Chunxia Wang‡
a,
Fan Yang‡*b,
Wang Yangb,
Liang Renb,
Yunhan Zhangb,
Xilai Jiab,
Liqiang Zhangb and
Yongfeng Li*b
aBeijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical Chemistry for Living Biosystems, Institute of Chemistry, the Chinese Academy of Science, Beijing 100190, China
bState Key Laboratory of Heavy Oil Processing, China University of Petroleum, Changping 102249, Beijing, China. E-mail: yangfan@cup.edu.cn; yfli@cup.edu.cn; Fax: +86-010-89739028; Tel: +86-010-89739028
First published on 10th March 2015
In this work, we demonstrate that the presence of PdO nanoparticles can significantly enhance the catalytic performance of Pd catalysts for the reduction of 4-nitrophenol (4-NP). Heterogeneous Pd/PdO nanoparticles supported on an oxidized multi-walled carbon nanotube (OCNTs) catalyst is prepared by a one-pot gas–liquid interfacial plasma (GLIP) method with the precursor Pd(NO3)2·2H2O. The Pd/PdO catalysts with uniform size distribution exhibit remarkable catalytic activity during the reduction of 4-NP to 4-aminophenol (4-AP) in neat water at room temperature. The turnover frequency (TOF) value is up to 750 h−1, which shows much higher catalytic activity than single Pd nanoparticles supported on OCNTs. Our results indicate that the Pd/PdO catalyst can be readily recovered and reused 10 times.
Due to the high surface energy of free nanoparticles tend to aggregate, it is difficult to handle in catalytic applications. Many inert solid materials such as mesoporous solids (MCM-41, SBA-15 or related mesoporous silicas),19–21 polymers,22,23 metal oxides24–26 and metal–organic framework (MOF)27–29 have been successfully applied to stabilize the particles. However, most of these supports are synthetic materials which require laborious and time-consuming efforts for their synthesis as well as surface functionalization to preserve high reactivity of catalyst. The carbon nanotubes (CNTs) have been normally used as a material support for the dispersion and stabilization of metal nanoparticles due to their large chemically active surface, unique physical properties, inherent size, hollow geometry and stability at high temperatures.30 Moreover, CNTs have been mass-produced by a CVD method.31 Therefore, the development of the nanoparticles and CNTs hybrid materials as high reactivity catalyst is highly required.
To effectively synthesize the Pd nanoparticles, several synthetic strategies including electrodeposition, electroless deposition, arc-discharge in solution, sonochemical method, microwave-assisted, and chemical reduction in supercritical CO2 solution have been developed.23–27 Whereas, the PdO nanoparticles are usually synthesized by calcinating Pd nanoparticles in air, or by stirring the Pd salt under the oxidation conditions.32–36 However, the synthesis of Pd/PdO nanoparticles is still challenging at the present time, for the reason that the formation of Pd nanoparticles need reduction conditions, whereas the formation of PdO nanoparticles need oxidation conditions. Therefore, successfully solving the above contradiction is the key to the synthesis of the Pd/PdO nanoparticles. Until now, the most common method to synthesize the Pd/PdO nanoparticles is calcination treatment of the Pd nanoparticles, however, the stabilizer or surfactant is usually needed to avoid the over oxidation or aggregation of the particles.37–40 As is known to all, the stabilizer and surfactant are not usually indispensable for catalysis. Therefore, development of a facile method to synthesize the Pd/PdO nanoparticles is of great significance.
Herein, we report for the first time that the controllable synthesis of Pd/PdO and Pd nanoparticles functionalized OCNTs (Pd/PdO/OCNTs and Pd/OCNTs) by the GLIP method under low argon pressure with using different Pd salt precursors. Recently, researchers focused on the fabrication of metal nanoparticles by using the GLIP method,41,42 which show unique properties of high process rate, preparation of nanomaterials in large scale, avoiding the use of toxic stabilizers and reducing agents, ambient reaction temperature, and no need to stir during the nanoparticle formation process. All the above advantages provide a facile and low-cost alternative way to prepare Pd nanoparticles.43–45 More importantly, it can provide weaker reduction conditions than NaBH4 and hydrazine hydrate reduction systems, and the emission of argon is produced along with glow.46,47 Therefore, we treat the Pd(NO3)2·2H2O in GLIP condition so that the Pd(NO3)2·2H2O can be reduced to Pd nanoparticles along with decomposing to PdO by the glow, and the formed PdO nanoparticles cannot be reduced by the glow system, which provides an efficient pathway to synthesize the Pd/PdO nanoparticles. Furthermore, the catalytic performance of the resultant hybrid materials is assessed by studying the reduction of 4-NP reaction, and the catalytic activity of Pd/PdO/OCNTs is better than Pd/OCNTs which could be ascribed to the existence of PdO active species. To the best of our knowledge, we report for the first time that the PdO species in the presence of Pd nanoparticles can enhance the reactivity for reduction of 4-NP.
