Monodisperse Pd Nanotetrahedrons on Ultrathin MoO3-x Nanosheets

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Monodisperse Pd Nanotetrahedrons on Ultrathin MoO Nanosheets as Excellent Heterogeneous Catalyst for Chemoselective Hydrogenation Reactions x

Xiao Zhou, Hong-Yan Zhou, Tuck-Yun Cheang, Zhi-Wei Zhao, Cong-Cong Shen, Kuang Liang, Ya-Nan Liu, Zheng-Kun Yang, Muhammad Imran, and An-Wu Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10279 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 27, 2017

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The Journal of Physical Chemistry

Monodisperse Pd Nanotetrahedrons on Ultrathin MoO3-x Nanosheets as

Excellent

Heterogeneous

Catalyst

for

Chemoselective

Hydrogenation Reactions Xiao Zhou,a Hong-Yan Zhou,b Tuck-Yun Cheang,*,b Zhi-Wei Zhao,a Cong-Cong Shen,a Kuang Liang,a Ya-Nan Liu,a Zheng-Kun Yang,a M. Imran a and An-Wu Xu*,a

a

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical

Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China b

Department of Breast and Thyroid Surgery, The First Affiliated Hosptital of Sun

Yat-Sen University, Guangzhou 510080, China

1

ACS Paragon Plus Environment

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ABSTRACT Two-dimensional (2D) metal oxides are ideal host supports for constructing active hybrid catalysts in various organic reactions as they have large specific surface area, strong adsorption ability and excellent thermal stability, which could serve as support and promoter to anchor noble metal nanoparticles with enhanced catalytic activity. Herein, a simple one-pot wet-chemical approach is developed for the synthesis of well-defined monodisperse Pd nanotetrahedrons on ultrathin MoO3-x nanosheets (Pd/MoO3-x). Novel 2D oxygen-deficient MoO3-x nanosheets are formed in the synthetic process, which can work as robust catalyst support. Furthermore, in situ growth route is beneficial for boosting chemical attachment and electronic communication between Pd nanotetrahedrons and MoO3-x nanosheets. Such a composite system that combines the advantages of electronic effect and strong metal-support interaction contributes to excellent performance for selective hydrogenation of α, β-unsaturated aldehydes with high conversion (97%) and selectivity (96%) to its saturated aldehydes. Catalytic activity of Pd/MoO3-x nanocatalysts is improved through electronic structure regulation of Pd nanocrystals via electron-donating effect of oxygen vacancies in MoO3-x nanosheets. The high catalytic activity of our obtained hybrid catalysts associated with a synergy of electronic effect and strong metal-support interaction inspires the future exploration for other 2D oxides as functional supports to fabricate hybrid nanocatalysts for selective hydrogenation of α, β-unsaturated aldehydes.

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INTRODUCTION With the global boom in graphene research, recent years have witnessed a growing interest in two-dimensional (2D) inorganic materials. In particular, 2D metal oxides including TiO2, MoO3, Co3O4, etc. are among the most studied materials due to their large surface area, superior stability at high temperature and a wide variety of applications.1-4 Ultrathin nanosheets that have an exceptionally small thickness and one-dimensional quantum confinement give rise to unique physical and chemical properties. Besides serving as an ideal model for providing insights into structure-property relationships in the solid state nanochemistry field, ultrathin 2D materials also have promising applications in catalysis, energy storage, gas sensors and biocompatible materials. Molybdenum trioxide (MoO3) nanosheets (NSs) are of particular interest due to promising application in heterogeneous catalysis. As they tend to partially lose lattice oxygen at high temperature to become oxygen-deficient and they have strong adsorption of organic molecules, MoO3 material is one of the most competitive candidates as metal support in catalysis.5, 6 In the construction of hybrid nanostructures, MoO3 NSs can be used as important building blocks in view of the large specific surface area, strong adsorption ability and excellent thermal stability of 2D nanomaterials.7-9 Meanwhile, surface modification of 2D nanomaterials with noble metals has been considered as an effective strategy to obtain an advanced catalyst for practical applications. As representative 2D nanomaterials, 2D oxides have been widely utilized as fascinating support for preparation of metal-support composites.10,11 The high 3



