cooperative Jahn-Teller activity (a distinguishing feature) [23], which is able to satisfy almost all the practical requirements of a low-temperature SCR catalyst. CuMn2O4 spinel has been developed as the SCR catalyst, and significantly promotes the reduction of NO by NH3 [24, 25]. The catalytic reaction mechanism of NO reduction and the active sites of solid surface are closely associated with the SCR rate-enhancing capacity of CuMn2O4 catalyst. However, the active sites which are responsible for NH3-SCR of NO over CuMn2O4 catalyst surface have yet to be identified. In addition, the catalytic chemistry of the SCR process is rather complex, because a number of possible reactions associated with the formation of N2, NO2 and N2O are involved in the SCR reaction system. To date, to the best of the authors’ knowledge, limited attempts have been made to reveal the NH3-SCR reaction mechanism of NO over CuMn2O4 catalyst. In this work, theoretical calculations based on density functional theory were carried out to identify the active sites for NO reduction by NH3 over CuMn2O4 catalyst. The progressive dehydrogenation process of NH3 dissociation over CuMn2O4 surface was discussed by analyzing the activation energy barrier. A skeletal reaction scheme including the possible elementary steps was proposed for NH3-SCR of NO over CuMn2O4 catalyst. Reaction pathway analysis was performed to determine the dominant channel of NO reduction over CuMn2O4 surface. This study provides a fundamental understanding of the chemistry involved in NH3-SCR reaction over CuMn2O4 catalyst surface, and provides a direction for the rational design of Cu-Mn spinel catalyst.

3

2. Theoretical method All calculations were carried out using DMol3 code [26], in which Perdew-Burke-Ernzerhoff (PBE) [27] functional in the generalized gradient approximation (GGA) [28] scheme was used to calculate the exchange-correlation potential. The molecular orbitals were expanded using a double numerical basis set with polarization functions (DNP). The range of integration for charge density and functional was confined within a global orbital cutoff value of 4.7 Å. The core electrons of Cu and Mn atoms were treated using the effective core potentials (ECP) method [29], in which the relativistic effect of core electrons was replaced with a simple potential. The all-electron method was used to treat the electrons of O, N and H atoms. The (4×4×4) Monkhorst-Pack k-point grid [30] was used for unit cell optimization. The following three convergence criteria were used for the geometry optimization and energy calculation: the atomic forces (2.0×10−3 Hartree/Å), maximum displacement (5.0×10−3 Å), and total energy variation (1.0×10−5 Hartree). High-resolution neutron scattering analysis suggested that the inversion parameter of CuMn2O4 spinel is less than 30% [23], implying that the tetrahedral and octahedral sites of CuMn2O4 spinel are mainly occupied by Cu2+ and Mn3+ cations, respectively. Therefore, CuMn2O4 unit cell with a normal spinel structure is used in our calculations, as shown in Fig. 1(a). Mn and Cu atoms are coordinated with six and four O atoms, respectively (Fig. 1(b) and 1(c)). The unit cell parameters of CuMn2O4 spinel were optimized using spin-polarized method to be a = b = c = 8.315 Å, deviating from the experimental values (a = b = c = 8.327 Å) [31] by 0.14%. It can be seen that the developed catalyst model and the calculations in this study are reliable. CuMn2O4(100) is a typical low-index surface which easily exposes to ambient atmosphere. Consequently, CuMn2O4(100) surface was used to investigate the NH3-SCR reaction mechanism of 4

NO over CuMn2O4 catalyst, as shown in Fig. 1(d). CuMn2O4(100) surface simulated by a periodic p(2×1) slab with nine atomic layers was constructed by cleaving the optimized unit cell. The bottom five layers were fixed in their original bulk positions, whereas the top four layers were fully relaxed during the calculations. The slab model was separated by a 20 Å-thick vacuum region to avoid the interactions between two periodic slabs. The adsorption energy of different species on CuMn2O4(100) surface is defined as follows: Eads = E(CuMn2O4–adsorbate) – (ECuMn2O4 + Eadsorbate)

(1)

where E(CuMn2O4–adsorbate) is the total energy of CuMn2O4/adsorbate adsorption system. ECuMn2O4 is the total energy of clean CuMn2O4 surface. Eadsorbate is the total energy of isolated gas-phase adsorbate molecule. Commonly, the adsorption process with a adsorption energy less than −30 kJ/mol is controlled by physisorption, whereas the adsorption process with a adsorption energy higher than −50 kJ/mol is dominated by chemisorption [32-34]. Different elementary reaction steps are responsible for NO reduction by NH3 over CuMn2O4 catalyst. These reactions involve different intermediates (IM), transition states (TS) and final states (FS). The SCR products can be formed through different reaction pathways including different elementary steps. All transition states along the different NH3-SCR reaction pathways are searched using the linear synchronous transit/quadratic synchronous transit (LST/QST) method [35]. Subsequently, vibrational frequency calculation is performed to confirm the obtained transition state structure. The activation energy barrier (Ea) of elementary reaction step is defined as the energy difference between transition state and intermediate according to the following equation [36, 37]: Ea = ETS – EIM where ETS and EIM represent the total energy of transition state and intermediate, respectively. 5

