Electromagnetic field-assisted low-temperature ammonia synthesis

doi:10.21203/rs.3.rs-2835841/v1

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Electromagnetic field-assisted low-temperature ammonia synthesis

https://doi.org/10.21203/rs.3.rs-2835841/v1

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Conventional ammonia synthesis (Haber-Bosch method) requires high temperature and high pressure, leading to extensive energy consumption and carbon emissions. Herein, an electromagnetic field (EMF)-assisted technique is developed for ammonia synthesis under mild conditions. With the assistance of EMF, commercial Fe-based catalysts exhibit obvious enhancement in ammonia yield (5 times) and energy efficiency (2.7 times) at 200°C and 1 MPa, originating from the effective activation of the N≡N bond.

Physical sciences/Chemistry/Chemical synthesis/Synthetic chemistry methodology

Physical sciences/Chemistry/Green chemistry/Sustainability

Ammonia (NH₃), as one of the most common industrial chemicals, is essential for nitrogenous fertilizer production and shows potential as a next-generation green fuel^1,2. Industrial ammonia synthesis relies on the reaction of fossil-fuel-derived hydrogen and nitrogen (Haber-Bosch method) under high temperature (~500 °C) and high pressure (>15 MPa), which consumes ∼2% of global power and discharges ∼1.5% of global greenhouse gas³^,⁴. Electrolysis of water driven by renewable energy such as wind/solar power can provide carbon-free hydrogen for ammonia synthesis. However, the harsh conditions of the Haber-Bosch (H-B) process induce continuous operation with a high load, which mismatches the volatility of wind/solar power. Thus, the development of advanced technology for ammonia synthesis under mild conditions, especially low pressure, is promising to broaden the loading range with low energy consumption.

Ammonia synthesis is an exothermic process accompanied by a stoichiometric number decrease; that is, low temperature and high pressure are favorable for increasing the equilibrium concentration. At present, the ammonia synthesis concentration is ~14% at 15 MPa and 500 °C, close to the equilibrium concentration (14.71%)⁵. From the point of thermodynamics, lowering the temperature is the key to simultaneously decreasing the operation pressure and maintaining a high ammonia concentration. However, the nonpolar N≡N triple bond with a large bonding energy (941 kJ mol^-¹) is difficult to activate and break. As a result, high temperature is required to accomplish high yield from the point of dynamics view. This conflict has retarded the development of ammonia synthesis in recent decades. Herein, we develop an electromagnetic field (EMF)-assisted H-B technique for ammonia synthesis under mild conditions by adopting commercial iron-based catalysts (Fig. 1a). The onset temperature with EMF assistance is 100 °C, which is obviously lower than that without EMF assistance (300 °C). In a typical scale-up experiment with 80 g commercial catalysts, the EMF-assisted H-B technique obviously increases the ammonia yield (~5 times) and decreases the energy consumption (~2.7 times). The enhanced catalytic performance can be ascribed to the EMF inducing more electron transfer from Fe orbitals to N≡N orbitals in both side-on and end-on adsorption modes.

EMF-assisted ammonia synthesis technique

A homemade EMF-assisted ammonia synthesis system is constructed as shown in Fig. 1b. An AC power supply outputs a high voltage to create an EMF. The current, voltage, and frequency of the applied electric field as well as the intensity and frequency of the induced magnetic field are recorded on an oscilloscope and magnet meter, respectively (Supplementary Figs. 1–2 and Supplementary Notes 1–2). A digital photograph of the key EMF reactor is provided in Fig. 1c, in which a quartz tube with an internal metal-rod electrode and an external metal-mesh electrode is surrounded by a heating apparatus. The commercial Fe-based catalyst is filled in the quartz tube (Fig. 1d, Supplementary Fig. 3 and Supplementary Note 3). When the external heating apparatus is set at 200°C, infrared thermal imaging of the reaction zone shows 205°C after the introduction of the EMF (Fig. 1e), excluding the effect of the electromagnetic system on the reaction zone temperature because the applied average current is very low (< 10 mA).

Ammonia synthesis performance.

The ammonia synthesis test is conducted with and without EMF assistance (Fig. 2a, Supplementary Fig. 4 and Supplementary Note 4). The onset temperature under EMF assistance is 100°C at 1 MPa, obviously lower than that without EMF (300°C), indicating the function of EMF in decreasing the activation temperature of nitrogen. Under 350°C and 1 MPa, the EMF assistance strategy produces ammonia with a concentration of 6270 ppm, approximately 9 times higher than that of the H-B technique (688 ppm). Moreover, a scale-up experiment with 80 g Fe-based catalysts is conducted at 200°C and 1 MPa to prove the application prospects of the EMF assistance technique (Fig. 2b and Supplementary Fig. 5). Impressively, the EMF assistance technique obviously increases the ammonia yield (~ 5 times) and decreases the energy consumption (~ 2.7 times) in the scale-up experiment. In particular, the produced ammonia concentration surpasses the state-of-the-art low-temperature ammonia synthesis bulk catalyst of Ni/LaN (Supplementary Table 1)⁸. Note that we also construct a pilot-scale EMF assistance system for green ammonia synthesis with a production capacity of 10000 kg year⁻¹ (Supplementary Fig. 6 and Supplementary Note 5). Furthermore, a series of comparison experiments exclude the possible effect of plasma on the EMF assistance technique for ammonia synthesis (Supplementary Fig. 7, Supplementary Table 2 and Supplementary Note 6).

