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Influence of Ce doping on catalytic oxidation of NO on LaCoO3 (011) surface: A DFT study
Applied Surface Science 499 (2020) 143866
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Influence of Ce doping on catalytic oxidation of NO on LaCoO3 (011) surface: A DFT study Xiaochen Lib, Hongwei Gaoa,
T
⁎
a
School of Life Science, Ludong University, Yantai, Shandong 264025, China Key Laboratory of Plant Resources and Chemistry in Arid Regions, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: DFT calculations Adsorption energy Oxygen chemistry NO oxidation Ce doping
The catalytic oxidation of NO on the undoped and Ce-doped LaCoO3 (011) surfaces has been systematically studied by utilizing the density functional theory (DFT) method. The extra-lattice oxygen and Mars−van Krevelen (MvK) mechanisms of NO oxidation are considered. According to our calculations, the extra-lattice oxygen mechanism is more likely to occur on the undoped and Ce-doped LaO-terminated LaCoO3 (011) surfaces, while the MvK mechanism is more suitable for the undoped and Ce-doped CoO2-terminated LaCoO3 (011) surfaces. By comparing and analyzing the same NO oxidation pathway on the undoped and Ce-doped surfaces, it is proved that the Ce doping is favorable for the LaO-terminated LaCoO3 (011) surface, but harmful for the CoO2terminated LaCoO3 (011) surface. In our research on NO oxidation, the addition of Ce dopant contributes to the LaO-terminated LaCoO3 (011) surface by reducing the energy barriers of O2 dissociation and NO2 desorption.
1. Introduction Nitrogen oxides (NOx, x = 1, 2) which are mainly emitted from motor vehicles and coal-fired power factories have brought tremendous threats to human health and environment [1,2]. To reduce the harm caused by NOx, various aftertreatment technologies have been developed, such as selective catalytic reduction (SCR) [3,4] and lean NOx traps (LNT) [5,6]. NO oxidation to NO2 is regarded as a key step in the two technologies mentioned above, since presence of NO2 in the NO/ NO2 mixture can substantially improve the reaction rate of SCR and NO2 is easier to be adsorbed on the surface of LNT catalysts than NO [6–10]. Platinum-based catalysts show promising catalytic performance for oxidation of NO to NO2 [11–14]. However, they are high in price and easily agglomerated during the reaction [15,16], which restricts seriously their large-scale application. This has stimulated a number of researchers to make great efforts on finding substitute materials with low cost and high stability for NO oxidation. In recent decades, perovskites with ABO3 structure, especially the LaCoO3, have received great attention as a substitute for Platinumbased catalysts. According to the paper [17], LaCoO3 achieves maximum NO conversion of 71% at 300 °C. Its catalytic performance can be improved by doping suitable transition metals. For example, Wen et al. [18] have reported that the activity of LaCoO3 was substantially enhanced with 20% substitution of cerium (Ce), reaching the highest ⁎
conversion of 80% at about 300 °C. Kim et al. [15] studied the NO oxidation activity of strontium (Sr) doped LaCoO3 samples and found that when Sr doping amount is 0.1, it achieved better oxidation activity than the commercial Pt/γ-Al2O3 catalysts. It is clear that understanding how doping improves the catalytic activity of LaCoO3 for NO oxidation can provide valuable referencing information for the development of perovskite-type catalysts. Therefore, more and more researchers began to study oxidation processes of NO on the undoped and doped LaCoO3 surfaces by using experimental and theoretical methods [18–22]. This is not an easy task, as many reaction routes are feasible in theory. In general, the oxidation mechanisms of NO on LaCoO3 surface can be summarized to two kinds, i.e., extra-lattice oxygen mechanism, in which an adsorbed O2 is dissociated into two isolated O adatoms and then NO molecules bind to each O adatom; and Mars−van Krevelen (MvK) mechanism, in which NO molecules are oxidized by lattice oxygen atoms and the role of O2 is to refill the vacancies by adsorption and solid state diffusion [19–26]. Choi et al. [19] proposed that the extra-lattice oxygen of LaO-terminated surface was likely available for NO oxidation via the first mechanism and its activity was enhanced by Sr doping. An et al. [22] pointed out that for Fe-doped LaCoO3, NO oxidation reaction proceeded via the second mechanism, where NO molecule was bounded to the lattice oxygen and the formed NO2 molecule was subsequently desorbed from the surface. However, to our knowledge, it is never clearly explained why the catalytic activity of
Corresponding author. E-mail address: [email protected] (H. Gao).
https://doi.org/10.1016/j.apsusc.2019.143866 Received 14 March 2019; Received in revised form 4 July 2019; Accepted 3 September 2019 Available online 04 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 499 (2020) 143866
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LaCoO3 for NO oxidation can be enhanced by doping Ce. In this paper, we first investigated the adsorption behaviors of oxygen molecule (O2) and atomic oxygen (O) on the undoped and Ce-doped surfaces. Secondly, the formation of surface oxygen vacancies and the dissociation of adsorbed O2 could be discussed in detail. Finally, we systematically investigated the possible oxidation pathways and made further explanation on the reasons why catalytic activity of LaCoO3 was improved after Ce doping.
Eads (O2) = Eslab + O2 − Eslab − EO2
(1)
Eads (O) = Eslab + O − Eslab − EO2 /2
(2)
In the above two formulas, Eslab refers to the energy of undoped or Ce-doped LaCoO3 slab; Eslab+O2 and Eslab+O are the total energies of a slab with an adsorbed O2 and a slab with an adsorbed O, respectively; EO2 represents the energy of a free O2 molecule. According to the above definition, when the value of Eads is positive, it means that the adsorption process is endothermic and adsorption structure is unstable; when the value of Eads is negative, it means that the adsorption process is exothermic and adsorption structure is stable. In addition, the smaller the value of Eads, the more stable the adsorption structure. Fig. 2 shows the most stable adsorption structures on the perfect LaO-terminated and CoO2-terminated surfaces. After doping Ce, the corresponding adsorption structures are displayed in Fig. 3. The calculated Eads and Mulliken charge (Q) are summarized in Table 1 and Fig. 4, respectively. The detailed calculation results can be found in the Supporting Information (SI). For the perfect LaO-terminated surface, both O2 and O preferentially adsorb at the hollow-top site. Fig. 2(a) shows the most stable adsorption configuration for molecular oxygen. The O-O bond length (dO-O) for adsorbed O2 is found to be 1.419 Å. The Eads(O2) calculated by Eq. (1) is −1.572 eV. Atomic O also prefers to associate with the hollow-top site of perfect LaO-terminated surface, as shown in Fig. 2(b). According to our calculations, the Eads(O) is −1.300 eV. Next we substitute one La atom of the outmost LaO slab with one Ce atom, corresponding to the dopant concentration of 0.042, to explore the influences of Ce doping. The hollow-O2 and hollow-O configurations of Ce-doped LaO-terminated surface are displayed in the Fig. 3(a) and (b), respectively. The dO-O of adsorbed O2 is 1.440 Å. After Ce doping, the absolute value of Eads for O2 and O is increased by 0.583 eV and 0.658 eV, respectively, indicating that the reaction species in our study are