Programmed synthesis of Pd@hTiO₂ hollow core–shell nanospheres as an efficient and reusable catalyst for the reduction of p-nitrophenol

Abstract

Here, we report a facile synthesis approach to obtain a Pd@hTiO₂ hollow mesoporous nanocomposite composed of tiny Pd nanoparticles cores encapsulated within hollow TiO₂ mesoporous shells. The core–shell strategy efficiently prevents the aggregation of Pd NPs (Pd nanoparticles) in the high temperature calcination process and the leaching of Pd NPs for the catalytic reaction in a liquid phase. The catalyst was characterized by transmission electron microscopy (TEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N₂ adsorption/desorption, elemental analysis and inductively coupled plasma atomic emission spectrometry (ICP-AES). The synthesized catalyst exhibited high catalytic activity in the reduction of p-nitrophenol with NaBH₄ aqueous solution at room temperature. Furthermore, Pd@hTiO₂ had an excellent recyclability, evidenced by being extensively reused for eight times without any substantial loss of activity.

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Introduction

The development of high-performance and stable catalysts has remained a significant objective because of its paramount importance for energy conversion.^1–3 Noble metal nanoparticles (NMNPs) have outstanding catalytic performance in catalysis due to their versatile, tunable, size-, shape- and composition-dependent properties.⁴ However, the migration, sintering or aggregation of NMNPs often lead to rapid decay of catalytic activity and stability.^5,6 Therefore, developing various strategies to effectively synthesis and utilize NMNPs catalysts with desirable dimensions and improved catalytic performance represents a technologically crucial but challenging issue.^7–9 Recently, hollow core–shell nanostructures with desirable shell components, variable hollow space sizes, and different encapsulated species in the interior spaces have attracted a great deal of interest.^10–15 Core–shell structured materials with hollow space provide an efficient solution to prevent the undesirable sintering and aggregation of the noble metal particles.^16–21 In addition, the core–shell structures have the possibilities for new physical and chemical characteristics, often different and improved from those of the constituent components. This opens new opportunities for their successful application in various fields like catalysis, optoelectronics, separation technology, coatings, and additives.²²

Many researchers have reported noble metal related core–shell materials (e.g. Pt@C, Pt@SiO₂, Pd@SiO₂) which showed superior catalytic activity in hydrogenation, oxidation, reforming and coupling reactions.^17,19,23,24 However, the unreactive silica or carbon shells only play a dispersion role of the active component, and the synergistic effect between the NMNPs and the shells was quite weak.²⁵ Replacing the shells with reducible oxides, such as CeO₂²⁶ and TiO₂,^26,27 to enhance the synergistic effect between the NMNPs and the shells may further improve the catalytic ability. TiO₂ is attractive due to its fascinating features such as various polymorphs, good chemical and thermal stability, low cost, low toxicity, and excellent electronic and optical properties.²⁸ In addition, TiO₂ has advantageous properties for practical catalytic applications.²⁹ TiO₂ hollow nanostructures possess high active surface area, reduced diffusion resistance, and improved accessibility, which provide many new opportunities for the design of a highly active nanostructure.³⁰ Because of these properties, the preparation of metal core/TiO₂-shell structures is potentially of great importance. In particular, dispersible TiO₂ encapsulated Pd has not been reported to date, although this system is very interesting for its numerous applications especially in catalysis.

Herein, we report the synthesis of Pd@TiO₂ core–shell nanocomposites with inner hollow space (denoted as Pd@hTiO₂) via a facile sol–gel method followed by calcination at high temperature in air and etched by NaOH to remove the template. And they were confirmed by corresponding characterization means. The hollow nanospheres showed good dispersity of NMNPs, enhanced synergistic effect, and decreased leaching of noble metals, leading to extremely high activity and stability for catalytic application. In addition, Pd@hTiO₂ exhibited an excellent reusable performance. Its catalytic activity and recyclability were evaluated through the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP).

Experimental

Preparation of the catalyst Pd@hTiO₂

The preparation of the Pd@hTiO₂ catalyst follows the steps described in Scheme 1. Firstly, SiO₂ were prepared. Secondly, load Pd nanoparticles on surface of SiO₂. Thirdly, the Pd/SiO₂@TiO₂ core–shell structures were prepared. Fourthly, the as-prepared Nanocomposites were calcinated at high temperature in air and etched by NaOH to remove the template. The amount of Pd in the obtained catalyst was found to be 2.45 wt% based on ICP analysis.