The morphology of Pd-n has been examined by TEM, as shown in Fig. S2.† The distribution histograms of the diameter are summarized in Fig. S3,† and the representative TEM images of Pd-1 are shown in Fig. 1a and b. These results demonstrate that all the synthesized Pd-n catalysts exhibit uniform morphologies, and the average particle diameters of Pd-1, Pd-2, Pd-3 and Pd-4 are 3.5, 3.7, 8.6 and 3.6 nm, respectively. The Pd loading content of Pd-1, Pd-2, Pd-3 and Pd-4 are 9.5, 10.0, 5.1 and 8.5 wt%, respectively, as summarized in Table S1.† It is found that the particle size of Pd nanoparticles except for Pd-3 is below 4 nm. The large particle size of Pd-3 is probably due to the introduction of potassium ions which leads to the size increase of Pd nanoparticles. In order to check the crystallinity of the synthesized Pd catalysts, a relatively large Pd nanoparticle of Pd-1 has been examined by the high-resolution TEM (HRTEM), as seen in Fig. 1c, where the interfinger distances of 0.225, 0.195 and 0.340 nm are found, corresponding to the (111), (200) lattice plane of the face-centred-cubic (fcc) palladium and shells separation of CNTs, respectively.
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Fig. 1 A TEM image of Pd-1 (a) and (b), HRTEM image of Pd-1 (c), EDX spectrum (d) and SAED image (e) of Pd-1. |
In addition, EDX spectra of Pd-1 is indicated in Fig. 1d, in which elements C and O origin from OCNTs, and Cu element is attributed to Cu grid of TEM, Pd element is considered to be originated from Pd nanoparticles. What's more, SAED pattern provides quick and easy crystal orientation information of the obtained Pd-1, as shown in Fig. 1e. The lattice spacing measured from the diffraction rings were 0.35, 0.23, 0.21, 0.20, 0.17, 0.13, 0.12 and 0.11 nm, corresponding to reflections C(002), Pd(111), C(100), Pd(200), C(004), Pd(220), C(110) and Pd(311), respectively. Furthermore, XRD spectra of Pd-n is depicted in Fig. 2 which shows the diffraction peaks at 25.9°, 40.2°, 42.6°, 46.5°, 53.3°, 68.1° and 82.0°, corresponding to C(002), Pd(111), C(100), Pd(200), C(004), Pd(220) and Pd(311), respectively. The lattice spacing of the C and Pd is calculated by the Bragg's formula, which is consistent with the measured results from the SAED pattern (Fig. 1e). These results have confirmed that the Pd-n possess a face-centered crystalline structure. The XRD image of Pd-1 also shows one extra peak at 33.8° which can be attributed to the reflection of PdO(101). In order to verify the presence of both Pd and PdO in Pd-1, we also synthesized the Pd-5, and the TEM image, XPS spectra and XRD of Pd-5 are shown in Fig. S4 and S5.† The XRD pattern of Pd-5 (Fig. S5d†) indicates the characteristic peak at 33.8° is attributed to the reflection of PdO(101), which is consistent with the XRD results of Pd-1.
