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Rational Design of Ag38 Cluster Supported by Graphdiyne for Catalytic CO Oxidation Z. W. Chen, Z. Wen,* and Q. Jiang* Key Laboratory of Automobile Materials, Ministry of Education, and School of Materials Science and Engineering, Jilin University, Changchun 130022, China ABSTRACT: Clusters with precise numbers of atoms can exhibit unique and unexpected properties due to their size-dependent active sites. Besides, to obtain the superior stability and catalytic activity, an appropriate substrate can prevent the metal clusters aggregating as well as change the geometric and electronic structures of metal clusters. In this study, the catalytic oxidation of CO on the Ag38 cluster supported by graphdiyne (Ag38−GDY) is investigated by density functional theory (DFT) and molecular dynamics (MD) simulations, which provide an intensive understanding of its catalytic properties. Moreover, the process of CO oxidation on the Ag38−GDY system has a high activity with low energy barrier (0.26 eV), which originates from the intrinsic activity of Ag38 cluster and the vital role of GDY.

1. INTRODUCTION Developing highly efficient and stable metal catalysts is important in chemistry and materials science due to both energy and environmental problems.1−3 Among the metallic catalysts, metal clusters occasionally exhibit significant catalytic activity on a subnanometer size scale4−7 due to their high surface/volume ratio, smaller coordination number,8 different active sites,9 superatomic character,10 and quantum confinement. 11 The rapid development in novel preparation techniques enable one to control the optimal size and morphology of a cluster, which significantly affects the activity and selectivity of a reaction.12−15 However, metal clusters always suffer from activity degradation due to their aggregation. To solve this issue, the appropriate substrate is of significantly importance to improve the stability. Furthermore, the presence of the substrate would alter electronic and geometric structures of metal clusters, boosting the catalytic activity. Thus, it is urgent to elaborate the synergistic effect between the metal cluster and the corresponding substrate for designing a catalyst with elite performance. With the rapid development of industry, the amount of carbon monoxide (CO) emission drastically increases, causing severe harm to human health and the environment.16 Currently, the most effective way in reducing CO is CO oxidation. The activity of O2 plays a vital role in determining the energy barrier of CO oxidation.17,18 A mechanism for the reaction is collection of a three processes: (1) O2 gradually approaches and bonds with the C atom of CO where the O−O bond length (lO−O) of O2 become longer; (2) the O−O bond breaks and the newly generated CO2 moves away from the catalyst surfaces; (3) the second CO reacts with the remaining O atom to form CO2.9 For the first process, the longer the lO−O of the adsorbed O2, the smaller the corresponding energy © 2017 American Chemical Society

barrier for the transition. For the last two processes, moving away from the catalyst surfaces needs weak adsorption of the product CO2, as well as O2. However, the weaker adsorption of O2 is related to the shorter lO−O,19 which restricts the development of catalysts for CO oxidation until now. Recently, Chorkendorff’ group20 and Cui’ group21 successively proved that using Pt-lanthanide alloys or battery electrode materials as a substitute to directly and continuously control the lattice strain of Pt catalyst and thus tune its catalytic activity for the oxygen reduction reaction. And the lattice compression can weaken the Ead−OH and Ead‑O. Besides, fewer electrons on active sites lead to weak adsorption of O2 because the adsorbed O2 would capture electrons from active sites. In light of the above results, we choose Ag38 cluster with the unique square Ag4 lattice (squ-Ag4) as a catalyst for CO oxidation, which is based on two aspects: the number of metal atoms and the morphology of metal cluster. First, the cluster with less atoms is difficult to prepare in experiments due to serious aggregation by their large surface energy. Also, the influences of stress and charge transfer between the metal cluster with more atoms and the substrate would be very small, the similar results as reported by Chorkendorff et al.20 Second, the metal cluster with 38 atoms has more square lattices, which is a fragment of (100) surface of face-centered cubic metal bulk. The (100) surfaces of Ag and Pd have shown the elite performance for the activation of O2.22,23 While the substrate would satisfy the following three criteria: (1) compress Ag38 cluster lattice; (2) get electrons from Ag38 cluster; (3) fix Ag38 cluster. Received: December 10, 2016 Revised: January 23, 2017 Published: January 26, 2017 3463

