High Stability and Reactivity of Single-Metal Atom Catalysts Supported

Subscriber access provided by the University of Exeter

Article

High Stability and Reactivity of Single-Metal Atom Catalysts Supported on Yttria-Stabilized Zirconia: the Role of the Surface Oxygen Vacancy Shan Dong, Yanxing Zhang, Xilin Zhang, Jianjun Mao, and Zongxian Yang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10355 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

High Stability and Reactivity of Single-Metal Atom Catalysts Supported on Yttria-Stabilized Zirconia: the Role of the Surface Oxygen Vacancy Shan Donga, Yanxing Zhanga†, Xilin Zhanga, Jianjun Maoa, Zongxian Yanga†,b,c a

College of Physics and Materials Science, Henan Normal University, Xinxiang, Henan 453007, China b

National Demonstration Center for Experimental Physics Education (Henan Normal University), Xinxiang, Henan 453007, China

c

Collaborative Innovation Center of Nano Functional Materials and Applications, Henan Province, China

ABSTRACT: Structural stability and catalytic reactivity of single noble metals (NMs) (NM = Pd, Pt, Rh and Ir) supported on yttrium-stabilized zirconia(YSZ) are systematically investigated using ab initio density functional theory calculations with the particular focus on the influence of the surface oxygen vacancy (Ov). Compared with the bare YSZ(111) system, adsorption of NM atoms can activate surface oxygen and the positions of Ov. The Ir/YSZ(111) and Pt/YSZ-Ov(111) are testified as the most stable ones among the NM/YSZ(111) and NM/YSZ-Ov(111) systems, respectively. To further probe their activity and the role of the Ov, the adsorption and dissociation of O2 on Ir/YSZ(111) and Pt/YSZ-Ov(111) are studied and compared. It is found that the Ov can promote the charge transfer from the YSZ to the NM adatoms (~1 e) which is beneficial to the bond breaking of O2 . The Pt/YSZ-Ov(111) exhibits high stability and reactivity for activating O2 molecule, which may be of beneficial to the oxygen reduction reactions (ORR) and initiate a clue for fabricating highly efficient single-metal atom catalysts on YSZ.

†

Correspondence author. E_mail: [email protected] (Y Zhang), [email protected] (Z

Yang)

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 27

1. INTRODUCTION Single atomic metal catalyst (SAC) anchored to support has attracted rapidly increasing attention due to its high utilization rate of metal atoms and high catalytic performance

1-3

. In

heterogeneous catalysis, precious metals that are atomically dispersed on the metal oxide supports provide a unique opportunity to tune active sites and are extensively used as important catalysts in industry4-8. Both theoretical and experimental researches have indicated that the single noble metal atoms such as Pd, Pt, Rh and Ir supported on oxide catalysts exhibit extraordinary catalytic activity in a large number of reactions, including CO oxidation, methanol oxidation reactions and more complex oxygen reduction reactions (ORRs)

4-6, 8-9

.

The dissociation of O2 is a crucial reaction step for all these chemical processes, and has been an attractive research topic for years9-12. An appropriate metal oxide support is important for dispersion of the precious metal atoms and enhancing the catalytic activity as well as for the reduction of catalyst cost. Among the numerous oxide supports, yttria-stabilized zirconia (YSZ) finds special attention owing to its stability over a wide temperature range and its resistance to catalytic poisoning13-15. Besides, YSZ has been widely used for various applications including catalyst, automotive gas sensor and solid oxide fuel cells (SOFC)16-18. Oxide surfaces are rarely perfect. Various defects may appear, including steps, kinks and vacancies depending on different preparation conditions. These defect sites can function as anchoring sites for individual metal atoms and enhance the interaction between single metal atoms and the support 19-21. Joon et al. found that YSZ surfaces contained high density of grain boundaries and those charge carriers (i.e. oxygen vacancies) at the grain boundary could enhance the activity of ORRs22. Jihwan et al. also observed oxide-ion vacancies on the YSZ surfaces23 by compositional mapping using energy dispersive X-ray spectroscopy in scanning transmission electron microscope (STEM-EDS)24, molecular simulations25 and secondary ion mass spectroscopy (SIMS) 22.The fuel oxidation is believed to take place at the metal/YSZ interface, which would create oxygen vacancy at the interface

26-28

. Most of the oxidation reactions can take place via a

Mars–van Krevelen mechanism on the surfaces of YSZ. In the Mars–van Krevelen 2


Page 3 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mechanism, fuel gas is oxidized by lattice oxygen ions. The lattice oxygen ions are incorporated into the products which result in the formation of surface oxygen vacancies. The cycle for catalytic partial oxidation can supplement the extracted lattice oxygen ions through the bulk oxygen ion transport and dissociative adsorption of molecular oxygen at the surface29-33. Shishkin and Ziegler’s studies on the hydrogen oxidation at the Ni/YSZ interface indicated that the most active sites are located at the Ni/YSZ interface 34. Zhang et al. showed that the O vacancy sites at the Ni/YSZ interface are highly active compared with the ideal Ni (111) surface 35-36. Although Ni particles have many desirable advantages, several barriers still exist, including sulfur poisoning, carbon deposition, and the inadvertent oxidation of Ni in the anode

30, 37

. Besides, in our previous work, we found that the dissociation of O2 is a

endothermic reaction with a large activation barrier on Ni/YSZ(111)

