One-Step Rapid and Facile Synthesis of Subnanometer-Sized Pd6

Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

One-Step Rapid and Facile Synthesis of Subnanometer-Sized Pd6(C12H25S)11 Clusters with Ultra-High Catalytic Activity for 4‑Nitrophenol Reduction Zhihua Zhuang,†,‡ Qin Yang,§ and Wei Chen*,†,‡

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/20/19. For personal use only.

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State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, Jilin, China ‡ School of Applied Chemistry and Engineering, University of Science and Technology of China, No. 96, JinZhai Road, Hefei 230026, China § School of Science, Xi’an University of Architecture & Technology, No. 13, Yanta Road, Xi’an 710055, China S Supporting Information *

ABSTRACT: Palladium has aroused multitudinous concerns in recent years because of its highly catalytic activity with applications in many reactions. However, the synthesis of atomically precise Pd nanoclusters (Pd NCs) under mild conditions is still challenging. Here, a new facile method is developed to synthesize subnanometer-sized Pd clusters with the molecular formula of Pd6(C12H25S)11 under very mild conditions. The composition, morphology, and optical properties of the Pd6 clusters are characterized by MS, XPS, HRTEM, FT-IR, and UV−vis spectroscopy. The Pd NCs loaded on carbon black show an ultrahigh catalytic activity for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). After annealing treatment, the Pd NCs can catalyze the reduction of 4-NP to 4-AP with a 100% conversion in only 0.5 min, which is the highest efficiency so far among the reported Pd-based catalysts. The significantly enhanced catalytic activity can be ascribed to the large fraction of Pd atoms on the surface of subnanoclusters with low coordination and the clean surface after the removal of ligands. The present work not only provides a facile method for synthesizing ultrasmall metal clusters but also indicates that subnanometersized Pd clusters can be used as highly efficient catalyst for the degradation of environmental pollutants. KEYWORDS: Nanocluster, Palladium, Catalysis, Catalyst, Degradation, 4-Nitrophenol reduction

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reaction was kept at 0 °C prior to the addition of a reducing agent (NaBH4). The obtained Pd nanoparticles are very stable with good solubility in apolar solvents. Later, Sharma et al. synthesized water-soluble Pd nanoparticles with H2PdCl4 as the palladium precursor, glutathione as the ligand, and NaBH4 as the reducing agent.14 The obtained product contains differently sized Pd nanoparticles with the size distribution from 1 to 4 nm. In another study, Kawaski et al. synthesized Pd clusters (1−1.5 nm) with the surface capped by N,Ndimethylformamide (DMF) molecules. In this method, DMF

INTRODUCTION Ultra-tiny monolayer ligand-protected noble metal nanoclusters usually have precise atom arrangement, and they attract much attention in different research fields due to the size-related optical,1,2 electronic,3 and catalytic properties.4−6 Among the metals, Pd is an important precious metal with wide applications in catalysis. However, compared to extensively studied gold,7,8 silver,9,10 and copper11,12 clusters, studies on Pd nanoclusters (Pd NCs) are still scarce, partly due to the lack of effective synthesis processes to lower the core size. In a previous report, Chen et al. synthesized Pd nanoparticles (ca. 1−5 nm) in a biphasic system (the aqueous and the toluene phases) with PdCl2 as the metal precursor and n-hexanethiol as the stabilizing ligand.13 In the preparation, the © XXXX American Chemical Society

Received: December 18, 2018 Revised: January 15, 2019 Published: January 16, 2019 A

DOI: 10.1021/acssuschemeng.8b06637 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Letter

ACS Sustainable Chemistry & Engineering

Figure 1. (A) Preparation process of Pd NCs. (B) UV−vis spectra of Pd(acac)2, Pd NCs, and 1-dodecanethiol (DT). (C) FTIR spectra of DT ligands and Pd6(C12H25S)11 clusters. (D) 1H NMR of DT ligands (black line) and Pd6(C12H25S)11 (red line); the insets give the molecular structure of DT (right) and the zoomed-in spectrum in the rectangle. (E) XRD patterns of Pd6(C12H25S)11, Pd NCs/CB-on, and Pd NCs/CB-off.

