Structures and Electronic Properties of Au Clusters Encapsulated ZIF

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

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 Wei-Dong Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12480 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 5, 2018

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The Journal of Physical Chemistry

Structures and Electronic Properties of Au Clusters Encapsulated ZIF-8 and ZIF-90 Li Dou,1‡ Shengnan Wu,1‡ De-Li Chen,1* Sihui He,2 Fang-Fang Wang,2* and Weidong Zhu1 1

Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Zhejiang Normal

University, 321004 Jinhua, China 2

College of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China

E-mail: [email protected]; [email protected] ‡

These authors contribute equally to this work.

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ABSTRACT The zeolitic imidazolate frameworks (ZIFs) are chemically and thermally stable microporous materials, being considered as ideal supports for 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, and surprisingly the pore-B with 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, while the well-known pore-A with 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, while the imidazole rings were identified as the binding sites in ZIF-8. Compared to ZIF-90, the binding of Au clusters in ZIF-8 was stronger, accompanies with larger electrons transfer from the frameworks to the confined Au clusters. Finally, the computed energy barriers for the CO oxidation using Au cluster confined ZIFs as catalysts were found to be smaller than those for isolated Au cluster.

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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, firstly synthesized by Park et al.1 and then extended by Banerjee and coworkers2. 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 high thermal stability up to 420 ºC; ZIF-8 is composed of cavities with pore diameter of about 11 Å that are connected by small pore with size of 3.4 Å.1,15 Therefore, ZIF-8 has been investigated as an appropriate template to disperse MNPs, while 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 large size nanoparticle. ZIFs have been investigated for their applications on catalysis by the open metal sites on their surface,16 catalytic active functional groups on imidazolate linkers,17 and 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 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 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 3



connected by small pores. Many other 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 as the decrease of 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 2D to 3D when the number of Au atoms n reaches 9, while for anionic Au cluster the transition starts at n = 14.20 In addition, the Au cluster loaded on support such as metal oxide tends to have distorted structure comparing 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 have been made to the MOFs based Au catalysts, especially the structures and the electronic properties of the confined Au clusters are unclear, which are the key to understand the mechanism of reactions occurred on these catalysts. Understanding on the formation of the MNPs inside the cavities of MOFs is challenging. Lots of theoretical progress have 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 thus it is a great challenge to simulate, until recently some progress 4


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have been made to investigate the encapsulation of small Au and Pd metal clusters confined into the cavities of MOF-74 and UiO-66(-NH2) using density functional theory (DFT) method combined with genetic algorithm.28-30 In addition, the defects are very popular in various MOFs and may play important role not only in gas adsorption but also in catalysis, thus the mechanism of the formation of defects have attracted great interests. DFT calculations have been performed to reveal the formation of defects inside ZIF-8 by Zhang and coworkers.31 The removal of BDC linkers in UiO-66 was known to create actives sites for catalysis and 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. 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-Ernzerhof (PBE)33 implemented in the Vienna

Ab-initio

Simulation

Package34-37 was

employed

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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 stable configurations for Aun@ZIF-8 and Aun@ZIF-90 (n up to 20), with 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 two dimensional and three dimensional clusters, which should reflect the evolution of small Au cluster aggregating to relatively large Au cluster, although the size of Au20 is still smaller than the actual size (10 to 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 500 K should be a reasonable value. The final temperature for the annealing simulations was set to 300 K since our calculations suggested that the diffusion of the gold cluster was dramatically 6


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decreased when the temperature is 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 is smaller than 0.03 eV/Å.

