Structures and Electronic Properties of Au Clusters Encapsulated ZIF

ZIF-8, and isolated Au cluster, respectively. The plot of charge density difference for. Au2@ZIF-8 system is shown in Figure 5A, where the cyan and ye...
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Article Cite This: J. Phys. Chem. C 2018, 122, 8901−8909

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Structures and Electronic Properties of Au Clusters Encapsulated ZIF‑8 and ZIF-90 Li Dou,†,§ Shengnan Wu,†,§ De-Li Chen,*,† Sihui He,‡ Fang-Fang Wang,*,‡ and Weidong Zhu† †

Key Laboratory of the Ministry of Education for Advanced Catalysis Materials and ‡College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China ABSTRACT: The zeolitic imidazolate frameworks (ZIFs) are chemically and thermally stable microporous materials that are being considered as ideal supports for the uniform encapsulation of noble metal nanoparticles. Our theoretical investigations started from the adsorption of the molecular precursor Au(CO)Cl in both ZIF-8 and ZIF-90; surprisingly, pore-B, with the diameter less than 2.2 Å in the two ZIFs, dramatically expanded as an energetically most favorable site for the location of Au(CO)Cl, whereas the well-known pore-A, with a diameter of about 3.5 Å, is less favorable. Then, ab initio molecular dynamics simulations showed that the confined Aun cluster has a transition from two-dimensional to three-dimensional structures when n is larger than 12 in both ZIFs. Interestingly, the aldehyde groups in ZIF-90 were computed to be the main binding sites for Au clusters, whereas the imidazole rings were identified as the binding sites in ZIF8. Compared to ZIF-90, the binding of Au clusters in ZIF-8 was stronger, accompanied by transfer of larger electrons from the frameworks to the confined Au clusters. Finally, the computed energy barriers for the CO oxidation using Au clusters confined in ZIFs as catalysts were found to be smaller than those for isolated Au clusters.

1. INTRODUCTION The zeolitic imidazolatic frameworks (ZIFs) are a subclass of metal−organic frameworks (MOFs) composed of tetrahedrally coordinated metal ions connected by imidazolate linkers, first synthesized by Park et al.1 and then extended by Banerjee and coworkers.2 Due to their high chemical and thermal stability, ZIFs were initially studied for their applications in CO2 gas adsorption and separation, and recently were also investigated as hosts for the encapsulation of metal nanoparticles (MNPs).3−14 ZIF-8 possesses a symmetrical porous structure analogous to zeolite and has a large surface area of 1400 m2/g and a high thermal stability at up to 420 °C; ZIF-8 is composed of cavities with a pore diameter of about 11 Å that are connected by a small pore of size 3.4 Å.1,15 Therefore, ZIF-8 has been investigated as an appropriate template to disperse MNPs, whereas the MOFs with straight channels are less appropriate candidates because the nanoparticles can easily move through the channel and migrate to the external surface, forming a nanoparticle of large size. The catalytically active sites in ZIF materials include the open metal sites on their surface,16 the functional groups on imidazolate linkers,17 and the MNPs confined in cavities.4−7,18,19 Jiang et al.7 successfully deposited Au nanoparticles to the host of ZIF-8 using a simple solid grinding method, and the catalyst was found to have a high catalytic activity for CO oxidation; the estimated diameter for the Au nanoparticle is about 34 ± 14 and 31 ± 9 Å for 1.0 wt % Au@ZIF-8 before and after the reaction, both of which are larger than the pore diameter of about 11 Å for ZIF-8, indicating a possible existence of defect sites or an occupation of neighboring cavities connected by small pores. Many other © 2018 American Chemical Society

MNPs such as Pd, Pt, Ir, and bimetallic alloys were also successfully deposited into the pores of ZIF-8 and the obtained catalysts showed excellent catalytic activity for different chemical reactions.4−6,18,19 Gold has attracted tremendous interests due to its wide applications in catalysis, and the properties of Au nanoparticles can vary widely with a decrease in the system size and the change of support such as zeolite, activated carbon, and metal oxide.20−23 It has been reported both experimentally and theoretically that the cationic Au cluster has a transition from two-dimensional to three-dimensional when the number of Au atoms n reaches 9, whereas for anionic Au cluster, the transition starts at n = 14.20 In addition, the Au cluster loaded on the support such as a metal oxide tends to have a distorted structure compared to the isolated Au cluster, and its electronic properties also change because of the strong interaction between the Au cluster and the support.24,25 The supported gold clusters have been widely investigated as potential catalysts for CO oxidation, selective oxidations, and so on.26,27 In contrast, less progress has been made in the MOFs-based Au catalysts, especially where the structures and the electronic properties of the confined Au clusters are unclear, which are the key to understand the mechanism of reactions occurring in these catalysts. Understanding of the formation of the MNPs inside the cavities of MOFs is challenging. Lots of theoretical progress have Received: December 21, 2017 Revised: March 8, 2018 Published: April 4, 2018 8901

DOI: 10.1021/acs.jpcc.7b12480 J. Phys. Chem. C 2018, 122, 8901−8909

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below 300 K. During all of the AIMD simulations, the kinetic energy cutoff of 300 eV and the time step of 2 ps was used, and the total simulation time for each system was set to 40 ps. The obtained structures from the AIMD simulations were further optimized using a higher value of 400 eV to make sure a local minima was found. Only Gamma point was used to sample the Brillouin zone for all of the calculations in this study, and the optimized structures were relaxed until the absolute force on each atom was smaller than 0.03 eV/Å.