DOI: 10.1021/acs.jpcc.6b12434 J. Phys. Chem. C 2017, 121, 3463−3468

Article

The Journal of Physical Chemistry C

Figure 1. Geometric configurations of Ag38−GDY (a) and the O2 adsorption on Ag38−GDY (c). The blue, gray, and red balls represent the Ag, C and O atoms. The corresponding spin-polarized partial density of states (PDOS) are shown in the right (b, d). The vertical black dotted lines denote the Fermi level.

energy convergence criteria is 1.0 × 10−5 hartree (Ha) for the energy change, 2.0 × 10−3 Ha/Å for the gradient, and 5.0 × 10−3 Å for the displacement, respectively. The smearing parameter is set to be 0.005 Ha in the geometric optimization. For transition states (TS) searching, the calculation first performed a linear synchronous transit (LST) maximum, which is followed by an energy minimization in directions conjugating to the reaction pathway.36 The TS approximation obtained via LST/optimization is then used to perform a quadratic synchronous transit (QST) maximization to find more accurate transitional states. The convergence tolerance of the rootmean-square (RMS) force is 2.0 × 10−3 Ha/Å and the maximum number for QST step is set as 10. In the simulation, the 4 × 4 single-layer GDY supercell with a vacuum width of 20 Å and three-dimensional periodic boundary conditions are taken, which leads to negligible interactions between the system and their mirror images. To accurate describe the van der Waals forces, we use TS method for DFT-D correction,37 which has been successfully applied in 2D materials, such as boron nitride,38 MoS2,39 graphene40 and boron monolayer.41 After structure relaxations, the density of states (DOS) is calculated with the empty bands of 12. Concerning with the properties of charge transfers, atom charges would be calculated via the Hirshfeld population analysis.42 We also investigated the stability of the Ag38−GDY by using molecular dynamics (MD) simulations. The interatomic interactions in MD are described by the force field of a condensed-phase optimized molecular potential for atomistic simulation studies (COMPASSII), and most parameters are derived by ab initio parametrization and empirical optimiza-

Graphdiyne (GDY) is a variant of graphyne with two acetylenic linkages in each unit cell, which has been fabricated in 2010.24 The two acetylenic linkages of GDY double the length of the carbon chains connecting the hexagonal ring.25,26 Compared to other 2D materials, GDY (with large pores)27,28 has great potential to substitute graphene as a substrate in catalytic reactions29−31 by fixing Ag clusters and promoting the catalytic activity. The elevated performance is mainly due to the charge transfer between Ag clusters and GDY as well as the generation of compressive stress. It is noteworthy that Ag38 cluster (about 1 nm in diameter) dispersed on GDY is experimentally possible because the Pd clusters with the size of 1 nm doped on GDY are successful fabricated.29 In view of the above discussion, the Ag38 cluster supported by single-layer GDY for CO oxidation is investigated by DFT and MD simulations. The energy barriers for the CO oxidation are studied and the corresponding mechanisms are analyzed through understanding the synergistic effect between GDY and Ag38 cluster.

2. COMPUTATIONAL DETAILS All calculations are performed using the spin-unrestricted density functional theory (DFT) as implemented in the DMol3 code.32,33 Exchange-correlation functions are taken as a generalized gradient approximation (GGA) with Perdew− Burke−Ernzerhof (PBE).34 A DFT semicore pseudo potentials (DSPPs) core treatment35 is taken for relativistic effects, which replaces core electrons by a single effective potential. In addition, double numerical plus polarization (DNP) is chosen as the basis set and the quality of orbital cutoff is fine.33 The 3464

