Coating of Pt-Loaded Mesoporous Silica Layers on Ceramics

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Coating of Pt-Loaded Mesoporous Silica Layers on Ceramics Scaffolds for Practical Preservation System for Greengrocery Jongbeom Na,†,‡,§ Dehua Zheng,† Jeonghun Kim,§ Yohei Jikihara,∥ Tsuruo Nakayama,∥ Youngsang Ko,⊥ Jungmok You,⊥ Yusuke Yamauchi,*,†,‡,§,⊥ and Jianjian Lin*,†

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†

Key Laboratory of Eco-chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China ‡ International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan § School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072, Australia ∥ NBC Meshtec Inc., 2-50-3 Toyoda, Tokyo 191-0053, Japan ⊥ Department of Plant & Environmental New Resources, Kyung Hee University, 1732 Deogyeong-daero, Giheung-gu, Yongin-si, Gyeonggi-do 446-701, South Korea S Supporting Information *

ABSTRACT: To enhance the decomposition properties of ethylene, Pt-loaded mesoporous silica (SiO2) was coated onto the scaffolds of cordierite (Mg2Al4Si5O18) membranes (CM) and glass fibers (GF). The surface areas of the mesostructured SiO2 layers coated on CM (CM/meso-SiO2) and GF (GF/ meso-SiO2) were, respectively, 118 and 323 times higher than those of the CM and GF. In addition, Pt nanoparticles were homogeneously loaded in the mesopores, which acted as a catalyst. The prepared CM/Pt@meso-SiO2 and GF/Pt@ meso-SiO2 showed an efficient and high-performing ethylene decomposition. In particular, the flowers, fruits, and vegetables were well stored with the use of CM/Pt@meso-SiO2 and GF/ Pt@meso-SiO2, which effectively decomposed ethylene gas they generated. As a result, Pt@meso-SiO2 with scaffolds (CM and GF) are expected to be effective in commercial application for a practical system of preservation. KEYWORDS: ethylene decomposition, Pt-loaded catalyst, mesoporous silica, scaffolds, preservation system

1. INTRODUCTION Ethylene, a hydrocarbon and natural hormone, is produced in natural sources, such as plants, vegetables, and fruits. It plays a positive role as a ripening agent under controlled conditions.1 However, exposure to excess ethylene can cause food waste and loss of fresh vegetables and fruits, even at low temperatures,2 which is one of the most critical problems facing our society.3,4 To prolong storage life, greengroceries are usually harvested and stored before ripening. However, both the degree of ripening and the sensitivity to ethylene can vary depending on the types of greengroceries. To solve these problems, many researchers from all over the world have developed advanced technologies to maintain the original freshness of greengroceries. Among them, ethylene decomposition photocatalysts always require exposure to light sources such as ultraviolet and visible-light lamps for activation.5−10 However, these methods require the installation of facilities in the refrigerated storage area with a continuous supply of electrical power. Another method is to decompose ethylene using porous materials such as activated carbon, zeolite, and mesoporous materials.11−13 The amount of ethylene gas © XXXX American Chemical Society

decomposed, however, is rather limited. As an alternative way, well-known adsorbents must be exchanged and disposed of when the effect is lost, making them somewhat cumbersome. Recently, many efforts have been made to use various nanostructured metal and alloy materials in a wide range of catalytic reactions.14−16 Among them, platinum (Pt) nanoparticles supported on a mesoporous silica (SiO2) powder were reported to oxidize ethylene at room temperature.17,18 However, nanosized powders are seriously harmful to human health. Keeping these powders on a solid substrate remains an important challenge when fabricating functional devices. Therefore, it is urgent to develop a highly active material that efficiently decomposes ethylene and commercialize it for an actual industry. Herein, we introduce a novel ethylene-decomposing material, which consists of a mesoporous SiO2 (meso-SiO2) coated onto the scaffolds of the cordierite (Mg2Al4Si5O18) membranes Received: April 22, 2019 Accepted: July 31, 2019

