Role of Phenolic Groups in the Stabilization of Palladium

Jun 27, 2013 - For carbon supported Pd catalysts, the surface properties of activated carbons (ACs) are closely related with the Pd particle size. In ...
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Role of Phenolic Groups in the Stabilization of Palladium Nanoparticles Tie-yong Xu, Qun-feng Zhang, Hua-feng Yang, Xiao-nian Li,* and Jian-guo Wang* Industrial Catalysis Institute of Zhejiang University of Technology, State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Hangzhou, 310032, P. R. China S Supporting Information *

ABSTRACT: For carbon supported Pd catalysts, the surface properties of activated carbons (ACs) are closely related with the Pd particle size. In this study, phenolic groups were adjustably introduced on ACs by hydrothermally treating ACs under different temperatures (433−513 K). Pd/ACs catalysts were prepared by the wetness impregnation method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), CO chemisorption, and H2-temperature-programmed reduction (TPR). The results revealed that the size of Pd nanoparticles (NPs) was highly dependent upon the amount of phenolic groups. The density functional theory (DFT) study suggested that the enhanced binding between palladium clusters and surface functional groups (SFGs) modified carbon nanotubes (CNTs) in the sequence: CNTs−O > CNTs−OH > CNTs−COOH > CNTs. Both the experimental and theoretical results suggested that phenolic groups on the surface of ACs play a vital role in the stabilization of Pd NPs, which provides insight into how to treat/or choose carbon supports for the preparation of small noble metal particles. carbons with low acidity; while for Cu,15 the strong acidic group (carboxyl group) resulted in the largest increase of binding energies between Cu and carbon nanotubes (CNTs). However, Bulushev et al.16 reported a negative role of carboxylic groups; they would decompose after the deposition of Au on the carbon surface, resulting in the agglomeration of Au nanoparticles (NPs). Our previous work indicated that it is still difficult to understand which oxygen-containing species play a vital role in stabilizing the noble metal particles in the case of coexistence of various SFGs, because various SFGs (−O, −OH, and −COOH) could be created by the conventional oxidation treatments.13 Recently, hydrothermal treatment (HT) was widely used to modify carbon materials in an aqueous phase. The HT would introduce phenolic groups on carbon surface,17 which may provide the possibility for us to study the effect of phenolic groups on the interaction between noble metals and carbon materials. Carbon supported Pd catalysts, particularly the Pd/ ACs catalyst, have been extensively used in hydrogenation,18 dehydrogenation,19 and oxidation20 for the synthesis of fine chemicals. The high dispersion of Pd on the carbon surface is essential for saving noble metals and improving the catalytic activity. This work continues along the lines of related studies in our group, in which activated carbons were treated by nitric acid and the Pd particle size was largely dependent upon the density of SFGs on ACs,13 while the interaction between palladium and each type of SFG was still unclear. The present work was motivated by our interest in the role of different SFGs for the

1. INTRODUCTION Carbon materials, such as activated carbons (ACs), are excellent supports owing to their strong chemical stability both in acid and basic environments, availability, and easiness for recycling of noble metals by a simple burning-off technique.1 The interaction between the activated carbon and metal precursors could be greatly enhanced by creating defects or introducing surface functional groups (SFGs) on the surface of activated carbon, which affects the deposition of noble metals significantly. The defects on carbon supports lead to the improvement of the covalent bond between carbon and noble metal atoms, such as Pt,2 Pd,3 Ag,4 Au,5 and Ru.6 It is generally believed that SFGs on activated carbon also play an important role for the adhesion and dispersion of noble metals. Thus, treatments by various oxidants, such as nitric acid (HNO3),7 potassium permanganate (KMnO4),8 hydrogen peroxide (H2O2),9 and O2,10 have been employed to introduce SFGs, such as carboxyl, lactonic, and phenolic groups onto the activated carbon. Due to the complexity of the surface properties of activated carbon, i.e., the amount and distribution of SFGs on activated carbon are difficult to determine. Therefore, the understanding of the enhancement effect of SFGs on carbon surface for the dispersion of noble metals is usually ambiguous and even controversial. Chen et al.11 reported that the presence of SFGs could improve the hydrophilicity of carbon materials, which is beneficial for the adsorption of metal precursors on the carbon surface. Further studies show that the particle size of noble metals is related to the amount of SFGs, which could be tailored by using various oxidants12 or changing the reaction conditions, such as temperature and concentration of oxidants.13 There is some controversy on which type of SFGs would result in a stronger interaction between metal precursors and carbons. Pt14 showed a stronger interaction between the metallic precursor and the © 2013 American Chemical Society

