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NO adsorption and dissociation on palladium clusters: The importance of charged state and metal doping
Chemical Physics Letters 658 (2016) 7–11
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NO adsorption and dissociation on palladium clusters: The importance of charged state and metal doping Yang Gao a,b, Li Mei Zhang a,b, Chun Cai Kong b, Zhi Mao Yang b,⇑, Yong Mei Chen a,b,⇑ a State Key Laboratory for Strength and Vibration of Mechanical Structures, International Center for Applied Mechanics and School of Aerospace, Xi’an Jiaotong University, Xi’an 710049, China b School of Science, MOE Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China
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Article history: Received 9 March 2016 In final form 24 May 2016 Available online 30 May 2016 Keywords: NO dissociation Pd13 Nanoclusters DFT Charge transfer
a b s t r a c t The NO adsorption and dissociation on neutral, charged and Ni-doped Pd13 clusters were studied by using density functional calculations. Our results revealed that NO always prefers to adsorb on the hollow site rather than the top or bridge sites. However, the charge state and Ni doping remarkably influence NO adsorption energy, dissociation barrier and reaction energy. The reaction on Pd 13 has the lowest energy barrier and largest reaction energy. The Hirshfeld charge analysis discloses that the origin of the catalytic activity difference is the charge transfer from clusters to NO in the metastable NO adsorption state. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen monoxide (NO) is an important pollutant produced from industrial and automotive exhausts. It is the major component of photochemical smog, which can cause acid rain, the ozone layer destruction and global warming [1,2]. Therefore, the removal of NO from automotive exhaust gases, which usually involves the conversion NO to molecular dinitrogen, has attracted wide attention in the last several decades. The three-way catalysts (TWC), in which precious metals Pt, Pd and Rh serve as efficient catalysts, are usually used to complete the catalytic reaction of NO [3–6]. Because Pd is cheaper, highly abundant and has good thermal stability, many experimental studies have been performed on the selective NO reduction using Pd as the catalysis up to now [7–18]. For example, de Wolf et al. studied the NO–H2 reaction on the clean Pd(1 1 1) surface [16]. Ma and Matsushima investigated the surface nitrogen removal pathways of NO + H2 reaction on Pd(1 1 0) [17]. Moreover, with the infrared reflection absorption spectroscopy, Goodman and coworkers focused the reaction of NO and CO on Pd single crystals, and revealed that the activity is ⇑ Corresponding authors at: State Key Laboratory for Strength and Vibration of Mechanical Structures, International Center for Applied Mechanics and School of Aerospace, Xi’an Jiaotong University, Xi’an 710049, China (Y.M. Chen). E-mail addresses: [email protected] (Z.M. Yang), [email protected]. cn (Y.M. Chen). http://dx.doi.org/10.1016/j.cplett.2016.05.048 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
increasing from more open surfaces to closed packed ones (Pd (1 0 0) < Pd(1 1 0) < Pd(1 1 1)) [7]. Compared with the Pd bulk, Pd nanoclusters have quite different structures and electronic properties, which makes them attractive for catalysis [19–22]. Lots of experimental and theoretical investigations have been devoted to the NO adsorption, dissociation and reduction reaction on Pd nanoclusters. For example, the interaction of NO with Pd metal cluster models (up to 13 atoms) with different size and geometry was studied by Duarte and Salahub [10]. Heiz et al. have extended these measurements to Pdn (n 6 30) clusters in reaction of NO and CO [23]. Very recently, Liu and coworkers theoretically studied the NO adsorption and dissociation on Pdn (n = 8, 13, 19, 25) clusters [24]. Three important findings are disclosed through above research: (i) the dissociation of NO bond is the key step in the NO reduction; (ii) the NO reduction activity is highly sensitive to the cluster geometries; (iii) the charge transfer takes place from the Pdn clusters to NO and increases upon NO dissociation. On the other hand, with ion beam and ion cyclotron resonance methods, many cationic and anionic Pd clusters have been synthesized experimentally and used to investigate their reactions with molecules like oxygen, nitrogen oxides, and n-butane [25–31]. Furthermore, Pd clusters supported on the metal oxides or coated by ligands are revealed to be charged rather than neutral [32,33]. With theoretical calculations, adsorption of NO or CO on charged clusters has been widely studied [34–38]. For example, Kalita
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Y. Gao et al. / Chemical Physics Letters 658 (2016) 7–11
and Deka performed density functional calculations (DFT) on CO adsorption on neutral and charged Pdn (n = 1–7) clusters [35]. Castro and coworkers calculated the adsorption energies of CO on neutral and charged Pd13 clusters and found that the Pd 13 can increase the adsorption energy [37]. Very recently, the nitric oxides adsorption on neutral and charged Pdn (n = 1–5) clusters were theoretically investigated [36]. The adsorption energy trend of NO on clusters is revealed to be anionic > neutral > cationic. Furthermore, Su et al. reported a DFT study on the catalytic reduction of NO with CO on positively charged Rh+4 clusters [34]. Even though several theoretical studies focused on the structural sensitivity of NO reduction, as far as we know, the effect of the charged states of Pd clusters on NO dissociation has never been discussed up to now. It has been found that the charge transfer takes place from the Pdn clusters to NO, which activates the adsorbed NO molecule [24]. Therefore, the charged state of Pd clusters might significantly influence NO dissociation. Moreover, it has been revealed in the experiments that Pd nanoclusters can be doped by other transition metals like Ni. However, whether the metal like Ni doping can increase the catalytic activity for palladium clusters or not is also unclear. In the present work, we performed a theoretical investigation on NO adsorption and dissociation on the neutral and charged Pd13 as well as Ni-doped NiPd12 clusters with DFT method. The calculation results show that even though the cluster is charged or doped, the best NO adsorption site is still the hollow site. Moreover, the charged state and Ni doping influence the N–O dissociation process. Specially, the energy barrier of N–O dissociation on cationic Pd 13 cluster is significantly reduced comparing with those on other clusters, revealing that charge state could remarkably affect the catalytic activity.
