Solid solution for catalytic ammonia synthesis from nitrogen and hydrogen gases at 50 °C.

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 and CaH2 formed at low temperatures. The catalyst produced 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+ and H− ions in the solid solution and the facile release of hydrogen atoms from H− sites.

The authors applied a solid mixture of CaF 2 and CaH 2 in ammonia synthesis, which showed small activity at 50 o C with a low activation energy barrier of 20 kJ mol -1 . But, there are many issues yet remained to answer than it can offer. In its present state, the manuscript is not considered suitable for publication in Nature Communications for the following reasons: 1. It is certainly 'eye-catching' and attractive to develop ammonia synthesis as low as 50 o C at ambient temperature, instead of using conventional 400-450 o C of over 150 bars. However, there have been numerous issues that cannot be achieved such goal in the past. First, the authors used thermodynamics equilibrium and heat of reactions to justify the needs for low temperature development of this reaction using the wind energy. However, from catalysis point of view, their argument does not carry any useful sense to lead to the catalysis development. For all exothermic catalytic reactions including ammonia synthesis, if one can achieve equilibrium, there is of course to argue that the equilibrium will become unfavourable at higher temperatures, hence using low temperatures would be an advantage.
But, the problem is that whether one can overcome the kinetic barrier at low temperature to achieve the equilibrium. Many industrial processes are exothermic but still be carried out at elevated temperatures due to unfavourable low rate at low temperature rather than their equilibrium position. If the conventional catalysts can get higher rates for ammonia synthesis at lower temperature, then the present extreme conditions would not be used: whether this is for non-renewables or wind energy, it is not the key factor. Using pressure not only affect the equilibrium position but also increase the kinetics. I have found that their rate of ammonia production at low temperature is very poor (many folds below the conventional catalysts at elevated temperature). Can they challenge the conventional catalysts? I really doubt it. Any defects/ impurities to create excited state in the catalyst can give miniscule activity at low temperatures but can they obtain an acceptable yields without much recycling ammonia in technical feasibility way, there is no evidence on this paper. At elevated temperatures whether CaFH survives and still dominants the activity (they seemed to have made this assumption) over other higher activated sites? There is also no discussion.
2. The relationship between the Ru nanoparticles and ionic support is not discussed. The author used the bulk mechanism to argue the formation of H donator sites and electron donator sites (low work function). The DFT calculations and XRD studies to imply their thermodynamic trend to contribute to Ru is not useful. But, how can these ionic centers move from bulk ionic compound to the Ru surface? If only the surface contact, then some surface studies are required.
3. H2-TPD was the only experimental evidence to prove the electron trapping at low temperature. However, the experiment was very roughly performed. The baseline was not even stabilised ( Fig. 3 and Fig. S2). The poor signal to noise level cannot be used to claim the onset temperature of H 2 desorption.
4. More experiments for example in-situ EPR, XPS et al. are required to support the existence of vacancy and electron trapping.
5. Quality of SEM images is also poor for statistical counting of particle size.
6. Catalyst weight required for equilibrium yield (CWEY) was employed to show the activity of catalysts: it is a crude and can easily mislead readers. Many other factors such as catalyst porosity, particle size and support can affect the activity when amplifying the catalysts amount.
7. The FT-IR spectra for N 2 adsorption was employed to show the back donation of electron from Ru to N 2 . As discussed, the H in the support CaFH can be used for hydrogenation of adsorbed N 2 . To illustrate their exchange of atomic/molecular species at both low and high temperature, the authors should label the NH species to demonstrate the interaction between the ionic phase and Ru.
8. HF may be formed under the reaction conditions, can the CaFH be stable at high temperature?
9. There are some format and spelling mistakes. We accept the reviewer's criticism concerning surface N species formed on Ru/CaFH by preparation using N 2 -H 2 gas at 340 °C. According to the reviewer's suggestion, the catalyst was prepared in a flow of pure H 2 gas alone for each measurement of the ammonia formation. It was confirmed by X-ray photoelectron spectroscopy (XPS) measurements that the prepared Ru/CaFH samples had no surface N species. The reduction of deposited Ru species into metallic Ru requires heating at ≥300 °C in the presence of H 2 . There was no difference in activity and stability at each temperature (50-340 °C) between the catalysts pretreated with only H 2 and with the N 2 -H 2 mixture; therefore, no N species are formed by activation in a flow of N 2 -H 2 at 340 °C and there is no contribution from surface N species to low temperature ammonia synthesis over Ru/CaFH. We suppose that the role of alkaline earth metal nitrides in Ru catalytic systems, where Ru has a strong affinity for N, is distinct from those in other transition metal catalytic systems. According to reviewer's suggestion, we try catalytic test at different temperature with the suggested manner. There is no significant difference in ammonia formation rate between the sample pretreated in N 2 -H 2 and H 2 .
The following has been added to "1.2. N species formed on catalysts by activation at 340 °C before low temperature ammonia synthesis" in 1. Additional discussion of Supplementary Information in the revised manuscript.
Page 2, line 12-17. "In this study, low temperature ammonia synthesis was carried out through ammonia synthesis in a flow of N 2 −H 2 at 340 °C, followed by cooling down below 20 °C in a flow of N 2 . It was confirmed by XPS measurements that the prepared Ru/CaFH samples had no surface N species. Moreover, when the tested catalysts, including Ru/CaFH, cooled down below 20 °C were heated in a flow of H 2 or He at 1 °C min -1 , the desorption of ammonia and N 2 was not detected at all even by a mass spectrometer." The rates of ammonia formation were measured under steady-state, as pointed out by the reviewer.
The following has been added to "Methods" in the revised manuscript.
Page 11, line 25-27. "The rate of ammonia formation was repeatedly measured more than 3 times after the ammonia formation rate remained constant for over 1 h."

