Enhanced Catalytic Ammonia Synthesis with Transformed BaO - ACS

Oct 18, 2018 - We report that a simple mixture of BaO and CaH2 powders with ruthenium nanoparticles acts as an effective heterogeneous catalyst for ...
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Enhanced Catalytic Ammonia Synthesis with Transformed BaO Masashi Hattori, Taiyo Mori, Tomohiro Arai, Yasunori Inoue, Masato Sasase, Tomofumi Tada, Masaaki Kitano, Toshiharu Yokoyama, Michikazu Hara, and Hideo Hosono ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02839 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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ACS Catalysis

Enhanced Catalytic Ammonia Synthesis with Transformed BaO Masashi Hattori,† Taiyo Mori,† Tomohiro Arai,† Yasunori Inoue,† Masato Sasase,‡ Tomofumi Tada,‡ Masaaki Kitano,‡ Toshiharu Yokoyama,‡,§ Michikazu Hara,*,†,§ and Hideo Hosono*,†,‡,§ †Laboratory

for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226– 8503, Japan, ‡Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226−8503, Japan, §ACCEL, Japan Science and Technology Agency, 4–1–8 Honcho, Kawaguchi, Saitama 332–0012, Japan. ABSTRACT: We report that a simple mixture of BaO and CaH2 powders with ruthenium nanoparticles acts as an effective heterogeneous catalyst for low temperature ammonia synthesis with a very small activation energy (41 kJ mol –1). The high catalytic performance of this material are not due to either the original CaH2 or BaO particles but to BaO transformed by reaction with CaH2. The transformed BaO contains BaH2, a stable and strong electron donating material that facilitates a reversible hydrogen storage-release reaction and significantly enhances electron donation to Ru, which results in high catalytic performance. KEYWORDS: ammonia synthesis, heterogeneous catalyst, CaH2, transformed BaO, ruthenium, 1. INTRODUCTION The mass production of ammonia by the “Haber-Bosh process” with iron-based catalysts1 has supported an increase in the human population and modern civilization for over 100 years. The population has been projected to increase to 10 billion in the near future, which will require more ammonia production. Ammonia has also attracted attention as a promising energy carrier to store H2 produced from natural gas and renewable energy sources, due to its high capacity for hydrogen storage (17.6 wt%) and facile liquefaction under mild conditions. The role of the HaberBosch process has further increased in recent wind-to-ammonia on-site ammonia production using H2 produced from a wind power station because the Haber-Bosch process comprises 40–50% of the total energy consumption.2 Such demands have made efficient ammonia production one of our greatest concerns. An increase in the rate of ammonia formation at low temperatures is the basic strategy for efficient ammonia production as an exothermic reaction, and facile cleavage of N2 molecules with strong N≡N bonds is the first priority for such a strategy.3 Predecessors revealed that electron donation into antibonding π-orbitals (π*) in N2 molecules through transition metals facilitates N 2 cleavage. This has developed various efficient ammonia synthesis catalysts such as promoted iron and Ru-based catalysts.4-10 One of the greatest achievements of Fritz Haber through his research on ammonia synthesis was to find an equilibrium dominating chemical reaction.11 As written in all textbooks that describe ammonia synthesis, temperature and pressure unmistakably determine the theoretical ammonia yield according to the equilibrium of the ammonia synthesis reaction (N2 + 3H2 ↔ 2NH3). This equilibrium significantly decreases the ammonia yield as the reaction temperature increases. While an ammonia yield of 80% is achieved under 4 MPa at 127 °C, the maximum ammonia yield at 400 °C cannot transcend 40%, even under a high pressure of 20 MPa.

For this reason, commercial ammonia processes that use iron-based catalysts with high working temperatures over 400 °C require compression from tens to a few tens of megapascals to achieve ammonia yields of ca. 30%, which requires large energy consumption and heavy manufacturing plants. Therefore, further decreases in catalyst working temperatures for increased ammonia yields has remained a challenge in the development of ammonia production by the Haber-Bosch process. We have found that Ru nanoparticles deposited on Ca materials with a small work function and reversible exchange properties between H– and electrons such as [Ca24Al28O64]4+(e–)4 (C12A7:e–), Ca2N:e–, Ca(NH2)2 and CaH2 exhibit high catalytic performance for ammonia synthesis with small activation energies, and are distinct from conventional heterogeneous catalysts.12-14 The high catalytic performance of these catalysts can be attributed to extremely strong electron donating capability based on the small work functions of these Ca materials. For example, C12A7:e– with a small work function comparable to that of metallic K (2.3 eV) enhances electron-donation to Ru immobilized on C12A7:e–, which facilitates N2 cleavage by electron donation into the π* orbitals of N2 molecules via the d orbitals of Ru, so that this step is no longer the rate-determining step for ammonia synthesis.15 These catalysts not only exhibit high catalytic activities for ammonia synthesis, but suggest that the activities for ammonia synthesis and the lower working temperatures are largely dependent on the hydrogen release capability of these Ca compounds to allow for the hydrogen storage-release reaction via H– ions (H + e– → H–).13 More recently, we reported that the addition of Ba species to Ca(NH2)2 significantly improves the ammonia synthesis activity of Ru-loaded Ca(NH2)2 (Ru/Ca(NH2)2).16 The addi-