A

DOI: 10.1021/acsami.9b07011 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces (CM) (CM/meso-SiO2) and glass fibers (GF) (GF/mesoSiO2). The surface areas of CM/meso-SiO2 and GF/mesoSiO2 were, respectively, 118 and 323 times higher than those before coating, which can dramatically enhance the exposure to ethylene. To further improve the catalytic activity of ethylene decomposition, Pt nanoparticles were homogeneously loaded in meso-SiO2 as the prototype catalyst. The Pt-loaded mesoSiO2 with CM (CM/Pt@meso-SiO2) and GF (GF/Pt@mesoSiO2) showed enhanced ethylene decomposition activities of 80 and 99.8%, respectively, at low temperature (5 °C). We reveal that CM/Pt@meso-SiO2 and GF/Pt@meso-SiO2 are highly efficient materials in the ethylene decomposition process, and thus are promising catalysts in the field of ethylene decomposition to freshen fruits and vegetables in refrigeration storage.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (PEO20-b-PPO70-b-PEO20, P-123) was purchased from Polymer Source. Tetraethyl orthosilicate and ethanol were purchased from Nacalai Tesque. All chemicals were used without further purification. Diamminedinitritoplatinum(II) was purchased from Tanaka Kikinzoku Kogyo. 2.2. Preparation of Pt@meso-SiO2 at scaffolds. P-123 containing a precursor solution for the preparation of a meso-SiO2 layer was prepared in accordance with our previous report.19 The CM and GF scaffolds were immersed in the precursor solution to coat the mesostructured SiO2 layer onto their surface. The precursor-coated CM and GF were calcined at 450 °C for 4 h to remove the P-123 template. The obtained CM/meso-SiO2 and GF/meso-SiO2 were further immersed into a diamminedinitritoplatinum(II) aqueous solution to load the Pt nanoparticles. Then, the excess amount of solution was removed by blowing of air. Finally, the CM and GF were heat-treated at 250 °C for 1 h in a mixture gas of 10% hydrogen and 90% nitrogen after drying at 300 °C for 3 h. 2.3. Characterization. The morphologies of the prepared materials were characterized by using a field emission scanning electron microscope (FE-SEM), (HITACHI SU-8000) with a 5 kV accelerating voltage. The Pt-loaded mesoporous structures with CM and GF were characterized by using a transmission electron microscope (TEM) in a 200 kV accelerating voltage (Talos F200X, FEI). The TEM samples were prepared by scratching and dispersing Pt@meso-SiO2. First, CM/Pt@meso-SiO2 and GF/Pt@meso-SiO2 were scratched to detach Pt@meso-SiO2 from the scaffolds. The detached powders were dispersed in ethanol through sonication. The prepared solution was dropped onto a TEM grid. The threedimensional (3D) tomography was characterized using a high-angle annular dark-field detector with a maximum tilt angle of ±70°. The nitrogen adsorption−desorption isotherms were obtained by using a Belsorp apparatus (Bel Japan, Inc.) at 77 K. The platinum contents were measured by using an inductively coupled plasma mass spectrometer (ICP-MS). The ethylene concentration was measured by using an integrated gas detector tube system (GASTEC Corp.).

Figure 1. Schematic illustration of the preparation for meso-SiO2 and Pt nanoparticles onto (a) CM (CM/Pt@meso-SiO2) and (b) GF (GF/Pt@meso-SiO2).

GF/meso-SiO2 were further immersed in the Pt precursor solution to synthesize the Pt nanoparticles homogeneously inside the mesopores (Figures S1 and S2). This synthesis strategy prohibited the aggregation of Pt nanoparticles in meso-SiO2 even after high-temperature treatment. The morphologies of the Pt-loaded mesoporous SiO2 (Pt@ meso-SiO2) with CM (CM/Pt@meso-SiO2) and GF (GF/ Pt@meso-SiO2) were characterized by field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), as shown in Figure 2. In the case of CM/ Pt@meso-SiO2, the original structure of CM was wellmaintained even after coating with Pt@meso-SiO2 (Figure 2a). GF/Pt@meso-SiO2 maintained the spaces that were formed by the glass fiber after coating with Pt@meso-SiO2 (Figure 2b). The coating of mesoporous layers on the scaffolds can increase the possibility of ethylene contact. The TEM images of CM/Pt@meso-SiO 2 and GF/Pt@meso-SiO 2 (Figures 2c,d and S3) demonstrated that the Pt nanoparticles were present homogeneously inside the mesopores (yellow arrows: meso-SiO2). Such a uniform distribution of nanoparticles without aggregations plays an important role in improving the catalytic effect.20−23 The loaded Pt nanoparticles were clearly observed as dark spots at meso-SiO2 with CM and GF. The average diameters of the Pt nanoparticles were estimated to be 2.5 ± 0.6 nm (CM/Pt@meso-SiO2) and 3.2 ± 0.8 nm (GF/Pt@meso-SiO2) by measuring more than 100 Pt nanoparticles (Figure 2e,f). To further analyze the diameter and distribution of Pt nanoparticles in meso-SiO2, three-dimensional (3D) TEM was carried out, which was prepared by the reconstruction of TEM images with surface modeling (Video S1), as shown in Figure 3a,b. The blue region corresponded to meso-SiO2, while the yellow corresponded to Pt. Pt nanoparticles were well-located not only on the surface but also inside meso-SiO 2 . Furthermore, the meso-SiO2 region (blue) had many mesopores, although, unfortunately, well-ordered mesoporous structures were not formed due to the serious hindrance to the self-assembly of surfactants in confined spaces. We expect that the resulting meso-SiO2 layers have a large surface area that can improve the catalytic effect.

