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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

High Electron Density on Ru in Intermetallic YRu: The Application to Catalyst for Ammonia Synthesis Takaya Ogawa, Yasukazu Kobayashi, Hiroshi Mizoguchi, Masaaki Kitano, Hitoshi Abe, Tomofumi Tada, Yoshitake Toda, Yasuhiro Niwa, and Hideo Hosono J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02128 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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The Journal of Physical Chemistry

High Electron Density on Ru in Intermetallic YRu2: The Application to Catalyst for Ammonia Synthesis Takaya Ogawa, †,‡,+,⊥ Yasukazu Kobayashi, †,‡,⊥Hiroshi Mizoguchi, †,‡ Masaaki Kitano, † Hitoshi Abe, §,ǁ,‡ Tomofumi Tada, † Yoshitake Toda, †,‡ Yasuhiro Niwa,§ Hideo Hosono, †,‡,¶,* †

Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259

Nagatsuta, Midori-ku, Yokohama, Kanagawa, 226-8503, Japan ‡

ACCEL, Japan Science and Technology Agency, 4−1−8 Honcho, Kawaguchi, Saitama 332-

0012, Japan. §

Institute of Materials Structure Science, High Energy Accelerator Research Organization

(KEK), 1−1 Oho, Tsukuba, Ibaraki 305−0801, Japan. ǁ

Department of Materials Structure Science, School of High Energy Accelerator Science,

SOKENDAI (the Graduate University for Advanced Studies), 1−1 Oho, Tsukuba, Ibaraki 3050801, Japan. ¶

Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-

ku, Yokohama, Kanagawa, 226-8503, Japan

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ABSTRACT

Ruthenium is the most effective catalyst reported to date for ammonia synthesis under mild conditions, especially when an electron promoter is used. However, electron donation from the promotor has not been sufficient because the promotor contacts with Ru only through their surface. Here, we report a Laves phase intermetallic bulk catalyst, YRu2, which has higher electron density on Ru. This is derived from large electron transfer from Y to Ru, which is firstly confirmed by X-ray absorption fine structure measurements and theoretical calculations. In addition, YRu2 has high hydrogen solubilities leading to suppression of hydrogen poisoning, a common drawback of Ru-based catalysts. Consequently, YRu2 exhibits higher catalytic activity for ammonia synthesis over 300 times that with pure ruthenium. The present results suggest a simple concept for ammonia synthesis: Laves phase intermetallic compounds of Ru and more electropositive metals are more efficient catalysts than pure Ru because of large electron promotion via intermetallic bonds and suppression of hydrogen poisoning.

1. INTRODUCTION Industrial ammonia synthesis has long been an essential process for obtaining the starting material for fertilizer production. Recently, ammonia has attracted attention as a renewable energy carrier because liquid ammonia can be easily obtained at ca. 1 MPa and 25 °C, i.e., NH3 can be used as a portable fuel. The energy density of NH3 is notably large at 22.5 kJ g–1, which constitutes a high heating value. Moreover, there is no need to recycle nitrogen as a reactant. These features make NH3 suitable as an easily transportable energy carrier even between countries. Ammonia has been industrially synthesised at 400−500 °C and 10−30 MPa using an Fe-based catalyst for over 100 years.1−4 However, these severe conditions consume the 2 ACS Paragon Plus Environment

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renewable energy that could be converted to NH3 and require a large, robust and expensive plant. Meanwhile, the landscape of the ammonia industry is changing rapidly; the demand for small scale, on-site synthesis is significantly increasing because of regulations, and the rising costs of transportation and storage. The on-site process makes it possible to reuse waste H2 from the whole process of ammonia synthesis for other processes, such as steel and glass production among other applications. Furthermore, on-site synthesis is suitable for the utilization of renewable energy generated at dispersed sites. Approaches to explore novel catalysts for ammonia synthesis have been based on the chemisorption energy of nitrogen over the catalyst surface and promotion by electropositive metals.3,5 The general rate-determining step is dissociative N2 adsorption due to its rigid triple bond (N≡N).3 The dissociation barrier decreases with increasing nitrogen chemisorption energy on metal surfaces.5 In contrast, a high chemisorption energy induces NHx (x = 1−3) species to become strongly bound to the catalyst surface, which impedes quick removal of the species and reduces the turnover frequency (TOF).3,5 Therefore, one approach to exploring an active catalyst is to find a catalyst that has an optimal nitrogen chemisorption energy. Another approach is the addition of electropositive metals such as alkali compounds to donate electrons to the catalysts. Electrons from electropositive metals increase the electron density of the catalyst and facilitate both dissociative N2 adsorption and NHx desorption.3,6 Since the 1970s, Ru has been the focus of attention as the most active catalyst reported to date for ammonia synthesis due to its optimal nitrogen chemisorption energy. Ru-based catalysts exhibit much higher activity for NH3 synthesis than industrial Fe-based catalysts under mild conditions when electropositive metals such as Cs are added near/on the Ru surface.7−13 However, the electron density on Ru is not enough large because the promoters contact with Ru