2. Computational methodology All DFT calculations were performed by using DMOL3 software package [39,40]. The PerdewBurkeErnzerhof (PBE) [41] functional within the generalized gradient approximation (GGA) was used to describe the exchange–correlation interaction. The DFT semi-core pseudopotential with the double-numerical polarized basis set (DNP) was used for all the atoms. The geometries were optimized without any restriction. To locate the transition states (TS), a combination of the linear synchronous transit and the quadratic synchronous transit methods (LST/QST) [42,43] was used. Vibrational frequencies were calculated to confirm the local minima and transition states. In all above calculations, the electronic self-consistence field (SCF) tolerance is set to 106 Ha, while the convergence tolerance are set to 105 Ha for the energy, 2 103 Ha/Å for the forces, and 5 103 Å for the displacement. The GGA/DNP method have been widely used to deal with adsorption energies of small molecules on metal clusters. For example, Deka et al. used BLYP/DNP to investigate the CO and NO adsorption on neutral and charged Pd clusters [35,36]. PBE usually slightly overestimates adsorption energies. Therefore, the rPBE developed by Hammer et al. was also employed to optimize geometries [44]. However, it’s necessary to use the thermal smearing of 0.0001 Ha to reach the final convergence in rPBE calculations. In our present work, both PBE and rPBE energies are discussed below and the geometries and charge information are got from PBE results. The results revealed that both functionals gave the similar trend. For a given structure, the ground state is predicted to be the same with different PBE and rPBE. The adsorption energy is calculated as follows
Ead ¼ EadsorbatePdn EPdn Eadsorbate
ð1Þ
where EadsorbatePdn , EPdn , and Eadsorbate are the total energy of the adsorbed system, the Pdn clusters, and the corresponding gas phase species, respectively. It’s worthy to note that a negative value of Ead indicates that the adsorption is exothermic. 3. Results and discussion 3.1. Geometries and electronic properties of Pd13, Pd+13, Pd 13, and NiPd12 clusters In this work, icosahedral (ICO) Pd13 clusters is chosen as the model, which has been extensively studied in previous theoretical calculations [23,24,32,37]. Even though a C3v structure is disclosed to be most stable structure of Pd13 cluster recently [32], the ICO structure has high symmetry and is a suitable model to investigate the effect of charge. Moreover, due to its core–shell structure, the Ni can be doped by replacing the core Pd atom. The geometries of the neutral Pd13, cationic Pd+13, anionic Pd 13, and bimetallic NiPd12 clusters are shown in Fig. 1. The average bond distances of Pd–Pd (dPd–Pd) and Pd–Ni (dNi–Pd) bonds are collected in Table 1. For the neutral Pd13 cluster, the average bond distance is 2.751 Å, which agrees well with the previous results [24,37]. The charged state has an effect on the average bond distance. For the positively charged Pd+13, dPd–Pd increases to 2.768 Å, whereas in the case of Pd 13, dPd–Pd reduces to 2.744 Å. The introduction of Ni into the Pd cluster doesn’t not change the dPd–Pd so much. Compared with average Pd–Pd bond lengths, the average Ni–Pd bond length is evidently shorter, as a consequence of the smaller size of Ni atom. The ground state of the neutral Pd13 is predicted to be nonet, which is same with the result from Fresch et al. [32]. The NiPd12 is also nonet. On the other hand, the ionic Pd13 has a ground state of octet. As the surface Pd atoms play an important role in the interaction between Pd and NO, the average Hirshfeld charges of the surface 12 Pd atoms are collected in Table 1. The average charge of the surface Pd atoms in the neutral Pd13 is almost zero. For positively and negatively charged clusters, the average charges are 0.076 and 0.074 |e|, respectively. Interestingly, the average charge of surface Pd atoms in the NiPd12 is 0.025 |e|, revealing the evident charge transfer from Pd to Ni.
Fig. 1. Schemes of Pd13, Pd+13, Pd 13, and NiPd12 clusters.
Table 1 The average bond distances of Pd–Pd (dPd–Pd) and Pd–Ni (dNi–Pd) bonds and the average Hirshfeld charge of the surface 12 Pd atoms. System
Spin state (S)
dPd–Pd (Å)
dNi–Pd (Å)
Hirshfeld charges (|e|)
Pd13 Pd+13 Pd 13 NiPd12
9 8 8 9
2.751 2.768 2.744 2.747
– – – 2.612
0.01 0.076 0.074 0.025
Y. Gao et al. / Chemical Physics Letters 658 (2016) 7–11
3.2. Adsorption of NO on the Pd13, Pd+13, Pd 13, and NiPd12 clusters The bond length of the isolated NO molecule is calculated to be 1.167 Å, which is close to the experimental value (1.151 Å) [45] and previous theoretical results (1.172 Å and 1.173 Å) [11,24]. There are three possible adsorption sites (top, bridge, and hollow sites) [24] for the NO molecule on the neutral and charged Pd13 as well as bimetallic NiPd12, as shown in Fig. 2. The bond lengths (Å) and adsorption energies Ead (eV) of NO adsorbed on top, bridge, and hollow sites were collected in Table 2. As shown in Table 2, the adsorption energy at the rPBE level is obviously lower than the corresponding PBE result, indicating that PBE might overestimate the adsorption energy. However, the trend of adsorption energies on different sites is predicted to be the same with two different functionals. For the neutral Pd13, the Ead of NO on three different sites follows the trend: hollow site > bridge site > top site, which is in conformity with the theoretical results from Liu et al. [24]. It’s revealed before that the adsorption energies of the NO molecule on top, bridge, and hollow sites in the Pd(1 1 1) surface follows the same trend and NO prefers to be adsorbed on the hollow site [46]. Moreover, even though it’s charged or Ni doping, the stability trend doesn’t change too. As the NO is a radical, the ground state of all the clusters change when NO is adsorbed. Interestingly, the ground state is also relative to the adsorbed site. For example, it’s octet when NO is adsorbed on top site while its sextet for hollow and bridge sites. Furthermore, the most stable structure with NO adsorption on hollow site always has relatively low spin state.
