Stanford researchers develop copper-based catalyst that produces ethanol from CO at room temperature; potential for closed-loop CO2-to-fuel process
11 April 2014
Researchers at Stanford University have developed a nanocrystalline copper material that produces multi-carbon oxygenates (ethanol, acetate and n-propanol) with up to 57% Faraday efficiency at modest potentials (–0.25 volts to –0.5 volts versus the reversible hydrogen electrode) in CO-saturated alkaline water.
The material’s selectivity for oxygenates, with ethanol as the major product, demonstrates the feasibility of a two-step conversion of CO2 to liquid fuel that could be powered by renewable electricity, the team suggests in their paper published in the journal Nature. Ultimately, this might enable a closed-loop, emissions free CO2-to-fuel process.
We have discovered the first metal catalyst that can produce appreciable amounts of ethanol from carbon monoxide at room temperature and pressure—a notoriously difficult electrochemical reaction.—Matthew Kanan, an assistant professor of chemistry at Stanford and coauthor of the Nature study
Two years ago, Kanan and Stanford graduate student Christina Li created a novel oxide-derived copper electrode material. While conventional copper electrodes consist of individual nanoparticles that just sit on top of each other, oxide-derived copper, is made of copper nanocrystals that are all linked together in a continuous network with well-defined grain boundaries. The process of transforming copper oxide into metallic copper creates the network of nanocrystals, Kanan explained.
For the Nature study, Kanan and Li built an electrochemical cell: two electrodes placed in water saturated with carbon monoxide gas. When a voltage is applied across the electrodes of a conventional cell, a current flows and water is converted to oxygen gas at one electrode (the anode) and hydrogen gas at the other electrode (the cathode). The challenge was to find a cathode that would reduce carbon monoxide to ethanol instead of reducing water to hydrogen.
The electrochemical conversion of CO2 and H2O into liquid fuel is ideal for high-density renewable energy storage and could provide an incentive for CO2 capture. However, efficient electrocatalysts for reducing CO2 and its derivatives into a desirable fuel are not available at present. Although many catalysts can reduce CO2 to carbon monoxide (CO), liquid fuel synthesis requires that CO is reduced further, using H2O as a H+ source. Copper (Cu) is the only known material with an appreciable CO electroreduction activity, but in bulk form its efficiency and selectivity for liquid fuel are far too low for practical use. In particular, H2O reduction to H2 outcompetes CO reduction on Cu electrodes unless extreme overpotentials are applied, at which point gaseous hydrocarbons are the major CO reduction products.—Li et al.
In the Nature experiment, Kanan and Li used a cathode made of oxide-derived copper, with the resulting high yield of ethanol and acetate at 57% Faradaic efficiency (i.e., 57% of the electric current went into producing these two compounds from carbon monoxide). By comparison, conventional Cu nanoparticles with an average crystallite size similar to that of oxide-derived copper produced nearly exclusive H2 (96% Faraday efficiency) under identical conditions.
The researchers attributed the ability to change the intrinsic catalytic properties of Cu for this notoriously difficult reaction to the growth of the interconnected nanocrystallites from the constrained environment of the oxide lattice.
The Stanford team has begun looking for ways to create other fuels and improve the overall efficiency of the process.
In this experiment, ethanol was the major product. Propanol would actually be a higher energy-density fuel than ethanol, but right now there is no efficient way to produce it.—Matthew Kanan
In the experiment, Kanan and Li found that a slightly altered oxide-derived copper catalyst produced propanol with 10% efficiency. The team is working to improve the yield for propanol by further tuning the catalyst’s structure. Ultimately, Kanan would like to see a scaled-up version of the catalytic cell powered by electricity from the sun, wind or other renewable resource.
For the process to be carbon neutral, scientists will have to find a new way to make carbon monoxide from renewable energy instead of fossil fuel, the primary source today. Kanan envisions taking carbon dioxide (CO2) from the atmosphere to produce carbon monoxide, which, in turn, would be fed to a copper catalyst to make liquid fuel. The CO2 that is released into the atmosphere during fuel combustion would be re-used to make more carbon monoxide and more fuel—a closed-loop, emissions-free process.
Technology already exists for converting CO2 to carbon monoxide, but the missing piece was the efficient conversion of carbon monoxide to a useful fuel that's liquid, easy to store and nontoxic. Prior to our study, there was a sense that no catalyst could efficiently reduce carbon monoxide to a liquid. We have a solution to this problem that’s made of copper, which is cheap and abundant. We hope our results inspire other people to work on our system or develop a new catalyst that converts carbon monoxide to fuel.—Matthew Kanan
The Nature study was coauthored by Jim Ciston, a senior staff scientist with the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory.
The research was supported by Stanford University, the National Science Foundation and the US Department of Energy.
Christina W. Li, Jim Ciston & Matthew W. Kanan (2014) “Electroreduction of carbon monoxide to liquid fuel on oxide-derived nanocrystalline copper,” Nature doi: 10.1038/nature13249
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