Catalysts are at the heart of many chemical reactions involving carbon dioxide. These catalysts often initiate a process known as chemical reduction that forms either carbon monoxide or formic acid (HCO2H) and can be used to produce fuel. The only problem is that they often lack the activity and selectivity to make suitable fuels, such as methanol, ethanol or methane.
Now (2020), scientists at the Argonne National Laboratory, in collaboration with Northern Illinois University, have designed a new electrocatalyst that can turn carbon dioxide (CO2) and water into ethanol with very high energy efficiency, high selectivity and low cost. Ethanol is widely used in many fuels and also as an intermediate product in the chemical industries. This new process is able to electrochemically convert the CO2 released during industrial processes, such as fossil-fuel production, into valuable end products at reasonable cost. The catalyst consists of atomically dispersed copper on a carbon-powder support. By an electrochemical reaction, it breaks down CO2 and water molecules and selectively reassembles the broken molecules into ethanol under an external electric field.
The discovery was made possible using a high photon flux of the X-ray beams, which allowed the scientists to analyse the reversible transformation from atomically dispersed copper to clusters of three copper atoms each on application of a low voltage. The CO2-to-ethanol catalysis occurred on these tiny copper clusters. This finding might prove an important step towards further improving the catalyst through rational design.
Scientists have long sought to utilize the CO2 emitted during industrial processes and turn it into useful products. In 2017, a membrane was developed, made of a compound of lanthanum, calcium and iron oxide, which allowed oxygen from a stream of carbon dioxide to migrate through to the other side, leaving carbon monoxide behind. The membrane, with a structure known as perovskite, was 100% selective for oxygen. The separation took place under temperatures of up to 990 degrees Celsius. Key to the process was to keep the oxygen that separated from carbon dioxide flowing through the membrane until it reached the other side. To this end, the researchers used a stream of fuel such as hydrogen or methane as these materials are easily oxidized and can transport the oxygen atoms through the membrane without needing a pressure difference. The membrane also prevented the oxygen from migrating back and recombining with the carbon monoxide. Finally, a combination of vacuum and fuel was used to reduce the energy required to drive the process and produce a useful product. The energy input needed to keep the process going was heat.
In 2019, engineers at Rice University in Houston, Texas, came up with a process to easily generate formic acid and to do away with some of the more involved steps, making the process far more efficient. They replaced the electrolyte with a solid matrix made of insoluble polymers or inorganic compounds. They also found a robust catalyst to speed up the conversion process: bismuth. Bismuth is bulkier than other metals capable of the same task, and therefore it cannot move about as easily. The resulting device was engineered to channel the carbon dioxide through the catalyst where it transformed into a negatively charged molecule called formate. From there it diffused into the solid electrolyte core where it combined with hydrogen ions released from a second catalytic reaction with water, resulting in a highly concentrated solution of formic acid.
The advantages of the new catalyst are that the electrochemical process of CO2-to-ethanol conversion could be coupled to the electric grid and the low-cost electricity available from renewable sources such as solar and wind could be used efficiently during off-peak hours. Because the process runs at low temperature and pressure it can start and stop rapidly in response to the intermittent supply of the renewable electricity. The electrocatalytic selectivity, or Faradaic efficiency, of the process is over 90 percent, much higher than any other reported process. What is more, the catalyst operates stably over extended operation at low voltage.
So far, the scientists have prepared several new catalysts using this approach and found that they are all highly efficient in converting CO2 to other hydrocarbons. They plan to continue this research in collaboration with industry to advance this promising technology.