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Effect of shell thickness of Pd core-porous SiO2 shell catalysts on direct synthesis of H2O2 from H2 and O2
Accepted Manuscript Title: Effect of shell thickness of Pd core-porous SiO2 shell catalysts on direct synthesis of H2 O2 from H2 and O2 Author: Myung-gi Seo Ho Joong Kim Sang Soo Han Kwan-Young Lee PII: DOI: Reference:
S1381-1169(16)30497-6 http://dx.doi.org/doi:10.1016/j.molcata.2016.11.021 MOLCAA 10118
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
25-10-2016 8-11-2016 14-11-2016
Please cite this article as: Myung-gi Seo, Ho Joong Kim, Sang Soo Han, KwanYoung Lee, Effect of shell thickness of Pd core-porous SiO2 shell catalysts on direct synthesis of H2O2 from H2 and O2, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.11.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Manuscript for Journal of Molecular Catalysis A: Chemical
Effect of shell thickness of Pd core-porous SiO2 shell catalysts on direct synthesis of H2O2 from H2 and O2
Myung-gi Seo a, Ho Joong Kim a, Sang Soo Han b, Kwan-Young Lee a, c, *
a
Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seoul 02841,
Republic of Korea b
Computational Science Research Center, Korea Institute of Science and Technology (KIST), Hwarangno
14-gil 5, Seoul 02792, Republic of Korea c
Green School, Korea University, 145 Anam-ro, Seoul 02841, Republic of Korea
*Corresponding author. Tel: +82-2-3290-3299; fax: +82-2-926-6102. E-mail address: [email protected]
1
Graphical abstract
Highlights Pd@SiO2 core-shell catalyst was tested in direct hydrogen peroxide synthesis. Mass transfer resistance increased with increasing shell thickness. H2O2 production rate was correlated closely with the Pd surface area. H2O2 formation was influenced by Pd surface area and shell thickness.
Abstract In our previous study, we applied Pd@SiO2 core-shell catalysts to hydrogen peroxide synthesis and obtained a higher yield of hydrogen peroxide than that obtained with the use of general supported-catalysts (Pd/SiO2). As an extension of the previous study on Pd@SiO2 catalysts, the effects of the core-shell thickness on the hydrogen peroxide synthesis reaction were examined in this study. A shell below a certain thickness in the core-shell structure of the Pd nanocatalyst results in a decrease in the catalytic activity. Overall, a volcano curve is observed for the hydrogen peroxide production rate as a function of the shell thickness. Through N22
adsorption and desorption, TEM, CO-chemisroption, and XRD analyses, we identify the causes for the improved direct hydrogen peroxide synthesis yields and later optimize the shell thickness for the efficient utilization of Pd.
Keywords: Palladium catalyst, Core–shell structured catalyst, Palladium nanoparticle, Direct hydrogen peroxide synthesis
1. Introduction
Hydrogen peroxide is used in various applications, incuding as an eco-friendly oxidant, bleaching agent, rocket fuel, and oxidant and cleaning agent for the manufacture of semiconductors [1-4]. The so-called Anthraquinone Process is a commercial process that produces hydrogen peroxide using the redox reaction of ethyl antraquinine. The use of toxic organic solvents (octanol, naphthalene) and the need for separation contribute to the high operating cost of this process [5]. To replace this commercial process, many studies are being actively conducted on the synthesis of hydrogen peroxide by directly reacting hydrogen with oxygen [6-8]. Direct synthesis of hydrogen peroxide was first proposed by Henkel and Weber in 1914, which would allow for the chemical synthesis (e.g., propylene oxide) process to occur on-site without the use of hazardous organic materials [9, 10]. Direct synthesis of hydrogen peroxide consists of the following four reactions: (reaction 1) (reaction 2)
3
(reaction 3) (reaction 4) The hydrogen peroxide formation (reaction 1) is accompanied by side reactions that generate water (reactions (2), (3), and (4)). These undesirable, voluntary side reactions significantly reduce the hydrogen peroxide yield, and because of this, direct synthesis of hydrogen peroxide has notreached a level of commercialization [6, 11]. Acids and/or halides have been added to the reaction solvent to mitigate the low yield issue [12-17]. Because expensive precious metals such as Pd can increase the hydrogen peroxide yield, many studies are geared toward efficient utilization of Pd [15, 18-21]. Pd can increase the selectivity for hydrogen peroxide when a halide is added to the solvent. In particular, the selectivity toward hydrogen peroxide is substantially increased when Br- ions are added compared to when other halides (F-, Cl- and I-) are added [12, 14, 16, 21, 22]. Active sites on the Pd surface are classified into energetic sites with more dangling bonds (corner/edge) and less energetic sites with fewer dangling bonds (terrace), which feature high selectivity for the formation of water and hydrogen peroxide, respectively [12, 16]. When Br- is added, it is preferentially adsorbed at and occupies the more energetic sites [23], thereby increasing hydrogen peroxide selectivity. Therefore, when a small amount of Br- is added to the solvent, the selectivity can be substantially increased [6, 15, 24-26]. However, an excessive amount of Brwould lead to adsorption also occurring at the favored sites and reduce the overall hydrogen conversion [25]. Thus, it is important to carefully adjust the amount of Br- added to efficiently utilize the active Pd sites. Recently, there have been many studies on using various nanoparticles and nanostructures as catalysts [27-32]. In particular, some reactions have shown that catalytic
4
activities are significantly affected by the structure of the nano-catalysts [19, 33]. In our previous study, we controlled the fraction of energetic sites/less energetic sites by changing the size of the Pd nanoparticles and showed that hydrogen peroxide selectivity can indeed be enhanced [19, 24]. The effects of the Pd crystal facets on hydrogen peroxide selectivity were also examined by using cubic Pd nanoparticles surrounded by {100} facets and octahedral Pd nanoparticles surrounded by {111} facets [18]. Pd {111} facets exhibited a higher selectivity toward hydrogen peroxide than Pd {100} facets because undissociative chemisorption predominantly occurred when O2 was adsorbed on the Pd surface; this result was consistent with the DFT simulation [34]. In addition, a core-shell catalyst was introduced to the hydrogen peroxide synthesis reaction and showed higher hydrogen conversion and hydrogen peroxide selectivity than general supported catalysts [20, 35, 36]. The superior hydrogen conversion of the core-shell structure is ascribed to a higher Pd dispersion because the shell physically blocks sintering of encapsulated Pd during calcination at high temperature [35, 37]. In this study, we attempted to find the optimal shell thickness for efficient use of precious metal Pd by adjusting the shell thickness of the nanocatalyst within the core-shell structure.
