APTES-functionalized Fe3O4 microspheres supported Cu atom-clusters with superior catalytic activity towards 4-nitrophenol reduction

Accepted Manuscript Title: APTES-functionalized Fe3 O4 microspheres supported Cu atom-clusters with superior catalytic activity towards 4-nitrophenol reduction Authors: Yuanhong Zhong, Yan Gu, Lin Yu, Gao Cheng, Xiaobo Yang, Ming Sun, Binbin He PII: DOI: Reference:

S0927-7757(18)30185-7 https://doi.org/10.1016/j.colsurfa.2018.03.015 COLSUA 22340

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

5-12-2017 7-2-2018 6-3-2018

Please cite this article as: Zhong Y, Gu Y, Yu L, Cheng G, Yang X, Sun M, He B, APTESfunctionalized Fe3 O4 microspheres supported Cu atom-clusters with superior catalytic activity towards 4-nitrophenol reduction, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2010), https://doi.org/10.1016/j.colsurfa.2018.03.015 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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APTES-functionalized Fe3O4 microspheres supported Cu atom-

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clusters with superior catalytic activity towards 4-nitrophenol

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reduction

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Yuanhong Zhong a, Yan Gu a, Lin Yu a,*, Gao Cheng a, Xiaobo Yang b, Ming Sun a,

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Binbin He a

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a

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Institutions, School of Chemical Engineering and Light Industry, Guangdong University of

Key Laboratory of Clean Chemistry Technology of Guangdong Regular Higher Education

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Technology, Guangzhou, 510006, P. R. China.

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b

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Graphical abstract

Waygreen Technologies, Inc., Guangzhou, 511441, P. R. China.

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*

Corresponding author. Present address: No.100 Waihuan Xi Road, Guangzhou

Higher Education Mega Center, Panyu District, Guangzhou 510006, China Tel: +86 20 39322202; Fax: +86 20 39322231 E-mail: [email protected] (L. Yu) 1

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ABSTRACT

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A kind of APTES-functionalized Fe3O4 microspheres supported Cu atom-clusters (Cu-

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APTES@Fe3O4) has been successfully synthesized. The composite material was

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monodispersed with particle size of ca. 170 nm. Cu clusters (ca. 8-10 nm) containing about

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30-40 Cu atoms were trapped by the amino groups of APTES on the surface of Fe3O4

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microspheres. The reduction catalysis of Cu-APTES@Fe3O4 has been investigated using 4-

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nitrophenol as a probe molecule. Under the optimum conditions, i.e., 0.17 mmol L-1 of 4-

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nitrophenol, 0.067 mol L-1 of NaBH4 and 9.9 mg L-1 of Cu-APTES@Fe3O4, the as-obtained Cu-

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APTES@Fe3O4 catalyst showed superior reducibility. Almost complete reduction of 4-

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nitrophenol to 4-aminophenol was accomplished within 10 min reaction. The kinetic

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constant kapp was 0.27 min-1, and the calculated apparent activation energy was about 50.5

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kJ/mol, both of which were comparable to those for the reported noble metal-based

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catalysts. The desirable catalytic activity was attributed to the facilitation of electron transfer

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process by Cu atom-clusters, and the improvement of surface functionalization by APTES.

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Furthermore, the magnetic composite catalyst has satisfactory stability and reusability,

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which is a promising reducing agent for reduction of 4-nitrophenol to 4-aminophenol.

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Keywords: Cu-APTES@Fe3O4; Cu atom-clusters; Heterogeneous catalyst; Reduction; 4-

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nitrophenol

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1. Introduction

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Due to the excellent catalytic activities, noble metals, such as Au, Pt, Ru and Pd,

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have been widely used in catalytic reactions. However, the scarcity and corresponding

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high prices of noble metals make it difficult to meet the increasing demands[1, 2]. To

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reduce production cost and maximize the utilization of resources, considerable efforts

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to replace precious metals with naturally abundant nonprecious metals have been

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made using various approaches. Due to the excellent physicochemical properties and

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promising application in a variety of fields, transition metal nanoparticles have

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attracted intense scientific interest in the past few decades. As an important transition

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metals, with the specific surface structures and redox properties, the applications of

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copper (Cu) and Cu-based nanoparticles in the field of catalysis (including C-N, C-O

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and C-C coupling reactions [3], oxidations [4], and reductions [5]) have received

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widespread attention [6-8]. However, during the application processes, the colloidal

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Cu nanoparticles often suffer from the following drawbacks: (i) low stability in

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agglomeration and air-oxidation, (ii) difficult to be separated from the reaction media

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and recycled after the catalytic reaction. To overcome these disadvantages, a variety

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of supporting materials, such as charcoal [9], active carbon [10], ZnO/Al2O3 [7],

