Self-assembly of hollow spherical nanocatalysts with encapsulated Pt NPs and the effect of Ce-dipping on catalytic activity

Abstract

This article reports a facile and controllable one-step method to construct Pt@hollow mesoporous SiO₂ (Pt@HMSiO₂) nanoparticles (NPs). To enhance the catalytic activity, cerium species were impregnated into Pt@HMSiO₂ NPs, fabricating highly reactive Pt–CeO₂@HMSiO₂ NPs. To verify the successful synthesis of the Pt@HMSiO₂ and Pt–CeO₂@HMSiO₂ NPs, and study the influence of CeO₂ species on the catalytic performance, the as-prepared NPs were characterized by several techniques, including SEM, TEM, EDX, FTIR, XRD, BET and UV-vis analyses. In this work, the reduction of 4-NP was employed as a model reaction to test the catalytic performance. Compared to Pt@HMSiO₂, the Pt–CeO₂@HMSiO₂ NPs show higher catalytic activity, because of the co-catalysis of CeO₂ NPs. However, the excess amount of CeO₂ NPs would lead to a lower catalytic activity, due to the blocking of the catalyst pore. In addition, the Pt–CeO₂@HMSiO₂ NPs show a high thermal stability due to the protection of the SiO₂ shell. Meanwhile, we have also used the reaction of propane dehydrogenation to further verify the excellent catalytic stability of Pt–CeO₂@HMSiO₂ NPs. This strategy is novel, albeit efficient, and can be extended to the preparation of other nanocatalysts.

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Introduction

Recently, hollow nanomaterials have been intensively investigated,^1–9 due to the possession of the functionalities of both cores and shells.^9–11 Meanwhile, hollow nanostructures decorated with functional nanoparticles, which are known as core–shell or yolk–shell nanostructures, have aroused the concern of researchers.^6,12–14 In previous studies, the hard-templating method can control the structure parameters (e.g., shape and size) of the as-synthesized hollow nanostructures by tuning the pre-fabricated templates.¹⁵ In a typical hard-templating approach, the template is first coated with a layer of silica precursors followed by removal of the templates. As a result, the core–shell structures can be further converted into hollow nanostructures.^15–20 However, during the hard-templating synthesis process, the nanostructure and active sites of the NPs are easily altered during removal of the templates.^21,22 Therefore, a novel method for the preparation of hollow nanostructures is meaningful and promising, but challenging.^23,24

It is well known that the electron affinity and electron transfer of neutral noble metal NPs largely depend on their size.^25–28 But, due to high surface energy, noble metal NPs tend to aggregate during the high-temperature catalytic reaction, which results in the rapid catalyst deactivation. Up to now, the studies found the oxide shell structure can not only prevent sintering, but also give rise to other novel properties.^10,29,30 In this regard, fabricating core–shell nanostructures has been extensively known as an efficient and promising strategy to prevent aggregation of noble metal NPs.^31–34 Therefore, in this work, we demonstrate a facile one-pot strategy to the preparation of Pt@HMSiO₂ nanostructures.

Ceria had been studied extensively both in synthesis and applications for the strong metal-support interaction.^35–38 Because of high oxygen storage capacity and abundant oxygen vacancy defects between III and IV oxidation states, the ceria based oxide–metal nanocomposites have become one of the frontiers in the study of hybrid materials.^35–41 Heretofore, most of the reported nanostructures were based on loading noble metal nanoparticles on a large CeO₂ matrix, such as big CeO₂ NPs or hollow structures, which would lead to less contact area.^41,42 In this case, the less ceria–metal interface would be disadvantageous for their catalytic activity. Besides, the segregation of components would occur during reactions, and the activity would decline after several catalytic cycles.⁴³ However, preparing small and well dispersed noble metal NPs around small CeO₂ NPs is still difficult because of the hydrophilic nature of CeO₂. So look, the construction of ceria-based nanocatalysts with abundant cerium–metal interface and good durability is still a great challenge. In our study, we have fabricated the Pt–CeO₂ NPs dispersed uniformly into hollow SiO₂ nanospheres by impregnation. Moreover, we can acquire the highest-performance catalysts by varying cerium solution concentration.

In this article, we have fabricated uniform Pt–CeO₂@HMSiO₂ NPs. The finally obtained Pt–CeO₂@HMSiO₂ NPs exhibit a good performance in the reduction of 4-nitrophenol (4-NP). In addition, benefitting from the separation role of SiO₂ shells, the finally obtained NPs showed good high-temperature stability during the propane dehydrogenation reaction. The detailed structure and synthetic procedures are depicted in Fig. 1. First, Pt@HMSiO₂ NPs were synthesized by a solvothermal method. In the synthesis, H₂PtCl₆ aqueous solution as precursor of Pt, and the boiling citrate solution as the solvent, and polyvinyl pyrrolidone (PVP) as surfactant were used to form Pt–PVP aqueous solution. Then, TEOS was used as precursor, the isopropyl alcohol as the solvent, added into the above Pt–PVP aqueous solution dropwise to construct the designed Pt@HMSiO₂ NPs. Second, the synthesized Pt@HMSiO₂ NPs were deposited in cerium nitrate aqueous solution followed by calcining to fabricate the Pt–CeO₂@HMSiO₂ NPs.

