Monodisperse MetalâOrganic Framework Nanospheres with

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Monodisperse metal–organic framework nanospheres with encapsulated core–shell nanoparticles Pt/Au@Pd@{Co2(oba)4(3bpdh)2}4H2O for the highly selective conversion of CO2 to CO Xi Zhao, Haitao Xu, Xiaoxiao Wang, Zhizhong Zheng, Zhen-Liang Xu, and Jianping Ge ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03561 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 12, 2018

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ACS Applied Materials & Interfaces

Monodisperse nanospheres nanoparticles

metal–organic with

encapsulated

framework core–shell

Pt/Au@Pd@{Co2(oba)4(3-

bpdh)2}4H2O for the highly selective conversion of CO2 to CO Xi Zhao,[a] Haitao Xu, [a]* XiaoXiao Wang,[a] Zhizhong Zheng,[a] Zhenliang Xu,[a] and Jianping Ge[b] a

State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab,

Chemical Engineering Research Center, East China University of Science and Technology (ECUST), Shanghai 200237,China，E-mail: [email protected] b

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry

and Molecular Engineering, East China Normal University, Shanghai 200062, China. KEYWORDS：Metal–organic framework, nanospheres, Au@Pd NP encapsulation, imparting functionality, RWGS catalyst

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ABSTRACT ： A new microporous metal–organic framework (MOF) with formula {Co2(oba)4(3-bpdh)2}4H2O (1Co) [oba = 4,4' -oxybis(benzoic acid); 3-bpdh = N,N' -bis-(1pyridine-3-yl-ethylidene)-hydrazine] was assembled, and its morphology was found to undergo a microrod-to-nanosphere transformation with temperature variation. Core–shell Au@Pd functional nanoparticles (NPs) were successfully encapsulated in the center of the monodisperse nanospheres, and Pt NPs were well dispersed and fully immobilized on the surface of Au@Pd@1Co to build the Pt/Au@Pd@1Co composites, which exhibited NPs catalytic activity for the reverse water gas shift (RWGS) reaction. The core–shell Au@Pd NPs in MOF significantly enchanced the CO selectivity of the catalyst, and the Pt NP loading on the surface of the nanosphere afforded a desirable CO2 conversion.

INTRODUCTION The consumption of fossil fuels has dramatically increased the atmospheric concentration of CO2 in the past centuries, contributing to the global warming and climate change. To stabilize the atmospheric CO2 levels, three strategies can be employed: emission reduction, CO2 sequestration, and CO2 conversion.1–2 The CO2 conversion process is considered more desirable than sequestration since it not only contributes to solve the global climate problem but also provides products that can be used as fuel or as precursors for the synthesis of more complex chemicals or fuels.3 The reverse water gas shift (RWGS) reaction is generally considered as one of the most promising processes for CO2 conversion, and the design of catalysts for this reaction has attracted considerable attentions.4 Among most studies in this area, noble metals such as Pt, Pd, Ru, Rh, and Au have been widely used in the synthesis of RWGS catalysts.5–9 Despite the


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great efforts taken, the hydrogenation of CO2 offers challenging opportunities in the fields of energy and environmentally sustainable development.10 Metal–organic frameworks (MOFs), self-assembled by organic linkers and inorganic secondary building units (SBUs)11–13 are promising candidates for various applications, including gas storage14 and separation,15 ion exchange,16 luminescence,17 magnetism,18 electrochemistry,19 drug delivery,20 and catalysis.21 Especially, in the field of catalysis, MOFs constitute unique host matrices that can be integrated with metal nanoparticles (MNPs) to build complexes with novel chemical and physical properties.22–25 Abundant of active research in this field have shown that the rational integration of MNPs and MOFs can not only prevent the MNPs from aggregation but also effectively synergize their respective strengths and offsets their drawbacks, and the functional synergy between MNPs and MOFs is of great significance for enhanced catalysis.26-41 Bimetallic core–shell NP catalysts exhibit enhanced activity and stability compared to their monometallic counterparts, mainly due to lattice effects and subtle electronic interplay in bimetallic NPs.42–48 Furthermore, bimetallic NPs have been reported to enhance the selectivity and efficieyncy of CO2 conversion processes.49–51 Hence, the construction of well-defined noble metal bimetallic NPs/MOF nanostructures using MOF as host matrices is highly desirable for the implementation of the RWGS reaction to prevent the aggregation of noble metal NPs in the of CO2 hydro-conversion procedure.