Fig. 2. Scheme of three possible NO adsorption geometries on the neutral and charged Pd13 as well as bimetallic NiPd12. The yellow atom represents Pd for Pd13 and Ni for NiPd12, respectively.
Table 2 The spin states (S), NO (dN–O) and NPd (dN–Pd) bond lengths (Å), and adsorption energies Ead (eV) of NO adsorbed on top, bridge, and hollow sites of Pd13, Pd+13, Pd 13, and NiPd12 clusters. System
Sites
Spin state (S)
Ead (eV) PBE/rPBE
dN–O (Å)
dN–Pd (Å)
Pd13
Top Bridge Hollow
8 6 6
2.17/1.88 2.81/2.09 3.01/2.30
1.177 1.198 1.217
1.873 1.958/1.960 2.016/2.017/2.034
Pd+13
Top Bridge Hollow
7 7 7
2.03/1.90 2.36/2.12 2.76/2.23
1.164 1.186 1.205
1.872 1.958/1.960 2.021/2.023/2.042
Pd 13
Top Bridge Hollow
7 5 5
2.37/2.04 2.68/2.23 2.90/2.41
1.192 1.211 1.229
1.873 1.958/1.958 2.024/2.028/2.038
NiPd12
Top Bridge Hollow
8 6 6
2.02/1.57 2.36/2.02 2.55/2.09
1.179 1.198 1.214
1.848 1.975/1.975 2.013/2.035/2.039
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Previously, the preferred NO adsorption sites are found to vary with cluster geometry [24]. For example, the NO prefers a hollow site to bridge and top sites on the Pd13 cluster, whereas for the Pd8 cluster, the adsorption energy for NO on the bridge site is higher than those on top and hollow sites [24]. However, our results here disclose that the charge state or Ni doping has little influence on the preferred adsorption site. On the other hand, adsorption energies Ead (eV) is evidently effected by the charge state or Ni doping. The Ead of the hollow site of Pd+13, Pd 13, and NiPd12 Clusters are evidently lower than that of neutral Pd13. Specially, when the center atom is replaced by Ni, the Ead of the hollow site reduces significantly from 3.01 to 2.55 eV at the PBE level and 2.30 to 2.07 eV at the rPBE level. The careful examination on the N–O bond shows that for a given cluster, the N–O bond length is in the good relationship with Ead. The bond lengths on three different sites also follow the trend of hollow site > bridge site > top site. Interestingly, despite the smaller adsorption energy of NO on the hollow site of Pd 13 compared with that of Pd13, the N–O bond length on the hollow site of Pd 13 (1.229 Å) is even larger than that of Pd13 (1.217 Å), suggesting that NO on Pd 13 is remarkably activated. Nevertheless, the N– Pd bond lengths don’t change so much for different clusters. 3.3. Dissociation process of NO on the Pd13, Pd+13, Pd 13, and NiPd12 clusters It has been revealed before by many studies that NO dissociation is the key step for NO reduction [15,18,24,34,47]. Therefore, we mainly focused on NO dissociation on four different clusters. Only the hollow site is chosen as it’s the preferred site over top and bridge sites. Fig. 3 shows the energy profiles of NO dissociation on four different clusters. The adsorption energies Ead, dissociation energy barriers DE–, reaction energies DEr, and the important distances are collected in Table 3. The INT, TS and PRO represent the metastable NO adsorption state, transition state, and product, respectively. As shown in Fig. 3, the NO is firstly adsorbed on the hollow site, which is an exothermic step. The elongated N–O bonds suggests that they are evidently activated. Then, NO dissociation step occurs through the transition state with an energy barrier. In the transition state, the NO bond length is significantly stretched, and the N atom is bound to the hollow site while the O atom bridges the PdPd edge. Finally, the O adatom diffuses and is adsorbed on the adjacent hollow sites. The adsorptiondissociation process here is same with the previous calculation by Liu and coworkers [24]. The further examination on the neutral Pd13 showed that the relative energies here are quite close to those from Liu et al. [24]. For instance, the dissociative energy barrier here is 2.70 eV, slightly larger than that (2.58 eV) from Liu. The slight difference might be ascribed to the fact that we used a different basis set in our present work. Although PBE and rPBE provide different adsorption energies and reaction energies, the energy barriers at the PBE and rPBE level are quite close. For all four clusters, NO adsorption–dissociation are exothermic with the reaction energy ranging from 1.03 to 1.51 eV at the PBE level and from 0.47 to 0.98 eV at the rPBE level when the isolated NO and clusters are taken as the references. This result suggests that NO adsorption–dissociation on those clusters are energetically favorable. Interestingly, the NO dissociation on negatively charged Pd 13 has the largest reaction energy of 1.51 eV. On the other hand, the energy barriers are also different for four clusters. Specially, the Pd 13 has the lowest energy barrier of 2.53 eV, smaller than that for neutral Pd13. This agrees with the fact that N–O bond length (1.229 Å) in INT of Pd 13 is much larger than others. Therefore, from both thermodynamic and kinetic points of view, the NO adsorption–dissociation should be the favored on
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Y. Gao et al. / Chemical Physics Letters 658 (2016) 7–11
+ Fig. 3. Predicted relative energy profiles of NO adsorptiondissociation on (a) Pd13, (b) Pd13 , (c) Pd13 , and (d) NiPd12 clusters. The PBE and rPBE results are in black and red, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 3 Calculated adsorption energy Ead, energy barrier DE–, and reaction energy DEr of NO adsorptiondissociation on Pd13, Pd+13, Pd 13, and NiPd12 clusters. The corresponding N–O distances (dN–O) of INT, TS and PRO and spin states (S) of INT and PRO are also included. System