Fig. 1 Schematic illustration for the preparation of a novel Pt–CeO₂@HMSiO₂ nanocatalyst.

Experimental section

Cerium(III) nitrate hexahydrate (Ce(NO₃)₃·6H₂O, 99.99%) was purchased from Aladdin Chemistry Co., Ltd. H₂PtCl₄·6H₂O (≥99.9%), and 4-nitrophenol (≥99%) were purchased from Aldrich. Tetraethyl orthosilicate (TEOS), sodium citrate, isopropanol, ethanol and ammonia solution (28 wt%) were of analytical grade and all of them were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium borohydride (NaBH₄) was purchased from the China National Pharmaceutical Group Corp. Deionized water was used in all experiment. All chemicals were used as received.

Synthesis of monodispersed Pt@HMSiO₂ NPs

Pt@HMSiO₂ NPs were synthesized by a solvothermal method. In the synthesis, 0.05 g trisodium citrate was dissolved in 50 mL boiling deionized water under continuous magnetic stirring to form a clear solution. After 30 min, H₂PtCl₆ aqueous solution as precursor of Pt (0.5 mL, 14 mg mL⁻¹) was added to the above solution under continuous magnetic stirring for 1 hour, and the solution was cooled naturally for 1 hour. Subsequently, the PVP solution (0.5 mL, 12.8 mg mL⁻¹) was added to the above solution. After another hour, the isopropyl alcohol (100 mL) was added into the above solution. Then, 0.5 mL TEOS was added dropwise to construct the designed Pt@HMSiO₂ NPs under continuous stirring for 12 h. After reaction, the solution was centrifuged and washed with isopropyl alcohol and methanol for five times, respectively. The products were then dried under vacuum at 60 °C for 12 h.

Synthesis of Pt–CeO₂@HMSiO₂ NPs

The CeO₂ NPs were added into the Pt@HMSiO₂ nanospheres by impregnation method. In short, the synthesized Pt@HMSiO₂ NPs were dissolved in the different concentrations of Ce(NO₃)₃·6H₂O (50 mL) aqueous solution, respectively. After being stirred for 5 h, the resulting solutions were carefully transferred into the 50 mL centrifuge tubes, centrifuged and washed with deionized water. Then the final products were calcined at 300 °C for 3 h in air.

Catalytic reduction of p-nitrophenol

The reduction of 4-NP by NaBH₄ was chosen as a model reaction to study catalytic properties of the Pt–CeO₂@HMSiO₂ NPs. In this experiment, the activities of the Pt–CeO₂@HMSiO₂ NPs were tested by comparison with different cerium content. Note that 5 mg of the catalyst was dispersed in 10 mL deionized water, and then NaBH₄ aqueous solution (10 mL, 1.2 M) and 4-NP aqueous solution (10 mL, 3.4 mM) were added. The reaction solution was stirred at room temperature, and a UV-vis spectrometer was used to monitor the progress of the reaction at regular intervals.

Catalytic reaction of propane dehydrogenation

Propane dehydrogenation was carried out in a conventional quartz tubular micro-reactor. Prior to testing, the catalyst was reduced in H₂ at 500 °C for 8 h to fully reduce it. The catalyst (mass 1.0 g) was placed into the center of the reactor. Reaction conditions were as follows: 590 °C for reaction temperature, 0.1 MPa pressure, n(H₂)/n(C₃H₈) = 0.25 and the propane weight hourly space velocity (WHSV) was 1.5 h⁻¹. The reaction products were analyzed with an online GC-14C gas chromatography equipped with an activated alumina packed column and a flame ionization detector (FID).

Instrumentation

Transmission electron microscopy (TEM) experiments were conducted on a JEM-1230 microscope operated at 100 kV. The samples for the TEM measurements were suspended in ethanol and supported onto a Cu grid. Scanning electron microscope (SEM) was performed on a Hitachi S-3400N scanning electron microscope and energy dispersive X-ray spectroscopy (EDX) analysis were conducted on a JEM-1230 microscope operated at 100 kV. The powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance Diffractometer (Germany) with Cu Kα radiation (γ = 1.5406 Å). Fourier trans-form infrared (FT-IR) spectra were measured on a Nicolet Magna-IR 750 spectrophotometer. The surface areas were calculated by the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was calculated from the desorption branch using the Barrett–Joyner–Halenda (BJH) theory. UV-vis spectra were recorded on a Shimadzu UV 3600 spectrometer.