(2)

3. Results and discussion In order to understand the NH3-SCR reaction mechanism of NO over CuMn2O4 catalyst, the adsorption characteristics of reactants, possible intermediates and products were first investigated. All possible adsorption sites (Cu2, bridge, Mn, O3, Cu4, hollow, and O4) and orientations (parallel and perpendicular) were considered during the calculations. The active sites for the adsorption of different species were identified through analyzing adsorption energy, structures and equilibrium distance. Based on dissociation barrier, the dehydrogenation reaction of NH3 over CuMn2O4 catalyst was analyzed. The energy barrier of different reactions was compared to determine the dominant reaction pathway of N2 formation. The possible elementary reaction steps involved in NH3-SCR of NO over CuMn2O4 catalyst are proposed and listed in Table 1.

3.1. Adsorption characteristics of NH3 and NHx species The most stable adsorption configurations of NHx species (NH3, NH2, NH, N and H) and the corresponding adsorption energy are shown in Fig. 2. NH3 is strongly adsorbed on Cu site (Lewis acid site) with N atom toward surface Cu atom, yielding an adsorption energy of −130.85 kJ/mol. A Cu-N bond with an equilibrium distance of 2.115 Å is formed during NH3 adsorption, which indicates that the stronger interaction between NH3 and CuMn2O4 surface is closely associated with the formation of Cu-N bond. It was experimentally found that NH3 is coordinated with Lewis acid site during NH3 adsorption on CuMn2O4 catalyst [24]. Therefore, DFT calculation is consistent with the experimental results. NH2 can be adsorbed on both surface Cu and Mn sites. However, NH2 prefers to adsorb on 6

surface Cu site compared to Mn site, because the adsorption energy (−314.70 kJ/mol) of NH2 on Cu site is higher than that (−290.55 kJ/mol) of NH2 on Mn site. Thus, Cu site is more active for NH2 adsorption than Mn site. In the most stable adsorption structure of NH2 (Fig. 2), NH2 is adsorbed on Cu atom in a parallel adsorption manner. During this adsorption process, N atom of NH2 simultaneously approaches surface Cu atom to form Cu-N bond. The bond length of Cu-N is 1.911 Å. It was reported that NH2 serves as an important intermediate during NH3-SCR reaction and easily reacts with NO to form N2 and H2O via NH2 + NO → N2 + H2O [38]. Consequently, surface Cu atom plays an important role in NO reduction due to its high activity for NH2 adsorption. NH strongly binds to Cu and Mn sites in a perpendicular adsorption way, forming a bridged connection Cu-N-Mn structure. N atom of NH is coordinated with surface Cu and Mn atoms to form two Cu-N and Mn-N bonds. The bond lengths of Cu-N and Mn-N are 1.962 and 2.060 Å, respectively. In the most stable adsorption structure of NH (Fig. 2), the adsorption process of NH on CuMn2O4(100) surface is highly exothermic by 404.12 kJ/mol. In the most stable adsorption configuration of N, the adsorption energy is −200.25 kJ/mol. N atom is adsorbed on surface Cu and Mn atoms to form a similar bridged connection Cu-N-Mn structure. The bond lengths of Cu-N and Mn-N are 1.963 and 1.952 Å, respectively. The adsorption energy of N is much lower than that (−404.12 kJ/mol) of NH. Consequently, the dehydrogenation of NH weakens the interaction between N and CuMn2O4 catalyst surface. In the most stable adsorption configuration of H, H prefers to adsorb on 3-fold coordinated O atom to form surface hydroxyl. The equilibrium distance of H-O bond of hydroxyl is 0.989 Å. The energy released in the adsorption reaction of H is −174.20 kJ/mol. Temperature-programmed desorption (TPD) and DRIFTS experiments indicated that N 7

is formed when CuMn2O4 catalyst

was pretreated by NH3 [24], because NH3 could be protonated to N Consequently, the existence of hydroxyl can provide a proton for N

by Brønsted acid site. formation when gas-phase

NH3 is adsorbed on Brønsted acid site.

