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A Novel Magnetically Recoverable Ni-CeO2-x/Pd Nanocatalyst with Superior Catalytic Performance for Hydrogenation of Styrene and 4-Nitrophenol Yi-Fan Jiang, Cheng-Zong Yuan, Xiao Xie, Xiao Zhou, Nan Jiang, Xin Wang, Muhammad Imran, and An-Wu Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00293 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on March 1, 2017

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ACS Applied Materials & Interfaces

A Novel Magnetically Recoverable Ni-CeO2-x/Pd Nanocatalyst with Superior Catalytic Performance for Hydrogenation of Styrene and 4-Nitrophenol

Yi-Fan Jiang, Cheng-Zong Yuan, Xiao Xie, Xiao Zhou, Nan Jiang, Xin Wang, Muhammad Imran and An-Wu Xu*

Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Deparment of Chemistry, University of Science and Technology of China, Hefei 230026, PR China

*To whom correspondence should be addressed. Email: [email protected]

ACS Paragon Plus Environment


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ABSTRACT Metal/support nanocatalysts consisting of various metals and metal oxides not only retain the basic properties of each component, but also exhibit higher catalytic activity due to their synergistic effects. Herein, we report the creation of a highly efficient, long-lasting and magnetic recyclable catalyst, composed of magnetic nickel (Ni) nanoparticles (NPs), active Pd NPs and oxygen deficient CeO2-x support. These hybrid nanostructures composed of oxygen deficient CeO2-x and active metal nanoparticles could effectively facilitate diffusion of reactant molecules and active site exposure that can dramatically accelerate the reaction rate. Impressively, the rate constant k and k/m of 4-nitrophenol reduction over 61 wt%Ni-CeO2-x/0.1 wt%Pd catalyst are respectively 0.0479 s−1 and 2.1 × 104 min−1 g−1, and the reaction conversion shows negligible decline even after 20 cycles. Meanwhile, the optimal 61 wt%Ni-CeO2-x/3 wt%Pd catalyst manifests remarkable catalytic activity towards styrene hydrogenation with a high TOF of 6827 molstyrene molPd-1 h-1 and a selective conversion of 100% to ethylbenzene even after eight cycles. The strong metal-support interaction (SMSI) between Ni NPs, Pd NPs and oxygen deficient CeO2-x support is beneficial for superior catalytic efficiency and stability toward hydrogenation of styrene and 4-nitrophenol. Moreover, Ni species could boost the catalytic activity of Pd due to their synergistic effect and strengthen the interaction between reactant and catalyst, which seems responsible for the great enhancement of catalytic activity. Our findings provide a new perspective to develop other high-performance and magnetically recoverable nanocatalysts, which would be widely applied to a variety of catalytic reactions.

Keywords: magnetic recyclable, synergistic effects, oxygen deficient, strong metal-support interaction, hydrogenation

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INTRODUCTION In recent years, much attention has been focused on noble metal nanoparticles (NMNPs) such as Au, Pt and Pd owing to their outstanding catalytic activity in various chemical reactions, especially hydrogenation,1 oxygenation,2 and coupling reactions.3 Especially, various palladium (Pd) based catalysts have been extensively investigated due to their wide applications in organic synthesis, such as Suzuki-Miyaura, Heck, Negishi couplings and CO2 electrohydrogenation.4,5 It is proved that the catalytic activity of NMNPs depends not only on their size but also on their shape, crystallinity and surface state.6 Moreover, NMNPs with an extremely small size and high surface area usually exhibit very fantastic size-dependent catalytic properties, which could not be realized by their respective bulk materials.7 Diverse Pd based nanocatalysts have been confirmed to be highly active and selective in heterogeneous catalytic reactions. However, suffering from the difficulty of recycling, efficient application of Pd based materials still remains a serious challenge. At present, the separation of nanocatalysts depends on complex methods, such as tedious centrifugation and filtration. Moreover, catalyst recovery is particularly important in sense of their economically accessible supplies of Pd and other noble metals at the risk of their gradually decreasing content. The consumption of precious metals at a large-scale may raise concerns about the sustainability of environments and economy due to their high cost. Therefore, it remains a great need for developing simple and easy control routes to fabricate low cost, highly active, stable and conveniently recyclable nanocatalysts. Magnetically active hybrid support for NMNPs could be an effective approach to overcome these issues due to easy separation from the reaction systems. Hybrid nanocatalysts consist of various metals and metal oxides are interesting kinds of materials that can display a combination of properties associated with each component, as well as boost catalytic activity due to their synergistic effect.8,9 Compared with other non-noble transition metal, nickel NPs are considered to be a special material due to its magnetic property and catalytic activity, for example in hydrogenation10 and oxidation reactions.11 Ni is a most promising candidate to partially substitute for Pd in catalytic reactions, which could reduce the cost of catalyst greatly. More interestingly, in many cases, the combination of two metal 2