surface area of MoO3 NSs can enable abundant anchoring sites for noble nanoparticles. Additionally, the rigid 2D structure makes MoO3 NSs allow most surface of the attached nanoparticles to be exposed to the environments.12 As such, MoO3 NSs can serve as ideal supports for high-performance hybrid catalysts as they could offer an extremely large proportion of surface atoms, thus serving as active adsorption sites to efficiently adsorb reactant molecules. As compared to traditional deposition techniques, an in situ growth route is beneficial for promoting activity of the hybrid nanocatalysts due to strong electron coupling between noble metals and supports, and a synergetic effect. This one-pot process produces finer and better-distributed nanoparticles, which is more time- and cost-effective, no need for multiple deposition steps or expensive precursors.13-15 Furthermore, in situ anchored noble metal nanoparticles are probably immobilized on the support surface through active groups at active sites, which can efficiently suppress the migration and aggregation of nanoparticles during catalytic process, thus leading to the increased catalytic activity and stability of metal/support nanocatalysts. One fourth of marketed drugs as well as clinical drug candidates involved at least one hydrogenation step in their synthetic route in 2013.16 It is an important but particularly challenging transformation for chemoselective hydrogenation of olefinic bonds with retention of other reducible functional groups such as aldehydes or ketones. Palladium nanocrystals (NCs) with well-controlled size and shape have continually attracted extensive attention in catalysis due to their promising properties and practical applications, in various organic reactions.17-19 More specifically, 4


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Pd-based catalysts play a central role in the hydrogenation of α, β-unsaturated aldehydes to saturated aldehydes.20-22 However, it is still difficult to obtain the corresponding saturated aldehydes with high selectivity and conversion. Toward improving the properties of Pd NCs, much progress has been achieved through regulating electron density of catalytic sites on Pd-based catalysts by exploiting support effects.23-25 Therefore we integrate the advantages of 2D MoO3-x NSs and active Pd NCs to fabricate hybrid nanocatalyst with high performance for the chemoselective hydrogenation of olefinic compounds. Herein, we demonstrate that strongly coupled Pd nanotetrahedrons/ Molybdenum trioxide nanosheets (Pd/MoO3-x) can be prepared via a simple one-pot wet-chemical synthetic method. The unique 2D structure of MoO3-x NSs offers a moderate surface area and plenty of anchoring sites for the in situ growth and subsequently immobilizing Pd NCs. Owing to the growth confinement effect of MoO3-x NSs, the migration and aggregation of Pd nanotetrahedrons is minimized. Pd NCs directly grow on MoO3-x NSs, which efficiently maximizes the support-catalyst contact and thus achieves excellent catalytic activity in a hydrogenation of α, β-unsaturated aldehydes. Our study indicates that the loading of noble metal NSs on 2D MoO3-x NSs may obtain a highly efficient catalyst for potential applications. EXPERIMENTAL SECTION Chemicals and Materials. Sodium tetrachloropalladate (II) (Na2PdCl4, 98%), molybdenum hexacarbonyl (Mo(CO)6, 98%), cetyltrimethyl ammonium bromide (CTAB, 99.0%) were purchased from Sigma-Aldrich. Poly (vinylpyrrolidone) (PVP, 5



