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Converting CO2 Back to Fuel

ELCAT uses catalysts within carbon nanotubes for the photoelectrochemical conversion of CO2 to hydrocarbon fuels.

Most of the work on reducing the concentration of anthropogenic carbon dioxide in the atmosphere is focused on either reducing the emissions from fossil fuel combustion or capturing and sequestering the resulting carbon dioxide. There is, however, a third possible path: the conversion of CO2 back to a hydrocarbon fuel.

In an invited talk at this week’s National Meeting of the American Chemical Society, Professor Gabriele Centi from the University of Messina provided an overview of an ambitious EU-funded project to use solar energy to power the photoelectrochemical gas-phase conversion of CO2 back to hydrocarbon fuels.

It is feasible to convert CO2 to fuel. There is still a long way to go to practical application, but it is a good and interesting direction to go.

—Prof. Gabriele Centi, University of Messina

There have been a number of attempts over the past decades to use solar energy to reduce carbon dioxide (CO2) and water (H2O) into a variety of products, including hydrogen and carbon monoxide for use as a syngas for further processing (e.g., Fischer-Tropsch) as well as direct hydrocarbon products.

Past efforts have found that the rate of recombination is not very high and productivity is very low, according to Prof. Centi. The products formed were lower carbon hydrocarbons—CH4 (methane) and CH3OH (methanol) for example. No hydrocarbon greater than C3 was obtained.

These aqueous phase processes found that the photoreduction of carbon dioxide was in competition with the formation of other reaction products, the formation of which would need to be blocked to develop higher carbon hydrocarbons—i.e., hydrocarbons closer to the liquid fuels used in most engines.

There were also a number of other limits on the processes. But not much had been done in exploring a gas-phase conversion.

The EU provided €875,246 ((US$1.1 million) in funding for ELCAT—electrocatalytic gas-phase conversion of CO2 in confined catalysts—a three-year project under the Sixth Framework Program (6FP) to focus on the gas-phase electrocatalysis of CO2 to Fischer-Tropsch (FT)-like products (C1-C10 hydrocarbons and alcohols). Work began in 2004.

The project was born from the observation that with carbon dioxide confined inside carbon micropores, and electrons and protons allowed to flow to an active catalyst of noble metal nanoclusters, that gaseous carbon dioxide was reduced to a series of hydrocarbons and alcohols. The reaction products were remarkably similar to those of the Fischer-Tropsch (FT) process in which synthetic gas is converted to a series of hydrocarbons (alkanes, alkenes and so on) and water.

Three organizations are involved in addition to the University of Messina, Italy: Fritz-Haber-Institut der Max-Planck-Gesellschaft in Berlin, Germany; Université Louis Pasteur in Strasbourg, France; and University of Patras in Patras, Greece.

The ELCAT approach confines the catalyst particles within carbon nanotubes. The catalyst particles need to be quite small, due to the fact of the high number of electrons that must be transferred to generate the higher hydrocarbons. The number of electrons required is quite high—on the order of 24 for a butanol product, and an average of 46 for C8 to C9.

There is no evolution of hydrogen in this process.

The ELCAT team has found that it is possible to produce higher carbon hydrocarbons (C8 to C9), with productivity depending upon a number of factors such as catalyst, electrolyte and flow rates.

As a closing note, Prof. Centi observed that in addition to its utility on Earth, such a process would be of use for Mars missions that could use Martian resources (CO2 and water) to produce propellant for Earth return as well as life-support consumables.




Might be that I'm missing the point, but this sounds very much like they are trying to reinvent photosynthesis.


Depending on how carbon trading pans out it might be more profitable to suck CO2 out of the atmosphere in a stable form and bury it. On the other hand a hydrocarbon fuel is only recycling CO2 as the previous commentator suggests. Hopefully the carbon nanotubes would last for many years since their manufacture probably involves fossil carbon.

richard schumacher

It is a little like re-inventing photosynthesis, but at a much greater energy density and much greater production rate than we can get with agriculture and thus without using millions of hectares of land. (While not the focus of this particular research effort one can also make artificial hydrocarbons using energy sources other than Sunlight.) "Only recycling CO2" from the atmosphere would be infinitely better than releasing billions of tonnes of fossil-derived CO2 as we do now. When we finally stop adding more CO2, natural processes which sequester carbon will eventually remove the excess.


Can I conclude that this process will require large amounts of energy, more or less the same amount as was released upon burning the fossil fuels that put the CO2 into the atmosphere? Then we're back to square one, because that is the big issue humanity is facing right now: clean energy. So if we somehow have large amounts of clean energy available to do the processes described above, then we can use that energy also to power our economies. Problem solved.

I really don't see what problem they are trying so solve.