Figure 1. The optimized crystal structures of ZIF-8 and ZIF-90 with (111) and (110) surfaces, where the C, H, N, O, Zn atoms are shown with gray, white, blue, red, and green balls. The green and red circles represent the pore-A and pore-B, respectively, for both ZIF-8 and ZIF-90. 3. RERESLTS AND DISCUSSION Our calculations indicated the ZIF-8 and ZIF-90 exihibt the same sodalite-like structure with small pore aperture size of 3.5 and 3.8 Å, respectively, connecting the 7



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 6 imidazole rings, while the pore-B consists of 4 imidazole rings and the pore size is much smaller, i.e., 0.2 (2.2) Å. Note that the pore-A and pore-B are the same to the reported 6-ring and 4-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, while 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 were 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 much larger size of metal nanoparticles than the main cavities. In this study, we focus on theoretical investigations on the structures and electronic properties of the confined Au metal clusters in pristine ZIF-8 and ZIF-90, while the formation of defect sites and its 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 8


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distribution of Au particles,7,39 suggesting the growth of metal cluster are 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 the pore-A (see Figure 1) with diameter of 3.5 Å was previously considered as the only pathway for the diffusion of gas molecules,1,40,41 the 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: Eb = Emolecule/ZIF – EZIF – Emolecule

where the 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 was 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 the 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.

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The diameters of pore-A and pore-B in pristine ZIF-8 and ZIF-90 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 the pore-B of ZIF-8 is –0.82 eV, which is smaller than the value of –1.05 eV in the pore-B of ZIF-90. As shown in Figure 2A and 2B, 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 Wars interaction between the guest molecule and four rings. It is noteworthy to mention that the 4 imidazole rings surrounding the pore-B in pristine ZIF-8 are rotated by about 23 degree after the encapsulation of Au(CO)Cl, opening the pore-B by forcing the 4 rings parallel to the direction of pore-B, as shown in Figure 2A. This leads to a distortion to the ZIF-8 framework, with an energy penalty of 0.39 eV. As a comparison, the 4 rings are only rotated by about 13 degree in ZIF-90 10


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after the loading of Au(CO)Cl molecule at the pore-B, and 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 making the pore size increased and thus allowing molecules enter the main cavities. To the best of our knowledge, the results presented in this study are the first evidence showing the pore-B could also be an important pathway for the adsorption and diffusion of small molecule.

Besides of pore-B, the 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 and 2D), the dynamic diameter only slightly increase from 3.5 to 3.7 Å in ZIF-8, while in ZIF-90 the pore size (3.8 Å) is unchanged. The binding energy of Au(CO)Cl molecule at the pore-A in ZIF-8 is –0.82 eV, the same 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), while in the main cavity the binding energies 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 the Au(CO)Cl in ZIF-90 indicate that this molecular precursor is energetically more favorable to be adsorbed in ZIF-90 comparing to ZIF-8 under the same experimental conditions. Compared to the main cavities, both the pore-A and pore-B should be energetically favorable for the 11



adsorption of Au(CO)Cl, and thus the diffusion barrier between the neighboring cavities should be overcome in order 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

Figure 2. The 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 ZIF-8, and (D) pore-A in ZIF-90; the values of diameters for the pore-A or pore-B of Au(CO)Cl@ZIFs are shown in each picture, and the numbers in parenthesis are shown for the pristine ZIFs.

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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 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 13



pore-B), and thus the main adsorption sites for the confined Au cluster are the 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 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 to 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 since the peaks (red and blue lines) correspond to large distances of 3.0 and 4.8 Å, respectively. Additionally, it is worthwhile to mention that the relative height for 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 imidazole rings, forming Au-C bonds with 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 since the planar Aun structures were 14


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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.

Figure 4. The 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. Note that only a few atoms surrounding the encapsulated Aun cluster are shown for clarity. 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 planar structure (see Figure 4), the same to the previously reported structures,47-49 but different to the confined Aun clusters to some extent. For example, compared to the isolated Au4 cluster, the confined Au4 15



cluster has a different planar structure with two edge Au atoms of the confined Au4 cluster (as shown in Figure 4) bind to two neighboring imidazole rings. The encapsulated Au8 cluster in ZIF-8 has a very similar structure comparing 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 on the bonding pattern between the Au cluster and ZIF-8 framework. The charge density difference was defined as ∆ρ = 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 transfer from the C atoms of imidazole ring to the Au2 cluster occurs, confirming the formation of bond between the Au2 and ZIF-8 framework.

