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surfactant to mesoporous metal oxides with enhanced thermal stability
Microporous and Mesoporous Materials xxx (xxxx) xxx
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Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Soft-to-hard consecutive templating one-pot route from metal nitrate/ phenol resin/surfactant to mesoporous metal oxides with enhanced thermal stability Su-Kyung Lee a, b, Changbum Jo c, Jaeheon Kim d, **, Ryong Ryoo a, d, * a
Department of Chemistry, Korea Advanced Institute for Science and Technology (KAIST), Daejeon, 34141, Republic of Korea Research Group of Green Carbon Catalysis, Korea Research Institute of Chemical Technology(KRICT), 141 Gajeong-Ro, Yuseong, Daejeon, 305-600, Republic of Korea Department of Chemistry and Chemical Engineering, Inha University, Incheon, 22212, Republic of Korea d Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon, 34141, Republic of Korea b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Mesoporous metal oxides Nanocrystalline Surfactant Gold-supported ceria Water-gas shift reaction
Mesoporous materials with crystalline ZrO2, Y2O3, and CeO2 frameworks were one-pot synthesized from a clear solution of ethanol containing metal nitrates, organic surfactant (i.e., Pluronic® F-127), formaldehyde, and phloroglucinol. The solution was converted to a mesostructured nanocomposite of metal nitrate/phenol resin/ surfactant via a solvent evaporation-induced self-assembly process. The obtained nanocomposite was calcined at 800 � C in a N2 atmosphere. X-ray powder diffraction and electron microscopic investigation revealed that the calcination caused amorphous-to-crystalline transformations in the metal oxide frameworks, while sintering to bulk metal oxides was prevented by the in-situ generated carbon component. Mesoporous metal oxides with a crystalline framework were obtained when the carbon skeleton was burnt off. The mesoporous metal oxides exhibited high BET surface areas, narrow pore-size distributions, and enhanced thermal stability. A practical benefit of the mesoporous metal oxides was demonstrated with Au/CeO2 exhibiting high catalytic activity in the water-gas-shift reaction.
1. Introduction There have been many efforts to synthesize mesoporous metal oxides possessing high specific surface area, pore volume, and stability, due to their applicability to catalysis, sensing, electronic devices, and photo voltaic solar cells [1–6]. A common bottom-up method to synthesize mesoporous metal oxides is to use surfactant micelles as a soft template, which is well-known as an effective route to mesoporous silicas such as MCM-41 and SBA-15 [7–9]. In the soft-templating synthesis for meso porous silica, the hydrothermal synthesis gives a mesostructured as sembly of surfactant micelles and amorphous silica layers. The amorphous silica layers are self-retained as a stable wall to support mesopores, i.e., a mesopore wall, during the removal of the surfactant by calcination at 450 � C, and even upon further heating to 800 � C. In the case of the surfactant-templated synthesis for most other metal oxides (e. g. TiO2, ZrO2, etc.), amorphous walls are also generated at the surfaces surrounding the surfactant micelles. However, these metal oxide walls
are normally unstable under the high-temperature calcination condi tions used to remove the surfactant template [10–16]. Most metal oxides have a tendency to crystallize, which often causes undesirable sintering into large particles and consequent loss of mesoporosity. There are particular cases of successful removal of surfactants without pore collapse via low-temperature calcination or a solvent extraction process [17]. Even in these cases, however, the metal oxide frameworks often suffer from amorphous-to-crystal or crystal-to-crystal phase trans formations at higher temperatures. Therefore, this soft-templating is inappropriate to yield thermally stable mesoporous materials for most metal oxides. The use of hard templates (e.g., CMK-type ordered mesoporous car bons, and amorphous mesoporous silicas) is another common bottom-up synthesis method for mesoporous metal oxides, and it can result in enhanced thermal stability in comparison to the aforementioned softtemplating method [18–23]. A typical hard templating route involves metal precursor infiltration, conversion of loaded-metal precursors into
* Corresponding author. Department of Chemistry, KAIST, Daejeon, 34141, Republic of Korea. ** Corresponding author. E-mail addresses: [email protected] (J. Kim), [email protected] (R. Ryoo). https://doi.org/10.1016/j.micromeso.2019.109767 Received 15 May 2019; Received in revised form 19 September 2019; Accepted 25 September 2019 Available online 30 September 2019 1387-1811/© 2019 Published by Elsevier Inc.
Please cite this article as: Su-Kyung Lee, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109767
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metal oxides, and template removal by air calcination or chemical etching. Prior to the template removal, the infiltrated metal precursors are converted into metal oxides via a heat treatment in air or inert gas. When the heat treatment temperature is above a certain high tempera ture, the infiltrated metal-oxide species undergoes crystallization. Nevertheless, the mesostructure can be retained because the hard tem plates are thermally stable and rigid enough even at the high tempera ture [21–23]. Therefore, mesoporous metal oxides with a crystalline framework can be obtained by hard-templating in this manner, and they show superior thermal stability to amorphous or semi-crystalline metal oxides hydrothermally synthesized by soft templating. However, the hard-templating route has a significant drawback in the pre-synthesis of hard templates, which is inefficient and tedious. Recently, considerable efforts have been devoted to developing a synthesis route that can give mesoporous metal oxides without presynthesized hard templates [2,24–26]. In this regard, Wiesner et al. synthesized mesoporous metal oxides by using a specially designed block copolymer as a soft-to-hard consecutive template [24]. The block copolymer was composed of hydrophilic poly (ethylene oxide) and hy drophobic poly (isoprene) blocks. The block-copolymers were readily assembled with metal precursors into a mesostructured organic-inorganic nanocomposite, similar to soft-templating. Upon further high temperature treatment under inert gas, however, the soft template of the block copolymer in-situ generated a carbon skeleton, which can be regarded as a hard template, via a pyrolysis process of the isoprene block in the block copolymer. Simultaneously, the metal con tent was also converted into nanocrystalline metal oxides, while retaining the entire mesostructure supported by the in-situ generated carbon skeleton. Finally, mesoporous metal oxides with a crystalline framework could be obtained after removal of the carbon skeleton by air calcination. The mesoporous crystalline metal oxides prepared by this soft-to-hard consecutive templating showed high thermal stability, comparable to the case of the hard templating method. Accordingly, various mesoporous metal oxides with nanocrystalline frameworks were synthesized in a similar manner [25,26]. Nevertheless, this route also had a similar problem to the hard-templating method, where the specially designed block-copolymer must be pre-synthesized via difficult anionic polymerization [24]. This was because the commonly used and commercially available surfactants (e.g., Pluronic® F-127, P-123, etc.) are not suitable for this method in terms of providing satisfactory amounts of pyrolyzable organic content as they usually decompose under the pyrolysis conditions due to high oxygen content. We undertook this work to establish a facile synthesis route to mesoporous metal oxides via the aforementioned soft-to-hard consecu tive templating even without the pre-synthesis of any templates. To this end, we used a combination of commercial surfactant and phenol resin as the soft-to-hard consecutive template. This idea was inspired by the previous work from Zhao et al., which reported that a 1000 C� -pyrolysis with a mesostructured assembly of Pluronic® F-127, phenol resin, and TiCl4 gave a highly mesoporous nanocomposite of carbon skeleton/ anatase TiO2 [27,28]. Even though Pluronic® F-127 decomposed in the 1000 C� -pyrolysis, the phenol resin could in-situ generate the carbon skeleton to retain the mesostructure. Their work was mainly focused on the mesoporous carbon skeleton/anatase TiO2 nanocomposite, but we expected that this method would also be suitable to obtain mesoporous crystalline metal oxides upon further removal of the carbon skeleton by air calcination at a high temperature. Based on this expectation, in this work, we prepared mesostructured ternary nanocomposites via the use of metal nitrates, commercial surfactants (i.e. Pluronic® F-127), and an additional carbon source (i.e., phloroglucinol/formaldehyde-derived resin). The nanocomposites were then pyrolyzed at high temperatures and further calcined in air to remove the templates. As a result, we were able to easily synthesize mesoporous materials built with various kinds of metal-oxide nanocrystallites (e.g., ZrO2, Y2O3, and CeO2) by only using commercially available and cheap reagents in one pot. Never theless, our mesoporous metal oxides showed high thermal stability,