been made to reveal the mechanism of gas adsorption and separation using MOFs; however, little is known about the structures and electronic properties of MNP@MOFs due to their complicated structures, and are thus a great challenge to simulate. Only recently, some progress has been made to investigate the encapsulation of small Au and Pd metal clusters confined in the cavities of MOF-74 and UiO-66(-NH2) using the density functional theory (DFT) method combined with genetic algorithm.28−30 In addition, the defects are very popular in various MOFs and may play an important role not only in gas adsorption but also in catalysis, thus the mechanism of the formation of defects has attracted great interests. The DFT calculations have been performed to reveal the formation of defects inside ZIF-8 by Zhang and co-workers.31 The removal of 1,4-benzene-dicarboxylate linkers in UiO-66 was known to create actives sites for catalysis. Vandichel et al.32 systematically computed the reaction pathways for the formation of defects by modulating different linkers. In this study, we focus on investigating the thermodynamically stable structures and the electronic properties of the small Au metal cluster encapsulated ZIF-8 and ZIF-90, as well as their adsorption for the CO molecule. Additionally, adsorption of the molecular precursor Au(CO)Cl in pristine ZIF-8 and ZIF-90 was studied to better understand the formation of the confined Au clusters.

3. RESULTS AND DISCUSSION Our calculations indicated that ZIF-8 and ZIF-90 exhibit the same sodalite-like structure with a small pore aperture size of 3.5 and 3.8 Å, respectively, connecting the neighboring main cavities. The (110) and (111) surfaces for both ZIF-8 and ZIF-90 are shown in Figure 1. The pore-A of ZIF-8 (ZIF-90) consists of six

2. THEORETICAL METHODS The confinement effect of the cavities should play a key role in determining the structures and the electronic properties of the encapsulated metal cluster/nanoparticle, thus it is dispensable to employ the periodic structure of ZIF-8 (ZIF-90) with a total number of 276 (252) atoms in our calculations. The generalized gradient approximation functional of Perdew−Burke−Ernzerhof33 implemented in the Vienna ab initio simulation package34−37 was employed to describe the exchange−correlation functional. The optimized lattice parameters for the cubic cells of ZIF-8 and ZIF-90 were computed to be 17.13 and 17.35 Å, respectively, using a kinetic energy cutoff of 500 eV, slightly larger than the experimental values of 16.99 and 17.27 Å.1,38 The computed lattice parameters were fixed in the following ab initio molecular dynamics (AIMD) simulations, and the structural optimizations for the Au clusters encapsulated ZIFs. The AIMD simulations combined with the annealing technique has been recently carried out to search for the thermodynamically stable Pdm clusters (m up to 32) confined within the nanopores of UiO-66-NH2.30 Herein, the same method was adopted to search for the stable configurations for Aun@ZIF-8 and Aun@ZIF-90 (n up to 20), with the functional group of methyl and aldehyde, respectively. Note that we do not attempt to locate thermodynamically most stable Aun structures confined in the cavities of ZIF-8 and ZIF-90 in this study, but instead try to explore the factors (such as the functional group) affecting the structures of the confined Aun and their electronic properties. The Aun clusters with n up to 20 include twodimensional and three-dimensional clusters, which should reflect the evolution of small Au cluster aggregating to a relatively large Au cluster, although the size of Au20 is still smaller than the actual size (10−20 Å) of Au nanoparticle in ZIFs.39 Different starting temperatures for the annealing simulations were carefully tested to avoid any potential collapse of the frameworks, and our results suggested that 500 K should be a reasonable value. The final temperature for the annealing simulations was set to 300 K because our calculations suggested that the diffusion of the gold cluster was dramatically decreased when the temperature is

Figure 1. Optimized crystal structures of ZIF-8 and ZIF-90 with (111) and (110) surfaces, where the C, H, N, O, and Zn atoms are shown with gray, white, blue, red, and green balls. The green and red circles represent pore-A and pore-B, respectively, for both ZIF-8 and ZIF-90.

imidazole rings, whereas pore-B consists of four imidazole rings and the pore size is much smaller, i.e., 0.2 (2.2) Å. Note that poreA and pore-B are the same to the reported six ring and four ring, respectively, defined by Park et al.1 Considering its small pore aperture, it is plausible to confine a metal cluster inside the main cavity if the framework is intact and rigid. However, it was reported that the confined gold nanoparticles inside ZIF-8 have a wide range size from 10 to 100 Å, whereas the gold nanoparticles in the ZIF-90 have a much narrower range size from 10 to 20 Å,39 respectively, suggesting the formation of Au nanoparticles was dramatically influenced by the chemical environment of the cavities. More specifically, several factors including the diffusion of both gold precursors and small Au cluster may play a key role in the aggregation of the confined Au nanoparticles. The defects sites in the ZIFs may be another important factor leading to a much larger size of metal nanoparticles than the main cavities. In this study, we focus on the theoretical investigations on the 8902

