Structure, Bonding, and Catalytic Properties of Defect Graphene

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Structure, Bonding, and Catalytic Property of Defect Graphene Coordinated Pd-Ni Nanoparticles Shiuan-Yau Wu, and Hsin-Tsung Chen J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 09 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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The Journal of Physical Chemistry

Structure, Bonding, and Catalytic Property of Defect Graphene Coordinated Pd-Ni Nanoparticles

Shiuan-Yau Wu and Hsin-Tsung Chen* Department of Chemistry, Chung Yuan Christian University, Chung Li District, Taoyuan City, 32023, Taiwan

*Corresponding

author. E-mail: [email protected]; Tel: +886-3-265-3324

1

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Abstract By means of periodic density functional theory calculations, we have investigated the structure, bonding, and catalytic property of defect graphene decorated Pd-Ni nanoparticles. According to systematical structure calculations for Pd-Ni nanoparticle, we found that Pd atoms tend to occupy the positions with lower coordination number and Ni atoms prefer to locate at the position with high coordination number resulting in the formation of core-shell Ni-Pd nanoparticles which agrees with the experimental observation. The lowest-energy Pd6Ni4 and Pd4Ni6 isomers are selected to examine the interaction between Pd-Ni nanoparticle and defect graphene. The adsorption energies are calculated to be –5.26 and –4.22 eV/atom for the Pd6Ni4 and Pd4Ni6 on the defect graphene, respectively. In addition, we found that more Ni-C bonding formation in the interface region could enhance the interaction between Pd-Ni nanoparticle and the defect graphene. Through the analysis of the electronic structures of Pd6Ni4 and Pd4Ni6 on the defect graphene, it is found that by using Ni atoms to combine with the defect site could prevent the electron missing from Pd atoms directly benefit the cluster dispersion without sacrificing the overall catalytic performance. The resulted Pd-Ni/defect graphene catalyst exhibits highly activity and selectivity for the formation of formic acid from CO2 and H2.

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1. Introduction Because of higher CO resistance of palladium and its higher abundance than platinum, palladium is a suitable alternative for fuel cells,1, 2 hydrogen storage,3, 4 hydrogen-related catalytic reactions5-7, catalytic converter for car exhaust8, 9, and C-C coupling reactions.10, 11 Over the last decade, the use of transition metal nanoparticles as catalysts has been attracted great attention due to their large surface areas and specific activity. Recent experiments have investigated the effectiveness of Pd nanoparticles1, 2, 11-13 and Pd-based alloys,14-22 such as Pd–Ni, Pd–Au, Pd–Au and Pd– Cu. It was proposed that the Pd catalysts have high activity toward ethanol oxidation reaction (EOR) in high pH media and the Pd–Ni nanoparticles is able to further promote the overall ethanol oxidation kinetics.15 The Pd–Ni alloy catalysts was also found to be an active catalyst in Suzuki–Miyaura cross-coupling reactions16 and enhanced the catalytic activity for CO2 hydrogenation into formic acid as compared to monometallic Pd catalyst by increase the adsorption ability of CO2.22 Carbon materials have been extensively employed as catalyst supports in fuel cells, sensors, and solar cell applications because of its high area for the dispersion of metal nanoparticles, high electrical conductivity and low cost. Pd-based nanoparticles on active carbon and graphene materials show good performance for hydrogen storage in both experimental and theoretical investigations.13, 21-26 The specific activity 3



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of a catalyst is related to the particle size and dispersion. The defect sites in the carbon-based materials could suppress the coalescence of metal particle and less particle size by increasing the interaction between metal nanoparticle and the support.24 However, there are still several controversial points: (1) the fine structure of Pd-Ni nanoparticle decoration on the defect graphene is completely unknown; (2) the coordination of Pd-Ni nanoparticle on the defect and the interaction between metal nanoparticle and the defect are unexamined; (3) the nature of Pd–Ni/defect-graphene catalysts remain ambiguous. The formation and reactivity of double vacancies (DV) graphene have been studying theoretically and experimentally.27, 28 Among the DV graphene, the 555-777 defect is the most stable one with a lower formation energy of 0.9 eV than that of 5-8-5 defect.26 However, the creating of 555-777 defect through the rotation of one C-C bond in octagon of 5-8-5 defect needs to overcome the energy barrier of ca. 5eV or proceed above 3800K in MD simulation.27 Consequently, we select the 5-8-5 defect graphene sheet as the representative model for our calculation. In this work, we have systematically performed the density functional theory calculations to investigate the formation of Pd-Ni clusters from tricapped PBP Pd10 cluster, which is convenient to control the ratio between Pd and Ni, and selected the Pd6Ni4 and Pd4Ni6 clusters to represents the Pd-rich and Ni-rich alloyed clusters. The 4


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structures and adsorption behaviors of Pd10, Pd6Ni4, Pd4Ni6 and Ni10 clusters on 5-8-5 defect graphene are then examined. The nature of Pd10, Pd6Ni4, Pd4Ni6, Ni10 clusters and those clusters on the 5-8-5 defect graphene are illustrated by d-band center calculation for the catalytic property. Finally, hydrogen adsorption and CO2 hydrogenation on Pd-Ni/defect-graphene are also investigated to explore the catalytic performance.

