Synthesis of Rare-Earth-Based Metallic Electride Nanoparticles and

The obtained particle sizes are 10−50 nm in diameter, and their surface areas are order ... that the Ar/H2 arc evaporation technique offers wide ver...
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Synthesis of Rare-Earth-Based Metallic Electride Nanoparticles and Their Catalytic Applications to Selective Hydrogenation and Ammonia Synthesis Yangfan Lu, Jiang Li, Tian-Nan Ye, yasukazu kobayashi, Masato Sasase, Masaaki Kitano, and Hideo Hosono ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03743 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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ACS Catalysis

Synthesis of Rare-Earth-Based Metallic Electride Nanoparticles and Their Catalytic Applications to Selective Hydrogenation and Ammonia Synthesis Yangfan Lu†,#,*, Jiang Li†,#, Tian-Nan Ye†,#, Yasukazu Kobayashi†,#,‡, Masato Sasase†, Masaaki Kitano† and Hideo Hosono†,* †Materials

Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ABSTRACT: We report the successful synthesis of LaCu0.67Si1.33 and Y5Si3 electride NPs using the Ar/H2 arc evaporation technique. The obtained particle sizes are 10−50 nm in diameter, and their surface areas are order of magnitude enhanced than hand-milled samples. Their catalytic performances were indeed improved by a factor of 60 for the hydrogenation of nitrobenzene (LaCu0.67Si1.33) and 3 for ammonia synthesis (Ru-loaded Y5Si3). This marked difference in the enhancement of catalytic activities between each system can be attributed to the geometric structure of active sites. These results show that the Ar/H2 arc evaporation technique offers wide versatility for the preparation of rare-earth-based electride NPs with enhanced catalytic activities. KEYWORDS. electrides, nanoparticles, rare-earth elements, ammonia synthesis, selective hydrogenation Electrides, which accommodate anionic electrons in the interstitial sites, are attracting considerable attention due to their unique catalytic performance towards various important chemical reactions.1,2 For example, Ru-loaded C12A7:e− (Ru/C12A7:e−) realized efficient ammonia production with reduced activation energy.2 It was discussed that the highly activated electrons, associated with the low work function property, enhanced N2 dissociation, known to be the rate-determining step (RDS) in the traditional ammonia synthesis process.2 Consequently, Ru/C12A7:e− enabled ammonia synthesis under lower temperature and pressure conditions.3 After the discovery of Ru/C12A7:e−, rare-earth-based electrides, such as Y5Si3, were also reported to be effective for ammonia synthesis upon loading Ru (Ru/Y5Si3).3 Interestingly, some of rare-earth-based electrides are chemically stable in air and water, rendering them applicable to a wide range of reactions. In particular, an efficient hydrogenation of nitroarenes using LaCu0.67Si1.33 has provided access to new liquid-phase reactions of water-compatible electride catalysts.4 In both hydrogenation of nitroarenes and ammonia synthesis, the turnover frequencies (TOF) of electrides catalysts are orders of magnitude higher than those of the traditional catalysts due to their strong electron donation ability, allowing various reactions to proceed under milder conditions.2-4 However, since most of the rareearth-based electrides are synthesized using arc melting or solid state reactions at high temperatures, their surface areas are limited to a few m2·g−1, which prevents further improvement of their catalytic activities.2-4 Moreover, the differences in size effects between metal-loaded and

unloaded catalysts are still unclear, hindering the design of new electride catalysts. Therefore, the synthesis of electride nanoparticles (NPs) has been strongly required to further enhance their catalytic activities as well as unveiling the characteristics of loaded and unloaded electride catalysts with the size of dozens of nm. Among the reported electrides, LaCu0.67Si1.33 and Y5Si3 co-exist strong electron donation ability and chemical stability, allowing applications under air and liquid phase reactions.3,4 However, both of them contain rare-earth elements, exhibiting strong oxygen affinity, and their NPs is difficult be synthesized using traditional chemical processes. Herein, we report that Ar/H2 arc evaporation technique offers water, organic solvents and reducing agents free environment, and effective to synthesize rareearth-based metallic compounds, such as LaCu0.67Si1.33 NPs and Y5Si3 NPs. The obtained NPs were well crystallized, and their surface areas are order of magnitude enhanced than those of the hand-milled samples. The catalytic activities of LaCu0.67Si1.33 NPs in the hydrogenation reaction and of Ru(10wt%)/Y5Si3-NPs in ammonia synthesis were enhanced by a factor of 60 and 3, respectively, compared to hand-milled samples. The enhancement of the catalytic activity is less significant than that of the surface area for Ru(10wt%)/Y5Si3-NPs due to the saturation of the numbers of active B5-type site of loaded Ru. These results demonstrate that the Ar/H2 arc evaporation technique offers high versatility in the preparation of rare-earth-based electride NPs, which exhibit enhanced catalytic activities, particularly in the case of unloaded catalysts.

