July 10, 2008

Also appeared in print Jul. 14, 2008, p. 9

Catalysis

Catalyst In A Bottle Works Better

Trapping metal particles inside nanotubes alters physical and chemical properties

Mitch Jacoby

Confining metallic nanoparticles inside carbon nanotubes alters the redox properties of the particles and can enhance their effectiveness as catalysts, according to a new study (J. Am. Chem. Soc., DOI: 10.1021/ja8008192). The investigation highlights a novel procedure for tailoring electronic properties of nanoparticles and may lead to various applications in catalysis, gas sensing, and magnetic devices.

Xinhe Bao/Dalian Institute of Chemical Physics View Enlarged Image

IN OR OUT? A new preparation method can selectively deposit roughly 5-nm catalytic particles inside carbon nanotubes (left). Compared with particles on the outside surfaces (right), confined particles exhibit enhanced catalytic properties.

"Carbon nanotubes are similar in some ways to activated carbon, used commercially as a catalyst support, yet they have electronic properties that make them a better support material for some types of reactions. Researchers are particularly interested in the effect of catalyst confinement achieved by attaching particles to interior nanotube surfaces, which differ electronically from nanotube exteriors. Most preparation methods, however, place the particles on the outer surfaces.

Now, Xinhe Bao, Xiulian Pan, Wei Chen, and Zhongli Fan at Dalian Institute of Chemical Physics, in China, have shown that a solution-phase method aided by ultrasonication selectively deposits iron oxide nanoparticles inside carbon nanotubes. These particles are precursors for catalyst used in Fischer-Tropsch (FT) synthesis, a carbon-coupling method for making synthetic fuels from mixtures of CO and hydrogen.

The team finds that confined particles are more prone to chemical reduction than particles attached to the exterior of the nanotubes. Specifically, they report that reduction of the particles with hydrogen and CO, which converts the precursors to the working form of the catalyst, doubles the ratio of iron-carbide/iron-oxide surface species on catalyst particles inside a nanotube compared with particles on the outside. A high relative concentration of iron carbide species is believed to be essential for high FT activity.

In FT synthesis tests, the team monitored the concentration of five-carbon and larger hydrocarbons. They found that, compared with exterior particles and particles supported on activated carbon, nanotube-confined FT catalyst particles produced twice and six times the yield, respectively.

Concerned initially about diffusion limitation problems, the Dalian team developed methods for cutting the tubes into short segments to minimize the distance that reactant and product molecules would need to travel to get in and out of the nanotubes. Judging by the enhanced reducibility of the confined particles, however, Bao notes that diffusion is not a severe impediment. The group now plans to conduct diffusion simulation studies.

"This is exciting work," comments Charles H. F. (Chuck) Peden, a senior scientist at Pacific Northwest National Laboratory. Clearly, there are dramatic differences in the chemical and physical properties of the catalyst particles when they are put inside or outside the nanotube channels, he says.

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