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 16


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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 the same to Figure 4 except for Au atoms, which are shown by purple balls. Although the topology and structural characteristics of ZIF-90 are the same 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 RDFs between Au atoms and the atoms (C, N, and Zn) in the ZIF-90 framework, as shown in Figure 3D-3F. The first peaks of the RDFs of Aun@ZIF-90 (n = 2, 4, and 8) correspond to Au-O atomic pair with distance of about 2.2 Å. In contrast, a much lower height of peak corresponds to the Au-C with distance of about 2.3 Å was found for Au4@ZIF-90, while no obvious Au-C peak (less than 3.0 Å) was found for Au2@ZIF-90 and Au8@ZIF-90. 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 planar structure. For Au2@ZIF-90, only one Au atom from the Au2 cluster binds to the O atom with bond length of 2.16 Å, and the other Au atom pointing to the center of main cavity. In Au4@ZIF-90, one Au atom attaches to the O atom from aldehyde group and another Au atom binds to two C atoms from one imidazole ring, with 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 17



a similarly planar structure to that in Au8@ZIF-8 with two edge Au atoms binding to two neighboring aldehyde groups. Additionally, the charge density difference of Au2@ZIF-90 was computed and plotted in Figure 5B. Compared to Au2@ZIF-8, there is a much smaller depletion and aggregation of electrons between the 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 18


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confined Au cluster.

Figure 6. The obtained stable structures of the isolated Aun, Aun@ZIF-8, and Aun@ZIF-90 with n = 12, 16, and 20. 3.3 Stable Structures of Aun (n = 12, 16, and 20) Cluster 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 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 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 similar structures with the previously reported structures.50-52 The isolated Au12 possesses planar structure, which is also maintained when it is confined 19



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 cage-like Au16 structure is only slightly distorted in Au16@ZIF8 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, 20) clusters in ZIF-8 and ZIF-90 are similar to the small Aun (n = 2, 4, 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. 3.4 Electronic Properties of Aun@ZIF-8 and Aun@ZIF-90 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 regards, it is reasonable to compute the average binding energy EAu(N) to reflect the stability of different sized Aun cluster confined ZIFs:30 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 ZIF-8. Compared to Aun@ZIF-8, the EAu(N) values for Aun@ZIF-90 are relatively smaller by 0.01 to 0.25 eV. Moreover, the stronger binding of Aun cluster inside ZIF-8 accompanies with larger charge transfer between the host and the guest. Bader charge analysis shows that only 0.08 electrons transferred from ZIF-90 framework to Au 20


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dimer in Au2@ZIF-90, while this value is 0.21 electrons in Au2@ZIF-8, indicating more electrons transferred from the ZIF-8 framework to the Au2 cluster and thus leading to stronger binding in Au2@ZIF-8. This trend holds true for relatively larger Au cluster confined composites. As shown in Figure 7B, the electrons transferred from ZIF-90 to Au cluster 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), while in ZIF-8 the transferred electrons 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 comparing to Aun@ZIF-90, which is 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 as discussed above in Section 3.2, the relatively larger Aun clusters also bind to the C-C bond in imidazolate ring and the aldehyde group from ZIF-8 and ZIF-90, respectively. We expect 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 future.

