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Understanding the role of Ru dopant on selective catalytic reduction of NO with NH3 over Ru-doped CeO2 catalyst
Chemical Engineering Journal 369 (2019) 124–133
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Understanding the role of Ru dopant on selective catalytic reduction of NO with NH3 over Ru-doped CeO2 catalyst
T
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Chirawat Chitpakdeea, Anchalee Junkaewa, , Phornphimon Maitaradb,c, Liyi Shib, ⁎ Vinich Promarakc, Nawee Kungwand,e, Supawadee Namuangruka,b, a
National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, 111 Thailand Science Park, Pathum Thani 12120, Thailand Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, PR China c Department of Material Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Wangchan, Rayong 21210, Thailand d Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand e Center of Excellence in Materials Science and Technology, Chiang Mai University, Chiang Mai 50200, Thailand b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
dopant changes electronic charge • Ru property and enhances the Lewis acidity of CeO2.
reaction favors to proceed on the • The Ru Lewis acid site. acid site suppresses the NH • Brønsted dissociation but enhances the water 3
formation.
water formation is the rate de• The termining step for the overall reaction. of catalytic performance • Enhancement can be focused on water formation aspect.
A R T I C LE I N FO
A B S T R A C T
Keywords: NH3-SCR Ru-doped CeO2 NO reduction mechanism DFT
Reaction mechanism of the selective catalytic reduction of nitric oxide (NO) by ammonia (NH3-SCR of NO) on the Ru-doped CeO2(111) surface was investigated using density functional theory calculation corrected by onsite Coulomb interactions (DFT + U) to understand the role of Ru dopant toward the catalytic performance of CeO2 based catalysts. The NH3-SCR of NO mechanisms on Ru-CeO2, which consisted of two consecutive NO reduction pathways, were systematically examined. Each NO reduction consists of important elementary steps such as NH3 adsorption/dissociation and water formation/desorption. The calculated results reveal that the Ru dopant substantially affects on the electronic charge property and enhances the Lewis acidity of the CeO2 surface. The NH3 adsorption and dissociation take place at the Lewis acid site of the catalyst. The first NO reduction via the NHNO intermediate is facile when the Ru dopant presents on the catalyst surface. The presence of Brønsted acid on surface catalyst suppresses the NH3 adsorption and dissociation but helps in promoting the water formation, which is the rate-determining step of overall reaction. Thus, the performance of this catalyst can be further enhanced by improving the water formation aspect. The obtained results deepen the fundamental understanding of the role of the different active sites on the crucial steps during the reaction and are useful for guiding the way to develop catalysts used in this application.
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Corresponding authors. E-mail addresses: [email protected] (A. Junkaew), [email protected] (S. Namuangruk).
https://doi.org/10.1016/j.cej.2019.03.053 Received 9 October 2018; Received in revised form 24 February 2019; Accepted 7 March 2019 Available online 08 March 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
of Ru-O-Ce bond and changing the oxidation state of Ce facilitate the formation of oxygen vacancy in CeO2 [30]. Very recently, doping Ru as a single Ru atom, Ru nanocluster, and large Ru nanoparticle forms into CeO2 can be successfully synthesized experimentally [27]. The experimental results showed that the single Ru atom and Ru nanocluster (ca. 1.2 nm) deposited CeO2 provided greater catalytic efficiency toward low-temperature CO2 methanation than the large Ru nanoparticle (ca. 4.0 nm) deposited CeO2. The study also suggested that Ru and Ce3+–OH sites are active sites for CO2 methanation [27]. Recently, we studied the NH3-SCR of NO reaction on the CeO2-based catalysts by using theoretical and experimental methods [31]. Our results demonstrated that the NH3 and NO adsorption energies as well as the oxygen vacancy formation energy are key catalytic indicators of the catalytic activity of metal doped on CeO2 (110) (metal = Zr, Ru, Cu, Mn) for NH3-SCR of NO. The Structure-activity relationship (SAR) analysis showed the very good relationship between the key indicators and catalytic performance of the metal doped on CeO2 catalysts in low temperature range (100 °C–200 °C). Our study also revealed that the Ru dopant enhances the catalytic performance of CeO2 for the NH3-SCR of NO reaction [31]. However, the role of Ru dopant on the reaction and how it enhances the performance of the catalyst have not been reported yet. From aforementioned advantages, it is interesting to examine the mechanistic insight of NH3-SCR of NO over the Ru-doped CeO2 catalyst. In this work, we employed DFT calculations to systematically investigate the reaction mechanisms of NH3-SCR of NO on the Ru single atom doped-CeO2 catalyst denoted as Ru-CeO2(111). We also explored the roles of the active sites (i.e. Ru dopant as the Lewis acid site, oxygen vacancies, and Brønsted acid site) on the reaction mechanism. The deep information can provide the important knowledge for design and enhancement the catalytic performance of CeO2 catalysts.
Nitrogen oxides (NOx) are known as the major cause of air pollution, acid rain, ozone depletion and the greenhouse effect. Most of NOx compounds in the atmosphere are released from combustion processes of fossil fuels in vehicles and power plants. The NOx emission has been continuously increasing every year. Pre-combustion and combustion controls were used to minimize NOx emission (deNOx), but both processes are still insufficient to reduce exhausted NOx [1–4]. A selective catalytic reduction (SCR) technique is one of the attractive methods for eliminating NO due to its low operation cost and high efficiency. Among several choices of reducing agents, ammonia reducing agent is widely used in the SCR of NO technology called NH3-SCR of NO. This NH3-SCR of NO method provides high conversion rate and reliable catalytic systems [5–7]. The NH3-SCR of NO reaction converts NO to N2 and H2O as described in the following equation:
4NO + 4NH3 + O2 → 4N2 + 6H2 O
(1)
The well-known commercial catalysts for NH3-SCR of NO reaction are V2O5/TiO2 promoted by WO3 or MoO3. Even if they are efficient catalysts for the reaction, they are active only in a narrow temperature window of 300–400 °C [8,9]. They also tend to oxidize SO2 to SO3 which causes a corrosion of downstream equipment and pore plugging of catalysts [10,11]. Moreover, vanadium species, which are toxic to environment, could leak from the reaction system at high temperature. Therefore, the development of new catalysts, which improve those limitations, is still challenging in this field. In literature, various metal oxide based catalysts have been modified and explored for this deNOx application [12,13]. Cerium oxide (CeO2)-based catalysts are potential candidates for NH3-SCR of NO, since they show good oxygen storage-, redox-abilities and good efficiency for this reaction at low temperature [14–16]. Several metaldoped and mixed oxide forms of CeO2 such as W-CeO2/TiO2 [17], MoO3-CeO2/TiO2 [18], Ca-MnOx/TiO2 [19], MnO2-CeO2 [20], and Ti/ Sn-MnOx/CeO2 [21] have been reported for the NH3-SCR of NO reaction. Those CeO2 modifications have shown the remarkable improvement of the catalytic activity at low temperatures and have provided the better resistance of SO2 poisoning [22]. In addition to experimental studies, theoretical methods have been used to investigate the reaction mechanisms of NH3-SCR of NO on many transition metal-doped CeO2(111) surfaces. The detailed reaction mechanism in molecular level provides very important information for understanding the nature of catalyst towards specific reaction, which is useful for designing better catalytic systems. For instance, Song et al. proposed the reaction mechanism of SCR of NO by NH3 on Mn-doped CeO2(111) or MnCeO2(111). They reported that NH3 is preferentially adsorbed on the Mn site and one N–H is dissociated to form the key NH2 intermediate and OH species on the surface. This active NH2 readily combines with NO via the Eley–Rideal (ER) mechanism to form nitrosamine (NH2NO) and then the second N–H cleavage forms another OH group on the surface [23]. Their calculation corresponds with the results from the Fourier transform infrared spectroscopy (FTIR) showing that the existence of NH2 species after NH3 adsorption and those NH2 species on the Lewis acid sites react with NO molecules to produce N2 and H2O [24]. In another case, Liu et al. investigated the mechanism of this reaction on the W-doped CeO2 catalyst or W-CeO2 (111) using DFT method. In contrast to Mn-CeO2, the NH2 species on the W-CeO2(111) surface is very unstable, which limits the ER mechanism. The reaction is found to proceed through the formation of adsorbed N2O22− species on W-CeO2 which promotes the SCR reaction via Langmuir–Hinshelwood (LH) mechanism [25]. From literature, the Ru dopant enhances catalytic-efficiency and -reactivity of CeO2 catalyst toward many catalytic reactions such as CO2 methanation [26–28], CO oxidation [29] and ammonia synthesis [30]. The experimental work claimed that Ru can promote the formation of oxygen vacancy in CeO2 [28,30]. Lin et al. proposed that the formation
2. Computational methods The reaction mechanism of NO reduction based on NH3-SCR process on the Ru-CeO2 catalyst was investigated by using a plane-wave-based DFT with a Perdew-Burke-Ernzerhof (PBE) functional implemented in Vienna Ab-initio Simulation Package (VASP) [32,33]. The interaction between ions and electrons is described by using the projector augmented wave (PAW) method [34]. A kinetic energy cutoff of 400 eV was used. To correct on-site Coulomb interactions of localized electrons, the DFT + U approach of Dudarev et al. [35], was employed, in which a Hubbard U-like term is the difference between the Coulomb U and exchange J parameters (i.e. Ueff = U − J). Herein, a value of Ueff = 4.5 eV is applied to the Ce 4f state. This Ueff value is reasonable and used by others such as works of Fabris et al. [36] and Cococcioni and de Gironcoli [37]. This value is within the range of 3.0–5.5 eV reported to provide electron localization Ce 4f [38]. For Ru atom, a value of Ueff = 4.5 eV was used on its d orbital. For the catalyst models, (3 × 3) periodic slabs with a dimension of a = b = 11.64 Å, c = 20.00 Å were constructed, (see Fig. 1). The surface model contains 81 atoms of tri-layers, a vacuum space of approximately 12 Å was introduced on top of the surface to separate the slab and its replicas. During optimization, six topmost atomic layers of the CeO2(111) slab were allowed to relax, while three bottom layers were fixed to their bulk parameters. The 3 × 3 × 1 of k-point was sampled by MonkhorstPack method [39,40]. Optimized structures were obtained by minimizing the forces on each ion until they were less than 0.05 eV/Å. The adsorption energy (Eads) can be used to describe the adsorption strength between adsorbate and surface. This term can be calculated from Eq. (2).