Received: Revised: Accepted: Published: 9783

January 25, 2013 June 20, 2013 June 27, 2013 June 27, 2013 dx.doi.org/10.1021/ie401454n | Ind. Eng. Chem. Res. 2013, 52, 9783−9789

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filtrate was titrated by HCl. The amount of SFGs was calculated by the amount of alkali consumption. X-ray diffraction (XRD) measurements of the Pd/ACs catalysts were performed using an X′Pert diffractometer (PNAlytical Co.) equipped with a Cu K radiation source that was operated at 60 kV and 55 mA. The Pd NPs morphology and size distribution of Pd/ACs catalysts were determined by a Tecnai G2 F30 S-Twin TEM at an operating voltage of 300 kV. About 400−500 particles were randomly counted to determine Pd particle size distribution. Temperature-programmed reduction (TPR) was performed in a quartz microreactor equipped with a thermal conductivity detector (TCD). Typically, 0.1 g of sample was loaded in the isothermal zone of the reactor and was first pretreated in flowing Ar at 383 K for 2 h; the sample was cooled down to room temperature, and then, H2-TPR was performed in a 5%H2/Ar (25 mL·min−1) flow by heating the sample up to 1123 K at a rate of 10 K/min. The size of Pd NPs on the ACs was also determined by CO uptake through a pulse chemisorption method with a mass spectrometry (Omnistar) at ambient temperature and pressure. The determination of Pd content was as follows: the Pd/ACs samples were calcined at 1073 K for 8 h (to burn off the carbon support); then, they was dissolved by aqua regia. Finally, the Pd content in solution was determined by an Elan DRC-e ICP-MS elemental analyzer. 2.4. Density Functional Theory (DFT) Calculations. Density functional theory calculations were performed using the Vienna Ab Initio Simulation package (VASP).22 A planewave basis set with a cutoff energy of 400 eV and ultrasoft Vanderbilt pseudopotentials (U.S.-PP)23 was employed. The Perdew−Burke−Ernzerhof (PBE) form of the generalized gradient approximation (GGA) exchange24 and correlation functional was used in all calculations reported herein. The adhesion of noble metals on different carbon materials is similar; for example, whether graphene4 or CNTs5 is used as carbon support, the point defects are always the anchoring sites for Au nanoparticles. Due to the fact that it is a great challenge to build the structure and simulate the activated carbons, carbon nanotubes were used as the model in our DFT study. Herein, the (5, 5) carbon nanotubes (CNTs) were used. The periodic supercell with the length of a, b, and c being 20, 25, and 9.92 Å, respectively, was adopted to avoid interactions among repeating slabs. The Brillouin zone integration was carried out with 1 × 1 × 4 k-point sampling, where the force threshold for the optimization was 0.01 eV/Å. Adsorption energies of all the related structures reported in this paper were calculated in their most favorable adsorption modes. The adsorption energy, Eads, was calculated by the following equation:

stabilization of Pd NPs. We developed a novel and facile method to introduce phenolic groups on the surface of ACs by HT of ACs in an aqueous phase. The hydrothermal pretreatment keeps porous structure intact, and the major SFGs on carbon surface are phenolic groups. Then, a series of highly dispersed Pd/ACs catalysts were prepared. The Pd NPs showed a higher dispersion on treated ACs than on untreated ACs. X-ray diffraction (XRD), transmission electron microscopy (TEM), chemical titration, CO chemisorption, and H2temperature-programmed reduction (TPR) characterizations showed that the Pd particle size was largely dependent on the amount of phenolic groups. Furthermore, the adhesion of Pd NPs on different SFG modified CNTs was investigated by the density functional theory (DFT) method. This work is to provide a fundamental understanding of the role of different SFGs (including atomic oxygen (−O), phenolic (−OH), and carboxyl (−COOH) groups) in the stabilization of Pd NPs. It should be noted that the HT is environmentally friendly, simplifying the washing procedure of ACs and avoiding the waste acid treatment process.