large specific surface area/pore volume, and uniform pore size distri bution. Herein, we report the synthesis procedure as well as various synthetic factors affecting the product quality. In addition, we show the potential applicability of gold-supported mesoporous CeO2 to the water-gas-shift reaction. 2. Experimental 2.1. Reagents All reagents used in the present work were commercially available and used as received without any purifications. Pluronic® F-127 (EO106PO70EO106, average molecular weight: 12600 g mol 1), Plur onic® P-123 (EO20PO70EO20, average molecular weight: 5800 g mol 1), poly(ethylene glycol) methyl ether [CH3(OCH2CH2)nOH, average mo lecular weight: 550 g mol 1], Brij®-58 [HO(EO)20C16H33, average mo lecular weight: 1124 g mol 1], phloroglucinol (C6H6O3), cerium(III) nitrate hexahydrate [Ce(NO3)3⋅6H2O], zirconium(IV) oxynitrate hy drate [ZrO(NO3)2⋅x H2O], yttrium(III) nitrate hexahydrate [Y (NO3)3⋅6H2O], and gold(III) chloride trihydrate (HAuCl4⋅3H2O) were purchased from Sigma-Aldrich. Phenol (C6H5OH, 99%) and formalde hyde solution (HCHO, 36%) were purchased from Junsei. Nitric acid solution (HNO3, 35%) and ethanol (99.5%) were purchased from Sam chun Chemical. A commercial CeO2 support (99.9%) was used as received from Sigma-Aldrich. 2.2. One-pot synthesis procedure for mesoporous metal oxides The mesoporous metal oxide samples were synthesized using metal nitrates, surfactants, and phloroglucinol-formaldehyde. In a typical procedure using Pluronic® F-127, 10 mmol of phloroglucinol, 0.3 mmol of Pluronic® F-127, and 10 mmol of metal nitrate precursors [e.g. ZrO (NO3)2⋅xH2O, Ce(NO3)3⋅6(H2O) and Y(NO3)3⋅6H2O] were dissolved in 25 mL of ethanol at room temperature in a quartz beaker. After complete dissolution, 1.1 mmol of nitric acid was dropwise added into the ethanol solution under vigorous stirring. After continuous stirring for 0.5 h at room temperature, 15 mmol of formaldehyde was dropwise added into the solution. The final ethanol solution had a molar ratio of phlor oglucinol: metal nitrate: Pluronic® F-127: HCHO: HNO3: EtOH ¼ 10: 10: 0.3: 15: 1.1: 440. This solution was aged under magnetic stirring for 2 h at room temperature. The ethanol in the solution was then completely evaporated under air flow at 50 � C for 24 h and subsequently at 100 � C for 24 h. After the evaporation, the solution was converted into a brownish film. The dried film was pyrolyzed at 800 � C for 3 h (ramping rate 2.5 � C min 1) in a tubular furnace under a flow of nitrogen (flow rate ¼ 100 mL min 1, WHSV ¼ 10–14 mL g 1). The pyrolyzed product was calcined under air flow (flow rate ¼ 100 mL min 1, WHSV ¼ 30–42 mL g 1) at 450 � C for 3 h (a ramping rate of 1.4 � C min 1) to remove the organic templates. When the used surfactant was changed from Pluronic® F-127 to another surfactant, the surfactant weight used was fixed and the molar ratio for other reagents was kept. 2.3. Au-loading to CeO2 The gold was supported on ceria using a deposition-precipitation method with ammonium carbonate as the precipitation agent and HAuCl4⋅3H2O. In a typical procedure, 20 mL of distilled water contain ing 0.1 g of CeO2 powder was sonicated for 0.5 h. Then, 0.012 g of HAuCl4⋅3H2O was added at once to this suspension and aged under magnetic stirring for 0.1 h. After the agitation, typical pH was about 2.2. Then, 1 M (NH4)2CO3 aqueous solution was slowly dropped under vigorous stirring until the pH of the suspension reached 7. The pHadjusted suspension was then aged at 50 � C with vigorous stirring for 1 h, cooled to room temperature, and then centrifuged at 8000 rpm for 10 min. The precipitates were washed with distilled water twice. Sub sequently, the precipitates were dried overnight at room temperature 2
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under vacuum. The gold supported on the ceria samples were usually stored in a vacuum chamber.
fixed-bed, quartz reactor (inner diameter ¼ 6 mm) at atmospheric pressure. Prior to the loading the catalyst, the powder form of catalyst was shaped into particles with a size range of 0.10–0.43 mm. Then, 0.15 g of the shaped catalyst was homogeneously mixed with 0.35 g of acid purified SiO2 (0.10–0.43 mm). The catalyst containing mixture was loaded into the reactor and then catalytic reaction tests were started with a feeding of reaction gas flow (18 cm3 min 1) with a molar ratio of 8.3% CO/16.67% H2O/5.8% CO2/21.67% H2/47.5% Ar). The catalytic reaction test was performed as increasing the reaction temperature. The product gas was analysed using a mass spectrometer (Pfeiffer-Vacuum, Inc.). The converted CO was almost equal to the produced CO2 and no CH4 was yielded in the WGS tests (see Fig. S14). From this, the H2 production was assumed to be equal to the CO conversion.
2.4. Characterization Powder X-ray diffraction (PXRD) patterns were obtained using a Rigaku Multiflex diffractometer equipped with Cu Kα radiation (λ ¼ 0.1541 nm) at 40 kV and 30 mA. N2 adsorption-desorption iso therms were measured at liquid nitrogen temperature ( 196 � C) using a Micromeritics Tristar 3020 instrument. Prior to the sorption measure ment, samples were degassed for 3 h at 300 � C. The specific surface areas were calculated using Brunauer-Emmett-Teller (BET) equation from the adsorption data in the relative pressure of 0.05–0.20. The pore size distributions were derived from the adsorption branch using the BarrettJoyner-Halenda (BJH) method. The total pore volumes were calculated from the amount of nitrogen adsorbed at a relative pressure of 0.95. A Titan ETEM G2 operated at 300 kV was used to obtain the transmission electron microscopy (TEM) images. Scanning transmission electron microscopy (STEM) images were taken with a Titan Double Cs corrected TEM instrument with an accelerating voltage of 300 kV. The elemental compositions were determined using an ICP-AES with an OPTIMA 4300 DV instrument (PerkinElmer). A thermogravimetric analyser (Sinco TGA N-1000/1500) was used for the thermogravimetric analysis (TGA). Prior to the TEM and STEM analyses, the sample was sonicated as dispersed in a proper amount of acetone, and subsequently immobilized onto the holey carbon grid (Electron Microscopy Sciences, 300 mesh, copper) via sprinkling/drying. In a typical TGA experiment, 10 mg of the sample was placed on a platinum pan. The weight changes were monitored as the temperature was increased from room temperature to 800 � C under N2 or air flow with a rate of 30 cm3 min 1. X-ray photoelectron spectra (XPS) were obtained using an K-alpha XPS system (Thermo VG Scientific).
3. Results and discussion 3.1. One-pot synthesis of mesoporous metal oxides As shown in Scheme 1, our typical synthesis process involves three steps as follows: 1. Evaporation-induced self-assembly (EISA), 2. Py rolysis, 3. Template removal. In the EISA step, all the precursors for the metal oxide and templates were clearly dissolved in ethanol, and sub sequently solvents were evaporated to generate a mesostructured as sembly as guided by the surfactant. The pyrolysis step was then performed at 800 � C to convert the obtained mesostructured assembly into a mesostructured nanocomposite of nanocrystalline metal oxide/ carbon skeleton. Finally, in the template removal step, we removed the carbon skeleton by air calcination at 450 � C to yield template-free mesoporous metal oxides. In the following section, the detailed syn thesis results and principle will be discussed with one-pot synthesis for a mesoporous ZrO2 as a representative example. 3.1.1. Synthesis for mesoporous zirconia Fig. 1 shows PXRD patterns and TEM images for zirconia samples collected after each synthesis step described in Scheme 1. In this syn thesis, zirconium oxynitrate, Pluronic® F-127, and phloroglucinol/
2.5. Water-gas-shift reaction The water-gas-shift reaction was carried out in a continuous flow
Scheme 1. A schematic representation of one-pot synthesis process, which was performed in this work, for mesoporous crystalline metal oxides by using commercial surfactant, phenol resin, and metal nitrates. 3
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formaldehyde were used as a metal precursor, surfactant, and additional carbon source, respectively. After the EISA step, a mesostructured as sembly was obtained, as confirmed by a sharp peak at 0.6� in the lowangle PXRD (Fig. 1a). In addition, the zirconium-containing inorganic wall portion in this sample appeared to be amorphous, as confirmed by the absence of noticeable peaks in the wide-angle PXRD of Fig. 1b. When the pyrolysis step was performed on the mesostructured as sembly obtained by the EISA step, several peaks appeared in the wideangle PXRD, as shown in Fig. 1b. The wide-angle PXRD peaks were assigned to reflections corresponding to crystalline zirconia with a tetragonal structure and were quite broad, indicating crystallization of amorphous zirconium layers into nanocrystalline zirconia. Notably, the mode value of the sharp peak at 0.6� in the low-angle PXRD was shifted to 0.9� after the pyrolysis step, implying a decrease of d-spacing. This may be due the condensation and decomposition of organic content of Pluronic® F-127 and phenol resin generated by acid polymerization of phloroglucinol/formaldehyde. In good agreement, the sample upon the pyrolysis step showed a deep black colour. Moreover, the organic con tent was decreased from 80 to 64 wt% after the pyrolysis step, as confirmed by TGA experiments (Fig. S1). Notably, the organics in the sample after the pyrolysis started to be burnt at around 300 � C (see Fig. S1). Consequently, the pyrolysis step successfully generated a bi nary nanocomposite of carbon skeleton/nanocrystalline zirconia while retaining the mesostructure.