Pd13 Pd+13 Pd 13 NiPd12
INT
TS
PRO
Ead (eV) PBE/rPBE
S
dN–O (Å)
DE– (eV) PBE/rPBE
dN–O (Å)
DEr (eV) PBE/rPBE
S
dN–O (Å)
3.01/2.30 2.76/2.23 2.90/2.41 2.55/2.09
6 5 5 6
1.217 1.205 1.229 1.214
2.70/2.71 2.90/2.93 2.53/2.56 2.77/2.78
1.926 1.901 1.899 1.898
1.49/0.74 1.19/0.62 1.51/0.98 1.03/0.47
6 5 7 6
2.928 2.915 2.894 2.870
the Pd 13. Our result here revealed for the first time that the charged state of clusters significantly influences the energy barrier and reaction energy of NO adsorption–dissociation. Moreover, the negative charge of clusters evidently enhances its catalytic activity toward the NO reduction. 3.4. Origin of the catalytic activity difference To further find out the reason why charge state and metal doping influence the catalytic activity, we summarized the Hirshfeld
charges on all the atoms in INT, TS, and PRO, which are shown in Table 4. When NO is adsorbed on the clusters, charge transfer from the clusters to the NO happens, which agrees with analysis of the charge difference in INT. It’s well known that the charge transfer to NO molecule goes to its p⁄ molecular orbital, resulting in the N–O bond elongation and N–O bond activation. Therefore, the more the charge transfer, the longer the N–O bond length. The NO with the long N–O bond will be easily broken, leading to the NO dissociation. In Fig. S1, it’s found that there is a good relationship between the Hirshfeld
Y. Gao et al. / Chemical Physics Letters 658 (2016) 7–11 Table 4 Hirshfeld charges on N and O atoms and clusters of INT, TS, and PRO for the Pd13, Pd+13, Pd 13, and NiPd12 clusters. Hirshfeld charges (|e|) System
Pd13 Pd+13 Pd 13 NiPd12
INT
TS
PRO
NO
Cluster
N+O
Cluster
N+O
Cluster
0.24 0.16 0.32 0.23
0.24 1.15 0.68 0.23
0.55 0.47 0.62 0.55
0.55 1.47 0.38 0.55
0.66 0.59 0.73 0.65
0.66 1.59 0.27 0.65
charges on NO in INT and energy barriers of TS. The more charge transfer on NO in INT, the lower the energy barrier of TS. For Pd13-INT, the electron transfer is predicted to be 0.24 e, which is similar with Liu et al.’s result (0.26 e) [24]. For Pd 13-INT, this value is calculated to be 0.32 e, much larger than other three systems. The large Hirshfeld charge in Pd 13-INT is in good agreement with its large N–O bond length, indicating the N–O bond is highly activated. Thus, the NO dissociation on Pd 13 has the lowest energy barrier. The molecular orbital analysis also revealed that p⁄ orbital has important contribution to the highest-occupied molec+ ular orbitals for Pd 13-INT (see Fig. S2). However, in the case of Pd13, since it is positively charged, only 0.16 e transfer to the NO, which leads to its high energy barrier (2.91 eV). For NiPd12, the presence of Ni doesn’t change the charge transfer so much and its energy barrier is similar with that of Pd13. When the NO dissociation takes place, the more charge will go from the clusters to the N and O atoms. Specially, the charge transfer increases up to more than 0.59 e for PRO, which is ascribed to the fact that both N and O atom are coordinated to the Pd atoms directly. 4. Conclusions By using density functional calculations, we theoretically investigated NO adsorption and dissociation on the neutral Pd13, cationic Pd+13, anionic Pd 13, and bimetallic NiPd12 clusters. The preferred adsorption site of NO is the hollow sites, no matter whether the cluster is neutral, charged or doped by Ni. Even though the neutral Pd13 shows the largest adsorption energy for NO, Pd 13 has the lowest energy barrier of NO dissociation. Moreover, the reaction energy of NO dissociation on Pd 13 also is the largest compared to other three clusters. On the other hand, the positively charged Pd13 and Ni-doped NiPd12 exhibit even worse activity than Pd13. Overall, the negatively charged Pd 13 is predicted to have high catalytic activity toward NO reduction, suggesting the important effect of the charge state. The Hirshfeld charge analysis revealed that the charge transfer is from the clusters to NO and increases upon NO dissociation. The large charge transfer from Pd 13 to NO in the adsorption state is the reason why Pd13 has high catalytic activity. Our work not only discloses the important roles of charge state and metal doping of Pd clusters on the catalytic activity, but also will assist experimental and theoretical scientists to find out more efficient nanocatalysis. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. Grant 51272209), International Science & Technology Cooperation Program supported by Ministry of
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Research paper
NO adsorption and dissociation on palladium clusters: The importance of charged state and metal doping Yang Gao a,b, Li Mei Zhang a,b, Chun Cai Kong b, Zhi Mao Yang b,⇑, Yong Mei Chen a,b,⇑ a State Key Laboratory for Strength and Vibration of Mechanical Structures, International Center for Applied Mechanics and School of Aerospace, Xi’an Jiaotong University, Xi’an 710049, China b School of Science, MOE Key Laboratory for Non-Equilibrium Synthesis and Modulation of Condensed Matter, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e
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Article history: Received 9 March 2016 In final form 24 May 2016 Available online 30 May 2016 Keywords: NO dissociation Pd13 Nanoclusters DFT Charge transfer