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Moreover, the XPS measurements revealed that the observed peaks of each NPs are consistent with bulk samples, demonstrating their surface properties were retained in NPs (Figure S4). These results revealed that Ar/H2 arc evaporation technique not only realized the formation of NPs in a clean environment, but also reduced damages on their surfaces.

Figure 1. SEM images of (A) LaCu0.67Si1.33 NPs and (B) Y5Si3 NPs. High resolution HAADF-STEM images of LaCu0.67Si1.33 NPs and Ru(10wt%)/Y5Si3 are shown in (C) and (D). The small white particles represent loaded Ru in (D). Powder XRD patterns of (E) LaCu0.67Si1.33 NPs and (F) Y5Si3 NPs. The reliability parameters are Rwp = 17.776 % (S = 0.9153) for (E) and Rwp = 7.116 % (S = 1.468) for (F). Experimental details are mentioned in the Supporting Information. Figure 1 A−D shows the SEM and HAADFSTEM images of the obtained NPs. The observed particle sizes of LaCu0.67Si1.33 NPs and Y5Si3 NPs were orders of magnitude smaller than those of hand-milled powder (Figure S2). The measured BET surface areas (SBET) were 65.0 m2·g−1 for LaCu0.67Si1.33 NPs and 56.6 m2·g−1 for Ru(10wt%)/Y5Si3-NPs. which supports the formation of NPs. High resolution STEM observations showed that both of LaCu0.67Si1.33 NPs and Y5Si3 NPs are well crystallized, and their lattice constants can be assigned to corresponding unit cells (Figure 1 and S3). The powder XRD paterns further provide the macroscopic evidences of the formation of LaCu0.67Si1.33 NPs and Y5Si3 NPs (Figure 1 E and F). The obtained peaks could be refined using their corresponding structures. For Y5Si3 NPs, Y5Si3Hx, which stems from performing the arc evaporation technique under Ar/H2 mixed gas atmosphere, was confirmed, and its hydrogen content was estimated to be 2.2 per f.u. from H2-TPD measurement. It is worth noting that the hydrogen incorporation should not affect the catalytic activity because these hydrogen atoms can be reversibly absorbed/desorbed in the Y5Si3 lattice.3 Impurity phases, could not observed at least in the powder XRD patterns.

Figure 2. (A) Time-dependent catalytic activity and (B) reaction rate of hand-milled LaCu0.67Si1.33 and LaCu0.67Si1.33 NPs in the hydrogenation of nitrobenzene. (C) Recycling experiments over 10 runs. (D) Recycling experiments over 10 runs at low conversion level. Reaction times were 2 h for (C) and 0.2 h for (D). The red and blue bars represent conversion and selectivity. Standard conditions: catalyst (10 mg), nitrobenzene (0.5 mmol), methanol (5 mL), 120 °C, H2 (3 MPa). Motivated by the powder XRD and XPS data, showing successful preparation of these NPs, we then investigated their catalytic performance in the hydrogenation of nitrobenzene and ammonia synthesis. Figure 2 displays the comparison of the catalytic activity of hand-milled LaCu0.67Si1.33 and LaCu0.67Si1.33 NPs towards the hydrogenation of nitrobenzne. The reaction rates were 0.95 mmol·g−1·h−1 for hand-milled LaCu0.67Si1.33 (SBET = 0.9 m2·g−1) and 61.2 mmol·g−1·h−1 for LaCu0.67Si1.33 NPs (SBET = 65.0 m2·g−1), showing that the catalytic activities improved lineally with increasing surface area. The obtained reaction rate of LaCu0.67Si1.33 NPs are much improved than famous 3d transition metal catalysts under the similar reaction temperature (110−120 °C) and H2 pressure (3−5 MPa) conditions.5 The catalytic activities of LaCu0.67Si1.33 NPs did not degrade in at least over 6 cycles, which confirmed their enhanced stability compared to the traditional metal loaded catalysts (Figure 2C).6 Moreover, the initial conversions also did not degrade, showing that LaCu0.67Si1.33 NPs is chemically stable to the reaction (Figure 2D). Similar to bulk LaCu0.67Si1.33,4 catalytic activity of LaCu0.67Si1.33 NPs start degrading after 7 cycles due to hydrogen incorporation, which results in suppressed electron donation ability, but it recovers by annealing used catalyst at 400 °C under Ar flow (Figure 2 C).