Eads = Esurf/gas − Esurf − Egas
(2)
Esurf/gas, Esurf and Egas are the total energies of the gas adsorbed surface, clean surface and isolated gas systems, respectively. A climbing image nudged elastic band (CI-NEB) method [41,42] and DIMER 125
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Fig. 1. Side view (a) and top view (b) of optimized structure of Ru-CeO2 where the bond distances are in Å. The biggest green ball represents Ru atom, while the big white ball represents Ce atom, big and small red balls present surface- and subsurface O atoms, respectively.
method [43,44] were employed to locate the transition state (TS). Each transition state was confirmed by a single imaginary frequency. The energy profiles of proposed mechanisms are represented as the relative energy of initial, intermediate and final states compared to the reference state. The activation energy barrier (Ea) is the energy difference between the transition state and its corresponding initial state. 3. Results and discussions 3.1. Reactant and intermediate adsorption on Ru-CeO2 As presented in the introduction session, there are many possible mechanisms of the NH3-SCR of NO on metal oxide catalysts. To narrow down the possible pathways, the adsorption of reactant and key intermediates on the optimization of the Ru-CeO2 surface was determined first. For the catalyst model, the Ru-CeO2 surface was constructed by replacing one surface Ce atom by the Ru atom. After optimization, Ru doping leads to structural distortion compared with the clean CeO2. The optimized structure of the Ru-CeO2 surface is shown in Fig. 1. The side view and top view of surface- and subsurface-atoms including the selected bond lengths are illustrated in Fig. 1a and b, respectively. The Ru atom is coordinated with two neighboring surface oxygen atoms and two neighboring subsurface oxygen atoms at the distances of 2.00 Å and 2.10 Å, respectively. The Ru-O distances are shorter than the original Ce-O bond distance (2.37 Å) of the clean CeO2 due to the smaller atomic radius of Ru compared to Ce. On the other hand, the neighboring oxygen atoms move closer to the Ru dopant resulting in the weakening of the neighboring Ce-O bonds around the doping site. The surface distortion upon adding dopant was also found in other metal-doped CeO2 systems reported in the literature [45,46].
Fig. 2. Optimized structures of NH3 adsorption at (a) the Ru atom, (b) the Ce atom near Ru dopant (Ce1 site), (c) the Ce atom away from Ru dopant (Ce2 site) and (d) Brønsted acid site of Ru-CeO2. The bond distances are in Å.
We found that the most favorable adsorption site for NH3 is the top Ru site with the Eads of −98 kJ/mol. The adsorption ability of NH3 on the Ru atom is higher than those on the Ce1 site (−57 kJ/mol), the Ce2 site (−55 kJ/mol), and the Brønsted acid site (−34 kJ/mol). Note that in Fig. 2c, the Eads value of NH3 at the Ce2 site of Ru-CeO2 equals to that on the Ce atom of bare CeO2 (Eads ∼ −55 kJ/mol) shown in Fig. S1e. The Eads value of NH3 on bare CeO2 in this work agrees well with the reported Eads values on clean CeO2 (Eads ∼ −46 to −55 kJ/mol) from other theoretical studies [23,25]. As the result, Ru-CeO2 shows the similar behavior as Mn-CeO2 that NH3 prefers the Lewis acid metal dopant site [23]. In contrast of Ru-CeO2 and Mn-CeO2 cases, NH3 prefers the Ce site adjacent to the W dopant in W-CeO2 [25]. According to the Eads of the most preferable site, the adsorption strength of NH3 at the Lewis acid Ru site is relatively weaker than that on Mn-CeO2 (−125 kJ/ mol) [23], but it is stronger than that on W-CeO2 (−68 kJ/mol) [25]. The bond distance of Ru⋯NH3 (2.10 Å) is also shorter than Ce-NH3 (2.66 Å). The shorter distance corresponds to the stronger adsorption strength. In addition, the adsorption results imply that the introduction of Ru dopant enhances Lewis acidity and improves the NH3 adsorption ability of the CeO2 catalyst.
3.1.1. NH3 adsorption The NH3 adsorption is known as the important initial step for the NH3-SCR of NO reaction [47,48]. In literature, the preference site of NH3 adsorption can be Lewis acid sites or Brønsted acid sites depending on the types of catalysts [12,49,50]. The most preferable site of NH3 adsorption is required to be tested first before investigating reactions in further steps. Herein, we examined the NH3 adsorption over various adsorption sites on the Ru-CeO2 surface. Three different Lewis acid metal sites are considered: (a) the Ru site, (b) the Ce atom near Ru dopant (Ce1 site) and (c) the Ce atom away from Ru dopant (Ce2 site). In addition, the NH3 adsorption on the Brønsted acid site near Ru is also considered since the Brønsted acid is formed after the first NO reduction occurs. The optimized geometries and adsorption energies of different adsorption sites on bare Ru-CeO2 are shown in Fig. 2a to c. The top views of these NH3 adsorption configurations are given in Fig. S1 in Supplementary Information (SI).
3.1.2. NO adsorption after NH2 formation To reduce choices of several possible mechanisms to be tested, the adsorption and stability of possible key intermediates after the NH3 dissociation step were determined and compared. The NO adsorption on the NH2-adsorbed Ru-CeO2 surface is discussed in this part. The optimized structures and the NO adsorption energies at the four different active sites are given in Fig. 3. The calculations reveal that the 126
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Scheme 1. The catalytic cycle for NH3-SCR of NO on Ru-CeO2.
around the OH bond can be observed. Fig. 2f shows the Bader charge of NH3 adsorption on CeO2 system; the electrons transfer from CeO2 to adsorbed NH3 of this system is 0.10|e|. Comparison of the charge transfer difference between the pure CeO2 and Ru-CeO2 systems clearly shows that Ru dopant greatly enhances amount of charge transfer from catalyst to NH3, which directly correlates to the adsorption strength of NH3 on the surface. It can be concluded that the Ru dopant affects on the electronic charge nature of CeO2 resulting in the change catalytic property of CeO2.
Fig. 3. Optimized structures of NO adsorption after NH2 formation on the RuCeO2 surface and the adsorption energy at the different adsorption sites: (a) NH2 species, (b) adjacent oxygen atom, (c) adjacent Ce atom and (d) Brønsted acid site of Ru-CeO2. The bond distances are in Å.
