Ammonia (NH
3) is a major nitrogen source for the production of fertilizers and other nitrogen-containing chemicals [
1,
2]. Furthermore, it is also increasingly recognized as an alternative hydrogen energy vector [
3-
5]. The current industrial NH
3 production heavily relies on the well-established Haber-Bosch (H-B) process using iron-based catalyst, which requires high temperatures and pressures (400−500 ℃, 10−30 MPa) [
6-
8]. Additionally, the supply of hydrogen (H
2) primarily comes from the steam reforming of natural gas or coal accompanying by high energy consumption, accounting for 1%−2% of the global energy supply and at least 500 million tons of greenhouse gas emissions per year [
1,
9,
10]. Therefore, under the background of energy conservation and carbon neutrality, it is essential to develop an energy-saving and environmentally friendly NH
3 synthesis process [
11]. Currently, the technology for producing H
2 via water electrolysis using renewable energy is becoming increasingly mature. Coupling water electrolysis with the H-B process for NH
3 synthesis can significantly reduce energy consumption and carbon emissions [
12,
13]. To achieve this, the development advanced catalysts that can produce NH
3 efficiently under mild conditions is crucial for NH
3 synthesis.
It is well known that the NH
3 synthesis reaction consists of several key steps, such as the activation of the reaction molecules N
2 and H
2, the formation of the intermediate NH
x species and the desorption of NH
3 [
14]. Among them, the dissociation of N
2 is generally considered to be the rate-determining step (RDS) for NH
3 synthesis. Therefore, extensive efforts have been dedicated to the development of novel catalysts aimed at investigating N
2 activation pathways to reduce the reaction energy barrier. In recent years, various types of support materials, including electride materials [
15], nitrides [
16-
18], hydrides [
19,
20], perovskite oxynitride-hydrides [
21], and intermetallic compounds [
22], supported transition metal catalysts have been reported to exhibit excellent catalytic activities for NH
3 synthesis under mild conditions. Compared to oxide supports, these hydrides and nitrides provide advantages in NH
3 synthesis due to their anion vacancies, which facilitate the activation or reaction with N
2. For example, Zhou
et al. [
23] investigated the impact of metal-anion interactions on NH
3 synthesis performance in different Zr materials (
i.e., ZrH
2, ZrN, and ZrO
2)-supported Ru catalysts. The results indicated that the moderate Ru-H interaction and easy hydrogen spillover in ZrH
2, which facilitates the activation of N
2 at the Ru sites. In addition to modifying the support composition, promoters also play a crucial role in enhancing the catalytic activity of metal catalysts. In the absence of electronic promoters, even industrially utilized Ru or Fe-based catalysts exhibit negligible catalytic performance [
24,
25]. At present, the promoters that are extensively studied in NH
3 synthesis primarily include alkali metals, alkaline earth metals, and rare earth elements [
26]. These element promoters facilitate unidirectional electron transfer to the surfaces of active metals, thereby increasing their electron density and enhancing catalytic performance. Unlike elemental promoters, fullerene C
60 has been reported to function as an "electron buffer" in metal-based catalysts, which can dynamically store and release electrons, thereby helping to balance the electron density of active metal species [
27,
28]. Additionally, the C
60 possesses a unique geometric structure and the ability to reversibly store and release hydrogen, which leads to the development of C
60-modified, oxide-supported Ru-based catalysts that mitigate the effects of hydrogen poisoning [
28]. Anchoring a controllable C
60 layer on Mo
2CT
x as a second H
2 adsorption site enhances their electronic interaction, substantially improving NH
3 synthesis performance [
29]. These unique properties enable C
60 to overcome the limitations of traditional element promoters and therefore exhibit potential for NH
3 synthesis under mild conditions.
Herein, C60-modified Ru/ZrH2 catalysts were prepared by a straightforward solvent-free ball milling method. Experimental characterizations demonstrate that the introduction of C60 decreases the size and increases the dispersion of Ru species, leading to the formation of more B5 active sites, which is beneficial to the activation of N2. Meanwhile, the interaction between Ru and C60 results in a significant decrease in the work function and an increase in the electron density of Ru atoms, as well as induces a shift in the d-band center. This promotes the activation of N2 and the desorption of produced NH3. Moreover, the presence of C60 promotes the migration of hydrogen atoms and accelerates the exchange of hydrogen species in ZrH2 with gaseous H2. Therefore, the C60-modified Ru/ZrH2 catalyst shows a higher NH3 synthesis rate than free-C60 sample. Our findings could stimulate broad interest in using C60 for NH3 synthesis under mild conditions.