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tion of Ba species to conventional Ru-based catalysts has often been reported to enhance catalytic activity for ammonia synthesis.17–23 While the role of added Ba species is considered as a structural promoter17,18 or an electronic promoter,23 this remains to be clarified. An understanding of the role and working principle of Ba species in Ru-based catalysts would lead to the development of more efficient catalysts for ammonia synthesis. In this paper, we report ammonia synthesis over a simple mixture of BaO and CaH2 powders, loaded with Ru nanoparticles (Ru/BaO-CaH2). 2. EXPERIMENTAL SECTION 2.1. Preparation of Ru catalyst on various supports. Ru nanoparticles were loaded on CaH2 (Sigma-Aldrich), BaH2 (Stream Chemicals) or a mixture of CaH2 and BaO (Kojundo Chemical) by chemical vapor deposition using ruthenium acetylacetonate (Ru(acac)3 (Sigma-Aldrich)) for the deposition of 10 wt% Ru.24,25 The mixture was heated at 260 °C for 1 h with an extra pure (>99.99995%) N2 flow (2.5 mL min– 1), at 260 °C for 1 h, and then at 340 °C for 5 h with an extra pure H2 flow (2.5 mL min–1). 2.2. Ammonia synthesis reaction. Ammonia synthesis was conducted in a silica glass or stainless-steel fixed bed reactor with an extra pure mixture of H2 and N2 (H2:N2 = 3:1) at a flow rate of 60 mL min–1. The reaction temperature was varied from 127 to 340 °C and the pressure was in the range of 0.1 to 0.9 MPa. All experiments were conducted at the weight hourly space velocity (WHSV) of 36000 mL gcat–1 h– 1. The ammonia produced was trapped in 5 mM H2SO4 aqueous solution and the amount of NH4+ generated in the solution was estimated using an ion chromatograph (LC-2000 plus, Jasco) equipped with a conductivity detector. TOF was calculated from the rate of ammonia synthesis divided by the number of exposed surface Ru atoms. The apparent activation energy (Ea) was calculated from Arrhenius plots for the reaction rate in each temperature ranges. The reaction orders with respect to N2 and H2 were obtained at a constant flow rate (60 mL min–1) using Ar gas as a diluent, and that for NH3 was determined by changing the flow rate of the H2 and N2 mixture (H2:N2 = 3:1). All kinetic results were measured well below equilibrium conversion. 2.3. Characterization. X-ray photoelectron spectroscopy (XPS) (Mg Kα, 8 kV, 25 mA) was performed in ESCA-3200 (Shimadzu) equipped with an Ar-filled glovebox. The samples were moved to the UHV XPS apparatus through the Arfilled glovebox without exposure to the ambient air. The binding energy was corrected with respect to the Au 4f 7/2 peak of Au-deposited samples. Powder X-ray diffraction (XRD; D8 Advance, Bruker) patterns were obtained using Cu Ka radiation. Nitrogen adsorption–desorption isotherms were measured at –196 °C with a surface-area analyzer (BELSORP-mini ΙΙ, MicrotracBEL) to estimate the BrunauerEmmett-Teller (BET) surface areas. The morphology of the samples was obtained using scanning electron microscopy (SEM; S-5500, Hitachi) equipped with an energy dispersive X-ray spectroscopy (EDX; EMAX EX-250, Horiba) detector. H2 temperature programmed reduction (TPR) measurements were conducted by heating (1 °C min–1) a sample (ca. 150 mg) in a stream of a mixture of H2 (4.8%) in Ar, and the concentration of H2 was monitored with a thermal conductivity detector (TCD) and a mass spectrometer (BELMass,

MicrotracBEL, Japan). H2 temperature programmed desorption (TPD) measurements were conducted by heating (1 °C min–1) a sample in Ar (30 mL min–1) using the same instrument as the TPR experiment. Fourier transform infrared (FT-IR) spectrum of Ru/CaH2 were measured using a spectrometer (FT/IR-6100, Jasco) equipped with a mercury–cadmium–tellurium detector at a resolution of 4 cm−1. Samples were pressed into self-supported disks. A disk was placed in a sealed and Ar filled silica-glass cell equipped with NaCl windows. The infrared spectrum of KBr was used as the background for difference spectra obtained by subtracting the backgrounds from the spectra of Ru/CaH2. The microstructural characteristics of the samples were determined from the transmission electron microscopy (TEM; JEM-ARM 200F, Jeol) images. 2.4. DFT calculations. Total energy calculations and structural relaxations of BaH2 and CaH2 with/without surface H defects were determined from density functional theory (DFT) calculations implemented in VASP first principles code. The Perdew−Burke−Ernzerhof (PBE) exchange-correlation functional in DFT was adopted. The convergence criteria of energy and force were respectively 0.5×10−5 eV and 1.0×10−2 eV/Å for all models. The core electrons were handled with the projector augmented wave (PAW) method. The k-point mesh was created to keep a single k-point per 1/40 (Å-1) in the reciprocal space. In both BaH2 and CaH2, (0 0 1), (0 1 0), (1 0 0), (0 1 1), (1 0 1), (1 1 0), and (1 1 1) surface models were relaxed using DFT, and the (1 0 0) surface was found to be the most stable one for both BaH2 and CaH2. It should be noted that a vacuum region of 20 Å is maintained in the unit cell to avoid artificial interaction between BaH2/CaH2 slabs. Calculated surface energies are listed in Figure S1. 3. RESULTS AND DISCUSSION 3.1. Catalytic Performance. Figure 1a shows time courses of ammonia formation rates over Ru/CaH2 and Ru/BaO-CaH2 catalysts under ambient pressure (0.1 MPa) at 340 °C.26 The former and latter were prepared by the pyrolysis of ruthenium acetylacetonate (Ru(acac)3) over CaH2

Figure 1. (a) Time courses for ammonia synthesis over Ru (10 wt%)/CaH2 and Ru (10 wt%)/ BaO-CaH2 catalysts (reaction conditions: catalyst (0.1 g), synthesis gas (H2/N2 = 3, 60 mL min–1), temperature (340 °C), pressure (0.1 MPa)). (b) XRD patterns for Ru (2 wt%)/CaH2 and Ru (2 wt%)/BaO-CaH2 after ammonia synthesis reaction (reaction conditions: catalyst (0.1 g), synthesis gas (H2/N2 = 3, 60 mL min–1), temperature (340 °C), pressure (0.9 MPa), time (100 h)).