It is quite strange that the support CaFH was prepared with CaH 2 and BaF 2 . Why not CaH 2 and CaF 2 ? What is the activity of Ru/CaFH for which the CaFH was made from a mixture of CaH 2 and CaF 2 ? It is well known that Ba is an excellent promoter for Ru-based catalysts. Is it possible that Ba plays an important role in ammonia synthesis? This should be clarified because it is essentially relevant to the proposed reaction mechanism.
We appreciate the reviewer's comment, and this is one of the highlights in this study, although we did not emphasize it in the manuscript. The surface areas of the supports and catalytic activities of Ru/CaFH, Ru/CaF x H 2-x -CaF 2 (x=1) and Ru/BaH 2 at 340 °C have been added to the revised manuscript as Supplementary Table 4. Ru/CaF x H 2-x -CaF 2 (x=1) prepared from CaH 2 and CaF 2 may have potential to act as an effective electron-donating material. However, the surface area of the material obtained by conventional solid-state reaction at the required high heating temperature (≥550 °C) for 20 h is so small (1 m 2 g -1 ) that it cannot exhibit sufficient catalytic performance. In this study, the addition of a small amount of BaF 2 to CaH 2 was found to form a CaFH solid solution on CaH 2 at lower temperatures, which resulted in a surface area of 10 m 2 g -1 . The catalytic performance of Ru/CaFH can be partly attributed to the surface area achieved with the new method.
For the Ru/CaFH system, the added BaF 2 is converted into BaH 2 with the formation of the CaF 1.0 H 1.0 solid solution, as shown in Fig. 2b, and Ru nanoparticles with deposited BaH 2 (Ru/BaH 2 , Supplementary Table 4 in the revised manuscript) showed a much smaller catalytic activity than Ru/CaF x H 2-x -CaF 2 (x=1); therefore, the catalytic performance of Ru/CaFH can be attributed to the CaF 1.0 H 1.0 solid solution, rather than to Ba species. The surface areas of the supports and catalytic activities of Ru/CaFH, Ru/CaF x H 2-x -CaF 2 (x=1) and Ru/BaH 2 at 340 °C have been added to the revised manuscript as Supplementary Table 4. Furthermore, the following has been added to "Catalytic performance for ammonia synthesis at low temperatures" in the revised manuscript. was formed on the surface with a surface area of 10 m 2 g -1 . This is considered to be the reason for the difference in activity between Ru/CaFH and Ru/CaF x H 2-x -CaF 2 (x=1)." Furthermore, "Preparation of Ru/CaH 2 and Ru/BaH 2 -BaO" in "Methods" (Page 10, line 30-Page 11, line 1.) has been replaced by the following in the revised manuscript.
"Preparation of Ru/CaH 2 , Ru/BaH 2 and Ru/BaH 2 -BaO. According to previous reports 1 , Ru/CaH 2 , Ru/BaH 2 and Ru/BaH 2 -BaO were prepared by heating CaH 2 , BaH 2 and a mixture of 3 mol% BaO (Kojundo Chemical) and 97 mol% CaH 2 , respectively, with Ru(acac) 3 corresponding to 10 wt% Ru at 260 °C in a flow of H 2 (2.5 mL min -1 ). After 2 h, the samples were heated at 340 °C for 10 h in the H 2 flow. The activities of both catalysts for ammonia synthetic increased with the Ru loading and reached their respective maximum at a loading of 10 wt% Ru."

The apparent activation barrier (20 kJ/mol) derived from the Arrhenius plot, with only three points is highly questionable. In general, such a low apparent activation energy suggests the experiments might be measured at transport-limited regimes.
As pointed out by the reviewer, the explanation for Table 1 was not clear. The E a in Table 1 was estimated from the ammonia formation rates at 50, 75, 100 and 125 °C although this was described as 50-150 °C written in the original manuscript. The coefficient of determination (r 2 ) for the Arrhenius equation obtained from the rates was 0.992. The temperature range "50-150 °C" has been replaced by "50-125 °C" in Table 1, and the ammonia formation rates for Ru/CaFH at 50, 75, 100 and 125 °C (50, 75, 120 and 190 μmol g -1 h -1 ) have been added to the footnote of Table 1 in the revised manuscript.
The followings imply that the experiments are not in the mass transport regime. 1. The rate of ammonia formation over Ru/RuCaFH was measured at various catalyst amounts and flow rates, but at the same WHSV (36000 mL gcat -1 h -1 ), and it was confirmed that the same WHSV gives the same ammonia formation rate. This indicates that external diffusion limitation can be eliminated from the reaction system. Nitrogen adsorption-desorption isotherm and Barrett-Joyner-Halenda (BJH) pore-size distribution revealed that Ru/CaFH has no small pore structures (Supplementary Table 2 in the revised manuscript), causing internal diffusion limitation. 2. The apparent activation energy for ammonia formation over Cs-Ru/MgO, a benchmark catalyst, was estimated at 94 kJ mol -1 (200, 250, 300 and 340 °C) under our reaction conditions. This was the same as those of Cs-Ru/MgO reported by other groups (for example, Reference 21 in the main text). 3. In general, reaction at higher temperatures is subject to mass transport regime.

Did they have any information on the chemical state of Ru? Could they provide more information on the electron transfer from CaFH support to Ru metal?
According to the reviewer's comment, the similarity of XPS Ru 3p 3/2 spectra for Ru/CaFH and Ru-deposited SiO 2 (the average Ru particle size (3.2 nm) to that for Ru/CaFH (3.4 nm)) has been shown as Supplementary Fig. 9 in the revised manuscript. Ru 3p 3/2 for Ru/CaFH appeared at a slightly lower binding energy than that of Ru/SiO 2 , which indicates that Ru on CaFH is more negative than Ru on SiO 2 . However, the difference is so small that we cannot evaluate the electron donation from CaFH to Ru. This is because XPS reflects only Ru atoms near the edges of Ru particles connected to CaFH from the point of view of the escape depth of photoelectrons. For this reason, we have adopted Fourier transform infrared (FT-IR) spectroscopy measurements using N 2 as a probe molecule, which is a more sensitive method. νN 2 for N 2 adsorbed on SiO 2 was observed at >2200 cm -1 , whereas that for Ru/CaFH appears below 2150 cm -1 .
The following has been added to "Electron-donating capability and reaction mechanism for Ru/CaFH" in the revised manuscript.
Page 8, line 19-26. "XPS Ru 3p 3/2 of Ru/CaFH was used to evaluate the electron-donating capability from CaFH to Ru ( Supplementary Fig. 9). Ru 3p 3/2 for Ru/CaFH appeared at a slightly lower binding energy than that for metallic Ru particles deposited on SiO 2 (Ru/SiO 2 ), which indicates that Ru on CaFH is more negative than Ru on SiO 2 . However, the difference was not so large, because XPS reflects only Ru atoms near the edges of Ru particles connected to CaFH from the point of view of the escape depth of photoelectrons. For this reason, we have adopted Fourier transform infrared (FT-IR) spectroscopy measurements using N 2 as a probe molecule, which is a more sensitive method (FT-IR, Fig. 4)."

Reviewer #2 (Remarks to the Author): The authors report a catalyst for ammonia synthesis that displays activity at much lower operating temperatures (50 C) than conventional ammonia catalysts. The activation barrier for ammonia formation is lowered due to a CaFH phase that acts as a strong electron donor to the Ru catalyst. This study will be important for the catalysis community and provides a deeper understanding of catalytic processes. I therefore support the acceptance of this paper with modest revisions.
Specific comments:

1) Why did you not just use the pure CaFH phase for ammonia synthesis? Is there a reason that the CaF 2 -BaF 2 (98:2 ratio) material performed better? This needs to be explained. If your current hypothesis is that the workfunction of Ru is lowered by CaFH because of its ability to donate electrons readily, pure CaFH should be a great catalyst.
We appreciate the reviewer's comment, and this is one of the highlights of this study. The surface areas of the supports and catalytic activities of Ru/CaFH, Ru/CaF x H 2-x -CaF 2 (x=1) and Ru/BaH 2 at 340 °C have been added to the revised manuscript as Supplementary Table 4. Ru/CaF x H 2-x -CaF 2 (x=1) prepared from CaH 2 and CaF 2 may have the potential to act as an effective electron-donating material. However, the surface area of the material obtained by conventional solid-state reaction at the required high heating temperature (≥550 °C) for 20 h is so small (1 m 2 g -1 ) that it cannot exhibit sufficient catalytic performance. In this study, the addition of a small amount of BaF 2 to CaH 2 was found to form a CaFH solid solution on CaH 2 at lower temperatures, which resulted in a surface area of 10 m 2 g -1 . The catalytic performance of Ru/CaFH can be partly attributed to the surface area achieved with the new method.
The surface areas of the supports and catalytic activities of Ru/CaFH, Ru/CaF x H 2-x -CaF 2 (x=1) and Ru/BaH 2 at 340 °C have been added to the revised manuscript as Supplementary Table 4. Furthermore, the following has been added to "Catalytic performance for ammonia synthesis at low temperatures" in the revised manuscript.
Page 7, line 19-35. "The surface areas of the supports and ammonia formation rates (340 °C) of Ru/CaFH, Ru-deposited BaH 2 (Ru/BaH 2 ) and Ru/CaF x H 2-x -CaF 2 (x=1) where Ru nanoparticles are deposited on the CaF 1.0 H 1.0 solid solution formed by heating mixtures of orthorhombic CaH 2 and cubic CaF 2 at 550 °C are summarized in Supplementary Table 4. BaH 2 and CaF 1.0 H 1.0 solid solution are expected to be formed on Ru/CaFH, and either or both of them can contribute to the catalytic performance of Ru/CaFH. However, Ru/BaH 2 had a much smaller catalytic activity for ammonia synthesis than Ru/CaF x H 2-x -CaF 2 (x=1), which indicates that the catalysis of Ru/CaFH is derived from the CaF 1.0 H 1.0 solid solution. Supplementary Table 4 also shows Ru/CaF x H 2-x -CaF 2 (x=1) to be inferior to Ru/CaFH with respect to ammonia synthesis. This can be attributed to the conventional preparation method for the CaF 1.0 H 1.0 solid solution. Ru/CaF x H 2-x -CaF 2 (x=1) was prepared by the deposition of Ru nanoparticles. CaF x H 2-x -CaF 2 (x=1) synthesized by high temperature solid-state reaction at ≥550 °C for 20 h and the resultant surface area was very small (1 m 2 g -1 ), which limits the catalytic activity of Ru/CaF x H 2-x -CaF 2 (x=1). On the other hand, in the case of CaFH prepared by the new method, CaFH solid solution was formed on the surface with a surface area of 10 m 2 g -1 . This is considered to be the reason for the difference in activity between Ru/CaFH and Ru/CaF x H 2-x -CaF 2 (x=1)."

2) More data is needed to claim that CaFH exists on the surface of the CaF 2 -BaF 2 (98:2 ratio) material. Your XRD does not show a peak for CaFH, which might be resolved with longer scan times. Also, your XPS and EDS results only show that there is fluorine on the surface, not that the signal is due to CaFH. Please provide your XPS data in the SI and provide further evidence that CaFH is present on the surface.
We could obtain a clearer CaF 1.0 H 1.0 (200) diffraction peak on the sample of Ca:Ba = 98:2, according to the reviewer's suggestion. The enlarged diffraction peak has been added to Fig. 2c in the revised manuscript. We also measured Ca 2p XPS spectra for heated CaH 2 -BaF 2 mixtures (Ca/Ba atomic ratio of 98:2), CaF x H 2-x -CaF 2 (x=1) and CaH 2 according to the reviewer's suggestion and have added the results to the revised manuscript as Supplementary Fig. 3. The following description has been also added to the revised manuscript.

. What was the ammonia formation rate of Ru/CaFH at higher temperatures? I would suggest adding this information to Supplementary Table 1.
I would like to express our gratitude to the reviewer for this comment. We have added the physicochemical information (surface area, porosity and Ru particle size) and the rates of ammonia formation for Ru/CaFH, Ru/CaH 2 (comparison catalyst), Ru/BaO-BaH 2 , Ru/Ba-Ca(NH 2 ) 2 , Ru/C12A7, and Cs-Ru/MgO and a commercial Fe catalyst (benchmark catalysts) at 100-340 °C to the revised manuscript as Supplementary Table 2, taking care to make the table easy to read.
Page 6, line 19-25. "Supplementary Table 2 shows the physicochemical information (surface area, porosity, and Ru particle size) and the rates of ammonia formation for Ru/CaFH, Ru/CaH 2 , Ru/Ba-Ca(NH 2 ) 2 , Ru/BaO-BaH 2 , Ru/C12A7 19 , Cs-Ru/MgO and a commercial Fe catalyst as benchmark catalysts at 100-340 °C. It was confirmed that the catalytic activities of the commercial Fe catalyst and Cs-Ru/MgO benchmark catalysts were comparable to those reported by other groups 19-21 . These conventional catalysts did not exhibit activity for ammonia synthesis below 100-200 °C," Fig. 3, please label the onset of H 2

desorption for the CaFH material. At first glance, it is not apparent that there is a difference with CaH 2 .
Each H 2 -TPD profile with the background profile was measured again and has been added to Fig. 3a and Supplementary Fig. 4 in the revised manuscript. According to the reviewer's suggestion, H 2 desorption from CaFH has been labeled in Fig. 2a of the revised manuscript. No desorption of molecular species giving the m/z=2 signal, such as H 2 O, was observed.

5) In line 132 of the main text, you claim that the XRD peaks for BaH 2 can be observed in Fig. 4a. This is not apparent. I would suggest zooming in on the BaH 2 peaks just as you have done for the CaFH peaks or improve the resolution of your diffractogram.
We appreciate the reviewer's suggestion. The diffraction due to BaH 2 in CaFH was measured again with high resolution and has been enlarged in Fig. 2b of the revised manuscript.

6) The authors have provided limited evidence for formation of electride-type electrons on the surface of CaFH. I agree that this is a plausible hypothesis. However, in absence of better support, the authors should re-write their abstract, as it overstates the evidence. Specifically, the authors wrote: "the high electron-donating capability due to the low work function (2.2 eV) of H − vacancies that trap electrons." This is stated with too much certainty.
The reviewer's comment is justified. We accept the reviewer's criticism concerning the abstract and have deleted a portion in the revised manuscript.

But, the problem is that whether one can overcome the kinetic barrier at low temperature to achieve the equilibrium. Many industrial processes are exothermic but still be carried out at elevated temperatures due to unfavorable low rate at low temperature rather than their equilibrium position. If the conventional catalysts can get higher rates for ammonia synthesis at lower temperature, then the present extreme conditions would not be used: whether this is for non-renewables or wind energy, it is not the key factor. Using pressure not only affect the equilibrium position but also increase the kinetics. I have found that their rate of ammonia production at low temperature is very poor (many folds below the conventional catalysts at elevated temperature). Can they challenge the conventional catalysts? I really doubt it. Any defects/impurities to create excited state in the catalyst can give miniscule activity at low temperatures but can they obtain an acceptable yield without much recycling ammonia in technical feasibility way, there is no evidence on this paper. At elevated temperatures whether CaFH survives and still dominants the activity (they seemed to have made this assumption) over other higher activated sites? There is also no discussion.
We should have paid attention to the problems pointed out by the reviewer and have provided sufficient explanation with appropriate results in the original manuscript. We accept the reviewer's criticism concerning the lack of explanation and discussion in the original manuscript, and have thus made corrections.
We have added the physicochemical information (surface area, porosity and Ru particle size) and the rates of ammonia formation for all tested catalysts (Ru/CaFH, Ru/CaH 2 (comparison catalyst), Ru/BaO-BaH 2 , Ru/Ba-Ca(NH 2 ) 2 , Ru/C12A7, Cs-Ru/MgO (benchmark catalyst) and commercial Fe catalyst (benchmark catalyst)) at 100-340 °C to the revised manuscript as Supplementary Table 2. The table clearly indicates that lowering the reaction temperature to 100-200 °C brings the rate of ammonia formation on all tested catalysts close to "zero". Ru/Ba-Ca(NH 2 ) 2 has been reported to have the highest catalytic performance for ammonia synthesis by our group (Angew. Chem. 130, 2678 (2018)) 6 ; the ammonia formation rates are much higher than those of conventional catalysts reported as highly active catalysts, as shown in the literature and Supplementary Table 3 added to the revised manuscript. The catalytic performance of Ru/BaO-BaH 2 that was also reported by our group (ACS Catal. 8, 10977 (2018)) 18 is close to Ru/Ba-Ca(NH 2 ) 2 (Supplementary Tables 2 and 3). It was confirmed that the catalytic activities of the two benchmark catalysts were comparable to those reported in the literature (see below). This implies that conventional catalysts equally lose activity for ammonia synthesis at 100-200 °C, even if they act as highly active catalysts above these temperatures. No catalyst that just manages to be active for ammonia synthesis from N 2 and H 2 below 100 °C has been reported. As pointed out by the reviewer, more efficient ammonia synthesis from N 2 and H 2 is required to overcome the kinetic barrier at lower temperature to achieve the equilibrium. On the other hand, the present results suggest that the conventional approach, based on the expectation that catalysts with high activities at high reaction temperatures would also act as highly active catalysts at low temperatures, cannot perform at such lower reaction temperatures. It would be difficult to enhance the activity of catalyst at low temperatures without catalyst which can function for the reaction at the temperatures. In this study, we have adopted a new strategy to lower the temperature as the first step to achieve the desired catalytic system. This may be a shortcut to highly active catalysts that exceed all conventional catalysts in all temperature ranges, whereby the conventional operating temperature and pressurization are decreased, although the guiding principles to lower the lowest catalyst working temperature have yet to be clarified. The simply prepared Ru/CaFH presented in this study not only works for ammonia synthesis at 50 °C but exceeds conventional highly active catalysts in activity in all temperature ranges, despite the low pressurization and weight hourly space velocity (Supplementary Tables 2 and 3). In addition, Ru/CaFH also produces ammonia for over 100 h without any decrease in activity even at 340 °C ( Supplementary Fig. 8 added to the revised manuscript). 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 amount of Ru/CaFH effective for the reaction (i.e., amount of substrate near the Ru/CaFH surface) is less than several percent of the Ru/CaFH used. Furthermore, there was no significant difference in the XRD patterns and surface F concentrations of Ru/CaFH before and after reaction. These results indicate that Ru/CaFH acts as a highly active and stable catalyst for ammonia synthesis.
According to the reviewer's comment, the following has been revised.
(1) Page 2, lines 23-29 in the original manuscript has been replaced by the following, in accordance with the reviewer's comments.
Page 2, line 23-29. "Thus, a lower temperature is favorable for ammonia production with respect to yield and energy consumption, and more efficient ammonia production is required to overcome the kinetic barrier at lower temperature to achieve the equilibrium. However, conventional catalysts equally lose the catalytic activity for ammonia formation from N 2 -H 2 at 100-200 °C, even if they exhibit high catalytic performance at high temperatures, as shown in Fig. 1a (see below). In addition, guiding principles to lower the temperature for a loss of activity have yet to be clarified," (2) "at 50 °C " (Page 2, line 36 in the original manuscript) has been replaced with "at lower temperatures".
"Supplementary Table 2 shows the physicochemical information (surface area, porosity, and Ru particle size) and the rates of ammonia formation for Ru/CaFH, Ru/CaH 2 , Ru/Ba-Ca(NH 2 ) 2 , Ru/BaO-BaH 2 , Ru/C12A7 19 , Cs-Ru/MgO and a commercial Fe catalyst as benchmark catalysts at 100-340 °C. It was confirmed that the catalytic activities of the commercial Fe catalyst and Cs-Ru/MgO benchmark catalysts were comparable to those reported by other groups 19-21 . These conventional catalysts did not exhibit activity for ammonia synthesis below 100-200 °C,"  Page 6, line 36-page 7, line 6. " Table 1 gives the catalyst weight required for the equilibrium yield (CWEY) of ammonia at 200 °C (See Supplementary Section 1.3). The rates of ammonia formation and CWEYs for all tested catalysts, including Ru/Ba-Ca(NH 2 ) 2 , and the recently reported highly active catalysts at 200-350 °C are summarized in Supplementary Table 3 22-25 . Although Ru/Ba-Ca(NH 2 ) 2 and Ru/BaO-BaH 2 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." Furthermore, the following has been added to Table 1 and "1.3. CWEY at each pressure" of "1. Additional discussion" in Supporting Information. "CWEYs were estimated from the rates of ammonia formation at each reaction temperature. It was confirmed that the rates of ammonia formation (μmol h -1 ) for Ru/CaFH, Ru/BaO-BaH 2 and Ru/CaH 2 increased in direct proportion to the catalyst weight (0.05-5.00 g)." (5) A time course of the ammonia formation rate for Ru/CaFH at 340 °C has been added to the revised manuscript as Supplementary Fig. 8 with the following caption. "The amount of ammonia produced by Ru/CaFH at 340 °C exceeded the amount of the used catalyst (ca. 24 mmol) within 100 min." Furthermore, Page 6, line 29-page 7, line 1 in the original manuscript has been replaced by the following, according to the addition of Supplementary Fig. 8.
Page 7, line 6-18. "In addition, Ru/CaFH produced ammonia without a decrease in activity for long periods of time and at higher temperatures (200 and 340 °C) (Supplementary Figs. 7 and 8). Ammonia formation over Ru/CaFH was close to 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 test 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 and 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 reaction. These results are clearly indicative of the stability of the Ru/CaFH catalyst."

