Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen

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Article

Direct synthesis of hydrogen peroxide from hydrogen and oxygen over mesoporous silica-shell-coated, Pd-nanocrystal-grafted SiO nano-beads 2

Myung-gi Seo, Dae-Won Lee, Sang Soo Han, and Kwan-Young Lee ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00388 • Publication Date (Web): 20 Mar 2017 Downloaded from http://pubs.acs.org on March 22, 2017

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Direct synthesis of hydrogen peroxide from hydrogen and oxygen over mesoporous silica-shell-coated, Pd-nanocrystal-grafted SiO2 nano-beads Myung-gi Seo a, Dae-Won Lee b, *, Sang Soo Han c, Kwan-Young Lee a, d, **

a

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seoul 02841, Republic

of Korea b

Department of Chemical Engineering, Kangwon National University, 1 Kangwondaehak-gil, Chuncheon-si,

Gangwon-do 24341, Republic of Korea c

Computational Science Research Center, Korea Institute of Science and Technology (KIST), Hwarangno 14-gil

5, Seoul 02792, Republic of Korea d

Green School, Korea University, 145 Anam-ro, Seoul 02841, Republic of Korea

*Corresponding author. Tel: +82-33-250-6331; fax: +82-33-3658. E-mail address: [email protected] (D.-W. Lee) **Corresponding author. Tel: +82-2-3290-3299; fax: +82-2-926-6102. E-mail address: [email protected] (K.-Y. Lee)

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Abstract Many studies have been conducted on core-shell structured, nanocatalysts thanks to their high thermal and physical stability. However, for a typical core-shell structure, shell thickness and pore size that affect mass transfer through the shell are difficult to control. Herein, we synthesized a different type of core-shell catalyst, in which a mesoporous silica shell encapsulates the Pdnanocrystals-grafted-SiO2 nano-beads. With the preparation method introduced, we successfully controlled the thickness of shell-layer and generated a mesoporous texture over the shell layer. In activity tests, the production rate of hydrogen peroxide significantly increased when using the mesoporous shell catalyst over the microporous shell catalyst of similar shell thickness. The thickening of mesoporous shell-layer reduced the production rate of hydrogen peroxide. Thus, the thinner the thickness of a mesoporous shell, the more favorable in terms of pore-diffusion rate. However, the shell thickness should be adequately adjusted, because an extremely thin shell-layer cannot protect the core Pd crystals from thermal agglomeration in a calcination and reduction process.

Keywords: Palladium, nanometal catalyst, core-shell structure, direct hydrogen peroxide synthesis, mesoporous shell


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1. Introduction In recent years, many studies have been conducted on the direct use of nano-sized transition or precious metal crystals as a catalyst for various kinds of reaction. In most cases, the nanocrystals are prepared well in the size and morphology as intended for application 1. The nanocrystals usually show very high catalytic activities, which are primarily attributed to their high surface-to-volume ratios 1. However, the nanocrystal catalysts lose their advantages under high-temperature reaction/pretreatment conditions, because the particles are thermally agglomerated and/or transformed into different morphologies 2. Such thermal vulnerability may be one of the major obstacles in expanding the applications of nanocrystal catalysts. The most potent solution is to encapsulate the nanocrystals with a porous inorganic oxide such as SiO2, TiO2, Al2O3, SnO2, ZnO and CeO2, by which the thermal agglomeration of nanocrystals can be effectively prevented 3. Many studies presented promising results by adopting a ‘core-shell’ or ‘yolk-shell’ structure 4. The pore paths in a shell oxide layer, which are required for the diffusional transport of reactant/product molecules to/from core metals, can be generated by removing the surfactant or polymer molecules that are added together when synthesizing the core metal crystals 2c, 3b, 5. Recently, the direct synthesis of hydrogen peroxide has attracted many interests, attributed to its advantages in a standpoint of green chemistry