10

. Some experimental

and theoretical researches have studied the atomic noble metals on ZrO2 surface 38-39. The high oxygen-ion conduction and possible vacancy effects in YSZ present great opportunity to improve the catalytic activity of the supported atomic noble metals catalysts. Several experimental researches have studied the noble metals (NMs) supported on YSZ substrate. Kotsionopoulos et al. 40 found the complete oxidation of propane on Pt or Rh films interfaced to YSZ. Carmen et al.41 found that Pd supported over ceria and yttria-stabilized zirconia showed the highest activity for completing oxidation of methane. Teiichi et al.42 showed that the interfacial conductivity for the Ir–YSZ electrodes was almost 1000 times higher than that of sputtered-Pt electrodes. Sung et al. found that the oxygen vacancies on the Pt/YSZ surface can be provided as adsorption sites and the oxygen molecules dissociated on oxygen vacancies43-47. However, the detailed mechanisms for the interfacial interaction and the high activity of these NM/YSZ systems are still lacking. In this work, we perform density functional theory (DFT) calculations to study the adsorption of noble metal atoms such as Pd, Pt, Rh, Ir on YSZ(111) surface and on the defected YSZ(111) surface with oxygen vacancies. The dissociation of O2 on the NM atoms loaded YSZ(111) is used to probe their activity and the role of the surface O vacancy (Ov) is investigated in detail. Our results indicate a credible possibility to design highly efficient single metal catalysts for various reactions using YSZ as support. 3



Page 4 of 27

2. MODEL AND COMPUTATIONAL DETAILS All the spin-polarized calculations are performed using periodic DFT method as implemented in the Vienna Ab-Initio Simulation Package (VASP)48. The exchange and correlation functional of Perdew-Burke-Ernzerhof (PBE) within the generalized gradient approximation (GGA) is used for the structural relaxation and energy minimization49. The electron-ion interactions are treated using the projector augmented wave (PAW) method50-51. The number of plane-waves depends on the cutoff energy which is tested and chosen as 408 eV in the calculations as we used before 10. The YSZ(111) surface is modeled as that used in the Shishkin and Ziegler's work13, 16. In this model (Figure 1(a) and (b)), a ZrO2(111) slab including three O-Zr-O triple layers is modeled first with the bottom three layers fixed during the structural optimization. Then two of the zirconium atoms in the slab are substituted with two yttrium atoms and one of the oxygen atoms is removed from the pure zirconia cell to ensure the stoichiometry, which are corresponding to a 9% mol concentration of yttria in YSZ. Experimentally, Abe et al.26 and other researchers27-28 found a clear absence of amorphous phases at the YSZ surface by using the transmission electron microscopy(TEM) technique. Zhang et al. also modeled the O vacancy (Ov) on the Ni/YSZ(111) systems36, 52-53. A vacuum layer of 15 Å is used to separate the periodic images in the direction perpendicular to the surface. The Monkhorst–Pack k-point mesh of 2×3×1 is used for the Brillouin zone (BZ) sampling as we used before10. Calculations for the free gaseous molecules are performed using periodic cells of 10×10×10 Å. The structures are optimized until the forces on each atom are less than 0.02 eV/ Å. The transition states and dissociation barriers are calculated by using the climbing image nudged elastic band (CI-NEB) method54. The adsorption energy is used to quantify the adsorption strength, which is defined by

Eads = ESupport + Ex − Ex / Support

(1),

where ESupport , E x and Ex / Support represent the total energies of the optimized support with (YSZ-Ov or NM/YSZ-Ov) and without an interface oxygen vacancy (YSZ or NM/YSZ), the adsorbate (x), and the optimized x/support system, respectively. The charge density difference 4


Page 5 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(CDD, △ρ) is used to measure the charge redistribution induced by the adsorbate-support interaction and defined as

∆ρ ( r ) = ρ ( X + Support ) - ρ ( Support ) - ρ ( X )

(2),

where ρ( X + Support ) , ρ (Support ) and ρ ( X ) are total charge density of the optimized x/support system, the optimized support and the adsorbate (x), respectively. The activation energies in the reaction processes are calculated from total energy of the transition states ETS relative to that of the initial EIS:

Ea = ETS − EIS

(3)

3. RESULTS AND DISCUSSIONS 3.1 The Adsorption of NM Atoms on the YSZ(111) Surface Firstly, we consider the adsorption of NM atoms on the YSZ(111) surface. The possible adsorption sites, including the hollow, top positions of Zr, O, and Y, as well as the bridge positions of Zr-O, Zr-Y, and O-Y, have been considered10. The most stable optimized configurations for the NM/YSZ(111) are shown in Figure 2 and the adsorption properties are summarized in Table 1. On YSZ(111), it is found that the Pd, Pt, Rh and Ir atoms prefer to be adsorbed over the intrinsic oxygen vacancy (IV), which is similar to the adsorption of single Ni atom on YSZ17. The calculated adsorption energies are 1.81, 3.30, 3.14, 4.21 eV, respectively. Bader analysis shows that the adatoms (Pd, Pt, Rh and Ir) get about 0.22, 0.35, 0.23, 0.34 electrons, respectively. Although the adsorption of Pt and Rh is stronger than that of Pd, it is much weaker relative to Ir, which has the largest adsorption energy among the adatoms, indicating that the Ir/YSZ(111) surface is energetically more stable than the corresponding Pd/YSZ (111), Pt/YSZ(111) and Rh/YSZ(111) surfaces. Table 1 The adsorption properties of NM/YSZ(111) and NM/YSZ-Ov(111) (NM = Pd, Pt, Rh and Ir), including the adsorption energy of NM (Eads, in eV), the total charge on the adsorbed atom (NM-chg, in e), the d-band centers of the adsorbed atoms (εd, in eV) and the diffusion 5



Page 6 of 27

barrier of NM (Eb in eV). The negative and positive numbers of NM-chg represent the loss and gain of electrons, respectively.