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only through their surface. To achieve higher catalytic activity, more concentrated electron should be on Ru although negatively charged Ru in Ru-based catalyst has not been reported in previous researches. In addition, Ru has a fatal drawback, hydrogen poisoning, where hydrogen covers the active sites on the Ru surface under high hydrogen pressure.1,14,15 To make matters worse, electron donation from the promoter reinforces the binding of Ru with hydrogen, which further enhances hydrogen poisoning16 and results in a negative reaction order for hydrogen.14−16 Hydrogen poisoning limits the total pressure, even though an elevated pressure is thermodynamically favourable and industrially efficient, because the ammonia produced can be collected as a liquid. Recent studies on Ru-loaded C12A7 electride catalyst have clarified that the activity is distinctly enhanced by depositing Ru-nanoparticles on support materials with low work functions (WFs), such as electrides, and hydrogen poisoning can be suppressed by employing supports that have a reversible and rapid exchangeability between electrons and hydrogen.4,17 These conclusions have led us to a straightforward concept for efficient catalysts; intermetallic Ru compounds composed of rather electropositive metals and with high hydrogen solubility. Here, we report a Laves phase Ru intermetallic catalyst, yttrium ruthenium (YRu2), where high electron density is on Ru, and which exhibits almost no hydrogen poisoning at high pressure. A Laves phase is an intermetallic compound that is described as AB2, where the A/B size ratio is from 1.05 to 1.67.18,19 This type of intermetallic phase typically has hydrogen storage capacity, which leads to high hydrogen solubility in YRu2 and suppression of hydrogen poisoning.20,21 Furthermore, the rather electropositive nature of Y increases the electron density of Ru as the catalyst, which results in the formation of a negatively charged Ru species, i.e., Y2+(Ru–)2, which has not been observed for supported Ru catalysts reported to date. The catalytic activity of YRu2 4 ACS Paragon Plus Environment

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is consequently two orders of magnitude higher than that of pure ruthenium. A similar intermetallic material, CeRu2, previously investigated as a catalyst is different from our concept because CeRu2 is completely decomposed to Ru metal and CeH2+x under ammonia synthesis conditions.22,23 For practical applications, the surface area of YRu2 catalysts must be increased. It is generally challenging to prepare intermetallic nanoparticles containing early transition metals due to their high oxygen affinity and negative reduction potentials.24 For example, only a few successful methods have been reported so far for YPt3 nanoparticles.25−27 In this study, we proposed the utilization of hydrogen embrittlement (HE) and laser ablation in argon atmosphere to fabricate nanoparticles of YRu2. The nanoparticles prepared by the laser ablation enhanced activity for ammonia synthesis almost linearly with increased surface area. This result validates that YRu2 nanoparticles keep their high activity when YRu2 is dispersed on catalyst supports in nanoparticle form.

2. EXPERIMENTAL AND THEORETICAL METHODS 2.1 Preparation of bulk and nanoparticles of YRu2. YRu2 was prepared from a stoichiometric mixture of Ru (99.95%, Rare Metallic Co. Ltd.) and Y (99.9%, Rare Metallic Co. Ltd.) via arcmelting on a water-cooled Cu hearth under a high purity Ar atmosphere. Ru powder (99.9%) was purchased for comparison as a reference. HE was performed by incubating YRu2 in a 1 MPa H2 atmosphere and the temperature was increased to 500 °C with a rate of 200 °C h–1 and held for 2 h. Hydrogen adsorbed on YRu2 was evacuated under vacuum at 300 °C for 30 min. Ball-milling was performed first at 200 rpm for 100 min, followed by five HE steps. The prepared YRu2 using both ball-milling and HE was utilised for analysis of reaction orders, Ea, stability tests, and X-ray absorption fine structure (XAFS) measurements. The YRu2 employed for comparison of 5 ACS Paragon Plus Environment


