Topotactic Synthesis of Mesoporous 12CaO·7Al2O3 (C12A7

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Topotactic Synthesis of Mesoporous 12CaO·7AlO (C12A7) Mesocrystalline Microcubes towards Catalytic Ammonia Synthesis George Hasegawa, Shizuka Moriya, Miki Inada, Masaaki Kitano, Masaaki Okunaka, Takahisa Yamamoto, Yuko Matsukawa, Kazuma Nishimi, Kazunari Shima, Naoya Enomoto, Satoru Matsuishi, Hideo Hosono, and Katsuro Hayashi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01819 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Chemistry of Materials

Topotactic Synthesis of Mesoporous 12CaO—7Al2O3 (C12A7) Mesocrystalline Microcubes towards Catalytic Ammonia Synthesis George Hasegawa,*,† Shizuka Moriya,† Miki Inada,† Masaaki Kitano,‡ Masaaki Okunaka,‡ Takahisa Yamamoto,§ Yuko Matsukawa,† Kazuma Nishimi,† Kazunari Shima,† Naoya Enomoto,† Satoru Matsuishi,‡ Hideo Hosono,‡ Katsuro Hayashi*,† †

Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan ‡ Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan § Department of Materials Design Innovation Engineering, Graduate School of Engineering, Nagoya University, Furo-cho, Chigusa-ku, Nagoya, 464-8603, Japan ABSTRACT: Material design from both crystallographic and morphological aspects is a key in a vast field of application. An inorganic crystal composed of abundant main-group elements, 12CaO·7Al2O3 (C12A7) with nanocage structure, shows multiple functions due to the incorporation of a wide variety of extraframework anions inside its subnanometer-size cages, yet very few are reported on the morphological design of this versatile and ubiquitous material. Here, we demonstrate a facile approach that allows for the formation of well-defined cubic-shaped crystallites and the introduction of porous structure. The synthesis strategy is based on the topotactic conversion from the hydrogarnet precursor obtained by a simple solution process, providing mesoporous C12A7 microcubes with a single-crystalline nature. This conversion is achieved by the crystallographic similarity between hydrogarnet and C12A7 and the phase separation most likely initiated by the spinodal decomposition. The catalytic investigation unveils a high potential of the mesoporous C12A7 microcubes embedded with Ru nanoparticles for NH3 synthesis under ambient pressure in terms of the synthetic rate over 5 mmol g–1 h–1 and the durability beyond 140 h at 400 ºC.

Since the first report on the nanocage structure of mayenite, 12CaO·7Al2O3 (C12A7), which has been commonly known as a constituent of alumina cement,1 broad researches have disclosed its unique characteristics regarding anion exchangeability2–8 and ionic conductivity.9,10 The nanocage is derived from the positively-charged framework with the unit cell formula of [Ca24Al28O64]4+⋅4X−, and accommodates anionic species (4X−) that compensate the positive charge. An intriguing feature of this ubiquitous material is to stably accommodate an electron (e–) in the lattice at ambient condition, which was recognized as the first inorganic “electride”, (C12A7:e–).11–14 The multifunctionalities of C12A7 and its derivatives open avenues for a variety of applications, such as anion electrolytes,11 ion and electron emitters,15,16 low-work function cathodes17 and catalysts.18–24 Recently, the catalytic synthesis of NH3 from N2 and H2 over Ru-loaded C12A7:e– has been reported as an environmentally benign protocol that might be substituted for the Haber-Bosch process.21–24 It is understood that C12A7:e– acts as a co-catalyst for the N2 dissociation on a Ru surface, enhances the activity of Ru by donating electron, and further protects active sits on Ru from hydrogen poisoning by capturing hydrogen species in the nanocages, allowing for the ammonia synthesis under milder conditions. Morphological design of catalysts is of great importance because a catalytic activity can be effectively enhanced by increasing surface area and improving mass transport.25,26 In inorganic crystals, the former is associated with reducing crystalline size while the latter is achieved by forming a well-

