With the help of the right catalyst, carbon dioxide emitted by fossil-fuel power stations could be used as a chemical feedstock, rather than contributing to greenhouse gas emissions. Researchers have now found that tiny clusters of copper atoms can generate methanol from CO2 at an unusually low pressure (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b03668).
Copper is already used to catalyze industrial methanol production from syngas—a mixture of carbon monoxide, CO2, and hydrogen—at pressures of 10 to 100 atm and temperatures of a few hundred degrees C. But these conditions make the process energy-intensive and costly. “We wanted to use lower pressure to save energy,” says Larry A. Curtiss of Argonne National Laboratory.
Curtiss—working with Peter Zapol, Stefan Vajda, and colleagues—calculated that copper clusters containing four atoms would offer higher catalytic activity than larger clusters or copper surfaces. The smaller clusters have more coordination sites available to bind reaction intermediates, offering a lower-energy reaction pathway.
The team peppered a thin film of alumina with Cu4+ clusters, and then monitored reactions over this catalyst in a gas stream containing 1% CO2, 3% H2, and 96% helium at an overall pressure of 1.25 atm. As they increased the temperature to 125 ºC, the H2 reduced the copper clusters to catalytically active Cu40. Methanol production peaked at 225 ºC, giving the highest reported activity for CO2 reduction to methanol at such a low CO2 partial pressure, a pressure around one-twentieth of that in work with previous catalysts.
Although promising, it remains to be seen whether this would be a chemically—or economically—viable way of utilizing CO2 waste from power plants. For now, Curtiss and his colleagues are looking for clusters that produce longer-chain hydrocarbons from CO2, potentially offering a renewable source of liquid fuels. Preliminary calculations suggest that a Cu20 cluster might be ideal, he says.