3.2. Dehydrogenation reaction of NH3 As mentioned above, NH2 + NO → N2 + H2O plays a significant role in NH3-SCR reaction of NO. Therefore, NH2 derived from the dehydrogenation reaction of NH3 is regarded as an important intermediate in NH3-SCR reaction. The progressive dehydrogenation reaction of NH3 is investigated from the most stable adsorption structure of NH3, and occurs via the following reactions: NH3 + * → NH3*

(R2)

NH3* + * → NH2* + H*

(R3)

NH2* + * → NH* + H*

(R4)

NH* + * → N* + H*

(R5)

The energy diagram and optimized structures for the dehydrogenation reaction of NH3 over CuMn2O4(100) surface are presented in Fig. 3. Gaseous NH3 molecule is first adsorbed on surface Cu atom via NH3

*→N

3*.

This adsorption step is exothermic by 130.85 kJ/mol. Subsequently,

the adsorbed NH3 is decomposed into NH2 fragment and H atom through transition state TS1 (NH3* + * → NH2* + H*). Meanwhile, the resulting H atom is coordinated with surface lattice O atom to form hydroxyl group (–OH). The NH2 fragment is strongly adsorbed on Cu site. The activation energy barrier and reaction heat of NH3 dehydrogenation reaction over CuMn2O4(100) surface are 36.69 kJ/mol and 35.84 kJ/mol, respectively. After the first dehydrogenation step of NH3, NH2 migrates from Cu site to Mn site. This migration step is slightly endothermic by 25.00 kJ/mol, and is 8

characterized by the change in optimized structure (Cu-NH2* → Mn-NH2*). In the second dehydrogenation step of NH3 (NH2* + * → NH* + H*), 4-fold coordinated O atom strips an H atom from NH2 fragment to form hydroxyl group (–OH) through transition state TS2, leaving a NH fragment adsorbed on Mn site (see NH*). This dehydrogenation step is activated by 275.30 kJ/mol and endothermic by 132.09 kJ/mol. In this dehydrogenation step, H atom first migrates from NH2 fragment to surface Cu atom (see TS2) and subsequently to 4-fold coordinated O atom (see NH*), but at the same time breaks the Cu-O bond. In addition, obvious surface reconstruction occurs over CuMn2O4(100) surface, which may be responsible for the higher activation energy barrier of the second hydrogen abstraction step of NH3. In the third dehydrogenation step of NH3 (NH* + * → N* + H*), H atom of NH fragment is further abstracted by 3-fold coordinated O atom to form hydroxyl group. N atom is coordinated with Cu and Mn atoms, forming two Mn-N and Cu-N bonds (see N*). The bond lengths of Mn-N and Cu-N are 1.983 and 1.947 Å, respectively. The H-stripping step of NH is an endothermic process with reaction heat of 45.13 kJ/mol, and is activated by 99.66 kJ/mol. The first dehydrogenation step of NH3 has a relatively lower energy barrier of 36.69 kJ/mol, indicating that NH2 intermediate can be easily formed from the first dehydrogenation reaction of NH3. However, the second dehydrogenation step is very difficult to be activated due to the high energy barrier of 275.30 kJ/mol. In addition, the dehydrogenation reaction of NH2 has a significant effect on the surface structure of CuMn2O4 catalyst. Consequently, NH2 can stably exist on CuMn2O4 surface to serve as an important intermediate in NH3-SCR reaction rather than dissociation into NH fragment. Even though the energy barrier (99.66 kJ/mol) of the third dehydrogenation step of NH3 is lower than that of the second step, the third dehydrogenation reaction of NH3 is controlled by NH 9

formation from the second step. Therefore, the third dehydrogenation step of NH3 is not very likely.

3.3. Adsorption characteristics of O2, N2 and NOx The adsorption energies and geometric parameters for the most stable adsorption configurations of O2, NO, NO2, N2O and N2 are shown in Fig. 4. O2 molecule is chemically adsorbed on Mn and Cu sites, forming two Mn-O and Cu-O bonds. The bond lengths of Mn-O and Cu-O are 2.149 and 2.060 Å, respectively. The adsorption energy of O2 is −321.69 kJ/mol, implying that O2 adsorption on CuMn2O4(100) surface is an exothermic process. Furthermore, Mulliken charge of the adsorbed O2 molecule is calculated. About 0.164 and 0.165 e charges are transferred from surface Mn and Cu atoms to O2 molecule, respectively. Thus, the adsorbed O2 molecule becomes negatively charged, and can provide active oxygen species for NH3-SCR reaction of NO. As mentioned previously, NH3-SCR reaction of NO over CuMn2O4 catalyst follows a Langmuir-Hinshelwood mechanism, where adsorbed NO reacts with adsorbed NH3 species. Therefore, the adsorption of NO plays an important role in NH3-SCR reaction over CuMn2O4 catalyst. In the most stable adsorption configuration of NO, NO is preferably adsorbed on Cu site with N atom toward 2-fold coordinated Cu atom. N atom of NO molecule is coordinated with surface Cu atom to form Cu-N bond with an equilibrium distance of 1.999 Å. The adsorption energy of NO is −301.38 kJ/mol, indicating a strong interaction between NO and CuMn2O4 catalyst surface. This strong interaction may be responsible for the excellent SCR performance of CuMn2O4 spinel. NO2 is involved in the Fast SCR reaction (2NH3 + NO + NO2 → 2N2 + 3H2O) which is inherently faster than the Standard SCR reaction (4NH3 + 4NO + O2 → 4N2 + 6H2O). The existence of NO2 accelerates the NH3-SCR reaction. The interaction between NO2 and CuMn2O4 catalyst 10