Initially, pristine C
60 was investigated for NH
3 synthesis, and the corresponding result is depicted in
Fig. 1a. At 400 ℃ and 1 MPa, pristine C
60 exhibits inactivity, resulting in a negligible rate of NH
3 synthesis. Meanwhile, the NH
3 synthesis rate of pure ZrH
2 is only 0.1 mmol
NH3 g
cat-1 h
-1. Anchoring Ru onto ZrH
2 support (Table S1 in Supporting information) increases the NH
3 synthesis rate to 6.4 mmol
NH3 g
cat-1 h
-1. Further anchoring C
60 onto Ru/ZrH
2 results in an NH
3 synthesis rate of 12.5 mmol
NH3 g
cat-1 h
-1, which is about ~2 times as high as that of the Ru/ZrH
2 catalyst (
Fig. 1a). Moreover, a volcano-type dependence between N
2/H
2 vol ratio and NH
3 synthesis rate over C
60-Ru/ZrH
2 is observed, exhibiting the maximum of 15.6 mmol
NH3 g
cat-1 h
-1 at ~1/1 (Fig. S1 in Supporting information). To evaluate the intrinsic activity of Ru/ZrH
2 and C
60-Ru/ZrH
2 catalysts, the total turnover frequency (TOF
Ru total) and the surface turnover frequency (TOF
Ru surface) were calculated based on the total and surface Ru atom counts, respectively. Remarkably, the TOF
Ru total and TOF
Ru surface of C
60-Ru/ZrH
2 reach values as high as 7.80 × 10
–3 and 0.35 s
-1 at 400 ℃ and 1 MPa (
Fig. 1b), respectively. These values are significantly higher than those of Ru/ZrH
2, highlighting the crucial role of C
60 in enhancing intrinsic activity.
Fig. 1c displays the variation in the NH
3 synthesis rate as a function of reaction pressure for both Ru/ZrH
2 and C
60-Ru/ZrH
2 at 400 ℃. Clearly, the NH
3 synthesis rate over Ru/ZrH
2 and C
60-Ru/ZrH
2 exhibits an approximately linear increase with pressure rises from 0.2 MPa to 5.0 MPa, implying that utilizing ZrH
2 hydride as a support can mitigate the hydrogen poisoning effect caused by strong hydrogen absorption on Ru-based catalysts. Stability is a crucial factor for evaluating the catalytic activity of the C
60-promoted Ru/ZrH
2 catalyst in NH
3 synthesis. No significant deactivation of the NH
3 synthesis rate was observed during continuous operation for 67 h at 400 ℃ and 1 MPa (
Fig. 1d), confirming the long-term stability of the C
60-Ru/ZrH
2 catalyst. Moreover, the concentration of methane discerned in the exhaust is negligible and quite low (Fig. S2 in Supporting information), indicating that C
60 is stable under the adopted conditions for NH
3 synthesis. Additionally, high-performance liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (HPLC-APCI-MS) analysis of the used C
60-Ru/ZrH
2 catalyst shows a sharp peak located at ~720
m/
z (Fig. S3 in Supporting information), which is attributed to the C
60 molecule [
30]. Notably, most of the C
60 in the used C
60-Ru/ZrH
2 sample can be recovered after the reaction through the condensation reflux of the toluene solution. Subsequent XRD analysis further confirms that the structure of the recycled C
60 is similar to that of the fresh sample (Fig. S4 in Supporting information). All these observations indicate that the C
60-modified Ru/ZrH
2 sample is highly active and stable under the adopted NH
3 synthesis conditions.