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mesoporous silica materials [11-13], NaY zeolites [14], CeO2 [15] and so on, have

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been successfully applied to avoid the aggregation, and thus improved the catalytic

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properties of Cu nanoparticles. Currently, numerous synthetic methods have been

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developed to fabricate various Cu and Cu-based nanomaterials, including co4

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precipitation, impregnation, microwave irradiation method, wet-chemical,

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photochemical, sonochemical and thermal decomposition [6]. As we know, the

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rational choice of the synthetic route is becoming increasingly important to regulate

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nanoparticles with desirable shape and size for catalysis application. Previous studies

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have largely focused on the varieties and properties of supporting materials, the

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interaction between Cu nanoparticles and supports, and some applications in catalysis

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fields [6, 16, 17]. Although the reported Cu-based composites have good catalytic

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activity, they were difficult to be recycled, which restricted their sustainable

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application in catalysis. Therefore, a recyclable and catalytically stable Cu catalyst is

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highly desired.

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Because the magnetic materials have unique magnetic separation property and inherent

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high thermal and mechanical stability, recently, new measures have been applied to stabilize

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Cu nanoparticles onto the surfaces of magnetic supports [18, 19]. Magnetite (Fe3O4) is one

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of the most important magnetic materials, which is a promising natural mineral. Due to its

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rich resources, high surface redox activity, strong electron transport capability, good

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compatibility with different substrates, and environmental friendly, the application of

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magnetite as supports in heterogeneous catalyst has become a hot topic [18, 20-22]. For

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instance, Colombo’s group [23] revealed that a kind of dumbbell-like Au0.5Cu0.5@Fe3O4

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nanocrystals exhibited high catalytic activity in CO oxidation reaction. Liu et al. [24] indicated

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that Cu nanoparticles anchored magnetic carbon materials (Cu&Fe3O4-mC) have favorable

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activity and separability on the catalytic reduction of 4-nitrophenol (4-NP), and could 5

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maintain good stability and reusability. Similarly, some magnetically retrievable nano-

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catalysts based on noble metals, e.g., Ag-Fe3O4 [25], Au-Fe3O4 [26, 27], Pt-Fe3O4 [28] and Pd-

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Fe3O4 [29] have also been widely reported. In recent years, many efforts have been

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progressively devoted to downsizing Cu particles to decrease cost and enhance catalytic

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activities [30], which still remains a major challenging problem in related fields. To the best

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of our knowledge, little literature is available till date regarding the work with Cu

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atoms/nano-clusters supported on (3-aminopropyl) triethoxysilane (APTES) functionalized

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Fe3O4 particles. APTES compound is an important silane coupling agent and is a widely used

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grafting agent to promote interfacial behavior of inorganic oxides, including magnetic iron

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oxide nanoparticles [31, 32]. Furthermore, APTES also known to have a preferential binding

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to Cu [33]. Thereby, APTES could serve as a “link-bridge” between Fe3O4 and Cu

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atoms/clusters, which may provide plentiful anchoring sites to trap single atom/cluster and

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prevent the agglomeration.

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Herein, we present an approach to synthesize Cu-APTES@Fe3O4, a composite material

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with Cu atom-clusters supported on Fe3O4 microspheres through APTES linkers. The micro-

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structure and physicochemical properties of Cu-APTES@Fe3O4 were evaluated in detail via

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various techniques. The heterogeneous catalytic activity of the catalyst was investigated by

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reduction of 4-NP to 4-aminophenol (4-AP) using NaBH4 as a reducing agent. As a phenolic

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compound, 4-NP is extensively used as a raw material in the production of insecticides,

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herbicides, textiles, petrochemical and pharmaceutical industries. It is carcinogenic and

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genotoxic to human beings and wildlife, which has been listed as a priority pollutant by US 6

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Environmental Protection Agency (EPA) [24, 34]. It has been reported that 4-NP was difficult

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to be removed through the traditional oxidizing methods, but it can effectively reduce to 4-

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AP to improve its biodegradability and abate its toxicity [35]. In addition, 4-AP has been

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known as a precious intermediate compound for synthesizing various kinds of

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pharmaceutical and plastic products [36]. Thus, it is of great practical value and important

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environmental significance for conversion of 4-NP to 4-AP [37]. Furthermore, the reusability

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of Cu-APTES@Fe3O4 was also discussed. The obtained results are of great significance for

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promoting the application of non-noble metal nanoparticles as heterogeneous catalyst, and

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providing a novel way to design the highly stable and active magnetic metal atom-clusters.