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species may achieve enhanced physical and chemical properties and even produce new catalytic properties due to synergistic effect.12 What's more, covering the surface of Ni NPs with noble metal can not only protect Ni from aerobic oxidation, but also maintain the magnetic characteristic of nickel and promote the excellent catalytic activity of Pd.13,14 And Ni is able to form charge-transfer complex with benzene ring,15 which could enhance the interaction of reactant with hybrid catalysts, so that the catalytic effect can be boosted up. However, Ni NPs tend to aggregate because of magnetic dipole interactions and the high electron affinity.16 So, how to suppress the aggregation of Ni NPs is of great significance to improve the properties and stability of Ni-based catalysts. Anchoring Ni NPs into porous supports, which provide large surface area and multiple pores, is a possible way to improve Ni NPs dispersion and suppress its aggregation.17 However, it was reported that Ni NPs might diffuse out from the pores to the surface of the supports with the increasing temperature owing to the internal weak interaction between Ni NPs and the supports, leading to the loss of the confinement effect and subsequent sintering.18,19 An effective solution is to use promotional oxides, such as TiO2, ZrO2 and CeO2 to strengthen the interactions with Ni NPs.20, 21 Among various promotional oxides, CeO2, which can alter its oxidation states between Ce4+ and Ce3+, is a promising substitute due to its low cost, excellent oxygen storage/release ability and high oxygen mobility.22 Recent studies have proved that CeO2 could be able to protect metal particles from thermal sintering because of the strong metal-support interaction (SMSI).23 The SMSI between metal species and CeO2 can modify the structural and electronic properties of metal, which is beneficial to improve the catalytic performance and stability of Ni NPs.24 Herein, we report the first example of a novel magnetically recoverable Ni-CeO2-x/Pd nanocatalyst composed of active Pd NPs, magnetic Ni NPs and oxygen deficient CeO2-x support. These magnetic hybrid nanocatalysts are prepared by adsorbing PdCl42− onto the surface of Ni(OH)2-CeO2 precursor, which is synthesized by one-step hydrothermal method, followed by reduction of metal ions via calcination in H2/Ar atmosphere. The obtained nanocatalysts maximize the catalytic activity and stability of Pd. The catalysts can be well dispersed in the reaction solution, and easily separated from the system by an external permanent magnet. The strong interactions 3


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between Pd, Ni NPs and oxygen vacancy-rich CeO2-x support are beneficial for superior catalytic performance and stability of the nanocatalysts. The catalytic activity of Ni-CeO2-x/Pd catalysts is systematically tested using the hydrogenation of 4-nitrophenol (4-NP) and styrene as model reactions. Results indicate that the weight ratios of Ni to CeO2 affect the catalytic activity of catalysts. The 61 wt%Ni-CeO2-x/0.1 wt%Pd catalyst manifests superior catalytic performance and stability towards catalytic reduction of 4-NP. The 61 wt%Ni-CeO2-x/3 wt%Pd catalyst exhibits outstanding catalytic and recycle performance in the hydrogenation of styrene. In this work, we have developed a highly efficient, durable and easily magnetically separated nanocatalyst, which could endow it with extensive potential applications in catalytic reactions, such as hydrogenation, oxidative reactions and cross-couplings.