surface can be estimated by calculating the adsorption energy of NO2. In the most stable adsorption configuration of NO2, NO2 is strongly adsorbed on CuMn2O4(100) surface and occupies two active sites (Cu and Mn sites). This adsorption process of NO2 is highly exothermic by 354.61 kJ/mol, which is much higher than the adsorption energy of NO. Therefore, the interaction between NO2 and CuMn2O4 surface is stronger than that between NO and CuMn2O4 surface. The strong interaction is beneficial for the enhancement of low-temperature SCR performance. N2O is a kind of undesired products formed in SCR reaction. As shown in the most stable adsorption configuration of N2O (see Fig. 4), N2O is chemically adsorbed on CuMn2O4 catalyst surface, yielding an adsorption energy of −228.66 kJ/mol. N and O atoms of N2O molecule are coordinated with surface Cu and Mn atoms, respectively, to form Cu-N and Mn-O bonds. The bond lengths of Cu-N and Mn-O are 1.946 and 2.131 Å, respectively. N2O, NO and NO2 occupy the same active site (Cu site), leading to the competitive adsorption among N2O, NO and NO2. However, the adsorption energies of NO and NO2 are much higher than that of N2O. Therefore, Cu site is preferred for the adsorption of NO and NO2. N2 is the desired product of NH3-SCR reaction. In the most stable structure of N2 adsorption, N2 molecule weakly chemisorbs on surface Cu site in a perpendicular adsorption manner. The adsorption energy of N2 in this structure is −71.46 kJ/mol. Compared to other N-containing species, N2 adsorption on CuMn2O4(100) surface shows the lowest adsorption energy. Thus, a weak chemisorption mechanism is responsible for the interaction between N2 and CuMn2O4 catalyst surface. It is reasonable to expect that N2 produced from NH3-SCR reaction of NO over CuMn2O4 spinel releases easily into flue gas.

11

3.4. Reaction pathway of NH3-SCR 3.4.1. N2 formation pathway The energy diagram and optimized structures of N2 formation are presented in Fig. 5. The desirable product N2 can be formed from NH3-SCR reaction of NO over CuMn2O4 catalyst via the following possible elementary reaction steps: NH2* + NO* → N2* + H2O*

(R6)

NO* + 2* → N** + O*

(R7)

N* + N** → N2** + *

(R8)

NH* + N* + OH* → N2* + H2O* + *

(R9)

As shown in Fig. 5(a), NH2* + NO* → N2* + H2O* is exothermic by 65.73 kJ/mol. The activation energy barrier is 6.87 kJ/mol. The lower energy barrier indicates that the adsorbed NO molecule can be easily and selectively reduced by NH2 into N2. In the configuration of initial state, NO and NH2 are adsorbed on Cu and Mn sites, respectively. Two N atoms are separated by a distance of 3.689 Å. During this reduction reaction, N atom of NH2 moves to NO molecule to break the N-O bond, forming N2 molecule adsorbed on Cu site. Meanwhile, the O atom of NO molecule interacts with two H atoms to form H2O. N* + N** → N2** + * is a possible reaction step for N2 formation. N2 molecule produced from SCR reaction contains a N atom from NH3 and another N atom from NO. Thus, the reaction pathway of N formation from NH3 and NO is first investigated. N formation pathway in the dehydrogenation reaction of NH3 has been discussed in section 3.2. N formation pathway in NO dissociation (NO* + 2* → N** + O*) is shown in Fig. 5(b). The adsorbed NO molecule dissociates into N and O atoms through transition state NO(#). The resulting O atom is adsorbed on surface Cu atom. The N atom is 12

