CHAMPAIGN, Ill. - Activated carbon is commonly used to trap contaminants in a variety of products ranging from air to whiskey. Despite its popularity, little was known about the inner workings of this natural adsorbent until scientists at the University of Illinois recently took a much closer look at this versatile material to understand why it works the way it does.
"In the past, pore sizes and distributions within an activated carbon sample were measured using indirect techniques, so there were lots of theories and assumptions embedded in the results," said Chris Mangun, a U. of I. doctoral student in materials science. "Now for the first time, we actually have seen and measured the pores."
Using a scanning tunneling microscope, the researchers were able to see the porous microstructure within an activated carbon fiber, and relate this microstructure to the fiber1s adsorption properties. The team1s findings will appear in the October issue of the journal Carbon.
"The microscope revealed a labyrinth of narrow tubes twisting and turning through the fiber, creating an interconnected, microporous network," said James Economy, a U. of I. professor of materials science. "The large macropores on the surface branch into smaller mesopores, which branch into smaller micropores - much like the tapering passages in a large cave."
As the researchers "spelunked" down the fiber, they encountered multitudes of ellipsoid-shaped micropores and mesopores. Many of the pores, which ranged in size from less than 1 nanometer in width to more than 50 nanometers, formed tubes that extended more than 30 times their width. (A nanometer is 1 billionth of a meter.) Adsorption takes place within the narrow confines of these pores and tubes.
The adsorption properties of a carbon fiber are directly related to the size and distribution of the pores, Mangun said. Activated carbons with smaller pore sizes, for example, are much more effective at removing contaminants at low concentrations.
"For a long time, surface area was presumed to be the measure of carbon1s ability to adsorb contaminants," Mangun said. "Now we are saying that pore size, pore shape and pore-surface chemistry are just as important. And these characteristics can be altered to meet specific adsorption requirements."
The commercial fibers used in the study were spun from a phenolic polymer. The fibers become "activated" when some of the carbon is etched away through an oxidation reaction. The nature of the reaction - which includes such factors as reagent, time and temperature - controls the pore structure and surface chemistry, which in turn control the adsorption properties of the fibers.