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A novel 2-dimensional catalytic system was developed in which platinum nanoparticles (Pt NPs) were immobilized on exfoliated MgAl-layered double hydroxide (LDH) nanosheets through an electrostatic self-assembly between negatively charged Pt NPs and positively charged LDH nanosheets. The LDH nanosheets effectively provided the large double sides of hydroxide functionality to absorb the Pt NPs, as well as fast diffusion rates of the incoming reactants into catalyst surfaces. This new nanostructure improved the rate of reaction, turnover frequency and reaction durability of Pt NPs on LDH nanosheet without significant loss in conversion efficiencies for the reduction of p-nitrophenol into p-aminophenol by NaBH4, maintaining more than 97% of catalytic conversions compared to free Pt NPs as well as commercial Pt/C catalyst.
Owing to thermodynamically unstable surface atoms and high surface-to-volume ratio of nanomaterials, transition-metal nanoparticles (NPs) have been used in the field of the heterogeneous catalysts over the past several decades.1 The unique characteristics of nanomaterials have consistently required developments in the surface stabilization of the individual NPs with organic molecules.2 However, organic stabilizers could hinder most active surface sites of the metal NPs to block their catalytic functions. Immobilizations of the metal NPs on desired solid supports such as metal oxides,1c graphitic carbons,3 and porous silica4 prevent agglomeration of the metal NPs, which has led to the poisoning of catalytic activities.5 Metal NPs on supports function in repeated recycles without organic stabilizers, maintaining high performance as heterogeneous catalysts. Nevertheless, the following common problems still exist in the development of new catalysts: (1) the use of covalent chemical linkers to bind metal NPs on the surface of solid supports, (2) loading of the metal NPs by impregnation onto the limited areas of mesoscopic supports, which produced irregular-size NPs, and (3) low dispersion capability of solid supports in solution that can restrict the practical applications of the metal NPs. Thus, the development of new types of solid supports needs a large open surface and reactive surface functionalities, which could bind to metal NPs, such as layered double hydroxides (LDH).6 In previous reports, we demonstrated the useful application of surface potentials for the charged particles, such as zeolite crystals, proteins, polymer beads, and surface-modified LDHs, which drove their electrostatic assemblies on the as-prepared or chemically modified LDH surface to produce complex nanostructures.7
To date, powdery LDHs as catalyst supports have been widely reported with polyoxometalates8 and transition-metal NPs.9 Metal NPs (e.g., Pt, Pd) impregnated into the LDHs by in situ chemical reduction of intercalated metal chlorides9a,b were formed on the edge surfaces of the LDH particles with an irregular size distribution, which is attributed to restricted interlayer galleries. These metal NPs may be readily removed from the LDHs during catalytic reactions, causing a significant loss in their catalytic abilities. In contrast, 2 D nanosheets of exfoliated LDHs were qualified as ideal catalytic supports toward metal NPs, as well as 2 D building blocks for the construction of various functional materials,10–13 because of the large surface area and ultrathin polar layer. The exfoliation of LDHs produces 1–2 nm thick nanosheets with ultimately 2 D anisotropy and extremely high surface charge density.14 The 2 D LDH nanosheets effectively provide the large double sides of hydroxide functionality to adsorb metal NPs, as well as fast diffusion rates of the incoming reactants into the catalytic surfaces thanks to Lewis base characteristics. Particularly, a strong electric field of the LDH nanosheets improves the loading capability for the metal NPs.
In this study, we explore the idea of electrostatic assembly between Pt NPs and exfoliated LDH nanosheets that were completely delaminated from LDH crystals (Mg2Al(OH)6(CO3)0.5⋅nH2O). This new nanocomposite catalyst exhibits high catalytic activity and remarkable durability in solution reactions. The hybrid catalysts (ex-LDH–Pt) were prepared by the careful mixing of two solutions of exfoliated LDH nanosheets and Pt NPs, as schematically described in Scheme 1 a. The ex-LDH–Pt could be dispersed in formamide and water. Two samples had similar stability in solutions without phase segregation, supporting the strong adhesion of the negatively charged Pt NPs15 on the positively charged LDH nanosheets (+29 mV of zeta potential). The catalytic functions of two supplementary systems were also evaluated: the Pt NPs without solid supports, and the Pt NPs in LDH (in-LDH–Pt). The later samples were prepared by the in situ chemical reduction of [PtCl6]2− ions partially incorporated into the interlayer spaces of LDHs (LDH–[PtCl6]2−), as schematically described in Scheme 1 b.