DOI: 10.1021/acs.jpcc.7b12480 J. Phys. Chem. C 2018, 122, 8901−8909

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The Journal of Physical Chemistry C structures and electronic properties of the confined Au metal clusters in pristine ZIF-8 and ZIF-90, whereas the formation of defect sites and their influence on the formation of the metal clusters are out of scope. 3.1. Comparison of the Adsorption of Au(CO)Cl in ZIF-8 and ZIF-90. Ideally, it is possible to completely confine the metal clusters in the cavities of MOFs with small pore apertures, which would limit the further growth of metal clusters. However, many reported composites possess metal clusters with a broad size distribution of Au particles,7,39 suggesting the growth of metal cluster is influenced by many effects such as the synthesis conditions, synergy between the metal precursors and host, and the interaction between the metal clusters and frameworks. Therefore, it is instructive to explore the adsorption of molecular precursor Au(CO)Cl inside the cavity of ZIFs. Many different potential adsorption sites inside the cavity were considered for structural optimization. Although pore-A (see Figure 1), with the diameter of 3.5 Å, was previously considered as the only pathway for the diffusion of gas molecules,1,40,41 pore-B was also considered as adsorption sites for Au(CO)Cl in our work. The binding energy of the Au(CO)Cl molecule confined in the framework, Eb, was computed to analyze its adsorption affinity at various adsorption sites inside the cavity. The Eb is defined as

Figure 2. Optimized structures of Au(CO)Cl molecule confined at the center of (A) pore-B in ZIF-8, (B) pore-B in ZIF-90, (C) pore-A in ZIF8, and (D) pore-A in ZIF-90; the values of diameters for pore-A or poreB of Au(CO)Cl@ZIFs are shown in each picture, and the numbers in parenthesis are shown for the pristine ZIFs.

E b = Emolecule/ZIF − EZIF − Emolecule

where Emolecule/ZIF, EZIF, and Emolecule represent the energies of Au(CO)Cl/ZIF, pristine ZIF, and isolated Au(CO)Cl molecule, respectively. The Au(CO)Cl is a linear molecule and computed to adsorb at various adsorption sites inside the cavities of ZIF-8 and ZIF-90 via physisorption. Unexpectedly, our calculations show that the energetically most favorable site is located at the center of pore-B rather than pore-A in both ZIF-8 and ZIF-90. To explore the adsorption mechanism, we analyzed the structures of pristine ZIFs and the energies for frameworks after Au(CO)Cl adsorption. The diameters of pore-A and pore-B in pristine ZIF-8 and ZIF90 are shown in Figure 2. The computed pore-A of pristine ZIF-8 has a dynamic diameter of 3.5 Å, which is slightly smaller than 3.8 Å in ZIF-90; the pore-B of ZIF-8 has a very narrow dynamic diameter of 0.2 Å, much smaller than 2.2 Å in ZIF-90. Obviously, the sizes of pore-A in both ZIF-8 and ZIF-90 are moderate and slightly larger than that of linear small gas molecule (e.g., 3.3 Å for CO2), but the sizes of pore-B are too small to accommodate even a small linear molecule if the framework is rigid. Surprisingly, structural optimizations suggest that the pore-B is highly flexible, i.e., the dynamic diameter of pore-B in ZIF-8 greatly increases from 0.2 to 3.1 Å and the dynamic diameter of pore-B in ZIF-90 increases from 2.2 to 3.4 Å (see Figure 2 for the optimized structures). Strikingly, for both ZIF-8 and ZIF-90, the center of pore-B was found to be the energetically most favorable sites for Au(CO)Cl. The computed binding energy of Au(CO)Cl in pore-B of ZIF-8 is −0.82 eV, which is smaller than the value of −1.05 eV in pore-B of ZIF-90. As shown in Figure 2A,B, the Au(CO)Cl molecule is tightly confined at the center of pore-B in both ZIF-8 and ZIF-90, with the linear molecule parallel to the four imidazole rings at pore-B, leading to a strong van der Waals interaction between the guest molecule and four rings. It is noteworthy to mention that the four imidazole rings surrounding pore-B in pristine ZIF-8 are rotated by about 23° after the encapsulation of Au(CO)Cl, opening pore-B by forcing the four rings parallel to the direction of pore-B, as shown in Figure 2A. This leads to a distortion in the ZIF-8 framework, with an energy

penalty of 0.39 eV. As a comparison, the four rings are only rotated by about 13° in ZIF-90 after the loading of Au(CO)Cl molecule at pore-B; not surprisingly, the deformation energy is much smaller, i.e., 0.17 eV. This phenomenon is similar to the well-known “gate-opening” phenomenon in ZIF-7,42−44 where the benzimidazole linkers are rotated over a certain angle, increasing the pore size and thus allowing molecules to enter the main cavities. To the best of our knowledge, the results presented in this study are the first evidence showing pore-B could also be an important pathway for the adsorption and diffusion of small molecule. Besides pore-B, pore-A sites are another important adsorption sites for Au(CO)Cl molecule. When the Au(CO)Cl molecule is confined in pore-A (see Figure 2C,D), the dynamic diameter only slightly increases from 3.5 to 3.7 Å in ZIF-8, whereas the pore size (3.8 Å) is unchanged in ZIF-90. The binding energy of Au(CO)Cl molecule at pore-A in ZIF-8 is −0.82 eV, similar to that at pore-B. In contrast, the binding energy of Au(CO)Cl at the main cavity of ZIF-8 is much smaller, from about −0.4 to −0.7 eV. Compared to ZIF-8, the binding energy at pore-A in ZIF-90 is larger, i.e., −0.92 eV (smaller than −1.05 eV at pore-B of ZIF-90), whereas the binding energies in the main cavity are from about −0.5 to −0.8 eV. The relatively larger binding energy in ZIF-90 should be ascribed to the existence of aldehyde groups, where the oxygen atoms form stronger van der Waals interaction with the Au(CO)Cl molecule. The computed larger binding energy of Au(CO)Cl in ZIF-90 indicates that this molecular precursor is energetically more favorable to be adsorbed in ZIF90 compared to ZIF-8 under the same experimental conditions. Compared to the main cavities, both pore-A and pore-B should be energetically favorable for the adsorption of Au(CO)Cl; thus, the diffusion barrier between the neighboring cavities should be overcome to have a uniform distribution in the ZIFs. In contrast to ZIF-8, the stronger binding energy of Au(CO)Cl in ZIF-90 may play an important role in the narrower distribution of particle size for Au in ZIF-90.39 8903

DOI: 10.1021/acs.jpcc.7b12480 J. Phys. Chem. C 2018, 122, 8901−8909

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Figure 3. Radial distribution function of the atom pairs between the Au atoms and atoms (C, N, and Zn) in the frameworks for various systems, i.e., (A) Au2@ZIF-8, (B) Au4@ZIF-8, (C) Au8@ZIF-8, (D) Au2@ZIF-90, (E) Au4@ZIF-90, and (F) Au8@ZIF-90.