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the TOF with other catalysts and activity measurements under high pressure were prepared by only hand-milling with an alumina mortar and pestle. These simply hand-milled YRu2 and HE performed YRu2 were confirmed to have same properties and similar surface areas, and thus we named them as bulk YRu2. YRu2 nanoparticles were prepared by laser ablation in argon. First, a YRu2 pellet of a 10 mm diameter used as a target in laser-ablating was shaped by pressing YRu2 powder prepared by arcmelting mentioned above at 20 MPa for 10 min. Next, the obtained pellet was placed on the bottom of a closed glass vessel filled with argon. Finally, focused pulse lasers (13 ns, 532 nm, 10 Hz) were repeatedly irradiated onto the pellet surface by a Q-switched Nd:YAG pulsed laser (Spectron Laser Systems Ltd., SL8585G) with a laser fluence of 31 J/cm2/pulse, which allowed for the production of the final nanoparticles at a rate of ca. 100 mg/hr. For some samples, the obtained nanoparticles were further refined by a sedimentation technique as a post-treatment. At first, approximate one gram of the laser-ablated powder was put into a 50 mL sample bottle filled with 20 mL acetonitrile and then the closed bottle was sonicated for 10 minutes to obtain good dispersed suspension liquid. Finally, the supernatant liquid was taken out within a few tens of minutes and dried in order to obtain the more refined samples. A series of samples with different surface areas were obtained by controlling the holding time after the sonication treatment. We refer the YRu2 prepared by the laser ablation techniques as a laser-ablated YRu2. 2.2 Characterization. The BET specific surface areas of the samples were determined from nitrogen adsorption–desorption isotherms measured at −196 °C using an automatic gasadsorption instrument (NOVA-4200e, Quantachrome) after evacuation of the samples at 130 °C. Powder X-ray diffraction (XRD) patterns were recorded using a D8 Advance diffractometer (Bruker) with monochromated Cu Kα radiation (λ = 0.15418 nm, 40 kV, 40 mA). Thermal 6 ACS Paragon Plus Environment

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desorption spectroscopy (TDS, 1400TV, ESCO) was measured with the temperature increased at 1 °C s–1 until 800 °C and the sample held for 10 min at 800 °C. Work function (WF) measurements were conducted using a hemispherical analyser (DA30, Scienta Omicron, Inc.) with a monochromated He I light source (MB Scientific AB). The measured surface was prepared by rupturing polycrystalline pellets under ultrahigh vacuum. XAFS measurements were performed at the AR-NW10A beamline of PF-AR, the Photon Factory at the Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Japan. The AR-NW10A beamline has a Si(311) double-crystal monochromator, a focusing mirror, and two flat mirrors to remove higher harmonics. The higher-harmonic reduction mirrors are not required for Ru K-edge XAFS measurements. The slit aperture was 1×1 mm2. Athena and Artemis software28 and the FEFF6 code29 were used for analysis of the XAFS spectra. EXAFS analysis was performed with the k range of 2.5–15.2 Å–1 and the R range of 2.0–3.1 Å. Transmission electron microscopy (TEM) images of laser-ablated YRu2 nanoparticles were taken using a microscopy (JEM-ARM 200F, JEOL) operated at 200kV. 2.3 Ammonia synthesis reaction. Ammonia synthesis was conducted in a silica glass or a stainless-steel flow reactor that operated up to 1.5 MPa with a supply of an ultrapure (99.99995%) mixture of H2:N2 (3:1). Prior to the reactions, all catalysts were treated in a stream of N2 + 3H2 at 0.1 MPa with heating to the target temperature at a rate of 200 °C h–1 and then holding at temperature for 30 min. The concentration of ammonia in the stream that left the catalyst bed (0.030 g and 0.50 g for YRu2 and Ru, respectively) was monitored under steadystate conditions of temperature, gas flow rate (60 mL min−1) and pressure (0.1 to 0.8 MPa). Reaction orders were estimated from the equation = k , where r is the ammonia synthesis rate, k is the reaction rate constant, PX is the partial pressure of gas X, and α, 7 ACS Paragon Plus Environment


