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[Shun Sato](https://orcid.org/0009-0004-2269-6552), Masayoshi Miyazaki, [Satoru Matsuishi](https://orcid.org/0000-0001-8905-0255), [Hideo Hosono](https://orcid.org/0000-0001-9260-6728), [Masaaki Kitano](https://orcid.org/0000-0003-4466-7387)

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[Ammonia Synthesis over Ruthenium Supported on Metastable Perovskite Oxyhydrides Ba<i>RE</i>O<sub>2</sub>H (<i>RE</i> = Y, Sc) Prepared by Mechanochemical Method](https://mdr.nims.go.jp/datasets/f9bbe5d3-9a0b-498b-a0f5-589a33973e54)

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Ammonia Synthesis over Ruthenium Supported on Metastable Perovskite Oxyhydrides BaREO2H (RE = Y, Sc) Prepared by Mechanochemical MethodRESEARCH ARTICLEwww.advenergymat.deAmmonia Synthesis over Ruthenium Supported onMetastable Perovskite Oxyhydrides BaREO2H (RE = Y, Sc)Prepared by Mechanochemical MethodShun Sato, Masayoshi Miyazaki, Satoru Matsuishi,* Hideo Hosono,*and Masaaki Kitano*Oxyhydrides have attracted attention as materials with various uniqueproperties derived from lattice hydride ions (H−). However, their instabilitymakes synthesis by conventional thermal synthesis methods difficult, so anappropriate synthesis strategy is required. Here, the mechanochemicalsynthesis of perovskite oxyhydrides BaREO2H (RE = Y, Sc) for catalystapplications is reported. The formation of BaYO2H is known to bethermodynamically unstable; however, a mechanochemical process thatinevitably proceeds under non-equilibrium conditions enables the synthesis ofsuch a metastable oxyhydride material without any heat treatment.Furthermore, BaScO2H, which is typically obtained at very high temperatures(1000 °C) and pressure (>4 GPa), is successfully synthesized at roomtemperature by the mechanochemical method. The ammonia synthesisreaction over these oxyhydrides supporting Ru is significantly enhanced atlow temperatures, and the ammonia synthesis rates are significantly higherthan conventional oxide-supported Ru catalysts. The mechanochemicallysynthesized BaREO2H has many anionic electrons with low work function atthe site of H− vacancies, which enables strong electron donation to Ru andthe storage of excess hydrogen adatoms from the Ru surface that results inhigh catalytic performance.1. IntroductionOxyhydrides, in which oxide ions (O2−) and hydride ions (H−)form an anion sublattice, are novel inorganic compounds thathave attracted attention in the field of materials science dueto their unique ionic conductivity, fluorescence properties, andS. Sato, M. Miyazaki, H. Hosono, M. KitanoMDX Research Center for Element StrategyInternational Research Frontiers InitiativeTokyo Institute of Technology4259 Nagatsuta, Midori-ku, Yokohama 226–8503, JapanE-mail: hosono@mces.titech.ac.jp; kitano.m.aa@m.titech.ac.jpThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/aenm.202402353© 2024 The Author(s). Advanced Energy Materials published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution License, which permits use, distributionand reproduction in any medium, provided the original work is properlycited.DOI: 10.1002/aenm.202402353catalytic properties.[1–4] These propertiesare mainly due to the unique coordina-tion environment and the specific chem-ical and physical properties of H−, whileO2− is responsible for the constructionand stabilization of the crystal frame-work. Oxyhydrides can be synthesizedby H− doping of typical oxides suchas BaTiO3−xHx and LaH3−2xOx by ex-ploiting the similar ionic radius of O2−and H− (135 pm).[5–7] It is also pos-sible to obtain novel compounds withunique chemical compositions, such asLa2LiHO3 and A3MO4H (A=Rb, Cs; M=Mo, W), which allows for a wide vari-ety of material designs.[8,9] Although anumber of oxyhydrides have been pre-dicted by theoretical calculations, it is dif-ficult to synthesize oxyhydrides by con-ventional high-temperature solid-state re-actions because of the chemical andthermal instability of H−. On the otherhand, it has been reported that high-pressure synthesis and topochemical re-actions are effective for the synthesis ofoxyhydride materials.[10] The most advantageous point of high-pressure synthesis is the capability to confine hydrogen in thesolid phase by ultra-high external pressure (2–8 GPa), which al-lows hydrogen to react and remain as H− even at high tem-peratures (800–1200 °C) if the sample is oxygen deficient. Insome cases, crystal phases known as pressure-stabilized phases,S. Matsuishi, H. HosonoResearch Center for Materials NanoarchitectonicsNational Institute for Materials Science1-1 Namiki, Tsukuba, Ibaraki 305-0044, JapanE-mail: matsuishi.satoru@nims.go.jpM. KitanoAdvanced Institute for Materials Research (WPI-AIMR)Tohoku UniversitySendai 980–8577, JapanAdv. Energy Mater. 