2. EXPERIMENTAL SECTION 2.1. Hydrothermal Pretreatment of ACs. The hydrothermal treatment of ACs was carried out in a high-pressure autoclave. 10.0 g of ACs (Shaowu Xinsen Chemical Industry Co. Ltd.) and 200 mL of deionized water were introduced into a 500 mL high pressure autoclave, respectively; then, it was heated to the desired temperature (T = 433−513 K, the pressure is self-generated and changes with temperature, P = 0.35−2.80 MPa) and held at that temperature for 2 h and subsequently cooled to room temperature. The treated ACs were filtered from the slurry and dried at 383 K overnight. The pretreated ACs were denoted as ACs-X, where X (K) was the pretreatment temperature. 2.2. Preparation of Pd/ACs Catalyst. Pd/ACs catalyst was prepared by adding a desired volume of H2PdCl4 aqueous solution (0.05 gmetal·mL−1) into an aqueous slurry of treated ACs (the ratio of ACs to water was 1 g/10 mL) to obtain a Pd nominal loading of 5 wt %. The slurry was vigorously stirred at 353 K for 5 h; then, the slurry solution pH of 8−9 was reached by dropwise addition of NaOH aqueous solution (10%). Eventually, the precipitated PdO·H2O supported on ACs was reduced by ∼2−3% hydrazine hydrate at 308 K for 2 h, filtered, and dried in vacuum at 383 K overnight. 2.3. Characterization of ACs and Pd/ACs Catalyst. The BET surface area and porous structure of ACs were determined by nitrogen physical adsorption−desorption at 77 K. Typically, a 50 mg sample was heated to 453 K and held at that temperature for 12 h to remove the adsorbed species. Nitrogen adsorption isothermal was measured using a NOVA 1000e surface area analyzer (Quantachrome Instruments Corp.). The surface area was calculated by the BET equation, and the external or nonmicropore surface area was obtained by the tplot method. The amount of SFGs was measured by the Boehm’s method.21 According to the acidities of SFGs, i.e., the amount of carboxyl groups, lactones/lactols and phenolic groups, and all oxygen-containing groups on the surface of ACs was determined by adsorption neutralization with NaHCO3, Na2CO3, and NaOH solutions, respectively. Typically, 0.5 g of ACs was immersed into 50 mL of 0.04 mol/L NaOH, Na2CO3, and NaHCO3 solutions for 48 h, respectively. Then, the ACs were filtered from the slurry, and the concentration of

Eads = ECNT − O*/Pdx − ECNT − O* − E Pdx

where ECNT−O*/Pdx is the total energy of the perfect CNTs modified by oxygen-containing groups (−O, −OH, −COOH) with Pdx (x = 1, 4); ECNT−o* is the total energy of perfect CNTs with certain oxygen-containing groups bound to them; and EPdx (x = 1, 4) is the energy of isolated Pd atom or Pd tetramer, respectively.

3. RESULTS AND DISCUSSION 3.1. Physicochemical Properties of ACs. The metallic dispersion is highly dependent upon the textural characteristics of carbon materials, such as the specific surface area and the 9784

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porous structure.25 The structural parameters of activated carbons were listed in Table 1. The significant effects of

ditions.26 The fully dissociative adsorption of water on the defective cluster resulted in the formation of carbonyl groups,27 and the carbonyl groups were further transformed into phenolic groups,28,29 which were confirmed by Boehm’s chemical titration and XPS technique. Thus, the defective sites created by the decomposition of carboxyl groups might be responsible for the introduction of phenolic groups on the AC surface. In fact, the oxidations of carbon materials by HNO3, H2O2, O3, O2, and KMnO4 have been used to tailor the distribution of SFGs. However, the distribution of SFGs on the carbon surface is extremely complicated, and it is difficult to remove some types of SFGs from the carbon surface selectively by these methods. Fortunately, the carboxyl groups could be selectively removed from the carbon surface by the HT method, and it is also easy to reach a desired amount of phenolic groups by changing the temperatures. The results of nitrogen physical adsorption and Boehm’s method test indicated that the hydrothermal pretreatment keeps porous structure intact, and the major SFGs on the carbon surface are phenolic groups. It becomes possible for us to investigate which kind of SFG plays a vital role in the dispersion of Pd NPs. 3.2. Pd NPs Size and Distribution of Pd/ACs Catalysts. The measured loadings of Pd in the Pd/ACs-untreated and Pd/ ACs-X catalysts were listed in Table 3. The experimental results

Table 1. Structural Parameters of Hydrothermally Treated ACs at Various Temperaturesa sample ACsuntreated ACs-433 ACs-453 ACs-473 ACs-493 ACs-513 a

