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Different on the surface
RESEARCH NEWS
Different on the surface SURFACE SCIENCE Researchers from the University of Aarhus, Denmark have used timeresolved scanning tunneling microscopy (STM) to observe the diffusion behavior of adsorbates on a metal oxide [Wahlström et al., Science (2004) 303, 511]. “We followed the motion of individual oxygen molecules on the rutile TiO2(110) surface as a function of temperature (between -100°C and -20 °C) with a STM,” says Flemming Besenbacher. The ordinary diffusion model cannot account for the researchers’ observations. “Instead, we found strong evidence that the molecular diffusion is linked to the electronic properties of the substrate in the surface region,” he explains. Charge transfer from the conduction band of the substrate to molecules on the surface induces them to ‘hop’ and diffuse across the surface. The hopping rate of oxygen molecules is, therefore, directly proportional to the number of electrons available at the surface. The results have implications for the
Making effective use of pores POROUS MATERIALS
Researchers at Rutgers University, National Energy Technology Laboratory, and the University of Pittsburgh have synthesized a novel microporous metal coordination material (MOMM) that absorbs hydrogen at room temperature [Pan et al., J. Am. Chem. Soc. (2004), doi: 10.1021/ja0392871]. “Hydrogen storage is a difficult problem because enough gas to provide a reasonable driving range must be packed into as small a space as possible without using excessively high pressures or low temperatures,” says Jing Li of Rutgers. Last year, Rosi et al. [Science (2003) 300, 1127] reported the first metal organic materials for hydrogen storage. Now Li and coworkers have synthesized a MOMM with a different structure that is also promising for storage. The material has micropores that closely fit the dimensions of adsorbed hydrogen to maximize the interaction energy between the gas and the pore walls. “The approach appears to be successful in that a higher volumetric storage density was achieved,” says Li. Although the amount was only 1 wt% at 48 atm, which is comparable to the best carbon nanotubes, it remained stable during the duration of the experiment. “This step forward is significant in that it illustrates a strategy that can be employed to optimize new structures,” says Li.
View of the structure: three-dimensional packing of microtubes. (Courtesy of Jing Li.)
Tailoring materials to have a smaller pore size similar to the dimensions of hydrogen gas, while increasing the overall pore volumes, appears a promising strategy. Such rational design of new materials could enable MOMMs to approach the Department of Energy target value of 0.036 g H2/cm3 by 2005 without resorting to high pressures. Li and coworkers are now investigating MOMMs of different structures to identify useful properties. “There are open fundamental questions about how hydrogen actually interacts with the metal organic materials, where hydrogen adsorbs, and how it adsorbs through narrow channels,” she says.
understanding of oxidation processes
Nanotubes reduce fuel cell costs
on metal oxides, as well as for the
NANOTECHNOLOGY
R&D of heterogeneous catalysts, photocatalysts, and gas sensors. “Understanding the mobility of oxygen molecules on such substrates is of crucial importance to improve or develop new oxidation catalysts,” says Besenbacher. But the results also indicate that diffusion on metal oxides may be completely different from diffusion on metal surfaces. The researchers are now investigating whether chemical reactions occurring on metal oxides are also coupled to a charge transfer mechanism. They will also be trying to tailor the diffusivity and reactivity of metal oxide-based catalysts by modifying the surface electronic properties with doping.
Schematic of a PEM fuel cell and MWNT-based electrode catalyst layer. The MWNT film was deposited by CVD using an electrodeposited catalyst and the fuel cell catalyst Pt was then selectively electrodeposited on top. (Courtesy of Yushan Yan.)
“It has been a long-standing wish to cut the use of Pt so that fuel cells can be made cheaper,” says Yushan Yan of the University of California, Riverside. He and coworkers have found that replacing carbon powder with multiwalled carbon nanotubes (MWNTs) as the catalyst support can cut the amount of Pt required by proton exchange membrane (PEM) fuel cells [Wang et al., Nano Lett. (2004), doi: 10.1021/nl034952p]. The MWNTs provide a large surface area for the deposition of the Pt catalyst by chemical vapor deposition (CVD). “Our approach has the potential to reduce the Pt use by 3-4 times,” says Yan. Although the performance of the MWNT-based fuel cell is not as good as conventional ones, further improvements can be made. “I think we are well positioned to optimize our processing steps and very hopefully to be able to significantly reduce the Pt use in fuel cells in the near future,” says Yan.