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ACS Catalysis Table 1. Catalytic Properties of Various Ru Catalysts under a Pressure of 0.1 MPa. Ru loading SBET rNH3 D d TOF Eae Ref. 2 –1 –1 –1 –1 (wt%) (m g ) (mmol g h ) (%) (nm) (s ) (kJ mol–1) Ru/BaO-CaH2 10 20 10.5a 32b 4.0b 0.0092b 41 (320–270 °C) This work a b b Ru/CaH2 10 13 7.4 47 2.7 0.0044b 68 (340–270 °C) This work Ru/BaH2 10 – 0.2a – – – – This work Ru/Ba-Ca(NH2)2 10 101 12.4a 48b 2.7b 0.0073b 44 (320–260 °C) This work Ru-Cs/MgO 2.0 12 3.2a 50.4c 2.5c 0.0195c 120 (340–250 °C) 13 Ru/C12A7:e– 1.8 1 2.0a 4.7c 28.7c 0.0675c 51 (400–320 °C) 12 Ru/Ca2N:e– 1.8 2 3.4a 3.1c 42.8c 0.1851c 60 (340–250 °C) 13 Ru/Ca(NH2)2 10 101 9.5a 61b 2.1b 0.0044b 57 (340–280 °C) 14 a NH3 synthesis rate (rNH3); reaction conditions: catalyst (0.1 g), synthesis gas (H2/N2 = 3, 60 mL min–1), temperature (340 °C), pressure (0.1 MPa).b Dispersion (D), particle size (d) and TOF were estimated by averaging the particle size distribution measured using STEM. c These values were estimated from active site numbers determined by the CO pulse chemisorption method (Ru/CO = 1). Ea is the apparent activation energy calculated from Arrhenius plots for the reaction rate in each temperature ranges. Catalyst

and CaH2-BaO mixture, respectively. The XPS spectra for Ru/BaO-CaH2 and Ru/CaH2 after reaction for 20 h at 340 °C showed that the Ru 3p3/2 peaks appear at ca. 461.6 eV in both spectra and Ru0 metal particles are formed on both catalysts (Figure S2). One feature of note in Figure 1S is that the Ru 3p3/2 peak of Ru/BaO-CaH2 has a higher intensity at lower binding energies (458-461 eV) than that of Ru/CaH2. The physicochemical and catalytic properties, including TOF and apparent activation energy, for the tested catalysts are summarized in Table 1. Ru/CaH2 exceeded Ru-loaded C12A7:e– (Ru/C12A7:e–) and Ca2N:e– (Ru/Ca2N:e–) in terms of ammonia formation immediately after the start of the reaction (Table 1). This is attributed to its high electron donation capability based on the reversible hydrogen storage-release reaction over CaH2 (CaH2 ↔ Ca2+H–(2-x)e–x + xH),13,27 whereby H– ions are removed from CaH2 as H atoms to form hydride defects (e– trapped at H– site). For this reason, CaH2 with hydride defects is a strong electron donor to Ru, which allows for efficient ammonia formation.13,27 However, the reaction deactivated Ru/CaH2 over 10 h and ammonia formation decreased with the reaction time. XRD peak at 2θ = 35.6○ in the pattern for Ru/CaH2 after the reaction for 100 h (Figure 1b) was assigned to the (2 0 0) plane of calcium imide (CaNH).28,29 FT-IR measurements also supported the formation of CaNH during the catalysis reaction because NH stretching vibration was observed at ca. 3100 cm–1 for Ru/CaH2 after the reaction (Figure S3). CaNH loaded with Ru has moderate catalytic activity for ammonia synthesis; therefore, it does not have high electron donating capability.13 This result indicates that CaNH formation cannot be avoided for CaH2 in the presence in ammonia, and the resultant CaNH degrades the catalytic activity of Ru/CaH2. In contrast, Ru/BaO-CaH2, a simple mixture of BaO and CaH2 powders loaded with Ru, exhibited a high ammonia formation rate over 180 h without degradation of activity, despite its small surface area. The ammonia formation rate over Ru/BaO-CaH2 increased with the amount of BaO mixed with CaH2 and reached a maximum at 3 mol%, while further BaO addition decreased the catalytic activity (Figure S4). Table S1 compares the ammonia synthesis performance (0.9–1.0 MPa) for highly efficient catalyst typically reported for ammonia synthesis. The performance of Ru/BaO-CaH2 for ammonia synthesis was close to that for Ru-loaded Ba-

containing Ca(NH2)2 (Ru/Ba-Ca(NH2)2) which was by far the most efficient catalyst, although the surface area of Ru/BaOCaH2 (20 m2 g–1) was quite smaller than that of Ru/BaCa(NH2)2 (101 m2 g–1). Figure 2a shows the ammonia formation rate with respect to the reaction temperature for several tested catalysts, and Arrhenius plots for the tested catalysts are shown in Figure 2b. Ammonia synthesis proceeds over Ru/BaO-CaH2 down to below 150 °C, as for tran-

Figure 2. (a) Catalytic activity for ammonia synthesis over various Ru catalysts as a function of reaction temperature (reaction conditions: catalyst (0.1 g), synthesis gas (H2/N2 = 3, 60 mL min–1), pressure (0.1 MPa)), and (b) Arrhenius plots of those data.