2024, 14, 2402353 2402353 (1 of 11) © 2024 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbHhttp://www.advenergymat.demailto:hosono@mces.titech.ac.jpmailto:kitano.m.aa@m.titech.ac.jphttps://doi.org/10.1002/aenm.202402353http://creativecommons.org/licenses/by/4.0/mailto:matsuishi.satoru@nims.go.jphttp://crossmark.crossref.org/dialog/?doi=10.1002%2Faenm.202402353&domain=pdf&date_stamp=2024-08-03www.advancedsciencenews.com www.advenergymat.dewhich form only above critical pressure, are obtained underhigh-pressure conditions. Topochemical reactions are typicallyconducted at low temperatures (<600 °C) by the reaction ofhost oxide materials with hydride materials such as CaH2 overa long time (>50 h). This technique limits cation diffusionat low temperatures so that the lattice O2− ions can be re-placed by H− without changes in the crystal framework of theoxide.However, some compounds have been reported to be im-possible to synthesize by these methods, such as BaYO2H.BaYO2H has been predicted to be within 100 meV per atomfrom the convex hull based on a machine-learning model thatassumes the formation of a tetragonal perovskite structure.[11]Ba2YO3H with a K2NiF4-type structure has been produced byhigh-pressure synthesis (4 GPa)[12]; however, the high-pressuresynthesis of BaYO2H has not been reported to date. It is alsodifficult to synthesize BaYO2H by topochemical reaction dueto the difficulty of producing the metastable Ba2Y2O5 precur-sor as a pure phase without any impurities.[13] It is impor-tant to overcome such metastability of oxyhydrides in devel-oping new materials, which requires constructing a suitablesynthesis strategy. Therefore, we have adopted mechanochem-ical methods in which chemical reactions are induced by me-chanical energy under non-equilibrium conditions as an effec-tive method for synthesizing oxyhydrides. The accumulationof strain energy generates a high-energy intermediate state inthe material, whereby metastable phases with high Gibbs freeenergies can be formed.[14] Mechanochemical methods havebeen conventionally used to synthesize metastable alloys andhydrides, and there have been some reports on the synthesisof multi-anionic hydrides in recent years.[15,16] There have alsobeen several reports on the synthesis of oxyhydrides such asBaTiO3−xHx, Sr5(PO4)3H, and Sr5(BO3)3H, which shows the sig-nificant potential of this method for the synthesis of metastableoxyhydrides.[17–19]Here, we report the mechanochemical synthesis of ametastable perovskite oxyhydride, BaYO2H, and BaScO2H,and their catalytic activity for NH3 synthesis. Since the powdersamples obtained by mechanochemical methods are expectedto exhibit excellent catalytic activity and ionic conductivity attheir surfaces and grain boundaries,[20] revealing their func-tions at the interfaces is an important issue in evaluating theavailability of the oxyhydrides obtained by mechanochemicalmethods. Most recently, the mechanochemical synthesis ofperovskite oxyhydrides, including these two oxyhydrides, havebeen reported before our report.[21] The paper reported a rela-tionship between the feasibility of mechanochemical reactionsand the calculated shear modulus of the starting reagents,while there is no application based on the interfacial or sur-face properties of these oxyhydrides. In the present work, thecatalytic properties toward ammonia synthesis are examinedfor Ru catalysts supported on the synthesized BaREO2H (RE =Y, Sc) and compared with those of Ru-loaded oxides with thesame cation composition and other reported oxyhydride-basedcatalysts. In addition, the surface structure of Ru/BaREO2His investigated in detail to clarify the role of lattice H− ions.In this catalyst system, hydride-deficient interfaces specificallygenerated by mechanochemical methods contribute to catalyticperformance.2. Results2.1. Synthesis and Characterization of BaREO2HFigure 1a shows powder XRD patterns for the oxyhydrides aftermechanochemical synthesis. The XRD patterns for the samplesjust after mechanochemical milling of a mixture of the precur-sors were simple due to the cubic perovskite structure, and nopeak splitting or other peaks due to impurity phases were ob-served. The resultant materials were heated at 500 °C for 72 h un-der vacuum to improve the crystallinity for Rietveld analysis. Thestructural data for BaScO2H obtained from ICSD (No.257866)were used to refine the XRD patterns, and in the case of BaYO2H,the Sc symbol in this data was replaced by the Y symbol (Tables S1and S2, Supporting Information). As shown in Figure 1b,c, themain peaks for both samples are well-fitted to BaREO2H with acubic perovskite structure (Pm-3m). There were no impurities inthe BaYO2H sample, while small impurity peaks were observedonly for the BaScO2H sample. The refined phase fractions for theBaScO2H sample indicate that the perovskite phase contributes98.5 wt.%, whereas the Sc2O3 phase remains at 1.5 wt.%. The re-fined lattice parameters for BaREO2H are summarized in Table 1.The lattice constant for BaScO2H (a = 4.1468(2) Å) is close tothe reported value (a = 4.15034(3) Å) within an error of lessthan 0.1%.[22] The volume of the unit cell for BaYO2H is ≈1.14times larger than that for BaScO2H, which is close to the volumeexpansion rate (15.0%) observed in GdFeO3-type orthorhom-bic perovskite LaREO3 (RE = Sc, Y).[23,24] BaREO3−𝛿 with thesame crystal structure and cation composition as BaREO2H wassynthesized by the mechanochemical method using only oxideprecursors (Figure S1, Supporting Information). The lattice con-stant for BaScO3−𝛿 (a = 4.132(2) Å) is similar to that reported forBaScO3−𝛿 (a = 4.13 Å) obtained by firing Ba3Sc2(OH)12.