Article pubs.acs.org/IECR
Enhanced Catalytic Properties of Palladium Nanoparticles Deposited on a Silanized Ceramic Membrane Support with a Flow-Through Method Hanyang Li,†,‡ Hong Jiang,†,‡ Rizhi Chen,*,†,‡ Yong Wang,† and Weihong Xing† †
State Key Laboratory of Materials-Oriented Chemical Engineering, and ‡Jiangsu Key Laboratory of Industrial Water-Conservation & Emission Reduction, Nanjing University of Technology, Nanjing 210009, PR China ABSTRACT: A flow-through method was developed for the deposition of palladium nanoparticles on a ceramic membrane support modified with aminofunctional silane to fabricate a Pd-loaded ceramic membrane support. The as-fabricated Pd-loaded ceramic membrane support was extensively characterized by energy-dispersive X-ray spectroscopy (EDS), inductively coupled plasma (ICP) emission spectroscopy, X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM), and temperature-programmed reduction (TPR), and its catalytic properties were evaluated in the reduction of p-nitrophenol to p-aminophenol with sodium borohydride. For comparison, the palladium nanoparticles were also deposited on a silanized ceramic membrane support by a traditional impregnation method. Superior p-nitrophenol conversion and catalytic stability are observed on the Pd-loaded ceramic membrane support prepared by the flow-through method. In the flow-through method, the synthesis solution is forced to flow through the membrane pores, thus the palladium nanoparticles can be deposited both on the membrane surface and in the membrane pores, resulting in an increased loading amount of palladium nanoparticles and an enhanced p-nitrophenol conversion. The superior catalytic stability is related to the preparation process: the palladium nanoparticles deposited on the membrane support will be scoured by the synthesis solution, some palladium nanoparticles having poor interaction with the membrane support may fall off during the preparation stage, and the remaining palladium nanoparticles have stronger interactions with the membrane support and do not easily fall off during the continuous reaction cycles, leading to better catalytic stability.
1. INTRODUCTION
The surface property of the support is one of the key factors affecting the amounts and quality of deposited metal particles. Generally, the metal particles are physically bonded to the support, and as a result, the metal particles show poor adhesion to the support and are easily leached from the support during the reactions, resulting in poor catalytic stability.11 It has been reported that the aminofunctional silane can be adopted to modify the support surface and then increase the number of deposited metal particles and improve the adhesion of the metal particles to the support.15−18 In our previous work,11 an aminofunctional silane of γ-aminopropyltriethoxy silane (3APTS) was used to modify the ceramic membrane support surface and then the palladium nanoparticles were loaded on the modified ceramic membrane support, and a comparative study was also done with palladium nanoparticles deposited on the ceramic membrane support without silanization. It was found that the catalytic activity and stability of palladium nanoparticles deposited on the silanized ceramic membrane support were obviously better than those deposited on the unmodified ceramic membrane support in the hydrogenation of p-nitrophenol. The preparation method is another key factor affecting the amounts and quality of deposited metal particles. For a traditional impregnation method, the metal particles are mainly
Metal nanoparticles have attracted considerable attention because of their specific catalytic properties compared to their bulk counterparts and hold great potential in many applications, such as the selective hydrogenation of nitroaromatic compounds, the oxidation of benzyl alcohol, and the Fischer−Tropsch process.1−4 One of the challenges in the further development of the metal nanoparticles as catalysts is to separate them from the reaction mixture.5 Coupling the nanocatalysis with the membrane separation to construct a membrane reactor can realize the in situ separation of metal nanoparticles from the reaction mixture.5,6 However, it was found that metal nanoparticles such as nickel were easily adsorbed on the surfaces of pipelines, tanks, and membranes during the recovery of catalysts. The adsorption of metal nanoparticles resulted in a decrease in the catalyst concentration in the reaction slurry and a consequent decline in the reaction rate and membrane flux, which would make the membrane reactor system unstable.7 One of the promising methods for solving the above problem is to load metal nanoparticles in or on membranes.8−13 Contrary to free powder catalysts, membrane catalysts can be easily reused in liquidphase reactions.14 Compared to polymeric membranes, ceramic membranes display many advantages such as good chemical stability and favorable mechanical strength and therefore are attractive alternatives as nanocatalyst supports and can be used under extreme reaction conditions including high temperature and the presence of erosive organic solvents.10−13 © 2013 American Chemical Society
Received: Revised: Accepted: Published: 14099
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with a flow-through method was designed and constructed and mainly consisted of a solution storage tank, a feed pump, and a membrane module. The glass solution storage tank had a working volume of 1 L and was equipped with an external jacket for temperature control. A peristaltic pump (Baoding Longer Precision Pump Co., Ltd., China) was used to pump the solution through the membrane module. The membrane module (14 mm outer diameter, 13 mm inner diameter) was made of stainless steel and also equipped with an external jacket for temperature control. The tubular membrane support was fixed in the membrane module with sealing washers. 2.2. Preparation of Pd-Loaded Ceramic Membrane Supports. A tubular alumina ceramic membrane support (CM) with a 12 mm outer diameter, an 8 mm inner diameter, and an 18 cm length was provided by Nanjing Jiusi High-Tech Co. Ltd., China, and used as the starting material. The ceramic membrane support had a symmetric pore structure and a nominal pore size of 3 μm. N-(2-Aminoethyl)-3-aminopropyltrimethoxysilane (AAPTS, industrial product) was used to modify the ceramic membrane support, and its structure is shown in Figure 2; it was purchased from Nanjing XinHuai Chemical Co. Ltd., China.