2. Computational methods All geometric optimizations were performed by the Vienna ab initio simulation package (VASP),29-31 based on spin-polarized density-functional theory (DFT) with the projector-augmented wave method (PAW).30, 32 The Kohn-Sham equations were solved in a self-consistent method under the generalized gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional.33 The Monkhorst-Pack mesh k-points34 (1 × 1 × 1) and (5 × 5 × 1) was used for isolated clusters and surface calculations, respectively, with energy truncated at 400 eV.

All slabs were separated

by a vacuum spacing greater than 20 Å, which ensures no interaction between the slabs. The 5-8-5 defect graphene was built by removing two adjacent carbon atoms from a 6 × 6 graphene supercell (72 atoms) with periodic boundary condition. The 5



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geometry of isolated Pd10, Pd6Ni4, Pd4Ni6 and Ni10 clusters was optimized in a 20 × 20 × 20 Å3 cubic cell. The cohesive energy35 (Ecoh) was employed to identify the stability of different clusters and was defined as follow: Ecoh = [E(PdxNiy) – xE(Pd)– yE(Ni)]/(x + y) in which E(PdxNiy) is the total energy of Pd-Ni clusters, E(Pd) and E(Ni) are the atomic energies for bulk Pd and Ni, respectively. In different isomers for Pd6Ni4 (or Pd4Ni6) the more stable one in potential energy (more positive E(PdxNiy)) would exhibit less cohesive energy as compare to the Pd and Ni bulk. DFT molecular dynamic (MD) simulation was also carried to test the stability of adsorbed clusters on defect graphene with the k-points of 3 × 3 × 1 in a canonical (NVT) ensemble condition; all MD simulations were performed using 1fs time step for 1ps at 1000K. The adsorption energy was computed to identify the stability of different clusters on defect sites of graphene by the following equation: Eads = E(cluster/defect graphene) – E(defect graphene) – E(cluster) where E(cluster/defect graphene), E(defect graphene) and E(cluster) are the total energy of adsorbed clusters on 5-8-5 defect graphene, 5-8-5 defect graphene, and isolated cluster, respectively. The climbing image nudged-elastic-band (CI-NEB) method36 was employed to locate transition structures, and minimum energy pathways (MEP) were constructed accordingly. 6


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3. Results and discussion 3.1 Isolated Pd10, Pd6Ni4, Pd4Ni6 and Ni10 clusters Geometry analysis The structures of Pd10 and Ni10 clusters are modeled as tricapped PBP structure since the tricapped PBP structure has been demonstrated as the low-lying one of Pd10 and Ni10 clusters in previous studies.37-40 As shown in Figure 1a and 1b, four kinds of position metal with coordination numbers of 6, 5, 5 and 4 in Pd10 and Ni10 clusters are evaluated and characterized as A, B, C and D sites, respectively. The cohesive energy, magnetic moment, and mean bond length of Pd10 are calculated to be 1.39 eV/atom, 6 µB, and 2.69 Å, respectively. For the lowest-energy Ni10 cluster, the cohesive energy, magnetic moment, and mean bond length are 1.74 eV/atom, 8 µB, and 2.34 Å, respectively. The corresponding bond lengths and magnetic moments for the structures in Figure 1 are tabulated in Table 1.

For the Pd-Ni nanoparticle, the Pd6Ni4 and Pd4Ni6 clusters are selected and represented as the Pd-rich and Ni-rich compositions, respectively. Pd6Ni4 (Pd4Ni6) isomers are generated by replacing 4 Pd to Ni atoms (4Ni to Pd atoms) in the tricapped PBP of Pd10 (Ni10). Resulted 47 possible isomers of Pd6Ni4 and Pd4Ni6 clusters are found out and the optimized results are shown in Figures S1 and S2, 7



