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Tailored Catalysts for fuel production from CO2

Source: Jynto, CC0

Electrochemical reduction of carbon dioxide is one of the possibilities to produce fuels or other chemicals by converting CO2 into organic feedstocks, including formic acid, carbon monoxide, methane or ethanol. To do so, various catalysts are used depending on the desired resulting product. Some of the more effective metallic catalysts for this process are made of tin, silver or copper.

The process was first applied in the 19th century when scientists reduced CO2 to CO with the help of a zinc cathode. In the 1980s this field was afforded renewed scientific attention and until now several companies have already developed pilot carbon dioxide electrochemical reduction processes, including Siemens. For example, this is reflected in the patent EP3307924B1 issued by Siemens AG in 2019:

Method for preparing hydrocarbon-selective gas diffusion electrodes based on copper-containing catalysts

Image: US20180230612A1

Now (2021), a group of scientists has found a method to enhance copper electrocatalysts for multi-carbon production by means of improving the microenvironment close to the catalyst surface. In their previous research, the scientists had already discovered which chemical and electrical environments were best for the desired carbon-rich products. Building on their gained knowledge, they now used thin layers of ionomers for their catalyst design which consisted of polymers that allowed certain ions to pass through while blocking others. This highly selective chemistry of ionomers could strongly influence the microenvironment in a catalyst. The two common ionomers the scientists used for the cooper catalyst were Nafion and Sustainion. These ionomers, the scientist assumed, would manipulate the environment in the vicinity of the catalyst to enable production of carbon-rich products which could then be converted into chemicals and fuels.

A thin layer of each ionomer was applied to copper films supported by a polymer material and formed membranes that could be inserted at one side of the electrochemical cell. When the CO2 was fed into the cell and a voltage applied, the total current was seen flowing through the cell. Then the gases and liquids that were collected in adjoining reservoirs during the reaction were also measured. In the two-layer coating, carbon-rich products accounted for 80% of the energy consumed by the reaction as opposed to 60% when using the uncoated catalyst. This was due to the high CO2 concentration accumulating in the coating layer on top of the copper. Also, negatively charged molecules assembling in the region between the two ionomers created a low local acidity. In order to further increase the reaction efficiency, the scientists used a technique called pulsing the voltage. This enabled them to achieve a 250% increase in carbon-rich products.

For many years, scientists have tried to convert CO2 into environmentally-friendly and sustainable fuels. In 2020, a new iron-based catalyst was created which could convert carbon dioxide into jet fuel. The catalyst was tested on carbon dioxide in a small reaction chamber which was operating at 300° Celsius and under a pressure of 10 times the air pressure at sea level. In the course of 20 hours, 38 percent of the carbon dioxide in the chamber were transformed into new chemical products. About 48 percent of those products were jet fuel hydrocarbons. Other by-products included similar petrochemicals, such as ethylene and propylene, which can be used to make plastics.

In 2021, a team of scientists designed a new catalyst which can work with either heat or electricity. It was based on nickel atoms and could speed up the reaction for turning carbon dioxide into carbon monoxide. For their experiments, the team chose a catalyst called NiPACN. The active parts of the catalyst consisted of individual nickel atoms with nitrogen atoms that were distributed throughout the carbon material. To prove the activity of the catalyst, they took the powdered catalyst to SLAC's Stanford Synchrotron Radiation Lightsource (SSRL) and set up a small reactor where the catalyst could start a reaction between hydrogen and carbon dioxide at high temperatures and pressure. A small window into the reactor was installed which enabled them to shine X-rays into the reaction through a window and watch the reaction proceed.

There are several advantages to using tailored catalysts: the sandwich coating of this particular catalyst can achieve high product selectivity and high activity. The bi-layered surface can produce not only carbon-rich products, but at the same time create a strong electrical current. Also, the unique method of controlling the environment at the surface of the catalyst might open up new possibilities of catalyst production, as only the kinetics of a reaction are used to change the environment at the catalyst site.

The second phase in the research will be to increase the catalyst production to make them suitable for commercial use. This will involve the challenging task of having to coat large amounts of small copper spheres. Also, there is the necessity of adding a second coating over the first one which makes this process even more difficult. If the scientists succeed, this will give society a cheap and efficient catalyst that can be used for different applications, including energy production.