One ceramic, two jobs: sodium manganates that trap CO₂ and burn CO
Updated: Oct 15
Before my postdoc at SLAC, I spent a few months at the Instituto de Investigaciones en Materiales (IIM) of UNAM, in Mexico City, in the group of Dr. Heriberto Pfeiffer. The materials were familiar to me. Sodium manganates such as NaMnO₂ are studied as cathodes for sodium-ion batteries. The question was not: can they clean a gas stream?
Why trap carbon oxides with a ceramic?
Most of our energy still comes from burning fossil fuels, and CO₂ is the main greenhouse gas they release. One way to reduce it is to capture it before it reaches the atmosphere. At high temperature, ceramics that contain an alkali metal (lithium or sodium) do this well. Their surface is basic, so it reacts with acidic gases such as CO₂ and keeps them as a solid carbonate.
Lithium ceramics capture a lot of CO₂, but lithium has become expensive, partly because batteries need it. Sodium is more abundant and cheaper, so sodium ceramics are a natural alternative.
Some of these materials can do a second job. Besides capturing CO₂, they can turn carbon monoxide (CO) into CO₂ (CO oxidation). A material that does both is called bifunctional. It is useful in processes such as syngas cleaning, where CO and CO₂ have to be removed together.
What did we make?
We prepared sodium manganates by two routes and compared them.
Co-precipitation (CPM): sodium and manganese are mixed in solution, so they meet at the ionic level. The product was 78 wt% α-NaMnO₂ and 22 wt% Na₂Mn₃O₇.
Solid-state reaction (SSM): the two oxides are ground together and heated. Part of the manganese oxidized on the way, and the proportions came out reversed, about 23 wt% α-NaMnO₂ and 77 wt% Na₂Mn₃O₇. A version with 15% extra sodium gave almost the same result.
Both routes ended with the same heating steps, up to 940 °C, so the main difference between the samples is the ratio of the two phases.
How much did they capture?
We followed the capture by thermogravimetry. The sample sits on a balance while it is heated in a gas, and it gains weight as it captures.
Under pure CO₂ the co-precipitated sample gained up to 9.5 wt% at 598 °C. The solid-state samples gained only about 2 wt%. Under CO the difference was similar: 10.4 wt% at 816 °C for CPM, against 1.1–1.4 wt% for the solid-state samples.
That last result is the one I find most interesting. Among the sodium ceramics reported so far, sodium manganate is the only one that captures CO without oxygen in the gas. The CO takes oxygen from the crystal itself, and the manganese is reduced.
The isotherms showed two stages. At 500 °C the sample captured 5.3 wt% in less than 5 minutes. This fast stage is a reaction at the surface, which forms a shell of sodium carbonate. After that, sodium and oxygen have to diffuse from the inside of the particle through that shell, and the capture slows down. The Avrami–Erofeev kinetic model describes both stages.
And the CO oxidation?
With a mixture of CO and O₂, the co-precipitated sample converted about 65% of the CO to CO₂ between 300 and 450 °C, and about 95% above 550 °C. The solid-state samples only started to work at about 400 °C.
The oxidation follows a Mars–van Krevelen mechanism. CO reacts with an oxygen atom of the crystal surface and leaves a vacancy, and the oxygen in the gas fills it again. This also explains why the material keeps converting some CO when there is no oxygen in the gas.
What does each phase do?
Comparing the samples shows a division of work.
α-NaMnO₂ gives the capacity. It captures more CO₂ and keeps forming carbonate for longer.
Na₂Mn₃O₇ activates at lower temperature, especially for CO.
To test this, we mixed equal parts of the two samples. The mixture started converting CO at about 300 °C instead of 400 °C, and reached about 90% conversion from around 400 °C. Neither sample alone did both.
What does it mean?
For these materials, the synthesis route matters as much as the composition, because it sets the ratio of the two phases. A sorbent-catalyst for a given process can be tuned by choosing that ratio.
The open questions are cycling stability (how many capture and release cycles the material survives) and a kinetic model for CO in the presence of O₂, which the usual models could not describe.
Full paper: E. Espinosa-Villatoro, D. G. Araiza, H. Pfeiffer, "Exploring the capture and catalytic properties of sodium manganates: bifunctional ceramics for CO/CO₂ chemisorption and CO oxidation", Journal of Environmental Management 401 (2026) 129026. DOI: 10.1016/j.jenvman.2026.129026
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