Copper−Aluminum Fluorides as Active Catalysts for Nitrate Reduction to Ammonia

Fluorinated multimetal materials have recently attracted attention as catalysts for nitrate reduction to ammonia (NO 3 − RR). However, their rational design and activity remain largely unexplored. Here, we report, for the first time, the NO 3 − RR performance of a hydrated fluorinated copper−aluminum, Cu 3 Al 2 F 12 (H 2 O) 12, previously reported as a precursor of an oxyfluorinated electrocatalyst prepared by calcination. Co-precipitation synthesis, through optimization of the synthesis parameters, significantly improves its purity compared to evaporation-based synthesis. Thermal monitoring of the decomposition process of Cu 3 Al 2 F 12 (H 2 O) 12 by X-ray diffraction (XRD) and thermogravimetric analysis, complemented by 19 F nuclear magnetic resonance measurements, made it possible to establish the reaction scheme involving the formation of AlF 3− x (OH) x and Cu(OH)F. The comparative study of the electrocatalytic properties of the fluorinated materials demonstrated that neither the synthesis method nor the calcination process significantly influenced NO 3 − RR performance. This led to the selection of the hydrated fluoride Cu 3 Al 2 F 12 (H 2 O) 12 prepared via the simplest and scalable synthesis route, exhibiting a Faradaic efficiency of about 70% and an ammonia yield of 31 mg·h −1 ·mg cat −1, after 24 h in 0.1 M KNO 3 at neutral pH. Postmortem characterization (electronic microscopies and XRD) reveals that the catalytically active phase generated under electrochemical conditions consists of a core−shell morphology, with smaller nanoparticles decorating the surface of the larger particles. These findings highlight the intrinsic potential of hydrated fluorinated multimetal materials as NO 3 − RR precatalysts.