Scheme 1 Programmed synthesis of Pd@hTiO₂.

Synthesis of SiO₂ nanospheres³¹

In a typical synthesis, 4.5 mL of tetraethoxysilane (TEOS) was rapidly added into a mixture solution of 62 mL of ethanol, 25 mL of H₂O, and 1.5 mL of ammonium. After stirring for 2 hours, the SiO₂ particles were collected by centrifugation, and washed with ethanol. This process was repeated three times before drying the SiO₂ particles at 50 °C.

Synthesis of Pd/SiO₂³²

200 mg SiO₂ precursor sphere powder was added to 35 mL distilled water and stirred for 10 min as part A. 200 mg SnCl₂ was dissolved in 35 mL 0.02 M HCl solution as part B. Parts A and B were mixed together under stirring, ten minutes later, the precipitate was recovered by centrifugation, followed by washing with distilled water five times and was dispersed into 40 mL distilled water. Then, 19 mg PdCl₂ solution was added to the above mixture. Ten minutes later, 20 mL of 0.15 M sodium formate solution was added. After stirring for 8 h, the black precipitate was recovered by centrifugation and washing with distilled water five times and then dried at 50 °C.

Synthesis of Pd@hTiO₂³³

Typically, 100 mg Pd/SiO₂ particles were dispersed in the mixture solution of 40 mL of ethanol and 0.25 g of Cetyltrimethyl Ammonium Bromide (CTAB). After this, the mixture solution was homogenized by ultrasonication for 30 min. Then, 0.5 mL Tetrabutyl titanate (TBOT) was added to the suspension under vigorous mechanical stirring. After being stirred at room temperature for 4 h, 15 mL of ethanol and 3 mL of deionized water were introduced to the system and the resulting intermixture was stirred continuously for 8 h. After that, the products separated by centrifugation and washed with water and ethanol, respectively. The final product was dried in vacuum at 50 °C and calcined in air for 3 h at desired temperatures. Next, 0.4 g of Pd/SiO₂@TiO₂ spheres was dissolved in 40 mL of NaOH aqueous solution (2.5 mol L⁻¹). The Pd@hTiO₂ was obtained after etching for 8 h, and washed five times with distilled water and dried.

Sample characterization

The synthesized catalyst Pd@hTiO₂ was confirmed by corresponding characterization means. The morphology and microstructure of Pd@hTiO₂ were characterized by transmission electron microscopy (TEM). The TEM images were obtained through Tecnai G2 F30 electron microscope operating at 300 kV. X-ray powder diffraction (XRD) measurements were carried out at room temperature and performed on a Rigaku D/max-2400 diffractometer using Cu-K_α radiation as the X-ray source in the 2θ range of 10–90°. X-ray photoelectron spectroscopy (XPS) was recorded on a PHI-5702 and the C1S line at 291.4 eV was used as the binding energy reference. Specific surface areas were calculated by the Brunauer–Emmett–Teller (BET) method and pore sizes by the Barrett–Joyner–Halenda (BJH) methods using Brunauer–Emmett–Teller (Tristar II 3020). Inductive coupled plasma atomic emission spectrometer (ICP-AES) analysis was conducted with Perkin Elmer (Optima-4300DV).The ultraviolet visible (UV-vis) spectroscopy measurement was conducted with the UV2800PC UV-vis spectrophotometer.

Catalytic activity and recyclability

The catalytic properties of Pd@hTiO₂ nanocomposites were investigated via the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with NaBH₄ as the reductant under ambient temperature as a model reaction. Firstly, 4-NP (187 μL, 1 mM) was added to the fresh NaBH₄ aqueous solution (2.5 mL, 0.1 M) in a quartz cuvette, and then Pd@hTiO₂ nanocomposites aqueous suspension (25 μL, 1 mg mL⁻¹) was added to the above solution, which gradually changed from bright yellow to colorless. The reaction progress was monitored by measuring the UV-vis absorption spectra of the reaction solutions.

To determine the catalytic recycling properties, the catalyst was separated by centrifugation after the reaction completed, and washed thoroughly with water and ethanol, followed by drying overnight at 30 °C in vacuum oven. Finally, the catalyst was redispersed in a new reaction system for subsequent catalytic experiments under the same reaction conditions.