a b s t r a c t The NO adsorption and dissociation on neutral, charged and Ni-doped Pd13 clusters were studied by using density functional calculations. Our results revealed that NO always prefers to adsorb on the hollow site rather than the top or bridge sites. However, the charge state and Ni doping remarkably influence NO adsorption energy, dissociation barrier and reaction energy. The reaction on Pd 13 has the lowest energy barrier and largest reaction energy. The Hirshfeld charge analysis discloses that the origin of the catalytic activity difference is the charge transfer from clusters to NO in the metastable NO adsorption state. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen monoxide (NO) is an important pollutant produced from industrial and automotive exhausts. It is the major component of photochemical smog, which can cause acid rain, the ozone layer destruction and global warming [1,2]. Therefore, the removal of NO from automotive exhaust gases, which usually involves the conversion NO to molecular dinitrogen, has attracted wide attention in the last several decades. The three-way catalysts (TWC), in which precious metals Pt, Pd and Rh serve as efficient catalysts, are usually used to complete the catalytic reaction of NO [3–6]. Because Pd is cheaper, highly abundant and has good thermal stability, many experimental studies have been performed on the selective NO reduction using Pd as the catalysis up to now [7–18]. For example, de Wolf et al. studied the NO–H2 reaction on the clean Pd(1 1 1) surface [16]. Ma and Matsushima investigated the surface nitrogen removal pathways of NO + H2 reaction on Pd(1 1 0) [17]. Moreover, with the infrared reflection absorption spectroscopy, Goodman and coworkers focused the reaction of NO and CO on Pd single crystals, and revealed that the activity is ⇑ Corresponding authors at: State Key Laboratory for Strength and Vibration of Mechanical Structures, International Center for Applied Mechanics and School of Aerospace, Xi’an Jiaotong University, Xi’an 710049, China (Y.M. Chen). E-mail addresses: [email protected] (Z.M. Yang), [email protected]. cn (Y.M. Chen). http://dx.doi.org/10.1016/j.cplett.2016.05.048 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
increasing from more open surfaces to closed packed ones (Pd (1 0 0) < Pd(1 1 0) < Pd(1 1 1)) [7]. Compared with the Pd bulk, Pd nanoclusters have quite different structures and electronic properties, which makes them attractive for catalysis [19–22]. Lots of experimental and theoretical investigations have been devoted to the NO adsorption, dissociation and reduction reaction on Pd nanoclusters. For example, the interaction of NO with Pd metal cluster models (up to 13 atoms) with different size and geometry was studied by Duarte and Salahub [10]. Heiz et al. have extended these measurements to Pdn (n 6 30) clusters in reaction of NO and CO [23]. Very recently, Liu and coworkers theoretically studied the NO adsorption and dissociation on Pdn (n = 8, 13, 19, 25) clusters [24]. Three important findings are disclosed through above research: (i) the dissociation of NO bond is the key step in the NO reduction; (ii) the NO reduction activity is highly sensitive to the cluster geometries; (iii) the charge transfer takes place from the Pdn clusters to NO and increases upon NO dissociation. On the other hand, with ion beam and ion cyclotron resonance methods, many cationic and anionic Pd clusters have been synthesized experimentally and used to investigate their reactions with molecules like oxygen, nitrogen oxides, and n-butane [25–31]. Furthermore, Pd clusters supported on the metal oxides or coated by ligands are revealed to be charged rather than neutral [32,33]. With theoretical calculations, adsorption of NO or CO on charged clusters has been widely studied [34–38]. For example, Kalita
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and Deka performed density functional calculations (DFT) on CO adsorption on neutral and charged Pdn (n = 1–7) clusters [35]. Castro and coworkers calculated the adsorption energies of CO on neutral and charged Pd13 clusters and found that the Pd 13 can increase the adsorption energy [37]. Very recently, the nitric oxides adsorption on neutral and charged Pdn (n = 1–5) clusters were theoretically investigated [36]. The adsorption energy trend of NO on clusters is revealed to be anionic > neutral > cationic. Furthermore, Su et al. reported a DFT study on the catalytic reduction of NO with CO on positively charged Rh+4 clusters [34]. Even though several theoretical studies focused on the structural sensitivity of NO reduction, as far as we know, the effect of the charged states of Pd clusters on NO dissociation has never been discussed up to now. It has been found that the charge transfer takes place from the Pdn clusters to NO, which activates the adsorbed NO molecule [24]. Therefore, the charged state of Pd clusters might significantly influence NO dissociation. Moreover, it has been revealed in the experiments that Pd nanoclusters can be doped by other transition metals like Ni. However, whether the metal like Ni doping can increase the catalytic activity for palladium clusters or not is also unclear. In the present work, we performed a theoretical investigation on NO adsorption and dissociation on the neutral and charged Pd13 as well as Ni-doped NiPd12 clusters with DFT method. The calculation results show that even though the cluster is charged or doped, the best NO adsorption site is still the hollow site. Moreover, the charged state and Ni doping influence the N–O dissociation process. Specially, the energy barrier of N–O dissociation on cationic Pd 13 cluster is significantly reduced comparing with those on other clusters, revealing that charge state could remarkably affect the catalytic activity.
