metal-organic frameworks: Synthesis, applications and challenges

Applied Materials Today 19 (2020) 100564

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Bimetallic nanoparticles/metal-organic frameworks: Synthesis, applications and challenges Mengbiao Duan a,b,1 , Longbo Jiang a,b,1 , Guangming Zeng a,b , Dongbo Wang a,b , Wangwang Tang a,b,∗ , Jie Liang a,b , Han Wang a,b , Di He c , Zhifeng Liu a,b , Lin Tang a,b,∗ a

College of Environmental Science and Engineering, Hunan University, Changsha, 410082, China Key Laboratory of Environmental Biology and Pollution Control (Hunan University), Ministry of Education, Changsha, 410082, China c Institute of Environmental and Ecological Engineering, Guangdong University of Technology, Guangzhou, 510006, China b

a r t i c l e

i n f o

Article history: Received 23 November 2019 Received in revised form 5 January 2020 Accepted 7 January 2020 Keywords: Bimetallic nanoparticles Metal-organic frameworks Controllable integration Synergistic effects Heterogeneous catalysis

a b s t r a c t Heterogeneous catalysis plays an important role in world economic development and has good application prospects in many industries. The major task in developing heterogeneous catalysts are fulfilling the rapid conversion of catalyzed reactions and ensuring high selectivity to target products. Recently, a novel type of composites, denoted as bimetallic nanoparticle (NP)/metal-organic framework (MOF), has attracted widespread attention in heterogeneous catalysis. The combination of bimetallic NPs with MOF materials is considered to be one of the most effective strategies to obtain enhanced catalytic activity and/or expand reaction scope. Due to synergistic effects between different components, the combined composites exhibit enhanced activity toward redox catalytic reactions, tandem reactions and photocatalytic reactions. Even though the development of bimetallic NP/MOF composites is still in its infancy, the increasing number of researches clearly indicates its huge potential in various practical applications. In this work, we systematically summarized the development of bimetallic NP/MOF composites, including their versatile fabrication strategies and their applications in various heterogeneous catalysis. The fundamental mechanisms of synergistically enhanced performance in heterogeneous catalysis were discussed in detail. Finally, the remaining challenges and prospects related to bimetallic NP/MOF for heterogeneous catalysis have been proposed. © 2020 Elsevier Ltd. All rights reserved.

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Fabrication strategies for bimetallic NP/MOF composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.1. Ship in the bottle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 2.1.1. Simple liquid impregnation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1.2. Double solvent method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1.3. Seed-mediated growth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1.4. Chemical vapor deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2. Ship around the bottle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3. One-pot method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Enhanced catalytic performance of bimetallic NP/MOF for oxidation reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1. Oxidation of CO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.2. Hydrolytic dehydrogenation from ammonia borane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3. Aerobic oxidation of alcohol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Enhanced catalytic performance of bimetallic NP/MOF for reduction reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

∗ Corresponding authors: College of Environmental Science and Engineering, Hunan University, Changsha, 410082, China. E-mail addresses: [email protected] (W. Tang), [email protected] (L. Tang). 1 These authors contribute equally to this article. https://doi.org/10.1016/j.apmt.2020.100564 2352-9407/© 2020 Elsevier Ltd. All rights reserved.

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Enhanced catalytic performance of bimetallic NP/MOF for tandem reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Enhanced catalytic performance of bimetallic NP/MOF for photocatalytic reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Summary and opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

1. Introduction Recently, bimetallic nanoparticles (NPs) have attracted increasing research attention in various fields, such as plasmonics [1,2], sensing [3,4], electrocatalysis [5–8] and, particularly, heterogenous catalysis [9–19]. Bimetallic NPs are a new kind of nanomaterials, which are composed of two different metal elements. Typically, they would exhibit a combination of the properties derived from two different metals [20,21]. Moreover, there is a great enhancement in the specific physical and chemical properties of bimetallic NPs due to synergistic effects, including electron effect, lattice strain, bifunctional effect, and ensemble effect [22–24]. Specifically, the combination of two distinct metals causes electron transfer and charge redistribution inside the bimetallic materials, resulting in a change in d-band center and state density of metal atoms [25]. Lattice strain could also lead to the similar results in core-shell bimetallic NPs. These synergistic effects guarantee the enhanced catalytic activity of bimetallic NPs. Moreover, in practical applications, the incorporation of two distinct metals could not only bring them bifunctional performance, but also endow them with new properties and capabilities [26–28]. Despite the greater difficulties of bimetallic NPs in synthesis control and characterization than monometallic NPs, bimetallic NPs have many advantages in heterogeneous catalysis. First, the introduction of inexpensive secondary metals, such as Fe, Co and Ni, can reduce the cost of hybrid bimetallic materials. Second, some properties of bimetallic NPs, like the physical, chemical and optical features, not only relate to two individual metals, but also can be modified by adjusting the size, shape and composition of bimetallic NPs. Particularly, an optimal property and a maximized atomic utilization of bimetallic NPs could be achieved once the optimum composition and structure were found. However, due to the existence of high surface energy, they have thermodynamic instability and have a tendency to aggregate with each other to reduce the surface energy of the entire system [29]. Thus, the nanoparticles are prone to irreversible agglomeration during catalytic process, which leads to a loss of high catalytic activity. Similarly, the controllable integration of two or more functional components to build a versatile composite with advanced performance has been a feasible way to achieve material multifunctionality and its wider application in many fields. The combination of bimetallic NPs and porous MOFs has drawn considerable attention in heterogeneous catalysis recently. Metal-organic frameworks (MOFs), also called porous coordination polymers, are porous crystalline materials possessing periodic multi-dimension network structures. Owing to the excellent properties of porous MOF materials, such as large internal surface area, uniform and tailorable porous structure as well as stable chemistry, MOF materials are considered as promising candidates to replace traditional porous materials, such as inorganic silicon oxide, aluminum oxide, and carbon materials [30–39]. Due to synergistic effects between bimetallic NPs and MOFs, the bimetallic NP/MOF composites exhibit significantly enhanced catalytic performance. Briefly, the porous structures and the large specific surface area provide a natural space for loading the highly dispersed bimetallic nanocatalysts, which can effectively prevent the agglomeration and leaching of the bimetallic NPs, and the porous structure allows the cata-

lysts to be more fully contacted with the substrates, facilitating the reaction [40–42]. In addition, several multi-functional MOFs could endow the resulting composites with more active sites or photoactive property [43–45]. Up to now, the excellent catalytic activity of obtained composites has been demonstrated in many applications, including carbon monoxide (CO) oxidation [46,47], hydrolytic dehydrogenation [48–51], selective alcohol oxidation [52,53], reduction reactions [54,55], tandem reactions [56–59] and photocatalysis [60–62], etc. In the past few years, the synergistic effects between metal nanocatalysts and MOF have attracted great interest in heterogeneous catalysis. At present, only a few reviews on monometallic NP/MOF composites in the application of heterogeneous catalysis have been published [40–42,63–66]. There is no published review totally focusing on bimetallic NP/MOF composites for applications of heterogeneous catalysis. Therefore, a comprehensive review of bimetallic NP/MOF composites, including their fabrication methods and their applications in various heterogeneous catalysis, is highly desired. In this work, we presented a systematic introduction of the combination of bimetallic NPs with MOFs. Then an overview on the fabrication methods of bimetallic NP/MOF composites was summarized. Next, we discussed various heterogeneous catalytic applications of bimetallic NP/MOF composites in detail, including the redox reactions, tandem reactions and photocatalytic reactions. Finally, the existing challenges and opportunities in this research area are proposed. 2. Fabrication strategies for bimetallic NP/MOF composites In order to realize the good combination of inorganic bimetallic NPs and MOFs, various fabrication strategies have been developed. According to the different sequence of inorganic bimetallic NPs and MOFs in the preparation process of composite materials, the fabrication methods can be divided into three major types. The first is called the “ship in the bottle” method, which is the most widely used method. This fabrication strategy mainly uses pre-synthesized MOFs as the host material, and the different metal precursors can be controlled in porous framework of MOFs by impregnation and subsequent decomposition or reduction processes. The second method is called the “bottle around the ship” method. This method constructs a composite material mainly by adding a previously prepared nanoparticle stabilized with a protective agent such as organic molecules, a surfactant or a polymer into the preparation system of metal-organic framework. The third method is called “one-pot synthesis” method, which combines the different metal precursors with the reaction precursors required for the synthesis of MOFs into the same reaction system. The three fabrication strategies for the bimetallic NP/MOF composites were vividly depicted in Fig. 1. Moreover, some important bimetallic NP/MOF composites were summarized in Table 1 at the end of this section with information of precursors, synthesis methods and their applications. 2.1. Ship in the bottle As mentioned previously, assembling the functional species inside the pores of the supportive porous matrix is called “ship in the bottle” method. The encapsulation of active metal parti-

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Fig. 1. Main approaches for the fabrication of bimetallic NP/MOF composites: (a) ship in the bottle; (b) bottle around the ship; and (c) one-pot method. (d) active bimetallic NPs defined inside the framework of MOF. (e) active bimetallic NPs immobilized on the surface of MOF.