K-30), N, N-dimethylformamide (DMF, 99.5%), toluene (99.5%), ethanol (EtOH, 99.5%), acetone (99.5%) and other reagents used in the hydrogenation reaction were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. (China). All chemicals are of analytical grade and were used as received without further purification. Synthesis of the Pd/MoO3-x Hybrid Nanomaterials. In a typical synthesis, 8 mL DMF solution containing 8 mg of Na2PdCl4 in a 20 ml vial under magnetic stirring, 2 mL solution of toluene containing 10 mg of Mo(CO)6 was added. After vigorous stirring for 15 min, the mixture solution color changed from brown to light yellow. Then, 50 mg of PVP and 50 mg of CTAB were added to the above mixture at room temperature, followed by the injection of 1 mL of ethanol and stirred for another 15 min. The resulting solution was transferred into a Teflon-lined stainless steel autoclave (25 mL). The autoclave was sealed, heated to 180 °C and maintained for 4 h in an oven. The reactor was then naturally cooled to ambient temperature. The final product was collected by centrifugation and washed several times with an ethanol-acetone mixture. Finally, the obtained products were redispersed in 6 mL of ethanol for further use. Synthesis of Supportless Pd Nanocrystals. Typically, 8 mg of Na2PdCl4 was dissolved in 10 mL DMF solution. 50 mg of PVP and 50 mg of CTAB were added to the above solution, followed by the injection of 1 mL ethanol and stirred for 15 min. Then the resulting homogeneous solution was transferred into a Teflon-lined stainless steel autoclave and heated at 180 °C for 4 h before it was cooled to room temperature. The final product was collected by centrifugation and washed several times with an ethanol-acetone mixture. Finally, the obtained products were redispersed in 6 mL 6


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ethanol for further use. Characterization. The morphologies of nanocrystals were examined by transmission electron microscopy (TEM), using a Hitachi-7700 microscope with an accelerating voltage of 100 kV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images, elemental mapping were performed using a JEOL JEM12100F field-emission high-resolution transmission electron microscope operated at 200 kV. The thickness analysis of MoO3-x nanosheets on a Si substrate was performed by AFM using an Asylum Research Instruments MFP-3D AFM system. The X-ray photoelectron spectroscopy (XPS) was performed at a Perkin-Elmer RBD upgraded PHI-5000C ESCA system. The actual concentration of Pd was measured with a Thermo Scientific Plasma Quad 3 inductively-coupled plasma mass spectrometry (ICP-MS) after dissolving sample with a mixture of HCl and HNO3 (3:1, volume ratio). The X-ray powder diffraction (XRD) patterns of the samples were carried out on a Rigaku/Max-3A X-ray diffractometer (operation voltage and current were maintained at 40 kV and 200 mA, respectively) with Cu Kα radiation (λ = 1.54178 Å) and analyzed in the range of 10° ≤ 2θ ≤ 70°. The electron paramagnetic resonance (EPR) spectra were recorded on a JEOL JES-FA200 EPR spectrometer (298 K, 9064 MHz, 0.998 mW, X-band). The UV/Vis-NIR absorbance spectra were examined by a Shimadzu spectrophotometer (Model 2501 PC). The reaction conversion was measured by using gas chromatography (Agilent 7890B). Selective Hydrogenation of α, β-unsaturated Aldehyde. The liquid-phase hydrogenation of 3-methylcrotonaldehyde was carried out in a Teflon-lined stainless 7



steel autoclave (Wattecs Parallel Reactor) equipped with a pressure gauge and a magnetic stirrer. In a typical procedure, a reaction tube was charged with 20 µL of 3-methylcrotonaldehyde, 2 mL ethanol, and 0.2 mL of Pd/MoO3-x nanocatalyst suspension. The autoclave was purged with H2 six times at 10 bar to remove air, and the mixture was then magnetically stirred at 600 rpm under 20 bar of H2 at 313 K for 10 h. During the reaction, the products were analyzed by gas chromatography at specific times. The control experiment was carried out with the same procedure, except that the supportless Pd nanoparticles were utilized. The recovery and reuse of the Pd/MoO3-x nanocatalyst is described below. Upon completion of a reaction cycle, the reaction mixture in the test tube was centrifuged at 11000 rpm for 3 min. The supernatant was discarded and the precipitate was rinsed with fresh ethanol and acetone by re-suspending the catalyst, centrifuging the mixture at 11000 rpm for 3 min, followed by discarding the supernatant. Fresh 3-methylcrotonaldehyde was added, and a new reaction was performed by using the procedure for selective hydrogenation of 3-methylcrotonaldehyde described above. This procedure was repeated for three times. RESULTS AND DISCUSSION Synthesis and Characterization of Pd/MoO3-x Hybrid Nanocatalyst. The Pd nanotetrahedrons/Molybdenum trioxide nanosheets hybrid nanomaterials (Pd/MoO3-x) were prepared by one-pot solvothermal method. Palladium (I) carbonyl complex bearing two bridging CO ligands, [Pd2(µ-CO)2Cl4]2−, was used for fabricating nanostructures with well-defined shape.14,26 This high reactive carbonyl complex 8


