Aminosilane decorated carbon template-induced in situ encapsulation of multiple-Ag-cores inside mesoporous hollow silica

Abstract

A dispersed and stable catalytic active phase and a protective reaction environment are acknowledged as ideal metal catalyst characteristics. In this paper, a cores@shell structure of microreactor with a well-dispersed active phase of multiple free-Ag-cores, hollow cavity and protective mesoporous shell was prepared by a simple and novel construction approach. The organic ligand of aminosilane (APTES) was directly incorporated on carbon nanospheres to anchor Ag ions as a metallotemplate, which avoids the tedious steps of conventional methods, and then the sacrificed metallotemplate was employed for directly fabricating the special hollow mesoporous silica microreactors. As a result, multiple free active Ag-cores were in situ produced and encapsulated in the cavity of hollow mesoporous silica during the thermal process. The important evidence of the configuration including a big hollow cavity containing active multiple-Ag nanoparticles and a mesoporous SiO₂-shell can be demonstrated with efficient techniques including XRD, FT-IR, XPS, BET, SEM and TEM. Just as expected, the catalyst as a functional microreactor exhibited a high catalytic activity for the liquid-reduction of 4-nitrophenol, and the increasing dosage of used catalyst contributes to the enhancing of catalytic activity. The methodology demonstrated here provides a new insight for the fabrication of versatile functional nanomaterials with noble or transition metals inside a hollow shell.

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1. Introduction

The design and fabrication of nanospherical materials with hollow interiors exhibited huge potential uses as low-density capsules for the controlled release of drugs,^1,2 development of artificial cells,³ protection of enzymes,⁴ proteins⁵ and DNA,⁶ and especially for catalysis.^7–9 Hollow mesoporous silica spheres are regarded as ideal nanoreactors due to their narrow mesochannels in the shell and large hollow intra-cavities, which have attracted considerable attention in nanomaterials fields.^10–13 This special morphology of carrier can accommodate the catalytic active phase and reactants and allow the chemicals to be transported in and out of the reactor via mesochannels, which could efficiently improve the conversion of reactants and selectivity of products.

Generally, contact and collision between a reactant and catalytic active sites and diffusion of chemicals in microscale reactors are regarded as key factors in determining the conversion and selectivity of chemicals. Therefore, a functional nanoreactor, possessing a dispersed active phase and a large diffused region, would be expected to obtain high catalytic activity. Fully functional nanoreactors can be realized using porous hollow spheres and attaching a highly-efficient catalytic active phase. So far, many attempts have been employed to encapsulate some special chiral metal complexes or noble metals into mesoporous silica nanocages just like a ship-in-a-bottle.^11,14–17 The resulting catalytic system shows remarkable activity and recyclability for asymmetric synthesis and other kinds of reactions. Noble metal nanoparticles (NPs), especially silver nanoparticles, have been widely studied due to their unique catalytic activities in hydrogenation reduction and compelling cost-efficiency in practical applied processes as compared to other counterparts.^18,19

A study found that restricting noble metal NPs in an isolated nanocavity can effectively avert some defects such as noble metals’ aggregation, sintering and rapid decay of catalytic activity.¹⁵ In order to obtain the isolated active phase, single core@porous-shell architectures have been proposed and well applied in heterogeneous catalysis.^12,20–22 The results revealed that the freely movable core contributes to high catalytic activity, and its hollow cavity and the protective shells provide a homogenous environment for the heterogeneous catalysis. However, a more attractive structure possessing multiple active cores, a hollow cavity and a porous shell is obviously superior to the previous one due to the large contribution of multiple free active cores for enhancing the collision frequency with reactant molecules.²³ Some strategies have been developed to combine these advantages for obtaining the expected functional catalysts.^23–25 The synthesis of these special nanoreactors mainly involves the modification of a sacrificial template through depositing noble metal particles on their surface.^15,26,27 However, there is still plenty of room for further improvement using new synthetic technology to simplify the tedious steps of conventional methods. As we know, the surface of a carbon sphere from the hydrothermal method is enriched with hydroxide radicals, and these active groups on the carbon sphere can be well utilized by tethering other special species to obtain functional carbon spheres. However, so far, there is still not any report involving organic modification of carbon spheres and their further application to anchor metal ion species. Inspired by these thoughts, a new tactic involving organic modification of carbon spheres and their further utilization as functional hard templates to prepare hollow microreactors is desirable to develop.