RESULTS AND DISCUSSION Ni-CeO2-x/Pd nanoparticles were synthesized by impregnation and reduction method (see experimental section). The crystal structures of obtained precursor and Ni-CeO2-x/Pd samples were examined using powder X-ray diffraction (XRD) measurements. As shown in Figure S1, the precursor was a mixture of CeO2 and Ni(OH)2. The XRD patterns of nanocatalysts with different Ni weight contents are presented in Figure S2. The diffraction peaks at 2 θ = 28.6o, 33.1o, 47.5o and 56.4o can be indexed to (111), (200), (220) and (311) of cubic flurite CeO2 (JCPDS card No. 43−1002). Other two strong peaks are assigned to lattice planes of (111) (2 θ = 44.6o) and (200) (2 θ = 51.9o) of a cubic structure of metallic nickel (JCPDS card No. 04−0850). The intensities of these two peaks at 44.6o and 51.9o increase with the increasing content of Ni in the catalysts. No obvious signals of Pd are observed due to a low actual loading content and highly dispersed state, which can be responsible for the excellent catalytic performance. These XRD results confirm that we have successfully synthesized the final Ni-CeO2-x/Pd products. The morphology and structure of 61 wt%Ni-CeO2-x/3 wt ％ Pd sample were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A typical SEM image (Figure 1a) displays that as-prepared catalyst consists of a large amount of irregular tiny nanoparticles. TEM image 4



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indicates that Ni NPs are uniformly mixed with small CeO2-x NPs, whose sizes are 5.6 nm (Figure 1b). Closer examination of 61 wt%Ni-CeO2-x/3 wt%Pd catalyst by high-resolution TEM (HRTEM) image clearly reveals the lattice fringes of Pd, metallic nickel and ceria NPs (Figure 1c). The lattice fringes with measured spacing of 3.09 Å and 2.73 Å correspond to the (111) and (200) atomic planes of CeO2-x, respectively. The resolved lattice fringes of (111) planes (d = 2.09 Å) are assigned to metallic nickel, while the lattice fringes of (111) planes (d = 2.23 Å) and (200) planes (d = 1.92 Å) are attributed to Pd NPs with a size of 3.8 nm. In order to further confirm the composition and architecture of our catalyst, the spatial distribution of different elements in the Ni-CeO2-x/Pd was visualized by the elemental mappings. The TEM image and associated elemental mappings in Figure 1d exhibit the homogeneous distribution of Ce, O, Ni and Pd elements, indicating the successful fabrication of a novel Ni-CeO2-x/Pd nanocatalyst composed of magnetic Ni NPs, active Pd NPs and CeO2-x support.

(a)

(b) (b)

(c) (d) Pd (111) 2.23Å

500 nm

30 nm

5 nm

Pd (200) 1.92Å Ni (111) 2.09 Å

nm CeO0.21 2 (111) 3.09Ni(111) Å

CeO2 (200) 2.73Å

(d)

500 nm

Ce

O

Ni

Pd

Figure 1. (a) SEM, (b) TEM, (c) HRTEM images of 61 wt%Ni-CeO2-x/3 wt%Pd catalyst, (d) TEM image and the corresponding elemental mappings of 61 wt%Ni-CeO2-x/3 wt%Pd.