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Fig. 2 The XRD patterns of Pd-1 (black curve), Pd-2 (olive curve), Pd-3 (red curve) and Pd-4 (blue curve). |
In order to certify the chemical element states of the Pd catalysts, XPS, a powerful technique for the investigation of CNT-based hybrid materials was used to confirm the component of catalysts. Wide XPS scan from Pd-n (Fig. 3a) shows the presence of Pd, C and O derived from OCNTs, which is in agreement with the EDX measurement. The peaks at 284.08 and 532.08 eV clearly indicate that the chemical components are C1s and O1s, respectively. A small peak corresponding to Pd3p is observed at 562.08 eV. Fig. 3b shows the enlarged XPS scan of Pd3d, the two peaks of Pd-2, Pd-3, Pd-4 for Pd3d appeared at 336.32 and 341.37 can be assigned to Pd3d5/2 and Pd3d3/2, which indicates the presence of Pd metal (Pd0) in the catalyst. By comparison, XPS-peak-imitating analysis of Pd-1 shows two peaks for Pd3d5/2 and Pd3d3/2 which are split into two types of Pd electronic states (Pd and PdO) centred at 334.78, 337.48, 340.18 and 342.58 eV, as shown in Fig. 3c. Moreover, the XPS spectra of Pd-5 indicates that we have successful synthesized the PdO/OCNTs with a little amount of Pd0, as shown in Fig. S5a and b.† Fig. S5c† shows the XPS spectra of Pd-5 and Pd-n, indicating clearly that the PdO nanoparticles are present in Pd-1 and absent from Pd-2, Pd-3 and Pd-4. Also, the O1s XPS can be divided into four components, including a PdO bond at 530.08 eV, a CO bond at 531.18 eV, a O–H bond at 532.28 eV and a C–O bond at 533.68 eV (Fig. 3d). What's more, the spectra of C1s XPS is split into four functional groups, including a C sp2 bond at 284.3 eV, a C–C bond at 285.08 eV, a C
O bond at 286.68 eV, and a C–C
O bond at 289.68 eV, as indicated in Fig. S6.† These XPS results suggest that the Pd-1 consists of both Pd and PdO active species,39 which is probably owing to using Pd(NO3)2·2H2O as Pd precursor. The synergistic effect between Pd and PdO may play an important role in the high activity of catalyst.
As a type of arene, 4-nitrophenol (4-NP) is highly toxic and has detrimental effect on human health and environment as toxic and mutagenic substances.49 On the other hand, aside from being an important intermediate in the preparation of several analgesic and antipyretic drugs, 4-AP has also been applied as photograph developer, corrosion inhibitor, anti-corrosion lubricant and hair-dyeing agent.50 Therefore, we evaluate the catalytic activity of as-prepared Pd-n by reducing 4-NP to 4-AP, which is an ideal reaction for characterizing the activity of Pd-n catalysts. 4-NP is pale yellow in aqueous solution which shows plasmon bond absorption centred around 317 nm, as shown in Fig. 4a curve B. After the addition of NaBH4, the color of the solution changes to yellow, which means that phenolate ions have been produced, as a result, a new absorption band appears at 400 nm, as shown in Fig. 4a curve C. Although NaBH4 is well known as a strong reductant, the reduction of 4-NP is quite slow (Fig. 4a curve D), even though OCNTs were introduced into the above reaction solution (Fig. 4a curve E), the reaction rate does not show any considerable progress. Thus, catalysts are usually needed to accelerate the conversion rate of 4-NP to 4-AP.
Initially, the Pd-n are used as catalysts, and their catalytic performance is evaluated by using model reaction of the reduction of 4-NP to 4-AP in aqueous solution at room temperature in quartz cuvette (method A). The evolution of UV-vis spectra with reaction time for the reduction of 4-NP to 4-AP by using different catalysts is monitored, as shown in Fig. S7.† The intensity of the absorption band at 400 nm decreases gradually with time increasing. At the same time, a new absorption band at 300 nm appears as shoulders, being ascribed to the formation of 4-AP (Fig. 4a curve A). The UV-vis spectra are not very irregular since the reaction proceeds rather fast, meanwhile, the reaction leads to the generation of a large amount of hydrogen, which affects the UV-vis spectra. However, the apparent rate constant of Pd-n catalysts for 4-NP reduction reaction could be assessed easily by the reduction of the intensity of the phenolate ions absorption band at 400 nm, as described in Fig. S9,† and the apparent reaction rates Kapp of Pd-n were 0.6, 0.25, 0.15 and 0.1 min−1, respectively (Table S2†). According to the calculated Kapp values, Pd-1 catalyst shows much higher activity than Pd-2, Pd-3 and Pd-4 catalysts.