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precursor was obtained by treating Na2PdCl4 with molybdenum hexacarbonyl (Mo(CO)6) in DMF solution at room temperature, then the mixture was treated by solvothermal reduction of carbonyl complex at 180 °C in a autoclave with poly(vinylpyrrolidone) (PVP ) and cetyltrimethyl ammonium bromide (CTAB) as surfactants (see Experimental section for details). As a result, we successfully prepared ultrathin MoO3-x nanosheets (NSs) with decorated uniform Pd tetrahedral nanocrystals (NCs). The as-prepared Pd/MoO3-x hybrid materials were characterized by transmission electron microscopy (TEM) measurements. As shown in Figure 1a and 1b, the product consists of uniform tetrahedral Pd NCs and ultrathin MoO3-x NSs. Additionally, the ultrathin MoO3-x nanosheets are evenly decorated with monodisperse Pd tetrahedrons. Figure 1b and 1c reveal the uniform free-standing and large-area MoO3-x NSs with lateral size ranging from 50 to 100 nm. Their bright contrast in TEM images implies the nature of the ultrathin nanosheets, which was further verified by atomic force microscopy (AFM) measurement (Figure 3). From the higher magnification TEM image of individual Pd NCs (Figure 1d), Pd tetrahedrons have an average edge length of 13.5 ± 1.0 nm. The powder X-ray diffraction (XRD) pattern of the obtained sample presents characteristic peaks matching well with metallic Pd diffractions (see Supporting Information, Figure S1), confirming the formation of Pd NCs. Besides diffraction peaks from Pd NCs, the diffraction peaks centered at 23.32° and 27.33° can be readily assigned to Molybdenum trioxide (JCPDS no. 05-0508). The XRD result provides clear evidence for the successful synthesis of Pd/MoO3-x hybrid material. 9



Figure 1. Representative TEM images of the Pd/MoO3-x hybrid nanomaterials at different magniﬁcations. The hybrid nanostructure of ultrathin MoO3-x NSs with Pd tetrahedrons was further confirmed by the high-resolution TEM (HRTEM) (Figure 2a-2c) and high-angle annular dark-field scanning TEM (HAADF-STEM) images (Figure 2d-2h). The lattice spacing of an individual Pd tetrahedron shown in the HRTEM image is 0.24 nm (Figure 2b), corresponding to the (111) plane of Pd fcc NCs,18 which is consistent with XRD results. A 2D structure of MoO3-x NSs is demonstrated in Figure 2c. The measured lattice distance of MoO3-x NSs is 0.26 nm, which is assigned to the (111) plane of the orthorhombic phase structure.27 From HRTEM images, it is found that most MoO3-x NSs exhibit a disordered structure, indicating their ultrathin nature. The AFM image, corresponding height distribution and height profiles exhibits that the 10


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overwhelming majority of MoO3-x NSs possess an average height of about 7 nm (Figure 3). The corresponding elemental mapping clearly confirms the present of Pd, Mo, O element, and tetrahedral Pd NCs are well dispersed on MoO3-x NSs (Figure 2e-2h).

Figure 2. TEM and HRTEM images of the as-made Pd/MoO3-x (a, b, c), the inset in (c) shows the magnified HRTEM image of MoO3-x nanosheets. HAADF-STEM images of the Pd/MoO3-x sample at different magnifications (d, e), and the corresponding elemental mapping of (f) Pd, (g) Mo, (h) O.