The compositions and valence states were determined by X-ray photoelectron spectroscopy (XPS) analysis. The representative XPS survey scan spectrum (Figure S3) of 61 wt%Ni-CeO2-x/3 wt%Pd sample clearly displays the existence of Ce, Ni, O and Pd elements. Figure 2a presents the high-resolution Ce 3d XPS spectrum. There are four main peaks of Ce 3d3/2 featuring at 916.8, 907.5, 902.9 and 900.8 eV, corresponding to the α1, α2, α3 and α4 components; while the peaks of Ce 3d5/2 at 898.3, 888.7, 884.5 and 882.5 eV are assigned to the β1, β2, β3 and β4 constituents, 5


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respectively.25 The signals of α3 and β3 are characteristics of Ce3+, clearly demonstrating the existence of Ce3+ in CeO2-x NPs with oxygen vacancies, which is attributed to the strong interactions between CeO2-x and the surrounding atoms.26 The formation of oxygen vacancy abundant CeO2-x is caused by Pd catalyzed instant hydrogenation of ceria. In the annealing process of H2 reduction, adsorbed Pd2+ was first reduced to Pd(0), and then H2 spontaneously dissociates on Pd surface to produce highly active atomic hydrogen species, which could diffuse into and interact with CeO2 lattices to form durable point defects (Ce3+, oxygen vacancies) and surface disorder.27 Recent researches have proved that CeO2-x NPs with abundant oxygen vacancies and single electron defects of Ce3+ ion are beneficial for boosting the catalytic activity.28 Figure 2b shows the doublet of Ni 2p1/2 and 2p3/2 transitions in the range of 850−870 eV. Taking consideration of spin-orbit split and shake-up satellites, two kinds of nickel species are observed, that is Ni2+ and metallic Ni. The peak around 852.6 eV for Ni 2p3/2 can be attributed to metallic Ni peak.29 The main peak at 855.4 eV with an intense satellite at 861.1 eV for Ni 2p3/2 is the characteristic of Ni2+ in nickel oxide.30 The existence of nickel oxide could be attributable to the surface oxidation upon exposure to air. The O 1s spectrum (Figure 2c) reveals that the primary peak located at 529.4 eV is mainly ascribed to the lattice oxygen of CeO2-x support. The additional peak at about 531.4 eV is denoted as adsorbed oxygen, likely introduced by H2O and CO2 molecules adsorbed on the surface of catalysts. As shown in Figure 2d, the spectrum of Pd 3d5/2 exhibits two different chemical states of Pd. The main peak at 337.5 eV could be ascribed to Pd2+, and the other peak centered at 335.8 eV proves the existence of metallic Pd. As compared to unsupported Pd NPs, the position of the two peaks shifts toward higher binding energy values,31 suggesting the formation of positively charged Pd species on oxygen deficient CeO2-x support, this result implies that strong metal-support interaction (SMSI) occurs between Pd NPs and CeO2-x support.23 Energy dispersive spectroscopy (EDS) analysis of 61 wt%Ni-CeO2-x/3 wt%Pd confirms Ce, Ni, O and Pd elements exist in the catalyst (Figure S4), in consistent with the result of XPS analysis.

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(a)

(b)

(c)

(d)

Figure 2. XPS spectra of 61 wt%Ni-CeO2-x/3 wt%Pd sample: Ce 3d (a), Ni 2p (b), O 1s (c) and Pd 3d (d).

The magnetic property of the Ni-CeO2-x/Pd nanocatalysts was investigated via a vibrating sample magnetometer (VSM). The magnetization curves shown in Figure 3 reveal that the catalysts show a strong magnetism with negligible coercivity and remanence at room temperature. For 61 wt%Ni-CeO2-x/3 wt%Pd nanoparticles, the saturation magnetization (Ms) value was 26.9 emu g−1. As shown in the photographs (inset in Figure 3), the Ni-CeO2-x/Pd catalysts can be well dispersed in reaction mixtures without external magnetic field, and rapidly separated from the mixtures in a few seconds with the help of a magnet, which makes it possible for the catalysts to be recycled for reuse. This good magnetic response of Ni-CeO2-x/Pd nanocatalysts could facilitate rapid catalyst separation and the throughput enrichment in large-scale industrial manufacture.

