(129f) Mesoporous Metal Sulfides and Carbides

Mesoporous materials have attracted attention due to their great potential as catalysts, electrodes, and magnetic materials. Since the nanocasting method was introduced, a wide range of mesoporous transition metal oxides, such as Fe₂O₃, MnO₂, Co₃O₄, NiO, and TiO₂, with ordered pores and crystalline walls have been synthesized using mesoporous SBA-15 and KIT-6 silica as hard templates. Some unique behaviors of nanocast materials have been found for a number of applications. For example, we recently showed a photocatalytically active ordered mesoporous Co₃O₄ with extremely high surface area by nanocasting a mesoporous MgCo₂O₄ and then selectively leaching the magnesium. Through manipulation of the synthetic conditions and elemental compositions the resulting Co₃O₄ material exhibited a much higher activity in the photocatalytic water oxidation reaction compared to its bulk counterpart. This study demonstrated the importance of synthesis in preparing mesoporous materials with the desired composition and porosity. To continue addressing critical challenges with mesoporous materials new strategies to access materials that could not be synthesized previously are necessary.

Among all the transition-metal-containing materials, metal sulfides and carbides are of particular interest because of their potential applications in sensors, separations, catalysis, solar cells, fuel cells and batteries. A few attempts to synthesize mesoporous metal sulfides and carbides have been made. Pioneering work by Shi et al. showed the possibility to synthesize ordered mesoporous metal sulfides via the nanocasting method. In their report of mesoporous WS₂ and MoS₂, phosphotungstic acid (H₃PW₁₂O₄₀·6H₂O) and phosphomolybdic acid (H₃PMo₁₂O₄₀·6H₂O) were loaded into a mesoporous silica template, followed by a high temperature thermal treatment (600â??) under a flow of mixed H₂S and H₂. Although some important progress has been made, the range of ordered mesoporous metal sulfides and carbides that can be synthesized is still very limited.

In this presentation, we will discuss an oxide-to-sulfide transformation approach as a general synthetic method to prepare ordered mesoporous metal sulfides. Three first-row transition metal sulfides, FeS₂, CoS₂, and NiS₂, with highly ordered mesoporous structures and crystalline walls have been successfully synthesized for the first time. Preliminary investigation has been performed to explore the photocatalytic properties of the newly synthesized mesoporous metal sulfides. All the mesoporous metal sulfides exhibited higher activity than their bulk counterparts for the photocatalytic degradation of methylene blue dye under visible light.

In addition to metal sulfides, we will also show the syntheses of two ordered mesoporous metal carbides, Mo₂C and W₂C, using a nanocasting approach coupled with a simultaneous decomposition/carbonization process under a continuous methane flow. The as-synthesized mesoporous Mo₂C and W₂C have three-dimensionally ordered porous structures, large surface areas (70-90 m² g^-1), and crystalline walls. In vapor-phase anisole hydrodeoxygenation (HDO) reactions, they exhibited excellent turnover frequencies under relatively low reaction temperatures (423-443 K) and ambient hydrogen pressures. Notably, the ordered mesoporous W₂C catalyst showed a greater than 96% benzene selectivity in anisole HDO, the highest benzene selectivity reported to date.