most preferable site for the NO molecule is the NH2 site since the NHNO intermediate is formed simultaneously with the Eads of −161 kJ/mol. The adsorption complex is illustrated in Fig. 3a. The H-bonding between the N of NO and the dissociated H on the catalyst surface has the distance of 1.68 Å. This H-bonding formation stabilizes the NHNO intermediate on the surface. We found that once NO is approaching the activated NH2, the NH2NO species is unstable on the surface since one of H atom of NH2 is transferring to an oxygen atom on the surface and form NHNO, see Figs. S9-S10 for explanation. In Fig. 3b and c, NO on the adjacent O and Ce sites result in −113 kJ/mol and −27 kJ/mol, respectively. Moreover, the NO adsorption on the Brønsted acid site is the least favorable configuration as its Eads is only −5 kJ/mol (see Fig. 3d). These results can be concluded that the reaction mechanism of NH3-SCR of NO reaction on the Ru-CeO2 surface will preferably proceed via the ER mechanism due to strong interaction between NO and NH2 species and forming the stable NHNO intermediate. Consequently, the detailed ER mechanism will be focused and investigated in this work. In addition, the Bader charge calculations were performed to understand the electronic charge nature of pure CeO2, Ru-CeO2, reduced Ru-CeO2 or H/Ru-CeO2 and NH3 adsorption on Ru-CeO2 or NH3/RuCeO2. The Bader charge change of each atom/molecule was calculated by comparing with the number of valence electrons of a neutral atom/ molecule. The positive and negative values in |e| denote the decrement and increment of valence electrons compared to the neutral state of each atom/molecule, respectively. The Bader charge change of selected atoms are given in Fig. S2 in SI. In pure CeO2 presented in Fig. S2a, Ce atoms on the surface present positive charge approximately +2.97|e|. The O atoms on the topmost layer and sub-surface layer have negatively charged about −1.45|e| and −1.48|e|, respectively. In Fig. S2b, the Ru atom reveals positive charge approximately +2.22|e|, when Ru is substituted into CeO2. The Bader charges of the neighboring oxygen atoms around Ru have less negative charge (−1.2|e|) compared to the oxygen atoms located far from Ru (−1.4|e|), which are similar to O atoms in pure CeO2. When one H presents on the surface, Ru have less negative charge compared to Ru-CeO2 (see Fig. S2c). In the NH3 adsorption on the Ru-CeO2 case, electrons transfer from Ru-CeO2 to NH3 approximately 0.43|e| showing that NH3 acts as electron acceptor. The Bader charge of each individual atom is illustrated in Fig. S2d. The Bader charge change of the dissociated NH3 to NH2 with one Brønsted site (OH) is presented in Fig. S2e. By comparing to non-dissociated NH3 adsorption in Fig. S2d, the increment of electrons of the neighboring Ce
3.2. Reaction mechanism of NH3-SCR of NO The reaction mechanism of SCR of NO with NH3 on the Ru-CeO2 catalyst focused in this work consists of two consecutive NO reduction reactions, see Scheme 1. The first NO reduction is composed of two possible pathways: (i) NO reduction via the N2O intermediate formation represented as pathway ABC (ii) NO reduction via N2OH intermediate formation represented as pathway AD. This first NO reduction generates N2 and H2O as products, while one H is left on the surface to produce the Brønsted acid surface see IM:13 in Scheme 1. Next, the second NO reduction proceeds on that Brønsted acid surface via pathway E to produce N2 and two H2O with one oxygen vacancy, denoted by V, left on the surface. The oxygen vacancy is healed by feeding O2 at the last step [25]. Eventually, the Ru-CeO2 catalyst is completely recovered. The relevant chemical reaction equations are explained by Eqs. (3) to (5). The first NO reduction on Lewis acid site via ABC and AD pathways:
NH3 + NO → H2 O+ N2 + H∗
(3)
The second NO reduction on Brønsted surface via E pathway:
NH3 + NO + H∗ → H2 O+ N2 + 2H∗
(4)
2 H∗ + Osurface → H2 O
(5)
The surface regeneration pathway can be done by feeding O2 [25]. The detailed mechanisms are systematically determined in the following part. The 2D-intermediate structures of all elementary steps and catalytic pathways are summarized in Fig. S4. The top view of 3D-intermediate structures are depicted in Figs. S5–S7. 3.2.1. First NO reduction The first NO reduction starts from the adsorption and dissociation of NH3 followed by the NHNO formation that is described by step A. Next, the N2 and H2O are produced by two possible pathways; (i) NO reduction via N2O intermediate described by steps B and C (ii) NO reduction via N2OH intermediate described by step D. The mechanistic details are discussed in this part. 127
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occurs to form NH2 on Ru and OH group at the IM:3A state. The NH3 dissociation requires an Ea of 17 kJ/mol to surmount the TS1A state. The NH3 dissociation on the pure CeO2 surface was also calculated to see the effect of Ru doping on the NH3 dissociation. As presented in Fig. S8 in SI, the NH3 dissociation on pure CeO2 is endothermic process which consumes energy approximately 74 kJ/mol. Hence doping Ru greatly promotes the NH3 dissociation ability of the CeO2 catalyst. Next, the incoming NO molecule interacts with NH2 following the ER mechanism as mentioned in section 3.1.2. When NO interacts with NH2, one N–H bond of NH2 is broken immediately. The dissociated H atom simultaneously forms a new bond with a neighboring O atom. NHNO and the second OH groups are formed on the surface, spontaneously (see IM:4A in Fig. 5). The NHNO intermediate is stable on top of the Ru dopant with a high exothermic reaction energy of −161 kJ/mol as shown in Fig. 4. The simple N–H cleavage of NH2 and high stability of NHNO indicate that neighboring O atoms around the Ru dopant are reactive to H atoms. From the calculated results, we concluded that the NHNO formation step is very facile and occurs rapidly on the Ru-CeO2 surface. The production of N2 and H2O will be proceeded after this step. Two possible competitive routes (BC or D) after step A are proposed in the following part. Steps B and C: N2 and H2O production via N2O intermediate The first possible route is described by steps B and C as shown in Scheme 1. The calculated energy profiles are shown in Fig. 6. The side view of relevant intermediate structures are depicted in Fig. 7a while the top view is provided in Fig. S5 for clear understanding. For step B, the N–H bond of the NHNO intermediate is broken and the dissociated H migrates to bind with surface O atom, while the produced N2O remains at the Ru site (IM:5B). The IM:4A → TS2B → IM:5B step requires a small energy barrier of 41 kJ/mol for the N–H cleavage. This elementary step is exothermic with −75 kJ/mol. Subsequently, desorption of N2O from the Ru site requires an energy of 23 kJ/mol (IM:6B). At the IM:7B state, the Ru-CeO2 surface contains three hydroxyl groups around the Ru site as shown in Fig. 7a. Next, the H of OH and another OH group forms H2O. This H2O formation requires energy of 118 kJ/mol (IM:8B). At the end of step B, H2O desorption from the surface consumes energy of 42 kJ/mol to be the IM:9B state. There is one oxygen vacancy site left on the surface (see IM:9B in Figs. 7a and S4). Our calculated H2O desorption energy on Ru-CeO2 (42 kJ/mol) is in a range of 32 kJ/mol to 101 kJ/mol found in other CeO2-based catalysts (see Table 1) [23,51–54]. The H2O formation is the rate determining step of this pathway.
Fig. 4. Calculated energy profile of step A.
Fig. 5. Calculated structures of the intermediates and the transition state of step A.
Step A: NH3 dissociation and NHNO formation The calculated energy profile and relevant intermediate structures in step A are depicted in Figs. 4 and 5, respectively. The relative energy (ΔE) of each state is compared with the energy summation of bare Ru-CeO2 (IM:1), isolated NO and isolated NH3. First, the NH3 adsorbs on the Ru site with an Eads of −98 kJ/mol (IM:2A). At this IM:2A state, one of three N–H bonds of NH3 slightly elongates from 1.02 Å to 1.04 Å due to the interaction of the H atom with the neighboring surface O atom (see Fig. 5). Then, the NH3 dissociation
Fig. 6. Calculated energy profiles of step B and step C. 128
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Fig. 7. Optimized structures of the intermediates and the transition states of (a) steps BC and (b) step D.
literature, there are theoretical and experimental studies reported that the surface oxygen vacancy of CeO2 based catalysts is responsible for decomposition of N2O gas [23,55,56]. In this study, N2O adsorption at the vacancy site is very strong with the Eads of −107 kJ/mol (IM:10C),
Next, the oxygen vacancy on the surface can be healed by re-adsorbing N2O and releasing N2 as a product. This process is described by step C regarding the IM:10C to IM:13C step. The energy profile and relevant configurations are presented in Figs. 6, 7a and S4. In the
Table 1 Comparison of energy and Ea (kJ/mol) in the important steps in CeO2 (1 1 1) based catalysts. Parentheses represent the metal sites which interact with gas molecules. Catalysts
NH3 adsorption
Ea of NH3 dissociation
H2O formation
H2O desorption
Rate-determining step and (Ea)
Refs.
Ru-CeO2 Reduced Ru-CeO2 Pure CeO2 Mn-CeO2
−98 (Ru) −32 (Ru) −55 (Ce) −125 (Mn)
17 (Ru) 22 (Ru)a 74 (Ce) N/A
118 107
42b 83-88b
H2O formation (118)
This work
H2O desorption
W-CeO2
−13(W) −68 (Ce)
175 (W) 30c
62b 101 (Mn) 37d 32e 51f
a b c d e f
24 (Mn) 33 (Ce)
NH3 dissociation occurs near Brønsted acid site. H2O desorbs and creates a vacancy site. NH3 dissociation by NO2 abstraction. H2O desorbs with the presence of NHNO on the surface. H2O desorbs with the presence of N2H on the surface. H2O desorbs with the presence of NH on the surface. 129
NH2 and N2O2H formation (166)
This work [23] [54] [25]
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approaching of surface H atom to the OH group of N2OH. H2O is formed and N2 molecule is simultaneously generated after the N-OH cleavage at the IM:7D state. This product formation is exothermic with the reaction energy of −177 kJ/mol. The desorption energy of H2O and N2, IM:8D to IM:9D, is approximately 46 kJ/mol. Similar to path BC, the surface OH group and Ru dopant rearrange themselves into more stable Brønsted acid surface as shown in IM:13C in Fig. 7a. Based on the Ea of the rate determining steps, path D is less preferable than path BC. The potential energy diagram clearly shows that the first NO reduction occurs via pathway ABC rather than pathway AD on the Ru-CeO2 surface. 3.2.2. Second NO reduction and surface regeneration The second NO reduction continues after the first NO reduction. This step represented by step E in which NO reduction proceeds on the Brønsted Ru-CeO2 surface. The calculated energy profile and intermediate structures of step E are depicted in Figs. 9 and 10, respectively. Similar to the clean Ru-CeO2 surface, NH3 still prefers to adsorb at the Lewis acid Ru site with the Eads of −32 kJ/mol (IM:14E). The NH3 adsorption strength is weaker when the Brønsted acid site presents on the catalyst surface. After the NH3 adsorption, one N–H bond is dissociated easily with an Ea of 22 kJ/mol to form NH2 on the Ru dopant (IM:15E). At the transition state (TS3E), the N–H bond is breaking while the O–H bond is forming with the distances of 1.27 Å and 1.26 Å, respectively. This NH3 dissociation step is slightly exothermic reaction with −2 kJ/mol compared with preceding state (IM:14E). Next, the NH2 species on this surface is reactive to the incoming NO gas to form NHNO as same as step A. NO interacts with NH2 leading to the N–H bond cleavage and the second H atom dissociates towards the bridging O atom of the surface. Then H2O and NHNO species are formed simultaneously (IM:16E). This IM:15E to IM:16E step has a reaction energy of −50 kJ/mol. Then H2O is released as presented in the IM:17E state. Next, the H dissociation from NHNO results in N2O and another OH group on the Ru-CeO2 surface (IM:18E). This IM:18E structure is more energetically stable than the IM:17E state approximately −113 kJ/mol. The vacancy is healed by N2O in the IM:19E → TS4E → IM:20E step. At the TS4E state, the N–O bond is elongated to 1.40 Å before cleavage and the vacancy site is filled by O atom. This step requires small energy of 24 kJ/mol to overcome the barrier. The formed N2 molecule is desorbed with the energy of 28 kJ/mol at IM:21E* (see Fig. S6). Then the Ru dopant rearrange into the most stable site as shown in IM:21E. There are two Brønsted (OH) sites on the surface at this state. The final process is the H2O formation and desorption represented by the IM:22E to IM:23E states. The surface regeneration step proceeds next for recovering the clean Ru-CeO2 (see Scheme 1). Experiment reported that the reduced surface of CeO2–based catalyst can
Fig. 8. Calculated energy profiles of step D.