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(a)

(b)

Figure 3. Magnetization curves of Ni-CeO2-x/Pd measured at room temperature. Inset shows photographs of sample dispersed in aqueous solution without (a) and with (b) an external magnetic field.

To evaluate the catalytic hydrogenation performance of Ni-CeO2-x/Pd catalysts, we chose the reduction of p-nitrophenol (4-NP) to p-aminophenol (4-AP) in NaBH4 aqueous solution at room temperature as a model reaction (Scheme S1). The reduction of 4-NP is of significance in pharmaceutical industries, photographic developer, corrosion inhibitor, etc.6 The reduction process was monitored by successive UV-vis spectroscopy (Figure 4a). The absorption peak of 4-NP at 400 nm rapidly decreased in intensity together with a new absorption peak emerging at 300 nm, which indicates that 4-NP transforms into 4-AP. The UV-vis spectra clearly show that the reaction finished within 100 s (Figure 4a) and the color quickly fades from bright yellow to colorless. The reaction was so rapid that the conversion was more than 50% within 20 s. Since the amount of NaBH4 is excess, the concentration of BH4− can be regarded constant throughout the reaction. That is to say only 4-NP and 4-AP as two significant species influence the reaction kinetics.32 For comparison, a blank test was performed with the same concentration of 4-NP and NaBH4 reducing agent in the absence of catalyst under the same ultrasonic condition (Figure S5). As we can see that the conversion of 4-NP was less than 20% even after 60 min, indicating that the catalytic reduction of 4-NP proceed very slowly without catalyst. As shown in Figure 4b, the relationship between ln (Ct/C0) (Ct and C0 are 4-NP concentrations at time t and 0, respectively) and reaction time t tends to be linear, indicating that the reaction obeyed 8



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pseudo-first-order kinetics. Since the amount of the used catalyst would influence the rate constant k, we evaluated the catalytic activity of the catalyst by the rate constant k and k/m (the reaction rate constant per the total weight of the used catalyst). The rate constant k and k/m of 61 wt%Ni-CeO2-x/0.1 wt%Pd catalyst towards 4-NP reduction were calculated to be 0.0479 s−1 and 2.1×104 min−1 g−1, respectively. Additionally taken in consideration that the ratio of Ni to CeO2-x may be an important factor for catalytic activity, and the optimum weight content of Ni is 61 wt% based on our experiments, as shown in Table S1. The reason for this phenomenon may be that Ni is likely to form charge-transfer complex with benzene ring,15 which could enhance the interaction of reactant with catalysts. Moreover, the neighboring Ni atoms can provide a synergistic effect upon the catalysis of Pd. Therefore, the catalytic activity of Ni-CeO2-x/Pd catalysts increases with the increasing Ni content. However, the further increase of Ni content may result in aggregation and consequently decrease the catalytic

activity.

Recently,

Zhang

et

al.

reported

dual-heterostructured

Fe3O4@CeO2/Pd (3 wt%) catalysts, which exhibited superior catalytic performance towards reduction of 4-NP due to the synergistic catalytic eﬀect between the NMNPs and CeO2.33 However, compared with Fe3O4@CeO2/Pd (3 wt%) and other catalysts reported in the literature, our Ni-CeO2-x/Pd catalyst has a very low content of Pd loading (0.1 wt%), while exhibiting excellent catalytic performance for the hydrogenation of 4-NP (Table S2).

(a)

(b)

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Figure 4. (a) Successive UV-vis spectra for the reduction reaction of 4-NP with 61 wt%Ni-CeO2-x/0.1 wt%Pd catalyst. (b) Kinetic curve for the reduction of 4-NP catalyzed by 61 wt%Ni-CeO2-x/0.1 wt%Pd. Ct and C0 are 4-NP concentrations at time t and 0, respectively.