Figure 7. Comparison of (A) the computed average binding energies for the confined Au cluster in ZIF-8 and ZIF-90 as a function of the number of Au atoms and (B) the 21



computed Bader charge for the confined Aun cluster. 3.5 Comparison of CO Oxidation Reactions with Different Catalyst Models Although many experiments have been performed investigating 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 better understanding of the catalytic reactions using metal nanoparticle confined MOFs. Herein, adsorption sites analysis with CO was first carried out 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. The structures of the catalyst models were almost unchanged upon the adsorption of CO molecule. For the isolated Au4 cluster, the CO molecule binds to the edge Au with coordination number of 3 in planar Au4 structure, while in Au4@ZIF-8 and Au4@ZIF-90 the CO molecule adsorbs at the edge Au with coordination number of 2. The edge Au sites in Au16, Au16@ZIF-8, 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@ZIF-90, respectively. Bader charge analyses show that the CO molecule donates 0.03 |e| to the Au in isolated Au4 cluster, 22


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while the CO molecule obtains a 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 to the previously reported results.53

Figure 8. The 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. Jiang et al.7 have confirmed that the Au@ZIF-8 catalysts had stable activity at room temperature and the sizes of Au nanoparticles were almost unchanged before and after the reactions. In order to illustrate structure property relationship for the Au cluster confined ZIFs theoretically, the three Au16 clusters, i.e., the isolated Au16 and 23



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 for the calculations are the same to our 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 catalyst, the O2 molecule was initially adsorbed closed to the CO molecule, which was considered as initial state (CO+O2 in Figure 9) for the reactions, and an intermediate (O-C-O-O*) was formed with relative energy of -0.01 eV comparing to the initial state. The second step reaction is exothermic with 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, for the case of Au16@ZIF-90, the two energy barriers reduce to 0.20 and 0.22 eV, respectively, indicating the Au16 cluster confined in ZIF-90 have a better catalytic activity for the CO oxidation. Furthermore, it is thermodynamically more favorable since 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, 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, suggesting the two step reactions for Au16@ZIF-8 are even more exothermic processes. 24


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The smaller energy barriers in Au16@ZIF-8 accompany with a more negatively charged Au16 cluster, suggesting the more negatively charged Au cluster confined in the ZIF is more favorable for the CO oxidation. Obviously, the comparison of the reaction pathways for the three cases discussed above suggest that the confined Au16 cluster in both ZIF-8 and ZIF-90 have smaller reaction barriers than those for isolated Au16 cluster. Therefore, we can conclude that the negatively charged Au clusters in ZIFs should have 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 confined Au cluster in ZIFs were used as catalysts. However, with the very limited data above, we can not conclude that the more negatively charged Au cluster confined in the ZIFs would always lead to a smaller energy barrier for CO oxidation. In order to have a more convincing conclusion, a systematic investigation using different sized Aun cluster confined in the ZIFs as catalysts should be performed for their catalytic activities over CO oxidation reactions.

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Figure 9. The 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. 4. CONCLUSIONS It is unexpected that the pore-B with four imidazole rings is highly flexible since the pore size greatly increases from 0.2 to 3.1 Å upon the loading of molecular precursor Au(CO)Cl in ZIF-90, and furthermore, the expanded pore-B is the 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 ZIF-90 matrix comparing to ZIF-8, which may lead to 26


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different sizes of the Au clusters confined in ZIFs. Then, a combined AIMD simulations and DFT calculations was performed to obtain the thermodynamically stable Aun cluster confined 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 composites. The average binding energy of Au cluster in ZIF-8 is stronger than that in ZIF-90, which accompanies with a larger electron transfer from the host to the guest in ZIF-8. Finally, CO molecule was 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 on 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 have a contribution to the enhancement of catalytic activities.

ACKNOWLEDGEMENTS D.-L.C and F.-F.W. gratefully acknowledge the support from the National Natural Science Foundation of China (21403198, 21303165) and Natural Science Foundation of Zhejiang Province (LQ14B030001, LY17B060001).

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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 the same to Figure 4 except for Au atoms, which are shown by purple balls. 331x166mm (95 x 95 DPI)



Figure 8. The 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. 264x206mm (95 x 95 DPI)


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Figure 9. The 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. 129x174mm (95 x 95 DPI)



TOC Graphic 99x64mm (300 x 300 DPI)


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Structures and Electronic Properties of Au Clusters Encapsulated ZIF

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