3. RESULTS AND DISCUSSION The synthesis involved the surface coating of meso-SiO2 and Pt nanoparticles, followed by immersion in a precursor solution and calcination. Figure 1 shows a schematic illustration of the preparation of the Pt@meso-SiO2-coated CM (CM/Pt@mesoSiO2) and GF (GF/Pt@meso-SiO2). The CM and GF scaffolds with lattice and fiber structures were immersed in the meso-SiO2 precursor solution. After calcination (450 °C), meso-SiO2 was well-coated onto the CM/GF surface (CM/ meso-SiO2 and GF/meso-SiO2). The specific surface area dramatically increased through the coating of meso-SiO2 onto the scaffolds with mesoporous structures. CM/meso-SiO2 and B

DOI: 10.1021/acsami.9b07011 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

meso-SiO2 are shown in Figure 3c,d. They appeared to be typical isotherms for mesoporous materials with hysteresis loops.24 Clear uptakes of N2 were observed in a range from 0.5 to 0.7 (=P/P0), which is an important evidence of the formation of uniformly sized mesopores. As shown in the SEM images in Figure 2a,b, GF/Pt@meso-SiO2 had a smaller and denser structure than that of CM/Pt@meso-SiO2, which results in a higher surface area. The detailed structural parameters of the samples obtained from the isotherm as well as the size distribution of pores and Pt nanoparticles are summarized in Table 1. The BET surface areas (SBET) were dramatically increased after Pt@meso-SiO2 coating onto CM (24 m2 g−1) and GF (97 m2 g−1), 118 and 323 times higher, respectively, than those of CM (0.2 m2 g−1) and GF (0.3 m2 g−1) before coating. It is, therefore, revealed that meso-SiO2 coating is an effective method of increasing the surface area by improving the possibility of an ethylene contact. The average diameters of Pt nanoparticles (Figure 2e,f) were smaller than those of the mesopores; therefore, they could be located inside the meso-SiO2. CM/Pt@meso-SiO2 and GF/Pt@meso-SiO2 were applied to ethylene decomposition with prototype devices such as a fixed-bed flow reactor (CM: 150 × 150 × 10 mm3, flow rate: 0.6 m3 min−1) and patch (GF: 100 × 100 × 0.2 mm3) (Figure S4). The catalytic activities were measured at 5 °C and 90% relative humidity with different volumes of 100 L (CM) and 5 L (GF). The obtained data are summarized in Figure 4. Using CM/Pt@meso-SiO2, the ethylene decomposition activity was dramatically enhanced as compared to that of pristine CM (Figure 4a). The percentage of ethylene decomposition reached 80% after 400 min (Figure 4d). It can be seen that the existence of Pt@meso-SiO2 plays a great role in promoting the activity. More interestingly, GF/Pt@meso-SiO2, however, showed a higher activity than CM/Pt@meso-SiO2 (Figure

Figure 2. SEM images of the prepared (a) CM/Pt@meso-SiO2 and (b) GF/Pt@meso-SiO2 (inset: photographic images). TEM images of (c) CM/Pt@meso-SiO2 and (d) GF/Pt@meso-SiO2. Pt nanoparticle size distributions in (e) CM/Pt@meso-SiO2 and (f) GF/Pt@mesoSiO2 (inset: TEM images).

