In Situ Generation of Active Pd Nanoparticles within a Macroreticular

May 11, 2010 - An acidic resin bearing SO3H functional groups within its reticular structure acts as an efficient support for the in situ formation of...
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In Situ Generation of Active Pd Nanoparticles within a Macroreticular Acidic Resin: Efficient Catalyst for the Direct Synthesis of Hydrogen Peroxide Kohsuke Mori,† Akihiro Hanafusa,† Michel Che,‡ and Hiromi Yamashita*,† †

Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, eactivit e de Surface, Suita, Osaka 565-0871, Japan, and ‡Institut Universitaire de France and Laboratoire de R Universite Pierre et Marie Curie-Paris 6, CNRS-UMR 7197, Paris, France

ABSTRACT An acidic resin bearing SO3H functional groups within its reticular structure acts as an efficient support for the in situ formation of highly active Pd nanoparticles (NPs) responsible for the direct synthesis of hydrogen peroxide from H2 and O2. Characterization by means of X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM) reveals that not only the acidity of the resins but also the mean size of the Pd NPs are crucial factors in achieving efficient catalytic performance. The acidic resins act effectively as supports, while basic resins hardly activate the reaction and do not induce any structural change around the Pd center. SECTION Surfaces, Interfaces, Catalysis

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unctional resins have been widely utilized in fundamental academic research and industrial applications.1,2 The unique surface properties of such resins, including hydrophilic/hydrophobic characteristics, the ease of introducing a range of functional groups, and their ability to swell in a reaction medium, make these materials very attractive from the viewpoint of catalyst design.3,4 Moreover, these resins can be used to stabilize highly dispersed metal nanoparticles (NPs) inside of the macroreticular domain with controllable morphology, along with reasonable thermal and mechanical stabilities, offering promising catalyst supports in a number of industrial processes.5 It is well-known that the location of suitable surface functional groups in the vicinity of active metal centers exerts a profitable influence on the catalyst activities. Swelling properties also dramatically improve accessibility of reactants in the liquid phase. As a consequence, catalytically active species generated within the functional resin matrix essentially differ from the conventional solid supported metal catalysts. Hydrogen peroxide (H2O2) is increasingly used in industry because the only byproduct of its reaction is water. This chemical is currently produced by the so-called anthraquinone method, involving sequential hydrogenation and oxidation of an alkyl anthraquinone as an intermediate.6 This indirect process requires the use of toxic solvents and is energy-intensive, resulting in a high overall cost for industry. Recently, the direct synthesis of hydrogen peroxide from H2 and O2 has attracted much attention because it leads to fewer side products and is significantly cheaper. A diverse array of intriguing Pd-based catalysts has been developed on various supports, such as titania, silica, and carbon, to achieve effective dispersion of the active metal components.7-17 In the direct synthesis of hydrogen peroxide, several undesired

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reactions occur simultaneously, including formation of water, hydrogenation, and decomposition of hydrogen peroxide, which are thermodynamically favorable and highly exothermic.18-20 It is well-known that acids inhibit the decomposition of hydrogen peroxide, while halides prevent the formation of water.18-20 In order to minimize the amount of mineral acids and halides being used, acid-functionalized solid supports have been extensively studied for the direct synthesis of hydrogen peroxide.21-23 In the present study, we have successfully utilized an acidic resin as a platform for the direct synthesis of hydrogen peroxide, in which a monomeric PdII species is readily transformed into catalytically active Pd NPs with a mean diameter of ∼20 nm during the course of reaction. The acidity of these resins and the mean size of the Pd NPs have a great impact on their activities, while basic resins do not provide efficient catalysts and do not lead to the formation of detectable Pd NPs. Several Pd catalysts were prepared from commercially available resins (Amberlite; Rohm and Haas) with different chemical properties. The characteristics of employed resins are summarized in Table 1. Resins 200CTNa (resin 2) and 15DRY (resin 3) are strongly acidic, and FPC3500 (resin 4) is weakly acidic. Resin 200CTNa has a higher water absorption capacity (swelling property) than 15DRY. Resin 200CTH (resin 1) was prepared by the simple treatment of 200CTNa with an aqueous HCl solution. IRA96SB (resin 5) and IRA900JCl (resin 6) are utilized as weakly and strongly basic Received Date: April 21, 2010 Accepted Date: May 6, 2010 Published on Web Date: May 11, 2010