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Herein, a novel microporous metal–organic framework {[Co2(oba)4(3–bpdh)2] ⋅ 4H2O (1Co), [oba = 4,4' -oxybis(benzoic acid) ], [3 -bpdh = N,N' -bis-(1-pyridine-3-yl-ethylidene)hydrazine]}, was assembled by a solvothermal synthesis using Co(NO3)2 ⋅6H2O and the organic linkers oba and 3-bpdh. Monodispersed 1Co microspheres were controllably fabricated with unified morphology and size and were further treated to encapsulate Au@Pd nanoparticles in their center and to immobilize Pt NPs on their surface. The as-prepared composite Pt/Au@Pd@1Co exhibited highly selective catalytic effectiveness for the RWGS reaction, with selectivity for CO up to 96.0% and a conversion of CO2 to CO of 18.2%, which stem from the synergistic effect of Au@Pd, Pt NPs, and 1Co. Our strategy for the catalyst assembly is shown in Figure 1.

Figure 1. Design and assembly strategies for synthesis of 1Co, Pt/1Co, Au@Pd@Co and Pt/Au@Pd@Co.


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RESULTS AND DISCUSSION As shown in Fig. 1, 1Co crystallized in the monoclinic space group P2(1)/n (a = 10.494(18) Å, b = 10.310(18) Å, c = 22.350(4) Å, and β = 92.853(18)°, Z = 4). Crystallographic Co(II) ions are independently arranged in a hexacoordinate environment, in which the Co(II) center is bonded to five O atoms from the carboxylate groups of four oba ligands and one N atom from the 3-bpdh ligand in a distorted octahedron. The Co–O bond length ranges from 2.006 to 2.178 Å, and the Co–N bond length is 2.062 Å. The O–Co–O and O–Co–N angles vary from 85.66° to 86.64° and 96.28° to 102.50°, respectively. The carboxylic groups of the oba ligand exhibit both chelate and bridging modes: one oxygen atom from one carboxylic group in tridentate fashion bridged two Co atoms, its two oxygen atom chelated one Co atoms, the other carboxylic group in bidentate fashion bridged two Co atoms, forming Co2(oba)2 units to get a double chains. The 3– bpdh units corresponding to the Co2+ ions link the double chains to yield a 3D framework with lattice water molecules, possessing 1D channels with size of 8.3 Å × 8.3 Å along the b axis.The thermogravimetric curve displayed in Fig. S4 shows that the first weight loss of 7.54% (calculated: 7.65%) occurs in 120°C–200°C, which corresponds to the departure of four lattice water molecules per unit cell. When heated beyond 350°C, the skeleton of 1Co was found to change dramatically due to its thermal degradation. No weight loss was observed upon heating till 900°C, which suggests that the CoO phase was the final residue. The purple-red sample resulting from the preparation of 1Co microspheres and nanospheres were subjected to scanning electron microscopy. The corresponding images shown in Fig. 2 (a, b) revealed that the monodispersed 1Co microspheres and nanospheres were uniform in shape and size with a particle average size of 1.7 µm and 500nm, respectively.



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Figure 2. a) SEM image of 1Co microspheres ( DMF:EtOH = 2, 100°C); b) SEM image of1Co nanospheres ( DMF:EtOH = 2, 0.6g PVP, 100°C); c) SEM image of Au@Pd@1Co (DMF:EtOH = 2, 0.2 g PVP, 4ml Au@Pd NPs, 100°C); d) HRTEM image of Au@Pd NPs; e–f) HRTEM image of Au@Pd@1Co (DMF:EtOH = 2, 0.2 g PVP, 4 ml Au@Pd NPs, 100°C); g) HRTEM image of Pt NPs; h–i) HRTEM image of Pt/Au@Pd@1Co (DMF:EtOH=2, 0.2 g PVP, 4ml Au@Pd NPs, 4 ml Pt NPs, 100°C). Under the DMF solvothermal condition, temperature is a key factor for the control of the morphology of 1Co from pointed rods to spheres (Fig. S14). Thus, at 100°C, 1Co exhibited a rod-like morphology, whereas the latter changed significantly and very rough spheres with an average diameter of 8 µm were observed when the temperature increased to 120°C. Upon further heating to 140°C, the number of spheres increased, dominating the number of rods At 160°C, the