Table 1 Summary of important bimetallic NP/MOF with information of precursors, synthesis methods and their applications. Composites

Metal precursor

NP size (nm)

Synthesis method

Application

Ref.

CuPd@MIL-101 Au@Ag/ZIF-8 AuPd/MIL-101 NiPt@ZIF-8 AuNi/MIL-101 PdAg@MIL-101 Pd@Co@MIL-101 PdCu@MIL-101 PdNi@UiO-67 Pd@Ag@UiO-67 CoAl@NU-1000 NiPd@MIL-101 PtNi@MOF-74 NiCu@MOF-74. AuPd/UiO-66-NH2 PtCo@UiO-66

Pd(NO3 )2 , Cu(NO3 )2 ·H2 O HAuCl4 ·4H2 O, AgNO3 HAuCl4, PdCl2 NiCl2 , PtCl4 HAuCl4 , NiCl2 Pd(NO3 )2 , AgNO3 Pd(NO3 )2 ·2H2 O, CoCl2 ·6H2 O H2 PtCl6 ·6H2 O, Cu(NO3 )2 ·3H2 O Pd(OAc)2 , Ni(NO3 )2 Pd(NO3 )2 , AgNO3 AlMe3, CoCl(PPh3 )3 [(C5 H5 )Pd(C3 H5 )], [(C5 H5 Ni] Pt(acac)2 , Ni(acac)2 Ni(acac)2 , Cu(acac)2 HAuCl4 , H2 PdCl4 Pt(acac)2 , Co(acac)2

2–3 2–6 2.59 2 1.8 1.5 2.5 1.7 2.5 2.6–3.1 – 2–3 – 7 5.3 2

Simple liquid impregnation Simple liquid impregnation Simple liquid impregnation Simple liquid impregnation DSM DSM DSM DSM Seed-mediated growth Seed-mediated growth CVD CVD Ship around the bottle Ship around the bottle One-pot method One-pot method

CO oxidation Reduction of 4-NPh Oxidation of cyclohexane Hydrolysis of ammonia borane Hydrolysis of ammonia borane Tandem reaction Hydrolysis of ammonia borane Oxidation of benzyl alcohol Hydrogenation of nitrobenzene. Hydrogenation of phenylacetylene Oxidation of benzyl alcohol Hydrogenation reactions Hydrogenation reactions Oxygen evolution reaction Reductive amination of nitrobenzene Hydrogenation of 1-hexene and tetraphenylethylene

[45] [47] [43] [50] [51] [76] [53] [77] [59] [58] [78] [70] [72] [79] [80] [81]

cles in the pores demands the growth of the entrapped metal ions and reductants should be restrained. Hence this synthesis method is to use MOF materials as stabilizing host materials, which can provide a relative confined space for nucleation process. The intermediates mentioned above are subsequently converted to bimetallic NP/MOF with reductive agents (including hydrogen, NaBH4 , hydrazine, methanol and so on). Considering the different synthesis conditions which will be tolerated by the structure of host materials, the precursors should be selected carefully. Consequently, the thermal stability of MOF materials should match the operational temperature during the decomposition toward the precursor, which is helpful to the encapsulation of metal particles inside the pore of matrices. Moreover, because the porous framework of MOFs can provide a defined space to prevent the overgrowth and agglomeration of the metal nanoparticles, the size and shape of the nanoparticles prepared by this method can be finely controlled. According to the multiple characteristics of the preparation method, we divide it into several approaches, such as simple liquid impregnation, double solvent method (DSM), seedmediated growth, and chemical vapor deposition (CVD). 2.1.1. Simple liquid impregnation Solution phase infiltration, namely impregnation, generally consists of several procedures. Firstly, the porous MOF materials are engaged in an aqueous phase solution of pre-selected metal precursors prepared with deionized water (HHitech, China). Then the

metal precursors are loaded into MOF matrix via capillary effect, and the metal precursors can be reduced to metal particles in the cavities of MOFs throught subsequent treatment. To the best of our knowledge, liquid impregnation is one of the most widely used methods to introduce metal NPs into the MOF materials owing to its simplicity and practicality. A facile and effective approach for introducing Pd-Cu NPs inside the mesoporous MIL-101 was developed [45]. Different from other general impregnation methods, chemical reduction of incorporated metal precursors was processed by microwave irradiation in the presence of hydrazine hydrate as reagent. By utilizing microwave irradiation to heat the reaction mixture, the coordinated water molecules around the active metal center of MIL-101 can be removed. Thus, the vacated sites can be assigned to the metal particles for nucleation growth. Moreover, owing to its large pore sizes, unique structure as well as thermostability and chemical stability, MIL-101 can also serve as support to hold bimetallic Au-Pd NPs for typical heterogeneous catalysis [43]. In order to promote the metal loading inside the cavities of MIL-101 via facilitating the interaction between the metal precursors and the host, the support could be modified with the electron-rich functional moiety ethylenediamine [46]. Apart from the efforts mentioned above, the construction of core-shell bimetallic NPs embedded inside porous MOFs has been opening a new avenue for developing heterogeneous catalysis of polymetallic NPs since the pioneering work in 2011 (Fig. 2a) [47]. With the limitation effect of unique structure of ZIF-8, different

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Fig. 2. (a) Schematic illustration of the preparation for Au@Ag core-shell NPs stabilized on ZIF-8. Representative TEM images for (b) 2 %Au/ZIF-8 (I20 ), (c) 2 %Ag/ZIF-8 (II02 ), (d) 2 %Au@2 %Ag/ZIF-8 (I22 ), and (e) 2 %Ag-2 % Au/ZIF-8 (II22 ) and HAADF-STEM images for (f) I22 and (g) II22 . Reprinted with permission [47]. Copyright 2011 American Chemical Society.

metal NPs have similar sizes and Au-Ag core-shell nanoparticles were perfectly defined to similar size (2–6 nm) with few aggregates (Fig. 2b–e), demonstrating the role of porous ZIF-8 in the confinement toward metal NPs. The pattern of HAAD-STEM showed the bright and dark structure in each particle, demonstrating the formation of Au@Ag (Fig. 2f, g). In this work, two operations with reversed order can obtain similar composition owing to the different reduction potential of the two metal salts. The Au3+ /Au couple has a higher reduction potential than Ag+ /Ag couple. Compared with approach I, in approach II, part of the nucleated Ag particles were engaged in the reduction of Au3+ to Au0+ as a new core, and the rest of Ag+ and Au3+ formed a special shell via coreduction treatment. In the next work of this research, the activity of catalytic reaction was obviously improved in the presence of synthesized Au-Ag core-shell immobilized on porous ZIF-8. In addition, this simple liquid impregnation method has been used for confining various bimetallic NPs inside unique and tunable MOFs, such as Ag-Pd [48], Ni-Rh [49] and Ni-Pt [50]. 2.1.2. Double solvent method Double solvent method (DSM) is a new way developed on the basis of solution impregnation, which can further prevent the aggregation of metal nanoparticles on the external surface of MOF materials. As the name suggests, this synthesis approach is processed by utilizing the capillary action and two types of solvents with reversed polarities. The result is that the metal precursors were completely introduced into the pores of MOFs. Typically,

MOFs with the hydrophilicity inside the pores are selected as the main body, which was highly dispersed in non-polar organic solvents, and then the metal precursors solution was added. Water acts as the hydrophilic while hexane acts as the hydrophobic. It should be noted that, in this process, the amount of water used for the preparation of metal precursors was less than or equal to the volume of pores inside the MOFs. The inner surface area of MOFs is much larger than the outer surface area, implying that the amounts of precursors loaded on the outer surface will be relatively small, so that the occurrence of aggregation can be avoided. Recently, the complete confinement of AuNi alloy nanoparticles inside the cavities of MIL-101 without aggregation on the external surface was realized via DSM with a liquid-phase concentrationcontrolled reduction strategy [51]. The AuNi alloy nanoparticles were completely encapsulated inside the mesoporous materials by utilizing an overwhelming reduction method during the reduction phase, while the situation was not very ideal through a moderate reduction operation (Fig. 3a). It is because the agglomeration of alloy NPs was observed on the external surface of MIL-101 under the moderate reduction condition. AuNi alloy particles with an average size larger than 5.0 nm were formed in the presence of 0.2 M NaBH4 solution, while AuNi alloy particles with sizes ranging from 2.0–5.0 nm were obtained by 0.4 M NaBH4 solution (Fig. 3b, c). When the concentration of NaBH4 solution was further increased to 0.6 M, AuNi alloy particles with an average size of 1.8 nm were fully encapsulated inside the cavities of MIL-101 without aggregation and deposition on the surface of MIL-101 (Fig. 3d, e). NiRu alloy