SBET (m2/g)

Smic (m2/g)

Vmic (cm3/g)

Vtotal (cm3/g)

D (nm)

2216

1602

0.634

1.505

2.772

2035 2068 2101 2229 2129

1480 1518 1554 1614 1548

0.584 0.610 0.610 0.647 0.621

1.384 1.402 1.523 1.523 1.450

2.754 2.750 2.754 2.770 2.764

S: specific surface area; V: pore volume; D: pore diameter.

hydrothermal pretreatment on the surface area, the pore volume, and the average pore diameter (all around 2200 m2/g, 1.500 cm3/g, and 2.77 nm, respectively) were not observed, which indicated that the structures of ACs were not damaged using H2O as oxidant.25 Furthermore, the porous size distributions between untreated ACs and hydrothermally treatment ones are very similar (see Figure S1, Supporting Information). Therefore, under our hydrothermal conditions, the structure of ACs was not significantly influenced. On the contrary, the collapse of structures of carbon materials is always observed using nitric acid (HNO3),7 potassium permanganate (KMnO4),8 and hydrogen peroxide (H2O2)9 as oxidants due to the strong oxidative capacity, especially at high temperature and/or using high concentration of oxidant.13 Thus, the hydrothermal treatment is favorable for us to study the effect of SFGs on the Pd NPs size, regardless of the influence of the structure of ACs. The amounts of oxygen-containing functional groups, such as carboxyl, lactonic, and phenolic groups on the surface of activated carbon were listed in Table 2. Their amount and

Table 3. Practical Pd Loading of the Obtained Pd/ACs and Pd/ACs-X Samplesa

untreated 433 453 473 493 513

carboxyl group (mmol/g)

lactonic group (mmol/g)

phenolic group (mmol/g)

total acidity (mmol/g)

0.105 0.113

0.078 0.057 0.049 0.099

0.009

0.178

0.106 0.109 0.168 0.223 0.252 0.290

0.289 0.279 0.217 0.322 0.252 0.475

Pd/ ACs433

Pd/ ACs453

Pd/ ACs473

Pd/ ACs493

Pd/ ACs513

Pd loading (%)

4.65

4.62

4.52

4.67

4.85

4.72

a

Practical Pd loading was determined by ICP-MS.

indicated that their practical Pd loadings are close to the nominal loading (5%), whether ACs were hydrothermally treated or not. No significant effect of surface properties of ACs on the practical Pd loading was observed, when ACs were treated hydrothermally. Figure 1 shows the TEM images and corresponding Pd NPs size distribution of Pd/ACs-untreated and Pd/ACs-513 samples, respectively. In the case of hydrothermally treated ACs, the morphology and thickness of ACs are almost similar to those of untreated ACs. In Figure 1a,c, it could be observed that there are heavy agglomerations of Pd NPs and almost 30% Pd NPs are larger than 15 nm on Pd/ACs-untreated sample, which might be attributed to the strong hydrolyzable nature of H2PdCl4. The highly insoluble hydroxides are generally formed during the catalyst preparation process, which results in the serious agglomeration of Pd NPs, as shown in Figure 1a. On the contrary, the Pd NPs of the Pd/ACs-513 sample are much more uniform than that of Pd/ACs-untreated sample, and over 70% Pd NPs are smaller than 5 nm as shown in Figure 1b,d. After hydrothermal pretreatment of ACs at 513 K, the percentage of Pd NPs smaller than 5 nm was increased from 42.9 to 72.4% while that of those bigger than 10 nm was decreased from 43.6 to 12.3%, respectively. These results indicated that the hydrothermal pretreatment would influence the interaction between Pd NPs and the surface of ACs significantly. In addition, a decrease of Pd particle size was observed with the increase of temperature (see Figure S2, Supporting Information). Therefore, the hydrothermal treat-

Table 2. SFGs Amount on the Surface of Various ACs temperature (K)

sample

Pd/ACsuntreated

distribution on the AC surface are highly dependent upon the hydrothermal treatment temperatures. No significant changes of the SFGs amount (∼7−9%, see Table S1, Supporting Information) and distribution of ACs treated hydrothermally at 433 K were observed. However, the carboxyl groups were almost removed from the surface of ACs treated hydrothermally at above 453 K, and the amount of phenolic groups was increased with the increasing temperature. Consequently, phenolic groups are the major SFGs on ACs after hydrothermal treatment and the amount of which can be easily tailored by changing temperature. The stability of carboxyl groups is not as good as that of phenolic groups, and the decomposition of which could generate defective sites on ACs under hydrothermal con9785