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50 nm

12 10 8

7 nm

Height (nm)

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6 4 2 0 0

20

40

60

Distance (nm)

80

Figure 3. AFM image of Pd/MoO3-x NSs deposited on a Si substrate and the inset shows cross-sectional profile. X-ray photoelectron spectroscopy (XPS) analysis was performed to determine the chemical composition and chemical states of Pd/MoO3-x nanocatalyst. Survey XPS spectrum confirms the presence of palladium, molybdenum, and oxygen in as-synthesized sample (Figure 4a). The Pd 3d signal of Pd/MoO3-x nanocomposites is fitted to two pairs of doublets, as shown in Figure 4b. The peaks at the high binding energy region, appeared at 335.8 and 341.1 eV, correspond to the 3d5/2 and 3d3/2 levels of Pd2+,28 which can be rationalized by Pd oxide shell formed on Pd tetrahedral NCs. The main peaks centered at 335.0 and 340.3 eV are attributed to the 3d5/2 and 3d3/2 binding energies of metallic Pd (0), respectively.29 The atomic ratio of Pd (0)/Pd2+ was calculated to be about 3.5. The corresponding binding energy of Pd 3d has a positive shift as compared to unsupported bare Pd (see Supporting Information, Figure S3), indicating strong metal-support interaction (SMSI) occurs between Pd NCs and 12


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MoO3-x NSs.30,31 The strong electronic communication was also supported by the UV/Vis-NIR spectrum of Pd/MoO3-x sample (Figure S4, Supporting Information). It can be seen that Pd/MoO3-x sample displays considerably large absorption tail in the visible and NIR regions, indicating the presence of oxygen vacancies. 27 This SMSI can effectively enhance the catalytic performance of the Pd/MoO3-x nanocatalyst. Meanwhile, the Mo 3d peak can be deconvoluted into two pairs of doublets (Figure 4c). The peaks at 232.4 and 235.5 eV are attributed to Mo6+, while those centered at 231.7 and 234.9 eV are assigned to Mo5+.27,32 Furthermore, the Mo6+ and Mo5+ cations account for 44.8 % and 55.2 % of the total Mo states, respectively, according to the XPS peak areas of Mo 3d. The average oxidation state of Mo was determined to be 5.45, thus demonstrating the existence of abundant oxygen vacancies in Pd/MoO3-x sample. This is in good agreement with the electron paramagnetic resonance (EPR) measurements (Fig. S5). The Pd/MoO3-x nanocatalyst gives rise to a very strong EPR signal at g = 2.003, which was previously identified as electrons trapped on surface oxygen vacancies.33 The O 1s electron binding energy peak, as seen from Figure 4d, can be fitted well by three peaks. Moreover, the peak with binding energy located at 530.2 eV could be assigned to O 1s levels of oxygen atom O2− in the lattice of MoO3-x,34 and the peak at 533.2 eV may be attributed to hydroxyl groups (−OH) on the oxide surface.35 The binding energy at 531.8 eV corresponds to adsorbed oxygen species, CO molecules for example were produced from the decomposition of Mo(CO)6 and/or DMF.14 The weight ratio of MoO3-x to Pd was measured be about 1:3 by inductively-coupled plasma mass spectrometry (ICP-MS), which is lower than the 13



nominal ratio (1:1) in the experiment.

Figure 4. XPS spectra of Pd/MoO3-x nanocatalyst: (a) the survey spectrum; (b) Pd 3d spectrum; (c) Mo 3d spectrum; (d) O1s spectrum. Catalytic Performance in the Selective Hydrogenation of α, β-unsaturated Aldehydes. 2D metal oxides have aroused tremendous interest as an ideal substrate for anchoring noble metal nanoparticles with respect to their large surface area, thermal stability and promoter effect. The 2D MoO3-x NSs decorated with Pd NCs were used as nanocatalyst with high activity for organic transformations. Among the diverse catalytic reactions employed, the selective hydrogenation of α, β-unsaturated aldehydes reaction has been recently widely used as a model reaction to evaluate the catalytic performance of Pd-based catalysts. The possible pathways the reaction can take are illustrated in Scheme 1. 3-methylcrotonaldehyde (3-MeCal), as typical α, β-unsaturated aldehyde, can result in various product distributions in its hydrogenation route. During the reaction, the hydrogenation of carbonyl (C=O) group 14