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β, and γ, are the reaction orders for N2, H2, and NH3, respectively. For measurement of the reactivity under high pressure, the flow rate was kept constant with a fixed composition of N2:H2 = 1:3, and the temperature at 400 °C for YRu2 and at 450 °C for Ru. The ammonia produced was trapped in 5 mM sulphuric acid solution and the amount of NH4+ generated in the solution was determined using ion chromatography (LC-2000 plus, Jasco) with a conductivity detector. Ea for catalysis was calculated from Arrhenius plots measured in the ranges of 320−400 °C and 400−480 °C for YRu2 and Ru, respectively. 0.10-0.20 g of YRu2 was utilised for comparison of the TOFs. 2.4 Computational conditions. Density functional theory (DFT) computational methods implemented in the Vienna ab initio simulation program (VASP) code were adopted for the electronic structure calculations and structural relaxations of YRu2.30,31 The 4s, 4p, 4d, 5s electrons of Y, and 4p, 4d, 5s electrons of Ru are handled as valence electrons (i.e., these are represented with wave functions), and core electrons are handled using the projector augmented wave (PAW) method. The exchange-correlation functional in DFT was described using the Perdew-Burke-Ernzerhof (PBE) type generalized gradient approximation (GGA). The cut-off energy adopted in this study was 500 eV. The convergence criteria of energy and force were 1.0×10−6 eV and 1.0×10−2 eV/Å, respectively.

3. RESULTS 3.1 Characterization of YRu2. Figure 1 shows the crystal structure of YRu2 prepared by an arcmelting method and the results of XRD measurements. The synthesized material was confirmed to be YRu2 with a hexagonal C14 Laves-phase structure when referenced to a previously reported pattern.32


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Figure 1 XRD patterns for reference32 and YRu2 prepared by an arc-melting method. Plane index are shown on the main peaks. The inset shows the crystal structure of YRu2. The solid lines indicate the unit cell.

The WF of bulk YRu2 was evaluated using ultraviolet photoelectron spectroscopy measurements of the secondary electron energy cut-off. The WF of YRu2 was approximately 3.6 eV, which is between that for Y (3.1 eV) and Ru (4.7 eV).33 This result suggests partial electron transfer from Y to Ru, which is consistent with the results of X-ray absorption near edge structure (XANES) measurements. The XANES spectrum for bulk YRu2 at the Ru K-edge is shown in Figure 2 together with those for Ru and RuO2 as references. Positively charged species generally exhibit higher energy edge positions, while negatively charged species exhibit lower energy. The absorption edge for YRu2 shifts to lower energy than that for Ru by ca. 2 eV, while that for RuO2 is higher than that for Ru by 10 eV. Here, the lower edge position for YRu2 relative to Ru indicates Yδ+Ru2δ− as the charge state. Thus, the WF and XANES results confirm that electron transfer from Y to Ru in YRu2 occurs and Ru has an anionic state. 9 ACS Paragon Plus Environment


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Figure 2 XANES spectra for YRu2, Ru, and RuO2. The absorption edge of YRu2 is shifted to lower energy than that of Ru and RuO2, which reveals that electron density is transferred from Y to Ru in YRu2.

The intermetallic structure of YRu2 was also examined by extended X-ray absorption fine structure (EXAFS) analysis. Fourier transforms (FTs) of EXAFS oscillations are shown in Figure 3. The FT peak structure for the YRu2 sample differs from those for Ru and RuO2. The first peak was fitted by the intermetallic YRu2 structure model. The average measured bond distances for Ru-Ru and Ru-Y were determined to be 2.64 and 3.10 Å, respectively, which are consistent with the reported crystal structure, 2.65 and 3.10 Å, respectively.34 This confirms that Y-Ru intermetallic bonds were formed (see Table S1).


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Figure 3 Fourier transforms of the EXAFS oscillations. The average measured bond lengths for Ru-Ru and Ru-Y were determined to be 2.64 and 3.10 Å, respectively.

The degree of electron transfer from Y to Ru in YRu2 was estimated based on the WF and XANES edge energy shift data. In principle, the 1s energy level is proportional to the valence number of the electron transfer. The difference in the 1s energy level between two species can be estimated from the total energy shift of the K-edge energy position and the difference in the lowest unoccupied energy level. Firstly, Ru(0) and RuO2(IV) were investigated to determine the quantitative relation between the valence electron transfer and the 1s energy level for the ruthenium species (Figure 4). RuO2 belongs to the class of conducting transition metal oxides. RuO2 exhibits electrical conductivity that is almost metallic35,36 and has no band gap above its Fermi level.37 Hence, RuO2 is in a metal state, and the lowest unoccupied energy level of RuO2 can be regarded as its Fermi level. The WF of RuO2 is 6.1 eV,38 and that of Ru is 4.7 eV; therefore, the difference between their lowest unoccupied energy levels is 1.4 eV. The K-edge energy shift between Ru and RuO2 was measured to be 10 eV. The 1s binding energy shift for 11 ACS Paragon Plus Environment


