My money's on hydrocabrons created from CO2+H2O when there's enough power available. You could use these in any internal combustion engine on the market right now.
Well, if you have the cheap energy....
NewScientist 16th September 2006
IT IS the biggest contributor to climate change. Now chemists are hoping to convert carbon dioxide into a useful fuel, with a little help from the sun.
If they succeed, it will be possible to recycle the greenhouse gas produced by burning fossil fuels. The work could also lead to a way for future Mars missions to generate fuel for their return journey from carbon dioxide in the planet's atmosphere.
Chemists have long hoped to find a method of bringing the combustion of fuel full circle by turning CO2 back into useful hydrocarbons. Now researchers at the University of Messina in Italy have developed an electro-catalytic technique they say could do the job. "The conversion of CO2 to fuel is not a dream, but an effective possibility which requires further research," says team leader Gabriele Centi.
The researchers chemically reduced CO2 to produce eight and nine-carbon hydrocarbons using a catalyst of particles of platinum and palladium confined in carbon nanotubes. These hydrocarbons can be made into petrol and diesel.
To begin with, the researchers used sunlight plus a thin film of titanium dioxide to act as a photocatalyst to split water into oxygen gas plus protons and electrons. These are then carried off separately, via a proton membrane and wire respectively, before being combined with CO2 plus the nano-catalyst to produce the hydrocarbons.
Although the nano-catalysts produced two or three times more hydrocarbons than a commercially available catalyst, the process converted only about 1 per cent of the CO2 at room temperature. Centi believes it will be possible to improve on that by using higher temperatures and a larger surface area of catalyst. It will also be necessary to boost the efficiency of the solar water-splitting, he says. With the right research, Centi believes that an efficient solar-powered reactor for converting CO2 into fuel could be available "within a decade".
He presented his latest work, which is funded by the European Union, at a meeting of the American Chemical Society in San Francisco on 13 September. Other chemists reacted positively, but cautiously, to the findings. "It sounds feasible," says John Turner from the US National Renewable Energy Laboratory in Golden, Colorado. "The solar-to-hydrocarbon conversion efficiency is pretty small, but it sounds like they are just getting started."
Ian Plumb, who researches water-splitting reactions at the Australian national research institute CSIRO Industrial Physics, says that unless the efficiency is improved it will be too expensive to implement. "But there is no doubt that what they are trying to achieve is very worthwhile."
New scientist 18th February 2009
Powered only by natural sunlight, an array of nanotubes is able to convert a mixture of carbon dioxide and water vapour into natural gas at unprecedented rates.
Such devices offer a new way to take carbon dioxide from the atmosphere and convert it into fuel or other chemicals to cut the effect of fossil fuel emissions on global climate, says Craig Grimes, from Pennsylvania State University, whose team came up with the device.
Although other research groups have developed methods for converting carbon dioxide into organic compounds like methane, often using titanium-dioxide nanoparticles as catalysts, they have needed ultraviolet light to power the reactions.
The researchers' breakthrough has been to develop a method that works with the wider range of visible frequencies within sunlight.
The team found it could enhance the catalytic abilities of titanium dioxide by forming it into nanotubes each around 135 nanometres wide and 40 microns long to increase surface area. Coating the nanotubes with catalytic copper and platinum particles also boosted their activity.
The researchers housed a 2-centimetre-square section of material bristling with the tubes inside a metal chamber with a quartz window. They then pumped in a mixture of carbon dioxide and water vapour and placed it in sunlight for three hours.
The energy provided by the sunlight transformed the carbon dioxide and water vapour into methane and related organic compounds, such as ethane and propane, at rates as high as 160 microlitres an hour per gram of nanotubes. This is 20 times higher than published results achieved using any previous method, but still too low to be immediately practical.
If the reaction is halted early the device produces a mixture of carbon monoxide and hydrogen known as syngas, which can be converted into diesel.
"If you tried to build a commercial system using what we have accomplished to date, you'd go broke," admits Grimes. But he is confident that commercially viable results are possible.
"We are now working on uniformly sensitising the entire nanotube array surface with copper nanoparticles, which should dramatically increase conversion rates," says Grimes, by at least two orders of magnitude for a given area of tubes.
This work suggests a "potentially very exciting" application for titanium-dioxide nanotubes, says Milo Shaffer, a nanotube researcher at Imperial College, London. "The high surface area, small critical dimensions, and open structure [of these nanotubes] apparently provide a relatively high activity," he says.
New scientist 27th February 2008
CARBON dioxide is the devil molecule of our time. Belched out from vehicle exhausts and power stations, it is the biggest contributor to global warming. As such it is universally recognised as a Bad Thing. Yet a pioneering band of researchers would like us to see it differently - as a valuable resource. They are developing a collection of technologies to retrieve some of the CO2 that would otherwise pollute the atmosphere, using its carbon atoms to form hydrocarbons. These could then be used as vehicle fuel, or as a feedstock to make plastics and other materials we now derive from oil. So could the expanding clouds of CO2 in our atmosphere really have a silver lining?
The idea is simple. Find a way of removing an oxygen atom from a CO2 molecule and you are left with carbon monoxide (CO). From there it is but a short step to hydrocarbon riches. Mix CO with hydrogen, pass the mixture over a catalyst, and out comes liquid hydrocarbon fuel. This reaction, called the Fischer-Tropsch process, was invented as long ago as the 1920s. It was used by Germany during the second world war, when oil was in short supply, to make petrol from gasified coal, and apartheid-era South Africa did the same when sanctions blocked oil imports.
The hard part is the first step: finding a cost-effective energy-efficient way of creating CO from CO2. The simplest route is to heat CO2 molecules to around 2400 °C, at which point they spontaneously split into CO and oxygen. The problem is finding the energy to do this.
