Methane evolution from blue titania decorated with Pt nanoparticles.
Methane evolution from blue titania decorated with Pt nanoparticles.

Turning carbon dioxide (CO2) into usable fuel could provide both a sustainable energy source and reduce atmospheric CO2. Now researchers from DGIST in Korea have developed a light-activated catalyst based on titanium dioxide (TiO2) that could make the process accessible on a large scale [Sorcar et al., Materials Today (2017), doi: 10.1016/j.mattod.2017.09.005].

“We have developed a titanium dioxide (TiO2) based photocatalyst that can efficiently convert carbon dioxide into methane,” explains Su-Il In of DGIST. “Our main motivation was to utilize excess carbon dioxide in the atmosphere and convert it into usable fuel.”

The photocatalyst consists of nanoparticles of blue titania, which is fabricated by exposing TiO2 to NaBH4 at low temperatures (350°C) for half an hour. This process is a significant improvement on previous methods of reducing TiO2 nanoparticles, which require harsh conditions and high temperatures.

When sensitized with Pt nanoparticles, the photocatalyst promotes light-activated photoconversion of CO2 into methane (CH4). The conversion is highly efficient, points out In, reaching a quantum yield of 12.4%, which the researchers believe is a record for photocatalytic-based CO2 reduction.

The improvement in yield is down to changes in surface structure that are produced during the fabrication process. TiO2 nanoparticles are a large bandgap semiconductor, which limits absorption to the ultraviolet region of the solar light spectrum. But by introducing defects into the titania nanoparticles, via the reduction process, the band gap can be modified to improve the catalyst’s light absorption properties.

“With the addition of a very small amount of the noble metal Pt, the photocatalyst shows excellent and stable solar light driven CO2 photoreduction into methane,” says In.

The researchers believe the reduction process creates a disordered shell on the outside of the nanoparticles. Along with the presence of Ti3+ ions, the combination shifts the valence band edge upwards and the conduction band downwards to reduce the overall bandgap. This bandgap engineering enhances the absorption of the visible portion of the light spectrum.

But despite the promising results for the reduced TiO2 photocatalyst, stability is a major concern for this application.

“Although the synthesized photocatalyst showed stable performance for 30 hours, right now we are still focusing on further enhancing the stability,” In told Materials Today.

The team would also like to replace the Pt nanoparticles with a cheaper co-catalyst to reduce costs further and make the photocatalyst more commercially viable. Modifying the blue titania nanoparticles with other co-catalyst species could also yield other hydrocarbons such as ethane.

“Photocatalytic conversion of CO2 to higher hydrocarbons is quite challenging because of back reactions and product selectivity,” cautions In.