simultaneously coordinated with Mn and Cu atoms to form Mn-N and Cu-N bonds. The bond lengths of Mn-N and Cu-N are 1.948 Å and 1.958 Å, respectively. The dissociation process of NO is an endothermic reaction with energy barrier of 217.17 kJ/mol and reaction heat of 91.27 kJ/mol. The energy profile of N2 formation via N* + N** → N2** + * is shown in Fig. 5(c). N atom from NH3 dehydrogenation interacts with N atom from NO dissociation to form N2 molecule. The distance between two N atoms decreases gradually: 4.690 Å → 2.724 Å → 1.189 Å. Furthermore, an open Cu site is exposed for the next SCR reaction when N2 is formed (see the structure of surface product in Fig. 5(c)). This reaction is activated by 58.26 kJ/mol, and is highly exothermic by 234.19 kJ/mol. Another elementary reaction step for N2 formation is NH* + N* + OH* → N2* + H2O* + *, as seen in Fig. 5(d). During this reaction process, N atom of NH fragment migrates from surface Mn site to another N atom to form N2 molecule. Meanwhile, H atom of NH fragment reacts with surface hydroxyl (–OH) to form H2O. This reaction is an exothermic process with activation energy barrier of 360.42 kJ/mol and exothermicity of −204.54 kJ/mol. The higher energy barrier implies that NH* + N* + OH* → N2* + H2O* + * is not an optimal reaction pathway for N2 formation. 3.4.2. NO2 formation pathway As mentioned above, NO2 is the reactant of the Fast SCR reaction. NO2 formation during NO reduction can accelerate the SCR reaction. Therefore, NO2 formation pathway is explored to evaluate oxidation activity of CuMn2O4 spinel. NO can be oxidized into NO2 via a series of reactions: NO + * → NO*

(R10)

O2 + 2* → O2**

(R11)

O2** → O* + O*

(R12)

13

NO* + O* → NO2**

(R13)

The adsorption reactions (R10) and (R11) of NO and O2 have been investigated in section 3.3. O* is predominantly formed from the dissociation of gas-phase O2. The dissociation pathway of gas-phase O2 is first investigated, as shown in Fig. 6(a). The adsorbed O2 molecule dissociates into surface chemisorbed O atom through transition state O2(#). The activation energy barrier and reaction heat of O2 dissociation are 13.90 kJ/mol and −77.73 kJ/mol, respectively. The lower activation energy barrier indicates that the dissociation of gas-phase O2 is practically spontaneous. This can be verified by X-ray photoelectron spectroscopy (XPS) [31] and O2-TPD-MS [24] results indicating that a large number of surface chemisorbed oxygen atoms exist on CuMn2O4 surface. The energy profile of NO2 formation pathway is presented in Fig. 6(b). It can be seen that NO molecule adsorbed on octahedral Mn site is oxidized by active O atom into NO2 species. In the structure of surface product, the formed NO2 molecule is strongly adsorbed on Mn and Cu sites. NO2 formation pathway is an endothermic reaction with a relatively higher activation energy barrier of 145.10 kJ/mol. Furthermore, this reaction process is endothermic by 57.16 kJ/mol. 3.4.3. N2O formation pathway As mentioned above, N2O produced from SCR reaction is an undesired by-product. Therefore, a possible reaction step of N2O formation is proposed and investigated for the evaluation of N2 selectivity of CuMn2O4 catalyst: NO* + N* → N2O**

(R14)

The energy profile of N2O formation pathway is shown in Fig. 7. In the structure of initial state, NO molecule is adsorbed on Mn site with N atom toward Mn atom. Another N atom generated from NO dissociation is strongly bound to 2-fold coordinated Cu atom. This N atom reacts with the 14

adsorbed NO molecule to form N2O through transition state NO-N(#). During this reaction, the distance between two N atoms decreases gradually: 4.467 Å → 2.472 Å → 1.234 Å. The formed N2O molecule occupies simultaneously surface Cu and Mn sites. Moreover, it can be seen that N2O formation is exothermic by 85.57 kJ/mol and has an activation energy barrier of 61.84 kJ/mol.

3.5. Reaction pathway analysis of NH3-SCR Reaction pathway analysis [39, 40] can provide a visual representation for the elementary steps of N2, NO2 and N2O formation during NH3-SCR reaction. A skeletal reaction scheme including the possible elementary steps (see Table 1) considered in this study is outlined and presented in Fig. 8. Based on the activation energy barriers and reaction heat, Fig. 8 compares the different pathways for NH3-SCR of NO over CuMn2O4(100) surface. During NH3-SCR reaction, N2 can be formed via three different reaction steps: NH2* + NO* → N2* + H2O*

(R6)

N* + N** → N2** + *

(R8)

NH* + N* + OH* → N2* + H2O* + *

(R9)

As shown in Fig. 8, the activation energy barrier (6.87 kJ/mol) of NH2* + NO* → N2* + H2O* is much lower than that of N* + N** → N2** + * (58.26 kJ/mol) and NH* + N* + OH* → N2* + H2O* + * (360.42 kJ/mol). N2 formation via NH2* + NO* → N2* + H2O* is kinetically more accessible compared to other two elementary steps. Therefore, NH2* + NO* → N2* + H2O* is dominant for N2 formation of NH3-SCR reaction. In this dominant reaction step, NH2 comes from the first dehydrogenation reaction (NH3* + * → NH2* + H*) of NH3. These indicate that the optimal reaction pathway of N2 formation in NH3-SCR of NO is a two-step process: 15