DOI: 10.1021/acs.jpcc.6b12434 J. Phys. Chem. C 2017, 121, 3463−3468

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The Journal of Physical Chemistry C tion.43 This COMPASSII force field has been proven to enable accurate and simultaneous prediction of structural, conformational, vibrational, cohesive, thermophysical, and various gasphase properties for a broad range of compounds in isolation and in condensed phases.44 The vdW interactions are calculated within a cutoff distance of 18.5 Å and the Ewald method is applied for electrostatic interactions.45 Then, we put the model into an NVT ensemble (constant particle number, volume and temperature condition) with a fixed time step of 1 fs. The total simulation time is 10 ns while the temperature of the system is controlled by the Anderson thermostat method. MD simulations are performed in three-dimensional periodic boundary conditions using the Forcite code of embedded in Materials Studio software. For one molecule (CO, O2, CO2) adsorbed on the catalyst, the adsorption energy values Ead‑M (M denotes the corresponding molecule) are determined by, Ead − M = Emol/cat − (Ecat + Emol)

Figure 2. Dynamic processes of 9 Ag38 clusters doped in GDY (28, 112, and 250 nm2) and GP (250 nm2). The total simulation time is 10 ns at the temperature of 400 K. Three snapshots for each process are shown in parts a−d.

(1)

where Emol/cat, Ecat, and Emol are total energies of the molecules/ catalyst system, the catalyst (isolate Ag38 cluster or Ag38 with the substrate of GDY) and a molecule in the same slab, respectively. For CO and O2 coadsorbed on the catalyst, the adsorption energy value ECO‑ad is determined by, ECO − ad = Emol/cat − (Ecat + EO2 + ECO)

most stable adsorption structure is present in Figure 1c. The O2 prefers to be adsorbed on the square site for Ag38−GDY due to less coordination number (6) of Ag atoms on the square site and the related Ead‑O2 value is −0.71 eV.48 To gain more insight into charge transfers between O2 and Ag38−GDY, the PDOS of O2 molecule, adsorbed O2 and Ag atoms are calculated and the results are shown in Figure 1d. The 2π* antibond orbitals of O2 molecule are half filled, which is consistent with literature data.49 When O2 is adsorbed on Ag38−GDY, significant electron transfers (0.464 e‑) from Ag38−GDY to O2 are found, which occupy the initial empty component of O2-2π* orbitals and lead to the elongation of lO−O from 1.22 to1.43 Å. The hybridization between Ag atoms and O2-2π* orbitals are located near Fermi level, which indicates that the adsorbed O2 has been activated, and CO molecule is more easy to be oxidized.50 The adsorptions of CO and CO2 also play important roles in CO oxidation reaction. In our system, the CO molecule prefers adsorbing on the top site of Ag clusters. Ead‑CO = −0.58 eV is weaker than Ead−O2 = −0.71 eV, indicating the absence of CO poisoning phenomenon.16,51 For the adsorption of CO2, Ead‑CO2 = −0.17 eV, denoting the physical adsorption. Thus, desorption of the product CO2 is easy. These adsorption conditions completely accord with the criteria as CO oxidation catalyst, as reported by Deng et al.51 We also calculate the Ead of CO, O2, and CO2 on GDY in the Ag38−GDY system, which are −0.23 eV, −0.26 eV and −0.04 eV respectively. The smaller values are weaker than that of CO (−0.58 eV), O2 (−0.71 eV) on Ag38 in the Ag38−GDY system, indicating that the CO and O2 prefer to adsorb on Ag38 cluster in the Ag38−GDY system. Also, the small Ead‑CO2 value means that desorption of CO2 is easy. Now we begin to study CO oxidation on Ag38−GDY with the Langmuir−Hinshelwood (LH) mechanism. Because the Ecoad value (−1.16 eV) is stronger than the corresponding Ead‑O2 value (−0.71 eV), coadsorption of CO and O2 is advantageous. For Ag38−GDY, there are always three steps reaction, CO* + O2* = OOCO*, OOCO* = CO2 + O* and CO + O* = CO2. The reaction paths of Ag38−GDY and the corresponding energy barriers are shown in Figures 3 and 4a.

(2)

where EO2 and ECO are the total energy of O2 and CO.