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rods were scarcely present, and the spheres exhibited a smaller size. The PXRD patterns (Fig. S5) of the samples were consistent with the simulated XRD patterns of the single crystal of 1Co, indicating that their crystalline phase comprised 1Co. As mentioned above, the morphology and size of 1Co could be effectively controlled by the temperature. Thus, the spheres became smoother upon increasing the temperature, exhibiting much smaller sizes and narrower size distribution. This may be attributed to the high temperature altering the relative kinetics for nucleation and nanocrystal growth in favor of the formation of uniform nanomaterials under solvothermal conditions.52–53 The solvents and their ratio also play a decisive role in the resulting morphology and size of the 1Co microcrystals. Accordingly, no 1Co crystals were observed when using water, ethanol, or their mixture as the solvent. However, using a mixture of DMF and EtOH attained 1Co in its microspheric morphology at 100°C (Fig. S15). The size uniformity of the microspheres was found to correspond with the ratio of DMF and EtOH, the optimal ratio being 2:1 (DMF:EtOH) leading to the smallest uniform, monodisperse 1Co microspheres. This can be attributed to the different dipolar moments of the solvents (µ = 3.86 D for DMF and µ = 1.68 D for EtOH). A high dipolar moment can change both the solvent−NP interface and the interfacial tension, therefore strongly affecting the particle size and morphology.54 When EtOH was introduced, the PXRD patterns of all samples were very weak and broad, indicating that the samples were amorphous, as reported previously by Meiting Zhao and coworkers.55 The addition of EtOH may be assumed to cause 1Co to become amorphous and the amorphous coordination polymers adopting a spherical morphology to minimize the interfacial free energy between particles and solvent.56 This would explain the need for adding EtOH to DMF for the generation of the microspheres at 100°C. The increase in the size of the 1Co microspheres corresponded with the



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temperature increasing to 160°C (Fig. S16), which can be attributed to suppressed crystal growth kinetics at a lower temperature.54 A number of surfactants including OP-10, PVP, CTAB, and TWEEN were introduced to control 1Co microspheres (Fig. S17), among which PVP and OP-10 were proven to be more effective. When increasing the concentration of OP-10 (Fig. S18) and PVP (Fig. S19) at 100 °C, the microsphere size decreased significantly, illustrating that the absorption on the crystal surface increases at high concentration, which stabilizes the growing particles and influences their growth rate.57–58 Smaller 1Co nanospheres are thereby obtained. The abovementioned results proved that EtOH and surfactants not only accelerated the nucleation rate, but also affected the morphology and size of the 1Co microcrystals. We also evaluated the microporous properties of 1Co by a N2 adsorption–desorption experiment [Figs. S (9–11)], which exhibit typical type I microporous isotherms and a low surface area. In addition, the surface area was found to increase with the introduction of EtOH and OP-10, attributable to their smaller size of the particle obtained under these conditions. The efforts directed toward the control of the 1Co morphology through the reaction temperature, solvents, and surfactant aim to obtain nanocomposites with a uniform, well-dispersed morphology and size. This would render the well-dispersed NPs as ideal catalyst carriers while enhancing their catalytic effectiveness. The results suggest that the 1Co nanospheres would be the most appropriate catalyst carrier candidates. The core−shell Au@Pd@1Co composite (Fig. 2) was prepared by solvothermal treatment in DMF–EtOH from the 1Co precursors, PVP and Au@Pd NPs. The Au particles served as both the seed and the catalyst for the formation of the palladium layer, and the Pd2+ ions were reduced by sodium citrate, resulting in the core–shell Au@Pd NPs with subnanometer-thick palladium shells.59 PVP not only acts as a stabilizer to ensure the satisfactory dispersion of the metal NPs