Atom

Surface

Eads(eV)

NM-chg(e)

εd(eV)

Eb(eV)

Pd

YSZ

1.81

0.22

-2.43

0.54

YSZ-Ov

4.61

1.41

-1.93

1.42

YSZ

3.30

0.35

-3.04

1.12

YSZ-Ov

6.86

1.56

-1.29

1.76

YSZ

3.14

0.23

-1.99

1.05

YSZ-Ov

5.51

1.49

-1.61

1.29

YSZ

4.21

0.34

-2.48

1.82

YSZ-Ov

6.49

1.59

-1.53

1.45

Pt

Rh

Ir

3.2 Vacancy Formation on the YSZ(111) and NM/YSZ(111) Surfaces On YSZ(111) surface, oxygen vacancies can be classified into two types: the intrinsic oxygen vacancy (IV) (Figure 1(a) and(b)) due to the introduction of Y and the surface oxygen vacancy (Ov) (Figure 1(c)-(e)) due to the consumption of surface oxygen in fuel oxidation. Here, we focus on the surface oxygen vacancy. On YSZ(111) surface, the Ov locations at the positions of different oxygen ions (V1, V2 and V3) in the first layer are considered. The vacancy formation energy ( E f ) defined as

E f = ESupport −Ov + 1 / 2 EO2 − ESupport

(4),

is used to describe to feasibility of the vacancy formation. In the equation (4),

ESupport and ESupport −Ov are the total energies of the system before and after the vacancy formation, respectively, whereas EO2 is the energy of an isolated oxygen molecule. The calculated formation energies are 6.16, 6.29, 6.48 eV for the Ov at V1, V2 and V3, respectively. The slightly smaller vacancy formation energy suggests that the V1 site is the most stable position. 6


Page 7 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 The formation energy (Ef in eV) of Ov in the YSZ(111) and NM/YSZ(111) systems. The Ef1 , Ef2 and Ef3 are for the Ov located at the V1, V2 and V3 positions, respectively. Ef1 (eV)

Ef2 (eV)

Ef3 (eV)

YSZ

6.16

6.29

6.48

Pd/YSZ

3.32

3.35

3.48

Pt/YSZ

2.55

2.57

2.68

Rh/YSZ

4.07

3.74

3.81

Ir/YSZ

4.11

3.84

4.57

On the NM/YSZ(111) surfaces, we consider the Ov locating at the positions of the oxygen atoms that bind both to nickel and zirconium. The most stable configurations and vacancy formation energies are given in Figure 2 and Table 2, respectively. It is interesting that the position of Ov can be tuned by adsorption of NM atoms. On the Pd/YSZ(111) and Pt/YSZ(111) surfaces, the Ov favors to locate at the V1 site with lower vacancy formation energy (3.32 and 2.55 eV). In contrast, the Ov favors to locate at the V2 site with the slight higher vacancy formation energy (3.74 and 3.84eV) on the Rh/YSZ(111) and Ir/YSZ(111) surfaces, respectively. The different locations of Ov can be explained by the fact that the Pd and Pt atoms have almost similar d orbital, which is different with that of the Rh and Ir atoms, indicating that one can control the locations of Ov for YSZ systems through adsorbing of an appropriate NM atom. Among the four NM atoms, the Pt atom shows prominent performance on the activity of surface oxygen, which has the lowest vacancy formation energy. This value is much less than that in Ni/YSZ(111)34. Although the Ir/YSZ(111) surface has the bigger vacancy formation energy, it is still much lower than that in the bare YSZ(111) surface. Such a drastic difference in vacancy formation energies can be explained by the fact that the valence of oxygen atoms in YSZ(111) surface is saturated, whereas the valence of oxygen atoms in NM/YSZ(111) surface is undersaturated. These results suggest that the NM atom has a significant effect on the activity of surface oxygen and the positions of Ov. 3.3 Electronic Properties and Structural Stability 7



Page 8 of 27

As a single atom catalyst, the stability of the NM/YSZ(111) systems is greatly important. The adsorption properties for the NM/YSZ(111) and the NM/YSZ-Ov(111) systems are shown in Table 1. It is found that the NM (Pd, Pt, Rh and Ir) atoms prefer to be adsorbed over the Ov on NM/YSZ-Ov(111) with larger adsorption energies (4.61, 6.86, 5.19, 6.23 eV) and get much more electrons (1.41, 1.56, 1.47, 1.64 e) compared with the NM/YSZ(111) systems, indicating that the Ov can stabilize the single NM atoms and cause extra charge transition from YSZ to the NM adatoms (~1 e). To gain more insight into the stability of the NM/YSZ(111) systems and the role of the surface oxygen vacancy (Ov), the charge density difference (CDD) and the density of states (DOS) are investigated. According to the CDD maps in Figure 3, compared with the original NM/YSZ(111) systems shown in Figure 3(a)-(d), the introduction of Ov results in an evident electrons accumulation around the adatoms as depicted in Figure 3(e)-(h), indicating a higher amount of charge transferred to the adatoms. This makes the Ov site an active surface site and would have positive contribution to enhance the interaction between the adatoms and the support. The DOS plots of the d and s states of the four kinds of adsorbed metal atoms on the YSZ and YSZ-Ov substrates are depicted in Figure 4. Obviously, the d orbitals of the NM atoms spread a wide range of energy below the Fermi level on the NM/YSZ supports. Comparatively, the DOS peaks of the adsorbed NM atoms show higher main peaks below and near the Fermi energy, which are consistent with the larger amount of charge transfer (Table 1) on the NM/YSZ-Ov (111) systems. For Pd/YSZ-Ov(111) and Pt/YSZ-Ov(111) systems, we note one main peak below the Fermi level. But for Rh/YSZ-Ov(111) and Ir/YSZ-Ov(111) systems, we observe two main peaks below the Fermi level. The different contributions of the d states of the adsorbed NM atoms are consistent with the different locations of Ov for NM/YSZ systems. Because of the change in the d-band width, the d-band centers (εd) of the NM/YSZ-Ov change correspondingly. Compared with the calculated d-band centers (εd =-2.43, -3.04, -1.99, -2.48 eV) of the NM (Pd, Pt, Rh, Ir) atoms in NM/YSZ systems, the d-band centers of the NM atoms shift closer to the Fermi level (εd =-1.93,-1.29,-1.61,-1.53 eV) when the Ov is introduced. The results show that the Ov would serve as an electron donator, which is benefit for stabilizing the single NM adatoms and changing their d-band centers and, 8