3.2. Stable Structures of Aun@ZIF-8 and Aun@ZIF-90 (n = 2−8). It is instructive to start our investigations from very small Au metal clusters confined in ZIFs. The initial configurations of Au2@ZIF-8, Au4@ZIF-8, and Au8@ZIF-8 were built by placing the small Au clusters at the center of their main cavities and then the AIMD simulations combined with annealing technique30 were performed to search for thermodynamically stable structures. Our calculations suggest that AIMD simulations starting from initial configuration with small Au cluster at other sites lead to almost the same structure. This is probably due to their very high symmetries of both ZIF-8 and ZIF-90, where each imidazolate ring connects two different pores (pore-A and poreB); thus, the main adsorption sites for the confined Au cluster are pore-A and pore-B. To better understand the structures of Au@ ZIFs during the AIMD simulations, it is helpful to analyze the radial distribution function (RDF). The RDFs for the three atom pairs, i.e., Au−C, Au−N, and Au−Zn, for the three systems, Au2@ZIF-8, Au4@ZIF-8, and Au8@ZIF-8, were analyzed using on the trajectories of the AIMD simulations, as shown in Figure 3. The similarity of the three systems is that all of the first peaks for the Au−C atomic pair (black line in Figure 3) correspond to a distance of about 2.3 Å, which is close to the Au−C bond distance (2.3−2.5 Å) in other systems,45,46 indicating the C atoms in the imidazole ring are favorable binding sites for gold cluster. Moreover, the RDF analyses show that there is no bonding for Au−N and Au−Zn pairs because the peaks (red and blue lines) correspond to large distances of 3.0 and 4.8 Å, respectively.

Additionally, it is worth mentioning that the relative height of the first peaks of the three atomic pairs decrease as the number of gold atoms increase from 2 to 8, indicating that the ratio of bonding between Au atoms and ZIF-8 framework decreases as the size of Au cluster grows, which will be discussed later. The above observations from the RDF analyses were further confirmed by the final optimized structures, as shown in Figure 4. All of the three encapsulated Aun (n = 2, 4, and 8) clusters have planar structures with a few edge Au atoms simultaneously binding to the imidazole rings, forming the Au−C bonds with a bond length (2.29 and 2.31 Å) close to the value of 2.3 Å from the RDF analyses. However, as the cluster size grows, the ratio of the Au atoms binding to the framework over the total number of Au atoms gradually decreases because planar Aun structures were formed inside the cavity and only a few edge Au atoms bind to the framework of ZIF-8, which is in good agreement with the decreased relative height of the first peaks of the RDFs discussed above. Therefore, the structural characteristics of the obtained stable structures for the small Aun (n = 2, 4, and 8) cluster encapsulated ZIF-8 agrees well with the conclusions from the RDF analyses. As a comparison, AIMD simulations were also performed to search for stable configurations for the neutral Au clusters. The calculations indicated that all of the isolated Aun clusters with n up to 8 possess a planar structure (see Figure 4), similar to the previously reported structures47−49 but different to the confined Aun clusters to some extent. For example, compared to the 8904

