Monodispersed Ni Nanoparticles Supported on Porous Glass

Mar 4, 2015 - We have put forward a facile and efficient strategy to immobilize Ni nanoparticles on porous glass by integrating two processes of ion-e...
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Monodispersed Ni Nanoparticles Supported on Porous Glass: Composition and Size Controllable Synthesis C. Shen, Y. Li, Y. J. Wang,* J. H. Xu, and G. S. Luo* State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: We have put forward a facile and efficient strategy to immobilize Ni nanoparticles on porous glass by integrating two processes of ion-exchange and reduction in one step. The in situ method avoids the use of high temperature and stabilizers which may reduce catalytic activity. The adsorption process could be described by a Langmuir model with the adsorption capacity of 46.4 ± 0.9 mg/g. Monodispersed Ni nanoparticles with a mean diameter of 1.8 nm showed high catalytic activity for cyclohexene hydrogenation. When the Ni content was 1.144 wt %, 100% conversion was obtained with the space time of only 17.7 s. Notably, the strategy may also be applied to other base metal nanoparticles. Cu and Co nanoparticles with the mean diameter of 5.7 and 3.4 nm were successfully immobilized, respectively. This method may be readily changed to prepare other supported transition-metal nanoparticles with catalytic properties and opens new avenues to coordination catalysis.

1. INTRODUCTION Metal nanoparticles have attracted increasing attention for application in a wide range of areas as they exhibit unique electronic, optical, and catalytic properties.1−5 Although the noble metal catalysts (e.g., Pd, Pt, and so on) have shown high activity, the high cost and limited availability discourage their further application. Much more intensive studies have been conducted on base metals to find alternative components to reduce or even replace the noble metals. A Ni-based catalyst which is cheap and needs mild reaction conditions for high yields is considered to be the most promising candidate.6−13 It is well-known that the synthesis method which results in different structural and textural properties of the catalyst plays an important role in the catalytic activity. Hence, a variety of methods, including precipitation, sonication, homogeneous deposition−precipitation, and sol−gel have been developed.11−22 Because of the simplicity in practical execution and facility in controlling the loading amount of active ingredient, impregnation has become the most popular method to prepare Ni nanoparticles.23−31 Nonetheless, the impregnation method generally requires an elevated temperature to complete the reduction of soluble metals and the use of stabilizers (polymers and some ligands) to inhibit agglomeration. Unfortunately, due to difficulties in the complete removal, the use of stabilizers always may result in deactivation and loss of catalytic activity of nanoparticles. Besides, as proven in our previous work,32 Pd nanoparticles synthesized by the one-step method in the presence of alcohol at room temperature showed smaller mean particles size compared with those prepared via the two-step method. In the later method, a reduction step conducted at high temperature with hydrogen gas is needed and it tends to result in the aggregation of metal nanoparticles and high energy consumptions. However, the developed one-step method does not test for the preparation of Ni nanoparticles. Other subsidiary conditions are still needed for reduction of Ni2+ ions. Therefore, apart from all the advantages of Ni nanoparticles, there has emerged a great challenge: how to prepare highly © 2015 American Chemical Society

dispersed Ni nanoparticles with high catalytic activity in a controllable and facile way. Herein we explored the feasibility of developing a novel in situ method to prepare monodispersed Ni nanoparticles supported on porous glass beads with an eggshell structure. To the best of our knowledge, this is the first report on the in situ synthesis of Ni nanoparticles supported on egg-shell structured porous glass beads. It is well-known that the support has significant influences on the characteristics of the prepared catalyst, for example by changing the average particles size and size distribution and also by varying the morphology.24,31 There may be three possible advantages for choosing porous glass beads as the support: first, as proven in our previous work,32,33 porous glass beads showed good ion exchange ability with heavy metal ions, and because of the strong interaction between the support and metal ions, it is easier to obtain smaller nanoparticles with uniform size distribution. Second, alkaline and alkaline earth elements which are contained in porous glass are reported to be good promoters for heterogeneous catalyst by increasing both the activity and stability.25,28 Third, porous glass beads are thermally and chemically stable, inexpensive, and environmentally friendly, offering a more efficient alternative as a catalyst support. Porous glass beads supported with Ni nanoparticles were characterized by scanning electron microscopy (SEM), highresolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and inductively coupled plasma atomic emission spectrometry (ICP, IRIS Intrepid II XSP from ThermoFisher Corp., America). The catalytic activity of the as-prepared composite material was measured by the hydrogenation of cyclohexene. The effects of residence time, catalyst content, and temperature on conversion have been investigated systematically. Received: Revised: Accepted: Published: 2910