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Fig. 3. (a) Schematic illustration of the preparation for AuNi@MIL-101 using the DSM combined with a liquid-phase concentration-controlled reduction strategy. TEM images of (b) AuNi@MIL-101 (0.2 M NaBH4 solution) and (c) AuNi@MIL-101(0.4 M NaBH4 solution); and (d) TEM and (e) HAADF-STEM images of AuNi@MIL-101 (0.6 M NaBH4 solution). Reprinted with permission [51]. Copyright 2013 American Chemical Society.

nanoparticles were embedded inside MIL-101 by using the same approach [52]. The different bimetallic NP/MOF catalysts could be fabricated by DSM with different procedures and reducing agents [53]. Moreover, NH2 -UiO-66(Zr) was also chosen as a perfect support to encapsulate the CuPd bimetallic NPs via DSM [56]. 2.1.3. Seed-mediated growth Seed-mediated growth is an effective method for fabricating bimetallic core-shell NPs in a fully confined state inside the mesoporous materials, including MIL-101 [57] and UiO-67 [57–59]. Compared to typical synthesis, the size, structure and morphology of synthesized bimetallic NPs can be effectively controlled. In general, this novel method for synthesizing bimetallic core-shell NPs involves two major steps: (1) synthesis of the first metal NPs as

seeds with uniform and small sizes followed by (2) nucleation and homogeneous deposition of secondary metals on the surface of the preformed seeds. It is worthy to note that the reaction parameters must be carefully considered in actual operation. On the one hand, owing to the existence of free-energy barrier in the nucleation of secondary metal and the bigger sizes of introduced metal seeds than the magic size [60], the heterogeneous nucleation process is more thermodynamically favored. Meanwhile, the homogeneous nucleation can be avoided when the practical reaction condition can be controlled to meet the thermodynamic requirements for bimetallic NPs preparation. On the other hand, benefiting from the competing crystal growth on different facet of preformed seeds, the formation of NPs can be realized to some extent. Moreover, the rate of simple growth on different facet is also controllable, contribut-

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Fig. 4. (a) Schematic illustration of the formation of Pt@Pd core-shell tetrahedral NPs on MIL-101 in the presence of CO and H2 . (b) TEM, (c, d) HAADF-STEM, and (e) high resolution HADDF-STEM of Pt@Pd/MIL-101. Reprinted with permission [57]. Copyright 2011 American Chemical Society.

ing to the realization of pre-designed morphology [67]. In general, a suitable stabilizing agent is employed to change the dynamic stability of a particular facet by using different interactions between the stabilizer and the surface. A pioneering effort for the preparation of surfactant-free bimetallic NPs supported on MOFs in the presence of CO and H2 at the solid-gas interface was made in 2013 [57]. In this pioneering work, the Pt@Pd core-shell nanocrystals (NCs) were successfully immobilized on the surface of MIL-101 by a two-step gas-phase reduction in presence of CO–H2 –He (Fig. 4a). The polyhedral bimetallic PtPd NCs with sizes ranging from 3 to 9 nm were formed and uniformly distributed on the framework of MIL-101 (Fig. 4b, c). About 70 % of them were tetrahedral or truncated tetrahedral nanocrystals, and some of them were by-products in the forms of sphere, cube, and octahedra. Due to the different adsorption enthalpy of CO on Pt and Pd (the former is bigger than the latter), polyhedral bimetallic PtPd NCs showed metal segregation resulting in a core-shell-like structure with Pt-rich shell and Pd-

rich core (Fig. 4d, e). The use of H2 –He in place of CO–H2 –He under the same conditions just results in spherical Pt loaded on MI-101. Thus, what we should pay attention to is the reduction method. In addition, the phenomenon of metal segregation and core-shell structure of metal under gas-phase reduction conditions pointed out a clear load for heterogeneous catalysis. 2.1.4. Chemical vapor deposition Chemical vapor deposition (CVD) is a kind of gas-phase loading method involving an encapsulation of metal precursors into porous frameworks followed by reduction under thermal or photochemical conditions. The most striking feature of this method in comparison to liquid impregnation is solvent-free. Owing to the absence of unfavorable competition between solvent and metal precursor molecules, a high amount of embedded metal nanoparticles can be achieved. No additional usage of metal precursors is the most important merit compared with other synthetic approaches for bimetallic NP/MOF composites. Certainly, the overgrowth and

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Fig. 5. (a) Size-selectivity cavity loading of MIL-101 (left: cavity-conform; center: undersized cavity loading; right: introduction of a second metal). TEM pictures of Pd2.5 Ni2.5 @MIL-101: (b) Successive loading of Pd and Ni shows slightly bigger metal nanoparticles (MNP), which are randomly located. (c) Simultaneous loading of Pd and Ni shows smaller particles within the cavities of MIL-101. Reprinted with permission [70]. Copyright 2012 Wiley Online Library.

aggregation phenomenon can be suppressed to some extent. It is worthy to note that the MOFs and volatile precursors are typically placed in sealed Schlenk tube separately and simultaneously. Furthermore, an appropriate reaction temperature and static pressure is also necessary during the whole process. This technology can achieve the prefect distribution of metal nanoparticles in the cavities of MOFs. Since the pioneering work was completed for introducing monometallic NPs inside the structure of MOF-5 [68], similar research works regarding the incorporation of bimetallic NPs into MOFs have begun in succession. The first preparation of bimetallic NPs encapsulated into the cavities of MOF-5 was successfully processed through the simultaneous CVD in 2009 [69]. In this research, PtRu alloy nanoparticles were defined inside the pores of MOF-5 through the cohydrogenolysis of the immobilized [Ru(cod)(cot)] and [Pt(cod)Me2]. However, the partial hydrogenation of the 1,4-benzenedicarboxylate linkers of MOF-5 resulted in a severe distortion of the support materials and a loss of catalytic activity. Considering the higher stability of MIL-101 toward hydrogenation reaction, MIL-101 was chosen as the host materials to define the generated Ni/Pd NPs via CVD [70] (Fig. 5a). Compared with the difference between two gasphase versions (successively and simultaneously), simultaneous gas-phase loading was employed to generate the well-defined Ni/Pd NPs inside the host materials, while successive loading method can lead to the formation of monometallic NPs with more oversize comparably. Taking Pd2.5 Ni2.5 @MIL-101 as an example, the successive loading of two different metals contributed to relatively larger PdNi alloy NPs which were distributed randomly (Fig. 5b). However, smaller alloy NPs were fully confined inside the cavity of MIL-101 in a simultaneous loading (Fig. 5c).

2.2. Ship around the bottle As the name suggests, “bottle around the ship” means that MOFs are constructed around the as-prepared bimetallic nanoparticles through different synthesis protocols. Typically, the pre-synthesized metals are added into the solution of MOF precursors including metal ions and organic linkers. Subsequently, the nucleation and successive growth of MOFs are proceeding. It is worth noting that the introduced bimetallic nanoparticles are acting as seeds or nucleation centers for framework structure in this technique. Furthermore, in order to anchor the heterogeneous growth of MOF materials, the “binder” on the surface of prepared bimetallic nanoparticles is imperative, such as polyvinylpyrrolidone (PVP). Comparing to “ship in the bottle” method, the most notable feature of this method is that the size, morphology and composition of the nanoparticles can be precisely regulated in the process of pre-synthesis. In the meantime, the shape and architectures of MOFs are able to control via adjusting the special reaction parameters. Moreover, this method can effectively avoid the agglomeration of nanoparticles on the outer surface of MOFs and the damage caused by the reducing agent to the MOF materials during the reduction of the metal precursors. Recently, this novel technique of “bottle around the ship” to embed bimetallic alloy nanoparticles into MOFs was reported [71]. In this experiment, the PtPd alloy NPs with tunable composites were fully confined in microporous zeolitic imidazolate framework ZIF-8. The encapsulated alloy metallic NPs (average size of 6.5 nm) were in a stable state, and the pattern of TEM indicated no occurrence of agglomerate phenomenon. Some similar research works have also been conducted. For example, a bimetallic platinum-nickel frame was incorporated into a functionalized MOFs through a novel in situ

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Fig. 6. (a) Schematic illustration of the coordination-assisted oxidative etching process: (1) Initial solid Pt-Ni polyhedral, (2) Pt-Ni frame@MOF intermediates I, (3) Pt-Ni frame@MOF intermediates II, and (4) Final Pt-Ni frame@MOF. Reprinted with the permission [72]. Copyright 2015 Wiley Online Library. (b) Encapsulation of PdNi bimetallic NPs in UiO-67 via one-pot method. Reprinted with permission [75]. Copyright 2015 Wiley Online Library.

approach involving etching and coordination synthesis [72]. The resultant was obtained through two major steps: 1) the solution of 2,5-dioxidoterephthalate was added into a dimethylformamide solution of the polyvinylpyrrolidone coated alloy NPs followed by 2) the solvothermal treatment of mixture. Meanwhile, it can also promote the oxidative etching of the Pt-Ni bimetallic NPs. The specific chemical process may follow equations below: 1 O2 + H2 O + 2e− ↔ 2OH− 2