The XRD pattern of the LDH–CO3 indicated a well-crystallized rhombohedral phase with lattice parameters of a=3.04 Å (a=2d(1 1 0)) and c=22.68 Å, as shown in Figure 1 a. The LDH-NO3 and LDH-Cl had basal spacings of 8.96 Å and 7.65 Å, respectively. The exfoliated LDH in formamide exhibited no peak in the XRD patterns, owing to the amorphous random orientation of LDH nanosheets.16 The LDH–[PtCl6]2− phase gave two peaks at 2θ values of 17.2° (5.16 Å) and 25.6° (3.47 Å). The absence of (0 0 1) was ascribed to the increase of electron density in the mid-plane after the intercalation of [PtCl6]2−.17 The XRD pattern in Figure 1 d also indicated the partial formation of the LDH–[PtCl6]2− phase, which disappeared after the reduction of [PtCl6]2−. The XRD patterns of in-LDH–Pt were similar to those of the corresponding LDH–Cl phase, and a broad diffraction for metallic Pt was also found, as shown in Figure 1 e.
Figure 1. XRD patterns of a) LDH–CO3, b) LDH–NO3, c) LDH–Cl, d) LDH–[PtCl6]2−, and e) in-LDH–Pt. In part d, • indicates LDH–[PtCl6]2− phase. Unit of the d-values is Å.
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The TEM data (Figure 2) demonstrated the successful synthesis of Pt NPs and monodisperse distribution of Pt NPs on ex-LDH–Pt nanosheets. The ex-LDH–Pt had plate-like morphology and the same 2 D structure as the samples before the formation of hybrid composites. The high-resolution TEM (HRTEM) image in Figure 2 d indicated that the Pt NPs were kept intact during the recovery process, which was ascribed to the hydrophilic surface of LDH nanosheets, and the strong binding of Pt NPs. Notably, the aqueous solution of the hybrid catalyst was monodisperse and stable for up to approximately 6 h, although Pt NPs and aqueous LDH nanosheets in solution were not redispersed once they were precipitated, as shown in Figure 2 g. This feature of common colloid nanomaterials limits their practical application in catalytic reactions.
Figure 2. TEM images of a) Pt NPs prepared by methanol/citrate method, b) ex-LDH–Pt; c) enlarged image of selected area of part b; d) HRTEM image of Pt NPs in ex-LDH–Pt; TEM images of e) LDH–[PtCl6]2− and f) in-LDH–Pt. Inset of part e is enlarged image and inset of (f) is washed sample. g) Photograph of colloids of Pt NPs in water (1, 2), exfoliated LDH nanosheets in formamide (3), and ex-LDH–Pt in formamide (4) and in water (5, 6).
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The uniform Pt distribution in LDH–[PtCl6]2− phase (Figure 2 e) supported the exchange of [PtCl6]2− anions in the interlayer space of LDH. However, we note that a large portion of Pt NPs in the in-LDH–Pt samples after reduction steps was located at the edge-on surface of LDH particles with irregular size distribution, as shown in Figure 2 f. This observation demonstrated that the in situ reduction was inappropriate for the well-dispersed Pt NPs on the interlayer surface of LDH, which was supported by the broad (1 1 1) reflection peak of the XRD for platinum metal in Figure 1 e. The [PtCl6]2− ions were released from the interlayer spaces of LDH hosts during the course of in situ reduction to form Pt NPs, which could be readily removed during the washing steps, and during the catalytic reactions.
The Raman data in Figure 3 give complementary evidence for the chemical and structural modifications of the samples described above. The as-prepared LDH–CO3 showed sharp absorption bands attributable to aluminum hydroxide at 557 cm−1, and to carbonate at 1064 cm−1.18 The strong doublet peaks of the LDH–[PtCl6]2− sample at 332 and 338 cm−1 were assigned as vibration modes of intercalated [PtCl6]2− ions,19 which disappeared after the reduction of [PtCl6]2−, as shown in Figure 3 c. The two absorption bands of the Pt NPs at 1016 and 1456 cm−1 were attributed to citrate ions on the Pt surface, which eventually disappeared after the formation of ex-LDH–Pt, as shown in Figure 3 e. The absence of the capping molecule of Pt NPs in ex-LDH–Pt might lead to a high catalytic activity of the ex-LDH-Pt catalyst.
Figure 3. Raman spectra of a) LDH–CO3, b) LDH–[PtCl6]2−, c) in-LDH–Pt, d) Pt NPs, and e) ex-LDH-Pt (absorption band of ▪: AlOH, ▴: CO32−, ▾: [PtCl6]2−, •: citrate, ○: Cl−).
