Catalytic reactions are important processes that are at the heart of various strategies to resolve current energy and environment issues. The construction of new structures for use as novel nanomaterials is critical for important catalytic reactions such as catalytic cracking [1], the water-gas shift reaction [2] and hydrogen generation [3] amongst others. Ceria (CeO2) is regarded as a suitable material for various catalytic applications because of its strong oxidizing power, low cost, and long-term stability [4]. CeO2 is a typical multifunctional rare earth oxide and it has attracted much attention because of its wide range of applications in the chemistry [5], engineering [6], catalysis [7] and fuel cells [8]. For a catalytic system, it is crucial to fabricate novel nanomaterial structures with high activity to act as efficient catalysts. Therefore, the design and synthesis of CeO2 with various morphologies and nanostructures remains a great challenge. The synthesis of a core-shell or a yolk-shell structure is of great significance because these materials exhibit unique physical and chemical properties [9]. Current methods for the synthesis of metal oxide coated noble metal nanocomposites with well-controlled core-shell structures mostly rely on complicated or environmentally unfriendly processes. They involve either the use of heterogeneous hard/soft templates or etching processes with toxic or corrosive agents such as potassium cyanide and sodium hydroxide [10, 11, 12, 13, 14, 15]. To our knowledge most reports about Au/CeO2 composites describe their behavior during CO oxidation [13, 16, 17, 18, 19, 20, 21]. Attention has been given to the core-shell structures of metal@CeO2 such as Pd@CeO2 and Pt@CeO2 [22, 23, 24, 25, 26, 27]. However, there are some inherent drawbacks including low efficiency, a complicated and tedious process and difficulties in ensuring that the etching reaction occurs exclusively within the shells.
In this study, we report an environmentally friendly, low cost, and template-free synthetic route for the preparation of multiple Au cores in CeO2 hollow sphere (MACCHS) core-shell structures with multiple gold nanoparticle cores embedded in a porous shell. The unique hollow structure of CeO2 can efficiently trap Au nanoparticles in the porous shell and the cavities allow the Au nanoparticles to disperse more easily. A facile route in the aqueous phase was used to synthesize hybrid MACCHS materials with a well-controlled multi-cores-in-shell structure by simply adjusting the ratio of the HAuCl4 solution that was reacted with sodium citrate. Finally, we show for the first time that the incorporation of several Au cores into the shell of the CeO2 semiconductor, using NaBH4 as a reductant, enhances the performance of CeO2 toward the reduction of p-nitrophenol to p-aminophenol. This represents an important catalytic process in green chemistry. We hope this work will offers useful directions for the scale-up fabrication of other metal oxide coated noble metal materials with tunable core-shell structures. This novel structure may be used as part of an ideal catalytic reaction method because the noble metal nanoparticles do not aggregate and a synergistic effect is exploited because of interactions between the metal nanoparticles.
The reagents (CeCl3·7H2O, ethanol, H2O2 (30 wt%), sodium citrate dihydrate, HAuCl4·3H2O, urea and p-nitrophenol, analytical reagent) were bought from Beijing Chemical Reagent Factory and used without further purification.
The hollow CeO2structure was prepared as our previous report [7]. In a typical procedure, 1.08 g urea was dissolved in 171 mL deionized water to form a clear solution. A 72 mL sodium citrate solution (10 mmol/L) was added to this solution and stirred for 10 min. CeCl3·7H2O (0.89 g) was then added to the solution, and the resulting mixture was stirred for 15 min. Subsequently, 1.08 mL H2O2 (30 wt%) was added to this solution dropwise with vigorous stirring for 30 min. Finally, the mixture was transferred to Teflon-lined stainless steel autoclaves (200 mL) and heated to 180 °C for 22 h. The resultant solid product was centrifuged, washed three times with distilled water and ethanol and dried at 100 °C for 12 h to give a light yellow powder.