2. Experimental 2.1. Materials L-ascorbic acid (Sigma-Aldrich, ACS reagent, > 99%), polyvinylpyrrolidone (SigmaAldrich, PVP, 55,000 g/mol), potassium bromide (Sigma-Aldrich, KBr, ACS reagent, > 99%), and sodium tetrachloropalladate (Aldrich, Na2PdCl4, 98%) were used to synthesize the Pd nanoparticles. Ethanol (Sigma-Aldrich, ACS reagent, > 99.5%, absolute), ammonium hydroxide solution (Sigma-Aldrich, 28 - 33% NH3 basis), and tetraethylorthosilicate (Sigma-Aldrich, TEOS,
5
ACS reagent, 98%) were used to synthesize the core-shell catalysts. All of the materials were used without purification.
2.2 Synthesis of Pd@SiO2 Synthesis of Pd@SiO2 catalyst could be divided into two stages as shown in Scheme 1: (1) Synthesis of Pd nanoparticles; and (2) Encapsulation of the nanoparticles with SiO2. Preparation of the Pd@SiO2 nanoparticles is described in detail in our previous papers [20, 35, 36]. First, Pd nanoparticles were synthesized, and the shells were formed using the Stӧber method. The Pd nanoparticles were synthesized by reducing Na2PdCl4 precursors with Lascorbic acid in a solution containing KBr and PVP. After nanoparticle recovery via centrifugation and washing with water, the nanoparticles were dispersed in ethanol. After adding NH4OH and water to the ethanol solution with the dispersed Pd nanoparticles, the solution was stirred at room temperature for 3 hours. Next, 5, 10, 15, 20, 25 or 30 ml of TEOS was added. The solution was stirred for 24 hours and the nanoparticles were collected with a centrifugal separator. After washing with a water-ethanol mixture several times, they were dried at 60 °C. The prepared core-shell catalyst was then calcined at 500 °C for 6 hours and reduced by H2 gas (10 vol% N2 balance, 50 ml/min) at 350 °C for 2 hours. Based on the volume of TEOS added, we named the catalysts Pd@S(5), Pd@S(10), Pd@S(15), Pd@S(20), Pd@S(25) and Pd@S(30).
2.3 Catalyst characterization Transmission electron microscopy (TEM) analysis was performed using a Tecnai G2 F30 instrument (FEI Company, USA). The catalyst was dispersed in ethanol and then dropped over a
6
Cu grid, which was dried prior to analysis. TEM analysis was performed at the KBSI (Seoul center of the Korea Basic Science Institute). The crystalline structures were analyzed by X-ray diffraction (XRD: D/MAX-2500/PC, Rigaku), and the XRD patterns were measured at a scanning speed of 1°/min using Cu Kα1 irradiation (λ = 1.5406 Å). N2 adsorption-desorption isotherms were obtained using a BELSORP-max instrument (BEL Japan Inc., Osaka, Japan) at -196 °C. Specific surface area and total pore volume of the catalysts were calculated by applying the Brunauer-Emmett-Teller (BET) theory. The microporous surface area and micropore volume were estimated by subtracting t-plot data from the BET data. The mass fractions of Pd in the catalysts were estimated by inductively coupled plasma optical emission spectrometry (ICP-OES) using a JY Ultima2C (Jobin Yvon, France). Before each measurement, each catalyst was dissolved in a liquid mixture (nitric acid, hydrochloric acid and hydrofluoric acid) at 200 °C. The Pd surface area of the catalysts was determined using the CO chemisorption method. Measurements were performed on an ASAP 2020 chemisorption analyzer (Micrometrics Inc., USA) at 35 °C. The N2 adsorption-desorption, TEM, XRD, ICP-OES and CO-chemisorption experiments were performed on the reduced catalysts.
