Ammonia Synthesis by a Supported Iron-Lithium Hydride Precatalyst: Silicon Nitride Support Enabled Synthesis and Nitrogen Reservoir Dynamics

Journal Article · Thu Jan 01 00:00:00 UTC 2026 · ACS Catalysis

DOI:https://doi.org/10.1021/acscatal.6c01546· OSTI ID:3374262

Deng, Kaixi; BRUENING, MEAGHAN A.; Hall, Jacklyn N.; Norris, Andrew; Dogan, Fulya; Snyder, Benjamin; Kropf, A. Jeremy; Delferro, Massimiliano; Kaphan, David M.

Amorphous silicon nitride (Si3N4) is an unconventional support for the chemisorption of organometallic complexes and offers potential improvements in active site stability and reactivity through enhanced metal-nitrogen covalency and orbital overlap in bonding interactions with the nitride framework. In this study, we show that silicon nitride-supported iron mesityl complexes display divergent reactivity compared to their silica-supported homologues, resisting metallic particle formation under reducing pretreatment conditions (exposure to excess organolithium reagents) and maintaining active iron/lithium speciation under ammonia synthesis conditions that is absent on the oxide support. When the organometallic iron complex on silicon nitride is exposed to excess n-butyllithium, iron remains isolated, catalyzing the conversion of butyllithium to lithium hydride, resulting in a divalent iron site in a polyhydride environment. In contrast, the silica-supported complex is converted to reduced iron clusters without forming persistent isolated hydrides. These structural differences lead to markedly different catalytic behaviors under ammonia synthesis conditions. The Li/Fe/Si3N4 catalyst is highly active (7.5 mol NH3/mol Fe/h at 300 °C, 10 bar, or 46 mol NH3/mol Fe/h at 400 °C, 10 bar), while both the silica-supported analog and the nonlithiated Si3N4-supported species are inactive. Notably, this activity is enhanced relative to previously reported iron-lithium hydride composite catalysts (0.43–4.1 mol NH3/mol Fe/h at 300 °C, 10 bar) and relative to the industrial benchmark promoted iron catalyst KM1 (3.0 mol NH3/mol Fe/h at 400 °C, 10 bar). The catalyst activation and LiH/LiNHx nitrogen reservoir dynamics for Li/Fe/Si3N4 are studied by X-ray Absorption, Mössbauer, and in situ DRIFT spectroscopies and isotopic exchange kinetics.

• Research Organization: Argonne National Laboratory (ANL)
• Sponsoring Organization: US Department of Energy; USDOE Office of Science - Office of Basic Energy Sciences; USDOE Office of Science - Office of Basic Energy Sciences - Chemical Sciences, Geosciences, and Biosciences Division; USDOE Office of Technology Transitions (OTT); University of Illinois – Urbana-Champaign
• Grant/Contract Number: AC02-06CH11357
• OSTI ID: 3374262
• Journal Information: ACS Catalysis, Journal Name: ACS Catalysis; ISSN 2155-5435
• Country of Publication: United States
• Language: English