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respectively. Among these possible Pd6Ni4 and Pd4Ni6 isomers, the lowest-energy structure of Pd6Ni4 with the cohesive energy of 1.54 eV/atom and magnetic moment of 6 µB is depicted in Figure 1c while those are 0.99 eV/atom and 8 µB for the lowest-energy structure of Pd4Ni6 (see Figure 1d). The cohesive energy and average coordination number of all Pd6Ni4 and Pd4Ni6 isomers with partial corresponding structures are plotted in Figures 2a and 2b. It is found that the increasing average coordination number of Ni with decreasing average coordination number of Pd could stabilize the isomers energetically with the gradually enhanced cohesive energy of Pd6Ni4 and Pd4Ni6 clusters, indicating the Pd atoms tend to occupy the positions with lower coordination number (cn =4) and Ni atoms prefer to locate at position with high coordination number, such cn = 6 and 7. This finding explains that the similar distortion occurs in the most stable Pd6Ni4 and Pd4Ni6 (see Figures 1c and d) which has more low coordination-number for Pd (cn =4 for 5) and high coordination-number for Ni (cn = 7 for 2) compared to initial tricapped PBP structures (cn = 4 for 3 and cn = 6 for 1) of Pd10 and Ni10. We also considered DFT-MD simulation in a canonical (NVT) ensemble for further testing of stability of Pd6Ni4 and Pd4Ni6 clusters at 1000K, and calculation results are shown in Figure S3, in which both clusters would be slight distort and Ni atoms would segregate together obviously. This phenomenon is consistence with the experimental observation16 that Pd atoms expose to the corner or 8


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as a shell in the formation of core-shell Ni-Pd nanoparticles and the theoretical study on Pd9Mn cluster reported by Yuewen et. al.41 Electronic structural analysis

We calculated the individual d-band center42-44of Pd and Ni atoms (including the spin up , spin down and total ) in Pd10, Pd6Ni4, Pd4Ni6, and Ni10 clusters by analyzing the projected density of states as tabulated in Table 2. Compared d-band center of the alloyed clusters (Pd6Ni4 and Pd4Ni6) with that of pure Pd10 and Ni10, we found the major changes occur on Pd and Ni sections.

The

Bader charge calculations45, 46 show that the net charge transfer from Ni to Pd atoms in alloyed clusters (Pd6Ni4 and Pd4Ni6) leading to the average charges of Pd in Pd6Ni4 and Pd4Ni6 are −0.162 and −0.218 |e|, respectively.

The individual Pd of Pd10,

Pd6Ni4 and Pd4Ni6 are −1.08, −1.53 and −1.65 eV relative to Fermi energy, respectively, indicating that the charge redistribution of Pd6Ni4 and Pd4Ni6 causes the downward shift of Pd d-band with respect to Pd10 clusters, which is consistent with the other DFT calculation on Pd-based alloys.47, 48 In addition, the Ni in Pd6Ni4 and Pd4Ni6 are −1.94 and −1.71 eV, respectively, lower than that of pure Ni10 clusters (−1.40 eV).

It is due to the formation of Ni-Pd bonds in Pd6Ni4 and Pd4Ni6, causing

the hybridization of d-band of Ni and Pd, so that the downshift of Pd d-band would induce downshift of Ni d-band, simultaneously. Accordingly, the average coordination 9



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number of Ni becomes an important fact to affect the Ni; the average coordination numbers of Ni10, Pd4Ni6 and Pd6Ni4 are 4.8, 5.67 and 6.75, which is found correlated well with Ni.

3.2. Adsorption of single Ni and Pd atoms on defect graphene

In this section, we considered the single Ni and Pd as dopants to incorporate with 5-8-5 defect graphene and investigated the bonding characteristics between metal atoms and carbon atoms by using the electronic-localization function (ELF) and projected density of the states (pDOS). The 5-8-5 defect graphene (Figure 3a) is a double vacancies structure which can be generated either by removing two neighboring or coalescence of two single vacancies, and the carbon atom neighboring the vacancies would reconstruct to form two pentagons and one octagon. As compared to pristine graphene, the defect position serves as an active site due to increased intensity of the p-band in the vicinity of Fermi energy (in Figure 3d, and compared with pristine graphene showed in Figure S3). Figures 3b and 3c show the calculated structures of single Ni and Pd atoms adsorbed on 5-8-5 defect graphene, the anchoring of Ni and Pd would break the C-C bonds and form four covalent bonds (NiC4 and PdC4, the ELF diagram in Figures 3e and 3f) with the calculated adsorption energies of −5.21 and −4.06 eV, respectively, indicating that Ni-C bonds are stronger

10


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that Pd-C bonds. As the atomic radii of Ni and Pd are larger than carbon atom, Ni and Pd are displaced out of the initial plane by 0.57 and 1.10 Å, respectively. Furthermore, as shown in the pDOS diagram for Ni- and Pd-doped graphene (see Figures 3e and f), the embedded d-electrons into graphene cause the enhanced intensities of p-band and the appearance of s-band in the vicinity of Fermi energy (in the range of 0 to −2 eV), revealing the changed hybridization type (from sp2 to sp3) of carbon atoms, and then induce the slight increase of Ni- and Pd-graphene surface areas (1.99 and 3.11 % relative to the area of 5-8-5 defect graphene, in Table 3).