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sition metals-deposited LiH with the lowest working temperature among the heterogeneous catalysts.30 A characteristic of Ru/BaO-CaH2 is the small apparent activation energy (41 kJ mol-1) as shown in Figure 2b and Ta ble 1. While other Ca-based materials have been reported as efficient catalysts with the lowest ever activation energies for ammonia synthesis, Ru/BaO-CaH2 has the smallest activation energy among the Ca-based catalysts.12,13,14,16 The reaction orders for N2, H2 and NH3 over each tested catalyst are summarized in Table S1. The reaction order for N2 over Ru/BaO-CaH2 is ca. 0.5, as were those over Ru/CaH2, Ru/Ca(NH2)2, Ru/Ca2N:e– and Ru/C12A7:e–. Ru/C12A7:e– with a small work function comparable to that of potassium metal has strong electron-donating power, which facilitates N2 cleavage by electron donation.15 Efficient ammonia synthesis over Ru/C12A7:e– is evident as a small apparent activation energy (50 kJ mol–1) and a reaction order of 0.5 for N2.15 Another feature of note in Table S1 is that the reaction orders for H2 are positive for the tested Ca-based catalysts, including Ru/BaO-CaH2. Dissociative adsorption of H2 is preferred over N2 cleavage on Ru with high affinity to hydrogen, which suppresses efficient ammonia synthesis under high pressures.7,31 This “hydrogen poisoning” is a common and serious drawback of conventional Ru catalysts and is represented as a reaction order between –1 and 0 with respect to H2. On the other hand, Ru/C12A7:e–, Ru/Ca2N:e–, Ru/Ca(NH2)2, Ru/Ba-Ca(NH2)2 and Ru/CaH2 can incorporate H adatoms on the Ru surface into the Ca compounds as H– ions on the basis of the reversible hydrogen storage–release reaction (H + e– ↔ H–), which thereby decreases hydrogen poisoning and appears as positive reaction orders for H2, as shown in Table S2. Therefore, Ru/BaO-CaH2 with a positive reaction order with respect to H2 also decreases hydrogen poisoning as well. Figure S5 shows the correlation between the ammonia formation rate and pressure over Ru/BaO-CaH2 and Cs-Ru/MgO. The ammonia formation rate over Cs-Ru/MgO, which is one of the most efficient catalysts among the conventional Ru catalysts, is independent of pressure by hydrogen poisoning. In contrast, the catalytic activity of Ru/BaO-CaH2 increases with the total pressure, which suggests the reversible hydrogen storage–release reaction occurs over the catalyst. Ru/BaO-CaH2 has high catalytic performance without degradation of activity. One reason for this is to the ability of the catalyst surface to preclude CaNH formation. The intense peaks of CaH2 in the XRD pattern of the Ru/BaO-CaH2 catalyst after reaction (Figure 1b) indicates that Ru/BaOCaH2 consists mainly of CaH2 as well as Ru/CaH2. No diffraction peaks arising from Ba and Ru related crystals were detected because of the small amount. In addition, no strong diffractions related to CaNH were detected in the XRD pattern of Ru/BaO-CaH2 after reaction (Figure 1b). The addition of BaO appears to suppress the conversion of CaH2 to CaNH, so that the high catalytic performance is maintained. Nevertheless, catalysis over Ru/BaO-CaH2 cannot be simply explained by only the suppression of CaNH formation. The rate of ammonia formation per specific surface area for Ru/CaH2 at the early stage of the reaction (< 10 h) is comparable to that of Ru/BaO-CaH2 at steady state as shown in Figure 1a and Table 1. However, the latter surpasses the former in terms of the apparent activation energy and TOF, in

addition to lifetime. Although Ru/CaH2 is gradually deactivated during the reaction, the activity remains unchanged at the early stage of reaction (within 10 h). The ammonia formation rate, TOF and apparent activation energy over CaH2 were measured during the early stage of the reaction; therefore, catalysis over Ru/BaO-CaH2 is distinct from that over Ru/CaH2, not in terms of specific surface area and catalytic durability but in the intrinsic characteristics of the material, such as electron donating capability. 3.2 BaO Transformed with CaH2. As the first step to clarify the role of BaO, the microscopic morphology of Ru/BaOCaH2 and Ru/CaH2 was examined. Figure S6 shows high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images and the Ru particle size distributions for Ru/CaH2 and Ru/BaO-CaH2 before and after reaction for 100 h. While the Ru particle size was somewhat increased during the reaction in both catalysts, Ru/BaO-CaH2 has a slightly larger Ru particle size than Ru/CaH2 both before and after reaction. Improvement in the catalytic activity of Ru-based ammonia synthetic catalysts by the addition of Ba species has been reported and, in most cases, is attributed to a high dispersion of Ru nanoparticles on the particle surfaces.13,17,18,22 However, no such effect was observed in Ru/BaO-CaH2. Next, the macroscopic morphology of Ru/BaO-CaH2 was studied using backscattered electron (BSE) imaging and EDX. Figure 3 shows BSE images and EDX spectra for a BaO-CaH2 mixture before and after vacuum heating at 340 °C. When 10 wt% Ru was deposited on a BaO-CaH2 mixture (3 mol% Ba, 97 mol% Ca) that was vacuum-heated in a similar manner, the resultant material exhibited catalytic activity similar to Ru/BaO-CaH2 shown in Figure 1. Figure 3a shows a BSE image for the BaOCaH2 mixture before heating, where BaO particles (6–10 μm) are observed as bright portions in a large number of dark CaH2 particles.32 Figure 3b shows a BSE image after BaO-CaH2 mixture was heated at 340 °C for 50 h under vacuum.33 The morphology of the mixture was changed significantly by heating; the original BaO particles were not recognizable in this BSE image with larger irregular particles. The

Figure 3. (a) BSE images of the BaO and CaH2 mixture before and (b) after vacuum heating at 340 °C, and (c) Ru/BaO-CaH2 after ammonia synthesis for 100 h at 340 °C. (d) SEM-EDX spectra for the regions indicated in Figure 3c.

