The lack of efficient catalysts for ammonia synthesis from N2 and H2 gases at the lower temperature of ca. 50 °C has been a problem not only for the Haber–Bosch process, but also for ammonia production toward zero CO2 emissions. Here, we report a new approach for low temperature ammonia synthesis that uses a stable electron-donating heterogeneous catalyst, cubic CaFH, a solid solution of CaF2 và CaH2 formed at low temperatures. The catalyst produced ammonia from N2 và H2 gases at 50 °C with an extremely small activation energy of 20 kJ mol−1, which is less than half that for conventional catalysts reported. The catalytic performance can be attributed lớn the weak ionic bonds between Ca2+ & H− ions in the solid solution and the facile release of hydrogen atoms from H− sites.
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The Haber–Bosch process currently enables the provision of food for over 70% of the world’s population, consuming 2% of global energy và generating 3% of global CO2 emissions1,2. These values would steeply increase by a rapid increase of the human population. Highly efficient conversion of N2 & H2 to ammonia with low-energy consumption has remained a challenge since the creation of the Haber–Bosch process, where sustainable ammonia production using natural energy is the ultimate goal. Ammonia is equilibrated in N2 and H2. Figure 1a demonstrates the correlation of the theoretical ammonia yield with reaction pressure và temperature. As the reaction temperature increases, ammonia decomposition as an endothermic reaction exceeds ammonia formation, an exothermic reaction, which decreases the ammonia yield. For this reason, the reaction system must therefore be further pressurized with an increase in the reaction temperature to lớn obtain the same ammonia yield; high reaction temperature causes high pressurization, which requires large energy consumption for both heating & pressurization. The iron-based catalysts used in the present Haber–Bosch process are effective for ammonia synthesis above 350 °C, so that the maximum ammonia yield is at most 30−40%, despite excess pressurization (>10–20 MPa) accompanied by large energy consumption. This is a serious drawback in sustainable ammonia production without the use of fossil fuels. While wind power generation has been reported khổng lồ be compatible with the Haber–Bosch process, the process itself consumes 40–50% of the electric power generated by wind turbine, và thereby electric power available for H2 production is considerably limited3. Figure 1a also indicates that the ammonia yield exceeds 98% at ca. 50 °C regardless of pressures, & there is no significant difference in ammonia yield among pressures below this temperature. Thus, a lower temperature is favorable for ammonia production with respect khổng lồ yield & energy consumption, & more efficient ammonia production is required to overcome the kinetic barrier at lower temperature lớn achieve the equilibrium. However, conventional catalysts equally thua trận the catalytic activity for ammonia formation from N2 and H2 at 100–200 °C, even if they exhibit high catalytic performance at high temperatures, as shown in Fig. 1a. Lowering the temperature for a loss of activity below 50 °C would largely enhance the catalytic activity for ammonia synthesis at low-temperature range below 300 °C. While there has been significant progress in homogeneous catalytic systems to lớn synthesize ammonia from N2 và H+ activated by specific & nonreusable reagents below room temperature4,5, guiding principles to lớn lower the temperature for a loss of activity on ammonia synthesis from N2 and H2 have yet lớn be clarified.
a Correlation of ammonia yield with temperature & pressure. b Arrhenius plots for ammonia synthesis over a commercial fe catalyst.