3.3. Adsorption of Pd10, Pd6Ni4, Pd4Ni6 and Ni10 clusters onto defect graphene

Geometry analysis

In order to find out the suitable models of cluster on defect graphene, the models in our studying are calculated under three procedures (shown in Figure 4). Take the Pd10 adsorption on defect graphene for instant, we selected atop site of PdB (cn = 5 in Figure 1a) to embed the Pd10 cluster onto the 5-8-5 defect graphene and obtained the calculated structure of Pd10-Gra, showing in the left of Figure 4a. Then we used the optimized structure as the initial configuration to perform the DFT-MD simulation in a canonical (NVT) ensemble for further testing of stability at 1000 K. The calculated structure after MD simulation for 1000 fs was gathered (see the middle of Figure 4a) 11



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and imported into DFT calculations; the re-optimization structure was obtained and shown on the right of Figure 4a. Comparison of the configurations of these three steps reveals that the average Pd-Pd bond length of Pd10 cluster is elongated at 1000 K by 0.06 Å and two Pd-Pd bonds were broken in the top region. The graphene sheet becomes wrinkled accompanying the elevation of Pd cluster off the plane about 0.6 Å. The similar adsorption energy of Pd10 cluster for the initial and re-optimized configurations is slightly different by 0.003 eV reveals the Pd10 cluster is stable and easily to recover its original structure after the structure distortion.

Figure 4b shows

the same calculation process for Pd6Ni4 cluster adsorption by using a Ni atom (cn= 5 in Figure 1b) of Pd6Ni4 to interact with defect site of graphene to get the configurations of charts a optimized, MD at 1000 K, and re-optimized configurations of Pd6Ni4-Gra.

At the initial structure of Pd6Ni4-Gra, the average bond length is 2.57

Å. In MD simulation at 1000 K, all Ni-Ni and Ni-Pd bonds were elongated but the average bond length shortened to 2.50 Å due to the scission of Pd-Pd bond. Furthermore, the re-optimized Pd6Ni4-Gra becomes more stable than initial structure with a lower relative energy of −0.115 eV.

To investigate the adsorption behaviors

of Pd10, Pd6Ni4, Pd4Ni6 and Ni10 clusters onto defect graphene, we considered several adsorption modes by using the different coordination types of Pd or Ni to bind with the defect site of graphene, and selected the re-optimized configurations after MD 12


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simulation as the low-lying structures to deal the following study.

In Figure 5, according to atomic species located at the defect site, the adsorption modes can be distinguished into Ni- and Pd-orientations by the formation of NiC4 or PdC4 when clusters adsorbed on the defect graphene.

However, the formation of

NiC4 or PdC4 is not the only fact affecting the stability of adsorbed clusters on graphene; as the pDOS shown in Figures 3e and f, the stronger p-band with the appearance of s-band of carbon atom near the Fermi energy indicates that these four carbon (surrounding the defect sites) are active and able to bind with metal atoms. Therefore, the number of formation of Ni-C and Pd-C bonds (besides NiC4 or PdC4) when clusters on the defect graphene is another factor to affect the interaction between clusters and graphene.

In Figure 5a, the Ni10 is adsorbed on the defect

graphene by the formation of NiC4 + 4NiC (labeled 4Ni in the chart) with the largest adsorption energy of −5.34 eV. The charts of Figures 5b ~ 5c show the adsorption energies of Pd4Ni6 and Pd6Ni4 on the defect graphene by using Ni orientation toward the defect site with various coordination modes, in which the largest adsorption models of alloyed cluster on the defect graphene is through the formation of NiC4 + 3NiC + 1PdC (labeled 3Ni1Pd in the chart) with the adsorption energy of −5.26 eV. The charts of Figures 5d-f show the Pd orientation for Pd10, Pd4Ni6 and Pd6Ni4 adsorbed on the defect graphene with different modes and the largest adsorption 13



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energies are −4.22, −3.88 and −3.89 eV, respectively. The lowest-energy configurations of charts a ~ f are shown in the lower region and the relevant calculated results are tabulated in Table 3. For pure clusters, the adsorption energy of lowest-energy configuration of adsorbed Ni10 is larger than Pd10; for Pd6Ni4-Gra and Pd4Ni6-Gra, the largest adsorption energies of Ni-orientation are −5.26 and −4.21 eV, respectively, which are both larger than that of Pd-orientation (−3.89 and −3.88 eV, respectively), indicating the Ni-orientation of Pd-Ni clusters to bind with the defect site of graphene is favorable. In addition, comparison of Ni10-, Pd4Ni6- and Pd6Ni4-Gra (both Ni- and Pd-orientations) shows that more Ni-C bond formation results in more stable adsorbed configuration. In the other hand, the existence of Pd atoms surrounding the interface would cause the large distortion of Pd-Ni clusters, leading to a reduced performance of the adsorption energies.