This is what biodiesel and biobutanol/ethanol are about. Turning CO2 into fuel. That's a form of CO2 recycling. If you want to more permanently sequester the CO2 you've got to turn the CO2 into something distant than fuel.

Growing wood, then making permanent value products out of wood, is an excellent way of sequestering CO2. All that cellulose tied up in a quasi-permanent structure. Recent work by materials scientists in making car panels and other structural products from wood is intriguing.

We're not going to stop adding more CO2 for decades, especially with India and China. No, we've got to learn to sequester CO2.


Seems like nothing more than an exercise for researchers


This is what you call enviromentally benign fuels and chemicals that basically incorporate man into the carbon cycle. Rather than have man pump more carbon into the atmosphere, it is theoretically and technically feasible to use carbon dioxide directly to produce methanol or other lower chemicals then convert these into higher products. If you heat municipal solid waste at 1000 degrees in a carbon dioxide environment you get a gas that is predominantly carbon monoxide; carbon monoxide can then be reoxidized to operate turbines, engines, make fuels through FT synthesis and the enflluent can undergo the process infinately.


Can I conclude that this process will require large amounts of energy, more or less the same amount as was released upon burning the fossil fuels that put the CO2 into the atmosphere?

The idea is to use solar power.

I really don't see what problem they are trying so solve.

The hope is that economies of scale and technological improvements will make it at least competitive with if not better than sticking a seed in the dirt. You can also tune the output so that you get CH4, CO, C2H4, whatever directly without further biomass -> output processing.

So far, we're nowhere close. We're not even nowhere far. The more you understand green plant photosynthesis, the more mind-bogglingly miraculous it becomes.


I think the problem that is being addresses is the storage and transportation of energy. The sun doesn't shine and wind doesn't blow when and where you need energy. You produce some energy product when and where it is best. Then you take it to where it will be used.


Let's see, you replace fossil fuel with a renewable fuel, and some of you don't understand what the excitement is about?

Dang, perhaps we are doomed...

Hydrogen Fan

I have question. Would it not be cheaper and a far quicker process to suck the billions of tonnes of CO2 hovering in the atmoshere, liquify it, and then bury it in the dried up oil wells from where it came. Seal the oil wells and be done with it?

G. R. L. Cowan, former hydrogen fan

To be really useful, the process would need to convert CO2 and water into C8-9 hydrocarbons and oxygen. If oxygen is not being freed, it is not at all like photosynthesis.

An Engineer

Hydrogen Fan,
Where would the energy come from to do all those wonderful things? Oil? Coal? Natural gas? (Get it?) Do you have any idea how much energy is required to "suck" the CO2 out of the atmosphere (concentration <<0.1%), liquify it (won't it form dry ice first?) and then pump it underground? But then again, as a hydrogen fan, thermodynamics can't be high on your list of specialties...

GRL Cowan,
Why oxygen too? At least you came to your senses about hydrogen...

Tom Catino

I like the CO2 Bioreactor MUCH BETTER...
and the technology is ready NOW...


Once I read about DOE envelope of R&D of so called advanced photovoltaic. The idea was to incorporate in single solar panel PV electricity generation, overnight energy storage, and ultimately elements for synthesis of hydrocarbon fuel. As I remember numbers right, they stated that energy conversion of natural photosynthesis is about 2% at some record plant (by the way sugar cane is pretty close to this limit), so artificial hydrocarbons synthesis should be way more effective that natural photosynthesis in order to compete with regular PV with about 20% of conversion efficiency. BTW, this 2% max efficiency – to total carbon mass, not only for small part we use to extract sugars for ethanol production or oil for biodiesel – made me big skeptic of feasibility of energy crops. Waste derived fuel, like corn stove ethanol – different matter.

shaun mann


plants are better at this and contribute to the economoy in a meaningful manner. if we really want to sequestor carbon, why don't we suck it out of the air and convert it to a stable form (by growing plants), then sequestor it in a proven manner (such as land fills that have been proven to prevent decomposition for decades). or, some people have compared improving soil by encouraging organic matter as a way to sequestor carbon.


>> I like the CO2 Bioreactor MUCH BETTER...

There's also or growing the algea at city sewage plants. These offer factories and cities, respectively, monetary incentive as well. Very space efficient.

Having lots of options is good.


I've googled around a bit, and it seems that the maximum efficiency of photosynthesis is 11%. In practice it will be around 3% and 6%. Source:

So indeed, setting up an artificial process could yield more biofuel per sq m than plants. Advantage is that you can place these solar power plants in deserts where the sun always shines and no plants can grow. But the costs will be staggering. Imagine, say 1.000.000 sq km of photovoltaic cells. Who's gonna pay for that?

@ dt:
using solar power to do the process seems a bit odd. I think it is more efficient to use the solar power directly to power our society. When renewable sources cover all our needs, we can begin thinking about using any surplus energy to do CO2 sequestration.