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RuO2 with respect to Ru can be estimated as the sum of 10 eV and 1.4 eV, i.e., 11.4 eV. The 11.4 eV shift results from the oxidation state of the Ru4+ species of RuO2. We can thus consider that a shift of 2.9 (= 11.4/4) eV is caused by one valence electron transfer. The lowest unoccupied energy level of YRu2 can also be estimated from its WF of 3.6 eV (Figure 4). The WF of Ru is 4.7 eV, so that the difference in WF between YRu2 and Ru is 1.1 eV. The K-edge energy difference between YRu2 and Ru was measured to be 2.0 eV. Thus, the sum of 2.0 eV and 1.1 eV indicates the 1s binding energy shift between Ru and YRu2 is 3.1 eV. A one valence electron transfer was considered to cause a shift of 2.9 eV; therefore, the shift of 3.1 eV indicates one valence electron transfer from Y to Ru, i.e., Y2+(Ru–)2 as a first approximation.

Figure 4 Energy diagram of the 1s energy level, Fermi level (Ef) and vacuum level (Evac) in YRu2, Ru and RuO2. These relations are depicted based on XANES edge energies and WF measurements.

Density functional theory (DFT) calculations were conducted to confirm the charge states of YRu2. A Bader charge analysis was adopted for bulk YRu2, which confirmed that the average number of electrons transferred from Y to Ru was 1.52 e− per Y atom (i.e., 0.76 e− is donated to 12 ACS Paragon Plus Environment

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each Ru atom). Thus, the electron transfer in YRu2 qualitatively corresponds well to that estimated from the XANES spectra. The charge states of the YRu2 surface were also calculated using DFT. The adopted surface models were all stoichiometric YRu2 (i.e., the ratio of Ru/Y is equal to 2), and the most stable surface was determined based on the surface energy, which is defined as, = { YRu − YRu }/2, where Es and Eb are respectively the total energies of the surface and bulk YRu2, and S is the surface area. Figures 5a and 5b show the most stable surface YRu2(0001) model, where Figure 5a shows a top view parallel to the c-axis, and Figure 5b shows a side view parallel to the b-axis, in which the upper half of the YRu2(0001) model is depicted. Figure 5b clearly shows that the amount of charge transfer between Y and Ru at the surface (i.e., the hatched atoms in Fig. 5a and 5b) is almost the same as that in the bulk. The computational models and surface energies for other surface models are given in Figure S1 and Table S2.



a

b

Figure 5 Calculated YRu2(001) surface model; a top- and b side-views. The colour scheme is the same as in Figure 1. The atoms at the topmost layers are shown with a hatched symbol. The numbers in b are the calculated Bader charges for Y and Ru, which are layer-averaged charges.

The hydrogen capacity of YRu2 was evaluated from TDS measurements. Prior to TDS measurements, bulk YRu2 was held at 500 °C and 1 MPa under a hydrogen atmosphere for 1 h (the same conditions for HE). The TDS results in Figure 6 indicate that YRu2 was converted to YRu2H2.4 during the hydrogen treatment. YRu2 can also absorb hydrogen, even under the reaction conditions for ammonia synthesis that were employed in the following sections. The inset in Figure 6 also shows TDS results for the same weight amounts of bulk YRu2 and Ru after incubation for 20 h under reaction conditions of 400 °C and 0.1 MPa with a gas flow of N2:H2 (= 14 ACS Paragon Plus Environment

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1:3). YRu2 has a peak for hydrogen desorption, whereas Ru does not have any such obvious peaks. The TDS results showed that YRu2 was converted to YRu2H0.3 under these reaction conditions. Since the dissolved hydrogen likely forms H− and hydrides in YRu2H0.3 can occupy 0.3 e− at most, the Ru in YRu2H0.3 remains negatively charged as Y2+(Ru0.85−)2(H−)0.3 during reaction.

Figure 6 TDS results for YRu2 after incubation under a hydrogen atmosphere at 1 MPa and 500 °C for 1 h, which are the same conditions for HE. The inset shows TDS results for the same weight amounts of YRu2 and Ru under the reaction conditions for ammonia synthesis.