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The catalytic activities of three different catalysts were evaluated by employing the reduction of p-nitrophenol into p-aminophenol by NaBH4, which is a useful system for the analysis of the catalytic performance of Pt NPs.20 In this study, excess NaBH4 compared to p-nitrophenol ([NaBH4]/[p-nitrophenol]=96) was added so that the reduction rate could be assumed to be independent of the concentration of NaBH4. The Pt contents in 1 mL of ex-LDH–Pt, Pt NPs, and in-LDH–Pt were 185, 189, and 431 ppm, respectively, as determined by inductively coupled plasma mass spectrometry (ICP–MS). For comparison, the catalytic activity of commercial Pt on activated carbon (Pt/C, ≈200 ppm) was also investigated. After the addition of the catalysts, the intensity of the absorption peak at 400 nm associated with p-nitrophenolate21 was gradually dropped as the reaction proceeded (Figure 4 a–c), demonstrating similar evolution trends in absorbance bands for the ex-LDH–Pt, Pt NPs, and commercial Pt/C catalysts. Considering the linear relationships between ln(Ct/C0) (C=concentration) and reaction time, all of these reduction reactions exhibited first-order reaction kinetics (Figure 4 d). The Pt NPs exhibited slightly higher catalytic activity in reaction rate and turnover frequency (TOF) than those of the supported catalysts (Table 1). As the ex-LDH–Pt possesses the same amount of Pt as the free Pt NPs, this is dominantly ascribed to the partially blocked surfaces of immobilized Pt NPs by the LDH nanosheet, which leads to less-accessible surfaces for the diffusion of reactants than in the case of the free Pt NPs. Importantly, the ex-LDH–Pt exhibited more competitive catalytic activity than other supported Pt catalysts (clay/Pt,20d TOF=0.1 min−1; CNT/Pt,20b TOF=0.67 min−1) toward the reduction of p-nitrophenol.
Figure 4. UV/Vis spectra for the reduction of p-nitrophenol with excess NaBH4 using a) ex-LDH–Pt, b) Pt NPs, and c) commercial Pt/C; d) the relationship between ln(Ct/C0) and reaction time (t), in which the ratio of p-nitrophenol (Ct at time t) to its initial value C0 was directly obtained by the relative intensity of the respective absorbance At/A0 with absorption peaks at 400 nm; e) recycling conversion percentages and f) recycling catalytic activity profiles for ex-LDH–Pt (•), Pt NPs (▾), in-LDH–Pt (▪), and commercial Pt/C (○). The reaction profiles in part f were obtained after the 4th catalytic reaction. Arrows 1–4 indicate the reaction profiles for each recycling step of the ex-LDH–Pt catalyst.
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Table 1. Rates of reaction and TOFs for the reduction of p-nitrophenol by the Pt catalysts.
|Pt catalysts||Rate constant [min−1]||TOF [min−1]|
After the completion of the reaction, the catalysts were recovered by centrifugation and subsequently washed with water several times. All samples had similar catalytic conversion efficiencies in the first reaction cycle up to 97 % for the same reaction time (30 min), as shown in Figure 4 e. However, the activity of the Pt NPs was dramatically decreased as the running cycles increased, because of the agglomeration of the free Pt NPs, which eventually lost their catalytic activity. This behavior is strongly related to the TEM image in Figure 5 a. The in-LDH–Pt lost catalytic activity in the third cycle, presumably because the Pt NPs on the edge-on surface of LDH particles were removed shortly, as shown in Figure 2 f. The Pt/C gradually lost its catalytic activity over the course of repeated reactions for the same reaction time (30 min). The rate constant of the Pt/C in fourth cycle significantly decreased to 0.014 min−1 (Figure 4 f and Table 1), suggesting a poor stability and reusability. This can be attributed to poisoning of the negatively charged surface of Pt NPs on activated carbon by adsorption of p-aminophenol with a protonated amino group.20a To our surprise, the ex-LDH–Pt held high activity with more than 97 % conversion efficiencies even after the fourth cycle. The catalytic reaction profiles for the ex-LDH–Pt (Figure 4 f) showed similar reaction slopes for all the recycling steps, demonstrating a higher resistance of the Pt NPs supported on LDH nanosheets than of the Pt/C against the adsorption of p-aminophenol. Moreover, the TEM and HRTEM inspections in Figure 5 b strongly demonstrate that the Pt NPs immobilized on the LDH nanosheet were kept intact during the recycle reactions, and the Pt NPs were rigidly bonded with the LDH, which effectively prevented the agglomeration and poisoning of the Pt NPs. Therefore, our current catalyst exhibited much higher catalytic activity and durability than the supported Pt/C catalyst. In addition, a simple centrifugation could be used to recover ex-LDH–Pt from the reaction pot.
Figure 5. TEM images of a) Pt NPs and b) ex-LDH–Pt recovered after the 4th cycle. Inset of part b is an HRTEM image of Pt NPs anchored on the exfoliated LDH nanosheets.
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In summary, we have demonstrated a novel hybrid catalyst consisting of Pt NPs and exfoliated LDH nanosheets as an inorganic support to stabilize the Pt NPs, and its use as a recyclable catalyst. The exfoliated LDH nanosheets led to not only a high dispersion of the hybrid catalyst, but also a strong affinity with the Pt NPs, which met the significant requirements for ideal heterogeneous catalysts. The hybrid catalysts allowed the surfactant-free Pt NPs on the exfoliated LDH to provide superior recyclability performance with 99 % conversion efficiency, in contrast to the cases of Pt NPs, in-LDH–Pt, and Pt/C. The adoption of various inorganic nanosheets is expected to enhance the catalytic activities of LDH–Pt nanocomposite catalysts, which is attributed to the variable compositions of the LDH, such as transition metals that could be electronically connected with the metal NPs. The suggested strategy would be beneficial to the construction of hybrid materials with nanosheets and nanoparticles to improve their functional properties.