"Let's see, you replace fossil fuel with a renewable fuel, and some of you don't understand what the excitement is about?"

Those are two completely different things you are mentioning. It is not the idea of creating a renewable fuel, it is the way these scientists are thinking to produce it. That is creating the polemic. See it as a positive sign of the opposite: everybody is very much interested in renewable energy.

richard schumacher

Finally, it is dawning on people that we cannot replace all fossil-derived vehicle fuels with biofuels: there simply isn't enough land available for it, even if we were to plow under every rainforest, prairie, and park in the world and replace them with oil palm and sawgrass farms. Making vehicle fuels from coal releases more greenhouse gas and further worsens global warming, unless CO2 sequestration can be shown to work on a usefully large scale. Lacking that, and without some enormous improvement in energy storage, part of the solution will have to be making vehicle fuels from atmospheric CO2 and .

richard schumacher

[D'ohh!! Stupid HTML tags. The last post should end with "insert your favorite non-fossil carbon-neutral energy source here".

Paul Dietz

Where would the energy come from to do all those wonderful things? Oil? Coal? Natural gas? (Get it?) Do you have any idea how much energy is required to "suck" the CO2 out of the atmosphere (concentration <<0.1%), liquify it (won't it form dry ice first?) and then pump it underground? But then again, as a hydrogen fan, thermodynamics can't be high on your list of specialties...

As a supposed expert in thermodynamics, you should realize that the theoretical minimum energy needed to concentrate a dilute gas increases only slowly (logarithmically) with dilution. The energy needed to further compress and pump the CO2 after it has been separated is also a small fraction of the chemical energy liberated when the CO2 was formed. So it's not obviously absurd to think about schemes for extracting CO2 from the air, although clearly practice and theory are not the same.

Klaus Lackner at Columbia has been looking into atmospheric CO2 extraction; he points out that the combustion energy density represented by the CO2 in the wind at a typical location is orders of magnitude higher than the wind energy itself. He points out that while sequestration of concentrated CO2 from central sources will always be easier, there will also be leakage and distributed sources, so atmospheric capture would still be a good thing to have.

Oh, and refamiliarize yourself with the phase diagram of CO2, mister. Compressed CO2 is often handled in liquid form. It doesn't form a liquid at 1 bar, but then we wouldn't be injecting it into the ground at 1 bar.


I think it is more efficient to use the solar power directly to power our society.

Copy that! I think this industrial photosynthesis is worth following up on at some level, but I'm not at all hopeful for the near term, and much will have to be shown to convince me for the longer term.


You guys know nuclear power is the real deal, right?

G. R. L. Cowan, former hydrogen fan

Uranium has been being consumed at a rate close to 30 billion barrels, oil equivalent, per eight years in the last eight years, and in that time, the IAEA Red Book says, reserves have gone from 330 billion BOE to 470 billion BOE. The actual amount in the Earth, mostly as near its surface, proportionally, as the skin of an apple, is 3500000000 billion BOE. It is abundant on a many-centuries-to-many-millennia timescale.

So yes, we know it's the real deal. It wouldn't be so controversial if it were not.

Rafael Seidl

Anne -

"Imagine, say 1.000.000 sq km of photovoltaic cells. Who's gonna pay for that?"

Considering sunshine is free of charge, the investment decision would have to be based on the alternative: 20+ years of buying fossil fuels, funding military protection to ensure a ready supply and, funding global warming mitigation projects. On that basis, solar would already be competitive today. Consider the hundreds of billions of dollars spent/wasted to "bring democracy to Iraq". After all, decades of Western support for repressive regimes flush with petrodollars have arguably contributed to the rise of al Qaeda and WMD programs (real or imagined).

The big problem with solar-derived electricity is that it's very hard to buffer the energy collected during the daytime to meet nighttime demand. Sure, some can be buffered in existing, fully amortized hydro dams. Some PHEV advocates call for charging up the high-capacity batteries during the workday.

However, using solar energy to produce a chemical energy carrier, e.g. hydrogen or preferably, a liquid hydrocarbon fuel, would more completely eliminate the circadian and logisitics drawbracks of solar power.

For now, biotechnology - including intensive biofuel farming based on algal oil - seems a promising route to achieve this technologically. But I've learnt not to underestimate the capacity of materials scientists and chemical engineers to pull a rabbit out of hats. Relying on biology comes with significant drawbacks. Note that either way, your inputs are CO2, H20 and solar energy.

As for carbon sequestration, the CO2 concentration in the oceans is two orders of magnitude greater than that in the atmosphere. If you could reduce ocean concentrations even just a little, e.g. by growing vast quantitites of limestone using solar power an let that sink to the ocean floor, plain old diffusion would eventually reduce that in the atmosphere as well. Currently, the biorock production process still requires a metal scaffolding, though.

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