NH3* + * → NH2* + H*

(R3)

NH2* + NO* → N2* + H2O*

(R6)

Compared with NH2* + NO* → N2* + H2O*, NH3* + * → NH2* + H* presents a relatively higher activation energy barrier of 36.69 kJ/mol. It can be concluded that NH3* + * → NH2* + H* is the rate-determining step of N2 formation. As shown in Fig.3, NH3 dehydrogenation is mediated by surface oxygen of catalyst. Therefore, this rate-determining step can be controlled through tuning the concentration of surface oxygen. NO2 can be formed via an elementary reaction step: NO* + O* → NO2**

(R13)

The activation energy barrier of NO* + O* → NO2** is 145.10 kJ/mol. NO is difficultly oxidized into NO2 under typical low-temperature SCR conditions, because NO* + O* → NO2** needs to surmount a significant activation energy barrier. Therefore, the contribution of the Standard SCR reaction to NO reduction over CuMn2O4 spinel is larger than that of the Fast SCR reaction. N2O can be produced through the following elementary reaction steps: NO* + 2* → N** + O*

(R7)

NO* + N* → N2O**

(R14)

NO* + N* → N2O** is activated by 61.84 kJ/mol. N of NO* + N* → N2O** is produced from the dissociation reaction of NO. The dissociation step (NO* + 2* → N** + O*) of NO presents a high activation energy barrier of 217.17 kJ/mol, limiting N formation from NO dissociation. Therefore, even though N2O formation via NO* + N* → N2O** has a relatively lower activation energy barrier of 61.84 kJ/mol, N2O is very difficult to be formed. Based on the above analysis, CuMn2O4 spinel can show a good N2 selectivity for NO reduction. 16

4. Conclusions Density functional theory calculations were performed to identify the active sites for NO reduction by NH3 over CuMn2O4 catalyst. A skeletal reaction scheme including the possible elementary steps was proposed for NH3-SCR of NO over CuMn2O4 catalyst. The activation energy barriers and reaction heat of these elementary steps were calculated to explore the contribution of each reaction to N2, NO2 and N2O formation. We found that surface Cu atom plays a crucial role in NH3-SCR of NO, because surface Cu atom is the active site for NH3 and NO adsorption. NH2 produced from NH3 dehydrogenation is identified as a key reactive intermediate of SCR reaction. The optimal reaction pathway of N2 formation in NH3-SCR of NO is a two-step process: (1) NH2 formation from NH3 dehydrogenation (NH3* + * → NH2* + H*), and (2) the reaction between NH2 and NO (NH2* + NO* → N2* + H2O*). The rate-determining step of N2 formation during NO reduction by NH3 is NH3* + * → NH2* + H* which is activated by 36.69 kJ/mol. N2O formation pathway needs to surmount significant activation energy barrier compared to N2 formation pathway, and NH3-SCR reaction prefers N2 formation pathway.

Acknowledgments This work was supported by National Natural Science Foundation of China (51661145010), Enterprise Technological Innovation Project of Wuhan (2018060402011256), and National Postdoctoral Program for Innovative Talents (BX20180108).

References 17

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22

Table 1 The possible elementary reaction steps for NH3-SCR of NO over CuMn2O4(100) surface. No.

Reactions

N2 formation pathway

No.

Reactions

8

NH* + N* + OH* → N2* + H2O* + *

1

NH3 + * → NH3*

NO2 formation pathway

2

NO + * → NO*

9

O2 + 2* → O2**

3

NH3* + * → NH2* + H*

10

O2** → O* + O*

4

NH2* + * → NH* + H*

11

NO* + O* → NO2**

5

NH* + * → N* + H*

N2O formation pathway

6

NH2* + NO* → N2* + H2O*

12

NO* + 2* → N** + O*

7

N* + N** → N2** + *

13

NO* + N* → N2O**

* indicates an unoccupied active site. 2* denotes two unoccupied active sites. A* and B** denote the adsorbed A and B species, respectively. A* and B** occupy one and two active sites, respectively.