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2. Materials and Methods

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2.1. Materials and reagent

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All chemicals and reagents employed in this study were analytical grade and used as received

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without further purification, including iron chloride hexahydrate (FeCl36H2O), trisodium citrate

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(Na3C6H5O72H2O), urea, ethylene glycol (EG), 3-amino-propyl-tri-ethoxyl-silane (APTES), absolute

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ethyl alcohol, ammonia (28wt% aq.), copper (II) chloride (CuCl2), sodium borohydride (NaBH4) and

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4-nitrophenol (4-NP).

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2.2. Preparation of Fe3O4 microparticles

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Fe3O4 microspheres were prepared and used as the support material, which were

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synthesized by a modified solvothermal method as described in previous literatures [38, 39].

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FeCl36H2O was used as the only iron source, while EG acted as both solvent and reductant. The

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detailed experimental procedures for preparation of Fe3O4 microparticles are described in the

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Supporting Information (Text S1). The obtained Fe3O4 product was suspended in ethanol to form a

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ferrofluid, and stored in refrigerator (4-8 oC) before usage.

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2.3. Synthesis of Cu-APTES@Fe3O4 nanocomposite

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The synthetic procedure is as follows: 0.012 g CuCl2 was dissolved in 8 mL absolute ethanol at

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room temperature. 0.8 g APTES was added and stirred to form solution A. 2.0 mL of the prepared

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ferrofluid was diluted to 100 mL with absolute ethanol (0.25wt%), followed by ultrasonic

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dispersion for 2-3 min. The obtained colloidal solution was transferred to a 250 mL three-neck

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round- bottom flask. After adding 0.66 mL aqueous ammonia (28%) under ultrasonic condition, 8

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the solution A was added dropwisely to the above reaction system, and then stirred for 12 h at

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room temperature. Subsequently, 5 mL NaBH4 (0.1 mol L-1) was added slowly into the mixture, and

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stirred with ultrasonic for another 15 min. Finally, the black product was collected and separated

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by a magnet, and dried at 60 oC in vacuum for 12 h. In addition, the synthetic Cu/Fe3O4

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nanoparticles and APTES@Fe3O4 microspheres were applied as reference materials to compare

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the catalytic activities. The procedure for Cu/Fe3O4 was the same as for Fe3O4 microspheres except

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that CuCl2 (0.012 g) was added simultaneously with FeCl36H2O and urea. APTES@Fe3O4 was

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synthesized using the same procedure as for Cu-APTES@ Fe3O4 but CuCl2 was not added.

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2.4. Catalyst characterization

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X-ray diffraction (XRD) was collected on a RIGAKU ULTIMA-III instrument using Cu Kα

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radiation at room temperature. The recorded angular range was from 10o to 80o (2θ) with a

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scanning step width of 0.02o and speed of 4o min-1. Scanning electron microscopy (SEM) using JEOL

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JSM-7001F was to observe the morphology and particle size of the samples. Further structure

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information was analyzed with a High resolution transmission electron microscopy (HRTEM) using

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FEI Tecnai G20 S-Twin operating at 200 kV. Nanocrystal morphology, size distributions, and lattice

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fringes were analyzed with a Gatan software Digital Micrograph (TM) 3.7.4. The chemical

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elemental composition was analyzed with X-Max50 energy-dispersive X-ray analyzer (EDS, Oxford,

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UK) attached to HRTEM. X-ray photoelectron spectroscopy (XPS) was taken on a Thermo ESCALAB

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250XI multifunctional imaging electron spectrometer with monochromatic Al Kα radiation. N2

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adsorption and desorption isotherms were measured using Micromeritics ASAP 2020M. Brunauer-

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Emmett-Teller (BET) method was applied to estimate the specific surface area and pore volume. 9

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Pore size distributions were analyzed by Barrett-Joyner-Haleda (BJH) method using data of the

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desorption branch. Magnetic property tests were carried out on Lake Shore7410 vibrating sample

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magnetometer (VSM) at room temperature.

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2.5. Catalytic reduction of 4-nitrophenol

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The heterogeneous catalytic activity of Cu-APTES@Fe3O4 was evaluated by reduction of 4-NP to

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4-AP as a probe reaction. The reactions were carried out in a quartz cuvette and in-situ monitored

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using UNICO UV2800 UV-Vis spectrophotometer. In a typical test, 0.5 mL of 4-NP solution (1.0

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mmol L-1) was mixed with 2 mL of NaBH4 solution (0.1 mol L-1) and 0.5 mL deionized water at room

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temperature. Subsequently, 30 µL of Cu-APTES@Fe3O4 suspension (1.0 g L-1) was added to the

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reaction system. The residual 4-NP and generated 4-AP concentrations were simultaneously

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monitored by UV-Vis spectral changes in the range of 200-600 nm. The kinetic process of

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reduction reaction was fitted by pseudo-first order rate equation: −ln (At/A0) =kappt, where At and

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A0 are the absorbance at λ = 400 nm at time t and zero, respectively, and kapp is the apparent

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kinetic constant. After reaction, the catalyst Cu-APTES@Fe3O4 was separated from the reaction

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solution with a magnet, and washed three times with deionized water, then reused for

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subsequent 4-NP reduction runs under identical reaction conditions.