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but also provides coordination interaction between the C=O and metal ions and hydrophobic interaction between its apolar groups and the nitrogen-containing organic linkers, which provides the MOF precursors with enhanced affinity toward the NPs surfaces.41, 55 In the absence of PVP, the NPs would exhibit inferior diffusivity, and the Au@Pd NPs (Fig. S22) would lead to aggregates in the blocking-like MOFs. When 0.2 g or 0.4 g PVP was added into the sample, the core–shell Au@Pd NPs were encapsulated in the center of the 1Co nanospheres. However, naked Au@Pd NPs were observed after increasing the amount of PVP to 0.6 g, owing to the competitive absorption of 1Co and excess PVP on the surface of the PVP-stabilized NPs. We found that the Au@Pd NPs are also of great importance to achieve an efficient control of the morphology of the 1Co nanospheres. In the absence of Au@Pd NPs, 1Co microspheres with an average size of 1 µm were obtained at the same condition (Fig. S19b). However, after adding 2 ml of Au@Pd NPs, some 1Co nanospheres with encapsulated NPs were observed, although the 1Co nanospheres without NPs were still predominant (Fig. S24). Only the addition of 4 ml of Au@Pd NPs yielded a much more uniform morphology that comprised Au@Pd@1Co nanospheres, exhibiting one or two Au@Pd NPs at their center. Interestingly, further increasing the amount of Au@Pd NPs resulted in NPs aggregation, which led to an unsatisfactory morphology and nonuniform size of Au@Pd@1Co. From these results, it can be concluded that monodispersed Au@Pd@1Co nanospheres (Fig. 2) with encapsulated core–shell Au@Pd NPs of about 17 nm in their centers could be yielded using 0.2 g of PVP, and 4 ml of Au@Pd NPs. Furthermore, core–shell Au@Pd play a key role in contracting the 1Co morphology from the 1Co microspheres of about 1 µm to form Au@Pd@1Co nanospheres exhibiting a size of about 288 nm. Next, different amounts of Pt NPs ( 4, 8, or 12 ml)29 were uniformly adsorbed and loaded onto the surface of the core–shell Au@Pd@1Co composites (4-Pt/Au@Pd@1Co, 8-



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Pt/Au@Pd@1Co, and 12-Pt/Au@Pd@1Co, respectively) through a straightforward stirring method. As shown in Fig. 3, high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM), energy-dispersive X-ray (EDX) elemental mapping, and line-scan EDX analysis were conducted to ascertain the composition of the Pt/Au@Pd@1Co composites.

Figure 3. a) STEM image and b) HADDF-STEM image of Pt/Au@Pd@1Co NP; c) and d) enlarged detail image marked in a) and b), respectively; e–l) EDX elemental mapping of the Pt/Au@Pd@1Co NP in (a); m) and n) The selected region of line-scan EDX and corresponding spectra across Au@Pd NP; o) HRSTEM image of the edge of a Pt/Au@Pd@1Co NP. The STEM and HAADF-STEM images [Fig. 3 (a–d)] demonstrate that the Pt/Au@Pd@1Co morphology was that of a typical core–shell nanosphere; the corresponding EDX elemental mapping show that Au is distributed only in the core and enclosed with Pd; C, O, N and Co of 1Co are homogeneously distributed throughout the nanosphere [Fig. 3(e–l)]; Pt is distributed on


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the nanosphere surface. As seen in Fig.3n, most Pd is located at the two ends of the blue line and Au is mainly in the middle for Au@Pd NP, further proving the formation of the core–shell Au@Pd NP. The HRSTEM image (Fig. 3o) of the edge of the Pt/Au@Pd@1Co nanosphere shows that the Pt NPs are well immobilized on the 1Co shell.

Figure 4. a) CO2 conversion and CO selectivity of the different catalysts for RWGS; b) CO2 conversion and CO selectivity at different reaction conditions for RWGS; c) Yield of the different catalysts for RWGS (catalyst: 0.2g 4-Pt/Au@Pd@1Co; space velocity: 12000 mL h-1 gcat-1 ).