2.4 Hydrogen peroxide synthesis The catalysts were tested in a double-jacketed glass reactor for direct hydrogen peroxide synthesis. The catalytic reactions were conducted at 20 °C and 1 atm for 3 hours. Both a 150 ml ethanol-water mixture (containing potassium bromide (0.15 mM) and phosphoric acid (0.03 M),
7
20 vol% ethanol) were fed into the reactor. We adjusted the amount of the catalyst such that a total of 1 mg of Pd would be added to the reaction. The gas feed ratio of H2/O2 was 1:10, and the total flow rate was 22 ml/min. The hydrogen peroxide content was measured by iodometric titration. H2 conversion, H2O2 selectivity and the H2O2 production rate were calculated by equations (1), (2) and (3), respectively. H2 conversion (%) =
(1)
H2O2 selectivity (%) =
(2)
H2O2 production rate (mmol/g-Pd·h)
(3)
3. Results and Discussion 3.1 Characterization of the Pd nanoparticles and core-shell catalysts Figure 1(a) shows a TEM image of the synthesized Pd nanoparticles. A total of 100 measurements taken from different regions of the sample, which showed uniformly-sized nanoparticles with an average size of 9.7 nm (see Figure 1(b)). Figure 2 shows the TEM images of the Pd@SiO2 catalysts synthesized with different amounts of TEOS. When a small amount of TEOS was added (Pd@S(5), Pd@S(10)), the proportion of the core-shell nanoparticles containing multiple (up to 4-5) Pd nanoparticles within a single shell was higher. The estimates for the average shell thickness are listed in Table 1. The values were obtained by measuring the shell thickness of 100 nanoparticles per catalyst. We confirmed that the shell of the Pd@SiO2 nanocatalyst became thicker as more TEOS was added.
8
The XRD analysis results are shown in Figure 3. In the XRD analysis, a Pd metal peak was observed, but no PdO peak was observed. All three Pd peaks detected were more pronounced in samples with thinner shell thicknesses, which is in agreement with the relative metal loading listed in Table 1.
Figure 4 shows the N2-adsorption and desorption isotherms of the core-shell catalysts. The corresponding micro surface areas and micropore volumes determined from the t-plot results are listed in Table 2. N2-adsorption and desorption measurements revealed that the shell thicknesses are negatively correlated with the pore volume and the inner surface area. For most of the catalysts (Pd@S(5) - Pd@S(20)), micropores developed in the shell and resulted in a large specific surface area. However, when the shell was thick (Pd@S(25), Pd@S(30)), pores did not develop in the shell during the calcination process, and the specific surface area was sharply reduced.
The exposed Pd surface areas from the CO-chemisorption measurements are listed in Table 1. A volcano curve was obtained when plotting the exposed Pd specific surface area (m2/gPd) as a function of the shell thickness. As seen from Figure 2, catalysts with thinner shells (Pd@S(5) and Pd@S(10)) are characterized by a higher proportion of the individual shell structures encapsulating multiple (4-5) Pd nanoparticles. This ‘bundling’ reduced the proportion of surface area which is exposed because the adjacent Pd nanoparticles are in contact with one another. Thus, the exposed Pd specific surface area (m2/g-Pd) increased with increasing shell thickness, as more Pd nanoparticles were individually encapsulated by a single shell.
9
3.2 Reaction results of the hydrogen peroxide synthesis The results of applying core-shell nanocatalysts to the hydrogen peroxide synthesis reaction are shown in Figure 5. Figure 5 (a) and Figure 5 (b) show the hydrogen conversion rate and hydrogen peroxide yield, and the hydrogen peroxide production rate, respectively, as a function of the SiO2 thickness. A volcano curve as a function of the shell thickness was observed for all three and is similar to the curve for the Pd exposed surface area as a function of the shell thickness. The largest exposed surface area was obtained for Pd@S(20), but the Pd@S(15) catalyst showed a higher hydrogen conversion rate. This discrepancy is ascribed to the increased mass transfer resistance, which would affect the overall reaction rate. The change in the mass transfer resistance with varying shell thickness was examined. The number of moles of reacted hydrogen was divided by time and the Pd surface area in the reaction medium as shown in Figure 6. Hydrogen conversion rate (reacted H2 mmoles divided by time and Pd area) decreased as the shell thickness increased, and a notable, sharp decrease and flattening out at Pd@(25) and Pd@(30). In the absence of well-developed pore channels that provide passage to the catalyst with much less resistance, the shell thickness would be negatively correlated with the catalytic performance. Although thinner shells would be preferred due to the advantage of smaller mass transfer resistance, shell precursors below a certain limit during shell formation resulted in bundled encapsulation and insufficient Pd exposure area. Therefore, it is suitable to use a catalyst with a moderate shell thickness for hydrogen peroxide synthesis.
