Mechanism Switching of Ammonia Synthesis Over Ru-Loaded Electride Catalyst at Metal–Insulator Transition

The substitution of electrons for O^2– anions in the crystallographic cages of [Ca₂₄Al₂₈O₆₄]⁴⁺(O^2–)₂ was investigated to clarify the correlation between the electronic properties and catalytic activity for ammonia synthesis in Ru-loaded [Ca₂₄Al₂₈O₆₄]⁴⁺­(O^2–)_2–x(e^–)_2x (0 ≤ x ≤ 2). This catalyst has low catalytic performance with an electron concentration (N_e) lower than 1 × 10²¹ cm^–3 and a high apparent activation energy (E_a) for ammonia synthesis comparable to that for conventional Ru-based catalysts with a basic promoter such as alkali or alkaline earth compounds. Replacement of more than half of the cage O^2– anions with electrons (N_e ≈ 1 × 10²¹ cm^–3) significantly changes the reaction mechanism to yield a catalytic activity that is an order higher and with half the E_a. The metal–insulator transition of [Ca₂₄Al₂₈O₆₄]⁴⁺­(O^2–)_2–x(e^–)_2x also occurs at N_e ≈ 1 × 10²¹ cm^–3 and is triggered by structural relaxation of the crystallographic cage induced by the replacement of O^2– anions with electrons. These observations indicate that the metal–insulator transition point is a boundary in the catalysis between Ru-loaded [Ca₂₄Al₂₈O₆₄]⁴⁺(O^2–)₂ and [Ca₂₄Al₂₈O₆₄]⁴⁺(e^–)₄. It is thus demonstrated that whole electronic properties of the support material dominate catalysis for ammonia synthesis.