DOI: 10.1021/acs.jpcc.7b12480 J. Phys. Chem. C 2018, 122, 8901−8909

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atoms of imidazole ring to the Au2 cluster, confirming the formation of a bond between the Au2 and ZIF-8 framework. Although the topology and structural characteristics of ZIF-90 are similar to those of ZIF-8, the replacement of the methyl groups in ZIF-8 with the aldehyde groups in ZIF-90 leads to different chemical environment, which should finally affect the formation of Au metal clusters. In the following, the structures and electronic properties of Aun@ZIF-90 are discussed. The trajectory of each AIMD simulation was analyzed to obtain the RDFs between Au atoms and the atoms (C, N, and Zn) in the ZIF-90 framework, as shown in Figure 3D−F. The first peaks of the RDFs of Aun@ZIF-90 (n = 2, 4, and 8) correspond to the Au−O atomic pair with a distance of about 2.2 Å. In contrast, no obvious peak corresponds to Au-C was found for Au2@ZIF-90 and Au8@ZIF-90 with distance less than 3.0 Å, and only a small Au-C peak with distance of about 2.3 Å was found for Au4@ZIF90. These indicate that the O atoms from the aldehyde groups mainly dominate the binding of Au cluster. Indeed, as shown in Figure 4, the O atoms from the aldehyde groups act as anchors binding small Aun (n = 2, 4, and 8) cluster, and all of the three Au cluster maintain a planar structure. For Au2@ZIF-90, only one Au atom from the Au2 cluster binds to the O atom with a bond length of 2.16 Å, and the other Au atom points to the center of the main cavity. In Au4@ZIF-90, one Au atom attaches to the O atom from the aldehyde group and another Au atom binds to two C atoms from one imidazole ring, with the Au−O distance of 2.16 Å and Au−C bond lengths of 2.40 and 2.47 Å. The confined Au8 in ZIF-90 has a planar structure similar to that in Au8@ZIF-8 with two edge Au atoms binding to two neighboring aldehyde groups. Additionally, the charge density difference of Au2@ZIF90 was computed and is plotted in Figure 5B. Compared to Au2@ZIF-8, there is a much smaller depletion and aggregation of electrons between Au cluster and oxygen atom in Au2@ZIF-90, in good agreement with the relatively smaller electrons transferred from the ZIF-90 framework to the Au cluster, which will be discussed in Section 3.4. This different charge density should arise from the different adsorption sites for Au2 cluster, i.e., aldehyde group site (head of imidazolate ring) and C−C site (tail of imidazolate ring) for Au2@ZIF-90 and Au2@ ZIF-8, respectively, leading to a different interaction energy between the Au cluster and the host. This phenomenon holds true as the cluster size grows to 20 atoms, which will be discussed later. In addition, the deformation energies of the ZIFs frameworks were computed and our calculations indicated that the deformation energies are 0.25−0.44 eV (Aun@ZIF-8) and 0.27−0.50 eV (Aun@ZIF-90), respectively, much smaller than those in Pd@UiO-66-NH2, i.e., 0.43−2.13 eV. This indicates that the loading of Au cluster does not greatly affect the structures of frameworks and explains the fact that the confined Aun clusters have similar structures to the isolated Au clusters. In conclusion, the RDFs and stable structures of Aun@ZIF-8 and Aun@ZIF-90 with n up to 8 suggest that the replacement of methyl group with aldehyde groups greatly changes the chemical environment of the main cavities, making the aldehyde groups as the energetically more favorable sites for Au clusters binding. It is reasonable to speculate that a larger confined Au cluster should have a similar bonding pattern to the small confined Au cluster. 3.3. Stable Structures of Aun (n = 12, 16, and 20) Clusters Confined in ZIF-8 and ZIF-90. The planar structures of small Aun clusters with n up to 8 are smaller than the main cavities of both ZIF-8 and ZIF-90, and thus the confinement effect is not obvious. Nevertheless, as the cluster size grows and

Figure 4. Optimized structures of the isolated Aun, Aun@ZIF-8, and Aun@ZIF-90 with n = 2, 4, and 8, where the Au, C, H, N, O, and Zn atoms are represented by yellow, gray, white, blue, red, and green balls, respectively. Note that only a few atoms surrounding the encapsulated Aun cluster are shown for clarity.

isolated Au4 cluster, the confined Au4 cluster has a different planar structure with two edge Au atoms of the confined Au4 cluster (as shown in Figure 4) bound to two neighboring imidazole rings. The encapsulated Au8 cluster in ZIF-8 has a very similar structure compared to the isolated Au8 cluster, with only minor distortion due to the two edge Au atoms in Au8@ZIF-8 binding to two different imidazole rings. The charge density difference could be analyzed to obtain insights into the bonding pattern between the Au cluster and the ZIF-8 framework. The charge density difference was defined as Δρ = ρAu/ZIF‑8 − (ρZIF‑8 + ρAu), where ρAu/ZIF‑8, ρZIF‑8, and ρAu represent the charge density for Au/ZIF-8, ZIF-8, and isolated Au cluster, respectively. The plot of charge density difference for Au2@ZIF-8 system is shown in Figure 5A, where the cyan and yellow color represent the completion and accumulation of electrons. The plot shows that with the adsorption of Au dimer, the charge density for the system is redistributed and a charge is transferred from the C

Figure 5. Comparison of the computed charge density difference of (A) Au2@ZIF-8 and (B) Au2@ZIF-90. The cyan and yellow color represent the completion and accumulation of electrons, respectively, and the isosurface values for the two systems are all set to 0.03 e/Å3. The representation of the atoms are same as in Figure 4 except for Au atoms, which are shown by purple balls. 8905

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3.4. Electronic Properties of Aun@ZIF-8 and Aun@ZIF90. Understanding the thermodynamics of the aggregation of metal cluster from single metal atom should be helpful to explore the stability of the catalyst. In this regard, it is reasonable to compute the average binding energy EAu(N) to reflect the stability of differently sized Aun cluster confined in ZIFs30

reaches the size of the main cavity, the confinement effect of the frameworks becomes stronger and finally affects the structure and electronic property of the encapsulated Au cluster. Figure 6

