Iron to catalyze ammonia synthesis at low temperature

doi:10.21203/rs.3.rs-1426351/v1

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Iron to catalyze ammonia synthesis at low temperature

https://doi.org/10.21203/rs.3.rs-1426351/v1

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posted 12 Apr, 2022

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Ammonia is used to provide food for over 5 billion people; however, it is currently required to be produced without the use of fossil fuels to reduce global CO2 emissions by 3% or more. To significantly decrease the energy consumed to produce ammonia from H2 and N2 with heterogeneous catalyst by the Haber-Bosch (HB) process, both H2 production from water and the HB process could be driven with only renewable electricity toward ammonia production without CO2 emissions. It is thus indispensable to devise heterogeneous catalysts for the synthesis of ammonia below 100‒150 °C to minimize the energy consumption of the process. Furthermore, the desired catalyst should be produced from ubiquitous and abundant resources on a large scale to maintain ammonia production that would reach 170 million tons per year. Here we report metallic iron as a catalyst for ammonia synthesis at 100 °C in combination with an electron-donating material as a new approach. The iron catalyst revealed that iron can exhibit a few hundred to several thousand times higher efficiency (turnover frequency) for ammonia synthesis than other transition metals used in highly active catalysts because of the intrinsic nature of iron to desorb adsorbed hydrogen atoms as hydrogen molecules at low temperature.

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Catalyst preparation.

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 (N_s) estimated on the basis of CO chemisorption values, assuming spherical metal particles. N_s 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 N_s is generally equal to that estimated by TEM observations.²⁹ However, the iron metal particle size of BaH₂-BaO/Fe was estimated from N_s to be larger than that (10‒30 nm) expected from HAADF-STEM and EDX images (Fig. 2b) because N_s 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 4f_7/2 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.

References

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30. Inoue, Y. et al. Direct activation of cobalt catalyst by 12CaO‧7Al₂O₃ electride for ammonia synthesis. ACS Catal. 9, 1670–1679 (2019).

Acknowledgments

This work was supported by a fund from the 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 (17H06153).

Author contribution

M. Hattori designed the study, carried out catalyst synthesis, catalyst evaluation and mechanistic study and composed the manuscript. N.O. and H.K. performed catalyst evaluation and characterization. M. Hara supervised the study, planned the experiments and analyzed the data.

There is NO Competing Interest.

Extended220302.docx
Extended Data Fig. 1, Extended Data Fig. 2, Extended Data Fig. 3, Extended Data Table 1, Extended Data Table 2

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Iron to catalyze ammonia synthesis at low temperature

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