biological degradability.32 In particular, 4-nitrophenol (4-NP) is a harmful species to vital organs of both animals and humans.33,34 Meanwhile, 4-NP is also an important raw material for the 4-aminophenol (4-AP) synthesis. 4-AP has been widely used as a photographic developer, hair-dyeing agent, and anticorrosion−lubricant agent.35 So far, many methods have been reported to improve the conversion efficiency of 4-NP to 4-AP.35−38 Among of them, the chemical reduction method by using NaBH4 as a reducing agent is considered to be a facile and highly efficient process. It should be noted that to speed up the reduction reaction highly active catalysts are usually required. Therefore, searching for suitable catalysts for the direct conversion of 4-NP to 4-AP by the reduction of NaBH4 has attracted increasing attention in recent years. Compared with other catalysts, noble metals, especially Pd catalysts, exhibit much better catalytic activities for the conversion from 4-NP to 4-AP. Based on the above-mentioned perspective, in this study, we synthesized atomically precise Pd6(C12H25S)11 clusters by a one-step chemical reduction process using Pd(acac)2 as the precursor, 1-dodecanthiol (DT) as both protecting ligand and the reducing agent, and oleylamine (OAm) as the solvent. By loading on carbon black and calcination treatment, the synthesized Pd NCs showed high catalytic activity and recyclability for the conversion of 4-NP to 4-AP.

serves as both solvent and reducing agent, and Pd clusters were prepared under refluxing at 140 °C.15 This reported method has also been used to synthesize Au,16 Cu,17 Pt,18 and Ir19 and even many bimetallic nanoclusters.20 Meanwhile, an interesting process was reported recently for preparing small Pd clusters (ca. 2 nm).21 The synthesis was performed by using different thiol molecules as protecting ligands. On one hand, different methods have also been reported for the preparation of atomically precise Pd clusters.22−28 For instance, Yang et al. successfully synthesized [Pd(SC12H25)2]6 clusters by using Na2PdCl4 as the metal precursor and 1dodecanethiol as the stabilizing ligand.29 Zhao et al. synthesized Pd NCs with a similar method for the preparation of Au NCs.30 In this method, Pd13‑17(SR)18‑22 clusters were obtained with bulky thiolate as the protecting ligand and NaBH4 as the reducing agent. In another report, Gao et al. synthesized Pd6(SR)12 clusters by following the previously reported process for the synthesis of Ni clusters with a minor modification.31 In this study, PdCl2 was used as the Pd source and tetrahydrofuran (THF) as the solvent, under the aid of a phase-transferring reagent (tetraoctylammoniu bromide). It should be pointed out that most of the synthesis procedures for the preparation of Pd NCs are complicated and time consuming under rigorous conditions. Therefore, developing a novel strategy for the preparation of Pd NCS is of importance for the application of highly active Pd-based catalysts. On the other hand, green chemistry has attracted increasing attention due to the serious environmental pollution with the explosive growth of global population. Nitrophenol (NP) is one of the most toxic pollutants in refractory water with poor

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RESULTS AND DISCUSSION As observed from Figure 1A, upon the addition of Pd(acac)2 into the oleylamine solvent, a colorless solution can be obtained. As soon as 1-dodecanthiol is added, the solution B


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Figure 2. (A, B) High resolution XPS of Pd 3d and S 2p of the Pd6(C12H25S)11, respectively. (C) Comparison of the Pd 3d XPS from Pd NCs/CBon and Pd NCs/CB-off. (D) MALDI-TOF MS of the Pd6(C12H25S)11 NCs.