In this study, we reported an approach, for the first time, to in situ encapsulate multiple active Ag-core nanoparticles inside hollow mesoporous silica spheres for further improving the efficiency of the core@shell approach. The carbon sphere was first decorated with a metal-capturing agent of aminosilane (APTES) to anchor Ag ions. Then, the Ag–aminosilane functional carbon sphere acted as a sacrificial template for preparing the special hollow mesoporous microreactors. In this structure, the free Ag nanoparticles within the inner cage of the hollow mesoporous silica sphere as catalytic active sites were fully accessible for reactants. The specific synthetic route is illustrated in Scheme 1. The involved samples were characterized using different techniques for demonstrating their morphology and structure, the status of the Ag NPs and the proposed synthetic method. Finally, the obtained hollow Ag@meso-SiO₂ was applied for the catalytic reduction of 4-nitrophenol to evaluate its catalytic performance and different amounts of Ag@meso-SiO₂ were utilized to investigate the effects on the reaction.

Scheme 1 Illustration of the preparation of Ag@meso-SiO₂.

2. Experimental

2.1. Chemicals

Glucose, tetraethylorthosilicate (TEOS), ammonia solution (25 wt%), silver nitrate (AgNO₃), L-arginine and cetyltrimethylammonium chloride (CTAC) were purchased from Sinopharm Chemical Reagent Co., Ltd. 3-Aminopropyltriethoxysilane (APTES) was purchased from the Tokyo TCI (Shanghai) Development Co., Ltd. 4-Nitrophenol and sodium borohydride were purchased from Aladdin. Commercial Ag nanoparticles were purchased from Zhuhai Najin Technology (Guangdong) Co., Ltd. All the chemicals were used as received without further purification.

2.2. Synthesis of multiple-Ag-core hollow microreactors

2.2.1. Synthesis of carbon nanospheres (CNs). The synthesis of the carbon spheres is based on that in ref. 15. Typically, glucose (9 g) was dissolved in 60 mL of water to form a clear solution and then transferred into an 80 mL Teflon-sealed autoclave. The autoclave was maintained at 190 °C for 5 h. The products were separated by centrifugation, followed by washing three times using water and ethanol and finally oven-dried at 80 °C for further use.

2.2.2. Silanization of CNs and anchoring of Ag ions. In this typical process, 100 mg of carbon nanospheres was dispersed in 100 mL of deionized water with ultrasonication for 45 min as part A. 0.2 mL of APTES was dissolved in 6 mL of deionized water as part B. Parts A and B were mixed together and reflux condensed in a 75 °C water bath for 6 h. Then the suspension was centrifuged and washed with distilled water five times. The resulting product was designated as NH₂-CNs. Then, the NH₂-CNs composite obtained from the last step was redispersed in the aqueous solution containing 0.03 g of AgNO₃ and stirred for 4 h. Finally, the suspension was centrifuged and washed again and the final product was labelled as Ag/NH₂-CNs.

2.2.3. Preparation of Ag@meso-SiO₂. Subsequently, the Ag/NH₂-CNs composite was redispersed in 40 mL of H₂O, 30 mL of ethanol, 0.132 g of CTAC and 568 μL of NH₃·H₂O with ultrasonication for 20 min. Then 150 μL of TEOS was added, and the mixture was vigorously stirred for 6 h. The precipitate was harvested after centrifugation and washed with distilled water and with ethanol three times, then dried at 60 °C for 6 h. Then the product was calcined at 550 °C for 6 h, then in an air atmosphere for 6 h to remove carbon spheres, the CTAC template and other organic species. The finally obtained Ag@meso-SiO₂ product was further employed as a catalyst for the liquid-phase reduction of 4-nitrophenol reaction.

2.2.4. Synthesis of comparative catalysts. Ag-supported SBA-15 materials were synthesized according to the reported literature.²⁸ Typically, 1 g of P123, 30 g of deionized water and 3 g of nitric acid were mixed and stirred for 1 h at 35 °C until a homogenous solution formed under acidic conditions. Meanwhile, 0.03 g of silver nitrate was added to the solution and stirred for 1 h. Then, 2.13 g of TEOS was added into the solution and stirred for 20 h at 35 °C in the dark. The derived solution was aged at 100 °C for 48 h. The product was filtered off and rinsed with deionized water several times. The resulting powders were calcined at about 550 °C for 5 h in air to obtain Ag/SBA-15.