3. RESULTS AND DISCUSSION The most stable morphology of Ag38 cluster on GDY is shown in Figure 1a, where the cluster shows the cuboctahedral morphology.13,46 The Ag38 cluster is grown on the holes of GDY and its Ead‑Ag38 value is −6.99 eV. This large value indicates that the GDY fastens the clusters well and prevents the agglomeration of clusters. This strong interaction is also confirmed by the partial density of states (PDOS) of Ag38 cluster and GDY as shown in Figure 1b, where significant hybridization arises between Ag and the adjacent C-2p, denoting the stability of the system. Furthermore, the d band center changes from −4.24 eV of Ag38 to −4.33 eV of Ag38− GDY. The d bands of the Ag38 cluster on GDY are moved away from the Fermi energy EF; namely, GDY reduces the adsorption ability of the Ag38 cluster.47 More important, 1.303 e− transfers from the Ag38 cluster to GDY in light of the Hirshfeld charge analysis, which can weaken the adsorption energy of O2 molecule. The direction of the electron transfer is in agreement with the Pauling electronegativity values of Ag (1.93) and C (2.55). Furthermore, MD simulation is taken to compare the stability of Ag38−GDY and Ag38−GP systems (GP denotes the graphene), which are given in Figure 2. As shown in Figure 2, parts a and b, GDY could, while GP could not, stabilize Ag38 cluster. Different doping densities of Ag38 clusters (9 Ag38 clusters on surface areas of 250, 112, and 28 nm2), shown in Figure 2b−d, are also calculated in the NVT ensemble. The cluster aggregation occurs when the density reaches about 1/3 nm2 where the distance between adjacent clusters is only a lattice constant. The possible adsorption configurations (side-on and end-on) and different adsorption sites (top, bridge, triangular and square sites) of O2 molecule on Ag38−GDY are considered and the 3465

DOI: 10.1021/acs.jpcc.6b12434 J. Phys. Chem. C 2017, 121, 3463−3468

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Figure 3. CO oxidation reaction processes on Ag38−GDY, including the initial state (IS), transition state (TS), intermediate state (MS), and final state (FS). The red, blue, and gray balls represent the O, Ag, and C atoms.

For the steps OOCO* = CO2 + O* and CO + O* = CO2, the reaction processes are almost the same as those in the literature.6,52 In light of Figure 3, the O−O bond of OOCO* breaks and CO2 molecule is formed with the energy barrier of Ebar2 = 0.26 eV. Meanwhile, the rest O atom is adsorbed on the square site of Ag38 cluster. Then, the second CO molecule goes close to the O atom to form the second CO2 molecule with the energy barrier of Ebar3 = 0.12 eV. In the two steps, the adsorbed O2 escapes from the square Ag4 active site to generate two CO2 molecules. Thus, the weaker Ead‑O2 value of −0.71 eV leads to the smaller Ebar2 = 0.26 eV and Ebar3 = 0.12 eV. The contributions of vdw force for Ebar1, Ebar2, and Ebar3 are also determined, the corresponding values are 0.004, 0.002, and 0.003 eV, indicating the negligible contributions, as reported by our previous work.50 The reaction mechanism of CO oxidation on Ag38 cluster is the same as that of Ag38−GDY and the corresponding energy barriers are shown in Figure 4b. The Ebar2 and Ebar3 on Ag38 cluster calculated are 0.30 and 0.29 eV, being larger than those on Ag38−GDY. Meanwhile, we find that Ead‑O2 of Ag38 cluster is equal to −0.94 eV, which is stronger than that (−0.71 eV) of Ag38−GDY. It also proves that weaker adsorption of O2 leads to the decrease of Ebar2 and Ebar3. The rate-determining step of CO oxidation determines the reaction rate, which is the most important indicator of catalysts. By comparing the energy barriers in Figure 4a, the second step (OOCO* = O* + CO2) with the Ebar2 = 0.26 eV is the ratedetermining step. This datum is better than that in other systems (ligand-protected Au clusters,6 graphene/Pt(111),16 Au rod supported TiO2,53 Au-doped h-BN monolayer54 and Cu4-doped monolayer MoS250), as shown in the inset of Figure 4a. For the system of the ligand-protected Au clusters, the ratedetermining step is the first step (CO* + O2* = OOCO*) with Ebar1 = 0.60 eV due to the short lO−O (1.31 Å). While lO−O (1.47 Å) is long enough in the system of Cu4-doped monolayer MoS2, the Ead‑O2 value (−1.75 eV) is so strong that the ratedetermining step shifts from the first step (2CO* + O2* = COOOCO*) to the second one (COOOCO* = CO2 + OCO*) with Ebar2 = 0.37 eV. To further explore the role of GDY, we analyze and compare the adsorption of O2 on Ag38 cluster and Ag38−GDY. For Ag38 cluster, there are six square Ag4 active sites, while there are five square Ag4 active sites in Ag38−GDY, as shown in Figure 5,