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ACS Catalysis bright patches derived from Ba appear on the dark irregular Ca particles after vacuum heating. It was confirmed that there was no significant difference in the morphology and XRD pattern between Ru/BaO-CaH2 (Figure 1 and Table 1) and this BaO-CaH2 mixture heated under vacuum. Figures 3c and d show a magnified BSE image and EDX spectra for Ru/BaO-CaH2, respectively.34 There are bright islands of Ba species on a dark Ca-based surface in Figure 3c. The characteristic X-ray (0.8 eV) of Ba was not detected (Figure 3d Spectrum 1) in Region 1 of Figure 3c, as evident by the lack of bright Ba islands. The EDX spectra show the characteristic X-ray of Ru (2.6 eV) is detected only near Ba islands. Thus, most Ru particles are in contact with the Ba species, which implies that heating BaO with CaH2 causes significant morphological change whereby deposited Ru particles are localized near the Ba species on CaH2. Figures S7a and S7b show H2-TPD and H2-TPR for Ru/BaO-CaH2 after Ru deposition, respectively. Ru/BaO-CaH2 was confirmed to desorb H2 at 150–350 °C by TPD. TPR revealed that the catalyst starts to adsorb H2 over 50 °C. The heated BaO-CaH2 mixture without Ru deposition did not desorb or adsorb H2. Thus, Ru/BaOCaH2 allows the reversible hydrogen storage-release reaction as with other Ru-deposited Ca compounds. However, it was confirmed that such hydrogen storage and release do not occur on BaO loaded with Ru. Ru is preferentially deposited on the Ba species in Ru/BaO-CaH2, which suggests that the reversible hydrogen storage-release reaction primarily originates not from CaH2, but from the Ba species formed through the significant morphological change. CaO formation on Ru/BaO-CaH2 would be a key to understanding the Ba species formed on CaH2. Diffraction peaks due to CaO were observed in both of the XRD profiles for Ru/BaO-CaH2 after reaction for 100 h (Figure 1b) and the BaO-CaH2 mixture heated under vacuum (Figure S8); therefore, the reaction of CaH2 with BaO forms CaO and decreases oxygen atoms in BaO. To further understand the Ba species on the catalyst, the vacuum-heated BaO-CaH2 mixture was examined using XPS. Figure 4 shows XPS spectra (Ca 2p and Ba 3d5/2) for the BaO-CaH2 mixture after vacuum heating at 340 °C. These spectra were measured without exposure of the samples to the ambient atmosphere by using an XPS apparatus equipped with an Ar-filled glove box. The Ca 2p3/2 peaks for the BaO-CaH2 mixture were shifted to lower binding energy from that of the original CaH2 at 347.5 eV by heating. The Ca 2p3/2 peak of CaO was observed at 346.9 eV in the XPS spectra, which indicates that the reaction with BaO converts a large part of surface CaH2 into CaO and forms an oxygen-deficient BaO, which is consistent with the

XRD results. The resulting CaO layer prevents bulk CaH2, which comprises the majority of the catalyst, from reacting with ammonia to form CaNH. For this reason, CaNH, which degrades the activity for the formation of ammonia is not formed in Ru/BaO-CaH2. The Ba 3d5/2 peak of the mixture before heating appears at 780.3 eV, and there is no difference in the Ba 3d5/2 peak between the original BaO particles and the BaO-CaH2 mixture before heating. After heating the mixture for 2 h, the Ba 3d5/2 peak is observed at 781.5 eV (Figure 4a). Further heating did not change the peak position. The Ba 3d5/2 peak is generally insensitive to the electronic states of Ba compounds, and the binding energy is ca. 780 eV in most Ba species, which includes metallic Ba and BaO, except for a few rare exceptions.35,36 One of a few rare exceptions is BaH2, for which the binding energy of the Ba 3d5/2 peak is ca. 782.9 eV.37 The Ba 3d5/2 peak for the heated BaO-CaH2 mixture is located at a much higher binding energy than those of most Ba species; however, the Ba 3d 5/2 peak cannot be simply assigned to BaH2, because the binding energy is much lower than the Ba 3d5/2 of BaH2. One possible explanation for the Ba species is a BaO containing BaH2-like species. Such a material would also allow the reversible hydrogen storage-release reaction via H– ions. CaH2 is thermodynamically able to convert BaO into BaH2, as shown in the following replacement reaction. CaH2 + BaO → CaO + BaH2, ∆G0 = –74.7 kJ mol-1 The solid-state reaction between simply mixed BaO and CaH2 particles at low temperature can thus form a mixture of BaO and BaH2 (BaO-BaH2). Figure S9 shows XRD patterns for BaO-CaH2 mixtures (the atomic ratio of Ca to Ba (Ca/Ba) = 9 and 4) heated for 20 h at 340 °C under ammonia reaction conditions, which revealed that weak diffraction peaks assignable to BaH2 (2 0 0) emerge around 2θ = 26.2° and increase with the amount of Ba. The XPS and XRD measurements clearly demonstrate that BaH2 can be formed on Ru/BaO-CaH2 during reaction. The binding energy (781. 5 eV) of the Ba 3d5/2 peak for the BaO-CaH2 mixture in Figure 4 is the median value between those of BaO (780.3 eV) and BaH2 (782.9 eV). The same can be said of the Ca 2p3/2 peak (347.2 eV) for the mixture (Figure 4(a)) because the binding energies of CaO and CaH2 are 346.9 and 347.5 eV, respectively. The deconvolution analysis of XPS results also roughly estimated about half those of CaH2 and BaO surfaces to be converted into CaO and BaH2. (CaH2•CaO and BaH2•BaO). The formation of BaO-BaH2 can drastically change the morphology of the original BaO particles, as shown in Figure 3, because orthorhombic BaH2 has a distinct crystal structure from that of cubic BaO. It is also difficult for Ca species to dissolve Ba species with large ionic radii. These would form striped BaO-BaH2 islands on the Ca phase. Table 2. Calculated Work Functions.