Nitrogen (N2) adsorption−desorption isotherms and poresize distribution curves for CM/Pt@meso-SiO2 and GF/Pt@

Figure 3. (a, b) Reconstructed 3D TEM images from TEM images with surface modeling: (a) reconstructed image with Pt nanoparticles and meso-SiO2 (inset: reconstructed images of only meso-SiO2 (left bottom) and Pt nanoparticles (right top)) and (b) magnified 3D TEM image of (a). (c) N2 adsorption−desorption isotherm for CM and GF/Pt@meso-SiO2. (d) Pore size distribution for CM and GF after coating of meso-SiO2 (inset: magnified data of GF/meso-SiO2). C

DOI: 10.1021/acsami.9b07011 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 1. Structural Parameters for CM/Pt@meso-SiO2 and GF/Pt@meso-SiO2 sample

SBETa (m2 g−1)

CM CM/Pt@meso-sSiO2 GF GF/Pt@meso-SiO2

0.2 24 0.3 97

Db (nm)

average Pt diameterc (nm)

meso-SiO2 loadingd (wt %)

Pt loadinge (wt %)

5.4

2.5 ± 0.6

4.5

1.8

6.2

3.2 ± 0.8

30

5.0

a

BET surface area. bAverage pore diameter estimated using the Barrett−Joyner−Halenda (BJH) method. cCalculation based on Pt particle size distribution. dRelative weight percentage to the amount of scaffold materials. eRelative weight percentage to the amount of mesoporous SiO2 (according to the ICP-MS analysis).

Figure 4. Ethylene decomposition properties of (a) CM/Pt@meso-SiO2 (initial ethylene concentration: 5 ppm; volume: 100 L; temperature: 5 °C; relative humidity: 90%) and GF/Pt@meso-SiO2 in a closed reactor with two initial ethylene concentrations of (b) 10 ppm and (c) 1 ppm (volume: 5 L; temperature: 5 °C; relative humidity: 90%). (d, e) Time versus percentage of ethylene conversion with and without catalyst of (d) CM/Pt@ meso-SiO2 in a flow reactor (concentration: 5 ppm; volume: 100 L; temperature: 5 °C; relative humidity: 90%) and (e) GF/Pt@meso-SiO2 in a closed reactor (concentration: 10 ppm; volume: 5 L; temperature: 5 °C; relative humidity: 90%). (f) Time-dependent ethylene concentration calculated based on (d) and (e), showing the decomposition kinetic behavior of CM/Pt@meso-SiO2 and GF/Pt@meso-SiO2 with linear fitting.

4b,c). As shown in Figure 4e, ethylene was rapidly decomposed by ∼50% only after 60 min. After 300 min, the percentage of ethylene decomposition reached as high as 99.8%, which was much higher than that of CM/Pt@mesoSiO2 (70% after 300 min). Furthermore, GF/Pt@meso-SiO2 showed a high ethylene decomposition at lower concentrations (e.g., 1 ppm) of ethylene gas (Figure 4c). We considered that the loading amount of Pt@meso-SiO2 would affect the catalytic activity. The loaded meso-SiO2 wt % in GF is 30 wt %, which is 6.7 times higher than that of CM (4.5 wt %), as shown in Table 1. This meso-SiO2 loading wt % difference can lead to a larger specific surface area, increase the loading catalytic active sites of Pt nanoparticles, and enhance the possibility of contact with ethylene. Thus, GF/Pt@mesoSiO2 showed a much higher ethylene decomposition activity than CM/Pt@meso-SiO2. This ethylene decomposition proceeded through the following proposed mechanism. First, ethylene and oxygen are adsorbed on platinum particles. At that time, oxygen molecules (O2) are dissociated and produce atomic oxygen species (O) onto platinum. Second, the CC bond of ethylene is dissociated and reacted with atomic oxygen to form formaldehyde (CH2O). The formed formaldehyde is decomposed to CO and atomic hydrogen (H), which react with atomic oxygen to be converted to CO2 and water.18,25

The ethylene decomposition process follows quasi-firstorder kinetics with eq 1. C = C0·e−kt

(1)

where C0 is the initial concentration of ethylene, C is the concentration after ethylene decomposition at time t, and k is a kinetic constant of reduction, which well represents the decomposition activity of the catalyst. Figure 4f shows the time-dependent ethylene concentration (from 0 to 4 h), showing the decomposition kinetic behavior of CM/Pt@mesoSiO2 and GF/Pt@meso-SiO2. The kinetic constant of reduction (k) for GF/Pt@meso-SiO2 (0.43) was much higher than that of CM/Pt@meso-SiO2 (0.15), indicating that GF/ Pt@meso-SiO2 showed a better ethylene decomposition performance, which is also consistent with the results discussed above. Fruit and vegetable storage simulation experiments were carried out to confirm the feasibility of commercialization with the flow and closed reactors. Flowers, bananas, and avocados were used for these simulated experiments. Each simulated experiment was conducted without ethylene injection. In this condition, ripening occurred by self-generated ethylene. Flowers were exposed to a flow reactor (100 L, Figure S4) with or without CM/Pt@meso-SiO2. After 9 days, the flowers D