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ACS Catalysis Catalytic activity did not degrade by the heat treatment since active Cu sites are incorporated in the lattice of compounds, preventing metal aggregation and leaching of active sites. Decomposition of LaCu0.67Si1.33 NPs could not be confirmed on the basis of the STEM observations and powder XRD data (Figure S5 and S6). Then, a variety of substituted nitrobenzenes bearing either electron-donating or -withdrawing functional groups were subjected to the reaction, which showed improved catalytic activities and selectivity (Table 1). The obtained yields were higher than 92% and the reaction times were much shorter than hand-milled sample, demonstrating the good performance and high versatility of LaCu0.67Si1.33 NPs in the hydrogenation of nitroarenes. Table 1. LaCu0.67Si1.33 NPs catalyzed hydrogenation of substituted nitroarenesa

~20 % larger than that of hand-milled Ru(10wt%)/Y5Si3, but much smaller than that of the traditional catalysts (~100 kJ·mol−1).2 These results suggest that the RDS is the formation of N–Hx species, instead of the N2 dissociation. It seems reasonable to assume that a strong electron donation property is retained in Y5Si3 NPs, and the N–N bond of absorbed N2 molecules is weakened by electron transfer. Decomposition of Ru(10wt%)/Y5Si3 NPs were not observed on the basis of the TEM and powder XRD data (Figure S5 and S6). The promoted N2 dissociation on the Ru(10wt%)/Y5Si3 NPs catalysts was supported further by a reaction kinetic analysis. We constructed the corresponding equations assuming the dissociation of N2 or the formation of N–Hx species as the RDS, and evaluated the differences between the constructed and modeled values (see Supporting Information).6 It can be clearly seen in Figure S7 and S8 that, the good of fitness (R2) is better when considering N−Hx formation as the RDS for Ru(10wt%)/Y5Si3-NPs, same to that of hand-milled Ru(10wt%)/Y5Si3. These results are consistent with the reduced activation energy, in which N2 dissociation is promoted significantly by the strong electron donation property of Y5Si3. Thus, the improvement of the catalytic activity of Ru(10wt%)/Y5Si3 NPs without changing the reaction mechanism was confirmed. Significant particle growth could not be confirmed for Ru(10wt%)/Y5Si3 and Ru(10wt%)/Y5Si3-NPs after the reacctions (Figure S5 and S9).

aStandard

conditions: catalyst (10 mg), nitroarenes (0.5 mmol), methanol (5 mL), 120 °C, H2 (3 MPa). Encouraged by the improved catalytic activities of LaCu0.67Si1.33 NPs, subsequently, we applied Ru(10wt%)/Y5Si3 NPs in the ammonia synthesis reaction. According to TEM observations, loaded Ru on Y5Si3 NPs were well fixed and electron donation from Y5Si3 to Ru can be expected (Figure S3). In Figure 3 A, the catalytic performance of Ru(10wt%)/Y5Si3 is compared to that of Ru(10wt%)/Y5Si3-NPs. At 340 °C and 0.1 MPa of reaction pressure, the ammonia production was as high as 1450 mol·g−1·h−1 for the hand-milled sample, whereas it reached 4448 mol·g−1·h−1 when using Ru(10wt%)/Y5Si3NPs. The ammonia production rate of Ru(10wt%)/Y5Si3NPs was further enhanced with increasing reaction pressure and reached 7835 mol·g−1·h−1 at 0.9 MPa (Figure 3 B), suggesting that hydrogen poisoning was avoided via the reversible hydrogen absorption/desorption properties of Y5Si3. The Ru(10wt%)/Y5Si3 NPs produced ammonia continuously at least for 24 hours (Figure 3 C). As summarized in Figure 3 D and Table S1, the activation energy (Ea) was determined to be 68.5 kJ·mol−1, which is