One obvious candidate is sunlight. Los Alamos Renewable Energy (LARE), a company based in Pojoaque, New Mexico, has built a small-scale prototype reactor that demonstrates how it can be harnessed. In the LARE reactor, CO2 is fed into a reaction chamber that is sealed at one end by a quartz window 8 centimetres in diameter. The chamber is fixed at the focal point of a mirrored dish that concentrates sunlight through the chamber's window onto a ceramic rod set inside the chamber to collect the heat. As the gas comes into contact with the rod its temperature rises to around 2400 °C, causing the molecules to break down and release CO and oxygen. Reed Jensen, LARE's managing director, says a larger prototype reactor will be ready for trials in a year's time, though he is not saying how big this reactor will be, nor how much CO it will produce.
One of the drawbacks of this approach is the high operating temperature, says Nathan Siegel of Sandia National Laboratories in Albuquerque, New Mexico, where a rival team is at work. High temperatures lead to heavy thermal losses, which in turn can reduce efficiency. Though the sun's energy is free, equipment to generate and withstand these temperatures is expensive to build, making efficient operation vital if the process is to be cost-effective.
With this in mind, the Sandia team is developing a rival system known as CR5 (short for counter-rotating ring receiver reactor recuperator) which operates at less extreme temperatures. Like the LARE reactor it has a concentrator dish that focuses the sun's rays. In this case, the high temperatures are generated on one side of a stack of 14 rings made of a cobalt ferrite ceramic, a material that when heated releases oxygen from its molecular lattice without destroying the lattice's integrity. The rings, which are about 30 centimetres in diameter, rotate at around one revolution per minute inside a sealed double chamber. Sunlight focused through a window in the hot side of the chamber heats the rings to 1500 °C, causing the ceramic lattice to liberate oxygen atoms. As the rings rotate, the heated section passes to the rear of the chamber, where it cools to around 1100 °C as it is bathed in CO2. At this temperature the deoxygenated ceramic reacts with the CO2 molecules to grab back the oxygen atom missing from its lattice, leaving behind a molecule of CO.
As the ring continues to rotate, the reoxygenated section passes back into the hot side of the chamber and the cycle begins again. Proper heating and cooling of the rings is crucial to the operation of the process. On the heated side, the rings must reach the correct temperature for the ceramic to liberate oxygen, and they must cool by several hundred degrees by the time they reach the cool side in order to react with the CO2. To help achieve this, alternate rings rotate in opposite directions, so as the hot section of each ring moves towards the cool side of the chamber it is cooled by neighbouring rings moving in the opposite direction. Both hot and cooler sides of the chamber are maintained at equal pressure to minimise the flow of gases between them.
The CR5 was originally developed as a way to produce hydrogen, using steam in the cool chamber rather than CO2, but its inventor, Rich Diver, reckoned that splitting CO2 would offer a more efficient way of capturing solar energy. Burning the CO formed in the solar reactor should deliver 10 per cent of the energy that was required to produce it, and in April he and his colleagues will switch on a prototype reactor to put their predictions to the test. They calculate it should be able to produce about 100 litres of CO per hour.
Reversing the fuel cell
The idea of using solar energy to convert CO2 into a carbon-based fuel is being taken a step further by Gabriele Centi at the Department of Industrial Chemistry and Engineering of Materials at the University of Messina, Italy. Rather than producing CO with a view to turning that into something more useful, he is building an electrochemical cell that produces hydrocarbon molecules such as nonane and ethylene - important chemical building blocks for plastics and other materials currently derived from oil.
Centi's cell is a distant cousin of the fuel cells that generate electricity by reacting hydrogen or methanol with oxygen, but with the chemical reaction running in reverse. On one side of the cell is a titanium dioxide catalyst that encourages water molecules to split when hit by photons of sunlight, producing hydrogen ions and oxygen gas. The hydrogen ions migrate through a proton exchange membrane to the other side of the cell, where a catalyst containing platinum nanotubes facilitates the reaction with CO2 to produce hydrocarbons.
The energy that would be liberated by using these hydrocarbons as fuel amounts to just under 1 per cent of the solar energy needed to produce it. This may not seem like much but it's better than the energy conversion rate that plants achieve through photosynthesis, and Centi says there is room for improvement by tweaking the catalysts.
So how do these technologies stack up against biofuels as a way of using solar energy to capture atmospheric carbon and turn it into fuel? Ellen Stechel, manager of Sandia's Fuels and Energy Transitions department, estimates that enough CR5 plants to fuel 100 million domestic vehicles with synthetic gasoline could be accommodated on about 5800 square kilometres of land. "That's actually not very much," she says. A recent survey of seven states in the US Southwest revealed that more than 135,000 square kilometres of suitable land were available there. "This is land that's not being used for anything else," she says.
By contrast, biofuels compete with food crops for fertile land. What's more, the percentage of the solar energy that is available from the fuel is staggeringly small - about 0.1 per cent if you take into account the irrigation, harvesting, transportation and refinery process, Stechel says.
To make the most of the available land, Jensen suggests coupling LARE's carbon capture reactor with an electricity generating station that would use the heat wasted by the reactor itself. He reckons the combined installation could convert as much as 48 per cent of the solar energy into usable energy.
As oil and natural gas become more expensive and scarce, petrochemical companies are increasingly interested in finding new raw materials to replace them. Centi is now working with one French firm to explore the use of recycled CO2 to meet this demand, though he refused to name the company. If competitively priced, hydrocarbons produced from industrial sources of CO2 could one day be used to make plastics and other products, where it would remain fixed for years rather than being pumped out into the atmosphere. The devil molecule may yet redeem itself.