immobilized on the membrane surface11 and cannot be loaded in the pores possibly because of the higher surface tension, resulting in lower catalyst loading and poor catalytic properties. Bruening and co-workers developed a layer-by-layer adsorption technique to prepare catalytic membranes and found that a high density of unaggregated metal nanoparticles could be loaded both on the surface and in the pores of the membranes via alternating adsorption of polyelectrolytes and negatively charged metal nanoparticles.8,19,20 In the present study, a flow-through method was developed to fabricate Pd-loaded ceramic membrane supports, where the synthesis solution was pumped through the pores of a ceramic membrane to overcome the tension of the membrane surface and the palladium nanoparticles could be deposited on the surface and in the pores to enhance the loading amount of palladium nanoparticles and the catalytic properties of Pd-loaded ceramic membrane supports. The aim of present work was to fabricate Pd-loaded ceramic membrane supports with a flow-through method. In continuing our work in the area of membrane modification,10,11 the ceramic membrane support was modified with an aminofunctional silane of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPTS) to improve the loading of palladium nanoparticles. The as-fabricated Pd-loaded ceramic membrane support was extensively characterized by EDS, ICP, XPS, HRTEM, and TPR, and its catalytic properties were tested in the reduction of p-nitrophenol to p-aminophenol using sodium borohydride (NaBH4) as a reductant. A comparative study was also made with palladium nanoparticles deposited on the silanized ceramic membrane support by a traditional impregnation method.
2. EXPERIMENTAL SECTION 2.1. Apparatus. An experimental setup, shown in Figure 1, for the preparation of Pd-loaded ceramic membrane supports
Figure 2. Schematic representation of the reaction steps in the preparation of the Pd-loaded ceramic membrane support.
As presented in Figure 2, the procedure for preparing Pdloaded ceramic membrane supports mainly included three steps: modification, impregnation, and reduction. All steps were carried out in the setup as shown in Figure 1. During each step, the solution was pumped from the outer surface of the ceramic membrane support to the inner surface and then back to the solution storage tank; the flux through the ceramic membrane support was controlled at 0.7 mL·cm−2·min−1. First, the ceramic membrane support was modified with 100 mL of a 6 g·L−1 solution of AAPTS in dichloromethane at room temperature
Figure 1. Experimental setup for the preparation of a Pd-loaded ceramic membrane support with a flow-through method. 14100
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programmed reduction (TPR) experiments were performed using a BELCAT-A analyzer. Prior to the reduction, the sample was pretreated in an Ar stream at 300 °C for 2 h and then cooled to 50 °C. Then a H2/Ar mixture (10% H2 by volume) was switched on, and the temperature was raised to 800 °C at a rate of 10 °C·min−1. The consumption of H2 in the reactant stream was detected with a thermal conductivity cell. For the XPS, HRTEM, or TPR analysis, the samples were obtained by scraping off the powder from the surface and cross-section of membrane supports. 2.4. Reduction of p-Nitrophenol. The p-nitrophenol reduction to p-aminophenol with NaBH4, as depicted in Scheme 1, was used as a model reaction to evaluate the catalytic
for 1 h. It was found, with further increasing modification time, that the condensation between AAPTS molecules would aggravate and a solid phase was formed, which was not beneficial to the loading of palladium nanoparticles. A similar result was also reported by Xie et al.,21 so a modification time of 1 h may be enough. AAPTS has two functional groups: −OH groups and −NH2 or −NH groups. As illuminated in our previous work,11 the −OH groups in AAPTS can react with the −OH groups on the ceramic membrane support via condensation as shown in step 1 in Figure 2, forming the covalent bonds of strong −Si−O−Al− between the ceramic membrane support and AAPTS. After modification, the ceramic membrane support was rinsed thoroughly with ethanol to desorb physisorbed AAPTS molecules. Thus, the interaction between AAPTS molecules and the ceramic membrane support was mainly chemical