Figure 3. (A) Ammonia production rate using Ru(10wt%)/Y5Si3 and Ru(10wt%)/Y5Si3 NPs under 0.1 MPa and (B) under high pressure. (C) Time course of ammonia production of Ru(10wt%)/Y5Si3 NPs. (D) Arrhenius plot of ammonia production in the temperature range of 260−320 °C. The reaction rates of the points used in (D) are smaller than 20 % of corresponding thermodynamic equivalent values. Although the reaction mechanism is retained in ammonia synthesis, the improvement of the catalytic activity is less significant for Y5Si3 NPs, which is likely to originate from metal loading. It is widely known that, in various metal loaded catalysts, TOF can be strongly

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affected by the size of loaded metals.7-9 For example, in Fe/MgO with a Fe size of ca. 1.5 nm, the TOF for ammonia synthesis is an order of magnitude smaller than that for Fe/MgO having a Fe size of ca. 30 nm.7 Similar tendencies can be also seen in other catalysts and reactions, such as Ru/C and Co/CNT.8,9 Since B5-type sites of loaded Ru have been discussed to be responsible to ammonia synthesis, the decrease in TOF can be attributed to numbers of exposed B5-type site saturating and even decreasing in the scale of a few nm scale.7-9 Therefore it is not surprising that the TOF of Ru(10wt%)/Y5Si3-NPs (Ru: ca. 4 nm) becomes an order of magnitude smaller than that of hand-milled Ru(10wt%)/Y5Si3 (Ru: ca. 160 nm), leading to the less pronounced improvement of the catalytic activity. Indeed, we confirmed that TOF of Ru/Y5Si3-NPs recovered with the increase in Ru particle size by controlling the support sizes via heat treatment (Table S2). In LaCu0.67Si1.33 NPs, by contrast, TOF is comparable to that of the hand-milled sample, and catalytic activity and surface area are ca. 60 times enhanced (Table S3). One of the differences between Ru(10wt%)/Y5Si3-NPs and LaCu0.67Si1.33 NPs is the geometric environment of the active sites. While Ru metal must be loaded on Y5Si3 NPs, no metal loading is required for LaCu0.67Si1.33 NPs because the active Cu sites are already incorporated in the lattice frameworks, constituting honeycomb network together with Si.4 The geometric structure of the Cu sites thus retained, and the role of nano-size effects can be considered secondary. This feature contrasts with the previously reported loaded catalysts Pd/CeO2 for the hydrogenation of nitroarenes, in which TOF is affected by the particle size of loaded metals.10 The catalytic activity of LaCu0.67Si1.33 can be then considered to enhance with increasing surface area. In summary, using the Ar/H2 arc evaporation technique, we successfully obtained rare-earth-based electride (LaCu0.67Si1.33 and Y5Si3) NPs under oxygen, water and organic solvent free environment. The surface areas of the NPs were an order of magnitude higher than those of the hand-milled samples were, and their catalytic activities were much enhanced, while their chemical stability was retained. The improvement of the catalytic activity was more significant for LaCu0.67Si1.33 NPs because the geometric structure of the active sites is protected by the lattice framework. Ar/H2 arc evaporation technique therefore offers wide versatility for the preparation of rare-earth-based metallic electride with improved catalytic activities.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. Experimental details, supporting figures and table, including XPS, SEM and TEM data.

AUTHOR INFORMATION Corresponding Author

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*Email: [email protected] *Email: [email protected]

Present Addresses ‡Department

of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

Author Contributions #These

authors contributed equally.

ACKNOWLEDGMENT This work was supported by funds from Kakenhi Grant-inAid (No. 17H06153) from the Japan Society for the Promotion of Science (JSPS) and MEXT Element Strategy Initiative to form a research core. Y. F. Lu is supported by JSPS fellowship for young scientists (No. 18J00745). T. N. Ye is supported by JSPS fellowship for International Research Fellow (No. P18361). Prof. M. Hara (Tokyo Institute of Technology) is greatfully acknowledged for use of XPS instrument.

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