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CO2 is not only a greenhouse gas. Scientists make plastics and gasoline from it. A good catalyst is key to this and new materials are needed.
Riming Wang in his laboratory in front of the gas electrolysis setup. Illustration shows crystal structure of Metal-organic Framework (MOF). (Photo & illustration: Sija van den Beukel)

CO2 is not only a greenhouse gas. Scientists make plastics and gasoline from it. A good catalyst is key to this and new materials are needed.

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Dr. Riming Wang defended his doctoral thesis on January 8that Catalysis Engineering (Applied Sciences) on using a new sort of catalyst for CO2 reduction. The initial idea started with reducing CO2. One way to do this is to initiate a reaction at 1,000 degrees Celsius. This process is called thermal catalysis. Because the high temperature has several disadvantages, Wang opted for a chemical reaction driven by an electric current: electrolysis.

Schematic electrolysis of aqueous solution. (Illustration: Sija van den Beukel)
Schematic electrolysis of aqueous solution. (Illustration: Sija van den Beukel)

Energy storage
The electric current needed for electrolysis could come from solar and wind energy. Because the amount of sun and wind changes, there is an energy shortage one moment and a surplus the next. Good batteries to store this energy are not yet available. However, a surplus of energy could be used directly to reduce CO2 through electrolysis. CO2 reduction to be used as a form of energy storage. To do this efficiently, CO2 gasses from industry are used as they contain more CO2 than the air at only 0.04 percent.

A soccer field on a teaspoon
For electrolysis, the catalyst - the compound that can speed up a reaction without being consumed itself - is crucial. Wang obtained his doctorate for using a new synthetic material as a catalyst in electrochemical CO2 reduction.

This new material is an artificial crystalline substance that contains metals bound together by organic linkers called Metal-organic framework (MOF). The way in which the metals and organic molecules are organized creates an open crystal structure, a kind of sponge through which liquids and gases can easily pass. Due to this open crystal structure, the internal surface of a teaspoon of this material (1 gramme) is larger than the surface area of a football field. This capacity has great potential for many applications. Also, it makes MOFs good catalysts.

In his speech at TU Delft’s birthday last Friday, the Rector Tim van der Hagen, Chair of TU Delft, mentioned that material sciences are key for energy transition. He explained that: “The success of the energy transition will rely on new ways to convert, store and transport energy. We need advanced materials if we are to achieve it.”

Lego
Three years of Wang’s PhD was spent on designing the catalyst, the bottleneck in CO2 reduction projects. MOFs are highly designable. Comparable to Lego, they can be used to build lots of structures. Wang tried various metal-organic linker combinations for MOFs and discovered several efficient catalysts for CO and ethylene (C2H4) production out of CO2.

Schematic gas electrolysis. (Illustration: Sija van den Beukel)
Schematic gas electrolysis. (Illustration: Sija van den Beukel)

Gas electrolysis
After optimising the catalyst, Wang realised that the bigger picture - how the catalyst combines with the electrolysis - is also quite important. As CO2 is the most stable carbon that contains molecules, it is hard to reduce. Also, it has a low solubility in an aqueous phase. So for aqueous electrolysis the efficiency was low compared to thermal catalysis. Wang explains that “Only when we started using a pure gas electrolysis did we make a huge advance. The efficiencies are now getting close to thermal catalysis.”

Gasoline
Wang produces two products from CO2. “In the first part of my project, we reduced CO2 to CO. Then we also produced ethylene. This is a building block for polymers, in other words plastic.” Formic acid is also a promising product of CO2 reduction, although not included in Wang’s project. The formic acid from CO2 reduction could be economically competitive with the petrochemical industry. Formic acid is widely used for pharmaceuticals. Wang continues, “From CO and hydrogen we can easily make gasoline. In the Second World War, gasoline was already produced this way.” If you burn the gasoline, it will release CO2. “But as long as we can reduce CO2 again, the cycle will be complete.”

Wang just left for China where he will work on electronic material development at research institute, Shenzhen Institute of Advanced Technology.

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