Nanocasting synthesis of an ordered mesoporous CeO₂-supported Pt nanocatalyst with enhanced catalytic performance for the reduction of 4-nitrophenol

Abstract

Ordered mesoporous ceria (meso-CeO₂) was fabricated by nanocasting employing Ia3d mesoporous silica KIT-6 as the template. For comparison, ceria nanoparticles (nano-CeO₂) with non-ordered mesoporous were also synthesized via a sol–gel method. Polyamidoamine (PAMAM) dendrimers were used as stabilizing agents to prepare a dispersed Pt nanoparticle colloidal solution. Afterward, the obtained well-dispersed Pt nanoparticles were immobilized on meso-CeO₂ and nano-CeO₂, respectively. The prepared samples were characterized through several techniques, such as X-ray diffraction (XRD), nitrogen adsorption–desorption isotherms, transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and energy dispersion X-ray analysis (EDX) with mapping. The results revealed that the as-prepared meso-CeO₂ has high crystallinity, a relatively small crystalline size, a well-ordered mesoporous structure and high surface area of 115.3 m² g⁻¹. In addition, the Pt/meso-CeO₂ catalyst showed relatively uniform distribution of Pt nanoparticles with small sizes (∼4 nm). The catalytic performances of the as-synthesized catalysts were evaluated relying on the reduction of 4-nitrophenol monitored by UV-vis spectra. It was found that Pt/meso-CeO₂ exhibited better catalytic activity compared with Pt/nano-CeO₂. Besides, Pt/meso-CeO₂ possessed good reusability and maintained a conversion of no less than 90% even after five cycles.

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1. Introduction

Recently, ordered mesoporous transition metal oxides have received great attention owing to their unique physicochemical properties such as a regular network structure.^1–3 Although it is common and facile to obtain mesoporous silica or mesoporous alumina via a soft template method,^4,5 this method is rather difficult to synthesize ordered mesoporous metal oxides including ceria owing to the difficult control of the hydrolysis and condensation of metal precursors. More seriously, the mesoporous structure will collapse when the temperature exceeds 400 °C.⁶ As a result, it is difficult to obtain highly crystallized and ordered mesoporous metal oxides. Fortunately, this inconvenience can be overcome by a nanocasting (so-called hard templates) approach using mesoporous silica as templates. This approach involves the filling of the channels of porous silica by the metal oxide precursor and forming the corresponding metal oxide via calcination. After removal of silica templates via etching, ordered mesoporous metal oxide with crystalline are achieved. Among various mesoporous silica templates, the cubic mesoporous silica (KIT-6) is often chosen as the hard template because its three-dimensional cubic Ia3d symmetric structure can provide large pore space for the fabricating of nanostructured objects.⁷ In recent years, various ordered mesoporous metal oxides such as TiO₂, NiO₂, Co₃O₄ and In₂O₃ have been successfully synthesized by using the silica template.^3,8–12 However, taking into account that the excess precursors facilely lead to partial crystalline oxide may be located outside of pores of the templates, the appropriate ratio between the metal oxide precursors and the templates still needs to be investigated.

Transition metal nanoparticles (NPs) are widely applied in the field of catalysis because their high surface area-to-volume ratios promote efficient use of transition metal nanoparticles.^13,14 For example, Pt NPs are used to catalyze selective oxidation of alcohols, propane dehydrogenation, and reduction of 4-nitrophenol.^15–18 According to the reported literatures,^19,20 colloidal metal NPs can be employed as precursors to synthesize heterogeneous catalyst. Nevertheless, the naked metal NPs facilely aggregate and form larger nanoparticles due to high active surface atoms. As a result, some surfactants, micelles, ligands and dendrimers are employed as protective and stabilizing agents to prevent aggregation of metal NPs.^21–25 In particular, polyamidoamine (PAMAM) dendrimers posses promising properties to control the size and stability of metal NPs, which is attributed to their hyper-branched, three-dimensional structure with fine monodispersity.²⁶ For instance, Kuhn and co-workers synthesized Pt NPs using PAMAM dendrimers and polyvinylpyrrolidone (PVP) as stabilized agents, and they found that dendrimers-stabilized Pt NPs exhibited smaller size and higher dispersion.²⁷ Consequently, it was a good choice for synthesizing dispersed colloidal Pt NPs using PAMAM dendrimers.