Figure 4. Corresponding energy barriers of CO oxidation on Ag38− GDY and Ag38 are described in parts a and b, where the red value represents the energy barrier of the rate-determining step. The inset in part a has shown the energy barrier of the rate-determining step of other systems and our work (red). The reaction paths include CO* + O2* = OOCO*, OOCO* = O* + CO2 and CO + O* = CO2, which are shown in the bottom right corner of part b.

For the step of CO* + O2* = OOCO*, as reported by our previous work,50 the O atom in adsorbed O2 molecule approaches the CO molecule and bonds with the C atom of the CO molecule to form the OCOO* intermediate state (MS), which has been described in Figure 3. Simultaneously, the lO−O is elongated from 1.42 to1.47 Å where the Ebar1 (the energy barrier of CO* + O2* = OOCO*) value is 0.07 eV. The small Ebar1 value is induced by a little elongation of lO−O (0.05 Å). The large lO−O is derived from the unique square active site of Ag38 cluster, which is similar to Long’s study.23 3466

DOI: 10.1021/acs.jpcc.6b12434 J. Phys. Chem. C 2017, 121, 3463−3468

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The Journal of Physical Chemistry C Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to thank the National Natural Science Foundation of China (No. 51631004) and the computing resources of the High Performance Computing Centers of Jilin University and Jinan, China.

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Figure 5. Square Ag4 active sites of isolated Ag38 cluster and Ag38− GDY and their average Ag−Ag bond length (lAg−Ag), average change transfer (Δq) and their adsorption energy of O2 molecule (Ead‑O2). The yellow balls represent the square Ag4 active sites.

where the average Ag−Ag bond lengths (lAg−Ag) and average transferred electrons (Δq) of the square Ag4 active site for Ag38 cluster and Ag38−GDY are compared. The lAg−Ag value of square Ag4 active sites for isolated Ag38 cluster is 2.86 Å, being larger than 2.84 Å for Ag38−GDY, it corresponds to the fact that the d band center moves toward the lower-energy range from −4.24 eV of Ag38 to −4.33 eV of Ag38−GDY, which implies that the presence of GDY reduces the adsorption ability.55,56 More importantly, the Δq value increases from 0.001 e‑ of Ag38 to −0.020 e‑ of Ag38−GDY where more electrons are transferred from Ag38 to GDY. When the O2 molecule is adsorbed on the surface of catalysts, it needs to get electrons. Thus, fewer electrons in the square Ag4 active site of Ag38−GDY will weaken the adsorption. Consequently, both atomic and electronic structures of Ag38−GDY support the Ead‑O2 value changing from −0.94 eV to −0.71 eV.

4. CONCLUSIONS In summary, our comprehensive DFT and MD studies of CO oxidation on Ag38−GDY suggest that the GDY is a valid substrate to support the Ag38 cluster catalyst for CO oxidation where Ag38−GDY without poisoning problems shows the lowest Ebar of 0.26 eV. These outstanding performances benefit from the active site selection of clusters and the role of GDY. It is found that a weak Ead‑O2 value is conducive for CO oxidation. Also, the GDY regulates the geometrical structure and electronic structure of Ag38 cluster with square Ag4 active sites to reduce the adsorption ability of Ag38 cluster. As a result, the Ag38−GDY could be a good candidate for CO oxidation with low cost and high activity.

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AUTHOR INFORMATION

Corresponding Authors

*(Z.W.) E-mail: [email protected]. *(Q.J.) E-mail: [email protected]. ORCID

Q. Jiang: 0000-0003-0660-596X 3467

DOI: 10.1021/acs.jpcc.6b12434 J. Phys. Chem. C 2017, 121, 3463−3468

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DOI: 10.1021/acs.jpcc.6b12434 J. Phys. Chem. C 2017, 121, 3463−3468