A new, improved approach to making porous carbon materials for sequestering the greenhouse gas carbon dioxide has been devised by researchers at Rice University in Houston, Texas, USA. Rice chemist Andrew Barron and his team have found that the conditions in which such materials are synthesized affects their efficacy as carbon capture agents. [Ghosh, et al., J. Mater. Chem. A (2016) DOI: 10.1039/C6TA04936B]
The researchers assessed the various properties of porous carbon manufactured as pellets - temperature, pressure, the material's surface area, the size of its pores and what elements are added - and how they affect adsorption. Barron explains that that the property map that emerged could influence how carbon capture research is carried out from now on.
"The traditional sense has been the more surface area and the greater the porosity of the material, the better it will adsorb," he explains. "So people have been synthesizing materials to maximize both. It turns out that's kind of a dead area of research because once you get to a critical number, no matter how high you get after that, they don't improve absorption." He and his colleagues have essentially cooked up a recipe for how to make the optimal carbon capture materials.
The team's experimental data was based on a range of porous carbons made from sources as diverse as pulverized coconut shells and sawdust and treated them with potassium hydroxide to pit these grains with nanoscopic pores. Some batches were treated with nitrogen others with sulfur as additives aimed at making them more adsorbent. Chemical activation was carried out at a range of temperatures from 500 to 800 degrees Celsius. Carbon dioxide adsorption capacity was measured at pressures from zero to 30 times atmospheric pressure.
Tests suggest adsorption plateaus with materials with a minimum surface area of 2800 square meters per gram and a pore volume of 1.35 cubic centimeters. "Once you get to a certain point, no matter what you do, you're not going to get any better with a certain material," Barron says.
They also found that a material with less than 90 percent carbon and enhanced by oxygen, rather than nitrogen or sulfur, worked best for both carbon capture and methane selectivity, especially for materials activated at close to 800 degrees Celsius. Indeed, there is a tradeoff between adsorption of carbon dioxide as opposed to methane. An ideal material would capture all the carbon dioxide and let all the energy-containing methane pass through for use as fuel. "The barrier where it doesn't help you any more is different for the total uptake of carbon dioxide than it is for the selectivity between carbon dioxide and methane," Barron adds.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".