The synthesis of the single core Ag@SiO₂ material is based on the reported literature.²⁹ 7.5 mL of an aqueous solution of glucose (30 mg mL⁻¹) was added into a mixture of water and CTAB (50 mg in 40 mL) previously heated at 80 °C for 30 min followed by dropwise addition of an aqueous solution of silver nitrate and arginine (54 mg and 56 mg, respectively, in 1.5 mL of water). After 3 min, an additional amount of CTAB (50 mg) was added to the solution, followed by dropwise addition of TEOS (1060 μL). The resulting brown solution was stirred at 500 rpm at 80 °C for 3 h. Finally, the resulting sample was collected, washed and calcined at 550 °C for 5 h in air to obtain single core Ag@SiO₂.

2.3. Characterization

X-ray diffraction patterns were recorded using a Smartlab™ 9 kW powder diffractometer with a monochromatic Cu K_α radiation source (λ = 1.5406 Å).

Fourier transform infrared (FT-IR) spectra of the samples were obtained in the range of 400–4000 cm⁻¹ with powders dispersed in KBr on a Bruker VECTOR22 spectrometer.

The X-ray photoelectron spectra (XPS) were performed on a PHI 5000 Versa Probe X-ray photoelectron spectrometer equipped with Al K_α radiation (1486.6 eV). The C 1s peak at 284.6 eV was used as the reference for binding energies.

The N₂ (77.4 K) adsorption–desorption measurements were carried out with a Micromeritics ASAP-2000 instrument in a relative pressure range P/P₀ from 0.01 to 0.99, and before the measurements, the calcined samples were outgassed under vacuum at 150 °C for 5 h. The specific surface area and pore size distribution were calculated by the Brunauer–Emmett–Teller (BET) method and Barrett–Joyner–Halenda (BJH) method, respectively, wherein the BET method is based on the multilayer molecular adsorption theory and the BJH method is based on a developed Kelvin capillary condensation theory.³⁰

Field emission scanning electron microscopy (FE-SEM) was performed on a Hitachi S4800 Field Emission Scanning Electron Microscope.

High-resolution transmission electron microscopy (HRTEM) images were recorded on a JEM-2010 EX microscope equipped with X-ray microanalyzer EDX, which was operated at an accelerating voltage of 200 kV. The samples were crushed in A.R. grade ethanol and the resulting suspension was allowed to dry on a carbon film supported on copper grids.

The catalytic reduction of 4-nitrophenol was detected by UV-visible absorption spectra and its results were recorded using a BLV-GHX-V ultraviolet spectrophotometer.

The silver content was analyzed using a Jarrell-Ash 1100 Inductively Coupled Plasma (ICP) spectrometer. Before analysis, the silver NPs in the sample were dissolved by adding nitric acid, and then hydrofluoric acid was added to dissolve the mesoporous silica.

2.4. Catalytic test

The catalytic performance of the Ag@meso-SiO₂ was evaluated using the liquid-phase reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with an NaBH₄ aqueous solution at room temperature. The catalytic reaction was carried out in a quartz cuvette and monitored by in situ measuring the UV-vis absorption spectra. Typically, 0.1 mL of the aqueous solution of 4-NP (0.005 M), 1 mL of deionized water and 1 mL of aqueous solution of fresh NaBH₄ (0.15 M) were added into a quartz cuvette, followed by the addition of 0.5 mL of Ag@meso-SiO₂ nanocatalyst (0.2, 0.3 and 0.5 mg mL⁻¹ in aqueous solution). In addition, the different systems for this reduction reaction based on the pure mesoporous silica, commercial Ag NPs, Ag/SBA-15 and single core Ag@SiO₂ were investigated under the same conditions. The gradual change of the solution color from light bright yellow to colorless was observed during the reaction. Finally, recycling tests were conducted to evaluate the stability of the catalysts.