Thus, the lack of catalysts that are workable at lower temperatures has remained a problem for the Haber–Bosch process for over a century, & has also prevented sustainable ammonia production toward zero CO2 emissions. We have begun to lớn re-examine the low-temperature kinetics of ammonia synthesis catalysts to find a route for low-temperature ammonia production. Figure 1b shows Arrhenius plots for a commercial sắt catalyst6 with ammonia formation rates that were both measured (rMNH3) and estimated from the Arrhenius equation (rENH3). The reaction rate follows the Arrhenius equation as long as the reaction mechanism is unchanged in the temperature range; therefore, the reaction rate at a specific temperature was estimated using the Arrhenius equation. The difference between rMNH3 & rENH3 increases with a decrease in the temperature below 300 °C. The Arrhenius equation predicted a sufficient amount of ammonia to form at 100 °C. However, no ammonia formation was detected below 150 °C, which brought the natural logarithm of the rate close lớn –∞. Even if the catalyst amount và space velocity were increased significantly, ammonia formation was not observed below 150 °C. Taking into trương mục the detection sensitivity of the ammonia analysis methods, the rate of ammonia formation was expected khổng lồ be less than nano mol h−1 g−1. This means that the catalyst cannot act for ammonia synthesis at all below the temperature. The same phenomenon was confirmed in several representative catalytic systems for the synthesis of ammonia from N2 and H2 (Supplementary Table 1). This cannot be simply attributed khổng lồ deactivation by ammonia adsorbed on the catalyst, because ammonia that adsorbs on transition metals (TMs) such as Fe và Ru will desorb below room temperature7. Ammonia formation from N2 and H2 over catalysts proceeds through the dissociative adsorption of N2 (N2 → 2 N), followed by the hydrogenation of nitrogen adatoms (N → NH3). The former step has a higher energy barrier than the latter that proceeds on TM surfaces at room temperature khổng lồ 100 °C8. It is well-known that the cleavage of N2 molecules with ammonia synthesis catalysts is largely enhanced by electron donation from electron-donating materials into the π* orbitals of N ≡ N via the d-orbitals of TMs9,10,11, và the electron-donating capability of the TM itself is almost independent of the temperature below 200 °C12,13 (see Supplementary Discussion). A decrease in the electron-donating capability has been purported as one possible explanation for the lack of ammonia synthesis by catalysis at low temperatures. Therefore, stable materials that exhibit high electron-donating capability at low temperatures may lead khổng lồ the realization of low-temperature ammonia synthesis.
Here we present a new approach for low-temperature ammonia synthesis that uses a stable electron-donating heterogeneous catalyst, Ru nanoparticle-deposited cubic CaFH solid solution. The catalyst produces ammonia from N2 and H2 gases at 50 °C with an extremely small activation energy of 20 kJ mol−1, which is less than half that for conventional catalysts reported. The catalytic performance can be attributed to the weak ionic bonds between Ca2+ & H− ions in the solid solution & the facile release of hydrogen atoms from H− sites.
CaFH solid solution as a strong electron-donating material
As a first step to lớn verify the working hypothesis và to prepare such electron-donating materials, we have focused on calcium hydride (CaH2), a familiar dehydrating agent, because of its simplicity. TM nanoparticles deposited on CaH2 abstract H atoms from the near-surface CaH2 due khổng lồ substantial interaction between the TM and H–, & the H atoms move on khổng lồ the metal nanoparticles và desorb as H2 molecules, leaving electrons in the H– vacancy of CaH2 (CaH2 → Ca2+H–(2–x)e–x + xH)14. The resulting Ca2+H–(2–x)e–x behaves as a stable surface electride with a small work function (Φ = 2.7 eV) comparable with that of metallic Li15. The strong electron donation from Ca2+H–(2–x)e–x lớn the TM nanoparticles enhances the cleavage of N2 molecules, which leads lớn high catalytic performance for ammonia synthesis14. Supplementary Table 1 shows that the difference between the rMNH3 & rENH3 rates for the formation of ammonia over Ru-deposited CaH2 (Ru/CaH2) was not negligible at ≤ 200 °C, as with other catalysts; however, the catalyst was not at all active for ammonia formation at 150 °C. The temperature that eliminates the activity of Ru/CaH2 is therefore between 150 và 200 °C, which is similar khổng lồ that for conventional catalysts (Supplementary Table 1). A H2-temperature-programmed desorption (TPD) profile for Ru/CaH2 (Fig. 2a) revealed that the H2 desorption-onset temperature was almost identical lớn the temperature where the catalytic activity of Ru/CaH2 is lost. Formation of a strong electron-donating material is a determinant for ammonia formation over Ru/CaH2 at low temperatures. Therefore, lowering of the onset temperature for H2 desorption would lead to a catalytic system for ammonia synthesis at lower temperatures.