Page 9 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


therefore, their activity. An ultimate prerequisite for single-atom metal species to be an effective catalyst is to avoid their aggregation. It is necessary to investigate the diffusion of the single NM atom on the defective surface to verify its stability. To clarify the effect of Ov on improving the stability, the diffusion of the single NM atom on the YSZ(111) and YSZ(111)-Ov surfaces are comparatively investigated. We select the most stable adsorption configuration as the initial states (IS) and the minor stable adsorption configuration as the final states (FS). The minimum energy paths (MEP) for the diffusion of NM atoms on YSZ(111) surface are shown in Figure 5. NEB calculations show that the single Pd atom could easily diffuse on YSZ(111) surface with small energy barrier 0.54 eV along a pathway between equilibrium states. But the other single NM atoms (Pt, Rh and Ir) diffuse with higher energy barriers (1.12, 1.05, 1.82 eV) on YSZ(111) surface. These results show that the Pd atom has the lowest stability and the other atoms have higher stability. This is understandable because these atoms (Pt, Rh and Ir) have undersaturated d-shells and interact with their neighbors with strong bonds. In contrast, the isolated Pd atom has a full d-shell and interact with its neighbors weakly. Comparatively, on the YSZ-Ov(111) surface, the Pd, Pt and Rh adatoms have larger diffusion barriers (1.42,1.76 and 1.29 eV). Although the Ir adatom has a large diffusion barrier (1.45eV) on YSZ-Ov(111), it is still lower than that on YSZ (111) (1.82eV). It is worth noting that the Ov moves along with the NM atom during the diffusion. In these stable adsorption configurations (ISs and FSs), the NM atom always stays at the Ov. Therefore, for Ir/YSZ-Ov(111), the diffusion of the Ov exacerbates the Ir atom diffusing. But for Pd/YSZ-Ov(111), Pt/YSZ-Ov(111) and Rh/YSZ-Ov(111), the Ov weaken the diffusion of the NM atom. To better appreciate the qualitative trends, we summarize the key results in Figure 6. In Figure 6 (a), one may see that the adsorption strength of NM atoms on the YSZ (111) surface (Eads) and the diffusion barriers (Eb) follow the same trend: Ir>Pt>Rh>Pd, which indicates that the stronger adsorption of the adsorbed NM atom, the higher stability on YSZ (111) surface. The highest point of the line suggests that the Ir/YSZ(111) could be the most stable one among the NM/YSZ(111) systems. The vacancy formation energies (Ef), adsorption energies 9



Page 10 of 27

and diffusion barriers of NM/YSZ-Ov (111) are depicted in Fig. 6(b). It is worth noting that a highest point appears at Pt/YSZ-Ov (111) for the line of adsorption energy and diffusion barrier, respectively. Besides, the corresponding line of vacancy formation energy reaches the lowest point for Pt/YSZ-Ov (111). The largest values of adsorption energy and diffusion barrier and the lowest vacancy formation energy all suggest that the Pt/YSZ-Ov (111) system would be a good catalyst with higher stability. In general, a good catalyst should have high stability and activity. In the above, we have showed that the Ov is benefit for stabilizing the single NM adatoms and changing their d-band centers and their activity. The Pt/YSZ-Ov (111) system shows the strongest stability and activity compared with the other systems, which may lead to higher activities in dissociation of O2. 3.4 Adsorption and Dissociation of O2 Molecule As shown above, the Ir/YSZ(111) and Pt/YSZ-Ov (111) have the highest stability among the NM/YSZ(111) and NM/YSZ-Ov (111) systems, respectively. In the following, we select these two systems to probe their activity and the role of the Ov. We investigate and compare the adsorption and dissociation of O2 on the Ir/YSZ(111) and Pt/YSZ-Ov (111) supports. The stable adsorption configurations for O2 on Ir/YSZ(111) and Pt/YSZ-Ov supports are shown in Figure 7(a) (IS) and Figure 7(b) (IS), respectively. The corresponding adsorption energies are summarized in Table 3. It is found that O2 prefers to be adsorbed on the top of the Ir atoms and forms two bonds with the Ir adatoms. The O-O bond is parallel to the Ir/YSZ(111) surface on the adsorbed Ir with bond lengths of 1.39 Å and an adsorption energy of 2.63 eV. On the Pt/YSZ-Ov(111) surface, however, the O2 prefers to be adsorbed near the Ov and forms one bond with the Pt adatom. The O2 molecule has a lower adsorption energy (1.70 eV) and a longer bond (1.50 Å) when it adsorbed on the Pt/YSZ-Ov(111) surface. This is beneficial for the Pt/YSZ-Ov(111) to be a better catalyst, because if oxygen binds too strongly to the catalyst, it would poison the surface and reduce its activity. On the other hand, if oxygen binds too weakly to the catalyst, the interaction between oxygen and catalyst may be too weak to have high reaction rate.. Table 3 The adsorption and dissociation properties of O2 on the surfaces of Ir/YSZ(111) and 10


Page 11 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt/YSZ-Ov(111), including the adsorption energy of O2 (Eads, in eV), the total charges of the adsorbed NM atom (NM-chg, in e) and that of the adsorbed O2 (O2-chg, in e), the bond length of the absorbed O2 atoms (dO-O, in Å) and the activation energy for O2 dissociation (Ea, in eV). The negative and positive numbers of the charges represent the loss and gain of electrons, respectively.