Due to the unique features of dendrimers, Pt NPs stabilized by PAMAM dendrimers have been used as precursors and immobilized on various solid supports, including TiO₂,²⁸ SiO₂ (ref. 29) and ZrO₂.³⁰ Such supported catalysts can not only further stabilize the metal NPs relying on the interaction between metal particles and supports, but also facilitate catalyst recovery in solution. However, to the best of our knowledge, there are rare reports that the dendrimers-stabilized Pt NPs are deposited on ordered mesoporous CeO₂ supports. Actually, as an important functional rare earth oxide, ceria are widely applied in fuel cells and catalytic field due to unique redox property and high oxygen storage capacity.^31–33 There is a potentially synergistic effect in catalytic reaction when noble metals are combined with CeO₂. For instance, oxygen vacancies in ceria can strongly adsorb gold species and further stabilize gold NPs, improving the catalytic activity.^34,35 Moreover, the fabricated ordered mesoporous ceria exhibited regular network structure and large surface area, which is beneficial to improve the dispersion of platinum nanoparticles.

In this work, PAMAM dendrimers-stabilized Pt NPs were immobilized on ordered mesoporous CeO₂. This Pt-based composite material combined the merits of hyperbranched structure of dendrimers and the unique physicochemical properties of ordered mesoporous CeO₂. Moreover, as a heterogeneous catalyst, it is also facile to be separated from solution. The prepared processes were briefly illustrated in Scheme 1. Ceria precursors were immersed into pores of KIT-6, and calcined to turn into the corresponding ceria oxides. Ordered mesoporous structure was obtained via etching silica templates with base solution. Notably, the optimum mesoporous CeO₂ with regular texture were fabricated via tuning the amounts of precursors. PAMAM-stabilized Pt NPs were immobilized on prepared ceria, and the dendrimers were removed via slow calcination to form mesoporous CeO₂-supported Pt catalyst. It was found that the obtained catalyst exhibited good catalytic performance in the reduction of 4-nitrophenol by NaBH₄.

Scheme 1 Schematic illustration for the preparation of Pt nanoparticles immobilized on ordered mesoporous CeO₂.

2. Experimental

2.1 Materials

P123 block copolymer (EO₂₀PO₇₀EO₂₀, MW = 5800) and 4-nitrophenol (≥99%) were purchased from Sigma-Aldrich Corporation. G4-OH PAMAM dendrimers were purchased from Chen Yuan Molecular New Materials Co., Ltd (Weihai, China). Prior to use, it was diluted to 0.34 mM with deionized water. Cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.5%) and citric acid (99%) were obtained from the Aladdin Reagent Co. Ltd. Ethanol, n-butanol, tetraethyl orthosilicate (TEOS, 98%), hydrochloric acid (36–38%), sodium borohydride, sodium hydroxide, potassium tetrachoroplatinate (K₂PtCl₄, ≥96%) were all purchased from Sinopharm Chemical Reagent Co. Ltd. Deionized water was used in all the experiments.

2.2 Catalyst preparation

(1) Preparation of KIT-6. Mesoporous silica KIT-6 was prepared according to that report by Ryoo et al.⁷ Briefly, P123 (3.0 g) were dissolved in water and concentrated HCl (5.7 g) at room temperature. Afterward, n-butanol (3.0 g) was injected and stirred for 1 h. TEOS (6.45 g) was added dropwise and stirred for 24 h at 35 °C. After the hydrothermal treatment for another 24 h at 100 °C, the slurry was filtered, dried and calcined at 550 °C for 6 h in air.

(2) Preparation of CeO₂. As-obtained KIT-6 was used as silica templates to synthesize ordered meso-CeO₂. Typically, Ce(NO₃)₃·6H₂O (0.8 g) was dissolved in ethanol (10 mL). Afterward, KIT-6 (0.3 g) was dispersed in the solution and stirred for 2 h at room temperature. Subsequently, the mixed solution was transferred to a dish and dried overnight at 50 °C to evaporate the solvent. The dried samples were then calcined at 300 °C for 4 h to pyrolyze the nitrate. In order to obtain higher loadings, the calcined sample was impregnated again with Ce(NO₃)₃·6H₂O (0.6 g) dissolved in ethanol. After evaporation of the solvent, the sample was calcined again at 500 °C for 5 h in air. Finally, the silica templates were removed through etching three times with 2 M NaOH aqueous solution at 60 °C, followed by washing with water and ethanol several times, and drying at 70 °C. Other samples were similarly prepared through tuning the mass of ceria precursors.

