Efficient and Stable Ammonia Synthesis by Self-Organized Flat Ru Nanoparticles on Calcium Amide

Transcription

1 Supporting Information Efficient and Stable Ammonia Synthesis by Self-Organized Flat Ru Nanoparticles on Calcium Amide Yasunori Inoue 1, Masaaki Kitano 2, Kazuhisa Kishida 2, Hitoshi Abe 3,4,5, Yasuhiro Niwa 3, Masato Sasase 2. Yusuke Fujita 1, Hiroki Ishikawa 1, Toshiharu Yokoyama 2,5, Michikazu Hara 1,5,6,*, Hideo Hosono 2,5,6,*. 1. Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, , Japan. 2. Materials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, , Japan. 3. High Energy Accelerator Research Organization, KEK, 1 1, Oho, Tsukuba, Ibaraki , Japan. 1

2 4. Department of Materials Structure Science, School of High Energy Accelerator Science, SOKENDAI (the Graduate University for Advanced Studies), 1-1 Oho, Tsukuba, Ibaraki , Japan. 5. ACCEL, Japan Science and Technology Agency, Honcho, Kawaguchi, Saitama, , Japan. 6. Laboratory for Materials and Structures, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, , Japan. Corresponding Author * mhara@msl.titech.ac.jp * hosono@msl.titech.ac.jp 2

3 Method Ru-loaded 12CaO 7Al 2 O 3 electride (Ru/C12A7:e ) C12A7:e was synthesized in accordance with a previously reported procedure. 1 The catalyst was prepared by a vapor deposition method according to a previous report. 2 Briefly, C12A7:e and Ru 3 (CO) 12 were mixed in an Ar-filled glovebox, and the mixture was then sealed in a silica ampoule and evacuated with a turbo molecular pump. Ru 3 (CO) 12 was decomposed by heating the silica tube with a multi-stage temperature program: 2 K min 1 up to 313 K, hold for 1 h; 0.25 K min 1 up to 343 K, hold for 1 h; 0.4 K min 1 up to 393 K, hold for 1 h; 0.9 K min 1 up to 523 K, hold for 2 h, cooling to an ambient temperature. Ru-Cs/MgO Synthesis of the Cs- promoted Ru/MgO catalyst was conducted according to a previous report. 3 MgO (Ube, 500A) was pre-heated at 773 K under vacuum for 12 h, followed by impregnation with a dehydrated tetrahydrofuran (THF) solution of Ru 3 (CO) 12. The amount of Ru loading was 10 wt%. After stirring for 4 h at room temperature, the solvent was evaporated and the resultant sample (RuO x /MgO) was heated at 673 K under vacuum to decompose the carbonyl species. Both Cs 2 CO 3 (Kanto Chemical Co., 99.9%) and Ru/MgO (atomic ratio: Ru/Cs = 1) were soaked in dehydrated ethanol and 3

4 stirred for 3 h, then the solvent was evaporated and the catalyst dried under vacuum overnight. Characterization XRD X-ray powder diffraction (XRD; D8 ADVANCE, Bruker) patterns of 10 wt% Ru-loaded Ca(NH 2 ) 2 and Ca(NH 2 ) 2 after reaction were obtained using Cu K radiation with a rotating anode at 45 kv and 360 ma in the 2θ range from 10 to 80 in 0.02 steps. XAFS Curve-fitting analysis of X-ray absorption fine structure (XAFS) was performed for the 2 wt% Ru/Ca(NH 2 ) 2 catalyst using the Athena and Artemis analysis software packages, and the FEFF6 code. 4,5 Ru-Ru, Ru-N, and Ru-O paths were obtained by calculations of Ru bulk metal, Ru-N 6 cluster, and Ru-O 6 cluster models, respectively. Fourier transformations of k 2 -weighted EXAFS oscillations were performed over the range of Å 1. The Ru-Ru, Ru-N, and Ru-O interactions were fitted in the range of Å. The R-factor of this fitting was