more easily adsorbed on the doped surface. When reaction species are adsorbed on the Ce-doped LaO-terminated surface, the Ce-top site should also be considered as a possible adsorption site. Therefore, we construct the initial adsorption configurations that O2 and O are attached to Ce-top sites. Interestingly, the initial Ce-O2 configuration becomes the hollow-O2 configuration after optimization, implying that hollow-top site, as compared to Ce-top site, has stronger ability for adsorbing O2. Fig. S3 shows the optimized Ce-O configuration. From our calculated results, the absolute value of Eads for the Ce-O configuration is 1.666 eV, which is smaller than that for the hollow-O configuration of Ce-doped LaOterminated surface (1.958 eV). In other words, O is only weakly adsorbed at the Ce-top site when compared to hollow-top site. According to our findings, it is concluded that the most favorable adsorption site of LaO-terminated surface for O2 and O is not affected by Ce doping. For O2 and O adsorption on the perfect CoO2-terminated surface, the lowest energy structures are shown in Fig. 2 (c) and (d), respectively. The preferred adsorption site for both O2 and O is Co-top. The O2 and O bind to the surface Co atom with the binding distance of 1.843 Å and 1.656 Å. The dO-O is increased from 1.225 Å (free O2) to 1.273 Å in the Co-O2 configuration. As seen from the Table 1, the values of Eads for Co-O2 and Co-O configurations are −0.709 eV and − 0.223 eV, respectively. Fig. 3 presents the optimized Co-O2 and Co-O configurations of Ce-doped CoO2-terminated surface. After the substitution of La with Ce, the dO-O of adsorbed O2 molecule is slightly increased (1.277 Å vs 1.273 Å). The calculated values of Eads for the Co-O2 and Co-O configurations are −0.825 eV and − 0.248 eV, respectively, which is relatively more negative as compared to the undoped cases. This is to say that the Ce doping can improve the adsorption ability to O2 and O. It is generally known that the adsorption process is always accompanied by charge transfer [41–43]. Therefore, the charge difference between adsorbed reaction species and free reaction species is analyzed by calculating Mulliken charge (Q) and the results are summarized in Fig. 4. In this section, only Q of the lowest energy structures (i.e., hollow-O2, hollow-O, Co-O2, and Co-O) is calculated and the effect of
2. Computational methods and models In our research, all the spin-polarized density functional theory (DFT) calculations were carried out using the DMol3 modular of Materials Studio (MS 8.0) [27,28]. The exchange and correlation energy was treated with the generalized gradient approximation (GGA) and the Perdew−Burke−Ernzerhof (PBE), which was proved to be efficient in calculating the NO oxidation on the surfaces of perovskites [29–31]. Based on our previous test results [32], a global orbital cutoff of 5.0 Å and a k-point mesh of 3 × 3 × 1 were chosen and employed in the entire calculations. A double numerical plus polarization (DNP), as the basis set, was used to describe the valence orbital of all the atoms. The spin states of Co3+ ion in LaCoO3 are diverse, including low spin (LS), intermediate spin (IS) and high spin (HS) [33,34]. According to previous experiments [15,18,20], it is found that LaCoO3 can exhibit catalytic activity for NO oxidation only when the temperature is above 500 K. The spin state of Co3+ corresponds mainly to the HS for the T > 500 K range [35,36]. So it is reasonable that only the HS of Co3+ was applied in our calculations. When the convergence criteria with respect to the energy, force, and displacement, i.e., 1.0 × 10−5 Ha, 2.0 × 10−3 Ha/Å, and 5.0 × 10−3 Å, respectively, were satisfied, the structure optimization would be considered to be complete. All the transition states (TS) in the research could be obtained by using linear synchronous transit (LST)/quadratic synchronous transit (QST) method provided by the DMol3 modular. LaCoO3 (011) surface was chosen to carry out a series of studies as our previous research had showed that the (011) was the most stable among the three surfaces (i.e., (011), (111), and (010)) [32]. The three surfaces mentioned above were frequently discussed in the research of perovskites [37–39]. To model the LaCoO3 (011) surface, a 2 × 2 slab with three LaO layers and three CoO2 layers was built and a 15 Å vacuum layer was added to avoid the interaction between periodic slabs, as shown in Fig. 1(a). The top views of exposed LaO and CoO2 terminations are displayed in the Fig. 1(b) and (c), respectively. For LaOterminated surface, the adsorption sites considered in our study are Latop, O-top, and hollow-top (see Fig. 1(b)). Similarly, there are three possible adsorption sites on CoO2-terminated surface: Co-top, O-top, and hollow-top (see Fig. 1(c)). One La atom in the outmost layer was substituted with one Ce atom to model Ce-doped species. The calculated bond length for the isolated NO and O2 are 1.163 Å and 1.225 Å, respectively, which are in excellent agreement with the corresponding experimental values [40]. 3. Results and discussion 3.1. Adsorption of molecular and atomic oxygen on the undoped and Cedoped LaCoO3 (011) surfaces Before exploring the oxidation mechanism of NO, we need to investigate the adsorption behaviors of reaction species (i.e., NO, O2, and O), since it can offer valuable information about the oxidation process. The adsorption behavior of NO had been discussed in detail in our previous work [32]. Therefore, we only investigate the adsorption behavior of O2 and O in this section. In our research, the adsorption energies of O2 (Eads(O2)) and O (Eads(O)) are calculated by the following formulas: 2
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Fig. 1. (a) The optimized slab model of LaCoO3 with three LaO layers and three CoO2 layers along the (011) direction; the top views of exposed LaO termination (b) and CoO2 termination (c).
Fig. 2. The most stable adsorption structures of O2 and O on the perfect LaO-terminated surface: (a) hollow-O2 configuration and (b) hollow-O configuration. The most stable adsorption structures of O2 and O on the perfect CoO2-terminated surface: (c) Co-O2 configuration and (d) Co-O configuration.
3
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Fig. 3. The optimized (a) hollow-O2 and (b) hollow-O configurations of the Ce-doped LaO-terminated surface; the optimized (c) Co-O2 and (d) Co-O configurations of the Ce-doped CoO2-terminated surface.
According to our previous discussion, the addition of Ce can enhance the Eads of hollow-O2 and hollow-O configurations. So we figure that the Ce-doped LaO-terminated surface may achieve an increase of total transferred charge by adsorbing more O2 and O. For the Co-O2 and CoO configurations of the undoped and Ce-doped CoO2-terminated surfaces, both O2 and O also act as acceptor and electrons transfer from surface to adsorbates. The number of the electrons transferred in adsorption process for the Ce-doped CoO2-terminated surface is more than that for the perfect CoO2-terminated surface. According to the analysis result, it can be concluded that the Ce doping can improve gas sensing performance of CoO2-terminated surface due to the fact that the more of the charge-transfer in the adsorption process means the better gas sensing performance of the perovskite oxides [43]. Furthermore, it is seen from the Fig. 4 that when O2 and O are adsorbed on the undoped and Ce-doped LaO-terminated surfaces, they can cause more chargetransfer than on the undoped and Ce-doped CoO2-terminated surfaces.