defined porous architecture. In this context, several approaches beyond the conventional solid-state reaction have been proposed for producing nanostructured C12A7 and its derivatives.22,27,28 One of the common synthetic methods to obtain nanocrystals is a solution process represented by the hydrothermal synthesis and the sol–gel process. When the desired crystal phase cannot be directly obtained by the solution process, the calcination of the as-dried products at relatively low temperatures enables us to obtain the target phase without sacrificing the nanostructure significantly. In the previous reports,22,28 the C12A7 fine particles with high specific surface area up to 46 m2 g–1 were prepared from the hydrothermallyproduced precursors. However, their crystal morphologies were found to be thoroughly irregular in shape, which are typical for the synthetic C12A7, and contained substantial impurities of CaO and 3CaO·Al2O3 (C3A), both of which are unfavorable for the ammonia catalysis. An appropriate calcination process, even accompanying with a phase transformation, preserves the morphology of a solution-process derived precursor, allowing for the morphological design of a crystal phase of interest. This process is known as “topotactic synthesis” or “topotactic conversion”.29– 33 Nanostructured materials prepared by this methodology range from binary metal oxides to more complex crystals, such as zeolites29 and spinels.32 In this report, we demonstrate a novel topotactic synthesis of mesoporous mesocrystalline microcubes of C12A7 from the

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hydrogarnet (3CaO·Al2O3·6H2O; C3AH6) precursor, as shown in Figure 1 (a). This process includes the conversion of C3AH6 to biphasic composite of C12A7 and CaO and the selective removal of CaO.34 As a first step, we have developed a simple solution process to fabricate C3AH6 powders in a well-defined cubic morphology. The rearrangement of C3AH6 into C12A7 and CaO is accompanied by the formation of mesopores, as schematically illustrated in Figure 1 (b). The formation mechanism of the mesoporous C12A7 will be discussed and its application to the catalytic NH3 synthesis is exemplified.

Figure 1. (a) Crystal structures of hydrogarnet and mayenite. (b) Schematic illustration of the topotacitic synthesis of the mesoporous C12A7 microcube from the C3AH6 precursor. (c) (top) TG-DTA curves of the precursor powder composed of C3AH6 and Ca(OH)2 for air and He atmospheres; (middle) MS spectrum of the precursor powder under He atmosphere; (bottom) Crystal phase change on heating confirmed by XRD analysis. The precursor C3AH6 microcubes (Figure 2 (a)) were synthesized by the reaction between Ca(OH)2 and boehmite (γAlO(OH)) with a molar ratio of Ca:Al = 3:1 in H2O at 100 °C for 5 h. The as-dried products consisted of C3AH6 microcubes, unreacted Ca(OH)2 and small amount of carbonated crystallites (C3A·CaCO3·H11), as confirmed in Figure S1. Although the carbonated impurities are readily formed in the