5 Kinetic analysis Reaction order Kinetic analyses of Ru/Ca(NH 2 ) 2 were performed according to a procedure in the literature. 2 The reaction orders with respect to N 2 and H 2 were evaluated at a constant flow rate (60 ml min 1 ) using Ar gas as a diluent, and that for NH 3 was determined with a 3H 2 +N 2 gas mixture by changing the synthesis gas flow rate. The constituent reactant gases (N 2, H 2, Ar) were as follows (in ml min 1 ): (5, 15, 0), (10, 30, 0), (15, 45, 0), and (20, 60, 0) for NH 3 order; (6, 30, 24), (10, 30, 20), (15, 30, 15), and (20, 30, 10) for N 2 order, (10, 20, 30), (10, 25, 25), (10, 30, 20), and (10, 40, 10) for H 2 order. These analyses were performed for Ru/Ca(NH 2 ) 2 under 0.1 MPa. 5

6 NH 3 synthesis rate (mmol g 1 h 1 ) Supplementary Results (a) Ca(NH 2 ) 2 H Ca (b) N Ru loading (wt%) Figure S1. (a) Crystal structure of Ca(NH 2 ) 2 was visualized using the CrystalMaker program. (b) NH 3 synthesis rate over Ru/Ca(NH 2 ) 2 as a function of the Ru-loading amount. Each reaction rate was recorded after 24 h of reaction at 613 K. Reaction conditions: catalyst weight, 0.1 g; synthesis gas, H 2 /N 2 = 3, 60 ml min 1 ; temperature, 613 K; pressure, 0.1 MPa; WHSV = ml g 1 h 1. 6

7 NH 3 production (mmol) Reaction time (h) Figure S2. Reaction time profile for NH 3 synthesis over 10 wt% Ru/Ca(NH 2 ) 2 at 473 K. Reaction conditions: catalyst weight, 0.1 g; synthesis gas, H 2 /N 2 = 3, 60 ml min 1 ; pressure, 0.1 MPa. 7

8 FT magnitude (arb. units) Ru RuO 2 3 Ru/Ca(NH 2 ) Distance (Å) Figure S3. FTs of EXAFS oscillations for 2 wt% Ru/Ca(NH 2 ) 2, Ru, and RuO 2. 8

9 FT magnitude (a) (b) (c) Fitting curve Ru N Ru Ru Ru-O Distance (Å 1 ) Figure S4. Procedures for curve-fitting analysis of EXAFS spectrum for 2 wt% Ru/Ca(NH 2 ) 2. (a) Normalized XAFS spectrum, (b) k 2 -weighted EXAFS oscillation, k 2 (k), and (c) FT of the sample (filled circle) and fitting curve for Ru-N, Ru-Ru and Ru-O interactions. The EXAFS data show the Ru catalyst is strongly bonded to the N atoms of the support. A Ru-O path was not yielded by the fitting result. 9

10 Intensity (arb. unit) Ru/Ca(NH 2 ) 2 after Ru/Ca(NH 2 ) 2 Ca(NH 2 ) 2 after Ca(NH 2 ) 2 syn. CaNH (PDF ) Ca(NH 2 ) 2 (PDF ) theta (degree) 60 Figure S5. XRD patterns for Ca(NH 2 ) 2 and Ru/Ca(NH 2 ) 2 before and after ammonia synthesis reaction at 613 K for 50 h. Standard diffraction patterns for CaNH and Ca(NH 2 ) 2 are provided for reference. 10

11 NH 3 synthesis rate (mmol g 1 h 1 ) Ru/Ca(NH wt%ru(nh2)2 2 ) 2 5 wt%ru/canh derived from Figure S6. Reaction rate for ammonia synthesis over 5 wt% Ru/Ca(NH 2 ) 2 and 5 wt% Ru/CaNH derived from Ca(NH 2 ) 2. CaNH was prepared from Ca(NH 2 ) 2 heated at 613 K for 12 h under N 2 and H 2 flow (H 2 /N 2 = 3, 60 ml min 1 ). Reaction conditions: catalyst weight, 0.1 g; synthesis gas, H 2 /N 2 = 3, flow rate, 60 ml min 1 ; pressure, 0.1 MPa; temperature, 613 K. 11