To eliminate the effect of the hydrogen produced during the reaction in UV-vis spectra, the catalytic performances of Pd catalysts are assessed in micro-reaction vial (method B), as shown in Fig. 4b–e, compared with Fig. S4,† two isosbestic points are observed at 280 and 312 nm. The catalytic activity tendency of Pd catalysts is consistent with that in method A, and the apparent reaction rates Kapp of Pd-n are 1.00, 0.56, 0.50 and 0.21 min−1, respectively (Table S2†). Compared with method A, the precise amount of catalysts and the stir process are two main reasons for getting high Kapp. Additionally, the catalytic activity of the Pd-5 have been investigated by 4-NP reduction reaction, the results are shown in Fig. S8.† The apparent reaction rates Kapp of Pd-5 is 0.7 min−1, compared with that of Pd-n, the catalytic activity is lower than Pd-1 and a little higher than Pd-2, Pd-3 and Pd-4. It can be estimated that the presence of PdO active species is beneficial for the improvement of Pd catalytic activity. Two possible reasons can be used to explain the better catalytic activity of Pd-1. First, the complex electron interaction between the Pd and PdO component makes the H− easy exist on the surface of Pd/PdO nanoparticles, leading to the reactive intermediates more stable. Second, the proceed of reduction reaction of 4-NP is probably more easy to on the phase boundary of Pd and PdO due to the reaction activity on the Pd surface is usually not as high as at the phase boundary of Pd and PdO component.
Usually, Kapp is dependent on the amount of catalyst, but it is not directly related to the catalytic activity of the catalysts, whereas, the turnover frequency (TOF) is a significant parameter for evaluating the catalytic activity in heterogeneous catalytic reaction. The TOF values of various catalysts are 750, 429, 375 and 200 h−1 for Pd-1, Pd-2, Pd-3 and Pd-4 catalysts, respectively. In comparison with other Pd heterogeneous catalysts reported recently, such as Fe3O4/Pd (300 h−1 of TOF),51 PEDOT–PSS–Pd (13 h−1 of TOF),52 SPB/Pd (6 h−1 of TOF),53 PPy/TiO2/Pd (326 h−1 of TOF),54 SBA-15/Pd (300 h−1 of TOF),55 @Pd/CeO2 (1068 h−1 of TOF),56 the Pd-1 catalyst demonstrates obviously superior catalytic activity. It is clear that the Pd-1 catalyst prepared in this work is much more active than CNT/PiHP/Pd (300 h−1 of TOF). In addition, it is worthy to mention that the synthesis of previously reported Pd catalysts involves a multistep process, which limits their practical applications.
Reusability is one of the most important features for a heterogeneous catalysts, which is better than a homogenous one. First, in order to confirm that the reaction is indeed catalyzed by solid Pd-1 rather than homogenous Pd species, leaching experiments have been carried out after the catalytic reduction of 4-NP proceeded for 1 min under standard conditions, then the supernatant was separated, and no further reaction took place after removing the catalyst. In addition, the leaching of the Pd has been also examined using ICP-OES, and no leaching of Pd was detected. To assess the recyclability of Pd-1, multiple 4-NP reduction cycles have been carried out, the Pd-1 can be repeatedly used for 10 times, and the conversion of 4-NP can be totally completed with the increase of reaction time. It is interesting to find that the catalytic activity decreased after the 1st cycle, and the conversion of 4-NP can be completed with increasing of the reaction time from the 2nd to the 10th cycle (Fig. 5a and b), which is possibly due to the PdO active species losing during the reduction reaction after the 1st cycle reaction, this phenomenon also indicates that the existence of PdO active species in Pd-1 can greatly promote the 4-NP reduction reaction. Moreover, the TEM image of the catalyst after the reactions for 10 cycles indicates that particle size is a little bit increased, as shown in Fig. S10.†
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Fig. 5 (a) Plots of ln(C/C0) versus time for 10 cycles of the conversion from 4-NP to 4-AP with Pd-1 catalyst; (b) the Kapp of the Pd-1 during 10 cycles of the same reduction reaction. |
Footnotes |
† Electronic supplementary information (ESI) available: Materials, experimental setup, and UV-vis spectra. See DOI: 10.1039/c4ra16792a |
‡ These authors contributed equally. |
This journal is © The Royal Society of Chemistry 2015 |