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.1,2 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.3-5 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.
Driving both hydrogen production by water electrolysis and the HB process with only renewable electricity, such as wind power generation, is a promising way to produce ammonia without CO2 emissions. Nevertheless, it is significantly inferior to the conventional ammonia production using H2 obtained from fossil fuels in terms of energy consumption because the HB process operates at high temperature and pressure, and is driven only by electricity (Fig. 1a).3-6 Any catalyst that can allow the operating temperature to be decreased below 100‒150 °C would reduce the energy consumed for the HB process by 70 to 80% (Fig. 1b) and decrease the difference in energy consumption between conventional and CO2-free methods (Fig. 1a). The issue with CO2-free ammonia production has been also a significant issue in the HB process for over 100 years and has pushed recent research toward low temperature ammonia synthesis. Most of the recently reported highly active catalysts use Ru, Co, and Ni as reaction sites because these can be easily deposited on supports as highly dispersed metal nanoparticles that exhibit high catalytic performance.7-11 While we have also discovered a unique Ru-based heterogeneous catalyst to synthesize ammonia from H2 and N2, even below 100 °C,12 we have questioned whether catalysts that use these transition metals have significant potential to increase the catalytic activity at the desired reaction temperature of ≤100‒150 °C from the perspective of the H2 adsorption/desorption equilibrium. The dissociative adsorption of H2 and H2 desorption from the resulting adsorbed H atoms (H adatoms) are in an equilibrium on all transition metal surfaces in the synthesis of ammonia as well as N2 adsorption/desorption. It is well-known for Ru catalysts that adsorption of H adatoms onto the transition metal surfaces is preferential, so that the H2 adsorption/desorption equilibrium shifts toward H2 adsorption, which decreases ammonia formation by the decrease in adsorption sites for N adatoms (Fig. 1c).13,14 Such “hydrogen-poisoning” can affect all transition metals used in the synthesis of ammonia. Attributing hydrogen-poisoning to the bond strength between H adatoms and the surface transition metal atoms, it is anticipated that this effect is enhanced with a decrease in ammonia synthesis temperature because a larger amount of H adatoms would be more tightly adsorbed on the transition metal surfaces with a decrease in temperature (Fig. 1c). As a result, hydrogen-poisoning would be a major obstacle to the desirable low temperature ammonia synthesis over catalysts that employ transition metals as the active sites. In the present study, we have again focused on iron used as the reaction sites since the beginning of the HB process. Iron has been regarded as a classical transition metal that is inferior to other transition metals for ammonia synthesis. Several catalysts that use other transition metals exhibit much higher catalytic performance for ammonia synthesis than iron-based catalysts used in the present HB process.8,12 In addition, no effective ways to use iron for ammonia synthesis at low temperatures as low as 100 °C have been found to date. On the other hand, ammonia synthesis over iron-based catalysts has not been reported to be strongly influenced by hydrogen-poisoning. This suggests that the H2 adsorption/desorption equilibrium on iron surfaces is shifted more toward H2 desorption, and thus a decrease in the H adatom concentration than with other transition metals. Therefore, iron may be used for ammonia synthesis while preventing hydrogen-poisoning, even at low temperatures, if iron can be combined with an appropriate promoter in an appropriate manner. The use of ubiquitous, abundant, and inexpensive iron is also a significant advantage with respect to the environment and economy.
In several tested catalyst designs, only a new type of catalyst design, metallic iron particles loaded with a mixture of BaO and BaH2 (BaH2-BaO/Fe), was effective for low temperature ammonia synthesis. Conventional supported iron catalysts, where iron nanoparticles are deposited on support particles with electron-donating capability, could not act as catalysts for ammonia synthesis below 200 °C. BaH2-BaO/Fe was readily prepared by heating Ba(NO3)2-impregnated Fe2O3 particles (20‒40 nm) with CaH2 particles at 300 °C in a flow of H2. Figs. 2a and b for the resulting material display that Ba species composed of BaO and BaH2 are segregated on metallic iron particles (>10‒30 nm) in contact with CaH2. Therefore, the preparation procedure readily leads to iron particles loaded with a mixture of BaO and BaH2, as schematically shown in Fig. 2c. We reported that BaH2 can exhibit strong electron-donating capability in BaH2-BaO with deposited Ru nanoparticles (Ru/BaH2-BaO).15 A part of the BaH2 in BaH2-BaO releases H atoms from H– anions as H2 molecules through transition metal surfaces, so that electrons remain in BaH2 to release H atoms (BaH2 ↔ Ba2+H–(2–x)e–x + xH↔Ba2+H–(2–x)e–x + x/2H2). The strong electron donation from the BaH2 species (Ba2+H–(2–x)e–x) to N2 adsorbed on the transition metal surfaces facilitates N2 cleavage. The temperature at which H2 begins to desorb corresponds to the lowest catalyst working temperature. It was confirmed that ammonia synthesis over Ru nanoparticles deposited on Ca2+F–H– (Ru/CaFH) also proceeded at 50 °C in a similar manner to that over Ru/BaH2-BaO.12 H2-temperature programmed desorption (TPD) revealed that BaH2-BaO/Fe desorbs H2 above ca. 100 °C (Fig. 2d). As a result, BaH2 species (Ba2+H–(2–x)e–x) with strong electron-donating capability are formed that can donate electrons to metallic Fe at this temperature. The electron-donating capability of Ba2+H–(2–x)e–x was estimated by density functional theory (DFT) calculations. The work functions for the (001) surfaces of Ba2+H–2 and Ba2+H–(2–x)e–x (x=1/9, Ba2+H–(2–1/9)e–1/9) were estimated to be Φ=4.2 and 2.6 eV, respectively, by DFT computation (Extended Data Fig. 1). Thus, the removal of H atoms from BaH2 can form BaH2 species with high electron-donating capability, comparable to those of metallic K and Na (Φ=2.3 and 2.4 eV, respectively).