a H2-TPD profiles for Ru/CaH2 và Ru/CaFH after ammonia synthesis reaction at 340 °C, followed by cooling down below 20 °C. TPD measurements were performed under Ar flow (1 °C min−1). b XRD pattern for CaH2–BaF2 mixture (Ca:Ba =98:2) after heating at 340 °C for 10 h in H2. c Narrow-range XRD patterns for CaH2–BaF2 mixtures (Ca:Ba = 98:2, 9:1, 5:1, and 3:1) after heating at 340 °C for 10 h in H2.
Here, we have adopted a new strategy based on classical theory lớn lower the electride formation temperature: the introduction of F– anions into CaH2. F– is an extremely hard base in hard & soft acids & bases (HSAB), and the Ca−F ionic bond (529 kJ mol−1) is harder than the Ca−H bond (224 kJ mol−1)16. Replacing a part of H– in CaH2 with F– would weaken ionic bonds between Ca2+ and H–, thereby lowering the temperature for the release of H atoms from the material. This replacement would increase the energy of electrons trapped at H– vacancies due to electron repulsion between the electron và F–, which would cause a reduction in the work function of the surface region và in turn enhance the electron-donating power. In this study, F– was introduced into CaH2 by heating a simple mixture of CaH2 & BaF2 powders in a flow of H2 at 340 °C. A wide-range X-ray diffraction (XRD) pattern (Fig. 2b) of the resultant sample (Ca/Ba atomic ratio = 98:2) indicated that the sample is mainly composed of CaH2. Diffraction peaks of BaF2 or CaF2 were not observed in the XRD pattern, whereas the diffraction peaks due to BaH2 were apparent (Supplementary Fig. 1). Consequently, CaF2 is not formed in the heated mixture, despite the complete replacement of F– in BaF2 with H– derived from CaH2. CaH2–BaF2 mixtures in various CaH2/BaF2 ratios were heated in H2 to lớn identify the material formed in the heated CaH2–BaF2 mixture. Figure 2c shows narrow-range XRD patterns (2θ = 31–35°) of heated CaH2–BaF2 mixtures (Ca/Ba atomic ratios of 98:2, 9:1, 5:1, và 3:1), where an asymmetrical diffraction assignable khổng lồ (200) of the cubic CaF1.0H1.0 solid solution appears at 2θ = 32.7°17. It is well-known that orthorhombic CaH2 is transformed into a cubic structure in the formation of cubic CaFH solid solution17. The diffraction peak intensity increased, but was not shifted with an increase in the tía content. In order to clarify the characteristics of CaF1.0H1.0 solid solution formed on CaH2–BaF2 mixtures, CaFxH2–x solid solution (denoted as CaFxH2–x–CaF2) was prepared by heating mixtures of orthorhombic CaH2 và cubic CaF2 at 550 °C, a conventional method17. XRD patterns of CaFxH2–x–CaF2 (1.0 ≤ x ≤ 1.6) (Supplementary Fig. 2) elucidated that the (200) diffraction for the cubic CaFxH2–x–CaF2 is sensitive khổng lồ the F− concentration và shifts