Results and discussion

Characterization of Pt@HMSiO₂ and Pt–CeO₂@HMSiO₂ NPs

The as-synthesized NPs were characterized by SEM and TEM. As shown in Fig. 1a, SEM image shows that the size of the as-synthesized Pt@HMSiO₂ is quite uniform with an average diameter of ∼160 nm. The monodisperse spherical morphology of Pt@HMSiO₂ can be further confirmed by the TEM image (Fig. 2c). The TEM image for Pt–PVP NPs is shown in Fig. 2b. The Pt–PVP NPs exhibit an average particle diameter around 2–3 nm. The surfaces of these Pt cubes were stabilized by the bilayer of the PVP surfactants.⁴⁴ These surface-capping PVP surfactants were also used as structure-directing templates for the polymerization of tetraethyl orthosilicate (TEOS) by a sol–gel process, as demonstrated in the synthesis of MCM-41-like ordered mesoporous silicas.^45,46 The TEM image (Fig. 2c) of as-synthesized Pt@HMSiO₂ shows that each hollow silica sphere encapsulated some Pt NPs uniformly. The histogram of Fig. 2c shows that the average thickness of the silica shell was around 17 nm. The small inset at the top right of Fig. 2c clearly shows the surfaces of Pt@HMSiO₂ are rough, because of the existence of hydroxyl groups on the edge of SiO₂ compound (demonstrated in Fig. S2b†). A representative HRTEM image (inset of Fig. 2b) of Pt NPs showed a lattice spacing of 0.218 nm. The X-ray diffraction (XRD) patterns of Pt@HMSiO₂ NPs (Fig. 2d) revealed that the crystal structure of the Pt NPs was (111), (200), (220), and (311) planes of Pt diffraction and the SAED pattern (Fig. S1a†) describes a dim ring corresponding to the (111) plane of face centered cubic platinum, which demonstrates the successful synthesis of Pt NPs. Fig. S1b† is a HAADF-STEM image of the obtained Pt–PVP NPs. Because the HAADF image is a Z-contrast image, the higher contrast parts in the image indicate the Pt NPs. The TEM image (Fig. 2b) and HAADF-STEM image (Fig. S1b†) confirm all Pt NPs are located near the center of the PVP uniformly, which suggests the successful formation of the Pt–PVP. EDX spectroscopy was applied to analyze the Pt@HMSiO₂ obtained. Fig. S3a† verifies the existence of Si, Pt and O elements, further confirming the successful synthesis of the Pt@HMSiO₂ NPs.

Fig. 2 (a) SEM image of Pt@HMSiO₂ NPs. (b) TEM image of Pt–PVP NPs, the inset at the top represents a high resolution TEM image of the Pt NPs. (c) TEM image of Pt@HMSiO₂ NPs. The inset at the top represents the higher magnification of the corresponding nanoparticle. The inset at the bottom is the size distribution histograms of the wall thickness of Pt@HMSiO₂ NPs. (d) XRD pattern of Pt@HMSiO₂ NPs.

An FTIR instrument is also employed to characterize the prepared NPs. Fig. S2a† shows three characteristic band for C–N stretching vibration, C–H bending vibration, and C=O stretching vibration at 1291 cm⁻¹, 1423 cm⁻¹ and 1641 cm⁻¹, respectively, suggesting the existence of PVP species. Fig. S2b† shows characteristic absorption peaks at 467 cm⁻¹, 799 cm⁻¹, 958 cm⁻¹, and 1093 cm⁻¹, corresponding to Si–O bending vibration, Si–O stretching vibration, Si–OH bending vibration, and Si–O–Si antisymmetric stretching vibration, respectively, further confirming the successful preparation of Pt@HMSiO₂ nanospheres. Fig. S2a† demonstrates not only the characteristic band for Pt–PVP, but also a new absorption peak corresponding to the absorption of chloridum at 700 cm⁻¹, suggesting the remnant of chloridum in the Pt–PVP solution. Fig. S2b† displays the broad absorption peaks which appeared at 1420 cm⁻¹ and 1640 cm⁻¹ can be assigned to O–H, and C–H bending vibrations, respectively, providing additional evidence of superfluous organic species of Pt@HMSiO₂ NPs uncalcined.

