Essential role of hydride ion in ruthenium-based ammonia synthesis catalysts

Abstract

The efficient reduction of atmospheric nitrogen to ammonia under low pressure and temperature conditions has been a challenge in meeting the rapidly increasing demand for fertilizers and hydrogen storage. Here, we report that Ca₂N:e^-, a two-dimensional electride, combined with ruthenium nanoparticles (Ru/Ca₂N:e^-) exhibits efficient and stable catalytic activity down to 200 °C. This catalytic performance is due to [Ca₂N]⁺·e_1-x^-H _x^- formed by a reversible reaction of an anionic electron with hydrogen (Ca₂N:e^- + xH ↔ [Ca₂N]⁺·e_1-x^-H _x^-) during ammonia synthesis. The simplest hydride, CaH₂, with Ru also exhibits catalytic performance comparable to Ru/Ca₂N:e^-. The resultant electrons in these hydrides have a low work function of 2.3 eV, which facilitates the cleavage of N₂ molecules. The smooth reversible exchangeability between anionic electrons and H^- ions in hydrides at low temperatures suppresses hydrogen poisoning of the Ru surfaces. The present work demonstrates the high potential of metal hydrides as efficient promoters for low-temperature ammonia synthesis.

Fig. 1. (a) Catalytic activity for ammonia synthesis over various Ru catalysts (2 wt%) as a function of reaction temperature (reaction conditions: catalyst, 0.1 g; WHSV, 36 000 mL g_cat^–1 h^–1; reaction pressure, 0.1 MPa). (b) Reaction time profile for ammonia synthesis over Ru (5 wt%)/Ca₂N at 340 °C (reaction conditions: catalyst, 0.1 g; WHSV, 36 000 mL g_cat^–1 h^–1; reaction pressure, 1.0 MPa). (c, d) Dependence of ammonia synthesis on the partial pressure of (c) N₂ and (d) H₂ using various Ru catalysts (2 wt%) at 340 °C under atmospheric pressure.

Fig. 2. (a) XRD patterns for Ru/Ca₂N:e^– before and after ammonia synthesis reaction at 340 °C for 20 h. Standard JCPDS diffraction patterns for Ca₂N (space group R3[combining macron]m, PDF: 70-4196), CaNH (space group Fm3[combining macron]m, PDF: 75-0430), and Ca₂NH (space group Fd3[combining macron]m, PDF: 76-608) are provided for reference. (b–d) Crystal structures of Ca₂N (b), Ca₂NH (c), and CaNH (d) were visualized using the VESTA program. (b) Ca₂N:e^– has a hexagonal layered structure with anionic electron layers between the cationic framework layers ([Ca₂N]⁺) composed of edge-sharing NCa₆ octahedra. (c) Ca₂NH is composed of Ca²⁺, N^3–, and H^– ions, where Ca atoms form a slightly distorted cubic close packed structure, and N and H are ordered in each anion layer. (d) CaNH, an inorganic imide compound with a cubic structure, consists of Ca²⁺ and NH^2– ions. (e) In situ Raman spectra for Ru/Ca₂N:e^– measured under ammonia synthesis conditions (N₂ : H₂ = 1 : 3, 0.1 MPa, 60 mL min^–1). The Raman spectrum for Ca₂NH is also shown as a reference. (f) H₂ TPA profiles for Ru/Ca₂N:e^– and Ru/CaNH catalysts. The TPA measurements were performed (1 °C min^–1) with a dilute mixture of H₂ (5%) in Ar. (g) H₂ TPD profiles for Ru/Ca₂N:e^– and Ru/CaNH after ammonia synthesis reaction at 340 °C for 10 h. The TPD measurements were performed (1 °C min^–1) under Ar flow.

Fig. 3. (a) Computational model used in the calculation of the work function of Ca₂NH(100). The vacuum region (transparent gray) is included in the model to determine the vacuum level from the electrostatic potential profile (solid thick line) in the region. (b) Surface structure and spin-averaged DOS of Ca₂NH(100), where an energy of 0.0 eV corresponds to the vacuum level. (c) Surface structure and spin-averaged DOS of Ca₂NH(100) with a hydrogen vacancy (V_H), indicated with a red dotted circle. The energy of 0.0 eV corresponds to the vacuum level. Inset: the local density (yellow) of anionic electron states just below E_F (a red arrow) depicted with an isosurface value of 0.015 e^– bohr^–3. The anionic electron states are purely spin polarized states. (d) Surface structure and spin-averaged DOS of Ru-loaded Ca₂NH(100). The vacuum level was not determined in this model; therefore, the DOS were represented to match the VBM of nitrogen with those of Ca₂NH(100) with/without V_H. Inset: DOS of H bonded with Ru(H^b) and H on the free surface (H^s). Green, blue, white, and gray atoms in the atomistic models correspond to Ca, N, H, and Ru, respectively. These crystal structures and charge distributions were visualized using the VESTA program.

Fig. 4. (a) Reaction time profiles for ammonia synthesis from N₂ and D₂ over Ru/Ca₂N:e^– at 340 °C (reaction conditions: catalyst, 0.2 g; reaction gas, N₂ : D₂ = 1 : 3; reaction pressure, 60 kPa). Prior to reaction, Ru/Ca₂N:e^– was heated under N₂ + H₂ flow (N₂ : H₂ = 1 : 3) at 340 °C for 10 h to form Ru/Ca₂NH. (b) TPD profiles of Ru/Ca₂N:e^– after the reaction (a). TPD measurements were performed (10 °C min^–1) with Ar flow. (c) Schematic illustration of ammonia synthesis over Ru/Ca₂N:e^–. During ammonia synthesis over Ru/Ca₂N:e^–, H₂ is incorporated into Ca₂N:e^– as H^– ions to form Ca₂NH (reaction 1). The H^– ions are released from Ca₂NH, which leaves electrons to form a hydrogen vacancy near the Ru-support interface (reaction 2). The cleavage of N₂ proceeds effectively on Ru surfaces due to electron injection from [Ca₂N]⁺·e_1–x^–H_x^– and the nitrogen adatoms react with H radicals to form ammonia (reaction 3).