Iron particles loaded with and without BaH2-BaO (BaH2-BaO/Fe, Fe particles). BaO-BaH2/Fe and Fe particles. 7.0 g of Fe(NO3)3∙9H2O (Kanto Chemical) was dissolved in 30 mL of distilled water. Water was removed from the solution by a rotary evaporator at 70 °C, and the remaining red solid was heated at 300 °C for 10 h in air to yield α-Fe2O3 powder. After 0.20 g of α-Fe2O3 and 0.052 g of Ba(NO3)2 (High Purity Chemicals) were stirred in 10 mL of distilled water for 30 min, water was evaporated at 70 °C. 0.023 g of the resultant Ba(NO3)2-impregnated α-Fe2O3 and 0.077 g of CaH2 were mixed in an alumina mortar inside an Ar-filled glovebox. The atomic ratio of Fe:Ba:Ca in the mixture was 12.5:1:100. The mixture was heated in a flow of H2 (45 mL min-1) at 300 °C for 2h, which resulted in BaO-BaH2/Fe. Fe particles, i.e., metallic iron particles without BaO-BaH2, were obtained by heating 0.019 g of α-Fe2O3 with 0.081 g of CaH2 in a flow of H2 (45 mL min-1) at 300 °C for 2h.
Ru or iron nanoparticles deposited on BaH2-BaO (Ru/BaH2-BaO). According to previous reports,15 Ru/BaH2-BaO was prepared by heating a mixture of 3 mol% BaO (Kojundo Chemical) and 97 mol% CaH2 with Ru(acac)3 corresponding to 10 wt% Ru or Fe at 260 °C in a flow of H2 (2.5 mL min–1). After 2 h, the samples were heated at 340 °C for 10 h in a H2 flow. In the resulting materials, transition metal nanoparticles are deposited on BaH2-BaO phase formed on CaH2 large particles (several 30 μm).15
Evaluation of catalytic performance. Ammonia synthesis over each catalyst was examined in a stainless steel fixed bed reactor (catalyst; 0.1 g) at 300 °C under a flow of N2−H2 (N2:H2 = 1:3, 60 mL min–1, weight hourly space velocity (WHSV): 36000 mL g cat–1 h–1) at 0.9 MPa. After no increase or decrease in activity was observed for over 20 h, the catalyst was cooled down to below 20 °C in a flow of N2 at a flow rate of 60 mL min–1 and then held under this flow for 5 h. After no ammonia formation was confirmed, the catalyst was heated at specific temperatures in a flow of N2−H2. 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 thermal conductivity detector. The rate of ammonia formation was repeatedly measured more than 3 times after the ammonia formation rate remained constant for over 1 h. It was verified that the measured rate of ammonia formation had an error of less than 5%.
TOF was calculated from the ammonia formation rate and the number of surface zero-valent transition metal atoms (Ns) estimated on the basis of CO chemisorption values, assuming spherical metal particles. Ns for each tested catalysts was measured by CO-pulse chemisorption (BELCAT-A, BEL, Japan) at 50 °C using a He flow of 30 mL min−1 and pulses of 0.09 mL (9.88% CO in He).7 Prior to these measurements, the catalyst after reaction was heated with flowing He (50 mL min−1) at 300 °C for 1 h. The stoichiometry of the transition metal/CO was assumed to be 1. The transition metal particle size calculated from Ns is generally equal to that estimated by TEM observations.29 However, the iron metal particle size of BaH2-BaO/Fe was estimated from Ns to be larger than that (10‒30 nm) expected from HAADF-STEM and EDX images (Fig. 2b) because Ns with the BaH2-BaO layer on iron particles taken into account is necessarily smaller than the number of surface iron atoms calculated only from particle size.
14N2-15N2 isotropic exchange reaction
N2 isotopic exchange was examined in a U-shaped glass reactor connected with a closed gas circulation system. A mixture of 15N2 and 14N2 (total pressure: 20.0 kPa, 15N2:14N2 = 1:4) was adsorbed on the catalyst without circulation at the reaction temperature until adsorption/desorption was in equilibrium. The change in the composition of circulating gas was monitored by a quadrupole mass spectrometer (M-101QA-TDM, Canon Anelva Co.). The m/z = 28, 29 and 30 signals were monitored as a function of time to follow the exchange.
Characterization. Powder X-ray diffraction (XRD; Miniflex600C, Rigaku) patterns were obtained using Cu Kα radiation. Nitrogen adsorption–desorption isotherms were measured at –196 °C with a surface-area analyzer (BELSORP-mini ΙΙ, MicrotracBEL) to estimate the Brunauer-Emmett-Teller (BET) surface areas. The morphologies of the samples were observed using HAADF-STEM and EDX (JEM-ARM 200F, Jeol). H2 and D2-TPD were measured by heating (1 °C min–1) a sample (ca. 100 mg) in a flow of Ar (30 mL min–1), and the concentration of H2 and D2 was monitored with a mass spectrometer (BELMass, MicrotracBEL, Japan). FT-IR spectra 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 to a closed gas-circulation system. The disk was heated under vacuum at 200 °C for 90 min. After the pretreatment, the disk was cooled to 25 °C under vacuum to obtain a background spectrum from the spectra of the N2-adsorbed samples. Pure N2 (99.99995%) was introduced into the system through a liquid-nitrogen trap. X-ray photoelectron spectroscopy (XPS; ESCA-3200, Shimadzu, Mg Kα, 8 kV, 30 mA) was performed in conjunction with an Ar-filled glovebox, where the samples were moved to the ultra-high vacuum XPS apparatus through the Ar-filled glovebox without exposure to the ambient air. The binding energy was corrected with respect to the Au 4f7/2 peak of Au-deposited samples.
DFT computations. Total energies and structural relaxations of BaH2 with/without surface H‒ anion defects were estimated from density functional theory (DFT) computation based on VASP first-principles code. We adopted the Perdew–Burke–Ernzerhof (PBE) exchange-correlation functional in DFT. The convergence criteria of energy and force were, respectively, 0.5 × 10–4 eV and 1.0 × 10–1 eV nm-1 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 1/4 (nm-1) in the reciprocal space. In BaH2, (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 the most stable surface for BaH2 (0.37 J m-2). A notable feature in the computation is that a vacuum region of 2 nm is maintained in the unit cell.
29. Larichev, Y. V. et al. XPS and TEM studies on the role of the support and alkali promoter in Ru/MgO and Ru-Cs+/MgO catalysts for ammonia synthesis. Appl. Surf. Sci. 111, 9427–9436 (2007).
30. Inoue, Y. et al. Direct activation of cobalt catalyst by 12CaO‧7Al2O3 electride for ammonia synthesis. ACS Catal. 9, 1670–1679 (2019).