Converting CO2 into fuel

Scientists at Strathclyde University are investigating the feasibility of converting CO2 into a fuel by mimicking the photosynthetic system of green plants. The project, supported by catalyst specialist Johnson Matthey, will look at creating a sunlight-powered aerogel photocatalytic diode which will chemically reduce carbon dioxide, and convert it into fuels such as methanol (for fuel cells) and methane (for cooking). “The idea is you have two reactants, carbon dioxide and water, and you reduce the CO2 to methanol or methane and simultaneously oxidise water to oxygen,” said principal researcher, Professor Andrew Mills.

He admitted that the idea of reacting carbon dioxide with water, two of the major components used to extinguish fire, and turning them into a fuel seemed “a bit outrageous. The only things that usually can do that are green plants, which do it by using light. The plants do not produce methane or methanol, but a reduced form of carbon in the form of carbohydrates,” he said. While this is a clear difference between nature and the new technology, Mills said that the process could still be considered similar because there would be a reduction of CO2 triggered by sunlight. “What we are looking at is an artificial photosynthesis system, the basic component of which will be an aerogel. This has a high surface area that will allow it to carry out many reactions a square centimetre and will absorb sunlight and drive the reaction forward.” According to Mills, the aerogel diode will look like a thin, almost colorless slab and be extremely light. “You can make an aerogel appear solid, but in fact 99 per cent of that will be air. It is a honeycomb of very small nanoparticles, all joined up to form something that is almost as light as air. For example, I could have something the size of a printer made of an aerogel which could be put on the back of a fly,” he said.

Despite being so light, Mills said the aerogel has a number of special properties, such as being thermally tough. This means that it can withstand high temperatures — a Bunsen burner flame, for example, would have no effect on it — which would be useful if the diode were to be installed in a hot environment. The structure of aerogel also has certain advantages. “The honeycomb structure means that gases can get in them and you can get reactions inside them as well as on the surface,” he said. The team will explore semiconductor materials called photocatalysts to make up the aerogel diode. Titanium dioxide, a semiconductor commercially used on self-cleaning glass, is one example of a photocatalyst. Moreover, the scientists will put catalysts, such as copper metal deposits, on each side of the diode to oxidise water and encourage CO2 reduction. “A diode is something that allows effluence to flow only in one direction. The good thing about the diode we are constructing is that these reactions occur on different sides, so you do not produce mixtures. You should be producing pure methanol on one side and pure oxygen on the other,” said Mills.

With a boiling point just below that of water, Mills said that the methanol coming out of the diode would be easy to capture for use. “Because it is a gas phase, what we are hoping for is that as you produce these products [the oxygen and methanol] they will be driven off by the new reactants that come in, so in the effluent from the diode there should be a lot of methanol. This forms a liquid very easily, so even if it hits something quite cool, like a cool surface at room temperature, what you would expect is the methanol to just fall out,” he said. While the diode would benefit from being installed in systems where there is a high concentration of CO2, for example, at gas power stations, the scientists said the only condition is that there must be sunlight. “You would install the diode anywhere you would put silicon solar cells. Anywhere you think there is going to be sufficient light to generate a fuel, such as roofs,” said Mills. One of the main objectives of the project is to produce something that is scalable and useful for industry. NASA is said to use large-scale aerogel monoliths to capture small comet particles.