which is much stronger than that in Mn-CeO2 (−16 kJ/mol) [23]. From state IM:10C to IM:11C, N2O linear adsorption mode changes to bent mode by changing the ∠NNO angle from 180° (IM:10C) to approximately 119°(IM:11C). This step requires the small energy of 14 kJ/mol. The bent N2O is decomposed to form N2 at the IM:12C state. This structure is stable with the exothermic energy of −156 kJ/mol compared with IM:11C. Next, the N2 molecule desorbs easily from the surface with very small energy of 7 kJ/mol and there is one OH group left on the surface (see Fig. 7a). The facile N2 desorption from Brønsted acid site on the Ru-CeO2 surface is similar with that of W-CeO2 (4 kJ/mol) [25]. Eventually, route BC produces N2, H2O and one Brønsted acid site (OH) on surface. Step D: N2 and H2O production via N2OH intermediate Another possible N2 and H2O production is proposed via N2OH intermediate represented by step D. The energy profile of step D and corresponding configurations are shown in Figs. 8, 7b and S5. The reaction of this step starts from IM:4A, in which intramolecular H transfer from the N terminal to the O terminal of NHNO results in the N2OH intermediate (IM:5D). This step requires a large energy of 135 kJ/mol to overcome the barrier at the transition state (TS2D). The N2OH intermediate is formed and adsorbed on the Ru site (see IM:5D). This process is slightly endothermic approximately 19 kJ/ mol. This large Ea and slightly endothermic feature suggests that this route is less favorable than path BC. Then, the rotation of the N2OH complex from IM:5D to IM:6D occurs as presented in Fig. 7b. The rearrangement of the N2OH structure facilitates the H2O and N2 formation. The N2OH complex is decomposed easily with the
Fig. 9. Calculated energy profile of step E. 130
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Fig. 10. Calculated structures of the intermediates and the transition states of step E.
clean Ru-CeO2 catalyst is dominated by the ER reaction mechanism. Next, the second NO reduction occurs on the Brønsted acid surface via step E. The NO reduction by NH3-SCR process occurs at the Lewis acid Ru site located near the Brønsted acid site. The NH3 dissociation on the Brønsted acid surface is the clean Ru-CeO2 surface in term of reaction mechanism. However, the calculated adsorption energy and energy barrier reveal that the Brønsted acid surface has less reactivity than the clean surface. Compared with the clean Ru-CeO2 surface, the NH3 adsorption and dissociation on the Brønsted acid surface are slightly more difficult, but the H2O formation and desorption are significantly improved. Our investigation reveals that the H2O formation on the clean surface (IM:7B → IM:8B), see Fig. 11, is the rate determining step in the
be easily reoxidized by O2 in the NH3-SCR of NO process [57]. Feeding oxygen is proposed to recover the clean Ru-CeO2 catalyst [25,54]. 3.2.3. Summary of all pathways and comparison with other ceria-based catalysts The calculated energy profiles of all pathways proposed in this work are depicted in Fig. 11. The first NO reduction by NH3-SCR on the RuCeO2 surface can proceed through two possible pathways, ABC or AD. NHNO and N2O are the key intermediates of pathway ABC, while N2OH is the key intermediate of AD pathway. Pathway ABC is energetically preferable than path AD. Finally, N2, H2O and the Brønsted surface are produced. Therefore, the NO reduction by NH3-SCR process on the 131
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Fig. 11. Calculated energy profiles of two competitive catalytic pathways D (red line) and ABCE (blue line) pathways. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
key NHNO intermediate, while H2O and N2O are formed simultaneously. We found that the presence of the Brønsted acid site near Ru dopant reduces the NH3 adsorption strength, but promotes the water formation. Interestingly, the H2O formation in the first NO reduction is the rate limiting step of overall reaction in Ru-CeO2, while the ratedetermining step in W-CeO2 and Mn-CeO2 is found as NH3 dissociation and water desorption, respectively. Thus, the improvement of water formation would enhance the catalytic performance of Ru-CeO2. Compare to pure CeO2 and some metal doped CeO2 catalysts, doping of Ru improves the reactivity of the catalyst toward this reaction. The atomic-scale investigation in this work provides the fundamental understanding of reaction mechanisms of NO reduction by NH3-SCR on the Ru-CeO2 catalyst. The obtained information is useful for the further design and improve performance of ceria-based catalysts for the NH3SCR of NO reaction.
NH3-SCR reaction over the Ru-CeO2 catalyst. This aspect was also observed in other catalysts as discussed in the following part. In order to determine the catalytic performance of Ru-doped CeO2, the mechanistic results of NH3-SCR of NO on Ru-CeO2 are compared with those in other CeO2-based catalysts reported by other theoretical works. The relative energy, the rate-determining step with its relevant Ea of selected important steps are compared in Table 1. From our results, Ru-CeO2 shows the improvement of NH3-adsorption and -dissociation ability compared to the un-doped CeO2 catalyst. Obviously, the NH3 dissociation on the Ru site is more facile than the Ce sites, the energy barrier decreases from 74 kJ/mol to 17 kJ/mol. Therefore, NH3 adsorption and dissociation ability is enhanced in cases of Ru- and Mndoped CeO2. In the case of Mn-CeO2, the water desorption to create O vacancy is the limiting step for NH3-SCR of NO [23]. Gao et al. [58] proposed that the presence of water greatly hinders the NH3 adsorption on the Ce-Cu-Ti oxide catalyst. Thus, in case of Mn-CeO2, avoiding water in the system could be the way of improvement of its catalytic performance. Contrast to other CeO2 based catalysts, the reaction on WCeO2 mostly proceeds on the Ce sites due to the low adsorption strength of NH3 (Ead ∼ −13 kJ/mol) and the high energy barrier of NH3 dissociation (Ea ∼ 175 kJ/mol) on the W site. The H2O-adsorption and -desorption on W-CeO2 requires lower energy than those on Ru- and Mn-CeO2 catalysts. However, NH3 dissociation and N2O2H formation proceed simultaneously and this step is the limiting step of the overall reaction on W-CeO2. This step consumes a very high energy of 166 kJ/ mol. In summary, Ru-CeO2 shows superior catalytic activity for NH3SCR of NO compared to pure CeO2 and W-CeO2. The tendency of NH3SCR of NO performance of these metal-doped CeO2 is similar to the NH3-SCR of NO on metal-doped CeO2 observed by our experimental results of our previous work [31]. From the calculation results presented in this work, the further design of the active site of Ru-CeO2 to enhance the water formation would help in the improvement of its catalytic performance for NH3-SCR of NO.
Acknowledgements The authors are grateful the financial support from National Nanotechnology Center (NANOTEC), Chiang Mai University, and the Thailand Research Fund (RSA6180080 and RTA6080005). P.M. would like to thank Natural National Science Foundation of China (NFSC) for Research Fellowship for International Young Scientists (No. 21650110450). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.03.053. References [1] K. Skalska, J.S. Miller, S. Ledakowicz, Sci. Total Environ. 408 (2010) 3976–3989. [2] M.V. Twigg, Appl. Catal. B Environ. 70 (2007) 2–15. [3] P. Schmitt, T. Poinsot, B. Schuermans, K.P. Geigle, J. Fluid Mech. 570 (2007) 17–46. [4] T.V. Johnson, Int. J. Eng. Res. 10 (2009) 275–285. [5] P. Maitarad, J. Han, D.S. Zhang, L.Y. Shi, S. Namuangruk, T. Rungrotmongkol, J. Phys. Chem. C 118 (2014) 9612–9620. [6] F. Nakajima, I. Hamada, Catal. Today 29 (1996) 109–115. [7] S. Roy, M.S. Hegde, G. Madras, Appl. Energy 86 (2009) 2283–2297. [8] C. Liu, L. Chen, J. Li, L. Ma, H. Arandiyan, Y. Du, J. Xu, J. Hao, Environ. Sci. Technol. 46 (2012) 6182–6189. [9] Y. Peng, Z. Liu, X. Niu, L. Zhou, C. Fu, H. Zhang, J. Li, W. Han, Catal. Commun. 19 (2012) 127–131. [10] G. Qi, R.T. Yang, Appl. Catal. B Environ. 44 (2003) 217–225. [11] M. Moliner, C. Franch, E. Palomares, M. Grill, A. Corma, Chem. Commun. 48 (2012)
4. Conclusions In summary, the Ru dopant exhibits as the Lewis acid site which greatly improves the NO reduction efficiency compared with the clean CeO2 catalyst. The NO reduction on the Ru-CeO2 proceeds via the ER mechanism. From the calculated energy profiles, the reaction mechanism is favorable to proceed through the NHNO/N2O intermediate pathway than the N2OH intermediate pathway. H2O and N2 are released as products and the Brønsted acid site is formed on Ru-CeO2 surface. For the second NO reduction, the reaction proceeds through the 132
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Understanding the role of Ru dopant on selective catalytic reduction of NO with NH3 over Ru-doped CeO2 catalyst
T
⁎
Chirawat Chitpakdeea, Anchalee Junkaewa, , Phornphimon Maitaradb,c, Liyi Shib, ⁎ Vinich Promarakc, Nawee Kungwand,e, Supawadee Namuangruka,b, a
National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, 111 Thailand Science Park, Pathum Thani 12120, Thailand Research Center of Nano Science and Technology, Shanghai University, Shanghai 200444, PR China c Department of Material Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Wangchan, Rayong 21210, Thailand d Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand e Center of Excellence in Materials Science and Technology, Chiang Mai University, Chiang Mai 50200, Thailand b
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
dopant changes electronic charge • Ru property and enhances the Lewis acidity of CeO2.
reaction favors to proceed on the • The Ru Lewis acid site. acid site suppresses the NH • Brønsted dissociation but enhances the water 3
formation.
water formation is the rate de• The termining step for the overall reaction. of catalytic performance • Enhancement can be focused on water formation aspect.