3. Results and discussion

Scheme 1 illustrates the synthetic procedure of the multiple-Ag-core hollow mesoporous silica spheres, Ag@meso-SiO₂. The synthetic method is based on a modified sacrificial template route involving the Ag-aminosilane modification of carbon spheres, cladding of mesoporous silica, and removal of the template. Firstly, the carbon nanospheres are prepared via the hydrothermal method of glucose. In the next step, the aminosilane of APTES is tethered onto the carbon nanospheres by reacting with the hydroxyl of the carbon sphere surface to endow the surface of the carbon spheres with abundant ligands. The presence of amino groups on the carbon spheres could efficiently anchor metal ions. Subsequently, Ag ions are anchored on the surface of the aminosilane modified carbon spheres via coordinated interaction with the –NH₂ of APTES. After that, siliceous oligomers attach a cationic micelle coat onto the functionalized carbon spheres to obtain the initial microreactors. Finally, the organic species of the micelles and carbon sphere are removed by the heat treatment, and the multiple Ag NPs, hollow cavity and mesoporous shell are in situ produced during the hollowing process.

3.1. Wide-angle XRD results

The wide-angle XRD patterns of all the samples are presented in Fig. 1. All samples exhibit a broad diffraction peak from 20° to 30°. The broad peaks presenting on the curves of the samples of the CNs and Ag/NH₂-CNs are ascribed to amorphous carbon. Meanwhile the broad peaks of other two samples are in accordance with that of amorphous silica. Additionally, it is worth noting that there are no other peaks detected in the curves of the sample of Ag/NH₂-CNs and the as-synthesized Ag/NH₂-CNs@SiO₂, suggesting no formation of the crystalline Ag NPs before the hollowing process. With regard to the calcined Ag@meso-SiO₂, several newly emerging crystal diffraction peaks at 38°, 44°, 64° and 77° are observed and indexed to the (111), (200), (220) and (311) crystal diffraction for Ag nanocrystals. The result demonstrates the formation of Ag cores within the silica shell during the thermal process. In addition, on the basis of the Scherrer equation, the calculated average size of the generated Ag NPs is 17.4 nm.

Fig. 1 Wide-angle XRD patterns of CNs, Ag/NH₂-CNs, Ag/NH₂-CNs@SiO₂ and Ag@meso-SiO₂.

3.2. FT-IR results

FT-IR spectra were employed for testing different modified samples and the results are exhibited in Fig. 2. As shown above, as compared with bare carbon nanospheres, aminosilane-modified carbon spheres show a set of additional peaks at 1090 and 460 cm⁻¹ which are ascribed to the asymmetric stretching vibrations and bending vibrations of Si–O.³¹ These new absorption peaks confirm the presence of aminosilane and the successful grafting of APTES onto the surface of the carbon spheres. With regard to the sample of Ag/NH₂-CNs@SiO₂, the absorption peaks ascribed to Si–O obviously became stronger as compared with the carbon spheres modified with APTES. This phenomenon supports the deposition and coating of siliceous species on the carbon spheres. Finally, with the heat treatment, it is evident that the absorption bands of APTES, the micelle template and carbon spheres obviously disappear. The remaining peaks coincide with the absorption of typical mesoporous silica, which indicates that the organic species and carbon spheres are mostly removed.

Fig. 2 FT-IR spectra of corresponding samples of CNs, NH₂-CNs, Ag/NH₂-CNs@SiO₂ and Ag@meso-SiO₂.

3.3. XPS results

In order to demonstrate the chemical environment of the elements, XPS spectra were utilized to test the samples of NH₂-CNs and Ag/NH₂-CNs. As shown above, Fig. 3 shows the representative survey of the XPS spectrum of N 1s. The N 1s spectrum (Fig. 3a) for the NH₂-CNs shows two clear peaks of binding energy at about 399.75 and 401.65 eV, which are attributed to the free amine groups and protonated amine groups of APTES, respectively.³² The presence of a protonated amine group is due to the protonation of the aminopropyl groups of APTES in the presence of H⁺, whereas the N 1s spectrum (Fig. 3b) for the Ag/NH₂-CNs obviously gives three characteristic peaks of binding energy located at 406.91 eV, 401.47 eV and 399.75 eV. The presence of the peak at 399.75 eV should be attributed to free amino groups. It is noticeable that another peak at 401.47 eV apparently differs from its counterpart in the N 1s spectrum of the NH₂-CNs with an obvious shift to a lower binding energy. Meanwhile, the enhanced intensity of this characteristic peak is observed as compared to the peak of the free amino group. This result indicates that the presence of this peak should be associated with coordination of Ag ions on the amino group of APTES. Furthermore, based on this result, the amino groups coordinated with Ag⁺ are predominantly in the surface of the modified carbon spheres. Finally, it should be noted that an additional peak at 406.91 eV is observed and according to ref. 32 the presence of the peak should be ascribed to nitrate ions matched with Ag ions present on the surface of the functional carbon spheres. Taken together, these results strongly support the coordination of Ag ions on the aminosilane modified carbon spheres.