The MACCHS nanomaterials were prepared by the impregnation method. In a typical procedure, 2 mL of HAuCl4 solution (10 mmol/L) and 2 mL of sodium citrate solution (10 mmol/L) were dissolved in 76 mL deionized water to form a clear solution and it was stirred for 10 min. We then added 0.046 g of the hollow structure CeO2 to the above-mentioned solution, and the resulting mixture was stirred for 40 min and ultrasonicated for 10 min. The obtained mixture was then centrifuged at 6500 rpm twice to ensure that unreacted HAuCl4 and sodium citrate were removed completely. Finally, the above-mentioned solid product was added slowly to a 26 mL deionized water and 2 mL sodium citrate solution. This was followed by the addition of 1 mL NaBH4 solution (10 mmol/L) dropwise with vigorous stirring for 12 h. After centrifugation at 5000 rpm the final purple MACCHS products were dried at 60 °C for 6 h.
Powder XRD patterns were recorded on a Panalytical X’Pert-pro MPD X-ray powder diffractometer using Cu Kα radiation (λ = 0.154056 nm). Scanning electron microscopy (SEM) was performed on a Hitachi S-4800. Transmission electron microscopy (TEM) was performed on a FEI Tecnai G2 F20 electron microscope operating at 200 kV with a software package for automated electron tomography. The BET (Brunauer-Emmett-Teller) surface area, pore volume and pore size of the catalysts were measured using a Quadrasorb SI-MP instrument. Before each measurement the sample was heated to 300 °C and kept at this temperature for 3 h. XPS spectra were obtained using an ESCALAB 250 Xi XPS system from Thermo Scientific. The analysis chamber was at 1.5×10−9 mbar and the X-ray spot was 500 µm. UV-Vis diffuse reflection absorption spectra (UV-Vis/DRS) of the samples were recorded using a UV-vis spectrometer (Lambda 950, PE, USA) equipped with an integrating sphere accessory in diffuse reflectance mode (R) and BaSO4 as the reference material.
The catalytic properties of the MACCHS nanomaterials were investigated by the reduction of p-nitrophenol to p- aminophenol with NaBH4 solution as the reductant at ambient temperature as a model reaction. First, p-nitrophenol (10 mL, 0.034 mmol/L) was added to a NaBH4 solution (2 mL, 0.1 mol/L) and the mixture was stirred for 10 min. The MACCHS nanomaterial aqueous suspension (10 mL, 1 mg/mL) was then added to the above-mentioned solution, which was stirred until it changed from bright yellow to colorless. The reaction progress was monitored by the UV-Vis absorption spectra of the reaction solutions. To determine the catalytic recycling properties, the catalyst was separated after reaction for 2 min and washed thoroughly with water and ethanol followed by drying at 60 °C for 12 h in a vacuum oven. Finally, the catalyst was redispersed in a new reaction system for subsequent catalytic experiments under the same reaction conditions.
Scheme 1 shows a typical two-step procedure for the synthesis of the catalysts. In the first step, the synthesis of hollow CeO2 involves the hydrothermal treatment of the solution containing the mixed sodium citrate, urea, H2O2 and CeCl3·7H2O. When the solution was heated to 180 °C the urea hydrolyzes and reacted with Ce3+ and H2O2 to form Ce(OH)4 (2Ce3+ + H2O2 + 6H2O = 2Ce(OH)4 + 6H+). Ce(OH)4 then transforms into CeO2 hollow CeO2 spheres. In the second step, the synthesized hollow CeO2 with adsorbed AuCl4- ions were separated from the solution by centrifugation and washed twice with pure water to remove the incompletely absorbed AuCl4- ions. The AuCl4- ions in the CeO2 hollow spheres then mixed with the sodium citrate solution. The NaBH4 solution was then added leading to the formation of the MACCHS. The final products were purified by centrifugation and washing with pure water. The desired multi-core Au nanoparticles in hollow CeO2 were thus obtained.
