Map helps maximize carbon-capture material

Map helps maximise carbon-capture material

4:49 AM, 25th August 2016
Map helps maximize carbon-capture material
High-resolution scanning electron microscope (top) and transmission microscope images of a porous carbon sample studied for its ability to capture carbon dioxide from natural gas.

HOUSTON, US: A careful balance of the ingredients in carbon-capture materials would maximise the sequestration of greenhouse gases while simplifying the processing – or “sweetening” – of natural gas, according to researchers at Rice University.

The lab of Rice chemist Andrew Barron led a project to map how changes in porous carbon materials and the conditions in which they’re synthesised affect carbon capture. They discovered aspects that could save money for industry while improving its products.

The research appears in the journal Materials Chemistry A.

The lab compared how characteristics of porous carbon, often manufactured in pellet form, affect carbon dioxide capture. Temperature, pressure, the material’s surface area, the size of its pores and what elements are added all impact results, Barron said. He said the map will 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 absorb,” Barron said. “So people have been synthesising materials to maximise 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.

“What we’ve done is provide a recipe to make carbon capture materials the best they can be,” he said.

The researchers made a variety of porous carbon materials from sources like pulverised coconut shells and sawdust and treated them with potassium hydroxide to give the grains nanoscale pores. Some batches were enhanced with nitrogen and some with sulphur; these have been studied as additives to make materials more adsorbent. The researchers used a variety of precursors to synthesise porous carbon-based sorbent materials chemically activated at temperatures between 500 and 800 degrees Celsius (932 to 1472 degrees Fahrenheit) and carefully measured their carbon dioxide-capturing capacities at pressures between 0 and 30 bar. (One bar is slightly less than the average atmospheric pressure at sea level.)

Regardless of the functional additives, experiments showed that once a sorbent material achieved a surface area of 2,800 square meters per gram and a pore volume of 1.35 cubic centimetres per gram, neither more surface area nor larger pores made it more efficient at capturing carbon dioxide.

“Trying to make something with a higher pore volume doesn’t help,” Barron said. “Higher surface area doesn’t help. Once you get to a certain point, no matter what you do, you’re not going to get any better with a certain material.”

The researchers also discovered the best conditions for carbon capture aren’t the same as those that achieve the best trade-off between carbon and methane selectivity. An ideal material would capture all the carbon dioxide and let all the energy-containing methane pass through, Barron said.

“The barrier where it doesn’t help you anymore is different for the total uptake of carbon dioxide than it is for the selectivity between carbon dioxide and methane,” he said. “Industry doesn’t have to be making the highest-surface-area material. They just have to make it with a surface area that reaches maximum production.”

They determined a material with less than 90 percent carbon and enhanced by oxygen, rather than nitrogen or sulphur, worked best for both carbon capture and methane selectivity, especially for materials activated at temperatures approaching 800 degrees Celsius. Materials with a surface area of 2,800 square meters per gram excelled at absorbing carbon dioxide at pressures of 30 bar, but the advantages of such high surface area diminished at lower pressures.

The presence of oxygen, added by the pore-inducing potassium hydroxide, was far more relevant to the results than either nitrogen or sulphur, they found.

“We understand oxygen is important,” Barron said. “We don’t understand why. Does it stabilise certain pore structures? Is it because it stabilises the pore neck? Is it changing the shape of pores? We don’t know whether it’s a chemical or physical issue, but now we know what we should study next.”

Rice postdoc researcher Saunab Ghosh is the lead author of the paper. Co-authors are researcher Marta Sevilla and professor Antonio Fuertes of the Spanish National Research Council, Spain; senior lecturer Enrico Andreoli of the Energy Safety Research Institute, Swansea University; and Jason Ho, an engineer at the Apache Corp, Houston.

The research was supported by Apache, the Welsh Government Ser Cymru Program, the Robert A Welch Foundation, the Ministry of Economy and Competitiveness of Spain and the European Regional Development Fund.

© Rice University News

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