3.2 Catalytic activity of YRu2. The catalytic activity of YRu2 for ammonia synthesis was examined at 400 °C under atmospheric pressure (see experimental section). Pure Ru metal was also investigated for comparison. The BET surface areas for bulk YRu2 and Ru after reaction were 0.72 and 3.08 m2 g−1, respectively. The reaction rate over bulk YRu2 reached 486 µmol g−1 h−1, which was much higher than that over Ru at 12 µmol g−1 h−1. In terms of the TOF, YRu2 15 ACS Paragon Plus Environment


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achieved 1.9×10−2 s−1, while that for Ru was only 0.6×10−4 s−1, which demonstrates that the activity of YRu2 is over 300 times higher than that of Ru (see Table S3. Each ruthenium atom on the surface is assumed to be an active site). Moreover, YRu2 possesses a smaller Ea (73 kJ mol−1) than Ru (122 kJ mol−1), as shown in Figure 7a. The reduced Ea is considered to be due to the strong electron donation power of YRu2 by electron transfer from Y to Ru. Anionic Ru can donate electrons to N2 adsorbed on the catalyst surface to facilitate the dissociative adsorption of N2 and enhance the catalytic activity. This observation is consistent with that reported for Ruloaded electride catalysts.4,17 Figure 7b shows the activity of bulk YRu2 held at 400 °C and under atmospheric pressure for ammonia synthesis over two days. The activity decreased during the initial 24 h but then remained constant. 3.3 Kinetic study and catalytic activity under high pressure. The kinetics for ammonia production over bulk YRu2 and Ru was investigated according to a previously reported method (Figure 7c).39 Ru has a negative reaction order of −0.54 for hydrogen, which is reasonable, as reported in the previous work.40 In contrast, YRu2 has a high positive reaction order of 0.81 for hydrogen. Therefore, hydrogen poisoning on YRu2 is suppressed. Although electron donation generally reinforces stronger binding of Ru to hydrogen, hydrogen poisoning over the YRu2 surface is suppressed. This can be attributed to the hydrogen capacity of YRu2, which was confirmed from TDS measurements that show YRu2 can absorb hydrogen under these reaction conditions. Consequently, the activity of bulk YRu2 was enhanced significantly with an increase in the reaction pressure and was twice as high at 0.8 MPa than at 0.1 MPa, while the activity of Ru did not show such a significant change due to hydrogen poisoning (see Figure S2). Other reaction orders are shown in Table S4. 16 ACS Paragon Plus Environment

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3.4 Synthesis of YRu2 Nanoparticles. In order to obtain small-sized YRu2 nanoparitcles, YRu2 powder was prepared by HE or laser ablation in argon atmosphere. Figure S3 shows XRD patterns of YRu2 before and after HE and laser ablation. The diffraction peaks arising from YRu2 phase were broadened and weak peaks due to Ru were observed after HE and laser ablation. These results show the formation of smaller YRu2 particles and the partial decomposition of YRu2 to Ru. The surface area of YRu2 treated by one step of HE increased up to 0.77 m2 g−1 from 0.52 m2 g−1 (1.5 times). The surface area of bulk YRu2 slightly increased after reaction test from 0.65 m2 g−1 to 0.72 m2 g−1. The increase after reaction can be an error, otherwise, attributed to the effect of HE because of hydrogen pressure in reaction condition. Laser-ablated YRu2 particles increases its surface area to 9.2 m2 g−1, which is 18 times larger than that of original YRu2. As shown in TEM images (Figure S4) of laser-ablated YRu2 with the largest surface area (9.2 m2 g−1), spherical YRu2 particles are formed after laser ablation and the particle size is widely distributed in the range of 4−40 nm. This size range is much smaller than the size expected from the surface area, and thus it is speculated that some small particles could attach onto each other to reduce the effective surface area. The laser-ablated YRu2 nanoparticles with various surface areas were tested for NH3 synthesis and the results are summarized in Figure 7d. The NH3 synthesis rate was enhanced almost linearly with an increase in the surface area of the laser-ablated YRu2, indicating all TOF values remain almost unchanged (see Table S3 for more details). These results demonstrate that YRu2 keeps its high activity in case that YRu2 particle size is reduced to nano-size. Surface area of laser-ablated YRu2 decreases after reaction as shown in Table S5. It should be because small particle of YRu2 easily sinter in the reaction condition, which can be suppressed when nano-particle is dispersed on supports.