Fig. 3 XPS N 1s spectra of NH₂-CNs (a) and silver-loaded Ag/NH₂-CNs (b).

3.4. Low-angle XRD results

The low-angle XRD pattern of Ag@meso-SiO₂ is displayed in Fig. 4. A diffraction peak at 2θ = 2–3° can be observed which is ascribed to the typical 100 peak of the mesophase. The presence of this characteristic peak indicates the mesostructure of the material.

Fig. 4 Low-angle pattern of the sample of Ag@meso-SiO₂.

3.5. N₂-adsorption/desorption results

The structural properties of the sample of Ag@meso-SiO₂ hollow spheres were further investigated using N₂-adsorption/desorption analysis and the results are shown in Fig. 5. The isotherm exhibits type IV curves with an H1 hysteresis loop, which are typical characteristics of mesoporous materials. This result suggests that the sample of Ag@meso-SiO₂ preserves a mesoporous structure after removing the template of carbon spheres and surfactants. Additionally, the surface area and total pore volume are calculated to be as large as 622 m² g⁻¹ and 0.59 cm³ g⁻¹, respectively. Additionally, according to the BJH model, the Ag@meso-SiO₂ hollow nanospheres have a pore size distribution of approximately 3.29 nm (Fig. 5b, inset). This result reveals that the functional microreactors possess a large surface area and pore volume which would provide a broad diffused passageway for the chemicals to be transported in and out.

Fig. 5 N₂ adsorption/desorption isotherms (a) and pore size distribution (b, inset) of Ag@meso-SiO₂.

3.6. SEM results

The representative SEM images of the samples of Ag/NH₂-CNs, as-synthesized Ag/NH₂-CNs@SiO₂ and Ag@meso-SiO₂ are displayed in Fig. 6. As described above, Fig. 6a gives the image for the sample of Ag/NH₂-CNs, indeed, the obvious spherical morphology can be observed even with modification with Ag ions and aminosilane. Meanwhile, associating with the EDX result of the sample of Ag/NH₂-CNs, the signal peaks of elements C, O, Si and Ag can be obviously detected. The presence of elements Si and Ag in the EDX pattern strongly demonstrates the successful grafting of APTES and anchoring of Ag ions. Additionally, no aggregated Ag nanoparticles are detected on the surface of the Ag/NH₂-CNs, which indicates that Ag species should exist in the form of ions on the surface of the carbon spheres. When the sample of Ag/NH₂-CNs is coated by mesoporous siliceous species, the spherical morphology has not been changed. After calcining under air, the round morphology for the sample of Ag@meso-SiO₂ is exhibited in Fig. 6c, and the size of the core–shell Ag@meso-SiO₂ is about 400 nm. Additionally, an obvious hollow status of the spheres can be observed in every image (wherein, an obvious broken sphere is marked with a yellow circle). It is noteworthy that a corrugated and shrinking silicate shell is present in the hollow spheres. The surface of Ag@meso-SiO₂ in a wrinkled state should be due to the shrinking of the silicate shell during the annealing process. However, the resulting product with hollow cavities does not have a collapsed or fractured status. This phenomenon strongly demonstrates the good structural integrity of the hollow and multi-core spheres. Meanwhile, it is observed that the synthesized mesoporous spheres also possess a well-dispersed state without obvious aggregation and crosslinking.

Fig. 6 Representative SEM images of samples of Ag/NH₂-CNs (a), Ag/NH₂-CNs@SiO₂ (b) and Ag@meso-SiO₂ (c) and EDX pattern (d) of a sample of Ag/NH₂-CNs.