2, 5-6

. Hydrogen peroxide (H2O2) is used as an

oxidizing agent widely in many industrial or green chemistry applications, attributed to its high activity, selectivity and most of all, environmental benignness (only water is produced as a byproduct in oxidation applications). Currently, over 3.0 million metric tonnes of hydrogen peroxide is being produced annually by anthraquinone oxidation process. However, an alternative process is required, because the anthraquinone process operates using lots of hazardous organic chemicals and is unsuitable for the small/midium sized production of hydrogen peroxide 7. The direct synthesis of hydrogen peroxide from hydrogen and oxygen (the reaction route I, Scheme 1) is received as the only alternative process which satisfies the requirements. The direct synthesis of hydrogen peroxide has the virtues as a green chemistry process: It is a single-step reaction which can occur under ambient


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temperature and pressure without a requirement and emission of environmentally hazardous chemicals. However, it has not been established as a commercial process yet. A safety issue is related to it, because a hydrogen-oxygen mixture is explosive under wide composition range 8. However, a more fundamental reason is an insufficient selectivity (or yield) of hydrogen peroxide due to the occurrence of side reactions producing water. All of the (three) side reactions (the routes II, III and IV, Schematic 1) are spontaneous with negative Gibbs energies of reaction

7, 9

, which thereby compete

with the hydrogen peroxide-production reaction, making negative influences on obtaining a high selectivity of hydrogen peroxide.

Scheme 1. The reaction routes involved in direct hydrogen peroxide synthesis and their standard enthalpies and Gibbs energies of reaction

Such low activities have been improved continuously over decades owing to the advancement of catalyst technology 10. In obtaining a high selectivity of hydrogen peroxide, it is very effective to use Pd or Pd-Au as the active metal component

10f, 10h, 11

, with the addition of mineral acids and/or halide

ions to a reaction medium 9, 10d, 11b, 12. In particular, the Hutchings group has recently developed a PdSn catalyst displaying a maximum hydrogen peroxide selectivity of 96% without the use of halides13. Mechanism-wise, the hydrogen peroxide selectivity is determined according to the adsorption mode of oxygen molecules on a Pd surface. An oxygen molecule chemisorb dissociatively on an ‘energetic’ site (e.g. corner and edge) while molecularly on a ‘less energetic’ site (i.e., terrace)

6b, 14

. It is

generally received that the dissociated O radicals lead mostly to the production of water, while the molecularly-adsorbed O2 reacts with H2 (in a Eley-Rideal mode) to produce hydrogen peroxide 10b, 15. If a proper amount of halide ions (e.g., Br– ion) are added to the aqueous media, the ions adsorb


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selectively onto the corner and edge sites of Pd crystals, which inhibits the O-O bond dissociation and consequently the selectivity of hydrogen peroxide is increased. The addition of mineral acids (H+ ion) also results in the improvement of hydrogen peroxide selectivity, by accelerating the desorption of hydrogen peroxide formed over Pd sites

10d

. In contrast to previous reports, the Flaherty group has

suggested and experimentally proved that H+ is a reactant in the hydrogen peroxide synthesis reaction16. In addition, the added H+ lowers the isoelectronic point of catalyst surface, which enhances the adsorption of halide ions (Br–) on the energetic Pd sites 10h. Meanwhile, the density functional theory (DFT) studies have provided many useful insights in developing a (Pd-based) catalyst with the elevated hydrogen peroxide selectivity

17

. However, in

the simulation studies, it was generally assumed that the adsorption sites are on a single crystal Pd surface, which is not the case for the Pd metal-loaded catalysts via conventional preparation methods (impregnation, precipitation and etc.). Therefore, to verify the DFT results about the relative activities of single crystal Pd surfaces, it is worth measuring the actual catalytic activities of single Pd nanocrystals in the direct hydrogen peroxide synthesis. According to the findings of the Mavrikakis group, the surface structure of Pd and the O*/OH* coverage of the Pd surface affect the synthesis and degradation of hydrogen peroxide, as shown in their comparative analysis of the catalytic activity of the Pd {111} and Pd {100} faces18. According to the DFT study by Tian et al., a Pd{111} face is more active than a Pd{100} face in terms of hydrogen peroxide selectivity 17a. In our previous study, we had reached the same conclusion as that of Tian et al. via an experimental route: Pd octahedrons (enclosed by {111} facets) were higher than Pd cubes (enclosed by {100} facets) nanocrystals in hydrogen peroxide selectivity and productivity 6a. It was also proved experimentally that the activities of Pd nanocrystals are dependent on their dimensions, which was because the number ratio of energetic sites to less-energetic sites changes directly according to the dimension of nanocrystals 5, 6b, 6c