For acquiring the excellent catalytic performance, based on the synthesized Pt@HMSiO₂ NPs, the Pt–CeO₂@HMSiO₂ NPs were obtained via a facile impregnation method. TEM images of the Pt–CeO₂@HMSiO₂ NPs are shown in Fig. 3a and b, respectively. A representative HRTEM image (inset of Fig. 3a) of CeO₂ NPs showed a lattice spacing of 0.312 nm. The SAED pattern (inset of Fig. 3a) described a dim ring corresponding to the (111) plane of the cubic CeO₂ phase, which further proves that the CeO₂ NPs have been developed after the impregnation process. The existence of Ce and Pt elements of the nanocomposites could also be proved by the EDX analysis (Fig. S3†). Meanwhile, the chemical composition of the surfaces of the synthesized NPs was investigated by EDX spectroscopy analysis (the insets of Fig. S3†). As shown in the inset of Fig. S3b,† the cerium content, the Pt content and Si content can be observed to be 0.2%, 0.7% and 26.6% in mean atomic value, respectively. Besides, the inset of Fig. S3c† shows the cerium content, the Pt content and Si content are 4.4%, 0.6% and 22.0% in mean atomic value, respectively. To further confirm the successful synthesis of the Pt–CeO₂@HMSiO₂ NPs, the XRD pattern of the as-prepared sample is investigated, shown in Fig. 3c. As can be noted, the diffraction peaks of (111), (200), (220), (311), (222), (400), (331) and (420) planes can be indexed to the cubic fluorite-type CeO₂ structure, respectively (JCPDS card no. 65-2975). Apparently, the strong diffraction peaks confirm the good crystallinity of the samples. Meanwhile, combining with the Fig. 2d, no additional peaks can be observed, which implies the phase purity of the as-prepared CeO₂ NPs. As a result, the constructed Pt–CeO₂@HMSiO₂ NPs may act as a good catalyst, due to the characteristics of small particle size, planes of CeO₂ and plenty of Pt–CeO₂ interfaces. Besides, the silica shell also favors catalytic reactions, particularly, durability of Pt–CeO₂@HMSiO₂ as nanocatalyst.⁴³

Fig. 3 TEM images of (a) PtCe-0.2 NPs and (b) PtCe-4.4 NPs. The SAED pattern and high resolution TEM image were obtained from PtCe-0.2 NPs sample in the corresponding insets. (c) XRD pattern of Pt–CeO₂@HMSiO₂ NPs.

A new mechanism for the formation of Pt@HMSiO₂ NPs has been schematically illustrated in Fig. 4. As is well known, the ammonia content controls the growth rate and self-assembly process of forming the SiO₂ nanospheres. So, by adding different dosage of ammonia water, the three different NPs are formed. When PVP : NH₃·H₂O = 1 : 6, like reversed micelle as Fig. 4a, the Pt@mSiO₂ NPs were formed. The reason is that the excess addition of ammonia contributes to the ionic strength increase of the solution due to the dissociation of ammonia, which leads to the aggregation of Pt NPs.⁴⁷ As shown in Fig. 4b, when the PVP : NH₃·H₂O = 1 : 3, the even role of PVP and NH₃·H₂O on Pt NPs leads to the Pt NPs uniformly dispersed in the inner of silica spheres, forming the (Pt/SiO₂)@mSiO₂ NPs. The formation of the Pt@HMSiO₂ NPs is because the more content of PVP which is linking the Pt NPs gathers inward, like micelle, as shown in Fig. 4c. In this case, the hydrolysis of TEOS would proceed around the PVP. As a result, the structure of hollow nanospheres which are composed of the Pt NPs and mesoporous SiO₂ shells can be formed. Moreover, the size of the Pt@HMSiO₂ can increase by controlling the content of TEOS. The result indicates that the addition of different ammonia water content is not only as reaction media but also plays an important role on the formation of hollow SiO₂ spheres. The TEM images of the products, forming schematic and simulation drawings are shown in Fig. 4, respectively, and the corresponding dosage of the ammonia is 3 mL (a), 1.5 mL (b), and 1 mL (c). Hence, the right usage of ammonia is the key to synthesizing the desired sample.

Fig. 4 Speculated mechanism of forming Pt@HMSiO₂ NPs. The figure includes TEM images, forming schematic and simulation drawings of Pt@mSiO₂ (a), (Pt/SiO₂)@mSiO₂ (b), Pt@HMSiO₂ (c) NPs; the corresponding dosage of adding the ammonia is 3 mL (a), 1.5 mL (b), and 1 mL (c), respectively.

Fig. 5 TEM images of (a) Pt@HMSiO₂ NPs and (b) PtCe-0.2 NPs, calcined at 550 °C, ​respectively.