from 2θ = 32.7° to lower angles with decreasing F− concentration17. These results suggest that heating a CaH2–BaF2 mixture at 340 °C forms the most stable CaF1.0H1.0 on CaH2. In X-ray photoelectron spectroscopy (XPS) measurements for heated CaH2–BaF2 mixtures (Ca/Ba atomic ratio of 98:2), CaFxH2–x–CaF2 (x = 1) and CaH2 (Supplementary Fig. 3), the Ca 2p peaks for both heated CaH2–BaF2 mixtures và CaFxH2–x–CaF2 (x = 1) appeared at 346.5 eV, which is lower than that for CaH2 (347.3 eV) and supports the formation of the CaF1.0H1.0 solid solution on the surface of a small amount of BaF2-added CaH2. Next, Ru nanoparticles (12 wt%) were deposited on CaF1.0H1.0 phase obtained by heating a CaH2–BaF2 (Ca:Ba=98:2) mixture at 340 °C (denoted as Ru/CaFH), & Ru/CaFH after ammonia synthesis reaction over 30 h at 340 °C was examined by H2-TPD (Fig. 2a). The starting temperature for H2 desorption was lowered to the range of room temperature khổng lồ 50 °C, compared with that for Ru/CaH2. Supplementary Fig. 4 is the H2-TPD profile of the Ru-deposited CaFxH2–x–CaF2 (denoted as Ru/CaFxH2–x–CaF2 (x = 1)), which also indicates that H2 begins to desorb from the material at ca. 50 °C. This implies that the formation of the CaFH solid solution, i.e., the formation of Ca2+−F− ionic bonds, clearly weakens the Ca2+−H− bond and lowers the temperature for the hydrogen release reaction. A notable feature in the H2-TPD profile for Ru/CaFH is that it overlaps the H2-TPD profiles of Ru/CaH2 (Fig. 2a) & Ru/CaFxH2–x–CaF2 (x = 1) (Supplementary Fig. 4). As a result, the CaFH solid solution may coexist with CaH2 on the surface of Ru/CaFH.
To evaluate the electron-donating capability of CaFH, the work function of CaFH with H‒ vacancies was estimated by density-functional theory (DFT) computations (“Methods”, Supplementary Fig. 5). The work function on the most stable surface (111) of CaFH calculated by DFT was 2.2 eV, which is smaller than that (2.7 eV) for CaH2 with H− defects because electron repulsion between electrons & F– increases the energy of electrons trapped at H– vacancies. These results indicate that the abstraction of H atoms from CaFH forms a strong electron-donating material, which has a work function comparable to lớn that of metallic potassium (Φ = 2.3 eV).
Morphological information for Ru/CaFH is summarized in Supplementary Fig. 6. Ru/CaFH with a surface area of 30 m2 g−1 consists of irregular-shaped particles of 0.5–3 μm in diameter. The particle size of Ru deposited on CaFH was estimated to be 3–4 nm. XPS measurements for F 1 s & Ca 2p revealed that the surface atomic ratio of F lớn Ca (F/Ca) in Ru/CaFH was 0.08, which is much smaller than that expected from CaF1.0H1.0, & also supports the coexistence of CaFH và CaH2.