The difficulty in separation and recovery of Pd nanoparticles from the reaction system has hindered its large scale application in catalysis. However, our obtained catalysts could be easily separated from the system with the help of a magnet. And it exhibited relatively stable catalytic performance with a slight decrease even after 20 cycles (Figure 5). Such result confirms that oxygen deficient CeO2-x support can stabilize catalytically active metal nanoparticles by restricting their aggregation and leaching, consequently leading to the high catalytic performance and outstanding stability.

Figure 5. Reusability of the 61 wt%Ni-CeO2-x/0.1 wt%Pd catalyst for the reduction of 4-nitrophenol.

As well-known, the catalytic hydrogenation of olefins is a common industrial process.34 Therefore, the catalytic activity of as-synthesized Ni-CeO2-x/Pd catalysts was also investigated using the hydrogenation of styrene as a model reaction. We chose 1,3,5-trimethylbenzene as internal standard and ethanol as solvent at room temperature under 1 atm H2 atmosphere (Scheme S2). From Figure 6 it can be seen that the weight ratio of Ni to CeO2-x affects the catalytic activity of catalyst, the catalytic activity increased with an increase of the Ni content, and then decreased with further increasing the Ni content while the nominal content of Pd was kept constant based on calculation (3 wt%). Since it is preferential to form charge-transfer complex 10



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between benzene ring and Ni,15 the increase of Ni content may strengthen the interaction between reactant and catalyst surface and increase the synergistic effect between Ni and Pd, which consequently improve the activity. However, the further increase of Ni content may lead to aggregate and decrease catalytic active sites on catalyst surface, which finally decrease the activity. The results of styrene conversion and the turnover frequency (TOF) values are summarized in Table S3.

Figure 6. Total styrene conversion as a function of time over Ni-CeO2-x/3 wt%Pd nanocatalysts with different weight ratio of Ni:CeO2-x, (■) 25 wt%, (●) 49 wt%, (▲) 61 wt%, (▼) 68 wt% and (◆) 73 wt%.

The optimum weight content of Ni in the catalyst is 61 wt% for styrene hydrogenation, in agreement with the result of 4-NP reduction. Figure 7a exhibits the relationship between kinetics of styrene hydrogenation and time for 61 wt%Ni-CeO2-x/3 wt%Pd catalyst. As expected, the linearity of the plots confirms zero-order kinetics characteristic of the hydrogenation reaction with respect to styrene, whose reaction rate is independent of the styrene concentration, in accordance with the previous report.35 The results show that the hydrogenation of styrene proceeded smoothly and thoroughly to produce ethylbenzene without any detectable byproducts and surplus styrene for 25 min. To evaluate the catalytic activity of 61 wt%Ni-CeO2-x/3 wt%Pd nanocatalysts, the TOF (molstyrene molPd-1 h-1): moles of styrene produced on per mole of Pd per hour and catalytic activity (molstyrene h-1 gcatalyst-1): moles of styrene produced on per mass of catalyst per hour were calculated to be 6827 molstyrene molPd-1 h-1 (The actual Pd amount was detected by ICP-AES 11


