Converting CO2 Back to Fuel
14 September 2006
|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.
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