indicated in the spectrum. Note that the purified Pd NCs show a set of broadened resonances with a slight shift as compared to those of free 1-dodecanethiol. Moreover, the NMR peak from the thiol group (−SH) at 1.31 ppm is not present in the spectrum of Pd NCs, which can also be ascribed to the breaking of the S−H bond during the formation of Pd clusters. On the other hand, since C-1 is linked to the mercapto group, once the S atom is attached on the surface of the clusters to form the Pd−S bond, the NMR peak of the protons from C-1 are significantly broadened and shift to downfield (Figure 1D, inset). For the protons farther away from the Pd core, a smaller effect can be observed. The protons on C-2 and C-3 are also slightly affected by the formation of the Pd−S bond, and the corresponding peaks get broadened and move up slightly. The protons on other carbons bring about the peaks at 0.88 and 1.26 ppm. The 1H NMR spectra demonstrate again that the Pd cores are capped with DT ligands through the binding of S on the Pd cluster surface. Figure 1E shows the XRD patterns of Pd NCs, Pd NCs/CBon, and Pd NCs/CB-off. For comparison, the standard XRD diffraction pattern from Pd is also included (Pd 46-103). In the spectra of Pd NCs/CB-on and Pd NCs/CB-off, the broad peaks around 25° and 44° can be ascribed to the (002) and (101) planes of carbon black. However, the diffraction peaks from Pd are not present in all the XRD patterns, indicating the formation of small nanoclusters with no crystal structure and the absence of large metal particles. Meanwhile, such results also suggest that the following calcination treatment will not result in the agglomeration of clusters and the generation of large palladium particles. XPS measurements were then carried out to examine the composition and oxidation states of the formed Pd nanoclusters and the influence of annealing treatment. The existence of elements of palladium, carbon, and sulfur in the produced Pd nanoclusters can be determined from the survey spectrum in Figure S2. The peaks at around 162, 226, 283,

color changes immediately, suggesting the occurrence of the reduction reaction of the Pd precursor and the formation of Pd nanoclusters. The UV−vis absorption property of the product was first studied. It can be seen clearly from Figure 1B that different from the Pd(acac)2 precursor and 1-dodecanthiol ligand, the prepared Pd clusters have three apparent absorption bands around 274, 320, and 410 nm. The three absorption peaks could be ascribed to the ligand-to-metal transition, interband transition, and ligand-centered transition, respectively. Such UV−vis absorption features are in accordance with those of the previously reported metal nanoclusters.29,31 After loading on carbon black (Pd NCs/CB-on), the UV−vis absorption peaks of Pd nanoclusters can still be observed clearly (Figure S1), indicating that the structure of Pd NCs can be still well kept after the loading on carbon black. To analyze the surface chemistry of the produced clusters, FTIR and 1H NMR characterizations were also carried out. Figure 1C compares the FT-IR spectra of 1-dodecanthiol molecules and the as-synthesized Pd clusters. In the FTIR spectrum of 1-dodecanthiol, the IR absorption at 2575 cm−1 can be ascribed to the stretching vibration of the S−H functional group. However, this peak cannot be observed in the FTIR curve of the as-prepared Pd clusters. Meanwhile, in both spectra, the signals originated from the stretching and bending modes of C−H bonds can be found at 2848, 2919, 2955, 1464, and 721 cm−1, respectively. Such different FTIR characteristics from the ligands and cluster product suggest that the protecting thiol molecules have been capped on the surface of clusters with the cleavage of the S−H bond and the subsequent formation of Pd−S bonds. The 1H NMR spectrum of Pd NCs in CDCl3 is compared with that of pure DT ligands. As shown in Figure 1D, on the basis of the structure of DT (Figure 1D, inset on the right), the NMR signals from 1-dodecanthiol can be assigned to the protons bound on different carbons, as C


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Figure 3. HRTEM images of Pd6(C12H25S)11 (A), Pd NCs/CB-on (B), Pd NCs/CB-off (C), and Pd NCs/CB-off after eight cycles of catalytic recycling (D).