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leads to 3-methylcrotonalcohol (3-MeCol), that of the olefinic (C=C) group yields 3-methylbutyraldehyde (3-MeBal), and hydrogenation of the both groups produces 3-methyl-1-butanol (3-MeBol). Under ideal circumstances, the only reacting pathway would be the selective reduction of the C=C double bond, leading to exclusive formation of 3-methylbutyraldehyde (3-MeBal). The catalytic performances were investigated for the selective hydrogenation of 3-MeCal over Pd/MoO3-x nanocatalyst.

H2 3-methylcrotonaldehyde (3-MeCal)

3-methylbutyraldehyde (3-MeBal)

H2

H2 H2

3-methylcrotonalcohol (3-MeCol)

3-methyl-1-butanol (3-MeBol)

Scheme 1. Reaction pathways for 3-methylcrotonaldehyde hydrogenation. According to Figure 5, Pd/MoO3-x nanocatalyst exhibited superior performance for the selective hydrogenation of 3-methylcrotonaldehyde. In the presence of the hybrid nanocatalyst, the selectivity to 3-MeBal was up to 96% at 97% 3-MeCal conversions. The by-product, 3-MeBol (from the reduction of C=C and C=O bonds) was formed in only 4% with no detectable amount of 3-MeCol (product of the C=O reduction). Compared to reported data, Pd/MoO3-x nanocatalyst showed satisfying conversion and selectivity for selective hydrogenation of α, β-unsaturated aldehydes at relative low temperature (as shown in Table S2). To better study the selective hydrogenation properties, unsupported Pd NCs were also prepared by the same procedure only 15



without the addition of Mo(CO)6, and used as a reference to compare the catalytic activity. In comparison, unsupported Pd NCs performed 81.3% 3-MeCal conversion and 52.6% selectivity to 3-MeBal. It is generally accepted that metallic Pd with high electron density would accelerate its performance on chemoselective hydrogenation of α, β-unsaturated carbonyls.21, 25 It is not surprising that catalysts which have strong electron-donating part to increase electron density on the palladium d-band center tend to exhibit good activity. It has been proved that O-vacancies of metal oxides can act as a promising candidate for electron donating.36, 37 Due to the high surface Mo5+ fraction and concentration of O-vacancies (as confirmed by XPS analysis, Mo5+ cations account for 55.2 % of the total Mo states) in MoO3-x support, the electrons transfer from MoO3-x donor to Pd nanotetrahedrons with increased electron density. This electron communication and strong metal-support interaction between Pd and MoO3-x enhance the activation of Pd NCs. The incorporation of MoO3-x NSs alters the electronic structure of Pd, which also facilitates the disassociation of molecular hydrogen,38 and subsequently boosts the catalytic activity of Pd NCs for the selective hydrogenation reaction. This result indicates that the strong interaction between Pd NCs and MoO3-x support is in favor of improving the catalytic activity of Pd/MoO3-x nanocatalyst. We further calculated the total numbers of the surface Pd atoms of Pd/MoO3-x nanocatalyst by the method reported in the literature.39,

40

The catalytic efficiency of Pd/MoO3-x

nanocatalyst was evaluated with respect to the amount of surface Pd atoms by the calculation of turnover frequency (TOF, defined as the moles of 3-MeCal per mole of 16


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Pd catalyst per hour), resulting in a TOF value of 120 h-1. The actual molar content of Pd was determined to be 0.3% by inductively-coupled plasma mass spectrometry (ICP-MS). We also investigated the effect of the ratio of Pd/Mo on the selective hydrogenation reaction (as shown in Table S1). Pd/MoO3-x nanocatalysts with different Pd/Mo ratios were prepared via increasing the Mo(CO)6 amount in the synthesis process. It can be seen that with the increase of Mo(CO)6 amount from 5 mg to 10 mg, the conversion and the selectivity increased from 26 to 97% and 70 to 96%, respectively. Moreover, further increase of Mo(CO)6 amount resulted in little influence on conversion and selectivity for selective hydrogenation of α, β-unsaturated aldehydes.