EAu(N ) = (Etot − EZIF − N ·EAu)/N

where Etot, EZIF, and EAu represent the energies of Aun@ZIF, ZIF, and a single Au atom, respectively. As shown in Figure 7A, the computed EAu(N) for both Aun@ZIF-8 and Aun@ZIF-90 monotonically increases, e.g., from −1.71 to −2.35 eV for ZIF8. Compared to Aun@ZIF-8, the EAu(N) values for Aun@ZIF-90 are relatively smaller by 0.01−0.25 eV. Moreover, the stronger binding of Aun cluster inside ZIF-8 is accompanied by a larger charge transfer between the host and the guest. Bader charge analysis shows that only 0.08 electrons are transferred from the ZIF-90 framework to Au dimer in Au2@ZIF-90, whereas this value is 0.21 electrons in Au2@ZIF-8, indicating more electrons are transferred from the ZIF-8 framework to the Au2 cluster, thus, in turn, leading to a stronger binding in Au2@ZIF-8. This trend holds true for a relatively larger Au cluster confined in the composites. As shown in Figure 7B, the electrons transferred to Au cluster in ZIF-90 increase from 0.08 (the number of Au atoms n = 2) to 0.20 (n = 12) and then slightly decrease to 0.17 (n = 20), whereas the transferred electrons in ZIF-8 increase from 0.20 (n = 2) to 0.47 (n = 16) and then decrease to 0.44 (n = 20). Overall, the charge transfer between the host and the guest is larger in Aun@ZIF-8 compared to Aun@ZIF-90, in good agreement with their host−guest interactions. With the discussions above, it is reasonable to conclude that the different electronic properties for Aun@ZIF-8 and Aun@ZIF-90 should arise from the different adsorption sites for Au clusters in ZIF-8 and ZIF-90. Similar to Au2@ZIF-8 and Au2@ZIF-90 discussed above in Section 3.2, the relatively larger Aun clusters also bind to the C−C bond in the imidazolate ring and the aldehyde group from ZIF-8 and ZIF-90, respectively. We expect that the different adsorption sites for Au clusters in ZIF-8 and ZIF-90 could be experimentally verified by examining the vibration of Au−C and Au−O bonds in the future. 3.5. Comparison of CO Oxidation Reactions with Different Catalyst Models. Although many experiments have been performed to investigate CO oxidation using the confined Aun cluster,7 many questions including the adsorption of reactants and the role of framework during the reactions remain unclear and are a great challenge to simulate due to the limited knowledge of the structures for catalysts. Therefore, theoretical investigations are requisite for a better understanding of the catalytic reactions using metal nanoparticle confined

Figure 6. Obtained stable structures of the isolated Aun, Aun@ZIF-8, and Aun@ZIF-90 with n = 12, 16, and 20.

shows the stable structures of Aun@ZIF-8 and Aun@ZIF-90 with n = 12, 16, and 20, where the isolated Aun clusters are also presented for comparison. The three isolated Aun clusters have structures similar to the previously reported structures.50−52 The isolated Au12 possesses a planar structure, which is also maintained when it is confined in both ZIF-8 and ZIF-90. The only difference is that the Au12 cluster binds to the C sites and O sites in ZIF-8 and ZIF-90, respectively. The isolated cagelike Au16 structure is only slightly distorted in Au16@ZIF-8 and Au16@ ZIF-90 due to the host−guest interaction. It is interesting that the magic Au20 cluster maintains a pyramidal structure when confined in ZIF-8; however, the pyramidal structure is distorted in ZIF-90, which is probably due to its stronger confinement effect. The binding sites for the three Aun (n = 12, 16, and 20) clusters in ZIF-8 and ZIF-90 are similar to the small Aun (n = 2, 4, and 8) cluster, i.e., the C atoms from imidazole rings in ZIF-8 and the O atoms from aldehyde groups in ZIF-90 are the main binding sites.

Figure 7. Comparison of (A) the computed average binding energies for the Au cluster confined in ZIF-8 and ZIF-90 as a function of the number of Au atoms and (B) the computed Bader charge for the confined Aun cluster. 8906

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The Journal of Physical Chemistry C MOFs. Herein, the CO molecule adsorption sites were analyzed to understand the difference of the electronic properties of Aun@ ZIF-8 and Aun@ZIF-90 (n = 4 and 16), which were also compared to the isolated Aun clusters. A number of different initial configurations for each system were considered for structural optimizations, and the most stable structures are shown in Figure 8.

Figure 8. Computed stable structures for CO adsorption on isolated Aun, Aun@ZIF-8, and Aun@ZIF-90 with n = 4 and 16, where the frameworks were omitted for clarity. Figure 9. Energy profiles are shown for the CO oxidation using isolated Au16 (black line), Au16@ZIF-8 (red line), and Au16@ZIF-90 (blue line) as catalysts, respectively. The configurations for various stages during the two-step reactions using the isolated Au16 as catalyst are presented.

The structures of the catalyst models were almost unchanged upon the adsorption of CO molecule. In the isolated Au4 cluster, the CO molecule binds to the edge Au with the coordination number of 3 in a planar Au4 structure, whereas in Au4@ZIF-8 and Au4@ZIF-90, the CO molecule adsorbs at the edge Au with the coordination number of 2. The edge Au sites in Au16, Au16@ZIF8, and Au16@ZIF-90 are energetically preferential sites for the adsorption of CO molecule, as shown in Figure 8. The binding energy of CO on the isolated Au4 cluster is about −1.76 eV, larger than those of −1.16 and −1.42 eV for Au4@ZIF-8 and Au4@ZIF90, respectively. Bader charge analyses show that the CO molecule donates 0.03 |e| to the Au in an isolated Au4 cluster, whereas the CO molecule obtains very small electrons of 0.01 |e| from the catalysts in both Au4@ZIF-8 and Au4@ZIF-90. The charge transfer between the CO molecule and the Au16 catalyst models are similar, i.e., the CO molecule has a Bader charge of 0.02, −0.02, and −0.04 |e| for isolated Au16, Au16@ZIF-8, and Au20@ZIF-90, respectively. Compared to the small Au4 models, the binding energies of CO adsorption on Au16 models are remarkably smaller, i.e., −1.07 (Au16), −0.86 (Au16@ZIF-8), and −1.23 (Au16@ZIF-90) eV, respectively. The relatively larger Aun cluster having a smaller adsorption energy for CO molecule in this study is the same as that in the previously reported results.53 Jiang et al.7 have confirmed that the Au@ZIF-8 catalysts had a stable activity at room temperature and the sizes of Au nanoparticles were almost unchanged before and after the reactions. To illustrate the structure−property relationship for the Au cluster confined in ZIFs theoretically, the three Au16 clusters, i.e., the isolated Au16 and two confined Au16 clusters, were selected for studying the CO oxidation reactions in this work. A two-step reaction with the conventional Langmuir− Hirshelwood mechanism was proposed and discussed. All of the transition states were identified with the climbing image nudged elastic band method, and the details of the calculations are same as that in previous work.54 As shown in Figure 9, the computed

