Synthesis and Atmosphere‐Driven Phase Evolution of Ultrafine Mn 3 O 4 ‐Based Ceramics
Dmytro Demirskyi, Minoru TakedaABSTRACT
We report a surfactant‑controlled synthesis of ultrafine Mn 3 O 4 and a systematic study of phase formation and microstructure evolution as a function of surfactant content and annealing conditions. We consider Mn 3 O 4 to be a promising catalyst for accelerating ortho–para hydrogen conversion. Our approach involves synthesizing Mn(OH) 2 precursors that convert to Mn 3 O 4 under normal atmospheric conditions. This change has been verified through in situ x‐ray diffraction experiments. To control particle size and phase purity, we use sandalwood oil as a surfactant. An optimal ratio of 1:3 yields single‐phase Mn 3 O 4 but produces the coarsest particles (20–60 nm). Calcination at 260°C is conducted in argon or hydrogen. Argon annealing promotes the time‐dependent growth of MnO and the development of fine oxide crystallites in the surface layer, with sizes ranging from 5–12 nm at 260°C to 3–6 nm at 160°C. Processing in an inert atmosphere or in hydrogen can significantly alter the composition of the catalyst and its surface conditions prior to ortho–para hydrogen conversion testing at the catalyst activation stage via formation of MnO. This should directly impact the catalytic characteristics and is relevant to other oxide/hydroxide catalysts.