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Figure 7. a Arrhenius plots for the rate of ammonia synthesis over bulk YRu2 (320−400 °C) and Ru (400−480 °C) under atmospheric pressure. b Reactivity of bulk YRu2 at 400 °C and under atmospheric pressure for ammonia synthesis over two days. c Dependence of ammonia synthesis rate on the partial pressure of H2 over bulk YRu2 (400 °C) and Ru (450 °C) under atmospheric pressure and with a total flow rate of 60 mL min−1. The reaction rate C, is defined as = k . d NH3 synthesis activities of the YRu2 nanoparticles prepared by laser ablation with various surface areas.


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4. DISCUSSION The WF, XANES, and DFT calculation results reveal that electron transfer from Y to Ru does occur, forming Y2+(Ru−)2 approximately. The electron transferred to Ru in YRu2 facilitates N2 dissociation and results in a reduced Ea. Although electron donation generally reinforces stronger binding of Ru to hydrogen, hydrogen poisoning over the YRu2 surface is suppressed. This can be attributed to the hydrogen capacity of YRu2, which was confirmed from TDS measurements that show YRu2 can absorb hydrogen under these reaction conditions. We have previously clarified the relationship between the hydrogen absorption capacity of the support material and the hydrogen reaction order.41 Ru supported on C12A7:e− with high hydrogen storage capacity has a positive reaction order with respect to H2, whereas Ru/CaO·Al2O3 without hydrogen absorption ability has a negative reaction order. Therefore, it was concluded that the hydrogen solubility of YRu2 results in a positive reaction order for H2, i.e., suppression of hydrogen poisoning. The catalytic activity at 0.8 MPa was increased to twice that at 0.1 MP, which confirms that the drawback typically observed with the pure ruthenium catalyst was overcome. The catalytic activities of YRu2 laser-ablated YRu2 were compared with those of other ammonia synthesis catalysts including Co3Mo3N, a benchmark bulk catalyst for its high activity42 and Cs-Ru/MgO, one of the most active Ru-supported catalysts reported to date4 (Table 1). The TOFs with YRu2 and laser-ablated YRu2 were two orders of magnitude higher than that of pure Ru. The promotion effects for reported Ru catalysts are summarized in Table S6. The activities of Ru catalysts with alkali promoters are generally enhanced by a factor of 10–20 compared with non-promoted Ru catalysts. Then, the promotion effect observed for YRu2 (two orders magnitude higher TOF than Ru) is significant. Moreover, both YRu2 exhibit catalytic activity that is over 30 and 15 times higher activity than that of Co3Mo3N and Cs-promoted 19 ACS Paragon Plus Environment


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Co3Mo3N,42 respectively. Furthermore, support-free YRu2 has a higher TOF than Cs-Ru/MgO (Table 1). Such a high TOF with YRu2 is attributed to the formation of negatively charged Ru, i.e., Y2+(Ru–)2, as demonstrated by the XANES results and DFT calculations. The electron transfer in YRu2 is much more efficient than that reported for Ru catalysts. The negative shift (to neutral Ru) of the Ru absorption edge in the XANES spectrum was not observed for the RuCs/MgO catalyst (see Figure S5). Negatively charged Ru corresponds to the ultimate case of a Ru catalyst loaded on a support with electron donating power. To the best of our knowledge, YRu2 is the first example to drastically enhance the intrinsic activity of the Ru catalyst by forming intermetallics. Therefore, Laves phase compounds of Ru and more electropositive metals are more effective catalysts than pure Ru because of large electron promotion via intermetallic bonds and suppression of hydrogen poisoning. The compounds including YRu2 can be utilized instead of pure Ru and be combined with various promotors and catalyst supports.