For comparison, nano-CeO₂ was synthesized through the sol–gel method.³⁶ Ce(NO₃)₃·6H₂O, water, citric acid were mixed at a molar ration of 1 : 100 : 2. Afterward, the solution was stirred at 80 °C until a yellow gel remained, which was dried overnight at 110 °C. At last, the obtained gel was calcined at 500 °C for 5 h in air.

(3) Preparation of catalysts. PAMAM dendrimers-stabilized platinum nanoparticles (Pt DSNs) were prepared according to the previous procedures.³⁷ Briefly, appropriate aliquots of aqueous solution of K₂PtCl₄ were mixed with G4-OH solution, and stirred three days at 40 °C in nitrogen, followed by reduction using at least 10-fold molar excess of NaBH₄ ice-solution.

The mesoporous CeO₂-supported Pt catalyst was synthesized via an impregnation method. For this process, the desired amounts of meso-CeO₂ powders were dispersed in dendrimers-stabilized Pt NPs solution and stirred for 12 h at room temperature. After the extra water was evaporated in a vacuum oven at 50 °C, the dried samples were calcined at 550 °C for 2 h (3 °C min⁻¹ heating ramp) to decompose the dendrimers. The as-obtained catalyst was denoted as Pt/meso-CeO₂, and Pt content was 1.5 wt% by inductively coupled plasma (ICP) measurement. Similarly, Pt NPs were deposited on nano-CeO₂ support, which was marked as Pt/nano-CeO₂ with 1.6 wt% of Pt content by ICP measurement.

2.3 Catalyst characterization

Transmission electron microscopy (TEM) and energy dispersion X-ray analysis (EDX) were conducted on a FEI Tecnai G20 microscope operated at 100 kV. Wide-angle powder X-ray diffraction patterns were recorded on a Bruker D8 Advance Diffractometer (Germany) with Cu Kα radiation (λ = 1.5406 Å). The ordered structures of the samples were characterized by small-angle X-ray diffraction (Rigaku, RINT-Ultima III) with Cu Kα radiation with 40 kV and 200 mA. Field emission scanning electron microscopy (FESEM) images were performed on a scanning electron microscope (Zeiss, Ultra Plus) unit operating at 20 kV. The nitrogen adsorption–desorption isotherms were carried out at 77 K by using ASAP 2020 (Micrometrics USA). The surface area and the pore size distribution were calculated using the Brunauer–Emmett–Teller (BET) method and Barrett–Joyner–Halenda (BJH) theory (using the desorption branch of the isotherms), respectively. X-ray photoelectron spectra (XPS) measurements were carried out in a Thermo ESCALAB 250 instrument (USA) using non-monochromatic Al Kα 1486.6 radiation. UV-vis spectra analysis was performed on a Shimadzu UV 3600 spectrometer. The Pt content of samples was measured by means of an inductivity coupled plasma mass spectrometry (ICP-MS, Thermo Elemental X7 series).

2.4 Catalytic reduction reaction of 4-nitrophenol

In order to investigate the catalytic performance of prepared catalysts, we chose the reduction of 4-nitrophenol (4-NP) by NaBH₄ at room temperature as a probe reaction. Typically, freshly prepared 1.0 mL of NaBH₄ ice-solution (0.25 M) was mixed with 30 μL of 4-NP solution (0.01 M) and 1.7 mL of water in a quartz cuvette. Then, 0.3 mL of the dispersed catalyst solution (1.0 g L⁻¹) was infused into the cuvette to start the reaction. The progress of the reduction was recorded using UV-vis spectra at a regular time. For the recycling experiment, the catalysts were collected by centrifugation, washed with water, dried at 60 °C and reused in the next cycles.