(1)

Ni (0) − 2e− ↔ Ni(II)

(2)

2Ni (II) + dobdc

4−

↔ Ni2 (dobdc)

(3)

Since Pt is relatively inert to oxygen compared with Ni, the diffusion rate during the oxidative etching process is different. Thus, the Kirkendall effect can explain the intrinsic formation mechanism of bimetallic nano-frames [73]. It should be noted that the equilibrium of etching reactions would be shifted towards the generation of Ni2+ once the formed Ni-MOF-74 started precipitating. Thus, the rate of etching reactions would be intensively accelerated. As shown in the Fig. 6a, it took a few coordination-assisted oxidative etching steps to get the particular Pt-Ni frame@MOF. 2.3. One-pot method Different from all the fabrication strategies mentioned above, “one-pot” method means that different metal precursors and MOF precursors (including metal ions or clusters and polytopic bridging ligands) were directly mixed in aqueous solution. On this basis, some reaction conditions could be changed such as the temperature of reaction system, the type and concentration of the selected surface stabilizer, and the concentration and ratio of each reaction precursors in the reaction system, etc. Consequently, effective regulation and control of the competition among the growth of inorganic nanoparticles, MOF self-nucleation and controlled preparation of assembled product inorganic particle/MOF compos-

ites could be achieved. Furthermore, the self-assembly product of bimetallic NP/MOF composites can be controlled effectively. In order to achieve precise control of the bimetallic nanoparticle core and MOF shell directly from the source, a core-shell AgPd@MIL-100(Fe) catalyst by a facile “one-pot” strategy was fabricated [74]. Two kinds of metal precursors and MOF precursors were added into a solution containing polyvinylpyrrolidone, dimethylformamide and ethanol. Firstly, the Ag-Pd alloy nanoparticles were formed by the reduction of dimethylformamide. Then, the polyvinylpyrrolidone-modified AgPd bimetallic NPs induced the growth of MOF shell on its surface. After a series of research work, the size of bimetallic alloy NPs can be controlled in a range of 14–86 nm, and the MIL-100 (Fe) shell was in the 7–118 nm thickness range. Notably, all of these can be realized by changing the amount of the added metal precursors in reaction system. More recently, a facile protocol for the in situ incorporation of bimetallic precursors into porous frameworks of UiO-67 was presented [75]. And, the different metal precursors were added to a mixture solution of 2,2’-bipyridine-5,5’-dicarboxylic acid and ZrCl4 , so the incorporation of metal precursors and synthesis of MOF can be integrated into one step (Fig. 6b). Finally, bimetallic NPs were successfully encapsulated inside the frameworks of UiO-67 with the reduction of NaBH4 . In this work, the sizes of particle exceeded the diameter of cavity in UiO-67. Researchers ascribed this weird phenomenon to local defects/deformation of support materials by the growth of metal NPs or the aggregation of metal NPs caused by TEM electron beam. 3. Enhanced catalytic performance of bimetallic NP/MOF for oxidation reactions The rapidly increasing number of published literatures in this field suggests the promise of combination of bimetallic NPs and MOFs. Bimetallic NP/MOF composites exhibit synergistically enhanced catalytic activity for oxidation reactions in comparison to corresponding metal NPs (MNPs) and monometallic NP/MOF

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Table 2 Catalytic conditions and activities of bimetallic NP/MOF in heterogeneous oxidation reactions. Composite

Application

Condition

Result

Ref.

AgPd@UiO-66-NH2

Hydrolysis of ammonia borane

293–308 K, 1:4 ratio of Ag/Pd

[100]

AuNi@MIL-101 CuCo/MIL-101 PdPt@UiO-66

Hydrolysis of ammonia borane Hydrolysis of ammonia borane Hydrogenation of nitrobenzene

AuCo@MIL-101 Pd@Co/MIL-101 RhNi@ZIF-8 NiRu@MIL-101 RuCo@MIL-101

Hydrolysis of ammonia borane Hydrolysis of ammonia borane Hydrolysis of ammonia borane Hydrolysis of ammonia borane Hydrolysis of ammonia borane

RT, 7:93 ratio of Au/Ni RT, ambient atmosphere, 3:7 ratio of Au/Ni 25 ◦ C, p(H2 ) = 0.2 MPa, 1:1 ratio of Pd/Pt and 4.4 wt% metal content RT, 6:94 ratio of Au/Co RT, 0.3 ratio of Pd/Co RT, 85:15 ratio of Ni/Rh RT, 7:3 ratio of Ni/Ru RT, 1:1 ratio of Ni/Ru

TOF = 90 min−1 , activation energy = 51.77 kJ mol−1 TOF = 66.2 h−1 , TOF = 19.6 min−1 TOF = 790.4 min−1

[87] [53] [103] [52] [104]

AuPd@MIL-101

Oxidation of cyclohexane

AuPd@MIL-101 PdCu@MIL-101 PtCu@MIL-101

Oxidation of toluene Oxidation of CO Oxidation of benzyl alcohol

Solvent-free, 150 ◦ C, 1.0 MPa O2 , 1.4:1 ratio of Au/Pd 120 ◦ C, 1.0 MPa O2 – 373 K, O2 (0.5 MPa), 0.5 ratio of Pt/Cu

PdCe/MIL-101 AuNi@MIL-101

Oxidation of glycerol Oxidation of benzyl alcohol

60 ◦ C, pressure at 20 psi 80 ◦ C,1 bar

TOF = 23.5 min−1 TOF = 51 h−1 TOF = 58.8 min−1 TOF = 272.72 min−1 TOF = 320 min−1 , activation energy = 36.0 kJ · mol−1 TOF = 19000 h−1 , 28.4% conversion 96.9 % conversion – >99 % conversion, 100 % selectivity 55 % conversion TOF = 15.4 h−1 , 1:0.35 ratio of Au/Ni

composites. In this area, many important reactions are involved, including CO oxidation [57,82], dehydrogenation reaction [83–89], selective oxidation [78,90,91]. These common oxidation reactions will be introduced in detail and a general summary was given in Table 2 at the end of this section. 3.1. Oxidation of CO Carbon monoxide (CO), named silent killer, is a typical toxic gas and it does not have taste, color or smell. Carbon monoxide is produced from the incomplete combustion of carbon-containing compounds, especially fossil fuels. Since the affinity of carbon monoxide with hemoglobin in the body is 200–300 times greater than that of oxygen with hemoglobin, the dissociation rate of carboxyhemoglobin is 3600 times slower than that of oxyhemoglobin. A small exposure to CO can cause gas poisoning. Owing to its close relevance in practical applications, for example, air purification, harmful gas sensors, automobile exhaust treatment, oxidation of CO has been widely studied in the heterogeneous catalysis. In the past few years, the development of bimetallic NP/MOF composites in the application of CO oxidation has made some progresses. The Pd-Cu alloy nanoparticles with different proportions were successfully combined with MIL-101 via a simple liquid impregnation method under microwave irradiation process [45]. It was found that the CO conversion of the mixed Pd-Cu nanoparticles catalysts hosted by MIL-101 was obviously better than that of the corresponding monometallic Cu/MIL-101catalysts. Anomalously, the selective oxidation of CO was more thoroughly converted in the presence of Pd/MIL-101 than Pd-Cu/MIL-101, suggesting that the catalytic activity may primarily derive from nano-sized Pd clusters. A similar study was conducted by selecting MIL-101 as the host material to hold PtPd bimetallic polyhedral nanocrystals [57]. The Pt@Pd core-shell nanocrystal was also constructed by using a seed-mediated two-step growth method and then successfully immobilized on the surface of MIL-101. Results revealed that MIL-101 possessed no catalytic activity for CO oxidation while Pt/MIL-101, PtPd/MIL-101 and Pt@Pd/MIL-101 began exhibiting the activity at 100 ◦ C with complete conversion of CO to CO2 at 175, 175, 200 ◦ C, respectively. Nevertheless, Pd/MIL-101 began exhibiting the activity at 125 ◦ C with complete CO conversion at 200 ◦ C, corresponding to a slightly lower catalytic activity for CO oxidation. In terms of the inconsistency between the two studies, more

[51] [101] [102]

[43] [105] [45] [77] [106] [107]

research efforts should be made to explore the fundamental mechanisms of CO oxidation over bimetallic NP/MOF composites. 3.2. Hydrolytic dehydrogenation from ammonia borane Hydrogen is considered as an alternative energy carrier to fossil because of its green, efficient, safe, sustainable characteristics and abundant reserves. In hydrogen energy systems, safe and efficient hydrogen storage materials are one of the most critical aspects [92]. Among the hydrogen storage compounds, ammonia borane (NH3 BH3 ) has the characteristics of light weight and high hydrogen content (19.6 wt%), and is non-toxic, environmentally friendly, stable at room temperature and highly soluble in water [93]. It has aroused great interest among researchers. Ammonia borane hydrolysis can also release three equivalents of hydrogen (Eq. (4)) [94,95]. However, in the absence of a catalyst, the rate of hydrogen evolution at room temperature is very slow [96]. Therefore, various metal catalysts for catalyzing the hydrogen evolution from NH3 BH3 are prepared, wherein both the noble metal and the transition metal have a catalytic hydrogen release effect [97,98]. Furthermore, the various bimetallic NPs incorporated in porous MOFs were also used to catalyze hydrolytic dehydrogenation of NH3 BH3 , and this has become a new research direction [85,87]. NH3 BH3 + 2H2 O → NH+ + BO− 2 + 3H2 4