We studied the catalytic effectiveness of 1Co, Au@Pd@1Co, m-Pt/Au@Pd@1Co (m = 4, 8, and 12), and Pt/1Co (4ml of Pt) in the RWGS reaction. As shown in Fig. 4a, the conversion of CO2 at 400°C was about 12.4% for Pt/1Co, 15.6% for 4-Pt/Au@Pd@1Co, 18.2% for 8-



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Pt/Au@Pd@1Co, 8.88% for 12-Pt/Au@Pd@1Co, and only 2.40% for Au@Pd@1Co. The activity of 1Co was negligible for the same condition. The conversion was higher for 8Pt/Au@Pd@1Co than that for the other analogs. This could be explained in terms of an excess Pt NPs loadding reducing the number of active sites and consequently decreasing the catalytic effectiveness due to migration and sintering of the metal NPs at high temperature.60 This finds support in the values obtained for their respective BET surface area (Table S4, 40.29 m2g−1 for 4Pt/Au@Pd@1Co, 42.44 m2g−1 for 8-Pt/Au@Pd@1Co, and 25.54 m2g−1 for 12-Pt/Au@Pd@1Co). The trend observed for the selectivity for CO was similar to that of the CO2 conversion with increasing Pt NPs except for Au@Pd@1Co. The CO selectivity data were 99.9% for Au@Pd@1Co, 39.6% for Pt/1Co, 87.5% for 4-Pt/Au@Pd@1Co, 96.1% for 8-Pt/Au@Pd@1Co, and 98.2% for 12-Pt/Au@Pd@1Co, respectively. In the absence of Au@Pd NPs, Pt/1Co exhibited lower CO selectivity than Pt/Au@Pd@1Co. The yield diagram (Fig. 4c) clearly revealed the effects of both Pt and Au@Pd NPs in the hydrogenation of CO2: Pt NPs enhanced the conversion of CO2, while Au@Pd NPs dramatically changed the proportion of CO to CH4 in the product. This catalytic-synergistic effect of Au@Pd and Pt NPs in 1Co plays a very important role in the improvement of the catalytic effectiveness of Pt/Au@Pd@1Co for the conversion of CO2 to CO. These results demonstrate that Pt/Au@Pd@1Co represents an ideal catalyst for the selective CO2 conversion. The reaction mechanism for CO formation from RWGS has two widely accepted pathways. One is the redox mechanism of active metal centers (CO2 + Mn+ → MOx + CO, H2 + MOx → Mn+ + H2O). The other one is the formate decomposition mechanism in which CO2 is first hydrogenated into formate, followed by cleavage of the C=O bond.6 In our study, the microporous 1Co is not good for the formate formation but good for CO formation owing that


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formate molecules are larger than CO molecules. If the formate decomposed to CO, its intermediates are easy to further produce methane so as to increase methane product,61-62 which opposes the experimental result of increased CO selectivity. Considering all above, the redox mechanism of active metal centers would be becoming to explain the role of Au@Pd NPs and Pt NPs. CO2 molecules were initially adsorbed on 1Co and then reach the two active sites (Au@Pd and Pt NPs), C=O was dissociated on the metal active site and reduced to CO: CO2 + Au@Pd(Pt) →Au@Pd(Pt)Ox + CO, H2 + Au@Pd(Pt)Ox →Au@Pd(Pt) + H2O. Bimetallic Au@Pd NPs would have subtle electronic interplay which correlated with the activity for C=O bond hydrogenation and the decomposition of oxygenate molecules, and then can enhance the selectivity towards CO.3 The microporous effect may also be an important factor that results in the main product CO when those CO2 reduced on Au@Pd NPs@1Co, because the steric hindrance of the tetrahedral CH4 is larger than that of the linear CO when they were in the micropores. The CO2 conversion is not high enough for Au@Pd@1Co without Pt NPs, attributing to its low pore volume limiting the amount of CO2 reaching inner Au@Pd NPs. The low Au@Pd NPs content in the Au@Pd@1Co may also account for the low conversion of CO2. The high hydrogenation activity63 Pt NPs could also enhance the hydro-conversion of CO2. The formed CO can be further hydrogenated to CH4 on the Pt NPs, either by the initial C−O bond breaking or by initial CO hydrogenation and subsequent CO bond breaking.6, 10 Furthermore, Ningmu Zou