Eads(eV)

NM-chg(e)

O2-chg(e)

dO-O(Å)

Ea(eV)

Ir/YSZ

2.63

-0.53

0.72

1.39

1.04

Pt/YSZ-Ov

1.70

0.38

1.37

1.50

0.33

According to the Bader charge analysis, the Ir adatom loses about 0.53 e, while the adsorbed O2 molecule gains about 0.72 e on Ir/YSZ(111) surface, indicating that electronic charge transfers from Ir adatoms to O2. The charge redistribution due to the adsorption of O2 on the Ir/YSZ(111) in Figure 7(c) indicates that electrons deplete from the vicinity of the Ir atoms and accumulate in the vicinity of the Ir–O2 interfaces. Since the positive charge on the adsorbed Ir atom would reduce the ability of the metal centers to donate electrons into the empty 2π* orbital of O2, the single Ir atom supported by YSZ(111) surface would not be favorable for the adsorption and activation of O2. In contrast, on Pt/YSZ-Ov(111) surface, O2 molecule gains more electrons, 1.37 e, which are almost 2-fold compared with the original value on the Ir/YSZ(111) surface, while the Pt atom still possesses about 0.38 e. The CDD map for O2 on the Pt/YSZ-Ov system is shown in Figure 7(d). Compared with Ir/YSZ(111) system, more electrons accumulate at the O and Pt atoms near the vacancy. The much more pronounced charge density redistribution and the bond elongation of O2 on the Pt/YSZ-Ov system (Table 3) as compared with those on the Ir/YSZ(111) system indicate the adsorbed O2 is much more activated on the Pt/YSZ-Ov system. This result is in accordance with the results of the d-band center analysis. Therefore, the Ov in the NM/YSZ(111) systems would influence the charge states of the NM catalysts and then influence their activities toward O2 dissociation. The Pt/YSZ-Ov system could be a good catalyst with high stability and activity. To further clarify the effect of Ov on improving the activity of the NM/YSZ systems, the 11



Page 12 of 27

dissociation of O2 on the Ir/YSZ(111) and Pt/YSZ-Ov(111) surfaces are comparatively investigated. The climbing image nudged elastic band method (CI-NEB)54 is adopted to identify reaction paths and transition states. The potential energy surfaces for the dissociation of O2 are calculated and shown in Figure 7(b) and (d), in which the optimized adsorbed O2 configurations are chosen as the IS states. On Ir/YSZ(111) system (Figure 7(b)), the dissociative O2 geometry with one O atom binding with Ir and Y atoms, the other O binding with Ir atom and over the O top site is chosen as the FS. The calculated activation barriers for O2 dissociation is 1.04 eV with about 2.24 eV reaction energy released on Ir/YSZ(111). Although the Ir/YSZ(111) has higher stability compared with the other NM/YSZ(111) systems, the activation barrier for O2 dissociation (1.04 eV) is still big, indicating that the cleaving of O-O bond is difficult on the NM/YSZ(111) systems. Interestingly, on the Pt/YSZ-Ov(111) surface, the optimized dissociative O2 geometries (FS) have one O atom filling in the Ov site and the other O atom binding on the Zr top site. During the dissociation process, the O2 approaches the Ov site and then the O-O bond is cleaved with the Zr-O bonds formed with the Ov healed and the original YSZ surface recovered. The dissociation of O2 needs a rather small barrier of 0.33 eV and releases a large amount of energy (2.03 eV) on the Pt/YSZ(111)-Ov surface. It is obvious that the dissociation of O2 on the Pt/YSZ(111)-Ov surface is favorable and the Ov acts as an active center for O2 dissociation, which ensure the high efficiency and activity of the Pt/YSZ(111)-Ov system in the catalytic process. Therefore, the Pt/YSZ(111)-Ov system is the best catalysts for the dissociation of O2 with the highest stability and the smallest barrier. 4. CONCLUSIONS To shed light for the search of single-atom metal species with high stability and activity for ORR in fuel cells, we performed DFT calculations to investigate novel catalysts that consist of single noble metal (NM = Pd, Pt, Rh and Ir) atoms on the pristine YSZ(111) and the YSZ(111) surface with O vacancy (Ov). It is interesting that the locations of Ov in the YSZ systems can be regulated through the adsorption of an appropriate NM atom. The different NM/YSZ substrates (with or without Ov) would modify the electronic structure of 12


Page 13 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the NM catalysts, activate the surface oxygen, changing their d-band centers and, therefore, their activity. According to the analysis of the adsorption energy, diffusion barrier and vacancy formation energy we summarize that the Ir/YSZ(111) and Pt/YSZ-Ov (111) systems could be the most stable systems compared with other NM/YSZ(111) and NM/YSZ-Ov (111) systems, respectively. Besides, the analysis of the d-band center, DOS and CDDs indicates that the Pt/YSZ-Ov (111) system could have the higher catalytic activities for the adsorption and dissociation of O2 molecule and may be beneficial to ORR. To further probe their activity and the role of the Ov, we investigated the adsorption and dissociation of O2 on the Ir/YSZ(111) and Pt/YSZ-Ov (111) systems, respectively. It is found that the cleaving of O-O bond is difficult on Ir/YSZ(111). The dissociation of O2 has much smaller energy barrier on Pt/YSZ-Ov (111), and the Ov significantly promotes the interfacial charge transfer and facilitates the bond breaking of O2 on Pt/YSZ-Ov (111), which is predicted as the highly efficient and stable catalyst for O2 dissociation and would be beneficial to ORR. The results validate the stability and reactivity of novel catalysts on the atomic scale supported on YSZ.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 11174070, 11474086 and 11704100). Parts of the simulations are performed on computer resources provided by the High Performance Computing Center of Henan Normal University and the National Super Computing Center of Shenzhen.