In summary, the order

of interaction energy49 with the corresponding coordinated type in table 3 is following: Ni10 (NiC4+ 4NiC) > Pd4Ni6 (NiC4+ 3NiC+1PdC) > Pd6Ni4 (NiC4+ 1NiC+3PdC) > Pd4Ni6 (PdC4+ 3NiC+1PdC) > Pd6Ni4 (PdC4+ 3NiC+1PdC) > Pd10Ni4 (PdC4+ 4PdC) > single Ni (NiC4) > single Pd (PdC4), indicating that Ni-C bond dominates the adsorption behavior of Pd-Ni alloyed clusters on the defect graphene.

Electronic analysis of Graphene supported Pd10, Pd6Ni4, Pd4Ni6 and Ni10 clusters Here we selected the lowest-energy configurations of Pd10, Pd6Ni4, Pd4Ni6 and 14


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Ni10 clusters adsorbed on the defect graphene to analyze the detailed interaction between each cluster and defect graphene. In Figure 6, we calculated the Bader charges to quantify the charge of each atom and plotted charge different of each model to observe the charge redistribution after cluster adsorption, in which the yellow and cyan areas represent the charge accumulation and depletion, respectively. The significant redistribution of charge of each model indicates the great interaction between the cluster and the defect graphene. The net charges of C-layers for Ni10-Gra, Pd6Ni4-Gra and Pd4Ni6-Gra (−1.66, −1.37 and −1.89 |e|) are larger than that of Pd10-Gra (−0.98 |e|) implying that greater extent of electron from the cluster to the defect graphene sheet accompanying the formation of Ni-C bonds. In addition, compared the pure and alloyed cluster system, the most difference is the electronic redistribution within clusters, in which the slight negative charge located on top four atoms of adsorbed Pd10 and Ni10 cluster, but obvious negative charge only located on the Pd atoms of adsorbed Pd6Ni4 and Pd4Ni6 clusters.

We also calculated the

individual d-band center of Pd and Ni atoms of adsorbed Pd10, Pd6Ni4, Pd4Ni6 and Ni10 clusters and the difference before and after adsorbed on the defect graphene (compared to the values for isolated clusters in Table 2), and tabulated in Table 4. Comparison the d-band center of isolated and adsorbed Pd10 clusters, the total d-band shift Pd is –0.26 eV, indicating the downwardly shifted of Pd d-band away 15



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from Fermi energy after adsorbed on graphene due to 0.98 electron transfer from Pd10 to the defect graphene, similar to the other DFT results about Pd13 on various defect graphene (–0.23 ~ –0.43 eV).50 According to the Nørskov’s d-band center theory, the position of d-band center correlates well with the adsorption energy of different molecules42-44. In general, the electron transfer from transition metal cluster to support would cause the downshifted of the d-band of metal cluster, but for the Ni-Pd alloyed system in this work, we found that major electron transfer is from Ni atom to graphene, the downwardly shifted effect is mainly occurring on the d-band of Ni atoms. In other words, when Ni and Pd mix to form an alloyed cluster, the Ni-Pd cluster prefer using the Ni-orientation to bind with the graphene. It could reduce the amount of Pd-C bond and prevent the electron flow form Pd atom to graphene sheet directly.

3.4 Adsorption and catalytic behaviors on Defect Graphene supported Pd10, Pd6Ni4, Pd4Ni6. Finally, we performed the H2 adsorption on Pd10-gra, Pd6Ni4-gra and Pd4Ni6-gra and found that the largest adsorption energies of single H2 molecule on Pd sites are −0.70, −0.56 and −0.61 eV, respectively. The adsorption energy for single H2 on graphene supported Pd10 cluster (−0.70 eV) is close to the results by other DFT results, in which the H2 adsorption energy on graphene supported Pd4 and Pd5 cluster are – 16


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0.60 and −0.69 eV, respectively.51,

