Copper foam catalyst yields different product slate from CO2 than smooth electrodes; importance of catalyst architecture
A catalyst made from a foamy form of copper has different electrochemical properties from catalysts made with smooth copper in reactions involving carbon dioxide, according to a new study by a team from Brown University. The research, reported in the journal ACS Catalysis, suggests that copper foams could provide a new way of converting excess CO2 into useful industrial chemicals.
The researchers showed that the electrochemical reduction of CO2 at copper foams yields formic acid at a lower onset potential with faradaic efficiencies that are 10−20% higher than other reported values. In comparison to smooth copper electrodes, the faradaic efficiencies of CO, methane, and ethylene are reduced significantly, whereas C2 and C3 products such as ethane and propylene are produced in small but detectable quantities—overall, a very different product outcome than obtained from planar electrodes.
The Brown team attributed the differences to high surface roughness, hierarchical porosity, and confinement of reactive species in the foam.
Electrochemical reduction of CO2 has been investigated at a variety of metallic electrodes, and a number of reports and reviews have been published on this subject. Among the metals studied, copper generates significant quantities of hydrocarbons such as methane and ethylene in aqueous media. Hori et al. conducted extensive studies on the electrochemical reduction of CO2 and CO at copper electrodes and concluded that the product distribution reflected a sensitivity of adsorbed hydrogen species to the underlying structure of the copper electrode and that “surface roughening likely introduced surface defects such as steps and vacancies that are favorable for reaction of adsorbed hydrogen atoms”.
… Recent work that describes a novel approach to the fabrication of metal foams with hierarchical porosity provides an excellent opportunity for testing the effect of three-dimensional nanostructured metal surfaces and their corresponding cavities on the products produced during the electrochemical reduction of CO2.—Sen et al.
Copper foam, which has been developed only in the last few years, provided the surface roughness that Tayhas Palmore, professor of engineering and senior author, and her colleagues were looking for. The foams are made by depositing copper on a surface in the presence of hydrogen and a strong electric current. Hydrogen bubbles cause the copper to be deposited in an arrangement of sponge-like pores and channels of varying sizes.
After depositing copper foams on an electrode, the researchers set up experiments to see what kinds of products would be produced in an electrochemical reaction with CO2 in water.
The experiments showed that the copper foam converted CO2 into formic acid—a compound often used as a feedstock for microbes that produce biofuels—at a much greater efficiency than planar copper. The reaction also produced small amounts of propylene, a useful hydrocarbon that’s never been reported before in reactions involving copper.
The product distribution was unique and very different from what had been reported with planar electrodes, which was a surprise. We’ve identified another parameter to consider in the electroreduction of CO2. It’s not just the kind of metal that’s responsible for the direction this chemistry goes, but also the architecture of the catalyst.—Tayhas Palmore
Now that it’s clear that architecture matters, Palmore and her colleagues are working to see what happens when that architecture is tweaked. It’s likely, she says, that pores of different depths or diameters will produce different compounds from a CO2 feedstock. Ultimately, it might be possible to tune the copper foam toward a specific desired compound.
The work in the study is part of a larger effort by Brown’s Center for the Capture and Conversion of CO2. The Center, funded by the National Science Foundation, is exploring a variety of catalysts that can convert CO2 into usable forms of carbon.
The Center for Capture and Conversion of CO2 is a Center for Chemical Innovation funded by the National Science Foundation (CHE-1240020).
Sujat Sen, Dan Liu, and G. Tayhas R. Palmore (2014) “Electrochemical Reduction of CO2 at Copper Nanofoams,” ACS Catal., 4, pp 3091–3095 doi: 10.1021/cs500522