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Figure 1. TEM images and corresponding Pd NPs distributions of Pd/ACs-untreated (a, c) and Pd/ACs-513 (b, d).

uniform dispersion of Pd NPs just by the hydrothermal treatment of ACs at various temperatures. 3.3. Interaction of Pd NPs with SFGs Modified Carbons. It is well-known that Pd dispersion is highly dependent upon the surface properties of carbon support,13,31,32 among which SFGs would significantly affect the adhesion behavior of Pd on carbon support. As shown in Table 2, the major SFGs of ACs pretreated hydrothermally are phenolic groups. The relationship between the amount of phenolic groups and the mean Pd particle size measured by CO chemisorption in Figure 3 indicated that the size of Pd NPs will be decreased with the increase of the concentration of phenolic groups on carbon surface, which could be attributed to the strong interaction between Pd and phenolic groups of ACs. In contrast, for untreated ACs, a considerable number of carboxyl groups are left on the surface of ACs, which may contribute to the agglomeration of Pd particles. The results of our study for

ment of ACs is an effective way to improve Pd NPs dispersion on ACs. The XRD patterns of Pd/ACs-untreated and Pd/ACs-X are shown in Figure 2. Three peaks at 2θ of ∼40, 46, and 68

Figure 2. XRD patterns of Pd/ACs-untreated and Pd/ACs-X catalysts.

corresponding to the (111), (200), and (220) reflections of Pd metal with a face-centered cubic (fcc) structure are observed, respectively. The diffraction intensity of Pd/ACs-X (X = 433− 513) is much smaller than that of Pd/ACs-untreated, and with the increase of temperature, the peak width (which is inversely proportional to the size of Pd NPs as Scherrer formula indicated) becomes broader gradually, and the diffraction peak becomes asymmetric. The size of Pd NPs on ACs-513 is too small to be detected by XRD. Additionally, especially for Pd/ ACs-473, there is another crystalline phase besides bulk Pd. According to the literature,30 this broad peak near Pd bulk peak could be attributed to PdHx phase, which forms on small Pd NPs more easily. The results of XRD and TEM clearly demonstrate the facile controllability of Pd NPs size and more

Figure 3. Relationship between Pd particle size and the amount of phenolic groups. 9786

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Pd are similar to that for Au,16 in which carboxyl groups decompose during interaction with [Au(en)2]Cl3 solution, forming metallic Au clusters. These Au clusters serve as nucleation centers for the formation of big agglomerates during the reduction process. To further study the enhancement effect of phenolic groups on the interaction between palladium species and carbon support, the reduction performance of PdO/ACs-untreated and PdO/ACs-513 were investigated by H2-TPR, as shown in Figure 4. The H2-TPR results indicated that the maximum

support. Besides, the amount of hydrogen consumption of PdO/ACs-untreated is much smaller than that of PdO/ACs513, indicating that more palladium precursor was reduced to metallic Pd by carbon support itself on ACs-untreated than on ACs-513. Considering that the practical loading of Pd on Pd/ ACs-untreated and Pd/ACs-513 are close (as shown in Table 3), the difference may be explained by the fact that the palladium precursor could result in the decarboxylation of carboxylic groups, and the palladium precursor is reduced to Pd(0) at the same time. The formation of Pd(0) would serve as nucleation centers for the formation of bigger agglomerations of Pd NPs.16 Consequently, the reduction of palladium precursor is retarded on the ACs-513 surface, leading to a higher Pd dispersion on ACs-513. In addition, there is a negative peak at 360−363 K in the H2-TPR curves of both catalysts, which might be attributed to the dissociation of palladium hydride. A much sharper negative peak was observed in the H2-TPR curves of PdO/ACs-513, also approved the smaller Pd particle size.28 To further reveal the effect of different SFGs on the distribution of Pd NPs, the DFT method was employed to compare the adhesion behavior of Pd on different SFG modified CNTs, as shown in Figure 5. The most stable adsorption site is the bridge site for one Pd atom on the surface of perfect CNTs as well as for SFG modified CNTs. For four Pd atoms, it tends to form a tetrahedron structure with three Pd atoms directly binding to carbons. Obviously, on one hand, for either one Pd atom or Pd tetrahedron, the existence of SFGs enhances the binding energy between Pd and CNTs. This result is in accordance with the case reported by Lee et al. that SFGs improve the interaction between Cu and CNTs.15 On the other hand, for one Pd atom system, the −O, −OH, and −COOH show the different enhancement extent, with the order of adsorption energy being CNT−O > CNT−OH >