reaction pathways were compared for the three systems, i.e., isolated Au16, Au16@ZIF-8, and Au16@ZIF-90. When the isolated Au16 was employed as a catalyst, the O2 molecule was initially adsorbed close to the CO molecule, which was considered as the initial state (CO + O2 in Figure 9) for the reactions, and an intermediate (O−C−O−O*) was formed with a relative energy of −0.01 eV compared to the initial state. The second step reaction is exothermic, with the relative energy for the final product (CO2 + O*) of −2.02 eV, and its energy barrier of 0.29 eV is much smaller than 0.50 eV for the first step reaction. As a comparison, in the case of Au16@ZIF-90, the two energy barriers reduce to 0.20 and 0.22 eV, respectively, indicating that the Au16 cluster confined in ZIF-90 has a better catalytic activity for the CO oxidation. Furthermore, it is thermodynamically more favorable because the relative energies for the intermediate and final products are −1.12 and −3.16 eV, respectively, both of which are more exothermic than those in the case of isolated Au16. Interestingly, the energy barriers corresponding to the TS1 and TS2 of Au16@ZIF-8 are only 0.02 and 0.21 eV, respectively, even smaller than those in the case of Au16@ZIF-8. In addition, the relative energy for the intermediate and the final product is −0.99 and −3.16 eV, respectively, suggesting the two-step reactions for Au16@ZIF-8 are even more exothermic. The smaller energy barriers in Au16@ZIF-8 are accompanied by a more negatively charged Au16 cluster, suggesting that the more negatively charged Au cluster confined in the ZIF is more favorable for the CO oxidation. Obviously, a comparison of the reaction pathways for the three cases discussed above suggests that the confined Au16 clusters in both ZIF-8 and ZIF-90 have smaller reaction barriers than those for isolated Au16 cluster. 8907

DOI: 10.1021/acs.jpcc.7b12480 J. Phys. Chem. C 2018, 122, 8901−8909

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The Journal of Physical Chemistry C

21303165) and Natural Science Foundation of Zhejiang Province (LQ14B030001 and LY17B060001).

Therefore, we can conclude that the negatively charged Au clusters in ZIFs should have a higher catalytic activity for CO oxidation based on the computed data in this study. In addition, compared to the isolated Au16 cluster, the reactions are more exothermic and thus thermodynamically more favorable when the Au cluster confined in ZIFs was used as a catalyst. However, with the very limited data above, we cannot conclude that the more negatively charged Au cluster confined in the ZIFs would always lead to a smaller energy barrier for CO oxidation. To have a more convincing conclusion, a systematic investigation using differently sized Aun cluster confined in the ZIFs as catalysts should be performed for their catalytic activities over CO oxidation reactions.