3.7. TEM results

The morphologies and structure of the prepared Ag@meso-SiO₂ nanospheres and the status of the Ag nanoparticles were characterized using transmission electron microscopy (TEM) and the results are shown in Fig. 7. The cores@shell structure can be demonstrated in the TEM images. The representative TEM images of Ag@meso-SiO₂ (Fig. 7a and b) reveal that the as-prepared Ag@meso-SiO₂ particles possess well-defined core–shell structures with multiple free Ag cores nicely encapsulated in hollow mesoporous shells. Additionally, the free Ag cores are well separated in the hollow cavity. It is well-documented that several-in-one encapsulation of noble metal nanoparticles in hollow shells can effectively prevent the aggregation of active nanoparticles and therefore significantly improve their catalytic stability. Besides, the shell of Ag@meso-SiO₂ with thickness of ca. 50 nm exhibits a slightly corrugated surface. The phenomenon should be associated with shrinking of the silicate shell during heat treatment. However, it is worth noting that no massive collapsed hollow mesoporous spheres are detected in the TEM images, which indicates that the shrinking during the heat treatment does not result in the deterioration of the whole structure. By comparing the TEM (Fig. 7) and XRD (Fig. 1) results, it can be found that these results favor the formation of small Ag particles. And the average size calculated from the TEM result is 17.6 nm, which is in agreement with the calculated result from wide-angle XRD.

Fig. 7 Representative TEM images of Ag@meso-SiO₂ (a and b), particle size distribution of Ag (c) in image a, and an image of the designed target product (d).

3.8. Catalytic analysis

The catalytic performance of the Ag@meso-SiO₂ hollow microreactors was investigated using the liquid-phase reduction of 4-NP by NaBH₄ to 4-AP (Fig. 8a). Adding NaBH₄ into the aqueous solution of 4-NP will lead to a color change of the solution from light yellow to light bright yellow. Moreover, the UV-vis absorption peak shifting from 317 to 400 nm can be observed in Fig. 8a because of the deprotonation of 4-NP and the formation of 4-AP. No obvious change in the UV-vis absorption spectra was measured after 24 h, suggesting that the reduction reaction did not start without the Ag@meso-SiO₂ hollow nanosphere catalysts (not shown). Additionally, a similar result is also observed for the system just using pure mesoporous silica as a catalyst (Table 1), which indicates that Ag NPs act as the catalytic active centers for the reduction of 4-NP. Meanwhile, the mesoporous shell just provides a protective environment for this reaction. After the addition of Ag@meso-SiO₂, the reduction reaction started immediately, and the color of the reaction solution turned lighter and lighter. The absorption intensity at 400 nm became weaker and weaker along with the increase in reaction time. At the same time, an absorption peak around 317 nm appeared due to the formation of 4-AP (Fig. 8a).

Fig. 8 Time-dependent UV-vis spectral changes of the reaction mixture catalyzed using 0.2 mg L⁻¹ of Ag@meso-SiO₂ microreactors (a). C/C₀ versus reaction time for the reduction of 4-NP using different amounts of Ag@meso-SiO₂ as catalysts (b). Plot of ln(C/C₀) versus reduction time for the reduction of 4-NP using different amounts of Ag@meso-SiO₂ catalyst (c). Recycling test results for different catalysts of single core Ag@SiO₂, Ag/SBA-15 and Ag@meso-SiO₂ (d).

Table 1 Comparison of the reaction parameters with different catalyst systems

Samples	Ag/Si (%)	Dose (mg mL^−1)	Reaction time (min)	k/min^−1	Normalized kapp (min^−1 mg^−1)
Meso-SiO2	0	0.3	8	0	0
Ag@meso-SiO2	1.56	0.2	8	0.399	3.99
	1.56	0.3	5	0.727	4.84
	1.56	0.4	4	0.825	4.13
	1.56	0.5	4	0.829	3.32
Ag NPs	—	0.3	15	0.167	1.11
Ag/SBA-15	1.49	0.3	4	0.865	5.76
Ag@SiO2	3.5	0.3	10	0.306	2.04