March 2004
15
Different on the surface SURFACE SCIENCE Researchers from the University of Aarhus, Denmark have used timeresolved scanning tunneling microscopy (STM) to observe the diffusion behavior of adsorbates on a metal oxide [Wahlström et al., Science (2004) 303, 511]. “We followed the motion of individual oxygen molecules on the rutile TiO2(110) surface as a function of temperature (between -100°C and -20 °C) with a STM,” says Flemming Besenbacher. The ordinary diffusion model cannot account for the researchers’ observations. “Instead, we found strong evidence that the molecular diffusion is linked to the electronic properties of the substrate in the surface region,” he explains. Charge transfer from the conduction band of the substrate to molecules on the surface induces them to ‘hop’ and diffuse across the surface. The hopping rate of oxygen molecules is, therefore, directly proportional to the number of electrons available at the surface. The results have implications for the
Making effective use of pores POROUS MATERIALS
Researchers at Rutgers University, National Energy Technology Laboratory, and the University of Pittsburgh have synthesized a novel microporous metal coordination material (MOMM) that absorbs hydrogen at room temperature [Pan et al., J. Am. Chem. Soc. (2004), doi: 10.1021/ja0392871]. “Hydrogen storage is a difficult problem because enough gas to provide a reasonable driving range must be packed into as small a space as possible without using excessively high pressures or low temperatures,” says Jing Li of Rutgers. Last year, Rosi et al. [Science (2003) 300, 1127] reported the first metal organic materials for hydrogen storage. Now Li and coworkers have synthesized a MOMM with a different structure that is also promising for storage. The material has micropores that closely fit the dimensions of adsorbed hydrogen to maximize the interaction energy between the gas and the pore walls. “The approach appears to be successful in that a higher volumetric storage density was achieved,” says Li. Although the amount was only 1 wt% at 48 atm, which is comparable to the best carbon nanotubes, it remained stable during the duration of the experiment. “This step forward is significant in that it illustrates a strategy that can be employed to optimize new structures,” says Li.
View of the structure: three-dimensional packing of microtubes. (Courtesy of Jing Li.)
Tailoring materials to have a smaller pore size similar to the dimensions of hydrogen gas, while increasing the overall pore volumes, appears a promising strategy. Such rational design of new materials could enable MOMMs to approach the Department of Energy target value of 0.036 g H2/cm3 by 2005 without resorting to high pressures. Li and coworkers are now investigating MOMMs of different structures to identify useful properties. “There are open fundamental questions about how hydrogen actually interacts with the metal organic materials, where hydrogen adsorbs, and how it adsorbs through narrow channels,” she says.
understanding of oxidation processes
Nanotubes reduce fuel cell costs
on metal oxides, as well as for the
NANOTECHNOLOGY
R&D of heterogeneous catalysts, photocatalysts, and gas sensors. “Understanding the mobility of oxygen molecules on such substrates is of crucial importance to improve or develop new oxidation catalysts,” says Besenbacher. But the results also indicate that diffusion on metal oxides may be completely different from diffusion on metal surfaces. The researchers are now investigating whether chemical reactions occurring on metal oxides are also coupled to a charge transfer mechanism. They will also be trying to tailor the diffusivity and reactivity of metal oxide-based catalysts by modifying the surface electronic properties with doping.
Schematic of a PEM fuel cell and MWNT-based electrode catalyst layer. The MWNT film was deposited by CVD using an electrodeposited catalyst and the fuel cell catalyst Pt was then selectively electrodeposited on top. (Courtesy of Yushan Yan.)
“It has been a long-standing wish to cut the use of Pt so that fuel cells can be made cheaper,” says Yushan Yan of the University of California, Riverside. He and coworkers have found that replacing carbon powder with multiwalled carbon nanotubes (MWNTs) as the catalyst support can cut the amount of Pt required by proton exchange membrane (PEM) fuel cells [Wang et al., Nano Lett. (2004), doi: 10.1021/nl034952p]. The MWNTs provide a large surface area for the deposition of the Pt catalyst by chemical vapor deposition (CVD). “Our approach has the potential to reduce the Pt use by 3-4 times,” says Yan. Although the performance of the MWNT-based fuel cell is not as good as conventional ones, further improvements can be made. “I think we are well positioned to optimize our processing steps and very hopefully to be able to significantly reduce the Pt use in fuel cells in the near future,” says Yan.
March 2004
15