Table 1 The calculated adsorption energies (eV) of the most stable adsorption configurations for O2 and O adsorption on the undoped and Ce-doped LaCoO3 (011) surfaces. Adsorbate
O2 O
Undoped LaCoO3
Ce-doped LaCoO3
LaO
CoO2
LaO
CoO2
−1.572 −1.300
−0.709 −0.223
−2.155 −1.958
−0.825 −0.248
3.2. Oxygen chemistry on the undoped and Ce-doped LaCoO3 (011) surfaces It is essential to understand the surface oxygen chemistry of LaCoO3 for elucidating the origin of its catalytic oxidation activity. In this section, we will focus our attention on the formation of surface oxygen vacancies and the dissociation of adsorbed O2. The oxygen vacancy formation energy (EVO) is defined as
E VO = Eslab − O − Eslab + EO2 /2
(3)
In Eq. (3), Eslab-O is the energy of LaCoO3 slab with an oxygen vacancy. Eslab and EO2 are the same as in Eqs. (1) and (2). The definition states that if the value of EVO is lower, the O atoms can be more easily removed from the corresponding surface. The calculated values of EVO are listed in the Table 2. From our calculations, the EVO of the perfect CoO2-terminated surface is 1.271 eV, which is much lower than that of the perfect LaO-terminated surface (4.704 eV). The result points to the fact that the O atoms of perfect CoO2-terminated surface escape more easily than those of perfect LaO-terminated surface. After Ce doping,
Fig. 4. The calculated Mulliken charge for O2 (blue circles) and O (red diamonds).
doping on the charge transfer is also discussed. From our results, when O2 and O are adsorbed on the perfect LaO-terminated surface, they act as the electron acceptor and get 0.877e and 0.860e from the surface respectively. After Ce doping, the number of electrons transferred in the adsorption process is reduced by about 0.050e and 0.179e, respectively. 4
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and hope to ascertain the contributions of doping. Fig. 6 displays the energy diagrams of NO catalytic oxidation on the undoped and Cedoped LaO-terminated surfaces based on the extra-lattice oxygen mechanism. On the perfect LaO-terminated surface, an O2 is first attached to the hollow-top site (A1 → A2) and then dissociated into two adsorbed O atoms after overcoming the TSA2,3 with energy barrier of 1.162 eV. The dissociation process of O2 (A2 → A3) is endothermic by 0.164 eV. Subsequently, the first NO molecule is bound to one O adatom and the formed NO2 is detached from surface with the desorption energy of 1.775 eV (A3 → A4 and A4 → A5). In the next step, the second NO molecule combines with the remaining O adatom to form NO2 (A5 → A6). Finally, the second NO2 overcomes a high barrier of 2.626 eV to escape from the surface, regenerating the initial surface (A6 → A1). Clearly, the desorption of the second NO2 is the rate-determining step (RDS) since its energy barrier is the highest. When the same reaction process occurs on the Ce-doped LaO-terminated surface, the dissociation process of O2 is exothermic by 0.804 eV and the energy barrier is reduced to just 0.466 eV (B2 → B3). With regard to the NO oxidation on the undoped LaO-terminated surface, the step is endothermic by 0.164 eV and the energy barrier is as high as 1.162 eV. Obviously, this step is much more easier to occur on the Ce-doped LaOterminated surface. Besides, Ce doping has lead to a reduction in the energy barrier of NO2 desorption. The desorption energies of the first and second NO2 molecules are reduced by about 0.240 eV and 0.130 eV, respectively. This is because doping weakens the adsorption ability of corresponding surface to NO2 molecule. More importantly, it is found that the desorption process of the second NO2 molecule is the RDS on the Ce-doped LaO-terminated surface, of which activation barrier is relatively lower in comparison with that on the undoped LaO-terminated surface (2.496 eV vs 2.626 eV). So it is drawn that the catalytic activity of Ce-doped LaO-terminated surface for NO oxidation is better than that of perfect LaO-terminated surface in terms of extra-lattice oxygen mechanism. A scheme of MvK mechanism for NO oxidation reaction is sketched in Fig. 7, where the energetic routes on the undoped and Ce-doped CoO2-terminated surfaces are compared to obtain further understanding of the effect of doping. For the perfect CoO2-terminated surface, the NO molecule is most likely to be adsorbed at Co-top site [32,44], which is the first step of oxidation reaction (C1 → C2). Then the adsorbed NO snatch one lattice oxygen atom to form Co-nitrite (-NO2) after overcoming the TSC2,3 with a high barrier of 2.223 eV. The reaction process (C2 → C3) corresponds to the formation of the first NO2 and it is endothermic by 0.460 eV. The subsequent step is that a NO2 molecule desorbs from the surface. The desorption process of NO2 molecule corresponds to C3 → C4 in the Fig. 7 and it is endothermic by 1.143 eV. Subsequently, the formed oxygen vacancy is occupied by an O2 molecule, recovering the defective surface and adding a protruding O atom (C4 → C5). The SI has showed that when O atom is attached to the O-top site of the perfect CoO2-terminated surface, the system is unstable. Thus, we hold that the protruding O atom should be dissociated and then binds to Co atom to form the Co-O configuration (C5 → C6). The calculated energy barrier is 0.535 eV and the reaction energy is −0.510 eV. The low energy barrier and exothermic reaction mean that the step, C5 → C6, is kinetically and thermodynamically favored. Then the second NO molecule is attached to the protruding O atom and the Co-O bond is stretched from 1.656 Å to 1.861 Å (C6 → C7). Finally, the adsorbed NO2 molecule overcomes an energy barrier of 0.954 eV to escape from the surface so that the surface returns to its initial state (C7 → C1). In the NO oxidation route of the perfect CoO2terminated surface, the RDS is the formation of the first NO2 as the activation barrier of this step is highest (2.223 eV). Next, we discuss the same oxidation pathway occurred on the Ce-doped CoO2-terminated surface. As depicted in the Fig. 7, the energy profile of the Ce-doped CoO2-terminated surface has only a slight change compared with that of the perfect surface. When NO is adsorbed on the Ce-doped surface (D2), it abstracts a surface O atom to generate D3 after overcoming a
Table 2 The oxygen vacancy formation energy (EVO) of the undoped and Ce-doped LaCoO3 (011) surface. Surface
EVO (eV)
Undoped LaCoO3
Ce-doped LaCoO3
LaO
CoO2
LaO
CoO2
4.704
1.271
4.537
1.796
the EVOof LaO-terminated surface is slightly decreased (about 0.167 eV), while that of CoO2-terminated surface is increased by about 0.525 eV. The phenomenon is similar to previous study by Choi et al. [19]. All the results lead to a clear conclusion that the formation of oxygen vacancies of the LaO-terminated surface generally consumes more energy than that of the CoO2-terminated surface. Therefore, for the LaO-terminated surface, NO oxidation via the MvK mechanism is not an appropriate choice. Next, we discuss how adsorbed O2 molecule is dissociated to produce extra-lattice oxygen atoms. Firstly, O2 dissociation processes on the perfect LaO-terminated and CoO2-terminated surfaces are discussed. In our research, the initial state (IS) in each system is the most stable adsorption structure of O2 molecule on the corresponding surface. For the perfect LaO-terminated surface, the structure that two isolated O atoms are attached to hollow-top sites is chosen as the final state (FS) due to the fact that the O-O configuration is unstable and the La-O configuration becomes hollow-O configuration after structural optimization (as discussed in SI). Similarly, the model that two dissociated O atoms bind to two surface Co atoms is selected as the FS for the perfect CoO2-terminated surface. In the Fig. 5, it can be seen that that the dissociation barrier (Edis) for the perfect LaO-terminated surface is 1.162 eV, which is lower than that for the perfect CoO2-terminated surface (2.615 eV); meanwhile, the reaction energy of this process for the perfect LaO-terminated surface is 0.164 eV, which is smaller than that for the perfect CoO2-terminated surface (0.642 eV). Therefore, we can reach the following conclusion that the dissociation on the perfect LaO-terminated surface is easier than that on the perfect CoO2terminated surface. Then, the effect of doping on O2 dissociation is investigated by exploring the same path on the Ce-doped LaO-terminated and CoO2-terminated surfaces. For easy comparison, the calculated results are also summarized in the Fig. 5. It is easy to see that the Ce doping helps reduce the Edis of LaO-terminated surface to 0.466 eV, which is beneficial for the produce of dissociated O atoms. What is more important is that the O2 dissociation becomes exothermic by 0.804 eV for the Ce-doped LaO-terminated surface. Therefore, the O2 dissociation on the Ce-doped LaO-terminated surface is kinetically and thermodynamically favored due to low Edis and high exothermicity. For the CoO2-terminated surface, doping has an adverse effect on oxygen dissociation process (see Fig. 5(b)). Compared to the perfect CoO2-terminated surface, the O2 dissociation process on the Ce-doped CoO2-terminated surface requires relatively high Edis (2.689 eV) and reaction energy (0.709 eV). Moreover, comparing Fig. 5(a) and (b), it is found that the LaO-terminated surface presents lower Edis for O2 dissociation than the CoO2-terminated surface, whether they are doped or not. So we can obtain the conclusion that the LaO-terminated surface is more likely to have activity for NO oxidation via the extra-lattice oxygen mechanism than the CoO2-terminated surface.