23

List of Figures Captions Fig. 1. (a) The face-centered cubic crystal structure of CuMn2O4. (b) 6-fold coordinated Mn atom locating at octahedral site. (c) 4-fold coordinated Cu atom locating at tetrahedral site. (d) Slab model of CuMn2O4(100) surface. a, b, c, d, e, f, and g represent the adsorption sites of Cu2, bridge, Mn, O3, Cu4, hollow, and O4, respectively. Cu2 and Cu4 denote 2-fold and 4-fold coordinated Cu, respectively. O3 and O4 denote 3-fold and 4-fold coordinated O, respectively. Fig. 2. Adsorption energies and geometric parameters of the most stable configurations of NHx adsorption on CuMn2O4(100) surface. Fig. 3. Reaction energy profile and optimized structures of intermediates, transition states, and products of NH3 dehydrogenation over CuMn2O4(100) surface. The reaction energy profile is plotted relative to the total energy of gaseous NH3 molecule and clean CuMn2O4(100) surface. Fig. 4. Adsorption energies and geometric parameters of the most stable configurations of O2, NO, NO2, N2O and N2 adsorption on CuMn2O4(100) surface. Fig. 5. Energy profiles and optimized structures of intermediates, transition states, and products of N2 formation reaction over CuMn2O4(100) surface. (a) Reaction step of NH2* + NO* → N2* + H2O*. (b) Reaction step of NO* + 2* → N** + O*. (c) Reaction step of N* + N** → N2** + *. (d) Reaction step of NH* + N* + OH* → N2* + H2O* + *. Fig. 6. Energy profiles and optimized structures of intermediates, transition states, and products of NO2 formation reaction over CuMn2O4(100) surface. (a) Reaction step of O2**→ O* + O*. (b) Reaction step of NO* + O* → NO2**. Fig. 7. Energy profile and optimized structures of intermediates, transition states, and products of N2O formation reaction over CuMn2O4(100) surface. 24

Fig. 8. A skeletal reaction scheme for NH3-SCR of NO over CuMn2O4 surface. N atoms of NO and NH3 are highlighted with red and blue, respectively. The activation energy barrier and reaction heat of all elementary steps are given in kJ/mol. The optimal reaction pathway for NH3-SCR of NO is represented by green arrow.

25

(a)

(d)

Cu Top view

g

Mn

f e

a b

Z Y X

(b)

c

O

d 1 2 3 4

(c)

Side view

5 6 7 8 9

Fig. 1. (a) The face-centered cubic crystal structure of CuMn2O4. (b) 6-fold coordinated Mn atom locating at octahedral site. (c) 4-fold coordinated Cu atom locating at tetrahedral site. (d) Slab model of CuMn2O4(100) surface. a, b, c, d, e, f, and g represent the adsorption sites of Cu2, bridge, Mn, O3, Cu4, hollow, and O4, respectively. Cu2 and Cu4 denote 2-fold and 4-fold coordinated Cu, respectively. O3 and O4 denote 3-fold and 4-fold coordinated O, respectively.

26

-600

NH3

2.115 Å

NH

NH2 1.911 Å

1.962 Å

1.963 Å 2.060 Å

N

1.952 Å

H

Adsorption energy (kJ/mol)

0.989 Å

-450 Ea= 404.12 kJ/mol Cu

Mn

O

N

H

Ea= 314.70 kJ/mol -300 Ea= 200.25 kJ/mol Ea= 174.20 kJ/mol -150

Ea= 130.85 kJ/mol

0 NH3

NH2

NH

N

H

Fig. 2. Adsorption energies and geometric parameters of the most stable configurations of NHx adsorption on CuMn2O4(100) surface.

27

400

1.620 Å 1.900 Å

0.969 Å

2.016 Å 2.139 Å

NH3 205.29

Relative energy (kJ/mol)

200

161.74 107.21

1.280 Å

99.66

1.267 Å

1.983 Å

275.30

0.00

N

1.947 Å

62.08

0

−94.16

−130.85

NH3

0.990 Å

−95.01

36.69

0.989 Å

NH2

−200

NH

−70.01

2.006 Å 0.991 Å

2.006 Å

1.605 Å

Cu

1.055 Å

Mn

2.115 Å

O H −400 NH3

NH3*

TS1

Cu-NH2*

Mn-NH2* TS2

NH*

TS3

N*

Reaction pathway

Fig. 3. Reaction energy profile and optimized structures of intermediates, transition states, and products of NH3 dehydrogenation over CuMn2O4(100) surface. The reaction energy profile is plotted relative to the total energy of gaseous NH3 molecule and clean CuMn2O4(100) surface.

28

-600

O2

2.060 Å

NO 1.999 Å

Adsorption energy (kJ/mol)

2.149 Å

NO2

N2O 2.040 Å

N2

1.946 Å

2.165 Å

1.948 Å

2.131 Å

-450 Cu

Mn

O

N

Ea= 354.61 kJ/mol Ea= 321.69 kJ/mol

Ea= 301.38 kJ/mol

-300 Ea= 228.66 kJ/mol

-150 Ea= 71.46 kJ/mol

0 O2

NO

NO2

N2 O

N2

Fig. 4. Adsorption energies and geometric parameters of the most stable configurations of O2, NO, NO2, N2O and N2 adsorption on CuMn2O4(100) surface.