The crystal structure of the MACCHS was further investigated by XRD. As shown in Fig. 1, two series of peaks are present and these are attributed to face-centered-cubic Au and fluorite CeO2, respectively. The characteristic 2θ peaks at ca. 28.60°, 33.09°, 47.56°, 56.39°, 59.14°, 69.53°, 76.78°, 79.14° and 88.75° correspond to the (111), (200), (220), (311), (222), (400), (331), (420) and (422) crystal facets of the CeO2 fluorite structure. The two weak peaks belong to the (111) and (220) lattice planes of Au nanoparticles.
The morphology and structure (Fig. 2) of the CeO2 hollow spheres were investigated by different characterization techniques including SEM, TEM, high resolution TEM (HRTEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). We found that the products were roughly spherical with a size distribution of about 500-600 nm in diameter (Fig. 2(a)). Each sphere was characteristic of the hollow structure (Fig. 2(b)). Elemental mapping by HAADF-STEM revealed that Ce and O were homogenously distributed within the hollow spheres (Fig. 2(d)) while the edge of the structure was enriched with Ce and O. This is consistent with STEM-EDS line scans (Fig. 2(d)). The selected area electron diffraction (SAED) image demonstrates the polycrystalline nature of the hollow CeO2 and the HRTEM image further indicates that each nanoparticle in the shell is highly crystalline with interplanar spacings of 0.31 and 0.19 nm corresponding to the (111) and (220) planes of CeO2, respectively (Fig. 2(c)).
As shown in Fig. 3, the products are similar to the former CeO2 hollow spheres (Fig. 3(a)), and each sphere is characteristic of a single-shelled hollow structure with a multicore-shell nanostructure morphology (Fig. 3(b)). Elemental mapping by HAADF-STEM revealed that Ce and O distributed within the single-shelled hollow sphere while the edge of all shells were enriched in Ce and O. Interestingly, the Au in the cores (diameter of the Au cores is about 34.58 nm) as well as in a few smaller nanoparticles were distributed in the shell of the CeO2 hollow spheres, which is also consistent with the STEM-EDS line scans (Fig. 3(d)). The SAED image shows the polycrystalline nature of the hollow CeO2 and the HRTEM image further indicates that the unique structure is a nanocomposite of Au and CeO2 (Fig. 3(c)). Elemental mapping by HAADF-STEM also reveals that Ce and O are homogeously distributed within the MACCHS.
A detailed X-ray photoelectron (XPS) analysis of the surface element composition of the MACCHS shows clearly that the shell is composed of Au, Ce and O. As shown in Fig. 4, there are strong Ce 3d peaks and noble metal Au peaks in the XPS spectra of the MACCHS nanomaterials. The XPS spectrum of the Au 4f of the MACCHS has two peaks indicative of metallic Au at binding energy of 83.7 and 87.3 eV for the Au 4f7/2 and Au 4f5/2 levels. This implies the complete in situ reduction of Au3+ ions by NaBH4. Regarding the typical signals of the Ce 3d XPS spectrum, the peaks located around 881.6 and 899.9 eV can be attributed to Ce 3d5/2 and Ce 3d3/2, respectively. The four additional peaks in the spectrum come from satellite peaks of Ce 3d. The TEM, energy dispersive X-ray spectroscopy (EDS) and XPS results confirm the formation of a multi-cores-in-shell hollow structure for our as-synthesized MACCHS nanomaterials by a simple and green route in the aqueous phase.
As shown in Fig. 5(a), the EDS analysis confirms that the MACCHS nanomaterial consists of Ce, O and Au that originate from CeO2 and Au, respectively. More importantly, the Au nanoparticles were successfully coated by a CeO2 cavity structure and the Au content is about 0.6 wt%. Interestingly, the MACCHS has a stronger absorption of visible light compared with hollow CeO2, as shown in Fig. 5(b). This comes from the plasmonic effect of the Au nanoparticles. A clear adsorption peak is present from 500 to 600 nm [28]. This raises the potential for MACCHS to be used for photocatalytic reactions such as photocatalytic phenol oxidation [29, 30].