adsorption. Then the silanized ceramic membrane support was impregnated in 100 mL of a 0.01 M solution of Pd(OAc)2 in acetone at 30 °C for 12 h. −NH2 or −NH is a strong electron donor having a strong capability for chelating to transition metal ions, so the Pd2+ ions can react with the amine groups (−NH2) and imino groups (−NH) in AAPTS (step 2 in Figure 2) to form a chelate complex and be bound to the ceramic membrane support.11 Finally, the Pd2+ ions were reduced by 100 mL of a 0.015 M hydrazine hydrate (N2H4·H2O) alkaline solution at 30 °C for 1 h to form metallic Pd particles (step 3 in Figure 2). After Pd2+ ions were reduced to Pd nanoparticles, there were also chemical bonds between Pd nanoparticles and AAPTS.22 During the reduction, the color of the membrane support surface became black, indicating the formation of metallic palladium nanoparticles on the membrane support. After each step, the ceramic membrane support was taken out of the membrane module and rinsed thoroughly with ethanol, followed by drying at room temperature for 12 h, and the setup was cleaned with deionized water and then used for next-step operation. The as-fabricated Pd-loaded ceramic membrane support is marked as Pd-CM-FT. For comparison, the ceramic membrane support was modified with AAPTS, impregnated with Pd(OAc)2, and reduced with hydrazine hydrate under the same conditions by a traditional impregnation method as presented in our previous work;11 the Pdloaded ceramic membrane support is marked as Pd-CM-TI. The reproducibility was checked by repeating the experiments at least three times, and the error in the catalytic performance was found to be within acceptable limits (±3%). 2.3. Characterization of Pd-Loaded Ceramic Membrane Supports. The content of palladium was determined by inductively coupled plasma emission spectroscopy (ICP-AES, Optima 7000DV). The measurements were performed at the Pd standard (340.458 nm). For ICP analyses, the samples were digested in 10% (v/v) nitric acid solution at 60 °C for 1 h. The distribution of palladium nanoparticles was analyzed using energy-dispersive X-ray spectroscopy (EDS, NORAN System Six). The element composition and chemical state were investigated by X-ray photoelectron spectrometry (XPS, Thermo ESCALAB 250) with monochromatized Al Kα radiation (hν = 1486.6 eV) at 15 kV. The residual pressure in the analysis chamber was about 10−10 mbar. The C 1s signal (284.6 eV) was used to calibrate the binding energies. The morphology and particle size of the palladium nanoparticles were examined on a JEM-2010 HRTEM instrument operated at 200 kV. For HRTEM observations, the samples were prepared by sonication in ethanol for 10 min and then deposited onto carbon-coated copper grids. Temperature-
Scheme 1. Reduction of p-Nitrophenol to p-Aminophenol with NaBH4
properties of as-fabricated Pd-loaded ceramic membrane supports. The catalytic reaction was also carried out in the setup as shown in Figure 1. For each reduction experiment, 2.5 g (18 mmol) of pnitrophenol and 2.0 g (53 mmol) of NaBH4 were dissolved in 500 mL of an ethanol−water mixture (20/480 mL) in the solution storage tank, and then the system was heated with circulating hot water. When the temperature reached the setting value of 30 °C, the pump was turned on, the solution was forced to flow through the pores of the Pd-loaded ceramic membrane support and then back into the solution storage tank, and the reaction started as presented in Figure 3. After 1 h of reaction, the Pd-loaded ceramic membrane support was removed from the membrane module, thoroughly washed with ethanol, and dried at room temperature for the next run. The products were collected from the outlet of the reactor at set intervals and analyzed by a high-performance liquid chromatography (HPLC) system (Agilent 1200 series, USA) equipped with a diode array detector (DAD) and an autosampler. Chromatographic separations were performed at 35 °C with a ZORBAX Eclipse XDB-C18, 5 μm, 4.6 mm × 250 mm column. A mobile phase composed of 80% methanol and 20% water at a flow rate of 1 mL/min was used. The automatic injection volume was 2 μL per sample. To evaluate the extent of catalyst leaching during the reaction, the amount of palladium in the reaction mixture was determined using inductively coupled plasma optical emission spectroscopy (ICP-AES, Optima 7000DV).