To gain the mechanism information for NH
3 synthesis, kinetic analyses were conducted over Ru/ZrH
2 and C
60-Ru/ZrH
2. The apparent activation energy (
Ea) of the Ru/ZrH
2 catalyst is estimated to be 84 kJ/mol (
Fig. 2a), which decreases to 68 kJ/mol after the addition of C
60. Such low
Ea value for the C
60-Ru/ZrH
2 catalyst is significantly lower than that of conventional Ru-based catalysts (90–120 kJ/mol, Table S2 in Supporting information). These observations reveal that the introduction of C
60 could decrease the reaction barrier for NH
3 synthesis. The N
2 reaction order (
α) of the C
60-Ru/ZrH
2 catalyst is 0.79 (
Fig. 2b), which is remarkably lower than that of the Ru/ZrH
2 (1.36), proving that the addition of C
60 can promote the adsorption and activation of N
2 [
31]. The distinct differences in the
α value observed between the Ru/ZrH
2 and C
60-Ru/ZrH
2 catalysts can be ascribed to the variation of the size and electronic structure of the Ru entities after C
60 doping (
vide infra). Interestingly, both Ru/ZrH
2 and C
60-Ru/ZrH
2 catalysts exhibit a positive reaction order with respect to H
2 (
Fig. 2c). This observation suggests that the ZrH
2 support effectively avoids hydrogen poisoning effect on Ru catalysts [
32], which are consistent with the finding that the NH
3 synthesis rate increases monotonically with reaction pressure (
Fig. 1c). The absence of hydrogen poisoning effect may result from ZrH
2 is a strong electron-donating support material that has reversible hydrogen storage-and-release ability [
33,
34].
Moreover, the reaction order of NH
3 over Ru/ZrH
2 is −2.03 (
Fig. 2d), implying that the adsorbed NH
x species are too strong to desorb from the catalyst surface at low temperature, resulting in the requirement for high temperature to desorb the adsorbed NH
3 [
16,
35]. The addition of C
60 to the Ru/ZrH
2 catalyst results in the increase of the NH
3 reaction order to −1.38, indicating that C
60 induces weak adsorption of NH
3 on the catalyst surface, possibly due to its electronic effect (
vide infra). Overall, these kinetic analyses indicate that the introduction of C
60 could simultaneously enhance the activation of N
2 and weaken the adsorption of the product NH
3 on the catalyst surface.
To elucidate the impact of C
60 on the chemical state over the Ru/ZrH
2, the physicochemical properties of Ru/ZrH
2 and C
60-Ru/ZrH
2 were investigated. X-ray diffraction (XRD) was used to analyze the crystal structure of the prepared catalysts. As shown in Fig. S5a (Supporting information), the XRD patterns of the Ru/ZrH
2 and C
60-Ru/ZrH
2 mainly show the characteristic diffraction peak of ZrH
2 (JCPDS No. 073–2076). Two additional diffraction peaks at 38.38° and 44.02° for the Ru/ZrH
2 catalyst are attributed to the hexagonal Ru (100) and (101) crystal planes (JCPDS No. 089–4903) (Fig. S5b in Supporting information). However, the diffraction peaks related to Ru species disappear after the doping of C
60, indicating that the addition of C
60 decreases the size and promotes the dispersion of Ru species. Moreover, no diffraction peaks related to C
60 are observed, primarily due to the high dispersion of C
60 on the support or because the size of the C
60 layer falls below the detection limit of the XRD technique. Besides, the
13C-labeled solid-state nuclear magnetic resonance (ssNMR) spectrum of the C
60-Ru/ZrH
2 shows a peak at 143.8 ppm, corresponding to the characteristics of underivatized C
60 with
Ih symmetry [
36,
37], indicating that C
60 has been successfully loaded onto the catalyst (Fig. S6 in Supporting information).
Raman spectrum of pure C
60 shows three distinct bands at 267, 493 and 1457 cm
-1, which are attributed to the A
g(1), H
g(1) and A
g(2) vibrational modes of the fullerene C
60 [
38], respectively (Fig. S7a in Supporting information). All of the Raman bands of C
60 are also observed in the case of C
60-Ru/ZrH
2, proving that C
60 has been successfully incorporated into the Ru/ZrH
2 catalyst. Notably, the position of the A
g(2) peak is an important index of the charge transfer between C
60 and transition metals [
39,
40]. Compared to pure C
60 (1457 cm
-1), the A
g(2) peak of C
60-Ru/ZrH
2 shifts to 1462 cm
-1 (Fig. S7b in Supporting information), implying that the existence of a strong electronic interaction between Ru atom and C
60 clusters. The specific surface area of Ru/ZrH
2 is 5.8 m
2/g, which increases to 33.8 m
2/g for C
60-Ru/ZrH
2 (Table S1 in Supporting information). The increase of the specific surface area can be attributed to the addition of C
60, which improves the pore structure of the catalyst.
