Core–Shell Nanostructured Catalysts

Abstract

Novel nanotechnologies have allowed great improvements in the syn-thesis of catalysts with well-controlled size, shape, and surface properties. Transition metal nanostructures with specific sizes and shapes, for instance, have shown great promise as catalysts with high selectivities and relative ease of recycling. Researchers have already demonstrated new selective catalysis with solution-dispersed or supported-metal nanocatalysts, in some cases applied to new types of reactions. Several challenges remain, however, particularly in improving the structural stability of the catalytic active phase. Core-shell nanostructures are nanoparticles encapsulated and protected by an outer shell that isolates the nanoparticles and prevents their migration and coalescence during the catalytic reactions. The synthesis and characterization of effective core-shell catalysts has been at the center of our research efforts and is the focus of this Account. Efficient core-shell catalysts require porous shells that allow free access of chemical species from the outside to the surface of nanocatalysts. For this purpose, we have developed a surface-protected etching process to prepare mesoporous silica and titania shells with controllable porosity. In certain cases, we can tune catalytic reaction rates by adjusting the porosity of the outer shell. We also designed and successfully applied a silica-protected calcination method to prepare crystalline shells with high surface area, using anatase titania as a model system. We achieved a high degree of control over the crystallinity and porosity of the anatase shells, allowing for the systematic optimization of their photocatalytic activity. Core-shell nanostructures also provide a great opportunity for controlling the interaction among the different components in ways that might boost structural stability or catalytic activity. For example, we fabricated a SiO₂/Au/N-doped TiO₂ core-shell photocatalyst with a sandwich structure that showed excellent catalytic activity for the oxidation of organic compounds under UV, visible, and direct sunlight. The enhanced photocatalytic efficiency of this nanostructure resulted from an added interfacial nonmetal doping, which improved visible light absorption, and from plasmonic metal decoration that enhanced light harvesting and charge separation. In addition to our synthetic efforts, we have developed ways to evaluate the accessibility of reactants to the metal cores and to characterize the catalytic properties of the core-shell samples we have synthesized. We have adapted infrared absorption spectroscopy and titration experiments using carbon monoxide and other molecules as probes to study adsorption on the surface of metal cores in metal oxide-shell structures in situ in both gas and liquid phases. In particular, the experiments in solution have provided insights into the ease of diffusion of molecules of different sizes in and out of the shells in these catalysts.

Magnetically Separable and Sustainable Nanostructured Catalysts for Heterogeneous Reduction of Nitroaromatics

“…The syntheses, characterizations and practical applications of effective core-shell and yolk-shell nanostructured catalysts have been introduced as a unique strategy to resolve the aforementioned problems. Core-shell nanostructure catalysts are generally encapsulated and protected by an outer shell that isolates the catalytic NPs and prevents their migration and coalescence during the catalytic reactions [65]. Yolk-shell nanostructured catalysts consist of a core encapsulated in a hollow capsule with a permeable shell, and catalytic NPs are designed as an alternative tactic to meet an excellent catalyst [66].…”

Nanocatalysis I: Synthesis of Metal and Bimetallic Nanoparticles and Porous Oxides and Their Catalytic Reaction Studies

“…Core/shell nanoparticles have been designed for a stable catalyst against coalescence and sintering during catalytic reactions by encapsulating the core nanoparticles with porous shells [24]. Joo et al developed a Pt/SiO 2 core/shell as a thermally stable catalyst, in which mesoporous silica shells encaged Pt cores and the Pt nanoparticles maintained their structure up to 750°C in air without aggregation (Fig.…”

“…The shells overgrown on the surface of core nanoparticles become porous during the calcinations by removing organic surfactants. Recently, an alternative strategy was developed to control the porosity of the shells systematically by a surface-protected etching process [24,26]. The coated oxide shells resulted in porous layers by removing organic capping ligands in the shells by an appropriate etching agent.…”

“…However, preparation of non-siliceous porous oxide shells is still challenging, because the calcination process for crystallization causes the loss of porosity. For the creation of high crystalline oxide shells, an alternative method was developed by silica-protected calcinations [24,27]. Figure 3b shows Pt/TiO 2 core/shell structures synthesized by the growth of a TiO 2 layer on top of the given Pt/SiO 2 core/ shells.…”

Two-Step Fabrication of Porous a#945;-Fe2O3@TiO2 core-shell Nanostructures with Enhanced Photocatalytic Activity

“…The synergetic effects between the core and shell materials may drastically improve the overall performance of the hybrid composites, which results in a range of potential applications, including biomedical application [3,4], catalysis [5,6], electronics [7] and chemical sensors [8]. The properties of core-shell nanostructures can be adjusted by changing either the constituting materials or the core to shell ratio [9].…”