. However, the use of nanocrystals suspended in a (aqueous) slurry state is not desirable in a

standpoint of catalyst recovery. Thus, it was tried to immobilize the Pd nanocrystals on silica with impregnation method, but the detachment of Pd nanoparticles from support was unavoidable, which was estimated to be 25-70 % on a weight basis 6a, 6b. We then tried to immobilize the Pd crystals over


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silica in a core- and yolk-shell structure 2a, 2b, 6c. The attempts were successful, which resulted in high yields of hydrogen peroxide without loss of Pd crystals from catalyst structure. The core-shell catalysts was very effective in suppressing the thermal agglomeration of Pd nanoparticles (core), which might occur during activation of Pd metal via high-temperature calcination and/or reduction procedures 2c. However, a problem for the core- and yolk-shell structured Pd@SiO2 catalysts is the difficulty in the control of the thickness and pore size of silica shell, which are necessary for securing a desired level of mass transfer efficiency. In this study, it was tried to overcome this problem by applying a different structure of Pd nanocrystal-immobilized catalyst, in which a Pd nanocrystals-grafted SiO2 nano-bead was encapsulated by porous silica shell layer 19. In this structure, the immobilization of Pd crystals was as efficient as in the previously studied, core- and yolk-shell structured catalysts, but it was possible to control the thickness and pore size of exterior shell which provides the pore paths for molecular delivery to or from core-Pd metal. The structure of catalyst was verified by applying X-ray and electron-beam based characterization techniques and BET analysis. The change of hydrogen peroxide production rate was monitored with changing the thickness of exterior silica shell layer, by which it was tried to reveal how the activity was influenced by pore-diffusion resistance.


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2. Experimental The procedure for catalyst synthesis consisted of five stages as shown in Scheme 2: (1) the synthesis of Pd nanocrystals; (2) the synthesis of SiO2 nano-beads; (3) surface modification of SiO2 nano-beads with NH2 functional groups (-NH2-terminated silica nano-spheres); (4) the grafting of Pd nanocrystals onto SiO2 nano-beads via coordination to terminal amide groups (Pd nanocrystal-grafted SiO2, Pd/SiO2) ; (5) the encapsulation of Pd/SiO2 nanoparticles with porous SiO2 shells.

Scheme 2. Synthesis of nanocatalysts consisting of Pd/SiO2 nanoparticles coated with a mesoporous silica shell.

2.1. Synthesis of Pd nanocrystals The Pd nanocrystals were synthesized following the procedure described in our previous publications

5, 6b

.

It was aimed to prepare the crystals in average size of 4.5 nm, which was the

smallest we could achieve in the previous study. The preparation procedure was as follows: A 16 mL aqueous solution containing 0.212g of polyvinylpyrrolidone (PVP, 55,000 g/mol, Sigma-Aldrich), 0.12g of L-ascorbic acid (ACS reagent,

99%, Sigma-Aldrich), 0.003g of KBr (ACS reagent,

Sigma-Aldrich), and 0.097g of KCl (ACS reagent,

99%,

99%, Sigma-Aldrich) was heated to 353 K with


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stirring. Upon reaching 353 K, 6 mL of 64 mM Na2PdCl4 aqueous solution (98%, 35.3–37.1% Pd basis, Sigma-Aldrich) was added to the flask. The mixture was stirred for 3 hours to achieve the Pd nanocrystals. Acetone was subsequently added to the slurry and the nanocrystals were recovered (33 mg) using a centrifuge. The recovered nanocrystals were washed with water several times and dispersed in 40 mL of ethanol (ASC reagent, 99.5%, absolute, Sigma-Aldrich).

2.2. Synthesis of SiO2 nano-beads We synthesized silica nano-beads and -NH2-terminated silica nano-beads according to the method reported by Bian et al. 20. A 6 mL solution of tetraethyl orthosilicate (TEOS, ACS reagent, 99%, Sigma-Aldrich) was added into a solution containing 74 mL of ethanol (ASC reagent, 99.5%, absolute, Sigma-Aldrich), 10 mL of deionized (DI) water, and 3.15 mL of NH4OH (28–33% NH3 basis, Sigma-Aldrich). The mixture was stirred for 12 hours and the SiO2 nano-beads were recovered (1.5 gram) using a centrifuge. The nano-beads were washed repeatedly with DI water and propanol (anhydrous 2-propanol, 99.5%, Sigma-Aldrich).