Fig. 3a shows the catalytic performance for ammonia synthesis (100-200 °C, 0.9 MPa) over BaH2-BaO/Fe and Ru/BaH2-BaO. In Ru/BaH2-BaO, Ru metal nanoparticles (2-4 nm) are deposited on BaH2-BaO phase formed on CaH2 large particles (several 30 μm).15 The results for a commercial promoted iron catalyst (promoted-Fe) that consists of Fe, K2O, Al2O3 and CaO, MgO loaded with Ru nanoparticles and Cs oxide species (Cs-Ru/MgO), and Ru-nanoparticles deposited on [Ca24Al28O64]4+(e–)4 (Ru/C12A7:e–) as benchmark catalysts are also shown. Extended Data Table 1 summarizes the ammonia synthesis activities of the tested materials and representative catalysts with physicochemical information, including metal particle sizes and surface areas. The promoted-Fe catalyst, which was first found by Haber, Bosch and Mittasch more than 100 years ago and has been improved since then, is not inferior to most of the recently reported catalysts; only a handful of recent catalysts surpass the iron catalyst in terms of ammonia synthesis activity (Extended Data Table 1).8,15-17 Nevertheless, promoted-Fe could not form ammonia below 200 °C. Cs-Ru/MgO and Ru/C12A7:e– also did not work for ammonia synthesis below 200 °C. In contrast, BaH2-BaO/Fe first revealed that iron can catalyze ammonia synthesis at 100 °C. The rate of ammonia formation increased with the reaction temperature. Ammonia formation proceeded over BaH2-BaO/Fe without a significant decrease in activity for over 100 h (Fig. 3b). Taking into account the number of zero-valent Fe atoms on the iron surface of the catalyst, the reaction for 100 h was estimated to give a turnover number of ca. 600 thousand (6.12×105); BaH2-BaO/Fe with a new type of catalyst structure acts as a stable catalyst.
Ru/BaH2-BaO was inferior to BaH2-BaO/Fe in ammonia formation rate below 200 °C, even although both catalysts use the same electron-donating material. The difference is further emphasized with respect to the ammonia formation turnover for the surface zero-valent Fe and Ru atoms (turnover frequency; TOF). In Fig. 3a, BaH2-BaO/Fe exhibits TOFs that are several thousand times higher than Ru/BaH2-BaO. In heterogeneous catalysts that adopt Ru, Co and Ni as the active sites for ammonia synthesis, the maximum TOF is at most less than 0.17 s-1, even at 400 °C (Extended Data Table 2).11 However, the TOF of the iron catalyst was 0.23 s-1 at 100 °C and reached 12.3 s-1 at 300 °C. To understand the difference in catalysis between Fe and Ru promoted by the same electron-donating material, the correlation of ammonia formation rate with total pressure and the reaction orders in the rate equation on BaH2-BaO/Fe and Ru/BaH2-BaO were measured at 200 °C and are summarized in Fig. 3c. The ammonia formation rate for Ru/BaH2-BaO is independent of the pressure or gradually decreased with the pressure, and the reaction order for H2 showed a negative value of -0.79; an increase in reactant concentration cannot lead to an increase in product formation. These results are inconsistent with kinetics theory and are clearly due to hydrogen-poisoning,13,14 where the Ru surface on Ru/BaH2-BaO is severely poisoned by H adatoms and cannot exhibit satisfactory catalytic performance for ammonia formation as shown in Fig. 1c. Such a hydrogen-poisoned Ru surface would not show a high TOF. On the other hand, the rate of ammonia formation increased in proportion to the pressure over BaH2-BaO/Fe to show a positive value of +0.47 as the reaction order for H2. As a result, the iron catalyst is not strongly affected by hydrogen poisoning, even at 200 °C, which results in a much higher TOF than those of Ru-based catalysts. One possible explanation for hydrogen-poisoning due to the H2 adsorption/desorption equilibrium on transition metals is the density and strength of the bonds between H adatoms and surface transition metal atoms. Because a larger amount of H adatoms is expected to be more tightly adsorbed on transition metal surfaces with a decrease in temperature, we may clarify the effect of hydrogen-poisoning by observing H2 desorption from BaH2-BaO/Fe and Ru/BaH2-BaO at low temperature. Fig. 3d shows molecular deuterium (D2)-TPD measurements for both the catalysts. The desorption of D adatoms due to gas phase D2 can