A R T I C LE I N FO
A B S T R A C T
Keywords: NH3-SCR Ru-doped CeO2 NO reduction mechanism DFT
Reaction mechanism of the selective catalytic reduction of nitric oxide (NO) by ammonia (NH3-SCR of NO) on the Ru-doped CeO2(111) surface was investigated using density functional theory calculation corrected by onsite Coulomb interactions (DFT + U) to understand the role of Ru dopant toward the catalytic performance of CeO2 based catalysts. The NH3-SCR of NO mechanisms on Ru-CeO2, which consisted of two consecutive NO reduction pathways, were systematically examined. Each NO reduction consists of important elementary steps such as NH3 adsorption/dissociation and water formation/desorption. The calculated results reveal that the Ru dopant substantially affects on the electronic charge property and enhances the Lewis acidity of the CeO2 surface. The NH3 adsorption and dissociation take place at the Lewis acid site of the catalyst. The first NO reduction via the NHNO intermediate is facile when the Ru dopant presents on the catalyst surface. The presence of Brønsted acid on surface catalyst suppresses the NH3 adsorption and dissociation but helps in promoting the water formation, which is the rate-determining step of overall reaction. Thus, the performance of this catalyst can be further enhanced by improving the water formation aspect. The obtained results deepen the fundamental understanding of the role of the different active sites on the crucial steps during the reaction and are useful for guiding the way to develop catalysts used in this application.
⁎
Corresponding authors. E-mail addresses: [email protected] (A. Junkaew), [email protected] (S. Namuangruk).
https://doi.org/10.1016/j.cej.2019.03.053 Received 9 October 2018; Received in revised form 24 February 2019; Accepted 7 March 2019 Available online 08 March 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.
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1. Introduction
of Ru-O-Ce bond and changing the oxidation state of Ce facilitate the formation of oxygen vacancy in CeO2 [30]. Very recently, doping Ru as a single Ru atom, Ru nanocluster, and large Ru nanoparticle forms into CeO2 can be successfully synthesized experimentally [27]. The experimental results showed that the single Ru atom and Ru nanocluster (ca. 1.2 nm) deposited CeO2 provided greater catalytic efficiency toward low-temperature CO2 methanation than the large Ru nanoparticle (ca. 4.0 nm) deposited CeO2. The study also suggested that Ru and Ce3+–OH sites are active sites for CO2 methanation [27]. Recently, we studied the NH3-SCR of NO reaction on the CeO2-based catalysts by using theoretical and experimental methods [31]. Our results demonstrated that the NH3 and NO adsorption energies as well as the oxygen vacancy formation energy are key catalytic indicators of the catalytic activity of metal doped on CeO2 (110) (metal = Zr, Ru, Cu, Mn) for NH3-SCR of NO. The Structure-activity relationship (SAR) analysis showed the very good relationship between the key indicators and catalytic performance of the metal doped on CeO2 catalysts in low temperature range (100 °C–200 °C). Our study also revealed that the Ru dopant enhances the catalytic performance of CeO2 for the NH3-SCR of NO reaction [31]. However, the role of Ru dopant on the reaction and how it enhances the performance of the catalyst have not been reported yet. From aforementioned advantages, it is interesting to examine the mechanistic insight of NH3-SCR of NO over the Ru-doped CeO2 catalyst. In this work, we employed DFT calculations to systematically investigate the reaction mechanisms of NH3-SCR of NO on the Ru single atom doped-CeO2 catalyst denoted as Ru-CeO2(111). We also explored the roles of the active sites (i.e. Ru dopant as the Lewis acid site, oxygen vacancies, and Brønsted acid site) on the reaction mechanism. The deep information can provide the important knowledge for design and enhancement the catalytic performance of CeO2 catalysts.
Nitrogen oxides (NOx) are known as the major cause of air pollution, acid rain, ozone depletion and the greenhouse effect. Most of NOx compounds in the atmosphere are released from combustion processes of fossil fuels in vehicles and power plants. The NOx emission has been continuously increasing every year. Pre-combustion and combustion controls were used to minimize NOx emission (deNOx), but both processes are still insufficient to reduce exhausted NOx [1–4]. A selective catalytic reduction (SCR) technique is one of the attractive methods for eliminating NO due to its low operation cost and high efficiency. Among several choices of reducing agents, ammonia reducing agent is widely used in the SCR of NO technology called NH3-SCR of NO. This NH3-SCR of NO method provides high conversion rate and reliable catalytic systems [5–7]. The NH3-SCR of NO reaction converts NO to N2 and H2O as described in the following equation:
4NO + 4NH3 + O2 → 4N2 + 6H2 O
(1)
The well-known commercial catalysts for NH3-SCR of NO reaction are V2O5/TiO2 promoted by WO3 or MoO3. Even if they are efficient catalysts for the reaction, they are active only in a narrow temperature window of 300–400 °C [8,9]. They also tend to oxidize SO2 to SO3 which causes a corrosion of downstream equipment and pore plugging of catalysts [10,11]. Moreover, vanadium species, which are toxic to environment, could leak from the reaction system at high temperature. Therefore, the development of new catalysts, which improve those limitations, is still challenging in this field. In literature, various metal oxide based catalysts have been modified and explored for this deNOx application [12,13]. Cerium oxide (CeO2)-based catalysts are potential candidates for NH3-SCR of NO, since they show good oxygen storage-, redox-abilities and good efficiency for this reaction at low temperature [14–16]. Several metaldoped and mixed oxide forms of CeO2 such as W-CeO2/TiO2 [17], MoO3-CeO2/TiO2 [18], Ca-MnOx/TiO2 [19], MnO2-CeO2 [20], and Ti/ Sn-MnOx/CeO2 [21] have been reported for the NH3-SCR of NO reaction. Those CeO2 modifications have shown the remarkable improvement of the catalytic activity at low temperatures and have provided the better resistance of SO2 poisoning [22]. In addition to experimental studies, theoretical methods have been used to investigate the reaction mechanisms of NH3-SCR of NO on many transition metal-doped CeO2(111) surfaces. The detailed reaction mechanism in molecular level provides very important information for understanding the nature of catalyst towards specific reaction, which is useful for designing better catalytic systems. For instance, Song et al. proposed the reaction mechanism of SCR of NO by NH3 on Mn-doped CeO2(111) or MnCeO2(111). They reported that NH3 is preferentially adsorbed on the Mn site and one N–H is dissociated to form the key NH2 intermediate and OH species on the surface. This active NH2 readily combines with NO via the Eley–Rideal (ER) mechanism to form nitrosamine (NH2NO) and then the second N–H cleavage forms another OH group on the surface [23]. Their calculation corresponds with the results from the Fourier transform infrared spectroscopy (FTIR) showing that the existence of NH2 species after NH3 adsorption and those NH2 species on the Lewis acid sites react with NO molecules to produce N2 and H2O [24]. In another case, Liu et al. investigated the mechanism of this reaction on the W-doped CeO2 catalyst or W-CeO2 (111) using DFT method. In contrast to Mn-CeO2, the NH2 species on the W-CeO2(111) surface is very unstable, which limits the ER mechanism. The reaction is found to proceed through the formation of adsorbed N2O22− species on W-CeO2 which promotes the SCR reaction via Langmuir–Hinshelwood (LH) mechanism [25]. From literature, the Ru dopant enhances catalytic-efficiency and -reactivity of CeO2 catalyst toward many catalytic reactions such as CO2 methanation [26–28], CO oxidation [29] and ammonia synthesis [30]. The experimental work claimed that Ru can promote the formation of oxygen vacancy in CeO2 [28,30]. Lin et al. proposed that the formation
2. Computational methods The reaction mechanism of NO reduction based on NH3-SCR process on the Ru-CeO2 catalyst was investigated by using a plane-wave-based DFT with a Perdew-Burke-Ernzerhof (PBE) functional implemented in Vienna Ab-initio Simulation Package (VASP) [32,33]. The interaction between ions and electrons is described by using the projector augmented wave (PAW) method [34]. A kinetic energy cutoff of 400 eV was used. To correct on-site Coulomb interactions of localized electrons, the DFT + U approach of Dudarev et al. [35], was employed, in which a Hubbard U-like term is the difference between the Coulomb U and exchange J parameters (i.e. Ueff = U − J). Herein, a value of Ueff = 4.5 eV is applied to the Ce 4f state. This Ueff value is reasonable and used by others such as works of Fabris et al. [36] and Cococcioni and de Gironcoli [37]. This value is within the range of 3.0–5.5 eV reported to provide electron localization Ce 4f [38]. For Ru atom, a value of Ueff = 4.5 eV was used on its d orbital. For the catalyst models, (3 × 3) periodic slabs with a dimension of a = b = 11.64 Å, c = 20.00 Å were constructed, (see Fig. 1). The surface model contains 81 atoms of tri-layers, a vacuum space of approximately 12 Å was introduced on top of the surface to separate the slab and its replicas. During optimization, six topmost atomic layers of the CeO2(111) slab were allowed to relax, while three bottom layers were fixed to their bulk parameters. The 3 × 3 × 1 of k-point was sampled by MonkhorstPack method [39,40]. Optimized structures were obtained by minimizing the forces on each ion until they were less than 0.05 eV/Å. The adsorption energy (Eads) can be used to describe the adsorption strength between adsorbate and surface. This term can be calculated from Eq. (2).