3. RESULTS AND DISCUSSION 3.1. Characterization of Pd-Loaded Ceramic Membrane Supports. In the present work, EDS was used to analyze the distribution of palladium nanoparticles on the membrane support, and in order to investigate the effect of the preparation method on the loading of palladium nanoparticles, the scan area and time were the same for Pd-CM-FT and PdCM-TI. The surface EDS images in parts a and b of Figure 4 14101
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Figure 3. Schematic diagram of the reduction of p-nitrophenol with NaBH4 in a Pd-loaded ceramic membrane support.
spectrum of ceramic membrane support, some new peaks including Pd 3d and N 1s can be observed for Pd-CM-FT and Pd-CM-TI. The Si 2p peak can be found for all samples; however, the content of Si in Pd-CM-FT (1.9 at. %) or Pd-CMTI (2.3 at. %) is obviously higher than the value of 1.6 at. % in CM. These results confirm the presence of the silane coupling agent on the silanized ceramic membrane support and the loading of palladium nanoparticles on the membrane support with chemical bonds. As presented in Figure 6, for Pd-CM-FT, the Pd 3d5/2 and Pd 3d3/2 electronic states corresponding to the Pd(0) species are observed at 335 and 340.15 eV, respectively, similar to the reported values.23 At the same time, the Pd 3d5/2 and Pd 3d3/2 electronic states for Pd(II) species are also detected at 337.6 and 342.85 eV, respectively. Similar results are also found with respect to Pd-CM-TI. These results suggest that Pd(II) is only partially reduced to Pd(0) in the present work, irrespective of the preparation method. The presence of Pd(II) species might be due to the AAPTS-Pd(II) complex. A similar result was reported in the work of Jayamurugan et al.24 HRTEM was performed to analyze the morphology and particle size of the palladium nanoparticles in the two types of Pd-loaded ceramic membrane supports. One can find from Figure 7 that there are palladium nanoparticles loaded on the ceramic membrane support; however, the palladium nanoparticles tend to aggregate together. The mean particle size of palladium particles is about 5 nm. The characteristic lattice fringe of 2.2 Å corresponds to the (111) planes of the facecentered-cubic Pd(0) structure.25 No obvious difference is observed for the morphology and particle size by comparing the HRTEM images of Pd-CM-FT and Pd-CM-TI. Figure 8 shows the TPR profiles of the powder taken from the Pd-loaded ceramic membrane supports, and the result of the ceramic membrane support is also given for comparison. Negative peaks at 76.5 and 74.7 °C are observed for Pd-CM-FT and Pd-CM-TI, respectively, corresponding to the hydrogen evolution resulting from the decomposition of the β-PdH phase,26 which further confirms the presence of palladium nanoparticles on the membrane support. The reduction peaks observed from 250 to 700 °C might be ascribed to the reduction of the ceramic membrane support as presented in Figure 8. 3.2. Examination of the Catalytic Properties of PdLoaded Ceramic Membrane Supports. The reduction of pnitrophenol to p-aminophenol with NaBH4 was used as a model reaction for examining the catalytic properties of Pd-
show that there are many palladium nanoparticles on the membrane surface, and there is no obvious difference between the two types of Pd-loaded ceramic membrane supports. Some blank area can be observed, possibly as a result of the existence of membrane pores. Parts c and d of Figure 4 give the EDS images of the entire cross-section of Pd-CM-FT and Pd-CMTI, respectively. It is interesting to find that the distribution of palladium nanoparticles is completely different: for Pd-CM-FT, many palladium nanoparticles uniformly distribute in the entire cross-section, whereas with respect to Pd-CM-TI, there are nearly no palladium nanoparticles in the cross-section. These results indicate that the preparation method greatly affects the loading of palladium nanoparticles on the membrane support. For the traditional impregnation method, the solution is not easy to insert in the membrane pores because of the higher surface tension during preparation, and almost all of the palladium nanoparticles deposit only on the membrane surface, as in our previous work.11 With respect to the developed flowthrough method, the solution is forced to flow through the membrane pores during preparation, thus the palladium nanoparticles can deposit both