be distinguished from the desorption of H adatoms originated from H‒ anions in BaH2-BaO through transition metals by the use of D2-TPD. Most D adatoms desorbed as D2 from the iron catalyst at 40‒100 °C. It was confirmed that D2 desorption from the iron catalyst is consistent with H2 desorption from a single crystal iron surface.19 This means that the iron surface provides sufficient adsorption sites for N2 and N adatoms without a strong influence of hydrogen-poisoning under ammonia synthesis conditions above 100 °C. In the case of Ru/BaH2-BaO, a large D2 desorption peak was observed at 125‒225 °C; Ru binds to H adatoms more tightly than iron, and these H adatoms can cause hydrogen-poisoning on Ru over a broad temperature range. Co and Ni surfaces have also been found to strongly adsorb H adatoms that are desorbed as H2 at > 150‒200 °C as with Ru.20-22 These results imply that many transition metals used to synthesize ammonia can be significantly affected by hydrogen-poisoning at low temperatures and iron is an exceptional transition metal that prevents hydrogen-poisoning. The difference in hydrogen-poisoning may be expressed as the difference in TOF among BaH2-BaO/Fe and the other transition metal-based catalysts.
Ammonia synthesis over the iron catalyst was further studied to understand the reaction mechanism. The energy diagram obtained from the heat of dissociative adsorption of N2 (∆H N2→2N), the apparent activation energies for ammonia synthesis (Ea NH3) and N2 desorption from N adatoms (Ea 2N→N2) on BaH2-BaO/Fe is shown in Fig. 4a with the details. From these thermodynamic data, the activation energy for N2 cleavage (Ea N2→2N) on BaH2-BaO/Fe was estimated to be below 16 kJ mol-1. This value is too small compared with the apparent activation energy (Ea NH3=40 kJ mol-1) for ammonia synthesis over BaH2-BaO/Fe if we regard Ea N2→2N as the apparent activation energy, i.e., the activation energy for the rate-determining step (Fig. 4a-A). Consequently, it is consistent that the rate-determining step of ammonia synthesis on BaH2-BaO/Fe is not N2 cleavage, but the formation of subsequent N-Hn species such as NH, NH2 and NH3 (Fig. 4a-B). In ammonia formation composed of N2 decomposition and N-Hn species formation, the former step has been regarded as the rate-determining step in most catalysts.23 To decrease the activation energy for the former below those of the latter by weakening strong N≡N triple bonds requires electron donation from the electron-donating materials to the antibonding π* orbitals of adsorbed N2 via transition metal d-orbitals to be boosted. In the Fourier transform infrared (FT-IR) spectrum (Fig. 4b) for N2 adsorbed on Ru/C12A7:e−, the νN2 band appeared at 2100-2200 cm-1, lower than that of N2 (2150-2250 cm-1) adsorbed on Ru nanoparticles-deposited Al2O3 support (Ru/Al2O3) because C12A7:e− (work function Φ=2.1 eV) has a stronger electron-donating capability than Al2O3 (Φ=4.7 eV).7 Ru/C12A7:e− is an exceptional catalyst to shift the rate-determining step from N2 cleavage to N-H species formation by the strong electron-donating capability of C12A7:e− as with BaH2-BaO/Fe.24 Fig. 4b also displays that νN2 for N2-adsorbed BaH2-BaO/Fe is observed as a broad band in the range of 2000-2175 cm-1, which is lower than of Ru/C12A7:e−, and disappears after the removal of N adatoms by evacuation. Thus, the iron catalyst is more electron-donating than Ru/C12A7:e−, which reduces the activation energy for N2 decomposition below those for the formation of N-Hn species.
As a summary of these results, the mechanism postulated for ammonia synthesis over BaH2-BaO/Fe is shown schematically in Fig. 4c. The shift of the H2 adsorption/desorption equilibrium toward H2 desorption on the iron surfaces does not only give the strong electron-donating capability to BaH2-BaO, but also provides sufficient adsorption sites for N2 molecules on iron surface. This results in a high TOF for ammonia synthesis over the iron catalyst to prevent hydrogen-poisoning. Therefore, Iron, which was first found to catalyze ammonia formation at high temperature by Haber, Bosch and Mittasch over a century ago, may be also effective for low temperature ammonia synthesis.
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