64

reported nanoparticles can communicate to each other through a mechanism

which is related to spillover effect:65 the reaction product desorbs from the surface of a nanocatalyst, diffuses out of the shell and then affects the reactions at a nearby nanocatalyst, even these particles are separated from one another and encapsulated into a core-shell structure. We thought that such communication mechanism existed between Au@Pd NPs and Pt NPs, and



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the cooperative effect improved the catalytic effectiveness of Pt/Au@Pd@1Co for the conversion of CO2 to CO. Moreover, it was found that increasing the reaction temperature led to a gradual increase of the CO2 conversion and CO selectivity. Thus, when increasing the temperature from 300°C to 400°C, the CO2 conversion with 4-Pt/Au@Pd@1Co increased from 4.38% to 15.6%, and the CO selectivity increased from 73.4% to 87.5%. The CO2 conversion was relatively low at 300°C due to the chemical inertness of CO2, which renders the RWGS reaction (CO2 + H2 → CO + H2O; ∆H298.15K = 41.2 kJmol-1) endothermic, whereas the side reaction (methanation reaction: CO2 + 4H2 →CH4 + 2H2O; ∆H298.15K = −252.9 kJmol−1) is exothermic. These results suggest that both the CO selectivity and CO2 conversion improve at a high temperature. Finally, the effects of reaction pressure and the volume ratio of H2 and CO2 (1:1, 3:1, and 5:1) were investigated. The results revealed an increase in the CO2 conversion and CO selectivity with increasing H2:CO2 volume ratio. One explanation is that the excess H2 has a promotional effect on the generation of intermediate species with highly reactivity on the catalyst surface 66. What’s more, the excess H2 can also facilitate reductive step in redox mechanism of RWGS reaction. Furthermore, as the reaction pressure increased, the CO2 conversion underwent a sharp increase, approaching a high of 24.0% when the pressure was 4 Mpa. However, the CO selectivity started decreasing after reaching a maximum peak value. The improvement of the CO2 conversion could be explained in terms of the higher pressure resulting in a larger adsorption volume of the reactant gas, which would increase the number of reactant molecules that could interact with the NP active sites. In contrast, the generation of CH4 from CO2 also improves under high pressure. The combination of these two effects would account for the


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selectivity fluctuation. The collective results obtained indicate that 8-Pt/Au@Pd@1Co is a promising candidate as a catalyst for the conversion of CO2 to CO (Fig. 4). CONCLUSION In summary, we successfully synthesized a new microporous MOF 1Co, which underwent a microrod-to-nanosphere transformation and was used for the assembly of monodisperse metal– organic framework nanospheres Pt/Au@Pd@1Co with encapsulated nanoparticles. These nanospheres allowed the incorporation of multiple NPs in a non-agglomerated fashion and the control of the spatial distribution of NPs within the MOF matrix. These as-prepared core–shell composites exhibited selective catalytic effectiveness for the RWGS reaction, which derive from the synergistic effect of Au@Pd and Pt NPs, as well as the immobilization function of 1Co. The core–shell Au@Pd NPs significantly enhanced the CO selectivity of the catalyst, and the Pt NP loading on the surface of the Au@Pd@1Co composites results in a desirable CO2 conversion. Such a rational design of a NP/MOF composite and its application may inspire the design and fabrication of complicated catalysts for the hydro-conversion of CO2. EXPERIMENTAL SECTION Single–crystals of {Co2(oba)4(3-bpdh)2}⋅4H2O(1Co): A mixture of Co(NO3)2⋅6H2O (0.4 mmol, 0.1172 g), {4,4' -oxybis(benzoic acid), oba} (0.4 mmol, 0.1032 g), and 3-bpdh (0.4 mmol, 0.0956 g) was added into 10 ml DMF and sonicated to ensure the uniform dispersion of all solids in the solution. The solution was then transferred into a 30 ml Teflon-lined stainless steel container and heated to 100°C for 72 h. After cooling slowly to room temperature, orange– yellow rods of single–crystals of {Co2(oba)4(3-bpdh)2}⋅4H2O suitable for X-ray diffraction were collected.