13



Page 14 of 27

REFERENCES (1) Yang, X.F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T., Single-Atom Catalysts: A New Frontier in Heterogeneous Catalysis. Accounts Chem. Res. 2013, 46, 1740-1748. (2) Hu, P.; Huang, Z.; Amghouz, Z.; Makkee, M.; Xu, F.; Kapteijn, F.; Dikhtiarenko, A.; Chen, Y.; Gu, X.; Tang, X., Electronic Metal–Support Interactions in Single-Atom Catalysts. Angew. Chem. Int. Edit. 2014, 53, 3418-3421. (3) Mao, K.; Li, L.; Zhang, W.; Pei, Y.; Zeng, X. C.; Wu, X.; Yang, J., A Theoretical Study of Single-Atom Catalysis of Co Oxidation Using Au Embedded 2d H-Bn Monolayer: A Co-Promoted O2 Activation. Sci. Rep. 2014, 4, 5441. (4) Liang, J.X.; Lin, J.; Yang, X.F.; Wang, A.Q.; Qiao, B.T.; Liu, J.; Zhang, T.; Li, J., Theoretical and Experimental Investigations on Single-Atom Catalysis: Ir1/Feox for Co Oxidation. J. Phys. Chem. C 2014, 118, 21945-21951. (5) Li, X.N.; Yuan, Z.; Meng, J.H.; Li, Z.Y.; He, S.-G., Catalytic Co Oxidation on Single Pt-Atom Doped Aluminum Oxide Clusters: Electronegativity-Ladder Effect. J. Phys. Chem. C 2015, 119, 15414-15420. (6) Hackett, S. F. J.; Brydson, R. M.; Gass, M. H.; Harvey, I.; Newman, A. D.; Wilson, K.; Lee, A. F., High-Activity, Single-Site Mesoporous Pd/Al2O3 Catalysts for Selective Aerobic Oxidation of Allylic Alcohols. Angew. Chem. Int. Edit 2007, 119, 8747-8750. (7) Zhang, X.; Shi, H.; Xu, B.Q., Catalysis by Gold: Isolated Surface Au3+ Ions Are Active Sites for Selective Hydrogenation of 1,3-Butadiene over Au/ZrO2 Catalysts. Angew. Chem. Int. Edit 2005, 44, 7132-7135. (8) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T., Single-Atom Catalysis of Co Oxidation Using Pt1/Feox. Nat Chem 2011, 3, 634-641. (9) Ghosh, T. K.; Nair, N. N., Rh1/Γ-Al2O3 Single-Atom Catalysis of O2 Activation and Co Oxidation: Mechanism, Effects of Hydration, Oxidation State, and Cluster Size. ChemCatChem 2013, 5, 1811-1821. (10) Dong, S.; Zhang, Y.; Zhang, X.; Mao, J.; Yang, Z., A Possible Highly Active Supported Ni Dimer Catalyst for O2 Dissociation: A First-Principles Study. Appl. Surf. Sci. 2017,402,168-174. (11) Fajín, J. L.; Cordeiro, M. N. D.; Gomes, J. R., Dft Study on the Reaction of O2 Dissociation Catalyzed by Gold Surfaces Doped with Transition Metal Atoms. Applied Catalysis A: General 2013, 458, 90-102.

14


Page 15 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12) Deng, X.; Min, B. K.; Guloy, A.; Friend, C. M., Enhancement of O2 Dissociation on Au (111) by Adsorbed Oxygen: Implications for Oxidation Catalysis. J. Am. Chem. Soc. 2005, 127, 9267-9270. (13) Shishkin, M.; Ziegler, T., The Oxidation of H2 and CH4 on an Oxygen-Enriched Yttria-Stabilized Zirconia Surface: A Theoretical Study Based on Density Functional Theory. J. Phys. Chem. C 2008, 112, 19662-19669. (14) Breedon, M.; Spencer, M.; Miura, N., The Adsorption of NO on YSZ (111) and Oxygen-Enriched YSZ (111) Surfaces. Chem. Phys. Lett. 2014, 593, 61-68. (15) Chu, X.; Lu, Z.; Zhang, Y.; Yang, Z., Can H2S Poison the Surface of Yttria-Stabilized Zirconia? Int. J. Hydrogen Energ. 2013, 38, 8974-8979. (16) Shishkin, M.; Ziegler, T., Oxidation of H2, CH4, and Co Molecules at the Interface between Nickel and Yttria-Stabilized Zirconia: A Theoretical Study Based on DFT. J. Phys. Chem. C 2009, 113, 21667-21678. (17) Cadi-Essadek, A.; Roldan, A.; de Leeuw, N. H., Ni Deposition on Yttria-Stabilized ZrO2 (111) Surfaces: A Density Functional Theory Study. J. Phys. Chem. C 2015, 119, 6581-6591. (18) Janardhanan, V. M.; Heuveline, V.; Deutschmann, O., Three-Phase Boundary Length in Solid-Oxide Fuel Cells: A Mathematical Model. J. Power Sources 2008, 178, 368-372. (19) Frondelius, P.; Häkkinen, H.; Honkala, K., Adsorption of Small Au Clusters on MgO and MgO/Mo: The Role of Oxygen Vacancies and the Mo-Support. New J.Phys. 2007, 9, 339. (20)Del Vitto, A.; Pacchioni, G.; Delbecq, F.; Sautet, P., Au Atoms and Dimers on the MgO (100) Surface: A DFT Study of Nucleation at Defects. J. Phys. Chem. B 2005, 109, 8040-8048. (21) Ganduglia-Pirovano, M. V.; Hofmann, A.; Sauer, J., Oxygen Vacancies in Transition Metal and Rare Earth Oxides: Current State of Understanding and Remaining Challenges. Surf. Sci. Rep. 2007, 62, 219-270. (22)Shim, J. H.; Park, J. S.; Holme, T. P.; Crabb, K.; Lee, W.; Kim, Y. B.; Tian, X.; Gür, T. M.; Prinz, F. B., Enhanced Oxygen Exchange and Incorporation at Surface Grain Boundaries on an Oxide Ion Conductor. Acta Mater. 2012, 60, 1-7. (23)An, J.; Park, J. S.; Koh, A. L.; Lee, H. B.; Jung, H. J.; Schoonman, J.; Sinclair, R.; Gür, T. M.; Prinz, F. B., Atomic Scale Verification of Oxide-Ion Vacancy Distribution near a Single Grain Boundary in YSZ. Sci. Rep. 2013, 3, 2680. (24) Lee, W.; Jung, H. J.; Lee, M. H.; Kim, Y.-B.; Park, J. S.; Sinclair, R.; Prinz, F. B., Oxygen Surface Exchange at Grain Boundaries of Oxide Ion Conductors. Adv.Funct.Mater. 2012, 22, 965-971. (25) Lee, H. B.; Prinz, F. B.; Cai, W., Atomistic Simulations of Grain Boundary Segregation in Nanocrystalline Yttria-Stabilized Zirconia and Gadolinia-Doped Ceria Solid Oxide Electrolytes. Acta Mater. 2013, 61, 3872-3887. 15