Eads = Esurf/gas − Esurf − Egas
(2)
Esurf/gas, Esurf and Egas are the total energies of the gas adsorbed surface, clean surface and isolated gas systems, respectively. A climbing image nudged elastic band (CI-NEB) method [41,42] and DIMER 125
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Fig. 1. Side view (a) and top view (b) of optimized structure of Ru-CeO2 where the bond distances are in Å. The biggest green ball represents Ru atom, while the big white ball represents Ce atom, big and small red balls present surface- and subsurface O atoms, respectively.
method [43,44] were employed to locate the transition state (TS). Each transition state was confirmed by a single imaginary frequency. The energy profiles of proposed mechanisms are represented as the relative energy of initial, intermediate and final states compared to the reference state. The activation energy barrier (Ea) is the energy difference between the transition state and its corresponding initial state. 3. Results and discussions 3.1. Reactant and intermediate adsorption on Ru-CeO2 As presented in the introduction session, there are many possible mechanisms of the NH3-SCR of NO on metal oxide catalysts. To narrow down the possible pathways, the adsorption of reactant and key intermediates on the optimization of the Ru-CeO2 surface was determined first. For the catalyst model, the Ru-CeO2 surface was constructed by replacing one surface Ce atom by the Ru atom. After optimization, Ru doping leads to structural distortion compared with the clean CeO2. The optimized structure of the Ru-CeO2 surface is shown in Fig. 1. The side view and top view of surface- and subsurface-atoms including the selected bond lengths are illustrated in Fig. 1a and b, respectively. The Ru atom is coordinated with two neighboring surface oxygen atoms and two neighboring subsurface oxygen atoms at the distances of 2.00 Å and 2.10 Å, respectively. The Ru-O distances are shorter than the original Ce-O bond distance (2.37 Å) of the clean CeO2 due to the smaller atomic radius of Ru compared to Ce. On the other hand, the neighboring oxygen atoms move closer to the Ru dopant resulting in the weakening of the neighboring Ce-O bonds around the doping site. The surface distortion upon adding dopant was also found in other metal-doped CeO2 systems reported in the literature [45,46].
Fig. 2. Optimized structures of NH3 adsorption at (a) the Ru atom, (b) the Ce atom near Ru dopant (Ce1 site), (c) the Ce atom away from Ru dopant (Ce2 site) and (d) Brønsted acid site of Ru-CeO2. The bond distances are in Å.
We found that the most favorable adsorption site for NH3 is the top Ru site with the Eads of −98 kJ/mol. The adsorption ability of NH3 on the Ru atom is higher than those on the Ce1 site (−57 kJ/mol), the Ce2 site (−55 kJ/mol), and the Brønsted acid site (−34 kJ/mol). Note that in Fig. 2c, the Eads value of NH3 at the Ce2 site of Ru-CeO2 equals to that on the Ce atom of bare CeO2 (Eads ∼ −55 kJ/mol) shown in Fig. S1e. The Eads value of NH3 on bare CeO2 in this work agrees well with the reported Eads values on clean CeO2 (Eads ∼ −46 to −55 kJ/mol) from other theoretical studies [23,25]. As the result, Ru-CeO2 shows the similar behavior as Mn-CeO2 that NH3 prefers the Lewis acid metal dopant site [23]. In contrast of Ru-CeO2 and Mn-CeO2 cases, NH3 prefers the Ce site adjacent to the W dopant in W-CeO2 [25]. According to the Eads of the most preferable site, the adsorption strength of NH3 at the Lewis acid Ru site is relatively weaker than that on Mn-CeO2 (−125 kJ/ mol) [23], but it is stronger than that on W-CeO2 (−68 kJ/mol) [25]. The bond distance of Ru⋯NH3 (2.10 Å) is also shorter than Ce-NH3 (2.66 Å). The shorter distance corresponds to the stronger adsorption strength. In addition, the adsorption results imply that the introduction of Ru dopant enhances Lewis acidity and improves the NH3 adsorption ability of the CeO2 catalyst.
3.1.1. NH3 adsorption The NH3 adsorption is known as the important initial step for the NH3-SCR of NO reaction [47,48]. In literature, the preference site of NH3 adsorption can be Lewis acid sites or Brønsted acid sites depending on the types of catalysts [12,49,50]. The most preferable site of NH3 adsorption is required to be tested first before investigating reactions in further steps. Herein, we examined the NH3 adsorption over various adsorption sites on the Ru-CeO2 surface. Three different Lewis acid metal sites are considered: (a) the Ru site, (b) the Ce atom near Ru dopant (Ce1 site) and (c) the Ce atom away from Ru dopant (Ce2 site). In addition, the NH3 adsorption on the Brønsted acid site near Ru is also considered since the Brønsted acid is formed after the first NO reduction occurs. The optimized geometries and adsorption energies of different adsorption sites on bare Ru-CeO2 are shown in Fig. 2a to c. The top views of these NH3 adsorption configurations are given in Fig. S1 in Supplementary Information (SI).
3.1.2. NO adsorption after NH2 formation To reduce choices of several possible mechanisms to be tested, the adsorption and stability of possible key intermediates after the NH3 dissociation step were determined and compared. The NO adsorption on the NH2-adsorbed Ru-CeO2 surface is discussed in this part. The optimized structures and the NO adsorption energies at the four different active sites are given in Fig. 3. The calculations reveal that the 126
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Scheme 1. The catalytic cycle for NH3-SCR of NO on Ru-CeO2.
around the OH bond can be observed. Fig. 2f shows the Bader charge of NH3 adsorption on CeO2 system; the electrons transfer from CeO2 to adsorbed NH3 of this system is 0.10|e|. Comparison of the charge transfer difference between the pure CeO2 and Ru-CeO2 systems clearly shows that Ru dopant greatly enhances amount of charge transfer from catalyst to NH3, which directly correlates to the adsorption strength of NH3 on the surface. It can be concluded that the Ru dopant affects on the electronic charge nature of CeO2 resulting in the change catalytic property of CeO2.
Fig. 3. Optimized structures of NO adsorption after NH2 formation on the RuCeO2 surface and the adsorption energy at the different adsorption sites: (a) NH2 species, (b) adjacent oxygen atom, (c) adjacent Ce atom and (d) Brønsted acid site of Ru-CeO2. The bond distances are in Å.