52

We also calculated the co-adsorption of

multiple H2 molecules by placed 9H2, 6H2 and 4H2 onto each Pd site of Pd10-Gra, Pd6Ni4-Gra and Pd4Ni6-Gra. The average adsorption energies are calculated to be −0.43, −0.46 and −0.40 eV/Pd-atom for 9H2-Pd10-Gra, 6H2-Pd6Ni4-Gra and 4H2-Pd4Ni6-Gra, respectively (configurations are shown in Figure 7). These results show that embedding of Ni atoms into clusters do not change the behavior of hydrogen storage but can reduce the cost. Similar phenomenon was also appeared for the adsorption of atomic oxygen on graphene supported MPd12 (M = Fe, Co, Ni, Cu, Zn, and Pd), in which graphene supported NiPd12 and Pd13 has the closest adsorption energy for oxygen atom as compared to other graphene supported MPd12 clusters.53 Then, we considered the H2 dissociative adsorption by calculating the dissociation path (H2 → 2H) on Pd10-Gra, Pd6Ni4-Gra and Pd4Ni6-Gra and the tabulated in Table 5. The small activation energies of 0.20, 0.35, and 0.33 eV with exothermicities of –0.58, –0.12, and –0.19 eV for Pd10-Gra, Pd6Ni4-Gra and Pd4Ni6-Gra, respectively, indicate that H2 molecule is easy to dissociate into two H atoms on the three systems which can enhance their hydrogen storage capacity. However, the smaller activation energies of the reserved reaction (2H → H2) for Pd6Ni4-Gra and Pd4Ni6-Gra reveal that remove of atomic H from alloyed clusters become easier than pure system, which enhances the fast hydrogen kinetics or can be applied to the further hydrogenation reactions. 17



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The adsorption energies of CO2 on Pd10-Gra, Pd6Ni4-Gra and Pd4Ni6-Gra are −0.46, −0.66 and −0.62eV, respectively. For Pd6Ni4-Gra and Pd4Ni6-Gra the charge for the Pd atoms at the top region are –0.14 ~ –0.25 |e|, and Ni atoms are +0.16 ~ +0.23 |e| (shown in Figure 6c and d). In addition, the charge distribution for CO2 molecule is positive at carbon atom and negative at oxygen atoms, so that the CO2 prefer to be adsorbed on Pd6Ni4-Gra and Pd4Ni6-Gra by the formation the C-Pd and O-Ni bond simultaneously, and exhibited greater adsorption energies than that on pure Pd10 counterpart. Accordingly, the electronic distribution between Pd (electron rich) and Ni (electron deficient) could enhance the adsorption energies of CO2 on alloyed system, which is consistence with experimental observation.28 Due to the largest adsorption energies of CO2, we performed the calculations for reaction mechanism of CO2 hydrogenation to formic acid on Pd6Ni4-Gra as shown in Figure 8.

After the

dissociative adsorption of H2 on Pd6Ni4-Gra with energy barrier of 0.35 eV, the adsorption energy of CO2 on 2H-Pd6Ni4-Gra slight reduce to −0.53 eV. The calculation energy barrier for hydrogenation of adsorbed CO2 on 2H-Pd6Ni4-Gra is 0.45 eV, being less than the energy barrier of CO2 dissociation (0.72eV), indicating the formation and selectivity of HCOOH from CO2 + H2 is more favorable on Pd6Ni4-Gra.

As compared to pure Pd10-Gra, alloyed Ni-Pd cluster could enhance the

CO2 adsorption, catalytic activity for the formation of HCOOH (for CO2 on Pd10-Gra, 18


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Eads of CO2 = −0.46 and Ea = 0.56eV for HCOO formation).

In addition, comparison

of the barrier of CO2 dissociation on Pd6Ni4-Gra and Ni10-Gra (Ea = 0.72 versus 0.31eV) indicates that the existence of Pd could avoid the CO2 composition, which benefits the selectivity for the formation of HCOOH.

Conclusion In this work, we have systematically calculated the geometries of Pd6Ni4 and Pd4Ni6 isomers and identify the stability of each isomer by using the cohesive energy. Our study presents an evidence that Ni atoms prefer to bind with adjacent atoms to enhance its coordination number and eject Pd atoms into the positions with lower coordination numbers in the Pd-Ni nanoparticle. Compared the d-band for lowest-energy isomers of Pd10, Pd6Ni4 and Pd4Ni6 and Ni10 clusters, we found charge redistribution between Pd and Ni would cause downwardly shifted d-band of Ni and Pd. We also investigated the adsorption of Pd10, Pd6Ni4 and Pd4Ni6 and Ni10 on defect graphene, and found that Ni atom could provide stronger bonding with the defect sites of graphene. The larger adsorption energy of Pd4Ni6 than Pd6Ni4 on defect graphene suggests that Ni dopant is more conducive to the clusters dispersion onto the graphene sheet, due to the more possibility to from Ni-C bond at interface region. Finally, we proved the adsorption and catalytic behaviors of defect graphene supported Pd-Ni 19



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nanoparticle by studying H2 adsorption and CO2 hydrogenation reaction on the Pd10-Gra and Pd6Ni4-Gra catalysts. The calculation results show that the Pd6Ni4/defect graphene catalyst exhibits high gravimetric capacity and fast kinetics for hydrogen storage and highly activity and selectivity towards the formation of formic acid from CO2 and H2 in the heterogeneous catalytic system. This theoretical insight should provide important guideline for designing better catalysts for hydrogen storage and CO2 conversion.