The carbon skeleton was completely removed by air calcination at 450 � C (see the TGA result in Fig. S1), while mostly retaining the mes ostructure of nanocrystalline zirconia walls. As shown in Fig. 1b, wideangle PXRD peaks became slightly sharper upon the template removal, indicating that sintering of zirconia nanocrystallites occurred to some degree. However, the template-free mesoporous zirconia still showed very small particle diameters of about 10 nm, as shown in TEM images of Figs. 1e and d. This was not much larger than the sizes before the template removal step (see a TEM image in Fig. S2). On the other hand, the low-angle PXRD peak was broadened and left-shifted after the template removal, indicating disordering and increase of d-spacing in the mesostructure. This may be also due to the slight sintering of zir conia walls and collapse of the mesostructure from the template removal. Nevertheless, our template-free zirconia product showed a high-quality porous textural property, as confirmed by N2 sorption re sults in Fig. 2. Our mesoporous zirconia showed high BET surface area and pore volume (138 m2 g 1 and 0.21 cm3 g 1, respectively). Further more, it was quite interesting that our mesoporous zirconia showed very uniform mesopore diameters centred at about 3 nm, as shown in Fig. 2b. In good agreement, uniformly sized mesopores were observed throughout the template-free zirconia sample in the low-magnification TEM image in Fig. 1c. Consequently, we were able to synthesize highquality mesoporous zirconia by using very cheap commercial pre cursors and a one-pot route. To understand the importance of the pyrolysis step in our zirconia synthesis process, we investigated the formation of mesoporous zirconia and product quality by varying the pyrolysis temperatures (0–900 � C) in the aforementioned representative zirconia synthesis route and the related results are summarized in Table 1. Notably, when the pyrolysis step was performed at 500 � C or omitted, the resulting nanocomposites had semi-crystalline and amorphous zirconia frameworks and thereby gave poor mesoporosity upon further template removal by air calcina tion at 450 � C due to the sintering of zirconia walls. When the pyrolysis steps were performed at temperatures from 600 to 800 � C, a meso structured nanocomposite of nanocrystalline zirconia and carbon skel eton was obtained, and the template removal by air calcination resulted in highly mesoporous zirconia samples. Within these zirconia samples, the BET surface area was increased with the higher pyrolysis tempera ture. This may be due to the higher pyrolysis temperature affording better rigidity to the in-situ generated carbon skeleton for retaining the mesostructure. When the pyrolysis step was performed at 900 � C, the resulting nanocomposite showed mixed phases of tetragonal and monoclinic zirconia structure. Upon further template removal by air calcination at 450 � C, the zirconia product was still a dark grey colour, indicating carbonaceous species remained to a significant degree. This may be due to excessively high pyrolysis temperature that could generate thermally stable graphitic carbon under the 450 � C-air calci nation. To clearly remove the remaining graphitic carbon, we recalcined the 900 � C-pyrolyzed sample at 600 � C under air flow but the resulting zirconia exhibited a very low BET surface area due to severe
Fig. 1. Low-angle (a) and wide-angle (b) PXRD results for zirconia samples, which were collected after the EISA (black line), pyrolysis (blue line), and template removal step (red line) in the synthesis for mesoporous ZrO2 by using Pluronic® F-127 and phenol resin as a soft-to-hard consecutive template. TEM images with low-(c) and high-magnification (d) for the template-free ZrO2 product. The synthesis gel composition was 10 phloroglucinol/0.3 zirconium oxynitrate/15 Pluronic® F-127/10 HCHO/1.1 HNO3/1440 EtOH. (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. N2 sorption isotherm (a), and pore size distribution (b) for the templatefree mesoporous ZrO2 sample. 4
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appropriate soft-to-hard consecutive hard template for the mesoporous zirconia.
Table 1 Physicochemical properties for ZrO2 products with varying temperature employed for the pyrolysis step. The ZrO2 synthesis started from a solution with a molar composition of 10 phloroglucinol/0.3 zirconium oxynitrate/15 Plur onic® F-127/10 HCHO/1.1 HNO3/1440 EtOH. Pyrolysis temperature (� C)
Zirconia phase after pyrolysisa
SBETb (m2g 1)
Vtotald (cm3g
Not processed 500 600 700 800 900e
amorphous amorphous þ tetragonal tetragonal tetragonal tetragonal tetragonal þ monoclinic
45 57 86 102 138 9e
0.03 0.18 0.07 0.10 0.21 0.06e
1
3.1.2. Thermal stability of mesoporous ZrO2 We investigated whether our optimized one-pot synthesis route could give enhanced thermal stability in mesoporous metal oxide products, compared to other representative synthesis methods. To confirm this, we evaluated the porous textural properties of our meso porous zirconia sample after heat treatments at 450 and 600 � C in air, in comparison to those of mesoporous zirconia samples synthesized by other representative soft- and hard-templating methods. To prepare a soft-templated zirconia counterpart, we synthesized mesoporous zirco nia by following the soft-templating method using Pluronic® P-123 re ported in a previous work by Stucky et al. [13]. In the case of the hard-templated counterpart, we referred to results from the literature by Kang et el., reporting mesoporous zirconia synthesized by a hard-templating method using CMK-3 ordered mesoporous carbon [21]. The synthesis of the hard-templated counterpart involved heat-treatment at 800 � C in N2 prior to the template removal, similar to the pyrolysis step of our procedure. Table 3 shows the results of thermal stability tests with various zir conia samples. Notably, when the heat treatment temperature was increased from 450 to 600 � C, the BET surface areas were decreased (entry 1 and 2 in Table 3). This implies that the 600 � C heat treatment caused sintering of nanocrystalline zirconia walls. Nevertheless, our zirconia sample with 450 � C heat treatment showed higher BET surface area (entry 2 in Table 3) even when compared with that (entry 4 in Table 3) of the soft-templated zirconia sample at 450 � C treatment temperature, indicating the much better thermal stability of our zirconia sample. In the case of the soft-templated zirconia, most of the porosity collapsed upon 600 � C heat treatment (entry 4 in Table 3). Furthermore, our zirconia sample with 600 � C heat treatment (entry 2 in Table 3) showed similar BET surface area to that of the CMK-3-templated zirconia sample (entry 5 in Table 3) treated at 550 � C. This indicated that our procedure was effective to obtain enhanced thermal stability on meso porous zirconia relative to soft-templating and was comparable to the hard-templating method.
)
a Judged by wide-angle PXRD measurements with ZrO2 sample just after the pyrolysis step. b SBET is the BET specific surface area for the 450 � C-air-calcined ZrO2 samples. d Vtot is the pore volume calculated at a relative pressure of 0.95 with the calcined ZrO2 samples. e In case of 900 � C pyrolysis temperature, the air calcination for template removal was performed at 450 � C.
sintering by the high heat from burning off the carbon. We confirmed that the pyrolysis step plays an important role in determining the template-free zirconia product. To obtain highly mesoporous zirconia with a phase-pure tetragonal structure, pyrolysis temperature of 800 � C was the most suitable. On the other hand, we also investigated whether Pluronic® F-127 and phloroglucinol could be replaced by other chemical compounds in the mesoporous zirconia synthesis. To confirm this, various surfactants of Pluronic® P-123, Brij®-72, and poly (ethylene glycol) ethyl ether were tested as a surfactant instead of Pluronic® F-127 in the one-pot synthesis of mesoporous zirconia. The BET surface area of the final zirconia product was highly dependent on the kind of surfactant (see entries 2–5 in Table 2). Notably, the most effective surfactant was Pluronic® F-127, amongst the tested surfactants. In addition, we tested phenol instead of phloroglucinol to generate phenol resin with formal dehyde. The results showed that phloroglucinol gave a better meso porous structure (see entry 2 and 6 in Table 2). Also, if the phenol resin precursor was excluded in the synthesis gel and solely a surfactant was used, the product showed poor BET surface area and pore volume (entry 1 in Table 2). The detailed synthesis and characterization results are described in Section. S3 of Supplementary data. Consequently, the combination of Pluronic® F-127 and phloroglucinol was the most
3.1.3. Extension to other mesoporous metal oxides Our synthetic strategy in Scheme 1 was readily extended to the synthesis of mesoporous metal oxides with various compositions such as Table 3 Porous textural properties of various mesoporous ZrO2 samples after heat treatment at 450–600 � C in air.
Table 2 Porous textural properties for mesoporous zirconia samples, which were syn thesized by using various surfactants and phenol resin precursors. In a synthesis using Pluronic® F-127 and phloroglucinol, the starting mixture composition was 10 phloroglucinol/0.3 zirconium oxynitrate/15 Pluronic® F-127/10 HCHO/1.1 HNO3/1440 EtOH. The synthesis procedure corresponded with Scheme 1. In the other synthesis, phloroglucinol and Pluronic® F-127 were replaced with other components while keeping the used weight. Entry
Phenol resin precursor
surfactant
SBETa (m2 g 1)
WBJHb (nm)
Vtotc (cm3 g 1)
1 2 3 4 5
Not used phloroglucinol phloroglucinol phloroglucinol phloroglucinol
50 138 80 76 107
3 3 3.2 2.3 5.7
0.04 0.21 0.09 0.06 0.19
6
phenol
Pluronic® F-127 Pluronic® F-127 Pluronic® P-123 Brij-72 Poly (ethylene glycol) methyl ester Pluronic® F-127
116
13
0.42
Entry
Templating method
Used templates
Treatment Temperature (� C)
SBETa (m2 g 1)
Vtotb (cm3 g 1)
1
Soft-to-hardc
450
138
0.21
2
Soft-to-hardc
600
72
0.25
3
Soft templating Soft templating Hard templating
Pluronic® F127 /phenol resin Pluronic® F127 /phenol resin Pluronic® P123 Pluronic® P123 CMK-3
450
60
0.43
600
21
0.07
550
74
0.21
4 5d a
SBET is the apparent BET specific surface area. WBJH is a mode value of mesopore diameter derived from the adsorption branch using the BJH method. c Templating method performed in this work using Pluronic® F-127 and phloroglucinol. d All results in this entry was from Ref. [21]. b
a
SBET is the apparent BET specific surface area. WBJH is a mode value of mesopore diameter derived from the adsorption branch using the BJH method. c Vtot is the pore volume calculated at the relative pressure of 0.95. b
5
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CeO2, Y2O3, and ZrCeOx (mixed bimetallic oxides). Except for the kind of metal nitrates that were used, the same synthesis procedures as for the mesoporous zirconia in Section 3.1.1 were employed for these various metal oxides. As shown in the PXRD results of Fig. 3a, all the samples with various framework compositions showed nanocrystalline metal oxides. In good agreement, high resolution TEM images indicated that all the samples were composed of agglomerated metal oxide nano crystallites. Notably, in the case of ZrCeOx, the high resolution STEMEDS mapping (Fig. S9) displayed that two metal oxide components were homogeneously mixed through the entire particles, and the PXRD patterns (Fig. 3) indicated that the mixed metal oxide had a phase-pure cubic structure. Based on the EDS-STEM and PXRD results, the meso porous ZrCeOx seemed to be a solid solution. The porous textural properties of these various mesoporous metal oxide samples were also investigated by N2 sorption analysis (see N2 sorption isotherms, pore size distribution in Fig. S10, and result sum maries in Table S1). All three samples had a uniform mesopore size distribution, indicating effective mesopore generation by our soft-tohard consecutive templates. Moreover, these samples showed quite high BET surface area and pore volume. In particular, the mesoporous ceria synthesized in this work showed markedly high BET surface area even though the ceria is a well-known oxide that readily undergoes the thermal sintering process. In conclusion, our one-pot synthetic strategy was very effective to prepare various mesoporous metal oxides with nanocrystalline frameworks.