December 4, 2014 March 3, 2015 March 4, 2015 March 4, 2015 DOI: 10.1021/ie5047522 Ind. Eng. Chem. Res. 2015, 54, 2910−2918

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2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Soda-lime glass microbeads with diameters ranging from 75 to 150 μm and composed of 59.7 wt % SiO2, 25.1 wt % Na2O, 9.8 wt % MgO, and 4.9 wt % CaO were obtained from Hebei Chiye Corporation. The beads were sieved before application, and beads with sizes ranging from 95 to 105 μm were taken as samples. Nickel acetate tetrahydrate was purchased from Sinopharm Chemical Reagent Co., Ltd., sodium hydroxide and cyclohexene were purchased from Aladdin Chemistry Co., Ltd. Analytical grade ethanol and hydrazine hydrate (80 wt %) were purchased from Tongguang Chemical Plant in Beijing, China. All chemicals were used as received without further treatment. 2.2. Preparation and Characteristic of Porous Glass Beads Supported with Ni Nanoparticles. The porous glass beads were prepared by the subcritical water treatment method described in our previous study.32,34 First, 200 g of water and 5 g of glass beads were placed in a tank reactor with a volume of 250 cm3. The reactor was then gradually heated to 573 ± 0.1 K, and the pressure was increased from atmospheric pressure to almost 8 MPa. The subcritical state was maintained for 60 min, and then the reactor was cooled naturally to room temperature. Afterward, the porous glass beads were washed several times with deionized water. It was proven in our previous work32 that palladium, silver, and gold ions could be reduced in the presence of alcohol under such mild conditions because they are good catalysts for oxidation, and Ni nanoparticles which show little catalytic activity for oxidation could not be reduced under the mild conditions. Hence, hydrazine hydrate which is a widely used reducing agent is applied in this work to get Ni nanoparticles. Different amounts of nickel acetate were dissolved in 40 g of ethanol with the initial concentrations of Ni2+ ranging from 100 to 700 ppm. Then 1.5 g of the porous glass beads, 0.20 g of NaOH, and 5 mL of hydrazine hydrate were added to the solution. The mixture was shaken for 2−8 h at 160 rpm in a temperature-controlled shaker at 333 K. The porous glass beads were separated from the solution by filtration and then washed with deionized water several times. Ni nanoparticles in the suspension mode were prepared in the same way, but there were no porous glass beads added. The two-step method is described as follows: 1.5 g of the prepared porous glass beads were added into 50 mL of the solution composed of nickel acetate and ethanol. The mixtures were shaken for 24 h at 160 rpm in a temperature-controlled shaker at 313 K. Then porous glass beads supported with Ni nanoparticles were reduced with hydrogen gas at a flow of 290 mL/min for 3 h at 573 K. The morphology of the porous glass beads supported with Ni nanoparticles was observed via SEM (HITACHI S-4500, HITACHI Ltd., Japan). TEM images were generated with a JEOL JEM-2011 high-resolution transmission electron microscope. Samples were characterized on a Bruker D8 Advanced powder X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). The amount of Ni element in the whole composite material was detected through ICP (ICP, IRIS Intrepid II XSP from ThermoFisher Corp., USA). The concentration of metal ions was measured using an atomic-absorption spectrophotometer (AAS, Z5000, Hitachi) with an air−acetylene flame. The amount of adsorbed metal is calculated as described in our previous work.35 2.3. Hydrogenation of Cyclohexene. Cyclohexene hydrogenation was performed in a tubular reactor which was

made of stainless steel with an inner diameter of 3 mm and the total length of 10 cm. In a typical run, the liquid reagent, cyclohexene dissolved in ethanol with an initial concentration of 1000 ppm was pumped at a predetermined flow rate of 0.4 mL/min by an advection pump. Hydrogen gas flowed from a cylinder through a mass flow controller and was mixed with the liquid reagent in a 1/16 in. tee. Then the mixture flowed into the tubular reactor packed with 1.2 g of the as-prepared catalyst. The reactor was immersed in a water bath to keep the temperature constant, varied from 313 to 343 K. The reaction pressure was adjusted at 0.2 MPa by a back pressure regulator (BPR). The reactor was connected to a phase separator with an inner diameter of 10 mm and height of 10 cm. The gas−liquid system flowed into the middle position of the phase separator and was separated into two phases immediately for the large density difference. The liquid product was analyzed by gas chromatography (GC) 7890A with a FID detector made by Agilent. The chromatographic column is HP-INNOWAX polyethylene glycol, model 19091N-133. The temperature of the injector, detector, and the oven is 533 K, 493 K, and 333 K, respectively. Nitrogen gas is used as the carrier gas with a flow rate of 30 mL/min. The flow rate of hydrogen gas and air is 30 and 300 mL/min, respectively. The yield of cyclohexane is calculated as eq 1, and the space time is defined as eq 2. Y=

τ=

Ccyclohexane C0cyclohexene

(1)

V v0

(2)

where Ccyclohexane means the concentration of cyclohexane after the reaction, while C0cyclohexene means the initial concentration of cyclohexane. V means the volume of the reactor, and v0 means the volume flow rate at the entrance.