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3. Results and Discussion 10

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3.1. Microstructure and morphology of Cu-APTES@Fe3O4

Scheme 1 illustrates the possible processes for the formation of Cu-APTES@Fe3O4

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nanocomposites. It means that, in ethanol solution, Cu2+ bonded with the amino group on one end

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of APTES molecule to form a complex compound, while in the alkaline Fe3O4 suspension, the other

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end of APTES molecule (i.e., the ethoxyl-silane group) reacted with the surface hydroxyl (−OH) of

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Fe3O4 particles through dehydration condensation, forming −Si−O−Fe− bonds [40, 41].

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Consequently, Cu2+ cations anchored to the surface of Fe3O4 particles through Cu−N−C3–Si− chain,

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and they were reduced to Cu0 by reducing agent NaBH4. Generally speaking, the APTES

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functionalized the surface of Fe3O4 microspheres with ethoxyl-silane groups through dehydration

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condensation, and trapped the Cu nanoparticles with amino groups. A similar nanocomposite was

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reported by Mayne [33], in which a modified nanoparticle with Cu2+ bound to APTES, termed Si-

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APTES-Cu.

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Scheme. 1. Schematic image of the fabrication of Cu-APTES@Fe3O4. 11

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Fig. 1 shows the powder XRD patterns of naked Fe3O4 nanoparticles and Cu-APTES@Fe3O4

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nanocomposites. Both patterns exhibited the diffraction peaks at 2θ = 18.3°，30.2°, 35.5°, 43.3°,

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53.6°, 57.1° and 62.7°, which fitted well with the spinel structure corresponding to magnetite

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(JCPDF 19-629). A slightly broad (2θ =17-30°) and relatively weak intensities of the diffraction

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peaks can be seen in XRD pattern of Cu-APTES-Fe3O4, which may be attributed to the amorphous

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silane shell formed surrounding Fe3O4 core. It is noteworthy that the loading amount of Cu

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particles in the as-obtained Cu-APTES-Fe3O4 composites (2.53wt%) were far beyond the detection

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limit of XRD instrument (generally ca. 5wt%). Therefore, the diffraction peaks for Cu metal

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particles were unobserved.

Fig. 1. XRD patterns of Fe3O4 microspheres and Cu-APTES@Fe3O4 composites.

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The size and micro-morphology of the samples were further revealed by SEM and HRTEM. As

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shown in SEM image (Fig. 2a), Cu-APTES@Fe3O4 microcrystals were highly monodispersed,

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revealing a uniform spherical morphology with very narrow diameter distributions. The average

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diameter of these spheres was about 170 nm. Obviously, the surfaces of these spheres were

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highly rough, and each Fe3O4 microparticle was composed of many tiny Fe3O4 nanocrystals (top-

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right inset in Fig. 2a). TEM image (Fig. 2b) also exhibits the particles sized in ca. 170 nm with good

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dispersion, in accordance with the SEM results. Fig. 2c shows the interface between Fe3O4

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microsphere and one Cu atom-cluster. From the inset in Fig. 2c, the clear atomic lattice fringes

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could be observed. The measured lattice spacing (d values) of the crystallographic planes was

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approximately ca. 0.293 nm (Fig. S1), rather close to the {220} lattice planes of Fe3O4 crystal

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(JCPDS: 19-0629). It demonstrated that the microparticles were composed of Fe3O4 nanocrystal

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with high crystallinity. Additionally, a few tiny nanoparticles scattered onto the boundary of Fe3O4

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can been found, which were marked with red circles in Fig. 2b and 2c. The observed d values were

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approximately 0.21 nm (Fig. 2d and S1), and similar to that of {111} lattice plane of Cu crystal.

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These nanoparticles were 8~10 nm in size, suggesting that the formation of Cu atom-clusters

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deposited on Fe3O4 surface. Supposing that copper atoms were tightly stacked, the estimated

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number of Cu atoms (0.255 nm) [42] in each atom-cluster was in the range of 30-40.

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Fig. 3 displays the elemental mapping images across one individual Cu-APTES@Fe3O4

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microparticle, obtained by an energy-dispersive X-ray spectroscopy (EDS). The low resolution TEM

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image (Fig. 3a) illustrates the spheroidal structure of Cu-APTES@Fe3O4 microparticle with very

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rough surface.