Acknowledgement. This study was supported by the Chung Yuan Christian University (CYCU), Ministry of Science and Technology (MOST) and National Center for Theoretical Sciences (NCTS), Taiwan, under Grant Numbers MOST 106-2113-M-033-003,

105-2113-M-033-008,

104-2113-M-033-010

and

103-2632-M-033-001-MY3 and the use of facilities at the National Center for High-Performance Computing, Taiwan.

Supporting Information Structures for Pd6Ni4 and Pd4Ni6 isomers (Figure S1 and Figure S2), DFT-MD energy trajectories for Pd6Ni4 and Pd4Ni6 clusters (Figure S3), project density of states diagram of pristine graphene (Figure S4) and DFT-MD energy trajectories for Pd10-Gra and Pd6Ni4-Gra clusters (Figure S5). 20


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Table 1. Binding Energies (Ecoh), total magnetic moments (M), and the average bond length (Dcluster) of the lowest-energy isomers of Pd10, Pd6Ni4, Pd4Ni6 and Ni10 (corresponding structures are shown in Figure 1)

a

cluster

Ecoh (eV/atom)

M (µB)

Dcluster (Å)a

Pd10 Pd6Ni4 Ni6Pd4 Ni10

1.39 1.54 0.99 1.74

6 6 8 8

2.67 (2.61 ~ 2.74) 2.50 (2.28 ~ 2.72) 2.47 (2.29 ~ 2.67) 2.34 (2.30 ~ 2.38)

The values in parentheses are the shortest and longest bond lengths of each cluster.

29



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Table 2. The electronic analysis for the lowest-energy isomers of Pd10, Pd6Ni4, Pd4Ni6 and Ni10 (corresponding structures are shown in Figure 1), including the total charge (Q) and average charge (Q/atom) of Pd and Ni atoms, and individual d-band center of Pd and Ni atoms (including the spin up , spin down and total ) isolated cluster

Q

Q/atom

εd ↑

εd ↓

εd ↑ ↓

Pd10

0

0

Pd6Ni4 Pd4Ni6

−0.97 −0.87

−0.162 −0.218

−1.41 −1.48 −1.62 —

−1.08 −1.53 −1.65

−1.34 −1.50 −1.63

Ni10

Pd

Ni Q

Q/atom

εd ↑

total

εd ↓

εd ↑ ↓

εd ↑ ↓

−1.38 −1.25 −1.14

−1.34 −1.45 −1.40 −1.14

— +0.97 +0.87 0

+0.243 +0.145 0

30


−1.94 −1.71 −1.40

−0.66 −0.60 −0.64

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Table 3. Adsorption (Eads) of single Pd, Ni and Pd10, Pd6Ni4, Pd4Ni6 and Ni10 on defect graphene (notation (a)-(f) according to Figure 5), the changes after adsorption, (a) the deformation energy (Edef) of graphene, the increase ratio of surface area (∆A) and The elevation (DH) for graphene sheet, and (b) the deformation energy (Edef) of cluster, average bond length(Dcluster) and the shortest to longest bond length (d) of each adsorbed cluster, and the interaction energies (Eint) between cluster and graphene sheet. Cluter-Gra

Eads (eV)

Single Pd Single Ni Ni10 Pd4Ni6-Ni Pd6Ni4-Ni Pd10 Pd6Ni4-Pd Pd4Ni6-Pd

−4.06 −5.21 −5.34 −5.26 −4.22 −4.22 −3.89 −3.88

(a) Graphene

(b) adsorbed cluster a

Edef (eV)

Dclustera (Å)

Edef (eV)

∆A(%)

DH (Å)

d (Å)

(a)

+5.36 +4.98 +7.44

3.11 1.99 2.37

1.10 0.57 1.34

+0.56

2.42 (2.34)

2.27 ~ 2.69

(b) (c) (d) (f) (e)

+5.74 +6.79 +7.99 +7.27 +7.08

2.07 2.18 2.07 2.64 1.61

0.65 1.12 1.62 1.17 1.09

+1.52 +1.37 +0.09 +1.18 +1.41

2.51 (2.47) 2.57 (2.50) 2.71 (2.67) 2.55 (2.50) 2.51 (2.47)

2.26 ~ 2.66 2.29 ~ 2.88 2.56 ~ 2.87 2.36 ~ 2.78 2.29 ~ 2.80

Eintb (eV) −9.42

a. The values in parentheses are the average bond lengths of isolated clusters. b. Eint = Eads − [Edef of graphene + Edef of cluster]

31


−10.19 −13.34 −12.52 −12.38 −12.30 −12.34 −12.37


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Table 4. The electronic structural analysis for the lowest-energy configurations of Graphene supported Pd10, Pd6Ni4, Pd4Ni6 and Ni10 clusters and the difference before and after adsorbed on graphene ( as compared the value of adsorbed clusters and the value in Table 2): the total charge (Q) and average charge (Q/atom) of Pd and Ni atoms, and individual d-band center of Pd and Ni atoms (including the spin up , spin down and total ) adsorbed

