Surface activation of cobalt oxide nanoparticles for photocatalytic carbon dioxide reduction to methane

Climate change, global warming and energy crisis due to the rapid increase in the global carbon footprint are among the main challenges facing our globe. So far, various mitigation and regulation measures have been enacted to reduce the emission of greenhouse gases. Particularly, the conversion of carbon dioxide into other valuable chemicals has attracted significant attention from researchers.

Among the available methods for carbon dioxide conversion, the photochemical approach is highly preferred considering its ability to effectively utilize pollutant-free renewable and solar energy. On the other hand, metal oxide semiconductors are preferred as photocatalytic materials owing to their excellent properties. In particular, cobalt oxide is expected to exhibit superior catalytic activities due to suitable band gap and carbon dioxide adsorption capacity, which makes it suitable for photocatalytic reduction of carbon dioxide. However, recent attempts have not produced the desired results. To overcome its low activity in various reactions, the surface activation of nanoparticles by appropriate surface treatments can be one of the solutions.

In a recent paper published in Journal of Materials Chemistry A, Dr. Ji Yong Choi (Postdoctoral fellow), Chan Kyu Lim, Bumjin Park, Minjun Kim and led by Professor Hyunjoon Song from Korea Advanced Institute of Science and Technology in collaboration with Aqil Jamal from Saudi Aramco investigated the feasibility of using cobalt oxide nanoparticle in the photocatalytic reduction of carbon dioxide. Specifically, the cobalt oxide nanoparticles were synthesized through thermal decomposition of cobalt acetylacetonate and oleylamine. Subsequently, the obtained cobalt oxide nanoparticles were subjected to surface treatment with N-bromo-succinimide to enhance its properties.

Through surface activation, cobalt oxide nanoparticles enhanced the photocatalytic reduction of carbon dioxide with remarkably high efficiency and selectivity in aqueous solution, which methane was formed as major product with 0.94% of quantum efficiency and 98% of selectivity and carbon monoxide and hydrogen were generated as minor products. This was attributed to the formation of Co₃O₄, surfactant removal, and bromine coordination on the surface, which the synthesized catalysts generally outperformed the previously used catalysts in an aqueous medium. Additionally, the photocatalysts showed high stability considering that the particle morphology and mixture composition of CoO and Co₃O₄ remained unchanged.

The surface activation strategy was extended to modify cobalt oxide nanoparticles using sulfur. Sulfur increases carbon dioxide binding on the surface and cobalt sulfide can be worked as cocatalyst. After treatment with N-bromo-succinimide under the same condition, the sulfur doped cobalt oxide nanocatalyst exhibited a remarkable activity with a quantum efficiency of 2.3%. It is expected that this surface modification approach reported by Professor Hyunjoon Song and his research team can be extended to other metal oxide semiconductors, and this would be a key to enhancing photocatalytic performance in various reactions.