3.3. NO oxidation on the undoped and Ce-doped LaCoO3 (011) surfaces According to the above discussion (Section 3.2), we can draw the conclusion that the extra-lattice oxygen mechanism is more likely to occur on the undoped and Ce-doped LaO-terminated LaCoO3 (011) surfaces, while the MvK mechanism is more suitable for the undoped and Ce-doped CoO2-terminated LaCoO3 (011) surfaces. Next, we will further study the reaction routes of NO oxidation on different surfaces 5
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Fig. 5. The potential energy profiles and associated structures for the dissociation of O2 on different surfaces: (a) the undoped and Ce-doped LaO-terminated surfaces, (b) the undoped and Ce-doped CoO2-terminated surfaces.
formation of the first NO2 is still the RDS, which forms an oxygen vacancy on the Ce-doped CoO2-terminated surface. As presented in Fig. 7, it is very clear that Ce doping is bad for the whole NO oxidation, in terms of comparing the energy barrier of RDS with that on the perfect CoO2-terminated surface (2.559 eV vs 2.223 eV). The Section 3.2 has showed that Ce doping makes it more difficult for oxygen atoms to be removed from the CoO2-terminated surface, which is the main reason for the increase of energy barrier of RDS. From what has been discussed above, we can draw the conclusion that the perfect CoO2-terminated surface has better catalytic activity for NO oxidation than the Ce-doped CoO2-terminated surface in terms of MvK mechanism. From the above analysis, it is shown that the Ce-doped LaO-terminated surface has higher catalytic activity for NO oxidation than the
2.559 eV activation energy through TSD2,3. For the same step, the activation energy of the perfect surface is only 2.223 eV. Apparently, the Ce-doped surface shows lower activity than perfect surface in the process, which is greatly related to the phenomenon that the Ce doping increases the EVo of the CoO2-terminated surface. Besides, the Ce doping makes the direct desorption of two NO2 molecules a bit more difficult. The desorption energies of the first and second NO2 molecule are increased to 1.284 eV (D3 → D4) and 1.078 eV (D7 → D1), respectively, which is due to the fact that Ce doping improves adsorption ability of corresponding surface to NO2 molecule. Interestingly, the migration of the protruding O atom (D5 → D6) becomes easier compared to the perfect surface due to the lower activation energy (0.391 eV) and higher exothermicity (−0.756 eV). Obviously, the 6
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Fig. 6. The calculated potential energy diagrams of NO oxidation on the undoped and Ce-doped LaO-terminated surface based on the extra-lattice oxygen mechanism.
4. Conclusions
perfect LaO-terminated surface by assuming extra-lattice oxygen mechanism. However, assuming the MvK mechanism, the Ce-doped CoO2terminated surface is less active for NO oxidation than the perfect CoO2terminated surface. When the extra-lattice oxygen mechanism is considered on the undoped and Ce-doped LaO-terminated surface, the RDS is the desorption of the second NO molecule. All three energy barriers of NO oxidation are reduced after Ce doping. So it is demonstrated that Ce doping can improve the catalytic activity of LaCoO3 for NO oxidation through the extra-lattice oxygen mechanism on the LaO-terminated surface. When the MvK mechanism is considered on the undoped and Ce-doped CoO2-terminated surface, the formation of the first NO2 is the RDS. The RDS on the Ce-doped CoO2-terminated surface has a higher barrier compared with the undoped case. The result shows that when NO oxidation occurs on the CoO2-terminated surface via the MvK mechanism, Ce doping has a negative effect on the enhancement of surface reactivity. In other words, the effective way that Ce doping improves the catalytic performance of LaCoO3 for NO oxidation is not through the MvK mechanism on the CoO2-terminated surface. Obviously, all the analysis results manifest that the Ce doping contributes to improve catalytic activity of LaCoO3 for NO oxidation through extra-lattice oxygen mechanism on the LaO-terminated surface.
Using the DFT method, we have clarified the oxidation mechanism of NO on the undoped and Ce-doped LaCoO3 (011) surfaces. Both extralattice oxygen and MvK mechanisms were considered. The adsorption behaviors of reaction species (i.e., O2 and O) on different surfaces were first studied. The calculated results show that when reaction species are adsorbed on the perfect LaO-terminated and CoO2-terminated LaCoO3 (011) surface, the most stable adsorption sites are hollow-top and Cotop, respectively. After Ce doping, the most favorable adsorption sites are not changed, but the adsorption ability to reaction species is improved. After that, we focused our attention on the formation of surface oxygen vacancies and the dissociation of adsorbed oxygen molecule. The CoO2-terminated surface generally consumes less energy to form oxygen vacancies than the LaO-terminated surface, whether they are doped or not. The calculated activation barrier for the dissociation of O2 on the perfect LaO-terminated surface is lower than that on the perfect CoO2-terminated surface. After Ce doping, the O2 dissociation on the LaO-terminated surface becomes easier, because the dissociation barrier is reduced and the reaction becomes slightly exothermic. At the same time, the Ce doping of CoO2-terminated surface results in a slight 7
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Fig. 7. The calculated potential energy diagrams of NO oxidation on the undoped and Ce-doped CoO2-terminated surface based on the MvK mechanism.
end Talent Introduction "two-hundred plans" Foundation of Yantai.
increase of dissociation barrier. The above results demonstrate that extra-lattice oxygen mechanism is feasible on the undoped and Cedoped LaO-terminated surfaces, while MvK mechanism is more preferable on the undoped and Ce-doped CoO2-terminated surfaces. Based on the above, we further investigated the reaction pathways of NO catalytic oxidation on these four surfaces. The RDS steps for the perfect LaO-terminated and CoO2-terminated surfaces are the desorption of the second NO2 molecule and formation of the first NO2, respectively. The RDS on the Ce-doped surface is the same as that on the perfect surface. Moreover, our findings in this study suggest that the Ce doping contributes to improve catalytic activity of LaCoO3 for NO oxidation through extra-lattice oxygen mechanism on the LaO-terminated surface.
Declaration of competing interest The authors declare no competing financial interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.143866. References [1] S. Roy, A. Baiker, NOx storage-reduction catalysis: from mechanism and materials properties to storage-reduction performance, Chem. Rev. 109 (2009) 4054–4091. [2] F. Chen, H. Zhou, J. Gao, P.K. Hopke, A chamber study of Secondary Organic Aerosol (SOA) formed by ozonolysis of d-limonene in the presence of NO, Aerosol Air Qual. Res. 17 (2017) 59–68. [3] N. Takahashi, H. Shinjoh, T. Iijima, T. Suzuki, K. Yamazaki, K. Yokota, H. Suzuki, N. Miyoshi, S. Matsumoto, T. Tanizawa, T. Tanaka, S. Tateishi, K. Kasahara, The new concept 3-way catalyst for automotive lean-burn engine: NOx storage and reduction catalyst, Catal. Today 27 (1996) 63–69.