Transmission electron microscopy (TEM,
Fig. 3a) and high-angle annular dark-field scanning-TEM (HAADF-STEM, Figs. S8a-c in Supporting information) images of the Ru/ZrH
2 catalyst reveal the presence of large agglomerated Ru nanoparticles, with an average particle size of ~6.5 nm on the ZrH
2 support. Interestingly, the addition of C
60 leads to a decrease in the size of Ru, resulting in a uniform size of approximately 3.6 nm (
Fig. 3b and Figs. S8d-f in Supporting information). This observation further indicates that the introduction of C
60 facilitates the dispersion of the Ru nanoparticles, as confirmed by the results of the CO pulse experiment (Table S1). The TEM images of Ru/ZrH
2 and C
60-Ru/ZrH
2 primarily display two distinct lattice fringes with spacings of 0.21 and 0.28 nm (Figs. S9 and S10 in Supporting information), corresponding to the (101) crystal plane of metallic Ru and (101) crystal plane of ZrH
2 [
41,
42], respectively. Notably, the line scan profiles of C
60-Ru/ZrH
2 display that C
60 layers are wrapped around the periphery of the Ru/ZrH
2 (
Fig. 3c). Such continuous carbon shells could accelerate the charge transport [
43]. Besides, the energy-dispersive X-ray spectroscopy (EDX) mappings of C
60-Ru/ZrH
2 reveal the relatively homogeneous dispersion of Ru, Zr, and C elements throughout the catalyst (
Fig. 3d).
Previous studies have shown that the amount of exposed surface-active sites, such as B
5 sites, gradually increases as the size of Ru particles decreases [
44-
46]. Compared to Ru/ZrH
2, C
60-Ru/ZrH
2 exhibits a smaller Ru particle size, suggesting that more active sites are available for N
2 activation. To investigate the effect of C
60 on Ru adsorption sites,
in situ diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) measurement using CO as a probe over Ru/ZrH
2 and C
60-Ru/ZrH
2 was performed. As shown in
Fig. 4a, in addition to the two gaseous CO adsorption peaks at 2175 and 2115 cm
-1, a shoulder peak at 2066 cm
-1 observed over Ru/ZrH
2 can be attributed to multicarbonyl species adsorbed on oxidized Ru sites ([Ru
δ⁺-(CO)
x]) [
45,
46]. Two peaks located at 2019 and 1989 cm
-1 are attributed to the mono-adsorption of CO at the step site [
12] and the bonding of CO to the Ru bridge site at the Ru-support interface [
47,
48], respectively. Interestingly, the intensity of the absorption peaks for C
60-Ru/ZrH
2 is significantly higher than that of Ru/ZrH
2. This observation indicates that the addition of C
60 results in a significant increase in the number of exposed step and interface Ru sites, which can be attributed to the reduction in Ru particle size after C
60 doping. Compared to Ru/ZrH
2, the disappearance of the peak at 2066 cm
-1 and the slight redshift of the other two peaks (2012 and 1980 cm
-1) in the C
60-Ru/ZrH
2 catalyst indicate that the addition of C
60 is favourable for stabilizing low-valence state Ru
0 species [
49].
Moreover, the chemical states of the Ru species on Ru/ZrH
2 and C
60-Ru/ZrH
2 were investigated using X-ray photoelectron spectroscopy (XPS). The Ru 3p XPS spectrum of Ru/ZrH
2 shows two peaks at ~462.1 and 484.3 eV, corresponding to Ru 3p
3/2 and Ru 3p
1/2, respectively (
Fig. 4b) [
50]. Clearly, the Ru 3p spectrum of C
60-Ru/ZrH
2 is negatively shifted by 0.4 eV compared to that of Ru/ZrH
2, indicating that the Ru species in C
60-Ru/ZrH
2 exhibit a state of electron enrichment. Furthermore, the Ru 3p XPS spectra of these two catalysts can be further deconvoluted into multiple peaks, which can be assigned to Ru
0 and Ru
δ+ [
51]. Notably, the ratio of Ru
0/(Ru
0+Ru
δ+) for C
60-Ru/ZrH
2 is 52.4%, which is higher than the ratio for Ru/ZrH
2 at 46.7%. This indicates that the addition of C
60 leads to the formation of a greater number of metallic Ru sites.
To further identify the electronic structure of the Ru sites, synchrotron radiation-based near-edge X-ray absorption fine structure spectroscopy (NEXAFS) was conducted. The white lines observed at the Ru L
3- and L
2-edges in the NEXAFS primarily reflect electron transitions from the Ru core 2p
3/2 and 2p
1/2 to the unoccupied 4d
5/2 and 4d
3/2 orbitals [
52,
53], respectively. Therefore, the intensity of the white line can be utilized as a metric for assessing the empty state density of the Ru d orbitals or the quantity of 4d holes [
54]. As shown in
Fig. 4c, the C
60-promoted Ru/ZrH
2 catalyst exhibits a significantly lower white-line intensity compared to that of Ru/ZrH
2, indicating that more occupied states are formed in the Ru d orbitals after C
60 doping. This phenomenon arises from the strong electronic coupling interaction between the C
60 and Ru atoms, resulting in the formation of fewer 4d holes, which is consistent with the XPS results. The extent of electron transfer is influenced by the number of unoccupied d orbitals (
KhT) at the Ru sites, which can be semi-quantitatively assessed using the Ru L-edge NEXAFS spectra [
54]. The
KhT value for C
60-Ru/ZrH
2 (0.53) is obviously lower than that of Ru/ZrH
2 (0.74) (
Fig. 4d and Table S3 in Supporting information). This indicates that the introduction of C
60 induces a strong interaction between the C
60 π-orbitals and the Ru 4d orbitals, resulting in a substantial decrease in the amount of unpaired Ru 4d charge.
To further investigate the impact of C
60 addition on the electronic structure of these catalysts, ultraviolet photoelectron spectroscopy (UPS) measurements were conducted (
Fig. 5). The work function (
ΦWF), determined from the cutoff of the secondary electron emission (magnified image i), is 3.79 eV for Ru/ZrH
2 and 3.66 eV for C
60-Ru/ZrH
2, respectively. The doping of C
60 decreases the work function, enabling that the trapped electrons at the C
60-Ru/ZrH
2 interface are loosely bound. This results in a decreased barrier for electron donation from the Ru sites to the anti-bonding
π orbitals of the adsorbed N
2 [
55,
56]. Consequently, this weakens the N≡N bond and promotes the activation of N
2. Moreover, upon the addition of C
60, the d-band center (ε
d, as shown in the magnified image ii) of the Ru/ZrH
2 catalyst shifts downward from −1.51 eV to −1.64 eV with respect to the Fermi energy. This downshift in ε
d for the C
60-Ru/ZrH
2 indicates a decrease in the antibonding energy state, which results in a weakened interaction between the adsorbed intermediate species NH
x (where
x = 0–3) and Ru metal. This weakening is beneficial for the NH
x desorption process, as evidenced by the significantly higher positive NH
3 reaction order observed with C
60-Ru/ZrH
2 compared to Ru/ZrH
2. These findings suggest that the introduction of C
60 plays a crucial role in reducing the work function and lowering the d-band center of the Ru/ZrH
2 catalyst. This adjustment simultaneously meets the electronic requirements for promoting N
2 activation and facilitating NH
x desorption. This effect may be attributed to the function of C
60 as an electron buffer, which helps balance the electron density of the active metal [
27,
28].
Previous studies have shown that negative hydrogen (H
-) ions in hydrides are reactive during NH
3 synthesis process [
57-
59]. To determine whether H
- ions in ZrH
2 can participate in the formation of NH
3, we exposed the C
60-Ru/ZrH
2 catalyst to pure N
2 to detect the NH
3 yield at 400 ℃ and 1 MPa. With the progress of the reaction time, the accumulated amount of NH
3 at the outlet gradually increased and approached a steady state after 12 h (Fig. S11 in Supporting information), indicating that the H
- ions in ZrH
2 can combine with the N species to form NH
3. To further investigate the reactivity of H species in C
60-Ru/ZrH
2, an isothermal surface reaction experiment was performed at 400 ℃. As shown in Fig. S12 (Supporting information), the NH
3 signal gradually decreases with the introduction of 5%N
2/Ar gas, indicating the depletion of H species and the subsequent formation of H vacancies. When the gas was switched to 10%H
2/Ar, the NH
3 signal was largely restored, indicating that the H species in ZrH
2 can be replenished and react with the adsorbed N species to form NH
3. A similar isothermal surface reaction experiment was performed with isotopic D
2 to further verify that H species in the catalyst can be supplemented (Fig. S13 in Supporting information). When D
2 gas was introduced, signals for ND
3 (
m/
z = 20) and ND
2H (
m/
z = 19) were detected, revealing that the D species in the replenished C
60-Ru/ZrH
2 originated from gas-phase D
2 [
60]. Additionally, a stronger HD (
m/
z = 3) signal is observed from the C
60-modified Ru/ZrH
2 catalyst compared to that from the Ru/ZrH
2 (Fig. S14 in Supporting information). This observation demonstrates that the presence of C
60 enhances the activation of H
2 and facilitates the exchange of lattice H species with gaseous H
2. H
2-temperature programmed desorption (H
2-TPD) curve (Fig. S15 in Supporting information) for Ru/ZrH
2 shows two distinct H
2 desorption peaks: one in the range of 420–600 ℃ (
α) and another in the range of 600–800 ℃ (
β), corresponding to the desorption of H
2 from the Ru surface or subsurface sites and the ZrH
2 support, respectively. Notably, the introduction of C
60 into Ru/ZrH
2 increases the intensity of the two H
2 desorption peaks and shifts these peaks to lower temperature (
α peak), indicating that the presence of C
60 enhances the spillover effect of hydrogen species while weakening the adsorption strength at the subsurface sites. Moreover, the presence of C
60 accelerates the migration and spillover of hydrogen species, as further demonstrated by the WO
3 color change experiment [
28,
61]. As shown in Fig. S16 (Supporting information), pure WO
3 does not exhibit significant color changes. However, a more pronounced color change is observed in the C
60-modified Ru/ZrH
2 compared to the unmodified Ru/ZrH
2 during hydrogen treatment. This indicates that the addition of C
60 enhances the migration of adsorbed hydrogen species. Moreover, the addition of C
60 significantly increases the amount of NH
3 desorption compared to the C
60-free Ru sample (Fig. S17 in Supporting information). This suggests that the presence of C
60 facilitates the formation of more active sites with weaker NH
3 adsorption, which is consistent with the remarkable increase in the NH
3 reaction order after C
60 doping (
Fig. 2d).
To elucidate the reaction pathway for NH
3 synthesis, we conducted isotope reaction experiments using a C
60-Ru/ZrH
2 catalyst with a feedstock gas mixture of
15N
2–
14N
2H
2 at a temperature of 400 ℃. If the dissociation of N
2 is the rate-determining step (RDS), then the dissociated nitrogen (N) atoms can immediately react with hydrogen species to form NH
3. Conversely, if the formation of N—H
x is the RDS, the dissociated N atoms may accumulate and exchange to form
14N
15N species [
21,
42]. As shown in Fig. S18 (Supporting information), a distinct signal for
14N
15N (
m/
z = 29) is observed in the C
60-Ru/ZrH
2 catalyst compare to that of Ru/ZrH
2, which proves the shift of RDS from the dissociation of N
2 to the formation of the N—H
x bond. This may be attributed to the lower activation energy (
Fig. 2a) and the smaller reaction order for N
2 (
Fig. 2b) associated with the C
60 modification. As a result, the C
60-Ru/ZrH
2 catalyst can efficiently activate N
2, leading to outstanding NH
3 synthesis performance (
Scheme 1).
To summarize, we developed a C60-modified Ru/ZrH2 catalyst that serves as an efficient catalyst for NH3 synthesis at mild conditions. Our results demonstrate that the strong interaction between the C60 and Ru entities not only regulates the charge density of Ru atoms but also promotes the dispersion of Ru species to expose more step sites. Meanwhile, the presence of C60 facilitates the exchange of lattice H species in ZrH2 with gaseous H2 and accelerates the hydrogen spillover from Ru metal to the catalyst surface. Therefore, the NH3 synthesis rate of the C60-modified Ru/ZrH2 catalyst is approximately 2 times as high as that of the C60-free Ru sample at 400 ℃ and 1 MPa. This enhancement is attributed to the co-optimization of nitrogen activation and hydrogen migration resulting from the addition of C60. Moreover, the incorporation of C60 adjusts the d-band center, facilitating the desorption of NH3 from the catalyst surface and thereby freeing up additional active sites for the continuous activation of N2. These findings broaden the range of promoters available for the NH3 synthesis reaction.
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