3. Results and discussion

3.1 Characterizations of synthesized catalyst

The morphology of synthesized KIT-6 template was characterized by TEM. As can be observed in Fig. 1a, KIT-6 showed well-ordered cubic network structure with uniform pore size of around 7 nm. On the other hand, the N₂ adsorption–desorption isotherm and the corresponding pore size distributions of KIT-6 were presented in Fig. 1b. Obviously, KIT-6 exhibited a type IV isotherm and H1 hysteresis loop with a sharp capillary condensation step at high relative pressures, which is attributed to channel-like pores in a narrow range of size.⁷ The measured BET specific surface area and pore volume were as high as 728.2 m² g⁻¹ and 1.1 cm³ g⁻¹, respectively. According to the inset in Fig. 1b, the synthesized KIT-6 revealed the narrow pore size distribution with maximum at 7.7 nm, which is consistent with the observation of TEM.

Fig. 1 (a) TEM image of KIT-6 and (b) N₂ adsorption–desorption isotherms of KIT-6 (inset is the corresponding BJH pore size distribution curve).

Small-angle XRD patterns of KIT-6 and as-prepared various ceria samples were illustrated in Fig. 2A. The small-angle XRD pattern of KIT-6 (inset in Fig. 2A) well showed diffraction peaks at 2θ = 0.95° and 2θ = 1.2° corresponding to (211) and (220) crystal planes, which are characteristic of cubic Ia3d symmetrical structure.¹⁰ To determine the optimum precursor/template ratio for the ordered mesoporous CeO₂, different mass of ceria precursor (including 1.82 g, 1.40 g, 0.98 g and 0.56 g) were immersed into KIT-6 template (0.3 g), and fabricated meso-CeO₂ were characterized by small-angel XRD test. As presented in Fig. 2A(b–e), all the prepared meso-CeO₂ via nanocasting method approximately exhibited one peak at around 2θ = 1° attributable to (211) plane, indicating structural regularity. Furthermore, the intense of the diffraction peaks became stronger with increasing mass of precursors. Presumably, this phenomenon can be explained by the fact that more ceria may sustain well mesoporous structure after etching. However, if the precursors were excessive, partial ceria would be formed outside pores of the template, bringing down the regularity partially.³⁸ In this experiment, when the precursor was 1.40 g, the produced CeO₂ gave the strongest diffraction peak as shown in Fig. 2A(d), suggesting the optimal well-defined structure. This meso-CeO₂ was employed as the desired support. In comparison with KIT-6, obviously, the intensity of the diffraction peak of meso-CeO₂ was weaker, indicating meso-CeO₂ was less-ordered than their silica template. Besides, from Fig. 2A, the diffraction peaks of meso-CeO₂ slightly shifted to a higher angle, which revealed small shrinkage of the mesostructure after replication.³⁹ As for Fig. 2A(a), it involved the pattern of nano-CeO₂ without visible peaks, indicating nano-CeO₂ had disordered structure. After the silica template was etched, according to the EDX analysis, the weight ratio of remaining Si with 0.9% was much lower than the stoichiometic ratio (Fig. 2B), confirming almost complete removal of the silica template.

Fig. 2 (A) Small-angle XRD patterns of (a) nano-CeO₂, (b)–(e) meso-CeO₂ prepared with different mass of ceria precursor including (b) 0.56 g, (c) 0.98 g, (d) 1.40 g, (e) 1.82 g. The inset shows the small-angle XRD pattern of KIT-6. (B) EDX analysis of meso-CeO₂ corresponding to (d) in left image.