Pd

Ni

Q

Q/atom

εd ↑

εd ↓

εd ↑ ↓

Pd10

+0.98

+0.098

−1.60

−1.57

−1.60

Pd6Ni4 Pd4Ni6 Ni10

−0.34 −0.38

−0.057 −0.095

−1.59 −1.67

−1.59 −1.66

−1.59 −1.66

Before and after

Pd

Pd10 Pd6Ni4 Pd4Ni6 Ni10

Q

total Q/atom

εd ↑

εd ↓

εd ↑ ↓

−1.60 +1.71 +2.27 +1.66

+0.427 +0.378 +0.166

−1.78 −1.21 −1.43

−0.79 −0.98 −0.91

−1.40 −1.08 −1.24

Ni

∆ QPd

∆Q/atom

∆εd ↑

∆εd ↓

∆εd ↑ ↓

+0.98 +0.63 +0.49

+0.098 +0.105 +0.123

−0.19 −0.11

−0.49 −0.06

−0.26 −0.09

−0.05

−0.01

−0.03

εd ↑ ↓

∆Q

−1.51 −1.39 −1.24 total

∆Q/atom

∆εd ↑

+0.74 +1.40

+0.185 +0.233

−0.16 +0.50

+1.66

+0.166

−0.03

32


∆εd ↓ −0.13 −0.38 −0.27

∆εd ↑ ↓

∆εd ↑ ↓

−0.02 +0.17

−0.26 −0.06 −0.01

−0.10

−0.10

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Table 5 The largest adsorption energies of single H2 on Pd10-Gra, Pd6Ni4-Gra and Ni6Pd4-Gra and the activation energy (Ea), reverse activation energy (Ea, reverse) and ∆H for the H2 dissociation (H2(a) → 2H(a)), all unit in eV.

H2(a) → 2H(a) Eads of H2 Pd10-Gra −0.70 Pd6Ni4-Gra −0.56 Ni6Pd4-Gra −0.61

Ea 0.20 0.35 0.33

Ea, reverse ∆H 0.78 −0.58 0.47 −0.12 0.52 −0.19

33



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Figure 1. The low-lying isomers of (a) Pd10, (b) Ni10, (c) Pd6Ni4 and (d) Pd4Ni6 clusters. The coordination numbers of each atom are given; the corresponding bond lengths and magnetic moments are tabulated in Table 1.

34


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Figure 2. The isomers of (a) Pd6Ni4 isomers and (b) Pd4Ni6 isomers versus the corresponding cohesive energy (up) and average coordination numbers (down).

35



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Figure 3. (a) 5-8-5 Defect graphene, (b) single Ni- and (c) single-Pd doped graphene. (d)- (f) Project density of states (right) and electronic localization function (ELF) diagrams (left). The elevation out of graphene is also given (HNi and HPd).

36


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Figure 4. The morphological changes of (a)Pd10-Gra and (b)Pd6Ni4-Gra configuration at three steps (i) DFT calculation (ii) DFT-MD at 1000K for 1000fs (the energy trajectories were shown in Figure S5) and (iii) re-optimization of the MD structure by DFT calculation.

37



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Figure 5. Adsorption energy (Ead) versus the various orientations and coordinated atoms of Pd10, Pd6Ni4, Pd4Ni6, and Ni10 clusters on defect graphene sheets, including Ni-orientation of (a) Ni10, (b) Pd4Ni6, (c) Pd4Ni6 and Pd-orientation of (d) Pd10, (e) Pd4Ni6, (f) Pd6Ni4. The lowest-energy configurations of each orientation are shown in the below region.

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Figure 6. Charge-density different plot of (a) Pd10-Gra, (b) Ni10-Gra, (c) Pd6Ni4-Gra and (d) Pd4Ni6-Gra, the yellow and cyan areas represent the charge accumulation and depletion, respectively. (Isosurfaces are at 0.002 e/Å3) The Bader charge on each metal atoms and carbon-layer are also given.

39



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Figure 7. The configurations of (a) 9H2-Pd10-Gra, (b) 6H2-Pd6Ni4-Gra and (c) 4H2-Pd4Ni6-Gra, in which the H2 molecules occupied each exposed Pd atoms with the average adsorption energy of 0.43, −0.46 and −0.40eV/Pd-atom, respectively.

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Figure 8. Calculated potential energy diagram for CO2 hydrogenation to HCOOH on Pd6Ni4-Gra and catalyst regeneration.

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TOC Graphic

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Structure, Bonding, and Catalytic Properties of Defect Graphene

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