Room‐Temperature Catalytic Reduction of Aqueous Nitrate to Ammonia with Ni Nanoparticles Immobilized on an Fe₃O₄@n‐SiO₂@h‐SiO₂–NH₂ Support

Efficient and selective catalytic reduction of aqueous nitrate to ammonia was achieved over Ni nanoparticles immobilized on Fe 3 O 4 @n‐SiO 2 @h‐SiO 2 –NH 2 [a magnetic hierarchical mesoporous amine‐functionalized (M‐HMAF) silica] by using hydrazine hydrate as a reducing agent. The high hierarchical mesoporous structure (surface area of 416 m 2 g –1 and pore size of 3.7 nm) and Fe 3 O 4 core (ca. 7 nm) of the M‐HMAF silica support resulted in a high dispersion of Ni nanoparticles over the support and easy recovery of the catalyst, respectively. Interestingly, the Ni/M‐HMAF silica catalyst exhibited an excellent catalytic turnover (275 mmol g metal –1 h –1 ) compared with most of the existing catalysts for the conversion of nitrate ions at room temperature. The mechanistic study using UV/Vis spectroscopy revealed that the catalytic conversion of nitrate ions to ammonia proceeded through in situ generated nitrite ions. Subsequently, the ammonia produced from nitrate ions was isolated and analyzed by 1 H and 15 N NMR spectroscopy, whereas the N 2 gas released as a byproduct of hydrazine was characterized by GC–MS.