applicable to practical catalytic applications. Among the various kinds of metal oxides synthesized in this work and industrially important catalytic reactions, we decided to show the advantages of mesoporous CeO2 as a supporting material for Au catalysts in the water-gas shift (WGS) reaction, because Au-supported CeO2 catalysts are known to exhibit high catalytic performance. In addition, for the WGS reaction thermally stable supports are preferable in terms of avoiding sintering by exothermic reaction heat [29]. For the WGS reaction tests, we supported about 5 wt% Au via a deposition-precipitation technique on the mesoporous CeO2, which was synthesized by our one-pot procedure described in Scheme 1. The resulting supported-Au catalyst is denoted by ‘Au/mesoporous-CeO2’. For comparison, a ceria nanoparticle agglomerate, which was prepared by a procedure reported in another work using urea as a precipitation agent [30], was supported with 5 wt% Au in the same manner. This sample is denoted by ‘Au/nano-CeO2’. In addition, a commercially available ceria was also supported with 5 wt% Au, and the resulting catalyst is denoted by ‘Au/bulk-CeO2’. Fig. 4 shows representative STEM images for the three CeO2-sup ported catalysts, Au/mesoporous-CeO2, Au/bulk-CeO2 and Au/nanoCeO2. As shown in the low-magnification STEM images in Fig. 4a, b, and c, Au nanoparticle sizes appear to be similarly distributed in a range from 0.5 to 2.0 nm in all three Au/CeO2 samples. In the high resolution STEM images, however, we detected extremely tiny Au species smaller than 0.5 nm, which could be considered a single atomic species, in the Au/mesoporous-CeO2 sample (see STEM image of Fig. 4d). Except for the Au/mesoporous-CeO2, it was difficult to find Au species that could be considered a single atom. However, when the volume-weighted Au particle size distribution was obtained from these STEM investigations, the Au nanoparticle dispersion appeared to be comparable in the three
3.2. Water-gas shift reaction over Au/mesoporous-CeO2 In this section, we demonstrate that the one-pot synthesized meso porous metal oxides described in the previous sections would be highly
Fig. 3. PXRD patterns (a) for mesoporous CeO2 (black line), Y2O3 (blue line), ZrCeOx (red line), synthesized by using Pluronic® F-127 and phenol resin as a soft-tohard consecutive template. High resolution TEM images for CeO2 (b) Y2O3 (c) and ZrCeOx (d). Insets in each TEM image are FFT images for the red-squared region. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 6
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Fig. 4. STEM images for (a, d) Au/mesoporous-CeO2, (b, e) Au/bulk-CeO2, (c, f) Au/nano-CeO2. White circles in STEM images indicate a gold species, which seemed to be a single atom.
Au/CeO2 samples, except that the single atomic Au species was detected only in the Au/mesoporous CeO2. This contrary result might come from the unsatisfactory resolution of the high resolution STEM investigation that did not allow detection of all of the single atoms from the ceria matrix. To clarify the presence of single atomic Au species, we obtained XP spectra of the Au4f for the Au/CeO2 samples. In previous studies, the Au1þ species in Au/CeO2, observed via XPS measurement, are typically regarded as a single atomic Au species stabilized by a ceria support [31]. As shown in Fig. S13, it was noteworthy that the atomic fraction of Au1þ increased in the order of Au/bulk-CeO2 < Au/nano-CeO2 < Au/meso porous-CeO2. Therefore, we could conclude that the Au/mesopor ous-CeO2 sample had a relatively greater fraction of single atomic Au1þ species compared to that of the other Au/CeO2 samples. This could be attributed to the high BET surface area of the mesoporous CeO2 sample (see Table S2), which would be beneficial to accommodate a more efficient metal-support interaction. Fig. 5 shows catalytic results for the water-gas-shift reaction over the three aforementioned supported Au-ceria catalysts. The water-gas-shift reaction converts carbon monoxide and water in the reformate stream into carbon dioxide and hydrogen. This reaction is intended to be used for hydrogen enrichment and CO reduction of the reformate gas for hydrogen production. Fig. 5 displays the conversion of CO as a function of the reaction temperature over the three supported gold catalysts. As the results show, the catalytic CO conversion at similar reaction tem peratures decreases in the following order: Au/mesoporous-CeO2 > Au/ nano-CeO2 > Au/bulk-CeO2. The Au/mesoporous-CeO2 sample exhibi ted 15% CO conversion at 150 � C and the CO conversion continuously increased up to 47% at 360 � C. On the other hand, Au/nano-CeO2 and Au/bulk-CeO2 showed very low levels of CO conversion over the entire temperature range. The CO conversion at 150 � C was only 4% for Au/ nano-CeO2 and 1% for Au/bulk-CeO2, which were four times and 15 times lower, respectively, than the CO conversion achieved by Au/ mesoporous-CeO2 at 150 � C. Since it is often reported that atomically dispersed Au1þ on the ceria could exhibit distinctively high intrinsic activity for the WGS reaction, the high catalytic activity of Au/meso porous-CeO2 might be attributable to the larger number of Au1þ species, which was confirmed in the XPS results (Fig. S13). Furthermore, the thermally stable ceria support in the Au/mesoporous CeO2 could be beneficial to obtain high CO conversion even at the high reaction tem perature region. In addition, high oxygen-storage capacity for the ceria
Fig. 5. CO conversion in the water-gas-shift reaction over Au/mesoporous CeO2 (●), Au/bulk-CeO2 (▴) and Au/nano-CeO2 (■), plotted as a function of the reaction temperature. (Reaction condition: the flow rate of the reaction CO2/21.67% H2/47.5% mixture (8.3% CO/16.67% H2O/5.8% Ar) ¼ 18 cm3 min 1, catalyst ¼ 150 mg, and reaction pressure ¼ 1 bar).
support in the Au/mesoporous CeO2 sample might be another reason for the high water-gas-shift activity. It is known that, as the particle size of ceria decreases, the oxygen storage capacity is enhanced, resulting in an increase of the water-gas-shift activity by providing oxygen more readily. 4. Conclusions We one-pot synthesized mesoporous ZrO2, Y2O3, CeO2, and ZrCeOx by using only a commercially available inexpensive organic surfactant, carbon source, and metal precursors. To this end, the pyrolysis step was necessary to in-situ generate carbon skeletons to prevent collapse of the mesostructure during the crystallization of the metal precursors. The mesoporous ZrO2 synthesized in this manner showed an enhanced thermal stability compared to the representative mesoporous ZrO2 prepared by hydrothermal synthesis using soft templates, and was comparable to that of CMK-3-replicated mesoporous ZrO2 with a 7
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crystalline framework. Furthermore, we demonstrated that goldsupported mesoporous CeO2, synthesized in this work exhibited signif icantly high catalytic CO conversion through a wide range of reaction temperature in the water-gas-shift reaction. We believe that our results suggest a facile and valuable synthesis route that can yield thermally stable mesoporous metal oxides with a crystalline framework.
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Acknowledgements This work was supported by the Institute for Basic Science, Daejeon, South Korea [IBS-R004]. Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi .org/10.1016/j.micromeso.2019.109767. References [1] F. Schüth, Chem. Mater. 13 (2001) 3184–3195. http://doi.org/10.1021/ cm011030j. [2] J.N. Kondo, K. Domen, Chem. Mater. 20 (2008) 835–847. http://doi.org/10.1021/ cm702176m. [3] D. Wang, Z. Ma, S. Dai, J. Liu, Z. Nie, M.H. Engelhard, Q. Huo, C. Wang, R. Kou, J. Phys. Chem. C 112 (2008) 13499–13509. http://doi.org/10.1021/jp804250f. [4] L. Qi, J. Ma, H. Cheng, Z. Zhao, Langmuir 14 (1998) 2579–2581. http://doi.org/ 10.1021/la971077c. [5] E.L. Crepaldi, G.J. de A.A. Soler-Illia, A. Bouchara, D. Grosso, D. Durand, C. Sanchez, Angew. Chem. Int. Ed. 42 (2003) 347–351. http://doi.org/10. 1002/anie.200390113. [6] D. Li, H. Zhou, I. Honma, Nat. Mater. 3 (2004) 65–72. http://doi.org/10.1038 /nmat1043. [7] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T. W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834–10843. http://doi.org/10.1021/ja00053a 020. [8] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024–6036. http://doi.org/10.1021/ja974025i. [9] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710–712. http://doi.org/10.1038/359710a0.