2.3. Synthesis of -NH2-terminated SiO2 nano-beads The 1.5 gram of synthesized SiO2 nano-beads were dispersed in 320 mL of propanol, and the slurry was heated to 353 K. Upon reaching 353 K, 2.65 mL of (3-aminopropyl)triethoxysilane (APTS, 98%, Sigma-Aldrich) was added and the slurry was stirred for 2 hours with maintaining the temperature. The -NH2-terminated silica nano-beads were then recovered (1.47 gram) using a centrifuge and washed several times with ethanol.

2.4. Synthesis of Pd nanocrystal-grafted SiO2 nano-beads (Pd/SiO2) The 1.47 gram of synthesized -NH2-terminated silica nano-beads were dispersed in 160 mL of ethanol, to which the 33 mg of synthesized Pd nanocrystals were added. The slurry was stirred for


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30 minutes, and then sonicated for 30 minutes. The resulted solids were recovered (ca. 1.5 gram) using a centrifuge to afford the synthesized Pd/SiO2 nanoparticles.

2.5. Encapsulation of Pd nanocrystal-grafted SiO2 nano-beads with porous silica shell The 1.45 gram of Pd/SiO2 nano-beads were first dispersed in 160 ml of ethanol, followed by addition of 1.1 g of hexadecyltrimethylammonium bromide (CTAB,

98%, Sigma-Aldrich), 5.76 ml

of DI water, and 2.5 ml of NH4OH. The slurry was then stirred for an hour. A pre-determined quantity of TEOS (1.2, 2.5, 4.5, or 6.5 ml) was added to an equal volume of the original slurry, and the mixture was stirred for 24 hours. It has to be mentioned that the difference in TEOS amount used for shell layer formation resulted in the different Pd wt.% for each of catalyst (Table 1). The resulted solids were recovered (ca. 2 gram) using a centrifuge and then washed with ethanol. The solids were then dried at 333 K for 24 hours and calcined in air at 773 K for 10 hours. Finally, the catalysts were activated via reduction at 623 K for 1 hour under the flow of 50 ml/min of diluted hydrogen gas (10 vol% H2/N2). The final catalysts were designated as M(1), M(2), M(3), and M(4) according to the order in the quantity of TEOS used for shell-layer formation. Some catalysts were prepared without addition of CTAB, and those catalysts were named as S(1) and S(2) according to the quantity of TEOS (1.2 and 2.5 ml, respectively) used for shell-layer formation.

2.6. Catalyst characterization The transmission electron microscopy (TEM) study was performed using a Tecnai G2 F30 transmission electron microscope (FEI Company, USA) operating at 300 kV. The analysis was performed at the Korea Basic Science Institute (KBSI), Seoul. The average dimension of a sample was determined by measuring the 100 entities captured on the TEM images. The particle-size distributions of various samples were measured by dynamic light scatting (DLS) method using ELS-2000ZS (Otsuka electronics, Japan). The zeta potential of a sample was measured using the same apparatus. The measurements were performed with the samples dispersed in


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deionized water, which was obtained after 1 hour of sonication. The pH of slurry was adjusted with aqueous solutions of hydrocrhloric acid and potassium hydroxide. The pore characteristics of a catalyst were analyzed via N2 adsorption-desorption method using a BELSORP-max instrument (BEL Japan Inc., Japan). The specific surface area of catalysts was estimated by BET (Brunauer–Emmett–Teller) method. The exposed area of Pd crystal was determined via the CO chemisorption method using ASAP 2020 chemisorption analyzer (Micrometrics Inc., Norcross, USA). The content of Pd in a catalyst was determined via inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a Polyscan 61E spectrometer (Thermo Scientific, Waltham, USA). The X-ray diffraction (XRD) patterns of a sample were obtained using the ATX-G X-ray powder diffractometer (Rigaku) using Cu Kα (λ = 1.5406 Å) irradiation. The data were collected in the 2θ range of 10° to 80° with a scanning speed of 1°/min.