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Catalytic performance for ammonia synthesis at low temperatures
The catalytic performance for ammonia formation from N2 and H2 for tested catalysts is summarized in Fig. 3. Figure 3 also contains the results for CaH2 (Ru/CaH2), a BaH2–BaO mixture (Ru/BaH2–BaO)18, and Cs-doped MgO (Cs–Ru/MgO)6,10 loaded with Ru nanoparticles, và a commercial sắt catalyst6 for comparison. Cs–Ru/MgO has a higher catalytic activity for ammonia synthesis than the commercial fe catalyst above 300–400 °C. Ru/BaH2–BaO acts as a highly active ammonia synthesis catalyst18 that is comparable with Ru nanoparticles immobilized on Ca(NH2)2 containing Ba2+ (Ru/Ba–Ca(NH2)2), which exhibits the highest catalytic performance for ammonia synthesis among the reported catalysts over wide temperature (200–450 °C) and pressure ranges (0.1–0.9 MPa)6. Supplementary Table 2 shows the physicochemical information (surface area, porosity, & Ru particle size) và the rates of ammonia formation for Ru/CaFH, Ru/CaH2, Ru/Ba–Ca(NH2)2, Ru/BaO–BaH2, Ru/C12A719, Cs–Ru/MgO, & a commercial sắt catalyst as benchmark catalysts at 100–340 °C. It was confirmed that the catalytic activities of the commercial fe catalyst và Cs–Ru/MgO benchmark catalysts were comparable with those reported by other groups19,20,21. These conventional catalysts did not exhibit activity for ammonia synthesis below 100–200 °C, whereas Ru/CaFH synthesized ammonia at 50 °C. The ammonia formation over the catalyst at 50 °C was confirmed by both direct mass spectrometry & ion chromatography, & is not derived from N species formed on the catalyst during the catalyst activation at 340 °C (see Supplementary Discussion). The ammonia formation rate of Ru/CaFH increased with the temperature and was unchanged even after the rate measurement was repeated, which indicates that Ru/CaFH is a stable catalyst. There was no significant difference in XRD pattern between Ru/CaFH after reaction và CaH2–BaF2 mixture heated at 340 °C (Ca/Ba atomic ratio = 98:2, Fig. 2b, c). The apparent activation energy for ammonia synthesis over the catalyst in the range of 50–150 °C was estimated to lớn be 20 kJ mol−1, which is less than half that of reported catalysts (from 40 kJ mol−1)6,14,18,19,21. Furthermore, Ru/CaFH as a stable catalyst surpasses conventional catalysts at higher temperatures. Figure 3 gives the catalyst weight required for the equilibrium yield (CWEY) of ammonia at 200 °C (see Supplementary Discussion). The rates of ammonia formation & CWEYs for all tested catalysts, including Ru/Ba–Ca(NH2)2, & the recently reported highly active catalysts at 200–350 °C are summarized in Supplementary Table 322,23,24,25. Although Ru/Ba–Ca(NH2)2 & Ru/BaO–BaH2 have had much smaller CWEYs among the reported highly active catalysts, the CWEY of Ru/CaFH was only half that of both catalysts at 200 °C. In the case of Ru/CaFH, the apparent activation energy (Ea = 20 kJ mol−1) and coefficient due to the collision frequency (A = 10) in the Arrhenius equation estimated from the rates of ammonia formation at 50, 75, 100, & 125 °C were almost the same as those (Ea = 23 kJ mol−1, A = 12) obtained by the ammonia formation rates at 275–340 °C. In addition, there was no significant difference in Ea và A for ammonia formation over Ru/CaFH at 0.1 & 0.9 MPa. This suggests that the same active sites on Ru/CaFH size ammonia through a reaction mechanism in a wide range of reaction conditions (≥ 50 °C, ≥ 0.1 MPa). It was also confirmed in ammonia synthesis (240–400 °C) over a commercial sắt catalyst used in this study that Ea & A at 0.1 MPa are identical with those at 0.9 MPa6. Furthermore, Ru/CaFH produced ammonia without a decrease in activity for long periods of time & at higher temperatures (200 và 340 °C) (Supplementary Figs. 7 & 8). Ammonia formation over Ru/CaFH was close khổng lồ the equilibrium yield, even at ca. 300 °C, because of the high catalytic performance. As a result, ammonia synthesis over Ru/CaFH in Supplementary Fig. 8 reaches the equilibrium. Despite such equilibrium conversion (i.e., catalyst deactivation thử nghiệm conditions), the rate of ammonia formation over Ru/CaFH was constant for over 100 h. The amount of ammonia produced by Ru/CaFH at 340 °C exceeded the amount of the used catalyst (ca. 24 mmol) within 100 min. The XRD pattern & surface atomic ratio of F to Ca (F/Ca = 0.08) for Ru/CaFH were unchanged after reaction for 100 h, which was consistent with the lack of F species such as HF detected during the reaction. These results are clearly indicative of the stability of the Ru/CaFH catalyst.