on the membrane surface and in the membrane pores, which is expected to enhance the loading number of palladium nanoparticles and their catalytic properties. ICP was used to measure the loading amount of palladium nanoparticles in Pd-CM-FT and Pd-CM-TI. According to the ICP analysis, the weight percentage of palladium is 0.14 wt ‰ (corresponding to 5.0 mg of palladium mounted in the reactor for the catalytic reactions) with respect to Pd-CM-FT, which is significantly higher than the value of 0.095 wt ‰ for Pd-CM-TI (corresponding to 3.3 mg of palladium mounted in the reactor for the catalytic reactions). As expected, the flow-through method can noticeably increase the loading amount of palladium nanoparticles on the ceramic membrane support compared to the traditional impregnation method because the palladium nanoparticles can be loaded in the pores of the ceramic membrane with the flow-through method, in good agreement with the EDS results. XPS analysis was carried out to confirm the presence of the silane coupling agent and palladium in the as-fabricated Pdloaded ceramic membrane supports and for the semiquantitative analysis of the relative atomic concentration of different elements. Figure 5 shows the XPS survey spectra of the powder taken from the Pd-loaded ceramic membrane supports and the ceramic membrane support. Compared to the 14102
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Figure 4. EDS analysis of (a) the surface of Pd-CM-FT, (b) the surface of Pd-CM-TI, (c) the cross-section of Pd-CM-FT, and (d) the cross-section of Pd-CM-TI.
Figure 6. Pd 3d XPS spectra of the powder taken from Pd-CM-FT and Pd-CM-TI.
Figure 5. XPS survey spectra of the powder taken from Pd-CM-FT, Pd-CM-TI, and CM.
loaded ceramic membrane supports. For comparison, the catalytic properties of blank ceramic membrane supports were 14103
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Figure 7. HRTEM images of the powder taken from (a) Pd-CM-FT and (b) Pd-CM-TI.
membrane pores as confirmed by the EDS results, leading to higher palladium loading and a better p-nitrophenol conversion. It is also found from Figure 9 that the p-nitrophenol conversion first increases and then remains stable possibly because of the decomposition of NaBH4.8,27 Although the initial concentration of NaBH4 greatly exceeded that of p-nitrophenol as presented in the Experimental Section, NaBH4 is easy to decompose, and as a result, the relative concentration of NaBH4 would decrease with time and some p-nitrophenol could not be reduced, leading to a stable p-nitrophenol conversion. A series of catalytic reaction cycles were carried out to investigate the catalytic stability of the as-fabricated Pd-loaded ceramic membrane supports. In the present work, the catalytic stability is expressed by the ratio of the p-nitrophenol conversion 60 min after a certain number of reaction cycles to that at the first reaction cycle. The correlation between the relative decrease degree in the p-nitrophenol conversion and the number of catalytic reaction cycles is shown in Figure 10.
Figure 8. TPR profiles of the powder taken from Pd-CM-TI, Pd-CMFT, and CM.
also tested. It is found that no obvious reduction reaction happens on the blank ceramic membrane supports, indicating that the palladium nanoparticles are the active species that are necessary for the reduction of p-nitrophenol to p-aminophenol with NaBH4. Figure 9 shows the change in p-nitrophenol
Figure 10. Catalytic stability investigations of Pd-CM-FT and Pd-CMTI.
During each reaction cycle, both Pd-CM-FT and Pd-CM-TI are undergoing deactivation. Throughout six continuous reaction cycles, the Pd-CM-FT suffers about 9.2% deactivation, whereas the Pd-CM-TI suffers about 12.4% deactivation. These results indicate that the catalytic stability of Pd-CM-FT is superior to that of Pd-CM-TI, which might be related to the preparation method. For Pd-CM-FT, during the preparation the solution is forced to flow through the membrane pores with the help of a pump as presented in the Experimental Section, thus the palladium nanoparticles deposited on the membrane support will be scoured by the solution. As a result, some palladium nanoparticles having poor interactions with the membrane support may fall off during the preparation stage, and the remaining palladium nanoparticles have stronger interactions
Figure 9. Variation of the p-nitrophenol conversion with time.