[25] Thelarger lattice constant for BaREO2H than for BaREO3−𝛿 can beunderstood by the introduction of H− into the lattice, i.e., H−with the same ionic size as O2− (135 pm)[7] occupies the oxygenvacancy sites in oxygen-deficient cubic perovskite BaREO3−𝛿 toform BaREO2H.TDS measurements were conducted to determine the amountof H− in the oxyhydrides. Two H2 desorption peaks (m/z =2) were observed in the TDS profiles for BaYO2H (Figure 2a)≈150 °C and above 300 °C. The sharp desorption peak ≈150 °Cis attributed to the H2 desorption from the surface, whereas thebroad desorption peaks above 300 °C are the desorption of hydro-gen from bulk since the diffusion of bulk H− ion to the surfacerequires higher temperature. Signals with m/z = 18 were alsoobserved below 400 °C, attributed to H2O physically adsorbedon the sample holder and the sample’s surface. However, the in-tensity of the H2O signal was only ≈1/100 that of the H2 sig-nal. BaScO2H showed desorption profiles similar to those forBaYO2H (Figure 2b). These results suggest that hydrogen ex-ists in similar environments in the crystals of both compounds.Table 1. Refined lattice constants for BaREO2H and BaREO3−𝛿 .BaYO2H BaYO3−𝛿 BaScO2H BaScO3−𝛿a (Å) 4.3294(5) 4.2979(7) 4.1468(2) 4.132(2)Adv. Energy Mater. 2024, 14, 2402353 2402353 (2 of 11) © 2024 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202402353 by National Institute For, Wiley Online Library on [17/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deFigure 1. Crystal structure analysis of BaREO2H. a) XRD patterns for BaREO2H before and after heat treatment. Experimental XRD patterns and Rietveldrefinement for b) BaYO2H and c) BaScO2H samples after heat treatment.The amounts of H2 released from BaYO2H and BaScO2H were1.12 and 1.06 mmol g−1, respectively. The chemical compositionwas estimated to be BaYO2H0.58 and BaScO2H0.46, which sug-gests that the obtained oxyhydrides contain significant amountsof H− vacancies. When anion vacancies are formed, optical ab-sorption of low-energy light should be observed, and the samplewas gray in color (Figure S2, Supporting Information). The sam-ple color is caused by the broad and weak absorption band ≈1.7–3.5 eV (Figure S3, Supporting Information). This band is similarto the simulated optical absorption spectra of H-deficient modelsFigure 2. Detection and quantification of hydrogen incorporated in BaREO2H. TDS spectra of a) BaYO2H and b) BaScO2H. c) FT-IR spectra of BaYO2Hand BaYO3−𝛿 . d) FT-IR spectra of BaScO2H and BaScO3−𝛿 . Calculated spectra for BaREO2H (RE = Y or Sc) are shown for comparison.Adv. Energy Mater. 2024, 14, 2402353 2402353 (3 of 11) © 2024 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202402353 by National Institute For, Wiley Online Library on [17/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deFigure 3. Investigation of the thermodynamic phase stability of BaYO2H. a) Equilibrium phase diagram of the Ba─Y─O─H system at 0 K when thechemical potential of hydrogen is −0.3 eV. b) XRD patterns for BaYO2H samples after heat treatment at various temperatures under an H2 atmosphere.The sharp peaks ≈37° and 45° marked by * are attributed to the Al sample holder.calculated by DFT (Figure S4, Supporting Information). More-over, it was predicted that the electrons are localized at the hy-drogen vacancy sites, which gives rise to a mid-gap state inthe bandgap (Figure S5, Supporting Information). These resultsclearly show the presence of electrons trapped at hydrogen vacan-cies in the synthesized BaREO2H.Figure 2c,d show FT-IR spectra for BaREO2H and BaREO3−𝛿measured in the 400–2000 cm−1 region. Based on a compari-son with the computational spectra and phonon density of states(Figures S6 and S7, Supporting Information), the broad peaks ob-served ≈1250 cm−1 in the BaREO2H spectra are attributed to thehydride vibration mode at the anion sites in the perovskite struc-ture (Figure S8, Supporting Information). In contrast, no suchvibration was observed in the BaREO3−𝛿 spectra, and only a vibra-tion attributed to the carboxylate structure (O─C═O) ≈1400 cm−1was observed.[26] Therefore, H− in the BaREO2H samples occu-pies the same anion sites to form metal-hydrogen bonds.The formation enthalpy (ΔEF) and the equilibrium phase di-agram for the Ba─Y─O─H system at 0 K were predicted fromDFT calculations. The ΔEF for BaYO2H from BaO, BaH2, andY2O3 was estimated to be +13.0 kJ mol−1, which indicates thatthe formation of BaYO2H is thermodynamically unstable. More-over, BaYO2H did not appear on the convex hull in any re-gions in the Ba─Y─O─H phase diagram, with the chemical po-tential of hydrogen as the projection axis (Figure S9, Support-ing Information). Figure 3a shows the Ba─Y─O─H phase di-agram, where the hyperplane corresponding to the stable re-gion of BaYO2H is clearly drawn. The hyperplane is discontin-uous with the convex hull, which indicates that BaYO2H is ametastable phase. When BaYO2H is heated above 600 °C, it isdecomposed to form the raw materials and BaY2O4, consistentwith its thermodynamic instability (Figure 3b). For this reason,BaYO2H could not be synthesized by conventional thermal syn-thesis methods. On the other hand, Ba2YO3H can be synthe-sized by the conventional thermal synthesis method under high-pressure conditions, as predicted from the equilibrium diagram(Figure 3a).[12]Sirota proposed that a thermodynamically unfavorable reac-tion can be allowed under a nanostructural excited state causedby strain energy accumulated by mechanical stress and the in-sertion of atoms into interstitial spaces.