2. Computational methodology All DFT calculations were performed by using DMOL3 software package [39,40]. The PerdewBurkeErnzerhof (PBE) [41] functional within the generalized gradient approximation (GGA) was used to describe the exchange–correlation interaction. The DFT semi-core pseudopotential with the double-numerical polarized basis set (DNP) was used for all the atoms. The geometries were optimized without any restriction. To locate the transition states (TS), a combination of the linear synchronous transit and the quadratic synchronous transit methods (LST/QST) [42,43] was used. Vibrational frequencies were calculated to confirm the local minima and transition states. In all above calculations, the electronic self-consistence field (SCF) tolerance is set to 106 Ha, while the convergence tolerance are set to 105 Ha for the energy, 2 103 Ha/Å for the forces, and 5 103 Å for the displacement. The GGA/DNP method have been widely used to deal with adsorption energies of small molecules on metal clusters. For example, Deka et al. used BLYP/DNP to investigate the CO and NO adsorption on neutral and charged Pd clusters [35,36]. PBE usually slightly overestimates adsorption energies. Therefore, the rPBE developed by Hammer et al. was also employed to optimize geometries [44]. However, it’s necessary to use the thermal smearing of 0.0001 Ha to reach the final convergence in rPBE calculations. In our present work, both PBE and rPBE energies are discussed below and the geometries and charge information are got from PBE results. The results revealed that both functionals gave the similar trend. For a given structure, the ground state is predicted to be the same with different PBE and rPBE. The adsorption energy is calculated as follows
Ead ¼ EadsorbatePdn EPdn Eadsorbate
ð1Þ
where EadsorbatePdn , EPdn , and Eadsorbate are the total energy of the adsorbed system, the Pdn clusters, and the corresponding gas phase species, respectively. It’s worthy to note that a negative value of Ead indicates that the adsorption is exothermic. 3. Results and discussion 3.1. Geometries and electronic properties of Pd13, Pd+13, Pd 13, and NiPd12 clusters In this work, icosahedral (ICO) Pd13 clusters is chosen as the model, which has been extensively studied in previous theoretical calculations [23,24,32,37]. Even though a C3v structure is disclosed to be most stable structure of Pd13 cluster recently [32], the ICO structure has high symmetry and is a suitable model to investigate the effect of charge. Moreover, due to its core–shell structure, the Ni can be doped by replacing the core Pd atom. The geometries of the neutral Pd13, cationic Pd+13, anionic Pd 13, and bimetallic NiPd12 clusters are shown in Fig. 1. The average bond distances of Pd–Pd (dPd–Pd) and Pd–Ni (dNi–Pd) bonds are collected in Table 1. For the neutral Pd13 cluster, the average bond distance is 2.751 Å, which agrees well with the previous results [24,37]. The charged state has an effect on the average bond distance. For the positively charged Pd+13, dPd–Pd increases to 2.768 Å, whereas in the case of Pd 13, dPd–Pd reduces to 2.744 Å. The introduction of Ni into the Pd cluster doesn’t not change the dPd–Pd so much. Compared with average Pd–Pd bond lengths, the average Ni–Pd bond length is evidently shorter, as a consequence of the smaller size of Ni atom. The ground state of the neutral Pd13 is predicted to be nonet, which is same with the result from Fresch et al. [32]. The NiPd12 is also nonet. On the other hand, the ionic Pd13 has a ground state of octet. As the surface Pd atoms play an important role in the interaction between Pd and NO, the average Hirshfeld charges of the surface 12 Pd atoms are collected in Table 1. The average charge of the surface Pd atoms in the neutral Pd13 is almost zero. For positively and negatively charged clusters, the average charges are 0.076 and 0.074 |e|, respectively. Interestingly, the average charge of surface Pd atoms in the NiPd12 is 0.025 |e|, revealing the evident charge transfer from Pd to Ni.
Fig. 1. Schemes of Pd13, Pd+13, Pd 13, and NiPd12 clusters.
Table 1 The average bond distances of Pd–Pd (dPd–Pd) and Pd–Ni (dNi–Pd) bonds and the average Hirshfeld charge of the surface 12 Pd atoms. System
Spin state (S)
dPd–Pd (Å)
dNi–Pd (Å)
Hirshfeld charges (|e|)
Pd13 Pd+13 Pd 13 NiPd12
9 8 8 9
2.751 2.768 2.744 2.747
– – – 2.612
0.01 0.076 0.074 0.025
Y. Gao et al. / Chemical Physics Letters 658 (2016) 7–11
3.2. Adsorption of NO on the Pd13, Pd+13, Pd 13, and NiPd12 clusters The bond length of the isolated NO molecule is calculated to be 1.167 Å, which is close to the experimental value (1.151 Å) [45] and previous theoretical results (1.172 Å and 1.173 Å) [11,24]. There are three possible adsorption sites (top, bridge, and hollow sites) [24] for the NO molecule on the neutral and charged Pd13 as well as bimetallic NiPd12, as shown in Fig. 2. The bond lengths (Å) and adsorption energies Ead (eV) of NO adsorbed on top, bridge, and hollow sites were collected in Table 2. As shown in Table 2, the adsorption energy at the rPBE level is obviously lower than the corresponding PBE result, indicating that PBE might overestimate the adsorption energy. However, the trend of adsorption energies on different sites is predicted to be the same with two different functionals. For the neutral Pd13, the Ead of NO on three different sites follows the trend: hollow site > bridge site > top site, which is in conformity with the theoretical results from Liu et al. [24]. It’s revealed before that the adsorption energies of the NO molecule on top, bridge, and hollow sites in the Pd(1 1 1) surface follows the same trend and NO prefers to be adsorbed on the hollow site [46]. Moreover, even though it’s charged or Ni doping, the stability trend doesn’t change too. As the NO is a radical, the ground state of all the clusters change when NO is adsorbed. Interestingly, the ground state is also relative to the adsorbed site. For example, it’s octet when NO is adsorbed on top site while its sextet for hollow and bridge sites. Furthermore, the most stable structure with NO adsorption on hollow site always has relatively low spin state.