conversion with time for the two types of Pd-loaded ceramic membrane supports. As expected, the p-nitrophenol conversion for Pd-CM-FT is significantly higher than that for Pd-CM-TI under the same reaction conditions. For example, the pnitrophenol conversion at 60 min is 97% for Pd-CM-FT whereas it is only 69% for Pd-CM-TI. Together with the previous characterization results, one can find that higher palladium loading should be one of the main reasons for the higher p-nitrophenol conversion for Pd-CM-FT. Through the flow-through method, the palladium nanoparticles can be loaded not only on the membrane surface but also in the 14104
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HPLC analysis shows that the product is only p-aminophenol in each case (data not shown here), indicating that the asfabricated Pd-loaded ceramic membrane supports have high catalytic selectivity in the p-nitrophenol reduction to paminophenol.
with the membrane support and do not easily fall off during the continuous reaction cycles, leading to superior catalytic stability. With respect to Pd-CM-TI, the palladium nanoparticles deposited on the membrane support are not scoured by the solution during the preparation stage, thus some palladium nanoparticles having poor interactions with the membrane support remained and fall off easily during the continuous reaction cycles, resulting in lower catalytic stability. This can be verified by ICP analyses of the used Pd-loaded ceramic membrane supports. As shown in Table 1, after six
4. CONCLUSIONS In this work, palladium nanoparticles were deposited on a silanized ceramic membrane support to fabricate a Pd-loaded ceramic membrane support with a flow-through method in which the palladium nanoparticles can be loaded both on the membrane surface and in the membrane pores with increased palladium loading amount and without additional separation steps for the catalyst particles. The as-fabricated Pd-loaded ceramic membrane support displays significantly higher pnitrophenol conversion and catalytic stability in the pnitrophenol reduction to p-aminophenol compared to the Pdloaded ceramic membrane support prepared by a traditional impregnation method. The present work demonstrates that the developed flow-through method is beneficial for enhancing the loading amount of palladium nanoparticles and their catalytic properties.
Table 1. ICP Analysis of Palladium Content sample
palladium content/wt ‰
palladium leaching/%
Pd-CM-FT (fresh) Pd-CM-TI (fresh) Pd-CM-FT (used six times) Pd-CM-TI (used six times)
0.14 0.095 0.12 0.069
14.3 27.4
continuous reaction cycles, the leaching degree of palladium is 14.3% for Pd-CM-FT, which is obviously lower than the value of 27.4% for Pd-CM-TI. The ICP results also indicate that palladium leaching should be one of the main reasons for the deactivation of the Pd-loaded ceramic membrane support. Metal leaching in the reaction medium was considered to be the main cause for the reactivity decrease in liquid-phase reactions.28,29 However, by correlating the catalyst deactivation and the palladium leaching, we find that these two do not match and the degree of catalyst deactivation is lower than that of palladium leaching. For example, only 72.6% of the initial palladium is present in the used Pd-CM-TI, but 87.6% of its initial p-nitrophenol conversion is maintained. This may be caused by the fact that the catalyst performance depends on its surface area and the percentage of surface area loss is not the same as the mass loss. In addition, other parameters such as the morphology evolution of palladium nanoparticles also play certain roles in determining the catalytic stability of the Pdloaded ceramic membrane support. To investigate the influence of the morphology evolution of palladium nanoparticles, the palladium nanoparticles on the membrane support after six reaction cycles were examined by HRTEM as shown in Figure 11. By comparing Figures 7 and 11, it is seen that after six reaction cycles the palladium particle size decreases and the palladium dispersion improves especially for Pd-CM-TI, possibly as a result of palladium leaching, resulting in a higher p-nitrophenol conversion and a lower deactivation degree compared to the leaching degree.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation (21106061, 21125629, and 21306081), the National Key Science and Technology Program (2011BAE07B05), and the National High Technology Research and Development Program (2012AA03A606) of China is gratefully acknowledged.
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REFERENCES
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Figure 11. HRTEM images of the powder taken from (a) Pd-CM-FT and (b) Pd-CM-TI after six catalytic reaction cycles. 14105
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dx.doi.org/10.1021/ie401903v | Ind. Eng. Chem. Res. 2013, 52, 14099−14106