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Fig. 2. SEM (a) and HRTEM images of Cu-APTES@Fe3O4 (b-d).

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The summation of all measured elemental mapping is presented in Fig. 3b, while the

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mapping results of specific elements (including Cu, C, N and Si) are illustrated in Fig. 3(c-h).

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Obviously, the Fe atoms were mainly distributed in the core of the microparticle. And the other

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elements, i.e., Cu, C, N and Si, were evenly distributed in a slightly bigger area. These

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phenomenons demonstrated that Cu atoms and APTES have been successfully deposited on the

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surface of Fe3O4 microspheres.

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Fig. 3. EDS elemental mapping analysis of Cu-APTES@Fe3O4

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The surface property of microparticles was investigated using XPS based on the specific binding

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energy. Fig. 4a delineates the full-range XPS spectrum of Cu-APTES@Fe3O4 composites. The

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composites contained elements of Fe, O, C, N, Cu and Si, showing characteristic peaks of each

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element as Fe 2p, O 1s, C 1s, N 1s, Cu 2p and Si 2p, respectively. Due to the characteristic doublet

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corresponded to Cu 2p3/2 and Cu 2p1/2 of Cu0 and Cu2 O are pretty closed, it is difficult to

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distinguish between Cu0 and Cu2O based on XPS spectral features. Therefore, the Cu 2p spectrum

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was fitted in the assignment of Cu0 and CuO. As showed in Fig. 4, the measured Cu 2p3/2 (Cu 2p1/2)

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binging energies were 932.7 (952.4 eV) for Cu0 and 934.7 (954.6 eV) for CuO. In addition, there

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were two small shake-up satellite peaks located at ca. 942.0 and 962.7 eV, which corresponded to

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unfilled Cu 3d shell of CuO [43]. It means Cu0 and CuO species coexisting on the nanocomposite

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surfac e. The occurrence of the CuO was possibly attributed to the air-oxidized surface of Cu to

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CuxO (Cu2O and CuO) during sample storage and the XPS preparation procedure. Combined the

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XPS fitting results (Fig. 4) with the above HRTEM analysis (Fig. 2), it can be inferred that Cu0 in

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nanocomposite played the dominant role. These results are in good agreement with the previous

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reports that the CuxO were usually coexisting on a Cu0 surface [44-46]. Fig. 4c exhibits the N 1s

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spectra of of Cu-APTES@Fe3O4 and APTES@Fe3O4. The N 1s peak shifted to higher energy by 0.27

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eV after Cu loading. The N 1s spectrum can be resolved into two peaks at about 399.8 eV and

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398.8 eV, respectively (Fig. 4c, i). Peak 1 located at 399.8 eV was close to the N 1s spectrum of

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pure APTES, corresponding to free amino groups without bonded to Cu in the composite

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materials. With the introduction of Cu, peak 2 located at 398.8 eV showed an obvious shift of

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binding energy, which can be attributed to the linkage of Cu nano-cluster and amino group. It

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revealed that two types of N atoms co-existed in the composite materials, and the amino group of

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APTES should be combined with Cu atom-cluster. Considering that the observed N/Cu atomic ratio

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in Fig. 4a was approximately 3:1 (data not shown), it is reasonable that amino groups existed

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simultaneously in bonded and unbonded form. As for Fe 2p, the doublet peaks located at 723.7 eV

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and 710.6 eV were ascribed to Fe 2p1/2 and Fe 2p3/2 of Fe3O4, respectively. These results are in

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accordance with the previously reported spectra of Fe3O4 [47-49]. It further implies that the bulk

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of the composite material retained the highly crystalline structure of Fe3O4, but not γ-Fe2O3.

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Fig. 4. The XPS spectra of Cu-APTES@Fe3O4, (a) survey spectrum (b) Cu 2p of Cu-APTES@Fe3O4, (c, i) N 1s of Cu-APTES@Fe3O4, (c, ii) N 1s of APTES@Fe3O4 (d) and Fe 2p of Cu-APTES@Fe3O4.

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Fig. 5 displays the N2 adsorption-desorption isotherms and BJH pore size distribution analysis

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of Fe3O4 microspheres and Cu-APTES@Fe3O4 composites, respectively. Both the isotherms

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represented the typical type IV curves accompanied by a type H1 hysteresis loop, suggesting the

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presence of interparticle and non-ordered mesoporous networks in the samples. The BET specific

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surface area and the total pore volume of Fe3O4 microspheres were 83.12 m2 g-1 and 0.07 cm3 g-1,

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respectively. These values of Cu-APTES@Fe3O4 were 88.61 m2 g-1 and 0.15 cm3 g-1. The specific

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surface area of Fe3O4 after loading Cu-APTES did not change obviously, but the pore volume was

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improved greatly. The pore sizes of Fe3O4 microsphere were narrowly distributed around 3.6 nm

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according to the BJH model (inset of Fig. 5a), suggesting the creation of mesoporosity. While in

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sample Cu-APTES@Fe3O4, a wide range of pore distribution, i.e., 20-90 nm, can be observed (inset 17

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of Fig. 5b). It is suggested that the mesoporous Fe3O4 spheres became swelled after functionalizing

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by APTES with the C3 chains.