The morphology of the as-obtained ceria is further investigated by the TEM analyses. Fig. 3a displayed the morphology of CeO₂/KIT-6 before etching. This phenomenon was also observed for the preparation of mesoporous Co₃O₄.⁴⁰ This behavior occurs because the volume of ceria compound will remarkably diminish after thermal treatment. After the removal of the silica template (Fig. 3b), the as-obtained meso-CeO₂ replica had inherited the ordered structure of the silica template. Especially, the enlarged view of orange area clearly exhibited ordered 3D cubic structure, which was in good agreement with the results of small-angle experiments. This regular structure still retained even after calcination at 500 °C, indicating good thermal stability via hard template method. Meanwhile, it was noticed that the prepared meso-CeO₂ showed approximate spherical shape, which was also confirmed by SEM test (see the following discussions). Similar morphology can also be observed in prepared Co₃O₄ and In₂O₃.⁴¹ The above mentioned result of EDX also confirmed the silica template was almost removed (Fig. 2B). The SAED pattern of meso-CeO₂ (inset in Fig. 3b) implied the synthesized meso-CeO₂ preserved high crystallinity, in good consistence with wide-angle XRD (see discussions below). Besides, in Fig. 3c, the high-resolution transmission electron microscopy (HRTEM) of meso-CeO₂ revealed a well-defined (111) crystalline planes with a 0.31 nm distance of CeO₂ (JCPDS PDF # 43-1002). Fig. 3d presented nano-CeO₂ similar to a flake composed of ceria particles forming, but ordered structure was invisible, in consistence with the result of small-angle XRD. The SAED patterns in Fig. 3d showed multiple bright diffraction rings, suggesting the presence of high crystalline of nano-CeO₂.

Fig. 3 TEM images of (a) CeO₂/KIT-6, (b) meso-CeO₂ (the insets are local enlargements of selected areas and the SAED pattern), (c) high-magnified TEM image of meso-CeO₂, (d) nano-CeO₂ (the inset shows the SAED pattern).

To further investigate the pore structures of as-synthesized ceria, N₂ adsorption analysis was performed. N₂ adsorption–desorption isotherms and corresponding BJH pore size distributions of meso-CeO₂ and nano-CeO₂ were shown in Fig. 4. It can be noted that the meso-CeO₂ displayed a type of IV curves with a H3 hysteresis loop, corresponding to the characteristic of mesoporous structure.^42,43 Moreover, the pore size distribution cures of the meso-CeO₂ exhibited an obvious strong and narrow peak at 3.2 nm (right panel), confirming relatively narrow mesoporous size distribution.⁴⁴ Obviously, these results are coincident with the ones observed from TEM and small-angle XRD analysis of the desired meso-CeO₂ prepared with precursor of 1.40 g, suggesting a prefect replica structure of KIT-6 template. On the other hand, from the N₂ adsorption–desorption isotherm of nano-CeO₂ (Fig. 4 (left panel)), a clear H3-type hysteresis loop was visible in the P/P₀ range from 0.80 to 0.99, which was probably attributed to interparticle porosities between ceria crystal particles. According to the pore size distribution, nano-CeO₂ showed a weak peak centered at 3.9 nm accompanied with inconspicuously wide peak at around 14 nm, implying relatively wide pore size distribution. Additionally, meso-CeO₂ presented the BET specify surface area of 115.3 m² g⁻¹ and pore volume of 0.21 cm³ g⁻¹, which were much larger than those of nano-CeO₂ with 21.6 m² g⁻¹ and 0.05 cm³ g⁻¹, respectively. These results are consistent with the TEM observations of meso-CeO₂ with ordered mesoporous structure and nano-CeO₂ with non-ordered mesoporous structure. Ordered mesoporous structure distinctly increases the surface area of ceria, which is great potential to act as supports and expected to improve catalytic performance.

Fig. 4 N₂ adsorption–desorption isotherms (left panel) and corresponding BJH pore size distribution curve (right panel) of nano-CeO₂ (a) and meso-CeO₂ (b).

UV-vis absorption spectra of nano-CeO₂ and meso-CeO₂ materials were displayed in Fig. 5. It can be seen that both of the samples have intense absorption in the UV that trails into the visible region of the spectrum. Meanwhile, UV-vis spectra for meso-CeO₂ and nano-CeO₂ all exhibited two peaks around 270 nm and 350 nm in the UV region, which originated from the charge-transfer transitions from O 2p to Ce 4f in O²⁻ and Ce⁴⁺.⁴⁵ The onset of absorption of meso-CeO₂ was about 437 nm corresponding to the bandgap energy of 2.83 eV (E_BG = 1240/λ_Absorp.Edge), which is in agreed with the characteristic value of ceria material. In comparison, in the case of the nano-CeO₂, the absorption edge of meso-CeO₂ was blue-shifted, which may be due to the smaller size of mesoporous ceria.⁴⁶ Actually, after roughly calculating the crystalline sizes of different ceria by XRD pattern diffraction peaks, the result also demonstrated that meso-CeO₂ possessed smaller crystalline size (see the following discussion).