4. Conclusions When synthesizing Pd@SiO2 core-shell nanoparticles using the Stöber method, the shell thickness is easily adjusted above a minimum thickness. Adding a small amount of TEOS to
10
synthesize a thin shell (Pd@S(5), Pd@S(10)) confirmed the presence of a higher proportion of core-shell nanoparticles containing multiple Pd nanoparticles within a single shell, thereby reducing the Pd weight-specific exposed surface area of the nanoparticle (m2/g-Pd). In addition, although the exposed Pd surface area was not significantly reduced for thick shells (Pd@S(25) and Pd@S(30)), the specific surface area of the core-shell catalyst sharply decreased due to calcination in formation of micropores. When applying catalysts with thin shells (Pd@S(5) and Pd@S(10)) to direct hydrogen peroxide synthesis reaction, the small exposed Pd surface area (m2/g-Pd) resulted in a reduction of hydrogen conversion and the hydrogen peroxide production rate. However, when the shell was thick (Pd@S(25) and Pd@S(30)), the hydrogen conversion rate sharply decreased due to underdeveloped pores and hence increased mass transfer resistance. Catalysts with intermediate shell thicknesses (Pd@S(15) and Pd@S(20)) achieved high hydrogen conversion and hydrogen peroxide yield owing to the high Pd specific surface areas. The thinner the shell, the better for hydrogen peroxide synthesis, but a minimum thickness is necessary to achieve the maximum Pd specific surface area in the core-shell synthesis process.
Acknowledgments This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF-2016M3D1A1021143). This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MSIP) (2016, University-Institute cooperation program).
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Figure 1. (a) TEM image of Pd nanoparticles and (b) their particle size distribution.
14
Figure 2. TEM images of (a) Pd@S(5), (b) Pd@S(10), (c) Pd@S(15), (d) Pd@S(20), (e) Pd@S(25) and (d) Pd@S(30).
15
Pd(111) Pd(200)
Pd(220)
Intensity (A.U.)
Pd@S(5)
Pd@S(10) Pd@S(15) Pd@S(20) Pd@S(25) Pd@S(30)
10
20
30
40
50
60
70
80
2 theta (degree)
Figure 3. XRD results of Pd@SiO2 catalysts.
1200
Volume adsorbed (ml/g)
1000 Pd@(5) 800 Pd@(10) 600 Pd@(15) 400 Pd@(20) 200
Pd@(25) Pd@(30)
0 0.0
0.2
0.4
0.6
Relative pressure (p/p0) Figure 4. N2 adsorption-desorption isotherms of Pd@SiO2 catalysts.
16
0.8
1.0
25
H2 conversion
H2 conversion & H2O2 yield (%)
(a)
H2O2 yield
20
15
10
5
2 1 0
Pd@S(5)
Pd@S(10) Pd@S(15) Pd@S(20) Pd@S(25) Pd@S(30)
H2O2 production rate (H2O2mmol/g-pd h)
1000
(b) 800
600
400
20
0 Pd@S(5)
Pd@S(10)
Pd@S(15)
Pd@S(20)
Pd@S(25)
Pd@S(30)
Figure 5. Direct synthesis of hydrogen peroxide using Pd@SiO2 catalysts. (a) H2 conversion and H2O2 yield; (b) H2O2 productivity (Pd weight specific H2O2 production rate); Test conditions: 293 K, 1 atm, 1 mg of Pd metal, 150 mL of ethanol/water (4:1, v/v) mixture (containing 0.15 mM KBr and 0.03 M H3PO4), Stirring rate = 1,200 rpm, Total gas flow rate = 22 mL/min, H2/O2 = 1:10 (v/v).
17
2
Hydrogen reactin rate (H2mmol/m Pdh)
35
30
25
20
15
10
5
0
Pd@S(5)
Pd@S(10)
Pd@S(15)
Pd@S(20)
Pd@S(25)
Pd@S(30)
Figure 6. Hydrogen conversion rate of Pd@SiO2 catalysts (Hydrogen conversion rate: reacted H2 moles divided by time and Pd area in the reaction medium)
18
Scheme 1. Pd@SiO2 catalyst synthesis scheme.
19
Table 1. Pd content, Pd surface area and shell thickness of the catalysts.
a
Catalyst
Pd loading (wt.%) a
Pd@S(5) Pd@S(10) Pd@S(15) Pd@S(20) Pd@S(25) Pd@S(30)
3.50 1.78 0.90 0.68 0.54 0.49
Pd
were
loadings
Exposed Pd area b m2/g-cat m2/g-Pd 0.976 27.9 0.654 36.7 0.434 48.2 0.417 61.3 0.281 52.0 0.241 49.2
calculated
from
Shell thickness (nm) c 37.4 42.4 44.3 53.3 57.9 62.8
the
ICP-OES
data.
b
Exposed Pd areas and Pd dispersion were calculated based on the CO chemisorption results.
c
The average shell thickness was determined by measuring 100 core-shell nanocatalysts from the
TEM images.
Table 2. Surface area and pore volume of the catalysts. Catalyst
Surface area (m2/g-cat.)
Pore volume (ml/g-cat.)