29

Relative energy (kJ/mol)

30

(a) NH2*+NO*→N2*+H2O* NH2*+NO*

0 -30

NH2-NO(#) 6.87

0.00 3.689 Å

NO

NH2

N2*+H2O* -65.73

-60 Cu Initial state 300

Relative energy (kJ/mol)

H2O

2.231 Å

-90

O

1.206 Å

Surface product

NO

2.042 Å

N**+O* 91.27

100

4.704 Å

O

1.948 Å

0

NO*+2* 0.00

Cu

Initial state 100

H

N

NO(#) 217.17

(b) NO*+2*→N**+O*

200

Mn

Transition state

Mn

Surface product

N-N(#) 58.26

N*+N**

1.958 Å

N

H

Transition state

(c) N*+N**→N2**+*

0

N

O

-100

Relative energy (kJ/mol)

1.171 Å

N2

1.189 Å

N2

2.724 Å

0.00 4.690 Å

-100

N

N

-200

N2**+*

Cu

Mn

O

N

-234.19

H

-300 Initial state

Relative energy (kJ/mol)

400

(d) NH*+N*+OH*→ N2*+H2O*+*

200 0

Surface product

Transition state

NH*+N*+OH* 0.00 N

4.117 Å

1.176 Å

NH-N-OH(#) 360.42

H2 O

N2

2.286 Å

OH

N2*+H2O*+*

NH -200 Cu

-400 Initial state

Transition state

Mn

O

-204.54 H N

Surface product

Reaction pathway

Fig. 5. Energy profiles and optimized structures of intermediates, transition states, and products of N2 formation reaction over CuMn2O4(100) surface. (a) Reaction step of NH2* + NO* → N2* + H2O*. (b) Reaction step of NO* + 2* → N** + O*. (c) Reaction step of N* + N** → N2** + *. (d) Reaction step of NH* + N* + OH* → N2* + H2O* + *.

30

Relative energy (kJ/mol)

50 0

O2(#) 13.90

(a) O2**→O*+O*

6.161 Å

O

1.847 Å

1.907 Å

O2** 0.00

-50

O 1.848 Å

1.361 Å

O2 -100

Cu

Mn

O*+O* –77.73 N O

-150 Initial state Relative energy (kJ/mol)

150 100

Transition state

(b) NO*+O*→NO2** O 4.612 Å

Surface product

NO-O(#) 145.10

1.186 Å

NO

1.771 Å

NO2** 57.16

1.230 Å

50

1.348 Å 1.252 Å

0

NO*+O* 0.00

-50 Initial state

Cu

Mn

O

NO2

N

Transition state

Surface product

Reaction pathway

Fig. 6. Energy profiles and optimized structures of intermediates, transition states, and products of NO2 formation reaction over CuMn2O4(100) surface. (a) Reaction step of O2**→ O* + O*. (b) Reaction step of NO* + O* → NO2**.

31

2.472 Å

NO*+N*

150

1.176 Å

N2O**

Relative energy (kJ/mol)

100 1.234 Å

50 NO*+N* 0.00

1.335 Å

N2O

NO-N(#) 61.84

0

-50

N

4.467 Å

1.186 Å

N2O** –85.57

NO -100 Cu

Mn

O

N

-150 Initial state

Transition state

Surface product

Reaction pathway

Fig. 7. Energy profile and optimized structures of intermediates, transition states, and products of N2O formation reaction over CuMn2O4(100) surface.

32

NH3 ΔE=−130.85

H H

N

H

CuMn2O4

H

H

N

H

CuMn2O4

Ea=36.69 ΔE=35.84

H

H

CuMn2O4

Ea=275.30 ΔE=132.09

NO NO O NH2

N CuMn2O4

N

Ea=6.87 ΔE=−65.73

H

CuMn2O4

CuMn2O4

N

O Ea=145.10 ΔE=57.16

N

Ea=58.26 ΔE=−234.19

CuMn2O4

Ea=217.17 ΔE=91.27

N

N N

Ea=360.42 ΔE=−204.54

O O

Ea=58.26 ΔE=−234.19

H

O

CuMn2O4

N

OH

N

N

Ea=99.66 ΔE=45.13

N

Ea=6.87 ΔE=−65.73

ΔE=−301.38

H

N

N

NO

O

Ea=61.84 ΔE=−85.57

CuMn2O4

N

O

CuMn2O4

Fig. 8. A skeletal reaction scheme for NH3-SCR of NO over CuMn2O4 surface. N atoms of NO and NH3 are highlighted with red and blue, respectively. The activation energy barrier and reaction heat of all elementary steps are given in kJ/mol. The optimal reaction pathway for NH3-SCR of NO is represented by green arrow.

33

Highlights  A skeletal reaction scheme was proposed for NH3-SCR of NO over CuMn2O4 surface.  NH2 is identified as a key reactive intermediate of NH3-SCR reaction.  The optimal reaction pathway of NO reduction by NH3 is a two-step process.  The rate-determining step of NO reduction is the first dehydrogenation step of NH3.

34

Graphical Abstract

35