Results and discussion

Characterizations of the catalyst Pd@hTiO₂

The morphologies and structures of the products at different synthetic steps were observed by TEM. Fig. 1(A) shows the typical TEM image of SiO₂ nanoparticles. As can be seen from the image, the average diameter of the as-synthesized spherical particles was ∼250 nm and they were nearly monodisperse. Fig. 1(B) displays the TEM image of Pd/SiO₂ nanospheres with Pd nanoparticles well dispersed on the outer surfaces of SiO₂ nanospheres. It is proven by the magnified TEM image in the inset of Fig. 1(B) that the Pd nanoparticles were successfully loaded on the surfaces of SiO₂. The sizes of Pd nanoparticles are estimated to be ∼6 nm. Fig. 1(C) shows the TEM images of Pd/SiO₂@TiO₂ nanospheres. The Pd/SiO₂@TiO₂ nanospheres with diameters of about ∼310 nm have relatively smooth surfaces and better dispersibility. After calcined at various temperatures, the SiO₂ core is removed using concentrated NaOH aqueous solution, leaving only Pd nanoparticles inside the hollow TiO₂, resulting in the formation of Pd@/hTiO₂ hollow spheres. The TEM images of Pd@/hTiO₂ shown in Fig. 1(D–J) clearly display the hollow mesoporous structures of TiO₂ with a shell thickness of ∼28 nm.

Fig. 1 TEM images of (A) SiO₂, (B) Pd/SiO₂, (C) Pd/SiO₂@TiO₂, (D–F) Pd@hTiO₂ catalysts treated at 298 K, 673 K, 723 K, 773 K, 823 K, 873 K, respectively.

The elemental composition of the Pd@hTiO₂ samples was determined by EDX analysis. The result shown in Fig. 2 reveals that the as-prepared products contain Pd, Cu, Ti, C and O. Among these elements, Cu, C and O are generally influenced by the copper network support films and their degree of oxidation, Ti, O and Pd signals result from the Pd@hTiO₂. And from the EDX result, the signal of Si element has not been found, revealed that the SiO₂ was totally removed.

Fig. 2 EDX spectrum of Pd@hTiO₂.

Because of the encapsulated Pd nanoparticles can't be visualized very evidently in the TEM images of Pd@hTiO₂ hollow core–shell nanocomposites. Therefore, the elemental mapping has been performed to reveal the element distribution in the Pd@hTiO₂ spheres. The mapping results [Fig. 3(B–D)] indicate that the elements Ti, O and Pd spread evenly in the whole spheres and the Pd nanoparticles are not located on the outermost surface of the hollow spheres, which confirms the Pd core@TiO₂ shell structure as obtained by the step-by-step assembly procedures illustrated in Scheme 1.

Fig. 3 (A) the high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) image and mapping results of the elements (B) Ti, (C) O and (D) Pd for the Pd@hTiO₂.

Fig. 4 shows the XRD patterns between 10° and 90° of the samples calcined at various temperatures. The sharp, strong peaks confirm the products are well crystallized. The XRD patterns show that the appearance of the peaks at 2θ = 40.1°, 46.5° and 68.0°corresponding to the reflections of (111), (200) and (220) crystal planes of palladium respectively, and mostly corresponded to face-centered cubic metallic palladium diffraction. For the sample before calcination, diffraction peaks due to the crystalline phase are not observed, suggesting that the sample is still in the amorphous phase. When the sample was calcined at 400 °C, weak and broad peaks at 2θ = 25.5°, 37.9°, 48.2°and 53.8° were observed. These peaks represent the indices of (101), (004), (200) and (105) planes of anatase phase, respectively.³⁴ The average crystallite size of Pd in Pd@hTiO₂ calculated using Scherrer's equation, is about 6 nm. When the calcination temperature was increased, the diffraction peaks due to anatase phase became narrow and intense in intensity. This indicates that the crystallinity of the anatase phase is further improved.³⁵ When the sample was calcined at 773 K, the new weak peak was observed at 2θ = 27.6°, which correspond to the indices of (110) planes of rutile phase.³⁶ This indicates that the anatase phase starts to transform into the rutile phase at 773 K.

Fig. 4 XRD patterns of Pd@hTiO₂ catalysts calcined at 673 K, 723 K, 773 K, 823 K and 873 K and dried at 298 K.

In addition, the surface area and porosity of the Pd@hTiO₂ hollow core–shell nanocomposites have been characterized. Fig. 5 displayed the N₂ adsorption–desorption isotherms and the corresponding pore-size distribution curve for Pd@hTiO₂ calcined at 773 K. The pore size was calculated by using the BJH method, as shown in the inset of Fig. 5. As a result, average pore size was about 5 nm for Pd@hTiO₂. The BET surface areas and the cumulative pore volumes of Pd@hTiO₂ were 105 m² g⁻¹ and 0.37 cm³ g⁻¹, respectively.