Fig. 2. Scheme of three possible NO adsorption geometries on the neutral and charged Pd13 as well as bimetallic NiPd12. The yellow atom represents Pd for Pd13 and Ni for NiPd12, respectively.
Table 2 The spin states (S), NO (dN–O) and NPd (dN–Pd) bond lengths (Å), and adsorption energies Ead (eV) of NO adsorbed on top, bridge, and hollow sites of Pd13, Pd+13, Pd 13, and NiPd12 clusters. System
Sites
Spin state (S)
Ead (eV) PBE/rPBE
dN–O (Å)
dN–Pd (Å)
Pd13
Top Bridge Hollow
8 6 6
2.17/1.88 2.81/2.09 3.01/2.30
1.177 1.198 1.217
1.873 1.958/1.960 2.016/2.017/2.034
Pd+13
Top Bridge Hollow
7 7 7
2.03/1.90 2.36/2.12 2.76/2.23
1.164 1.186 1.205
1.872 1.958/1.960 2.021/2.023/2.042
Pd 13
Top Bridge Hollow
7 5 5
2.37/2.04 2.68/2.23 2.90/2.41
1.192 1.211 1.229
1.873 1.958/1.958 2.024/2.028/2.038
NiPd12
Top Bridge Hollow
8 6 6
2.02/1.57 2.36/2.02 2.55/2.09
1.179 1.198 1.214
1.848 1.975/1.975 2.013/2.035/2.039
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Previously, the preferred NO adsorption sites are found to vary with cluster geometry [24]. For example, the NO prefers a hollow site to bridge and top sites on the Pd13 cluster, whereas for the Pd8 cluster, the adsorption energy for NO on the bridge site is higher than those on top and hollow sites [24]. However, our results here disclose that the charge state or Ni doping has little influence on the preferred adsorption site. On the other hand, adsorption energies Ead (eV) is evidently effected by the charge state or Ni doping. The Ead of the hollow site of Pd+13, Pd 13, and NiPd12 Clusters are evidently lower than that of neutral Pd13. Specially, when the center atom is replaced by Ni, the Ead of the hollow site reduces significantly from 3.01 to 2.55 eV at the PBE level and 2.30 to 2.07 eV at the rPBE level. The careful examination on the N–O bond shows that for a given cluster, the N–O bond length is in the good relationship with Ead. The bond lengths on three different sites also follow the trend of hollow site > bridge site > top site. Interestingly, despite the smaller adsorption energy of NO on the hollow site of Pd 13 compared with that of Pd13, the N–O bond length on the hollow site of Pd 13 (1.229 Å) is even larger than that of Pd13 (1.217 Å), suggesting that NO on Pd 13 is remarkably activated. Nevertheless, the N– Pd bond lengths don’t change so much for different clusters. 3.3. Dissociation process of NO on the Pd13, Pd+13, Pd 13, and NiPd12 clusters It has been revealed before by many studies that NO dissociation is the key step for NO reduction [15,18,24,34,47]. Therefore, we mainly focused on NO dissociation on four different clusters. Only the hollow site is chosen as it’s the preferred site over top and bridge sites. Fig. 3 shows the energy profiles of NO dissociation on four different clusters. The adsorption energies Ead, dissociation energy barriers DE–, reaction energies DEr, and the important distances are collected in Table 3. The INT, TS and PRO represent the metastable NO adsorption state, transition state, and product, respectively. As shown in Fig. 3, the NO is firstly adsorbed on the hollow site, which is an exothermic step. The elongated N–O bonds suggests that they are evidently activated. Then, NO dissociation step occurs through the transition state with an energy barrier. In the transition state, the NO bond length is significantly stretched, and the N atom is bound to the hollow site while the O atom bridges the PdPd edge. Finally, the O adatom diffuses and is adsorbed on the adjacent hollow sites. The adsorptiondissociation process here is same with the previous calculation by Liu and coworkers [24]. The further examination on the neutral Pd13 showed that the relative energies here are quite close to those from Liu et al. [24]. For instance, the dissociative energy barrier here is 2.70 eV, slightly larger than that (2.58 eV) from Liu. The slight difference might be ascribed to the fact that we used a different basis set in our present work. Although PBE and rPBE provide different adsorption energies and reaction energies, the energy barriers at the PBE and rPBE level are quite close. For all four clusters, NO adsorption–dissociation are exothermic with the reaction energy ranging from 1.03 to 1.51 eV at the PBE level and from 0.47 to 0.98 eV at the rPBE level when the isolated NO and clusters are taken as the references. This result suggests that NO adsorption–dissociation on those clusters are energetically favorable. Interestingly, the NO dissociation on negatively charged Pd 13 has the largest reaction energy of 1.51 eV. On the other hand, the energy barriers are also different for four clusters. Specially, the Pd 13 has the lowest energy barrier of 2.53 eV, smaller than that for neutral Pd13. This agrees with the fact that N–O bond length (1.229 Å) in INT of Pd 13 is much larger than others. Therefore, from both thermodynamic and kinetic points of view, the NO adsorption–dissociation should be the favored on