Acknowledgements This work was financially supported by the High-end Talent Team Construction Foundation (Grant No. 108-10000318 and Natural Science Foundation of Shandong, China (Grant No. ZR201808030032). This work was also financially supported by the High8
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Influence of Ce doping on catalytic oxidation of NO on LaCoO3 (011) surface: A DFT study Xiaochen Lib, Hongwei Gaoa,
T
⁎
a
School of Life Science, Ludong University, Yantai, Shandong 264025, China Key Laboratory of Plant Resources and Chemistry in Arid Regions, Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Urumqi 830011, China
b
A R T I C LE I N FO
A B S T R A C T
Keywords: DFT calculations Adsorption energy Oxygen chemistry NO oxidation Ce doping
The catalytic oxidation of NO on the undoped and Ce-doped LaCoO3 (011) surfaces has been systematically studied by utilizing the density functional theory (DFT) method. The extra-lattice oxygen and Mars−van Krevelen (MvK) mechanisms of NO oxidation are considered. According to our calculations, the extra-lattice oxygen mechanism is more likely to occur on the undoped and Ce-doped LaO-terminated LaCoO3 (011) surfaces, while the MvK mechanism is more suitable for the undoped and Ce-doped CoO2-terminated LaCoO3 (011) surfaces. By comparing and analyzing the same NO oxidation pathway on the undoped and Ce-doped surfaces, it is proved that the Ce doping is favorable for the LaO-terminated LaCoO3 (011) surface, but harmful for the CoO2terminated LaCoO3 (011) surface. In our research on NO oxidation, the addition of Ce dopant contributes to the LaO-terminated LaCoO3 (011) surface by reducing the energy barriers of O2 dissociation and NO2 desorption.
1. Introduction Nitrogen oxides (NOx, x = 1, 2) which are mainly emitted from motor vehicles and coal-fired power factories have brought tremendous threats to human health and environment [1,2]. To reduce the harm caused by NOx, various aftertreatment technologies have been developed, such as selective catalytic reduction (SCR) [3,4] and lean NOx traps (LNT) [5,6]. NO oxidation to NO2 is regarded as a key step in the two technologies mentioned above, since presence of NO2 in the NO/ NO2 mixture can substantially improve the reaction rate of SCR and NO2 is easier to be adsorbed on the surface of LNT catalysts than NO [6–10]. Platinum-based catalysts show promising catalytic performance for oxidation of NO to NO2 [11–14]. However, they are high in price and easily agglomerated during the reaction [15,16], which restricts seriously their large-scale application. This has stimulated a number of researchers to make great efforts on finding substitute materials with low cost and high stability for NO oxidation. In recent decades, perovskites with ABO3 structure, especially the LaCoO3, have received great attention as a substitute for Platinumbased catalysts. According to the paper [17], LaCoO3 achieves maximum NO conversion of 71% at 300 °C. Its catalytic performance can be improved by doping suitable transition metals. For example, Wen et al. [18] have reported that the activity of LaCoO3 was substantially enhanced with 20% substitution of cerium (Ce), reaching the highest ⁎
conversion of 80% at about 300 °C. Kim et al. [15] studied the NO oxidation activity of strontium (Sr) doped LaCoO3 samples and found that when Sr doping amount is 0.1, it achieved better oxidation activity than the commercial Pt/γ-Al2O3 catalysts. It is clear that understanding how doping improves the catalytic activity of LaCoO3 for NO oxidation can provide valuable referencing information for the development of perovskite-type catalysts. Therefore, more and more researchers began to study oxidation processes of NO on the undoped and doped LaCoO3 surfaces by using experimental and theoretical methods [18–22]. This is not an easy task, as many reaction routes are feasible in theory. In general, the oxidation mechanisms of NO on LaCoO3 surface can be summarized to two kinds, i.e., extra-lattice oxygen mechanism, in which an adsorbed O2 is dissociated into two isolated O adatoms and then NO molecules bind to each O adatom; and Mars−van Krevelen (MvK) mechanism, in which NO molecules are oxidized by lattice oxygen atoms and the role of O2 is to refill the vacancies by adsorption and solid state diffusion [19–26]. Choi et al. [19] proposed that the extra-lattice oxygen of LaO-terminated surface was likely available for NO oxidation via the first mechanism and its activity was enhanced by Sr doping. An et al. [22] pointed out that for Fe-doped LaCoO3, NO oxidation reaction proceeded via the second mechanism, where NO molecule was bounded to the lattice oxygen and the formed NO2 molecule was subsequently desorbed from the surface. However, to our knowledge, it is never clearly explained why the catalytic activity of
Corresponding author. E-mail address: [email protected] (H. Gao).
https://doi.org/10.1016/j.apsusc.2019.143866 Received 14 March 2019; Received in revised form 4 July 2019; Accepted 3 September 2019 Available online 04 September 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 499 (2020) 143866
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LaCoO3 for NO oxidation can be enhanced by doping Ce. In this paper, we first investigated the adsorption behaviors of oxygen molecule (O2) and atomic oxygen (O) on the undoped and Ce-doped surfaces. Secondly, the formation of surface oxygen vacancies and the dissociation of adsorbed O2 could be discussed in detail. Finally, we systematically investigated the possible oxidation pathways and made further explanation on the reasons why catalytic activity of LaCoO3 was improved after Ce doping.
Eads (O2) = Eslab + O2 − Eslab − EO2
(1)
Eads (O) = Eslab + O − Eslab − EO2 /2
(2)
In the above two formulas, Eslab refers to the energy of undoped or Ce-doped LaCoO3 slab; Eslab+O2 and Eslab+O are the total energies of a slab with an adsorbed O2 and a slab with an adsorbed O, respectively; EO2 represents the energy of a free O2 molecule. According to the above definition, when the value of Eads is positive, it means that the adsorption process is endothermic and adsorption structure is unstable; when the value of Eads is negative, it means that the adsorption process is exothermic and adsorption structure is stable. In addition, the smaller the value of Eads, the more stable the adsorption structure. Fig. 2 shows the most stable adsorption structures on the perfect LaO-terminated and CoO2-terminated surfaces. After doping Ce, the corresponding adsorption structures are displayed in Fig. 3. The calculated Eads and Mulliken charge (Q) are summarized in Table 1 and Fig. 4, respectively. The detailed calculation results can be found in the Supporting Information (SI). For the perfect LaO-terminated surface, both O2 and O preferentially adsorb at the hollow-top site. Fig. 2(a) shows the most stable adsorption configuration for molecular oxygen. The O-O bond length (dO-O) for adsorbed O2 is found to be 1.419 Å. The Eads(O2) calculated by Eq. (1) is −1.572 eV. Atomic O also prefers to associate with the hollow-top site of perfect LaO-terminated surface, as shown in Fig. 2(b). According to our calculations, the Eads(O) is −1.300 eV. Next we substitute one La atom of the outmost LaO slab with one Ce atom, corresponding to the dopant concentration of 0.042, to explore the influences of Ce doping. The hollow-O2 and hollow-O configurations of Ce-doped LaO-terminated surface are displayed in the Fig. 3(a) and (b), respectively. The dO-O of adsorbed O2 is 1.440 Å. After Ce doping, the absolute value of Eads for O2 and O is increased by 0.583 eV and 0.658 eV, respectively, indicating