Active and stable Fe-based catalyst, mechanism, and key role of alkali promoters in ammonia synthesis

Worldwide NH3 production reached 0.18 Gton in 2019, and 1–2% of the global CO2 emissions are due to large-scale synthesis (1 billion tons / year). A catalyst for ammonia has been obtained by pyrolysis iron phthalocyanine (FePc) precursor under N2, followed impregnation with alkali metals (Na, Li, K, Cs) H2 treatment. Characterization (XPS, XRD, HR-TEM, ICP-OES, TGA, CHNS analysis, BET) revealed nano-sized core–shell structures formed during treatment, Fe core promoters (“Cs2O” “K2O”) carbon on shell. The partially inhibit methanation process carbon. These NPs were found be very active stable catalysts, as compared commercial iron-based KM1 (Haldor-Topsoe). Activities promoted catalysts follow order: K > Cs Na ~ more than 6% at 400 °C 7 MPa, contact time (WHSV) 12000 ml g?1 h?1 K. apparent activation energy was 31 kJ mol?1 34 3-K-FePc700 10-Cs-FePc700 suggesting facile N2 surface. DFT-based predicted atomic electronic reveal a similarity partial charge distribution surface species or Cs. Surprisingly main effect is related geometrical repartition alkali, leading larger number exposed atoms, sites, case (present metal oxide) leaves medium coverage some Fe(0) non-dissociative chemisorption (end-on type). free profile demonstrates that thermodynamic stability reaction intermediates nitrogen reduction (NRR) increases pressure indicating better feasibility higher pressures.