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+ Fig. 3. Predicted relative energy profiles of NO adsorptiondissociation on (a) Pd13, (b) Pd13 , (c) Pd13 , and (d) NiPd12 clusters. The PBE and rPBE results are in black and red, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 3 Calculated adsorption energy Ead, energy barrier DE–, and reaction energy DEr of NO adsorptiondissociation on Pd13, Pd+13, Pd 13, and NiPd12 clusters. The corresponding N–O distances (dN–O) of INT, TS and PRO and spin states (S) of INT and PRO are also included. System
Pd13 Pd+13 Pd 13 NiPd12
INT
TS
PRO
Ead (eV) PBE/rPBE
S
dN–O (Å)
DE– (eV) PBE/rPBE
dN–O (Å)
DEr (eV) PBE/rPBE
S
dN–O (Å)
3.01/2.30 2.76/2.23 2.90/2.41 2.55/2.09
6 5 5 6
1.217 1.205 1.229 1.214
2.70/2.71 2.90/2.93 2.53/2.56 2.77/2.78
1.926 1.901 1.899 1.898
1.49/0.74 1.19/0.62 1.51/0.98 1.03/0.47
6 5 7 6
2.928 2.915 2.894 2.870
the Pd 13. Our result here revealed for the first time that the charged state of clusters significantly influences the energy barrier and reaction energy of NO adsorption–dissociation. Moreover, the negative charge of clusters evidently enhances its catalytic activity toward the NO reduction. 3.4. Origin of the catalytic activity difference To further find out the reason why charge state and metal doping influence the catalytic activity, we summarized the Hirshfeld
charges on all the atoms in INT, TS, and PRO, which are shown in Table 4. When NO is adsorbed on the clusters, charge transfer from the clusters to the NO happens, which agrees with analysis of the charge difference in INT. It’s well known that the charge transfer to NO molecule goes to its p⁄ molecular orbital, resulting in the N–O bond elongation and N–O bond activation. Therefore, the more the charge transfer, the longer the N–O bond length. The NO with the long N–O bond will be easily broken, leading to the NO dissociation. In Fig. S1, it’s found that there is a good relationship between the Hirshfeld
Y. Gao et al. / Chemical Physics Letters 658 (2016) 7–11 Table 4 Hirshfeld charges on N and O atoms and clusters of INT, TS, and PRO for the Pd13, Pd+13, Pd 13, and NiPd12 clusters. Hirshfeld charges (|e|) System
Pd13 Pd+13 Pd 13 NiPd12
INT
TS
PRO
NO
Cluster
N+O
Cluster
N+O
Cluster
0.24 0.16 0.32 0.23
0.24 1.15 0.68 0.23
0.55 0.47 0.62 0.55
0.55 1.47 0.38 0.55
0.66 0.59 0.73 0.65
0.66 1.59 0.27 0.65
charges on NO in INT and energy barriers of TS. The more charge transfer on NO in INT, the lower the energy barrier of TS. For Pd13-INT, the electron transfer is predicted to be 0.24 e, which is similar with Liu et al.’s result (0.26 e) [24]. For Pd 13-INT, this value is calculated to be 0.32 e, much larger than other three systems. The large Hirshfeld charge in Pd 13-INT is in good agreement with its large N–O bond length, indicating the N–O bond is highly activated. Thus, the NO dissociation on Pd 13 has the lowest energy barrier. The molecular orbital analysis also revealed that p⁄ orbital has important contribution to the highest-occupied molec+ ular orbitals for Pd 13-INT (see Fig. S2). However, in the case of Pd13, since it is positively charged, only 0.16 e transfer to the NO, which leads to its high energy barrier (2.91 eV). For NiPd12, the presence of Ni doesn’t change the charge transfer so much and its energy barrier is similar with that of Pd13. When the NO dissociation takes place, the more charge will go from the clusters to the N and O atoms. Specially, the charge transfer increases up to more than 0.59 e for PRO, which is ascribed to the fact that both N and O atom are coordinated to the Pd atoms directly. 4. Conclusions By using density functional calculations, we theoretically investigated NO adsorption and dissociation on the neutral Pd13, cationic Pd+13, anionic Pd 13, and bimetallic NiPd12 clusters. The preferred adsorption site of NO is the hollow sites, no matter whether the cluster is neutral, charged or doped by Ni. Even though the neutral Pd13 shows the largest adsorption energy for NO, Pd 13 has the lowest energy barrier of NO dissociation. Moreover, the reaction energy of NO dissociation on Pd 13 also is the largest compared to other three clusters. On the other hand, the positively charged Pd13 and Ni-doped NiPd12 exhibit even worse activity than Pd13. Overall, the negatively charged Pd 13 is predicted to have high catalytic activity toward NO reduction, suggesting the important effect of the charge state. The Hirshfeld charge analysis revealed that the charge transfer is from the clusters to NO and increases upon NO dissociation. The large charge transfer from Pd 13 to NO in the adsorption state is the reason why Pd13 has high catalytic activity. Our work not only discloses the important roles of charge state and metal doping of Pd clusters on the catalytic activity, but also will assist experimental and theoretical scientists to find out more efficient nanocatalysis. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. Grant 51272209), International Science & Technology Cooperation Program supported by Ministry of
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