(4)

In 2003, AuNi nanoparticles were successfully embedded inside the pores of MIL-101 with an average size of 1.8 ± 0.2 nm through double solvent method and sequential overwhelming reduction approach [51]. It is for the first time that the size and location of formed bimetallic NPs can be controlled by this novel reduction strategy. In this research, the obtained Au0.07 Ni0.93 @MIL101catalyst with an Au/Ni atomic ratio of 7:93 presented the optimal catalytic activity for hydrogen evolution from ammonia borane among different Au/Ni compositions, achieving a turnover frequency (TOF) value of 66.2 molH2 ·molcat −1 min−1 . Additionally, the excellent catalytic activity of AuNi@MIL-101 remained almost unchanged after five operating cycles. Apart from the bimetallic alloy NPs encapsulated inside the pores of mesoporous MIL-101 for hydrolytic dehydrogenation of ammonia borane at room temperature, a new attempt for fabricating the core-shell structure of bimetallic NPs was conducted by the same group [56]. The tiny Pd@Co core-shell NPs were fully confined inside the cavities of

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Fig. 7. (a) Schematic illustration for the synthesis of Pd@Co@MIL-101, Pd@Co/MIL-101, and PdCo@MIL-101 by different procedures and reducing agents. (b) Plots of volume of hydrogen evolution versus time from ammonia borane hydrolysis at room temperature catalyzed by different catalysts. (c) Durability test for the hydrogen generation from aqueous ammonia borane solution catalyzed by Pd@Co@MIL-101 ((Pd + Co)/ammonia borane = 0.011) at 30 ◦ C. Reprinted with permission [56]. Copyright 2014 Wiley Online Library.

MIL-101 via a similar method. Differently, various synthesis procedures and reducing agents were employed for the preparation of diverse catalysts including Pd@Co@ MIL-101, Pd@Co/MIL-101, PdCo@MIL-101, Pd@MIL-101 and Co@MIL101 (Fig. 7a). During the synthesis process, the formation of bimetallic core-shell structure relied on the different reduction potentials between Pd2+ and Co2+ . As shown in Fig. 7b, it is evident that the catalytic performance of bimetallic core-shell NP/MIL-101 (including Pd@Co@MIL-101 and Pd@Co/MIL-101) for hydrolytic hydrogen evolution of NH3 BH3 is superior to that of monometallic NP/MIL-101 and alloy NP/MIL101. For the most excellent type of catalyst Pd@Co@MIl-101, the catalyst with a Pd/Co ratio of 0.3 exhibited the superior catalytic performance and its corresponding turnover frequency (TOF) value was up to 51 molH2 ·molcat −1 min−1 . Notably, the resulting Pd@Co@MIL-101 also possessed excellent recyclability in hydrolytic hydrogen evolution of NH3 BH3 (Fig. 7c). These wonderful performance of Pd@Co@MIL-101 for heterogeneous catalytic reaction mentioned above could be explained by the perfect confinement of MIL-101 and the synergistic effect between Co and Pd. 3.3. Aerobic oxidation of alcohol The selective oxidation of alcohol to the corresponding aldehyde or ketone is an important unit reaction in organic synthesis, which is of great significance in both laboratory and industrial production. Currently, the trend in the development of this field is to oxidize the alcohol under the action of an oxidizing agent such as

hydrogen peroxide or air. Among them, the concentration of hydrogen peroxide used is strictly limited and the usage cost is relatively high, which restricted its extensive applications. Oxygen and air are cheaper, more efficient, and more versatile. However, most substrates react slowly by using only oxygen or air at room temperature without any catalyst because the reaction requires higher activation energy. Therefore, in the field of selective oxidation of alcohol, it is a hot research topic to find a suitable catalyst and develop a mild, efficient, highly selective, economical and environmentally friendly catalytic oxidation system. At present, the world is keen on the research of metal-catalyzed alcohol selective oxidation. In this section, we will review the research progress of alcohol selective oxidation over bimetallic NP/MOF composites. In the early stage, the Au-Pd bimetallic NPs coated by PVP were embedded into the zinc-imidazolate-based framework ZIF-8 via “bottle around ship” method, and the resultant Au-Pd@ZIF-8 catalyst was employed for the selective aerobic alcohol oxidation [90]. The high catalytic activity of the free bimetallic NPs was restrained by the encapsulation of ZIF-8. The oxidation rate of liquid-phase aerobic alcohol to aldehyde or ketone by Au-Pd@ZIF-8 was only 1.9 %, while that by Au-Pd NPs reached 82.2 %. According to the reported literatures [99], due to the presence of methyl group, the pores of ZIF-8 exhibit strong hydrophobicity. Therefore, no access of hydrophilic reductant to the encapsulated Au-Pd catalysts might be the main reason for the poor conversion of aerobic alcohol oxidation. The work of Rösler’s group gave the researchers a good design guideline for the development of bimetallic NP/MOF composites in spite of its inferior catalytic activity for aerobic alcohol

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oxidation. Very recently, the Fe-MIL-101-NH2 was chosen as host matrix to support Pd-Ce NPs for the selective glycerol oxidation because of its good stability and high porosity [91]. Different from the general experimental concept, the bimetallic NPs were immobilized after the Fe-MIL-101-NH2 was functionalized by neocuproine, and neocuproine ligand was attached to Fe-MIL-101-NH2 by forming an amide (CO NH) bond in this work. It turned out that the Pd-Ce/Fe-MIL-101-NHCO-neocuproine exhibited the best catalytic activity for aerobic glycerol conversion to 1,3-dihydroxyacetone in comparison to other control groups. Furthermore, the resulting catalyst has good cyclability. In 2019, Chen et al. creatively used 129 Xe NMR and PAS techniques to achieve the determination for metal NPs relative to the MOF [77]. In this work, bimetallic PtCu NPs with size of 1.7 nm were successfully incorporated into porous MIL-101 via DSM. The resultant PtCu@MIL-101 exhibited excellent catalytic activity, selectivity, and recyclability toward the aerobic oxidation of benzyl alcohol. This work is expected to open a new avenue for the detection of guest species location relative to various host porous materials and thus greatly promote the development of related host-guest nanocomposites for the applications, especially in catalysis.

4. Enhanced catalytic performance of bimetallic NP/MOF for reduction reactions In addition to the application in catalytic oxidation reactions, the bimetallic NP/MOF composites are also widely used in the application of catalytic reduction reactions, due to the excellent catalytic activity of metal nanoparticles and the confinement effect of porous MOFs. These catalytic reduction reactions are mainly focused on the hydrogenation reaction of organic compounds, such as nitrophenol, nitrobenzene and alkyne [108–111]. The hydrogenation reduction of aryl nitro compounds is of great industrial significance, and the nitro-hydrogenated product of the nitro group is an important raw material for the synthesis of medicines, pesticides, dyes and fine chemicals. Further hydrogenation of the aromatic amine compound can produce an alicyclic compound that is also an important chemical raw material, especially as a synthetic raw material for isocyanate and polyurethane. In the field of hydrogenation reactions of nitro compounds to prepare aromatic amines, it has always been one of the research focuses to improve the activity of inexpensive metal catalysts and reduce the use of precious metals. A number of works have reported the synergistic effects of the bimetallic nanoparticles and the host MOF materials. In 2010, Jiang et al. [43] reported the immobilization of bimetallic Au@Ag core-shell nanoparticles on the outer surface of ZIF-8 via a typical liquid-phase impregnation combined with a reduction process. This is the first attempt to select the porous materials as the support to hold the bimetallic core-shell NPs. By successful coupling the bimetallic catalysis with the porous frameworks, the prepared Au@Ag/ZIF-8 exhibited better activity than those monometallic and alloy nanoparticles for the catalytic reduction of 4-nitrophenol. According to the Arrhenius plot of the reaction rate constants over the 2Au@2Ag/ZIF-8, the apparent activation energy Ea was calculated to be 14 kJ/mol, which is evidently lower than those of reported Au- or Ag-based catalysts [112,113]. This further demonstrated the synergistic catalytic effect of two distinct Au and Ag NPs. In addition, some researchers have focused their efforts on the encapsulation of bimetallic NPs in the cavities of MOFs to improve the catalytic reductive performance [70,111]. For example, using the different metal precursors, [(C5 H5 )Pd(C3 H5 )] and [(C5 H5 )2 Ni], NiPd@MIL-101 via a facile CVD method was successfully synthesized [70]. The synergistically promoted catalytic activity of the formed NiPd@MIL-101 (conversion rate up to 80 %) toward the reduction of 3-heptanone was observed, while the