Figure 4. H2-TPR profiles of (a) PdO/ACs-untreated and (b) PdO/ ACs-513. Reduction conditions: 0.1 g of sample, heated at a ramp of 10 K/min from 303 to 1123 K, 25 mL/min 5%H2 in Ar.

characteristic reduction peak at 377 K of PdO/ACs-untreated catalyst could be attributed to the reduction of PdO species; the corresponding reduction peak shifts to 447 K for PdO/ACs513 catalyst, which suggested the presence of a stronger interaction between palladium species and ACs-513 carbon

Figure 5. The most stable structures of (a) Pd1 and (b) Pd4 on the different oxygen-containing group modified CNTs. The gray, blue, red, and white atoms are carbon, palladium, oxygen, and hydrogen, respectively. 9787

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Industrial & Engineering Chemistry Research CNT−COOH, which is 2.24, 1.83, and 1.69 eV, respectively. Interestingly, the Pd tetrahedron system also follows the same regularity with the energies of 2.72, 1.73, and 1.47 eV, respectively. From these results, we can reasonably suppose that the binding energy between Pd and CNTs could be enhanced by oxygen groups on the AC surface. Particularly, the enhanced effect of isolated oxygen groups for binding energy between Pd and CNTs is the largest, which may be on the AC surface or generated during the impregnation process. It should be pointed out that the interaction between palladium precursor and phenolic groups (−OH) would result in the formation of oxygen anion (−O−) by the deprotonation process,16 which is the most stable oxygen species for the stabilization of Pd NPs (as Figure 5 indicated). Thus, it is clear that SFGs on carbon surface are beneficial for the Pd dispersion and the enhancement effect was largely dependent on the type of SFGs. For carboxyl groups, the palladium precursors could be easily reduced to its metallic Pd(0) due to decarboxylation reaction, leading to the agglomeration of palladium NPs

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REFERENCES

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However, for phenolic groups, the strong anchoring effect of oxygen on Pd would contribute to the high dispersion of Pd NPs. We believe that our study would provide a quite general rule which could be applied in many other noble metals and carbon materials. ©−OH + Pd2 + → ©−O−Pd2 + + H+

4. CONCLUSIONS Hydrothermal pretreatment of activated carbons could keep the porous structure intact and introduce a desired amount of phenolic groups as the major SFGs on carbon surface at the same time. The size of Pd NPs is highly dependent upon the amount of phenolic groups: the greater amount of the phenolic groups, the smaller will be the Pd NPs size. Both experimental and DFT calculated results showed that SFGs on carbons enhanced the binding energy between palladium precursor and carbons, and the binding energies of Pd tetrahedron on different SFG modified CNTs are 2.72, 1.73, and 1.47 eV for CNT−O, CNT−OH, and CNT−COOH, respectively. The different binding energies of Pd on different SFG modified carbons are responsible for the improvement of Pd dispersion after hydrothermal pretreatment of activated carbons. For over 70% Pd NPs smaller than 5 nm on activated carbon, the hydrothermal treatment might be an effective method to prepare highly dispersed Pd/ACs catalyst. ASSOCIATED CONTENT

S Supporting Information *

Details concerning pore size distribution, TEM images, and XPS spectra. This information is available free of charge via the Internet at http://pubs.acs.org/.

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ACKNOWLEDGMENTS

This work was supported by National Basic Research Program of China (973 Program) (2013CB733501 and 2011CB710800), National Natural Science Foundation of China (NSFC-20976164, 21176221, and 211360001), Zhejiang Provincial Natural Science Foundation of China (ZJNSFR4110345), and the New Century Excellent Talents in University Program (NCET-10-0979).

©−COOH + Pd2 + → © + Pd0 + H+ + CO2 ↑

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.-n.L.); [email protected] (J.-g.W.). Fax: +86-571-88320667 (X.-n.L.); +86-571-88871037 (J.-g.W.). Tel.: +86-571-88320002 (X.-n.L.); +86-571-88871037 (J.g.W.). Notes

The authors declare no competing financial interest. 9788

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dx.doi.org/10.1021/ie401454n | Ind. Eng. Chem. Res. 2013, 52, 9783−9789