[14] Quantitative stud-ies on the accumulation of internal energy using shaker millsand planetary mills have already been performed, which have re-vealed that internal enthalpies accumulate ≈0.8–10 kJ mol−1 forFe, Al, and AlRu alloys.[27,28] Therefore, the high-energy millingperformed in this study enabled the formation of a metastablephase, BaYO2H, with a positive formation enthalpy. On theother hand, the kinetic effects are expected to be important inthe formation of BaScO2H from BaO, BaH2, and Sc2O3 be-cause of its negative formation enthalpy (−26.9 kJ mol−1). Themechanochemical method is expected to be effective toward theinhibition of hydrogen desorption at low temperatures becauseof the high local pressure caused by mechanical stress, similar tothat of a high-pressure synthesis process.[29] Therefore, it wouldbe reasonable for BaScO2H to form under mechanochemicalconditions.2.2. Catalytic Performance for Ammonia SynthesisThe results have clarified that BaREO2H with many hydro-gen defects easily releases hydrogen even at low temperaturesbelow 200 °C, which is similar to the reported highly activehydride-based ammonia synthesis catalysts.[30–32] Therefore, theAdv. Energy Mater. 2024, 14, 2402353 2402353 (4 of 11) © 2024 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202402353 by National Institute For, Wiley Online Library on [17/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deFigure 4. Catalytic stabilities of Ru-loaded BaREO2H. a) Stability test for NH3 synthesis over Ru-loaded BaREO2H at 300 °C and 0.9 MPa. b) XRD patternsfor Ru/BaREO2H before and after ammonia synthesis reaction at 300 °C and 0.9 MPa for 70 h.synthesized oxyhydrides were applied as catalyst supports of Rufor the ammonia synthesis reaction, and their catalytic proper-ties were evaluated. All catalytic reactions were conducted in afixed bed flow system as shown in Figure S10 (Supporting Infor-mation). The ammonia produced was trapped in an H2SO4 solu-tion, and the amount of generated ammonia was quantified usingion chromatography with the calibration curve method (FiguresS11,S12, and S13, Supporting Information). The Ru-loaded oxy-hydrides worked as catalysts for ammonia synthesis for over70 h without significant loss of activity (Figure 4a). The crystalstructures of BaREO2H remained unchanged after the reaction(Figure 4b), and of particular note, BaYO2H exhibited high dura-bility, despite being a metastable phase. XPS measurements wereconducted to investigate the chemical stability of these catalystsduring ammonia synthesis reactions. Figure S14 (SupportingInformation) shows the wide-range XPS spectra of Ru/BaREO2Hbefore and after the catalytic reaction. All peaks are assignableto the constituent elements of the catalysts. After the catalyticreaction at 300 °C under 0.9 MPa for 24 h, the Ru 3d5/2peaks for Ru/BaREO2H shifted to a lower binding energy sidethan the zero-valent Ru peak (280 eV) observed on originalRu/BaREO2H (Figure 5a,b). This result suggests that the strongRu-BaREO2H interaction occurs during ammonia synthesis, re-sulting in electron transfer from BaREO2H to Ru. As shownin Figure 5c,d, both Sc 2p3/2 and Y 3d5/2 peaks shift to lowerbinding energy after the catalytic reaction, which may be dueto the reduction of metal cation sites by replacing lattice O2−with H− ions or by forming hydrogen vacancy sites. Anyway,the catalyst surface was not oxidized during the reaction, andtherefore, the Ru/BaREO2H exhibited stable catalytic activity(Figure 4a).Figure 6a shows the temperature dependence of the ammo-nia synthesis rate over various Ru catalysts at 0.9 MPa. TheRu-loaded oxyhydrides exhibited higher activity than the Ru-loaded oxides over the entire temperature range of 200–400 °C.Ru/BaScO2H showed the highest catalytic performance amongthe Ru catalysts measured in this study (Table 2). Figure 6b sum-marizes the ammonia synthesis rates and apparent activationenergies for the various catalysts (Figures S15 and S16, Sup-porting Information). The activities of the Ru/BaREO2H cata-lysts were superior to the conventional oxide-supported Ru cat-alysts with comparable or higher catalytic activities than those ofother oxyhydride-based Ru catalysts such as Ru/BaAl2O4−xHy.[33]The apparent activation energies for Ru/BaREO2H (49–67 kJmol−1) are clearly lower than those for conventional Ru cat-alysts (120 kJ mol−1) and Ru/BaREO3−𝛿 (88–91 kJ mol−1),and close to those for the electride- and hydride-based cata-lysts (45–69 kJ mol−1).[31–33] These electride and hydride sup-ports have strong promoting effects that lower the energy bar-rier for ammonia synthesis on Ru; therefore, the results in-dicate that the BaREO2H supports also have such promotingeffects.2.3. Analysis of Reaction Kinetics and Surface ElectronicPropertiesKinetic analyses were conducted to obtain information on the re-action mechanism for ammonia synthesis. Table 3 summarizesthe reaction orders with respect to N2, H2, and NH3 (Figure S17,Supporting Information). The N2 orders were close to +1or higher for all the catalysts, which suggests that the rate-determining step is still the dissociation of N2. In contrast,there was a significant difference in the H2 orders betweenthe oxide and oxyhydride supports; positive H2 orders were ob-tained with Ru/BaREO2H, whereas negative H2 orders wereobtained with Ru/BaREO3−𝛿 , as for conventional Ru catalysts.Conventional Ru catalysts exhibit negative H2 orders because ad-sorption of dissociated H2 prevents N2 adsorption on the activesites on Ru at high H2 pressure.