The porosity and the pore size distribution of the as-prepared Pt–CeO₂@HMSiO₂ hollow nanospheres were determined using N₂ adsorption–desorption isotherm. Fig. S4† displays the N₂ adsorption–desorption isotherm and pore size distribution of the mesoporous hollow spheres. As can be observed, the obtained isotherm (Fig. S4a†) shows the special surface area of Pt–CeO₂@HMSiO₂ NPs was 127 m² g⁻¹. Meanwhile, from the results of pore size distribution (Fig. S4b†), the Pt–CeO₂@HMSiO₂ NPs exhibits a bimodal distribution with one centered in the mesoporous range (3.5 nm) and the other around 55 nm. Usually, a large BET surface area and the hierarchical mesoporous architectures are beneficial to enhance the catalytic ability of catalyst.²¹

Catalytic activity characterization

Herein, the reduction reaction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) was employed as a model reaction to test the catalytic activity of the prepared nanocatalyst.^48–52 In the reaction, to avoid the influence of the concentration of NaBH₄ on the reduction rate, an excess amount of NaBH₄ was added, and the reduction process could be considered as a first-order reaction. With the addition of NaBH₄, the absorption maximum of 4-NP at 317 nm shifts to 400 nm due to the formation of 4-nitro-phenolate ion.⁴⁹ After adding a little bit of catalysts into the solution, the adsorption peak point lowered observably as the reaction proceeds (Fig. 6a). Moreover, because of the inertia of 4-NP to NaBH₄, the reduction reaction would not proceed without catalyst.³² For reducing errors, the dosage of every reagent kept constant in all catalytic runs.

Fig. 6 (a) Successive UV-vis adsorption spectra of the reduction of 4-NP by NaBH₄ in the presence of PtCe-0.2 catalyst. (b) Plots of ln(C_t/C₀) of 4-NP against time: (a) PtCe-0, (b) PtCe-0.2 and (c) PtCe-4.4. (c) Rate constants as a function of cycle numbers in seven successive reduction recyclings using PtCe-0.2 nanocatalyst. (d) Propane conversion vs. time on stream of the PtCe-0.2 and Pt/SiO₂ nanocatalyst.

As shown in Fig. 6b, in the reaction catalyzed by different samples, the linear relationships between ln(C_o/C_t) and reaction time (C_t is the ordinate values of the absorption peak at 400 nm) well match the first-order reaction kinetics. Considering the dosage of NaBH₄ is enough, the reaction rate can be assumed to be dependent of the concentration of the reactants. Thus, the catalytic rate with regard to 4-NP can be evaluated by using a pseudo-first-order rate kinetics. The rate constant k_app is calculated from the slope of the linear relationships between ln(C_o/C_t) and reaction time. The apparent rate constants (k_app) of the as-prepared PtCe-0 (pure Pt@HMSiO₂ sample), PtCe-0.2 (the content of CeO₂ in Pt–CeO₂@HMSiO₂ sample: 0.2%), and PtCe-4.4 (the content of CeO₂ in Pt–CeO₂@HMSiO₂ sample: 4.4%) samples are 0.1817 min⁻¹, 0.3014 min⁻¹ and 0.0785 min⁻¹, respectively. However, in sharp contrast, the sample of PtCe-0.2 shows the higher catalytic activity (0.3014 min⁻¹) in this work which is roughly 2 times higher than that of the PtCe-0 sample. In addition, the sample of PtCe-4.4 shows the lower catalytic activity. The blocking of the catalyst pore by the excess CeO₂ NPs which are formed in the process of impregnation is the reason for the deactivation. The fact reveals that the dosage of cerium species plays a key role in the reaction rate. With this experiment, the incorporation of SiO₂ shell would also affect the diffusion of reactants inside the shell, limiting their access to the catalytic surface. As is well known, if not the protection of SiO₂ shell, Pt nanoparticles would aggregate easily in the reaction. It is worth mentioning that the SiO₂ shell is mesoporous, which would decrease the limiting of reactants access to the catalytic surface.

To explain the improved catalytic activity of the Pt–CeO₂@HMSiO₂ NPs compared with Pt@HMSiO₂, we considered the band structure of CeO₂ and the Fermi level of Pt. As is well known, CeO₂ is an n-type semiconductor and electrons would transfer from CeO₂ to Pt, when they were in contact with each other. Meanwhile, a band bending would arise at the surface of the CeO₂, which restraints the transfer of more electrons followed by forming a Schottky barrier.⁴³ As shown in Fig. 7, Pt (5.65 eV) has a higher work function than CeO₂ (4.64 eV), so electrons leave from the CeO₂ into the near Pt NPs, which leads to an electron-enriched region. The electrons on the conductive band of CeO₂ would transfer to the Fermi level of platinum through conquering the Schottky barrier, which leads to effective charge separation and suppresses the recombination of electrons and holes, followed by improving catalytic activity of Pt–CeO₂@HMSiO₂ NPs. When CeO₂ absorbs BH₄⁻¹, electrons on the valance band could be excited to the conductive band, and holes left on the valance band, where BH₄⁻¹ is oxidized to BO₂⁻¹ by giving away the electron. In other words, the Pt–CeO₂ interface serves as the electron relay for accepting electrons from BH₄⁻¹ ions and conveys them to 4-NP molecules, which ultimately results in the reduction of 4-NP. The present studies found charge separation on the interface between oxide and noble metal is important for excellent catalytic activity, which was observed not only in ceria, but also in other semiconductor based noble-metal nanocatalysts.^43,53–55

Fig. 7 Speculated mechanism of the catalytic reduction of 4-NP with the PtCe-0.2 nanocatalyst.