Fig. 5 UV-vis diffuse reflectance spectra of (a) nano-CeO₂ and (b) meso-CeO₂.

In the present work, the supported Pt NPs on ceria nanocrystals catalysts were characterized by TEM and SEM measurements. Pt NPs were stabilized by PAMAM dendrimers and used as precursors. As shown in Fig. S1,† Pt NPs (dark particles) exhibited monodispersion and small size of around 2 nm. The prepared precursor solution exhibited dark brown (inset in Fig. S1†). After Pt NPs were deposited on meso-CeO₂, as displayed in Fig. 6a, the support retained well-ordered structure and Pt NPs showed good dispersion and small size of around 4 nm. However, after the loading of Pt NPs on the nano-CeO₂, relatively large size with around 10 nm and partial aggregation could be observed (Fig. S2a†). The SAED patterns (insets in Fig. 6a and S2a†) exhibited a dim ring assigning to the (111) plane of face-centered cubic platinum, implying the load of Pt NPs. From the HRTEM of Pt/meso-CeO₂ in Fig. 6b, it was clear that the lattice spacing d value of 0.35 nm and 0.21 nm, assigning to (111) plane of CeO₂ (JCPDS PDF # 43-1002) and (111) plane of platinum (JCPDS PDF # 04-0802), respectively. These observations indicated the existence of cubic crystal ceria and platinum. Furthermore, EDX analysis also certified the prepared catalysts contained Ce and Pt elements (Fig. 6d). According to the SEM image, the synthesized Pt/meso-CeO₂ displayed the approximate sphere morphology (Fig. 6c), which were different from the KIT-6 template with amorphism (Fig. S3†). By comparison, various flakes were clear visible as for the Pt/nano-CeO₂ (Fig. S2b†). In addition, compared with the Pt/nano-CeO₂, EDX mapping images indicated that Pt atoms relatively uniformly dispersed over the observed area of Pt/meso-CeO₂. It was noticed that Pt map was not as dense as Ce, which was owing to low content of Pt in the catalyst. In fact, the total Pt content of Pt/meso-CeO₂ was only 1.5 wt% as measured by ICP-MS. And the Pt content of Pt/nano-CeO₂ was only 1.6 wt%. Consequently, the Pt content of two kinds of catalysts can be considered to be almost no difference.

Fig. 6 (a) TEM image of Pt/meso-CeO₂ (the inset shows the SAED pattern); (b) high-magnified TEM image of Pt/meso-CeO₂; (c) SEM image of Pt/meso-CeO₂ (the insets are the EDX mapping analysis of the Ce, Pt); (d) EDX analysis of Pt/meso-CeO₂.

The XPS spectra were employed to analyze the chemical status of the sample. As observed from Fig. 7a, Ce 3d spectra can be fitted into four peaks at binding energies of 900.9, 898.1, 888.6 and 882.2 eV corresponding to Ce 3d_5/2, whereas the two peaks located at 916.3 and 907.4 eV are attributed to Ce 3d_3/2.⁴⁷ It was also noted that a shoulder peak at about 885.1 eV and a tiny peak at about 905 eV corresponding to the Ce³⁺ state, indicating the existence of oxygen vacancies in as-prepared CeO₂.⁴⁷ In Fig. 7b, the spectra of Pt 4f revealed the two peaks indicative of Pt⁰ at 75.4 eV and 71.1 eV corresponding to Pt 4f_5/2 and Pt 4f_7/2, which also indicated Pt NPs were loaded on ceria.⁴⁸

Fig. 7 XPS spectra of (a) Ce 3d and (b) Pt 4f of the as-prepared Pt/meso-CeO₂.