[34] Therefore, the positive H2orders for Ru/BaREO2H indicate that these catalysts overcomesuch hydrogen poisoning. The NH3 orders for Ru/BaREO2H arelargely more negative than Ru/BaREO3−𝛿 , which indicates thatNHx species derived from the reverse reaction are more denselypresent on Ru/BaREO2H than on Ru/BaREO3−𝛿 . The accumu-lated NHx species may prevent H2 adsorption, which has anAdv. Energy Mater. 2024, 14, 2402353 2402353 (5 of 11) © 2024 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202402353 by National Institute For, Wiley Online Library on [17/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deFigure 5. Chemical states of Ru/BaREO2H surface before and after ammonia synthesis reaction. XPS spectra for Ru 3d5/2 of a) Ru/BaScO2H and b)Ru/BaYO2H. c) XPS spectra for Sc 2p3/2 of Ru/BaScO2H. d) XPS spectra for Y 3d5/2 of Ru/BaYO2H.Figure 6. Comparison of catalytic activities. a) Temperature dependence of NH3 synthesis activity for BaREO2H and BaREO3−𝛿 with Ru metal at 0.9 MPa.b) NH3 synthesis rates and apparent activation energies for various Ru catalysts at 300 °C and 0.1 MPa (lower part), and at 0.9 MPa (top part).Adv. Energy Mater. 2024, 14, 2402353 2402353 (6 of 11) © 2024 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202402353 by National Institute For, Wiley Online Library on [17/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deTable 2. Catalytic performance of various Ru catalysts measured in this work.Catalyst SBETa) [m2 g−1] Ru loading b) [wt.%] Pressure [MPa] rNH3c) [mmol g−1 h−1] Ead) [kJ mol−1]Ru/BaYO2H 2.2 1.8 0.1 2.77 66.00.9 4.35 54.2Ru/BaScO2H 4.4 1.7 0.1 4.09 66.80.9 7.87 48.9Ru/BaYO3−𝛿 – 1.5 0.9 0.39 91.0Ru/BaScO3−𝛿 – 1.7 0.9 1.36 88.0a)Specific surface area of various support materials determined by BET analysis;b)Determined by XRF;c)Ammonia synthesis rate measured at 300 °C;d)Activation energyfor ammonia synthesis was estimated from Arrhenius plots in the temperature range of 200—400 °C.Table 3. Reaction order for various Ru catalysts measured at 0.9 MPa.Catalyst Temperature [°C] N2 order (𝛼) H2 order (𝛽) NH3 order (𝛾) Refs.Ru/BaYO2H 300 +1.20 +0.34 −1.20 This workRu/BaScO2H 300 +1.05 +0.30 −0.83 This workRu/BaYO3−𝛿 320 +1.08 −0.04 −0.32 This workRu/BaScO3−𝛿 320 +1.47 −0.09 −0.42 This workRu/BaAl2O4−xHy 340 +0.72 +1.02 −1.00 [33]influence on the positive H2 order.[35] However, the H2 ordersfor the oxyhydride-supported Ru catalysts cannot be simply ex-plained by the NH3 orders. For example, Ru/BaYO2H witha larger negative NH3 order shows a smaller H2 order thanRu/BaAl2O4−xHy catalysts. Accordingly, other factors would con-tribute to the positive H2 orders for the Ru/BaREO2H catalysts.Positive H2 orders have been reported for electride- and hydride-supported Ru catalysts,[30–32] where reversible exchange betweenanionic electrons and H− occurs at the metal-support interface,which leads to a high tolerance toward H2 poisoning. BaREO2Hhas a large amount of H− vacancies so that H adatoms on the Rusurface can react with electrons at the anion vacancies to formH−. Subsequently, the regeneration of anion vacancies occurswhen H− reacts with nitrogen adsorbed on the Ru surface or des-orbs as H2 from the support surface, and this cycle is expected tosuppress hydrogen poisoning. Therefore, we tentatively considerthat the above mechanism suppresses hydrogen poisoning onRu/BaREO2H, which results in high ammonia synthesis rates,even at temperatures as low as 300 °C.Furthermore, the superior catalytic activity of Ru/BaREO2Hcan be explained by the electron-donating properties of the oxyhy-dride supports. Nitrogen bond dissociation has been reported tobe promoted in electride- and hydride-supported Ru catalysts bythe enhancement of electron donation to the antibonding orbitalof N2 adsorbed on Ru, which is attributed to the charge trans-fer of loosely bound electrons at the support surface to Ru.[30,32]Figure 7a shows the work functions (WFs) for the most stable sur-faces of BaREO2H and BaREO3−𝛿 (Table S3 and Figures S18,S19,S20, and S21, Supporting Information). The WFs for BaScO2Hand BaYO2H for the most stable surface without anion vacanciespredicted by DFT calculations are respectively 3.2 eV and 4.2 eV,which are similar to those for BaScO3−𝛿 and BaYO3−𝛿 . However,Figure 7. Surface electronic properties. a) WFs of BaREO2H and BaREO3−𝛿 . b) The most stable geometrical configurations of Ru4 cluster onBaScO2H(100) surface with hydrogen vacancy with electrons on the surface (left) and the bulk (right). ΔQ(Ru) represents the Bader charge on theRu4 cluster.Adv. Energy Mater. 2024, 14, 2402353 2402353 (7 of 11) © 2024 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202402353 by National Institute For, Wiley Online Library on [17/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergymat.dewww.advancedsciencenews.com www.advenergymat.dethe WFs for BaScO2H and BaYO2H were significantly decreasedby the formation of H vacancies on the top surface, and the lowestWFs were 1.9 and 2.2 eV, respectively. These values are compara-ble to that for C12A7:e− (2.4 eV), which has been experimentallyconfirmed to exhibit high electron donation ability to Ru and N2on Ru.