2.7. Activity test: direct synthesis of hydrogen peroxide The activity of a catalyst was measured using a double jacket glass reactor, in which catalysts were suspended in a 150 mL of ethanol/water mixture (4/1, v/v) containing potassium bromide (KBr, 0.4 mM) and phosphoric acid (H3PO4, 0.03 M). The reaction tests were performed under atmospheric pressure and 293 K, with the reactor stirred at 1200 rpm under the continuous supply of hydrogen and oxygen (22 mL/min, H2/O2 ratio = 1/10). The catalyst was added into the reactor and the reaction was carried out for 3 hours. The amount of added catalyst was basically 0.1 g. Otherwise, it was changed to adjust the total exposed Pd area of catalyst to 0.02, 0.04 or 0.065 m2, for which the CO chemisorption data (m2 Pd/g-catalyst) were utilized. On the other hand, the KBr concentration is important in measuring catalytic activity of Pdbased catalysts in direct hydrogen peroxide

2c, 6c, 8

. The given KBr concentration (0.4 mM) was

determined from a set of optimization tests, which results are provided in supporting information.

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Hydrogen concentration in a gas flow was measured using gas chromatography (Younglin, ACME6000; the gas chromatograph was equipped with a Carbosieve SΙΙ-packed column), while hydrogen peroxide concentration was measured via iodometric titration method. Hydrogen conversion, hydrogen peroxide selectivity and hydrogen peroxide production rate were calculated by equation (1), (2) and (3), respectively. Hydrogen conversion (%) =

(1)

Hydrogen peroxide selectivity (%) = Hydrogen peroxide production rate (mmol/g-Pd·h)


(2) (3)

3. Results and Discussion 3.1. Characterization results

The TEM images of Pd nanocrystals were presented in Figure 1(a), in which the average size of the crystals was estimated to be 4.7 nm. Figure 1(b) shows a TEM image of the synthesized silica nanobeads. The samples were close to spheres and 195 nm in average size. The TEM images in Figure 1 (c) and (d) were for Pd-nanocrystal-grafted SiO2 nano-beads, which show that the Pd crystals were well dispersed over silica surface.

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size estimated from TEM images (195 nm). The average hydrodynamic diameter was increased to 400 nm with the amine functionalization of silica nano-beads, which was due to the enhanced ethanol solvation via hydrogen bond formation of –NH2 terminals. The profile of distribution was almost identical between before and after amine functionalization, which implies a homogeneous –NH2 functionalization over nano-bead surfaces. The zeta potential data in Figure 3 show that the isoelectric point of silica nano-beads was greatly increased from 2.3 to 7.6 after amine functionalization. Finally, with the grafting of Pd crystals on the –NH2 functionalized silica, the hydrodynamic diameter distribution becomes broader and more asymmetrical with the increase of the average diameter to 474 nm.

2.5

-2

Normalized Intensity (x 10 )

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SiO2 2.0

(RD,avg = 211 nm)

SiO2-NH2 (RD,avg = 400 nm)

1.5

Pd/SiO2nanoparticles

1.0

(RD, avg = 474 nm)

0.5

0.0

200

400 600 Hydrodynamic diameter (RD, nm)

800

Figure 2. Hydrodynamic diameter distributions of SiO2 nano-beads, -NH2-terminated SiO2 (SiO2NH2) and Pd-grafted-SiO2 nanobeads. (DLS analysis results, RD, avg = weighted average of hydrodynamic diameter)


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SiO2 -NH2

80 60

Zeta potential (mV)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40 Isoelectric point 20

