Use of copper nanofoams to electrochemically reduce carbon dioxide

Dr. Neil Canter, Contributing Editor | TLT Tech Beat November 2014

A new catalyst converts carbon dioxide to a hydrocarbon derivative not previously seen with other copper species.

KEY CONCEPTS
• A new three-dimensional copper nanofoam catalyst converts carbon dioxide to formic acid at higher efficiencies than previously seen.
• The nanofoam contains a series of hierarchical cavities that have pore diameters ranging from 20 to 50 microns.
• Besides the formation of formic acid, simple hydrocarbons were formed including propylene, which had not previously been detected in the electrochemical reduction of carbon dioxide with copper.

THE GROWING PRESENCE OF CARBON DIOXIDE IN OUR ENVIRONMENT is leading to many different approaches to find ways to convert it to useful derivatives such as hydrogen and simple hydrocarbons. These derivatives can be used as building blocks to manufacture fuels and more complex organic compounds used in manufacturing.

In a previous TLT article, a new approach was provided for more efficiently converting carbon dioxide to carbon monoxide in a process known as the Boudouard reaction (1). Microwave radiation was found to reduce the reaction temperature from 700 C to 400 C.

Metal foams are of interest to researchers because they retain the excellent mechanical properties of metals but are less dense on a per volume basis. The foam contains a porous structure that has a metal present with a large volume fraction of gas-filled pores. G. Tayhas Palmore, professor of engineering and chemistry at Brown University in Providence, R.I., says, “Metal foams are porous networks of cavities within which electrochemical reactions can occur.”

A previous TLT article discussed the development of polycrystalline metal foams based on a combination of nickel, manganese and gallium (2). These metal foams change shape when a magnetic field is applied and have been characterized as smart foams.

Copper is of interest as a catalyst in the electrochemical reduction of carbon dioxide because the resulting products formed in an aqueous environment include hydrocarbons such as methane and ethylene. Nanoparticles and nanoporous electrode surfaces of copper have been found to be more effective catalysts in generating hydrocarbons. Palmore explains, “Nanoparticles have more step sites and are not just the flat planes seen with macroscopic copper. These step sites create more roughness on the surface, leading to greater reactivity.”

Research has now shown that a new copper catalyst can more efficiently convert carbon dioxide to useful derivatives through electrochemical reduction.

HIERARCHICAL CAVITIES
Palmore and her associates Sujat Sen and Dan Liu have prepared three-dimensional copper nanofoams that produce formic acid from carbon dioxide at higher levels than previously seen. Methane and ethylene were also detected at lower percentages along with propylene, a product of carbon dioxide reduction not previously seen with copper.

The researchers prepared copper nanofoams through electrodeposition of copper on a mechanically polished copper substrate. This reaction is conducted by applying a current in excess of 0.5 Amperes per square centimeter to a solution of copper sulfate and sulfuric acid. Reduction of the copper salts lead to the formation of the metal in combination with evolution of hydrogen gas. The resulting nanofoam contains copper pores and hydrogen bubbles.

In Figure 3, the top scanning electron microscope image shows the nanofoam 60 seconds after the electrodeposition was completed. The bottom image shows the nanostructure of the electrodeposited foam.


Figure 3. The top scanning electron microscope images shows the formation of a three-dimensional copper nanofoam catalyst 60 seconds after the electrodeposition process is completed. The nanostructure of the electrodeposited foam is shown in the bottom image. (Courtesy of Brown University)

Palmore says, “The copper nanofoam formed is a series of hierarchical cavities with pore diameters ranging from 20 to 50 microns. You can think of this structure as a kind of cone with the pour size decreasing as you move deeper into the three-dimensional structure.”

The researchers used faradaic efficiency to evaluate the effectiveness of the copper nanofoams. Palmore says, “Faradaic efficiency is a measure of how many electrons are converted into chemical bonds. This term can be thought of as a return-on-investment on the potential applied.”

A second parameter that is equally important to efficiency is selectivity. Side reactions such as the reduction of water to form hydrogen gas need to be minimized, according to Palmore.

One of the biggest surprises seen by the researchers was the selective formation of formic acid. At an applied potential of -1.5 volts, a maximum faradaic efficiency of 37 percent was obtained for formic acid, the highest value ever seen for the reaction at ambient pressure.

Palmore says, “For an unknown reason, the pathway to formic acid was favored, while the reduction of carbon dioxide to hydrocarbons was virtually shut down. We believe that foam thickness and reaction kinetics are two reasons why formic acid formation was favored.”

Propylene formation was a second surprise for the researchers. Palmore says, “We are not sure how propylene formed but believe it may be due to longer residence times of adsorbed carbon intermediates in the nanoscale cavities of the foam, which then react with other adsorbed carbon intermediates to yield multicarbon species such as propylene.”

The fundamental mechanisms for how these products are formed are not known at this point. A color change seen in the nanofoam over time is due to the oxidation of copper through reaction with the oxygen in the air. This does not appear to impact the short-term effectiveness of the copper nanofoam, though long-term stability of the nanofoam and durability of the catalyst are two factors that will need to be studied in the future.

Additional information on this work can be found in a recent paper (3), as well as reviewing the research underway in Palmore’s group at the following Website: click here or by contacting Palmore at Tahyas_Palmore@brown.edu.

REFERENCES
1. Canter. N. (2014), “Use of microwaves to prepare carbon monoxide,” TLT, 70 (3), pp. 10-11.
2. Canter, N. (2010), “Smart metal foam,” TLT, 66 (3), pp. 12-13.
3. Sen. S., Liu, D. and Palmore, G. (2014), “Electrochemical Reduction of CO₂ at Copper Nanofoams,” ACS Catalysis, 4 (9), pp. 3091-3095.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.