Wide-angle XRD patterns of prepared samples were reported in Fig. 8. The XRD pattern of KIT-6 presented a broad peak at 2θ = ∼23°, which is ascribed to amorphous silica.⁴⁹ In Fig. 8A, synthesized meso-CeO₂ (b) and nano-CeO₂ (c) were well-resolved and all displayed diffraction peaks at 2θ of 28.6°, 33.2°, 47.4°, 56.4°, 59.1°, 69.4°, 76.8° and 79.1°, which can be indexed to (111), (200), (220), (311), (222), (400), (331) and (420) reflections planes of the FCC fluorite CeO₂ phase (JCPDS PDF # 43-1002). For the (111) plane of ceria, one can see that the peak of meso-CeO₂ was wider than that of nano-CeO₂, implying the smaller nanoparticles for meso-CeO₂.⁵⁰ The particle size of the crystalline meso-CeO₂ was calculated to be 8.5 nm by Scherrer's formula, whereas the particles size was 14.5 nm for nano-CeO₂. Therefore, it can be speculated herein that growth of particles was partially limited by framework of silica template during the formation process of meso-CeO₂. After Pt NPs were immobilized on CeO₂, it was observed that a new weak peak at 39.5° corresponding to (111) plane of platinum in the XRD patterns (Fig. 8B). The extremely weak intensity of the peak was maybe ascribed to homogeneous dispersion on ceria or the small amount of Pt NPs.⁵¹

Fig. 8 (A) XRD patterns of (a) KIT-6, (b) meso-CeO₂ and (c) nano-CeO₂; (B) XRD patterns of (a) Pt/meso-CeO₂ and (b) Pt/nano-CeO₂.

3.2 Catalytic performance

As mentioned above, in our experiments, the catalytic performance of different samples were evaluated employing the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with an excess of NaBH₄ as a test reaction. It is well-known that 4-NP is one of the most common organic pollutants in industrial, while 4-AP has high-value applications include preparation of analgesic and antipyretic drugs, corrosion inhibitor, and so on.^52,53 Generally, 4-NP solution has a distinct absorption peak centered at 317 nm in neutral or acidic situation, while the absorption peak shift to 400 nm immediately upon the addition of NaBH₄ solution, owing to the formation of 4-nitrophenolate ion as the alkalinity of the solution increased (Fig. 9a).⁴⁸ In view of the high kinetic barrier between 4-nitrophenolate ion and BH₄⁻ ion, the reduction of 4-NP proceed extremely slowly without catalyst.⁵⁴ In the absence of catalyst, as expected, the absorption spectra of 4-nitrophenolate ion at 400 nm almost preserved unchanged in intensity for a long duration and the solution still remained yellow, indicating the reduction hardly processed (Fig. 9a). In contrast, when a small amount of Pt/meso-CeO₂ catalyst was added to the solution, the absorption peak at 400 nm dropped successively over the processes and a weak peak at 307 nm concomitantly appeared (Fig. 9b). Meanwhile, the yellow solution faded gradually with time and became colorless eventually, reflecting the successful conversion of 4-NP to 4-AP. This outstanding reaction process is ascribed to that the catalyst acts as an electronic relay system to bring down the kinetic barrier from large potential difference between reaction substrates.⁵⁵ In addition, it was noted that the isosbestic point at 313 nm was displayed in the UV-vis spectra (Fig. 9b), indicating that the catalytic reduction of 4-NP produce 4-AP only without any byproduct.^56,57

Fig. 9 (a) UV-vis absorption spectra of 4-NP before and after the addition of NaBH₄ solution without catalyst; (b) successive UV-vis absorption spectra of 4-NP in the presence of Pt/meso-CeO₂ catalyst; (c) relationship of ln(C_t/C₀) and reaction time for the reduction of 4-NP catalyzed by different samples. Error bars in each point is shown. (d) The reusability test of Pt/meso-CeO₂ catalyst.

In this study, given that the amount of NaBH₄ was in excess compared to 4-NP, thus the concentration of BH₄⁻ nearly remained constant during the reaction. In this way, the reduction process could be reasonably regarded as pseudo-first-order reaction relative to 4-NP.⁵⁸ Fig. 9c showed the linear correlation of ln(C_t/C₀) as a function of time (t) for the reaction catalyzed by different samples, where C_t and C₀ denoted the concentrations of 4-NP at intervals and the initial stage, respectively. The fine linear relationship also certified the reaction followed the first order kinetics. The kinetic rate constants (k), calculated using the rate equation ln(C_t/C₀) = −kt, were 6.03 × 10⁻³ s⁻¹ for the Pt/meso-CeO₂ and 4.07 × 10⁻³ s⁻¹ for the Pt/nano-CeO₂ catalyst, respectively. As a contrast, when pure mesoporous CeO₂ without platinum was chosen to catalyze the same reaction, it was not observed that measurable catalytic activity (blue line in Fig. 9c), suggesting platinum nanoparticles were main in charge of the driving the reduction reaction process. The Pt/meso-CeO₂ exhibited excellent catalytic activity, which demonstrated the utilization of mesoporous supports improved the catalytic activity significantly. In comparison with nano-CeO₂, meso-CeO₂ possessed small size and well-ordered mesoporous structure as presented in XRD patterns and TEM images. In addition, the meso-CeO₂ also had larger surface area than nano-CeO₂. As catalyst supports, such porous structures are beneficial to disperse and stabilize platinum nanoparticles on supports, as well as facilitate the adsorption and diffusion of reactant molecules, enhancing the catalytic activity.