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(1) Park, K. S.; Ni, Z.; Cote, A. P.; Choi, J. Y.; Huang, R.; Uribe-Romo, F. J.; Chae, H. K.; O’Keeffe, M.; Yaghi, O. M. Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10186−10191. (2) Banerjee, R.; Phan, A.; Wang, B.; Knobler, C.; Furukawa, H.; O’Keeffe, M.; Yaghi, O. M. High-Throughput Synthesis of Zeolitic Imidazolate Frameworks and Application to CO2 Capture. Science 2008, 319, 939−943. (3) Whitford, C. L.; Stephenson, C. J.; Gómez-Gualdrón, D. A.; Hupp, J. T.; Farha, O. K.; Snurr, R. Q.; Stair, P. C. Elucidating the NanoparticleMetal Organic Framework Interface of Pt@ZIF-8 Catalysts. J. Phys. Chem. C 2017, 121, 25079−25091. (4) Jia, X.; Wang, S.; Fan, Y. A Novel Strategy for Producing Highly Dispersed Pd Particles on ZIF-8 Through the Occupation and Unoccupation of Carboxyl Groups and Its Application in Selective Diene Hydrogenation. J. Catal. 2015, 327, 54−57. (5) Huang, Y.; Zhang, Y. H.; Chen, X. X.; Wu, D. S.; Yi, Z. G.; Cao, R. Bimetallic Alloy Nanocrystals Encapsulated in ZIF-8 for Synergistic Catalysis of Ethylene Oxidative Degradation. Chem. Commun. 2014, 50, 10115−10117. (6) Dang, T. T.; Zhu, Y. H.; Ngiam, J. S. Y.; Ghosh, S. C.; Chen, A. Q.; Seayad, A. M. Palladium Nanoparticles Supported on ZIF-8 As an Efficient Heterogeneous Catalyst for Aminocarbonylation. ACS Catal. 2013, 3, 1406−1410. (7) Jiang, H.-L.; Liu, B.; Akita, T.; Haruta, M.; Sakurai, H.; Xu, Q. Au@ ZIF-8: CO Oxidation over Gold Nanoparticles Deposited to MetalOrganic Framework. J. Am. Chem. Soc. 2009, 131, 11302−11303. (8) Tran, U. P. N.; Le, K. K. A.; Phan, N. T. S. Expanding Applications of Metal-Organic Frameworks: Zeolite Imidazolate Framework ZIF-8 as an Efficient Heterogeneous Catalyst for the Knoevenagel Reaction. ACS Catal. 2011, 1, 120−127. (9) Wang, C.; Tuninetti, J.; Wang, Z.; Zhang, C.; Ciganda, R.; Salmon, L.; Moya, S.; Ruiz, J.; Astruc, D. Hydrolysis of Ammonia-Borane over Ni/ZIF-8 Nanocatalyst: High Efficiency, Mechanism, and Controlled Hydrogen Release. J. Am. Chem. Soc. 2017, 139, 11610−11615. (10) Lin, L.; Liu, H. O.; Zhang, X. F. ZnO-Template Synthesis of Rattle-Type Catalysts with Supported Pd Nanoparticles Encapsulated in Hollow ZIF-8 for Liquid Hydrogenation. Chem. Eng. J. 2017, 328, 124− 132. (11) Ding, S. S.; Yan, Q.; Jiang, H.; Zhong, Z. X.; Chen, R. Z.; Xing, W. H. Fabrication of Pd@ZIF-8 Catalysts with Different Pd Spatial Distributions and Their Catalytic Properties. Chem. Eng. J. 2016, 296, 146−153. (12) Rösler, C.; Esken, D.; Wiktor, C.; Kobayashi, H.; Yamamoto, T.; Matsumura, S.; Kitagawa, H.; Fischer, R. A. Encapsulation of Bimetallic Nanoparticles into a Metal-Organic Framework: Preparation and Microstructure Characterization of Pd/Au@ZIF-8. Eur. J. Inorg. Chem. 2014, 5514−5521. (13) Kuo, C. H.; Tang, Y.; Chou, L. Y.; Sneed, B. T.; Brodsky, C. N.; Zhao, Z. P.; Tsung, C. K. Yolk-Shell Nanocrystal@ZIF-8 Nanostructures for Gas-Phase Heterogeneous Catalysis with Selectivity Control. J. Am. Chem. Soc. 2012, 134, 14345−14348. (14) Lu, G.; Li, S. Z.; Guo, Z.; Farha, O. K.; Hauser, B. G.; Qi, X. Y.; Wang, Y.; Wang, X.; Han, S. Y.; Liu, X. G.; et al. Imparting Functionality to a Metal-Organic Framework Material by Controlled Nanoparticle Encapsulation. Nat. Chem. 2012, 4, 310−316. (15) Phan, A.; Doonan, C. J.; Uribe-Romo, F. J.; Knobler, C. B.; O’Keeffe, M.; Yaghi, O. M. Synthesis, Structure, and Carbon Dioxide Capture Properties of Zeolitic Imidazolate Frameworks. Acc. Chem. Res. 2010, 43, 58−67. (16) Chizallet, C.; Lazare, S.; Bazer-Bachi, D.; Bonnier, F.; Lecocq, V.; Soyer, E.; Quoineaud, A. A.; Bats, N. Catalysis of Transesterification by a Nonfunctionalized Metal-Organic Framework: Acido-Basicity at the External Surface of ZIF-8 Probed by FTIR and ab Initio Calculations. J. Am. Chem. Soc. 2010, 132, 12365−12377.

4. CONCLUSIONS It is unexpected that pore-B with four imidazole rings is highly flexible because the pore size greatly increases from 0.2 to 3.1 Å upon the loading of molecular precursor Au(CO)Cl in ZIF-90; furthermore, the expanded pore-B is an energetically most favorable site for the adsorption of Au(CO)Cl. In addition, the adsorption energy of Au(CO)Cl in ZIF-90 is larger than that in ZIF-8, which is a result of the existence of aldehyde groups along the channel of pore-B. This could result in a better distribution of Au(CO)Cl molecules in the ZIF-90 matrix compared to ZIF-8, which may lead to different sizes of the Au clusters confined in ZIFs. Then, a combination of AIMD simulations and DFT calculations was performed to obtain the thermodynamically stable Aun clusters confined in ZIF-8 and ZIF-90 with n up to 20. The RDF analyses of the trajectory for the systems indicate that the imidazole rings and the aldehyde groups are the main adsorption sites for Au cluster in ZIF-8 and ZIF-90, respectively. The calculations suggest that the different functional groups in ZIF-8 and ZIF-90 indeed lead to different electronic properties of Au cluster confined in composites. The average binding energy of Au cluster in ZIF-8 is stronger than that in ZIF-90, which is accompanied by a larger electron transfer from the host to the guest in ZIF-8. Finally, CO molecules were selected for their adsorption on the selected catalyst models, i.e., Aun (n = 4 and 16) in ZIF-8 and ZIF-90, and the computational results show that ZIF-8-based catalyst has a smaller adsorption affinity for CO molecule, suggesting the host for the loading of Au cluster plays a key role in the adsorption of gas molecules. Finally, the activation energies of CO oxidation using Au16@ZIF-8 and Au16@ZIF-90 were computed to be smaller than those for isolated Au16, suggesting that the ZIFs contribute to the enhancement of catalytic activities.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.-L.C.). *E-mail: wangff@zjnu.cn (F.-F.W.). ORCID

De-Li Chen: 0000-0002-3550-5119 Fang-Fang Wang: 0000-0001-5626-314X Author Contributions §

L.D. and S.W. contribute equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.-L.C and F.-F.W. gratefully acknowledge the support from the National Natural Science Foundation of China (21403198 and 8908

DOI: 10.1021/acs.jpcc.7b12480 J. Phys. Chem. C 2018, 122, 8901−8909

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