Total a
Micropore b (%) c
Total a
Micropore b (%) c
Pd@S(5)
295
242 (82)
0.61
0.50 (82)
Pd@S(10)
272
221 (81)
0.57
0.47 (82)
Pd@S(15)
234
191 (82)
0.49
0.40 (82)
Pd@S(20)
216
174 (80)
0.45
0.37 (82)
Pd@S(25)
55
19 (35)
0.40
0.40 (100)
Pd@S(30)
42
10 (24)
0.40
0.40 (100)
a
The total specific surface area and pore volume were estimated by the BET method. Microporous surface area and micropore volume were estimated by subtracting t-plot data from BET data. c The percentage of the total surface area or total pore volume due to micropores. b
20
S1381-1169(16)30497-6 http://dx.doi.org/doi:10.1016/j.molcata.2016.11.021 MOLCAA 10118
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date: Revised date: Accepted date:
25-10-2016 8-11-2016 14-11-2016
Please cite this article as: Myung-gi Seo, Ho Joong Kim, Sang Soo Han, KwanYoung Lee, Effect of shell thickness of Pd core-porous SiO2 shell catalysts on direct synthesis of H2O2 from H2 and O2, Journal of Molecular Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.11.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Manuscript for Journal of Molecular Catalysis A: Chemical
Effect of shell thickness of Pd core-porous SiO2 shell catalysts on direct synthesis of H2O2 from H2 and O2
Myung-gi Seo a, Ho Joong Kim a, Sang Soo Han b, Kwan-Young Lee a, c, *
a
Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seoul 02841,
Republic of Korea b
Computational Science Research Center, Korea Institute of Science and Technology (KIST), Hwarangno
14-gil 5, Seoul 02792, Republic of Korea c
Green School, Korea University, 145 Anam-ro, Seoul 02841, Republic of Korea
*Corresponding author. Tel: +82-2-3290-3299; fax: +82-2-926-6102. E-mail address: [email protected]
1
Graphical abstract
Highlights Pd@SiO2 core-shell catalyst was tested in direct hydrogen peroxide synthesis. Mass transfer resistance increased with increasing shell thickness. H2O2 production rate was correlated closely with the Pd surface area. H2O2 formation was influenced by Pd surface area and shell thickness.
Abstract In our previous study, we applied Pd@SiO2 core-shell catalysts to hydrogen peroxide synthesis and obtained a higher yield of hydrogen peroxide than that obtained with the use of general supported-catalysts (Pd/SiO2). As an extension of the previous study on Pd@SiO2 catalysts, the effects of the core-shell thickness on the hydrogen peroxide synthesis reaction were examined in this study. A shell below a certain thickness in the core-shell structure of the Pd nanocatalyst results in a decrease in the catalytic activity. Overall, a volcano curve is observed for the hydrogen peroxide production rate as a function of the shell thickness. Through N22
adsorption and desorption, TEM, CO-chemisroption, and XRD analyses, we identify the causes for the improved direct hydrogen peroxide synthesis yields and later optimize the shell thickness for the efficient utilization of Pd.
Keywords: Palladium catalyst, Core–shell structured catalyst, Palladium nanoparticle, Direct hydrogen peroxide synthesis
1. Introduction
Hydrogen peroxide is used in various applications, incuding as an eco-friendly oxidant, bleaching agent, rocket fuel, and oxidant and cleaning agent for the manufacture of semiconductors [1-4]. The so-called Anthraquinone Process is a commercial process that produces hydrogen peroxide using the redox reaction of ethyl antraquinine. The use of toxic organic solvents (octanol, naphthalene) and the need for separation contribute to the high operating cost of this process [5]. To replace this commercial process, many studies are being actively conducted on the synthesis of hydrogen peroxide by directly reacting hydrogen with oxygen [6-8]. Direct synthesis of hydrogen peroxide was first proposed by Henkel and Weber in 1914, which would allow for the chemical synthesis (e.g., propylene oxide) process to occur on-site without the use of hazardous organic materials [9, 10]. Direct synthesis of hydrogen peroxide consists of the following four reactions: (reaction 1) (reaction 2)
3
(reaction 3) (reaction 4) The hydrogen peroxide formation (reaction 1) is accompanied by side reactions that generate water (reactions (2), (3), and (4)). These undesirable, voluntary side reactions significantly reduce the hydrogen peroxide yield, and because of this, direct synthesis of hydrogen peroxide has notreached a level of commercialization [6, 11]. Acids and/or halides have been added to the reaction solvent to mitigate the low yield issue [12-17]. Because expensive precious metals such as Pd can increase the hydrogen peroxide yield, many studies are geared toward efficient utilization of Pd [15, 18-21]. Pd can increase the selectivity for hydrogen peroxide when a halide is added to the solvent. In particular, the selectivity toward hydrogen peroxide is substantially increased when Br- ions are added compared to when other halides (F-, Cl- and I-) are added [12, 14, 16, 21, 22]. Active sites on the Pd surface are classified into energetic sites with more dangling bonds (corner/edge) and less energetic sites with fewer dangling bonds (terrace), which feature high selectivity for the formation of water and hydrogen peroxide, respectively [12, 16]. When Br- is added, it is preferentially adsorbed at and occupies the more energetic sites [23], thereby increasing hydrogen peroxide selectivity. Therefore, when a small amount of Br- is added to the solvent, the selectivity can be substantially increased [6, 15, 24-26]. However, an excessive amount of Brwould lead to adsorption also occurring at the favored sites and reduce the overall hydrogen conversion [25]. Thus, it is important to carefully adjust the amount of Br- added to efficiently utilize the active Pd sites. Recently, there have been many studies on using various nanoparticles and nanostructures as catalysts [27-32]. In particular, some reactions have shown that catalytic
4
activities are significantly affected by the structure of the nano-catalysts [19, 33]. In our previous study, we controlled the fraction of energetic sites/less energetic sites by changing the size of the Pd nanoparticles and showed that hydrogen peroxide selectivity can indeed be enhanced [19, 24]. The effects of the Pd crystal facets on hydrogen peroxide selectivity were also examined by using cubic Pd nanoparticles surrounded by {100} facets and octahedral Pd nanoparticles surrounded by {111} facets [18]. Pd {111} facets exhibited a higher selectivity toward hydrogen peroxide than Pd {100} facets because undissociative chemisorption predominantly occurred when O2 was adsorbed on the Pd surface; this result was consistent with the DFT simulation [34]. In addition, a core-shell catalyst was introduced to the hydrogen peroxide synthesis reaction and showed higher hydrogen conversion and hydrogen peroxide selectivity than general supported catalysts [20, 35, 36]. The superior hydrogen conversion of the core-shell structure is ascribed to a higher Pd dispersion because the shell physically blocks sintering of encapsulated Pd during calcination at high temperature [35, 37]. In this study, we attempted to find the optimal shell thickness for efficient use of precious metal Pd by adjusting the shell thickness of the nanocatalyst within the core-shell structure.