most preferable site for the NO molecule is the NH2 site since the NHNO intermediate is formed simultaneously with the Eads of −161 kJ/mol. The adsorption complex is illustrated in Fig. 3a. The H-bonding between the N of NO and the dissociated H on the catalyst surface has the distance of 1.68 Å. This H-bonding formation stabilizes the NHNO intermediate on the surface. We found that once NO is approaching the activated NH2, the NH2NO species is unstable on the surface since one of H atom of NH2 is transferring to an oxygen atom on the surface and form NHNO, see Figs. S9-S10 for explanation. In Fig. 3b and c, NO on the adjacent O and Ce sites result in −113 kJ/mol and −27 kJ/mol, respectively. Moreover, the NO adsorption on the Brønsted acid site is the least favorable configuration as its Eads is only −5 kJ/mol (see Fig. 3d). These results can be concluded that the reaction mechanism of NH3-SCR of NO reaction on the Ru-CeO2 surface will preferably proceed via the ER mechanism due to strong interaction between NO and NH2 species and forming the stable NHNO intermediate. Consequently, the detailed ER mechanism will be focused and investigated in this work. In addition, the Bader charge calculations were performed to understand the electronic charge nature of pure CeO2, Ru-CeO2, reduced Ru-CeO2 or H/Ru-CeO2 and NH3 adsorption on Ru-CeO2 or NH3/RuCeO2. The Bader charge change of each atom/molecule was calculated by comparing with the number of valence electrons of a neutral atom/ molecule. The positive and negative values in |e| denote the decrement and increment of valence electrons compared to the neutral state of each atom/molecule, respectively. The Bader charge change of selected atoms are given in Fig. S2 in SI. In pure CeO2 presented in Fig. S2a, Ce atoms on the surface present positive charge approximately +2.97|e|. The O atoms on the topmost layer and sub-surface layer have negatively charged about −1.45|e| and −1.48|e|, respectively. In Fig. S2b, the Ru atom reveals positive charge approximately +2.22|e|, when Ru is substituted into CeO2. The Bader charges of the neighboring oxygen atoms around Ru have less negative charge (−1.2|e|) compared to the oxygen atoms located far from Ru (−1.4|e|), which are similar to O atoms in pure CeO2. When one H presents on the surface, Ru have less negative charge compared to Ru-CeO2 (see Fig. S2c). In the NH3 adsorption on the Ru-CeO2 case, electrons transfer from Ru-CeO2 to NH3 approximately 0.43|e| showing that NH3 acts as electron acceptor. The Bader charge of each individual atom is illustrated in Fig. S2d. The Bader charge change of the dissociated NH3 to NH2 with one Brønsted site (OH) is presented in Fig. S2e. By comparing to non-dissociated NH3 adsorption in Fig. S2d, the increment of electrons of the neighboring Ce
3.2. Reaction mechanism of NH3-SCR of NO The reaction mechanism of SCR of NO with NH3 on the Ru-CeO2 catalyst focused in this work consists of two consecutive NO reduction reactions, see Scheme 1. The first NO reduction is composed of two possible pathways: (i) NO reduction via the N2O intermediate formation represented as pathway ABC (ii) NO reduction via N2OH intermediate formation represented as pathway AD. This first NO reduction generates N2 and H2O as products, while one H is left on the surface to produce the Brønsted acid surface see IM:13 in Scheme 1. Next, the second NO reduction proceeds on that Brønsted acid surface via pathway E to produce N2 and two H2O with one oxygen vacancy, denoted by V, left on the surface. The oxygen vacancy is healed by feeding O2 at the last step [25]. Eventually, the Ru-CeO2 catalyst is completely recovered. The relevant chemical reaction equations are explained by Eqs. (3) to (5). The first NO reduction on Lewis acid site via ABC and AD pathways:
NH3 + NO → H2 O+ N2 + H∗
(3)
The second NO reduction on Brønsted surface via E pathway:
NH3 + NO + H∗ → H2 O+ N2 + 2H∗
(4)
2 H∗ + Osurface → H2 O
(5)
The surface regeneration pathway can be done by feeding O2 [25]. The detailed mechanisms are systematically determined in the following part. The 2D-intermediate structures of all elementary steps and catalytic pathways are summarized in Fig. S4. The top view of 3D-intermediate structures are depicted in Figs. S5–S7. 3.2.1. First NO reduction The first NO reduction starts from the adsorption and dissociation of NH3 followed by the NHNO formation that is described by step A. Next, the N2 and H2O are produced by two possible pathways; (i) NO reduction via N2O intermediate described by steps B and C (ii) NO reduction via N2OH intermediate described by step D. The mechanistic details are discussed in this part. 127
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occurs to form NH2 on Ru and OH group at the IM:3A state. The NH3 dissociation requires an Ea of 17 kJ/mol to surmount the TS1A state. The NH3 dissociation on the pure CeO2 surface was also calculated to see the effect of Ru doping on the NH3 dissociation. As presented in Fig. S8 in SI, the NH3 dissociation on pure CeO2 is endothermic process which consumes energy approximately 74 kJ/mol. Hence doping Ru greatly promotes the NH3 dissociation ability of the CeO2 catalyst. Next, the incoming NO molecule interacts with NH2 following the ER mechanism as mentioned in section 3.1.2. When NO interacts with NH2, one N–H bond of NH2 is broken immediately. The dissociated H atom simultaneously forms a new bond with a neighboring O atom. NHNO and the second OH groups are formed on the surface, spontaneously (see IM:4A in Fig. 5). The NHNO intermediate is stable on top of the Ru dopant with a high exothermic reaction energy of −161 kJ/mol as shown in Fig. 4. The simple N–H cleavage of NH2 and high stability of NHNO indicate that neighboring O atoms around the Ru dopant are reactive to H atoms. From the calculated results, we concluded that the NHNO formation step is very facile and occurs rapidly on the Ru-CeO2 surface. The production of N2 and H2O will be proceeded after this step. Two possible competitive routes (BC or D) after step A are proposed in the following part. Steps B and C: N2 and H2O production via N2O intermediate The first possible route is described by steps B and C as shown in Scheme 1. The calculated energy profiles are shown in Fig. 6. The side view of relevant intermediate structures are depicted in Fig. 7a while the top view is provided in Fig. S5 for clear understanding. For step B, the N–H bond of the NHNO intermediate is broken and the dissociated H migrates to bind with surface O atom, while the produced N2O remains at the Ru site (IM:5B). The IM:4A → TS2B → IM:5B step requires a small energy barrier of 41 kJ/mol for the N–H cleavage. This elementary step is exothermic with −75 kJ/mol. Subsequently, desorption of N2O from the Ru site requires an energy of 23 kJ/mol (IM:6B). At the IM:7B state, the Ru-CeO2 surface contains three hydroxyl groups around the Ru site as shown in Fig. 7a. Next, the H of OH and another OH group forms H2O. This H2O formation requires energy of 118 kJ/mol (IM:8B). At the end of step B, H2O desorption from the surface consumes energy of 42 kJ/mol to be the IM:9B state. There is one oxygen vacancy site left on the surface (see IM:9B in Figs. 7a and S4). Our calculated H2O desorption energy on Ru-CeO2 (42 kJ/mol) is in a range of 32 kJ/mol to 101 kJ/mol found in other CeO2-based catalysts (see Table 1) [23,51–54]. The H2O formation is the rate determining step of this pathway.
Fig. 4. Calculated energy profile of step A.
Fig. 5. Calculated structures of the intermediates and the transition state of step A.
Step A: NH3 dissociation and NHNO formation The calculated energy profile and relevant intermediate structures in step A are depicted in Figs. 4 and 5, respectively. The relative energy (ΔE) of each state is compared with the energy summation of bare Ru-CeO2 (IM:1), isolated NO and isolated NH3. First, the NH3 adsorbs on the Ru site with an Eads of −98 kJ/mol (IM:2A). At this IM:2A state, one of three N–H bonds of NH3 slightly elongates from 1.02 Å to 1.04 Å due to the interaction of the H atom with the neighboring surface O atom (see Fig. 5). Then, the NH3 dissociation
Fig. 6. Calculated energy profiles of step B and step C. 128
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Fig. 7. Optimized structures of the intermediates and the transition states of (a) steps BC and (b) step D.
literature, there are theoretical and experimental studies reported that the surface oxygen vacancy of CeO2 based catalysts is responsible for decomposition of N2O gas [23,55,56]. In this study, N2O adsorption at the vacancy site is very strong with the Eads of −107 kJ/mol (IM:10C),
Next, the oxygen vacancy on the surface can be healed by re-adsorbing N2O and releasing N2 as a product. This process is described by step C regarding the IM:10C to IM:13C step. The energy profile and relevant configurations are presented in Figs. 6, 7a and S4. In the
Table 1 Comparison of energy and Ea (kJ/mol) in the important steps in CeO2 (1 1 1) based catalysts. Parentheses represent the metal sites which interact with gas molecules. Catalysts
NH3 adsorption
Ea of NH3 dissociation
H2O formation
H2O desorption
Rate-determining step and (Ea)
Refs.
Ru-CeO2 Reduced Ru-CeO2 Pure CeO2 Mn-CeO2
−98 (Ru) −32 (Ru) −55 (Ce) −125 (Mn)
17 (Ru) 22 (Ru)a 74 (Ce) N/A
118 107
42b 83-88b
H2O formation (118)
This work
H2O desorption
W-CeO2
−13(W) −68 (Ce)
175 (W) 30c
62b 101 (Mn) 37d 32e 51f
a b c d e f
24 (Mn) 33 (Ce)
NH3 dissociation occurs near Brønsted acid site. H2O desorbs and creates a vacancy site. NH3 dissociation by NO2 abstraction. H2O desorbs with the presence of NHNO on the surface. H2O desorbs with the presence of N2H on the surface. H2O desorbs with the presence of NH on the surface. 129
NH2 and N2O2H formation (166)
This work [23] [54] [25]
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approaching of surface H atom to the OH group of N2OH. H2O is formed and N2 molecule is simultaneously generated after the N-OH cleavage at the IM:7D state. This product formation is exothermic with the reaction energy of −177 kJ/mol. The desorption energy of H2O and N2, IM:8D to IM:9D, is approximately 46 kJ/mol. Similar to path BC, the surface OH group and Ru dopant rearrange themselves into more stable Brønsted acid surface as shown in IM:13C in Fig. 7a. Based on the Ea of the rate determining steps, path D is less preferable than path BC. The potential energy diagram clearly shows that the first NO reduction occurs via pathway ABC rather than pathway AD on the Ru-CeO2 surface. 3.2.2. Second NO reduction and surface regeneration The second NO reduction continues after the first NO reduction. This step represented by step E in which NO reduction proceeds on the Brønsted Ru-CeO2 surface. The calculated energy profile and intermediate structures of step E are depicted in Figs. 9 and 10, respectively. Similar to the clean Ru-CeO2 surface, NH3 still prefers to adsorb at the Lewis acid Ru site with the Eads of −32 kJ/mol (IM:14E). The NH3 adsorption strength is weaker when the Brønsted acid site presents on the catalyst surface. After the NH3 adsorption, one N–H bond is dissociated easily with an Ea of 22 kJ/mol to form NH2 on the Ru dopant (IM:15E). At the transition state (TS3E), the N–H bond is breaking while the O–H bond is forming with the distances of 1.27 Å and 1.26 Å, respectively. This NH3 dissociation step is slightly exothermic reaction with −2 kJ/mol compared with preceding state (IM:14E). Next, the NH2 species on this surface is reactive to the incoming NO gas to form NHNO as same as step A. NO interacts with NH2 leading to the N–H bond cleavage and the second H atom dissociates towards the bridging O atom of the surface. Then H2O and NHNO species are formed simultaneously (IM:16E). This IM:15E to IM:16E step has a reaction energy of −50 kJ/mol. Then H2O is released as presented in the IM:17E state. Next, the H dissociation from NHNO results in N2O and another OH group on the Ru-CeO2 surface (IM:18E). This IM:18E structure is more energetically stable than the IM:17E state approximately −113 kJ/mol. The vacancy is healed by N2O in the IM:19E → TS4E → IM:20E step. At the TS4E state, the N–O bond is elongated to 1.40 Å before cleavage and the vacancy site is filled by O atom. This step requires small energy of 24 kJ/mol to overcome the barrier. The formed N2 molecule is desorbed with the energy of 28 kJ/mol at IM:21E* (see Fig. S6). Then the Ru dopant rearrange into the most stable site as shown in IM:21E. There are two Brønsted (OH) sites on the surface at this state. The final process is the H2O formation and desorption represented by the IM:22E to IM:23E states. The surface regeneration step proceeds next for recovering the clean Ru-CeO2 (see Scheme 1). Experiment reported that the reduced surface of CeO2–based catalyst can
Fig. 8. Calculated energy profiles of step D.