These results clearly show that the calcination process can successfully get rid of the protecting 1-dodecanethiol molecules on the cluster surface, and therefore, the obtained Pd clusters with clean surfaces are in tight contact with carbon black. Mass spectrometry is proved to be an effective analytical tool to determine the molecular structure of a synthesized metal nanocluster. Specifically, MALDI-TOF and ESI have been applied to the structure analyses of almost all the reported metal clusters. The maximum mass detection limit of the available ESI-MS is ∼3 kDa, and the relative molecular mass of our synthesized Pd nanoclusters is larger than this. In order to check whether there is the presence of larger Pd clusters or nanoparticles in the product or not, MALDI-TOF MS was used in this study. The exact formula of the prepared Pd NCs can be determined by MALDI-TOF MS. It can be seen from Figure 2D that in the wide MS range the largest peak at m/z = 3096.3 can be from the species of [Pd6(C12H25S)11+DCTB]+. Other distinct peaks at 2546.4, 1740.6, and 1572.5 can be ascribed to (Pd5(C12H25S)10+H)+, Pd5(C12H25S)6+, and [Pd3(C12H25S)5+DCTB+H]+, respectively. These lower mass cluster species could be ascribed to the fragments of the parent

337, 532, and 561 eV are assigned to S 2p, S 2s, C 1s, Pd 3d, O 1s, and Pd 3p, respectively. From the corresponding high resolution Pd 3d spectrum in Figure 2A, the peaks at 336.8 and 342.2 eV can be assigned, respectively, to Pd 3d5/2 and 3d3/2. Meanwhile, the four binding energy peaks fitted from the Pd 3d spectrum are from metallic Pd and the surface oxidation states. On the other hand, as shown in Figure 2B, two peaks at 162.2 and 163.3 eV were deconvoluted from the S 2p XPS spectrum, which correspond to S 2p2/3 and S 2p1/2. The survey spectra of Pd NCs/CB-on and Pd NCs/CB-off (Figure S3A and B) have the Pd 3d signals, suggesting the successful loading of Pd NCs on carbon black. However, from Pd 3d XPS shown in Figure 2C, after removal of ligands by the annealing treatment, the Pd 3d binding energies show an obvious blue shift. Such results indicate the strong effect of 1-dodecanthiol ligands on the electronic structure of the Pd core. It can be also seen from Figure S3C that after the pyrolysis the Pd NCs/CBoff sample shows much decreased S 2p peaks. It should be noted that the signals of the C−H bond stretching at 2848, 2919, and 2955 cm−1 can be still observed in the FTIR spectrum of Pd NCs/CB-on, as shown in Figure S4; however, the peaks do not appear in the spectrum of Pd NCs/CB-off. D