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Microporous and Mesoporous Materials journal homepage: http://www.elsevier.com/locate/micromeso
Soft-to-hard consecutive templating one-pot route from metal nitrate/ phenol resin/surfactant to mesoporous metal oxides with enhanced thermal stability Su-Kyung Lee a, b, Changbum Jo c, Jaeheon Kim d, **, Ryong Ryoo a, d, * a
Department of Chemistry, Korea Advanced Institute for Science and Technology (KAIST), Daejeon, 34141, Republic of Korea Research Group of Green Carbon Catalysis, Korea Research Institute of Chemical Technology(KRICT), 141 Gajeong-Ro, Yuseong, Daejeon, 305-600, Republic of Korea Department of Chemistry and Chemical Engineering, Inha University, Incheon, 22212, Republic of Korea d Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon, 34141, Republic of Korea b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Mesoporous metal oxides Nanocrystalline Surfactant Gold-supported ceria Water-gas shift reaction
Mesoporous materials with crystalline ZrO2, Y2O3, and CeO2 frameworks were one-pot synthesized from a clear solution of ethanol containing metal nitrates, organic surfactant (i.e., Pluronic® F-127), formaldehyde, and phloroglucinol. The solution was converted to a mesostructured nanocomposite of metal nitrate/phenol resin/ surfactant via a solvent evaporation-induced self-assembly process. The obtained nanocomposite was calcined at 800 � C in a N2 atmosphere. X-ray powder diffraction and electron microscopic investigation revealed that the calcination caused amorphous-to-crystalline transformations in the metal oxide frameworks, while sintering to bulk metal oxides was prevented by the in-situ generated carbon component. Mesoporous metal oxides with a crystalline framework were obtained when the carbon skeleton was burnt off. The mesoporous metal oxides exhibited high BET surface areas, narrow pore-size distributions, and enhanced thermal stability. A practical benefit of the mesoporous metal oxides was demonstrated with Au/CeO2 exhibiting high catalytic activity in the water-gas-shift reaction.
1. Introduction There have been many efforts to synthesize mesoporous metal oxides possessing high specific surface area, pore volume, and stability, due to their applicability to catalysis, sensing, electronic devices, and photo voltaic solar cells [1–6]. A common bottom-up method to synthesize mesoporous metal oxides is to use surfactant micelles as a soft template, which is well-known as an effective route to mesoporous silicas such as MCM-41 and SBA-15 [7–9]. In the soft-templating synthesis for meso porous silica, the hydrothermal synthesis gives a mesostructured as sembly of surfactant micelles and amorphous silica layers. The amorphous silica layers are self-retained as a stable wall to support mesopores, i.e., a mesopore wall, during the removal of the surfactant by calcination at 450 � C, and even upon further heating to 800 � C. In the case of the surfactant-templated synthesis for most other metal oxides (e. g. TiO2, ZrO2, etc.), amorphous walls are also generated at the surfaces surrounding the surfactant micelles. However, these metal oxide walls
are normally unstable under the high-temperature calcination condi tions used to remove the surfactant template [10–16]. Most metal oxides have a tendency to crystallize, which often causes undesirable sintering into large particles and consequent loss of mesoporosity. There are particular cases of successful removal of surfactants without pore collapse via low-temperature calcination or a solvent extraction process [17]. Even in these cases, however, the metal oxide frameworks often suffer from amorphous-to-crystal or crystal-to-crystal phase trans formations at higher temperatures. Therefore, this soft-templating is inappropriate to yield thermally stable mesoporous materials for most metal oxides. The use of hard templates (e.g., CMK-type ordered mesoporous car bons, and amorphous mesoporous silicas) is another common bottom-up synthesis method for mesoporous metal oxides, and it can result in enhanced thermal stability in comparison to the aforementioned softtemplating method [18–23]. A typical hard templating route involves metal precursor infiltration, conversion of loaded-metal precursors into
* Corresponding author. Department of Chemistry, KAIST, Daejeon, 34141, Republic of Korea. ** Corresponding author. E-mail addresses: [email protected] (J. Kim), [email protected] (R. Ryoo). https://doi.org/10.1016/j.micromeso.2019.109767 Received 15 May 2019; Received in revised form 19 September 2019; Accepted 25 September 2019 Available online 30 September 2019 1387-1811/© 2019 Published by Elsevier Inc.
Please cite this article as: Su-Kyung Lee, Microporous and Mesoporous Materials, https://doi.org/10.1016/j.micromeso.2019.109767
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metal oxides, and template removal by air calcination or chemical etching. Prior to the template removal, the infiltrated metal precursors are converted into metal oxides via a heat treatment in air or inert gas. When the heat treatment temperature is above a certain high tempera ture, the infiltrated metal-oxide species undergoes crystallization. Nevertheless, the mesostructure can be retained because the hard tem plates are thermally stable and rigid enough even at the high tempera ture [21–23]. Therefore, mesoporous metal oxides with a crystalline framework can be obtained by hard-templating in this manner, and they show superior thermal stability to amorphous or semi-crystalline metal oxides hydrothermally synthesized by soft templating. However, the hard-templating route has a significant drawback in the pre-synthesis of hard templates, which is inefficient and tedious. Recently, considerable efforts have been devoted to developing a synthesis route that can give mesoporous metal oxides without presynthesized hard templates [2,24–26]. In this regard, Wiesner et al. synthesized mesoporous metal oxides by using a specially designed block copolymer as a soft-to-hard consecutive template [24]. The block copolymer was composed of hydrophilic poly (ethylene oxide) and hy drophobic poly (isoprene) blocks. The block-copolymers were readily assembled with metal precursors into a mesostructured organic-inorganic nanocomposite, similar to soft-templating. Upon further high temperature treatment under inert gas, however, the soft template of the block copolymer in-situ generated a carbon skeleton, which can be regarded as a hard template, via a pyrolysis process of the isoprene block in the block copolymer. Simultaneously, the metal con tent was also converted into nanocrystalline metal oxides, while retaining the entire mesostructure supported by the in-situ generated carbon skeleton. Finally, mesoporous metal oxides with a crystalline framework could be obtained after removal of the carbon skeleton by air calcination. The mesoporous crystalline metal oxides prepared by this soft-to-hard consecutive templating showed high thermal stability, comparable to the case of the hard templating method. Accordingly, various mesoporous metal oxides with nanocrystalline frameworks were synthesized in a similar manner [25,26]. Nevertheless, this route also had a similar problem to the hard-templating method, where the specially designed block-copolymer must be pre-synthesized via difficult anionic polymerization [24]. This was because the commonly used and commercially available surfactants (e.g., Pluronic® F-127, P-123, etc.) are not suitable for this method in terms of providing satisfactory amounts of pyrolyzable organic content as they usually decompose under the pyrolysis conditions due to high oxygen content. We undertook this work to establish a facile synthesis route to mesoporous metal oxides via the aforementioned soft-to-hard consecu tive templating even without the pre-synthesis of any templates. To this end, we used a combination of commercial surfactant and phenol resin as the soft-to-hard consecutive template. This idea was inspired by the previous work from Zhao et al., which reported that a 1000 C� -pyrolysis with a mesostructured assembly of Pluronic® F-127, phenol resin, and TiCl4 gave a highly mesoporous nanocomposite of carbon skeleton/ anatase TiO2 [27,28]. Even though Pluronic® F-127 decomposed in the 1000 C� -pyrolysis, the phenol resin could in-situ generate the carbon skeleton to retain the mesostructure. Their work was mainly focused on the mesoporous carbon skeleton/anatase TiO2 nanocomposite, but we expected that this method would also be suitable to obtain mesoporous crystalline metal oxides upon further removal of the carbon skeleton by air calcination at a high temperature. Based on this expectation, in this work, we prepared mesostructured ternary nanocomposites via the use of metal nitrates, commercial surfactants (i.e. Pluronic® F-127), and an additional carbon source (i.e., phloroglucinol/formaldehyde-derived resin). The nanocomposites were then pyrolyzed at high temperatures and further calcined in air to remove the templates. As a result, we were able to easily synthesize mesoporous materials built with various kinds of metal-oxide nanocrystallites (e.g., ZrO2, Y2O3, and CeO2) by only using commercially available and cheap reagents in one pot. Never theless, our mesoporous metal oxides showed high thermal stability,