CaO–Al2O3–H2O system,35 the reaction under CO2-free conditions minimize the formation of such impurities, for example, by degassing the solution and substituting the atmosphere to N2. Microcubes with a well-defined cubic shape (see Figure S2) were obtained when the amount of Ca(OH)2 was doubled with respect to the stoichiometric ratio of Ca and Al sources for C3AH6. The choice of Al source was crucial for the morphology of products as well. When Al(OH)3 was employed instead of γ-AlO(OH), the shape and size of the C3AH6 crystallites were hardly controlled, as indicated in Figure S3. Since C3AH6 crystallites are formed in this solution process via the dissolution and reprecipitation, the difference in solubility of the Al sources influences markedly on the crystal morphology; the more soluble γ-AlO(OH)36 with lower crystallinity (Figure S4) is advantageous for dissolution, thereby more efficiently supplying the Al component for the reprecipitation of C3AH6. With the enough Al supply, the C3AH6 crystallite likely grows with the automorphology of cubic {100} faces, reflecting its innate crystal structure with the cubic Ia d symmetry.34 The formation of C3AH6 microcubes was also probed in terms of the reaction temperature (Figure S5 and S6). At room temperature (~25 °C), the Ca-Al crystals other than C3AH6 were generated in an irregular shape as reported previously.34 A drastic change was observed when the reaction was carried out at 50 °C so that the C3AH6 microcubes emerged within ten minutes. The C12A7 microcubes were obtained by firing the C3AH6 precursor followed by washing with 0.1 M NH4Cl/methanol. The transition from C3AH6 to C12A7 was monitored by the thermogravimetric analysis/mass spectroscopy (TG-DTA/MS) and X-ray diffraction (XRD), as displayed in Figure 1 (c) and S7, respectively. These results indicate that, on heating C3AH6, the dehydration takes place at around 250 °C in concert with the transition into C12A7, which is probably in a hydrated state described as 12CaO·7Al2O3·H2O (C12A7H), concurrently spinning off Ca(OH)2, which converts to CaO (and CaCO3 in air) above ~350 °C. Reaction between C12A7 and CaO to form 3CaO·Al2O3 (C3A)34 was not observed up to 1000 °C. The subsequent washing process eliminated the residual CaO, ending up with the single-phase C12A7 microcubes, as shown in Figure 2 (b,c) and S7 (b). It is noteworthy that the calcined C12A7 microcubes were interspersed with mesopores and that the pore size appeared to be enlarged as increasing the calcination temperature. Since the crystal sizes in the microcubes approximated from the XRD patterns by the Le Bail method were typically smaller than 50 nm (see Table S1), those mesoporous morphologies were made up of aggregates of C12A7 crystallites. Figure 2 (d) and (e) display the high-resolution transmission electron microscopy (HRTEM) images of the representative C12A7 microcube. The observed porous morphology is reminiscent of a co-continuous structure formed during the spinodal decomposition, where the two phases are individually interconnected in three dimensions.37 Notably, the wellordered lattice image can be observed in the whole crystallite (see also the corresponding fast Fourier transform (FFT) images in Figure S8), and the selected area electron diffraction (SAED) pattern (Figure 2 (f)) are indexed to the body-centered cubic symmetry (I 3d) of C12A7 crystal. Slight rotational deviation in the diffraction spots indicates that the microcube consists of single-orientation grains with very small misfit angles estimated to be less than 1.5°, which underpins the

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Chemistry of Materials

Figure 2. (a-c) Field emission-scanning electron microscopy (FE-SEM) images of the C3AH6 and C12A7 samples calcined at different temperatures for 1 h: (a) as-dried, (b) 700 °C and (c) 900 °C. The insets of (b) and (c) are magnified images of the surface. (d,e) HRTEM images of the slice of the C12A7 microcube calcined at 700 °C prepared by the focused ion beam (FIB) and Ar ionmilling technique. (f) SAED pattern of the whole slice of microcube indexed as the hk0 projection. (g) N2 physisorption isotherms and the corresponding mesopore size distributions (insets) for the C12A7 microcubes calcined at different temperatures for 1 h. mesocrystalline nature of the microcube.38 The mesocrystal alignment is deduced to inherit the orientation of C3AH6 single crystal due to the similarity in the crystal structure with the near-agreement in the lattice constants of C3AH6 and C12A7 (see Figure 1 (a)). The N2 physisorption isotherms of the C12A7 microcubes treated at different temperatures are given in Figure 2 (g) and S9. The detailed pore properties are listed in Table S1. All the isotherms exhibit the steep N2 uptake near p/p0 = 1, which is not derived from pores in a C12A7 microcube but from interparticle spaces. The samples treated above 400 °C adsorbed substantial amount of N2 in the lower relative pressure region (p/p0 ~ 0.6–0.8) as presented in Figure S9 (a), implying the development of mesopores in each C12A7 microcube. The mesopore size with relatively narrow distribution became enlarged as increasing the calcination temperature beyond 600 °C. Owing to the mesoporous structures, the C12A7 microcubes possessed high specific surface area, reaching >60 m2 g– 1 for the specimens calcined at 600 and 700 °C. In addition, the mesopore size can be controlled by prolonging the calcination time, as shown in Figure S9 (b) and S10. It was figured out that the mesopore formation is not traced solely to the removal of CaO. No mesopores were found in the sample heated at 400 °C before the washing process, which consisted of C12A7 and Ca(OH)2, whereas the relatively small volume (0.107 cm3 g–1) of mesopores was formed in the sample calcined at 700 °C even before the washing (see Figure S11). In the latter case, the microcube was composed of C12A7 and CaO, and thereby the volume shrinkage from Ca(OH)2 to CaO is responsible for the pore formation. The removal of CaO augmented the mesopores so that the mesopore volume became more than double. The mechanism for the formation of mesoporous C12A7 mesocrystalline microcubes is summarized as follows (refer to Figure 1). On heating the precursor, the single C3AH6 phase is initially separated into the two phases of C12A7H and Ca(OH)2 in a microcube, which is probably mediated by the spinodal decomposition because of the characteristic co-