Figure 5. The conversion of 3-MeCal as a function of time (black curve) and the selectivity of three products (blue curve). The error bars result from averaging experiments of three repeated runs. Reaction conditions: 0.2 mmol of 3-MeCal, 2 mL EtOH, and 0.2 mL suspension of Pd/MoO3-x catalyst (0.3 mol% relative to substrate), reaction time, 10 h, 20 bar H2, temperature, 313 K. Reusability of the Pd/MoO3-x nanocatalyst. The superiority of heterogeneous 2D 17



material supported catalysts lies in their good stability, dispersity and recyclability. To investigate the stability of Pd/MoO3-x nanocatalyst, the used catalyst was recycled by centrifugation and drying. The catalyst was tested for continuous four cycles under the same reaction condition. Compared with the first run, the catalyst displays no appreciable loss of its catalytic activity or selectivity for the following three cycles (Figure 6), suggesting the excellent stability of this catalyst. TEM images of Pd/MoO3-x nanocatalyst after running for four times also reveal that the recycled catalyst shows only a small change in morphology and no aggregation of Pd NCs (Figure S6), thus implying that MoO3-x NSs as a support can effectively immobilize Pd NCs to inhibit their aggregation during the catalytic reaction process. Apparently, the good reusability of as-prepared Pd/MoO3-x nanocatalyst results from the strong interaction between Pd nanotetrahedrons and the surface of MoO3-x support. Conversion

100

Con. and sel. / %

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Figure 6. Evaluation of the conversion and selectivity in the selective hydrogenation of 3-MeCal over Pd/MoO3-x nanocatalyst for four catalytic cycles.

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CONCLUSIONS To summarize, we have developed a novel approach to in-situ growth of Pd nanotetrahedrons on ultrathin MoO3-x nanosheets in one-pot fashion. Novel ultrathin MoO3-x nanosheets are formed through molybdenum hexacarbonyl involved solvothermal process, affording abundant oxygen vacancies, anchoring sites and high structural stability. Well-defined monodisperse Pd nanotetrahedrons grow on MoO3-x nanosheets in the reaction system. The one-pot synthetic process endows strong metal support interaction between metallic Pd and MoO3-x support, as revealed by XPS analysis. The MoO3-x nanosheets support with abundant oxygen vacancies can improve the catalytic activity of Pd nanocatalyst for chemoselective hydrogenation reactions through electronic structure change of Pd nanocrystals via electron-donating effect. The strongly coupled Pd/MoO3-x hybrid nanomaterials demonstrate high activity and selectivity for selective hydrogenation of α, β-unsaturated aldehydes reaction and are reusable for at least four cycles. These enhanced catalytic performances of Pd/MoO3-x nanocatalyst can be attributed to the synergistic effect of Pd nanotetrahedrons and MoO3-x nanosheets. Briefly, our work introduces a simple strategy to one-pot in-situ growth of metal nanoparticles on 2D oxide surfaces, affording strong metal-support interaction and electron communication between metallic Pd and MoO3-x support. We anticipate that other 2D materials supported metal nanoparticles could be explored as hybrid nanocatalysts for a wide range of organic transformations.

ASSOCIATED CONTENT 19



Supporting Information Additional figures are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (A. W. X.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the special funding support from the National Natural Science Foundation of China (51572253, 21271165, 81372821, 21771171), Scientific Research Grant of Hefei Science Center of CAS (2015SRG-HSC048), and cooperation between NSFC and Netherlands Organization for Scientific Research (51561135011).

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TOC Graphic:

Selective hydrogenation

O

H2

Pd/MoO3-x nanocatalyst

27


Monodisperse Pd Nanotetrahedrons on Ultrathin MoO3-x Nanosheets

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