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analysis) and 2.43 molstyrene h-1 gcatalyst-1, respectively. The high hydrogenation activity of our Ni-CeO2-x/Pd catalyst is even superior to many state-of-the-art noble metal catalysts reported previously (see Table S4 for comparison). Our catalyst with low Pd loadings is one of the best catalysts for the hydrogenation of styrene, at least 7-fold higher than commercial 26 wt% Pd/C.35 The CeO2-x/3 wt%Pd, Ni/3 wt%Pd and 61 wt%Ni-CeO2 nanocatalysts were tested for the hydrogenation of styrene under the same conditions for comparison (Figure S6). However, compared with 61 wt%Ni-CeO2-x/3 wt%Pd, Ni/3 wt%Pd and 61 wt%Ni-CeO2-x proved to have negligible activity, while CeO2-x/3 wt%Pd proved to have poorer activity. This strongly corroborates that the synergistic effect among Pd, Ni and the CeO2-x support could enhance the catalytic activity. The catalytic activity of 61 wt%Ni-CeO2/3 wt%Pd without defect was also tested, as shown in Figure S7. It can be seen that the hydrogenation performance of 61 wt%Ni-CeO2/3 wt%Pd without defect is much poorer as the conversion of styrene is only 73% after 40 min. Therefore, such excellent catalytic activity could be attributed to surface defect sites (oxygen vacancies) of CeO2-x, which could improve the efficiency of H2 dissociation.36 Collecting the catalyst from the reaction system and reusing it for subsequent reaction cycles are indispensable in practical catalytic applications. Compared with homogeneous catalysts, the heterogeneous catalysts can be separated from the reaction system by complex methods, such as tedious centrifugation and filtration, which will inevitably suffer from progressive loss of catalysts. In this work, we successfully integrate magnetic Ni NPs into hybrid materials as a magnetically recoverable catalyst, which makes it easier for catalyst separation. The results of recycling use of 61 wt%Ni-CeO2-x/3 wt%Pd catalyst for styrene hydrogenation are presented in Figure 7b. The catalyst was successfully recycled for eight runs and displayed a negligible decline in activity for styrene hydrogenation, giving 100% conversion to ethylbenzene. To gain a deeper insight into the stability of the catalysts, TEM images of the Ni-CeO2-x/Pd catalysts before and after cycling test were shown in Figure S8. Only a little aggregation of the particles was observed after cycling test, which further confirms the good stability of our Ni-CeO2-x/Pd catalysts. The highly efficient activity and excellent reusability of our novel catalysts could be applied for other catalytic reactions. 12



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(a)

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(b)

Figure 7. (a) Kinetics of styrene hydrogenation over 61 wt%Ni-CeO2-x/3 wt%Pd catalyst. (b) Recycling of 61 wt%Ni-CeO2-x/3 wt%Pd catalyst used for hydrogenation of styrene.

CONCLUSIONS In conclusion, we have successfully fabricated a novel Ni-CeO2-x/Pd nanocatalyst composed of magnetic Ni NPs, oxygen deficient CeO2-x and active Pd NPs, which can be conveniently separated from the reaction system by a magnet. The optimal 61 wt%Ni-CeO2-x/3 wt%Pd catalyst shows outstanding catalytic activity and excellent reusability compared to other catalysts towards the hydrogenation of 4-NP with a rate constant of 0.0479 s−1 and styrene with a TOF of 6827 molstyrene molPd-1 h-1. Impressively, the catalysts exhibit superior stability when separated from the reaction system and recycled for 20 times for 4-NP and 8 times for styrene with negligible decline in activity. The excellent performance of Ni-CeO2-x/Pd nanocatalyst can be mainly attributed to the following aspects: (a) These hybrid nanostructures composed of oxygen deficient CeO2-x and metal nanoparticles are beneficial for reactant diffusion and active site exposure. (b) Abundant oxygen vacancies produced by Pd-catalyzed instant hydrogenation of CeO2 and the strong metal-support interaction between Ni NPs, Pd NPs and oxygen deficient CeO2-x support facilitates electron transfer during the hydrogenation reactions. (c) Ni species could form charge-transfer complex with benzene ring, which could strengthen the interaction between reactant and catalyst surface, thus promoting the hydrogenation performance of catalyst. Therefore, the synthetic strategy developed in this work 13


could be extended to

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prepare highly efficient, long-lasting, and easy recyclable catalysts for organic transformations. The combination of oxygen vacancy abundant CeO2-x support, magnetic Ni NPs and Pd active species may endow a wide range of potential applications in catalysis, especially in the recyclable and efficient catalytic production in organic chemistry, environmental and biological chemistry.

ASSOCIATED CONTENT Supporting Information Experimental details, material characterizations and 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] Notes The authors declare no competing financial interest.

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

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Graphical abstract:

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Pd Nanocatalyst with

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