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ACS Sustainable Chemistry & Engineering metal clusters. Based on the MALDI-TOF MS results, the obtained Pd NCs have the composition of Pd6(C12H25S)11. To check the purity of the Pd NCs, thin-layer-chromatography (TLC) analysis was also performed (Figure S5). It can be seen that the TLC plate has only one yellow dot, suggesting the monodispersity of the Pd NCs. Meanwhile, ICP-AES measurements showed that 1.3% Pd was loaded in Pd NCs/CB-off. Figure S6 displays the thermogravimetric measurement result of the obtained nanoclusters. Clearly, the decomposition of 1-dodecanthiol starts approximately from 230 °C and is almost finished at about 300 °C. To get surface-clean Pd nanoclusters with removal of the protecting ligands and meanwhile prevent the clusters from agglomeration, an annealing process at 300 °C was performed. Similarly, the previous study39 also showed that 300 °C is the best calcination temperature to remove 1-dodecanthiol on Au25 nanoclusters supported on carbon nanotubes. Meanwhile, the residue mass after removal of 1-dodecanthiol is approximately 21.7%, from which the molar ratio of Pd to 1-dodecanthiol should be 6:11. This result is in consistence with the formula of the Pd6(C12H25S)11 NCs determined by MALDI-TOF MS. The size and morphology of Pd6(C12H25S)11 were also characterized by HRTEM. Figure 3A−C shows the HRTEM images of Pd6(C12H25S)11 NCs, Pd NCs/CB-on, and Pd NCs/ CB-off, respectively. From Figure 3A, the Pd6(C12H25S)11 nanoclusters have a tiny core size smaller than 1 nm. The HRTEM image in Figure 3B indicates that the ultrafine Pd6(C12H25S)11 have been dispersed on a carbon black support with a homogeneous distribution. Moreover, as shown in Figure 3C, no obvious aggregation can be observed after the annealing process, which can be attributed to the applied relatively low-temperature annealing treatment and the loading of only a small amount of Pd NCs on carbon black. Interestingly, in the preparation, if oleylamine (OAm) is replaced by oleic acid (OAc) or 1-octadecene (ODE), Pd nanoclusters can still be obtained, and the products have the similar UV−vis absorption profiles to that of the clusters prepared in oleylamine (Figure S7). Figure S8A and B displays the high-resolution TEM of the Pd NCs using OAc and ODE as the solvent, respectively. Such control experiments indicate that in addition to OAm, oleic acid and 1-octadecene can also be used as solvents for the synthesis of Pd NCs. Meanwhile, 1dodecanthiol ligands can also be changed to other alkyl hydrosulfide, such as 1-hexanethiol, 1-octanethiol, and 1octadecanethiol. These experimental results suggest that the present work provides a general procedure for the synthesis of Pd nanoclusters by using various solvents and protecting ligands. Therefore, this method could be developed to synthesize differently sized Pd and other metal nanoclusters. The catalytic conversion from 4-NP to 4-AP has been extensively studied as a probe reaction to assess the catalytic properties of various nanostructured catalysts.40,41 Meanwhile, 4-NP degradation by the reduction of NaBH4 aided with a catalyst is one of the important pathways to obtain 4-AP.42 Here, to examine the catalytic properties of the synthesized Pd nanoclusters, the catalytic degradation of 4-NP is investigated on Pd NCs/CB-on, Pd NCs/CB-off, and the carbon black support. The catalytic reaction is tracked by the change of the UV−vis absorption spectrum in the conversion reaction process. Figure 4A shows the UV−vis spectra of the 4-NP solution with and without the addition of NaBH4. Apparently, the 4-NP aqueous solution shows a strong absorption peak at 317 nm.

Figure 4. (A) UV−visible absorption spectra of 4-NP with and without the presence of NaBH4. The change of UV−visible spectrum of 4-NP with reaction time using (B) carbon black, (C) Pd NCs/CBon, and (D) Pd NCs/CB-off as catalysts, respectively. (E) Dependence of the UV−vis absorption intensity of the reaction solution at 400 nm on the reduction time with the presence of different materials. (F) Recyclability of the Pd NCs/CB-off toward the conversion of 4-NP in eight cycles.

After adding 2 mL of 0.1 M NaBH4 into the 4-NP solution, the solution shows an obvious color change, and the absorption shows a remarkable red shift from 317 to 400 nm, which can be ascribed to the generation of 4-nitrophenolate ions.43−46 However, only with the presence of NaBH4, the absorption spectrum and the color of the solution will not change with increasing the reaction time even to 60 min (Figure S9), which can be ascribed to the very high kinetic energy barrier between the mutually repelling anions of 4-NP and BH4−.42 The catalytic efficiencies of the prepared different samples for the conversion of 4-NP to 4-AP were studied by comparing the time-dependent changes of the absorption at 400 nm (Figure 4B−D). As shown in Figure 4B, with the presence of carbon black, 4-NP cannot be degraded, indicating the negligible catalytic activity of carbon black for the reduction of 4-NP. By contrast, as shown in Figure 4C, with the presence of Pd NCs/ CB-on, the absorption intensity of 4-NP anions at 400 nm decreases rapidly with reaction time, and the conversion from 4-NP to 4-AP can finish in 5 min. Meanwhile, the bright yellow color of the solution faded to colorless gradually. However, from the change of UV−vis absorption in Figure 4D, upon the addition of Pd NCs/CB-off into the solution, the complete degradation of 4-NP into 4-AP can be finished in only 0.5 min, indicating the excellent catalytic activity of Pd nanoclusters with removing protecting ligands for the reduction of 4-NP. Figure 4E compares the degradation rate of 4-NP (absorption intensity versus reduction time) with the prepared Pd cluster E