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Table 1. Textural Properties of Resin Supported Pd Catalysts catalyst

resina

functional group

exchange capacity (meq/mL)

property

water absorption capacity (%)

reduction

average Pd size (nm)b

in situ NaBH4

20.3 8.0

Pd/resin 1a Pd/resin 1b

200CTH 200CTH

-SO3H -SO3Na

g1.8 g1.8

strongly acidic strongly acidic

46-51 46-51

Pd/resin 1c

200CTH

-

-

-

-

in situ

Pd/resin 2

200CTNa

-SO3H

g1.8

strongly acidic

46-51

in situ

23.4

Pd/resin 3

15Dry

-SO3H

g2.6

strongly acidic

1-2

in situ

16.3

Pd/resin 4

FPC3500

-COOH

g2.0

weakly acidic

60-70

in situ

16.3

Pd/resin 5a

IRA96SB

-NMe2

g1.3

weakly basic

57-63

in situ

n.d

Pd/resin 5b

IRA96SB

-NMe2

g1.3

weakly basic

57-63

NaBH4

7.2

Pd/resin 6a Pd/resin 6b

IRA900JCl IRA900JCl

-NMe3Cl -NMe3Cl

g1.0 g1.0

strongly basic strongly basic

56-65 56-65

in situ NaBH4

n.d. 8.8

a

Styrenic matrix, except for resin 4 (acrylic). b Determined from the TEM image.

resins, respectively. All resins showed moderate BET specific surface areas of approximately 45 m2 g-1. Prior to the Pd deposition, resins were crushed by a ball mill (650 rpm for 15 min). A suspension containing the resin (5.0 g) and 200 mL of aqueous PdCl2 solution (11.3  10-3 M) was stirred at room temperature for 24 h. The mixture was filtered, and the obtained solid was washed repeatedly with distilled water and air-dried overnight. The obtained catalysts are denoted as Pd/resins 1-6. Induction coupled plasma (ICP) analysis determined that the Pd species were easily attached to the resins at the same level of loading (0.5 Pd wt%). As detailed below, the catalysts subjected to catalytic reaction without any treatments were denoted as Pd/resins 1a, 5a, and 6a. Some samples were prereduced with NaBH4 to produce Pd NPs (Pd/resins 1b, 5b, and 6b). The mean diameter of the Pd NPs of Pd/resins 1b, 5b, and 6b was determined to be 8.0, 7.2, and 8.8 nm, respectively, by TEM analysis. In the case of resin 1, the sample was heated at 923 K under an Ar atmosphere to afford Pd/resin 1c. Figure 1A shows the Pd K-edge XANES spectra of the assynthesized Pd/resins, together with PdCl2 and Pd foil as reference samples. The shapes of the XANES spectra and the edge position of all of the as-synthesized Pd/resins (c-e) are identical to those of PdCl2 (a) but differ from those of the Pd foil (b), thus revealing that most of Pd species are in the II oxidation state. The FT-EXAFS data are depicted in Figure 1B. The Pd species entrapped within the acidic resins exhibit a main peak due to the Pd-O bond at around 1.5 Å, corresponding to PdII ions interacting with -SO3- or COO- groups of the resin matrix (c and d). An additional small peak due to a Pd-Pd bond in the second coordination sphere is observed in the case of Pd/resin 1a despite the fact that no Pd nanoparticles are detected by TEM (detection limit 1 nm). These results suggest that extremely small sub-nano-ordered Pd nanoclusters are formed during preparation. XRD also does not evidence peaks attributable to Pd NPs, similarly suggesting that only small clusters are formed. On the other hand, basic Pd/resin 6a (e) exhibits only a Pd-Cl bond at around 1.9 Å, indicating that the PdII species may exist in this case as monomeric [PdCl4]2-. To evaluate their catalytic ability, the Pd/resins were used in the direct synthesis of hydrogen peroxide from H2 and O2. In a

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Figure 1. (A) Pd K-edge XANES spectra and (B) FT-EXAFS spectra for (a) PdCl2, (b) Pd foil, (c) as-synthesized Pd/resin 1a, (d) assynthesized Pd/resin 4, (e) as-synthesized Pd/resin 6a, (f) recovered Pd/resin 1a, and (g) recovered Pd/resin 6a.

typical run, the resin catalyst (0.05 g) was placed into a reaction vessel (80 mL) containing 0.01 M HCl aqueous solution (50 mL). The resulting mixture was treated by flowing gaseous H2 and O2 (20 mL 3 min-1, H2/O2 = 1:1) for 6 h under magnetic stirring at room temperature under atmospheric pressure to ensure operational simplicity. The concentration of H2O2 formed was monitored using a hydrogen peroxide counter (HP-300, Hiranuma Sangyo Co. Ltd.). The results are summarized in Figure 2. The Pd/resin 1a exhibits the highest activity among those examined, in which H2O2 productivity reaches 75.7 mol(H2O2) h-1 mol(Pd)-1. The catalytic activity is greatly influenced by the acidity of the resins and decreases in the order of Pd/resin 1a > Pd/resin 2 > Pd/resin 3 > Pd/resin 4. The relatively low catalytic activity of Pd/resin 3, despite its significant acidity, can be explained by its low swelling ability, which may circumvent smooth transfer of H2 and O2 gases. In contrast, the use of basic Pd/resins (5a and 6a) significantly