Durability of Pt–CeO₂@HMSiO₂ as nanocatalyst

The durability is the other important character of a good nanocatalyst. To study the durability of the Pt–CeO₂@HMSiO₂, a cyclic test was executed. The previous studies found small NPs tend to aggregate to minimize the surface energy during catalytic reactions and the composite structure may be destroyed, followed by the deactivation.^56–60 As shown in Fig. 6c, the Pt–CeO₂@HMSiO₂ nanocatalysts could be successfully recycled and reused for at least seven times with a stable transformation rate. TEM measurement (Fig. S6†) of the final nanocatalyst reveals that the particles size of Pt NPs and the structure of the nanocatalyst were well retained after seven repeating catalytic cycles. As shown in Fig. 5, it is obvious that Pt@HMSiO₂ and Pt–CeO₂@HMSiO₂ NPs remain virtually unchanged upon calcination at 550 °C. Compared to Pt/SiO₂ NPs (Fig. S5†), the good durability of these NPs is due to the separation role of SiO₂ shells, which prevents the aggregation of the active Pt NPs and composite structure breakage during the catalytic reactions, despite high-temperature reaction.⁶¹ Meanwhile, we have also used the reaction of propane dehydrogenation to further verify the excellent catalytic stability of Pt–CeO₂@HMSiO₂ NPs. As presented in Fig. 6d, the initial conversions of propane catalyzed by PtCe-0.2 and Pt/SiO₂ NPs are 21.5 and 25.5%, respectively. Moreover, the deactivation parameter D for these catalysts are 9.85 and 35.07%, respectively (defined as D = [X₀ − X_f] × 100%/X₀, where X₀ is the initial propane conversion and X_f is the final propane conversion). These findings clearly suggest that PtCe-0.2 NPs possess the higher catalytic stability and thermal stability due to the help of SiO₂ shell. Furthermore, as shown in Fig. S7,† after reaction for 8 h, nanostructure of PtCe-0.2 almost keeps unchange. Therefore, the good durability and high catalytic activity strongly suggest that the finally obtained Pt–CeO₂@HMSiO₂ NPs would have potentials in catalytic oxidation of carbon monoxide applications.^62–64

Conclusions

In this article, Pt@HMSiO₂ NPs have been synthesized by a novel one-step method. Meanwhile, to enhance the catalytic activity, cerium species have been impregnated into Pt@HMSiO₂ NPs to fabricate the Pt–CeO₂@HMSiO₂ NPs. Compared to Pt@HMSiO₂, the Pt–CeO₂@HMSiO₂ NPs show higher catalytic activity, because of the existence of CeO₂ NPs. However, the excess CeO₂ NPs would lead to the blocking of the catalyst pore, which is the reason for the deactivation of PtCe-4.4 sample. The fact reveals that the dosage of cerium species plays a key role on the reaction rate. In addition, the Pt@HMSiO₂ and Pt–CeO₂@HMSiO₂ NPs show the high thermal stability and catalytic stability due to the protection of SiO₂ shell. This work developed a novel method to synthesize Pt@HMSiO₂, and can be extended to other composites of noble metals and oxides. Finally, we can speculate the obtained Pt–CeO₂@HMSiO₂ NPs would have potentials in catalytic oxidation of carbon monoxide applications.

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 (Grant No. BK20131288), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2014100), Qing Lan Project of Jiangsu Province, The Fundamental Research Funds for the Central Universities (3207045421, 3207046302, 3207046409) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) (1107047002).