2. Experimental 2.1. Materials L-ascorbic acid (Sigma-Aldrich, ACS reagent, > 99%), polyvinylpyrrolidone (SigmaAldrich, PVP, 55,000 g/mol), potassium bromide (Sigma-Aldrich, KBr, ACS reagent, > 99%), and sodium tetrachloropalladate (Aldrich, Na2PdCl4, 98%) were used to synthesize the Pd nanoparticles. Ethanol (Sigma-Aldrich, ACS reagent, > 99.5%, absolute), ammonium hydroxide solution (Sigma-Aldrich, 28 - 33% NH3 basis), and tetraethylorthosilicate (Sigma-Aldrich, TEOS,
5
ACS reagent, 98%) were used to synthesize the core-shell catalysts. All of the materials were used without purification.
2.2 Synthesis of Pd@SiO2 Synthesis of Pd@SiO2 catalyst could be divided into two stages as shown in Scheme 1: (1) Synthesis of Pd nanoparticles; and (2) Encapsulation of the nanoparticles with SiO2. Preparation of the Pd@SiO2 nanoparticles is described in detail in our previous papers [20, 35, 36]. First, Pd nanoparticles were synthesized, and the shells were formed using the Stӧber method. The Pd nanoparticles were synthesized by reducing Na2PdCl4 precursors with Lascorbic acid in a solution containing KBr and PVP. After nanoparticle recovery via centrifugation and washing with water, the nanoparticles were dispersed in ethanol. After adding NH4OH and water to the ethanol solution with the dispersed Pd nanoparticles, the solution was stirred at room temperature for 3 hours. Next, 5, 10, 15, 20, 25 or 30 ml of TEOS was added. The solution was stirred for 24 hours and the nanoparticles were collected with a centrifugal separator. After washing with a water-ethanol mixture several times, they were dried at 60 °C. The prepared core-shell catalyst was then calcined at 500 °C for 6 hours and reduced by H2 gas (10 vol% N2 balance, 50 ml/min) at 350 °C for 2 hours. Based on the volume of TEOS added, we named the catalysts Pd@S(5), Pd@S(10), Pd@S(15), Pd@S(20), Pd@S(25) and Pd@S(30).
2.3 Catalyst characterization Transmission electron microscopy (TEM) analysis was performed using a Tecnai G2 F30 instrument (FEI Company, USA). The catalyst was dispersed in ethanol and then dropped over a
6
Cu grid, which was dried prior to analysis. TEM analysis was performed at the KBSI (Seoul center of the Korea Basic Science Institute). The crystalline structures were analyzed by X-ray diffraction (XRD: D/MAX-2500/PC, Rigaku), and the XRD patterns were measured at a scanning speed of 1°/min using Cu Kα1 irradiation (λ = 1.5406 Å). N2 adsorption-desorption isotherms were obtained using a BELSORP-max instrument (BEL Japan Inc., Osaka, Japan) at -196 °C. Specific surface area and total pore volume of the catalysts were calculated by applying the Brunauer-Emmett-Teller (BET) theory. The microporous surface area and micropore volume were estimated by subtracting t-plot data from the BET data. The mass fractions of Pd in the catalysts were estimated by inductively coupled plasma optical emission spectrometry (ICP-OES) using a JY Ultima2C (Jobin Yvon, France). Before each measurement, each catalyst was dissolved in a liquid mixture (nitric acid, hydrochloric acid and hydrofluoric acid) at 200 °C. The Pd surface area of the catalysts was determined using the CO chemisorption method. Measurements were performed on an ASAP 2020 chemisorption analyzer (Micrometrics Inc., USA) at 35 °C. The N2 adsorption-desorption, TEM, XRD, ICP-OES and CO-chemisorption experiments were performed on the reduced catalysts.