that the reaction species in our study are more easily adsorbed on the doped surface. When reaction species are adsorbed on the Ce-doped LaO-terminated surface, the Ce-top site should also be considered as a possible adsorption site. Therefore, we construct the initial adsorption configurations that O2 and O are attached to Ce-top sites. Interestingly, the initial Ce-O2 configuration becomes the hollow-O2 configuration after optimization, implying that hollow-top site, as compared to Ce-top site, has stronger ability for adsorbing O2. Fig. S3 shows the optimized Ce-O configuration. From our calculated results, the absolute value of Eads for the Ce-O configuration is 1.666 eV, which is smaller than that for the hollow-O configuration of Ce-doped LaOterminated surface (1.958 eV). In other words, O is only weakly adsorbed at the Ce-top site when compared to hollow-top site. According to our findings, it is concluded that the most favorable adsorption site of LaO-terminated surface for O2 and O is not affected by Ce doping. For O2 and O adsorption on the perfect CoO2-terminated surface, the lowest energy structures are shown in Fig. 2 (c) and (d), respectively. The preferred adsorption site for both O2 and O is Co-top. The O2 and O bind to the surface Co atom with the binding distance of 1.843 Å and 1.656 Å. The dO-O is increased from 1.225 Å (free O2) to 1.273 Å in the Co-O2 configuration. As seen from the Table 1, the values of Eads for Co-O2 and Co-O configurations are −0.709 eV and − 0.223 eV, respectively. Fig. 3 presents the optimized Co-O2 and Co-O configurations of Ce-doped CoO2-terminated surface. After the substitution of La with Ce, the dO-O of adsorbed O2 molecule is slightly increased (1.277 Å vs 1.273 Å). The calculated values of Eads for the Co-O2 and Co-O configurations are −0.825 eV and − 0.248 eV, respectively, which is relatively more negative as compared to the undoped cases. This is to say that the Ce doping can improve the adsorption ability to O2 and O. It is generally known that the adsorption process is always accompanied by charge transfer [41–43]. Therefore, the charge difference between adsorbed reaction species and free reaction species is analyzed by calculating Mulliken charge (Q) and the results are summarized in Fig. 4. In this section, only Q of the lowest energy structures (i.e., hollow-O2, hollow-O, Co-O2, and Co-O) is calculated and the effect of
2. Computational methods and models In our research, all the spin-polarized density functional theory (DFT) calculations were carried out using the DMol3 modular of Materials Studio (MS 8.0) [27,28]. The exchange and correlation energy was treated with the generalized gradient approximation (GGA) and the Perdew−Burke−Ernzerhof (PBE), which was proved to be efficient in calculating the NO oxidation on the surfaces of perovskites [29–31]. Based on our previous test results [32], a global orbital cutoff of 5.0 Å and a k-point mesh of 3 × 3 × 1 were chosen and employed in the entire calculations. A double numerical plus polarization (DNP), as the basis set, was used to describe the valence orbital of all the atoms. The spin states of Co3+ ion in LaCoO3 are diverse, including low spin (LS), intermediate spin (IS) and high spin (HS) [33,34]. According to previous experiments [15,18,20], it is found that LaCoO3 can exhibit catalytic activity for NO oxidation only when the temperature is above 500 K. The spin state of Co3+ corresponds mainly to the HS for the T > 500 K range [35,36]. So it is reasonable that only the HS of Co3+ was applied in our calculations. When the convergence criteria with respect to the energy, force, and displacement, i.e., 1.0 × 10−5 Ha, 2.0 × 10−3 Ha/Å, and 5.0 × 10−3 Å, respectively, were satisfied, the structure optimization would be considered to be complete. All the transition states (TS) in the research could be obtained by using linear synchronous transit (LST)/quadratic synchronous transit (QST) method provided by the DMol3 modular. LaCoO3 (011) surface was chosen to carry out a series of studies as our previous research had showed that the (011) was the most stable among the three surfaces (i.e., (011), (111), and (010)) [32]. The three surfaces mentioned above were frequently discussed in the research of perovskites [37–39]. To model the LaCoO3 (011) surface, a 2 × 2 slab with three LaO layers and three CoO2 layers was built and a 15 Å vacuum layer was added to avoid the interaction between periodic slabs, as shown in Fig. 1(a). The top views of exposed LaO and CoO2 terminations are displayed in the Fig. 1(b) and (c), respectively. For LaOterminated surface, the adsorption sites considered in our study are Latop, O-top, and hollow-top (see Fig. 1(b)). Similarly, there are three possible adsorption sites on CoO2-terminated surface: Co-top, O-top, and hollow-top (see Fig. 1(c)). One La atom in the outmost layer was substituted with one Ce atom to model Ce-doped species. The calculated bond length for the isolated NO and O2 are 1.163 Å and 1.225 Å, respectively, which are in excellent agreement with the corresponding experimental values [40]. 3. Results and discussion 3.1. Adsorption of molecular and atomic oxygen on the undoped and Cedoped LaCoO3 (011) surfaces Before exploring the oxidation mechanism of NO, we need to investigate the adsorption behaviors of reaction species (i.e., NO, O2, and O), since it can offer valuable information about the oxidation process. The adsorption behavior of NO had been discussed in detail in our previous work [32]. Therefore, we only investigate the adsorption behavior of O2 and O in this section. In our research, the adsorption energies of O2 (Eads(O2)) and O (Eads(O)) are calculated by the following formulas: 2
Applied Surface Science 499 (2020) 143866
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Fig. 1. (a) The optimized slab model of LaCoO3 with three LaO layers and three CoO2 layers along the (011) direction; the top views of exposed LaO termination (b) and CoO2 termination (c).
Fig. 2. The most stable adsorption structures of O2 and O on the perfect LaO-terminated surface: (a) hollow-O2 configuration and (b) hollow-O configuration. The most stable adsorption structures of O2 and O on the perfect CoO2-terminated surface: (c) Co-O2 configuration and (d) Co-O configuration.
3
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Fig. 3. The optimized (a) hollow-O2 and (b) hollow-O configurations of the Ce-doped LaO-terminated surface; the optimized (c) Co-O2 and (d) Co-O configurations of the Ce-doped CoO2-terminated surface.
According to our previous discussion, the addition of Ce can enhance the Eads of hollow-O2 and hollow-O configurations. So we figure that the Ce-doped LaO-terminated surface may achieve an increase of total transferred charge by adsorbing more O2 and O. For the Co-O2 and CoO configurations of the undoped and Ce-doped CoO2-terminated surfaces, both O2 and O also act as acceptor and electrons transfer from surface to adsorbates. The number of the electrons transferred in adsorption process for the Ce-doped CoO2-terminated surface is more than that for the perfect CoO2-terminated surface. According to the analysis result, it can be concluded that the Ce doping can improve gas sensing performance of CoO2-terminated surface due to the fact that the more of the charge-transfer in the adsorption process means the better gas sensing performance of the perovskite oxides [43]. Furthermore, it is seen from the Fig. 4 that when O2 and O are adsorbed on the undoped and Ce-doped LaO-terminated surfaces, they can cause more chargetransfer than on the undoped and Ce-doped CoO2-terminated surfaces.