SiO2

0 -20 -40

Isoelectric point

-60

2

3

4

5

6

7

8

9

10

pH Figure 3. Zeta potential curves of SiO2 nano-beads (SiO2) and NH2-terminated SiO2 (SiO2-NH2) The TEM images of the final catalysts, Pd-grafted SiO2 nano-beads which are encapsulated by porous silica shells, are presented in Figure 4. M(1), M(2), M(3), and M(4) are the samples which we intended to induce mesoporous textures over silica shell via thermal dissociation of CTAB surfactant during shell formation, while S(1) and S(2) are the samples synthesized without addition of CTAB surfactant (hence, the formation of microporous shell layer was expected.). The TEM images show that silica shells were formed in uniform thickness, except S(1) and M(1), for both of which the smallest volume (1.2 ml) of TEOS was used. As shown in the images of M(2), M(3), and M(4), the shell thickness increases from 12 to 31 nm depending on the amount of TEOS amount (from 2.5 to 6.5 ml). S(2) and M(2) were similar in shell thickness (~ 12 nm), which implies the addition of CTAB did not influence on shell thickness. The shell layers of S(1) and M(1) were noticeably different to those of other samples: The shell layer of S(1) was thin, rugged and uneven, and judging from the contrast of its image, the layer seems to be less dense than those of other samples. In case of M(1), the shell formation was negligible, without being able to call it a core-shell structure. Consequently, M(1) failed in protection of Pd crystals from thermal agglomeration: The Pd crystals of M(1) were comparatively large and uneven in size, implying the Pd crystals were agglomerated during


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calcination. On the contrary, it was found that the sizes of the Pd crystals in core-shell structures (M(2), M(3), M(4), and in a less strict sense, S(1)) are relatively changed little after calcination.

Figure 4. TEM images of (a) S(1), (b) S(2), (c) M(1), (d) M(2), (e) M(3) and (f) M(4) (*: the shell pore diameters were the averages of BJH data in Figure 6(b))


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Figure 5 shows the XRD analysis results of the S and M catalysts. All the samples were similar in diffraction pattern, showing a single broad peak of SiO2 and a couple of peaks assigned to Pd metal. Palladium oxide peaks were not observed for any of the catalysts. The Pd metal peaks were comparatively intense for M(1), which might be attributed to the relatively large mass fraction of Pd (1.87 wt.%, Table 1) and not to mention, to the large size of agglomerated Pd crystals. The latter can be verified by the fact that the Pd peaks of M(1) were narrower than those of S(1) which was a bit higher in mass fraction of Pd (1.93 wt.%, Talbe 1).

SiO

2

Pd(111)

M(4)

Intensity (A.U.)

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M(3) M(2) Pd(200)

M(1) S(2) S(1)

10

20

30

40

50

60

70

80

2 theta (degree) Figure 5. XRD results of S and M catalysts

Table 1. Pd contents, exposed Pd area, shell thickness and BET surface area of catalysts Catalyst

Mass fraction of Pd (wt.%) a

Exposed Pd area b 2

m /g-cat

2

m /g-Pd

BET area (m2/g-cat)

1.93 0.57 29.6 30.7 S(1) 1.63 0.60 37.0 93.4 S(2) 1.87 0.57 30.3 131.7 M(1) 1.50 0.65 43.0 221.6 M(2) 0.85 0.36 42.3 164.1 M(3) 0.81 0.35 43.0 192.1 M(4) a Mass fractions of Pd were calculated from the ICP-AES data. b Exposed Pd areas and Pd dispersion were calculated based on the CO chemisorption results.


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The exposed Pd surface areas of the samples were measured by CO chemisorption method and the results are listed with gram-catalyst- and gram-Pd-specific values in Table 1. Because the measurement was based on a chemisorption amount of CO in equilibrium, the resulted data might not be influenced by the kinetic effects such as pore-diffusion resistance. The gram-Pd surface Pd areas of M(2), M(3), and M(4) were similar to one another (~ 43 m2/g-Pd), but that of M(1) was comparatively smaller (30.3 m2/g-Pd), which might be related to the agglomeration of Pd crystals in M(1). Meanwhile, the g-Pd specific Pd areas of S(1) and S(2) samples are smaller (29.6 and 37.0 m2/g-Pd) than those of M(2), M(3), and M(4). It cannot be attributed to thermal agglomeration of Pd, because a clear evidence was not found in the TEM images of S(1) and S(2). It was supposed that the relatively low gram-Pd specific Pd areas of S catalysts are associated with their low BET areas. As shown in the last column of Table 1, S catalysts were very smaller than M catalysts in BET area, which might be because the S catalysts were prepared without addition of CTAB molecules. Because of relatively low porosity, S catalysts would be higher in the portion of Pd area screened by silica shell than M catalysts, which then necessarily influences on the values of exposed Pd areas. Figure 6(a) presents the nitrogen absorption-desorption isotherms of prepared catalysts. It was observed that the adsorption branches of the isotherms for S(1) and S(2) samples increased sharply around the zero relative pressure mark, and then remained mostly unchanged until the relative pressure registered a reading of 0.9. Such a pattern is attributed to the development of a microporous texture. In the range of relative pressure between 0.1 and 0.9, the isotherms were flat, implying the absence of mesopores. The Barrett-Joyner-Halenda (BJH) analysis results of S(1) and S(2) samples in Figure 6 (b) do not show any appreciable sign of volume accumulation in the mesoporous range (2~50 nm) but show the accumulating signs, albeit slightly, in the micoporous range (< 2nm), which supports the fact the samples have microporous textures only.


