Bifunctional Heterogeneous Catalysts for Selective Epoxidation and Visible Light Driven Photolysis: Nickel Oxide‐Containing Porous Nanocomposite

Abstract

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 nanosheets.[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 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. Figure 1 represents the powder X-ray diffraction (XRD) patterns of the protonated titanium oxide and the NT nanocomposites. The as-prepared NT nanocomposite exhibits a series of equally spaced (0k0) reflections at lower angles than the protonated titanium oxide, [10] ind...

Unique Properties of 2 D Layered Titanate Nanosheets as a Building Block for the Optimization of the Photocatalytic Activity and Photostability of TiO2‐Based Nanohybrids

“…The very thin thickness, wide 2D surface, and layered structure of TiO 2 2D nanosheets are mainly responsible for the observed strong chemical interaction in the 2D-TOAP nanohybrids. [10,11,33] Of prime importance is that all the coordinatively unsaturated oxygen ions of these layered TiO 2 nanosheets are exposed to surface and can interact strongly with hybridized silver phosphate species ( Figure S1 in the Supporting Information). Thus, a strong electronic coupling with Ag 3 PO 4 nanoparticles can occur in the entire part of TiO 2 2D nanosheets, see the right panel of Figure 1.…”

“…For the synthesis of the 2D-TOAP nanohybrids, the exfoliated TiO 2 2D nanosheets with lepidocrocite-type structure was applied as TiO 2 precursor. [10,40] First the pristine layered cesium titanium oxide with lepidocrocite-type structure was prepared by conventional solid-state reaction and its protonated derivative was obtained by the acid treatment of the pristine layered cesium titanium oxide. The exfoliated TiO 2 nanosheets was prepared by the reaction of protonated titanium oxide with TBA ions, leading to the formation of stable colloidal suspension of titanium oxide nanosheets.…”

“…[8,9] Among various nanostructured titanium oxides, the exfoliated 2D nanosheets of layered titanium oxide are fairly unique in terms of the subnanometer-level thickness, wide 2D surface, and high flexibility. [10,11] Such characteristics of TiO 2 2D nanosheets render this material a useful building block for novel hybrid-type photocatalysts via the immobilization of semiconductor nanoparticles on the expanded surface area. Furthermore, since most of component ions in the monolayered nanosheets are exposed on the surface, they can be strongly affected by the hybridization with other components via a direct and strong bond formation.…”

Thermostable nitrogen-doped HTiNbO5 nanosheets with a high visible-light photocatalytic activity

“…In the past few years, some metallic elements such as Fe [25], Cr [26], and Ni [27] have been employed to tune the electronic structure and enhance the photocatalytic activity of layered compounds under visible light irradiation. Unfortunately, doping with metallic elements can result in a decrease in thermal stability and an increase in carrier trapping, leading to a decrease in photocatalytic efficiency [28].…”

Construction of inorganic nanoparticles by micro-nano-porous structure of cellulose matrix

“…Recently, considerable efforts have been focused on developing visible-light-driven photocatalysts, such as limited mixed oxides (Kim et al 2008a) (oxy)nitrides , and (oxy)sulfides (Ishikawa et al 2003), capable of using the less energetic but more abundant visible light that accounts for about 43% of the solar spectrum. Among these catalysts, CdS is one of the most active.…”