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July 2012

Redefining Benchmarks for Carbon Dioxide Conversion to Fuel

New molecular catalyst boosts fuel production efficiency from carbon dioxide

Ralph L. House

Proposed mechanism for electrocatalytic CO2 reduction in 5 percent H2O/acetonitrile.

A new molecular catalyst orchestrates a chemical reduction process where, by definition, electrons react with CO2 to form the molecule formate (HCOO-), which can serve as a fuel or a precursor to fuels such as methanol. The catalyst is highly efficient, creating formate 85 percent of the time at an unprecedentedly fast rate, twenty CO2 molecules converted per second, which is 380 percent greater than previous reports. Designed by the Solar Fuels Energy Frontier Research Center at the University of North Carolina at Chapel Hill, the catalyst contains a central iridium atom that drives the reduction reaction and sets a new standard for converting CO2 to formate.

Water Plays a Key Role: When the catalyst is placed in acetonitrile with 5 percent H2O, CO2 reduction occurs at about -1.2 V vs a normal hydrogen electrode. This relatively low potential is partially because water stabilizes, via a process called solvation, the negatively charged formate product, generating a positively charged iridium intermediates that is easier to reduce. This increases the catalytic rate and lowers the amount of energy required to reduce the CO2.

Why Formate? To become a primary source of energy, solar radiation will need to be stored on massive scales. The only realistic option is storing the energy in chemical bonds with the energy of the sun used to carry out chemical reactions that produce "solar fuels" for storage and later use when the sun is unavailable. Formate is a desirable target because it can serve multiple purposes, either as a fuel in its own right, a material to store hydrogen fuel or as a precursor to methanol fuel production.

Until now, catalytic reduction of CO2 to formate using molecular catalysts was a largely inefficient process often accompanied by byproducts such as carbon monoxide and hydrogen, leading to poor overall yields. Hydrogen is an especially common byproduct because water is much easier to reduce than CO2. The iridium catalyst described in this work overcomes many of these hurdles, making formate a now viable option towards the realization of an alternative energy economy.

More Information

Kang P, C Cheng, ZF Chen, CK Schauer, TJ Meyer, and M Brookhart. 2012. "Selective Electrocatalytic Reduction of CO2 to Formate by Water-Stable Iridium Dihydride Pincer Complexes." Journal of the American Chemical Society 134(12):5500-5503. DOI: 10.1021/ja300543s

Acknowledgments

The UNC EFRC: Center for Solar Fuels, an Energy Frontier Research Center funded this work. The center is funded by the U.S. Department of Energy, Office of Basic Energy Sciences. The research team also acknowledges the National Science Foundation for supporting two researchers as part of the Center for Enabling New Technologies through Catalysis.

About the author(s):

  • A member of the Solar Fuels Energy Frontier Research Center, Ralph is a Research Associate specializing in the use of multiple spectroscopic techniques to analyze the steps leading to the generation of solar fuels. Ralph is also involved with coordinating projects leading to the fabrication of a prototype solar fuels device and is the UNC-EFRC Liaison for External Outreach and Collaboration.

Designer Molecule Turns Pollutant into Fuel

New iridium-centered catalyst proves fast and efficient

Designer material quickly and efficiently produces formate. This reaction could, one day, be used to turn intermittent solar energy and carbon dioxide into liquid fuel.

Instead of being a reviled “greenhouse gas” product of gasoline engines, coal-fired power plants, and other sources, carbon dioxide could be an important source of fuel. Using solar power to add electrons to carbon dioxide, scientists could create the fuels formate or formic acid, which are also precursors to other fuels like methanol. Creating formate using solar power circumvents the intermittent nature of sunshine as a power source -- the sun goes down at night, allowing the energy gathered to be stored and used on demand. Traditional catalysts, which speed up the rates of reactions, typically produce a lot of carbon monoxide but little formate. UNC scientists devised an iridium-based catalyst that turns carbon dioxide into formate by a fast and efficient process which converts 20 molecules a second with 85% efficiency. Further, the catalyst is specific, resulting in no carbon monoxide production. This research raises the bar in terms of speed and efficiency for formate-creating catalysts. The Solar Fuels Energy Frontier Research Center, led at the University of North Carolina at Chapel Hill, headed this study.

More Information

Kang P, C Cheng, ZF Chen, CK Schauer, TJ Meyer, and M Brookhart. 2012. "Selective Electrocatalytic Reduction of CO2 to Formate by Water-Stable Iridium Dihydride Pincer Complexes." Journal of the American Chemical Society 134(12):5500-5503. DOI: 10.1021/ja300543s

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.