Silicon Zintl Intermetallics With Low Work Functions for Ammonia Synthesis

ABSTRACT Barium‐based Zintl‐phase intermetallic supports (BaAlSi and BaSi 2 ) deliver significantly higher ammonia synthesis rates than their calcium‐based counterparts (CaAlSi, CaSi 2 , and CaSi), owing to their stronger electron‐donating (lower work function) character. These Zintl compounds have low work functions, ranging from 2.6 – 3.5 eV as calculated by DFT, and below 3.0 eV when UPS is measured on Ar‐sputtered powders. Under Haber‐Bosch reaction conditions at 5 MPa, BaAlSi and CaAlSi form hydrides, but kinetic analysis reveals that this does not alter the catalytic mechanism. Reaction orders and activation energies remain essentially unchanged, and nonhydride‐forming silicides (BaSi 2 and CaSi 2 ) show similarly high activity, indicating that hydride formation has minimal impact on catalysis. At 5 MPa, 400°C, Co/BaAlSi achieved activities of 55 mmol∙g − 1 ∙h − 1 , exceeding the commercial Fe‐based KM‐1 catalyst (41 mmol∙g − 1 ∙h − 1 ). Moreover, the BaAlSi‐ and BaSi 2 ‐supported Co and Ru catalysts achieved NH 3 production rates of 500–700 mmol∙g ( Ru/Co ) − 1 ∙h − 1 , surpassing the performance of many previously reported ammonia synthesis catalysts. Previous intermetallic catalysts for ammonia synthesis focused on specific properties such as electride nature, electron‐excess, or hydride formation; our results show that a wider range of Zintl compounds have adequately low work functions and reasonably high activities overall.