3. RESULTS AND DISCUSSION 3.1. Loading Mechanism of Nickel Element. The change in colors of the porous glass beads before and after the loading process is shown in Figure 1. White porous glass beads turned to black after 2 h of shaking.

Figure 1. Change in colors of the porous glass before and after the loading process: (a) porous glass before the loading process; (b) porous glass after the loading process.

It is assumed that Ni2+ ions had been immobilized on the surface of porous glass via ion exchange. To prove the hypothesis, two groups of experiments were designed. In the 2911

DOI: 10.1021/ie5047522 Ind. Eng. Chem. Res. 2015, 54, 2910−2918

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Industrial & Engineering Chemistry Research first group, concentrations of both metal ions contained in porous glass (namely Na+, Mg2+, and Ca2+) and Ni2+ before and after the shaking process were detected on an atomicabsorption spectrophotometer (AAS, Z5000, Hitachi). Results are shown in Table 1. Before the shaking process, there were Table 1. Changes in Concentrations of Metal Ions before and after the Shaking Processa element

concentration before the shaking process (mg/kg) [mmol/kg]

concentration after the shaking process (mg/kg) [mmol/kg]

Ni Na Ca Mg

100.31 [1.71] / [/] / [/] / [/]

1.56 [0.03] 66.93 [2.91] 7.20 [0.18] 0.96 [0.04]

a

Figure 4. XRD patterns of Ni nanoparticles supported on porous glass.

The slash (/) indicates values below the detection limit.

were adsorbed onto porous glass through ion exchange with an adsorption capacity of 46.4 ± 0.9 mg/g. 3.2. Characteristics of Ni Nanoparticles Supported on Porous Glass. Surface morphologies of the support namely porous glass beads are shown in Figure S1 panels a and b in the Supporting Information, and surface morphologies after the immobilization of Ni nanoparticles are shown in Figure 3 panels a and b. Porous glass beads supported with Ni nanoparticles retained their spherical shape and were covered by a layer of uniform flakes. The as-prepared composite material is egg-shell structured as shown in Figure S2 in the Supporting Information, the inner part is compact (the density is 2.5 g/cm3) and the outer part is covered with glass flakes. Characterization of Ni supported on porous glass with X-ray diffraction is presented in Figure 4. The peaks are observed at around 44.5° and 51.8°, indexed to Ni(1 1 1) and (200) planes, excellent reflections of successful immobilization of Ni nanoparticles.

only Ni2+ ions in the solution, and after the shaking process, the concentration of Ni2+ ions declined sharply, while concentrations of Na+, Ca2+, and Mg2+ increased. According to our previous work,32 one Ni2+ ion would substitute two Na+ ions or one Ca2+ ion or one Mg2+ ion, thus, the result of the first group of experiments suggests that Ni2+ ions were immobilized onto porous glass via the ion exchange process. In the second group of experiment, the sorption isotherm was investigated. As shown in Figure 2a, with the increasing initial concentration of Ni2+, the loading amount increased steadily; besides, the adsorption capacity measured by experiment was 46.4 ± 0.9 mg/g. It is well-known that the adsorption should be a monolayer for the ion exchange process, and the adsorption of Ni2+ ions behaves in accordance with the Langmuir isotherm with a correlation coefficient of 0.999 as shown in Figure 2b, demonstrating that the adsorption is monolayer. In a nutshell, the experimental results confirmed the hypothesis that Ni2+ ions

Figure 2. (a) Adsorption capacities under different initial concentrations at 333 K; (b) adsorption isotherm fitting.