Fig. 5. The N2 adsorption-desorption isotherms and (insets) pore size distribution from the desorption branch of (a) Fe3O4 micro (b) and Cu-APTES@Fe3O4.

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The magnetic properties of mesoporous Fe3O4 microspheres and Cu-APTES@Fe3O4 composites

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have been measured (Fig. S2). Both of them displayed the typical superparamagnetic

266

(ferromagnetic) behavior, suggesting that the introduction of Cu-APTES did not destroy the

267

inherent magnetic property of Fe3O4 microspheres. However, the saturation magnetization value

268

decreased from 67.4 emu g-1 to 39.3 emu g-1, indicating the non-magnetic Cu-APTES parts led to

269

the decrease of saturation magnetization of Cu-APTES@Fe3O4 composites. The magnetic property

270

enables the materials to be easily and completely separated from suspensions in aqueous or

271

organic solvents using a magnet, which is of great significance for repeated recycling.

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3.2. Catalytic activity of Cu-APTES@Fe3O4 composites

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The reduction of 4-NP was used as a probe reaction to investigate the catalytic performances

274

of Cu-APTES@Fe3O4 composites (Fig. 6). Generally, the reduction of 4-NP to 4-AP in the presence 18

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of NaBH4 is a thermodynamically feasible process, i.e., E0 for H3BO3/BH4− = -1.33 V and for 4-NP/4-

276

AP=-0.76 V versus NHE [50], but is a kinetically unfavorable process owing to the large potential

277

difference between donor and acceptor couples decreases the feasibility of this reaction.

278

Reportedly, without a catalyst, this reaction did not carried out even in two days [50], while the

279

appropriate catalyst can overcome the kinetic restrictions by facilitation electron relaying from the

280

donor BH4− to the acceptor 4-NP [51]. As shown in Fig. 6a, the light yellow 4-NP aqueous solution

281

exhibited a maximum absorbance at 317 nm. After adding NaBH4, the aqueous solution changed

282

to deep yellow along with a red shift of the absorbance peak to 400 nm, which was ascribed to the

283

formation of the 4-nitrophenolate anions in alkaline solution [52, 53]. With the addition of Cu-

284

APTES@Fe3O4 catalyst, the reduction reaction was significantly activated (Fig. 6b, 6c). 4-NP was

285

rapidly reduced as the absorbance at λ=400 nm diminished with fading of the yellow color of the

286

solution, and simultaneously a new peak at λ = 295 nm appeared and gradually increased. This

287

phenomenon revealed the formation of reduction product 4-AP (λmax= 295 nm). From Fig. 6c, it

288

can be seen that the peaks at 400 nm decreased rapidly, and totally disappeared within 10 min

289

after adding Cu-APTES@Fe3O4. Due to the concentration of BH4− was much higher than 4-NP, the

290

reaction rate constant can be assumed to be independent of the concentration of BH4−. The good

291

linear correlation of −ln (At/A0) versus time plot (Fig. 6d) indicated the reduction 4-NP process

292

followed pseudo-first-order kinetic under the tested conditions. The fitted apparent kinetic

293

constant kapp was 0.27 min-1 at room temperature. The catalytic efficiency was more active than

294

the previous Cu- and noble metal- based catalysts, and was comparable to that of the magnetic Au

295

nanocatalyst reported by Chen’s group [54] (Table 1). 19

Fig. 6. UV-vis spectra of 4-NP before and after the addition of NaBH4 (a), reduction of 4-NP by CuAPTES@Fe3O4 catalysts with NaBH4 (b and c), plots of −ln (At/A0) versus time (d). Conditions: 0.5 mL of 1.0 mmol L-1 4-NP, 2 mL of 0.1 mol L-1 NaBH4 and 30 µL of 1.0 g L-1 catalyst, 25 oC.