N2 adsorption-desorption results, i.e., the specific surface area, pore volumes and pore diameters are shown in Table 1. By comparison with the hollow structured CeO2 catalyst, a small amount of post-added Au (e.g., 0.6 wt% Au) does result in changes. On the other hand, the pore size distributions move toward smaller pores for the MACCHS. This observation suggests that a certain amount of Au-species were located in the larger pores of the hollow structured CeO2 catalyst, which decreased the size of the pores. The holes on the surface of the hollow MACCHS were large enough for p-nitrophenol diffusion and the Au nanoparticles were not completely coated by CeO2. Therefore, the reactant may easily come into contact with the Au nanoparticles. The large specific surface area of the MACCHS is an advantage for the catalytic reduction of p-nitrophenol.
To determine the advantages of the novel nanostructure MACCHS catalyst, we investigated the reduction of p-nitrophenol to p-aminophenol by NaBH4 at room temperature as a model reaction [31, 32]. The reduction of aromatic nitro compounds to amines is an important process in synthetic organic chemistry and in the industrial production of many industrially important chemicals. The reduction process was monitored by UV-Vis absorption spectroscopy (Fig. 6). A yellow solution was obtained after mixing the p-nitrophenol solution with NaBH4 solution. This was characterized by the 400 nm p-nitrophenol absorption peak. When the MACCHS catalyst was added, the absorption peak at 400 nm quickly disappeared while a median absorption peak appeared at 300 nm. This is evidence for the reduction of p-nitrophenol to p-aminophenol. This change is shown by the UV-Vis spectra in Fig. 6. The reaction occurred rapidly completing within 2 min. The Au nanoparticles that are trapped in the CeO2 hollow spheres play a critical role in the p-nitrophenol reduction reaction. UV-Vis spectra of the reaction solutions showed that the use of a small amount of the MACCHS nanomaterial as a catalyst resulted in the reaction being complete within 2 min. The solution color changed from bright yellow to colorless.
As shown in Fig. 7, the MACCHS exhibited high activity and good recyclability. The conversion decreased slightly after 5 successive cycles. For comparison, when bare Au nanoparticles were used as the catalyst the catalytic activity decreased rapidly. This behavior indicates severe aggregation. These results indicate that a CeO2 core-shell support is required to stabilize the Au nanoparticles by preventing their aggregation and leaching. This produces an excellent catalyst with high activity and good recyclability [33, 34]. The prepared CeO2 core-shell support may be considered to be a reasonable MACCHS for the encapsulation of small Au nanoparticles. Fine tuning the synergistic interactions between the Au nanoparticles and the CeO2 support is important in achieving high activity. The results indicate that the MACCHS structure is an ideal candidate for nanoreactor.
Scheme 2 shows a proposed mechanism for the reduction of p-nitrophenol to p-aminophenol with NaBH4 solution catalyzed by MACCHS. “Three-dimensional” intimate contact between the multiple Au cores and the CeO2 shell maximizes the metal-support interaction, which facilitates interfacial charge transfer. A uniform structural composition wherein several Au cores are encapsulated by a CeO2 shell provides a homogeneous environment for the reduction reaction. All these features along with our preliminary promising results indicate significant potential for the scale-up synthesis of metal core@semiconductor oxide shell nanocomposites. In heterogeneous catalysis, further investigations for its applications are thus warranted.
A simple and facile template-free approach was developed to prepare a MACCHS catalyst using a combination of hydrothermal and impregnation treatments. The reduction of p-nitrophenol to p-aminophenol using a NaBH4 solution as a model reaction showed that the MACCHS nanomaterials can effectively promote catalytic reduction reactions and retain their excellent stability. These materials may find a wide range of potential application such as in CO oxidation, gas sensing, removal of pollutants in waste water, and even in biological and medical fields. Our synthesized hollow core-shell materials with a multi-core-in-shell structure are a powerful platform for nanoreactors and we believe that this synthetic strategy can be extended to the production of new composite materials with enhanced properties for advanced applications.
2015, Vol. 36 Issue (3): 261-267
PDF (2091 KB)
2015, Vol. 36 Issue (3): 261-267
PDF (2091 KB)