11

pure Ni or Pd failed to drive the reduction reaction under the similar reaction conditions. More recently, NiPd bimetallic NPs were also chosen as catalysts for the hydrogenation reactions, but the difference is that the support matrix was replaced by UiO-67 [75]. The resulting NiPd@UiO-67 through the “one-pot” method exhibited superior performance towards the reductive hydrogenation of nitrobenzene compared with pure Pd@UiO-67 and Ni@UiO-67. The synergistically enhanced catalytic activities of Pdx Niy @UiO-67 were strongly dependent on the composition of Pd/Ni. Among them, Pd7 Ni3 @UiO-67 presented the highest activity towards nitrobenzene hydrogenation reaction with the complete conversion of nitrobenzene achieved in 3 h, while the Ni@UiO-67 catalyst showed no catalytic activity and the Pd@UiO-67 catalyst completed the conversion of nitrobenzene within 18 h. Compared with the reported structures of bimetallic NPs including alloy and core-shell, the novel crown-jewel bimetallic NPs were firstly fabricated inside the pores of UiO-67 by Chen et al. [59], where the encapsulation of bimetallic NPs was processed through a hydride-induced-reduction strategy. As shown in Fig. 8a, Pd metal was selected as the crown, while the transition metal NPs serve as jewels. The second transition metal was successfully introduced after two processes: 1) the H decomposed by H2 adsorbs on the surface of Pd followed by 2) the second metal precursors were reduced in the presence of H. Remarkably, the prepared Pd-based bimetallic NPs@UiO-67 showed high catalytic activity and stability toward the hydrogenation of nitrobenzene. In order to figure out the mechanism of synergistic effect of the crown-jewel bimetallic NPs and porous support, the interaction between mono- or bimetallic NPs and the nitrobenzene molecules was deeply investigated. In the previous stage, the distance between the nitrobenzene molecules and the pure metal surface was around 2.36 Å. After the Ni metal was introduced for the formation of crown-jewel structure, the distance decreased to 2.11 Å, which means a stronger interaction between nitrobenzene molecules and the catalyst. Furthermore, the relationship between the bimetallic composition and reactivity was elucidated by calculating the d-band center of bimetallic NPs and the binding energy of nitrobenzene. Evidently, a change of the d-band of bimetallic NPs occurred, owing to the addition of the Ni metal on the surface of Pd (Fig. 8b). Meanwhile, the binding energy of nitrobenzene adsorbing on catalysts presented an alteration from 1.66 eV to —2.22 eV. According to the fundamental chemical reaction theory, when the binding energy of nitrobenzene on metal catalysts is positive, the adsorption process of nitrobenzene is endothermic. Thus, the reductive catalytic activity of pure Pd@UiO-67 was suppressed in thermodynamics. In contrast, the adsorption of nitrobenzene on Pd54 Ni and Pd53 Ni2 was thermodynamically favorable, providing the enhanced catalytic activity on Pd-based crown-jewel bimetallic NPs encapsulated inside the cavities of UiO-67. However, excessive addition of Ni would cause difficult desorption of nitrobenzene from the catalyst, which could keep the reactants away from the nitrobenzene and slow down the catalytic reaction.

5. Enhanced catalytic performance of bimetallic NP/MOF for tandem reactions In recent years, bimetallic NP/MOF composites have received great attention toward tandem reactions due to their excellent synergistic catalytic activities [76,105,114–119]. Tandem reactions, also called cascade reactions, consist of two or more successive independent reactions in a single synthetic step without separating and purifying the intermediates. Compared with typical independent reactions, the tandem reaction is conducive to reducing capital investment and eliminating unnecessary and expensive intermediate purification and isolation processes, thereby enhanc-

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Fig. 8. (a) Schematic diagram of the deposition of transition metal atoms (M = Ni, Cu, Fe) on Pd NPs by the hydride-induced-reduction strategy. (I) Splitting of H2 into H atoms on Pd surfaces. (II) Reductive deposition of M on Pd by chemisorbed H atoms. (b) Plot of binding energy of nitrobenzene adsorbing on catalysts versus d-band center of prepared PdNi-in-UiO-67. Adapted with permission [59]. Copyright 2017 Royal Chemical Society.

ing its economic competitiveness in the production of desired compounds. In addition, high selectivity and few by-products are also the obvious advantages of the tandem reaction. It is believed that the key to improving tandem reactions is exploration of multifunctional catalysts, which can facilitate all individual reactions involved in the tandem process. The introduction of bimetallic NPs inside the framework of porous MOF materials for tandem reactions appears promising. On the one hand, the nano-sized metal particles with different structures have excellent catalytic properties. On the other hand, MOFs play important roles in the catalytic tandem reactions, since they not only provide a special porous framework to hold embedded bimetallic NPs catalysts, but also afford the diverse and tunable active sites inside porous structure. Therefore, the resulting bimetallic NP/MOF composites, which consists of the bimetallic NPs and multifunctional MOF, may realize the enhanced catalytic performance and selectivity. From a typical chemistry viewpoint, the applications of bimetallic NP/MOF composites toward tandem reactions are considered efficient and environmentally friendly. In 2012, Liu et al. [114] reported the deposition of Au-Pd alloy nanoparticles on the external surface of MIL-101. The synthesized Au-Pd/MIL-101 catalyzed the selective oxidation of C H bond to obtain alcohols and aldehydes, and then the products of the previous step further underwent aldol condensation. It was demonstrated that the incorporation of Au-Pd alloy NPs in the MIL-101(Cr) could contribute to synergistically enhanced catalytic performance, and, when the molar ratio of Au/Pd was 1.5, the catalytic reaction of toluene to benzyl benzoate could be realized with higher conversion and selectivity. The possible mechanism of Au-Pd/MIL-101(Cr)

for improvement in catalyzing the formation of benzyl benzoate from toluene is shown in Fig. 9a. The interaction between Lewis acidic Cr sites of the MIL-101 and the aromatic ring of toluene influences the electron distribution of the methyl group of toluene, making the carbon atoms of the methyl group more easily attacked by the activated oxygen species produced from Au-Pd NPs, and benzaldehyde is formed quickly. Subsequent acid–base interaction between benzaldehyde and the Lewis centre inhibits further oxidation to benzoic acid, and makes the carbonyl group in benzaldehyde more active to undergo an aldol condensation reaction to produce benzyl benzoate. Apart from this bifunctional catalytic system, a multifunctional catalyst over bimetallic NP@MOF with three active sites (Lewis acid-Pd-Ag) was constructed in 2015 [76]. In this research, the PdAg alloy NPs were embedded into MIL101 via a double-solvent method, and the cooperative catalysis of obtained PdAg@MIL-101 was tested through one-pot tandem reaction for the synthesis of value-added secondary arylamines from nitroarene. It was found that the PdAg@MIL-101exhibited good catalytic activity and selectivity in the tandem reactions under mild conditions, on the basis of cooperation between the MOF host and the metal NP guest as well as the synergistic catalysis between bimetallic Pd and Ag NPs. Specifically, the MOF host offers Lewis acidic sites, Pd affords the catalytic activity for the hydrogenation reaction, and Ag improves selectivity towards the desired product (Fig. 9b, c). Herein, it is worth pointing out that there exists an optimum Pd/Ag molar ratio because excessive Ag contents would lead to sluggish overall reaction rates though increasing the ratio of Ag in PdAg alloy allows a higher selectivity to the target product. Recently, first-row transition metal NPs, such as Co and Ni,

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Fig. 9. (a) Plausible mechanism for the selective aerobic oxidation of toluene using Au-Pd/MIL-101. Reprinted with permission [114]. Copyright 2012 Royal Chemical Society. (b) Schematic illustration of the multistep reaction over PdAg@MIL-101 involving Lewis acid and Pd/Ag sites. (c) Synthesis of the secondary arylamine through hydrogenation of nitrobenzene and reductive amination of benzaldehyde. Reprinted with permission [76]. Copyright 2015 American Chemical Society. (d) Schematic diagram showing introduction and in situ reduction of Cu2+ and Ni2+ encapsulated into MIL-101 for cascade catalysis involving dehydrogenation of NH3 BH3 and hydrogenation of nitroarenes. Reprinted with permission [117]. Copyright 2017 Royal Chemical Society.

