Reversing the process of combustion may beco₂, and not oxygen, as its input. Over millennia, carbon got stored as vegetation, and then as coal underground, while the atmosphere received oxygen, for living things to flourish. And it happened thanks to the energy from the sun.

It was when humans discovered fire that it became possible to carry stored energy, in the form of wood, or later, as coal, to where it was needed, and this fuelled the growth of civilization The crisis created by CO2 pumped so back into the atmosphere is common knowledge. The worldwide effort, now, apart from containing generation, is to capture CO₂ and put it away.

Dorna Esrafilzadeh, Ali Zavabeti, Rouhollah Jalili, Paul Atkin, Jaecheol Choi, Benjamin J. Carey, Robert Brkljac, Anthony P. O’Mullane, Michael D. Dickey, David L. Officer, Douglas R. MacFarlane, Torben Daeneke and Kourosh Kalantar-Zadeh, from the Universities, of New South Wales, Melbourne, Wollongong, the Queensland University of Technology and Monash University, in Australia, Nanjing University of Aeronautics, and Astronautics, China and University of Münster, Germany, write in the journal, Nature Communications, that they have tested a low temperature process of converting CO2 back to carbon. This would have great potential, where untapped, non-polluting energy sources like wind and solar could be quietly reducing the carbon content of the atmosphere.

The reason that burning carbon gives off heat is that the energy state of the carbon and oxygen atoms, together, as they are in CO₂, is lower than wh,en the atoms are separate. There is thus surplus energy when the atoms combine, and this is given off as heat. It is like water at higher altitudes can do work, like generating electricity, while it flows down to a lower altitude. We can imagine that at least equal work needs to be done to raise the water back up. In the same way, it takes energy to split the CO2 molecule to get back to free carbon and oxygen.

In fact, it takes a lot more than just the energy difference, because the low energy state of free carbon and oxygen is hidden behind a barrier of even higher energy, and it takes high temperatures before the separation can happen. A great many chemical processes, manufacture of fertilizer, for instance, are similar, high temperature processes. The energy for these generally comes from burning of coal. If recovery of carbon from CO₂ were to join the list, we can see that there would be nothing gained.

Photosynthesis, in green plants, gets around the high temperature demand with the help of chlorophyll. Chlorophyll provides an intermediate energy state, a by-pass route, to use energy from sunlight to go from the low energy state of CO₂ to the higher energy state of hydrocarbons, without the need for high temperature. There is much research going on to mimic the way of plants, of ‘fixing’ CO₂. with the help of sunlight. One such is the scaffold of titanium dioxide and silicon nanowires, with the help of a specific bacterium, which breaks the CO₂ molecule using electrons that sunlight ejects from silicon.

The work of the group writing in Nature Communications is of using electricity to reduce CO2, at room temperature, with the help of a metallic catalyst, or agent that helps reactions get going in the same way as chlorophyll does in green plants. The catalyst used is a soft, silvery-white metal, cerium, whose oxide has surfaces which act as landing stages for chemical conversions. A reaction of interest is that the oxide, Ce₂O₃ changes to CeO₂, or an increase in the level of oxidation, accompanied by reduction of CO₂, to carbon monoxide (CO) or carbon.

The trouble with using a coat of cerium on a metal surface, to support use of electricity to reduce CO₂ to carbon, is that the carbon particles adhere to the active surface and block the access of the medium that contains CO₂ ., a process known as coking. The surface gets rapidly coated with carbon and the conversion stops.

The team has dealt with this by two devices. The first is that the cerium is in the form of nanoparticles and the second is that the particles are not coated on a metal surface, but are suspended in a metal in liquid form. A common metal that is ordinarily a liquid is mercury. Another such, which is more active, is gallium, a metal that melts at 29.76°C. Gallium is thus solid on a cool day, but would melt at body temperature, if we held it in our hand.

Now, the reason that flakes of carbon, which are formed at the surface of a metal, stick to the surface is that there are weak forces, called Van der Waals forces, which act very near the surface. When the metal itself is in liquid form, there are no surfaces and hence no forces, and the phenomenon of coking does not take place. The paper in Nature Communications says that trials with liquid gallium, saturated with cerium, with dissolved CO₂, readily passed electric current, with very efficient conversion of CO₂ to solid carbon, all at room temperature.

The strength of the current that passed was seen to indicate the extent of the electrolytic action. As the quantity of the products were too small for reliable measurement, it was this current that served to measure the efficacy of the catalyst. And it was found, paper says, that a very substantial current was supported by the cerium in the liquid metal. To eliminate other reasons for the flow of the current, there was a control experiment with nitrogen gas taking the place of CO₂, and it was confirmed that it is the breakdown of CO₂ that gives rise to the current.

To study the mechanism of the specific exchanges that took place at the cerium nanoparticle surface, the particles were bathed in laser light and Raman Effect changes in the scattered light were examined. The Raman Effect is the slight changes that come about in photons of light scattered by molecules of a substance. Molecules consist of atoms connected by electrical forces, and they vibrate with specific frequencies. The energy that a molecule would absorb or impart to a scattered photon is thus characteristic. Observing the scattered frequencies during the catalytic action provided a sensitive window to identify molecules in the process, and the action of cerium to reduce CO₂ was established.

The process of reducing CO₂ to carbon, or reversing the process of burning, will always consume energy, as it is the CO₂ state that is at the lower level. The methods available so far, however, have involved very high temperatures, calling for high energy use. The work now reported suggests that a room temperature method, which can be sustained by renewable energy sources, even off-grid sources, which are not available for other applications, could be the means of extracting carbon from the atmosphere. If it is not just carbon, for storage, but other chemicals that are produced, the method could be regarded as a negative carbon technology, the paper says.

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