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Figure 6. (a) N2 adsorption-desorption isotherms of catalysts; (b) pore size distribution of catalysts (BJH analysis) On the contrary, the nitrogen adsorption-desorption isotherms of M(1), M(2), M(3), and M(4) samples (Figure 6 (a)) showed in common a sharp increase of adsorption around zero pressure, followed by a gradual increase of adsorption in the pressure range of 0.1~0.5, which implies the M catalysts had mesoporous textures plus microporous ones. The BJH analysis results of the M catalysts (Figure 6 (b)) showed volume increment signs in the range of diameter 1~5 nm, which matches well with the conclusion previously made based on the nitrogen absorption-desorption isotherms. The last thing which needs to be mentioned is the BET area of M(2), which is exceptionally high compared to those of other M catalysts (Table 1): Except M(2), the M catalysts showed a trend of increasing BET area along with the increase in shell thickness. The high BET area of M(2) may be associated with the relatively high proportion of PVP molecules contained in the shell layer before calcination: The Pd crystals were initially suspended in a slurry state via interaction with surfactant (PVP) molecules. When preparing Pd-grafted SiO2 nano-beads, the Pd crystals were dragged in a state adhering to PVP molecules to the surface of SiO2 nano-beads. Then, the Pd/PVP-grafted nano-beads were encapsulated by silica shell. Thus, the mass fraction of PVP molecules enclosed in the shell layer is proportional to the mass fraction of Pd in the catalyst. As shown in the second column of Table 1, Pd wt.% of M(2) was about twice as that of M(3), which means M(2) was higher as such than M(3) in the mass fraction of enclosed PVP. At the stage of calcination, the PVP molecules (and CTAB) were degraded with leaving porous textures in the shell layer. Then, M(2) became higher than


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ACS Catalysis

M(3) in the PVP-induced pore areas, which must have reflected in the BET area results. On the other hand, M(2) was also higher than M(1), though the former was lower in Pd wt.% (thus, PVP amount) than the latter. It might be due to the negligible thickness of shell-layer in M(1), for which BET area could not build up as did in M(2) or other M catalysts.

3.2. Activity test results in direct hydrogen peroxide synthesis Figures 7 presents the results of activity test using a fixed amount (0.1 g) of catalysts. Pd-grafted SiO2 nano-beads with no silica shell were tested in direct synthesis of H2O2 after calcination and reduction, but it showed very low H2 conversion (6.6%) and H2O2 yield (6.2%), compared to those of M(1), M(2), and S(1). It might be due to the absence of shell structure which protects Pd from thermal agglomeration during calcination and reduction. When comparing S(2) and M(2) catalysts which were similar in g-catalyst-specific exposed Pd area (0.60 and 0.65 m2/g-cat, Table 1) and shell thickness (ca. 12 nm, Figure 4), M(2) was much higher than S(2) in hydrogen conversion. The noticeable difference of two catalysts in hydrogen conversion could be explained by their difference in shell layer porosity: The reaction should initiate with adsorption of H2 and/or O2 on the surface of Pd metal. Under the given reaction conditions (293 K and 1 atm), the aqueous solubility of H2 and O2 are relatively low, for which a pore-diffusion resistance would become more pronounced compared to the reactions progressed under high temperature and/or pressure conditions. As a result, if the compared catalysts are similar in total exposed Pd area and shell thickness, hydrogen conversion could be determined according to the average pore size of a catalyst, as revealed in the difference of hydrogen conversion between mesoporous M(2) and microporous S(2) catalysts.


