Nanodiamonds could turn methanol into an industrial raw material

Rather than being released into the atmosphere and exacerbating the problem of climate change, CO2 can also be used as a raw material for substances required in industrial processes, such as formic acid or methanol.

To do so, scientists have developed a process that relies on using nanodiamonds as a catalyst and irradiating them with short-wave UV-C light in a liquid environment.

The diamonds used in this research are not the jewellery-grade kind. Instead, the team used what is known as a "detonation diamond", which is produced on an industrial scale and is therefore relatively inexpensive as a catalyst.

As diamonds are largely made of carbon, the material can be considered a "green" catalyst, the team said. 

"Up to now, the experiments have been carried out in a batch reactor; i.e. a stirred flask. There are certain disadvantages to this method," says Thomas Rehm, one of the scientists at Fraunhofer IMM.

"Firstly, the contacting between the gas and liquid phase and the catalyst is less than ideal; secondly, the catalyst - i.e. the nanoparticles that are floating around - needs to be separated from the solution after the reaction."

The process the scientists have developed until now relies on placing all the components in a flask, but the team is now looking at ways that the reaction can be done in larger areas, such as reaction plates measuring around 5cm by 9cm.

To this end, the researchers have developed a microreactor with an upright standing reaction plate which features microchannels coated with the diamond catalyst. At the top of the plate is a slit into which water is constantly being pumped and then runs through the plate. 

Due to the capillary forces at play, the process results in a liquid film with a thickness of 10 to 50 micrometres, which constantly coats the microchannels. The CO2 is then directed over the reaction plate from below in a counterflow configuration.

"In this way, we can apply much higher quantities of carbon dioxide directly to the catalyst film and in a smaller volume of solution. This improves the gas-liquid-solid contacting, which can result in higher CO2 conversion and hence a larger quantity of formic acid," says Rehm. 

In addition, the researchers are no longer using energy-intensive UV-C light - as in the case of the nanoscale catalyst - and are instead using visible light which is more inexpensive and easier to handle.

This process has, however, required a modification to the diamond surface as it needs to capture visible light but still trigger the same reaction as the nanoscale diamond.

"What we have here is a light-powered electron pump," confirms Rehm. In order to supply more electrons, the team can apply a low electrical voltage to the diamond surface.

One aspect that the research team is still working on is the low contact time. The CO2, water and diamond layer currently only have 10 to 15 seconds for the reaction - not enough time to produce the amount of formic acid required for real-world applications.

To solve this issue, researchers are looking at two solutions: more efficient metal complexes in order to increase the reaction speed, and adapting the reactor to enable longer contact times.