Elemental boron is an effective photothermocatalyst for the conversion of carbon dioxide

April 11, 2017
Elemental boron is an effective photothermocatalyst for the conversion of carbon dioxide

A "self-heating" boron catalyst that makes particularly efficient use of sunlight to reduce carbon dioxide (CO2) serves as a light harvester, photothermal converter, hydrogen generator, and catalyst in one. In the journal Angewandte Chemie, researchers introduce a photothermocatalytic reaction that requires no additives beyond water. This could form the basis of a new, more efficient process for converting the greenhouse gas CO2 into a useful carbon source for the production of fuels and chemical products.

The ideal route for making CO2 useful is considered to be reduction aided by a photocatalyst to use sunlight as the only source of energy—a process that corresponds to the first step of photosynthesis. Despite decades of research, processes for converting CO2 are still too inefficient. "This is largely due to the insufficient utilization of solar light, the high energy barrier for CO2 activation, and the sluggish kinetics of the multiple electron and proton transfer processes," explains Jinhua Ye.

Working with a team for the National Institute for Materials Science (NIMS) in Tsukuba, Ibaraki, and Hokkaido University in Sapporo (Japan), as well as Tianjin University and Nanjing University of Aeronautics and Astronautics (China), Ye is now pursuing a strategy that uses both the light and thermal energy provided by sunlight. When the sun shines on a surface, it is heated. The researchers want to use this ordinary photothermic effect to increase the efficiency of catalytic systems. Their material of choice is powdered elemental boron, which very strongly absorbs sunlight and efficiently converts it photothermically, heating itself up remarkably. This allowed the team to carry out the efficient reduction of CO2 to form carbon monoxide (CO) and methane (CH4) under irradiation in the presence of water, with no additional reagents or co-catalysts.

Irradiation causes the boron particles to heat up to about 378 °C. At this temperature it reacts with water, forming hydrogen and boron oxides in situ. The boron oxides act as "traps" for CO2 molecules. The hydrogen is highly reactive and, in the presence of the light-activated boron catalyst, efficiently reduces the CO2 by providing the necessary protons (H+) and electrons.

"The key to our success lies in the favorable properties of the powder, which make it an all-in-one catalyst: light harvester, photothermic converter, hydrogen source, and ," says Ye. "Our study confirms the highly promising potential of a photothermocatalytic strategy for the conversion of CO2 and potentially opens new vistas for the development of other solar-energy-driven reaction systems."

Explore further: Hydrogen from sunlight—but as a dark reaction

More information: Guigao Liu et al. Elemental Boron for Efficient Carbon Dioxide Reduction under Light Irradiation, Angewandte Chemie International Edition (2017). DOI: 10.1002/anie.201701370

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9 comments

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Mark Thomas
not rated yet Apr 11, 2017
Potentially useful on Mars? The methane end product certainly would be, at least as rocket fuel.
gkam
1.8 / 5 (5) Apr 11, 2017
That requires an oxidizer.

Simply splitting the water into its constituents provides both fuel and oxidizer.
Mark Thomas
not rated yet Apr 11, 2017
Dr. Zubrin's two books, both titled, The Case for Mars, use a simple chemical reaction to separate O2 from atmospheric CO2, so I am not particularly concerned about that being doable. I just wonder how well the process mentioned above would function on Mars? It superficially sounds promising, but I realize there are a thousand ways the wheels can come off the cart (so to speak), so maybe it is not the best choice for any one of those reasons.
gkam
1.8 / 5 (5) Apr 11, 2017

I hope to still be around when we do send some brave people to Mars.
RealScience
5 / 5 (3) Apr 11, 2017
That requires an oxidizer.

Simply splitting the water into its constituents provides both fuel and oxidizer.


So does the reaction cited above - they left out a lot of details in the description...

The article is pay-walled, but the most likely balanced reaction can be worked out.
To get 4 hydrogens for the methane, and the two carbons (one for CO and one for CH4) needs two CO2s and two H2Os to start with, so there are five oxygens left over.

While it is possible that there are unlisted reactants (after all, oxygen as a product was not listed), or that ozone is produced, these are unneeded and unlikely. Oxygen atoms generally pair up, and simple doubling the reactants gives:
4*CO2 + 4* H2O = 2*CO + 2*CH4 + 5*O2.

In any case, there should be enough oxygen liberated to turn it all back into CO2 and H2O.

The main detail left out is the energy efficiency.
Mark Thomas
5 / 5 (3) Apr 12, 2017
Thanks for your thoughts on this RealScience. It makes a lot of sense to me that energy efficiency could make or break the desirability of this photothermocatalyst process.

The initial attraction here was the recited combination of functions, "light harvester, photothermal converter, hydrogen generator, and catalyst in one." It sounds promising to combine so many functions from a weight and component count perspective.
RealScience
5 / 5 (2) Apr 12, 2017
... "light harvester, photothermal converter, hydrogen generator, and catalyst in one." It sounds promising to combine so many functions from a weight and component count perspective.


And in something robust like elemental boron rather than some fancy/fragile organic or scarce/expensive ruthenium- or platinum-containing compound!

At even 10% efficiency in converting solar energy to fuel energy it would make captured-carbon fuels competitive with fossil fuels. These advances usually start out well below 1% efficient and it is year of hard work to claw each percentage point out in the lab, and then another struggle to make the system robust enough for outdoor use. But again the simplicity would help, so I am more optimistic than with most such processes.
gkam
1 / 5 (4) Apr 12, 2017

What is the partial pressure of CO2 on Mars?
RealScience
5 / 5 (4) Apr 12, 2017

What is the partial pressure of CO2 on Mars?


Roughly 600 Pa.
(Versus about 40 Pa on earth)

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