have captured extensive interest due to their low cost and special catalytic capability toward dehydrogenation of NH3 BH3 [120]. It has reported that the precursors of first-row transition metal could be reduced in the presence of Cu(II) and moderate reductants, such as NH3 BH3 [53,121]. Once the copper precursor was reduced, the Cu-H species generated during the hydrolysis reaction of NH3 BH3 afforded intense reducibility, which can induce the reduction of transition metals. Based on this premise, precursors of Cu and Ni inside MIL-101 via a typical double solvent method were introduced, and the CuNi@MIL-101 was obtained after in situ reduction process [117]. Remarkably, this system showed powerful catalytic activity, selectivity and recyclability toward cascade reactions involving NH3 BH3 dehydrogenation and the subsequent selective reduction of nitroarenes (Fig. 9d). Further studies indicated that the hydrogen, obtained from the dehydrogenation of NH3 BH3 , was mainly responsible for the efficient nitroarene reduction through its sufficient contact with the substrate. In addition to the porous MIL-101, other MOF materials were also explored for the combination with bimetallic NPs. For instance, the combination of PtPd alloy NPs with the hierarchically microand mesoporous MIL-53(Al) was reported [115]. The prepared system, denoted as PtPd/MM-MIL-53(Al), presented synergistically enhanced catalysis for the oxidant-free dehydrogenation of alcohols (Fig. 10a). The frameworks of microporous MIL-53(Al) were established by the interconnection of infinite trans chains of corner-sharing AlO4 (OH)2 octahedra while microporous MIL53(Al) crystals form the walls of the mesoporous MM-MIL-53(Al). This MOF’s mesopores facilitated diffusion and accessibility of substrates and products, while the micropores provided high surface areas and numerous active sites to enhance the catalytic activity. The acidic and basic active site on the MM-MIL-53(Al) stimulated the alcohol to release proton to form an alkoxide group. Then, the dissociation of alkoxide C H bond occurred on the bimetallic NPs with the formation of PtPd-H, and the desired ketone product was obtained with the release of H2 . More recently, NiPt alloy NPs were encapsulated inside porous framework of ZIF-8, and the

resulting catalyst, denoted as NiPt@ZIF-8, was employed in one-pot tandem hydrogenation reaction [50]. In this tandem reaction, the obtained composite firstly catalyzed the hydrolytic dehydrogenation reaction of ammonia borane, and then the hydrogen generated from NH3 BH3 was utilized for the hydrogenation of 4-nitroohenol, styrene and benzonitrile in the presence of NiPt@ZIF-8. The plausible mechanism of the hydrolysis of NH3 BH3 in the presence of NiPt@ZIF-8 was clearly shown in Fig. 10b. As can be seen, H2 O formed a hydrogen bond [H3 NBH2 H] H OH due to the hydridic character of B H bond. The presence of hydrogen bonds reduces the energy barrier of the reaction, making it easier for the oxidative addition of O H bond on the alloy NPs surface. Apart from the open metal sites, various functional groups in MOFs can play important roles in tandem reactions by acting as active sites. For instance, the one-pot cascade reactions from disaccharides (sucrose, cellobiose) and polysaccharides (starch, cellulose) to 2,5-dimethylfuran (2,5-DMF) were investigated over Cu-Pd/amino-functionalized Zr-based MOF incorporated into sulfonated graphene oxide (Cu-Pd/UiO-66(NH2 )@SGO or Cu-Pd/US) catalyst [118] (Fig. 11). Sulfonated graphene oxide contained Brønsted acid site (-SO3 H groups) and UiO-66(NH2 ) contained both Brønsted acid sites (Zr(IV)-OH species) and Lewis acid sites (Zr4+ species). There are a series of tandem reactions: 1) glycosidic bond cleavage of polysaccharide over Brønsted acid site to produce glucose, 2) isomerization of glucose over Lewis acid sites to produce fructose, 3) dehydration of fructose to produce 5-(hydroxymethyl)furfural (5-HMF) over Brønsted acid site, and 4) hydrogenolysis/hydrogenation of 5-HMF over metallic active sites to produce the target product 2,5-DMF. The synergy between the components of Cu-Pd/UiO-66(NH2 )@SGO contributed to the high yield of 2,5-DMF: Balancing the ratio of Brønsted and Lewis acidity in the UiO-66(NH2 )@SGO supports was beneficial to reactions 1–3 and thereby the formation of 5-HMF, while bimetallic Cu-Pd facilitated reaction 4 and thereby the production of 2,5-DMF. By contrast, the use of monometallic Cu/UiO-66(NH2 )@SGO or Pd/UiO66(NH2 )@SGO resulted in low sucrose conversion and 2,5-DMF

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Fig. 10. (a) Oxidant-free alcohol dehydrogenation catalyzed by Pt-Pd alloy NPs on the MM-MIL-53 support. Reprinted with permission [115]. Copyright 2015 Elsevier. (b) Plausible mechanism of the hydrolysis of NH3 BH3 in the presence of NiPt@ZIF-8. Reprinted with permission [50]. Copyright 2018 American Chemical Society.

yield, which was caused by rate-limiting reaction intermediates and side reactions. Moreover, the presence of the amino functional groups (-NH2 ) in the UiO-66 framework was conducive to improving the feed conversion due to the thermodynamically preferable adsorption of 5-HMF on the UiO-66(NH2 ) as compared to UiO-66. 6. Enhanced catalytic performance of bimetallic NP/MOF for photocatalytic reactions In recent years, bimetallic NP/MOF composites have been regarded as excellent photocatalysts for various reactions owing to their unique properties and advantages. The multidentate organic linkers and metallic ions or clusters in MOFs play the role of valence band (VB) and conduction band (CB), respectively. The high porosity of composites facilitates the fast reaction of photo-induced charge carriers with the substrates that are accessible to active sites inside the porous frameworks, thus greatly restraining the recombination of holes and electrons. In this part, we would focus on the recent studies of bimetallic NP/MOF composites on light-driven photocatalytic reaction. In 2014, Huang et al. [71] firstly prepared PtPd@ZIF-8 catalyst via “bottle around the ship”, and then the synergistic photocatalytic activity was tested for the photo-driven oxidative degradation of ethylene. Ethylene is a kind of plant hormone, which is not conducive to the freshness preservation of fruits and vegetables. Results indicated that the bimetallic alloy NPs encapsulated in the MOFs exhibited a higher catalytic activity in comparison to the pure MOF material, bimetallic alloy NPs and monometallic NPs@ZIF-8 composites. It was suggested that the MOF not only accelerated the adsorption of ethylene as a porous material, but also prevented the aggregation of alloy metallic NPs. More recently, the immobilization of AuPd alloy NPs on the sur-

face of chromium-based amine-functionalized MOF via a simple impregnation method was reported [122]. The high photocatalytic performance of AuPd/NH2 -MIL-101 was investigated towards Suzuki-Miyaura coupling reaction. Au is a typical photo-responsive metal, which can be utilized as effective plasmonic photocatalyst through the formation of bimetallic structure with other metals [123–125]. Generally, it releases hot electrons under visible light irradiation. Due to the lower electronegativity of Pd than Au, the photo-induced electrons would transfer to Pd leading to enhanced inherent catalysis of Pd. Moreover, the amine-functionalized MIL101 was proved to be photosensitive material, which can obviously improve the photocatalytic activity of supported inorganic metal NPs [126]. As a consequence, the enhanced photocatalytic activity of AuPd/NH2 -MIL-101 for Suzuki-Miyaura coupling reaction was derived from the synergistic effect between AuPd bimetallic catalyst and photoactive NH2 -MIL-101 under visible light irradiation. The induced electrons transferred from plasmonic catalyst and photoreactive MIL-101 to Pd active sites, thereby promoting the Suzuki-Miyaura coupling reaction. CuPd@NH2 -UiO-66(Zr) catalytic system was also fabricated for photo-driven Suzuki coupling reaction through double solvent method [56]. It is worth noting that the encapsulation of bimetallic catalyst into the cavities of MOF opens a new avenue for designing multifunctional MOF-based catalysts for light-driven organic reactions. The enhanced photocatalytic performance of as-obtained CuPd@NH2 -UiO-66(Zr) takes advantage of both MOF and different metal NPs. NH2 -UiO-66(Zr) is a photoactive material that can generate electrons with incident light irradiation, and Cu serves as an electron mediator to accelerate the transportation of induced electrons to Pd, as shown in Fig. 12a. In this way, the formed electron-rich Pd as active sites would boost the catalytic activity towards Suzuki coupling reaction. Compared with CuPd/NH2 -UiO-66(Zr), the CuPd@NH2 -UiO-66(Zr) showed better

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Fig. 11. (a) Sucrose conversion over the monometallic Cu/U50 S50 and Pd/U50 S50 and the bimetallic Cu-Pd/U50 S50 catalysts. (b) Plausible transformation of 5-HMF into 2,5-DMF over the bimetallic Cu-Pd/US catalyst. (i) sequential fructose dehydration into 5-HMF and direct adsorption of 5-methylfurfural (5-MFA) onto the adjacent Cu-Pd metallic phase and (ii) sequential hydrogenation of 5-methylfurfural (5-MFA) into 5-methyl-2-furanmethanol (5-MFM) and hydrogenolysis of 5-MFM into 2,5-DMF. Reprinted with permission [118]. Copyright 2019 Elsevier.