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Due to the influence of the number of catalytic active centers for reaction activity in the catalytic process, enough active sites would strongly promote the reaction rate of the reduction reaction. This is due to the presence of increasing unsaturated catalytic active adsorbed sites in the reaction. Therefore, herein, the effect of the amount of the Ag@meso-SiO₂ catalyst on the reduction reaction was investigated. The C/C₀ versus reaction time for the reduction of 4-NP using different amounts of Ag@meso-SiO₂ as catalysts is displayed in Fig. 8b, where C and C₀ are the concentration of 4-NP at different times and the initial time, which can be measured from the relative intensity of the absorbance C and C₀, respectively. As shown in Fig. 8b, the biggest amount of Ag@meso-SiO₂ used obviously shows the fastest reaction rate in the reduction process. Additionally, the linear relationship using different amounts of catalyst between ln(C/C₀) and the reaction time of the reduction reaction is demonstrated in Fig. 8c. It is obvious that using higher amounts of catalyst leads to a quicker reaction rate in the reaction process. This result suggests that the concentration of Ag@meso-SiO₂ has certain effects on the catalytic reaction rate. However, at the time of the initial reaction, the reaction is dominated by the diffusion process, therefore, the linear relationship at the initial point does not locate well in the fitting line between ln(C/C₀) and reaction time. A similar result could be observed in the reported literature.³³ We suggest that the reactants may firstly diffuse across the mesopores of the silica shells and subsequently interact with the Ag cores with catalytic activity. Herein, the liquid-phase reduction reaction can be assumed to follow a pseudo-first-order expression:³⁴ ln(C₀/C) = kt, where C₀/C is the normalized organic compounds concentration and k is the apparent reaction rate (min⁻¹). By plotting ln(C/C₀) as a function of reduction time through linear regression starting from the second point, the k constant can be obtained from the slopes of the simulated straight lines. The k constants from the calculation corresponding to different amounts of catalyst from 0.2 mg L⁻¹, 0.3 mg L⁻¹ and 0.5 mg L⁻¹ are 0.399, 0.727 and 0.829 min⁻¹. Judging from the calculated result of the reaction rate constant, the reduction reaction rate of 4-NP is obviously influenced by the amounts of Ag@meso-SiO₂ used. The catalytic reaction rate of 4-NP reduction reaches the maximum when the amount of Ag@meso-SiO₂ used is largest. However, it should be noted that the increasing catalyst amounts do not lead to a linear increase in the reaction rate. The influence of catalyst concentration on the reaction rate has tended to be saturated when using 0.3 mg mL⁻¹ of Ag@meso-SiO₂. Therefore, an added catalyst concentration of 0.4 mg mL⁻¹ was applied in this reaction and its reaction rate was calculated. For a quantitative comparison, the activity parameter k_app = k/M(mass) was introduced and defined as the ratio of the rate constant k to the weight of the catalyst added.^34,35 By comparison, it was discovered that the excess catalysts lead to a decrease in the normalized k_app of Ag@meso-SiO₂ even with a close reaction rate for the different catalyst concentrations of 0.4 and 0.5 mg mL⁻¹. This may be attributed to the presence of unsaturated adsorbed sites on the Ag cores due to the excess of catalyst.

In addition, in order to investigate the specificity of the Ag@meso-SiO₂ system, several different catalytic systems using commercial Ag NPs, Ag-supported SBA-15 and single core Ag@SiO₂ were compared. Obviously, the Ag-supported SBA-15 exhibits the highest catalytic reaction rate as compared with commercial Ag NPs, Ag@meso-SiO₂ and single core Ag@SiO₂. This result is attributed to the largest exposure of Ag active particles on the surface of the mesoporous silica, therefore the contact of reactant molecules with Ag is more accessible. Meanwhile, as compared to the single core Ag@SiO₂, the Ag@meso-SiO₂ obviously exhibits a larger superiority in catalytic reaction rate. This should be assignable to the presence of multiple Ag cores, which extremely increases the contact chance between reactant molecules and Ag cores. In addition, the recycling experiments for the reduction of 4-NP using Ag@meso-SiO₂, Ag/SBA-15 and single core Ag@SiO₂ were conducted and the results are displayed in Fig. 8d. Just as observed in this figure, even with six reuses in the reduction reaction, no obvious drop in catalytic activity is observed for the Ag@meso-SiO₂. However, it is noteworthy that the Ag/SBA-15 shows a reduced trend in catalytic activity. This is due to the missing Ag species in the reaction process without the protection of a mesoporous shell.

4. Conclusions

In summary, we successfully fabricated a noble metal functionalized catalyst with desired dispersed Ag cores, hollow cavities and mesoporous shell structures using a new construction approach, which is regarded as an ideal candidate for microreactors just like cores@shell architecture. The strategy offered the possibility for confining noble metal NPs in a host material by a novel approach. The as-prepared Ag@meso-SiO₂ has been successfully used as a reduction catalyst and exhibited high catalytic performance in the liquid-phase reduction of 4-NP and the increasing dosage of the catalyst facilitated enhanced catalytic performance. The present method is versatile, and potentially many noble metals, transition metals and their bimetallic nanoparticles can be situated inside hollow mesoporous silica spheres via this process.

Acknowledgements

The authors acknowledge the project being funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the financial support of the National Natural Science Foundations of China (21276125, 20876077 and 21476108).

Notes and references

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