Oak Ridge National Laboratory research on magnesium oxide for carbon capture shows that absorption rates slow down over time due to the formation of surface layers, challenging economic viability and providing guidance for future solution-focused research. Magnesium oxide is a promising material for capturing carbon dioxide directly from the atmosphere and injecting it deep underground to limit the effects of climate change. However, for this approach to be cost-effective, it will be necessary to discover how quickly carbon dioxide is absorbed and how environmental conditions affect the associated chemical reactions.

In one proposed carbon capture method, magnesium oxide crystals on the ground combine with carbon dioxide molecules in the surrounding air to form magnesium carbonate. The magnesium carbonate is then heated, converting it back into magnesium oxide and releasing the carbon dioxide, which is placed underground or sequestered. Source: AdamMalin/ORNL, U.S. Department of Energy

Scientists at the U.S. Department of Energy's Oak Ridge National Laboratory (ORNL) analyzed a sample of magnesium oxide crystals that had been exposed to the atmosphere for decades, and another sample of magnesium oxide crystals that had been exposed to the atmosphere for days to months to determine reaction rates. They found that carbon dioxide was absorbed more slowly over longer periods of time due to the reactive layer that formed on the surface of the magnesium oxide crystals.

"This reactive layer is a complex mixture of different solids, limiting the ability of carbon dioxide molecules to find fresh magnesium oxide to react with. To make this technology cost-effective, we are now working on ways to overcome this armor effect," said ORNL's Juliane Weber, a principal investigator on the project. "If we can do that, this process may be able to achieve the goal of a carbon-negative Earthshot, which is to capture gigatons of carbon dioxide from the air for less than $100 per metric ton of carbon dioxide."

Most previous studies aimed at understanding how quickly magnesium oxide and carbon dioxide react chemically relied on back-of-the-envelope calculations rather than material testing. The ORNL study marks the first decades-long test to determine the speed of responses over long periods of time. Using transmission electron microscopy at ORNL's Center for Nanomaterials Science (CNMS), the researchers found that a reactive layer had formed. This layer consists of a complex variety of crystalline and amorphous hydrated and carbonate phases.

"In addition, by doing some reaction transport model computer simulations, we determined that as the reaction layer forms, it better prevents the carbon dioxide from finding new magnesium oxide to react with," said Vitaliy Starchenko, a researcher at ORNL. "So we are looking at how to bypass this process and allow the carbon dioxide to find new surfaces to react with."

Computer simulations help scientists and engineers understand how the reaction layer evolves and how the way matter moves through it changes over time. Computer models allow predictions of the reaction and movement of materials in natural and engineered systems such as materials science and geochemistry.

Compiled source: ScitechDaily