Table 1 The calculated adsorption energies (eV) of the most stable adsorption configurations for O2 and O adsorption on the undoped and Ce-doped LaCoO3 (011) surfaces. Adsorbate
O2 O
Undoped LaCoO3
Ce-doped LaCoO3
LaO
CoO2
LaO
CoO2
−1.572 −1.300
−0.709 −0.223
−2.155 −1.958
−0.825 −0.248
3.2. Oxygen chemistry on the undoped and Ce-doped LaCoO3 (011) surfaces It is essential to understand the surface oxygen chemistry of LaCoO3 for elucidating the origin of its catalytic oxidation activity. In this section, we will focus our attention on the formation of surface oxygen vacancies and the dissociation of adsorbed O2. The oxygen vacancy formation energy (EVO) is defined as
E VO = Eslab − O − Eslab + EO2 /2
(3)
In Eq. (3), Eslab-O is the energy of LaCoO3 slab with an oxygen vacancy. Eslab and EO2 are the same as in Eqs. (1) and (2). The definition states that if the value of EVO is lower, the O atoms can be more easily removed from the corresponding surface. The calculated values of EVO are listed in the Table 2. From our calculations, the EVO of the perfect CoO2-terminated surface is 1.271 eV, which is much lower than that of the perfect LaO-terminated surface (4.704 eV). The result points to the fact that the O atoms of perfect CoO2-terminated surface escape more easily than those of perfect LaO-terminated surface. After Ce doping,
Fig. 4. The calculated Mulliken charge for O2 (blue circles) and O (red diamonds).
doping on the charge transfer is also discussed. From our results, when O2 and O are adsorbed on the perfect LaO-terminated surface, they act as the electron acceptor and get 0.877e and 0.860e from the surface respectively. After Ce doping, the number of electrons transferred in the adsorption process is reduced by about 0.050e and 0.179e, respectively. 4
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and hope to ascertain the contributions of doping. Fig. 6 displays the energy diagrams of NO catalytic oxidation on the undoped and Cedoped LaO-terminated surfaces based on the extra-lattice oxygen mechanism. On the perfect LaO-terminated surface, an O2 is first attached to the hollow-top site (A1 → A2) and then dissociated into two adsorbed O atoms after overcoming the TSA2,3 with energy barrier of 1.162 eV. The dissociation process of O2 (A2 → A3) is endothermic by 0.164 eV. Subsequently, the first NO molecule is bound to one O adatom and the formed NO2 is detached from surface with the desorption energy of 1.775 eV (A3 → A4 and A4 → A5). In the next step, the second NO molecule combines with the remaining O adatom to form NO2 (A5 → A6). Finally, the second NO2 overcomes a high barrier of 2.626 eV to escape from the surface, regenerating the initial surface (A6 → A1). Clearly, the desorption of the second NO2 is the rate-determining step (RDS) since its energy barrier is the highest. When the same reaction process occurs on the Ce-doped LaO-terminated surface, the dissociation process of O2 is exothermic by 0.804 eV and the energy barrier is reduced to just 0.466 eV (B2 → B3). With regard to the NO oxidation on the undoped LaO-terminated surface, the step is endothermic by 0.164 eV and the energy barrier is as high as 1.162 eV. Obviously, this step is much more easier to occur on the Ce-doped LaOterminated surface. Besides, Ce doping has lead to a reduction in the energy barrier of NO2 desorption. The desorption energies of the first and second NO2 molecules are reduced by about 0.240 eV and 0.130 eV, respectively. This is because doping weakens the adsorption ability of corresponding surface to NO2 molecule. More importantly, it is found that the desorption process of the second NO2 molecule is the RDS on the Ce-doped LaO-terminated surface, of which activation barrier is relatively lower in comparison with that on the undoped LaO-terminated surface (2.496 eV vs 2.626 eV). So it is drawn that the catalytic activity of Ce-doped LaO-terminated surface for NO oxidation is better than that of perfect LaO-terminated surface in terms of extra-lattice oxygen mechanism. A scheme of MvK mechanism for NO oxidation reaction is sketched in Fig. 7, where the energetic routes on the undoped and Ce-doped CoO2-terminated surfaces are compared to obtain further understanding of the effect of doping. For the perfect CoO2-terminated surface, the NO molecule is most likely to be adsorbed at Co-top site [32,44], which is the first step of oxidation reaction (C1 → C2). Then the adsorbed NO snatch one lattice oxygen atom to form Co-nitrite (-NO2) after overcoming the TSC2,3 with a high barrier of 2.223 eV. The reaction process (C2 → C3) corresponds to the formation of the first NO2 and it is endothermic by 0.460 eV. The subsequent step is that a NO2 molecule desorbs from the surface. The desorption process of NO2 molecule corresponds to C3 → C4 in the Fig. 7 and it is endothermic by 1.143 eV. Subsequently, the formed oxygen vacancy is occupied by an O2 molecule, recovering the defective surface and adding a protruding O atom (C4 → C5). The SI has showed that when O atom is attached to the O-top site of the perfect CoO2-terminated surface, the system is unstable. Thus, we hold that the protruding O atom should be dissociated and then binds to Co atom to form the Co-O configuration (C5 → C6). The calculated energy barrier is 0.535 eV and the reaction energy is −0.510 eV. The low energy barrier and exothermic reaction mean that the step, C5 → C6, is kinetically and thermodynamically favored. Then the second NO molecule is attached to the protruding O atom and the Co-O bond is stretched from 1.656 Å to 1.861 Å (C6 → C7). Finally, the adsorbed NO2 molecule overcomes an energy barrier of 0.954 eV to escape from the surface so that the surface returns to its initial state (C7 → C1). In the NO oxidation route of the perfect CoO2terminated surface, the RDS is the formation of the first NO2 as the activation barrier of this step is highest (2.223 eV). Next, we discuss the same oxidation pathway occurred on the Ce-doped CoO2-terminated surface. As depicted in the Fig. 7, the energy profile of the Ce-doped CoO2-terminated surface has only a slight change compared with that of the perfect surface. When NO is adsorbed on the Ce-doped surface (D2), it abstracts a surface O atom to generate D3 after overcoming a
Table 2 The oxygen vacancy formation energy (EVO) of the undoped and Ce-doped LaCoO3 (011) surface. Surface
EVO (eV)
Undoped LaCoO3
Ce-doped LaCoO3
LaO
CoO2
LaO
CoO2
4.704
1.271
4.537
1.796
the EVOof LaO-terminated surface is slightly decreased (about 0.167 eV), while that of CoO2-terminated surface is increased by about 0.525 eV. The phenomenon is similar to previous study by Choi et al. [19]. All the results lead to a clear conclusion that the formation of oxygen vacancies of the LaO-terminated surface generally consumes more energy than that of the CoO2-terminated surface. Therefore, for the LaO-terminated surface, NO oxidation via the MvK mechanism is not an appropriate choice. Next, we discuss how adsorbed O2 molecule is dissociated to produce extra-lattice oxygen atoms. Firstly, O2 dissociation processes on the perfect LaO-terminated and CoO2-terminated surfaces are discussed. In our research, the initial state (IS) in each system is the most stable adsorption structure of O2 molecule on the corresponding surface. For the perfect LaO-terminated surface, the structure that two isolated O atoms are attached to hollow-top sites is chosen as the final state (FS) due to the fact that the O-O configuration is unstable and the La-O configuration becomes hollow-O configuration after structural optimization (as discussed in SI). Similarly, the model that two dissociated O atoms bind to two surface Co atoms is selected as the FS for the perfect CoO2-terminated surface. In the Fig. 5, it can be seen that that the dissociation barrier (Edis) for the perfect LaO-terminated surface is 1.162 eV, which is lower than that for the perfect CoO2-terminated surface (2.615 eV); meanwhile, the reaction energy of this process for the perfect LaO-terminated surface is 0.164 eV, which is smaller than that for the perfect CoO2-terminated surface (0.642 eV). Therefore, we can reach the following conclusion that the dissociation on the perfect LaO-terminated surface is easier than that on the perfect CoO2terminated surface. Then, the effect of doping on O2 dissociation is investigated by exploring the same path on the Ce-doped LaO-terminated and CoO2-terminated surfaces. For easy comparison, the calculated results are also summarized in the Fig. 5. It is easy to see that the Ce doping helps reduce the Edis of LaO-terminated surface to 0.466 eV, which is beneficial for the produce of dissociated O atoms. What is more important is that the O2 dissociation becomes exothermic by 0.804 eV for the Ce-doped LaO-terminated surface. Therefore, the O2 dissociation on the Ce-doped LaO-terminated surface is kinetically and thermodynamically favored due to low Edis and high exothermicity. For the CoO2-terminated surface, doping has an adverse effect on oxygen dissociation process (see Fig. 5(b)). Compared to the perfect CoO2-terminated surface, the O2 dissociation process on the Ce-doped CoO2-terminated surface requires relatively high Edis (2.689 eV) and reaction energy (0.709 eV). Moreover, comparing Fig. 5(a) and (b), it is found that the LaO-terminated surface presents lower Edis for O2 dissociation than the CoO2-terminated surface, whether they are doped or not. So we can obtain the conclusion that the LaO-terminated surface is more likely to have activity for NO oxidation via the extra-lattice oxygen mechanism than the CoO2-terminated surface.