Figure 4. (a) XPS Ca 2p and (b) Ba 3d spectra for the BaOCaH2 mixture after vacuum heating at 340 °C for 2 h and 50 h.

Compound

Surface index

WF (eV)

CaH2

(1 0 0)

4.9

CaH2-1/9

(1 0 0)

3.3

BaH2

(1 0 0)

4.2

BaH2-1/9

(1 0 0)

2.6

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Figure 6. Proposed mechanism for the ammonia synthesis reaction over the Ru/BaO-CaH2 catalyst. Figure 5. H2-TPD profiles for Ru (10 wt%)/CaH2, Ru (10 wt%)/BaH2 and Ru (10 wt%)/BaO-CaH2 after ammonia synthesis reaction at 340 °C for 50 h. TPD measurements were performed under Ar flow (1 °C min–1). 3.3. Role of BaO Phase on Ru/BaO-CaH2. The results so far reported suggest the formation of a BaO phase involving BaH2 species on Ru/BaO-CaH2. Ru/BaO-CaH2 is superior to Ru/CaH2 in terms of catalytic properties for ammonia synthesis, such as activation energy, ammonia formation rate and TOF. Therefore, such a BaO phase would exceed CaH2 in electron donating capability. We further examined Ru/BaOCaH2 through density functional theory (DFT) calculations and H2 TPD measurements to understand the role and working principle of the BaO phase in the catalyst. First, DFT was conducted to evaluate the electron donating power of CaH2 and BaH2 (detailed conditions are described in Methods). The calculated work functions for CaH2(1 0 0), BaH2(1 0 0) and their derivate surfaces are summarized in Table 2. The CaH2(1 0 0) surface has a large work function that reaches 4.9 eV, which is in good agreement with the experimental values of 4.8–5.1 eV measured for powder samples of CaH2 and indicates the reliability of the DFT calculations.38,39 When a part of the H– ions are removed as H atoms and hydride defects remain in CaH2, the work function of the resultant CaH2-1/9 e–1/9 (3.3 eV) is much below that of CaH2. For this reason, Ru/CaH2 exhibits high catalytic activity with a low activation energy.13 In the case of BaH2, the work function of BaH2-1/9 e–1/9 (2.6 eV) drops below that of CaH2-1/9 e– 1/9, which means that BaH2 with hydride defects transcends CaH2 with hydride defects in electron donating capability. Moreover, it was reported that Ba-N–H species, which are formed as an intermediate in ammonia synthesis with BaH2containing catalysts, have an essential role for efficient ammonia synthesis.40 Therefore, BaH2 loaded with Ru (Ru/BaH2) is considered to be a more efficient catalyst. However, the catalytic activity of Ru/BaH2 was so moderate that it could not be compared with those of the tested catalysts shown in Table 1. Figure 5 shows the H2-TPD profiles for Ru/CaH2, Ru/BaOCaH2 and Ru/BaH2 after reaction.41,42 Ru/CaH2 began to desorb H2 at 170 °C, and H2 desorption reached a maximum at ca. 230 °C. H2 desorbs from the CaH2 surface in the vicinity of the Ru particles and does not desorb from bare CaH2 in such a temperature range.13 Ru/BaH2 desorbed H2 at higher temperatures (peak top, 360 °C). Ru/BaH2 thus has a higher

energy barrier for hydride defect formation than Ru/CaH2. On the other hand, H2 desorbs from Ru/BaO-CaH2 at 150– 350 °C. Assuming H2 desorption from Ru/BaO-CaH2 occurs from CaH2 and BaH2, Ru/BaO-CaH2 is expected to facilitate H2 desorption from BaH2. Our previous studies demonstrated that ammonia synthesis activities at low temperatures are determined by the temperatures that H 2 is released from Ru-deposited Ca compounds such as Ru/C12A7:e–, Ru/Ca2N:e–, Ru/Ca(NH2)2 and Ru/CaH2.13 H– ions are stored in these catalysts via the hydrogen storage reaction (H + e– → H–) with electrons in the Ca-based materials under ammonia synthesis conditions. These electrons that react with H atoms exist as “cage electrons” and hydride defects in C12A7:e– and other Ca compounds, respectively. The hydrogen storage reaction decreases H adatoms on the Ru surfaces and prevents hydrogen poisoning. On the other hand, the accumulation of H– ions significantly decreases the electron density, which leads to high electron donating capability for Ru in these Ca compounds. Ru/C12A7:H–, where all cage electrons in Ru/C12A7:e– are replaced with H–, so that is much more inferior to Ru/C12A7:e– in terms of catalytic activity.15 H– ions must therefore be decreased by subsequent ammonia formation or H2 desorption via the hydrogen release reaction (H– →H + e–) to maintain the high catalytic performance based on strong electron-donating capability. In Ru/C12A7:e–, most ammonia had been confirmed to be formed from hydrogen atoms derived from H– ions.15 Hydride formation is thermodynamically more favorable than hydride defect formation, so that the activity for ammonia synthesis at lower reaction temperatures is largely dependent on the hydrogen release temperature. In this study, the DFT calculations and TPD measurements indicate that BaH2 can inherently exhibit high electron donating capability with the formation of hydride defects; however, it requires higher energy to produce hydride defects. Thus, BaH2 itself cannot function as an effective electron donating material under ammonia synthesis conditions. On the other hand, the BaH2 phase formed on Ru/BaO-CaH2 allows the hydrogen release reaction to occur at lower temperatures, so that high electron donating power is realized by the formation of BaH2-x. 3.4. Catalytic Mechanism. To summarize the foregoing results, a possible mechanism for the ammonia synthesis reaction over the Ru/BaO-CaH2 catalyst (Figure 6) is proposed as follows. By heating a simple mixture of a Ru source,