[30,36] As additional information, the surface hydrogen va-cancy formation energy ΔEVH, at the most stable surfaces was0.69–1.31 eV for BaScO2H and 0.51–1.25 eV for BaYO2H (TableS3, Supporting Information), which are close to the reported val-ues for Ca2NH and CaH2 (0.88–1.11 eV) that are predicted toexhibit low WFs (2.3 eV) by the formation of H vacancies.[32,37]These calculation results indicate that BaREO2H exhibits highelectron-donating properties due to easily formed H vacancies.TDS results also support the ease of hydrogen vacancy forma-tion. The electron-donating ability of BaREO2H to Ru was ex-perimentally confirmed by the XPS measurements. As shownin Figure 5a,b, the Ru 3d5/2 peak of Ru/BaREO2H after the cat-alytic reaction appears at a lower binding energy side than thatof metallic Ru (280.0 eV).[38] This is clear experimental evidencefor the electron transfer from the oxyhydride support to Ru. Inparticular, the Ru 3d5/2 peak for Ru/BaScO2H (279.4 eV) was lo-cated at a lower binding energy side than that for Ru/BaYO2H(279.6 eV), meaning that BaScO2H has higher electron donat-ing ability than BaYO2H. As shown in Table S4 (Supporting In-formation), there is no significant difference in the particle sizeof Ru between Ru/BaScO2H and Ru/BaYO2H, which indicatesthat the superior catalytic activity of Ru/BaScO2H is not due tothe dispersibility of Ru. Therefore, the efficient electron dona-tion ability of BaScO2H accounts for the superior catalytic ac-tivity of Ru/BaScO2H than that of Ru/BaYO2H. Additional DFTcalculations were performed on the Ru-loaded model to investi-gate the electron-donating properties of BaScO2H to Ru in de-tail (Figure S22, Supporting Information). Figure 7b shows thecharge transfer between Ru cluster and BaScO2H, which indi-cates that the electron donation from the surface hydrogen va-cancy with electrons is more efficient than that from the bulkhydrogen vacancy. The ΔEVH in the bulk models of BaScO2H(1.50 eV) is much larger than the surfaceΔEVH, which means thatthe hydrogen vacancies are formed more easily on the surfacethan in the bulk. Therefore, the high electron-donating abilityof BaScO2H mainly originates from the surface hydrogen vacan-cies. Moreover, the amount of hydrogen vacancies in BaScO2H ismore than three times higher than that of BaAl2O4-xHy (Table S4,Supporting Information). Given the similar Ru particle size be-tween these two catalysts, their activity difference is attributed tothe difference in the electron-donating ability between BaScO2Hand BaAl2O4-xHy. We consider that these electron donation prop-erties of BaREO2H contribute to the promotion of N2 cleavageover Ru, which reduces the activation barrier for ammonia syn-thesis (49–67 kJ mol−1). From these results, the reaction mecha-nism of ammonia synthesis over Ru/BaREO2H is illustrated inFigure 8. The BaREO2H support has numerous hydrogen va-cancy sites (Figure 2), among which the surface hydrogen vacan-cies mainly contribute to electron donation into Ru nanoparticlesto form negatively charged Ru sites (Figure 5a,b, and Figure 7b).N2 dissociation is facilitated on negatively charged Ru nanopar-ticles on BaREO2H. The surface hydrogen vacancy sites can cap-ture the hydrogen atoms on the Ru surface, preventing hydro-gen poisoning on the Ru surface (Table 3). The surface latticeFigure 8. Schematic illustration of ammonia synthesis over Ru/BaREO2H.H− ions react with nitrogen species on the Ru surface, and atthe same time, hydrogen vacancy sites are regenerated at the Ru-support interface. Thus, the reversible exchange between anionicelectrons and H− takes place at the Ru-BaREO2H support in-terface. As a result, nonstoichiometric BaREO2-xHx is preservedduring ammonia synthesis as confirmed by TDS experiments(Figure S23, Supporting Information).3. ConclusionMechanochemical synthesis of perovskite-type oxyhydridesBaYO2H and BaScO2H was successfully achieved. In particular,BaYO2H, which is a metastable phase with a thermodynamicallyunstable crystal framework, could be synthesized only by themechanochemical method. The synthesized oxyhydrides wereused as supports for ammonia synthesis catalysts that exhibitedhigh catalytic performance comparable to those of previously re-ported efficient support materials at low temperatures. The highcatalytic performance is attributed to the inhibition of hydrogenpoisoning at the active sites and the high electron-donatingability induced by H− vacancies formed on the oxyhydrides.These results are expected to encourage further syntheses ofmetastable oxyhydrides and the development of promisingcatalyst supports for ammonia synthesis.4. Experimental SectionSynthesis of Support Materials: BaO (99.99%, Sigma–Aldrich), BaH2,Y2O3 (99.99%, Kojundo), and Sc2O3 (>99.9%, Kojundo) were used asstarting materials for the synthesis of the BaREO2H (RE = Y, Sc) oxy-hydrides. BaH2 was prepared by heating Ba metal (99.99%, Sigma–Aldrich) under an H2 (>99.99%) atmosphere at 600 °C for 12 h. Themechanochemical synthesis was conducted with reference to a previouslyreported method for the synthesis of oxyhydrides.[18,19] The starting mate-rials (total weight:2 g, BaO:BaH2:RE2O3 = 1:1:1 molar ratio) were added toa 20 mL grinding bowl made of WC with ten balls (10 mm diameter) underan Ar atmosphere. Mechanochemical synthesis was then conducted usinga planetary ball mill (Pulverisette 7 Premium Line, Fritsch) at a speed of800 rpm for a total of 120 cycles (4 times 30 cycles). One cycle consistedof 2 min of milling and 3 min pauses to reduce the heating of the sam-ple. After 30 cycles, the sample was removed from the grinding bowl andAdv. Energy Mater. 