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Figure 2. The effect of catalyst on the concentration of generated H2O2.

retards the reaction. Notably, the catalytic activity of Pd/resin 1a is higher by a factor of 1.5-4 than those of our prepared conventional catalysts such as Pd/TiO2 (rutile), Pd/SiO2, and Pd/Al2O3 with the same Pd contents. The average mean diameter of Pd NPs is determined by CO adsorption to be 3.4, 4.5, and 4.7 nm for Pd/TiO2, Pd/SiO2, and Pd/Al2O3, respectively. The importance of the macroreticular structure in combination with the strong acidity is also demonstrated by the drastic decrease in catalytic activity of Pd/resin 1c, which is calcined under inert conditions after the deposition of Pd. Unfortunately, the catalysts reduced by NaBH4 before catalytic reaction (Pd/resins 1b, 5b, and 6b) are found to be less active than their analogues reduced in situ. The acidic Pd/resin 1 showed a color change from yellow to light gray in the very initial stages of the reaction, whereas no color change occurred in the case of the inactive basic Pd/ resins. These observations suggest that the reaction medium transforms the putative catalyst into the real one due to significant chemical reduction in the attached Pd species. To elucidate the structure-activity relationship in the direct synthesis of hydrogen peroxide, the recovered Pd/resins were characterized by XAFS and TEM during the catalytic reaction. As shown in Figure 1A, the Pd K-edge XANES spectrum (f) of the recovered Pd/resin 1a was similar to that of the Pd foil (b). The FT-EXAFS spectra exhibit a single peak at approximately 2.5 Å due to Pd-Pd bonds found in the metal ((f) in Figure 1B). The XRD pattern shows the peak characteristic of the fcc Pd lattice at around 2θ = 40°. These results are in good agreement with those obtained from TEM, in which relatively large Pd NPs with a mean diameter of ∼20.3 nm are found with a narrow size distribution (Figure 3). Pd NPs with similar size are also observed in other acidic resins (Table 1). On the other hand, the monomeric PdII species in the basic resins are exceptionally stable even in the presence of H2. The XANES spectrum of the recovered Pd/ resin 6a is identical to those of the fresh catalyst and PdCl2, revealing that the electronic configuration of the Pd species does not change ((g) versus (a) in Figure 1A). In the FT-EXAFS spectra, there are no peaks due to the Pd atoms bonded each other ((g) in Figure 1B). TEM examination also confirms that there are no Pd particles formed after the reaction. It can be concluded that the Pd NPs generated in situ within the

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Figure 3. TEM image and size distribution diagram of Pd/resin 1a after the catalytic reaction.

macroreticular domain during the course of the reaction act as promising catalytically active species. We also wish to point out that both the nature of the resin and the size of the Pd NPs determine the catalytic activity for the reaction. As shown above, the basic Pd/resins exhibit negligible activity even with Pd NPs formed by reduction with NaBH4 (Pd/resins 5b and 6b), which can be ascribed to the occurrence of undesired base-catalyzed side reactions. The prereduced acidic resin-supported Pd NPs (Pd/resin 1b) are unfavorable despite their smaller size (∼8 nm) compared to the in-situgenerated Pd NPs (Pd/resin 1a) of larger size (∼20 nm). This indicates that the relatively larger size of the Pd NPs is suitable for this reaction, and such NPs are self-assembled within the acidic macroreticular domain during the reaction. The reason for the high catalytic activity of the in-situ-generated Pd NPs is unclear now, but the electronic, rather than geometric, differences may be a significant factor which suppress the nonproductive decomposition of H2O2. In summary, we have utilized an acidic resin as a support to produce a promising Pd catalyst for the direct synthesis of hydrogen peroxide from H2 and O2. The in-situ-generated Pd NPs offer a simple and efficient catalytic system with a high activity under atmospheric pressure as compared to conventional Pd catalysts. The acidity of the resins and the mean size of the Pd NPs are found to be key factors in achieving efficient catalytic performances.

SUPPORTING INFORMATION AVAILABLE Experimental details and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. Fax and Tel: þ816-6879-7457. E-mail: [email protected].

ACKNOWLEDGMENT Part of this work was financially supported by the Industrial Technology Research Grant Program in 2007 from

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the New Energy and Industrial Technology Development Organization (NEDO) of Japan. We acknowledge Dr. Eiji Taguchi and Prof. Hirotaro Mori at the Research Center for Ultra-High Voltage Electron Microscopy, Osaka University, for their assistance with the TEM measurements. XAFS spectra were recorded at the beamline 01B1 station in SPring-8, JASRI, Harima, Japan (Prop. No. 2009B1127). H.Y. acknowledges his invited professorship at UPMC.

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DOI: 10.1021/jz100509x |J. Phys. Chem. Lett. 2010, 1, 1675–1678