Figure 3. (a,b) Surface morphologies of porous glass supported with Ni nanoparticles. 2912

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Figure 5. continued

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Figure 5. TEM images of Ni nanoparticles supported on porous glass bead: Ni content of (a) 0.102, (b) 0.448, and (c) 1.144 wt %; histograms of particle size distribution for Ni nanoparticles supported on porous glass beads with Ni content of (d) 0.102, (e) 0.448, and (f) 1.144 wt %; TEM mages of the Ni-decorated porous glass beads with shaking time (g) 2, (h) 4, (i) 6, and (j) 8 h; histograms of particle size distribution for Ni-decorated porous glass beads with shaking time (k) 2, (l) 4, (m) 6, and (n) 8 h, respectively; (o) TEM image and (p) histograms of particle size distribution of supported Ni particles prepared by the two-step method (Ni content: 1.144 wt %); (q,r) TEM images of Ni nanoparticles in the suspension mode.

might be of great help for improving the catalytic activity. Besides, the mean diameter of Ni nanoparticles remained almost constant around 1.8 nm and did not increase obviously with the increase in Ni content. However, the nanocrystal of nickel supported on porous glass beads with different shaking time showed different mean particle size. TEM images of supported Ni nanoparticles (Ni content is 1.144 wt %) with the shaking time of 2 h, 4 h, 6 h, and 8 h are shown as Figure 5 panels g, h, i, and j, respectively, and the corresponding histograms of particle size distribution of samples are shown in Figure 5 panels k, l, m, and n, respectively. With the increase of treating time, the mean diameter of Ni nanoparticles increased

TEM images of supported Ni nanoparticles with the Ni content of 0.102, 0.448, and 1.144 wt % are shown as Figure 5 panels a, b, and c, respectively, and the corresponding histograms of particle size distributions are given in Figure 5 panels d, e, and f, respectively. The histograms were derived from the TEM images by surveying more than 150 particles. The average Ni particle size was quantified based on a number-weighted diameter (d̅ = Σnidi/Σni,ni is the number of counted Ni particles with a diameter of di) with values of 1.8, 1.9, and 1.8 nm with Ni contents of 0.102, 0.448, and 1.144 wt %, respectively. Obviously, Ni nanoparticles prepared in this work exhibited small mean particle size and narrow size distribution which 2914

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and gas was gradually intensified by increasing the gas flow rate. And better mixing of the two kinds of reagents resulted in higher yield. However, the space time decreased with the increase of gas/liquid ratio at the same. When the gas/liquid ratio is larger than 5, the space time is too short resulting in the decrease in yield due to the insufficient contact between the mixture and the catalyst. Ni content plays a more significant role in the catalytic activity as shown in Figure 7. With the increase in Ni content,

gradually. Combined with the change in the loading amount as stated above, the increased treatment time makes both the mean diameter and the loading amount of Ni nanoparticles become more controllable. To prove that the black points are Ni nanoparticles, TEM images with EDS are provided as Supporting Information, Figure S3 and the element contents determined by EDS are listed in Table S1. Ni content determined by EDS is 4.68 wt % which is much bigger than that determined by ICP (because the Ni content here is referred to the mass ratio of Ni nanoparticles to the glass flakes instead of the whole composite material). Porous glass beads supported with Ni nanoparticles were also prepared through the two-step method which was composed of ion exchange process and reduction with hydrogen gas at 573. The TEM image and the corresponding histogram of particle size distribution of the supported Ni nanoparticles prepared by the two-step method (with the Ni content of 1.144 wt %) are shown as Figure 5 panels o and p, respectively. Obviously, it is much easier to obtain Ni nanoparticles with smaller mean particles size through the in situ method. Avoidance of reduction at high temperature contributes to smaller particle size and less energy cost. To take a deeper insight into the effect of the support, Ni nanoparticles in the suspension mode were prepared in this work. And the TEM images are shown in Figure 5 panels q and r. An Ni nanoparticle without support showed much larger size distribution and bigger mean particle size. As for Ni nanoparticles in the immobilization mode, due to the strong interaction between the support and metal ions, it is easier to obtain smaller nanoparticles with uniform size distribution. However, the agglomeration is caused easily in the absence of stabilizers as for Ni nanoparticles in the suspension mode. Magnetic studies of Ni nanoparticles in the suspension mode and the immobilization mode are both investigated shown in Figure S4 in the Supporting Information. Other results are listed in Table S2. 3.3. Catalytic Activity of Supported Ni Nanoparticles for Cyclohexene Hydrogenation. To determine the catalytic performance of Ni nanoparticles supported on porous glass, effects of space time, Ni content, and reaction temperature have been studied systematically in relation to the change in yields. As shown in Figure 6, with the increase in ratio of gas to liquid, the space time decreased from 35.3 to 6.6 s, and the

Figure 7. Effect of Ni content on yields. Reaction temperature, 333 k; reaction pressure, 0.2 MPa; liquid flow rate, 0.4 mL/min; initial concentration of cyclohexene, 1000 ppm.

the yield increased steadily. When the Ni content increased from 0.226 wt % to 1.144 wt %, an increase in the yield from 70.6% to 100% was observed with a space time of only 17.7 s. The effect of reaction temperature was investigated, and the results are shown in Figure 8. The reaction rate was faster

Figure 8. Effect of reaction temperature on yields. Reaction pressure, 0.2 MPa; liquid flow rate, 0.4 mL/min; gas/liquid ratio, 5; initial concentration of cyclohexene, 2000 ppm.

at higher temperatures. For the catalyst with Ni content of 1.144 wt %, the yield increased from 58% to 100% when the temperature varied from 303 to 333 K, with fixed space time of 17.7 s. The hydrogenation of cyclohexene has also been investigated in other works.36−41 Nanoparticles of Ru, Rh, and Ir prepared in ionic liquid were used as the catalyst.36 A total time of 216 min was needed to achieve 95% conversion at 363 K using Ru nanoparticles as the catalyst, and the conversion decreased sharply to 42% when the reaction temperature changed to 348 K. Ir nanoparticles behaved the worst with a conversion of 36% after 205 min at 363 K. Co nanoparticles as one kind of base metals showed very little catalytic activity for cyclohexene

Figure 6. Effect of space time on yields. Reaction temperature, 333 k; reaction pressure, 0.2 MPa; Ni content, 1.144 wt %; liquid flow rate, 0.4 mL/min; initial concentration of cyclohexene, 1000 ppm.

yield increased first and then decreased. The maximal yield was 100% when the gas-to-liquid ratio was 5. The mixing of liquid 2915

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Figure 9. TEM images of Cu (a) and Co (b) nanoparticles supported on porous glass, respectively; histograms of particle size distribution for Cu (c) and Co (d) nanoparticles, respectively.

hydrogenation as reported by Redel.37 Only 0.8% of cyclohexene was converted after 3 h at 353 K, and the conversion increased to 1.1% after 18 h. Hydrogenation of cyclohexene over 10%Co/γ-Al2O3 and 10%Ni/γ-Al2O3 have been tested in the work of Lu and co-workers.38 However, the catalytic activity was almost zero. Therefore, supported Ni nanoparticles prepared in this work showed high catalytic activity. 3.4. Application for Other Metal Nanoparticles. To verify the universality of this method, it had been also applied to some other base metal nanoparticles. As shown in Figure 9, Cu nanoparticles with a mean diameter of 5.7 nm and Co nanoparticles with a mean diameter of 3.4 nm supported on porous glass beads would be obtained using this in situ method too.

about the in situ process, more research should be conducted in the future, such as the effects of the chemical properties of target ions and dynamic of crystal growth.

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ASSOCIATED CONTENT

* Supporting Information S

Surface morphologies of porous glass (Figure S1a,b); cross-section of porous glass supported with Ni nanoparticles (Figure S2); Ni nanoparticles immobilized on porous glass characterized by TEM and EDS, (Figure S3a,b); other results of EDS (Table S1); magnetic studies of Ni nanoparticles in the suspension mode and the immobilization mode (Figure S4); other results (Table S2). This material is available free of charge via the Internet at http:// pubs.acs.org.

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4. CONCLUSION A facile in situ strategy in which high temperature and stabilizers are avoided to immobilize Ni nanoparticles on porous glass beads with an egg-shell structure has been developed. Nickel ions were adsorbed on the surface of porous glass via ion exchange process and then reduced to metal nanoparticles. The ion adsorption process fit the Langmuir isotherm model well with a maximal adsorption amount of 46.4 ± 0.9 mg/g. On the basis of the strong interaction between nickel ions and the support, it is easier to obtain small nanoparticles with narrow size distribution. The monodispersed Ni nanoparticles prepared in this work show a mean diameter of 1.8 nm. Notably, the strategy reported here may also be applied to prepare other supported base metal nanoparticles with catalytic properties. For example, Cu nanoparticles with a mean diameter of 5.7 nm and Co nanoparticles with a mean particle size of 3.4 nm were successfully immobilized on the surface of porous glass, respectively. The as-prepared supported Ni nanoparticles showed high catalytic activity for cyclohexene hydrogenation. By using the catalyst with the Ni content of 1.144 wt %, 100% conversion of cyclohexene was obtained with a space time of only 17.7 s at 333 K. To understand deeply

AUTHOR INFORMATION

Corresponding Authors

*Tel: 86-010-62788568. Fax: 86-010-62788568. E-mail: wangyujun@ mail.tsinghua.edu.cn. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the support of the National Basic Research Foundation of China (Grant No. 2013CB733600) and the National Natural Science Foundation of China (91334201, U1463208, 21276140).

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DOI: 10.1021/ie5047522 Ind. Eng. Chem. Res. 2015, 54, 2910−2918