2024, 14, 2402353 2402353 (8 of 11) © 2024 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202402353 by National Institute For, Wiley Online Library on [17/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergymat.dewww.advancedsciencenews.com www.advenergymat.decrushed using an agate mortar. A yttrium-stabilized zirconia grinding bowland balls were also available and when used, the number of reaction cy-cles was increased, typically to ≈240–300 cycles at a speed of 850 rpm. Toexamine the crystal structure, the crystallinity of the milled sample was im-proved by annealing in an evacuated glass tube at 500 °C for 72 h. The syn-thesized compounds were highly air-sensitive; therefore, they were storedand handled in an Ar-filled glove box.BaREO3−𝛿 oxides were also synthesized using the same procedure forthe synthesis of BaREO2H (total weight:2 g, BaO:RE2O3 = 2:1 molar ratio).Mechanochemical synthesis was conducted at a speed of 850 rpm for atotal of 90–150 cycles (3–5 times 30 cycles). An annealing process wasalso conducted using the same conditions as those for BaREO2H.Preparation of Catalysts and Catalytic Reaction: Ru3(CO)12 (99%,Sigma–Aldrich) was used as the Ru precursor for Ru/BaREO2H andRu/BaREO3−𝛿 . The amount of Ru was fixed at 2 wt.%, and Ru loadingwas conducted according to a previously reported method.[32] The actualamount of Ru loaded on the support was determined by X-ray fluorescencespectroscopy (S8 TIGER, Bruker).Ammonia synthesis was conducted in a stainless-steel reactor with thecatalyst (0.1 g) under a flow of N2 (purity >99.99995%, 15 mL min−1) andH2 (purity >99.99999%, 45 mL min−1) with a H2/N2 flow ratio of 3 at200–400 °C and 0.1–0.9 MPa. The ammonia produced was trapped in anH2SO4 solution, and the amount of NH4+ ions generated in the solutionwas determined using ion chromatography (Prominence, Shimadzu) witha conductivity detector. The apparent activation energies were calculatedfrom Arrhenius plots of the ammonia synthesis rates at various tempera-tures. Ammonia synthesis rates that were less than ca. 15% of the equilib-rium conversion were used to evaluate the apparent activation energies.Kinetic Analysis: The reaction orders of the catalysts were measuredusing a previously reported method.[39] The reaction orders with respectto N2 (𝛼) and H2 (𝛽) were calculated from the gas partial pressure depen-dence on the ammonia synthesis rate at a constant flow rate of mixed gasof Ar, H2, and N2 (60 mL min−1). The catalyst (0.1 g) was kept at 0.9 MPaand at a temperature where the outlet ammonia concentration was lessthan 10% of the equilibrium conversion. The ammonia synthesis rate (r)over various catalysts can be expressed by a power law equation using thepartial pressures of nitrogen (PN2), hydrogen (PH2), and ammonia (PNH3):r = k P𝛼N2 P𝛽H2 P𝛾NH3 (1)where k is a rate constant, and 𝛼, 𝛽, 𝛾 are the reaction orders with respect toN2, H2, and NH3, respectively. The reaction orders were calculated usingEquations. (1–5):r =( 1W)dy0∕d (1∕q) (2)log y0 = log(Cq)1∕m(3)r =( 1W) ( Cm)y1−m0 (4)C = k2P𝛼N2 P𝛽H2 (5)where y0, W, q, (1–m) and k2 represent the mole fraction of outlet ammo-nia, the weight of the catalyst, the total flow rate, the NH3 order (𝛾), andthe rate constant respectively.Characterization: The crystal structures of the oxyhydrides were iden-tified using powder X-ray diffraction (XRD; MiniFlex600, Rigaku) with CuK𝛼 radiation (𝜆 = 0.15418 nm). The sample was sealed in an X-ray trans-mitting container to protect it from oxidation and hydration. Rietveld re-finements were performed using the TOPAS (Bruker AXS, version 4.2)software. Hydrogen desorption from the support materials was detectedusing thermal desorption spectroscopy (TDS; TDS-1400TV, ESCO). Pow-der samples placed on a SiO2 holder were heated to 1200 °C under vac-uum conditions, and desorbed hydrogen ions (m/z = 2) and water ions(m/z = 18) were counted as an electronic current by a quadrupole massspectrometer. The numbers of hydrogen molecules and water moleculeswere calculated based on the area of the desorption peak corrected usinga silicon wafer containing implanted hydrogen. The Brunauer–Emmett–Teller (BET) specific surface area of the oxyhydrides was obtained fromnitrogen adsorption isotherms measured at liquid-nitrogen temperatureusing an automatic gas adsorption instrument (BELSORP-mini II, Mi-crotracBEL). The vibrational spectra were recorded with a Fourier trans-form infrared spectrometer (FT-IR; Frontier DTGS, PerkinElmer). The sam-ples were sealed in KBr plates to avoid oxygen and water contamination.Measurements were conducted between 4000 and 500 cm−1 with a spec-tral resolution of 4 cm−1. Optical diffuse reflectance spectra were mea-sured at room temperature with an UV–vis spectrometer (UV–vis; U-4000,Hitachi). X-ray photoelectron spectroscopy (XPS; KRATOS ULTRA2, Shi-madzu) was conducted using an apparatus equipped with a charge neu-tralization system. The binding energy of all spectra was calibrated withreference to the C 1s peak. All samples were delivered from the Ar-filledglovebox to the XPS analysis chamber through the vacuum transfer cham-ber to avoid oxidation and decomposition of the sample surface upon con-tact with air. The average particle size of Ru was determined by CO pulsechemisorption at 50 °C with a He flow of 30 mL min−1 and 0.031 mL pulsesof 9.51% CO in He using a catalyst analyzer (BELCAT-A, MicrotracBEL);spherical metal particles and a stoichiometry of Ru/CO = 1 were assumed.DFT Calculations: Density functional theory (DFT) calculations wereconducted using the Vienna Ab initio Simulation Package (VASP)code.