continuous mesoporous structure. The relative thermodynamic stability of the fully-hydrated C3AH6 phase becomes less against the binary phases of C12A7H and Ca(OH)2 at relatively low temperature, which allows for the preservation of good crystal orientation alignment in the C12A7H phase throughout a microcube. In the microscopic scale, the microcube shrinks due to the volume reduction by the partial dehydration, while each crystal domain grows up as the phase separation proceeds at higher temperature. The further heating completes the dehydration of Ca(OH)2 and C12A7H into CaO and C12A7, respectively. Since the C12A7 domain in a microcube is fairly rigid at this stage, the volumetric stress arising from the crystal volume reduction from Ca(OH)2 to CaO engenders voids in the CaO domains.33 The following removal of CaO opens more mesopores, resulting in the mesoporous C12A7 microcubes.

Figure 3. (a) Time dependence of NH3 synthesis rates over the Ru@C12A7 (open square) and the Ru@re-C12A7 (solid triangle) at 400 °C. (b) Comparison of the NH3 synthesis rates as a function of temperature between the Ru-loaded C12A7 with and without the reduction with Ti. The mesoporous C12A7 microcubes have a high potential for catalysis due to their high surface area offering many catalytic reaction sites as well as the well-defined mesopores contributing to efficient mass transport. Here, the catalytic activities of the C12A7 microcubes incorporated with Ru nanoparti-

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cles (5 wt%) are demonstrated in terms of NH3 synthesis under ambient pressure. Figure 3 (a) shows the NH3 synthesis rates over the Ru catalysts deposited on the C12A7 microcubes (700 °C for 1 h) at 400 °C as a function of the reaction time. As the C12A7 surface was hydrated by the CaO removal process (see Figure S12), they were annealed at 700 °C for 30 min prior to the Ru deposition. Ru nanoparticles were deposited on the microcubes through the gas phase by reacting with Ru3(CO)12 at 250 °C in a vacuum-closed tube21–24 to obtain Ru@C12A7. For comparison, the C12A7 microcubes were reduced with Ti metal14 at 700 °C for 24 h followed by the Ru deposition to be Ru@re-C12A7. The details about the reduction of C12A7 microcubes are described in the supporting information. While the reduction process diminished the specific surface area of the microcubes (63 m2 g–1 for C12A7 compared with 26 m2 g–1 for re-C12A7), the Ru metal surface areas per the whole catalyst (Ru and C12A7) were assessed to be similar according to the CO chemisorption results (11 m2 g–1 for Ru@C12A7 and 9.8 m2 g–1 for Ru@re-C12A7), as listed in Table S2. It should be highlighted that the good metal dispersion was achieved for the both catalysts (60% and 54%) even with the high Ruloading ratio of 5 wt%, which is in stark contrast to only 2% of dispersion for the 4 wt% Ru-loaded conventional C12A7:e–.21 It is plausible that the well-defined mesoporous morphology consisting of nanocrystallites offers abundant nucleation sites for Ru deposition, preventing the discrete particle growth and thereby leading to the good metal dispersion.39 In both cases, the synthesis rate increased during the first 20 h followed by the leveling-off. The initial increment of catalytic activity is interpreted as the exclusion of inactive species and the activation of catalyst surface under the H2/N2 gas flow condition.21 It is notable that not only Ru@re-C12A7 but also Ru@C12A7 showed good catalytic performance even at atmospheric pressure (> 5 mmol g–1 h–1), which are by far higher than those of the Ru catalyst on the conventional C12A7 with low surface area.21 By contrast to the previous study using the conventional C12A7, where the catalytic activity of the Ru@C12A7:O2– (unreduced C12A7) was about 20% of that of the Ru@C12A7:e– under the same condition,21 the two samples apparently exhibited the similar catalytic capabilities. It is also worth noting that they exhibited the high durability for the continuous NH3 synthesis beyond 140 h, signifying a high potential for a sustainable catalyst. Figure 3 (b) shows the NH3 synthesis rates as a function of temperature for the two samples. The corresponding Arrhenius plots (the inset of Figure 3 (b)) give the similar activation energy values (83 kJ mol–1 and 77 kJ mol–1 for Ru@C12A7 and Ru@re-C12A7, respectively). These values are noticeably lower than those for the Ru@C12A7:O2– (104 kJ mol–1) and the Ru@C12A7:H– (154 kJ mol–1)23 and comparable to that for the Ru@C12A7 electride with relatively low concentration less than 1 × 1021 cm–3, where the metal-insulator transition does not take place, (~83 kJ mol–1).24 Considering the low activation energy shown in Figure 3 (b), it is deduced that extraframework e– were somewhat generated at the microcube surface through the NH3 synthesis process. In the case of Ru@re-C12A7, the abundant cage H– near Ru would be predominantly expended for NH3 production leaving e– in the cage. The generated cage e–subsequently act as H2 storage-release sites,23,40 suppressing the H2 poisoning of Ru catalysts.21–24