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ACS Catalysis CaH2 and BaO particles (a), BaO reacts with CaH2. A considerable part of the CaH2 surface is converted into CaO during reaction, which forms the BaO-BaH2 phase (b). While this reaction is accompanied by significant morphological change, probably due to the difference in the crystal structures between BaH2 and BaO, and the difficulty for Ca species to dissolve Ba species, the resultant material bulk is mainly composed of CaH2. Ru particles are preferentially deposited on the BaO-BaH2 phase (b). The BaO-BaH2 phase can allow hydrogen release and storage reactions, which lead to the formation of hydride defects and hydrides, respectively, through Ru particles (c). These functionalities are not due to the original BaO itself but probably to the BaH2 species. Hydride defect formation causes high electron donating capability originated from BaH2 and facilitates the dissociative adsorption of N2, which leads to high catalytic performance for ammonia synthesis with a small activation energy. The hydrogen storage reaction decreases hydrogen adatoms on the Ru surfaces, which represses hydrogen poisoning. BSE, EDX and XPS measurements indicate that most Ru nanoparticles in Ru/BaO-CaH2 are immobilized onto the BaO phase on Ca-based surfaces that mainly consist of CaO. Most Ru nanoparticles are not expected to contact with CaH2, whereas the H2-TPD for Ru/BaO-CaH2 (Figure 5) shows a H2 desorption peak at ca. 230 °C, which corresponds to the H2 desorption temperature of Ru/CaH2, although the H2 desorption peak of Ru/BaO-CaH2 is somewhat smaller than that of Ru/CaH2. This suggests that CaH2 links to Ru nanoparticles, possibly through BaO-BaH2.43 4. CONCLUSIONS A simple mixture of BaO and CaH2 powders acts as a stable and strongly electron donating material. The high electron donating capability of the material originates from BaH2 formed through the solid-state reaction of BaO with CaH2. BaH2 formed with significant morphological change allows the reversible hydrogen storage reaction and enhances catalytic ammonia synthesis through the strong electron donating capability without a decrease in activity and with a small activation energy.

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ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Experimental method about Kinetic analysis and DFT calculation, the catalytic activities of various catalysts, orders of reaction for ammonia synthesis, calculated surface energies, results of XPS, FT-IR, XRD and H2-TPD, and STEM images (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by a fund from the Accelerated Innovation Research Initiative Turning Top Science and Ideas into High-Impact Values (ACCEL) program of the Japan Science and Technology Agency (JST) and Grants-in-Aid for Japan Society for the Promotion of Science (JSPS) Fellows and for Scientific Research from the Ministry of Education, Culture, Science, Sports, and Technology (MEXT) of Japan (18H05251).. Discussions with K. Kamata and Y. Kita are acknowledged. We thank E. Sano and N. Watanabe for their technical assistance.

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Shifts the Bottleneck in Ammonia Synthesis. Nat. Commun. 2015, 6, 6731. Kitano, M.; Inoue, Y.; Sasase, M.; Kishida, K.; Kobayashi, Y.; Nishiyama, K.; Tada, T.; Kawamura, S.; Yokoyama, T.; Hara, M.; Hosono, H. Self-Organized Ruthenium-Barium Core-Shell Nanoparticles on a Mesoporous Calcium Amide Matrix for Efficient Low-Temperature Ammonia Synthesis. Angew. Chem. 2018, 130, 2678–2682. Bielawa, H.; Hinrichsen, O.; Birkner, A.; Muhler, M. The Ammonia-Synthesis Catalyst of the Next Generation: Barium-Promoted Oxide-Supported Ruthenium. Angew. Chem. Int. Ed. 2001, 40, 1061–1063. Szmigie, D.; Bielawa, H.; Kurtz, M.; Hinrichsen, O.; Muhler, M.; Rarog, W.; Jodzis, S.; Kowalczyk, Z.; Znak, L.; Zielinski, J. The Kinetics of Ammonia Synthesis over Ruthenium-Based Catalysts: the Role of Barium and Cesium. J. Catal. 2002, 205, 205– 212. Rosseti, I.; Pernicone, N.; Forni, L. Promotes Effect in Ru/C Ammonia Synthesis Catalyst. Appl. Catal. A 2001, 208, 271– 278. Truszkiewicz, E.; Pilecka, W. R.; Szalowski, K. S.; Jodzis, S.; Wilczkowska, E.; Lomot, D.; Kaszkur, Z.; Karpinski, Z.; Kowalczyk, Z. Barium-Promoted Ru/Carbon Catalyst for Ammonia Synthesis: State of the System when Operating. J. Catal. 2009, 265, 181–190. Fishel, C. T.; Davis, R. J.; Garces, J. M. Ammonia Synthesis Catalyzed by Ruthenium Supported on Basic Zeolites. J. Catal. 1996, 265, 148–157. Liang, C.; Li, Z.; Qiu, J.; Li, C. Graphitic Nanofilaments as Novel Support of Ru-Ba Catalysts for Ammonia Synthesis. J. Catal. 2002, 211, 278–282. Siporin, S. E.; Davis, R. J. Use of Kinetic Models to Explore the Role of Base Promoters on Ru/MgO Ammonia Synthesis Catalysts. J. Catal. 2004, 225, 359–368. For the preparation of Ru/CaH2-BaO, CaH2 and BaO were mixed, and then Ru(acac)3 was added to the resulting mixture by using an alumina mortar in an Ar-filled glovebox. In the case of Ru/CaH2 preparation, Ru(acac)3 was added to CaH2 in a similar manner as above. For the preparation of Ru (2 wt%)/CaH2-BaO and CaH2, the support and Ru3(CO)12 were mixed in an Ar-filled glovebox, and the mixture was then sealed in a silica tube and evacuated with a turbo molecular pump. Ru3(CO)12 was decomposed by heating the silica tube with a multi-stage temperature program: 2 °C min−1 up to 40 °C, hold for 1 h; 0.25 °C min−1 up to 70 °C, hold for 1 h; 0.4 °C min−1 up to 120 °C, hold for 1 h; 0.9 °C min−1 up to 250 °C, hold for 2 h, cooling to ambient temperature. Even though the catalyst amount and gas flow rate were altered, it was confirmed that a same WHSV gave the same ammonia formation rate. Ong, Phoung-V.; Johnson, L. E.; Hosono, H.; Sushko, P. V. Structure and Stability of CaH2 Surfaces: on the Possibility of Electron-Rich Surfaces in Metal Hydrides for Catalysis. J. Mater. Chem. A 2017, 5, 5550–5558. The diffraction peak assignable to CaNH (2 0 0) in the XRD pattern for Ru/CaH2 has a higher diffraction angle (2θ = 35.6°) than that of the reference CaNH. It was reported that CaNH mixed with CaH2 can be converted into unknown phase through thermal treatment and the diffraction peak of CaNH (2 0 0) appears at 2θ = 35.5° in such phase. The same thing is therefore expected to happen in Ru/CaH2. Hino, S.; Ichikawa, T.; Leng, H.; Fujii, H. Hydrogen Desorption