[40,41] The electron exchange and correlation were described usinga Perdew–Burke–Ernzerhof type generalized gradient approximation.[42]The 5s2, 5p6, and 6s2 electrons of Ba, 4s2, 4p6, and 5s2 electrons of Y, 3s2,3p6, and 4s2 electrons of Sc, 2s2 and 2p4 electrons of O, 1s1 electron of H,and 4p6, 5s2, and 4d6 electrons of Ru were handled as valence electrons.The core electrons were described using the projector augmented wave(PAW) method.[43,44] A cutoff energy of 550 eV for BaYO2H and 500 eVfor BaScO2H were used. 4 × 4 × 7 k-point meshes were used for struc-tural optimization of the bulk, while the Γ-point-only approach was ap-plied for the slab. Electrostatic potentials applied for evaluation of the WFwere calculated using 2 × 3 × 1 k-point meshes for the optimized slabmodels. The convergence criteria for energy and force were respectively1.0 × 10−5 eV and 1.0 × 10−2 eV Å−1 for other than phonon calculations.For the phonon calculations, 7 × 7 × 15 k-point meshes were used andconvergence criteria of 1.0 × 10−6 eV and 1.0 × 10−4 eV Å−1 were usedfor energy and force, respectively. To determine the stable anion arrange-ment of BaREO2H with the lowest energy, 35 structures with different an-ion arrangements in a 2 × 2 × 1 supercell based on a cubic unit cell weregenerated using the Supercell program and optimized.[45] Stable struc-tures of oxygen-deficient perovskite BaREO3−𝛿 were also determined bythe same procedure as that for BaREO2H. The composition of BaREO3−𝛿was assumed to be BaREO2.5 to satisfy the charge neutrality. Phonon cal-culations were performed based on the finite displacement method, andthe displaced structures were generated using the Phonopy module.[46]IR spectra with fixed line widths of 16.5 cm−1 were calculated using thePhonopy-Spectroscopy package.[47] Slab models with non-polar and sto-ichiometric surfaces were generated by the tsubo.pl, module with primi-tive cells of the structure with the most stable anion arrangement as theinput.[48,49] The slabs were constructed in 13 atomic layers, and a 20 Å vac-uum layer was introduced between the slabs to eliminate the interactionsbetween the slabs. Surface energies were calculated for all optimized slabmodels and the most stable facets were determined as (111) for BaYO2H,(100) for BaScO2H, (010) for BaYO2.5, and (111) for BaScO2.5. The for-mation enthalpies for the BaREO2H oxyhydrides ΔEF, at 0 K were definedas 2E(BaREO2H) − {E(BaO) + E(BaH2) + E(RE2O3)}, where E is the to-tal energy per compositional formula. Structural information for variousphases that contained the elements of Ba, Y, O, and H was obtained fromthe ICSD and Materials Project databases. The formation enthalpies foreach element were calculated after optimization of all the structures, andthe equilibrium phase diagram for the Ba─Y─O─H system was createdusing the Chesta program with the calculated enthalpies. The WF was de-fined as the difference between the Fermi energy for the slab models andthe electrostatic potential in the vacuum gap of the slab model. We used aAdv. Energy Mater. 2024, 14, 2402353 2402353 (9 of 11) © 2024 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202402353 by National Institute For, Wiley Online Library on [17/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergymat.dewww.advancedsciencenews.com www.advenergymat.deBader analysis of the charge density distributions for an atomic charge.[50]All crystal structures for the bulk and slab models were visualized usingthe VESTA software package.[51]Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work was supported by the FOREST Program (No. JPMJFR203A), theJST-Mirai Program (JPMJMI21E9) and the JST SPRING (No. JPMJSP2106)of the Japan Science and Technology Agency (JST), the Kakenhi Grant-in-Aid (No. JP22H00272, JP24H02204) from the Japan Society for thePromotion of Science (JSPS), and the Tokyo Tech Advanced Researchers[STAR] Grant funded by the Tokyo Institute of Technology Fund (Tokyo TechFund).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the cor-responding author upon reasonable request.Keywordsammonia synthesis, mechanochemical method, metastable oxyhydride,Ru-based catalystReceived: May 31, 2024Revised: July 11, 2024Published online: August 3, 2024[1] K. 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Energy Mater. 2024, 14, 2402353 2402353 (11 of 11) © 2024 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH 16146840, 2024, 47, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/aenm.202402353 by National Institute For, Wiley Online Library on [17/02/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.advenergymat.de Ammonia Synthesis over Ruthenium Supported on Metastable Perovskite Oxyhydrides BaREO2H (RE &#x003D; Y, Sc) Prepared by Mechanochemical Method 1. Introduction 2. Results 2.1. Synthesis and Characterization of BaREO2H 2.2. Catalytic Performance for Ammonia Synthesis 2.3. Analysis of Reaction Kinetics and Surface Electronic Properties 3. Conclusion 4. Experimental Section Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords