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1 DOI: /NCHEM.2595 Breaking scaling relations to achieve low-temperature ammonia synthesis through LiH-mediated nitrogen transfer and hydrogenation Peikun Wang 1,2, Fei Chang 1,2, Wenbo Gao 1,2, Jianping Guo 1 *, Guotao Wu 1, Teng He 1, Ping Chen 1,3,4 * 1 Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian , China. 2 University of Chinese Academy of Sciences, Beijing , China. 3 State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian , China. 4 Collaborative Innovation Center of Chemistry for Energy Materials, Dalian , China. *Correspondence to: guojianping@dicp.ac.cn and pchen@dicp.ac.cn Table of Contents 1. Materials and Methods Supplementary Table Supplementary Figures 1 to Supplementary References...14 NATURE CHEMISTRY 1

2 1. Materials and Methods: Materials. All sample handlings were carried out in a glove box filled with Argon. VCl 3 (Alfa, 99%), CrCl 2 (Alfa, 97%), MnCl 2 (Alfa, 97%), FeCl 2 (Alfa, 99.5%), CoCl 2 (Aldrich, 97%), NiCl 2 (Alfa, 98%), LiH (Aldrich, 97%), LiNH 2 (Aldrich, 95%), Li 2 O (Aladdin, 99%), V 2 O 5 (Alfa, 99.6%), Cr 2 O 3 (Aladdin, 99.95%), Ru 3 (CO) 12 (Aladdin, 99%), CsNO 3 (Aladdin, 99%), Fe 2 (CO) 9 (Aladdin, 99%), Co(NO 3 ) 2 6H 2 O (Kermel, 99.0%), Ni(NO 3 ) 2 6H 2 O (Kermel, 98.0%), Mg(CH 3 COO) 2 4H 2 O (Kermel, 99.0%), oxalic acid (Kermel, 99.5%), FeC 2 O 4 xh 2 O (Kermel, 99%) were used as supplied. Preparation of Mn 2 N-LiH sample. MnN was firstly synthesized following the procedure described previously 50. Mn 2 N was prepared by heating MnN sample in a hydrogen flow under ambient pressure at 673 K for 2 hours. X-ray diffraction (XRD) characterization clearly shows the presence of Mn 2 N. Then Mn 2 N (100 mg) and LiH (32 mg) were ball milled (150 rpm for 3 hours) and collected for use. The molar ratio of Mn 2 N to LiH is ca. 1:5. Preparation of Fe 4 N-LiH sample. Fe 4 N was prepared from FeC 2 O 4 xh 2 O by temperature-programmed reaction method. Typically, the precursor (FeC 2 O 4 xh 2 O, 1.8 g) was heated in a mixture of NH 3 :H 2 (1:1.2) flow from room temperature to 773 K and held at that temperature for 8 hours. The formation of Fe 4 N was confirmed by XRD. Then Fe 4 N (100 mg) and LiH (17 mg) were ball milled (150 rpm for 3 hours) and collected for use. The molar ratio of Fe 4 N to LiH is ca. 1:5. Preparation of Mn 4 N catalysts. Mn 4 N was prepared by heating MnN to 673 K and held for 1 hour under 20 bar of hydrogen pressure. Clear Mn 4 N phase was observed NATURE CHEMISTRY 2

3 by XRD. Preparation of VN catalyst. VN was prepared by heating V 2 O 5 powder (100 mg) to 973 K and held for 10 hours under a flow of ammonia gas. The black powders obtained were confirmed by XRD to be VN. Preparation of CrN catalyst. CrN was prepared by heating Cr 2 O 3 powder (100 mg) to 1073 K and held for 10 hours under a flow of ammonia gas. The formation of CrN was confirmed by XRD. Preparation of Ru/MgO catalyst. MgO was obtained according to the literature report 51. The MgO supported Ru catalyst was prepared by the incipient wetness impregnation of MgO with a THF solution of Ru 3 (CO) 12. After removing THF, the powder was reduced by the syngas (N 2 :H 2 =1:3) to 773 K at a ramping rate of 2 K min -1 and held for 2 hours. 5.4 wt% Ru was loaded as determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, optima 7300DV, Perkin Elmer, USA). Preparation of Cs-Ru/MgO catalyst. Cs-Ru/MgO catalyst was prepared by the incipient wetness impregnation of Ru/MgO catalyst with an ethanol/water solution of CsNO 3. After removing the solvent, the powder was reduced by the syngas (N 2 :H 2 =1:3) to 773 K at a ramping rate of 2 K min -1 and held for 2 hours. The weight ratio of Cs to Ru is 1:1. Preparation of Fe/MgO catalyst. Fe/MgO catalyst was prepared by the incipient wetness impregnation of MgO with a THF solution of Fe 2 (CO) 9. After removing THF, the powder was heated to 773 K at a ramping rate of 2 K min -1 and held for 10 hours NATURE CHEMISTRY 3

4 in the flow of syngas (N 2 :H 2 = 1:3). The weight ratio of Fe to MgO is ca Preparation of Co/MgO and Ni/MgO catalysts. MgO supported Co or Ni catalyst was prepared by the incipient wetness impregnation of MgO with ethanol solution of Co(NO 3 ) 2 or Ni(NO 3 ) 2, respectively. The weight ratio of Co or Ni to MgO is ca After removing ethanol, the powder was heated to 773 K in a flow of hydrogen at a ramping rate of 2 K min -1 and held for 2 hours. Dynamic N 2 activation and transfer. The dynamic N 2 activation and transfer on TM-LiH composites (TM = Cr, Mn, Fe and Co) was performed on a homemade quartz-lined stainless steel reactor. After heating the TM-LiH samples under pressurized N 2 (5 bar) at 548 K for 40 hours, the solid products were weighed and characterized by XRD or FTIR. The nitridation of LiH follows the similar procedure. Hydrogenation of LiNH 2. The hydrogenation of LiNH 2 was performed on the quartz-lined stainless steel reactor. The exhaust gas was monitored by a conductivity meter (Mettler Toledo SevenMulti). Temperature-Programmed reactions. Temperature-programmed reactions of transition metal nitride-lih samples were performed in the quartz-lined stainless steel reactor. 30 mg sample was heated in a H 2 flow (40 ml min -1 ) from room temperature to desired temperature at a ramping rate of 5 K min -1. The exhaust gases were analyzed by an on-line mass spectrometer (Hiden HPR20). XRD measurements. XRD patterns were recorded on a PANalytical X pert diffractometer using a homemade sample cell covered with KAPTON film to avoid air contamination. NATURE CHEMISTRY 4

5 FTIR measurements. Fourier transform infrared measurements were conducted on a Varian 3100 FT-IR spectrometer in the diffuse re ection (DRIFT) mode. 2. Supplementary Table 1 Average crystallite size Supplementary Table 1. BET specific surface area and particle size of TM-LiH catalyst. catalysts BET specific surface area (m 2 g -1 ) a (nm) b Fe-LiH Mn-LiH Co-LiH Cr-LiH a The samples were collected after catalyst test at 573 K. b The average crystallite sizes were calculated by the Debye-Scherrer equation. NATURE CHEMISTRY 5

6 3. Supplementary Figures 1 to 8 Supplementary Figure 1 Hydrogenation of LiNH 2 at 15 bar and H 2 flow rate of 40 ml min -1. (A) Temperature dependence of NH 3 formation rate. The derived E a (ca. 49 kj mol -1 ) is close to the reported value (56 kj mol -1 ) 43. (B) Dependence of NH 3 formation rate on the partial pressure of H 2 (P total = 10 bar, T = 573 K). The derived reaction order of H 2 (γ) is NATURE CHEMISTRY 6

7 Supplementary Figure 2 The adsorption energies (ΔE) of NH x (x = 0, 1 and 2) species on Fe and Co surfaces, respectively (data taken from ref. 19 ). Those energies are TM-dependent and scaled with each other as evidenced by the inserted equations in the figure. However, for the TM-LiH composite catalysts (see Figure 1b in the main text), the ΔE of NH and NH 2 species are independent of TM, so that there is no longer linear correlations among adsorption energies of N, NH and NH 2 (Figure 1b). NATURE CHEMISTRY 7

8 Supplementary Figure 3 (A) The XRD patterns of fresh (a) and heated (b) Fe 4 N-LiH (1:5) samples. The absence of LiH phase in the XRD pattern (b) is likely due to the poor crystallinity (sample was pretreated by ball milling) and the low X-ray sensitivity of LiH. (B) The FTIR spectrum of the heated Fe 4 N-LiH sample. The signal is rather weak due to the relatively low content of Li 2 NH and the dark color of the sample. Trace amount of H 2 was observed during the heating process, which may be due to the reaction of LiNH 2 with excess LiH to form Li 2 NH. So only the N-H stretch of Li 2 NH can be observed. NATURE CHEMISTRY 8

9 Supplementary Figure 4 XRD patterns of the fresh-made Mn-LiH (a) and Fe-LiH (b) samples. The absence of LiH phase in the XRD pattern is likely due to the poor crystallinity (prepared by ball milling) and the low X-ray sensitivity of LiH. NATURE CHEMISTRY 9

10 Supplementary Figure 5 (A) The XRD patterns of the post N 2 treated samples (a, Cr-LiH, b, Mn-LiH, c, Fe-LiH and d, Co-LiH), where the formation of Li 2 NH/LiNH 2, Mn 2 N and CrN phases can be clearly seen. Conditions: 5 bar of N 2 at 548 K. (B) The FTIR spectra of the post N 2 treated samples, showing the presence of the characteristic vibrations of LiNH 2 and Li 2 NH. NATURE CHEMISTRY 10

11 Supplementary Figure 6 H 2 temperature-programmed-reaction (H 2 -TPR) profiles showing the formation of NH 3 from neat LiNH 2 and the post-n 2 treated Cr-, Fe-, and Co-LiH samples. The main feature of all the TM-LiH samples, in principle, resembles that of LiNH 2. NATURE CHEMISTRY 11

12 Supplementary Figure 7 The XRD patterns of a, Cr-LiH, b, Mn-LiH, c, Fe-LiH and d, Co-LiH samples after being tested under the conditions of 10 bar and 598 K. The low N content nitrides, i.e., CrN n (0.5 < n < 1) and Mn 4 N, as well as metallic Fe and Co are observed. Noted that due to low X-ray diffraction sensitivity and poor crystallinity of the sample, LiNH x phases are very weak and LiH phase is undetectable. NATURE CHEMISTRY 12

13 Supplementary Figure 8 Ammonia synthesis rate as a function of temperature over the CrN-LiH prepared by the method described in the manuscript (in red) and by ball milling CrN and LiH (in black). NATURE CHEMISTRY 13

14 4. Supplementary References 51. H. B. Guan et al., Preparation of nanometer magnesia with high surface area and study on the influencing factors of the preparation process. Acta Phys. Chim. Sin. 22, (2006). NATURE CHEMISTRY 14