large specific surface area/pore volume, and uniform pore size distri bution. Herein, we report the synthesis procedure as well as various synthetic factors affecting the product quality. In addition, we show the potential applicability of gold-supported mesoporous CeO2 to the water-gas-shift reaction. 2. Experimental 2.1. Reagents All reagents used in the present work were commercially available and used as received without any purifications. Pluronic® F-127 (EO106PO70EO106, average molecular weight: 12600 g mol 1), Plur onic® P-123 (EO20PO70EO20, average molecular weight: 5800 g mol 1), poly(ethylene glycol) methyl ether [CH3(OCH2CH2)nOH, average mo lecular weight: 550 g mol 1], Brij®-58 [HO(EO)20C16H33, average mo lecular weight: 1124 g mol 1], phloroglucinol (C6H6O3), cerium(III) nitrate hexahydrate [Ce(NO3)3⋅6H2O], zirconium(IV) oxynitrate hy drate [ZrO(NO3)2⋅x H2O], yttrium(III) nitrate hexahydrate [Y (NO3)3⋅6H2O], and gold(III) chloride trihydrate (HAuCl4⋅3H2O) were purchased from Sigma-Aldrich. Phenol (C6H5OH, 99%) and formalde hyde solution (HCHO, 36%) were purchased from Junsei. Nitric acid solution (HNO3, 35%) and ethanol (99.5%) were purchased from Sam chun Chemical. A commercial CeO2 support (99.9%) was used as received from Sigma-Aldrich. 2.2. One-pot synthesis procedure for mesoporous metal oxides The mesoporous metal oxide samples were synthesized using metal nitrates, surfactants, and phloroglucinol-formaldehyde. In a typical procedure using Pluronic® F-127, 10 mmol of phloroglucinol, 0.3 mmol of Pluronic® F-127, and 10 mmol of metal nitrate precursors [e.g. ZrO (NO3)2⋅xH2O, Ce(NO3)3⋅6(H2O) and Y(NO3)3⋅6H2O] were dissolved in 25 mL of ethanol at room temperature in a quartz beaker. After complete dissolution, 1.1 mmol of nitric acid was dropwise added into the ethanol solution under vigorous stirring. After continuous stirring for 0.5 h at room temperature, 15 mmol of formaldehyde was dropwise added into the solution. The final ethanol solution had a molar ratio of phlor oglucinol: metal nitrate: Pluronic® F-127: HCHO: HNO3: EtOH ¼ 10: 10: 0.3: 15: 1.1: 440. This solution was aged under magnetic stirring for 2 h at room temperature. The ethanol in the solution was then completely evaporated under air flow at 50 � C for 24 h and subsequently at 100 � C for 24 h. After the evaporation, the solution was converted into a brownish film. The dried film was pyrolyzed at 800 � C for 3 h (ramping rate 2.5 � C min 1) in a tubular furnace under a flow of nitrogen (flow rate ¼ 100 mL min 1, WHSV ¼ 10–14 mL g 1). The pyrolyzed product was calcined under air flow (flow rate ¼ 100 mL min 1, WHSV ¼ 30–42 mL g 1) at 450 � C for 3 h (a ramping rate of 1.4 � C min 1) to remove the organic templates. When the used surfactant was changed from Pluronic® F-127 to another surfactant, the surfactant weight used was fixed and the molar ratio for other reagents was kept. 2.3. Au-loading to CeO2 The gold was supported on ceria using a deposition-precipitation method with ammonium carbonate as the precipitation agent and HAuCl4⋅3H2O. In a typical procedure, 20 mL of distilled water contain ing 0.1 g of CeO2 powder was sonicated for 0.5 h. Then, 0.012 g of HAuCl4⋅3H2O was added at once to this suspension and aged under magnetic stirring for 0.1 h. After the agitation, typical pH was about 2.2. Then, 1 M (NH4)2CO3 aqueous solution was slowly dropped under vigorous stirring until the pH of the suspension reached 7. The pHadjusted suspension was then aged at 50 � C with vigorous stirring for 1 h, cooled to room temperature, and then centrifuged at 8000 rpm for 10 min. The precipitates were washed with distilled water twice. Sub sequently, the precipitates were dried overnight at room temperature 2
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under vacuum. The gold supported on the ceria samples were usually stored in a vacuum chamber.
fixed-bed, quartz reactor (inner diameter ¼ 6 mm) at atmospheric pressure. Prior to the loading the catalyst, the powder form of catalyst was shaped into particles with a size range of 0.10–0.43 mm. Then, 0.15 g of the shaped catalyst was homogeneously mixed with 0.35 g of acid purified SiO2 (0.10–0.43 mm). The catalyst containing mixture was loaded into the reactor and then catalytic reaction tests were started with a feeding of reaction gas flow (18 cm3 min 1) with a molar ratio of 8.3% CO/16.67% H2O/5.8% CO2/21.67% H2/47.5% Ar). The catalytic reaction test was performed as increasing the reaction temperature. The product gas was analysed using a mass spectrometer (Pfeiffer-Vacuum, Inc.). The converted CO was almost equal to the produced CO2 and no CH4 was yielded in the WGS tests (see Fig. S14). From this, the H2 production was assumed to be equal to the CO conversion.
2.4. Characterization Powder X-ray diffraction (PXRD) patterns were obtained using a Rigaku Multiflex diffractometer equipped with Cu Kα radiation (λ ¼ 0.1541 nm) at 40 kV and 30 mA. N2 adsorption-desorption iso therms were measured at liquid nitrogen temperature ( 196 � C) using a Micromeritics Tristar 3020 instrument. Prior to the sorption measure ment, samples were degassed for 3 h at 300 � C. The specific surface areas were calculated using Brunauer-Emmett-Teller (BET) equation from the adsorption data in the relative pressure of 0.05–0.20. The pore size distributions were derived from the adsorption branch using the BarrettJoyner-Halenda (BJH) method. The total pore volumes were calculated from the amount of nitrogen adsorbed at a relative pressure of 0.95. A Titan ETEM G2 operated at 300 kV was used to obtain the transmission electron microscopy (TEM) images. Scanning transmission electron microscopy (STEM) images were taken with a Titan Double Cs corrected TEM instrument with an accelerating voltage of 300 kV. The elemental compositions were determined using an ICP-AES with an OPTIMA 4300 DV instrument (PerkinElmer). A thermogravimetric analyser (Sinco TGA N-1000/1500) was used for the thermogravimetric analysis (TGA). Prior to the TEM and STEM analyses, the sample was sonicated as dispersed in a proper amount of acetone, and subsequently immobilized onto the holey carbon grid (Electron Microscopy Sciences, 300 mesh, copper) via sprinkling/drying. In a typical TGA experiment, 10 mg of the sample was placed on a platinum pan. The weight changes were monitored as the temperature was increased from room temperature to 800 � C under N2 or air flow with a rate of 30 cm3 min 1. X-ray photoelectron spectra (XPS) were obtained using an K-alpha XPS system (Thermo VG Scientific).
3. Results and discussion 3.1. One-pot synthesis of mesoporous metal oxides As shown in Scheme 1, our typical synthesis process involves three steps as follows: 1. Evaporation-induced self-assembly (EISA), 2. Py rolysis, 3. Template removal. In the EISA step, all the precursors for the metal oxide and templates were clearly dissolved in ethanol, and sub sequently solvents were evaporated to generate a mesostructured as sembly as guided by the surfactant. The pyrolysis step was then performed at 800 � C to convert the obtained mesostructured assembly into a mesostructured nanocomposite of nanocrystalline metal oxide/ carbon skeleton. Finally, in the template removal step, we removed the carbon skeleton by air calcination at 450 � C to yield template-free mesoporous metal oxides. In the following section, the detailed syn thesis results and principle will be discussed with one-pot synthesis for a mesoporous ZrO2 as a representative example. 3.1.1. Synthesis for mesoporous zirconia Fig. 1 shows PXRD patterns and TEM images for zirconia samples collected after each synthesis step described in Scheme 1. In this syn thesis, zirconium oxynitrate, Pluronic® F-127, and phloroglucinol/
2.5. Water-gas-shift reaction The water-gas-shift reaction was carried out in a continuous flow
Scheme 1. A schematic representation of one-pot synthesis process, which was performed in this work, for mesoporous crystalline metal oxides by using commercial surfactant, phenol resin, and metal nitrates. 3
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formaldehyde were used as a metal precursor, surfactant, and additional carbon source, respectively. After the EISA step, a mesostructured as sembly was obtained, as confirmed by a sharp peak at 0.6� in the lowangle PXRD (Fig. 1a). In addition, the zirconium-containing inorganic wall portion in this sample appeared to be amorphous, as confirmed by the absence of noticeable peaks in the wide-angle PXRD of Fig. 1b. When the pyrolysis step was performed on the mesostructured as sembly obtained by the EISA step, several peaks appeared in the wideangle PXRD, as shown in Fig. 1b. The wide-angle PXRD peaks were assigned to reflections corresponding to crystalline zirconia with a tetragonal structure and were quite broad, indicating crystallization of amorphous zirconium layers into nanocrystalline zirconia. Notably, the mode value of the sharp peak at 0.6� in the low-angle PXRD was shifted to 0.9� after the pyrolysis step, implying a decrease of d-spacing. This may be due the condensation and decomposition of organic content of Pluronic® F-127 and phenol resin generated by acid polymerization of phloroglucinol/formaldehyde. In good agreement, the sample upon the pyrolysis step showed a deep black colour. Moreover, the organic con tent was decreased from 80 to 64 wt% after the pyrolysis step, as confirmed by TGA experiments (Fig. S1). Notably, the organics in the sample after the pyrolysis started to be burnt at around 300 � C (see Fig. S1). Consequently, the pyrolysis step successfully generated a bi nary nanocomposite of carbon skeleton/nanocrystalline zirconia while retaining the mesostructure.