Fig. 12. (a) Cu-mediated electron transfer process over CuPd@NH2 -UiO-66(Zr) for enhanced light-induced Suzuki coupling reaction. Reprinted with permission [56]. Copyright 2018 Wiley Online Library. (b) The mechanism of photocatalytic hydrogen evolution reaction catalyzed by EY-sensitized NiMo@MIL101 under visible illumination. Reprinted with permission [127]. Copyright 2016 American Chemical Society.

performance for photo-induced Suzuki coupling reaction since the encapsulated bimetallic NPs are smaller than those distributed on the surface of MOF. Apart from these photo-driven reactions mentioned above, the photocatalytic hydrogen evolution reaction can also be promoted by heterogeneous bimetallic NPs based MOF catalyst. Zhen et al. [127] successfully prepared a highly active noble-metal-free Ni-Mo

cluster cocatalyst in MOFs (NiMo@MIL-101) for visible photocatalytic hydrogen evolution using the double solvent method. In comparison to Ni@MIL-101 and Mo@MIL-101, the obtained NiMo@MIL-101 showed outstanding performance for photocatalytic hydrogen evolution (740.2 ␮mol h–1 ), stability, and high apparent quantum efficiency (75.7 %) under 520 nm illumination at neutral pH. The fundamental mechanism of photocatalytic hydro-

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gen evolution over NiMo@MIL-101 in the Eosin Y-sensitized system was clearly displayed in the Fig. 12b. Firstly, Eosin Y (EY) converts to the intermediate EY1 * by adsorbing a photon and then further forms a triplet intermediate of the lowest-excited-lying state (EY3 *) by • intersystem crossing (ISC). Secondly, the EY3 * is quenched and EY− is generated with triethanolamine (TEOA) as a sacrificial molecule. Afterwards, due to the excellent properties of MOF in electron capture and transport, the induced electrons pass over MIL-101, and then transfer to NiMo active sites located inside the MOF. Finally, H+ reacts with electrons from NiMo to produce H2 . It is clear that the highly efficient photocatalytic performance for hydrogen evolution is derived from the synergistic effects between different components, in which MIL-101 acts as electron mediator to restrain the recombination of photo-induced carriers. 7. Summary and opportunities In this review, we have summarized the fabrication of bimetallic NPs supported by MOFs through various synthesis methods, and their applications in a diverse range of heterogeneous catalysis, including oxidation reactions, reduction reactions, tandem reactions and photocatalytic reactions. So far, various fabrication strategies have been developed for the support of bimetallic NPs on the porous MOFs, which generally could be divided into three classes according to the different order of synthesis of inorganic bimetallic NPs and MOFs: (1) “ship in the bottle”; (2) “bottle around the ship”; (3) “one-pot method”. MOFs are a kind of excellent host materials to support the active bimetallic NPs. MOF materials often play one or more roles in heterogeneous catalytic reactions, and these could be summarized as: (1) supporting and confining the introduced bimetallic NPs; (2) offering additional active sites by exposing internal unsaturated metal sites or/and grafting functional groups; (3) accelerating the transport of the substrates and products; (4) improving the size-selectivity of catalytic reactions; (5) restraining the recombination of photo-induced carries. Therefore, the combination between inorganic bimetallic NPs and MOFs endows the resulting composites wonderful properties, leading to the synergistically enhanced heterogeneous catalytic performance. Bimetallic NP/MOF composites are receiving increasing attention due to their potential application in the field of heterogeneous catalysis. As a new type of composite material, although significant progress has been made, there are still many challenges. 1) Although various strategies for the controllable construction of bimetallic NP/MOF composites have been developed, how to coordinate the self-nucleation of MOF due to that lattice mismatch between MOF materials and inorganic bimetallic NPs is still a challenge that researchers have to face. Consequently, for a long period time in the future, researchers still need to conduct a series of studies on the subject that “how to achieve the controllable growth of MOFs on the surface of inorganic bimetallic NPs” while maintaining the size, morphology and dispersibility of bimetallic NPs. 2) The composition of bimetallic NP/MOF composite materials needs to be further expanded, and the structure can be further regulated. In addition to inorganic bimetallic NPs, many organometallic compounds with application value in the field of devices and biology can be introduced into the bimetallic NP/MOF composite material. Moreover, the heterogeneous catalysis of bimetallic NP/MOF composites is still in the early stage of exploration, and the catalytic reactions chosen were usually some mature catalytic reactions. For instance, the heterogeneous reductions and photocatalytic reactions over bimetallic NP/MOF composites are limited to only several applications. The obtained composites are not comparable to

traditional metal catalysts in terms of the diversity of reaction types and the universality of substrates. This requires a more detailed development of bimetallic NP/MOF composites with a wide range of applications, such as Sonogashira coupling reaction reaction, Heck reaction, C H coupling reaction. 3) More attention should be devoted to investigating the mechanism of enhanced catalytic activity over prepared bimetallic NP/MOF catalysts. In other words, it is indispensable to identify the roles of different components, including two metal elements and porous framework, and figure out synergistic effects on the high catalytic activity. Evidently, the relationship between structure and catalytic activity could provide forward-looking guidance for the structural design of catalyst composites. 4) Up to now, the fabrication of bimetallic NP/MOF composites is still at the small-scale production stage. Most of the current mainstream synthetic strategies for desired composites are not conducive to industrial applications. More facile synthetic approaches like “one-pot method” are urgently desired to accelerate the pace of bimetallic NP/MOF composites going out of the laboratory and moving towards practical applications. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors are grateful for financial supports from the National Natural Science Foundation of China (51809088), the National Innovative Talent Promotion Program of China (2017RA2088), the Funds for Innovative Province Construction of Hunan Province of China (2019RS1025, 2019RS3012), and the Fundamental Research Funds for the Central Universities (531118010106). References [1] Y.C. Tsao, S. Rej, C.Y. Chiu, M.H. Huang, Aqueous phase synthesis of Au-Ag core-shell nanocrystals with tunable shapes and their optical and catalytic properties, J. Am. Chem. Soc. 136 (2014) 396–404. [2] C.Y. Chiu, M.Y. Yang, F.C. Lin, J.S. Huang, M.H. Huang, Facile synthesis of Au-Pd core-shell nanocrystals with systematic shape evolution and tunable size for plasmonic property examination, Nanoscale 6 (2014) 7656–7665. [3] M. Janyasupab, C.W. Liu, Y. Zhang, K.W. Wang, C.C. Liu, Bimetallic Pt-M (M = Cu, Ni, Pd, and Rh) nanoporous for H2 O2 based amperometric biosensors, Sens. Actuators B Chem. 179 (2013) 209–214. [4] C. Chen, Q. Xie, D. Yang, H. Xiao, Y. Fu, Y. Tan, S. Yao, Recent advances in electrochemical glucose biosensors: a review, RSC Adv. 3 (2013) 4473–4491. [5] B.A. Kakade, H. Wang, T. Tamaki, H. Ohashi, T. Yamaguchi, Enhanced oxygen reduction reaction by bimetallic CoPt and PdPt nanocrystals, RSC Adv. 3 (2013) 10487–10496. [6] D. Wang, H.L. Xin, R. Hovden, H. Wang, Y. Yu, D.A. Muller, F.J. Disalvo, H.D. ˜ Structurally ordered intermetallic platinum-cobalt core-shell Abruna, nanoparticles with enhanced activity and stability as oxygen reduction electrocatalysts, Nat. Mater. 12 (2013) 81–87. [7] Y. Bing, H. Liu, L. Zhang, D. Ghosh, J. Zhang, Nanostructured Pt-alloy electrocatalysts for PEM fuel cell oxygen reduction reaction, Chem. Soc. Rev. 39 (2010) 2184–2202. [8] B.Y. Xia, H. Bin Wu, X. Wang, X.W. Lou, One-pot synthesis of cubic PtCu3 nanocages with enhanced electrocatalytic activity for the methanol oxidation reaction, J. Am. Chem. Soc. 134 (2012) 13934–13937. [9] W. Ye, S. Kou, X. Guo, F. Xie, H. Sun, H. Lu, J. Yang, Controlled synthesis of bimetallic Pd-Rh nanoframes and nanoboxes with high catalytic performances, Nanoscale 7 (2015) 9558–9562. [10] S. Iihama, S. Furukawa, T. Komatsu, Efficient catalytic system for chemoselective hydrogenation of halonitrobenzene to haloaniline using PtZn intermetallic compound, ACS Catal. 6 (2016) 742–746. [11] D.A. Hansgen, D.G. Vlachos, J.G. Chen, Using first principles to predict bimetallic catalysts for the ammonia decomposition reaction, Nat. Chem. 2 (2010) 484–489. [12] A.U. Nilekar, S. Alayoglu, B. Eichhorn, M. Mavrikakis, Preferential CO oxidation in hydrogen: reactivity of core-shell nanoparticles, J. Am. Chem. Soc. 132 (2010) 7418–7428.

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