296

It has been widely demonstrated that the heterogeneous reduction of 4-NP is surface-

297

controlled and can be explained in terms of the Langmuir-Hinshelwood mechanism [55, 56]. It

298

means the reduction reaction probably proceeded in three steps: (i) BH4− ions were first adsorbed

299

on the surface of Cu atom-clusters, then the activated hydrogen atom formed from hydride after

300

the electron transfer to the Cu atom-clusters; (ii) the 4-NP was diffused and reversibly adsorbed

301

onto the surface of Cu atom-clusters; (iii) The electron transferred from Cu to the nitro group of 4-

302

NP, meanwhile, the adsorbed 4-NP was also attacked by the activated hydrogen atom, then 4-AP

303

was formed after several steps of hydrodeoxygenation reactions. The reaction of adsorbed 4-NP

20

304

with the surface-hydrogen species has been identified as the rate-determining step [57]. Thus, the

305

high activity of Cu-APTES@Fe3O4 can be mainly ascribed to the small size of Cu nanoclusters (ca. 8-

306

10 nm) on Fe3O4 surface, which could remarkably facilitate the transfer of electrons and further

307

help to overcome the kinetic barrier of the reduction reaction.

308

During the pre-experiment, the control experiment on bare Cu atom-clusters for the catalytic

309

reduction of 4-nitrophenol has been performed (Data not showed). The bare Cu atom-clusters

310

showed a pretty good catalytic activity, and even more efficient than that of the catalyst Cu-

311

APTES@Fe3O4. However, the bare Cu atom-clusters showed low stability and very difficult to be

312

recovered from the liquid-phase reaction medium, which would seriously affect the sustainable

313

utilization of the Cu-clusters and lead to high cost. The catalytic activities of mesoporous Fe3O4

314

microspheres and co-precipitated Cu/Fe3O4 have also been tested comparably under the same

315

conditions (Fig. S3). It can be seen that the concentration of 4-NP did not obviously decrease in the

316

presence of Fe3O4 microspheres, indicating the reduction of 4-NP to 4-AP was hardly activated by

317

Fe3O4 alone. In contrast, the catalytic activity of Cu/Fe3O4 was greatly improved by loading Cu on

318

Fe3O4 surface (Fig. S3b), but the conversion efficiency of 4-NP was also much lower than that of

319

Cu-APTES@Fe3O4. The kinetic constant kapp (0.021 min-1) in Cu/Fe3O4 system was found to be one

320

order of magnitude smaller than the later one. The enhancement of catalytic activity of Cu-

321

APTES@Fe3O4 may be also attributed to the contribution of APTES on Fe3O4 surface: (i) the APTES

322

played an important role as “link-bridge” to preserve the copper atom-clusters highly dispersed;

323

(ii) the introduction of APTES led to a richer pore structure of the catalyst, which was supported by

324

the pore size distribution analysis; (iii) the amino group in APTES was hydrophilic and weakly basic, 21

325

which not only improved the hydrophilicity of the magnetite surface but also improved the

326

adsorption of 4-NP onto the surface of catalyst.

327

Fig. 7 illustrates the kinetics process of 4-NP reduced by NaBH4 with Cu-APTES@Fe3O4

328

catalyst in the temperature (T) range of 25-50 oC. The kinetics constants k from Fig. 7a were read

329

directly from the slope of the fitted lines, then the Arrhenius plot of lnk versus T-1 can be obtained

330

(Fig. 7b), which revealed to be a satisfactory linear relationship. The calculated apparent activation

331

energy (Ea) was about 50.5 kJ/mol. This Ea value was comparable to those of the reported noble

332

metal-based catalysts, e.g., 44 kJ mol-1 for Pd nanoparticles [58], 52 kJ mol-1 for Au nanoparticles

333

[54], 55 kJ mol-1 for partially hollow Au nanoboxes, 44 kJ mol-1 for hollow Au nanoboxes and 28

334

mol-1 for Au nanocages [59]. It is suggested that the Cu-APTES@Fe3O4 catalyst without using a

335

precious metal but with a considerable activation energy, is a promising reducing agent for

336

reduction of 4-NP to 4-AP.

Fig. 7. Plots of −ln (At/A0) versus time for the reduction of 4-NP by NaBH4 at different temperature (a), and Plot of lnk versus 1/T (b). Conditions: 0.5 mL of 10-3 mol L-1 4-NP, 2.5 mL of 0.1 mol L-1 NaBH4 and 30 µL of 1.0 g L-1 Cu-APTES@Fe3O4 catalyst.