2.4 Hydrogen peroxide synthesis The catalysts were tested in a double-jacketed glass reactor for direct hydrogen peroxide synthesis. The catalytic reactions were conducted at 20 °C and 1 atm for 3 hours. Both a 150 ml ethanol-water mixture (containing potassium bromide (0.15 mM) and phosphoric acid (0.03 M),
7
20 vol% ethanol) were fed into the reactor. We adjusted the amount of the catalyst such that a total of 1 mg of Pd would be added to the reaction. The gas feed ratio of H2/O2 was 1:10, and the total flow rate was 22 ml/min. The hydrogen peroxide content was measured by iodometric titration. H2 conversion, H2O2 selectivity and the H2O2 production rate were calculated by equations (1), (2) and (3), respectively. H2 conversion (%) =
(1)
H2O2 selectivity (%) =
(2)
H2O2 production rate (mmol/g-Pd·h)
(3)
3. Results and Discussion 3.1 Characterization of the Pd nanoparticles and core-shell catalysts Figure 1(a) shows a TEM image of the synthesized Pd nanoparticles. A total of 100 measurements taken from different regions of the sample, which showed uniformly-sized nanoparticles with an average size of 9.7 nm (see Figure 1(b)). Figure 2 shows the TEM images of the Pd@SiO2 catalysts synthesized with different amounts of TEOS. When a small amount of TEOS was added (Pd@S(5), Pd@S(10)), the proportion of the core-shell nanoparticles containing multiple (up to 4-5) Pd nanoparticles within a single shell was higher. The estimates for the average shell thickness are listed in Table 1. The values were obtained by measuring the shell thickness of 100 nanoparticles per catalyst. We confirmed that the shell of the Pd@SiO2 nanocatalyst became thicker as more TEOS was added.
8
The XRD analysis results are shown in Figure 3. In the XRD analysis, a Pd metal peak was observed, but no PdO peak was observed. All three Pd peaks detected were more pronounced in samples with thinner shell thicknesses, which is in agreement with the relative metal loading listed in Table 1.
Figure 4 shows the N2-adsorption and desorption isotherms of the core-shell catalysts. The corresponding micro surface areas and micropore volumes determined from the t-plot results are listed in Table 2. N2-adsorption and desorption measurements revealed that the shell thicknesses are negatively correlated with the pore volume and the inner surface area. For most of the catalysts (Pd@S(5) - Pd@S(20)), micropores developed in the shell and resulted in a large specific surface area. However, when the shell was thick (Pd@S(25), Pd@S(30)), pores did not develop in the shell during the calcination process, and the specific surface area was sharply reduced.
The exposed Pd surface areas from the CO-chemisorption measurements are listed in Table 1. A volcano curve was obtained when plotting the exposed Pd specific surface area (m2/gPd) as a function of the shell thickness. As seen from Figure 2, catalysts with thinner shells (Pd@S(5) and Pd@S(10)) are characterized by a higher proportion of the individual shell structures encapsulating multiple (4-5) Pd nanoparticles. This ‘bundling’ reduced the proportion of surface area which is exposed because the adjacent Pd nanoparticles are in contact with one another. Thus, the exposed Pd specific surface area (m2/g-Pd) increased with increasing shell thickness, as more Pd nanoparticles were individually encapsulated by a single shell.
9
3.2 Reaction results of the hydrogen peroxide synthesis The results of applying core-shell nanocatalysts to the hydrogen peroxide synthesis reaction are shown in Figure 5. Figure 5 (a) and Figure 5 (b) show the hydrogen conversion rate and hydrogen peroxide yield, and the hydrogen peroxide production rate, respectively, as a function of the SiO2 thickness. A volcano curve as a function of the shell thickness was observed for all three and is similar to the curve for the Pd exposed surface area as a function of the shell thickness. The largest exposed surface area was obtained for Pd@S(20), but the Pd@S(15) catalyst showed a higher hydrogen conversion rate. This discrepancy is ascribed to the increased mass transfer resistance, which would affect the overall reaction rate. The change in the mass transfer resistance with varying shell thickness was examined. The number of moles of reacted hydrogen was divided by time and the Pd surface area in the reaction medium as shown in Figure 6. Hydrogen conversion rate (reacted H2 mmoles divided by time and Pd area) decreased as the shell thickness increased, and a notable, sharp decrease and flattening out at Pd@(25) and Pd@(30). In the absence of well-developed pore channels that provide passage to the catalyst with much less resistance, the shell thickness would be negatively correlated with the catalytic performance. Although thinner shells would be preferred due to the advantage of smaller mass transfer resistance, shell precursors below a certain limit during shell formation resulted in bundled encapsulation and insufficient Pd exposure area. Therefore, it is suitable to use a catalyst with a moderate shell thickness for hydrogen peroxide synthesis.