Fig. 5 Nitrogen adsorption–desorption isotherm of the Pd@hTiO₂ calcined at 773 K (inset: the corresponding pore diameter distribution).

Fig. 6 shows the XPS spectrum of the synthesized Pd/hTiO₂ nanoreactor calcined at 773 K. Peaks corresponding to O, C, Pd, Ti and Sn are observed. However, typical peaks of Sn and Pd elements are not obviously found. This indicates that Sn, and Pd particles have been coated by TiO₂, since the analysis depth of XPS is only several nanometers. To ascertain the oxidation state of the Pd, X-ray photoelectron spectroscopy (XPS) studies were carried out. The XPS signature of Pd 3d doublet for the Pd nanoparticles was given in the inset figure. The Pd 3d3/2 and Pd 3d5/2 peaks appeared at 342.6 and 337.2 eV respectively were the characteristic peaks for the metallic Pd. It can be seen that there is no oxidation state of Pd species in the catalyst. In the experiment, the reducing agent is excessive that can reduce the Pd(II) into Pd(0) completely, which is beneficial for the reaction. So the oxidation state of Pd species is hard to find in the figure.

Fig. 6 XPS wide-scan spectrum of the Pd@hTiO₂ calcined at 773 K (inset: high-resolution spectrum of Pd3d).

To investigate the catalytic characteristics of the new Pd@hTiO₂ nanostructures, the reduction of 4-NP to 4-AP by NaBH₄ at room temperature was chosen as a model reaction. As is well known, 4-NP can cause water pollution, which has attracted a wide public concern, while its derivative, 4-AP, is very important in synthetic organic chemistry and many industrial applications such as analgesic and anti-pyretic drugs, photographic developers, corrosion inhibitors, and so on.³⁷ The color changes of the solution in the cuvette at different time could be observed visually, which was from bright yellow to colorless (Fig. 7) (Fig. 8).

Fig. 7 The reduction of p-nitrophenol to p-aminophenol by NaBH₄ at the present of Pd@hTiO₂.

Fig. 8 UV-vis spectra of catalytic reduction of 4-NP to 4-AP by NaBH₄ with Pd@hTiO₂ as catalyst calcined at various temperature: (B)673 K, (C)723 K, (D)773 K, (E) 823 K, (F) 873 K, and dried at (A)298 K.

The reduction process was monitored by UV-vis absorption spectroscopy. The reduction reaction did not proceed in the absence of Pd nanoparticles, which was evidenced by a constant absorption peak at 400 nm. The intensity of the characteristic absorption peak of 4-NP at 400 nm quickly decreases, and the characteristic absorption of 4-AP at around 300 nm also appears rapidly. This indicates that 4-NP is reduced to 4-AP quickly with discoloration of the solution from bright yellow to colorless. Fig. 9 showed the ln(C_t/C₀) versus reaction time for the reduction of 4-NP over the Pd@hTiO₂ composite catalyst used for the first time, where C_t and C₀ are the concentrations of 4-NP at intervals and the initial stage, respectively. (A_t/A₀) was defined as the ratio of maximal absorbance A_t of 4-NP at 400 nm at time t to its value.

Fig. 9 The corresponding plot of ln(C_t/C₀) against the reaction time of Pd@hTiO₂ calcined at (B) 673 K, (C) 723 K, (D) 773 K, (E) 823 K, (F) 873 K and dried at (A) 298 K.

A₀ at t = 0, which could be directly interpreted as the ratio of the respective concentrations C_t/C₀. Therefore, the reaction conversion at time t was calculated according to eqn (1):

Conversion (%) = (1 − C_t/C₀) × 100 = (1 − A_t/A₀) × 100 (1)

In this reaction system, the concentration of NaBH₄ was much higher than that of 4-NP and could be considered as constant during the reaction, thus pseudo-first-order kinetics could be applied to evaluate the kinetic rate constant (k) of the current reaction [eqn (2)]:

dC_t/dt = −kt, or ln(C_t/C₀) = ln(A_t/A₀) = −kt (2)

Meanwhile, the expected linear relationship between ln(C_t/C₀) and reaction time (t) confirmed the pseudo-first-order kinetics. Since the rate constant k is influenced by the amount of the used catalyst, two kinds of the reaction rate constant were used to compare the catalytic activity.

