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–<i>x</i>(e^–)_2<i>x</i> (0 ≤ <i>x</i> ≤ 2). This catalyst has low catalytic performance with an electron concentration (<i>N</i>_e) lower than 1 × 10²¹ cm^–3 and a high apparent activation energy (<i>E</i>_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 (<i>N</i>_e ≈ 1 × 10²¹ cm^–3) significantly changes the reaction mechanism to yield a catalytic activity that is an order higher and with half the <i>E</i>_a. The metal–insulator transition of [Ca₂₄Al₂₈O₆₄]⁴⁺­(O^2–)_2–<i>x</i>(e^–)_2<i>x</i> also occurs at <i>N</i>_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