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 (Ne) lower than 1 × 10(21) cm(-3) and a high apparent activation energy (Ea) 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 (Ne ≈ 1 × 10(21) cm(-3)) significantly changes the reaction mechanism to yield a catalytic activity that is an order higher and with half the Ea. The metal-insulator transition of [Ca24Al28O64](4+)(O(2-))2-x(e(-))2x also occurs at Ne ≈ 1 × 10(21) 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.

Electride support boosts nitrogen dissociation over ruthenium catalyst and shifts the bottleneck in ammonia synthesis.

Novel approaches to efficient ammonia synthesis at an ambient pressure are actively sought out so as to reduce the cost of ammonia production and to allow for compact production facilities. It is accepted that the key is the development of a high-performance catalyst that significantly enhances dissociation of the nitrogen-nitrogen triple bond, which is generally considered a rate-determining step. Here we examine kinetics of nitrogen and hydrogen isotope exchange and hydrogen adsorption/desorption reactions for a recently discovered efficient catalyst for ammonia synthesis--ruthenium-loaded 12CaO·7Al2O3 electride (Ru/C12A7:e(-))--and find that the rate controlling step of ammonia synthesis over Ru/C12A7:e(-) is not dissociation of the nitrogen-nitrogen triple bond but the subsequent formation of N-Hn species. A mechanism of ammonia synthesis involving reversible storage and release of hydrogen atoms on the Ru/C12A7:e(-) surface is proposed on the basis of observed hydrogen absorption/desorption kinetics.

Ammonia synthesis using a stable electride as an electron donor and reversible hydrogen store.

Industrially, the artificial fixation of atmospheric nitrogen to ammonia is carried out using the Haber-Bosch process, but this process requires high temperatures and pressures, and consumes more than 1% of the world's power production. Therefore the search is on for a more environmentally benign process that occurs under milder conditions. Here, we report that a Ru-loaded electride [Ca(24)Al(28)O(64)](4+)(e(-))(4) (Ru/C12A7:e(-)), which has high electron-donating power and chemical stability, works as an efficient catalyst for ammonia synthesis. Highly efficient ammonia synthesis is achieved with a catalytic activity that is an order of magnitude greater than those of other previously reported Ru-loaded catalysts and with almost half the reaction activation energy. Kinetic analysis with infrared spectroscopy reveals that C12A7:e(-) markedly enhances N(2) dissociation on Ru by the back donation of electrons and that the poisoning of ruthenium surfaces by hydrogen adatoms can be suppressed effectively because of the ability of C12A7:e(-) to store hydrogen reversibly.