The apparent rate constant k and k/(μmol Pd) (the reaction rate constant per the Pd content of the used catalyst) were calculated and showed in Table 1. The catalyst that calcined at 500 °C showed the highest catalytic activity under the present conditions (Table 1, entry 4), because the TiO₂ have completely transformed into the anatase phase and the anatase shows higher catalytic activity than other crystalline phase.³⁸ Compared with the literature,^39–41 this synthesized catalyst Pd@hTiO₂ exhibited comparable or much better catalytic activity for 4-NP reduction.

Table 1 The apparent rate constant k and k/(μmol Pd) (the reaction rate constant per the Pd content of the used catalyst)

Entry	Calcination temperature T (K)	K (s^−1)	k per Pd content(s^−1 μmol Pd^−1)
1	298	4.77 × 10^−4	0.0825
2	673	1.32 × 10^−3	0.229
3	723	3.23 × 10^−3	0.56
4	773	4.77 × 10^−3	0.826
5	823	4.02 × 10^−3	0.696
6	873	9.03 × 10^−4	0.156

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For comparing the catalytic activity, the corresponding Pd/SiO₂, hollow TiO₂ and Pd/TiO₂ catalysts were also prepared and evaluated using model reaction of the reduction of 4-NP to 4-AP with NaBH₄ aqueous solution at room temperature as showed in Fig. 10. The Pd@hTiO₂ hollow structure that calcined at 773 K exhibit better catalytic activity than the corresponding Pd/SiO₂, hollow TiO₂ and Pd/TiO₂, due to its unique hollow structures with active NMNPs uniformly dispersed inside, which favor the quick diffusion of reactants, decreased depletion of catalytic active species, and effective contact of reactants with catalytic active species, eventually helping to improve their catalytic performance.

Fig. 10 UV-vis spectra of catalytic reduction of 4-NP to 4-AP by NaBH₄ over various catalysts: (A) Pd/SiO₂, (B) hollow TiO₂, (C) Pd/TiO₂ and the corresponding Plot of ln(C_t/C₀) against the reaction time of (a)Pd/SiO₂, (b) hollow TiO₂ and (c) Pd/TiO₂.

The separation and recovery of the Pd nanoparticles from the reaction system is one of the most important issues in the Pd-catalysis practical applications, the reusability of Pd@hTiO₂ was further investigated. The catalyst was recycled in the reduction of 4-NP with NaBH₄ and the catalytic process was achieved within 10 minutes under each cycle. As illustrated in Fig. 11, the catalyst was reused for at least eight times without a significant loss of activity.

Fig. 11 The recycling experiments of the catalyst Pd@hTiO₂.

The catalyst Pd@hTiO₂ demonstrating excellent activity and reusability in the reduction of 4-NP was attributed to several factors. Firstly, the catalysts with a thin mesoporous shell and the desired large inner space favors reactant diffusion and exposure of active sites, leading to the improvement of catalytic activity. Secondly, the election affinity and electron transfer of neutral Pd particles largely depend on their size and small Pd nanoparticles encapsulated in the support behave more like individual Pd atoms, which are highly active. In our case, the size of Pd nanoparticles was mostly smaller than 6 nm; besides, they were excellent accessibility, good dispersion and high stability, so the catalyst exhibited very efficient. Moreover, the hollow TiO₂ shell not only avoided Pd nanoparticles leaching from the support, but also improved the homogenous distribution of Pd nanoparticles on the support and their catalytic activity for the reduction.

Conclusion

In conclusion, we demonstrated a successful synthesis of multicomponent Pd@hTiO₂ with well-defined core–shell nanostructures. Tiny Pd nanoparticles were successfully encapsulated within TiO₂ hollow shells. The obtained Pd@hTiO₂ catalyst exhibited high catalytic activity in the reduction of 4-NP by NaBH₄. The Pd@hTiO₂ had an excellent recyclability, evidenced by being extensively reused for eight times without any substantial loss of activity, which was mainly attributed to the small size of Pd nanoparticles and the hollow structure of TiO₂. Therefore, this functional nanostructure holds great promise as a novel Pd-based catalyst system for various catalytic reactions. Additionally, the synthetic methodology may be an effective strategy for preparation of highly efficient hollow catalysts.

Acknowledgements

The authors are grateful to Projects in Gansu Province Science and Technology Pillar Program (1204GKCA047).

Notes and references

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