12 Number of particles (a) (b) d avg = 2.8±0.9 nm 20 nm Particle size (nm) Figure S7. (a) HAADF-STEM image of 2 wt% Ru-Cs/MgO catalyst. (b) Mean particle size distribution of 2 wt% Ru-Cs/MgO. The number of particles measured from STEM images was more than

13 Number of particles Number of particles Number of particles (a) (b) 1 wt% Ru 5 wt% Ru d avg = 0.8±0.2 nm Particle size (nm) d avg = 2.0±0.5 nm Particle size (nm) (c) 10 wt% Ru 5 nm d avg = 2.1±1.0 nm Particle size (nm) Figure S8 TEM and HAADF STEM images of Ru-loaded Ca(NH 2 ) 2 catalysts with various amounts of Ru after NH 3 synthesis at 613 K under 0.1 MPa. The insets show mean particle size distributions. The number of particles measured from STEM images was more than 100. The average particle sizes were d avg = 0.8±0.2 nm (1 wt%), 2.0±0.5 nm (5 wt%) and 2.1±1.0 nm (10 wt%). 13

14 Table S1. Results for curve-fitting analysis of Ru K-edge EXAFS for Ru-loaded Ca(NH 2 ) 2 catalysts with various amounts of Ru after ammonia synthesis at 613 K for 50 h. Sample Ru (wt%) Shell CN a d b (Å) σ 2 c (Å 2 ) TOF ( 10 3 s 1 ) 1 Ru N Ru Ru Ru N Ru Ru Ru/Ca(NH 2 ) 2 5 Ru N Ru Ru N/A Ru N Ru Ru N/A a CN = average coordination number; b d = average interatomic distance; c σ 2 = Debye Waller factor. 14

15 Table S2. Reaction order and apparent activation energy for various Ru catalysts. α, β and γ are exponents of the equation r = kp α N 2 P β H 2 P γ NH 3. a Apparent activation energies are derived from Arrhenius plots in the given temperature range using a total flow of 60 ml min 1 (H 2 /N 2 = 3). Catalyst T range (K) α (N 2 ) β (H 2 ) γ (NH 3 ) E a (kj mol 1 ) Ref. Ru/MgO , 6 Cs-Ru/MgO Ru/C12A7:e Ru/Ca 2 N:e Ru/Ca(NH 2 ) This work a The reaction conditions were far from equilibrium: WHSV = ml g 1 h 1 ; temperature, 613 K; catalyst weight, 0.05 g. References (1) Matsuishi, S.; Nomura, T.; Hirano, M.; Kodama, K.; Shamoto, S.-i.; Hosono, H. Chem. Mater. 2009, 21, (2) Kitano, M.; Inoue, Y.; Yamazaki, Y.; Hayashi, F.; Kanbara, S.; Matsuishi, S.; Yokoyama, T.; Kim, S.-W.; Hara, M.; Hosono, H. Nature. Chem. 2012, 4, (3) Rosowski, F.; Hornung, A.; Hinrichsen, O.; Herein, D.; Muhler, M.; Ertl, G. Appl. Catal. A 1997, 151, (4) Ravel, B.; Newville, M. J. Synchro. Rad. 2005, 12, (5) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. Rev. B 1995, 52, (6) Bielawa, H.; Hinrichsen, O.; Birkner, A.; Muhler, M. Angew. Chem. Int. Ed. 2001, 40, (7) Kitano, M.; Inoue, Y.; Ishikawa, H.; Yamagata, K.; Nakao, T.; Tada, T.; Matsuishi, S.; Yokoyama, T.; Hara, M.; Hosono, H. Chem. Sci. 2016, 7,