The good catalytic activities of Ru@C12A7 on par with Ru@re-C12A7 in terms of the NH3 synthesis rate, activation energy and durability suggests the possibility that the porous C12A7 microcube surface was in-situ reduced by H2 gas yielding H– in the cage11 under the NH3 synthesis condition even at the low temperature of 400 °C. This is likely to be allowed by the high surface area of the C12A7 microcubes composed of small nanocrystallites (~30 nm), where the substitution of H– possibly takes place at the outermost crystal surface. In summary, we have successfully prepared the mesoporous C12A7 mesocrystalline microcubes via the topotactic synthesis approach, which could be a versatile platform for fabricating mesoporous inorganic mesocrystals. The single-phase C3AH6 is separated into Ca(OH)2 (to CaO afterward) and C12A7 by means of the spinodal decomposition on heating, giving rise to the co-continuous mesoporous structure in a cubic mesocrystal after removing the byproducts. The mesopore size can be tuned simply by varying calcination temperature and/or duration time. The catalytic study reveals that the mesoporous C12A7 microcubes can work out as a good support for Ru catalyst even without reduction.

ASSOCIATED CONTENT Supporting Information. Experimental procedures; FE-SEM images, XRD patterns, TG-DTA curves, a FFT pattern from the HRTEM image, N2 sorption isotherms, CO chemisorption results, UV-Vis spectra and a EPR spectrum of the samples; Details about the reduction of mesoporous C12A7 microcubes.

AUTHOR INFORMATION Corresponding Author * Dr. George Hasegawa e-mail: [email protected] Tel/Fax: +81 92 802 2862 * Prof. Katsuro Hayashi e-mail: [email protected] Tel/Fax: +81 92 802 2859

Present Addresses †Dr. Miki Inada Center of Advanced Instrumental Analysis, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan †Prof. Naoya Enomoto National Institute of Technology, Ariake College, Omuta-shi, Fukuoka 836-8585, Japan

Funding Sources JSPS KAKENHI (Grant No. JP16K05935, JP16H006439 and JP16H06440)

ACKNOWLEDGMENT This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant-in-Aid for Scientific Research (C) (Grant No. JP16K05935) for GH, Grant-in-Aid for Scientific Research on Innovative Areas “Mixed Anion” (No. JP16H06439 and JP16H06440) for MI, SM and KH, and the Elements Strategy Initiative to Form Core Research Center, Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan for MK, SM, HH and KH. The TEM analysis was support by Nagoya University “Advanced Characterization Nanotechnology Platform” by MEXT, Japan.

REFERENCES

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