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samples as catalysts. Obviously, Pd NCs/CB-off shows much enhanced catalytic activity with 10 times higher catalytic efficiency than that of NCs/CB-on. Such ligand influence on the catalytic properties of Au clusters have also been reported recently.47−49 Moreover, the recyclability of Pd NCs/CB-off was also studied. As demonstrated in Figure 4F, after six catalytic cycles, Pd NCs/CB-off still shows 100% catalytic efficiency for the reduction of 4-NP, indicating the extremely high catalytic stability. Moreover, from the HRTEM of Pd NCs/CB-off after eight cycles of catalytic recycling (Figure 3D), the Pd nanoclustes have still a uniform distribution on the carbon black support with no obvious size change and aggregation, further indicating the high durability of the Pd NCs/CB-off. The catalytic performances of different previously reported Pd-based catalysts and present Pd NCs/CB-off for the conversion of 4-NP are listed in Table S1. Obviously, in comparison with other Pd-based nanocatalysts, our prepared Pd NCs/CB shows the highest catalytic efficiency with the shortest time (0.5 min) for the complete conversion of 4-NP to 4-AP. Normally, the catalytic degradation of 4-NP involves two processes. First, 4-NP molecules are diffused and adsorbed on the surface of a catalyst. Second, on the surface of a catalyst, an electron transfer process occurs from BH4− to 4-NP. Therefore, the conversion efficiency from 4-NP to 4-AP is significantly affected by the adsorption capability and the surface and electronic structures of the used catalyst. Here, with Pd NCs/CB as the catalyst, abundant anions can be absorbed. At the same time, Pd NCs serve as an electronic relaying system for the electron transfer from BH4− to −NO2 of 4-NP, which can largely reduce the energy barrier and thus enhance the catalytic degradation of 4-NP. In this work, after loading Pd NCs on carbon black, the thiol ligands capped on the cluster surface were removed in a mild condition with a relatively low temperature and N2 flow. After the annealing treatment, the surface-cleaned Pd NCs can be stabilized by metal−support interactions, and meanwhile, more surfaceactive sites can be assessable for the reactants. Moreover, the electron transfer property of the catalyst can be enhanced after the removal of the ligands because of the poor conductivity of 1-dodecanthiol. Attributed to the above advantages, Pd NCs/ CB-off exhibited extremely high catalytic performance for the conversion from 4-NP to 4-AP.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b06637.

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Detailed experimental procedures, additional UV−vis, XPS, FTIR, TGA, HRTEM, and characterizations and catalysis measurements. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Wei Chen: 0000-0001-5700-0114 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21575134, 21633008, 21773224, 21605006), the National Key Research and Development Plan (2016YFA0203200), and K. C. Wong Education Foundation.

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REFERENCES

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CONCLUSIONS In summary, by using a one-pot, facile, and fast method, subnanometer-sized Pd clusters with precise atoms have been successfully synthesized. The composition of the Pd cluster is determined to be Pd6(C12H25S)11 by MALDI-TOF MS. By loading the Pd NCs on carbon black and removing the thiol ligands capped on the cluster surface through an annealing process, the obtained surface-cleaned Pd clusters exhibited ultrahigh catalytic activity and stability for the reduction reaction of 4-NP. The results showed that the conversion from 4-NP to 4-AP can be completed in only 0.5 min with Pd NCs/ CB as the catalyst, and such catalytic efficiency is 10 times more than Pd NCs/CB-on and higher than the reported Pdbased catalysts. The enhanced catalytic efficiency of the Pd clusters can be ascribed to the highly active Pd atoms with low coordination in the subnanoclusters, increased exposure of active sites on the cluster surface, and improved electron transfer property of the ligand-removed catalyst. F


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One-Step Rapid and Facile Synthesis of Subnanometer-Sized Pd6

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