Notes and references

1. S. Q. Zhao, T. M. Liu, S. J. Zheng, W. Zeng, T. F. Li, Y. Zhang, S. H. Hussain, D. W. Hou and X. H. Peng, Mater. Lett., 2016, 168, 13–16.
2. K. Z. Qi, R. Selvaraj, T. Al Fahdi, S. Al-Kindy, Y. Kim, C. W. Tai and M. Sillanpaa, Mater. Lett., 2016, 166, 116–120.
3. S. B. Atla, Y. J. Chen, H. W. Chiu, C. C. Chen, J. C. Shu and C. Y. Chen, Mater. Lett., 2016, 167, 238–241.
4. V. Nagarajan and R. Chandiramouli, J. Inorg. Organomet. Polym., 2016, 26, 394–404.
5. J. B. Joo, H. Y. Liu, Y. J. Lee, M. Dahl, H. X. Yu, F. Zaera and Y. D. Yin, Catal. Today, 2016, 264, 261–269.
6. L. Dubau, M. Lopez-Haro, J. Durst and F. Maillard, Catal. Today, 2016, 262, 146–154.
7. H. M. Ma, Y. Li, Y. L. Wang, L. H. Hu, Y. Zhang, D. W. Fan, T. Yan and Q. Wei, Biosens. Bioelectron., 2016, 78, 167–173.
8. S. K. Wu, X. P. Shen, G. X. Zhu, H. Zhou, Z. Y. Ji, K. M. Chen and A. H. Yuan, Appl. Catal., B, 2016, 184, 328–336.
9. P. M. Arnal, M. Comotti and F. Schuth, Angew. Chem., Int. Ed., 2006, 45, 8224–8227.
10. J. Lee, J. C. Park and H. Song, Adv. Mater., 2008, 20, 1523–1528.
11. S. H. Wu, C. T. Tseng, Y. S. Lin, C. H. Lin, Y. Hung and C. Y. Mou, J. Mater. Chem., 2011, 21, 789–794.
12. P. P. Yang, Y. Xu, L. Chen, X. C. Wang and Q. Zhang, Langmuir, 2015, 31, 11701–11708.
13. J. Y. Gu, X. M. Wang, L. Tian, L. Feng, J. Y. Qu, P. G. Liu and X. Zhang, Langmuir, 2015, 31, 12530–12536.
14. G. J. Cui, Z. B. Sun, H. Z. Li, X. N. Liu, Y. Liu, Y. X. Tian and S. Q. Yan, J. Mater. Chem. A, 2016, 4, 1771–1783.
15. A. H. Lu, T. Sun, W. C. Li, Q. Sun, F. Han, D. H. Liu and Y. Guo, Angew. Chem., Int. Ed., 2011, 50, 11765–11768.
16. M. Kim, K. Sohn, H. Bin Na and T. Hyeon, Nano Lett., 2002, 2, 1383–1387.
17. B. Zhao and M. M. Collinson, Langmuir, 2012, 28, 7492–7497.
18. T. Harada, S. Ikeda, F. Hashimoto, T. Sakata, K. Ikeue, T. Torimoto and M. Matsumura, Langmuir, 2010, 26, 17720–17725.
19. S. Ikeda, S. Ishino, T. Harada, N. Okamoto, T. Sakata, H. Mori, S. Kuwabata, T. Torimoto and M. Matsumura, Angew. Chem., Int. Ed., 2006, 45, 7063–7066.
20. T. Harada, S. Ikeda, Y. H. Ng, T. Sakata, H. Mori, T. Torimoto and M. Matsumura, Adv. Funct. Mater., 2008, 18, 2190–2196.
21. Y. Xu, Y. Zhang, Y. Zhou, S. Xiang, Q. Wang, C. Zhang and X. Sheng, RSC Adv., 2015, 5, 58237–58245.
22. M. Zhang, Y. Zhang, X. Sheng, Y. Zhou, S. Zhao, X. Fu and H. Zhang, RSC Adv., 2016, 6, 35076–35085.
23. N. Zhang and Y.-J. Xu, Chem. Mater., 2013, 25, 1979–1988.
24. Y. Zhang, S. Xiang, Y. Zhou, Y. Xu, Z. Zhang, X. Sheng, Q. Wang and C. Zhang, RSC Adv., 2015, 5, 48187–48193.
25. J. L. Gong, Chem. Rev., 2012, 112, 2987–3054.
26. H. B. Chong, P. Li, J. Xiang, F. Y. Fu, D. D. Zhang, X. R. Ran and M. Z. Zhu, Nanoscale, 2013, 5, 7622–7628.
27. Y. Liu, P. She, J. Gong, W. P. Wu, S. M. Xu, J. G. Li, K. Zhao and A. P. Deng, Sens. Actuators, B, 2015, 221, 1542–1553.
28. B. Zhang, H. Asakura, J. Zhang, J. Zhang, S. De and N. Yan, Angew. Chem., Int. Ed. Engl., 2016, 55, 1–6.
29. Y. Hu, Y. Wang, Z.-H. Lu, X. Chen and L. Xiong, Appl. Surf. Sci., 2015, 341, 185–189.
30. C. Zhang, Y. Zhou, Y. Zhang, Z. Zhang, Y. Xu and Q. Wang, RSC Adv., 2015, 5, 64951–64960.
31. C. Zhang, Y. Zhou, Y. Zhang, Z. Zhang, Y. Xu and Q. Wang, Powder Technol., 2015, 284, 387–395.
32. Z. Zhang, Y. Zhou, Y. Zhang, S. Zhou, S. Xiang, X. Sheng and P. Jiang, J. Mater. Chem. A, 2015, 3, 4642–4651.