The carbon skeleton was completely removed by air calcination at 450 � C (see the TGA result in Fig. S1), while mostly retaining the mes ostructure of nanocrystalline zirconia walls. As shown in Fig. 1b, wideangle PXRD peaks became slightly sharper upon the template removal, indicating that sintering of zirconia nanocrystallites occurred to some degree. However, the template-free mesoporous zirconia still showed very small particle diameters of about 10 nm, as shown in TEM images of Figs. 1e and d. This was not much larger than the sizes before the template removal step (see a TEM image in Fig. S2). On the other hand, the low-angle PXRD peak was broadened and left-shifted after the template removal, indicating disordering and increase of d-spacing in the mesostructure. This may be also due to the slight sintering of zir conia walls and collapse of the mesostructure from the template removal. Nevertheless, our template-free zirconia product showed a high-quality porous textural property, as confirmed by N2 sorption re sults in Fig. 2. Our mesoporous zirconia showed high BET surface area and pore volume (138 m2 g 1 and 0.21 cm3 g 1, respectively). Further more, it was quite interesting that our mesoporous zirconia showed very uniform mesopore diameters centred at about 3 nm, as shown in Fig. 2b. In good agreement, uniformly sized mesopores were observed throughout the template-free zirconia sample in the low-magnification TEM image in Fig. 1c. Consequently, we were able to synthesize highquality mesoporous zirconia by using very cheap commercial pre cursors and a one-pot route. To understand the importance of the pyrolysis step in our zirconia synthesis process, we investigated the formation of mesoporous zirconia and product quality by varying the pyrolysis temperatures (0–900 � C) in the aforementioned representative zirconia synthesis route and the related results are summarized in Table 1. Notably, when the pyrolysis step was performed at 500 � C or omitted, the resulting nanocomposites had semi-crystalline and amorphous zirconia frameworks and thereby gave poor mesoporosity upon further template removal by air calcina tion at 450 � C due to the sintering of zirconia walls. When the pyrolysis steps were performed at temperatures from 600 to 800 � C, a meso structured nanocomposite of nanocrystalline zirconia and carbon skel eton was obtained, and the template removal by air calcination resulted in highly mesoporous zirconia samples. Within these zirconia samples, the BET surface area was increased with the higher pyrolysis tempera ture. This may be due to the higher pyrolysis temperature affording better rigidity to the in-situ generated carbon skeleton for retaining the mesostructure. When the pyrolysis step was performed at 900 � C, the resulting nanocomposite showed mixed phases of tetragonal and monoclinic zirconia structure. Upon further template removal by air calcination at 450 � C, the zirconia product was still a dark grey colour, indicating carbonaceous species remained to a significant degree. This may be due to excessively high pyrolysis temperature that could generate thermally stable graphitic carbon under the 450 � C-air calci nation. To clearly remove the remaining graphitic carbon, we recalcined the 900 � C-pyrolyzed sample at 600 � C under air flow but the resulting zirconia exhibited a very low BET surface area due to severe
Fig. 1. Low-angle (a) and wide-angle (b) PXRD results for zirconia samples, which were collected after the EISA (black line), pyrolysis (blue line), and template removal step (red line) in the synthesis for mesoporous ZrO2 by using Pluronic® F-127 and phenol resin as a soft-to-hard consecutive template. TEM images with low-(c) and high-magnification (d) for the template-free ZrO2 product. The synthesis gel composition was 10 phloroglucinol/0.3 zirconium oxynitrate/15 Pluronic® F-127/10 HCHO/1.1 HNO3/1440 EtOH. (For inter pretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 2. N2 sorption isotherm (a), and pore size distribution (b) for the templatefree mesoporous ZrO2 sample. 4
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appropriate soft-to-hard consecutive hard template for the mesoporous zirconia.
Table 1 Physicochemical properties for ZrO2 products with varying temperature employed for the pyrolysis step. The ZrO2 synthesis started from a solution with a molar composition of 10 phloroglucinol/0.3 zirconium oxynitrate/15 Plur onic® F-127/10 HCHO/1.1 HNO3/1440 EtOH. Pyrolysis temperature (� C)
Zirconia phase after pyrolysisa
SBETb (m2g 1)
Vtotald (cm3g
Not processed 500 600 700 800 900e
amorphous amorphous þ tetragonal tetragonal tetragonal tetragonal tetragonal þ monoclinic
45 57 86 102 138 9e
0.03 0.18 0.07 0.10 0.21 0.06e
1
3.1.2. Thermal stability of mesoporous ZrO2 We investigated whether our optimized one-pot synthesis route could give enhanced thermal stability in mesoporous metal oxide products, compared to other representative synthesis methods. To confirm this, we evaluated the porous textural properties of our meso porous zirconia sample after heat treatments at 450 and 600 � C in air, in comparison to those of mesoporous zirconia samples synthesized by other representative soft- and hard-templating methods. To prepare a soft-templated zirconia counterpart, we synthesized mesoporous zirco nia by following the soft-templating method using Pluronic® P-123 re ported in a previous work by Stucky et al. [13]. In the case of the hard-templated counterpart, we referred to results from the literature by Kang et el., reporting mesoporous zirconia synthesized by a hard-templating method using CMK-3 ordered mesoporous carbon [21]. The synthesis of the hard-templated counterpart involved heat-treatment at 800 � C in N2 prior to the template removal, similar to the pyrolysis step of our procedure. Table 3 shows the results of thermal stability tests with various zir conia samples. Notably, when the heat treatment temperature was increased from 450 to 600 � C, the BET surface areas were decreased (entry 1 and 2 in Table 3). This implies that the 600 � C heat treatment caused sintering of nanocrystalline zirconia walls. Nevertheless, our zirconia sample with 450 � C heat treatment showed higher BET surface area (entry 2 in Table 3) even when compared with that (entry 4 in Table 3) of the soft-templated zirconia sample at 450 � C treatment temperature, indicating the much better thermal stability of our zirconia sample. In the case of the soft-templated zirconia, most of the porosity collapsed upon 600 � C heat treatment (entry 4 in Table 3). Furthermore, our zirconia sample with 600 � C heat treatment (entry 2 in Table 3) showed similar BET surface area to that of the CMK-3-templated zirconia sample (entry 5 in Table 3) treated at 550 � C. This indicated that our procedure was effective to obtain enhanced thermal stability on meso porous zirconia relative to soft-templating and was comparable to the hard-templating method.
)
a Judged by wide-angle PXRD measurements with ZrO2 sample just after the pyrolysis step. b SBET is the BET specific surface area for the 450 � C-air-calcined ZrO2 samples. d Vtot is the pore volume calculated at a relative pressure of 0.95 with the calcined ZrO2 samples. e In case of 900 � C pyrolysis temperature, the air calcination for template removal was performed at 450 � C.
sintering by the high heat from burning off the carbon. We confirmed that the pyrolysis step plays an important role in determining the template-free zirconia product. To obtain highly mesoporous zirconia with a phase-pure tetragonal structure, pyrolysis temperature of 800 � C was the most suitable. On the other hand, we also investigated whether Pluronic® F-127 and phloroglucinol could be replaced by other chemical compounds in the mesoporous zirconia synthesis. To confirm this, various surfactants of Pluronic® P-123, Brij®-72, and poly (ethylene glycol) ethyl ether were tested as a surfactant instead of Pluronic® F-127 in the one-pot synthesis of mesoporous zirconia. The BET surface area of the final zirconia product was highly dependent on the kind of surfactant (see entries 2–5 in Table 2). Notably, the most effective surfactant was Pluronic® F-127, amongst the tested surfactants. In addition, we tested phenol instead of phloroglucinol to generate phenol resin with formal dehyde. The results showed that phloroglucinol gave a better meso porous structure (see entry 2 and 6 in Table 2). Also, if the phenol resin precursor was excluded in the synthesis gel and solely a surfactant was used, the product showed poor BET surface area and pore volume (entry 1 in Table 2). The detailed synthesis and characterization results are described in Section. S3 of Supplementary data. Consequently, the combination of Pluronic® F-127 and phloroglucinol was the most
3.1.3. Extension to other mesoporous metal oxides Our synthetic strategy in Scheme 1 was readily extended to the synthesis of mesoporous metal oxides with various compositions such as Table 3 Porous textural properties of various mesoporous ZrO2 samples after heat treatment at 450–600 � C in air.