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Figure 7. Activity test results using a fixed amount (0.1 g) of S and M catalysts: (a) H2 conversion and H2O2 selectivity; (b) H2O2 production rate (Pd-area-specific H2O2 production rate). Test conditions: 293 K, 1 atm, 0.1 g catalyst, 150 mL of ethanol/water (4:1, v/v) mixture (containing 0.4 mM KBr and 0.03 M H3PO4), stirring rate = 1200 rpm, total gas flow rate = 22 mL/min, H2/O2 = 1:10 (v/v) For mesoporous M catalysts, the hydrogen conversion decreased rapidly from 32% to 5% as the applied catalyst was changed from M(2) to M(4) (Figure 7(a)). The H2O2 yield (Figure 7(a)) and Pd-area-specific H2O2 production rate (Figure 7(b)) decreased similarly according to the change of catalyst. It is supposed that the rapid decrease of hydrogen conversion might be associated not only with the decrease in gram-catalyst-specific Pd area (0.65 to 0.35 m2/g-cat, Table 1), but also with the increase in shell-layer thickness (i.e., the increase in pore-diffusion resistance). In Figure 8(a), the activities of M catalysts are presented with respect to total exposed Pd area, which was adjusted by changing the amount of each catalyst on the basis of m2/g-cat data in Table 2. With this figure, it could be explained more clearly about the influence of shell-layer thickness (pore diffusion resistance) on catalytic activity. The figure revealed two facts distinctively: (1) if the exposed Pd areas are identical, H2 conversion and H2O2 yield decreased with the increase of shell-layer thickness. (2) For any catalyst, H2 conversion and H2O2 yield increased with the increase of exposed Pd area. However, as the exposed area of Pd increases, the hydrogen peroxide yield of the thick shell-layer catalysts (M(3) and M(4)) increases more gradually than that of the thin shell-layer catalysts (M(1) and M(2)). As shown in Figure 8(b), Pd-area-specific H2O2 production rate of the thin shell-layer catalysts (M(1), M(2)) were nearly constant (~26 mmol/m2Pd h) regardless of total exposed Pd area, while the thick shell-layer catalysts (M(3), M(4)) showed a decreased rate of H2O2


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ACS Catalysis

production with respect to total exposed Pd area. It implies the Pd areas in M(3) and M(4) were not fully utilized for H2O2 production and it might be due to comparatively high degrees of pore-diffusion resistance working on those catalysts. Therefore, the shell-layer thickness became an important factor for the activity of core-shell catalysts. In terms of activity, the thinnest shell-layer catalyst, M(1) is the most desirable to use. However, the shell layer of M(1) catalyst was proved not effective in protecting core-metals from thermal agglomeration (Figure 4(c)). Considering Pd-area-specific activity, porediffusion rate and core-metal protection efficiency together, M(2) is thought to have the highest potential use as catalyst among the samples prepared.

Figure 8. Activity of M catalysts with respect to the total exposed Pd surface area: (a) H2 conversion and H2O2 selectivity; (b) H2O2 production rate (Pd-area-specific H2O2 production rate): Test conditions were identical to those of Figure 7, except the amount of catalyst, which are provided in Table 2

Table 2. The amount of catalysts used in the activity tests of Figure 8 Total Exposed Pd area

Amount of Catalyst (g-catalyst)

(m2-Pd)

M(1)

M(2)

M(3)

M(4)

0.02

0.035

0.031

0.056

0.057

0.04 0.065

0.070 0.114

0.062 0.100

0.111 0.180

0.114 0.185

In order to determine the stability and reusability of our catalytic systems, the M(2) catalyst, which exhibited the highest activity, was evaluated in reusability tests. The M(2) catalyst was subjected to three reaction cycles and the hydrogen peroxide production rate at each cycle is shown in


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Figure 9. The experimental results show that the hydrogen peroxide production rate of the M(2) catalyst was maintained throughout the experiments. After the reaction, the structure and composition of the M(2) catalyst were analyzed by TEM (Figure 9b) and ICP (Table 3). The results showed that the structure of the M(2) catalyst was maintained and that there was no Pd leaching (the overall Pd loss was

Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen

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