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Tiny Au takes on the pollution giants
RESEARCH NEWS
Amazing ‘breathing’ nanoporous frameworks POROUS MATERIALS
of these solids, A team of researchers ranging from 85% from France, the UK, expansion up to and the European an unprecedented Synchrotron Radiation 230%. This Facility (ESRF) have reversible discovered remarkable, ‘breathing’ is similar giant and reversible to human lung swelling in a family of function, except nanoporous materials that normal lung [Serre et al., Science Structures (along the c-axis) of the MIL-88A, B, C, D series in expansion is only (2007) 315, 1828]. The their dry forms (top) and open forms (bottom). (Courtesy of the ~40%. The swelling materials are flexible, Institut Lavoisier.) effect is achieved by highly selective, and simply immersing show promise for the the MIL-88 material into solvents, which enter the environmentally friendly and economically feasible cavities and open the framework without breaking separation, recovery, and reuse of vapors and any bonds. The crystallinity of the material is retained greenhouse gases. throughout the process. The swelling process can be Porous hybrid solids have attracted much attention reversed by heating the solvated form to recover the in recent years as a possible source of new materials ‘dry’ one. This ‘dry’ form has closed pores with almost for environmental clean-up. A family of metal-organic no accessible porosity, while the closed hydrated form three-dimensional structures, MIL-88, with cages and shows remarkable selectivity in absorption of polar and channels has been developed. The compounds contain nonpolar gases. CrIII or FeIII ions with organic linkers, creating a flexible The next step is to investigate how hydrogen or structure that can easily change shape. External factors greenhouse gases can be stored in these kinds of such as pressure, temperature, light, or the influence materials, which could open the door to applications of gases and solvents can cause the framework to such as H2 fueled cars or CO2 capture in the future. open up or close down. For the first time, researchers have tracked a reversible, giant increase in the volume Catherine Reinhold
When these atoms become depleted, they are replaced by O2 present in air, which can dissociate on the surface of the nanoparticles and penetrate the lattice. The effectiveness of these oxides as purifiers, both with and without nanoparticles, has been tested using the pollutants acetaldehyde, toluene, and hexane. Mesoporous γ-MnO2 eliminates acetaldehyde in under an hour at room temperature, producing CO2 as a by-product. Results indicate that at 2 ppm concentrations, mesoporous γ-MnO2 has a removal capacity that is 25 times higher than activated carbon. On addition of Au nanoparticles, this value increases to 50. Toluene and hexane are less easy to remove. The undecorated oxide can absorb 60% of toluene at room temperature and less than 2% of hexane at elevated temperatures. When the material is functionalized with Au nanoparticles, the removal capacity improves dramatically, with figures reaching 95% and 30%, respectively, at room temperature. The new materials could find applications in energy devices, oxidation catalysts, and adsorbents.
Katerina Busuttil
Cordelia Sealy
POROUS MATERIALS
10
JUNE 2007 | VOLUME 2 | NUMBER 3
POROUS MATERIALS Researchers from Southern Illinois University have synthesized alumina films with novel nonlinear pores of varied and complex shapes from curves to bends to dendritic structures [Zakeri et al., Chem. Mater. (2007), 19, 1954]. “The controlled and reproducible synthesis of complex shaped nanotubes and nanowires is crucial to further advance applications in the fields of nanoelectronics, biosensing, bioseparations, etc.,” says Punit Kohli. He suggests that the pores of alumina films could be used as templates to fabricate such structures. “Using the nonlinear nanoporous alumina films… as templates, one could reproducibly and inexpensively synthesize nanotubes and nanowires of complex shape in large quantities that may otherwise be difficult to prepare using conventional methods.” The researchers used alumina films to produce silica nanotubes in the standard manner. However, by varying the geometric shape of the Al substrate (flat/planar, rectangular, cylindrical, hexagonal), the pore characteristics such as orientation, density, merging with other pores, and curvature can be determined. As expected, flat/planar and rectangular substrates produce wellordered, hexagonally packed pores perpendicular to the surface. However, cylindrical substrates produce nanopores that merge with each other and are curved. Using beveled faces of cylindrical substrates produces Y-shaped and dendritic nanopores. Finally, the researchers also observed pores with 90º bends. The complex structures of nanopores on cylindrical substrates result from the combined effects of spatial factors, kinetics, and the strength and direction of the electric field.
Tiny Au takes on the pollution giants Researchers at Toyota Central R&D Labs, Japan have developed a mesoporous γ-MnO2 material with catalytic Au nanoparticles that can absorb and decompose volatile organic compounds (VOCs), which are environmental pollutants [Sinha et al., Angew. Chem. Int. Ed. (2007) 46, 2891]. The team used surfactant-assisted wet chemistry techniques to synthesize γ-MnO2 with a >300 m2g-1 surface area, increasing the availability of adsorption sites for VOCs. Vacuum ultraviolet radiation (VUV)-assisted laser ablation is used to evaporate and deposit 3-6 nm Au particles into the γ-MnO2. The highly energetic ablated Au particles can penetrate deep into the lattice of the γ-MnO2 or into the mesoporous channels in the material. X-ray photoelectron spectroscopy binding energies confirm strong attachment of the ablated metal particles to the oxide support. VUV photons produced during this process can induce thermal activation and lattice defects in the oxide, increasing the number of vacant sites available for pollutant adsorption. A large number of oxygen atoms are contained in the γ-MnO2 lattices that can take part in the breakdown of pollutants.