Based on the obtained results, a possible catalytic process was illustrated in Scheme 2. Commonly, in the reduction of 4-AP, the electron transfer from BH₄⁻ (donor) to 4-nitrophenolate ion (acceptor) is depended on metal NPs in the reaction.⁵⁹ Femi level alignment occurs when metal particles contact semiconductor, inducing the charge redistribution.⁶⁰ Pt (5.65 eV) has higher work function than semiconductor of CeO₂ (4.64 eV), thus CeO₂ render the electron to Pt NPs leading to electron-enriched region.⁶¹ Pt NPs accepting electron show surplus electrons effect, which is beneficial to uptake of electrons by adjacent 4-NP molecules to produce 4-AP. At last, 4-AP molecule desorbs from the adsorbed surface quickly. As a result, the synergy effect between platinum NPs and meso-CeO₂ may increase the rate of electron transfer across the interface of the CeO₂–Pt hybrid to facilitate efficient electron transferring through the nanocatalyst.⁶² Additionally, the size of Pt NPs significantly influences transporting efficiency of electron to affect the catalytic efficiency.⁵⁹ As commented before, the meso-CeO₂ can control size and restrict aggregation of Pt NPs relying on unique network structures. That is to say, Pt NPs could exhibit smaller size and fine dispersion for Pt/meso-CeO₂ catalyst. Thus, the improved catalytic performances were not surprising. Besides, the highly branched structure of dendrimers also played a role in determining the dispersion of Pt NPs.

Scheme 2 A probable mechanism of the reduction of 4-nitrophenol over the Pt/meso-CeO₂ catalyst.

In the practical application of catalysis, as we all know, the reusability of catalyst is significant since it is beneficial to make the process cost-effective and eco-friendly. In order to study the reusability of the fabricated Pt/meso-CeO₂ catalyst, the reuse cycles were performed for the reduction of 4-AP. At the end of catalytic reaction, the catalysts were filtered, washed with water, dried and then was used for the next recycle. As shown in Fig. 9d, the Pt/meso-CeO₂ catalyst exhibited similar catalytic performance without visible decrease. Even after five successive recycles, the conversion still maintained more than 90%, indicating good recyclability.

4. Conclusions

We fabricated well-ordered mesoporous CeO₂ with high surface area (115.3 m² g⁻¹) and high crystalline using KIT-6 as hard templates. Particularly, it was ascertained that the optimum ratio of precursor (1.40 g)/KIT-6 (0.3 g) through exploring the effect of mass of impregnant on ceria. Pt NPs stabilized by PAMAM dendrimers were deposited on meso-CeO₂ supports, and exhibited fine dispersion and relatively small size of Pt NPs. When applying in the reduction of 4-nitrophenol, compared to Pt NPs immobilized on nano-CeO₂, Pt/meso-CeO₂ catalyst showed higher catalytic activity corresponding to 6.03 × 10⁻³ s⁻¹. Besides, the fabricated Pt/meso-CeO₂ was reused in the reaction and preserved converse yield of more than 90% after five cycles.

Acknowledgements

The authors are grateful to the financial supports of National Natural Science Foundation of China (Grant No. 21376051, 21106017 and 21306023), Natural Science Foundation of Jiangsu Province of China (Grant No. BK20131288), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2014100), Fundamental Research Funds for the Central Universities (No. 3207045421) and Instrumental Analysis Fund of Southeast University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23472g

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