Mechanistic studies.

To gain insight into the EMF assistance mechanism, density functional theory calculations are carried out. An external electric field and spin-polarized simulations are applied to mimic the EMF effects on ammonia synthesis. As shown in Fig. 2c, N₂ adsorption on the Fe catalyst with EMF assistance is stronger than that without EMF assistance for both side-on and end-on N₂ adsorption modes. Meanwhile, the N‒N bond length is slightly longer under EMF assistance (Figs. 2d-g), implying that the N\(\equiv\)N bond is weakened under EMF. Electron transfer between the Fe catalyst and adsorbed N₂ is displayed via the charge density difference (Figs. 2h-2k and Supplementary Fig. 8). For the side-on adsorption mode, the π bonding and anti-bonding orbitals of N ≡ N play an important role in the “acceptance-donation” activation mechanism⁹. Unoccupied π anti-bonding orbitals of N ≡ N accept electrons from occupied Fe d orbitals, and occupied π bonding orbitals of N ≡ N donate available electrons to unpaired d orbitals of Fe. Moreover, the σ anti-bonding orbital of N₂ also accepts electrons from Fe d orbitals in side-on adsorption mode under an external electric field (Fig. 2h, 2i and Supplementary Fig. 8a, 8b). For the end-on adsorption mode, the σ orbitals of N₂ obtain more electrons under an external electric field (Fig. 2j, 2k and Supplementary Fig. 8c, 8d). Overall, N₂ can attract more electrons under an external electric field, facilitating N₂ activation. The adsorption becomes stronger with increasing electric field intensity (Supplementary Fig. 9 and Supplementary Note 7). The positive influence of the magnetic field on N₂ adsorption is also proven by the calculations without the consideration of spin polarization (Supplementary Fig. 10 and Supplementary Note 8).

In conclusion, we developed a distinctive strategy for ammonia synthesis under mild conditions by introducing an alternating EMF. By adopting 80 g commercial Fe-based material as a model catalyst, EMF assistance obviously increases the ammonia yield (~ 5 times) and energy efficiency (~ 2.7 times) at 200°C and 1 MPa. The applied EMF induces the transfer of more electrons from Fe to N ≡ N in both side-on and end-on adsorption modes, promoting the adsorption and activation of inert N₂. This work offers an industry-ready avenue for low-temperature and low-pressure ammonia synthesis in a sustainable way.

Materials

Commercial Fe-based catalysts with particle sizes of 1–2 mm were obtained from Linqu FengTai Chemical Co., Ltd.

Ammonia synthesis test. A homemade electromagnetic field-assisted fixed-bed quartz reactor mounted in a tube furnace was used for ammonia synthesis. The reactor structure is illustrated in Fig. 1c. A stainless steel rod located inside the quartz reactor served as the high-voltage electrode, while a stainless steel mesh twined (length is 5 cm) outside the quartz reactor served as the low-voltage electrode. Commercial Fe-based catalysts were filled into the reactor. A mixture of H₂ and N₂ (volume ratio of 3:1 with 99.99% purity) was adopted as the feeding gas. Before the reaction, the Fe-based catalysts (5 g) were prereduced at 500°C for 72 h with a heating rate of 2°C min⁻¹ by the aforementioned H₂ and N₂ mixture (volume ratio of 3:1 with 99.99% purity). Then, the ammonia synthesis reaction was conducted under the following conditions with a space velocity of 12000 h_cat⁻¹, pressure of 1 MPa, temperature range of 100–400°C, power of 40 W, root mean square of voltage (U_RMS) of 7800 V, and high-frequency electric field of 8.6 kHz. The concentration of produced ammonia was detected via an online ammonia analyser until the reaction leveled off. The parameters of the applied alternating electric field were recorded by an oscilloscope equipped with a high voltage probe and current probe. The parameters of the alternating magnetic field were recorded by a magnet meter with a Hall probe.

Scale-up experiment. Commercial Fe-based catalysts of 80 g and a stainless steel mesh of 15 cm were used in the scale-up experiment at 200°C. The power and pressure employed in the scale-up experiment were 80 W and 1 MPa, respectively. The applied U_RMS is 12000 V, and the frequency of the electric field is 8.6 kHz. Other conditions were the same as the previous tests.

Pilot-scale test. The pilot-scale EMF assistance system consists of water electrolysis (3 Nm³ h⁻¹ for hydrogen production), air pressure swing adsorption (PSA) separation (5 Nm³ h^− 1 for nitrogen production) and a green ammonia synthesis reactor (10000 kg year⁻¹ for green ammonia production).

Other experimental details have been added to the Supplementary Information, including Materials characterizations, determination and quantitation of ammonia, and computational methods.

Acknowledgement

We acknowledge the National Natural Science Foundation of China (Nos. 22071173 and 22003047), the Haihe Laboratory of Sustainable Chemical Transformations for financial support and the Natural Science Foundation of Tianjin City (No. 20JCJQJC00050).

Author Contributions

B.Z., Y.Y., and B.S.Z. designed the experiments. B.S.Z., J.Z, Y.W and W.J. carried out experiments and measurements. N.Z. and X.C. performed theoretical calculations. B.S.Z., J.Z. and F.C. drew the schematic diagram. Y.W, Y.Y., B.S.Z. and B.Z. wrote the paper with comments from all authors.

Competing interests

The authors declare no competing interests.

Data and materials availability

All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

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Electromagnetic field-assisted low-temperature ammonia synthesis

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