DOI: 10.1021/acsami.9b07011 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 2. Freshness Test Results of Flowers, Fruit, and Vegetables with Different Catalysts sample flowers

reactor type flow reactor (100 L)

period (days)

temperature (°C)

catalyst

Ca (ppm)

9

25

without catalyst CM/Pt@meso-SiO2 without catalyst GF/Pt@meso-SiO2 without catalyst GF/Pt@meso-SiO2

0.2 0.0 3.0 0.0 60 0.0

bananas

closed reactor (5 L)

14

25

avocado

closed reactor (5 L)

8

25

a

Final ethylene concentration (ppm) after 9, 14, and 8 days.

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became brown without a catalyst and the ethylene concentration of ∼0.2 ppm was detected (Table 2). By contrast, those with a catalyst maintained their initial state very well (Figure S5a,b), and ethylene was not detected. This result shows that CM/Pt@meso-SiO2 can efficiently decompose ethylene generated by flowers. GF/Pt@meso-SiO2 and closed reactor (5 L) were used in simulation experiments for bananas and avocados. Bananas became black very quickly without GF/Pt@meso-SiO2 after 14 days, with 3 ppm of ethylene detected (Figure S5c and Table 2). However, the color changed much slower than that in the catalyst-free condition (Figure S5d). Avocados also showed brown color without a catalyst (Figure S5e), with a very high ethylene concentration of 60 ppm detected after 8 days (Table 2). Interestingly, with GF/Pt@meso-SiO2, however, the avocados remained very stable, and no ethylene was detected (Figure S5f and Table 2). These results demonstrate that our catalysts show very high ethylene decomposition properties and are expected to be effective in the application for commercial fruit and vegetable storage.

Corresponding Authors

*E-mail: [email protected] (Y.Y.). *E-mail: [email protected] (J.L.). ORCID

Jongbeom Na: 0000-0002-3890-7877 Jeonghun Kim: 0000-0001-6325-0507 Youngsang Ko: 0000-0003-4817-8185 Jungmok You: 0000-0002-9583-2242 Yusuke Yamauchi: 0000-0001-7854-927X Jianjian Lin: 0000-0002-9424-3323 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the China Postdoctoral Science Foundation (2019M650160), the National Natural Science Foundation of China (NSFC) (61604070), and the National Natural Science Foundation of Jiangsu Province (BK20161000). This work was supported by Australian Research Council (ARC) Future Fellowship (FT150100479). This work was performed in part at the Queensland node of the Australian National Fabrication Facility, a company established under the National Collaborative Research Infrastructure Strategy to provide nano- and micro-fabrication facilities for the researchers of Australia.

4. CONCLUSIONS Pt-loaded mesoporous SiO2 (Pt@meso-SiO2) was homogeneously coated onto the scaffolds of cordierite membranes (CM) and glass fibers (GF) with well-maintained original shapes. CM/Pt@meso-SiO2 and GF/Pt@meso-SiO2 showed 118 and 323 times greater surface areas than those of CM and GF, respectively. Our catalysts showed very high ethylene decomposition performance due to the large surface area and loaded Pt nanoparticles in mesopores. Flowers, fruit, and vegetables were well-stored in simulated experiments with the use of CM/Pt@meso-SiO2 and GF/Pt@meso-SiO2, which effectively decomposed the self-generated ethylene gas. As a result, our catalysts are expected to have effective applications in commercial preservation system for greengroceries.

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AUTHOR INFORMATION

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07011. Photographic images of CM after coating of meso-SiO2 and platinum (Figure S1); photographic images of the prepared CM/Pt@meso-SiO2 and GF/Pt@meso-SiO2 (Figure S2); TEM images of the prepared CM/Pt@ meso-SiO2 and GF/Pt@meso-SiO2 (Figure S3); photographic images of the structure of the prototype catalyst with CM/Pt@meso-SiO2 (Figure S4); photographic images of simulation experiments for the storage of fruits and vegetables (Figure S5) (PDF) Reconstructed 3D TEM images by using the HAADF detector (MPG) E

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DOI: 10.1021/acsami.9b07011 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX