Bifunctional heterogeneous catalysts for selective epoxidation and visible light driven photolysis: Nickel oxide-containing porous nanocomposite

Over the last decades, intense research activities have been devoted on the development of efficient heterogeneous catalysts because of their convenient use and facile recycling.([1]) In particular, porous transition metal oxides have attracted special attention as promising catalyst materials since transition metal ions can show high catalytic activity for redox- and photoinduced-reactions, and moreover the expansion of surface area through the fabrication of porous structure can provide a large number of catalytically active sites.([2]) Among various synthetic routes to porous inorganic solids, one of the most powerful methods is the self-assembly of inorganic precursors with the help of surfactant molecules or block copolymers.([3]) This synthetic route is very effective for creating mesoporous silica-based catalysts like MCM-41 and MCM-48, however it is less successful in preparing thermally stable porous transition metal oxides. As an alternative route, the intercalation reaction of metal oxide into layered inorganic solids is useful for synthesizing porous transition metal oxides.([4]) Furthermore, this method can provide a unique merit of hybridizing two different metal oxides into a porous lattice, leading to the synergetic combination of two physicochemical properties. Recently nanoporous nickel phosphate prepared by microwave irradiation has been reported to show high catalytic activity for the epoxidation of olefin molecules.([5]) Moreover, nanocrystalline nickel oxide has been widely used as a co-catalyst for improving the photocatalytic activity of semiconductive materials.([6]) In this regard, nickel oxide-containing porous compound is expected to be active not only as a redox catalyst but also as a photocatalyst. Up to date, however, there have been no reports on the synthesis of porous intercalation compound of nickel oxide nanoparticle and layered inorganic solids. A previous attempt to dope a small amount of nickel oxide (< 1wt%) into the interlayer space or on the surface of layered perovskite metal oxide was unsuccessful in producing porous NiO-based materials.([7]) Recently it was reported that the intercalation of bulky organic molecules leads to the exfoliation of transition metal oxides into colloidal nanoshects.([8]) Since the resultant individual monolayers are negatively charged, they are expected to easily be reassembled with positively charged nickel hydroxide nanoclusters,([9]) leading to the formation of porous nickel oxide-based materials (Scheme 1). In the present work, we are successful in synthesizing for the [GRAPHICS] first time porous nickel oxide-titanium oxide (hereafter it is called as NT) nanocomposites with exfoliated titanium oxide nanosheets and characterized their multifunctional catalytic activities and chemical bonding nature.