4. Conclusions When synthesizing Pd@SiO2 core-shell nanoparticles using the Stöber method, the shell thickness is easily adjusted above a minimum thickness. Adding a small amount of TEOS to
10
synthesize a thin shell (Pd@S(5), Pd@S(10)) confirmed the presence of a higher proportion of core-shell nanoparticles containing multiple Pd nanoparticles within a single shell, thereby reducing the Pd weight-specific exposed surface area of the nanoparticle (m2/g-Pd). In addition, although the exposed Pd surface area was not significantly reduced for thick shells (Pd@S(25) and Pd@S(30)), the specific surface area of the core-shell catalyst sharply decreased due to calcination in formation of micropores. When applying catalysts with thin shells (Pd@S(5) and Pd@S(10)) to direct hydrogen peroxide synthesis reaction, the small exposed Pd surface area (m2/g-Pd) resulted in a reduction of hydrogen conversion and the hydrogen peroxide production rate. However, when the shell was thick (Pd@S(25) and Pd@S(30)), the hydrogen conversion rate sharply decreased due to underdeveloped pores and hence increased mass transfer resistance. Catalysts with intermediate shell thicknesses (Pd@S(15) and Pd@S(20)) achieved high hydrogen conversion and hydrogen peroxide yield owing to the high Pd specific surface areas. The thinner the shell, the better for hydrogen peroxide synthesis, but a minimum thickness is necessary to achieve the maximum Pd specific surface area in the core-shell synthesis process.
Acknowledgments This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (NRF-2016M3D1A1021143). This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (MSIP) (2016, University-Institute cooperation program).
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Figure 1. (a) TEM image of Pd nanoparticles and (b) their particle size distribution.
14
Figure 2. TEM images of (a) Pd@S(5), (b) Pd@S(10), (c) Pd@S(15), (d) Pd@S(20), (e) Pd@S(25) and (d) Pd@S(30).
15
Pd(111) Pd(200)
Pd(220)
Intensity (A.U.)
Pd@S(5)
Pd@S(10) Pd@S(15) Pd@S(20) Pd@S(25) Pd@S(30)
10
20
30
40
50
60
70
80
2 theta (degree)
Figure 3. XRD results of Pd@SiO2 catalysts.
1200
Volume adsorbed (ml/g)
1000 Pd@(5) 800 Pd@(10) 600 Pd@(15) 400 Pd@(20) 200
Pd@(25) Pd@(30)
0 0.0
0.2
0.4
0.6
Relative pressure (p/p0) Figure 4. N2 adsorption-desorption isotherms of Pd@SiO2 catalysts.
16
0.8
1.0
25
H2 conversion
H2 conversion & H2O2 yield (%)
(a)
H2O2 yield
20
15
10
5
2 1 0
Pd@S(5)
Pd@S(10) Pd@S(15) Pd@S(20) Pd@S(25) Pd@S(30)
H2O2 production rate (H2O2mmol/g-pd h)
1000
(b) 800
600
400
20
0 Pd@S(5)
Pd@S(10)
Pd@S(15)
Pd@S(20)
Pd@S(25)
Pd@S(30)
Figure 5. Direct synthesis of hydrogen peroxide using Pd@SiO2 catalysts. (a) H2 conversion and H2O2 yield; (b) H2O2 productivity (Pd weight specific H2O2 production rate); Test conditions: 293 K, 1 atm, 1 mg of Pd metal, 150 mL of ethanol/water (4:1, v/v) mixture (containing 0.15 mM KBr and 0.03 M H3PO4), Stirring rate = 1,200 rpm, Total gas flow rate = 22 mL/min, H2/O2 = 1:10 (v/v).
17
2
Hydrogen reactin rate (H2mmol/m Pdh)
35
30
25
20
15
10
5
0
Pd@S(5)
Pd@S(10)
Pd@S(15)
Pd@S(20)
Pd@S(25)
Pd@S(30)
Figure 6. Hydrogen conversion rate of Pd@SiO2 catalysts (Hydrogen conversion rate: reacted H2 moles divided by time and Pd area in the reaction medium)
18
Scheme 1. Pd@SiO2 catalyst synthesis scheme.
19
Table 1. Pd content, Pd surface area and shell thickness of the catalysts.
a
Catalyst
Pd loading (wt.%) a
Pd@S(5) Pd@S(10) Pd@S(15) Pd@S(20) Pd@S(25) Pd@S(30)
3.50 1.78 0.90 0.68 0.54 0.49
Pd
were
loadings
Exposed Pd area b m2/g-cat m2/g-Pd 0.976 27.9 0.654 36.7 0.434 48.2 0.417 61.3 0.281 52.0 0.241 49.2
calculated
from
Shell thickness (nm) c 37.4 42.4 44.3 53.3 57.9 62.8
the
ICP-OES
data.
b
Exposed Pd areas and Pd dispersion were calculated based on the CO chemisorption results.
c
The average shell thickness was determined by measuring 100 core-shell nanocatalysts from the
TEM images.
Table 2. Surface area and pore volume of the catalysts. Catalyst
Surface area (m2/g-cat.)
Pore volume (ml/g-cat.)
Total a
Micropore b (%) c
Total a
Micropore b (%) c
Pd@S(5)
295
242 (82)
0.61
0.50 (82)
Pd@S(10)
272
221 (81)
0.57
0.47 (82)
Pd@S(15)
234
191 (82)
0.49
0.40 (82)
Pd@S(20)
216
174 (80)
0.45
0.37 (82)
Pd@S(25)
55
19 (35)
0.40
0.40 (100)
Pd@S(30)
42
10 (24)
0.40
0.40 (100)
a
The total specific surface area and pore volume were estimated by the BET method. Microporous surface area and micropore volume were estimated by subtracting t-plot data from BET data. c The percentage of the total surface area or total pore volume due to micropores. b
20