which is much stronger than that in Mn-CeO2 (−16 kJ/mol) [23]. From state IM:10C to IM:11C, N2O linear adsorption mode changes to bent mode by changing the ∠NNO angle from 180° (IM:10C) to approximately 119°(IM:11C). This step requires the small energy of 14 kJ/mol. The bent N2O is decomposed to form N2 at the IM:12C state. This structure is stable with the exothermic energy of −156 kJ/mol compared with IM:11C. Next, the N2 molecule desorbs easily from the surface with very small energy of 7 kJ/mol and there is one OH group left on the surface (see Fig. 7a). The facile N2 desorption from Brønsted acid site on the Ru-CeO2 surface is similar with that of W-CeO2 (4 kJ/mol) [25]. Eventually, route BC produces N2, H2O and one Brønsted acid site (OH) on surface. Step D: N2 and H2O production via N2OH intermediate Another possible N2 and H2O production is proposed via N2OH intermediate represented by step D. The energy profile of step D and corresponding configurations are shown in Figs. 8, 7b and S5. The reaction of this step starts from IM:4A, in which intramolecular H transfer from the N terminal to the O terminal of NHNO results in the N2OH intermediate (IM:5D). This step requires a large energy of 135 kJ/mol to overcome the barrier at the transition state (TS2D). The N2OH intermediate is formed and adsorbed on the Ru site (see IM:5D). This process is slightly endothermic approximately 19 kJ/ mol. This large Ea and slightly endothermic feature suggests that this route is less favorable than path BC. Then, the rotation of the N2OH complex from IM:5D to IM:6D occurs as presented in Fig. 7b. The rearrangement of the N2OH structure facilitates the H2O and N2 formation. The N2OH complex is decomposed easily with the
Fig. 9. Calculated energy profile of step E. 130
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Fig. 10. Calculated structures of the intermediates and the transition states of step E.
clean Ru-CeO2 catalyst is dominated by the ER reaction mechanism. Next, the second NO reduction occurs on the Brønsted acid surface via step E. The NO reduction by NH3-SCR process occurs at the Lewis acid Ru site located near the Brønsted acid site. The NH3 dissociation on the Brønsted acid surface is the clean Ru-CeO2 surface in term of reaction mechanism. However, the calculated adsorption energy and energy barrier reveal that the Brønsted acid surface has less reactivity than the clean surface. Compared with the clean Ru-CeO2 surface, the NH3 adsorption and dissociation on the Brønsted acid surface are slightly more difficult, but the H2O formation and desorption are significantly improved. Our investigation reveals that the H2O formation on the clean surface (IM:7B → IM:8B), see Fig. 11, is the rate determining step in the
be easily reoxidized by O2 in the NH3-SCR of NO process [57]. Feeding oxygen is proposed to recover the clean Ru-CeO2 catalyst [25,54]. 3.2.3. Summary of all pathways and comparison with other ceria-based catalysts The calculated energy profiles of all pathways proposed in this work are depicted in Fig. 11. The first NO reduction by NH3-SCR on the RuCeO2 surface can proceed through two possible pathways, ABC or AD. NHNO and N2O are the key intermediates of pathway ABC, while N2OH is the key intermediate of AD pathway. Pathway ABC is energetically preferable than path AD. Finally, N2, H2O and the Brønsted surface are produced. Therefore, the NO reduction by NH3-SCR process on the 131
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Fig. 11. Calculated energy profiles of two competitive catalytic pathways D (red line) and ABCE (blue line) pathways. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
key NHNO intermediate, while H2O and N2O are formed simultaneously. We found that the presence of the Brønsted acid site near Ru dopant reduces the NH3 adsorption strength, but promotes the water formation. Interestingly, the H2O formation in the first NO reduction is the rate limiting step of overall reaction in Ru-CeO2, while the ratedetermining step in W-CeO2 and Mn-CeO2 is found as NH3 dissociation and water desorption, respectively. Thus, the improvement of water formation would enhance the catalytic performance of Ru-CeO2. Compare to pure CeO2 and some metal doped CeO2 catalysts, doping of Ru improves the reactivity of the catalyst toward this reaction. The atomic-scale investigation in this work provides the fundamental understanding of reaction mechanisms of NO reduction by NH3-SCR on the Ru-CeO2 catalyst. The obtained information is useful for the further design and improve performance of ceria-based catalysts for the NH3SCR of NO reaction.
NH3-SCR reaction over the Ru-CeO2 catalyst. This aspect was also observed in other catalysts as discussed in the following part. In order to determine the catalytic performance of Ru-doped CeO2, the mechanistic results of NH3-SCR of NO on Ru-CeO2 are compared with those in other CeO2-based catalysts reported by other theoretical works. The relative energy, the rate-determining step with its relevant Ea of selected important steps are compared in Table 1. From our results, Ru-CeO2 shows the improvement of NH3-adsorption and -dissociation ability compared to the un-doped CeO2 catalyst. Obviously, the NH3 dissociation on the Ru site is more facile than the Ce sites, the energy barrier decreases from 74 kJ/mol to 17 kJ/mol. Therefore, NH3 adsorption and dissociation ability is enhanced in cases of Ru- and Mndoped CeO2. In the case of Mn-CeO2, the water desorption to create O vacancy is the limiting step for NH3-SCR of NO [23]. Gao et al. [58] proposed that the presence of water greatly hinders the NH3 adsorption on the Ce-Cu-Ti oxide catalyst. Thus, in case of Mn-CeO2, avoiding water in the system could be the way of improvement of its catalytic performance. Contrast to other CeO2 based catalysts, the reaction on WCeO2 mostly proceeds on the Ce sites due to the low adsorption strength of NH3 (Ead ∼ −13 kJ/mol) and the high energy barrier of NH3 dissociation (Ea ∼ 175 kJ/mol) on the W site. The H2O-adsorption and -desorption on W-CeO2 requires lower energy than those on Ru- and Mn-CeO2 catalysts. However, NH3 dissociation and N2O2H formation proceed simultaneously and this step is the limiting step of the overall reaction on W-CeO2. This step consumes a very high energy of 166 kJ/ mol. In summary, Ru-CeO2 shows superior catalytic activity for NH3SCR of NO compared to pure CeO2 and W-CeO2. The tendency of NH3SCR of NO performance of these metal-doped CeO2 is similar to the NH3-SCR of NO on metal-doped CeO2 observed by our experimental results of our previous work [31]. From the calculation results presented in this work, the further design of the active site of Ru-CeO2 to enhance the water formation would help in the improvement of its catalytic performance for NH3-SCR of NO.
Acknowledgements The authors are grateful the financial support from National Nanotechnology Center (NANOTEC), Chiang Mai University, and the Thailand Research Fund (RSA6180080 and RTA6080005). P.M. would like to thank Natural National Science Foundation of China (NFSC) for Research Fellowship for International Young Scientists (No. 21650110450). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.03.053. References [1] K. Skalska, J.S. Miller, S. Ledakowicz, Sci. Total Environ. 408 (2010) 3976–3989. [2] M.V. Twigg, Appl. Catal. B Environ. 70 (2007) 2–15. [3] P. Schmitt, T. Poinsot, B. Schuermans, K.P. Geigle, J. Fluid Mech. 570 (2007) 17–46. [4] T.V. Johnson, Int. J. Eng. Res. 10 (2009) 275–285. [5] P. Maitarad, J. Han, D.S. Zhang, L.Y. Shi, S. Namuangruk, T. Rungrotmongkol, J. Phys. Chem. C 118 (2014) 9612–9620. [6] F. Nakajima, I. Hamada, Catal. Today 29 (1996) 109–115. [7] S. Roy, M.S. Hegde, G. Madras, Appl. Energy 86 (2009) 2283–2297. [8] C. Liu, L. Chen, J. Li, L. Ma, H. Arandiyan, Y. Du, J. Xu, J. Hao, Environ. Sci. Technol. 46 (2012) 6182–6189. [9] Y. Peng, Z. Liu, X. Niu, L. Zhou, C. Fu, H. Zhang, J. Li, W. Han, Catal. Commun. 19 (2012) 127–131. [10] G. Qi, R.T. Yang, Appl. Catal. B Environ. 44 (2003) 217–225. [11] M. Moliner, C. Franch, E. Palomares, M. Grill, A. Corma, Chem. Commun. 48 (2012)
4. Conclusions In summary, the Ru dopant exhibits as the Lewis acid site which greatly improves the NO reduction efficiency compared with the clean CeO2 catalyst. The NO reduction on the Ru-CeO2 proceeds via the ER mechanism. From the calculated energy profiles, the reaction mechanism is favorable to proceed through the NHNO/N2O intermediate pathway than the N2OH intermediate pathway. H2O and N2 are released as products and the Brønsted acid site is formed on Ru-CeO2 surface. For the second NO reduction, the reaction proceeds through the 132
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