33. Y. Zhang, Y. Zhou, Z. Zhang, S. Xiang, X. Sheng, S. Zhou and F. Wang, Dalton Trans., 2014, 43, 1360–1367.
34. H. Zhang, Y. Zhang, Y. Zhou, C. Zhang, Q. Wang, Y. Xu and M. Zhang, RSC Adv., 2016, 6, 18685–18694.
35. G. Cheng, J. L. Zhang, Y. L. Liu, D. H. Sun and J. Z. Ni, Chem. Commun., 2011, 47, 5732–5734.
36. P. Lakshmanan, J. E. Park, B. Kim and E. D. Park, Catal. Today, 2016, 265, 19–26.
37. C. Zhang, Y. Zhou, Y. Zhang, Q. Wang and Y. Xu, RSC Adv., 2015, 5, 12472–12479.
38. Q. Wang, Y. Zhang, Y. Zhou, Z. Zhang, J. Xue, Y. Xu, C. Zhang, X. Sheng and N. Kui, RSC Adv., 2016, 6, 730–739.
39. P. Sudarsanam, B. Mallesham, P. S. Reddy, D. Grossmann, W. Grunert and B. M. Reddy, Appl. Catal., B, 2014, 144, 900–908.
40. N. Ta, J. Y. Liu, S. Chenna, P. A. Crozier, Y. Li, A. L. Chen and W. J. Shen, J. Am. Chem. Soc., 2012, 134, 20585–20588.
41. J. A. Rodriguez, R. Si, J. Evans, W. Q. Xu, J. C. Hanson, J. Tao and Y. M. Zhu, Catal. Today, 2015, 240, 229–235.
42. S. Q. Liu, M. J. Xie, X. F. Guo and W. J. Ji, Mater. Lett., 2013, 105, 192–195.
43. H. S. Wu, L. D. Sun, H. P. Zhou and C. H. Yan, Nanoscale, 2012, 4, 3242–3247.
44. H. Lee, S. E. Habas, S. Kweskin, D. Butcher, G. A. Somorjai and P. Yang, Angew. Chem., Int. Ed. Engl., 2006, 45, 7824–7828.
45. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 1992, 359, 710–712.
46. A. Corma, Chem. Rev., 1997, 97, 2373–2419.
47. D. D. Lekeufack, A. Brioude, A. Mouti, J. G. Alauzun, P. Stadelmann, A. W. Coleman and P. Miele, Chem. Commun., 2010, 46, 4544–4546.
48. X. Chen and S. S. Mao, Chem. Rev., 2007, 107, 2891–2959.
49. P. Herves, M. Perez-Lorenzo, L. M. Liz-Marzan, J. Dzubiella, Y. Lu and M. Ballauff, Chem. Soc. Rev., 2012, 41, 5577–5587.
50. N. Pradhan, A. Pal and T. Pal, Colloids Surf., A, 2002, 196, 247–257.
51. K. Esumi, R. Isono and T. Yoshimura, Langmuir, 2004, 20, 237–243.
52. Q. Zhang, T. R. Zhang, J. P. Ge and Y. D. Yin, Nano Lett., 2008, 8, 2867–2871.
53. Y. Q. Cai, Z. Q. Bai, S. Chintalapati, Q. F. Zeng and Y. P. Feng, J. Chem. Phys., 2013, 138, 154711.
54. R. L. Park, Science, 1988, 241, 1839.
55. J. M. Song, S. Y. Chen, Y. L. Shen, C. H. Tsai, S. W. Feng, H. T. Tung and I. G. Chen, Appl. Surf. Sci., 2013, 285, 450–457.
56. R. Guttel, M. Paul, C. Galeano and F. Schuth, J. Catal., 2012, 289, 100–104.
57. R. X. Jin, Y. Yang, Y. F. Li, L. Fang, Y. Xing and S. Y. Song, Chem. Commun., 2014, 50, 5447–5450.
58. Y. Ming, L. Sha, W. Yuan, A. H. Jeffrey, X. Ye, F. A. Lawrence, L. Sungsik, H. Jun, M. Manos and F.-S. Maria, Science, 2014, 346, 1498–1501.
59. C. Liu, Y. Xu, L. Wu, Z. Jiang, B. Shen and Z. Wang, J. Mater. Chem. A, 2015, 3, 10566–10572.
60. H. Zhang, Y. Zhang, Y. Zhou, C. Zhang, M. Zhang, S. Zhao, J. Fang and X. Sheng, J. Alloys Compd., 2016, 688, 23–31.
61. S. H. Joo, J. Y. Park, C. K. Tsung, Y. Yamada, P. Yang and G. A. Somorjai, Nat. Mater., 2009, 8, 126–131.
62. Y. W. Zhang, Y. M. Zhou, J. J. Shi, X. L. Sheng, Y. Z. Duan, S. J. Zhou and Z. W. Zhang, Fuel Process. Technol., 2012, 96, 220–227.
63. A. U. Nilekar, S. Alayoglu, B. Eichhorn and M. Mavrikakis, J. Am. Chem. Soc., 2010, 132, 7418–7428.
64. X. Liang, J. Xiao, B. Chen and Y. Li, Inorg. Chem., 2010, 49, 8188–8190.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13245f

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