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Table 1. Comparison of bulk and laser-ablated YRu2 with other bulk catalysts and supported Ru catalysts for NH3 synthesisa Total weight of NH3 synthesis rate

Surface area

TOFb

ENH3c

catalyst (g)

(µmol g−1 h−1)

(m2 g−1)

(s−1)

(vol%)

Bulk YRu2

0.2

486

0.72

0.01918

0.066

Laser-ablated YRu2

0.1

3318

6.4

0.01473

0.23

Ru

0.5

12

3.08

0.00006

0.004

Co3Mo3Nd

0.4

652

21

0.00049

0.18

Co3Mo3N−Cs(0.2 wt%)d

0.4

986

16

0.00098

0.27

Cs-Ru(1.0 wt%)/MgOe

0.2

2264

12

0.013

0.31

a

Other reaction conditions: flow rate 60 mL min−1 (H2/N2 = 3), 0.1 MPa, temperature 400 °C

b

TOFs of bulk catalysts are estimated using the surface area and atomic radius (see Table S3)

c

ENH3 is the effluent NH3 concentration. Thermodynamic limits of effluent NH3 concentration

under 0.1 MPa are 0.44 vol%, respectively. dFrom Ref. 42 eFrom Ref. 4



5. CONCLUSIONS The concept of an efficient catalyst for ammonia synthesis was proposed with the Laves phase of a Ru intermetallic, YRu2, which exhibits hydrogen storage capacity and has a higher electron concentration around Ru. Electron density transferred from Y to Ru enhances the catalytic activity, which would typically induce further hydrogen poisoning. However, hydrogen poisoning on YRu2 is suppressed by the hydrogen storage capacity, and the reaction order of hydrogen over YRu2 has a high positive value. In particular, the electron density of Ru in YRu2 is much larger than that of other Ru catalysts on supports such as electrides with very low WFs, as evidenced by the XANES results. These two features of YRu2 result in a TOF that is 300 times higher than that of Ru, and high performance under high pressure conditions. Moreover, synthesis of YRu2 nanoparticles to obtain high surface area, which is generally an obstacle for utilization of intermetallic catalysts, has been overcome as demonstrated by application of laser ablation. This indicates that the intermetallic catalysts remain active in the case that the intermetallics are nanoparticles. The next step of this research is to fabricate a catalyst comprised of YRu2 nanoparticles deposited on an appropriate support with a large surface area. The advantages with this catalyst are expected to lead to more efficient ammonia synthesis under milder conditions, which would reduce the enormous energy consumption associated with industrial ammonia synthesis and allows for low cost and small scale operation. These benefits should be suitable for the low density of renewable energy per land area, and provide a promising way to convey renewable energy in the form of a portable fuel. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 22 ACS Paragon Plus Environment

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The structural parameters extracted with the EXAFS software; The details of the computational conditions; additional discussion on negatively charged Ru in YRu2; Number of active sites for relevant catalysts; Reaction orders of relevant catalysts for nitrogen, hydrogen, and ammonia; The catalytic activities at high pressure; XRD patterns for YRu2 prepared by arc-melting, HE and laser ablation; TEM images of YRu2 nanoparticles prepared by laser ablation in argon and the histogram of the particle size distribution; NH3 synthesis activities of YRu2 nanoparticles prepared by laser ablation with various surface areas; XANES Ru K-edge spectra for YRu2, RuO2, Ru-Cs/MgO, and Ru; Promotion effects in conventional Ru catalysts. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. Present Addresses +

Graduate School of Energy Science, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto

606-8501, Japan Author Contributions ⊥

T.O and Y.K. contributed equally.

ACKNOWLEDGMENT



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This work was supported by funds from the Accelerated Innovation Research Initiative Turning Top Science and Ideas into High-Impact Values (ACCEL) program of the Japan Science and Technology Agency (JST). A portion of this work was supported by a Kakenhi Grant-in-Aid (No. 15H04183 & 17H06153) from the Japan Society for the Promotion of Science (JSPS). This work was supported by a JSPS postdoctoral fellowship for T.O. XAFS measurements were performed under the approval of PF-PAC No. 2013S2-002. The authors acknowledge Soshi Iimura for assistance with the Rietveld analysis and Toshiharu Yokoyama for valuable comments. We also thank Associate Professor Hiroyuki Wada for the cooperation in laser ablation experiments to obtain YRu2 nanoparticles. ABBREVIATIONS TOF, turn over frequency; HE, hydrogen embrittlement; WF, work function; XRD, X-ray diffraction; XANES, X-ray absorption near edge structure, EXAFS, X-ray absorption fine structure; FT Fourier transforms; DFT, density functional theory; VASP, Vienna ab initio simulation program; TDS, thermal desorption spectroscopy; TEM, Transmission electron microscopy; XAFS, X-ray absorption fine structure; BET, Brunauer-Emmett-Teller REFERENCES (1)

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TOC graphic


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XANES result that shows high electron density on Ru. 128x128mm (144 x 144 DPI)

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