The relationship between the Ru nanoparticles and ionic support is not discussed. The author used the bulk mechanism to argue the formation of H donator sites and electron donator sites (low work function). The DFT calculations and XRD studies to imply their thermodynamic trend to contribute to Ru is not useful. But, how can these ionic centers move from bulk ionic compound to the Ru surface? If only the surface contact, then some surface studies are required.
We appreciate the reviewer's comment for the clarification of the reaction mechanism. We studied the early stage of ammonia synthesis from N 2 −D 2 over the catalyst at 180 °C by additional experiments. The results have been added to the revised manuscript as Fig. 3 in the main text. Ru/CaFH prepared in a flow of pure H 2 at 340 °C was cooled down from the temperature to 180 °C in a flow of Ar and held under this flow for 5 h. After each mass signal intensity remained constant, N 2 −D 2 was passed into Ru/CaFH at the temperature under atmospheric pressure. First, NH 3 was detected, and then NDH 2 began to form soon after. ND 2 H was observed after NDH 2 generation; ammonia species containing H are produced at the early stage of the reaction. Because H adatoms are not expected to be on the Ru surfaces immediately before the introduction of N 2 -D 2 , these results imply that H in the CaFH bulk is used for ammonia formation at the early stage of the reaction and the ionic centers can move from the bulk ionic compound to the Ru surfaces.
The following revisions have been made in the revised manuscript.
(1) These experimental results have been added to the main text in the revised manuscript as Fig.  3 "Reaction time profiles for ammonia synthesis from N 2 −D 2 over Ru/CaFH at 180 °C".
(2) The following "Ammonia synthesis from N 2 and D 2 " has been added to "Methods" in the revised manuscript.

Ammonia synthesis from N 2 and D 2 .
A mixture of 98 mol% CaH 2 , 2 mol% modified BaF 2 and Ru(acac) 3 corresponding to 12 wt% Ru was heated in the reactor at 260 °C for 2 h and at 300 °C for 10 h in a flow of H 2 (2.5 mL min -1 ). The resultant Ru/CaFH was cooled down from that temperature to 180 °C in a flow of Ar at a flow rate of 30 mL min -1 . After each mass signal intensity was kept constant, N 2 −D 2 (N 2 : 5 mL min -1 , D 2 : 15 mL min -1 ) was passed into Ru/CaFH at that temperature under atmospheric pressure. The outlet gas from the reactor was analyzed using mass spectrometry (BELMass, MicrotracBEL, Japan).
(3) The following has been added to "Electron-donating capability and reaction mechanism of Ru/CaFH" in the revised manuscript.
Page 8, line 2-18. "Ammonia synthesis from N 2 and D 2 over Ru/CaFH was examined to clarify the reaction mechanism. Ru/CaFH prepared at 340°C in a flow of H 2 alone was cooled down from 340 °C to 180 °C in a flow of Ar, and then N 2 -D 2 was passed into Ru/CaFH at temperature under atmospheric pressure (See Methods). The experimental reaction time profiles for ammonia synthesis from N 2 −D 2 over Ru/CaFH are shown in Fig. 3. Soon after an increase in the m/z=17 signal (NH 3 and NDH as fragments of ND 2 H and NDH 2 ), the signal of m/z=18 (NDH 2 and ND 2 as a fragment of ND 3 ) increased. The m/z= 19 (ND 2 H) signal was observed after ca. 3 min from the introduction of N 2 −D 2 . The fragment ratio of m/z=17 (NH 3 ), 16 (NH 2 ) and 15 (NH) in pure NH 3 was ca. 100:80:8, so that each fragment intensity did not exceed the parent intensity. The formation of ND 3 (m/z=20) was not observed within 10 min from the beginning of the reaction. As a result, NH 3 was first formed, followed by the formation of NDH 2 and ND 2 H; ammonia species containing H was formed at the early stage of ammonia synthesis from N 2 −D 2 over Ru/CaFH. These results indicate that H in the CaFH bulk is used for ammonia formation at the early stage of reaction and H can move from CaFH bulk to the Ru surface to react with N adatoms. The same phenomenon had been observed on Ru nanoparticle-deposited Ca 2 NH (calcium nitride hydride). 14 " 3. H 2 -TPD was the only experimental evidence to prove the electron trapping at low temperature. However, the experiment was very roughly performed. The baseline was not even stabilised (Fig. 3 and Fig. S2). The poor signal to noise level cannot be used to claim the onset temperature of H 2 desorption. Figure 3a and Supplementary Fig. 4 were unclear due to the lack of a background profile with m/z=2 obtained from a blank cell. Each H 2 -TPD profile with a background profile was measured again and are shown in Fig. 3a and Supplementary Fig. 4 of the revised manuscript. No desorption of molecular species giving an m/z=2 signal, such as H 2 O, was observed. There is a clear difference in the H 2 -TPD profiles among Ru/CaFH, R/CaH 2 and the blank.

More experiments for example in-situ EPR, XPS et al. are required to support the existence of vacancy and electron trapping.
According to the reviewer's comment, XPS Ru 3p 3/2 spectra for Ru/CaFH and Ru-deposited SiO 2 (the average Ru particle size (3.2 nm) was similar to that of Ru/CaFH (3.4 nm)) are shown as Supplementary Fig. 9 in the revised manuscript. The Ru 3p 3/2 spectra for Ru/CaFH appeared at a slightly lower binding energy than that of Ru/SiO 2 , which indicates that Ru on CaFH is more negative than Ru on SiO 2 . However, the difference was so small that we could not evaluate the electron donation from CaFH to Ru. This is because XPS reflects only Ru atoms near the edges of Ru particles connected to CaFH with respect to the escape depth of photoelectrons. For this reason, we have adopted FT-IR measurements using N 2 as a probe molecule, a more sensitive method. νN 2 for N 2 adsorbed on SiO 2 was observed at >2200 cm -1 , whereas that for Ru/CaFH appears below 2150 cm -1 .
The following has been added to "Electron-donating capability and reaction mechanism for Ru/CaFH" in the revised manuscript.
Page 8, line 19-26. "XPS Ru 3p 3/2 of Ru/CaFH was used to evaluate the electron-donating capability from CaFH to Ru ( Supplementary Fig. 9). Ru 3p 3/2 for Ru/CaFH appeared at a slightly lower binding energy than that for metallic Ru particles deposited on SiO 2 (Ru/SiO 2 ), which indicates that Ru on CaFH is more negative than Ru on SiO 2 . However, the difference was not so large, because XPS reflects only Ru atoms near the edges of Ru particles connected to CaFH from the point of view of the escape depth of photoelectrons. For this reason, we have adopted Fourier transform infrared (FT-IR) spectroscopy measurements using N 2 as a probe molecule, which is a more sensitive method (FT-IR, Fig. 4)."

Quality of SEM images is also poor for statistical counting of particle size.
According to the reviewer's comment, Supplementary Fig. 6c has been replaced with a clearer STEM image in the revised manuscript.

Catalyst weight required for equilibrium yield (CWEY) was employed to show the activity of catalysts: it is a crude and can easily mislead readers. Many other factors such as catalyst porosity, particle size and support can affect the activity when amplifying the catalysts amount.
We accept the reviewer's criticism concerning Table 1 and Supplementary Table 2 in the original manuscript. The physicochemical information (surface area, porosity, Ru particle size) and the rates of ammonia formation for Ru/CaFH, Ru/CaH 2 (comparison catalyst), Ru/BaO-BaH 2 , Ru/Ba-Ca(NH 2 ) 2 , Ru/C12A7, and a commercial Fe catalyst and Cs-Ru/MgO (benchmark catalysts) at 100-340 °C are shown as Supplementary Furthermore, the following has been added to Table 1 and "1.3. CWEY at each pressure" of "1. Additional discussion" in Supporting Information. "CWEYs were estimated from the rates of ammonia formation at each reaction temperature. It was confirmed that the rates of ammonia formation (μmol h -1 ) for Ru/CaFH, Ru/BaO-BaH 2 and Ru/CaH 2 increased in direct proportion to the catalyst weight (0.05-5.00 g)." 7. The FT-IR spectra for N 2 adsorption was employed to show the back donation of electron from Ru to N 2 . As discussed, the H in the support CaFH can be used for hydrogenation of adsorbed N 2 . To illustrate their exchange of atomic/molecular species at both low and high temperature, the authors should label the NH species to demonstrate the interaction between the ionic phase and Ru.
The FT-IR spectrum noted by the reviewer is an important result in this study. We have added an FT-IR spectrum (1550-1700 cm -1 ) for N-adsorbed Ru/CaFH at 25 °C to the revised manuscript as Supplementary Fig. 10 with the following.
" Supplementary Fig. 10 shows an FT-IR spectrum (1550-1700 cm -1 ) for N-adsorbed Ru/CaFH at 25 °C (N 2 : 12 kPa) and a band assignable to δNH bending, which can be attributed to adsorbed ammonia from the broad band at ca. 1600 cm -1 . This indicates that N 2 molecules are dissociated into N adatoms, which react with H from CaFH even at 25 °C, and is consistent with the H 2 -TPD results and ammonia synthesis from N 2 −D 2 ."

HF may be formed under the reaction conditions, can the CaFH be stable at high temperature?
Ru/CaFH produced ammonia without a decrease in activity for over 100 h at 340 °C without decrease in activity, and F species such as HF were not detected during the reaction.
A time course of the ammonia formation rate for Ru/CaFH at 340 °C has been added to the revised manuscript as Supplementary Fig. 8 with the following caption. "The amount of ammonia produced by Ru/CaFH at 340 °C exceeded the amount of the used catalyst (ca. 24 mmol) within 100 min." Furthermore, Page 6, line 29-page 7, line 1 in the original manuscript has been replaced with the following.
Page 7, line 6-18. "In addition, Ru/CaFH produced ammonia without a decrease in activity for long periods of time and at higher temperatures (200 and 340 °C) (Supplementary Figs. 7 and 8). Ammonia formation over Ru/CaFH was close to 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 test 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 and 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 reaction. These results are clearly indicative of the stability of the Ru/CaFH catalyst." 9. There are some format and spelling mistakes.