337 22

338

Table 1. Comparison of the catalytic performance of the reported catalysts for the reduction of 4-NP to 4-AP in excess NaBH4 Catalyst Cu-APTES@Fe3O4 CuO nanostructures with different morphology Co (1.0-5.1%) doped CuO Cu&Fe3O4-mC CuO Ag/PAN CFN Calcium-Alginate-Stabilized Au Nanoparticles Calcium-Alginate-Stabilized Ag Nanoparticles Ag/TiO2 composites (TPHtAg2), under visible light Ag nanoparticles on tea polyphenols-modified graphene magnetically recoverable Au nanocatalyst Pd immobilized in colloidal carrier Au nanoparticles with different morphology

340

C0(4-NP), mol L-1 0.17×10-3 6.6 ×10-5

C0(catalyst), mg L-1 9.9 33.1

C0(NaBH4), mol L-1 0.067 0.099

Reaction time/min 10 20-34

Reduction efficiency 100% >90%

kappa, min-1 0.27 0.072-0.117

E a, kJ mol-1 50.5 15.36-26.24

Reference

0.12×10-3 2.0×10-3 0.25×10-3 6.5×10-2 1.0 ×10-4

2.0 500 0.02-0.1mol % 166.7 1.2

0.008 0.019 0.5 1.47×10-3 0.1

35-3.5 30 25-30 70 50

100% 100% 100% 100% 92%

0.2-2.63 — 0.0781-0.0885 0.038-0.085 0.14-0.20×10-5

— — — — 4.9

[43] [24] [60] [61] [50]

1.0 ×10-4

1.2

0.1

8

99%

1.03-1.04×10-5

3.3

[50]

1.8×10-4

1.0

3.3×10-3

10

100%

1.5

—

[35]

1.0 ×10-4

0.5

1.0×10-2

12

100%

0.20

—

[62]

1.0×10-3

0.98

3.3×10-3

6-10

100%

0.748

52

[54]

1.0×10-4

0.25-2.88

1.0×10-2

20

100%

1.5-4.41×10-3

44

[58]

1.4×10-4

—

4.2×10-2

8

100%

0.20-2.83

28-55

[59]

339

a:

This study [52]

Measured at 25 oC or room temperature.

23

341

Using the 4-NP as a probe reaction, we also studied the stability and recyclability of Cu-

342

APTES@Fe3O4. The used Cu-APTES@Fe3O4 nanocomposite can be easily recovered and

343

reused for more than nine consecutive recycles (Fig. 8). It still retained the highly efficient

344

catalytic activity, and more than 92% 4-NP was reduced in the 9th run. It should be

345

mentioned that, in each cycle, the catalyst was separated from the reaction media, rinsed

346

with water, and subjected to fresh 4-NP solution again, which would result in a slight loss of

347

the catalyst. This was probably the main reason for the slight decrease of efficiency in

348

reduction of 4-NP. It is clear that Fe and Cu leaching was quite important for the application

349

of Cu-APTES@Fe3O4 as a reusable catalyst. In the present study, the Fe and Cu leaching from

350

the catalyst in the reaction solution was also tracked. It showed that the final concentration

351

of dissolved Fe and Cu in reaction system was below the detection limit. These results

352

demonstrated that the Cu-APTES@Fe3O4 nanocomposites have satisfactory stability and

353

reusability.

354 355

Fig. 8. Conversions of 4-NP to 4-AP over recovered Cu-APTES@Fe3O4 catalyst.

24

356

357

4. Conclusions

A magnetically recoverable catalyst consisting of Cu atom-clusters supported on

358

APTES@Fe3O4 has been fabricated. The mesoporous Fe3O4 microparticles used as a support

359

material were highly dispersed and sized in about 170 nm. Each microparticle was composed

360

of many tiny Fe3O4 nanocrystals. The Cu clusters (ca. 8-10 nm) containing about 30-40 Cu

361

atoms were anchored on Fe3O4 microspheres through the amino groups of APTES linkers.

362

Cu-APTES@Fe3O4 composites exhibited excellent catalytic activities for reduction of 4-NP to

363

4-AP. The reduction 4-NP process followed the pseudo-first-order kinetic process under the

364

tested conditions, which can be explained in terms of the Langmuir-Hinshelwood

365

mechanism. The high activity of Cu-APTES@Fe3O4 can be ascribed to two main reasons: (1)

366

the small size of Cu clusters on Fe3O4 surface could remarkably facilitate the transfer of

367

electrons and help to overcome the kinetic barrier of the reduction reaction; (2) APTES

368

played an important role as “link-bridge” to preserve the copper self-agglomeration, and

369

simultaneously improved the adsorption of 4-NP onto the catalyst surface. In addition, Cu-

370

APTES@Fe3O4 has satisfactory stability and reusability, and its Ea value was comparable to

371

those for the reported noble metal-based catalysts. It is a promising catalyst for reduction of

372

4-NP to 4-AP or other organic transformation reactions.

373

374

375

Acknowledgements 25

376

We gratefully acknowledge the financial support from the National Natural Science

377

Foundation of China (No. 41602031, 21576054), the Scientific Program of Guangdong

378

Province (No. 2016B020241003), and the China Postdoctoral Science Foundation

379

Funded Project (No. 2016M592464).

380 381 382

Appendix A. Supplementary data The supplementary materials can be found in the online version.

383

26

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31