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Properties of the Ca-N-H system. J. Alloys Compd., 2005, 398, 62−66. (30) Wang, P. K.; Chang, F.; Gao, W. B.; Guo, J. P.; Wu, G. T.; He, T.; Chen, P. Breaking Scaling Relations to Achieve Low-temperature Ammonia Synthesis through LiH-Mediated Nitrogen Transfer and Hydrogenation. Nat. Chem. 2017, 9, 64–70. (31) Siporin, S. E.; Davis, R. J. Use of Kinetic Models to Explore the Role of Base Promoters on Ru/MgO Ammonia Synthesis Catalysts. J. Catal. 2004, 225, 359–368. (32) A mixture of BaO (10 mol%) and CaH2 (90 mol%) was used in BSE observation experiment to facilitate BaO particle observation. (33) A mixture of BaO (3 mol%) and CaH2 (97 mol%) was used in BSE observation. (34) Ru/BaO-CaH2 used in BSE observation was obtained by heating 10% of Ru loaded on a mixture of BaO (10 mol%) and CaH2 (90 mol%) at 340 °C for 100 h under NH3 synthesis conditions. (35) Lampert, W. V.; Rachocki, K. D.; Lamartine, B. C.; Haas, T. W. Electron-Spectroscopic Investigations of Ba and Ba Compounds. J. Electron Spectrosc. Relat. Phenom. 1982, 26, 133– 145. (36) Koenig, M. F.; Grant, J. T. XPS Studies of the Chemical State of Ba on the Surface of Impregnated Tungsten Dispenser Cathodes. Appl. Surf. Sci. 1985, 20, 481–496. (37) Franzen, H. F.; Merrick, J.; Umana, M.; Khan, A. S.; Peterson, D. T. XPS Spectra and Crystalline Potentials in Alkaline-Earth Chalcogenides and Hydrides. J. Electron Spectrosc. Relat. Phenom. 1977, 11, 439–443. (38) Kawano, H.; Serizawa, N.; Takeda, M.; Maeda, T.; Tanaka, A.; Zhu, Y. Desorption Energy of H– from Heated Saline Hydrides and Their Work Function Effective for Thermal Electron Emission. Thermochim. Acta, 1997, 299, 81–85. (39) Kawano, H.; Serizawa, N.; Takeda, M. Work Function and Desorption Energy of H– from Heated CaH2. Appl. Phys. Lett., 1995, 67, 3904–3905. (40) Gao, W. B.; Wang, P. K.; Guo, J. P.; Chang, F.; He, T.; Wang. Q.; Wu, G. T.; Chen, P. Barium Hydride-Mediated Nitrogen Transfer and Hydrogenation for Ammonia Synthesis: a Case Study of Cobalt. ACS Catal. 2017, 7, 3654–3661. (41) Each catalyst after reaction at 340 °C for 50 h was moved to a TPD cell made of quartz glass in the Ar-filled glovebox, and the TPD cell was attached to a TPD apparatus (BELCAT, MicrotracBEL) without exposure of the catalyst to the ambient air. The H2-TPD for the sample was measured in an Ar flow without any treatment. (42) H2-TPD was also examined for the sample after treating H2 atmosphere. Before TPD experiment, the sample was heated in a H2 flow (15 mL min-1) at 340 °C for 5 h and was cooled down to room temperature in an Ar flow. The H2-TPD profiles for Ru/CaH2 with and without H2 treatment are shown in Figure S10. There is no large difference in TPD profile between Ru/CaH2 samples with and without H2 treatment. (43) One possible explanation for the phenomenon is that Ru particles are located on the interface between CaH2 and BaO-BaH2. Another possibility is H— conduction from CaH2 through BaOBaH2. The details are currently under investigation.

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