Page 16 of 27

(26) Abe, H.; Murata, K.; Fukui, T.; Moon, W. J.; Kaneko, K.; Naito, M., Microstructural Control of Ni–Ysz Cermet Anode for Planer Thin-Film Solid Oxide Fuel Cells. Thin Solid Films 2006, 496, 49-52. (27) Muñoz, M. C.; Gallego, S.; Beltrán, J. I.; Cerdá, J., Adhesion at Metal–ZrO2 Interfaces. Surf. Sci. Rep. 2006, 61, 303-344. (28) Sasaki, T.; Matsunaga, K.; Ohta, H.; Hosono, H.; Yamamoto, T.; Ikuhara, Y., Atomic and Electronic Structures of Ni/YSZ (111) Interface. Mater. Trans.2004, 45, 2137-2143. (29) Zhu, J.; van Ommen, J. G.; Bouwmeester, H. J. M.; Lefferts, L., Activation of O2 and CH4 on Yttrium-Stabilized Zirconia for the Partial Oxidation of Methane to Synthesis Gas. J. Catal. 2005, 233, 434-441. (30) Wang, W.; Su, C.; Wu, Y.; Ran, R.; Shao, Z., Progress in Solid Oxide Fuel Cells with Nickel-Based Anodes Operating on Methane and Related Fuels. Chem.Rev. 2013, 113, 8104-8151. (31) Zhu, J.; van Ommen, J. G.; Lefferts, L., Partial Oxidation of Methane by O2 and N2O to Syngas over Yttrium-Stabilized ZrO2. Catal. Today 2006, 112, 82-85. (32) Xia, X.; Oldman, R.; Catlow, R., Computational Modeling Study of Bulk and Surface of Yttria-Stabilized Cubic Zirconia. Chem. Mater. 2009, 21, 3576-3585. (33) Cucinotta, C. S.; Bernasconi, M.; Parrinello, M., Hydrogen Oxidation Reaction at the Ni/YSZ Anode of Solid Oxide Fuel Cells from First Principles. Phys. Rev. Lett. 2011, 107, 206103. (34) Shishkin, M.; Ziegler, T., Hydrogen Oxidation at the Ni/Yttria-Stabilized Zirconia Interface: A Study Based on Density Functional Theory. J. Phys. Chem. C 2010, 114, 11209-11214. (35) Zhang, Y.; Fu, Z.; Wang, M.; Yang, Z., Oxygen Vacancy Induced Carbon Deposition at the Triple Phase Boundary of the Nickel/Yttrium-Stabilized Zirconia (YSZ) Interface. J. Power Sources 2014, 261, 136-140. (36) Zhang, Y.; Yang, Z., Resistance to Sulfur Poisoning of the Gold Doped Nickel/Yttria-Stabilized Zirconia with Interface Oxygen Vacancy. J. Power Sources 2014, 271, 516-521. (37) Kim, S.D.; Moon, H.; Hyun, S.H.; Moon, J.; Kim, J.; Lee, H.W., Performance and Durability of Ni-Coated Ysz Anodes for Intermediate Temperature Solid Oxide Fuel Cells. Solid State Ionics 2006, 177, 931-938. (38) Jung, C.; Ishimoto, R.; Tsuboi, H.; Koyama, M.; Endou, A.; Kubo, M.; Del Carpio, C. A.; Miyamoto, A., Interfacial Properties of Zro 2 Supported Precious Metal Catalysts: A Density Functional Study. Appl. Catal. A: Gen.2006, 305, 102-109.

16


Page 17 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) Jung, C.; Ito, Y.; Endou, A.; Kubo, M.; Imamura, A.; Selvam, P.; Miyamoto, A., Quantum-Chemical Study on the Supported Precious Metal Catalyst. Catal. Today 2003, 87, 43-50. (40) Kotsionopoulos, N.; Bebelis, S., Electrochemical Promotion of the Oxidation of Propane on Pt/YSZ and Rh/YSZ Catalyst-Electrodes. J. Appl. Electrochem. 2005, 35, 1253-1264. (41) Jiménez-Borja, C.; Dorado, F.; de Lucas-Consuegra, A.; García-Vargas, J. M.; Valverde, J. L., Complete Oxidation of Methane on Pd/YSZ and Pd/CeO2/YSZ by Electrochemical Promotion. Catal. Today 2009, 146, 326-329. (42)Kimura, T.; Goto, T., Ir–Ysz Nano-Composite Electrodes for Oxygen Sensors. Surf. Coat. Tech.2005, 198, 36-39. (43) Yoon, S. P.; Nam, S. W.; Kim, S.G.; Hong, S.A.; Hyun, S.H., Characteristics of Cathodic Polarization at Pt/YSZ Interface without the Effect of Electrode Microstructure. J. Power Sources 2003, 115, 27-34. (44) Opitz, A. K.; Fleig, J., Investigation of O2 Reduction on Pt/Ysz by Means of Thin Film Microelectrodes: The Geometry Dependence of the Electrode Impedance. Solid State Ionics 2010, 181, 684-693. (45) Beck, G.; Fischer, H.; Mutoro, E.; Srot, V.; Petrikowski, K.; Tchernychova, E.; Wuttig, M.; Rühle, M.; Luerßen, B.; Janek, J., Epitaxial Pt(111) Thin Film Electrodes on YSZ(111) and YSZ(100) — Preparation and Characterisation. Solid State Ionics 2007, 178, 327-337. (46) Mutoro, E.; Luerssen, B.; Günther, S.; Janek, J., Structural, Morphological and Kinetic Properties of Model Type Thin Film Platinum Electrodes on Ysz. Solid State Ionics 2008, 179, 1214-1218. (47) Mitterdorfer, A.; Gauckler, L. J., Identification of the Reaction Mechanism of the Pt, O2(G)|Yttria-Stabilized Zirconia System: Part I: General Framework, Modelling, and Structural Investigation. Solid State Ionics 1999, 117, 187-202. (48) Kresse, G.; Furthmüller, J., Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169. (49) Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple. Phys. Rev. lett. 1996, 77, 3865. (50) Blöchl, P. E., Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953. (51) Kresse, G.; Joubert, D., From Ultrasoft Pseudopotentials Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758.

