Tiered Electron Anions in Multiple Voids of LaScSi and Their Applications to Ammonia Synthesis

Abstract

Electrides-compounds in which electrons localized in interstitial spaces periodically serve as anions-have attracted broad attention for their exotic properties, such as extraordinary electron-donating ability. In our efforts to expand this small family of phases, LaScSi emerges as a promising candidate. Its electron count is 2e^- f.u.^-1 in excess of that expected from the Zintl concept, while its structure offers interstitial spaces that can accommodate these extra electrons. Herein, this potential is explored through density functional theory (DFT) calculations and property measurements on LaScSi. DFT calculations (validated by heat capacity and electrical transport measurements) reveal electron density peaks at two symmetry-distinct interstitial sites. Importantly, this electride-like character is combined with chemical stability in air and water, an advantage for catalysis. Ru-loaded LaScSi shows outstanding catalytic activity for ammonia synthesis, with a turnover frequency (0.1 s^-1 at 0.1 MPa, 400 °C) an order of magnitude higher than those of oxide-based Ru catalysts, e.g., Ru/MgO. As with other electrides, LaScSi's ability to reversibly store hydrogen prevents the hydrogen poisoning of Ru surfaces. The better performance of LaScSi, however, hints at the importance of the high concentration (>1.6 × 10²² cm^-3 ) and tiered nature of its anionic electrons, which offers guidance toward new catalysts.

Keywords: RTX compounds; ammonia syntheses; catalysts; electrides; tiered electron anions.