3.3. NO oxidation on the undoped and Ce-doped LaCoO3 (011) surfaces According to the above discussion (Section 3.2), we can draw the conclusion that the extra-lattice oxygen mechanism is more likely to occur on the undoped and Ce-doped LaO-terminated LaCoO3 (011) surfaces, while the MvK mechanism is more suitable for the undoped and Ce-doped CoO2-terminated LaCoO3 (011) surfaces. Next, we will further study the reaction routes of NO oxidation on different surfaces 5
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Fig. 5. The potential energy profiles and associated structures for the dissociation of O2 on different surfaces: (a) the undoped and Ce-doped LaO-terminated surfaces, (b) the undoped and Ce-doped CoO2-terminated surfaces.
formation of the first NO2 is still the RDS, which forms an oxygen vacancy on the Ce-doped CoO2-terminated surface. As presented in Fig. 7, it is very clear that Ce doping is bad for the whole NO oxidation, in terms of comparing the energy barrier of RDS with that on the perfect CoO2-terminated surface (2.559 eV vs 2.223 eV). The Section 3.2 has showed that Ce doping makes it more difficult for oxygen atoms to be removed from the CoO2-terminated surface, which is the main reason for the increase of energy barrier of RDS. From what has been discussed above, we can draw the conclusion that the perfect CoO2-terminated surface has better catalytic activity for NO oxidation than the Ce-doped CoO2-terminated surface in terms of MvK mechanism. From the above analysis, it is shown that the Ce-doped LaO-terminated surface has higher catalytic activity for NO oxidation than the
2.559 eV activation energy through TSD2,3. For the same step, the activation energy of the perfect surface is only 2.223 eV. Apparently, the Ce-doped surface shows lower activity than perfect surface in the process, which is greatly related to the phenomenon that the Ce doping increases the EVo of the CoO2-terminated surface. Besides, the Ce doping makes the direct desorption of two NO2 molecules a bit more difficult. The desorption energies of the first and second NO2 molecule are increased to 1.284 eV (D3 → D4) and 1.078 eV (D7 → D1), respectively, which is due to the fact that Ce doping improves adsorption ability of corresponding surface to NO2 molecule. Interestingly, the migration of the protruding O atom (D5 → D6) becomes easier compared to the perfect surface due to the lower activation energy (0.391 eV) and higher exothermicity (−0.756 eV). Obviously, the 6
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Fig. 6. The calculated potential energy diagrams of NO oxidation on the undoped and Ce-doped LaO-terminated surface based on the extra-lattice oxygen mechanism.
4. Conclusions
perfect LaO-terminated surface by assuming extra-lattice oxygen mechanism. However, assuming the MvK mechanism, the Ce-doped CoO2terminated surface is less active for NO oxidation than the perfect CoO2terminated surface. When the extra-lattice oxygen mechanism is considered on the undoped and Ce-doped LaO-terminated surface, the RDS is the desorption of the second NO molecule. All three energy barriers of NO oxidation are reduced after Ce doping. So it is demonstrated that Ce doping can improve the catalytic activity of LaCoO3 for NO oxidation through the extra-lattice oxygen mechanism on the LaO-terminated surface. When the MvK mechanism is considered on the undoped and Ce-doped CoO2-terminated surface, the formation of the first NO2 is the RDS. The RDS on the Ce-doped CoO2-terminated surface has a higher barrier compared with the undoped case. The result shows that when NO oxidation occurs on the CoO2-terminated surface via the MvK mechanism, Ce doping has a negative effect on the enhancement of surface reactivity. In other words, the effective way that Ce doping improves the catalytic performance of LaCoO3 for NO oxidation is not through the MvK mechanism on the CoO2-terminated surface. Obviously, all the analysis results manifest that the Ce doping contributes to improve catalytic activity of LaCoO3 for NO oxidation through extra-lattice oxygen mechanism on the LaO-terminated surface.
Using the DFT method, we have clarified the oxidation mechanism of NO on the undoped and Ce-doped LaCoO3 (011) surfaces. Both extralattice oxygen and MvK mechanisms were considered. The adsorption behaviors of reaction species (i.e., O2 and O) on different surfaces were first studied. The calculated results show that when reaction species are adsorbed on the perfect LaO-terminated and CoO2-terminated LaCoO3 (011) surface, the most stable adsorption sites are hollow-top and Cotop, respectively. After Ce doping, the most favorable adsorption sites are not changed, but the adsorption ability to reaction species is improved. After that, we focused our attention on the formation of surface oxygen vacancies and the dissociation of adsorbed oxygen molecule. The CoO2-terminated surface generally consumes less energy to form oxygen vacancies than the LaO-terminated surface, whether they are doped or not. The calculated activation barrier for the dissociation of O2 on the perfect LaO-terminated surface is lower than that on the perfect CoO2-terminated surface. After Ce doping, the O2 dissociation on the LaO-terminated surface becomes easier, because the dissociation barrier is reduced and the reaction becomes slightly exothermic. At the same time, the Ce doping of CoO2-terminated surface results in a slight 7
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Fig. 7. The calculated potential energy diagrams of NO oxidation on the undoped and Ce-doped CoO2-terminated surface based on the MvK mechanism.
end Talent Introduction "two-hundred plans" Foundation of Yantai.
increase of dissociation barrier. The above results demonstrate that extra-lattice oxygen mechanism is feasible on the undoped and Cedoped LaO-terminated surfaces, while MvK mechanism is more preferable on the undoped and Ce-doped CoO2-terminated surfaces. Based on the above, we further investigated the reaction pathways of NO catalytic oxidation on these four surfaces. The RDS steps for the perfect LaO-terminated and CoO2-terminated surfaces are the desorption of the second NO2 molecule and formation of the first NO2, respectively. The RDS on the Ce-doped surface is the same as that on the perfect surface. Moreover, our findings in this study suggest that the Ce doping contributes to improve catalytic activity of LaCoO3 for NO oxidation through extra-lattice oxygen mechanism on the LaO-terminated surface.
Declaration of competing interest The authors declare no competing financial interest. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apsusc.2019.143866. References [1] S. Roy, A. Baiker, NOx storage-reduction catalysis: from mechanism and materials properties to storage-reduction performance, Chem. Rev. 109 (2009) 4054–4091. [2] F. Chen, H. Zhou, J. Gao, P.K. Hopke, A chamber study of Secondary Organic Aerosol (SOA) formed by ozonolysis of d-limonene in the presence of NO, Aerosol Air Qual. Res. 17 (2017) 59–68. [3] N. Takahashi, H. Shinjoh, T. Iijima, T. Suzuki, K. Yamazaki, K. Yokota, H. Suzuki, N. Miyoshi, S. Matsumoto, T. Tanizawa, T. Tanaka, S. Tateishi, K. Kasahara, The new concept 3-way catalyst for automotive lean-burn engine: NOx storage and reduction catalyst, Catal. Today 27 (1996) 63–69.
Acknowledgements This work was financially supported by the High-end Talent Team Construction Foundation (Grant No. 108-10000318 and Natural Science Foundation of Shandong, China (Grant No. ZR201808030032). This work was also financially supported by the High8
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