to

the

Projector

(52) Zhang, Y.; Lu, Z.; Yang, Z.; Woo, T., The Mechanism of Sulfur Poisoning on the Nickel/Yttrium-Stabilized Zirconia Anode of Solid Oxide Fuel Cells: The Role of the Oxygen Vacancy. J. Power Sources 2013, 237, 128-131.

17



Page 18 of 27

(53) Zhang, Y.; Yang, Z., The Origin of the Low Efficiency of Carbon Removal from the Nickel/Yttrium–Stabilized Zirconia Triple-Phase Boundary by Adsorbed Water. J. Power Sources 2015, 279, 224-230. (54) Henkelman, G.; Uberuaga, B. P.; Jónsson, H., A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113, 9901-9904.

18


Page 19 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Captions Figure 1 The YSZ model: (a) side views, (b) top views, the IV symbols represent the intrinsic oxygen vacancies in the YSZ lattice, (c)-(e) top views of the YSZ surfaces with a surface vacancy at the V1, V2 and V3 locations, respectively. Figure 2 The optimized adsorption configurations of NM/YSZ(111) (NM=Pd, Pt, Rh and Ir) and NM/YSZ-Ov(111) systems. Figure 3 Calculated charge density differences (CDD) for the most stable adsorption configurations of NM/YSZ(111) (NM=Pd, Pt, Rh and Ir) ((a), (b), (c) and (d)) and NM/YSZ-Ov(111) ((e), (f), (g) and (h)), respectively. Color scheme is labeled in the figure. The isosurface value used is ±0.02 e/Å3. Figure 4 The d and s partial density of states (PDOS) of the NM atoms in the NM/YSZ(111) and NM/YSZ-Ov(111) systems, respectively. Figure 5 The minimum energy paths (MEPs) for the diffusion of the single NM atom on the YSZ (111) ((a), (b), (c) and (d)) and YSZ-Ov(111) surfaces ((e), (f), (g) and (h)), respectively. Figure 6 (a) Adsorption energies and diffusion barriers of the NM atom in the NM/YSZ(111) systems, (b) adsorption energies, diffusion barriers and vacancy formation energies of the NM atom in the NM/YSZ-Ov(111) systems. Figure 7 The minimum energy paths (MEPs) and CDD for the dissociation process of O2 on the Ir/YSZ(111) ((a) and (c)) and Pt/YSZ-Ov(111)((b) and(d)) surfaces.

19



Page 20 of 27

TOC Graphic:

The Pt/YSZ-Ov(111) exhibits high stability and reactivity for ORR.

20


Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


IV

IV IV

IV

(b)

(a)

V3 V1

V2

(c)

O

(e)

(d)

Zr


Y

SubO Figure 1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Pd/YSZ

Pt/YSZ

Pd/YSZ-Ov

Pt/YSZ-Ov

Rh/YSZ

Rh/YSZ-Ov

Page 22 of 27

Ir/YSZ

Ir/YSZ-Ov


Figure 2

Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


O

0.02

Pd O

O

O

Pt

Rh

Ir O

O

O

(a)

0

O

(c)

(b)

(d)

O

V1

Pd

Zr

Pt

V1

Zr Zr

O Zr

Rh V2

Ir V2

Zr

Zr

-0.02

(e)

(f)


(g)

(h) Figure 3

The Journal of Physical Chemistry Pd-d 4 Pd-s

6

2

Pd/YSZ

0

0

-3

-2

DOS(states/eV)

DOS(states/eV)

3

-6 6

Pd/YSZ-Ov

3

4

-3

-2

-6

-4

-2

0

2

Pt/YSZ-Ov

2 0

-4

Pt/YSZ

-4

0

-6

4

-8

-6

-4

-2

0

E(eV)

E(eV)

(a)

(b)

2

4

4

Rh-d Rh-s

4 2

Ir-d Ir-s

2

Ir/YSZ

Rh/YSZ 0

DOS(states/eV)

0

DOS(states/eV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 24 of 27 Pt-d Pt-s

-2 -4 4

E(eV)

-2 -4 4 2

Ir/YSZ-Ov

Rh/YSZ-Ov

2

0

0

-2

-2

-4

-4 -6

-4

-2

0

E(eV)

(c)

2

4

6

-8

-6

-4

-2

0

2

4

6

E(eV) ACS Paragon Plus Environment

(d)

Figure 4

Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


TS(0.54)

(a)

IS(0.00)

TS(1.12)

FS(0.31)

FS(0.04)

TS(1.76) FS(0.29)

(f)

TS(1.05)

(c) IS(0.00)

TS(1.82)

IS(0.00)

(e) IS(0.00)

FS(0.77)

(b) IS(0.00)

(d)

TS(1.42)

FS(0.59)

IS(0.00)

TS(1.29) FS(0.07)

(g) IS(0.00)

TS(1.45)

FS(1.74) ACS Paragon Plus Environment IS(0.00)

(h)

FS(0.27)

Figure 5

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Page 26 of 27

(b)

(a)


Figure 6

Page 27 of 27

O TS(1.04)

O Ir

IS(0.00)

O

O

FS(-2.24) (c)

O

(a)

O TS(0.33)

Pt

IS(0.00) FS(-2.03) (b)


O

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41


(d)

Figure 7

High Stability and Reactivity of Single-Metal Atom Catalysts Supported

Recommend Documents