Table 2 Porous textural properties for mesoporous zirconia samples, which were syn thesized by using various surfactants and phenol resin precursors. In a synthesis using Pluronic® F-127 and phloroglucinol, the starting mixture composition was 10 phloroglucinol/0.3 zirconium oxynitrate/15 Pluronic® F-127/10 HCHO/1.1 HNO3/1440 EtOH. The synthesis procedure corresponded with Scheme 1. In the other synthesis, phloroglucinol and Pluronic® F-127 were replaced with other components while keeping the used weight. Entry
Phenol resin precursor
surfactant
SBETa (m2 g 1)
WBJHb (nm)
Vtotc (cm3 g 1)
1 2 3 4 5
Not used phloroglucinol phloroglucinol phloroglucinol phloroglucinol
50 138 80 76 107
3 3 3.2 2.3 5.7
0.04 0.21 0.09 0.06 0.19
6
phenol
Pluronic® F-127 Pluronic® F-127 Pluronic® P-123 Brij-72 Poly (ethylene glycol) methyl ester Pluronic® F-127
116
13
0.42
Entry
Templating method
Used templates
Treatment Temperature (� C)
SBETa (m2 g 1)
Vtotb (cm3 g 1)
1
Soft-to-hardc
450
138
0.21
2
Soft-to-hardc
600
72
0.25
3
Soft templating Soft templating Hard templating
Pluronic® F127 /phenol resin Pluronic® F127 /phenol resin Pluronic® P123 Pluronic® P123 CMK-3
450
60
0.43
600
21
0.07
550
74
0.21
4 5d a
SBET is the apparent BET specific surface area. WBJH is a mode value of mesopore diameter derived from the adsorption branch using the BJH method. c Templating method performed in this work using Pluronic® F-127 and phloroglucinol. d All results in this entry was from Ref. [21]. b
a
SBET is the apparent BET specific surface area. WBJH is a mode value of mesopore diameter derived from the adsorption branch using the BJH method. c Vtot is the pore volume calculated at the relative pressure of 0.95. b
5
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CeO2, Y2O3, and ZrCeOx (mixed bimetallic oxides). Except for the kind of metal nitrates that were used, the same synthesis procedures as for the mesoporous zirconia in Section 3.1.1 were employed for these various metal oxides. As shown in the PXRD results of Fig. 3a, all the samples with various framework compositions showed nanocrystalline metal oxides. In good agreement, high resolution TEM images indicated that all the samples were composed of agglomerated metal oxide nano crystallites. Notably, in the case of ZrCeOx, the high resolution STEMEDS mapping (Fig. S9) displayed that two metal oxide components were homogeneously mixed through the entire particles, and the PXRD patterns (Fig. 3) indicated that the mixed metal oxide had a phase-pure cubic structure. Based on the EDS-STEM and PXRD results, the meso porous ZrCeOx seemed to be a solid solution. The porous textural properties of these various mesoporous metal oxide samples were also investigated by N2 sorption analysis (see N2 sorption isotherms, pore size distribution in Fig. S10, and result sum maries in Table S1). All three samples had a uniform mesopore size distribution, indicating effective mesopore generation by our soft-tohard consecutive templates. Moreover, these samples showed quite high BET surface area and pore volume. In particular, the mesoporous ceria synthesized in this work showed markedly high BET surface area even though the ceria is a well-known oxide that readily undergoes the thermal sintering process. In conclusion, our one-pot synthetic strategy was very effective to prepare various mesoporous metal oxides with nanocrystalline frameworks.
applicable to practical catalytic applications. Among the various kinds of metal oxides synthesized in this work and industrially important catalytic reactions, we decided to show the advantages of mesoporous CeO2 as a supporting material for Au catalysts in the water-gas shift (WGS) reaction, because Au-supported CeO2 catalysts are known to exhibit high catalytic performance. In addition, for the WGS reaction thermally stable supports are preferable in terms of avoiding sintering by exothermic reaction heat [29]. For the WGS reaction tests, we supported about 5 wt% Au via a deposition-precipitation technique on the mesoporous CeO2, which was synthesized by our one-pot procedure described in Scheme 1. The resulting supported-Au catalyst is denoted by ‘Au/mesoporous-CeO2’. For comparison, a ceria nanoparticle agglomerate, which was prepared by a procedure reported in another work using urea as a precipitation agent [30], was supported with 5 wt% Au in the same manner. This sample is denoted by ‘Au/nano-CeO2’. In addition, a commercially available ceria was also supported with 5 wt% Au, and the resulting catalyst is denoted by ‘Au/bulk-CeO2’. Fig. 4 shows representative STEM images for the three CeO2-sup ported catalysts, Au/mesoporous-CeO2, Au/bulk-CeO2 and Au/nanoCeO2. As shown in the low-magnification STEM images in Fig. 4a, b, and c, Au nanoparticle sizes appear to be similarly distributed in a range from 0.5 to 2.0 nm in all three Au/CeO2 samples. In the high resolution STEM images, however, we detected extremely tiny Au species smaller than 0.5 nm, which could be considered a single atomic species, in the Au/mesoporous-CeO2 sample (see STEM image of Fig. 4d). Except for the Au/mesoporous-CeO2, it was difficult to find Au species that could be considered a single atom. However, when the volume-weighted Au particle size distribution was obtained from these STEM investigations, the Au nanoparticle dispersion appeared to be comparable in the three
3.2. Water-gas shift reaction over Au/mesoporous-CeO2 In this section, we demonstrate that the one-pot synthesized meso porous metal oxides described in the previous sections would be highly
Fig. 3. PXRD patterns (a) for mesoporous CeO2 (black line), Y2O3 (blue line), ZrCeOx (red line), synthesized by using Pluronic® F-127 and phenol resin as a soft-tohard consecutive template. High resolution TEM images for CeO2 (b) Y2O3 (c) and ZrCeOx (d). Insets in each TEM image are FFT images for the red-squared region. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) 6
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Fig. 4. STEM images for (a, d) Au/mesoporous-CeO2, (b, e) Au/bulk-CeO2, (c, f) Au/nano-CeO2. White circles in STEM images indicate a gold species, which seemed to be a single atom.
Au/CeO2 samples, except that the single atomic Au species was detected only in the Au/mesoporous CeO2. This contrary result might come from the unsatisfactory resolution of the high resolution STEM investigation that did not allow detection of all of the single atoms from the ceria matrix. To clarify the presence of single atomic Au species, we obtained XP spectra of the Au4f for the Au/CeO2 samples. In previous studies, the Au1þ species in Au/CeO2, observed via XPS measurement, are typically regarded as a single atomic Au species stabilized by a ceria support [31]. As shown in Fig. S13, it was noteworthy that the atomic fraction of Au1þ increased in the order of Au/bulk-CeO2 < Au/nano-CeO2 < Au/meso porous-CeO2. Therefore, we could conclude that the Au/mesopor ous-CeO2 sample had a relatively greater fraction of single atomic Au1þ species compared to that of the other Au/CeO2 samples. This could be attributed to the high BET surface area of the mesoporous CeO2 sample (see Table S2), which would be beneficial to accommodate a more efficient metal-support interaction. Fig. 5 shows catalytic results for the water-gas-shift reaction over the three aforementioned supported Au-ceria catalysts. The water-gas-shift reaction converts carbon monoxide and water in the reformate stream into carbon dioxide and hydrogen. This reaction is intended to be used for hydrogen enrichment and CO reduction of the reformate gas for hydrogen production. Fig. 5 displays the conversion of CO as a function of the reaction temperature over the three supported gold catalysts. As the results show, the catalytic CO conversion at similar reaction tem peratures decreases in the following order: Au/mesoporous-CeO2 > Au/ nano-CeO2 > Au/bulk-CeO2. The Au/mesoporous-CeO2 sample exhibi ted 15% CO conversion at 150 � C and the CO conversion continuously increased up to 47% at 360 � C. On the other hand, Au/nano-CeO2 and Au/bulk-CeO2 showed very low levels of CO conversion over the entire temperature range. The CO conversion at 150 � C was only 4% for Au/ nano-CeO2 and 1% for Au/bulk-CeO2, which were four times and 15 times lower, respectively, than the CO conversion achieved by Au/ mesoporous-CeO2 at 150 � C. Since it is often reported that atomically dispersed Au1þ on the ceria could exhibit distinctively high intrinsic activity for the WGS reaction, the high catalytic activity of Au/meso porous-CeO2 might be attributable to the larger number of Au1þ species, which was confirmed in the XPS results (Fig. S13). Furthermore, the thermally stable ceria support in the Au/mesoporous CeO2 could be beneficial to obtain high CO conversion even at the high reaction tem perature region. In addition, high oxygen-storage capacity for the ceria
Fig. 5. CO conversion in the water-gas-shift reaction over Au/mesoporous CeO2 (●), Au/bulk-CeO2 (▴) and Au/nano-CeO2 (■), plotted as a function of the reaction temperature. (Reaction condition: the flow rate of the reaction CO2/21.67% H2/47.5% mixture (8.3% CO/16.67% H2O/5.8% Ar) ¼ 18 cm3 min 1, catalyst ¼ 150 mg, and reaction pressure ¼ 1 bar).
support in the Au/mesoporous CeO2 sample might be another reason for the high water-gas-shift activity. It is known that, as the particle size of ceria decreases, the oxygen storage capacity is enhanced, resulting in an increase of the water-gas-shift activity by providing oxygen more readily. 4. Conclusions We one-pot synthesized mesoporous ZrO2, Y2O3, CeO2, and ZrCeOx by using only a commercially available inexpensive organic surfactant, carbon source, and metal precursors. To this end, the pyrolysis step was necessary to in-situ generate carbon skeletons to prevent collapse of the mesostructure during the crystallization of the metal precursors. The mesoporous ZrO2 synthesized in this manner showed an enhanced thermal stability compared to the representative mesoporous ZrO2 prepared by hydrothermal synthesis using soft templates, and was comparable to that of CMK-3-replicated mesoporous ZrO2 with a 7
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crystalline framework. Furthermore, we demonstrated that goldsupported mesoporous CeO2, synthesized in this work exhibited signif icantly high catalytic CO conversion through a wide range of reaction temperature in the water-gas-shift reaction. We believe that our results suggest a facile and valuable synthesis route that can yield thermally stable mesoporous metal oxides with a crystalline framework.
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