The shape of pores to come
Amazing ‘breathing’ nanoporous frameworks POROUS MATERIALS
of these solids, A team of researchers ranging from 85% from France, the UK, expansion up to and the European an unprecedented Synchrotron Radiation 230%. This Facility (ESRF) have reversible discovered remarkable, ‘breathing’ is similar giant and reversible to human lung swelling in a family of function, except nanoporous materials that normal lung [Serre et al., Science Structures (along the c-axis) of the MIL-88A, B, C, D series in expansion is only (2007) 315, 1828]. The their dry forms (top) and open forms (bottom). (Courtesy of the ~40%. The swelling materials are flexible, Institut Lavoisier.) effect is achieved by highly selective, and simply immersing show promise for the the MIL-88 material into solvents, which enter the environmentally friendly and economically feasible cavities and open the framework without breaking separation, recovery, and reuse of vapors and any bonds. The crystallinity of the material is retained greenhouse gases. throughout the process. The swelling process can be Porous hybrid solids have attracted much attention reversed by heating the solvated form to recover the in recent years as a possible source of new materials ‘dry’ one. This ‘dry’ form has closed pores with almost for environmental clean-up. A family of metal-organic no accessible porosity, while the closed hydrated form three-dimensional structures, MIL-88, with cages and shows remarkable selectivity in absorption of polar and channels has been developed. The compounds contain nonpolar gases. CrIII or FeIII ions with organic linkers, creating a flexible The next step is to investigate how hydrogen or structure that can easily change shape. External factors greenhouse gases can be stored in these kinds of such as pressure, temperature, light, or the influence materials, which could open the door to applications of gases and solvents can cause the framework to such as H2 fueled cars or CO2 capture in the future. open up or close down. For the first time, researchers have tracked a reversible, giant increase in the volume Catherine Reinhold
When these atoms become depleted, they are replaced by O2 present in air, which can dissociate on the surface of the nanoparticles and penetrate the lattice. The effectiveness of these oxides as purifiers, both with and without nanoparticles, has been tested using the pollutants acetaldehyde, toluene, and hexane. Mesoporous γ-MnO2 eliminates acetaldehyde in under an hour at room temperature, producing CO2 as a by-product. Results indicate that at 2 ppm concentrations, mesoporous γ-MnO2 has a removal capacity that is 25 times higher than activated carbon. On addition of Au nanoparticles, this value increases to 50. Toluene and hexane are less easy to remove. The undecorated oxide can absorb 60% of toluene at room temperature and less than 2% of hexane at elevated temperatures. When the material is functionalized with Au nanoparticles, the removal capacity improves dramatically, with figures reaching 95% and 30%, respectively, at room temperature. The new materials could find applications in energy devices, oxidation catalysts, and adsorbents.
Katerina Busuttil
Cordelia Sealy
POROUS MATERIALS
10
JUNE 2007 | VOLUME 2 | NUMBER 3
POROUS MATERIALS Researchers from Southern Illinois University have synthesized alumina films with novel nonlinear pores of varied and complex shapes from curves to bends to dendritic structures [Zakeri et al., Chem. Mater. (2007), 19, 1954]. “The controlled and reproducible synthesis of complex shaped nanotubes and nanowires is crucial to further advance applications in the fields of nanoelectronics, biosensing, bioseparations, etc.,” says Punit Kohli. He suggests that the pores of alumina films could be used as templates to fabricate such structures. “Using the nonlinear nanoporous alumina films… as templates, one could reproducibly and inexpensively synthesize nanotubes and nanowires of complex shape in large quantities that may otherwise be difficult to prepare using conventional methods.” The researchers used alumina films to produce silica nanotubes in the standard manner. However, by varying the geometric shape of the Al substrate (flat/planar, rectangular, cylindrical, hexagonal), the pore characteristics such as orientation, density, merging with other pores, and curvature can be determined. As expected, flat/planar and rectangular substrates produce wellordered, hexagonally packed pores perpendicular to the surface. However, cylindrical substrates produce nanopores that merge with each other and are curved. Using beveled faces of cylindrical substrates produces Y-shaped and dendritic nanopores. Finally, the researchers also observed pores with 90º bends. The complex structures of nanopores on cylindrical substrates result from the combined effects of spatial factors, kinetics, and the strength and direction of the electric field.
Tiny Au takes on the pollution giants Researchers at Toyota Central R&D Labs, Japan have developed a mesoporous γ-MnO2 material with catalytic Au nanoparticles that can absorb and decompose volatile organic compounds (VOCs), which are environmental pollutants [Sinha et al., Angew. Chem. Int. Ed. (2007) 46, 2891]. The team used surfactant-assisted wet chemistry techniques to synthesize γ-MnO2 with a >300 m2g-1 surface area, increasing the availability of adsorption sites for VOCs. Vacuum ultraviolet radiation (VUV)-assisted laser ablation is used to evaporate and deposit 3-6 nm Au particles into the γ-MnO2. The highly energetic ablated Au particles can penetrate deep into the lattice of the γ-MnO2 or into the mesoporous channels in the material. X-ray photoelectron spectroscopy binding energies confirm strong attachment of the ablated metal particles to the oxide support. VUV photons produced during this process can induce thermal activation and lattice defects in the oxide, increasing the number of vacant sites available for pollutant adsorption. A large number of oxygen atoms are contained in the γ-MnO2 lattices that can take part in the breakdown of pollutants.
The shape of pores to come