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1Co microspheres: 1Co microspheres were synthesized typically following the same procedure as that described for the single-crystals of 1Co using a mixture of the DMF–EtOH as the solvent and heating at 100°C for 24 h instead of DMF with 72 h of heating at 100°C, aside from the reagents decreased. Au@Pd@1Co microspheres: Au@Pd@1Co microspheres were synthesized following the same procedure as that of 1Co microspheres by adding 4 ml of a Au@Pd NPs suspension and 0.2 g PVP in a mixture of DMF–EtOH. Pt /Au@Pd@1Co microspheres: A solution of Pt NPs solution (0.6 mM; 4 ml, 8 ml, or 12ml) was added into Au@Pd@1Co (30 mg) dispersed into 4 ml DMF under stirring at room temperature for 6 h, and then the product was collected by centrifugation at 8000 rpm for 3 min. Catalytic effectiveness of 1Co, Au@Pd@1Co, Pt/Au@Pd@1Co, and Pt/1Co for the RWGS reaction was measured in a continuous flow fixed bed reactor. Pressure was controlled by YB150A precision pressure gauge provided by Shanghai automation instrument co. LTD. CO2 and H2 flow were controlled by D08-1F mass flow controller provided by Beijing seven star hua chuang electronic co. LTD. Temperature was controlled by AI-516 temperature controller provided by Xiamen yudian automation technology co. LTD. The outlet products were analyzed using a gas chromatograph [Model GC7890II, a TM-1 column ( length = 30 m, inner diameter = 0.25 mm, film thickness = 0.5 µm )] equipped with thermal conductivity. Contents were quantified with external standard method. The operating parameters are as follows: column temperature, 80 ºC; injection port temperature, 120 ºC; detector temperature, 120 ºC; detector electricity, 60mA; carrier gas, hydrogen. All the samples were activated with H2 at 200 ºC for 3 h before catalytic test. CO2 conversion was calculated on a carbon atom basis according to the following equation:


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‫ܱܥ‬ଶ Conversion =

‫ܱܥ‬ଶ ௜௡௟௘௧ − ‫ܱܥ‬ଶ ௢௨௧௟௘௧ × 100% ‫ܱܥ‬ଶ ௜௡௟௘௧

CO selectivity was calculated according to: CO Selectivity =

CO୭୳୲୪ୣ୲ × 100% CO୭୳୲୪ୣ୲ + ‫ܪܥ‬ସ ௢௨௧௟௘௧

Yield was calculated according to : COyield = COଶ Conversion × CO Selectivity CHସ yield = 1 − COyield where CO2 outlet denotes moles of CO2 at the outlet, COoutlet and CH4outlet represent moles of CO and CH4 at the outlet, respectively.

ASSOCIATED CONTENT (detailed synthesis method, XRD patterns, TG data, BET data, SEM and TEM image, catalytic data, Crystallographic data) is available in the Supplementary material online version. AUTHOR INFORMATION Corresponding Authors [email protected] ( Haitao Xu) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes



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The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21371058). REFERENCES [1]

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Table of content Title ： Monodisperse metal–organic framework nanospheres with encapsulated core–shell nanoparticles Pt/Au@Pd@{Co2(oba)4(3-bpdh)2}4H2O for the highly selective conversion of CO2 to CO Xi Zhao,[a] Haitao Xu, [a]* XiaoXiao Wang,[a] Zhizhong Zheng,[a] Zhenliang Xu,[a] and Jianping Ge[b]

TOC graph

New microporous MOF {Co2(oba)4(3-bpdh)2}4H2O(1Co)

[oba = 4,4'-oxybis(benzoic

acid), 3-bpdh = N,N'-bis-(1-pyridine-3-yl-ethylidene)-hydrazine] were assembled and different morphologies of 1Co were obtained by modulating the synthesis conditions. Au@Pd NPs were encapsulated into 1Co nanosphere, and Pt NPs were deposited on its surface. Catalytic effectiveness of different composites were investigated with respect to RWGS reaction.


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